Compressed air energy storage batteries to store more renewable energy

An energy storage startup called Hydrostor is planning to build an advanced compressed air energy storage (A-CAES) project in Australia, using a defunct underground zinc mine as a container for compressed air.

Hydrostor announced the plans last week after receiving A$9 million ($6.4 million) in funding from an Australian government agency.

Compressed air energy storage (CAES) is a physical battery (as opposed to a chemical battery) that uses excess electricity to compress air. The compressed air is stored in tanks, balloons, or underground caverns. When more electricity is needed, the compressed air is heated, which drives a turbine to expand.

Ideally, this allows more renewable energy to be added to the grid: for example, if there's a lot of wind in the evening and not a lot of people are using electricity, the excess power can be used to run compressors on the CAES system. The stored energy can then be used 12 hours later when the wind dies down.

Conventional CAES generally cannot be carbon neutral because it uses natural gas to heat the compressed air. However, such a device uses much less natural gas than simply adding another gas turbine to the grid, so conventional CAES is seen as a way to reduce carbon emissions from the grid.

Remarkably, the A-CAES system from Hydrogen Resistors is able to retain the heat removed from the surrounding air when it is compressed. The so-called adiabatic system stores the heat separately from the compressed air, and when it is time to deliver the electricity back to the grid, the compressed air is heated using the stored heat.

The Second Life of Zinc Mine

The project announced by Hydrostor this week is relatively small, rated at just 5 MW and with a discharge time of 2 hours (10 MWh). But A-CAES systems are rare, and the project, expected to be completed in 2020, could pave the way for larger such projects.

This particular project will be built at the Angas zinc mine outside of Adelaide, South Australia. The device will compress air and inject it into the mine, using a column of water to hold the air in place. "During the charging process, the compressed air draws water out of the caverns, through the water column and into a surface reservoir," a press release from Hydrostor states. "During the discharge process, water flows back into the caverns, forcing air to the surface under pressure, where it is reheated using stored heat and then expanded through a turbine to generate electricity on demand."

"The project was sized to utilize the full underground space at the required depth at the Angas mine without any additional mining to increase storage capacity, thereby minimizing costs," Alex Fuentes, Hydrostor's vice president of business development and marketing, told Ars in an email.

Money is the most thought-provoking part

Of course, CAES systems aren't cheap. So far, very few nuclear plants have been built, and at this point, economies of scale seem far-fetched.

Hydrostor has several projects in the pipeline, but so far it has only one facility operating and delivering electricity to the grid. That project started up in 2015 in Toronto, Canada.

Additionally, the grants Hydrostor received did not cover the cost of building the entire system. "These grants covered a portion of the total project cost, with the rest being provided by Hydrostor," Fuentes told Ars.

Storage companies seeking financing for an unconventional battery might do well to choose Australia as a location to make it. The country, and South Australia in particular, has suffered devastating heatwaves in recent years, and officials are eager to invest in cutting-edge storage technology. Besides, there’s precedent: Tesla’s big South Australian battery has been a neat piece of cash to pair with wind farms, and the Australian Energy Market Operator (AEMO) recently praised these batteries for providing a direct method of frequency regulation for Australia’s grid that’s superior to traditional frequency services from fossil-fuel-powered turbines.

AEMO even released a report last year saying it was open to possible improvements in how storage providers are compensated, given that storage can often generate electricity faster than fossil-fuel turbines. (A-CAES are also turbine-driven, however, so it’s unclear whether such a system would outperform gas turbines on speed, as chemical batteries can.)

But Hydrostor has big ambitions for the Angas project. Fuentes said Angas will be a "showcase for A-CAES technology," which Hydrostor sees as promising in transforming so-called brownfields, such as abandoned mines and oil wells, "into clean energy hubs."