As energy storage is demanded on a vast scale, the possibilities of converting mines into pumped-hydro batteries are being investigated.

What do you do with a big hole in the ground? With a portfolio that includes five operating coal mines, two on “care and maintenance” and others undergoing rehabilitation, Centennial Coal has to think well ahead about these things.

Centennial supplies coal for about 30% of NSW’s coal-fired generation, but the energy mix is changing and storage of energy has quickly emerged as a valuable service. With help from the Australian Renewable Energy Agency and NSW Government ($995,000 and $4.16 million respectively), the miner has started a technical feasibility study into using an underground coal mine as part of a 600MW pumped hydro energy storage facility.

The Newstan Colliery in Fassifern, south-west of Newcastle, has been on care and maintenance since 2014, which means the owner keeps it in reserve should they decide to scrape more coal out of it. But the site stacks up well for pumped hydro – there are already an upper and a lower reservoir and the site is already connected to the grid and close to a secure water source – so Centennial thinks $13 million spent on a study will be worth it.

If all goes well, other disused brownfield sites and coal mines may be reborn as hydro plants. Underground pumped hydro energy storage is new concept, however, and the project will begin as desktop research into the potential for a pilot-scale trial, to evaluate how coal seams react to water movement and to determine what would be needed for a plant to work.

Sun underground

The project proposes a closed-loop operation where water in flooded mined seams about 50 metres below ground is transferred to seams about 250m farther down, or 300m underground. Other options for the upper reservoir include a traditional ground-level reservoir, other underground coal seams or open cut voids.

A closed-loop system would suffer minimal water loss. If the water is pumped to the upper reservoir when solar energy is abundant and cheap, the pumped hydro system essentially becomes a storage of that solar power which can be dispatched when the stored energy can be sold for a profit.

The mine is a neighbour to the 2.88GW Eraring coal-fired power station, the state’s largest, so a vast undergound battery would have access to top quality transmission.  

Centennial is a sister company of Banpu Energy Australia, the local arm of a Thailand-listed energy multinational that owns about 900MW of renewable energy capacity throughout Asia.

The Eraring coal station is scheduled to retire around 2032 and the Lake Macquarie region where it is sited will be transformed as the NSW Government’s Hunter Renewable Energy Zone is planned and developed. The transition is part of the state’s Electricity Infrastructure Roadmap, which has targeted new renewables generation, transmission and long-duration storage, with the potential for up to 2GW of pumped hydro.

A coal mine reborn as an out-of-sight battery that draws in and deploys clean energy would made a great addition to the network.