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Appendix A – CO2 as a Working Fluid in Heat Pumps The thermal drawback caused by a pinch point inside the heat exchanger is illustrated in the T-Q diagram in Figure A15. In this example water is heated from 25°C to 50°C (set-point), and the initial high-side pressure in the gas cooler is 7.5 MPa. The diagram does not show the actual tempe- rature profiles for the flows as a function of the relative gas cooler length. 100 80 60 40 20 Figure A15 QH = CPH (TH,i - TH,o) QC = CPC (TC,o - TC,i) 7.5 MPa TH,i ∆TH TC,o ∆TC pinch point dT TH,o dH QH = QC TC,i Transferred Heat [kW] Illustration of the thermal drawback caused by a pinch point inside the gas cooler, when water is to be heated from 25oC to 50oC, and the high-side pressure is 7.5 MPa. The inverse slope of the T-Q curve for the water (broken line) represents the theoretical maximum CP value for the flow, since it is tangent to the CO2 isobar at the pinch point. Above the pinch CPH < CPC, and CPH > CPC below the pinch. If the water flow rate is increased above this maximum value, the outlet water temperature will drop below the set-point, which is normally undesirable. By increasing the high-side pressure, the isobar will become straighter and the water flow rate can theoretically be increased until the T-Q curve for the water becomes tangential to the CO2 isobar. The increased water flow rate will in turn reduce the CO2 outlet tempe- rature from the gas cooler and increase the heating capacity of the heat pump. Nekså et al. (1998), Rieberer et al. (1998), Saikawa and Hashimoto (2000) and Adriansyah (2001) discussed the existence of a gas cooler pinch point and the effect of high-side pressure control for CO2 heat pump water heaters. Figure A16 illustrates the result of increasing the high-side pressure from 8 to 11 MPa for a CO2 heat pump water heater. The city water temperature is 5oC and the set-point for the hot water is 70oC. A20 Temperature [°C]PDF Image | Residential CO2 Heat Pump System for Combined
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