LOS ANGELES - California is a leader in renewable energy, and the state has pledged to use only clean sources for electricity, including wind and solar power, by 2045. One hurdle is energy storage, but an old solution involving water may help the state reach its goal of zero emissions.

The solution is “pumped storage,” which uses water in reservoirs at different elevations to smooth the fluctuations of intermittent power from the wind and sun, and makes electricity available when it is needed.

California has mandated 60% renewable energy sources for its power generation by 2030, and all zero-emission sources by 2045, which could include nuclear generation along with renewables.

How it works

The Lake Hodges facility near San Diego, a relatively small 40 megawatt generating station, is one of 40 pumped storage facilities around the United States, and its operator says it is helping the state meet its ambitious goals. San Diego is planning a larger system at another site, the San Vicente reservoir, again using two water sources at different elevations.

“When you have excess energy during the day, you pump the water up a hill,” explained Gary Bousquet, deputy director of engineering at the San Diego County Water Authority. “At the end of the day (when solar generation has stopped), you’re able to bring that generating unit online, and then supply that energy back into the grid,” he said. The water runs downhill and activates generators.

At Castaic Lake north of Los Angeles, a 1,250-megawatt power plant with a double reservoir is helping that city meet its power needs. Again, pump-turbines move water uphill during peak production times, then generate power when the water runs downhill at night.

Ninety-five percent of utility-scale power is stored this way, using water.

Pumped storage sites in the United States produce more than 20 gigawatts of power. It’s just 2%, however, of the national output. That compares to 5% in parts of Europe and 10% in Japan.

More plants planned

More plants are planned, however. Los Angeles power officials and their partners hope to build a major pumped storage project at Lake Mead near Las Vegas, where the Hoover Dam already generates hydroelectric power. The largest hydroelectric faculty in the United States, it sends power to Arizona, Nevada and California.

“If you could add some pumps and pipelines after it goes through the dam at some point down river, pump it back upstream, back into Lake Mead,” said Reiko Kerr, senior assistant general manager for the power system at the Los Angeles Department of Water and Power, “you could actually increase the efficiency and optimize that asset” by adding a pumped storage component to the existing plant, she said.

Many utilities, like hers, store also small amounts of power in lithium-ion batteries.

“But they’re expensive,” said research professor Sadrul Ula of the Center for Environmental Research and Technology at the University of California, Riverside. “The challenge,” he said, “is to make them larger … and make it cost effective.”

Technology improvements

He says battery technology is improving, and so is computer-based integration of the electrical grid. He foresees a future when electric vehicles are tapped for power when not in use, so-called “V to G,” or battery to grid, with vehicles sending power to the electrical system when they’re parked and plugged and fully charged. That level of integration of vehicles with home and business micro-grids is many years in the future, he said.

Pumped storage is about 80% efficient because of system losses, Gary Bousquet said. The experts say, however, it’s the best solution now to store large amounts of energy for high-demand periods and regulate a clean power grid.