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152 M.-H. Kim et al. / Progress in Energy and Combustion Science 30 (2004) 119–174 Fig. 42. Heat exchangers of CO2 mobile air-conditioning system prototype (right) compared to R-134a baseline (left). (a) Evaporator, (b) condenser or gas cooler. Test matrices were defined for the purpose of developing and validating component and system simulation models, as well as supporting data-to-data comparisons of CO2 and R-134a at normal, seasonal and extreme operating conditions. The results demonstrated that comparable cooling COPs could be obtained at most operating conditions (Fig. 43). The CO2 system was sized to provide approximately equal capacity at the extreme high-temperature (54.4 8C) idling condition, as shown in Fig. 44. However, its COP fell 10% short of the baseline system at that point. At outdoor ambient temperatures below 40 8C where most air-con- ditioning operation occurs, the CO2 system COP exceeded that of the baseline R-134a system up to 40%. The lines in Fig. 44 connect points having equal outdoor air flow rates, Vc; while the indoor air temperatures and air flow rates are shown in boxes. The 22 data points represent only a subset of the conditions investigated. Several factors were found to be responsible for these results: (1) higher real compressor efficiency due to lower compression ratio; (2) higher evaporating temperature due to superior thermophysical properties and greater tolerance of pressure drop due to the slope of the vapor pressure curve; (3) closer approach temperature differences at the gas cooler outlet. More detailed data can be found in Refs. [133,135 – 137]. Experiments were conducted on both systems in both steady state and cycling modes. Cycling behavior is very sensitive to the type of expansion device used. A fixed orifice is simplest, but is unable to maintain the high-side pressure at its COP-optimizing level during the on-cycle as reflected by the large differences between refrigerant flow rates shown in Fig. 45 [137]. It is also clear that the backpressure valve Fig. 43. Measured performance of automotive air-conditioning systems.PDF Image | CO2 Vapor Compression Systems
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