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CHAPTER 2: LITERA TURE STIJDY 43 Figure 2.27: Simulated relative COPs for a CO2 heat pump water heater as a function of the inlet water temperature to the gas cooler at 60 and ao°c set point temperature for domestic hot water. (Stene, 2007). 0 0.85 () CD a::: > .. 1.00 I I II II III ------1""--- ---1""-- 1 ' -/ +-- --+ fff CD I I I fII 0.80 ---/------- .f..- --/-0 t\l II - - - - - -- - 0.95 , 0.90 --------r- IIf II I -1" III 0fI f IIf ...... ......, 0.75 0.70 0.65 ~DHW80°C --6-DHW 60"C II II L_ --.1..-- If II __L .1..- IJ 5 10 15 20 25 30 Inlet Water Temperature [OeJ The optimum gas cooler pressure was found to be between 9 and 11MPa. CO2 heat pumps for domestic hot water production ranging between S to 20 kW have since been tested by various researchers including Rieberer and Halozen, Hwang a.nd Radermacher and Saikawa and Hashimoto. These systems had the same typical measured COPs than for the SOkW machine built by Neksa (2004) which will be discussed later on. According to Stene (2007) the Denso Corporation Ltd. in Japan was the first company to start production of a residential air source heat pump water heater in 2001. This machine is a 4.SkW system able to produce hot water at temperatures of up to 8SoC. There are three specific distinctions favouring a CO2 system above systems using HCFC and CFC refrigerants. The first is the fact that heat is rejected at a supercritical pressure. This classifies the system as a transcritical system because it operates above the critical point of the refrigerant. In a transcritical system the high side pressure is determined by the refrigerant charge and not the saturation pressure as with conventional systems. This means that the systems design should take into account the possibility of controlling the high side pressure to ensure a good COP. An example of the importance of manipulating the discharge pressure is given in Figure 2.28 (Neksa et a/., 1998). Figure 2.28 shows how the heating capacity, A Techno-Economical Analysis of a C02 Heat Pump. School ofMechanical Engineering, North-West UniversityPDF Image | CO2 HEAT PUMP Analysis
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