Hydrogen Storage
What is hydrogen storage?
Producers can separate hydrogen from water through electrolysis, powered by solar cells or wind turbines. Later, on converting hydrogen into electricity, the only by-product is water.
Between plant production and the fuel cell, safe and efficient hydrogen storage is essential for this energy source to become practicable and economical.
How does hydrogen storage work?
Presently, gas pressurization is the most widespread method. Its advantages include lower capital outlay and operating costs than other hydrogen energy storage options.
Depending on the type of material, hydrogen storage vessels weigh between 20 and 100 kilograms for every kilogram of gas storage capacity.[1]
What are three methods of hydrogen storage?
Broadly, the types of hydrogen storage are:
- High-pressure gas compression.
- Liquefaction, which requires cryogenic temperatures below minus 252.9 degrees Celsius.
- Solid state storage, using metal hydride hydrogen storage or carbon nanotube adsorption.
Liquefaction is preferable in space applications because of its high volumetric and gravimetric efficiency. The disadvantages are low energy efficiency and high cost.
Given their high volumetric efficiency, metal hydrides and carbon nanotubes are promising hydrogen storage technologies. They look set to play a vital role in future energy development. While these complex chemical compounds are dense and have high fuel efficiency, reaction kinetics are slow. The technique is increasingly common and relatively safe; it is usually possible to handle the metal alloys directly in air.[2]
What are the advantages of hydrogen storage?
Achieving extended autonomy is a technical challenge in electrically-powered vehicles. To run significant distances on rechargeable batteries, the weight of sufficient cells loads the vehicle chassis and affects its maneuverability.
In contrast, hydrogen fuel cells enable the direct generation of electricity thanks to the chemical reaction between hydrogen and oxygen. Gravimetric capacity – i.e., the energy yield for a specific weight – is higher than with batteries. Thus, vehicles are lighter and can travel further between top-ups.[3]
An example of a metal hydride-based hydrogen storage system was the Japanese long-range autonomous underwater vehicle named Urshima, built in 1998. The sub-sea vehicle then used AB5 alloy hydride fuel cells to cruise 317 kilometers in sixty hours.
Liquid hydrogen has been in use for years on NASA space missions. Under atmospheric conditions, liquid hydrogen storage delivers an energy density three times higher than gaseous hydrogen compressed at 350 bars. However, due to the subzero temperatures, liquefaction is energy-intensive and the process requires adequate thermal insulation.
What materials can store hydrogen?
A popular hydrogen storage option is gas cylinders at pressures of 350, 700 or 900 bars. Hydrogen storage tank materials are steel, aluminum, carbon fiber, epoxy resins or tough polyethylene.[4]
The selection of the hydrogen storage method is mission-specific. Although thermally insulated and efficient, a liquid hydrogen storage tank loses around one percent of its contents per day due to evaporation, also termed boil-off.
In contrast, solid-state hydrogen storage involves the gas reacting to create hydride compounds. Each metallic compound behaves differently depending on its chemical structure, ranging from salt-like hydrides to molecular and metallic hydrides.
Certain types of metal hydride are rechargeable and, therefore, reusable.
Because efficiency is relatively high, these methods offer several benefits. They could play a significant role in achieving a sustainable hydrogen economy. The choice of metal hydride type depends on usage requirements.[5]
Hydrogen storage cost
Pressure vessel technology is inexpensive and easy to implement, but has relatively low volumetric storage density. Nevertheless, safer operation at higher storage pressure could lessen this drawback. In the future, economical underground hydrogen storage could be possible on an industrial scale within salt caverns and subsurface shafts.[6]
From a life cycle point of view, solid hydrogen storage is environmentally friendly. In addition, advances in hydrogen storage technology are bringing costs down.
Liquefaction is suitable for space applications due to its high volumetric efficiency. However, process costs are high. Also, it is often impractical because the energy necessary to produce liquid hydrogen is about a third of the power that the liquefied gas later provides.
What else should I know about hydrogen storage?
Hydrogen is much lighter than air, a mix of approximately one-fifth oxygen and four-fifths nitrogen. Therefore, leaks from hydrogen storage flow upward and tend to disperse quickly.
Because the accumulation of hydrogen around the source of a leak is unlikely compared to heavier fuel gases, it is less hazardous. However, sufficient ventilation is advisable, with immediate dilution to less than 1 percent by volume.[7]
Conclusion
Hydrogen storage containers are neither hazardous nor environmentally detrimental, but the safety concerns for hydrogen storage are similar to those for other fuel gases.
In conclusion, metal hydride and carbon adsorption are proven hydrogen storage technologies. Moreover, hydride hydrogen storage is a promising solution where safety is paramount. To discover more or to discuss your hydrogen fuel cell requirements, please go here.