Evaluation of defrosting control performance for air-source heat pump hot water units

Air-source heat pumps (ASHPs) will encounter different degrees of frosting problem during space heating in winter. Reasonable defrosting control performance can effectively mitigate the influence of frosting on the unit’s energy efficiency. Due to the various types of ASHPs as well as the obvious differences in frosting characteristics, there is no quantifiable defrosting control performance index, leading to the lack of design guidelines for defrosting control methods for manufacturers. This further results in the frequent occurrence of “mal-defrosting” accidents in the actual space heating. In order to obtain a general evaluation index that can effectively quantify the defrosting control performance of the unit, experimental studies are conducted based on nine air-source heat pump hot water units from different manufacturers, and the characteristic indexes for the configuration and operation (CICO) of these units are various. By analysing the experimental results, the relationship between the optimal defrosting initiating time point and the value of CICO is revealed, and the predictive models of the optimal defrosting initiating time point under standard frosting condition and general frosting condition are established. Based on the variations of the heating performance at the optimal defrosting initiating time point and different reduction rates of heating capacity, the “offsetting optimal initiating time point -15%” (lower limit) and “heating capacity reduction rate 20%” (upper limit) are proposed as the index range for evaluating the defrosting control performance of ASHPs.