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Appendix A – CO2 as a Working Fluid in Heat Pumps A2.3 Optimum High-side Pressure at Constant CO2 Outlet Temperature from the Gas Cooler Both the heating capacity and the coefficient of performance (COP) of a transcritical CO2 heat pump cycle are affected by the high-side pressure (Lorentzen and Petttersen, 1993). In Figure A10, the transcritical cycle is illustrated in a temperature-enthalpy diagram for high-side pressures ranging from 8 to 11 MPa. The evaporation temperature is -5oC, the superheating is 5 K, the isentropic compressor efficiency is 60%, and the CO2 outlet temperature from the gas cooler is kept constant at 35oC. 2a - 3a: 8 MPa 2b - 3b: 9 MPa 2c - 3c: 10 MPa 2d - 3d: 11 MPa 35oC 2d 2c 2b 2a 3dcb 3a COP = q = (h2 - h3) w (h2 - h1) 4dcb 4a -5oC 1 1' ηis=0.6 0oC 110 90 70 50 30 10 -10 500 550 600 650 700 750 800 850 Specific Enthalpy [kJ/kg] Figure A10 The transcritical CO2 heat pump cycle operated at four different high-side pressures. The CO2 outlet temperature from the gas cooler is assumed to be constant at 35oC. With reference to Figures A7 and A10, the inlet enthalpy to the gas cooler increases and the outlet enthalpy decreases when the high-side pressure is raised. Due to the great variations in the specific heat capacity at pressures and temperatures above and near the critical point, the slope (∂T/∂h)p is not constant and the isobars are not parallel. As a consequence, the change in the specific enthalpy difference in the gas cooler is not proportional to the change in the specific compressor work, and for each fixed outlet temperature from the gas cooler there will therefore be an optimum high- side pressure leading to a maximum COP. A14 Temperature [°C]PDF Image | Residential CO2 Heat Pump System for Combined
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