Performance Analysis of a CO_2 Heat Pump Water Heating System Under a Daily Change in a Simulated Demand

摘要

Air-to-water heat pumps using CO_2 as a refrigerant have been developed. In addition, water heating systems each of which combines a CO_2 heat pump with a hot water storage tank have been commercialized and widespread. They are expected to contribute to energy saving in residential hot water supply. It has become more and more important to enhance the system performance. In this paper, the performance of a CO_2 heat pump water heating system is analyzed under a daily change in a simulated hot water demand by numerical simulation. A static model of a CO_2 heat pump and a dynamic model of a storage tank result in a set of differential algebraic equations, and it is solved numerically by a hierarchical combination of Runge-Kutta and Newton-Raphson methods. Daily changes in the temperature distributions in the storage tank and the system performance criteria such as volumes of stored and unused hot water, coefficient of performance, and storage and system efficiencies are clarified under a series of daily hot water demands during a month.