An equivalent temperature drop method for evaluating the operating performances of ASHP units jointly affected by ambient air temperature and relative humidity

•An equivalent temperature drop (ETD) method was developed.•It considered the joint effect of Ta and RH on the performances of ASHPs.•The applicability of the ETD method was verified by a field test.•The performances of ASHPs may be conveniently evaluated.•Impacts from either Ta or RH may be individually assessed. It has been known that the actual operating performances of a space heating ASHP unit are significantly impacted by the joint effect of ambient air temperature, Ta, and relative humidity, RH. To enable a convenient evaluation of the operating performances of a space heating ASHP unit with a reasonable accuracy, an equivalent temperature drop (ETD) method has been developed. The ETD method transferred the impacts of frosting-defrosting on the operating performances of ASHP units into that of an equivalent ambient air temperature drop, so that the joint effects of Ta and RH on the actual operating performances of the ASHP units may be assessed using a single parameter of equivalent ambient air temperature. In this paper, the models for predicting the output heating capacity of an ASHP unit operated at both frosting and non-frosting conditions were firstly established through either regression analysis or referring to previous related studies. Secondly, with the availability of the models, the ETD method was developed for evaluating the actual operating performances of ASHP units at different ambient conditions. Thirdly, the developed ETD method was verified by a field study on the operating performances from an ASHP unit installed in Beijing. The field study results demonstrated that for any frosting operating conditions, using the ETD method, a corresponding non-frosting condition at which the field ASHP would output the same heating capacity may be identified. Furthermore, the use of ETD method can also distinguish the individual impacts from either Ta or RH on the actual operating performances of an ASHP unit.