New research could improve solar power dramatically

Artist rendition of a field of SunBots with sunflowers in the foreground Courtesy Ximin He

For cottagers on solar power, there’s a research study just published that may mean big improvements in sun harvesting, especially in high latitudes. 

Solar panels work well when the sun shines directly on them, but their efficiency drops as the sun sets, or seasonally, as the angle of the sun becomes lower to the horizon. Existing solutions that adjust the angle of the panel require an electromechanical device, something to move the panel, and a computer chip and preset computer program set to the position of the sun in the sky at a specific time on a specific day of the year. “We wanted a system that doesn’t require human direction—that was the challenge. These systems are costly, and in need of a gigantic mechanical system. They’re not as smart as natural plants,” says Ximin He, an assistant professor of Materials Science and Engineering at the University of California, Los Angeles. 

Enter the new research, inspired by the ability of plants to orient toward light, called phototropism.“Plants have this strategy that uses a self-control mechanism, without any programming,” says He. “We thought it would be really advantageous to engineer self-tracking abilities in a man-made material in order to enhance the harvesting of solar energy,” she says.

Inspired by sunflowers, He and her team’s research developed a new use for hydrogels, a type of cross-linked polymer familiar to most people in the form of contact lenses and blister bandages. Their application, called SunBots, uses columns of this material that exhibit self-regulating autonomous motion using environmental stimuli to allow columns of the hydrogel to orient themselves towards light.

The technology works like this: when light comes in at an angle and hits the side of a column made of this photoresponsive hydrogel, the light stimulates the material to shrink and therefore causes the pillar to bend toward the light. “Different photo-responsive soft materials have different shrinkage mechanisms,” she explains. “For example, hydrogels become hydrophobic and expel water out of the polymer network upon illumination.” Another key challenge they faced was figuring out how to make the pillars know when to stop bending. “We found that as the pillar bends, the tip of the pillar blocks the light and causes a self-shadowing effect that mechanically blocks the light. The area starts to cool down,” and because it is a reversible material, “there’s a little recovery.” The material is able to adjust itself between overbending and under bending and, she says, “it will find the most comfortable spot. As the sun changes angle, it continuously adjusts.” How fast does the material respond? “It takes seconds or milliseconds to follow the light. That’s much faster than real sunflowers.”

How much more light are they able to harvest? He’s team tested it across a range of angles, comparing non-tracking surfaces with theirs. As the light source gets lower, the difference between the harvesting capability rises. For example “when the angle of the light came in from 25 degrees above the surface, the non-tracking surface harvests 24 per cent of total input energy. With our surface, it tracks 90 per cent,” she says. “That’s almost a four-fold enhancement.” In theory, says He, the improvement could be as much as 1200 per cent, when the sun is very close to the horizon and non-tracking solar panels are least efficient. 

No products are yet on the market, as the research was only published earlier this month, but the team is working towards developing a coating to apply to existing solar panels.If we can make the material itself have a self-control mechanism that is more efficient, and that also can apply light-tracking microstructures on all solar panels, windows, cars,” she says, “it could completely change the game.” 


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