Green hydrogen has great potential to contribute to the global effort of reducing carbon dioxide (CO2) emissions. However, developing green hydrogen as a major energy source requires resolution of many technical and economic challenges, and storage is one of the greatest. Hydrogen produced in processes that utilize renewable energy assets is generally categorized as green hydrogen.
Hydrogen is the lightest and one of the most abundant chemical elements on Earth. Though it is an energy-dense fuel by weight, it has poor energy density by volume. For comparison, it has approximately one-third the volumetric energy density of natural gas, which means it must be compressed and stored at ultrahigh pressures — up to 900 bar — to achieve the same relative energy output. Alternatively, hydrogen can be stored in a liquefied state, but this requires it to be chilled to minus 423 degrees Fahrenheit, a temperature that is close to absolute zero.
Due to these factors, storage is key to unlocking hydrogen’s potential.
Currently, there are several hydrogen storage methods in various stages of research and development. Among those, underground or undersea storage is getting significant attention.
Salt caverns, deep saline aquifers, rock caverns, and depleted oil and gas fields — both onshore and offshore — are considered viable possibilities for storage. In salt formations, storage can be created via hot water injections. Salt layers exist in most parts of the world, but only a small percentage have the purity and thickness necessary to be used as viable storage facilities.
There is a wealth of geological data available on underground storage facilities that have been used for natural gas, and these formations have proven to present low risk of contamination. However, when converting these facilities from natural gas to hydrogen storage, complex engineering challenges should be anticipated. Proper surveys, studies and testing of these formations must be conducted to analyze how hydrogen will react to subsurface minerals and fluids as well as the capacity to maintain hydrogen at proper temperature and pressure limits.
Additionally, designing pipelines and transporting hydrogen from onshore and offshore facilities adds to the complexity of these options.
Both high-pressure and medium-pressure above-ground storage tanks are proven for industrial applications. Tanks have long been viable for refineries, fertilizer plants and other industrial facilities. However, if tanks are scaled up for large-volume storage, significant challenges must be addressed.
High-pressure tanks can store compressed hydrogen up to 700 bar (approximately 10,000 psig), but the process can consume up to 30% of the hydrogen being stored. Storage in medium-pressure tanks designed for pressures of around 138 bar (approximately 2,000 psig) comes with a lower energy penalty, though the hydrogen stored does not have the same energy density.
Cryogenic storage of liquid hydrogen is feasible within tanks as well, though the process of super cooling hydrogen to ultra-cold temperatures comes with an energy penalty of approximately 50%.
Testing Feasibility in Los Angeles
A project being developed by the Los Angeles Department of Water and Power (LADWP) could become an important demonstration of how hydrogen can reduce carbon emissions in power production. LADWP is participating in a plan to build a combined-cycle replacement for the coal-fired Intermountain Power Plant in Utah that could burn a blend of 30% hydrogen/natural gas on startup, gradually increasing to 100% hydrogen firing. Nearby salt domes in Utah would be developed for bulk energy storage with capacity to store many weeks of fuel.
The LADWP project gives a glimpse into a possible future for hydrogen as an energy storage medium. The amount of renewable generation that will be required to meet zero-carbon legislative mandates will trigger the need for dispatchable, load-following generation with days or weeks of capacity to maintain production during periods when renewable output is low or during periods of high grid demand.
Areas like California with significant excess renewable capacity can direct that production toward electrolysis rather than simply curtailing it. Green hydrogen, or synthetic fuels produced from it, can meet the need for flexible generation in a way most other energy storage options cannot, simply by virtue of scale.
Electric utilities are well positioned to be major participants in the evolution of hydrogen as an energy source that helps reduce carbon emissions from fossil fuels.