Abstract
Heat pumps using CO_2 as a natural refrigerant have been developed, and are expected to contribute to energy saving in domestic hot water supply. The objective of this paper is to analyze the performance of a CO_2 heat pump water heating system using a storage tank with a movable insulating plate by numerical simulation. A simulation model based on variable control volumes and a relevant solution method are proposed for this objective. A numerical study is conducted to compare the performance of the conventional and proposed systems. It is shown that the temperature in the middle part of the storage tank in the proposed system becomes higher than that in the conventional one, which reduces the volume occupied by water with middle temperatures. It is also shown that as a result, the performance of the proposed system is superior to that of the conventional one from the viewpoint of the trade-off relationship between the system efficiency and the volume of unused hot water.
