A truck tire supporting a 36,300-kilogram load repeatedly traverses an 18-meter stretch of road, day in and day out, rolling up 483,000 kilometers on the odometer at the U.S. Department of Transportation's (DoT) testing facility in Virginia. The goal is to thoroughly challenge any new paving techniques and see how the road surface holds up. Now imagine putting a solar panel under there.



That's exactly what Scott Brusaw of Sagle, Idaho–based Solar Roadways hopes to do next February. The electrical engineer is currently at work building a prototype of his so-called "Solar Road Panel" with the help of a $100,000 small business contract from the DoT.



"We're building solar panels that you can drive on," Brusaw says. "The fact that it's generating power means it pays for itself over time, as opposed to asphalt."



There are about 260,000 kilometers of roadway in the U.S. National Highway System alone, and thousands more in state highways, suburban thoroughfares and rural roads. Could all that asphalt be replaced with a solar technology that would also double as the nation's power grid?



The key to making this work will be the glass: The solar road panel prototype is 1,024 modules—each containing a solar cell, a light-emitting diode and, someday, an ultracapacitor for storage—sandwiched between a layer of some yet-to-be developed glass and a layer of conducting material. "Nobody's tried to drive on glass long-term," Brusaw says.



In addition to needing strength, this glass will be textured to allow tires to grip and water to run off. It will also be embedded with heating elements—like a car's rear windshield—to melt snow or ice. And it will need to be self-cleaning, coping with the grit and grime of an endless procession of tires as well as dust, dirt and other highway detritus. Needless to say, such glass does not exist yet but Brusaw hopes to partner with researchers at The Pennsylvania State University's Materials Research Institute to develop it.



"Glass theoretically can have a very high strength, provided there are no flaws," says materials scientist John Hellmann of Penn State, a glass expert. But "can you keep the proper optical properties to transmit light to the PV [photovoltaics, or solar cell] and still not weather or change with that traffic going over it? … We make some pretty doggone good glass for structural applications but we're not driving trucks on them."



The engineering challenges are immense, adds materials scientist Richard Brow of the Missouri University of Science and Technology, another glass expert. But glass can be strengthened by compressing its surface using special heating techniques or, at a molecular level, swapping ions in the glass itself. Such enhanced glass is 10 times stronger than the conventional variety and is used, for example, in smart phones to withstand the pressures of texting. "Can you go from a teenager's thumb to a truck? That's a pretty big leap, but 10 years ago we didn't think you could make a 15-micron piece of glass for what's relatively rough handling in a PDA," Brow says.



Glass has been used to build footbridges, such as the Chihuly Bridge of Glass in Tacoma, Wash. And new glass ceramic composites with increased toughness have been developed for the photovoltaics industry, Brow adds—but that might boost the price of the resulting panel.



In the meantime, Brusaw is spending $40,000 of the DoT's money to build a prototype from chemically hardened glass panels that can be purchased today. He will experiment with various types of solar cells, from thin-film to traditional monocrystalline silicon photovoltaics, and he will try to strike the right balance between transparency—so the panel works to deliver at least several thousand kilowatt-hours of electricity each day—and road-gripping texture, which will block some of the light. "If you have perfectly clear glass, you get perfect PV efficiency. But [with] perfectly smooth glass, everybody slips off the road," he notes. "Glass manufacturers can cut grooves into the glass in a hatch-type pattern. We'll try various methods and see what holds up."



Cost will be a factor: "The cost to develop a glass that will hold up in the fast lane of a highway? Fifteen [million] to 25 million dollars over three to five years," Brusaw says. "The cost in mass production? About $1 per square foot." The goal is to produce a 12-foot by 12-foot panel for $10,000 that is capable of producing 7,600 watt-hours of electricity daily; it would take roughly six panels to match the electricity demand of one average U.S. home, which use 936 kilowatt-hours per month, according to the Energy Information Administration.*



In addition to requiring a yet-to-be-invented form of glass, solar roadways would need some form of energy-storage capability—whether batteries or some not-yet-devised ultracapacitor. The goal is to create a cross-country highway system that can also serve as an national electricity generator and power grid. And paired with wind turbines to generate electricity at night, Brusaw estimates replacing the nation's highways with his solar roadways could eliminate the need for fossil fuel–fired power plants. "Based on my calculations, at 15 percent efficiency [from the photovoltaics] we produce more than three times the electricity we have ever produced," he says. Even with cars constantly casting shade over the road surface, along with other challenges, "we think we can make enough to meet the nation's energy needs," he adds.



Other companies, such as the England's Invisible Heating Systems, have developed roads that use embedded water pipes to harvest some of the sun's ample energy that also bathes U.S. roads.



The solar roadway will also offer embedded LEDs to illuminate the road and display information, whether the actual traffic directions, such as lane markers, or messages such as "SLOW DOWN." And, should electric cars become popular, powered pavement could also offer recharging stations wherever such panels are installed.



The first test of Brusaw's crystalline vision will be when the prototype is delivered to the DoT on February 12, 2010. And the DoT's challenges will be followed by some durability testing by the inventor with a pickax, sledgehammer and, depending on the prototype's fortitude, guns. Then it's on to parking lots and perhaps fast food restaurants. "Parking lots are much better than going right out onto the highway," Brusaw says. "You have slow-moving, lightweight vehicles. We can learn all the lessons there before moving into the fast lane."



*This sentence was changed to reflect actual power output of the panels, which is in watt-hours not kilowatt-hours as originally written.