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

Jul 15, 2025

Delphine Chérel-Sparham, Managing Director of ENGIE’s pumped hydro business, brings over 30 years of experience in energy to the conversation. She discusses the critical role of pumped hydro in grid stability, highlighting its unmatched capacity for long-duration energy storage. Delphine details the modernization efforts for iconic assets like Dinorwig and Ffestiniog, and compares pumped hydro’s advantages over batteries. She also shares insights on her engineering journey, the importance of diversity in the field, and the exciting future of energy technologies.

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INSIGHT

Why Pumped Hydro Scales Where Batteries Don’t

Pumped hydro stores large volumes by moving water between two reservoirs and delivers power quickly by releasing it through turbines.
Dinorwig's underground caverns and 1.8 GW capacity illustrate pumped hydro's unique long-duration and high-power capability.

ANECDOTE

A Cathedral-Sized Cavern Inside Dinorwig

Dinorwig was carved into the mountain with 16 km of tunnels and caverns large enough to host a cathedral.
The scale of civil works houses the mechanical and electrical systems that enable massive pumped storage operations.

INSIGHT

Spin Mode Enables Sub-20 Second Response

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

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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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