To do this, the researchers embed tiny multi-junction solar cells, roughly half a millimeter square, into a sheet of glass that slides between a pair of plastic lenslet arrays. The whole arrangement is about two centimeters thick and tracking is done by sliding the sheet of solar cells laterally between the lenslet array while the panel remains fixed on the roof. An entire day's worth of tracking requires about one centimeter of movement, which is practically imperceptible.
"Our goal in these recent experiments was to demonstrate the technical feasibility of such a system," said Giebink. "We put together a prototype with a single microcell and a pair of lenses that concentrated sunlight more than 600 times, took it outdoors and had it automatically track the sun over the course of an entire day."
Because the team needed to know exactly how much direct and diffuse sunlight there was during the test, they set up at the Russell E. Larson Agricultural Research Center at Penn State where there is a National Oceanic and Atmospheric Administration Surface Radiation monitoring site. Graduate students Jared Price and Alex Grede worked together with post-doctoral researcher Baomin Wang to test the system over two sunny days from dawn to dusk right alongside a commercial silicon solar cell.
The researchers report today (July 17) in Nature Energy, that the CPV system reached 30 percent efficiency, in contrast to the 17 percent efficiency of the silicon cell. All together over the entire day, the CPV system produced 54 percent more energy than the silicon and could have reached 73 percent if microcell heating from the intense sunlight were avoided.
According to Giebink, this embedded tracking CPV technology would be perfect for places with lots of direct sunlight, such as the southwestern U.S. or Australia.
Giebink notes that major challenges still lie ahead in scaling the system to larger areas and proving that it can operate reliably over the long term, but he remains optimistic.
"With the right engineering, we're looking at a step-change in efficiency that could be useful in applications ranging from rooftops to electric vehicles — really anywhere it's important to generate a lot of solar power from a limited area."
Other researchers working on this research from Penn State are Jared Price and Alex Grede, graduate students in electrical engineering, and Baomin Wang, postdoctoral scholar in electrical engineering, all first authors; Michael Lipski, undergraduate student in electrical engineering; and Chris Rahn, professor, Greg Brulo, graduate student, and Xiaokun Ma, graduate student, all in mechanical engineering.
Researchers from other institutions include Brent Fisher and Scott Burroughs, Semprius Inc.; Kyu-Tae Lee and John A. Rogers, Department of Materials Science and Engineering; and Junwen He and Ralph G. Nuzzo, Department of Chemistry, all at University of Illinois at Urbana-Champaign.
The Advanced Research Projects Agency-Energy and the National Science Foundation supported this work.