Why Upgrading Pumped Hydro Is Key to Grid Flexibility with Delphine Chérel-Sparham

Jul 15, 2025

Delphine Chérel-Sparham, the Managing Director of ENGIE’s pumped hydro business in the UK, brings over 30 years of energy sector experience to the table. In this engaging discussion, she delves into the unmatched capabilities of pumped hydro for long-duration energy storage and grid stability. Delphine explains ongoing refurbishments at iconic sites like Dinorwig and Ffestiniog, the modernization of vast underground infrastructures, and how these upgrades can enhance renewable energy capacity. She also shares insights on her engineering journey and offers advice for aspiring professionals.

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INSIGHT

Scale And Principle Of Dinorwig

Dinorwig is a massive underground pumped hydro plant built into 16km of tunnels and huge caverns.
It stores renewable electricity by pumping water uphill and can release it rapidly to provide grid power.

INSIGHT

Fast Response Through Spin Generation

Dinorwig can go from stopped to full power in 30–60 seconds and under 20 seconds from spin generation.
Keeping turbines spinning synchronized with the grid enables near-instantaneous power injection.

INSIGHT

High Cycling Causes Mechanical Wear

The plants receive very frequent mode-change instructions: about 25,000 per year.
That high cycling rate drives mechanical wear, so full replacement (not simple refurbishment) is required.

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As the UK accelerates its transition to a net-zero electricity system, the challenge of maintaining grid stability is growing.  A key part of a balanced system is large-scale, long-duration flexibility: assets that can store energy when it’s abundant and release it when it’s needed most.

But much of the capacity that can provide it already exists.

One of the most mature and proven forms of energy storage - pumped hydro is able to deliver gigawatts of power in seconds, store energy for hours, and provide inertia and frequency response that batteries alone can't match. Despite being decades old, they remain some of the fastest, most reliable, and highest-capacity tools for balancing supply and demand. In a world aiming for net zero, understanding and upgrading these assets is no longer optional, it’s essential.

In this episode, Delphine Chérel-Sparham - Engie’s Managing Director of Hydro UK joins Ed Porter to discuss how ENGIE is breathing new life into two of the UK’s most iconic pumped storage assets: Dinorwig and Ffestiniog. Over the conversation, they discuss: 

Why pumped hydro remains unmatched for long-duration storage and grid inertia.
How ENGIE is modernising vast underground infrastructure.
The role of fast-responding, synchronous machines in providing inertia and flexibility to the grid.
How repowering legacy assets creates headroom for more renewables on the system.
Delphine’s engineering journey and her advice for young people considering careers in energy infrastructure.

About our guest

Delphine is Managing Director of ENGIE’s pumped hydro business, where she leads the strategy, operations, and refurbishment of some of Europe’s most iconic energy infrastructure. With over 30 years’ experience in the energy sector, Delphine has worked across gas, LNG, oil, and renewables. Today, she’s focused on revitalising legacy pumped hydro assets like Dinorwig and Ffestiniog, to support the evolving needs of a flexible, decarbonised grid.

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