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160 M.-H. Kim et al. / Progress in Energy and Combustion Science 30 (2004) 119–174 Fig. 53. System design for a combined space and water heating system. The process is also illustrated in the T – s diagram. temperature. In order to simplify the system design, the tap water heating part could also be implemented as a separate system or covered when space heating is not required. Compared to a system using R-134a as working fluid the proposed CO2 system showed favorable seasonal perform- ance when more than 30% of the power demand for space heating was covered by the air heating system. The rest was then covered by the radiator system. A 70/50 8C radiator system and heat recovery efficiency of the balanced ventilation system of 60% was assumed. In larger buildings in Norway typically more than 50% of the heating demand is air heating and this percentage is increasing due to better insulation and increased air quality requirements. This indicates that CO2 may be a promising candidate for this application. Enkemann et al. [160] made a theoretical study on CO2 heat pumps for retrofit in typical hydronic heating systems in Western Europe. A system originally designed for temperatures 70/50 8C was modified by reducing the mass flow rate of water to obtain a 93/40 8C system, which should give a corresponding heat output using the existing radiator system. The seasonal performance was then increased from 2.8 to 3.2. In addition, this system will be able to supply hot tap water without any loss in energetic efficiency. Experimental results from two prototype systems for this application are reported in Ref. [161]. Efficiency figures in the same range as the calculated were reported. Professor Gustav Lorentzen published several papers describing the possibilities of using CO2 as working fluid in heat pumps and refrigeration systems. Lorentzen [32,162] outlined possible system design of large heat pumps for district heating. This is a high-capacity application where turbo expanders may be possible to realize in a cost efficient manner. Also described is the possibility to combine refrigeration/freezing and tap water heating, which will give very high overall system efficiency. 8.5. Water heating The first application of CO2 systems on the market is heat pump water heaters, where the thermodynamic proper- ties are very favorable Fig. 54 shows, in a temperature– entropy diagram, how the temperature characteristics of the transcritical cycle matches the temperature profiles of the heat source and heat sink, giving small heat transfer losses and high efficiency. A pre-condition for high efficiency is a low water inlet temperature, giving a low refrigerant inlet temperature to the throttling device. Thus, the design of the hot water accumulating system for temperature stratification is essential in order to achieve high heating COP. Studies on CO2 heat pump water heaters were initiated at SINTEF/NTNU from the late 1980s, and a full-scale prototype system of 50 kW heating capacity was completed in 1996 as shown in Fig. 55 [108]. Results from extensive measurements on this prototype showed that a COP above 4PDF Image | CO2 Vapor Compression Systems
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