Unearthing clean energy storage

Solar panels and wind turbines provide clean, renewable power when weather conditions are right, but people still need electricity at night and on windless days.

There needs to be some type of storage system to capture energy for future use. One solution is to deploy lithium-ion batteries for renewable energy storage, but they require the use of mining and eventually degrade. Underground gravity energy storage (UGES) is a proposed solution to this problem.

Abandoned mines pockmark the world, vestiges of more prosperous days. They have no current use and in many cases are even dangerous.  But they also quite literally hold potential. Instead of sitting empty, they can serve as sites for giant, potential-energy-based storage systems.

Although underground gravity energy storage is still in the conceptual stage, researchers have fleshed out what it might look like in a 2022 publication. Each mine shaft could hold a large container of sand occupying 50% of the volume of the shaft. The other half of the mineshaft would be empty. The UGES system would include the mineshaft, motor, cable, and containers for the sand.

In times of excess energy production, such as during very sunny or windy weather, the motor-powered cable would hoist the sand container to a high point. Like a ball poised on the edge of a skyscraper, the higher the container sits, the more potential energy it will hold — in other words, the harder it will fall.

This controlled fall would power a turbine during times of peak energy demand or low energy generation, just as water powers a turbine in hydroelectric dams. This stage is called discharge and works on the concept of regenerative braking.

Like a compressed air storage system, the power would come from the release of stored potential energy being converted into kinetic energy, and the system would absorb excess renewable energy during times of increased output.

Once at the bottom of the mine, the cable would once again lift the sand to a high point and the process would start over. This stage is called charging.

Benefits

Underground gravity energy storage could offer widespread solutions for some of the biggest issues with renewable power by storing energy in sand.

It could revitalise unused space – there’s little infrastructure building needed as abandoned mines could be utilised. It would also reduce the time, cost and environmental damage associated with building new energy storage centres.

UGES can also overcome the battery challenges associated with renewable energy storage. Battery metals are finite and acquiring them can be an arduous process involving poorly regulated mining.

This mining can harm the environment and the people who participate in it. There are also few resources for recycling spent battery metals. Unlike batteries, sand doesn’t degrade over time, making it a long-lasting or even permanent energy storage solution.

Gravity storage can also minimise wasted energy. In many cases, wind turbines generate electricity day and night, degrading them quickly and shortening their life span. That’s fine if all the energy is being put to use, but if it has nowhere to go, it is essentially wasted and shortening the wind turbines’ life span for no reason.

UGES would minimise wasted energy by storing it underground. Deeper mines could store and generate more energy. The investment costs of UGES would likely hover around $1/kWh and the technology could account for seven to 70 TWh globally, with most of it based in Russia, India, China and the US.

Potential downsides

While underground gravity energy storage looks promising, it might also run into some challenges. Long-abandoned mineshafts may not be ready for immediate use. Many mines could require reinforcement to make them safe for workers and machinery to enter, and the cost of renovation must be less than what the UGES system will be worth to make it economically viable.

And not all mine sites may be suitable. Deep, vertical mineshafts are the ideal location for underground energy storage systems. Although there are countless unused mines around the world, only some of them will have the right conditions to allow UGES solutions.

Equipment maintenance also needs to be a consideration. Compared to hydroelectric power plants — which often have a life span of over 100 years — researchers expect UGES sites to last only two to three decades. It’s still longer than the typical lithium battery’s life span, but technicians will have to take care to maintain equipment in the harsh, sandy conditions of the mines.

Harnessing gravity

Like an enormous underground hourglass, UGES could store renewably generated energy in the sand, raising and lowering the earth to capture and release the energy. It offers a unique solution for power storage compared to batteries, which degrade and slowly discharge energy over time.

The technology also looks promising for its ability to utilise previously abandoned mines. It could revitalise economies that previously collapsed after mine closures, bringing much-needed jobs to struggling cities. Time will tell if the energy storage solution of the future rests in the world’s empty mines.