Optimisation of Brayton cycle CO2-based binary mixtures: An application for waste heat recovery of marine low-speed diesel engines exhaust gas

The energy-saving capabilities and efficient operation of marine low-speed diesel engines (MLDE) is a key emphasis for the main power source for ocean transportation. The purpose of the supercritical carbon dioxide recompression Brayton cycle (SCRBC) is to capture and utilise waste heat emitted by the engine's exhaust gas. However, the SCRBC performance will be severely affected by the large temperature fluctuations of ocean-going vessels during operation and high ambient temperatures in the cabin. A SCRBC model was built using the exhaust gas test data as the boundary conditions and validated using the Sandia National Laboratory (SNL) test data. The physical characteristics of the working fluids were evaluated by adding other fluids to CO2 in specific proportions to modify the critical point and increase cycle efficiency. The results demonstrated that employing CO2-based binary working fluids with low alkane and hydrogen sulfide (H2S) enhanced the recovery power, with the most significant increase obtained by the addition of 16.48 % H2S, which increased the power by 9.72 kW and improved the Brayton cycle efficiency by 3.31 %. Compared to the MLDE at 100 % load, the total efficiency increased by 1.77 % and the BSFC decreased by 6.76 (g kW- 1 h- 1) using CO2-H2S as the working fluid. The analysis of the SCRBC system component exergy losses showed that the cooler had the highest exergy losses. Adding other fluids to CO2 reduced the exergy losses of each component with the SCRBC system exergy losses decreasing from 162.97 to 129.90 kW and the exergy loss efficiency decreasing from 24.24 % to 22.65 %. The use of CO2-based binary working fluids specifically designed for ambient temperature may be expanded to other engines to enhance the efficiency of waste heat recovery.