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CHAPTER 2: LITERA TURE STUDY 46 heater without operational problems and only a sma!! !oss in the systems efficiency (Neksa, 2002). Measured results from the lab prototype heat pump showed that increasing the water outlet temperature from 60°C to 80°C resulted in a decrease in the heating COP from 4.3 to 3.6 at an evaporating temperature of O°C. This lead to one of the biggest advantages of a CO2 heat pump system running at supercritical conditions, namely the ability to produce high temperature water with a good COP (Neksa et al., 1998). This made CO2 a feasible possibility as refrigerant in heat pump water heaters for domestic use including hotels and restaurants. According to Neksa et al. (1998) the temperature difference between the outlet of the gas cooler and the secondary fluid is determined by the high-side pressure. When operating at high compressor outlet pressures the temperature difference is small and stays more or less the same but as the pressure decreases the temperature difference increases. This is because of a temperature pinch point occurring in the gas cooler. The reason for the pinch point occurring at lower operating temperatures is because of the fact that the temperature glide becomes more curvaceous as it reaches the critical point (Neksa et al., 1998). Figure 2.30: Measured heating COP of prototype system with an inlet water temperature of 10°C (Neksa et ai, 1998). ~ +-> 5 ~ 6 I 0~ 0 .~~ H i-i ...-; (,) 4 q~ '--' ~i Ii -,.-1 3 ~ 0-: 02 u 1 p.., I I I I I II I ,.r::::: 1 °-25 -20 -15 -10 -5 o 5 10 15 I -' . Hot Water Temperature = 60°C I IjIJ Evaporating Ten1perature, [DC] The implementation of an internal heat exchanger is for further cooling down the high pressure CO2 and to vaporise any CO2 liquid entering the low pressure receiver (if A Techno-Economical Analysis of a CO2 Heat Pump. School ofMechanical Engineering, North-West UniversityPDF Image | CO2 HEAT PUMP Analysis
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