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Generation from solar farms roughly tripled between 2018 and 2023, while the output of wind turbines almost doubled. That provided enough extra clean energy to power Japan and Russia, put together. Meanwhile, the hydroelectric dams that generate about one-seventh of the world’s electricity stood still: The additional 52 terawatt-hours in 2023 was only enough to fuel Greece.
That’s not overly surprising. Hydro’s problems resemble those of the other veteran form of clean power, nuclear. Unlike wind and solar, it’s been an established technology for decades, so much of its growth potential has already been exploited. Like nuclear, it’s dependent on the building of costly and time-consuming projects to move forward, and shows little sign of reducing its (already low) costs. Compounding the problems, a two-year drought connected to the La Niña climate cycle during 2022 and 2023 caused a collapse of generation in China, home to about a third of the world’s hydro dams.
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The productivity of a power plant can be measured by its capacity factor — the number of kilowatt-hours it generates as a percentage of a theoretical maximum. For nuclear facilities, which run around the clock, this number is close to 90%, while for solar panels, which mostly operate for a few hours around midday, it’s between 15% and 20%.
Hydro facilities typically have capacity factors in the region of 40% — but it’s a mistake to assume that figure must remain fixed in the long term. The US study found that four out of five plants saw declines between 1980 and 2022, with the cumulative fall representing nearly a quarter of generation, equivalent to closing down a Hoover Dam-sized project every two to three years.
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Why should this be happening? One well-known factor is that climate change itself may be reducing the availability of the fresh water on which hydro dams feed. In the Alps, hotter, drier summers are likely to lead to thinner snow cover in the coming decades. In the Himalayas, glaciers that currently provide vast natural batteries trickling water into the catchments of hydro reservoirs from China to Pakistan are receding.
The most worrying fact about the US study is that it doesn’t even depend much on this effect. Water shortage explained the measured decline at only a fifth of the plants studied. A substantial share of the shortfall, in fact, appears to derive from a change in the way dams are put to use. Non-energy considerations — such as river ecology, flood management, boating and fishing, and the supply of water for irrigation, industry, and homes — appear to be growing in importance in recent years.
A plant operator may want to run their dam just for electricity production — but plenty of other interest groups may regard a reservoir as a shared public asset, and want a say in how it’s managed. That’s likely to be a particularly fraught issue in places like South Asia, where farmers are already voracious consumers of water and are will demand more allocations as temperatures rise and the climate dries out.
To that group can now be added another: renewable power generators. As fossil fuels are pushed out of the generation mix — falling to less than a third of the total in the European Union last year, for instance — hydro will more and more be called on to balance out fluctuations from wind and solar power.
In Europe, North America and the Pacific, almost all dams currently under construction are for so-called pumped storage, which can force water uphill with cheap midday power before discharging it back to generate electricity in peak hours. Even conventional dams will find themselves used more flexibly to back up wind and solar. As a result, they’re likely to generate less than they would if run flat-out.
That’s going to be a challenge. If we’re going to zero out our emissions, the world has three decades to roughly double the fleet of dams that it has been building up since the 19th century. The pace of construction is less than half of what’s needed — and the shortfall will be even worse if current plants are operating less efficiently. Hydro is probably the most controversial and most neglected form of renewable power. We can’t afford to ignore it, though.
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