Hydrogen Storage

Compact hydrogen storage is a critical component of hydrogen T&S networks, ensuring that hydrogen produced from various methods and processes may be effectively utilised and transported to the ‘end-use’ applications. However, efficient hydrogen storage has proved to be problematic and exhibits unique technical challenges owing to the low volumetric energy density of hydrogen and its high diffusivity nature, requiring advanced and technologically innovative storage solutions. Current hydrogen storage technologies allow for hydrogen to be stored as a compressed gas or cryogenic liquid for vessel-based storage applications, chemically bonded or physically absorbed in solid-state storage solutions, and as a derivative in Liquid Organic Hydrogen Carriers (LOHCs).

In gaseous terms, compressed vessel-based storage is a relatively well-developed hydrogen storage technology. To maximise storage capacity and enable fast refuelling, hydrogen is compressed to very high pressures and charged into carbon fibre composite or metal alloy tanks, specifically designed to withstand high pressures. Although attractive for fuel cell vehicles, compressed hydrogen storage tanks are hindered with technical challenges, mainly stemming from material compatibility issues, weight, high energy consumption, and safety concerns arising from leaks and explosion risks. For long-term gaseous hydrogen storage, underground cavernous storage has been discussed, which leverages natural geological formations for large-scale compressed hydrogen storage. While a promising solution, the process is still in its infancy, and pilot projects are required to address concerns regarding leakage, contamination, and cost-effectiveness.

Liquid cryogenic hydrogen storage provides a compact solution in limited space applications, and involves compression, heat rejection, and rapid expansion of hydrogen gas to produce a cryogenic liquid at atmospheric pressure. Cryogenic hydrogen is stored in specially constructed cryogenic tanks, usually double super vacuum insulated, that minimise heat transfer from the environment. Although offering many advantages over compressed hydrogen storage, such as higher volumetric energy density, cryogenic liquid hydrogen storage still has a long way to go before it can be safely commercialised. Challenges include an energy intensive liquification process (which significantly increases costs), safe and efficient boil-off management (dealing with the evaporated hydrogen after charging), and safety risks posed by cryogenic burns, leaks, and explosions.

Solid-state hydrogen storage has emerged as a promising technology, involving the storage of hydrogen within solid materials, usually through chemical absorption or physical adsorption. Chemical absorption involves hydride formation, where hydrogen reacts with certain metals or metal alloys to form solid hydride compounds. Physical adsorption is achieved when hydrogen is stored on the surface of materials with a high surface area (metal-organic frameworks, carbon nanostructures, and zeolites). Solid-state storage is perhaps the safest hydrogen storage method, as hydrogen is released instantaneously, which reduces the risk of leaks and explosions. Yet, several critical challenges need to be overcome, such as high temperature hydrogen release (hydrides) or ultra-low temperature operation (adsorption) requirements, weight, cost-effectiveness, and process reversibility.

Liquid Organic Hydrogen Carriers (LOHCs) are the latest development in hydrogen storage technologies, leveraging hydrogenation to chemically bind hydrogen to organic molecules, effectively creating a non-flammable liquid transportation medium. Common examples of LOHCs are toluene and methylcyclohexane. Catalyst induced dehydrogenation releases the bonded hydrogen, ready to be used upon reaching the ‘end-use’ application. The biggest advantage LOHCs offer are their non-flammability and non-explosiveness at ambient conditions, making them a safer choice over compressed or cryogenic liquid hydrogen storage. However, high costs and concerns regarding hydrogen purity are hindering their widespread adoption.

Hydrogen storage infrastructure is pivotal within the hydrogen supply chain, enabling the safe and efficient containment of hydrogen for various applications, achievable only through effective collaborations between the government, stakeholders, and the wider public. Ensuring a reliable supply involves addressing significant challenges such as material compatibility, energy density limitations, and cost-effectiveness.