Exergetic efficiency and exergy-based ecological function performance optimizations for two irreversible simple Brayton refrigeration cycle models

Abstract

Air (Brayton) refrigerator has some application potentials nowadays. Based on two irreversible models of constant- and variable-temperature heat reservoir simple Brayton refrigerators established in previous references, this paper further derives exergetic efficiencies and ecological functions, and provide exergy-based comprehensive comparative analyses and optimizations for the two cycle models. Comparative studies among coefficient of performance (COP), cooling load, exergetic efficiency and ecological function characteristics are performed, including general performance analyses for fixed effectivenesses of heat exchangers and performance optimization by optimizing heat conductance distribution and optimizing heat capacity rate marching between heat-reservoir and working substance. For constant-temperature heat reservoir cycle, when select optimal pressure ratio, exergetic efficiency indicator is more reasonable than cooling load and ecological function indicators. When optimizing heat conductance distribution, cooling load indicator is almost the same as exergetic efficiency indictor. For variable-temperature heat reservoir cycle, the reasonable range of pressure ratio is that slightly larger than optimal one at maximum COP. For fixed pressure ratio, the difference between two optimal heat conductance distributions corresponding maximum cooling load and maximum exergetic efficiency is small. Various fixed design parameters have their influence features on the general performances and optimal performances of the cycle models, they should be considered specifically and pointedly. The major difference comparing with classical thermodynamic analysis is the consideration of heat-transfer loss in heat exchangers herein.