Review on the optimization studies of reverse cycle defrosting for air source heat pump units with multi-circuit outdoor coils

When an air source heat pump (ASHP) operates at low ambient temperature and high relative humidity, its outdoor coil faces frosting problem. Defrosting becomes necessary and reverse cycle defrosting (RCD) has drawn extensive attention due to its specific advantages. To fully understand the research trends of defrosting topic and further improve the RCD efficiency for ASHP, reported experimental and numerical studies around the optimization of RCD were reviewed. As summarized, RCD coupled with phase change energy storage technology is the most promising one, which resolves the fundamental problem of insufficient thermal energy during defrosting. Numerical studies mainly focused on the divisions of defrosting stages, the negative effects of downward flowing melted frost on defrosting efficiency, the energy transfer process in the metal of heat exchangers during RCD, and the validation and extension of defrosting models at different stages. For the optimization of defrosting initiation and termination control strategies, more accurate and economic measurement methods of frost layer thickness and total mass are necessary. Finally, the outlooks for future studies on RCD for ASHP are given. This work contributes to guiding the optimization and design of the ASHP, and providing insight into the investigation of frosting and defrosting mechanisms.