Experimental Investigation Of Active Cooling Of Photovoltaic Cells
Experimental Investigation of Active Cooling of
H.G. Teo, P.S. Lee , MNA Hawlader
Department of Mechanical Engineering, National University of Singapore
9 Engineering Drive 1, Singapore 117576
Absorption of solar radiation increases the temperature of photovoltaic (PV) cells,
resulting in a drop of electrical efficiency. A hybrid photovoltaic/thermal (PV/T) solar
system was designed, fabricated and experimentally investigated in this work. To
actively cool the PV cells, a parallel array of ducts with inlet/outlet manifold was
designed for uniform airflow distribution and attached to the back of the PV panel.
Experiments were performed with and without active cooling. A linear trend between
the efficiency and temperature was found. Without active cooling, the temperature of
the panel was high and solar cells can only achieve an efficiency of 8 to 9%. However,
when the panel was operated under active cooling condition, the temperature dropped
significantly leading to an increase in efficiency of solar cells between 12 and 14%.
Keywords: Hybrid photovoltaic /thermal system; manifold design; active cooling;
operating temperature; cell efficiency
In recent years, renewable energy is widely advocated by many developed country.
PV cell is one of the most popular renewable energy products. It can directly convert
the solar radiation into electricity which can be utilised to power household appliances.
However, during the operation of the PV cell, only around 15% of solar radiation is
converted to electricity with the rest converted into heat. The electrical efficiency will
decrease when the operating temperature of the PV module increases.
Therefore, decreasing the temperature of PV module can boost the electrical
efficiency. Generally speaking, some techniques, like air cooling and water cooling,
are utilised to cool the PV module to maintain lower operating temperature. Many
numerical and experimental studies have been conducted to find out the most efficient
and low cost hybrid PV/T system. Sometimes, the thermal energy extracted from the
PV module can also be utilised for low temperature applications e.g. water and air
﹡Corresponding author. Tel.: +65 65164187
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Ac Aperture area of PV module (m2)
p Packing factor
G (t) Solar radiation (W/m2)
m Mass flow rate (kg/s)
cp Specific heat capacity (J/kg．k)
Tg Front glass temperature (℃)
Tc Cell Temperature (℃)
Tb Temperature of backsheet (℃)
Ta Temperature of air flow (℃)
Tin Inlet temperature of air (℃)
Tout Outlet temperature of air (℃)
hg Heat transfer coefficient of front glass (W/m2．k)
hc Heat transfer coefficient in air duct (W/m2．k)
σ Stefan-Boltzmann constant (W/m2．k4)
εg Emittance of glass
αT Tedlar absorptivity
ηo Nominal efficiency of cell
αc Cell absorptivity
τg Fraction transmitted through the front glass
ηe Cell efficiency
β Temperature coefficient (℃-1)
In some cases of study, attention is focused on modifying the configuration of PV
panel. By changing the structure of the panel, the variation of performance of the
system can be observed. Dubey et al.  reported the efficiency of different
configurations of PV/T-air collector (Case A-Glass to glass PV module with duct;
Case B-Glass to glass PV module without duct; Case C-Glass to tedlar PV module
with duct; Case D-Glass to tedlar PV module without duct). It was indicated that case
A can give the highest efficiency among the all four cases. The annual average
efficiency of case A and B is 10.41% and 9.75%, respectively.
In order to enhance the heat transfer from the PV panel thereby effectively
reducing the temperature and improving the efficiency of the PV module, Prasad and
Saini  artificially increase the roughness of absorber plate and wall of the channel.
However, increased roughness of wall and absorber will incur a pressure drop penalty
and, therefore, requiring a higher pumping power. Han et al.  and Gupta et al. 
showed that several types of ribs in the air channel can provide a better performance
in heat extraction but it is also accompanied by a significant increase in frictional
losses. Garg and Datta  suggested several practical modifications to enhance the
heat transfer in the air duct.
Garg et al.  presented a study of a PV/T air hybrid system, this system
comprised a plane booster and a flat plat collector mounted with PV cells. An
optimization study of the absorber geometry for solar air heating collector has been
investigated by Pottler . Naphon  carried out a study on the performance and
entropy generation of the double pass solar air heater with longitudinal fins. The result
of this study showed that the thermal efficiency of PV panel increases with increasing
the flow rate, as the heat transfer coefficient increases with increased Reynold
Tonui and Tripanagnostopoulos  also reported a study that an improvement of
heat extraction has been achieved by modifying the channels of PV/T air system in
low cost. Three different configurations of air ducts (simple air channel, thin
aluminum sheet and rectangular fin) were investigated experimentally and
numerically. Sopian et al.  presented a steady state simulation on single and
double pass combined PV/T air collector. The results of simulation indicated that the
double pass PV/T collector has superior performance during the operation. Joshi et al.
 carried out an evaluation of a hybrid PV/T system. Two types of PV module
(glass to tedlar and glass to glass) were utilized to investigate the performance under
the meteorological conditions of New Delhi. The results showed that the overall
performance of hybrid thermal collector with PV module glass-to-glass is better than
In this study, the main focus will be on the electrical efficiency of the PV panel
with and without cooling. By varying the air flow rate through the duct, the electrical
performance will be investigated.
A test set up was designed to investigate the thermal and electrical performances
of the PV/T air system. This system was built on the roof top of EA building at the
National University of Singapore. A schematic diagram of the complete experimental
set-up is shown in Fig. 1.
This experimental set up is designed to investigate how the temperature affects the
efficiency and power output of PV panel during the operation. Four polycrystalline
solar panels were used in the experiment to generate electricity. The electricity
generated by the solar panels was stored in four deep cycle gel batteries. An array of
air ducts were used for the air to pass through and it was attached underneath the PV
panels. Fins were fitted in the duct to increase the heat transfer rate from the PV panel
to the moving fluid. A parallel array of ducts with inlet/outlet manifold was designed
for uniform airflow distribution.
A direct current blower, which was connected to the batteries, extracted air from
the surrounding to cool the panels. During the operation, a maximum power point
tracker (MPPT) was used to modulate the power output from solar panel to ensure
that the maximum electrical power is extracted. Another alternating current (AC)
blower was also used in this experiment because it could function as variable speed
blower, to control the flow rate passing through the duct. The experiments were
normally conducted from 9:30 am to 5:00 pm. In the experiment, PV current, PV
voltage, temperature of panels, temperatures air at inlet and outlet manifolds, wind
speed and solar irradiation were collected during the operation of system.
Figure 1. A schematic diagram of the experimental set-up.
3. Mathematical Formulation
The energy balance equations of the PV module are modified from Cox and
Ec = pα cτ g G (t ) (1)
Ec is the total energy absorbed by the PV cell
Ece = ηe pτ g G (t ) (2)
Ece is electrical energy produced by the PV cell.
Ect = (1 − ηe / α c ) pα cτ g G (t ) (3)
Ect is thermal energy released by PV cell. p is the cell packing factor which defined as
the ratio of area of solar cell to the area of blank absorber ηe which is the cell
efficiency can be represented as a function of the module temperature .
ηe = ηo [1 − β (Tc − To )] (4)
ET = τ g (1 − p )α T G (t ) (5)
ET is the rate of solar energy absorbed by tedlar (material of backsheet) after
transmission from EVA (polymer encapsulatant of solar cell). Principle of energy
conservation is applied to the components of the PV module as shown below:
(1 − ηe / α c ) pα cτ g G (t ) + τ g G (t )α T (1 − p ) = Eloss + qc (6)
Eloss = hg [Tg − Ta (t )] + ε g σ Tg 4 − α g σ [Ta (t ) − 6]4 (7)
Eloss is the energy losses from the front glass to environment through the forced
and free convection and radiation.
The convective heat transfer from the back of module can be presented by
Newton’s law of cooling:
qc = hc [Tb − Ta (t )] (8)
The energy balance of the air flow
m cp dx = qc (9)
The thermal efficiency can be computed with the following equation:
m c p ∫ (To − Ti )dt
ηth = (10)
Ac ∫ G (t )dt
The electrical efficiency of the PV module can be described as following
A∫ G (t )dt
The total efficiency of the hybrid PV/T system is:
m c p ∫ (To − Ti )dt + ∫ VIdt
ηtotal = ηth + ηe == (12)
Ac ∫ G (t )dt
4. Results and Discussion
The electrical efficiency of the PV module is presented in Fig. 2. It can be observed
that the electrical efficiency is a linear function of module temperature. The electrical
efficiency of PV module declines with the rise in PV module temperature. During the
experiment, cooling and non-cooling cases were considered. The impact of cooling is
also shown in Fig. 2. Under the same meteorological condition, the temperature of
non-cooling case is much higher than the cooling one and this is also reflected in the
electrical efficiency of the PV module.
) gnilooC hti W
21 n gnilooC tuohti W
00.53 00.04 00.54 00.05 00.55 00.06 00.56 00.07
Figure 2.Electrical efficiency as a function of PV temperature.
00.0 20.0 40.0 60.0 80.0 01.0 21.0 41.0 61.0 81.0
)s/gk( etaR wolF
Figure 3. Influence of flow rate on electrical efficiency.
The influence of flow rate on electrical efficiency is presented in Fig. 3. The
electrical efficiency of the PV module increases with the flow rate until the flow rate
reaches about 0.055 kg/s. The electrical efficiency of PV module will be maintained at
a relatively constant value after the flow rate reaches about 0.055 kg/s. This could be
explained in association with the thermal efficiency of collector. When the flow rate
increases to around 0.055 kg/s, the thermal efficiency of the collector will be
maintained at a relatively constant level. In other words, the heat extracted by the
cooling fluid has reached a saturated level and it can no longer be increased by
increasing the flow rate. Thus, the electrical efficiency of PV module will also be
maintained at a relatively constant value after the flow rate reached about 0.055 kg/s.
Temperature difference between inlet and outlet air is also investigated in this
study and is given in Fig. 4. The electrical efficiency of PV module will decrease
when the temperature difference between the inlet and outlet air increases. This could
be explained by the occurrence of hot spot in the PV module due to the high
temperature gradient. Therefore, the overall electrical efficiency can be dragged down.
The inlet and outlet air temperature difference should be controlled to an optimum
range to ensure that the electrical efficiency of the PV module can be maintained at
the desired level.
Figure 2 provides an indicative trend in the relation of electrical efficiency and
operating temperature. A linear relation can be obtained:
ηel = 0.1577 − 0.0009Tpanel (13)
The theoretical efficiency of PV module can be obtained from the Eq. 4. From the
theoretical deduction, the electrical efficiency of the module can be written as the
ηel = 0.1664 − 0.0007Tpanel (14)
Based on the experimental data, Fig. 5 showed that the theoretical electrical
efficiency is about 1 to 2% higher than experimental electrical efficiency. This
discrepancy can be attributed to the module to module connection which will result in
a drop in the electrical efficiency. This result can be used to compare with result from
Tonui et al. . A linear correlation the electrical efficiency and module temperature
was found from their experiments.
η el = 0.147 − 0.0008Tpanel (15)
The efficiency of the system shown in the Fig. 6 indicated that the electrical
efficiency seems to be more stable than the thermal efficiency. The average electrical
efficiency range is around 10.1% to 10.9%. However, the thermal efficiencies of the
system are much higher than electrical efficiency of the system; it is about 40% higher.
This showed that most of the solar irradiation is converted into the heat and the
thermal efficiency which obtained from the experiment is significant compared to
electrical efficiency. The total efficiency of the system is around 50% to 70%. The
cooling mechanism not only can enhance the electrical performance of PV cell but
also increase the total efficiency of the system.
Fig. 7 shows that the peak of total energy output will be at second day and fourth
day. This can be attributed to the meteorological condition on these days. The ambient
temperature on these two days was relatively high and the solar irradiation was also
very intense. Therefore, it can be concluded that under the proper operation of the
system, the output energy can be generated proportional to the solar power input. To
ensure that the household appliances can be operated under the low irradiation
meteorological condition, the auxiliary power is needed to supply the power when the
power from the PV module is not sufficient to operate the appliances. The battery
bank of the system is also needed to be charged when the solar irradiation is very
intense as the battery bank can be also supply the power to the appliances during the
53 1. 0
52 1. 0 f
21.0 a c
ri s/g k 38 70.0 =etaR w olF ssaM
51 1. 0 el s/g k 67 60.0 =etaR w olF ssaM
E s/ gk 98 30.0= etaR w olF ssaM
0 1 2 3 4 5 6 7 8 9 01
)C( ecnereffiD erutarepmeT
Figure 4. Influence of temperature difference (To-Ti) on electrical
efficiency for different flow rates.
Figure 7. Electrical and thermal energy and the total energy gain over the
5 4 3 2 1
0 0. 0
0 0. 5
0 0 . 5 1M
niaG y grenE latoT
ygre nE lamrehT 00.02
Figure 6. A comparison of thermal and electrical efficiency over 5 days.
5 4 3 2 1
ycneiciffE latoT %00.02
ycneiciffE lacirtcelE f
Figure 5. A comparison between theoretical and experimental results
)C( erutarepmeT eludoM
57 07 56 06 55 05 54 04 53 03
80 .0 E
90 .0 el
1. 0 ri
11 .0 l
21 .0 E
41 .0 n
Both electrical and thermal energy are generated through the hybrid PV/T system.
From the experiment result, it shows that the effect of using the cooling mechanism.
Under the situation where no cooling was used, the operating temperature of PV
module attained a value as high as 68℃ and the electrical efficiency dropped
significantly to 8.6%. By using the blower to cool the PV module, the operating
temperature of module could be maintained at 38℃ and the electrical efficiency
could also be kept at around 12.5%. Besides, an optimum flow rate was also found in
this study. Air flow rate with 0.055kg/s is sufficient to absorb the maximum heat from
the PV module. When the flow rate exceeds this value, the thermal and electrical
energies are no longer affected. This helps in choosing the power rating of blower in
order to avoid wasting unnecessary energy. Temperature gradient over the different
PV module when connected together is also a key factor to affect the electrical
performance. To boost the electrical efficiency of the PV module, temperature and
temperature gradient over the PV module are critical. The increased efficiency of the
air cooled PV/T systems will significantly contribute to the applications of PV system.
Furthermore, increasing the performance of the PV systems can also reduce the
energy supply to buildings leading to lower CO2 emissions.
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