State of the Art of Hydrogen Storage and Transportation

The global energy transition is driving the adoption of hydrogen as a key vector, especially green hydrogen produced from renewable sources. For large-scale implementation, it is essential to address the challenges associated with its efficient and safe storage and transportation.

 Hydrogen Storage

Hydrogen can be storage using different methods, each with its own particular characteristics, advantages, and challenges. Choosing the appropriate method depends on the scale, storage duration, end-use, and geographical conditions.

Gaseous Hydrogen (GH2)

Hydrogen storage in the gaseous phase is the most common and relies on compression to increase its density. One kilogram of hydrogen at ambient pressure occupies approximately 11,000 liters, while when compressed to 350 bar, the volume is reduced to 38 liters, and at 700 bar, it is around 20 liters. To achieve these pressures, mechanical compression systems (such as piston, diaphragm, and ionic compressors) and adsorption or metal hydride technologies are used. GH2 is stored in pressurized tanks of various sizes, from small cylinders to large-capacity tanks. Underground storage in salt caverns is also being investigated, offering high capacity, safety, and potential profitability. However, this technique depends on the geological availability of suitable formations and managing of the purity of the stored hydrogen.

Liquid Hydrogen (LH2)

Hydrogen is brought to a liquid state by cooling it to -253°C, which increases its volumetric density to 71 kg/m³. LH2 is stored in cryogenic tanks designed to minimize evaporation and loss of hydrogen due to boiling. This method is suitable for applications where energy density and mobility are essential, although its high energy consumption in the liquefaction process (approximately 25-30% of its contained energy) poses a challenge to optimization.

Hydrogen Carriers

Chemical hydrogen carriers allow for H2 storage and transport in more stable forms and under less extreme conditions than GH2 or LH2.

• Ammonia (NH): Obtained from hydrogen and nitrogen through the Haber-Bosch process. It can be stored in liquid form at -33°C and 1 bar or at room temperature under moderate pressure. Its hydrogen density makes it attractive for large-scale applications, although its conversion back to hydrogen requires additional processes.
• Methanol (CH): It is liquid at room temperature and can be used as a hydrogen carrier through reforming processes. Its ease of handling and compatibility with existing infrastructure make it an option to be studied.
• Liquid Organic Hydrogen Carriers (LOHC): Compounds such as toluene hydrogenated to methylcyclohexane allow hydrogen to be safely stored at ambient temperature and pressure. The main advantage of such compounds is that they can use conventional liquid fuel transport infrastructure and be stored for long periods without significant losses.

Storage in Solids

Some technologies allow hydrogen storage in solid materials, offering advantages in safety and volumetric density.

• Metal Hydrides: Elements such as magnesium (Mg), boron (B), and aluminum (Al) can form compounds with hydrogen, allowing its storage in a solid state. Hydrogen is released through controlled temperature (up to 300°C in the case of MgH₂) and pressure.
• Carbon Frameworks and MOFs: Nanomaterials such as graphene, carbon nanotubes, and metal-organic frameworks (MOFs) have hydrogen storage capacity through adsorption (currently <5% by weight at room temperature), although large-scale implementation is still under development.

 Hydrogen Transport

Hydrogen transport is another crucial aspect for its widespread adoption. Several options are under development for their implementation.

Maritime Transport

Maritime transport is essential for international hydrogen trade, especially for long distances. The main alternatives considered are:

• Liquid Hydrogen (LH2): Transporting LH2 in specialized tankers is an option, although it requires maintaining cryogenic temperatures. Projects such as HESC are exploring this option. It is compared to transporting LNG (Liquefied Natural Gas), although there are technical differences.
• Ammonia (NH3): Transported in vessels designed for different pressure and refrigeration levels, using infrastructure similar to that for liquefied natural gas (LNG).

• LOHC: They allow safe transport at ambient temperature and pressure, with a lower risk of flammability, high chemical stability, and the advantage of being reused in multiple cycles without significant degradation.

Pipeline Transportation

Pipeline transportation is an efficient option for transporting large volumes of hydrogen over medium and long distances.

• Hydrogen Pipelines: New hydrogen-dedicated pipelines (hydroducts) are being planned and built worldwide. It is estimated that there are around 91 such projects (according to the 2023 European Hydrogen Backbone report), totaling 30,300 kilometers, which will begin operating around 2035.
• Converting Natural Gas Pipelines: The feasibility of adapting existing infrastructure for hydrogen transportation is being investigated, which would allow the use of existing networks and reduce implementation costs. However, hydrogen presents challenges such as the embrittlement of metallic materials, permeability in certain pipelines, and the need for special coatings to minimize losses and ensure safety. Furthermore, compatibility with existing compression and distribution systems is a key aspect of these studies.

• Blending: Hydrogen can be blended with natural gas in controlled proportions (typically ≤20% by volume) for distribution in existing networks, although this requires adaptations to end applications and possible subsequent separation processes.

Road Transport

Hydrogen can also be transported by tanker truck, either as compressed hydrogen in high-pressure cylinders or as liquid hydrogen in cryogenic tanks. This option is more flexible for short and medium distances, although it is less efficient for large volumes compared to other alternatives.

Conclusion

The storage and transportation of hydrogen is constantly evolving. Various technologies are being optimized, from gaseous and liquid storage to innovative solutions such as chemical carriers and solid storage. The choice of the most appropriate solution will depend on technical, economic, and logistical factors specific to each application. Continued research and development are essential to overcome existing challenges and enable the large-scale implementation of hydrogen as a fundamental pillar of a sustainable energy system.

For more information:

Hydrogen Technologies: A Practical Approach