Performance optimization of ORC-based waste heat recovery system integrated with marine engine using alternative fuels under different operating conditions

Improving ship energy efficiency is becoming increasingly important in reducing global warming. Waste heat recovery (WHR) is one of the effective methods to improve the energy efficiency of ships. In this study, energetic, exergetic and economic performances of two recuperative organic Rankine cycle (ORC) systems recovering exhaust gas waste heat from liquefied natural gas (LNG), liquefied petroleum gas (LPG), marine diesel oil (MDO) and methanol (MeOH) fueled engines with the same brake power are investigated under various engine and ORC operating conditions. Low global warming potential (GWP) and zero ozone depletion potential (ODP) working fluids R1233zd(E), R1336mzz(Z), RE245fa2, R245ca, and R365mfc are selected for the ORC WHR systems. The genetic algorithm is used for thermo-economic performance optimization using the specific investment cost (SIC) as the objective function. Moreover, the sensitivity of the objective function to the variables of engine load, exhaust gas temperature, fuel consumption, turbine inlet pressure, and condensation temperature are determined. According to the results, the integration of ORC WHR into engines, significantly improves brake thermal efficiency by more than 5 %, particularly for the methanol-fueled engine having relatively lower efficiency and higher fuel consumption compared to other engines. Moreover, R365mfc shows the best thermodynamic and economic performance as the SIC of ORC WHR integrated MeOH-fueled engine is approximately 7.9 % less than other ORC systems. Besides, R1336mzz(Z) emerges as a promising candidate for onboard exhaust gas ORC WHR systems requiring a very low GWP (<5).