Proposal and evaluation of a combined refrigeration system for engine waste heat recovery based on a supercritical CO2 Brayton cycle

In recent years, engine waste heat utilization technology has become one of the essential directions for green energy-saving development. In this study, the design of a combined refrigeration system for engine waste heat recovery under a two-stage supercritical carbon dioxide Brayton cycle is carried out, and based on the engine top-cycle energy flow and reactive flow characteristics, combined with thermodynamic and economic analysis models, the thermal and reactive efficiencies of the S-CO_2 Brayton cycle system are investigated. The multi-objective optimization model for waste heat recovery was created with hydropower efficiency and investment return as its objectives. The results showed that the minimum values of annual operating cost and initial investment cost of the combined system under the optimized conditions were 104.8 and 505.2 thousand RMB, respectively, and the total system energy loss was 7.25%. By further analyzing the top cycle parameters of the combined system, it is concluded that the combined system has better thermo-economic performance, and the results can provide some references for the combined refrigeration with engine waste heat recovery.