Energy, Environment, and Sustainability

Series Editor: Avinash Kumar Agarwal

Himanshu Tyagi
Prodyut R. Chakraborty
Satvasheel Powar
Avinash Kumar Agarwal Editors

Solar Energy
Systems, Challenges, and Opportunities
Energy, Environment, and Sustainability

Series Editor
Avinash Kumar Agarwal, Department of Mechanical Engineering, Indian Institute
of Technology Kanpur, Kanpur, Uttar Pradesh, India
This books series publishes cutting edge monographs and professional books
focused on all aspects of energy and environmental sustainability, especially as it
relates to energy concerns. The Series is published in partnership with the
International Society for Energy, Environment, and Sustainability. The books in
these series are edited or authored by top researchers and professional across the
globe. The series aims at publishing state-of-the-art research and development in
areas including, but not limited to:
• Renewable Energy
• Alternative Fuels
• Engines and Locomotives
• Combustion and Propulsion
• Fossil Fuels
• Carbon Capture
• Control and Automation for Energy
• Environmental Pollution
• Waste Management
• Transportation Sustainability

More information about this series at http://www.springer.com/series/15901

Himanshu Tyagi Prodyut R. Chakraborty
• •

Satvasheel Powar Avinash Kumar Agarwal

•

Editors

Solar Energy
Systems, Challenges, and Opportunities

123
Editors
Himanshu Tyagi Prodyut R. Chakraborty
Department of Mechanical Engineering Department of Mechanical Engineering
Indian Institute of Technology Ropar Indian Institute of Technology Jodhpur
Rupnagar, India Jodhpur, India

Satvasheel Powar Avinash Kumar Agarwal

School of Engineering Department of Mechanical Engineering
Indian Institute of Technology Mandi Indian Institute of Technology Kanpur
Mandi, India Kanpur, India

ISSN 2522-8366 ISSN 2522-8374 (electronic)

Energy, Environment, and Sustainability
ISBN 978-981-15-0674-1 ISBN 978-981-15-0675-8 (eBook)
https://doi.org/10.1007/978-981-15-0675-8
© Springer Nature Singapore Pte Ltd. 2020
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Preface

Energy demand has been rising remarkably due to increasing population and
urbanization. Global economy and society are significantly dependent on the energy
availability because it touches every facet of human life and activities.
Transportation and power generation are two major examples. Without the trans-
portation by millions of personalized and mass transport vehicles and availability of
24  7 power, human civilization would not have reached contemporary living
standards.
The International Society for Energy, Environment and Sustainability (ISEES)
was founded at Indian Institute of Technology Kanpur (IIT Kanpur), India, in
January 2014, with an aim to spread knowledge/awareness and catalyze research
activities in the fields of energy, environment, sustainability, and combustion. The
Society’s goal is to contribute to the development of clean, affordable, and secure
energy resources and a sustainable environment for the society and to spread
knowledge in the above-mentioned areas and create awareness about the environ-
mental challenges, which the world is facing today. The unique way adopted by the
society was to break the conventional silos of specializations (engineering, science,
environment, agriculture, biotechnology, materials, fuels, etc.) to tackle the prob-
lems related to energy, environment, and sustainability in a holistic manner. This is
quite evident by the participation of experts from all fields to resolve these issues.
The ISEES is involved in various activities such as conducting workshops, semi-
nars, and conferences in the domains of its interests. The society also recognizes the
outstanding works done by the young scientists and engineers for their contribu-
tions in these fields by conferring them awards under various categories.
Third International Conference on “Sustainable Energy and Environmental
Challenges” (III-SEEC) was organized under the auspices of ISEES from December
18 to 21, 2018, at Indian Institute of Technology Roorkee. This conference provided
a platform for discussions between eminent scientists and engineers from various
countries including India, USA, Norway, Finland, Sweden, Malaysia, Austria, Hong
Kong, Bangladesh, and Australia. In this conference, eminent speakers from all over
the world presented their views related to different aspects of energy, combustion,
emissions, and alternative energy resource for sustainable development and cleaner

v
vi Preface

environment. The conference presented five high-voltage plenary talks from globally
renowned experts on topical themes, namely “The Evolution of Laser Ignition Over
more than Four Decades” by Prof. Ernst Wintner, Technical University of Vienna,
Austria; “Transition to Low Carbon Energy Mix for India” by Dr. Bharat Bhargava,
ONGC Energy Center; “Energy Future of India” by Dr. Vijay Kumar Saraswat, Hon.
Member (S&T) NITI Aayog, Government of India; “Air Quality Monitoring and
Assessment in India” by Dr. Gurfan Beig, Safar; and “Managing Large Technical
Institutions and Assessment Criterion for Talent Recruitment and Retention” by
Prof. Ajit Chaturvedi, Director, IIT Roorkee.
The conference included 24 technical sessions on topics related to energy
and environmental sustainability including 5 plenary talks, 27 keynote talks, and
15 invited talks from prominent scientists, in addition to 84 contributed talks and
50 poster presentations by students and researchers. The technical sessions in the
conference included Advances in IC Engines, Solar Energy, Environmental
Biotechnology, Combustion, Environmental Sustainability, Coal and Biomass
Combustion/Gasification, Air and Water Pollution, Biomass to Fuels/Chemicals,
Combustion/Gas Turbines/Fluid Flow/Sprays, Energy and Environmental Sustain-
ability, Atomization and Sprays, Sustainable Transportation and Environmental
Issues, New Concepts in Energy Conservation, and Waste to Wealth. One of the
highlights of the conference was the Rapid-Fire Poster Sessions in (i) Engine/
Fuels/Emissions, (ii) Renewable and Sustainable Energy, and (iii) Biotechnology,
where 50 students participated with great enthusiasm and won many prizes in a
fiercely competitive environment. 200+ participants and speakers attended this
four-day conference, which also hosted Dr. Vijay Kumar Saraswat, Hon. Member
(S&T) NITI Aayog, Government of India, as the chief guest for the book release
ceremony, where 14 ISEES books published by Springer, Singapore, under a
special dedicated series “Energy, environment and sustainability” were released.
This was second time in a row that such significant and high-quality outcome has
been achieved by any society in India. The conference concluded with a panel
discussion on “Challenges, Opportunities and Directions for National Energy
Security,” where the panelists were Prof. Ernst Wintner, Technical University of
Vienna; Prof. Vinod Garg, Central University of Punjab, Bhatinda; Prof. Avinash
Kumar Agarwal, IIT Kanpur; and Dr. Michael Sauer, Boku University of Natural
resources, Austria. The panel discussion was moderated by Prof. Ashok Pandey,
Chairman, ISEES. This conference laid out the road map for technology devel-
opment, opportunities and challenges in energy, environment, and sustainability
domain. All these topics are very relevant to the country and the world in present
context. We acknowledge the support received from various funding agencies and
organizations for the successful conduct of the Third ISEES Conference
(III-SEEC), where these books germinated. We would, therefore, like to
acknowledge NIT Srinagar, Uttarakhand (TEQIP) (special thanks to Prof. S. Soni,
Director, NIT, UK), SERB, Government of India (special thanks to Dr. Rajeev
Sharma, Secretary); UP Bioenergy Development Board, Lucknow (special thanks
to Sh. P. S. Ojha), CSIR, and our publishing partner Springer (special thanks to
Swati Meherishi).
Preface vii

The editors would like to express their sincere gratitude to large number of
authors from all over the world for submitting their high-quality work in a timely
manner and revising it appropriately at a short notice. We would like express our
special thanks to all the reviewers who reviewed various chapters of this mono-
graph and provided their valuable suggestions to improve the manuscripts.

Rupnagar, India Himanshu Tyagi

Jodhpur, India Prodyut R. Chakraborty
Mandi, India Satvasheel Powar
Kanpur, India Avinash Kumar Agarwal
Contents

Part I General
1 Introduction to Solar Energy: Systems, Challenges,
and Opportunities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Himanshu Tyagi, Prodyut R. Chakraborty, Satvasheel Powar
and Avinash Kumar Agarwal
2 Sustainable Development Goals in Context to BRICS
Countries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Bhabajit Baruah and Rakesh Nath
3 Installations of Solar Systems in Remote Areas of Himachal
Pradesh, INDIA: Challenges and Opportunities . . . . . . . . . . . . . . . 23
Santosh B. Bopche
4 Utilising Passive Design Strategies for Analysing Thermal
Comfort Levels Inside an Office Room Using PMV-PPD
Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
Sana Fatima Ali and Dibakar Rakshit

Part II Solar Thermal Systems: Heating

5 Design and Development of a Concentrated Solar Water
Heating System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
Bandi Sai Mukesh, Ravi Teja Parella, Sudipto Mukhopadhyay
and Laltu Chandra
6 Multi-objective Performance Optimization of a Ribbed
Solar Air Heater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
Naveen Sharma and Rajesh Choudhary
7 Mathematical Modelling of Solar Updraft Tower . . . . . . . . . . . . . . 95
K. V. S. Teja, Kapil Garg and Himanshu Tyagi

ix
x Contents

Part III Solar Thermal Systems: Cooling

8 Solar Thermal-Powered Adsorption Chiller . . . . . . . . . . . . . . . . . . 117
Mahbubul Muttakin, Kazuhide Ito and Bidyut Baran Saha
9 TEWI Assessment of Conventional and Solar Powered
Cooling Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
Md. Amirul Islam and Bidyut Baran Saha
10 Thermodynamic Analysis of Activated Carbon–Ethanol
and Zeolite–Water Based Adsorption Cooling Systems . . . . . . . . . . 179
Satish Sangwan and Prodyut R. Chakraborty

Part IV Energy Storage

11 PCM-Metal Foam Composite Systems for Solar Energy Storage . . . 207
Anirban Bhattacharya
12 Direct Photo-Thermal Energy Storage Using Nanoparticles
Laden Phase Change Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235
Deepak Moudgil and Vikrant Khullar
13 Review on PCM Application for Cooling Load Reduction
in Indian Buildings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247
Rajat Saxena, Dibakar Rakshit and S. C. Kaushik
14 Fabrication and Thermal Performance Evaluation of Metastable
Supercooled Liquid PCM Based Heat Pack . . . . . . . . . . . . . . . . . . 277
Rohitash Kumar, Sumita Vyas, Bobin Mondal, Ravindra Kumar
and Ambesh Dixit

Part V Solar Cells

15 Yet to Be Challenged: TiO2 as the Photo-Anode Material
in Dye-Sensitized Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285
Janethri B. Liyanage, Ishanie Rangeeka Perera
and R. J. K. U. Ranatunga
16 p-Type Dye Sensitized Solar Cells: An Overview of Factors
Limiting Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315
Sasanka Peiris, R. J. K. U. Ranatunga and Ishanie Rangeeka Perera
17 Conducting Polymers as Cost Effective Counter Electrode
Material in Dye-Sensitized Solar Cells . . . . . . . . . . . . . . . . . . . . . . 345
Shanal Shalindra Bandara Gunasekera, Ishanie Rangeeka Perera
and Samodha Subhashini Gunathilaka
18 Interfacial Materials for Organic Solar Cells . . . . . . . . . . . . . . . . . 373
Amaresh Mishra
Editors and Contributors

About the Editors

Dr. Himanshu Tyagi is currently working as an

Associate Professor in the School of Mechanical,
Materials and Energy Engineering at IIT Ropar. He
has previously worked at the Steam Turbine Design
Division of Siemens (Germany and India) and at the
Thermal and Fluids Core Competency Group of Intel
Corp (USA). He received his Ph.D. from Arizona State
University, in the field of heat transfer and specifically
looked for the radiative and ignition properties of
nanofluids. He and his co-workers proposed the
concept of direct absorption solar collectors using
nanofluids which won the Best Paper Award at the
ASME Energy Sustainability Conference at Long
Beach, CA. He obtained his master’s degree from
University of Windsor, Canada, and his bachelor’s
from IIT Delhi, in Mechanical Engineering. At present,
he is working to develop nanotechnology-based clean
and sustainable energy sources with a team of several
Ph.D., postgraduate, and undergraduate students.
Among other awards, he has received Summer
Undergraduate Research Award (SURA) from IIT
Delhi, International Graduate Student Scholarship from
University of Windsor Canada, Indo-US Science and
Technology Forum (IUSSTF) grant awarded for orga-
nizing an Indo-US Workshop on ‘Recent Advances in
Micro/Nanoscale Heat Transfer and Applications in
Clean Energy Technologies’ at IIT Ropar.

xi
xii Editors and Contributors

Dr. Prodyut R. Chakraborty is an Assistant

Professor in the Mechanical Engineering, IIT Jodhpur
since February 2013. He received his Bachelor degree
of Mechanical Engineering from the North Bengal
University in 2000, and his M.Sc Engineering in 2004,
and PhD in 2011 both from the Department of
Mechanical Engineering, Indian Institute of Science
Bangalore. Prior to his joining at IIT Jodhpur, he
worked for two years at the Department of Material
Physics in Space in German Aerospace Center
(DLR) Cologne as a postdoctoral research fellow. He
also worked as a Research Analyst at the Applied CFD
Lab, G.E. Global Research Centre Bangalore from
2004 to 2005. His primary area of research is numerical
modeling of alloy solidification, latent heat based
energy storage systems for high temperature applica-
tions, Thermal management and thermal comfort, and
sorption cooling.

Dr. Satvasheel Powar is an Assistant Professor in the

School of Engineering, IIT Mandi since June 2015. He
received his Bachelors (Production Engineering) from
Shivaji University in 2003, and Masters (Mechanical
Engineering) from the Dalarna University, Sweden in
2005. He then worked with Greatcell Solar S.A.,
Switzerland, and G24i, the UK. He received his Ph.D.
in Chemistry/Materials Engineering from the Monash
University, Australia in 2013. Before joining at IIT
Mandi, he worked for two and half years at the
Nanyang Technological University, Singapore as a
postdoctoral research fellow. His primary area of
research is new generation solar photovoltaic and solar
thermal utilization. He was recently awarded the
Bhaskara Advanced Solar Energy fellowship by
Indo-US Science and Technology Forum (IUSSTF) to
visit Lawrence Berkeley National Laboratory,
University of California, Berkeley, USA for three
months.
Editors and Contributors xiii

Prof. Avinash Kumar Agarwal joined IIT Kanpur in

2001. He worked at the Engine Research Center,
University of Wisconsin at Madison, USA as a
Post-Doctoral Fellow (1999–2001). His interests are
IC engines, combustion, alternate and conventional
fuels, lubricating oil tribology, optical diagnostics, laser
ignition, HCCI, emissions and particulate control, and
large bore engines. Prof. Agarwal has published 270+
peer reviewed international journal and conference
papers, 35 edited books, and 63 books chapters. He is
an associate editor of ASME Journal of Energy
Resources Technology, and has edited the Handbook
of Combustion, Wiley VCH, Germany. Prof. Agarwal
is a Fellow of SAE, ASME, NASI, Royal Society of
Chemistry, ISEES, and INAE. He has been the
recipient of several prestigious awards such as
Clarivate Analystics India Citation Award-2017 in
Engineering and Technology, NASI-Reliance
Industries Platinum Jubilee Award-2012; INAE Silver
Jubilee Young Engineer Award-2012; Dr. C. V. Raman
Young Teachers Award: 2011; SAE Ralph R. Teetor
Educational Award -2008; INSA Young Scientist
Award-2007; UICT Young Scientist Award-2007;
INAE Young Engineer Award-2005. Prof. Agarwal
received Prestigious Shanti Swarup Bhatnagar
Award-2016 in Engineering Sciences.

Contributors

Avinash Kumar Agarwal Department of Mechanical Engineering, Indian

Institute of Technology Kanpur, Kanpur, Uttar Pradesh, India
Sana Fatima Ali Centre for Energy Studies, Indian Institute of Technology Delhi,
New Delhi, India
Bhabajit Baruah Department of Mechanical Engineering, Girijananda
Chowdhury Institute of Management and Technology, Guwahati, India
Anirban Bhattacharya School of Mechanical Sciences, IIT Bhubaneswar,
Bhubaneswar, Odisha, India
Santosh B. Bopche Department of Mechanical Engineering, NIT Hamirpur,
Hamirpur, India
Prodyut R. Chakraborty Department of Mechanical Engineering, Indian Institute
of Technology Jodhpur, Jodhpur, Rajasthan, India
xiv Editors and Contributors

Laltu Chandra Department of Mechanical Engineering, IIT BHU, Varanasi, India

Rajesh Choudhary Lovely Professional University, Phagwara, Punjab, India
Ambesh Dixit Department of Physics, Indian Institute of Technology Jodhpur,
Jodhpur, India
Kapil Garg School of Mechanical, Materials and Energy Engineering, Indian
Institute of Technology Ropar, Rupnagar, Punjab, India
Shanal Shalindra Bandara Gunasekera Department of Chemistry, Faculty of
Science, University of Peradeniya, Peradeniya, Sri Lanka
Samodha Subhashini Gunathilaka Department of Chemistry, Faculty of
Science, University of Peradeniya, Peradeniya, Sri Lanka
Md. Amirul Islam Kyushu University Program for Leading Graduate School,
Green Asia Education Center, IGSES, Kyushu University, Fukuoka, Japan;
International Institute for Carbon-Neutral Energy Research (WPI-I²CNER), Kyushu
University, Fukuoka, Japan;
Department of Electronics and Telecommunication Engineering, Bangabandhu
Sheikh Mujibur Rahman Science and Technology University, Gopalganj,
Bangladesh
Kazuhide Ito Interdisciplinary Graduate School of Engineering Sciences, Kyushu
University, Kasuga-shi, Fukuoka, Japan
S. C. Kaushik Centre for Energy Studies, Indian Institute of Technology Delhi,
New Delhi, India
Vikrant Khullar Mechanical Engineering Department, Thapar Institute of
Engineering and Technology Patiala, Patiala, Punjab, India
Ravindra Kumar Defence Laboratory, Jodhpur, India
Rohitash Kumar Defence Laboratory, Jodhpur, India;
Department of Physics, Indian Institute of Technology Jodhpur, Jodhpur, India
Janethri B. Liyanage Department of Chemistry, University of Peradeniya,
Peradeniya, Sri Lanka
Amaresh Mishra School of Chemistry and Nano Research Centre, Sambalpur
University, Sambalpur, India
Bobin Mondal Defence Laboratory, Jodhpur, India
Deepak Moudgil Mechanical Engineering Department, Thapar Institute of
Engineering and Technology Patiala, Patiala, Punjab, India
Bandi Sai Mukesh Department of Mechanical Engineering, Indian Institute of
Technology Jodhpur, Jodhpur, Rajasthan, India
Editors and Contributors xv

Sudipto Mukhopadhyay Department of Mechanical Engineering, Indian Institute

of Technology Jodhpur, Jodhpur, Rajasthan, India
Mahbubul Muttakin International Institute for Carbon-Neutral Energy Research
(WPI-I²CNER), Kyushu University, Nishi-ku, Fukuoka, Japan;
Interdisciplinary Graduate School of Engineering Sciences, Kyushu University,
Kasuga-shi, Fukuoka, Japan
Rakesh Nath Department of Mechanical Engineering, Girijananda Chowdhury
Institute of Management and Technology, Guwahati, India
Ravi Teja Parella Department of Mechanical Engineering, Indian Institute of
Technology Jodhpur, Jodhpur, Rajasthan, India
Sasanka Peiris Sri Lanka Institute of Nanotechnology, Homagama, Sri Lanka
Ishanie Rangeeka Perera Department of Chemistry, Faculty of Science,
Postgraduate Institute of Science, University of Peradeniya, Peradeniya, Sri Lanka
Satvasheel Powar School of Engineering, Indian Institute of Technology Mandi,
Mandi, Himachal Pradesh, India
Dibakar Rakshit Centre for Energy Studies, Indian Institute of Technology Delhi,
New Delhi, India
R. J. K. U. Ranatunga Department of Chemistry, Faculty of Science, Postgraduate
Institute of Chemistry, University of Peradeniya, Peradeniya, Sri Lanka
Bidyut Baran Saha Kyushu University Program for Leading Graduate School,
Green Asia Education Center, IGSES, Kyushu University, Fukuoka, Japan;
International Institute for Carbon-Neutral Energy Research (WPI-I²CNER), Kyushu
University, Fukuoka, Japan;
Mechanical Engineering Department, Kyushu University, Nishi-ku, Fukuoka,
Japan
Satish Sangwan Department of Mechanical Engineering, Indian Institute of
Technology Jodhpur, Jodhpur, Rajasthan, India
Rajat Saxena Centre for Energy Studies, Indian Institute of Technology Delhi,
New Delhi, India
Naveen Sharma DVR & Dr. HS MIC College of Technology, Kanchikacherla,
Andhra Pradesh, India
K. V. S. Teja School of Mechanical, Materials and Energy Engineering, Indian
Institute of Technology Ropar, Rupnagar, Punjab, India
Himanshu Tyagi Department of Mechanical Engineering, School of Mechanical,
Materials and Energy Engineering, Indian Institute of Technology Ropar,
Rupnagar, Punjab, India
Sumita Vyas Defence Laboratory, Jodhpur, India
 Part I
General
Chapter 1
Introduction to Solar Energy: Systems,
Challenges, and Opportunities

Himanshu Tyagi, Prodyut R. Chakraborty, Satvasheel Powar

and Avinash Kumar Agarwal

Abstract Solar energy-based technologies have seen remarkable development in

the last decades. This is observed all across the world. The concerns related to
cost and availability of fossil fuels, effects of climate change due to use of fossil
fuels are the primary drivers behind the usages of these technologies. With such
motivation, several research groups around the world are working in the field of
solar energy. Since this field is quite diverse, it includes academicians of diverse
background such as physics, chemistry, materials science, mechanical engineering,
electrical engineering etc. This monograph is an effort to present the collection of
such effort with an aim to present the developments at the system-level as well as
the challenges and opportunities that are present at the technical level. It includes
chapters covering the following themes—overview of energy usage in developing
countries, solar thermal systems (heating and cooling), energy storage, and solar
cells.

Keywords Solar energy · Energy storage · Heating · Cooling · Phase change

materials · Solar cells · Sustainability

H. Tyagi (B)
Department of Mechanical Engineering, Indian Institute of Technology Ropar, Rupnagar,
Punjab 140001, India
e-mail: himanshu.tyagi@iitrpr.ac.in
P. R. Chakraborty
Department of Mechanical Engineering, Indian Institute of Technology Jodhpur, Jodhpur,
Rajasthan 342037, India
e-mail: pchakraborty@iitj.ac.in
S. Powar
School of Engineering, Indian Institute of Technology Mandi, Mandi, Himachal Pradesh
175005, India
A. K. Agarwal
Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur,
Uttar Pradesh 208016, India

© Springer Nature Singapore Pte Ltd. 2020 3

H. Tyagi et al. (eds.), Solar Energy, Energy, Environment,
and Sustainability, https://doi.org/10.1007/978-981-15-0675-8_1
4 H. Tyagi et al.

This chapter presents an outline of the whole monograph, along with collection of
the abstract of each chapter. The whole monograph has been divided into five parts.
Each part deals with a unifying theme. The monograph begins with Part I where the
overall usage of energy is put into context, especially with a focus on the progress of
developing countries (such as India, China, Brazil). It also includes chapters which
highlight the usage of solar energy-based technologies in remote areas of India, and
also the optimum usage of energy in buildings for thermal comfort. This part consists
of four chapters (including this Introduction chapter). The abstracts of each of these
chapters are as follows.
Chapter 2: Sustainable Development Goals calls for a substantial increase in the
share of solar energy in the global energy mix, as well as for a more efficient use
of energy. With the considerable cost of degradation in the environment at USD80
billion annually, which is equivalent to 5.7% of GDP in 2009, a major constraint in
sustaining future economic growth could be the environment. In this present work, an
attempt has been made to outline and focus on the goals set by BRICS countries with
the aid of the major financial institutions and the socio economic groups for these five
countries and the action plans taken as an initiative for the sustainable development
of the energy sector at large and solar energy in specific. India is planning to increase
the production of renewable energy by 40% within 2030 and in turn reduce emissions
intensity by 33–35% over 2005 levels. China on the other hand intends to reduce
emission intensity by 60–65% over 2005 levels by increasing its solar energy capacity
from 43 GW at the end of 2015 to 127 GW by 2020. Russia’s current emissions are
around 40% lower than 1990 levels which are greater than the target set of 25–30%
over 1990 levels. More emphasis is given in developing sustainable renewable green
energy. Brazil is arguably in the best position to achieve its goals in renewable energy
sphere, as in 2015, 74% of its energy came from renewable sources. South Africa
has a very far way to go in the field of renewable energy as at present more than 90%
of the energy are still generated from fossil fuels though the country has targets to
commission another 17.8 GW of renewable energy capacity by 2020.
Chapter 3: The solar energy is available at free of cost and cleanest source of
renewable type energy that can be utilized as a better substitute to the fossil fuel
energy. These days lot of research is going on in extracting maximum possible heat
energy from the solar irradiation. From solar systems practicability and remotely
located societal upliftment point of view, it’s a big challenge to erect and run con-
tinuously or effectively the solar energy systems in remotely located or hilly areas
of Himachal Pradesh, India. This chapter discusses various applications of the solar
concentrating collector technology that may help remotely located people get rid of
their day to day problems e.g. water freezing, water pumping, cooking, drying of
agricultural products, air conditioning etc. The challenges to be faced by the commu-
nity people to meet household and agricultural demands and ways to get better the
performance of solar-energy based systems along with opportunities for solar energy
systems to meet the requirements of remotely located people are also discussed in
this chapter.
Chapter 4: Energy efficiency and conservation measures in buildings are the focus
in today’s design and construction practices. One of the major reasons for energy
1 Introduction to Solar Energy … 5

consumption in buildings is maintaining thermal comfort. Providing a thermally

comfortable environment with an energy efficient design will not only lead to energy
and cost savings, but will also have other intangible benefits, such as enhanced
productivity, and health and well-being of the occupants. Studies have reported that
buildings have 50–60% energy saving potential by means of an efficient design. This
study aims at utilizing passive design strategies, such as provision of insulation and
window glazing, to analyse their effects on thermal comfort of the occupants inside an
office room. Measurements of indoor environmental quality parameters was done for
the room, and Predicted Mean Vote and Percentage People Dissatisfied models, given
in ASHRAE Standard 55, have been used in this study to assess the existing comfort
levels of the occupants. A parametric study to examine their influence on the thermal
environment using these models has been done using IDA ICE Beta 4.7 software.
It was observed that application of the passive techniques although enhanced the
thermal environment of the room, the comfort levels were still not within the ideal
range specified by ASHRAE. The study concluded that more passive strategies can
be employed to enhance the comfort levels. This would help in reducing the need for
alternate methods of space conditioning, hence, leading to energy conservation.
The next part in this monograph, (Part II) consists of three chapters which deal
with the heating aspects of solar thermal systems. It contains chapters which study
the solar water heating system as well as solar air heaters. Moreover it contains
a chapter highlighting the mathematical model of a solar updraft tower where a
buoyancy driven-flow (heated by solar energy) drives turbine for power generation.
The abstracts of each of these three chapters are as follows.
Chapter 5: Solar energy is a promising renewable source to support the grow-
ing energy demand. This energy is widely harnessed for solar water heating sys-
tems to provide hot water for both domestic and industrial sectors thus reducing
use of conventional energy sources. In this work, a concentrated solar water heater
(CSWH) system is designed and fabricated at IIT Jodhpur. The main objectives are
development of a point focus based direct solar water heating system and prelimi-
nary experiment based evaluation of the designed system. The system envisages a
flux concentration of 100 Suns, which will enable receiver area reduction and the
use of other heat transfer fluids like oil in future. The CSWH system consists of
(a) receiver and (b) parabolic dish with two-axis sun tracking provision. In the con-
ventional solar water heater system the irradiance from sun is directly collected by
the collector whereas in concentrated solar water heater the reflected irradiance is
received by the receiver. The reflector consists of a reflecting surface mounted on a
parabolic structure and the cavity receiver consists of consists of a serpentine copper
tube exposed to concentrated irradiance. The receiver will be insulated from top in
order to prevent heat loss from one of its surface. An optical model of parabolic
dish and receiver has been developed using TracePro software. This model is used as
reference to generate the flux density distribution. The experimental setup consists of
a parabolic dish, a receiver with thermocouples, a Coriolis flow meter, pump, water
tank and NI DAQ. Coriolis flow meter is used to measure the mass flow rate in the
system. K-type thermocouples are attached on to the receiver and the temperature
6 H. Tyagi et al.

is recorded using NI DAQ system. The theoretical geometric concentration ratio

predicted is 115 but from the experiment a flux concentration ratio 94 is measured.
Chapter 6: To achieve maximum thermohydraulic performance, by maximizing
the heat transfer and minimizing the pumping power, for a ribbed solar air heater, a
Taguchi method has been used to predict the optimal set of design and flow param-
eters. An L16 (43 ) orthogonal array is selected as an investigational plan to perform
the computational fluid dynamics (CFD) simulations for investigating the effect of
design parameters, i.e. Reynolds number (4000–16,000), rib pitch to height ratio
(3–12), and rib geometry due to change in inclination angle of front face of the rib
(α = 45°–90°) on the effectiveness of a solar air heater. Nusselt number, friction fac-
tor and thermohydraulic performance are considered as performance indexes, and the
minimization of pumping power (minimum pressure drop) and the maximization of
heat transfer and overall performance are taken as the optimization criteria. Results
show that Reynolds number has the greatest influence on the heat transfer as well as
thermohydraulic performance, while rib spacing on the friction factor ratio. The best
combinations of design factors for heat transfer, pressure drop, and thermohydraulic
performance are A4 B4 C1 , A4 B1 C1 , and A1 B4 C1 , respectively. Numerical results val-
idate the aptness of the proposed methodology, so, this investigation suitably offers
an improved rib design for internal fluid flow investigations related to effective heat
transfer applications.
Chapter 7: Solar updraft tower power plant is a way to harness energy from the
sun. It is a simple concept which requires low maintenance and utilises land that
is already being used for growing plants, and generates power from it. A prototype
plant was setup in Manzanares, Spain. Numerical analysis on power generation is
performed for a similar plant assuming it is setup in Ropar. By considering losses via
convection and radiation through the top surface of the collector, collector efficiency
is calculated. Two cases arise here, 1. With 100% collector efficiency and 2. Collector
efficiency is obtained after subtracting convection and radiation losses. The influx of
solar radiation is highest in June. Hence, the variation of parameters like temperature,
velocity, power output, efficiency with time of the day is done by taking averages
for the month of June. Next the impact of physical parameters like chimney height,
chimney radius and collector radius are studied on 21st June 11:00 to 12:00. How
each parameter impacts the output of the plant is studied by creating a mathematical
model of the power plant. Methods to improve the power output are discussed.
Part III of the monograph contain three chapters which discuss the possibilities
of space cooling using non-conventional sources of energy, such as solar thermal
powered adsorption cooling. The abstracts of each of these three chapters are as
follows.
Chapter 8: Adsorption based cooling systems are gaining considerable attention
since it can utilize low grade thermal energy, which otherwise could go as a waste.
Heat sources possessing a temperature of as low as 60 °C can drive an adsorption
chiller and that temperature requirement is even lower in the case of multi-stage
adsorption cooling systems. A typical flat plate solar collector can provide hot water
having a temperature of 65 °C in most of the countries in the Asian region. The tem-
perature of evacuated tube collectors’ water outlet can reach above 95 °C. In order
1 Introduction to Solar Energy … 7

to make use of such collectors, in conjunction with other auxiliary heat sources,
for providing heat to power an adsorption chiller, it is imperative to have a proper
mathematical model. This can aid in designing the network and predicting the per-
formance of the whole system, prior to installation. This chapter focuses on the
modelling of a system that incorporates flat plate collectors, evacuated tube collec-
tors and a thermally powered adsorption chiller. Here, mathematical equations to
calculate the efficiency of flat plate and evacuated tube collectors are presented; pro-
cesses that are involved in a typical two bed adsorption cooling system are explained
in brief, and a mathematical model of an adsorption chiller, that employs mass and
heat recovery schemes is developed. Finally, the simulation results of the model are
presented, and the performance of the chiller is investigated to demonstrate a clear
understanding of its operation.
Chapter 9: Conventional cooling and refrigeration systems already evolved to
efficient design, have higher COP and compact size. However, the compressor part
of such system consumes a tremendous amount of electricity and contribute indirectly
to global warming. The working fluids of these systems are typically HFC or HFC
blends which possess very high global warming potential. A significant percentage
of working fluid is leaked from the high-pressure side of the system and directly
contribute to global warming. The summation of indirect and direct warming impact,
namely, total equivalent warming impact (TEWI) of the vapour compression cooling
systems are significantly high. Adsorption cooling system (ACS) can resolve this
critical issue. In ACS, the mechanical compressor of the traditional cooling system
is replaced by a thermal compressor, namely, a pair of adsorption beds. Highly
porous adsorbent material (silica gel, activated carbon, zeolite and so forth) is the
key component of an adsorption bed. These materials have the capability to capture
and hold certain types of fluid. This phenomenon is known as adsorption. Upon
heating, the adsorbed fluid is liberated from the pores (desorption process) and gets
thermally compressed. Solar thermal energy is the most prospective option for the
desorption process to occur. Since there is no mechanical compressor, electricity
consumption is deficient, which significantly minimizes the indirect warming impact.
Moreover, natural or alternative refrigerants are used as the working fluid, which
has zero/negligible GWP. Hence, the direct warming impact is also shallow. In this
chapter, the working principle and governing equations of a solar energy driven
adsorption cooling system will be elaborated. Besides, TEWI assessment procedure
will be explained and compared for both vapour compression and adsorption cooling
systems.
Chapter 10: The present study focuses on the thermodynamic analysis of
zeolite-water and activated carbon-ethanol based adsorption cooling systems. The
performance of the system depends critically on four operating temperatures namely
maximum desorption temperature, minimum adsorption temperature, condensing
temperature, and evaporating temperature and also on the ratio of specific heat of
structural material and the specific heat of adsorbent. Dubinin-Astakhov equation
is used to estimate the equilibrium uptake of working pairs. A comparative study is
made between these working pairs for the air-conditioning applications.
8 H. Tyagi et al.

The next part in this monograph, (Part IV) consists of four chapters which deal
with the critical issue of energy storage. Phase change materials (PCM) can be used
for the purpose of storing thermal energy. This allows the end-user to store energy
for those periods where direct solar energy may not be available. Moreover PCMs
can also be used to offset cooling loads for buildings in warmer climatic zones. The
abstracts of each of these four chapters are as follows.
Chapter 11: Efficient storage of solar thermal energy has been a key research area
in recent years. Among the various methods for energy storage, phase change mate-
rial (PCM) based latent heat systems have shown a lot of promise due to their high
energy storage densities and smaller system sizes. However, the low thermal con-
ductivities of PCM pose a significant challenge in designing such systems, therefore,
augmentation with suitable thermal conductivity enhancers becomes necessary to
improve their energy charging and discharging performances. The use of metal foam
structures embedded in PCM to form composite PCM-metal foam energy storage
system can improve the effective thermal conductivity remarkably due to the high
surface area for heat transfer between the metal foam and the PCM. This chapter
presents a study of PCM-metal foam composite systems for solar energy storage. At
first, a brief overview of the relevant thermal enhancement methods with particular
emphasis on metal foam systems is presented. This is followed by the description of a
typical PCM-metal foam composite system and the important parameters governing
its energy storage performance. Different modelling approaches for such systems and
their advantages and disadvantages are presented. The effect of important factors for
metal foam-PCM composite systems are analyzed by performing pore-scale simu-
lations. It is shown that factors such as metal foam porosity, pore size distribution,
foam material, phase change material and overall system size contribute significantly
towards the melting pattern and energy storage characteristics of these systems.
Chapter 12: In the present work, we propose thermal energy storage by direct
photo-thermal energy conversion (referred to as optical charging) using nanoparticles
laden phase change materials (PCMs). In the conventional thermal storage systems,
the absorbed solar energy is indirectly transferred to the PCM (primarily through
conduction and convection heat transfer mechanisms) and is subsequently stored in
the form of latent heat of the PCM (referred to as thermal charging). Opposed to
the conventional thermal storage strategies; optical charging involves direct interac-
tion of the sunlight with the phase change material (radiation being the predominant
heat transfer mechanism). Broad absorption-based nanoparticles (amorphous carbon)
have been seeded into the pristine phase change material (paraffin wax) to enhance
photo-thermal conversion efficiency. Particularly, we investigate the effect of adding
nanoparticles to conventional PCMs during optical charging process. To understand
the role of nanoparticles; samples of pristine paraffin wax and nano-PCMs [different
concentrations of nanoparticles (0.05, 0.1, 0.2, 0.4, wt%) dispersed in the pristine
paraffin wax] have been optically heated. Furthermore, optical charging has been
compared with the conventional thermal charging process. As per the experimental
observations, the optical charging scheme significantly improves the thermal charg-
ing rate (by more than 157%) at optimum nanoparticle concentration (0.2%, in the
present study) as compared to conventional thermal charging.
1 Introduction to Solar Energy … 9

Chapter 13: Buildings use more than 40% of the total power consumed in India.
Therefore, implementing energy conservation within buildings is of prime concern.
Utilization of passive design parameters, such as Phase Change Material (PCM)
incorporation, for energy conservation in buildings is thus a lucrative option. Incor-
porating PCMs within elements result in lowering of heat gain and temperature within
the building. A number of simulation and experimental studies on PCM incorporated
buildings, have been carried out. The advancements made in last forty years in the
field of PCMs and their utilization as Thermal Energy Storage (TES) medium for
buildings have been reviewed and presented in this study. This study focuses on
PCM incorporation which is sensitive to its properties and climatic parameters of
the location. Thus, there is a need of benchmarking the PCM for their application in
buildings. Focus is on buildings in tropical hot climatic conditions, where reduction in
cooling load is a challenge. This study lays emphasis on using an appropriate PCM
selection. Thus, phase change temperature forms and important criteria for PCM
selection. Thermal conductivity, specific heat and latent heat are other properties
which must be evaluated before PCM selection and implementation within build-
ings. The study also encompasses different methods of PCM incorporation being
implemented across the world and have marked advantages and disadvantages of
each followed by their impact in terms of energy savings.
Chapter 14: Metastable supercooled liquid phase change material (MSLPCM)
is prepared by homogeneous mixing of sodium acetate trihydrate (SAT), water and
ethylene glycol (EG) in 92:5:3 weight fractions, respectively. A stainless steel (SS-
306) triggering disk of size 20 mm (L) × 18 mm (W) × 0.2 mm (T) is fabricated
by engraving of grooves on SS disk for initiating nucleation in heat pack and release
of heat at the time of requirement. The PCM heat pack containing 300 g PCM and
a triggering disk is fabricated using high frequency PVC sealing machine. Thermal
performance of identical PCM heat pack and water pack (300 g water) is carried
out inside temperature history measuring setup at 0 °C ambient temperature. The
temperature of PCM heat pack and water heat pack reduces from 70 to 30 °C in 210
and 48 min, respectively at 0 °C ambient temperature. The heating time of PCM heat
pack is ~4.4 times compared to water heat packs.
The last part of this monograph, (Part V) consists of four chapters deal with the
technological advancements in the field of solar cells. New types of solar cells (DSSC
and OSC, which stand for ‘Dye Sensitized Solar Cells’ and ‘Organic Solar Cells’,
respectively) have the potential to generated electricity at very low cost, however
they require further improvements. The abstracts of each of these four chapters are
as follows.
Chapter 15: Utilization of renewable sources can reduce the impact of increasing
global energy demand on the rate of depletion of fossil fuels. One of the most stud-
ied and implemented routes to meet this energy demand is to harvest solar energy.
Among solar-energy harvesting devices, dye-sensitized solar cells have been recog-
nized as some of the cheapest and most environment friendly technologies, since they
do not require high purity of starting materials or advanced fabrication techniques.
A dye-sensitized solar cell is composed of a working electrode, in which the light
10 H. Tyagi et al.

absorbing sensitizer is chemisorbed onto the surface of a wide bandgap semicon-

ductor; a redox electrolyte, that is placed in between two electrodes and functions to
regenerate the sensitizer; and a counter electrode, which is a catalyst which accel-
erates a redox reaction with the electrolyte. Titanium dioxide (anatase phase) is the
most widely used semiconductor material in dye-sensitized solar cells due to its
low cost, chemical stability and optical properties. In this chapter, the literature on
optimizing TiO2 as a semiconductor material for n-type DSCs is reviewed. The evo-
lution of TiO2 nanostructures and techniques such as doping, composite preparation
and surface modification are elaborated on. These methods have enhanced both the
chemical and physical properties of TiO2 nanostructures. Moreover, despite good
overall performance, rapid recombination kinetics are a major disadvantage inher-
ent in TiO2 . Thus, research has been carried out to substitute TiO2 with alternative
semiconductor. In view of this, other potential competitors for photo-anode material
are reviewed and assessed. Finally, the prospects of an ideal semiconductor material
for dye-sensitized solar cells is discussed.
Chapter 16: The energy crisis is a global problem that drives investment on renew-
able energy sources worldwide. Utilization of solar energy has become an effective
strategy for sustainable energy generation, as it has the potential to fill the energy
gap created due to the depletion of fossil fuel. On the ever-extending ladder of solar
harvesting technologies, third generation dye-sensitized solar cells (DSCs) have the
advantages of better cost effectiveness and environmental footprint when compared to
the first-generation silicon solar cells and second-generation thin film photovoltaics.
The ultimate goal of constructing high-efficiency multi-junction devices has set the
target of improving single junction components of DSCs (n- and p-type), separately.
The pace of development of single junction p-DSCs has been much slower than
that of n-DSCs. Discovery of suitable materials and techniques have lifted the per-
formance of n-DSCs to more than 14% since it was first reported in 1991. On the
other hand, p-DSCs have a maximum efficiency of 2.51%. It is important to bridge
the gap between the efficiencies of these single junction configurations, in order to
adopt the concept of multi-junction/tandem-DSCs that have the potential to reach
higher efficiencies by harvesting a larger fraction of the solar spectrum. This chapter
focuses on reviewing literature on development of p-DSCs. First, as an introduction,
the structure, function and kinetics of p-DSCs are described. Next, the two main
factors that affect the overall performance of a p-DSC; light harvesting capacity, and
energy loss within the device, are comprehensively discussed.
Chapter 17: Dye-sensitized solar cells (DSCs) are third generation photovoltaic
devices capable of harvesting solar energy to generate electricity. DSCs have gained
significant research interest during past decades due to its high theoretical power
conversion efficiencies and most importantly the cost effectiveness and environ-
ment friendly fabrication process. Firstly, in this chapter, the function of the counter
electrode (CE) in a DSC has been discussed in brief. The CE participates in the
electron transfer from the external circuit back to the redox mediator thereby cat-
alyzing its regeneration reaction. In the state of art DSCs, Pt has been the preferred
CE material. Properties such as promising conductivity and high electrocatalytic
activity towards the process of reduction of I3 − to I− which is the typical redox
1 Introduction to Solar Energy … 11

mediator, contributes to its applicability as the preferred CE material. However, the

use of Pt CE adhere major drawbacks such as the high cost and its susceptibility to
undergo corrosion. These limitations have emphasized the importance of exploring
alternative cost effective functional materials with better conductivity and electro-
catalytic properties. Subsequently, the limitations of using Pt as the CE materials,
and the advantages and challenges associated with alternative materials have been
elaborated. Conducting polymers with extended conjugate electron systems are a
promising substitute material for Pt in DSCs. A wide range of conducting polymers
and polymer hybrid composites have been investigated for their applicability as CEs
in DSCs. These polymers have gained popularity not just due to cost effectiveness
compared to Pt but also due to their promising conductivity, superior electrocatalytic
properties, easy preparation and fabrication. Different types of conjugated polymers
and polymer hybrid composites, their synthetic methods, fabrication processes and
their respective photovoltaic performances are then reviewed. Finally, the future
prospects of conducting polymers as CE material has been discussed.
Chapter 18: Organic solar cell (OSC) is one of the promising photovoltaic technol-
ogy for next generation low-cost renewable energy sources. The power conversion
efficiencies (PCE) of OSCs have reached above 14% in single-junction and ~17%
in tandem OSCs. This rapid increase in the performance is mostly profited from the
synergetic advances in rational molecular design, device processing and interfacial
layer modifications. In addition to the development of efficient photoactive materials,
interfacial design plays a crucial role in the improvement of device performance and
stability. Most importantly, the interfacial layer is responsible for establishing good
ohmic contact in the device, thus minimize the resistance, interfacial recombination
and improve charge selectivity. In this chapter, we present the recent development
in the electron and hole transporting interfacial materials design for both single-
junction and tandem OSCs. Special attention will be paid to the design principles
of interfacial materials which includes inorganic metal oxides, composite materials,
oligomeric and polymeric molecules and their use as cathode and anode interlayer
for high efficiency devices. The structure-property relationships of various interfacial
materials will be analyzed as an approach towards high performance OSCs. Finally,
we will discuss the current challenges with possible solutions and perspectives for
performance enhancement in OSCs.
The lists of specific topics included in this monograph are as follows:
• Sustainable Development Goals in context to BRICS Countries
• Installations of Solar Systems in Remote Areas of Himachal Pradesh, India: Chal-
lenges and Opportunities
• Utilising Passive Design Strategies for Analysing Thermal Comfort Levels inside
an Office Room using PMV-PPD Models
• Design and Development of a Concentrated Solar Water Heating System
• Multi-objective Performance Optimization of a Ribbed Solar Air Heater
• Mathematical Modelling of Solar Updraft Tower
• Solar Thermal-Powered Adsorption Chiller
• TEWI Assessment of Conventional and Solar Powered Cooling Systems
12 H. Tyagi et al.

• Thermodynamic Analysis of Activated Carbon-Ethanol and Zeolite-water Based

Adsorption Cooling Systems
• PCM-Metal Foam Composite Systems for Solar Energy Storage
• Direct Photo-Thermal Energy Storage Using Nanoparticles Laden Phase Change
Materials
• Review on PCM application for cooling load reduction in Indian buildings
• Fabrication and thermal performance evaluation of Metastable supercooled liquid
PCM based Heat pack
• Yet to be Challenged: TiO2 as the Photo-anode Material in Dye-sensitized Solar
Cells
• p-type Dye Sensitized Solar Cells: An Overview of Factors Limiting Efficiency
• Conducting Polymers as Cost Effective Counter Electrode Material in Dye-
sensitized Solar Cells
• Interfacial Materials for Organic Solar Cells.
In this monograph the various chapters are arranged into five different parts:
(i) General, (ii) Solar Thermal Systems: Heating, (iii) Solar Thermal Systems: Cool-
ing, (iv) Energy Storage, and (v) Solar Cells.
Chapter 2
Sustainable Development Goals
in Context to BRICS Countries

Bhabajit Baruah and Rakesh Nath

Abstract Sustainable Development Goals calls for a substantial increase in the

share of solar energy in the global energy mix, as well as for a more efficient use
of energy. With the considerable cost of degradation in the environment at USD80
billion annually, which is equivalent to 5.7% of GDP in 2009, a major constraint
in sustaining future economic growth could be the environment (Griffith-Jones in A
BRICS development bank: a dream coming true? (No. 215), 2014). In this present
work, an attempt has been made to outline and focus on the goals set by BRICS
countries with the aid of the major financial institutions and the socio economic
groups for these five countries and the action plans taken as an initiative for the
sustainable development of the energy sector at large and solar energy in specific.
India is planning to increase the production of renewable energy by 40% within 2030
and in turn reduce emissions intensity by 33–35% over 2005 levels (Schmidt and
Sewerin in Nat Energy 2(6):17084, 2017). China on the other hand intends to reduce
emission intensity by 60–65% over 2005 levels by increasing its solar energy capacity
from 43GW at the end of 2015 to 127GW by 2020. Russia’s current emissions are
around 40% lower than 1990 levels which are greater than the target set of 25–30%
over 1990 levels. More emphasis is given in developing sustainable renewable green
energy. Brazil is arguably in the best position to achieve its goals in renewable energy
sphere, as in 2015, 74% of its energy came from renewable sources (Schmidt and
Sewerin in Nat Energy 2(6):17084, 2017). South Africa has a very far way to go
in the field of renewable energy as at present more than 90% of the energy are still
generated from fossil fuels though the country has targets to commission another
17.8 GW of renewable energy capacity by 2020.

Keywords Smart grid · Renewable energy · Solar photovoltaic · Sustainable

energy

B. Baruah (B) · R. Nath

Department of Mechanical Engineering, Girijananda Chowdhury Institute of
Management and Technology, Guwahati 781017, India
e-mail: bhabajit_me@gimt-guwahati.ac.in

© Springer Nature Singapore Pte Ltd. 2020 13

H. Tyagi et al. (eds.), Solar Energy, Energy, Environment,
and Sustainability, https://doi.org/10.1007/978-981-15-0675-8_2
14 B. Baruah and R. Nath

List of Symbols

GDP Gross domestic product

USD United States dollar
BRICS Acronym for Brazil, Russia, India, China and South Africa
GW Giga watt
MW Mega watt
NDB New Development Bank
SE for All Sustainable energy for All
SDG Sustainable development goals
HIO High impact opportunities
PV Photovoltaic
kWh Kilowatt hour
IEEFA Institute for energy economics and financial analysis
NOx Nitrogen oxide
SO2 Sulphur dioxide
PFF Project financing facility
EDB Eurasian Development Bank
CO2 Carbon dioxide
T&D Transmission and distribution
IRP Integrated resource plan
RE Renewable energy

2.1 Introduction

2.1.1 Economic Analysis

Sustainable Development Goals calls for a substantial increase in the field of renew-
able sources i.e. solar, wind, geothermal and hydropower in the global energy mix,
as well as for more efficient use of energy. With the considerable cost of degrada-
tion in the environment at USD 80 billion annually, which is equivalent to 5.7% of
GDP in 2009, a major constraint in sustaining future economic growth could be the
environment (Griffith-Jones 2014). It is worthy to note that, it might be practically
impossible or too expensive to clean up later. Certain model simulations indicate that
policy interventions such as taxes in the environment could be used to render posi-
tive environmental and health benefits with minimum economic costs (Griffith-Jones
2014). In this context, BRICS energy ministers are committed to:
• Emphasizing for the use of natural resources;
• Reducing the use of fossil fuels by promoting energy efficient technologies;
• Strengthening energy security cooperation with the aid of joint research on strate-
gic reserves, energy efficiency and renewable energy;
2 Sustainable Development Goals in Context to BRICS Countries 15

• Seeking investment opportunities for the New Development Bank (NDB) in the
fields of energy efficiency and renewable energy.
Several initiatives were set up to pursue these goals. In 2017, countries launched
an Energy Research Cooperation Platform to support their work on energy efficiency
and energy more widely. Its purpose is to conduct research and analysis, contribute to
implementing BRICS investment projects in the energy sector, develop cooperation
on energy technology, and improve training for personnel in BRICS countries. The
platform is supported by two additional initiatives: the BRICS Network University
and the BRICS Think Tank Council. To mobilize investments, Energy and Green
Economy Working Group was established to encourage public-private partnerships
for energy-efficient technologies. The Working Group is part of the BRICS Business
Council, which was created in 2013 with a vision to strengthen and promote business,
trade, and investment amongst the BRICS business communities.
In 2014, the BRICS forum set up the New Development Bank to assist fund
infrastructure in the BRICS countries. One of the key objectives of the NDB’s job
is to lay energy targets for the BRICS countries to provide reliable and sustainable
investment for the BRICS countries to build their independent renewable energy
capacity (Council 2015). The targets laid down by the NDB triggered the BRICS
countries to develop plans and execute new and existing renewable capacity. In
this regard to achieve these goals, the bank offer loans quickly and flexibly to the
BRICS countries. It’s a great step towards sustainability in the energy field. And
these concerns of BRICS counties can lead to better cooperation between them in
the future.
Though the NDB has brought remarkable advances in the field of renewable
capacity for the BRICS countries, some of them have been seen to miss their targets.
Thereby demand has been placed for the NDB and other multilateral development
banks and financial institutions to increase investment such that the BRICS could
have a massive impact on the environmental damage currently being created by their
energy systems (Council 2015). One of the most perspective and rapidly developing
fields in renewable energy is the Smart grid communications market. It has been
predicted that the Smart grid communications market would reach USD 9.5 billion
by 2020 among the BRICS countries (News on Renewable 2019). The market is
expected to grow rapidly over the next eight years which would be followed by
three distinct phases. In the first phase, a two-way communication system would
be established between utilities and subscribers by the deployment of smart meters.
That would be followed by the incorporation of the new sensors and other devices at
prime junctures of the network. This would help the utilities to develop value-added
services to aid their smart grid infrastructure (News on Renewable 2019; Trindade
et al. 2017). The final phase would comprise of the development of new services and
software for the optimization of smart meter establishment and overall grid.
16 B. Baruah and R. Nath

2.2 BRICS: Role of India

The Sustainable Energy for All (SEforALL) report on India reflects that the Gov-
ernment has launched numerous schemes for its cities and villages to transform to
meet the Sustainable Development Goals (SDGs) for energy. As per the report, over
300 million Indians in rural areas are having no power connections. The government
of India had set a target for universal household electrification within December
2018. The report also identifies short, medium and long term High Impact Oppor-
tunities (HIO’s) which would support sustainable energy for all sectors. Some of
the HIO’s include renewable energy sources, storage facilities of these energies and
implementation of smart grids, solar energy efficient pumps for agriculture, etc.
India, being one of the fastest growing countries in the field of renewable energy,
is planning to increase the production of renewable energy by 40% within 2030
(Chu and Majumdar 2012). This would increase the capacity of renewable energy to
175 GW. The incorporation of such huge capacity in the field of renewable energy
would, in turn, reduce emissions intensity by 33–35% over 2005 levels (Solar Power
in India 2019). To reach this rather ambitious aim India undertakes the following
steps:
In June 2018, Siemens Gamesa, Renewable solution provider, addressed that it has
bagged an order in the field of Wind Energy from India’s largest renewable energy
Independent Power Producer, Re New Power, for the construction of a 150 MW
wind farm located in the Kutch district of Gujarat. Since 2009, Siemens Gamesa has
installed over 5 GW of renewable energy in various parts of the country. One of the
fastest developing energy sectors in India is Solar Energy. The Government of India
had a target to increase the capacity by 20 GW by the year 2022. According to the
report by the Indian Government, this target was fulfilled way before four years than
the stipulated time and as of 31 March 2019; the country’s solar installed capacity
already reached 28.18 GW. With the increasing capture of the market by the solar
sector, this target was raised to 100 GW of solar capacity by 2022 of which 40 GW
was the target set for rooftop solar (News on Renewable 2019). The budget set for
the total investment was USD100 billion. Following the target set, 3 GW of solar
capacity was commissioned in 2015–2016, 5 GW in 2016–2017 and over 10 GW in
2017–2018 which bought down the price of solar electricity by 18% below the price
of the electricity produced by coal (Solar Power in India 2019).
In addition to its solar photovoltaic (PV) large scale grid connection, India is
expanding its off-grid solar energy local urban and rural energy needs. The rapid
growth of solar energy has reduced drastically the use of kerosene in rural areas.
Developer Azure Power has commissioned its largest solar-power project in the
Indian state of Punjab, with a capacity of 150 MW (Goswami and Zhao 2009). The
project occupies 713 acres of land and will cater to the power requirements of the
local community while generating an estimated 1000 jobs within the community. The
new plant constitutes a portfolio of three projects each with a capacity of 50 MW.
The company noted that the weighted average tariff on these projects is INR 5.63
(USD0.083) per kWh and Azure Power will supply Punjab State Power Corporation
2 Sustainable Development Goals in Context to BRICS Countries 17

for 25 years. Achieving these impressive targets, additional funding was required. In
this regard, Canara Bank came forward to finance renewable energy projects through
Canara Renewable Energy Financing Scheme. The objective of the scheme was
similar to that of the New Development Bank’s (NDB) to provide green financing
and increase the rate of renewable energy development. The loans provided for
the infrastructural development of the renewable energy sector by NDB would be
on-lent through Canara Bank to the renewable energy sub-projects that primarily
include solar, wind, biomass, geothermal, small hydropower, waste-to-energy, and
other projects. This will, in turn, mobilize long term financing to renewable energy
projects. Canara Renewable Energy Financing Scheme is estimated to have an overall
capacity of USD 500 million (New Development Bank 2019).

2.3 BRICS: Role of China

China’s target to reduce emission intensity by 60–65% over 2005 levels (News on
Renewable 2019) makes it even greater than any other country, according to the
IEEFA report. China has targeted increasing its solar capacity from 43 GW at the
end of 2015 to 127 GW by 2020, and wind capacity to 250 GW by 2020 from 145 GW
in 2015. Since the forecasting of China’s development of renewable energy is rather
high, the issue of financing the majority of the deal is also significantly important
which makes it unavoidable to turn to the NDB that is currently engaged in three big
energy ventures in China. China follows its age-old tradition of driving the agenda
forward, as the country edges closer to a more sustainable pattern of growth (Liu
et al. 2010). China’s desire for a significant rise in renewable energy in the energy
mix market makes China’s aim to increase the use of renewables to 15% of its energy
consumption by 2020. By 2020, China should be declared as a nation of 50 GW solar
powers as addressed by the National Energy Administration. In this context, roof-top
solar power technology was designed and supported by the Lingang Distributed Solar
Power Project. The project with the aid of the NDB has accelerated green financing
and promotes clean energy (New Development Bank 2019).
The prime objective of this project is to promote roof-top solar energy by incor-
porating solar photovoltaic power technology for the generation of electricity in
Shang-hai Lingang Industrial Area and reduce carbon emission. The project aims to
generate electricity through 100 MW roof-top solar photovoltaic powers by reducing
73,000 tons of carbon emission every year. The project also aids in saving the cost of
losses in potential transmission by importing energy from places outside Shanghai.
Subsequently, the project has been divided into many sub-projects to be implemented
within 3 years until the end of 2019. To prove the concept, an onsite 3 MW pilot
project has already been implemented successfully. The agreement has been made
and the state grid would procure the electricity generated from the roof-top by solar
photovoltaic power technology. The project aligns with the NDB’s focus to support
projects that aim at developing renewable energy sources. The project also estimates
to reduce carbon dioxide emissions by approximately 73,000 tons per year and NOx
18 B. Baruah and R. Nath

emissions by 1300 tons per year. The project will meet the desire need of saving gas
consumption by 23,000 tons per year and coal consumption by about 32,000 tons
per year (New Development Bank 2019).
Putian Pinghai Bay Offshore Wind Power Project is the second major initiative
of the Government of China to increase offshore wind power capacity and provides
sufficient electricity supply to Fujian and to stimulate the development of offshore
wind energy with technological advances. Its focus aligns with NDB to offer financial
support to projects aiming at developing renewable energy sources. The NDB will
also provide financial support to the cost incurred for the procurement of equipment
and other civil works. The project has been estimated to provide an effective elec-
tricity generation of 3490 h per year. Apart from providing electricity of 873 million
kWh per year to meet the increasing demand for power consumption in Fujian, the
capacity would also have a total targeted capacity of 700 MW offshore wind power
(New Development Bank 2019). The increasing demand for offshore wind power
would help China to sustain a greener and healthier environment and thus reducing
carbon emissions with a target of avoiding 869,900 tons of carbon emissions per
year. Meanwhile, the project also estimates the elimination of harmful components
of emissions such as 26,175 tons of SO2 , 13,090 tons of NOx, and 237,300 tons
of flue gas. It would also avoid the consumption of coal by 314,100 tons. With a
vision to create employment opportunities and help the local economy to grow, a
new industrial cluster has been initiated keeping in mind the socio-economic aspects
of the society.
The third project, the Jiangxi Industrial Low Carbon Restructuring and Green
Development Pilot Project aims at upgrading the traditional industries to achieve
energy conservation and reduce emissions. Financial assistance would be provided
to the Project by NDB through a Project Financing Facility (PFF) loan of up to
USD 200 million. The Project comprises lengthy sub-projects, which as a whole will
promote conservation of energy, regeneration of waste and emission of pollutants
will be reduced. It will also promote the reutilization of industrial water in the Jiangxi
Province.
The approved subprojects will acknowledge the contribution to an energy saving
level of 95,118 tons of coal equivalents per annum and a carbon emission reduction
of 263,476 tons per annum on an aggregate level, through increased recycling ratios,
upgraded factory machinery, increased utilization of heat waste and improved energy
conservation.

2.4 BRICS: Role of Russia

Russia’s advancement amongst other BRICS nations is slightly above par as it has
already met the expectations defined in its annual forum. The goal was to reduce
emissions over 1990 levels by 25–30%, and Russia’s emissions are around 40%
lower than 1990 levels currently. However, by the end of 2020, the country is plan-
ning a production hike of 4.5% in the amount of renewable energy. The progress is
2 Sustainable Development Goals in Context to BRICS Countries 19

predominantly sluggish as compared to what has already been predicted due to the
lack of investment from the country itself. It has allocated a share of USD 1billion for
renewable technologies in all 17 Russian states in 2014 (Cherepovitsyn and Tcvetkov
2017).
Russia accounts for untapped renewable resources of energy. The non-fossil fuel
based energy of Russia contributes to only 3% of the total primary energy con-
sumption of the country (Kutsenko 2015). Karelia, a federal subject of Russia, has a
low capacity of energy generation. It is still not sufficient as it imports power from
other regions of Russia. Hence, the Nord-Hydro model project is designed for the
enhancement in the capacity of power generation in the region and to facilitate the
development of renewable energy. The project in alignment with the New Develop-
ment Bank has accelerated green financing and promotes clean energy development.
Two loans will be provided by the NDB in order to support the Eurasian Develop-
ment Bank (EDB) and International Investment Bank for renewable energy projects.
The loans will accelerate the business scenario of the Nord-Hydro project to increase
the supply of energy through the renewable energy source in the Karelia region. The
Russian government supports the project with a preferential tariff. This project ini-
tiates the construction of a small dam and two hydroelectric, providing an installed
capacity of 49.8 MW in total. As much as 48,000 tons of carbon dioxide emissions
per year will be avoided with the proposed power generation.
Meanwhile, a rather challenging project is being developed in the Russian Fed-
eration Territory. The project is designed in January 2015 to run smart grids in Ufa
which slated to continue till 2019 as was planned. Concerning the expectation, after
the smart grids are deployed, the Bashkiria project implementation will raise the
region’s power supply to a whole new qualitative level without a surge in the invest-
ment planned. It is further expected to reduce power losses in Ufa by a factor of two
which in money terms would translate to 400 million rubles annually (Cherepovit-
syn and Tcvetkov 2017). The experience is extensive and can be replicated in any
region where the power grid infrastructure properties are the same as Russia’s. The
prospects of exporting this technology are significant.

2.5 BRICS: Role of Brazil

Brazil, the energy surplus country, is undoubtedly in a comfortable position to achieve

its goals in the renewable energy horizon. In 2015, about 74% of the total energy gen-
erated came from renewable sources. Brazil has been one of the largest nations since
2007, contracting renewable energy through auction bidding. A pool of sophisticated
and new technologies has been adopted at regular intervals by the auction bidding
mechanism (La Rovere et al. 2011).
According to the IEEFA’s report, “Brazil’s 2024 Energy Plan envisages an increase
in total installed renewable capacity, including large hydropower, from 106.4 GW in
2014 to 173.6 GW in 2024.” But the current political situation prevents the devel-
opment of this field. With the declined GDP growth, the country had to witness a
20 B. Baruah and R. Nath

major drop in electric power load projections. It has been predicted that the demand in
energy load would reduce to an extent of 3480 MW in 2019 which can be reflected by
the fall in electricity consumption by 0.9% in 2016. The dip in electricity consump-
tion sends negative feedback to the industries. In December 2016, the solar and wind
energy auction was canceled which seemed to shake the confidence of the investors
as 1260 projects were registered for the auction out of which 841 numbers were
under wind energy and 419 numbers were under solar photovoltaic (PV), totaling to
35,147 MW of installed capacity (New Development Bank 2019). The cancellation
decision by the government brought doubt on the minds of investor’s on the intention
of the government to support energy projects which were going well forward. Since
that was the only tender in the year for renewable energy, its cancellation brought
a halt in the process of commissioning wind and solar capacity for that year. The
investors felt that this decision rather would bring a long-term impact on renewable
energy investment in the country and hence slowing down the investment process in
that sector.
The New Development Bank hasn’t financed any renewable energy project in
Brazil since 2017 which also illustrates the complicities in the renewable energy field.
With the growing pace, Brazil is expected to have a jump of 44% in solar installed
capacity in 2019, which would enhance the solar capacity by another 3.3 GW. With
an expected demand in electricity consumption between 2018 and 2022 at an average
of 3.8% annually, the need for further investment in infrastructure in the renewable
energy sector becomes more prominent (New Development Bank 2019).
Brazil has been a great supporter and promoter of renewable energy for years. But
due to the inadequate infrastructure in transmission lines, several projects have been
delayed. This made the Brazilian Government set prior conditions for the investors
to have secure transmission lines before participation in the auctions. This would not
only reduce the problem of delays due to insufficient transmission infrastructure but
would also accelerate to drive the market for T&D equipment.

2.6 BRICS: Role of South Africa

The most developed economy in Sub-Saharan Africa is the Republic of South Africa,
yet the slow growth is the strong headwinds the country is facing at present. Frequent
disruption in the electricity seems to complicate and bring challenges for the eco-
nomic growth of the country (Conway et al. 2015). Moreover, the grid facilities also
need up gradation as they are outdated. As per the National Treasury of South Africa,
if the issue of electricity shortage is well addressed, then GDP growth is expected to
increase by 2% roughly. Therefore it has become a major matter of concern for the
government to secure energy supply and develop renewable energy (Martin 2017).
As per the national commitment for the transition from high carbon to low carbon
economy, IRP was formulated to set an ambitious target of 17 800 MW of renewable
energy in 2010 to be achieved by 2030 (Wentworth 2014). About 5000 MW of renew-
able energy was planned to be operational by 2019 within this frame time of 20 years.
2 Sustainable Development Goals in Context to BRICS Countries 21

This would be followed by another 2000 MW of RE by 2020. Ministerial Determi-

nations are entrusted for the proper implementation of the IRP 2010 that is being
regulated by Act No. 4 of Electricity Regulations, 2006 (Kok 2014). It is through
the bidding windows that the country has procured 6 422 MW of electricity from
112 Renewable Energy Independent Power Producers in 2017. By the end of June
2017, a total of 3 162 MW of electricity generation capacity was already connected
to the national electricity grid. In this regard, the NDB’s PFF supported the infras-
tructural development of the grid connection. With such an objective, NDB would
provide a PFF loan to Eskom Holdings State Owned Company Limited of USD180
million. This would, in turn, reduce the dependency of the country on fossil fuels
and thus would aid towards the sustainable development of energy and increasing
electricity supply. Considering the volume of projects to be funded, the PFF has been
divided into sub-projects. Currently, the sub-projects under commissioning include
infrastructural development of expedited independent power producer project for
Upington, the establishment of substation and transmission lines for Soweto area
and Ankerlig-Sterrekus (Martin 2017). With the approval and selection criteria from
NDB, future sub-projects would be proposed to ensure the overall development
objective of the project.
Through the implementation of the project, Eskom is expected to generate a total
of 670 MW of RE to the grid. This would meet 10% of the national target for RE
capacity from 2020 to 202 (Solar Power in India 2019). Once the transmission lines
are developed to meet the demand for electricity, prospects in the development of
RE could be established.
It is to be noteworthy that out of all the BRICS countries, South Africa has a
very far way to go in the field of renewable energy as at present more than 90%
of the energy are still generated from fossil fuels through the country has targets to
commission another 17.8 GW of renewable energy capacity by 2020 (Solar Power
in India 2019).

2.7 Conclusion

In the context of the BRICS countries based on their geographic advantages shown by
the perceptible economic growth in the early 21st century, the current piece of work
tries to analyze the possibility of the BRICS countries for sustainable development
and discussed the necessary conditions to be fulfilled. Every effort should be made
to focus on the proper utilization of their abundant potential natural resources, land
and population by effective technological advancement to encourage sustainable
development. India, China, and South Africa being the energy deficit countries, has
a great role to play in the field of solar renewable energy to overcome the shortage of
energy in the context of the global energy mix. On the other hand, Brazil and Russia
being the energy surplus countries could utilize the excess form of renewable energy
in exporting to the other nations to meet the demand for energy in the global energy
mix.
22 B. Baruah and R. Nath

References

Cherepovitsyn A, Tcvetkov P (2017) Overview of the prospects for developing a renewable energy
in Russia. In: 2017 International conference on green energy and applications (ICGEA). IEEE,
pp 113–117
Chu S, Majumdar A (2012) Opportunities and challenges for a sustainable energy future. Nature
488(7411):294
Conway D, Van Garderen EA, Deryng D, Dorling S, Krueger T, Landman W, Lankford B, Lebek K,
Osborn T, Ringler C, Thurlow J (2015) Climate and southern Africa’s water–energy–food nexus.
Nat Clim Chang 5(9):837
Council BTT (2015) Towards a long-term strategy for BRICS: a proposal by the BRICS think tanks
council
Goswami DY, Zhao Y (Eds) (2009) Proceedings of ISES world congress 2007, vol. 1–5. Solar
Energy and Human Settlement. Springer Science & Business Media
Griffith-Jones S (2014) A BRICS development bank: a dream coming true? (No. 215). In: United
Nations conference on trade and development
Kok N (2014) South Africa’s peacebuilding and PCRD activities: the role of IBSA and BRICS
Kutsenko E (2015) Pilot innovative territorial clusters in Russia: a sustainable development model.
Forsat 9(1 (eng))
La Rovere EL, Pereira AS, Simões AF (2011) Biofuels and sustainable energy development in
Brazil. World Dev 39(6):1026–1036
Liu LQ, Wang ZX, Zhang HQ, Xue YC (2010) Solar energy development in China—A review.
Renew Sustain. Energy Rev 14(1):301–311
Martin B (2017) The politics of electricity planning in South Africa: a review of dominant advocacy
coalitions seeking to influence the Integrated Resource Plan of 2010 (IRP2010), and its update
in 2013 (Doctoral dissertation, University of Cape Town)
New Development Bank (NDB) (2019) https://www.ndb.int/. Accessed on 15th June 2019
News on Renewable (2019) Nuclear, fossil, technology, market data—power technology, https://
www.power-technology.com/. Accessed on 15th June 2019
Schmidt TS, Sewerin S (2017) Technology as a driver of climate and energy politics. Nat Energy
2(6):17084
Solar Power in India (2019) https://en.wikipedia.org/wiki/Solar_power_in_India. Accessed on 15th
June 2019
Trindade EP, Hinnig MPF, Moreira da Costa E, Marques J, Bastos R, Yigitcanlar T (2017) Sustain-
able development of smart cities: A systematic review of the literature. J. Open Innov.: Technol.,
Mark., Complex. 3(3):11
Wentworth L (2014) Creating incentives for green economic growth: green energy in South Africa
Chapter 3
Installations of Solar Systems in Remote
Areas of Himachal Pradesh, INDIA:
Challenges and Opportunities

Santosh B. Bopche

Abstract The renewable energy is an energy obtained from natural replenishable

resources. It consists of wind, water power, tidal power, solar, biomass and energy
recovered from wastes. The solar energy is available at free of cost and cleanest
source of renewable type energy that can be utilized as a better substitute to the
fossil fuel energy. These days lot of research is going on in extracting maximum
possible heat energy from the solar irradiation. From solar systems practicability and
remotely located societal upliftment point of view, it’s a big challenge to erect and run
continuously or effectively the solar energy systems in remotely located or hilly areas
of Himachal Pradesh, INDIA. The present chapter discusses various applications of
the solar concentrating collector technology that may help remotely located people
get rid of their day to day problems e.g. water freezing, water pumping, cooking,
drying of agricultural products, air conditioning etc. The challenges to be faced by
the community people to meet household and agricultural demands and ways to get
better the performance of solar-energy based systems along with opportunities for
solar energy systems to meet the requirements of remotely located people are also
discussed in this chapter.

Keywords Solar energy · Systems · Challenges · Opportunities · Rural · Remote

3.1 Introduction

The distantly located and interior/disconnected community of Himachal Pradesh

faces so many problems in their day to day routine endeavors. It comprises; for
cooking they have to still rely on conventional fuels e.g. woods, kerosene etc. For
cooking purposes bio gas can be generated more effectively with the assistance of
solar energy. Solar cookers energized by concentrating collectors can be made easily
available to such communities. During winter, at hill top locations, the pipe lines
meant for water supply gets chocked due to formation of ice. This type of problem
can completely be overcame by employing solar collector cum thermal energy storage

S. B. Bopche (B)
Department of Mechanical Engineering, NIT Hamirpur, Hamirpur 177001, India
e-mail: santoshbopche@nith.ac.in
© Springer Nature Singapore Pte Ltd. 2020 23
H. Tyagi et al. (eds.), Solar Energy, Energy, Environment,
and Sustainability, https://doi.org/10.1007/978-981-15-0675-8_3
24 S. B. Bopche

systems, so that water shall remain in circulation all times. Solar driven pumps can
be erected in such areas to fulfill the household as well as irrigation water supply
necessities. Solar dryers can be employed for drying of agricultural products. The
people can be made aware about the construction of their houses so that cool air shall
remain in circulation and serve the purpose of air conditioning.
Conventional means for production of electricity to meet the energy demands is
Diesel Generator which is considered as an economic burden to remotely located
poor and backward communities. These systems are non-environment friendly and
less reliable. Solar dependent energy systems could be better option to meet routine
energy demands (Khatib et al. 2018).
An anticipated solar energy potential of INDIA is about 748 GW, according
to MNRE, INDIA (Padmanathan et al. 2018). In India, about 61% of the overall
installed electricity generation is generated using coal/lignite however contribution
of renewable energy resources is only about 13%. Despite growth in power generation
capacity of about 272.5 GW, around 75.02 million inhabitants of rural and remote
areas are deprived of electricity. According to Census 2011, among 75.02 million
inhabitants, 72.04 million people uses kerosene as a primary illumination source
and 0.92 million households use renewable (solar) energy as a primary light source.
The success with erection of solar photovoltaic (PV) technology is quite limited
although it is a better alternative in terms of quality of lighting, permanence and
resourcefulness. The challenges faced in deploying solar PV panels in remotely
located areas contains unavailability of solar PV products market in order to build
connectivity between service providers and the end user(s), unavailability of active
financing agencies in order to make the products quite affordable to the consumers,
and unawareness among the society members about the solar based technologies
e.g., PV or other renewable energy technology based appliances like solar cookers,
solar water heaters, solar air heaters etc. (Anand and Rao 2016).

3.2 Challenges/Limitations of Concentrating Solar Power

Technology in Remote Regions

For individual house power generation, concentrating solar power technology is

realistic and convincing in remote areas. Lots of challenges are to be faced for effi-
cient and uninterrupted operation of such systems. The limitations comprise: water
consumption, water availability, transmission of electricity, power supply security,
materials design, selection of heat transfer fluids, viability of thermal energy storage
& design of receiver subsystems in addition to commercial feasibility and ecological
effects. Some of the technical challenges are discussed as follows (Xu et al. 2016).
3 Installations of Solar Systems in Remote Areas … 25

1. Water consumption

The availability and easy reach to water is a basic concern in remotely located
areas. Large quantity of water is necessary for extracting energy from solar ener-
gized absorbers/receivers and for cleaning of reflecting surfaces e.g. mirrors, pol-
ished aluminium or stainless steel. The availability of water is a prime concern in
some remotely located regions. The predicted water requirement ranges from 3 to
3.50 m3 /kW. The 95% of total water requirement is generally utilized at cooling tow-
ers and 5% is consumed in general for reflective surfaces cleaning. The implementa-
tion of dry mirror cleaning techniques/technologies are really helpful in mitigating
the usage of water consumption in solar-based power generation units.

2. Drycooling

Lots of concentrating solar based power units are working on Rankine cycle with
steam as a working fluid. In steam condenser, steam cooling is achieved using
ordinary cooling water. Better cycle efficiency can be attained at lower steam-
condensation temperature. Studies of NREL (National Renewable Energy Labo-
ratory) have revealed that dry-cooling would preserve water usage by more than
90%. However, the difference of temperature between the cooing water and the dry
cooling medium may differ the yield of cycle thermal efficiency.

3. Dust-cleaning

The collection/optical efficiency of concentrating collectors depend on the reflectivity

of surfaces e.g. parabolic dish/cylindrical concentrator, heliostats and transmissivity
of the solar absorber cover tubes. Optical efficiency drops due to accumulation of
dust particles and therefore the overall performance of the systems. The studies have
disclosed that about 40% of the solar-thermal to electric power conversion decreases
due to formation of a dust layer of size 4.84 g/m2 . It also influences the optical-
performance of a concentrating solar power reflective surfaces. Is also observed that
1% drop in the reflectivity raises the levelized cost of electricity by 1%. Regularly
washing mirrors with the help of water and maintaining reflective surface’s parabolic
profile/orientation of heliostats are the most effective ways maintaining collector’s
performance. At remotely located/rural areas it’s a big challenge to arrange water for
the cleaning task (Xu et al. 2016).
4. Concentrating Solar Power (CSP) desalination cogeneration

This is nowadays an attractive option in remotely situated rural/agricultural areas. The

steam generated using CSP cycle is utilized to support the desalination processes. It
may also fulfill the water demands of the remotely situated population facing dearths.

5. Heat transfer/working fluid

The heat exchange fluid is an important component of CSP technologies, which is

doing a job of collecting energy effectively from the concentrated solar radiation to
the steam driven prime mover. These are water/steam, air, thermal-oils, organic as
26 S. B. Bopche

well as molten-salts. It also serves the purposes of energy storage. It may need to
remain stable or sustain temperatures up to 700 °C and must be corrosion resistant.
Higher viscosity increases the pumping power whereas higher specific heat is good
to make the systems more compact. The air is reported as a best option for heat
transfer fluid meeting all above discussed criterias.
6. Environmental impacts

It is well known that CSP systems are climate friendly. The researches in this area
and widespread of this CSP technology may fulfill household power needs in a more
effective way by capturing free and clean source of solar irradiation.
Living creatures e.g. flying birds gets burnt badly due to accidental exposure to
concentrated solar radiation beams focussing at absorber locations.
After usage, the heat transfer fluids like synthetic-oils, organic fluids, hydraulic
fluids, molten salts, lubricant as well as coolants in CSP collecting systems, exposing
it to the clean outer ambience may not be climate friendly. Better reuse/effluent prac-
tices need to be pursued in order to minimize the environmental impacts caused using
such hazardous fluids/compounds. The organo-compounds e.g., Biphenyl/Diphenyl
oxide used in CSP systems are very poisonous and inflammable also.

7. Higher capital costs

The initial cost of CSP technology based power unit is higher. The pursual of mass
production techniques, availability of sales and services in remote locations and
research innovations in heat transfer fluids, higher temperature range storage systems
and thermodynamic cycles may decrease the capital, running and maintenance cost
by around 60% in coming future.
So, it can be concluded that setting up of solar thermal power units in remotely
situated areas for a bigger capacity, say above 100 kW is problematic to maintain and
operate. In such areas, solar thermal power plant of less than or equal to 5 kW capacity
for an individual home may prove beneficial to meet the routine/daily household
needs.
Interruptions in the power supply may occur due to natural constraints like cloudy
weather, rains, heavy rainfall, heavy wind, snowfall, fall of bigger sized ice pellets that
may cause damage to the renewable energy systems, failure of solar tracking systems,
instrumentation devices. In addition to this, interruptions in electricity supply to the
accessories associated with the solar collectors may occur due to longer hours load
shedding.
The use of locally available renewable energy options e.g., solar, wind and biomass
would be extra efficient than long distance transmission of electric power for supply
to remotely located agricultural areas, in order to evolve self-sustained methods to
cope up with the crisis of fossil based fuel depletion and weather change in long run.
It leaves smaller carbon footprints behind.
The solar thermal based systems have poor solar thermal efficiency. The disad-
vantage associated with power transmission through longer distances is transmission
loss and towers as well as cable installation cost. For such circumstances, erection
3 Installations of Solar Systems in Remote Areas … 27

of a local standalone microgrid sourced by hybrid renewable energy system (HRES)

could be the best alternative of electricity production (Li et al. 2018).

3.3 Opportunities for Renewable Energy Installations

In order to electrify remotely located regions, economically and efficiently there are
lots of prospects for renewable energy based installations.
Bioenergy is a renewable energy which is generally obtained from biomass pro-
cessed by various biochemical as well as thermochemical actions in order to convert
it as a syngas, biooil and biochar. The thermochemical processes involves combus-
tion, pyrolysis and gasification to transform agricultural waste biomass to electricity.
Biochar is a Carbon rich solid residue obtained from pyrolysis and gasification. An
availability of biomass is an important parameter to judge production of Biochar as
well of energy generation (Li et al. 2018).
The solar home system (SHS) is also an important option for meeting cooking
and illumination needs of remote locations. It involves; solar cooker, solar powered
LED lamps, and solar charged battery to run home appliances. The disadvantages
associated with parabolic concentrating energized cooker comprise potentially risky
and faster operation due to concentrated radiation and uncontrolled heating rate.
An Off -grid PV (Photovoltaic) system may supply electricity/power to meet daily
basic domestic needs. However, solar home systems cover illuminating, refrigerating,
charging, pumping, electric mosquito shocking, fanning, air conditioning needs most
of which are not extensively examined and evolved. Using SHSs would save time
spend for doing domestic basic tasks and would preserve a healthy environment. It
would promote educational creative activities and finally would take advantage of
scoring innovations in energy efficiency and developing cost effective systems.
Khatib et al. (2018) have developed a product called Solar Box (Solenx) serves
three basic applications e.g., lightening LED lamps, a mini Insulin-refrigerator, two
USB ports, electric mosquito shocker or a battery charger. A mini Insulin refrigerator
is used to keep Insulin (used for Diabetic patients) under suitable thermal conditions
(Within 2–8 °C for 25 °C ambient temperature). It can also be used to store Veteri-
nary medicines and various animal health products. An electric mosquito shocker
consisting of electric grid and a light source used to attract flying objects which may
be harmful causing Leishmaniasis, Malaria as well as Dengue Fever. These insects,
gets burned after coming in contact with its wire mesh due to higher grid voltage
(Khatib et al. 2018).
For energizing an individual home, a solar home system for fulfilling daily power
demand of about 0.75 kW-h is also suggested by Zubi et al. (2019), for illuminating
LED lamps and cooking with the help of energy proficient multicooker.
Energy scarcity alleviation can be done by adopting solar house technology to
renovate a rural home. In solar house, solar-air source heating system and floor
radiative heating systems are installed (Liu et al. 2018).
28 S. B. Bopche

About 20% of the world’s residents live in remotely and electrically disconnected
areas. Electrifying these areas by conventional ways imply huge capital investments
and intense infrastructures. However, Microgrids (DC microgrids) based on renew-
able energy sourced generation and storage systems may be appropriate and cost
effective solution. The systems working on direct current supply have its own bene-
fits like lesser complexity and affordable operating systems. The difficulties involved
in case of AC based electrical generators are as: synchronizing complexity, optional
requirement of last stage inverter, huge investment and losses associated with it. All
these shortcomings are totally removed in case of DC generators. The DC based
home appliances comprise LED lighting, TVs, Laptops, charging units etc. The AC
based appliances include fans, refrigerators/air conditioning systems. Such efficient
appliances drastically reduce the domestic electricity consumption leading to cheaper
and compact systems. Electrification of rural and remotely based locations is a big
and inevitable challenge in order to empower the life of millions of inhabitants of
such places, in a maintainable ways (Gandini and Almeida 2017).
Wazed et al. (2017) have evolved and examined the Solar Powered Irrigation
Systems energized by Photo-Voltaic and solar based thermal technologies, which
may be utilized by individual agriculturalists in small-scale remote rural agricultural
lands. The remote hilly areas of Himachal Pradesh have been facing inconsistent
water availability and supplies issues since longer time. The only dependable source
is ground water supply. The remotely located farmers and localities have to cope
up with larger installation investments required for the erection of bore wells and
purchase of generator and pumps. The operational costs are also higher due to day by
day hiking of conventional fuel prices and routine maintenance requirements. The
technology desires to be evolved and importance/stress/emphasis should be put on
the renewables based energy options.
An irrigation system based on renewable energy option uses PV panels for gen-
erating electrical power by extracting energy from solar power. It may be used for
energizing electric motor in order to run/drive a pump. The system may be improved
with the employ of batteries and storage water tanks. Solar PV panels produce DC
power, which needs to be supplied to AC motor pump unit via an inverter. In modern
times the PV based irrigation systems become more reachable to the remotely located
agriculturists, by mitigating the installation as well as operational costs of PV based
power generation technology. It is reported that in order to irrigate one hectare farm
land a power of about 1 kW is required, which can be meet through the use of PV
panels. The permanent magnet type DC motors are preferred for irrigation purposes
owing to its high efficiency, torque and easy starting capabilities as compared to AC
motors. The positive displacement pump and diaphragm type pump are generally
used for higher and lower water head requirements, respectively.
A PV power based solution scheme proposed for retrieving ground water for the
purpose of irrigation of farm land is as shown in Fig. 3.1.
The drawbacks of PV technology comprise: (1) degradation of PV cells due to
long term exposure (0.8% per year), (2) harmful manufacturing technology of PV
cells, (3) use of batteries and transportation may promote carbon footprints of the
technology and (4) performance loss due to accumulation of dust (Wazed et al. 2017).
3 Installations of Solar Systems in Remote Areas … 29

Water Storage
for usage

PV panels Permanent Pump

magnet type DC
Motor

Water Storage

Fig. 3.1 PV power based water pumping system

Solar Driven Heat Engines: Another technology proposed is solar thermal based
irrigation system, as depicted in Fig. 3.2. This technology utilizes concentrated solar
energy to produce mechanical power via an engine operated on Stirling, Rankine or
Brayton engine. The power/work generated may be directly used to drive pumps or
to produce electrical power for driving pump motors (Wazed et al. 2017).
The solar irrigation can be a best alternative to increase the production of agricul-
tural products by not levying extra burden on the power grid or diesel consumption.
It also helps keeping clean an environment. The diesel based pumps planted on agri-
cultural lands may need to be replaced by solar driven irrigations pumps all over the
country. Providing subsidy and related incentives from the government will encour-
age and give confidence to farmers to cultivate more and more land using solar based
irrigation technique (Islam et al. 2017).

Fig. 3.2 Solar thermal Water Storage tank

power based irrigation
system
Stirling engine
driven pump
assembly

Water sink/supply
tank
30 S. B. Bopche

PV Panels DC
Pump
(Idc, Vdc) Motor

Fig. 3.3 Directly coupled DC motor based Solar-irrigation-system. Adapted from Islam et al.
(2017)

A Photovoltaic array system is used to energize motors of the irrigation pumps.

In order to transform variable magnitude DC voltage into a constant magnitude AC
or DC power electronic inverter circuit is generally used, before energizing AC or
DC motors. For solar irrigation system dynamic as well as displacement type pumps
are preferred. The displacement type pumps are reported to be more efficient than
the dynamic type. The performance of dynamic pump is solar insolation dependent.
The additional unutilized energy obtained from solar irrigation systems can be used
for providing energy/work for rice-parboiling-systems (Islam et al. 2017).
The schematic diagram of directly coupled solar-irrigation-system is as depicted
in Fig. 3.3.
Irrigation using Solar based Thermal Systems: Thermal energy can be obtained
from the Sun with the help of a solar-concentrating surface. It can be transferred to
the working fluid, which directly or indirectly executes Rankine, Brayton or Stirling
cycle engines. Mechanical work is obtained from these heat engines used to drive
the generator in order to drive the motors for energizing various household and
agricultural appliances. This solar thermal technology can also be used to drive the
pumps for irrigation purposes, which has not been researched extensively in last
10–15 years. A schematic showing, solar driven irrigation pump run by Stirling
engine is as depicted in Fig. 3.4 (Wazed et al. 2018).

Concentrating
collector cum
receiver

Hot end Stirling engine

reservoir operated
Cold end pumping
reservoir system
Water Suction Water delivery

Fig. 3.4 Schematic of solar driven stirling engine. Adapted from Wazed et al. (2018)
3 Installations of Solar Systems in Remote Areas … 31

A metal hydride based pumping system, wherein, a metal hydride is heated using
solar energy which produces a Hydrogen gas. It then pressurizes a piston which
ultimately produces a water-head up to 15 m on Sunny days (Wazed et al. 2018).
The solar thermal based pumping systems exhibited an efficiency of about 3%
in comparison to 6% of that of Photovoltaic systems. But efficiency shall not be
the prime concern in case the systems working on renewable sources. For remotely
located society irrigation purposes, a locally evolved solar thermal based irrigation
system is reported to be more cost effective and ecofriendly as compared to PV
technology (Wazed et al. 2018).
Electricity generation through solar based Micro-grid: The lowest ever rural
electricity tariff in the world is at Myanmar. Xu et al. (2019) have provided an
economic comparison of various microgrid systems e.g., Solar PV based microgrid,
diesel based microgrid, biogas based microgrid, solar PV cum diesel based microgrid,
and solar PV cum biogas based microgrid. In their study, rice husk is used as a source
for biogas energized micro-grid. Among these five types of microgrids, solar PV
based microgird generated an excess electricity of about 30% more than the local
requirements. It is attributed to the hourly variation of the incoming solar radiation
(Xu et al. 2019).
Energy Generation from Biogas: Energy can also be obtained from biogas, which
can be produced using household waste products. The disadvantage of having lesser
energy production efficiency can be improved by uniting it with a solar power. This
technology is termed as solar assisted biogas power generation unit. Biogas is gen-
erally produced from biodegradation of organic wastes/resources, by bacteria under
anaerobic condition. Such biogas generation units can be installed anywhere either
in urban area or in remotely located hilly rural area. It comprises Methane with some
amount of Carbon Dioxide, and little proportion of Hydrogen Sulphide. Based on a
large scale production of biogas, electricity generation unit can also be evolved. The
overall performance of the biogas generation unit can be controlled by maintaining
suitable range of parameters e.g., pH, system temperature, loading rate and agitation
etc. The correct temperature range is 30–60 °C for anaerobic bio-degradation pro-
cess and pH range need to be maintained at optimum loading rate is 6.8–7.2. Among
all parameters, temperature is the main concern contributing in overall production
of biogas. The length of fermentation also depends on temperature of the biogas
generation system. This temperature can also be maintained using external sources
e.g. heat of exhaust gases leaving from power generating engine driven by biogas,
electrical heat and heat yielded from any adjacent combustion unit or from biofuel
or from any of the renewable sources i.e., solar energy. The heat obtained from solar
energy, can be utilized to heat biogas digester above the ambient, may be by means
of hot water production. It is circulated through the jacket of the biogas digester.
It may reduce the time required for attaining the optimum temperature required for
biogas production. At lower temperatures, production of biogas is reduced and may
even discontinue.
An advantage associated with solar assisted biogas generating unit is that it
improves the production rate of biogas due to solar assistance. The installation cost
of such modified systems is more. It needs special attention for the operation and
32 S. B. Bopche

maintenance of the solar panels due to dust accumulation and atmospheric calami-
ties like thunderstorm, heavy winds and heavy snow-ball falls. Solar energy is not a
continuous source. During nights, it is very difficult to maintain performance of the
biogas generation unit (Mahamudul et al. 2019).
There are other problems that reduce the performance of the Solar Collector based
systems. These can be overcame by routine and timely look-after and maintenance.

3.4 Improvement of the Performance of Solar-Based Power

Units/Set Ups

The methods that may increase the efficiency of the solar based power units are as
dictated follows.
(1) In order to pick the power plant performance up, dust accumulation on the
reflecting surfaces need to be prevented, by regular dry/wet cleaning. Since, it
is observed that the glass plates transmittance reduces by 39.4%, on an average
on account of exposure to the solar radiation for say about 38 days. Super-hydro-
phobic materials are specifically recommended to inherit the self-cleaning and
anti-contamination properties in the concentrating solar power mirror (Xu et al.
2016).
(2) The heat transfer fluids which are thermally more stable even at higher tem-
peratures also mend the solar energy system’s performance. The requirement
of pumping power reduces for lesser viscous heat transfer fluids. The sizes of
thermal energy storage tanks can be mitigated by using working fluids having
higher values of specific heats. These two factors help increasing the efficiency
of a solar collector system (Xu et al. 2016).
(3) The performances of solar systems can also be improved by replacing the alu-
minium concentrator by galvanized steel structure. In addition, Aluminium
polymeric reflector be replaced by thin-silvered glass reflector and by provid-
ing selective surface coatings on the absorber surface of a receiver in a way
to improve the outcome of the system. A practice of using of steel and thin
glass reflector also raise the performance of the system by about 12% that may
diminish the cost of energy to be supplied by about 25% (Njoh et al. 2019).
(4) The use mirrors having high reflective qualities allow more than 98.5% reflec-
tivity to the solar rays incident on the reflector surface, also assist enhancing the
system performance (Price et al. 2002).

3.5 Concluding Remarks

In order to cope up domestic energy poverty, the developing countries should keep on
high priority an access to electricity to each and every remotely located region. It may
3 Installations of Solar Systems in Remote Areas … 33

be by adopting grid expansion, distributed smaller power capacity units, micro-grid

projects and solar home systems. Therefore, serious attention may needs to be focused
to give more and more renewable energy solutions, specifically to tackle power
supply, power security and local (in-house) as well as global climatic concerns. Every
household should have access to lower cost/affordable electricity. Such conditions
may definitely progress living standards, economic increase, development of human
as a whole, mitigate gender-inequality, pick up an initial age education standards,
protect the atmosphere and circumvent the negative health consequences of indoor
flames/fires (Zubi et al. 2019).
A Solar Photovoltaic-biomass-biogas hybrid system as presented by Ganthia et al.
(2018) has been proved to be an easy solution for meeting daily electricity needs
of remotely located societies. Its advantages are: (i) lesser maintenance cost, (ii)
emission-less, (iii) no requirement of conventional fuels, (iv) easy installation and
(v) easy operation. The disadvantage of Solar PV water pumping system reported is
having higher Initial cost. In addition to all above-discussed systems, windmill based
water pumping system can also be employed for irrigation purposes. Its advantages
comprise; no emission, no requirement of conventional fuels and longer working
life. Its disadvantages are; higher initial/maintenance cost and limited availability of
market for sales and service. This energy is wind speed dependent (Rathore et al.
2018).
The unavailability of market and skilled manpower for sales, supplies and mainte-
nance of these solar energy systems at such remotely located areas is a big challenge
in a path of making local communities aware of using renewable energy technologies
for fulfilling their daily energy needs.
The challenges/problems faced in remote locations comprise: variation of solar
radiation topography, wind speed (lesser wind speeds means no possibility of wind
propelled turbines in order to assist solar based power generation), rainfall (cloudy
weather hinders solar irradiation), community spirit, unorganized administrative
structure, lack of respect for nature, self-help ethos, sparsely distributed houses
(increases the cost of cabling etc.), lack of renewable energy infrastructure (It incurs
extra high initial cost), Human capital (lack of skilled workers for initial installation
and post installation routine maintenances), small market size, no funding sources,
scarce income opportunities (Njoh et al. 2019).
The adaption to the solar energy by remotely located homes depends on social-
economic, demographic and approach of the government towards rural, remotely
located electrification. Solar home systems have assisted socio-economic activities
and have improved the living standards of rural localities especially house women. It
also has reduced the house pollution which used to occur due to use of Kerosene and
other conventional fuels. This technology adaptation is constrained by unavailability
of skilled personnel (Mishra et al. 2016).

Acknowledgements This chapter work is motivated by the research project, entitled ‘Solar Ther-
mal Collection Efficiency Improvement using Multistaging of Parabolic Dish Collector’, by State
Council for Science, Technology & Environment, Shimla, H.P. INDIA (Grant No. SCST&E
(R&D)/2017–18).
34 S. B. Bopche

References

Anand S, Rao AB (2016) Models for deployment of solar PV lighting applications in rural India.
Energy Proc 90:455–462
Gandini D, Almeida ATD (2017) Direct current microgrids based on solar power systems and storage
optimization, as a tool for cost-effective rural electrification. Renew Energy 111:275–283
Ganthia BP, Sasmita S, Rout K, Pradhan A, Nayak J (2018) An economic rural electrification study
using combined hybrid solar and biomass-biogas system. Mater Today Proc 5:220–225
Islam MR, Sarker PC, Ghosh SK (2017) Prospect and advancement of solar irrigation in Bangladesh:
a review. Renew Sustain Energy Rev 77:406–422
Khatib T, Shaar A, Alhamad F, Dwikat H, Shunnar R, Othman M (2018) Development of solenx:
a reliable and cost effective solar aid box for underserved and rural areas in Palestine. Energy
Convers Manag 174:863–873
Li L, You S, Wang X (2018) Optimal design of standalone hybrid renewable energy systems with
biochar production in remote rural areas: a case study. In: 10th International conference on applied
energy. ICAE 2018. Hong Kong, China, 22–25 Aug 2018
Liu Z, Wu D, He BJ, Liu Y, Zhang X, Yu H, Jin G (2018) Using solar house to alleviate energy
poverty of rural Qinghai-Tibet region, China: a case study of a novel hybrid heating system.
Energy Build 178:294–303
Mahamudul HM, Rasul MG, Akbar D, Mofijur M (2019) Opportunities for solar assisted biogas
plant in sub-tropical climate in Australia, a review. Energy Proc 160:683–690
Mishra P, Behera B (2016) Socio-economic and environmental implications of solar electrification:
experience of rural Odisha. Renew Sustain Energy Rev 56:953–964
Njoh AJ, Etta S, Ngyah-Etchutambe Ijang B, Enomah Lucy ED, Tabrey HT, Essia U (2019) Oppor-
tunities and challenges to rural renewable energy projects in Africa: lessons from the Esaghem
village, cameroon solar electrification project. Renew Energy 131:1013–1021
Padmanathan K, Govindarajan U, Ramachandaramurthy VK, Rajagopalan A, Pachaivannan N,
Sowmmiya U, Padmanaban S, Nielsen JBH, Xavier S, Periasamy SK (2018) A sociocultural
study on solar photovoltaic energy system in India: stratification and policy implication. J Cleaner
Prod. Accepted manuscript. https://doi.org/10.1016/j.jclepro.2018.12.225
Price H, Lupfert E, Kearney D, Zarza E, Gee GCR, Mahone R (2002) Advances in parabolic trough
solar power technology. J Sol Energy Eng ASME 124:109–125
Rathore PKS, Das SS, Chauhan DS (2018) Perspectives of solar photovoltaic water pumping for
irrigation in India. Energy Strategy Rev 22:385–395
Wazed SM, Hughes BR, O’Connor D, Calautit JK (2017) Solar driven irrigation systems for remote
rural farms. In: 9th international conference on applied energy, ICAE 2017, 21–24 Aug 2017,
Cardiff UK. Energy Proc 142:184–191
Wazed SM, Hughes BR, O’Connor D, Calautit JK (2018) A review of sustainable solar irrigation
systems for Sub-Saharan Africa. Renew Sustain Energy Rev 81:1206–1225
Xu X, Vignarooban K, Xu B, Hsu K, Kannan AM (2016) Prospects and problems of concentrating
solar power technologies for power generation in the desert regions. Renew Sustain Energy Rev
53:1106–1131
Xu D, Mumata M, Mogi G (2019) Economic comparison of microgrid systems for rural electrifi-
cation in Myanmar. Energy Proc 159:309–314
Zubi G, Fracastoro GV, Lujano-Rojas JM, Bakari KE, Andrews D (2019) The unlocked potential
of solar home systems; an effective way to overcome domestic energy poverty in developing
regions. Renew Energy 132:1425–1435
Chapter 4
Utilising Passive Design Strategies
for Analysing Thermal Comfort Levels
Inside an Office Room Using PMV-PPD
Models

Sana Fatima Ali and Dibakar Rakshit

Abstract Energy efficiency and conservation measures in buildings are the focus
in today’s design and construction practices. One of the major reasons for energy
consumption in buildings is maintaining thermal comfort. Providing a thermally
comfortable environment with an energy efficient design will not only lead to energy
and cost savings, but will also have other intangible benefits, such as enhanced
productivity, and health and well-being of the occupants. Studies have reported that
buildings have 50–60% energy saving potential by means of an efficient design. This
study aims at utilizing passive design strategies, such as provision of insulation and
window glazing, to analyse their effects on thermal comfort of the occupants inside an
office room. Measurements of indoor environmental quality parameters was done for
the room, and Predicted Mean Vote and Percentage People Dissatisfied models, given
in ASHRAE Standard 55, have been used in this study to assess the existing comfort
levels of the occupants. A parametric study to examine their influence on the thermal
environment using these models has been done using IDA ICE Beta 4.7 software.
It was observed that application of the passive techniques although enhanced the
thermal environment of the room, the comfort levels were still not within the ideal
range specified by ASHRAE. The study concluded that more passive strategies can
be employed to enhance the comfort levels. This would help in reducing the need for
alternate methods of space conditioning, hence, leading to energy conservation.

Keywords Thermal comfort · Passive design · IDA ICE

S. F. Ali · D. Rakshit (B)

Centre for Energy Studies, Indian Institute of Technology Delhi, Hauz Khas,
New Delhi 110016, India
e-mail: dibakar@ces.iitd.ac.in
S. F. Ali
e-mail: esz178538@ces.iitd.ac.in

© Springer Nature Singapore Pte Ltd. 2020 35

H. Tyagi et al. (eds.), Solar Energy, Energy, Environment,
and Sustainability, https://doi.org/10.1007/978-981-15-0675-8_4
36 S. F. Ali and D. Rakshit

Notations

A Du Dubois area (m2 )

Cr es Heat exchange by convection in breathing (W/m2 )
Ec Heat exchange by evaporation on skin (W/m2 )
Er es Evaporative heat exchange in breathing (W/m2 )
f cl Clothing surface area factor
H Sensitive Heat Losses (W/m2 )
H t. Height of a person (cm)
hc Convective Heat Transfer Coefficient (W/m2 K)
Icl Clothing insulation (m2 K/W)
L Thermal load on the body of an occupant (W/m2 )
M Metabolic Rate (W/m2 )
PMV Predicted Mean Vote
PPD Percentage People Dissatisfied
pa Water Vapor Partial Pressure (kPa)
ps Partial Vapor Pressure of Saturated Air (kPa)
RH Relative Humidity
ta Air Temperature (°C)
tcl Surface temperature of clothing (°C)
tg Globe temperature (°C)
tr Mean Radiant Temperature (°C)
tsk External skin temperature (°C)
tW Wet Bulb temperature (°C)
va Velocity of air (m/s)
var Relative velocity of air (m/s)
W Effective Mechanical Power (W/m2 )
W t. Weight of a person (kg)

4.1 Introduction

4.1.1 Energy Conservation in Buildings

Buildings are necessary to provide shelter to the occupants, to provide a safe environ-
ment and for carrying out various activities inside a space. Thus, growth in building
sector is at an all-time rise as a result of the increase in population. National Statisti-
cal Organization reported that currently, buildings stand at consuming approximately
40% of the total electricity and one of the major factors in consumption of electricity
is maintaining comfort of the occupants, be it thermal or visual (02 Energy and Build-
ings 2014). Generation of electricity still relies on non-renewable energy sources and
hence, to cater to the increasing demand, resource depletion would take place along
4 Utilising Passive Design Strategies for Analysing Thermal … 37

with leading to an unnecessary wastage of money. Bureau of Energy Efficiency

reports that buildings can potentially save up to almost 50% of the electricity (02
Energy and Buildings 2014).
The energy expenditure in buildings can be truncated to a large extent by putting
into practice measures such as climate responsive design, modifying the construction
of building, making use of local resources, etc. Such measures form a part of the
passive techniques in buildings, which do not rely on any supporting electrical or
mechanical systems. Apart from this, efficient space conditioning, or lighting systems
can be employed in buildings, as part of the active measures in buildings, which
involve the utilisation of building services systems.
Several studies have been carried out which focus on employing passive tech-
niques to alleviate the energy expenditure. Sharma et al. evaluated the impact of the
orientation of the building on its energy expenditure (Sharma et al. 2015). One such
study by Kumar et al. focused on changing the construction material, the platform
area and the orientation of the building to examine their effect on the energy need
(Sushil Kumar et al. 2016). Another such study by Lee et al. concentrated on optimis-
ing a window system on the basis of its type and properties to examine their effects on
the total load of the building, including thermal and lighting, and thus minimizing it
(Lee et al. 2013). Such studies prove that implementing passive measures in buildings
as a first step has a lot of potential to curb the energy requirements in buildings, and
hence, such measures should be encouraged. However, employing such techniques,
should also have a positive effect on the comfort level of the occupants, apart from
its objective of saving energy in buildings. Therefore, the effect of the techniques
employed should be examined not only for the energy requirements of the building
but also for the comfort requirement of the occupants.

4.1.2 Meeting the Indoor Environmental Quality (IEQ)

In addition to meeting the energy requirements in a building, the indoor environment

should also meet the comfort criteria since it affects the health, comfort, safety
and productivity of the occupants, making it an intangible asset. It incorporates
distinct parameters such as air quality, odor, lighting, noise, and thermal comfort
(ISHRAE 2015). Fulfilling the requirements of the indoor environmental quality
leads to augmenting occupant’s life span, benefits the resale worth of the building,
and brings down the accountability and liability of the owners. Whilst the elements
of the IEQ should be restricted to their allowable limits, such as noise and odor,
there are still some facets which cannot be eluded, and thus, can only be regulated
according to the requirements. One of the parameters fitting to this description is the
thermal comfort, which also utilises a major portion of the electricity in buildings.
The aim of regulating the parameter implies optimising the conditions of the most
favorable environment for occupants within available funds and resources.
38 S. F. Ali and D. Rakshit

4.1.3 Thermal Comfort: A Necessity

ASHRAE Standard 55 (ASHRAE 2010) defines thermal comfort as the condition of

the occupants in agreement with their thermal environment, estimated subjectively. It
represents a balanced state between the occupants and their environment determined
when heat by occupant’s metabolism process is dispersed into the environment.
Nowadays, designing an efficient building solely on the basis of energy require-
ments is not sufficient. The thermal comfort of the occupants, along with other
comfort criteria should be met for a building to be called efficient. The occupants
incline towards easier methods of retaining a thermally comfortable environment,
hence opt for auxiliary systems of space conditioning. This results in an increase in
the consumption of energy in buildings. Hence, passive methods should be adopted
in buildings to the greatest extent possible not only for lowering down the demand
for energy, but also for sustaining the recommended thermal comfort. Studies have
proven that passive techniques are useful in achieving comfort conditions. There
have been many such studies, such as that done by Haase and Amato (2009) wherein
they checked the influence of natural ventilation and orientation in improving the
thermal comfort of the occupants, by Ravikumar and Prakash (2009) wherein the size
of the openings were adjusted in order to achieve thermal comfort conditions, by Liu
et al. (2018) where an optimum window-to-wall ratio and optimum thicknesses of
the insulation for the building envelope for meeting the required comfort was found,
by Berkovic et al. (2012) where a particular configuration as well as orientation of
a building was proposed for thermal comfort, or study of different arrangements for
window-door by Daghigh et al. (2009), or provision of a partition wall for comfort
improvement, as shown by Aryal and Leephakpreeda (2016), etc. The various passive
techniques that can be employed in buildings for maintaining comfort conditions for
the occupants are (Haase and Amato 2009; Ravikumar and Prakash 2009; Liu et al.
2018; Daghigh et al. 2009; Aryal and Leephakpreeda 2016; Gadi 2010; Zahiri and
Altan 2016; Kamal 2012; Berkovic et al. 2012):
• Orientation
• Building Form
• Window-to-Wall Ratio
• Arrangement and Size of Openings
• Envelope Construction
• Envelope Materials
• Thermal Insulation
• Glazing
• Provision of Shading Devices
• Provision of Partition Walls
• Induced Ventilation Techniques (such as solar chimneys, wind towers, etc.)
• Earth Coupling Techniques (such as provision of earth air tunnels, etc.)
• Provision of Thermal Storage Components (such as Trombe walls, water walls,
etc.).
In case if the passive techniques employed are not able to meet the required
comfort, then efficient active techniques should be implemented in order to fill the
4 Utilising Passive Design Strategies for Analysing Thermal … 39

gap. Standards recommend that a minimum of 80% of the occupants need to be

thermally satisfied with their environment (ASHRAE 2010; ISO 2005).

4.1.4 Thermal Comfort Evaluation

A variety of factors dictate the occupant comfort conditions, which may differ from
one person to another. These factors include rate of metabolism of human beings
(M), clothing insulation (fcl ), air speed (var ), air temperature (ta ), mean radiant tem-
perature (tr ) and relative humidity (RH), which establish the heat exchange between
the occupants and surroundings.
Numerous models are available to determine the comfort level of the occupants,
formulated on the basis of the factors affecting it, such as Effective Temperature
(ET), Corrected Effective Temperature (CET), Operative Temperature (OT), Tropical
Summer Index (TSI), etc. (Auliciems and Szokolay 1997). However, two of the
most recognised models for determining thermally comfortable environment are the
Predicted Mean Vote (PMV) and the Percentage People Dissatisfied models, which
are also accepted as the standard indices for evaluating thermal comfort by ASHRAE
Standard 55 (ASHRAE 2010), and ISO 7730 (ISO 2005). Studies by researchers
such as Pourshaghaghy and Omidvari (2012), Azad et al. (2018), Nematchoua et al.
(2017), Rupp and Ghisi (2017), Katafygiotou and Serghides (2014), Calis and Kuru
(2017), Pazhoohesh and Zhang (2018), Stamou et al. (2007) etc., have successfully
utilised these PMV-PPD models for predicting the comfort conditions within the
indoor spaces.
Predicted Mean Vote (PMV). The standards (ASHRAE 2010; ISO 2005) describe
the predicted mean vote index for evaluating thermal comfort of the occupants in
a space, according to which, the condition of the comfort can be envisaged by the
PMV values. This index, developed by P. O. Fanger, represents a scale of thermal
perception, having 7 points representing sensations of the occupants. It spans from a
value of −3.0, representing a cold environment to +3.0, representing a hot environ-
ment, as shown in Table 4.1 (ASHRAE 2010; ISO 2005). The ideal thermal comfort
conditions are represented by the PMV range of −0.5 to +0.5, as recommended by
the standards (ASHRAE 2010; ISO 2005).
The different environmental factors (air temperature—ta , mean radiant tempera-
ture—tr , air velocity—var and relative humidity—RH), which can be measured, as
well as the physiological factors (rate of metabolism—M and clothing insulation—
fcl ), which are the expected parameters, form the main basis of assessing the PMV
values.

Table 4.1 ASHRAE scale of thermal sensation of occupants

Thermal Cold Cool Slightly cool Neutral Slightly warm Warm Hot
sensation
PMV scale −3.0 −2.0 −1.0 0.0 1.0 2.0 3.0
40 S. F. Ali and D. Rakshit

The PMV values of a space for occupants can be assessed from the heat transfer
balance equations, as given by Eq. (4.1) (ASHRAE 2010; ISO 2005):

P M V = (0.303e−0.036M + 0.028) × L (4.1)

Here, L illustrates the different thermal loads on the human body, which can be
evaluated using Eq. (4.2)

L = M − W − H − E c − Cr es − Er es (4.2)

In Eq. (4.2), M can be taken from the table given in Annexure A of ASHRAE
standard 55 (ASHRAE 2010), found to be 1.2 met for the present case. Additionally,
the remaining terms on the right hand side of the equation can be calculated using
the following equations:

H = 3.96 × 10−8 × f cl × [(tcl + 273)4 − (tr + 273)4 ] − f cl h c (tcl − ta ) (4.3)

E c = 3.05 × [5.73 − 0.007 × (M − W ) − pa ] − 0.42[(M − W ) − 58.15] (4.4)

Cr es = 0.0014 × M × (34 − ta ) (4.5)

Er es = 0.0173 × M × (5.87 − pa ) (4.6)

These equations also constitute a term fcl , which can be evaluated using Eq. (4.7) or
Eq. (4.8):

f cl = 1 + 1.29Icl ; If Icl ≤ 0.078 m2 K/W (4.7)

f cl = 1.05 + 0.645 Icl ; If Icl > 0.078 m2 K/W (4.8)

Here, the value of Icl can be obtained from another table given in annexure B of
ASHRAE Standard 55 (ASHRAE 2010), which was found to be 1.2 clo for the
present study. Apart from this, the values of hc and pa can be obtained from Eq. (4.9)
or Eqs. (4.10) and (4.11) respectively.
   √ 
h c = 2.38 × |tcl − ta |0.25 ; If 2.38 × |tcl − ta |0.25 > 12.1 × var (4.9)

√    √ 
h c = 12.1 × var ; If 2.38 × |tcl − ta |0.25 ≤ 12.1 × var (4.10)

RH
pa = 1000 × × ps (4.11)
100
4 Utilising Passive Design Strategies for Analysing Thermal … 41

where,
 
16.6536− 4030.183
ps = e ta +235
(4.12)

Another term, var mentioned in the above equations can be found from Eq. (4.13):
 
M
var = va + 0.005 − 58.15 (4.13)
A Du × 58

Here, the term ADu can be attained using Eq. (4.14):

A Du = (H t.)0.725 × (W t.)0.425 × 0.007184 (4.14)

Additionally, another term used in the above mentioned equations is tcl , the value of
which, can be obtained using the iterative Eq. (4.15):

tcl = tsk − Icl × 3.96 × 10−8 × f cl × [(tcl + 273)4 − (tr + 273)4 ]

(4.15)
−Icl × f cl × h c × (tcl − tr )

where,
tsk = 35.7 − 0.028(M − W ) (4.16)

Percentage People Dissatisfied (PPD). Whilst determining the PMV value is cru-
cial to acquire an idea about the thermal perception of the occupants of their environ-
ment, it is also critical to analyse whether the occupants are feeling content with their
environment or not. This led P. O. Fanger to devise another model: The Percentage
People Dissatisfied (PPD) Model. This index establishes a relationship between the
occupant’s satisfaction with their thermal environment and the PMV values attained.
Standards recommend a PPD value of ≤20% (ASHRAE 2010; ISO 2005). More-
over, the minimum PPD value is 5%, lying at 0 PMV value, which suggests that at
least 5% of the occupants feel discomfort even in a thermally neutral environment
(ASHRAE 2010; ISO 2005).

The PPD values rely solely on the PMV values, and hence, can be found using
Eq. (4.17) (ASHRAE 2010; ISO 2005):

P P D = 100 − 95 × e[−(0.3353P M V +0.2179P M V 2 )]

4
(4.17)

4.2 Methodology

A small office room, established on the first floor of a G + 1 building in Delhi, has
been selected for the study of thermal comfort. The room has dimensions 7.62 ×
6.1 × 3 m3 , with the floor area being 37.2 m2 . The room has three windows with
42 S. F. Ali and D. Rakshit

dimensions 1.52 × 1.52 × 0.02 m3 each, with two windows in west direction and
one window in east direction, along with two doors of dimensions 2.32 × 0.91 ×
0.04 m3 each, where one faces the north-west direction, while the entrance lies in the
north-eastern direction, as shown in Fig. 4.1. The thickness of the walls is 0.23 m,
while 12 mm thick clear glass single pane windows have been used. The building
element details and associated properties have been mentioned in Tables 4.2 and 4.3
(SP 1987).
The study has been carried out for a period of two months (December to February).
An IEQ sensor, called Active Space Indoor Environmental Quality Sensor, consist-
ing of different industrial sensors together, was stationed at the occupant’s working
table surface to continuously measure and record the readings of the environmental
parameters, such as relative humidity, air velocity, air temperature and mean radi-
ant temperature, at time steps of 1–2 min. The readings were registered for all the
occupancy days (weekdays) and occupancy hours (10:00–18:30 h) for the period of
study.
The equations for assessing the PMV and PPD indices were then used to evaluate
the existing comfort level of the occupants analytically on the basis of the observa-
tions taken.

Fig. 4.1 Plan of the office room (dimensions in m)

4 Utilising Passive Design Strategies for Analysing Thermal … 43

Table 4.2 Building element details

Building Layer Thickness Thermal Specific heat Density
element (m) conductivity capacity (kg/m3 )
(W/m K) (kJ/kg K)
External Gypsum 0.012 0.22 1.09 970
walls plaster
Bricks 0.230 0.58 0.84 1500
Gypsum 0.012 0.22 1.09 970
plaster
Roof Gypsum 0.012 0.22 1.09 970
plaster
Concrete 0.127 1.70 0.88 2300
slab
Cement 0.100 0.72 0.92 1648
mortar slurry
Mud phuska 0.102 0.52 0.88 1622
Brick tiles 0.038 0.79 0.88 1892
Floor Concrete 0.13 1.7 0.88 2300
slab
Gypsum 0.012 0.22 1.09 970
plaster
Marble tiles 0.039 3.00 0.88 2300
Doors Wood 0.039 0.14 0.50 2300

Table 4.3 Window properties of the building

Properties Thickness Solar heat Transmittance U-value Emissivity
(m) gain Solar Visible (W/m2 K) Internal External
coefficient
(SHGC)
Values 0.012 0.68 0.60 0.74 1.90 0.84 0.84

In addition to the experimental work, a simulation study analysis has been per-
formed to investigate the impact of different passive parameters on the occupant’s
thermal comfort, for which, IDA ICE 4.7 Beta software has been used. The design
input parameters of the study include the time zone of the location—+5.5 h and the
latitude and longitude of the place, being N 28° 36 E 77° 12 respectively. The office
room has been modeled as a single zone in the software and the construction and
material details, as given in Tables 4.2 and 4.3, have also been specified (SP 1987).
The model of the office room is shown in Fig. 4.2.
Other software inputs required were the no. and schedule of the occupants as
well as the lighting and equipment, which were the occupancy hours of the office
room, as mentioned above. The lighting and equipment load included taking into
44 S. F. Ali and D. Rakshit

Fig. 4.2 Model of the office room

consideration 11 PCs, 3 tube-lights and 4 bulbs. Furthermore, the infiltration rate,

holiday list, the simulation period, etc., also needs to be provided as an input.
The thermal comfort of the occupants in terms of the PMV and PPD values have
been determined with the help of the software as well and validated against the exper-
imental results. The changes in building construction along with their corresponding
properties was later carried out in order to analyse the impact of these modifications
on the comfort levels of the occupants.

4.3 Results and Discussion

4.3.1 Experimental Results

The experimental observations led to determining the PMV and PPD values analyti-
cally at each instant. Results suggested that the PMV values obtained has a minimum
value of −1.21 and a maximum value of 1.11, and the rest of the values at other
instances vary between these two extents. This implies that the mostly cooler con-
ditions were persisting in the office room, since the minimum PMV value is farther
from the −0.5 to +0.5 ideal range recommended by standards (ASHRAE 2010; ISO
2005). This is in agreement with the aspect that the experiments were conducted
in the winter season in an unconditioned building. However, there were only a few
instants of time when the PMV values ranged in the ideal zone, while at other instants
the values were far from the values corresponding to the comfort conditions. Hence,
suggesting that measures should be taken to improve the comfort conditions of the
occupants.
4 Utilising Passive Design Strategies for Analysing Thermal … 45

Similarly, PPD values were also estimated corresponding to each of the PMV
value obtained for each instant of time. With the minimum PPD value being 5%,
the maximum PPD value reached 36.01%, which is again much beyond the 20%
restriction, as recommended by the standards (ASHRAE 2010; ISO 2005). The PPD
results also suggest that though there have been instances where the PPD values lie
within the 20% range, but there are other cases as well wherein the PPD values have
crossed the 20% cap, suggesting that for the study duration, a maximum of 36.01% of
the occupants are dissatisfied with their thermal environment. Therefore, this value
should also be tried to be brought down in order to reach the ideal conditions.
A graph between the two values has also been plotted, as shown in Fig. 4.3, which
agrees well with the ideal PMV versus PPD curve, that can be obtained from any
literature (ASHRAE 2010; ISO 2005). The graph between the two values obtained
indicates that the minimum PPD value of 5% corresponds to the 0 PMV value,
implying that even at thermally neutral conditions, at least 5% of the occupants
remain dissatisfied, as stated in Sect. 4.1.4. The graph also depicts that moving away
from the 0 PMV value, towards either negative or positive sides of the graph, the
percentage of dissatisfied occupants is increasing, while only a very small percentage
of people are feeling comfortable with their thermal environment, and hence, lie
within the zone of −0.5 to +0.5. It also implies that some percentage of occupants
still feel thermally satisfied with their environment on moving only a little away from
the ideal zone. However, as the threshold crosses 20%, measures need to be taken to
maintain the thermal environment within the ideal range and reduce the percentage
of people dissatisfied with their thermal environment.

Fig. 4.3 Experimental 42

results 39
36
33
30
27
PPD (%)

24
21
18
15
12
9
6
3
0
-1.4 -1.2 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4
PMV
46 S. F. Ali and D. Rakshit

Fig. 4.4 Simulation 42

software results 39
36
33
30

PPD (%)
27
24
21
18
15
12
9
6
3
0
-1.4 -1.2 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4
PMV

4.3.2 Analysis Led Design Results and Validation

The PMV and PPD values estimated with the help of the software were then tabulated
and plotted in the form of a graph, as shown in Fig. 4.4. Simulation results yielded the
minimum PMV value as −1.22 and the maximum PMV value as 1.11. These results
too indicate thermally cooler conditions prevailing in the office room for most of the
cases. The corresponding maximum PPD value reached is 36.61% in this case, which
also indicates the percentage of people not feeling comfortable with their thermal
environment.
On comparing the experimental results with that obtained from simulation, it was
found that the two curves were approximately identical with each other and with
the ideal PMV versus PPD curve. However, a little deviation in the outcomes was
found, with the maximum deviation between the experimental and simulation results
being 2.78%, which falls within the range of permissible errors, i.e., 15%. It implies
the validation of the simulation results with the experimental ones. Hence, it was
suggested that the simulation software could be used to proceed further with the
study.

4.3.3 Passive Design Strategies for Thermal Comfort Analysis

The results of the study indicated that the thermal environment of the office room for
the period of study was not favorable to the occupants, and lied in the zone of a cooler
environment, with the PMV values being within −1.22 and 1.11. This is in conformity
with the case that experiments were performed on an unconditioned building in
the winter season. The results also concluded that the percentage of occupants not
comfortable with their thermal environment was high (36.61%), suggesting that
measures should be taken to bring down this value and make the indoor zone within
4 Utilising Passive Design Strategies for Analysing Thermal … 47

Table 4.4 Insulation

Material Specific heat Thermal Density
material’s properties
(kJ/kg K) conductivity (kg/m3 )
(W/m K)
Cellulose 2.02 0.035 57
foam
Mineral wool 0.84 0.038 40
Fiber glass 0.85 0.040 48
Polyurethane 1.80 0.026 30
foam
Expanded 1.30 0.038 15
polystyrene

a comfortable range. Thus, passive design strategies should be employed to the

building to tend to this issue. Though numerous strategies can be applied to the
building, however, this study has been conducted for incorporating provision of wall
insulation, provision of roof insulation and provision of window glazing.
Provision of Wall Insulation. Insulation acts as a barrier between the ambient con-
ditions and the indoor conditions of a space and maintains an appropriate required
temperature inside in both summers as well as in winters (https://www.greenmatch.
co.uk/blog/2014/08/insulation-why-is-it-important). It basically restricts the heat
flow inside or outside of the space, thus, making a building energy efficient (https://
www.usiinc.com/blog/insulation/why-you-should-insulate-your-walls/).
There are various insulation materials available in the market, however, the most
commonly employed insulation materials are the cellulose foam, mineral wool, fiber
glass, polyurethane foam and expanded polystyrene (EPS), which have been selected
for the purpose of this study for analysing their effects on the comfort level of the
occupants. The properties of these materials have been depicted in Table 4.4 (http://
www.greenspec.co.uk/building-design/insulation-materials-thermal-properties/):
In the simulation software, the construction of the wall was changed by employing
each of these insulations one by one with a constant thickness 0.04 m for the sake
of drawing comparisons between these materials and their impact on the thermal
comfort (Kapoor et al. 2016). Simulation was run for each of the case by providing
the values of the rest of the input parameters same as that of the base case.
Figure 4.5 depicts the results of the case when cellulose foam has been used as
the insulation material. It can be observed from the plot of PMV versus PPD for
this material that in contrast to the base case, the minimum PMV value has reduced
from −1.22 to −0.43 and the maximum PMV value has reduced from 1.11 to 0.98.
This implies that provision of this insulation has led to causing the PMV range of
the office room closer to the ideal range, though it still does not make for the ideal
comfort conditions. However, another point to be noted in this is that in comparison
to the base case, the curve has shifted towards the warmer side of the PMV scale,
denoting that cellulose foam insulation of 40 mm thickness is creating a barrier
for the heat inside the office room to cross and flow towards the ambient, hence,
48 S. F. Ali and D. Rakshit

Fig. 4.5 PMV versus PPD Cellulose Foam

curve for cellulose foam wall 42
insulation 39
36
33
30

PPD (%)
27
24
21
18
15
12
9
6
3
0
-1.4 -1.2 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4
PMV

creating a zone of warmer environment. It can also be spotted that the PPD value has
decreased as well, from a value of 36.61 to 25.24%, indicating that even though the
environment is warmer, more percentage of occupants are feeling satisfied with their
thermal environment, as during the winter season, people tend to opt for warmer
indoor spaces.
Likewise, each of the material was applied to the wall construction and similar
results were obtained, as that in Fig. 4.4, for each of the case, with slight deviations
in the extents of the PMV and PPD ranges, which can be observed from Table 4.5.
It can be established from the results that there is a reduction in the minimum and
maximum PMV values in all the cases. These materials brought the range of PMV of
the room closer to the ideal range, and in each of the case, curves of PMV versus PPD
have shifted from a cooler side of the scale to the warmer side of the PMV scale owing
to the reason that the colder conditions inside the unconditioned building due to the
ambient are transforming into warmer conditions because of the restriction to the heat
flow from the inside to the outside due to provision of insulation. Consequently, the
PPD value too has decreased in each case. Additionally, these results also suggest that
after provision of these insulating materials, people might feel discomfort because
of the resulting warmer environment rather than the colder one. On comparing the
results of the comfort indices for each of the insulating materials, it can be perceived

Table 4.5 Results of provision of wall insulation

Insulation Cellulose Mineral wool Fiber glass Polyurethane Expanded
material foam foam polystyrene
Minimum −0.43 −0.46 −0.45 −0.41 −0.44
PMV
Maximum 0.98 0.97 0.97 1.01 0.98
PMV
Maximum 25.24 25.13 24.82 26.54 25.19
PPD
4 Utilising Passive Design Strategies for Analysing Thermal … 49

Fig. 4.6 PMV versus PPD Cellulose Foam

curve for cellulose foam roof 42
insulation 39
36
33
30

PPD (%)
27
24
21
18
15
12
9
6
3
0
-1.4 -1.2 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4
PMV

that though they are all following the same trend, still there are slight differences in
the magnitude of these materials, which might be possible because of the different
thermal properties they possess, as mentioned in Table 4.4. On assessing the results in
these cases against each other, it is observed that the fiber glass insulation for the same
thickness is performing better in improving the comfort conditions of the occupants
inside the office space, as compared to the base case, while the least enhancement
was seen in the case of polyurethane foam insulation.
Provision of Roof Insulation. The materials selected for studying the effect of pro-
vision of wall insulation have also been applied for analysing the effect of provision
of roof insulation on the comfort of the occupants. Similar to the previous case, each
of the insulation with a constant thickness of 0.04 m was applied to the existing roof
construction and the rest of the input parameters same as the base case, so that their
effects on the comfort levels can be analysed and compared with each other (Kapoor
et al. 2016).
The results of the case when cellulose foam has been applied as the roof insulation
can be seen in Fig. 4.6. The results clearly indicate an improvement in the comfort
levels as against the base case. In this case, the minimum PMV value has reduced
from −1.22 to −0.64, while there is only a slight reduction in the maximum PMV
value from 1.11 to 1.06, implying that there is a shift in the curve towards the warmer
side of the PMV scale, in comparison to the base case. This indicates that a 40 mm
thick cellulose foam roof insulation is acting as a barrier between the ambient and the
indoor conditions of the space, and hence, restricts the inside heat to cross and flow
towards the ambient, thus, leading to a zone of warmer environment. This warmer
zone, however, is still an improvement over the base case, as the maximum PMV
corresponding to this zone is still less than the maximum PMV value corresponding
to the warmer zone of the base case. This case also signifies that the PMV range has
come closer to the ideal range, in comparison to the base case, still it is far from the
ideal comfort range. In addition to this, it can be seen from Fig. 4.6 that the PPD
value too has decreased from 36.61 to 28.52%, suggesting that bringing the maximum
PMV value closer to ideal range has also led to a decrease in the percentage of people
50 S. F. Ali and D. Rakshit

feeling dissatisfied with their thermal environment, since during the winter season,
people tend to opt for warmer indoor spaces.
In a similar manner, each of the material was applied to the roof construction,
which yielded similar results for each of the case, as that in Fig. 4.6, but with slight
variations in the maximum and minimum PMV as well as PPD values, as shown in
Table 4.6. The results clearly indicate that there is a reduction in the minimum and
maximum PMV values in all the cases. Each of these materials brought the range of
PMV of the room closer to the ideal range, and in each of the case, curves of PMV
versus PPD have shifted from a cooler side of the scale to the warmer side of the PMV
scale because of the fact that the colder conditions inside the unconditioned building
due to the ambient are converting into warmer conditions because of the barrier to the
heat flow from the inside to the outside due to provision of insulation. Subsequently,
the PPD value as well has decreased in each case. In addition to these, the results also
indicate that in comparison to the base case, where occupants were uncomfortable
due to the colder environment persisting inside the office room, the occupants would
now feel discomfort due to resulting warmer environment persisting in the room
due to the application of these insulation materials on roof. However, the percentage
of people dissatisfied with their thermal environment would still be less in each of
these cases, as against the base case, hence, signifying improvements in the comfort
conditions. The comparison of the results of the comfort indices for each of the
insulating materials demonstrate that though they are all following the same trend,
there are still some minor differences in the magnitudes of these materials, which
might be because of the different thermal properties they possess, as mentioned in
Table 4.4. On evaluating these case results against each other, it is observed that
in this case as well, the fiber glass insulation for the same thickness, is performing
better in improving the comfort conditions of the occupants inside the office space,
as compared to the base case, while the least enhancement was seen in the case of
mineral wool roof insulation.
Provision of Window Glazing. Glazing has an impact on the flow of heat across
windows in buildings as the heat is directly transferred through the window glaz-
ing, along with absorbing some portion of it which is consequently transmitted
to the inside space through radiation and convection (http://www.greenspec.co.uk/
building-design/windows/). Hence, the type of window and the glazing is another

Table 4.6 Results of provision of roof insulation

Insulation Cellulose Mineral wool Fiber glass Polyurethane Expanded
material foam foam polystyrene
Minimum −0.64 −0.66 −0.46 −0.64 −0.66
PMV
Maximum 1.06 1.07 0.80 1.04 1.06
PMV
Maximum 28.52 29.30 18.61 27.92 28.65
PPD
4 Utilising Passive Design Strategies for Analysing Thermal … 51

Table 4.7 Window glazing properties

Parameters Solar heat gain U value (W/m2 Emissivity Solar
coefficient K) Internal External transmittance
(SHGC) (T)
Double pane 0.76 2.9 0.84 0.84 0.70
clear glass
Triple pane 0.68 1.9 0.84 0.84 0.60
clear glass
Tinted glass 0.60 5.7 0.5 0.1 0.47
Low-e glass 0.62 3.8 0.1 0.5 0.58

important parameter for analysing heat exchange through fenestrations. This study
has been done considering four different types of glazing systems—double pane clear
glass, triple pane clear glass, tinted glass and low-emissivity (low-e) glass, with their
properties mentioned in Table 4.7 (http://glassed.vitroglazings.com/topics/how-low-
e-glass-works; EQUA 2013; https://www.commercialwindows.org/tints.php).
The input parameters for each of the three windows were modified according to
each of the case mentioned above and simulations were run for the cases to obtain the
thermal comfort indices for each of them throughout the simulation period. Figure 4.7
represents the PMV versus PPD curve for the case of double pane windows with clear
glass. The graph depicts that similar to the case of provision of wall insulation, the
PMV as well as PPD values have decreased in this case too. It can be observed that
the minimum PMV value has reduced from −1.22 to −0.61 and the maximum PMV
value has decreased from 1.11 to 0.85. The possible reason behind this could be that
its SHGC as well as transmittance value is comparatively high while its U value is
comparatively lower, hence admitting the solar heat to the inside while restricting
the heat from the inside to the outside. Correspondingly, the PPD value obtained in
this case has also reduced from 36.61 to 20.55%. It can be seen in this case as well
that the PMV as well as PPD ranges have been brought closer to the ideal ranges

Fig. 4.7 PMV versus PPD 42

curve for double pane clear 39
glass window 36
33
30
27
PPD (%)

24
21
18
15
12
9
6
3
0
-1.4 -1.2 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4
PMV
52 S. F. Ali and D. Rakshit

Table 4.8 Results of provision of window glazing

Type Double pane clear Triple pane clear Tinted glass Low-e glass
glass glass
Minimum PMV −0.61 −0.59 −0.81 −0.63
Maximum PMV 0.85 0.82 1.06 0.77
Maximum PPD 20.55 19.25 28.86 17.47

recommended by standards (ASHRAE 2010; ISO 2005) and the curve dominates
towards the zone of positive PMV values, making the environment comparatively
warmer, and resulting in attaining a maximum PPD value because of that. However,
that maximum PPD value is still approximately equal to the upper limit of the required
PPD, even though the PMV value is a little beyond the maximum tolerable PMV
value.
Analogously, the window properties were modified according to the cases under-
taken for the study and the simulation was run for each of them. The thermal comfort
indices obtained using the simulation analysis for each of the case were plotted
against each other and it was noticed that the PMV versus PPD curves of the cases
followed the same trend as depicted in Fig. 4.7, though with different PMV as well as
PPD ranges. The results of these cases are tabulated in Table 4.8. Both the PMV and
PPD values in each of the cases were observed to have comparatively shrunk down
and tended towards the ideal range, with a considerable shift from a cooler envi-
ronment to a warmer one. This suggests that changing the glazing type to the cases
chosen can result in admitting the heat inside and restricting its flow to the ambient.
Moreover, comparison of the results of these cases revealed that the difference in
their magnitudes are due to the different thermal properties they possess, enlisted in
Table 4.7. Comparison of results against each of the cases suggests that the low-e
glass has a better performance since the maximum reduction in values were observed
in this case, while minimum improvement was observed in the case of tinted glass.

4.3.4 Optimal Solution for Maximum Thermal Comfort

Implementing passive design technologies in buildings have a huge impact on the

energy consumption as well as maintaining comfort of occupants. Section 4.3.3 dealt
with examining the impact of different passive strategies applied to the building, for
which, other parameters were kept constant as the base case, on the basis of which,
a certain strategy giving the finest outcome is chosen. However, if more than one
strategy is applied to a building, the result of these techniques do not just add up
to each other. They have different interactions with the environment. They might
behave differently when alone or when in conjunction with other parameters. Hence,
the effects of these strategies should be studied when applied together in order to
analyse their inter relationships and to come to a conclusion on the basis of their
4 Utilising Passive Design Strategies for Analysing Thermal … 53

Fig. 4.8 PMV versus PPD 42

curve for optimised case 39
compared to base case 36
33
30
27

PPD (%)
24
21
18
15
12
9
6
3
0
-1.4 -1.2 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4
PMV
Base Case Optimised Case

objectives. The study involved analysing the effect of provision of wall insulation
and provision of roof insulation, both of which yielded fiber glass insulation in each
of their cases as the best among other options on the basis of maximum improvement
in thermal comfort of the occupants. Similarly, low-e glass windows provided better
results as compared to the other cases. However, for analysing the inter relationships
of these parameters, a combination of each of the type of window glazing was checked
and simulated with each of the case of provision of insulations in order to maximize
the thermal comfort goal. The simulation was run for the each combination and
the results in the form of thermal comfort indices was obtained. On comparing the
simulation results with each other, it was learnt that although individual strategies
applied showed different results, but if those strategies are used in conjunction, the
maximum improvement in comfort conditions was for the case of cellulose foam
wall insulation with fiber glass roof insulation of 40 mm thicknesses each, along with
triple pane clear glass window. The results of this case has been plotted in Fig. 4.8,
which depicts that there is a reduction in the minimum as well as maximum PMV
values, as compared to the base case. The minimum PMV value has reduced from −
1.22 to −0.31, while the maximum PMV value has decreased from 1.11 to 0.84. It
can be observed that when only individual strategies were taken into consideration,
the results with respect to the type of the insulation or glazing provisions were
different. However, when these strategies are combined, better results were obtained
with different type of insulation and glazing combination, rendering it as an optimal
solution for the cases studied. The combined effect of these strategies was that it
reduced the coolness in the inside zone and brought its minimum value down to −
0.31, which is within the ideal environmental condition (ASHRAE 2010; ISO 2005),
whereas, while doing that, the combined strategy led to shifting of the zone more
towards a warmer side for most of the instances, hence, most of the values of the
curve lie on the positive side of the PMV scale. Consequently, the PPD value too
dropped substantially, reaching to a value of 19.81% from 36.61%, indicating that
a larger percentage of occupants would be satisfied with their thermal environment.
54 S. F. Ali and D. Rakshit

Moreover, this has resulted in bringing the PPD value within the range specified by
standards, however, the value is nearer to the upper limit of the PPD range (ASHRAE
2010; ISO 2005). It is worth mentioning here that this forms the optimal solution
for the cases explored in this study, which has tried to improve the thermal comfort
conditions of the office room, still has not been successful in completely bringing it
within the ideal range for a comfort zone, hence, more passive strategies should be
explored and employed in order to maintain a thermally comfortable environment
for the occupants.

4.4 Conclusions

Building designers should focus on the comfort requirements of the occupants as well,
in addition to other goals. Employing passive design strategies can not only lead to
lowering the energy requirements of the building, but can also aid in improving the
thermal comfort in addition to other comfort parameters. The existing comfort level
for an office room has been determined, using the PMV and PPD models, and the
impact of different passive strategies on the comfort levels of the occupants have been
assessed. It was observed that application of different passive technologies resulted in
an improvement in the comfort conditions of the occupants. As compared to the base
case, the curve of PMV versus PPD approached closer to the ideal comfort range,
however, it was noticed that in each case, the curve shifted towards the positive side of
the PMV scale, indicating a zone of warmer environment. This resulted from the fact
that the cooler conditions inside the unconditioned building due to the ambient were
transforming into warmer conditions because of the heat being admitted inside from
the windows and the restriction to the heat flow from the inside to the outside due to
provision of different passive techniques. This, however, is still an improvement over
the base case as the maximum values of PMV as well as PPD was found to decrease,
since the study was done in the winter season, during which, a warmer environment
is preferred by the occupants.
An optimal solution with respect to the strategies explored in the study has been
proposed for the office room under analysis, which resulted in enhancement of the
comfort conditions. It has been observed that it is not necessary that the solution
obtained from employing individual strategies shall correspond to the optimal solu-
tion when a combination of the different passive techniques are used. When the
passive strategies were employed individually, the optimal solution for provision of
wall insulation was the fiber glass insulation of thickness 40 mm, which resulted in
percentage improvements of 63.11% and 12.61% on the negative and positive sides
of the PMV scale respectively, and 32.20% improvement in PPD value. Optimal roof
insulation, obtained with 40 mm thick fiber glass resulted in percentage improve-
ments of 62.29% and 27.93% in minimum and maximum PMV values respectively,
and 49.17% in PPD. Similarly, low emissivity glazing was yielded as the optimal
solution out of the different window glazing studied, which resulted in percentage
improvements of 48.36%, 30.63% and 52.28% in the values of minimum PMV,
4 Utilising Passive Design Strategies for Analysing Thermal … 55

maximum PMV and PPD respectively. However, it was observed that the comfort
indices and the optimal solutions obtained in the cases when individual techniques
were employed in the building were different from the optimal solution obtained
when these techniques were applied in conjunction with each other. The optimised
case resulted when cellulose foam wall insulation was applied with fiber glass roof
insulation of 40 mm thicknesses each, along with triple pane clear glass windows,
which caused the percentage improvements of 74.59% and 24.32% on the negative
and positive sides of the PMV scale respectively, while 45.89% improvement in the
PPD value. This shows the different behavior and interaction of these parameters
with the environment and their interdependency which led to a change in the final
result of the comfort indices. It has also been seen that the optimal solution on the
basis of the strategies undertaken in this study though improved the conditions as
compared to the base case, but were still not enough to completely transform the
indoor space into a comfortable environment. Thus, additional passive strategies can
be employed to bring the comfort indices within the ideal range, and hence maintain
a thermally comforting environment.

Acknowledgements Authors are thankful to Global Evolutionary Energy Design (GEED), New
Delhi, India, where the experimental work for this study was conducted.

References

02 Energy and Buildings (2014) New Delhi

Aryal P, Leephakpreeda T (2016) Effects of partition on thermal comfort, indoor air quality, energy
consumption, and perception in air-conditioned buildings. J Sol Energy Eng 138:051005. https://
doi.org/10.1115/1.4034072
ASHRAE (2010) ANSI/ASHRAE standard 55-2010, thermal environmental conditions for human
occupancy, 1st edn. American Society of Heating, Refrigerating and Air-conditioning Engineers,
Inc., Atlanta, Georgia, USA
Auliciems A, Szokolay SV (1997) Thermal comfort. PLEA in association with Department of
Architecture, University of Queensland
Azad AS, Rakshit D, Wan MP, Babu S, Sarvaiya JN, Kumar DEVSK, Zhang Z, Lamano AS,
Krishnasayee K, Gao CP, Valliappan S, Goh A, Seoh A (2018) Evaluation of thermal comfort
criteria of an active chilled beam system in tropical climate: a comparative study. Build Environ
145:196–212. https://doi.org/10.1016/J.BUILDENV.2018.09.025
Berkovic S, Yezioro A, Bitan A (2012) Study of thermal comfort in courtyards in a hot arid climate.
Sol Energy 86:1173–1186. https://doi.org/10.1016/J.SOLENER.2012.01.010
Calis G, Kuru M (2017) Assessing user thermal sensation in the Aegean region against standards.
Sustain Cities Soc 29:77–85. https://doi.org/10.1016/J.SCS.2016.11.013
Daghigh R, Adam N, Sopian K, Sahari B (2009) Thermal comfort of an air-conditioned office
through different windows-door opening arrangements. Build Serv Eng Res Technol 30:49–63.
https://doi.org/10.1177/0143624408099448
EQUA (2013) User manual, IDA indoor climate and energy, version 4.6. EQUA Simulation AB,
Sweden
Gadi MB (2010) Application of design and passive technologies for thermal comfort in buildings
in hot and tropical climates. Mater Energy Effic Therm Comf Build 681–708. https://doi.org/10.
1533/9781845699277.3.681
56 S. F. Ali and D. Rakshit

Greenspec: Windows: heat loss & heat gain. http://www.greenspec.co.uk/building-design/

windows/. Accessed 1 Jun 2019
Haase M, Amato A (2009) An investigation of the potential for natural ventilation and building
orientation to achieve thermal comfort in warm and humid climates. Sol Energy 83:389–399.
https://doi.org/10.1016/J.SOLENER.2008.08.015
How low-e glass works. http://glassed.vitroglazings.com/topics/how-low-e-glass-works. Accessed
15 Feb 2017
Insulation materials and their thermal properties. http://www.greenspec.co.uk/building-design/
insulation-materials-thermal-properties/. Accessed 9 Feb 2017
Insulation—why is it important? | GreenMatch. https://www.greenmatch.co.uk/blog/2014/08/
insulation-why-is-it-important. Accessed 2 Jun 2019
ISHRAE (2015) ISHRAE position paper on indoor environmental quality. New Delhi
ISO (2005) Ergonomics of the thermal environment—analytical determination and interpretation
of thermal comfort using calculation of the PMV and PPD indices and local thermal comfort
criteria. ISO, Switzerland
Kamal MA (2012) An overview of passive cooling techniques in buildings: design concepts and
architectural interventions. Acta Tech Napocensis Civ Eng Archit 55
Kapoor R, Roulet C-A, Maithel S, Bhanware P (2016) Thermal insulation of buildings for energy
efficiency
Katafygiotou MC, Serghides DK (2014) Thermal comfort of a typical secondary school building
in Cyprus. Sustain Cities Soc 13:303–312. https://doi.org/10.1016/J.SCS.2014.03.004
Lee JW, Jung HJ, Park JY, Lee JB, Yoon Y (2013) Optimization of building window system in Asian
regions by analyzing solar heat gain and daylighting elements. Renew Energy 50:522–531. https://
doi.org/10.1016/J.RENENE.2012.07.029
Liu S, Huang C, Liu Y, Shen J, Li Z (2018) Retrofitting traditional Western Hunan dwellings with
passive strategies based on indoor thermal environment. J Archit Eng 24:04018017. https://doi.
org/10.1061/(ASCE)AE.1943-5568.0000316
Nematchoua MK, Ricciardi P, Reiter S, Asadi S, Demers CM (2017) Thermal comfort and compar-
ison of some parameters coming from hospitals and shopping centers under natural ventilation:
the case of Madagascar Island. J Build Eng 13:196–206. https://doi.org/10.1016/J.JOBE.2017.
07.014
Pazhoohesh M, Zhang C (2018) Investigating occupancy-driven air-conditioning control based
on thermal comfort level. J Archit Eng 24:04018003. https://doi.org/10.1061/(ASCE)AE.1943-
5568.0000295
Pourshaghaghy A, Omidvari M (2012) Examination of thermal comfort in a hospital using
PMV–PPD model. Appl Ergon 43:1089–1095. https://doi.org/10.1016/J.APERGO.2012.03.010
Ravikumar P, Prakash D (2009) Analysis of thermal comfort in an office room by varying the
dimensions of the windows on adjacent walls using CFD: a case study based on numerical
simulation. Build Simul 2:187–196. https://doi.org/10.1007/s12273-009-9317-7
Rupp RF, Ghisi E (2017) Predicting thermal comfort in office buildings in a Brazilian temperate and
humid climate. Energy Build 144:152–166. https://doi.org/10.1016/J.ENBUILD.2017.03.039
Sharma P, Azad AS, Rakshit D (2015) Quantitative evaluation of directional influence on build-
ing performance in India. In: International conference on advances in power generation from
renewable energy sources (APGRES 2015), Kota, Rajasthan
SP 41 (1987) Handbook on functional requirements of a building (other than industrial buildings)
(parts 1–4). Bureau of Indian Standards, New Delhi
Stamou AI, Katsiris I, Schaelin A (2007) Evaluation of thermal comfort in indoor stadiums of the
Athens 2004 Olympic Games with CFD models: case of Nikea Indoor Stadium. J Archit Eng
13:130–135. https://doi.org/10.1061/(ASCE)1076-0431(2007)13:3(130)
Sushil Kumar TVK, Chandrasekar J, Moorthy SK, Sakthikala A, Arvind Bharath SR (2016) Opti-
mization of building envelope to reduce air conditioning. Indian J Sci Technol 9. https://doi.org/
10.17485/ijst/2016/v9i4/79072
4 Utilising Passive Design Strategies for Analysing Thermal … 57

Why you should insulate your walls | USI Building Solutions. https://www.usiinc.com/blog/
insulation/why-you-should-insulate-your-walls/. Accessed 3 May 2019
Windows for high-performance commercial buildings. https://www.commercialwindows.org/
tints.php. Accessed 29 Jan 2017
Zahiri S, Altan H (2016) The effect of passive design strategies on thermal performance of female
secondary school buildings during warm season in a hot and dry climate. Front Built Environ
2:3. https://doi.org/10.3389/fbuil.2016.00003
 Part II
Solar Thermal Systems: Heating
Chapter 5
Design and Development
of a Concentrated Solar Water
Heating System

Bandi Sai Mukesh, Ravi Teja Parella, Sudipto Mukhopadhyay

and Laltu Chandra

Abstract Solar energy is a promising renewable source to support the growing en-
ergy demand. This energy is widely harnessed for solar water heating systems to
provide hot water for both domestic and industrial sectors thus reducing use of con-
ventional energy sources. In this work, a concentrated solar water heater (CSWH)
system is designed and fabricated at IIT Jodhpur. The main objectives are devel-
opment of a point focus based direct solar water heating system and preliminary
experiment based evaluation of the designed system. The system envisages a flux
concentration of 100 Suns, which will enable receiver area reduction and the use of
other heat transfer fluids like oil in future. The CSWH system consists of (a) receiver
and (b) parabolic dish with two-axis sun tracking provision. In the conventional so-
lar water heater system the irradiance from sun is directly collected by the collector
whereas in concentrated solar water heater the reflected irradiance is received by the
receiver. The reflector consists of a reflecting surface mounted on a parabolic struc-
ture and the cavity receiver consists of consists of a serpentine copper tube exposed
to concentrated irradiance. The receiver will be insulated from top in order to prevent
heat loss from one of its surface. An optical model of parabolic dish and receiver has
been developed using TracePro software. This model is used as reference to generate
the flux density distribution. The experimental setup consists of a parabolic dish, a
receiver with thermocouples, a Coriolis flow meter, pump, water tank and NI DAQ.
Coriolis flow meter is used to measure the mass flow rate in the system. K-type

B. S. Mukesh · R. T. Parella · S. Mukhopadhyay

Department of Mechanical Engineering, Indian Institute of Technology Jodhpur,
Jodhpur, Rajasthan 342037, India
e-mail: bandisaimukesh@gmail.com
R. T. Parella
e-mail: teja.1@iitj.ac.in
S. Mukhopadhyay
e-mail: smukhopadhyay@iitj.ac.in
L. Chandra (B)
Department of Mechanical Engineering, IIT BHU, Varanasi 221 005, India
e-mail: chandra.mec@iitbhu.ac.in

© Springer Nature Singapore Pte Ltd. 2020 61

H. Tyagi et al. (eds.), Solar Energy, Energy, Environment,
and Sustainability, https://doi.org/10.1007/978-981-15-0675-8_5
62 B. S. Mukesh et al.

thermocouples are attached on to the receiver and the temperature is recorded using
NI DAQ system. The theoretical geometric concentration ratio predicted is 115 but
from the experiment a flux concentration ratio 94 is measured.

5.1 Introduction

Solar energy is a promising renewable source to meet the growing energy demand.
From the direct normal irradiance (DNI) map of India the abundance of solar ra-
diation in Rajasthan, Gujarat, and Ladakh is obvious with an availability of more
than 5.5 kWh/m2 /day NREL (2019). This energy can be harnessed for solar water
heating systems to provide hot water for both domestic and industrial sectors which
will reduce the electrical energy consumption. The solar water heater has many ad-
vantages, such as negligible global warming potential, lower payback period, ease
of manufacturing, and less maintenance Singh et al. (2016).
A large variety of solar water heater designs exist each with its own set of ad-
vantages and disadvantages. Flat plate solar water heaters consist of pipes welded
onto a metal plate collector, which is placed inside a rectangular case covered by
a glass with efficiency as high as 44% Ayompe and Duffy (2013a). Sae-Jung et al.
(2015) conducted experiments on a thermos-syphon based solar water heater, which
resulted in enhanced efficiency of 56%. Evacuated tube collectors consist of glass
vacuum-sealed metal absorber tube. Ayompe and Duffy (2013b) studied the thermal
performance of solar water heating system with evacuated heat-pipe tubular collector
and achieved efficiency of 62%. In general, evacuated tube collectors have higher
efficiency in comparison to flat-plate collectors. Diego-Ayala and Carrillo (2016)
compared performance of flat-plate water heating systems with thermos-syphon
and forced flow. They reported higher efficiency with forced flow in comparison
to thermos-syphon based system. Kakaza and Folly (2015) compared the alternative
energy sources to be provided for a particular demand. Results revealed that heat
pump based solar water heaters have short payback period even though the initial
investment is high. Papadimitratos et al. (2016) developed a new design by integrat-
ing evacuated tube solar collectors with phase change material (PCM) based thermal
energy storage. Here, heat pipe is immersed inside the phase change material, where
heat is effectively accumulated and stored for an extended period of time. Evacu-
ated tube collectors also have better thermal insulation in comparison to a flat-plate
collector, which enhances the overall efficiency. The efficiency of the Dual-PCM
solar water heater system has increased 26% compared with standard system. Xue
(2016) reported experimental investigation of a domestic solar water heater coupled
with phase change material based energy storage. Robles et al. (2014) conducted
experiments on a non-evacuated aluminium based mini channel solar water heater,
which resulted in an average increase in thermal efficiency of 13% compare to flat
plate collector. Deng et al. (2015) presented a novel flat plate solar water heater using
micro heat pipe array sprayed with solar selective coating and arranged closely as
the absorber of collector.The experiment was conducted on a 2 m2 flat plate collector
5 Design and Development of a Concentrated Solar … 63

and showed an efficiency of 64.25%. Some of the involved process parameters are
summarized in Table 5.1. Currently flat-plate solar water heater systems are widely
utilized. However, flat plate collectors suffer from disadvantages like occupying
more roof area and are corrosion prone. This has led to design and development of
concentrated solar heating systems.
In concentrated solar water heating systems, compound parabolic concentrators
(CPCs) are one of the majorly used collectors for domestic water heating purposes
after conventional flat plate collectors. Hadjiat et al. (2018) proposed design and
analysis of a novel integrated collector storage (ICS) solar water heater. The system
consists of CPC reflectors where solar collector and water storage are integrated as
single unit with the geometric concentration ratio of collector being around 1.22
and the highest temperature of water in tank is around 50 ◦ C. Benrejeb et al. (2015)
proposed an ICS solar water heater system with improved optical and thermal effi-
ciencies. The geometric concentration ratio was improved from 1.05 in old design
to 1.34 in new design and the highest temperature attained was 65 ◦ C. Harmim et al.
(2019) proposed a design of ICS solar water heater for integration into building fa-
cade. A linear parabolic reflector was used as concentrator, with the maximum water
temperature obtained varying from 40 to 49 ◦ Cand the geometric concentration ratio
reported is 3.3. Chong et al. (2012) investigated solar water heater using stationary
V-trough collector. The geometry is simpler as compared to that of a CPC. The set up
was designed for solar concentration ratio of 1.8 suns whereas from the experiments
it achieved a concentration ratio of 1.41 suns and the highest water temperature at-
tained in the tank is 85.9 ◦ C. Rajamohan et al. (2017) carried out analysis on a solar
water heater with parabolic dish concentrator and a conical absorber. Water is the
working fluid, and as solar radiation is concentrated onto the absorber the water in
the absorber evaporates and converts into vapor phase. The vapor is passed into a
heat exchanger where heat transfer between vapor and fresh cold water occurs with
the vapor condensing to liquid phase. The working fluid returns to the bottom of the
conical absorber under gravity and continues as a cyclic process inside the system.
The highest water temperature achieved is 65 ◦ C and the average system efficiency
achieved is 55.40%. The available literature in concentrated solar water system is not
abundant. Further, design of simple systems which can be easily fabricated and can
produce a high concentration ratio is desirable. In this work, a concept of concen-
trating solar water heating (CSWH) system is proposed and evaluated. This system
is expected to be versatile with multifaceted applications such as:
• Reduce the rate of heat loss by means of reducing the surface area while keeping
the water temperature comparable to conventional systems. This is expected to
increase the overall system efficiency.
• The proposed CSWH system envisages a flux concentration of about 100 Suns.
This may allow in future evaluation of other heat transfer fluids like oil by elevating
the operating temperature.
• The reduction of receiver area may allow implementation of such a system in cold
deserts or high-altitude by mitigating the freezing related issues.
 64

Table 5.1 Parameters of various solar water heater systems

Reference/ AP16 AR14 Robles et al. SX16 Xue (2016) UDA16 LMA13 Ayompe AD13 Ayompe and SJ15 Sae-Jung
parameters Papadimitratos (2014) Diego-Ayala and and Duffy (2013a) Duffy (2013b) et al. (2015)
et al. (2016) Carrillo (2016)
Power output, Pout (kW) 0.4 1.8 0.5 0.5 1 1.2 0.8
Power input, Pin (kW) 1.5 2.6 0.8 1.1 2.4 2.1 1.4
Solar irradiance, q (W/m2 ) 947 822 700 1200 633 701 700
Inlet temperature, T i (◦ C) 25–42 28–53 16–35 40 23–60 23–51 44
Outlet temperature, T o (◦ C) 33–50 33–58 21–40 48.6 28–66 29–57 69
Ambient temperature, T a (◦ C) 25 25 16 26 19 16 34
Thermal efficiency, η (%) 27.8 68 56.8 47 44 56 56.4
Mass flow rate, ṁ (kg/s) 0.0126 0.109 0.02 0.014 0.047 0.047 0.008
Tube diameter, D (mm) – 1.42 8 16 12 – –
Length of tube (mm) – 2912 1800 – – – 1800
Collector area (m2 ) 1.6 3.2 1.27 0.9 4 3 2.16
B. S. Mukesh et al.
5 Design and Development of a Concentrated Solar … 65

Most of the CSWH systems reported in literature are mostly based on a parabolic
trough concentrator with oil as working fluid to best of our knowledge. Water is used
as the secondary fluid, which is indirectly heated via a thermal energy storage, see
e.g. Prasartkaew (2018). In the proposed system, water serves as heat transfer fluid
as well as the storage. Thus, the present system is different and more compact.
With the above mentioned objectives, the design of the proposed CSWH system
is discussed followed by the experimental set up description. Finally results of the
experiments are presented.

5.2 Design of Concentrated Solar Water Heating System

The CSWH system consists of a receiver and a parabolic dish with two-axis sun
tracking mechanism. The schematic of the proposed receiver assembly is shown in
Fig. 5.1. In conventional solar water heater system, the irradiance from sun is directly
collected by the collector whereas in concentrated solar water heater the irradiance
reflected from the reflector is received by the receiver. Thus, the amount of heat flux
incident on the receiver in CSWH is greater than that collected by the collector in
conventional solar water heater. A parabolic dish reflector consists of a reflecting
surface mounted on a parabolic profile reflector. The surface of parabolic dish is
covered with aluminum foil which is a good reflector available at low cost. A two-
axis sun tracking system mechanism tracks the sun such that the sunrays are always
incident normally on the parabolic reflector. The reflected DNI from the parabolic
dish reflector is concentrated onto the focal point of parabolic dish where the receiver
is placed to heat the water. The area of receiver is 400 cm2 , which is substantially
lower compare to flat-plate solar collector, which is typically around 2–3 m2 for
similar thermal power. The receiver has square-shaped aperture of 20 cm × 20 cm.
Thus, the thermal losses will be reduced substantially as compared to a flat-plate
type system under the same operating conditions.

Fig. 5.1 Schematic of

concentrated solar water
heating (CSWH) system
66 B. S. Mukesh et al.

Fig. 5.2 Top view of

serpentine tube receiver

Figure 5.2 depicts the proposed receiver assembly comprising of serpentine tubes
inside a cuboid shaped receiver. The selection of receiver dimension is based on a
preliminary analysis of the spot size of concentrated solar irradiance from parabolic
dish. The receiver will be insulated from top in order to prevent heat loss from one
of its surface. The serpentine tube and receiver is made of copper with high thermal
conductivity of 401 W/mK at 300 K Incropera (2006). For higher absorption of
solar flux, the serpentine pipe is painted with black colour whose absorptivity is
0.98 Incropera (2006). The entire receiver is enclosed inside a cavity made up of
glass as depicted in Fig. 5.1. Due to the presence of cavity the ambient air will not
be in direct contact with the base of the receiver where the reflected irradiation is
concentrated. This will reduce losses by convective heat transfer. The glass may be
coated with a solar selective anti-reflective coating to reduce transmission of reflected
thermal radiation. Such a design is envisaged to be useful for high-altitude areas with
low ambient temperature, to avoid reaching freezing point temperature of water.
According to the NASA Surface meteorology, the monthly averaged DNI is 5–
6 kWh/m2 /day NREL (2019) in desert and high-altitude regions of India. Taking 9
hours of effective sun per day, the average solar irradiation on the dish is more than
600 W/m2 . In CSWH, the geometric concentration ratio (C) is defined as the ratio of
reflector (A pd ) to the receiver area (A R ) as given in Eq. (5.1). In the present case, C
∼ 115 and the corresponding heat flux onto receiver is given by Eq. (5.2).

A pd
C= (5.1)
AR

q R = C × q pd (5.2)

5 Design and Development of a Concentrated Solar … 67

5.2.1 Parabolic Dish Design

The parabolic dish reflector is designed for an input power of 2 kW thermal power
i.e. the reflector area collects 2 kJ/s from DNI. Aluminum foil used as the reflector
material on the dish has a reflectivity 0.85. For an average DNI 641.1 W/m2 , the
required minimum area of reflector is 4.3 m2 . For a parabolic dish with aperture of
W = 2.4 m and focal length f = 1.5 m where long focal length is chosen such that
the shadow effects are minimized, the depth of the parabolic dish Z R is calculated
as 0.24 m using Eq. (5.3).
W2
f = (5.3)
16Z R

The central role of the parabolic dish in solar concentrator systems is its ability to
focus parallel rays to a point, at distance f from its vertex as shown in Fig. 5.3. The rim
angle φ R is the angle between the axis and a line from the focus to physical edge of
the concentrator. Together the focal length and rim angle of a parabolic concentrator
completely define its cross-sectional geometry. The rim angle (φ R ) is given by

0.5W
tan φ R = (5.4)
f − ZR

where W is the width of parabola and Z R is the depth of the parabola.

The parabolic effect of focusing to a single point only occurs with perfectly
parallel incoming rays. Each point on the parabolic mirror will reflect a cone of rays
that matches the angular distribution of the solar source (half angle size θs ). The size
of the spot formed by the cone of rays reflected from the points on the mirror, when
incident on a flat target placed in the focal plane is shown in Fig. 5.3. The rays from
the rim will form the widest spot on the flat receiver. The reflected cone from the

Fig. 5.3 Parabolic dish with

ray tracing for calculation of
spot diameter on the receiver
68 B. S. Mukesh et al.

single spot on the mirror will actually form an elliptical spot on the target with a
major axis length of d Lovegrove and Pye (2012). The width of the focal spot on the
focal plane is given by Eqs. (5.5) and (5.6)

2P sin θs
d= (5.5)
cosφ R

where
0.5W
P= (5.6)
sinφ R

A parabolic dish of focal length (f ) = 1.5 m, aperture (W ) = 2.4 m and depth of

parabola (Z R ) = 0.24 m has been designed with a collector aperture area of ∼4.6 m2 .
This gives an input power of ∼2.5 kW. For the calculation of input power an average
irradiance of 641.1 W/m2 is taken. The sun rays reflected from the parabolic dish is
concentrated onto the receiver. These rays form an elliptical spot. Taking the value
of angular distribution of the solar source (half angle θs ) as 0.0046 rad, and using
Eq. (5.4), φ R comes out to be 0.76 rad. Now using Eq. (5.5), the width of the focal
spot obtained is 2.25 cm. To increase the width of spot on the receiver, flexibility is
provided in the design for a movement of 5–10 cm along the vertical direction. The
optical model as shown Fig. 5.4 is designed using TracePro software to calculate the
receiver travel so that the width of the spot is 20 cm.
The solar radiation incident on the parabolic dish is reflected on to the receiver
at the focus. This model is used as reference to generate flux density distribution.
Figure 5.5 shows the flux distribution and width size at focal point i.e., receiver is
placed at 1.5 m vertical from the vertex of parabolic dish. To get a spot of width 20 cm,
the receiver is moved down by ∼10 cm. The spot of width 20 cm covering the receiver

Fig. 5.4 Optical model for

ray-tracing
5 Design and Development of a Concentrated Solar … 69

Fig. 5.5 Flux distribution on receiver at focus

Fig. 5.6 Gaussian flux distribution on the receiver

70 B. S. Mukesh et al.

is shown in Fig. 5.6. In parabolic collector based systems, the flux concentrating on
to the receiver is not constant, the flux is distributed on the surface with a Gaussian
profile. The Gaussian flux distribution on the surface of the receiver is shown in
Fig. 5.6 having a spot diameter of 20 cm which is modeled in the ray tracing software,
TracePro. It is observed from Fig. 5.6 that the flux at the center of the receiver
surface is high and decreases with the increasing radius of spot. Here all the TracePro
simulations are done for average DNI of 641.1 W/m2 for Jodhpur.

5.2.2 Receiver Design

As shown in Fig. 5.2, the receiver consists of a serpentine tube. The surface of the
receiver, which is exposed to concentrated solar irradiance, is 20 cm × 20 cm in
dimensions. Let, the outer diameter of pipe be Do = 10 mm. Thus, maximum of 20
parallel pipes can be fitted on receiver with surface area of 20 cm × 20 cm. However,
the direct contact of pipe must be avoided to allow flow development and heating up
with distance. Consequently, there must be a gap between these pipes. For n number
of parallel pipes there will be n − 1 number of gaps in the serpentine tube receiver
as in Fig. 5.2, let x is the size of gap between the pipes, y is the length of the parallel
pipes with uniform spacing. Therefore, the total length of receiver is given by:

Do n + x(n − 1) = 20 (5.7)

y + x + 2Do = 20 (5.8)

The total length of pipe is given by:

π
L = ny + (n − 1) (x + Do ) + x + 2Do (5.9)
2
For the current purpose Re D >5000 is considered in view of turbulent flow and the
associated higher heat transfer compared to laminar flow. This will mitigate to some
extent the heat loss by reduced surface temperature of copper pipe. The temperature
difference between the inlet and outlet of the receiver for a total tube length of L
is given by Eq. (5.6) Incropera (2006) and the efficiency of CSWH is given by Eq.
(5.11).
q  π Do L
T = R (5.10)
ṁc p

ṁc p T
η= (5.11)
q  A pd
5 Design and Development of a Concentrated Solar … 71

Table 5.2 Variation of T with length of pipe

n gap, x(mm) Length, L(m) T (◦ C) for ṁ = T (◦ C) for ṁ =
0.03 kg/s 0.035 kg/s
11 9.5 2.2 9 7.6
12 7.8 2.4 9.7 8.3
13 6.3 2.6 10.5 9
14 5.1 2.8 11.3 9.7
15 4.1 3.0 12.2 10.4

Table 5.3 Receiver geometry design parameters

Parameter Value
width of absorber, w 20 cm
Length of pipe, L 2.6 m
Outer diameter of pipe, Do 9.525 mm
Inner diameter, Di 7.747 mm
No of loops, n 13

Table 5.2 shows the variation of temperature rise with length of pipe. The length
of 2.6 m is chosen for the design as it gives a temperature rise of 10 ◦ C and also
for ease of manufacturing of the serpentine receiver. The parameters of serpentine
receiver are shown in Table 5.3.

5.3 Manufacturing of CSWH

The development of the CSWH system involved the fabrication of parabolic dish,
copper tube receiver, selection of required accessories and assembling the system.
At first, mild steel strips with the required co-ordinates are forged and assembled
into a parabolic skeleton. Channels are fitted on to the skeleton and aluminium sheets
(reflectivity 0.85) are fixed on the channels with screws, forming the parabolic dish of
required depth and aperture. The receiver is made by using copper tube. A copper tube
of length 2.6 m and outer diameter 9.525 mm is cut and brazed with elbows to form the
required shape according to the design specifications. The receiver is insulated with
a pack of glass wool on the backside. A cavity is made with aluminium sheets on the
receiver to reduce the losses and increase efficiency. Cavity receiver is mounted on
to the parabolic dish with steel angles. Manual tracking mechanism is incorporated
on the parabolic dish for tracking the sun. Hose pipes of diameter 12.7 mm are used
to connect the inlet and outlet of the receiver to the reservoir tank of capacity 200 L.
An electric pump is installed for circulating the water in the system along with a seat
valve to control the mass flow rate at the inlet (Figs. 5.7 and 5.8).
72 B. S. Mukesh et al.

Fig. 5.7 Manufacturing of parabolic dish and receiver

Fig. 5.8 Experimental layout of concentrated solar water heating (CSWH) system

5.4 Experimental Setup

The experimental setup consists of a parabolic dish, a receiver with thermocouples, a

Coriolis flow meter, a 0.5 hp pump, 200 L tank and data acquisition system (make NI).
Coriolis flow meter is used to measure the mass flow rate in the system. The K-type
thermocouples are attached on to the receiver and the temperature is recorded using
NI DAQ system. The water is pumped using a 0.5 hp pump from the reservoir filled
with 200 L of water and mass flow rate is controlled using a seat valve. The mass flow
rate in the experiment is 0.03 kg/s and the parabolic dish is tracked manually with
respect to sun. Solar radiation is reflected by the parabolic dish on to the receiver
and this radiation is absorbed by the serpentine copper tubes. The water is kept
circulating inside the copper tubes with the help of pump. The heat transfer takes
place between the copper tube and the water flowing through it and the temperature
rises. It is to be noted here that the system is single pass and hence the temperature
rise is limited by that. The serpentine pipe is coated with black paint for higher
5 Design and Development of a Concentrated Solar … 73

absorptivity. The storage tank is also an insulated tank to minimize the heat loss to
the ambient. The experiment is carried out from 4:05 p.m. to 5:15 p.m. in the month
of April at IIT Jodhpur and the DNI data at the time of experiment is obtained from
the DNI measuring station at IIT Jodhpur. The experiments are performed manually
by aligning the dish at an interval of 15 min following the shadow of receiver at the
center of dish. The day time was selected in view of clear sky, low wind speed and low
dust condition. Further, experiments at different times of day will be conducted in
future. The actual experimental setup and the zoomed in view of the focused receiver
is shown in Fig. 5.9.

5.5 Results and Discussions

From the DNI data on 16-04-2017, the measured average flux is 322.08 W/m2 (from
4:25 p.m. to 4:30 p.m.). The ambient temperature at the time of the experiment is
314 K and and mass flow rate of water is 0.03 Kg/s. It is observed in the experiments
that the spot size on the receiver is 30 cm, which is larger than the theoretical spot
size. So reflectors are folded about the receiver as shown in Fig. 5.9 which resem-
bles a cavity to reflect the solar radiation on the serpentine copper tube. The outlet
temperature and inlet temperature of fluid obtained from the experiment is shown in
Fig. 5.10. It must be mentioned here that since the system is single pass, the outlet
temperature attained is relatively low. The highest temperature difference between
inlet and outlet attained is 6.6 and the average temperature difference is 6 ◦ C. A
multipass system can easily produce hot water for domestic use within reasonable
time duration. The flux estimated from the water inlet and outlet experimental tem-
peratures is 30.58 kW/m2 . In the experiments, the power input to CSWH is around
1.4kW and the efficiency of CSWH excluding the pump power is 59%.

Fig. 5.9 Parbolic dish and receiver with zoomed in view of receiver
74 B. S. Mukesh et al.

Fig. 5.10 Inlet and outlet temperature from the single pass CSWH system

The theoretically predicted geometric concentration ratio is 115 but from the ex-
periment the flux concentration ratio obtained is 94. The obtained outlet experimental
temperature can be used for validation of the numerical model of this system.

5.6 Conclusions

A concept of concentrated solar water heater (CSWH) system is proposed, designed

and developed from easily available and low cost materials. This is based on the point
focusing concept using parabolic dish onto a small receiver. This aims to reduce the
exposed surface area of receiver to ambient and thus, reduce heat loss. The presented
CSWH system shows a thermal efficiency of about 59% at a lesser power input and
smaller HTF mass flow rate as compared to system LMA13 as in Table 5.1 Ayompe
and Duffy (2013a), which is a non-concentrating solar water heating system with
thermal efficiency of 44%. The theoretical geometric concentration ratio is 115 and
from the experiments conducted a flux concentration ratio of 94 is obtained. The first
level design with the measured efficiency of about 59% compares well with that of
the existing systems. Further improvements are foreseen with a cavity to mitigate the
direct heat loss. Due to the ease of the fabrication, easy availability of the materials
used and low cost the presented CWSH system can be widely used.
5 Design and Development of a Concentrated Solar … 75

References

Ayompe L, Duffy A (2013) Analysis of the thermal performance of a solar water heating system
with flat plate collectors in a temperate climate. Appl Therm Eng 58(1–2):447–454
Ayompe L, Duffy A (2013) Thermal performance analysis of a solar water heating system with heat
pipe evacuated tube collector using data from a field trial. Solar Energy 90:17–28
Benrejeb R, Helal O, Chaouachi B (2015) Optical and thermal performances improvement of an
ics solar water heater system. Solar Energy 112:108–119
Chong K, Chay K, Chin K (2012) Study of a solar water heater using stationary v-trough collector.
Renew Energy 39(1):207–215
Deng Y, Zhao Y, Quan Z, Zhu T (2015) Experimental study of the thermal performance for the
novel flat plate solar water heater with micro heat pipe array absorber. Energy Procedia 70:41–48.
International Conference on Solar Heating and Cooling for Buildings and Industry, SHC 2014
Diego-Ayala U, Carrillo J (2016) Evaluation of temperature and efficiency in relation to mass flow
on a solar flat plate collector in mexico. Renew Energy 96:756–764
Hadjiat M, Hazmoune M, Ouali S, Gama A, Yaiche M (2018) Design and analysis of a novel ics
solar water heater with cpc reflectors. J Energy Storage 16:203–210
Harmim A, Boukar M, Amar M, Haida A (2019) Simulation and experimentation of an integrated
collector storage solar water heater designed for integration into building facade. Energy 166:59–
71
Incropera FP (2006) Fundamentals of heat and mass transfer. Wiley
Kakaza M, Folly K (2015) Effect of solar water heating system in reducing household energy
consumption. IFAC-PapersOnLine 48(30):468–472. 9th IFAC Symposium on Control of Power
and Energy Systems CPES 2015
Lovegrove K, Pye J (2012) 2 - fundamental principles of concentrating solar power (csp) systems.
In: Lovegrove K, Stein W (eds) Concentrating solar power technology. Woodhead Publishing
Series in, Energy, Woodhead Publishing, pp 16–67
NREL. DNI Map of India
Papadimitratos A, Sobhansarbandi S, Pozdin V, Zakhidov A, Hassanipour F (2016) Evacuated tube
solar collectors integrated with phase change materials. Solar Energy 129:10–19
Prasartkaew B (2018) Efficiency improvement of a concentrated solar receiver for water heating
system using porous medium. In: IOP conference series: materials science and engineering, vol
297, Jan, p 012059
Rajamohan G, Kumar P, Anwar M, Mohanraj T (2017) Analysis of solar water heater with parabolic
dish concentrator and conical absorber. In: IOP conference series: materials science and engi-
neering, vol 206, Jun, p 012030
Robles A, Duong V, Martin AJ, Guadarrama JL, Diaz G (2014) Aluminum minichannel solar water
heater performance under year-round weather conditions. Solar Energy 110:356–364
Sae-Jung P, Krittayanawach T, Deedom P, Limmeechokchai B (2015) An experimental study of
thermo-syphon solar water heater in thailand. Energy Procedia 79:442–447. 2015 International
Conference on Alternative Energy in Developing Countries and Emerging Economies
Singh R, Lazarus IJ, Souliotis M (2016) Recent developments in integrated collector storage (ics)
solar water heaters: a review. Renew Sustain Energy Rev 54:270–298
Xue HS (2016) Experimental investigation of a domestic solar water heater with solar collector
coupled phase-change energy storage. Renew Energy 86:257–261
Chapter 6
Multi-objective Performance
Optimization of a Ribbed Solar Air
Heater

Naveen Sharma and Rajesh Choudhary

Abstract To achieve maximum thermohydraulic performance, by maximizing the

heat transfer and minimizing the pumping power, for a ribbed solar air heater, a
Taguchi method has been used to predict the optimal set of design and flow param-
eters. An L16 (43 ) orthogonal array is selected as an investigational plan to perform
the computational fluid dynamics (CFD) simulations for investigating the effect of
design parameters, i.e. Reynolds number (4000–16000), rib pitch to height ratio
(3–12), and rib geometry due to change in inclination angle of front face of the rib (α
= 45°–90°) on the effectiveness of a solar air heater. Nusselt number, friction factor
and thermohydraulic performance are considered as performance indexes, and the
minimization of pumping power (minimum pressure drop) and the maximization of
heat transfer and overall performance are taken as the optimization criteria. Results
show that Reynolds number has the greatest influence on the heat transfer as well as
thermohydraulic performance, while rib spacing on the friction factor ratio. The best
combinations of design factors for heat transfer, pressure drop, and thermohydraulic
performance are A4 B4 C1 , A4 B1 C1 , and A1 B4 C1 , respectively. Numerical results val-
idate the aptness of the proposed methodology, so, this investigation suitably offers
an improved rib design for internal fluid flow investigations related to effective heat
transfer applications.

Keywords Computational fluid dynamics · Heat transfer · Pumping power ·

Optimization · Solar air heater · Thermohydraulic performance

6.1 Introduction

In today’s scenario, the demand of energy is increasing continuously owing to the

industrial development and increasing population. With limited and rapidly decreas-
ing conventional energy sources, the supply of consistent and continual energy has

N. Sharma (B)
DVR & Dr. HS MIC College of Technology, Kanchikacherla, A.P 521180, India
e-mail: sharma.naveen28@yahoo.com
R. Choudhary
Lovely Professional University, Phagwara, Punjab 144411, India
© Springer Nature Singapore Pte Ltd. 2020 77
H. Tyagi et al. (eds.), Solar Energy, Energy, Environment,
and Sustainability, https://doi.org/10.1007/978-981-15-0675-8_6
78 N. Sharma and R. Choudhary

turned into a global concern. Therefore, use of renewable energy sources and design-
ing compact and efficient thermal systems are urgent requirements to have a sustain-
able future. Solar air heater (SAH), act as heat exchangers, uses solar energy to
produce low to moderate temperatures which have a number of applications such
as solar water heaters, in preserving industrial and agricultural products and heat-
ing/cooling of buildings (Siddhartha et al. 2012). Admittedly, the solar air heaters
have simple design and easy operation, but also have fewer problems, for instance,
such as less heat storage capacity and low performance. Therefore, substantial efforts
are required to make these thermal systems more appropriate and cost-effective.
For improving the performance of heat exchanging devices, the researchers
and scientists working in the pertinent field have suggested numerous heat trans-
fer enhancement (active/passive) techniques (Webb 1994). Generally, heat transfer
enhancement techniques such as rib turbulators, winglets and extended surfaces con-
trol the flow passively and cause turbulence in the near wall region which leads to
reduction in the thermal resistance compared to a conventional heat exchanger and
provide enhanced heat transfer. Along with enhanced heat transfer rates, the pres-
sure penalty also increases with the introduction of rib turbulators (Tariq et al. 2018).
These conflicting perspectives motivate the researchers and designers to find the solu-
tion how much should be the modification required in the rib geometry for achieving
the best performance. Significant amount of research activity has been devoted to
the understanding of detailed heat transfer and fluid flow in rib turbulated ducts for
different applications (Kumar et al. 2019; Sharma et al. 2018; Jin et al. 2019). Nowa-
days, researchers have more focused towards employing soft computing approaches
such as ANN, RSM, Jaya algorithm, GA, and Taguchi method for selecting optimal
blends of design parameters in different heat transfer applications (Rao et al. 2018;
Aidinlou and Nikbakht 2017; Sharma et al. 2017; Nagaiah and Geiger 2019).
It is evident that for predicting the optimal sets of design factors corresponding
to maximum heat transfer, minimum friction factor and highest thermohydraulic
performance, there is no need to perform all the simulations/experiments (an = a ×
a × …n times). Conducting all the experiments/CFD simulations consumes too much
time and also costly affair; but, the Taguchi method effectively predicted the optimum
values with a few experiments/simulations and short span of time. Therefore, in the
present work also Taguchi method has been successfully applied with a very limited
number CFD simulations for prediction of the best sets of design parameters.
Numerous investigations have been performed to design and optimize the design
parameters of heat exchangers mounted with ribs, wire inserts, fins for various heat-
ing/cooling applications using Taguchi Method (Yun and Lee 2000; Bilen et al. 2001;
Wang et al. 2009; Aghaie et al. 2015; Chamoli 2015; Caliskan et al. 2016; Kotcioglu
et al. 2018; Sahin et al. 2019). Yun and Lee (2000) systematically analyzed the
influence of slit fins on the aerothermal characteristics using the Taguchi method.
Four design parameters among considered eight parameters have significant contri-
butions, i.e. 39%, 28%, 20% and 9% contributions of fin pitch, slit pattern angle,
length and height of the slit, respectively, on the performance of the slit finned heat
exchanger. The contribution of Reynolds number and rectangular block positions,
i.e. angular displacement, span wise and stream wise disposition, placed on a heat
6 Multi-objective Performance Optimization of a Ribbed Solar … 79

transferring surface on heat transfer enhancement have been studied using Taguchi
Method (Bilen et al. 2001). The most significant parameter influencing heat transfer
is the Reynolds number, which is followed by the turning angle of the block. It was
also reported that the heat transfer rate increases with increasing Reynolds number
and turning angle of the block. The cooling effectiveness of a reactor with/without
noise sinking shield was studied by Wang et al. (2009) for avoiding the overheat-
ing problem using a systematic CFD-Taguchi approach. The radius of the bottom
and the top opening of noise reducing cover are found to be the most significant
with contribution ratios of 49.5% and 23%, respectively, to the natural convection
cooling performance. The performance of a ribbed SAH was studied by Aghaie
et al. (Aghaie et al. 2015) using CFD-Taguchi approach at a Reynolds number of
10000. The authors optimized a general rib geometry, which can produce triangular,
trapezoidal and rectangular geometries by varying its parameters, while considering
maximization of thermal performance as the criteria of optimization. It was found
that relative rib pitch and height have the greatest influence on performance improve-
ment. Triangular rib geometry was found to be the optimum configuration. Chamoli
(Chamoli 2015) optimized different design parameters, i.e. Reynolds number (Re),
open perforation ratio (β), relative baffle height (e/H) and relative baffle pitch (p/e), of
a rectangular duct roughened with perforated V-down baffle. Maximum heat transfer
and minimum friction factor conditions were found at p/e = 2, e/H = 0.4, β = 12%,
Re = 18600 and p/e = 4, e/H = 0.285, β = 44%, Re = 14800 respectively. Caliskan
et al. (2016) studied the impact of design parameters on heat transfer distribution
for a surface attached with V-shaped and convergent-divergent shaped ribs. Results
revealed that Reynolds number was the most dominant factor influencing heat trans-
fer, and the highest thermal performance was obtained for V-shaped ribs at Reynolds
number of 10000. The aerothermal features of a cross flow heat exchanger with
pin-fins have been studied by Kotcioglu et al. (2018) using Taguchi method with L25
orthogonal array. The best thermal performance was observed for hexagonal pin-fins,
followed by square-angle pin-fins. Recently, the design parameters, i.e. corner angle,
inclination angle, baffle height, baffle length, baffle width and Reynolds number, of
a heat sink fixed with hollow trapezoidal baffles have been optimized to enhance the
performance (Sahin et al. 2019). The length of the baffle is the greatest influenc-
ing factor, with contribution ratio of 32.5%, on the pumping power and Reynolds
number, with influence ratio of 80%, for heat transfer.
The above discussed studies reveal that the rib design parameters i.e. shape, size,
spacing, inline or staggered arrangement, number of ribbed walls, perforation, rib
inclination, and Reynolds number, have significant impact on the overall perfor-
mance of the heat exchanger mounted with ribs. It has also been reported that the
CFD-Taguchi scheme can be employed easily and also economically for a reli-
able and robust optimization (Wang et al. 2009; Aghaie et al. 2015). Therefore, the
present research work is primarily concentrated on the prediction of best rib design
parameters for a ribbed SAH using Taguchi method. The ribs have been provided
on the bottom surface, which was exposed to constant heat flux. Firstly, the pro-
found impacts of design parameters, i.e. rib geometry (see Fig. 6.1), rib spacing and
80 N. Sharma and R. Choudhary

Fig. 6.1 Details of SAH and considered rib geometries

Reynolds number, on flow structures variation has been studied from CFD simula-
tions. Subsequently, the performance indexes, i.e. heat transfer in terms of Nusselt
number, pumping power in terms of friction factor, and effectiveness in terms of ther-
mohydraulic performance, have been determined. The proposed innovative geometry
(Fig. 6.1), that can produce right angle triangular, square and trapezium ribs, has been
optimized using Taguchi method, and then the order of most influential parameters
on performance indexes has been assessed.
6 Multi-objective Performance Optimization of a Ribbed Solar … 81

6.2 Geometric Modeling

A 2-dimensional model of a SAH inserted with periodic ribs, whose front side is
inclined at different angles (45°, 60°, 75°, and 90°), has been developed to understand
the flow and heat transfer features (see Fig. 6.1).
The desired airflow, at atmospheric temperature and pressure, is provided by
selecting suitable Reynolds number (Re = 4000, 8000, 12000 and 16000) in order
to deduce forced convective heat transfer in the duct. The lower ribbed surface is
provided with continuous heat flux of 4000 W/m2 , while the other sides are adiabatic
in nature. The cross-section of the rib is square, i.e. e = w = 8 mm, while the
duct height (H) and hydraulic diameter (Dh ) are 40 and 80 mm (2H). The spacing
between two consecutive ribs is varied as 3e, 6e, 9e, and 12e. Depending upon front
face inclination, the type of rib geometry changes from triangular at inclination angle
of 45° to trapezoidal for inclination angles of 60° and 75°, and square at inclination
angle of 90° (Fig. 6.1).
The computational domain, as shown in Fig. 6.1, is judiciously decided so as to
settle a uniform flow at inlet and fully developed flow at outlet (Aghaie et al. 2015).
Numerical simulations have been carried out with certain assumptions, i.e. the flow is
steady and fully developed turbulent flow, pressure variation and shear forces in wall
normal direction are considered as zero, body forces due to gravity are neglected,
working fluid is considered as an incompressible, and the axial heat conduction in
the fluid is negligible.

6.2.1 Mathematical Modeling

The flow and heat transfer behaviour inside a ribbed SAH are mathematically
described by some governing equations, i.e. continuity, momentum and energy equa-
tions, as given below in Cartesian coordinate system (Fluent 2006):
Mass conservation (Continuity equation):

∂
(ρu i ) = 0 (6.1)
∂ xi

Momentum equation:
   
∂   ∂ ∂u i ∂u j 2 ∂u k ∂  ∂P
ρ ui u j = μ + − δi j + −ρu i u j −
∂x j ∂x j ∂x j ∂ xi 3 ∂ xk ∂x j ∂ xi
(6.2)
82 N. Sharma and R. Choudhary

Energy Equation:
 
∂   ∂ ∂T
ρ μj T = ( + t ) (6.3)
∂x j ∂x j ∂x j

RNG k-ε model with “enhanced wall treatment” has been reported as the finest
turbulence model to foresee the aerothermal characteristics inside a duct with attached
mechanical devices (Wang et al. 2009; Aghaie et al. 2015; Akcayoglu and Nazli
2018). Therefore, the RNG k-ε model, which holds turbulent kinetic energy (k) and
turbulence dissipation rate (ε) is employed here (Fluent 2006):
 
∂ ∂ ∂k
(ρ k u i ) + ρ ε = μe f f αk + Gk (6.4)
∂ xi ∂x j ∂x j
 
∂   ∂ ∂ε E ε2
ρ εuj − μe f f αE + RE = C1ε (G k ) − C2ε ρ (G k ) (6.5)
∂ xi ∂x j ∂x j k k

where, αk and αε are the effective turbulent Prandtl number for k and ε.
μt (=ρCμ (k 2 /ε)) is turbulent viscosity and μe f f is effective turbulent viscosity.

6.2.2 Numerical Solution Procedure

The commercial software ANSYS 15.0 has been used for solving the govern-
ing equations by using Finite Volume Method (FVM) with segregated solution
approach. The second order upwind scheme has been used to the governing equations.
QUICK differencing scheme has been employed for solving momentum equation.
The pressure–velocity-coupled solution is obtained by SIMPLE algorithm (Fluent
2006). Air is considered as working fluid. The convergence conditions of 10−3 and
10−6 have been applied for momentum and energy equations, respectively. After
grid independency, a mesh grid consisting of 1.81 × 105 cells has been found to be
suitable for CFD simulations, and further increase in number of cells has negligible
effect on heat transfer results with a maximum deviation of about 1.5%.
The averaged heat transfer distribution, Nu, is assessed from the local convective
heat transfer coefficient, h(x), and expressed as (Aghaie et al. 2015):

1 h(x)Dh
Nu = (6.6)
L k

The friction factor, f, is the parameter refers to pressure penalty (P) and measured
over the length of test section, L, with a flow velocity of u as (Webb 1994; Tariq et al.
2018):
6 Multi-objective Performance Optimization of a Ribbed Solar … 83

2 × P × Dh
f = (6.7)
ρu 2 L

To study the combined effect of turbulence on heat transfer augmentation and

increase in pressure drop, the thermohydraulic performance is introduced as (Webb
1994; Tariq et al. 2018),

N u/N u s
η= (6.8)
( f / f s )1/3

here, subscript s refers to the smooth duct results.

Further, Dittus-Boelter and modified Blasius correlations are used to find the
Nusselt number and friction factor inside a smooth SAH, respectively, as (Aghaie
et al. 2015):

N u s = 0.024 × Re0.8 × Pr 0.4 (6.9)

f s = 0.085 × Re−1/4 (6.10)

6.3 Taguchi Approach

Taguchi approach is initially proposed as a method for enhancing the quality of

products. It is usually embraced for optimizing the design parameters, in any case,
by using two basics ideas. First idea is that the quality loss should be characterized
as deviances from the major goals, not conformity to random details, and the other
idea is accomplishing high system quality ranks, from financial aspects, allocated to
the quality products. To accomplish attractive product quality, Taguchi recommends
a three-stage process, i.e. design of system, parameter and tolerances.
Since the experimental techniques are costly and tedious, the requirement to fulfill
the design goals with the minimum number of assessments (experimental/simulation)
is evidently a vital prerequisite. Therefore, Taguchi robust design strategy, which
utilizes the potential of numerical tools called Orthogonal Array (OA) and signal to
noise (S/N) ratio, can be suitably used for a wide range of process parameters with few
experiments. The careful selection of controlled parameters and response parameters
is essential for accurate results. In the present work, the controllable parameters are
Reynolds number, relative rib pitch and inclination angle (α) as described in Table 6.1.
The response factors are heat transfer (Nu), friction factor (f ) and thermohydraulic
performance (η). Taguchi investigation is executed with Minitab 17.0 software. A
L16 (43 ) OA is utilized, which inferred completing 16 tests with 3 factors of 4 levels
that are documented in Table 6.2. Based on the proposed OA, the CFD simulations
have been performed to obtain the values of response parameters, as presented in
Table 6.2.
84 N. Sharma and R. Choudhary

Table 6.1 Taguchi parameters and their levels

Code Parameters Levels
1 2 3 4
A Reynolds Number (Re) 4000 8000 12000 16000
B Rib pitch to height ratio (p/e) 3 6 9 12
C Inclination angle (α) 45° 60° 75° 90°

Table 6.2 Chosen L16 (43 ) orthogonal array and corresponding simulation results
Number of test Factors and their levels Simulation results
A (Re) B (p/e) C (α) Nu f η
1 1 1 1 40.98 0.010555451 2.58
2 1 2 2 38.459 0.013026767 2.26
3 1 3 3 39.701 0.015456197 2.20
4 1 4 4 41.014 0.017508645 2.18
5 2 1 2 54.02 0.009832906 1.89
6 2 2 1 64.803 0.010942903 2.19
7 2 3 4 65.44 0.011844515 2.15
8 2 4 3 66.955 0.013529408 2.11
9 3 1 3 68.318 0.009229039 1.71
10 3 2 4 75.682 0.010104006 1.83
11 3 3 1 90.06 0.011697562 2.08
12 3 4 2 89.525 0.012511095 2.02
13 4 1 4 83.63 0.008236498 1.68
14 4 2 3 93.063 0.008874747 1.83
15 4 3 2 99.059 0.010604145 1.83
16 4 4 1 111.21 0.012571249 1.94

Taguchi method strongly recommends the use of signal-to-noise (S/N) ratios.

Maximizing the S/N ratio results in minimization of the losses related with the
process. There are numerous S/N ratios accessible relying upon the sort of charac-
teristics. The S/N ratio attributes can be partitioned into three classifications, stated
by Eqs. (6.11)–(6.13) when the characteristic is continuous (Ross 1995):
Smaller is better:
 
S y2
= −10 log (6.11)
N n

Nominal is best:
6 Multi-objective Performance Optimization of a Ribbed Solar … 85

S Y /SY2
= −10 log (6.12)
N n

Larger is better:
 
S 1/y 2
= −10 log (6.13)
N n

where, y and Y is the local and averaged observed data of n number of observations,
respectively, while SY2 is variance of y. Suitably “larger is better” characteristic is
used for maximization of heat transfer (Nu) and performance (η), while “smaller is
better” characteristic is employed for minimization of pumping power (f ).

6.4 Results and Discussion

Before performing simulations for all the cases as mentioned in Table 6.2, the numer-
ical results obtained using RNG k-ε turbulence model for smooth duct are compared
with Dittus-Boelter (Eq. 6.9) and modified Blasius (Eq. 6.10) correlations and pre-
sented in Fig. 6.2. The CFD simulation results are showing good agreement with
standard correlations with the maximum deviation of ±5.23% for Nu and ±7.84%
for f.
The profound impact of rib installation, which plays a decisive role in varying
the flow patterns, has been shown in Fig. 6.3. From computational results, a trapped
vortex is found at the lowest rib spacing (p/e ≤ 6), whereas the flow reattaches on the

Fig. 6.2 Validation of computational results with standard correlations

86 N. Sharma and R. Choudhary

Fig. 6.3 Effect of rib

spacing on flow structures

surface, but without boundary layer redevelopment, at the intermediate rib spacing
(6 < p/e < 9). Markedly, flow reattachment along with a fresh boundary layer restora-
tion is observed at higher rib spacing (p/e ≥ 9).
Figure 6.4 shows the definable mean flow patterns in the enclosure between ribs.
Evidently, the global flow structures, i.e. corner eddies (secondary recirculation bub-
ble at the downstream rib corner and a separation bubble at upstream rib corner) and
recirculation bubbles, and sizes (i.e. reattachment length, L r ) are strongly dependent
on rib configurations (see Fig. 6.4). The reattachment length increases with decrease
in inclination angle. Interestingly, the separation bubble has been appeared for the
ribs with higher inclination angle i.e. α ≥ 75°.
The outcomes of CFD simulations, as reported in Table 6.2, are converted into S/N
ratios by Taguchi analysis. Assigned enactments with their conforming outcomes are
displayed in Table 6.3 for all performance indexes. The general means of S/N ratios
for performance indexes have been calculated and found to be 36.44 dB for Nusselt
number, 38.84 dB for friction factor and 6.097 dB for performance factor.
The response tables of S/N ratios for heat transfer, pressure drop and performance
factor are presented in Tables 6.4, 6.5 and 6.6, and plotted in Figs. 6.5, 6.6 and 6.7,
respectively. The importance of design factors for the studied performance indexes
is rated in the last penultimate row of the tables (see Tables 6.4, 6.5 and 6.6). The
parameter having the maximum difference between the highest and the lowest values
of S/N ratio has the greatest influence on the performance indexes.
The heat transfer augments with the increase of average fluid velocity, i.e. param-
eter Re (A), as expected (Fig. 6.5). Therefore, heat transfer can suitably be con-
trolled by the flow Reynolds number. The heat transfer increases with increasing p/e
6 Multi-objective Performance Optimization of a Ribbed Solar … 87

Fig. 6.4 Effect of

inclination angle on flow
structures

Table 6.3 Taguchi S/N ratios for performance indexes

Number of test Factors and their levels Signal to noise (S/N) ratios
A B C S/N(Nu) S/N(f) S/N(η)
1 4000 3 45° 32.25 39.53 8.24
2 4000 6 60° 31.70 37.70 7.08
3 4000 9 75° 31.98 36.22 6.86
4 4000 12 90° 32.26 35.13 6.78
5 8000 3 60° 34.65 40.15 5.53
6 8000 6 45° 36.23 39.22 6.80
7 8000 9 90° 36.32 38.53 6.65
8 8000 12 75° 36.52 37.37 6.47
9 12000 3 75° 36.69 40.70 4.64
10 12000 6 90° 37.58 39.91 5.27
11 12000 9 45° 39.09 38.64 6.35
12 12000 12 60° 39.04 38.05 6.11
13 16000 3 90° 38.45 41.69 4.52
14 16000 6 75° 39.38 41.04 5.23
15 16000 9 60° 39.92 39.49 5.26
16 16000 12 45° 40.92 38.01 5.77
88 N. Sharma and R. Choudhary

Table 6.4 Response table for

Levels A B C
Nusselt number
1 32.05 35.51 37.12
2 35.93 36.22 36.33
3 38.1 36.83 36.14
4 39.67 37.18 36.15
Delta 7.62 1.67 0.98
Rank 1 2 3
Contribution ratio (%) 74.20 16.26 9.54

Table 6.5 Response table for

Levels A B C
friction factor
1 37.15 40.51 38.85
2 38.82 39.47 38.85
3 39.32 38.22 38.83
4 40.06 37.14 38.81
Delta 2.91 3.37 0.03
Rank 2 1 3
Contribution ratio (%) 46.12 53.41 0.47

Table 6.6 Response table for

Levels A B C
thermohydraulic performance
1 7.241 5.731 6.79
2 6.362 6.094 5.993
3 5.591 6.281 5.8
4 5.194 6.282 5.805
Delta 2.047 0.55 0.991
Rank 1 3 2
Contribution ratio (%) 57.05 15.33 27.62

(B), because with increasing p/e the flow reattaches on the heat transferring surface
(Fig. 6.3), thereby providing more heat transfer rate. The heat transfer decreases with
increase in inclination angle (C) of the front face of the rib. Further inspection reveals
the blends of design parameters, which result in better heat transfer rate, are as fol-
lows: Re = 16000 (A4 ), p/e = 12 (B4 ), and α = 45° (C1 ). Consequently, A4 B4 C1
is found to be the ideal sets of design parameters, which provide the highest heat
transfer (Table 6.7).
Figure 6.6 illustrates the pronounced influence of control parameters on friction
factor. It increases with increase in Re (A). The pressure drop decreases significantly
with increasing rib spacing (B), while reduces slightly with rise in inclination angle
(C). The optimal sets of the design factors corresponding to minimum pumping power
6 Multi-objective Performance Optimization of a Ribbed Solar … 89

A B C
40

39

38
Mean of SN ratios

37

36

35

34

33

32

1 2 3 4 1 2 3 4 1 2 3 4

Fig. 6.5 Taguchi—S/N ratios for Nusselt number

A B C
41

40
Mean of SN ratios

39

38

37
1 2 3 4 1 2 3 4 1 2 3 4

Fig. 6.6 Taguchi—S/N ratios for friction factor

are as follows: Re = 16000 (A4 ), p/e = 3 (B1 ), and α = 45° (C1 ). Consequently,
A4 B1 C1 is found as the best grouping of design factors associated with the lowest
pressure drop (see Table 6.7).
Figure 6.7 elucidates the prominent impact of design parameters on thermohy-
draulic performance factor. Thermohydraulic performance decreases with increase in
Re (A), because of significant increase in pressure penalty. While η tends to increase
with the increase of p/e (B). The reason can be attributed to enhancement in heat
transfer and also decrease in friction factor with the increase in rib spacing. How-
ever, the η shows same trend as that of Nu with inclination angle, it decreases with
90 N. Sharma and R. Choudhary

A B C
7.5

7.0
Mean of SN ratios

6.5

6.0

5.5

5.0
1 2 3 4 1 2 3 4 1 2 3 4

Fig. 6.7 Taguchi—S/N ratios for thermohydraulic performance

Table 6.7 Optimum conditions and performance values

Levels Factors Value
A (Re) B (p/e) C (α)
Nu Best level 4a 4b 1c 111.2
Best value 16000 12 45°
f Best level 4b 1a 1c 0.00812
Best value 16000 3 45°
η Best level 1a 4c 1b 2.71
Best value 4000 12 45°
a, b, c represents the order of most dominant factor

increase in the inclination angle (C). The optimal values of the design parameters
for the maximum performance are as follows: Re = 4000 (A1 ), p/e = 12 (B4 ), and
α = 45° (C1 ). Consequently, A1 B4 C1 is found as the optimal combination of design
parameters associated with the highest performance as per the “larger is the better”
condition for thermohydraulic performance factor (see Table 6.7).
The delta is the difference of the greatest and the least value of S/N ratios for each
design factor. The contribution ratio is equivalent to the ratio of delta estimations of
individual factor to the cumulative delta estimation, and here introduced in the last
line of Tables 6.4, 6.5 and 6.6.
The contribution ratio of each controllable design factor to performance indexes
is depicted in Fig. 6.8. Figure 6.8 shows that the factor A (Re) contributes to 74.2%
of the aggregate impact, thereby signifying that Re has the greatest influence on
heat transfer. It can be seen that factors B (p/e) and C (α) are the second and third
most dominant factors, respectively. In view of Fig. 6.8, the factor B (p/e), having
6 Multi-objective Performance Optimization of a Ribbed Solar … 91

Fig. 6.8 Impact of each factor on performance indexes

a contribution ratio of 53.41%, is considered as the most dominant factor affecting

pressure penalty. The factors A and C add to 46.12% and 0.47% of the aggregate
impact on friction factor, respectively.
Further analysis reveals that Re has the greatest impact, around 57.05%, on the
thermohydraulic performance factor. This implies that the Reynolds number (param-
eter A) is the most influential one on η, followed by inclination angle (C) and p/e (B)
with contribution ratio of 27.62% and 15.33%, respectively. The ideal level of design
parameters for heat transfer, pressure penalty and thermohydraulic performance are
A4 B4 C1 , A4 B1 C1 and A1 B4 C1 , respectively depicted in Table 6.7. The coefficients
a, b, and c signify the first, second, and third most dominant factor, respectively.

6.5 Confirmation Tests

The confirmation test, last step in DOE process, has been performed at the optimum
settings of the design factors, as described in Table 6.7, to confirm the interpretation
made from analysis of the results. Table 6.8 depicts the results of confirmation tests
for all the performance indexes, where the predicted S/N ratios have been estimated
using Eqs. (6.14), (6.15), and (6.16), respectively (Ross 1995).
     
χ N u = T + A4 − T + B4 − T + C1 − T (6.14)

     
χ f = T + A4 − T + B1 − T + C1 − T (6.15)

     
χη = T + A1 − T + B4 − T + C1 − T (6.16)
92 N. Sharma and R. Choudhary

Table 6.8 Confirmation results for performance indexes

Initial parameter Optimum factors
Prediction Simulation
Nu Level A1 B1 C1 A4 B4 C1 A4 B4 C1
S/N ratio (dB) 31.81 41.10 40.92
f Level A1 B1 C1 A4 B1 C1 A4 B1 C1
S/N ratio (dB) 38.84 41.75 41.81
η Level A1 B1 C1 A1 B4 C1 A1 B4 C1
S/N ratio (dB) 7.57 8.12 8.66

where χ and T refer to the predicted and overall averaged values of S/N ratios of
16 simulations, respectively, while A, B and C indicate towards the average S/N
ratios of considered parameters at selected levels. From Table 6.8, it is clear that
the predicted and confirmation tests results show a good agreement with maximum
deviation of ±1, ±1% and ±6.5% in the heat transfer, friction factor, and thermo-
hydraulic performance factor, respectively. Therefore, it can be concluded that the
Taguchi method can be considered as a reliable soft computing tool for optimization
in heat transfer researches.

6.6 Conclusions

In this research work, the effects of the rib geometry and rib pitch to height ratio
are investigated by using CFD-Taguchi approach at Reynolds number varying from
4000 to 16000 on the aerothermal characteristics. The significant results can be
summarized as follows:
1. The flow patterns between two neighboring ribs are dependent on rib configura-
tion, rib spacing as well as Reynolds number.
2. With increase in inclination angle, from 45° to 90°, the size of primary recircula-
tion bubble decreases. In addition, a separation eddy has been appeared in front
of the ribs with higher inclination angle i.e. α ≥ 75°.
3. Reynolds number is the most dominant parameter in respect of heat transfer
and thermohydraulic performance. The optimal conditions of design parameters
for maximization of heat transfer and thermal performance are A4 B4 C1 (Re =
16000, p/e = 12, and α = 45°) and A1 B4 C1 (Re = 4000, p/e = 12, and α = 45°).
4. For friction factor, the rib spacing is the key parameter having contribution ratio of
53.41%, which is followed by the Reynolds number (46.12%) and have negligible
effect of rib configuration (0.47%). Ideal set of design parameters providing
minimum friction factor is A4 B1 C1 (Re = 16000, p/e = 3, and α = 45°).
5. Rib with inclination angle of 45° provides the highest average heat transfer and
the lowest friction along with the best thermohydraulic performance among the
6 Multi-objective Performance Optimization of a Ribbed Solar … 93

tested configurations. So, this investigation offers an improved rib design suitable
in various heat exchanging devices for improved overall performance.

References

Aghaie AZ, Rahimi AB, Akbarzadeh A (2015) A general optimized geometry of angled ribs for
enhancing the thermo-hydraulic behavior of a solar air heater channel—a Taguchi approach.
Renew Energy 83:47–54
Aidinlou HR, Nikbakht AM (2017) Intelligent modeling of thermohydraulic behavior in solar air
heaters with artificial neural networks. Neural Comput Appl 1–15
Akcayoglu A, Nazli C (2018) A comprehensive numerical study on thermohydraulic performance of
fluid flow in triangular ducts with delta-winglet vortex generators. Heat Transf Eng 39(2):107–119
Bilen K, Yapici S, Celik C (2001) A Taguchi approach for investigation of heat transfer from a
surface equipped with rectangular blocks. Energy Convers Manag 42:951–961
Caliskan S, Nasiri Khalaji M, Baskaya S, Kotcioglu I (2016) Design analysis of impinging jet array
heat transfer from a surface with V-shaped and convergent–divergent ribs by the Taguchi method.
Heat Transf Eng 37(15):1252–1266
Chamoli S (2015) A Taguchi approach for optimization of flow and geometrical parameters in a
rectangular channel roughened with V-down perforated baffles. Case Studies Therm Eng 5:59–69
Fluent (2006) Fluent 6.3-User’s guide. Fluent Inc.
Jin D, Quan S, Zuo J, Xu S (2019) Numerical investigation of heat transfer enhancement in a solar
air heater roughened by multiple V-shaped ribs. Renew Energy 134:78–88
Kotcioglu I, Khalaji MN, Cansiz A (2018) Heat transfer analysis of a rectangular channel hav-
ing tubular router in different winglet configurations with Taguchi method. Appl Therm Eng
132:637–650
Kumar R, Kumar A, Goel V (2019) Simulation of flow and heat transfer in triangular cross-sectional
solar-assisted air heater. J Solar Energy Eng 141(1):011007-1-12
Nagaiah NR, Geiger CD (2019) Application of evolutionary algorithms to optimize cooling chan-
nels. Int J Simul Multi Design Optim 10:A4
Rao RV, Saroj A, Ocloń P, Taler J, Taler D (2018) Single-and multi-objective design optimization
of plate-fin heat exchangers using Jaya algorithm. Heat Transf Eng 39(13–14):1201–1216
Ross PJ (1995) Taguchi techniques for quality engineering, 2nd edn. McGraw-Hill, New York
Sahin B, Ates I, Manay E, Bayrakceken A, Celik C (2019) Optimization of design parameters for
heat transfer and friction factor in a heat sink with hollow trapezoidal baffles. Appl Therm Eng
154:76–86
Sharma N, Sharma V, Tariq A (2017) Performance optimization of trapezium rib parameters using
response surface methodology. In: ASME 2017 gas turbine India conference, American Society
of Mechanical Engineers, Bangalore, India, , pp. V001T03A016-1-8
Sharma N, Tariq A, Mishra M (2018) Detailed heat transfer and fluid flow investigation in a rect-
angular duct with truncated prismatic ribs. Exp Thermal Fluid Sci 96:383–396
Siddhartha V, Sharma N, Varun G (2012) A particle swarm optimization algorithm for optimization
of thermal performance of a smooth flat plate solar air heater. Energy 38:406–413
Tariq A, Sharma N, Mishra M (2018) Aerothermal characteristics of solid and slitted Pentagonal
rib turbulators. J Heat Transf 140(6):061901-1-14
Wang Q, Chen Q, Zeng M (2009) A CFD-Taguchi combined method for numerical investigation
of natural convection cooling performance of air-core reactor with noise reducing cover. Numer
Heat Transf Part A Appl 55(12):1116–1130
Webb RL (1994) Principles of enhanced heat transfer. Wiley, New York
Yun JY, Lee KS (2000) Influence of design parameters on the heat transfer and flow friction char-
acteristics of the heat exchanger with slit fins. Int J Heat Mass Transf 43(14):2529–2539
Chapter 7
Mathematical Modelling of Solar
Updraft Tower

K. V. S. Teja, Kapil Garg and Himanshu Tyagi

Abstract Solar updraft tower power plant is a way to harness energy from the
sun. It is a simple concept which requires low maintenance and utilises land that
is already being used for growing plants, and generates power from it. A prototype
plant was setup in Manzanares, Spain. Numerical analysis on power generation is
performed for a similar plant assuming it is setup in Ropar. By considering losses via
convection and radiation through the top surface of the collector, collector efficiency
is calculated. Two cases arise here, 1. With 100% collector efficiency and 2. Collector
efficiency is obtained after subtracting convection and radiation losses. The influx of
solar radiation is highest in June. Hence, the variation of parameters like temperature,
velocity, power output, efficiency with time of the day is done by taking averages
for the month of June. Next the impact of physical parameters like chimney height,
chimney radius and collector radius are studied on 21st June 11:00 to 12:00. How
each parameter impacts the output of the plant is studied by creating a mathematical
model of the power plant. Methods to improve the power output are discussed.

Keywords Solar energy · Solar updraft tower · Collector · Chimney · Wind turbine

Nomenclature

A Area (m2 )
C Power coefficient for wind turbine, C = 0.45
Cp Specific heat at constant pressure for air (J/kg K), C p = 1007 J/kg K
g Acceleration due to gravity (m/s2 ), g = 9.81 m/s2
Gsc Solar constant (W/m2 ), Gsc = 1367 W/m2
h Convective heat transfer coefficient (W/m2 K)
H Height (m)
Io Hourly incident solar energy on an extra-terrestrial horizontal surface (J/m2 )

K. V. S. Teja (B) · K. Garg · H. Tyagi

School of Mechanical, Materials and Energy Engineering, Indian Institute of
Technology Ropar, Rupnagar, Punjab 140001, India
e-mail: 2018mem1016@iitrpr.ac.in

© Springer Nature Singapore Pte Ltd. 2020 95

H. Tyagi et al. (eds.), Solar Energy, Energy, Environment,
and Sustainability, https://doi.org/10.1007/978-981-15-0675-8_7
96 K. V. S. Teja et al.

m Refractive index
ṁ Mass flow rate of air (kg/s)
n Day of the year
p Pressure (Pa)
P Power (kW)
Q Heat (W)
Q Hourly average incident heat flux (W/m2 )
r Radius (m)
R Characteristic gas constant (J/kg K), R = 287 J/kg K
T Temperature (°C)
v Velocity (m/s)

Greek symbols

ε Emissivity
η Efficiency (%)
ρ Density (kg/m3 )
σ Stefan-Boltzmann’s constant (W/m2 K4 ), σ = 5.67 × 10−8 W/m2 K4
τ Transmissivity

Subscripts

chim Chimney
coll Collector
conv Convection
i Inlet of turbine/chimney
o Ambient
ovr Overall
rad Radiation
turbine Turbine

7.1 Introduction

Over the past few decades, the population has been rising at a very steep rate. This
leads a proportional growth in the energy requirement. Current major sources of
energy production have been through fossil fuels like coal, oil, natural gas. These
fossil fuels account for about 80% of total energy supply. But these resources are
not infinite and are going to deplete sometime in the near future. So, in order to
7 Mathematical Modelling of Solar Updraft Tower 97

tackle such a huge demand in energy, we need to start using renewable sources like:
hydro-electric, wind, solar etc. Among the renewable energy sources, solar energy
and wind energy are the most abundant, easily available and environment friendly.
Without a doubt, fossil fuels are much more efficient and effective ways to produce
power. But keep in mind that these sources are going to deplete one day and they
also have huge adverse impact on the environment. Hence, the need to find more and
better ways to use these forms of energy arises (Shahzad 2012). Solar updraft tower
is one such way to harness energy from the sun. It is a very simple construction.
The power plant consists of three major components: 1. Greenhouse (collector),
2. Chimney, 3. Wind turbine.
The greenhouse is made of glass or plastic. These materials act as transparent
medium for shorter-wavelengths and opaque for longer-wavelengths. Hence, the heat
from the sun gets trapped inside the greenhouse. A chimney is placed at the centre.
During the day, the solar irradiation falling onto the collector heats up the air inside
the greenhouse. Since, temperature and density are inversely related for constant
pressure (from ideal gas equation); density of air is lower inside when compared to
the ambient air. This density difference creates pressure gradient between the base
and ambient air inside the chimney. This pressure gradient when created over a large
area, gets converted in kinetic energy, and used to run a turbine and extract energy.
For the purpose of this plant, a vertical axis wind turbine is used to generate power.
During the night, the ground is hot because it absorbs the sun’s radiation during
the day. The hot ground heats up the air inside the greenhouse at night. Hence, this
type of plant can operate for 24 h a day. The land can be utilised by planting crops.
Even though a very large area is required for such a plant to generate significant
power output, the land is not wasted and can be utilised in a number of ways such
as, planting crops, solar panels can also be set up on the ground.
A solar updraft tower power plant prototype has been set up in Manzanares,
Spain. This working prototype used the above-mentioned principles. The plant in
Manzanares operated between 1981 and 1989. Various parameters of this plant have
been listed in Table 7.1 (Schlaich et al. 2005; Agarwal et al. 2018). Average power
output of 50 kW has been reported for this plant. Despite being a very simple con-
cept, it involves a lot of fluid mechanics and heat transfer phenomenon. There are
numerous parameters like height of the chimney, chimney radius, collector radius,

Table 7.1 Various physical

Chimney height 194.6 m
parameters of the Manzanares
plant Chimney radius 5.08 m
Mean collector radius 122 m
Mean roof height 1.85 m
Number of turbine blades 4
Typical collector air temperature increase T = 20 °C
Average power output 50 kW
Turbine blade profile FX W-151-A
98 K. V. S. Teja et al.

Table 7.2 Correlation between plant dimensions and power output

Capacity MW 5 30 100 200
Chimney height m 550 750 1000 1000
Chimney radius m 45 70 110 120
Collector radius m 1250 2900 4300 7000

greenhouse height, chimney profile, inclination of the greenhouse etc., which affect
the output power that can be extracted from the system. Various studies, simulations
and experiments were carried out to analyse these parameters. By increasing the
height of chimney and collector diameter, the power output can be increased by a
large factor. In Table 7.2 (Schlaich et al. 2005; Agarwal et al. 2018), the relation
between the physical parameters of the plant and its power output are listed. The
date was not specified for this data hence, it can be assumed as the averaged values
taken over the year.
Haaf et al. (1983) studied the working principle and construction of the prototype
located in Manzanares, Spain. They were the first to realise that increase in collector
area increases the power output but reduces the efficiency of the plant (Haaf et al.
1983; Haaf 1984). Increasing chimney height leads to an increase in the velocity of
air, which causes an increase in the mass flow rate (Pasumarthi and Sherif 1998a, b).
A detailed journal on the power generation, efficiency and costs involved in setting
up the plant were studied by Lodhi (1999). A similar study on the working and eco-
nomics was done on the Manzanares plant by Schlaich et al. (1996). Koonsrisuk and
Chitsomboon (2013) determined that the efficiency and power output vary linearly
with chimney height. It has also been observed by Chitsomboon (2001) that the effi-
ciency has almost no correlation with solar insolation, height of the greenhouse roof
and the chimney diameter.
Optimal dimensions of the plant can be obtained only by considering economic
constraints because increase in solar insolation, collector diameter and chimney
height will also increase the power output. Fasel et al. (2013) studied the plant
using ANSYS Fluent, which is a commercial CFD tool. They have performed a
CFD analysis to determine temperatures and air velocities in the system. Agarwal
et al. (2018) also did a CFD analysis on ANSYS Fluent for the plant at Manzanares,
Spain. They have considered a full scale 3-D model, performed simulation for steady
state with/without radiation and transient state with/without thermal storage on 8th
June. Results show that the simulation with radiation model showed very close val-
ues of various parameters when compared to the actual prototype’s observed data.
Also using water as thermal storage lowers the velocity of air but compensates for
the intermittent availability of solar insolation. The Manzanares plant operates for
about 8–9 h a day and requires a minimum velocity of 2.5 m/s for the turbine to be
operational.
Overall efficiency of the system is the product of turbine efficiency, chimney
efficiency and the collector efficiency. For a wind turbine, the Betz limit is 59.25%.
Which means that the maximum amount of power extracted is 59.25% of the kinetic
7 Mathematical Modelling of Solar Updraft Tower 99

energy of air at the inlet. Actual efficiency is always less than this limit. Generally,
it lies between 35 and 45%. For this study, the turbine efficiency is taken as 45%.
Two different cases were analysed using the dimensions of the Manzanares plant.
In case 1, the collector efficiency is assumed to be 100%. And in case 2, both con-
vection and radiation losses through the top surface of the greenhouse roof were
accounted and calculated the collector efficiency. The ground is assumed to be a
black body to calculate the tower efficiency.

7.2 Mathematical Modelling of the Power Plant

For this analysis, consider a hypothetical solar updraft tower power plant located in
Ropar, India (Latitude = 30.97° N). Assume the dimensions to be similar to the plant
in Manzanares, Spain (Agarwal et al. 2018). The aim is to calculate the power that
can be extracted using this setup.
Initially it is assumed that the collector is circular, with a radius of 122 m. The
height and radius of the chimney are taken as 194.6 m and 5.08 m respectively. A
vertical axis wind turbine is set up inside the chimney. The blade radius of the turbine
is assumed to be equal to the chimney radius. The turbine efficiency is taken as 45%.
All calculations are done from sunrise to sunset.
The schematic diagram of the setup is shown in Fig. 7.1. For simplifying the
calculations, the greenhouse roof is taken as horizontal with respect to the ground.
Chimney radius is constant throughout its height. For better efficiency and power
generation, a sloped roof and a chimney with a tapered or hyperbolic profile is taken.
Another novel concept of combining a thermal power plant’s cooling tower with

Fig. 7.1 Schematic diagram of the Manzanares plant showing physical dimensions of the plant
100 K. V. S. Teja et al.

a solar chimney was proposed by Zandian and Ashjaee (2013). By doing so, the
efficiency of solar updraft towers was improved. Also, the power generated by these
units was around 10 times more than the one located in Manzanares.
By using water or some other fluid as thermal storage medium, plant can be
operational even during the night. But here, the plant is assumed to be operational
during the day only (i.e. from sunrise to sunset). Also, neglect the losses due to
atmospheric interactions like fog, rain, clouds. Power output was calculated for sunny
day. Ambient temperature is taken as the monthly average temperature of Ropar from
sunrise to sunset. The ground is assumed to be a black body.
In order to calculate the power output of the system, heat flux incident on the
ground must be calculated. The incident solar energy per unit area from ω2 to ω1
hour angles is obtained using the following relation (Duffie and Beckman 2003).
  
24 ∗ 3600 ∗ G sc 360 ∗ n
Io = ∗ 1 + 0.033 ∗ cos
π 365
 
π ∗ (ω2 − ω1 )
∗ cos φ ∗ cos δ ∗ (sin ω2 − sin ω1 ) + ∗ sin φ ∗ sin δ (7.1)
180

Solar constant is taken as 1367 W/m2 . Latitude angle for Ropar is +30.97°. n ranges
from 1 to 365. By taking ω2 and ω1 in 15° intervals, the hourly incident solar energy
per unit area from sunrise to sunset can be calculated.Declination is given by (Duffie
and Beckman 2003)
 
284 + n
δ = 23.45 ∗ sin 360 ∗ (7.2)
365

Once Io is calculated, incident heat flux can be calculated using the following equation

(ω2 − ω1 )
Q  = Io ∗ (7.3)
15 ∗ 3600
Ambient temperature is taken as the monthly average temperature from sunrise to
sunset for Ropar. Collector is assumed to be a circle with radius 122 m. Chimney
radius is taken as 5.08 m. Areas of collector and chimney are 46,759.465 m2 and
81.073 m2 respectively.

7.2.1 Case 1 (Without Losses)

In case 1, the collector efficiency is taken as 100%. This means all the incident flux
is used to raise the temperature of the air inside the greenhouse i.e.

Q  ∗ Acoll = ṁC p (Ti − To ) (7.4)

7 Mathematical Modelling of Solar Updraft Tower 101

Here, mass flow rate of air unknown. It can be calculated using

ṁ = ρi ∗ Achim ∗ vi . (7.5)

In order to simplify the calculations, approximate ρi = ρo . Mass flow rate of air after
approximation is given by

ṁ ≈ ρo ∗ Achim ∗ vi (7.6)

By taking the above approximation, the equation to obtain temperature at turbine

inlet becomes a linear equation rather than a cubic equation. This simplifies the
calculations significantly. Velocity at the bottom of the chimney is given by (Schlaich
et al. 2005).

Ti − To
vi = 2 ∗ g ∗ Hchim ∗ (7.7)
To

Put the values of velocity in Eq. (7.6) to get the mass flow rate. Substitute ṁ in
Eq. (7.4). Here, Ti is the only unknown. Making Ti the subject gives us a relation to
obtain the average temperature of air at chimney entrance.
  1/3 
(Q  ∗ Acoll ∗ R)2
Ti = To 1 + (7.8)
(Po ∗ Achim ∗ C p )2 ∗ 2 ∗ g ∗ Hchim

Once Ti is obtained, velocity of air at chimney entrance (vi ) can be calculated

using Eq. (7.7). Using air velocity, the power generated by the wind turbine is given
by

1
Pout = C ∗ ∗ ρi ∗ Achim ∗ vi3 (7.9)
2
where C is assumed to be 0.45 for this wind turbine. Efficiency of the chimney is
defined as the ratio of kinetic energy of air at chimney entrance to the heat absorbed
by the air.
1
∗ ρi ∗ Achim ∗ vi3
ηchim = 2
(7.10)
ṁ ∗ C p (Ti − To )

Putting the values of ṁ and vi as defined earlier in Eqs. (7.5) and (7.7), this can be
simplified to

g ∗ Hchim
ηchim = (7.11)
C p ∗ To
102 K. V. S. Teja et al.

Clearly, it can be observed that the efficiency of the plant is a function of the chimney
height. Depending on mechanical and economic constraints, the chimney height must
be kept as high as possible.

7.2.2 Case 2 (with Losses)

For case 2, the collector losses from the top surface are taken into account. Addi-
tionally, the reflectivity and absorptivity of glass is taken into account. Assuming
refractive index of glass (m) as 1.5+10−7 i, the average wavelength of solar spectrum
is assumed to be 0.5 µm. The corresponding reflectivity, absorptivity and transmis-
sivity are calculated. Transmissivity is found to be around 0.912, which shows the
fraction of solar radiation that actually is incident on the ground.
Accordingly, the temperature of the glass roof is calculated. From the top surface
of the greenhouse roof, convective and radiative losses are calculated and subtracted
from the incident heat flux and the power output is calculated again. The results
obtained in case 1 and case 2 are compared.
In order to calculate the roof temperature, certain assumptions have to be taken,
as analytical solution for this is not easily obtained. First of all, the roof temperature
is assumed to be constant throughout. This temperature is assumed to be equal to
the average of ambient temperature and mean air temperature inside the plant. It
is known that the temperature at the periphery is equal to the ambient atmospheric
temperature and temperature at the centre is equal to the temperature at the chimney
entrance. So, assuming the temperature distribution along the radial direction to be
linear, assume the mean temperature of the air is the temperature of air at the Acoll /2
mark along the radial direction. At radius rcoll = 86.267 m, the area = Acoll /2.
Using linear interpolation, temperature at rcoll = 86.267 m can be calculated. Now,
roof temperature can be determined.

Convection losses
Assuming wind velocity as 0, the convection losses take place via natural convection
only. But in reality, the wind velocity cannot be determined theoretically and is con-
stantly fluctuating. So, if only natural convection is considered, the Nusselt number
is calculated and then heat transfer coefficient is obtained to be around 1.8 W/m2 K
(Incropera et al. 1993). Actual heat transfer coefficient is obviously higher than that,
hence it is assumed to be around 5 W/m2 K.

Q conv = h ∗ Acoll ∗ (Tcoll − To ) (7.12)

Radiation losses
Emissivity of glass roof is taken as 0.95 to determine radiation losses. Assume sky
temperature and ambient temperatures are equal.
7 Mathematical Modelling of Solar Updraft Tower 103

Q rad = εcoll ∗ Acoll ∗ σ ∗ (Tcoll

4
− To4 ) (7.13)

After calculating the losses from the top surface of the roof, new value of heat input
to the system is given by

Q = τ ∗ Q  ∗ Acoll − Q conv − Q rad (7.14)

This is the actual input to the system. Using this, new value of temperature at turbine
inlet is calculated, which is obviously less than the old value. There is a significant
difference in both these values, hence new value of roof temperature is calculated
and 6 such iterations are performed until almost no change in old and new values of
chimney inlet temperature is observed.
Using the final temperature at chimney inlet, determine other parameters like
velocity, power output and efficiency. New parameter collector efficiency is also
obtained in this case.
 
Q Q conv + Q rad
ηcoll =  = τ− (7.15)
Q ∗ Acoll Q  ∗ Acoll

Overall efficiency is given by

ηovr = ηtur bine ∗ ηchim ∗ ηcoll (7.16)

7.3 Results and Discussions

7.3.1 Impact of Solar Radiations on Performance of the Plant

This setup relies almost exclusively on solar radiations to generate power. Other
forms such as wind have a small impact in air velocity at the inlet and losses through
the collector roof. Since solar radiation is the major factor here, its variation must
be thoroughly studied. If we consider the hour of the day for any particular day,
then the hourly heat flux from the sun starts at minimum in the morning and reaches
maximum at solar noon and then symmetrically reaches minimum again at sunset.
But the same cannot be said about the temperature distribution. It is significantly
hotter in the afternoon session when compared to forenoon. Due to this the plant
generates more power in the afternoon session. The ambient temperature here is
taken as the monthly average temperature of Ropar taken on an hourly interval.
However, if the day off the year is varied keeping the hour of the day constant, it is
observed that since Ropar lies in the Northern hemisphere, solar radiation is highest
in the month of May and June. Hence the power output is maximum on these months.
And expectedly, the solar radiation is minimum in December and January. Hence the
104 K. V. S. Teja et al.

output of the plant reaches minimum in these months. However, the variation is not
exactly smooth. The increment is very steep and then it remains almost constant for
a few months before it decreases in a similar manner and again remains constant for
a few more months.
In order to study the variation in output parameters, June month is selected here
and studied. There are 4 major parameters that affect the performance of the plant.
1. Hour of the day
2. Collector radius
3. Chimney radius
4. Collector height.
Other parameters like slope of the collector roof, profile of the chimney, wind velocity,
cloud coverage, day of the year etc. also affect the performance. But the main focus
here was the physical parameters like collector radius, chimney radius, chimney
height and hour of the day. Since the calculations were performed for June, day of
the year is not a major factor because the temperature range over one month is not
much.
4 different cases are now created. The aim here is to keep 3 of the above listed
parameters as constants and vary one parameter. For both case 1 and case 2 all
parameters are varied and compared.

7.3.2 Hour of the Day

Using Eq. (7.1), the hourly incident heat flux was calculated for everyday of the year.
Since the maximum heat flux is incident in the month of May and June, the power
output is also highest for these months. All calculations were performed focussing
majorly on the month of June. Using the heat flux, the rise in temperature within the
system was calculated. The variation of temperature at the chimney inlet versus time
of the day can be observed in Fig. 7.2. It shows the variation between case 1 and case
2. Variation not symmetric between forenoon and afternoon for temperature because
the ambient temperature is higher afternoon compared to forenoon even though the
heat flux distribution is symmetric about 12:00 noon.
From Eq. (7.7), velocity at chimney inlet for every hour was calculated for June.
Corresponding power output was calculated. Monthly averaged values of velocity
and power output were calculated. Variation of velocity at chimney inlet and power
output against time of the day can be observed in Fig. 7.3. Power output is proportional
to the cube of velocity. Hence, their trends are similar when varied with time. Start
low at sunrise and reach maximum around midday and then decrease till sunset.
Distribution is almost symmetric between forenoon and afternoon.
The above curves are similar of case 1 and case 2 but efficiency is very different.
This is because the collector efficiency is also accounted in case 2. This can be seen
in Fig. 7.4. Maximum chimney efficiency is observed when ambient temperature is
minimum i.e. at sunrise. But collector efficiency is maximum at 12:00 noon. This
7 Mathematical Modelling of Solar Updraft Tower 105

Ambient temperature
80 Temperature at turbine inlet (without losses)
Temperature at turbine inlet (with losses)
70
Temperature (oC)

60

50

40

30

6:00 9:00 12:00 15:00 18:00

Time of the day (hr)

Fig. 7.2 Variation of ambient temperature and temperature at turbine inlet for both cases from
sunrise to sunset for June

24 Velocity at turbine inlet (without losses) 240

Velocity at turbine inlet (with losses)
22 Power output (without losses) 220
Power output (with losses)
20 200
Velocity at turbine inlet (ms-1)

18 180
Power output (kW)

16 160
14 140
12 120
10 100
8 80
6 60
4 40
2 20
0 0
6:00 9:00 12:00 15:00 18:00
Time of the day (hr)

Fig. 7.3 Variation of velocity at turbine inlet and power output from sunrise to sunset for June
106 K. V. S. Teja et al.

Fig. 7.4 Comparison of

without losses
overall efficiency for case 1 0.28
and case 2 from sunrise to with losses
sunset for June
0.26

Overall Efficiency (%)

0.24

0.22

0.20

0.18
6:00 9:00 12:00 15:00 18:00
Time of the day (hr)

Fig. 7.5 Comparison of

Temperature at turbine inlet (oC) Collector efficiency (%)
various parameters versus 140
Velocity at turbine inlet (ms-1) Power output (kW)
time of the day for June
when convection and 120
radiation losses through
collector roof are accounted 100

80

60

40

20

0
6:00 9:00 12:00 15:00 18:00
Time of the day (hr)

makes it difficult to predict when the overall efficiency will be maximum. Variation
of all the parameters with time for case 2 can be observed in Fig. 7.5.

7.3.3 Collector Radius

Keeping chimney height as 200 m, chimney radius as 5 m, varying the collector

radius from 100 to 300 m, all the parameters were calculated for 21st June 11:00 to
12:00. From Figs. 7.6, 7.7 and 7.8 it is observed that as collector radius increases, the
temperature and velocity at the chimney inlet increase almost linearly. Also, power
7 Mathematical Modelling of Solar Updraft Tower 107

60
Temperature at turbine inlet (without losses)
140 Temperature at turbine inlet (with losses)
Velocity at turbine inlet (without losses) 55
Velocity at turbine inlet (with losses)
Temperature at turbine inlet (oC)

Velocity at turbine inlet (ms-1)

120 50

45
100
40

80 35

30
60

25
40
20

20 15
100 150 200 250 300
Collector radius (m)

Fig. 7.6 Variation of velocity and temperature at turbine inlet calculated on June 21st between
11:00 and 12:00 for both cases when collector radius is varied

Fig. 7.7 Variation of power 800

without losses
output calculated on June
700 with losses
21st between 11:00 and
12:00 for both cases when
collector radius is varied 600
Power output (kW)

500

400

300

200

100

0
100 150 200 250 300
Collector radius (m)

output increases as well but efficiency of the plant decreases. This shows that the
collector radius must be as large as possible. The only constraint is the availability
of land and the costs involved.
The incident solar radiation flux remains constant regardless of the plant dimen-
sions but, when the collector radius increases, the heat input increases because the
area increases. But for a given increase in heat flux, the corresponding velocity incre-
ment is much lower. Hence, the power output increases at a slower rate compared
108 K. V. S. Teja et al.

Fig. 7.8 Variation of overall

without losses
efficiency calculated on June 0.26 with losses
21st between 11:00 and
12:00 for both cases when
collector radius is varied

Overall Efficiency (%)

0.24

0.22

0.20

0.18

100 150 200 250 300

Collector radius (m)

to the input heat. Due to this reason the ratio between these two i.e., the efficiency
decreases with increase in collector radius.

7.3.4 Chimney Radius

Here, chimney height is kept at 200 m, collector radius is kept at 150 m and the power
output, temperature and velocity variations are observed as chimney radius varies
from 2 to 10 m on 21st June 11:00 to 12:00. Figures 7.9 and 7.10 show these variations
for both case 1 and case 2. While the velocity and temperature at chimney inlet
decrease as the radius is increased, the power output as well as efficiency increases.
The variation is very steep when the radius is small, but as radius increases, the slope
of all these curves approaches zero. This implies that the radius of tower should be
significantly large, but increasing it beyond a certain point will yield poor cost to
power ratio.

7.3.5 Chimney Height

Finally, chimney height is varied from 100 to 300 m keeping collector radius at 300 m,
and chimney radius at 5 m on 21st June 11:00 to 12:00. The temperature of the air
at chimney entrance decreases as height increases. Velocity of air, power output and
efficiency increase with increase in height. Which means the chimney height must be
as high as possible keeping structural and cost constraints into account. Figures 7.11
and 7.12 show the variation of various parameters with height.
7 Mathematical Modelling of Solar Updraft Tower 109

60
Temperature at turbine inlet (without losses)
180
Temperature at turbine inlet (with losses) 55
Velocity at turbine inlet (without losses)
160
Temperature at turbine inlet (oC)
Velocity at turbine inlet (with losses) 50

Velocity at turbine inlet (ms-1)

140
45
120
40
100
35
80 30
60 25

40 20

20 15

0 10
2 4 6 8 10
Chimney radius (m)

Fig. 7.9 Variation of temperature and velocity at turbine inlet calculated on 21st June between
11:00 and 12:00 for both cases when chimney radius is varied

Power output (without losses)

260 0.34
Power output (with losses)
Overall Efficiency (without losses)
240 0.32
Overall Efficiency (with losses)
220 0.30
Overall Efficiency (%)
Power output (kW)

200 0.28
180
0.26
160
0.24
140
0.22
120
0.20
100
0.18
80
0.16
2 4 6 8 10
Chimney radius (m)

Fig. 7.10 Variation of power output and overall efficiency calculated on 21st June between 11:00
and 12:00 for both cases when chimney radius is varied
110 K. V. S. Teja et al.

40
Temperature at turbine inlet (without losses)
90 Temperature at turbine inlet (with losses) 38
Velocity at turbine inlet (without losses)
85 Velocity at turbine inlet (with losses) 36
Temperature at turbine inlet (oC)

Velocity at turbine inlet (ms-1)

80 34
32
75
30
70
28
65 26

60 24
22
55
20
50
18
45 16
100 150 200 250 300
Chimney height (m)

Fig. 7.11 Variation of temperature and velocity at turbine inlet calculated on 21st June between
11:00 and 12:00 for both cases when chimney height is varied

0.60
Power output (without losses)
350
Power output (with losses) 0.55
Overall Efficiency (without losses)
300 0.50
Overall Efficiency (with losses)
0.45 Overall Efficiency (%)
Power output (kW)

250
0.40

200 0.35

0.30
150
0.25
100
0.20

50 0.15

0.10
0
100 150 200 250 300
Chimney height (m)

Fig. 7.12 Variation of output power and overall efficiency calculated on 21st June between 11:00
and 12:00 for both cases when chimney height is varied
7 Mathematical Modelling of Solar Updraft Tower 111

Additionally, it is observed that the velocity of air reaches a maximum at the top
of the chimney. But it is infeasible to install a turbine at such a large height.

7.3.6 Comparison of Power Output Between 21st June

and 21st December

In order to get an idea about the average performance of the plant, a comparison
between 21st June and 21st December is necessary because, these are the days with
the maximum and minimum incident solar radiation respectively. By doing an anal-
ysis on 21st December similar to the earlier one, the range of power outputs that can
be obtained using the plant is calculated. It is found that the power output between
11:00 and 12:00 ranges from 118.81 to 145.43 kW in case 1 and from 91.41 to
120.67 kW. Also, by varying the physical parameters like collector radius, chimney
radius and chimney height, the variation between the power outputs on 21st June and
21st December for case 2 can be observed in Figs. 7.13, 7.14 and 7.15. Obviously, the
trend remains the same, but the rate of change varies slightly. The minimum power
output is generally observed to be 75–80% of the maximum power output. From
these figures, average power output over the year can be estimated theoretically.
Since, the power output increases with increase in each of the 3 mentioned physi-
cal parameter, there is no optimum dimension that can be theoretically obtained. As
the plant dimensions increase, the power output also continuously increases. How-
ever, by considering the limiting temperature at which plants can grow healthily
inside the greenhouse and by performing an economic analysis and considering the
cost and material constraints, it is possible to arrive at an optimum value of the
plant dimensions. These dimensions can vary vastly depending on the requirement,
climatic conditions etc.

Fig. 7.13 Comparison 200

between power output 21st June
21st December
generated on 21st June and
180
21st December 11:00 to
12:00 for case 2 when
Power output (kW)

chimney radius is varied 160

140

120

100

80
2 4 6 8 10
Chimney radius (m)
112 K. V. S. Teja et al.

Fig. 7.14 Comparison 300

21st June
between power output
21st December
generated on 21st June and
21st December 11:00 to 250
12:00 for case 2 when

Power output (kW)

chimney height is varied
200

150

100

50
100 150 200 250 300
Chimney height (m)

Fig. 7.15 Comparison 700

21st June
between power output
21st December
generated on 21st June and 600
21st December 11:00 to
12:00 for case 2 when 500
Power output (kW)

collector radius is varied

400

300

200

100

0
100 150 200 250 300
Collector radius (m)

7.4 Conclusions

Mathematical modelling of a hypothetical solar updraft tower with dimensions iden-

tical to the prototype located in Manzanares, Spain was done. Numerical analysis
on various parameters like temperature, velocity, power output, efficiency was per-
formed. From the above analysis, it can be inferred that
1. For case 1, no losses through the top surface of the collector were accounted.
The power output for this case was found to be around 75 kW when averaged
from sunrise to sunset for all days of the year. For the month of June, the average
power output was around 90 kW.
7 Mathematical Modelling of Solar Updraft Tower 113

2. In case 2, convective as well as radiative losses through the collector roof were
calculated. Also, the absorptivity and reflectivity of the roof was determined.
Thus, the actual heat flux involved in power generation was calculated for June
and corresponding power output was estimated to be around 74 kW and the
collector efficiency turned out to be nearly 80%. Theoretically, when averaged
over the year, the power output could be around 60 kW, which is around 20%
more than the plant in Manzanares.
3. The peak power output for case 1 was found to be nearly 145 kW and for case
2, it was around 120 kW.
4. The prototype in Manzanares was producing around 50 kW on average. The
difference in power output is due to many factors like the cloud coverage, latitude,
actual collector area, the ground not being a black body, effect of shadowing,
difference in the ambient temperature etc.
5. Physical parameters like collector radius, chimney radius and chimney height
were varied and their impact on the power output was observed.
6. Power output increases with increase in each of the above-mentioned parameters,
but the nature of the increment is different. The increment is linear when chimney
height is increased. When plotted against collector radius, the graph is concave
upwards and against chimney radius, it is convex upwards.
7. This type of plant can be established wherever plants are being grown and thus
the land is fully utilised. Also, it can be run at night by storing the heat from
the sun using water or some other means. By doing so, the power generation
during the day would be less when compared to a plant that does not utilise
thermal storage. But the plant can run day and night and produce more power
when averaged over the entire day.

Acknowledgements The authors (K. V. S. T., K. G. and H. T.) wish to express their gratitude to
the School of Mechanical Material and Energy Engineering at Indian Institute of Technology Ropar
for their support.

References

Agarwal A, Kumar P, Mehta B (2018) Solar updraft tower—a potential for future renewable power
generation: a computational analysis. In: Tyagi H, Agarwal AK, Chakraborty PR, Powar S (eds)
Applications of solar energy. Springer, Singapore, pp 319–339
Chitsomboon T (2001) A validated analytical model for flow in solar chimney
Duffie JA, Beckman WA (2003) Solar radiation
Fasel HF, Meng F, Shams E, Gross A (2013) CFD analysis for solar chimney power plants. Sol
Energy 98:12–22
Haaf W (1984) Solar chimneys. Part II: preliminary test results from the Manzanares pilot plant.
Int J Sol Energy 2(2):141–161
Haaf W, Friedrich K, Mayr G, Schlaich J (1983) Part I: principle and construction of the pilot plant
in Manzanares. Int J Sol Energy 2(1):3–20
Incropera FP, Dewitt DP, Bergman TL, Lavine AS (1993) Fundamentals of heat and mass transfer
114 K. V. S. Teja et al.

Koonsrisuk A, Chitsomboon T (2013) Mathematical modeling of solar chimney power plants.

Energy 51:314–322
Lodhi MAK (1999) Application of helio-aero-gravity concept in producing energy and suppressing
pollution. Energy Convers Manag 40(4):407–421
Pasumarthi N, Sherif SA (1998a) Experimental and theoretical performance of a demonstration
solar chimney model—Part II: experimental and theoretical results and economic analysis. Int J
Energy Res 22(5):443–461
Pasumarthi N, Sherif SA (1998b) Experimental and theoretical performance of a demonstration solar
chimney model—Part I: mathematical model development. Int J Energy Res 22(3):277–288. Fuel
Energy Abstr 39(3):201
Schlaich J (1996) The solar chimney: electricity from the sun, p 55
Schlaich J, Bergermann R, Schiel W, Weinrebe G (2005) Design of commercial solar updraft tower
systems—utilization of solar induced convective flows for power generation. J Sol Energy Eng
127(1):117
Shahzad U (2012) The need for renewable energy sources. ITEE J 1(1):30–38
Zandian A, Ashjaee M (2013) The thermal efficiency improvement of a steam rankine cycle by inno-
vative design of a hybrid cooling tower and a solar chimney concept. Renew Energy 51:465–473
 Part III
Solar Thermal Systems: Cooling
Chapter 8
Solar Thermal-Powered Adsorption
Chiller

Mahbubul Muttakin, Kazuhide Ito and Bidyut Baran Saha

Abstract Adsorption based cooling systems are gaining considerable attention since
it can utilize low grade thermal energy, which otherwise could go as a waste. Heat
sources possessing a temperature of as low as 60 °C can drive an adsorption chiller
and that temperature requirement is even lower in the case of multi-stage adsorption
cooling systems. A typical flat plate solar collector can provide hot water having a
temperature of 65 °C in most of the countries in the Asian region. The temperature of
evacuated tube collectors’ water outlet can reach above 95 °C. In order to make use of
such collectors, in conjunction with other auxiliary heat sources, for providing heat
to power an adsorption chiller, it is imperative to have a proper mathematical model.
This can aid in designing the network and predicting the performance of the whole
system, prior to installation. This chapter focuses on the modelling of a system that
incorporates flat plate collectors, evacuated tube collectors and a thermally powered
adsorption chiller. Here, mathematical equations to calculate the efficiency of flat
plate and evacuated tube collectors are presented; processes that are involved in a
typical two bed adsorption cooling system are explained in brief, and a mathemat-
ical model of an adsorption chiller, that employs mass and heat recovery schemes
is developed. Finally, the simulation results of the model are presented, and the per-
formance of the chiller is investigated to demonstrate a clear understanding of its
operation.

M. Muttakin · B. B. Saha (B)

International Institute for Carbon-Neutral Energy Research (WPI-I2 CNER), Kyushu
University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
e-mail: saha.baran.bidyut.213@m.kyushu-u.ac.jp
M. Muttakin
e-mail: muttakin@kyudai.jp
M. Muttakin · K. Ito
Interdisciplinary Graduate School of Engineering Sciences, Kyushu University,
Kasuga-Koen 6-1, Kasuga-shi, Fukuoka 816-8580, Japan
e-mail: ito@kyudai.jp
B. B. Saha
Mechanical Engineering Department, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka
819-0395, Japan

© Springer Nature Singapore Pte Ltd. 2020 117

H. Tyagi et al. (eds.), Solar Energy, Energy, Environment,
and Sustainability, https://doi.org/10.1007/978-981-15-0675-8_8
118 M. Muttakin et al.

Keywords Adsorption chiller · Evacuated tube collector · Mathematical

modelling · Solar collector

8.1 Introduction

Global environmental concern and the increasing cost of fossil fuels press scientists
to make renewable energy sources more sustainable and cost effective. Solar energy
has been found to be the most widely used renewable energy sources over the last
few decades. The early researches on solar energy had been conducted in the early’
40s by Hottel (1942). He (Hottel 1954; Zarem and Erway 1963; Hottel and Whillier
1955) continued his extensive research on solar energy, and all the early works are
summarized by Duffie and Beckman (1974) in his book. Later Kalogirou (2004)
presented a summary on various types of solar thermal collectors and their applica-
tions. Solar thermal collectors convert solar radiation into heat energy, which can be
utilized in various applications. One of the promising application of thermal energy
is cooling and dehumidification through the utilization of adsorption refrigeration
technology.
With the rapid growth of the economy and development of infrastructure, energy
consumption is increasing at a fast pace, and it is estimated to rise 28% within the
next 20 years (Conti et al. 2016). Furthermore, according to the estimation of the
International Institute of Refrigeration in Paris (Coulomb 2006), 15% of the world’s
total produced electricity is spent on air-conditioning and refrigeration processes of
various kinds. In the present scenario, vapor compression refrigeration technology
is predominant in the market, which is highly energy intensive. Furthermore, it uses
CFC, HCFC based refrigerants, which are being considered as the major culprits for
global warming and ozone depletion. Adsorption based refrigeration technology is
environmentally benign, as it can use natural refrigerants (e.g. water, CO2 ). Instead
of electricity, it is primarily driven by low grade heat having a temperature of as low
as 60 °C (Muttakin et al. 2018; Mitra et al. 2017; Saha et al. 2006). A typical low cost
flat plate solar collector is able to provide heat at such low temperature. Hence solar
thermal powered adsorption chiller is gaining considerable attention, particularly by
the environmental scientists.
Scientists (Muttakin 2013) worked on the modeling and optimization of solar
thermal systems utilizing various computer based simulation programs. Modelling
of adsorption chiller (Wang 2001) is also necessary to make the system efficient
and economically viable. The computer modelling of any system has numerous
advantages (Kalogirou and Papamarcou 2000),
• Prediction of system performance.
• Optimization of the system to make it more economical and energy efficient.
• Reduction or elimination of the cost of building prototypes.
• Estimation of system performance under variable operating conditions.
• Sensitivity analysis of different design parameters.
8 Solar Thermal-Powered Adsorption Chiller 119

In this book chapter model equations of a solar thermal powered adsorption chiller
are presented. First, the system is briefly explained in Sect. 8.2. In the next section, the
construction of a flat plate collector is described, which is followed by the derivation
of different heat transfer components to calculate its efficiency. Section 8.4 models an
evacuated tube collector to estimate its dynamic performance. Next model equations
of a typical commercial adsorption chiller are explained, and simulation results of a
10-RTon chiller are presented under specific operating conditions. Finally, the chapter
ends with a conclusion.

8.2 System Description

An adsorption chiller requires low grade heat for its operation. Hence the heat from
flat plate collectors or evacuated tube collectors can drive the system. The main
components of a two-bed adsorption chiller are two ad/de-sorption beds with adsor-
bents, one evaporator and one condenser. A schematic of a solar thermal powered
adsorption chiller is shown in Fig. 8.1.
A typical ad/de-sorption bed has finned tube heat exchanger, and the adsorbents
are placed between the fins. The chiller works in cycles. The four major steps of
an adsorption chiller are adsorption, mass recovery, heat recovery and desorption.
During the adsorption phase, cold water flows through the tubes of the adsorbent bed
and during the desorption phase, hot water from the hot water tanks flows through
those tubes. The difference between the temperatures of hot and cold water is known

Condenser chamber

Desorber bed
Adsorber bed
Solar thermal
collector

Adsorbent
Hot water tank Cooling water tank
Return through

Evaporator chamber
expansion

Fig. 8.1 Schematic diagram of a solar thermal powered adsorption chiller

120 M. Muttakin et al.

as temperature swing. For a typical single stage silica gel—water adsorption chiller,
the temperature of the hot water requires to be 60–80 °C. However, a multistage
adsorption chiller can be operated at a temperature swing of 10 °C (Saha and Kashi-
wagi 1997). Theoretically, a temperature swing of 2.2 °C can be utilized to drive a
10-stage adsorption chiller (Saha et al. 2007).
In a solar thermal-powered adsorption chiller, heat is obtained primarily from
the solar thermal collectors. However, an auxiliary electrical heater is often used to
maintain the temperature in the hot water tank. This is required as the performance
of the solar thermal collector strongly depends on the environmental condition. It
is obvious that during rainy days, the output from the collector will be significantly
lower when compared to the same during bright sunny days. The poor solar irradiance
in rainy days results in lower absorbed energy by the solar thermal collector, which
in turn reduces its output.

8.3 Model Equations of a Flat Plate Collector

A typical solar thermal system includes solar thermal collectors, water storage tanks
and necessary piping. An adsorption chiller requires a low temperature heat source
to be driven efficiently, and flat plate collectors (FPC) are the most popular solar
thermal collectors considering low temperature application.

8.3.1 Components of an FPC

As can be seen in Fig. 8.2, the major components of a flat plate collector are;
• Glazing: A glazing material is placed on the top of the FPC. Glass is the most
widely used glazing material because of its ability to transmit 90% of the incoming
shortwave solar irradiation while being opaque to longwave radiation emitted
outward by the absorber plate (Duffie and Beckman 1974).
• Absorber plate: The role of an absorber plate is to support the tubes and fins.
In some collectors, it may be integrated with the tubes. The three most important
materials that are used as absorber plates are copper, aluminium and stainless steel.
• Tubes: Tubes provide the path for the flow of heat-transfer fluid from inlet to outlet.
• Header: The header provides the passage for cold water inlet and hot water outlet.
• Insulation: Insulation prevents the loss of heat from the collector through its bottom
and its side.
• Housing: The housing holds all the components as well as protects the system
from moisture, dust etc.
An FPC is a non-concentrating type of solar thermal collector, i.e., its concentra-
tion ratio is one. The concentration ratio signifies the amount of transmitted solar
8 Solar Thermal-Powered Adsorption Chiller 121

Solar
Radiation
Glazing
Copper
risers
Cold
water in

Housing
Hot water
out
Absorber Header
Insulation tube

Fig. 8.2 Components of a flat plate collector

energy that causes the rise of temperature of the heat-transfer fluid and can be defined
as the ratio of aperture area of the collector to its absorber area.
Prior to modelling a flat plate collector, it is important to define two parameters
that play vital roles in the performance of the collector; these are, absorptance α
and emittance ε. The monochromatic, directional absorptance is a surface property,
defined as the fraction of the incident radiation of wavelength ψ from the direction
μ, ϕ, that is absorbed by the surface. Mathematically it can be shown as,

Iψ,abs (μ, ϕ)
αψ (μ, ϕ) = (8.1)
Iψ,inc (μ, ϕ)

Here μ is the cosine of the polar angle and ϕ is the azimuth angle; I represents
the radiant exposure and subscripts abs and inc stand for absorbed and incident,
respectively.
On the other hand, the monochromatic directional emittance, of a surface is the
ratio of the monochromatic intensity emitted by the surface in a specific direction to
the same that would be emitted by a blackbody maintained at the same temperature
(Duffie and Beckman 1980). Mathematically it can be written as,

Iψ (μ, ϕ)
εψ (μ, ϕ) = (8.2)
Iψ,b

where subscript b represents blackbody.

122 M. Muttakin et al.

8.3.2 Efficiency of an FPC

An efficient solar collector should possess high absorptance for radiation in the solar
energy spectrum. At the same time, in order to minimize the losses, it must have low
emittance for long wave radiation. The efficiency (η) of an FPC is defined as the ratio
of useful gain (Qu ) to the incident solar radiation power (QT ),

Qu Qu
η= = (8.3)
QT G Ac

where G represents the solar irradiance in W/m2 , and Ac represents an aperture area
of the collector. With Gs being the absorbed energy, the useful energy gain can be
defined by,

Q u = Ac [G S − U L (Tc − Ta )] (8.4)

where U L is the overall heat transfer coefficient, T c and T a are the mean absorber
plate temperature and ambient air temperature, respectively. Hence, U L (T c – T a )
is the thermal energy lost from the collector to the ambient through conduction,
convection and infrared radiation.
In order to determine the efficiency of the collector, all the parameters of Eq. (8.4)
must be known. The parameters G, Ac , T c and T a can be measured through experi-
ments. The absorbed energy Gs can be calculated from,

G S = G(τ α)e f f (8.5)

where (τ α)eff is called the effective transmittance-absorptance coefficient. It can be

approximated from the known value of incidence angle modifier of a collector, as
defined by,

(τ α)e f f
Kτ α =
(τ α)n

where (τ α)n is transmittance-absorptance normal to the collector surface. For FPC

with flat covers, the incidence angle modifier depends on the angle of incidence θ
following the below equation (Souka and Safwat 1966),
 
1
K τ α = 1 + b0 −1
cos θ

where b0 is the incidence angle modifier constant, and it has a positive value.
The thermal network of an FPC with two covers is shown in Fig. 8.3. The absorbed
energy Gs is converted to useful energy gain Qu after losing a portion to the ambient
environment through the top and bottom of the collector. At some typical location, let
T p be the absorber plate temperature. From the top of the collector, heat loss is due to
8 Solar Thermal-Powered Adsorption Chiller 123

Gs
Bottom Top
1 1 1 1
hc , b − a hc , p − c1
Cover 1 hc ,c1− c 2 Cover 2 hc , c 2 − a
Ambient Ambient
R4
R5 R3 R2 R1
Ta Tb Tp Tc1 Tc2 Ta
1 1 1 1
hr ,b − a hr , p − c1 hr ,c1− c 2 hr ,c 2 − a

Qu

Fig. 8.3 Thermal model of an FPC having two covers

convection and radiation heat transfer to the ambient. This heat loss is essentially the
same as the steady state energy transfer between the plate at T p and the first cover
at T c1 and is equal to the energy transfer between any other two adjacent covers.
Hence, the heat loss through the top of the FPC can be expressed by,

σ (T p4 − Tc14 )
Q top,coll = h c, p−c1 (T p − Tc1 ) + (8.6)
1
εp
+ 1
εc1
−1

where hc,p–c1 is the convection heat transfer coefficient between two inclined par-
allel plates, σ is the Stefan-Boltzmann constant which is equal to 5.6697 × 10−8
W/(m2 °C4 ), and εp and εc1 are the directional emittances of absorber plate and
cover 1, respectively. Equation (8.6) can be written in terms of radiation heat transfer
coefficient hr,p–c1 as,

Q top,coll = (h c, p−c1 + h r, p−c1 )(T p − Tc1 ) (8.7)

hr,p–c1 can be calculated from,

σ (T p + Tc1 )(T p2 + Tc12 )

h r, p−c1 = (8.8)
1
εp
+ 1
εc1
−1

Thus the resistance R3 can be written as,

1
R3 = (8.9)
h c, p−c1 + h r, p−c1

The resistance, R2 , between the two covers can have a similar expression, and
the expression is essentially of the same form for further adjacent covers. However,
considering practical applications, the maximum limit of the number of covers of an
FPC is two. The resistance to heat loss, R1 , from the top cover to the ambient, can
be represented following the similar expression,
124 M. Muttakin et al.

1
R1 = (8.10)
h w + h r,c2−a

where resistance to radiation heat transfer accounts for radiation exchange with the
sky having a temperature T sky ;

σ εc (Tc2 + Tsky )(Tc22 + Tsky

2
)(Tc2 − Tsky )
h r,c2−a = (8.11)
Tc2 − Ta

For free-convection conditions, the minimum values of convection heat transfer

coefficient are 5 and 4 W/(m2 °C) for temperature differences of 25 and 10 °C,
respectively. But for forced-convection conditions, it can be calculated following the
expression obtained from Mitchell’s (1976) experimental results,
 0.6
c0 vv0
h w =  0.4 (8.12)
L
L0

where v0 = 1 m/s, c0 = 8.6 W/(m2 °C) and L 0 = 1 m. L is the cubic root of the collector
house volume in m and v represents the wind speed in m/s. For the simultaneous
occurrence of free and forced convections, it is recommended (McAdams 1954)
to use the larger value of the convection heat transfer coefficients; hence it can be
expressed as,
⎡  0.6 ⎤
8.6 vv0
⎢ ⎥
h w = max⎣5,  0.4 ⎦W/(m2 ◦ C) (8.13)
L
L0

Thus, for an FPC having two covers, the mathematical expression of the top loss
coefficient from the collector to the surroundings becomes,

1
Utop,coll = (8.14)
R1 + R2 + R3

On the other hand, losses through the back of the collector can be obtained from,

1 κins,coll
Ubot,coll = = (8.15)
R4 δins,coll

where κ ins,coll and δ ins,coll are the thermal conductivity and thickness of the insulation,
respectively.
It needs mentioning that, for a well-designed system, the edge loss from the
collector can be neglected as its value is very small compared to other heat losses
from the collector. With edge loss coefficient-area product of (UA)edge , the losses
8 Solar Thermal-Powered Adsorption Chiller 125

through the edges of the collector can be calculated from,

(U A)edge,coll
Uedge,coll = (8.16)
AC

The overall heat transfer coefficient, U L , is essentially the summation of all the
loss coefficients. Adding Eqs. (8.14)–(8.16),

U L = Utop,coll + Ubot,coll + Uedge,coll (8.17)

Putting the value of U L into Eq. (8.4), one can determine the useful energy gain
Qu , which can be utilized to determine the FPC efficiency η using Eq. (8.3).

8.4 Model Equations of an Evacuated Tube Collector

An evacuated tube collector (ETC) comprises an absorber plate attached to a heat

pipe, placed inside a vacuum-sealed tube, as can be seen in Fig. 8.4. Since the plate
and the heat pipe are surrounded by the vacuum, the heat loss to the environment by
convection and conduction is very minimal. This results in higher efficiency of ETC
when compared to FPC.
The solar energy absorbed by the plate, through both direct and diffuse radiation,
is transferred to the heat-transfer fluid kept inside the heat pipe. Receiving heat, the
fluid, e.g. methanol, evaporates and rises upward to the heat sink where it condenses
again releasing heat to the flow-fluid, e.g. water. After condensing, the heat transfer

Heated
Manifold water
Heat
transfer Insulation

Heat
transfer Solar
Radiation

Vapor rises Evacuated

to top heat pipe
Evacuated
glass tube
Condensate
returns to bottom

Fig. 8.4 Schematic of a heat pipe evacuated tube collector

126 M. Muttakin et al.

fluid returns to the collector for receiving heat from the absorbed solar energy again.
Thus it undergoes an evaporating—condensing cycle.

8.4.1 Efficiency of an ETC

The efficiency and useful energy gain of such a collector can be obtained using the
same equations as used for FPC, i.e. Eqs. (8.3) and (8.4).

Qu Qu
η= =
QT G Ac
Q u = Ac [G S − U L (Tc − Ta )]

Also from Eq. (8.5), we know, GS = G(τ α)eff . For evacuated tube collectors,
the overall incidence angle modifier K τ α is equal to the product of incidence angle
modifier in transverse plane K t and that in longitudinal plane K l ,

(τ α)e f f
K τ α = K t .K l =
(τ α)n

In order to determine the efficiency and useful energy gain, it is important to know
the thermal model of an ETC, which is shown in Fig. 8.5. The energy absorbed by
the plate is first transmitted to the heat-transfer fluid (e.g., methanol) placed inside
the heat pipe, which transfers it to the manifold fluid (e.g., water). This causes the
rise of the temperature of the manifold fluid.
Now, the steady state heat transfer rate from plate to heat transfer fluid can be
represented by,

Q c−h = h e AC (Tc − Th ) (8.18)

where T h represents the mean temperature of the heat-transfer fluid, and he is the
heat transfer coefficient. This heat transfer rate Qc–h is essentially the same as the

Fig. 8.5 Thermal model of a

typical ETC
Gs
QL Qu

1 1 1
U L Ac he Ac hh − m Ah − m

Ta Tc Th Tw
8 Solar Thermal-Powered Adsorption Chiller 127

useful energy gain Qu , i.e. Qc–h = Qu . Eliminating T c and from Eqs. (8.4), (8.5) and
(8.18), we can write,

h e /U L
Q c−h = AC [G(τ α)e f f − U L (Th − Ta )] (8.19)
h e /U L + 1

The steady state energy transfer from the heat-transfer fluid to the manifold fluid
can be expressed by,

Q h−m = h h−m Ah−m Th − T f (8.20)

where, hh–m represents the heat transfer coefficient for this heat flow, Ah–m is the area
of the heat pipe in contact with the manifold fluid and T f is the mean temperature of
the working fluid.
Again, since Qc–h = Qh–m , eliminating T h from Eqs. (8.19) and (8.20) we can
write,

AC
Q h−m = [G(τ α)e f f − U L (T f − Ta )]
(U L AC / h h−m Ah−m ) + (U L / h e + 1)
⇒ Q h−m = Fr AC [G(τ α)e f f − U L (T f − Ta )] (8.21)

where
1
Fr = (8.22)
(U L AC / h h−m Ah−m ) + (U L / h e + 1)

F r is known as the heat removal factor and can be defined as the ratio of the actual
amount of heat transferred to the manifold fluid to the heat that would be transferred
if the entire collector were at the fluid inlet temperature. From Eq. (8.22), it can be
said that the value of F r is dependent on three ratios, U L /he , U L /hh–m and Ah–m /Ac .
Since Qh–m = Qu , from Eqs. (8.3) and (8.21), collector efficiency can be written
as,

(T f − Ta )
η = Fr (τ α)e f f − Fr U L (8.23)
G
Thus the steady state efficiency of an ETC has a linear form; however, in real
applications, it may not be linear and may be difficult to obtain solving all the
parameters. To overcome this shortcoming, Cooper and Dunkle (1981) proposed a
second order efficiency equation with the assumption,

Fr U L = a + b(T f − Ta ) (8.24)

Substituting Eq. (8.24) into the Eq. (8.23), we obtain,

128 M. Muttakin et al.

2
T f − Ta T f − Ta
η = Fr (τ α)e f f −a −b (8.25)
   G G
η0

where η0 , a and b are constants and are usually furnished by the collector manufac-
turer. The mean manifold fluid temperature T f can be considered as an average of
the fluid temperatures at the inlet (T i ) and at the outlet (T 0 ), i.e.,

Ti + T0
Tf =
2
and thus the efficiency can be calculated from,
2
T f − Ta T f − Ta
η = η0 − a −b (8.26)
G G
It needs mentioning that the efficiency of an FPC can be expressed by the same
equation as Eq. (8.26) shown above.

8.4.2 Dynamic Modelling of an ETC

In an evacuated tube collector, there are two fluids, heat transfer fluid or refrigerant
and manifold fluid (water is considered in the current modelling). The current model
is based on the assumption that there is no refrigerant present in the ETC, rather,
water is assumed to flow directly through the heat pipes. The major assumptions are
summarized below,
• There is no presence of refrigerant. Water flows directly through the heat pipes.
• The flow of water is only in the positive x direction.
• The heat conduction in the fluid moving direction is neglected.
• The temperature dependence of thermo-physical properties of water is considered.
• Properties of glass and absorber do not depend on temperature and are assumed
to be constant.
• The infrared emissivity of the sky is unity (εsky = 1).
Based on the stated assumptions, the heat influx to various components of an ETC
is depicted in Fig. 8.6. As shown in the figure, the model (Praene et al. 2005) consists
of 3 thermal nodes. These are, the transparent glass cover, the absorber plate and
the fluid (water) having temperatures T g , T c and T f , respectively. The heat transfer
between the sky and the glass cover is by radiation only. Again, since the absorber
plate is surrounded by the vacuum, heat transfer between the glass cover and absorber
plate, is entirely by radiation. Convective heat transfer is existent between the glass
cover and the ambient environment, and between the absorber plate and water.
8 Solar Thermal-Powered Adsorption Chiller 129

Tsky
Ta
Absorber
Glass cover
Radiation Convection
Tg

G(τα) Tc

Tf Water flow direction

x

Fig. 8.6 Assumed model of ETC

Now, the governing equation that explains the time dependence of the temperature
of the glass cover is,

dTg
C pg δg ρg = εg σ (Tsky
4
− Tg4 ) + h g,a (Ta − Tg )
dt
εc εg
+ σ (Tc4 − Tg4 ) (8.27)
εc + εg − εc εg

where subscripts g, a, c and sky stand for glass cover, ambient, absorber plate and the
sky, respectively. Cp represents the specific heat capacity, T is the temperature in K;
δ stands for the thickness, and ρ represents the density. Using Swinbank’s formula
(Swinbank 1963) Tsky = pTa1.5 where p = 0.0552 K−1/2 , the sky temperature can be
obtained from the ambient temperature.
Now, for the absorber plate, the governing equation can be expressed as,

dTc εc εg
C pc δc ρc = G(τ α) + σ (Tg4 − Tc4 ) + h f,c (T f − Tc ) (8.28)
dt εc + εg − εc εg

where subscript f stands for fluid (water). The plate receives radiative heat from the
glass cover and transfers it to the working fluid predominantly in convective mode.
Finally, the temperature of the fluid, having a velocity of u along the positive x
axis, depends on time and its position in the flow channel. The governing equation
to describe the change in fluid temperature with time and position is,
2  
π din dT f dT f
Cp f ρ f +u = π din h f,c (Tc − T f ) (8.29)
4 dt dx
130 M. Muttakin et al.

where d in represents the diameter of the absorber tube that contains the fluid.
Thus Eqs. (8.27)–(8.29) are the governing dynamic equations to determine the
temperatures of the glass cover, absorber plate and working fluid (with both time and
position) respectively.

8.5 Modelling of an Adsorption Chiller

The major components of a two-bed adsorption chiller are evaporator, condenser and
two adsorber beds. An adsorber bed is necessarily a heat exchanger, filled with the
adsorbent material, e.g., silica gel, packed between the fins of that finned tube heat
exchanger. Adsorption and desorption are exothermic and endothermic processes,
respectively. To extract the heat of adsorption, it is necessary to circulate cooling water
through the tubes of the adsorber bed during the adsorption process. Furthermore,
during this process, the bed is connected to the evaporator in order to receive the
adsorbate vapor, e.g., water vapor, to be adsorbed by the adsorbent. Consequently,
the other bed goes through the desorption process and is connected to the condenser.
The hot water, flowing through its tubes, provide the necessary heat required for
desorption.
Thus an adsorption chiller works in a cyclic manner. Each cycle consists of three
operating modes (a) adsorption/desorption, (b) mass recovery and (c) heat recov-
ery. This section will state the lumped analytical simulation model equations of an
adsorption chiller that uses silica gel as the adsorbent and water as the adsorbate.
The model is based on the following assumptions,
• The porous properties of the adsorbent are constant.
• The kinetics parameters, i.e. the diffusivity and activation energy, are independent
of temperature and pressure.
• At the first time step of any phase, the model does not consider the water flow
condition within the bed in the previous time step.
• The isotherm and kinetics equations for adsorption and desorption are same.

8.5.1 Description of Operating Modes

8.5.1.1 Mode (a): Adsorption/Desorption

In the operating mode (a) (see Fig. 8.7), Bed 1 goes through the desorption phase,
and adsorption is taking place in Bed 2. Bed 1 is heated up by the hot water streams,
which also raises the pressure of Bed 1.
The connecting valve between Bed 1 and condenser is open at this stage, and
the desorbed vapor is then condensed in the condenser and returned back to the
evaporator through an expansion valve. The other bed (Bed 2) is connected to the
8 Solar Thermal-Powered Adsorption Chiller 131

Cooling
Condenser
water out

Condensate
return
Hot water out

Cooling Hot water in

water in Bed 2 Bed 1

Chilled
water out
Chilled
water in Evaporator

Fig. 8.7 Schematic of an adsorption chiller. Mode (a): Bed 1 goes through desorption and Bed 2
goes through adsorption

evaporator to adsorb the evaporated vapor, and this evaporation produces the desired
cooling effect. The cooling water exiting from Bed 2 flows through the condenser
to condense the desorbed vapor. The heated cooling water is usually cooled by a
cooling tower.

8.5.1.2 Mode (b): Mass Recovery

At the end of ad/de-sorption, pressure in Bed 1 becomes higher than that in Bed
2. In the mass recovery mode (see Fig. 8.8), the bypass valve between the two
beds is opened which causes flow of water vapor from hot Bed 1 to comparatively
cooler Bed 2 by means of pressure swing. The mass recovery time is adjusted to
attain the mechanical equilibrium between the two beds, while the excess time is not
recommended.

8.5.1.3 Mode (c): Heat Recovery

Due to the flow of hot and cooling water through Bed 1 and Bed 2, respectively, the
temperature of Bed 1 becomes higher than that of Bed 2.
In the heat recovery mode (see Fig. 8.9) the cooling water first flows through Bed
1, extracting heat from the bed the water releases it to Bed 2. This causes an increase
and reduction of temperatures of Bed 2 and Bed 1, respectively. Thus heat recovery
reduces the total amount of heat required, which ultimately increases the system’s
coefficient of performance (COP).
132 M. Muttakin et al.

Cooling
Condenser
water out

Condensate
return

By-pass
Hot water out

opened
Cooling Hot water in
water in Bed 2 Bed 1

Chilled
water out
Chilled
water in Evaporator

Fig. 8.8 Schematic of an adsorption chiller. Mode (b): Mass recovery. Water vapor flows from Bed
1 to Bed 2 due to the difference in pressure

Cooling
Condenser
water out

Condensate
return
Hot water out

Hot water in
Cooling Bed 2 Bed 1
water in

Chilled
water out
Chilled
water in Evaporator

Fig. 8.9 Schematic of an adsorption chiller. Mode (c): Heat recovery. Cooling water extracts heat
from Bed 1 and releases it to Bed 2 before flowing through the condenser

Thus one cycle is complete and in the next stage, the cycle is repeated, and the beds
perform an alternating role. Bed 1 starts adsorbing and is connected to the evaporator
and Bed 2 starts desorbing and gets connected to the condenser. The pressure of Bed
1 becomes essentially the same as that of evaporator and Bed 2 attains the same
pressure as of condenser. Hence in the mass recovery mode, the desorbed vapor of
Bed 2 is further passed to Bed 1 in order to achieve the mechanical equilibrium
between the two beds. The heat is extracted from the hot Bed 2 by the cooling water
and released to comparatively cooler Bed 1, during the heat recovery mode.
8 Solar Thermal-Powered Adsorption Chiller 133

8.5.2 Model Equations for the Adsorbent—Adsorbate Pair

Prior to modelling different components (beds, evaporator, condenser) of an adsorp-

tion chiller, it is required to define the models of two most important parameters of
an adsorbent—adsorbate pair; these are isotherm and kinetics.

8.5.2.1 Isotherm Model

Adsorption isotherm defines the maximum amount of adsorbate that can be adsorbed
by the adsorbent at a particular pressure. The values of isotherm parameters are deter-
mined by correlating the experimental data of equilibrium uptake with the model.
For silica gel—water pair, S-B-K model (Saha et al. 1995) can be used to determine
the equilibrium adsorption uptake,
  B(Tads )
Psat Tr e f
w∗ = A(Tads ) (8.30)
Psat (Tads )

where,

A(Tads ) = A0 + A1 Tads + A2 Tads

2
+ A3 Tads
3

B(Tads ) = B0 + B1 Tads + B2 Tads

2
+ B3 Tads
3
.

T ads and T ref are the adsorption temperature and saturation temperature, respec-
tively, and Psat is the saturation pressure. The values of the parameters A0 , A1 , A2 ,
A3 , B0 , B1 , B2 , B3 are determined by fitting the model with experimental uptake data.

8.5.2.2 Kinetics Model

Adsorption kinetics can be defined as the rate of adsorption, which essentially deter-
mines the cycle time of the adsorption chiller. The faster the kinetics, the smaller
the cycle time required, hence the greater the specific cooling produced. The most
widely used kinetics models are linear driving force (LDF) model, Fickian diffu-
sion (FD) model, Langmuir model, semi-infinite model etc. Some models are also
proposed by implementing suitable modifications of the above mentioned models.
In the current modelling of the adsorption chiller LDF model is utilized to simulate
the adsorption kinetics, which assumes that the adsorption rate is proportional to the
difference between the equilibrium uptake (w*) and the instantaneous uptake (w),

dw
= ks av [w ∗ −w(t)] (8.31)
dt
134 M. Muttakin et al.

where k s av is the overall mass transfer coefficient. Due to the spherical shape of silica
gel adsorbent, the overall mass transfer coefficient can be written as,

15Ds
k s av = ,
R 2p

where Ds is the diffusion time constant, and Rp is the adsorbent particle radius. Ds
depends on the temperature T, following equation,
 
−E a
Ds = Dso exp
RT

where Dso is the pre-exponential constant, E a is the activation energy, and R is the uni-
versal gas constant. The values of Dso and E a can be obtained from the experimental
data by plotting lnDs against 1/T as described by the equation stated below,
 
−E a
ln Ds − ln Dso =
RT

This plot is known as the Arrhenius plot. The slop yields −E a /R and the intercept
provides the constant, Dso . Hence, for an adsorption chiller comprising silica gel—
water pair, Eq. (8.31) can be rewritten as,
  
dw 15Dso −E a
= exp (w ∗ −w) (8.32)
dt R 2p RT

8.5.3 Model Equations of the Chiller

The energy balance equations of different components (beds, evaporator, condenser)

can be generalized as follows,

  dTk dw  
MC p H.E x
= Mbed h ads + ṁC p (Tin − Tout ) H.E x. f luid (8.33)
dt dt
where M is the mass, C p is specific heat capacity, M bed is the mass of adsorbent used,
and hads is the isosteric heat of adsorption/desorption. k indicates various compo-
nents of adsorption chiller, i.e., adsorbing/desorbing bed, evaporator and condenser.
Subscripts H.Ex and H.Ex.fluid stand for heat exchanger and heat transfer fluid
respectively; in and out represent inlet and outlet respectively. The left-hand side of
Eq. (8.33) represents the rate of change of enthalpy of the particular heat exchanger
component (adsorbing/desorbing bed, evaporator, condenser). The first term on the
right-hand side indicates the amount of heat released or absorbed by the adsorbent
8 Solar Thermal-Powered Adsorption Chiller 135

during the process of adsorption or desorption. And the last term indicates the sen-
sible heat transfer between the heat transfer fluid and the respective component.
The solution of Eq. (8.33) provides the temperature T k of the heat exchanger
components. But that requires the outlet temperature T out to be known. In order to
determine T out , the log mean temperature difference (LMTD) of the energy balance
equation for heat transfer fluid is employed. According to that method, the amount
of heat transfer,

Q̇ = U ATln (8.34)

where U and A are the overall heat transfer coefficient and heat transfer surface area
of the heat exchanging component respectively, and T ln represents LMTD. Again
Q̇ = ṁC p (Tout − Tin ) gives,

ṁC p (Tout − Tin )

Tln = (8.35)
UA
But T ln is defined by the equation,

(T − Tin ) − (T − Tout )
Tln =   (8.36)
−Tin
ln TT−T out

Hence, from Eqs. (8.35) and (8.36), we can determine T out , as given in the equation
below,
  
UA
Tout = T − (T − Tin ) exp − (8.37)
ṁC p

Substituting T out into Eq. (8.33), it can be written as,

   
  dTk dw UA
MC p H.E x = Mbed h ads + ṁC p (Tin − Tk ) 1 − exp −
dt dt ṁC p H.E x. f luid
(8.38)

Thus, Eq. (8.38) is the energy balance equation, which is valid for all the heat
exchanger components of the chiller.

8.5.3.1 Modelling of Adsorber/Desorber Bed

An adsorber/desorber bed comprises several modules where each module is neces-

sarily a finned tube heat exchanger, and the adsorbents are packed between the fins.
The mass of adsorbent loaded in each bed depends on the cooling capacity of the
chiller. Figure 8.10 represents a typical module of a finned-tube heat exchanger.
136 M. Muttakin et al.

Lm

Hf

Heat transfer
fluid inlet

Lf

Wf

Fig. 8.10 A module of a finned-tube heat exchanger

Let us consider a bed containing N m number of modules. Length of each module

is L m, and each module contains N t a number of tubes having outside and inside
diameters Dm,o and Dm,i respectively. If the tube material density is denoted by ρ t ,
the total mass of the tubes can be determined by,
π 2
Mt = D − Dm,i
2
L m Nt ρt (8.39)
4 m,o
If L f , H f and W f are the fin’s length, height and width respectively, the fin surface
area that is in contact with the adsorbent can be calculated from,
π 2
A f = L f Hf − D Nt (8.40)
4 m,o
If there are N f number of fins in each module and the density of the fin material
is ρ f , the total mass of the fins can be expressed by,

Mf = Af Wfρf Nf (8.41)

Now, let us assume C p,t , C p,f and C p,s are the specific heat capacities of tube
material, fin material and silica gel. If M bed is the amount of adsorbent used in each
bed, we can write,
 
MC p bed
= Mt C p,t + M f C p, f Nm + Mbed Cs (8.42)

The adsorbent bed goes through adsorption, desorption, mass recovery and heat
recovery phases. Hence the model equations for each phase is narrated in the follow-
ing sub-sections.
8 Solar Thermal-Powered Adsorption Chiller 137

Bed Equations During Adsorption-Desorption

Let us assume, at a certain point of time, Bed 1 is at the desorption phase, and
Bed 2 is at adsorption phase. The desorber bed, i.e., Bed 1, is then connected to
the condenser. Hot water flows through the tubes of the heat exchanger, providing
the necessary heat for the process of desorption. The hot water outlet temperature
depends on the rate of heat transfer between the tubes and the adsorbent bed. For the
desorber bed, Eq. (8.38) can be written as,
 
  dTbed,des dw
MC p bed,des = Mbed h ads
dt dt des
  
(U A)bed
+ ṁ hot C p,hot (Tin,hot − Tbed,des ) 1 − exp −
ṁ hot C p,hot
(8.43)

where, T bed,des is the temperature of the desorbing bed, ṁ hot and C p,hot are the flow
rate and specific heat capacity of hot water. dw dt des
is the rate of desorption and
can be calculated using Eq. (8.32). The equilibrium uptake w* at any time step can
be determined using Eq. (8.30). It needs mentioning that in the case of a desorber
bed, Psat (T ref ) is essentially the condenser pressure and
 Psat (T
 ads ) corresponds to the
saturation pressure of water at the bed temperature. MC p bed,des can be obtained
from,
   
MC p bed,des
= MC p bed + wMbed C p,des

where, C p,des is the specific heat capacity of water vapor at the desorber bed tempera-
ture. T in,hot is the inlet temperature of hot water, and the hot water outlet temperature
can be determined from,
  
(U A)bed
Tout,hot = Tbed,des − Tbed,des − Tin,hot exp − (8.44)
ṁ hot C p,hot

Similar to desorber bed, the model equation for adsorber bed, i.e., for Bed 2, can
be written as,
 
  dTbed,ads dw
MC p bed,ads = Mbed h ads
dt dt ads
  
(U A)bed
+ ṁ cool C p,cool Tin,cool − Tbed,ads 1 − exp −
ṁ cool C p,cool
(8.45)

where subscripts ads and cool represent adsorption and cooling water, respectively.
The outlet temperature of cooling water can be calculated from,
138 M. Muttakin et al.
  
(U A)bed
Tout,cool = Tbed,ads − Tbed,ads − Tin,cool exp − (8.46)
ṁ cool C p,cool

Since the adsorber bed is connected to the evaporator, the equilibrium uptake
w* is a function of evaporator pressure and saturation pressure of water at the bed
temperature.

Bed Equations During Mass Recovery

In the mass recovery stage, there is no water flow through any of the bed, as can be
seen in Fig. 8.8. Hence the outlet temperatures of hot and cooling water will remain
the same as their inlet temperatures. The beds are also isolated from the evaporator
and condenser. At the end of the ad/de-sorption phase, the pressure at Bed 1 becomes
higher than that at Bed 2. The opening of the by-pass valve in the mass recovery
stage initiates the flow of excess vapor from Bed 1 to Bed 2 due to pressure swing,
and the process is usually continued (accomplished by appropriate determination of
cycle time) until the mechanical equilibrium between the two beds is attained. The
flow rate of vapor, from Bed 1 to Bed 2, during this stage can be determined from
Thu et al. (2017),

dw Pρ
= 1.41576Y Abp (8.47)
dt K

where Y is the expansion factor (Shashi Menon 2015), Abp is the cross-section area
of the bypass valve, ΔP is the pressure difference between the two beds and K is the
total resistance coefficient. It needs mentioning that the reversed vapor flow due to
pressure changes is also accounted for in the above model. K can be calculated from,

L 
K = f + ξi
D i

where ξ is the minor loss coefficient for different components, L and D are the length
and width of the bypass valve, respectively. f is the friction factor given as,
⎧
⎨ Re
64
f or Re < 2300
f = #  1.325 $2 f or 5000 ≤ Re ≤ 108 &10−6 ≤ e
≤ 10−2
⎩ D
3.7D + Re0.9
e 5.74
ln

The energy balance equations for mass recovery can be written as follows;
for Bed 1,

  dTbed,des dw
MC p bed,des
= −Mbed h ads (8.48)
dt dt
8 Solar Thermal-Powered Adsorption Chiller 139

and for Bed 2,

  dTbed,ads dw
MC p bed,ads
= Mbed h ads (8.49)
dt dt

Bed Equations During Heat Recovery

In the heat recovery stage, the beds are isolated from evaporator, condenser and also
from each other. As a result, there is no flow of water vapor through the beds. The
temperature of hot water at the outlet remains the same as it is at the inlet since there
is no flow of hot water through the bed (see Fig. 8.9). The cooling water extracts heat
from the desorbed bed (Bed 1) and heats up the adsorbed bed (Bed 2) on its way to
the condenser. The cooling water outlet temperature from Bed 1 can be written as,
  
(U A)bed
Tout,des = Tbed,des − Tbed,des − Tin,cool exp − (8.50)
ṁ cool C p,cool

And the cooling water outlet temperature from Bed 2 can be calculated from,
  
(U A)bed
Tout,ads = Tbed,ads − Tbed,ads − Tout,des exp − (8.51)
ṁ cool C p,cool

The energy balance equations for the two beds can be expressed by;
for Bed 1,
  
  dTbed,des (U A)bed
MC p bed,des = ṁ cool C p,cool Tin,cool − Tbed,des 1 − exp −
dt ṁ cool C p,cool
(8.52)

for Bed 2,
  
  dTbed,ads (U A)bed
MC p bed,ads = ṁ cool C p,cool Tbed,des − Tbed,ads 1 − exp −
dt ṁ cool C p,cool
(8.53)

8.5.3.2 Modelling of the Evaporator

An evaporator is usually a shell and tube type heat exchanger comprising a series of
tubes. Chilled water flows through the tubes while the refrigerant vapor evaporates
from the shell side. During the ad/de-sorption phase, the evaporator is connected
to the adsorber bed, and the evaporated vapor gets adsorbed by the adsorbent. Let
us assume, the evaporator has N t,evap number of tubes and mass, length and inside
diameter of each tube are M t,evap , L t,evap , and De,i respectively. Hence, at any instant,
140 M. Muttakin et al.

the amount of chilled water present within the tubes of the evaporator is,
π 2
Mch,evap =
D L t,evap Nt,evap ρch (8.54)
4 e,i
 
where ρ ch is the density of chilled water. MC p bed,des can be calculated from,
 
MC p evap
= Mch,evap C p,ch + Mt,evap Nt,evap C p,t,evap (8.55)

where, C p,ch and C p,t,evap are the specific heat capacities of chilled water and evapo-
rator tube material respectively.
Let hfg,evap be the latent heat of evaporation of water at the evaporation temperature.
The energy balance equation for the evaporator during ad/de-sorption phase can be
written as,
 
  dTevap dw % &
MC p evap = −Mbed h f g,evap + C p,ads Tbed,ads − Tevap
dt dt ads
  
(U A)evap
+ ṁ ch C p,ch Tin,ch − Tevap 1 − exp − (8.56)
ṁ ch C p,ch

where subscript ch stands for chilled water. UA value of the evaporator, i.e., (UA)evap ,
is determined to utilize the LMTD method, as mentioned in Sect. 5.3.
The chilled water outlet temperature is determined from,
  
(U A)evap
Tout,ch = Tevap − Tevap − Tin,ch exp − (8.57)
ṁ ch C p,ch

During mass and heat recovery stages, the evaporator is isolated from the beds.
Hence, the temperature of the evaporator and chilled water remain unchanged at
these stages.

8.5.3.3 Modelling of the Condenser

Cooling water flows through the tubes of the condenser condensing the desorbed
vapor that comes from the desorber bed. Similar to evaporator modelling, let us
assume, the condenser has N t,cond number of tubes and mass, length and inside
diameter of each tube are M t,cond , L t,cond , and Dc,i respectively. Then for condenser,
Eqs. (8.54) and (8.55) can be written as,
π 2
Mcool,cond = D L t,cond Nt,cond ρcool (8.58)
4 c,i
and,
8 Solar Thermal-Powered Adsorption Chiller 141
 
MC p cond
= Mcool,cond C p,cool + Mt,cond Nt,cond C p,t,cond (8.59)

where subscripts cool and cond represent cooling water and condenser, respectively.
During the ad/de-sorption stage, the cooling water from the outlet of Bed 1 enters
the condenser. Hence, the energy balance equation can be written as,
 
  dTcond dw % &
MC p cond = −Mbed h f g,cond + C p,des Tbed,des − Tcond
dt dt des
  
(U A)cond
+ ṁ cool C p,cool Tout,cool − Tcond 1 − exp −
ṁ cool C p,cool
(8.60)

The water temperature at the outlet of the condenser can be expressed as,
  
(U A)cond
Tout,cond = Tcond − Tcond − Tout,cool exp − (8.61)
ṁ cool C p,cool

In the mass recovery stage, the condenser is not connected to any bed. Thus, the
temperatures of the condenser and cooling water don’t change at this stage.
During the heat recovery stage, the condenser is isolated from the beds and water
from Bed 1 first flows through Bed 2, before entering the condenser. Thus the energy
balance for this stage can be modeled as,
  
  dTcond (U A)cond
MC p cond
= ṁ cool C p,cool Tout,ads − Tcond 1 − exp −
dt ṁ cool C p,cool
(8.62)

The water temperature at the outlet of condenser is,

  
(U A)cond
Tout,cond = Tcond − Tcond − Tout,ads exp − (8.63)
ṁ cool C p,cool

8.5.3.4 Performance Parameters

An adsorption chiller is usually specified by its cooling capacity and coefficient of

performance (COP). The cooling capacity can be defined as the amount of cooling
effect that an adsorption chiller is able to produce. For a chiller having a cycle time
of t cycle , the cooling capacity can be defined by,
' tcycle
ṁ ch C p,ch Tin,ch − Tout,ch dt
Qc = 0
(8.64)
tcycle
142 M. Muttakin et al.

The rate of heat input to drive an adsorption chiller can be expressed by,
' tcycle
ṁ hot C p,hot Tin,hot − Tout,hot dt
Qh = 0
(8.65)
tcycle

The COP of the chiller is defined by the ratio of useful cooling capacity produced
to the heat input to the chiller. Mathematically it can be shown as,

Qc
COP = (8.66)
Q h − Q r ec

where Qrec is the rate of heat recovered during the heat recovery stage.

8.5.4 Typical Simulation Results

In this subsection, simulation results of a typical 10-ton capacity adsorption chiller are
shown. Let us consider the inlet temperatures of the cold water, hot water and chilled
water are 27, 80 and 13 °C respectively and their flow rates are 10, 15 and 6 m3 /h
respectively. These values are assumed to be constant throughout the operation of the
chiller. However, they may not be such a uniform in real-life application. The chiller
uses silica gel as the adsorbent and water as the refrigerant. The isotherm parameters
of the S-B-K model are furnished in Table 8.1 (Rezk and Al-Dadah 2012). The
isosteric heat of ad/de-sorption for the pair is found to be, hads = 2.8 × 104 J/kg.
The particle radius of the silica gel adsorbent is considered as Rp = 0.17 mm.
The adsorption rate is determined using the LDF model and the values of its kinetics
parameters (Saha et al. 1995) are Dso = 2.54 × 10−4 m2 /s and E a = 4.2 × 104
J/mol. The assumed specifications of different components of the chiller are shown
in Table 8.2.
In the current simulation, the time required for the ad/de-sorption, mass recovery
and heat recovery are considered as 300 s, 20 s and 10 s, respectively. The typi-
cal simulation results are shown in Figs. 8.11, 8.12 and 8.13. The average cooling
capacity of the chiller is found to be 10.03 RTon with a COP of 0.51.
Figure 8.11 shows the variation of temperature of different components of the
chiller obtained from the simulation results. The experimental data can be obtained

Table 8.1 Isotherm properties of the silica gel—water pair

Parameter Value Parameter Value
A0 −6.5314 B0 −15.587
A1 0.7245 × 10−1 B1 0.15915
A2 −0.23951 × 10−3 B2 −0.50612 × 10−3
A3 0.25493 × 10−6 B3 0.53290 × 10−6
8 Solar Thermal-Powered Adsorption Chiller 143

Table 8.2 Specifications of

Ad/de-sorber bed
different components of the
adsorption chiller Mass of adsorbent used, M bed 80 kg
Number of modules, N m 10
Length of each module, L m 3.65 m
Tube material Copper
Number of tubes in each module, N t 3
Outside diameter of the tube, Dm,o 16.6 mm
Inside diameter of the tube, Dm,i 15 mm
Fin material Aluminium
Number of fins in each module, N f 3310
Length of the fin, L f 342 mm
Height of the fin, H f 30 mm
Width of the fin, W f 0.105 mm
Evaporator
Tube material Copper
Number of tubes, N t,evap 115
Length of the tube, L t,evap 3.9 m
Mass per tube, M t,evap 1.4 kg
Inner diameter of the tube, De,i 17.65 mm
Condenser
Tube material Copper
Number of tubes, N t,cond 220
Length of the tube, L t,cond 2.65 m
Mass per tube, M t,cond 0.96 kg
Inner diameter of the tube, De,i 17.65 mm

from the test results of commercial adsorption chiller and can be utilized to validate
the model. The variation of temperature and uptake of the beds is depicted in Fig. 8.12.
As expected, the bed temperature increases during desorption and decreases during
adsorption. The instantaneous cooling capacity at different stages of cycle time can
be seen in Fig. 8.13. It needs mentioning that in determining the average cooling
capacity and COP, the simulation result of the first cycle is ignored. It is done as the
initial results are strongly influenced by the values used for initialization, and the
model requires a few time steps to stabilize.
144 M. Muttakin et al.

90

80

70

60
Temperature [°C]

Chilled water out Chilled water in

50 Cooling water out Cooling water in
Hot water out Hot water in
40

30

20

10

0
0 250 500 750 1000 1250 1500 1750 2000
Time [s]

Fig. 8.11 Typical simulation results showing the variation of temperature with time of different
components of an adsorption chiller

90 Temperature Bed 1 Temperature Bed 2

0.50
Uptake Bed 1 Uptake Bed 2
80 0.45

0.40
70
0.35
60
0.30
Temperature [°C]

Uptake [kg/kg]

50
0.25
40
0.20
30
0.15
20
0.10

10 0.05

0 0.00
0 250 500 750 1000 1250 1500 1750 2000
Time [s]

Fig. 8.12 Simulation results showing the temperature of beds and uptakes at different stages of
operation of an adsorption chiller
8 Solar Thermal-Powered Adsorption Chiller 145

16

14

12
Chilling capacity [RTon]

10

0
0 250 500 750 1000 1250 1500 1750 2000
Time [s]

Fig. 8.13 The instantaneous cooling capacity of the chiller

8.6 Conclusion

Flat plate collectors have widely been used for domestic water heating. On the other
hand, the evacuated tube collector is the cheapest and most preferred option for
boiler water preheating. In this chapter, the model equations of these two types of
collectors are presented. Furthermore, a commercial adsorption chiller is modelled,
and the simulation results of the model are presented. The design parameters of the
chiller may be varied to observe their effects on its performance. The model can be
utilized to predict the performance of the chiller under variable operating conditions.
It can also play a vital role in designing different components of the chiller to make
the system more efficient and cost effective.

References

Conti J, Holtberg P, Diefenderfer J, LaRose A, Turnure JT, Westfall L (2016) International energy
outlook 2016 with projections to 2040. USDOE Energy Information Administration (EIA), Wash-
ington, DC, USA
Cooper PI, Dunkle RV (1981) A non-linear flat-plate collector model. Sol Energy 26:133–140
Coulomb D (2006) Statement by the Director of the International Institute of Refrigeration. In:
United Nations climate change conference, Nairobi. http://www.un.org/webcast/unfccc/2006/
statements/061117iir_e.pdf
Duffie JA, Beckman WA (1974) Solar energy thermal processes. University of Wisconsin-Madison,
Solar Energy Laboratory, Madison, WI
146 M. Muttakin et al.

Duffie JA, Beckman WA (1980) Solar engineering of thermal processes. Willey, New York
Hottel HC (1942) Performance of flat-plate solar heat collectors., Trans ASME 64:91
Hottel C (1954) Performance of flat plate energy collectors: space heating with solar energy. In:
Course symposium. Cambridge
Hottel H, Whillier A (1955) Evaluation of flat-plate solar collector performance. In: Transdisci-
plinary conference on the use of solar energy
Kalogirou SA (2004) Solar thermal collectors and applications. Prog Energy Combust Sci 30:
231–295. https://doi.org/10.1016/j.pecs.2004.02.001
Kalogirou SA, Papamarcou C (2000) Modelling of a thermosyphon solar water heating system and
simple model validation. Renew Energy 21:471–493
McAdams WH (1954) Heat transmission. McGraw-Hill, New York, pp 252–262
Mitchell JW (1976) Heat transfer from spheres and other animal forms. Biophys J 16:561–569
Mitra S, Thu K, Saha BB, Dutta P (2017) Performance evaluation and determination of minimum
desorption temperature of a two-stage air cooled silica gel/water adsorption system. Appl Energy
206:507–518. https://doi.org/10.1016/j.apenergy.2017.08.198
Muttakin M (2013) Optimization of solar thermal collector systems for the tropics. National Uni-
versity of Singapore. https://scholarbank.nus.edu.sg/handle/10635/47365
Muttakin M, Mitra S, Thu K, Ito K, Saha BB (2018) Theoretical framework to evaluate minimum
desorption temperature for IUPAC classified adsorption isotherms. Int J Heat Mass Transfer
122:795–805. https://doi.org/10.1016/j.ijheatmasstransfer.2018.01.107
Praene J-P, Garde F, Lucas F (2005) Dynamic modelling and elements of validation of solar evac-
uated tube collectors. In: Ninth international IBPSA conference
Rezk ARM, Al-Dadah RK (2012) Physical and operating conditions effects on silica gel/water
adsorption chiller performance. Appl Energy 89:142–149. https://doi.org/10.1016/j.apenergy.
2010.11.021
Saha BB, Kashiwagi T (1997) Experimental investigation of an advanced adsorption refrigeration
cycle. ASHRAE Trans 103:50–58
Saha BB, Boelman EC, Kashiwagi T (1995) Computational analysis of an advanced adsorption-
refrigeration cycle. Energy 20:983–994
Saha BB, El-Sharkawy II, Chakraborty A, Koyama S, Banker ND, Dutta P, Prasad M, Srinivasan
K (2006) Evaluation of minimum desorption temperatures of thermal compressors in adsorption
refrigeration cycles. Int J Refrig 29:1175–1181. https://doi.org/10.1016/j.ijrefrig.2006.01.005
Saha BB, Chakraborty A, Koyama S, Srinivasan K, Ng KC, Kashiwagi T, Dutta P (2007) Thermo-
dynamic formalism of minimum heat source temperature for driving advanced adsorption cooling
device. Appl Phys Lett 91. https://doi.org/10.1063/1.2780117
Shashi Menon E (2015) Meters and valves. Trans Pipeline Calc Simul Manual 431–471. https://
doi.org/10.1016/b978-1-85617-830-3.00012-2
Souka AF, Safwat HH (1966) Determination of the optimum orientations for the double-exposure,
flat-plate collector and its reflectors. Sol Energy 10:170–174
Swinbank WC (1963) Long-wave radiation from clear skies. Q J R Meteorol Soc 89:339–348
Thu K, Saha BB, Mitra S, Chua KJ (2017) Modeling and simulation of mass recovery process in
adsorption system for cooling and desalination. Energy Procedia 105:2004–2009. https://doi.org/
10.1016/j.egypro.2017.03.574
Wang RZ (2001) Performance improvement of adsorption cooling by heat and mass recovery
operation  lioration de la performance d’ un syste Á me de Ame Á adsorption a Á l’ aide de re
 cupe  ration de masse refroidissement a et de chaleur. Int J Refrig 24:602–611. https://doi.
org/10.1016/j.ijrefrig.2014.04.018
Zarem A, Erway D (1963) Introduction to the utilization of solar energy. McGraw-Hill, New York
Chapter 9
TEWI Assessment of Conventional
and Solar Powered Cooling Systems

Md. Amirul Islam and Bidyut Baran Saha

Abstract Conventional cooling and refrigeration systems already evolved to effi-

cient design, have higher COP and compact size. However, the compressor section of
such system consumes a tremendous amount of electricity and contribute indirectly
to global warming. The working fluids of these systems are typically HFC or HFC
blends which possess very high global warming potential. A significant percentage
of working fluid is leaked from the high-pressure side of the system and directly con-
tribute to global warming. The summation of indirect and direct warming impact,
namely, total equivalent warming impact (TEWI) of the vapour compression cool-
ing systems are significantly high. Adsorption cooling system (ACS) can resolve this
critical issue. In ACS, the mechanical compressor of the traditional cooling system is
replaced by a thermal compressor, namely, a pair of adsorption beds. Highly porous
adsorbent material (silica gel, activated carbon, zeolite and so forth) is the key com-
ponent of an adsorption bed. These materials have the capability to capture and hold
certain types of fluid. This phenomenon is known as adsorption. Upon heating, the
adsorbed fluid is liberated from the pores (desorption process) and gets thermally
compressed. Solar thermal energy is the most prospective option for the desorption
process to occur. Since there is no mechanical compressor, electricity consumption is
deficient, which significantly minimizes the indirect warming impact. Moreover, nat-
ural or alternative refrigerants are used as the working fluid, which has zero/negligible
GWP. Hence, the direct warming impact is also shallow. In this chapter, the working

Md. A. Islam · B. B. Saha (B)

Kyushu University Program for Leading Graduate School, Green Asia Education Center,
IGSES, Kyushu University, Kasuga-Koen 6-1, Kasuga-shi, Fukuoka 816-8580, Japan
e-mail: saha.baran.bidyut.213@m.kyushu-u.ac.jp
Md. A. Islam
e-mail: toha_apece@yahoo.com
International Institute for Carbon-Neutral Energy Research (WPI-I2 CNER), Kyushu University,
744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
Md. A. Islam
Department of Electronics and Telecommunication Engineering, Bangabandhu Sheikh Mujibur
Rahman Science and Technology University, Gopalganj 8100, Bangladesh

© Springer Nature Singapore Pte Ltd. 2020 147

H. Tyagi et al. (eds.), Solar Energy, Energy, Environment,
and Sustainability, https://doi.org/10.1007/978-981-15-0675-8_9
148 Md. A. Islam and B. B. Saha

principle and governing equations of a solar energy driven adsorption cooling sys-
tem will be elaborated. Besides, TEWI assessment procedure will be explained and
compared for both vapour compression and adsorption cooling systems.

Keywords Adsorption cooling · GWP · HFC · Solar thermal energy · TEWI ·

Vapor compression refrigeration

9.1 Introduction

The main objectives behind the invention of cooling systems are to attain thermal
comfort during summer and to preserve various goods (fish, meat, vegetable, and so
forth) that usually rots very fast at ambient conditions (Arora 2010; Islam et al. 2017).
The conventional cooling systems have four major building blocks: evaporator, com-
pressor, condenser and expansion device. A working fluid (namely refrigerant) flows
through these components, change the phase due to pressure variation and removes
heat from a low-temperature source to a relatively higher temperature sink (Wang
2000). Electricity is required to run this cooling system, and the mechanical compres-
sor consumes about 90% of that electricity. Electricity generation sources are mainly
fossil fuel based and release greenhouse gases during electricity production (Rah-
man and de Castro 1995; Bose 2010; Cherp et al. 2017). Briefly, a cooling system
indirectly contributes to global warming by using electricity. Moreover, refrigerant
leakage is inevitable from joints and seals, mechanical failure or during servicing
(Tassou and Grace 2005; Francis et al. 2016). Previous generation refrigerants such
as ethers, NH3 , SO2 have toxicity problem; H2 O has narrow operating temperature
range; CFCs and HCFCs have ozone depletion potential and high global warming
impact; HCs are unsafe due to their high flammability issue although they have very
low GWP and wider operation range (Calm 2008; Sarbu 2014). The properties of
some refrigerants are shown in Table 9.1. Currently employed refrigerants in the cool-
ing systems are mostly HFCs and have a very high global warming impact. Hence,
the leaked refrigerant contributes directly to global warming. The summation of indi-
rect and direct warming impact, namely, total equivalent warming impact (TEWI) of
the conventional cooling systems are significantly high (Calm 2002; Makhnatch and
Khodabandeh 2014; Davies and Caretta 2004; Kruse 2000). Hence, the adsorption
cooling system is becoming popular, which can be driven by solar thermal energy
or any low-temperature heat source (Saha et al. 2006; Kayal et al. 2016; Thu et al.
2013; El-Sharkawy et al. 2008). Electricity consumption of this system is very low.
Moreover, natural refrigerants can be used as working fluids which have zero or neg-
ligible global warming impact (Habib et al. 2014; Kasaeian et al. 2018; Saha et al.
2001). According to history, Faraday discovered the adsorption cooling phenomenon
in 1948. He observed that NH3 adsorption onto AgCl could produce cooling. G. E.
Hulse proposed an adsorption refrigeration system with silica gel as adsorbent and
SO2 as the refrigerant in the 1920s (Wang et al. 2014). Nowadays, many renowned
Table 9.1 Properties of the selected refrigerants
R22 R32 R134a R404A R410A R717 R718 Methanol
Chemical CHClF2 CH2 F2 CH2 FCF3 R125 (44%) R32 (50%) NH3 H2 O CH3 OH
formula R134a (4%) R125 (50%)
(Kohler et al. R143a (52%)
2016)
GWP 1760 677 (Stocker 1300 (Stocker 3922 (Vaitkus 2088 0 0 0
(kg–CO2 eq.) (Mota-Babiloni et al. 2019) et al. 2019) and Dagilis (Mota-Babiloni
et al. 2017) 2017) et al. 2017)
ODP 0.055 (Deveciog 0 0 0 0 0 0 0
and Oruç 2017)
Critical 4.99 5.78 4.06 3.73 4.9 1.14 2.21 8.10
pressure
(MPa)
(Lemmon
et al. 2018)
Critical 96.15 78.11 101.06 72.12 71.34 132.41 373.95 239.45
temperature
(°C)
(Lemmon
et al. 2018)
Normal – 40.81 – 51.65 – 26.07 – 46.22 – 51.44 – 33.32 99.97 64.48
boiling point
9 TEWI Assessment of Conventional and Solar Powered Cooling Systems

(°C)
(Lemmon
et al. 2018)
(continued)
149
Table 9.1 (continued)
150

R22 R32 R134a R404A R410A R717 R718 Methanol

Latent heat of 182.74 270.91 177.79 138.99 190.6 1165.8 2441.7 1169.0
evaporation at
25 °C (kJ/kg)
(Lemmon
et al. 2018)
ASHRAE A1 A2L A1 A1 A1 B2L A1 N/A
safety group
(Kohler et al.
2016)
Flammability No Yes (mild) No No No Yes (mild) No Yes (Machiele
(Kohler et al. 1987)
2016)
Toxicity No No No No No Yes No Yes (Machiele
(Kohler et al. 1987)
2016)
Md. A. Islam and B. B. Saha
9 TEWI Assessment of Conventional and Solar Powered Cooling Systems 151

industries such as Mitsubishi (2012), Tokyo Boeki Machineries (2019), Bry-Air

(2019), Mayekawa (2019), and SorTech (2009) are commercially manufacturing
high capacity adsorption chillers.

9.2 Cooling Systems

The refrigerant fluid changes its phase between vapour and liquid in various sections
of the cooling system due to pressure variation, which is the fundamental working
principle of any cooling system (Ahamed et al. 2010; Gill and Singh 2018; Yumrutaş
et al. 2002; Yang and Yeh 2015). Most common types of cooling system utilize elec-
trical energy to achieve the desired pressure. Moreover, researchers have developed
new systems that are capable of doing the same job using thermal energy such as
(solar energy, waste heat, geothermal energy, and so forth) (Carotenuto et al. 2017;
Ghaebi et al. 2018; Ng et al. 2001, 2006; Habib et al. 2013; Choudhury et al. 2013;
Alahmer et al. 2016; Khan et al. 2007).

9.2.1 Vapor Compression Refrigeration (VCR)

A conventional VCR system comprises of four major parts: evaporator, compressor,

condenser and expansion device. A refrigerant (pure/blends) is the primary work-
ing fluid which changes phase in evaporator and condenser to perform the cooling
(Cengel 2004). The schematic and working principle of a VCR is shown in Fig. 9.1.

Warm
Subcooling surroundings
circuit at TH °C
Removed
Liquid Condenser
Hot
Vapor
QH heat

Expansion
Win
Compressor Required
valve work input

Evaporator
Vapor Qc Target
output

Liquid + vapor Superheating Cooled space

circuit
at Tc °C

Fig. 9.1 Schematic and norm of a conventional vapor compression system

152 Md. A. Islam and B. B. Saha

9.2.1.1 Evaporator

The liquid refrigerant evaporates at low pressure in the evaporator. During evapora-
tion, liquid refrigerant accumulates latent heat from the surroundings. Evaporation
is an isothermal process. A heat exchanger fluid (air/water) is flown to the evaporator
coil for efficient cooling. Most of the evaporator is followed by an auxiliary circuit
for superheating the vapour which ensures that the liquid refrigerant is completely
evaporated before leaving the evaporator. Superheating prevents the damage of the
compressor and increases the efficiency of the system.

9.2.1.2 Compressor

Vapour refrigerant is fed into the suction line of the compressor and compressed until
the condenser pressure. The compression process of an ideal compressor is isentropic.
However, in practice, there is no idea compressor. Hence, the compression is non-
isentropic. According to Gay-Lussac’s gas law, the temperature increases during the
compression. Modern compressors are variable in speed and electricity consumption
is very low.

9.2.1.3 Condenser

Hot vapour refrigerant discharges from compressor outlet and flows through the
condenser coil. The coil has fins, and the air is flown for faster heat transfer. Heat
is released from the refrigerant to the surroundings and gets liquefied. Released
refrigerant from the condenser is passed through an additional circuit which subcools
the liquid refrigerant to ensure that all the refrigerant is in the liquid state before
entering expansion device.

9.2.1.4 Expansion Device

The high-pressure liquid refrigerant is forced through a small orifice of the expan-
sion valve, which causes a sudden pressure and temperature drop. Refrigerant is in
two-phase condition after the expansion device. This refrigerant reenters into the
evaporator and the cycle continues to repeat.

9.2.2 Solar Cooling

Unlike the VCR system, work input of a solar cooling system is thermal energy
instead of electricity. The mechanical compressor of a VCR is replaced with a
thermal compressor, namely adsorption bed (Sayigh and McVeigh 1992; Chang
9 TEWI Assessment of Conventional and Solar Powered Cooling Systems 153

Hot water Condenser Cooling

Vapor tower
reservoir
V3 V2 Cooling water in
Hot water in
Liquid
Hot Expansion
water valve Adsorption
out Desorption
bed bed
Liquid
+
Vapor Cooling water out
Hot water out V4 V1

Vapor
Solar Evaporator
collector Ambient
water in

Fig. 9.2 Adsorption cooling system by using solar thermal energy

et al. 2007; Cho and Kim 1992; Bassols-Rheinfelder 1985; Buonomano et al. 2018;
Tangkengsirisin et al. 1998; Al Mers et al. 2006; Chang et al. 2009). Other three
components of the system are the same as VCR. Some additional components are
also required in the solar cooling system for the cooling to happen. The complete
system is entitled as adsorption cooling system (ACS), which is shown in Fig. 9.2.
The main component of an adsorption bed is the adsorbent. An adsorbent material
contains millions of pores and capable of adsorbing (capture and hold) the refrigerant
(also called adsorbate) molecules. Choosing the appropriate adsorbent/adsorbate is
crucial to design an efficient adsorption cooling system.

9.2.2.1 Adsorption/Desorption

Adsorption is the phenomenon to trap and retain refrigerant molecules into the adsor-
bent pores. Adsorption capacity depends on the porous properties (surface area, pore
width, pore volume) of the adsorbent and also surrounding conditions (temperature
and pressure). Hence, choosing the adsorbent is crucial for a solar cooling system.
Properties of some common adsorbents are shown in Table 9.2.
The refrigerant molecules need to be regenerated and recirculated to complete the
refrigeration cycle. The regeneration process is called desorption. Hot water is passed
through the heat exchanger coil, which is placed inside the adsorption/desorption bed
for the regeneration. Required hot water for the desorption is obtained from solar
thermal energy.

9.2.2.2 Solar Collector

A solar collector is an apparatus that accumulates or concentrates solar insolation.

There are various types of solar collectors such as flat plate collector, parabolic
collector, direct absorption collector, photovoltaic thermal (PVT) collector, hybrid
photovoltaic and thermal collector (Khullar et al. 2012; Phelan et al. 2013; He et al.
154 Md. A. Islam and B. B. Saha

Table 9.2 Porous properties of some well-known adsorbents

Adsorbent name BET surface Pore width, Pore volume,
area, A (m2 g–1 ) w υ
(Å) (cm3 g–1 )
Activated carbon (Maxsorb III) 3045 11.20 1.70
(El-Sharkawy et al. 2014)
Activated carbon from waste 2927 16.80 2.41
palm trunk (Pal et al. 2017)
Activated carbon from Mangrove 2924 14.70 2.13
(Pal et al. 2017)
Activated carbon fiber (A20) 1900 21.60 0.765
(El-Sharkawy et al. 2006)
Activated carbon fiber (A15) 1400 21.75 1.028
(El-Sharkawy et al. 2006)
Silica gel (RD) (Ng et al. 2001) 650 21.00 0.35
Silica gel (type A) (Ng et al. 650 22.00 0.36
2001)
Zeolite (AQSOA Z01) (Kayal 132 7.40 0.087
et al. 2016)
Zeolite (AQSOA Z02) (Kayal 590 3.70 0.2769
et al. 2016)

2006; Fudholi et al. 2014; Tyagi et al. 2012; Khelifa et al. 2015). Necessary heat for
desorption can be arranged by employing one of those collectors.

9.3 Total Equivalent Warming Impact (TEWI)

Every refrigeration system contributes to global warming, either directly or indirectly.

TEWI is the overall quantity of direct and indirect GWP released from a system.
The compressor (desorption bed for a solar cooling system) and condenser sections
operate at a high temperature and pressure than the atmospheric. Although all the
sections are properly sealed, a significant amount of leakage (about 20%) can occur
from joints and seals, due to mechanical failure or during servicing (Tassou and Grace
2005; Reddy et al. 2012). This leakage will lead to performance degradation and will
increase energy consumption (Du et al. 2016; McIntosh et al. 2000). Moreover,
the leaked refrigerants have high GWP, which will eventually contribute directly to
global warming (Davies and Caretta 2004).
Indirect emission occurs due to the electricity consumption of the refrigeration
system since a major share of the electricity is generated from fossil fuel.
9 TEWI Assessment of Conventional and Solar Powered Cooling Systems 155

A cooling system is mostly made with metals such as stainless steel, aluminium,
and copper. These materials production requires a lot of energy which indirectly
contribute to global warming.
Hence, TEWI can be assessed from the following equation:

T E W I = dir ect emission + indir ect emission

= (GW Pr × L) × n + [(E × ε) × n + m × GW Pm ] (9.1)

Here,
GWPr Global warming potential of the selected refrigerant (kg-CO2 eq.)
L Annual leakage rate of charged refrigerant (%)
n Duration to calculate TEWI (year)
E Electricity consumption (kWh)
ε CO2 emission factor for per unit electricity generation (kg-CO2 eq./kWh)
m Mass of the raw materials that are used to build the cooling system (kg)
GWPm CO2 emission factor for per unit raw material production (kg-CO2 eq./kg).

9.3.1 TEWI of Conventional Cooling System

Operating conditions and assumptions are required to set before the TEWI assessment
of a conventional VCR system. The complete set of parameters are listed in Table 9.3.

9.3.1.1 Assessment Parameters

Evaporation Temperature

Different cooling applications require a different degree of cooling. For example, in

summer, the temperature of the room air-conditioning system is set to 20–25 °C for
thermal comfort. However, beverages, vegetables, fruits and many other products
are preserved at 2–5 °C to prevent from rotting. These products are called medium
temperature (MT) cooling loads. Moreover, Fish, meat, ice-cream requires much
lower (about −20 °C) temperature for preservation. These are called low temperature
(LT) cooling loads. Another important application of low-temperature requirement
is data storage centers.
Evaporation temperature of the VCR is set about 5°–10° lower than the target
temperature. Hence, three different evaporation temperature is considered (Table 9.3)
for three types of cooling applications.
156 Md. A. Islam and B. B. Saha

Table 9.3 Operating conditions and assumptions for a conventional cooling system
Parameter Quantity
Evaporation temperature for specific 12 °C (room air-conditioning)
applications, T eva – 7 °C (medium temperature applications)
– 25 °C (low temperature applications)
Condensation temperature, T con 40 °C
Selected refrigerants R32 (Difluoromethane: CH2 F2 )
R134a (1,1,1,2-Tetrafluoroethane: C2 H2 F4 )
R404A (44% R125, 4% R134a, 52% R143a)
(Lemmon et al. 2018)
GWP and ODP of the considered refrigerants, Refer to Table 9.1
GWPr and ODPr
Cooling load, Qc 10 kW
Suction gas superheat, T sup 8 °C
Degree of subcooling, T sub 5 °C
Isentropic efficiency of the compressor, ηisen 70% (room air-conditioning)
65% (medium temperature applications)
60% (low temperature applications)
Initial refrigerant charging amount 1 kg for per kW room air-conditioning
cooling load
2 kg for per kW medium temperature load
3 kg for per kW room air-conditioning load
(Poggi et al. 2008)
Annual leakage rate of refrigerant, La 15% (Tassou and Grace 2005; Francis et al.
2016)
GWP for electricity generation, ε 0.518 (kg-CO2 /kWh) (Electricity Review
Japan 2018)
Product weight (indoor + outdoor unit), m 60 kg (10 kW room air-conditioning) (Hitachi
Room Aircon Product Brochure 2019;
General 2018)
80 kg (10 kW medium temperature system)
100 kg (10 kW low temperature system)
Weight percentage of raw materials to build a Stainless steel: 50%
cooling system Copper: 20%
Aluminium: 20%
Others (refrigerant, plastic, paint etc.): 10%
GWP of per kg material production, GWPm Stainless steel: 2.13 kg-CO2 eq. (Chisalita
et al. 2019)
Copper: 4.97 kg-CO2 eq. (Kosai and Yamasue
2019)
Aluminum: 9.17 kg-CO2 eq. (Kosai and
Yamasue 2019)
System lifespan 15 years
System runtime 12 h/day
9 TEWI Assessment of Conventional and Solar Powered Cooling Systems 157

Condensation Temperature

The assessment has been performed for the summer season when the ambient tem-
perature is around 30 °C. The condensation temperature is always higher than the
ambient temperature so that heat exchange can occur naturally. In this assessment,
the condensation temperature is considered constant (40 °C).

Selected Refrigerants and Their Environmental Impact

Global warming potential (GWP) is the quantity of heat that a greenhouse gas can
grasp in the atmosphere for a specific duration, relative to CO2 . Each refrigerant has a
certain amount of GWP when released to the environment. Previously used CFCs and
HCFCs have very high GWP. Those refrigerants are also responsible for ozone layer
depletion (ODP), which is located at the lower portion of the stratosphere. ODP value
of a fluid is the measure of destructive effects compared to the reference substance
R11 (or CFC-11 or CCl3 F or Trichlorofluoromethane). HFC based refrigerants are
used in modern domestic air-conditioning and refrigeration systems. Most frequently
used refrigerants for room air-conditioning systems are R134a and R32. However,
R22 is still used in room air-conditioning system in some developing and underde-
veloped countries. Besides, HFC blend R404A and R410A are predominantly used
in commercial refrigeration systems and low-temperature applications. Hence, these
three refrigerants (R32, R134a and R404A) are selected for the assessment. Analysis
with R22 (for room air-conditioning application) and R410A (for medium and low
temperature applications) are also presented in the appendix section for the readers’
convenience.

Cooling Load

The cooling load is the quantity of heat energy that needs to be removed from
the cooling space (evaporation surroundings) to maintain the target temperature.
Usually, a standard 20 m2 room (typical height −2.44 m) can be cooled with a
5 kW air-conditioner (Quick calculations for walk-in coolers and freezers 2019).
A typical two-room house with 10 kW air-conditioning cooling load is considered
for this assessment. Medium temperature (MT) and low temperature (LT) cooling
loads are also considered 10 kW. Since, MT and LT loads are mostly used for food
preservation; the cooled space is smaller compared to room air-conditioning system.

Superheat and Subcool

A certain degree of superheating is applied after the evaporator to ensure that the
liquid refrigerant is completely vaporized before it reaches the suction line of the
compressor. Likewise, subcooling is performed after condenser to confirm that the
158 Md. A. Islam and B. B. Saha

refrigerant is completely liquefied before entering the expansion device. Superheat-

ing and subcooling of the refrigerant prevent damage of the components and increase
the efficiency of the system. The typical value of superheating and subcooling is in
the range of 5–10 K. In this assessment, superheating has been considered 8 K and
subcooling is 5 K.

Isentropic Efficiency

When a thermodynamic process is adiabatic and completely reversible, the process

is called isentropic. Isentropic efficiency of an ideal compressor is hundred-percent.
An ideal compressor has to be frictionless, and there should be no heat transfer
during the operation. In reality, there is no ideal compressor, and we consider a cer-
tain value for the isentropic efficiency. In principle, the compression ratio increases
when the difference between the evaporation temperature and condensation tem-
perature increases. Therefore, the isentropic efficiency of the compressor is lower
for low-temperature applications. Hence, isentropic efficiency values are different in
Table 9.3, depending on the application.

Refrigerant Charging

Initially, each cooling system requires a certain amount of refrigerant charging. The
actual initial charging amount depends on the evaporator cooling capacity, condenser
capacity, evaporator and condenser heat exchanger size, heat transfer coefficients,
operating conditions, refrigerant type, and so forth. Approximately, the required
refrigerant amount can be expressed in terms of cooling capacity. For domestic
cooling systems, the amount is 0.24–1 (in kg) times of the cooling load (kW). For
medium and low-temperature applications, the amount is higher (2–3.4 times) (Poggi
et al. 2008). In order to simplify the calculation, a certain amount of refrigerant charge
has been considered and shown in Table 9.3.

Leakage Rate

Although the compressor is hermetically sealed, 15–20% of the charged refriger-

ant might be leaked from joints and seals, mechanical failure or during servicing
(Tassou and Grace 2005; Francis et al. 2016). This leakage will lead to performance
degradation and will increase energy consumption. Modern refrigeration system
manufacturers claim that their system has a very low leakage rate, which could be
as low as 5% per year. Hence, the sensitivity of direct emission with the leakage rate
is presented in the appendix section.
9 TEWI Assessment of Conventional and Solar Powered Cooling Systems 159

GWP for Electricity Generation

Electricity is required to drive the VCR, and most of the electricity generation sources
are fossil fuel based. For example, in Japan, the most share of the electricity is gener-
ated from gas (39.2%), coal (33.7%) and oil (8.2%). Other sources are hydro (8.2%),
solar PV (4.9%), waste (1.8%), nuclear (1.7%), biofuels (1.4%), wind (0.6%), and
geothermal (0.2%) (IEA 2018). Combined CO2 emission factor for per unit electric-
ity generation is 0.518 kg for Japan (Electricity Review Japan 2018). Developing
and underdeveloped countries have much higher GWP for per unit of electricity
generation.

Weight of the Cooling System

Choosing raw materials is crucial to building various parts of an air-conditioner. The

objective is to reduce the mass, increase the efficiency and decrease the cost. Differ-
ent parts of the air conditioners are made of different metals, plastic and steel (Rude
2019). Frames and base structures are usually made of corrosion resistant stainless
steel. Heat exchangers and fins are typically made of Aluminum. Additionally, cop-
per is chosen for tubing of condenser and evaporator. Copper has relatively higher
thermal conductivity, higher tensile strength, lower thermal expansion coefficient
than Aluminum. However, Aluminum is lighter and cheaper than Copper. In this
assessment, we have considered that 50% is stainless steel, 20% is copper, 20% is
Aluminum and 10% other materials of total weight.

GWP of Raw Materials Production

Building materials of a cooling system are mostly metals. These metals are usually
extracted from ores. The extraction process includes heating in high temperature
furnace, electrolysis of molten compounds, refining, shaping, cutting, and so forth.
A large amount of energy is required to obtain the metals in usable form to build a
cooling system. Hence, the raw materials that are used in the cooling system also
contribute to global warming.

System Lifespan

An average life expectancy of the cooling system is considered 15 years. Since TEWI
is usually calculated on per year basis, lifespan is necessary to calculate the annual
indirect warming impact to build the system.
160 Md. A. Islam and B. B. Saha

System Runtime

A cooling system is not always running. Either the system is turned off by the user or
the system turns itself off when it reaches the desired temperature. Hence, a certain
runtime is considered for this assessment.

9.3.1.2 Assessment Results

In order to determine the indirect emission, electricity consumption is required.

Electricity consumption depends on the COP of the system, which is ultimately
obtained from the thermodynamic properties of the refrigerant by using REFPROP
(Lemmon et al. 2018) and the equations from (9.2) to (9.10).

Pd  = Pd = Pa = Pa  (9.2)

Pb = Pb = Pb = Pc = Pc (9.3)

Ta = Teva (9.4)

Ta  = Teva + Tsup (9.5)

h c = h d  (9.6)

sa  = sb (9.7)

Tb = Tc = Tcon (9.8)

Tc = Tcon − Tsub (9.9)

h b − h a 
ηisen = (9.10)
h b − h a 

Here, T, P, s, h, and η represents temperature, pressure, entropy, enthalpy and

isentropic efficiency, respectively.
Table 9.4 shows thermodynamic quantities of room air-conditioning system. A
similar data set can be evaluated for the medium and low-temperature system. Refrig-
eration cycles for these three systems are drawn in Fig. 9.3 from those three sets of
thermodynamic data.
The cooling cycle consists of isobaric, isentropic, isenthalpic and isothermal pro-
cesses. State lines of the cooling cycle is explained in Table 9.5.
9 TEWI Assessment of Conventional and Solar Powered Cooling Systems 161

Table 9.4 Thermodynamic quantities at state points of room air-conditioning system

State point Explanation Temperature Pressure Enthalpy Entropy
T P h s
(°C) (kPa) (kJ kg–1 ) (kJ kg−1 K−1 )
a Saturated 12.00 1174.18 516.80 2.111
vapor
a Superheated 20.00 1174.18 527.36 2.148
vapor
b Hot vapor 78.97 2478.31 571.15 2.186
with non-
isentropic
compression
b Hot vapor 68.91 2478.31 558.02 2.148
with
isentropic
compression
b Saturated 40.00 2478.31 512.71 2.009
vapor
c Saturated 40.00 2478.31 275.61 1.252
liquid
c Subcooled 35.00 2478.31 265.08 1.218
liquid
d Expanded 12.00 1174.18 265.08 1.229
two-phase
refrigerant
when
subcooled
d Expanded 12.00 1174.18 275.61 1.266
two-phase
refrigerant
without
subcooling

Theoretical maximum COP of the system is denoted as Carnot COP and can be
expressed by the following equation,

273.15 + Teva
C O Pcar not = (9.11)
Tcon − Teva

A practical system has frictional and other losses in various sections. Hence, the
actual COP is lower than COPcarnot . The ratio of cooling load and work of compression
is the practical COP of the system, which can be calculated by the following equation.

h a − h d 
COP = (9.12)
h b − h a 
162 Md. A. Islam and B. B. Saha

100
7000 Critical Point,
Critical Point, b" 414.15, 5782.64
80 1.649, 78.105
b'
Temperature (°C)

60

Pressure (kPa)
c b c' c b b' b"
40 c'
20 a'
d' d a
0 d' d a a'

-20 700
0.50 0.80 1.10 1.40 1.70 2.00 2.30 2.60 150 250 350 450 550 650
Entropy (kJ kg-1 K-1) Enthalpy (kJ kg-1)

(a) Room air-conditioning cycle with R32.

10000
120 Critical Point,
Critical Point, 390.94, 4059.02
90 1.559, 101.059
Temperature (°C)

b"
60 b' Pressure (kPa) c' c b b' b"
c b 1000
c'
30

0 a'
d' d a d' d a a'
-30 100
0.60 0.90 1.20 1.50 1.80 2.10 100 200 300 400 500
Entropy (kJ kg-1 K-1) Enthalpy (kJ kg-1)

(b) Medium temperature (–7 °C) refrigeration cycle with R134a.

100 10000
Critical Point, b" Critical Point,
1.445, 72.117 345.21, 3734.70
70
b'
Temperature (°C)

c b c' c b b' b"

Pressure (kPa)

40 c'
1000
10

-20 a'
d' d a a'
d' d a
-50 100
0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00 100 150 200 250 300 350 400 450
Entropy (kJ kg-1 K-1) Enthalpy (kJ kg-1)

(c) Low temperature (–25 °C) refrigeration cycle with R404A.

Fig. 9.3 T-s and P-h diagram for various cooling applications

Electric power consumption is dependent on the COP of and can be determined

by the following equation,

Qc
Ep = (9.13)
COP
The mass flow rate of the refrigerant can be calculated by the following equation,

h a − h d 
ṁ = (9.14)
Qc
9 TEWI Assessment of Conventional and Solar Powered Cooling Systems 163

Table 9.5 Illustration of the cooling cycle

State line Description
a–a This is the superheating line, which is an isobaric process. 8 °C temperature
increases for the current application
a –b Non-isentropic compression happens along this line. Discharged vapour refrigerant
has a very high temperature and pressure at b (if we consider an ideal case, the
compression process is isentropic, and the state line will be a –b )
b –b Isobaric cooling of compressed hot vapour refrigerant until it reaches condensation
temperature in the outdoor unit (for ideal case, the line will be b –b)
b–c The phase change of saturated vapour to saturated liquid through the two-phase
region. The process is isothermal and isobaric
c–c Isobaric liquid subcooling process. In this case, 5 °C temperature decrement from
condensation temperature
c –d Isenthalpic expansion of liquid refrigerant
d –a Isobaric evaporation of liquid refrigerant. The process requires latent heat for phase
change, and hence, there is no temperature change along this line

The performance of a system can be estimated by the equations from (9.11) to

(9.14). All these parameters can be obtained from assumptions and state diagrams.
The results clearly indicate that the performance of the system deteriorates when
the temperature requirement is lower. It happens due to the higher compression
ratio requirement to reach a lower evaporation temperature. Hence, the electricity
requirement and indirect warming impact both increases. Indirect warming impact
is lower than direct warming for room air-conditioning system and medium tem-
perature system. However, the value exceeds the indirect warming impact on the
low-temperature system (Table 9.6). Higher GWP value of the refrigerant R404A
is responsible for that. Direct warming impact will substantially increase if R404A
is used in the medium temperature system or room air-conditioning system. How-
ever, the application of R404A is limited in the commercial and low-temperature
refrigeration system due to their thermodynamic properties.
A small 10 kW conventional cooling system can produce 4.816–31.346 tonne
equivalent CO2 emission every year depending on the application. The emission
amount could be higher if the system is situated in an underdeveloped or developing
country where electricity is primarily generated from coal, gas and other fossil fuel
based sources. Additionally, TEWI would increase for tropical countries where the
exterior temperature is higher than the considered temperature. Environmental impact
would further increase, (i) for lower temperature applications because COP and
isentropic efficiency is lower, (ii) with aging of the system because the system would
be more prone leakage and will consume more electricity, (iii) if selected refrigerant
have higher GWP or poor thermodynamic property in the operation condition.
164 Md. A. Islam and B. B. Saha

Table 9.6 Results of assessment: conventional cooling system

Parameter System type
Room Medium temperature Low-temperature
air-conditioning refrigeration (R134a) refrigeration
(R32) (R404A)
COPcarnot 10.184 5.663 3.818
COP 5.989 3.029 1.660
Refrigerant flow rate, 137.257 236.251 143.344
ṁ (kg h–1 )
Discharge 78.971 69.950 160.416
temperature of
refrigerant (°C)
Discharge pressure of 2478.310 1016.592 2478.31
refrigerant (kPa)
Compression ratio 2.111 4.509 7.408
Annual electricity 7312.923 14,462.196 26,392.087
consumption (kWh)
Annual indirect 3.788 7.491 13.671
warming impact on
electricity
consumption (t-CO2
eq.)
Total indirect 0.234 0.311 0.389
warming impact for
building the cooling
system (t-CO2 eq.)
Annual indirect 0.016 0.021 0.026
warming impact for
building the cooling
system (t-CO2 eq.)
Annual direct 1.013 3.900 17.649
warming impact
(t-CO2 eq.)
Annual TEWI 4.816 11.412 31.346
(t-CO2 eq.)

9.3.2 TEWI of Solar Cooling System

Operating conditions and assumptions for the solar cooling system is summarized in
Table 9.7. Same operating temperatures (Teva and Tcon ), operation hours and cool-
ing capacity have been set for the adsorption cooling system to compare the results
with the conventional cooling system. Natural refrigerants (GWP = 0) have been
chosen to decrease environmental impact. Three different functional pairs have been
selected for three different applications. Silica gel/water pair is suitable for the tem-
perature range of room air-conditioning systems. Moreover, medium temperature
9 TEWI Assessment of Conventional and Solar Powered Cooling Systems 165

Table 9.7 Operating conditions and assumption for the solar cooling system
Parameter Quantity
Evaporation temperature for specific 12 °C (room air-conditioning)
applications, T eva −7 °C (medium temperature applications)
−25 °C (low temperature applications)
Condensation temperature, T con 40 °C
Selected refrigerants R717 (Ammonia: NH3 ), R718 (Water: H2 O),
MetOH (Methanol: CH3 OH)
Functional adsorbent/adsorbate pairs Silica gel/water (room air-conditioning)
(Tokyo Boeki Machineries 2019; Bry-Air
2019)
Activated carbon/methanol (medium
temperature applications) (Attalla et al. 2018)
Activated carbon/ammonia (low temperature
applications) (Wang et al. 2009; Askalany
et al. 2013)
GWP of the considered refrigerants, GWPr GWPwater : 0 kg-CO2 eq. (Bolaji and Huan
2013), GWPmethanol : 0 kg-CO2 eq. (Rosset
et al. 2018), GWPammonia : 0 kg-CO2 eq. (Abas
et al. 2018)
Cooling load, Qc 10 kW
Thermal co-efficient of performance, 0.6 (room air-conditioning) (Bry-Air 2019;
COPthermal Stryi-Hipp 2016)
0.4 (medium temperature applications)
0.2 (low temperature applications)
Electricity consumption (for pumps and 0.8 kW (room air-conditioning) (Bry-Air
valves) 2019)
1.4 kW (medium temperature applications)
2.0 kW (low temperature applications)
Required adsorbent amount (two beds) 50 kg (room air-conditioning) (Bry-Air 2019)
70 kg (medium temperature applications)
90 kg (low-temperature applications)
Initial refrigerant charging amount 30 kg (room air-conditioning) (Bry-Air 2019)
50 kg (medium temperature applications)
70 kg (low-temperature applications)
The annual leakage rate of refrigerant, L a 15%
GWP for electricity generation, ε 0.518 (kg-CO2 /kWh) (Electricity Review
Japan 2018)
Product weight, m 900 kg (10 kW room air-conditioning)
(Bry-Air 2019)
1200 kg (10 kW medium temperature system)
1500 kg (10 kW low-temperature system)
(continued)
166 Md. A. Islam and B. B. Saha

Table 9.7 (continued)

Parameter Quantity
The weight percentage of raw materials to Stainless steel: 60%
build a cooling system Copper: 10%
Aluminum: 10%
Others (refrigerant, adsorbent, cooling/chilled
water etc.): 20%
GWP of per kg material production, GWPm Stainless steel: 2.13 kg-CO2 eq. (Chisalita
et al. 2019)
Copper: 4.97 kg-CO2 eq. (Kosai and Yamasue
2019)
Aluminum: 9.17 kg-CO2 eq. (Kosai and
Yamasue 2019)
System lifespan 25 years (Bry-Air 2019)
System runtime 12 h/day

and low-temperature applications require different working fluids, such as methanol

and ammonia (Ebrahimi and Keshavarz 2014). Activated carbon is suitable for the
adsorption of these refrigerants.
COP of the adsorption cooling system can be expressed in two ways which are
shown in Eqs. (9.15) and (9.16). Thermal input is very high compared to cool-
ing output for adsorption cooling system. Hence, thermal COP is usually very low.
Moreover, electricity input is much lower for a solar cooling system, and therefore
electrical COP is very high. Thermal COP of the chiller for room air-conditioning
system is the range of 0.5–0.6 (Bry-Air 2019; Stryi-Hipp 2016). COP decreases for
lower evaporation temperature. In this assessment, we have considered COP values
0.6, 0.4 and 0.2 for room air-conditioning, medium temperature and low-temperature
cooling systems, respectively.

Q eva
C O Pther mal = (9.15)
Q des
Q eva
C O Pelctrical = (9.16)
E in

Here,
Qeva Evaporator cooling capacity (kJ)
Qdes Required thermal input for desorption (kJ)
E in Required electrical power input (kWh) or (kJ).
The adsorption chillers require a small amount of electricity to run the pumps
(chilled water, cooling water) and control the valves. A 10 kW chiller requires about
0.8 kW electricity according to the manufacturer’s specification (Bry-Air 2019).
Medium and low-temperature applications require a considerably higher amount
9 TEWI Assessment of Conventional and Solar Powered Cooling Systems 167

of electricity, which is shown in Table 9.7. The required amount of adsorbent and
refrigerant also increases for lower temperature applications.
An adsorption system has two or more beds. The frames of the beds are usually
made of stainless steel. The beds contain adsorbent and heat exchangers. Hence, the
weight of an adsorption system is ten times or higher than the conventional system
of the same cooling capacity. The weight of a 10 kW solar room air-conditioning
cooling system is about 900 kg according to manufacturer’s specification. Weight
of medium and low-temperature systems have been considered higher due to the
higher adsorbent and heat exchanger requirement. The weight percentage of steel,
aluminum, copper and other materials have been modified due to the constructional
difference with the conventional cooling system. The system has a higher lifetime
and requires lower maintenance than the conventional systems because there are very
few moving parts in this system.
The results of the assessment for the solar cooling system is summarized in
Table 9.8. Thermal COP, electricity consumption, indirect emission due to electric-
ity consumption, direct emission and TEWI of the considered solar cooling systems

Table 9.8 Results of assessment: solar cooling system

Parameter System type
Room Medium temperature Low temperature
air-conditioning refrigeration refrigeration
(Silica gel/water) (Activated (activated
carbon/methanol) carbon/ammonia)
COPthermal 0.6 0.4 0.2
COPelectrical 12.5 7.143 5
Annual electricity 3504 6132 8760
consumption (kWh)
Annual indirect 1.815 3.176 4.538
warming impact on
electricity
consumption (t-CO2
eq.)
Total indirect 2.423 3.230 4.038
warming impact for
building the cooling
system (t-CO2 eq.)
Annual indirect 0.097 0.129 0.162
warming impact for
building the cooling
system (t-CO2 eq.)
Annual direct 0 0 0
warming impact
(t-CO2 eq.)
Annual TEWI 1.912 3.306 4.699
(t-CO2 eq.)
168 Md. A. Islam and B. B. Saha

are lower than the conventional system. Since the weight of the adsorption cooling
system is much higher, the indirect warming impact for building the cooling sys-
tem is higher. However, TEWI of the solar cooling systems is much lower than the
conventional systems.

9.4 Results Comparison and Discussions

The conventional vapour compression system is very compact in size and COP is
also high. COP of adsorption cooling system is very low because of the huge loss of
thermal energy conversion during operation. Hence, the COP of adsorption cooling
system is often referred to as thermal COP. Electrical COP (ratio of cooling capacity
to electricity input) is much higher for the solar cooling system, which is shown in
Fig. 9.4. COP decreases for both conventional and solar cooling system when the
evaporation temperature requirement is lower.
In Fig. 9.5, electricity consumption is compared between the conventional and
solar cooling system. Inevitably the consumption is lower for the solar cooling system
since the desorption (often referred to as thermal compression) is carried out by solar
thermal input.
Indirect and direct emissions are individually drawn in Fig. 9.6 for conventional
and solar cooling system for three different applications. Last two bars of each appli-
cation are the TEWI for that particular application which is obtained by aggregating
the direct and indirect emissions.

14.0
COP of Conventional cooling system
12.5
12.0 Thermal COP of Solar cooling system

Electrical COP of Solar cooling system

10.0
COP [–]

8.0
7.143
5.989
6.0
5

4.0
3.029

2.0 1.54
0.6 0.4 0.2
0.0
Room air-conditioning Medium temperature Low temperature
system (at 12 °C) application (at –7 °C) application (at –25 °C)

Fig. 9.4 COP variation for different applications: conventional vs solar cooling system
9 TEWI Assessment of Conventional and Solar Powered Cooling Systems 169

30000
Conventional cooling system 26392
25000 Solar cooling system
Annual Electricity Consumption (kWh)

20000

15000 14462

10000 8760
7313
6132
5000 3504

0
RoomSolar
Conventional air-conditioning
cooling
cooling
systemsystem Medium temperature Low temperature
system (at 12 °C) application (at –7 °C) application (at –25 °C)

Fig. 9.5 Electricity consumption for various applications: conventional vs solar cooling

35 ④
Conventional Cooling System
31.346
30 Solar Cooling System
Global Warming Impact (t-CO2 eq.)

① electricity consumption
Indirect emission for

25
Indirect emission to

③
③ refrigerant leakage

① ②
Direct emission for

11.412 ④ TEWI
② build the system

17.649

20
13.671

15
① ② ③ ④
7.491

10
4.816

4.699
3.788

4.538
3.306
3.900
3.176
1.912

5
1.815

1.013

0.162
0.129
0.097

0.026
0.021
0.016

0 Conv ention al Cooling

Sy stem
Sol a rCo ol n
i g Sy stem

Room air-conditioning Medium temperature Low temperature

system (at 12 °C) application (at –7 °C) application (at –25 °C)

Fig. 9.6 Indirect, direct and total equivalent warming impact comparison
170 Md. A. Islam and B. B. Saha

Since the solar cooling system has low electricity consumption, indirect emission
is also lower for all three applications. However, the dimension and weight of the
solar cooling system are much higher than the conventional system. Hence, indirect
emission for raw materials is higher for the adsorption cooling system. Additionally,
direct emission for the solar cooling system is zero because of deploying natural
refrigerants as working fluid.
Total equivalent warming impact is much higher for conventional cooling systems,
as shown in Fig. 9.6. Lower evaporation temperature causes higher TEWI.

9.5 Conclusions

In this assessment, performance and environmental impact of conventional and solar

cooling system have been evaluated and compared. Three different applications:
room air-conditioning (T eva at 12 °C), medium temperature application (T eva at −
7 °C) and low-temperature applications (T eva at −25 °C) have been considered for
both the systems. R32, R134a and R404A have been selected as working fluid for
the conventional system. Whereas, silica gel/water, activated carbon/methanol and
activated carbon/ammonia pairs have been considered for the solar cooling system.
A constant cooling load of 10 kW is considered for all the applications. The results
indicate that COP of the conventional cooling system is higher than thermal COP
of the solar cooling system. However, electrical COP of the solar cooling system is
considerably high due to the thermal input for desorption (thermal compression).
Indirect emission for electricity consumption is definitely higher for the con-
ventional cooling system. It happens due to the huge electricity consumption by the
mechanical vapour compressor. However, the weight of the used raw materials is very
high that build the solar cooling system. Hence, annual indirect warming impact to
build the system is relatively higher for the solar cooling system. In the case of direct
emission, the solar cooling system is better because it uses natural refrigerants, and
the GWP of those refrigerants are zero. Currently deployed refrigerants (R32, R134a,
R404A, etc.) in conventional systems have high GWP, and hence, the direct warming
impact is higher. However, the situation might change if low GWP refrigerants are
introduced in conventional systems.
Currently, TEWI (summation of indirect and direct emissions) is very high for
conventional cooling systems. Therefore, now is the appropriate time to switch from
conventional systems to solar cooling technology.

Appendix 1

The Sensitivity of TEWI with a Refrigerant Leakage Rate

The refrigerant leakage rate of a conventional cooling system could vary for several
reasons such as evaporation and condensation temperature, selected refrigerant and
9 TEWI Assessment of Conventional and Solar Powered Cooling Systems 171

Table 9.9 Emission sensitivity with leakage rate

System type Emission Annual leakage rate from the initial amount
L a = 5% L a = 10% L a = 15% L a = 20%
Room Direct emission 0.338 0.675 1.013 1.350
air-conditioning (t-CO2 eq.)
system (R32) TEWI (t-CO2 eq.) 4.141 4.479 4.816 5.154
Change of TEWI −14.02% −7.00% 0 +7.02%
Medium Direct emission 1.3 2.6 3.9 5.2
temperature (t-CO2 eq.)
refrigeration TEWI (t-CO2 eq.) 8.812 10.112 11.412 12.712
(R134a)
Change of TEWI −22.78% −11.39% 0 +11.39%
Low temperature Direct emission 5.883 11.766 17.649 23.532
refrigeration (t-CO2 eq.)
(R404A) TEWI (t-CO2 eq.) 20.646 26.529 31.346 38.295
Change of TEWI −36.30% −18.15% 0 +18.15%

system-manufacturing process. When the difference between evaporation and con-

densation temperature is large, the compression ratio is high, which leads to a higher
leakage rate. Besides, each refrigerant possesses different pressure at the operating
conditions. Hence, the leakage rate also depends on the selected refrigerant type.
Moreover, the leakage rate depends on the sealing method of refrigerant compres-
sion chamber and airtightness of the refrigerant flowing channels. This part is solely
dependent on the design of the system manufacturer. The annual leakage rate is var-
ied from 5 to 20%, and the assessment result is compared in Table 9.9. One percent
leakage rate reduction can decrease the total equivalent warming impact 1.4% for
room air-conditioning system with R32 working fluid. The reduction is about 2.28%
and 3.63% for medium and low temperature applications with R134a and R404A,
respectively.
The solar cooling system would not be affected by the leakage amount since all
the selected refrigerants have zero GWP.

Appendix 2

Effect of Refrigerant Type on TEWI

The current assessment is carried out considering R32, R134a and R404A as a refrig-
erant for room air-conditioning, medium temperature and low temperature systems,
respectively. Once the refrigerant is changed, the refrigeration cycle will be affected,
which would ultimately have an impact on the indirect emission. Moreover, GWP of
the refrigerants are different, and hence, the direct emission will be changed. In this
section, the assessment is performed with R22 for room air-conditioning system, and
172 Md. A. Islam and B. B. Saha

Table 9.10 Results of assessment with different refrigerant: conventional cooling system
Parameter System type
Room Medium temperature Low-temperature
air-conditioning refrigeration refrigeration
(R22) (R410A) (R410A)
COPcarnot 10.184 5.663 3.818
COP 6.261 2.828 1.621
Refrigerant flow rate, 208.749 211.302 221.665
ṁ (kg h–1 )
Discharge 69.396 86.749 115.333
temperature of
refrigerant (°C)
Discharge pressure of 1533.570 2418.695 2418.695
refrigerant (kPa)
Compression ratio 2.122 3.813 7.343
Annual electricity 6995.385 15,488.595 27,018.872
consumption (kWh)
Annual indirect 3.624 8.023 13.996
warming impact on
electricity
consumption (t-CO2
eq.)
Total indirect 0.234 0.311 0.389
warming impact for
building the cooling
system (t-CO2 eq.)
Annual indirect 0.016 0.021 0.026
warming impact for
building the cooling
system (t-CO2 eq.)
Annual direct 2.64 8.352 9.396
warming impact
(t-CO2 eq.)
Annual TEWI 6.279 16.396 23.418
(t-CO2 eq.)

R410A is considered for both medium and low temperature system. All the other
parameters are the same, as shown in Table 9.3. Result of the assessment is shown
in Table 9.10.
COP of the room air-conditioning system is higher when R22 is deployed instead
of R32. Hence, electricity consumption and direct emission due to electricity con-
sumption are lower. However, GWP of R22 is much higher than R32, which results
in about 23% higher TEWI for room air-conditioning system.
9 TEWI Assessment of Conventional and Solar Powered Cooling Systems 173

Contrarily, replacing R134a with R410A for medium temperature applications

gives a huge hike in TEWI, which is about 30%. However, TEWI could be decreased
by about 25% by deploying R410A in exchange for R404A for low temperature
applications.
Most of the adsorption cooling systems are using natural refrigerants. However,
conventional refrigerants such as R32 and R134a have higher adsorption capacity,
which could lead to develop an efficient adsorption cooling system (Saha et al. 2012;
Askalany and Saha 2015). In that case, there would be direct warming impact from
the adsorption cooling system. Since currently available systems only use natural
refrigerants, the direct emission is considered to be zero.

References

Abas N, Kalair AR, Khan N, Haider A, Saleem Z, Saleem MS (2018) Natural and synthetic refriger-
ants, global warming: a review. Renew Sustain Energy Rev 90:557–569. https://doi.org/10.1016/
j.rser.2018.03.099
Adsorption Chiller: Adref-Noa; Friendly to mankind and to the earth (2019). Mayewaka-Mycom;
Mayekawa Mfg. Ltd. http://www.mayekawa.co.id/pic/AdRef-Noa—English-828.pdf. Accessed
24 Apr 2019
Ahamed JU, Saidur R, Masjuki HH (2010) A review on exergy analysis of vapor compression
refrigeration system. Renew Sustain Energy Rev 15:1593–1600. https://doi.org/10.1016/j.rser.
2010.11.039
Al Mers A, Azzabakh A, Mimet A, El Kalkha H (2006) Optimal design study of cylindrical finned
reactor for solar adsorption cooling machine working with activated carbon–ammonia pair. Appl
Therm Eng 26:1866–1875. https://doi.org/10.1016/j.applthermaleng.2006.01.021
Alahmer A, Wang X, Al-Rbaihat R, Amanul Alam KC, Saha BB (2016) Performance evaluation of
a solar adsorption chiller under different climatic conditions. Appl Energy 175:293–304. https://
doi.org/10.1016/j.apenergy.2016.05.041
Arora RC (2010) Refrigeration and air conditioning, 1st edn. PHI Learning Pvt., Ltd., New Delhi
Askalany AA, Saha BB (2015) Experimental and theoretical study of adsorption kinetics of Diflu-
oromethane onto activated carbons. Int J Refrig 49:160–168. https://doi.org/10.1016/j.ijrefrig.
2014.10.009
Askalany AA, Salem M, Ismael IM, Ali AHH, Morsy MG, Saha BB (2013) An overview on
adsorption pairs for cooling. Renew Sustain Energy Rev 19:565–572. https://doi.org/10.1016/j.
rser.2012.11.037
Attalla M, Sadek S, Salem Ahmed M, Shafie IM, Hassan M (2018) Experimental study of solar
powered ice maker using adsorption pair of activated carbon and methanol. Appl Therm Eng
141:877–886. https://doi.org/10.1016/j.applthermaleng.2018.06.038
Bassols-Rheinfelder J (1985) Solar cooling using an open-cycle adsorption system with adiabatical
mass transfer and external heat exchange. Sol Energy 35:93–104. https://doi.org/10.1016/0038-
092X(85)90040-4
Bolaji BO, Huan Z (2013) Ozone depletion and global warming: case for the use of natural refriger-
ant—a review. Renew Sustain Energy Rev 18:49–54. https://doi.org/10.1016/j.rser.2012.10.008
Bose B (2010) Global warming: energy, environmental pollution, and the impact of power elec-
tronics. IEEE Ind Electron Mag 4:6–17. https://doi.org/10.1109/MIE.2010.935860
Buonomano A, Calise F, Palombo A (2018) Solar heating and cooling systems by absorption and
adsorption chillers driven by stationary and concentrating photovoltaic/thermal solar collectors:
Modelling and simulation. Renew Sustain Energy Rev 81:1112–1146. https://doi.org/10.1016/j.
rser.2017.07.056
174 Md. A. Islam and B. B. Saha

Calm JM (2002) Emissions and environmental impacts from air-conditioning and refrigeration
systems. Int J Refrig 25:293–305. https://doi.org/10.1016/S0140-7007(01)00067-6
Calm JM (2008) The next generation of refrigerants—historical review, considerations, and outlook.
Int J Refrig 31:1123–1133. https://doi.org/10.1016/j.ijrefrig.2008.01.013
Carotenuto A, Figaj RD, Vanoli L (2017) A novel solar-geothermal district heating, cooling
and domestic hot water system: dynamic simulation and energy-economic analysis. Energy
141:2652–2669. https://doi.org/10.1016/j.energy.2017.08.084
Cengel YA (2004) Thermodynamics, an engineering approach, 5th edn. McGraw-Hill, New York
Chang W-S, Wang C-C, Shieh C-C (2007) Experimental study of a solid adsorption cooling system
using flat-tube heat exchangers as adsorption bed. Appl Therm Eng 27:2195–2199. https://doi.
org/10.1016/j.applthermaleng.2005.07.022
Chang W-S, Wang C-C, Shieh C-C (2009) Design and performance of a solar-powered heating and
cooling system using silica gel/water adsorption chiller. Appl Therm Eng 29:2100–2105. https://
doi.org/10.1016/j.applthermaleng.2008.10.021
Cherp A, Vinichenko V, Jewell J, Suzuki M, Antal M (2017) Comparing electricity transitions:
a historical analysis of nuclear, wind and solar power in Germany and Japan. Energy Policy
101:612–628. https://doi.org/10.1016/j.enpol.2016.10.044
Chisalita D-A, Petrescu L, Cobden P, van Dijk HA, Cormos A-M, Cormos C-C (2019) Assessing
the environmental impact of an integrated steel mill with post-combustion CO2 capture and
storage using the LCA methodology. J Clean Prod 211:1015–1025. https://doi.org/10.1016/j.
jclepro.2018.11.256
Cho S-H, Kim J-N (1992) Modeling of a silica gel/water adsorption-cooling system. Energy
17:829–839. https://doi.org/10.1016/0360-5442(92)90101-5
Choudhury B, Saha BB, Chatterjee PK, Sarkar JP (2013) An overview of developments in adsorption
refrigeration systems towards a sustainable way of cooling. Appl Energy 104:554–567. https://
doi.org/10.1016/j.apenergy.2012.11.042
Davies TW, Caretta O (2004) A low carbon, low TEWI refrigeration system design. Appl Therm
Eng 24:1119–1128. https://doi.org/10.1016/j.applthermaleng.2003.12.026
Deveciog AG, Oruç V (2017) The influence of plate-type heat exchanger on energy efficiency and
environmental effects of the air-conditioners using R453A as a substitute for R22 2017. https://
doi.org/10.1016/j.applthermaleng.2016.10.180
Du Z, Domanski PA, Payne WV (2016) Effect of common faults on the performance of differ-
ent types of vapor compression systems. Appl Therm Eng 98:61–72. https://doi.org/10.1016/j.
applthermaleng.2015.11.108
Ebrahimi M, Keshavarz A (2014) Combined cooling, heating and power. Elsevier. https://doi.org/
10.1016/C2013-0-18763-6
Electricity Review Japan (2018) The federation of electric power companies 2018. http://www.fepc.
or.jp/english/library/electricity_eview_japan/index.html. Accessed 14 Feb 2019
El-Sharkawy II, Kuwahara K, Saha BB, Koyama S, Ng KC (2006) Experimental investigation of
activated carbon fibers/ethanol pairs for adsorption cooling system application. Appl Therm Eng
26:859–865. https://doi.org/10.1016/j.applthermaleng.2005.10.010
El-Sharkawy II, Saha BB, Koyama S, He J, Ng KC, Yap C (2008) Experimental investigation on
activated carbon–ethanol pair for solar powered adsorption cooling applications. Int J Refrig
31:1407–1413. https://doi.org/10.1016/j.ijrefrig.2008.03.012
El-Sharkawy II, Uddin K, Miyazaki T, Saha BB, Koyama S, Miyawaki J et al (2014) Adsorption
of ethanol onto parent and surface treated activated carbon powders. Int J Heat Mass Transf
73:445–455. https://doi.org/10.1016/j.ijheatmasstransfer.2014.02.046
Francis C, Maidment G, Davies G (2016) An investigation of refrigerant leakage in commercial
refrigeration. Int J Refrig 74:12–21. https://doi.org/10.1016/j.ijrefrig.2016.10.009
Fudholi A, Sopian K, Yazdi MH, Ruslan MH, Ibrahim A, Kazem HA (2014) Performance analysis
of photovoltaic thermal (PVT) water collectors. Energy Convers Manag 78:641–651. https://doi.
org/10.1016/j.enconman.2013.11.017
9 TEWI Assessment of Conventional and Solar Powered Cooling Systems 175

General (2018) Product catalogue 2018—air conditioners lineup. https://www.fujitsu-general.com/

shared/g-eu/pdf-geur-support-ctlg-3eg019-1801e-01.pdf. Accessed 8 Apr 2019
Ghaebi H, Shekari Namin A, Rostamzadeh H (2018) Performance assessment and optimization of
a novel multi-generation system from thermodynamic and thermoeconomic viewpoints. Energy
Convers Manag 165:419–439. https://doi.org/10.1016/j.enconman.2018.03.055
Gill J, Singh J (2018) Component-wise exergy and energy analysis of vapor compression refriger-
ation system using mixture of R134a and LPG as refrigerant. Heat Mass Transf Und Stoffueber-
tragung 54:1367–1380. https://doi.org/10.1007/s00231-017-2242-x
Habib K, Choudhury B, Chatterjee PK, Saha BB (2013) Study on a solar heat driven dual-mode
adsorption chiller. Energy 63:133–141. https://doi.org/10.1016/j.energy.2013.10.001
Habib K, Saha BB, Koyama S (2014) Study of various adsorbent-refrigerant pairs for the application
of solar driven adsorption cooling in tropical climates. Appl Therm Eng 72:266–274. https://doi.
org/10.1016/j.applthermaleng.2014.05.102
He W, Chow TT, Ji J, Lu J, Pei G, Chan LS (2006) Hybrid photovoltaic and thermal solar-collector
designed for natural circulation of water. Appl Energy 83:199–210. https://doi.org/10.1016/j.
apenergy.2005.02.007
Hitachi Room Aircon Product Brochure (2019) http://www2.tochi.hitachi.co.jp/ra.drowing/zumen/
JT90J2E7.PDF. Accessed Apr 8 2019
IEA (2018) International Energy Agency. Key world energy statistics. https://www.iea.org/
countries/Japan/. Accessed 13 Feb 2019
Islam MA, Srinivasan K, Thu K, Saha BB (2017) Assessment of total equivalent warming impact
(TEWI) of supermarket refrigeration systems. Int J Hydrogen Energy 42:26973–26983. https://
doi.org/10.1016/j.ijhydene.2017.07.035
Kasaeian A, Hosseini SM, Sheikhpour M, Mahian O, Yan WM, Wongwises S (2018) Applications
of eco-friendly refrigerants and nanorefrigerants: a review. Renew Sustain Energy Rev 96:91–99.
https://doi.org/10.1016/j.rser.2018.07.033
Kayal S, Baichuan S, Saha BB (2016) Adsorption characteristics of AQSOA zeolites and
water for adsorption chillers. Int J Heat Mass Transf 92:1120–1127. https://doi.org/10.1016/j.
ijheatmasstransfer.2015.09.060
Khan MZI, Saha BB, Alam KCA, Akisawa A, Kashiwagi T (2007) Study on solar/waste heat driven
multi-bed adsorption chiller with mass recovery. Renew Energy 32:365–381. https://doi.org/10.
1016/j.renene.2006.02.003
Khelifa A, Touafek K, Ben Moussa H, Tabet I, Hocine HB, Cheikh El, Haloui H (2015) Analysis
of a hybrid solar collector photovoltaic thermal (PVT). Energy Proc 74:835–843. https://doi.org/
10.1016/j.egypro.2015.07.819 (Elsevier)
Khullar V, Tyagi H, Phelan PE, Otanicar TP, Singh H, Taylor RA (2012) Solar energy harvesting
using nanofluids-based concentrating solar collector. In: ASME 2012 third international con-
ference on micro/nanoscale heat mass transfer, vol 3. ASME, p 259. https://doi.org/10.1115/
mnhmt2012-75329
Kohler JA, Briley GC, Clark EM, Dorman DR, Duda SW, Halel DM, et al (2016) ANSI/ASHRAE
standard 15-2016. Safety standard for refrigeration systems
Kosai S, Yamasue E (2019) Global warming potential and total material requirement in metal
production: identification of changes in environmental impact through metal substitution. Sci
Total Environ 651:1764–1775. https://doi.org/10.1016/j.scitotenv.2018.10.085
Kruse H (2000) Refrigerant use in Europe. ASHRAE J 42:16–24. https://www.usgbc.org/drupal/
legacy/usgbc/docs/LEED_tsac/Energy/ASHRAEJournal09-00RefrigerantUseinEurope.pdf
Lemmon EW, Bell IH, Huber ML, McLinden MO (2018) NIST standard reference database 23: ref-
erence fluid thermodynamic and transport properties-REFPROP, version 10.0. National Institute
of Standards and Technology. https://dx.doi.org/10.18434/T4JS3C
Machiele PA (1987) Flammability and toxicity tradeoffs with methanol fuels, vol 2018. https://doi.
org/10.4271/872064
176 Md. A. Islam and B. B. Saha

Makhnatch P, Khodabandeh R (2014) The role of environmental metrics (GWP, TEWI, LCCP)
in the selection of low GWP refrigerant. Energy Proc 61:2460–2463. https://doi.org/10.1016/j.
egypro.2014.12.023
McIntosh IBD, Mitchell JW, Beckman WA (2000) Fault detection and diagnosis in chillers—part
I: model development and application. ASHRAE Trans 106:268
Mitsubishi Plastics launches a compact adsorption chiller with the highest cooling capacity per
volume in the world (2012) https://www.m-chemical.co.jp/en/news/mpi/201203140474.html.
Accessed 24 Mar 2019
Mota-Babiloni A, Navarro-Esbrí J, Makhnatch P, Molés F (2017) Refrigerant R32 as lower GWP
working fluid in residential air conditioning systems in Europe and the USA. Renew Sustain
Energy Rev 80:1031–1042. https://doi.org/10.1016/j.rser.2017.05.216
Ng KC, Chua HT, Chung CY, Loke CH, Kashiwagi T, Akisawa A et al (2001) Experimental investi-
gation of the silica gel-water adsorption isotherm characteristics. Appl Therm Eng 21:1631–1642.
https://doi.org/10.1016/S1359-4311(01)00039-4
Ng KC, Wang X, Lim YS, Saha BB, Chakarborty A, Koyama S et al (2006) Experimental study on
performance improvement of a four-bed adsorption chiller by using heat and mass recovery. Int
J Heat Mass Transf 49:3343–3348. https://doi.org/10.1016/j.ijheatmasstransfer.2006.01.053
Pal A, Thu K, Mitra S, El-Sharkawy II, Saha BB, Kil HS et al (2017) Study on biomass derived
activated carbons for adsorptive heat pump application. Int J Heat Mass Transf 110:7–19. https://
doi.org/10.1016/j.ijheatmasstransfer.2017.02.081
Phelan P, Otanicar T, Taylor R, Tyagi H (2013) Trends and opportunities in direct-absorption solar
thermal collectors. J Therm Sci Eng Appl 5:021003. https://doi.org/10.1115/1.4023930
Poggi F, Macchi-Tejeda H, Leducq D, Bontemps A (2008) Refrigerant charge in refrigerating
systems and strategies of charge reduction. Int J Refrig 31:353–370. https://doi.org/10.1016/j.
ijrefrig.2007.05.014
Quick calculations for walk-in coolers and freezers (2019). Heatcraft Refrigeration Products LLC.
http://www.uscooler.com/blog/load-calc.pdf. Accessed 9 Apr 2019
Rahman S, de Castro A (1995) Environmental impacts of electricity generation: a global perspective.
IEEE Trans Energy Convers 10:307–314. https://doi.org/10.1109/60.391897
Reddy VS, Panwar NL, Kaushik SC (2012) Exergetic analysis of a vapour compression refrigeration
system with R134a, R143a, R152a, R404A, R407C, R410A, R502 and R507A. Clean Technol
Environ Policy 14:47–53. https://doi.org/10.1007/s10098-011-0374-0
Rosset K, Mounier V, Guenat E, Schiffmann J (2018) Multi-objective optimization of turbo-ORC
systems for waste heat recovery on passenger car engines. Energy 159:751–765. https://doi.org/
10.1016/j.energy.2018.06.193
Rude J (2019) How products are made. http://www.madehow.com/Volume-3/Air-Conditioner.html.
Accessed 10 Apr 2019
Rupp J (2009) SolarCombi+, SorTech package solution description. http://www.solarcombiplus.
eu/docs/d44_sortech_v02_english.pdf. Accessed 23 Apr 2019
Saha BB, Akisawa A, Kashiwagi T (2001) Solar/waste heat driven two-stage adsorption chiller: the
prototype. Renew Energy 23:93–101. https://doi.org/10.1016/S0960-1481(00)00107-5
Saha BB, El-Sharkawy II, Chakraborty A, Koyama S, Banker ND, Dutta P et al (2006) Evaluation
of minimum desorption temperatures of thermal compressors in adsorption refrigeration cycles.
Int J Refrig 29:1175–1181. https://doi.org/10.1016/j.ijrefrig.2006.01.005
Saha BB, El-Sharkawy II, Thorpe R, Critoph RE (2012) Accurate adsorption isotherms of R134a
onto activated carbons for cooling and freezing applications. Int J Refrig 35:499–505. https://doi.
org/10.1016/j.ijrefrig.2011.05.002
Sarbu I (2014) A review on substitution strategy of non-ecological refrigerants from vapour
compression-based refrigeration, air-conditioning and heat pump systems. Int J Refrig
46:123–141. https://doi.org/10.1016/j.ijrefrig.2014.04.023
Sayigh AAM, McVeigh JC (1992) Solar air conditioning and refrigeration, 1st edn. Pergamon Press,
New York
9 TEWI Assessment of Conventional and Solar Powered Cooling Systems 177

Standard specification of adsorption chiller (2019). Tokyo Boeki Machinery, Japan. http://www.
tokyo-boeki-machinery.jp/adsorption_chiller/siyou.html. Accessed 23 Apr 2019
Stocker TF, Qin D, Plattner GK, Tignor M, Allen SK, Boschung J, Nauels A, Xia Y, Bex V,
Midgley PM (eds) (2019) Contribution of working group i to the fifth assessment report of the
intergovernmental panel on climate change. Cambridge University Press, Cambridge. http://www.
climatechange2013.org/images/report/WG1AR5_ALL_FINAL.pdf. Accessed 23 Apr 2019
Stryi-Hipp G (2016) Renewable heating and cooling: technologies and applications, 1st edn. Wood-
head Publishing, Amsterdam
Tangkengsirisin V, Kanzawa A, Watanabe T (1998) A solar-powered adsorption cooling sys-
tem using a silica gel-water mixture. Energy 23:347–353. https://doi.org/10.1016/S0360-
5442(98)00002-4
Tassou SA, Grace IN (2005) Fault diagnosis and refrigerant leak detection in vapour compression
refrigeration systems. Int J Refrig 28:680–688. https://doi.org/10.1016/j.ijrefrig.2004.12.007
Technical specification of 10 kW Bry-Chill adsorption chiller (2019) Bry-Air (Asia)
Pvt. Ltd. https://www.bryair.com/products-solutions/adsorption-chillers/brychilltm-adsorption-
chiller/. Accessed 23 Apr 2019
Thu K, Chakraborty A, Saha BB, Ng KC (2013) Thermo-physical properties of silica gel
for adsorption desalination cycle. Appl Therm Eng 50:1596–1602. https://doi.org/10.1016/j.
applthermaleng.2011.09.038
Tyagi VV, Kaushik SC, Tyagi SK (2012) Advancement in solar photovoltaic/thermal (PV/T) hybrid
collector technology. Renew Sustain Energy Rev 16:1383–1398. https://doi.org/10.1016/j.rser.
2011.12.013
Vaitkus L, Dagilis V (2017) Analysis of alternatives to high GWP refrigerants for eutectic refriger-
ating systems. Int J Refrig 76:160–169. https://doi.org/10.1016/j.ijrefrig.2017.01.024
Wang SK (2000) Handbook of air conditioning and refrigeration, 2nd edn. McGraw-Hill, New York
Wang LW, Wang RZ, Oliveira RG (2009) A review on adsorption working pairs for refrigeration.
Renew Sustain Energy Rev 13:518–534. https://doi.org/10.1016/j.rser.2007.12.002
Wang R, Wang L, Wu J (2014) Adsorption refrigeration technology—theory and application. Wiley,
Singapore
Yang MH, Yeh RH (2015) Performance and exergy destruction analyses of optimal subcooling
for vapor-compression refrigeration systems. Int J Heat Mass Transf 87:1–10. https://doi.org/10.
1016/j.ijheatmasstransfer.2015.03.085
Yumrutaş R, Kunduz M, Kanoğlu M (2002) Exergy analysis of vapor compression refrigeration
systems. Exergy An Int J 2:266–272. https://doi.org/10.1016/S1164-0235(02)00079-1
Chapter 10
Thermodynamic Analysis of Activated
Carbon–Ethanol and Zeolite–Water
Based Adsorption Cooling Systems

Satish Sangwan and Prodyut R. Chakraborty

Abstract The present study focuses on the thermodynamic analysis of zeolite–water

and activated carbon–ethanol based adsorption cooling systems. The performance
of the system depends critically on four operating temperatures namely maximum
desorption temperature, minimum adsorption temperature, condensing temperature,
and evaporating temperature and also on the ratio of specific heat of structural material
and the specific heat of adsorbent. Dubinin–Astakhov equation is used to estimate
the equilibrium uptake of working pairs. A comparative study is made between these
working pairs for the air-conditioning applications.

Keywords Adsorption · Zeolite · Activated carbon · Ethanol

10.1 Introduction

The world is getting warmer, the average global temperature of earth has increased
by 0.8 ◦ C since 1880 and most of this increase occurred since 1975 (Hansen et al.
2010). Due to this increase in average temperature, demand of refrigeration and
air-conditioning is increasing continuously. It is approximated by the international
Institute of Refrigeration in Paris that 15% of the total electricity production across
the world is used for cooling applications and it is also estimated that the 45% of
the total households and commercial buildings energy consumption is consumed
for air-conditioning applications (Choudhury et al. 2013). Traditional methods of
cooling uses CFC, HCFC and HFC as a refrigerant which are the major cause of
ozone layer depletion and global warming. So the demand of environment friendly
refrigeration and air-conditioning technologies is under continuous research focus.
One more problem with the traditional refrigeration systems is energy consumption.
Conventional vapor compression cooling systems uses electricity as source of energy.
The energy efficiency of electricity production is about 40–50%, so most of the
energy is released in atmosphere as waste heat at a temperature of 70–200 °C (Wang

S. Sangwan · P. R. Chakraborty (B)

Department of Mechanical Engineering, Indian Institute of Technology Jodhpur, Jodhpur
342037, Rajasthan, India
e-mail: pchakraborty@iitj.ac.in

© Springer Nature Singapore Pte Ltd. 2020 179

H. Tyagi et al. (eds.), Solar Energy, Energy, Environment,
and Sustainability, https://doi.org/10.1007/978-981-15-0675-8_10
180 S. Sangwan and P. R. Chakraborty

et al. 2014). Energy consumption problems can be solved by developing cooling

technologies which utilizes low grade heat as source of power.
Adsorption cooling has gained attention as a novel technology. The main advan-
tage with this technology is having zero ODP (ozone depleting potential) and GWP
(global warming potential) values as it uses natural ingredients as refrigerants alter-
native to refrigerants containing CFC, HCFC, or HFC (Saha et al. 2007). Since 1970s,
adsorption technology have been focused more among other sorption technologies. In
comparison to other sorption systems which uses low grade energy as source of heat,
adsorption systems has variety of adsorbents such as physical, chemical and com-
posite adsorbents and these can be used over a range of temperatures (50–400 °C).
Secondly, this technology doesn’t require solution pump and rectification equipment.
Adsorption cooling systems are devoid of problems faced by absorption systems like
solution crystallization and use of pollution prone refrigerants. But adsorption refrig-
eration systems are less efficient as compared to absorption refrigeration, and another
disadvantage this technology have, is that the systems are of large volume. Due to
these merits and demerits, this technology is given recognition by academicians as
a complementary technology for absorption refrigeration.
Adsorption systems uses zeolite–water, silica gel–water, and activated car-
bon–methanol/ethanol/ammonia, as working pairs (Lu et al. 2006). Many researchers
have studied adsorption systems based on these working pairs.

10.2 History of Adsorption Cooling Technology

In 1848, Faraday discovered that the cooling effect can be produced when AgCl
adsorbed NH3 (Wang et al. 2014). In the 1920s, Hulse suggested a silica gel–SO2
based cooling system for storing the food and heat source for this system was com-
bustion of propane and air was used for cooling. Lowest temperature achieved was
12 ◦ C (Hulse 1929). Activated carbon–methanol based cooling system was intro-
duced by Plank and Kuprianoff (Wang et al. 2014). Adsorption refrigeration was
not given importance by scientists and researchers for a long period due to improve-
ment in the efficiency of vapor compression refrigeration systems and adsorption
technology was not able to compete with efficient CFCs systems.
Due to energy crisis which occurred in the 1970s, adsorption refrigeration tech-
nology got a chance for the development because adsorption systems can be driven
by waste heat sources and solar energy. The problem of environmental pollution
became very serious from 1990 onward with the major drawbacks of CFC being
the significant contributor to ozone layer depletion and global warming. Because of
this adsorption cooling technology was given recognition by researchers and aca-
demicians. Till now air conditioning systems for automobiles, marine refrigeration
and heat pump systems are focused more under adsorption technology (Suzuki 1993)
because these systems have no moving parts and perform well in vibrating conditions
(Jones and Golben 1985).
10 Thermodynamic Analysis of Activated Carbon–Ethanol … 181

In early years, focus of research was on the performance of the working pairs.
The main objective was to use this technology to practical applications. Adsorbents
which were studied are silica gel, activated carbon, zeolite, CaCl2 , hydride and the
refrigerants under focus were methanol, ammonia, water, hydrogen (Critoph and
Vogel 1986). Initial research work suggested improvements in adsorption refrig-
eration cycles are required especially to address the intermittent behaviour of the
cycle, and low COP and SCP issues. Other major issue was low heat and mass trans-
fer performance inside the adsorption bed. For solving the above stated problems,
researchers mainly focused on aspects like adsorption properties, heat and mass trans-
fer, and advanced adsorption refrigeration cycles were proposed e.g. heat recovery
cycle, mass recovery cycle, thermal wave cycle (Wang 2001b), convective thermal
wave cycle (Critoph 1994) and cascading cycle (Douss and Meunier 1989). In some
references (Wang et al. 2004; Aristov et al. 2002; Mauran et al. 1993) working pairs
with good adsorption characteristics e.g. composite adsorption working pairs were
proposed and adsorption cycles were analysed for the proposed working pairs.
Till 1992, most of the work was based on theoretical analysis of different adsorp-
tion cycles and mainly on the effect of cycle parameters on the performance of
adsorption cycle (Luo and Feidt 1992; Hajji et al. 1991; Cacciola and Restuccia
1995). Based on the theoretical and experimental analysis researchers focused on
the design of an adsorbent bed which enhances the heat and mass transfer perfor-
mance (Restuccia et al. 1988).
In the 1990s, European Union listed a project on the adsorption (JOULE0046F)
into their research and the members of the research group for this project were
Meunier from France (zeolite–water), Cacciola from Italy (zeolite–water), Critoph
from England (activated carbon–ammonia), Groll from German (metal hydrides-
Hydrogen), Spinner from France, Zigler from German (nickel chloride-ammonia).
In 1999, International Journal of Refrigeration published the results of the JOULE
research plan (Wang et al. 2014).

10.3 Current Research on Adsorption Refrigeration

Countries like Italy, France, USA, UK, and India are working on adsorption heat
pump and cooling systems, and, the main focus is on the heat and mass transfer
performance of adsorbent beds, working pairs, and adsorption cycles. The develop-
ment in this field can be summarized as working pairs; structure of cooling system;
enhancement in the heat and mass transfer performance of adsorbent bed.
A. Adsorption working pairs: Working pair used in adsorption refrigeration is a
key element for the system. Coefficient of performance of the system is influ-
enced by the thermal properties of the working pair. For the better performance
of the system selection of a suitable working pair is required and this selection
must be based on the available heat source temperature and application require-
ments. Commonly used working pairs are activated carbon–ammonia, activated
182 S. Sangwan and P. R. Chakraborty

carbon–ethanol, silica gel–water, zeolite–water, metal hydrides–hydrogen, cal-

cium chloride–ammonia and strontium chloride–ammonia etc. (Srivastava and
Eames 1998). Activated carbon–methanol has high adsorption and desorption
capacity with low heat of adsorption of the order of 1800–2000 kJ/kg. Desorp-
tion temperature cannot exceed 120 ◦ C, because above this temperature methanol
will start decomposing. Activated carbon–ammonia with similar heat of adsorp-
tion as activated carbon–methanol pair, has the advantage of low evaporation
temperature of refrigerant and is used for high temperature heat source. Silica
gel–water is generally used for low temperature heat source because silica gel
will be destroyed at temperature higher than 120 ◦ C. Zeolite–water has wide
range of desorption temperature from 70 to 250 ◦ C and, the heat of adsorption is
around 3200–4200 kJ/kg, which is a disadvantage of this working pair. Activated
carbon–ethanol has the advantage of high adsorption/desorption capacity but has
low value of latent heat of vaporization approximately 30% lower than methanol.
It has saturation pressure similar to methanol. Activated carbon–ethanol works
best around desorption temperature of 100 ◦ C.
B. Low grade heat utilization: Adsorption Refrigeration Technology is suitable for
recovery of thermal energy from low grade heat sources by using working pairs
such as Activated carbon–methanol. On the other hand, activated carbon–am-
monia can be used for high temperature heat source. As compared to absorption
systems, adsorption refrigeration technology is more suitable in vibratory sys-
tems because of solid adsorbent. Therefore in the recent past, researchers focused
on the use of low grade heat in adsorption cooling.
Suzuki (1993) worked on zeolite–water based automobile air conditioner and
observed that heat and mass transfer properties of bed plays an important role
in minimizing the cycle time and mass of the system. Lavan (Mei et al. 1979)
worked out on the probability of a system that utilizes the exhaust gas of trucks.
Zhu et al. (1992) worked on the adsorption system used for fish storage on boats.
SJTU also developed some adsorption cooling systems (Wang et al. 2004; Wang
2001a) mainly activated carbon–ammonia based 5 kW air conditioner, zeo-
lite–water based 5 kW system for locomotive cab and an ice maker based on
activated carbon–methanol.
Saha et al. (2007) studied an adsorption system based on activated carbon
fiber–ethanol which works on low temperature waste heat source (60–95 °C).
COP of the system is 0.6 with cycle time of 600–700 s.
C. Solar energy utilization: Solar energy based sorption cooling systems got atten-
tion due to the seasonal matching of cooling requirements and heat supply. Use of
low temperature heat as source of energy by adsorption cooling systems makes
them more suitable for solar energy applications as compared to absorption
systems.
Tchernev (Wang et al. 2014) developed a zeolite–water based adsorption cool-
ing system, and from then onwards, researchers focused on the adsorption sys-
tems driven by solar energy. In France, Guilleminot and Pons (Grenier et al.
1988) investigated adsorption cooling systems which utilizes solar energy as heat
source, working pairs used were activated carbon–methanol and zeolite–water,
10 Thermodynamic Analysis of Activated Carbon–Ethanol … 183

and it is observed that the COP of zeolite–water system is 0.10 having col-
lector area of 20 m2 and mass of adsorbent is 360 kg. The COP of activated
carbon–methanol system (Pons and Guilleminot 1986) was found out to be
0.12–0.14 having collector area of 6 m2 and mass of adsorbent is 20–24 kg/m2 .
Sumathy et al., studied an activated carbon–methanol system driven by solar
energy, and observed that output of the system is 4–5 kg of ice per day and the
COP is about 0.1–0.2 with collector area of 0.92 m2 (Sumathy and Zhongfu
1999). Li et al., at Guangzhou Institute of Energy Conversion and Tan at South
China University of Technology developed a refrigeration system powered by
solar energy having similar performance to the system developed by Sumathy
(Wang et al. 2014).
To enhance the energy efficiency a compound refrigerator and water heater
system based on solar energy was developed by SJTU (Wang et al. 2000) and in
2004, SJTU also developed a solar energy based adsorption chiller which uses
silica gel–water as working pair for building and grain storage hall.
El-Sharkawy et al. (2008) worked on activated carbon–ethanol based solar pow-
ered adsorption system. Ideal COP of the system is 0.8 at an evaporator temper-
ature of 15 ◦ C and desorption temperature of 80 ◦ C.

10.4 Adsorption Process and Selection Criteria

for Working Pairs

Adsorption is a surface phenomenon in which molecules of one substance adheres

to the surface of another. This results in an increase in concentration at the inter-
face. The solid which adsorbs the molecules called adsorbent and the liquid or gas
which is getting adsorbed is called adsorbate. Adsorption is of mainly two types,
physical adsorption (Ponec et al. 1974) and chemisorption (Wang 2005). Driving
force in case of physisorption is weak wander wall force of attraction whereas in
case of chemisorption adsorbent and adsorbate undergo a chemical reaction (Gasser
and Ehrlich 1987). Chemisorption is an endothermic process and is not reversible.
Physisorption is an exothermic process and the heat released during this process is
known as adsorption heat. This process is reversible in nature, means desorption
occurs when heat is supplied to the adsorbent and this heat can be acquired using
solar energy and waste heat from automobiles or any other sources.
Basic requirements for the selection of adsorbent are as follows:
1. Should desorb large amount of adsorbate over a given range of temperature.
2. Should have low specific heat.
3. Should have high conductivity.
4. Should be stable with adsorbate/refrigerant used.
5. Should be easily available and have low cost.
184 S. Sangwan and P. R. Chakraborty

Basic requirements for the selection of adsorbate are as follows:

1. Should have high latent heat of vaporization to minimize the amount of adsorbent
needed.
2. The saturation pressure corresponding to the evaporating temperature should be
above atmospheric pressure so that the system always runs on positive pressure.
3. The saturation pressure corresponding to the condensing temperature should be
as low as possible.
4. Should be thermally and chemically stable.
5. Non-toxic, non-flammable and should have low cost.

10.5 Adsorption Refrigeration Cycle

An adsorption refrigeration cycle consist of adsorber/desorber, a condenser, an evap-

orator and an expansion valve (Meunier 2001). The mechanical compressor which
is an integral part of traditional VCR systems is replaced by low grade energy driven
adsorbent bed. Single bed adsorption refrigeration cycle is intermittent in nature and
consists of four processes.
I. Sensible Heating
II. Desorption and Condensation
III. Sensible Cooling
IV. Adsorption and Evaporation
The desorption and adsorption occurs at constant pressure (isobaric process), whereas
the sensible heating and cooling occurs at constant concentration (isosteric process).
To deal with the problem of intermittent nature of single bed cycle, we are consid-
ering two bed system in which both the beds will work simultaneously, one will
undergo adsorption process, at the same time other will undergo desorption process.
Schematic of two bed adsorption refrigeration system (Basic cycle) with uniform
bed temperature assumption and representative ideal thermodynamic cycle for the
beds is shown in Fig. 10.1, the system operates in the following way.

Sensible Heating
This is the period from a to b as shown in Fig. 10.1. During this process concentration
of refrigerant or adsorbate remains constant or at maximum (xa = xb ) and the bed is
sensibly heated from Ta to Tb , resulting an increase in pressure from Pe (evaporating
pressure) to Pc (condensing pressure).

Heating, Desorption and Condensation

This is the period from b to c as shown in Fig. 10.1. At Tb pressure in the adsorber
is equal to the condensing pressure Pc and desorption starts. During desorption
temperature of the bed increases along isobar until the refrigerant concentration
reaches X min . This desorbed refrigerant get condensed in the condenser releasing
the latent heat of condensation to atmosphere.
10 Thermodynamic Analysis of Activated Carbon–Ethanol … 185

Fig. 10.1 Schematic of two bed adsorption cooling system (basic cycle) and representative ideal
thermodynamic cycle for the beds

Cooling and Depressurization

This is the period from c to d as shown in Fig. 10.1. After the completion of desorption
process, the bed is at highest temperature and needs to be cooled for re-adsorption of
refrigerant. So the bed is sensibly cooled resulting in decrease in temperature from
Tc to Td and pressure for Pc to Pe while the concentration of refrigerant remains
constant (xc = xd ).

Cooling, Adsorption and Evaporation

This is the period from d to a as shown in Fig. 10.1. During this process refrigerant
get evaporated inside the evaporator absorbing the heat from the surroundings. Then
refrigerant comes out of evaporator and enters into the adsorbent bed, where re-
adsorption of refrigerant occurs and as adsorption is an exothermic process the bed
needs to be cooled simultaneously. This process continues until the cycle is completed
and the system is returned to its original temperature of Ta .
186 S. Sangwan and P. R. Chakraborty

10.6 Heat Recovery Cycle

The schematic of two bed adsorption cooling system with heat recovery and associ-
ated thermodynamic cycle for the beds is shown in Fig. 10.2. At the completion of
each half of the cycle, desorption bed will be at highest temperature (Tc ) and adsorp-
tion bed will be at lowest temperature (Ta ). The bed at lowest temperature requires
heat to undergo desorption process while the bed at highest temperature needs to be
cooled, so that it can undergo adsorption process. Part of the heat rejected during
cooling (process c-d-a) can be utilized to preheat the desorbing bed (process a-b-c),
reducing the total amount of external heating and cooling requirements. It will also
result in the improvement of COP and second law efficiency of the system.
The heat recovery can be achieved by circulating a heat transfer fluid between
both the beds as shown in Fig. 10.3. But heat can only be recovered till both the beds
attains thermal equilibrium (point e and f in thermodynamic cycle).

Fig. 10.2 Schematic of two bed adsorption cooling system with heat recovery and associated
thermodynamic cycle for the beds
10 Thermodynamic Analysis of Activated Carbon–Ethanol … 187

10.7 Mathematical Modelling

The mathematical model is derived on the basis of simple thermodynamic cycles

described in Fig. 10.1. This model is based on the adsorption equilibrium equations of
the working pair and heat flows. The assumptions made to simplify the calculations
are as follows. (i) Equilibrium condition is reached in adsorbent bed during both
adsorption and desorption process, it implies that the amount of adsorbate adsorbed
or desorbed is calculated using equilibrium adsorption equation. (ii) The pressure,
temperature and concentration of adsorbate are uniform throughout the adsorbent
bed. (iii) No change in the concentration of adsorbate during sensible heating and
cooling of adsorbent bed. (iv) Specific heat of refrigerant is considered to be constant
over the working temperature range. (v) Temperature difference between the heat
transfer fluid and the adsorbent bed is considered to be negligible. (vi) Temperature
difference between the heat transfer fluid and the refrigerant is considered to be
negligible in the evaporator/condenser. And, (vii) system is well insulated i.e., no
heat loss from the system to the environment.
The Dubinin–Astakhov (D-A) equation is used to calculate sorption uptake of
adsorbate in the adsorbent bed as a function of temperature (T ) and pressure (P)
(Tamainot-Telto and Critoph 1997; Critoph and Turner 1988; Saha et al. 2009, 2011).
  n 
T
x(T, T psat ) = x0 exp −k −1 (10.1)
T psat

where, xo is the maximum concentration of adsorbate obtained under saturated con-

centration, k and n are empirical coefficients. Values xo , k, and n depends on the
nature of adsorbent-adsorbate pair and are constants for a given adsorbent-adsorbate
pair. T is the temperature of the bed in Kelvin and T psat is the saturation temperature
(K) of adsorbate at the bed pressure (p).

Heat analysis of basic cycle

Sensible heat (Q sh ) is the energy required to increase the bed temperature from
minimum adsorption temperature Ta to minimum desorption temperature Tb and
pressure from pe to pc prior to the onset of desorption process (process a-b on
thermodynamic cycle in Fig. 10.1). Q sh heat supply per unit kg of adsorbent is given
by:

Tb Tb Tb

ref
Q sh = cad (T ) dT + Rm cad dT + cvr xa dT (10.2)
Ta Ta Ta

ref
where, cad is the specific heat of activated carbon, cad is the specific heat of activated
carbon at reference temperature (300 K) and cvr is average constant volume specific
ref
heat of liquid and gaseous refrigerant. Rm = (cm m m )/(cad m ad ) is defined as the
ratio of heat capacity of the combined container material of the adsorber bed and HTF
188 S. Sangwan and P. R. Chakraborty

with that of adsorbent. cm is the combined specific heat capacity of the structural
material of bed and HTF, and m m is the combined mass of the same (cm m m =
cst m st +c H T F m H T F and m m = m st +m H T F ). xa = xb is the maximum concentration
of adsorbate adsorbed in the bed.
The heat input per unit kg of adsorbent during the desorption process Q de (process
b–c in Fig. 10.1) can be estimated as:

Tc Tc Tc xc

ref
Q de = cad (T ) dT + Rm cad dT + c pr x(T, Tcsat ) dT − h d d x (10.3)
Tb Tb Tb xb

where, c pr is average constant pressure specific heat of liquid and gaseous refrigerant.
Once again, coexistence of gaseous and liquid refrigerant at equilibrium in the bed
leads to estimation of c pr on the basis of the average between gas-phase and liquid
phase constant pressure specific heats of refrigerant. Last term in Eq. 10.3, represents
the amount of heat required for desorption as desorption is an endothermic process
and the negative sign is due to the fact that the term dx is negative during desorption.
The sensible cooling per unit kg of adsorbent (Q sc ) of the bed prior to onset of
adsorption process (process c–d in Fig. 10.1) brings down the pressure of the bed
from condensing pressure pc to evaporating pressure pe accompanied by a decrease in
bed temperature from maximum desorption temperature (Tc ) to maximum adsorption
temperature (Td ). Q sc can be estimated as:

Td Td Td

ref
Q sc = cad (T ) dT + Rm cad dT + cvr xc dT (10.4)
Tc Tc Tc

Process c–d being isosteric xc = xd , xc = xd is the minimum concentration of

refrigerant adsorbed in the bed.
Re-adsorption of adsorbate occurs at evaporation pressure pe (isobaric process d-
an in Fig. 10.1). Adsorption being exothermic process, heat removal from the bed is
essential during this process. The bed temperature reduces from maximum adsorption
temperature Td to minimum adsorption temperature Ta during this process. The heat
removal (Q ad ) required per kg of adsorbent can be found out to be:

Ta Ta ref Ta xa

Q ad = cad (T ) dT + Rm cad dT + c pr x(T, Tesat ) dT − ha d x
Td Td Td xd
(10.5)
xa
+ c pr (T − Tesat ) d x
xd

Last term in Eq. 10.5, is included to take care of the cooling effect provided by the
refrigerant coming from evaporator as it will be at a lower temperature compared to
bed temperature. Second last term is adsorption heat.
10 Thermodynamic Analysis of Activated Carbon–Ethanol … 189

During the cooling process of adsorption bed, heat can be directly rejected to the
ambient till the adsorption bed reaches the ambient temperature. If the minimum
adsorption temperature (Ta ) is lower than that of ambient temperature, cooling of the
adsorber bed below the ambient will require a special cooling arrangement. In order
to be on the safer side, cooling of the adsorption bed from condensation temperature
Tcsat to minimum adsorption temperature Ta is considered as the external cooling
requirement. If we denote cooling requirement (per kg of adsorbent) of the adsorption
bed below Tcsat as Q <Tcsat , it can be expressed as:

Ta Ta Ta

ref
Q <Tcsat = cad (T ) dT + Rm cad dT + c pr x(T, Tesat ) dT
Tcsat Tcsat Tcsat
xa xa
− ha d x + c pr (T − Te ) d x (10.6)
xcsat xcsat

If Ta is greater than Tcsat then Q <Tcsat = 0.

After desorption gaseous refrigerant at high temperature and superheated state
enters the condenser, and condenses to a state of saturated liquid at condensing
pressure pc by releasing heat to the ambient. The heat released to the atmosphere
(per kg of adsorbent) can be estimated as:
⎡ ⎤
xc
Q cond = −⎣(xb − xc )h f g + c pr (T − Tcsat ) d x ⎦ (10.7)
xb

An additional cooling (per unit kg of adsorbent) of refrigerant is required during the

free expansion of saturated liquid from pc to pe as the refrigerant flows through the
expansion valve between the condenser and evaporator, which is given by:

Tesat
Q co = crl (T )(xb − xc ) dT (10.8)
Tcsat

where, crl is the temperature dependent specific heat of liquid refrigerant.

Refrigeration effect per kg of adsorbent Q evap can be estimated as

Q evap = (xb − xc )h f g (10.9)

It is pertinent to mention here that the signs of estimated values of Q sh , Q de , and

Q evap is positive and the signs of estimated values of Q sc , Q ad , Q <Tcsat , Q cond , Q co
are negative. The SCE and COP of the system can be estimated as:

SC E = Q evap − Q co − Q <Tcsat (10.10)

190 S. Sangwan and P. R. Chakraborty

SC E
COP = (10.11)
Q de + Q sh

Heat recovery cycle (analysis)

The thermodynamic cycle of the heat recovery process is shown in Fig. 10.2. The
heat recovery can be attained till thermal equilibrium is established between the beds
so that both the beds reach a common temperature (described by sates e and f on
the thermodynamic cycle in Fig. 10.2). The estimation of equilibrium temperature of
the two beds Te and T f (Te = T f = Teq ) is be based on two alternative formulations
based on relative values of Tb and Td . If the condition Tb ≥ Td is satisfied, then Teq
can be estimated from the following formulation.

T T
Q sh |Taeq = Q sc |Teqc (10.12)

On the other hand if the condition Tb < Td is satisfied, then Teq needs to be estimated
from the following formulation.

T T
Q sh + Q de |Teqb = Q sc + Q ad |Teqd (10.13)

T T T T
Q sh |Taeq , Q de |Teqb , Q sc |Teqc , and Q ad |Teqd can be obtained from Eqs. 10.2–10.5 by chang-
ing the upper limits of integrations to e and f respectively. Although the temperatures
Te and T f are equal at state points e and f, the concentrations of refrigerant at des-
orbing and adsorbing beds are not equal, i.e. xe (Te , Tcsat ) = x f (T f , Tesat ); and one
T T
should be careful while calculating Q de |Teqb and Q ad |Teqd from Eqs. 10.3 and 10.5.
T T T
By substituting appropriate expressions of Q sh |Taeq , Q sc |Teqc , Q sh , Q sc , Q de |Teqb , and
T
Q ad |Teqd in Eq. 10.12 or 10.13, we obtain an equation with a single unknown Teq and
Eq. 10.12 or 10.13 can be rewritten as:

F(Teq ) = 0 (10.14)

where, F(Teq ) is a nonlinear function of Teq . Equation 10.14 can be solved by using
iterative secant method to determine the value of Teq in the following manner.

Teq(i−1) − Teq(i−2)
Teq(i) = Teq(i−1) − F (i−1) (10.15)
F (i−1) − F (i−2)

where, i represents the iteration step. The two initial guesses to initiate the iteration
has been considered to be Tb , and Td .
If Tb ≥ Td , regenerated or recovered heat per kg of adsorbent Q r eg can be simply
estimated as:
T T
Q r eg = Q sh |Taeq = Q sc |Teqc (10.16)
10 Thermodynamic Analysis of Activated Carbon–Ethanol … 191

On the other hand, for Tb < Td , Q r eg can be simply estimated as:

T T
Q r eg = Q sh + Q de |Teqb = Q sc + Q ad |Teqd (10.17)

The net heat input from the external heat source per kg of adsorbent Q in is estimated
as:

Q in = Q sh + Q de − Q r eg (10.18)

Since Teq > Tcsat , the net external cooling requirement per kg of adsorbent Q out is:

Q out = Q <Tcsat (10.19)

The SCE is same as given by Eq. 10.10, however, the COP of the system can be
estimated as:
SC E
COP = (10.20)
Q in

Second Law Efficiency

The analysis will remain incomplete if we do not discuss the second law efficiencies
of each of these adsorption refrigeration cycles. From the first law of thermodynamics
for the half cycle we obtain the following heat balance equation.

(Q de + Q sh ) + (Q evap − Q co − Q <Tcsat ) = Q cond + (Q ad + Q sc − Q <Tcsat )

(10.21)

From the second law: Suni = Ssys + Ssur ≥ 0. Noting that Ssys = 0 for a
closed cycle, net entropy increase of the surrounding must be greater than or equal
to zero (Ssur ≥ 0). The change in entropy of the surrounding can be expressed as.

(Q de + Q sh ) (Q evap − Q co − Q <Tcsat ) Q cond (Q ad + Q sc − Q <Tcsat )

− − + + ≥0
Tc Tesat Tcsat Tamb
(10.22)

where, Tamb is the ambient temperature, and all the temperatures values are in absolute
temperature scale (K). It is desired that the ambient temperature should be less than
saturation temperature at condenser pressure (Tamb < Tcsat ). However, in worst case
scenario, Tamb should at least be equal to Tcsat . Putting this condition in Eq. 10.22,
we obtain the following expression for entropy generation of the surrounding.

(Q de + Q sh ) (Q evap − Q co − Q <Tcsat ) Q cond (Q ad + Q sc − Q <Tcsat )

− − + + ≥0
Tc Tesat Tcsat Tcsat
(10.23)
192 S. Sangwan and P. R. Chakraborty

Finally, combining Eqs. 10.23 and 10.21, and rearranging the terms we obtain the
expression for maximum coefficient of performance (C O Pmax ) given by:
  
Q evap − Q co − Q <Tcsat Tc − Tcsat Tesat
C O Pmax = ≤ (10.24)
(Q de + Q sh ) Tc Tcsat − Tesat

The second law efficiencies of the cycles described earlier are defined as:

C O Pcycle
ηI I = (10.25)
C O Pmax

10.8 Results

COP, SC E, η I I values of the two cycles for both the working pairs are plotted against
five input parameters (maximum desorption temperature (Tc ), minimum adsorption
temperature (Ta ), condensing temperature (Tcsat ), evaporating temperature (Tesat )
and the heat capacity ratio (Rm ) between structural material and adsorbent). Activated
Carbon–Ethanol and Zeolite–Water are used as working pairs for the present analysis.
Adsorption parameters such as xo , k, and n and thermos-physical properties used for
the analysis are given in Tables 10.1 and 10.2 respectively.

Table 10.1 Adsorption parameters of working pairs

Working pair x0 (kg of refrigerant/kg of adsorbent) k n
Activated carbon–ethanol (Uddin 1.2 25.587 1.8
et al. 2014)
Zeolite–water (Wang et al. 2014) 0.302 4.40 2

Table 10.2 Thermo-physical properties

Property Unit Activated carbon–ethanol Zeolite–water
cad (Turner 1992; Sharafian and kJ/kg K 0.175 + 2.245 × 10−3 × T 0.836
Bahrami 2015)
crl (Sharafian and Bahrami 2015) kJ/kg K 2.87 4.185
c pr (Sharafian and Bahrami kJ/kg K 1.687 1.880
2015)
cvr (Sharafian and Bahrami 2015) kJ/kg K 1.5063 1.419
h f g (Wang et al. 2014) kJ/kg 842 2258
R kJ/kg K 0.1807 0.462
C K 4659 4897
10 Thermodynamic Analysis of Activated Carbon–Ethanol … 193

Figure 10.3 shows the effect of varying maximum desorption temperature (Tc )
and heat capacity ratio (Rm ) on COP of the basic cycle keeping all other input
parameters constant i.e. Ta = Tcsat = 300 K and Tesat = 273 K for both the working
pairs. From Fig. 10.3a and b, it is clear that with the increase in maximum desorption
temperature, COP also increases and attains maxima for a particular value of Tc
(370 K for Activated carbon ethanol and around 450 K for Zeolite–water pair) but
with further increase in temperature it starts decreasing because a fixed amount of
refrigerant is adsorbed inside the adsorbent and no more desorption occurs after a
particular temperature. With the increase heat capacity ratio more amount of input
heat will be absorbed by the structural material and it will affect the COP adversely
which can also be seen from Fig. 10.3.
Figure 10.4 shows the effect of varying maximum desorption temperature (Tc ) and
heat capacity ratio (Rm ) on COP of the heat recovery cycle keeping all other input
parameters constant i.e. Ta = Tcsat = 300 K and Tesat = 273 K for both the working
pairs. Heat recovery cycle also follows the same trend with respect to maximum
desorption temperature as followed by the basic cycle. Heat recovery cycle resulted
in higher COP as compared to basic cycle. From Fig. 10.4a and b, it can be noticed
that there is a sudden change in the COP gradient with respect to Tc at Tc = 365 K
(activated carbon–ethanol) and Tc = 360 K (zeolite–water). This change is due to the
fact that condition Tb ≥ Td prevails till Tc = 365 K (activated carbon–ethanol) and
Tc = 360 K (zeolite–water), and the estimation of equilibrium bed temperature Teq
is obtained from Eq. 10.16 pertaining to sensible heating and cooling. On the other
hand, for Tc > 365 K (activated carbon–ethanol) and Tc > 360 K (zeolite–water)
the condition Tb < Td gets satisfied and estimation of Teq is obtained from Eq. 10.17
involving adsorption and desorption cooling along with sensible heating or cooling
process.
Figure 10.5 shows the effect of varying minimum adsorption temperature on
COP while all other parameters are kept constant i.e. Tc = 370 K (activated car-
bon–ethanol), Tc = 450 K (Zeolite–water), Tcsat = 300 K, and Tesat = 273 K. It is
evident from Fig. 10.5a and b that with the increase in minimum adsorption temper-
ature COP, increases and is maximum at Ta = Tcsat = 300 K. With further increase
in temperature COP starts decreasing, this can be explained using Eqs. 10.6 and
10.8–10.10, when Ta approaches Tcsat , Q <Tcsat approaches zero and at the same time
the value of (xb − xc ) also reduces which causes decrease in Q evap (Eq. 10.9) and
Q co (Eq. 10.8) but the net effect is increase in SCE and COP values. For the case
when Ta is greater than Tcsat , Q <Tcsat = 0 and the net effect of increase in Ta will be
decrease in SCE and COP values. It can be concluded that for maximum COP both
Ta and Tcsat should be kept same.
194 S. Sangwan and P. R. Chakraborty

Fig. 10.3 Variation of COP of basic cycle with maximum desorption temperature (Tc ) and Rm ,
a activated carbon–ethanol and b zeolite–water
10 Thermodynamic Analysis of Activated Carbon–Ethanol … 195

Fig. 10.4 Variation of COP of heat recovery cycle with maximum desorption temperature (Tc ) and
Rm , a activated carbon–ethanol and b zeolite–water
196 S. Sangwan and P. R. Chakraborty

Fig. 10.5 Variation of COP with minimum adsorption temperature (Ta ) a activated carbon–ethanol
and b zeolite–water
10 Thermodynamic Analysis of Activated Carbon–Ethanol … 197

Figure 10.6 shows the effect of varying condensing temperature (Tcsat ) on COP,
keeping all other parameters constant i.e. Rm = 2.5, Tc = 400 K (activated car-
bon–ethanol), Tc = 450 K (Zeolite–water), Ta = 300 K, and Tesat = 273 K. Con-
densing temperature has an adverse effect on COP, with increase in Tcsat value
of COP decreases monotonically. It can be explained physically as well as using
Eqs. 10.1 and 10.8–10.10. From Eq. 10.1 it is clear that higher the Tcsat higher will
be the xc and hence lower the difference xb − xc , this will decreases the value of SCE
and COP.
Figure 10.7 shows the effect of varying evaporating temperature (Tesat ) on COP,
keeping all other input parameters constant, i.e. Rm = 2.5, Tc = 400 K (activated
carbon–ethanol), Tc = 450 K (Zeolite–water), Ta = Tcsat = 300 K. Evaporating
temperature has positive impact on COP, means increase in Tesat increases the value
of COP. It can be explained the same way as the effect of condensing temperature.
Figure 10.8 shows the variation of SCE with respect to maximum desorption
temperature (Tc ) at Rm = 2.5, Ta = Tcsat = 300 K, and Tesat = 273 K. SCE
increases with increase in maximum desorption temperature (Tc ).
Figure 10.9 shows the effect of variation of maximum desorption temperature on
second law efficiency for two cycles under consideration. From Figs. 10.3, 10.4, and
10.9, it is clear that maxima of COP and maxima of second law efficiency (η I I ) are
not in the same range. That is why there needs to be a trade off in choosing Tc in
the best possible way such that neither COP nor η I I is sacrificed too much in the
interest of improving the individual entities. In order to obtain a common scale for
COP and η I I both the parameters are normalized within a range of zero to one by
dividing COP and η I I with corresponding maximum values C O Pmax and η I I max
respectively. C O P/C O Pmax and η I I /η I I max are then plotted simultaneously for
varying Tc values (Fig. 10.10). The intersection point of C O P/C O Pmax versus Tc
plot with η I I /η I I max versus Tc plot provides the required optimized values of COP,
η I I , and Tc .
From Figs. 10.3, 10.4, 10.5, 10.6, 10.7, 10.8 and 10.9, it is evident that activated
carbon–ethanol working pair has higher values of COP, SCE, and η I I as compared
to zeolite–water. From Fig. 10.2, we can observe that maxima for activated carbon
ethanol occurs in the range 360–375 K, whereas for zeolite–water maxima occurs
around 445–460 K. So zeolite–water working pair can be used at higher desorption
temperatures while activated carbon–ethanol is suitable for lower desorption temper-
ature. Typical COP values heat recovery cycle for both the working pairs for air con-
ditioning applications (Tesat = 283 K) at desorption temperature of 100 ◦ C (373 K)
and at condensing temperature of 27, 37, and 47 ◦ C are given in Table 10.3.
198 S. Sangwan and P. R. Chakraborty

Fig. 10.6 Variation of COP with condensing temperature (Tcsat ) a activated carbon–ethanol and
b zeolite–water
10 Thermodynamic Analysis of Activated Carbon–Ethanol … 199

Fig. 10.7 Variation of COP with evaporating temperature (Tesat ) a activated carbon–ethanol and
b zeolite–water
200 S. Sangwan and P. R. Chakraborty

Fig. 10.8 Variation of SCE with maximum desorption temperature (Tc), a activated carbon–ethanol
and b zeolite–water
10 Thermodynamic Analysis of Activated Carbon–Ethanol … 201

Fig. 10.9 Variation of second law efficiency η I I with Tc for two cycles a activated carbon–ethanol;
b zeolite–water
202 S. Sangwan and P. R. Chakraborty

Fig. 10.10 Optimized COP, η I I , and Tc obtained from intersection point of C O P/C O Pmax versus
Tc and η I I /η I I max versus Tc plots a activated carbon–ethanol; b zeolite–water

Table 10.3 COP values for

Tcsat = Ta (K) Tesat = 283 K
air conditioning applications
at different condensing Activated carbon–ethanol Zeolite–water
temperatures 300 0.9040 0.5712
310 0.6341 0.3938
320 0.4704 0.2147
10 Thermodynamic Analysis of Activated Carbon–Ethanol … 203

10.9 Conclusion

Thermodynamics analysis of two cycles namely basic cycle and heat recovery cycle
is presented for activated carbon–ethanol and zeolite–water. Coefficient of perfor-
mance (COP), Specific cooling effect (SCE), and Second law efficiency (η I I ) are
studied with respect to five input i.e. maximum desorption temperature, minimum
adsorption temperature, condensing temperature, evaporating temperature, and heat
capacity ratio. Maximum desorption temperature plays significant role in finding
the maxima of COP and Second law efficiency (η I I ). But the maxima of both the
parameters found to be at different range of temperature. A graphical method to find
common temperature so that neither COP nor second law efficiency is sacrificed
in maximizing the individual entities is discussed. Activated carbon–ethanol per-
forms better at low desorption temperature while zeolite–water is suitable for high
desorption temperature applications.

References

Aristov YI, Restuccia G, Cacciola G, Parmon VN (2002) A family of new working materials for
solid sorption air conditioning systems. Appl Therm Eng 22(2):191–204
Cacciola G, Restuccia G (1995) Reversible adsorption heat pump: a thermodynamic model. Int J
Refrig 18(2):100–106
Choudhury B, Saha BB, Chatterjee PK, Sarkar JP (2013) An overview of developments in adsorption
refrigeration systems towards a sustainable way of cooling. Appl Energy 104:554–567
Critoph RE (1994) Forced convection enhancement of adsorption cycles. Heat Recovery Syst CHP
14(4):343–350
Critoph RE, Turner HL (1988) Performance of ammonia–activated carbon and ammonia–zeolite
heat pump adsorption cycles. In: Proceedings of the international conference on Pompes a Chaleur
Chimiques de Hautes Performances, Perpignan, France, pp 202–11
Critoph RE, Vogel R (1986) Possible adsorption pairs for use in solar cooling. Int J Ambient Energy
7(4):183–190
Douss N, Meunier F (1989) Experimental study of cascading adsorption cycles. Chem Eng Sci
44(2):225–235
El-Sharkawy II, Saha BB, Koyama S, He J, Ng KC, Yap C (2008) Experimental investigation on
activated carbon–ethanol pair for solar powered adsorption cooling applications. Int J Refrig
31(8):1407–1413
Gasser RPH, Ehrlich G (1987) An introduction to chemisorption and catalysis by metals. Phys
Today 40:128
Grenier P, Guilleminot JJ, Meunier F, Pons M (1988) Solar powered solid adsorption cold store. J
Sol Energy Eng 110(3):192–197
Hajji A, Worek WM, Lavan Z (1991) Dynamic analysis of a closed-cycle solar adsorption refrig-
erator using two adsorbent-adsorbate pairs. J Sol Energy Eng 113(2):73–79
Hansen J, Ruedy R, Sato M, Lo K (2010) Global surface temperature change. Rev Geophys 48(4)
Hulse GE (1929) Freight car refrigeration by an adsorption system employing silica gel. Refrig Eng
17(2):41–53
Jones JA, Golben PM (1985) Design, life testing, and future designs of cryogenic hydride refriger-
ation systems. Cryogenics 25(4):212–219
204 S. Sangwan and P. R. Chakraborty

Lu ZS, Wang RZ, Wang LW, Chen CJ (2006) Performance analysis of an adsorption refrigerator
using activated carbon in a compound adsorbent. Carbon 44(4):747–752
Luo L, Feidt M (1992) Thermodynamics of adsorption cycles: a theoretical study. Heat Transfer
Eng 13(4):19–31
Mauran S, Prades P, L’haridon F (1993) Heat and mass transfer in consolidated reacting beds for
thermochemical systems. Heat Recovery Syst CHP 13(4):315–319
Mei V, Chaturvedi SK, Lavan BZ (1979) A truck exhaust gas operated absorption refrigeration
system. ASHRAE Trans 85:66–76
Meunier F (2001) Adsorptive cooling: a clean technology. Clean Prod Process 3(1):8–20
Ponec V, Knor Z, Černý S, Smith D, Adams NG (1974) Adsorption on solids. Butterworths, London
Pons M, Guilleminot JJ (1986) Design of an experimental solar-powered, solid-adsorption ice
maker. J Sol Energy Eng 108(4):332–337
Restuccia G, Recupero V, Cacciola G, Rothmeyer M (1988) Zeolite heat pump for domestic heating.
Energy 13(4):333–342
Saha BB, El-Sharkawy II, Chakraborty A, Koyama S (2007) Study on an activated car-
bon fiber–ethanol adsorption chiller: Part I—system description and modelling. Int J Refrig
30(1):86–95
Saha BB, Habib K, El-Sharkawy II, Koyama S (2009) Adsorption characteristics and heat of adsorp-
tion measurements of R-134a on activated carbon. Int J Refrig 32(7):1563–1569
Saha BB, Jribi S, Koyama S, El-Sharkawy II (2011) Carbon dioxide adsorption isotherms on
activated carbons. J Chem Eng Data 56(5):1974–1981
Sharafian A, Bahrami M (2015) Critical analysis of thermodynamic cycle modeling of adsorption
cooling systems for light-duty vehicle air conditioning applications. Renew Sustain Energy Rev
48:857–869
Srivastava NC, Eames IW (1998) A review of adsorbents and adsorbates in solid–vapour adsorption
heat pump systems. Appl Therm Eng 18(9):707–714
Sumathy K, Zhongfu L (1999) Experiments with solar-powered adsorption ice-maker. Renew
Energy 16(1):704–707
Suzuki M (1993) Application of adsorption cooling systems to automobiles. Heat Recovery Syst
CHP 13(4):335–340
Tamainot-Telto Z, Critoph RE (1997) Adsorption refrigerator using monolithic carbon–ammonia
pair. Int J Refrig 20(2):146–155
Turner L (1992) Improvement of activated charcoal-ammonia adsorption heat pumping/refrigeration
cycles: investigation of porosity and heat/mass transfer chacteristics
Uddin K, El-Sharkawy II, Miyazaki T, Saha BB, Koyama S (2014) Thermodynamic analysis of
adsorption refrigeration cycles using parent and surface treated Maxsorb III/ethanol pairs
Wang RZ (2001a) Adsorption refrigeration research in Shanghai Jiao Tong University. Renew
Sustain Energy Rev 5(1):1–37
Wang RZ (2001b) Performance improvement of adsorption cooling by heat and mass recovery
operation. Int J Refrig 24(7):602–611
Wang LW (2005) Performances, mechanisms and application of a new type compound adsorbent
for efficient heat pipe type refrigeration driven by waste heat. Doctor Dissertation, Shanghai Jiao
Tong University, Shanghai, China
Wang RZ, Li M, Xu YX, Wu JY (2000) An energy efficient hybrid system of solar powered water
heater and adsorption ice maker. Sol Energy 68(2):189–195
Wang LW, Wang RZ, Wu JY, Wang K (2004) Compound adsorbent for adsorption ice maker on
fishing boats. Int J Refrig 27(4):401–408
Wang R, Wang L, Wu J (2014) Adsorption refrigeration technology: theory and application. Wiley
Zhu R, Han B, Lin M, Yu Y (1992) Experimental investigation on an adsorption system for producing
chilled water. Int J Refrig 15(1):31–34
 Part IV
Energy Storage
Chapter 11
PCM-Metal Foam Composite Systems
for Solar Energy Storage

Anirban Bhattacharya

Abstract Efficient storage of solar thermal energy has been a key research area in
recent years. Among the various methods for energy storage, phase change mate-
rial (PCM) based latent heat systems have shown a lot of promise due to their high
energy storage densities and smaller system sizes. However, the low thermal con-
ductivities of PCM pose a significant challenge in designing such systems, therefore,
augmentation with suitable thermal conductivity enhancers becomes necessary to
improve their energy charging and discharging performances. The use of metal foam
structures embedded in PCM to form composite PCM-metal foam energy storage
system can improve the effective thermal conductivity remarkably due to the high
surface area for heat transfer between the metal foam and the PCM. This chapter
presents a study of PCM-metal foam composite systems for solar energy storage. At
first, a brief overview of the relevant thermal enhancement methods with particular
emphasis on metal foam systems is presented. This is followed by the description of a
typical PCM-metal foam composite system and the important parameters governing
its energy storage performance. Different modelling approaches for such systems and
their advantages and disadvantages are presented. The effect of important factors for
metal foam-PCM composite systems are analyzed by performing pore-scale simu-
lations. It is shown that factors such as metal foam porosity, pore size distribution,
foam material, phase change material and overall system size contribute significantly
towards the melting pattern and energy storage characteristics of these systems.

Keywords Solar energy storage · Phase change material · Metal foam · Thermal
enhancement

11.1 Introduction

Solar energy is one of the most promising sources of renewable energy. It is uni-
versally available, can be harnessed safely and relatively easily, and is unlimited in
nature. Solar energy can be harnessed at various scales starting from small roof-top

A. Bhattacharya (B)
School of Mechanical Sciences, IIT Bhubaneswar, Bhubaneswar, Odisha 752050, India
e-mail: anirban@iitbbs.ac.in
© Springer Nature Singapore Pte Ltd. 2020 207
H. Tyagi et al. (eds.), Solar Energy, Energy, Environment,
and Sustainability, https://doi.org/10.1007/978-981-15-0675-8_11
208 A. Bhattacharya

units to large scale solar power plants. One of the major drawbacks of solar energy is
its intermittent nature. Not only the solar energy is unavailable at night, during day-
time also the availability of solar insolation varies significantly depending on local
weather pattern and cloud formation. This necessitates the use of suitable energy
storage devices which can distribute the available energy evenly over the required
time period.

11.1.1 Types of Energy Storage

In the recent past, several forms of energy storage devices have been proposed. Energy
storage devices can use different mechanisms for energy storage and can store the
energy in different forms such as chemical energy, mechanical energy, thermal energy
and electrical energy. The suitability of the type of energy storage device depends
on the mechanism of solar energy collection. For example, thermal energy storage
devices may be better suited for solar thermal power plants while chemical energy
based batteries or electrical storage units may be more suitable for direct electricity
generation through the use of photovoltaic (PV) cells. Other important factors such
as safety, ease of installation and operation and maintenance costs also need to be
accounted for. Out of all the different modes of energy storage proposed, thermal
energy storage systems show a lot of potential and have been studied widely.

11.1.2 Thermal Energy Storage Systems

Thermal energy storage systems are systems which store energy in the form of heat.
Depending on the form of energy storage, thermal energy storage systems can be
classified into sensible heat based systems and latent heat based systems. Sensible
heat based systems, such as molten salt thermocline systems, store the entire energy
by temperature increase of the storage medium. Various designs have been studied
such as single medium storage where the heat transfer fluid itself acts as the storage
medium. On the other hand dual media systems have a filler material in addition to
the charging and discharging fluid which is cost effective for large systems. Latent
heat based systems store energy in terms of latent heat transfer during phase change
process. It consists of a suitable material which melts and solidifies at the required
temperature and stores energy during melting and releases it during solidification.
Compared to other forms of energy storage, latent heat energy storage (LHES)
systems have high storage densities which leads to smaller overall system sizes.
Also, as the phase change process occurs at a constant temperature or over a narrow
band of temperature, LHES systems can be designed to operate at specific required
temperatures. Various materials have been used for LHES systems ranging from
organic compounds such as fatty acids and esters to inorganic salts, collectively
called as phase change materials (PCM). A classification of different types of PCM
11 PCM-Metal Foam Composite Systems for Solar Energy Storage 209

is present in Pielichowska and Pielichowski (2014), Zalba et al. (2003) and Nazir
et al. (2019). The main factor for the selection of storage medium is the operating
temperature of the system. PCM based energy storage systems have been discussed
in details in Sharma et al. (2009), Kenisarin and Mahkamov (2007), Farid et al.
(2004), Agyenim et al. (2010) and Xu et al. (2015).

11.1.3 Thermal Enhancement of Latent Heat Energy Storage

Systems

Among all the energy storage media, PCMs, in particular organic PCMs, have been
studied and used most widely. PCMs have high latent heat of fusion and thus can
store large quantities of energy in a limited size. However, PCMs typically have low
thermal conductivities. As a result, the full storage potential of a system cannot be
used if the operating time cycle is of short duration as the entire PCM cannot be
melted in that time. Various mechanisms for increasing the heat transfer in the PCM
has been proposed such as the use of high thermal conductivity fins, use of flow
channels, use of filler materials, nanoparticles, encapsulated PCM distributed in the
heat transfer fluid and porous metal matrix structures such as metal foam. Extensive
discussion about various thermal enhancement mechanisms have been presented in
Fan and Khodadadi (2011), Liu et al. (2016), Ibrahim et al. (2017), Qureshi et al.
(2018) and Lin et al. (2018).

11.2 PCM-Metal Foam Hybrid System

11.2.1 Concept

The main concept of thermal enhancement of PCM based LHES systems involves
the use of a high thermal conductivity secondary material whose main function is
to distribute the heat quickly to different parts of the energy storage unit. Increasing
the amount of this material increases the overall heat transfer rate of the system but
at the cost of reduced energy storage due to the reduced amount of PCM present
in the system. Another important parameter for designing these systems is the area
of contact between the PCM and the high thermal conductivity material. Based on
these criteria, metal foam structures are among the best mechanisms for thermal
conductivity enhancement of PCM based energy storage systems.
Metal foams are metallic structures with high porosity. As a result, they have low
weight but very high surface area. Metal foam structures with different geometries can
be produced such as structures with regular geometries consisting of arranged array
of cells or irregular geometries with randomly distributed pores. At lower porosity,
the foam structure may consist of large number of pores embedded within a metallic
210 A. Bhattacharya

Fig. 11.1 Typical metal

foam structure with irregular
geometry

base while at very high porosity the foam structure resembles an intricate network
of metallic wires. Metal foam structures also depend on the manufacturing method.
Metal foams are typically produced by foaming agents generating gas bubbles during
solidification of metal from a melt pool or by passing a foaming gas during solidifi-
cation. This results in an irregular geometry with pores of various sizes interlinked
with each other. Foam with regular geometry can be created through direct molding
or additive manufacturing techniques. Figure 11.1 shows the structure of a typical
metal foam with irregular geometry.
These metallic foam structures may be filled with PCM to develop metal foam-
PCM composite structures for thermal energy storage. Due to the large contact area,
significant amount of heat transfer can occur between the metal foam and PCM.
Also due to the low volume fraction of metals in metal foams, the quantity of PCM
can be relatively high resulting in higher energy storage capacity as compared to
other thermal enhancement mechanisms such as use of fins. The intricate network
of metallic structure can transfer heat to all parts of the PCM such that there are no
large areas of continuous PCM, unlike metal fin systems.

11.2.2 Design and Construction

Composite metal foam-PCM systems typically have a cylindrical or rectangular

cuboid geometry. The external container contains the metal foam whose pores are
filled with PCM. Air gaps need to be removed to preserve high effective thermal
conductivity of the system. The entire structure is insulated to prevent heat loss.
11 PCM-Metal Foam Composite Systems for Solar Energy Storage 211

Energy is transferred to the system by heating it from one side. This may be directly
heated by incoming solar radiation or more typically heated by the heat transfer
fluid circulated from the collector. Multiple metal foam-PCM composite systems are
needed to limit the size of each storage unit.

11.2.3 Materials

Various materials have been proposed and tested for the metal foam structure as well
as for the PCM. Aluminum and copper are the most widely tested material for metal
foam as they offer excellent thermal conductivity (Hong and Herling 2006; Siahpush
et al. 2008; Wang et al. 2016; Mancin et al. 2015; Xiao et al. 2013). Graphite foams,
although not technically metal, have also been tested (Ji et al. 2014; Lafdi et al.
2008; Zhang et al. 2012; Zhao et al. 2014). For the PCM, the choice of material
depends on the temperature range of application as well as its suitability for use with
a metallic foam. The PCM must not react with the metal foam structure or corrode
the surface of the foam. Typical materials which can be used are different types of
organic compounds such as paraffin.

11.3 Important Design Parameters for a PCM-Metal Foam

System

The performance of a composite metal-foam PCM system depends on various factors.

The important factors are: thermo-physical properties of the foam material and PCM,
the foam structure, foam porosity and overall storage unit size and aspect ratio. Each
of these is discussed in the following sub-sections.

11.3.1 Thermo-physical Properties of Foam Material

The properties of the foam material play a vital role in the performance of the storage
system. The foam material should have very high thermal conductivity and high
specific heat for better heat transfer and sensible energy storage characteristics. Low
density of the material is desirable for reducing the overall weight of the system.
However, low density also leads to lower sensible heat storage and thus reduces
the energy storage capacity of the system. Low thermal coefficient of expansion
is necessary so that the foam structure does not change much during the charging
or discharging process. Relatively good mechanical properties such as strength are
212 A. Bhattacharya

also desirable to maintain the integrity of the system. The foam material and PCM
combination should also have high wettability to prevent formation of air pockets
due to shrinkage of PCM during solidification.

11.3.2 Thermo-physical Properties of PCM

The most important property of PCM is high latent heat of fusion as this is the critical
factor which governs the total energy storage capacity of the system. PCM should
also have relatively high thermal conductivity for better heat transfer to the interior
regions and high specific heat for better sensible heat storage. As mentioned before,
PCM should also have high wettability with the metal foam structure. Another impor-
tant property is the change in density during solidification and melting. Typically,
PCMs undergo shrinkage due to volume contraction during solidification. Ideally,
shrinkage should be as minimum as possible. Shrinkage can be very detrimental
to the performance of energy storage system as it can lead to the formation of air
pockets between the metal foam and PCM thus drastically reducing the effective
thermal conductivity of the system. Other important properties of PCM are chemical
inertness and stability. Initial filling up of the metal foam with PCM also requires
the PCM to have lower viscosity.

11.3.3 Foam Porosity

The overall porosity of the metal foam has to be carefully chosen based on detailed
analysis. Lower porosity results in higher heat transfer rates leading to faster charging
and discharging of energy. On the other hand, higher porosity, leads to larger volume
fraction of PCM leading to higher overall energy storage capacity. The sensible heat
capacity of the metal foam and PCM needs to be considered to calculate the required
porosity of the system.

11.3.4 Foam Structure

The structure of the metal foam plays an important role in the performance of a
metal foam-PCM energy storage system. It has been shown that foams with lower
pore size and higher pore density have higher heat transfer rates due to the more
intricate network of metal structure (Lafdi et al. 2007; Ren et al. 2017; Dinesh and
Bhattacharya 2019). Also, open cell foams with high porosity enables significant
convective heat transfer which contributes towards the overall heat transfer rate. On
the other hand, closed foams or nearly closed foam structures have conduction as
the dominant heat transfer mechanism. Foams with regular structure and irregular
structure may also have different heat transfer characteristics.
11 PCM-Metal Foam Composite Systems for Solar Energy Storage 213

11.3.5 Overall Size and Aspect Ratio

The overall dimensions of the storage unit has to be designed such that the energy
charging and discharging can be completed during the required time period. How-
ever, if the storage unit is too small, a large number of units will be necessary thus
increasing the overall cost. Aspect ratio also is an important parameter as it defines
the farthest distance of PCM from the heat transfer surface. A broad and shallow
design where the wall opposite to the heat transfer surface is relatively near is better
as energy can be transported to the entire PCM in less time.

11.4 Previous Studies on PCM-Metal Foam Hybrid

Systems

11.4.1 Experimental Studies

A number of experimental investigations have been performed to study the effect

of aluminum, copper and nickel foam on the heat transfer characteristics of PCM
based storage systems (Siahpush et al. 2008; Xiao et al. 2013, 2014; Zhao et al. 2010;
Chen et al. 2014; Zhu et al. 2018; Zheng et al. 2018). The main findings from the
experimental studies can be summarized as follows.
• Comparison of pure PCM systems and metal foam-PCM systems show that use
of metal foam drastically improves the heat transfer characteristics of the system
and the effective thermal conductivity of the composite structure is many times
higher than that of the PCM.
• Metal foam structure plays an important role in determining the system character-
istics. Lower porosity leads due to higher heat conduction whereas higher porosity
leads to higher convection and energy storage capacity.
• Foam material has a strong influence on the heat transfer.

11.4.2 Numerical Modelling

Numerical modelling of metal foam-PCM composite systems mainly follows two

approaches: (a) Volume averaged modelling and (b) Pore-scale modelling.
Volume averaged models. Volume averaged models (Mesalhy et al. 2005; Tian
and Zhao 2011; Srivatsa et al. 2014; Zhang et al. 2017; Yang et al. 2018; Sundarram
and Li 2014; Kumar and Saha 2018) assume that the entire composite structure is
a porous medium and formulates the governing equations for energy balance for
this porous medium. Two different approaches are taken. Basic models assume that
214 A. Bhattacharya

the metal foam and PCM are in local thermal equilibrium and thus a single nodal
temperature variable can specify the temperature evolution of the system. All the
properties at the discretized nodes are calculated based on volume averaging the
foam and PCM properties. For these models, a single energy balance equation in
terms of the average temperature is sufficient to simulate the system. The volume
averaged energy equation for this case can be written as

∂(C pav T + ε fl L) 
ρav + ερ PC M ∇.(U C p PC M T ) = ∇.(ke f f ∇T ) (11.1)
∂t

In Eq. 11.1, T is the temperature, ρ is the density, C p is the specific heat, U is the
velocity of liquid PCM, f l is the liquid fraction of PCM, L is the latent heat of fusion
of PCM, and subscripts ‘av’ and ‘PCM’ denote the average values and values for
PCM, respectively. k eff is the effective thermal conductivity which can be formulated
in terms of the metal foam porosity ε. The main drawback of the single equation
approach is that it does not capture the local thermal non-equilibrium between the
metal and PCM and thus cannot calculate the heat transfer between the metal foam
and PCM.
The other approach for volume averaged models considers separate energy bal-
ance equations for the metal foam and for the PCM (Yang et al. 2018; Kumar and
Saha 2018). The energy equation for metal foam can be written as follows.

∂(C pm Tm )
(1 − ε)ρm = ∇.(km,e f f ∇Tm ) − h int aint (Tm − TPC M ) (11.2)
∂t
The energy equation for the PCM can be written as follows.

ερ PC M ( p PC M ∂tPC M l ) + ερ PC M ∇.(U C p PC M TPC M )

∂ C T +f L 

(11.3)
= ∇.(k PC M,e f f ∇TPC M ) − h int aint (TPC M − Tm )

In Eqs. 11.2 and 11.3, T m and T PCM denote the temperature of the metal foam and
PCM, respectively. Subscripts ‘m’ and ‘PCM’ denote the properties of metal and
PCM. The last terms in each equation represent the interface heat transfer between the
metal foam and PCM. hint is the interface heat transfer coefficient and aint is the spe-
cific area of the interface. For equilibrium models, it is assumed that the temperature
of PCM and metal at the PCM-metal foam interface is equal, i.e. T m = T PCM . On the
other hand, non-equilibrium models assume that the temperature of PCM and metal
are different at the interface. For this case, the two energy equations are coupled by
using the interface heat transfer term, h int aint (Tm − TPC M ), as given in Eqs. 11.2 and
11.3. Although these models can capture the local temperature differences between
the metal and PCM, defining the interface heat transfer coefficient (h int ) between
the metal and PCM is challenging. Different models have been proposed for metal
11 PCM-Metal Foam Composite Systems for Solar Energy Storage 215

foam-PCM systems using the two equation non-equilibrium approach with different
interface heat transfer coefficients (Mesalhy et al. 2005; Srivatsa et al. 2014; Yang
et al. 2018).
Volume averaged models can predict the overall heat transfer characteristics and
melting pattern. However, they cannot capture the localized heat transfer between the
metal foam and PCM accurately. This is particularly problematic for systems which
have similar porosities but different foam structures as the governing equations for
the volume averaged models are functions of porosities. These models also may not
capture the convection pattern accurately as convection in metal foams is dependent
on the foam structure itself.

Pore-scale models. The primary distinguishing feature of pore-scale models (Ren

et al. 2017, 2018; Dinesh and Bhattacharya 2019; Abishek et al. 2018; Deng et al.
2017) is that they resolve the metal foam structure in the model. Metal foam geometry
is modelled by using various geometry creation techniques (Dinesh and Bhattacharya
2019; Boomsma et al. 2003; Wang and Pan 2008; Abishek et al. 2017). It is assumed
that the porous region in the metal foam is filled with PCM. These models can capture
the heat transfer between the metal foam and PCM accurately as the effect of foam
structure is present in the model. Simulations based on these models have shown
that the pore structure plays an important role in determining the energy transfer
characteristics of the system (Ren et al. 2017; Dinesh and Bhattacharya 2019). The
main drawback of pore-scale models is the increased computational effort required
as very fine numerical grid is necessary to capture the foam structure. A typical
pore-scale model (Dinesh and Bhattacharya 2019) has been discussed in details in
Sect. 11.5.1.

11.5 Case-Studies

11.5.1 Numerical Model and Problem Description

In this section, the effects of important design parameters described in Sect. 11.3
are studied by performing pore-scale simulations of melting for a metal foam-PCM
composite energy storage system. Five different comparative studies are performed to
see the effect of different foam material, different PCM, change in porosity, change
in pore size and overall system size. For performing the simulations, the model
described in (Dinesh and Bhattacharya 2019) is used. The main characteristics of the
model are described briefly in the subsequent paragraphs. For more details, one can
refer to Dinesh and Bhattacharya (2019).
Problem description. For the comparative studies, a cuboidal domain with heating
from the bottom is considered. It is assumed that all the other sides are insulated.
The domain is initially held at a certain temperature below the melting temperature
216 A. Bhattacharya

of the PCM. Heat transfer occurs due to heating from the bottom boundary and at
first sensible energy absorption occurs. Subsequently, the PCM starts melting and
energy is absorbed as latent heat.

Geometry creation model. For simulating the stated problem a coupled method is
used (Dinesh and Bhattacharya 2019). The method combines a geometry creation
model with a melting and solidification solver. The geometry creation model assumes
that the 3-dimensional cuboidal domain is filled with overlapping pores of different
sizes. The entire geometry creation process is performed using the following algo-
rithm.
• At first, the cuboidal domain is created based on the length of its three sides.
• Pores are represented by spheres. Each sphere is defined by its radius (r) and
co-ordinates of its center (x c , yc , zc ). The minimum and maximum sphere radius
(r min , r max ) are input parameters specified by the user. To insert a sphere in the
domain, a random location within the domain is selected as the center coordinates
for the sphere. The radius of the sphere is randomly selected using a random
number generator subjected to the upper and lower bounds of r max and r min , i.e.
rmin ≤ r ≤ rmax . After the coordinates of its center (x c , yc , zc ) and radius (r) are
fixed, a sphere can be considered to be generated in the domain.
• To insert the subsequent sphere, another parameter, the sphere overlap, needs to
be defined. The sphere overlap parameter denotes whether two spheres intersect
each other or not. The  distance between the centers of the two spheres can be
calculated using d = (xn − xc )2 + (yn − yc )2 + (z n − z c )2 where x n , yn , zn are
the coordinates of the center of the new sphere. Sphere overlap is defined as
q = d/(r + rnew ) where rnew is the radius of the new sphere. If q is less than 1, it
means that the two spheres intersect each other. This type of interaction between
the spheres will lead to open cell type foam structure. If q is greater than 1, it
means that the two spheres do not intersect each other leading to closed cell foam
structure. At the limiting value of 1, the two spheres touch each other at a single
point. For the geometry creation model, the minimum and maximum values of
pore overlap are specified by the user.
• To insert a second sphere, the sphere radius is randomly assigned using the criteria
rmin ≤ rnew ≤ rmax and a random location is selected as the coordinates for the
center of the second sphere with random overlap subjected to the minimum and
maximum overlap bounds specified previously.
• Subsequently, spheres are generated one by one in a sequential manner following
the same procedure. For each sphere, its overlap with all the existing spheres has
to be checked. However, to make the algorithm computationally efficient, only the
spheres within a certain distance from the center of the new spheres are considered
for this overlap calculation.
• After generation of each sphere, the total volume occupied by the spheres is cal-
culated and the resultant porosity is checked. If the resultant porosity reaches or
exceeds the specified foam porosity, the generation of spheres is stopped.
11 PCM-Metal Foam Composite Systems for Solar Energy Storage 217

• After the spheres are generated, the generated geometry needs to be mapped to a
metal volume fraction parameter, (ϕ), to differentiate the metal and PCM in the
discretized problem domain. To do this, the entire domain is divided into a very
fine uniform grid structure. The grid points which are within any of the spheres
are assigned a value of 0 and the grid points outside all the spheres are assigned
a value of 1. Subsequently, a new relatively coarse mesh is generated. Each grid
point in the new mesh correspond to several grid points in the original mesh. The
value of metal volume fraction (ϕ) is calculated for each grid point of the new
mesh by summing all the values of 1 and 0 for the corresponding grid points in the
original mesh and dividing by the number of original grid points corresponding to
a single grid point in the new mesh.
• The value of ϕ for all the grid points in the new mesh denote the metal fraction
for the entire geometry. This new mesh is used for all the subsequent calculations
and the old mesh is discarded. The volume fraction of PCM at each node is equal
to (1 − ϕ).
Figure 11.2a shows a typical metal foam surface generated using this method.
Figure 11.2b shows the corresponding metal foam structure filled with PCM.

Phase change model. Melting and solidification of PCM is simulated by using

the enthalpy method (Voller 2008; Bhattacharya and Dutta 2013). The process is
governed by the energy conservation equation (Eq. 11.4) which is formulated in
terms of volume averaged enthalpy as given in Eq. 11.5 (Dinesh and Bhattacharya
2019).

∂H
ρ = ∇.(K ∇T ) (11.4)
∂t

Fig. 11.2 Sample geometry generated by the model for 75% porosity. a Foam structure. b PCM-
metal foam composite (red represents the metal foam and blue represents the PCM)
218 A. Bhattacharya

The volume average enthalpy is defined in terms of the sensible and latent heat as

H = [C p pcm (1 − ϕ) + C pm ϕ]T + (1 − ϕ) f L (11.5)

The volume fraction of liquid PCM is tracked using the nodal melt fraction param-
eter f. In Eqs. 11.4 and 11.5, ρ is the average density, K is the average thermal
conductivity and C p pcm and C pm are the specific heat of PCM and metal, respectively.
The detailed algorithm for solving the governing equation is described in Dinesh
and Bhattacharya (2019). The model has been previously validated with experimen-
tal results given in Chen et al. (2014) and verified with 1D analytical solutions for
melting and solidification (Dinesh and Bhattacharya 2019).

Standard case. In the present work, the model is applied to study the effect of the
5 different parameters described in Sect. 11.3. For all the 5 cases, a base system is
considered with aluminum foam and paraffin as PCM and with the specifications
given in Table 11.1. The thermo-physical properties of aluminum is specified in
Table 11.2 while that for paraffin is presented in Table 11.3. A cuboidal domain is
taken with a constant high temperature boundary condition of 373 K at the bottom
surface. All the other sides are kept adiabatic. The entire domain is initially at 303 K.
For each case, one of the parameter is changed while keeping all the other parameters
constant and simulation results are compared to see its effect on temperature evolution
and melting pattern. The total energy absorbed and the latent heat stored are also
compared.

Table 11.1 Specifications for

Parameter Specification
the base case
Foam material Aluminum
PCM Paraffin
Foam porosity (%) 75
Minimum pore radius (cm) 0.5
Maximum pore radius (cm) 3
Domain side length (cm) 20
Initial temperature (K) 303
Hot boundary temperature (K) 373
Grid spacing for geometry model (mm) 0.625
Grid spacing for phase change model (mm) 2.5
Time step (s) 0.01

Table 11.2 Thermo-physical

Property Aluminum Copper
properties of aluminum (Al)
and copper (Cu) Thermal conductivity (W/m K) 205 401
Specific heat (J/kg K) 910 385
Density (kg/m3 ) 2830 8960
11 PCM-Metal Foam Composite Systems for Solar Energy Storage 219

Table 11.3 Thermo-physical

Property Paraffin Naphthalene
properties of paraffin and
naphthalene Latent heat (kJ/kg) 169 147.7
Thermal conductivity (W/m K) 0.2 0.132
Specific heat (J/kg K) 2100 1563
Density (kg/m3 ) 880 976
Melting temperature (K) 335 353

11.5.2 Effect of Foam Material

For this comparison, two different cases are considered—one with aluminum foam
and the other with copper foam. The thermo-physical properties of copper is specified
in Table 11.2. All the other parameters, such as porosity, pore size, PCM and domain
size are kept same as that mentioned in Table 11.1. Figure 11.3 shows the temperature
contours for the two systems at time t = 500 and 1000 s. It can be seen that for
the copper foam system the temperature evolution is faster. This is because of the

Fig. 11.3 Temperature contours at the mid-section. a Aluminum foam at t = 500 s. b Aluminum
foam at t = 1000 s. c Copper foam at t = 500 s. b Copper foam at t = 1000 s
220 A. Bhattacharya

significantly higher thermal conductivity of copper. The density of copper is about

3 times as that of aluminum and thus the mass of copper is higher for the same
overall porosity. However, copper has significantly lower specific heat as compared
to aluminum. Thus the overall sensible heat absorption in copper is not markedly
higher. The main difference in the system comes from the faster heat transfer to the
PCM with use of copper foam which results in faster melting of PCM. This is evident
from the liquid fraction contours for the two systems shown in Fig. 11.4.
The variation of total energy absorption and latent heat absorption with time is
compared in Fig. 11.5a. It can be observed that use of Cu foam results in increase
of approximately 20% in total heat absorption and approximately 23% in latent heat
absorption for the given configuration. The variation of overall liquid fraction with
time, as shown in Fig. 11.5b, confirms the higher melting rate of PCM with embedded
Cu foam.

Fig. 11.4 Liquid fraction at the mid-section. a Aluminum foam at t = 500 s. b Aluminum foam at
t = 1000 s. c Copper foam at t = 500 s. d Copper foam at t = 1000 s
11 PCM-Metal Foam Composite Systems for Solar Energy Storage 221

Fig. 11.5 Comparison of aluminum and copper foam. a Variation of energy absorption with time.
b Variation of overall melt fraction with time

11.5.3 Effect of Phase Change Material

For this comparison, two different PCMs are considered—paraffin wax and naph-
thalene. All the other parameters are same as that stated in Table 11.1. The thermo-
physical properties of naphthalene is specified in Table 11.3. Figure 11.6 shows the
temperature contours for the two systems at time t = 500 and 2000 s. It can be seen
that the temperature is higher in the naphthalene based system. This is because of the
higher melting temperature of naphthalene. Figure 11.7 shows the liquid fraction of
PCM at time t = 500 and 2000 s. It can be seen that melting occurs at a faster rate for
the paraffin based system. For this system, due to the lower melting temperature of
paraffin, less sensible heating is required to reach the melting temperature and more
energy can be stored as latent heat leading to higher melting rate.
The variation of total energy absorbed and the latent heat absorbed for the two
systems with time are compared in Fig. 11.8a. It is seen that both the quantities are
higher for the paraffin based system. However, the difference in latent heat absorption
is significantly higher as compared to the total energy absorption. This suggest that
the sensible heat absorption is higher for the naphthalene based system. For this
case, most of the energy is absorbed as sensible heat during the initial period as the
melting temperature of naphthalene is higher. Figure 11.8b shows a comparison of the
variation of overall liquid fraction for the two systems with time. It confirms that the
rate of melting is significantly higher for the paraffin based system. This study shows
that the PCM should be selected depending on the energy absorption characteristics
required and the necessary temperature range, as systems based on different PCM
have significantly different heat absorption characteristics and melting pattern.
222 A. Bhattacharya

Fig. 11.6 Temperature contours at the mid-section. a With paraffin at t = 500 s. b With paraffin at
t = 2000 s. c With naphthalene at t = 500 s. d With naphthalene at t = 2000 s

11.5.4 Effect of Porosity

To study the effect of porosity, two systems are considered—one with 75% porosity
and the other with 50% porosity. Although the porosity is different, the pore size
variation is kept at the same range. The geometries generated by the model for the
two systems are presented in Fig. 11.9. It is observed that the 75% porosity system
has considerably more number of pores. All the other parameters are kept same as
that specified in Table 11.1. Figure 11.10 shows the temperature contours for the
two systems at time t = 500 and 2000 s. It can be observed that the temperature is
higher for the 50% porosity system. Also, there are localized low temperature regions
which correspond to the interior sections for large pores filled with PCM. For the
75% porosity system, the temperature profile has a more uniform gradient from the
bottom surface to the top surface. The corresponding liquid fraction contours are
presented in Fig. 11.11. It is seen that, for the 50% porosity system, melting near the
top surface starts faster. The 50% system has lower amount of PCM. As a result more
energy can contribute towards sensible heating and the system reaches the melting
11 PCM-Metal Foam Composite Systems for Solar Energy Storage 223

Fig. 11.7 Liquid fraction at the mid-section. a With paraffin at t = 500 s. b With paraffin at t =
2000 s. c With naphthalene at t = 500 s. d With naphthalene at t = 2000 s

Fig. 11.8 Comparison of paraffin and naphthalene as PCM. a Variation of energy absorption with
time. b Variation of overall melt fraction with time
224 A. Bhattacharya

Fig. 11.9 Geometries generated for a 75% porosity; b 50% porosity (red represents the metal foam
and blue represents the PCM)

Fig. 11.10 Temperature contours at the mid-section with a 75% porosity at t = 500 s; b 75%
porosity at t = 2000 s; c 50% porosity at t = 500 s; d 50% porosity at t = 2000 s
11 PCM-Metal Foam Composite Systems for Solar Energy Storage 225

Fig. 11.11 Liquid fraction at the mid-section a 75% porosity at t = 500 s; b 75% porosity at t =
2000 s; c 50% porosity at t = 500 s; d 50% porosity at t = 2000 s

temperature of PCM faster. Also, in this case due to the higher amount of metal, the
effective thermal conductivity of the system is higher leading to higher heat transfer
rate.
The variation of total energy absorbed and the latent heat absorbed for the two
systems with time are compared in Fig. 11.12a. It is observed that the rate of energy
transfer is higher for the 50% porosity system. However, although the rate of latent
heat absorption is initially higher for the 50% porosity system, at later time period, the
75% porosity system has higher latent heat transfer. This shows that sensible heating
is significantly higher for the lower porosity system as larger volume fraction of
metal is present in this case. Figure 11.12b shows the variation of liquid fraction
with time for the two systems. It is seen that initially the liquid fraction increases
at a faster rate for the 50% porosity system. However towards the end, it saturates
as most of the PCM melts and only centers of large pores still remain solid. For the
75% porosity system the liquid fraction keeps on increasing steadily for the given
time period. It should be noted here that, although the liquid fraction of PCM for
the 75% porosity system is lower at the end of the simulation, the actual latent heat
226 A. Bhattacharya

Fig. 11.12 Comparison of systems with 75% porosity and 50% porosity. a Variation of energy
absorption with time. b Variation of overall melt fraction with time

absorption is higher as the total volume of PCM is higher for this system. Higher
heat transfer rate with lower porosity of metal foam has been previously observed
experimentally by Lafdi et al. (2007).

11.5.5 Effect of Pore Size

To study the effect of pore size, two systems are considered with varying pore
sizes. The geometry for the first system is generated with a pore radius (r) range
of 0.5–3.0 cm while for the second system, the geometry is created with a pore

Fig. 11.13 Geometries generated for a pore size 0.5 < r < 3 cm; b pore size 0.5 < r < 1 cm (red
represents the metal foam and blue represents the PCM)
11 PCM-Metal Foam Composite Systems for Solar Energy Storage 227

Fig. 11.14 Temperature contours at the mid-section with a 0.5 < r < 3 cm at t = 500 s; b 0.5 < r <
3 cm at t = 2000 s; c 0.5 < r < 1 cm at t = 500 s; d 0.5 < r < 1 cm at t = 2000 s

radius range of 0.5–1.0 cm. Hence the second system has significantly lower average
pore size. It should be noted here that although the pore size is varied for the two
systems the porosity is kept constant and equal to 75%. The geometries generated
by the model for the two systems are presented in Fig. 11.13. It is observed that for
the second system large pores are not present. All the parameters are kept same as
that given in Table 11.1. Figure 11.14 shows the temperature contours for the two
systems at time t = 500 and 2000 s. It is seen that the first system has localized low
temperature regions as some of the pores are significantly large and energy transfer
for those pores is very slow due to lower thermal conductivity of PCM. The sec-
ond system shows uniform temperature gradient from the bottom surface to the top
surface. For this case, as all the pores are smaller, all the PCM inside the pores are
relatively near to metal foam structures and thus energy transfer can occur faster in
this case. Figure 11.15 shows the corresponding liquid fraction contours. It confirms
that the larger pores still contain a significant amount of solid PCM.
The variation of total energy absorbed and the latent heat absorbed for the two
systems with time are compared in Fig. 11.16a. It is seen that both the quantities
are slightly higher for the second system. As explained previously, the smaller pore
228 A. Bhattacharya

Fig. 11.15 Liquid fraction at the mid-section a 0.5 < r < 3 cm at t = 500 s; b 0.5 < r < 3 cm at t =
2000 s; c 0.5 < r < 1 cm at t = 500 s; d 0.5 < r < 1 cm at t = 2000 s

Fig. 11.16 Comparison of systems with different pore size distribution. a Variation of energy
absorption with time. b Variation of overall melt fraction with time
11 PCM-Metal Foam Composite Systems for Solar Energy Storage 229

size of the second system results in faster energy transfer to the PCM. The variation
of liquid fraction with time for the two systems are compared in Fig. 11.16b which
confirms that melting occurs at slightly faster rate if the average pore size is less.
This study is important as it shows that although two systems may have exactly same
material and porosity, variation of pore size leads to different energy absorption rates.
Hence for designing a composite metal foam-PCM system, the pore size of the foam
should also be taken into consideration. Similar results, predicting the increase in
heat transfer with decrease in pore size, has been obtained by pore scale modelling
by Ren et al. (2017).

11.5.6 Effect of Overall System Size

For this study two different cubic domains are considered—with side lengths of
20 and 10 cm, respectively. All the other parameters are same as that specified in
Table 11.1. Figure 11.17 shows the temperature contours for the two systems at time

Fig. 11.17 Temperature contours at the mid-section for domain of side length a 20 cm at t = 500 s;
b 20 cm at t = 2000 s; c 10 cm at t = 500 s; d 10 cm at t = 2000 s
230 A. Bhattacharya

t = 500 and 2000 s. Although the size is different, the plots for the small system are
zoomed to the same size as those of the larger system. It is seen that the temperature
is significantly higher for the smaller system as heat transfer occurs at a faster rate
towards the top surface due to the lesser distance from the heating surface. This is
also evident from the liquid fraction contours shown in Fig. 11.18 which shows that
the initial melting rate is very high for the smaller system. It should be noted here that
although the overall volume has been reduced by a factor of 8 for the second system,
the heat transfer surface area is only 4 times smaller. Hence more heat transfer can
occur per unit volume for the second case.
The variation of total energy absorbed and the latent heat absorbed for the two
systems with time are compared in Fig. 11.19a. It is seen that both the quantities
are higher for the larger system. However, although the system volume is 8 times
more, the rate of energy absorption is significantly less than 8 times as that for
the smaller system. This means that 8 smaller systems, which will have the same
volume of PCM and metal as that for the larger system, will have significantly higher

Fig. 11.18 Liquid fraction at the mid-section for domain of side length a 20 cm at t = 500 s;
b 20 cm at t = 2000 s; c 10 cm at t = 500 s; d 10 cm at t = 2000 s
11 PCM-Metal Foam Composite Systems for Solar Energy Storage 231

Fig. 11.19 Comparison of systems with different overall size. a Variation of energy absorption
with time. b Variation of overall melt fraction with time

energy absorption rate. However, more heat transfer area will be required for this
case. Figure 11.19b shows the variation of overall liquid fraction for the two systems
which confirms that the rate of melting is higher for the smaller system.

11.6 Summary

This chapter presents a study of metal foam-PCM composite systems for energy
storage. It has been previously shown that metal foams can be very effective in
increasing the overall heat transfer rate for PCM based energy storage systems due
to their high conductivity, intricate network and large surface area. In this chapter,
various factors which can affect the energy absorption characteristics of metal foam-
PCM systems are considered and analyzed. Pore-scale simulations of melting in metal
foam-PCM systems are performed by varying parameters such as foam porosity,
pore size distribution, foam and energy storage material and overall system size.
The energy absorption characteristics, temperature evolution and melting pattern
are compared to quantify the effect of these parameters. Results show that the each
of these factors is important and hence should be taken into consideration while
designing such systems.
232 A. Bhattacharya

References

Abishek S, King AJC, Mead-Hunter R, Golkarfard V, Heikamp W, Mullins BJ (2017) Generation

and validation of virtual nonwoven, foam and knitted filter (separator/coalescer) geometries for
CFD simulations. Sep Purif Technol 188:493–507
Abishek S, King AJC, Nadim N, Mullins BJ (2018) Effect of microstructure on melting in metal-
foam/paraffin composite phase change materials. Int J Heat Mass Transf 127:135–144
Agyenim F, Hewitt N, Eames P, Smyth M (2010) A review of materials, heat transfer and phase
change problem formulation for latent heat thermal energy storage systems (LHTESS). Renew
Sustain Energy Rev 14(2):615–628
Bhattacharya A, Dutta P (2013) An enthalpy-based model of dendritic growth in a convecting binary
alloy melt. Int J Numer Methods Heat Fluid Flow 23(7):1121–1135
Boomsma K, Poulikakos D, Ventikos Y (2003) Simulations of flow through open cell metal foams
using an idealized periodic cell structure. Int J Heat Fluid Flow 24(6):825–834
Chen Z, Gao D, Shi J (2014) Experimental and numerical study on melting of phase change materials
in metal foams at pore scale. Int J Heat Mass Transf 72:646–655
Deng Z, Liu X, Zhang C, Huang Y, Chen Y (2017) Melting behaviors of PCM in porous metal foam
characterized by fractal geometry. Int J Heat Mass Transf 113:1031–1042
Dinesh BVS, Bhattacharya A (2019) Effect of foam geometry on heat absorption characteristics of
PCM-metal foam composite thermal energy storage systems. Int J Heat Mass Transf 134:866–883
Fan L, Khodadadi JM (2011) Thermal conductivity enhancement of phase change materials for
thermal energy storage: a review. Renew Sustain Energy Rev 15(1):24–46
Farid MM, Khudhair AM, Razack SAK, Al-Hallaj S (2004) A review on phase change energy
storage: materials and applications. Energy Convers Manag 45(9–10):1597–1615
Hong ST, Herling DR (2006) Open-cell aluminum foams filled with phase change materials as
compact heat sinks. Scripta Mater 55(10):887–890
Ibrahim NI, Al-Sulaiman FA, Rahman S, Yilbas BS, Sahin AZ (2017) Heat transfer enhancement of
phase change materials for thermal energy storage applications: a critical review. Renew Sustain
Energy Rev 74:26–50
Ji H, Sellan DP, Pettes MT, Kong X, Ji J, Shi L, Ruoff RS (2014) Enhanced thermal conductivity of
phase change materials with ultrathin-graphite foams for thermal energy storage. Energy Environ
Sci 7(3):1185–1192
Kenisarin M, Mahkamov K (2007) Solar energy storage using phase change materials. Renew
Sustain Energy Rev 11(9):1913–1965
Kumar A, Saha SK (2018) Latent heat thermal storage with variable porosity metal matrix: a
numerical study. Renew Energy 125:962–973
Lafdi K, Mesalhy O, Shaikh S (2007) Experimental study on the influence of foam porosity and
pore size on the melting of phase change materials. J Appl Phys 102(8):083549
Lafdi K, Mesalhy O, Elgafy A (2008) Graphite foams infiltrated with phase change materials
as alternative materials for space and terrestrial thermal energy storage applications. Carbon
46(1):159–168
Lin Y, Jia Y, Alva G, Fang G (2018) Review on thermal conductivity enhancement, thermal proper-
ties and applications of phase change materials in thermal energy storage. Renew Sustain Energy
Rev 82:2730–2742
Liu L, Su D, Tang Y, Fang G (2016) Thermal conductivity enhancement of phase change materials
for thermal energy storage: a review. Renew Sustain Energy Rev 62:305–317
Mancin S, Diani A, Doretti L, Hooman K, Rossetto L (2015) Experimental analysis of phase change
phenomenon of paraffin waxes embedded in copper foams. Int J Therm Sci 90:79–89
Mesalhy O, Lafdi K, Elgafy A, Bowman K (2005) Numerical study for enhancing the thermal
conductivity of phase change material (PCM) storage using high thermal conductivity porous
matrix. Energy Convers Manag 46(6):847–867
11 PCM-Metal Foam Composite Systems for Solar Energy Storage 233

Nazir H, Batool M, Osorio FJB, Isaza-Ruiz M, Xu X, Vignarooban K, Kannan AM (2019) Recent

developments in phase change materials for energy storage applications: a review. Int J Heat Mass
Transf 129:491–523
Pielichowska K, Pielichowski K (2014) Phase change materials for thermal energy storage. Prog
Mater Sci 65:67–123
Qureshi ZA, Ali HM, Khushnood S (2018) Recent advances on thermal conductivity enhancement
of phase change materials for energy storage system: a review. Int J Heat Mass Transf 127:838–856
Ren Q, He YL, Su KZ, Chan CL (2017) Investigation of the effect of metal foam characteristics on
the PCM melting performance in a latent heat thermal energy storage unit by pore-scale lattice
Boltzmann modeling. Numer Heat Transf Part A: Appl 72(10):745–764
Ren Q, Meng F, Guo P (2018) A comparative study of PCM melting process in a heat pipe-
assisted LHTES unit enhanced with nanoparticles and metal foams by immersed boundary-lattice
Boltzmann method at pore-scale. Int J Heat Mass Transf 121:1214–1228
Sharma A, Tyagi VV, Chen CR, Buddhi D (2009) Review on thermal energy storage with phase
change materials and applications. Renew Sustain Energy Rev 13(2):318–345
Siahpush A, O’Brien J, Crepeau J (2008) Phase change heat transfer enhancement using copper
porous foam. J Heat Transf 130(8):082301
Srivatsa PVSS, Baby R, Balaji C (2014) Numerical investigation of PCM based heat sinks with
embedded metal foam/crossed plate fins. Numer Heat Transf Part A: Appl 66(10):1131–1153
Sundarram SS, Li W (2014) The effect of pore size and porosity on thermal management per-
formance of phase change material infiltrated microcellular metal foams. Appl Therm Eng
64(1–2):147–154
Tian Y, Zhao CY (2011) A numerical investigation of heat transfer in phase change materials (PCMs)
embedded in porous metals. Energy 36(9):5539–5546
Voller VR (2008) An enthalpy method for modeling dendritic growth in a binary alloy. Int J Heat
Mass Transf 51(3–4):823–834
Wang M, Pan N (2008) Modeling and prediction of the effective thermal conductivity of random
open-cell porous foams. Int J Heat Mass Transf 51(5–6):1325–1331
Wang C, Lin T, Li N, Zheng H (2016) Heat transfer enhancement of phase change composite
material: copper foam/paraffin. Renew Energy 96:960–965
Xiao X, Zhang P, Li M (2013) Preparation and thermal characterization of paraffin/metal foam
composite phase change material. Appl Energy 112:1357–1366
Xiao X, Zhang P, Li M (2014) Effective thermal conductivity of open-cell metal foams impregnated
with pure paraffin for latent heat storage. Int J Therm Sci 81:94–105
Xu B, Li P, Chan C (2015) Application of phase change materials for thermal energy storage in con-
centrated solar thermal power plants: a review to recent developments. Appl Energy 160:286–307
Yang X, Bai Q, Guo Z, Niu Z, Yang C, Jin L, Yan J (2018) Comparison of direct numerical simulation
with volume-averaged method on composite phase change materials for thermal energy storage.
Appl Energy 229:700–714
Zalba B, Marın JM, Cabeza LF, Mehling H (2003) Review on thermal energy storage with phase
change: materials, heat transfer analysis and applications. Appl Therm Eng 23(3):251–283
Zhang Z, Zhang N, Peng J, Fang X, Gao X, Fang Y (2012) Preparation and thermal energy stor-
age properties of paraffin/expanded graphite composite phase change material. Appl Energy
91(1):426–431
Zhang Z, Cheng J, He X (2017) Numerical simulation of flow and heat transfer in composite PCM
on the basis of two different models of open-cell metal foam skeletons. Int J Heat Mass Transf
112:959–971
Zhao CY, Lu W, Tian Y (2010) Heat transfer enhancement for thermal energy storage using metal
foams embedded within phase change materials (PCMs). Sol Energy 84(8):1402–1412
234 A. Bhattacharya

Zhao W, France DM, Yu W, Kim T, Singh D (2014) Phase change material with graphite foam for
applications in high-temperature latent heat storage systems of concentrated solar power plants.
Renew Energy 69:134–146
Zheng H, Wang C, Liu Q, Tian Z, Fan X (2018) Thermal performance of copper foam/paraffin
composite phase change material. Energy Convers Manag 157:372–381
Zhu ZQ, Huang YK, Hu N, Zeng Y, Fan LW (2018) Transient performance of a PCM-based heat sink
with a partially filled metal foam: effects of the filling height ratio. Appl Therm Eng 128:966–972
Chapter 12
Direct Photo-Thermal Energy Storage
Using Nanoparticles Laden Phase
Change Materials

Deepak Moudgil and Vikrant Khullar

Abstract In the present work, we propose thermal energy storage by direct photo-
thermal energy conversion (referred to as optical charging) using nanoparticles laden
phase change materials (PCMs). In the conventional thermal storage systems, the
absorbed solar energy is indirectly transferred to the PCM (primarily through con-
duction and convection heat transfer mechanisms) and is subsequently stored in
the form of latent heat of the PCM (referred to as thermal charging). Opposed to
the conventional thermal storage strategies; optical charging involves direct interac-
tion of the sunlight with the phase change material (radiation being the predominant
heat transfer mechanism). Broad absorption-based nanoparticles (amorphous carbon)
have been seeded into the pristine phase change material (paraffin wax) to enhance
photo-thermal conversion efficiency. Particularly, we investigate the effect of adding
nanoparticles to conventional PCMs during optical charging process. To understand
the role of nanoparticles; samples of pristine paraffin wax and nano-PCMs [different
concentrations of nanoparticles (0.05, 0.1, 0.2, 0.4%, wt%) dispersed in the pristine
paraffin wax] have been optically heated. Furthermore, optical charging has been
compared with the conventional thermal charging process. As per the experimental
observations, the optical charging scheme significantly improves the thermal charg-
ing rate (by more than 157%) at optimum nanoparticle concentration (0.2%, in the
present study) as compared to conventional thermal charging.

Keywords Optical charging · Phase change material (PCM) · Nanoparticles

D. Moudgil · V. Khullar (B)

Mechanical Engineering Department, Thapar Institute of Engineering and Technology Patiala,
Patiala, Punjab 147004, India
e-mail: vikrant.khullar@thapar.edu
D. Moudgil
e-mail: dmoudgil_me17@thapar.edu

© Springer Nature Singapore Pte Ltd. 2020 235

H. Tyagi et al. (eds.), Solar Energy, Energy, Environment,
and Sustainability, https://doi.org/10.1007/978-981-15-0675-8_12
236 D. Moudgil and V. Khullar

Nomenclature

English symbols

m Mass
Tavg Average temperature
Tatm Atmospheric temperature

Subscripts

f Fraction

Abbreviations

PCM Phase change material

NPs Nanoparticles
TES Thermal energy storage
OC Optical charging
TC Thermocouple

12.1 Introduction

In today’s world the energy demand in the industrial, household, agriculture and ser-
vice sector vary on daily, weekly and seasonal basis. Thermal energy storage (TES)
provides a simple an economic means of utilizing solar energy even during non-
sunshine hours to satisfy our growing needs for heating and cooling. TES essentially
involves three processes: charging, storage and discharging. Furthermore, thermal
storage process could be either sensible TES or latent TES. In sensible heat storage,
the material (concrete, rock, etc.) undergoes temperature change during charging and
discharging processes; whereas in latent TES, the material (ice, salts, etc.) under-
goes phase transformation during the charging and discharging processes (Dincer
and Rosen 2002). Latent TES (employing PCMs) in particular offers range of capac-
ities (quantity of energy stored) and capability of storing energy ranging from few
hours to a few days at relatively high energy stored to volume ratio. PCMs could
be classified into three basic types: organic (paraffins, fatty acids), inorganic (salt
hydrates), eutectics (organic, inorganic) (Paksoy 2007). Conventionally, the charg-
ing process (melting of PCM) involves bleeding a part of the heated fluid from the
12 Direct Photo-Thermal Energy Storage Using Nanoparticles … 237

outlet of solar thermal system and circulating it in a pipe through the PCM contain-
ing storage tank. Herein, the heat of the fluid gets transferred to the PCM primarily
through conduction and therefore the pipe material is invariable a highly conducting
material (such as copper). Furthermore, researchers have used pipes with fins and
employed various agitation techniques to enhance the heat transfer. Recently, the
photon-transport based optical charging (OC) wherein, the solar irradiance directly
interacts with the PCM has proven to significantly improve the charging process.
Herein, nanoparticles have been seeded in the conventional PCMs to enhance its
absorption capability; and the nanoparticle-laden PCM directly interacts with the
incident sunlight through absorption and scattering mechanisms. Although, seeding
nanoparticles into a transparent PCM is the most effective proposition but researchers
have revealed that photon-transport based optical charging can still be applicable to
nanoparticles-laden opaque PCMs (Wang et al. 2017). To enhance the charging rate
researchers have dispersed various nanoparticles in PCMs (Cu NPs, Au NPs, Al2 O3
NPs, metal foam copper, carbon nanotubes sponge, hybrid NPs etc.)—these could
be broadly classified into plasmonic metallic nanoparticles or carbon-based broad
absorption nanoparticles.
With the objective of optimizing the optical charging process, there have been
efforts to understand the photon-nanoparticle laden PCM interaction mechanism
and subsequently the melting process. Richardson et al., studied the light interaction
with gold nanoparticles dispersed in ice matrix (Richardson et al. 2006). Chen et al.,
enhanced the enthalpy and thermal conductivity of PCM through usage of deformable
carbon nano-tube sponge wherein energy storage is driven by illumination (Chen
et al. 2012). Lin and Al-Kayiem used copper NPs to improve the thermophysical
properties of PCMs for thermal energy storage (Lin and Al-Kayiem 2012). Usage
of copper NPs in solar pond has been proven to reduce the charging and discharging
time significantly (Karunamurthy et al. 2012). Alumina NPs dispersed into PCM to
form a nano-PCM which has been used to increase the charging rate as compare to
pure PCM (Pise et al. 2013). Hybrid nanoparticles have been used in solar heating
systems to enhance the storage and nanofillers of graphene NPs have been found
to enhance the thermal conductivity with increase in loading of NPs (Harikrishnan
et al. 2014; Fan et al. 2013). Ultra-thin foam dispersed into PCM have shown to
significantly increase the thermal conductivity at even low volume fractions (up to
18 times) and PCM filled spherical capsules have also been used to increase the
thermal storage, cyclohexane with copper nanoparticles have been dispersed into
PCM with porous medium to increase the energy storage (Ji et al. 2014; Khot et al.
2012; Hossain et al. 2015). The addition of metal foams has shown to increase the
overall heat transfer up to 3–10 times. Researchers have also shown that usage of
liquid metal gallium as nano-PCM reduces the melting time and improves thermal
storage (Zhao et al. 2010; Wang et al. 2014; Alomair et al. 2017; Salyan and Suresh
2018).
The present work is essentially an attempt to further understand the optical charg-
ing and discharging process. This shall help in identifying the key parameters and
processes impacting the melting rate of nanoparticle-laden PCMs. Firstly, we prepare
the samples of nano PCM by using ultra-sonication method. Two set of experimental
238 D. Moudgil and V. Khullar

set-ups have been designed, which are similar in all respects except that one sim-
ulates the surface heating i.e. the conventional charging mechanism and the other
simulates the optical charging mode. Subsequently, charging rate of nano PCM at
different concentrations have been carefully measured under optical charging mode.

12.2 Experimental Materials and Methods

In order to clearly understand the heat transfer mechanisms involved during thermal
and optical heating two sets of experiments have been carefully designed. In the
present work paraffin wax has been used as PCM and amorphous carbon nanoparti-
cles (<100 nm SIGMA ALDRICH) of desired amount have been dispersed into PCM
to form nano-PCM. All materials have been used without any further purification. For
temperature measurements, K-type thermocouples were employed. In experiments
glass and solar selective surface (black chrome coated copper sheet, 0.019 cm thick,
Solchrome) were used as interface in two set-ups. Various nano-PCMs containing
paraffin wax (10 g) and amorphous carbon nanoparticles (ACNs) with different mass
fractions have been prepared and thermal conductivity has been measured using heat
flow method (P.A Hilton Ltd., H112N) (see Table 12.1).

12.2.1 Ultra-sonication Process

Initially, paraffin wax has been melted in water bath and subsequently amorphous car-
bon nanoparticles of desired mass have been added to melted paraffin wax as shown
in Fig. 12.1. Paraffin wax and amorphous carbon has been mixed by ultra-sonication
process. All samples are prepared by using sonication process in which ultra-sonic
waves leads to uniform mixing of the nanoparticles into the melted medium. This

Table 12.1 Nano-PCMs with

Phase change Mass fractions, mf Thermal
different concentrations
material % conductivity
(W m−1 K−1 )
Paraffin wax (10 g) 0 0.278 ± 0.011
Paraffin wax (10 g) 0.05 0.324 ± 0.010
+ ACNs (0.005 g)
Paraffin wax (10 g) 0.1 0.388 ± 0.013
+ ACNs (0.01 g)
Paraffin wax (10 g) 0.2 0.359 ± 0.014
+ ACNs (0.02 g)
Paraffin wax (10 g) 0.4 0.329 ± 0.012
+ ACNs (0.04 g)
12 Direct Photo-Thermal Energy Storage Using Nanoparticles … 239

Fig. 12.1 a Schematic showing ultra-sonication process for preparing nano-PCM; b showing pre-
pared samples of nano-PCM after ultra-sonication process

method is indirect method of sonication and the advantage of this method is that
sonic energy is transmitted from horn through the water and into multiple samples
i.e. a large number of nano-PCM samples could be prepared simultaneously. We
use ultra-sonicator bath model SU-323 with load 3A by filling ¾ liquid in the tank
(water). The process has been carried out for 15 min, subsequently, the as-prepared
nano-PCM samples were cool down to atmospheric conditions. Figure 12.1b shows
the picture of as-prepared nano-PCM samples after the ultra-sonication process.

12.3 Experimental Set-Up

Figure 12.2 shows the experimental set up consisting of 0.8 cm diameter optical light
guide transporting the light from halogen lamp (250 W, 24 V Philips) connected with
transformer and power supply. Intensity of light as measured by thermopile detector
and power meter (1918-R, Newport) is 12.5 W/cm2 . Holding clamp is used to hold
the optical light guide and the cylindrical tube housing the PCM is fixed at the base of
240 D. Moudgil and V. Khullar

Fig. 12.2 Schematic showing experimental set-up for charging process of nano-PCM

the stand. Four equally spaced (0.5 cm apart) and calibrated K-type thermocouples
have been placed in the cylindrical tube (1.7 cm × 1 cm) housing the PCM and
fifth thermocouple is kept in open to record ambient temperature. As mentioned
earlier, two sets of experimental were carried out i.e. volumetric (simulating optical
charging) and surface (simulating thermal charging) heating. In volumetric heating,
the cylindrical tube is covered with glass (2.5 cm × 2.1 cm) from both sides and in
surface heating top is covered by solar selective surface (2.5 cm × 2.1 cm), bottom
is covered with glass.
In both set of experiments, we fix the cylindrical tube at the base of the stand and
also fix the positions of the thermocouples and optical light guide (see Fig. 12.3).
Tube is filled with PCM/nano-PCM. Heating or charging process has been carried out
for 1800 s, after which the power supply is turned off and the melted PCM/nano-PCM
is allowed to cool for 1380 s (to ensure that the solidification process is completed
and the PCM/nano-PCM is in equilibrium with the atmospheric conditions).
Each experiment has been performed for at least three times (to ensure repeatabil-
ity of the results). Figure 12.3 shows the schematics of set-ups employed for surface
and volumetric heating. The temperature field as measured by the thermocouples
was logged using data acquisition (DAQ) system (NI).

12.4 Results and Discussion

Charging rates for two cases (namely interface-glass and interface-solar selective
material) at various concentrations of the nanoparticles have been plotted in Figs. 12.4
and 12.5. Graphs clearly reveal that for a given illumination period (30 min in the
12 Direct Photo-Thermal Energy Storage Using Nanoparticles … 241

Fig. 12.3 Schematics showing the position of optical light guide and thermocouples for a volu-
metric heating, and b surface heating

present study), charging rate increases with increase in concentration of nanoparti-

cles. Furthermore, volumetric heating is more effective as compare to surface heating.
In order to clearly understand the reasons for charging rate enhancements tem-
perature distribution in each case was also analyzed. To understand the effect of
nanoparticles in volumetric heating; samples of pristine paraffin wax and nano-PCMs
[different concentrations of nanoparticles (0.05, 0.1, 0.2, 0.4%, wt%)] have been opti-
cally heated. From experimental observations we note that charging rate enhances,
with increase in concentration. To understand the effect of nanoparticles in surface
heating; samples of pristine paraffin wax and nano-PCMs [different concentrations
of nanoparticles (0.05, 0.4%, wt%) have been heated. In case of surface heating
(solar selective material used as interface) overall temperature distribution is less as
compare to volumetric heating.
The temperature difference between the initial state and final state shows the
charging process of nano-PCM. Four thermocouple positions give the temperature
profiles to understand the thermal behavior of nano-PCM. These observations reveal
that, the optical charging scheme significantly improves the thermal charging rate
(by more than 157%) at optimum nanoparticle concentration (0.2%, in the present
study) as compared to conventional thermal charging.
It may be noted that in case of thermal charging (surface heating), the light does
not interact directly with the nanoparticles, instead it is absorbed by the solar selective
surface and subsequently transferred to nanoparticle laden PCM through conduction.
Thermal conductivity being nearly insensitive to the addition of nanoparticles, we
need not carry out experiments for all mass fractions in case of surface heating.
This is also confirmed through careful observation of the temperature distribution
242 D. Moudgil and V. Khullar

Fig. 12.4 Temperature field under OC mode as a function of time for four different concentrations
of nanoparticles a pure paraffin wax, b NPs with concentration 0.05%, c NPs with concentration
0.1%, d NPs with concentration 0.2%, and e NPs with concentration 0.4%
12 Direct Photo-Thermal Energy Storage Using Nanoparticles … 243

Fig. 12.5 Temperature field under thermal charging mode as a function of time for two different
concentrations of nanoparticles a pure paraffin wax, b NPs with concentration 0.05%, and c NPs
with concentration 0.4%

curves for various nanoparticle laden PCMs in case of surface heating (see Fig. 12.5).
Finally, thermal images of various samples both under optical charging as well ther-
mal charging modes have been captured (see Fig. 12.6) using IR camera (Keysight
U5855A TrueIR thermal imager).
244 D. Moudgil and V. Khullar

Fig. 12.6 Time-sequential thermal IR images of a pure paraffin and four different concentrations
of nanoparticles [volumetric heating], and b pure paraffin and two different concentrations of
nanoparticles [surface heating]
12 Direct Photo-Thermal Energy Storage Using Nanoparticles … 245

12.5 Conclusions

Experimental results reveal that addition of amorphous carbon nanoparticles can

effectively improve the charging rate of PCMs. Furthermore, temperature distribution
measurements show that optical heating of PCM in presence of nanoparticles could
achieve more distributed energy absorption and hence proving to be an efficient and
reliable method for charging PCMs. Volumetric heating gives more effective results
as compared to conventional thermal charging under similar operating conditions.
As a part of the future work, we intent to develop a comprehensive theoretical model
simulating the optical charging process.

Acknowledgements The authors gratefully acknowledge the support provided by Mechanical

Engineering Department, Thapar Institute of Engineering and Technology, Patiala, India.

References

Alomair MA, Alomair YA, Abdullah HA, Mahmud S, Tasnim S (2017) Nanoparticle enhanced
phase change material in latent heat thermal energy storage system: an experimental study
Chen L, Zou R, Xia W, Liu Z, Shang Y, Zhu J, Wang Y, Lin J, Xia D, Cao A (2012) Electro- and
photodriven phase change composites based on wax-infiltrated carbon nanotube sponges. ACS
Nano 6(12):10884–10892
Dincer I, Rosen MA (2002) Thermal energy storage: systems and applications. Wiley, New York
Fan LW, Fang X, Wang X, Zeng Y, Xiao YQ, Yu ZT, Xu X, Hu YC, Cen KF (2013) Effects of various
carbon nanofillers on the thermal conductivity and energy storage properties of paraffin-based
nanocomposite phase change materials. Appl Energy 110:163–172
Harikrishnan S, Deepak K, Kalaiselvam S (2014) Thermal energy storage behavior of com-
posite using hybrid nanomaterials as PCM for solar heating systems. J Therm Anal Calorim
115(2):1563–1571
Hossain R, Mahmud S, Dutta A, Pop I (2015) Energy storage system based on nanoparticle-enhanced
phase change material inside porous medium. Int J Therm Sci 91:49–58
Ji H, Sellan DP, Pettes MT, Kong X, Ji J, Shi L, Ruoff RS (2014) Enhanced thermal conductivity of
phase change materials with ultrathin-graphite foams for thermal energy storage. Energy Environ
Sci 7(3):1185–1192
Karunamurthy K, Murugumohankumar K, Suresh S (2012) Use of CuO nano-material for the
improvement of thermal conductivity and performance of low temperature energy storage system
of solar pond. Dig J Nanomater Biostruct 7(4):1833–1841
Khot SA, Sane NK, Gawali BS (2012) Thermal energy storage using PCM for solar domestic hot
water systems: a review. J Inst Eng (India): Ser C 93(2):171–176
Lin SC, Al-Kayiem HH (2012) Thermophysical properties of nanoparticles-phase change material
compositions for thermal energy storage. In: Applied mechanics and materials, vol 232. Trans
Tech Publications, pp 127–131
Paksoy HÖ (ed) (2007) Thermal energy storage for sustainable energy consumption: fundamentals,
case studies and design, vol 234. Springer
Pise AT, Waghmare AV, Talandage VG (2013) Heat transfer enhancement by using nanomaterial in
phase change material for latent heat thermal energy storage system. Asian J Eng Appl Technol
2(2):52–57
246 D. Moudgil and V. Khullar

Richardson HH, Hickman ZN, Govorov AO, Thomas AC, Zhang W, Kordesch ME (2006) Ther-
mooptical properties of gold nanoparticles embedded in ice: characterization of heat generation
and melting. Nano Lett 6(4):783–788
Salyan S, Suresh S (2018) Liquid metal gallium laden organic phase change material for energy
storage: an experimental study. Int J Hydrogen Energy 43(4):2469–2483
Wang Z, Tao P, Liu Y, Xu H, Ye Q, Hu H, Song C, Chen Z, Shang W, Deng T (2014) Rapid charging
of thermal energy storage materials through plasmonic heating. Sci Rep 4:6246
Wang Z, Tong Z, Ye Q, Hu H, Nie X, Yan C, Shang W, Song C, Wu J, Wang J, Bao H (2017)
Dynamic tuning of optical absorbers for accelerated solar-thermal energy storage. Nat Commun
8(1):1478
Zhao CY, Lu W, Tian Y (2010) Heat transfer enhancement for thermal energy storage using metal
foams embedded within phase change materials (PCMs). Sol Energy 84(8):1402–1412
Chapter 13
Review on PCM Application for Cooling
Load Reduction in Indian Buildings

Rajat Saxena, Dibakar Rakshit and S. C. Kaushik

Abstract Buildings use more than 40% of the total power consumed in India. There-
fore, implementing energy conservation within buildings is of prime concern. Uti-
lization of passive design parameters, such as Phase Change Material (PCM) incor-
poration, for energy conservation in buildings is thus a lucrative option. Incorporating
PCMs within elements result in lowering of heat gain and temperature within the
building. A number of simulation and experimental studies on PCM incorporated
buildings, have been carried out. The advancements made in last forty years in the
field of PCMs and their utilization as Thermal Energy Storage (TES) medium for
buildings have been reviewed and presented in this study. This study focusses on
PCM incorporation which is sensitive to its properties and climatic parameters of
the location. Thus, there is a need of benchmarking the PCM for their application in
buildings. Focus is on buildings in tropical hot climatic conditions, where reduction
in cooling load is a challenge. This study lays emphasis on selection of appropriate
PCMs based on its phase change temperature. Thermal conductivity, specific heat
and latent heat are other properties which must be evaluated before PCM selection
and implementation within buildings. The study also encompasses different meth-
ods of PCM incorporation being implemented across the world and have marked
advantages and disadvantages of each followed by their impact in terms of energy
savings.

Keywords Latent thermal energy storage · PCM · Passive system ·

Thermophysical properties · Building energy conservation

R. Saxena · D. Rakshit (B) · S. C. Kaushik

Centre for Energy Studies, Indian Institute of Technology Delhi, Hauz Khas,
New Delhi 110016, India
e-mail: dibakar@iitd.ac.in
R. Saxena
e-mail: rajat.saxena@ces.iitd.ac.in
S. C. Kaushik
e-mail: kaushik@ces.iitd.ac.in

© Springer Nature Singapore Pte Ltd. 2020 247

H. Tyagi et al. (eds.), Solar Energy, Energy, Environment,
and Sustainability, https://doi.org/10.1007/978-981-15-0675-8_13
248 R. Saxena et al.

13.1 Introduction

Energy exists in many forms and keeps transforming from one form to another.
Energy utilization per person determines the economic and living standard of people
within any country. Therefore, countries are aiming at increased rate of energy pro-
duction year after year. The major problem to this is that there are limited resources
available; thus, it is insufficient to just produce and use more energy. Thus, there is
a need to utilize the available knowledge and technology for extracting energy from
alternate sources, efficiently. The aim is to reduce the use of fossil fuel and replace
them with utilization of alternate sources of energy.
Energy storage (ES) systems are an important component of alternate energy
harnessed through solar, due to its intermittency. These systems store solar energy
in different forms i.e. heat, chemical, biological, mechanical etc. Of these thermal
(heat) energy storage systems are most studied and implemented world-wide for
various applications. TES incorporation results in energy savings for cold climatic
conditions as reported by Mehling and Cabeza (2008). With buildings having 40%
share of the total electricity consumption (Saxena 2018), the focus is on reducing the
power consumption of buildings in India. For tropical countries like India, the major
power consumption during the greater part of the year is due to the cooling loads.
There are several technologies being studied for cooling load reduction in buildings.
This present study investigates the PCM implementation for buildings, as a passive
measure, to reduce the cooling loads during peak hours of the day.
This chapter aims at providing a formidable solution to rapidly increasing building
energy demands. The research emphasizes on careful selection of PCMs, as a passive
storage option, for heating/cooling load abatement in buildings. PCMs are carefully
selected based on their thermal characteristics, and mapped based on the climatic
conditions of the place. This study also brings out how researchers have tried to
incorporate PCMs within building elements followed by their assessment based on
temperature reduction and energy conserved.
It is observed that PCMs in general have lower thermal conductivity thus, reducing
their charge and discharge time. In order to ensure high rate of dispatchability of
PCMs, it is necessary to investigate methods to increase their thermal conductivity.
The nanoparticle dispersion is one such option which has also been discussed in this
study.

13.2 TES Materials for Application in Buildings

A number of studies are available right from 1981, when concept of TES implemen-
tation in buildings was conceptualized and numerical model for a room with phase
changing material (Kaushik et al. 1981) was developed. Further, a study in 1983, dis-
cussed the availability and application of different PCMs for storage of heat at low
temperatures (0–120 °C). This study also discussed about the corrosion resistance
13 Review on PCM Application for Cooling Load Reduction … 249

Fig. 13.1 TES material classification (Sharma et al. 2009)

and long term stability of these materials (Abhat 1983). Cabeza et al. (2011), Farid
et al. (2004), Khudhair and Farid (2004), Sharma et al. (2009), Zalba et al. (2003)
provides a detailed review on various types of PCMs and its applications. Material
thermal properties, cost and compatibility are vital for them to be rendered suitable
as TES material for any desired application. TES materials can be categorized as
shown in Fig. 13.1.

13.2.1 Sensible Heat Storage Materials

Materials which absorb heat with rise in temperature and visa-versa, without under-
going phase change (in working range of temperatures) are referred to as sensible
heat storage materials. Stored heat (Qsensible ) depends on specific heat of material as
shown in Eq. 13.1. For example, in solar water heaters, the solar heat is utilized for
heating water which subsequently results in its rise in temperature.

T f
Q sensible = m C p .dT (13.1)
Ti

Water maybe be referred to as best sensible heat storage materials owing to its
highest specific heat. However, for temperature above 100 °C different salts, oils etc.
maybe considered. Few sensible heat storage materials are as shown in Table 13.1
(Sharma et al. 2009; Solutia 1998; Benoit et al. 2016).
For high temperature solar thermal applications, molten metals and eutectic mix-
tures are also used. Heat stored per unit mass for sensible heat storage materials is
relatively less followed by significant change in temperature, is observed. To over-
come these drawbacks latent heat storage materials can be used.
 250

Table 13.1 Different sensible heat storage solid and liquid materials
S. No. Materials Type Specific heat (J/kg K) Temperature/Range (°C) Density (kg/m−3 )
1 Rock Solid 879 20 2560
2 Concrete Solid 880 20 1900–2300
3 Brick Solid 840 20 1600
4 Water Solid 4190 0–100 1000
5 Caloriea HT43 Oil 2200 12–260 867
6 Diphenyl oxide (therminol Oil 1523–2600 12–393 1071–700
VP-1)
7 Ethanol Organic liq. 2400 <79 790
8 Propanol Organic liq. 2500 <98 800
9 Butanol Organic liq. 2400 <119 810
10 Isobutanol Organic liq. 3000 <100 808
11 Octane Organic liq. 2400 <126 704
12 Hitec Molten salt – 142–535 1740
13 Hitec XL Molten salt – 120–500 1700
14 Solar salt Molten salt 1553 220–600 1868
15 Liq. Sodium Molten metal 1280 98–883 –
16 Lead bismuth eutectic (LBE) Eutectic mixture 1260 125–1670 10529
R. Saxena et al.
13 Review on PCM Application for Cooling Load Reduction … 251

13.2.2 Latent Heat Storage Materials

These materials have high specific heat capacity thus high concentration of energy
is stored per unit mass due to latent heat involved. Another main advantage of using
these materials is that they operate between small temperature differences while
storing or retrieving the energy. These undergo solid- liquid or liquid-gas phase tran-
sition, in working range of temperature. The volumetric transition during liquid-gas
phase change is quiet high as compared to solid-liquid PCM, hence are less suitable
for building storage applications. Thus, solid-liquid transition storage systems are
focused in this chapter. The heat stored within PCM storage is given in Eq. 13.2.

Tmelt T f
Q=m C ps .dT + m.L + m C pl .dT (13.2)
Ti Tmelt

The solid-liquid PCMs are further classified as inorganic and organic. Example
of organic latent heat storage materials are paraffins, sugar alcohols, fatty acids etc.
and example of inorganic PCMs are water, hydrated salts, molten salts, aqueous
solutions etc. Moreover, two more miscible pure PCM constituents, having single
melting/solidifying temperature are referred as eutectic mixtures. Use of hydrated
salts as thermal energy storage materials is preferred, as they possess high energy
density owing to higher density and latent heat. However, they show tendency of
sub-cooling (or super-cooling) due to low nucleation rate thus, diminishing their
usability significantly. Different PCMs are studied as potential TES materials, for
temperature reduction within buildings and are illustrated in Table 13.2.
A significant number of researchers are working on organic PCMs i.e. on methods
to increase their thermal conductivity which is a major drawback of using organic
PCM. The research is going on to dope the nanoparticles into the PCM matrix to
enhance their thermal conductivities. The methodology of mixing and characterizing
the nano-enhanced PCMs is discussed in detail under Sect. 13.6.

13.2.3 Chemical Heat Storage Materials

Research is still under process to explore materials storing thermal energy as enthalpy
change of a reaction. For e.g.:

A + B + energy → C + D (13.3)

In this interaction A and B absorb energy and convert into C and D thus the energy
is stored as enthalpy change. It is important for reaction to be reversible so that energy
can be retrieved when required. The challenge lies in finding such reactions that may
hold true, must be stable and can be triggered as and when required. A perfect
Table 13.2 Solid-liquid PCMs for cooling load reduction in buildings
252

Material Melting temperature (°C) Thermal conductivity (W/m K) Heat of fusion (kJ/kg) Density (kg/m3 ) References
LiClO3 3H2 O (Hydrated salt) 8 – 253 1720 (solid) Agyenim et al. (2010)
8 155 1530 (liq.) Agyenim et al. (2010)
Polyglycol E400 (organic) 8 0.187 (liq., 38.6 °C) 99.6 1125 (liq., 25 °C) Cabeza and Mehling (2003)
8 0.19 100 1228 (solid) Agyenim et al. (2010)
Paraffin C15 10 – 205 – Sharma et al. (2009)
n-Pentadecane 10 – 770 (liq.) Cabeza and Mehling (2003)
9.9 193.9 Agyenim et al. (2010)
Caprylic acid (fatty acid) 16 0.149 148.5 901 (liq., 30 °C) Zhou et al. (2012)
Paraffin C16 16.7 – 237.1 – Sharma et al. (2009)
45% capric acid + 55% lauric acid 17–21 – 143 – Kant et al. (2017)
(organic)
Glycerin (organic) 17.9 0.143 198.7 – Zhou et al. (2012)
n-Heptadecane (organic) 19 0.21 240 760 (liq.) Cabeza and Mehling (2003)
20.8–21.7 – 171–172 – Cabeza and Mehling (2003)
Butyl stearate (organic) 19 – 140 760 (liq.) Cabeza and Mehling (2003)
18–23 0.21 123–200 – Sharma et al. (2009)
OM 21 22 0.14 174 891 Pluss Polymers (2017)
FeBr3 ·6H2 O (Hydrated salt) 21 – 105 – Zhou et al. (2012)
Octadecyl 3-mencaptopropylate 21 – 143 – Sharma et al. (2009)
(organic)
(continued)
R. Saxena et al.
Table 13.2 (continued)
Material Melting temperature (°C) Thermal conductivity (W/m K) Heat of fusion (kJ/kg) Density (kg/m3 ) References
Paraffin C17 (organic) 21.7 – 213 – Zhou et al. (2012)
HS 22 23 0.56 167.6 1540 De Gracia and Cabeza (2015)
34% C14 H28 O2 + 66% 24 – 147.7 – Zhou et al. (2012)
C10 H20 O2
HS 24 26 0.55 1510 – De Gracia and Cabeza (2015)
Mn(NO3 )·6H2 O (hydrated salt) 25.5 – 125.9 1738 (liq., 20 °C) (Sharma et al. 2009)
25.8 125.9 1728 (liq., 40 °C) (Cabeza and Mehling 2003)
Lactic acid (organic acid) 26 – 184 – Sharma et al. (2009)
50% CH3 CONH2 + 50% 27 163 Zhou et al. (2012)
NH2 CONH2 (organic eutectic)
n-Octadecane (organic) 28 0.148 (liquid) 244 774 (liq.), 814 (solid) Zhou et al. (2012)
HS 29 29 0.382 190 1530 De Gracia and Cabeza (2015)
LiNO3 ·2H2 O (hydrated salt) 30 – 296 – Zhou et al. (2012)
LiNO3 ·3H2 O (hydrated salt) 30 – 296 – Sharma et al. (2009)
67% Ca(NO3 )2 + 33% Mg(NO3 )2 30 – 136 1670 Cabeza and Mehling (2003)
(Inorganic eutectic)
60% Na(CH3 COO)·3H2 O + 40% 31.5 – 226 – Cabeza and Mehling (2003)
13 Review on PCM Application for Cooling Load Reduction …

CO(NH2 )2 (Inorganic eutectic)

OM 32 33 0.145 157 870 De Gracia and Cabeza (2015)
Na2 SO4 ·3H2 O (Hydrated salt) 32 – 251 – Cabeza and Mehling (2003)
32.4 241 Zhou et al. (2012)
Na2 CO3 ·10H2 O (hydrated salt) 32–36 – 246.5 1442 Cabeza and Mehling (2003)
(continued)
253
Table 13.2 (continued)
254

Material Melting temperature (°C) Thermal conductivity (W/m K) Heat of fusion (kJ/kg) Density (kg/m3 ) References
Paraffin C19 (organic) 32 – 222 – De Gracia and Cabeza (2015)
Capric acid (fatty acid) 32 0.153 152.7 878 (liq., 45 °C) De Gracia and Cabeza (2015)
30.1–31.4 150–158 Sharma et al. (2009)
31.5 0.152–0.149 (liquid) 153 886–1004 Cabeza and Mehling (2003)
62.6% lauric acid + 37.4% 32.6 – 156 – Cabeza and Mehling (2003)
myristic acid
64% lauric acid + 36% palmitic 32.8 – 165 – Sharma et al. (2009)
acid
CaBr2 ·6H2 O (hydrated salt) 34 – 115.5 1956 (liq., 35 °C) Cabeza and Mehling (2003)
Lauric acid + stearic acid (organic) 34 – 150 – Cabeza and Mehling (2003)
66% lauric acid + 34% myristic 34.2 – 166.8 – Cabeza and Mehling (2003)
acid
Na2 HPO4 ·12H2 O (hydrated salt) 35–44 0.514 (solid) 280 1522 (solid) Cabeza and Mehling (2003)
36 – 280 – Farid et al. (2004)
40 – 279 – De Gracia and Cabeza (2015)
OM 35 35 0.16 171 870 De Gracia and Cabeza (2015)
Paraffin C20 36.7 – 246 – Farid et al. (2004)
OM 37 37 – 231 – Pluss Polymers (2017)
Mn(NO3 )·6H2 O (hydrated salt) 37.1 – 115 1738 (liq., 20 °C) Cabeza and Mehling (2003)
– – 1795 (liq., 5 °C) Farid et al. (2004)
1-Tetradecanol 38 – 205 – Cabeza and Mehling (2003)
50% Na(CH3 COO)·3H2 O + 50% 40.5 – 255 – Liu et al. (2012)
HCONH2
(continued)
R. Saxena et al.
Table 13.2 (continued)
Material Melting temperature (°C) Thermal conductivity (W/m K) Heat of fusion (kJ/kg) Density (kg/m3 ) References
Heptadecanone 41 – 201 – De Gracia and Cabeza (2015)
KF·2H2 O (Hydrated salt) 41.4 – – Farid et al. (2004)
K(CH3 COO)·(1/2) H2 O (hydrated 42 – – – De Gracia and Cabeza (2015)
salt)
Lauric acid 42–44 0.147 178 870 (liq.) Cabeza and Mehling (2003)
13 Review on PCM Application for Cooling Load Reduction …
255
256 R. Saxena et al.

example is lead acetate batteries however, long term stability of these storage is still
a topic of research. The advantage of these materials is high storage density near
ambient temperature; thus, losses are greatly subsided.
This study however, deals with PCMs for application in Indian climatic conditions.

13.3 PCM Properties and Methods of Characterization

13.3.1 Desired PCM Properties and Their Inherent Problems

The PCM selection for a particular application is to be made judiciously, with the
help of the following parameters (Mehling and Cabeza 2008):
• Suitable phase change temperature and high latent heat; specifically, temperature
must be determined with high accuracy (as melting and solidification temperature
plays a critical role in storage and release of heat at required temperature which is
a pre-requisite for any thermal energy storage system);
• Other thermophysical properties: large heat capacity, low volume change ratio,
large density and thermal conductivity, no phase segregation and lower super-
cooling;
• Chemical parameters: high chemical stability, no corrosion, low toxicity, non-
inflammable, compatible and non-polluting;
• Economic parameters: good recyclability, proper availability and cheap.
PCMs also possess certain inherent problems that should to be addressed before
their application. Some of the major issues are as follows:
• Super-cooling (also referred as sub-cooling). It is the difference in melting
and solidification temperature; PCMs are to be cooled below their solidifica-
tion onset temperature to initiate nucleation which is followed by crystalliza-
tion/solidification. For inorganic PCMs, sub-cooling is significantly higher.
• Phase segregation. PCMs (generally hydrated salts and eutectic mixtures) gener-
ally exhibit phase segregation. They separate out during melting thus making it
inhomogeneous.
• Low thermal conductivity. It is problem for most of the PCMs. For organic PCMs
the thermal conductivity is below 0.5 W/m K. For inorganic PCMs it is generally
below 1 W/m K.
To overcome the above disadvantages the foremost priority is to assess the PCM
properties with high accuracy and precision as there is divergence in data or data mis-
match in the available literature followed by measures to enhance the PCM properties
as per their specific field of application.
13 Review on PCM Application for Cooling Load Reduction … 257

13.3.2 Material Characterization

The characterization plays a significant role in material selection for TES systems.
It is necessary for design, modelling and implementation of these systems. The
available knowledge on PCMs is limited and in some cases even found to be inac-
curate/incomplete (Saxena et al. 2017). PCMs (salts in particular) show incongruent
melting (with melting occurring non-uniformly) and also showing sub-cooling. Thus,
there is a need to characterize these PCMs to determine the temperature dependence
of different thermophysical properties Energy efficient building constructions, incor-
porating PCM storage require accurate assessment of PCM properties with respect
to temperature as single data point may not define the system with accuracy. The
specific heat, latent heat, phase change temperature, sub-cooling has been accurately
determined using differential scanning calorimeter. Thermal conductivity is mea-
sured using thermal conductivity meter.
Differential Scanning Calorimetry: Differential Scanning Calorimetry is a
method in which heat transfer to sample and reference pan is measured along with
rise in temperature. The difference in heat flow and rise in temperature profile gives
the latent heat, phase change temperature, specific heat of the sample. It also deter-
mines the sub-cooling as well. Differential scanning calorimeter (DSC) is used to
accurately evaluate these thermal properties and their temperature dependence.
Thermal Conductivity Meter: It determines the dispatchability of the TES sys-
tems, which is one of the most important characteristic property for any energy
storage system. The thermal conductivity measurement in different phases is gen-
erally done by two methods, that is steady-state and transient. The heat flow meter
and guarded hot plate techniques are steady-state methods both based on Fourier’s
law, which suits the solid phase measurements, but requires a lot of time attain stable
temperature gradient within the specimen. The transient techniques on the other hand
measures thermal conductivity within minutes, therefore researchers now often use
this method for PCMs. Transient plane source, transient hot wire and laser flash are
the example of transient methods (Khodadadi et al. 2013).

13.4 Methods of PCM Incorporation

The integration of TES in buildings can be of two types i.e. active and passive. In
Active systems, energy required is often derived from renewable resources whereas,
passive systems concentrate on energy saving using seasonal shadings, blinds,
increasing thermal mass or thermal insulation, focusing on reducing the energy
demand. The thermal mass may be increased by PCM incorporation in building
components. Thus, it is necessary to understand different methods that may be used
for the purpose and their respective merits and demerits as discussed in Navarro et al.
258 R. Saxena et al.

For integrating PCMs into any building, five methods have been suggested, they are,
direct incorporation, immersion, vacuum impregnation, encapsulation, shape and
form stabilized materials (Navarro et al. 2016a; b).

13.4.1 Direct Incorporation

It deals with adding PCM directly to the building material such as concrete etc. Feld-
man et al. carried out direct incorporation of butyl stearate by 22% in the gypsum.
The energy storage capacity was reported to have increased by ten-fold times (Feld-
man et al. 1991). However, there can be leaking problems as reported by Soares et al.
(2013). Direct incorporation in concrete is still a topic of research as there are two
major challenges first is that PCMs undergo phase change thus, they often leak or
separate out from the mixture. Second, they readily do not mix with concrete and sep-
arate out which is to be taken care of, while mixing. Thus, other techniques/methods
are used for PCM mixing.

13.4.2 Immersion

In this method, the construction elements are soaked or dipped in the container filled
with PCM, which thus gets absorbed within the construction material. Immersion
method requires many hours of soaking (Ling and Poon 2013). This method however
is often not used due to leakage problems (Soares et al. 2013) and due to compatibility
issues with certain building materials (Hawes et al. 1989). This method however, is
slightly different from direct incorporation however, it faces similar constraints and
problems in case of PCMs.

13.4.3 Vacuum Impregnation

In this method the air is first evacuated using the vacuum pump and soaked in the
PCM container thereby replacing the air in the pores by the PCM. There is an effective
reduction in the overall thermal conductivity of the sample. This method is gener-
ally suitable in case of foams etc. where thermal conductivity is to be reduced for
insulation purpose. This method however has similar shortcomings for solid-liquid
PCMs as previous cases.
13 Review on PCM Application for Cooling Load Reduction … 259

13.4.4 Encapsulation

It is the process of encapsulating the PCM before being incorporated in the buildings.
It is of two types: micro-encapsulation and macro-encapsulation and have been dealt
in detail by many researchers. Different researches are being carried out to find out
materials that are suitable for the sheath and also the size and shape to provide a
proper transfer surface and area (Su et al. 2015; Konuklu et al. 2015; Khadiran et al.
2015). Navarro et al. has provided with a number of examples of micro and macro-
encapsulated PCM test constructions in Spain, Paris and other parts of the world. For
Indian conditions PCMs within sheet metal casing have been tested experimentally
within bricks (Saxena et al. 2019). Cylindrical encapsulations are assumed by Kant
et al. (2017) for simulating PCM incorporated bricks for Rae Bareilly, India.

13.4.5 Shape and Form Stabilized Composites

These basically aim at mixing supporting materials like HDPE by melting it along
with the PCMs at higher temperature thereby creating composites on cooling and
could be used in the construction material and avoid leakage of the PCM on melting
(Navarro et al. 2016). These PCMs however, are still at the research stage and their
long-term impact, cost and compatibility are to be studied in detail before imple-
mentation for building applications.

13.5 Assessment of PCM Incorporation within Buildings

To assess the impact of PCM incorporated buildings first it is necessary to assess

the heat flow to non-conditioned room. This is followed by assessing the impact of
PCM incorporation within buildings. Kaushik et al. (1981) carried out the thermal
modelling of PCM incorporated room by assuming the solar influx with constant
ambient air temperature. Jan Kosny et al. (2012) have implemented PCM over roof
in East Tennessee and reported 30% reduction in heating load and 50% reduction
in cooling load. Similar findings were observed by Sleiti and Naimester (2016) for
USA and Entrop et al. (2016) for Netherlands, with stabilized inside temperature
with reduction in heating loads. For India, it may be noted that the conditions are just
the opposite of western countries, where outside temperature is higher and cooling
is required to maintain comfortable temperature inside a building.
Heat flow to the control volume adds to the cooling load requirement for space
conditioning. Thus, the heat, which is stored acts as waste heat. PCM incorporation
results in overall increase in the thermal mass of the building as it absorbs latent
heat by undergoing phase change, thus reducing the need of heavy construction.
Biplab and Rakshit (2017) compared insulations and PCMs for different climatic
260 R. Saxena et al.

conditions in India. PCMs showed better results in terms of temperature reduction

and heat transfer reduction. Pasupathy et al. (Pasupathy et al. 2008) have tested PCM
incorporation over roof for Chennai through direct incorporation. However, long
term impact was not carried out. Kant et al. (2017) carried out CFD simulation for
PCM integrated brick, for March 15, in Rae Barelli, India. Temperature of different
sections of bricks at different time of day was simulated. PCM suitability however,
has not been studied in any of these studies.
The testing of PCM bricks in actual conditions under various PCM configura-
tions has been carried out by Saxena et al. (2019). For this purpose, modified mud
bricks were prepared using conventional Indian brick making process, as it is well
established and could be scaled for future applications. The experiments were carried
out for peak summer conditions with ambient temperature of 40 °C and above. The
reduction of 6 °C was observed. Heat transfer reduction up to 12% was observed.

13.6 PCM Thermal Conductivity Enhancement

with Nanoparticle Dispersion

Owing to the low thermal conductivities of PCMs there has been a considerable effort
towards enhancement of thermal conductivity thereby increasing the dispatchability
of TES systems. Inorganic PCMs are cheap and have better thermal conductivities
than the organic PCMs however their stability over large number of cycles of charging
and discharging is still an issue. Tendency of super-cooling is also higher in the case
of inorganic salts. Due to super-cooling, the PCM is to be cooled below the freezing
point temperature before it actually starts freezing and release heat. More impor-
tantly, it would not solidify unless this lower (nucleating) temperature is reached.
However, the accompanying temperature in the new cycle it only absorbs the sensible
heat. Thus, the usability of the inorganic PCMs is significantly lowered. Apart from
this, inorganic materials also have compatibility issues with metal containers etc.
Table 13.3 compares inorganic and organic PCM.
The researchers across the globe are working to overcome the problem of low
thermal conductivities of PCMs. Particularly for organic PCMs, thermal conductivity
enhancement seems to be only issue that is to be addressed for which a number of
solutions have been suggested in the literature. The researchers are aiming at creating
composite materials by enhancing organic PCMs using metallic foam, nano-particles
of metal oxides and carbon. Babapoor and Karimi (2015) compared the results of
adding SiO2 , Al2 O3 , Fe2 O3 , ZnO nano to PCMs to create a nano enhanced PCM
(NEPCM). Of these, Al2 O3 nano was found to give best results in terms of thermal
conductivity enhancement. A comparison was made using DSC for different metal
oxide nanoparticles including Al2 O3 and TiO2 being added to medium temperature
PCM (Teng and Yu 2012). It was found that, of the five metal oxide nanoparticles,
TiO2 was the best.
13 Review on PCM Application for Cooling Load Reduction … 261

Table 13.3 Comparison of inorganic and organic PCM (Sharma et al. 2009)
Inorganic PCMs (−10 to 117 °C) Organic PCMs (−30 to 200 °C)
• Low thermal conductivity (K) • Low thermal conductivity; lower than
(0.3–0.8 W/mK) inorganic PCMs (0.13–0.4 W/mK)
• Storage capacity 105–300 kJ/kg • Storage capacity 120–210 kJ/kg
• Problem of sub-cooling exists nucleators • Low degree of sub-cooling hence no
required nucleators required
• Have sharp phase change temperatures • Lack sharp phase change temperatures. Fatty
acids show sharp phase change temperature
but are three times more expensive
• They break down into smaller compounds • High cycle stability
and hence are rendered useless after a few
cycles and get segregated
• Cause corrosion in metal containers • Chemically non-corrosive (exception: fatty
acids are mildly corrosive)

PCM has high viscosity thus, direct mixing of nano to PCMs are difficult to blend
and form a homogeneous composite. Thus, advanced wet impregnation method has
been used for this purpose (Shi et al. 2013). In this method PCMs are dissolved in a
solvent and nano particles are added and sonicated for around 30 min. Once homog-
enized the solvent is evaporated to form a homogenous nano enhanced PCM. Certain
researchers in the case of inorganic salts use water as solvent for mixing, however
to maintain the homogeneity, small amount of additive is mixed to this composite.
These are known as surfactants (Singh et al. 2019). Then the composite mixture is
heated to evaporate water thus inorganic NEPCM is left behind. For organic PCMs,
generally, toluene or acetone is used as solvent, based on the solubility and compat-
ibility. These composite PCMs are then characterized for determining the properties
and improvement with the help of DSC and thermal conductivity meter. In build-
ing perspective, the impact of enhanced thermal conductivity for better discharge
characteristics is yet to be tested experimentally. Apart from using composite PCMs
another solution is to use different designs to assist heat transfer like the study of
Hoogendoorn and Bart (Hoogendoorn and Bart 1992). This study discussed perfor-
mance and modelling of latent heat storage materials and suggested use of thin film
aluminum matrix embedded within PCM to reduce the solidification time due to
increase in effective thermal conductivity.

13.7 PCM Selection and Mapping for Building Application

in India

Extensive literature review is carried out on different types of PCMs, their thermo-
physical properties and specific application in buildings. Adding PCM to the building
envelop increases the heat storage capacity of the building elements. This chapter
262 R. Saxena et al.

is focused towards assessing the PCMs suitability for different Indian conditions,
find the impact of PCM incorporation within the building envelop. Before carrying
out a detailed analysis first it is necessary to assess the potential PCMs that may be
suitable for application, this is referred to as PCM mapping. This chapter assesses
the average temperature of the place for different places followed by mapping of
PCMs for different climatic conditions in India.
The weather of a place is determined based on its climatic conditions i.e. tempera-
ture, solar radiation falling, humidity, wind speed etc. Location, altitude, time of day
and year, orientation are other important parameters which affect the solar radiation
thereby controlling the temperature of a building. For example, buildings on a south
facing slope in Leh will receive more radiation compared to other orientations.
Based on these factors, weather and climatic condition in India is divided into five
zones. Table 13.4 provides description about each. Knowing the climatic condition,
it becomes easier to designate particular PCM to suit the existing daily conditions.
During the day, PCM will store most of the solar radiation. During the night, PCM
temperature is greater than ambient thus, the heat flow direction is reversed and
energy stored is now discharged to the ambient. It is important for a PCM to get
discharged so as to be able to absorb heat the next day. To ensure PCM charging
and discharging daily, it is necessary to carefully choose the PCM. The charging and
discharging of PCM depends upon the temperature fluctuation of the place which is
governed by its climatic condition and inherent PCM properties. Especially focusing
the temperate regions, the heat stored in the PCMs can be sufficiently high, thus,
would help in keeping the conditions comfortable or in some cases at least tolerable
for any person inside the room.
For this study, certain cities were selected from each climatic zone and data for
mean hourly temperature for all the months was taken from the literature for all these
places. The following case studies are presented for Delhi, Jaipur, and Chennai to
access the heat flow reduction during the peak hours. The surface is assumed to be
covered with particular mapped PCM, so as to absorb or entrap solar energy during
peak hours. A surface temperature of 27 °C is assumed on the other side (Twi ) of the
PCM incorporated surface as shown in Fig. 13.2.

Table 13.4 Specification for different Indian climatic zones (Nayak and Prajapati 2006)
Criteria Climate R.H. (%) Average monthly temperature (°C)
I Warm and humid >55 >30
>75 >25
II Hot and dry <55 >30
III Temperate <75 25–30
IV Cold All values <25
V Composite If do not fulfill any of the above criteria for more than
6 months
13 Review on PCM Application for Cooling Load Reduction … 263

Tsol

Twi

Fig. 13.2 Schematic geometry

13.8 Design Conditions

13.8.1 Case Study 1: Delhi (Altitude: 416° M, Latitude

28.6° N, Longitude 77.2° E)

It lies in type-V (composite) climatic zone. The mean average temperature in winter
can be as low as 6.6 °C in the month of Dec-Jan and can be as high as 40.5 °C in June
(Fig. 13.3). It is observed that significant amount of energy is utilized for cooling
purpose. The cooling loads in Delhi are at peak between 11 a.m. and 4 p.m. This
is when the solar intensity is at peak. Assuming the solar radiation to be 600 W/m2
(April-October during peak hours) (actual value normally varies between 580 to
680 W/m2 (Tyagi 2009) falling on a surface of dimension 4 × 2.5 m2 . For Delhi
mean wind velocity was taken to be 10.8 km/h.
The average monthly minimum and maximum temperature of a place, is key for
selecting the a PCM for that particular place. The phase change temperature must lie
within the range of maximum and minimum temperature or else it would be rendered
useless. Thus, the choice of the materials is made by mapping the minimum and
maximum temperature of Delhi for the months of April to October and the energy
264 R. Saxena et al.

45
New Delhi
40

35
Temperature (°C)

30
Mean Max.
25 Temp. (°C)

20 Mean Min.
Temp. (°C)
15

10

5
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Months

Fig. 13.3 Average monthly maximum and minimum temperature for New Delhi

savings per day was calculated on the basis of mean hourly temperature between
11 a.m. and 4 p.m. i.e. for five consecutive hours, in which cooling load is maximum.
This energy saving is consequence of the solar insolation which falls on the surface
and gets absorbed and is stopped from being transferred. The PCM is kept in a
rectangular tray (of insulated material) having internal dimensions of 4 m × 2.5 m ×
0.2 m. The PCM is filled in the tray and kept on the roof in the direct sunlight.
The solar air temperature is calculated from 11 a.m. to 4 p.m. Difference in sol air
temperature and ambient temperature is significant and determines the heat flow to
the inside. The total energy that could be transferred and stored in the different PCMs
is calculated and the energy saving in kWh (unit of electricity) per day is calculated.

13.8.2 Case Study 2: Jaipur (Altitude: 431° M, Latitude

28.9° N, Longitude 75.8° E)

It lies in type-II (hot and dry) climatic zone. The mean average temperature in winter
is 8.2 °C in the month of Dec-Jan and can be as high as 40.3 °C in June (Fig. 13.4).
Assuming the solar radiation to be 660 W/m2 (April-October during peak hours)
(actual value normally varies between 640 and 690 W/m2 (Tyagi et al. 2011) falling
on a surface of dimension 4 × 2.5 m2 . For Jaipur mean wind velocity was taken to
be 8 km/h.
Since the mean minimum temperature is below 30 °C hence same PCMs suit
for the weather conditions in Jaipur as well. The mean hourly sol air temperature
13 Review on PCM Application for Cooling Load Reduction … 265

45
Jaipur
40

35
Temperature (°C)

30
Mean Max.
25
Temp. (°C)
20 Mean Min.
Temp. (°C)
15

10

5
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Months

Fig. 13.4 Average monthly maximum and minimum temperature for Jaipur

is calculated and used in calculating heat absorbed by PCMs. The total energy that
could be stored in different PCMs is calculated and hence the energy savings in kWh.
There is a relative increase in energy stored in the PCMs owing to the fact that
the effective sol air temperature in Jaipur is higher, as ho value is less due to lower
wind speed than Delhi and higher solar insolation.

13.8.3 Case Study 3: Chennai (Altitude: 6° M, Latitude

13.1° N, Longitude 80.3° E)

It lies in type-I (warm and humid) climatic zone. The mean average temperature in
winter is about 20.4 °C in the month of Dec-Jan and can be as high as 37.4 °C in
May (Fig. 13.5). Assuming the solar radiation to be 630 W/m2 (April-October during
peak hours) (actual value normally varies between 620 and 660 W/m2 (Tyagi 2009)
falling on a surface of dimension 4 × 2.5 m2 . For Chennai, mean wind velocity is
much higher than Delhi and Jaipur, and is taken to be 20 km/h.
Same PCMs suit for the temperatures at Chennai as well. Owing to the higher
wind velocity in Chennai, the effective ho value is also much higher therefore sol
air temperature is much lower in Chennai. Based on these values of the amount of
energy that can be stored in the PCMs is calculated.
266 R. Saxena et al.

45
Chennai
40

35
Temperature (°C)

30
Mean Max.
25 Temp. (°C)

20 Mean Min.
Temp. (°C)
15

10

5
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Months

Fig. 13.5 Average monthly maximum and minimum temperature for Chennai

13.9 Thermal Calculations

In order to calculate the temperature difference, which would be driving the heat
transfer to take place, the sol-air temperature needs to be defined. It is an effective
outer surface temperature which considers both radiation and convection (Duffie and
Beckman 2013). The sol air temperature (T sol ) is as follows:
 
α I − ε R
Tsol = Ta + (13.4)
ho

where, ‘T a ’ is ambient temperature, ‘α’ and ‘ε’ are absorptivity and emissivity of
surface.
R = Long wavelength radiative exchange from surface to sky.
ε.R ≈ 60 W/m2 for horizontal and 0 for vertical surfaces.
‘I’ is the global solar insolation and ho is coefficient of heat transfer.
ho is evaluated from Eq. 13.5 (McAdams (1954) relation):

h o = 5.7 + 3.8 s for 0 < s < 5 m/s (13.5)

where, ‘s’ is wind speed.

Heat transfer (Q) is calculated using:

Q = Ue f A(Tsol − Twi ) (13.6)

where, ‘U ef ’ is overall heat transfer coefficient and ‘A’ is exposed area.

13 Review on PCM Application for Cooling Load Reduction … 267

1 1 δ1 δ2 δ3
= + + + (13.7)
Ue f ho k1 k2 k3

where, δ1 , δ2 , δ3 , k1 , k2 , k3 are thickness and the thermal conductivity (W/mK) of

PCM, tray material and brick respectively. k2 is assumed to be very high hence term
δ2
k2
≈ 0 (assumed).

13.10 Results and Discussion

The comparative values of mean ambient temperature and the sol air temperature for
the month of May are plotted and shown in Fig. 13.6. The value of sol air temperatures
for Chennai is relatively lower to that of Jaipur and New Delhi as the wind velocity
in Chennai is much higher than the other two places, resulting in the higher value of
convective heat transfer coefficient ho. This justifies the lower energy storage in the
PCMs in Chennai.
Figure 13.7 shows a comparison between two different PCMs and a sensible heat
storage material i.e. concrete for three different climatic conditions of New Delhi,
Jaipur and Chennai. The comparison clearly shows that more energy can be stored
using PCMs for all climatic conditions.
These initial calculations made affirms that PCM incorporation can save signifi-
cant amount of energy however, detailed thermal modelling is required, considering
a complete room incorporated with PCM.
A complete cyclic study is necessary as this study considers heat storage only
during the peak hours. The PCM temperatures has not been modelled and assumed

70
Average Hourly Ambient
temperature for Jaipur
60 Average Hourly Sol air
Temperature (°C)

temperature for Jaipur

Average Hourly Ambient
50 temperature for Chennai
Average Hourly Sol air
temperature for Chennai
40
Average Hourly Ambient
temperature for New Delhi

30 Average Hourly Sol air

11 hrs 12 hrs 13 hrs 14 hrs 15 hrs temperature for New Delhi
Time of the day

Fig. 13.6 Average hourly ambient and sol air temperatures for the month of May
268 R. Saxena et al.

14

12
kWh/day of savings
10

8 Capric Acid
Na2SO4. 10H2O
6
Concrete
4

0
New Delhi Jaipur Chennai

Fig. 13.7 Comparative kWh/day savings for different materials and places

to be constant at its phase change temperature as complete melting do not take place
during the period of five hours. Further there is a need to assess the discharging of the
PCM during the night, which is necessary for PCM to absorb heat on the subsequent
day.

13.10.1 Mapping/Selection of PCM

Adding PCM to the building envelop increases the storage capacity of walls/roof.
Mapping of different PCM is carried out for different climatic conditions in India.
Selection of PCM is done based on the melting temperature (as given in literature)
and the mean maximum and minimum temperature of the location. Both maximum
and minimum temperatures are considered as higher temperature during day melts
the PCM and when the temperature of the ambient is lower PCM releases heat and
gets discharged. It is necessary for the PCM to get solidified during the night or else it
will behave as a sensible heat storage, thus, reducing its storage capacity significantly.
Figure 13.8 and Table 13.5 show different PCMs that are selected based on melting
temperature and mean temperature of the places.
The PCMs are selected within the maximum and minimum temperature of the
places. However, they must be simulated for a PCM incorporated room to assess their
charging and discharging on implementation within buildings. This must be followed
by characterization of these PCMs to assess their thermophysical properties and their
temperature dependence to determine any mismatch in properties.
 PCM Mapping for different Climatic
Zones in India
13 Review on PCM Application for Cooling Load Reduction …

Fig. 13.8 PCM mapping for different climatic conditions in India

269
Table 13.5 PCM selected for different climatic conditions in India
270

Hot and dry Cold Moderate

Jodhpur Ahmedabad Srinagar Shimla Leh Bangalore Pune
Min avg 26.1 Min avg 26 Min avg temp 12.9 Min avg temp 13.9 Min avg 5 Min avg temp 20.8 Min avg 22
temp (°C) temp (°C) (°C) (°C) temp (°C) (°C) temp(°C)
Max avg 38.9 Max avg 38.4 Max avg temp 26.2 Max avg temp 21.8 Max avg 18.8 Max avg temp 31.6 Max avg 33.9
temp (°C) temp (°C) (°C) (°C) temp (°C) (°C) temp (°C)
PCM Melting PCM Melting PCM Melting PCM Melting PCM Melting PCM Melting PCM Melting
point point point point point point point
(°C) (°C) (°C) (°C) (°C) (°C) (°C)
Climsel 32 (De Na2 SO4 32 RT20 22 45–52% 17.2 Caprylic acid 16 Paraffin C18 28 (De Climsel C32 32 (De
C32 Gracia ·3H2 O [Sharma (Sharma LiNO3 ·3H2 O (Sharma (Sharma Gracia Gracia
and et al. et al. + 48–55% et al. et al. and and
Cabeza 2009] 2009) Zn(NO3 )2 2009) 2009) Cabeza Cabeza
2015) ·6H2 O 2015; 2015)
Mehling
and
Cabeza
2008;
Sharma
et al.
2009)
E32 32 (Liu 80% lauric 32.7 RT26 25 n- 19 (De Climsel C15 15 (De CaCl2 .12H2 O 29.8 E32 32 (Liu
et al. acid + 20% (Liu (Sharma Heptadecane Gracia Gracia (Mehling et al.
2012) palmitic acid et al. et al. and and and 2012)
2012) 2009) Cabeza Cabeza Cabeza
2015) 2015) 2008)

(continued)
R. Saxena et al.
Table 13.5 (continued)
Hot and dry Cold Moderate
Jodhpur Ahmedabad Srinagar Shimla Leh Bangalore Pune
Paraffin 36.7 Na2 HPO4 36 (De 45% 25 61.5 mol% 19 (Liu 48% butyl 17 (Liu LiNO3 .2H2 O 30 Na2 CO3 33 (De
C20 (Mehling ·12H2 O Gracia Ca(NO3 )2 (Saxena capric acid + et al. palmite + et al. (Mehling .10H2 O Gracia
and and ·6H2 O + 55% et al. 38.5 mol% 2012) 48% butyl 2012) and and
Cabeza Cabeza Zn(NO3 )2 2017) lauric acid stearate + 3% Cabeza Cabeza
2008) 2015) ·6H2 O other 2008) 2015)
69% 35.2 (Liu Zn(NO3 )2 36.4 Ethyl Palmitate 23 (Liu Glycerin 17.9 NaOH 15 (Liu Vinyl Sterate 27 62.6% lauric 32.6 (Liu
lauric acid et al. ·6H2 O (De et al. (Mehling ·(3/2)H2O et al. (Sharma acid + 37.4% et al.
+ 31% 2012) Gracia 2012) and 2012) et al. myristic acid 2012)
palmitic and Cabeza 2009)
acid Cabeza 2008)
2015)
CaBr2 34 (De LiBr2 ·2H2 O 34 34% 24 Capric acid + 18 (Liu 38.5% 14.4 (Liu TH29 29 (De Paraffin C19 32
·6H2 O Gracia (Mehling C14H28O2 + (Mehling lauric acid et al. trimethylole- et al. Gracia (Mehling
and and 66% and 2012) tane + 31.5% 2012) and and
Cabeza Cabeza C10H20O2 Cabeza water + 30% Cabeza Cabeza
2015) 2008) 2008) urea 2015) 2008)
FeCl3 37 (De Mn(NO3 ) 37.1 Climsel C24 24 45% capric 21 (Liu Dimethyl 16.5 (De RT30 28 (De Capric acid 31.5 (De
·6H2 O Gracia ·6H2 O (Mehling (Sharma acid + 55% et al. sulfoxide Gracia Gracia Gracia
and and et al. lauric acid 2012) (DMSO) and and and
Cabeza Cabeza 2009) Cabeza Cabeza Cabeza
2015) 2008) 2015; 2015) 2015)
Sharma
et al.
2009)
13 Review on PCM Application for Cooling Load Reduction …

(continued)
271
Table 13.5 (continued)
272

Warm and humid Composite

Trivandrum Chennai Guwahati New Delhi Lucknow Raipur
Min avg 24.2 Min avg 27 Min avg 23.1 Min avg 25.5 Min avg 24.4 Min avg 25.1
temp (°C) temp (°C) temp (°C) temp (°C) temp (°C) temp (°C)
Max avg 31.3 Max avg 35.7 Max avg 31.3 Max avg 37.3 Max avg 37.5 Max avg 38.1
temp (°C) temp (°C) temp (°C) temp (°C) temp (°C) temp (°C)
PCM Melting point PCM Melting point PCM Melting point PCM Melting point PCM Melting point PCM Melting point
(°C) (°C) (°C) (°C) (°C) (°C)
Vinyl Sterate 27 (Sharma Lauric acid + 34 (Liu et al. 4.3% NaCl + 27 (Sharma n-Eicosane 37 (Saxena Zn(NO3 )2 36.4 (De 77% lauric 33 (Liu et al.
et al. 2009) stearic acid 2012) 0.4% KCl + et al. 2009) et al. 2018) ·6H2 O Gracia and acid + 23% 2012)
48% CaCl2 + Cabeza 2015) palmitic acid
47.3% H2 O
Paraffin C18 28 (De Gracia LiBr2 .2H2 O 34 (Mehling LiNO3 ·3H2 O 30 (De Gracia E32 32 (Liu et al. 62% lauric 32.6 (Liu Capric acid 36 (De Gracia
and Cabeza and Cabeza and Cabeza 2012) acid + 38% et al. 2012) and Cabeza
2015; Sharma 2008) 2015; Sharma myristic acid 2015)
et al. 2009) et al. 2009)
CaCl2 .12H2 O 29.8 (Mehling Na2 CO3 33 (De Gracia 47% 30 (De Gracia Paraffin C20 36.7 (Mehling Mn(NO3 ) 37.1 (Mehling 64% lauric 32.8 (Liu
and Cabeza .10H2 O and Cabeza Ca(NO3 )2 and Cabeza and Cabeza ·6H2 O and Cabeza acid + 36% et al. 2012)
2008) 2015) ·4H2 O + 2015) 2008) 2008) palmitic acid
53%
Mg(NO3 )2
·6H2 O
Ga 30 (Mehling 62.6% lauric 32.6 (Liu LiNO3 .2H2 O 30 (Mehling OM32 33 (Mehling Polyglycol E 35–40 66% lauric 34.2 (Liu
and Cabeza acid + 37.4% et al. 2012) and Cabeza and Cabeza 1000 (Sharma et al. acid + 34% et al. 2012)
2008) myristic acid 2008) 2008) 2009) myristic acid
Methyl 29 (Liu et al. Ga + 29.8 (Mehling 62.5% 29.8 (Liu FeCl3 ·6H2 O 37 (De Gracia 80% lauric 32.7 (Liu CaBr2 ·6H2 O 34 (De Gracia
stearate 2012) [Ga-NH4 ] and Cabeza Trimethylole- et al. 2012) and Cabeza acid+20% et al. 2012) and Cabeza
2008) tane + 37% 2015) palmitic acid 2015)
water
n-Octadecane 28 (De Gracia Capric acid 31.5 (De RT30 28 (De Gracia Na2 SO4 ·10H2 O 32.4 (De CaBr2 ·6H2 O 34 (De Gracia Na2 HPO4 35.5 (De
and Cabeza Gracia and and Cabeza Gracia and and Cabeza ·12H2 O Gracia and
2015) Cabeza 2015) 2015) Cabeza 2015) 2015) Cabeza 2015)
R. Saxena et al.
13 Review on PCM Application for Cooling Load Reduction … 273

13.11 Conclusions

This chapter is key in selecting particular PCMs for heat flow reduction in build-
ings for different conditions in India. Studies carried out so far discuss about PCM
integration however, proper method for PCM selection has been found missing for
cooling load reduction of buildings in India. This study provides a comprehensive
description about different PCMs, their properties, advantages, challenges and meth-
ods of implementation and incorporation followed by their assessment in buildings.
The studies carried out in Indian context have been reviewed and impact of PCM
incorporation for different cities has been discussed. PCMs have been categorized
for cities lying in different climatic conditions based on their temperature variation
and phase change temperature of the PCMs.
Significant energy savings can be achieved with PCM incorporation as shown in
this study however, proper selection of PCMs is necessary in terms to their phase
change temperature, thermal stability and compatibility. This study envisages that
significant potential exits and more experimental studies need to be carried out to
assess the overall savings through PCM implementation within buildings in India.
This study can form the basis of initial PCM selection, benchmarking and their
implementation within Indian buildings.

References

Abhat A (1983) Low temperature latent heat thermal energy storage: heat storage materials. Sol
Energy 30:313–332. https://doi.org/10.1016/0038-092x(83)90186-x
Agyenim F, Hewitt N, Eames P, Smyth M (2010) A review of materials, heat transfer and phase
change problem formulation for latent heat thermal energy storage systems (LHTESS). https://
doi.org/10.1016/j.rser.2009.10.015
Babapoor A, Karimi G (2015) Thermal properties measurement and heat storage analysis of paraf-
finnanoparticles composites phase change material: comparison and optimization. Appl Therm
Eng 90:945–951. https://doi.org/10.1016/j.applthermaleng.2015.07.083
Benoit H, Spreafico L, Gauthier D, Flamant G (2016) Review of heat transfer fluids in tube-receivers
used in concentrating solar thermal systems: Properties and heat transfer coefficients. Renew
Sustain Energy Rev 55:298–315. https://doi.org/10.1016/j.rser.2015.10.059
Biplab K, Rakshit D (2017) Comparative assessment of thermal comfort with insulation and phase
change materials utilizations in building roofs and walls. Adv Mater Proc 2:393–397. https://doi.
org/10.5185/amp.2017/609
Cabeza LF, Mehling H (2003) Review on thermal energy storage with phase change : materials,
heat transfer analysis and applications
Cabeza LF, Castell A, Barreneche C, de Gracia A, Fernández AI (2011) Materials used as PCM in
thermal energy storage in buildings: a review. Renew Sustain Energy Rev 15:1675–1695. https://
doi.org/10.1016/j.rser.2010.11.018
De Gracia A, Cabeza LF (2015) Phase change materials and thermal energy storage for buildings.
Energy Build 103:414–419. https://doi.org/10.1016/j.enbuild.2015.06.007
Duffie JA, Beckman WA (2013) Solar engineering of thermal processes. Wiley, Fourth
274 R. Saxena et al.

Entrop AG, Halman JIM, Dewulf GPMR, Reinders AHME (2016) Assessing the implementation
potential of PCMs: the situation for residential buildings in the Netherlands. Energy Procedia
96:17–32. https://doi.org/10.1016/j.egypro.2016.09.090
Farid MM, Khudhair AM, Razack SAK, Al-Hallaj S (2004) A review on phase change energy
storage: materials and applications. Energy Convers Manag 45:1597–1615. https://doi.org/10.
1016/j.enconman.2003.09.015
Feldman D, Banu D, Hawes D, Ghanbari E (1991) Obtaining an energy storing building material
by direct incorporation of an organic phase change material in gypsum wallboard. Sol Energy
Mater 22:231–242. https://doi.org/10.1016/0165-1633(91)90021-C
Hawes DW, Banu D, Feldman D (1989) Latent heat storage in concrete. Sol Energy Mater
19:335–348. https://doi.org/10.1016/0165-1633(89)90014-2
Hoogendoorn CJ, Bart GCJ (1992) Performance and modelling of latent heat stores. Sol Energy
48:53–58. https://doi.org/10.1016/0038-092x(92)90176-B
Kant K, Shukla A, Sharma A (2017a) Advancement in phase change materials for thermal energy
storage applications. https://doi.org/10.1016/j.solmat.2017.07.023
Kant K, Shukla A, Sharma A (2017b) Heat transfer studies of building brick containing phase change
materials. Sol Energy 155:1233–1242. https://doi.org/10.1016/J.SOLENER.2017.07.072
Kaushik SC, Sodha MS, Bhardwaj SC, Kaushik ND (1981) Periodic heat transfer and load levelling
of heat flux through a PCCM thermal storage wall/roof in an air-conditioned building. Build
Environ 16:99–107. https://doi.org/10.1016/0360-1323(81)90026-3
Khadiran T, Hussein MZ, Zainal Z, Rusli R (2015) Encapsulation techniques for organic phase
change materials as thermal energy storage medium: a review. Sol Energy Mater Sol Cells
143:78–98. https://doi.org/10.1016/j.solmat.2015.06.039
Khodadadi JM, Fan L, Babaei H (2013) Thermal conductivity enhancement of nanostructure-based
colloidal suspensions utilized as phase change materials for thermal energy storage: a review.
Renew Sustain Energy Rev 24:418–444. https://doi.org/10.1016/j.rser.2013.03.031
Khudhair AM, Farid MM (2004) A review on energy conservation in building applications with
thermal storage by latent heat using phase change materials. Energy Convers Manag 45:263–275.
https://doi.org/10.1016/S0196-8904(03)00131-6
Konuklu Y, Ostry M, Paksoy HO, Charvat P (2015) Review on using microencapsulated phase
change materials (PCM) in building applications. Energy Build 106:134–155. https://doi.org/10.
1016/j.enbuild.2015.07.019
KoŚny J, Biswas K, Miller W, Kriner S (2012) Field thermal performance of naturally ventilated
solar roof with PCM heat sink. Sol Energy 86:2504–2514. https://doi.org/10.1016/j.solener.2012.
05.020
Ling T-C, Poon C-S (2013) Use of phase change materials for thermal energy storage in concrete:
an overview. Constr Build Mater 46:55–62. https://doi.org/10.1016/j.conbuildmat.2013.04.031
Liu M, Saman W, Bruno F (2012) Review on storage materials and thermal performance enhance-
ment techniques for high temperature phase change thermal storage systems. Renew Sustain
Energy Rev 16:2118–2132. https://doi.org/10.1016/j.rser.2012.01.020
McAdams WH (1954) Heat transmission. McGraw-Hill, New York, pp 252–262
Mehling H, Cabeza LF (2008) Heat and cold storage with PCM : an up to date introduction into
basics and applications. Springer
Mehling H, Cabeza LF (2008) Heat and cold storage with PCM an up to date introduction into
basics and applications. Springer
Navarro L, de Gracia A, Colclough S, Browne M, McCormack SJ, Griffiths P, Cabeza LF (2016a)
Thermal energy storage in building integrated thermal systems: a review. Part 1. Active storage
systems. Renew Energy 88:526–547. https://doi.org/10.1016/j.renene.2015.11.040
Navarro L, de Gracia A, Niall D, Castell A, Browne M, McCormack SJ, Griffiths P, Cabeza LF
(2016b) Thermal energy storage in building integrated thermal systems: a review. Part 2 inte-
gration as passive system. Renew Energy 85:1334–1356. https://doi.org/10.1016/j.renene.2015.
06.064
13 Review on PCM Application for Cooling Load Reduction … 275

Nayak JK., Prajapati JA (2006) Chapter -2 climate and buildings. in: handbook on energy conscious
buildings
Pasupathy A, Athanasius L, Velraj R, Seeniraj RV (2008) Experimental investigation and numerical
simulation analysis on the thermal performance of a building roof incorporating phase change
material (PCM) for thermal management. Appl Therm Eng 28:556–565. https://doi.org/10.1016/
j.applthermaleng.2007.04.016
Pluss Polymers (2017) Pluss PCM technical data sheet
Saxena R, Biplab K, Rakshit D (2017) Quantitative assessment of phase change material utiliza-
tion for building cooling load abatement in composite climatic condition. J Sol Energy Eng
140:011001. https://doi.org/10.1115/1.4038047
Saxena R, Rakshit D, Kaushik SC (2018) Experimental assessment of characterised PCMs for ther-
mal management of buildings in tropical composite climate. https://doi.org/10.11159/htff18.170
Saxena R, Rakshit D, Kaushik SC (2019) Phase change material (PCM) incorporated bricks
for energy conservation in composite climate: a sustainable building solution. Sol Energy
183:276–284. https://doi.org/10.1016/j.solener.2019.03.035
Sharma A, Tyagi VV, Chen CR, Buddhi D (2009) Review on thermal energy storage with phase
change materials and applications. Renew Sustain Energy Rev 13:318–345. https://doi.org/10.
1016/j.rser.2007.10.005
Shi J-N, Ger M-D, Liu Y-M, Fan Y-C, Wen N-T, Lin C-K, Pu N-W (2013) Improving the thermal
conductivity and shape-stabilization of phase change materials using nanographite additives.
Carbon N Y 51:365–372. https://doi.org/10.1016/j.carbon.2012.08.068
Singh RP, Xu H, Kaushik SC, Rakshit D, Romagnoli A (2019) Charging performance evaluation of
finned conical thermal storage system encapsulated with nano-enhanced phase change material.
Appl Therm Eng 151:176–190. https://doi.org/10.1016/J.APPLTHERMALENG.2019.01.072
Sleiti AK, Naimaster EJ (2016) Application of fatty acid based phase-change material to reduce
energy consumption from roofs of buildings. J Sol Energy Eng 138:051003. https://doi.org/10.
1115/1.4033574
Soares N, Costa JJ, Gaspar AR, Santos P (2013) Review of passive PCM latent heat thermal energy
storage systems towards buildings energy efficiency. Energy Build 59:82–103. https://doi.org/10.
1016/j.enbuild.2012.12.042
Solutia (1998) Technical bulletin 7239115C (supersedes 7239115B), therminol VP-1, heat transfer
fluid
Su W, Darkwa J, Kokogiannakis G (2015) Review of solid-liquid phase change materials and their
encapsulation technologies. Renew Sustain Energy Rev 48:373–391. https://doi.org/10.1016/j.
rser.2015.04.044
Teng T-P, Yu C-C (2012) Characteristics of phase-change materials containing oxide nano-additives
for thermal storage. Nanoscale Res Lett 7:611. https://doi.org/10.1186/1556-276X-7-611
Tyagi AP (2009) Solar radiant energy over India
Tyagi A, Singh OP, Singh SS, Kumar S (2011) Climate of Jaipur meteorological centre, Jaipur India
meteorological department ministry of earth sciences government of India
Zalba B, Marı́n JM, Cabeza LF, Mehling H (2003) Review on thermal energy storage with phase
change: materials, heat transfer analysis and applications. Appl Therm Eng 23:251–283. https://
doi.org/10.1016/s1359-4311(02)00192-8
Zhou D, Zhao CY, Tian Y (2012) Review on thermal energy storage with phase change materials
(PCMs) in building applications. Appl Energy 92:593–605. https://doi.org/10.1016/J.apenergy.
2011.08.025
Chapter 14
Fabrication and Thermal Performance
Evaluation of Metastable Supercooled
Liquid PCM Based Heat Pack

Rohitash Kumar, Sumita Vyas, Bobin Mondal, Ravindra Kumar

and Ambesh Dixit

Abstract Metastable supercooled liquid phase change material (MSLPCM) is pre-

pared by homogeneous mixing of sodium acetate trihydrate (SAT), water and ethy-
lene glycol (EG) in 92:5:3 weight fractions, respectively. A stainless steel (SS-306)
triggering disk of size 20 mm (L) × 18 mm (W) × 0.2 mm (T) is fabricated by
engraving grooves on SS disk for initiating nucleation in heat pack and release of
heat at the time of requirement. The PCM heat pack containing 300 g PCM and a
triggering disk is fabricated using high frequency PVC sealing machine. Thermal
performance of identical PCM heat pack and water pack (300 g water) is carried
out inside temperature history measuring setup at 0 °C ambient temperature. The
temperature of PCM heat pack and water heat pack reduces from 70 to 30 °C in 210
and 48 min, respectively at 0 °C ambient temperature. The heating time of PCM heat
pack is ~4.4 times more compared to water heat packs.

Keywords Phase change material · Supercooling · T-history · Latent heat · Heat

packs

14.1 Introduction

The prolonged exposure of the human body to very low ambient temperature con-
ditions may cause health problems such as frostbite and hypothermia. It is also very
difficult to go outside for work in such ambient conditions. Therefore, it is desirable
to use insulating clothing for thermal comfort and to reduce heat loss from the body
to the environment, maintaining the safe temperature limits for a human body in an
extreme cold environment. In the case of hypothermia and frostbite conditions, the
external heating source is required to provide heat to the body, maintaining the body
temperature in the safe limit for enhancing blood circulation and to reduce cold socks

R. Kumar · S. Vyas · B. Mondal · R. Kumar

Defence Laboratory, Ratanada, Jodhpur 342011, India
e-mail: dootrohit1976@gmail.com
R. Kumar · A. Dixit (B)
Department of Physics, Indian Institute of Technology Jodhpur, Jodhpur 342037, India
e-mail: ambesh@iitj.ac.in
© Springer Nature Singapore Pte Ltd. 2020 277
H. Tyagi et al. (eds.), Solar Energy, Energy, Environment,
and Sustainability, https://doi.org/10.1007/978-981-15-0675-8_14
278 R. Kumar et al.

and related adverse effects. Various technologies such as hot water bottles, electric
pads, electric blankets, and insulating clothing are being used to provide the ther-
mal comfort for inhabitants living in extremely cold conditions (Watson and Watson
1976). The insulating clothing maintains body temperature by reducing the rate of
heat transfer between the human body and surrounding ambient. This method may
be effective for a shorter time duration as there is no heating source. The relief time
using hot water bottles is less as it utilizes the specific heat of water (4 kJ kg−1 ) and
also provides heating in a large temperature window, not soothing to the human body.
The electric power based heating products utilizes a temperature controller such as
a thermostat to regulate the temperature of these devices. In the case of any failure
of the temperature controller, the temperature of the heating device may shoot and
cause serious skin burn causality. Further, these devices are not cost effective and not
suitable for outdoor applications. The coal-based heating devices such as Kangaries,
Bukharies release CO2 , CO gases and are dangerous for the health. These heating
systems are also prone to the skin cancer incidences (Wani 2010). The phase change
material (PCM) based technologies such as PCM heat packs are very promising for
thermal comfort under such conditions and therapeutic applications at the time of
requirement. It utilizes a phase change material as thermal energy storage media. The
thermal energy storage capacity of PCM is very high as compared to the sensible
storage materials such as water because of the latent heat of fusion of PCMs while
changing the phase of material from solid to liquid. For example, the latent heat val-
ues (and melting temperature) of some of salt hydrates CaCl2 .6H2 O, LiNO3 .3H2 O,
Na2 CO3 .10H2 O, Na2 SO4 .10H2 O are 174–191 kJ kg−1 (29–30 °C), 296 kJ kg−1
(30 °C), 267 kJ kg−1 (32 °C), and 241 kJ kg−1 (32.4 °C), respectively (Barrett et al.
1984; Barrett and Best 1985). Further, it releases heat in the very limited temperature
range (near phase change temperature of PCM), therefore it may be more effective
for body heating applications. The hydrated salt PCMs are being used in the PCM
heat packs. The hydrated salt PCMs are known for their large supercooling and may
remain in a metastable supercooled liquid state far below PCM’s melting tempera-
ture before solidification. This property of hydrated salts makes them very useful for
storing heat at the time of availability and releasing it later at the time of requirement
(Barrett et al. 1984; Barrett and Best 1985). These PCM heat packs are reusable,
reliable, and cost-effective, providing heat at body soothing temperature (Ulman and
Valentin 1983). Among numerous PCM based heat packs, the sodium acetate trihy-
drate (SAT) based heat packs has attracted more attention due to its high latent heat
(~270 kJ kg−1 ), suitable melting temperature (~58 °C), large supercooling (up to −
10 °C), and stability against large numbers of charging cycles, making it reusable,
and very cost effective (Kimura and Kai 1985). These PCM based heat packs use
a metallic disk containing multiple grooves to start nucleation in metastable super-
cooled liquid SAT for releasing heat at the time of requirement (Kapralis et al. 1990;
Sandnes and Rekstad 2006). These heat packs can be recharged by heating packs in
hot water for 25–30 min. Rohitash Kumar et al. developed SAT, ethylene glycol and
water-based PCM suitable for body warming in extremely cold climates and medical
applications such as treatment of muscle cramp, frostbite etc. (Kumar et al. 2017;
14 Fabrication and Thermal Performance Evaluation … 279

Kumar et al. 2016). In this study, authors have investigated the thermal performance
of PCM heat packs containing 300 g PCM at ambient temperature ~0 °C and the
results are compared with an identical water heat pack.

14.2 Materials and Methods

Ethylene glycol (EG) and Sodium acetate trihydrate (grade excel R) are procured
from Alfa Assar and used without any modification to prepare a metastable super-
cooled liquid PCM for fabricating a heat pack. SAT, EG and deionized (DI) water are
mixed in 92:5:3 weight fractions, respectively. The mixture is heated up to 80 °C and
stirred for 25–30 min using magnetic stirrer. After homogeneous mixing of samples,
the solution (~300 g) is filled in a PVC pack, as shown in Fig. 14.1a. Multiple grooves
are incorporated on a metallic disk made of Stainless Steel (SS)-304 for initiating heat
nucleation for releasing heat from PCM heat packs, shown in Fig. 14.1b. The size of
this metallic disk is 20 mm (L) × 18 mm (W) × 0.2 mm (T). An identical PVC pack
containing 300 g DI water is also fabricated to compare the thermal performance of
PCM heat pack with a similar water pack. In-house developed temperature-history
(T-history) setup is used to study the thermal performance of PCM and water heat
packs (Fig. 14.1c; Kumar et al. 2016). K-type thermocouples are used to measure
the temperature of packs and ambient temperature of the air chamber of T-history
setup by connecting to the data logger (model: 34972A, make: Agilent) for recording
temperature. The temperature data are recorded after every 10 s time intervals.

Fig. 14.1 Actual photograph of a PCM heat pack containing 300 g metastable supercooled liquid
PCM, b metallic disk consisting multiple grooves on its surface, and c a photograph of in-house
developed T-history measuring set up
280 R. Kumar et al.

14.3 Results and Discussion

14.3.1 Thermal Performance Evaluation of PCM Heat Pack

The metallic disk, consisting of multiple grooves, is used to start nucleation in the
PCM heat pack at the desired temperature. The process of the heat release from PCM
heat pack is explained in Fig. 14.2. The metastable supercooled liquid PCM based
heat pack containing 300 g MSLPCM and a metallic disk is shown in Fig. 14.2a. The
solid particles of SAT entrapped between two opposite surfaces of the metallic disk
is released after pressing the metallic disk and nucleation is started in metastable
supercooled liquid, shown in Fig. 14.2b. The nuclei in MSLPCM grow at a fast rate
due to the stability of SAT phase below its melting temperature i.e. solid phase.
Initially, the PCM heat pack is kept inside a T-history air chamber at 80 °C
temperature, above melting temperature ~56 °C of PCM for complete melting of
PCM. After ensuring the complete melting of PCM, the air chamber temperature is
reduced to 0 °C. The PCM pack surface temperature in conjunction with ambient
temperature is measured using K type thermocouple at every 10 s time interval, and
the recorded temperature-history data is plotted in Fig. 14.3.
The heat pack has shown initially cooling from 70 to 35 °C without any solid-
ification and at this temperature the nucleation is initiated by pressing the metallic
disk. The temperature of PCM heat pack is increased from 35 to 53 °C after initiating
solidification due to the heat release, stored in the form of latent heat of fusion in
SAT, Fig. 14.3. The MSLPCM solidifies in the phase change region and temperature
of heat pack remain in the phase change temperature range of MSLPCM (56–46 °C)
during phase change process. After complete solidification of PCM, the PCM pack
releases heat in the form of specific heat of PCM. The PCM heat pack took about
210 min to cool from 70 to 30 °C.

Fig. 14.2 Actual photographs of a PCM in metastable supercooled liquid state in a PVC pack, b ini-
tiating nucleation in MSLPCM by pressing metallic disk, and c growth of nucleation (solidification)
in PCM heat pack after 5 s
14 Fabrication and Thermal Performance Evaluation … 281

Fig. 14.3 Temperature

response of MSLPCM heat MSLPCM pack temperature
pack at ambient temperature 60 Ambient temperature
0 °C, recorded from in-house
developed T-history
measurement setup. The
small oscillations (<5 °C) in 40
ambient temperature are

T (º C)
attributed to the temperature
feedback control, causing
20
variations during its on-off
cycles

0 60 120 180 240 300 360

t (Minutes)

14.3.2 Thermal Performance Evaluation of Water Heat Pack

A DI water heat pack, identical to the MSLPCM heat pack, is fabricated to compare
the thermal performance with SAT based heat pack. The temperature of water heat
pack is measured from 70 °C at ambient temperature 0 °C, and the results are shown
in Fig. 14.4. The water heat pack took ~48 min for cooling from 70 to 30 °C, as
normal sensible heat release process.

Fig. 14.4 Temperature

response of water heat pack
at ambient temperature 0 °C, 60
used for comparison with Water pack temperature
SAT PCM heat pack. Here Ambient temperature
also small oscillations are
attributed to on-off cycles of 40
T ( º C)

temperature feedback control

while cooling the chamber
20

0 60 120 180 240 300 360

t (Minutes)
282 R. Kumar et al.

14.4 Conclusion

We designed MSLPCM based thermal heat packs for thermal comfort application
and the thermal performance of metastable supercooled liquid phase change material
based heat pack is evaluated against an identical water heat pack. The heating time
of MSLPCM based heat pack is ~4.4 times more than that of water based heat pack.
Thus, MSLPCM heat pack, showing excellent thermal performance, may be the
promising thermal energy storage systems for providing thermal comfort to human
beings in extreme cold environments in the form of body heating and also for medical
applications such as treatment of muscle cramps, frostbite etc.

Acknowledgements Authors acknowledge Defence Research and Development Organization

(DRDO) and Central Instrumentation Facility at IIT Jodhpur for supporting this work.

References

Barrett PF, Best BR (1985) Thermal energy storage in supercooled salt mixtures. Mater Chem Phys
12:529–536. https://doi.org/10.1016/0254-0584(85)90038-0
Barrett PF, Best BR, Oldham KB (1984) Thermal energy storage in supersaturated salt solutions.
Mater Chem Phys 10:39–49. https://doi.org/10.1016/0254-0584(84)90077-4
Kapralis GI, Kapralis JE, Lowther J (1990) Inventors assignee. Imperforate groove trigger. United
States patent US 4,899,727 A. 1990 Feb. 13
Kimura H, Kai J (1985) Phase change stability of sodium acetate trihydrate and its mixtures. Sol
Energy 35:527–534. https://doi.org/10.1016/0038-092x(85)90121-5
Kumar R, Dixit A, Chandra L, Vyas S, Kumar R (2016) An experimental set-up for measuring
thermodynamic response of low temperature phase change materials. In: 2016 First IEEE inter-
national conference on control, measurement and instrumentation (CMI), pp 107–109
Kumar R, Vyas S, Kumar R, Dixit A (2017) Development of sodium acetate trihydrate-ethylene
glycol composite phase change materials with enhanced thermophysical properties for thermal
comfort and therapeutic applications. Sci Rep 7:5203
Sandnes B, Rekstad J (2006) Supercooling salt hydrates: Stored enthalpy as a function of temper-
ature. Sol Energy 80:616–625. https://doi.org/10.1016/j.solener.2004.11.014
Ulman A, Valentin B (1983) Investigations of sodium acetate trihydrate for solar latent-heat stor-
age, controlling the melting point. Sol Energy Mater 92:177–181. https://doi.org/10.1016/0165-
1633(83)90040-0
Wani I (2010) Kangri cancer. surgery 147:586–588. https://doi.org/10.1016/j.surg.2009.10.025
Watson SS, Watson WKR (1976) Inventors; kay laboratories, Inc., assignee. Constant temperature
device. United States patent US 3,951,127A. Apr. 20
 Part V
Solar Cells
Chapter 15
Yet to Be Challenged: TiO2
as the Photo-Anode Material
in Dye-Sensitized Solar Cells

Janethri B. Liyanage, Ishanie Rangeeka Perera and R. J. K. U. Ranatunga

Abstract Utilization of renewable sources can reduce the impact of increasing

global energy demand on the rate of depletion of fossil fuels. One of the most stud-
ied and implemented routes to meet this energy demand is to harvest solar energy.
Among solar-energy harvesting devices, dye-sensitized solar cells have been recog-
nized as some of the cheapest and most environment friendly technologies, since they
do not require high purity of starting materials or advanced fabrication techniques.
A dye-sensitized solar cell is composed of a working electrode, in which the light
absorbing sensitizer is chemisorbed onto the surface of a wide bandgap semicon-
ductor; a redox electrolyte, that is placed in between two electrodes and functions to
regenerate the sensitizer; and a counter electrode, which is a catalyst which accel-
erates a redox reaction with the electrolyte. Titanium dioxide (anatase phase) is the
most widely used semiconductor material in dye-sensitized solar cells due to its
low cost, chemical stability and optical properties. In this chapter, the literature on
optimizing TiO2 as a semiconductor material for n-type DSCs is reviewed. The evo-
lution of TiO2 nanostructures and techniques such as doping, composite preparation
and surface modification are elaborated on. These methods have enhanced both the
chemical and physical properties of TiO2 nanostructures. Moreover, despite good
overall performance, rapid recombination kinetics are a major disadvantage inherent
in TiO2 . Thus, research has been carried out to substitute TiO2 with alternative semi-
conductors. In view of this, other potential competitors for photo-anode material are
reviewed and assessed. Finally, the prospects of an ideal semiconductor material for
dye-sensitized solar cells is discussed.

J. B. Liyanage · I. R. Perera (B) · R. J. K. U. Ranatunga (B)

Department of Chemistry, University of Peradeniya, Peradeniya 20400, Sri Lanka
e-mail: ishanieperera@pdn.ac.lk
R. J. K. U. Ranatunga
e-mail: udyranatunga@pdn.ac.lk
J. B. Liyanage
e-mail: janethriliyanage@gmail.com
I. R. Perera · R. J. K. U. Ranatunga
Postgraduate Institute of Science, University of Peradeniya, Peradeniya 20400, Sri Lanka

© Springer Nature Singapore Pte Ltd. 2020 285

H. Tyagi et al. (eds.), Solar Energy, Energy, Environment,
and Sustainability, https://doi.org/10.1007/978-981-15-0675-8_15
286 J. B. Liyanage et al.

Keywords Titanium dioxide · Photoanode · Dye-sensitized solar cells ·

Nanostructures

15.1 Introduction

The increasing demand for energy in conjunction with our dependence on fossil fuel
has fast-tracked the reduction of the oil reserves of the earth. Moreover, the use of
fossil fuel has resulted in severe ecological contamination and change. Renewable
energy sources promise to alleviate both these problems and among these sources,
solar energy has garnered much interest.
Photovoltaic cells, which convert solar radiation into electric current, are in the
forefront of plans for sustainable energy. Presently, first generation solar cells based
on crystalline or multi-crystalline Si hold a majority (>90%) of the market-share of
solar cells. However, the adoption of Si photovoltaic cells has not reached desired
levels, and research into more commercially viable technologies continues. Dye sen-
sitized solar cells (DSCs) are third-generation, semiconductor photovoltaic devices
that incorporate dyes to mediate efficient conversion of radiation. DSCs are consid-
ered a promising alternative (and complement) to single crystal silicon solar cells,
due to their lower manufacture cost, easy fabrication on flexible substrates, and per-
formance at ambient temperatures.
Although research into several materials has been reported, TiO2 remains the
most widely utilized photo-anode material for DSCs. In this chapter, we discuss
modifications to the TiO2 component which have been reported, and their effect on
DSC performance. We begin with a general treatise on DSCs, and then move on to
reviewing literature on modifications to TiO2 . Finally, we briefly discuss alternative
semiconductor materials to TiO2 , in terms of DSC function.
A contemporary DSC utilizes five components for operation, namely;
1. Transparent conductive oxide, the anode of the device. This is the side of the
device receiving solar radiation.
2. Semiconductor, deposited on the anode. This layer activates electronic conduc-
tion.
3. Sensitizer, a charge transfer dye bonded to the semiconductor.
4. Electrolyte, a redox mediator which restores the dye with each cycle.
5. Catalyst, coated on the cathode, which accelerates the redox reaction.
An illustration of the electron flow in a working DSC is given in Fig. 15.1. When
the DSC is exposed to sunlight, electrons in the sensitizer can absorb a photon with
energy hν, and excite to the LUMO. Dye sensitizers are chosen such that the LUMO
has high energy, and excited electrons can inject into the conduction band (CB) of
the working electrode (semiconductor) material. These electrons diffuse through the
anode and can be used to drive an external circuit. Electrons returning from the
counter electrode can reduce oxidized molecules of the redox-couple, which in turn
undergo a redox reaction with dye, regenerating it and completing the cycle.
15 Yet to Be Challenged: TiO2 as the Photo-Anode Material … 287

Fig. 15.1 Working diagram

of DSC

In DSCs, the semiconductor layer functions as the support for the sensitizer, and
as the transporter of photo-excited electrons from sensitizer to the external circuit.
The semiconductor/sensitizer interface provides the platform for charges to separate
after the photoexcitation of the dye, and consequently the injected charge carriers
diffuse to the external circuit. Therefore, an ideal semiconductor should have a large
surface area, for high dye loading; and fast charge transport, for efficient electron
collection (Ye et al. 2015). Furthermore, strong binding with the dye keeps the dye
in close proximity to the semiconductor; while suitable energy band potentials make
the electron transfer process more efficient. Therefore, these characteristics can be
considered qualifications for potential semiconductor materials (Gong et al. 2012).
The semiconductor is usually not fabricated as a monolith and is deposited as
separate dense and mesoporous layers (see Fig. 15.2). The dense layer, a fine layer
of the semiconductor material of about 100–200 nm thickness, prevents recom-
bination reactions. The mesoporous layer is deposited on the dense layer, and is
several micrometers thick. The mesoporous layer can be further divided into the
transparent layer and the scattering layer. The transparent layer, having a thickness
of about 1–15 μm, consists of particles of size 20–30 nm. A typical transparent layer
ensures a large surface area of the nanocrystalline semiconductor, which allows
higher dye adsorption. This leads to efficient light harvesting, especially for wave-
lengths under 600 nm. The scattering layer consist of much larger particles of a size
about 300–400 nm and is coated on the transparent conductive oxide layer. This size
of particles scatter light of longer wavelengths (red) back to the transparent layer, and
enhances the absorption of light by the sensitizer (Ghadiri et al. 2010). The thickness
288 J. B. Liyanage et al.

Fig. 15.2 Layered structure of material constituting a DSC

of the layer depends on the light absorbing ability of the dye and the deposition
technique used.
DSCs have been fabricated using both n-type semiconductors, where the majority
charge carriers are electrons, and p-type semiconductors, where the majority charge
carriers are holes. This chapter focuses on the n-type TiO2 semiconductor, which
is still considered the gold-standard in DSC devices, and has been the most investi-
gated semiconductor material since the 1970s. The long lifetime of excited electrons,
remarkable resistance to photocorrosion, low-toxicity, while being a cheaper alterna-
tive, have made TiO2 quite popular for solar energy applications (Roose 2015). TiO2
nanostructures can be synthesized using sol-gel methods, hydrothermal synthesis,
solvothermal synthesis, anodization, flame spray pyrolysis, among other methods.
Although the efficiency of the original DSC fabricated by Grätzel achieved only
an efficiency of 7% (O’Regan and Grätzel 1991), subsequent optimization of devices
and developments in the field have steadily improved efficiencies, until Chiba et al.
reported the highest certified efficiency of ~11% (Chiba et al. 2006). In this chapter
we review novel approaches in breakthroughs in optimizing the TiO2 semiconductor
material to produce better performing DSCs.

15.1.1 Characterizing DSSCs

Several parameters can be used to characterize the operation of a DSC:

Open circuit voltage (VOC ), the maximum voltage that the array provides when the
terminals are not connected to any load. According to band gap theory, the difference
between the quasi-Fermi level of the semiconductor layer and the electrolyte redox
potential, controls the highest voltage generated under illumination.
Short circuit current (JSC ), the maximum current generated by the photovoltaic
array when the output connectors are shorted together. JSC depends on the incident
photon to current efficiency (IPCE) and the incident photoflux. JSC can be improved
15 Yet to Be Challenged: TiO2 as the Photo-Anode Material … 289

Fig. 15.3 Characteristic current-voltage and power-voltage curves of DSCs

by increasing the IPCE value using panchromatic dyes which can absorb incident
light at a broader wavelength range.
Fill factor (FF), the ratio between maximum power the array can provide under
normal conditions, and the product of VOC and JSC . The FF is related to the electron
transfer and the internal resistance of the DSC, and can be optimized by minimizing
the internal series resistance.
V max × J max
Fill Factor(FF) =
V oc × J sc

Efficiency (η), which is the ratio between the maximum electric power that the array
can produce compared to the amount of solar irradiance incident hitting the array.

V max × J max
Efficiency (η) =
Pin
These parameters can be extracted from typical current-voltage (J–V) plots, as
shown in Fig. 15.3.

15.1.2 Modifications to TiO2

Throughout years of development, many modifications to TiO2 have been carried out
to increase the overall efficiency of DSCs through the enhancement of light harvest-
ing efficiency, enhancing charge collection efficiency, and photogenerated charge
transport efficiency (Kim et al. 2008). This chapter categorizes these modifications
into three categories,
290 J. B. Liyanage et al.

1. Morphology
2. Composites/hybrids
3. Doping.
In this chapter, we focus on changes in the structures and morphology, composites,
and doping, in terms of their influence on the properties of TiO2 DSCs. These three
categories will be discussed separately. Apart from these surface modifications aim to
improve the properties roughness, hydrophobicity, biocompatibility, surface energy,
gas diffusion barrier etc. (De Jonge et al. 2008). In this chapter a separate space has
not been allocated for the surface modifications done on TiO2 as these are discussed
under other modification techniques.

15.2 TiO2 Morphology

The availability, ease of synthesis, and the intrinsic energy band structure are some
reasons TiO2 remains the forerunner among semiconductor materials for n-type
DSCs. Apart from the material, the structure and morphology can have a consid-
erable influence on the photogenerated electrons produced by a semiconductor. The
multi-layered structuring of TiO2 in conventional DSCs has many advantages, as
described in the previous section. However, there are many facets of this architecture
which have been targeted for optimization.
Increasing total dye adsorbed on the TiO2 has been an objective in tuning the
morphology. Raising the number of sensitizer molecules in contact with the semi-
conductor material enhances the light harvesting efficiency (O’Regan and Grätzel
1991). Either improving the binding of dye to the semiconductor or increasing the
surface area of the material could elevate dye adsorption. Several studies have inves-
tigated the change in efficiency of cells when the porosity and particle size of TiO2
are varied.
O’Regan et al. studied colloidal TiO2 , of average particle size of 15 nm, which
was then deposited on a conducting glass sheet (O’Regan and Grätzel 1991). After
the deposition a monolayer of the Ru based dye, the film turned into a deep reddish
brown colour, shifting the absorption onset to 750 nm and giving a light harvesting
efficiency of almost 100% below 550 nm, in the visible range (η = 7.9%, at 10%
sunlight; 7.12% at 100% sunlight).
After Grätzel and O’Regan’s initial work the highest efficiency obtained so far,
of 11.1% was attained by Chiba et al. They used the strategy of trapping incident
light by incorporating submicron (400 nm) sized TiO2 nanoparticles together with
nanosized TiO2 particles. The investigators optimized the haze, which is the portion
of diffused light in total transmittance, at 800 nm to achieve high JSC .
After decades, a similar study has been done on flexible substrates by Pichot et al.,
using a low temperature sintering method, without the use of an organic surfactant,
which resulted in a high dye loading capacity, with respect to the film thickness, that
compensated the lower efficiency of the DSC. The lower efficiency can be attributed
15 Yet to Be Challenged: TiO2 as the Photo-Anode Material … 291

to the inefficient charge transport of electrons through the TiO2 film, due to the pores
of the film, causing recombination reactions before being collected (Pichot et al.
2000).
Another reason for optimizing the layer structure of TiO2 is to lessen the grain
boundaries of the material. This allows for faster transport of charges and lower
recombination. Yu et al. lowered the thickness of the mesoporous layer from the
conventional 10 μm, to 300 nm, leading to recombination only at the surface, and
eliminating recombination with the oxidized dye and redox electrolyte. An added
advantage was that at these thicknesses, the localized surface plasmon created at the
FTO/TiO2 interface was able to induce a relatively strong electric field at the TiO2 /dye
interface, enhancing light absorption by the dyes and increasing cell efficiency. How-
ever, in this study, oxygen vacancy-Ti3+ type surface defects were formed, decreas-
ing the efficiency almost linearly with the increased concentration of Ti3+ (Tributsch
2004). Defects can serve as recombination centers and pathways for electron back
transfer, and decreased VOC and FF. The lower efficiency can also be attributed to
the lower dye adsorption, due to the smaller mesoporous layer, leading to a lower
JSC (Yu et al. 2012).
To address both the surface area and grain boundaries, several groups have inves-
tigated fabricating TiO2 in nanostructures to yield materials with fewer grain bound-
aries, and well-defined conduction pathways. These materials have the potential of
(1) improving of electron transport, (2) enhancing dye adsorption through increas-
ing surface area, and (3) augmenting scattering of red light, where absorption of
most molecular sensitizers is weak (Ghadiri et al. 2010). The nanostructured mor-
phologies that have been tested for DSCs include hollow nanoparticles, nanorods
and nanofibers, anodized nanotube arrays, and hierarchical 3D nanostructures (see
Fig. 15.4).

Fig. 15.4 Modification of the morphology of TiO2

292 J. B. Liyanage et al.

Another nanostructure that was considered for the development of TiO2 , as a bet-
ter semiconductor material in DSCs, would be hollow nanoparticles. In 2008, Yang
et al. synthesized quasi-ordered TiO2 hollow hemispheres, using a colloidal template
and radio frequency-sputtering, in order to obtain fast electron transport and high
surface area in a thin photoelectrode, and obtained a photoconversion efficiency of
3.49%. The fact that the diameter, wall thickness and the height of these unique
microstructures could be easily changed and the enhanced surface activity, encour-
ages its application in DSCs (Yang et al. 2008). Koo et al. studied nano-embossed
hollow spherical TiO2 particles to obtain bifunctionality: light scattering and pho-
toexcited electron generation, to obtain an efficiency of 10.34%. It has been confirmed
that the nano-embossed hollow spheres containing layer was able to adsorb 5 times
more of the dye while acting as the scattering layer (Koo et al. 2008). Similarly, Yu
et al., studied hollow structured TiO2 due to its low density, high surface area and
its porous structure which affects the increase in light harvesting efficiency and fast
motion of charge carriers. Under the optimal conditions, they were able to generate
an efficiency of 4.82% (Yu and Zhang 2010).
Nanofibers have also been used as a structural modification implemented on to
TiO2 , in the aim of improving the photovoltaic properties. Ghadiri et al. used cellulose
fibers as a template material to synthesize the hollow TiO2 nanofibers, by a stepwise
hydrolysis and dehydration of a saturated Ti4+ solution. The unoriented nanostruc-
ture showed enhanced electron transport properties compared to the mesoporous
layer made of spherical nanoparticles, leading to 7.15% efficiency under optimal
conditions. The retardation of surface recombination, between the conduction band
electrons and oxidized species in the electrolyte, observed in nanostructured fiber-
based cells was able to balance out the adverse shift of the band edge toward positive
potential and produce an open-circuit photovoltage of 760 mV, which is a gain com-
pared to that of the DSC containing nanoparticles (Ghadiri et al. 2010), which is
seen with the decrease of the photoactive layer. This shift of the conduction band
edge towards the positive potential, has been generally observed for DSCs based
on nanotubes and nanoparticles (Kuang et al. 2008). TiO2 nanorods have been fur-
ther studied by Pandanga et al. to optimize the morphology and dye uptake and to
encourage fast electron transport avoiding recombination reactions (Pandanga et al.
2019).
Self-organized porous metal oxides with a nanotubular structure have attracted
significant interest in recent years (Fraoucene et al. 2019). It possible to apply anodi-
cally formed TiO2 nanotubes to photovoltaic devices, yielding semiconductor mate-
rial with a large interconnected internal surface area (O’Regan and Grätzel 1991).
Although considerable research has been concentrated on the development of nan-
otubular TiO2 layers, there is no simple synthesis route to obtain a uniformly stretched
nanotubular structure, other than by an electrochemical anodization process. How-
ever, electrons in the nanotube can also meet with many grain boundaries, defects,
and trap sites, becoming a factor that retards the electron transport time (Kang et al.
2007).
Kai Zhu et al. reported that the light-harvesting efficiencies of nanotube (NT)-
based DSCs were higher than those of nanoparticle (NP)-based DSCs. It was also
15 Yet to Be Challenged: TiO2 as the Photo-Anode Material … 293

shown that by reducing the disorder that is typically present at the top of the NT
layers, an increase in efficiency can be obtained as the dye uptake is increased, while
the internal light scattering effect is seen to be strongly (Zhu et al. 2006, 2007).
It was later discovered that some of these advanced morphologies can be directly
grown on the conductive substrate such as wires (Feng et al. 2008), templated rods
(Liu and Aydil 2009) or self-organized anodic structures (Macak and Schmuki 2006;
Stergiopoulos et al. 2008) that can be obtained as an oxide layer firmly attached to
the Ti-(metal) substrate.
In the aim of allowing a longer, uninterrupted path for the photogenerated elec-
trons, Kim et al. synthesized a bamboo type morphology, by controlled anodization
of a Ti plate, for the increase in the photovoltaic efficiency. However, it was observed
that there was no change in the transport properties or the recombination kinetics of
bamboo type NTs and normal NTs. The only improvement observed was the dye
loading capacity, as the bamboo type NTs allow the exterior as well as the interior
coverage which enhanced the generated current, giving a maximum efficiency of
2.98%, which is greater than that obtained for normal NTs (Kim et al. 2008). But
later on, several more studies have been done using this bamboo type NTs, in order to
further increase the dye loading capacity. Luan et al., studied a two-step method, to
form ridges on the surface of the bamboo type NTs and to obtain evenly lengthened
NTs. It was noted that the ridge density and the length of the NT had a significant
impact on the overall efficiency. Highest efficiency of 6.80% was obtained with the
bamboo type NTs synthesized under a lower water content, which led to a longer tube
length (Luan et al. 2012). Further studies were done by Ji et al. where they synthe-
sized double walled bamboo type NTs. The dye loading was found to be significantly
larger for the DSCs fabricated with NTs of higher ring density, leading to a better
conversion efficiency (Ji et al. 2012). Wang et al. too studied a similar morphology
which led to a reduced interfacial resistance and increased interfacial capacitance,
compared to that of smooth walled NTs, resulting in a higher dye loading capability
(Wang et al. 2014).
More recently, in 2019, research has been carried out to construct a hierarchical
hetero-structured TiO2 photoanode material, where anatase nano branches and rutile
nanorods were used, aiming for a higher surface area (Jin et al. 2019). Also, research
has been conducted to include TiO2 hollow spheres as well as nanorods as a composite
powder, to be applied as the light scattering layer in the photoanode of DSC. The
increase in efficiency, up-to 9.58%, can be attributed to the two different scattering
structures incorporated (Marandi et al. 2019) (Table 15.1).

15.3 TiO2 Composites/Hybrid Materials

The VB of TiO2 is composed of hybridized 2p orbitals of oxygen and the 3d orbitals

of titanium, while the CB is purely 3d orbitals of titanium, which causes a decrease
in the transition of electrons back to the VB, minimizing recombination reactions.
294 J. B. Liyanage et al.

Table 15.1 Table with the

Modification Efficiency (%) References
maximum efficiencies
obtained by modifications to TiO2 submicron par- 11.1 (Chiba et al. 2006)
the nanostructure or ticles/nanoparticle
morphology mix
TiO2 hollow spheres 9.58 (Marandi et al. 2019)
and nanorods
Colloidal TiO2 7.9 (O’Regan and
Grätzel 1991)
Nanofibres 7.15 (Ghadiri et al. 2010)

Due to the high surface area, TiO2 in leads to efficient dye loading, and consequently
high JSC . The high Fermi level leads to a higher Voc , which ultimately improves the
overall efficiency.
However, inefficient electron transfer through the FTO/TiO2 interface increases
the back-electron transfer to the electrolyte. The electron trapping and the high den-
sity of grain boundaries present with spherical TiO2 nanoparticles, also lowers con-
version efficiency (Kopidakis et al. 2003). Therefore, much effort has been made to
minimize the back-electron transfer and recombination. Using composites in DSC
is one of the modifications done in order to minimize these problems.
Composites in DSCs use combinations of one or more materials including, metals,
metal oxides, metal sulfides, etc. where the combination can be of the form of layered
or core shell structures. DSCs using TiO2 as a component can be produced by various
methods such as chemical synthesis, solution or gas phase synthesis and template
fabrication (Dahl et al. 2014). A composite material should have two main properties:
(1) a CB band edge with energy between the TiO2 CB and sensitizer LUMO (see
Fig. 15.6), to facilitate the electron injection from the dye to the electrode; and (2) a
high electron mobility (Mao et al. 2016). These properties would lead to a smoother,
stepwise electron transport from the dye to the working electrode. Composites of TiO2
can be tuned by controlling the composition of the material, in order to decrease the
charge carrier recombination rate or adjust the band gap. Here, discussion of TiO2
composites has been further broken down into subsections by the type of modifying
material.

15.3.1 Metals

The most commonly used metallic nanoparticles in preparing composites with TiO2
are silver and gold, as they can improve the optical absorption by inducing surface
plasmons (Deepa et al. 2012) and form Schottky barriers at the metal-TiO2 interface,
which can reduce electron recombination (Chou et al. 2009). These can also increase
the optical path length in electrodes, by reflecting and scattering incident light (Peng
et al. 2013). Another important factor is that they can form electron transfer networks.
15 Yet to Be Challenged: TiO2 as the Photo-Anode Material … 295

Fig. 15.6 Energy band

structure of a DSC utilizing a
TiO2 composite material for
the photoanode

Generally, there are two methods of preparing the metal composite. One is by
simply mixing the metal and TiO2 , the other method is immersing TiO2 in metal
solution and reducing the adsorbed metal ions using either thermal evaporation or
atomic laser deposition. Wu et al. have carried out a method to deposit the silver
nanoparticles by chemical reduction with the aid of NaBH4 . The highest efficiency
of 5.66% was observed at the highest concentration of NaBH4 leading to a larger
sized nanoparticle. The silver clusters formed on the surface TiO2 would act as a
scattering layer reflecting incident light, which increased the photocurrent. In this
study, the dye adsorption was increased by an acid treatment (Wu et al. 2017). Silver
has also been coated on TiO2 , using photo deposition and an overall efficiency of
6.86% was obtained. The investigators proposed that the silver layer lengthens the
optical pathway and suppresses the charge transfer from surface states (Peng et al.
2013). Chou et al. prepared two composite mixtures containing silver and gold, using
a dry coating method. This study concludes that the doping of metal nanoparticles
improves the overall efficiency but the process of fabrication is very important for
the overall efficiency of the DSC (Chou et al. 2009).
Similar studies have been done incorporating silver and gold as a composite mate-
rial using a hydrothermal process (Muduli et al. 2012) and one-step electrospinning
process (Nam et al. 2010). Studies have also been done using various nanostructures
of TiO2 along with metal, such as the use of hierarchical TiO2 spheres containing nee-
dles with gold nanoparticles for a plasmonic enhancement (Bai et al. 2016) and the
use of TiO2 hollow spheres, nanoparticles, nanosheets with the metal nanoparticles
and nanowires for an enhanced electron transport (Ran et al. 2018).
296 J. B. Liyanage et al.

One of the drawbacks of using metals as composites is that I− /I3 − , the most
commonly used redox electrolyte, is corrosive in the presence of metals, which
would affect the durability of the system (Chou et al. 2009).

15.3.2 Metal Oxides

Metal oxides can be considered as the most commonly used material in preparing
composites with TiO2 , for DSCs. Many have been studied so as to provide the
electrons an easier transport with a stepwise conduction band edge. Among all metal
oxides, Al2 O3 , ZnO, NiO, SiO2 and SnO2 have been studied extensively.
Zhang et al. studied the use of a blocking layer of Al2 O3 , so as to avoid recombi-
nation reactions, leading to a better VOC and FF. Although VOC and FF increase with
greater thickness of the Al2 O3 layer, confirming the function of the blocking layer,
JSC decreases. This may be due to the fact that the probability of electrons tunneling
through the barrier has an exponential relationship with the tunneling length; or due
to the decrease in dye adsorption due to the blocking layer. At an optimal coating
the highest efficiency was obtained was 2.59% (Zhang et al. 2003).
Similar studies have been done by coating Al2 O3 with reactive direct current
magnetron sputtering (Wu et al. 2008), by stepwise condensation (Choi et al. 2008),
using highly ordered TiO2 /Al2 O3 (Kim et al. 2010) and also using highly ordered,
vertically oriented TiO2 /Al2 O3 nanotubes (Kim et al. 2014), where they have also
made a modification to the structure.
Another popular material in preparing composites with TiO2 is ZnO, due to its
high electron mobility and band gap quite similar to that of TiO2 . However, using pure
is thought to cause degradation of the dye forming Zn2+ -dye complexes. Manthina
et al., studied the use of ZnO containing composites using 1D nanostructures. Overall
dye uptake was lowered, but even taking this into account, it was observed that
the performance of the pure TiO2 system was greater than that of the ZnO-TiO2
containing. The authors hypothesise that the transfer of electrons from TiO2 to ZnO
is attenuated as the CB edge of ZnO lies at a slightly more negative potential than
that of TiO2 , and more work is needed to remove this barrier (Manthina et al. 2012).
Self-organized nanotubular metal oxides have attracted much interest in the recent
past. But there is no simple synthesis method to obtain these structures other than
anodization of the metal (Beranek et al. 2005), also leading to many grain boundaries,
defects, and trap sites, becoming a factor that retards the electron transport time. Kang
et al. tried to suppress the charge recombination by applying a wide band gap metal
oxide, such as ZnO, to coat the TiO2 film. The formation of an energy barrier by
coating as a shell at the electrode/electrolyte interface was found to be essential for
increasing the physical separation of the injected electrons from the cations of the
redox electrolyte, thereby decreasing the rate of charge recombination. Here, in order
to improve the FF, an H2 O2 surface treatment was done, which reduced the thickness
of the TiO2 barrier layer. The ZnO coating was attributed to suppress electron flow
15 Yet to Be Challenged: TiO2 as the Photo-Anode Material … 297

to the back-direction, which enhanced the Voc , leading to a maximum efficiency of

0.906% after the H2 O2 treatment (Kang et al. 2007).
ZnO as a composite material with TiO2 has been studied by synthesizing a coaxial
structure (Williams et al. 2012), as a photoanode material for an effective UV-visible
active DSC (Noor et al. 2018), as multilayers (Bhatti et al. 2019) and as a core-shell
nanostructure (Rajamanickam et al. 2019).
SnO2 is another candidate for applications in DSCs. SnO2 exhibits a band gap of
3.8 eV, larger than that of TiO2 , generating much fewer oxidative holes under illu-
mination, therefore improving the long-term stability of the DSSCs and minimizing
the degradation rate of the dye (Qian et al. 2009). Also, electron mobility in SnO2
is much faster than in TiO2 . However, SnO2 has the disadvantages of having poor
dye loading capacity and fast charge recombination, when used in pure form within
DSCs (Gao et al. 2012). These shortcomings can be overcome by the use of a TiO2
barrier, which has been studied by Huo et al. The overall performance of the DSC
was increased to 6.98%, compared to that of P25 and pure SnO2 photoelectrodes,
due to the high light scattering effect, fast electron transport due to the 1D structure
and the enhanced dye loading capability (Huo et al. 2014).
Desai et al. has also studied similar hybrid materials using TiO2 and SnO2 (Desai
et al. 2013). Further research based on the TiO2 –SnO2 composite has been done using
TiO2 compact layer with nanocrystalline SnO2 (Yang et al. 2014), using a core-shell
structure (Knauf et al. 2015), by the synthesis of hybrid nanofibers (Wali et al. 2016)
and also by the sol-gel and coprecipitation method (Endarko and Adawiyah 2019).
TiO2 has also being incorporated with the p-type semiconductor NiO, by Hsu
et al. They have used a flower like structure in order to enhance the light scattering,
trapping for efficient photon harvesting and offering a large surface area for dye
adsorption, thereby improved power conversion efficiency. The p-type component in
TiO2 –NiO nanoparticles has significantly decreased the charge recombination, but
its photovoltaic efficiency remains below 4% (Hsu et al. 2015).
Maçaira et al. prepared a SiO2 /TiO2 composite photoelectrode to overcome the
high number of grain boundaries and morphological defects, that cause recombina-
tion reactions, of a usual TiO2 photoelectrode, while keeping the surface area at an
optimum level. The high efficiency of 9.20% is due to the high surface area, and the
low recombination (Maçaira et al. 2017). A similar study has been done by Cardoso
et al. tuning anatase and rutile phase transition temperature (Cardoso et al. 2019).
More recently, studies have been conducted using gold, SiO2 and TiO2 in a core/shell
structure (Fadhilah et al. 2019; Li et al. 2019).
Furthermore, Fe2 O3 has also been studied considering the cocktail effect of the
two conduction bands, to prevent electron recombination (Im et al. 2011).

15.3.3 Metal Nitrides

Huang et al. have studied the composite gallium nitride (GaN) with TiO2 as a pho-
toelectrode material in order to enhance the power conversion. Here a maximum
298 J. B. Liyanage et al.

efficiency of 4.90% has been obtained after optimizing the amount of GaN used in
the composite mixture, which is an increase of about 60% when compared with that
of a DSC of pure P25 nanoparticles. Here the photovoltaic performance has increased
due to the decrease in charge transfer resistance and the increase in charge recombi-
nation resistance, at the electrolyte/dye/semiconductor interface (Huang et al. 2014).
Another metal nitride composite that has been studied is titanium nitride (TiN)
with TiO2 . Higher content of TiN has shown a higher absorption of visible light. The
VOC has been increased as the flat band potential has shifted to a more negative value.
The increase in FF can be attributed to the high conductivity of the film. However,
this system yielded a decrease in Jsc which can be due to the low amounts of dye
adsorption. Here an efficiency of 7.27% was observed, which is much higher than
that obtained by simply using a P25 photoanode (Li et al. 2015).

15.3.4 Metal Sulfides

Photovoltaic properties of some metal sulfide-TiO2 composites have been studied,

several which have shown remarkable potential in solar cell applications. Ding et al.
studied a one step, high temperature, solvothermal method to synthesize TiO2 -sulfide
nanospheres. They were prepared using the respective hydrated sulfate. The consid-
ered composite materials are CdS (Wang et al. 2010), Cu2 S (Peng et al. 2014),
ZnS (Sadikin et al. 2019) and Co9 S8 (Yuan et al. 2017). These sulfide containing
composites absorbed visible light and the high reaction temperature improved the
crystallinity, making the composite mixtures suitable for DSSC application as well
(Ding et al. 2012).

15.3.5 Carbon Nanostructures

Carbonaceous nanomaterials are an economic option to composite with TiO2 , which

can result in enhanced efficiency of DSCs. The proposed activity of carbonaceous
materials is either by excitation of the carbonaceous compound followed by charge
injection to TiO2 (Wang et al. 2005) or by formation of a Ti–O–C bond which leads
to the creation of energy states within the band gap of TiO2 (Pyrgiotakis et al. 2005),
facilitating more efficient light absorption. The mechanism depends on the synthetic
technique used. These composites also show improvement in dye adsorption and
increased electron transport.
One of the most commonly used carbon nanostructures in preparing composites
with TiO2 are carbon nanotubes (CNT). As they not only have a large electrons-
storage capacity, but also can show electronic conductivity similar to that of metals
(Kongkanand et al. 2007). The 1D nano-structure and good electrical conductivity
of CNT are beneficial to transport the electrons within TiO2 films and enhance their
photocatalytic and photoelectric conversion efficiencies (Yu et al. 2007).
15 Yet to Be Challenged: TiO2 as the Photo-Anode Material … 299

Table 15.2 Table with the

Modification Efficiency (%) References
maximum efficiencies
obtained by SiO2 /TiO2 9.20 (Maçaira et al. 2017)
composites/hybrid materials TiN/TiO2 7.27 (Li et al. 2015)
SnO2 nanorods-TiO2 6.98 (Huo et al. 2014)
hybrid material
Ag coated TiO2 6.86 (Peng et al. 2013)

Yu et al. studied CNT with both P25 nanoparticles and TiO2 hollow spheres and
obtained an efficiency of 4.71%, under optimal conditions when hollow spheres
were used. The improvement in the conversion efficiency is due to the fact that CNT
can reduce the electrolyte/electrode interfacial resistance, the recombination rate of
excited electrons and holes, and enhance the transport of electrons from the films to
FTO substrates (Yu et al. 2011). More studies have been done using functionalized
single-walled CNTs (Jang et al. 2004), using low temperature fabrication methods
(Lee et al. 2008) and by the synthesis of a hybrid material with multi-walled CNTs
(Mehmood et al. 2015).
Incorporating carbon nanofibres (CF) in TiO2 has also been studied extensively.
The CF/TiO2 composite has been studied after using a spray-coating mechanism
(Sigdel et al. 2014), coaxial electrospinning (Hieu et al. 2014) and bilayer structuring
(TiO2 /CF and Ag@TiO2 core-shell structure) (Lu et al. 2018).
Research has also been done using graphene and TiO2 as the composite material,
as graphene has excellent optical and electrical properties. Studies have used electro-
spun TiO2 -graphene nanofibers (Anish Madhavan et al. 2012) and simple inclusion
of graphene in the composite material (Kusumawati et al. 2014; Zhu et al. 2014).
A unique study done with the use of graphene was by Chen et al., where they used
a TiO2 /graphene/TiO2 sandwich structures. The sandwich structure improved the
performance of the DSC (η = 3.93%) over pure TiO2 and TiO2 /graphene containing
photovoltaics. This improvement is associated with an increase in the absorption of
light, a wide range of absorption wavelengths, shorter charge transportation distances,
and the suppression of charge recombination when the graphene is applied (Chen
et al. 2014).
Since graphene has a higher work function than TiO2 (Dahl et al. 2014), it allows an
increase in charge separation by electron injection into the graphene sheets. Chemical
utilization of graphene has been established through reduction of graphene oxide
sheets and has been used in DSC for the improvement of its photovoltaic properties
(Low and Lai 2018; Suriani et al. 2019) (Table 15.2).

15.4 Modifications Done by Doping

Changes in the semiconductor electronic band structure can be accomplished by

substitutional doping, which is the deliberate replacement of semiconductor atoms
300 J. B. Liyanage et al.

in the host material with impurities, usually of similar atomic radius to the ion to
be replaced in the lattice. Here, either electrons are donated by the dopant (n-type
doping), where the valency is higher in the dopant than of the host material, or holes
would be donated by the dopant (p-type doping), where the valency is lower than that
of the host material. Figure 15.7 illustrates the changes in the energy band structure
due to doping.
Doping in TiO2 can be achieved by either the replacement of Ti4+ or O2− in the
lattice. Since the lower edge of the CB has a high contribution of Ti4+ 3d orbitals,
replacing Ti4+ by a different cation is expected to heavily affect the CB structure.
The upper edge of the VB consists of O2− p orbitals and replacing O2− by a different
anion affects the VB energy (Roose 2015). Moreover, electron traps are caused by
oxygen vacancies, titanium interstitials and the reduced crystal surface, which would
lead to a higher conductivity. A positive shift of the flat band potential from the CB
would lead to a better electron injection, giving a better JSC , but a lower VOC as
the gap between the Fermi level and the potential of the redox electrolyte becomes
smaller.
Even though adding an n-type dopant to the system would increase the number
of free electrons in the TiO2 system, these can also act as defects that trap charge
carriers, encouraging electron-hole recombination reactions, which would decrease
the photon-to-electron conversion efficiency (Duan et al. 2012) and VOC (Roose
2015) at high dopant content. Thus, dopants, at any concentration, simply cannot
improve the efficiency of a DSC. An improvement in the efficiency is only seen at
an optimum concentration of the dopant.
Furthermore, as the (sensitizer) dye molecules anchor to Ti atoms, the replacement
of Ti with another cation can also affect dye adsorption due to different binding
strengths between the dye and the dopant, or because the dopant induces oxygen
vacancies (De Angelis et al. 2010).
The most common method of depositing the dopant is by simply mixing the
dopant with TiO2 , using either a sol–gel, hydrothermal, solvothermal, spray pyroly-
sis, atomic layer deposition, electrochemical deposition, sonochemical, microwave

Fig. 15.7 Effects of doping on TiO2

15 Yet to Be Challenged: TiO2 as the Photo-Anode Material … 301

or electrospinning method. However, there are other methods of deposition possible,

namely; pulse laser deposition, immersing the final TiO2 sample in a solution of the
dopant and applying a voltage, anodization and thermal oxidation.
Dopants of TiO2 can be further categorized into four types: nonmetals, transition
metals, post-transition metals, lanthanides.

15.4.1 Nonmetals

Even though they are not studied much, nonmetals have been used as a dopant for
the improvement of TiO2 . Studies have been conducted by doping nitrogen, sulfur
and fluorine, either by itself or as a combination of dopants.
TiO2 is oxygen deficient, and produces species which can destroy both the dye
and redox electrolyte. Ma et al. doped TiO2 with nitrogen to overcome this problem.
Nitrogen doped TiO2 was synthesized by heating commercial anatase TiO2 in the
presence of N2 . The doped TiO2 system showed an outstanding efficiency of 8%
compared to those of the other two types of commercially available TiO2 nanoparti-
cles [P25 and Solaronix, Ti-Nanoxide D (SL-D)] included in the study. As the VOC
among the three systems had no drastic change, it can be assumed the N doping had
not significantly affected the CB edge. However, N doping not only enhanced the
effective surface area for dye adsorption, leading to an efficient photoresponse in the
visible region, N doped TiO2 was stable under extended thermal stress (Ma et al.
2005). It should be noted however that in this study the N doped TiO2 was in the
shape of needles, while the structure of P25 and SL-D were different.
Further, Simya et al. have shown that sulfur can be used as a co-dopant with
N doped TiO2 to decrease the band gap energy and improve light harvesting. DSCs
fabricated with S and N-doped TiO2 gave an efficiency of 1.80% in the visible region,
which is 50% more when compared to pure TiO2 (Simya et al. 2014). Fluorine has also
been used as a co-dopant along with Ho3+ and Yb3+ , to synthesize a doped TiO2 ,
which exhibited upconversion properties (Yu et al. 2014). This would be further
discussed under lanthanide dopants.

15.4.2 Transition Metals

Transition metal dopants can be considered as the most commonly used type of
dopant with TiO2 , as studies are still being conducted using these metals. The main
reason behind this could be the ability of most transition metals to shift the conduction
band edge to a more negative potential, for efficient electron transport.
Among all the considered transition metals, niobium has been studied the most.
Lee et al. coated Nb doped TiO2 on the transparent conducting oxide and found that,
under optimal amounts of the dopant, the Nb doped TiO2 layer acted as a blocking
layer, reducing the interfacial resistance compared to the undoped TiO2 compact
302 J. B. Liyanage et al.

layer by making an ohmic contact between FTO and the TiO2 film. This system gave
an efficiency of 6.58%, which is about 21.2% enhancement when compared with
bare FTO-based DSCs (Lee et al. 2009).
In 2010, Chandiran et al. studied doping of TiO2 with Nb5+ to create donor levels
below the conduction band and promote charge transport over recombination and
improving the transparency. Under optimal conditions, doped with 0.5% Nb and
post treatment with TiCl4 , an overall efficiency of 8.7% was obtained. Here, Nb5+
would replace the Ti4+ in the lattice structure as they have similar radii and form
strong hybridization with 4d and 3d orbitals. Due to the down-shift of the conduction
band edge, doping with Nb5+ allows to drastically slow the dynamics of electron
recombination with I3 − , resulting in an increase of electron lifetime. The TiCl4 post-
treatment also acts to increase the electron lifetime. On the other hand, the gain
achieved is partially compensated by a loss of electron transport (Chandiran et al.
2010). Lu et al. have performed a similar study, with the use of a water-soluble
precursor, and have obtained a positive shift in the flat band potential along with
an increase in the conductivity. An overall efficiency of 7.8% was obtained, and an
increase of 18.2% compared to pure TiO2 (Lü et al. 2010).
Another widely used transition metal used in doping is silver. Because the Nern-
stian potential of the conduction band in TiO2 is lower than the standard electrode
potential of Ag+ /Ag, the difference of potential is formed. Thus, the electron will
be transferred from the conduction band of TiO2 –Ag+ which was absorbed on the
surface of TiO2 nanospheres, and silver ions are reduced to silver atoms (Han et al.
2012).
Peng et al., studied the change in solar cell efficiency when silver is coated by
photo-deposition. At the deposition time of 10 min, the conversion efficiency was
improved from 5.97 to 6.86%. The Ag layer formed can reflect incident light and
lengthen the optical path in electrodes, and in this case lead to an increase in Jsc
up to 13.55 mA cm−2 . Furthermore, electron recombination was slowed down by
Ag deposition, which led to a larger VOC of 0.735 V. However, past the optimal
deposition time, there was an increase in the series resistance due to the formation
of the Ag barrier layer (Peng et al. 2013).
Wei and coworkers too have studied the use of the noble metal silver nanoparticles
in TiO2 nanotubes, using electrophoretic deposition. It was observed that the total
dye adsorption was reduced with the deposition time, as silver nanoparticles on
the surface of TiO2 , reduce adsorption capacity. The holes in the electrolyte and
photoelectrons in TiO2 recombine easily through the TiO2 –Ag-electrolyte interface
(Wei et al. 2017), which lead to an overall efficiency of 5.01%, under optimum
conditions of 30 min of deposition.
Similar doping methods have been done using zinc (Ghanbari Niaki et al. 2014),
copper (Navas et al. 2012; Wijayarathna et al. 2008), nickel (Archana et al. 2013)
and iron (Liu 2014), in order to improve photovoltaic properties.
Yttrium too has been used as a dopant as it is known to not change the band gap
of TiO2 (Li et al. 2009), even though the problems arise when considering the long-
term stability of the DSC. With 1% Y-TiO2 , an efficiency of 9.1% was observed.
15 Yet to Be Challenged: TiO2 as the Photo-Anode Material … 303

The improvement in performance can be attributed to the increase of electron lifetime

in the new photoanode, which affords close to unity charge collection efficiency
(Chandiran et al. 2011).

15.4.3 Post-transition Metal

As another part of the study done by Chandrian et al. in (2011), gallium too was doped
to TiO2 , to improve the charge collection efficiency and the electrode transparency.
The highest efficiency obtained with 1% gallium was 8.1%. With the increase in
percentage of the Ga dopant, an increase in the open circuit voltage from 732 to
768 mV was observed. In turn, lower short circuit current led to a smaller overall
efficiency. As the impurity levels increase the band gap of TiO2 would also increase,
making it favorable for electron transport. But having more positive conduction band
potential would make it unfavorable for the electron to transport. Then again, having a
more negative conduction band would also be unfavorable, which is why an optimum
amount of the dopant must be used (Chandiran et al. 2011).
Another widely used post transitional metal as a dopant in TiO2 is tin. Duan et al.
studied tin as a dopant so as to improve the charge collection and electron transport
while maintaining optimum JSC . The Sn-doped TiO2 nanoparticles showed a high
VOC of 722 mV, due to the negative shift of the flat band potential, and an enhanced
JSC of 16.01 mA cm−2 , due to the faster electron transport in the Sn-doped TiO2
films (Duan et al. 2012).
Despite the lack of agreement on aluminium doped TiO2 , the substitution of
Al into the TiO2 lattice was confirmed by Pathak et al. It was suggested that Al
doping reduces the number of sub-bandgap states, increasing the VOC and electron
conductivity, which led to an overall improvement in device performance of DSC
(Pathak et al. 2014).

15.4.4 Lanthanides

Ytterbium and holmium, have been used, together with fluorine, as an upconversion
material of TiO2 by Yu et al. Upconversion materials have the ability to convert
lower energy (near-infrared or infrared) radiation into high energy radiation (ultravi-
olet or visible) via multiphoton absorption and energy transfer (ET) processes (Zou
et al. 2012). The improved DSCs conversion efficiency of 8.93% observed is associ-
ated with closer attachment of the upconversion process, enhanced light harvesting,
and photogenerated electron–hole pair separation, as well as elevated Fermi level.
Ho3+ –Yb3+ –F− tridoped TiO2 had excellent NIR-to-green upconversion ability help-
ing dye sensitized solar cells to utilize more NIR light (Yu et al. 2014).
Erbium and ytterbium have also been used as co-dopants, in an upconversion
material in the work of Shan et al. (Shan and Demopoulos 2010).
304 J. B. Liyanage et al.

Table 15.3 Table with the

Modification Efficiency (%) References
maximum efficiencies
obtained by doping TiO2 1% Y3+ doped TiO2 9.00 (Chandiran et al.
2011)
Ho3+ –Yb3+ –F− 8.93 (Yu et al. 2014)
tridoped TiO2
0.5% Nb doped TiO2 8.7 (Chandiran et al.
2010)
1% Ga doped TiO2 8.1 (Chandiran et al.
2011)
N2 doped TiO2 8.00 (Ma et al. 2005)

Other lanthanides that have been used as dopant to improve photovoltaic properties
of TiO2 are neodymium (Yao et al. 2006), lanthanum (Zhang et al. 2010) and cerium
(Zhang et al. 2012) (Table 15.3).

15.5 Potential Competitors

After the first report of O’Regan and Grätzel in the application of nano-sized TiO2
porous film electrodes in DSCs (O’Regan and Grätzel 1991), TiO2 has been inves-
tigated under numerous modifications, some of which have been reviewed in this
chapter. Modifications attempted to improve the surface area, light scattering effect,
charge collection efficiency and interface quality. Although the development of TiO2
based DSCs have shown much promise, many more competitors have arisen in the
past decade, challenging TiO2 as the semiconductor material for DSCs.
Two of the main factors affecting performance in DSC is efficient electron injec-
tion from the dye and rapid electron transport. TiO2 nanoparticle based photoanodes
have some limitations due to the presence of a large number of grain boundaries, and
leads to electron recombination losses and also poor efficiency in the near infrared
region (Sugathan et al. 2015).
Some researchers consider ZnO as the most promising alternative to TiO2 . There
are several reasons behind this, such as the fact that both TiO2 and ZnO have similar
band gaps (~3.2 eV and ~3.3 eV, respectively) and electron affinities. However,
comparing the two, ZnO has a much higher electron diffusivity and electron mobility
of about 115–155 cm2 V−1 s−1 , which implies efficient electron transport and the
reduction of recombination rates (Vittal and Ho 2017). Apart from the large excitation
binding energy, low cost and stability against photocorrosion (Anta et al. 2012)
the crystalline structure of ZnO is conducive to anisotropic growth (Baxter et al.
2006), unlike that of TiO2 , making it a prime candidate for DSCs with photoanodes
consisting of nanorods, nanowires or nanosheets.
Many recent studies show the application of ZnO nanostructures for photo-
electrodes with enhanced photovoltaic performance of DSCs. Some studies have
15 Yet to Be Challenged: TiO2 as the Photo-Anode Material … 305

focused on the effect of morphologies such as nanofibre network mats (Kim et al.
2007), nano-sheets (Li et al. 2012) and among others (Giannouli and Spiliopoulou
2012; Zhang et al. 2009). While others have investigated synthesis procedures (Zhao
et al. 2008).
After much study, the maximum reported efficiency for pure ZnO based DSC,
using a liquid electrolyte is 7.5% (Memarian et al. 2011), which is much lower than
that of TiO2 based DSC. The low conversion efficiencies of ZnO-based systems
are most likely due to the dissolution of ZnO to Zn2+ by the adsorbed acidic dye,
followed by the formation of an insulating layer of Zn2+ and dye molecules, blocking
the injected electrons from the dye molecules to the semiconductor by the insulating
layer (Hiroaki Horiuchi et al. 2003). As ZnO is more basic than TiO2 , it is more
prone to be attacked by acidic dyes. In order to avoid the formation of the insulating
layer, many core-shell structures have been developed by coating a buffer layer on
the ZnO surface. SiO2 has been demonstrated to be a very effective shell material
on ZnO, which prevents the formation of aggregates through a strong interaction
between Si4+ and O2− ions (Shin et al. 2007). Many core–shell structures have been
developed using ZnO nanocrystals and ZnO nanowires, by coating a buffer layer of
Al2 O3 (Matt Law et al. 2006), TiO2 (Matt Law et al. 2006), and even ZnO (Guillén
et al. 2013) on their surfaces, to prevent the formation of the insulating Zn2+ /dye
complex. ZnO has also been studied with CuO, with and without TiO2 blocking
layer, in the presence of Co2+ /Co3+ redox electrolyte (Habibi et al. 2014).
Most nanostructures of the semiconductor material, have been synthesized so
as the final product would give a large surface area for the dye adsorption. ZnO
and Fe2 O3 have been taken into consideration as they are both suitable to prepare
thin films, have good electrical conductivity, are inexpensive, and have a very good
chemical stability. These reasons have led to use these materials in energy storage
and photoelectrochemistry (Livage and Ganguli 2001). Reda et al. have used ZnO
and Fe2 O3 as photoanode materials, and studied the photovoltaic characteristics
by changing the annealing temperature, in order to obtain a higher incident light to
electrical energy conversion and has obtained a maximum efficiency of 2.2% for ZnO
and 1.2% for Fe2 O3 (Reda 2010). Moreover, due to the suitable band gap of about
2.2 eV Fe2 O3 , various nanostructures have been synthesized, including nanocubes
(Ozaki et al. 1984), nano-rings (Jia et al. 2008), dendrites (Liang et al. 2010) and
polyhedron (Lv et al. 2010). Fe2 O3 has also been studied as a photoanode material in
DSCs, using a one-pot, low temperature synthesis method (Manikandan et al. 2014).
Another possible alternative to TiO2 is Nb2 O5 , due to the larger band gap of
3.49 eV, leading to a higher conduction band edge of −1.32 eV compared to that of
TiO2 (Ghosh et al. 2011), which could lead to higher VOC . Moreover, in terms of
IPCE Nb2 O5 has the second value of 18%, after that of TiO2 , which has 45% (Jose
et al. 2009). Ghosh et al. synthesized a nanoforest of Nb2 O5 using laser ablation,
under several gas compositions and pressure conditions, which was quite similar
to vertically aligned nanocrystals to Gratzel’s TiO2 nanoforest. This study has also
encouraged the growth of photoanode material, with high conduction band levels
such as Ta2 O5 and SrTiO3 , in a similar manner, for future studies in DSCs (Ghosh
et al. 2011).
306 J. B. Liyanage et al.

Another upcoming class of competitors for TiO2 , are metal-organic frameworks

(MOFs), which are porous crystalline material made up of inorganic nodes, which
are mostly metal cations or clusters; and organic linkers, which are organic ligands
(Li et al. 2014). The light harvesting properties of MOFs were found to depend on the
properties of the linker. Since the linkers are organic molecules having p-electrons,
they absorb energy from the UV and blue regions in the electromagnetic spectrum for
electronic transitions. As result, metal-to-ligand or ligand-to-metal energy transfer
processors are possible, depending on the selection of the linker and metal type. Apart
from this, the controllable architecture, adjustable pore size and high surface area
of MOFs encouraged the application of them in DSCs. Most MOFs are insulators.
However, MOF-5 has exhibited semiconductor behavior due to the geometric type
of metal clusters containing ZnO units in the network which can act as quantum dots
(Llabrés i Xamena et al. 2007). Various studies have been done by incorporating
MOFs into either the working electrode, counter electrode, dye or the electrolyte
medium, that is, any component of a DSC. The highest efficiency of 8.49%, has so
far been attained by the Al-MOG MOF used in the electrolyte medium by Fan et al.
(Fan et al. 2014). So far MOFs have only been incorporated into n-type DSCs and
studies are yet to be done for p-type DSCs.

15.6 Conclusion

This chapter has discussed four aspects of modifying TiO2 , in order to improve
the efficiency of a TiO2 based DSCs. All these four modifications have a differ-
ent approach to improve photovoltaic properties. The main purpose of changing the
nanostructure would be to increase the dye adsorption, by enhancing the effective
surface area. One-dimensional nanostructures are widely adopted to improve the
electron transport, and minimizing recombination reactions. Surface modifications
aim to decrease in grain boundaries which can act as trapping sites of the injected elec-
trons, leading to recombination; and also, to strengthen and increase the attachment
of the dye to the surface. Composites aim to provide a smoother, step-like electron

Table 15.4 Table with maximum efficiencies obtained by each modification

Modification Efficiencies (%) References
Structure and Nanoparticle (haze) 11.1 (Chiba et al. 2006)
morphology Nano-embossed hollow 10.34 (Koo et al. 2008)
spheres
Composites TiO2 –SiO2 composite 9.20 (Maçaira et al. 2017)
Doping Doping with 1% Y3+ 9.00 (Chandiran et al. 2011)
Optimizing the amount 8.93 (Yu et al. 2014)
of Ho3+ -Yb3+ -F− tri
doped TiO2 /pure TiO2
15 Yet to Be Challenged: TiO2 as the Photo-Anode Material … 307

transport from the dye to the working electrode, minimizing electron back transfer
and recombination. Doping can improve the photovoltaic properties in two meth-
ods: better electron injection, and improved dye adsorption. The highest efficiencies
obtained under these modifications have been tabulated in Table 15.4.
Even after extensive studies to improve the efficiency of DSCs, the highest effi-
ciency is still 11.1% obtained by tuning the used TiO2 nanoparticle sizes (Chiba et al.
2006). Even so, the improvement seen in the overall efficiency of DSCs along with
these modifications, can be said to be promising for a more efficient and greener
energy source.

References

Anish Madhavan A, Kalluri SK, Chacko D, Arun TA, Nagarajan S, Subramanian Nair KRV, Sreeku-
maran A, Nair SV, Balakrishnan A (2012) Electrical and optical properties of electrospun TiO2 -
graphene composite nanofibers and its application as DSSC photo-anodes. RSC Adv 2:13032
Anta JA, Guillén E, Tena-Zaera R (2012) ZnO-based dye-sensitized solar cells. J Phys Chem C
116:11413–11425
Archana PS, Naveen Kumar E, Vijila C, Ramakrishna S, Yusoff MM, Jose R (2013) Dalt Trans
42:1024–1032
Bai L, Liu X, Li M, Guo K, Luoshan M, Zhu Y, Jiang R, Liao L, Zhao X (2016) Plasmonic
enhancement of the performance of dye-sensitized solar cells by incorporating hierarchical TiO2
spheres decorated with Au nanoparticles. Electrochim Acta 190:605–611
Baxter JB, Walker AM, van Ommering K, Aydil ES (2006) Synthesis and characterization of ZnO
nanowires and their integration into dye-sensitized solar cells. Nanotechnol 17:S304–S312
Beranek R, Tsuchiya H, Sugishima T, Macak JM, Taveira L, Fujimoto S, Kisch H, Schmuki P (2005)
Enhancement and limits of the photoelectrochemical response from anodic TiO2 nanotubes. Appl
Phys Lett 87:243114
Bhatti KA, Khan MI, Saleem M, Alvi F, Raza R, Rehman S (2019) Analysis of multilayer based
TiO2 and ZnO photoanodes for dye-sensitized solar cells. Mater Res Express 6:075902
Cardoso BN, Kohlrausch EC, Laranjo MT, Benvenutti EV, Balzaretti NM, Arenas LT, Santos MJL,
Costa TMH (2019) Tuning anatase-rutile phase transition temperature: TiO2 /SiO2 nanoparticles
applied in dye-sensitized solar cells. Int J Photoenergy 2019:1–9
Chandiran AK, Casas-Cabanas M, Comte P, Zakeeruddin SM, Graetzel M (2010) Doping a TiO2
photoanode with Nb5+ to enhance transparency and charge collection efficiency in dye-sensitized
solar cells. J Phys Chem C 114:15849–15856
Chandiran AK, Etgar L, Graetzel M (2011) 9232–9240
Chen L, Hsu C, Chan P, Zhang X, Huang C (2014) Improving the performance of dye-sensitized
solar cells with TiO2 /graphene/TiO2 sandwich structure. Nanoscale Res Lett 9:1–7
Chiba Y, Islam A, Watanabe Y, Komiya R, Koide N, Han L (2006) Dye-sensitized solar cells with
conversion efficiency of 11.1%. Jpn J Appl Phys 45:L638–L640
Choi H, Kim S, Kang SO, Ko J, Kang M-S, Clifford JN, Forneli A, Palomares E, Nazeeruddin MK,
Grätzel M (2008) Stepwise cosensitization of nanocrystalline TiO2 films utilizing Al2 O3 layers
in dye-sensitized solar cells. Angew Chem Int Ed 47:8259–8263
Chou C-S, Yang R-Y, Yeh C-K, Lin Y-J (2009) Preparation of TiO2 /nano-metal composite particles
and their applications in dye-sensitized solar cells. Powder Technol 194:95–105
Dahl M, Liu Y, Yin Y (2014) Composite titanium dioxide nanomaterials. Chem Rev 114:9853–9889
De Angelis F, Fantacci S, Selloni A, Nazeeruddin MK, Grätzel M (2010) First-principles modeling
of the adsorption geometry and electronic structure of Ru(II) dyes on extended TiO2 substrates
for dye-sensitized solar cell applications. J Phys Chem C 114:6054–6061
308 J. B. Liyanage et al.

De Jonge LT, Leeuwenburgh SCG, Wolke JGC, Jansen JA (2008) Organic-inorganic surface mod-
ifications for titanium implant surfaces. Pharm Res 25: 2357–2369
Deepa KG, Lekha P, Sindhu S (2012) Efficiency enhancement in DSSC using metal nanoparticles:
a size dependent study. Sol Energy 86:326–330
Desai UV, Xu C, Wu J, Gao D (2013) Hybrid TiO2 –SnO2 nanotube arrays for dye-sensitized solar
cells. J Phys Chem C 117:3232–3239
Ding S, Yin X, Lü X, Wang Y, Huang F, Wan D (2012) One-step high-temperature solvothermal
synthesis of TiO2 /sulfide nanocomposite spheres and their solar visible-light applications. ACS
Appl Mater Interfaces 4:306–311
Duan Y, Fu N, Liu Q, Fang Y, Zhou X, Zhang J, Lin Y (2012) Sn-doped TiO2 photoanode for
dyesensitized solar cells. J Phys Chem C 116:8888–8893
Endarko E, Adawiyah SR (2019) Experimental study of TiO2 nanoparticles fabrication by sol-gel
and coprecipitation methods for TiO2 /SnO2 composite thin film as photoanode. J ILMU DASAR
20:61
Fadhilah N, Pratama DY, Sawitri D, Risanti DD (2019) Preparation of Au@TiO2 @SiO2 core-shell
nanostructure and their light harvesting capability on DSSC (dye sensitized solar cells). In: AIP
conference proceedings. AIP Publishing LLC, p 060007
Fan J, Li L, Rao H-S, Yang Q-L, Zhang J, Chen H-Y, Chen L, Kuang D-B, Su C-Y (2014) A novel
metal–organic gel based electrolyte for efficient quasi-solid-state dye-sensitized solar cells. J
Mater Chem A 2:15406
Feng X, Shankar K, Varghese OK, Paulose M, Latempa TJ, Grimes CA (2008) Vertically aligned
single crystal TiO2 nanowire arrays grown directly on transparent conducting oxide coated glass:
synthesis details and applications. Nano Lett 8:3781–3786
Fraoucene H, Sugiawati VA, Hatem D, Belkaid MS, Vacandio F, Eyraud M, Pasquinelli M, Djenizian
T (2019) Optical and electrochemical properties of self-organized TiO2 nanotube arrays from
anodized Ti−6Al−4V alloy. Front Chem 7:66
Gao C, Li X, Lu B, Chen L, Wang Y, Teng F, Wang J, Zhang Z, Pan X, Xie E (2012) A facile method
to prepare SnO2 nanotubes for use in efficient SnO2 –TiO2 core–shell dye-sensitized solar cells.
Nanoscale 4:3475
Ghadiri E, Taghavinia N, Zakeeruddin SM, Gra M (2010) Enhanced electron collection efficiency in
dye-sensitized solar cells based on nanostructured TiO2 hollow fibers. Nano Lett 10:1632–1638
Ghanbari Niaki AH, Bakhshayesh AM, Mohammadi MR (2014) Double-layer dye-sensitized solar
cells based on Zn-doped TiO2 transparent and light scattering layers: improving electron injection
and light scattering effect. Sol Energy 103:210–222
Ghosh R, Brennaman MK, Uher T, Ok MR, Samulski ET, McNeil LE, Meyer TJ, Lopez R (2011)
Nanoforest Nb2 O5 photoanodes for dye-sensitized solar cells by pulsed laser deposition. ACS
Appl Mater Interfaces 3:3929–3935
Giannouli M, Spiliopoulou F (2012) Effects of the morphology of nanostructured ZnO films on the
efficiency of dye-sensitized solar cells. Renew Energy 41:115–122
Gong J, Liang J, Sumathy K (2012) Review on dye-sensitized solar cells (DSSCs): fundamental
concepts and novel materials. Renew Sustain Energy Rev 16:5848–5860
Guillén E, Azaceta E, Vega-Poot A, Idígoras J, Echeberría J, Anta JA, Tena-Zaera R (2013) ZnO/ZnO
core–shell nanowire array electrodes: blocking of recombination and impressive enhancement of
photovoltage in dye-sensitized solar cells. J Phys Chem C 117:13365–13373
Habibi MH, Karimi B, Zendehdel M, Habibi M (2014) Preparation of nanostructure mixed copper-
zinc oxide via co-precipitation rout for dye-sensitized solar cells: the influence of blocking layer
and Co(II)/Co(III) complex redox shuttle. J Ind Eng Chem 20:1462–1467
Han Z, Zhang J, Yu Y, Cao W (2012) A new anode material of silver photo-deposition on TiO2 in
DSSC. Mater Lett 70:193–196
Hieu NT, Baik SJ, Jun Y, Lee M, Chung OH, Park JS (2014) Electrospun coaxial titanium
dioxide/carbon nanofibers for use in anodes of dye-sensitized solar cells. Electrochim Acta
142:144–151
15 Yet to Be Challenged: TiO2 as the Photo-Anode Material … 309

Horiuchi H, Katoh R, Hara K, Yanagida M, Murata S, Arakawa H, Tachiya M (2003) Electron

injection efficiency from excited N3 into nanocrystalline ZnO films: effect of (N3-Zn2+ ) aggregate
formation. J Phys Chem B 107:2570–2574
Hsu WWC, Chen MWC, Zhang X, Sutanto I, Taguchi T, Zhi J, Chen A, Cui H, Huang F, Williams
VO, Jeong NC, Prasittichai C, Farha OK, Pellin MJ, Hupp JT (2015) NiO-decorated mesoporous
TiO2 flowers for an improved photovoltaic dye sensitized solar cell. Phys Chem Chem Phys
80:6185–6196
Huang Y-R, Huang T-W, Wang T-H, Tsai Y-C (2014) Improved performance of dye-sensitized solar
cells using gallium nitride–titanium dioxide composite photoelectrodes. J Colloid Interface Sci
428:128–132
Huo J, Hu Y, Jiang H, Huang W, Li C (2014) SnO2 nanorod@TiO2 hybrid material for dye-sensitized
solar cells. J Mater Chem A 2:8266–8272
Im JS, Lee SK, Lee Y-S (2011) Cocktail effect of Fe2 O3 and TiO2 semiconductors for a high
performance dyesensitized solar cell. Appl Surf Sci 257:2164–2169
Jang S-R, Vittal R, Kim K-J (2004) Incorporation of functionalized single-wall carbon nanotubes
in dyesensitized TiO2 solar cells. Langmuir 20:9807–9810
Ji Y, Zhang M, Cui J, Lin K-C, Zheng H, Zhu J-J, Samia ACS (2012) Highly-ordered TiO2 nanotube
arrays with double-walled and bamboo-type structures in dye-sensitized solar cells. Nano Energy
1:796–804
Jia C-J, Sun L-D, Luo F, Han X-D, Heyderman LJ, Yan Z-G, Yan C-H, Zheng K, Zhang Z, Takano M,
Hayashi N, Eltschka M, Kläui M, Rüdiger U, Kasama T, Cervera-Gontard L, Dunin-Borkowski
RE, Tzvetkov G, Raabe J (2008) Large-scale synthesis of single-crystalline iron oxide magnetic
nanorings. J Am Chem Soc 130:16968–16977
Jin Z, Chen S, Zhang Y, Wang Y, Zhang X, Liu Y (2019) Construction of hierarchical hetero-
structured TiO2 photoanodes for dye-sensitized solar energy conversion: case study of anatase
nanobranches on rutile nanorod arrays. Chem Phys 522:129–133
Jose R, Thavasi V, Ramakrishna S (2009) Metal oxides for dye-sensitized solar cells. J Am Ceram
Soc 92:289–301
Kang SH, Kim J, Kim Y, Kim HS, Sung Y (2007) Surface modification of stretched TiO2 nanotubes
for solid-state dye-sensitized solar cells. J Phys Chem C 111:9614–9623
Kim I-D, Hong J-M, Lee BH, Kim DY, Jeon E-K, Choi D-K, Yang D-J (2007) Dye-sensitized solar
cells using network structure of electrospun ZnO nanofiber mats. Appl Phys Lett 91:163109
Kim D, Ghicov A, Albu SP, Schmuki P (2008) 16454–16455
Kim J-Y, Kang SH, Kim HS, Sung Y-E (2010) Preparation of highly ordered mesoporous Al2 O3
/TiO2 and Its application in dye-sensitized solar cells. Langmuir 26:2864–2870
Kim J-Y, Lee K-H, Shin J, Park SH, Kang JS, Han KS, Sung MM, Pinna N, Sung Y-E (2014) Highly
ordered and vertically oriented TiO2 /AlO3 nanotube electrodes for application in dye-sensitized
solar cells. Nanotechnology 25:504003
Knauf RR, Kalanyan B, Parsons GN, Dempsey JL (2015) Charge recombination dynamics in
sensitized SnO2 /TiO2 core/shell photoanodes. J Phys Chem C 119:28353–28360
Kongkanand A, Domínguez RM, Kamat PV (2007) Single wall carbon nanotube scaffolds for
photoelectrochemical solar cells. Capture and transport of photogenerated electrons. In: Nano
Letters. American Chemical Society, pp. 676–680
Koo H-J, Kim YJ, Lee YH, Lee WI, Kim K, Park N-G (2008) Nano-embossed hollow spherical TiO2
as bifunctional material for high-efficiency dye-sensitized solar cells. Adv Mater 20:195–199
Kopidakis N, Benkstein KD, van de Lagemaat J, Frank AJ (2003) Transport-limited recom-
bination of photocarriers in dye-sensitized nanocrystalline TiO2 solar cells. J Phys Chem B
107:11307–11315
Kuang D, Brillet J, Chen P, Takata M, Uchida S, Miura H, Sumioka K, Zakeeruddin SM, Grätzel M
(2008) Application of highly ordered TiO2 nanotube arrays in flexible dye-sensitized solar cells.
ACS Nano 2:1113–1116
Kusumawati Y, Martoprawiro MA, Pauporté T (2014) Effects of graphene in graphene/TiO2 com-
posite films applied to solar cell photoelectrode. J Phys Chem C 118:9974–9981
310 J. B. Liyanage et al.

Law M, Greene LE, Radenovic A, Kuykendall T, Liphardt J, Yang P (2006) ZnO-Al2 O3 and ZnO-
TiO2 coreshell nanowire dye-sensitized solar cells. J Phys Chem B 110:22652–22663
Lee K-M, Hu C-W, Chen H-W, Ho K.-C (2008) Incorporating carbon nanotube in a low-temperature
fabrication process for dye-sensitized TiO2 solar cells. Sol Energy Mater Sol Cells 92:1628–1633
Lee S, Noh JH, Han HS, Yim DK, Kim DH, Lee J-K, Kim JY, Jung HS, Hong KS (2009) Nb-
doped TiO2 : a new compact layer material for TiO2 dye-sensitized solar cells. J Phys Chem C
113:6878–6882
Li J, Yang X, Yu X, Xu L, Kang W, Yan W, Gao H, Liu Z, Guo Y (2009) Rare earth oxide-doped
titania nanocomposites with enhanced photocatalytic activity towards the degradation of partially
hydrolysis polyacrylamide.. Appl Surf Sci 255:3731–3738
Li H, Zhang Y, Wang J (2012) ZnO nanosheets derived from surfactant-directed process: growth
mechanism, and application in dye-sensitized solar cells. J Am Ceram Soc 95:1241–1246
Li Y, Chen C, Sun X, Dou J, Wei M (2014) Metal–organic frameworks at interfaces in dye-sensitized
solar cells. ChemSusChem 7:2469–2472
Li CT, Li SR, Chang LY, Lee CP, Chen PY, Sun SS, Lin JJ, Vittal R, Ho KC (2015) Efficient titanium
nitride/titanium oxide composite photoanodes for dye-sensitized solar cells and water splitting.
J Mater Chem A 3:4695–4705
Li Mingyue, Yuan N, Tang Y, Pei L, Zhu Y, Liu J, Bai L, Li Meiya (2019) Performance optimization
of dye-sensitized solar cells by gradient-ascent architecture of SiO2 @Au@TiO2 microspheres
embedded with Au nanoparticles. J Mater Sci Technol 35:604–609
Liang J, Li L, Song W, Fang J, Luo M, Li Y (2010) Rapid synthesis of dendrite- and platelet-like
α-Fe2 O3 via a hydrothermal oxidation route. Cryst Res Technol 45:405–408
Liu Q-P (2014) Analysis on dye-sensitized solar cells based on Fe-doped TiO2 by intensity-
modulated photocurrent spectroscopy and Mott–Schottky. Chin Chem Lett 25:953–956
Liu B, Aydil ES (2009) Growth of oriented single-crystalline rutile TiO2 nanorods on transparent
conducting substrates for dye-sensitized solar cells. J Am Chem Soc 131:3985–3990
Livage J, Ganguli D (2001) Sol-gel electrochromic coatings and devices: a review. Sol Energy Mater
Sol Cells 68:365–381
Llabrés i Xamena FX, Abad A, Corma A, Garcia H (2007) MOFs as catalysts: activity, reusability
and shapeselectivity of a Pd-containing MOF. J Catal 250:294–298
Low FW, Lai CW (2018) Reduced graphene oxide decorated TiO2 for improving dye-sensitized
solar cells (DSSCs). Curr Nanosci 14
Lu D, Qin L, Liu D, Sun P, Liu F, Lu G (2018) High-efficiency dye-sensitized solar cells based
on bilayer structured photoanode consisting of carbon nanofiber/TiO2 composites and Ag@TiO2
core-shell spheres. Electrochim Acta 292:180–189
Lü X, Mou X, Wu J, Zhang D, Zhang L, Huang F, Xu F, Huang S (2010) Improved-performance dye-
sensitized solar cells using Nb-doped TiO2 electrodes: efficient electron injection and transfer.
Adv Funct Mater 20:509–515
Luan X, Guan D, Wang Y (2012) Facile synthesis and morphology control of bamboo-type TiO2
nanotube arrays for high-efficiency dye-sensitized solar cells. J Phys Chem C 116:14257–14263
Lv B, Liu Z, Tian H, Xu Y, Wu D, Sun Y (2010) Single-crystalline dodecahedral and octodecahedral
α-Fe2 O3 particles synthesized by a fluoride anion-assisted hydrothermal method. Adv Funct
Mater 20:3987–3996
Ma T, Akiyama M, Abe E, Imai I (2005) High-efficiency dye-sensitized solar cell based on a
nitrogen-doped nanostructured titania electrode. Nano Lett 5:2543–2547
Maçaira J, Andrade L, Mendes A (2017) Highly efficient SiO2 /TiO2 composite photoelectrodes for
dyesensitized solar cells. Sol Energy 158:905–916
Macak JM, Schmuki P (2006) Anodic growth of self-organized anodic TiO2 nanotubes in viscous
electrolytes. Electrochim Acta 52:1258–1264
Manikandan A, Saravanan A, Antony SA, Bououdina M (2014) One-pot low temperature synthesis
and characterization studies of nanocrystalline α-Fe2 O3 based dye sensitized solar cells. J Nanosci
Nanotechnol 15:4358–4366
15 Yet to Be Challenged: TiO2 as the Photo-Anode Material … 311

Manthina V, Pablo J, Baena C, Liu G, Agrios AG (2012) ZnO-TiO2 nanocomposite films for high
light harvesting efficiency and fast electron transport in dye-sensitized solar cells. J Phys Chem
C 116:23864–23870
Mao X, Zhou R, Zhang S, Ding L, Wan L, Qin S, Chen Z, Xu J, Miao S (2016) High efficiency
dyesensitized solar cells constructed with composites of TiO2 and the hot-bubbling synthesized
ultra-small SnO2 nanocrystals. Sci Rep 6:19390
Marandi M, Bayat S, Naeimi Sani Sabet M (2019) Hydrothermal growth of a composite TiO2
hollow spheres/TiO2 nanorods powder and its application in high performance dye-sensitized
solar cells. J Electroanal Chem 833:143–150
Mehmood U, Hussein IA, Harrabi K, Mekki MB, Ahmed S, Tabet N (2015) Hybrid TiO2 –multiwall
carbon nanotube (MWCNTs) photoanodes for efficient dye sensitized solar cells (DSSCs). Sol
Energy Mater Sol Cells 140:174–179
Memarian N, Concina I, Braga A, Rozati SM, Vomiero A, Sberveglieri G (2011) Hierarchically
assembled ZnO nanocrystallites for high-efficiency dye-sensitized solar cells. Angew Chem Int
Ed 50:12321–12325
Muduli S, Game O, Dhas V, Vijayamohanan K, Bogle KA, Valanoor N, Ogale SB (2012) TiO2 –Au
plasmonic nanocomposite for enhanced dye-sensitized solar cell (DSSC) performance. Sol
Energy 86:1428–1434
Nam SH, Shim H-S, Kim Y-S, Dar MA, Kim JG, Kim WB (2010) Ag or Au nanoparticle-embedded
one-dimensional composite TiO2 nanofibers prepared via electrospinning for use in lithium-ion
batteries. ACS Appl Mater Interfaces 2:2046–2052
Navas J, Fernández-Lorenzo C, Aguilar T, Alcántara R, Martín-Calleja J (2012) Improving
open-circuit voltage in DSSCs using Cu-doped TiO2 as a semiconductor. Phys Status Solidi
209:378–385
Noor S, Sajjad S, Leghari SAK, Shaheen S, Iqbal A (2018) ZnO/TiO2 nanocomposite photoanode
as an effective UV-vis responsive dye sensitized solar cell. Mater Res Express 5:095905
O’Regan B, Grätzel M (1991) A low-cost, high-efficiency solar cell based on dye-sensitized colloidal
TiO2 films. Nature 353:737–740
Ozaki M, Kratohvil S, Matijević E (1984) Formation of monodispersed spindle-type hematite
particles. J Colloid Interface Sci 102:146–151
Pandanga JJ, Nursam NM, Shobih, Prastomo N (2019) Synthesis and application of TiO2 nanorods
as photoanode in dye-sensitized solar cells. J Phys Conf Ser 1191:012023
Pathak SK, Abate A, Ruckdeschel P, Roose B, Gödel KC, Vaynzof Y, Santhala A, Watanabe S-I,
Hollman DJ, Noel N, Sepe A, Wiesner U, Friend R, Snaith HJ, Steiner U (2014) Performance
and stability enhancement of dye-sensitized and perovskite solar cells by Al doping of TiO2 . Adv
Funct Mater 24:6046–6055
Peng W, Zeng Y, Gong H, Leng Y, Yan Y, Hu W (2013) Silver-coated TiO2 electrodes for high
performance dye-sensitized solar cells. Solid State Electron 89:116–119
Peng Z, Liu Y, Zhao Y, Chen K, Cheng Y, Chen W (2014) Incorporation of the TiO2 nanowire
arrays photoanode and Cu 2S nanorod arrays counter electrode on the photovoltaic performance
of quantum dot sensitized solar cells. Electrochim Acta 135:276–283
Pichot F, Pitts JR, Gregg BA (2000). Low-temperature sintering of TiO2 colloids: application to
flexible dyesensitized solar cells. Electrochim Acta 135:276–283
Pyrgiotakis G, Lee S-H, Sigmund W (2005) Advanced photocatalysis with anatase nano-coated
multi-walled carbon nanotubes. MRS Proc 876:R5.7
Qian J, Liu P, Xiao Y, Jiang Y, Cao Y, Ai X, Yang H (2009) TiO2 -coated multilayered SnO2 hollow
microspheres for dye-sensitized solar cells. Adv Mater 21:3663–3667
Rajamanickam N, Kanmani SS, Jayakumar K, Ramachandran K (2019) On the possibility of fer-
romagnetism and improved dye-sensitized solar cells efficiency in TiO2 /ZnO core/shell nanos-
tructures. J Photochem Photobiol A Chem 378:192–200
Ran H, Fan J, Zhang X, Mao J, Shao G (2018) Enhanced performances of dye-sensitized solar cells
based on Au-TiO2 and Ag-TiO2 plasmonic hybrid nanocomposites. Appl Surf Sci 430:415–423
312 J. B. Liyanage et al.

Reda SM (2010) Synthesis of ZnO and Fe2 O3 nanoparticles by solgel method and their application
in dyesensitized solar cells. Mater Sci Semicond Process 13:417–425
Roose B, Pathak S, Steiner U (2015) Doping of TiO2 for sensitized solar cells. Chem Soc Rev
44:8326–8349
Sadikin SN, Rahman MYA, Umar AA (2019) Zinc sulphide-coated titanium dioxide films as pho-
toanode for dye-sensitized solar cells: effect of immersion time on its performance. Superlattices
Microstruct 130:153–159
Shan G-B, Demopoulos GP (2010) Near-infrared sunlight harvesting in dye-sensitized solar cells
via the insertion of an upconverter-TiO2 nanocomposite layer. Adv Mater 22:4373–4377
Shin YJ, Lee JH, Park JH, Park NG (2007) Enhanced photovoltaic properties of SiO2 -treated ZnO
nanocrystalline electrode for dye-sensitized solar cell. Chem Lett 36:1506–1507
Sigdel S, Dubey A, Elbohy H, Aboagye A, Galipeau D, Zhang L, Fong H, Qiao Q (2014)
Dye-sensitized solar cells based on spray-coated carbon nanofiber/TiO2 nanoparticle composite
counter electrodes. J Mater Chem A 2:11448
Simya OK, Selvam M, Karthik A, Rajendran V (2014) Dye-sensitized solar cells based on visible-
light-active TiO2 heterojunction nanoparticles. Synth Met 188:124–129
Stergiopoulos T, Ghicov A, Likodimos V, Tsoukleris DS, Kunze J, Schmuki P, Falaras P (2008)
Dyesensitized solar cells based on thick highly ordered TiO2 nanotubes produced by controlled
anodic oxidation in non-aqueous electrolytic media. Nanotechnol 19:235602
Sugathan V, John E, Sudhakar K (2015) Recent improvements in dye sensitized solar cells: a review.
Renew Sustain Energy Rev 52:54–64
Suriani AB, Muqoyyanah, Mohamed A, Mamat MH, Othman MHD, Ahmad MK, Abdul Khalil
HPS, Marwoto P, Birowosuto MD (2019) Titanium dioxide/agglomerated-free reduced graphene
oxide hybrid photoanode film for dye-sensitized solar cells photovoltaic performance improve-
ment. Nano-Struct Nano-Object 18:100314
Tributsch H (2004) Dye sensitization solar cells: a critical assessment of the learning curve. Coord
Chem Rev 248:1511–1530
Vardys VS (1979) Democray in the baltic states 1918–1934: the stage and the actors. J Balt Stud
10:320–336
Vittal R, Ho KC (2017) Zinc oxide based dye-sensitized solar cells: a review. Renew Sustain Energy
Rev 70:920–935
Wali Q, Bakr ZH, Manshor NA, Fakharuddin A, Jose R (2016) SnO2 –TiO2 hybrid nanofibers for
efficient dye-sensitized solar cells. Sol Energy 132:395–404
Wang W, Serp P, Kalck P, Faria JL (2005) Visible light photodegradation of phenol on MWNT-TiO2
composite catalysts prepared by a modified sol–gel method. J Mol Catal A Chem 235:194–199
Wang H, Bai Y, Zhang H, Zhang Z, Li J, Guo L (2010) CdS quantum dots-sensitized TiO2 nanorod
array on transparent conductive glass photoelectrodes. J Phys Chem C 114:16451–16455
Wang S, Zhou X, Xiao X, Fang Y, Lin Y (2014) An increase in conversion efficiency of dye-
sensitized solar cells using bamboo-type TiO2 nanotube arrays. Electrochim Acta 116:26–30
Wei X, Nbelayim PS, Kawamura G, Muto H, Matsuda A (2017) Ag nanoparticle-filled TiO2 nan-
otube arrays prepared by anodization and electrophoretic deposition for dye-sensitized solar cells.
Nanotechnol 28:135207
Wijayarathna TRCK, Aponsu GMLP, Ariyasinghe YPYP, Premalal EVA, Kumara GKR, Tennakone
K (2008) A high efficiency indoline-sensitized solar cell based on a nanocrystalline TiO2 surface
doped with copper. Nanotechnol 19:485703
Williams VO, Jeong NC, Prasittichai C, Farha OK, Pellin MJ, Hupp JT (2012) Fast transporting
ZnOTiO2 coaxial photoanodes for dye-sensitized solar cells based on ALD-modified SiO2 aerogel
frameworks. ACS Nano 6:6185–6196
Wu S, Han H, Tai Q, Zhang J, Xu S, Zhou C, Yang Y, Hu H, Chen B, Zhao X (2008) Improvement
in dye-sensitized solar cells employing TiO2 electrodes coated with Al2 O3 by reactive direct
current magnetron sputtering. J Power Sources 182:119–123
Wu W-Y, Hsu C-F, Wu M-J, Chen C-N, Huang J-J (2017) Ag–TiO2 composite photoelectrode for
dyesensitized solar cell. Appl Phys A Mater Sci Process 123:357
15 Yet to Be Challenged: TiO2 as the Photo-Anode Material … 313

Yang S-C, Yang D-J, Kim J, Hong J-M, Kim H-G, Kim I-D, Lee H (2008) Hollow TiO2 hemi-
spheres obtained by colloidal templating for application in dye-sensitized solar cells. Adv Mater
20:1059–1064
Yao Q, Liu J, Peng Q, Wang X, Li Y (2006) Nd-Doped TiO2 nanorods: preparation and application
in dye-sensitized solar cells. Chem An Asian J 1:737–741
Ye M, Wen X, Wang M, Iocozzia J, Zhang N, Lin C, Lin Z (2015) Recent advances in dye-sensitized
solar cells: from photoanodes, sensitizers and electrolytes to counter electrodes. Mater. Today.
Yu J, Zhang J (2010) A simple template-free approach to TiO2 hollow spheres with enhanced
photocatalytic activity. Dalt Trans 39:5860. https://doi.org/10.1039/c0dt00053a
Yu H, Quan X, Chen S, Zhao H (2007) TiO2 -multiwalled carbon nanotube heterojunction arrays
and their charge separation capability. J Phys Chem C 111:12987–12991
Yu J, Fan J, Cheng B (2011a) Dye-sensitized solar cells based on anatase TiO2 hollow spheres/carbon
nanotube composite films. J Power Sources 196:7891–7898
Yu J, Fan J, Cheng B (2011b) Dye-sensitized solar cells based on anatase TiO2 hollow spheres/carbon
nanotube composite films. J Power Sources 196:7891–7898
Yu Y, Wu K, Wang D (2011) Dye-sensitized solar cells with modified TiO2 surface chemical states:
The role of Ti3. Appl Phys Lett 99:97–100
Yu J, Yang Y, Fan R, Liu D, Wei L, Chen S, Li L, Yang B, Cao W (2014) Enhanced near-infrared
to visible upconversion nanoparticles of Ho3+ -Yb3+ -F- tri-doped TiO2 and its application in
dye-sensitized solar cells with 37% improvement in power conversion efficiency. Inorg Chem
53:8045–8053
Yuan Z, Tang R, Zhang Y, Yin L (2017) Enhanced photovoltaic performance of dye-sensitized
solar cells based on Co9S8nanotube array counter electrode and TiO2/g-C3N4 heterostructure
nanosheet photoanode. J Alloys Compd 691:983–991
Zhang XT, Sutanto I, Taguchi T, Tokuhiro K, Meng QB, Rao TN, Fujishima A, Watanabe H,
Nakamori T, Uragami M (2003) Al2O3-coated nanoporous TiO2 electrode for solid-state dye-
sensitized solar cell. Sol Energy Mater Sol Cells 80:315–326
Zhang Q, Dandeneau CS, Zhou X, Cao G (2009) Nanostructures for dye-sensitized solar cells. Adv
Mater 21:4087–4108
Zhang J, Zhao Z, Wang X, Yu T, Guan J, Yu Z, Li Z, Zou Z (2010) Increasing the oxygen
vacancy density on the TiO2 surface by la-doping for dye-sensitized solar cells. J Phys Chem C
114:18396–18400
Zhang J, Peng W, Chen Z, Chen H, Han L (2012) Effect of cerium doping in the TiO2 photoanode
on the electron transport of dye-sensitized solar cells. J Phys Chem C 116:19182–19190
Zhao D, Peng T, Lu L, Cai P, Jiang P, Bian Z (2008) Effect of annealing temperature on the photoelec-
trochemical properties of dye-sensitized solar cells made with mesoporous TiO2 nanoparticles.
J Phys Chem C 112:8486–8494
Zhu K, Neale NR, Miedaner A, Frank AJ (2006)
Zhu K, Vinzant TB, Neale NR, Frank AJ (2007) Enhanced charge-collection efficiencies and light
scattering in dye-sensitized solar cells using oriented TiO2 nanotubes arrays. Nano Lett
Zhu M, Li X, Liu W, Cui Y (2014) An investigation on the photoelectrochemical properties of
dye-sensitized solar cells based on graphene–TiO2 composite photoanodes. J Power Sources
262:349–355
Zou W, Visser C, Maduro JA, Pshenichnikov MS, Hummelen JC (2012) Broadband dye-sensitized
upconversion of near-infrared light. Nat Photonics 6:560–564
Chapter 16
p-Type Dye Sensitized Solar Cells:
An Overview of Factors Limiting
Efficiency

Sasanka Peiris, R. J. K. U. Ranatunga and Ishanie Rangeeka Perera

Abstract The energy crisis is a global problem that drives investment on renew-
able energy sources worldwide. Utilization of solar energy has become an effective
strategy for sustainable energy generation, as it has the potential to fill the energy
gap created due to the depletion of fossil fuel. On the ever-extending ladder of solar
harvesting technologies, third generation dye-sensitized solar cells (DSCs) have the
advantages of better cost effectiveness and environmental footprint when compared to
the first-generation silicon solar cells and second-generation thin film photovoltaics.
The ultimate goal of constructing high-efficiency multi-junction devices has set the
target of improving single junction components of DSCs (n- and p-type), separately.
The pace of development of single junction p-DSCs has been much slower than
that of n-DSCs. Discovery of suitable materials and techniques have lifted the per-
formance of n-DSCs to more than 14% since it was first reported in 1991. On the
other hand, p-DSCs have a maximum efficiency of 2.51%. It is important to bridge
the gap between the efficiencies of these single junction configurations, in order to
adopt the concept of multi-junction/tandem-DSCs that have the potential to reach
higher efficiencies by harvesting a larger fraction of the solar spectrum. This chapter
focuses on reviewing literature on development of p-DSCs. First, as an introduction,
the structure, function and kinetics of p-DSCs are described. Next, the two main
factors that affect the overall performance of a p-DSC; light harvesting capacity, and
energy loss within the device, are comprehensively discussed.

Keywords p-type · Solar energy · Dye-sensitized solar cells

S. Peiris
Sri Lanka Institute of Nanotechnology, Homagama 10200, Sri Lanka
R. J. K. U. Ranatunga · I. R. Perera (B)
Department of Chemistry, Faculty of Science, University of Peradeniya, Peradeniya 20400,
Sri Lanka
e-mail: ishanieperera@pdn.ac.lk
Postgraduate Institute of Chemistry, University of Peradeniya, Peradeniya 20400, Sri Lanka

© Springer Nature Singapore Pte Ltd. 2020 315

H. Tyagi et al. (eds.), Solar Energy, Energy, Environment,
and Sustainability, https://doi.org/10.1007/978-981-15-0675-8_16
316 S. Peiris et al.

16.1 Introduction

Photovoltaic (PV) energy conversion strategies have attracted huge attention in the
scientific world today. With the depletion of fossil fuel deposits, the world is at the
brim of facing an energy crisis; Utilization of solar energy is a hot spot of current
research in this regard. Although photovoltaics date back nearly 175 years, develop-
ment of photovoltaic cells progressed sporadically, and only became a viable energy
source with the discovery of p-n junction silicon photocells, which could achieve an
efficiency of 6%. Advances have improved efficiencies to over 20%, and currently,
silicon based solar cells are a prime candidate for efficient photo current generation
(Nattestad et al. 2016; Fraas 2014; Hagfeldt et al. 2010; Venkatraman et al. 2018).
Silicon being the most abundant element on the earth’s crust facilitates easy access.
Moreover, using Si cells are stable to ambient temperatures (Chapin et al. 1957),
minimize reflection losses and monocrystalline silicon solar cells have relatively high
photo conversion efficiencies. These cells are thought of as first-generation solar cells.
Since the purification process of silicon is expensive, it has become the bottleneck
for large scale commercial application. As a result, thin film solar cells composed
of amorphous silicon, CIGS (copper indium gallium diselenide), and CdTe, which
belong to the second generation of solar cells, came into play. However, the photo
conversion efficiencies obtained with these devices are lower compared to those of
monocrystalline silicon (Hagfeldt et al. 2010). Among other thin film solar cells,
amorphous silicon is the most established technology due to advantages like lower
temperature coefficient for power loss, low toxicity and cost effectiveness (Shah
et al. 1995). A vast array of scientific approaches is being implemented to develop
affordable and clean energy based on silicon solar cells (Chapin et al. 1954, 1957;
Hagfeldt et al. 2010).
A subsequent stage in the field of photovoltaics was the development of the dye-
sensitized solar cell (DSC), which is one of the outstanding inventions with an envi-
ronmentally friendly, low cost and feasible practical application. DSCs are regarded
as a bridge that directs the functionality of solar cells towards the third generation
(O’regan and Grätzel 1991). Michael Grätzel and Brian O’Regan published their
seminal work on DSCs in 1991, reporting a 7% overall light-to-electricity energy
conversion yield under diffused sunlight (Hagfeldt et al. 2010; Baxter 2012). From
that point onwards many developments and advancements to DSCs have brought
about higher efficiencies. In 2006, Chiba et al. achieved an efficiency of 11.1%,
which is the highest certified energy conversion reported (Chiba et al. 2006). By 2013,
Kakiage et al. reported a conversion efficiency over 14% by co-photosensitization
with an alkoxy silyl-anchor dye and a carboxy-anchor organic dye (Kakiage et al.
2015). This illustrates the fast evolution of DSCs within a few years. Michael Grätzel
states “Our present needs could be met by covering 0.1% of the Earth’s surface with
PV installations that achieve a conversion efficiency of 10%” (Fraas 2014; Grätzel
2007). Consequently, it is possible PVs could dominate the world’s energy mar-
ket; bringing sustainable energy generation with low levels of pollution. Figure 16.1
shows renewable electricity generation, with projections to 2050, published by U.S.
16 p-Type Dye Sensitized Solar Cells … 317

Fig. 16.1 Renewable

electricity generation (in
billions of kilowatt hours) in
the United States of America,
including end-use. Source
U.S. Energy Information
Administration (2018)

Energy Information Administration. Accordingly, wind and solar power is expected

to monotonously increase, accounting for nearly 900 BkWh (94%) of the total growth.
Furthermore, it is expected that solar photovoltaic (PV) usage will increase due to
advancement in cost effectiveness. Figure 16.2 is also an extract from the above
report, representing the annual electricity generating capacity additions and retire-
ments, that predicts, solar PV will be a prominent generation mode by 2050 (Outlook
2018).
Although the first DSCs were fabricated, using a single n-type or p-type semicon-
ductor material (single-junction devices), a contemporary push is to develop cells
with both n-type and p-type concurrently functioning in a tandem (multijunction)

Fig. 16.2 Annual electricity generating capacity additions and retirements in USA. Source U.S.
Energy Information Administration (2018)
318 S. Peiris et al.

device. The performance of tandem devices depends on the performance of each

junction. Although n-type DSCs have been optimized extensively, the progress of
p-DSCs is as yet, lacking (Odobel and Pellegrin 2013; Yu et al. 2012; Yum et al.
2011).
This chapter reviews the factors that limit the achievement of high photon to
current conversion efficiencies in single junction p-DSCs. First a brief introduction
on the structure, function and kinetics of a p-DSC will be discussed. Secondly, an
overview of factors affecting the overall device performance such as light harvesting
efficiency and charge collection and transport efficiency will be discussed in detail.
Finally, the future prospects of p-DSCs will be elaborated.

16.2 Structure of DSCs

The art and the architecture of a typical DSC is interesting in its own right. Assembly
of a working electrode (WE) and a counter electrode (CE) formulates a single DSC,
in which an electrolyte is wedged between the two (Perera et al. 2019). Figure 16.3
illustrates the arrangement of basic components in a typical DSC. Proper fabrication
of the WE are crucial in producing high photo conversion efficiencies. A transparent
layer of a semiconductor material is placed on a glass substrate which is made
conductive by coating with fluorine-doped tin oxide (FTO) (Hagfeldt et al. 2010;
Zhang et al. 2016). The semiconductor material (with a wide bandgap) is deposited
as a thin film having a large surface area, by producing Nano crystallites 10–20 nm in
size, and then sintering to form a thin film of ~16 μm thickness (Baxter 2012; Perera
et al. 2019). A high surface area is desired, to improve the dye loading capability,
and thereby increase the light harvesting efficiency (Baxter 2012). The sensitizers
are generally organic, or metal based, having the ability to transition to excited states
with the absorption of solar energy. This will be elaborately discussed in the section b.

Fig. 16.3 Schematic Illustration of a typical DSCs

16 p-Type Dye Sensitized Solar Cells … 319

Fig. 16.4 Schematic diagram of components of a p-DSC (left), and n-DSC (right), respectively

The electrolyte containing a redox mediator, which is placed between two electrodes,
could be in the form of a liquid, a quasi-solid, or a solid (Hagfeldt et al. 2010; Perera
et al. 2019).
Based on the semiconductor material and the operation, DSCs can be categorized
as n-type or p-type; both of which are considered as single junction DSCs. Electrons
are considered as the major charge carriers in n-DSCs while holes are the major
charge carriers in p-DSCs. In the fabrication process, n-type semiconductor materials
such as TiO2 and ZnO are used in n-DSCs while p-type semiconductor materials
like NiO, Cu2 O and CuCrO2 are used in p-DSCs (Zhang et al. 2016; Gong et al.
2017). Figure 16.4 shows the arrangement of components in both n- and p-DSCs,
respectively. So far, n-DSCs have been developed and utilized more widely, reaching
efficiencies higher than 14% (Kakiage et al. 2015), while p-DSCs require further
advancement with the highest reported efficiency currently at 2.51% (Nattestad et al.
2016; Perera et al. 2015).

16.3 Kinetics of Single Junction DSCs

Kinetics and charge movement dynamics in DSCs have been investigated by making
use of experimental methods as well as computational modeling. The chemical kinet-
ics in DSCs are complex, to an extent that it is difficult to apply simple rate laws. In
spite of this, the protocol of expressing the half-life relevant to different processes is
the current practice of conveying the kinetics of DSCs (Baxter 2012). The processes
are interdependent and changes in one of them can influence the overall performance
of the cell. Figure 16.5 show the time scale corresponding to different reactions. The
injection rate of electrons into the semiconductor layer must be faster than the relax-
ation of dye molecules (Hagfeldt et al. 2010). For example, generally the relaxation
time of a typically used Ru dye is around 50 ns while the electron injection into
the semiconductor takes place in less than one picosecond (Baxter 2012). Another
320 S. Peiris et al.

Fig. 16.5 Time scale

corresponding to different
processes in a DSC

important requirement is the regeneration of dye from the redox mediator should
be much faster (microseconds) than that of the recombination, that occurs by back
transfer of electrons from the semiconductor to the dye (hundreds of microseconds)
reactions a, b, and c in Fig. 16.7 represents the possible recombination reactions
taking place in a DSC (Hagfeldt et al. 2010).
Rrec1 indicates the recombination reaction between the injected electrons at the
CB of the semiconductor and the oxidized dye molecules. Similarly, Rrec3 depicts
the recombination of injected electrons with the oxidized species of the electrolyte.
Rrec3 has been suppressed with the development of the device components, which
represents the relaxation of excited dye molecules prior to electron injection.

16.4 Current–Voltage Characteristics of a DSC

Photoelectrochemical parameters related with characterization techniques in DSCs

are short-circuit current density (JSC ), open-circuit voltage (VOC ), fill factor (FF), and
overall power conversion efficiency (η) (Gong et al. 2017). These are derived from
current density (J) vs voltage (V) curves (see Fig. 16.6). In a J-V curve the intercept
of Y axis which represents a zero-bias condition (short circuit condition) is the JSC of
the device which can be defined as the photocurrent per unit area under short-circuit
condition. The intercept of the X axis, which represents zero current density (open
circuit condition), is taken as the VOC , which is the working potential, produced by
the device. It is the difference between redox potential of the electrolyte and the
fermi level of the semiconductor material. The power conversion efficiency (η) is
16 p-Type Dye Sensitized Solar Cells … 321

Fig. 16.6 General current versus voltage curve for DSCs

given by the ratio of maximum power output (Pmax ) to the incident solar power (Pin ).
Maximum power output is the product of JSC , VOC and FF (Eq. 16.1). Also, it can
be calculated when the current and the voltage of the device reaches their maximum
possible values, Jmax and Vmax , respectively (Eq. 16.3). The value of the FF lies in
between 0 and 1 and it can be calculated using Eq. 16.2 (Perera et al. 2019). It can
also be defined as the rectangularity of the J-V curve (Perera et al. 2019; Grätzel
2007).

η = Pmax /Pin = JSC VOC FF/Pin (16.1)

FF = Pmax /JSC VOC (16.2)

Pmax = Jmax Vmax (16.3)

16.5 p-Type DSCs

The development and optimization of p-DSCs are still in its infancy. NiO has predom-
inantly been used as the semiconductor material, however, it suffers from low effi-
ciency. Therefore, novel p-DSCs semiconductor materials such as Cu2 O8 , CuCrO2
and CuGaO2 are being developed (Zhang et al. 2016). Further, the advance research
trends focusing on overall performance of p-DSC devices, pursue in tailoring novel
sensitizers, (triphenylamine dyes, ruthenium dyes, cyclometalated iridium dye) elec-
trolytes ([Co(en)3 ]2+/3+ , [Fe(acac)3 ]0/1− ) and CEs (CoS, NiCo2 S4 ) (Xu et al. 2014;
Xiong et al. 2013; Zhu et al. 2014; Lyu et al. 2016; Perera et al. 2015).
322 S. Peiris et al.

Fig. 16.7 Electron flow of a

functioning p-DSC

Functioning principle of p-DSC can be explained using a few separate reactions,

which are depicted in Fig. 16.7. The pilot reaction initiates with the photoexcitation
of sensitizer (dye) molecules followed by injection of holes into the semiconductor
layer, that leads to transfer electrons from the valence band (VB) of the semiconductor
to the HOMO level of the dye, relaxing the excited state. The holes then diffuse
through the mesoporous oxide film, and after passing through the external circuit,
finally they reach the CE. The oxidized species of the electrolyte regenerates the
reduced dye at CE by accepting the electrons and then, regenerate itself by collecting
holes at the CE (Nattestad et al. 2016; Grätzel 2009; Hagfeldt et al. 2010).

16.6 Tandem DSCs

Lindquist proposed that by replacing the platinized CE of a single junction DSC, with
a serial connection to another dye sensitized photo electrode, the photo conversion
efficiency could be boosted. By connecting both n- and p-type semiconductors in a
single device, tandem-DSCs (multi junction DSCs) are created (Odobel and Pellegrin
2013; Yu et al. 2012; Lefebvre et al. 2014). The theoretical photo conversion effi-
ciency for a such tandem DSC was calculated to be 43% (Hagfeldt et al. 2010; Gong
et al. 2017). The development of efficient tandem DSCs would be a breakthrough in
photovoltaics because it allows the collection of photons with higher energy at one
electrode and photons with lower energy at the other. This allows for a higher portion
of incident solar energy to be used by the device. Figure 16.8, shows the alignment
16 p-Type Dye Sensitized Solar Cells … 323

Fig. 16.8 Electron flow in a

tandem DSC

of energy bands of the components in a tandem DSC and the electron flow within
the device (Gong et al. 2017).
The VOC value of a tandem-DSC can be approximated by the sum of VOC values
of n- and p- counterparts, whereas the JSC is limited by the (low performing) photo-
electrode. Due to the enhancement in VOC the overall efficiency of a tandem-DSC
is expected to be higher than the single junction counterparts. However, in many
occasions the overall efficiency of a device does not fulfill its potential because of
competition between the two sensitizers to capture photons within the same spectral
range.
Although the promise of tandem DSCs is appealing, for these devices to be viable
the overall efficiency must be improved significantly. At present the overall device
performance of a tandem device which is limited to 2.42% (Yum et al. 2011), and
this value is due to lower efficiency of the p-junction. Therefore, developing p-DSCs
is important for the future of tandem DSCs (Hagfeldt et al. 2010; Yum et al. 2011;
Baxter 2012; Perera et al. 2019).

16.7 Factors Affecting Overall Performances of p-Type

DSCs

The developmental process of DSCs paves the way to create advanced technologies
for generating affordable, efficient and green energy. In the field of energy generation,
Thus, there is a concept been built known as the critical triangle, which is based on
three main determinants: light-to electric energy conversion efficiency, stability, and
cost. Among these determinants, the light-to-electricity energy conversion efficiency
is given the most attention, and evolution of tandem DSCs is a major push towards
fulfilling this requirement. Consequently, the development of p-DSCs is critical,
324 S. Peiris et al.

since contemporary tandem DSCs are limited by the power conversion efficiency
of p-DSCs. This could be achieved by focusing on to two main strategies, namely,
(1) maximizing the light harvesting, and (2) minimizing electron losses (Gong et al.
2017). Novel practices based on these strategies are elaborately discussed in this
review, in Sects. 16.8 and 16.9, respectively.
Stability of the device is another aspect identified in the critical triangle that
has been studied. A major hurdle in extending the lifetime of DSCs has been the
use of liquid electrolytes, which can evaporate. A strategy to avoid this problem is
to substitute liquid electrolytes with solid electrolytes, yielding in solid state dye-
sensitized solar cells (ssDSCs). These devices are briefly discussed in Sect. 16.10.

16.8 Maximizing Light Harvesting

The light harvesting efficiency is predominantly determined by the performance of

the photosensitizer. Photon capture is followed by excitation of the dye, causing injec-
tion of positive charge carriers into the valence band of the p-type semiconductor
material. An efficient photo sensitizer should capture a maximum amount of sun-
light, usually from photons with energy higher than the band gap; from wavelengths
corresponding to UV and visible spectral region and reaching the near infrared (NIR)
region of the electromagnetic spectrum. Since each absorption event can generate
and push a pair of charges into the external circuit, enhancing the light harvesting
capacity can increase the photocurrent density. An efficient sensitizer should also be
strongly adsorbed on to the semiconductor surface so that the charge injection into
the semiconductor is efficient. In addition, rapid regeneration of the sensitizer by the
redox mediator is essential to minimize the recombination process, and importantly,
the dye should be stable in both its excited and ground states. There are few novel sen-
sitizers that are being used in p-DSCs: donor-π-acceptor systems, push-pull dyes,
cyclometalated dyes, quaraines, and ku Quinones (Lyu et al. 2016; Ji et al. 2013;
Jiang et al. 2014; Bonomo et al. 2017b).
Donor-(π linker)-acceptor systems have common structural motif including the
integrants arranged in the order as donor–π–acceptor (see Fig. 16.9). The absorp-
tion and redox properties of the dye can be fine-tuned by changing the individual
components. For example, arylamines are common donors while, cyanoacrylates
are common acceptors, and the pi-bridge, which is the linker, may contain one or
more thiophene units. Scientists have developed novel dyes by modifying these
donor, linker and acceptor units. According to one investigation, when carboxylic
acid groups are used as the acceptor, surface protonation of NiO occurs, causing a
negative shift of the valence band edge. This leads to reduce the hole-injection and
the photovoltage (Cui et al. 2014). Jin Cui et al. have developed a novel donor-π-
acceptor organic sensitizer incorporating pyridine as the anchoring group that could
overcome this problem. Under full sunlight (AM 1.5G,100 mW cm−2 ) the reported
conversion efficiency was ~0.16%, while VOC is 117.9 mV, JSC is 4.05 mA cm−2 and
FF is 0.34. This causes positive shifting of VB edge of NiO, minimizing interfacial
16 p-Type Dye Sensitized Solar Cells … 325

Fig. 16.9 Typical arrangement of units in a donor-π-acceptor type sensitizer

charge recombination with binding linkage that improves the potential for efficient
p-DSCs (Cui et al. 2014).
Sheibani et al. also have produced two oligomer dyes for p-DSCs, incorporat-
ing different electron acceptor groups. Triphenylamine and oligothiophene with
bulky alkyl chains have been used as the electron donor and the linker, respectively
(Sheibani et al. 2016) Two different dyes were produced by including napthoilene-
1,2-benzimidazole (NBI) (E1) and malononitrile (E2) as electron acceptors which
are electron withdrawing. This is the first application of NBI as an electron acceptor
in organic dyes for p-DSCs. When investigating the charge-separation kinetics, an
important finding obtained from transient absorption spectroscopy (TAS) is that there
is a prolonged Dye-NiO(+) charge separation lifetime identified for the NBI unit.
According to the results, it shows beneficial characteristics for regeneration of the
dye, since the dye can be stabilised when it is reduced. The narrower light absorption
spectrum of E1 results in a relatively low photocurrent, which a drawback of the dye
synthesized (Sheibani et al. 2016) (Table 16.1).
When comparing the studies carried out by Sheibani et al. and Jin Cui et al. which
are based on modification of acceptors (in donor acceptor systems), work done by
Jin Cui et al. which is incorporated with a pyridine ring exhibits a relatively high
performance.

Table 16.1 Comparison of the photovoltaic performance of two dyes E1 and E2

Dye VOC /mV JSC /mA cm−2 Fill factor Efficiency/%
E1 320 0.93 0.44 0.130
E2 320 0.78 0.41 0.102
326 S. Peiris et al.

A study conducted by Zhu and coworkers has revealed how the bridging ligand
could be modified to develop p-DSCs with a higher performance. Increasing the num-
ber of ter-thiophene groups in between triphenylamine and carboxylic acid resulted
in power conversion efficiency of 0.19%, VOC of 144 mV, JSC of 4.01 mA cm−2 and
FF of 0.33. The author proposed that ter-thiophene groups strongly affect the hole
injection and prevent charge recombination, increasing power conversion efficiency.
The investigators designed hexyl chains on the bridged thiophene rings that could
avoid dye aggregation on the NiO film and block the electrolyte from approaching
the surface of NiO; minimizing recombination between nickel oxide, electrolyte and
semiconductor (Zhu et al. 2014).
It was reported that inclusion of Boradiazaindacene (bodipy) sensitizer is com-
patible with NiO based p-DSCs (Lefebvre et al. 2014). Bodipy dyes have large
extinction coefficients, tunable absorption properties (i.e. to optically match photoan-
odes in tandem cells) and electrochemical stability. A further step for the improve-
ment of charge separation has been taken by Lefebvre and coworkers. They have
achieved an increase in charge-separated state lifetime of three orders of magnitude,
by using triphenylamine-donor bodipy-acceptor design (JSC = 3 mA cm−2 , IPCE
= 28%). The investigators state that further increase of yield and lifetime of the
charge-separated state could be achieved by altering the electronic coupling through
modifying the substituents on the bodipy (Lefebvre et al. 2014).
Push pull dyes can be considered as a further modification of the donor-
(π linker)-acceptor system (Fig. 16.10). Insertion of a push-pull moiety into the
donor-π-acceptor system is a novel method of enhancing the light harvesting and
intermolecular charge transfer (ICT) between the electron donor and the electron
acceptor. Table 16.2 lists different donors, acceptors, p-conjugated linkers and push-
pull linkers that are frequently used. Donors like triphenylamine and diphenylamine
are electron rich lewis-bases. The electron donor properties can be improved by
increasing the number of methoxy substituents. Different combinations of these
units can be applied to optimize the maximum possible light harvested by the photo-
sensitizer in DSCs. The electron donor can be used in modification of the absorption
spectra (Hadsadee et al. 2017).

Fig. 16.10 Typical arrangement of units in a push-pull sensitizer, and examples of units used
16 p-Type Dye Sensitized Solar Cells … 327

Table 16.2 Examples representing Donors, Acceptors and linker

Donors Triarylamine, indoline, perylene, fluorine, coumarin, carbazole,
di(p-carboxyphenyl) amine
Acceptors Cyanoacrylic acid, carboxylic acid, perylene monoamide
p-conjugated linkers Thiophene, phenyl group
Push-Pull linkers N-annulated perylene base

Zonghao Liu et al. have synthesized a series of novel push-pull arylamine-fluorene

based organic dyes which are reported as, zzx-op1, zzx-op2, and zzx-op3 (featuring
one, two, and three fluorene units as shown in Fig. 16.11). When they were applied in
p-DSCs, with NiO nanoparticles as a semiconductor material and I− /I3 − as a redox
couple, a significant energy conversion efficiency of 0.184% has been observed
for zzx-op1 with a JSC of 4.36 mA cm−2 under AM 1.5G condition and VOC of
112 mV. The electron donor and acceptor of zzx-op1 are di(p-carboxyphenyl)amine
(DCPA) and perylenemonoimide (PMID), respectively. A fluorene (FLU) unit with
two aliphatic hexyl chains was used as a π-conjugated linker. According to the
results, zzx-op1 showed 90.3% charge injection efficiency, which is higher than the
other two dyes. In addition, when changing the linker from a single fluorene to a
combination with 3,4-ethylenedioxythiophene and thiophene units, the absorption
maximum was shifted towards longer wavelengths (300–420 nm), decreasing the
energy conversion efficiency. Their investigation also demonstrates that electron-
rich heterocyclic aromatic groups like 3,4-ethylenedioxythiophene and thiophene
have the ability to raise HOMO energy levels and decrease the driving force for hole
injection (Liu et al. 2014b).
Another paper published by Liu et al. on fine-tuning fluorene-based dye structures
for high-efficiency p-DSCs, reveals how the application of optimized conditions

Fig. 16.11 zzx-op type push-pull dyes

328 S. Peiris et al.

could affect performance. They serendipitously found that the solvent system used
to dissolve the dye could greatly influence cell performance. As a result of applying
acetonitrile/THF solvent system in 1:1 ratio, the dye zzx-op1-2 outperformed the
other two dyes (zzx-op1 and zzx-op1-3; Fig. 16.11) giving a photocurrent density of
7.57 mA cm−2 under full sun illumination (simulated AM 1.5G light illumination,
100 mW cm−2 , VOC = 117 mV, FF = 0.4, η = 0.353%). It is exciting that these cells
with zzx-op1-2 dye, have excellent long-term stability. Moreover, it was found that
this particular dye has comparatively longer hole lifetime, lower photocurrent losses
and higher photogenerated hole density (Liu et al. 2014a).
Two novel donor–π acceptor push-pull dyes based on organometallic ruthenium
di acetylide complexes, which could be applied for p-DSCs, were reported on by Lyu
et al. Though these dyes are still not optimized, it is observed that their photovoltaic
performance is relatively high compared to other ruthenium polypyridine complexes
(JSC = 2.25 mA cm−2 , VOC = 104 mV, FF = 0.34, η = 0.79%). According to the
investigators lowering the energy level the HOMO will yield ruthenium diacetylide
dyes with high performances due to enhancing injection Gibbs free enthalpy (Lyu
et al. 2016).
Vasilis Nikolaou et al. synthesised the first covalently linked zinc porphyrin-
fullerene (ZnP-C60) Donor-Acceptor dyads that can be used in p-DSCs. Three such
structures (C60ZnPCOOH, C60-trZnPCOOH and C60trZnPtrCOOH) were synthe-
sised and used as sensitizers in NiO-DSCs. Ultrafast transient absorption spec-
troscopy illustrates that there is a long-lived charge separation achieved with the
shift of electrons from the porphyrin core (reduced) to the C60. Further, the tran-
sient experiments show that triazole ring also could expand the lifetime of charge
separated state. In addition, they have examined the effect of introducing a spacer
group with different lengths in between the electron acceptor and porphyrin macro-
cycle in order to reduce the charge recombination. The photo conversion efficiency
of the best performing dyad with I3 − /I− electrolyte is 0.076% (VOC = 109 mV; JSC
= 1.86 mA cm−2 ; FF = 0.35) (Georgios Coutsolelos et al. 2018).
A zinc-porphyrin sensitizer has been also been designed by Jianfeng Lu et al.
They have developed a D-π-A system by connecting the electron donor-di(p-
carboxyphenyl)amine (DCPA) and the electron acceptor- perylenemonoimide (PMI)
through a zinc(II) porphyrin having alkyl chains which act as a π- conjugated
bridge. When the dye is fabricated into a p-DSC, with NiO semiconductor and
tris(acetylacetonato) iron(III/II) redox mediator a power conversion efficiency of
0.92% has been observed under simulated 100 mW cm−2 (AM 1.5G irradiation) (Lu
et al. 2018).
Hanni Wu et al. have developed Zn porphyrin-polyoxometalate hybrids incorpo-
rating different p-linkers which can be utilized as sensitizers in p-DSCs. They have
made use of density functional theory method and time dependant density functional
theory method to investigate the impact created on sensitizers by the p-linkers and
have observed higher delocalization with longer p-linkers (Wu et al. 2016).
The groups of Koten (He et al. 2014) and Grätzel (Bessho et al. 2009) separately
used cyclometalated Ru(II) complexes in n-DSCs. A significant recognition is given
to these complexes due to their electrochemical and photochemical stability, and
16 p-Type Dye Sensitized Solar Cells … 329

the presence of a broad metal-to-ligand charge transfer (MLCT) band of significant

intensity. Not like the other classes of sensitizers, cyclometalated [Ru(NˆN)2 (CˆN)] +
provide flexibility to be used in both n- and p-type DSCs by varying the functional
group. When it is used in a n-DSCs, the CˆN ligand bears an ancillary role (similar to
electron pushing in a ‘push-pull’ dye architecture) but when it comes to a carboxylic
or phosphonic acid group in p-DSC it acts as the anchoring unit (Marinakis et al.
2017). Through functionalization of the C,N-ligand in [Ru(NˆN)2 (CˆN)]+ complex
(NˆN ¼ bidentate N,N0 -ligand, CˆN4 ¼ cyclometalated C,N-ligand), the electronic
properties can be changed due to the localization of the highest-occupied molecular
orbital (HOMO) on the Ru/CˆN domain (Bomben et al. 2011, 2012; Ji et al. 2012,
43; He et al. 2014; Marinakis et al. 2017).
The first application of cyclometalated ruthenium complexes of the type Ru-
[(NˆN)2 (CˆN)]+ , as sensitizers for NiO p-DSCs, was reported by Ji et al. (2012).
These dyes demonstrated broad absorption in the visible region. Furthermore, hole
injection to the VB of the semiconductor is promoted by the carboxylic anchoring
group attached to the phenylpyridine ligand. Rigid phenyl linkers have been inserted
to systematically vary the distance between the Ru-[(NˆN)2 (CˆN)] + core and the
carboxylic anchoring group. Increasing the number of phenylene linkers depressed
charge recombination losses and thereby, improved the photogenerated hole lifetime.
Therefore, the devices fabricated by incorporating dyes with longer spacer groups
reached higher efficiencies as well as higher JSC and VOC (With O12 dye, VOC =
82 mV, JSC = 1.84 mA cm−2 , FF = 0.34 and η = 0.015%) (Ji et al. 2012).
Zhiqiang Ji et al. have also studied the behavior of the triphenylamino linker
attached to the para position of the ruthenium–carbon bond of the [NˆC] ligand. This
could improve the electronic coupling for hole injection and thereby generate compar-
atively higher JSC values than the prior sensitizers with phenylene linkers. They have
synthesized the sensitizers using 2,2 -bipyridine (O3), 1,10-phenanthroline (O13),
and bathophenanthroline (O17), in a manner that increases the conjugation of NˆN
ligand, leading to intensification of the extinction coefficient, and a concurrent red-
shift in absorption. Nevertheless, the O3 sensitizer exhibits comparatively higher Jsc
of 3.04 mA cm−2 . But the VOC is 93 mV which is low yielding a lower efficiency
of 0.099% and fill factor is 0.35. However, the results suggest that O3 can perform
efficient dye regeneration and slow charge recombination (Ji et al. 2013).
Another panchromatic cyclometalated Ru(II) complex (O18) is reported by He
et al. (2014). which has the capability of absorbing photons from visible to near-IR
region of the electromagnetic spectrum extending up to 800 nm. O18 has an improved
molar extinction coefficient (ε = 1.9 × 104 M−1 cm−1 at 593 nm in solution) that
could stabilize the LUMO level of the dye. Due to the use of a π-conjugated system of
2,2 -bipyridyl (bpy) ligands they have increased the absorption cross section. Overall
performance is relatively good (Jsc = 3.43 mA cm−2 VOC = 93 mV; FF = 0.33; η
= 0.104%) (He et al. 2014).
In addition to the ruthenium dyes, iridium complexes are also being used as
cyclometalated sensitizers for p-DSCs. Marcello Gennari and coworkers synthesised
three new cyclometalated iridium dyes and surprisingly they have revealed slow
charge recombination and a long-lived charge separation which is found to be unique
330 S. Peiris et al.

for these dyes. Thus, it is more compatible with other redox mediators allowing high
open circuit voltage. From the three iridium complexes investigated, IrDPQCN2
shows higher photovoltaic performances (VOC = 508 mV; JSC = 0.25 mA cm−2 , ff
= 0.54; η = 0.068%) (Gennari et al. 2014).
Squaraines (SQs) are a novel group of sensitizers that have the potential to be used
in p-DSCs. They are derived from squaric acid and generate intramolecular donor-
acceptor-donor charge transfer state in both the ground and the excited states (Law
and Bailey 1992). Squaraines have extremely high molar extinction coefficients, and
intense narrow absorbance/fluorescence in the red-IR. Since the reactivity of the
electrophilic cyclobutyl core is high, SQs easily react with nucleophiles, as well
as bases, limiting the application possibilities. However, recent modifications have
made them more stable (Jiang et al. 2014).
Three squaraines (VG1-C8, VG10-C8, VG11-C) have been synthesised and
applied in p-DSCs by Matteo Bonomo and coworkers. Among the three dyes, VG11-
C8 with a dicyano–vinyl substituent on the central squaric ring, produces compara-
tively higher conversion efficiency, than the other two dyes (VOC = 93 mV; JSC =
1.16 mA cm−2 ; FF = 0.36; η = 0.043%). This yields favourable external quantum
efficiency (EQE) of NiO and imbues characteristics of lower resistance of charge
transport, low interfacial recombination, shorter hole diffusion times, and larger hole
diffusion coefficient, in comparison to other two dyes (Bonomo et al. 2016).
A novel class of quinoid compounds known as KuQuinones (see Fig. 16.12) have
been applied to p-DSCs. The presence of a highly conjugated pentacyclic structure
results in a broad absorption in the visible region with two intense bands between
450 and 630 nm, making them worthy candidates for light harvesting in p-DSCs. In
addition, these dyes consist of a very low reduction potential (Sabuzi et al. 2016).
Bonomo and co-investigators have also incorporated KuQuinones (KuQ) as the
sensitizer in NiO based pDSCs. They state the dye possesses HOMO/LUMO states
that match with the redox potential of I− /I3 − and the upper edge of the NiO valence
band. They have reported on KuQ substituted with carboxylic acid and with different
alkyl chain. According to the results KuQ-sensitized cells show similarity to that of
the benchmark sensitizer erythrosine B (Ery B) other than the absence of electronic
conjugation between the light absorbing unit and the anchoring group. Due to this

Fig. 16.12 General structure

of KuQuinones
16 p-Type Dye Sensitized Solar Cells … 331

consequence, it is hypothesized, that chemical bonds are not involve in photoinduced

charge transfer between the excited KuQ dyes and the NiO electrode, instead, it
occurs through space. The KuQ-sensitized cells have a VOC of 92.2 mV, JSC of
0.74 mA cm−2 , FF of 35.9 and efficiency of 0.025% (Bonomo et al. 2017b).

16.9 Minimizing Electron Losses

After photoexcitation and injection of charge carriers into the semiconductor mate-
rial by the photosensitizer, fast dye recombination, comparatively slow regeneration
and lower charge mobility in the semiconductor material can contribute to loss of
electrons within a DSC. Proper manipulation of the electrolyte and the semiconduc-
tor material could improve photoconversion efficiency. Novel electrolytes have been
invented that could minimize the electron losses ensuring fast dye regeneration and
minimum recombinations.
Powar et al. have fabricated efficient p-DSCs based on Tris(1,2-diaminoethane)
Cobalt (II)/(III) electrolytes. They have observed [Co(en)3 ]2+/3+ redox couple could
render energy conversion efficiencies of 1.3% at 100% sun (1.67% at 10% sun), with
among the best performance indicators to date (VOC = 709 mV, JSC = 4.44 mA cm−2 ;
FF = 0.42). It is a transparent electrolyte, which can be easily applied into tandem
DSCs. This electrolyte is best applicable with the NiO semiconductor; when it was
used with TiO2 the VOC obtained was at a level around 475 mV (Powar et al. 2013).
Perera et al. have discovered a novel electrolyte based on tris(acetylacetonato)-
iron (III)/(II) redox couple ([Fe(acac)3 ]0/1− ) for p-DSCs. Interestingly they have
improved the photocurrent by introducing a blocking layer on top of the NiO semi-
conductor material in the working electrode and by using chenodeoxycholic acid
in the electrolyte. The device fabrication was done by incorporating perylene–thio-
phene–triphenylamine (PMI–6T–TPA) as the sensitizer and ([Fe(acac)3 ]0/1− ) as the
electrolyte. The best reported short circuit of JSC = 7.65 mA cm−2 up to date has
been achieved by them along with η = 2.51%, FF = 0.51, VOC = 645 mV. When
analysing the measurement of dye regeneration kinetics of the redox mediator, the
rate constant obtained (1.7 × 108 M−1 s−1 ) is close to the maximum theoretical rate
constant of 3.3 × 108 M−1 s−1 . As a result, considerably high dye-regeneration yield
(>99%) has been obtained by these devices (Perera et al. 2015).
Xu et al. have introduced disulfide/thiolate electrolytes to improve the efficiency
of p-DSCs. An organic redox couple of 1-methyl-1H-tetrazole-5-thiolate (T) and its
disulfide dimer (T2) redox shuttle were used as an electrolyte in a p-DSC, of CuCrO2
electrode and an organic dye (P1) sensitizer. Using this iodide-free transparent redox
electrolyte with the sensitized heterojunction, they have gained a comparatively high
VOC of over 300 mV. And also studied on the application of CoS as the counter
electrode and have observed an efficiency of 0.23%. This can be considered as the
maximum efficiency gained for an organic redox couple (Xu et al. 2013).
332 S. Peiris et al.

More recently Marinakis and coworkers have investigated performance and

long-term stability of p-DSCs with a cycloruthenated dye through electrolyte sol-
vent tuning. Fabrication of p-DSC using NiO as the semiconductor material along
with the zwitterionic cyclometalated ruthenium dye [Ru(bpy)2(H1)](H31¼(4-(2-
phenylpyridin-4-yl)phenyl)phosphonic acid and I3 − /I− /acetonitrile (AN) electrolyte,
produced a device with VOC = 95 mV, JSC = 4.06 mA cm−2 ; and η = 0.139%. With
the use of propionitrile (PN), valeronitrile (VN) and 3-methoxypropionitrile (MPN)
as electrolyte solvents, the trend of JSC was observed as AN > PN > MPN > VN >
NMP and VOC follows the trend VN > PN > MPN > AN > NMP. This means there
is an effect created by the electrolyte solvent on the efficiency of a DSC (Marinakis
et al. 2017).
Effort to enhance the photoconversion efficiency of p-DSCs is mainly directed
towards improving performance of individual components. Currently, numerous
researches based on novel p-type semiconductor materials, sensitizers and elec-
trolytes are underway. Though NiO is the primarily used semiconductor material
in p-DSCs, there are few drawbacks that have been encountered when it is utilized.
The VB of NiO lies at +0.50 V versus the NHE and if the typically used I− /I3 −
redox mediator (+0.35 V vs. NHE) is used as the electrolyte, the VOC will be limited
to about 150 mV leading to a poor overall performance of the device (Zhang et al.
2016). On the other hand, NiO is a dark coloured compound which absorbs a signif-
icant amount of solar radiation. It has been reported that nearly 30–40% of incident
photons are absorbed by a NiO film of thickness 2.3 μm (Yu et al. 2012). Further-
more, fast recombination reactions between the reduced sensitizers and the holes in
NiO, cause a significant loss in the number of holes injected into the semiconductor
(Ji et al. 2013), since the electronic conductivity of porous NiO is relatively low
compared to TiO2 . The magnitude of hole diffusion coefficient in NiO (10−8 to 10−7
cm2 s−1 ) is three orders lower than that of the electron diffusion in TiO2 . As a result,
the cathodic current is slowed down due to the delay of injection of holes and their
collection back into the electrolyte. NiO also possesses lower dielectric constant
and poor chromophore surface loading (Odobel and Pellegrin 2013). Scientists seek
novel approaches to overcome these issues related with NiO and on the other hand,
introduce different types of semiconductor material that can replace it. Flynn et al.
have taken attempts to control the morphology and microstructure of NiO and bring
about a methodology to synthesise high quality NiO hexagonal plates, which are
ultra-thin and composed of a high density of nanoscale pores. They have observed
more than a 10-fold improvement in the electrical mobility compared to conventional
nanoparticles. Further, they revealed an increment of 20–30% of the performance
of p-DSCs fabricated using these NiO plates. Although it is commonly suggested
in literature that the black colour of NiO is due to mixed valence Ni2+ /Ni3+ , a study
conducted by Adèle Renaud and coworkers show that it is due to the presence of
Ni0 and progressive oxidation could completely change the colour to pale green, and
also create larger particle sizes that could lower the efficiency. According to their
findings, the presence of elemental Ni does not seem to significantly affect the pho-
tovoltaic performances of p-DSCs (Renaud et al. 2013b). Zannotti et al. have tried
using Ni/Mg mixed metal oxides for p-DSCs. When the concentration of MgO is
16 p-Type Dye Sensitized Solar Cells … 333

increased, photovoltage was increased due to the positive shift in fermi level of the
semiconductor (NiO/MgO); however, the generated photocurrent drops with increas-
ing the concentration of Mg2+ . Still they were able to show that a constitution of 5%
MgO could balance the increased photo voltage and decreased photo current own-
ing a large pore volume in both meso and macropore range (BET surface area of
35 m2 g−1 ). The power conversion efficiency obtained by incorporating 5% MgO
was higher than using NiO alone (VOC = 123 mV; JSC = 5.09 mA cm−2 ; FF = 0.32;
η = 0.2%) (Zannotti et al. 2015).
Muhammad Awais et al. have revealed, that in addition to electrochemical parame-
ters of individual components, the deposition method of NiO nanoparticles on indium
tin oxide (ITO) substrate could affect the performance of p-DSCs. They have cho-
sen spray deposition followed by sintering of nickel oxide nanoparticles to acquire a
large surface area. A comparison of the J–V performance with NiO samples prepared
using sol–gel method exemplified an enhanced performance in the NiO sample pre-
pared using spray deposition due to higher mesoporosity obtained with this method
(Awais et al. 2013).
Plasma-assisted microwave sintering (or rapid-discharge sintering, RDS) is a
novel technique that was being employed by Awais et al. for the preparation of NiO
thin films which can be used to fabricate photoactive electrodes in p-DSCs (Awais
et al. 2014). The novelty of this method is the application of plasma bombardment
treatment prior to the step of spray-deposition and heating of the FTO substrate.
During the spray deposition, heating is controlled by the energy of microwaves. This
method could ensure more efficient charge collection due to improved adhesion and
electrical contact at the FTO/NiO interface. For comparison, they have used NiO
nanoparticles synthesised by conventional sintering in furnace or through sol–gel
procedures which are screen printed. They were able to produce NiO samples with
better performance using RDS method when applied to p-DSCs sensitized with ery-
throsine B (Awais et al. 2014; Novelli et al. 2015) .
Current p-type semiconductor materials have been maneuvered towards a novel
category of chemicals called delafossites. They belong to a family of transparent
conducting oxides. Low electrical resistivity (high conductivity), high transparency,
wide band gap, high energy conversion efficiency and low fabrication cost motivate
their use as the semiconductor material in p-DSCs. More recognition is given to Cu
(1) based CuMO2 (M = Al, Cr, Ga, Cr, B, In) delafossites. Strong hybridization of
3d orbitals of Cu with 2p orbitals of O facilitate the delocalization of hole charge
carriers, increasing the hole mobility (Ahmed et al. 2014; Jiang et al. 2016; Xu et al.
2014).
Yu et al. reported the first application of CuGaO2 delafossite into pDSCs. The
VB edge of CuGaO2 is +0.6 V v.s. NHE and they have achieved VOC of 357 mV
by using Co3+/2+ (dtb-bpy) as the electrolyte (1 Sun AM 1.5 illumination). Increasing
the illumination enhanced the VOC up to 464 mV, which is known as the saturation
photovoltage. Nano plates of CuGaO2 that are stable up to to 350 °C have been made
with an average diameter of 200 nm and thickness of 45 nm using a hydrothermal
method. Since there is an off white colour in these nano plates, no competition exists
against the sensitizer for capturing sunlight (Yu et al. 2012). Another research based
334 S. Peiris et al.

on CuGaO2 plates conducted by Zhen Xu et al. produced devices with a remarkably

high JSC = 2.05 mA cm−2 incorporating I− /I3 − electrolyte as the redox mediator.
Nanoplates of 100–200 nm diameter and thickness of 20–30 nm were obtained, and
optimization of temperature for the hydrothermal synthesis was done (Xu et al. 2014).
Adèle Renaud et al. have done further modification to CuGaO2 based p-DSCs by
doping Mg with assembly of PMI-NDI dyad as the dye, and tris(4,4 -bis-tert-butyl-
2,2 -bipyridine)cobalt(II/III) as the electrolyte. They found out that there is a 73%
increase in photovoltaic yield compared to undoped CuGaO2 , when doped with 1%
of Mg. But this performance is lost when the doping is increased above 1% of Mg
(VOC = 305 mV; JSC = 0.415 mA cm−2 ; FF = 0.35; η = 0.45) (Renaud et al. 2013a).
CuAlO2 is another type of delafossite which was synthesised by Ahmed and
coworkers under controlled oxygen pressure (pO2 ) of 105 atm and at a temperature
of 775 ° C. A uniform particle size of 35 nm was obtained and cathodic photocurrent
with JSC = 0.954 mA cm−2 under 1.5 sun illumination (Ahmed et al. 2014). Miclau
et al. have conducted a study based on the effect of polymorphism on photovoltaic
performance of CuAlO2 delafossite nanomaterials by synthesizing nanocrystalline
3R or mixture of 2H and 3R polytypes. Key factors that cause the formation of poly
types are temperature and autogenous pressure. Even a small concentration of 2H
polytype caused raise energy conversion efficiency by 49% compared to pure 3R-
CuAlO2 polytype. The reason for higher efficiencies may be because of the presence
of smaller Cu-Cu distance of 2H polytype compared to that of 3R polytype. As the
electrical conductivity predominantly occurs through the Cu+ plane, 2H polytype
generates a higher photocurrent density (JSC ) (Miclau et al. 2017).
The first application of hydrothermally synthesised, Mg doped CuCrO2 delafossite
material in p-DSCs was reported by Xiong et al. They have observed an enhancement
in optical transmittance and reduction of crystallite size, that leads to improvement
of JSC by approximately 27%, when CuCrO2 was doped with Mg. Lower valence
band edge and faster hole diffusion coefficient has been observed in CuCr0.9 Mg0.1 O2
(in comparison to NiO), producing a VOC of 201 mV and JSC of 1.51 mA cm−2 . The
efficiency obtained was 0.132% which is nearly three times higher compared to NiO-
based reference devices with a VOC of 0.449 (Xiong et al. 2013). Powar et al. have
also worked on CuCrO2 using [Co(en)3 ]2+/3+ based redox mediators (VOC = 734 mV;
JSC = 1.23 ± 0.17 mA cm−2 ; FF = 0.53 ± 0.04; η = 0.48 ± 0.08%). Though there is a
comparatively higher VOC value, the value of JSC has been drastically decreased, due
to insufficient hole-injection driving forces. Thus the efficiency was limited (Powar
et al. 2014). Daniel et al. used hydrothermal synthesis to obtain nanocrystals of (Al,
Mg)-Doped CuCrO2 with smaller size (and therefore larger surface areas) compared
with other delafossites. Doping of Al3+ causes shifting of the optical band gap by
0.05 eV, while dopic with Mg2+ causes it to shift by 0.27 eV. In the presence of
Coumarin C343 as the sensitizer, the short-circuit density (JSC ) is increased by 11%
for Al3+ doping and by 16% for Mg2+ doping (Daniel et al. 2017).
Ursu Daniela et al. have doped CuCrO2 with Co to produce non-stoichiometric
nanocrystals using a low-temperature hydrothermal method which increases the oxy-
gen content of the delafossite structure. The electrical resistivity has been decreased
16 p-Type Dye Sensitized Solar Cells … 335

due to Co-substitution that increases the JSC value 0.50 mA/cm2 , when using N719
and I− /I3 − as the sensitizer and the redox couple, respectively (Daniel et al. 2017).
Adèle Renaud et al. have successfully demonstrated the first DSC with p-type
LaOCuS nanoparticles as the photocathode. Transparent LaOCuS is an oxysulfide
material with similar VB potential to that of NiO, that enables effective hole injection
from the excited sensitizer. When using PMI-NDI as the sensitizer, it could attain
the following photovoltaic characteristics VOC = 150 mV, Jsc = 0.039 mA/cm2 , ff
= 26%, η = 0.002% (Renaud et al. 2015).
An attractive discovery by Ze Yu et al. was based on finding p-type semiconducting
property in degenerate n-type semiconductors, could open a novel avenue for research
on p-DSCs. Indium tin oxide (ITO) is a n-type degenerate semiconductor having
good charge transport properties, and sufficient transparency. With the application
of ITO semiconductor in place of highly coloured NiO, and [Fe(acac)3 ]0/− redox
mediator, along with a new organic sensitizer, they have achieved a photo conversion
efficiency of 1.96 ± 0.12% (JSC = 5.65 mA cm−2 , VOC = 700 mV). ITO owns a
significant local density of states below −4.8 eV, enabling transfer of electrons to
excited dye that produces a sustained photo cathodic current. A drawback of ITO is
faster recombination rate at the ITO–electrolyte interface. In order to overcome this
problem and attain higher efficiencies, the investigators have specifically designed
and synthesised an extended oligothiophene π-bridging sensitizer (PMI-8T-TPA).
Use of ITO as a blocking layer is another powerful modification that they have
implemented to achieve a VOC value of 758 mV in the presence of [Co(en)3 ]3+/2+ as
the electrolyte and 712 mV in the presence of [Fe(acac)3 ]0/− electrolyte (Yu et al.
2016).
Incorporation of porous metal–organic frameworks (MOFs) in fabrication of p-
DSCs is a novel approach by Junkuo Gao et al. They have reported a Ti(IV)-based
MOF, NTU-9, that shows p-type semiconductor behaviour with a bad gap of 1.72 eV.
It absorbs in the visible range up to 750 nm and has good photocatalytic activity.
Though there are numerous MOF applications related to n-DSCs, this is the first
novel MOF which has been developed with a potential as a semiconductor material
in p-DSCs. They also suggest that NTU-9 is a promising visible-light photocatalyst
for energy conversion and environmental remediation (Gao et al. 2014).
In addition to the Cu based delafossite materials, Shi et al. have successfully
introduced NiCo2 S4 nanosheet films obtained from NiCo2 O4 , to be applied as a
counter electrode in p-DSCs. Both NiCo2 S4 and NiCo2 S4 could be used to replace
Pt and NiO, respectively. Cost effectiveness is a major advantage. NiCo2 S4 is imbued
with high catalytic activity towards I− /I3 − electrolyte, generating an improved JSC
= 2.989 mA cm−2 and η = 0.248% compared to when Pt is the counter electrode
(1.824 mA/cm2 and 0.158%, respectively) but VOC is not improved (Shi et al. 2014).
Cuprous oxide (Cu2 O) is an important semiconducting material due to its high
electron transmission. Studies on investigating electrodes made of pure Cu2 O are
infrequent. Sisi et al. have demonstrated the possibility of applying Cu2 O into p-
DSCs. They have compared commercial Cu2 O particles and electrodes prepared
from Cu2 O powder. In comparison, prepared Cu2 O has shown better photovoltaic
performance than commercial Cu2 O (JSC = 1.3 mA cm−2 , VOC = 710 mV, FF =
336 S. Peiris et al.

0.46, η = 0.42%) due to high dye adsorption capability. They also verified factors
such as the thickness of prepared Cu2 O(2) film, uniformity and the grain size of
material Cu2 O, affects the electrode performances (Du et al. 2014).
Daniel Ursu et al. have synthesized different morphologies of cuprous oxide,
such as porous truncated octahedrons, and 3D hierarchical structures consisting of
the micrometer dendritic rods, employing copper (II) acetate and ethyl cellulose as
reactants. They have observed nearly 15% increase in VOC and JSC in the porous
structure over the 3D hierarchical structure due to high dye loading capacity and the
reduced recombination process at the oxide/dye/electrolyte interfaces (Ursu et al.
2018).
Despite the fact that attention given for the improvement of counter electrode is
not very prominent, Mirko Congiu et al. have studied a novel application of trans-
parent thin film of cobalt sulfide (CoS) as an anodic counter electrode in place of
a Pt electrode. CoS is preferred over Pt due to low cost of the starting material and
easier deposition. They have implemented a typical p-type cathode incorporated with
mesoporous NiO-deposited via discharge sintering, erythrosin B as the photo sensi-
tizer and I− /I3 − as the redox mediator along with the CoS anodic counter electrode.
They report similar efficiency, JSC , FF, and VOC values to those obtained for DSCs
with the Pt counter electrode (VOC = 74 V, JSC = −1.051 mA cm−2 , FF = 0.325,
η = 0.026%). Electrochemical impedance spectroscopy of CoS anodes in p-DSCs
have shown higher electrocatalytic efficiency and lower charge-transfer resistance in
comparison to platinized FTO anodes (Congiu et al. 2016).
The work done by Matteo Bonomo et al. has shown the limits on using cobalt
sulfide as the anode of p-DSCs by employing a film of CoS of thickness <10 μm as
the counter electrode with NiO photoactive cathode and I− /I3 − redox mediator. In
comparison with Pt in the counter electrode CoS is unable to sustain photocurrent
densities generated by NiO at a given potential, due to slower kinetics of iodide
oxidation at the CoS anode. This could create 30 times larger charge transfer resis-
tance of CoS with respect to Pt-FTO (130  vs. 4.5 ). Furthermore, the behavior of
CoS like a p-type semiconductor without a degeneracy, could induce the reverse bias
of the photoelectrochemical cell. This displays a lower photoconversion efficiency
(VOC = 128 mV, JSC = −1.70 mA cm−2 , FF = 0.317, η = 0.07%) that is about 35%
less than that of the analogous p-DSC with Pt-FTO anode (0.07 vs. 0.11%). However,
there is no significant effect identified on the open circuit potential (130 mV) or the
FF based on the nature of the anodic material (Bonomo et al. 2017a).

16.10 Solid State Dye Sensitized Solar Cells

One of the challenges encountered in liquid state DSCs is the instability of con-
ventional liquid electrolytes due to volatilization. This affects the durability and
the performance of solar cells. Consequently, scientists paid their attention towards
employing solid or quasi solid hole conducting materials in place of liquid elec-
trolytes like I− /I3 − . Both n-DSCs and p-DSCs are candidates for replacement of
16 p-Type Dye Sensitized Solar Cells … 337

liquid electrolytes. Among the p-semiconductors that act as hole transport materials,
CuI plays a key role due to its higher stability and conductivity (Kato et al. 2018).
In 1998, Nature has published a letter by Grätzel putting forward the concept of
a solid state dye-sensitized TiO2 solar cells that can achieve high photon to electron
conversion efficiency (Li et al. 2006; Bach et al. 1998). In 2014, Lei Zhang et al. fabri-
cated the first solid state p-type dye-sensitized solar cell (p-ssDSC) using the electron
conducting material, phenyl-C61-butyric acid methyl ester (PCBM) in the presence
of NiO semiconductor and organic dye sensitizer. They have observed an admirable
open circuit voltage (VOC = 620 mV) due to the suppression of charge recombina-
tion losses. However, the JSC value is very small (50 μA cm−2 ) due to unsatisfactory
lifetime of the excited state of the dye or slow regeneration of the dye. However, this
attempt intensified the development of tandem solid-state dye-sensitized solar cells
(t-ssDSCs) as well as dye-sensitized solar fuel devices (DSSFDs) (Zhang et al. 2016).
The same group of scientists have developed an Indacenodithieno[3,2-b]thiophene
(IDTT)-based organic dye (TIP) for further improvement of p-ssDSCs by minimizing
the drawbacks of earlier research work. More recent work has produced compara-
tively higher performance (η = 0.18%; VOC = 550 mV; JSC = 0.86 mA cm−2 ).
According to the charge lifetime experiments they have revealed that the TIP dye
could significantly minimise recombination losses (Xu et al. 2019).
Lei Tian et al. have developed a core-shell NiO-dye-TiO2 mesoporous film for
the first time using a newly designed triphenylamine dye. Fabrication of the film
has been done by employing atomic layer deposition technique. In this architecture,
NiO nanoparticles are covered by TiO2 layer that allows close contact of the dye
and the semiconductor. Further, the dye alignment is well oriented for favourable
electron/hole injection. This fabrication renders efficient and ultra-fast hole injection
into NiO (>98%, ≤200 fs), followed by faster dye regeneration (70–93%, ≤500 fs),
transferring electrons to TiO2 . This ensures much slower charge recombination than
that in the absence of TiO2 layer (t1/2 ≈ 100 ps). This is an interesting discovery
which is applicable in ssDSCs (Tian 2019; Tian et al. 2017).

16.11 Summary and Outlook

Dye-sensitized solar cells represent one of the brightest prospects to address the
energy crisis the world is facing. DSCs have the promise of being a commercially
viable renewable energy source, due to the Research throughout the world has been
directed towards a goal of discovering the best combination of components for
DSCs. Currently, p-DSCs have gained attention in this regard, since it entails much
more development to achieve the ultimate goal of fabricating efficient tandem-DSCs
(Table 16.3).
Three major hurdles remain for viable p-DSC device, namely: (1) energy losses
within the device, (2) coloration and high recombination rates of semiconductor
material, and (3) poor light harvesting ability of the photocathode.
338 S. Peiris et al.

Table 16.3 Summary of photoelectrical performances corresponding to different components of

pDSCs
Modifications done Voc/(mV) Jsc/mA cm−2 FF η/%
for each component
Semiconductor Ni, Mg mixed metal 123 5.09 0.32 0.2
oxides (Grätzel 2007)
NiO prepared via 125 −2.84 0.34 0.121
RDS (Awais et al.
2014)
Delafossite 199.3 2.05 0.45 0.182
(CuGaO2 ) nano
plates (Xu et al.
2014)
Mg dopped CuGaO2 305 0.415 0.35 0.045
(Renaud et al. 2013a)
Delafossite CuAlO2 155 0.954
nanoparticles
(Ahmed et al. 2014)
Delafossite 734 1.23 0.48 0.53
(CuCrO2 )
nanoparticles (Powar
et al. 2014)
Delafossite 201 1.51 0.45 0.132
(CuCrO2 ) dopped
with Mg (Xiong et al.
2013)
Delafossite 150 0.039 0.26 0.002
(LaOCuS)
nanoparticles
(Renaud et al. 2015)
ITO (Yu et al. 2016) 712 5.65 0.49 1.96
Cu2 O (Du et al. 710 1.3 0.46 0.42
2014)
Dye Triphenylamine dyes 144 4.01 0.33 0.19
(Zhu et al. 2014)
Oligomer-acceptor 320 0.93 0.44 0.13
dyes (Sheibani et al.
2016)
Pyridine Ring 117.9 4.05 0.16 0.34
Anchoring Group
(Cui et al. 2014)
Fluorene-based light 112 4.36 0.38 0.184
absorbers (Liu et al.
2014b)
(continued)
16 p-Type Dye Sensitized Solar Cells … 339

Table 16.3 (continued)

Modifications done Voc/(mV) Jsc/mA cm−2 FF η/%
for each component
Fluorene-based dye 117 7.57 0.4 0.353
structures (Liu et al.
2014a)
Push-pull 104 2.25 0.34 0.079
rhutheniumm
diacetylidedyes (Lyu
et al. 2016)
Cyclometallated 93 3.04 0.35 0.099
ruthenium dye with
triphenylamino group
(Ji et al. 2013)
Panchromatic 93 3.43 0.33 0.104
cyclometallated
ruthenium dye (He
et al. 2014)
Cyclometallated 508 0.25 0.54 0.068
iridium dye (Gennari
et al. 2014)
Squaraines (Bonomo 93 −1.16 0.36 0.043
et al. 2016)
KuQuinones 92.2 0.74 0.35 0.023
(Bonomo et al.
2017b; Powar et al.
2013)
Porphyrine (Georgios 109 1.86 0.35 0.076
Coutsolelos et al.
2018)
Electrolyte Tris(1,2- 709 4.44 0.42 1.3
diaminoethane)Cobalt
(II)/(III) (Powar et al.
2013)
Tris(acetylacetonato)- 645 7.65 0.51 2.51
iron (III)/(II) (Perera
et al. 2015)
Disulfide/thiolate 304 1.73 0.44 0.023
electrolytes (Xu et al.
2013)
ssDSCs (Xu et al. 2019) 550 0.86 0.4 0.18
Counter-electrode CoS anode (Congiu 74 −1.051 0.33 0.026
et al. 2016)
CoS anode (Bonomo 128 −1.7 0.32 0.07
et al. 2017a)
NiCo2 S4 (Shi et al. 148.6 2.989 0.56 0.248
2014)
340 S. Peiris et al.

Energy losses associated with the redox process of I− /I3 − , which is the typically
used electrolyte, have been overcome by discovering/designing novel redox medi-
ators such as Co (II/II) and Fe (II/II) systems. These materials have reached high
VOC, even though the slow diffusion of ions is still a significant issue for most of
the studied mediators. The developed Fe (II/III) systems have seemingly overcome
this limitation by exhibiting diffusion rates near the theoretical limit, but the light
absorption profiles of these colored redox mediators compete for light with the sensi-
tizer, lowering the overall device performance. Hence, further development of redox
mediators with high diffusion rates and low light absorption remains a challenge in
p-DSC research. Fine-tuning existing coordination complexes has been identified as
one possible pathway to overcome this challenge.
The second major issue is replacing the widely used p-semiconductor material,
NiO. It has been discussed here, and elsewhere, that the coloration of NiO due to the
presence of Ni0 is a major issue, again due to competing for light absorption with the
sensitizer. Further, it has been shown that the electrochromic nature of NiO reduces
the light harvesting efficiency of the p-DSC near VOC . Moreover, the existence of
trap states and the low conductivity of bulk NiO both contribute to unfavourable
charge recombination reactions. However, NiO is still under investigation due to its
capability of generating high photocurrents and the low-lying VB edge (−5.1 eV vs.
vacuum) compared to other p-type semiconductors. Other p-type semiconductors
with a VB edge between −5 and −6 eV (vs. Vacuum) are yet to be discovered. Thus,
extensive research on alternative p-type semiconductor materials may be required to
outperform NiO.
Further, in order to realize high light harvesting efficiencies it is important to
develop sensitizers with broader absorption wavelength rage that extends from visible
to near-IR region of the electromagnetic spectrum. Even though a lot of research has
been done developing p-type sensitizers, the main focus has been the improvement of
charge separation but not the light harvesting efficiencies. Since the charge separation
challenge has been overcome with novel sensitizers, shifting focus to enhancing
light harvesting efficiencies, while maintaining proper charge separation, for p-type
sensitizers.
As elaborated throughout this chapter, remarkable development has occurred
in the field of DSCs since the introduction of high-performance n-DSCs in 1999.
Although n-DSCs are far ahead of p-DSCs in terms of efficiency at present, the
historical improvements of n-DSC performance bode well for the advancement of
p-DSCs too, particularly because there is no fundamental limitation to achieve high
efficiencies. Although contemporary p-DSCs are limiting the overall performance of
tandem-DSCs, the opportunities and avenues to upgrade these devices are clear and
promising.

Acknowledgements Authors would like to express their gratitude to Mr. Dushan Wijewardena for
the extended support.
16 p-Type Dye Sensitized Solar Cells … 341

References

Ahmed J, Blakely CK, Prakash J, Bruno SR, Yu M, Wu Y, Poltavets VV (2014) Scalable synthesis
of delafossite CuAlO2 nanoparticles for p-type dye-sensitized solar cells applications. J Alloy
Compd 591:275–279
Awais M, Dowling DD, Rahman M, Vos JG, Decker F, Dini D (2013) Spray-deposited NiO x
films on ITO substrates as photoactive electrodes for p-type dye-sensitized solar cells. J Appl
Electrochem 43:191–197
Awais M, Gibson E, Vos JG, Dowling DP, Hagfeldt A, Dini D (2014) Fabrication of efficient
NiO photocathodes prepared via RDS with novel routes of substrate processing for p-type dye-
sensitized solar cells. ChemElectroChem 1:384–391
Bach U, Lupo D, Comte P, Moser J, Weissörtel F, Salbeck J, Spreitzer H, Grätzel M (1998) Solid-state
dye-sensitized mesoporous TiO2 solar cells with high photon-to-electron conversion efficiencies.
Nature 395:583
Baxter JB (2012) Commercialization of dye sensitized solar cells: present status and future research
needs to improve efficiency, stability, and manufacturing. J Vacuum Sci Technol Vacuum Surf
Films 30:020801
Bessho T, Yoneda E, Yum J-H, Guglielmi M, Tavernelli I, Imai H, Rothlisberger U, Nazeeruddin
MK, Grätzel M (2009) New paradigm in molecular engineering of sensitizers for solar cell
applications. J Am Chem Soc 131:5930–5934
Bomben PG, Thériault KD, Berlinguette CP (2011) Strategies for optimizing the performance
of cyclometalated ruthenium sensitizers for dye-sensitized solar cells. Eur J Inorg Chem
2011:1806–1814
Bomben PG, Robson KC, Koivisto BD, Berlinguette CP (2012) Cyclometalated ruthenium chro-
mophores for the dye-sensitized solar cell. Coord Chem Rev 256:1438–1450
Bonomo M, Barbero N, Matteocci F, Carlo AD, Barolo C, Dini D (2016) Beneficial effect of
electron-withdrawing groups on the sensitizing action of squaraines for p-type dye-sensitized
solar cells. J Phys Chem C 120:16340–16353
Bonomo M, Congiu M, de Marco ML, Dowling DP, di Carlo A, Graeff CF, Dini D (2017a) Limits
on the use of cobalt sulfide as anode of p-type dye-sensitized solar cells. J Phys D Appl Phys
50:215501
Bonomo M, Sabuzi F, di Carlo A, Conte V, Dini D, Galloni P (2017b) KuQuinones as sensitizers
for NiO based p-type dye-sensitized solar cells. New J Chem 41:2769–2779
Chapin DM, Fuller C, Pearson G (1954) A new silicon p-n junction photocell for converting solar
radiation into electrical power. J Appl Phys 25:676–677
Chapin DM, Fuller CS, Pearson GL (1957) Solar energy converting apparatus. Google Patents
Chiba Y, Islam A, Watanabe Y, Komiya R, Koide N, Han L (2006). Dye-sensitized solar cells with
conversion efficiency of 11.1%. Jap J Appl Phys 45:L638
Congiu M, Bonomo M, Marco MLD, Dowling DP, di Carlo A, Dini D, Graeff CF (2016) Cobalt
sulfide as counter electrode in p-Type dye-sensitized solar cells. ChemistrySelect 1:2808–2815
Cui J, Lu J, Xu X, Cao K, Wang Z, Alemu G, Yuang H, Shen Y, Xu J, Cheng Y (2014) Organic
sensitizers with pyridine ring anchoring group for p-type dye-sensitized solar cells. J Phys Chem
C 118:16433–16440
Daniel U, Anamaria D, Sebarchievicia I, Miclau M (2017) Photovoltaic performance of Co-doped
CuCrO2 for p-type dye-sensitized solar cells application. Energy Procedia 112:497–503
Du S, Cheng P, Sun P, Wang B, Cai Y, Liu F, Zheng J, Lu G (2014) Highly efficiency p-type
dye sensitized solar cells based on polygonal star-morphology Cu2 O material of photocathodes.
Chem Res Chin Univ 30:661–665
Fraas LM (2014) History of solar cell development. Springer, Berlin
Gao J, Miao J, Li P-Z, Teng WY, Yang L, Zhao Y, Liu B, Zhang Q (2014) A p-type Ti (IV)-based
metal–organic framework with visible-light photo-response. Chem Commun 50:3786–3788
342 S. Peiris et al.

Gennari M, Légalité F, Zhang L, Pellegrin Y, Blart E, Fortage J, Brown AM, Deronzier A, Collomb
M-N, Boujtita M (2014) Long-lived charge separated state in NiO-based p-type dye-sensitized
solar cells with simple cyclometalated iridium complexes. J Phys Chem Lett 5:2254–2258
Georgios Coutsolelos A, Nikolaou V, Plass F, Planchat A, Charisiadis A, Charalambidis G, Angaridis
P, Kahnt A, Odobel F (2018) The effect of the triazole ring in hybrid electron donor-acceptor
systems towards light harvesting in NiO-based devices
Gong J, Sumathy K, Qiao Q, Zhou Z (2017) Review on dye-sensitized solar cells (DSSCs): advanced
techniques and research trends. Renew Sustain Energy Rev 68:234–246
Grätzel M (2007) Photovoltaic and photoelectrochemical conversion of solar energy. Philos. Trans.
R. Soc. A: Math Phys Eng Sci 365:993–1005
Grätzel M (2009) Recent advances in sensitized mesoscopic solar cells. Acc Chem Res
42:1788–1798
Hadsadee S, Rattanawan R, Tarsang R, Kungwan N, Jungsuttiwong S (2017) Push-Pull N-
annulated perylene-based sensitizers for dye-sensitized solar cells: theoretical property tuning
by DFT/TDDFT. ChemistrySelect 2:9829–9837
Hagfeldt A, Boschloo G, Sun L, Kloo L, Pettersson H (2010) Dye-sensitized solar cells. Chem Rev
110:6595–6663
He M, Ji Z, Huang Z, Wu Y (2014) Molecular orbital engineering of a panchromatic cyclometalated
Ru (II) dye for p-type dye-sensitized solar cells. J Phys Chem C 118:16518–16525
Ji Z, Natu G, Huang Z, Kokhan O, Zhang X, Wu Y (2012) Synthesis, photophysics, and photovoltaic
studies of ruthenium cyclometalated complexes as sensitizers for p-type NiO dye-sensitized solar
cells. J Phys Chem C 116:16854–16863
Ji Z, Natu G, Wu Y (2013) Cyclometalated ruthenium sensitizers bearing a triphenylamino group
for p-type NiO dye-sensitized solar cells. ACS Appl Mater Interfaces 5:8641–8648
Jiang J-Q, Sun C-L, Shi Z-F, Zhang H-L (2014) Squaraines as light-capturing materials in photo-
voltaic cells. RSC Adv 4:32987–32996
Jiang T, Bujoli-Doeuff M, Farré Y, Blart E, Pellegrin Y, Gautron E, Boujtita M, Cario L, Odobel F,
Jobic S (2016) Copper borate as a photocathode in p-type dye-sensitized solar cells. RSC Adv
6:1549–1553
Kakiage K, Aoyama Y, Yano T, Oya K, Fujisawa J-I, Hanaya M (2015) Highly-efficient dye-
sensitized solar cells with collaborative sensitization by silyl-anchor and carboxy-anchor dyes.
Chem Commun 51:15894–15897
Kato N, Moribe S, Shiozawa M, Suzuki R, Higuchi K, Suzuki A, Sreenivasu M, Tsuchimoto K,
Tatematsu K, Mizumoto K (2018) Improved conversion efficiency of 10% for solid-state dye-
sensitized solar cells utilizing P-type semiconducting CuI and multi-dye consisting of novel
porphyrin dimer and organic dyes. J Mater Chem A 6:22508–22512
Law KY, Bailey FC (1992) Squaraine chemistry. Synthesis, characterization, and optical properties
of a class of novel unsymmetrical squaraines:[4-(dimethylamino) phenyl](4’-methoxyphenyl)
squaraine and its derivatives. J Organic Chem 57:3278–3286
Lefebvre J-F, Sun X-Z, Calladine JA, George MW, Gibson EA (2014) Promoting charge-separation
in p-type dye-sensitized solar cells using bodipy. Chem Commun 50:5258–5260
Li B, Wang L, Kang B, Wang P, Qiu Y (2006) Review of recent progress in solid-state dye-sensitized
solar cells. Sol Energy Mater Sol Cells 90:549–573
Liu Z, Li W, Topa S, Xu X, Zeng X, Zhao Z, Wang M, Chen W, Wang F, Cheng Y-B (2014a) Fine
tuning of fluorene-based dye structures for high-efficiency p-type dye-sensitized solar cells. ACS
Appl Mater Interfaces 6:10614–10622
Liu Z, Xiong D, Xu X, Arooj Q, Wang H, Yin L, Li W, Wu H, Zhao Z, Chen W (2014b) Modulated
charge injection in p-type dye-sensitized solar cells using fluorene-based light absorbers. ACS
Appl Mater Interfaces 6:3448–3454
Lu J, Liu Z, Pai N, Jiang L, Bach U, Simonov AN, Cheng YB, Spiccia L (2018) Molecular
engineering of zinc-porphyrin sensitisers for p-type dye-sensitised solar Cells. ChemPlusChem
83:711–720
16 p-Type Dye Sensitized Solar Cells … 343

Lyu S, Farré Y, Ducasse L, Pellegrin Y, Toupance T, Olivier C, Odobel F (2016) Push–pull ruthenium
diacetylide complexes: new dyes for p-type dye-sensitized solar cells. RSC Adv. 6:19928–19936
Marinakis N, Willgert M, Constable EC, Housecroft CE (2017) Optimization of performance and
long-term stability of p-type dye-sensitized solar cells with a cycloruthenated dye through elec-
trolyte solvent tuning. Sustain Energy Fuels 1:626–635
Miclau M, Miclau N, Banica R, Ursu D (2017) Effect of polymorphism on photovoltaic performance
of CuAlO2 delafossite nanomaterials for p-type dye-sensitized solar cells application. Mater
Today 4:6975–6981
Nattestad A, Perera I, Spiccia L (2016) Developments in and prospects for photocathodic and
tandem dye-sensitized solar cells. J Photochem Photobiol, C 28:44–71
Novelli V, Awais M, Dowling DP, Dini D (2015) Electrochemical characterization of rapid dis-
charge sintering (RDS) NiO cathodes for dye-sensitized solar cells of p-type. Am. J. Anal. Chem
6:176–187
Odobel F, Pellegrin Y (2013) Recent advances in the sensitization of wide-band-gap nanostruc-
tured p-type semiconductors. Photovoltaic and photocatalytic applications. J Phys Chem Lett
4:2551–2564
O’regan B, Grätzel M (1991) A low-cost, high-efficiency solar cell based on dye-sensitized colloidal
TiO2 films. Nature 353:737
Outlook AE (2018) With projection to 2050, US Energy Information Administration (EIA), 6 Feb
2018
Perera IR, Daeneke T, Makuta S, Yu Z, Tachibana Y, Mishra A, Bäuerle P, Ohlin CA, Bach U,
Spiccia L (2015) Application of the tris (acetylacetonato) iron (III)/(II) Redox Couple in p-Type
Dye-Sensitized Solar Cells. Angew Chem Int Ed 54:3758–3762
Perera I, Hettiarachchi C, Ranatunga R (2019) Metal–organic frameworks in dye-sensitized solar
cells. In: Advances in solar energy research. Springer, Berlin
Powar S, Daeneke T, Ma MT, Fu D, Duffy NW, Götz G, Weidelener M, Mishra A, Bäuerle P, Spiccia
L (2013) Highly efficient p-Type dye-sensitized solar cells based on tris (1, 2-diaminoethane)
cobalt (II)/(III) Electrolytes. Angew Chem Int Ed 52:602–605
Powar S, Xiong D, Daeneke T, Ma MT, Gupta A, Lee G, Makuta S, Tachibana Y, Chen W, Spiccia L
(2014) Improved photovoltages for p-type dye-sensitized solar cells using CuCrO2 nanoparticles.
J Phys Chem C 118:16375–16379
Renaud AL, Cario L, Deniard P, Gautron E, Rocquefelte X, Pellegrin Y, Blart E, Odobel F, Jobic
S (2013a) Impact of Mg doping on performances of CuGaO2 based p-type dye-sensitized solar
cells. J Phys Chem C 118:54–59
Renaud AL, Chavillon B, Cario L, Pleux LCL, Szuwarski N, Pellegrin Y, Blart E, Gautron E,
Odobel F, Jobic SP (2013b) Origin of the black color of NiO used as photocathode in p-type
dye-sensitized solar cells. J Phys Chem C 117:22478–22483
Renaud A, Cario L, Pellegrin Y, Blart E, Boujtita M, Odobel F, Jobic S (2015) The first dye-sensitized
solar cell with p-type LaOCuS nanoparticles as a photocathode. RSC Adv 5:60148–60151
Sabuzi F, Armuzza V, Conte V, Floris B, Venanzi M, Galloni P, Gatto E (2016) KuQuinones: a new
class of quinoid compounds as photoactive species on ITO. J Mater Chem C 4:622–629
Shah A, Platz R, Keppner H (1995) Thin-film silicon solar cells: a review and selected trends. Sol
Energy Mater Sol Cells 38:501–520
Sheibani E, Zhang L, Liu P, Xu B, Mijangos E, Boschloo G, Hagfeldt A, Hammarström L, Kloo L,
Tian H (2016) A study of oligothiophene–acceptor dyes in p-type dye-sensitized solar cells. Rsc
Adv 6:18165–18177
Shi Z, Lu H, Liu Q, Cao F, Guo J, Deng K, Li L (2014) Efficient p-type dye-sensitized solar cells
with all-nano-electrodes: NiCo2 S4 mesoporous nanosheet counter electrodes directly converted
from NiCo2 O4 photocathodes. Nanoscale Res Lett 9:608
Tian H (2019) Solid-state p-type dye-sensitized solar cells: progress, potential applications and
challenges. Sustain Energy Fuels
344 S. Peiris et al.

Tian L, Föhlinger J, Pati PB, Zhang Z, Lin J, Yang W, Johansson M, Kubart T, Sun J, Boschloo G
(2017) Ultrafast dye regeneration in a core–shell NiO–dye–TiO2 mesoporous film. Phys Chem
Chem Phys 20:36–40
Ursu D, Dabici A, Vajda M, Bublea N-C, Duteanu N, Miclau M (2018) Effect of Cu2 O morphology
on photovoltaic performance of p-type dye-sensitized solar cells. Ann West Univ Timisoara-Phys
60:67–74
Venkatraman V, Raju R, Oikonomopoulos SP, Alsberg BK (2018) The dye-sensitized solar cell
database. J Cheminform 10:18
Wu H, Zhang T, Wu C, Guan W, Yan L, Su Z (2016) A theoretical design and investigation on
Zn-porphyrin-polyoxometalate hybrids with different π-linkers for searching high performance
sensitizers of p-type dye-sensitized solar cells. Dyes Pigm 130:168–175
Xiong D, Zhang W, Zeng X, Xu Z, Chen W, Cui J, Wang M, Sun L, Cheng YB (2013) Enhanced per-
formance of p-type dye-sensitized solar cells based on ultrasmall Mg-doped CuCrO2 nanocrystals.
Chemsuschem 6:1432–1437
Xu X, Zhang B, Cui J, Xiong D, Shen Y, Chen W, Sun L, Cheng Y, Wang M (2013) Efficient p-type
dye-sensitized solar cells based on disulfide/thiolate electrolytes. Nanoscale 5:7963–7969
Xu Z, Xiong D, Wang H, Zhang W, Zeng X, Ming L, Chen W, Xu X, Cui J, Wang M (2014)
Remarkable photocurrent of p-type dye-sensitized solar cell achieved by size controlled CuGaO
2 nanoplates. J Mater Chem A 2:2968–2976
Xu B, Wrede S, Curtze A, Tian L, Pati P, Kloo L, Wu Y, Tian H (2019) An indacenodithieno [3, 2-b]
thiophene based organic dye for solid-state p-Type dye-sensitized solar cells. ChemSusChem
Yu M, Natu G, Ji Z, Wu Y (2012) p-type dye-sensitized solar cells based on delafossite CuGaO2
nanoplates with saturation photovoltages exceeding 460 mV. J Phys Chem Lett 3:1074–1078
Yu Z, Perera IR, Daeneke T, Makuta S, Tachibana Y, Jasieniak JJ, Mishra A, Bäuerle P, Spiccia L,
Bach U (2016) Indium tin oxide as a semiconductor material in efficient p-type dye-sensitized
solar cells. NPG Asia Mater 8:e305
Yum J-H, Baranoff E, Wenger S, Nazeeruddin MK, Grätzel M (2011) Panchromatic engineering
for dye-sensitized solar cells. Energy Environ Sci 4:842–857
Zannotti M, Wood CJ, Summers GH, Stevens LA, Hall MR, Snape CE, Giovanetti R, Gibson
EA (2015) Ni Mg mixed metal oxides for p-type dye-sensitized solar cells. ACS Appl Mater
Interfaces 7:24556–24565
Zhang L, Boschloo G, Hammarström L, Tian H (2016) Solid state p-type dye-sensitized solar cells:
concept, experiment and mechanism. Phys Chem Chem Phys 18:5080–5085
Zhu L, Yang HB, Zhong C, Li CM (2014) Rational design of triphenylamine dyes for highly efficient
p-type dye sensitized solar cells. Dyes Pigm 105:97–104
Chapter 17
Conducting Polymers as Cost Effective
Counter Electrode Material
in Dye-Sensitized Solar Cells

Shanal Shalindra Bandara Gunasekera, Ishanie Rangeeka Perera

and Samodha Subhashini Gunathilaka

Abstract Dye-sensitized solar cells (DSCs) are third generation photovoltaic

devices capable of harvesting solar energy to generate electricity. DSCs have gained
significant research interest during past decades due to its high theoretical power
conversion efficiencies and most importantly the cost effectiveness and environment
friendly fabrication process. Firstly, in this chapter, the function of the counter elec-
trode (CE) in a DSC has been discussed in brief. The CE participates in the electron
transfer from the external circuit back to the redox mediator thereby catalyzing its
regeneration reaction. In the state of art DSCs, Pt has been the preferred CE material.
Properties such as promising conductivity and high electrocatalytic activity towards
the process of reduction of I− −
3 to I which is the typical redox mediator, contributes to
its applicability as the preferred CE material. However, the use of Pt CE adhere major
drawbacks such as the high cost and its susceptibility to undergo corrosion. These
limitations have emphasized the importance of exploring alternative cost effective
functional materials with better conductivity and electrocatalytic properties. Sub-
sequently, the limitations of using Pt as the CE materials, and the advantages and
challenges associated with alternative materials have been elaborated. Conducting
polymers with extended conjugate electron systems are a promising substitute mate-
rial for Pt in DSCs. A wide range of conducting polymers and polymer hybrid com-
posites have been investigated for their applicability as CEs in DSCs. These polymers
have gained popularity not just due to cost effectiveness compared to Pt but also due
to their promising conductivity, superior electrocatalytic properties, easy preparation
and fabrication. Different types of conjugated polymers and polymer hybrid compos-
ites, their synthetic methods, fabrication processes and their respective photovoltaic
performances are then reviewed. Finally, the future prospects of conducting polymers
as CE material has been discussed.

S. S. B. Gunasekera · I. R. Perera · S. S. Gunathilaka (B)

Department of Chemistry, Faculty of Science, University of
Peradeniya, Peradeniya 20400, Sri Lanka
e-mail: subhashinig@pdn.ac.lk
S. S. B. Gunasekera
e-mail: shanalgunasekera@gmail.com
I. R. Perera
e-mail: ishanieperera@pdn.ac.lk

© Springer Nature Singapore Pte Ltd. 2020 345

H. Tyagi et al. (eds.), Solar Energy, Energy, Environment,
and Sustainability, https://doi.org/10.1007/978-981-15-0675-8_17
346 S. S. B. Gunasekera et al.

Keywords CE · Platinum · Conducting polymers · Dye-sensitized solar cells

17.1 Introduction

The demand for energy is increasing rapidly with the growth of population and
industrialization. According to International Energy Outlook 2016 by U.S. Energy
Information Administration, the world energy demand is expected to be increased
by 48% from 2012–2040 for a period of 28 years (DOE/EIA-0484 (2016)). The
world energy production mainly depends on primary energy sources including oil,
coal followed by natural gases. The statistics indicate that, oil contributes around
33% to the total energy demand being the world’s leading fuel. The global demand
for oil in 2014 was 92.4% which is projected to be increased up to 99.1% by 2020
(World Energy Resources 2016, World Energy Council). The second primary energy
source in terms of consumption is coal and the demand for coal to produce energy
has increased by 64% from 2000 to 2014 (World Energy Resources 2016, World
Energy Council). The third primary energy source; natural gasses contributes 24%
to the global energy demand (World Energy Resources 2016, World Energy Coun-
cil). Although the demand for the primary energy sources shows a dramatic increase,
these petroleum based sources are depleting at an alarming rate. Moreover, com-
bustion of such fuels has led to increase in harmful greenhouse gases emitted into
the atmosphere. These adverse effects have led the scientists to focus research on
harvesting energy from renewable energy sources such as wind, tide, geothermal and
solar energy (World-nuclear.org. (2019), Renewables 2014-Global Status report).
Among many renewable energy sources, the use of solar power has been expo-
nentially increased over the past decades as it is the most abundant, constant and least
polluting energy source which can be harvested (Lee 2019). The amount of energy
falling on the surface of the earth from the sun is about 3 × 1024 J year−1 or 104
times the consumption of the world. Covering 0.1% of the total area of the earth by
using 10% efficient solar cells (SCs) would be adequate to meet up the global energy
demand (Grätzel 2005).
Silicon based SCs are the most popular SCs in the market which give efficiencies
up to 15–17% (Lee et al. 2014). Major problem with SCs are their high manufacturing
cost, because it requires highly pure silicon (99.9999% of purity) making the SC
expensive (Braga et al. 2008). Promising solution for this problem was given by
Grätzel (2003) with the breakthrough on dye-sensitized solar cell (DSC) with many
desirable properties such as higher flexibility for the choice of starting materials,
higher transparency and less environmental pollution in addition to being low-cost
(O’Regan and Grätzel 1991; Grätzel 2003).
17 Conducting Polymers as Cost Effective Counter Electrode … 347

17.1.1 Operational Principle of Dye-Sensitized Solar Cells

A dye-sensitized solar cell (DSC) typically consists of three main components; the
dye sensitized semiconductor oxide photoanode/working electrode (WE), redox elec-
trolyte and a counter electrode (CE)/cathode (Grätzel 2001; Hagfelt and Grätzel
2000). The photoanode is a transparent electrode made out of transparent conductive
oxide (TCO) coated on a glass substrate, generally indium doped tin oxide (ITO)
(Tahar et al. 1998) or fluorine doped tin oxide (FTO) (Yang 2009) are used that
facilitate electrical conductivity and light transmittance. A mesoporous semiconduc-
tor layer (typically TiO2 ) is deposited on the TCO in order to enhance and activate
electronic conduction. Additionally the surface of the mesoporous oxide layer is
covalently bonded to a monolayer charge transfer dye which contributes to enhance
light absorption and eventually generate charge carriers. The porous nature and the
morphology of the TiO2 layer is selected such that it assures a greater degree of
absorption of the dye at the surface thus accommodating a greater area of reaction
sites (Gong et al. 2012).
Figure 17.1 depicts the function of a typical DSC. Upon illumination with sunlight
the sensitizer molecules harvest incoming photons and undergo excitation from the
ground state to the excited state. Subsequently the excited electron will be injected
into the conduction band of the TiO2 semiconductor material, which leads to the
formation of an oxidized sensitizer. These electrons generated will diffuse to the
anode and will be eventually migrated along an external circuit to the CE. The CE
is made out of a TCO glass substrate which is coated with a catalyst, most com-
monly Pt (Thomas et al. 2014; Maiaugree et al. 2015). This platinised CE facilitates
smooth electron flow and catalyse the redox reaction in the electrolyte. Pt is used

Fig. 17.1 Schematic diagram of typical DSC and the electron transfer process involved in energy
conversion
348 S. S. B. Gunasekera et al.

as a preferred CE material due to its favourable characteristics such as high cat-

alytic activity and good electrical conductivity (Yun et al. 2015). A liquid electrolyte
is present between the WE and the CE. This electrolyte contains a redox media-
tor and typically iodide/triiodide redox couple is used in liquid electrolyte (Chiba
et al. 2006) whereas organic hole transport materials such as spiro-OMeTAD (Bach
et al. 1998) and polymer materials such as polyethyleneglycol, polyacrylonitrile and
poly(vinylidine) fluoride-co-hexauoropropylene (Upadhyaya et al. 2006; Cao et al.
1995; Wang et al. 2003) are the common mediators in solid state and quasi-solid state
electrolytes respectively. The function of the electrolyte is the regeneration of the
oxidized dye via electrochemical redox reaction thus completing the cycle. The dye
molecules subjected to oxidation are been reduced by the reduced species migrating
through the electrolyte medium and meanwhile the redox couple regeneration occurs
by means of reduction at the CE (Hagfelt and Grätzel 2000).
The Eqs. (17.1)–(17.6) summarise the operational principle of a typical DSCs
fabricated with I− /I−
3 redox couple (Nogueira et al. 2004)

TiO2 |S + hν → TiO2 |S∗ (17.1)

TiO2 |S∗ → TiO2 |S+ + e−

CB (17.2)

TiO2 |S+ + e−
CB → TiO2 |S (17.3)

TiO2 |S+ + 3/2 I− → TiO2 |S + 1/2 I−

3 (17.4)

1/2 I− −
3 + e(Pt) → 3/2 I
−
(17.5)

I− −
3 + 2eCB → 3I
−
(17.6)

The substrate of the photo electrode plays a major role in the WE functioning;
acting as a charge collector and providing support for the semiconductor layer. They
need to possess high optical transparency such that natural sunlight is allowed to
pass down to the semiconductor material without unwanted absorption and high
electrical conductance to facilitate the electron transfer process (Gong et al. 2012).
ITO and FTO are commonly used as substrate materials (Sima et al. 2010). There are
drawbacks associated with their usage due to material’s brittle nature, high cost and
limited availability of indium, lack of sufficient transparency and the fact that they are
temperature and pH sensitive (Chen et al. 2001). This has led to the investigation of
novel material such as graphene films as substitute materials for the use as a substrate
(Zhu et al. 2009).
The semiconductor material at the WE which is under extensive investigation is
TiO2 (Park et al. 2000). In addition to this ZnO (Law et al. 2005), SnO2 (Fonstad and
Rediker 1971) and chalcogenides (Gong et al. 2012) are also commonly used. These
semiconductors have a characteristic conductive electronic structure consisting of
17 Conducting Polymers as Cost Effective Counter Electrode … 349

valence band and conduction band with a band gap having energy in the range of UV
region in the electromagnetic spectrum, hence a photon with energy that matches the
band gap can cause the excitation of an electron leaving behind a hole in the valence
band. These charges are been migrated to the external circuit to provide electrical
current. TiO2 is the semiconductor material most commonly used and it is been used
either in the form of rutile or anatase or a mixed composition of both these natural
forms. The morphology and network geometry has a significant impact on the effi-
ciency and photoelectrochemical performance of DSCs. A multilayered structure of
mesoporous network of TiO2 has yielded high cell efficiency (Wang et al. 2004).
Highly ordered nano structure of TiO2 such as network structure of TiO2 crystal like
nanowires (Adachi et al. 2004), structure consisting of submicrometer-sized meso-
porous TiO2 beads (Chen et al. 2009) and hollow TiO2 hemisphere films deposited
on substrate material (Yang et al. 2008) have been developed with high photovoltaic
performances. In addition plasma enhanced polymerized aniline/TiO2 DSCs with
favourable photovoltaic performances have been developed and investigated (Ameen
et al. 2009).
There are specific criteria to be satisfied by the dye sensitizer material. Most
importantly the LUMO level of the sensitizer material must be closer (about 150 mV)
in energy and must match the conduction band edge of the semiconductor in order
to decrease energetic potential losses associated with the transfer of electrons. The
HOMO level of the sensitizer material should be at a sufficient low level to enable
acceptance of electrons from an electrolyte or hole conducting material. Additionally
it needs to absorb all incident light below the near IR wavelength. The sensitizer
material should be composed of carboxylate/phosphate group for anchorage onto the
semiconductor oxide surface. Furthermore the sensitizer material needs to possess
the endurance to withstand multiple turnovers and prolonged exposure to sunlight
without degradation (Grätzel 2001). Initially ruthenium(II) based dyes such as the
N3 dye (Nazeeruddin et al. 1993) and the black dye (Nazeeruddin et al. 1997) had
been commonly used as the sensitizer material with considerable efficiency but their
usage had been restricted due to the high cost and limited availability of Ru metal.
Afterwards metal free organic dyes had been used (Mishra et al. 2009) and currently
the use of natural dyes (Zhou et al. 2011) with structure modification is investigated as
a substitute material for the sensitizer material with considerable efficiencies (Gong
et al. 2012).
The electrolyte which is responsible for the electron collection at the CE and elec-
tron transportation back to the oxidized dye molecule needs to satisfy specific criteria
such as rapid electron transfer kinetics, sufficient driving force to regenerate the dye,
low light absorption and ability to regenerate at minimum overpotential at the CE.
The most commonly used electrolyte is the iodide/triiodide (Boschloo et al. 2009)
redox couple in an organic matrix usually acetonitrile. Alternative redox couples
such a Br− /Br− − − −
3 , SCN /(SCN)2 , SeCN /(SeCN)3 , [Fe(CN)6 ]
3−/4−
, Co(II)/Co(III)
0/1+
(Daeneke et al. 2012), [Mn(acac)3 ] (Perera et al. 2014) and disulfide/thiolate
are used (Wang et al. 2010). Although liquid electrolytes as mentioned above offer
favourable kinetics there are challenges in its usage in terms of long term durability
350 S. S. B. Gunasekera et al.

and operational stability due to leakage of toxic organic solvent and increase in inter-
nal resistance due to evaporation of volatile components such as iodine. Hence as
an alternative the use of ionic liquid, quasi-solid state and solid state electrolytes
have been investigated. The use of room temperature ionic liquids such as 1,3-
dialkylimidazolium, 1,2,3-trialkylimidazolium and N-alkylpyridinium (Gorlov et al.
2007) are investigated and it had been possible to achieve significant efficiency with
ionic electrolytes of eutectic salts (Bai et al. 2008).
The CE is responsible for the electron transfer from the external circuit back to the
redox mediator thus completing the circuit. A CE material needs to possess excel-
lent catalytic activity for redox electrolyte reduction, low resistance, high chemical
stability and low production cost (Rahman et al. 2016). A typical CE is made out
of a transparent conductive oxide glass on which a catalyst is fabricated (Thomas
et al. 2014). The applicability of a wide variety of CE materials in DSCs has been
extensively investigated in the recent years, this is been discussed in detail under
Sect. 17.1.2 as this is the main focus of the chapter.
The preliminary characterization method for a DSC is current density-voltage
(J-V) curves. The overall performance of a DSC can be evaluated in terms of cell
efficiency (ï) and fill factor (FF) that can be derived from a typical J-V curve.
ï is described as the quantum yield of photogenerated electron for the incident
photonflux. The FF is another parameter that has an influence on the cell efficiency
which relies on internal resistance and electron transfer. In addition to these two
parameters the short circuit current density (J sc ) and open-circuit voltage V oc ) can
also be derived from J-V curves (Gong et al. 2012).
The main focus of this chapter is the applicability of conducting polymers as
promising substitute material for Pt CE in DSCs with a brief introduction on DSCs
and their operational principle. The advantages and the drawbacks associated with the
usage of the typical Pt CE is discussed followed by a description of other promising
candidates as a substitute for the Pt CE. Then the use of conducting polymers as a CE
material is reviewed focussing on three of the most promising conducting polymers
and polymer hybrid composites. These conducting polymers and polymer hybrid
composites are discussed in detail focussing on their synthetic methods, fabrication
processes and their respective photovoltaic performances.

17.1.2 CE Materials

Pt has been utilized as the preferred CE material with regard to its high conductivity,
high electrocatalytic activity towards reduction of I− −
3 to I which is the typical redox
mediator in DSCs and high light to electric energy conversion efficiency. A DSC
with an exceptionally high efficiency of 12.3% was reported with the use of thermally
decomposed platinum (Yella et al. 2011). The applicability of a variety of Pt materials
as CE in DSCs has been investigated. Pt nano particles synthesized by means of
decomposition of chloroplatinic acid thermally and electrochemical reduction of
hexachloro platinic acid (Fu et al. 2012) was found to be a promising CE material
17 Conducting Polymers as Cost Effective Counter Electrode … 351

due to favourable characteristics such as low charge transfer resistance, promising

electrical conductivity, high surface area and corrosion resistance. The applicability
of Pt composite materials as CE in DSCs has also been studied (Xu et al. 2008; Yen
et al. 2012). Although platinum based CEs yields high photovoltaic performances,
platinum is an expensive noble metal and its usage in DSCs makes it one of the most
expensive components in a DSC. In addition to its expensiveness, Pt is corrosive when
used with electrolytes consisting of the redox mediator I− −
3 /I . It has been discovered
that Pt decomposes into PtI4 in the presence of redox mediator I− −
3 /I which would
result in an adverse impact on the long term stability of the CE (Xiao et al. 2014).
The high cost and corrosion susceptibility has led to the exploration of substi-
tute materials in place of Pt as a CE material. Hence carbon based material such as
graphene (Xu et al. 2014a, b), conducting polymer-graphene nanocomposites (Huang
et al. 2011), metal organic frameworks (MOFs) (Sun et al. 2016), conducting poly-
mers and polymer hybrid composites (Rahman et al. 2016) have been investigated as
alternative CE materials and they have shown favourable photovoltaic performances.
Carbon is used as a promising counter electrode material due to favourable prop-
erties such as high conductivity, appreciable catalytic activity towards many redox
mediators, corrosion resistivity and cost effectiveness. The use of carbon based mate-
rials such as carbon black, graphene, mesoporous carbon, carbon nanotubes and
graphene based composite materials; as CE material has been explored. The use of
carbon black as a CE material was investigated and it yielded an efficiency around 9%
(Murakami et al. 2006). The highest efficiency achieved with the use of a graphene
CE was reported by Kakiage et al., in which an efficiency of 14.7% was yielded with
the use of a graphene/gold CE (Kakiage et al. 2015).
The applicability of MOFs as CE materials has been explored. The use of CoS2
embedded within carbon nanocages synthesized using ZIF-67 template achieved an
efficiency of 8.2% (Cui et al. 2016). An outstanding efficiency of 8.69% was yielded
by the use of CE material ZnSe embedded N-doped carbon cubes synthesized using
MOFs with Zn nodes and benzimidazole linkers (Jiang et al. 2018).

17.2 Conducting Polymers as CE Material

Conducting polymers are organic polymers that possess an orbital system that assures
the mobility of charge carriers. These polymers show extensive conjugation with
overlapping π orbitals along the polymer backbone of sp2 hybridized carbons. As
a result of the delocalization of the π electrons these polymers reveal appreciable
conductivity that ranges from 10−8 to 105 S/cm (Rahman et al. 2016). Conducting
polymers are considered as favourable candidates for CE materials in DSCs due to
characteristics such as efficient catalytic activity, promising conductivity and superior
stability. Moreover the usage of conducting polymers is further favoured due to high
surface area associated with the porous nature of the material thus enhancing catalytic
activity and their facile cost effective synthesis. Various conducting polymers such
as polypyrrole, polyaniline, polythiophene, poly(3,4-ethylenedioxythiphene) and
352 S. S. B. Gunasekera et al.

poly(styrene sulfonate) doped poly(3,4-ethylenedioxythiophene) have been inves-

tigated as cost effective Pt-free CEs (Thomas et al. 2014; Hou et al. 2012; Tang
et al. 2013). They have revealed promising electrocatalytic activities and low charge
transfer resistance. However conducting polymers associates a major challenge in
their applicability due to been insoluble and mechanically weak. Due to this reason
nanomaterials are been added such as carbon nanotubes, boron nanotubes, graphene,
fullerene in order to improve and enhance their mechanical, electrical properties (Lee
et al. 2008). The use of nanographite/polyaniline composites as CE in DSCs has
yielded an efficiency of 7.07% (Huang et al. 2011). The favourable properties and
appreciable efficiencies of conducting polymer based CEs have inspired extensive
investigation of them as promising CE materials in Pt free DSCs.

17.2.1 Polyaniline (PANI) as a CE in DSCs

PANI is a conducting polymer consisting of a reactive NH group with two phenyl

rings on either sides of it. The reactive NH group can undergo protonation and
deprotonation. Hence conductivity of PANI depends not only on the oxidation state
but also the degree of protonation and the dopant type. Figure 17.2 summarises the
different forms of PANI that exist with different oxidation states. PANI is a promising
material as a CE due to its favourable characteristics such as low cost, high electro
chemical activity and high thermal and chemical stability (Saranya et al. 2015). PANI

Fig. 17.2 Oxidative forms of PANI

17 Conducting Polymers as Cost Effective Counter Electrode … 353

Fig. 17.3 Doping of PANI with HClO4

can be synthesized using two distinct methods, chemical polymerization method and
electropolymerization method.
Microporous polyaniline synthesized using oxidative chemical polymerization
has been used as a substitute CE material with a size diameter of 100 nm; and a
higher overall energy conversion efficiency had been achieved (Li et al. 2008). Here
PANI had been synthesized by means of aqueous oxidative polymerization reaction
with ammonium persulfate using perchloric acid as the dopant (Fig. 17.3).
PANI nanoparticles synthesized using this methodology had been coated on a
conducting FTO glass by dip tugging method thus preparing a PANI CE. The PANI
nanoparticles with 100 nm size diameter and microporosity contributes to enhance
the surface area of the PANI CE thus improving the catalytic activity and trapping
liquid electrolytes in microporomerics in DSCs.
Cyclic voltagrams provide evidence for the fact that this PANI CE has a reduced
charge transfer resistance and high electricatalytic activity towards I− −
3 /I redox cou-
ple. Furthermore PANI CE had achieved a 7.15% overall energy conversion efficiency
which is greater with respect to the usage of Pt CE under same conditions (Li et al.
2008).
The electropolymerization method of synthesizing PANI is considerably simpler
and cost effective with respect to chemical polymerization. This method enables
the preparation of CE material with controllable surface morphology and it assures
effective adhesion to the substrate surface. Several techniques of electropolymer-
ization are used in the synthesis of PANI CE material such as cyclic voltammetry,
constant potential, constant current, pulse current, and pulse potentiostatic methods
(Xiao et al. 2013).
PANI CEs prepared using constant potential method is the conventional synthetic
methodology. PANI CE prepared using the synthetic methodology of pulse poten-
tiostatic polymerization has revealed high electrochemical stability and excellent
electrocatalytic activity for the I− −
3 /I redox reaction with respect to conventional
PANI CEs prepared using constant potential synthetic methodology. PANI CEs pre-
pared using pulse potentiostatic polymerization results in a higher cathodic peak
354 S. S. B. Gunasekera et al.

current revealed from CV tests and lower charge transfer resistance. These conduct-
ing polymers have a rough surface morphology and due to this high surface area the
electrocatalytic activity is been enhanced. Although the power conversion efficiency
is not as high as Pt CE (up to 90% PCE of DSCs using Pt CE) in this case of con-
ducting PANI polymers, it is a preferred CE material due to its cost effectiveness and
facile synthesis methodology (Xiao et al. 2013).
The two step cyclic voltammetry approach for the synthesis of PANI CE material
has also been able to achieve substantial power conversion efficiency. This method
enables the preparation of well controlled and short branched PANI nanofibres with
high performance. It involves a pre electropolymerization step in which a larger
potential range (0–1.3 V) is applied for few cycles (typically one) followed by PANI
electropolymerization subjected to a smaller potential range (0–0.9 V) for a larger
number of cycles (10 cycles). It reveals high electrocatalytic activity towards I− 3 /I
−

redox couple due to high cathodic peak current and lower charge transfer resistance
as evident from CV tests. This is also due to the enhanced surface area attributed
to PANI nanofibres due to short branched surface morphology. The conversion effi-
ciency of these two step PANI CEs can be enhanced to achieve up to 97% of the
power conversion efficiency observed when platinised CEs are been used. But their
applicability is favoured due to cost effective nature (Xiao et al. 2014).
The surface morphology of electropolymerized PANI has a significant impact
on the electrocatalytic activity and charge transfer resistance in DSCs. This surface
morphology of electropolymerized PANI CEs varies with the usage of different
− − −
dopant anions such as SO2− 4 , ClO4 , BF4 , Cl and p-toluene sulfonate (Rahman
et al. 2016).
The use of H2 SO4 -doped dense PANI nanoparticles CE has enabled a higher power
conversion efficiency of 7.30% in DSCs. The long conjugation structure, high level
of doping and extensive electron delocalization contributes for favourable electrical
conductance and enhanced electrocatalytic activity towards I− −
3 /I redox species thus
owing to an overall higher power conversion efficiency (Rahman et al. 2016).
The electrocatalytic activity and power conversion efficiency of sulfamic acid-
doped PANI nanofibres have been enhanced due to the fact that sulfamic acid con-
tributes to create voids in the fibrous network of PANI nanofibres. This doping allows
a 27% improvement in the photon-conversion efficiency (Ameen et al. 2010).
The usage of PANI nano wire arrays as a CE material enables a very high power
conversion efficiency of 8.24%. The oriented PANI nano wire arrays are prepared
by means of in situ polymerization owing to a greater surface area and hence a
high catalytic activity towards the redox mediator. It had been found that a greater
electrocatalytic activity is recorded towards Co2+ /Co3+ redox mediator. The usage
of PANI nano wire arrays also associates the advantage of been able to be grown on
flexible substrate material (Wang et al. 2013).
Furthermore transparent PANI CEs have been used for the fabrication of novel
bifacially active DSCs that can be illuminated from both sides. Such bifacially active
DSCs prepared using transparent PANI CEs enables high power generating efficiency
due to the ability of utilizing light from both sides. Bifacial DSCs require transparency
in the CEs; hence the use of a Pt CE is unfavoured due to its high reflectance of
17 Conducting Polymers as Cost Effective Counter Electrode … 355

Fig. 17.4 Schematic diagram of a Bifacial DSC with transparent PANI CE. Adapted from Tai et al.
(2011)

light. Carbon material based CEs cannot be used as it requires them to possess
substantial thickness to achieve desirable conductivity and electrocatalytic activity
which diminishes the transparency of the CE. However there are methods of preparing
high quality transparent PANI films that can be substituted as CEs in bifacial DSCc
such as in situ polymerization of aniline on FTO glass to produce novel transparent
PANI CE. In addition the emaralidine PANI possess complementary absorption in
the visible region of the electromagnetic spectrum with the N3 dye and this is crucial
for the high performance of the Bifacial DSCs. CV tests provide evidence to show
that PANI CEs in bifacial DSCs yields a greater peak current density than Pt CEs,
hence PANI is the more efficient CE in bifacial DSCs (Tai et al. 2011) (Fig. 17.4).
The use of PANI CE has enabled the yield of a power conversion efficiency of
6.54% which is closer to that of a DSC with a Pt CE which is 6.69% (Table 17.1).
But in the bifacial DSCs in addition, the rear illumination yields a power conversion
efficiency of 4.26%. This distinct feature of bifacial DSCs been able to utilize light
from both front illumination and rear illumination encourages its applicability in
building integrated photovoltaics such as power generating windows (Tai et al. 2011).
Table 17.2 summarises the photovoltaic parameters of DSCs with Pt free PANI
CEs having outstanding efficiencies. The number of active sites available for the
redox reaction can be improved by increasing the thickness. It is possible to enhance
the electrochemical catalytic properties of PANI CE by the use of special morpholo-
gies which can be attained using electrochemical polymerization method. The main
challenge associated with the use of PANI CE is the difficulty of coating a uniform

Table 17.1 Electrical impedance spectroscopy (EIS) and J-V parameters of DSCs with Pt and
PANI CEs
Electrodes V oc (mV) J sc (mA/cm2 ) FF PCE (%)
FTO/Pt 706 14.75 0.643 6.69
FTO/PANI 710 15.24 0.604 6.54
356 S. S. B. Gunasekera et al.

Table 17.2 Photovoltaic performances of different PANI CEs

Type of PANI Preparation FF PCE (%) Refs.
methodology
Microporous PANI Oxidative 0.69 7.15 Li et al. (2008)
polymerization
Transparent PANI Chemical deposition 0.65 8.35 Wu et al. (2014)
H2 SO4 doped PANI Chemical deposition 0.59 7.30 Xu et al. (2014a, b)
PANI film Technique employing 0.67 8.80 Chiang et al. (2013)
Hexafluoro-isopropanol
(HFIP) solution

film with appreciable conductivity and surface area. Therefore when considering the
use of dopants in efficiency enhancement, both its ability to enhance conductivity and
act as a pore former increasing surface area; needs to be taken into account (Saranya
et al. 2015).

17.2.2 Polypyrrole (PPy) as a CE in DSCs

PPy is another substitute material that can be used in place of Pt as the CE. Besides its
good mechanical, chemical stability and high conductivity, PPy shows good catalytic
behaviour for I−3 reduction when used as the CE but have slightly lower fill factor
and lower power conversion efficiency with regard to Pt CE (Rahman et al. 2016;
MacDiarmid 1997). However by means of controlling the morphology of the PPy
CE it is possible to improve the power conversion efficiency. The use of PPy as
a CE material is also favoured due to simple preparation procedure and low cost
(Fig. 17.5).
PPy nano particles synthesized using chemical oxidative method and coated on
conducting FTO glass reveals higher electrocatalytic activity and greater power con-
version efficiency in comparison to Pt CEs. PPy had been easily synthesized by

Fig. 17.5 Structure of polypyrrole

17 Conducting Polymers as Cost Effective Counter Electrode … 357

Table 17.3 Photoelectric properties of DSCs with PPy and Pt CEs

Electrode J sc (mA/cm2 ) FF PCE (%)
PPy 15.01 0.69 7.66
Pt 14.47 0.68 6.90

means of chemical oxidative polymerization in the presence of iodine and these PPy
nanoparticles (40–60 nm) were coated on FTO glass to prepare the CE (Wu et al.
2008).
Condition: Liquid electrolyte contains 0.1 M KI, 0.01 M I2 and 0.6 M tetrabutyl
ammonium iodide and acetonitrile.
The CV tests reveal that the use of PPy CEs results in smaller charge transfer
resistance and higher electrocatalytic activity towards I− −
3 /I redox reaction than the
Pt CE. The overall power conversion efficiency when using PPy CE is 7.66% which
is 11% greater than when using Pt CE provided the same conditions (Table 17.3).
The improved photovoltaic performances can be attributed to its high surface area,
low charge transfer resistance and high electrocatalytic activity. In addition to the
excellent photoelectric properties of PPy, its simple synthesis, fabrication and cost
effectiveness also contributes to its credibility as a substitute for Pt CEs (Wu et al.
2008).
The use of PPy synthesized using vapour phase polymerization, as CE material
in DSCs has been investigated. PPy synthesized using vapour phase polymerization
(VPP) shows favourable catalytic behaviour, but associates a slightly lower fill factor
and a lower power conversion efficiency compared to Pt CE material. The lower
power conversion efficiency can be attributed to insufficient contact between the
polymer films and FTO glass and also due to low oxidant concentrations. Therefore
it is possible to improve photovoltaic performances by manipulating the morphology
of PPy and its adhesion to the FTO substrate and the oxidant concentration (Rahman
et al. 2016). PPy was synthesized by exposing FTO glass spin coated with iron (iii)
p-toluenesulfonate (Fe-TsO) to pyrrole vapour which resulted in the preparation of
VPP-PPy homogenous individual particles demonstrating a particle size in the range
of 100–150 nm. The VPP-PPy CEs yield power conversion efficiency in the range
of 2.0–3.4% depending on the oxidant concentration which is lower than the power
conversion efficiency observed for Pt CE material which is 4.4% (Table 17.4). It was
discovered that the photovoltaic performances were improved with an increase in
Fe-TsO oxidant concentration (Xia et al. 2011).

Table 17.4 Photoelectric properties of DSCs with PPy and Pt CEs

Electrode J sc (mA/cm2 ) FF PCE (%)
Pt 9.5 0.65 4.4
5% VPP-PPy 8.3 0.51 2.8
20% VPP-PPy 9.2 0.54 3.4
358 S. S. B. Gunasekera et al.

There has been a recorded increase in the power conversion efficiency in VPP-PPy
CEs when the Fe-TsO oxidant concentration is increased from 5% (PCE of 2.8%)
to 20% (PCE of 3.4%). From cyclic voltammetry it has been revealed that VPP-PPy
possess high electrocatalytic activity for I− /I−
3 redox system. Despite its lower power
conversion efficiency its substitution of Pt CE is favoured due to its cost effectiveness
and facile synthesis (Xia et al. 2011).
Furthermore traditional electrochemical polymerization methods are employed
to synthesize PPy to be used as a CE in DSCs. The use of potentiostatic mode of
polymerization at 1.2 V (vs. Ag/AgCl) for polymerization of pyrrole in acetonitrile
solution to prepare EP-PPy had been investigated. This enables the synthesis of PPy
particles around the size of 300 nm and assures full coverage of FTO substrate.
PPy CE prepared by electropolymerization exhibits a power conversion efficiency
of 3.2% which is comparable with regard to optimized VPP-PPy CE’s power con-
version efficiency. Although its power conversion efficiency is less than that of Pt
CE material they show good catalytic behaviour with respectable Jsc . In terms of
electropolymerization the use of bulky sized inert doping anions such as ClO− 4 are
preferred in the preparation of PPy, due to their high mobility which contributes to
achieve high current density. In addition to its respectable photovoltaic performances;
feasibility of electropolymerization synthesis and inexpensiveness contributes to the
credibility of EP-PPy as a substitute material for CEs (Xia et al. 2011).
The use of novel self-assembled PPy nanotube membrane as CE material in Pt-free
DSCs has been reported by Peng et al. (2013). Free standing paper like membranes
composed of PPy nanotubes have been synthesized and used to prepare FTO and
Pt free CEs. Usually PPy CE is fabricated by deposition of the polymer on FTO
substrate and in situ polymerization and both these methods require FTO substrate.
This method allows the preparation of FTO free CEs consisting of PPy nanofibres
only, hence flexible DSCs with considerable mechanical properties. This facile syn-
thetic methodology involves heating of pulp like homogenous suspensions at low
temperature followed by doping in HCl solution. Finally the PPy nanotubes allowed
to self-assemble into paper like flexible PPy membranes under a vacuum atmosphere
at high temperature (Rahman et al. 2016).
The high surface area of the PPy nanofibres and good electrolyte penetration in
the poriferous PPy membrane contributes to the respectable electrocatalytic activity
as revealed from the photovoltaic performance data shown in Table 17.5. This PPy
membranes yield a power conversion efficiency of 5.27%. That is 84% of the power
conversion efficiency for platinised CE under same conditions. Despite the slightly
lower power conversion efficiency the use of self-assembled PPy nanotube mem-
branes as a substitute material for Pt CE material is favoured due to inexpensiveness,

Table 17.5 Photovoltaic performance data of DSCs with different CEs

Electrode J sc (mA/cm2 ) FF PCE (%)
Pt/FTO 13.41 0.65 6.25
PPy 13.10 0.56 5.27
17 Conducting Polymers as Cost Effective Counter Electrode … 359

facile synthesis and most importantly the ability to prepare flexible DSCs consisting
of FTO free CEs (Peng et al 2013).
Therefore it is possible to understand that PPy is a credible substitute material
for Pt as the CE in DSCs and the performance of PPy based CEs depends on factors
such as method of polymer synthesis and conditions, type of dopant anions, type of
oxidant species used and film morphology (Peng et al. 2013).
Furthermore the use of PPy CEs in quasi solid state DSCs has been investigated
by Makris et al. Quasi-solid state DSCs have been constructed using nanocrystalline
titania, ureasil based nanoconposite gel electrolyte and PPy functionalized CE. PPy
was electrodeposited on FTO substrate under potentiostatic conditions using the
pyrrole precursor monomer in aqueous medium thus synthesizing PPy functionalized
CE.
Despite the fact that DSC with Pt CE exhibits higher open circuit voltage and a
better fill factor relative to PPy functionalized CE, they possess higher short circuit
current density which accounts to its superior electrocatalytic activity. The thickness
of the PPy film plays a pivotal role in the photovoltaic performances of the DSCs and
this thickness is controllable by the deposition time. PPy films having a thickness of
750 nm gave rise to the optimum photovoltaic performances which are summarized
in Table 17.6. The employment of PPy functionalized CEs in quasi-solid state DSCs
is preferred due to inexpensiveness, durability feasibility of synthesis and the fact
that it can achieve substantial power conversion efficiency which is only 30% less
efficient with respect to platinised CEs (Makris et al. 2011).
Table 17.7 summarises the photovoltaic parameters of DSCs with Pt free PPy
CEs having outstanding efficiencies. The efficiency of PPy CE is highly dependent
on the nature of the dopant, synthetic methodology and surface morphology. They

Table 17.6 Photovoltaic performance data of DSCs with different counter electrodes
Electrode Jsc (mA/cm2 ) FF PCE (%)
Pt 13.5 0.69 6.7
PPy 15.9 0.45 4.6

Table 17.7 Photovoltaic performances of different PPy CEs

Type of PPy Preparation FF PCE (%) Refs.
methodology
PPy nanoparticles Synthetic chemical 0.69 7.66 Wu et al. (2008)
method
Ultra-thin PPy Organic single crystal 0.62 6.80 Hwang et al. (2014)
nanosheets surface induced
polymerization
Spherical PPy Chemical oxidative 0.64 7.73 Jeon et al. (2011)
polymerization
Porous PPy In situ polymerization 0.52 5.74 Bu et al. (2013)
360 S. S. B. Gunasekera et al.

tend to yield comparatively lower efficiencies associated with higher Rct and lower
conductivity (Saranya et al. 2015). But despite the lower efficiencies relative to Pt
CEs its applicability as a CE material is preferred due to cost effectiveness and facile
synthesis.

17.2.3 Poly(3,4-Ethylenedioxythiophene)/(PEDOT) as a CE
in DSCs

PEDOT is another type of conducting polymer which is extensively studied as a

substitute for Pt CE material. It is an efficient catalyst in DSCs due to its nanoporous
structure, high conductivity at room temperature, electrochemical reversibility and
high chemical and thermal stability (Fig. 17.6).
The use PEDOT polymerized via constant current electropolymerization as CE
material has been reported by Gao et al. (2014). PEDOT films were synthesized
on FTO substrate by means of aqueous galvanostatic polymerization technique at
current density of 0.5 mA cm−2 . These PEDOT CEs show variable photovoltaic
performances based on the surface morphologies which can be controlled using the
polymerization time.
From CV and EIS data it is evident that the polymerization time has an impact
on the photovoltaic performances of the DSCs. The PEDOT CEs showed faster
reaction rates and higher electrocatalytic activity as the polymerization time was
increased. The PEDOT CE polymerized at 40 s possessed higher electrocatalytic
activity towards I−3 and charge transfer resistance which is comparable with that
of Pt CE. It was possible to achieve a power conversion efficiency of 6.46% by
optimized PEDOT CE which is greater than the power conversion efficiency yielded
by Pt CE under same conditions which is 6.33% (Table 17.8). Therefore PEDOT

Fig. 17.6 Structure of Poly(3,4-ethylenedioxythiophene)/PEDOT

17 Conducting Polymers as Cost Effective Counter Electrode … 361

Table 17.8 Photovoltaic performances of DSCs of various CEs

Electrodes J sc (mA/cm2 ) FF PCE (%)
Pt 15.27 0.61 6.33
PEDOT-10s 15.58 0.54 5.38
PEDOT-40s 16.66 0.57 6.46

films are regarded as promising alternative materials that can be used in place of Pt
as the CE in DSCs (Gao et al. 2014).
PEDOT nanoporous layers synthesized via electro-oxidative polymerization using
room temperature hydrophobic ionic liquids as a medium; was investigated as a
source of CE material that can substitute Pt in DSCs (Rahman et al. 2016). In this
synthetic methodology room temperature ionic liquids are used as the polymeriza-
tion medium. This enables the provision of favourable growth conditions to produce
grains in the nanometer size range which is desirable as the grain size dictates the cat-
alytic behaviour of the polymer. The applicability of π-conjugated polymer electro-
chemical devices is limited due to poor environmental stability and electrochemical
cycling between oxidation states. These problems arise partly due to the electrolytes
used in these devices, but room temperature ionic liquids are electrochemically stable
and hence they are ideal solvents for long life electrochemical processes. Therefore
the use of PEDOT CEs associates the advantage of reproducible high cycling life
(Ahmad et al. 2010).
The photovoltaic performance has been monitored by increasing the thickness of
PEDOT film by means of increasing the polymerization time. As the thickness of
PEDOT film increased the J sc has increased but in contrast the open circuit voltage has
decreased. It could be understood from the above data tabulated that the photovoltaic
performance increases when the PEDOT film thickness decreases as thin films have
favourable catalytic properties. These PEDOT CEs achieved a power conversion
efficiency of 7.93% which is only slightly lower than when using platinised CE in
DSCs (Table 17.9). Therefore in terms of inexpensiveness, feasibility of synthesis,
reproducible high cycling life and respectable photovoltaic performances PEDOT is
a credible alternative for Pt CEs (Ahmad et al. 2010).
Furthermore a significant photovoltaic performance was reported with the use
of nanostructured PEDOT which were electrochemically prepared, as the CE mate-
rial in DSCs (Rahman et al. 2016). Here PEDOT nanofibres were synthesized hav-
ing high catalytic activity and they were used as the CE material in DSCs. The

Table 17.9 Photovoltaic performances of DSCs for various CEs

Electrode J sc (mA/cm2 ) FF PCE (%)
Pt 15.9 0.73 8.71
PEDOT-30s 15.0 0.76 7.93
PEDOT-60s 15.2 0.75 7.86
362 S. S. B. Gunasekera et al.

Fig. 17.7 Schematic illustration of the synthesis of PEDOT nanofibres. Adapted from Jeon et al.
(2013)

synthetic methodology consists of chemical oxidative polymerization in aqueous

medium in the presence of sodium dodecyl sulphate (SDS) and FeCl3 using 3,4-
ethylenedioxythiophene (EDOT) monomer. The schematic diagram above illustrates
the synthetic methodology employed to prepare the PEDOT nanofibres (Jeon et al.
2013) (Fig. 17.7).
The nanofibrous PEDOT CE with high electrical conductivity and large surface
area behaves as a CE material with high catalytic activity. The nanofibrous structure
is porous and possess a rough morphology which enables penetration of the elec-
trolyte to facilitate and catalyse redox reactions. This is evident from the photovoltaic
performance data obtained (Table 17.10) from CV and EIS tests (Jeon et al. 2013).
Both J sc and V oc recorded for PEDOT-NF CE is greater than in DSCs with Pt
CE resulting in a higher conversion efficiency of 8.34%. In comparison to platinised
CE PEDOT-NF CE shows higher redox current revealed from CV tests which is
attributed to the larger number of catalytic active sites associated with the large
surface area. Therefore due to its distinct photovoltaic performances PEDOT-NF
CEs are a credible alternative for platinised CEs (Jeon et al. 2013).
The polymer morphology plays a pivotal role in terms of efficiency of conducting
polymer CEs (Rahman et al. 2016). This is evident from the investigation of the use
of PEDOT films having nano-meadows morphology as CE material in DSCs. Here

Table 17.10 Photovoltaic performances of DSCs assembled with different CEs

Electrodes J sc (mA/cm2 ) FF PCE (%)
Pt 16.6 0.60 7.20
PEDOT-NF 17.3 0.67 8.34
17 Conducting Polymers as Cost Effective Counter Electrode … 363

Table 17.11 Photovoltaic performances of different PEDOT CEs

Type of PEDOT Preparation methodology FF PCE (%) Refs.
PEDOT film Electropolymerization 0.69 8.87 Li et al. (2014)
PEDOT-NFs Chemical oxidative 0.67 8.34 Jeon et al. (2013)
polymerization—organic
liquid electrolyte
PEDOT-EG film Electropolymerization 0.64 8.50 Zhang et al. (2013)
PEDOT film HFIP solution 0.67 9.00 Chiang et al. (2013)

PEDOT films were synthesized using electropolymerization employing pulse poten-

tiostatic method. The PEDOT nano-meadows were eletcropolymerized onto multi
walled carbon nanotubes (MWCNT) and fabricated onto FTO glass substrate. The
PEDOT/MWCNT CEs demonstrated greater power conversion efficiency (7.03%)
relative to DSCs with Pt CE (5.88%) under the same conditions. Therefore due to the
high electrocatalytic activity attributed to high specific surface area and high pho-
tovoltaic performances PEDOT CE with nano-meadows morphology is a promising
alternative CE material (Xiao et al. 2012).
Table 17.11 summarises the photovoltaic parameters of DSCs with Pt free PEDOT
CEs having outstanding efficiencies. PEDOT exhibits the highest electrocatalytic
activity towards I− −
3 /I ; amongst all conducting polymers (Bay et al. 2006). By dif-
ferent polymerization methods it is possible to synthesize PEDOT with high conduc-
tivity. Hence PEDOT CEs yield outstanding efficiencies relative to other conducting
polymer CEs (Saranya et al. 2015). Furthermore, its applicability as a CE is favoured
also due to its cost effectiveness and feasibility of synthesis.

17.2.4 Polymer Hybrid Composites as CEs in DSCs

Polymer hybrids are a blend of different types of polymers and their applicability as
CEs is under extensive investigation due to synergistic effect it generates in terms
of enhancing electrocatalytic activity (Yue et al. 2012). The preparation of CEs by
means of electrodeposition of PEDOT:PSS (polystyrenesulfonate) has yielded an
outstanding efficiency of 8.3% (Zhang et al. 2013).
Carbon is another cost effective substitute for Pt as a CE material. Nevertheless
its applicability as a CE material is limited due to its insolubility in most solvents. To
subdue this drawback carbon materials could be utilized as a composite material with
conducting polymers and conducting polymer blends thus giving rise to synergistic
effects. In addition, the usage of such composite materials contributes to favourable
mechanical properties. Therefore the applicability of conducting polymer/carbon
composites are extensively investigated as CE materials. Various forms of carbon
such as carbon black (CB), carbon nanotubes (CNT) and graphene have been used
364 S. S. B. Gunasekera et al.

to prepare composites with conducting polymers; that have emerged as promising

candidates for CE materials (Saranya et al. 2015).
Graphene has emerged as promising CE material in DSCs with very good electrical
conductivity and high electrocatalytic activity (Rahman et al. 2016). The surface
functionalization of such carbon materials with conducting polymers have found to
enhance the electrocatalytic activity, as conducting polymers such as PANI, PPy and
PEDOT are promising conducting polymer candidates to replace Pt CEs in DSCs.
The use of reduced graphene oxide/PPy/PEDOT composite films as substitute
material in place of Pt CE in DSCs has been investigated by Sekkarapatti et al. Here
moderately reduced graphene oxide (RGO)/PPy/PEDTO composite films were fabri-
cated employing a process of three steps. GO/PPy composites were initially prepared
via in situ polymerization. Afterwards uniform thin films of graphene oxide/PPy
were deposited on a transparent conductive electrode substrate and annealed at
high temperature (300 °C) which results in the preparation of moderately reduced
GO/PPy composite films by thermal reduction. Finally PEDOT was deposited on
the RGO/PPy film by one-step electrodeposition as illustrated by Fig. 17.8. The
RGO/PPy/PEDTO composites thus produced were employed as the CE in DSCs.
In the composite CE graphene and the conducting polymers, PPy and PEDOT
provides synergistic properties in terms of conductivity and electrocatalytic activity
which can be understood by the evaluation of data from CV and EIS analysis.
The composite CE yields J sc and V oc values that are comparable with Pt CEs
(Table 17.12). Furthermore it exhibits good power conversion efficiency (7.1%)

Fig. 17.8 Schematic illustration of the synthesis of RGO/PPy/PEDOT CE. Adapted from Sekkara-
patti et al. (2015)

Table 17.12 Photovoltaic performances of various different CEs

Electrodes J sc (mA/cm2 ) FF PCE (%)
Pt 19.2 0.62 9.3
RGO/PPy/PEDOT 17.0 0.55 7.1
17 Conducting Polymers as Cost Effective Counter Electrode … 365

Table 17.13 Photovoltaic performances of DSCs with different CEs

Electrodes J sc (mA/cm2 ) FF PCE (%)
Pt 17.9 0.64 8.5
MWCNT/PEDOT:PSS 15.5 0.63 6.5

which is slightly less than in DSCs with Pt CEs. Hence RGO/PPy/PEDOT com-
posite CEs reveal respectable electrocatalytic activity and sufficient photovoltaic
performances. In addition to this its inexpensiveness and facile synthesis assures its
credibility as a promising CE material (Sekkarapatti et al. 2015).
Furthermore conducting polymer/carbon nanotube composites are reported as
promising CE materials in DSCs. Carbon nanotubes have remarkable potential as
CE materials due to fast electron transfer and good conductivity. The composite
solution was prepared by dispersal of multiwall carbon nanotubes (MWCNTs) in
aqueous PEDOT:PSS solution via ultrasonication. The carbon nanotubes/conducting
polymer composites were fabricated on FTO substrate by means of spin coating (Fan
et al. 2008).
The photovoltaic performance data reveals that the composite CE exhibits compa-
rable J sc and V oc with respect to Pt CE (Table 17.13). The power conversion efficiency
demonstrated by DSCs with composite CE is only slightly less than that of DSCs
with Pt CEs. Therefore due to respectable photovoltaic performance, facile synthe-
sis and inexpensiveness, MWCNT/PEDOT:PSS composite material are credible as
promising CE materials that can be used in place of Pt (Fan et al. 2008).
Table 17.14 summarises the photovoltaic parameters of DSCs with Pt free con-
ducting polymer/carbon composite CEs having outstanding efficiencies. The use of
such composites enables favourable electrical, optical and mechanical properties
through synergistic effects (Saranya et al. 2015).

Table 17.14 Photovoltaic performances of different CP/carbon composite CEs

CP/carbon, CNT, Preparation FF PCE (%) Refs.
graphene composites methodology
PEDOT:PSS/carbon Doctor blade method 0.69 7.60 Yue et al. (2013a, b)
PEDOT:PSS/graphite Chemical 0.73 7.36 Sun et al. (2010)
polymerization
PPy/SWCNT Reflux method 0.71 8.30 He et al. (2014)
PEDOT:PSS/MWCNT Spin coating 0.71 8.30 Guan et al. (2013)
film
PPy/RGO composite Rapid mixing method 0.64 8.14 Gong et al. (2013)
Graphene/PEDOT:PSS Electropolymerization 0.65 7.86 Yue et al. (2013a, b)
366 S. S. B. Gunasekera et al.

17.3 Summary

The CE is a vital component in DSCs which is responsible for the transfer of electrons
from the external circuit back to the redox mediator and regeneration of redox couple.
The CE electrode typically comprises of a transparent conductive oxide glass on
which a catalyst is fabricated. The applicability of a wide variety of CE materials
has been investigated out of which Pt has been utilized as the preferred CE material
due to its high conductivity and high electrocatalytic activity towards I− −
3 /I . The
applicability of Pt as a CE is limited due to its high cost and susceptibility to undergo
corrosion. Hence the use of substitute materials in place of Pt as the CE; such as carbon
based materials, metal organic frameworks, conducting polymers and polymer hybrid
composites have been extensively investigated.
Conducting polymers are considered as promising CE material due to their high
conductivity, high electrocatalytic activity and good stability. Furthermore their
applicability as CE materials is favoured due to cost effectiveness and feasibility
of synthesis. Various conducting polymers such as PANI, PPy, PEDOT and polymer
hybrid composites have been extensively investigated as cost effective CE mate-
rials in Pt free DSCs. PANI based CE material is a promising prospect in DSCs
due to appreciable photovoltaic performances and cost effectiveness. PANI reveals
favourable characteristics such as low cost, high electrochemical activity, high envi-
ronmental stability and facile synthesis. PANI used as CE material is synthesized by
means of either chemical polymerization or electropolymerization method. It is pos-
sible to enhance the electrochemical catalytic properties of PANI CEs by the use of
special morphologies. In PANI CEs, dopants are selected such that it enhances con-
ductivity and increases surface area by acting as a pore former. The use of PPy based
CEs in DSCs yield relatively lower efficiencies due to low conductivity and high
Rct . Nevertheless its efficiency is improved by selection of proper dopants, effective
synthetic methodologies and manipulating surface morphologies. Despite lower effi-
ciencies their applicability as a CE material is preferred due to cost effective facile
synthesis and fabrication of PPy based CEs. PEDOT yields the highest efficiency
and electrocatalytic activity towards I− −
3 /I amongst other conducting polymers in
Pt free DSCs. The applicability of PEDOT CEs is limited due to its cost which is
comparable with that of Pt CE. Furthermore, carbon materials are used as composite
materials with conducting polymers and conducting polymer blends to prepare CEs
with enhanced conductivity, electrocatalytic properties and favourable mechanical
properties derived through synergistic effects. The use of composite materials consist-
ing of conducting polymers and carbon materials such as CB, CNTs and graphene; as
CE material has yielded outstanding efficiencies in Pt fee DSCs. Hence the prospect
of substitution of Pt CE with conducting polymers and polymer hybrid composites
and preparing cost effective environmentally friendly DSCs seems promising in the
future.
17 Conducting Polymers as Cost Effective Counter Electrode … 367

References

Adachi M, Murata Y, Takao J, Jiu J, Sakamoto M, Wamg F (2004) Highly efficient dye-sensitized
solar cells with a titania thin-film electrode composed of a network structure of single-crystal-like
TiO2 nanowires made by the oriented attachment mechanism. J Am Chem Soc 126:14943–14949
Ahmad S, Yum J-H, Xianxi Z, Gratzel M, Butt H-J, Nazeeruddin MK (2010) Gye-sensitized solar
cells based on poly (3,4,-ethylenedioxythiophene) counter electrode derived from ionic liquids.
J Mater Chem 20:1654–1658
Ameen S, Shaheer Akhtar M, Kim G-S, Kim YS (2009) Plasma-enhanced polymerized aniline/TiO2
dye-sensitized solar cells. J Alloy Compd 487:382–386
Ameen S, Akhtar MS, Kim YS, Yang O-B, Shin H-S (2010) Sulfamic acid-doped polyaniline
nanofibres thin-film based counter electrode: application in dye-sensitized solar cells. J. Phys.
Chem. 114(10):4760–4764
Bach U, Lupo D, Comte P, Moser JE, Weissortel F, Salbeck J, Spreitzer H, Gratzel M (1998)
Solid-state dye-sensitized mesoporous TiO2 solar cells with high photon-to-electron conversion
efficiencies. Nature 395(6702):583–585
Bai Y, Cao Y, Zhang J, Wang M, Li R, Wang P, Zakeeruddin SM, Gratzel M (2008) High-performance
dye-sensitized solar cells based on solvent-free electrolytes produced from eutectic melts. Nat
Mater 7:626–630
Bay L, West K, Winther-Jensen B, Jacobsen T (2006) Electrochemical reaction rates in a
dye-sensitised solar cell—the iodide/tri-iodide redox system. Solar Energy Mater Solar Cells
90(3):341–351
Boschloo G, Hagfeldt A (2009) Characteristics of the iodide/triiodide redox mediator in dye-
sensitized solar cells. Acc Chem Res 42(11):1819–1826
Braga A, Moreira S, Zampieri P, Bacchin J, Mei P (2008) New processes for the production of
solar-grade polycrystalline silicon: a review. Sol Energy Mater Sol Cells 92(4):418–424
Bu C, Tai Q, Liu Y, Guo S, Zhao X (2013) A transparent and stable polypyrrole counter electrode
for dye-sensitized solar cell. J Power Sources 221:78–83
Cao F, Oskam G, Searson PC (1995) A solid state, dye sensitized photoelectrochemical cell. J Phys
Chem 99(47):17071–17073
Chen Z, Cotterell B, Wang W, Guenther E, Chua SJ (2001) A mechanical assessment of flexible
optoelectronic devices. Thin Solid Films 394(1):201–205
Chen D, Huang F, Cheng Y, Caruso RA (2009) Mesoporous anatase TiO2 beads with high surface
areas and controllable pore sizes: a superior candidate for high-performance dye sensitized solar
cells. Adv Mater 21:2206–2210
Chiang C-H, Chen S-C, Wu C-G (2013) Preparation of highly concentrated and stable conducting
polymer solutions and their application in high-efficiency dye-sensitized solar cell. Org Electron
14(9):2369–2378
Chiba Y, Islam A, Watanabe Y, Komiya R, Koide N, Han LY (2006) Dye-sensitized solar cells with
conversion efficiency of 11.1%. Jpn J Appl Phys Part 2 45 (24–28):L638–L640
Cui X, Xie Z, Wang Y (2016) Novel CoS2 embedded carbon nanocages by direct sulfurizing
metal-organic frameworks for dye-sensitized solar cells. Nanoscale 8(23):11984–11992
Daeneke T, Uemura Y, Duffy NW, Mozer AJ, Koumura N, Bach U, Spiccia L (2012) Aqueous
dye-sensitized solar cell electrolytes based on the ferricyanideferrocyanide redox couple. Adv
Mater
Fan B, Mai X, Sun K, Ouyang J (2008) Conducting polymer/carbon nanotube composite as counter
electrode of dye sensitized solar cells. Appl. Phys. Lett. 93:143103
Fonstad C, Rediker R (1971) Electrical properties of high-quality stannic oxide crystals. J Appl
Phys 42(7):2911–2918
Fu D, Huang P, Bach U (2012) Platinum coated counter electrodes for dye-sensitized solar cells
fabricated by pulsed electrodeposition—correlation of nanostructure, catalytic activity and optical
properties. Electrochim Acta 77:121–127
368 S. S. B. Gunasekera et al.

Gao L, Mao X, Zhu H, Xiao W, Gan F, Wang D (2014) Electropolymerization of PEDOT on CNTs
conductive network assembled at water/oil interface. Electrochim Acta 136(1):97–104
Gong J, Liang J, Sumathy K (2012) Review on dye-sensitized solar cells (DSSCs): fundamental
concepts and novel materials. Renew Sustain Energy Rev 16:5848–5860
Gong F, Xu X, Zhou G, Wang Z-S (2013) Enhanced charge transportation in a polypyrrole counter
electrode via incorporation of reduced graphene oxide sheets for dye-sensitized solar cells. Phys
Chem Chem Phys 15(2):546–552
Gorlov M, Pettersson H, Hagfeldt A, Kloo L (2007) Electrolytes for dye-sensitized solar cells based
on interhalogen ionic salts and liquids. Inorg Chem 46(9):3566–3575
Grätzel M (2001) Photoelectrochemical cells. Nature 414(6861):338–344
Grätzel M (2003) Dye-sensitized solar cells. J Photochem Photobiol C 4(2):145–153
Grätzel M (2005) Solar energy conversion by dye-sensitized photovoltaic cells. Inorg Chem
44(20):6841–6851
Guan G, Yang Z, Qiu L, Sun X, Zhang Z, Ren J (2013) Oriented PEDOT:PSS on aligned carbon
nanotubes for efficient dyesensitized solar cells. J Mater Chem A 1(42):13268–13273
Hagfeldt A, Grätzel M (2000) Molecular photovoltaics. Acc Chem Res 33(5):269–277
He B, Tang Q, Luo J, Li Q, Chen X, Cai H (2014) Rapid charge-transfer in polypyrrole–single wall
carbon nanotube complex counter electrodes: improved photovoltaic performances of dyesensi-
tized solar cells. J Power Sour 256:170–177
Hou S, Cai X, Wu H, Lv Z, Wang D, Fu Y, Zou D (2012) Flexible, metal-free composite counter
electrodes for efficient fiber-shaped dye-sensitized solar cells. J Power Sour 215:164–169
Huang KC, Huang JH, Wu CH, Liu CY, Chen HW, Chu CW, Lin CL, Ho KC (2011)
Nanographite/polyaniline composite films as the counter electrodes for dye-sensitized solar cells.
J Mater Chem 21:10384–10389
Hwang DK, Song D, Jeon SS, Han TH, Kang YS, Im SS (2014) Ultrathin polypyrrole nanosheets
doped with HCl as counter electrodes in dye-sensitized solar cells. J Mater Chem A 2(3):859
Jeon SS, Kim C, Ko J, Im SS (2011) Spherical polypyrrole nanoparticles as a highly efficient counter
electrode for dye-sensitized solar cells. J Mater Chemi 21(22):8146–8151
Jeon N, Hwang DK, Kang YS, Im SS, Kim D-W (2013) Quasi-solid state dye-sensitized solar cells
assembled with polymeric ionic liquid and poly(3,4,-ethylenedioxythiophene) counter electrode.
Electrochem Commun 34:1–4
Jiang X, Li H, Li S, Huang S, Zhu C, Hou L (2018) Metal-organic framework-derived Ni–Co
alloy@carbon microspheres as high-performance counter electrode catalysts for dye-sensitized
solar cells. Chem Eng J 334:419–431
Kakiage K, Aoyama Y, Yano T, Oya K, Fujisawa J-I, Hanaya M (2015) Highly-efficient dye-
sensitized solar cells with collaborative sensitization by silyl-anchor and carboxy-anchor dyes.
Chem Commun 51(88):15894–15897
Law M, Greene LE, Johnson JC, Saykally R, Yang P (2005) Nanowire dye-sensitized solar cells.
Nat Mater 4(6):455–459
Lee P (2019) Solar energy. In: Managing global warming-an interface of technology and human
issues, pp 317–332
Lee C, Wei X, Kyssar JW, Hone J (2008) Measurement of the elastic properties and intrinsic strength
of monolayer graphene. Science 321:385–388
Lee Y, Kim B, Ifitiquar S, Park C, Yi J (2014) Silicon solar cells: Past, present and the future. J
Korean Phys Soc 65(3):355–361
Li Q, Wu J, Tang Q, Lan Z, Li P, Lin J, Fan L (2008) Application of microporous polyaniline counter
electrode for dye-sensitized solar cells. Electrochem Commun 10:1299–1302
Li C-T, Lee C-P, Fan M-S, Chen P-Y, Vittal R, Ho K-C (2014) Ionic liquiddoped poly(3,4-
ethylenedioxythiophene) counter electrodes for dye-sensitized solar cells: cationic and anionic
effects on the photovoltaic performance. Nano Energy 9:1–14
Macdiarmid AG (1997) Polyaniline and polypyrrole: where are we headed. Synth Met 84:27–34
Maiaugree W, Lowpa S, Towannang M, Rutphonsan P, Tangtrakarn A, Pimanpang S, Maiaugree P,
Ratchapolthavisin N, Sang-aroon W, Jarernboon W, Amornkitbamrung V (2015) A dye sensitized
17 Conducting Polymers as Cost Effective Counter Electrode … 369

solar cell using natural counter electrode and natural dye derived from mangosteen peel waste.
Sci Rep 5:15230
Makris T, Dracopoulos V, Sterigiopoulos T, Lianos P (2011) A quasi solid-state dye-sensitized solar
cell made of polypyrrole counter electrodes. Electrochim Acta 56:2004–2008
Mishra A, Fischer MKR, Bauerle P (2009) Metal-free organic dyes for dye-sensitized solar
cells: from structure: property relationships to design rules. Angewandte Chemie Int Ed
48(14):2474–2499
Murakami TN, Ito S, Wang Q, Nazeeruddin MK, Bessho T, Cesar I, Liska P, Humphry-Baker R,
Comte P, Péchy P, Grätzel M (2006) Highly efficient dye-sensitized solar cells based on carbon
black counter electrodes. J Electrochem Soc 153(12):A2255–A2261
Nazeeruddin MK, Kay A, Rodicio I, Humphry-Baker R, Mueller E, Liska P, Vlachopoulos N, Graet-
zel M (1993) Conversion of light to electricity by cis-X2bis (2,20-bipyridyl-4,40-dicarboxylate)
ruthenium (II) charge-transfer sensitizers (X¼ Cl– , Br– , I– , CN– , and SCN– ) on nanocrystalline
titanium dioxide electrodes. J Am Chem Soc 115(14):6382–6390
Nazeeruddin MK, Pechy P, Gratzel M (1997) Efficient panchromatic sensitization ¨ of nanocrys-
talline TiO2 films by a black dye based on atrithiocyanatoruthenium complex. Chem Commun
18:1705–1706
Nogueira AF, Longo C, De Paoli MA (2004) Polymers in dye sensitized solar cells: overview and
perspectives. Coord Chem Rev 248:1455–1468
O’Regan B, Grätzel M (1991) A low-cost, high-efficiency solar cell based on dye-sensitized colloidal
TiO2 films. Nature 353(6346):737–740
Park NG, van de Lagemaat J, Frank AJ (2000) Comparison of dye-sensitized rutile and anatase-
based TiO2 solar cells. J Phys Chem B 104(38):8989–8994
Peng T, Sun W, Huang C, Yu W, Sebo B, Dai Z, Guo S, Zhao XZ (2013) Self-assembled free-
standing polypyrrole nanotube membrane as an efficient FTO- and Pt-free counter electrode for
dye-sensitized solar cells. ACS Appl Mater Interfaces 6:14–17
Perera IR, Gupta A, Xiang W, Baeneke T, Bach U, Evans RA, Spiccia L (2014) Introducing man-
ganese complexes as redox mediators for dye-sensitized solar cells. Phys. Chem. Chem. Phys.
16(24):12021–12028
Rahman MS, Hammed WA, Yahya RB, Mahmud HNME (2016) Prospects of conducting polymer
and graphene as counter electrodes in dye-sensitized solar cells. J Polym Res 23:192–205
Saranya K, Rameez Md, Subramania A (2015) Developments in conducting polymer based counter
electrodes for dye-sensitized solar cells—an overview. Eur Polymer J 66:207–227
Sekkarapatti M, Nikolakapoulou A, Raptis D, Dracopoulos V, Paterakis G, Lianos P (2015) Reduced
graphene oxide/Polypyrrole/PEDOT composite films as efficient Pt-free counter electrode for
dye-sensitized solar cells. 173:276–281
Sima C, Grigoriu C, Antohe S (2010) Comparison of the dye-sensitized solar cells performances
based on transparent conductive ITO and FTO. Thin Solid Films 519(2):595–597
Sun H, Luo Y, Zhang Y, Li D, Yu Z, Li K (2010) In situ preparation of a flexible polyaniline/carbon
composite counter electrode and its application in dye-sensitized solar cells. J Phys Chem C
114(26):11673–11679
Sun X, Li Y, Dou J, Shen D, Wei M (2016) Metal-organic frameworks derived carbon as a high-
efficiency counter electrode for dye-sensitized solar cells. J Power Sources 232:93–98
Tahar RBH, Ban T, Ohya Y, Takahashi Y (1998) Tin doped indium oxide thin films: electrical
properties. J Appl Phys 83(5):2631–2645
Tai Q, Chen B, Guo F, Xu S, Hu H, Sebo B, Zhao X-Z (2011) In situ prepared transparent polyaniline
electrode and its application in bifacial dye-sensitized solar cells. ACS Nano 5:3795–3799
Tang Q, Cai H, Yuan S, Wang X (2013) Counter electrodes from double-layered polyaniline nanos-
tructures for dye-sensitized solar cell applications. J Mater Chem A 1(2):317–323
Thomas S, Deepak TG, Anjusree GS, Arun TA, Nair SV, Nair AS (2014) A review on counter
electrode materials in dye-sensitized solar cells. J Mater Chem A 2(13):4474–4490
370 S. S. B. Gunasekera et al.

Upadhyaya HM, Hirata N, Haque SA, de Paoli M-A, Durrant JR (2006) Kinetic competition in
flexible dye sensitised solar cells employing a series of polymer electrolytes. Chem Commun
8:877–879
U.S. Energy Information Administration (2016) International energy outlook-2016
Wang P, Zakeeruddin SM, Moser JE, Nazeeruddin MK, Sekiguchi T, Grätzel M (2003) A stable
quasi-solid-state dye-sensitized solar cell with an amphiphilic ruthenium sensitizer and polymer
gel electrolyte. Nat Mater 2(6):402–407
Wang Z-S, Kawauchi H, Kashima T, Arakawa H (2004) Significant influence of TiO2 photoelectrode
morphology on the energy conversion efficiency of N719 dye-sensitized solar cell. Coord Chem
Rev 248:1381–1389
Wang M, Chamberland N, Breau L, Moser JE, Humphry-Baker R, Marsan B, Zakeeruddin SM,
Gratzel M (2010) An organic redox electrolyte to rival triiodide/iodide in dye-sensitized solar
cells. Nat Chem 2(5):385–389
Wang H, Feng Q, Gong F, Li Y, Zhou G, Wang Z-S (2013) In situ growth of oriented polyaniline
nano wires array for efficient cathode of Co(III)/Co(II) mediated dye-sensitized solar cell. J Mater
Chem A 1:97–104
World Energy Council (2016). https://www.worldenergy.org/wp-content/uploads/2016/10/World-
Energy-Resources-Full-report-2016.10.03.pdf
World Nuclear Association (2019) Renewable Energy and electricity, sustainable energy, renew-
able energy. http://www.world-nuclear.org/information-library/energy-and-the-environment/
renewable-energy-and-electricity.aspx)
Wu J, Li Q, Fan L, Lan Z, Li P, Lin J, Hao S (2008) High-performance polypyrrole nanoparticles
CE for dye-sensitized solar cells. J Power Sources 181:172–176
Wu J, Li Y, Tang Q, Yue G, Lin J, Huang M (2014). Bifacial dyesensitized solar cells: a strategy to
enhance overall efficiency based on transparent polyaniline electrode. Sci Rep 4
Xia J, Chen L, Yanagida S (2011) Application of polypyrrole as a CE for a dye-sensitized solar
cell. J Mater Chem 21:4664–4649
Xiao YM, Lin JY, Wu JH, Tai SY, Chou S-W, Yue GT, Wu J (2012) Pulse electropolymerization
of high performance PEDOT/MWCNT CEs for Pt-free dye sensitized solar cells. J Mater Chem
22:19919–19925
Xiao Y, Lin J-Y, Wang W-Y, Tai S-Y, Yue G, Wu J (2013) Enhanced performance of low-cost
dye-sensitized solar cells with pulse-electropolymerized polyaniline CEs. Electrochim Acta
90:468–474
Xiao Y, Han G, Li Y, Li M, Chang Y (2014) High performance of Pt-free dye-sensitized solar cells
based on two-step electropolymerized polyaniline CEs. J Mat Chem A 2:3452–3460
Xu M, Li R, Pootrakulchote N, Shi D, Guo J, Yi Z, Zakeeruddin SM, Grätzel M, Wang P (2008)
Energy-level and molecular engineering of organic D-p-A sensitizers in dye-sensitized solar cells.
J Phys Chem C 112(49):19770–19776
Xu J, Li M, Wu L, Sun Y, Zhu L, Gu S, Liu L, Bai Z, Fang D, Xu W (2014a) A flexible polypyrrole-
coated fabric counter electrode for dye-sensitized solar cells. J Power Sour 257:230–236
Xu P, Tang Q, Chen H, He B (2014b) Insights of close contact between polyaniline and FTO
substrate for enhanced photovoltaic performances of dye-sensitized solar cells. Electrochim Acta
125:163–169
Yang F (2009) Thin film solar cells grown by organic vapor phase deposition. Princeton University
Yang S-C, Yang D-J, Kim J, Hong J-M, Kim H-M, Kim H-d, Lee H (2008) Hallow TiO2 hemi-
spheres obtained by colloidal templating for application in dye-sensitized solar cells. Adv Mater
20:1059–1064
Yella A, Lee HW, Tsao N, Yi C, Chandiran AK, Nazeeruddin MK, Diau EW, Yeh CY, Zakeeruddin
SM, Grätzel M (2011) Porphyrin-sensitized solar cells with cobalt (II/III)-based redox electrolyte
exceed 12% efficiency [science (629)]. Science 334(6060):1203
Yen Y-S, Chou H-H, Chen Y-C, Hsu C-Y, Lin JT (2012) Recent developments in molecule-based
organic materials for dye-sensitized solar cells. J Mater Chem 22(18):8734–8747
17 Conducting Polymers as Cost Effective Counter Electrode … 371

Yue G, Wu J, Xiao Y, Lin J, Huang M, Lan Z (2012) Application of poly(3,4- ethylenedioxythio-

phene):polystyrenesulfonate/polypyrrole counter electrode for dye-sensitized solar cells. J Phys
Chem C 116(34):18057–18063
Yue G, Wu J, Xiao Y, Lin J, Huang M, Fan L (2013a) A dye-sensitized solar cell based on
PEDOT:PSS counter electrode. Chin Sci Bull 58(4–5):559–566
Yue G, Wu J, Xiao Y, Lin J, Huang M, Lan Z (2013b) Functionalized graphene/poly(3,4-
ethylenedioxythiophene):polystyrenesulfonate as counter electrode catalyst for dye-sensitized
solar cells. Energy 54:315–321
Yun S, Freitas JN, Nogueira AF, Wang Y, Ahmad S, Wang Z (2015) Dye sensitized solar cells
employing polymers. Progr Polymer Sci 59:1–40
Zhang T-L, Chen H-Y, Su C-Y, Kuang D-B (2013) A novel TCO- and Pt-free counter electrode for
high efficiency dye-sensitized solar cells. J Mater Chem A 1(5):1724
Zhou H, Wu L, Gao Y, Ma T (2011) Dye-sensitized solar cells using 20 natural dyes as sensitizers.
J Photochem Photobiol A Chem 219(2–3):188–194
Zhu H, Wei J, Wang K, Wu D (2009) Applications of carbon materials in photovoltaic solar cells.
Sol Energy Mater Sol Cells 93(9):1461–1470
Chapter 18
Interfacial Materials for Organic Solar
Cells

Amaresh Mishra

Abstract Organic solar cell (OSC) is one of the promising photovoltaic technol-
ogy for next generation low-cost renewable energy sources. The power conversion
efficiencies (PCE) of OSCs have reached above 14% in single-junction and ~17%
in tandem OSCs. This rapid increase in the performance is mostly profited from the
synergetic advances in rational molecular design, device processing and interfacial
layer modifications. In addition to the development of efficient photoactive materials,
interfacial design plays a crucial role in the improvement of device performance and
stability. Most importantly, the interfacial layer is responsible for establishing good
ohmic contact in the device, thus minimize the resistance, interfacial recombination
and improve charge selectivity. In this chapter, we present the recent development
in the electron and hole transporting interfacial materials design for both single-
junction and tandem OSCs. Special attention will be paid to the design principles
of interfacial materials which includes inorganic metal oxides, composite materials,
oligomeric and polymeric molecules and their use as cathode and anode interlayer
for high efficiency devices. The structure-property relationships of various interfacial
materials will be analyzed as an approach towards high performance OSCs. Finally,
we will discuss the current challenges with possible solutions and perspectives for
performance enhancement in OSCs.

Keywords Interfacial material · Organic semiconductor · Cathode interlayer ·

Anode interlayer · Organic solar cell

18.1 Introduction

Solution-processed organic solar cells (OSC) using π-conjugated organic materials

as active layer components have shown significant advance in recent years with power
conversion efficiency (PCE) in the range from 10–14% in single bulk-heterojunction
devices (Mishra and Bäuerle 2012; Li et al. 2019; Zhang et al. 2018a, b; Xiao et al.

A. Mishra (B)
School of Chemistry and Nano Research Centre, Sambalpur
University, Jyoti Vihar, Sambalpur, India
e-mail: amaresh.mishra@suniv.ac.in

© Springer Nature Singapore Pte Ltd. 2020 373

H. Tyagi et al. (eds.), Solar Energy, Energy, Environment,
and Sustainability, https://doi.org/10.1007/978-981-15-0675-8_18
374 A. Mishra

2017; Mishra and Sahu 2019). In tandem cells the PCE of around 17% was recently
being reported (Meng et al. 2018). Theoretically it has been demonstrated that PCE
close to 20% is not so far to realize for tandem cell based on the optimized device
configuration, material bandgaps alignment and high achievable fill factors (FF)
(Li et al. 2014a). A typical bulk-heterojunction solar cells (BHJSC) comprise of
donor (D):acceptor (A) based organic photoactive layer sandwiched between two
electrodes. The excitons (electron-hole pair) generated in the photoactive layer after
light excitation needs to move towards D: A interface to separate holes and electrons
which then transport to the respective electrodes through highest occupied molecular
orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). The holes travel
towards the anode and electrons collected at the cathode (Fig. 18.1). In BHJ devices
the energy level offset between the D and A and their contact with the respective
electrodes plays a crucial role in driving the potential out of the device. Therefore,
a barrier-less contact (Ohmic contact) with the electrodes is necessary for efficient
charge transport and extraction, which is very challenging to manipulate between
organic and metal electrode interface. Generally, a Schottky barrier formed at the
electrodes creates loss of potential thus lower the open circuit voltage (V OC ) (Yip
and Jen 2012).

Fig. 18.1 Device architecture of a conventional OSC and b inverted OSC, c and d are the cor-
responding energy level alignment with interfacial layers providing Ohmic contacts and charge
selectivity at both electrodes
18 Interfacial Materials for Organic Solar Cells 375

In addition to the synthesis of novel active layer materials and updated device fab-
rication conditions, the design and implementation of interfacial materials is also very
crucial in the device performance improvement. An ideal interfacial design is most
important for efficient charge transport as they not only establish good Ohmic contact
but also regulate other device parameters like film morphology, control energy level
alignment, alter work functions (WF) of both anode and cathode, minimize resis-
tance with high charge selectivity, reduce charge recombination rate, enhance charge
extraction and improve device stability (Ma et al. 2010; Chen et al. 2010). The WF
is defined as the energy difference between Fermi energy and vacuum level which is
the minimum amount of energy necessary to withdraw an electron from the metal. In
order to match the energy levels for efficient charge transport and collections various
cathode and anode interfacial layers have been introduced between the active layer
and electrodes. To date, several reports have revealed the underlying mechanisms for
the tuning of WF by interfacial materials at the active layer/electrode interfaces (van
Reenen et al. 2014; Lee et al. 2014a; Wang et al. 2015). The interfacial modification
influence the formation of interfacial dipole resulting in permanent shifting of the
vacuum level to different degrees at the interface which depends on the direction of
the dipole (Fig. 18.2). The solar cell performance completely depends on the direc-
tion of the dipole. The net interfacial dipole directed towards the metal electrode
reduces the device performance, while when the net dipole directed away from the
metal improves the device performance due to increase in the built-in voltage.
In conventional OSCs, poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)
(PEDOT:PSS) has been widely used as anode interface layer (AIL) to modify indium
tin oxide (ITO)-coated glass surface for efficient hole extraction. In these devices LiF
or Ca generally are used to modify low WF metal cathode like Al. However, due to
polar and acidic nature of PEDOT:PSS they are very sensitive to low WF metal, oxy-
gen and moisture. Therefore, many groups have developed inverted device structure
where high WF metals such as Ag and Au used as top electrode and low WF metal

Fig. 18.2 Schematic representation of the band energy alignment for OSCs a without interfacial
layer b and c tuned by the introduction of dipolar CIL above the active layer surface with the
spontaneous dipole formation and work function modification of the metal cathode. The dipole
introduces an electrostatic potential shift for charges crossing the interface, shifting the vacuum
level between the metal and organic, b interfacial dipole directed towards metal surface c interfacial
dipole directed away from metal surface and increases the built-in voltage
376 A. Mishra

oxide such as TiO2 , ZnO as cathode interfacial layer (CIL) to modify ITO surface
(Fig. 18.2). Subsequently, various organic and polymeric materials were synthesized
for their use as interlayer material in solution-processed OSCs. The interfacial layer
is very important to fine tune the device performance such as V OC , short-circuit cur-
rent density (J SC ) and FF, therefore the overall PCE. Along with the photoactive
layers it is also highly important to optimize the charge extracting interfacial layer in
single-junction and charge recombination layer in tandem cell to achieve high per-
formance OSCs. The interconnecting layer (ICL) in tandem cell is highly responsible
for extraction of holes and electrons from the adjacent subcells. Hence, the choice
of ICL plays a detrimental role in the device performance improvement. Mihailetchi
et al. observed a variation in the V OC of polymer:fullerene-based conventional solar
cells (~0.4 to 0.85 V) by using metal cathodes of variable WFs which is due to the
Fermi level pinning of the electrodes to the active layer which enables Ohmic contact
at the cathode (Mihailetchi et al. 2003). The low WF metals (Ag, Al, Ca etc.) gener-
ally forms Ohmic contact with the LUMO of the acceptor, while the high WF metals
(Au, Pd etc.) forms Ohmic contact with the HOMO of the donor material. Jen group
demonstrated the influence of metal anodes on the V OC of an poly-(3-hexylthiophene)
(P3HT): [6,6]-phenyl C60 butyric acid methyl ester (PC61 BM) based inverted solar
cell using ZnO-NPs/C60 -self-assembled monolayer as interfacial material and found
that only Pd with high WF can form good Ohmic contact for hole extraction from
the donor (Hau et al. 2010).
In recent years major efforts have been devoted to develop interfacial layers for
single-junction and tandem OSCs. The interfacial layers are mostly used to modify
the energy level alignment between active layer and electrode, adjust the polarity
of the electrodes for charge selectivity, control the surface energy by modulating
active layer morphology, as well reduce carrier recombination and improve stabil-
ity of photoactive layer and electrodes interface by preventing the metal ions from
diffusion into the organic layer (van Reenen et al. 2014; Yip and Jen 2012; Manders
et al. 2013; Bilby et al. 2014; Jørgensen et al. 2012; Ma et al. 2010). The interfa-
cial materials have been designed in such a way that they can be processed from
orthogonal solvent relative to the active layer and thus, can be processed from solu-
tion. In the last few years, the number of publications on interfacial material design
and their implementation in various OSC devices have been considerably increased.
Recently, some of the reviews have comprehensively summarized the role of inter-
facial layers in OSCs (Lai et al. 2013; Chueh et al. 2015; Wang et al. 2015; Yin
et al. 2016; Yip and Jen 2012; Li et al. 2018a). Frey and co-workers discussed the
mechanistic studies on the driving force for interlayer formation and their influence
on device performance (Vinokur et al. 2016). In this chapter we systematically dis-
cuss some important progress on solution-processed interfacial materials including
organic-inorganic hybrids, transition metal oxide, composite materials, oligomeric
and polymeric molecules, and their implementation as cathode and anode interfa-
cial layer for high efficiency single-junction OSCs. Finally, structure-property and
device performance relationship and challenges of the interfacial materials towards
high performance devices will be deduced.
18 Interfacial Materials for Organic Solar Cells 377

18.2 Interfacial Design for Efficient Organic Solar Cells

The metal:organic interface plays a crucial role in overall device performance

improvement. Insertion of an interfacial layer between the metal and organics can
dramatically alter the interface properties. They can control the charge transfer and
recombination process and minimize the contact resistance in the device and lower
the series resistance thus improve the FF, thus overall PCE. The processing con-
ditions and wettability between the components are also very important for device
performance improvement. Another important aspect is the charge selectivity at the
electrode interface. The interfacial layer should ensure selective charge extraction by
blocking the flow of unfavorable charge carrier. The interfacial layers can improve
the charge selectivity at the organic: electrode interface by selectively transporting
either electrons or holes, thus acting as an electron-transport layer (ETL) and block
holes to flow through, and a hole-transport layer (HTL) by blocking electron to flow
through (Irwin et al. 2008; Walzer et al. 2007). Till date numerous small molecules
and polymers have been developed and used as interfacial materials in OSCs and
successfully improved the device PCE and most importantly the stability. Below
we will discuss the design and development some efficient CIL and AIL and their
performance in high efficiency OSCs.

18.2.1 Electron Transport Materials as Cathode Interface

Layers

The prerequisite for a cathode interfacial layer is low WF to match with the LUMO
of the organic acceptor for efficient charge extraction, electron transport with hole
blocking properties, lower the energy loss and reduce interfacial defects. The CIL
should have transparent in nature for inverted devices in order for efficient transmis-
sion of light and good stability to prevent metal electrode diffusion in conventional
devices. Till date a variety of ETL, such as low WF metal oxide, composite materials,
organic molecules and polymers, have been used in high performance OSCs. Here
we will focus some of the recent advance in the use of CIL including their devel-
opment and performance in OSC devices and deduce some structure-performance
relationships.

18.2.1.1 Metal Oxides as CIL

n-Type metal oxides such as TiO2 , ZnO, SnOx , Al2 O3 , ZrO2 were used as CIL
materials due to their low lying energy levels (~4.3 to 4.4 eV) (Waldauf et al. 2006;
Trost et al. 2012; Hau et al. 2008; Trost et al. 2015; Jheng et al. 2013). These
materials have good optical transparency in the visible region and efficiently transport
378 A. Mishra

electrons. Recently, some ternary oxides, like Al-doped ZnO, Mg-doped ZnO, Li-
doped ZnO, Ga-doped ZnO, In-doped ZnO) have also emerged as effective materials
for CIL (Shin et al. 2010; Yin et al. 2014; Soultati et al. 2019; Stubhan et al. 2013; Liao
et al. 2014). TiO2 and ZnO, are the most widely used CILs in OSCs because of their
good transparency, environmentally stable, they can be solution-processed, nontoxic
and low cost. The WF of TiOx (−4.3 eV, LUMO ~4.4 eV) and ZnO (−4.3 eV, LUMO
~4.1 eV) are suitable for collecting electron and efficiently block hole. The low WF
of about 4.3 eV is suitable to modify the WF of both the ITO in inverted devices
or metal electrodes in conventional devices. The photovoltaic parameters of some
representative devices using various ETLs are summarized in Table 18.1.
The TiOX layer was prepared by sol-gel method from tetrabutyl titanate in iso-
propanol by spin-coating and efficiently used as CIL in inverted OSCs (Bao et al.
2014; Liu et al. 2012b). It has been found that the precursor and annealing temperature
strongly influence the optoelectronic properties of the TiOX layer. In conventional
device TiOX layer acts as optical spacer to enhance light absorption and also acts as
ETL/HBL. Kim et al. reported that by exchanging the isopropyl ligands of titanium
isopropoxide with 2-methoxyethanol formed an ETLs that need a shorter illumina-
tion time to fill shallow electron traps and improve the PCEs due to increase in both
J SC and V OC (Kim et al. 2013).
Lee et al. presented very stable OSC devices using sol-gel derived TiOX as ETL
in conventional device. Although the devices with and without TiOx generated very
similar PCEs ~4.0%, the air stability with TiOX layer was significantly enhanced by
two orders of magnitude (Lee et al. 2007). Park et al. reported PCEs of 6.1% for
PCDTBT:PC71 BM based devices with internal quantum efficiency close to 100%
using TiOx as CIL (Park et al. 2009). Mor et al. demonstrated a PCE of 4.1%
in P3HT:PC61 BM-based device using vertically aligned transparent TiO2 nanotube
arrays. The pore size of the nanotube was tailored to infiltrate the polymer into the
nanotubes to form self-aligned aggregates. The devices displayed excellent external
quantum efficiency (EQE) up to 80% resulting from efficient charge separation at
both the P3HT-TiO2 and P3HT-PC61 BM interfaces (Mor et al. 2007). Sharma et al.
demonstrated an increased in the PCE from 2.8 to 4.1% by the introduction of TiO2
layer between active layer (P3HT:fullerene free acceptor CSORG5) and Al metal
electrode in conventional device (Sharma et al. 2014).
The performance improvement of TiOX based solar cells generally required light
soaking to improve carrier densities, and reduce oxygen defect. The light soaking
significantly reduced the s-shape of J-V curve enhancing the FF from 0.26 to 0.60
and PCE from 1.3 to 3.3% (Lin et al. 2013). It has been shown that the n-doped TiOX
synthesized via controlling the nitrogen doping concentration in sol-gel synthesis
overcome the light-soaking process. N-doping significantly reduced the WF of TiOX
on the ITO (from 4.8 to 4.2 eV) and improved the Ohmic contact with the active
layer, thus increased the PCE from 2.13% (for undoped TiOX ) to 8.82% without light
soaking (Kim et al. 2015).
Yan et al. reported a light soaking free inverted solar cell by doping TiO2 with
titanium oxide bis(2,4-pentanedionate) (TOPD). The treatment of TiO2 /TOPD film
with UV light and then with ethanolamine eliminate the light soaking of the device
Table 18.1 Summary of photovoltaic performance based on representative metal oxide based CILs
Cathode structure Active layer Anode structure V OC (V) J SC (mA cm−2 ) FF PCE (%) References
ITO/TiOX P3HT/PC71 BM MoO3 /Ag 0.63 11.8 0.63 4.65 Bao et al. (2014)
FTO/TiO2 NT P3HT:PC61 BM PEDOT:PSS/Au 0.64 12.4 0.51 4.07 Mor et al. (2007)
TiO2 /Au P3HT:CSORG5 ITO/PEDOT:PSS 0.98 7.28 0.56 4.16 Sharma et al. (2014)
TiOx/Al P3HT:PC61 BM ITO/PEDOT:PSS 0.62 10.8 0.61 4.10 Lee et al. (2007)
TiOx/Al PCDTBT:PC71 BM ITO/PEDOT:PSS 0.88 10.6 0.66 6.10 Park et al. (2009)
ITO/TiOx PCDTBT:PC71 BM MoO3 /Al 0.85 10.8 0.60 5.50 Liu et al. (2012b)
ITO/TiOx PTB7-Th:PC71 BM MoO3 /Ag 0.79 15.5 0.72 8.82 Kim et al. (2015)
ITO/TiO2 :TOPD PTB7-Th:PC71 BM MoO3 /Ag 0.79 18.8 0.71 10.6 Yan et al. (2017)
(UV and EA treat-
ments)
ITO/TiO2 PTB7:PC71 BM MoO3 /Ag 0.70 14.4 0.64 6.39 You et al. (2012)
18 Interfacial Materials for Organic Solar Cells

ITO/TiO2 -Cs PTB7:PC71 BM MoO3 /Ag 0.72 14.7 0.66 7.01 You et al. (2012)
ITO/ZnO P3HT:PC61 BM PEDOT:PSS/Ag 0.62 11.2 0.54 3.78 Hau et al. (2008)
ITO/ZnO P3HT:PC61 BM Ag 0.57 9.60 0.50 2.70 Takanezawa et al.
(2007)
ITO/ZnO PTB7:PC71 BM MoO3 /Ag 0.72 14.7 0.69 7.34 You et al. (2012)
ITO/ZnO PDTG–TPD:PC71 BM MoO3 /Ag 0.86 14.1 0.67 8.10 Chen et al. (2012)
ITO/ZnO PTB7:PC71 BM MoO3 /Ag 0.71 13.7 0.69 6.71 Lee et al. (2014b)
ITO/ZnO/MeOH PTB7:PC71 BM MoO3 /Ag 0.72 14.7 0.73 7.72 Lee et al. (2014b)
ITO/ZnO/2–ME + PTB7:PC71 BM MoO3 /Ag 0.71 16.8 0.73 8.69 Lee et al. (2014b)
EA(1%)
(continued)
379
Table 18.1 (continued)
380

Cathode structure Active layer Anode structure V OC (V) J SC (mA cm−2 ) FF PCE (%) References
ZnO/Al PTB7:PC71 BM ITO/PEDOT:PSS 0.73 14.6 0.62 6.66 Dkhil et al. (2014)
ZnO/EA(1%)/Al PTB7:PC71 BM ITO/PEDOT:PSS 0.75 15.5 0.66 7.60 Dkhil et al. (2014)
ITO/ZnO PCDTBT:PC71 BM MoO3 /Ag 0.88 10.4 0.69 6.33 Sun et al. (2011)
ITO/ZnO PffBT4T–2OD:TC71 BM MoO3 /Al 0.77 18.8 0.75 10.8 Liu et al. (2014)
ITO/ZnO PffBT4T–2OD:PC71 BM MoO3 /Al 0.77 18.4 0.74 10.5 Liu et al. (2014)
ITO/ZnO PIFTBT8:PC71 BM MoO3 /Ag 1.04 9.74 0.50 5.05 Yin et al. (2013)
ITO/ZnO (patterned) PTB7-Th:PC71 BM MoO3 /Al 0.78 19.5 0.67 10.1 Chen et al. (2015)
ITO/ZnO PTh4 FBT:PC71 BM MoO3 /Ag 0.77 13.5 0.66 6.82 Jheng et al. (2013)
ITO/ZnO + TiO2 PTB7:PC71 BM MoOX /Al 0.74 16.5 0.72 8.82 Li et al. (2014b)
ITO/SnOX P3HT:PC61 BM MoO3 /Ag 0.52 10.4 0.55 3.00 Trost et al. (2012)
ITO/SnOX PCDTBT:PC61 BM MoO3 /Ag 0.91 10.6 0.61 5.90 Trost et al. (2015)
ITO/SnO2 PBDTT-DPP:PC61 BM MoO3 /Al 0.73 11.7 0.61 5.24 Bob et al. (2013)
FTO/TiO2 /Al2 O3 PTB7:PC71 BM MoOX /Al 0.72 14.3 0.69 7.10 Vasilopoulou et al.
(2014)
FTO/TiO2 /ZrO2 PTB7:PC71 BM MoOX /Al 0.71 14.0 0.69 6.90 Vasilopoulou et al.
(2014)
ITO/Cs0.5 MoO3 PBDTDTTT–S–T:PC71 BM MoO3 /Ag 0.61 15.6 0.63 6.00 Li et al. (2014c)
ITO/CsV2 O5 PBDTDTTT–S–T:PC71 BM V2 O5 /Ag 0.63 15.8 0.61 6.08 Li et al. (2014c)
ITO/MoO3 PBDTDTTT–S–T:PC71 BM Cs0.5 MoO3 /Al 0.68 16.1 0.67 7.32 Li et al. (2014c)
ITO/V2 O5 PBDTDTTT–S–T:PC71 BM CsV2 O5 /Al 0.68 16.4 0.67 7.49 Li et al. (2014c)
ITO/Al doped MoO3 PCDTBT:PC71 BM MoO3 /Al 0.89 10.7 0.66 6.28 Liu et al. (2012a)
(continued)
A. Mishra
Table 18.1 (continued)
Cathode structure Active layer Anode structure V OC (V) J SC (mA cm−2 ) FF PCE (%) References
ITO/Mg doped ZnO PTB7:PC71 BM MoO3 /Ag 0.74 16.8 0.67 8.31 Yin et al. (2014)
ITO/Li doped ZnO PTB7-Th:PC71 BM MoO3 /Al 0.80 17.9 0.70 10.1 Soultati et al. (2019)
ITO/Li doped ZnO PTB7-Th:IT-4F MoOX /Al 0.83 16.1 0.67 8.96 Soultati et al. (2019)
ITO/Li doped ZnO PSEHTT:IC60 BA PEDOT:PSS/MoOX /Al 0.94 10.6 0.66 6.59 Yusoff et al. (2014)
ITO/Al doped ZnO PTB7-Th:PC71 BM MoOX /Al 0.80 17.9 0.72 10.4 Liu et al. (2016a)
ITO/In doped ZnO PTB7-Th:PC71 BM MoO3 /Ag 0.79 16.3 0.70 9.11 Liao et al. (2014)
ITO/AZO-Ti P3HT:PC61 BM WOX /Al 0.65 8.66 0.68 3.83 Gadisa et al. (2013)
ITO/ZnO/[BMIM]BF4 PTB7-Th:PC71 BM MoOX /Ag 0.72 17.2 0.74 9.12 Yu et al. (2015)
18 Interfacial Materials for Organic Solar Cells
381
382 A. Mishra

(Yan et al. 2017). Further analysis suggested that the addition of TOPD into the
TiO2 nanoparticle films reduced the WF from 4.43 to 4.23 eV (by Kelvin probe
measurements), which reduced the energy barrier for charge injection from active
layer to electrode. Similar effect was observed for UV and ethanolamine treatment
of TiO2 .
The optical properties of ZnO CILs can be tuned by preparation method, com-
position, film morphology, thickness of the layer. The most widely used preparation
method of ZnO for OSC applications are chemical deposition (Park et al. 2013) sol-
gel processing, (Kyaw et al. 2008; Yin et al. 2013) and nanoparticle approach. The
choice of precursor solution and annealing temperature strongly influence the device
performance of sol-gel prepared ZnO. Takanezawa et al. reported a PCE of 2.7% for
P3HT-PC61 BM devices using ZnO nanorod array as ETL (Takanezawa et al. 2007).
White et al. introduced a solution-processed ZnO as CIL in P3HT-PC61 BM devices
with Ag as hole extraction back contact exhibiting a PCE of 2.58%. (White et al.
2006) The authors suggested efficient electron transfer without loss of energy at the
PC61 BM:ZnO interface due to near equal LUMO of PC61 BM and conduction band
of ZnO. However, stability was the major issue in those devices due to desorption of
oxygen from ZnO in both air and inert atmosphere. Hau et al. revealed an excellent
device stability with PCE of 3.78% by incorporating PEDOT:PSS as AIL in inverted
device structure and ZnO nanoparticle as CIL processed by sol–gel method (Hau
et al. 2008). The PEDOT:PSS worked as an oxygen-diffusion barrier in inverted
devices. In addition, the WF with Ag was modified to −5.0 eV by oxidizing to Ag2 O
in presence of air, thus matching well with the PEDOT:PSS HOMO level (−5.1 eV)
and improve electrical contact at the interface.
You et al. used TiO2 , ZnO, or TiO2 :Cs with similar WF ~4.2 eV as CILs in
inverted device using PTB7:PC71 BM as active layer (You et al. 2012). PCE as high
as 7.3% with high stability have been achieved with ZnO nanoparticle. Chen et al.
reported a PCE of 8.1% using UV ozone treated ZnO nanoparticle as CIL (Chen
et al. 2012). The UV ozone treatment reduced the trap state and surface defects on
the nanoparticle surface.
Lee et al. demonstrated the reduction of energy barrier between the LUMO of
acceptor and conduction band of ZnO by treating the ZnO surface by polar sol-
vents like 2-methoxyethanol (2-ME) + ethanolamine (EA) as co-solvent (Lee et al.
2014b). The polar solvent lowers the contact barrier for electron transport and extrac-
tion, reduce contact resistance thus series resistance and bimolecular recombination
resulting in dramatic improvement in PCE from 6.71 to 8.69% for PTB7:PC71 BM
devices. The PCE improvement was due to increase in J SC and FF. The device with
Methanol treatment also improved the device PCE by about 1% compared to the
untreated ZnO. Using ethanolamine (EA) treated ZnO nanoparticle as optical spacer
and PEDOT:PSS as HTL in PTB7:PC71 BM based conventional devices the PCE
could be increased from 5.8 to 7.6%. (Dkhil et al. 2014) The EA treatment lowered
the WF from ~4.3 to 4.1 eV (calculated from XPS study), which can be assigned
to dipolar polarization of the ZnO surface via the adsorption of EA. The enhance-
ment was attributed to reduced contact barrier, reduced recombination and enhanced
electron extraction at the cathode.
18 Interfacial Materials for Organic Solar Cells 383

Using a low band gap polymer PCDTBT:PC71 BM blend in combination with

low temperature sol-gel prepared ZnO as CIL and MoOx as HTL in an inverted
device reached PCEs up to 6.33%. (Sun et al. 2011) Liu et al. reported further PCE
improvement to 10.8 and 10.5% with high FF of 0.77 using PffBT4T-2OD:TC71 BM
and PffBT4T-2OD:PC71 BM blend and sol-gel processed ZnO prepared from diethyl
zinc as CIL (Liu et al. 2014). Yin et al. demonstrated that using sol-gel processed
ZnO by controlling the film thickness and MoO3 as AIL PCE up to 5.05% could be
achieved with greater device stability compared to without ZnO. The V OC in these
devices was significantly enhanced from 0.22 V without ZnO to 1.04 V with ZnO
(Yin et al. 2013). The use of Cu2 O, NiO and WO3 as AIL showed comparatively lower
performance compared to MoO3 which is ascribed to their energy levels differences,
and interfacial contacts with the active layer materials and metal anode. The starting
precursor materials also make a significant difference in the device performance.
MacLeod et al. revealed that using diethylzinc-derived ZnO as CIL a lower PCE
but more stable device could be obtained compared to the device derived from zinc
acetate precursor (MacLeod et al. 2015).
When a patterned ZnO prepared from sol-gel techniques was employed as CIL
in PTB7-Th:PC71 BM based devices and MoOx /Al as top electrode the PCE could
be increased to 10.1% compared to 8.5% for the reference device without ZnO
(Chen et al. 2015). The improved performance was ascribed to collective effect
of the patterned-induced anti-reflection, light scattering, surface plasmon resonance,
reduced recombination probability and improved charge extraction at the ZnO:active
layer interface.
Using a combination of ZnO nanoparticles and TiO2 nanorods an enhancement in
the PCE (8.82%) could be observed compared to bare ZnO nanoparticles (7.76%) and
TiO2 nanorods (7.66%). This improvement was mainly due to interfacial contact and
reduced contact resistance and leakage current and facilitation of electron collection
and transport efficiency (Li et al. 2014b).
Trost et al. used solution-processed SnOx prepared from tetrakis(diethylamino)tin
as reactive precursor in isopropanol solution as electron extraction layer in inverted
OSC. The WF of SnOx was −4.1 eV similar to that of TiOx (−4.0 eV), thus generating
very similar PCE of around 3.0%. (Trost et al. 2012) However, the SnOx based devices
showed remarkable stability in contrast to TiOx at elevated temperature up to 80 °C in
air without encapsulation. Deposition of SnOX does not required any light soaking.
The deposition of a thin layer of SnOX (>20 nm) significantly change the WF from
4.8 eV of the ITO to the 4.2 eV of the SnOX -coated ITO which is attributed to the
formation of an interface dipole between ITO and SnOX . Using SnOX CILs prepared
by atomic layer deposition (ALD) at 80 °C without UV-treatment a high PCE of
5.9% could be achieved compared to 0.4% for OSCs based on TiOX and reduced the
s-shape for PCDTBT:PC71 BM devices. The results demonstrated that SnOX acts as
barrier-free electron extraction layer without UV-irradiation and does not change its
WF (4.2 eV) upon illumination (Trost et al. 2015). However, a significant lowering
of WF from 4.4 to 3.9 eV was observed for TiOX after illumination.
Yang and co-workers revealed that the use of nanostructured gelled SnO2 could
significantly enhanced the PCE of PBDTT-DPP:PC61 BM-based device to 5.24%
384 A. Mishra

(Bob et al. 2013). The PCE of bulk- or extended-SnO2 as CILs reduced to 0.7%
and 1.45%, respectively due to dramatic reduction in the V OC and FF. The lower
performance for bulk and extended SnO2 were due to rough thin film formation
(relatively large particles) that prevents electron transport and increased the shunting
pathways.
Insulating low WF Al2 O3 or ZrO2 were applied to the TiO2 layer by ALD method
to passivate the surface trap states followed by a downward shift of the conduction
band minimum. A range of polymer donors were tested and highest PCE of 6.9 and
7.1% were achieved for PTB7:PC71 BM device using Al2 O3 and ZrO2 nanolayers due
to significant suppression of charge recombination and enhanced electron extraction
at the TiO2 /Al2 O3 or ZrO2 /organic interface (Vasilopoulou et al. 2014).
By intercalating cesium into the V2 O5 or MoO3 , the WF of metal oxides can be
tuned over a wide range of 1.1 eV thus making them as both ETL and HTL (Li
et al. 2014c). Using MoO3 /V2 O5 and Cs0.5 MoO3 /CsV2 O5 as HTL and ETL, the
PBDTDTTT-S-T:PC71 BM-based devices exhibited PCEs of 6.0 and 6.08%, respec-
tively. The best performance were achieved with the Cs:Mo and Cs:V mole ratios
of 0.5:1. By using with 0.5 wt% of Cs the WF of MoO3 /V2 O5 were tuned from
5.32 eV/5.41 eV to 4.28 eV/4.19 eV, respectively.
Similar to Cs-doping, Al-doping also tuned the WF of MoO3 to acts as CIL.
Liu et al. demonstrated an improvement in PCE for PCDTBT:PC71 BM-based OSCs
using Al-doped MoO3 as CIL and MoO3 /Al as anode. A high FF of 0.66% and a PCE
of 6.28% was achieved. The MoO3 -Al composite films are highly transparent and
exhibit a high WF of 4.09 eV with 55% Al doping compared to 5.5 eV for neat MoO3 ,
thus able to form Ohmic contact with the LUMO of PC71 BM. (Liu et al. 2012a) Li-
doped ZnO (LZO) when used as CIL in PSEHTT:IC60 BA generated a PCE of 6.59%
compared to 5.36% for ZnO. LZO layer was further used as interconnecting layer in
tandem and triple cell generating PCE up to 10.4 and 11.83% (Yusoff et al. 2014).
Using Mg-doped ZnO as CIL, Yin et al. obtained a PCE of 8.31% which was
significantly higher than those of device without CIL (3.5%) or with only ZnO
(7.1%) as CIL. (Yin et al. 2014) The WF can be tuned by changing the amount of Mg
doping and also the device stability improved significantly. The insertion of various
metal carbonates such as Li2 CO3 , K2 CO3 , Na2 CO3 , Cs2 CO3 , and (NH4 )2 CO3 as
gradient doping agent for ZnO layer can improve the electron extraction properties by
modifying the energy levels without making any damage to the ZnO nanostructures
(Nho et al. 2016). When 5 wt% Li-doped ZnO was used as CIL in PTB7-Th:PC71 BM
based devices the PCE was improved from 8.59 to 10.05% (Soultati et al. 2019).
Fullerene-free solar cells based on the PTB7-Th:IT-4F blend exhibited PCEs up
to 8.96% under similar device structure. It has been proposed that the Li ions are
intercalated within the ZnO lattice as interstitial dopants and replace the defects
which acts as trap state in ZnO, improve the electron conductivity and alter the WF
of doped oxide.
The performance improvement was further visible with low temperature pro-
cessed Al-doped ZnO (AZO) CIL. High PCE of 10.42% was achieved with PTB7-
Th:PC71 BM blend device and was found to be insensitive to the thickness of AZO.
Additionally, due to low temperature processing flexible devices on poly(ethylene
18 Interfacial Materials for Organic Solar Cells 385

terephthalate)/indium tin oxide substrates were also prepared showing PCE of 8.93%
(Liu et al. 2016a). The conductivity of AZO was enhanced by three order of mag-
nitude compared to neat ZnO, therefore a thicker film can be prepared with AZO
(above 100 nm) (Stubhan et al. 2011). Similar enhancement in conductivity was also
observed for Ga-doped ZnO and In-doped ZnO thus enabling thicker film formation
up to 200 nm. (Shin et al. 2010; Puetz et al. 2011) The device performance was
improved after doping due to improved electron transport/hole blocking properties.
Liao et al. present a high efficiency and stable inverted OSCs using sol-gel pro-
cessed In–doped ZnO as CIL. The PTB7-Th:PC71 BM device exhibited a PCE of
9.11% compared to 8.25% for undoped ZnO (Liao et al. 2014). The LUMO energy
level lowered from 4.29 to 4.62 eV upon doping. In-doping also improved the electron
mobility from 8.25 × 10−5 to 9.5 × 10−3 cm2 V−1 s−1 and enhanced the conductivity
from 0.015 to 8.51 S cm−1 .
Gadisa et al. prepared AZO nanoparticle stabilized with a TiOX complex. The
P3HT:PC61 BM device prepared with modified CIL and WO3 as AIL generated high
FF of 0.68 due to efficient hole blocking property of AZO-Ti and could solve the issue
related to interfacial recombination in the devices (Gadisa et al. 2013). Brabec and
co-workers improved the device performance of P3HT:PC61 BM device by employ-
ing a C60 self-assembled monolayer (SAM) containing phosphonic acid anchoring
group (Stubhan et al. 2012). The series resistance in the device was reduced keep-
ing the shunt resistance high. The use of ZnO-poly(vinyl pyrrolidone) (PVP) based
composite films prepared using sol-gel method as ETL, demonstrated PCEs >8%
under AM 1.5G illumination at 100 mW cm−2 (Small et al. 2012). ZnO/ionic liquid
composite interlayer was also fabricated as CIL and show improved photovoltaic per-
formance over 9%. The ionic nature of [BMIM]BF4 form interfacial dipole between
active layer and ZnO and facilitate charge transport. (Yu et al. 2015) Electrochemical
impedance spectral analysis showed the reduction of charge transfer resistance from
ZnO to ZnO/[BMIM]BF4 .
It has been realized that the binary and ternary metal oxides as well their doped
state whenever required can be used as efficient CILs in BHJ solar cells. Most impor-
tantly, the low temperature processing methods using sol-gel approach is viable to
apply in other electronic devices including flexible substrate.

18.2.1.2 Organic Molecules as CIL

In this section we will summarize the current development on the use of some selec-
tive organic molecules as CIL and discuss their molecular design and implementa-
tion in OSCs. In order to improve the device performance along with new donor and
acceptor material design, it is important to get control over the interfacial properties.
Toward this goal, many new organic interfacial material systems are being designed
and synthesized to optimize the energy levels of electrodes in order to increase the
device performance. Earlier, BCP has been extensively used as an efficient EBL in
vacuum-processed devices (Peumans and Forrest 2001).
386 A. Mishra

Wide bandgap materials such as bathocuproine (BCP) and bathophenanthroline

(BPhen) were widely used as exciton blocking layer (EBL) to eliminate anomalous
exciton quenching and formed a passivating layer at the organic: cathode interface
(Peumans and Forrest 2001; Vogel et al. 2006; Chan et al. 2006). Gommans et al.
demonstrated a significant enhancement of V OC (from 0.07 V to 0.92 V) by the
incorporation of BCP between the active layer and Al in a planar heterojunction
solar cells containing subphthalocyanine (SubPc)/Buckminsterfullerene(C60 ) active
layer. The built-in potential in the device increased from 0.25 to 0.85 V showing a
PCE improved from 0.05 to 3.0%. BCP layer also acts as optical spacer to improve
the absorption of the active layer (Gommans et al. 2008). Zhao et al. used 4,7-bis(3,5-
bis(trifluoromethyl)phenyl)-2,9-dimethyl-1,10-phenanthroline (BCP-2CF3 ) as EBL
between PCDTBT:PC61 BM device. In comparison to the reference cell (3.3%) with-
out EBL, the use of BCP, BCP-2CF3 improved the PCE to about 4.3 and 4.6% (Zhao
et al. 2016). Furthermore, The BCP-2CF3 sustain to annealing temperature up to
100 °C, while BCP revealed a loss of about 90% to its original PCE. It has been
believe that the diffusion of EBL into the D: A layer is the main reason for device
degradation.

In OSCs the WF of ITO (4.5–4.7 eV) lies between the HOMO and LUMO of con-
jugated organic materials thus can collect either electrons or holes efficiently. Thus,
by using different interfacial layers the polarity of the ITO surface can be tuned to
collect either holes or electrons. Li et al. demonstrated that the ITO surface polar-
ity can be changed by using either cesium carbonate (Cs2 CO3 ) or vanadium oxide
(V2 O5 ) as interfacial layer on ITO in P3HT:PC61 BM blend. A PCE of 2.25% was
reported for ITO/Cs2 CO3 /active layer/V2 O5 /Al inverted device compared to 1.55%
for conventional device using ITO/PEDOT:PSS/active layer/Cs2 CO3 /Al structure (Li
et al. 2006).
Surface modification of interlayer using various self-assembled layers was
also found to be an efficient route for performance improvement by changing
the contact properties, phase morphology and manipulating the barrier height.
Brabec and co-workers used 0.1 wt% polyoxyethylene tridecyl ether (PTE)
as an organic interfacial layer between ITO and TiOx to improve the qual-
ity of the TiOx electron extraction layer. The P3HT:PC61 BM-based BHJ device
(ITO/PTE/TiOx /P3HT:PCBM/PEDOT:PSS/Ag) with modified interface generated
a PCE of 3.6% with improved FF of 0.64 compared to 3.1% (FF = 0.55) without
organic interlayer. Without any ETL the device performance was very poor with
18 Interfacial Materials for Organic Solar Cells 387

PCE of only 1.6% (FF = 0.36). The PTE layer clearly alter the electron selectiv-
ity of ITO/PTE/TiOx electrode. The use of Ag instead of Au top electrode gave
a low absorption loss (Steim et al. 2008). The modification of ZnO surface by
cesium stearate (CsSt) improve the surface microstructure, energy level, conduc-
tivity, exciton generation rate and dissociation probability. Inverted OSCs prepared
using ZnO/CsSt CIL layer in PTB7:PC71 BM devices a high PCE of 8.69% was
achieved which is about 20% higher than the ZnO-only CIL. (Wang et al. 2014a)
In another study an organic dye PAPTA was used as surface modifier for ZnO in
PBDTTT-C-T/PC71 BM device resulting in an improvement in the PCE from 3.27%
for bare ZnO to 7.11% for the modified ZnO with a significant increase in V OC and
J SC values. (Song et al. 2013) The dye layer reduced the leakage current and e− -h+
recombination at the cathode interface by blocking hole injection to ZnO layer.
The interfacial layer between P3HT and TiO2 was modified by cyanoacrylic acid
containing anchoring group in TBTDA (Yu et al. 2012). The surface modification
with TBTDA monolayer increased the electron affinity close to the TiO2 surface and
induced a molecular dipole oriented away from the surface of TiO2 enabling simul-
taneous improvement in V OC and J SC . The PCE was improved from 0.2 to 2.87%.
4-tert-Butyl-pyridine (TBP) was co-deposited to reduce the unfavorable protonation
effect of TiO2 and retard the charge recombination in P3HT/TiO2 interface.

1,4,5,8-Naphthalenetetracarboxylic dianhydride (NTCDA) was also used as n-

type material between organic/Ag interface and prevent electrical shorting of the cell
due to the migration of vacuum-deposited Ag to the organic layer (Suemori et al.
2005; Hiramoto et al. 2006). Singh et al. incorporate BPhen and BCP as additional
buffer layers between P3HT:PC61 BM and LiF/Al (Singh et al. 2016). The device
with BCP gave a PCE of 4.96% due to the combined effects of better hole-blocking
capacity of BCP and low work function provided by LiF/Al. The use of Bphen
and WO3 as EBL and AIL increased the PCE of the CuPc/C60 -based OSC devices
to 3.33% compared to the device without WO3 layer (2.64%). The better electron
transport and exciton blocking ability of Bphen were the major contribution to the
increased device performance. The device without Bphen and WO3 layer generated
very low PCE of 0.84% (Chan et al. 2006). Despite of similar optical transparency,
388 A. Mishra

the electron mobility of Bphen is two order of magnitude higher compared to BCP
thus led to better electron transport.
Zhao et al. used amino-group containing small molecules such as dicyandiamide
(DCDA) and urea as CIL in P3HT:PC61 BM BHJ solar cells. The PCE has been
enhanced from 3.35% for the reference device to 4.25% and 4.39% respectively,
due to increase in J SC and FF values (Zhao et al. 2014). Further investigation of
film morphologies revealed the interfacial dipole formation between the photoactive
layer and Al cathode which might have lowered the WF of Al and facilitate electron
extraction from PCBM. Also the amine groups can coordinate to the Al and prevent
its interaction with P3HT.
Nam et al. successfully used a combination of pyromellitic dianhydride
(PMDA)/LiF as CIL in P3HT:PC61 BM BHJ devices which showed superior photo-
voltaic performance (PCE = 3.9%) compared to the device with single CILs PMDA
(PCE = 1.8%) or with only LiF (PCE = 3.3%) due to reduced leakage current and
series resistance (Nam et al. 2012).
The use of organic dipolar interlayers for interfacial tuning of the electron-
collecting buffer layer and the photoactive layer has been projected as an effi-
cient way to improve the overall device performance. In this respect, various
perylene derivatives have been used as CIL to modify the interface between
cathode and organic layer. Chen et al. demonstrated the modification of ITO
WF by coating with a cationic N,N -bis[2-(trimethylammonium)ethylene]perylene-
3,4,9,10-tetracarboxyldiimide (PDIN+ I− ) and anionic PEDOT:PSS– composite pre-
pared by electrostatic layer by layer (eLbL) deposition technique to prepared
PDIN+ I− :PEDOT:PSS– composite and used as CIL (Fig. 18.3) (Chen et al. 2011).
The interlayer was prepared by sequential deposition of precleaned ITO electrode
into the cationic PDIN+ I− solution for 5 min followed by subsequent dipping in
anionic PEDOT:PSS solutions for 5 min with immediate rinsing steps using water.
Multiple repetition steps gave multilayer films on ITO surface. Odd-even effect of the
interlayer can be clearly visible in the device performance. The WF of the modified
ITO can be varied between 4.35 and 4.60 eV depending on the layer number. Li et al.
obtained a reduction of ITO WF by coating a PDIN+ I− film and used as CIL which

Fig. 18.3 Electrostatic layer by layer (eLbL) deposition technique to prepare multilayers of inter-
facial layer
18 Interfacial Materials for Organic Solar Cells 389

gave improved PCE of 7.0 to 7.18% when used without or with LiF layer than that
of device with bare Al cathode or only LiF/Al (Li et al. 2016).
Gregg and co-workers reported a self-doping, O2 -Stable water soluble dicationic
perylene bis(2-ethyltrimethylammonium hydroxide imide (PETMA+ -OH– ) salt as
CIL (Reilly III et al. 2012). The molecule doped by dehydration and de-doped by
hydration. It has been shown that the conductivity for the doped state could be
significantly increased from 4 × 10−3 to 2 × 10−3 S cm−1 .
Zhang et al. reported a thickness insensitive room temperature processed perylene
derivatives comprising dimethylamino (PDIN) or dimethylamino-N-oxide (PDINO)
as terminal groups and explore their use for CIL (Wang et al. 2014b). PDINO was
synthesized from PDIN by treatment with H2 O2 . The large interfacial dipole formed
by the perylene derivatives induced the vacuum level shift and change the WF of metal
electrode. The LUMO energy levels of PDIN and PDINO are estimated to be –3.63 eV
and –3.72 eV, respectively and close to the PC71 BM, while the HOMO levels are at
−6.05 and −6.21 eV lowered enough to block the holes from the donors. Ultraviolet
photoelectron spectroscopy (UPS) results revealed the shift of the WF of Al from
4.3 to 3.5 eV by deposition of a thin layer (5 nm) of either PDI derivatives. However,
the WF of Ag and Au strongly depend on the type of PDI layer. For both electrodes,
PDIN/PDINO treatment lowered the WF to ~3.93 eV/~3.60 eV allowing them to form
good ohmic contacts with the fullerene acceptor. This change in the WF is related
to the polar nature and dipole formation ability of the terminal amino or amino-
N-oxide groups in PDIN and PDINO. The lowered WFs of PDI/metal cathodes
also increase the built-in field used to break the electrical symmetry inside of the
cells, beneficial for charge extraction and reduce recombination losses. Conventional
device using PDIN/Al or PDINO/Al as CIL and PEDOT:PSS as AIL gave excellent
PCE of 7.68–8.24% comparatively higher than that of Ca/Al CIL (PCE = 6.98%).
Moreover, the perylene derivatives also lowered the WF of Au and Ag metal to use as
cathode. Due to lowering of the WF, Ag or Au were also used as cathode generating
high photovoltaic performance above 7% and high FF close to 0.73.
Min et al. used ZnO/PDINO as CIL in small molecule based OSC devices (BDTT-
S-TR:PC71 BM) and Ag as top electrode. The ZnO/PDINO bilayer not only served
as an effective cathode interlayer but also acts as a protective coating on top of the
active layer. The device gave a PCE of 8.2%. They further fabricate all solution-
processed OSCs using highly transparent Ag nanowire as top electrode resulting
in a PCE of 3.62% (Min et al. 2016). When PDINO interlayer was implemented
in conventional fullerene-free devices PCEs up to 14.04% have been achieved for
PBDB-TF:IDIC-C4Ph blend after thermal annealing at 130 °C (Li et al. 2019).
Yu et al. reported a high performance fullerene-free OSC using PDIN as CIL
and B-DIPDI:PTB7-Th as photoactive layer. PDIN helps in interfacial doping of
B-DIPDI acceptor and facilitate the charge transport and extraction (Yu et al. 2016).
The use of PDIN interlayer as a surface modifier on ZnO resulted in a ≈ 14%
enhancement of the PCE than that of the pristine ZnO-based device. The grazing
incident wide angle X-ray scattering (GIWAXS) study revealed a more crystalline
face-on orientation of the BHJ film achieved due to compatibility of both PDI acceptor
and interlayer. Lin et al. observed a PCE of 6.31% using PDIN CIL PTB7Th:IEIC
390 A. Mishra

based devices (Lin et al. 2015). A polar solvent soluble zwitterion perylene diimide
zwitterion (PDI-z) consisted of sulfobetaine ion as terminal units was prepared and
employed as CIL. (Song et al. 2018) The fullerene-free PBDB-T:IT-M based devices
under optical condition gave a PCE of 11.2%. PDI-z can also found to alter the high
WF metals, such as Au, Cu, Ag, where PDI-z/Ag gave the best PCE of 9.38%.
The device using PDIN as CIL also gave a high PCE of 10.25%. Fullerene-free
ternary solar cells using PDIN interlayer have been fabricated showing excellent
PCE of 11.6% (Hu et al. 2018). The enhanced performance was due to the strong
and complementary absorption of the active layer and interfacial modification by
PDIN layer improving the charge extraction and dissociation.
Sun et al. used a zwitterionic rhodamine 101 as CIL and tested the device perfor-
mance using various metal as top electrodes. The PCDTBT:PC71 BM device with Al
as top electrode gave the best PCE 6.15% compared to 4.57 for Ca/Al or 3.8% for Al
alone (Sun et al. 2012). Similar effect was also observed for other metal electrodes
(Ag, Au, Cu) in combination with rhodamine 101.
By employing a water/alcohol soluble pyridinium salt F8PS (HOMO = 5.71 eV
and LUMO = −3.31 eV) as CIL in PCDTBT:PC71 BM devices simultaneous
improvement of all photovoltaic parameters was observed. The PCE increased from
4.32% for the reference cell to 6.56% and V OC from 0.76 to 0.94 V (Ye et al. 2013).
The V OC improvement was due to the generation of interface dipoles and excellent
electron transfer/collection ability of the hydrophilic pyridinium salt.
Zhang et al. reported two hydrophobic materials FBF-N and FTBTF-N which
served CIL to modulate the atop-BHJ morphology (Zhang et al. 2014). Thus, the
PCE of PTB7:PC71 BM device could be enhanced to 7.97 and 9.22% from 1.18%
for bare ITO without interlayer. The hydrophobic nature of backbone formed better
morphology and helped to improve the J SC and FF.
Vacuum-deposition of N,N-Dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-
C8) over ZnO layer modified the interface properties, reduced the WF of ZnO and
improve device performance of PTB7-Th:PC71 BM device with PCE from 8.26 to
9.29% (Fan et al. 2017).
Ma and co-workers developed a CIL by doping sol-gel derived ZnO with a 1 wt%
perylene derivative PBI-H, which formed an N − Zn bond with ZnO by thermal
treatment and improved the electron mobility by an order of magnitude (5.10 ×
10−4 for neat ZnO to 2.02 × 10−3 cm2 V−1 s−1 ). (Nian et al. 2015) The conductivity
also improved to 4.50 × 10−3 S m−1 . OSC devices with PTB7:PC71 BM and PTB7-
Th:PC71 BM photoactive layer generated an excellent PCEs of 9.01% and 10.5%,
respectively, which is independent of CIL thickness and higher than the device with
pure ZnO (7.4 and 8.3%). The WF of modified ITO/ZnO:PBI-H decreased to 3.8 eV
compared to 4.2 eV for bare ITO/ZnO indicating increased electron population on
the conduction band of ZnO and also pinned the WF of ITO near LUMO of PC71 BM
forming good Ohmic contact with PC71 BM. PBI-H blocks holes due to low lying
HOMO energy level. Furthermore, the PBI-H interlayer form self-assembled J-type
aggregate with absorption maximum at 650 nm, 80 nm red-shifted than the monomer
and formed nanofibril networks by non-covalent interactions efficient for charge
transport (Xie et al. 2015).
18 Interfacial Materials for Organic Solar Cells 391

Hou and co-workers synthesized a water soluble interfacial material NDIO which
showed high transparency in the visible region, suitable work-function, low rough-
ness for excellent interface contact and aqueous processability. Incorporation of
NDIO in PBDT-TS1:PC71 BM based inverted devices gave high PCE of 9.51%, which
was significantly higher compared to the device without CIL (PCE = 5.33%) (Zhao
et al. 2015). Using ZnO or PEIE as CIL instead of NDIO in PBDT-TS1/PCBM-based
devices gave comparable PCEs of 9.67% and 9.32%, respectively.
Cao and co-workers used a star-shaped triazine based CIL TzPyBr containing
terminal pyridinium moieties. Inverted devices using PTB7:PC71 BM photoactive
layer gave a PCE of 6.84% almost double than that of reference device (3.5%) (Chen
et al. 2014). The low-lying HOMO energy level of TzPyBr effectively blocks the
holes travelling towards cathode and improve directional charge transport.
Recently, Wang et al. developed a series of star-shaped triphenylamine-based
materials (namely, TFN, TFB, TFO, TFS) containing various polar pendant
groups for CIL (Wang et al. 2016). The compounds were synthesized via Suzuki
cross-coupling reactions of boronic ester of triphenylamine with different bromo
derivatives. All compounds showed very similar HOMO/LUMO energy levels
(~−5.45/~−2.5 eV). Conventional OSCs with PTB7:PC71 BM using these CILs
exhibited PCE improvement in the order as cast (6.8%) < methanol (8.0%) < TFN
(8.6%) < PFN (8.7%) < TFS (8.7%) < TFO (9.7%) < TFB (10.1%). The results
showed that TFB as the best cathode modifier reaching highest PCE of 10.1%. The
392 A. Mishra

pendant ionic groups (quaternary ammonium bromide, amino N-oxide, and sulfobe-
taine ion) of ETMs formed strong interaction with Al electrode and induced dipole
orientation towards active layer, thus, reduces the Al WF (from -4.3 eV to between
−3.79 and −4.06 eV), lowering the energy barrier at the active layer: Al interface
and increased the built-in potential (V bi ) to improve the V OC values. It has also been
shown that the mobile Br− counterions of TFB can easily absorbed on the Al elec-
trode and form the stronger and more regular dipole moments than those zwitterionic
TFO and TFS with immobile counterions linked via covalent bond.
Wang et al. fabricated a CIL using a tetraphenylethylene-based small molecule
TPE-2 terminated with zwitterionic aminopropane sulfonate groups (Wang et al.
2017). TPE-based molecule has strong tendency to show aggregation-induced emis-
sion (AIE) effect. PTB7:PC71 BM-based conventional device with TPE-2/Al cathode
gave PCE of 8.94% compared to 3.89% for bare Al, 7.31% for Ca/Al, 7.83% for
MeOH/Al and 8.33% for PFN/Al. This strong improvement of PCE was attributed
to the well-organized lamellar structure, resulting from the self-assemble property
of TPE-2.

A series of benzophenone-based molecular CILs with different polar terminal

groups comprising hydroxyl (BPOH), neutral amino (BPN), amino N-oxide (BPO),
and sulfobetaine (BPS) ions were prepared. and used as CIL between PTB7:PC71 BM
and Al electrode. OSCs with solution-processed BPO-based interlayer showed a PCE
of 9.34% with the highest J SC and FF due to decreased series resistance and charge
recombination compared to the devices with BPN, BPOH and BPS interlayers. It was
18 Interfacial Materials for Organic Solar Cells 393

further noted that the methanol treatment plays an important role in performance
improvement (PCE = 7.97%) by increasing the V bi and passivating surface traps
(Liu et al. 2017).
Fused ring quinacridone derivatives were developed and tested as CIL materials.
Wudl and co-workers reported a water/alcohol soluble quinacridone-based molecule
QHSO3 Na which when used as CIL increased the device PCE from 4.34 to 5.17%
(Pho et al. 2011). A significant improvement in the FF from 0.53 to 0.63 was observed,
while the V OC and J SC values were similar to the reference cell. Chen et al. demon-
strated an improvement in the PCE (6.70%) of PCDTBT:PC71 BM devices using
4 nm thin QAPyBr CIL containing pyridinium pendent groups due to significant
increase in the FF along with V OC and J SC higher than the Al-based devices (5.13%)
(Chen et al. 2016). Further increase in CIL thickness (~20 nm) the PCE dropped to
around 1.7% due to significant reduction in the J SC and FF. Interestingly, the strong
electron accepting group dicyanomethylene group containing CIL, DCNQAPyBr
showed further improvement in the PCE due to enhanced electron transport ability
and conductivity as well insensitive to thickness of the CIL. The best device gave a
PCE of 6.96% using 13 nm thick DCNQAPyBr. The device gave maintain high PCE
of 5.8% even up to 40 nm thickness showing the importance of cyano groups on the
device performance improvement.
Indacenodithiophene (IDT), which has been successfully employed to develop
various fullerene-free acceptors, was used to synthesize two new cationic electrolyte
TBIDTD and TBIDTCN for CIL. Among the two CILs TBIDTCN gave the better
performance with PCE of 9.19% compared to 8.62% for TBIDTD due to dramatic
reduction of series resistance, lower LUMO energy level for efficient electron trans-
port and extraction (Miao et al. 2018). The reference device gave 1.66 times lower
performance.

18.2.1.3 Fullerene Derivatives as CIL

Various water/alcohol soluble fullerene derivatives have been prepared to use them
as CIL and reviewed recently (Lai et al. 2014; Chueh et al. 2015). Due to similar
energy levels with respect to PC61 BM/PC71 BM they have been widely employed
as CIL to modify the surface properties and enhance the photovoltaic performance.
Some representative examples are discussed below and the photovoltaic parameters
are presented in Table 18.2.
Zhang et al. synthesized an alcohol soluble poly(ethylene glycol) end-capped
fullerene derivative, PEGN-C60 , for its used as EBL with different active layers, such
as P3HT:PC61 BM, PBDTTT-C:PC71 BM, and PBDTTT-C-T:PC71 BM. The optimal
PCEs of the these devices reached values of up to 3.84%, 6.22%, 7.45%, respectively,
and close to the values obtained using Ca/Al based CIL and significantly higher
compared to without EBLs (Zhang et al. 2013).
Li and co-workers prepared two fullerene derivatives namely, PCBDAN and PCB-
DANI and studied their ability to act as CIL (Li et al. 2013). PCBDANI was synthe-
sized by the reaction of PCBDAN with methyl iodide. These CILs formed interfacial
Table 18.2 Summary of photovoltaic performance based on representative small molecule CILs
394

Cathode structure Active layer Anode structure V OC (V) J SC FF PCE (%) References
(mA cm−2 )
BCP/Al SubPc/C60 ITO 0.92 5.42 0.60 3.03 Gommans et al. (2008)
BCP/LiF/Al P3HT:PC61 BM ITO/PEDOT:PSS 0.60 13.5 0.61 4.96 Singh et al. (2016)
ITO/PETMA+ -OH− P3HT:PC61 BM V2 O5 /Al 0.51 9.87 0.51 2.52 Reilly III et al. (2012)
PDIN+ I− /LiF/Al PBTI3T: PC71 BM ITO/PEDOT:PSS 0.86 11.2 0.75 7.18 Li et al. (2016)
PDINO/Al PTB7:PC71 BM ITO/PEDOT:PSS 0.75 15.9 0.73 8.24 Wang et al. (2014b)
PDIN/Al PTB7:PC71 BM ITO/PEDOT:PSS 0.76 14.3 0.71 7.68 Wang et al. (2014b)
ZnO/PDINO/Ag BDTT-S-TR:PC71 BM ITO/PEDOT:PSS 0.96 13.1 0.65 8.21 Min et al. (2016)
PDINO/Al PBDB-TF:IDIC-C4Ph ITO/PEDOT:PSS 0.94 19.1 0.78 14.0 Li et al. (2019)
ITO/ZnO/PDIN PTB7-Th:B-DIPDI MoO3 /Ag 0.78 13.0 0.62 6.29 Yu et al. (2016)
PDIN/Al PBDB-T:IT-M ITO/PEDOT:PSS 0.94 16.1 0.68 10.3 Song et al. (2018)
PDIN-z/Al PBDB-T:IT-M ITO/PEDOT:PSS 0.94 16.1 0.74 11.2 Song et al. (2018)
PDIN/Al J71:IT-M:ITIC ITO/PEDOT:PSS 0.98 18.1 0.66 11.6 Hu et al. (2018)
Rhodamine 101/Al PCDTBT:PC71 BM ITO/PEDOT:PSS 0.94 11.1 0.59 6.15 Sun et al. (2012)
F8PS/Al PCDTBT:PC71 BM ITO/PEDOT:PSS 0.94 11.3 0.62 6.56 Ye et al. (2013)
ITO/FBF-N PTB7:PC71 BM MoO3 /Al 0.75 15.5 0.68 7.97 Zhang et al. (2014)
ITO/FTBTF-N PTB7:PC71 BM MoO3 /Al 0.74 17.2 0.72 9.22 Zhang et al. (2014)
ITO/ZnO/PTCDI-C8 PTB7-Th:PC71 BM MoO3 /Ag 0.81 18.3 0.63 9.29 Fan et al. (2017)
ITO/ZnO:PBI-H PTB7:PC71 BM MoO3 /Al 0.75 17.2 0.70 9.01 Nian et al. (2015)
(continued)
A. Mishra
Table 18.2 (continued)
Cathode structure Active layer Anode structure V OC (V) J SC FF PCE (%) References
(mA cm−2 )
ITO/ZnO:PBI-H PTB7-Th:PC71 BM MoO3 /Al 0.82 17.5 0.73 10.5 Nian et al. (2015)
ITO/NDIO PBDT-TS1:PC71 BM MoO3 /Al 0.79 18.02 0.67 9.51 Zhao et al. (2015)
ITO/TzPyBr PTB7:PC71 BM MoO3 /Al 0.70 15.6 0.62 6.84 Chen et al. (2014)
TFN/Al PTB7:PC71 BM ITO/PEDOT:PSS 0.73 15.6 0.72 8.60 Wang et al. (2016)
TFB/Al PTB7:PC71 BM ITO/PEDOT:PSS 0.78 17.6 0.74 10.1 Wang et al. (2016)
TFO/Al PTB7:PC71 BM ITO/PEDOT:PSS 0.76 17.4 0.73 9.70 Wang et al. (2016)
TFS/Al PTB7:PC71 BM ITO/PEDOT:PSS 0.75 16.4 0.71 8.70 Wang et al. (2016)
PFN/Al PTB7:PC71 BM ITO/PEDOT:PSS 0.75 17.1 0.68 8.70 Wang et al. (2016)
TPE-2/Al PTB7:PC71 BM ITO/PEDOT:PSS 0.76 16.8 0.70 8.94 Wang et al. (2017)
18 Interfacial Materials for Organic Solar Cells

BPN/Al PTB7:PC71 BM ITO/PEDOT:PSS 0.76 16.9 0.70 8.98 Liu et al. (2017)
BPO/Al PTB7:PC71 BM ITO/PEDOT:PSS 0.76 17.1 0.72 9.34 Liu et al. (2017)
BPS/Al PTB7:PC71 BM ITO/PEDOT:PSS 0.76 16.9 0.70 9.03 Liu et al. (2017)
QHSO3 Na/Al PCDTBT:PC71 BM ITO/PEDOT:PSS 0.84 9.83 0.63 5.17 Pho et al. (2011)
QAPyBr/Al PCDTBT:PC71 BM ITO/PEDOT:PSS 0.90 11.2 0.66 6.70 Chen et al. (2016)
DCNQAPyBr/Al PCDTBT:PC71 BM ITO/PEDOT:PSS 0.91 11.3 0.68 6.96 Chen et al. (2016)
TBIDTD/Al PTB7:PC71 BM ITO/PEDOT:PSS 0.77 16.2 0.69 8.62 Miao et al. (2018)
TBIDTCN/Al PTB7:PC71 BM ITO/PEDOT:PSS 0.77 16.6 0.72 9.19 Miao et al. (2018)
PEGN-C60 /Al P3HT:PC61 BM ITO/PEDOT:PSS 0.63 9.21 0.67 3.84 Zhang et al. (2013)
(continued)
395
Table 18.2 (continued)
396

Cathode structure Active layer Anode structure V OC (V) J SC FF PCE (%) References
(mA cm−2 )
PEGN-C60 /Al PBDTTT-C:PC71 BM ITO/PEDOT:PSS 0.74 14.0 0.61 6.22 Zhang et al. (2013)
PEGN-C60 /Al PBDTTT-C-T:PC71 BM ITO/PEDOT:PSS 0.79 14.8 0.63 7.45 Zhang et al. (2013)
PCBDAN/Al PBDTTT-C-T/PC71 BM ITO/PEDOT:PSS 0.78 17.4 0.57 7.70 Li et al. (2013)
PCBDANI/Al PBDTTT-C-T/PC71 BM ITO/PEDOT:PSS 0.78 17.3 0.57 7.69 Li et al. (2013)
C60 -N/Ag PTB7-Th:PC71 BM ITO/PEDOT:PSS 0.78 16.8 0.71 9.35 Page et al. (2014)
C60 -SB/Ag PTB7-Th:PC71 BM ITO/PEDOT:PSS 0.75 16.9 0.68 8.57 Page et al. (2014)
ITO/C60 -SB PTB7-Th:PC71 BM MoO3 /Ag 0.75 18.2 0.66 9.08 Liu et al. (2015a)
ITO/B-PCPO PCDTBT:PC71 BM MoO3 /Al 0.89 9.50 0.62 6.20 Duan et al. (2012)
ITO/ZnO-C60 PTB7:PC71 BM MoO3 /Ag 0.73 15.4 0.73 8.21 Liao et al. (2013)
ITO/ZnO-C60 PTB7-Th:PC71 BM MoO3 /Ag 0.80 15.7 0.74 9.35 Liao et al. (2013)
ITO/ZnO-C60 PBDT-BT:PC71 BM MoO3 /Ag 0.92 15.4 0.66 9.40 Subbiah et al. (2015)
A. Mishra
18 Interfacial Materials for Organic Solar Cells 397

dipole between the active layer and Al metal and demonstrated very similar OSC per-
formance with PCEs around 7.7% for PBDTTT-C-T/PC71 BM devices and showed
greater stability compared to Ca/Al or only Al.
The trade-off between stability due to oxidation and WF of electrode is a major
challenge in the field of OSCs. Page et al. used amine-functionalized C60 -N and
sulfobetaine-substituted zwitterionic C60 -SB as cathode independent buffer layer
in conventional device (Page et al. 2014). Specifically, using a thin layer of C60 -
N the effective WF of Ag, Cu, and Au electrodes can be significantly reduced to
3.65 eV. C60 -SB was prepared from C60 -N by reaction with 1,3-propanesultone via
ring opening. The insertion of C60 -N between active layer and metal electrodes
resulted in good Ohmic contact for electron injection, and a large built-in potential
difference for efficient charge extraction in OSCs. Devices with devices with C60 -N or
C60 -SB interlayers yielded PCE values 9.35% and 8.57%, respectively. Lower PCEs
were obtained for devices with Al (8.65%), Cu (8.67%) or Au (8.56%) cathodes,
relative to Ag (9.35%) cathodes, which can be credited to the lower reflectivity of
Cu and Au.
Russell and co-workers further implemented C60 -SB as CIL in an inverted device,
which showed exceptional improvement of PCE to 9.23% for PTB7-Th:PC71 BM
blend compared to 1.96% with bare ITO (Liu et al. 2015a). The CIL also acts as an
electron acceptor and modified the WF of ITO from 4.5 eV to ~ 4.0 eV as measured by
UPS. The highest performance was achieved with the CIL thickness of ~40 nm and
remain above 8.0% for the entire thickness range above 10 nm. The work function
modification of ITO by C60 -SB may arised from orientation of the permanent dipole
at the interface, due to the preferential interactions of the sulfobetaine zwitterion
with the ITO surface.
Cao group developed an alcohol soluble phosphate-containing fullerene bis-
adducts, B-PCPO as CIL in inverted OSCs to improve the electron transport and
collection efficiency (Duan et al. 2012). It was observed that the B-PCPO interlayer
could effectively decrease the WF of ITO to 3.9 eV and thereby enhance the electron
collection at the ITO electrode. The device resulted in PCE increased from 4.83 to
6.20% by using a B-PCPO interlayer, compared to 5.31% for ZnO and 4.83% for
device without interlayer.
Chen et al. developed a CIL ZnO-C60 by reacting zinc acetate with hydroxyl
containing fullerene derivative PCBE-OH (Liao et al. 2013). This ZnO-C60 CIL pro-
vides dual functionalities for enhanced electron collection, producing a fullerene-
rich cathode surface and promote compatibility of the BHJ layer at the interface.
The C60 -doped ZnO lower the LUMO level to −4.53 eV compared to pristine ZnO
(−4.1 eV) promoting better electron transport pathways. Inverted BHJ devices com-
prising PTB7-Th:PC71 BM blend exhibited excellent PCEs of 9.35% higher than that
bare ZnO. Subbiah et al. reported a PCE of 9.4% using a ZnO−C60 interlayer at the
cathode contact (Subbiah et al. 2015). The surface modification eliminates of oxy-
gen vacancies on the ZnO surface that can act as electron traps and reduced carrier
recombination.
398 A. Mishra

In summary, hydrophilic fullerene derivatives would be a promising family of

interfacial materials for highly efficient OSCs. Although the synthesis and purifica-
tion of these fullerene derivatives somehow complicated and for their implementation
as efficient CIL efficient preparation methods need to be developed.

18.2.1.4 Organic Polymers as CIL

The use of water/alcohol soluble polymers as CIL was widely interpreted as inter-
facial dipole formation model (He et al. 2011b, 2012). These CILs can effectively
modify the WF of the metal cathode by forming interfacial dipole between the oganic:
metal interface, thus improve the electron injection and transport in the device (Duan
et al. 2013c; Hu et al. 2014; Tang et al. 2014). Various water soluble non-conjugated,
such as PEIE and PEI as cathode modifier and π-conjugated polymers such as PFN,
PFN-Br and others have effectively implemented as CILs in various OSC devices
(He et al. 2011a, 2012; Zhou et al. 2012a; Kang et al. 2012). The conjugated chain
improve the delocalization of electrons and the polar pendant groups makes them
process from polar water/alcohol, thus does not affects the active layer processed
from complementary organic solvents (Duan et al. 2013c). The shift of the vacuum
level by interfacial dipoles could increase the V bi of the device and thus increases the
V OC . Depending on the types of interfacial materials the work function of ITO can be
reduced by 0.3−0.6 eV. The increased Vbi does not alter the energetics of the active
layer and therefore improvement in all photovoltaic parameters can be expected. The
photovoltaic data are presented in Table 18.3.
Shao et al. used a poly(ethylene oxide) (PEO)-modified ZnO composite as an
ETL (Shao et al. 2013). The surface modification using 0.05% of PEO effectively
passivate the surface trap state in ZnO, thus suppressed the recombination losses,
reduced series resistance, thus enhanced the PCE to 5.64 and 6.59% for TQ1:PC71 BM
and PCDTBT:PC71 BM, respectively. Using polyallylamine (PAA) as polyelectrolyte
Table 18.3 Summary of photovoltaic performance based on representative polymer CIL
Cathode structure Active layer Anode structure V OC (V) J SC (mA cm−2 ) FF PCE (%) References
ITO/PEO:ZnO TQ1:PC71 BM MoO3 /Ag 0.88 9.60 0.67 5.64 Shao et al. (2013)
ITO/PEO:ZnO PCDTBT:PC71 BM MoO3 /Ag 0.88 11.4 0.65 6.59 Shao et al. (2013)
ITO/ZnO/PEIE p–DTS(FBTTh2 )2 :PC71 BM MoO3 /Ag 0.77 15.2 0.67 7.88 Kyaw et al. (2013)
ITO/PEIE protonated PTB7-Th:IEICO-4F MoO3 /Ag 0.70 27.2 0.69 13.2 Xiong et al. (2019)
MSAPBS/Al PTB7:PC71 BM ITO/PEDOT:PSS 0.76 19.2 0.68 10.0 Ouyang et al.
(2015)
DSAPS/Al PTB7:PC71 BM ITO/PEDOT:PSS 0.76 18.2 0.70 9.79 Liu et al. (2016b)
ZnO/DSAPS PTB7:PC71 BM MoO3 /Al 0.75 17.0 0.70 9.10 Liu et al. (2016b)
ITO/PEI/F16 CuPc PTB7:PC71 BM MoO3 /Ag 0.72 20.2 0.66 9.51 Li et al. (2018b)
ITO/ZnO/APTES PTB7-Th:PC71 BM MoO3 /Ag 0.78 17.1 0.71 9.46 Fu et al. (2017)
18 Interfacial Materials for Organic Solar Cells

ITO/ZnO/APTMS PTB7-Th:PC71 BM MoO3 /Al 0.80 16.7 0.68 9.07 Wei et al. (2018)
ITO/ZnO/PDEPB PTB7:PC71 BM MoO3 /Ag 0.73 16.0 0.70 8.14 Nam et al. (2017)
ITO/ZnO/PDEPB PTB7-Th:PC71 BM MoO3 /Ag 0.79 18.6 0.71 10.4 Nam et al. (2016)
ITO/ZnO/PPy1 PTB7:PC71 BM MoO3 /Ag 0.75 15.3 0.72 8.29 Aryal et al. (2018)
ITO/ZnO/Ppy1 PTB7-Th:PC71 BM MoO3 /Ag 0.81 17.1 0.68 9.37 Aryal et al. (2018)
ITO/PEI/SnO2 /PFN PCDTBT:PC71 BM MoO3 /Ag 0.85 14.2 0.60 7.18 Guo et al. (2019)
ITO/P3TBPHT:PEDOTPSS PCDTTPD:PC71 BM V2 O5 /Al 0.91 11.2 0.55 5.60 Worfolk et al.
(2012)
P3TMAHT:SDS/Al P3HT:PC71 BM ITO/PEDOT:PSS 0.62 9.54 0.68 4.01 Chang et al. (2012)
(continued)
399
Table 18.3 (continued)
400

Cathode structure Active layer Anode structure V OC (V) J SC (mA cm−2 ) FF PCE (%) References
P3IMDHT/Al PCDTBT:PC71 BM ITO/PEDOT:PSS 0.87 12.0 0.59 6.69 Kesters et al.
(2013)
P3TMAHT/Al PCDTBT:PC71 BM ITO/PEDOT:PSS 0.88 11.8 0.58 6.48 Kesters et al.
(2013)
ITO/P3IMDHT PCDTBT:PC71 BM MoO3 /Ag 0.84 11.2 0.51 4.80 Zilberberg et al.
(2013)
PCDTBT-N/Al PCDTBT:PC71 BM ITO/PEDOT:PSS 0.92 10.2 0.57 5.32 Duan et al. (2013a)
ITO/ZnO/PBN PTB7:PC71 BM MoO3 /Ag 0.75 15.8 0.72 8.60 Kim et al. (2014)
PFN/Ca/Al PTB7:PC71 BM ITO/PEDOT:PSS 0.76 15.4 0.71 8.20 He et al. (2012)
ITO/PFN PTB7:PC71 BM MoO3 /Al 0.74 17.2 0.72 9.20 He et al. (2012)
PFNSO/Al PTB7:PC71 BM ITO/PEDOT:PSS 0.73 16.4 0.73 8.74 Duan et al. (2013b)
ITO/PFN-OX PBDT-DTNT: PC71 BM MoO3 /Ag 0.74 17.6 0.66 8.62 Hu et al. (2014)
PFN/Al PCDTBT:PC71 BM ITO/PEDOT:PSS 0.91 12.1 0.62 6.73 He et al. (2011b)
ITO/ZnO/PFN PTB7:PC71 BM:ICBA MoO3 /Ag 0.77 16.5 0.72 9.30 Han et al. (2016)
PFN-Br/Ag PTB7:PDIBDT-IT ITO/PEDOT:PSS 0.74 13.6 0.60 6.06 Liu et al. (2018)
ITO/ZnO/PFN-Br PBDB-TF:IT-4F MoO3 /Al 0.87 20.2 0.79 13.8 Zheng et al. (2019)
PFN/Al P1:DCI-2 ITO/PEDOT:PSS 0.8 13.8 0.63 6.94 Mishra et al.
(2017)
(continued)
A. Mishra
Table 18.3 (continued)
Cathode structure Active layer Anode structure V OC (V) J SC (mA cm−2 ) FF PCE (%) References
PFN/Al TPA-SN5-DCV:PC71 BM ITO/PEDOT:PSS 0.74 14.2 0.69 7.24 Mishra et al.
(2018)
ITO/PFS PDBT-TS1:PC71 BM PFS/Al 0.78 18.0 0.66 9.48 Xu et al. (2016)
PFN/Ag PBDB-T:ITIC ITO/PEDOT:PSS 0.91 15.8 0.58 8.60 Sun et al. (2017)
PFN-2TNDI/Ag PBDB-T:ITIC ITO/PEDOT:PSS 0.92 16.6 0.70 11.1 Sun et al. (2017)
PT2 NDISB/Ag PBDTT-TT:PC71 BM ITO/PEDOT:PSS 0.76 18.8 0.70 10.1 Liu et al. (2015b)
ITO/PNDIT10 N PTB7-Th:PC71 BM MoOx/Ag 0.73 16.4 0.55 7.0 Bjuggren et al.
(2018)
18 Interfacial Materials for Organic Solar Cells
401
402 A. Mishra

PCE of 6.3% was achieved with poly(thieno[3,4-b]thiophene-alt-benzodithiophene)

(PTB):PC71 BM devices. The lowering of the ITO WF was originated from the
strong electrostatic self-assembled dipoles formed by the presence of protonated
amines within the ITO/PAA cathodes, forming a good Ohmic contact with LUMO
of PC71 BM (Kang et al. 2012).
A ionic polyacetylene-based conjugated polymer electrolyte poly(N-dodecyl-2-
ethynylpyridiniumbromide) (PDEPB) was developed and used as interfacial dipole
layer to modify the ZnO surface and reduced its WF. Performance improvement was
observed for the ZnO/PDEPB modified interlayer due to lowering of interfacial resis-
tance (probed by impedance spectroscopy measurement) and surface defects on ZnO
(Nam et al. 2017). PDEPB layer lowered the WF of the electron transporting ZnO
layer (by UPS measurement) and increased the built-in potential, hence enabling
efficient charge transport/extraction. The PCE was further improved to over 10%
using PTB7Th:PC71 BM as photoactive layer and ZnO/PDEPB (0.5 mg/mL) inter-
layer (Nam et al. 2016). The increase in the V OC with PDEPB interlayers can be due
to the increased V bi caused by the dipolar interactions between PDEPB and ZnO,
basically contributing to accelerate the charge transport process.
Aryal et al. synthesized a series of polyacetylene-based pyridinium salts (PPy1-
PPy3) and used as cathode modifier of ZnO layer. Interfacial modification improved
the PCE from 7.46% for a ZnO-based control device to ~8.3% for PTB7:PC71 BM
and to ~9.3% for PTB7-Th:PC71 BM based devices (Aryal et al. 2018). Incorporation
of CIL suppressed the charge recombination and thus facilitated charge extraction.
The interlayer also protect the device from oxygen and humidity contamination and
improved the device stability (88% retention after ~1000 h).
After the successful use of non-conjugated polyelectrolytes PEI and PEIE as
neutral surface modifier by Kippelen and co-wokers, these materials have found
significant importance in the single junction and tandem photovoltaic device fabri-
cations (Zhou et al. 2012a, b). These materials successfully tune the WFs of metal
electrodes, reduce the interfacial energy barrier and increase the build-in potential.
For example the WF of ITO can be reduced from ~4.4 to ~3.3 eV (measured by
UPS) by depositing thin film of solution-processed PEI or PEIE, originating from
the strong electrostatic self-assembled dipoles formed due to polar amine groups.
Heeger and Bazan group introduced 80% ethoxylated PEIE on the top of the
ZnO forming a composite film to enhance the device efficiency by lowering the
work function of ZnO (Kyaw et al. 2013). Inverted OSCs using a small molecule
p-DTS(FBTTh2 )2 :PC71 BM active layer demonstrated PCEs up to 7.88% using
ZnO/PEIE CIL which is higher compared to bare ZnO (6.29%) or PEIE (5.18%).
PEIE coating on ZnO lowered the WF of ZnO from 4.5 to 3.8 eV by creating a
dipole moment at the interface and thus supresses the trap-assisted recombination.
The increased V OC for ZnO/PEIE based devices (0.77 V) compared to bare ZnO
(0.72 V) can be attributed to the reduced WF of cathode after PEIE deposition.
Recently, Zhou and co-workers demonstrated that the PEIE interlayer can reduce
the performance of fullerene-free devices by reacting with the acceptor molecules
with amine leading to s-shaped J-V curve. The authors deactivate the chemical inter-
action by protonating the PEIE electrolyte processing from aqueous solution. OSCs
18 Interfacial Materials for Organic Solar Cells 403

using PTB7-Th:IEICO-4F active layer and the protonated PEIE interfacial layer
exhibited an excellent PCE of 13.2%, which is higher than the reference devices
with ZnO interlayer (12.6%). Also the device processed from water eliminates fur-
ther thermal annealing treatment (Xiong et al. 2019). It is important to note that the
PEIE processed from alcohols such as isopropanol, ethanol, methanol gave <10%
PCE due to low FF of 0.53–0.57 arising from s-shape character of the J-V curve.

ooseness-1Ge and co-workers reported a non-conjugated zwitterionic MSAPBS

which was used to modify the Al cathode and offer good ohmic con-
tact for photogenerated charge carrier collection. Conventional device using
ITO/PEDOT:PSS/PTB7:PC71 BM/MSAPBS/Al device structure gave a high PCE
of 10.0%, much higher compared to PFN/Al (8.2%) and Ca/Al (7.99%) cathode
(Ouyang et al. 2015). It has been shown that treatment of zwitterionic interlayer
DSAPS dissolved in methanol eliminate the trade-off between V OC and J SC . The
MeOH treatment increased the built-in voltage and surface potential, thus contributes
to the V OC enhancement and the CIL improve the charge transport and thus the J SC
and FF. The PCE of both conventional and inverted devices with PTB7/PC71 BM
blend improved from 3.89 to 9.79% and 7.34 to 9.10% (Liu et al. 2016b).
Li et al. adopted a vacuum-evaporation method to deposit fluorinated copper
phthalocyanine (F16 CuPc) layer over the ITO/PEI buffer layer to modify the surface
properties. The composite layer increase the electronic coupling between the ITO
and organic layer and induce forward charge transfer by decreasing recombination
of charges (Li et al. 2018b). The F16 CuPc also regulates the growth of organic layer
and control morphology. Optimized inverted device displayed the best PCE of 9.52%
with 6 nm thick F16 CuPc layer over PEI compared to 7.147% of the device without
F16 CuPc. The PEI layer lowered the WF of ITO and reduce the Schottky barrier. The
introduction of F16 CuPc decreases the potential difference between ITO and active
layer and helps to transport electrons to ITO cathode and also acts as hole blocking
layer.
Fu et al. developed a modified ZnO ETL by surface ligand exchange of the
hydroxyl group with 3-aminopropyl triethoxysilane (APTES) (Fu et al. 2017). The
silane group can form crosslinking and modify the metal oxide surface by covalent
bonding. The ZnO/APTES modified CIL gave an improved PCE of 9.46% for device
404 A. Mishra

based on PTB7-Th/PC71 BM active layer, a higher value than the control device with-
out linker (8.47%). The aminosilane linker plays dual function to enhance the PCE,
which includes (1) passivating the ZnO surface via silane groups and decreasing
the surface WF of ZnO to 3.9 eV and (2) interaction with active layer to reduce the
interface contact resistance. Recently, Ma and co-workers used the similar strategy to
prepared air stable 3-aminopropyltrimethoxysilane (APTMS)-capped ZnO nanopar-
ticle, which reduced the surface adsorbed oxygen defects, improved the charge trans-
fer efficiency and most importantly suppressed the light-soaking effect of ZnO (Wei
et al. 2018). The PCE of PTB7-Th/PC71 BM using ZnO@APTMS gave a PCE of
9.07%, much higher than the bare ZnO (4.39%). It is interesting to note that the PCE
of modified ZnO-based device does not influenced by the light soaking, however,
the ZnO-based device was almost doubled after UV light soaking. The devices with
modified interlayer also showed excellent air stability for a year.
Various polythiophene derivatives were prepared using pendant amino function-
alities, such as t–butylpyridinium (P3TBPHT), trimethylammonium (P3TMAHT)
and imidazolium (P3IMDHT). The polymers were synthesized by quaternization
reaction of poly[3-(6-bromohexyl)thiophene] with the corresponding amines. The
P3TBPHT and PEDOT:PSS were deposited on the ITO surface using eLbL tech-
nique discussed above. The number of layers control the WF of ITO from 4.6 to
3.8 eV and successfully used as cathode buffer layer. The OSCs prepared using
P3TBPHT:PEDOT layer gave excellent PCEs of 5.6% and outstanding device sta-
bility retaining >90% of initial performance when stored over 1000 h in air compared
to other metal oxides (Worfolk et al. 2012). Yang and co-workers P3TMAHT:SDS
(sodium dodecylbenzene sulfonate) complex by simple mixing of both compounds
and successfully used as polyelectrolyte in P3HTP:C61 BM BHJ achieving PCE of
4.0% almost double compared to only Al (2.12%) due to large improvement in the
V OC and FF (Chang et al. 2012). Kesters et al. used an imidazolium-substituted poly-
thiophene P3IMDHT as ETL, which perform close to the value of other analogous
conjugated polyelectrolytes (CPE) (Kesters et al. 2013). The device performance
also depends on the molecular weight (MW) of P3IMDHT and high MW gave the
best performance. The interlayer creates an extra built-in electric field promoting
charge transfer from the BHJ layer into the interlayer as investigated by scanning
probe microscopy (Drijkoningen et al. 2014). Zilberberg et al. used an ultrathin layer
of P3IMDHT as CIL in an PCDTBT:PC71 BM inverted device which achieved very
similar PCE (~4.8%) to that obtained for device with TiO2 interlayer (Zilberberg
et al. 2013). P3MAHT CPE when used over ITO/AZO layer an improved device
performance was observed (Min et al. 2013).
18 Interfacial Materials for Organic Solar Cells 405

Cao and co-workers developed various alcohol-/water-soluble conjugated

polymers, poly[(9,9-bis(3 -(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-
(9,9–dioctyldioctylfluorene)] (PFN) and poly[(9,9-bis(3 -((N,N-dimethyl)-N-
ethylammonium)-propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] dibromide
(PFN-Br) and used as the ITO surface modifier (Huang et al. 2004). Using an inverted
device architecture a certified PCE of 9.2 and 8.2% for conventional structure was
achieved which offered Ohmic contact for photogenerated charge carrier collection
and improve photon harvesting (He et al. 2012). The same group further developed
an alcohol soluble polyelectrolyte poly[(9,9-bis((N-(4-sulfonate-1-butyl)-N,N-
dimethylammonium)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFNSO)
containing sulfobetaine zwitterionic groups on their side chains to use as CIL. (Duan
et al. 2013b) A PCE of 8.74% could be achieved using PTB7:PC71 BM photoactive
layer resulting from the small leakage current and lower series resistance. The
same group further developed alcohol soluble thermal cross-linkable, conjugated
polyfluorene, poly[(9,9-bis(6 -(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-
(9,9-bis(3-ethyl(oxetane-3-ethyloxy)-hexyl)-fluorene)] (PFN-OX). (Hu et al. 2014)
PFN-OX electron extraction layer can be completely crosslinked at 140 °C forming a
smooth layer over ITO and modify the WF of ITO to 3.5 eV to match with the LUMO
level of PC71 BM. Inverted BHJ device using PBDT–DTNT:PC71 BM exhibited a
PCE of 8.62% with a BHJ film thickness of 280 nm. While device without PFN-OX
layer gave very low PCE of 1.42% due to very low V OC (0.32 V) and FF (0.31).
Remarkable PCE of 7.24% has also been measured with BHJ thickness of 1000 nm
demonstrating high hole mobility and ordered molecular structure of donor polymer.
Cao and co-workers demonstrated the incorporation of PFN as CIL between
PCDTBT:PC71 BM blend and Al simultaneously improved the V OC , J SC and FF,
thus the PCE from 4.02 to 6.73% (He et al. 2011b). The V OC was improved from
0.71 to 0.91 V and FF from 0.49 to 0.62. It has been shown that the device with
PFN/Al exhibited a high turn-on voltage of 0.8–1.0 V compared to bare Al device
(0.5–0.6 V), indicating the enhancement of V bi across the device with PFN layer
which is key to the V OC improvement. The role of PFN layer was further studied by
406 A. Mishra

scanning Kelvin probe microscopy (SKPM) measurement, which revealed the PFN
interlayer can effectively prevent the built-up of space charge under light, thus have
a strong impact on the charge carrier transport. The PFN layer formed a microscopic
electric dipole and the direction of the dipole was aligned with the V bi .
Han et al. used ZnO/PFN bilayer as CIL in which the ZnO nanoparticles can
function as an efficient ETL to reduce series resistance, while the PFN can improve
the energy level alignment through the formation of an interfacial dipole between
ZnO and photoactive layer. The inverted device gave high performance using bilayer
ZnO/PFN compared to only ZnO or PFN-based devices (Han et al. 2016).
Guo et al. demonstrated an enhancement of the OSC performance with ternary
PEI/SnO2 /PFN composite interlayer by modulating the energy level between the
active layer and electrode and reduced the interfacial defects (Guo et al. 2019). The
inverted device using this ternary interlayer led to a dramatic increase in the device
performance with PCE of 7.18% compared to 5.5% larger shunt resistance (Rsh ) and
a decrease of series or contact resistance (Rs ) were observed, which could prevent
the leakage current, resulting in the increased J SC and FF.

Recently, using PFN-Br/Ag as CIL in PTB7-Th:PDIBDT-IT based fullerene-free

devices generated a PCE of 6.06% (Liu et al. 2018). PFN-Br was used to effectively
alleviate the interfacial energy barrier in the device. Hou and co-workers demon-
strated that by mixing ZnO nanoparticles with PFN-Br the surface free energy of
ZnO layers can be tuned from 51.23 to 76.62 mN m−1 (Zheng et al. 2019). BHJSCs
constructed from PBDB-TF:IT-4F photoactive layer excellent PCE of 13.82% was
achieved together with a record FF value of 0.79. Transient photovoltage measure-
ment revealed longer electron lifetime suggesting retardation of charge carrier recom-
bination as well boost the exciton dissociation efficiency. The introduction of PFN-Br
18 Interfacial Materials for Organic Solar Cells 407

into ZnO ETL, the stability of ETL increased and is related to the suppressed aggre-
gations of ZnO nanoparticles.
Mishra et al. demonstrated a low energy loss (E loss ) in conventional fullerene-free
solar cells using P1:DCI-2 blend and PFN as CIL. The device exhibited a PCE of
6.94% with low E loss of 0.39 V showing the importance of PFN layer on device
performance (Mishra et al. 2017). The authors further reported a PCE of 7.24%
using a D: A based organic molecule TPA-SN5-DCV:PC71 BM blend using PFN/Al
cathode after solvent additive treatment followed by thermal annealing (Mishra et al.
2018).
Duan et al. developed a conjugated alcohol soluble PCDTBT-N functionalized
with tertiaryamine pendant groups and used as buffer layer in metal: organic interface
to improve electron collection efficiency (Duan et al. 2013a). The amino groups
formed complex with C70 and act as hole traps as well shift the Fermi level from
about 4.6–4.2 eV. PCDTBT:PC71 BM-based devices gave PCE of 5.32% compared
to without interlayer due to the reduced transport loss for efficient electron collection
through the n-doping of PC71 BM.
Kim et al. prepared a conjugated polyelectrolyte PBN consisting of benzodithio-
phene and fluorine units as CIL for ZnO surface modification (Kim et al. 2014). PBN
creates an interfacial dipoles at the ZnO/organic interface and improve the contact
between the layers thus enhanced the PCE of PTB7:PC71 BM by about 21% i.e. from
7.1 to 8.6%.
Hou group developed a unique polymer poly[9,9-bis(1-sulfopropane-3-yl)-
fluorene-2,7-diyl-alt-(2,2 -bithiophene-5,5 )-diyl] (PFS) comprising sulfonic acid
containing fluorene and bithiophene unit which acts as both anode and cathode inter-
facial layers. The device with PDBT-TS1:PC71 BM achieved a high PCE of 9.48%
and comparable to device with PEDOT:PSS (9.76%) or Mg (9.63%) as anode and
cathode interlayers (Xu et al. 2016).
Sun et al. reported fullerene-free OSCs with a PCE of over 11% by introducing
n-type PFN or PFN-2TNDI as CIL (Sun et al. 2017). The results revealed that the
contact between the n-type interlayer and the donor provides an extra interface for
charge dissociation and the matching of energy levels between the interlayer and
the acceptor allows efficient electron extraction from the BHJ at the interface. Fur-
thermore, in contrast to PFN, PFN-2TNDI layer is suitable for exciton dissociation
due to appropriate energy level offset between the donor and interlayer resulting in
enhanced photocurrent generation.
A conjugated polymer PT2 NDISB containing napthalenediimide and bithiophene
with pendant zwitterionic groups was synthesized via Suzuki coupling reaction and
incorporated as interlayer between active layer and Ag in OSCs. A thin layer of
polymer interlayer (~8 nm) increased the device efficiency from 3.17 to 10.19%.
The interlayer lowered the WF to 3.8 eV, increase the V bi and reduces the pinholes
formation. The CIL act as optical spacers to enhance total photocurrent generated
within the active layer (Liu et al. 2015b). Using a NDI derivative PNDIT10 N as CIL
and MoOx as AIL, the inverted device using PTB7-Th:PC71 BM gave a PCE of 7.0%
(Bjuggren et al. 2018). UPS spectroscopy demonstrated reduction in the ITO work
function after surface modification which is related to the formation of interfacial
408 A. Mishra

dipole by the electron pair present in the pendant nitrogen atoms which facilitates
electron collection and transport to the ITO cathode (van Reenen et al. 2014; Lee
et al. 2018). The PNDIT10 N also acts as hole blocking layer due to its low lying
HOMO energy level.

18.2.2 Hole Transport Materials as Anode Interface Layers

For materials to be used as AIL, the WF should be high enough to match with
the HOMO of the donor for efficient hole extraction. An efficient hole transport
led to reduction in the series resistance. The AIL materials should possess high
optical transparency, good chemical stability, good optical transmittance, a high
ionization potential and suitable electron blocking ability. AIL has similar importance
as CIL in order to obtain high performance devices (Xu and Hou 2018). The reaction
between the active layer and metal electrode due to diffusion of metal ion resulted
in detrimental effect on device performance forming interfacial dipole barrier and
defect states. Earlier, it has been shown that the s-shape current density-voltage (J-
V ) curve resulted from the interfacial barrier (Trost et al. 2013; Tress et al. 2011).
Variation of hole selective layer can strongly affect the s-shape in the device. Sims
et al. reported that by introducing 1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile
(HATCN) to the N,N -bis(3-methylphenyl)-N,N -bis(phenyl)benzidine (TPD) layer
the WF and mobility of the hole selective layer can be tuned and effectively reduce
the s-shape and improve the FF (Sims et al. 2014). Here HATCN worked as inert
buffer layer which prevent diffusion and chemical reaction at the interface. Subbiah
et al. used a bilayer of MoO3 /MTDATA (4,4 ,4 -tris(3-methylphenylphenylamino)
triphenylamine) processed by vacuum-deposition as AIL and improved the device
performance of an inverted device 5.8–6.45% suggesting an improvement of the hole
extraction from the photoactive layer to the anode, effectively blocks the electrons
and allowed favourable vertical morphology of active layer blend (Subbiah et al.
2012). Various triarylamine based HTMs have been successfully utilized to improve
the hole transporting process in vacuum-process OSCs (Walzer et al. 2007; Mishra
et al. 2011; Fitzner et al. 2011, 2012; Schulze et al. 2006).
18 Interfacial Materials for Organic Solar Cells 409

Conventional solution-processed OSCs generally used PEDOT:PSS (WF ≈

5.1 eV) as AIL and many high performance devices have been reported. However,
PEDOT:PSS influences the device long term stability due to its acidic nature. To
improve the device performance and stability PEDOT:PSS has been modified with
metal nanoparticles, such as Ag and Au. The photovoltaic data of some representative
devices with various HTLs are presented in Table 18.4. Using colloidal Ag nanoprism
embedded with PEDOT:PSS as AIL and modified bis-C60 as CIL, enhancement in
the PCE from 7.7 to 9.0% has been observed due to enhanced optical absorption.
(Yao et al. 2014) Au@Ag core–shell nanocube (NC) showing strong plasmonic light
scattering effect than Au nanoparticles when used as AIL with PEDOT:PSS the PCE
of PCDTBT:PC71 BM solar cells improved from 5.3 to 6.3% and for PTB7:PC71 BM
PCE improved from 7.9 to 9.2% (Baek et al. 2014). The Ag shell acted to enhance
the plasmonic scattering and improve the absorption of Au@Ag nanocube. Solution-
processable MoOx as AIL have improved the photovoltaic performance and device
stability compared to PEDOT:PSS. (Tan et al. 2013) NiO nanoparticle when used as
AIL in PTB7:PC71 BM blend achieved a high PCE of 9.16% which is higher than
that of PEDOT:PSS based devices (8.6%) (Jiang et al. 2015).
Solution-processed OSCs using dithieno[3,2-b:2 ,3 -d]pyrrole (DTP)-based small
molecule donor and PC61 BM acceptor as active layer and either PEDOT:PSS or V2 O5
as AIL were prepared. Devices with V2 O5 gave much better performance with very
high FF up to ~0.60 compared to PEDOT:PSS (~0.50) was ascribed to the improved
hole collection at the anode associated with a better match between the WF of V2 O5
and the HOMO of the donor as well different surface energy at the interface. When
polydimethylsiloxane (PDMS) was used as solvent additive the PCE with V2 O5
AIL was improved to 5.3% (Schulz et al. 2016). Small molecule OSCs employing
CuSCN as AIL obtained a PCE of 8.22% (V OC = 0.86, FF = 0.68) compared to
7.54% (V OC = 0.84, FF = 0.65) for PEDOT:PSS AIL (Mishra et al. 2016). Both
device gave nearly similar photocurrent values which could be ascribed to the similar
transparencies in the visible region. The improvement in the FF was related to the
formation of better ohmic contact between the active layer and CuSCN, because of
the high WF of CuSCN than PEDOT:PSS. The high V OC was due to better alignment
of HOMO of the donor and valance band of CuSCN (−5.35 eV) which also acts as
good electron blocking layer.
Various p-type small molecules and polymers have been prepared and used in
conventional and inverted devices. Heeger and co-workers used chlorobenzoic acid
(CBA) as surface modifier of ITO electrode which produce a high WF electrode
with better transparency as an alternative to PEDOT:PSS. The benzoic acid bound
to the surface and chlorine atom pointed towards the organic layer forming a dipole.
The PTB7:PC71 BM based device achieved an improved PCE of 8.5% for CBA AIL
compared to 7.5% for PEDOT:PSS, ascribed improved light absorption and formation
of an Ohmic contact for hole transport (Choi et al. 2015). Huang et al. demonstrated
the enhancement of WF of ITO surface by self-assembling different benzoic acid and
sulfonic acids on the ITO surface (Huang et al. 2016). The WF of SAM-modified
ITO changed in the order CBSA (5.18 eV), CBA (4.91 eV), FBA (4.85 eV), and
BA (4.75 eV) when applied with same concentration. Among all derivatives CBSA
Table 18.4 Summary of photovoltaic performance based on representative AILs
410

Anode structure Active layer Cathode structure V OC (V) J SC (mA cm−2 ) FF PCE (%) References
MTDATA/MoO3 /Ag PDTS-BTD:PC71 BM ITO/ZnO 0.61 17.8 0.59 6.45 Subbiah et al. (2012)
ITO/PEDOT:PSS + Ag PIDTT-DFBT:PC71 BM Bis-C60 + Ag NPs/Ag 0.96 14.4 0.63 9.02 Yao et al. (2014)
NPs
ITO/PEDOT:PSS + PTB7:PC71 BM TiOX /Al. 0.75 17.5 0.70 9.19 Baek et al. (2014)
Au@Ag
ITO/MoOx P3HT:IC60BA Ca/Al 0.85 10.4 0.72 6.29 Tan et al. (2013)
ITO/NiOX PTB7-Th:PC71 BM Ca/Al 0.79 18.3 0.63 9.16 Jiang et al. (2015)
ITO/V2 O5 DTP2THex:PC61 BM LiF/Al 1.07 7.9 0.63 5.30 Schulz et al. (2016)
ITO/PEDOT:PSS 2IN:PC71 BM Al 0.84 13.8 0.65 7.54 Mishra et al. (2016)
ITO/CuSCN 2IN:PC71 BM Al 0.86 14.1 0.68 8.22 Mishra et al. (2016)
ITO/CBA PTB7:PC71 BM Al 0.74 16.5 0.70 8.48 Choi et al. (2015)
ITO/CBSA (0.1 mg/ml) PTB7-Th:PC71 BM PFN/Al 0.80 16.4 0.70 9.20 Huang et al. (2016)
ITO/CBA (1.0 mg/ml) PTB7-Th:PC71 BM PFN/Al 0.80 16.3 0.66 8.60 Huang et al. (2016)
ITO/FBA (1.0 mg/ml) PTB7-Th:PC71 BM PFN/Al 0.72 16.1 0.62 7.10 Huang et al. (2016)
ITO/BA (1.0 mg/ml) PTB7-Th:PC71 BM PFN/Al 0.66 15.2 0.61 6.10 Huang et al. (2016)
ITO/CBSC (1.0 mg/ml) PTB7-Th:PC71 BM PFN/Al 0.80 16.7 0.68 9.10 Huang et al. (2016)
ITO/PABTSi2 :TFB P3HT:PC61 BM LiF/Al 0.54 9.31 0.63 3.10 Hains et al. (2010)
ITO/TPDSi2 :TFB P3HT:PC61 BM LiF/Al 0.55 4.72 0.10 0.28 Hains et al. (2010)
(continued)
A. Mishra
Table 18.4 (continued)
Anode structure Active layer Cathode structure V OC (V) J SC (mA cm−2 ) FF PCE (%) References
ITO/TPDB P3HT:PC61 BM LiF/Al 0.55 8.62 0.64 3.03 Lu et al. (2013)
ITO/TPDB PBDTTPD:PC61 BM LiF/Al 0.87 11.5 0.65 6.51 Lu et al. (2013)
ITO/PCP-Na PBDT-TS1:PC71 BM Mg/Al 0.80 17.5 0.71 9.89 Cui et al. (2016)
ITO/PCF-Na PBDT-TS1:PC71 BM Mg/Al 0.80 17.5 0.68 9.45 Cui et al. (2016)
ITO/PFS-Na PBDT-TS1:PC71 BM Mg/Al 0.79 16.5 0.63 8.16 Cui et al. (2016)
18 Interfacial Materials for Organic Solar Cells
411
412 A. Mishra

showed the best PCE with 9.2% with PTB7–Th:PC71 BM using 0.1 mg/mL CBSA.
However, with increasing the concentration of CBSA the performance were reduces
to 7.7% for 1.0 mg/mL. All other derivatives gave best performance with 1.0 mg/mL
of surface modifiers. Considering the acidic nature of CBSA the authors also prepared
SAM with chlorobenzenesulfonyl chloride (CBSC) which gave similar PCEs of 9.1%
and significantly improved the device stability. A triple dipole effect for Cl-assisted
self-assembled molecule on ITO has been proposed.
Mark et al. developed two AILs (TPDSi2 , PBATSi2 ) and implemented in
P3HT:PC61 BM device (Hains et al. 2010). The molecules were anchor on the ITO
surface via SiCl3 groups and form cross linking by 1:2 blending with TFB poly-
mer. The devices based on PBATSi2 :TFB interlayer gave PCE of 3.14% similar to
PEDOT:PSS, while the TPDSi2 :TFB exhibited very poor PCE of only 0.28% and
FF of 0.10 due to s-shape curve. This poor performance of TPDSi2 the large HOMO
energy mismatch of TPDSi2 (HOMO = −5.3 eV) and P3HT (−5.0 eV) forming
a barrier to hole transport/collection at the anode. However, the HOMO energy of
PBATSi2 (−4.9 eV) align well with P3HT and the LUMO is high enough to block
the electrons.

Small molecule TPDB comprising carboxyl side chains was implemented as

AIL in P3HT:PC61 BM and PBDTTPD:PC61 BM based devices due to its high trans-
parency in the visible region and appropriate energy level alignment for hole trans-
porting and electron blocking capacity. The devices based on these new HTLs gave
improved PCEs up to 6.51% compared to PEDOT:PSS buffer layer and better device
stability (Lu et al. 2013).
Hou and co-workers developed three pH neutral conjugated polymers, namely
PCP-Na, PCF-Na, and PFS-Na and tested AILs in conventional OSCs. Among the
three polymers PCP-Na exhibited self-doping ability and with high electrical con-
ductivity of 1.66 × 10−3 S cm−1 . OSCs prepared with PCP-Na as AIL recorded
PCEs of up to 9.89% and possess thickness insensitivity over 300 nm, retaining FF
of 70% (Cui et al. 2016).
18 Interfacial Materials for Organic Solar Cells 413

18.3 Conclusions and Outlook

In this chapter we have explored some cathode and anode interfacial materials and
their role in solution-processed organic solar cells. Together with the development of
novel active layer materials (polymers or oligomers), the interfacial materials design
have found equal importance toward their role in device performance improvement.
These interlayer materials have gained tremendous interest in recent years and been
successfully employed in high performance conventional and inverted devices. A
structure-property device performance relationships have been established which
is beneficial for the development of new interlayer materials. Furthermore, with
an effort towards molecular design, from neutral to charged materials, the optical
properties, energy levels, charge carrier mobilities, conductivity, work function as
well as surface morphology and surface energy can be tuned, and therefore affect-
ing the OSC performance. When these materials are combined with suitable donor:
acceptor photoactive layers and device processing conditions high performance sin-
gle junction solar cells reaching PCEs >10% have been achieved. Recently, record
PCE exceeding 17% was reported for an inverted tandem cell using ZnO/PFN-Br as
CIL, PEDOT:PSS/ZnO as interconnecting layer and MoO3 /Ag as AIL showing the
importance of molecular design of different active layers, interlayers and device engi-
neering (Meng et al. 2018). Apart from device efficiency, the device stability is also
an important issue which was almost solved by the use of metal oxides as interlayer
in inverted devices. Simultaneously, together with the performance the use of organic
and polymeric interlayers have also shown tremendous improvement in device sta-
bility. Sometime very thin interlayer is sufficient to get high efficiency. However,
for practical applicability of the technology it is also important to develop thickness
insensitive interlayers which are very rare at this moment. Now it is also important to
exploit the potential of various interlayer materials in different device configuration
which could open up new opportunities towards performance improvement and PCE
of >20% is not far to reach.

Acknowledgements The authors would like to acknowledge the Sambalpur University for pro-
viding research infrastructure and Department of Science and Technology (DST), New Delhi
(DST/TMD/SERI/D05) for financial support.

References

Aryal UK, Chakravarthi N, Park H-Y, Bae H, Jin S-H, Gal Y-S (2018) Highly efficient poly-
acetylene–based polyelectrolytes as cathode interfacial layers for organic solar cell applications.
Org Electron 53:265–272
Baek S-W, Park G, Noh J, Cho C, Lee C-H, Seo M-K, Song H, Lee J-Y (2014) Au@Ag core-shell
nanocubes for efficient plasmonic light scattering effect in low bandgap organic solar cells. ACS
Nano 8(4):3302–3312
Bao X, Sun L, Shen W, Yang C, Chen W, Yang R (2014) Facile preparation of TiOX film as an
interface material for efficient inverted polymer solar cells. J Mater Chem A 2(6):1732–1737
414 A. Mishra

Bilby D, Frieberg B, Kramadhati S, Green P, Kim J (2014) Design considerations for electrode
buffer layer materials in polymer solar cells. ACS Appl Mater Interfaces 6(17):14964–14974
Bjuggren JM, Sharma A, Gedefaw D, Elmas S, Pan C, Kirk B, Zhao X, Andersson G, Andersson
MR (2018) Facile synthesis of an efficient and robust cathode interface material for polymer solar
cells. ACS Appl Energy Mater 1(12):7130–7139
Bob B, Song T-B, Chen C-C, Xu Z, Yang Y (2013) Nanoscale dispersions of gelled SnO2 : material
properties and device applications. Chem Mater 25(23):4725–4730
Chan MY, Lee CS, Lai SL, Fung MK, Wong FL, Sun HY, Lau KM, Lee ST (2006) Efficient organic
photovoltaic devices using a combination of exciton blocking layer and anodic buffer layer. J
Appl Phys 100(9):094506
Chang Y-M, Zhu R, Richard E, Chen C-C, Li G, Yang Y (2012) Electrostatic self-assembly con-
jugated polyelectrolyte-surfactant complex as an interlayer for high performance polymer solar
cells. Adv Funct Mater 22(15):3284–3289
Chen L-M, Xu Z, Hong Z, Yang Y (2010) Interface investigation and engineering—achieving high
performance polymer photovoltaic devices. J Mater Chem 20:2575–2598
Chen Q, Worfolk BJ, Hauger TC, Al-Atar U, Harris KD, Buriak JM (2011) Finely tailored per-
formance of inverted organic photovoltaics through layer-by-layer interfacial engineering. ACS
Appl Mater Interfaces 3(10):3962–3970
Chen S, Small CE, Amb CM, Subbiah J, Lai T-h, Tsang S-W, Manders JR, Reynolds JR, So F
(2012) Inverted polymer solar cells with reduced interface recombination. Adv Energy Mater
2(11):1333–1337
Chen D, Zhou H, Cai P, Sun S, Ye H, Su S-J, Cao Y (2014) A water-processable organic electron-
selective layer for solution-processed inverted organic solar cells. Appl Phys Lett 104(5):053304
Chen J-D, Cui C, Li Y-Q, Zhou L, Ou Q-D, Li C, Li Y, Tang J-X (2015) Single-junction polymer
solar cells exceeding 10% power conversion efficiency. Adv Mater 27(6):1035–1041
Chen W, Lv J, Han J, Chen Y, Jia T, Li F, Wang Y (2016) N-Type cathode interlayer based on
dicyanomethylenated quinacridone derivative for high-performance polymer solar cells. J Mater
Chem A 4(6):2169–2177
Choi H, Kim H-B, Ko S-J, Kim JY, Heeger AJ (2015) An organic surface modifier to produce a
high work function transparent electrode for high performance polymer solar cells. Adv Mater
27(5):892–896
Chueh C-C, Li C-Z, Jen AKY (2015) Recent progress and perspective in solution-processed Inter-
facial materials for efficient and stable polymer and organometal perovskite solar cells. Energy
Environ Sci 8(4):1160–1189
Cui Y, Xu B, Yang B, Yao H, Li S, Hou J (2016) A novel pH neutral self-doped polymer for anode
interfacial layer in efficient polymer solar cells. Macromolecules 49(21):8126–8133
Dkhil SB, Duché D, Gaceur M, Thakur AK, Aboura FB, Escoubas L, Simon J-J, Guerrero A, Bis-
quert J, Garcia-Belmonte G, Bao Q, Fahlman M, Videlot-Ackermann C, Margeat O, Ackermann
J (2014) Interplay of optical, morphological, and electronic effects of ZnO optical spacers in
highly efficient polymer solar cells. Adv Energy Mater 4(18):1400805
Drijkoningen J, Kesters J, Vangerven T, Bourgeois E, Lutsen L, Vanderzande D, Maes W, D’Haen J,
Manca J (2014) Investigating the role of efficiency enhancing interlayers for bulk heterojunction
solar cells by scanning probe microscopy. Org Electron 15(6):1282–1289
Duan C, Zhong C, Liu C, Huang F, Cao Y (2012) Highly efficient inverted polymer solar cells
based on an alcohol soluble fullerene derivative interfacial modification material. Chem Mater
24(9):1682–1689
Duan C, Cai W, Hsu BBY, Zhong C, Zhang K, Liu C, Hu Z, Huang F, Bazan GC, Heeger AJ, Cao
Y (2013a) Toward green solvent processable photovoltaic materials for polymer solar cells: the
role of highly polar pendant groups on charge carrier transport and photovoltaic behavior. Energy
Environ Sci 6:3022–3034
Duan C, Zhang K, Guan X, Zhong C, Xie H, Huang F, Chen J, Peng J, Cao Y (2013b) Conjugated
zwitterionic polyelectrolyte-based interface modification materials for high performance polymer
optoelectronic devices. Chem Sci 4:1298–1307
18 Interfacial Materials for Organic Solar Cells 415

Duan C, Zhang K, Zhong C, Huang F, Cao Y (2013c) Recent advances in water/alcohol-soluble

π-conjugated materials: new materials and growing applications in solar cells. Chem Soc Rev
42(23):9071–9104
Fan P, Zheng Y, Song J, Yu J (2017) N-type small molecule as an interfacial modification layer for
efficient inverted polymer solar cells. Sol Energy 158:278–284
Fitzner R, Reinold E, Mishra A, Mena-Osteritz E, Ziehlke H, Körner C, Leo K, Riede M, Weil M,
Tsaryova O, Weiß A, Uhrich C, Pfeiffer M, Bäuerle P (2011) Dicyanovinyl-substituted oligoth-
iophenes: structure-property relationships and application in vacuum-processed small molecule
organic solar cells. Adv Funct Mater 21(5):897–910
Fitzner R, Mena-Osteritz E, Mishra A, Schulz G, Reinold E, Weil M, Körner C, Ziehlke H,
Elschner C, Leo K, Riede M, Pfeiffer M, Uhrich C, Bäuerle P (2012) Correlation of π-conjugated
oligomer structure with film morphology and organic solar cell performance. J Am Chem Soc
134(27):11064–11067
Fu P, Guo X, Wang S, Ye Y, Li C (2017) Aminosilane as a molecular linker between the electron-
transport layer and active layer for efficient inverted polymer solar cells. ACS Appl Mater Inter-
faces 9(15):13390–13395
Gadisa A, Hairfield T, Alibabaei L, Donley CL, Samulski ET, Lopez R (2013) Solution processed
al-doped ZnO nanoparticles/tiox composite for highly efficient inverted organic solar cells. ACS
Appl Mater Interfaces 5(17):8440–8445
Gommans H, Verreet B, Rand BP, Muller R, Poortmans J, Heremans P, Genoe J (2008) On the role
of bathocuproine in organic photovoltaic cells. Adv Funct Mater 18(22):3686–3691
Guo J, Ren G, Han W, Sun Y, Wang M, Zhou Y, Shen L, Guo W (2019) Facilitating electron
extraction of inverted polymer solar cells by using organic/inorganic/organic composite buffer
layer. Org Electron 68:187–192
Hains AW, Ramanan C, Irwin MD, Liu J, Wasielewski MR, Marks TJ (2010) Designed bithiophene-
based interfacial layer for high-efficiency bulk-heterojunction organic photovoltaic cells. Impor-
tance of Interfacial Energy Level Matching. ACS Appl Mater Interfaces 2(1):175–185
Han C, Cheng Y, Chen L, Qian L, Yang Z, Xue W, Zhang T, Yang Y, Cao W (2016) Enhanced perfor-
mance of inverted polymer solar cells by combining ZnO nanoparticles and poly[(9,9-bis(3 -(N,
N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyfluorene)] as electron transport layer.
ACS Appl Mater Interfaces 8(5):3301–3307
Hau SK, Yip H-L, Baek NS, Zou J, O’Malley K, Jen AKY (2008) Air-stable inverted flexible
polymer solar cells using zinc oxide nanoparticles as an electron selective layer. Appl Phys Lett
92(25):253301–253303
Hau SK, O’Malley KM, Cheng Y, Yip H, Ma H, Jen AK (2010) Optimization of active layer and
anode electrode for high-performance inverted bulk-heterojunction solar cells. IEEE J Sel Top
Quantum Electron 16(6):1665–1675
He Z, Zhang C, Xu X, Zhang L, Huang L, Chen J, Wu H, Cao Y (2011a) Largely enhanced efficiency
with a PFN/Al bilayer cathode in high efficiency bulk heterojunction photovoltaic cells with a
low bandgap polycarbazole donor. Adv Mater 23(27):3086–3089
He Z, Zhong C, Huang X, Wong W-Y, Wu H, Chen L, Su S, Cao Y (2011b) Simultaneous enhance-
ment of open-circuit voltage, short-circuit current density, and fill factor in polymer solar cells.
Adv Mater 23(40):4636–4643
He Z, Zhong C, Su S, Xu M, Wu H, Cao Y (2012) Enhanced power-conversion efficiency in polymer
solar cells using an inverted device structure. Nat Photon 6:591–595
Hiramoto M, Suemori K, Matsumura Y, Miyata T, Yokoyama M (2006) P-I-N junction organic
solar cells. Mol Cryst Liq Cryst 455:267–275
Hu X, Yi C, Wang M, Hsu C-H, Liu S, Zhang K, Zhong C, Huang F, Gong X, Cao Y (2014) High-
performance inverted organic photovoltaics with over 1-μm thick active layers. Adv Energy
Mater 4(15):1400378
Hu Z, Zhang F, An Q, Zhang M, Ma X, Wang J, Zhang J, Wang J (2018) Ternary nonfullerene
polymer solar cells with a power conversion efficiency of 11.6% by inheriting the advantages of
binary cells. ACS Energy Lett 3(3):555–561
416 A. Mishra

Huang F, Wu H, Wang D, Yang W, Cao Y (2004) Novel electroluminescent conjugated polyelec-

trolytes based on polyfluorene. Chem Mater 16(4):708–716
Huang L, Chen L, Huang P, Wu F, Tan L, Xiao S, Zhong W, Sun L, Chen Y (2016) Triple dipole
effect from self-assembled small-molecules for high performance organic photovoltaics. Adv
Mater 28(24):4852–4860
Irwin MD, Buchholz DB, Hains AW, Chang RPH, Marks TJ (2008) p-type semiconducting nickel
oxide as an efficiency-enhancing anode interfacial layer in polymer bulk-heterojunction solar
cells. Proc Nat Acad Sci 105(8):2783–2787
Jheng J-F, Lai Y-Y, Wu J-S, Chao Y-H, Wang C-L, Hsu C-S (2013) Influences of the non-covalent
interaction strength on reaching high solid-state order and device performance of a low bandgap
polymer with axisymmetrical structural units. Adv Mater 25(17):2445–2451
Jiang F, Choy WCH, Li X, Zhang D, Cheng J (2015) Post-treatment-free solution-processed non-
stoichiometric NiOx nanoparticles for efficient hole-transport layers of organic optoelectronic
devices. Adv Mater 27(18):2930–2937
Jørgensen M, Norrman K, Gevorgyan SA, Tromholt T, Andreasen B, Krebs FC (2012) Stability of
polymer solar cells. Adv Mater 24(5):580–612
Kang H, Hong S, Lee J, Lee K (2012) Electrostatically self-assembled nonconjugated polyelec-
trolytes as an ideal interfacial layer for inverted polymer solar cells. Adv Mater 24(22):3005–3009
Kesters J, Ghoos T, Penxten H, Drijkoningen J, Vangerven T, Lyons DM, Verreet B, Aernouts T,
Lutsen L, Vanderzande D, Manca J, Maes W (2013) Imidazolium-substituted polythiophenes
as efficient electron transport materials improving photovoltaic performance. Adv Energy Mater
3(9):1180–1185
Kim JB, Ahn S, Kang SJ, Nuckolls C, Loo Y-L (2013) Ligand chemistry of titania precursor affects
transient photovoltaic behavior in inverted organic solar cells. Appl Phys Lett 102(10):103302
Kim HI, Bui TTT, Kim G-W, Kang G, Shin WS, Park T (2014) A benzodithiophene-based novel
electron transport layer for a highly efficient polymer solar cell. ACS Appl Mater Interfaces
6(18):15875–15880
Kim G, Kong J, Kim J, Kang H, Back H, Kim H, Lee K (2015) Overcoming the light-soaking problem
in inverted polymer solar cells by introducing a heavily doped titanium sub-oxide functional layer.
Adv Energy Mater 5(3):1401298
Kyaw AKK, Sun XW, Jiang CY, Lo GQ, Zhao DW, Kwong DL (2008) An inverted organic solar
cell employing a sol-gel derived ZnO electron selective layer and thermal evaporated MoO3 hole
selective layer. Appl Phys Lett 93(22):221107
Kyaw AKK, Wang DH, Gupta V, Zhang J, Chand S, Bazan GC, Heeger AJ (2013) Efficient solution-
processed small-molecule solar cells with inverted structure. Adv Mater 25(17):2397–2402
Lai T-H, Tsang S-W, Manders JR, Chen S, So F (2013) Properties of interlayer for organic photo-
voltaics. Mater Today 16(11):424–432
Lai Y-Y, Cheng Y-J, Hsu CS (2014) Applications of functional fullerene materials in polymer solar
cells. Energy Environ Sci 7:1866–1883
Lee K, Kim JY, Park SH, Kim SH, Cho S, Heeger AJ (2007) Air-stable polymer electronic devices.
Adv Mater 19(18):2445–2449
Lee BH, Jung IH, Woo HY, Shim H-K, Kim G, Lee K (2014a) Multi-charged conjugated polyelec-
trolytes as a versatile work function modifier for organic electronic devices. Adv Funct Mater
24(8):1100–1108
Lee BR, Jung ED, Nam YS, Jung M, Park JS, Lee S, Choi H, Ko S-J, Shin NR, Kim Y-K, Kim
SO, Kim JY, Shin H-J, Cho S, Song MH (2014b) Amine-based polar solvent treatment for highly
efficient inverted polymer solar cells. Adv Mater 26(3):494–500
Lee J-H, Jeong SY, Kim G, Park B, Kim J, Kee S, Kim B, Lee K (2018) Reinforcing the built-in
field for efficient charge collection in polymer solar cells. Adv Funct Mater 28(10):1705079
Li G, Chu C-W, Shrotriya V, Huang J, Yang Y (2006) Efficient inverted polymer solar cells. Appl
Phys Lett 88(25):253503
18 Interfacial Materials for Organic Solar Cells 417

Li S, Lei M, Lv M, Watkins SE, Za Tan, Zhu J, Hou J, Chen X, Li Y (2013) [6,6]-Phenyl-C61-

butyric acid dimethylamino ester as a cathode buffer layer for high-performance polymer solar
cells. Adv Energy Mater 3(12):1569–1574
Li N, Baran D, Spyropoulos GD, Zhang H, Berny S, Turbiez M, Ameri T, Krebs FC, Brabec CJ
(2014a) Environmentally printing efficient organic tandem solar cells with high fill factors: a
guideline towards 20% power conversion efficiency. Adv Energy Mater 4(11):1400084
Li P, Sun C, Jiu T, Wang G, Li J, Li X, Fang J (2014b) High-performance inverted solar cells based
on blend films of ZnO naoparticles and TiO2 nanorods as a cathode buffer layer. ACS Appl Mater
Interfaces 6(6):4074–4080
Li X, Xie F, Zhang S, Hou J, Choy WCH (2014c) Over 1.1 eV workfunction tuning of cesium
intercalated metal oxides for functioning as both electron and hole transport layers in organic
optoelectronic devices. Adv Funct Mater 24(46):7348–7356
Li Z, Yang D, Zhao X, Li Z, Zhang T, Wu F, Yang X (2016) New PDI-based small-molecule
cathode interlayer material with strong electron extracting ability for polymer solar cells. RSC
Adv 6(103):101645–101651
Li X, Zhang W, Usman K, Fang J (2018a) Small molecule interlayers in organic solar cells. Adv
Energy Mater 8(28):1702730
Li Z, Liu C, Zhang X, Guo J, Zhang X, Guo W (2018b) Boosting electron extraction in poly-
mer solar cells by introducing a N-Type organic semiconductor interface layer. J Phys Chem C
122(1):207–215
Li Y, Zheng N, Yu L, Wen S, Gao C, Sun M, Yang R (2019) A simple phenyl group introduced at
the tail of alkyl side chains of small molecular acceptors: new strategy to balance the crystallinity
of acceptors and miscibility of bulk heterojunction enabling highly efficient organic solar cells.
Adv Mater 31(12):1807832
Liao S-H, Jhuo H-J, Cheng Y-S, Chen S-A (2013) Fullerene derivative-doped zinc oxide nanofilm
as the cathode of inverted polymer solar cells with low-bandgap polymer (PTB7-Th) for high
performance. Adv Mater 25(34):4766–4771
Liao S-H, Jhuo H-J, Yeh P-N, Cheng Y-S, Li Y-L, Lee Y-H, Sharma S, Chen S-A (2014) Single
junction inverted polymer solar cell reaching power conversion efficiency 10.31% by employing
dual-doped zinc oxide nano-film as cathode interlayer. Sci Rep 4:6813. https://doi.org/10.1038/
srep06813
Lin Z, Jiang C, Zhu C, Zhang J (2013) Development of inverted organic solar cells with TiO2
interface layer by using low-temperature atomic layer deposition. ACS Appl Mater Interfaces
5(3):713–718
Lin Y, Zhang Z-G, Bai H, Wang J, Yao Y, Li Y, Zhu D, Zhan X (2015) High-performance fullerene-
free polymer solar cells with 6.31% efficiency. Energy Environ Sci 8:610–616
Liu J, Shao S, Fang G, Meng B, Xie Z, Wang L (2012a) High-efficiency inverted polymer solar cells
with transparent and work-function tunable MoO3 -Al composite film as cathode buffer layer. Adv
Mater 24(20):2774–2779
Liu J, Shao S, Meng B, Fang G, Xie Z, Wang L, Li X (2012b) Enhancement of inverted polymer
solar cells with solution-processed ZnO-TiOX composite as cathode buffer layer. Appl Phys Lett
100(21):213906
Liu Y, Zhao J, Li Z, Mu C, Ma W, Hu H, Jiang K, Lin H, Ade H, Yan H (2014) Aggregation and
morphology control enables multiple cases of high-efficiency polymer solar cells. Nat Commun
5:5293
Liu Y, Page Z, Ferdous S, Liu F, Kim P, Emrick T, Russell T (2015a) Dual functional zwitterionic
fullerene interlayer for efficient inverted polymer solar cells. Adv Energy Mater 5(14):1500405
Liu Y, Page ZA, Russell TP, Emrick T (2015b) Finely tuned polymer interlayers enhance solar cell
efficiency. Angew Chem Int Ed 54(39):11485–11489
Liu X, Li X, Li Y, Song C, Zhu L, Zhang W, Wang H-Q, Fang J (2016a) High-performance
polymer solar cells with PCE of 10.42% via Al-Doped ZnO cathode interlayer. Adv Mater
28(34):7405–7412
418 A. Mishra

Liu Z, Ouyang X, Peng R, Bai Y, Mi D, Jiang W, Facchetti A, Ge Z (2016b) Efficient polymer solar
cells based on the synergy effect of a novel non-conjugated small-molecule electrolyte and polar
solvent. J Mater Chem A 4(7):2530–2536
Liu Z, Li W, Peng R, Jiang W, Guan Q, Lei T, Yang R, Islam A, Wei Q, Ge Z (2017) Benzophenone-
based small molecular cathode interlayers with various polar groups for efficient polymer solar
cells. J Mater Chem A 5(21):10154–10160
Liu Y, Liu G, Xie R, Wang Z, Zhong W, Li Y, Huang F, Cao Y (2018) A rational design and
synthesis of cross-conjugated small molecule acceptors approaching high-performance fullerene-
free polymer solar cells. Chem Mater 30(13):4331–4342
Lu K, Yuan J, Peng J, Huang X, Cui L, Jiang Z, Wang H-Q, Ma W (2013) New solution-processable
small molecules as hole-transporting layer in efficient polymer solar cells. J Mater Chem A
1(45):14253–14261
Ma H, Yip H-L, Huang F, Jen AK-Y (2010) Interface engineering for organic electronics. Adv
Funct Mater 20(9):1371–1388
MacLeod BA, Tremolet de Villers BJ, Schulz P, Ndione PF, Kim H, Giordano AJ, Zhu K, Marder
SR, Graham S, Berry JJ, Kahn A, Olson DC (2015) Stability of inverted organic solar cells with
ZnO contact layers deposited from sol-gel precursors. Energy Environ Sci 8(2):592–601
Manders JR, Tsang S-W, Hartel MJ, Lai T-H, Chen S, Amb CM, Reynolds JR, So F (2013) Solution-
processed nickel oxide hole transport layers in high efficiency polymer photovoltaic cells. Adv
Funct Mater 23(23):2993–3001
Meng L, Zhang Y, Wan X, Li C, Zhang X, Wang Y, Ke X, Xiao Z, Ding L, Xia R, Yip H-L, Cao Y,
Chen Y (2018) Organic and solution-processed tandem solar cells with 17.3% efficiency. Science
361(6407):1094–1098
Miao Y, Yu H, Zhang Y, Yan X, Zhang J, Wang Y (2018) Efficient polymer solar cells based
on a cathode interlayer of dicyanomethylenated indacenodithiophene derivative with large π-
conjugation and electron-deficient properties. J Mater Chem C 6(1):57–65
Mihailetchi VD, Blom PWM, Hummelen JC, Rispens MT (2003) Cathode dependence of
the open-circuit voltage of polymer: fullerene bulk heterojunction solar cells. J Appl Phys
94(10):6849–6854
Min J, Zhang H, Stubhan T, Luponosov YN, Kraft M, Ponomarenko SA, Ameri T, Scherf U,
Brabec CJ (2013) A combination of Al-doped ZnO and a conjugated polyelectrolyte interlayer
for small molecule solution-processed solar cells with an inverted structure. J Mater Chem A
1(37):11306–11311
Min J, Bronnbauer C, Zhang Z-G, Cui C, Luponosov YN, Ata I, Schweizer P, Przybilla T, Guo
F, Ameri T, Forberich K, Spiecker E, Bäuerle P, Ponomarenko SA, Li Y, Brabec CJ (2016)
Fully solution-processed small molecule semitransparent solar cells: optimization of transparent
cathode architecture and four absorbing layers. Adv Funct Mater 26(25):4543–4550
Mishra A, Bäuerle P (2012) Small molecule organic semiconductors on the move: promises for
future solar energy technology. Angew Chem Int Ed 51(9):2020–2067
Mishra A, Sahu SN (2019) Fullerene-Free Molecular Acceptors for Organic Photovoltaics. In: Tyagi
H, Agarwal AK, Chakraborty PR, S. Powar (eds) Advances in Solar Energy Research, Energy,
Environment, and Sustainability. Springer Nature, Singapore Pte Ltd., pp 221–279
Mishra A, Uhrich C, Reinold E, Pfeiffer M, Bäuerle P (2011) Synthesis and characterization of
acceptor-substituted oligothiophenes for solar cell applications. Adv Energy Mater 1(2):265–273
Mishra A, Rana T, Looser A, Stolte M, Würthner F, Bäuerle P, Sharma GD (2016) High performance
A-D-A oligothiophene-based organic solar cells employing two-step annealing and solution-
processable copper thiocyanate (CuSCN) as an interfacial hole transporting layer. J Mater Chem
A 4(44):17344–17353
Mishra A, Keshtov ML, Looser A, Singhal R, Stolte M, Würthner F, Bäuerle P, Sharma GD (2017)
Unprecedented low energy losses in organic solar cells with high external quantum efficiencies
by employing non-fullerene electron acceptors. J Mater Chem A 5(28):14887–14897
18 Interfacial Materials for Organic Solar Cells 419

Mishra A, Wetzel C, Singhal R, Bäuerle P, Sharma GD (2018) Low energy gap tripheny-
lamine–heteropentacene–dicyanovinyl triad for solution-processed bulk-heterojunction solar
cells. J Phys Chem C 122(21):11262–11269
Mor GK, Shankar K, Paulose M, Varghese OK, Grimes CA (2007) High efficiency double hetero-
junction polymer photovoltaic cells using highly ordered TiO2 nanotube arrays. Appl Phys Lett
91(15):152111
Nam E, Oh S, Jung D, Kim H, Chae H, Yi J (2012) Organic photovoltaic devices with the bilayer
cathode interfacial structure of pyromellitic dianhydride and lithium fluoride. Semicon Sci Tech
27(10):105004
Nam S, Seo J, Han H, Kim H, Hahm SG, Ree M, Gal Y-S, Anthopoulos TD, Bradley DDC, Kim Y
(2016) >10% efficiency polymer: fullerene solar cells with polyacetylene-based polyelectrolyte
interlayers. Adv Mater Interfaces 3(23):1600415
Nam S, Seo J, Song M, Kim H, Ree M, Gal Y-S, Bradley DDC, Kim Y (2017) Polyacetylene-
based polyelectrolyte as a universal interfacial layer for efficient inverted polymer solar cells.
Org Electron 48:61–67
Nho S, Baek G, Park S, Lee BR, Cha MJ, Lim DC, Seo JH, Oh S-H, Song MH, Cho S (2016) Highly
efficient inverted bulk-heterojunction solar cells with a gradiently-doped ZnO layer. Energy Env-
iron Sci 9(1):240–246
Nian L, Zhang W, Zhu N, Liu L, Xie Z, Wu H, Würthner F, Ma Y (2015) Photoconductive cathode
interlayer for highly efficient inverted polymer solar cells. J Am Chem Soc 137(22):6995–6998
Ouyang X, Peng R, Ai L, Zhang X, Ge Z (2015) Efficient polymer solar cells employing a non-
conjugated small-molecule electrolyte. Nat Photon 9:520–524
Page ZA, Liu Y, Duzhko VV, Russell TP, Emrick T (2014) Fulleropyrrolidine interlayers: tailoring
electrodes to raise organic solar cell efficiency. Science 346(6208):441–444
Park SH, Roy A, Beaupre S, Cho S, Coates N, Moon JS, Moses D, Leclerc M, Lee K, Heeger AJ
(2009) Bulk heterojunction solar cells with internal quantum efficiency approaching 100%. Nat
Photon 3(5):297–302
Park H-Y, Lim D, Kim K-D, Jang S-Y (2013) Performance Optimization of Low-temperature-
annealed Solution-processable ZnO Buffer Layers for Inverted Polymer Solar Cells. J Mater
Chem A 1:6327–6334
Peumans P, Forrest SR (2001) Very-high-efficiency double-heterostructure copper
phthalocyanine/C60 photovoltaic cells. Appl Phys Lett 79(1):126–128
Pho TV, Kim H, Seo JH, Heeger AJ, Wudl F (2011) Quinacridone-based electron transport layers
for enhanced performance in bulk-heterojunction solar cells. Adv Funct Mater 21(22):4338–4341
Puetz A, Stubhan T, Reinhard M, Loesch O, Hammarberg E, Wolf S, Feldmann C, Kalt H, Colsmann
A, Lemmer U (2011) Organic solar cells incorporating buffer layers from indium doped zinc oxide
nanoparticles. Sol Energy Mater Sol Cell 95(2):579–585
Reilly TH III, Hains AW, Chen H-Y, Gregg BA (2012) A self-doping, O2 -stable, n-type interfacial
layer for organic electronics. Adv Energy Mater 2(4):455–460
Schulz GL, Kar P, Weidelener M, Vogt A, Urdanpilleta M, Lindén M, Mena-Osteritz E, Mishra
A, Bäuerle P (2016) The influence of alkyl side chains on molecular packing and solar cell
performance of dithienopyrrole-based oligothiophenes. J Mater Chem A 4(27):10514–10523
Schulze K, Uhrich C, Schüppel R, Leo K, Pfeiffer M, Brier E, Reinold E, Bäuerle P (2006) Efficient
vacuum deposited organic solar cells with high photovoltage based on a new low band-gap
oligothiophene and fullerene C60 . Adv Mater 18:2872–2875
Shao S, Zheng K, Pullerits T, Zhang F (2013) Enhanced performance of inverted polymer solar
cells by using poly(ethylene oxide)-modified ZnO as an electron transport layer. ACS Appl Mater
Interfaces 5(2):380–385
Sharma GD, Anil Reddy M, Ramana DV, Chandrasekharam M (2014) A novel carbazole-
phenothiazine dyad small molecule as a non-fullerene electron acceptor for polymer bulk hetero-
junction solar cells. RSC Adv 4:33279–33285
420 A. Mishra

Shin K-S, Lee K-H, Lee HH, Choi D, Kim S-W (2010) Enhanced power conversion efficiency of
inverted organic solar cells with a ga-doped ZnO nanostructured thin film prepared using aqueous
solution. J Phys Chem C 114(37):15782–15785
Sims L, Hörmann U, Hanfland R, MacKenzie RCI, Kogler FR, Steim R, Brütting W, Schilinsky P
(2014) Investigation of the s-shape caused by the hole selective layer in bulk heterojunction solar
cells. Org Electron 15(11):2862–2867
Singh A, Dey A, Das D, Iyer PK (2016) Effect of dual cathode buffer layer on the charge carrier
dynamics of rrP3HT:PCBM based bulk heterojunction solar cell. ACS Appl Mater Interfaces
8(17):10904–10910
Small CE, Chen S, Subbiah J, Amb CM, Tsang S-W, Lai T-H, Reynolds JR, So F (2012) High-
efficiency inverted dithienogermole-thienopyrrolodione-based polymer solar cells. Nat Photon
6(2):115–120
Song CE, Ryu KY, Hong S-J, Bathula C, Lee SK, Shin WS, Lee J-C, Choi SK, Kim JH, Moon S-J
(2013) Enhanced performance in inverted polymer solar cells with D–π–A-type molecular dye
incorporated on ZnO buffer layer. Chemsuschem 6(8):1445–1454
Song C, Liu X, Li X, Wang Y-C, Wan L, Sun X, Zhang W, Fang J (2018) Perylene diimide-based
zwitterion as the cathode interlayer for high-performance nonfullerene polymer solar cells. ACS
Appl Mater Interfaces 10(17):14986–14992
Soultati A, Fakharuddin A, Polydorou E, Drivas C, Kaltzoglou A, Haider MI, Kournoutas F, Fakis
M, Palilis LC, Kennou S, Davazoglou D, Falaras P, Argitis P, Gardelis S, Kordatos A, Chroneos
AI, Schmidt-Mende L, Vasilopoulou M (2019) Lithium doping of ZnO for high efficiency and
stability fullerene and non-fullerene organic solar cells. ACS Appl Energy Mater 2(3):1663–1675
Steim R, Choulis SA, Schilinsky P, Brabec CJ (2008) Interface modification for highly efficient
organic photovoltaics. Appl Phys Lett 92(9):093303
Stubhan T, Oh H, Pinna L, Krantz J, Litzov I, Brabec CJ (2011) Inverted organic solar cells using
a solution processed aluminum-doped zinc oxide buffer layer. Org Electron 12(9):1539–1543
Stubhan T, Salinas M, Ebel A, Krebs FC, Hirsch A, Halik M, Brabec CJ (2012) Increasing the fill
factor of inverted P3HT:PCBM solar cells through surface modification of Al-doped ZnO via
phosphonic acid-anchored C60 SAMs. Adv Energy Mater 2(5):532–535
Stubhan T, Litzov I, Li N, Salinas M, Steidl M, Sauer G, Forberich K, Matt GJ, Halik M, Brabec CJ
(2013) Overcoming interface losses in organic solar cells by applying low temperature, solution
processed aluminum-doped zinc oxide electron extraction layers. J Mater Chem A 1:6004–6009
Subbiah J, Amb CM, Irfan I, Gao Y, Reynolds JR, So F (2012) High-efficiency inverted polymer
solar cells with double interlayer. ACS Appl Mater Interfaces 4(2):866–870
Subbiah J, Purushothaman B, Chen M, Qin T, Gao M, Vak D, Scholes FH, Chen X, Watkins SE,
Wilson GJ, Holmes AB, Wong WWH, Jones DJ (2015) Organic solar cells using a high-molecular-
weight benzodithiophene–benzothiadiazole copolymer with an efficiency of 9.4%. Adv Mater
27(4):702–705
Suemori K, Miyata T, Yokoyama M, Hiramoto M (2005) Three-layered organic solar cells incor-
porating a nanostructure-optimized phthalocyanine: fullerene codeposited interlayer. Appl Phys
Lett 86(6):063509–063503
Sun Y, Seo JH, Takacs CJ, Seifter J, Heeger AJ (2011) Inverted polymer solar cells integrated with
a low-temperature-annealed sol-gel-derived ZnO film as an electron transport layer. Adv Mater
23(14):1679–1683
Sun K, Zhao B, Murugesan V, Kumar A, Zeng K, Subbiah J, Wong WWH, Jones DJ, Ouyang J
(2012) High-performance polymer solar cells with a conjugated zwitterion by solution processing
or thermal deposition as the electron-collection interlayer. J Mater Chem 22:24155–24165
Sun C, Wu Z, Hu Z, Xiao J, Zhao W, Li H-W, Li Q-Y, Tsang S-W, Xu Y-X, Zhang K, Yip H-L,
Hou J, Huang F, Cao Y (2017) Interface design for high-efficiency non-fullerene polymer solar
cells. Energy Environ Sci 10(8):1784–1791
Takanezawa K, Hirota K, Wei Q-S, Tajima K, Hashimoto K (2007) Efficient charge collection with
ZnO nanorod array in hybrid photovoltaic devices. J Phys Chem C 111(19):7218–7223
18 Interfacial Materials for Organic Solar Cells 421

Tang Z, Tress W, Bao Q, Jafari MJ, Bergqvist J, Ederth T, Andersson MR, Inganäs O (2014)
Improving cathodes with a polymer interlayer in reversed organic solar cells. Adv Energy Mater
4:1400643
Tress W, Leo K, Riede M (2011) Influence of hole-transport layers and donor materials on open-
circuit voltage and shape of I–V curves of organic solar cells. Adv Funct Mater 21(11):2140–2149
Trost S, Zilberberg K, Behrendt A, Riedl T (2012) Room-temperature solution processed SnOx as
an electron extraction layer for inverted organic solar cells with superior thermal stability. J Mater
Chem 22:16224–16229
Trost S, Zilberberg K, Behrendt A, Polywka A, Görrn P, Reckers P, Maibach J, Mayer T, Riedl T
(2013) Overcoming the “Light-Soaking” issue in inverted organic solar cells by the use of Al:ZnO
electron extraction layers. Adv Energy Mater 3(11):1437–1444
Trost S, Behrendt A, Becker T, Polywka A, Görrn P, Riedl T (2015) Tin oxide (SnOx) as universal
“Light-Soaking” free electron extraction material for organic solar cells. Adv Energy Mater
5(17):1500277
van Reenen S, Kouijzer S, Janssen RAJ, Wienk MM, Kemerink M (2014) Origin of work function
modification by ionic and amine-based interface layers. Adv Mater Interfaces 1(8):1400189
Vasilopoulou M, Georgiadou DG, Soultati A, Boukos N, Gardelis S, Palilis LC, Fakis M, Skoulatakis
G, Kennou S, Botzakaki M, Georga S, Krontiras CA, Auras F, Fattakhova-Rohlfing D, Bein
T, Papadopoulos TA, Davazoglou D, Argitis P (2014) Atomic-layer-deposited aluminum and
zirconium oxides for surface passivation of TiO2 in high-efficiency organic photovoltaics. Adv
Energy Mater 4(15):1400214
Vinokur J, Shamieh B, Deckman I, Singhal A, Frey GL (2016) Mechanisms for spontaneous gen-
eration of interlayers in organic solar cells. Chem Mater 28(24):8851–8870
Vogel M, Doka S, Breyer C, Lux-Steiner MC, Fostiropoulos K (2006) On the function of a
bathocuproine buffer layer in organic photovoltaic cells. Appl Phys Lett 89(16):163501
Waldauf C, Morana M, Denk P, Schilinsky P, Coakley K, Choulis SA, Brabec CJ (2006) Highly
efficient inverted organic photovoltaics using solution based titanium oxide as electron selective
contact. Appl Phys Lett 89(23):233517
Walzer K, Männig B, Pfeiffer M, Leo K (2007) Highly efficient organic devices based on electrically
doped transport layers. Chem Rev 107(4):1233–1271
Wang G, Jiu T, Tang G, Li J, Li P, Song X, Lu F, Fang J (2014a) Interface modification of ZnO-based
inverted PTB7:PC71BM organic solar cells by cesium stearate and simultaneous enhancement
of device parameters. ACS Sustainable Chem Eng 2(5):1331–1337
Wang J, Zhang Z-G, Qi b, Jin Z, Chi D, Qi Z, Li Y (2014b) Perylene diimides: a thickness-insensitive
cathode interlayer for high performance polymer solar cells. Energy Environ Sci 7(6):1966–1973
Wang F, Za Tan, Li Y (2015) Solution-processable metal oxides/chelates as electrode buffer layers
for efficient and stable polymer solar cells. Energy Environ Sci 8(4):1059–1091
Wang Z, Li Z, Xu X, Li Y, Li K, Peng Q (2016) Polymer solar cells exceeding 10% efficiency enabled
via a facile star-shaped molecular cathode interlayer with variable counterions. Adv Funct Mater
26(26):4643–4652
Wang C, Liu Z, Li M, Xie Y, Li B, Wang S, Xue S, Peng Q, Chen B, Zhao Z, Li Q, Ge Z, Li Z (2017)
The marriage of AIE and interface engineering: convenient synthesis and enhanced photovoltaic
performance. Chem Sci 8(5):3750–3758
Wei J, Ji G, Zhang C, Yan L, Luo Q, Wang C, Chen Q, Yang J, Chen L, Ma C-Q (2018) Silane-
capped ZnO nanoparticles for use as the electron transport layer in inverted organic solar cells.
ACS Nano 12(6):5518–5529
White MS, Olson DC, Shaheen SE, Kopidakis N, Ginley DS (2006) Inverted bulk-heterojunction
organic photovoltaic device using a solution-derived ZnO underlayer. Appl Phys Lett
89(14):143517
Worfolk BJ, Hauger TC, Harris KD, Rider DA, Fordyce JAM, Beaupré S, Leclerc M, Buriak JM
(2012) Work function control of interfacial buffer layers for efficient and air-stable inverted low-
bandgap organic photovoltaics. Adv Energy Mater 2(3):361–368
422 A. Mishra

Xiao Z, Jia X, Ding L (2017) Ternary organic solar cells offer 14% power conversion efficiency.
Sci Bull 62(23):1562–1564
Xie Z, Xiao B, He Z, Zhang W, Wu X, Wu H, Würthner F, Wang C, Xie F, Liu L, Ma Y, Wong W-Y,
Cao Y (2015) Self-assembled perylene bisimide J-aggregates as promising cathode modifiers for
highly efficient inverted polymer solar cells. Mater Horiz 2(5):514–518
Xiong S, Hu L, Hu L, Sun L, Qin F, Liu X, Fahlman M, Zhou Y (2019) 12.5% flexible nonfullerene
solar cells by passivating the chemical interaction between the active layer and polymer interfacial
layer. Adv Mater:1806616
Xu B, Hou J (2018) Solution-processable conjugated polymers as anode interfacial layer materials
for organic solar cells. Adv Energy Mater 8(20):1800022
Xu B, Zheng Z, Zhao K, Hou J (2016) A bifunctional interlayer material for modifying both the
anode and cathode in highly efficient polymer solar cells. Adv Mater 28(3):434–439
Yan Y, Cai F, Yang L, Li J, Zhang Y, Qin F, Xiong C, Zhou Y, Lidzey DG, Wang T (2017) Light-
soaking-free inverted polymer solar cells with an efficiency of 10.5% by compositional and
surface modifications to a low-temperature-processed TiO2 Electron-transport layer. Adv Mater
29(1):1604044
Yao K, Salvador M, Chueh C-C, Xin X-K, Xu Y-X, deQuilettes DW, Hu T, Chen Y, Ginger DS,
Jen AK-Y (2014) A general route to enhance polymer solar cell performance using plasmonic
nanoprisms. Adv Energy Mater 4(9):1400206
Ye H, Hu X, Jiang Z, Chen D, Liu X, Nie H, Su S-J, Gong X, Cao Y (2013) Pyridinium salt-based
molecules as cathode interlayer for enhanced performance in polymer solar cells. J Mater Chem
A 1:3387–3394
Yin Z, Zheng Q, Chen S-C, Cai D (2013) Interface control of semiconducting metal oxide layers
for efficient and stable inverted polymer solar cells with open-circuit voltages over 1.0 Volt. ACS
Appl Mater Interfaces 5(18):9015–9025
Yin Z, Zheng Q, Chen S-C, Cai D, Zhou L, Zhang J (2014) Bandgap tunable Zn1-x Mgx O thin films
as highly transparent cathode buffer layers for high-performance inverted polymer solar cells.
Adv Energy Mater 4:1301404
Yin Z, Wei J, Zheng Q (2016) Interfacial materials for organic solar cells: recent advances and
perspectives. Adv Sci 3(8):1500362
Yip H-L, Jen AKY (2012) Recent advances in solution-processed interfacial materials for efficient
and stable polymer solar cells. Energy Environ Sci 5:5994–6011
You J, Chen C-C, Dou L, Murase S, Duan H-S, Hawks S, Xu T, Son HJ, Yu L, Li G, Yang Y (2012)
Metal oxide nanoparticles as an electron-transport layer in high-performance and stable inverted
polymer solar cells. Adv Mater 24(38):5267–5272
Yu J, Shen T-L, Weng W-H, Huang Y-C, Huang C-I, Su W-F, Rwei S-P, Ho K-C, Wang L (2012)
Molecular design of interfacial modifiers for polymer-inorganic hybrid solar cells. Adv Energy
Mater 2(2):245–252
Yu W, Huang L, Yang D, Fu P, Zhou L, Zhang J, Li C (2015) Efficiency exceeding 10% for inverted
polymer solar cells with a ZnO/ionic liquid combined cathode interfacial layer. J Mater Chem A
3(20):10660–10665
Yu J, Xi Y, Chueh C-C, Zhao D, Lin F, Pozzo LD, Tang W, Jen AKY (2016) A room-temperature
processable pdi-based electron-transporting layer for enhanced performance in PDI-based non-
fullerene solar cells. Adv Mater Interfaces 3(18):1600476
Yusoff ARBM, Kim D, Kim HP, Shneider FK, da Silva WJ, Jang J (2014) A high efficiency solution
processed polymer inverted triple-junction solar cell exhibiting a power conversion efficiency of
11.83%. Energy Environ Sci 8:303–316
Za Tan, Qian D, Zhang W, Li L, Ding Y, Xu Q, Wang F, Li Y (2013) Efficient and stable polymer solar
cells with solution-processed molybdenum oxide interfacial layer. J Mater Chem A 1:657–664
Zhang Z-G, Li H, Qi Z, Jin Z, Liu G, Hou J, Li Y, Wang J (2013) Poly(ethylene glycol) modified
[60]fullerene as electron buffer layer for high-performance polymer solar cells. Appl Phys Lett
102(14):143902
18 Interfacial Materials for Organic Solar Cells 423

Zhang W, Wu Y, Bao Q, Gao F, Fang J (2014) Morphological control for highly efficient inverted
polymer solar cells via the backbone design of cathode interlayer materials. Adv Energy Mater
4(12):1400359
Zhang H, Yao H, Hou J, Zhu J, Zhang J, Li W, Yu R, Gao B, Zhang S, Hou J (2018a) Over 14%
efficiency in organic solar cells enabled by chlorinated nonfullerene small-molecule acceptors.
Adv Mater:1800613
Zhang S, Qin Y, Zhu J, Hou J (2018b) Over 14% efficiency in polymer solar cells enabled by a
chlorinated polymer donor. Adv Mater 30(20):1800868
Zhao X, Xu C, Wang H, Chen F, Zhang W, Zhao Z, Chen L, Yang S (2014) Application of biuret,
dicyandiamide, or urea as a cathode buffer layer toward the efficiency enhancement of polymer
solar cells. ACS Appl Mater Interfaces 6(6):4329–4337
Zhao K, Ye L, Zhao W, Zhang S, Yao H, Xu B, Sun M, Hou J (2015) Enhanced efficiency of polymer
photovoltaic cells via the incorporation of a water-soluble naphthalene diimide derivative as a
cathode interlayer. J Mater Chem C 3(37):9565–9571
Zhao Y, Schwab MG, Kiersnowski A, Pisula W, Baumgarten M, Chen L, Müllen K, Li C
(2016) Trifluoromethyl-functionalized bathocuproine for polymer solar cells. J Mater Chem C
4(21):4640–4646
Zheng Z, Zhang S, Wang J, Zhang J, Zhang D, Zhang Y, Wei Z, Tang Z, Hou J, Zhou H (2019)
Exquisite modulation of ZnO nanoparticle electron transporting layer for high-performance
fullerene-free organic solar cell with inverted structure. J Mater Chem A 7(8):3570–3576
Zhou Y, Fuentes-Hernandez C, Shim J, Meyer J, Giordano AJ, Li H, Winget P, Papadopoulos T,
Cheun H, Kim J, Fenoll M, Dindar A, Haske W, Najafabadi E, Khan TM, Sojoudi H, Barlow S,
Graham S, Brédas J-L, Marder SR, Kahn A, Kippelen B (2012a) A universal method to produce
low-work function electrodes for organic electronics. Science 336(6079):327–332
Zhou Y, Fuentes-Hernandez C, Shim JW, Khan TM, Kippelen B (2012b) High performance poly-
meric charge recombination layer for organic tandem solar cells. Energy Environ Sci 5:9827–9832
Zilberberg K, Behrendt A, Kraft M, Scherf U, Riedl T (2013) Ultrathin interlayers of a conju-
gated polyelectrolyte for low work-function cathodes in efficient inverted organic solar cells. Org
Electron 14(3):951–957