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Why pumped hydro is back in vogue for energy storage
10/9/2025
10 min read
Feature
In the ever more pressing pursuit of low-carbon energy storage in the era of intermittency, the venerable technology of pumped hydro is making a comeback. Although it’s already the most popular form of energy storage, accounting for over 90% of global capacity, most existing pumped hydro stations were built 40 or more years ago. But it is increasingly seen as an important standby technology when the sun doesn’t shine and the wind doesn’t blow. Selwyn Parker reports.
‘Energy storage is essential for electricity grids, particularly as the generation of renewable energy continues to rise,’ argues the Oxford Institute of Energy Studies (OEIS) in a paper issued in July. Other rival storage technologies such as batteries, compressed air and hydrogen are emerging, but pumped hydro has the virtue of being tried and tested as the kind of ‘long-duration’ backup that electricity grids will need.
‘Large-capacity, long-duration energy storage enhances grid resilience when used in combination with intermittent or highly variable generation,’ summarises engineering consultancy Arup, which is working on projects in Britain, including design and project management for the first large-scale battery energy storage system (BESS) in Northern Ireland, at Dunore Point for Northern Ireland Water, to be connected at a higher distribution voltage of 33 kV. ‘Stored energy can be dispatched when needed, improving grid reliability, especially during periods of high demand or during outages. Energy storage also reduces the need for curtailment of renewable energy during periods of high output and low demand,’ says the consultancy.
Essentially, a pumped hydro system (PHS) creates instant energy by sending huge volumes of water up and down pipes at high speed. According to one definition: ‘A PHS consists of two linked reservoirs, one upper and one lower. When energy demand is low, water is pumped from the lower reservoir to the higher one. When demand is high, water is released through pumps which act as turbines to generate electricity that can be released onto the grid in seconds.’