The Challenges of Underground Hydrogen Gas Storage

ABSTRACT: While hydrogen as a gas (H2) has been stored in salt caverns on the American Gulf Coast for the last 40 years, it’s attributes are a challenge for underground storage. Natural gas is a proxy in understanding the role and the place of hydrogen in the energy economy. Natural gas has been stored in the United States for over 100 years. Certainly, the argument can be made for such an understanding, but hydrogen has its own peculiarities which set it apart from other more traditional forms of energy. As the energy and electricity requirements for civilization continue to grow at an unrelenting pace, we will need to understand the best possible fit for each energy type in our suite of options. Storing hydrogen then, will be vitally important as we look for ways to reliably and affordably store electricity generated from solar and wind farms. The considerations for storing hydrogen are not limited to geologic features, i.e. domal salt, bedded salt, depleted oil and gas reservoirs, or saline aquifers. Underground storage must be understood both in terms of the rock itself and the surface system which it supports. Underground storage protects the product from inclement weather, acts of vandalism or terrorism, and provides volumes that would be spatially and economically prohibitive on the surface. Also needed for viable hydrogen storage are safety practices, substantial surface infrastructure buildout, consistent storage and transportation regulations both at the federal and at the state levels, adequate water supply for hydrogen production near the storage site, electricity for the electrolyzers, and the pathway from underground storage to end users. Depending on both the storage type and end use, there may be gas purity levels that must be attained prior to distribution. Assessing the geologic structures for storage, except for domal salts in Texas or the world class bedded salt in Utah, are currently being conducted at the bench scale. Should widespread hydrogen production and consumption take place in the United States and around the globe, field scale trials of different geologic structures and rock types will be necessary as uniform underground storage is not available across the continent. Hydrogen’s unique characteristics affect its storage potential: small sized molecule, unfavorable compressibility factor, tendency to permeate metals, wide flammability range, and potential for feeding hydrogen sulfide producing microbes all differentiate it from other gases stored below ground. Lastly, the storage requirements of hydrogen, compared to natural gas, will be on the order of four times the total volume. This means that in addition to suitable infrastructure on the surface, the underground storage types will need to be broadly available and reliable for hydrogen producers and consumers. Natural gas storage can serve as an initial proxy, but the unique qualities of hydrogen mean current underground natural gas storage cannot easily be converted to hydrogen gas storage. Hydrogen more than ten times costlier than natural gas when compared on the basis of energy content. The US stores between 9 and 10% of its annual natural gas consumption with approximately 87% of natural gas stored in depleted oil and gas reservoirs, 10% in salt caverns, and the remaining 3% in saline aquifers. Hydrogen is currently stored successfully in salt dome caverns in Texas, will soon be stored in bedded salt caverns in Utah, and is currently stored as a mix with methane in saline aquifers in Europe. If we are going to develop a robust hydrogen economy, considerably more storage capacity will need to be developed. Trials will need to move from bench scale to full scale. And as we forge ahead into the unknown, we will be ready to address the challenges of storing hydrogen gas - safely and reliably - underground.