Anti-frosting strategies of air source heat pump derived from homogenizing the uneven surface temperature of outdoor coils

In actual operation of an air source heat pump unit, the outdoor coils generally showcase uneven surface temperature distribution, and this leads to uneven frosting. Homogenizing the uneven frosting has been demonstrated to be helpful for anti-frosting. In this study, two strategies, i.e., airflow regulation and refrigerant flow regulation, were proposed to tackle the uneven frosting issue. Experimental tests with a newly designed heat exchanger were conducted to demonstrate these two strategies. The experimental results were depicted in a semi-experimental frosting evaluation plot, in which the anti-frosting potential were quantitatively clarified. Specifically, by implementing the proposed strategies, the critical air temperature for frosting could be reduced by 1.6 degrees C, and the critical relative humidity could be reduced by 4%-8%, respectively. Such optimization can significantly reduce the annual accumulated frosting duration. In detail, the annual frosting duration could be decreased by nearly 50% in Hohhot, Urumqi, and Xining; 35%-40% in Beijing and Lhasa, greater than 55% in Kunming; and nearly 45% in Wuhan and 80% in Shanghai. In Baise, the frosting may be avoided during operation throughout the year. The findings of this study can provide some useful clues for the anti-frosting design of air source heat pumps in different climatic regions.