Storing Renewables Via Regenerative Braking in Decommissioned Mines

Regenerative braking is an energy recovery mechanism that takes the wasted energy from the process of slowing down a vehicle. The proposed underground gravity energy storage (UGES) concept consists of lowering sand into an underground mine and lifting this sand to an upper reservoir via electric motors to store energy at moments of low power demand.

“UGES uses regenerative braking to adjust the descent speed of the sand in the mine shaft and generate electricity,” researcher Zakeri Benham told pv magazine. “It has the advantage of making use of existing infrastructure and providing clean, cheap, and long-duration energy storage with no material intensity, on top of being already close to the electricity grid and roads.”

The system consists of a shaft with variable depths and diameters, a motor/generator, upper and lower storage sites, and mining equipment.“To maximize power capacity, the sand containers in the shaft occupy approximately 50% of the shaft’s volume,” the scientists said. “The other 50% of space is required for filling and emptying the containers with sand.”

The charging mode consists of collecting sand from the lower storage site at the bottom of the mine with excavators and transporting the sand to the shaft with electric trucks or conveyor belts. Energy is stored using cheap or excess electricity from the grid or a large solar PV farm nearby to pull the sand up via the shaft with the motors/generators. After the sand reaches the top of the mine, the sand is stored in sand piles.

The discharging mode consists of collecting sand from the upper storage site on top of the mine with bucket wheel conveyor belts and transporting it to the mine shaft using a small amount of electricity. Electricity is generated by lowering the heavy volumes of sand in the shaft. Several motors/generators generate electricity with regenerative braking along the shaft. After the sand reaches the bottom of the shaft, it is transported by conveyor belts or electric trucks to fill up the entire mine caverns with sand.

“The investment costs of UGES are about 1 to 10 USD/kWh and power capacity costs of 2.000 USD/kW,” the scientists said. “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 USA.”

They presented the system in the paper “Underground Gravity Energy Storage: A Solution for Long-Term Energy Storage,” which was recently published in Energies. Other research groups at IIASA recently developed an energy storage technique based on the use of electric trucks for hydropower generation at high altitudes and a gravitational energy storage technology for weekly cycles in high-rise buildings in urban environments.