Advancing maritime decarbonisation: Design and optimisation of ammonia-fuelled propulsion systems

Innovative marine propulsion systems play a pivotal role for decarbonising global maritime transportation. This study investigated an ammonia-fuelled hybrid powertrain system that integrates ammonia internal combustion engines with a battery energy storage system for a large containership, which were based on the credible and validated physical engine and battery models and real operation profiles of the case ship. A multi-objective optimisation framework based on the Non-Dominated Sorting Genetic Algorithm II (NSGA-II) is developed for component sizing and selection. In parallel, a rule-based Energy Management Strategy (EMS) is employed to optimize power distribution and improve operational efficiency. The proposed hybrid configuration is benchmarked against a real-world powertrain system powered by heavy fuel oil engines and a scenario powertrain solely powered by the same type of ammonia engines. The case study on the representative containership demonstrates that the ammonia-fuelled propulsion system has significantly lower carbon emissions than the conventional propulsion system, which achieves a 79.36 % reduction in CO₂ emissions. The hybrid ammonia-fuelled powertrain system has further 0.15 % CO₂ emission reductions with an Energy Efficiency Existing Ship Index (EEXI) of 0.8085 gCO₂/(t∗nmi). Nevertheless, fuel costs account for over 86 % of lifecycle expenditure, and high ammonia genset prices increase Capital Expenditure (CAPEX), resulting in higher overall costs of the ammonia-fuelled propulsion systems. These results indicate that ammonia as a marine fuel offers substantial decarbonisation potential and its economic feasibility depends on future ammonia cost reductions and optimal marine propulsion system design.