Huge weights suspended in mine shafts could store renewable energy with gravity

They were dug to carry coal to the surface, but disused mine shafts could soon form part of a much more environmentally-friendly energy system.

The Gravitricity storage concept would use weights of up to 2,000 tonnes suspended in deep shafts. When there is excess electricity in the grid – on a windy day, for example – the weight would be winched to the top. When there is not enough electricity, the operators would release the weight, using its movement to power generators.

This week, a report by independent analysts at Imperial College London predicted that the concept could offer a lower cost of energy storage than all alternatives, including lithium-ion batteries.

After receiving a £650,000 grant from Innovate UK in February, Edinburgh start-up Gravitricity has signed an R&D agreement with Dutch lifting multinational Huisman to develop a 250kW concept demonstrator and test it in the Netherlands and Scotland early next year.

The idea itself is “fairly straightforward,” said Dr Jenifer Baxter, head of engineering at the IMechE, who compared it to similar systems which pump water uphill to release it when energy is needed.

Stability of land around the mine shafts will be a key issue to deal with, she told Professional Engineering – especially given the “quite significant” weight.

“The surface has to hold the total weight, and in mining areas there are a lot of different shafts and directions of tunnels, which means sub-surface movement is possible,” she said. “They would have to do a lot of work to ensure the stability.”

Mines are often in remote places, she added. “Some of these places where there are deep mine shafts might not be easy to connect to the grid... what is the cost of extending to these areas and how many substations would they have to have?”

Despite the potential hurdles, she said the concept could provide vital storage for the growing renewable energy sector and bring jobs to former mining areas. “I live in South Wales and there are a lot of disused mines,” she said. “It could bring jobs and opportunities to parts of the country that don’t have that.”

The report by researcher Oliver Schmidt compared storage techniques on a “level playing field,” considering factors such as system lifespans and expected cost reductions. It found the Gravitricity system could be a cheaper alternative to lithium-ion batteries, which degrade over time, thanks to zero degradation and the potential for multiple daily cycles.

In a scenario requiring 700 cycles of 15 minutes per year, at a power output of 4MW, the system had a predicted ‘levelised cost of storage’ of $141 per kilowatt-year, outperforming all alternatives. “I don’t expect Gravitricity to displace all lithium batteries on grids, but it certainly looks like a compelling proposition,” said Schmidt.

The price of battery storage is falling quickly but Gravitricity’s costs could drop even quicker, said managing director Charlie Blair. “The most encouraging conclusion is that we will be even more competitive in 2025 than we are now,” he added.