SSE Renewables has completed a significant phase in the development of the Coire Glas pumped storage project in the Scottish Highlands, marking the end of exploratory tunnelling at the site.

The project - the UK's first major pumped storage scheme in since 1984 - has now seen the successful construction of a 1.2km exploratory tunnel.

The Coire Glas scheme, located at Loch Lochy in the Great Glen, is designed to deliver 30 gigawatt hours (GWh) of long-duration electricity storage. This capacity can supply renewable power to three million homes in Britain for up to 24 hours continuously.

SSE Renewables said it is aiming for a final investment decision (FID) by late 2025 or early 2026, with the main construction potentially starting in the latter half of 2026, contingent upon securing a favourable cap and floor mechanism.

The exploratory works, which began in December 2022, are being carried out by mining and tunnelling contractor STRABAG UK.

In early 2023, SSE Renewables announced a £100m investment to advance the development of the storage facility, including the construction of the major exploratory tunnel.

The technical aspects of the project are overseen by partners Stantec and COWI, who have designed and supervised the complex ground investigation programme executed by Strabag.

The construction of the tunnel, measuring five metres in height and four-and-a-half metres in width, enables the project team to collect precise geological data crucial for the detailed design of the main construction works.

SSE Renewables Coire Glas development director Mike Seaton stated: “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.”

With the tunnel now complete, exploratory drilling and testing will continue to further assess the ground conditions around the proposed underground powerhouse area.

Stantec energy business director John Ord stated: “The Stantec team has thoroughly enjoyed working on this stage of the Coire Glas project, delivering award-winning work alongside COWI, Strabag and SSE Renewables.

“We look forward to continuing our support as it moves into the next phase, defining rock quality more than a kilometre inside the mountain.

“This important work will inform the design of the main pumped storage scheme, moving the project closer to offering a huge boost for the UK’s renewable energy storage.”