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132 M.-H. Kim et al. / Progress in Energy and Combustion Science 30 (2004) 119–174 Fig. 22. Relative change in heating capacity (left) and heating COP (right) for R-22, R-134a and CO2 at varying evaporating temperature, for a condenser/gas cooler exit temperature of 40 8C. Reference point: 0 8C evaporating temperature. Results for CO2 are shown at COP-optimum high-side pressure, and with relative data for other high-side pressures. Based on ideal cycle calculations without subcooling or superheating. pressure, maximum motor load, and compressor discharge temperature limitations. 3.5. Approach temperature and its importance In applications where the rejected heat is not needed, the thermodynamic losses in heat transfer can be limited by allowing the CO2 exit temperature from the gas cooler to approach the air- or cooling water inlet temperature as closely as possible. Heat exchanger design calculations and practical experience show that it is possible to obtain a temperature approach of a few degrees, even in air-cooled coils. Assuming that the mean temperature difference is approximately equal for a given heat exchanger size, the temperature approach must necessarily be lower when heat is rejected over a temperature glide than when it is rejected at constant temperature. Owing to the relatively high throttling loss and the gliding heat rejection temperature, the cooling COP for a CO2 system is very sensitive to the gas cooler refrigerant exit temperature. Fig. 23 shows the relative change in ideal cycle COP at varying condenser/gas cooler outlet tempera- ture, normalized by the COP at 40 8C [31]. While the ideal COP for R-22 and R-134a is increased by about 40% through a 10 K condenser outlet temperature reduction, the effect on the CO2 cycle COP is nearly twice as high (70%). The close temperature approach that is obtained in CO2 gas coolers therefore contributes significantly to practical COP improvement. 3.6. Analysis of transcritical system energy efficiency Comparisons of energy efficiency and/or TEWI (total equivalent warming impact) between baseline systems and CO2 systems have to account for two important factors: The effect of climate, i.e. a seasonal data for comparisons of energy consumption, and a system approach, including the effects of supplementary heat and secondary power requirements for fans or pumps. Most refrigeration, air-conditioning and heat pump systems are operated in a varying climate. Comparisons based on design point operation apply conditions that rarely occur—typically at an extreme ambient temperature. In order to obtain a realistic comparison of annual or seasonal energy consumption, realistic climatic data should Fig. 23. Relative change in cooling COP for R-22, R-134a and CO2 at varying refrigerant exit temperature from condenser/gas cooler (i.e. minimum heat rejection temperature). Evaporating temperature 0 8C. Reference point: 40 8C exit temperature. Based on ideal cycle calculations without subcooling or superheating.PDF Image | CO2 Vapor Compression Systems
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