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Appendix A – CO2 as a Working Fluid in Heat Pumps Consequently, in brine-to-water and water-to-water heat pump systems operating with moderate CO2 outlet temperatures from the gas cooler, the use of a suction gas heat exchanger will only marginally improve the COP. A2.4 Optimum High-Side Pressure when Incorporating Real Gas Cooler Performance In the previous section it was presupposed that the CO2 outlet temperature from the gas cooler was constant when varying the high-side pressure. However, in a real gas cooler the high-side pressure may have a conside- rable impact on the CO2 outlet temperature, which in turn will change the heating capacity of the system and alter the optimum high-side pressure. Pinch Point in the Gas Cooler The minimum temperature difference ∆Tm between the hot CO2 flow (H) and the cold water or air flow (C) in a counterflow gas cooler occurs at the pinch point. When the pinch point occurs inside the gas cooler it will represent a constraint for the heat transfer process, and therefore affect the heating capacity and the CO2 outlet temperature from the gas cooler. For a given gas cooler design, the temperature profiles and the location of the pinch point are determined by the inlet temperatures, the local heat capacity flow rates (CP-values) and the local heat transfer coefficient (U- value). The heat capacity flow rate is defined as (Linnhoff et al., 1984): CP ≡ m ⋅ c where m& is the mass flow rate and cP the local value of the specific heat capacity of the fluid. The temperature-heat (T-Q) diagram is suitable to represent the thermal characteristics for the hot and cold flows in the gas cooler. With reference to Eq. (A2), the slope of the curves in the T-Q diagram is expressed as: & p (A5) dT= 1 =1 (A6) dH m⋅c CP p & A low CP value leads to a steep T-Q curve and vice versa. Due to the considerable variations in the specific heat capacity of supercritical CO2 at relatively low high-side pressures, the isobars will appear as curves with a sway-backed shape. On the other hand, the T-Q curves for water and air (heat sink) will be more or less linear, since their specific heat capacities are virtually constant in the temperature range from 0 to 100oC (±0.4%). A19PDF 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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