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In this work, performances of conventional air source heat pump water heater (ASHPWH) and direct expansion solar-assisted heat pump water heater (DX-SAHPWH) under various operating conditions are analyzed to make comparison between the two systems. The DX-SAHPWH system in this study uses a plate type roll-bond solar collector with optimized flow channel as the evaporator, which can take both solar energy and ambient air as the heat source. Performances of both systems are investigated experimentally and numerically. The influences of operating conditions (air temperature, water temperature, solar radiation intensity, et al) are analyzed, and performance comparison under various operating conditions is made between the two systems. Results show that, in clear day conditions, the COP of DX-SAHPWH is obviously higher that of ASHPWH because the solar energy can improve the evaporating temperature of the heat pump obviously; in overcast day conditions (without frost formation), COP of both systems are almost the same; and in night conditions, especially in clear night conditions, the DX-SAHPWH shows poor performance because of the poor convectional heat exchanging performance of the solar collector/evaporator and the radiative heat loss to the night sky. And annual performance analysis shows that the average COP of the DX-SAHPWH system is remarkably higher than the conventional ASHPWH system, especially in winter season.
Sun et al. (Fri,) studied this question.