Water Batteries

https://www.science.org/content/article/how-giant-water-batteries-could-make-green-power-reliable

By ROBERT KUNZIG, Science. 

Excerpt: The machines that turn Tennessee’s Raccoon Mountain into one of the world’s largest energy storage devices—in effect, a battery that can power a medium-size city—are hidden in a cathedral-size cavern deep inside the mountain. But what enables the mountain to store all that energy is plain in an aerial photo. The summit plateau is occupied by a large lake that hangs high above the Tennessee River.... At night, when demand for electricity is low but TVA’s nuclear reactors are still humming, TVA banks the excess, storing it as gravitational potential energy in the summit lake. The pumps draw water from the Tennessee and shoot it straight up the 10-meter-wide shaft at a rate that would fill an Olympic pool in less than 6 seconds. During the day, when demand for electricity peaks, water drains back down the shaft and spins the turbines, generating 1700 megawatts of electricity—the output of a large power plant, enough to power 1 million homes. The lake stores enough water and thus enough energy to do that for 20 hours. Pumped storage hydropower, as this technology is called, is not new. Some 40 U.S. plants and hundreds around the world are in operation. ...Pumped storage, however, has already arrived; it supplies more than 90% of existing grid storage. China, the world leader in renewable energy, also leads in pumped storage, with 66 new plants under construction, according to Global Energy Monitor. ...In the Alps, where pumped storage was invented in the late 19th century, Switzerland opened a plant in 2022 called Nant de Drance that can deliver 900 megawatts for as long as 20 hours.... 

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