Salt caverns to store hydrogen?
Deep underground, France hides deposits of crystallized salts over a large part of its territory. In total, more than 20,000km² of salt, hundreds of meters thick, were deposited by the ocean before being covered by sediments and other rocks. For several decades, industrialists have been exploiting the properties of salt by digging caves to store gas. With the extremely low permeability and porosity of the crystallized salt, this solution could be adapted in order to meet the storage needs of hydrogen, the energy vector of the future.
Salt caverns are created artificially, simply by injecting water into the subsoil to dissolve the salt. With a two-pass pipe, it is then possible to recover the brine, i.e. the water saturated with salt, and gradually dig caves mesuring several hundred thousand cubic meters.
The cavern on which Pierre Bérest, Emeritus Professor at École Polytechnique within the Solid Mechanics Laboratory (LMS*), is currently working on is a "small cavity" located in Bresse (French region): only 8000m3. Eventually, the largest cavities will have a volume of 1 million m3 and will be able to contain 180 million m3 of gas under pressure.
Specific controls for hydrogen
Before setting up this device on an industrial scale, it is necessary to ensure its viability. Hydrogen is a very explosive gas. Therefore, its storage can cause concern and the solution of salt cavities has the advantage of isolation from ambient air, and strategic camouflage in case of conflict. Nevertheless, leak monitoring is necessary.
Pierre Bérest and his team at the LMS have already worked on extremely precise devices to detect tiny variations in pressure in this type of cavity.
But "leakage, which is the most obvious point of attention, is not the only thing that needs monitoring," explains Pierre Bérest. On the geological time scale, salt behaves like a liquid: over decades, we observe that salt flows and cavities can tend to close up. "This phenomenon can be accentuated by cycling (regularly filling and emptying the cavity with gas), thus reducing the volume of the cave initially created."
Finally, Pierre Bérest is interested in a phenomenon that has not yet been studied: thermodynamic exchanges within the cavern. The presence of humidity related to the creation of the cavity, combined with the difference in temperature and pressure between the top and bottom of the cavity, are likely to generate a real underground "meteorology" (rain, fog, etc.). In addition to their interest in the pure understanding of these phenomena, this research would ensure that the circulation of water does not risk humidifying the gas, thus making it less pure for subsequent uses of hydrogen.
A link in the large energy chain
The research work carried out at the LMS is a link in a more global energy chain dedicated to hydrogen and tested by the European HyPSTER project. As a consortium of European companies and organizations, this collaboration, led by Storengy (a subsidiary of ENGIE), aims to create an integrated pilot involving:
- The collect of green electricity
- The conversion of this electricity into hydrogen by water electrolysis
- Salt cavity storage discussed in this article
- Hydrogen recovery and distribution to end-users
This project has received funding from the European Commission to carry out the different stages of its creation as well as storage and exploitation of this energy vector, thus allowing to test and demonstrate its concrete interest for the future.
* LMS: A joint research unit of the CNRS and École Polytechnique - Institut Polytechnique de Paris