Hydrogen storage: going underground

Near the coastal city of Luleå, in northern Sweden, a 100-cu metre rock cavern, 30m below the surface is being filled and then emptied of hydrogen. The cavern is part pf the Hybrit project, an ambitious programme to produce steel using green hydrogen. A full-scale facility could see the construction of a cavern up to 120,000 cu m in size.

Sweden is one of the countries leading the charge in the production of green hydrogen, which is set to play an important part in the world’s transition away from fossil fuel to cleaner sources of energy. Other front runners include Brazil, China, India and Spain. Green hydrogen is produced from water by electrolysis, using renewable energy, and produces only water when it burns in a fuel cell.

The Hybrit project in Sweden, which aims to produce and store green hydrogen for steel making, has constructed a 100- cu m rock cavern below the surface.

Green hydrogen could be used in several ways: for industrial processes such as steel making, to store electricity created from solar or wind power, to fuel construction plant, larger vehicles or cars, to heat homes and other buildings. Exactly what mix of uses works best has yet to be seen and will vary from country to country.

Whatever green hydrogen strategies develop, an important element of all of them will be the means to store it. As a gas, hydrogen is lower density than natural gas, which means that for the same energy output, it takes up more volume. This means that the best place to store significant quantities of hydrogen will be underground.

“It is generally assumed that, because hydrogen is very low density, storing it on the surface is not going to be particularly easy so the subsurface comes into its own as a solution,” says Kevin Taylor, professor in energy geoscience at the University of Manchester in the UK.

“It is inherently safer to store underground than above ground,” says Jack Trevail, mechanical engineer at energy storage technology company Gravitricity. “And it takes up a lot less footprint: you would need a 40 times larger footprint for above ground storage, compared with our method underground.”

Gravitricity was set up ten years ago to develop technology for storing energy in disused mine shafts, lifting and lowering weights as a means of storing and releasing energy. In 2022, it started looking into hydrogen storage, working with Arup on a feasibility study.

For companies that have traditionally operated in the mining or tunnelling sectors, the energy transition phase offers new opportunities. In Australia, civil engineering contractor Abergeldie Complex Infrastructure is actively seeking new uses for its four massive blind boring machines that have traditionally been used for creating shafts for mines.

“Around 2018, considering the forecast decline in underground coal mining, the owner of Abergeldie began seeking alternative applications for the blind boring rigs,” says David Bentley, CEO of Ardent Underground, a joint venture between Abergeldie and ITP Thermal. “So, he set about looking for alternative uses for the rigs specifically to contribute to the clean energy transition. I was employed three years ago to see what we could shake out of the ideas that were floating around.

“The leading idea is to adapt proven shaft drilling techniques to provide a solution for the emerging green hydrogen industry. Blind boring is ideally suited to the construction of lined vertical shafts within natural rock structures,” says Bentley.

There are several other ways that hydrogen can be stored underground too, some tried and tested, others still under development.

The colours of hydrogen

Green hydrogen is so called because it is produced with ‘green’ energy. In 2022 around 1% of hydrogen produced was green, according to the International Energy Agency. Which means that 99% came from fossil fuels using processes which emit between 8 and 12kg of carbon for every 1kg of hydrogen produced.

Grey hydrogen, produced from natural gas through steam reforming is the most common. Brown hydrogen, which is the worst offender in terms of carbon emissions, comes from the gasification of coal. Blue hydrogen is grey hydrogen – but the carbon produced is captured and stored. There’s also purple hydrogen – made using nuclear power – and the emerging turquoise hydrogen, made from methane but with pure carbon as the byproduct.

The reason why we aren’t producing more green hydrogen is down to its cost, which is significantly higher than its polluting cousins. There are challenges around the availability of renewable energy for most countries, and the competing demand for it. However, the price to produce green hydrogen will come down as demand increases and technology develops, some commentators say by the end of this decade.

According to the International Energy Agency, China is leading the way by a considerable head in both its operational production capacity for green hydrogen and its planned capacity. Looking further ahead to 2030, an analysis of hydrogen mega projects by Swedish consultancy Rystad Energy puts Australia, the US and Spain in the lead.

One of the challenges for any company in any part of the green hydrogen supply chain is that hydrogen production in most places is still very much in the planning stages. And plans can change.

Australia has the ambitious goal of growing a viable hydrogen industry for its home markets by 2030. Other region’s plans are equally stretching. The European Union wants to produce 10 million tonnes a year by 2030 and import another 10 million tonnes. The US has said it wants to produce 10 million tonnes of green hydrogen by 2030, ramping up to 20 million by 2040 and 50 million by 2050.

The Brazilian government has said it wants to be among the most competitive green hydrogen producer by 2030. India is aiming to produce 5 million tonnes of green hydrogen by the same date.

Salt caverns

With these ambitious production plans must come ambitious storage plans.

The most straight-forward way of storing hydrogen underground is in salt caverns, which are created by solution mining: injecting water into the rock via a shaft and extracting saturated brine. “Rock salt is a crystalline rock which means there are no holes in it, so it is ideal for holding gas in place,” explains Taylor. “It’s probably the only rock you could use to store hydrogen without a lining in it.”

Salt caverns have been used to store hydrogen as early as the 1970s. There are salt caverns in the UK and Germany that are still used for the purpose. Pilot projects involving salt caverns are looking to investigate the impact of rapid cycling of hydrogen, as hydrogen at pressure is injected and then withdrawn.

In Etrez, France, the Hydrogen Pilot Storage for Large Ecosystem Replication (HyPSTER) is converting salt caverns which have been used to store natural gas. Launched in January 2021, the project moved into a testing phase late last year, cycling different pressures of hydrogen in the cavern over three months. This year the project aims to increase production at the site so that the full capacity of the cavern, 50 tonnes, can be used by 2026.

Similarly, projects to trial salt caverns for hydrogen storage are underway in Ruedersdorf in Germany, as part of the HyCAVmobil research project. The cavern will contain just 6 tonnes of hydrogen with testing focusing on the storage and retrieval processes, including drying systems and assessing the purity of the hydrogen when it comes out again.

Of course, salt caverns are only a possibility where there are big enough deposits of rock salt. But it isn’t uncommon; Taylor reckons rock salt lies beneath around 10 to 20% of Europe’s land area.

If the ground does make them viable, salt caverns are likely to be the most cost-effective underground be useful for when hydrogen production and use ramps up – but before it is big enough to warrant the massive investment that will be required in storage and distribution networks.

“Mid-scale storage volumes are critical for energy transition,” says Travail.

Ardent envisages its shafts being used to store between 10 and several thousand tonnes of hydrogen, using multiple shafts for the higher volumes. For requirements in this bracket, shafts will be more economically viable than above- ground storage in pressurised cylinders, says Bentley.

He illustrates this by referencing a company looking to convert its ammonia production from grey hydrogen feedstock to green hydrogen: “Storing around 50 tonnes, their consultants estimated they would need 278 ISO containers, each containing nine cylinders above ground. Ardent could do that in one shaft with a much smaller footprint and the inherent safety advantages.” says Bentley.

Underground lined shafts could be deployed to store green hydrogen destined for a variety of uses: as buffer storage for an industrial process such as cement production; to store energy from a renewable electricity source so that it can top up power during peak demand periods; or for a hydrogen hub feeding different users.

Gravitricity has designed its lined shafts to provide storage for mid- scale volumes of hydrogen. Ardent Underground is looking for new uses for its parent company’s drill rigs, which are currently used for coal mines.

The clever thing about these shafts is the lining design which must allow deformations and pressure cycling whilst transferring loads to the rock mass – which could vary in type over the depth of the shaft. Ardent’s design – patent pending – features a double-skinned wall with thin steel as its inner liner and a flexible membrane as its outer skin. Between the two skins is a fluid which is pumped in and out to change the volume as the rock mass deflects under the pressure forces.

This means the inner liner can be low-carbon steel, which hydrogen does not interfere with, and that it will not be under stress since the fluid-filled chamber will manage the deformations of the host rock structure. This system will also help manage differing deformations as the shaft passes through various strata, says Bentley.

Trevail can’t give too many details about the lining system for Gravitricity’s system but does say this: “There are structural elements and a gas tight liner to store the hydrogen safely. Hydrogen embrittlement effects mean that the material selection is more onerous than for other gases. The way the lining transfers the load to the rock is our intellectual property.”

Ardent Underground is looking for new uses for its parent company’s drill rigs, which are currently used for coal mines.

It sounds like the two systems will deploy different means of preventing the chambers from popping out of the ground due to uplift. Ardent’s has a shoulder at the top of the tank which will activate a cone of ground above it to keep it down. Gravitricity is considering combinations of ground mass and active measures, says Trevail.

For both Ardent and Gravitricity, the biggest hurdle they face is moving their technology into pilot projects in the ground. In addition to funding from Governments and others, both will need to join forces with industry partners.

The challenge here is that future producers and users of hydrogen don’t yet know how things will pan out from a policy perspective. “There is uncertainty in what the production capacity and offtake demand will be,” says Trevail. “There are lots of projects being planned and spoken about but what will take place in the future is unknown.”