Performances of a water-source CO₂ heat pump coupled with hot and cold thermal storage were investigated experimentally in this study. This combined system was tested by controlling compressor frequency, expansion valve opening, and hot and cold circulating water flow rates. Experimental results show that higher compressor frequency leads to a shorter energy charging time and a higher overall COP of the combined system during the charging process. Expansion valve opening affects the COPs significantly but affects the thermal stratification in thermal storage tanks slightly. Low hot and cold water flow rates lead to the good thermal stratification in storage tanks, which is beneficial for the overall COP of the combined system during the charging process, although high water flow rates are beneficial for the transient COP of heat pump at the beginning of the charging process. The overall COP reaches maximum when the hot and cold water flow rates were set as 0.1 m³/h (0.059 ft³/min) and 0.2 m³/h (0.118 ft³/min) respectively at the compressor frequency of 50 Hz and at the expansion valve opening of 330 pulses. Based on the test data, the correlations of COP with the outlet water temperature of thermal storage tanks were developed for further optimizing this combined system

Keywords: CO₂ heat pump; thermal storage; dual-mode; optimization