Optimal heat pump defrosting strategies for full-cycle capacity and COP maximization

The defrosting of air source heat pumps (ASHPs) is essential for melting the accumulated frost and is primarily affected by outdoor ambient conditions, along with the ASHP’s component speed. While existing studies propose various methods for optimum defrost initiation, there is limited research on developing optimum defrost cycle controls for variable-speed ASHPs under different ambient conditions. Hence, this study experimentally investigates key parameters for both defrost initiation and the defrosting process. First, multiple parameters were evaluated to identify an optimum defrost initiation method that preserves the heating capacity and adapts to changing ambient conditions. The approach temperature difference emerged as a reliable and effective indicator for defrost initiation. Next, an optimized defrosting cycle was developed using the indoor fan, outdoor fan, and compressor speed. Upon comparison with the conventional method of using superheat, the developed strategy showed an improvement of approximately 1.54%, 7.85%, 7.85%, and 7.4% over four consecutive frosting/defrosting cycles. Similarly, the energy consumption during the defrost cycle was compared to that during the frosting cycle, showing a difference of less than 3%, which highlights the effectiveness of the developed strategy. These cycles were conducted consecutively to closely replicate the continuous operation in the field.