An experimental investigation on hot gas defrost of air-source transcritical CO₂ heat pump systems

A hot gas defrosting method is investigated to solve the frost accumulation on the finned tube evaporator for transcritical CO₂ heat pump systems. It is based on the analysis of defrost cycle process, and an air-source transcritical CO₂ heat pump water heater test rig is built in an environmental laboratory. Based on the experimental results of the pressure and temperature variations that the defrost cycle process and defrosting characteristic in the evaporator are discussed, and it is found that the mass flow rate increases and the temperature difference between gas cooler inlet and outlet decreases during the defrosting process. It is also found that the key to improve the defrost efficiency is to increase the gas cooler outlet temperature of CO₂ gas. Meanwhile, instant defrosting pictures of evaporator surface are captured to record the dynamic defrosting process and to evaluate the defrost effect of hot gas defrosting. The whole defrosting process maintains 600 s and the defrosting efficiency of the hot gas defrosting method is 35.6%. It can be concluded from the experimental results that the hot gas defrosting method is a reasonable and practicable method for CO₂ heat pump systems.