Development and Experimental Investigation on Two-Stage Compression Variable Frequency Air Source Heat Pump

A two-stage compression variable frequency air source heat pump is proposed combining variable frequency technology with two-stage compression cycle. The empirical formula of optimal intermediate pressure is calculated, and two-control-model with the priority target as coefficient of performance (COP) or heating capacity is advanced in this paper. A test bench is developed for the two-stage compression variable frequency air source heat pump in order to observe the practical operation and obtain the measured data. The COP and compressor discharge temperature in the single-stage compression system and two-stage compression system at different evaporating temperatures are measured, and the variation of heating capacity in the two-stage compression system along with the evaporating temperature and power frequency of low pressure stage compressor is investigated experimentally too. The experimental results show that the COP is over 2, the compressor discharge temperature under 120 C, and the heating capacity can meet the heating load needed when the condensing temperature is 50 C and evaporating temperature -25 C. Also the experiment shows this system runs safely and steadily, which can be used in the cold regions where outdoor temperature is not lower than -18 C.