As intermittent renewables expand, so does the demand for long duration energy storage (LDES). However, different geologies and applications require tailored solutions, and increasingly developers are looking to the subterranean.
Underground storage bypasses some limitations of traditional electrochemical batteries, including degradation, thermal runway and supply chain constraints. However, compressed air energy storage (CAES) and underground gravity energy storage (UGES) bring their own techno-economic challenges.
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In this episode, we speak to two underground energy storage providers to understand how the sector is taking shape on, and under, the ground. Storelectric CEO Tallat Azad addresses CAES while Green Gravity founder and CEO Mark Swinnerton unpacks the technicalities of UGES. We also hear from Edward Barbour, associate professor of energy systems and storage at the University of Birmingham, about the broader landscape.
How CAES and UGES work
CAES works by storing air and heat. Barbour explains that “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 inevitable. While Storelectric hopes to achieve an efficiency of around 62%, Azad notes that its two CAES plants are currently at around 42%. “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 explains.
UGES is an entirely different concept, which relies on gravitational potential.
“UGES moves weights up and down,” says Barbour. “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 explains why mine shafts are a good location for UGES: “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.”
Assessing the economics of mines versus salt caverns
“Proponents of each technology will state that they are all likely to have certain locations where that specific technology is best”, says Barbour.
In CAES, it is geology that make salt caverns an ideal location. Barbour says that in areas where salt deposits have the right depth of around 500m, CAES becomes economical.
“In the right location, we might find that we can get the cost for CAES below £5 ($6.75) per kilowatt-hour (kWh).”
Azad adds that salt caverns offer longevity: “Once you have actually produced a salt cavern, there are a certain amount of ten-year maintenance checks that you have to make, but they will last for decades, if not hundreds of years.”
He adds that hard rock mines have been considered for CAES but that there is a leakage risk. Meanwhile, salt offers a unique advantage in that “it is basically self-sealing”.
Meanwhile, disused mines are “one of the best places you can find height”, according to Swinnerton, giving them a stronger techno-economic profile in UGES. Similar to salt caverns, mine shafts also offer longevity, with infrastructure lasting decades or longer.
There is also scaling potential. “There are nearly two million mines globally that are currently closed […] so the first reason to repurpose mines for energy storage is to repurpose mines for their land, their assets, their position”, says Swinnerton.
Potential capacities of CAES and UGES
In UGES, storage capacity depends on the mass and the height. Swinnerton says hundreds of thousands of disused mines would be classed as medium-sized mine shafts, offering 20 to 50 megawatt-hours (MWh) of potential energy storage.
In comparison, larger mine shafts, which are also “plentiful”, hold even more capacity potential: “We are talking between 150MWh and 350MWh of energy in a [large] shaft, so very large energy densities. Given that many mines that hold those shafts can have many shafts – we can talk about ten mine shafts at a single mine, often in that space – then we are talking about gigawatt-hour (GWh) storage potentials for our technology deployed to those.”
In CAES, storage capacity depends on the size of the cavern. Storelectric has recently acquired a site around 550 acres in Teesside, rich in caverns thanks to its history as a major industrial and chemical hub. Azad says the caverns average around about 60,000m³, which equates to around 250MWh of energy – “probably something like 70,000 or 80,000 homes for an hour using that cavern”.
However, he adds a crucial caveat: “That is the energy density in the cavern itself. When you actually come to release that energy through a turbine, there are then efficiency losses that you have to take into account. The actual energy that is usable on the other end in the grid would be less than that.”
Underground innovation
Underground LDES technologies are still nascent. Green Gravity commissioned its first pilot plant in 2023 while Storelectric’s recent acquisition of Teesside site marked the culmination of a three-year due diligence process.
But innovation in the space continues to grow momentum, driven by the increasing need for tailored storage solutions to support grid stability.
Barbour considers the ripest opportunities for salt caverns: “Hydrogen storage in salt caverns is, in my opinion, the leading technology. Biomethane in salt caverns is also very exciting.”
He also calls heat storage “some of the lowest hanging fruit”, noting that it “is of major interest because storing heat we can generally do for relatively long times quite cheaply, and therefore it may make sense as an option where the energy is required as heat later on anyway”.
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