Frosting performance variations of variable-frequency air source heat pump in different climatic regions

In practical projects, it is found that at the same outdoor temperature and relative humidity, the frosting performance of variable-frequency air source heat pumps (VFASHPs) in different climatic regions varies obviously, e.g. frost will be accumulated on its outdoor coil at the condition of 3 °C, 90 % in Beijing, but not in Harbin. To reveal the frosting performance variations of VFASHPs in different climatic regions, a mathematical model of VFASHPs was established, and its frosting maps in cold region and hot-summer and cold-winter region were developed. Combining its frosting map in severe cold region obtained from experiments, its frosting performance variations in three climatic regions were investigated. Results indicate that when a VFASHP is used in different climatic regions, its frosting region varies obviously. In different climatic regions, when the outdoor design temperature for heating is lower, its critical relative humidity for frosting is higher; while the upper and lower limit temperatures of frosting region are both lower. Taken 0 °C as an example, the critical relative humidity for frosting in Harbin, Beijing, and Shanghai are 79.3 %, 66.2 %, and 55.0 %, respectively. This work can provide guidance for defrosting control strategy design of VFASHP in different climatic regions.