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components. Furthermore, an operation in these conditions will end up in some components like the compressor underperforming and parts deteriorating. Refrigerant charge is an important parameter that affects the performance of a heat pump system. There is an optimum charge at which the system yields the best COP. Performance deterioration is more severe at undercharged condition than at overcharged condition mostly because of the effects of less mass flow rates and reduced compressor efficiency. The CO2 system performance is reported to be more sensitive to system refrigerant charge compared to that in conventional systems because of its low liquid to vapor density ratio [14]. Therefore, the refrigerant charge must be controlled more precisely in a CO2 system than in other conventional systems to achieve high performance under various operating conditions. Charge optimization of the trans-critical CO2 cycle is directly related with optimizing the gas cooling pressure to maximize the COP (or heating capacity) since the second derivative of the pressure with respect to the enthalpy at constant temperature for CO2 becomes zero in supercritical region near the critical point where the gas cooler outlet condition is approaching. This characteristic would result in the gas cooler capacity increase in superior degree to that of the compressor power increase with the gas cooling pressure increase until the gas cooling pressure reaches to the pressure corresponding to this reflection point. Since the higher refrigerant charge will result in higher system operating pressures and the higher ambient temperature will result in higher optimum gas cooling pressure [15, 16]. Entropy generation can be used to analyze the second law efficiency for the trans critical CO2 system under various charging conditions. Expansion loss is the dominant factor affecting system performance at undercharged condition while the gas cooler loss became the major parameter at overcharged conditions. The losses in the gas cooler increases as the amount of refrigerant charge increase due to the increase of the heat transfer rate. The enthalpy at the inlet of the evaporator significantly varies with a change of the gas cooler pressure; this change greatly varies the cooling capacity. Generally, the enthalpy at the exit of the gas cooler decreases with an increase of gas cooler pressure, resulting to lower quality at the inlet of the evaporator. Although the refrigerant temperature at the exit of the gas cooler gradually increase with the addition of refrigerant charge at undercharged conditions, the quality at the inlet of the evaporator decrease due to a reduction in the enthalpy at the exit of the gas cooler, thus increasing the cooling capacity. However, for overcharging conditions, the performance of the CO2 system decreases with refrigerant charge because the enthalpy difference across the compressor increase more than that across the gas cooler. Furthermore, compression ratio decrease with the addition of the refrigerant charge; this decrease increase the refrigerant mass flow rate. Therefore the compressor power consumption slowly but continuously increases with normalized charge, while the cooling capacity rapidly increases at lower normalized charges and then the slope gradually decreases with an increase of the normalized charge[14]. 324PDF Image | TYPICAL INITIAL OUTPUT OF A CO2 HEAT PUMP
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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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