Performance enhancement of solar photovoltaic cells using effective cooling methods: A review

Renewable and Sustainable Energy Reviews 64 (2016) 382–393

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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

Performance enhancement of solar photovoltaic cells using effective cooling methods: A review S. Sargunanathan a,n, A. Elango b, S. Tharves Mohideen c a

Department of Mechanical Engineering, Annamalai Polytechnic College, Chettinad, Tamilnadu 630 102, India Department of Mechanical Engineering, A C College of Engineering & Technology, Karaikudi, Tamilnadu 630 004, India c Department of Mechanical Engineering, Institute of Road Transport Technology, Erode, Tamilnadu 638 316, India b

art ic l e i nf o

a b s t r a c t

Article history: Received 4 April 2016 Accepted 16 June 2016

The Photovoltaic (PV) cells are sensitive to temperature variations. When the ambient temperature and the intensity of solar irradiance falling on the PV cells increases, the operating temperature of the PV cells also increases linearly. This increase in operating temperature of the PV cells leads to reduction in open circuit voltage, fill factor and power output for mono and polycrystalline PV cells which are used in most of the power applications. The net results lead to the loss of conversion efficiency and irreversible damage to the PV cells materials. Therefore, to overcome these effects and to maintain the operating temperature of the PV cells within the manufacturer specified value, it is necessary to remove heat from the PV cells by proper cooling methods. This review presents an overview on passive cooling (heat pipe based and by fins), active cooling (by spraying water), liquid immersion cooling and cooling by employing phase change material (PCM) to enhance the performance of the commercially available PV and concentrated photovoltaic (CPV) cells. & 2016 Elsevier Ltd. All rights reserved.

Keywords: Photovoltaic cells Passive cooling Active cooling Immersion cooling Phase change material Effective cooling

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Heat pipe passive cooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Other passive cooling techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Active cooling by flow of water over the front surface of the modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Liquid immersion cooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5. Active cooling by attaching air/water/fin cooling system on the backside of the module . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Numerical studies on PV cell cooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction The standard of living of people in any country mainly depends on the industrial growth of that country, which in turn depends on the energy availability and energy consumption. There is a wellestablished relationship that countries having higher per capita annual energy consumption have higher literacy rates than the countries having lower per capita energy consumption. The energy n

Corresponding author. E-mail address: [email protected] (S. Sargunanathan).

http://dx.doi.org/10.1016/j.rser.2016.06.024 1364-0321/& 2016 Elsevier Ltd. All rights reserved.

382 383 383 384 385 387 389 390 392 392

demands in most of the countries are met out by fossil fuels such as coal, oil and gas but their availability is limited. Conversion of energy stored in the fossil fuels into useful form produces harmful pollutants and they leads to global warming, which is one of the major threats for the entire world. One of the best alternative to the fossil fuels is the harnessing the solar energy into electrical energy. The power from the sun intercepted by the earth is about 1.8
1011 MW, which is many thousands times greater than the power consumption from all sources. Solar energy will not produce pollutants like fossil fuels during conversion into electricity and it is possible to protect the

S. Sargunanathan et al. / Renewable and Sustainable Energy Reviews 64 (2016) 382–393

world from the global warming and also possible to preserve the fossil fuels for our future generation. The technology of converting solar energy into electrical energy was invented by Charles Feritts and is referred to as photovoltaic (PV) cell. When the solar irradiance is made to fall on the PV cells, the photons are absorbed by the PV cell materials and the photons having the energy above the band gap of PV cell materials will constitute the flow of electric current from the PV cell to external load. In general the wave length of solar irradiance from 400 nm to 1200 nm are strongly absorbed by the PV cells and converted into the electric power. The conversion efficiency of the commercially available module ranges from 12% to 18% and the laboratory cells have a record efficiency of 24.7%. The remaining solar irradiance falling on the PV cells are converted in to heat, which in turn increases the operating temperature of the solar modules. The increase in operating temperature of the PV cells results in decrease of open circuit voltage (Voc), fill factor and power output of about 2–2.3 mV/°C, 0.1–0.2%/°C and 0.4–0.5%/°C respectively, with increase in short circuit current (Isc) of 0.06–0.1%/°C for mono and polycrystalline PV cells, which results in the loss of conversion efficiency and irreversible damage to the PV cells materials [1]. Radziemska [2] investigated the influence of temperature and wavelength on electrical parameters of crystalline silicon solar cell module. The single crystalline solar cell is exposed to the halogen lamp irradiation of intensity 618–756 W/m2. Figs. 1 and 2 showed that the maximum output voltage and power decreased with increase in operating temperature of the cell. The performance of the module was measured at module temperature of 25 °C and 60 °C. The results obtained indicated that the temperature co-efficient of the module was  0.66%/K. The fill factor and conversion efficiency was decreased by 0.2%/K and 0.08%/K respectively. Chander et al. [3] investigated the effect of cell temperature on the photovoltaic parameters of mono-crystalline silicon cell and reported that the open circuit voltage, maximum power, fill factor and efficiency were decreased with cell temperature. Zaoui et al. [4] studied experimentally and numerically, the effect of irradiance and temperature on the performance of PV modules and reported the similar results. Therefore to overcome the effects of cell temperature and to maintain the operating temperature of the PV cells within the manufacturer specified value, it is necessary to remove heat from the PV cells by proper cooling methods. Passive cooling and active cooling techniques are used to remove heat in order to enhance the performance of PV cells.

Fig. 1. Output power versus voltage of a single-crystalline silicon solar cell at various temperatures: 28 °C, 40 °C, 60 °C, 80 °C [2].

383

Fig. 2. Temperature dependence of the maximum output power Pm(T) [2].

2. Experimental studies 2.1. Heat pipe passive cooling Heat pipe is a device which is used to transport heat by two phase flow of working fluid from one place to other. The heat pipe consists of evaporator section, adiabatic section and condenser section. Heat absorption takes place in evaporator section; heat rejection at the condenser section and the adiabatic section is fully insulated. With vacuum pump the evacuation is made at the heat pipe to facilitate the filling of working fluid and the evaporator section of the heat pipe is attached to the back side of the PV cells to absorb the heat from them. Due to this, vaporization occurs so that the liquid inside the heat pipe vaporizes, hence the vapor carrying the latent heat of vaporization, flows towards the condenser section and gives up its latent heat to the surroundings by natural convection. Heat pipes are manufactured using envelop material, working fluid and wick material which must be compatible. For the temperature range of  20 °C to 100 °C, the two potential heat pipe wick and wall materials are copper and aluminum. The choice of working fluid for different heat pipe materials is given in Table 1. Akbarzadeh and Wadowski [7] introduced a passive cooling method based on thermosyphon, which can effectively cool the solar cells under concentrated light. The proposed system for cooling of the solar cells contains two heat exchangers piped together, initially evacuated and filled with refrigerant R-11. As the convection heat transfer co-efficient is low, the external heat transfer area of the condenser is extended by fins. Polycrystalline solar cells having dimensions of 25 mm by 20 mm were installed on both sides of the evaporating surface of the cooling system. The test results showed that the maximum temperature of the cells without cooling was 84 °C and with cooling was 46 °C. The maximum power output was 10.6 W and 20.6 W without and with cooling respectively. Cheknane et al. [8] experimentally investigated the role of passive cooling on silicon based concentrator solar cell performance. They designed gravity dependent copper heat pipe using water or acetone as working fluid. They measured open circuit voltage (Voc), short circuit current (Isc), fill factor (FF) and series resistance. The results showed that the Voc is smaller without concentration and increased more rapidly with intensity and its value is more for acetone than water. They also reported that the FF decreased with increasing intensity for both liquids and the efficiency increased with increase in intensity. Anderson et al. [9] successfully demonstrated the feasibility of a heat pipe passive cooling solution to CPV cell. Copper/water heat pipe with aluminum fins can be used to remove the heat from the CPV cell passively by natural convection. Copper and aluminum heat pipe with various working fluids were examined and copper

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Table 1 Choice of working fluid for heat pipes [11]. Heat pipe working fluid

Acetone

Operating temperature (°C)  48 to 125 Heat pipe shell materials Aluminum, Stainless steel

Ammonia

Ethane

 75 to 125 Aluminum, Stainless steel

 150 to 25  75 to 120 Aluminum Copper, Stainless steel

Fig. 3. Solar panel cooling by air with heat pipe and by water with heat pipe. [10].

heat pipe with water as working fluid was selected for their work. They found the optimum size of the fin and spacing for rejecting heat by natural convection using a series of CFD analysis. A prototype heat pipe heat sink was designed, fabricated, tested and the test result showed that with an input heat flux of 40 W/cm2, the heat pipe rejected the heat to the environment by natural convection, with the temperature difference of 40 °C. Tang et al. [10] used a novel micro heat pipe array for cooling the solar panel. They investigated both air and water cooling under natural convection condition. They concluded the following from the test results. For Air cooling when the daily radiation value of 26.3 MJ, the maximum temperature reduction was 4.7 °C and average temperature reduction was 1.5 °C. The maximum increase in conversion efficiency was 2.6% and the average increase in conversion efficiency was 0.4%. The average increase in power output was 6.3% as compared to without cooling. For water cooling when the daily radiation value of 21.9 MJ, the maximum temperature reduction was 8 °C and average temperature reduction was 2.7 °C, the maximum increase in conversion efficiency (Fig. 3) was 3% and the average increase in conversion efficiency was 0.5%, the average increase in power output was 9%, as compared to air cooling. Benuel Sathish Raj et al. [11] investigated the performance of CPV module with heat pipe cooling. A pulsating heat pipe filled with acetone was attached on the back side of the solar panel in order to provide cooling and improve the output. The pipes extended beyond the PV panel to dissipate the heat to surrounding. Experiments were conducted on the setup to determine the voltage generated by the PV panel without concentrator and cooler, with concentrator and without cooler and panel with concentrator and cooler. They found from the results that the PV panel without concentrator and cooler has the maximum output voltage of 21.03 V and maximum operating temperature of 31.48 °C. The PV panel with concentrator and without cooler has the maximum output voltage of 23.03 V and maximum operating temperature of 34.26 °C. The PV panel with concentrator and with cooler has the maximum output voltage of 22.23 V and maximum operating temperature of 30.32 °C. The previous works reported on heat pipe passive cooling of PV/CPV cells are given in Table 2. 2.2. Other passive cooling techniques Cuce et al. [12] experimentally investigated the effect of passive cooling on performance parameters of polycrystalline solar cells

Methanol

Water

R-11 and R-22

1 to 325 Copper, Monel, Nickel, Titanium

20 to 100 Copper

under indoor conditions. Current-voltage and power-voltage characteristics of photovoltaic cells with/without fins for 200 W/ m2 is shown in Fig. 4. The experiments were carried out for different ambient temperatures and various illumination intensities up to one sun under solar simulator. The experimental setup consists of a solar simulator, a control room and measurement devices. The experiments were carried out in the illumination intensity range from 200 W/m2 to 800 W/m2. Temperature of the control room was adjusted to 25 °C. Two identical polycrystalline silicon PV cells were analyzed. One of the PV cell was equipped with aluminum heat sink for passive cooling. Thermal grease was used in between the heat sink and the back surface of the PV cell. The results showed that increase in power output due to passive cooling were 8 mW, 27 mW, 46 mW and 65 mW for 200 W/m2, 400 W/m2, 600 W/m2 and 800 W/m2 respectively. They found from the results, the passive cooling increased the energy efficiency by 9%, power conversion efficiency by 13% and exergy efficiency by 20%. The level of cooling achieved for the intensity levels of 400 W/m2, 600 W/m2 and 800 W/m2 was 20.09%, 31.1% and 25.2% respectively. Wu et al. [13] proposed a passive cooling method for domestic house application that utilized rainwater as cooling media and gas expansion device to distribute the rain water. The proposed method reduced the operating temperature of the cells up to 19 °C and average electrical yield was increased by 8.3%. The cooling maintains the efficiency of cells above 14.5% each hour in a design day (Fig. 5), particularly between 12 pm and 2 pm during which the PV panel has very low efficiency of 13.3% without cooling. Chen et al. [14] investigated the performance of polycrystalline PV panel with passive fin cooling under natural ventilation. They carried out the comparative experimental study of the PV panels with and without fin cooling to investigate the effect of PV panel inclination, solar radiation, ambient temperature and wind velocity on the electrical efficiency and power output. Aluminum alloy sheet of 0.8 mm thick was made into ‘U’ shape and ‘L’ shape units, acting as cooling fins of the PV panel and those units were pasted evenly on the back of the PV panel with good thermal conductive glue. They proposed the four testing modes A, B, C and D to investigate the effect of PV panel inclination, ambient temperature, wind velocity and solar radiation, on the electrical efficiency and power output. The mode ‘A’ results showed that the electrical efficiency and power output decreased firstly and increased with increase in PV panel inclination. The average increase in efficiency and power output with fin cooling was 1.3% and 3.1% respectively. The mode ‘B’ results showed that the electrical efficiency and power output decreased with increase in ambient temperature. The average increase in efficiency and power output of fin cooling was 0.3% and 1.85% respectively. The mode ‘C’ results showed that higher wind velocity led to improved fin cooling and better electrical performance. The average increase in efficiency and power output of fin cooling was 1.8% and 11.8%. The mode ‘D’ results showed that the electrical efficiency decreased and the power output increased with increase in solar radiation. The average increase in efficiency and power output of fin cooling was 0.7% and 2.4%.

Temperature reduction was 4 °C

1 sun Water

Temperature reduction was 4.7 °C. Output power and efficiency were 8.4% & 2.6% higher than without cooling ΔT ¼ 8 °C ΔPmax ¼13.9% and Δŋmax ¼3% higher than air cooling Maintain operating temperature as 30 °C 1 sun Air

2 sun Acetone

1 sun Water

Benuel Sathish Raj et al. [11]

Copper heat pipe with aluminum fins Aluminum heat pipe Hughes et al. [37]

385

2.3. Active cooling by flow of water over the front surface of the modules

Solar panel its total radiation area was 0.2049 m2

Temperature difference between cell and ambient air was 40 °C 400 sun Water

Single cell

Temperature reduction was 38 °C and output power increased was 10 W Voc and efficiency increases with intensity Refrigerant R-11 20 sun Line concentrator Water or Acetone Up to 500 sun Single cell

Requires 1 m/s or higher wind speed Line concentrator 24 sun Benzene

Temperature difference between cell and ambient air was 30 °C Up to 700 sun Single cell Acetone or water

Copper heat pipe, soldered copper fins Feldman et al. [6] Aluminum heat pipe and integral fins Akbarzadeh and Wadowski [7] Copper Chekane et al. [8] Copper heat pipe with copper fins Anderson et al. [9] Copper heat pipe with aluminum fins Tang et al. [10] Beach et al. [5]

Heat pipe materials Work done by

Table 2 Works on heat pipe passive cooling of PV/CPV cells.

Working fluid

Irradiation

Type

Results

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Reflection of the sun irradiance typically reduces the electrical yield of PV modules by 8 – 15%. This loss was reduced by using anti-reflection coating but they are not durable. Structured surfaces were also used to reduce the reflection losses but they are expensive, accumulate dust and difficult to clean. Krauter [15] investigated the electrical yield of photovoltaic panels by spraying the water over the front surface. When water is sprayed over the PV panel, its refractive index is 1.3, and is in between refractive index of glass (1.5) and air (1.0), which reduces the reflection loss by 2–3.6% and keeps the panel clean and dust free. Due to the water flow and additional cooling by evaporation, the cells operating temperature was reduced up to 22 °C in comparison to panel without cooling. The increased electrical yield over the whole day was about 10.3% (Fig. 6). Abdolzadeh and Ameri [16] investigated the performance of photovoltaic cells which is used to drive the water pump by spraying water over the front surface of the PV panels. The experimental setup has two polycrystalline PV modules (45
2 W) with 13.5% efficiency and one positive displacement type water pump. The PV cells are fixed at 10° facing south and the power produced from the array was used to drive the DC motor of the water pump. To spray water over PV cells, a tube with small holes placed on the top of the PV module is used. Temperature sensors were installed on the back of the two modules where the actual temperature is about 1.5 °C below the temperature on front of the modules. Irradiance and pump flow rate were measured by pyranometer and flow meter respectively. Data were recorded for every 15 min and the results showed that the operating temperature of the module with spray water reduced up to 23 °C. The maximum operating temperature of the modules was 35 °C and 58 °C with and without spray water respectively. The mean power output of the panel with cooling increased to 66.9 W instead of 55.4 W without cooling. The mean flow rate of the pumping system was increased from 479 l/h to 644 l/h. The mean volume of water used for spraying over the cells was about only 50 l/h. The PV cells achieved 12.5% mean conversion efficiency during the test day. Water sprayed also improved the optical performance by 1.8%. Odeh and Behnia [17] experimentally investigated the performance of PV module using water cooling. The experimental setup used multi-crystalline PV module of 60 W maximum power output. The module was connected to a variable resistance to find I-V characteristics curve of the module. The cooling system consists of a water trickling tube (2.5 cm diameter and 65 cm length) fixed on the upper edge of the PV module, water conduit at the lower edge of the module and by pass to deliver cooling water from the submersible pump. The trickling tube has 32 holes of 5 mm diameter distributed evenly and the cooling water flow rate of 4 l/min was maintained. The results indicated that heat loss by convection due to water flow over the module upper surface cause the temperature reduction from 58 °C to 32 °C (the temperature reduction up to 26 °C) and the module output increased in the range of 4– 10% (Fig. 7). Kordzadeh [18] investigated the effects of nominal power of array and system head on the operation of photovoltaic water pumping set with array surface covered by a film of water. Two similar crystalline type arrays having nominal power of 90 W and 135 W used and tested in three different heads of 10 m, 12.5 m and 16 m. The results showed that the continuous film of water on the surface of PV array has two important effects on the operation of the system. First it reduced the reflection loss and then improved the optical properties of the array surface. The short circuit current Isc of the panel which is temperature dependent, increased due to thin water film. Second it reduced the cell temperature by absorbing the

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Fig. 4. Current-voltage and power-voltage characteristics of photovoltaic cells with/without fins for 200 W/m2. [12].

Fig. 5. Comparison of the efficiencies and power output between with cooling and without cooling to the PV panel [13].

heat generated by the array during the day. The temperature reduction exceeds 25 °C at noon and operating temperature graph of the panel with thin water film closes to the variable ambient temperature. Thin film of water over the array with nominal power of 90 W and 135 W were improved the power graph peak from 52 W to 78 W and 92 W to 98 W respectively. The average improvements in efficiency were about 3.66% and 0.69% for array having nominal of 90 W and 135 W respectively. The experiment results showed that the decreasing of array nominal power and increasing in system head with thin film of water increase the power generated. Hosseini et al. [19] investigated the combination of a photovoltaic system cooled by a thin film of water with an additional system to use heat transferred to the cooling water. The experimental setup was composed of two similar but separate solar PV panels each with area of 0.44 m2. The maximum power output was 60 W with the maximum output voltage and current of 23 V and 2.61 A respectively. One of the panels was used in a combined system with a film of water running over its top surface without front glass and an additional fabricated system to use the heat generated by the panel. The other panel is a conventional PV as a reference panel and to produce a film of water over the PV panel, a tube with slit was installed on the top end of the PV panel. Water pumped to the feeding tube leaves the slit and flows over the panel as a thin film. The water collected at the lower end of the

Fig. 6. Comparison of photovoltaic conversion efficiencies of the PV-modules [15].

panel passed through a finned tube used as heat exchanger to utilize the heat taken by the cooling water. Thermocouples were installed on the back surface of the panels to measure the panel operating temperature. The results showed that due to water flow

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Fig. 7. The effect of water cooling on voltage-power characteristic curve of the PV module. Radiation on PV module surface is equal 1000 W/m2 [17].

over the panel surface and additional cooling by water evaporation, the panel operating temperature measured was much lower in comparison to the conventional reference panel and a maximum temperature difference of 18.7 °C was observed. This temperature reduction has a noticeable improvement in electrical efficiency and the relative difference was more than 33%. Dorobantu et al. [20] proposed a system to increase the efficiency of PV panels that makes a water film on the front surface of panels. The experiment was conducted on monocrystalline panel of 75 W, which was cooled by a continuous film of water that pours on the working surface from the top of the panel. The cooling device for single module consists of a cylindrical tube with 25 holes, each with 1.5 mm diameter. The diameter of the tube is 20 mm and its length is equal to that of the panel. The panel was placed on the fixed frame with tilt angle of 35°. During the measurement the average radiation level was 780 W/m2 and rate of water flow was 33.3
10  6 m3/s. The results showed that the front surface temperature was between 38.5 °C and 41.5 °C, the rear surface temperature was in between 50 °C and 52 °C and the power output was 73.11 W for without cooling panel. For the panel with cooling, the front surface temperature was between 26 °C and 27 °C, the rear surface temperature was between 31 °C and 32.5 °C and the power output was 76.74 W. The reduction in temperature brings a gain of 1.5 V and drops of 0.2 A. The net results lead to overall increase in power output of 3.5 W and the percentage increase was 9.5%. Moharram et al. [21] developed a heating rate and cooling rate models to predict the commencement of cooling of solar module by water cooling and the duration for which the water was sprayed in order to enhance the performance of the PV module and also to reduce the amount of cooling water and electrical energy needed to provide cooling. The heating model determines the maximum allowable temperature (MAT) at which the water to be sprayed to cool the solar PV module. They found from the mathematical model that the heating rate of the solar cells was 6 °C/h and the cooling rate of the solar cells was 2 °C/min for the water flow of 29 l/min. The experimental setup used six monocrystalline module of power output 185 W each and 120 water nozzles to spray water over the front surface of the module and found that the cooling rate was 2.05 °C/min. They also observed that without cooling the temperature was increased from 35 °C to 45 °C and the efficiency dropped from 12% to 10.5%. The proposed cooling system reduced the operating temperature by 10 °C in 5 min and increased the solar module efficiency by 12.5%. The selection of MAT was based on maintaining the efficiency of the modules at an acceptable level with least amount of cooling water

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and energy usage. They found that from net output energy vs MAT, the optimum value of MAT to cool solar panels with least amount of water and energy usage was 45 °C. Aldihani et al. [22] reported that the dusty environmental conditions reduced the power output by 16% and the water cooling on the front surface of the module can be partially compensated the power reduction. Balamuralikrishnan et al. [23] investigated the performance improvement of solar PV panel by active cooling. The proposed cooling system consists of LM35 temperature sensor interfaced with PLC and placed over the panel. When the temperature of the panel reaches the 35 °C, the PLC controller actuate the water pump for 30 s, which will spray the water on the top surface of the panel and provide cooling to the solar panel. The temperature was sensed after water spray and the cycle was repeated in order to maintain the temperature of the panel within the predetermined temperature. The results showed that the operating temperature of the panel was reduced by 8 °C and the efficiency was improved by 3%. Irwan et al. [24] studied the performance of PV panel under indoor test conditions. They proposed a water cooling method by spraying water over the front surface of the panel. Four sets of average solar radiation at the test surface of the solar simulator were produced by the halogen lamps and were measured as 413, 620, 821 and 1016 W/m². A DC water pump is used to maintain the water flow over the front surface of PV panel and to provide cooling. The minimum temperature reduction of 5.03 °C for the intensity of 413 W/m² and maximum temperature reduction of 23.17 °C for the intensity of 1016 W/m² was found from the results with cooling. They also found that the cooling increased the maximum power output by 9.76%, 14.87%, 18.19% and 22.81% (Fig. 8) for the intensities 413, 620, 821 and 1016 W/m² respectively. The various works on active cooling of PV modules by spraying water over the front surface of the modules are shown in Table 3. 2.4. Liquid immersion cooling Abrahamyan et al. [25] investigated the performance of the common silicon solar cell covered with thin film of antireflection layer immersed in the dielectric liquids of glycerin, isopropyl alcohol, acetone, butanol and DI water. Silicon solar cells of the Soviet production with a conventional configuration of a contact grid were used during their research. The areas of the solar cells were 2, 4 and 20 cm2. After the installation of cells the measurements of the load current-voltage characteristics were carried out at various densities of solar radiation for diffused day light and direct solar radiation with the intensity 70–80 mW/cm2. They

Fig. 8. Maximum power output of PV panel with and without water cooling mechanism [24].

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Table 3 Works on active cooling of PV modules by spraying water over the front surface of the modules. Workdone by

Dimensions of water spray pipe

Krauter [15]

Water used

Type PV cell

Results

4.4 l/min-m2

M55 module

50 l/h

Polycrystalline [45
2 W]

Operating temperature reduced up to 22 °C Optical performance improved by 1.5% Electrical yield increased by 10.3% Operating temperature reduced up to 23 °C Optical performance improved by 1.8% Electrical yield increased from 55.4 W to 66.9 W Operating temperature reduced up to 26 °C Electrical yield increased in the range of 4–10%. Operating temperature reduction exceeds 26 °C Improved the power graph peak from 52 W to 78 W for 90 W array and 92 W to 98 W for 135 W array Efficiency improvement 3.66% Temperature reduction up to 18.7 °C Relative difference in electrical efficiency was more than 33% Temperature reduction on the front surface up to 13.5 °C Temperature reduction on the rear surface up to 19.3 °C Percentage increase in power output was 9.5% Temperature reduction up to 10 °C Efficiency increased by 12.5% Maximum allowable temperature was 45 °C Temperature reduction was 8 °C Efficiency increased by 3% The minimum temperature reduction of 5.03 °C and maximum temperature reduction of 23.17 °C The maximum power output increased by 9.76%, 14.87%, 18.19% and 22.81% for the intensities 413, 620, 821 and 1016 W/m² respectively

Abdolzadeh and Ameri [16]

0.25 in. diameter

Odeh and Behnia [17]

Diameter 2.5 cm, length 65 cm, no. 4 l/min of holes 32 of diameter 5mm.

Kordzadeh [18]

Hosseini et al. [19]

Multicrystalline PV module of 60 W Crystalline array of 90 W and 135 W

PV module of 60 W

Dorobanţu and Popescu [20]

Diameter 20 mm, length equal to panel length, no. of holes 25 of diameter 1.5 mm.

33.3
10  6 m3/s. Mono crystalline panel of 75 W

Moharram et al. [21]

120 water nozzles

29 l/min

Balamuralikrishnan et al. [23] Irwan et al. [24]

observed that there were greatest changes in the Isc and Voc of the cell immersed in the glycerin than that of cell with direct exposure to the solar radiation. The fill factor of I-V characteristics of solar cell has little changes. The growth of the Isc and Voc took place with further increase in the level of the liquid above the surface of the cell up to 5–6 mm. Further increase of liquid level resulted in some decrease in Isc. They also confirmed the fact that the deposition of a noticeable thin film of glycerin on the surface of the p–n junction of the cell increased the photo current by 1.5–1.8 times. The presence of dielectric thin film increased the solar cell efficiency by 40–60%. They also mentioned the reasons for such an increase, which include an increase in the barrier height of n–p junction, a decrease in the velocity of the surface recombination followed by an increase in the factor of the separation of charge carriers generated by light as well as a decrease in a part of the reflected radiation. Wang et al. [26] achieved the better performance when the bare silicon solar cells were immersed in liquids to enhance the heat removing. They examined the efficiency of the solar cells under simulated sun light. The iodine tungsten lamp was employed as the solar simulator with intensity of 999 W/m2. Eleven single crystalline solar cells with an area of 2.5
10  5 m2 were connected in series for the immersion of the bare solar cell test. Six solar cells were connected in series and encapsulated by EVA, which were used for the immersion solar module test. Polar ethanol and glycerin, non-polar benzene and silicon oil and inorganic distilled water and tap water were used as immersion liquids. The thickness of liquid cover film was adjusted to 3 mm, 6 mm and 9 mm to detect the changes in performance parameters. The results showed that the Voc of the module was almost constant, but there were certain changes in the Isc when the thickness of immersion liquids was increased. The maximum increase in power of solar cells immersed in silicon oil was up to 4.07%. The solar cells immersed in silicon oil have the best performance. The

Six monocrystalline module of 185 W each.

Two units of 50 W monocrystalline PV panels.

changes in the performance were more in solar cells than solar module immersed in liquids. Rosa-Clot et al. [27] investigated the behavior of single crystalline PV panel submerged in water. Three identical panels were studied and compared. Panel 1 was placed in air exposed to solar radiation. Panel 2, SP2 was submerged under 4 cm of water. Panel 3, SP2 was submerged under 40 cm of water. The temperature range in panel 1 was in between 70 °C and 80 °C. The panel 2 has a stable temperature of about 30 °C. They found that average increase in efficiency of the SP2 at the 4 cm depth was about 11%, whereas at 40 cm a reduction of 23% was reported. Han et al. [28] proposed a direct liquid immersion cooling of concentrator solar cells in order to maintain the low and uniform temperature across the solar cells. Bare solar cells were made to immerse in a circulating liquid. DI water, Isopropyl Alcohol (IPA), Ethyl acetate (EA) and Dimethyl Silicon Oil (DMSO) were used as potential immersion liquids. Optical transmittance of the liquids was measured with help of spectrometer and they observed that the four immersion liquids are quite transparent in the wave length range between 400 nm and 1200 nm, over which the silicon solar cells absorbs strongly. The photocurrent density of solar silicon cell when its front surface is encapsulated with 10mm of DI water, IPA, EA and DMSO were 0.938, 0.975, 0.984 and 0.987 respectively. These results showed that DI water immersion can cause largest power loss. The electrical characteristics analysis was carried out on the solar cell of 40mm width and 50mm length having cell aperture area of 19.5 cm2 by using constant voltage I – V flash tester. The test was carried out in both the absence of the immersion liquid and also with 1.5mm liquid thickness on top. Both the configuration was tested at 30 sun and 25 °C. They observed that the Isc and Voc of the concentrator cells in the liquids were larger than those in air but, the degree of change in Voc is relatively less than that of Isc. The liquid immersion cooling reduced the loss due to reflection and increased the Isc by 7%. The

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increase in efficiency of solar cell immersed in IPA is larger than that with other liquids. The efficiency was increased from 18.7% to 21.7%. The thermal performance analysis was carried out by CFD analysis. The results showed that DI water was the best option with respect to lowest operating cell temperature. Nikhil et al. [29] investigated the performance of amorphous silicon solar cell cooled by silicone oil for different thickness up to 6 mm.The system consists of solar module of 7 W rated power output surrounded by glass sheets on four sides so that module surface can retain liquid on it. The experiments were conducted for 0 mm, 2 mm, 3 mm, 4 mm and 6 mm thickness of silicon oil over the module. The efficiency and power output of the module without oil over it were 2.98% and 2.775 W respectively. The efficiency and power output at optimum thickness of 2 mm were about 3.5% and 3.397 W respectively. They found that the power output increased up to 2 mm thickness of oil and decreased above 2 mm thickness of oil over its surface. The percentage variation in output for 2 mm, 3 mm, 4 mm and 6 mm thickness of silicone oil were 23.29%, 20.05%, 10.803% and 13.032% respectively and the variation of module efficiency throughout the day is shown in Fig. 9. The various works on liquid immersion cooling of PV modules is given in Table 4.

Fig. 9. Variation of module efficiency throughout the day [29].

389

2.5. Active cooling by attaching air/water/fin cooling system on the backside of the module Teo et al. [30] developed a hybrid PV/T solar system to investigate the electrical efficiency of the PV module with and without active cooling. They investigated the effects of temperature on the efficiency and power output of the module. Four 55 W polycrystalline solar modules were used in the experiment. An array of air ducts that allowed air to pass through was attached underneath of the PV modules. Fins were fitted in the duct to increase the heat transfer rate from the PV panel to moving air. They observed that the electrical efficiency was a linear function of module temperature and decreased with increase in PV module temperature. Without active cooling increase in temperature was higher at 1.6 °C for every 100 W/m2 increment of solar radiation and attained maximum value of 68 °C. The efficiency dropped to 8.6%. However with active cooling the increment was about 1.4 °C for every 100 W/m2 increment of the solar irradiation. The operating temperature of the module could be maintained at 38 °C and the electrical efficiency could also be kept at 12.5%. Tarabsheh et al. [31] investigated the performance of photovoltaic modules with respect to temperature and proposed pipes layouts as shown in Fig. 10. The module was cooled by a fluid flowing through pipes underneath the PV module backside to improve the conversion efficiency of the module. The temperature of the cooling fluid at the outlet of the PV module is higher than that of inlet due to heat exchange between the backside of the module and pipes. Therefore, the temperature of the pipe increased gradually from the inlet towards the outlet resulting into a non-uniformly cooled PV module. In other words, each PV cell in the module has a different operation temperature leading to different I–V characteristics of each cell. In their work, each PV panel composed of N-series connected solar cells was evaluated using MATLABTM software, from which the I–V characteristics of each ‘n’ cell (1 r nZN) were calculated for different operating temperature Tn and it is provided in Eq. (1).

Table 4 Works on liquid immersion cooling of PV modules. Work done by

Type of PV cell

Intensity of solar irradiation

Immersion liquid

Height of liquid above Results the cell/module

Abrahamyan et al. [25] Anti-reflection coated solar cell

70–80 mW/cm2

Glycerin

5–6 mm

Wang et al. [26]

Bare silicon PV cell [simulated sunlight]

999 W/m2

Silicon oil

3 mm, 6 mm and 9 mm

Rosa-Clot et al. [27]

Single crystalline PV panel

Water

4 cm

40 cm Han et al. [28]

Nikhil et al. [29]

Bare PV cells

Amorphous PV module of 7 W rated power

Simulated sun light

DI water IPA, EA and DMSO

10 mm

Simulated sun light of 30 sun

DI water IPA, EA and DMSO

1.5 mm

Solar irradiation

Silicon oil

0 mm, 2 mm, 3 mm, 4 mm and 6 mm

Isc and Voc increased up to 5–6 mm Anti-reflection coating increased Photo current by 1.5–1.8 times Voc was almost constant Isc increased with depth Output power increased by 4.07% Temperature reduction up to 40 °C Stable operating temperature of 30 °C The efficiency increased by 11% Stable operating temperature The efficiency decreased by 23% DI water caused largest power loss [0.938] DMSO caused less power loss [0.987] Isc and Voc are higher than in air Isc increased by 7% The increase in efficiency is higher in IPA DI water has the lowest operating temperature Optimum thickness was 2 mm The efficiency increased by 17.5% at 2 mm thickness Power output increased by 22.4% at 2 mm thickness Above 2 mm thickness the performance decreased

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Fig. 10. Different design suggested by Anas Al Tarabsheh et al. [31].

Fig. 11. Comparison of maximum power output of the module during the day with and without cooling. [32].

Tn = Ta +

( Tb – Ta){ ( n − 1) / ( N − 1)}

(1)

Ta ¼cooling fluid inlet temperature, Tb ¼cooling fluid outlet temperature, and N ¼number of cells. Design ‘A’ used one continuous pipe. Design ‘B’ splits the cooling medium flow into 9 channels with each channel used for cooling of 4 cells in series. Design ‘C’ splits the cooling medium into 4 channels used for cooling of 9 cells in series. The efficiency of module without cooling was 14%. The efficiency of modules with cooling for design A, B and C were 15%, 16.4% and 16.25% respectively. Bahaidarah et al. [32] investigated the performance of hybrid PV water cooled system numerically and experimentally. The numerical model was developed by using EES [Engineering Equation Solver] software. The experimental setup composed of a monocrystalline PV module of 230 W rated power combined with solar thermal collector. The cooling system attached on the rear side of the module has the inlet and outlet port for water flow. The cooling water was stored in an insulated tank connected to the PV/ T system through PVC pipes. With active cooling technique, the maximum operating temperature of the module reduced from 45 °C to 34 °C. An overall reduction in operating temperature of about 20% and an increase of 9% in the electrical efficiency was observed due to the cooling. The electrical output with cooling at an irradiance of 900 W/m2 was (Fig. 11) about 211 W, whereas the PV system without cooling was 190 W. Kolhe et al. [33] evaluated the concentrated single crystalline silicon PV module with water cooling system for temperature, power output and efficiency. The effect of cooling water flow rate on the performance also evaluated. The water cooling system was attached to the back of the PV module. A fixed PV module without solar concentration and cooling was used for comparing the performance of it with CPV module with cooling and the concentration ratio was 8.5. They observed that the maximum temperature

difference between CPV module and fixed PV module was lower than 5 °C due to the water cooling of CPV module. The power output of the CPV with cooling was 71.13 W instead of 16.55 W of fixed PV module. At noon, the efficiency of CPV with cooling was 7.81% and for the fixed PV was 10.68%. The efficiency of CPV decreased after noon and the increase in cooling water flow rate increases the electrical efficiency. Chong and Tan [34] proposed an automotive radiator cooling system for the heat rejection of dense-array concentrator PV system. Theoretical modeling on integration of automotive radiator into the cooling system with a specially designed cooling block has been carried out. The proposed system has been constructed and tested at solar concentration ratio of 377 sun. During on-site measurement, it has been observed that the conversion efficiency of CPV module was improved from 22.39% to 26.85%, when CPV cell operating temperature was reduced from 59.4 °C to 37.1 °C. Tonui and Tripanagnostopoulos [35] investigated the performance of two low cost heat extraction improvements in the channel of PV/T air system to achieve higher thermal output and PV cooling so as to keep electrical efficiency at acceptable level during energy conversion. For the improved system the channels were modified as shown in Fig. 12 by suspending a thin aluminum sheet in the middle (TMS) or attaching rectangular fins at the opposite back wall of the air channel (FIN). The experimental model used polycrystalline silicon PV module of length 1 m, aperture area 0.4 m2 and rated power 46 W. The channel walls, the metal sheet and the fins surfaces were painted black to increase their absorptivity and emissivity. The air circulation in the channel was accomplished with help of air pump. Their test results showed that the FIN system has the much lower operating temperature than other two systems by up to 10 °C and the additional power required by the modified systems (TMS and FIN) were about 1% more than that of the reference (REF) system. But the increase in electrical efficiency due to the cooling was about 1% for TMS system and 6% for the FIN systems compared to the reference system. Thus, the net electrical gain for the FIN system is about 5%. The parametric results also confirmed that the FIN system was more superior to the other two configurations for collector length up to 3 m.

3. Numerical studies on PV cell cooling Gray et al. [36] carried out numerical analysis on the Amonix high concentration photovoltaic system [HCPV], to study the passive cooling aspect of this particular type of system. A model of the HCPV passive cooling system was made using Gambit and numerical results were computed using Fluent software. The simulation was

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Fig. 12. Different cross-sectional view of PVT/AIR collector models suggested by Tonui et al. [35].

carried out for different elevation angle in order to predict the temperature distribution and velocity profile. The temperature distribution and air velocity vector information gives the visual description of how the air is moving in the system. Hughes et al. [37] used computational fluid dynamics (CFD) to model the heat transfer from a standard PV panel in order to determine the rate of heat dissipation occurred in the PV panel. They proposed a finned copper heat pipe attached with aluminum fins to improve the heat dissipation and efficiency of the PV panel. CFD analysis was carried out by using ANSYS version 12.1. They successfully demonstrated that the proposed system has the cooling solution for the PV panels. They developed the mathematical model for temperature distribution and were reasonable agreement with the CFD predicted values. The CFD results were validated by the results of developed scaled prototype. They also determined the optimum temperature cooled under the UAE environmental conditions by CFD analysis. Natarajan et al. [38] numerically studied the solar cell temperature for concentrating PV system with the concentration ratio of 10. They developed the two dimensional thermal model and predicted the temperature for concentrator PV system with and without passive cooling arrangements. They also studied the effect of ambient temperature and solar radiation intensity on the solar cell temperature for the system with and without cooling fins. They found from the results that the fins attached at the base of the back of the plate effectively reduced the solar cell operating temperature than the cell without fins and the thermal conductivity of back plate played a key role in reducing the solar cell temperature. They also observed that the increase in the height of fins from 2.5 to 5 mm, the cell temperature was decreased by 4.3 °C and the temperature was reduced by 10 °C for the fin height of 5–20 mm (Fig. 13). Based on the simulation results they proposed two separate solar cell temperature correlation for the system with and without fins to predict the cell temperature. Micheli et al. [39] identified the most convenient micro fins geometry for CPV passive cooling. The micro-fin array was used to cool single CPV cell exposed to 500 sun, reproduced in COSMOL Multiphysics 5.0.They found from the results that micro-fins can improve the thermal performance and lower the weight of a system. They found that the proposed fin array able to enhance the mass specific power up to 50% compared to an unfinned surface. Wu et al. [40] proposed and described by selecting a wick heat pipe to absorb isothermally the excessive heat from solar PV cells in order to solve the non-uniform cooling of solar PV cells and control the operating temperature of solar PV cells conveniently. The results showed that the overall thermal, electrical and exergy efficiencies of heat pipe PV/T hybrid system could reach up to 63.65%, 8.45% and 10.26% respectively. The varying range of operating temperature for solar cell on the solar PV panel is less than 2.5 °C. The parametric analysis showed that decrease in inlet water temperature and increase in water mass flow rate, packing factor of solar cell and heat loss coefficient would increase the electrical efficiency of the hybrid system. Elmir et al. [41] studied the cooling of a solar cell under forced convection in the presence of nanofluid by numerical simulation. They used the model of Brinkman and Wasp for the physical parameters of Al2O3-Water nanofluid. The finite elements method was used to solve the system of differential equations that was based on the Galerkin method. They evaluated the possibilities for

Fig. 13. Variation of solar cell temperature with height of the fins [38].

improvement of the transfer of heat by the use of nanofluids, as well as the influence of the volume fraction of nanofluid. They found from the simulation, the structure of the flow was not affected by the addition of nanoparticles to a basic fluid, but the increase in the thermal conductivity of the mixture increases the rate of heat transfer. The presence of nanoparticles in the fluid increases the rate of transfer of heat in comparison with the basic fluid thus improving cooling of the solar cells which leads to have a better performance of the solar panel. Kerzmann and Schaefer [42] studied the system simulation to model a medium 2D solar concentration energy system with an active cooling. The simulation was coded in Engineering Equation Solver (EES) and was used to simulate the linear concentrating photovoltaic system (LCPV) under Phoenix, AZ, solar and climatic conditions for a full year. They used the output data from this simulation to evaluate the LCPV system from an economic and environmental perspective and they claimed that over one year a 6.2 kW LCPV system would save a residential user $1623 in electricity and water heating, as well as displace 10.35 t of CO2. Gardas and Tendolkar [43] designed and developed a system for cooling the solar cell in order to increase its electrical efficiency and also to extract the heat energy. A hybrid solar system which generates both electricity and heat energy consists of PV cells attached to an absorber plate with fins attached at the other side of the absorber surface. Simulation model for single pass, single duct solar collector with fins was prepared and performance curves obtained. Performance with seven different gases was analyzed for maximum heat transfer, minimum mass flow rate and minimum number of fins. They found that hydrogen was the most suitable option with the present. For hydrogen, the system requires a mass flow rate of 0.00275 kg/s, which was the least amongst all. Theoretical number of fins required in this case was found to be 3.46. Popovici et al. [44] numerically studied the efficiency improvement of the PV cell in the same condition of solar radiation and proposed an air cooled heat sink to improve their performance. The cooling efficiency was studied for different configurations of heat sink by using ANSYS Fluent software. The simulation results

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Drabiniok and Neyer [50] demonstrated a novel cooling system for PV cells using the bionic method of evaporation cooling using a porous compound polymer foil. The foil was laminated directly on silicon substrates providing good thermal contact with the water cooled down by evaporation. Once the cooling process is started, the system was solar driven without a request of additional energy support. Beyond this the cooling mechanism is self-regulating depending only on temperature and air velocity. A temperature reduction up to 11.7 °C was proved with perspective to further significant enhancement. Fig. 14. Average and maximum relative increase in efficiency for different transition temperatures. [49].

showed that the minimum temperature reduction was 10 °C and the maximum power produced increased by 4%. Siddiqui and Arif [45] developed a multiphysics model for estimating the three dimensional thermal, structural and electrical performance of a PV module under given meteorological conditions. ANSYS CFX CFD software was used for the thermal modeling, the structural modeling had been done in ANSYS Mechanical FEA code and the electrical modeling had been developed in MATLAB environment. The developed model was used to simulate the electrical, thermal and structural performance of a PV module with and without cooling for four different days representing different environmental conditions at Jeddah, Saudi Arabia. From the simulation they concluded that the effectiveness of cooling in improving the electrical conversion efficiency was strongly dependent on irradiance than ambient temperature. The cooled panel showed lower cell temperatures than the module without cooling. Reddy et al. [46] carried out numerical analysis of a heat sink based on micro-channels for efficient cooling of a commercial high concentration (500
) photovoltaic (HCPV) cell. The micro channels were found to better at cooling the module and pressure drop was found to be low in straight flow channels. A combinatory model of an array of micro-channels enclosed in a wide parallel flow channel design was developed. The optimized geometry of the micro-channel heat sink was found by using commercial CFD software ANSYS 13. Biwole et al. [47] investigated the use of phase-change materials (PCM) to maintain the temperature of the panels close to the ambient. They developed CFD modeling of heat and mass transfers in a system composed of an impure phase change material situated in the back of a solar panel (SP). The results showed that adding a PCM on the back of a solar panel can maintain the panel's operating temperature under 40 °C for around two hours under a constant solar radiation of 1000 W/m². When the panel temperature rises, the excess heat must be absorbed until the PCM has completely melted. When the panel temperature decreases, the solidification of the PCM should provide additional heat for the operating liquid in solar thermal panel, provide heat to the building or act as an insulation material. The SP/PCM solution is expected to be very useful for roof or facade integrated panels where space for ventilation is limited. Browne et al. [48] proposed a PV/T phase change material (PCM) system to provide cooling by absorbing the heat in PCM and were taken away by the water flows through a pipe network within the PCM. They found that heat gain by the water in PV/T-PCM system was approximately 6 °C higher than that of a PV/T system. Machniewicz et al. [49] carried out dynamic simulations of thermal and electrical performance of PV/PCM panels using ESP-r software in order to determine the transition temperature of PCM layer that allows avoiding rapid temperature fluctuations on the PV back surface. They concluded from the results that additional PCM layer on the back side of PV panel can effectively increase the efficiency of electricity production with PCM transition temperature about 20 °C (Fig. 14).

4. Summary This paper presents an overview of the experimental and numerical studies on performance enhancement of solar photovoltaic cells by using effective cooling methods. The experimental and numerical analysis showed that the passive and active cooling techniques can reduce the rate of increase of solar cell operating temperature with time, irradiation intensity and ambient temperature and also maintain the temperature of the solar cell within the manufacturer specified value. 1. Heat pipe based passive cooling can be used up to 700 sun for single cell configuration and reduction in operating temperature of about 30–40 °C was possible. However use of that concept in terms of economic viability for the large power generation modules may need further research. 2. Passive cooling with attaching the fins on back side of the module may effectively reduce the operating temperature; improve the power output and efficiency. But their performance mainly depends on the heat transfer area and wind velocity. Therefore further research is needed to find the dimensions of the fin and number of fins required for commercial modules. 3. Active cooling by spraying the water over the front surface of the module will yield very good performance. This method can reduce the operating temperature up to 26 °C and reduce the reflection losses by 2–4%. It improves the solar cell performance to near value of rated performance parameters. But, water and additional power needed for pumping must be taken to consideration while designing the system. 4. The liquid immersion cooling reduces the reflection losses, increases the power output and can maintain the stable operating temperature, but there is no clear information on effect of depth of immersion on the performance. 5. Additional PCM layer on the back side of PV panel can effectively increase the efficiency of electricity production by providing cooling. The transition temperature of the PCM layer temperature should be as low as possible for maximum performance improvement. 6. Bionic method of evaporation cooling using a porous compound polymer foil is also used for effective cooling and they need further research to enhance the effect.

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