Exergetic efficiency optimization for an irreversible Brayton refrigeration cycle

Exergetic efficiency optimization that combines exergy concept and finite-time thermodynamic theory has been carried out for an irreversible Brayton refrigeration cycle. Multi-irreversibilities considered in the system include finite rate heat transfer, internal dissipation of the working fluid and heat leak between heat reservoirs. Exergetic efficiency defined as the ratio of rate of exergy output to rate of exergy input of the system is considered as the objective index. The goal of exergetic efficiency optimization is to maximize this index. The maximum value of the exergetic efficiency can be determined analytically. The results are compared with those obtained from the traditional coefficient of performance. The influences of heat leak between heat reservoirs and temperature ratio of two reservoirs on the exergetic efficiency are investigated by numerical calculations. The allocation of a fixed total thermal conductance between the two heat exchangers is also discussed. The results show that the method of exergetic efficiency optimization is an important and effective criterion for the evaluation of an irreversible Brayton refrigeration cycle.