Duration statistics and grid-forming inverters

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.

About Modo Energy

Modo Energy helps the owners, operators, builders, and financiers of battery energy storage solutions understand the market - and make the most out of their assets.

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