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Appendix A – CO2 as a Working Fluid in Heat Pumps Figure A7 displays constant pressure lines (isobars) for supercritical CO2 in a temperature-enthalpy diagram. Since the slope of the isobars (∂T/∂h)p is the inverse of the isobaric specific heat capacity (∂h/∂T)p, the diagram reflects the variations in the specific heat capacity under supercritical conditions. 120 100 80 60 40 20 0 20 MPa 12 MPa 10 8 MPa MPa 7.5 MPa 0 50 100 150 200 250 300 350 Specific Enthalpy [kJ/kg] Figure A7 Supercritical isobars for CO2 (RnLib, 2003). At pressures and temperatures close to the critical point, the isobars are virtually horizontal. This indicates that the temperature will be almost constant during heat rejection, and the temperature development will be rather similar to a process with subcritical condensation of the working fluid. On the other hand, at high operating pressures or at temperatures below the critical point and above roughly 40 to 50oC, there will be a considerable temperature drop during heat rejection. A2.2 Methods of Controlling the High-side Pressure In a conventional subcritical heat pump process, the saturation pressure of the working fluid during heat rejection is inevitably linked to the satura- tion temperature, which in turn is determined by the heat balance in the condenser. In a transcritical CO2 system, however, the gas cooler (high- side) pressure and the temperature are independent variables, and the pressure can be calculated using the real gas equation: p⋅V=n⋅R⋅T⋅Z (A2) A11 Temperature [°C]PDF Image | Residential CO2 Heat Pump System for Combined
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CO2 Organic Rankine Cycle Experimenter Platform The supercritical CO2 phase change system is both a heat pump and organic rankine cycle which can be used for those purposes and as a supercritical extractor for advanced subcritical and supercritical extraction technology. Uses include producing nanoparticles, precious metal CO2 extraction, lithium battery recycling, and other applications... More Info
Heat Pumps CO2 ORC Heat Pump System Platform More Info
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