Performance improvement of vapor-injection heat pump system by employing PVT collector/evaporator for residential heating in cold climate region

The adoption of vapor injection (VI) cycle could overcome the defects of the conventional one-stage air source heat pump (ASHP) system under extremely low ambient temperature conditions. Nevertheless, the ASHP system with VI cycle still could not operate efficiently due to the low evaporating temperature in the fin-tube evaporator. For such a situation, the PVT (Photovoltaic/Thermal) collector/evaporator could reach higher evaporating temperature attributed to its physical structure, thereby improving the system COP. This study proposes the detailed mathematical model of the vapor-injection heat pump system by incorporating PVT collector/evaporator and verifies the effectiveness of the proposed system. Parametric studies have been then conducted. The proposed system can operate off-grid and its COP can reach 4.0 at the ambient temperature of −10 °C and the solar irradiation of 500 W/m2. The practical electrical efficiency of PV panels achieves at 15.1%, whereas the thermal efficiency of the system is 44.8%. A hybrid control method including three modes has been also proposed based on the results for improving the system performance. The levelized cost of heat (LCOH) of this system is 0.054 $/kWh, which is 51.5% lower than that of electric heating system (0.111 $/kWh).