Sains Malaysiana 47(5)(2018): 1061-1067

http://dx.doi.org/10.17576/jsm-2018-4705-24

Energy, Exergy and Efficiency Analysis of a Flat Plate

Solar Collector Used as Air Heater
(Analisis Tenaga, Eksergi dan Kecekapan Pengukur Solar Plat Rata yang Digunakan sebagai Pemanas Air)

MUHAMMAD HANIF*, MANSOOR KHAN KHATTAK, MAAZULLAH KHAN, MUHAMMAD RAMZAN & ABDURAB

ABSTRACT
Air heating by solar collectors is renewable technology providing hot air for different purposes. The present research
emphasizes on analysis of energy, exergy and efficiency of a flat plate solar air heater. The analysis model was tested
on five different air mass flow rates of 0.5 (Natural), 1.31, 2.11, 2.72 and 3.03 kgs-1 under three different tilt angles of
25, 35 (Recommended) and 50o. The data was replicated three times making a total of 45 treatments. A two factorial
completely randomized design was used to find if there is any significant difference among the treatments. The results
showed that the solar collector gave better performance at air mass flow of 3.03 kgs-1 under tilt of 35o. At maximum
air mass flow rate of 3.03 kgs-1and optimum tilt angle of 35o the maximum energetic efficiency of 51%, while minimum
exergetic efficiency of 24% and maximum overall efficiency of 71% were recorded. It was concluded that to get maximum
thermal efficiencies of 71% from flat plate solar collector used as an air heater must be operated at high air mass flow
rates of 3.03 kgs-1under 35o tilt angle at Peshawar, Pakistan.

Keywords: Efficiency; energy; exergy; flat plate solar collector

ABSTRAK
Pemanasan udara oleh pengumpul suria merupakan teknologi yang boleh diperbaharui yang menyediakan udara panas
untuk tujuan berbeza. Penyelidikan yang terkini menekankan tentang analisis tenaga, eksergi dan kecekapan plat rata
pemanas udara solar. Model analisis diuji pada lima kadar aliran jisim udara yang berbeza iaitu 0.5 (semula jadi), 1.31,
2.11, 2.72 dan 3.03 kgs-1 di bawah tiga sudut kecondongan 25, 35 (disyorkan) dan 50o. Data uji kaji diulang sebanyak
tiga kali merangkumi 45 rawatan secara total. Reka bentuk dua rawak faktorial digunakan untuk mencari perbezaan
yang signifikan antara rawatan. Keputusan uji kaji menunjukkan bahawa pengumpul suria memberikan prestasi yang
lebih baik pada aliran jisim udara 3.03 kgs-1 di bawah kecondongan 35o. Pada kadar aliran jisim maksimum udara 3.03
kgs-1 dan sudut kecondongan optimum 35o, keputusan kecekapan energetik maksimum ialah 51%, manakala kecekapan
eksergi ialah minimum 24% dan kecekapan keseluruhan maksimum pula ialah 71%. Kesimpulannya, untuk mendapatkan
kecekapan terma maksimum 71% daripada pengumpul suria plat rata yang digunakan sebagai pemanas udara, ia mesti
dikendalikan pada kadar aliran jisim udara tinggi 3.03 kgs-1 di bawah kecondongan 35o di Peshawar, Pakistan.

Kata kunci: Eksergi; kecekapan; pengumpul plat rata; tenaga

INTRODUCTION
maximum output (Koca et al. 2014). Renewable energy
Energy is the base of economic and social development. has many sources, among them solar energy is a clean,
The timely increasing demand of energy and limitations abundant and environment friendly source which has
of fossil fuels reservoirs is a serious issue for the world received considerable attention for generating heat and
particularly in Pakistan. Increase in the demand of fossil electricity. It can be converted into heat and electricity
fuels for energy production and their impact on the using different solar conversion technologies namely flat
environment is now a global problem which led to a plate solar collector, concentration solar collector and
significant enhanced trend of acquiring renewable energy photovoltaic technologies. For heating purposes flat plate
sources. Renewable energy sources have the potential to solar collectors and parabolic trough solar collectors are the
overcome the problems of energy demand and environment main component for solar thermal heating. The flat plate
destruction by greenhouse gases emitting by the using solar collectors are used mainly for low thermal systems of
fossil fuels. It is thus widely investigated that renewable agricultural and domestic heating requirements while the
energy can play a vital role in any field where energy is parabolic trough concentrating solar collector technology
a primary source. Renewable energy resources have a is used for high thermal systems like steam and power
potential to replace the non-renewable energy resources generation (Hanif et al. 2014; Kalogirou et al. 2016).
if properly worked out and installed according to the Before installing a thermal solar system for a specific
demand with all the engineering aspects needed for their purpose, it is most important to analyse the performance of
1062

these collectors. The efficiency for a solar thermal system are 34.0204o, 71.4822o and 448.3608 m respectively. The
is the key for its maximum thermal performance. But to site is said to be perfect because it receives maximum solar
determine the performance of the system the efficiency irradiance from dawn to dusk without interference of any
equation alone does not determines the internal losses shadow of a structure.
by the solar collectors. Only calculating efficiency is not
the sufficient criteria for finding optimum performance THE THERMAL SOLAR COLLECTOR
of a solar collector. For this, an analysis based on second
The environmental and design conditions and parameters
law of thermodynamics is best to determine the exergy
of the solar collector that will be used in the experiment
analysis of the solar system. Exergy analysis is a vital
are given in Table. 1. The flat plate solar collector that was
and most necessary parameter for optimization of design
used in the experiment is shown in Figure 1 and drawing
and future operation of the solar thermal collectors. Solar
of the flat plate solar collector assembly is given in Figures
thermal collectors must have been examined in terms of
2, 3 and 4.
their energy and exergy, efficiency and entropy generation
for optimal performance (Ge et al. 2014; Farahat et al.
2009), discussed the optimal and critical operations of RECORDING SOLAR IRRADIANCE
different solar collectors by means of energy and exergy The data of daily diffused solar irradiance was recorded by
analysis. They concluded that an optimal operational model the help of digital Pyranometer (Solar Power Meter, TES-
at any state is not useful for performance analysis. It’s not 1333, Taiwan) as shown in Figure 1. The environmental
a complete setup of generalizing and optimizing the solar conditions of the site were as given in Table 1.
collector performance. Hence an advanced model with
solving common errors for energy and exergy analysis
is required for computing efficiencies of solar collectors. TABLE 1. Average meteorological data of the site during the
The performance must be tested at dynamic models and time of recording of solar irradiance
energy and exergy analysis must be carried out at transient Local time Ambient temperature Air velocity
states. Many authors reported their work on optimizing the (h) (oC) (m.s-1)
performance of solar collectors and they have concluded
9:00 30.6 2.1
that solar collectors showed a lot of difficulties at steady
10:30 36.7 2.5
state conditions. Thus, it is recommended to develop and
11:00 40.1 2.4
test the solar thermal collectors at transient states for
12:00 (Noon) 40.5 2.7
achieving better results of energy and exergy analysis
1:00 pm 40.2 2.1
(Hamed et al. 2014).
2:00 38.3 2.6
To test a solar collector like concentrating dish or
3:00 37.5 2.2
parabolic trough type solar collectors for energy and exergy
4:00 36.6 3.1
analysis, it is necessary to keep in mind that the sky must be
5:00 31.5 3.3
clear, the flow rate must be constant, assume the properties
of material of construction of the solar collector as constant,
the glazing must have more than 85% transmittance, the
fluid velocity must be uniform, the losses by reflection
are assumed to be negligible and the air gap between the
absorber and glazing must be transparent. Keeping in view
these energy and exergy analysis for solar collectors must
be carried out (Jafarkazmi & Ahmadifard 2013).
Keeping in view the above-mentioned facts in
the literature, the present research study is designed to
investigate the energy and exergy analysis of flat plate
solar collector used as an air heater. The objective of the
research was to investigate the sole and interaction effects
of air mass flow rates and tilt angles of the solar collector
energy, exergy and efficiency.

MATERIALS AND METHODS

SITE SELECTION
The solar collector assembly was installed on the roof
of the Department of Agricultural Mechanization, The
University of Agriculture Peshawar, Khyber Pakhtunkhwa,
Pakistan. The Latitude, Longitude and elevation of the site FIGURE 1. Pyranometer used in the experiment
 1063

ENERGY AND EXERGY ANALYSIS Now we have to calculate each and every component
When the solar collector is at equilibrium with the of the (1). In detail to develop a model from the equation.
environment then the general form of energy and exergy
balance equation of the system is given by Bellos et al.
(2016),

Ein+ Es+ Eout+ El + Ed = 0 (1)

TABLE 2. Design and environmental conditions of

the flat plate solar collector

Parameters Value
Collector’s length, width and depth 3.40, 2.46 & 0.66m
Area 8.365 m2
Volume of the solar collector 5.520 m3
Absorber V-corrugated, Black
painted with header
raiser pipes FIGURE 2. The flat plate solar collector assembly
Absorber area 8.360 m2
Absorbance and transmittance of absorber 0.92 and 0.88
Glazing (single glass) 0.008 m (8 mm)
Agent fluids Air and water
Wind speed range 0.1 - 35 m s-1
Collector tilt angles 25 o, 35 o and 50o
Ambient temperature range 283 - 810oC
Apparent sun temperature 4320 to 4350oC
Absorber thickness 0.0024 m (2.4 mm)
Optical efficiency 0.88
Transmittance of glazing 0.89
Thickness of insulation 0.15 m
(Wood of deodar + polystyrene)
Thermal conductivity of absorber 380 W.m-1. o C-1
Solar irradiance range 0-850 W.m-2
Tubes/ Pipes diameter (steel) 0.025 m
Flow rates 0.1- 3.5 m s-1
Volume of storage tank 0.12 m3
Mass of water in storage tank 120 kg
Volume of head raised pipes 0.086 m3
Mass of water in pipes 86 kg
Total volume of water in solar collector 206 kg FIGURE 3. Front view of the flat plate solar collector

FIGURE 4. Side view of the flat plate solar collector

1064

ENERGY ANALYSIS OF A THERMAL SOLAR COLLECTOR where El represents the energy leaked by the solar collectors
To calculate energy available to the solar collector, it is due to the material characteristics to lost heat.
important to consider the energy gained equation by the
working fluid. The equation is given by Al-Sulaiman ENERGY DESTROYED BY THE SOLAR COLLECTOR
(2013),
The energy destroyed by the solar collector is given by
Islam et al. (2015),
Quse= maCpΔT here ΔT = Tout – Ta (2)

Ed = – mCPTa Ed = – mCPTa
Now expending the equation for considering heat
losses form the collector during the fluid flowing inside
the solar collector to the atmosphere is given by Hotted– (8)
Whillies as (Calise et al. 2016), where Ed is the lost heat energy caused by the difference
in temperature of absorber plate and fluids. It is the latent
Quse = AabsQr [Si – Ul (ΔT)] here ΔT = Tin – Ta (3) heat absorbed by the fluids in phase change.
where Si is the optical absorbed solar flux and Ul is the
heat lost by reflection, emittance and optical efficiency of EXERGY OF THE SOLAR COLLECTOR
glazing given by formula Farahat et al. (2009), Now the exergy of a solar collector is given by Kalogirou
et al. (2016),
Si = TαIT

QR = (4)

(9)
Now putting (3) in (2), we get the overall equation of
energy given to the collector and yielded by the absorber
where Eout represent the energy to be loaded to the fluids
plate and is given by Islam et al. (2015),
and is the exergy of the system. It is also called as entropy
of the solar collectors.
Quse = AabsSi – UlAabs(ΔT) here ΔT = Tabs – Ta (5)
EFFICIENCY OF A THERMAL SOLAR COLLECTOR
EXERGY ANALYSIS OF A THERMAL SOLAR COLLECTOR It is ratio of energy output and useful energy available to
Exergy is defined as the maximum amount of heat energy the solar collectors. Now from (10). The second law of
the solar collectors provide to driers by the help of fluid heat exchange efficiency equation of the solar collectors
(air, water) flowing inside the solar collector. is by taking the ratio of (4) and (7). The overall equation
Before calculating the exergy, we have to calculate is given by Ge et al. (2014),
the losses, stored or destroyed in the system.
ɳ=
ENERGY STORED BY FLUIDS FLOWING IN
THE SOLAR COLLECTOR
(10)
The energy stored by the solar collector fluids is given by
Jafarkazmi and Ahmadifard (2013),
DATA LOGGERS
Es = mwCpΔT (ΔT = Tw – Ta) (6) Data loggers were installed on the solar collectors that
recorded the solar irradiance, temperature of inlet and
where Es represents the energy stored by the fluids for a outlet, temperature inside the collector for 24 h.
specific period of time. In the solar collectors used in the
experiment the Es is only for the water to be heated and STATISTICAL ANALYSIS
stored in the water tank.
The data was analyzed using two factorial Completely
Randomized Design (C.R.D). The first factor was air mass
ENERGY LEAKED BY THE SOLAR COLLECTOR flow rates with five levels and second was tilt angle of the
The energy leaked from the solar collector is given by solar collector with three levels. Both the factors were
Jafarkazmi and Ahmadifard (2013) replicated three times (Hanif et al. 2016). The statistics
were applied on energy, exergy and efficiency of the solar
El= {UlAabs (Tabs – Ta) (1 – )} (7) collector.
 1065

RESULTS AND DISCUSSIONS of these authors. It is due to the size of the collector. They
used 1.4 and 2.1 m2 collector which is 5 and 4 times smaller
SOLAR RADIATION than the collector used in the present study having an area
of 8.34 m2. The results are also in accordance with findings
The data regarding the solar irradiation on the site and other of Calise et al. (2016), Farahat et al. (2009) and Hanif et
meteorological conditions that affected the performance of al. (2016) who reported the exergy increase with increase
the solar collector are shown in Figure 5. The data showed in air mass flow at tilt angle of 28 to 35o.
that during the experimental period the site received more
than 5000 kW.m-2 per month of solar irradiance at an
average. This accounts more than 600 W.m-2 of solar power
availability at the site. The data also shows that during the
experimental period there was 50% relative humidity and
38oC ambient temperature which is far more best for drying
different fruits and vegetables.

FIGURE 6. Energetic efficiencies of the flat plate solar collector

as affected by air mass flow rates and tilt at different
tilts of the solar collector

EXERGY EFFICIENCY
Experimental data regarding the exergy as affected by
different air mass flow rates and tilt angles is given in
Figure 7. The data shows that the exegetic efficiencies of
FIGURE 5. Relative humidity, temperature and solar the flat plate solar collector decreased by increasing air
irradiance of the site mass flow rates. Maximum efficiency of 51% was recorded
for a tilt angle of 35o at natural flow rate which decreased
24% by optimizing the flow to 3.03 kgs-1. The minimum
ENERGY EFFICIENCY
efficiency of 43% was recorded for tilt of 50o at natural
Experimental data regarding the energy as affected by flow which decreased to 19% at maximum flow rate of
different air mass flow rates and tilt angles is given in 3.03 kgs-1. The exergy decrease is due the heat flowing
Figure 6. The data in Figure 6 shows that increase in air towards the dryer and storage tank by agent fluids. Exergy
mass flow rates will increase the energy efficiency of the was maximum at 35o tilt angle because of the normality of
solar collector. The increase in energetic efficiencies is due solar irradiance at this angle on the absorber plate of the
to improvement in convective heat transfer at higher flow solar collector. The results are in argument of Farahat et
rates. At natural or passive flow more, heat leaked from the al. (2009) who reported decrease in exergetic efficiencies
collector as a result more energy is lost and solar collector
was having low energetic efficiencies but as soon as the
flow rates are made forced or convective, therefore, the
solar collector energy improved by reducing leakage and
loss of energy from the absorber through glazing to the
environment. On the other hand, in collector tilt, an angle
of 35o gave maximum energy gained by the solar collector
as compared to 25 or 50o tilt. At natural flow the recorded
energetic efficiencies were 33.3, 37.5 and 30.1% for 25,
35 and 50o tilt angles. As flow rates are increased and
reached to maximum of 3.03 kgs-1 therefore, the energetic
efficiencies reached to 67.0, 72.0 and 61.5% for 25, 35
and 50o tilt angles. The results are in accordance with the
findings of Al-Sulaiman (2014) and Bellos et al. (2016)
who reported increase in energetic efficiency with increase FIGURE 7. Exergetic efficiencies of the flat plate solar collector
in air mass flow through the solar collector. The efficiency as affected by air mass flow rates and tilt at different
of the present study is almost 10% higher than the finds tilts of the solar collector
1066

of a flat plate solar collector with increase in tilt angle collector must be operated at high air mass flow rate of 3.03
and increase in air mass flow rates. Ge et al. (2014) and kgs-1 under 35o tilt angle to achieve maximum performance
Hanif et al. (2016) also reported in their results that exergy from it as a dryer for drying carrots.
decreased with increased in air mass flow rates.
ACKNOWLEDGEMENTS
OVERALL EFFICIENCY The authors acknowledge the funding provided by the
Experimental data regarding the efficiency of flat plate Higher Education Commission of Pakistan (HEC-PAK)
solar collector to convert energy to useful exergy is given for the Research under the project “comparison of flat
in Figure 8. The data shows that the efficiencies of the plate and parabolic trough solar collector for drying and
flat plate solar collector increased by increasing air mass water heating purposes”. The authors also acknowledge
flow rates. Minimum efficiency of 59% was recorded for the support and help provided by the University of
a tilt angle of 35o at natural flow rate which increased to Agriculture Peshawar Pakistan and Nuclear Institute
71% by optimizing the flow to 3.03 kg. s-1. The minimum for Food and Agriculture Peshawar Pakistan during the
efficiency of 56% was recorded for tilt of 50o at natural research study.
flow which increased to 64% at maximum flow rate of
3.03 kgs-1. The increase in overall efficiency is due the
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change material for a solar collector. Renewable Energy Abdurab

33(4): 567-574. Department of Horticulture
The University of Agriculture Peshawar
25130 Khyber Pakhtunkhwa
Muhammad Hanif* Mansoor Khan Khattak & Pakistan.
Muhammad Ramzan
Department of Agricultural Mechanization *Corresponding author; email: hanif_mechanization@aup.edu.pk
The University of Agriculture Peshawar
25130, Khyber Pakhtunkhwa Received: 3 April 2017
Pakistan Accepted: 14 November 2017

Maazullah Khan
Nuclear Institute for Food and Agriculture
25130 Peshawar
Pakistan