Gravitricity H2 FlexiStore underground hydrogen storage

Going underground – a new approach to hydrogen storage

By Robin Lane, Commercial Director at Gravitricity.

This article first appeared in Energy Voice.

Large industrial hydrogen hubs are coming our way and will be key to decarbonising those ‘hard to reach’ sectors and activities. But storing hydrogen is hard, and existing approaches are unlikely to be a good fit in these settings. At Gravitricity, we are developing an approach to hydrogen storage that is specifically designed to deliver on what these industrial hubs will need.

The vital role of hydrogen

It’s clear that hydrogen produced from renewable energy will play an important role in supporting our transition away from fossil fuels to a low carbon energy system. But the precise nature of that role is more contentious. It is not obvious to everyone, for example, that the ‘hydrogen-ready’ domestic boilers we’re being promised will ever have these capabilities tested (much like my ‘Ferrari-ready’ driveway).

But while electrification, if practical, will often be the best route to decarbonisation, there are some applications – high grade industrial heat, large scale transportation and long duration energy storage among them – where electrification will be more challenging. It is here that hydrogen will step in, providing a low carbon option where there are few alternatives. It is likely that these use cases will largely drive the growth in global hydrogen demand, projected by the IEA to double to reach 180 Mega tonnes (Mt) by 2030.

If this is right, (and we’re not the only ones saying so) then it is likely that green hydrogen will be produced, stored and consumed close by at large, industrial sites with one or more significant users, often around the coast where cheap offshore wind generation can power the hydrogen electrolysers and compressors.

Hydrogen Storage

The ability to store hydrogen at these sites will be critical to ensure users have reliable flows when needed. So which hydrogen storage solutions are most suitable for the requirements of these industrial sites? Two alternatives are commonly proposed:

  1. First, Salt Caverns. These are very large underground spaces created by injecting water to dissolve geological rock salt. The storage potential of each cavern is large, but they can only be situated where suitable salt formations exist. To take the UK as an example, only areas within Cheshire and East Yorkshire are known to have these formations, so Southeast England, as well as existing industrial clusters in South Wales and in Scotland would be some distance from their nearest storage facility. What this means in practice is that storing hydrogen in salt caverns will mean expensive upgrades to the gas network infrastructure – a consideration which is invariably overlooked by analysis claiming that these caverns offer a low cost per unit of hydrogen stored. If you add in problems with hydrogen purity, slow access, and even slower lead times, it’s tempting to question whether salt caverns are the right storage solution for industrial hydrogen hubs. 1
  2. At the other end of the scale are Pressurised Metal Vessels. Unlike the caverns these vessels can be located anywhere. But hydrogen storage potential per cylinder is much smaller, and with all the metal required to contain the pressurised hydrogen, they are an expensive option which will take up valuable space on site while presenting a health and safety hazard to nearby (and not so nearby) infrastructure.

Between them these approaches span very large scale and relatively small-scale storage capacity. Both have significant shortcomings, and don’t offer the mid-scale storage solution most likely to be appropriate for the industrial hydrogen hubs of the future.

A new approach to hydrogen storage

Gravitricity is best known for the solid weight gravity-based energy storage technology which we’ll be deploying in old mine shafts. By finding new uses for old coal mines, we’re demonstrating how the infrastructure of the old energy system can be re-used to enable the new. But we’ve always believed that underground spaces have energy storage potential beyond gravity, and so more recently, alongside our gravity technology development programme, we’ve been focussing on how fuel gases, particularly hydrogen, can be stored safely and effectively underground.

Our solution, which we call H2 FlexiStore, is based on a gas tight metal liner within an underground shaft, where the surrounding geology will exert pressure on the container enabling higher pressures (and more hydrogen) at a lower cost. Crucially, the storage capacity of around 100 tonnes of hydrogen per shaft offers a ‘Goldilocks’ mid-scale option specifically calibrated to deliver the likely needs of industrial hydrogen hubs.

FlexiStore will have a number of significant advantages over traditional approaches to hydrogen storage.

FlexiStore is a mid-scale solution, storing hydrogen in the quantities required to match supply via electrolysis with the demand from onsite industrial activities.

H2 FlexiStore can be located where it is needed, and is not dependent on particular geological formations or expensive hydrogen transport infrastructure.

Limited transportation infrastructure and lower metal requirements make the H2 FlexiStore solution highly competitive in cost / unit of hydrogen stored.

By storing hydrogen underground H2 FlexiStore is inherently safer than above ground storage as no oxygen is present to create an explosive mix.

H2 FlexiStore’s steel lining means that hydrogen can be stored underground while maintaining purity. Alternative underground options will require expensive ‘scrubbing’ equipment to remove contaminants.

We have recently concluded a feasibility study which confirms the viability of this approach and are now building upon this work with an accelerated technology development project. At the same time, we are actively building partnerships with people and organisations who, like us, see the vital role which hydrogen will play in our decarbonised energy system, and who understand that the infrastructure to support this new hydrogen economy will need to fit the specific requirements of green hydrogen producers and consumers.

1 100 tonnes of hydrogen contains about 3GWh of energy, and if the system was fully charged and discharged each day it would absorb the average energy from a 500MW offshore wind farm.

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