A concentrating photovoltaic (CPV) system developed at Penn State, Pennsylvania (US), tracks the sun with nearly unnoticeable movement. The microcells convert 600-times concentrated sunlight with 30% efficiency, daily producing more than 50% more energy than an average silicon solar cell.
“In contrast to conventional CPV systems that track by rotating to face the sun, our system remains at fixed tilt and tracks the sun by translating the solar cells laterally,” says Chris Giebink, Charles K Etner Assistant Professor of electrical engineering at Penn State. “Because our solar cells are very small — about half a millimeter square — the concentrating optical system is also very compact.”
The system consists of a pair of lenslet arrays that sandwich an array of tiny solar cells that slide between them. This whole sandwich is about 2 cm thick. The solar cell array only needs to translate laterally by the size of one lenslet, which is about 1 cm in diameter. “We call this mode of tracking 'embedded microtracking,’ “ Giebink adds.
Tiny cells, micro-tracking
Existing CPV systems are typically very large — the size of a billboard — and rely on precision, dual-axis rotational tracking, which is a challenge with rooftop installations where space is constraint. However, “Our goal is to take the high efficiency of conventional CPV and put it in the form factor of a traditional solar panel,” says Giebink. What is more, the new system tracks the sun with practically imperceptible movement. “This opens up new market opportunities for CPV that have not been possible with the traditional rotational tracking approach,” the expert says, pointing to the obvious example of rooftops, where panels need to lie flat on the surface.
Giebink and his colleagues developed their system with applications in mind where space is limited yet more power is needed than today’s standard silicon panel can deliver. “For example, several hybrid and electric vehicles have photovoltaics integrated in their rooftops — our approach could deliver significantly more power in situations like this.”
Performance compared with previous CPV technologies
“The efficiency of our prototype system peaks at 30% efficiency, in contrast to the 15 to 20% typical of silicon and cadmiumtellurid thin-film panels,” says Giebink. “Traditional CPV systems operate in the 35 to 40% range, and we should be able to get there, if we can resolve issues like cell heating that were identified in our prototype testing.”
He suggests that ultimately, the most direct comparison between technologies is to look at how much energy each type of system would generate over the course of an entire day. “From this standpoint, our prototype generated 54% more energy than a typical commercial silicon cell.” He makes an important point, though: CPV technology requires direct sunlight and therefore performs poorly in predominantly cloudy areas.
The future of CPV
The Penn State researcher believes this new type of CPV technology may open up new markets not served by traditional CPV systems. “The goal is not to compete with but to complement traditional CPV. Our recent work demonstrates that embedded microtracking CPV is technically feasible, but there is still a great deal of engineering that stands between this result and a practical microtracking CPV panel.”
Major applications for embedded microtracking CPV would be rooftops and installation sites in urban environments where space is very limited vehicle rooftops.
Right now, Giebink’s team is aiming to scale up their prototype demonstration to make a 0.2 m by 0.2 m, fully automated panel based on plexiglas lenslet arrays.
The research is detailed in the article “High-concentration planar microtracking photovoltaic system exceeding 30% efficiency,” published in Nature Energy.
Written by Sandra Henderson, research editor Solar Novus Today