Salt domes as H₂ storage sites

A successful ramp-up of the hydrogen market would be impossible without a means of hydrogen storage, and salt caverns are ideally suited to the task. These artificial cavities, more than 1,000 meters (3,200 feet) deep in salt rock, can be primarily found in northwestern Germany. While previously they have contained fossil fuels such as crude oil and natural gas, in the future they are set to hold hydrogen.

Visitors to Harsefeld, a small community near Stade in Niedersachsen, will find themselves surrounded by fields and meadows lined with hedge banks known as “Knicks.” This is the scenery surrounding Storengy Deutschland’s natural gas storage facility which has been in operation since 1992. It is here that the subsidiary of French network operator Engie intends to create one of the first hydrogen storage reservoirs in Germany.

Underground salt caverns have long proved themselves safe places to store large quantities of gas, explains Gunnar Assmann, project manager for hydrogen storage at Storengy. “Storage caverns are cavities engineered in salt rock, which forms a tight, natural barrier.” Consequently, the company plans to create two salt caverns as part of its SaltHy project. The caverns would store hydrogen that can be produced regionally using green electricity generated by onshore or offshore wind turbines, thus emitting zero greenhouse gas emissions.

Northern Germany lends itself to the creation of hydrogen storage infrastructure for several reasons: Firstly, due to the proximity of onshore and offshore wind farms as well as future centers of industrial hydrogen use. In addition, the region holds 80 percent of Europe’s salt cavern storage capacity. And there are already a large number of long-distance gas pipelines that can be repurposed to convey hydrogen. This also explains why the installation of the European Hydrogen Backbone, the EU’s planned long-distance hydrogen pipeline, will kick off in northwestern Germany. The first cavern in SaltHy, which stands for Storage Alignment with Load and Transport of Hydrogen, is due to be linked to this network by a connecting pipeline.

Hydrogen for the steel and chemicals industries

In Harsefeld, the first cavern is expected to be up and running between 2030 and 2032. The decision about the construction of the second cavern will be taken by the company in 2028 and will depend on how the H₂ market has developed by that point. The second facility could then become operational in 2034. Each cavern is designed to contain up to 7,500 metric tons of hydrogen. “That would, for example, cover the needs of a regional steel plant that requires 140 metric tons of hydrogen a day for approximately two months,” explains Assmann.

The H₂ gas will be treated at the Harsefeld site in an overground facility before being stored below the surface. The storage pressure will be over 200 bar, depending on the quantity. The pressure in the transportation pipeline will, nevertheless, be lower at a maximum of just over 80 bar. The gas will therefore be compressed and cooled in the compressor station prior to storage. When required, the hydrogen can be removed, processed and fed into the grid for onward supply.

A feasibility study carried out by Storengy Deutschland in 2022 came to a positive conclusion, as did a market survey of companies in March this year. “Many of the announced H₂ projects for which a connection to a hydrogen storage facility will be relevant are at an advanced stage and are situated in northern Germany,” says the company, which is reassured of the need for new subterranean hydrogen reservoirs in Germany.

Mapping work and preparations for the approvals process are currently underway in Harsefeld. Underground storage facilities are subject to mining law and must be signed off by the relevant regional authorities. The planning, approval, construction and operation of such facilities are a complex matter, explains Assmann, a process engineer who has worked in the energy sector for over 30 years. The cost and effort involved should therefore not be underestimated. Storengy hopes to submit initial documents before the end of this year.

The investment decision on the underground part of the reservoir is expected at the beginning of next year. In view of the long time line for the project, investment will be phased over several stages in order to reduce the risk. If the company goes ahead, it will be spending upfront so it can cover the demand for hydrogen storage that emerged from the market survey.

H₂ reservoirs relieve power grid

Construction work could begin in 2026 with the drilling of the first boreholes, says Assmann. The process of debrining a salt cavern, i.e., flushing out the salt with water, takes three to five years depending on its size. Similar to the method used to build natural gas storage reservoirs, here, the intention is to create a roughly cylindrical void that is around 200 meters (650 feet) in height and approximately 60 to 70 meters (195 to 230 feet) in diameter. Thanks to high injection and withdrawal rates, the caverns would also help relieve the strain on the power grid.

Located in the area around Harsefeld and in the Hamburg metropolitan region are large industrial companies that will need considerable amounts of hydrogen in future to defossilize their production processes. This will be the case for both the metalworking industry and the chemicals sector. The Dow factory situated around 20 kilometers (12 miles) from Stade is a case in point. As a cooperation partner, the global corporation, which operates one of the biggest production facilities for chlorine chemicals in Europe on the lower reaches of the river Elbe, will process the salt resulting from debrining the cavern.

The port of Stade together with the planned ammonia terminal will make the town a hub for trade, logistics and industrial development and allow hydrogen to be imported in the form of ammonia, for example. This is why the region is being developed as a focal point for green H₂.

Politicians should devise demand schedule

Storengy Deutschland, which boasts a market share of 8 percent in Germany thanks to its six natural gas storage sites, is planning more hydrogen storage facilities besides those in Harsefeld. From a geological standpoint, the sites in Lesum and Peckensen in northern Germany would be suitable, according to Assmann. What is still lacking on the political side, in his opinion, is a schedule for at least the next 10 years which sets out the yearly requirement for converting storage reservoirs to hydrogen and the construction of new hydrogen storage facilities. Details of how much H₂ storage capacity should be available – and by when – are yet to be determined, he says. Similarly, questions remain about how the storage facilities will be funded and how access to them will be regulated.

In France, where the parent company has also been managing natural gas storage reservoirs for decades, Storengy is developing a large-scale demonstrator for green hydrogen alongside industrial partners. According to company information, a salt cavern in Étrez in the Auvergne-Rhône-Alpes region with a storage capacity of 44 metric tons of hydrogen is being set up in conjunction with an electrolyzer and applications in the chemicals industry and heavy-duty mobility in order to support the development of the area’s Zero Emission Valley.

Since it isn’t possible to relinquish use of fossil-based forms of energy entirely in the short term, it will not be possible to repurpose the necessary storage reservoirs immediately. “We will have to continue to safeguard supply with natural gas via existing storage facilities,” says Assmann, explaining that this is why it’s necessary to build new storage reservoirs for the emerging hydrogen market. Only when natural gas storage facilities are no longer needed can these be converted for the storage of green gases where required.