Scientists propose converting abandoned mines into gravity batteries

Called Underground Gravity Energy Storage, the new technique proposes an effective long-term energy storage solution utilizing now-defunct mines.
Deena Theresa
Illuminated night shot of open surface coal mining.
Illuminated night shot of open surface coal mining.

Christian Meurer/iStock 

There is no doubt that renewable energy sources are the future. They're available in all countries, but their potential is yet to be fully harnessed. According to The International Renewable Energy Agency, 90 percent of the world's electricity should come from renewable energy by 2050.

Now, sources like sunshine and wind are inconsistent so finding innovative ways to store energy in an accessible and efficient way is crucial. There are effective solutions for daily energy storage, like batteries, but a cost-effective long-term solution is lacking.

In a new International Institute for Applied Systems Analysis-led study, a team of researchers has developed a novel way to store energy by transporting sand into abandoned underground mines. Called Underground Gravity Energy Storage (UGES), the new technique proposes an effective long-term energy storage solution utilizing now-defunct mines, which number in the millions globally.

Their work is published in MDPI.

Scientists propose converting abandoned mines into gravity batteries
Underground Gravity Energy Storage system: a schematic of different system sections.

The potential energy of the sand is converted into electricity

"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," Julian Hunt, a researcher in the IIASA Energy, Climate, and Environment Program and the lead author of the study, said in a statement.

How does this method work? 

The primary components of UGES are the shaft, generator, upper and lower storage sites, and mining equipment. UGES generates electricity when the price is high by lowering sand into an underground mine. The potential energy of the sand is then converted into electricity via regenerative braking. Then, the sand is lifted from the mine to an upper reservoir with the help of electric motors to store energy when electricity is cheap.

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, as per the release.

No energy is lost to self-discharge

Since the energy storage medium of UGES is sand, there is zero energy lost to self-discharge, unlike normal batteries. This permits ultra-long time energy storage ranging from weeks to several years. 

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The researchers note the investment costs of UGES to be about one to 10 USD/kWh and power capacity costs of 2.000 USD/kW. And the technology is estimated to have a global potential of seven to 70 TWh, with most of this potential concentrated in China, India, Russia, and the USA.

"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," said Behnam Zakeri, study coauthor and a researcher in the IIASA Energy, Climate, and Environment Program.

Study Abstract:

Low-carbon energy transitions taking place worldwide are primarily driven by the integration of renewable energy sources such as wind and solar power. These variable renewable energy (VRE) sources require energy storage options to match energy demand reliably at different time scales. This article suggests using a gravitational-based energy storage method by making use of decommissioned underground mines as storage reservoirs, using a vertical shaft and electric motor/generators for lifting and dumping large volumes of sand. The proposed technology, called Underground Gravity Energy Storage (UGES), can discharge electricity by lowering large volumes of sand into an underground mine through the mine shaft. When there is excess electrical energy in the grid, UGES can store electricity by elevating sand from the mine and depositing it in upper storage sites on top of the mine. Unlike battery energy storage, the energy storage medium of UGES is sand, which means the self-discharge rate of the system is zero, enabling ultra-long energy storage times. Furthermore, the use of sand as storage media alleviates any risk of contaminating underground water resources as opposed to an underground pumped hydro storage alternative. UGES offers weekly to pluriannual energy storage cycles with energy storage investment costs of about 1 to 10 USD/kWh. The technology is estimated to have a global energy storage potential of 7 to 70 TWh and can support sustainable development, mainly by providing seasonal energy storage services.