One of the main solutions to the problem of large-scale hydrogen storage is to place it in salt caverns. While these solid, impermeable-walled caverns have long been used to store methane, this technology remains experimental for dihydrogen and will not be able to handle large amounts of H2 for several years.
Gas storage in salt caverns is something amazing. There are salt deposits buried in the ground and the idea is to inject water, using a tube, to dissolve the salt and then recover the brine. So you create a cavity with solid walls, made of sodium chloride. This used to be done mainly to recover the brine created. Now, it is also (mostly?) to create reservoirs.
One of the interests is that the storage can be done at high pressures: “up to 200 bars for a cavity located at 1100 m depth.” (Agnoletti et al. 2021, p.20) Salt, being non-reactive and impermeable, acts as an ideal liner.
While this is a technique that has long been used to store methane, it is still in the experimental stage for large-scale hydrogen storage.
Storing gas in salt caverns: a mature solution?
Mining salt by dissolution is an ancient practice: it was already practiced by the Qin dynasty in 3rd century BC China using bamboo. (Warren 2016, p.1304) Since 1949, the process has been used to create watertight cavities to store hydrocarbons. (Warren 2016, p.1319) There are currently several dozen storage sites in the world, including 2 in France. For example, you have the Géosel underground site, commissioned in 1969, which can store 9.2 million m3 of petroleum products between 350 and 750 meters deep. It is managed by Géostock. Another site, Géométhane in Manosque, operated by Storengy, stores up to 274 million cubic meters of natural gas. In France, however, storage in aquifers is generally preferred, in particular because the fact that salt is corrosive poses a problem.
Let’s talk about the risks associated with this type of storage, before studying whether it is a solution for storing hydrogen.
Risk of ground collapse
Perhaps the most obvious risk is the collapse of the area above the cavern. For example, the city of Tuzla, Bosnia-Herzegovina, saw its ground “sink” up to 12 meters between 1956 (~the start of a salt mine) and 2003 depending on the area. (Warren 2016, p.1336) As with any underground hydrogen storage, it is questionable whether seismic activity poses a risk depending on location.
The Verkhnekamsky mine in the Urals collapsed following an earthquake measuring 4.7 on the Richter scale, releasing 900,000m3 of gas. The second largest mine in the world, in the United States, also collapsed on March 12, 1994, following an earthquake of magnitude 3.6. (Warren 2016, p. 1337-8) It is likely that modern protocols take these risks into account, as they already exist for methane storage, but I think it is worth pointing out, not least because it perhaps limits the scale of viable sites.
The contraction of the cavity: the “salt creep
Finally, we must also mention the “salt creep”, the propensity of the cavity to shrink:
“Left alone, and not subject to freshwater incursion, salt (especially carnallite and bischofite salt) will creep into a sealed storage cavern until differential pressures are equalized.” Warren 2016, p. 1367
I’m not sure this is very problematic for seasonal use.
A “mature” solution nonetheless … for methane
There are also many, many other difficulties, such as the “non-uniform distribution of salt blocks” (Wanyan et al. 2018), but in the end, salt caverns would still be a very stable storage medium overall:
Nearly all of the problems documented in the United States and elsewhere have come from operations using caverns built using older, less precautionary methods. Even so, in all incidents related to various salt cavern storages the integrity of the cavern has never been threatened. All incidents to date can be attributed to poor management practices or the use of inadequate facilities. Purpose-built salt caverns are probably one of the safest ways to store hydrocarbons and dispose of hazardous waste.Warren 2016, p. 1374
“The technique of gas storage in salt caverns has now been mastered, it benefits from more than 50 years of feedback for natural gas, which makes it possible to very largely control the risks for populations and the environment.”Grégoire Hévin. https://firstname.lastname@example.org/vers-le-stockage-dhydrog%C3%A8ne-en-cavit%C3%A9-saline-7d7e5882589c
Nevertheless, if this is undoubtedly true for natural gas, it is not true for hydrogen.
Salt cavern storage: the challenges of hydrogen
“We have to check that the equipment used today with methane can be adapted for hydrogen, that the sealing is appropriate, and that all the surface equipment is in place. We need to make sure that it works, and we have European subsidies for this.Camille Bonenfant-Jeanneney, Managing Director of Storengy.
There are currently four hydrogen storage sites in salt caverns: there are sites in
- Teesside in the UK since 1972, operated by Sabic Petroleum,
- Moss Bluff in the USA since 2007, operated by Parxair and
- Clemens in the US since 1983 operated by Conoco Phillips (Tarkowski 2019)
- Spindletop in the United States (Agnoletti et al. 2021, p. 43)
They contain 210, 566, 580 and 906 thousand m3 of hydrogen, respectively. These are ‘strategic reserves for hydrocarbon refining uses.” (Engie) Nevertheless, this use does not correspond to what is planned: “The frequency and quantities used are low. For energy use, injection and withdrawal cycles are expected to be faster and of greater amplitude. (Engie) In France, there are two sites that are experimental (and which we will discuss later).
Large-scale industrial storage of hydrogen in salt caverns is therefore by no means mature. Hydrogen poses several specific challenges.
Adapting the equipment
As always, we find the main problem with hydrogen: it is tiny and it damages steel. Equipment (compressors, tubes, etc.) must be adapted.
But according to engineer Ludovic Leroy, who provides training to energy professionals, threshold studies have not yet begun. “They haven’t injected a single hydrogen molecule. They are trying to redefine the compressor [the device that increases the pressure of the gas by reducing its volume, editor’s note], as it is not adapted to hydrogen.https://archive.is/KgyNT#selection-2713.12-2713.13
The corruption of hydrogen
Hydrogen can be corrupted by micro-organisms during its stay. According to Agnoletti et al. (2021, p.24-5), “the impact of microorganisms on the geological storage of hydrogen in salt caverns is still unknown.” Nevertheless, it would be “legitimate to think that an external supply of hydrogen in environments such as the earth’s subsurface, where microbial growth is likely to be limited, is likely to exacerbate the activity of microorganisms.” One study even observed the “conversion of 45-60% of the hydrogen in city gas, stored for 7 months in a tank, to methane and incubated with an enrichment of methanogenic groundwater bacteria”.
The main risk appears to be the production of corrosive H2S.
In fact, this type of salt cavern storage has so far only worked in Texas and the United Kingdom, and only for a very short storage time,” he says. And for good reason: after only a few days, the hydrogen risks becoming loaded with sulfur due to the microorganisms in the cavity, requiring desulfurization downstream.https://archive.is/KgyNT#selection-2713.12-2713.13
The uses of hydrogen for mobility require 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. This brine contains sulfates from the anhydrite (H2S) frequently associated with underground salt. At its exit, the gas is wet and loaded with various impurities including H2S, which is particularly harmful for downstream uses of the gas. Purification can represent a significant expense.Pierre Bérest
The scarcity of suitable sites?
In order to make salt caverns, there must be a salt deposit to dig. According to Engie, their scarcity is the “main drawback of salt cavities”.
Caglayan et al (2019) estimate the theoretical storage capacity in Europe at 84.8 PWhH2.
Hydrogen storage projects in saline cavities
An example: H2 storage in saline cavities, the soil studies before testing have not started, the adaptation of compressors has not been done, so today we do not know if it is possible. The same goes for storage in aquifers.@princertitude, https://twitter.com/princertitude/status/1452978692821594113
More and more European funds are allocated to hydrogen and projects allowing its storage in salt caverns are developing.
The HyPSTER project
In France, there is a project whose engineering studies began in 2021 and which is led by Storengy: the HyPSTER project. With a budget of 13 million euros, it will be able to house up to 44 tons of hydrogen. There will also be a 1MW electrolyser, which should produce 400kg of hydrogen per day. This is a large-scale test of the viability of this storage method.
It is part of the Zero Emission Valley, a project in Auvergne-Rhône-Alpes.
The project involves a consortium of European players:
- Storengy, which is coordinating the project,
- Armine-École Polytechnique, which will help with the research
- Element Energy, ESK and Inovyn, each of which will address a specific technical topic.
- Ineris for risk assessment and regulation.
- Axelera to communicate with the scientific and industrial community
The Hygreen project
The Hygreen Provence project plans to produce hydrogen (thanks to photovoltaic) and to store it in 2 nearby salt caverns (Manosque, operated by Géométhane) not operated and able to contain up to 6000t. It was initiated in 2017 by the Durance-Luberon-Verdon agglomeration community and should start producing in 2024. The project seems to be mainly focused on electricity production for now.
Advanced Clean Energy Storage
In the United States, the Advanced Clean Energy Storage (ACES) project brings together Mitsubishi Power and Magnum Development to produce 1GW of green hydrogen and store some of it in salt caverns that can hold up to 5,500 tons of hydrogen (CNBC).
HYPOS (Hydrogen Power Storage and Solutions East Germany)
Hydrogen Power Storage & Solutions East Germany (HYPOS) is a project to deploy hydrogen production and storage capacities in East Germany, between Dresden, Magdeburg and Erfurt. One of the first projects would be a salt cavern storage with a capacity of 126 GWh of H2. The second is a 1.4GW electrolyser based on PEM technology to allow coupling with renewable energies. (Innovation&Strukturwandel)
- Warren J.K. (2016) Solution Mining and Salt Cavern Usage. In: Evaporites. Springer, Cham. https://link.springer.com/chapter/10.1007/978-3-319-13512-0_13#citeas
- AGNOLETTI et al. (2021) Etat des connaissances sur le stockage de l’hydrogène en cavité saline et apport du projet ROSTOCK- https://www.ineris.fr/fr/etat-connaissances-stockage-hydrogene-cavite-saline-apport-projet-rostock
- ENGIE, « H2 en sous-sol : les cavités salines, futur du stockage de l’hydrogène ? », https://innovation.engie.com/fr/news/actus/le-saviez-vous-/hydrogene-souterrain-stockage-sel-cavites-mines/25906
- An $11 trillion global hydrogen energy boom is coming. Here’s what could trigger it, https://www.cnbc.com/2020/11/01/how-salt-caverns-may-trigger-11-trillion-hydrogen-energy-boom-.html
- Pierre Bérest, Cavernes de sel : la clé pour stocker l’hydrogène ?, polytechnique-insights.com
- Innovation&Strukturwandel : https://www.innovation-strukturwandel.de/strukturwandel/de/unternehmen-region/die-initiativen/_documents/artikel/a-h/hypos