Following the successful completion of the conversion and leak tests with hydrogen in two caverns in 2024, STORAG ETZEL began injecting the first quantities of a planned total of 90 tonnes of hydrogen at the turn of the year. The injection is part of the ‘H2CAST Etzel’ research and development project, which is developing the technical implementation of hydrogen storage in salt caverns. ‘This represents another milestone in the conversion of existing caverns for the storage of hydrogen in Etzel,’ says Carsten Reekers, project manager of the “H2CAST Etzel” research and development project.

The gaseous hydrogen delivered at a pressure of 300 bar by truck trailers displaces the brine from the caverns during storage via the cavern head and borehole piping suitable for hydrogen. The brine is transported away via the STORAG ETZEL brine system.

‘After the expected start-up difficulties in the procurement of hydrogen - it is clear that the hydrogen market is still in its infancy - several tonnes have now been safely stored underground at a maximum pressure of 170 bar as of the end of January,’ Reekers continued. ‘We are proud of what we have achieved so far and have now shown that the existing facilities in the Etzel cavern field are suitable for hydrogen storage. Our specialist colleagues in gas operations have the process steps for hydrogen filling under control.’

STORAG Etzel is continuously pursuing its hydrogen storage strategy. The Etzel caverns are scalable and flexible, meaning that existing local underground gas and oil storage facilities can be quickly converted for the use of hydrogen. ‘We'll be ready to go when the H2 market is,’ says Reekers.

In 2025, STORAG Etzel and its project partners still have some challenging milestones ahead of them in the H2CAST project. The focus will then be on the construction and commissioning of the hydrogen purification plants. Different processes and their efficiency will be tested. Preparations for this are already in full swing. ‘We are very confident that we will also succeed in this step as a team effort with

with our employees and partners,’ continues Reekers. ‘The fact that we can manage the operation and maintenance of the systems with specialist personnel from our existing organisation is proving to be an advantage. Our decades of experience in underground storage will help us here,’ concludes Reekers.

Due to the lack of an H2 pipeline connection, the hydrogen still has to be delivered to Etzel by lorry. Up to three lorry loads of hydrogen will arrive in Etzel every week until the summer. A total of around 200 lorry trailer loads will be required to fill the roof area of the caverns with hydrogen. Smaller quantities of hydrogen were already injected into the caverns at the beginning of 2024 as part of upstream tests to prove the hydrogen-tightness of the caverns and the drilling equipment under maximum gas pressure. In future, the Etzel site will be supplied with hydrogen via pipeline after connection to the already approved H2 ‘core network’.

The storage of hydrogen was authorised by the supervisory authority, the State Office for Mining, Energy and Geology (LBEG).

The H2CAST Etzel project aims to demonstrate the feasibility of large-volume underground hydrogen storage and to prove the suitability of the salt caverns in Etzel for hydrogen storage. The operation of the hydrogen storage facility will be trialled and will serve to establish a hydrogen industry in Germany and Europe. H2CAST stands for ‘H2 CAvern Storage Transition’, i.e. the rededication of the existing large-volume caverns and technical facilities in Etzel for the future storage of hydrogen as a building block of a renewable and thus ecologically sustainable energy system.

The project is funded by the state of Lower Saxony and the Federal Ministry of Economics and Climate Protection (BMWK).

The H2CAST project envisaged the installation of a so-called production pipe tour in one of the access boreholes of the caverns, through which hydrogen is now injected and extracted. A sealing element, a so-called “packer”, at the lower end of the production pipe tour seals these welded pipes against the outer pipe tour cemented to the rock. 

In between, an annular space a few centimetres wide, comparable to a double-walled tank, was created over the hydrogen production pipe route, which runs to a depth of approx. 1 km. This annular space is filled with a liquid and forms a two-barrier system against the surrounding rock. Thanks to a special monitoring system, even very small quantities of hydrogen can be collected and measured in the annulus if it diffuses through the seals. 

A further developed cavern head manufactured by Hartmann Valves in Lower Saxony was installed as a bore closure, which has additional monitoring options. 

An additional pipe string with a smaller diameter has been installed in the delivery pipe run as a brine displacement string. This allows displaced brine to be extracted when the cavern is first filled with hydrogen. A special feature of the H2CAST project is that this process can also be reversed. This means that brine from the cavern field can be pumped back into the caverns until they are almost completely filled with brine again. The displaced hydrogen is then transferred to one of the two caverns, which acts as a “hydrogen pipeline” in this operating mode. By pumping or delivering brine, the usable storage volume (working gas volume) for hydrogen can be infinitely varied and the hydrogen gas remaining in the cavern to maintain the support pressure, the so-called “cushion gas”, can be minimized. In the following test operation, various pressure levels in the cavern can be tested, as expected in the future hydrogen economy, as well as H2 purification.

The components were largely based on suitable technology already available on the market, which has proven itself in the storage of natural gas over decades. The individual elements are tested here in real operation with hydrogen; they were previously tested in the laboratory by specialists for their H2 compatibility under artificial hydrogen loading. 

As part of the H2CAST demonstration operation, the long-term suitability is to be proven under real operating conditions. “In particular, we want to show that the existing facilities in the Etzel cavern field are suitable for hydrogen storage without the need to develop new components. This makes enormous economic sense for a Germany-wide market ramp-up, without any restrictions on plant safety,” says Carsten Reekers, head of the H2CAST joint project. 

STORAG ETZEL would like to expressly thank the H2CAST partner companies and institutions for their hard work and commitment in successfully realizing the project goals through teamwork. 

Following the filling with the first tranche of around 45 tons of hydrogen, the caverns are to be pressurized with the second tranche of 45 tons to a total of 90 tons of hydrogen up to the highest permissible pressure. Construction of an above-ground test facility will begin in spring. This is expected to enable gas storage operation from winter 2025 with the work steps of compression, gas drying and purification, pressure regulation, quantity and quality measurement.

The Etzel site should be “H2-ready” by 2026. Subsequently, the research results and technical standards of the H2CAST Etzel flagship project funded by the state of Lower Saxony and the federal government will be made available to other hydrogen cavern sites in the region, thus serving as a “blueprint” for the conversion of caverns for future hydrogen storage in Germany.