Experimental study on hot liquid subcooling defrosting of an air source heat pump with multi-connected outdoor units

Air source heat pumps (ASHPs) are widely used in clean heating because of the high efficiency, energy saving and economic advantages. Efficient defrosting technology for continuous heating is a key issue that must be addressed when operating the large ASHP at low temperatures. To solve the technical problem, this paper designed and built an experimental ASHP system with multi outdoor units connected in parallel, and used the hot liquid from the outlet of the condenser to defrost outdoor units in turn to study the change of heat pump per-formance with defrosting start time at different outdoor temperatures and outdoor humidity, and compared the system performance of the hot liquid subcooling defrosting (HLSD) with that of the hot gas bypass defrosting (HGBD). The experimental results showed that the HLSD enabled the heat pump to complete rotational defrosting with good defrosting effect in the case of continuous heating. The lower the outdoor ambient tem-perature and higher the air relative humidity, the worse the heating performance, and the longer the defrosting time. When using the HLSD, the heating capacity and energy efficiency ratio of heat pumps could be increased by 10% to 20% compared with the HGBD.