Thermo-economic analysis and multi-objective optimization of S-CO2 Brayton cycle waste heat recovery system for an ocean-going 9000 TEU container ship

Abstract

Supercritical CO2 recompression Brayton cycle (S-CO2 RBC) can be used as waste heat recovery system in a ship because of the advantages of high efficiency, compact size and lower cost. This paper proposes a modified S-CO2 RBC system, namely the dual turbine-alternator-compressor (TAC) S-CO2 RBC system, which to be used to recover the waste heat from the main engine exhaust gas of an ocean-going 9000 TEU container ship. Mathematical and simulation models for the modified system are established to investigate the improvement in the performance. Parametric study is conducted to analyze the effects of key thermodynamic parameters on the system performance. The Imperialist Competitive Algorithm - based multi-objective optimization is carried out to get the modified system’s optimal operating parameters, aiming to maximize the system net power output, energetic efficiency and exergetic efficiency as well as to minimize the heat exchanger area per unit power and the levelized cost of energy. The results show that the performance of the modified system is strengthened significantly, whose highest net power output, energetic efficiency and exergy efficiency are 452.2 kW, 24.53% and 41.47%, approximately 26.58%, 16.67% and 16.90% higher than that of the S-CO2 RBC system. Compared with the S-CO2 RBC system, the modified system is more compact because its heat exchanger area per unit power decreased by 44.08%. The modified system can contribute to decreasing the ship auxiliary engine fuel consumption and the Energy Efficiency Design Index by about 1.01% and 1.02%, and improving the thermal efficiency of the ship main engine system by 3.23%. The results of this study can provide theoretical support for the application of the dual TAC S-CO2 RBC waste heat recover system on ships.