Experimental investigation on the hot gas bypass defrosting in air source transcritical CO₂ heat pump water heater

The transcritical CO₂ heat pump is widely used due to its higher efficiency. Frost is inevitable at low ambient temperature, resulting in a deteriorated coefficient of performance and heating capacity. Considering the disadvantages and limitations of other defrosting methods, the hot gas bypass defrosting method was experimentally investigated in this paper. The dynamic characteristics of pressure and temperature during the defrosting process are studied in detail and the pressure-enthalpy diagrams are depicted. In addition, energy analysis of the heat supply and energy consumption is presented to evaluate the defrosting efficiency. Furthermore, the influence of ambient temperature is also discussed. The results show that the measured parameters change gradually during the defrosting process except for the initial stage and defrosting time is within the defrosting regulations. The defrosting efficiency is evaluated to be 41.16%–50.84% with variation of ambient temperatures, which indicates that the hot gas bypass defrosting method is suitable for the air source transcritical CO₂ heat pump.