A form of long-duration storage that can be sited wherever it’s needed is edging closer to reality at two sites on the NEM.

The menu or energy storage technologies is long and highly variable and just when you think there couldn’t possibly be any chemical or mechanical solution yet to be discovered, along comes another one blasting from the ground right in front of you.

Who would have thought that air pushed deep underground could be so useful when it’s suddenly allowed back to the surface? But this form of energy storage system has earned the Australian arm of Canada-based company Hydrostor a place on the NSW Government’s short list for pre-investment funding for a 200MW project in Broken Hill. If the feasibility study leads to a green light the site could be online by 2023, if not sooner.

It couldn’t happen in a better part of the world, according to Hydrostor CEO Curtis VanWalleghem, who points to his company’s advanced compressed air energy storage (A-CAES) technology as an ideal solution in a grid where surplus renewables will need to be put to work.

“We’ll just go where the grid needs power,” VanWalleghem tells EcoGeneration. “That’s our big advantage over pumped hydro.”

Under pressure

Here’s how it works. Up on the surface, air is sucked into a compressor to be sent underground. Heat that is produced during compression is stored to be used later. The pressurised air sent underground lifts water, which keeps the pressure up. When the owners want power, a valve is opened so the weight of that water pushes the air back up. The air recollects heat along the way and then goes through an air turbine. Voila! Electricity!

“It’s a lot simpler than you might think,” says VanWalleghem.

These underground batteries are also pretty easy to build. Once the relevant permits are in hand, a shaft about 2m across is sunk to about 500-600m, which takes about a year. Mining equipment is then sent down to hollow out an air tank. “It’s very simple mining,” he says. “You’re just making void space in the rock.” Then it’s time to clear all the equipment out and flood it with water.

Up at ground level, equipment which is usually used to pressurise natural gas is used to suck in atmospheric air and send it below, after the heat’s been stored. “Once it’s all set up you’ve got this really reliable equipment topside that’s been used for decades in oil and gas,” he says. “You’ve got to be at 100-200MW and larger for it to make sense.”

A legacy mine was used for Hydrostor’s first project in Canada because, at 5MW, “it wouldn’t make sense cost-effectively to sink a shaft at that smaller scale.” That plant has been in operation four years and another has recently been commissioned in Canada.

In South Australia, a Hydrostor project is under construction in a former zinc mine 50km southeast of Adelaide.

Going underground

Far, far underground, the mined cavity may require some sealing – although there isn’t much opportunity for air to escape. “When you’re down 600 metres you feel water and the earth trying to collapse,” VanWalleghem says. “The ground is under pressure. You’re very far down. The air is at similar pressure to the groundwater and the lateral earth pressure in and around it. We don’t really have to do a lot of sealing.”

Compressed air storage is similarly capable as pumped hydro or a gas plant, he says. “Response rates, turn up, turn down, the fact it doesn’t matter your depth of discharge, all that is very similar to pumped hydro,” he says. “It’s almost equivalent.”

“Blocks” of compressors would be deployed, so a 300MW project might use three 100MW trains, for instance. Like pumped hydro, the technology is happy to sit idle until the grid calls. “If they need you for an hour, great. If they need you for 12 hours, no problem,” he says. “When it comes to long-run capacity, that’s really what they provide. You have reserve capacity and you are synchronous, providing that inertia on the grid.”

Hydrostor guarantees a round-trip efficiency of 60% but VanWalleghem estimates more in the vicinity of two-thirds. For projects between 200MW and 500MW he estimates cost at roughly $US2,000/kW for a complete project with 10 hours of storage.

More storage

The world is entering an era of large-scale energy storage, where assets will be pitched into the fray to level out supply in markets where a high penetration of renewables sends power prices to zero or below.

“That’s kind of a perfect market. As that penetration [of renewables] gets up, storage makes more sense – and more sense – and the more renewables there are on the grid the more storage really brings it all together.”

In September the company finalised $US37 million in financing to help it push into its pipeline of large-scale applications of its A-CAES technology, which it claims sits at 2GW/16GWh of deployment potential.

VanWalleghem came across the storage solution while working for a utility with storage problem. “What I really liked about it was its flexible siting, it’s pretty easy to permit and it uses all oil and gas supply chains, meaning we could build a hundred of these simultaneously,” he says. “And it employs the oil and gas workforce that is going to need a new job as we save the planet. We’re making machines, we’re digging holes, we’re moving product. Same workforce, same skillset.”

He cofounded the company nine years ago with Cameron Lewis, who filed the original patent and is now chief technology officer.

Land down under

For Hydrostor Australia managing director Greg Allen, things are starting to get busy. The South Australia project will be commissioned by late 2020, he says, as work is undertaken on the feasibility study for Broken Hill – a region of the NEM that is similarly broken.

“Long duration storage is effectively a hedge product where you are taking low-cost energy, storing it and moving it to when the market needs it,” says Allen, mindful of negative pricing and curtailment events that have plagued large-scale solar plants this year.

“To be able to store that cost-effectively for longer durations than batteries you need technologies like pumped hydro … and we’re effectively an alternative to pumped hydro that can be effectively sited,” he says. “We can go to where the grid or generation needs it as opposed to where the topology dictates it should go.”

The Australian Energy Market Operator’s Integrated System Plan foretells dramatic augmentation to transmission, but it still calls out for about 17GW of energy storage with an average duration of 4-5 hours, Allen reminds EcoGeneration. “It will be challenging to do that at a competitive level of cost from a battery.”