A new performance index for constant speed air-source heat pumps based on the nominal output heating capacity and a related modeling study

Air source heat pump (ASHP) units have been widely used for space heating in recent years. While a space heating ASHP unit is normally rated at its nominal operating condition, however, during its actual space heating operation, it rarely works at the nominal condition. The actual output heating capacity can remarkably deviate from that at the nominal condition, due to the influences of ambient air temperature and frosting-defrosting operation. Therefore, to enable a comprehensive and convenient evaluation of the operating performance of ASHP units with frosting-defrosting operation and to provide designers with appropriate design guidelines to size a space heating ASHP unit, a new performance index has been proposed. The new performance index, ɛ_NL, or the loss coefficient in the nominal output heating energy, was actually based on the nominal output heating capacity of ASHP units, which was readily available and stayed unchanged irrespective when and where ASHP units were operated. In this paper, the definition of ɛ_NL was firstly given, following a detailed explanation of the frosting-defrosting operation of an ASHP unit. Secondly, a GRNN model for predicting the ɛ_NL of a field ASHP unit was established following a correlation analysis using a total of 473 groups of field measured data from the field ASHP unit. Finally, a modeling study using the developed GRNN model was carried out, and the study results suggested that when an ASHP unit was operated at a fixed ambient condition, defrosting initiating time would affect the ɛ_NL, and there may exist an optimal defrosting initiating time at which ɛ_NL was at its minimum. The model results also demonstrated that an increase in ambient air relative humidity or a decrease in ambient air temperature would result in an increase in ɛ_NL, when an ASHP unit was operated with a fixed defrosting initiating time.