Abstract: When the air-source heat pump system is operated under low temperature conditions, there exists some problems such as frost formation on the evaporator and deterioration of system heating performance. Considering the limitations of traditional defrosting methods applied in the transcritical CO2 heat pump system, the hot-gas bypass defrosting method was experimentally investigated. The platform of air-source transcritical CO2 heat pump system was designed and a copper bypass tube with an outer diameter of 12.7mm was used as the expansion device. The platform was tested under various conditions to analyze the dynamic parameters during defrosting process and the effect of ambient temperature on the defrosting time. Meanwhile, the defrosting process at different times was depicted in the pressure-enthalpy diagram. The experimental results show that the hot-gas bypass defrosting process is relatively stable, and the parameters of each measuring point change relatively gently. According to the experimental data, it can be found that the hot-gas bypass defrosting method can significantly increase the evaporator inlet temperature to about 30℃, effectively shortening the defrosting time. The defrosting time was greatly affected by the defrosting stability period. The decrease of environmental temperature or the increase of environmental humidity would extend the defrosting time of the system. The change trend of defrosting energy consumption ratio is basically consistent with the defrosting time ratio. The defrosting efficiency is calculated to be 46.5% and 33.62% higher than that of other defrosting methods and the defrosting time is shortened by 100s, which indicates that the hot-gas bypass defrosting method is more suitable for the air-source transcritical CO2 heat pump.

Key words: transcritical CO2 heat pump; hot-gas bypass defrosting method; defrosting time; defrosting efficiency