Design of a combined organic Rankine cycle and turbo-compounding system recovering multigrade waste heat from a marine two-stroke engine

As a mature technology for utilizing surplus exhaust pressure energy, turbo-compounding can significantly redistribute multigrade waste heat in marine two-stroke engines. This paper aims to provide a comprehensive thermodynamic study and design criteria for the combined organic Rankine cycle (ORC) and turbo-compounding system. Firstly, the turbo-compounding system applied to the 6EX340 two-stroke engine and ORC systems designed with double-source and three-source configurations are described. The thermodynamic performance of ORC systems using four working fluids is then analyzed, followed by a thermodynamic and environmental assessment of various combined systems. The results show that utilizing the power turbine bypass results in an additional 10.5 % increase in exhaust energy at the rated condition, while the scavenging air waste energy decreases by around 26.4 % compared to the base engine. For the base engine, the three-source ORC using R245ca shows the greatest potential with a 5.4 % improvement in fuel economy, a CO2 emissions reduction of 579.7 t, and a payback period of 7.4 years. Further combined with a power turbine, the double-source ORC using toluene as the working fluid outperforms the three-source ORC, resulting in a 9.3 % improvement in fuel economy, a CO2 emission reduction of 950.6 t, and a payback period of 4 years.