Salt caverns: the key to storing hydrogen?
Projections show that by 2050 the energy consumed in Europe will be divided equally between electricity and hydrogen – replacing fossil fuels that currently account for the largest share. It is therefore critical for us to start getting a hold of how to manage this gas on a large scale, now. In particular, the entire chain hydrogen energy (production, transport, distribution and end-use) will require significant storage capacity, particularly if the primary production methods are intermittent, as is the case for wind turbines and solar panels.
To do this, we will need a system capable of connecting the production equipment, such as electrolysers, with the distribution network. This system will need to be able to store hydrogen en masse when it is produced and consequently make it available when the grid requires. To achieve this, we propose to store H2 in deep saline cavities, and with the hydrogen industry gaining momentum, we estimate that by 2050 Europe could need several hundred of them.
An existing solution
Salt caverns are widely used in Europe today, particularly for storing methane (‘natural gas’). They take advantage of the presence of salt layers or domes in the subsoil several hundred metres thick that extend over large areas. In France, the total extension of salt caverns is currently the order of 20,000 km2. The salt, which is not very permeable, can be easily dissolved to create caverns with oil wells to dig the rock down to the salt formation. Then, after some time, a cavern with a typical size of 500,000m3 is available. In theory, we could simply use these pre-existing caverns to store hydrogen. In theory, they would be able to storing around 6,000 tonnes of hydrogen at varying pressures of 6–24MPa – operated like an oxygen tank for diving.
Today in France there are about thirty gas storage cavities, spread over three sites. Some have been in operation for about fifty years. There are several hundred in the world, including a few hydrogen storage caverns already used by the chemical industry. Several industrial pilot projects for the production and use of hydrogen are organised around some of these existing salt caverns. In France, there is the Hypster project led by Storengy in Etrez and supported by the European Union, Hygéo by Terega, HdF and BRGM in Carresse-Cassaber and Hygreen led in Manosque by Storengy-Geostock. If these projects exist, it is because there are still some challenges to overcome if we are to achieve large-scale hydrogen storage in salt caverns.
1/ Preventing leaks
Even for conventional structures, social acceptability is the major challenge posed by the construction of new large-scale energy facilities (nuclear power plants, dams, wind farms). As such, it is up to the designers to identify and explain to the public the particularities of the structures from the point of view of their safety and the solutions provided. Salt caverns are no exception. Since the hydrogen molecule is particularly mobile, the major problem is the sealing of the metal access shaft, which is several kilometres long. A number of accidents or incidents from the past are known and well described in the 2,000 or so salt caverns for the storage of liquid or gaseous hydrocarbons in operation around the world. For the most part, these incidents occurred a long time ago, as the industry progressively adopted a principle known as the “double barrier” whereby the pressure is continuously monitored to detect when the gas has breached a first barrier; thereby avoiding the risk of breaching the second.
Another essential check is the “leak test”. This consists of lowering a column of nitrogen a little above the roof of the cavern and monitoring the evolution of the gas-brine interface: a rapid rise is a sign of poor sealing. On the scale of the cave, the well is a very thin capillary, and the system resembles an extremely sensitive barometer or thermometer. The challenge is to track down small leaks, of the order of 10-4/year of the stored volume. An abundant literature, to which the Solid Mechanics Laboratory (LMS) has made a major contribution, is devoted to this test and we can expect, with hydrogen storage, important developments concerning the method, periodicity, and acceptability criteria.
2/ Controlling the behaviour of salt
Over large time scales, salt is a viscous liquid and therefore any cavity will gradually close. The resulting annual loss of volume must remain below one percent to prevent the storable volume from decreasing too quickly, especially for the deepest cavities. But also because it can cause damage to the wall or the salt-access shaft interface. The description of the behaviour of salt is old but has been profoundly renewed; Rock Physics establishes that there is a specific mechanism of deformation under very low stresses, which is difficult to measure because the deformation speeds involved are of the order of 10-12/s. However, salt is also a brittle material, which is susceptible to breakage – particularly under the effect of a sudden change in mechanical load. Sudden variations in stock and therefore in pressure can be expected if the cavities are fed by highly intermittent hydrogen production and must satisfy a demand that is also discontinuous.
3/ Understanding gas thermodynamics
The uses of hydrogen for mobility requires extreme purity. However, the gas will have to remain for a long time in a cavity at the bottom of which there are thousands of m3 of brine. The brine contains sulphates from the anhydrite (H2S) frequently associated with underground salt. The gas is wet and loaded with various impurities including H2S, which is particularly harmful for downstream gas uses. Purification can be a major expense.
Finally, given its large size, the cavern is a complex thermodynamic machine – one can speak of cave meteorology with rain, snow, and temperature inversion, made very exotic by the very large changes in pressure. The driving force behind this machine is the presence of a natural temperature gradient in the subsoil; it generates intense convection of gas in the upper part of the cave, countered in the lower part by the maintenance of a relatively cold temperature of the brine resulting from the succession of episodes of vaporisation and condensation of the water. These phenomena, highlighted by the LMS with Storengy, have consequences for the purity of the gas extracted that have yet to be fully explored.