As renewable energy sources continue to expand their share of the grid, the search for long-duration energy storage (LDES) solutions capable of smoothing out their intermittency has intensified.
While electrochemical batteries remain the most familiar option, they come with well-known drawbacks, including degradation over time, thermal runaway risk and supply chain bottlenecks.
A growing number of developers are instead looking underground, repurposing disused mines and salt caverns to store energy at scale.
That shift was the focus of a recent episode of the Energy Technology podcast, which explored two underground storage technologies: compressed air energy storage (CAES) and underground gravity energy storage (UGES).
The episode featured Storelectric CEO Tallat Azad, Green Gravity founder and CEO Mark Swinnerton, and Edward Barbour, associate professor of energy systems and storage at the University of Birmingham.
CAES works by storing air and heat together.
As Barbour explained on the podcast, “you essentially combine these two things when energy is required to generate hot, pressurised air, which is used to drive a turbine”.
Efficiency losses are unavoidable, however, Storelectric is targeting around 62 per cent efficiency, while Azad said its two existing CAES plants currently operate at closer to 42 per cent.
“The actual energy that is usable on the other end in the grid would be less than that [in the cavern] by virtue of what the efficiency of the plant is,” he told the podcast.
UGES relies on an entirely different principle: gravitational potential.
“UGES moves weights up and down,” Barbour said.
“You are storing gravitational potential energy when the weight is at the top, and then you are releasing that via a motor as the weight is lowered.”
Swinnerton explained why old mine shafts suit the technology so well: “The physics that drives that is energy equals mass times height. So, our technology is serviced by having a lot of mass and a lot of height.”
According to Barbour, each technology tends to favour different sites: “Proponents of each technology will state that they are all likely to have certain locations where that specific technology is best.”
For CAES, salt caverns roughly 500 metres deep offer the best economics, potentially bringing costs below £5 (US$6.75) per kilowatt-hour, as noted by Barbour.
Azad added that salt caverns can last for generations once built, noting that salt is basically self-sealing, unlike hard rock mines, which carry greater leakage risk.
For UGES, disused mine shafts are prized for their depth.
Swinnerton pointed to the sheer scale of the opportunity: “There are nearly two million mines globally that are currently closed.”
Medium-sized shafts could offer 20 to 50 megawatt-hours of storage, while larger shafts could hold between 150MWh and 350MWh of energy, with multiple shafts at a single site pushing potential into the gigawatt-hour range.
On the CAES side, Storelectric’s newly acquired 550-acre Teesside site offers caverns averaging around 60,000m³, equivalent to roughly 250MWh, or enough for 70,000 or 80,000 homes for an hour, according to Azad, before cautioning that turbine efficiency losses reduce the usable output.
Both technologies remain in early stages of deployment, but momentum is building.
Barbour flagged hydrogen and biomethane storage in salt caverns as particularly promising next steps, along with underground heat storage, which he described as some of the lowest hanging fruit for the sector.
