The San Diego County Water Authority has an unusual plan to use the city's scenic San Vicente Reservoir to store solar power so it's available after sunset. The project, and others like it, could help unlock America's clean energy future.

Perhaps a decade from now, if all goes smoothly, large underground pipes will connect this lake to a new reservoir, a much smaller one, built in a nearby canyon about 1100 feet higher in elevation. When the sun is high in the sky, California's abundant solar power will pump water into that upper reservoir.

It's a way to store the electricity. When the sun goes down and solar power disappears, operators would open a valve and the force of 8 million tons of water, falling back downhill through those same pipes, would drive turbines capable of generating 500 megawatts of electricity for up to eight hours. That's enough to power 130,000 typical homes.

"It's a water battery!" says Neena Kuzmich, Deputy Director of Engineering for the water authority. She says energy storage facilities like these will be increasingly vital as California starts to rely more on energy from wind and solar, which produce electricity on their own schedules, unbothered by the demands of consumers.

Californians learned this during a heat wave this past summer. "Everybody in the state of California, I believe, got a text message at 5:30 in the evening to turn off their appliances," Kuzmich says. The sun was going down, solar generation was disappearing, and the remaining power plants, many of them burning gas, couldn't keep up with demand. The alert worked; People stopped using so much power, and the grid survived.

Yet earlier on that same day, there was so much solar power available that the grid couldn't take it all. Grid operators "curtailed," or turned away, more than 2000 megawatt hours of electricity that solar generators could have delivered, enough to power a small city. That electricity was wasted, and there was no way to store it for later, when grid operators desperately needed it.

"We have a problem if we're going to have these continuous heat waves," Kuzmich says. "We need a facility to store energy so that we don't need to turn off our appliances."

Pumped hydro has a history

The technology that San Diego is proposing, called pumped hydro energy storage, is already operating at more than 40 sites in the United States. Some of the largest ones, which can generate more than 1000 MW for up to eight hours, were built during the 1970s and 1980s to store electricity that nuclear power plants generated during the night. But few new plants have been built over the past 30 years in the U.S. China has continued to build such plants.

Now, the need to store power from renewable sources is reviving interest in this old technology in the U.S.

"Just in the past several years, 92 new projects have come into the development pipeline," says Malcolm Woolf, president and CEO of the National Hydropower Association. Most of the projects, however, are in the planning stages and still need regulatory approval and financing.

Thanks to the climate bill that President Biden signed in August, these projects now qualify for the same 30 percent tax credit that solar and wind projects enjoy. "That is an absolute game-changer," Woolf says. "A number of these projects that have been in the pipeline for a number of years now suddenly are a whole lot more bankable."

Water batteries have a lot of competitors, when it comes to storing energy. Some companies, including the car company GM, are exploring ways for the electric grid to draw emergency power from the batteries in millions of privately owned electric cars. Others are working on ways to store electricity by compressing air or making hydrogen. Still others are focused on ways to manage the demand for electricity, rather than the supply. Electric water heaters, for instance, could be remotely controlled to run when electricity is plentiful and shut down when it's scarce.

Pumping water, however, has some advantages. It's a proven way to store massive amounts of power. The San Vicente project would store roughly as much electricity as the batteries in 50,000 of Tesla's long range Model 3 cars. Water batteries also don't require hard-to-find battery materials like cobalt and lithium, and the plants can keep working for more than a century.

The biggest problem with them, at least according to some, is that it's hard to find places to build them. They need large amounts of water, topography that allows construction of a lower and higher reservoir, and regulatory permission to disturb the landscape.

Woolf, however, says the perception of pumped hydro's limited prospects "is a myth that I am working hard to disabuse folks of." Pumped hydro facilities, he says, don't have to be as massive as those of the past century, and they don't need to disturb free-flowing streams and rivers. Many proposals are for "closed-loop" systems that use the same water over and over, moving it back and forth between two big ponds, one higher than the other, like sand in an hourglass.

Three of the proposed projects in the U.S. that appear closest to breaking ground, in Montana, Oregon, and southern California, all would operate as closed loops.

Kelly Catlett, director of hydropower reform at American Rivers, an environmental advocacy organization which has highlighted the environmental harm caused by dams, says that "there are good pumped storage projects, and there are not-so-good pumped storage projects."

Her group won't support projects that build new dams on streams and rivers, disrupting sensitive aquatic ecosystems. But San Diego's plan, she says, "looks like something that we could potentially support" because it uses an existing reservoir and doesn't disturb any flowing streams. Also, she says, "I'm unaware of any opposition by indigenous nations, which is another really important factor, as they have borne a lot of the impacts of hydropower development over the decades."

The board of the San Diego County Water Authority, and San Diego's city council, are expected to vote soon on whether to move ahead with a detailed engineering design of pumped hydro storage at the San Vicente reservoir. The state of California is chipping in $18 million. The design work, followed by regulatory approvals, financing, and actual construction, is likely to take a decade or more.