A new approach for concentrating photovoltaic systems gets rid of mechanical sun trackers, making this design a contender for sunny rooftops across the world, a recent report shows.

Current strategies for turning sunlight into electricity make trade-offs between performance, price and versatility. However, researchers will have to advance in all three areas to meet the Department of Energy's "SunShot" goal: cost parity with fossil fuels by 2020.

Conventional silicon-based cells are durable, but engineers are now hitting the upper performance limit of these designs, hovering around 25 percent power conversion efficiency.

Thin-film cells tend to be cheaper but also less efficient. Multi-junction solar cells, on the other hand, are setting efficiency records around the world, but remain very expensive (ClimateWire, Oct. 20, 2014).

To exploit their efficiency while bringing down their price, some solar companies are using concentrators to focus sunlight on tiny multi-junction cells. This approach cuts the materials cost for concentrating photovoltaics.

Fixing a tracking problem
However, as the sun moves across the sky, light hits solar panels at different angles, changing the amount of electricity they can produce. Concentrating photovoltaic panels have to sway back and forth in order to keep sunlight focused on the small cells (ClimateWire, Jan. 21). This makes them too expensive, too heavy and too big for rooftops, where most of the world's solar panels currently reside.

In a study published last week in the journal Nature Communications, scientists demonstrated a device that moves less than a centimeter to follow the sun throughout the day, solving part of the tracking problem.

The system sandwiches photovoltaic cells between miniature plastic lenses on top and small mirrors on the bottom, each separated by a thin layer of oil. The lenses and mirrors focus sunlight on the solar cell like a magnifying glass. With a gentle nudge, the concentrators move relative to the cells, keeping sunlight in focus all day.

"From a long distance, you wouldn't be able to tell it's tracking," said Chris Giebink, an assistant professor of electrical engineering at Pennsylvania State University and a co-author of the report.

Another advantage is that the design uses light, cheap materials. "The majority of the system is made out of acrylic plastic," Giebink said. This means that a commercial device could use well-established techniques like injection molding to bring down costs further.

In the prototype, researchers used 3-D printed lenses and found that they could achieve 70 percent optical efficiency, meaning they transmitted 70 percent of the light hitting the device to the cells. With further development, Giebink said he expects to reach 90 percent optical efficiency.

Power conversion efficiency depends on the solar cell itself, which is independent of the lens. The current record belongs to a cell built by the Fraunhofer Institute for Solar Energy in Germany, reaching 46 percent efficiency. Higher efficiency means solar panels can get more energy out of less space and, with economies of scale, reduce the overall price of electricity from the sun.

Going after more rooftops
"I think what this development does is it gives you the capability of having the high efficiency of concentrating photovoltaics in the form factor of standard photovoltaics," Giebink said, adding that this would be a good option for roofs.

Some companies are already developing and deploying concentrating photovoltaics in grid-scale solar farms, where space and weight aren't as big of a concern.

Semprius Inc., which received a $3 million award from DOE's SunShot incubator in 2009, demonstrated 43.9 percent efficiency in a four-junction cell last year.

Another company, Glint Photonics, received $2.7 million in 2012 from DOE under the Advanced Research Projects Agency-Energy to commercialize low-cost concentrating photovoltaic modules. "Our target is to be producing panels that are roughly equivalent, on a per-area cost, with silicon modules," said Glint CEO Peter Kozody at the ARPA-E summit this week. "You can get up to twice the energy output for roughly the same cost per square meter."

Michael Haney, a program director at ARPA-E, said this kind of performance improvement would increase the size of the photovoltaic market. "Now that you have that enhanced efficiency, you can think about expanding the rooftop market to roofs that would never have considered them before because they are too small now," he said.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500