Investigation into the switching mechanism of photovoltaic-thermal power heat pipe/heat pump composite cycle system in cogeneration mode

To refine the thermodynamic cycle of the existing vapor-compression photovoltaic-thermal (PVT) heat pump systems, a novel PVT power heat pipe/heat pump composite cycle system was proposed. The switching mechanism between the PVT power heat pipe cycle and PVT heat pump cycle was investigated to achieve optimal performance under complex and variable outdoor conditions. A simulation platform for the proposed system was established, and operating control strategies were proposed, employing system exergy efficiency and supply-return water temperature differentials (0.5, 1.0, and 1.5 °C) as the switching criteria between the PVT-PHP and PVT-HP cogeneration modes. Additionally, a comparative study was performed to evaluate the system's performance under various weather conditions and different operating control strategies. The research results demonstrate that using exergy efficiency as the switching criterion yields optimal performance under sunny conditions, with the respective values for hot water production, power generation, average exergy efficiency, and average COP reaching 5.0 tons, 42.4 kWh, 13.5 %, and 8.2; whereas a supply-return water temperature differential of 0.5 °C as the switching criterion is more effective under non-sunny conditions. Under cloudy conditions, the corresponding values for hot water production, power generation, average exergy efficiency, and average COP are 4.0 tons, 24.3 kWh, 13.2 %, and 5.3, respectively; under overcast conditions, the corresponding values for hot water production, power generation, average exergy efficiency, and average COP are 4.0 tons, 7.1 kWh, 14.9 %, and 3.9, respectively.