It doesn’t look like much right now, but Stanford University’s plywood-and-Mylar experimental solar cell apparatus could pave the way for a new, low-cost approach to boosting solar cell efficiency. The trick is to prevent the loss of efficiency that results when solar cells heat up. The solution, as described somewhat poetically by the research team, is to add a layer of material that enables the cell to access the “coldness of the universe.”
Cooling Off A Hot Solar Cell
The basic problem is that solar cells have two functions. They absorb photons to generate electricity, and they also retain heat. Only one of those two functions is desirable.
The skyward-facing position of solar cells does offer a way out, as described by the abstract for the new Stanford solar cell study:
Because a solar absorber by necessity faces the sky, it also naturally has radiative access to the coldness of the universe. Therefore, in these applications it would be very attractive to directly use the sky as a heat sink while preserving solar absorption properties.
The Stanford research team hammered together a prototype model to demonstrate that you could engineer a protective, transparent layer or “blackbody” that enables the solar cell to function efficiently while funneling off excess heat into the vast sink of the universe.
The Stanford team based its blackbody on an atomic-thin wafer made from silica photonic crystal (silica is fancyspeak for quartz). If you have the right equipment you could DIY the same thing yourself. To engineer the crystal, the researchers etched holes into it to a depth of about 10 micrometers, tapering the holes slightly to act as funnels, with this result:
When placed on a silicon absorber under sunlight, such a blackbody preserves or even slightly enhances sunlight absorption, but reduces the temperature of the underlying silicon absorber by as much as 13 °C due to radiative cooling.
In the image below, the Stanford logo is actually under the photonic crystal, so yes, when Standford says visibly transparent they mean visibly transparent.
Image (A) shows the two of the experimental apparatuses on a campus rooftop. For their experiment, instead of using actual solar cells the team deployed “mock” solar cells, designed to absorb solar energy without generating electricity (the next step will be to test the silica layer on solar cells).
The four dots in the two apparatuses are solar absorbers. Moving from left to right, the first dot is an absorber with a silica layer but without the blackbody, and the second is an absorber with the silica photonic crystal blackbody. The third and fourth dots are plain absorbers for comparison.
Images (C) and (D) are from scanning electron microscopes, showing the structure of the silica photonic crystal.
Solar Cell Efficiency: Let’s Hear It For The 1%
The study will be formally presented in June at the Conference on Lasers and Electro-Optics in San Jose, California, under the title, “Radiative cooling of solar absorbers using a transparent photonic crystal thermal blackbody,” by Linxiao Zhu, Aaswath P. Raman and Shanhui Fan.
Meanwhile, the folks at The Optical Society have provided a plain-language rundown of how the new apparatus performs in terms of solar cell efficiency:
Because heat makes solar cells less efficient, the researchers predict their cooling layer could help solar cells turn approximately 1 percent more sunlight into electricity, a big boost from a relatively simple add-on.
Okay, so a one percent improvement sounds less than impressive at first take, but it represents a big step forward in the context of solar research and solar cell efficiency, where progress is measured in fractions of a percent.
In addition, the research team expects that the cooling effect will prevent degradation and help the solar cell last longer, and that will help lower the lifecycle, bottom line cost of solar power. That’s an important point because “soft costs” still account for a large proportion of the overall cost of solar power. Progress on solar cell efficiency is important, but other factors have also been contributing to the precipitous drop in solar prices.
But why stop at solar cells? The team anticipates that its finely tuned silica layer could also be used to save energy by deflecting excess heat from cars, buildings, and other surfaces.
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Images: via US National Institutes of Health, PubMed.gov.
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