The plant, which will start to generate electricity for nearby Las Vegas in April, consists of approximately 184,000 mirrors arranged in long, parabolic arrays that focus the sun's energy on a receiver--a metal tube filled with oil that's encased in specialized glass--from German conglomerate Schott.
Sunlight heats the oil to 400 degrees Celsius (about 750 degrees Fahrenheit). The oil gets transferred to a heat exchanger where it makes steam, which then cranks a turbine to produce electricity. If the heat can't be used right away, it gets transferred to vats of molten salt which retain the heat for later use.
"The steam side, it is not rocket science. It has existed for more than 100 years. The solar side, we know it is going to work," said Gilbert Cohen, senior vice president of Acciona Solar Power, which owns the plant and will sell it to local utilities. "The potential is huge here."
The project underscores the resurgence that's taking place for a technology calledfor generating electricity. Solar thermal power plants began popping up in and the American Southwest in the '80s, but construction of new plants largely ground to a halt in the early '90s.
Now, solar thermal projects are under way--or at least on the white board--in Spain, Greece, Mexico, Iran, Algeria and parts of the U.S., among other places. When it goes live, Solar One will be the third largest solar thermal plant in the world with a 64 megawatt capacity. Potentially, the site could crank out 2,000 megawatts, or enough power for about a half-million people, Cohen said. The U.S. Southwest could ultimately produce 4,000 to 40,000 megawatts of solar thermal power, he speculated, enough for 1 million to 10 million consumers.
In California's Mojave Desert, already home to 354 megawatts of solar thermal facilities, Stirling Energy Systems in conjunction with utility company Southern California Edison is erecting a 500 megawatt plant to open in 2009.
One of the biggest solar projects in recent U.S. history is almost complete.
The driving force behind the demand for solar thermal power, besides global warming and fears about rising electrical prices, are state and federal laws aimed at curbing fossil fuels and coal. In Nevada, regulations require that utilities get 15 percent of their power from renewable resources and a total of 5 percent from solar power by 2015. Other southwestern states have passed similar laws.
Solar thermal plants aren't cheap. The construction tab for building Solar One will likely run about $250 million, said Cohen. The power generated by the plant, minus any subsidies, may not get to parity with electricity generated from conventional plants until around 2020, added Nikolaus Benz, a development manager for Schott. Solar thermal electricity, according to statistics from Schott's publications, will cost around 15 to 17 cents a kilowatt hour in the U.S. Residents of Las Vegas now pay around 7 cents a kilowatt hour.
Solar thermal fans, however, say the technology represents the most economical way to harness the sun's power on a large scale. The solar plants will last for decades, so by 2030, solar thermal will be a better buy than coal-fired electricity, which is expected to go up in price. To eliminate risks to its customers--namely large utilities--Acciona sets long-term fixed prices for electricity generated by its plant.
"The return is pretty good, but you have to take the first step," Cohen said.
The technology also has an advantage in age, added Cohen. During the 15-year lull when utilities weren't commissioning new plants, engineers had time to enhance the performance of their products and wring out operational inefficiencies. Schott, for example, has come up with a new coating for the glass tube on the receiver that lets 96 percent of the solar radiation through to the oil-filled metal tube sealed inside the glass. The coating also withstands abrasion better than earlier versions.
Solar One also makes for a pretty cool field trip. You're zipping along on an empty desert highway, and suddenly a field of kilometer-long rows of what look like white antenna dishes come into view. The fences and isolation immediately make you think: CIA. The mirrors are mounted on the arrays in four rows that form a near perfect parabola to reflect as much sunlight as possible onto the purple receiver, about the diameter of a can of pasta sauce.
Contrary to expectations, the mirrors won't sizzle birds or bugs: they only get as hot as the outside temperature, said site manager Bob Cable. When operational, the arrays will rotate with the sun--software and microcontroller adjusting the pace of their movements depending on the day of the year and the position of the sun.
Work on the massive plant is slightly ahead of schedule. Construction on Solar One--which includes over 7 million aluminum parts in the frame and 76 kilometers' worth of oil-filled receivers as well as a nearby electrical plant--began in February 2006. The field is now 90 percent done while the companion buildings are 70 percent there.
Although solar thermal systems and solar photovoltaic (PV) panels both transform energy from the sun into electricity, they work in vastly different manners. PV panels, which have , split photons from the sun into electrons and positive charges from the sun. The electrons are harvested and funneled into the electrical system of a building or the grid. In general, silicon PV panels convert 15 to 22 percent of the light that strikes them into electricity; mixing other materials into the panels can increase efficiency, but also adds cost.
Solar thermal plants are more efficient, said Cohen, with efficiencies ranging from around 20 percent to 40 percent, according to studies, in part because it's easier to extract heat from sunlight than electrons. Solar thermal water heaters--which heat water for--rely on the same principle.
The molten salt vats also give solar thermal systems insurance against cloudy days, something that PV doesn't have. One hundred thousand square feet of molten salt holds enough heat to provide electricity for four hours.
The big drawback is that solar thermal plants . They work best in warm, dry locations, unlike PV panels which even work well in Germany. Shadows from vapor trails and planes can curb their production. And dust is a major problem. To keep it off, a cart festooned with moist brushes that look like they came from a car wash hoses off the mirrors.
As a result, solar thermal mostly gets deployed for power plants, which cost hundreds of millions of dollars and take up hundreds, if not thousands of acres of land. An individual can put a PV system on a private home, but it will cost about $20,000. Until recently, financing for these projects has been nearly impossible to obtain. Security is a potential issue, too. Venture capitalist Vinod Khosla recently said during a panel discussion that a thermal plant occupying three percent of Morocco's land could provide Western Europe with all of its power. Maybe so, said other panelists, but that would also make it a potential target for terrorists, or even a political tool.
Solar One's location was the result of a host of factors, said Cohen. A lake about 18.5 miles away provides water to the station. Additionally, it's only 3 miles from three electrical substations. It costs about $1.5 million per mile to connect to a substation, so distance counts.
Plus, it's about the sunniest place in America.
"You have a site here that for 360 days is almost like today," Cohen said, nodding toward the bright blue sky. "We had to shut down construction three times because it was snowing. We talked to people who lived here, and they said they had never seen snow in their lives here."