Investigation of defrosting water retention on the surface of evaporator impacting the performance of air source heat pump during periodic frosting–defrosting cycles

To investigate the impact of defrosting-water retention on the surface of heat exchangers of the air source heat pump (ASHP) performance during frosting–defrosting cycles, an experimental study has been conducted. Testing was carried out in a controlled-environment chamber with ASHP operational data collected over 16 periodic frosting–defrosting cycles spanning a 360-min period. It was found that the defrosting-water retention undergoes three stages: single-phase water, a mixture of water and ice, followed by a predominance of ice. In this study, the third stage appears after the unit has been in operation for 256min. A “permafrost area” forms on the lower quadrant of the heat exchanger, which occupies over 20% of the total heat exchanger surface. This area remained continuously occupied regardless of whether the unit was in a frosting or defrosting cycle. The appearance of this “permafrost area” not only impacts the performance of the ASHP, it also indicates the beginning of repetitive “shutdown accidents” for the ASHP units. It is the first time that these phenomena, originally caused by the defrosting-water retention have been revealed. And a quantitative analysis is presented for the resulting performance degradation of the ASHP related to the “permafrost area” conditions and the related “repetitive shutdown accidents”.