TECH

Abandoned mines can become gravity batteries

Gravity Batteries...The concept of gravity batteries, or gravitational batteries, has been discussed at length as a way to store energy from renewable sources, typically intermittent, so that electricity can be injected into the distribution grid steadily and reliable.

The basic idea is that some substance - which can be water or sand - is placed in a higher position using electricity generated by solar or wind energy, among others; when electricity is needed, the substance sinks to a lower point by gravity, generating electricity similar to water stored in a hydroelectric dam.

Unlike the more well-known flow batteries - large tanks in which energy is stored in the form of chemical compounds -, which can be installed anywhere, gravity batteries depend on very specific terrain for them to become technically and economically viable.

An international team, which includes two Brazilian researchers, has now made the broadest and most complete proposal for using abandoned or exhausted mines as a structure for storing sand and using it to generate electricity.

The concept, named "Underground Energy Storage by Gravity" (ASEG), has the participation of professors Roberto Brandão, from UFRJ (Federal University of Rio de Janeiro), and Walter Leal Filho, graduated from UFBA (Federal University of Bahia ) and currently at the University of Hamburg, Germany.

Storing energy using sand...The proposal is an adaptation of an idea already disclosed by Professor Julian Hunt, from the IIASA (International Institute for Applied Systems Analysis), in Austria, to store energy in mountainous areas using sand, which has the great advantage of not evaporating, not leaking and not causing infiltration in structures.

Another great advantage over other energy storage methods, such as common batteries, is that there is no loss of energy through self-discharge over time. As the energy storage medium is sand, it will remain there intact, storing energy for weeks or years.

And it is still possible to dispense with the mountains in regions where there are already exhausted mines, where there is no more ore to be exploited, which can greatly lower the cost of the gravity battery.

The sand is stored in the mine tunnels, from where the ore was taken. A very common type of mining is known as "chamber and pillar", where one part of the ore is removed and another part is left, creating large halls with natural columns. This mine structure provides the most space for storing sand. But there are other advantages to using a mine.

"The mines already have basic infrastructure and are connected to the electrical grid, which significantly reduces the cost and facilitates the implementation of SEAG [Underground Gravity Energy Storage] plants," explains Hunt.

Reuse of abandoned mines...The team did extensive research worldwide, trying to map the abandoned mines and verifying those with potential to be used as gravitational batteries. As expected, countries where mining is strong are the ones that can benefit most from the technique.

"It is estimated that at least 50,000 of them exist in Australia; more than 550,000 have been found in the US, including more than 100,000 that pose a significant environmental risk; and more than 10,000 are known to exist in Canada. Old mines, exceptionally small mines and those located away from population centers, they are rarely documented, and some abandoned mines are only found when an accident occurs," the team said.

With so much imprecision, the researchers included the deepest documented mines in the world in their estimates. And, even with such a restriction, the potential of energy storage by gravity was impressive.

“The technology is estimated to have a global potential of 7 to 70 TWh, with most of that potential concentrated in China, India, Russia and the US. $1 to $10 per kWh, and power capacity costs around $2,000/kW," the team wrote.

"To decarbonize the economy, we need to rethink the energy system based on innovative solutions using existing resources. Turning abandoned mines into energy storage is an example of many solutions that exist around us, and we just need to change the way we implement them, " said Behnam Zakeri, member of the team.

Article: Underground Gravity Energy Storage: A Solution for Long-Term Energy Storage(Authors: Julian David Hunt, Behnam Zakeri, Jakub Jurasz, Wenxuan Tong, Pawel B. Dabek, Roberto Brandão, Epari Ritesh Patro, Bojan Durin, Walter Leal Filho, Yoshihide Wada, Bas van Ruijven, Keywan Riahi)