Heating performance analysis of quasi-two-stage compression variable refrigerant flow system in severe cold zone

To promote the application of variable refrigerant flow system (VRF) in the severe cold zone, a quasi-two-stage compression VRF test rig is built in Harbin, a segmented control strategy for the suction refrigerant state of the compressor based on outdoor temperature is adopted, and a 91-day test is conducted. The results show that by adopting the proposed control strategy, the experimental unit can operate stably during the whole test period, the highest discharge temperature is 90.6 ℃, the seasonal coefficient of performance reaches 2.40, and the indoor temperature can meet the heating demand. The main reason leading to the heating capacity shortage of the unit in the severe cold zone is mal-defrost, but not ultra-low outdoor temperature. The maximum hourly heating capacity shortage rate caused by the mal-defrost is 24.31%, while it is 7.74% caused by the ultra-low outdoor temperature. The heating effect of the experimental unit is better at the outdoor temperature of -22.5 to 7.5 ℃, and there is insufficient heating when the outdoor temperature is below -22.5 ℃. Due to the large selection of the unit in the severe cold zone and the limit of the lowest frequency of the compressor, the unit is frequently started and stopped when the outdoor temperature is higher than 7.5 ℃.