Experimental study on defrosting multi outdoor units in turn for air source heat pump using hot gas.

To improve the efficiency of air source heat pumps and eliminate the performance degradation caused by frost in winter, a hot gas bypass defrosting experimental platform was built and the hot gas bypass defrosting experiment under different outdoor temperature and different opening degree of hot gas bypass valve were carried out. The results showed that the defrosting of the heat pump evaporator can be completed well by adopting the hot gas bypass defrosting method without interrupting the heat pump heating. At outdoor temperatures of -10 °C, -5 °C and 0 °C, the most appropriate time for heat pump to activate hot gas bypass defrosting were 30 min, 40 min and 50 min, respectively. The corresponding peak value of average heating capacity were 7.96 kW, 10.16 kW, and 12.08 kW, and the heating COPC values were 3.03, 3.56, and 4.19, respectively. The increase of bypass exhaust volume would shorten the defrosting time but also reduce the heating capacity of the heat pump during the defrosting process. Hence the optimum defrosting start time of the system is different with different bypass valve opening. For the system built by this paper, the 20 % bypass valve opening had the best performance in defrosting, and the optimal defrosting start time was 40 min, during which the average heating capacity and COPC achieved peak values of 10.16 kW and 3.56, respectively. These results provide a reference for the defrosting design of air source heat pumps.