Experimental investigation on the large temperature lift heat pump with an integrated two-stage independently variable frequency compressor

A large temperature lift heat pump (LTLHP) is defined as a heat pump with a temperature lift exceeding 50 °C. The two-stage configuration represents a viable technological option for enhancing the energy efficiency of large temperature lift heat pumps. In this study, an integrated two-stage screw compressor with variable frequency for both high and low-stages is designed and fabricated for large temperature lift applications, and a corresponding LTLHP system is constructed. Experimental tests are carried out on both the designed screw compressor and the large temperature lift heat pump system under various evaporation and condensation temperatures. The results indicate that the coefficient of performance (COP) of this heat pump, operating at evaporation temperatures between −10 and 10 °C and condensation temperatures from 70 to 80 °C, has improved by up to 10.4 % and averaged 5.5 % compared to similar conventional systems available on the market. Additionally, this study investigated the effect of high-stage rotational speed on the system to develop control strategies. The technical prospect of the integrated two-stage independently variable frequency compressor in the large temperature lift heat pump system is validated.