BrightSource Energy has become the latest solar thermal power company to develop a system for generating power when the sun isn't shining. The company says the technology can lower the cost of solar power and make it more reliable, helping it compete with conventional sources of electricity.
Stored sunlight: A rendering shows BrightSource’s new thermal storage design. The two large tanks will store molten salt, which can be used to generate steam to drive a turbine.
The company, based in Oakland, California, is building one of the world's largest solar thermal power plants. The 392-megawatt solar plant in Ivanpah, California, however, will not include the storage technology. Instead, BrightSource is working with utilities to determine which future projects could best benefit from storage.
Solar thermal systems use mirrors to focus sunlight, generating temperatures high enough to produce steam to drive a turbine. One of the advantages of the solar thermal approach, versus conventional photovoltaics that convert sunlight directly into electricity, is that heat can be stored cheaply and used when needed to generate electricity. In all solar thermal plants, some heat is stored in the fluids circulating through the system. This evens out any short fluctuations in sunlight and lets the plant generate electricity for some time after the sun goes down. But adding storage systems would let the plant ride out longer periods of cloud cover and generate power well into, or even throughout, the night. Such long-term storage could be needed if solar is to provide a large share of the total power supply.
BrightSource is using a variation on an approach to storage that's a decade old: heating up a molten salt—typically, a combination of sodium and potassium nitride—and then storing it in a tank. To generate electricity, the molten salt is pumped through a heat exchanger to generate steam. BrightSource CEO John Woolard says one big factor in making this technology economically attractive is the use of power towers—in which mirrors focus sunlight on a central tower—that generate higher temperatures than other solar thermal designs. That higher temperature makes it possible to store more energy using a smaller amount of molten salt. "It's a much more efficient system and much more cost effective, overall," he says.
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."
By Kevin Bullis
From Technology Review