When Manuele Aufiero was a child, his parents would take him hiking along a reservoir in northern Italy. It wasn’t a typical reservoir, though. This one drained and refilled constantly, with pumps raising the water level when electricity was cheap. When nearby cities needed electricity, the pumps would reverse, turning into generators as the water drained out of the reservoir.

The technology, known as pumped-storage hydropower , or pumped hydro for short, has been around for over a century. Such facilities are some of the biggest “batteries” humans have ever built. Globally, pumped hydro reservoirs store 8,500 gigawatt-hours of electricity, according to the International Energy Agency.

Pumped hydro can generate electricity for hours on end, and the power plants have grown in importan

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