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Appendix A – CO2 as a Working Fluid in Heat Pumps Table A5 Measured heat transfer coefficients (hCO2) for in-tube cooling of pure supercritical CO2 in round tubes (RT) and micro- channel tubes (MPE) at various operating conditions. Authors Zingerli and Groll (2000) Dang and Hihara (2002) Yoon et al. (2003) Pettersen et al. (2000) [oC] [kW/(m2K)] ID Tube [mm] 6.4 (RT) 6.4 (RT) 7.7 (RT) 0.8 (MPE) M p [kg/(m2s)] [MPa] 930 8-12 950 8-10 225-450 7.5-8.8 600-900 8-10 T hCO2 &∫ & (A1) Q=m⋅ c dT=m⋅(h −h )=H −H & p 2323 T2 where m& is the CO2 mass flow rate and cp is the isobaric specific heat capacity. Subscripts 2 and 3 refer to the inlet and outlet of the gas cooler. As previously illustrated in Figure A4, the specific heat capacity of supercritical CO2 is virtually independent of temperature at high and low operating pressures. However, at pressures and temperatures close to the critical point, the property reaches tremendous values. As a consequence, there will be considerable variations in the temperature gradient for the CO2 gas during heat rejection at relatively low supercritical pressures. A10 20-95 5-35 15-80 3-14 30-65 2-21 15-70 3-17.5 Zingerli and Groll (2000) showed that 2 and 5% oil concentration in the supercritical CO2 reduced the heat transfer coefficient on average by 15 and 25%, respectively. Since a high pressure drop can be tolerated in the gas cooler and a high mass flux is beneficial for the heat transfer efficiency, Pettersen et al. (1998) concluded that air-cooled gas coolers can have mass fluxes typically ranging from 600 to 1200 kg/(m2s). Rieberer and Halozan (1997) recommended a mass flux of 1500 kg/(m2s) or even higher for water- cooled tube-in-tube gas coolers. A2 The Transcritical CO2 Heat Pump Cycle A2.1 Temperature Gradients During Heat Rejection Assuming isobaric conditions during the transcritical heat rejection, the heating capacity of the gas cooler is calculated as: T3PDF 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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