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Hydrogen caverns are a proven, inexpensive and reliable technology

An engineer by training, Louis Londe has been working at Geostock on underground storage since the early 1990s. Specializing in hydrocarbons from the start, Geostock is increasingly focusing on the energy storage possibilities offered by hydrogen caverns.

What are Geostock’s main activities?

Louis Londe, Technical Director at Geostock

Geostock is an engineering group, established just over fifty years ago. Historically, our core business has been designing underground storage facilities for gas, liquefied gas and liquid products.

How is gas stored underground?

There are three storage techniques: man-made cavities, pore storage, and salt caverns.

Man-made cavity storage involves digging a cavern in the rock at a depth of about 100 to 150 meters. The product is stored in direct contact with the rock, and the groundwater exerts a counter-pressure to hold the product inside the cavity.

Porous rocks provide a storage solution for gases. This requires a geological site capable of holding gases, for example a porous limestone layer covered by a dome-shaped impermeable layer of clay. We can drill through this clay layer and inject gas, which is stored in the pore spaces of the reservoir rock.

Three kind of underground storage techniques : salt caverns, man-made cavities and pore storage.

Finally, salt cavern storage requires a site with a layer of salt of sufficient thickness. A storage area is created in the shape of a large bottle by dissolving the salt. This is then filled with pressurized gas, or with liquid or liquefied hydrocarbons. One or more boreholes link the cavern to the outside world.

What is the most suitable technique for storing hydrogen?

In theory, man-made cavities and pore storage can be applied for projects involving hydrogen. A pilot pore storage project, combining methane and hydrogen, is underway in Austria.

However, salt caverns are by far the most promising technique. Most existing hydrogen projects today use this type of storage.

© Geostock

What are the characteristics of salt cavern storage?

To avoid damaging the cavity, it is important that the pressure is maintained between minimum and maximum levels. Indeed, salt is a rock with distinctive properties: while it appears solid, it has the rheology of a viscous liquid, and can therefore deform over time, like pressing down on some butter for example. We say that the salt “creeps”. This aspect defines the minimum operating pressure. Maximum pressure relates more to salt resistance considerations at the bottom of the borehole. Gas storage in salt caverns is typically carried out at pressures of 70 to 200 bar approximately, depending on the depth. The deeper the cave, the more one can increase the minimum and maximum pressures at which the gas is stored. The maximum pressure increases faster than the minimum pressure as we increase the depth, while the deeper the cavity, the greater its total storage capacity.

“These massive caverns are easily as high as the Eiffel Tower.”

The volume of gas corresponding to the minimum pressure is immobilized in the cavity. This is called the “cushion gas”. It acts as a counter pressure. It represents about 40% of the cavern’s total capacity.

How many storage facilities of this type exist today?

Salt cavern storage is very widespread: there are hundreds throughout the world. Regarding hydrogen, as far as I know there are six in operation today. Three are located in the North East of England, and three others in the United States, in the State of Texas.

Man-made cavity © Geostock

What are the advantages of salt cavern storage?

Salt caverns and pore storage are the only solutions for storing large volumes of gas. Salt caverns offer advantages in terms of flows and costs. It is said that salt caverns cost around 50 euros per cubic meter, including equipment. This is to give you an order of magnitude, but of course the actual figure can vary depending on many factors, such as the depth of the salt layer or its thickness, or the possibility of sharing certain surface equipment.

How big are salt caverns?

Some caverns are gigantic. The smallest are in the range of 50,000 to 100,000 cubic meters, while the largest (such as those found in the Gulf of Mexico) can exceed one million cubic meters. These massive caverns are easily as high as the Eiffel Tower.

Is there a risk of a shortage of suitable sites?

Many sites throughout the world have the potential to be used as storage caverns. This is well understood in the industry due to the availability of highly detailed global salt mapping. Indeed, for over a century, oil companies have carried out numerous drilling operations around the world. They particularly searched for salt layers, because salt naturally traps oil. Geophysics and drilling techniques have enabled us to refine the available information, in particular they facilitate the characterization of the geometry and purity of salt at a particular site, which is essential for the exploitation of a deposit.

“Hydrogen will play an increasingly important role in future mobility.”

The number of possible hydrogen storage sites can be further increased if we also consider the conversion of caverns used by the chlorine or sodium industries, once these caverns have reached their targeted size. On the global scale, there are hundreds of potential caverns.

How does a hydrogen cavern operate?

© Geostock

A salt cavern works like a lung: it is “inflated” then “emptied” as required. There are several applications for hydrogen. Hydrogen can be used directly at a cavern to generate electricity. It can also be injected into the natural gas network, up to a certain percentage, for heating (domestic or industrial usages) without the need to modify the burners of gas appliances. This percentage can be increased to even more significant proportions ​​if the burners are modified. In addition, hydrogen will play an increasingly important role in future mobility.

Hydrogen storage for both mobility and to supply the electricity network offers a way of addressing complex, varying and interlinking cycles of demand. Typically, cold countries consume more electricity in winter, while hot countries consume more in summer. Storage also provides a solution for responding to smaller oscillations that depend on weather events that vary on a weekly or daily basis.

“Hydrogen caverns are a proven, inexpensive and reliable technology.”

In comparison to the storage of compressed air, which is the other salt cavern storage solution for low carbon energy, hydrogen has an interesting feature: as a molecule with high energy value, its storage can be economically viable even for relatively low annual frequency cycles.

How much energy can be stored in a hydrogen cavern?

A 200,000 cubic meter cavern can contain about 18 million normal cubic meters of hydrogen stored between 70 and 200 bar. At a rate of 3 kWh per normal cubic meter of hydrogen, this represents approximately 50 GWh.

Could cities be supplied by these facilities?

I think in future we’ll see these types of caverns being used to feed grids of varying scales. Many R&D projects are emerging at the community and regional levels. These storage projects are often related to photovoltaic energy. Most often, they combine the provision of electrical power for the network as well as mobility. Personally, I believe this dual role of hydrogen is important.

Can this type of storage support the energy transition?

This is certainly our belief at Geostock. If it were not the case, we wouldn’t have chosen to focus on this area of development. I am convinced that energy storage is a necessity to overcome the intermittency of renewable energies, which are expected to become increasingly dominant in the energy mix. Today, pumped-storage plants (PSPs) occupy 99% of the market. We need to find alternative solutions: namely hydrogen and compressed air. Of these two solutions, hydrogen is admittedly the most expensive (because of the equipment needed to produce hydrogen and to produce electricity), but many analysts think that it is also the most advantageous since it offers more applications: in addition to energy storage, this gas can be used for mobility as well as in an industrial context.

Using hydrogen to store renewable energy before delivering it to the end-user.

How do you see the future for the hydrogen cavern solution?

Hydrogen caverns are a proven, inexpensive and reliable technology. At present, decision-makers are slow to implement this solution, but that will change. Today’s regulatory framework is probably too much in favor of fossil fuels. But the cost of CO2 is expected to increase in the coming years. Projects of this type could then emerge. We should expect to see the launching of small “pilot” units in the next five years, and the arrival of industrial projects within ten years.