STEAM AND MIRRORS: A compact linear Fresnel reflector, like Ausra's plant in Australia pictured here, uses lines of mirrors to focus the sun's rays on an overhead trough, turning water into steam to generate electricity. Image: COURTESY OF AUSRA
In the often cloudless American Southwest, the sun pours more than eight kilowatt-hours* per square meter of its energy onto the landscape. Vast parabolic mirrors in the heart of California's Mojave Desert concentrate this solar energy to heat special oil to around 750 degrees Fahrenheit (400 degrees Celsius). This hot oil transfers its heat to water, vaporizing it, and then that steam turns a turbine to produce electricity. All told, nine such mirror fields, known as concentrating solar power plants, supply 350 megawatts of electricity yearly.
In the face of mounting concern about climate change, alternatives to coal and natural gas combustion such as these never seemed more attractive. And with the bounty of the sun waiting to be captured near fast-growing major centers of electricity consumption—Las Vegas, Los Angeles and Phoenix, among others—interest in such solar thermal technology is on the rise. The first such plant to be built in decades started providing 64 megawatts of electricity to the neon lights of Vegas this summer.
But physicist David Mills, chief scientific officer and founder of Palo Alto, Calif.–based solar-thermal company Ausra, has bigger ideas: concentrating the sun's power to provide all of the electricity needs of the U.S., including a switch to electric cars feeding off the grid. "Within 18 months, with storage, we will not only reduce [the] cost of [solar-thermal] electricity but also satisfy the requirements for a modern society," Mills claims. "Supplying [electricity] 24 hours a day and effectively replacing the function of coal or gas."
The company insists it can do this at a cost of just 10 cents per kilowatt-hour, analogous to the price of electricity from burning natural gas in California if a cost was imposed for the emission of carbon dioxide, the leading greenhouse gas (as the state's Public Utilities Commission is considering).
Ausra will rely on a different type of concentrating solar power plant to deliver on this promise. French physicist Augustin Fresnel showed in the 19th century that a large lens, like the parabolic troughs of the existing solar-thermal plants, can be broken down into smaller sections that deliver the same focus. Applying this, Mills's design—a compact linear Fresnel reflector—allows for greater ground coverage, lower weight and greater durability than precision-shaped parabolic mirrors. "You can drop stones on it and they bounce off," Mills says. "We would be able to build these in Florida in the hurricane zone."
This Fresnel solar thermal plant also eliminates oil, directly heating water to a lower temperature of roughly 535 degrees F (280 degrees C) at a higher pressure, about 50 bars, or 50 times atmospheric pressure. Then, it uses the resultant steam to turn the same low-temperature turbines as those employed in nuclear reactors.
The amount of electricity produced is simply a function of the sun's bounty and the number of mirrors. "We're moving from 80- to 100-megawatt designs to 700 megawatts and above," says John O'Donnell, Ausra's executive vice president.
The key will be proving performance. Thus far, the company has exactly one solar array, hooked to a coal-fired power plant in Australia to provide extra steam that improves its efficiency at burning the dirty rock. At present, the Ausra mirrors produce just an additional 12 megawatts of extra heat, but there are plans to boost that as high as 38 megawatts thermal.