Harnessing sunlight on the cheap

For a project that could be on the very cutting edge of renewable energy, this one is actually decidedly low tech--and that's the point.

A team of students, led by mechanical engineering graduate student Spencer Ahrens, has spent the last few months assembling a prototype for a concentrating solar power system they think could revolutionize the field. It's a 12-foot-square mirrored dish capable of concentrating sunlight by a factor of 1,000, built from simple, inexpensive industrial materials selected for price, durability and ease of assembly rather than for optimum performance.

Rather than aiming for a smooth parabolic surface that would bring the sunlight to a perfect focus, the dish is being made with 10-inch-wide by 12-foot-long strips of relatively low-cost, lightweight bathroom-type mirror glass. The frame is assembled from cheap aluminum tubing, with holes drilled in precise locations using a simple jig for alignment, so that the struts can be assembled into a framework that passively snaps into just the right parabolic curvature.

The control mechanism, which allows the dish to track the sun automatically across the sky, is also remarkably simple--photocells mounted on each side of the dish with opaque baffles, which cast a shadow on the cell when it drifts out of alignment, connect to a simple circuit that turns on small electric motors to push the dish back into the right position.

"The technical challenge here is to make it simple," Ahrens explains. The team is keeping careful track of all the costs for parts and the time spent on assembly, to provide a baseline for figuring out what an eventual large-scale field of such dishes would cost. "We're using all commodity materials that are all in high production," he says.

That's in stark contrast to most attempts to build solar dish concentrating systems, which have tended to use expensive custom-made equipment to achieve high efficiency. A few large companies that have built such prototypes tend to "turn it into an ultimate high-tech, high-end project," says Jefferson Tester, HP Meissner Professor of Chemical Engineering, who has been advising the student-led group. "Then Spencer came along and said, 'We're going to fundamentally change this and make this an affordable technology for popular, widespread deployment.'"