Exploratory tunnelling at the site of what would be the UK’s first new large-scale pumped storage scheme in 40 years is complete.

Perth-based SSE hailed completion of the 1.2km tunnel as the latest milestone in the  energy giant’s Coire Glas project in the Highlands, which was undertaken as part of a package of site investigation works.

The proposed project, located at Loch Lochy in the Great Glen, would have an installed capacity of 1,300 megawatts (MW) and be capable of delivering 30 gigawatt hours (GWh) of long duration electricity storage. This would be enough to be able to provide “firm, flexible” renewable power for three million British homes for up to 24 hours non-stop.

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SSE noted that a programme of exploratory drilling and testing will take place over the next three months to further investigate the ground in the area of the proposed underground powerhouse excavations.

Mike Seaton, SSE renewables director of development for Coire Glas, said: “Completing the tunnelling is testament to the hard work and dedication of everyone involved and a positive step forward in de-risking the project.

“The works progressed to plan, and samples of the materials excavated from within the hillside are currently being analysed.

“This phase of the project is an engineering challenge in itself, and we are learning a huge amount as we progress the works. The findings of the tunnelling works, alongside our wider site ground investigation works completed by Fugro in December 2023, will be used to inform the final design, including the detailed design of underground structures, and will be a key consideration in any final decisions related to the project.”

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The Coire Glas milestone was announced shortly after SSE signalled that it would move forward with plans for a new pumped storage hydropower scheme at Loch Fearn in the Great Glen. It is part of a pipeline of pumped hydro projects the firm is developing, which also as well as Coire Glas include plans to convert SSE's existing Sloy Power Station on the shores of Loch Lomond into a pumped storage facility.

SSE Renewables hopes to make a final investment decision on the Coire Glas project in late 2025 or early 2026. That would allow the main construction to commence in the second half of 2026, providing the company is successful in the administrative allocation of an investable cap and floor mechanism.

SSE has previously made clear it will only commit to the Coire Glas project if the UK government guarantees a minimum price for its output. However, it has proposed using a cap and collar mechanism, under which it will make payments to the Government if prices exceed a set level.

Pumped storage hydropower projects work by using the flow of water between reservoirs and lochs to drive turbines.