The pumped hydro storage conversion could be completed by 2028. Image: SSE Renewables.

SSE Renewables has unveiled plans to convert Britain’s “largest conventional hydro power plant” into a new pumped hydro storage facility.

Confirmed on Monday (22 May), the energy supplier stated it will convert its conventional 152.5MW Sloy hydroelectric power station, situated on the shores of Loch Lomond in Argyll and Bute in central Scotland, to pumped hydro energy storage (PHES).

Subject to the final design of the project, SSE stated that the Sloy scheme would be capable of delivering up to 25GWh of long-duration electricity storage capacity. This means that flexible renewable energy could be provided for up to 160 non-stop hours.

“In converting our existing Sloy conventional hydro power plant to a pumped hydro storage facility, we can provide the additional large-scale, long-duration electricity storage we need as part of the country's future energy mix,” said Finlay McCutcheon, director of onshore Europe at SSE Renewables.

“With up to 25GWh of storage capacity, the scheme would be capable of powering 90,000 homes for an entire week, so bolstering our energy security and providing the balancing flexibility we need in a renewables-led energy system.

“The development of pumping capability at Sloy also complements our development plans for our other pumped hydro storage project at Coire Glas. Taken together and if approved for delivery, Coire Glas and Sloy can treble Britain’s current flexible electricity storage capacity. That’s why it’s crucial the UK Government urgently confirms its intention on exactly how they will help facilitate the deployment of pumped hydro storage projects as part of our future energy mix.”

The next steps for the project will see SSE refine its project design to convert the Sloy plant from conventional hydro power to pumped hydro storage technology ahead of a period of public consultation later in the year.

Subject to the scoping opinion, it’s expected a planning application could be submitted to the Scottish Government by late 2023 or early 2024, SSE said. Subject to a positive consenting outcome, it is hoped a final investment decision on Sloy could be made in late 2025 ahead of full commissioning by 2028.

Commenting on the pumped hydro scheme, Yousaf called on the UK Government to “provide an appropriate market mechanism for hydro power and other long duration energy storage technologies”.

Yousaf said: “Hydro power was the country’s original source of renewable energy and it has the potential to play a significantly greater role in the transition to net zero – both on a small-scale in co-operation with local communities and on a larger scale, to help to ensure a continued resilient and secure electricity supply.

“We continue to call for the UK Government to provide an appropriate market mechanism for hydro power and other long duration energy storage technologies, to ensure this potential is fully realised.”

SSE has been exploring pumped hydro across Scotland as a means to provide renewable flexibility to the grid. In March 2023, Solar Power Portal reported that SSE would allocate £100 million to boost Coire Glas, a pumped hydro storage project, with the potential to be the “biggest” in 40 years.

Situated near Loch Lochy, between Fort William and Inverness, the Coire Glas project could more than double Britain’s total current electricity storage capacity and could require a capital investment of over £1.5 billion to construct. It would be capable of delivering 30GWh of long duration energy storage.