Deep inside Elidir Fawr mountain, a bright green telephone connects workers manning the Dinorwig pumped hydropower station to grid operators hundreds of miles away, trying to keep Britain’s lights on by requesting last-minute adjustments to the electricity supply.
“If that rings, you answer it,” says Evan John Evans, shift manager at the station in north Wales.
Pumped hydropower stations store energy by using electricity to pump water from a reservoir at the bottom of the mountain up to one at the top. When electricity is needed, the water is released, flowing back down through turbines to generate power.
Built just over 40 years ago, Dinorwig was designed to store excess energy from the UK’s growing fleet of nuclear power stations — supply and demand need to be constantly balanced to avoid destabilising the grid.
“It used to be pump at night, generate during the day” so that nuclear plants did not have to turn off at night when demand is lower, adds Evans. “Now with wind and solar, we get [good] rates to take that energy off.”
Built by Britain’s former state-owned electricity board, the station is currently drained of water as part of a potential £1bn refurbishment by its owners, French state-backed Engie and CDPQ. The Canadian pension fund bought Brookfield’s 25 per cent stake earlier this year.
The investment is a welcome boost for the government as it tries to build a lower carbon electricity system.
The National Energy System Operator says technologies such as pumped hydropower are “particularly important” to balance the grid in an era of rising wind and solar generation, where output is volatile.
Dinorwig has a power output capacity of 1.8GW and storage capacity of 9.1GWh — enough to supply more than a million homes with electricity for about six hours.
At Evans’ fingertips is a 16-kilometre network of pipes that can unleash water from Lake Mawr at the top of the mountain at a rate of 85,000 gallons per second.
From here the water drops 500 metres into a cavern the size of a football pitch inside a former slate quarry in the Eryri or Snowdonia national park, where it spins six turbines. In the space of 16 seconds the system can go from stillness to producing electricity for up to 1.5mn homes.
New hydropower plants are expensive and difficult to build. Drax, the FTSE 250 energy company, earlier this month paused plans to expand its Cruachan station in Scotland by about 600MW, citing rising costs and uncertainty over the level of government support.
Meanwhile, its FTSE 100 rival SSE has expressed concerns over its Coire Glas pumped storage project in the Scottish Highlands. The project aims to have a power output capacity of 1.3GW and storage capacity of around 30GWh.
SSE said the “cap and floor” subsidy scheme for new power projects put forward by the government was “not yet investable for Coire Glas in its current form”.
By contrast, refurbishing an existing hydropower site should cost about one-third of the price of building one from scratch, according to Miya Paolucci, chief executive of Engie UK.
“It should almost be a no-brainer that we refurbish this,” she says of the Dinorwig site, highlighting the difficulties of building infrastructure in such prized landscapes. “We really have to cherish what we have.”
Engie and CDPQ have already refurbished two turbines at a smaller sister site, Ffestiniog, with another two under way.
But whether they replace the full six turbines at Dinorwig will partly depend on its success in Britain’s capacity market — where generators are paid to be on standby to supply electricity when needed.
Engie argues that the capacity market does not fully recognise the value of assets like Dinorwig, which can supply electricity for different lengths of time depending on how many turbines it runs.
“They need to be able to recognise the full flexibility of the assets,” said Delphine Cherel-Sparham, managing director of Engie’s pumped storage hydro team.
Engie’s backing for its UK project comes as the government is trying to attract investment to support its clean power ambitions.
Eluned Morgan, Wales’ first minister, who joined the visit to Dinorwig, said US President Donald Trump’s pullback on support for clean energy in America is redirecting attention towards the UK.
“Investors [at a recent meeting] were saying they’re running away from America because of the ‘drill, baby, drill’ approach, what have you got to offer here,” she said.
However, last week SSE said it would invest £3bn less than previously planned in the UK and Ireland by 2027. It blamed a “changing macroeconomic environment” and delays in the planning process.
Ørsted’s decision to halt work on its giant Hornsea 4 wind farm project in the North Sea has also caused concern.
Nonetheless, Engie’s Paolucci says the government’s clear sense of direction on developing a clean power system has helped build investors’ confidence.
“The country has shown a clear commitment,” she said. “Can we meet those ambitions or not is another question, but there’s a direction of travel.”
