Theoretical simulation models have been developed to analyze and evaluate the first and second-law based performance of basic and modified R-22, R-134a and transcritical CO_2 vapor compression refrigeration systems. The use of a two-phase expansion device as a throttle valve replacement is the only modification made to the basic system. This allows for an unbiased comparison of the different refrigeration technologies. The following conclusions can be made: 1-The optimum heat rejection pressure corresponds to a maximum system coefficient of performance and a median value of system irreversibility. For the operating conditions investigated, the optimum heat rejection pressure of a modified system is on average 5% lower than that corresponding to a basic transcritical CO_2 system.
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