A £1 billion refurbishment programme is underway to extend the life of Dinorwig and Ffestiniog pumped storage power plants in Wales. With 2.1GW of installed capacity the two facilities are the UK’s leading providers of power storage and flexibility, representing 5% of total installed power generation capacity and 74% of the UK’s pumped storage hydro capacity. Electric utility company ENGIE owns 75% of the plants via First Hydro Company, a 75:25 joint venture with Canadian investment group CDPQ.
With both plants now reaching the end of their operational lives, preparations for refurbishment at Dinorwig pumped storage station have begun – hot on the tail of a nine-year refurbishment at Ffestiniog.
ENGIE says that such ‘re-planting’ could see the complete refurbishment of up to all six generating units at Dinorwig – a final investment decision is still to be made on the number of units to replace – while the re-planting at Ffestiniog is set to be completed by July 2026.
The complexity and the scale of engineering involved, along with the necessity to keep the plant available to serve the electricity system, explains why the re-planting project for Dinorwig will take up to ten years. The programme involves replacement of the main inlet valves which are among some of the largest in the world, weighing in at over 160 tonnes. With full drain down of the station and detailed inspections of the water shafts required, all of this necessitate specialist technology, bespoke lifting systems, and expert contractors.
Indeed the valves themselves were modelled at a reduced scale in a lab and designed as fully working scale models to drive efficiencies. AI was used to optimise and improve the infrastructure and technology. These valves are now waiting to be installed at Dinorwig station, where engineers will lathe and fine tune the seal on site for up to two weeks.
Switched on
Earlier in the year, National Grid switched on new high voltage electricity cables as part of its Dinorwig to Pentir project, a scheme that will help futureproof vital infrastructure carrying clean electricity from ENGIE’s Dinorwig power station.
Since autumn 2021, National Grid has been upgrading the 12km connection between its 400kV Dinorwig substation which sits inside the same mountain as the power station, and its 400kV Pentir substation.
Engineers from National Grid, and contractors Morgan Sindall Infrastructure and Siemens Energy, have been working on the project to replace cables originally installed in the 1970s and which are now coming to the end of their operational life.
The first of three new circuits – comprising three 12.2km cables each made from 14 sections joined together and weighing an estimated 1500 tonnes – were installed under sections of main roads in nearby fields and inside Dinorwig.
Each new circuit takes a different route from the original cables, ensuring the existing connection can remain in service and continue transmitting vital clean power from Dinorwig while the upgrades take place. The new energised circuits will help create a more resilient power network for the UK, enabling it to better handle sudden changes in power demand and unexpected outages.
Phil Rose, National Grid’s Lead Project Manager for the Dinorwig to Pentir cable replacement project, said: “Completing the first of our three connections marks a really significant milestone in this crucial project to futureproof the infrastructure carrying clean power from Electric Mountain.
“It’s also a fantastic achievement for the team despite some very difficult conditions, with extremely hard rock and unprecedented rainfall challenging our progress at times,” he added.
Bat protection
As part of its work at Dinorwig, National Grid has also been safeguarding critical bat habitats in the cable tunnels. Dinorwig hydroelectric power station is made up of 16km of underground tunnels deep below Elidir Fawr mountain in Gwynedd.
Through the 40-year life of the power station, these tunnels have become an important maternity and hibernation roost for the rare lesser horseshoe bats and a day roost for Soprano and Pipistrelle bats. Several other species also use the tunnels. At its peak, 191 lesser horseshoe bats were recorded making the site regionally important and of high conservation significance.
Charlie Roberts at National Grid said: “Bats are a protected species in the UK. It’s amazing to think that the cable tunnels provide a safe haven for so many bats and we are committed to keeping it that way.
At Dinorwig several mitigation measures have been put in place over the years and, more recently, have included the installation of a permanent acoustic barrier alongside the maternity roost to reduce any noise and disruption during construction works.
Facts and figures from North Wales
Ffestiniog, one of the first pumped hydro systems in the UK, was opened in 1963 by Her Majesty, Queen Elizabeth II. As the older of the two plants, refurbishment is already underway.
Dinorwig, the largest and fastest-acting pumped storage station in Europe, followed in 1984 and was opened by HRH King Charles III. It is regarded as one of the world’s most imaginative engineering and environmental projects, capable of delivering full output in just 30 seconds
Situated in a former slate quarry, which is a UNESCO World heritage site, the station itself is built deep inside the Elidir Mountain in a network of tunnels. The lower reservoir, Llyn Peris, is positioned at the bottom of the quarry, while the upper Marchlyn Mawr reservoir sits higher up the mountain.
Dinorwig is capable of generating up to 1800MW of power – enough to supply approximately 1.5 million homes at peak output. The entire facility required the excavation of 16km of tunnels and the removal of 12 million tonnes of rock. The low pressure tunnel line is 1695m, transporting water between the reservoir and high pressure shaft at a maximum of 390M3/sec.
Over 180 people are employed across Dinorwig and Ffestiniog.
