Abstract
Abstract: The power generation efficiency of photovoltaic (PV) cells can be greatly reduced with the increase of working temperature. The PV-thermoelectric (PV-TE) technology can convert the excess heat in the PV power generation into the electric energy through the TE cells. As such, the temperature of PV cells can be reduced to realize the cascade utilization of solar energy. However, the temperature of PV-TE system can fluctuate with the change of irradiance, leading to a great influence on the solar energy utilization rate of the whole system. In this study, the phase change material (PCM) was introduced to design a PV-PCM-TE hybrid device system. The latent heat property of PCMs was utilized to improve the stability of the system under variable irradiance. The device test was carried out to evaluate the performance of PV-PCM-TE system in the Northeast Agricultural University, Xiangfang District, Harbin, China. The test date was set from October 1, 2020 to September 30, 2021, while the test time was 8 hours from 8:00 to 16:00 on the test day. The experimental results indicated that the maximum temperature of PV cells was 326.5 K during the period from 11:00 to 14:00, which was basically maintained at the phase change temperature of PCMs. Furthermore, there was only a few cooling effects on the performance of the PV cells, indicating the excellent latent heat characteristics of PCMs. Specifically, the maximum temperature difference of TE battery was 13.4 K during water cooling. By contrast, the maximum temperature difference of TE battery was 3.6 K during natural air cooling. The water cooling was effectively reduced the temperature of the cold end of the TE battery, in order to improve the temperature difference and power of the TE battery. When the direct contact, thermal conductive silicone grease 1 (2.0 W/(m·K)), and thermal conductive silicone grease 2 (5.15 W/(m·K)) were selected between the contact surfaces of the device, the maximum temperature difference of TE cells were 5.4, 11.2, and 13.4K, respectively, the average efficiencies of PV cells were 18.10%, 18.3%, and 18.38%, respectively, and the average total efficiencies of the system were 19.6%, 19.95%, and 20.18%, respectively. The best system performance was achieved in the thermal conductive silicone grease 2 with the higher thermal conductivity. Compared with the PV-TE system, the electrical efficiency and power of PV-PCM-TE system increased by 1.05% and 16.21%, respectively, during the instantaneous test period. Therefore, the optimal cooling mode was the water cooling with the cooling water flow rate of 6L/min during the whole year. The maximum performance indexes of PV-PCM-TE system was observed in summer, particularly with the strongest irradiance. The maximum electrical, thermal, and exergy efficiencies were 22.28%, 32.55%, and 27.32%, respectively. The PV-TE hybrid power generation device combined with PCMs can be expected for the excellent adaptability in the most areas and climatic conditions. At the same time, the output considerable heat and electric energy were obtained to supply the power for the environmental monitoring and lighting system of modern greenhouse, particularly for the part of the heat energy for the plant growth.
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