Tractebel supports ENGIE’s renewal of UK’s largest pumped-storage power plant

As the Dinorwig six-turbine hydropower plant nears the end of its design life, First Hydro Company (FHC), majority owned by ENGIE, has launched one of the biggest pumped storage renewal projects currently underway in Europe. Tractebel is providing Technical Assistance during the design stage.

Dinorwig Power Station in North Wales, widely known as the “Electric Mountain,” has been a cornerstone of the United Kingdom’s energy resilience since its commissioning in 1983. Carved deep into the mountainside above Llanberis, the underground complex houses six fixed-speed Francis pump turbines, each rated at 313.5 MW. Together, they deliver a combined generation capacity of around 1.8 GW, making Dinorwig the largest pumped-storage facility and fastest-responding generating asset in the UK.

After over four decades of reliable service, Dinorwig’s main electrical and mechanical plant components are approaching the end of their original design life. To secure the station’s future and enhance its capabilities, FHC, under 75% ownership by ENGIE and 25% by Caisse de dépôt et placement du Québec (CDPQ), has launched a full refurbishment project, known as “replanting.”

Inside the underground Dinorwig machine hall; spare runner stored between two units at turbine floor level

Tractebel providing technical assistance during design phase

Our team is contributing across conceptual, basic and detailed design review on the following fields of expertise:

• Turbine hydraulic model tests
• Hydraulic transient calculation
• Turbine mechanical design
• Mechanical Balance of Plant
• Generator design
• Electrical Balance of Plant
• Instrumentation and Control System
• Risk management and quality assurance

The project is divided into three stages. Stage 1 is mainly related to design and site survey. Stage 2 involves replanting of the first two units, as well as any remaining basic and detailed design not already carried out in Stage 1. Stage 3 is the replanting of the remaining units.

Stage 1 is currently ongoing, with full project completion expected for 2035. The replanting will replace and optimize key components with the latest-generation systems, ensuring improved efficiency, reliability, maintainability and operational flexibility.

Pumped storage key to the energy transition

More than just an equipment renewal project, the Dinorwig replanting is a major investment in the UK’s net-zero future. Pumped storage plays a critical role in enabling largescale renewable energy deployment. By storing excess electricity produced during periods of high wind or solar generation, stations like Dinorwig can release energy back into the grid precisely when it is needed most, ensuring stability, flexibility and reliability in an increasingly renewable-based power system.

Llyn Peris, the lower reservoir of the Dinorwig pumped-storage scheme

Key benefits include:

• Grid stability in a system with growing renewable penetration
• Massive short-response capability, essential for balancing intermittency
• Low-carbon flexibility, replacing fossil fuel-based balancing assets