Underground Storage of Hydrogen

Uniper natural gas storage facility in Epe

The storage of hydrogen is crucial for the energy transition. It allows for balancing between production and consumption. Hydrogen storage is therefore essential for the stability of an energy system and significantly contributes to security of supply. Hydrogen can be stored both underground and above ground. The choice of storage type depends on local conditions, required capacity, and purpose. To store large quantities of hydrogen, underground storage is needed. Due to their high capacity, hydrogen storage facilities can compensate for seasonal fluctuations in renewable energies and bridge supply bottlenecks. There are two forms of underground hydrogen storage: cavern and porous rock storage.

Graphic of the HPC Krummhörn cavern storage

Cavern Storage: Structure and Function

Cavern storage facilities are artificial cavities created in underground salt domes. They are formed in two steps. The first is drilling the cavity many hundreds of meters below the earth's surface – the deepest cavern storage facilities in Germany are up to 2.5 kilometers deep. The size of these cavities is also impressive: they can have diameters of up to 100 meters and heights between 50 and 500 meters. This results in a capacity of up to several hundred thousand cubic meters, providing ideal conditions for long-term storage.

The second step is solution mining. In this process, the salt present in the drilled cavities is removed by injecting water to dissolve it and then pumping it to the surface. What remains are cylindrical cavities referred to as "caverns". Due to the mineral properties of salt and the thickness of the salt domes, they provide a natural tightness for gases, making additional lining of the cavern wall unnecessary. The cavern storage is thus a tight, underground cavity into which gas can be filled like liquid into a bottle. This technology is already known from natural gas storage and is now being adapted for hydrogen.

Converting an existing cavern storage facility takes an average of up to five years, as replacing all components is almost as complex as building a new facility. Incidentally, building a new cavern storage can take up to ten years. Here, extensive geological preliminary work must first be carried out, numerous official permits obtained, and suitable locations for the above-ground facility found before the initial drilling can take place.

Graphic of the HyStorage porous rock storage

Porous Rock Storage: Structure and Function

Porous rock storage, also called "rock pore storage", "reservoir storage" or "storage in porous rock formations", are deposits in limestone and sandstone layers. Here, natural gas has accumulated over millions of years. Often, the rock formations used today are depleted natural gas or oil fields. After the end of extraction, these natural gas reservoirs can be converted into underground storage facilities. The porous rock, which previously contained natural gas, absorbs the stored gas like a sponge. The cap of the porous storage consists of a closed rock layer that prevents gas from escaping. Porous rock storage facilities have a storage volume of up to several million cubic meters.

While porous rock storage plays an important role in the long-term storage of natural gas, its use for hydrogen has hardly been investigated so far. The reason: Injection and long-term storage here are quite challenging. Hydrogen is very reactive to organic material and some minerals. This leads to it potentially reacting with surrounding substances and changing chemically during storage, or being contaminated by minerals or residual gases.

Advantages and Disadvantages of Storage Forms

The advantage of cavern storage is that gas can be injected or withdrawn quickly and flexibly. Additionally, one can assume that the stored hydrogen comes out exactly as it went in. Therefore, they are ideal for green hydrogen.

With porous rock storage, on the other hand, it is unclear how pure the withdrawn hydrogen still is. Since impurities can damage equipment in some hydrogen applications, the purity of the gas plays a significant role. Research on hydrogen quality after withdrawal from porous rock storage is still ongoing.

Comparison of gas storage capacities in the EU

The Country Needs New Storage Facilities

The market consultation published by Uniper in April 2024 shows a high demand for hydrogen storage facilities from 2029 onwards. It also signals how important the topic is for many actors: In addition to energy suppliers, power plant operators, hydrogen producers, industrial hydrogen consumers, and gas traders were involved in the survey.

The results aligned with those of other studies. For example, a recent analysis by the Gas Infrastructure Europe (GIE) association shows that EU-wide, around 45 TWh of hydrogen storage capacity is needed by 2030 and about 300 TWh by 2050. For comparison: Currently, only projects with around 9 TWh by 2030 and 21.5 TWh by 2050 are planned – the concretely planned storage volume is thus about three or 15 times lower than the expected demand.

In the coming years, huge underground gas storage facilities must therefore be built or upgraded for hydrogen storage throughout Europe. As Germany has the best geological conditions for gas storage, the market for conversions and new construction is likely to be particularly large here.

Above-ground facility at the Krummhörn storage site

Example Krummhörn

In August, the storage operator Uniper Energy Storage commissioned the hydrogen cavern storage in Krummhörn, Lower Saxony. The site of the "Hydrogen Pilot Cavern - HPC Krummhörn" project is located near Wilhelmshaven, where several electrolysis projects are planned and large quantities of hydrogen could potentially arrive in the form of ammonia.

For the project, Uniper used an existing borehole from which the pilot cavern was developed. Initially, the facility is still relatively small in scale. The gas trader's goal is to show how quickly a new cavern storage can actually be put into operation – this is what interests the first hydrogen customers the most.

Subsequently, the site is to be commercially further developed to provide the market with a storage capacity of 250 GWh in a first step.

You can learn more about the background of Uniper Energy Storage's storage projects and the role of hydrogen storage for the energy transition in general in the interview with Doug Waters, Managing Director of Uniper Energy Storage, by H₂News.

Inauguration of the hydrogen storage facility in Krummhörn on August 26, 2024