Performance Estimation and Optimal Operation of a CO2 Heat Pump Water Heating System

Abstract

IntroductionHot water demand occupies about one-third of the energy consumption in the residential sector in Japan, and energy saving in hot water supply has been an important issue. Under this situation, water heating systems each of which is composed of a heat pump using CO 2 as a natural refrigerant and a hot water storage tank have been developed and commercialized widely (Saikawa, 2004;Hashimoto, 2006). The performance of CO 2 heat pumps has been enhanced dramatically through the technological development of their components such as compressors and gas coolers. On the other hand, importance has also been given to the performance of water heating systems in case they are operated under a daily change in hot water demand.The performance of the CO 2 heat pump only, or coefficient of performance (COP) is affected by the air temperature as well as the inlet and outlet water temperatures. Many theoretical and experimental studies have been conducted for the performance analysis on CO 2 heat pumps (Hwang and Radermacher, 1998;Nekså et al., 1998; Saikawa et al., 1999;Saikawa and Hashimoto, 2001;Nekså, 2002;White et al., 2002;Skaugen et al., 2002;Richter et al., 2003;Yokoyama et al., 2006;Laipvadit et al., 2008;Yan et al., 2010;Sarkar et al., 2010;Yamaguchi et al., 2011). On the other hand, the performance of the water heating system composed of the CO 2 heat pump and storage tank is affected by many conditions. The ambient conditions such as air and feed water temperatures, the hot water demand, and the operating conditions such as startup and shutdown, and outlet water temperature during operation of the CO 2 heat pump affect the inlet water temperature and resultantly the COP through the temperature distribution in the storage tank. In addition to the COP, the storage and system efficiencies, and the volumes of stored and unused hot water are considered as system performance values, and these are also affected by the aforementioned various conditions through the temperature distribution in the storage tank. As a result, the system performance is affected by the operational history on past several days, and changes complexly with days. Therefore, in order to attain the maximum system performance, it is necessary to estimate the daily changes in system performance values accurately in relation to those in the ambient conditions, hot water demand, and operating conditions, and determine the operating conditions optimally based on them.Some studies have been conducted for the performance analysis on water heating systems (Cecchinato et al., 2005;Stene, 2005;Minetto, 2011). However, few studies have been conducted in consideration of daily changes in the aforementioned conditions. In order to investigate the daily changes in system performance values, laboratory and field tests have been tried under simulated and practical hot water demands, respectively. However, hot water demands depend on residential houses, and it takes extremely long time to conduct the tests. Thus, it is not necessarily easy to investigate the ...

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Numerical Analysis for Performance Evaluation of a Multi-Functional CO2 Heat Pump Water Heating System

“…Some review papers on the performance analysis of CO 2 enhance the performance by extracting tepid water from a storage tank [36]. In addition, 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.…”

Numerical analysis on performance enhancement of a CO2 heat pump water heating system by extracting tepid water

“…On the other hand, some studies have been conducted under unsteady states including charging and tapping modes: Fernandez et al have investigated the performance of a water heating system composed of a heat pump and a storage tank in the charging mode [26]; Yokoyama et al have conducted numerical simulations on such a system and have clarified the effects of the ambient and operation temperatures on system performance values under a daily cyclic change in hot water demand [27,28]; Yokoyama has conducted the exergy analysis of the system based on the results obtained by numerical simulation [29]; Yokoyama et al have also investigated daily changes in system performance values under daily changes in standardized and simulated hot water demands [30,31]. In addition, operation and design optimizations have been tried: Yokoyama et al have investigated the optimal conditions for operating temperatures under daily change in simulated hot water demand, which is based on the estimation of system performance values in place of the numerical simulation [32]; Deng et al have investigated performance analysis and optimization of a combination of a solar heater and a heat pump water heating system [33]. However, all the studies have been conducted by assuming that conventional storage tanks with outlets for hot water supply on their tops are installed.…”