A Cheaper Solar Concentrator
A new light-guiding optic combines low cost with high efficiency.
Nicolas Morgan holds up a square piece of clear, molded acrylic about a centimeter thick and shines a penlight directly at its flat surface. A green beam enters the acrylic and bends toward the center of the square. Morgan repeats the process at different points on the surface, and each time, the beam darts toward the center.
The acrylic component–called a Light-Guide Solar Optic (LSO)–is a new type of solar concentrator that could significantly lower the cost of generating electricity from the sun. Unlike existing designs, there’s no need for mirrors, complex optics, or chemicals to trap and manipulate the light. “It’s pure geometric optics,” says Morgan, director of business development at Toronto-based Morgan Solar.
Solar concentrators have emerged in recent years as a way to intensify the amount of sunlight hitting solar cells, which are the most expensive part of solar panels. To make solar power more affordable, engineers have sought to use less solar-cell material by concentrating sunlight onto much smaller spaces.
But this approach has its own challenges. Most concentrators tend to be complex systems that use special lenses, curved mirrors, and other optical components with a “nonzero” focal length. This means that there must be enough distance–an air gap–between the solar cell and the optic to properly focus the light. As a result, concentrator-based systems are usually packaged within bulky enclosures, with enough depth to accommodate the focal length and protect all components during shipping. This means higher material and assembly costs and more expensive shipping.
A couple of years ago, Nicolas’s brother John Paul Morgan came up with the idea of a solid-state solar concentrator system: a flat, thin acrylic optic that traps light and guides it toward its center. Embedded in the center of Morgan Solar’s concentrator is a secondary, round optic made of glass. With a flat bottom and convex, mirrored top, the optic receives the incoming barrage of light at a concentration of about 50 suns and amplifies it to nearly 1,000 suns before bending the light through a 90-degree angle.
Unlike other concentrators, the light doesn’t leave the optic before striking a solar cell. Instead, a high-efficiency cell about the size of an infant’s thumbnail is bonded directly to the center bottom of the glass optic, where it absorbs the downward-bent light. There’s no air gap, and there’s no chance of fragile components being knocked out of alignment.
“It’s all about critically controlling the angles once the light enters the first optic,” explains Nicolas Morgan. The design takes advantage of a phenomenon called total internal reflection–the angle at which a beam of light inside an optical material will reflect back into the material rather than escape.
The trick is to mold the acrylic such that it bends the light in a certain direction when it enters the first optic. It must assure that the light maintains that angle to prevent it from escaping and guide it to the glass optic at the center. Precision is crucial–not just in the design of the optics, but also in creating the molds to mass-produce them.
The company expects that its first commercial versions of the system will be composed of acrylic wafers about eight square inches in size containing a secondary glass optic that’s about twice the diameter of a nickel.
Ray LaPierre, a professor of engineering physics at McMaster University, in Ontario, Canada, and an expert in high-efficiency solar cells, first saw Morgan Solar’s LSO prototype in December at a Canadian solar conference and walked away impressed. “Their design is certainly novel, is physically sound, can be cheaply manufactured, and has a good chance to revolutionize concentrator [technology] ” says LaPierre.
But like other PV concentrators, Morgan Solar’s technology still requires a tracking system to keep it facing the sun. Researchers at MIT have eliminated the need for trackers by developing special dye coatings that can absorb diffuse light, but Morgan Solar’s technology is closer to market. Nicolas Morgan adds that trackers today are precise, reliable, and add “marginal” cost for 44 percent more power. Some business and engineering decisions must still be made, but he expects that the company will be able to build its system for less than $1 per watt by 2011–“and with some vertical integration, considerably less.” This would lead to a product close to 30 percent efficient at costs competitive with thin film.
“I think the concept should be pursued,” says engineering professor Roland Winston, an expert in nonimaging optics at the University of California, Merced. He does, however, question the use of acrylic as a concentrator material: “Acrylic has not been proven for long-term use, especially under concentrated sunlight.”
John Paul Morgan says that’s the main reason why the company is using both acrylic and glass in its system. The company has intentionally limited concentrations within the acrylic portion to 50 suns and has the smaller glass optic doing the heavy lifting. “We want this system to last for 25 years, so we’re trying to really understress the material,” he says. “Once we’ve proven we can push the acrylic further, we’re going to shrink the glass optic.”
A number of pilot projects planned for 2009 will test the concentrator in the field. The company expects commercial production to begin sometime in 2010.