Sand, gravity and abandoned mines to generate and store electricity

Antonio Sánchez Solís

Vienna, 18 Jan. A disused coal mine. Thousands of tons of sand. And the force of gravity. These are the essential elements of a scientific proposal to generate and store clean energy as part of efforts to combat climate change.

This novel proposal, still in its embryonic stage, has been dubbed "Underground Gravity Energy Storage" (UGES) by its creators at the international Iiasa institute near Vienna.

UGES proposes to use disused mine shafts as a kind of dam in which to place electricity generators that are activated by sand, not water.

The sand, a very cheap, abundant and durable raw material, would be placed in containers that would be dropped down the shaft, where electric generators would slow down and produce electricity in the process, applying what is known as "regenerative braking".

ELECTRICITY BY BRAKING

With this technique, used for example in electric vehicles, during braking the motor acts as a generator and transforms the kinetic energy into electricity, which can be used or stored.

"Instead of transforming braking into heat, it is converted into electricity," Julian Hunt, a researcher at the International Institute for Applied Systems Analysis (Iiasa) in Austria and lead author of this proposal, explains to EFE.

The deeper and wider the mine shaft, the more electricity-generating capacity the infrastructure will have.

The idea is that the UGES system will feed electricity into the grid when energy prices are high.

At times of cheaper prices, the sand that has been deposited at the bottom of the mine shaft is brought back up to the surface using electric motors.

In that way, the sand serves to "store" energy on a seasonal basis. It is always there, to be used when needed, just like water from a dam or pumping station, but without the problems of evaporation or dependence on rainfall.

In addition, Hunt explains, sand as a storage system has the advantage over batteries in that it does not discharge energy over time.

However, the researcher recognizes that, beyond the fact that the proposal is still at a very early stage, the UGES system presents other problems.

On the one hand, the initial investment to create the infrastructure is relatively high. The cost of adapting and equipping a mine about 1,000 meters deep and with the capacity to generate about 30 MW could be around $160 million, according to the study.

CHEAP STORAGE

Second, the price of the electricity generated could be up to four times higher than that of a pumped hydroelectric plant or wind power. However, the price of storage would be up to twenty times cheaper than doing it with batteries.

Hunt points out that this type of installation can be an alternative to pumped-storage power plants in areas where there are no mountains on which to build such facilities or where there is a shortage of rainfall.

In addition, UGES is much more effective as a long-term storage system, up to ten years, because unlike water, sand does not evaporate.

According to Iiasa, there are some 3,000 coal mines in the world where the UGES system could theoretically store between 7 and 70 TWh of energy.

By comparison, estimated solar energy production in the European Union currently exceeds 100 TWh per year.

Hunt explains that the ideal situation is not to reuse an abandoned coal mine, but to convert one while it is still in use, as this would lower costs.

The study focuses on the reuse of coal mines because, on the one hand, it abandons a fuel responsible for climate change.

On the other, it would mitigate somewhat the job losses and economic impact that closing a mine has on a community.

"The mines already have the basic infrastructure and are connected to the power grid, which significantly reduces the cost and facilitates the installation of UGES plants," Hunt concludes. EFE

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