What goes up must come down: the potential of gravity batteries

In essence, the process uses gravitational energy to fill batteries during peak conditions when energy derived from renewable sources is plentiful. Then, if demand for power outstrips that which renewable sources can provide, this stored energy can be released, providing power if, for instance, the sun does not shine, or the wind does not blow.

Specifically, this involves using excess renewable power to drive an electric motor, which draws a weight up to a height. When power is needed, the weight is lowered on a system of cables, and the movement of cables generates electricity, which is sent back to the grid.

The beauty of this idea is that it can either be housed in towers near existing renewable sites such as solar or wind farms, or alternatively in disused mine shafts, where the existing long drop provides the perfect infrastructure for such a system.

While it is debatable whether there would be cost savings associated with re-purposing mine shafts rather than constructing facilities above ground, what is clear is that in being out of sight, gravity battery systems are less of an imposition upon a local landscape, and so could be popular among host communities.

Indeed, a happy by-product of the below-ground approach would be to see heavily polluted areas cleaned up and made productive once again, acting to tick even more sustainable boxes.

As such, decommissioned coal mines in several jurisdictions spanning the globe are now being looked at by companies active in this new innovative field, such as Gravitricity. This affords the opportunity to design installations stretching down several hundred metres, while also providing scope for new employment opportunities in regions currently economically enslaved to the fossil fuel sector. 

For now, though, the focus is on determining whether the physical properties of the shafts lend themselves to retrofitting in this way, and on ascertaining the nature and extent of challenges around flooding and dangerous gases.

Should such obstacles prove too difficult or costly to overcome, there remains the option to sink new shafts, which would allow for bespoke engineering and likely pay dividends in the long run, although this would require more capital expenditure up front.

Other early participants in the fledgling gravity battery sector see greater potential in going up rather than down, with this theory assuming form in Switzerland, where the company Energy Vault has secured significant investment sufficient to go to market. Central to its plans is a design for a modular building where thousands of weights made from recycled material are housed in a trolley system.

With tests and simulations to date having shown that such a facility with a storage capacity of 100MWh could power 25,000 homes for 24 hours, it is small wonder interest has translated into concrete orders from jurisdictions across the globe, including the world’s biggest polluter, China.

While these installations would have a significant footprint, the suggestion is that they will be located near existing solar or wind farms, not only providing ready access to the clean energy needed to power the lifting of the weights, but also easing the planning approval process.

One significant advantage to gravity battery systems is that the speed of the drop is able to be regulated to help balance the grid, as necessary, by providing a lot or a little power through a swift or more controlled descent. This is particularly significant since the grid is predicated on the concept of aligning with thermal power stations, rather than renewable facilities, which can be marked by unpredictable surges associated with the vagaries of the weather.

Gravity batteries could prove integral to the world’s clean energy mix, especially in relation to other forms of renewable power storage. When compared to lithium-ion batteries which currently dominate the stored energy landscape, the environmental footprint of gravity batteries is light, with figures suggesting that the cost per MWh associated with gravity batteries could be half as much as their lithium counterparts, even factoring in maintenance, construction and running costs.

In time, should the idea catch on and gravity batteries become an integral part of the energy storage landscape, there is scope for the necessary infrastructure to be incorporated into the construction of new high-rise buildings to create truly mixed-use energy self-sufficient developments.

Of course, questions remain as to how effectively these processes can be incorporated into new builds and construction on a large scale, as this becomes a challenge of civic planning, rather than innovation in the energy sector.

However, continued innovation in the gravity batteries sector means that, for the time being at least, there is optimism regarding the future of the process. The German company New Energy Let’s Go has built on the pumped hydro idea with its gravity storage concept that places the fundamentals behind gravity batteries into a liquid setting. Its system sees water forced through a turbine by a large movable rock piston sited in underground shafts much like in a conventional hydroelectric plant.

While still early days for gravity batteries, encouraging results to date indicate this beautifully simple new technology, which seeks to exploit one of nature’s fundamental forces, has what it takes to be part of the mix, so helping to catalyse and deliver the clean energy transition.

// Main image: Hoover Dam. Credit: TMP - An Instant of Time via Shutterstock