The assessment of alternative fuel and engine power limitation utilisation in hybrid marine propulsion systems regarding energy efficiency metrics

The adoption of alternative fuels is vital for meeting maritime decarbonisation targets. While various options exist, liquefied natural gas (LNG) is the leading choice, with methanol gaining ground. Integrating LNG and methanol in hybrid propulsion systems (HPSs) improves operational efficiency and ensures compliance with energy efficiency standards. This study explores the environmental benefits of HPSs using a scenario-based approach in which the existing propulsion system of a Ro-Ro (Roll-on/Roll-off) cargo ship is replaced with HPSs powered by LNG, methanol, or conventional fuels. Results demonstrate that implementing HPS alone on the vessel reduces fuel consumption by up to 21% across all scenarios by managing power fluctuations in the main engines. The LNG-HPS scenario lowers the attained Carbon Intensity Indicator (CII) by 36% to 4.13, keeping the CII rating at level A until the end of 2026. The methanol-HPS scenario achieves a fuel reduction of up to 22%, yet none of the scenarios meets the Energy Efficiency Design Index (EEDI) Phase III threshold of 7.65. To surpass this threshold, vessel speed reduction applications with alternative fuel utilisation, are evaluated. This integration improves the case ship's EEDI to 6.2 for LNG and 7.3 for methanol scenarios, exceeding threshold values for energy efficiency metrics.