The completion of the 1.2-kilometre tunnel represents a significant milestone in the Coire Glas project, which, once fully constructed, could offer the UK 30 GWh of long-duration energy storage.

Located at Loch Lochy in the Great Glen, the proposed Coire Glas project would have an installed capacity of 1,300MW and be capable of delivering 30GWh of long duration electricity storage – enough to be able to provide firm, flexible renewable power for three million British homes for up to 24 hours non-stop.

SSE Renewables hopes to make a final investment decision on the project in late 2025 or early 2026, allowing for main construction to commence in the second half of 2026, subject to being successful in the administrative allocation of an investable cap and floor mechanism.

Exploratory works at Coire Glas commenced in December 2022 and are being conducted by STRABAG UK, a world leader in mining and tunnelling projects.

In early 2023, SSE announced a £100-million investment boost to further develop Coire Glas to include the construction of a major exploratory tunnel as part of a comprehensive package of site investigation works.

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SSE's technical partners, Stantec and COWI, have designed and supervised the unique and complex ground investigation programme which is being delivered by STRABAG.

The works included the creation of a tunnel approximately five metres high and four and a half metres wide which cuts into the hillside around the proposed location of the underground powerhouse complex. Construction of the tunnel will allow the project team to gather accurate information on the geological conditions to inform the detailed design for the main works.

Now that tunnel construction is complete, a programme of exploratory drilling and testing will continue for the next three months to further investigate the ground in the area of the proposed underground powerhouse excavations.

Once complete, Coire Glas would provide 30GWh of long duration electricity storage. The scheme would take excess energy from the grid and use it to pump water 500 meters uphill from Loch Lochy to an upper reservoir created by the construction of a 90m high dam. The vast reservoir could store enough water equivalent to nearly 11,000 Olympic-sized swimming pools. At the flick of a switch this water could be released to power the grid when required including at times when wind power output is low and customer demand is high.

Subject to being successful in the administrative allocation of an investable cap and floor mechanism for UK long-duration electricity storage projects, SSE Renewables hopes to make a final investment decision on Coire Glas (1,300MW) in late 2025 or early 2026, allowing for main construction to commence in the second half of 2026. Construction is expected to last up to seven years, which means the project could be operating in 2032 and fully completed during 2033.