Experimental analysis of an air-source transcritical CO2 heat pump water heater using the hot gas bypass defrosting method

When an air-source CO₂ heat pump water heater operates at low ambient temperatures in cold regions in winter, frost can form on the coil surface of its outdoor evaporator. The frost substantially affects the operating performance and energy efficiency of CO₂ heat pump water heaters and hence periodic defrosting is essential. In this paper, defrosting characteristics of an air-source CO₂ heat pump water heater using the hot gas bypass defrosting method is experimentally studied at different ambient conditions. An experimental setup is developed for this purpose and experimental procedures are detailed. Thereafter, the pressure and temperature in the outdoor evaporator, at the compressor and gas cooler outlets are evaluated during the defrosting period. An energy analysis is then performed of different system components during the defrosting process. Results indicate that 35% of the supplied energy is used for melting the frost, and 7.6% is used to heat the evaporator tubes and fins. About 57.4% of the supplied energy is consumed to increase the internal energy of the gas cooler. The typical efficiency of the hot gas bypass defrosting method applied in the CO₂ heat pump water heater ranges from 30 to 40%. It increases with increasing dry bulb temperature, and decreasing relative humidity.