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3 – Theoretical Background and System Analysis From Figures 3.11 and 3.12 as well as Appendix A2, The Transcritical CO2 Heat Pump Cycle, the following observations and comments can be made: ♦ At moderate high-side pressures, the CO2 isobar has a sway-backed shape, and the heat rejection process is quite similar to a conven- tional heat pump cycle with de-superheating (a-b), condensation (b-c) and sub-cooling (c-d) of the working fluid. ♦ The counter-flow tripartite gas cooler enables production of high- temperature DHW, and the CO2 outlet temperature before throttling may be considerably lower than the return temperature in the space heating system. ♦ For a fixed gas cooler design, the CO2 outlet temperature from the tripartite gas cooler, and with that the heating capacity and COP of the heat pump unit, is heavily affected by the temperature levels in the space heating and DHW systems. ♦ Due to the serial connection of the counter-flow gas cooler units, changes in heat transfer areas or boundary conditions for one of the gas cooler units will influence the heating capacity and the tempe- rature profiles for the two other units. ♦ As a result of the variation in the specific heat capacity at pressures and temperatures near to the critical point, the high-side pressure will affect on the temperature differences between the supercritical CO2 and the fluids to be heated, and with that the heating capacity of each of the three gas cooler units. Consequently, for an integrated CO2 heat pump system there is an optimum high-side pressure that will lead to a maximum COP (ref. Appendix A2.4). ♦ By using a tripartite gas cooler, a small temperature approach (∆TA) may be achieved at a moderate high-side pressure, which in turn will lead to a large heating capacity and a moderate power input to the compressor. As a consequence, the COP of an integrated CO2 heat pump in the combined mode may be even higher than that of the DHW mode, where the optimum high-side pressure typically ranges from 10 to 12 MPa (Nekså et al., 1998). Hot Water Heating Only (DHW Mode) The heat rejection process in the DHW mode is illustrated in a T-h dia- gram in Figure 3.13. The high-side pressure is 10 MPa, while the city water and DHW set-point temperature are 6.5oC and 70oC, respectively. 41PDF 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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