A novel waste heat recovery system combing steam Rankine cycle and organic Rankine cycle for marine engine

Aiming to reduce fuel consumption and pollutant emission, a new kind of waste heat recovery system (WHRS) is proposed to recycle the waste heat of marine engine. In the proposed system, steam and organic Rankine cycles are combined to convert the waste heat of the exhaust gas and jacket cooling water of marine engine into mechanical energy. A portion of the jacket cooling water is used as the working fluid for the steam Rankine cycle subsystem to efficiently utilize the heat of jacket cooling water and avoid increased ship weight caused by the extra water. The performance of the proposed system for recovering the waste heat of a 14-cylinder two-stroke marine engine was simulated and compared to performance of the WHRSs based on a single steam Rankine cycle (SSRC) and a dual pressure organic Rankine cycle (DPORC). The results show that the proposed system could improve the thermal efficiency of engine by 4.42% and reduce the fuel consumption by 9322 tons per year at an engine load of 100%, while a WHRS based on a SSRC and a DPORC can only increase the thermal efficiency by 2.68% and 3.42%, respectively. In addition, effects of evaporation pressure, the degree of superheat and engine load on the output work, the exergy destruction rate, and the thermal and exergy efficiency of the proposed system were analyzed to provide information for system optimization.