Australian gravity story hopeful locks in first underground trial with NSW coal mine

Australian gravity energy storage hopeful Green Gravity says it has locked down a Wollongong Resources coal mine shaft for its first underground trial of its technology.

The two companies started talking in 2023 and have now signed a binding lease after looking at a range of options and settling on a suitable location.

The arrangements mean that the two companies will share the technical risk of an untried technology that will be deployed at the #4 shaft at the Russell Vale mine. They will also share the upside should it succeed and prove its value, says Green Gravity CEO Mark Swinnerton.

The project will begin by installing safety equipment such as gas monitoring devices immediately, and Swinnerton expects mining and planning department approvals to be in place for construction to start in the new year. 

“We don’t have a track record of conducting these activities in a mine site so the risk profile is different,” the ex-BHP executive told Renew Economy on Monday.

“We’ll be commissioning for four to six months, then we want to trial it for several months.”

The technical trials at Russell Vale will include how to repurpose old site infrastructure at the site to work alongside new electrical and mechanical parts. The site won’t be grid connected during the tests to store 150 kilowatts (kW) of power using a depth of 400 metres.

The Green Gravity concept is basic: drop a weight down a mine shaft to generate electricity when power is most needed on the grid. Then when renewables are at their peak during the day, the system uses that cheap or free electricity to pull the weights up. 

In that way it works on the same principle as pumped hydro, but in this case it uses large mass. The company says a 40 tonne weight can “store” up to 10 kilowatt hours (kWh) of energy per 100m of depth. 

Gravity storage has been trialled by other companies, including the US-based Energy Vault, which built a large demonstration plant that used massive concrete blokes mounted in shafts built into a bespoke building. But it appears now totally focused on conventional battery storage.

Mt Isa still in play, as is $17m cap raise

Green Gravity has been trying to get its concept off the ground since 2022.  An initial memorandum of understanding that same year with Yancoal went nowhere.

But a deal with Glencore last year to turn one of its Mt Isa copper shafts into a potential 2 GWh commercial energy storage unit is still in play.

Swinnerton says they are working with the Queensland mines department on risk and other factors, and working up the necessary details around geotechnical data, grid integration, hydrology and other information needed for a commercial operation. 

Green Gravity has whittled down the original list of 175 potential mine shafts able to provide perhaps 3 gigawatt hours (GWh) of storage, to about 75 shafts and 10 GWh. 

Tests at the company’s Port Kembla above-ground lab helped it to land $9 million in funding last year from investors including HMC Capital, BlueScopeX, Pacific Channel, and Sumisho Coal.

But it will need more cash to build and run the under-ground test and Swinnerton is now in the market for another $17 million. 

He says raising capital for untried renewable energy technologies has been “tough”, but the money market is starting to improve. 

“There’s no doubt the geopolitical environment has impacted front flows and risk appetites. We’re seeing that ease up in the last month or two. There’s clear improvement in tone from the market,” he says.

“We believe it’s been due to international factors. The disruption to trade and financing flows… is slowly easing now.”

The Russell Vale mine in New South Wales was operating from the late 1800s until 2023 when it entered care and maintenance, another way of saying the owner is keeping the site stable just in case it can reopen it or sell it. 

One criticism of using old mine sites for other things is that it may allow the owner to avoid doing the rehabilitation they’re legally required to fund. 

Swinnerton says in Green Gravity’s case, they’re only taking over a small portion of a mine site.

“There’s a more nuanced argument needed in this area. A coal mine is a large, complex thing. We will be working on one portion of a mining lease and therefore… we don’t touch [the remaining portion] or change the pathway for any of that. We simply change it for the shaft and the areas we require,” he says.

“That’s a deferral while we operate but it doesn’t change the final approved state of the site. There’s a higher value use for longer, but it doesn’t change the ecological [requirement to rehabilitate].”

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