ENGIE and CDPQ Commit up to £1bn to Revamp Wales’ Dinorwig and Ffestiniog Hydro Storage Giants

A sweeping refurbishment initiative is underway at two of the UK’s most critical pumped storage hydroelectric facilities — Dinorwig and Ffestiniog — as ENGIE, through its 75:25 joint venture First Hydro Company with Canadian investor CDPQ, prepares these legacy assets for decades of low-carbon power generation.

Together, these two plants contribute a commanding 2.1GW of installed capacity—5% of the UK's total power generation base and 74% of the country’s pumped hydro storage. Their value in the energy transition is nothing short of foundational.

Located in the mountainous terrain of North Wales, Dinorwig and Ffestiniog serve as the silent giants of the grid, unseen but indispensable. These facilities don’t just store power; they provide system-critical flexibility, kicking into action within seconds when demand spikes or renewables dip. As the UK aims to deliver a fully decarbonised power grid by 2030, these ageing titans are being called upon once more — this time for a new mission: longevity.

Following the recent eight-year refurbishment at Ffestiniog, preparations have begun for an even more ambitious 10-year overhaul at Dinorwig. The work could ultimately involve the complete replanting and full replacement of all six of its generating units. However, a final investment decision on the scope is still pending.

By 2025, Ffestiniog’s upgrade — already well underway — will be complete. Two units have already been rebuilt; the remaining two will come online by the end of that year. Dinorwig, meanwhile, has already seen two of its six Main Inlet Valves replaced in 2023, with the remaining four set for installation in 2025.

But this isn’t a simple swap-out operation. Before valve installation can begin, the station must be drained. Not quickly. Not easily. This delicate, high-pressure task takes two full weeks — a rare undertaking performed perhaps once every 40 years. Once drained, engineers physically enter the water shafts to conduct intricate safety and structural assessments. The engineering scale is immense; the inlet valves, among the largest in existence, were first built as functioning scale models in a lab, then scaled up for real-world deployment. Even artificial intelligence was used to fine-tune performance, ensure reliability, and optimise the entire system.

Once on site, teams of expert engineers will lathe and precision-fit each valve, a process that can take up to two weeks per seal.

So why go to these lengths? Pumped hydro is unmatched in its ability to respond to demand in real time. When renewable output drops—the wind quiets, the sun sets—these plants step in. Water is released, turbines spin, and electricity flows. When renewables are abundant and demand is low, the same water is pumped back up the mountain, storing energy until the grid needs it again. There is no combustion, no emissions, just gravity, elevation, and engineering.

Ffestiniog, opened in 1963 by Queen Elizabeth II, was the UK’s first pumped hydro system—a blueprint of what clean, dispatchable energy storage could look like. Dinorwig followed in 1984, breaking records and pushing boundaries. At the time, it was hailed as one of the world’s most audacious engineering feats. Today, it remains the largest and fastest-responding pumped storage station in Europe.

This project is more than a refurbishment—it’s a recommitment. ENGIE, a global leader in the energy transition, operates in 30 countries and has 98,000 employees. Its portfolio spans renewables, flexibility assets like batteries and hydro, district heating networks, and green gas production. The Group invests over €10 billion annually toward achieving net zero by 2045.

In the UK, ENGIE employs 1,300 people and owns diverse low-carbon assets, including pumped hydro and offshore wind through Ocean Winds (with 950MW operational and 882MW under construction). It also supplies over 17,000 business customers. Over two decades, ENGIE has injected billions into Britain’s transition to a cleaner, more resilient energy future.