In a quest to find effective long-term energy storage solutions from variable renewable energy sources such as solar and wind, the International Institute for Applied Systems Analysis (IIASA) led a study with a team of international researchers. They came up with an innovative technique called underground gravity energy storage (UGES) that uses existing resources in a revolutionary manner to support the global transition to sustainable, clean energy production.

Batteries are an effective method of daily energy storage, but there are few accessible, cost-effective options for long-term storage. Because batteries lose energy over time through self-discharge, they are ineffective for long-term storage. Further complicating that prospect, lithium-ion batteries use nonrenewable, finite, already rare elements that are expected to become increasingly scarce in the future, according to the World Economic Forum. 

A recent report from market intelligence firm Infiniti Research Limited projects that the global off-grid energy storage system market will grow by $6.22 billion between 2022 and 2026—an acceleration rate of 7%. To reach a net-zero energy future, 6 terrawatt-hours of energy storage will be necessary.

Unlike other storage methods, this form of gravity storage uses sand as an energy-storage medium. Sand does not lose energy to self-discharge. Thus, energy can be stored anywhere from several weeks to several years.

Beyond efficient energy storage, this plan has several benefits. It converts decommissioned (and sometimes hazardous) mines into cost-effective, long-term energy storage facilities by transporting sand into abandoned underground mines. When the price of electricity is high, UGES generates electricity by converting the potential energy of the sand, first lowering it into mines, applying regenerative braking to convert kinetic energy into a useful form, and then using electric motors to lift the sand to an upper reservoir, where energy will be stored when prices are low.

An added benefit of this process is the creation of jobs to partially make up for the thousands of workers mines lay off when they shut down.

Investment costs for UGES amount to about $1–10 per kilowatt-hour.

“Mines already have the basic infrastructure and are connected to the power grid, which significantly reduces the cost and facilitates the implementation of UGES plants,” said Julian Hunt, a research scholar with IIASA’s Integrated Assessment and Climate Change Research Group and the lead author of the study. Components of a UGES system consist of a vertical shaft, a motor or generator, upper and lower storage sites and the mining equipment itself. Power capacity costs are roughly $2,000 per kilowatt. More power can be extracted from deeper and broader mineshafts. Larger mines have more storage capacity.

It’s thought that the number of defunct mines will be in the millions globally, offering ample possibilities to apply this technique. The United States, China, India and Russia have the highest potential of capitalizing on this process.