Energy, exergy, economic, and environmental (4E) analysis of S-CO2 Brayton cycle for low-speed engine exhaust gas under different conditions

To achieve energy efficiency, reduce emissions and optimize the operation of ocean-going vessels, five configurations of SCSBC (S-CO2 simple Brayton cycle), SCCBC (S-CO2 recuperative Brayton cycle), SCHBC (S-CO2 reheating Brayton cycle), SCIBC (S-CO2 intercooling Brayton cycle), and SCRBC (S-CO2 recompression Brayton cycle) were constructed for the exhaust gas waste heat recovery (WHR) under different loads of the marine LSE (low-speed engine) HHM-6EX340EF on the EBSILON platform using the bench test data. Experimental data from Sandia National Laboratories (SNL) was utilized to validated the accuracy of the model. Meanwhile, the effect of Brayton cycle parameters on exhaust gas WHR for different configurations was analyzed. The SCBC (S-CO2 Brayton cycle) was optimized from the perspectives of system configuration, cycle parameters, and working fluids preference. An indicator evaluation of the SCBC system was conducted from the perspectives of 4E analysis. Parameter optimization of the SCBC was performed using the MOGA and TOPSIS methods to improve the power, economy, and environmental performance of marine diesel engines. The results showed that the optimal configuration is a combination of using 100 % load exhaust gas as the input, CO2-H2S as the working medium, and the recompression cycle as the system configuration. The 4E analysis showed that the optimized SCRBC has a recovered power of 178.1 kW, Brayton cycle efficiency of 19.22 %, LCOE of 3.004 × 10−2 ($·kW−1·h−1), RCO2 of 9.68 × 105 (kg·a−1) and eeff of 23.29 %. A technical basis was provided for the application of the exhaust gas waste heat recovery S-CO2 Brayton cycle system in marine diesel engines.