Transparent coating keeps solar cells cool and efficient throughout the day

A transparent silicon coating that radiates heat into space can help solar cells maintain high levels of efficiency throughout the day(Credit: Stanford University)

Stanford engineers have developed a transparent silicon overlay that can increase the efficiency of solar cells by keeping them cool. The cover collects and then radiates heat directly into space, without interfering with incoming photons. If mass-produced, the development could be used to cool down any device in the open air for instance, to complement air conditioning in cars.

After a full day in the sun, solar cells in California can approach temperatures of 80° C (175° F), even in winter months. Excessive heat can pose problems because, while the cells need sunlight to harvest energy, they also lose efficiency as they heat up. A standard silicon cell, for example, will drop from 20 to 19 percent efficiency by heating up just 10° C (18° F) or so.

Laptops address the overheating problem with the help of carefully engineered fans and heat sinks, but for solar panels and other devices that work in the open air, space itself could serve as heat sink par excellence. The coolness of space, approaching absolute zero, would negate the need for elaborate and expensive heat dissipation contraptions if only we had a way to access it from the ground.

The silica (SiO2) solar panel cover devised by Prof. Shanhui Fan and colleagues at Stanford is successfully making use of space as the largest of heat sinks. It does so by collecting and then radiating heat as infrared electromagnetic waves, which can easily travel through the atmosphere, out into space. The coating is transparent, so it won't interfere with the solar cell's light collecting ability, and improves on the heat dissipation of the silicon found in most cells.

The researchers tested their technology on a solar thermal collector, comparing a "bare" collector with two possible heat radiating mechanisms that used, respectively, silica and photonic crystals (a nanoscale pattern affecting the motion of photons) for heat dissipation. They found the latter to be the most effective.

According to the study results, the overlay allowed visible light to pass through to the solar cells while cooling the underlying absorber by as much as 23° F (13° C). This translates to an absolute efficiency gain of over one percentage point which, although it may not sound like much, would add up to something substantial over the life of the cell.

There are a number of improvements that could further benefit the cell's cooling (and efficiency). The overlays are thought to work best in dry and clear environments, the ideal spots for large solar arrays. Also, because testing was done during winter, the collectors had to be tilted by 60 degrees toward the south to maximize solar irradiance, reducing access to the sky (and cooling ability with it). Finally, elements of the more conventional convective cooling could also be added to the silica cover.

Fan and team are optimistic about scaling up production for commercial applications. They believe the technology could apply to any instance where an outdoors system needs effective heat dispersal, such as to cool down cars to save on air conditioning (and mileage) without affecting aesthetics.

The advance is described in the current issue of the journal Proceedings of the National Academy of Sciences.

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