Exploratory drilling and testing will continue for the next three months. Image: SSE Renewables.

Exploratory tunnelling for SSE Renewables’ Coire Glas project, the UK’s first large-scale pumped hydro energy storage (PHES) scheme to be developed in 40 years, has been completed.

The proposed Coire Glas storage development would have an installed capacity of 1,300MW and be capable of delivering 30GWh of long-duration electricity storage. It is being developed by the renewables arm of UK power generator SSE.

Exploratory work at Coire Glas began in December 2022 and is being completed by Strabag UK, a construction company specialised in mining and tunnelling. The completion of the 1.2km tunnel is progress for the project, based in the Scottish Highlands at Loch Lochy.

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

Works included the creation of tunnel, measuring 5m by 4.5m, that cuts into the hillside where the underground powerhouse complex is proposed to go. SSE’s technical partners, environmental engineers Stantec and COWI, a UK consultancy, designed and supervised the ground investigation programme.

Exploratory drilling and testing will continue for the next three months. SSE Renewables hopes to make a final investment decision on the project in late 2025 or early 2026, with construction to begin in the second half of 2026 pending success in the administrative allocation of an investable cap and floor mechanism.

Mike Seaton, SSE Renewables director of development for Coire Glas, said: “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.”

SSE Renewables develops PHES

SSE Renewables is also developing a 1.8GW pumped hydro energy storage project at Loch Fearna, Scotland. The project has already secured a grid connection offer totalling 1.795GW and it is set to reach commercial operations in the mid-2030s.

Ross Turbet, head of investment management for hydro at SSE Renewables, highlighted that PHES projects will be “crucial for energy security and balancing the increasingly renewables-led energy system”.

“The proposed Fearna project is a welcome addition to our development pipeline of pumped storage hydro projects, which also includes our proposal to develop what could be one of Britain’s biggest pumped storage schemes in 40 years at Coire Glas and our intention to convert our existing Sloy Power Station into a PHES facility,” Turbet added.

In March of this year, Former Australian prime minister and current president of the International Hydropower Association (IHA) Malcolm Turnbull penned an open letter to Rishi Sunak stating that the UK has “almost 7GW of shovel-ready pumped storage hydropower projects with over 135GWh storage capacity”.

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