Experimental Study on Air-Source Heat Pump Alternate Defrosting with Multiple Outdoor Units in Parallel

An experimental system of an air-source heat pump with multiple outdoor units in parallel was designed and built. The outdoor units were defrosted in turn using hot subcooled liquid for defrosting. The heat pump heating performance variations in the course of frosting and defrosting under different outdoor temperature and humidity conditions were experimentally studied, and the heating performances of the hot liquid subcooling alternate defrosting system and hot gas bypass alternate defrosting system were compared and analyzed. The experimental results show that the hot liquid subcooling alternate defrosting can enable the heat pump to complete the defrosting under the condition of continuous heating and that the defrosting performance is good. When the outdoor air temperature is -10 ℃ and the relative humidity is 90%, the optimal time to start defrosting is 30 min. At this time, the system heating capacity decreases from 9.87 kW when there is no frost to 8.71 kW when the defrosting starts, and the heating COP decreases from 3.73 to 3.36. When the system operates defrosting under different humidity conditions, the minimum values of system heating capacity and heating COP can reach 5.42 kW and 2.28, respectively. Compared to the case of bypassing 20% of discharge gas for defrosting, the average heating capacity of hot liquid subcooling alternate defrosting is 13.8% higher, and the heating COP_C is 10.1% higher.