The energy stored and released process of cascade air source heat pump water heater with phase change material (PCM) was investigated experimentally. R404a was selected as refrigerant of low-temperature stage and R134a as refrigerant of high-temperature stage. At different ambient temperatures

heat transfer processes and variation of parameters for the single and cascade heat pump systems were compared

and the optimum ambient temperature

at which the cascade system should be switched to single stage system

was analyzed in order to guarantee the best system performance. The results showed that in the cascade system the heat transfer presented time-lag behavior and the difference of heat transfer from the condenser to PCM and to water was in the range of 7%—10%. When the opening of electronic expansion valve at high-temperature stage was increased by 10%

the heat transfer difference decreased from 18% to 6%

benefiting the heat balance of the condensation-evaporator and improving system efficiency. The cascade system took less time for energy storage than the single system. With the two cycles

the compression ratio and exhausting temperature increased as ambient temperature decreased. COP decreased as the water outlet temperature increased and increased with the ambient temperature. For this cascade system

the optimum ambient temperature for switching between the single and cascade systems was 7℃.