- Open Access
Parametric modeling and simulation of photovoltaic panels with earth water heat exchanger cooling
© The Author(s) 2016
Received: 26 June 2016
Accepted: 8 September 2016
Published: 16 September 2016
Photovoltaic (PV) systems work efficiently up to a certain cell temperature and exhibit efficiency losses and long-term degradation if the cell temperature exceeds a certain limit. The rise in temperature can be controlled with the help of cooling techniques to maintain cell temperature within limit. In the current research, a novel cooling technique termed earth water heat exchanger (EWHE) is designed and simulated by varying its operating parameters, which includes mass flow rate, length, pipe materials, and diameter of buried pipe. Results showed that peak PV panel temperature goes up to 79.31 °C without any cooling and drops to 47.13 °C with the help of EWHE cooling for optimum flow rate of 0.018 kg/s. At this flow rate, with decrease in panel temperature, the PV power also increased by 23.16 W with EWHE cooling. The comparative study between three different EWHE pipe material shows that the performance of coupled (PV/T + EWHE) system hardly depends on the properties of these materials. It is also observed that there is an inverse correlation between the EWHE pipe length. The variation in pipe diameter shows that the PV temperature decreases with an increase in pipe diameter. The PV/T system along with EWHE may be used for the purpose of PV power plants cooling in the hot and semi-arid regions of western Gujarat and Rajasthan (India), where solar irradiation is ample and ambient temperatures are very high.
Solar energy is considered as one of the most promising renewable energy sources due to the fact that it is widely available all over the world and is being used to generate electricity (Ummadisingu and Soni 2011; Gakkhar et al. 2016). PV systems are commercially proven technology for electrical power generation from solar radiation. However, only 10–20 % of incident solar radiation is converted into electrical energy, while the remaining radiation is absorbed as heat (Ozgoren et al. 2013). The absorbed radiation which is converted into heat results in an increase in the PV cells operating temperature. The rise in cell temperature beyond certain limit adversely impacts the efficiency and the life span of the cell (Jakhar et al. 2016b, c; Royne et al. 2005; Cabo et al. 2016). In fact, the PV electrical efficiency is highly dependent on the cell-operating temperature, and decreases with increasing temperature. From the literature review, it is observed that above a certain limit, the efficiency decreases by 0.45 % per unit rise in cell-operating temperature (Du et al. 2013). Therefore, PV temperature control with the help of cooling is necessary for its better performance. The literature discusses the work carried out on different PV systems and cooling techniques. One of the cooling technique, where thermal collectors are attached on the back side of PV panels to produce both electrical energy and thermal energy, is called photovoltaic/thermal (PV/T) cooling system (Chow 2010; Hegazy 2000). An experimental study was performed on a PV/T solar air heater system for indoor conditions (Solanki et al. 2009). It was found that the thermal, electrical, and overall efficiency of the system were 42, 8.4, and 50 %, respectively. A comparative study discussed a PV/T system and compared it with a conventional solar water heater and found that the primary energy saving efficiency of the system was about 60 %, which is higher than the conventional solar water heater (Huang et al. 2001). A novel PV/T was designed and tested which produced both electricity and hot water (Dubey and Tiwari 2008). An integrated PV and thermal solar water/air-heating system for the conditions of New Delhi was tested (Tiwari and Sodha 2006). They found the thermal efficiency for winter and summer as 77 and 65 %, respectively. An experimental study had been carried out on thermosyphon-based PV/T system with and without glass cover (Chow et al. 2009). An experimental study on a sheet and tube-type PV/T system with brine solution as a coolant was also reported in the literature (Saitoh et al. 2003). An experimental study was conducted using water spray to cool both the sides of PV panel (Nizetic et al. 2016a). Their results showed that the PV panel temperature decreased from 54 to 24 °C and an effective increase in electrical efficiency was measured as 5.9 % with cooling. Two PV panels (poly-Si and mono-Si) were tested numerically and experimentally with the backside convection cooling arrangement for the Mediterranean climatic conditions (Nizetic et al. 2016b). They found out that due to flow separation, there is increase in average panel temperature by 5–9 °C which results in the degradation of panel electrical efficiency from 2.5 to 4.5 %. It is also observed that the efficiency may improve if flow separation is removed.
Other cooling approaches which are used to cool down the system other than PV are also discussed by various researchers. Geothermal cooling is used for air conditioning which is based on the principle that at a depth of about 3.5 m or more, the soil temperature remains fairly constant throughout the year and is approximately equal to the average annual ambient air temperature (ASHRAE 1985). The concepts of earth air tunnel heat exchanger (EATHE) and earth water heat exchanger (EWHE) for air conditioning using the air and water as a cooling medium have been discussed in the literature (Sodha et al. 1985; Chel et al. 2015; Jakhar et al. 2016a). A numerical model for EATHE was presented and experimentally validated (Bansal et al. 2010). They found that the performance of EATHE does not depend on buried pipe material. The performance of the EATHE was evaluated for winter heating with solar air-heating duct (Jakhar et al. 2015). A one-dimensional heat transfer model of EATHE was developed to calculate the undisturbed temperature of soil, convective heat transfer coefficient of air, diameter of pipe, and pressure drop (Bisoniya 2015). In another study, the applicability of EATHE system for Chandigarh (India) based upon extensive literature review considering the soil properties was reported (Sobti and Singh 2015).
EWHE systems consist of buried pipe at a certain depth in which hot water is sent, through which heat is dissipated from the hot water to the earth resulting in decrease in the water outlet temperature. Such cooling system using buried pipes can be used for PV-cooling application also. The cooling of PV system has been investigated by many authors using various techniques, such as sheet-in-tube, heat sink, water spray, and parallel channel. However, no one has tried EWHE for the cooling of PV panel. The main objective of the present research work is to numerically investigate the performance of unglazed PV/T coupled with EWHE. For the same, the modeling and simulation of such a coupled system have been done in the transient analysis tool [TRNSYS (v17.0)] for the condition of Pilani, Rajasthan. The performance of this coupled system depends on various parameters which include mass flow rate of the water, pipe material, diameter, and length of pipe and hence evaluated accordingly. This research presents the variation in these parameters to achieve the optimum cooling effect for the PV panels. The proposed system would provide a great opportunity to utilize the geothermal cooling technique for PV cooling in the semi-arid regions of northwestern India, where high ambient temperature during peak summer leaves very small scope for convective cooling with heat rejection to ambient and also minimum scope of utilization of thermal energy.
Description of system and modeling
Physical and thermal parameters used in the simulation
PV/T collector length
PV/T collector width
Thermal conductivity of absorber plate
385 W/m K
Copper tube diameter (OD)
PV efficiency at reference condition
PV panel reference temperature
HDPE pipe thermal conductivity
0.40 W/m K
Galvanized Iron (GI) pipe thermal conductivity
16 W/m K
Steel pipe thermal conductivity
54 W/m K
Fluid thermal conductivity
0.55 W/m K
Buried pipe depth
The methodology of system design and its parametric variation for the EWHE is discussed in this section. The simulation of PV/T coupled with EWHE system is carried out for 10 h of system operation which is average sunshine hours as a conservative estimate during peak summer period (June 21). To optimize the design parameters of such a coupled system, the parametric simulation was performed for different mass flow rates for a fixed diameter and length of the high-density polyethylene (HDPE) pipe. This analysis gives the optimum flow rate (0.018 kg/s) for a 30 m HDPE pipe length and diameter of 12 mm. For three different EWHE pipe materials, i.e., galvanized iron (GI), HDPE, and steel pipe, the simulation was carried out that it shows that the performance of the coupled system hardly depends on the buried pipe material. Thus, among all the pipe materials discussed here, HDPE pipe is considered for the performance analysis, as it is economical as compared to other two. With HDPE as pipe material, variation in pipe length is analysed for a particular diameter and flow rate. Furthermore, the variation in pipe diameter is carried out by keeping pipe length and mass flow rate constant.
Results and discussion
The present paper discusses the analysis of unglazed PV/T-coupled EWHE system by varying different parameters, such as buried pipe diameter, pipe material, pipe length, and flow rate. The system is designed and simulated using the TRNSYS (v17.0) software for the conditions of Pilani, Rajasthan during peak summer day (21 June). The performance of such a system for various pipe materials, i.e., GI, HDPE, and steel, is compared. From the analysis, it is observed that there is marginally variation in the performance of the system for different pipe materials. Therefore, it is concluded that among three materials which are considered for the analysis, HDPE pipe may be used for practical applications because of economical reason. From the simulation results of variation in pipe lengths, it is observed that with increase in pipe length, the PV temperature decreases and power output increases. Results showed that maximum drop in PV temperature has observed from 10 to 50 m length as 60–42.89 °C. However, for the length of 60 m, the PV temperature is 41.59 °C which is little higher as compared to 50 m pipe length. Similar trend has been observed for the PV power output. Hence, the pipe length of 50 m will be sufficient for the system. Further analysis shows that with increase in the pipe diameter, the outlet temperature decreases gradually over a period of time, but at the peak simulation hour, the PV temperature for all the pipe diameters exhibits similar temperature drop. Thus, smaller pipe diameter, i.e., 12 mm, may be used for the practical applications. Therefore, finally, it is concluded that this combined system may be a better solution for rejecting the excess heat of PV panels for the semi-arid northwestern regions of India which are blessed with high solar insolation throughout the year.
All authors contributed according to their respective areas of expertise and experience for the parametric study of the present research. SJ carried out the literature review of the relevant research papers, identified the key parameters to optimized, performed modeling and simulation, and drafted the manuscript. MSS suggested the paper alignment, content of the paper, and proofread the draft. NG suggested the graphical representation and proofread the draft. All authors read and approved the final manuscript.
Authors gratefully acknowledge the support from the Center for Renewable Energy and Environment Development, Birla Institute of Technology and Science, Pilani, Rajasthan, for this research.
The authors declare that they have no competing interests.
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