The performance of EWHE and PV/T system is analysed by varying different parameters, which includes the type of pipe material, length, diameter of pipe, and mass flow rate of water. The effect of mass flow rate on the performance of PV/T along the EWHE pipe length of 30 m and diameter of 25 mm for the HDPE pipe is shown in Fig. 2. It reveals that the temperature of PV goes up to 79.31 °C without any cooling. In the case of EWHE cooling scenario, the PV temperature decreases significantly and it varies with different mass flow rates, i.e., 29.99–53.82 °C for 0.01 kg/s, 28.54–47.13 °C for 0.018 kg/s, and 28.33–46.29 °C for 0.026 kg/s. It is observed that with increase in mass flow rate, the PV temperature decreases and becomes almost the same for 0.018, 0.022, and 0.026 kg/s. For the practical applications, 0.018 kg/s flow rate could be used as with increase in mass flow rate, the pumping power required also increases. Figure 3 shows the PV temperature for three different EWHE pipe materials (i.e., GI, HDPE, and steel) for the diameter and length of 12 mm and 30 m, respectively, with the flow rate of 0.018 kg/s. It reveals that the temperature difference of PV for HDPE and GI is mere 1.04 °C, while in the steel and HDPE pipe is around 1.5 °C, for the same simulation conditions. Figure 4 shows the PV power output for three different pipe materials for the flow rate of 0.018 kg/s. It is observed from Fig. 4 that the difference in power output is just 0.97 W for HDPE and steel pipe, here in the analysis, power consumed by pump is not considered. This small variation is due to small coefficient of friction and lower thermal conductivity of HDPE pipes, while that of higher thermal conductivity and higher coefficient of friction for steel and GI. The higher heat transfer due to one better property is compensated by another poor property. Hence, there is marginal variation in the outlet temperature and power output for all the three materials. Thus, it can be concluded that the selection of pipe materials out of three materials has a small impact on the performance of the PV/T-coupled EWHE system. The same has also been discussed in the literature for EATHE (Bansal et al. 2010). This validates the selection of the HDPE pipe, as it is much cheaper than the other two.

Furthermore, the hourly variation of PV temperature is estimated for the different pipe lengths for HDPE pipe diameter (12 mm) and flow rate of 0.018 kg/s and has been represented in Fig. 5. From Fig. 5, it is observed that the PV temperature decreases with increase in pipe length from 10 to 60 m. It also observed that during peak sunshine hour, for the pipe length up to 50 m, the maximum PV temperature drops drastically, and with increase in pipe length beyond 50 m, this temperature drop is gradually. From Fig. 6, it is observed that for variation in pipe length from 10 to 50 m, the maximum PV power ranges from 151.64 to 167.36 W at peak sunshine hour. However, for the pipe length of 60 m, the maximum PV power is 168.60 W, which is just 1.24 W increase in power output for 10 m increase in pipe length as compared to 50 m. Thus, the pipe length of 50 m would be sufficient for such coupled systems.

The variation of PV temperature with respect to time for various diameters with the flow rate of 0.018 kg/s and pipe length of 50 mm is represented in Fig. 7. With increase in the pipe diameter, the PV temperature decreases gradually over a period of time. Initially, during the first hour of simulation, the PV temperature drop is less for smaller diameters, while it is more in the case of larger diameters. At the peak simulation hour, the PV temperature in all the pipe diameters exhibits almost similar temperature drop, with the variation of just 1.05 °C between 12 and 25 mm pipe diameters. Figure 8 shows the variation of PV power output with respect to time for various pipe diameters with the flow rate of 0.018 kg/s and the pipe length of 50 m. It is observed that at 1400 h, for 12 and 25 mm pipe diameter, PV power output is 167.36 and 168.30 W, respectively, which is a very small variation. From the analysis and the figures, it is observed that the variation in pipe diameters hardly affects the PV power output. Thus, it is concluded that 12 mm pipe may be considered for the practical applications because of economic reasons. The analysis of results of TRNSYS simulation of PV/T coupled with EWHE system as discussed above provides the optimum values of various parameters.

The variation of all the parameters combined together is represented in 3D surface plot using MATLAB and represented in Figs. 9 and 10. The variation in PV cell temperature with respect to variation in pipe diameter, pipe length, and mass flow rates is shown in Fig. 9. The comparative variation shows the direct correlation of each parameter with respect to PV cell temperature. Similar to this, another 3D surface plot has been mapped in Fig. 10 which depicts the variation in PV power output with respect to variation in pipe diameter, pipe length, and flow rates. It is observed that there is marginally temperature variation and power output for the flow rate of 0.022 kg/s, pipe diameter of 25 mm and pipe length of 60 m as compared to flow rate of 0.018 kg/s, pipe diameter of 12 mm, and pipe length of 50 m. Thus, the later case may be considered for practical application owing to economical reason.

From the literature review, it is observed that there is hardly any single study on PV/T coupled with EWHE using water as a heat transfer fluid. Studies on various PV/T systems and their applications vary and depend on the ambient condition, solar radiation, wind velocity, etc. Hence, it would be difficult to compare them on a common basis. Similarly, studies based on EWHE depend on soil properties and the location thus cannot be generalized. Moreover, a sufficient amount of research has been done on EATHE. Some researchers have done work using EATHE for air conditioning. One of the experimental studies was performed on the hybrid EATHE (Misra et al. 2013) for the conditions of Ajmer (India). In their experimental analysis, they used the thermal and physical parameters of various materials, such as soil density, soil thermal conductivity, soil specific heat, and pipe thermal conductivity as 2050 kg/m^{3}, 0.52 W/mK, 1.840 kJ/kg K, and 0.16 W/mK, respectively. Experimental study done by Misra et al. 2013 is simulated in TRNSYS on a model of EATHE to determine the precision of results for the same properties of materials. The variation of EATHE outlet air temperature with the length of EATHE pipe from both the simulation and experimental analyses is shown in Fig. 11. It is observed from Fig. 11 that the simulated results are within the close agreement with the experimental results with the variation of 0.8–7.93 %. This error may occur due to the improper insulation of pipes, variation in coefficient of friction of materials used in the simulation, and irregularities, such as fitting and joints in the experimental setup. Therefore, the comparison of the simulated model and experimental results in the literature with the air; as heat transfer fluid is showing small error for the same climatic conditions, it is expected that the study for the EWHE will also follow the same pattern with more precise results. Thus, the coupled system of EWHE and PV/T would provide better results in the present simulation.

The climatic condition for the simulation of the coupled system is taken as that of Pilani, Rajasthan which has semi-arid climate and has high ambient air temperature and high solar radiation during peak summer day (21 June, equinox day) and is shown in Fig. 12. It reveals that the ambient temperature and solar radiation range between 239–997.58 W/m^{2} and 33.40–39.58 °C, respectively, for the Pilani, Rajasthan. Figure 12 also shows the climatic data for the Jodhpur, Rajasthan and reveals that the solar radiation and ambient temperature vary between 261.70 and 987.90 W/m^{2}, and 33.50 and 43.40 °C, respectively. For an additional climatic information of western India, Fig. 13 shows the solar insolation and ambient temperature for the conditions of Bhuj and Ahmedabad (Gujarat). It is observed that maximum solar insolation and ambient temperature go up to 908.73 W/m^{2} and 40.89 °C, respectively. Since, the Rajasthan and Gujarat states have good solar insolation throughout the year, the technique of PV-cooling system with EWHE can prove to be suitable technique in specially arid region and when peak summer temperatures are about 45 °C. The weather conditions of Rajasthan and Gujarat are taken from the inbuilt meteornorm file in the TRNSYS.