Investigation into the Switching Mechanism of Photovoltaic-Thermal Power Heat Pipe/Heat Pump Composite Cycle System in Cogeneration Mode

Abstract

To refine thermodynamic cycle of existing  photovoltaic-thermal (PVT) heat pump systems, the 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. The 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. The research results demonstrate that using exergy efficiency as switching criterion yields optimal performance under sunny conditions, with the respective values for hot water production, power generation, exergy efficiency, and 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, exergy efficiency, and 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, exergy efficiency, and COP are 4.0 tons, 7.1 kWh, 14.5%, and 4.0, respectively.

Keywords: Switching mechanism, Photovoltaic-thermal, Power heat pipe, Heat pump, Composite cycle