Cooling-load density optimization for a regenerated air refrigerator

Abstract

A performance analysis and optimization of a regenerated air refrigeration cycle with variable-temperature heat-reservoirs is carried out by taking the cooling-load density, i.e., the ratio of cooling load to the maximum specific volume in the cycle, as the optimization objective using finite-time thermodynamics (FTT) or entropy-generation minimization (EGM). The model of a regenerated air refrigerator is presented, and analytical relationships between cooling-load density and pressure ratio, as well as between coefficient of performance (COP) and pressure ratio are derived. The irreversibilities considered in the analysis include the heat-transfer losses in the hot- and cold-side heat-exchangers and the regenerator, the non-isentropic compression and expansion losses in the compressor and expander, and the pressure-drop losses in the piping. The cycle performance comparison under maximum cooling-load density and maximum cooling-load conditions is performed via detailed numerical calculations. The optimal performance characteristics of the cycle are obtained by optimizing the pressure ratio of the compressor, and searching for the optimum distribution of heat-conductances of the hot- and cold-side heat-exchangers and regenerator for the fixed total heat-exchanger inventory. The effect of heat capacity rate matching between the working fluid and heat reservoirs on the cooling-load density is analyzed for the cycle. The influences of the effectiveness of the regenerator as well as the hot- and cold-side heat-exchangers, the efficiencies of the expander and the compressor, the pressure-recovery coefficient, and the temperature ratio of the heat reservoirs on the cooling-load density and COP are examined and illustrated by numerical examples.