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3 – Theoretical Background and System Analysis where the subscripts T-CW and T-m refer to the COP of the CO2 heat pump at the inlet water temperatures TCW and Tm for the gas cooler. The equality sign is only valid when V’CW=VCW, i.e. when VM=0. Figure 3.26 shows, as an example, the estimated percentage reduction in the COP for a residential CO2 heat pump water heater operating at differ- rent DHW temperatures, 10oC city water temperature and varying extent of the mixing zone. The COP data for the CO2 heat pump unit are from Figure 3.4 in Section 3.2.2.1. A relative extent of the mixing zone of 0.5, means that 50% of the city water volume has been mixed with an equal amount of DHW, and reached an intermediate temperature Tm. 30 25 20 15 10 5 0 80/10oC 70/10oC 60/10oC Variable hot water temperature 0.0 0.1 0.2 0.3 0.4 0.5 Relative Extent of Mixing Zone [-] Figure 3.26 Percentage reduction in the COP for a residential CO2 heat pump as a function of the relative extent of the mixing zone and the DHW temp. The city water temp. is 10oC . The mixing loss can be greatly reduced by installing adequate diffusers that decrease the inlet water velocities. A device that eliminates the mixing is presented in Section 3.3.4. 3.3.3.3 Conductive Heat Transfer Inside the DHW Tank Owing to the direct contact and the relatively high initial temperature difference between the hot and cold water reservoirs in the DHW tank during the tapping and charging periods, there will be considerable conductive heat transfer between the reservoirs. This will in turn increase the average inlet water temperature for the DHW preheating gas cooler unit during the charging period, and reduce the COP of the heat pump. 59 Reduction in COP [%]PDF Image | Residential CO2 Heat Pump System for Combined
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