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The Austar coal mine
The Austar coal mine is home to a number of decommissioned ventilation shafts.

Australian start-up Green Gravity recently announced it will work with coal miner Yancoal on a test site for the new green energy technology, but how does gravity energy storage work?

The elevator pitch for Green Gravity’s energy storage solution is fairly simple: “(it) harnesses the fundamental principles of gravity and kinetic energy to store and dispatch energy by lifting and lowering heavy-weighted objects”.

In other words, gravity energy storage involves lowering weights from a significant height to convert potential energy into carbon-free electricity.

“Like pumped hydro, we use the gravitational potential energy of a mass moving between two heights. However, rather than water between two dams, Green Gravity requires much less space by using very dense materials,” Green Gravity said on its website.

“To overcome friction, a vertical height available from a mine shaft is used rather than an incline on the side of a hill.”

The idea is to raise the weights when excess renewable energy is available (ie when solar power is at higher levels around the middle of the day), and then lower them at the moment the power is needed.

Green Gravity, a new company started by former BHP executive Mark Swinnerton, aims to produce energy by using the technology in the hundreds of old mine shafts that lay dormant across Australia.

The first cab off the rank may be the 108-year-old Austar coal mine near Cessnock in New South Wales, which has been in care and maintenance since 2020.

Green Gravity and Yancoal have signed a Memorandum of Understanding (MoU) to start a pre-feasibility study to determine whether the technology could work in decommissioned ventilation shafts. The study is expected to conclude next year.

“This study represents an outstanding opportunity for Green Gravity and Yancoal to identify

new economic potential at the former Austar mine site, add regional jobs, demonstrate re-use options for legacy coal mining, and deliver a better environmental outcome,” Swinnerton said.

“Successfully identifying the best method of fitting gravitational technology at the former Austar mine site will create a template for beneficial re-use of legacy mining sites for Yancoal, and for thousands of other mines around the country.

“By re-using mining assets, costs can be kept low. By using gravity as the fuel, we dispense with consuming the critical water, land and chemicals which other storage technologies rely on.”

Green Gravity’s technology is similar to concepts used by other companies, but according to Swinnerton, the fact his company uses disused mine shafts, rather than purpose-built towers, is an advantage.

“They have to build a tall building hundreds of metres high and suspend tens of thousands of tonnes at the top of the roof,” he said. “It is going to have to be a fairly big set of steel beams with good geotechnical engineering and everything else to make that work.

“I don’t have to do any of that. I can dispense with the construction of a building and simply use what was already there – a hole that somebody else already drilled and used and is now redundant.”

This type of renewable energy storage could represent something of a ‘holy grail’ for the resources sector in that it does not require the significant investment in land, water and chemicals that other storage technologies may need.

“Decarbonisation of the energy system needs rapid deployment of renewable energy. To manage the inherent variability of renewables, energy storage must be added to our electricity grids,” Green Gravity said.

“Our technology solution utilises conventional mechanical components to enable low-cost energy storage to be installed at legacy mine sites.”