Are salt caverns the UK’s answer to large-scale long-duration energy storage? They are vast underground chasms historically used to store natural gas and other hydrocarbons. These former fossil fuel assets can be repurposed to play a significant role in supporting the energy transition. Salt caverns can be repurposed into green energy storage facilities which hold huge quantities of compressed air or hydrogen, providing an abundant energy source with no harmful emissions.
How many Salt caverns does the UK need for net zero?
The UK is legally bound to reach net zero by 2050. To achieve this goal, Britain needs to increase renewable energy storage capacity significantly. A 2023 report by The Royal Society projects that to achieve its net zero goals, the UK will need 570TWh of storage by 2050. The UK was only projected to reach 11.6GWh storage by the end of 2024, so there is still a lot of progress to be made.
Currently, battery energy storage systems (BESS) dominate the energy storage market. These have a short duration and low capacity. It would take more than 600,000 of the largest Li Ion batteries to reach 100TWh, which is not feasible from a cost or space perspective.
These geological stores can be as deep as the Eiffel Tower is tall. It would theoretically only take 300 salt caverns filled with hydrogen to store the same amount of energy. Utilising geological energy storage alongside BESS infrastructure would push the UK much closer to this 570TWh target.
It is estimated that there are between 200-300 salt caverns in the UK, many of which have previously been used for natural gas storage. These caverns could be quickly repurposed for green energy storage. It is also a relatively easy process to create new salt caverns through solution mining into salt fields.
The Formation of Salt Fields
Salt fields are vast underground salt deposits that formed 200-300 million years ago, when they were inland seas. As the surrounding land rose over time, these seas were cut off from the ocean. Eventually, much of the water evaporated, leaving behind massive underground deposits of salt. These deposits hardened into a rock salt, also called halite, which over millennia have been buried under layers of earth.
These deposits can be found up to 300m underground and are detected using sonar and ultrasound.
There are large salt fields located in Teesside, Cheshire, and the East Riding of Yorkshire. Other notable sites include the Fylde coast, the Northern Ireland coastline, and Dorset.
Solution Mining: What is it?
Solution mining is a highly effective method for creating salt caverns. The process involves injecting pressurised water deep underground to dissolve water solvable minerals, like salt. As the salt is extracted, it leaves behind, large, airtight chasms, ideal for storing compressed air or hydrogen.
A deep, narrow well is drilled into the ground until it reaches the salt field. Water is pumped into the mine from the surface which dissolves the halite. This creates brine which is 10X saltier than seawater!
The brine is then extracted through a separate pipe and can be repurposed both commercially and industrially.
It can be used to thaw roads in icy weather, as an ingredient in water softeners and dishwasher salt, for chlorine and caustic soda production, and can even be used to flavour crisps!
Solution mining is not a new invention; Holford minefield in Cheshire has been using this process since the 1920s. Currently, INEOS maintains the site, and over its lifetime more than 200 salt caverns have been created there to store natural gases.
Salt cavern projects in the UK: Are there any Risks?
Solution mining is almost exclusively subterranean. This means that disturbances to the surface are minimal. Once all the boreholes are drilled and the pumping equipment removed, only a few trailer-mounted cabins remain to manage the wellheads, and the pipeline connected to them (often this pipe is buried too!).
Some have raised similarities between salt mining and fracking, when in reality, these processes are very distinct from each other. Fracking uses hydraulic fracturing fluid to create cracks in the targeted rock formations. This releases noxious chemicals, grit and sand underground, which can destabilise the rock and disturb the surrounding ecology. Fracking also requires ten times the pressure that solution mining does, which can cause earthquakes or other disturbances. Compressed Air Energy Storage stores air or hydrogen at the same pressure as the rock surrounding the caverns, keeping the geology stable.
Why the UK needs to utilise salt caverns
Without large-scale long-duration storage, there is no net zero, and no energy security. Geological storage in salt caverns provides storage at a scale that combats the intermittency of wind and solar power, enabling renewables to power the grid.
Through solution mining salt fields and repurposing old caverns, the UK can rapidly increase geological storage capacity for renewables. This will put the UK on track to reach its goal of 100TWh storage and allow us to slash fossil fuel use, bringing us closer to achieving net zero by 2050.
