Abstract
This paper describes a performance evaluation of CO_2 heat pump (HP) water heaters. We evaluated the influence of reheating operation and domestic hot water (DHW) load characteristics on the system performance. First, we carried out experiments by using the modified M1 mode for the DHW and reheating loads. The great differences about the control of the output hot water temperature of the HP unit, and the amount of hot water storage etc. are confirmed under the reheating operation compared with results of the modified M1 mode only. In the reheating operation, SCOP is lower than the value for the modified M1 mode by approximately 30% in winter. Secondly the Hot water whose amount of heat was equivalent to the reheating load was taken in a bathtub. Although differences in operation control compared to the modified M1 mode were not detected, the hot water supply temperature fell below 60°C several times. Within around 40% decrease or increase compared with the standard hot water usage, there were almost no differences on the operation control. SCOP at the lower hot water load fell below the standard. Therefore, it is necessary to consider a DHW usage model appropriate for the amount of a DHW load.
