Secure storage and movement of hydrogen molecules

Study findings show that challenges such as low storage density, high costs and inadequate infrastructure persist despite progress in high-pressure storage and cryogenic liquefaction

A key challenge to build out hydrogen infrastructure is the safe transport of the tiny molecules. Research papers show advancements are being made in the safe, efficient and cost-effect movement of hydrogen, whether as a gas or liquid.

Whether transporting hydrogen as a gas or liquid, the key safety concerns include material selection, hydrogen embrittlement, leak management, and infrastructure adaptation. 

As indicated in the research paper A Review of Hydrogen Storage and Transportation: Progresses and Challenges mastering the production of hydrogen is a prerequisite to its utilisation. 

For hydrogen energy to be widely adopted the cost competitiveness and development of an extensive hydrogen infrastructure (which includes production facilities and refuelling stations) are necessary. But the scarcity of infrastructure is only problem number one.

Problem number two is that hydrogen’s low energy density presents secure storage and safe transport concerns, requiring efficient and safe solutions. The researchers from Guangxi University, China look at some of the strengths, limitations and technological progress on various hydrogen storage methods. 

Study findings show that challenges such as low storage density, high costs and inadequate infrastructure persist despite progress in high-pressure storage and cryogenic liquefaction. 

Storage methodologies  

Hydrogen can be stored in several forms depending on its physical state and intended use. The two primary hydrogen categories are gaseous and liquid, each with its own storage methods, advantages, and challenges. 

Compressed gaseous hydrogen storage: This is one of the most common hydrogen storage methods in hydrogen fuel cell vehicles or hydrogen-powered vehicles. Stored in high-pressure cylindrical tanks made from steel, carbon fibre or composite materials (typically at 350–700 bar / 5,000–10,000 psi). 

This method, while relatively simple, allows for rapid refuelling but requires large volumes (low energy density by volume) and there are safety concerns due to the high pressure requirement. 

Cryogenic liquid hydrogen storage: Liquid hydrogen (LH2) is predominantly used in aerospace applications but has broad potential applications in civilian and industrial sectors. The LH2 is cooled −253°C (20K) and stored in insulated cryogenic tanks. 

“Currently, stainless steel is the prevalent material choice for LH2 storage and transportation due to its excellent low-temperature performance and resistance to hydrogen embrittlement,” state the researchers. 

Research into high specific strength cryogenic materials for storage and transportation containers for aerospace is advancing, but in the civilian sector it is nearly untouched. 

Organic liquid hydrogen storage: This involves two steps: hydrogenation of hydrogen-lean molecules and dehydrogenation of hydrogen-rich molecules. Hydrogen-lean organic liquids also referred to as Liquid Organic Hydrogen Carriers (LOHCs) have “more potential for the safety and transport efficiency” but this technology still needs a lot of development, though its characteristics render it advantageous for long distance transport. 

“Additionally, LOHCs are compatible with existing hydrocarbon infrastructure, including pipelines and fuel stations,” the researchers highlight. 

Solid material hydrogen storage: Currently, the most popular solid hydrogen storage materials include categories represented by magnesium hydride, sodium borohydride, and ammonia borane.

“The primary focus of current research is the identification of potential materials that offer both high hydrogen storage capacity and safety, with non-toxic characteristics.”

Underground hydrogen storage (UHS): This involves storing hydrogen gas in underground reservoirs such as depleted gas/oil fields or aquifers, or in salt caverns. Considered a potential solution for hydrogen energy storage and dispatchability, because the gas has a large volume at ambient conditions and requires high-pressure or cryogenic storage to meet energy demands.

“Its primary goal is to store surplus electricity generated from renewable sources, converting it into hydrogen and releasing it for energy use when needed. UHS facilities are capital-intensive investments, so the economic viability will be affected by the cost of electricity for water electrolysis and expenditure on the construction of storage sites.”

Metal-organic frameworks hydrogen storage: Metal-organic frameworks (MOFs) are porous materials with a periodic network structure formed by the assembly of metal ions and organic ligands through coordination interactions. In MOFs, metal ions typically serve as nodes or central points, while organic ligands generally function as linkers. While MOFs have been extensively studied for gas storage, their application in hydrogen storage still faces challenges. 

Hydrogen transportation options  

Because of the complex physical and chemical properties of hydrogen, any process involving hydrogen requires careful handling. 

Transportation takes place in an even less stable environment than storage, so prioritising safety is paramount. The cost of transporting the molecule, in whatever form, will form a large part of determining the feasibility of widespread adoption. 

Hydrogen is transported by pipeline or road, with rail considered a form of road transportation, or by sea. 

Seaborne transportation: Similar to the common transportation of petroleum and liquefied natural gas, the physical and chemical storage of LH2 has economic benefits for long distance oversea transportation due to its small footprint and the completeness of transportation infrastructure.

“In addition to traditional transportation of LH2, LOHCs and MOFs are also considered in oversea transportation for the future.”

However, for the current stage, the challenge of transporting LH2 lies not in the distance of transportation, but in the complexity of secure storage while in transit. 

Road transportation: Due to the limited load and capacity of trucks and trailers, two factors have a significant impact on the cost and energy consumption of transportation: the proportion of hydrogen in the total payload of a vehicle and the energy density.

Maximising the hydrogen storage capacity per unit volume and weight of the container is contingent on the hydrogen storage method. For high-pressure transport, hydrogen is usually moved on a truck or trailer, using a gas cylinder. In most cases it is considered highly desirable for the weight of the hydrogen to constitute around 2% of the total payload.

To increase the amount of transported hydrogen, lighter tank materials that can operate at higher pressures need to be manufactured and filling and venting processes of high-pressure hydrogen tanks also require special attention. 

Pipeline transportation: This is a relatively low-cost option for moving around large quantities of hydrogen gas, when compared to transport by road. But, if hydrogen energy is to become a viable alternative to fossil fuels, large-scale hydrogen pipeline infrastructure is essential. The existing hydrogen pipeline infrastructure is relatively small in scale and mostly operated by large commercial hydrogen suppliers for refineries and chemical plants.

The idea of blending hydrogen gas with natural gas and transporting it through natural gas pipelines is gathering steam, as these have been widely constructed and are operating worldwide, with mature technologies and broad coverage.

Using existing natural gas pipeline infrastructure to move around hydrogen could reduce the cost and time of constructing new pipelines, improve the efficiency of hydrogen energy infrastructure development and lower costs.

“Technically, natural gas pipelines share similarities with hydrogen transportation, such as pipeline conveyance and high pressure. Although hydrogen has smaller molecular size, appropriate modifications and technological measures can enable natural gas pipelines to meet the requirements for hydrogen transportation.” ESI

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