An Eight-Hour Compressed-Air Battery Clears Its Grid Connection

Hydrostor's Silver City Energy Storage Centre has received grid connection approval from the Australian Energy Market Operator and the transmission operator Transgrid, confirming compliance with Australia's Generator Performance Standards. The plant is rated 200 MW / 1,600 MWh — eight hours of discharge duration — and sits next to the Potosi Mine at Broken Hill in New South Wales.

Eight hours is the number that matters. The overwhelming majority of grid storage built to date is lithium-ion at one to four hours, which is well suited to frequency response and evening peaks and poorly suited to carrying a system through a long still night. Long-duration storage is the gap in most decarbonisation plans, and the technologies competing for it — flow batteries, iron-air, gravity, thermal and compressed air — are all still proving themselves at scale.

Hydrostor's approach is Advanced Compressed Air Energy Storage. The principle is old and the engineering is not: surplus electricity runs compressors, the heat of compression is captured and stored separately, and the compressed air is held in a water-compensated underground cavern that keeps it at constant pressure. On discharge the air is released and recombined with the stored heat to drive a turbine. Capturing the compression heat is what separates this from earlier compressed-air plants, which burned gas on the expansion side and were therefore only partly a storage technology.

Siting next to a mine is not incidental either — the geology and the excavation experience are half the project, and the co-location of long-duration storage with existing mining infrastructure is a pattern likely to repeat.

The local dimension is the most interesting part of the approval. Of the total capacity, 50 MW / 250 MWh is reserved as backup power for Broken Hill during network outages. Broken Hill sits at the end of a long single transmission line and has a history of being cut off from the rest of the grid, and a dedicated reserve behind an eight-hour asset is a materially different proposition from diesel standby. Fully charged, Hydrostor says the plant could supply more than 80,000 homes for a day.

The project already holds New South Wales planning approval and is designated a Priority Network Infrastructure Project. Construction is expected to create more than 700 jobs, with 35 permanent operational roles.