Rooftop Concentrating Photovoltaics Overshadow Silicon in Outdoor Testing

Penn State

July, 2017
Rooftop Concentrating Photovoltaics Overshadow Silicon in Outdoor Testing
The concentrating photovoltaic system showing the top lenslet; the little black square visible near the middle is the solar cell and the lines running away from it are the contact wires. Image: Giebink lab

At Penn State, engineers field-tested a prototype concentrating photovoltaic system (CPV) with embedded micro tracking that can produce over 50 percent more energy per-day than standard silicon solar cells.

“Solar cells used to be expensive, but now they’re getting really cheap,” said Chris Giebink, the Charles K. Etner Assistant Professor of electrical engineering. “As a result, the solar cell is no longer the dominant cost of the energy it produces. The majority of the cost increasingly lies in everything else – the inverter, installation labor, permitting fees, etc. – all the stuff we used to neglect.”

This shift of focus has put a premium on higher efficiency – conventional silicon solar panels achieve only 15–20 percent efficiency. In contrast, concentrating photovoltaics focus sunlight onto smaller but more efficient solar cells to boost efficiency to 35–40 percent, but come at the cost of size, with systems the size of billboards that have to rotate to track the sun during the day, requiring abundant space and direct sunlight.

“What we’re trying to do is create a high-efficiency CPV system in the form factor of a traditional silicon solar panel,” said Giebink.

Doing so, they embed tiny (about half a millimeter square) multi-junction solar cells into a sheet of glass that slides between a pair of plastic lenslet arrays. The entire arrangement is two centimeters thick, and solar tracking is done by sliding the sheet of cells laterally between the lenslet array while the panel remains fixed on the roof. A day’s worth of tracking requires a nearly imperceptible one centimeter of movement.

In their side-by-side test, the new system reached 30 percent efficiency while the traditional silicon cell hit a mere 17 percent efficiency, leading to the CPV producing 54 percent more energy, and could have reached 73 percent if microcell heating from the intense sunlight was avoided.

Giebink notes future challenges of scaling, but states that, “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.”

To learn more about the team’s future of solar energy capture, click here.

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