Renewable and Sustainable Energy Reviews 67 (2017) 641–650

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Renewable and Sustainable Energy Reviews

journal homepage: www.elsevier.com/locate/rser

A review on analysis and development of solar flat plate collector

Krishna Murari Pandey n, Rajesh Chaurasiya
Department of Mechanical Engineering, National Institute of Technology, Silchar, Assam 788010, India

art ic l e i nf o a b s t r a c t

Article history: Solar flat plate collectors are devices used to trap solar thermal energy and use it for heating applications
Received 20 January 2016 like water heating, room heating and other industrial applications. Flat plate collectors are popular for
Received in revised form low and medium heating applications and there are undergoing constant development in terms of size
19 May 2016
reduction and enhanced efficiency. This paper presents an overview on the different techniques that are
Accepted 14 September 2016
employed to enhance the efficiency of flat late collectors. Effect of using nanofluids as heat transfer fluid,
effect of altering absorber plate design for better capture of radiation, methods of heat loss reduction, use
Keywords: of polymer, employing mini channels for fluid flow, using PCM (phase changing materials) to provide
Flat plate collector heat during night without tank and effect of use of enhancement devices like inserts and reflector have
Efficiency of solar flat plate collector
been discussed in this paper. A brief insight on various techniques used to analyse the effects and various
Heat transfer
designs has also been presented with the development methodology. Some analytical studies and CFD
Numerical analysis
Nanofluids models have also been mentioned. This review paper also deals with the suggestions for the research
work which can be carried out in the direction of heat transfer from solar flat plate collectors.
& 2016 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 641
2. Analysis methods and techniques. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 642
3. Development of collectors: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 643
3.1. Use of polymer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 643
3.2. Mini and micro channels for heat transfer fluid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 644
3.3. Nanofluids as heat transfer fluid. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 644
3.4. Absorber plate designs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 646
3.5. Employing PCM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 647
3.6. Heat loss reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 647
3.7. Use of enhancement devices. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 648
4. Scope of further research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 648
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 650

1. Introduction concentrating type can be further divided in flat plate collector

and evacuated tube collectors. Flat plate collectors are the most
Solar collector is a kind of heat exchanger wherein heat ex- common type of collectors and the most primitive too. Work of
change takes between a distance source and a heat transfer fluid Hottel and Woertz [36] in 1942 and by Hottel and Whiller [37] in
flowing in the collector [35]. Solar radiation from sun hits the 1958 can be looked as a first work on solar flat plate collector. They
absorber plate of the collector and the thermal energy is then had developed the collectors consisting of a black flat plate ab-
transferred to the fluid. Based on their design, solar collectors can sorber, a transparent cover, heat transfer fluid and an insulating
be classified as concentrating and non-concentrating type. Non- case. Tabor [38] in 1955, employed selective black surfaces to in-
crease collector efficiency. His experiments on optical concentra-
n
Corresponding author. tion revealed the ability of optical concentration to produce high
E-mail address: kmpandey2001@yahoo.com (K.M. Pandey). pressure steam. Many studies have been done after that to analyse

http://dx.doi.org/10.1016/j.rser.2016.09.078
1364-0321/& 2016 Elsevier Ltd. All rights reserved.
642 K.M. Pandey, R. Chaurasiya / Renewable and Sustainable Energy Reviews 67 (2017) 641–650

and improve the thermal efficiency of the collector. analysis was done with cover plate and without cover plate, with
Due to growing energy problems, solar energy is been looked at water flow and without water flow. Solar radiation flux was varied
as source of infinite energy. Solar collector have been greatly in the CFD model analysis, keeping other parameters constant, so
studied in this matter. Many of the new designs have been de- as to make it more realistic as the experiment. Findings of this
veloped after 1990. Various research works are being carried out comparative study showed a good agreement in results from ex-
over the world to improve the thermal performance of flat plate periments and CFD despite the experimental imperfections. The
collectors. Polymers are used to build novel collectors so as to results showed that CFD can be confidently used to evaluate
reduce the weight. Use of nanofluids make the collectors compact thermal performance of flat plate collectors. Grine et al. [26]
by giving the same output as that of big collectors but in a com- worked with an air solar collector with a view to estimate the local
paratively smaller sizes [33]. Absorber plates have undergone and mean temperature field in the fluid, outlet fluid temperature,
many modifications with the help of new and better techniques in wall temperatures, heat flow from air to wall and coefficient of this
manufacturing and material science [9,25]. Studies are also fo- convective heat transfer. Due to complications in numerical
cused to find the optimum spacing between the glass covers in methods, authors developed an analytical model to simulate the
multi-glazed collector. Indoor and outdoor tests are carried out thermal behaviour of flat plate solar collector operating in forced
based on the operating and design parameters to obtain the best convection. A solution for energy equation was developed for a
possible design for desired working conditions. To increase the fluid flow inside the collector, based on the Green's function
efficiency, it is very much important to decrease the heat loss from method. With the help of this model, it is possible to estimate the
the collector. Many studies are oriented in this direction to study two dimensional air temperature profiles inside and outside the
and reduce heat loss, with increased glazing, honey comb maze collector and also, we may calculate the local coefficient of heat
based absorber plate, considering wind velocity in analysis of transfer. Design parameters such as solar collector wall tempera-
collector etc. ture, the flow temperature and the local Nusselt number can also
Analysing the performance of collector is equally important as be calculated by this model. According to the results obtained,
it helps to further develop and improve the design. In fact, de- Nusselt number showed a decreasing trend after 1.2 m from
velopment and analysis always go hand in hand. Experimental leading edge and hence authors decide to add fins to their model
analysis of all the different collectors developed is not feasible. after 1.2 m from the leading edge of the collector in order to in-
Hence analytical and numerical methods have been employed, to crease the performance. The analytical model was confirmed by
approximately analyse the behaviour of collector in different experimental results.
conditions. CFD codes have helped researchers to a great extent. Flat plate solar collectors are normally used for applications
Ability to reduce lead time, to study system under hazardous such as water heating, space heating, for providing process heat in
condition and study of controlled experiments of systems which industries, etc. In these practical applications, collectors are bound
are difficult or impossible to perform practically are some of the to work under dynamic conditions. For proper analysis of thermal
unique advantages of CFD [39]. performance of such system, dynamic analysis is thus important.
In the present study, authors have made an overview of dif- Steady state model derived from steady state test (SST) do not
ferent development and analysis techniques that have been em- consider dynamic conditions, hence it's necessary to build a dy-
ployed by the researchers to increase the overall performance of namic model. Many researchers have worked on this topic to build
collector. Studies include designing and analysis of novel absorber various models. One such model was built by Deng et al. [13].
plate designs, analysing the effect of nanofluids (as heat transfer Many of the models built earlier, were not so accurate in de-
fluid) on the efficiency of collector, ways to reduce heat loss to termining the momentary thermal characteristics of outlet tem-
surrounding, analysis of collectors made up of polymers and effect perature and useful heat gain when the instantaneous solar ra-
of add-ons like reflectors and inserts have been discussed here. diation changed sharply. Authors worked on the issue and built an
analytical model in the form of series expansion based on the
consideration of effective thermal capacitance, to determine mo-
2. Analysis methods and techniques mentary thermal characteristics of the collector. Model introduced
a thermal inertia correction along with steady state useful gain to
Yasin and Hakan [1] did a comparative analysis of natural accurately predict the instantaneous useful heat gain. Experiment
convection in flat plate and wavy solar collectors. Analysis was was performed on air solar collector and the data obtained was
based on Rayleigh number, inclination angle, aspect ratio and used to validate the model. Model was fund to accurately predict
wavelength. Laminar flow and thermal field simulation was done the momentary thermal characterises of air solar collector. Also, it
with the help of CFDRC commercial software. Mathematical model could also be used for other collectors provided, they are eligible
was developed neglecting the viscous dissipation and assuming for SST. Cerón et al. [2] developed a 3-D numerical model for flat
constant fluid properties. Vertical walls were considered as adia- plate collector for calculating the efficiency of collector. Different
batic. Results showed that shape and inclination angle of the col- heat interactions like solar radiation absorption, transmission and
lector greatly affected natural convection and heat transfer. Heat reflection;natural convection in the air cavity; heat conduction
transfer in case of wavy collector was more than that of flat plate across the tube-absorber welded junction; mixed convection flow
collector in all the cases. Variation of Nusselt number was found to in the risers; and heat losses by convection and radiation to the
be linear in case of flat plate collector while wavy in case of wavy ambient were taken into account and to check model reliability,
collector. With the decrease in wavelength, the mean Nusselt the heat transfer results inside the risers and in the air cavity were
number increased for same aspect ratio. Contrary to flat plate contrasted with well-known experimental correlations. The re-
collector, Rayleigh number was found to be highest at the highest sults obtained from the numerical model was validated from the
inclination angle. Selmi et al. [29] worked to find the usability and experimental data. Geometric model was based on the standard
validity of CFD models for evaluating solar collector on the basis of flat-plate solar thermal collector and was a reduced to 3 pipe for
their thermal performance. Authors obtained a 3-D temperature numerical analysis, maintaining the aspect ratio. A steady-state
distribution over the whole volume of collector and used it to uniform flow distribution at tube inlet was considered for nu-
validate the CFD model. An experimental setup was built in order merical modelling with three dimensional governing system of
to experimentally calculate the outlet temperature of the fluid equations based on RANS turbulence model. To account for
which may be used to compare it with CFD model's result. The buoyancy effects, Boussinesq approximation had been employed.
 K.M. Pandey, R. Chaurasiya / Renewable and Sustainable Energy Reviews 67 (2017) 641–650 643

Turbulence modelling was employed to account for free convec- buoyancy effect which dominates for a lower flow rate and has less
tion flow between glass cover and absorber plate and solar load influence at higher rates.
implemented combined Solar Ray Tracing (SRT) algorithm and
Surface to Surface (S2S) radiation model. Computational domain
was divided into two subdomains to reduce grid size. One of them 3. Development of collectors:
considered the working fluid tube-absorber assembly and the
corresponding insulation volumes and the other looked for air gap As discussed earlier, to effectively trap the heat from sun, new
and its lateral insulation. FLUENT was used to carry out calcula- techniques and methodology has to be applied. These techniques
tions based on FVM method. Discretization was done by second- aims at increasing the thermal efficiency and overall performance
order UPWIND scheme for momentum, energy and turbulent of the collector. Many studies have been done by various scholars
transport equations and the SIMPLE algorithm for velocity–pres- to enhance collector performance. The techniques include, use of
sure coupling. Results showed that the Nusselt number was found various different materials for the construction of collector, chan-
to be high in the fully developed region for the tube-side heat ges in absorber plate design, employing different heat transfer
transfer case as compared to horizontal pipe with uniformly he- fluids to absorb heat etc.
ated wall. Radiation model implemented, accurately reproduced
the process of absorption-transmission of solar load. Similar trend 3.1. Use of polymer
was found for averaged Nusselt number obtained inside the air
cavity with experimental correlation. Overall, numerical and ex- Martinopoulos et al. [3] worked on polymer solar collector.
perimental results showed a good agreement in solar collector Authors developed a polymer solar collector and investigated it
behaviour and accurate accordance in the optical factor and a good experimentally and with CFD. Advantage of polymer type over
trend for the thermal loss coefficient was achieved. metal type is its low cost manufacturing and light weight. About
Kessentini et al. [32] in their work demonstrated the perfor- 50% reduction in wt. can be achieved can be achieved by em-
mance of flat plate collector with plastic transparent insulated ploying this kind of collector and also, material cost is also greatly
material and economic overheating protection system designed to reduced. The polymer material used should be able to withstand
work in the temperature range of 80–120 oC. Protection system liquid pressure and vast temperature variations. For effective heat
consisted of a ventilation channel with a thermally actuated door transfer to the heat transfer fluid, which also acts as absorber, the
which would protect the system to move to stagnation condition. polymer material in which fluid is flowing should have high re-
Authors performed experiments on the collector prototype si- fractive index, comparable with glass or better than that, should
multaneously with a numerical modelling. Tests were carried out have low emissivity and durability to weathering from U.V. To
in indoor and outdoor conditions to observe the effectiveness of cater these needs, hydraulic channels of transparent, U.V. stabi-
the system and validate the model. Numerical model implemented lized, honeycombed LEXAN sheet was incorporated by the authors.
was based on the resolution of different components i.e. heat 1000/1 solution of black Indian ink was used as heat transfer fluid,
transfer analysis of separate components and then integrating sides were insulated by polyurethane and whole assembly was
each of them with modular object oriented tool. Main advantage of packed in aluminium casing. For measurements, an ISO 9806-1
this tool is that each object or component could be replaced by test bed was employed and data from various measuring devices
high level CFD object in order to parallelize whole system and and sensors was stored in computer. CFD modelling was done on a
decrease computational time. There was a good agreement in the fully structured grid of 1.2 million computational cells and 13
two results. Authors also carried out a parametric study to opti- computational block giving special attention to inlet and outlet
mise the collector design by altering component configuration of pipes and regions close to solid walls. Collector channel region was
FPC. 3125 configurations were tested by means of virtual proto- modelled as porous medium region to account for honey comb
typing and the most promising of them was spotted. In- structure and save computational power and time. Grid formation
stantaneous efficiency of this collector was 0.518 and stagnation was done with NUMECA/IGG and computation was done with
temperature of plastic transparent insulated material cover was FLUENT. Gravity was considered with buoyancy effect in the mo-
114.6 °C. mentum equation due to large size of collector. Experimental re-
Investigations have shown that flow distribution has a great sult of averaged efficiency showed great agreement with the
influence on efficiency of the solar collector. For a uniform flow, computational results. With the help of computational model, it
efficiency is high. In their study, Fan et al. [22], investigated the was found that there were certain problematic regions in the
flow distribution and temperature distribution of the collector model which led to decrease in efficiency. A further study is pos-
fluid, theoretically and experimentally. Authors studied the heat sible by modifying the computational model to remove or de-
transfer and fluid flow characteristics of propylene glycol/water crease such regions so as to increase overall efficiency of collector.
mixture, in the collector by means of a CFD model and experiment On similar grounds, Missirliset al. [11] investigated the heat
was carried out on a prototype of a proposed model to evaluate transfer behaviour of polymer solar collector for different manifold
the flow distribution through absorber from temperature dis- configurations. For full utilization of collectors with minimum cost,
tribution data obtained on the backside of the absorber tube. either the materials should be chosen wisely or the design of the
Parameters such as flow rate, properties of solar collector fluid, collector should be done so as to increase the performance. In
solar collector fluid inlet temperature and collector tilt angle were their work, authors varied the manifold configurations, i.e. they
taken into account for the analysis. Results from both, CFD simu- varies positions of inlet and outlet pipes and studied their effect
lations and thermal analysis, showed a good agreement for high with the help of CFD. CFD model used by the authors was the
flow rates, but large differences were obtained for the lower once. previously developed and validated model. By varying the inlet
It was revealed that for higher flow rates, flow distribution was temperatures such that the operational range is covered, they
uniform. With the decrease in flow rate and decrease concentra- analysed the thermal behaviour and efficiency of the polymer
tion of glycol in the mixture, the flow distribution became highly collector with the help of CFD. Aim of the study was to optimise
non uniform. Also, for increased tilt angle and inlet temperature, the flow field development and to improve thermal behaviour of
flow distribution turned worse, leading to increased risk of boiling the collector. The work revealed that by slight change in geometry,
in upper region of collector. But for higher flow rates, it did not the thermal energy exploited that the collector increased con-
affect much. The reason for this, according to authors may be the siderably with zero manufacturing cost. This shows that CFD is a
644 K.M. Pandey, R. Chaurasiya / Renewable and Sustainable Energy Reviews 67 (2017) 641–650

powerful tool to optimise the design leading to improved conventional collectors. At higher flow rates the thermal perfor-
performance. mance of the collector is improved on the expense of hydraulic
Mintsa Do Ango et al. [21] also worked on polymer solar col- performance. Deng et al. [17] investigated the potential of novel
lector and carried out numerical simulations with a view of opti- flat plate collector consisting of micro-channel heat pipe array.
mising the design of the collector. Conventional collectors are Heat pipe based collectors went through many studies and has
generally manufactured from copper or aluminium and are costly. proved its performance in terms of efficiency and temperature
Using polymer to manufactures collector can overcome this issue. distribution. But there are certain problems associated with it such
In their study, authors, studied the effect of design parameters as, high cost, technical difficulties in manufacturing, high thermal
such as air gap thickness and collector's length and operating resistance due to lower circular contact area, effect on heat
parameters such as mass flow rate, incident solar radiation and transfer due to fin efficiency, scaling problem etc. To get over these
inlet temperature on the efficiency of the collector. The collector drawbacks, microchannel heat pipe array based collector was de-
used for the study was made up of polycarbonate absorber, cov- veloped. These channels was made by thin aluminium sheets with
ered with polycarbonate glazing and insulated with glass wool. It micro grooves to increase the heat transfer. The characteristics
was operated with 45° angle of tilt. Numerical study was carried such as high heat transfer performance, high reliability makes
out assuming constant physical properties, grey and diffuse solid these array superior, high compressive strength, low cost and
surfaces and neglecting the buoyancy effect. Fluid was modelled as small contact resistance makes it superior to the conventional
incompressible Newtonian fluid and flow was treated as laminar. type. Authors firstly performed the preliminary test to verify
Air was considered as ideal transparent gas and system to be thermal performance of the proposed micro channel heat pipe
stationary. The results revealed that the length of the collector array. Results showed instant temperature variation along pipe
didn't have much influence on the efficiency, but air gap did, and with a response time of less than 2 min for different testing po-
the optimum air gap for this particular type of set up was found to sitions. In addition to quick thermal response, it also showed sa-
be 10 mm. increase in mass flow rate increased the efficiency but tisfactory isothermal behaviour with a temperature difference of
decreased the outlet temperature of fluid. Solar radiation did not 1 °C between evaporator and condenser. Preliminary test was thus
had much influence on this type of collector. Efficiency was greatly a success and was followed by testing of collector installed with
affected by the inlet temperature and for better performance, this these pipes. The setup was installed in Beijing and performance
parameter should be at least equal to ambient temperature. Peña test was conducted following the Chinese standard GB/T4271-
and Aguilar [44] studied a polymer solar collector developed by 2007. Results showed a maximum instantaneous efficiency of 80%
Modulo Solar, a Mexican company and found the thermal beha- which is 11.4% superior to required Chinese standard. Authors
viour of polymeric solar collector same as that of metallic collector compared their results with 6 groups of 15 samples and found that
for household applications. Also due to its high percentage of their proposed collector design's maximum instantaneous effi-
elongation, polymers can avoid usage of antifreeze external valve ciency surpasses 25% over avg. level of the selected samples. These
to resist low environmental temperatures. results clearly demonstrate the potential of micro channel heat
pipe array based collectors.
3.2. Mini and micro channels for heat transfer fluid A new structure of flat plate collectors was developed by Wei
et al. [7] in which one large wickless heat pipe was employed in-
Thermal analysis of minichannel-based solar flat-plate collector stead of separate heat pipes. There are many problems related to
was undertaken by Mansour [5] to study the heat transfer char- conventional type collectors in which heat transfer is done wither
acteristics and pressure drop of the working fluid. Collector was by forced convection or free convection. Freezing of water in
made up of an array of minichannels provided in the absorber colder climates, convective and radiative losses and deterioration
plate covered by glass cover. Heat transfer fluid was selected as of pipes due to corrosion are some of them. Heat pipes are now
water. Main advantage of the minichannel/microchannel heat ex- widely applied to flat plate designs to obtain better performance.
changer which pulls researcher is its high heat transfer potential The major advantage of the design provided by authors is the high
combining attributes of high surface area per unit volume, large stability and leak avoidance between the water cooling side and
heat transfer co-efficient and small working fluid inventory. Au- the solar heating side. Authors conducted an experiment to test
thors designed a square minichannel based solar collector with a the thermal performance of this new collector design and then
view to maximize thermal performance with minimum power theoretical energy balance analysis was done for each component
input to the circulating pump. Mathematical model developed for followed by a development of transient heat transfer model to
the analysis was divided into two parts. One for numerical mod- calculate collector efficiency, temperatures of water, the glass
elling for calculating heat transfer coefficient and pressure drop of cover and the absorber plate. For the experiment, the setup con-
working fluid and the other for thermal analysis for the collector sisted of solar heat collector, a water storage tank, a water pump, a
as a whole unit. The numerical model was build laminar flow of valve, and a flow meter. The collector was composed of a glass
Newtonian, incompressible fluid in steady state heat transfer ne- cover, an absorber plate, an insulation layer and an integrated heat
glecting viscous heat dissipation heat generation and any type of pipe in which had fifteen vertical pipes, two horizontal connected
body force or external force. Physical properties of solid material pipes on two ends of the vertical pipes and one working fluid-
and fluid were considered constant and flow was modelled as returning pipe. Alcohol was chosen as a working fluid. Experi-
hydrodynamically and thermally developing. ANYSIS FLUENT 12 ments revealed that a maximum efficiency of 66% was achieved in
was used to solve the set of equations numerically using finite- heating 200 kg of water by 25 °C and could be increased by en-
volume method. To solve pressure and velocity in continuity and hancing thermal insulation.
momentum equations, pressure-correlation scheme was em-
ployed using SIMPLE algorithm. Gauss-Seidel technique was used 3.3. Nanofluids as heat transfer fluid
as an iterative method. In thermal analysis, classical fin analysis
method is employed, in which the solid domain separating the There are many ways to increase the thermal performance of
two fluid channels is treated as thin film. EES was used for de- flat plate collectors. One of the way is to add nano-sized particles
veloping a program code to predict performance of the collector at of high thermal conductivity like carbon, metals, metal oxides etc.,
any operating conditions. Results revealed that minichannel based into the heat transfer fluid to increase the overall conductivity of
collector has higher heat removal factor as compared to the the working fluid. Thermal conductivity of water is low and hence
 K.M. Pandey, R. Chaurasiya / Renewable and Sustainable Energy Reviews 67 (2017) 641–650 645

it is important to find the alternative to improve performance of be increased surface area for smaller particles and also micro-
collector. With the advancements in nanotechnology, a new group convection phenomena taking place between water and smaller
of fluids came into existence, known as nanofluids which consists particles. Water temperature and heat gained also showed an in-
of a nano sized particles suspended in a base fluid. Effect of Cu crease of 12.24% and 24.52% respectively compared to water type.
nanoparticles on the efficiency of flat-plate solar collector was Frictional resistance was a little high for nanofluids. A numerical
investigated by Zamzamian et al. [8]. Cu nanoparticles were analysis of various nanofluids in heat transfer in solar collector
formed from the one step reduction of CuSO4
5H2O with was done by Nasrin et al. [20]. They selected four nanofluids: Ag-
NaH2PO2
H2O in ethylene glycol as a solvent. 10 nm particles were H2O, Cu-H2O, CuO-H2O, and Al2O3-H2O. Solar collector used was
used with a wt. fraction of 0.2% and 0.3%. Experimental rig used flat plate insulated metal box type with dark coloured sinusoidal
had two type of working fluid i.e. ethylene glycol and nanofluid. wavy absorber. Boussinesq model was used to approximate the
Experimentation was done with a variable flow rate to analyse the density of the nanofluid. Study was aimed at investigating the
effect and ASHRAE 93 standards was considered for testing the behaviour of the above nanofluids related to performance such as
performance of the collector. Results showed that the efficiency temperature and velocity distribution, heat transfer coefficients,
decreased with the decrease in volume flow rate and was max- mean velocity and temperatures with respect to the solid volume
imum at 1 l/min. There was an increase in efficiency with the in- fraction of the nanofluids. Numerical model developed based upon
crease in concentration of Cu particles and absorption parameter the heat transfer equation and Navier-Stokes equation. Governing
also showed an increment with use of nano particle. At 1.5 l/min, equation obtained was solved by Galerkin's finite element method.
absorption parameter was maximum. Optimum working condition The integrals of the obtained equation were solved by three points
was found to be 0.3 wt% Cu/EG nanofluid at 1.5 l/min. Another Gaussian quadrature while Newton-Raphson method was im-
work on nanofluids as heat transfer fluid was done by Alim et al. plemented to solve non-linear residual equation. Results showed,
[12] in which they investigated the rate of entropy generation, that highest rate of heat transfer was through the 5% Ag nano-
pressure drop and exergy destruction of flat plate collectors. particle. Collector efficiency raised from 65 to 85% for increase in
Theoretical analysis was done to analyse the effect of nano- Cu from 0 to 5%.
particles: Al2O3, CuO, SiO2 and TiO dispersed in liquid, on the basis Hordy et al. [23] worked on the stability of nanofluids for long
of capability of heat transfer enhancement, entropy generation duration and high temperature. Authors also studied the optical
and pressure drop. The flow of the nanofluid was considered characteristics of various nanofluids made up of a wide con-
steady, laminar and axial. To reduce complexity, parameters such centrationrange of plasma-functionalizedmicro-walled carbon
as nanofluid property, overall heat loss and solar radiation was nanotube (MWCNT) particles. Different nanofluids were made
considered as constant with time. Analysis was done by varying from base fluids like water, ethylene glycol, propylene glycol and
the nanoparticle concentration and nanofluid flow rate and en- Therminol VP-1. Authors found that glycol based nanofluids were
tropy generation and exergy destruction was then mapped as stable for a long duration i.e. about 8-months tests, while water
function of volume concentration of nanoparticles. The results based nanofluid showed gradual agglomeration while Therminol
showed that out of the four metal oxides, CuO could reduce the VP-1 based nanofluid showed high rate of degradation. At high
entropy generation by 4.34% and heat transfer enhancement is temperature i.e. about 85 °C for water and 170 °C for glycol based
brought about by 22.15%. However, there was a small 1.58% in- nanofluids, no agglomeration was detected. This shows that na-
crease in pumping power of CuO nanofluid which is tolerable. In nofluids are stable for long duration and at high temperature.
addition, they found improvement in heat transfer phenomena Optical study showed, that these MWCNT based nanofluids are
and superior convection coefficient with the use of nanoparticles. capable of absorbing solar radiation over a wide spectrum which
There was an improvement in heat transfer feature with an in- can lead to about 100% of solar energy absorption even at low
crease in nanoparticle concentration but no considerable change concentrations. Experimental investigation on MWCNT-H2O car-
was found in the friction factor as compared to the base fluid. It is ried out by Yousefi et al. [42] showed an increase in efficiency of
clear that nanofluid especially CuO based, can be used to effec- collector with the increase of weight fraction from 0.2 to 0.4%.
tively to improve heat transfer characteristics without much in- Experiments also concluded that use of surfactant also leads to
crease in cost. Moghadam et al. [15] also undertook experiments increase in overall efficiency.
on CuO-H2O nanofluid as a working fluid in collector to determine Effect of pH value of nanofluids on the efficiency of solar col-
its effect on efficiency on flat plat collector. Authors selected 99.9% lector was studied by Goudarzi et al. [24]. CuO-H2O and Al2O3-H2O
pure, 40 nm mean particle diameter and a volume fraction was with concentrations of 0.1 wt% and 0.2 wt% respectively were used
fixed to 0.4%. Operating range of mass flow rate was 1–3 kg/min. for the study. Nanofluids were prepared by a two-step process
ASHRAE standard 86–93 was used to evaluate the performance of involving mixing the particle in distilled water followed by ultra-
collector. With the increase in mass flow rate, efficiency initially sonic vibration to homogenize the mixture. To improve the sta-
increased, but after a certain point it showed a decreasing trend. At bility of the mixture, sodium dodecyl sulfonate was added to it as a
optimum mass flow rate, efficiency was found to increase by 16.7% surfactant. Experiment was done in a novel cylindrical solar col-
as compared to water type collector. lector consisting of a cylindrical glass tube cover which has a
He et al. [18] also worked on the nanofluid as working fluid in minimum reflectivity and maximum transmissivity and black he-
solar flat plate collectors. Authors used a Cu-H2O nanofluid as a lical pipe made up of copper as a receiver. Results showed that
working fluid to study the thermal performance. Tests were car- with an increase in a difference between the pH value of nanofluid
ried out using ASHRAE standard 86–93. Particle diameter were and its isoelectric point, the efficiency of the collector also in-
chosen to be 25 nm and 50 nm, mass flow rate of water was creased. For CuO nanofluid having isoelectric point of pH ¼ 9.5,
maintained at 140 l/h and test was carried out in time interval increasing in efficiency of about 52% was obtained at pH ¼3 as
from 9:00 to 16:00. Particle size and wt. fraction were the varying compared to that obtained at pH ¼10.5. The same goes for Al2O3
parameters in the tests. Results showed that the maximum in- having isoelectric point at pH ¼7.4. Efficiency obtained at pH ¼10.5
crease in efficiency obtained was 23.83% greater than that of water was 64.5% greater than that obtained at pH ¼ 9.2. Faizal et al. [33]
type. Efficiency with 25 nm, 0.2 wt% was lower as compared to evaluated nanofluids on the base of economic and environment.
that with 25 nm 0.1 wt%. This shows that efficiency shows a de- Increasing the area of the collector is one way to increase the
creasing trend with increase in wt. fractions of particles. Also with performance, but it would make the collector bigger, heavier and
the increase in particle size, efficiency decreased, the reason may bulkier. In their study, authors aimed to use nanofluids as heat
646 K.M. Pandey, R. Chaurasiya / Renewable and Sustainable Energy Reviews 67 (2017) 641–650

transfer fluid to make a smaller, light and compact collector, de- bond aluminium absorber plate was done by Del Col et al. [9] roll
livering same output temperature of that of bulker ones. With the bond absorber plates are canalised panels manufactured by ap-
use of numerical models, and previous literatures, size reduction, plying a special bonding technique in which a sandwich of two
efficiency, energy savings and cost savings was calculated for CuO, aluminium sheets is formed by a special hot/cold rolling. With the
SiO2, TiO2 and Al2O3, was calculated. It was found that there was use of serigraphic process, desired patterns of channels are printed
10,239 kg, 8625 kg, 8857 kg and 8618 kg reduction in weight for on the plate using a special ink which prevents the welding in
CuO, SiO2, TiO2and Al2O3 respectively for 1000 units of solar col- printed areas. After welding the two plates together, the un-
lector. Nanofluids with high density and low specific heat showed bounded region is inflated by passing high pressure air. A roll bond
higher thermal efficiency, CuO was winner in this segment. Area solar collector with integrated liquid channels is finally completed
reduction of 25.6%, 21.6%, 22.1% and 21.5% solar collector was with inlet and outlet connections. These type of collectors help us
achieved for CuO, SiO2, TiO2 and Al2O3 respectively. With the use to customize and optimise the flow patterns in the absorber
of nanofluids as working fluid, 220 MJ of embodied energy can be leading to a better and uniform temperature distribution across
saved with 170 kg less CO2 emissions in the manufacturing pro- absorber. In their study, authors did a comparative study of these
cess of these collectors. The payback period for these type of col- collectors one with black coating and another with semi-selective
lectors is 2.4 years while that of conventional ones is 2.49 years. coating, with the conventional sheet and tube type collectors, one
Thus the use of nanofluids, makes the collector, more environ- with copper black paint and another with aluminium selective
mental friendly, cost effective and compact. absorber. In all, four collectors were analysed, all having same
Shende, Sundara [34] studied the application of nanofluids in aperture area. Thermal efficiency tests were performed in both
direct absorption solar collectors (DASC). Carbon nanostructures steady-state and quasi dynamic conditions, according to the
like CNT and graphene have high thermal conductivity, unique standard EN 12975-2. In the numerical model, collectors were
optical properties, good mechanical strength, large surface area, treated as a series of overlapping parts and computational proce-
etc. which make them perfect to be used to prepare a nanofluid dure based on the steady state hypothesis was divided in two
working for DASC. In their study, authors synthesized a nitrogen steps: first, neglecting thermal conduction between adjacent
doped hybrid structure of reduced graphene oxide (rGO) and control elements and second, considering thermal conduction.
multi-walled carbon nanotubes (N-(rGO-MWNTs)), a nanoparticle Experiments showed that the efficiency curve of roll bond col-
to be used to create nanofluid for DASC. Base fluids for the nano-
lector with black coating was higher than copper collector for
fluids were de-ionised water (DI water) and ethylene glycol (EG).
same paint. With the increase in no. of tubes of the conventional
Due to the percolation network between MWNTs and rGO through
collectors, its efficiency increased and for twenty eight tube with
intercalation, the nanofluids were quite stable without any ag-
same diameter as that of roll bond absorber, efficiency was found
glomeration or sedimentation. Test based on absorption and
to be same. However, there are technical limitations for manu-
transmittance, was carried out and thermal conductivity of fluids
facturing of such collectors. Such type of collectors are not cost
was studied as a function of temperature. Results revealed a 17.7%
effective and not viable solution. Higher number of channels in roll
greater conductivity for 0.02% of DI and a 15.1% rise for EG. Due to
bond absorber, makes it better option. Numerical simulations have
black colour and increased surface area, these fluids have an ability
also revealed that ambient temperature also plays a vital role in
to absorb radiations ranging from far ultra-violet to far infra-red. A
thermal efficiency. With the increase in ambient temperature,
small fraction of these particles drastically enhance the optical
there is a decrease in efficiency. For proper evaluation of collectors,
properties of the fluid and with an increase in concentration, there
this factor should also be taken care of. Air solar collectors are best
was an increase in absorption. Also, with an increase in tem-
suited for low temperature applications such as space heating,
perature and concentration, thermal conductivity of fluids was
drying of agricultural products etc. Mostly, flat plate absorbers are
found to be increased.
employed in these collector. Design parameter which separates air
3.4. Absorber plate designs collector with water, is the low heat transfer coefficient of a sur-
face in contact in air type collector. Fluid-wall surface area is a
Absorber is the main component of the collector responsible minor concern in water type but is certainly the key parameter in
for absorbing solar radiation of different wavelength. To improve air type. Due to low heat transfer coefficient, the thermal efficiency
the performance of collector, selective coating on absorber is ne- of these collectors is lower. One of the solutions to improve ther-
cessary. Jyothi et al. [25] designed a new tandem absorber made mal efficiency is to increase the surface area and by increasing the
up of nanostructured TiAlC/TiAlCN/TiAlSiCN/TiAlSiCO/TiAlSiO for turbulence inside channel. Based on continuous folding technique,
high thermal power applications. First three layers of the tandem El-Sawi et al. [14] presented an innovative design of chevron
absorber acts as an absorbing layer while TiAlSiCO acts as semi- pattern absorber. Continuous folding technique is a technique
transparent layer and TiAlSiO acts as anti-reflecting layer. The wherein the metal sheet is pre-folded by passing it through a set
metallic behaviour decreases as we move from bottom to top. This of sequential and circumferential grooved rollers and then through
tandem absorber was deposited on a W coated stainless steel cross-folding rollers to create cross fold. The technique is eco-
substrate, having high absorptance of 0.961 and thermal stability nomical, continuous and produces minimum shredding. Authors
upto 650 °C. Authors found the optimum thickness for the tandem undertook experiments to check the reliability of these novel
layers with the help of cross-sectional images obtained from field- collectors and compare with flat plate type. Theoretical analysis
emission scanning electron microscope and later verified it by was done to compare flat plate, v-grooved and chevron type col-
transmission electron microscope. Performance of the absorber lectors. Mass flow rate and inlet temperature were the governing
was evaluated at different temperatures by heating it in air at parameters in the experiment. Experimental results revealed that
vacuum. It showed a high thermal stability in air upto 500 °C for chevron type led to 20% improvement in thermal efficiency and
2 h and long thermal stability upto 325 °C for 400 h. In vacuum, 10 oC increment in temperature, in comparison to flat plate col-
thermal stability upto 900 °C was obtained for 2 h and long term lector. Theoretical results also agreed with the experimental result
thermal stability upto 650 °C for 100 h was obtained under cyclic showing 10% and 20% improved thermal efficiency than v-grooved
heating conditions. type and flat plate type respectively. Chevron pattern, due to its
Experimental and computational analysis of a prototype of a low overall loss coefficient, proved to an economical, efficient and
special type of glazed flat plate collector was done having roll a better alternate design for air solar collectors.
 K.M. Pandey, R. Chaurasiya / Renewable and Sustainable Energy Reviews 67 (2017) 641–650 647

3.5. Employing PCM influence of convection is greater. Unglazed and single-glazed

collector are affected to a great extent by these natural winds, and
Main disadvantage of solar thermal systems is storage of solar to compute their performance accurately, it is necessary to find the
energy. Systems work during day, but to get hot water during convective heat transfer coefficient due to winds. Many wind
night time, storage tanks are needed to be fitted with these sys- tunnel test has been performed in order to investigate this para-
tems which not only make the system bulky, but also make it meter, but as these collectors are always exposed to solar radiation
costlier. PCM i.e. phase change material integrated solar collectors and thus the natural winds, the actual affect may differ from that
shows promising features which can eliminate the need of storage of wind tunnel. Kumar and Mullick [16] performed experiments on
units. The main advantages of these collectors involve large space unglazed solar collector to estimate the heat transfer coefficient
capacity and isothermal behaviour during melting. The drawback from upper region of collector. Experimental setup was mounted
is the low thermal conductivity. Lots of research is going on to on rooftop of building at IIT Delhi, which is a low wind region.
increase the effective thermal conductivity of these PCMs. A si- Experimental data was taken for the months from February to May
milar work is done by Chen et al. [10] to analyse the energy storage for 2 years. With the help of Sigma plot software, linear regression
process of solar collector with an integrated porous structure filled and power regression between wind velocity and heat transfer
with paraffin as the phase change medium. At day time the par- coefficient was made using experimental data. The correlation
affin will absorb the solar radiation and store within. At night, this obtained was compared with previous studies and theoretical
stored energy will be given to water in the capillary tubes which studies for different values of wind heat transfer coefficient at
are embedded in paraffin. Aluminium frame and paraffin ae se- same wind speed. There may be differences in correlation due to
parately modelled due to different thermal diffusivities. For the different experimental conditions and dimensions of collector, but
numerical analysis of this design, momentum conservation equa- authors could find good agreement with some of the previous
tion for paraffin was modelled with Darcy's law with Brinkmane- studies. The results by the author can be successfully used for the
Forchheimer's extension. Heat transfer in aluminium foam with estimation of wind heat transfer coefficient in outdoor conditions
melting paraffin was modelled with two temperature model. Due by taking care of collector size. An interesting study was under-
to its small size and little effect, serpentine pipe was neglected in taken by Vestlund et al. [4] regarding enhancing the performance
the mathematical model. Paraffin was modelled as isotropic and a of flat plate collector by replacing the air in between absorber and
Newtonian fluid when it's melted. Surface tension and curvature glass cover with some other gas, particularly some inert gas. The
effects were neglected and bottom and side walls were treated as main advantage of using the gas in the space is decrease in the
adiabatic. For single phase, physical properties were considered heat transfer rate and at the same time reduction in humidity
independent of temperature while for solid-liquid phase, proper- condensate and dust due the enclosed space filled with gas. The
ties varies linearly with temperature. Governing equation was main problem in using other gases lies in design complications.
discretized by FVM and SIMPLEC algorithm was used to solve the These gases require sealed spaces in which variation in pressure
coupled continuity momentum equation. Firstly, simulation was and volume of gas filling is needed to be controlled. Pressure and
done by the authors to investigate the characteristics of paraffin as volume of the gas to be filled greatly affects the heat transfer
heat storage medium and later investigation of aluminium foam properties and thus the performance of the collector. Authors did a
matrix saturated with paraffin was undertaken as an advanced computational heat transfer analysis of various gases in replace-
storage system. Studies revealed that aluminium has a great effect ment to air. A mathematical model was formulated using single
in increasing the heat transfer rate and melting of paraffin. Tem- dimension heat transfer equation considering convection, con-
perature distribution in aluminium foam with paraffin was more duction and radiation, and calculations were done in Matlab. Di-
uniform as compared to paraffin alone. Another study on appli- mensions and physical properties used for the collector was that of
cation of PCM in solar collector was carried out by Serale et al. [27]. a reference collector: a state of the art flat plate solar collector,
Authors used an earlier developed numerical approach to figure which has antireflective glass and fairly less heat losses. Reference
out the characteristics of slurry PCM based solar collector. Heat collector did not had side wall insulation and its heat transfer rate
carrier fluid used in this study was composed off from the mi- was accounted in the top and bottom heat transfer equation. To
croencapsulated PCM (mPCM) suspended in a mixture of water consider irregularities in the absorber plate, calculations were
and ethylene glycol, the whole mixture showing a constant mac- done by varying the distance between the absorber plate and glass
roscopic fluid properties. Due to suspension of PCM, the fluid cover, and final result was then averaged. Study showed that this
mixture shows enhanced thermal properties. Results from the technique of gas filling can be utilized to make a thinner collector
study demonstrates an increase in instantaneous efficiency upto with same performance or even better performance than the
0.08 compared to the conventional water based collector. There is conventional collectors. CO2 is a cheap gas and resulted in thinner
need of a careful optimisation of the climate and the nominal collector, but its performance was not upto the mark of conven-
melting point of the mPCM, as wrong value may lead to a dete- tional once. On the other hand, inert gases i.e. Argon, Krypton and
rioration in performance of the collector. To further improve the Xenon were far more attractive and showed better performance.
performance, optimisation of parameters like, locations, flow rate They formed thinner collectors as the amount used was reasonable
and mPCM can be done. Heat obtained is always between 20–40% despite their prices. Solar radiation is received to the absorber
(for various conditions) compared to the conventional ones. The plate after passing through glass covers of the flat plate collectors.
only limitation of the proposed system as mentioned by authors, is While it's passing through glass covers, some amount of radiation
that it is impossible to work with mPCM concentrations above 50% gets absorbed and the rest is transmitted to the absorber plate.
because the pumping energy demand might increase as compared This absorption leads to increase in surface temperature of glass
to a conventional system. covers which may alter the heat transfer coefficients. A numerical
study was undertaken by Akhtar and Mullick [19] to investigate
3.6. Heat loss reduction the effect of absorption on heat transfer coefficient single and
double gazed flat plate collectors. Study was focused in finding the
Thermal performance is very much affected by heat losses from effect of absorbed radiation on the inner and outer surface of the
various sections. The upward region is totally exposed to en- glass cover and thus on the convective and radiative heat transfer
vironment and hence, heat lost from this region is significant. Heat coefficients. The numerical model was developed on the basis of
lost is mostly through convection and radiation. Due to wind, the heat transfer equations considering conduction in glass cover(s),
648 K.M. Pandey, R. Chaurasiya / Renewable and Sustainable Energy Reviews 67 (2017) 641–650

convection between cover(s) and space and radiation. The factors numerical simulations of the collector. Operating parameters such
which affected the solar radiation incident and absorbed, such as, as local losses, friction coefficients, Nusselt number were studied
thickness of glass cover and incidence angle, hour angle, declina- as a function of Reynolds number to observe their effects on
tion, location of the place and the orientation of the collector, etc. thermohydraulic performance of collector. To take into account,
were also considered. Based on the numerical study, empirical internal heat transfer coefficient and friction factor, a new collector
relations for computing the inner and outer temperatures were model was developed by authors. All the simulations performed
developed for both, single and double gazed collectors. The results was based on UNE-EN 12975-2 standards. To evaluate pressure
from calculations, considering absorption in glass cover, and ne- drop and heat transfer in the tube, authors used their own cor-
glecting absorption were compared. Comparison revealed 6 °C relations and experimental data. Analysis was done on 2 working
increase in single glazed temperature while 14 °C and 11 °C in- fluids: water and propylene glycol/water mixture, with varied
crease in first and second glass covers of double glazed collectors mass flow rates. Results showed an increase of 4.5% in efficiency of
respectively. The difference between the heat transfer coefficients enhanced collector as compared to the standard collector. Pump-
of absorber plate for the two cases was as high as 49%. ing power for water increased for all the flow rates, but for pro-
Zhang et al. [30] carried out an experimental study considering pylene glycol 44%, there was no increase below 80 kg/h. the in-
heat shield in a direct-flow coaxial evacuated-tube solar collectors crease in pumping power is surely due to an increased friction
to find its effect on thermal efficiency of the collector. A heat shield because of the inserts. Overall, wire inserts are better option to
was added beneath the coaxial pipe so as to prevent the heat loss. enhance the thermal performance and make the collector com-
It was observed that using heat shield, thermal efficiency of col- pacts in warm climatic regions or for warm water, or for small
lector increased to 54.07% at the highest temperature of 123.9 °C applications like household water heating where the pump is
which is 31.49% higher than that of the solar collector without heat slight oversized and hence, there is no change in pumping power.
shield. Heat loss coefficient was improved by 50.08%. Thus heat Wire-coil insert was experimentally studied by García et al. [43], to
shield is a good option and also an economical mean to increase observe its heat transfer enhancement abilities. Experiments were
the performance of evacuated solar collectors. carried out on two collectors with 5 different mass flow rates.
Authors found that with the help of wire-coil insert, there was an
3.7. Use of enhancement devices average increase of efficiency from 14–31% and an increase in
useful power collected of upto 8–12%, with no additional pressure
Inserting a heat enhancement device inside a solar collector losses. The degree of enhancement due to inserts gets deteriorated
pipe is one of the method to improve thermal performance of the with the increasing mass flow rates.
collector without much modification and keeping size compact. Metal heat pipes by many researchers was reported as an ef-
These devices increase the turbulence in the flow thus increasing fective solution to increase the thermal performance of the col-
the heat transfer. A comparative study of few of insert devices with lectors. However, many of the collectors are heavy, non-versatile,
a combined effect of inclination on the efficiency of the collector have complex assembly and installation, possess high hydraulic
was carried out by Sandhu et al. [28]. Authors selected different resistance and low thermal efficiency and lack scalability and
insert devices with varied configuration: 3 types of twisted-tape adaptability for design. To overcome these shortcomings, Rassa-
inserts, 1. twisted tape with shortest pitch, 2. twisted tape with makin et al. [40] applied extruded aluminium alloy heat pipe with
medium pitch 3. twisted tape with largest pitch; 4. types of wire wide fins and longitudinal grooves to the solar collector. Number
coils inserts, 1. Simple coil, 2. Coil away from the tube wall, 3. of fins on absorber plate were taken as arbitrary. Opposite sides of
Concentric coils, 4. Conical coil and Mesh insert. The collector was the heat pipes had fins serving as a heat sink surface. Several tests
tested over a wide range of Reynold's no. 200–8000 and Prandtl were conducted on the new design. Results showed that with the
no. range 5–8, using water as a working fluid. Experiments showed help of this insert, it is possible to reduce the thermal and hy-
that all the used devices led to an increase in Nusselt no. of the draulic resistance. Thermal efficiency was also found to be high.
flow. Nusselt no. showed a heavy increase for transition and tur- The new light weight and inexpensive heat pipe showed high
bulent region while a little less in the laminar region. Considering thermal performance.
the wire family, concentric wire insert showed the best perfor- Tanaka [6] theoretically analysed the effect of using a bottom
mance and from twisted tapes, one with the smallest pitch ratio reflector on the performance of absorber. A gap between a col-
was more promising than the other two. Mesh inserts performed lector and reflector was maintained and were kept front facing to
best in laminar region, increasing the Nusselt no. by 270%, whereas each other at an angle such that the collector receives the reflected
concentric coils showed best results in turbulent region with a radiations from the reflector along with direct and diffused ra-
460% increase in Nusselt no. Mesh inserts showed a better per- diation. By a graphical method, the amount of radiation reflected
formance at laminar level, but it did increase the pumping power and then absorbed was calculated. It was absorbed that by placing
requirement of the system due to increased friction. The authors the bottom reflector at some distance, it is possible to increase the
thus recommended concentric coils as a better option with 110% amount of solar radiation absorbed. The distance between the two
enhancement in laminar region and 460% in turbulent. Test for the must be maintained such that the gap length is less than the
inclination showed that there is no significant enhancement of lengths of collector and reflector. With the change in gap in var-
Nusselt no. due to the channel inclination. On the similar grounds, ious seasons, optimum inclination of collector remains same while
Hobbi and Siddiqui [41] studied the effect of passive heat en- there is slight change in the inclination of reflector. There was a
hancement devices like twisted strip, coil-spring wire and conical decrease in absorbed radiation with the increasing gap length for
ridges. No significant difference in heat flux was observed to the optimum inclination, while the decrease was drastic for other
collector fluid. But a significant increase in Grash of, Richardson inclination.
and Rayleigh numbers was observed indicating that the heat A brief extract of all the development technologies is presented
transfer mode in the collector is of mixed convection type, free in Table 1 below.
convection being predominant. Authors concluded that due to the
high damping effect of shear produced turbulence by buoyancy 4. Scope of further research
forces, the applied inserts were ineffective in enhancing the heat
transfer to the collector. To see the effects of wire inserts in a liquid All the above studies refer to analysis and development of solar
solar collector, Martı´n et al. [31] carried out a TRNSYS based flat collectors. Various numerical models have been built to
 K.M. Pandey, R. Chaurasiya / Renewable and Sustainable Energy Reviews 67 (2017) 641–650 649

Table 1
Brief discussions on solar collector development.

Sr. No. Area of research Enhancements and discussions

1 Polymer as collector material Advantages: cheaper than metal collectors (material and manufacturing cost is low), lighter than metal collectors.
Desired properties: high reflective index, Low emissivity and durability to UV, able to withstand liquid pressure,
compatible with the HTF used.
Output of studies: similar performance as metal collectors.
Length of collector does not affect the efficiency but air gap does.
Increase in mass flow rate increases efficiency but decreases outlet temperature.
2 Mini and micro channels Advantages: high heat transfer potential combining attributes of high surface area per unit volume, large heat transfer
co-efficient and small working fluid inventory.
Desired properties: high heat transfer performance, high reliability, high compressive strength, low cost, small contact
resistance.
Output of studies: mini-channel based collector has higher heat removal factor. At higher flow rates, thermal perfor-
mance increases on expense of hydraulic performance.
Heat pipe channel array have low response time to temperature and show a very high efficiency as compared with the
traditional collectors.
3 Nanofluids as HTF Advantages: cost effective, environmental friendly, compact, light in weight collector can be built using nanofluids as
HTF. Higher efficiency than water based collectors.
Desired properties: nanofluid chosen should stable for long duration of time at varied temperatures i.e. they shouldn’t
go under agglomeration. Nanoparticles used for making nanofluids should have high thermal conduction, large surface
area and good mechanical and optical properties.
Output of studies: increase in efficiency with the increase in no. of nanoparticles. Decrease in efficiency with decrease in
volume flow rate of the nanofluid. Improved heat transfer phenomenon and superior convection co-efficient can be
obtained without a considerable increase in friction factor and pumping power. Efficiency decreases with increase in
weight fractions and size of particles. Overall heat gain increases.
Glycol based nanofluid stable for long duration.
CuO and Cu based nanofluid shows good thermal enhancements.
Area reduction of the collector can be achieved with the help of high density nanofluids.
4 Innovation in absorber plate Advantages: increase in heat absorbed from solar radiation.
design Desired properties: must be capable of absorbing of different wavelengths effectively.
Output of studies: five layered tandem absorber showed high thermal stability at high temperatures for long duration of
time.
Roll bond collector proved to be useful in customization and optimisation of flow patterns inside the absorber, however,
it's difficult to manufacture.
Chevron pattern absorber, due to its unique design have low overall loss co-efficient making it economical and efficient
than the conventional collectors.
5 Employing PCM Advantages: eliminates need for storage units making water heating systems less bulky. Provides large space capacity
and has isothermal behaviour during melting.
Desired properties: suitablephase-transition temperature, Highlatent heat of transition, High thermal conductivity in
both liquid and solid phases, High density, Small volume change, Low vapour pressure, chemically stability and cost
effective.
Output of studies: temperature distribution of aluminium foam matrix saturated with paraffin was better than that of
paraffin alone.
mPCM shows enhanced thermal properties. Have high thermal efficiency than the conventional PCM collectors. How-
ever, it is not possible to work with mPCM with concentrations above 50%.
6 Heat loss reduction techniques Advantages: increased efficiency due to decreased heat loss.
Desired properties: technique used must be cost-effective, must decrease the heat loss by convection and radiation to
the surroundings.
Output of studies: use of other inert gases in the glass-absorber gap, instead of air proves to be effective. Thinner
collectors can be manufactured at reasonable rates using this technique.
Double gazed collector is better than single gazed collector.
Adding heat shield beneath the fluid carrying tubes decreases the heat loss resulting in higher efficiency.
7 Use of enhancement devices Advantages: improvement in thermal performance without any modification and keeping size compact.
Increased turbulence in flow increasing heat transfer in fluid.
Desired properties: should be able to increase the turbulence in flow without much increase in pumping power.
Output of studies: insert devices leads to increase in Nu. no. in flow.
Twisted tape inserts with small pitch were promising.
Mesh inserts are better for laminar flow but increases pumping power.
Concentric coils showed enhanced performance in laminar and turbulence flow without any significant improvement in
pumping power.
Wire inserts improved the efficiency. When propylene glycol is used as HTF with wire insert, there is no increase in
pumping power below certain limits.
Degree of enhancement due to insert gets deteriorated with increasing mass flow rate.
Thermal and hydraulic resistance can be deceased by using heat pipes with groves and fins.
Use of reflectors to direct radiation on collector increases the overall performance of the collector.

accurately evaluate the performance of solar collectors. Analysis is all the above parameters or parameters which are more dominant.
generally based on operating and design parameters. Studies have Optimisation study can be undertook to find the optimum values
shown the effect of various parameters like solar radiation, in- of the design parameters and operating parameters which are
clination angle, ambient conditions, water inlet temperature, air under our control to improve the performance of the collector.
flow rate, etc. Based on these studies, a numerical model can be Development of flat plate collector for improved heat transfer
built considering each parameter as an independent variable and rate can be done by one or more of the following ways: improving
the effects of the required or the desire ones in a final equation to absorber design, choosing material with high thermal conductivity
evaluate efficiency of the collector, which would be a function of and absorptance capacity for absorber, increasing turbulence in
650 K.M. Pandey, R. Chaurasiya / Renewable and Sustainable Energy Reviews 67 (2017) 641–650

the flow by various means, using nanofluids for increased area and [18] He Q, Zeng S, Wang S. Experimental investigation on the efficiency of flat-plate
thermal conductivity, use of mini and micro channels for fluid solarcollectors with nanofluids. Appl Therm Eng 2014:1–7 xxx.
[19] Akhtar N, Mullick SC. Effect of absorption of solar radiation in glass-cover
flow, use of thermal enhancements and inserts, etc. Search for new (s) on heat transfer coefficientsin upward heat flow in single and double
materials for solar collector in a hot trend. In this context, gra- glazed flat-plate collectors. Int J Heat Mass Transf 2012;55:125–32.
phene, a carbon structure, may be a good option. Graphene/water [20] Nasrin R, Parvin S, Alim MA. Heat transfer by nanofluids through a flat plate
solar collector. Procedia Eng 2014;90:364–70.
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