Alternative large-scale storage comes to Broken Hill

Grid-scale, longer duration storage remains an important element in the transition of our grid to more renewables. Pumped hydro is proven technology that continues to be developed in Australia, while the potential of future hydrogen systems gains attention, there is also renewed interest in a storage technology developed more than 50 years ago for large scale supply, but which has had limited development to date: compressed air energy storage (CAES).

Recently the Australian Renewable Energy Agency (ARENA) announced a conditional grant of $45 million for development of the Silver City Energy Storage project, which aims to use compressed air energy storage (CAES) technology to help support network security and integration of renewable energy at Broken Hill. The 200MW/1600MWh project will use what is described as advanced CAES technology from Canadian company, Hydrostar, at a disused mine.

The CAES planned for Broken Hill has been cited as the best option for the town by TransGrid in its Project Assessment Conclusion Report into securing the area’s back-up power supply. Other options considered included a proposal for a battery energy storage system, refurbishing existing diesel generators or building a second transmission line.

So what is CAES and has it been used before or elsewhere.

Large-scale

The concept of large-scale compressed air energy storage has been considered since the middle of last century when the first patent for compressed air storage in artificially constructed cavities in deep underground, as a means of storing electricity was issued in the USA in 1948. Frazer W. Gay, the patent holder, said about it: “In the present invention, I propose to provide equivalent storage space for gas relatively close to the earth's surface and, furthermore, to make this storage space available for the storing of compressed air to be used for power generation purposes during periods of heavy power load, as well as for natural gas or manufactured gas, butane, propane or other fluids. The invention in general comprises the construction of huge caverns located comparatively close to the earth's surface”[i]

The world’s first commercial scale CAES plant was commissioned in Huntorf, Germany in 1978. The 290MW plant now provides reserves to the grid and balances shortfalls between day-ahead forecasts for wind power and actual generation. Another 110MW plant was commissioned in McIntosh in Alabama in the US in 1991 and its capacity increased in 1998 to 226MW. The McIntosh Power Plant is owned by PowerSouth Energy Cooperative (100%).

The Huntorf plant uses a 310,000 cubic metre cavern at a depth of 600metres converted from a solution mined salt dome. It runs on a daily charging cycle of 8 hours providing a peak output of 290MW for 2 hours. The McIntosh plant has a 538,000cubic metres salt cavern at a depth of 450m. Originally it provided an output of 110MW for 26 hours but in 1998 two extra generators were added and its total capacity is now 226MW[ii].

A further smaller plant was opened in Goderich in Canada in 2019 developed by Hyrdostor. While small - 1.75 MW (discharge), 2.2 MW (charge) / more than 10 MWh (contracted to 7 MWh) - it is considered to be the first commercially contracted A-CAES facility, according to the company’s website. The plant is contracted by Ontario’s Independent Electricity System Operator (IESO) for peaking capacity, ancillary services, and full participation in the merchant energy market. 

The first two commercial plants referred to above use compressed air energy storage (CAES) based around a gas turbine cycle - surplus power is used to compress air using a rotary compressor and it is then stored, often in an underground chamber, with salt caverns often using artificially constructed salt caverns in salt formations. When the power is required, it is released from the storage chamber and passed through an air turbine that generates electricity. Output from the plant can be boosted by heating the air by burning natural gas in the high pressure air before it enters the air turbine, the same as in a conventional gas turbine. The advanced plants store heat during air compression (for example in hot water storage) and then released during the expansion phase. It has the advantage of greater efficiency. Efficiencies of up to 70 per cent can be achieved if the heat of compression is recovered and used to reheat the compressed air during turbine operations.

Déjà vu?

This is not the first time that Australia has considered developing CAES project. In 2019 Hydrostor received grant funding of $9 million from both ARENA and the South Australian government for what would have been Australia’s first advanced CAES project. An abandoned zinc mine at Strathalbyn was earmarked for a 5MW/10MWh facility. The project would have employed Hydrostor’s technology to take off-peak, or surplus electricity from the grid or from renewable generation to compress air in a cavern.

Strathalbyn project was considered a commercial demonstration plant as a forerunner to other compressed air renewable energy storage facilities in Australia. Key factors behind the cancellation of the project appear to have been higher than expected electricity grid usage charges and capital costs along with disruptions as a result of COVID-19.

Goldman Sachs has shown interest in the A-CAES technology investing USD$250 million in Hydrostor to be paid out in tranches based on the progress of projects in development in Australia (Broken Hill) and California – the proposed 500MW Willow Rock Energy Storage Center and the Pecho Project.

Hydrostor stores heat created when compressing air in water for reuse and it also uses hard rock caverns rather than underground salt caverns. Hydrostor claims the caverns can be dug into “almost any rock”, making the storage more versatile.

ARENA’s funding for the Silver City Energy Storage Project, developed by Hydrostor, is conditional upon the project reaching financial close, which is expected to occur in late next year. The project was nominated as the preferred option for backup power for Broken Hill by TransGrid. Currently the town is supplied by one transmission line that runs 250km from Buronga, NSW near the Victorian border. The backup supply to the town when the transmission line is not available is provided by two diesel turbines, which TransGrid acquired from Essential Energy.

[i] Storing Energy: With Special Reference to Renewable Energy Sources, edited by Trevor M. Letcher, Elsevier 2022

[ii] Advances in Energy Storage: Latest Developments from R&D to the Market, Edited by Andreas Hauer, 2022