Experts have been working for almost two weeks to free Florence, a tunnel-boring machine wedged deep underground at Snowy 2.0, the project's boss says.
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Bogged a year ago, and the survivor of a surprise sinkhole, the latest partial collapse of a tunnel at a so-called pinch point prompted concerns at a late-night senate estimates session about another delay.
The cost of the nation-building project may have doubled in the past six years to $12 billion but its value has also increased in a changing electricity market, Snowy Hydro chief executive Dennis Barnes told the environment committee on Tuesday.
Mr Barnes said the project's completion was critical to supporting the national electricity market, decarbonisation and reliability targets.
Like other big batteries, Snowy 2.0 will keep the lights on when the wind is not blowing or the sun is not shining, and could power half a million homes for an entire week.
"The role of Snowy 2.0 in the market is to facilitate the delivery of the lowest-cost form of energy ... solar and wind," he said.
"We provide the backup for that low-cost energy ... It's just there for insurance."
Defending the machine nicknamed Flo, Mr Barnes said the rock had proved to be too soft and too hard over the years, which was not unusual for a mega project with geological risk.
Flo recently encountered very hard and abrasive rock that, when coupled with a curve in the tunnel, halted operations as the rock pinched the machine shield.
Very high-pressure water jets are being used to remove the rocks before restarting Flo can be considered, he said.
According to an updated business case, the targeted total cost of Snowy 2.0 remains $12 billion, with commercial operation of all units expected by December 2028.
"Some people would say we could do nuclear for the same price," Nationals senator Perin Davey told the committee as the coalition puts the finishing touches on its nuclear energy plans.
The latest Snowy 2.0 business case found storage value has increased significantly, driven by increased generation volumes on a 10 per cent increase in capacity from 2000 megawatts to 2200MW.
Increased generation volumes also reflect the greater penetration of wind and solar in the power grid, creating more opportunity to pump at low prices and generate at relatively higher prices during periods of high demand.
The combined effect was to increase generation from an average annual 3.5 terawatt hours to 5.3TWh, and boost the average annual price differential between pumping and generating from around $85 per megawatt hour to $100/MWh.
"Snowy 2.0 is now 57 per cent complete and our workforce of 3000 is achieving good progress across the many construction fronts," Mr Barnes said.
Tunnelling across the project was 30 per cent complete, he said, as the Australian government company considers buying another machine.
Australian Associated Press
