Abstract
In recent years, CO2 heat pump water heating systems have been developed, and their performances have been enhanced while their functions have been expanded. In a multi-functional system, used for both hot water supply and bath heating, hot water retrieved from the top of a storage tank is returned to its bottom or side after heat exchange for bath heating, which destroys the stratified temperature distribution in the storage tank and degrades the system performance. In this paper, the performance of a multi-functional CO2 heat pump water heating system has been evaluated by numerical simulation. A system model was created by combining component models for a CO2 heat pump, mixing valves, a storage tank, a heat exchanger, and a bathtub. Partly, perfect mixing by hot water return was assumed in the component model for the storage tank, and its validity was verified through experiments. A performance analysis has been conducted under daily repeated hot water and bath heating demands, and the system performance was evaluated at a periodically steady state. As a result, the system efficiency and the volume of unused hot water in the multi-functional system decreased by 4.9% and 16.3%, respectively, as compared to those in the uni-functional system, when hot water returned to the bottom of the storage tank. When the position for hot water return is heightened, the system efficiency becomes higher than that in the uni-functional system, while the volume of unused hot water decreases furthermore.
Highlights
In Japan, the energy consumption in the residential sector accounts for about 16% of the total one in all the sectors, and the energy consumption for hot water supply accounts for about 28% of that in the residential sector [1]
System M was investigated for when hot water returned to the bottom of the storage tank after heat exchange for bath heating
The performance of a multi-functional CO2 heat pump (HP) water heating systems (WHSs) with the functions of both hot water supply and bath heating has been analyzed by numerical simulation
Summary
In Japan, the energy consumption in the residential sector accounts for about 16% of the total one in all the sectors, and the energy consumption for hot water supply accounts for about 28% of that in the residential sector [1]. Water heating systems (WHSs), each of which is composed of a heat pump (HP) using CO2 as a natural refrigerant and a hot water storage tank, have been developed and commercialized widely [2] Their performances have been enhanced, while their functions have been expanded. Operation and design optimizations have been tried: Yokoyama et al have estimated system performance values and have clarified the optimal operating conditions under daily change in simulated hot water demand [37]; Deng et al have analyzed and optimized the performance of a combination of a solar heater and an HP WHS [38] These results have been obtained for uni-functional systems with the function only of hot water supply. The influence of the position for hot water return on the system performance was examined
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