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. Suc...