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Concentrating solar power (CSP) is a method of focusing the sun to produce power in less space. Focusing the sun converts more sunlight into usable energy. It's usually combined with mechanical tracking, to follow the sun and capture energy through more hours of the day.

The concentrating method has been used in large, utility-scale power plants, like Nevada Solar One, which covers hundreds of acres. Rooftop concentrating solar is a scaled-down version that operates on the same principles.

"We looked at the technology that was out there and thought, this technology is ready to go. What's not been done yet is really addressing the needs of the existing solar panel market. These people want access to the new technology, but they need it to be available in a form that they can use easily."

That's a quote from Brad Hines, CEO of Soliant Energy. His company is developing the Heliotube. It's a large, flat module with reflective troughs that track the sun. Other companies are working on their own ideas. Silicon Valley Solar makes the Sol-X2. And another company is developing the Sun Cube, with lenses to focus the sunlight. But it won't be exported to the U.S.

The first rooftop products will be available here in 2008. Concentrating solar can produce several times more energy per square foot of roof space, compared to conventional PV, and prices are expected to come down. But for now, the up-front investment per kilowatt is higher.

The moving parts in the trackers are likely to increase maintenance costs. And if the mounting hardware has to be bolted down, make sure the roof penetrations won't affect your commercial roof warranty.