Abstract
Attention has been paid to the enhancement of the performance of CO2 heat pump water heating systems. One of the ways to increase the heat pump coefficient of performance (COP) and system efficiency as well as the volumes of stored and unused hot water is to revise the system by extracting water with middle temperatures from the middle of the storage tank, and the performance of the revised system has been evaluated by numerical simulation. However, the volumes of stored and unused hot water have been evaluated in a conventional method. In this paper, they are evaluated in consideration of hot water supply procedure. An evaluation method by numerical analysis of differential equations is proposed to take account of the discontinuities of the temperature in the storage tank. Through evaluation examples, it turns out that the volumes of stored and unused hot water are evaluated accurately and efficiently in all the cases by the proposed method. In addition, it is clarified how the volumes of stored and unused hot water evaluated by the proposed method differ from those by the conventional method.
