Characteristics of the S-CO2 Brayton cycle for full-scale multi-condition diesel engines

With the diesel engine test data of the flue gas as the initial conditions and the boundaries, the arrangement of the S-CO2 recompression Brayton cycle (SCRBC) was established and the optimization of the S-CO2 Brayton cycle (SCBC) system for 6EX340EF, 6L16/24, and CHD622V20 diesel engine were studied. Additionally, the model accuracy was validated with the test data from Sandia National Laboratories (SNL). Meanwhile, the parameter optimization determined the optimal system operating conditions via a multi-objective genetic algorithm (MOGA). Finally, the feasibility of the SCBC applied in flue gas waste heat recovery (WHR) for marine engines was comprehensively assessed from the efficiency, net output power, fuel consumption rate, and exergy. The results showed that with the recompression Brayton cycle layout in the optimal configuration, the low-speed marine diesel engine could reach 1.68% of the maximum total efficiency improvement and 6.43 g/kWh of the fuel consumption rate reduction at 100% load, the maximum net output power increased to 178.14 kW; the medium-speed marine diesel engine could reach 2.37% of the maximum total efficiency improvement and 11.60 g/kWh of the fuel consumption rate reduction at 100% load, the maximum net output power increased to 31.69 kW; the high-speed marine diesel engine could reach 1.00% of the maximum total efficiency improvement and 5.58 g/kWh of the fuel consumption rate reduction at 25% load, the maximum net output power increased to 21.54 kW. By comparing the SCRBC of the marine diesel engine, the high-speed engine has the highest efficiency of flue gas WHR, followed by the medium-speed engine and finally, the low-speed engine. The cooler, flue gas heat exchanger, and high-temperature recuperator (HTR) modules were the weak points of the SCRBC system through the exergy analysis, and further optimization of the SCRBC system can be extended to other engines to improve efficiency and emission.