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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. [ABSTRACT FROM AUTHOR]

An effective supply mode, solar power is gradually gaining much attention for the environmental friendliness and convenience. Among them, a photovoltaic/thermal co-generation technology is usually utilized to improve the power generation efficiency of photovoltaic cells, as the operating temperature rises. As such, the redundant heat generated by the photovoltaic cells was reused as the heat source for the temperature difference of power generation system to realize secondary power generation. Moreover, the Maximum Power Point Tracking (MPPT) control is also required to achieve the optimal potential of co-generation system. In this study, a new MPPT control of photovoltaic/temperature difference was proposed further to combine the constant voltage and hyperbolic tangent type adaptive variable step size, in response to the oscillation and misjudgment caused by the fixed step size of traditional conductance increment. Two advantages were included here: First, the control was the fast tracking to the area near the nonlinear region of the maximum power point using 0.78 times of the system open circuit voltage, suitable for the great changing environmental conditions. Second, the step size was adjusted adaptively and quickly, according to the change of external environmental conditions, when the MPPT was tracking to the nonlinear region near the maximum power points. For instance, the light intensity was used to reduce the system oscillation, indicating the monotonic increase and fast variation in the hyperbolic tangent function. Furthermore, a simulation model was established to evaluate the performance of adaptive variable step conductance increment in the MPPT control of a combined photovoltaic/thermal power generation system using the Matlab/Simulink software. Specifically, Jinao JAMG-6-60-250/SI photovoltaic module was set as the photovoltaic cell model, and Xinghe F40550 was the thermoelectric chip model. Simulation results show that the step changes were consistent under the drastic variation in the light intensity, while the response speed was obviously improved with the rapid adjustment for tracking the maximum power point. At the same time, the step size was kept at 0, after the output power of system was stabilized. There were only small fluctuation and error in the steady-state output power, indicating that the MPPT control performed well. Correspondingly, a MPPT hardware experiment was conducted to further verify the feasibility at Northeast Agricultural University in Harbin in October 2020. Two periods A (8:00-9:00) and B (12:00-13:00) were selected, when the illumination and temperature were gradually enhanced to remain unchanged. The hardware experimental results show that the system was quickly tracked and stabilized at the maximum power point within 15ms, where the steady-state error was less than 0.3%, indicating more robust to external environmental disturbances and higher energy utilization. Consequently, an excellent balance was achieved in the system response speed and steady-state accuracy. The finding can provide a promising potential to the implementation on hardware, such as digital signal processors in practice. [ABSTRACT FROM AUTHOR]