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1260 J. Stene / International Journal of Refrigeration 28 (2005) 1259–1265 Nomenclature COP coefficient of performance SPF seasonal performance factor DHW domestic hot water Tripartite gas cooler three gas cooler units for preheat- ing of DHW, space heating and reheating of DHW Combined mode simultaneous space heating and hot water heating DHW mode hot water heating only SH mode space heating only heating of ventilation air [5], CO2 heat pump water heaters [6], CO2 heat pumps for retrofitting of boilers in high- temperature radiator systems [2], reversible split-type air-to- air CO2 heat pumps [1] and CO2 heat pumps in combination with low-temperature heat distribution systems [3]. Since previous work on residential CO2 heat pumps has been dealing with systems for either space heating or hot water heating, it was considered as interesting to study a residential CO2 heat pump system for combined space heating and hot water heating—a so called integrated CO2 heat pump system. 2. Characteristics of integrated CO2 heat pump systems Due to the low critical temperature of CO2 (31.1 8C), an integrated CO2 heat pump will reject heat by cooling of CO2 at supercritical pressure in a gas cooler. In order to achieve a high coefficient of performance (COP), it is essential that useful heat is rejected over a large temperature range, resulting in a relatively low CO2 outlet temperature from the gas cooler. A number of gas cooler configurations were evaluated. It was found that a counter-flow tripartite gas cooler for preheating of domestic hot water (DHW), low-temperature space heating and reheating of DHW, would enable production of DHW in the required temperature range from 60 to 85 8C, and contribute to the highest possible COP for the heat pump unit [7]. Fig. 1 shows the principle of an integrated CO2 heat pump system, where an expansion valve and a low-pressure liquid receiver (LPR) are used to control the supercritical (high-side) pressure in the tripartite gas cooler. Gas cooler units A and C are connected to a standard (single-shell) DHW storage tank and an inverter controlled pump by means of a closed water loop. Gas cooler unit B is connected to a low- temperature hydronic heat distribution system with floor heating, convectors or fan-coils. An integrated CO2 heat pump will be operating in three different modes: (1) space heating only (SH mode), (2) hot water heating only (DHW mode) and (3) simultaneous space heating and DHW heating (combined mode). When operating in the SH mode, there is no water circulation in the DHW circuit, and only gas cooler unit B is operative. During tapping of DHW, hot water from the top of the DHW tank is premixed with cold city water and delivered at the tapping site, while cold city water enters the bottom of the tank. During charging of the DHW storage tank in the combined mode and DHW mode, the cold city water from the bottom of the DHW tank is pumped through gas cooler units A and C, heated to the desired temperature level, and returned at the top of the tank. Fig. 1. The principle of an integrated residential CO2 heat pump system.PDF Image | Residential CO2 heat pump system space and hot water heating
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