Improving Photovoltaic Module Efficiency Using Water Cooling

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An effective way of improving efficiency and reducing the rate of thermal degradation of a photovoltaic (PV) module is by reducing the operating temperature of ...

Heat Transfer Engineering, 30(6):499–505, 2009 C Taylor and Francis Group, LLC Copyright  ISSN: 0145-7632 print / 1521-0537 online DOI: 10.1080/01457630802529214

Improving Photovoltaic Module Efficiency Using Water Cooling SAAD ODEH,1 and MASUD BEHNIA2 1 2

School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, Australia Dean of Graduate Studies, The University of Sydney, Australia

An effective way of improving efficiency and reducing the rate of thermal degradation of a photovoltaic (PV) module is by reducing the operating temperature of its surface. This can be achieved by cooling the module and reducing the heat stored inside the PV cells during operation. In this paper, long-term performance modeling of a proposed solar-water pumping system is carried out. The system, which is used for irrigation purposes, consists of a PV module cooled by water, a submersible water pump, and a water storage tank. Cooling of the PV panel is achieved by introducing water trickling configuration on the upper surface of the panel. An experimental rig is developed to investigate and evaluate PV module performance with the proposed cooling technique. The experimental results indicated that due to the heat loss by convection between water and the PV panel’s upper surface, an increase of about 15% in system output is achieved at peak radiation conditions. Long-term performance of the system is estimated by integrating test results in a commercial transient simulation package using site radiation and ambient temperature data. The simulation results of the system’s annual performance indicated that an increase of 5% in delivered energy from the PV module can be achieved during dry and warm seasons.

INTRODUCTION Different techniques have been used to improve the performance of photovoltaic (PV) modules and reduce the initial cost of the PV-driven systems. Some of these techniques are based on increasing the incoming radiation on the PV cells surface to reduce the PV panel area, which can be achieved by using solar concentrators, lenses, and/or using solar tracking. Using these techniques may reduce the PV system cost by 19% (for tracking technique) and 48% (for concentrating technique) [1]. The combined efficiency of a concentrating PV/thermal parabolic trough collector was studied and found almost equal to 58% [2]. However, the problem associated with these techniques is the increase in PV cell temperature above the operating limit and reduction in cell efficiency and probably cell damage in case of overheating. Therefore, PV cell requires an efficient cooling process, especially during hot weather. Cooling of PV panels is considered the less expensive technique that is used to improve PV panel performance. Most of the Address correspondence to Dr. Masud Behnia, Dean of Graduate Studies, The University of Sydney, NSW 2006, Australia. E-mail: [email protected]

research carried out on PV cooling is concentrated on building applications such as PV fa¸cade and PV thermal hybrid systems [3–11]. Such an application of PV in buildings can be used for the production of electricity and space heating. The dual system requires the optimization of air flow rate in order to achieve significant PV cell cooling. It was found that PV modules with air rear ducting can reduce operating temperature by 25◦ C [3]. The forced-air cooling technique with optimum flow rate for a PV array was found economical only for large-scale PV systems [4], and a temperature controller is required to adjust air flow rate. The width of air duct (air gap) behind the inclined PV modules has a significant effect on PV cell temperature. It was found that the greater the gap behind the modules, the greater the cooling due to natural convection [12]. Experimental results of commercial PV modules used in a hybrid thermal system of a building showed that PV cooling can increase the electrical efficiency and the total efficiency of the system [7]. The system performance can be improved by installing glass cover with air gap to a PV- building fa¸cade to allow for cooling by air flow on the upper surface of the PV module. Experimental results of a building fa¸cade with integrated PV module showed that when air flows over the two PV surfaces, the cell cooling improves and combined thermal-electric efficiency reaches

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70% [9]. Another type of PV fa¸cade cooling is by using water pipe network at the back of PV modules. This technique serves for heating up water, lowers the operating cell temperature by 20 K, and increases power output by 9% [5]. PV fa¸cade cooling techniques are built as curtain fa¸cades in front of thermally insulated buildings, with air ducts or a water tube network in between. This type of cooling increases the cost of the PV fa¸cade system significantly due to the additional cost of water tube network. Another appropriate technique for PV cooling is by using water flow over the upper surface of the PV cells. This technique was used in a PV fa¸cade, and a surplus of 10.3% in electric power is achieved [13]. DC motor water pump and small diameter nozzles are used to perform water layer along PV module upper surface. There are three main advantages of applying this technique: there is a drop in cell temperature, an increase in incident radiation due to radiation refraction by water, and continued surface cleaning by water flow. However, the drawback of the system is the power required by the pump to circulate cooling water. Thermal analysis and energy balance of PV cells is modeled based on climate variables such as cell temperature, ambient temperature, and solar radiation [14]. The model is found to be accurate to within 6 K of measured temperature values. PV-powered water pumping systems has been widely used in remote areas because its operating cost competes with conventional energy supply systems. Different models have been developed to optimize the size and cost of these systems, such as the photovoltaic project model given by Clean Energy Decision Support Centre [15]. The main idea of this paper is to apply PV water cooling in an irrigation system for urban and remote sites where water pumping from wells is required. Cooling is necessary in PV applications for two main reasons: increase the lifetime of the PV cells and reduce PV module area by increasing output power of the module. The developed cooling technique comprises water flowing on the upper surface of the PV module. Bypass water from the irrigation water pump flows under gravity on the PV module upper surface and is collected at the PV module lower edge by a conduit connected to an irrigation stream. The advantage of this cooling technique is the elimination of the circulating pump required for the cooling process and an increase in incident radiation due to refraction in water layer.

SPECIFICATION AND SETTING OF THE EXPERIMENTAL RIG A PV water pumping rig was constructed to perform outdoor tests under different climatic conditions. The main components of this system are as follows (see Figure 1): 1. Multicrystal photovoltaic module of 60 W maximum power. The module is connected to a variable resistance to find I-V characteristic curve of the module. heat transfer engineering

Figure 1 PV water cooling test rig.

2. Sun saver sensor, which regulates passing current and voltage from PV to the battery and load. 3. Submersible water-pump of 12 volts D.C. motor, flow capacity around 5 l/min and pumping head of 50 m. The pump is inserted into a water tank that represents an irrigation well. 4. 12-volt battery to run the water pump during indoor cooling system setting and water flow rate measurements. 5. Cooling water trickling configuration, which consists of a water trickling tube (D = 2.5 cm, L = 65 cm) fixed on the upper edge of the PV module, water conduit at the lower edge of the module (L = 65 cm), and a bypass to deliver cooling water from the submersible pump. The trickling tube has 32 holes of 5 mm diameter distributed evenly. 6. Pyranometer, which is mounted on the PV platform to measure the input solar radiation. 7. PV cell surface temperature measuring configuration. 8. The whole system is assembled on a mounted platform to study system performance under different radiation conditions.

OUTDOOR TEST PROCEDURE The rig was tested under desert climatic conditions at latitude (32◦ ), HU University, Jordan. To verify the manufacturer PV module electrical characteristics, two types of I-V curve test were performed: normal operation test and with water cooling test. In order to maintain steady-state conditions in each test, the following procedures were considered: 1. PV module horizontal direction and inclination are changed manually during the test to maintain steady incident radiation on the PV module surface. 2. Operating cell temperature is measured at constant radiation. 3. Cooling water flow rate is maintained constant (4 l/min). 4. Temperature of water in the tank was close to ambient; however, in an actual irrigation system, well water temperature vol. 30 no. 6 2009

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Figure 2 The effect of water cooling on voltage-power characteristic curve of the PV module. Radiation on PV module surface is equal 1000 W/m2 .

Figure 3 Maximum power of the PV module during surface cooling, and at different radiation levels. Surface temperature during cooling is 40◦ C.

(in summer) is always lower than ambient temperature [16]. 5. Cell temperature is measured before and after cooling at constant radiation. 6. A data logger (DL2e) was used to record radiation and temperature measurements over a period of one minute.

standard operating temperature (45◦ C) caused a drop of 5% in output power. When introducing water cooling technique (using water from the storage tank), a surplus in power of about 15% is achieved. If underground water is used directly in cooling, it will maintain PV module operating at almost constant temperature around the year. This is because underground water temperature does not experience a significant variation around the year [16]. Figure 4 shows that using an underground water temperature of around 25◦ C may cause an 8% surplus in PV module output power. Another advantage of using water for cooling the upper surface of the PV module is the increase in surface input radiation due to the refraction in water layer. This effect was reported by instantaneous readings of the PV module output power during the wet and dry surface of similar temperatures. First, the PV module power is measured during water flow; water flow is then stopped, and instantaneous power measurement is logged in. The

Typical summer days were selected for the test to achieve maximum PV cell operating temperature. To plot the I–V characteristics curve of the PV module, the current and voltage are measured for a range of resistances (0–2.07 k) and different cell temperatures, ambient temperatures, and solar radiation.

TEST RESULTS ANALYSIS Tests of PV module I-V characteristic curve were conducted with and without water cooling. The trends of PV module characteristic results of the developed system were found to be similar to those in the literature [17, 18]. Output power of the developed module was measured at different ambient temperatures and solar radiation. A significant difference in the power output (area under the curve) between the two module temperature tests were reported and shown in Figure 2. It is clear that heat loss by convection due to water flow above the module upper surface caused the significant decrease (about 26◦ C) in cell operating temperature at radiation level 1000 W/m2 . The module has been tested for different radiation levels during surface cooling to verify the linearity between maximum power and radiation level. As shown in Figure 3, maximum power voltage is constant for the different radiation level, unlike the maximum power current, which is directly proportional to radiation. The gain in output power of the PV module achieved by the different sources of cooling water is shown in Figure 4. The PV module surface temperature in a typical summer day was found around 58◦ C. The increase in cell temperature above the heat transfer engineering

Figure 4 Effect of using different sources of cooling water on the PV module output power. Radiation on surface is 1000 W/m2 .

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Figure 5 Effect of beam refraction in cooling water layer on PV module output power. Wet and dry surface temperatures are 33◦ C.

effect of using the developed cooling technique on PV module output power for a range of radiation, and module surface temperature equal to 33◦ C is shown in Figure 5. The increase in output power is due to two factors: the decrease in radiation incident angle (θ) due to refraction in water layer, and cooling by natural convection. It is clear from Figure 5 that radiation incident angle on dry surface is higher than with water drops (θ1 > θ2 ), which causes an increase in input radiation during PV module cooling. For a radiation level between 400–1000 W/m2 , PV module output increases in the range 4–10%. It is clear from Figure 5 that PV module output power is increased by using the developed cooling technique due to beam refraction. It was reported that there are no significant salt deposits on the PV module surface due to its inclination and effect of gravity, which allow continuous water flow and deposit removal. Bigger particle deposits are avoided by the water filter that comes with pump fitting. The major error in test results was found to be in the temperature measurement of the PV’s upper surface because thermocouples were fixed to the surface by glue, and there exists a possibility of imperfect contact with the surface. This error was found to be in the range of 0.1–0.3◦ C for measured temperatures (25–60◦ C). The effect of this uncertainty on the maximum PV power in Figure 3 was found to be 0.15–0.35%.

LONG-TERM OPERATION MODEL OF THE PV-WATER PUMPING SYSTEM The water pumping system of the proposed cooling technique is shown in Figure 6. Two configurations of PV module surface cooling are shown, using water directly from a well and using water from a storage tank. In both configurations, cooling water is collected at the lower part of the PV module and recombined with the main irrigation channel. Long-term performance of this system was estimated using the commercial transient simulaheat transfer engineering

Figure 6 Proposed PV- powered water pumping system configuration.

tion package TRNSYS Version 16 (TRaNsient SYstems Simulation, http://sel.me.wisc.edu/trnsys, accessed December 1, 2007). Hourly solar radiation data for different locations in Australia were used. Maximum power of the PV module was estimated during the simulation from the experimental relations between the PV module characteristics (Voc , Isc , Tm ) during the cooling process. Maximum power of a PV module operating at conditions other than standard test condition (given by manufacturer) is estimated by   Pmax Pmax = (1) Isc × Voc st Isc × Voc i where index i stands for the operating condition other than standard. Linear regression to the test data of PV module temperature (Tm ) versus open circuit voltage (Voc ) is found by V oc = 22.384 − 0.0627Tm

(2)

where Tm is in ◦ C. Also, the linearity between solar radiation (G) and short circuit current (Isc ) data is represented by Isc = 0.0967 + 0.0032G

(3)

where G is in W/m2 . Change of module temperature with ambient temperature can be expressed by the following linearity [19]:   Tm − Ta Tm = Ta + ×G (4) G s tan dard The value of the bracket depends on the type of PV module. In this study, this value was found equal to 0.022◦ Cm2 /W from tests at normal operating condition. However when the PV module water cooling technique was applied, the value of the bracket was found equal to 0.006◦ C m2 /W. The system performance model was applied to different sites in Australia (Sydney −33.8◦ , Perth −32◦ , and Darwin −12◦ ) vol. 30 no. 6 2009

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Maximum delivered energy (Wh)

Maximum delivered energy (kWh)

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Figure 8 Maximum delivered energy for different PV cooling arrangements, for a typical summer day in Darwin.

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(b) Perth 14 13 Darwin Maximum delivered energy (kWh)

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module cooling process is improved. It is clearly shown in Figure 7 that PV surface cooling is more effective all year round in warm sites, such as Darwin and the site at latitude 32◦ (desert area), than moderate weather sites shown in Figure 7a,b. The effect of PV cooling is steadier in Darwin than other sites such as Sydney and Perth (Figure 7a,b) because radiation and ambient temperature variation in Darwin is lower. Table 1 shows the yearly increase in PV module maximum delivered energy in (kWh) when using underground water for cooling and without cooling. At latitude (−12◦ ), a yearly increase in delivered energy of the 60 W module used in the test is equal to 6 kWh. The economic feasibility of the developed system is found by evaluating the cost of added cooling configuration, which was found equal to 1.7% of the initial cost of the water pumping system (by considering the extra length in pipes and fittings). The time for money payback of the added configuration to a larger scale system used for actual irrigation work (20 m3 /day) is found equal to 2.5 years by using Retscreen software [15]. Maximum energy delivered hourly by the different PV module arrangements is shown in Figure 8 for typical summer days in Darwin. It is clear that PV module cooling with underground water is more efficient during the middle of the day than cooling with storage tank water because the later temperature during this period is higher.

Cooling with storage tank water 5 Without cooling 4 1

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( c) Darwin, and site at latitude 32° Figure 7 PV module output at different sites and for different cooling arrangements.

to find monthly average output of the PV module of the water pumping system, as shown in Figure 7a–c. Maximum performance is achieved when cooling with underground water is adopted. This is because underground water temperature during warm seasons is lower than ambient temperature; therefore, PV heat transfer engineering

DISCUSSION AND CONCLUSIONS The surface cooling technique of the PV module was developed in this study to improve the performance of a PVpowered water pumping system. An arrangement of pipe fittings was used to allow water flow under gravity on the PV module upper surface. Tests under different weather conditions were conducted. The results showed an increase of system output in the range of 4–10% when the developed cooling technique was adopted. Part of this increase (50%) is due to cooling by direct contact between water and PV module surface; the other part is vol. 30 no. 6 2009

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Table 1 Monthly energy delivered by the 60W PV module with underwater cooling and without cooling at different latitudes Latitude = −33.8◦ Pmax (kWh)

Month 1 2 3 4 5 6 7 8 9 10 11 12 Total

Latitude = −31 .98◦ Pmax (kWh)

• • • •

Without Cooling

With Cooling

Without Cooling

With Cooling

Without Cooling

With Cooling

10.3 8.6 7.5 5.6 5.1 4.5 5.0 6.5 7.9 10.1 10.7 10.5 92.3

10.0 8.4 7.4 5.5 5.1 4.6 5.1 6.5 7.8 9.9 10.4 10.1 90.7

13.0 10.4 10.3 7.0 6.0 5.1 5.3 7.0 8.4 10.5 12.1 13.4 108.5

12.4 9.9 9.9 6.9 6.0 5.1 5.3 7.0 8.4 10.3 11.7 12.8 105.6

8.8 8.2 9.1 9.4 9.4 9.0 9.6 10.5 10.7 11.0 10.2 9.3 115.2

8.4 7.8 8.6 8.9 8.9 8.7 9.2 9.9 10.2 10.3 9.6 8.8 109.3

7.0 7.8 8.2 9.8 10.5 10.4 10.9 11.1 10.7 9.4 7.7 7.1 110.5

Increasing cooling efficiency due to the direct contact between water and PV module surface. Increasing incident solar radiation on PV module due to solar beam refraction in water layer. Maintaining the PV module upper surface free of dust due to continuous water flow. Elimination of the circulating pump required for the cooling process due flow under gravity. A cooling technique that is simple and can be added to any standard module without a significant increase in cost.

P T V

Without Cooling 7.1 7.7 8.1 9.6 10.1 9.9 10.3 10.5 10.2 9.1 7.6 7.1 107.3

PV module output power, W temperature, ◦ C PV module voltage, V

Greek Symbol solar radiation incident angle, degree

θ

Subscripts a m max oc sc st

ambient module PV module maximum power open circuit short circuit standard test condition

REFERENCES

ACKNOWLEDGMENT The authors would like to thank the Department of Mechanical Engineering and the engineer M. A. Hayeh at Hashemite University-Jordan for facilitating the assembly and the tests of the developed system.

NOMENCLATURE D G I L

Latitude = 32e Pmax (kWh)

With Cooling

due to refraction of the solar beam in water layer and the increase in incident radiation. Long-term performance analysis at different sites showed that a steady annual increase in PV module output was achieved at warm weather sites. At moderate weather sites in Australia, a significant increase in PV module output is reported only during a specific period between October and March. In conclusion, the major advantages of this technique are: •

Latitude = −12.25◦ Pmax (kWh)

diameter, cm solar radiation, W/m2 PV module current, Am length, cm heat transfer engineering

[1] Bione, J., Vilela, O. C., and Fraidenraich, N., Comparison of the Performance of PV Water Pumping Systems Driven by Fixed, Tracking and V Trough Generators, Solar Energy, vol. 76, pp. 703–711, 2004. [2] Coventry, J., Performance of a Concentrating Photovoltaic/Thermal Solar Collector, Solar Energy, vol. 78, pp. 211–222, 2005. [3] Brinkworth, J., Cross, M., Marshall, H., and Hongxing, Y., Thermal Regulation of Photovoltaic Cladding, Solar Energy, vol. 61, pp. 169–178, 1997. [4] Sweelem, E. A., Fahmy, F. H., Aziz, M. A., Zacharias, P., and Mahmoudi, A., Increased Efficiency in the Conversion of Solar Energy to Electric Power, Energy Sources, vol. 21, pp. 367–378, 1999.

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S. ODEH AND M. BEHNIA [5] Krauter, S., Ara´ujo, R. G., Schroe, S., Hanitsch, R., Salhi, M. J., Triebel, C., and Lemoine, R., Combined Photovoltaic and Solar Thermal Systems for Fa¸cade Integration and Building Insulation, Solar Energy, vol. 67, pp. 239–248, 1999. [6] Brinkworth, B. J., Marshall, R. H., and Ibrahim, Z., A Validated Model of Naturally Ventilated PV Cladding, Solar Energy, vol. 69, pp. 67–81, 2000. [7] Tripanagnostopoulos, Y., Nousia, T., Souliotis, M., and Yianoulis, P., Hybrid Photovoltaic—Thermal Solar Systems, Solar Energy, vol. 72, pp. 217–234, 2002. [8] Cucumo, M., De Rosa, A., Ferraro, V., Kaliakatsos, D., and Marinelli, V., Theoretical-Experimental Analysis of an AirCooled Thermophotovoltaic Collector, EuroSun 2004, vol. 1, pp. 199–206, Freiburg, Germany, 2004. [9] Athienitis, A., Tzempelikos, A., and Poissant, Y., Investigation of the Performance of a Double Skin Fa¸cade with Integrated Photovoltaic Panel, EuroSun 2004, vol. 2, pp. 529–536, Freiburg, Germany, 2004. [10] Pellegrino, M., Flaminio, G., Bolognesi, S., and Privato, C., What Has Been Wrong with the PV/T Technology, EuroSun 2004, vol. 1, pp. 551–557, Freiburg, Germany, 2004. [11] Tripanagnostopoulos, Y., Souliotis, M., Battisti, R., and Corrado, A., Application Aspects of Hybrid PVT/Air Solar Systems, EuroSun 2004, vol. 1, pp.744–753, Freiburg, Germany, 2004. [12] Martin, S., Seitz, C., and Saman, W., Techniques for Reducing the Operating, Temperature of Solar Cell Modules. International Solar Energy Society Conference, paper P43, Goteborg, Sweden, 2003. [13] Krauter, S., Increased Electrical Yield via Water Flow over the Front of Photovoltaic Panels, Solar Energy Materials & Solar Cells, vol. 82, pp. 131–137, 2004. [14] Jones, D., and Underwood, C., A Thermal Model for Photovoltaic Systems, Solar Energy, vol. 70, pp. 349–359, 2001. [15] Photovoltaic Project Model, Minister of Natural Resources, Ottawa, Canada, Available at: http://www.retscreen.net, 2005.

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[16] Burkhard, S., Shallow Geothermal Energy, Geo-Heat Center Bulletin, Oregon Institute of Technology, Klamath Falls, Ore., USA, vol. 22, pp. 19–25, 2001. [17] Kyocera Solar Electric Modules, Available at: http://www. kyocerasolar.com/solar/modules.html, accessed August 2007. [18] Meneses, D., Horley, P., Gonzalez, J, Vorobiev, Y., and Gorley, P., Photovoltaic Solar Cells Performance at Elevated Temperatures, Solar Energy, vol. 78, pp. 243–250, 2005. [19] Markvart, T., Solar Electricity, 2nd ed., Wiley, New York, pp. 81– 136, 2003.

Saad Odeh is an associate professor of mechanical engineering and research scholar at the University of Sydney, Australia. He received his Ph.D. in mechanical engineering from the University of New South Wales. His main research area is solar energy and sustainable energy systems. He has published more than 40 papers in international journals and conferences. He is a member of Association Building Sustainability Assessors, Sydney.

Masud Behnia obtained his BS, MS, and Ph.D. from Purdue University. Prior to starting his own academic career, he had an extensive period in the power industry. He has worked in experimental and numerical fluid mechanics and heat transfer for more than 25 years. Results of his research have been widely published in journals and conferences, and his career total publications include more than 300 refereed papers. He is currently the dean of Graduate Studies and professor at the University of Sydney, Australia.

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