Retrofitting diesel-powered ships to run on hydrogen presents a complex but increasingly viable pathway to decarbonizing maritime transport. The process involves significant technical modifications, economic considerations, and regulatory compliance. Unlike new-build hydrogen vessels, retrofits must work within existing structural and operational constraints, making the engineering challenges distinct.

The primary technical hurdle in converting diesel ships to hydrogen is the propulsion system. Diesel engines are not inherently compatible with hydrogen combustion due to differences in ignition properties and energy density. Hydrogen has a high auto-ignition temperature (around 585°C) compared to diesel (approximately 210°C), requiring modifications to the combustion process. Two main approaches exist: hydrogen-fueled internal combustion engines (ICEs) and hydrogen fuel cells. Retrofitting typically favors ICE adaptations due to lower costs and space requirements compared to fuel cell integration.

For hydrogen ICE retrofits, the engine must be adjusted to handle hydrogen’s rapid combustion characteristics. This involves modifying fuel injection systems, installing spark ignition (since hydrogen cannot rely on compression ignition like diesel), and reinforcing engine components to withstand higher peak pressures. The air-to-fuel ratio must also be carefully managed to prevent pre-ignition or knocking. Some retrofits use dual-fuel systems, blending hydrogen with diesel or other fuels to ease the transition, though this reduces emissions benefits.

Space constraints are a major challenge. Hydrogen storage demands significantly more volume than diesel for equivalent energy output. Compressed hydrogen gas (350–700 bar) requires bulky tanks, while liquid hydrogen (cryogenic at -253°C) needs insulated vessels to minimize boil-off. Retrofits often repurpose existing fuel tanks or cargo space, but this can compromise cargo capacity or operational range. For example, a mid-sized cargo ship retrofitted with compressed hydrogen may sacrifice 20–30% of its cargo hold for storage, depending on the desired range.

Safety modifications are critical. Hydrogen’s wide flammability range (4–75% in air) and low ignition energy necessitate robust leak detection, ventilation, and explosion-proof electrical systems. Existing ship structures must be evaluated for hydrogen embrittlement risks, particularly in pipelines and storage tanks. Fire suppression systems must be upgraded, as traditional water-based systems are less effective for hydrogen fires.

Classification society approvals add another layer of complexity. Organizations like DNV, ABS, and Lloyd’s Register have evolving guidelines for hydrogen retrofits, but standards are not yet fully codified. Shipowners must engage with these societies early to ensure compliance. Key considerations include risk assessments, material compatibility, and emergency protocols. The approval process can take 12–24 months, depending on the project’s novelty and regulatory jurisdiction.

Economic aspects of retrofits hinge on capital expenditures, operational costs, and fuel availability. Retrofitting a medium-sized vessel for hydrogen propulsion can cost $5–15 million, with fuel storage and engine modifications being the largest expenses. Operational savings depend on hydrogen fuel prices, which vary widely by region and production method. Green hydrogen (from renewables) remains costly ($4–8/kg), though gray hydrogen (from fossil fuels with carbon capture) is cheaper ($1–3/kg). Maintenance costs may decrease due to fewer moving parts in hydrogen ICEs compared to diesel engines, but fuel cell systems require specialized servicing.

Several retrofit case studies illustrate these challenges and solutions. In 2021, the European-funded HyShip project converted a short-sea cargo vessel to run on liquid hydrogen. The retrofit involved replacing one of the ship’s two diesel engines with a hydrogen ICE and installing a 200 m³ LH2 tank. The project highlighted the trade-offs between storage volume and range, with the vessel achieving a 300-nautical-mile range at reduced cargo capacity. Another example is the Norled ferry in Norway, which was retrofitted with a 200 kW fuel cell system alongside its existing diesel engines. The hybrid approach minimized space constraints while allowing zero-emission operation in protected fjord areas.

Infrastructure limitations also affect retrofit viability. Few ports currently offer hydrogen bunkering, restricting operations to specific routes. Shipowners must factor in additional costs for mobile refueling solutions or on-board hydrogen generation via electrolysis, though the latter increases energy demands.

Despite these hurdles, retrofits offer a pragmatic step toward maritime decarbonization, particularly for older vessels with remaining service life. The total cost of ownership over 10–15 years may become competitive as hydrogen production scales up and carbon pricing mechanisms tighten. However, the business case remains highly context-dependent, with favorable conditions in regions like the EU or California where subsidies and emissions regulations are strongest.

Future retrofits will benefit from advancing technologies such as metal hydride storage, which could reduce space requirements, and modular fuel cell systems that simplify integration. Standardization of retrofit protocols and classification rules will also lower barriers. For now, each project requires a bespoke approach, balancing technical feasibility, regulatory compliance, and economic viability. The lessons from early adopters will shape broader industry uptake as the hydrogen maritime sector matures.