Pascal Baylocq

In the context of the energy transition, hydrogen is poised to become a key energy source for power generation, heat generation, and mobility. For large-scale hydrogen storage, underground techniques offer several advantages over surface storage, including reduced environmental footprint, enhanced safety, and cost-effectiveness.

Underground storage of natural gas has been practiced for over a century, with more than 3,000 caverns/porous reservoirs in the world. Similarly, hydrogen storage in underground salt caverns has been used for industrial processes for several decades. In preparation for the energy transition, several pilot projects for hydrogen underground storage in salt, depleted fields, and saline aquifers have been developed.

There are five main techniques for underground hydrogen storage:

1. Salt Caverns: The most mature method, involving the creation of caverns by injecting freshwater into a geological layer of salt. The salt acts as a natural sealant, and this technique has been used for over 50 years.


2. Porous Rocks: Utilizing naturally porous rocks covered by impermeable rock to create a geological trap. This method offers high storage capacities and has been used in the past for hydrogen mixed with methane and carbon dioxide.


3. Hard Rock Caverns for Liquid Organic Hydrogen Carriers (LOHC): Constructing caverns in hard rock to store hydrogen converted into a liquid carrier, such as ammonia. Contact with water is to be avoided for some of these LOHC. This may require the installation of a liner.


4. Direct Injection of Gaseous Hydrogen into lined rock caverns: Involves injecting gaseous hydrogen into a rock cavern, with high pressure necessitating a liner. This technique is being actively developed in Europe.


5. Direct Injection of liquid hydrogen into lined rock caverns: Involves injecting liquid hydrogen into a lined rock cavern using cryogenic techniques. This method is still in the early stages of development.

The capital expenditure (CAPEX) for these techniques varies significantly based on geology, storage capacities, and operational requirements. For example, storage solutions based on porous reservoirs have an estimated cost of about 20€/kg, while salt caverns technology costs around 35€/kg. Storing gaseous hydrogen in mined, lined rock caverns is more challenging to assess, with costs ranging from 250€/kg to 500€/kg. There is no available data on the CAPEX for storing liquid hydrogen in mined, lined rock caverns, but costs are expected to be higher. For comparison, surface storage CAPEX varies between 1,000 and 2,000€/kg.

In conclusion, underground hydrogen storage has a history of over 50 years, and ongoing research and development efforts are necessary to mitigate risks and expand the solution portfolio.