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5 – Experimental Results for the water and the CO2. The reason was that the application of the logarithmic temperature difference (LMTD) in Eq. 5.1 for each gas cooler unit presupposes constant specific heat capacity for both fluids. Whereas the specific heat capacity (cp) of water is practically invariable between 5 and 90oC, i.e. 4.192 kJ/(kgK) ±0.5%, the specific heat capacity of super- critical CO2 is heavily affected by changes in the high-side pressure and the temperature as shown in Figure 5.46. 6 5 4 3 2 1 7.5 8 9 10 11 12 8 7.5 12 11 10 9 0 20 40 60 80 100 120 Temperature [°C] Figure 5.46 The specific heat capacity of supercritical CO2 at varying temperatures and high-side pressures (RnLib, 2003). During the design of the tripartite gas cooler, the Gnielinski correlation for single-phase flow (VDI, 1993) was used to calculate the convective heat transfer coefficient for the supercritical CO2. Figure 5.47 presents the results at the actual operating range for the prototype CO2 heat pump unit, i.e. 5 to 115oC fluid temperature, 8 to 11 MPa high-side pressure and a CO2 mass flux of 840 kg/(m2s). Zingerli and Groll (2000) measured the impact of oil on the heat transfer coefficient and the pressure drop during in-tube cooling of supercritical CO2 (ref. Appendix A1.6). The ID of the test tube was 6.35 mm and the mass flux was 950 kg/(m2s). A 5% oil concentration reduced the con- vective heat transfer coefficient on average by 25%, but the maximum value at each high-side pressure still occurred at the pseudocritical tempe- rature. Consequently, since the tube dimensions and the operating condi- tions of the prototype CO2 heat pump unit were virtually the same, it is reasonable to conclude that the drop in heat transfer efficiency in compa- rison to operation with pure CO2 was in the same order of magnitude. 153 Cp [kJ/kgK]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
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