Maritime transport of hydrogen is a critical component of the global energy transition, requiring efficient, safe, and scalable carrier solutions. Three leading options—liquid hydrogen (LH2), ammonia, and liquid organic hydrogen carriers (LOHCs)—are under evaluation for their suitability in shipping hydrogen over long distances. Each carrier presents distinct trade-offs in energy density, volumetric efficiency, handling complexity, and end-user readiness, with real-world insights drawn from projects like the Suiso Frontier.
**Energy Density and Volumetric Efficiency**
Energy density determines how much energy can be transported per unit volume or mass, directly impacting voyage economics.
- **LH2**: Liquid hydrogen offers high gravimetric energy density (120 MJ/kg), but its volumetric energy density is low (8.5 MJ/L) due to cryogenic storage requirements (-253°C). The Suiso Frontier, the world’s first LH2 carrier, demonstrated the challenges of maintaining ultra-low temperatures, requiring heavily insulated tanks that reduce cargo capacity.
- **Ammonia**: With a volumetric energy density of 11.5 MJ/L (at 20°C) and gravimetric density of 18.6 MJ/kg, ammonia outperforms LH2 in volume efficiency. It liquefies at -33°C or under moderate pressure (10 bar at 25°C), simplifying storage compared to LH2. However, its hydrogen content by weight is only 17.6%, necessitating cracking for end-use.
- **LOHCs**: Compounds like methylcyclohexane (MCH) or dibenzyltoluene offer near-ambient storage conditions (liquid at room temperature) but have lower gravimetric (2-3 wt% H2) and volumetric (5-6 MJ/L) densities. Their advantage lies in compatibility with existing oil tanker infrastructure, though dehydrogenation adds energy penalties.
**Handling Complexity**
The operational challenges of each carrier vary significantly in storage, transport, and conversion.
- **LH2**: Cryogenic storage demands advanced insulation and boil-off management. The Suiso Frontier reported boil-off rates of 0.2-0.3% per day, requiring reliquefaction or venting. Handling LH2 necessitates specialized training due to risks like embrittlement and flammability.
- **Ammonia**: Easier to store than LH2 but toxic, requiring strict safety protocols. Leak detection and mitigation are critical, as ammonia poses health and environmental risks. Existing ammonia shipping infrastructure (120+ ports globally) reduces adoption barriers, though cracking to extract hydrogen remains energy-intensive.
- **LOHCs**: Non-toxic and stable at ambient conditions, LOHCs leverage conventional tanker designs. However, dehydrogenation requires high temperatures (300°C) and catalysts, adding cost and complexity. Hydrogenation (loading) must occur before shipping, tying production to carrier availability.
**End-User Readiness**
The maturity of infrastructure and technology for each carrier influences their near-term viability.
- **LH2**: Limited to niche applications like aerospace or fuel cell vehicles, with minimal port infrastructure. Japan’s HySTRA project, involving the Suiso Frontier, aims to establish LH2 supply chains but faces scalability hurdles.
- **Ammonia**: Well-established global trade (20 million tons/year) for fertilizers ensures ready infrastructure. Pilot projects like Australia’s HESC project explore ammonia as a hydrogen carrier, leveraging existing terminals. However, cracking technology needs scaling for hydrogen markets.
- **LOHCs**: Compatible with oil infrastructure, enabling rapid deployment. Japan’s Chiyoda Corp demonstrated MCH-based hydrogen transport, but dehydrogenation plants are scarce. End-users must invest in release units, slowing adoption.
**Comparative Summary**
| Carrier | Gravimetric Energy Density | Volumetric Energy Density | Storage Conditions | Infrastructure Maturity |
|---------------|---------------------------|--------------------------|-------------------------|------------------------|
| LH2 | 120 MJ/kg | 8.5 MJ/L | -253°C, cryogenic | Low |
| Ammonia | 18.6 MJ/kg | 11.5 MJ/L | -33°C or 10 bar | High |
| LOHCs | 2-3 wt% H2 | 5-6 MJ/L | Ambient | Moderate |
**Conclusion**
Ammonia currently leads in maritime transport due to its balance of energy density, existing infrastructure, and handling feasibility, despite toxicity and cracking challenges. LH2 offers high purity but suffers from costly cryogenics and low volumetric efficiency, as seen in the Suiso Frontier trials. LOHCs provide a compromise with ambient storage and oil-compatible logistics but face energy penalties in hydrogen release. The choice depends on route-specific factors, end-use requirements, and infrastructure investments, with ammonia holding an early advantage for large-scale deployment.