Determination of the optimal defrosting initiating time point for an ASHP unit based on the minimum loss coefficient in the nominal output heating energy

When an air source heat pump (ASHP) is operated at frosting conditions, controlling its defrosting initiation is very important, which affects its operating efficiency and output heating capacity. Therefore, to improve the accuracy of defrosting initiation of ASHPs under frosting-defrosting operations, an investigation on determination of an optimal defrosting initiating time point (t_opt) for an ASHP unit based on the minimum loss coefficient in the nominal output heating energy (ε_NLmin) has been carried out and the investigation results are presented in this paper. Firstly, the existence of t_opt was verified through a field study. Secondly, the development of multiple-variable-nonlinear models for predicting ε_NLmin and t_opt is detailed. Finally, the results of a related modeling study using the developed multiple-variable-nonlinear model for t_opt is presented. The results from the investigation suggested that initiating defrosting for an ASHP unit at t_opt could result in the best possible operating performances under its frosting-defrosting operations. It was further demonstrated through the modeling study that at a fixed ambient relative humidity, an increase in ambient air temperature would lead to a decreasing-increasing variation trend in t_opt, and at a fixed ambient air temperature, t_opt was decreased with an increase in relative humidity.