Comparative Design Optimization of Low-Temperature Fuel Cell Systems Powered by Pure Hydrogen and Hydrogen Carriers for Marine Applications

The maritime sector is facing growing pressure to reduce greenhouse gas emissions, and short- to medium-range vessels, such as ferries and small cruise ships, are well-suited for driving this transition due to their routinely scheduled routes, regular port access, and moderate energy requirements. Among the promising zero-emission technologies, low-temperature proton exchange membrane fuel cells (PEMFCs) powered by hydrogen or hydrogen-based carriers offer a compelling alternative to conventional propulsion systems. However, the choice of the most suitable hydrogen vector - e.g. compressed hydrogen (CH₂), liquefied hydrogen (LH₂), ammonia (NH₃) or liquid organic hydrogen carriers (LOHC) - requires the identification of the optimal trade-off between technical feasibility, economic viability and ship design constraints. This paper presents a multi-objective mixed-integer linear programming (MILP) model developed to optimize the design and integration of PEMFC-based power systems on board representative vessel types: small to large ferries and a small cruise ship. The model incorporates detailed representations of hydrogen storage, fuel treatment units, PEMFC systems, and battery support to meet peak power requirements. The system simultaneously minimises the total life-cycle cost, including capital investment and operation and maintenance (O&M) expenses, and the cost due to payload reduction when the volume occupied by the new energy system exceeds that currently installed on board. The optimization framework enables direct comparison between different hydrogen carriers and conventional marine power systems under realistic operational conditions. Preliminary results indicate that no single hydrogen carrier offers a universal solution across all vessel types. LOHC, despite its ease of storage and transport, is always at a disadvantage due to high conversion energy requirements, high costs and complex processing infrastructure. Ammonia seems to be the most promising solution for large ships, especially when less stringent space constraints are required, due to the safety standards already in place for its use in shipping and the low cost of fuel storage and processing facilities to be hosted on board ship. For ferries, particularly those with shorter routes and space limitations, liquefied hydrogen (LH₂) emerges as the most balanced option in terms of cost, energy density and volume efficiency. It has been quantitatively demonstrated that battery systems play a crucial role in hybrid configuration by covering peak loads, thus allowing PEMFC installations to be downsized and improving the economy and resilience of the overall system. These results provide essential insights into the trade-offs that must be managed when choosing near-zero-emission marine fuels and the importance of adapting energy solutions to the specific characteristics of vessels.