Underground hydrogen storage

Calm waters that reflect the surrounding forest and the mountain peaks in the background... At first glance, pumped-storage hydropower plants (also referred to as pumped-storage hydropower station, or PSH for short) can appear to be naturally occurring features. Yet in many cases, the upper and lower reservoirs are anything but. In reality, they often consist of massive concrete structures constructed in otherwise unspoilt countryside areas that really ought to be protected. The banks are usually completely devoid of valuable vegetation, and the kinds of ecosystem that you would expect to see in lakes or ponds are unable to take hold. Regular fluctuations in the water levels are par for the course.

For this is how energy storage works: Excess electrical energy is used to pump water from the lower reservoir to the upper reservoir. If there is further demand for electrical energy, the stored potential energy is recovered using a water turbine and a generator (which can also operate as an electric motor for the pump). In simple terms, this means that the water is released from the upper reservoir and flows back down to the lower reservoir. Unfortunately, above-ground pumped-storage hydropower plants are an unsightly blot on the landscape. They are also associated with certain risks such as burst dams and ruptured pipes.

PSH plants are currently the only viable large-scale energy storage solution. So for now, the use of these plants is unavoidable when it comes to buffering electrical energy generated by intermittent wind and solar sources. But the transition to more sustainable energy sources is still in its infancy. Sooner or later, zero-carbon power plants will replace the existing coal- and gas-fired power plants. As a result, the need for storage solutions will skyrocket over the coming years and decades.

The idea that existing PSH plants should continue to operate is relatively uncontroversial. Many of these existing PSH plants have been upgraded to increase their capacity, and one or two new ones have popped up here and there. Across the world, a lot of regions have already exploited all that there is to exploit, however. It is becoming generally more difficult to find suitable new sites that possess the requisite geological attributes and where the environmental impact of a new plant would be acceptable. A promising new type of hydro-storage plant comes in the form of pumped-storage hydropower spheres: Hollow spheres measuring approximately 30 m in diameter are installed on the bed of a body of water (600 - 800 m deep), e.g. in the vicinity of an offshore wind farm. Excess electrical power is used to pump water from the hollow sphere into the surrounding water. This creates a negative pressure in the sphere. When electricity is needed, water is admitted into the sphere again, driving a turbine coupled to a generator as it flows in. This technology is currently still at the development stage (e.g. in the StEnSea ("Stored Energy in the Sea") project). According to Fraunhofer IEE, the concept has great potential for offshore sites next to densely populated regions, for example off the coast of Norway, Spain, the USA and Japan.

So how do we store excess energy from solar and wind sources in other parts of the world – let's say, for example, in desert regions, in largely flat landscapes or even in densely populated areas inland – where the possibilities for pumped-storage hydropower plants have already been exhausted? The majority of experts working in the energy transition sector believe that hydrogen technology will have an important role to play. Green hydrogen produced by electrolysing water can be used to store excess electricity. Fuel cells can be used to convert this energy back into electricity as needed. Besides this primary process, hydrogen can also be converted to other storable energy carriers such as methanol ("Power-to-X" concept) or used directly for powering machinery, vehicles and aircraft. It can replace coal in the extremely carbon-intensive steel production industry (by means of direct reduction), drastically reducing the resulting CO₂ emissions.

The hydrogen that is produced using excess energy can be buffered in above-ground pressure tanks – an incredibly safe alternative to this is to buffer it below ground, e.g. in salt caverns. When it comes to geology and technology, the underground storage of hydrogen has similar requirements to those that apply when storing natural gas. Most countries have adequate experience with storing natural gas in porous underground reservoir formations and storage caverns. For this purpose, they make use of depleted natural gas fields and salt caverns. However, the energy input required in order to store hydrogen is significantly greater than for natural gas or methane. Hydrogen's physico-chemical and biochemical behaviour also differs. For example, there are microorganisms that use hydrogen as their energy source. So there are a few issues that must be addressed before it becomes economical to use former natural gas fields for storage.

Aquifers (porous underground reservoir formations consisting of water-permeable layers that are confined by water-impermeable layers) have long been used for storing town gas with a high hydrogen content. Safety problems and gas leakage have not been observed with this type of storage, but nevertheless, it is certainly susceptible to intense bacterial activity and altered gas composition.

The situation is similar for natural gas fields, which are also porous underground reservoir formations. The H2STORE research project looked at various different reservoirs. Some of these turned out to be extremely promising. The most interesting options seemed likely to be well suited to use as seasonal hydrogen storage facilities. Some examples of large potential storage facilities in depleted natural gas fields are the gas fields within the UK Continental Shelf (6900 TWh potential storage capacity), along with numerous soon-to-be-depleted gas fields in the North Sea off the coast of the Netherlands (227 TWh) and on shore in the Netherlands (179 TWh).

Salt caverns, unlike the first two storage methods mentioned, accommodate the gas in cavities, which are larger than the pores in porous underground reservoir formations. These are created in salt deposits using a process called solution mining or in-situ leaching to dissolve the salt where the caverns are required. Rock salt is remarkably impermeable and does not react readily with methane or hydrogen. The output of storage caverns is up to ten times greater than porous underground reservoir formations, meaning that gas can be very rapidly injected and withdrawn. These gas storage capabilities suggest that they may be a worthy candidate for covering peak demand periods. So far, there are four sites globally where salt caverns are being used to store hydrogen, one in the UK and the rest in the USA. Salt caverns have also been used to store town gas comprising up to 60% hydrogen; two of these are located in Germany.

For a few years now, there have been a number of research projects within Europe focusing on hydrogen storage in salt caverns. As part of the EU's 'HyUnder' project (2014), researchers assessed the feasibility of storing renewable electricity in the form of hydrogen in large-scale underground storage facilities compared to other large-scale energy storage concepts. For Germany, salt caverns were found to be the best storage solution by far. Further pilot projects exploring underground hydrogen storage are taking place in the Netherlands (HaStock), Austria (Sun Storage), and further afield, in Argentina (Hychico).

Courtesy of the HyCavMobil project, Germany now has its very first test cavern for pure hydrogen at a site in Rüdersdorf (a municipality to the east of Berlin). Initial testing conducted by operator EWE in September 2022 before the cavity was created demonstrated that the supply line that would eventually feed gas into the cavity was leaktight to a depth of 1000 m. For the purpose of this test phase, the hydrogen was compressed to various pressure levels. A cavity approximately 500 m³ in size was then created by means of solution mining. According to the company's schedule, hydrogen will first be stored in it in spring 2023, which is when they will start testing the injection and withdrawal processes. The quality of the hydrogen after it is withdrawn from the mini cavern is of particular interest, especially if this green gas is to be used to power heavy goods vehicles in the future. The findings from this test phase should be transferable to larger caverns measuring 500,000 m³. Other, similar projects are under way elsewhere, including the Lesum storage facility near Bremen in Germany (Storengy), the Étrez storage facility in France, a storage facility in Groningen in the Netherlands, and the Stublach storage facility – the UK's largest natural gas storage facility – in the northwest of England. As early as 2028, locally produced hydrogen is set to be stored on a commercial scale in salt caverns in Manosque (located to the north of Marseille, France).

Geologically and financially speaking, it is already evident that salt caverns are, overall, the most promising method for large-scale hydrogen storage. But even depleted natural gas fields are suitable for certain purposes. And aquifers offer an alternative in areas where neither of these is available.