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Life span and overall performance enhancement of Solar Photovoltaic cell using water as coolant: A recent review
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 5 (2018) 18202–18210
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Life span and overall performance enhancement of Solar Photovoltaic cell using water as coolant: A recent review Rajat Sharmaa, Ayush Guptaa, Gopal Nandana, *, Gaurav Dwivedia, Satish Kumarb a
ME department, Amity University, Uttar Pradesh, Noida, India b ME Depertment, Thapar University,Patiyala, Punjab, India
Abstract In today’s world, as electricity consumption is increasing, people are more dependent on electricity. Solar Photovoltaic system is one method to generate electricity. The conversion efficiency of solar photovoltaic panel depends on atmospheric condition and reflection. The operating temperature of photovoltaic module plays an important role in performance of PV system as efficiency of PV system decreases when temperature module increases. The operating photovoltaic cells at high temperature degrades the material of it in long time. Operating solar photovoltaic at lower temperature will increase its lifespan. This will reduce module surface area by increasing overall output power. Researchers have proposed and tested several cooling techniques for the panel. One of the most common and effective way to cool PV module is used of water as coolant. In this paper, efforts made by various researchers to cool down solar photovoltaic module to increase the efficiency using water application have been discussed. The application of water on front surface, rear surface, both front and rear surface have been reviewed extensively. The performance of module by immersion in water also have been reported. © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Materials Processing and characterization. Keywords:Nanofluids, Solar Photovoltaics, Thermal conductivity, Overall efficiency, Electrical conversion efficiency
1. INTRODUCTION: The photovoltaics technology converts incident solar radiations falling on panel into electricity. The solar photovoltaics (SPV) systems are ideal, practical and economical to provide power for household applications as extension of power lines to rural areas. It has application in water pumps in remote areas, beacons, hydrological measuring stations, microwave repeaters, powering of lighting signs, meters etc. [1]. Maximum conversion
* Corresponding author. Tel.: 91-94 129 55789 E-mail address: [email protected] 2214-7853© 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Materials Processing and characterization.
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efficiency of SPV module is 4–17%. Rest of the solar energy incident on panel is either absorbed by PV panel or is lost to the surroundings. It has got more attention of engineers, researchers, business man and governments due to global environmental concern and fast depletion of fossil fuels. The installation rate of crystalline SPV modules are still very high worldwide. It is expected to increase further if the installation cost and cost of module is reduced. In past decade, photovoltaic market has known a growth of more than fifty times [2]. The global energy requirement is expected to increase by 33% in the period 2010-2030[3]. There are various factors affecting the operating conditions of the photovoltaic cell. Operating temperature of the PV module is found to be one of the most effective methods for cooling PV modules [4]. The factors on which the temperature of module depends are incident solar radiation, reflection of solar radiation, radiation and convection loss from PV module and various ambient conditions. Efficiency of PV system increase when temperature of PV module decrease therefore is important to cool the PV modules so that we can find maximum use of it. The conversion efficiency depends on the type of solar cells used and the ambient operating conditions. The conversion efficiency of SPV is influenced by the local climatic condition. As the operating temperature of the panel increases, the conversion efficiency decreases. The maximum SPV power decreases 0.4–0.5%/oC increase in cell temperature [5]. Apart from power output of panel, lifetime of the panels may also reduce. The conversion efficiency also depends on soiling of panel top, reduction in solar irradiance etc. The PVT devices simultaneously improves their performance and produce electrical as well as thermal energy. The thermal energy can be used for various useful purposes. The PVT system gives higher energy output per unit area than the standard PV system moreover they can be cost effective if the cost of the thermal units is low [68]. Various cooling techniques have been proposed in the literature to get maximum overall module efficiency. Various techniques used by researchers are applying water to front surface of the PV system, rear surface of PV system, both front and rear surface of PV module or completely immersing the PV module in water. The water application on SPV top maintains panel at low temperature. Besides the efficiency improvements due to cooling, the film of water also kept the panels clean, avoiding any reduced power output caused by panel soiling. This paper extensively covers the use of water to maintain PV at lower temperature so that an increase in the performance of the system can be achieved. 2. Application of water as cooling fluids: Researchers have reported the use of water as coolant to reduce the temperature of SPV panel. The technique used water to mainly in cooling front surface, rear surface. The performance of system by immersion cooling (both front as well as rear surface) have been reported in literature. These techniques have been elaborated in the succeeding sections. 2.1 Front surface cooling: Water is allowed to splash or sprayed over the top of the panel facing towards sun. Due to continuous wetting of front surface the temperature rise of SPV panel is controlled. Several authors have reported the use of water over the front surface. Krauter used twelve nozzles to spray water over top of modules and maintained water layer thickness was about 1mm [9]. The cells operating temperatures were reduced to 22oC as compared to conventional reference module and conversion efficiency was improved. The water evaporation was further cause the additional cooling. The increase of electrical power over the whole day was 10.3% which is shown in figure 1. A tube having small holes was fixed at the top of modules and head of 10m and 16m was maintained for water spraying [10]. The discharge rate of water was varied by conducting experiments at different pressure heads. Additional cooling of SPV was due evaporation apart from water spraying. Due to combined effect operating temperatures of modules reduced significantly as compared to uncooled SPV module. The temperature reduction was up to 23oC. The electrical power was increased by 17% over whole day and mean cell efficiency was increased to 3.26% at 16m pressure head. Even optical performance improved by 1.8%. For an indoor SPV panel’s performance study, Irwan et al. used DC power pumps experiments to spray water over the front panel. They conducted experiments using two set of monocrystalline SPV panels of 50 Watts. One panel was used as reference purpose while water flowing arrangement were made for another SPV panel. Through experimental results it was revealed that the operating temperature decreased by 5–23oC. The effective power output of panel increased by 9-22 % using cooling arrangement [11].
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Fig 1. Comparison of output power of the PV-modules [9] Dorobanțu et al [12] used a tube with 25 holes to produce a water film over panel. Diameter of holes was 1.5 mm and tube were fitted at the top end of the system. The flow rate of water was 2 lit/min. Fluke thermo-vision camera was used to measure temperature of front and back surfaces of the panel. The panel was oriented at 350o tilt angle. Although they haven’t discussed about the temperature of front panel, back temperature was reduced from 48°C to 35.5°C. Also owing to cooling, the electrical yield had exceeded about 8.4% which was sufficient to cover the power requirement for water circulation. An experimental analysis of comparative temperature and exergy of crystalline (c-Si) and amorphous (a-Si) using water cooling method was carried out by Kumar et al. [13] using panel area of 0.1455 m2. Water was made to circulate on the system using a 12 VDC pump. Uniform flow of water was maintained at the front surface of PV panel so as to cause uniform cooling on the surface of the panel (Refer figure 2). IR Thermometer was used to measure the front side and rear side temperature of PV system. Based on the concerned operating conditions, the cooling rate for the solar cells was 2oC/min. They concluded that SPV will give highest power output if cooling started when module temperature attained maximum allowable temperature. Similar water spraying arrangement using 120 water nozzles have been reported by Moharram et al [14]. They did mathematical modelling to predict cooling requirement of panel after achieving it maximum temperature. The results obtained were similar to Kumar et al. [13]. In their work, 45oC was selected to be optimum maximum allowable temperature as it yielded highest output energy by consuming least amount of water and energy [14]. The use of water was minimized,
Fig 2: Close-up of the flowing film of water at the PV module surface [13] Odeh and Behnia [15] introduced water trickling arrangement. Schematic diagram of test rig is depicted in figure 3. The system involved a water trickling tube fixed on the upper edge of the system module. The tube contains 32 holes (5 mm diameter) evenly distributed to maintain uniform and constant water flow. A submersible water pump is used to recirculate water, which is a drawback. In this arrangement, a overplus in power of about 15% is achieved.
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On using an underground water temperature (at 25oC) an average 8% improvement in SPV power is achieved. The effect of cooling in voltage power characteristics are shown in figure 4. The module output is in the range of 4–10% by this cooling. Part of this increase (50%) was concluded due to cooling by direct water with module whereas the other part was said due to refraction of the solar radiation in thin water layer which increased the incident radiation. TNANYS V16 were used to estimate long term performance of this system.
Figure 3: Schematic diagram of PV water cooling test rig [15]
Figure 4: Voltage-Power characteristic curve of Water cooled front Panel [15] Abdolzadeh et al. categorized three different operating conditions for determining the effect of sprinkling of water on front surface of PV modules. In first case, two modules were supplied with water having flow rate 25L/h/module and an increase in 1.35% of overall efficiency was observed. Second case had three modules supplied with water flow rate 5L/h/module and the third case also had three modules but flow rate was 25L/h/module which resulted in increase on overall efficiency to be 0.71% and 1.42% respectively. Cooling of modules was seen to be improved by improving the water flow [16]. An active water cooling system was built to spray water on front of panel surface. A water pump was used for water circulation. Humidity during testing time was found to be 20%-30%. In the midday, cooling rate was found to be 4oC/min. The average value of efficiency for spraying system along one day was 17.8 %. System output power with and without spraying is shown in figure 5. [17] Sukarno et. al. maintained laminar flow of over module surface [18]. They used simultaneous three similar panels in three different experimental conditions: without cooling system, continuous cooling and cooling system after gap of one hours. The cooling mechanism was fitted on the top of the SPV module. AS100 water pump was used to flow the water in the proposed study. The global solar radiation reached maximum value of 1052.9 W/m2. It was found that the efficiencies of panel in continuous cooling, intermittent cooling (every one hour) and without-cooling system was 16.7%, 14.4% and 13% respectively. A cooling system was designed to activate when the panel’s
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temperature reached max allowable temperature i.e. 35oC, and maintain the panel temperature at 25oC for max power output. Water pump was used to pump the water to the PVC pipe rails through the hosepipes and LM35 temperature sensor were used for temperature measurement. Those rails would drip the water onto the panel and eventually cool the panel. Cooling rate for the panel was 4.5oC/min, which meant that cooling apparatus would be operated each tie for 7.5 minutes for 10oC [19].
Figure 5: System output power with and without spraying [17] Another effort was made by Pradhan et al. to conduct a real-time experiment with two 20W/12V rated solar module having effective area of 0.136m2. They kept modules at 45o inclination. Wind speed was found to be measured at 11m/sec. A submersible DC motor pump was made to circulate the water. Total of 10 holes were made on the pipe so that there is even flow of water on the surface. It was found that the max power was obtained at max voltage and max current condition. With the given cooling system, temperature of the module was lowered by 5oC. The efficiency of cooling arrangement module was calculated as 17.8% as compared to the efficiency of noncooling panel which was 15.8% [20]. Saxena et al. used two panels one of which was used for performing the experiment [21]. Surface temperature was determined via a thermocouple that was installed at the back side of the panel. A water tank of capacity 50 litres was connected to water supply line for cooling purpose. As the heated water from panel surface did not flow back to supply tank, thus it was an open circuit flow system. A copper tube whose length was equal to width of PV panel containing 8 holes of 1mm each was fitted at the top of the panel horizontally. Volume flow rates of 3 lit/min, 5.3 lit/min, and 6.2 lit/min were used for intermittent cooling and volume flow rate of 0.6 lit/min was used for continuous water cooling. Cooling water would automatically start in all the intermittent cooling cases when the surface temperature reached 40oC and would cut off when the same decreased to 32oC. With intermittent cooling, max power achieved was 2.45W, which means an increase of 17.9%. In case of continuous cooling, total energy produced was 9.24 Wh i.e. an increase of 29.40% in total energy and 9.70% increase as compared to that of intermittent cooling. A clay pot evaporative cooling water was used to monitor thermal and electrical performance by using different water cooling systems which uses tap water and active pot evaporative cooling water system. Experimental setup is shown in the figure 6. The overall efficiency was better performance in pot water cooled system due to decrease in the temperature of cooling water due to evaporative. For pot water cooled SPV panel’s overall efficiency was 62% [4].
Figure 6: View of the experimental set up [4]
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2.2 Rear Surface Cooling: Various researchers used water flow at rear surface of SPV panel, to increase overall efficiency and to get thermal energy. A solar thermal collector was also attached at back surface of SPV module. The temperature drop of back surface of SPV was 20% and thus overall electrical efficiency increased by 9%. From their experiments, it was found that at an irradiance level of 900 W/m2, the PV only system collected nearly 190W of energy whereas nearly 750W of energy was captured by hybrid system [22]. For water cooled SPV system electrical and thermal has been developed using Engineering Equation Solver software to predicts electrical and thermal performance. The thermal and electrical performance of flat SPV module with cooling system using cotton wick structures in combination with water, water nanofluids based on metallic oxide (Al2O3, CuO). They used cotton wick od 7 mm and mounted in the circular ring shape (Figure 7) in the back side of rear side of SPV module. Free end of cotton wick was dipped in water reservoir for one set and in nanofluids for the another set of experiments. The maximum volumetric concentration of nanofluids was 0.1%. They observed that maximum efficiency of module without and with cotton wick cooling arrangement is 9% and 10.4%. As shown in below Table 1 Table 1: Comparison of results on using cotton wick structure under different conditions Output parameter Without cooling water CuO + Water Al2O3 + water Temperature 65oC 45oC 59oC 54oC Maximum power output 41W 47.5W 44.6W 44.6W Maximum module efficiency 9% 10.4% 9.5% 9.7% The use of cotton wick for cooling SPV module is better and more efficient with water in comparison to using cotton wick with nanofluids arrangement [1]. The arrangement of cotton wick is shown in figure 7.
Figure 7: Back surface of SPV module with cotton wick structure [1] Fujii et al compared two setups of solar PV module one placed on the roof in general state without cooling and other was being set up with cooling water on its rear side. Temperature of cooling water was reported 24oC at first and then gradually increases to 40oC during summers about every 2 hours and never touched 40oC during winters on initial temperature of 9oC. The total power consumption of solar module with water cooled equipment was about three percent higher than that of normal solar module in the summer and almost same as that of normal PV module during winters [23].
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Performance of a water-based PVT system was investigated in laminar and turbulent flow regime in [24]. effects of solar irradiation, packing factor, Reynolds number, collector length, pipes diameter and number of pipes were taken into consideration. A comparison was also made by covering the pv module with glass and one without it. Conclusions made from their work were that in general, the total energy efficiency reflected is higher by glazed PVT system in comparison to unglazed systems. The exergy efficiency for shorter lengths was found to be lower for glazed systems under laminar flow. Pipe diameter was found to enhance the total efficiency in turbulent region. Therefore, a total energy efficiency was found to be greater in turbulent region and exergy efficiency was found greater in laminar region. [24] Thermal–electrical model for energy estimation of a water cooled photovoltaic module was developed[25]. A conclusion made from their numerical work was that the coolant must be activated according to location and season of the year, and in some cases its use must be avoided. They also said that using a water-cooled PV module, regards the ability to reproduce the STC conditions which are not feasible with the PV modules subject to natural air circulation and thus proposed a technique to verify STC manufacturers data by conducting simple outdoor tests[25]. Shihabudheen and Arun used Monte Carlo Technique for the performance assessment. They concluded that annual solar fraction of the system [26]. Further, experiments were also conducted for code validation. Natural vapor is tried at the backside of module using circular holes of diameter of 8 mm [27]. The vapor temperature is in the range of temperature of natural vapor which is available from natural water bodies like rivers etc. The vapor simulator was used at discharge rates from 0–5 gram per min. The efficiency increased significantly using 8 vapor outlets. With 100% coverage of vapor, there was increase of 7.3% in the output. Aste et al used two modules to compare the performance [28]. The first module was a commercial product, which well represented the typical uncovered PVT modules available in the market. It integrated five strings of six poly-crystalline cells (mc-Si) connected in series (total power of 230Wp) placed in a glass-tedlar sandwich. On the back side of the PV module, a roll-bond aluminium heat exchanger was attached. The flow rate of the water was set at 0.055 kg/s. Second module, a covered PVT collector, was a prototype component which integrated a silicon thinfilm double-junction sandwich (stabilized peak power of 125 Wp) which is characterized by a low temperature coefficient, equal to -0.25 %/K. The flow rate of water here was 0.066 kg/s. The whole components were enclosed into an aluminium frame and were covered by a glass (4 mm thick). The air gap between the PV laminate and the glass cover was 20 mm. In addition, a thermal insulation of 50 mm thick mineral fiber material, was also used at the rear side of the absorber which further reduced thermal losses. The annual electrical efficiency of the covered PVT was 6.0% while the uncovered collector had a daily efficiency of 14.2%. However, during the year, the covered collector was able to produce more thermal energy, with an annual efficiency equal to 29.4%. They further showed that the covered collector was able to convert solar energy into primary energy with an annual efficiency of 42.3%, while the uncovered reached value of 52.6% [29]. Rawat et al fabricated solar PVT [30] of 37W polycrystalline silicon solar panel using copper sheets and copper tubes attached behind the panel, which was having area of 0.3216 square meter. Sheets attached would act as an absorber of heat from the panel and then transfer that heat to the water flowing in the tubes. Water was circulated in the system using an 18W A.C. pump at mass flow rate of 0.002 kg/sec [Figure 8]. It was seen that the electrical efficiency was obtained maximum at 25oC and 1000 W/m2. It was seen that the electrical efficiency, and daily thermal efficiency was7.57% and 50.1% respectively. The total efficiency of the system exceeded 73%. Also, the energy saving efficiency of the PVT system exceeded 68%.
Fig 8: Experimental setup of PV and PVT system [30]
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2.3 Immersion Cooling: Li Zhu et al used deionized water for immersion cooling of PV cells to improve the cell performance. The original module was made of Amonix Back Point-Contact (BPC) silicon cells as shown in figure 2. Another effort was made to improve the efficiency of PV system by water immersion technique in [31]. Tests were performed at seven different levels starting from 1 cm and ending at 7 cm and It was found that the efficiency for the PV panel gradually increased initially and reached its maximum value at 6 cm and therefore started decreasing further. Tina et al used crystalline silicon panels to investigate the performance of submerged photovoltaic panel under high irradiance and ambient temperature. They noticed the increase in energy performances under range of 10-15% [32]. 2.4 Both Front And Rear Surface Cooling: Nizetic et al. sprayed water using nozzles on both side of SPV monocrystalline module [33] Assembly of nozzles ensured proper distribution of water spray. Total of 20 nozzles were installed, 10 on each side. The back nozzles were installed perpendicular to the surface and front nozzles were at an angle of 40o, in order to avoid shadowing effect of wire frame and also it ensured wider distribution of water spray over the surface. The average panel temperature reduced to 24oC from 52oC. The electrical efficiency increased by 14.15%. 3. Conclusions The maximum conversion efficiency of solar photovoltaics technology module is 4–17%. Rest of the solar energy incident on panel is either absorbed by photovoltaics panel or is lost to the surroundings. As the operating temperature of the panel increases, the conversion efficiency decreases. The factors on which the temperature of module depends are incident solar radiation, reflection of solar radiation, radiation and convection loss from PV module and various ambient conditions. Efficiency of PV system increase when temperature of PV module decrease therefore is important to cool the PV modules to operate at maximum efficiency. The maximum power output decreases 0.4–0.5%/oC increase in cell temperature. The life span of panel also decreases due to operation at high temperature. Various cooling techniques have been proposed in the literature to get maximum overall module efficiency. The use of water as coolant has been used by various researchers to cool it. Researchers used water to front surface of the PV system, rear surface of PV system, both front and rear surface of PV module or completely immersed the module in water. This reduced panel operating temperature. Besides the efficiency improvements due to cooling, the film of water also kept the panels clean, avoiding any reduced power output caused by panel soiling. For cooling of solar photovoltaics panel active technique have been reported. The temperature drop rate of front panel were from 2oC/min to 4.5oC/min have been reported. References 1.
M. Chandrasekar, S. Suresh, T. Senthilkumar, and M. G. Karthikeyan, “Passive cooling of standalone flat PV module with cotton wick structures,” Energy Conversion and Management, vol. 71, pp. 43–50, Jul 2013. 2. G. Colt, “Performance evaluation of a PV panel by rear surface water active cooling,” in International Conference on Applied and Theoretical Electricity, (Craiova, Romania), 2016. 3. M. Hasanuzzaman, N. Rahim, M. Hosenuzzaman, R. Saidur, I. Mahbubul, and M. Rashid, “Energy savings in the combustion based process heating in industrial sector,” Renewable and Sustainable Energy Reviews, vol. 16, pp. 4527–4536, Sep 2012. 4. R. Ramkumar, M. Kesavan, C. Raguraman, and A. Ragupathy, “Enhancing the performance of photovoltaic module using clay pot evaporative cooling water,” in 2016 International Conference on Energy Efficient Technologies for Sustainability (ICEETS), IEEE, Apr 2016. 5. V. Dyk, S. B.J., E. Meyer, and L. A.W.R., “Temperature dependence of performance of crystalline silicon photovoltaic modules,” South African Journal of Science, vol. 96, no. 4, pp. 198–200, 2000. 6. C. Lamnatou and D. Chemisana, “Photovoltaic/thermal (PVT) systems: A review with emphasis on environmental issues,” Renewable Energy, vol. 105, pp. 270–287, may 2017. 7. N. Aste, C. del Pero, and F. Leonforte, “Water flat plate PV–thermal collectors: A review,” Solar Energy, vol. 102, pp. 98–115, apr 2014. 8. M. O. Lari and A. Z. Sahin, “Design, performance and economic analysis of a nanofluid-based photovoltaic/thermal system for residential applications,” Energy Conversion and Management, vol. 149, pp. 467–484, oct 2017. 9. S. Krauter, “Increased electrical yield via water flow over the front of photovoltaic panels,” Solar Energy Materials and Solar Cells, vol. 82, pp. 131– 137, may 2004. 10. M. Abdolzadeh and M. Ameri, “Improving the effectiveness of a photo- voltaic water pumping system by spraying water over the front of photovoltaic cells,” Renewable Energy, vol. 34, pp. 91–96, Jan 2009. 11. Y. Irwan, W. Leow, M. Irwanto, Fareq. M, A. Amelia, N. Gomesh, and I. Safwati, “Indoor test performance of PV panel through water cooling method,” Energy Procedia, vol. 79, pp. 604–611, Nov 2015. 12. L. Doroban¸tu, M. Popescu, C. Popescu, and A. Cr˘aciunescu, “Experimental assessment of PV panels front water cooling strategy,” Renewable Energy and Power Quality Journal, pp. 1009–1012, mar 2013.
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13. P. Kumar, V. Gupta, K. Sudhakar, and A. K. Singh, “Experimental analysis of comparative temperature and exergy of crystalline (c-Si) and amorphous (a-Si) solar PV module using water cooling method,” IOSR Journal of Mechanical and Civil Engineering, vol. 13, pp. 21–26, may 2016. 14. K. Moharram, M. Abd-Elhady, H. Kandil, and H. El-Sherif, “Enhancing the performance of photovoltaic panels by water cooling,” Ain Shams Engineering Journal, vol. 4, pp. 869–877, Dec 2013. 15. S. Odeh and M. Behnia, “Improving photovoltaic module efficiency using water cooling,” Heat Transfer Engineering, vol. 30, pp. 499–505, May 2009. 16. M. Abdolzadeh, M. Ameri, and M. A. Mehrabian, “Effects of water spray over the photovoltaic modules on the performance of a photovoltaic water pumping system under different operating conditions,” Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, vol. 33, pp. 1546– 1555, May 2011. 17. K. W. A. Salih Mohammed Salih, Osama Ibrahim Abd, “Performance enhancement of PV array based on water spraying technique,” International Journal of Sustainable and Green Energy, vol. 4, no. 3-1, pp. 8–13, 2015. 18. H. R. J. D. Kartini Sukarno, Abd Hamid Ag Sufiyan, “Evaluation on cooling effect on solar PV power output using laminar H2O surface method,” International Journal of Renewable Energy Research, vol. 7, no. 3, 2017. 19. A. A. Sequeira, S. Shetty, S. S. S, and C. P. S. M, “Improvement of power output from solar panel using water cooling system,” Global Journal of Advanced Engineering Technologies, vol. 5, no. 1, pp. 58–63, 2016. 20. A. Pradhan, S. K. S. Parashar, S. M. Ali, and P. Paikray, “Water cooling method to improve efficiency of photovoltaic module,” in 2016 International Conference on Signal Processing, Communication, Power and Embedded System (SCOPES), IEEE, oct 2016. 21. A. Saxena, S. Deshmukh, S. Nirali, and S. Wani, “Laboratory based experimental investigation of photovoltaic (PV) thermo-control with water and its proposed real-time implementation,” Renewable Energy, vol. 115, pp. 128–138, jan 2018. 22. H. Bahaidarah, A. Subhan, P. Gandhidasan, and S. Rehman, “Performance evaluation of a PV (photovoltaic) module by back surface water cooling for hot climatic conditions,” Energy, vol. 59, pp. 445–453, Sep 2013. 23. Masayuki Fujii et. al, “Improvement of conversion efficiency through water-cooled equipment in photo- voltaic system,” Journal of International Council on Electrical Engineering, vol. 3, no. 1, pp. 97–101, 2013. 24. F. Yazdanifard, E. Ebrahimnia-Bajestan, and M. Ameri, “Investigating the performance of a water-based photovoltaic/thermal (PV/t) collector in laminar and turbulent flow regime,” Renewable Energy, vol. 99, pp. 295– 306, Dec 2016. 25. F. Spertino, A. D’Angola, D. Enescu, P. D. Leo, G. V. Fracastoro, and R. Zaffina, “Thermal–electrical model for energy estimation of a water cooled photovoltaic module,” Solar Energy, vol. 133, pp. 119–140, Aug 2016. 26. M. Shihabudheen and P. Arun, “Performance evaluation of a hybrid photovoltaic-thermal water heating system,” International Journal of Green Energy, vol. 11, pp. 969–986, Feb 2014. 27. M. Ebrahimi, M. Rahimi, and A. Rahimi, “An experimental study on using natural vaporization for cooling of a photovoltaic solar cell,” International Communications in Heat and Mass Transfer, vol. 65, pp. 22–30, Jul 2015. 28. N. Aste, C. D. Pero, and F. Leonforte, “Water PVT Collectors performance comparison,” Energy Procedia, vol. 105, pp. 961–966, May 2017. 29. N. Aste, F. Leonforte, and C. D. Pero, “Design, Modelling and performance monitoring of a photovoltaic–thermal (PVT) water collector,” Solar Energy, vol. 112, pp. 85–99, Feb 2015. 30. S. M. Pratish Rawat, Mary Debbarma and K. Sudhakar, “Design, development and experimental investigation of solar photovoltaic/thermal (PV/T) water collector system,” International Journal of Science, Environment and Technology, vol. 3, no. 3, p. 1173-1183, 2014. 31. K. M. Y. Sayran A. Abdulgafar, Omar S. Omar, “Improving the efficiency of polycrystalline solar panel via water immersion method,” International Journal of Innovative Research in Science, Engineering and Technology, vol. 3, pp. 8127–8132, Jan. 2014. 32. G. Tina, M. Rosa-Clot, P. Rosa-Clot, and P. Scandura, “Optical and thermal behavior of submerged photovoltaic solar panel: SP2,” Energy, vol. 39, pp. 17–26, Mar 2012. ˇ oko, A. Yadav, and F. Grubiˇsi´c-C ˇ abo, “Water spray cooling technique applied on a photovoltaic panel: The performance 33. S. Niˇzeti´c, D. C response,” Energy Conversion and Management, vol. 108, pp. 287–296, Jan 2016.
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Life span and overall performance enhancement of Solar Photovoltaic cell using water as coolant: A recent review Rajat Sharmaa, Ayush Guptaa, Gopal Nandana, *, Gaurav Dwivedia, Satish Kumarb a
ME department, Amity University, Uttar Pradesh, Noida, India b ME Depertment, Thapar University,Patiyala, Punjab, India
Abstract In today’s world, as electricity consumption is increasing, people are more dependent on electricity. Solar Photovoltaic system is one method to generate electricity. The conversion efficiency of solar photovoltaic panel depends on atmospheric condition and reflection. The operating temperature of photovoltaic module plays an important role in performance of PV system as efficiency of PV system decreases when temperature module increases. The operating photovoltaic cells at high temperature degrades the material of it in long time. Operating solar photovoltaic at lower temperature will increase its lifespan. This will reduce module surface area by increasing overall output power. Researchers have proposed and tested several cooling techniques for the panel. One of the most common and effective way to cool PV module is used of water as coolant. In this paper, efforts made by various researchers to cool down solar photovoltaic module to increase the efficiency using water application have been discussed. The application of water on front surface, rear surface, both front and rear surface have been reviewed extensively. The performance of module by immersion in water also have been reported. © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Materials Processing and characterization. Keywords:Nanofluids, Solar Photovoltaics, Thermal conductivity, Overall efficiency, Electrical conversion efficiency
1. INTRODUCTION: The photovoltaics technology converts incident solar radiations falling on panel into electricity. The solar photovoltaics (SPV) systems are ideal, practical and economical to provide power for household applications as extension of power lines to rural areas. It has application in water pumps in remote areas, beacons, hydrological measuring stations, microwave repeaters, powering of lighting signs, meters etc. [1]. Maximum conversion
* Corresponding author. Tel.: 91-94 129 55789 E-mail address: [email protected] 2214-7853© 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Materials Processing and characterization.
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efficiency of SPV module is 4–17%. Rest of the solar energy incident on panel is either absorbed by PV panel or is lost to the surroundings. It has got more attention of engineers, researchers, business man and governments due to global environmental concern and fast depletion of fossil fuels. The installation rate of crystalline SPV modules are still very high worldwide. It is expected to increase further if the installation cost and cost of module is reduced. In past decade, photovoltaic market has known a growth of more than fifty times [2]. The global energy requirement is expected to increase by 33% in the period 2010-2030[3]. There are various factors affecting the operating conditions of the photovoltaic cell. Operating temperature of the PV module is found to be one of the most effective methods for cooling PV modules [4]. The factors on which the temperature of module depends are incident solar radiation, reflection of solar radiation, radiation and convection loss from PV module and various ambient conditions. Efficiency of PV system increase when temperature of PV module decrease therefore is important to cool the PV modules so that we can find maximum use of it. The conversion efficiency depends on the type of solar cells used and the ambient operating conditions. The conversion efficiency of SPV is influenced by the local climatic condition. As the operating temperature of the panel increases, the conversion efficiency decreases. The maximum SPV power decreases 0.4–0.5%/oC increase in cell temperature [5]. Apart from power output of panel, lifetime of the panels may also reduce. The conversion efficiency also depends on soiling of panel top, reduction in solar irradiance etc. The PVT devices simultaneously improves their performance and produce electrical as well as thermal energy. The thermal energy can be used for various useful purposes. The PVT system gives higher energy output per unit area than the standard PV system moreover they can be cost effective if the cost of the thermal units is low [68]. Various cooling techniques have been proposed in the literature to get maximum overall module efficiency. Various techniques used by researchers are applying water to front surface of the PV system, rear surface of PV system, both front and rear surface of PV module or completely immersing the PV module in water. The water application on SPV top maintains panel at low temperature. Besides the efficiency improvements due to cooling, the film of water also kept the panels clean, avoiding any reduced power output caused by panel soiling. This paper extensively covers the use of water to maintain PV at lower temperature so that an increase in the performance of the system can be achieved. 2. Application of water as cooling fluids: Researchers have reported the use of water as coolant to reduce the temperature of SPV panel. The technique used water to mainly in cooling front surface, rear surface. The performance of system by immersion cooling (both front as well as rear surface) have been reported in literature. These techniques have been elaborated in the succeeding sections. 2.1 Front surface cooling: Water is allowed to splash or sprayed over the top of the panel facing towards sun. Due to continuous wetting of front surface the temperature rise of SPV panel is controlled. Several authors have reported the use of water over the front surface. Krauter used twelve nozzles to spray water over top of modules and maintained water layer thickness was about 1mm [9]. The cells operating temperatures were reduced to 22oC as compared to conventional reference module and conversion efficiency was improved. The water evaporation was further cause the additional cooling. The increase of electrical power over the whole day was 10.3% which is shown in figure 1. A tube having small holes was fixed at the top of modules and head of 10m and 16m was maintained for water spraying [10]. The discharge rate of water was varied by conducting experiments at different pressure heads. Additional cooling of SPV was due evaporation apart from water spraying. Due to combined effect operating temperatures of modules reduced significantly as compared to uncooled SPV module. The temperature reduction was up to 23oC. The electrical power was increased by 17% over whole day and mean cell efficiency was increased to 3.26% at 16m pressure head. Even optical performance improved by 1.8%. For an indoor SPV panel’s performance study, Irwan et al. used DC power pumps experiments to spray water over the front panel. They conducted experiments using two set of monocrystalline SPV panels of 50 Watts. One panel was used as reference purpose while water flowing arrangement were made for another SPV panel. Through experimental results it was revealed that the operating temperature decreased by 5–23oC. The effective power output of panel increased by 9-22 % using cooling arrangement [11].
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Fig 1. Comparison of output power of the PV-modules [9] Dorobanțu et al [12] used a tube with 25 holes to produce a water film over panel. Diameter of holes was 1.5 mm and tube were fitted at the top end of the system. The flow rate of water was 2 lit/min. Fluke thermo-vision camera was used to measure temperature of front and back surfaces of the panel. The panel was oriented at 350o tilt angle. Although they haven’t discussed about the temperature of front panel, back temperature was reduced from 48°C to 35.5°C. Also owing to cooling, the electrical yield had exceeded about 8.4% which was sufficient to cover the power requirement for water circulation. An experimental analysis of comparative temperature and exergy of crystalline (c-Si) and amorphous (a-Si) using water cooling method was carried out by Kumar et al. [13] using panel area of 0.1455 m2. Water was made to circulate on the system using a 12 VDC pump. Uniform flow of water was maintained at the front surface of PV panel so as to cause uniform cooling on the surface of the panel (Refer figure 2). IR Thermometer was used to measure the front side and rear side temperature of PV system. Based on the concerned operating conditions, the cooling rate for the solar cells was 2oC/min. They concluded that SPV will give highest power output if cooling started when module temperature attained maximum allowable temperature. Similar water spraying arrangement using 120 water nozzles have been reported by Moharram et al [14]. They did mathematical modelling to predict cooling requirement of panel after achieving it maximum temperature. The results obtained were similar to Kumar et al. [13]. In their work, 45oC was selected to be optimum maximum allowable temperature as it yielded highest output energy by consuming least amount of water and energy [14]. The use of water was minimized,
Fig 2: Close-up of the flowing film of water at the PV module surface [13] Odeh and Behnia [15] introduced water trickling arrangement. Schematic diagram of test rig is depicted in figure 3. The system involved a water trickling tube fixed on the upper edge of the system module. The tube contains 32 holes (5 mm diameter) evenly distributed to maintain uniform and constant water flow. A submersible water pump is used to recirculate water, which is a drawback. In this arrangement, a overplus in power of about 15% is achieved.
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On using an underground water temperature (at 25oC) an average 8% improvement in SPV power is achieved. The effect of cooling in voltage power characteristics are shown in figure 4. The module output is in the range of 4–10% by this cooling. Part of this increase (50%) was concluded due to cooling by direct water with module whereas the other part was said due to refraction of the solar radiation in thin water layer which increased the incident radiation. TNANYS V16 were used to estimate long term performance of this system.
Figure 3: Schematic diagram of PV water cooling test rig [15]
Figure 4: Voltage-Power characteristic curve of Water cooled front Panel [15] Abdolzadeh et al. categorized three different operating conditions for determining the effect of sprinkling of water on front surface of PV modules. In first case, two modules were supplied with water having flow rate 25L/h/module and an increase in 1.35% of overall efficiency was observed. Second case had three modules supplied with water flow rate 5L/h/module and the third case also had three modules but flow rate was 25L/h/module which resulted in increase on overall efficiency to be 0.71% and 1.42% respectively. Cooling of modules was seen to be improved by improving the water flow [16]. An active water cooling system was built to spray water on front of panel surface. A water pump was used for water circulation. Humidity during testing time was found to be 20%-30%. In the midday, cooling rate was found to be 4oC/min. The average value of efficiency for spraying system along one day was 17.8 %. System output power with and without spraying is shown in figure 5. [17] Sukarno et. al. maintained laminar flow of over module surface [18]. They used simultaneous three similar panels in three different experimental conditions: without cooling system, continuous cooling and cooling system after gap of one hours. The cooling mechanism was fitted on the top of the SPV module. AS100 water pump was used to flow the water in the proposed study. The global solar radiation reached maximum value of 1052.9 W/m2. It was found that the efficiencies of panel in continuous cooling, intermittent cooling (every one hour) and without-cooling system was 16.7%, 14.4% and 13% respectively. A cooling system was designed to activate when the panel’s
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temperature reached max allowable temperature i.e. 35oC, and maintain the panel temperature at 25oC for max power output. Water pump was used to pump the water to the PVC pipe rails through the hosepipes and LM35 temperature sensor were used for temperature measurement. Those rails would drip the water onto the panel and eventually cool the panel. Cooling rate for the panel was 4.5oC/min, which meant that cooling apparatus would be operated each tie for 7.5 minutes for 10oC [19].
Figure 5: System output power with and without spraying [17] Another effort was made by Pradhan et al. to conduct a real-time experiment with two 20W/12V rated solar module having effective area of 0.136m2. They kept modules at 45o inclination. Wind speed was found to be measured at 11m/sec. A submersible DC motor pump was made to circulate the water. Total of 10 holes were made on the pipe so that there is even flow of water on the surface. It was found that the max power was obtained at max voltage and max current condition. With the given cooling system, temperature of the module was lowered by 5oC. The efficiency of cooling arrangement module was calculated as 17.8% as compared to the efficiency of noncooling panel which was 15.8% [20]. Saxena et al. used two panels one of which was used for performing the experiment [21]. Surface temperature was determined via a thermocouple that was installed at the back side of the panel. A water tank of capacity 50 litres was connected to water supply line for cooling purpose. As the heated water from panel surface did not flow back to supply tank, thus it was an open circuit flow system. A copper tube whose length was equal to width of PV panel containing 8 holes of 1mm each was fitted at the top of the panel horizontally. Volume flow rates of 3 lit/min, 5.3 lit/min, and 6.2 lit/min were used for intermittent cooling and volume flow rate of 0.6 lit/min was used for continuous water cooling. Cooling water would automatically start in all the intermittent cooling cases when the surface temperature reached 40oC and would cut off when the same decreased to 32oC. With intermittent cooling, max power achieved was 2.45W, which means an increase of 17.9%. In case of continuous cooling, total energy produced was 9.24 Wh i.e. an increase of 29.40% in total energy and 9.70% increase as compared to that of intermittent cooling. A clay pot evaporative cooling water was used to monitor thermal and electrical performance by using different water cooling systems which uses tap water and active pot evaporative cooling water system. Experimental setup is shown in the figure 6. The overall efficiency was better performance in pot water cooled system due to decrease in the temperature of cooling water due to evaporative. For pot water cooled SPV panel’s overall efficiency was 62% [4].
Figure 6: View of the experimental set up [4]
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2.2 Rear Surface Cooling: Various researchers used water flow at rear surface of SPV panel, to increase overall efficiency and to get thermal energy. A solar thermal collector was also attached at back surface of SPV module. The temperature drop of back surface of SPV was 20% and thus overall electrical efficiency increased by 9%. From their experiments, it was found that at an irradiance level of 900 W/m2, the PV only system collected nearly 190W of energy whereas nearly 750W of energy was captured by hybrid system [22]. For water cooled SPV system electrical and thermal has been developed using Engineering Equation Solver software to predicts electrical and thermal performance. The thermal and electrical performance of flat SPV module with cooling system using cotton wick structures in combination with water, water nanofluids based on metallic oxide (Al2O3, CuO). They used cotton wick od 7 mm and mounted in the circular ring shape (Figure 7) in the back side of rear side of SPV module. Free end of cotton wick was dipped in water reservoir for one set and in nanofluids for the another set of experiments. The maximum volumetric concentration of nanofluids was 0.1%. They observed that maximum efficiency of module without and with cotton wick cooling arrangement is 9% and 10.4%. As shown in below Table 1 Table 1: Comparison of results on using cotton wick structure under different conditions Output parameter Without cooling water CuO + Water Al2O3 + water Temperature 65oC 45oC 59oC 54oC Maximum power output 41W 47.5W 44.6W 44.6W Maximum module efficiency 9% 10.4% 9.5% 9.7% The use of cotton wick for cooling SPV module is better and more efficient with water in comparison to using cotton wick with nanofluids arrangement [1]. The arrangement of cotton wick is shown in figure 7.
Figure 7: Back surface of SPV module with cotton wick structure [1] Fujii et al compared two setups of solar PV module one placed on the roof in general state without cooling and other was being set up with cooling water on its rear side. Temperature of cooling water was reported 24oC at first and then gradually increases to 40oC during summers about every 2 hours and never touched 40oC during winters on initial temperature of 9oC. The total power consumption of solar module with water cooled equipment was about three percent higher than that of normal solar module in the summer and almost same as that of normal PV module during winters [23].
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Performance of a water-based PVT system was investigated in laminar and turbulent flow regime in [24]. effects of solar irradiation, packing factor, Reynolds number, collector length, pipes diameter and number of pipes were taken into consideration. A comparison was also made by covering the pv module with glass and one without it. Conclusions made from their work were that in general, the total energy efficiency reflected is higher by glazed PVT system in comparison to unglazed systems. The exergy efficiency for shorter lengths was found to be lower for glazed systems under laminar flow. Pipe diameter was found to enhance the total efficiency in turbulent region. Therefore, a total energy efficiency was found to be greater in turbulent region and exergy efficiency was found greater in laminar region. [24] Thermal–electrical model for energy estimation of a water cooled photovoltaic module was developed[25]. A conclusion made from their numerical work was that the coolant must be activated according to location and season of the year, and in some cases its use must be avoided. They also said that using a water-cooled PV module, regards the ability to reproduce the STC conditions which are not feasible with the PV modules subject to natural air circulation and thus proposed a technique to verify STC manufacturers data by conducting simple outdoor tests[25]. Shihabudheen and Arun used Monte Carlo Technique for the performance assessment. They concluded that annual solar fraction of the system [26]. Further, experiments were also conducted for code validation. Natural vapor is tried at the backside of module using circular holes of diameter of 8 mm [27]. The vapor temperature is in the range of temperature of natural vapor which is available from natural water bodies like rivers etc. The vapor simulator was used at discharge rates from 0–5 gram per min. The efficiency increased significantly using 8 vapor outlets. With 100% coverage of vapor, there was increase of 7.3% in the output. Aste et al used two modules to compare the performance [28]. The first module was a commercial product, which well represented the typical uncovered PVT modules available in the market. It integrated five strings of six poly-crystalline cells (mc-Si) connected in series (total power of 230Wp) placed in a glass-tedlar sandwich. On the back side of the PV module, a roll-bond aluminium heat exchanger was attached. The flow rate of the water was set at 0.055 kg/s. Second module, a covered PVT collector, was a prototype component which integrated a silicon thinfilm double-junction sandwich (stabilized peak power of 125 Wp) which is characterized by a low temperature coefficient, equal to -0.25 %/K. The flow rate of water here was 0.066 kg/s. The whole components were enclosed into an aluminium frame and were covered by a glass (4 mm thick). The air gap between the PV laminate and the glass cover was 20 mm. In addition, a thermal insulation of 50 mm thick mineral fiber material, was also used at the rear side of the absorber which further reduced thermal losses. The annual electrical efficiency of the covered PVT was 6.0% while the uncovered collector had a daily efficiency of 14.2%. However, during the year, the covered collector was able to produce more thermal energy, with an annual efficiency equal to 29.4%. They further showed that the covered collector was able to convert solar energy into primary energy with an annual efficiency of 42.3%, while the uncovered reached value of 52.6% [29]. Rawat et al fabricated solar PVT [30] of 37W polycrystalline silicon solar panel using copper sheets and copper tubes attached behind the panel, which was having area of 0.3216 square meter. Sheets attached would act as an absorber of heat from the panel and then transfer that heat to the water flowing in the tubes. Water was circulated in the system using an 18W A.C. pump at mass flow rate of 0.002 kg/sec [Figure 8]. It was seen that the electrical efficiency was obtained maximum at 25oC and 1000 W/m2. It was seen that the electrical efficiency, and daily thermal efficiency was7.57% and 50.1% respectively. The total efficiency of the system exceeded 73%. Also, the energy saving efficiency of the PVT system exceeded 68%.
Fig 8: Experimental setup of PV and PVT system [30]
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2.3 Immersion Cooling: Li Zhu et al used deionized water for immersion cooling of PV cells to improve the cell performance. The original module was made of Amonix Back Point-Contact (BPC) silicon cells as shown in figure 2. Another effort was made to improve the efficiency of PV system by water immersion technique in [31]. Tests were performed at seven different levels starting from 1 cm and ending at 7 cm and It was found that the efficiency for the PV panel gradually increased initially and reached its maximum value at 6 cm and therefore started decreasing further. Tina et al used crystalline silicon panels to investigate the performance of submerged photovoltaic panel under high irradiance and ambient temperature. They noticed the increase in energy performances under range of 10-15% [32]. 2.4 Both Front And Rear Surface Cooling: Nizetic et al. sprayed water using nozzles on both side of SPV monocrystalline module [33] Assembly of nozzles ensured proper distribution of water spray. Total of 20 nozzles were installed, 10 on each side. The back nozzles were installed perpendicular to the surface and front nozzles were at an angle of 40o, in order to avoid shadowing effect of wire frame and also it ensured wider distribution of water spray over the surface. The average panel temperature reduced to 24oC from 52oC. The electrical efficiency increased by 14.15%. 3. Conclusions The maximum conversion efficiency of solar photovoltaics technology module is 4–17%. Rest of the solar energy incident on panel is either absorbed by photovoltaics panel or is lost to the surroundings. As the operating temperature of the panel increases, the conversion efficiency decreases. The factors on which the temperature of module depends are incident solar radiation, reflection of solar radiation, radiation and convection loss from PV module and various ambient conditions. Efficiency of PV system increase when temperature of PV module decrease therefore is important to cool the PV modules to operate at maximum efficiency. The maximum power output decreases 0.4–0.5%/oC increase in cell temperature. The life span of panel also decreases due to operation at high temperature. Various cooling techniques have been proposed in the literature to get maximum overall module efficiency. The use of water as coolant has been used by various researchers to cool it. Researchers used water to front surface of the PV system, rear surface of PV system, both front and rear surface of PV module or completely immersed the module in water. This reduced panel operating temperature. Besides the efficiency improvements due to cooling, the film of water also kept the panels clean, avoiding any reduced power output caused by panel soiling. For cooling of solar photovoltaics panel active technique have been reported. The temperature drop rate of front panel were from 2oC/min to 4.5oC/min have been reported. References 1.
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