The purpose of this study is to clarify the operating conditions and the performance of hot water storage tank systems in houses. The authors measured the energy consumption and the loads of hot water consumption in houses and laboratories that had been set up with the model of a CO_2 heat pump water heater. In previous studies, the authors developed a method of estimating the hot water supply demand based on the measured results. Furthermore, a method of calculating the estimated energy consumption and efficiency performance of CO_2 heat pump water heaters was proposed. In this paper, factors such as the volume of hot water consumption, the climate conditions, and the characteristics of the equipment, all of which have an effect on energy consumption and efficiency performance, are analyzed by the simulation methods proposed in previous studies. The types of hot water loads, which are Large, Medium, and Small loads for a family composed of four persons, are prepared by using the results of the estimation method for hot water supply demands. Moreover, the energy consumption and efficiency performance of the CO_2 heat pump water heater are calculated according to the type of hot water load in different climate regions and seasons. As the result, it was clarified that the effect of the climate conditions and the characteristics of the equipment on the efficiency performance are larger than those of the hot water supply demand.
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