Investigation on the correlation between supply–return water temperature difference and frosting–defrosting performance of air source heat pumps

Air-source heat pumps (ASHPs) are increasingly used for cold-climate space heating owing to low resource demand. However, in some cold regions, sustained “low-temperature and high-humidity” conditions trigger frequent frosting–defrosting cycles and degrade energy efficiency of ASHPs. Previous studies emphasized defrosting strategies while overlooking the role of water-side operating parameters in delaying frosting and the comprehensive effects of their coupled interaction with environmental factors. This study demonstrated that increasing the supply–return water temperature difference (SRWTD) raised the outdoor coil temperature, delayed frosting, and provided clear advantages in cold, humid climates. Three quasi-two-stage ASHP test rigs were established in Weinan, Shaanxi Province, a cold-region city in China. SRWTDs of 5/7/10 °C were applied during the heating season. As the SRWTD increased, the frosting rate decreased significantly and the overall system performance improved. Compared with the 5 °C condition, the 7 °C and 10 °C conditions reduced the time during which the unit extracted heat from the water loop by 21.8% and 50.2%, respectively, and substantially shortened the period during which defrosting affected indoor temperature. The maximum heating power increased by 8.5% and 14.7%, and the total heating capacity increased by 11.0% and 15.7%, respectively. Meanwhile, the accumulated defrosting energy consumption decreased by 20.4% and 40.5%, and the total COP increased by approximately 8.7%. These results indicate that a large SRWTD offers significant advantages in improving ASHP energy efficiency. Considering both heating performance and overall system efficiency, adopting a large temperature difference is recommended in cold and humid regions.