Comparison between reverse cycle and hot gas bypass defrosting methods in a transcritical CO₂ heat pump water heater

Defrosting is an important issue that should be considered when an air source heat pump is operating in a cold climate. For the transcritical CO₂ heat pump (TCHP) water heater, the high operating pressure affects the application of defrosting methods. In this paper, the reverse cycle defrosting (RCD) and hot gas bypass defrosting (HGBD) methods were experimentally investigated in an air source TCHP water heater. The dynamic defrosting characteristics, influences on system operation and overall energy performances of the two methods were analyzed. The results suggested that the RCD method presented more complex parameter variations, lower power consumption, shorter defrosting time and larger influence on the system operation; while the HGBD method displayed less impact on the system heating capacity. The electric energy used for the RCD method was only 17.5% of that for the HGBD method. Moreover, the averaged COP with the RCD method was higher than that with the HGBD method by 4.61%. In summary, the RCD method showed evidently higher energy performance and could be a promising solution for the TCHP water heater, and the HGBD method demonstrated its robustness but had a limited energy performance.