The use of vertical shafts in decommissioned underground mines for energy storage using gravity could provide a viable alternative to battery energy storage (BESS) and underground pumped hydro storage systems, a new International Institute for Applied Systems Analysis (IIASA)-led study, with participation from an international team of researchers, said.

The study comes at a crucial time when the world is looking at accelerated solutions to tide over the intermittency of renewable sources with cost-effective and environment-friendly technology. Variable renewable energy (VRE) sources require energy storage options to reliably match energy demand at different time scales.

The system’s technical lifespan can range from 20-30 years, the study said. It added that its storage potential ranges from 7-70 TWh globally, with most of this potential concentrated in China, India, the U.S., and Russia.

The proposed technology called Underground Gravity Energy Storage (UGES) has a zero self-discharge rate, unlike BESS. Further, the technology uses sand which eliminates the prospects of contamination of underground water resources as opposed to the pumped hydro storage system (PHS).

The UGES technology in the vertical mine shafts would use electric motors and generators for lifting and dumping large volumes of sand. It 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.

Fig: A Schematic of Different Sections of the Underground Gravity Energy Storage System

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

The UGES technology can produce a modest but constant amount of energy for a long time. It could also be designed to store energy over weekly, monthly or seasonal scales, depending on the demand for energy storage.

“To offset the short-term changes in electricity consumption of solar and wind generation, this modest but consistent electricity generation might be supplemented with other storage technologies, such as batteries and PHS,” the study said.

It added that the cost of installed energy storage for UGES is estimated to vary from $1–10/kWh, assuming an average height difference between the upper and lower storage sites of 1500 and 200 meters, respectively.

Further, the study also said that the project is less expensive the more significant the height difference. The power generation capacity varies with the mine’s depths, the mine shaft’s diameter, and the sand moving speed.

“This paper proposed constructing several motors/generators along the shaft to reduce the cables’ costs and allow using smaller, more common/affordable motors/generators,” it said.

Recently, government-owned NTPC signed an agreement with Energy Vault Holdings to collaborate on deploying the latter’s gravity-based energy storage technology.

Further, SECI had sought proposals for research and development to demonstrate scalability in gravity-based energy storage systems.