Experimental studies on hot gas bypass defrosting control strategies for air source heat pumps

The air source heat pump (ASHP) is commonly used for improving indoor thermal comfort with its traditional defrosting strategy based on reverse circulation, leading to a widely fluctuating range of indoor air temperatures, which can drop dramatically when the defrosting is in operation and increase sharply after it stops. Based on a theoretical analysis of the characteristics of a hot gas bypass defrosting system, this study proposes a novel intelligent control strategy based on the maximum heating capacity of the ASHP system with appropriate start and end points for defrosting designed to operate accurately under different outdoor temperature ranges. This defrosting method was verified in a well-structured environmental laboratory and the performance was compared to the conventional system through experiments. The results show that the heating capacity of the hot gas bypass defrosting is 10.17% higher and the overall energy efficiency is 4.06% higher. The indoor air temperature fluctuates in the range of 1°C–1.6 °C, which is about 84% less than that under conventional defrosting conditions. Moreover, the temperature of the outlet air is decreased by about 7 °C, with the fluctuation range being 56% less than that of conventional defrosting. The maximum outlet temperature remains stable at 35.2 °C and the rising rates of indoor air temperature and outlet temperature are significantly improved, ensuring a comfortable indoor thermal environment. The proposed hot gas bypass control strategy enables a high energy efficiency for the ASHP for winter heating to be achieved whilst avoiding sharp fluctuations in the indoor air temperature during defrosting.