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Appendix A – CO2 as a Working Fluid in Heat Pumps Table A1 Important physical and thermophysical properties of CO2, propane (R-290) and selected HFCs (RnLib, 2003). Property CO2 R-290 R-407C3 R-410A R-134a Molar mass [kg/kmol] 44.01 Normal boiling point [oC] -78.41 Critical temperature [oC] 31.1 Critical pressure [MPa] 7.38 Saturation pressure at 0oC [MPa] 3.49 ∆h at 0oC [kJ/kg]2 231 Density, sat. liquid at 0oC [kg/m3] 928 Density, sat. vapour at 0oC [kg/m3] 97.8 44.10 86.20 -42.1 -43.8 96.8 87.3 4.25 4.63 0.48 0.57 375 209 493 1237 10.3 19.7 72.59 -51.6 -26.2 72.5 101.1 4.95 4.07 0.80 0.29 221 199 1171 1295 30.5 14.4 1) Sublimation temperature. Triple point 2) Specific enthalpy of evaporation 3) The mean of the bubble point and dew at -56.6°C and 0.518 MPa point has been used as the datum temperature A1.1 High Operating Pressure CO2 has an especially high critical pressure and low critical temperature. As a consequence, the operating pressure in CO2 heat pump systems will typically be 5 to 10 times higher than that of plants using propane or HFCs. This is demonstrated in Figure A1 on the following page, which compares the pressure-enthalpy diagrams of CO2 and HFC-134a. At 4.07 MPa, which is the critical pressure or HFC-134a, the saturation tempera- ture of CO2 is only 5.3oC. A1.2 Heat Rejection at Supercritical Pressure Due to the low critical temperature (31.1oC), most CO2 heat pumps will have to operate in a so-called transcritical cycle (Lorentzen, 1990). This means that the CO2 absorbs heat from the heat source at subcritical pressure, and gives off heat above the critical point at supercritical pressure (high-side pressure). Unlike conventional subcritical heat pump cycles, the heat is not given off by means of condensation of the fluid in a condenser but by cooling of the high-pressure CO2 gas in a gas cooler. The temperature drop for the CO2 gas during heat rejection is denoted the temperature glide. A2 102.03PDF 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
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