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| STEAM AND MIRRORS: A Compact linear Fresnel reflector, like Austra'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. - Courtesy of Austra |
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.
"The issue of the linear Fresnel concept is proof of performance of a large system, not just a prototype system in the field," says Mark Mehos, concentrating solar power program manager at the National Renewable Energy Laboratory (NREL) in Golden, Colo. Ausra and other companies that employ the same technology, such as New York City-based SkyFuel and Solar Power Group in Munich, Germany, "are making large claims," he says, "without testing in the field."
If those claims stand up, however, solar-thermal plants could provide a significant chunk of the Southwest's - and potentially the nation's - electricity. "The maximum you can get into the grid is about 25 percent from solar," including photovoltaics, Mills says. But "once you have storage, it changes from this niche thing to something that could be the big gorilla on the grid equivalent to coal."
Ausra claims to have solved the storage problem without using molten salts or other expensive means of conserving heat. In fact, the company estimates that the price of its electricity will drop to roughly 8¢ per kilowatt hour if it can store heat for 16 hours. "Thermal storage is generally considered to be quite a bit cheaper than electrical storage," says Nate Blair, a senior analyst at NREL. "There isn't a lot of power generation combined with storage systems that can take advantage of that. [Concentrated solar power] has a leg up on storage in the grid or flow batteries or even ultracapacitors."
The system will employ pressure and a steam accumulator to accomplish the trick. "You allow some of the steam to recondense," O'Donnell explains. "It flashes back to steam when you reduce the pressure just by opening the valve to the turbine."
Such long-term steam storage, however, is unproved. "Steam storage is currently feasible at small levels, for example, one hour or so," NREL's Mehos notes. "Due to large volumes and high pressures involved with steam storage, scaling up steam storage to baseload applications is very high risk."
Assuming that their storage system works, Mills and his colleagues calculated in a paper presented today at the Solar Energy Society World Congress in Beijing that such solar-thermal power plants could match the electricity needs of both California and Texas. And, by combining a system that would meet the needs of California and Texas, solar-thermal plants could supply 96 percent of the national electricity demand. "The entire energy use of 2006, the current technology including storage would use a patch of land 92 miles by 92 miles," O'Donnell says. "Ten percent of the [Bureau of Land Management] land in Nevada is enough."
Even adding a transition to electric-powered vehicles did not alter the sunny picture. "You have to generate more electricity," Mills says. But "it doesn't destroy the correlation" between solar output and electricity demand for things like air conditioning.
Such a solar-dominated grid could also tolerate intermittent resources like wind energy, as long as storage systems worked. "A lot of the [winter] heating load correlates with wind [resources]," Mills adds, and the fickle supply of wind generation can be smoothed with hydropower and solar, he argues.
Such a solar solution to the nation's energy needs would require a host of other investments, including high-energy, long-distance, direct current transmission lines from areas like the Southwest or Southeast with fewer clouds to areas like the Northwest and Northeast with too many. "To do it in the East would drive up the cost because the solar resource isn't as good," NREL's Blair says. "Or you could build some kind of massive transmission system to try and get that power up to the East."
But that technology already exists. "There's no new technology on the transmission side, there are megavolt transmission lines around the world today," O'Donnell says. "It is the cost of building electricity transmission compared to the cost and liability of nuclear waste disposal or cost and liability of long-term carbon sequestration."
Ausra hopes to announce several partnerships this fall and has already acquired the land to build one such solar-thermal plant at an undisclosed location in southern California. If its storage system works and proves cost-effective, Ausra might just help usher in a solar revolution. "We have the ability to transition to a zero-carbon electricity future without moving the electricity price around," O'Donnell says. "That hasn't been part of anybody's conventional wisdom."
* Correction: Initially, this read "kilowatts" rather than the correct unit of measure "kilowatt-hours," which reflects the averaging of the sun's energy over the course of a day.