Rooftop concentrating photovoltaics win big over silicon in outdoor testing

Rooftop concentrating photovoltaics win big over 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. Credit: Giebink Lab, Penn State

A concentrating photovoltaic system with embedded microtracking can produce over 50 percent more energy per day than standard silicon solar cells in a head-to-head competition, according to a team of engineers who field tested a prototype unit over two sunny days last fall.

"Solar cells used to be expensive, but now they're getting really cheap," said Chris Giebink, Charles K. Etner Assistant Professor of Electrical Engineering, Penn State. "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 changing economic landscape has put a premium on high efficiency. In contrast to silicon solar panels, which currently dominate the market at 15 to 20 percent efficiency, concentrating photovoltaics focus sunlight onto smaller, but much more efficient solar cells like those used on satellites, to enable overall efficiencies of 35 to 40 percent. Current CPV systems are large—the size of billboards—and have to rotate to track the sun during the day. These systems work well in open fields with abundant space and lots of direct sun.

"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.

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 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.

Rooftop concentrating photovoltaics win big over silicon in outdoor testing
The concentrating photovoltaic system operating near sunset. Credit: Giebink Lab

"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 .

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."


Explore further

High efficiency concentrating solar cells move to the rooftop (w/ Video)

More information: High-concentration planar microtracking photovoltaic system exceeding 30% efficiency, Nature Energy (2017).
nature.com/articles/doi:10.1038/nenergy.2017.113
Journal information: Nature Energy

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Jul 17, 2017
Although the article is behind a pay wall, there is a nice cross sectional figure of their collector at the Nature link. Nice, simple, mechanism they came up with.

I thought that increasingly cheap panels would make up for lower efficiency by covering every surface that was exposed to sunlight, but it seems concentrated solar is still alive, providing options when space is limited or one just wants More Power from a full roof installation.

Jul 18, 2017
"I thought that increasingly cheap panels would make up for lower efficiency by covering every surface that was exposed to sunlight, but it seems concentrated solar is still alive, providing options when space is limited"

The space needed is the same for the traditional kind and concentrated solar. Even though you're not covering the entire area with solar cell in a CPV setup you are covering the entire area with the lens array. (You need to capture the incident solar radiation on a certain area in either case)

The thing that changes is that you're replacing a medium cost substrate over a large area with a high cost substrate over a tiny area plus a low cost lens/motion setup over a large area - while increasing yield significantly.

"Why not just change the lens' refractive index electrically rather than physically moving?"
Changing refractive index would need constant power (reducing overall efficiency). Moving the lenses only requires power during motion.

Jul 24, 2017
A_P: The article talked about 50% more power in a head to head competition between systems. I presumed that this was for arrays of the same overall area, but I could be wrong. With CPS at 30% efficiency and silicon at 17% for the same of collector area, then it would be possible to get the same amount of power from less collector area. Or am I missing something??

Sep 06, 2017
Or am I missing something??


Non-tracking panels lose additional efficiency when the light isn't coming in dead straight due to the structure of the cells. It's a similiar effect to how a computer monitor's contrast changes when you view it from different angles.

What on paper is a 17% efficient panel will in practice turn out closer to 12% when you nail it down to your roof. With CPS you get automatic tracking because the lens system keeps the cell at an optimal angle to the light.

Sep 06, 2017
Now, the problem with CSP is that glass lenses are heavy and costly. Any practical system would need lenses made out of plastic.

But plastics are UV sensitive and break down or turn cloudy under sunlight.

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