Optimum thermodynamic conditions for upper pressure limits of transcritical carbon dioxide refrigeration cycle

It is well known that the choice of upper pressure limit in a transcritical CO2 refrigeration cycle is independent of the gas cooler approach temperature. This is in contrast to sub-critical cycles, where the condensing pressure is invariably governed by the ambient conditions. The criteria used for limiting the upper pressure limit in a transcritical cycle are the state of maximum COP for a given set of evaporating and gas cooler exit temperatures. The latter is governed by the local ambient conditions and the possible approach. This paper provides a thermodynamic basis for the evaluation of this pressure for the case of ideal compression and with some compressors available in the market. In addition, it also provides an additional criterion that minimizes the cycle irreversibility which is predominantly due to gas throttling. This paper evaluates the pressure limits for these two criteria for some typical evaporating temperatures and ambient conditions. The possible compressor discharge temperatures in each case are calculated and criteria for two-stage compression are identified.