Experimental analysis of reverse cycle defrosting and control strategy optimization for transcritical carbon dioxide heat pump water heater

Abstract

The heating performance of air source transcritical carbon dioxide heat pump will be dramatically reduced with frost formation when operating in cold regions. Although reverse cycle defrosting is widely used, no information is available in the transcritical carbon dioxide system. In this paper, the reverse cycle defrosting was experimentally investigated in a transcritical carbon dioxide heat pump with a microchannel evaporator. The effect of water flow rate, initial water tank temperature, compressor frequency and electronic expansion valve opening are respectively studied, and an optimized control strategy is proposed considering the defrosting time and stability. Results show that thermal energy is considerably affected by water side parameters, and stability is more sensitive to the water temperature and electronic expansion valve opening. With the optimized reverse cycle defrosting, defrosting time and total energy consumption are reduced by 95 s and 21.8% compared with the pre-set basic control strategy, and the total energy consumption is also saved by 38.6% compared with the other defrosting method. The optimized reverse cycle defrosting is verified to be preferable for the transcritical carbon dioxide system, and the presented discussion is beneficial for the application of reverse cycle defrosting in the air-to-water heat pump.