DAVID GREENE, HOST:
Solar power has become quite a force, especially here in California. Some days, in the middle of the day, it's delivering half the electricity in the state. And more solar farms are coming. Of course, there's just one problem. The sun goes down, and people still want their power. So now there's a search underway for a battery big enough to store all of that solar energy for when people need it. And it's reviving interest in an old technology, a way to store massive amounts of electricity. Dan Charles from NPR's Planet Money team visited what some are calling the biggest battery in the world.
DAN CHARLES, BYLINE: I drive into the Appalachian Mountains toward the border between Virginia and West Virginia to the Bath County Pumped Storage Station owned by Dominion Energy. The gate's closed, but there's a phone.
UNIDENTIFIED PERSON: So what we can do is - I'll open the gate for you. You come through that gate there.
UNIDENTIFIED PERSON: And then you go up the hill. OK?
CHARLES: I drive up the hill and realize, oh, this is a dam. And it's created a huge lake. It fills this entire valley. On the shore of the lake, there's a windowless concrete building, which is where I meet Sean Fridley, the station director. We get in an elevator and start going down.
And we're going down into the ground. Is that it?
SEAN FRIDLEY: The ground's on one side, and we've got the lower reservoir on our other side here. So we will be well underwater here.
CHARLES: We arrive at the beating heart of this place - a cavernous hall and six giant motors, the tops of them round like carousels, lined up in a row. These are pumps. They're pumping water from that lake on one side of us up through tunnels in the mountain on the other side - huge tunnels, big enough to drive a car through. The water is heading up toward a valley way up above us, where there's another dam. This water is filling a new lake way up there.
And the water has to go up to 1,200 feet. You seem kind of nonchalant about it.
FRIDLEY: It's a massive project.
CHARLES: The water in that upper lake represents stored power. The power that these motors consumed pumping it up there can be reclaimed later - most of it, anyway. And a couple of hours later, this is what happens. People up and down the East Coast are turning on their air conditioning, demanding power.
FRIDLEY: We'll watch things as they happen. And I'll try to talk loud and explain a little bit.
CHARLES: Below me, a valve opens. And the force of millions of tons of water in that upper lake hits the steel of these machines. You can't see the water, but you can feel the force. This whole place is vibrating. It's scary actually.
FRIDLEY: It's starting to turn.
CHARLES: Slowly at first, then faster and faster.
FRIDLEY: We are online.
CHARLES: The pump that pushed water up the mountain is now spinning in reverse. It's being driven by the water, and now it's generating electricity. When all six of these generators are operating, this is one of the ten biggest power plants in the country. There are other plants like this around the country. Most of them were built decades ago to store electricity from nuclear power plants during nighttime, when everybody was sleeping. And now they're getting a fresh look as a way to store power from the sun and the wind.
Scott Flake, an energy consultant in Sacramento, Calif., is working on plans for new pumped storage plants just like this one.
SCOTT FLAKE: This is a technology that could really move this idea of renewable energy forward.
CHARLES: They wouldn't be easy to build. They cost billions of dollars, involve dams and water - things people get in fights over. But along with that environmental cost, there's also a huge environmental opportunity, a chance to run entire cities all day long on solar power.
Dan Charles, NPR News.
(SOUNDBITE OF EMANCIPATOR'S "BARALKU") Transcript provided by NPR, Copyright NPR.