Storage will make concentrated solar power reliable after sunset

Reliability is the greatest weakness of solar power. The sun provides an abundant, clean source of power to run steam generators that produce electricity. But it is ineffective after the sun sets.

To address that shortcoming, concentrating solar thermal plant developers have plans to build facilities in Inland Southern California to store the sun’s power as heat that can be tapped to run steam generators in the late afternoon and evenings when demand for electricity is greatest.

BrightSource Energy Inc., based in Oakland, recently announced plans to add storage to three of six tower-style concentrating solar thermal power plants it will build in Riverside and San Bernardino counties to provide electricity for Southern California Edison customers.

The storage is expected to add enough power production capacity so that BrightSource can eliminate plans for a seventh 200-megawatt solar thermal plant and still meet its contract obligation to deliver 4 million megawatt hours of power annually to the utility.

If the contract amendment is approved by the California Public Utilities Commission, “it will represent the largest deployment of thermal energy storage in the world,” said Joe Desmond, former chairman of the California Energy Commission who is senior vice president at BrightSource.

The addition of solar thermal storage will also result in lower costs for utility ratepayers, the company said.

Another benefit will be 1,280 acres of desert land that can remain undisturbed if a seventh plant isn’t needed, said BrightSource.

BrightSource is altering its construction plans to accommodate storage because its utility customers including Southern California Edison are asking for it, said Desmond.

In its contract with Edison, BrightSource is proposing to add storage to plants that are not too far along in the government permitting process to make a design change, Desmond said.

Storage will be a component of two BrightSource concentrating solar power plants in San Bernardino County and one in Riverside County, the company said, declining to identify the exact locations. Those three concentrated solar power plants, each with 250 megawatts of power production capacity, are expected to start generating electricity in 2016 and 2017.

The way BrightSource technology works is that a vast array of mirrors concentrates sunlight on water-filled boilers atop towers to create high-temperature, pressurized steam. The steam is piped through a conventional steam turbine generator that produces electricity.

To store the sun’s power, some of the steam produced during the day at BrightSource plants will be used to super heat molten salts held in a tank. Molten salts are used as a storage medium because they can absorb and hold heat at extremely high temperatures without changing to steam. The heat retained in the molten salts later will convert water to steam to run the plants’ turbines and produce electricity when the sun isn’t shining.

The storage capacity of the BrightSource concentrated solar thermal plants will be three to six hours, meaning they could operate that long without daylight. The storage provided will be determined by Southern California Edison, BrightSource said.

Under a state-legislated mandate to reduce the state’s reliance on fossil fuels, California’s investor-owned utilities are required to increase the portion of electricity they generate from renewable resources like solar energy and wind power, both of which are variable and difficult to integrate into a utility transmission system without storage.

Also under state legislation enacted in 2010, the state Public Utilities Commission and publicly owned utilities have been instructed to consider setting goals for energy storage with the aim of making better use of renewable energy resources.

“If solar power is going to move beyond being a small percentage of electricity generation, you need to take a share away from natural gas and one way to better compete with natural gas would be if it had storage,” said Brett Prior, a senior analyst covering the solar industry for GTM Research in Boston. He added, however, that even with storage the cost of producing electricity with solar power is still about 70 percent higher than with natural gas.

A more immediate goal of the solar thermal industry is to offer storage to compete with photovoltaic power plants that convert sun directly to electricity without heating water or another medium, Prior said.

Since 2008, the price of photovoltaic panels has fallen so dramatically that concentrating solar thermal technology has lost most of its former price advantage or is now more expensive, Prior said. But with storage, he said, concentrated solar power plants can offer a different advantage in the form of longer hours of production than photovoltaic plants, which do not have an affordable form of electricity storage.

Bill Walsh, manager of renewable procurement for Southern California Edison, said solar thermal power plants are less interrupted by clouds than photovoltaic plants because the solar thermal process holds heat longer. Solar thermal power plants with storage will be even more valuable for offsetting and managing more intermittent renewable power resources, he said.

An example, he said, is that after sunrise when photovoltaic plants start to produce electricity, solar thermal plants could store the sun’s heat for use later in the day when consumer demand is greater and photovoltaic plants shut down.

SolarReserve, a Santa Monica-based developer of concentrating solar thermal power plants, plans to build the 150-megawatt Rice Solar Energy project in east Riverside County about 30 miles north of Blythe. That project includes 12 hours of storage. Its construction is scheduled to begin the first quarter of 2013.

Storage has been part of SolarReserve’s technology since the company was founded in January 2008, said its chief executive officer, Kevin Smith. “When we started we thought the right solution was storage,” he said.

Like BrightSource’s plants, SolarReserve’s have fields of mirrors that concentrate the sun’s power on a receiver at the top of a tower. But the sun is used to directly heat molten salt that can be stored, rather than water.

SolarReserve is building its first concentrated solar power plant in Nevada that is expected to go on line in December 2013, several years ahead of the BrightSource’s California solar storage projects. The 110-megawatt plant, located between Las Vegas and Reno, is planned with 10 hours of storage. Smith said it will help to keep the lights on in Las Vegas, where the highest demand for electricity spans from noon to 11 pm.