An international team of researchers has developed a novel way to store energy by transporting sand into abandoned underground mines. The new technique, called Underground Gravity Energy Storage (UGES), proposes an effective long-term energy storage solution while also making use of now-defunct mining sites.

In a paper published in the journal Energies, the scientists explain that UGES generates electricity when the price is high by lowering sand into an underground mine and converting the potential energy of the sand into electricity via regenerative braking and then lifting the sand from the mine to an upper reservoir using electric motors to store energy when electricity is cheap.

Regenerative braking is an energy recovery mechanism that slows down a moving vehicle or object, such as an elevator, by converting its kinetic energy into a form that can be either used immediately or stored until needed. In other words, the electric traction motor uses the vehicle’s momentum to recover energy that would otherwise be lost to the brake discs as heat. Regenerative braking system lifts are already applied in newly highly energy-efficient buildings.

Based on this principle, the main components of UGES are a vertical shaft, a motor/generator, upper and lower storage sites, and mining equipment. Using the shaft and electric motor/generators, large volumes of sand are lifted and dumped. The deeper and broader the mineshaft, the more power can be extracted from the plant, and the larger the mine, the higher the plant’s energy storage capacity.

“When a mine closes, it lays off thousands of workers. This devastates communities that rely only on the mine for their economic output. UGES would create a few vacancies as the mine would provide energy storage services after it stops operations,” Julian Hunt, lead author of the study and a researcher at the International Institute For Applied Systems Analysis, said in a media statement. “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.”

According to Hunt, other energy storage methods, like batteries, lose energy via self-discharge over long periods. The energy storage medium of UGES is sand, meaning that there is no energy lost to self-discharge, enabling ultra-long time energy storage ranging from weeks to several years.

The researcher noted that the investment costs of UGES are about 1 to 10 USD/kWh and power capacity costs of 2 USD/kW. The technology is estimated to have a global potential of 7 to 70 TWh, with most of this potential concentrated in China, India, Russia and the United States.

“To decarbonize the economy, we need to rethink the energy system based on innovative solutions using existing resources. Turning abandoned mines into energy storage is one example of many solutions that exist around us, and we only need to change the way we deploy them,” study coauthor Behnam Zakeri said.