Going underground: Gravitricity on the use of hydrogen for energy storage

Although green hydrogen is a major element in net zero plans, current and future salt caverns will be insufficient to meet all of our hydrogen storage needs. Additional new underground solutions will be required, says Martin Wright, founder and executive chairman of Gravitricity.

Green hydrogen is set to be a major component of low carbon energy systems of the future.

Although the debate is still open on the role this zero-carbon emissions gas will play in domestic settings, it is clear that green hydrogen – produced via the electrolysis of water using renewable energy – will have a significant role in industry, and as a strategic power reserve.

Hydrogen is already a major component in an array of industrial processes, from ammonia production for use in fertilisers, to refining, and myriad other uses – but in the vast majority of cases we still rely on what is known as ‘grey’ hydrogen produced via the highly polluting steam reforming process using methane or natural gas.

Worldwide hydrogen production is currently around 120 million tonnes a year, creating carbon emissions greater than that of aviation, and almost greater than that of aviation and shipping combined.

If we are going to have any hope of reaching net zero, our first step should be to replace this high carbon feedstock with its low carbon alternative.

Industrial users

In parallel, we should look to use green hydroegn as a high temperature fuel – for things that cannot be electrified or decarbonised in other ways – and again many of these will be industrial process, such as the production of green steel.

Furthermore, green hydrogen may have a strategic role to play in long duration energy storage – soaking up spare electrons when the grid is over capacity, and generating green power when renewables cannot. This could be day to day, or for weeks, or even longer.

So, whilst the full scale of green hydrogen’s role is still to be determined, it is clear that demand will be high. The respected Climate Change Committee’s 6th Carbon Budget suggests 250-460 TWh of hydrogen could be needed in 2050, making up 20-35 per cent of UK final energy consumption.

But if this is the case, we will need to find ways to store it – safely and affordably.

With natural gas – we are able to rely on the vast natural reserves beneath the sea, and on ‘linepack’ – basically storing gas within our existing pipe network.

However, hydrogen is different. Green hydrogen is a wonder gas – it can be created from water and renewable energy and produces zero emissions (when used in fuel cells). But on the flip side, it is highly flammable, with much smaller molecules than the natural gas on which we rely, and it cannot be stored in the same way without significant modifications to the current network.  

But as green hydrogen production ramps up, the ability to store large quantities of this gas for extended periods will become a critical building block in our low carbon future.

Salt caverns

At present, the only proven technology for storing large quantities of pure hydrogen is within underground salt caverns, in areas where there is suitable geology.

The UK currently has 0.025 TWh of salt cavern hydrogen storage, with two notable projects in development, namely HyKeuper in England’s northwest and Aldbrough near the strategically important Humber, that will add a further 1.3TWh and 0.5TWh storage capacity, respectively.

But these will take many years to build, and whilst the UK is fortunate to have suitable geology for salt cavern storage, there is simply not enough of it. Plus, they can only be built in very specific locations, which don’t always match up with where current or future storage demands will be situated.

A recent report by global energy consultants Arup in partnership with University of Edinburgh and supported by the British Geological Survey, suggests we may have overestimated UK salt cavern capacity by as much as 90 percent and states: “Without immediate action, hydrogen storage capacity limitations threaten UK progress to net zero.”

Mid-scale storage

Therefore, the future large-scale adoption of green hydrogen in industrial processes and for heat, transport and backup generation will also require safe and cost-effective mid-scale hydrogen ‘buffer’ storage – up to 100 tonnes per unit – which can be located wherever required, both in distribution networks and at point of use.

This could be for industrial off-takers – such as ammonia or steel producers – where there may be on-site requirements for 500-1,000 tonnes of hydrogen storage to provide one to two days buffer storage.

In addition, as parts of the GB gas network is repurposed for hydrogen, there will be requirements for localised nodal ‘linepack’ storage – in the 100s of tonnes, in tandem with the use of large-scale salt cavern storage.

In other words, it is not ‘either/or’ for large scale and mid-scale storage – we will need both.

Already National Gas – the UK’s gas transmission system owner and operator – is investigating the potential of hydrogen transportation using the current network infrastructure, with a requirement for mid-scale storage at its core.

This initiative, known as Project Union, is a pioneering project to create a British hydrogen backbone, capable of transporting 100 percent hydrogen, connecting hydrogen production and storage with industrial end users across Britain.

Through the phased repurposing of existing high pressure gas transmission network infrastructure – alongside the construction of selected new pipelines – Project Union aims to create a hydrogen network of up to 2,000km, equivalent to 25 per cent of Britain’s current methane transmission network.

As part of this, medium-scale hydrogen ‘buffer storage’ will be key.

The idea of buffer storage is not new.  In the last century, in the time of manufactured ‘town gas’, buffer storage came in the shape of the gigantic, drum-shaped gasometers which still feature in so many of our cities.

But in the modern era, above-ground storage is expensive and will face planning challenges – and could be a potential security risk at a time when bad actors are actively targeting energy infrastructure.

This means there are no clear and obvious ways for end users and for network operators to safely and affordably store significant quantities of green hydrogen as part of the new low carbon economy.

The race to net zero is a major challenge to all nations and there is no ‘one size fits all’ technology which will get us there.

Green hydrogen will certainly have a role to play – but to unlock its potential, we need to find new ways to store this flammable ‘wonder gas’. Salt caverns will provide large volume storage, but we will also need mid-scale storage which can be located in multiple nodes around the future gas network and also in close proximity to major industry off takers. I believe the answer to this lies right beneath our feet.