Thermodynamic analysis of a novel heat pump water heater with two-stage heating for a great rise of water temperature

Two-stage compression with refrigerant injection can effectively overcome the problems that high discharge temperature, low heating capacity and COP of heat pump water heaters in extreme condition. However, it still has the problem such as big temperature difference between water and refrigerant in condenser. In order to reduce the temperature difference, enhance the efficiency and decrease the discharge temperature, this paper put forward a novel heat pump water heater system with two-stage heating for a great rise of water temperature. A special two-stage compressor with two cylinders was introduced, and the thermodynamically analytical model for the new cycle was derived. The effects of condensing temperature of first stage, evaporating temperature and relative liquid injection mass on system performance were discussed. It was found that the new cycle system improved the COP by 17.43% compared to the traditional two-stage compression system at ambient temperature of 0 °C, inlet water temperature of 5 °C and outlet water temperature of 65 °C. The COP of both two cycle systems increased slowly with the increasing of the relative liquid injection mass, while its discharge temperature of second stage decreased faster than that of traditional two-stage compression system.