Experimental assessment of PV module cooling strategies A. Ozemoya, A.J Swart and HCvZ Pienaar Department of Electronic Engineering Vaal University of Technology, Private Bag X021, Vanderbijlpark, 1900 Corresponding author email:
[email protected] Abstract – The main limiting factors to the extensive use of Photovoltaic (PV) modules include the high initial investment cost and the relatively low conversion efficiency. The issue of increasing the PV efficiency has been of great interest since the 1950’s, both from a research and economic point of view. Temperature, however, exerts considerable influence on PV modules, with cell efficiencies decreasing as the cell’s operating temperature increases. Higher surface temperatures mean lower output voltages and subsequent lower output power. This paper focuses on cooling techniques for controlling a PV module's surface temperature and the effect of different cooling techniques on the output voltage of the PV module and subsequently on the output power. Two cooling systems were investigated; a water cooling system and a forced air cooling system. A comparison was made between three PV modules, with water cooling, forced air cooling and without cooling. The results show a direct correlation between temperature rise and voltage decrease. It further reveals that water cooling is more effective than air cooling, with a water cooling system producing 4% more than a system with no cooling. Keywords - Photovoltaic module, temperature degradation, tilt angle, ambient temperature, PV cooling. I. INTRODUCTION Solar power production using PV modules has increased and is currently one of the fastest growing energy technologies worldwide, leading to speculation that it will be the main source of electrical power in future [1]. Solar energy has been considered a promising solution to the global energy and environmental challenges facing mankind, including global warming [2]. Renewable energy sources contributed only about 13.1% of total primary energy supply in 2009; the share of solar PV was only 0.04% and is estimated to reach a maximum of 1% by 2030 [3]. Over the past five years, solar PV has averaged an annual growth rate of over 50%. Growth has generally been concentrated in a few countries, where solar PV currently generates only a low percentage of the total yearly electricity production [4]. This growth is driven not only by the progress in materials and processing technology, but also by market introduction programs in many countries around the world due to the increased volatility and mounting costs associated with fossil fuels [5]. However, owing to the recent economic slowdown and its associated developments, one might doubt the predicted magnitude or contribution of solar power to the global primary energy domain despite the proven potential of the sun in providing enough energy in one hour to meet the annual global consumption of energy. Achieving such ambitious targets demand reduction in the cost of solar
energy per kilowatt hour (kWh) when compared to those of conventional fossil fuels. The current cost of production of PV modules has prevented it from being widely used, especially in the public sector where home residents could benefit greatly from its use. This high cost can be attributed to the fact that presently PV modules are made of expensive semiconductor materials, which are most commonly crystalline silicon (c-Si) [5]. Moreover, low efficiencies of PV modules still exist. A typical PV module converts 6-20% of the incident solar radiation into electrical energy, depending upon the type of PV module and climatic conditions [6]. This means that a larger surface area PV cell is required to produce the same amount of electrical energy, which could otherwise be produced by a smaller and more efficient PV cell. Therefore, to address the high manufacturing and installation costs associated with PV modules, two different approaches have been adopted, namely: • •
Increase the efficiency of PV modules (PV efficiency). Use cheaper materials for the construction of PV modules.
The adverse effect of temperature increase on the performance of a PV module is a significant factor to take into consideration. A PV cell suffers from high temperatures reached under high irradiation conditions and can reach as high as 60-80°C [7]. An effective way of improving efficiency and reducing the rate of thermal degradation of a PV module is by reducing the operating temperature of its surface [8]. This can be achieved by cooling the module and reducing the heat stored inside the PV module during operation. Decreasing the temperature of PV module can boost the electrical efficiency [9]. The purpose of this paper is to optimize the available output power from a PV module by comparing the electrical performances of PV modules with and without cooling techniques. Literature relating to factors affecting PV module temperature is also discussed. The research methodology is then introduced and the practical set-up is explained. Initial results are presented in a number of graphs. Future work and conclusions are finally presented. II. FACTORS THAT CONTRIBUTE TO EFFICIENCY LOSSES IN PV MODULES
Factors that influence the operating condition of a PV cell are the total irradiance, the spectral distribution of the irradiance and the temperature [10]. Conditions in real life situations are not standard; instead, they vary strongly and influence the electrical performance of a PV cell, causing an efficiency loss with respect to the standard testing condition
(STC) nominal value [10]. This loss can be divided into different categories [11], namely: Angular distribution of light: Due to the movement of the sun and the diffuse components of the radiation, light does not fall perpendicular onto the PV module’s surface, as is the case when measurements are done and the nominal efficiency is determined.
crystalline PV module by SOLARWORLD with its peak efficiency of 13.12% under STC (25°C, 1000 W/m2). The system was installed on the roof of the S-Block at the Vaal University of Technology (VUT), with latitude 26°S and longitude 27°E . Figure 1 and 2 presents the block diagram of the practical set-up and experimental set-up inside the laboratory.
Spectral content of light: For the same power content, different spectra produce different cell currents according to the spectral response. The solar spectrum also varies with the sun’s position, weather, pollution, etc. and never exactly matches the AM1.5 standard. Irradiance level: For a constant cell temperature, the efficiency of the module decreases with diminished irradiance levels. This is primarily due to the logarithmic dependence of open-circuit voltage on photocurrent; at very low illumination the efficiency loss is faster and less predictable. Ambient temperature: The surface temperature of PV modules rises with longer exposure periods to sunlight and high ambient temperature. The elevated temperatures directly impact the PV modules efficiency. The ambient temperature changes because of the thermal insulation provided by the encapsulation. This is usually the most important performance loss. However, prediction of the module response under different conditions is required to correctly assess the yearly production of a PV system in the field [11]. Surface orientation of PV modules: The radiation falling on a tilted surface will be the sum of direct radiation, diffuse radiation and reflected radiation or albedo [12]. The sum of these three components is called global radiation. The correct installation of a collector can enhance its application advantage, as the amount of radiation flux incident upon the collector is mainly affected by the surface orientation i.e. azimuth and tilt angles of installation. In the northern hemisphere, the best azimuth is due south (facing equator), but the tilt angle varies with factors such as the geographic latitude, climate condition, utilization period of time, etc. [13]. Thermal effect: Thermal response of the PV module affects the electrical power output. The PV module receives the incident irradiation where a portion of it is converted to electrical energy in proportion to the module’s efficiency. The rest of the incident irradiation heats up the PV module and increases its operating temperature in relation to the PV material’s specific heat capacity [14]. PV module voltage is reduced when compared to the increase of current at higher operating temperatures, so the generated power is reduced. A portion of the absorbed heat is dissipated into the surroundings, occurring through conduction, convection and radiation [14]. III.
PROPOSED SYSTEM
The test was designed to investigate the electrical performance of a PV module with different cooling techniques. The practical set-up consists of a SW220 poly-
Figure 1: Block diagram of the practical set-up
Figure 2: Experimental set-up of the monitoring station The block diagram of the practical set-up comprises of a PV module connected to a data logging interface circuit (DLIC) via a circuit breaker. The data acquisition equipment consists of a DLIC and a PICOLOG 1216 data logger. The 12/24 V solar charger with maximum power point tracker (MPPT) was connected via several channels of the DLIC. A PICOLOG 1216 data logger was connected on each corresponding channel of the DLIC where it was used to record PV module output voltages and currents. A PICOLOG 1216 data logger was used as it has 16 analogue input channels that can accommodate all PV modules used in the set-up. The loads i.e., LED FLOOD LIGHT was coupled to the MPPT along with (RA12-100) lead acid discharge batteries (LADDB). Three identical PV systems are considered for this research. The first test was to investigate the factors that impact the PV module surface temperature as the transformation of solar energy into electrical energy depends on the operating temperature of the module [15]. Therefore, the surface temperature at different points was measured jointly with the air temperature. A pilot study was undertaken to investigate which tilt angles produce maximum PV module surface temperature and how it affects the output power. Results
show that a PV module’s surface temperature is highest at a tilt angle of 16° during the day and lowest at night time [15]. This tilt angle was therefore chosen in the main study with the intention of maximizing the effect of the cooling system. It further reveals that the cell temperature and back-surface module temperature can be distinctly different [15]. The back-surface temperature is a good approximation of the actual cell temperature, and studies have shown that cells typically run 3°C warmer than back-surface temperatures for glass–glass laminate construction [16]. The electrical performance and reliability of PV modules can be severely affected by elevating cell operating temperatures due to elevated ambient temperature.
The cooling circulation system consists of a brushless DC pump (12 VDC 10.5 W), a 12 V DC solenoid valve, sprayers, a garden hose and a 50 litre water tank. The pump is powered from a 12 V/ 1.4 A power supply. Its maximum head being 4 m with a maximum flow rate of 450 l/h. The water pump circulates the water through a spray system connected by a hose located between the aluminium frames that separate two PV modules (see Figure 3).
The study features three identical SW 220 poly-crystalline PV modules set to the same tilt angle of 16° with an orientation angle of 0° [15], connected to a data logging interface circuit (DLIC) via a circuit breaker (CB). The setup has been developed to study the effect of different cooling techniques on the output voltage of the PV module and subsequently on the output power. Table 1 indicates selected parameters for a PV module which was used in this research due to its lower cost and better performance in areas of direct solar radiation [17]. Table 1: Electrical characteristic of SOLAR WORLD SW220 poly-crystalline PV module Specification
Abbreviation
Value
Pmax
220 W
Open circuit voltage
Voc
36.6 V
Rated voltage
Vm
29.2 V
Short circuit current
Isc
8.08 A
Rated current
Im
7.54 A
NOCT
46°C
ŋ
13.12%
Maximum output
Nominal operating cell temperature Efficiency
A water cooling system is used with the first PV system, where water is sprayed onto the PV module’s front surface at specific time intervals. A forced air cooling system is used with the second PV system, where fans are used to blow air onto the back of the PV module. The third system has no cooling system employed as it serves as a reference to make a reasonable comparison to the other two cooling systems. The aim of using water or forced air is to keep the PV modules operating temperatures within limits so as to achieve higher cell efficiencies, since generation of heat within the PV cell accounts for the increase in cell temperature and the decrease in its conversion efficiency [9]. The surface temperature of the PV module, the output voltage and current are monitored using a data logger and all measurement is saved to an MS Excel file for further analysis to draw reasonable conclusions Water cooling setup Water is used as the cooling medium in the first PV system.
Figure 3: Water cooling system. (Outdoor setup 1 - PV module, 2 - pump, 3 - solenoid valve, 4 - sprayers, 5 - hose, 6 - water tank covered with cardboard box. The pump and solenoid valve are controlled by an electronic timer circuit giving a duty cycle cooling period of 10 seconds for every 5 minute intervals. Initial trials for spraying were 20 seconds every 10 minutes. However, this spray interval was not effectively keeping the temperature from decreasing, as the 20 second cooling effects drops off significantly after 5 minutes. Around 20 litres of water is used per day when the sprayers are activated for 10 seconds every 5 minutes. The electronic timer circuit runs from 09:00 am till 3:00 pm, this time period was considered because the PV module’s surface temperature climbs considerably as revealed in the pilot study [15]. To investigate the performance of the PV cell under different cooling interventions, the system was operated for 15 weeks from December 2013 to March 2014, as this is the hottest period of the year in Vanderbijlpark [18]. This design is employed to minimize the consumption of water which is crucial to the objective of the project. The water tank is enclosed by a cardboard box to avoid heating by solar irradiation. Having the precise pump selection is important in making this system viable. The right pump needs to be: • • •
Low powered; Sufficient flow rate (at the head pressure); and Reliable
Forced Air cooling setup The second cooling technique involves attaching eight DC brushless fans (SUNON 12 VDC 10 W, 120·120·38 mm) to the back of a PV module to improve the electrical performance of the module. Figure 4 shows the back of a PV module with the fans attached. The brushless fans are powered from a 12 VDC source and are connected in
parallel. The fans disperse the heat generated by the PV module to the surrounding areas. The experiments were conducted from 9:00 am until 3:00 pm. The fans are controlled by an electronic timer circuit giving a duty cycle cooling period of four minutes on and one minute off. Initial trial runs of air cooling set the interval to be 20 seconds for every 10 minutes. However, the results showed no significant temperature drop as compared to the PV module without cooling (reference system). Figure 4 shows the forced air cooling setup and the angle changing mechanism.
module surface than forced air. Table 2 shows the statistical data for a period of one full week, which indicates that there is a statistical significant relationship (p-value < 0.001) between the temperature rise and output voltage decrease (shown by the negative correlation value). Table 2: Average, mode, median, correlation and probability-value (p-value) statistics for temperature and voltage for different techniques Week 5 Data
December 29th - January 4th PV2 Water cooling
Figure 4: The back of a PV module with attached cooling fans IV. EXPERIMENTAL RESULTS The obtained results are presented to check the performance of the PV module under different cooling interventions. Figure 5 presents the relationship between the PV module surface temperature and its output voltage with the different cooling interventions applied. The difference between the PV module’s surface temperature and output voltage for a particular day is plotted. Average PV temperature vs voltage 70.00
Temperature
Voltage
Temp. (°C)
Vol. (V)
Temp. (°C)
Vol. (V)
Temp. (°C)
Vol. (V)
Average
47.57
32.91
63.38
32.43
63.91
31.73
Mode
46.13
33.08
63.32
32.55
61.00
31.90
Median
48.08
32.98
64.43
32.50
64.44
31.83
Correlation
-0.44
-0.40
-0.50
P-value
0.00
0.00
0.00
Result also shows that with forced air cooling, there is a substantial temperature rise along the PV module due to the low heat capacity of air. The thermal properties of air make it less effectual as a coolant medium than water [19]. More power fans will therefore be needed to achieve the same cooling performance. The water cooling system resulted in an average higher output voltage of 1.18 V and an average temperature reduction of 16.34°C (or 25.57%) when compared to a no cooling system. The water cooling system was able to maintain an average PV module surface temperature of below 50°C. Table 3 shows the results for the entire 15 week period, indicating that the water cooling system resulted in an average higher output voltage of 780 mV.
Data
15 weeks period
34.00 50.00
PV2
PV3
PV5
Water cooling
Forced Air cooling
No cooling
33.50 33.00
40.00
32.50 30.00
32.00 31.50
20.00
Voltage (V)
PV module temperature (°C)
PV5 No cooling
Table 3: Average voltage, correlation and p-value for 15weeks
35.00 34.50
60.00
PV3 Forced Air cooling
Temp. (°C) Average
39.22
Vol. (V) 31.30
Temp. (°C) 48.39
Vol. (V) 31.16
Temp. (°C) 49.10
Vol. (V) 30.59
31.00 10.00
Correlation
0.11
0.09
0.07
P-value
0.00
0.00
0.00
30.50
0.00
30.00 PV2 (Water Cooling)
PV3 ( Forced air Cooling)
PV5 (No Cooling)
PV modules cooling methods
Figure 5: Average PV temperature vs voltage for 31st Dec 2013. It shows that the output voltage of a PV module with cooling is slightly higher than the PV module without cooling. The results shows a direct correlation is observed between temperatures rise and voltage decrease. It also shows the advantage of using the water as a coolant since it can sustain a lower PV module surface temperature, providing better temperature uniformity along the PV
The effect of ambient temperature on the output power of a PV module can be clearly discerned in Figure 6. It shows the different combinations of temperature and power that can be produced by a given PV module under different cooling conditions. The operating temperature plays a central role in the PV conversion process. Both the electrical efficiency and hence, the power output of a PV module depend linearly on the operating temperature. The results indicate a higher current value of 65.30% (water cooling system) when compared to the reference system current (no
cooling system) which results in a 49.6% power difference between these two systems. Average temperature vs Average Power Temperature
Power
35.00 30.00
50.00
25.00 40.00 20.00 30.00 15.00 20.00 10.00 10.00
Average Power (W)
Average Temperature (ºC)
60.00
5.00
0.00
0.00 PV2 (Water Cooling)
PV3 (Air Cooling)
PV5 (No Cooling)
PV module cooling startegies
Figure 6: Average PV temperature vs power for a period of 15 weeks V. CONCLUSION The purpose of this paper was to optimize the available output power from a PV module by comparing the electrical performances of PV modules with and without cooling techniques. The variation of temperature between these cooling and no cooling techniques can be as high as 16ºC. Results indicate a direct correlation between PV surface temperature rise and voltage decrease. It further reveals that water cooling is more effect than air cooling, with a water cooling system producing 1.18 V more than a system with no cooling in the last week of December 2013. Weekly period percentages indicate a higher voltage value of 3.58% and 2.16% (water and forced air cooling respectively) when compared to a no cooling technique. However, water cooling of a PV module did reveal significant decolouration in the protective tempered glass layer. Future work exists to investigate ionized or deionized (distilled) water as a better option. V. REFERENCES [1] D. Redfield, "Solar energy: Its status and prospects," CSIT Newsletter, IEEE, vol. 4, pp. 15-19, 1976. [2] Y. Dajiang and Y. Huiming, "Energy Conversion Efficiency of a Novel Hybrid Solar System for Photovoltaic, Thermoelectric, and Heat Utilization," Energy Conversion, IEEE Transactions on, vol. 26, pp. 662-670, 2011. [3] M. Hoffman, "PV solar electricity industry: market growth and perspective.," Solar Energy Mater. Solar Cells, vol. 90, pp. 3285-3311, 2006. [4] IEA. [Online]. Available: http://www.iea.org/aboutus/faqs/renewableenergy/ [5] A. Jäger-Waldau, "1.09 - Overview of the Global PV Industry," in Comprehensive Renewable Energy, S. Editor-in-Chief: Ali, Ed., ed Oxford: Elsevier, 2012, pp. 161-177. [6] S. Dubey, J. N. Sarvaiya, and B. Seshadri, "Temperature Dependent Photovoltaic (PV) Efficiency and Its Effect on PV Production in the World – A Review," Energy Procedia, vol. 33, pp. 311-321, 2013.
[7] Z. Farhana, Y. M. Irwan, R. M. N. Azimmi, A. R. N. Razliana, and N. Gomesh, "Experimental investigation of photovoltaic modules cooling system," in Computers & Informatics (ISCI), 2012 IEEE Symposium on, 2012, pp. 165-169. [8] S. Odeh and M. Behnia, "Improving Photovoltaic Module Efficiency Using Water Cooling," Heat Transfer Engineering, vol. 30, pp. 499-505, 2009. [9] H. G. Teo, P. S. Lee, and M. N. A. Hawlader, "An active cooling system for photovoltaic modules," Applied Energy, vol. 90, pp. 309-315, 2012. [10] M. Chegaar and P. Mialhe, "Effect of atmospheric parameters on the silicon solar cells performance," Journal of Electron Devices, vol. 6, pp. 173-176, 2008. [11] A. Luque and S. Hededus, Handbook of Photovoltaic Science and Engineering. West Sussex: John Wiley & Sons Ltd, 2003. [12] C. S. Solanki, "Solar Photovoltaics: Fundamentals, technology and applications", 2nd ed. New Delhi: PHI learning private Ltd., 2011. [13] T. P. Chang, "Study on the Optimal Tilt Angle of Solar Collector According to Different Radiation Types," International Journal of Applied Science and Engineering, vol. 6, pp. 151-161, 2008. [14] S. Krauter and R. Hanitsch, "Actual optical and thermal performance of PV-modules," Solar Energy Materials and Solar Cells, vol. 41–42, pp. 557-574, 1996. [15] A. Ozemoya, A. J. Swart, C. Pienaar, and R. M. Schoeman, "Factors impacting on the surface temperature of a PV panel," presented at the Southern Africa Telecommunication Networks and Applications Conference (SATNAC) 2013 16th International Conference, Stellenbosch, 2013. [16] D. L. King, W. E. Boyson, and J. A. Kratochvil, "Photovoltaic array performance model," Sandia National Laboratories, Albuquerque, New Mexico2004. [17] O. Asowata, J. Swart, and C. Pienaar, "Optimum Tilt and Orientation Angles for Photovoltaic Panels in the Vaal Triangle," in Power and Energy Engineering Conference (APPEEC), 2012 Asia-Pacific, 2012, pp. 15. [18] A. J. Swart, R. M. Schoeman, and H. C. Pienaar, "Assessing the effect of variable atmospheric conditions on the performance of photovoltaic panels: A case study from the Vaal Triangle," in Energy Effciency Convention (SAEEC), 2011 Southern African, 2011, pp. 1-6. Ozemoya Augustine received his undergraduate degree in 2011 from Vaal of Technology University, South Africa. And he is presently studying towards his Master degree in electrical engineering at the Vaal University of technology. His research interests include how to improve the efficiency of photovoltaic modules by controlling the ambient temperature.
