Experimental study on operating performances of the low and high temperature stages for cascade air source heat pumps

Previous studies on cascade air source heat pumps (CASHPs) mainly focused on overall performances, while impacts of the low temperature stage (LTS) and high temperature stage (HTS) on overall performances were paid less attention to. Therefore, operating performances of a CASHP in LTS and HTS were investigated in this paper, to provide a solution for the optimization of the CASHP. A CASHP unit with a variable speed compressor in LTS and a constant speed compressor in HTS was adopted, and its operating performances were investigated under 26 cases. Results showed that the pressure ratio and COP in LTS and HTS were affected differently by the compressor speed. Specifically, as the speed of the compressor in LTS increased from 30 rps to 90 rps, P_rL increased by 151 % ∼ 178 %, and P_rH reduced by 38 % ∼ 54 %. Meanwhile, COP in LTS reduced by 21 % ∼ 52 %, but the COP in HTS rose by 54 % ∼ 96 %. Besides, it was found that adjustments of the compressor speed in LTS could improve heating performances of the CASHP. However, when the ambient air temperature and the supply water temperature were consistent, COP changes of the CASHP were limited, only by 0.3 % ∼ 9 %. Furthermore, empirical models of the COP in LTS and HTS were established to predict the COP of the CASHP on different operating conditions. Results of this study were helpful in promoting the configurations and operation control optimization of CASHPs.