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3 – Theoretical Background and System Analysis ♦ In order to achieve a high COP for the transcritical CO2 heat pump cycle, useful heat has to be rejected over a wide temperature range (i.e. a large temperature glide for the CO2), and the resulting CO2 outlet temperature from the gas cooler as well as the high-side pressure must be relatively low. ♦ At temperatures and high-side pressures relatively close to the critical values, the COP curves are very steep and even minor varia- tions in the CO2 outlet temperature from the gas cooler lead to a significant change in the COP (ref. Appendix A2.3, Optimum High- Side Pressure at Constant CO2 Outlet Temp. from the Gas Cooler). ♦ At CO2 outlet temperatures below 30oC, the COP curves are virtually linear, and the COP increases on average by roughly 1% per degree Kelvin drop in the CO2 outlet temperature. ♦ At CO2 outlet temperatures below 30°C, the COP increases by roughly 1.5 to 3.5% per 0.1 MPa drop in the high-side pressure. ♦ In order to achieve a COP of 3.5, the CO2 outlet temperature from the gas cooler must be lower than 8 to 24oC at high-side pressures ranging from 8 to 10 MPa, respectively. When assuming a minimum CO2 outlet temperature of 10oC, a COP above 4 can only be achieved at high-side pressures below approximately 8.2 MPa. The COP of a transcritical CO2 heat pump is heavily affected by the eva- poration temperature and the isentropic efficiency of the compressor. This is illustrated in Figures 3.5 and 3.6, where the high-side pressure is kept constant at 9 MPa and the other boundary conditions are as in Figure 3.4. At CO2 outlet temperatures below 30oC, the COP increases by roughly 2.5% per degree Kelvin rise in the evaporation temperature, whereas the COP increases on average by 1.2% per percentage points rise in the isen- tropic compressor efficiency. Since the COPs for the transcritical CO2 heat pump cycle and the conven- tional subcritical heat pump cycle are depending on the temperature requirements and heating demands for the space heating and DHW systems, the component performance (ref. Appendix A1.5, Compressor Performance, and A1.6, Heat Exchanger Performance) as well as the system design, it is impossible to make a general comparison of the system performance for the cycles. 31PDF 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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