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7 – Discussion and Analysis ♦ At 80oC DHW temperature, the difference between the average COP at varying temperature levels in the SH system and the maximum COP was approximately 7% (combined mode), 10% (DHW mode) and 18% (SH mode), when the CO2 heat pump unit was run at a constant high-side pressure of 10 MPa. Due to the relatively flat maximum points of the COP curves at 60 and 70°C DHW temperature, the COP of the prototype CO2 heat pump unit was only 5% less than the maximum COP when operating at constant high-side pressures. At 80oC DHW temperature, the drop in the COP was considerably larger. Consequently, at moderate DHW temperatures, an integrated CO2 heat pump unit can be operated at constant high-side pressure in all three modes with only a minor reduction in the COP. 7.1.4 Design of the Tripartite Gas Cooler 7.1.4.1 The Total and the Relative Heating Capacities The experimental results for the prototype CO2 heat pump unit regarding the heating capacities of the three gas cooler units during operation in the different modes, can be summarized as follows: ♦ Despite the relatively large variations in the set-point temperatures for the SH and DHW systems, the total heating capacity of the tripartite gas cooler at optimum high-side pressure was relatively invariable (6.8 kW ±6%) during operation in the different modes. ♦ The ratio of the heating capacity for the SH gas cooler during operation in the SH mode and the combined mode ranged from about 1.2 to 2.6, and the higher the temperature level in the SH system and the lower the DHW temperature, the larger the ratio. ♦ The ratio of the heating capacity for the DHW gas cooler units during operation in DHW mode and the combined mode ranged from about 1.4 to 2.6, and the higher the DHW temperature and the lower the temperature level in the SH system, the larger the ratio. ♦ The heat transfer surface of the DHW reheating gas cooler unit con- stituted less than 10% of the total heat transfer area of the tripartite gas cooler, but covered about 22 to 36% of the total DHW heat load during operation in the combined mode. This was due to the rela- tively large average temperature difference in the heat exchanger. 216PDF Image | Residential CO2 Heat Pump System for Combined
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