Daytime Radiative Cooling Tech Delivers Power-Free AC

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Ultrabroadband Photonic Structures To Achieve High-Performance Daytime Radiative CoolingTreehugger has an intriguing new article about a new radiative cooling solar structure developed by researchers at Stanford University that can provide cooling for buildings — even in direct sunlight — by reflecting energy back into space. Unfortunately, the article is a little light on actual scientific explanation, and the actual paper that it cites is behind a stinking paywall, but between the article and the abstract, I think I have a bit better understanding about how the technology works.

The story’s second paragraph had me a little confused at first:

The solar cooling device designed by the team reflects most of the sunlight hitting it, while also taking heat from the buildings or structures it is installed on and radiating it out into space, thereby improving the cooling efforts of those structures.

What makes this confusing is the fact that, while our atmosphere does a great job of letting light in, it tends to like to hang onto the heat that results. If you’ll forgive me the laborious science lesson: our warm (and ever-warming) atmosphere is a result of something called vibrational energy (and any of you actual scientists in the audience will have to forgive my gross simplification, but I’m trying to make this as palatable as possible). Sunlight strikes the earth, is absorbed by the surface, and radiates back outward as infrared energy. Certain molecules in the atmosphere (C02, for example) absorb that outgoing energy in sort of the same way a bass trap absorbs specific frequencies of sound. When that infrared energy is re-released, it isn’t released in the same direction it was originally heading (that is to say upward and outward, away from the Earth), and thus, we end up with more infrared energy (heat) sticking around.

That’s not a wholly inherently bad thing; without that mechanism, our planet would be a dreadfully chilly place and we wouldn’t be here. But add more greenhouse gases to the atmosphere — that is to say, gases whose molecules trap and return infrared energy to differing degrees — and we’ve got a mess on our hands.

My question, though, was how this new radiative cooling solar structure manages to get around that fact, returning heat to space without having it trapped and returned back to us in the process. The story does explain to a certain degree:

According to the team, the challenge of creating such a device is in developing a reflector capable of reflecting as much sun as possible, while also efficiently emitting thermal radiation within a specific range of wavelengths, so the heat energy can escape the Earth’s atmosphere.

The solution came from using nanostructured photonic materials that can either enhance or suppress certain wavelengths of light, enabling the team to combine the reflector and thermal emitter in one device that has no moving parts and no external energy demands. The team engineered nanophotonic materials from quartz and silicon carbide, which are both weak in absorbing sunlight, to build their device.

Combine that with the graphic included in the paper’s abstract, and the mechanism starts to make a lot more sense:

The structure behaves as a broadband mirror for solar light, while simultaneously emitting strongly in the mid-IR within the atmospheric transparency window, achieving a net cooling power in excess of 100 W/m2 at ambient temperature.

In other words, it sense the UV light from the sun straight back up before it has a chance to be absorbed and radiated as IR, and it emits IR in specific frequencies that aren’t absorbed and re-radiated by greenhouse gases in the atmosphere. Sneaky! The end result is that a house with only ten percent of its roof covered with this radiative cooling material could see 35 percent of its cooling needs in the hottest months covered by passive radiation, and areas of the world where air conditioning isn’t even a real thing could benefit from a radiative cooling effect using absolutely zero electricity.

Via: [Treehugger]

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