Storag Etzel GmbH has started work on retrofitting the salt caverns in Etzel. The work is taking place as part of the H2CAST project. H2CAST stands for "H2 CAvern Storage Transition", i.e. it deals with the conversion of existing large-volume caverns for the storage of hydrogen. The project is funded by the state of Lower Saxony and the German Federal Ministry of Economics and Climate Protection (BMWK). The injection of 80 t of H2 and the operational use of hydrogen storage are scheduled to start in summer 2024.
In one cavern, Storag Etzel is installing a gas storage completion and a brine pendulum string in the access well. In addition, a new cavern head suitable for hydrogen will be constructed. The company expects to complete the "workover operations" using a winch by the end of the year. The completion work was approved in advance by the regulatory authority, the State Office for Mining, Energy and Geology (LBEG).
Proven technology from natural gas storage
For the components, the storage company is largely using technology that is already available on the market and has proven itself over decades in the storage of natural gas. The individual elements are tested here in real operation with hydrogen; they were previously tested in the laboratory by specialists under artificial hydrogen loading for their H₂ compatibility.
H2CAST envisages that a so-called production pipe tour will be installed in one of the caverns' access holes, through which hydrogen can be admitted and discharged in the near future. A sealing element, a so-called "packer", at the lower end of the production pipe tour would seal these welded pipes against the outer pipe tour cemented to the rock. Between them, an annular space a few centimeters wide, comparable to a double-walled tank, is created over the hydrogen production pipe run that extends to a depth of about 1 km.
This annular space is filled with a liquid, forming a two-barrier system against the surrounding rock. According to Storag Etzel, a special monitoring system allows even very small amounts of hydrogen, should it diffuse through the seals, to be collected and measured in the annulus. A further developed cavern head from the company Hartmann Valves, manufactured in Lower Saxony, will be installed as the borehole closure, which has additional monitoring capabilities.
Infinitely variable storage volume
According to the company, an additional pipe string with a smaller diameter will be installed in the production pipe run to serve as a brine pendulum string. This will allow the brine displaced during the initial filling of the cavern with hydrogen to be brought to the surface. What is special about this is that in the H2CAST project this process can also be reversed, so that brine from the cavern field can be pumped back into the caverns until they are almost completely filled with brine again.
According to the storage company, the displaced hydrogen is then transferred to one of the two caverns, which acts as a "hydrogen pipeline" in this mode of operation. By pumping in or pumping out brine, the usable storage volume (working gas volume) for hydrogen can be varied continuously and the hydrogen gas remaining in the cavern to maintain the support pressure, the so-called "cushion gas," can be minimized. In the subsequent test operation, this could be used to test, among other things, various pressure levels in the cavern, as can be expected in the future hydrogen economy, as well as H2 purification.
