A giant hydro scheme in Scotland, which could power three million homes, is taking a leap forward with a £100million investment.
The proposed dam and two reservoirs at Coire Glas in the Highlands would be Britain's biggest hydro-electric project for 40 years.
Scottish ministers approved the 1.5GW pumped-storage facility in 2020. But power giant SSE wants assurances from the UK Government before finally signing it off.
A spokesperson for the Department for Energy Security and Net Zero (DESNZ) said it was "committed to supporting the low-carbon hydro sector, including hydro storage".
Perth-based SSE says the £1.5billion project would help tackle climate change and improve UK energy security.
The BBC says the concept of Coire Glas is simple.
Two reservoirs
It involves two reservoirs at different heights in the Great Glen. When power is plentiful and cheap, water would be pumped uphill for storage in the upper reservoir with the capacity of 11,000 Olympic-sized swimming pools.
When supply is tight and prices high, it would be released, using gravity to generate electricity by spinning four turbines way below before flowing into the lower reservoir.
By storing electricity generated in windy or sunny weather for use on cold, still or dark days, Coire Glas could help smooth the transition from oil, gas and coal to more sustainable but intermittent sources of energy such as wind and solar.
"We believe strongly it could play a huge role in enabling a decarbonised energy system," said Finlay McCutcheon, SSE's director of onshore renewables.
But Keith Bell, professor of future power systems at Strathclyde University, injected a note of caution.
He said: "We need a lot more energy storage capacity to get rid of fossil fuels completely - probably 10 to 50 times greater even than the capacity of Coire Glas."
500 construction jobs
Nonetheless it would still be the UK's largest hydro scheme since the electric mountain project was completed at Dinorwig in Snowdonia in 1984, and one of the biggest-ever engineering projects in the Highlands, creating up to 500 construction jobs at its peak.
SSE’s Mr McCutcheon insisted there was a clear case for the UK Government to support a strategic expansion in hydro capacity.
The firm's existing assets had been "absolutely critical" in keeping the lights on during a "full-blown energy crisis" in the UK and Europe this winter, he said.
Scotland's only other pumped storage scheme, operated by Drax Group, is housed in a giant artificial cavern inside Ben Cruachan on the shores of Loch Awe in Argyll.
The North Yorkshire-based company plans to more than double the generating capacity of its facility, nicknamed hollow mountain, to more than 1GW, with the construction of a new underground power station.
But the BBC says both Drax and SSE have been reluctant to press ahead without assurances from Whitehall.
Clarity needed
"SSE needs clarity around how the UK Government is going to support projects like Coire Glas," Mr McCutcheon explained.
"It was a key element of its energy-security strategy last year, but we need to see how that's going to work in practice."
He added: "It doesn't require subsidy, but they are enormous investments."
Specifically, SSE would like a commitment to a revenue-stabilisation mechanism and more assurances about how the regulated energy market would reward low-carbon power generation.
The DESNZ accepted that hydro would be "critical to delivering greater energy security and independence, economic growth, and our net-zero ambitions".
SSE says it hopes to make a final investment decision next year. If it goes ahead, completion is expected in 2031.
Rapid flexibility
Coire Glas could reach full generating capacity in under 60 seconds, providing rapid flexibility in the case of a loss of power elsewhere on the national grid.
Alternatively, operating at a more sustainable level, it could power three million homes for up to 24 hours.
The £100million outlay announced by the FTSE 100 company will be used for exploratory work, which includes boring a tunnel into the hillside to assess the geology of the site.