In conventional power plants, heat from burning fossil fuels is used to boil water and make steam, which then turns turbines to generate electricity. The same thing can happen when the heat comes from the sun. This method is called concentrating – or sometimes concentrated – solar power, and in sunny locations, it may prove more affordable than any other fuel. From sunny Las Vegas, Shelley Schlender reports.
The Las Vegas Strip is a fantasy of flashing neon, crowded theatres and high-tech hotels. Much of the electricity to keep it going comes from coal and natural gas-fossil fuels that are expensive, polluting… and running out.
But drive just half-hour into the desert and you come to a new electric power plant which uses a clean fuel that's free and won't run out.
From a distance, the Nevada Solar One plant makes a shimmer of blue, almost like a lake. Driving closer, the blue mirage turns into 200,000 mirrors that reflect the cloudless desert sky. Each mirror rises 6 meters high, tracking very slowly as it follows the sun across the sky.
Bob Cable, who manages this concentrated solar power plant, explains that the mirrors are capturing solar energy. "The large parabolic structure with the mirrors focuses its energy into the focal point where a receiver tube is, and we heat a heat transfer fluid up to 390 degrees C." The heat transfer fluid is an oil that is then used to boil water, which generates steam which turns a turbine and turns a generator and produces electricity.
It all begins with heating a black metal tube that runs above the middle of each mirror. Each mirror is curved to concentrate the solar thermal energy on this receiving tube, making it super hot. Thanks to concentrated solar power, or CSP, Cable says these 120 hectares of desert land make enough electricity for 14,000 households. "It's a very proven technology. I've been involved with this business 17 years now. I know it works. I'm comfortable with it. I would love to see a lot more of it."
So would Mark Mehos, the CSP expert at the National Renewable Energy Lab, in Golden, Colorado. Mehos says that the southwestern U.S. has plenty of flat, sunny, open space for making electricity this way. "With that optimal siting, we can supply more than 10 times the capacity used currently in the United States," he predicts.
Concentrating solar power has similar potential anywhere that's sunny, such as Spain, North Africa and the Middle East. But solar energy provides less than one percent of the world's electricity. To make more, Mehos says, we must lower costs, so that concentrated solar power is competitive with fossil fuels. He expects it will be, eventually. "We will get there. We need the policies in place and we need the research we do in the laboratory."
Much of the research is focused on the technological challenges throughout the system. Another scientist at the National Renewable Energy Lab, Cheryl Kennedy, is working on developing a better mirror. She points out various types around the lab, "Glass mirrors, very thin glass mirrors, aluminum mirrors, silver polymer mirrors, these are a multi-layer coating mirror…" Her goal is to cut the cost of the mirrors in half, at least, and improve them in other ways. "We would like a perfect mirror that lasts forever, doesn't cost anything, weigh anything and never gets dirty," she says with a laugh.
Currently, when the sun sets, CSP plants shut down, including Nevada Solar One, which – at 64 megawatts of power – is the world's third largest. In order to operate into the night, they need affordable ways to store the oil that's been superheated by the sun during the day. Adding to the challenge, one of the best fluids for retaining heat is molten salt, and salt is corrosive. So the inner coatings of the receiving tubes and thermal storage containers must be extremely durable, as well as insulating.
As scientists make that happen, Mark Mehos says that more plants will operate at night. "If we can achieve that goal of reducing the costs of thermal storage, along with these other components, concentrating solar power will be used widely, I believe, in the U.S. and internationally."
Currently, Spain leads the way in this technology, with strong governmental subsidies and support for research. A Spanish company, ACCIONA, runs Nevada Solar One. Meanwhile, in Spain, a new plant opening this summer can store seven hours of solar heat and run into the evening. And in Arizona, another Spanish company, Abengoa, is scheduled to open a 280 megawatt plant in 2011. It will be the world's largest, and will operate into the night.
The U.S. Department of Energy estimates that both concentrated and the more familiar photovoltaic solar power plants could replace fossil-fired ones, leading to near zero carbon emissions from electricity production in just a few decades. Mark Mehos is hopeful that the U.S. will invest more in this technology, to speed the transition to clean, solar fuel.
