Storage allows a thermal power plant to run more hours in the day, so they can more quickly recover the cost of expensive steam turbines and generators. Woolard says that while a solar thermal plant without storage can generate electricity about 2,700 hours a year, BrightSource’s storage system increases that to 4,300 hours. The increased output more than offsets the added cost of storage. A study from the National Renewable Energy Laboratory (NREL) in Golden, Colorado, estimates that storage in a power tower system could cut costs per kilowatt hour by 25 to 30 percent.

At least two other companies are pairing power tower technologies with molten salt storage. Torresol Energy has built such a system at a 19.9-megawatt solar thermal power plant near Seville, Spain, and demonstrated that it can run the power plant through the night using stored heat. In the United States, Solar Reserve plans to build a power tower with molten salt storage in Riverside County, California. 

In addition to lowering costs, the storage system also improves the economics of solar thermal power by increasing the price that utilities are willing to pay for the electricity. Storage decreases the need for utilities to invest in backup power for smoothing out variations in power. Utilities will also pay a higher price for power they can count on at any given moment to make up for increases in demand. And the storage system lets the plant sell power into the evening, when power prices are higher in some locations.

Storage technology may be essential if solar thermal technology is to compete with photovoltaic solar panels, which have been coming down in price, says Mark Mehos, an NREL researcher. “BrightSource plants that don’t have energy storage probably generate electricity at about the same price as a plant that uses photovoltaics,” he says. “So all things being equal, they would like to be able to deliver that at a higher value.”