It's pretty counter-intuitive, but a surface can lose heat even while being hit by sunlight. This is a big deal for buildings, especially the roofs of buildings, and especially when those buildings are in urban areas that experience the urban heat island effect.
We've been measuring the behavior of heat on a roof for a long time. It's called a measure of albedo, which takes into account both the ability of the surface to reflect heat, and also to dissipate it back into the atmosphere. These are two different things, and in this new technique, it is the second feature that makes it work.
Sure we can make things reflective. Painting them white is a good move. Shiny and white, even better. But how do we make it so that the roof also dissipates its heat better?
Think about the fins on an air conditioner. Their sole purpose is to rid of heat, a thousand strips of metal reaching into the air, far enough away from each other that they don't release their heat right back to themselves, and yet packed enough that they do a maximum job.
That's pretty much what this passive daytime radiative cooling technique does. It's more about the shape and structure and texture of the surface, which is a polymer coating made of foamlike nano-to-microscale air voids. I instead describe it as billions of little fingers, each one reaching out into the air to release the heat it carries from the mass behind it. They're also scattering and reflecting sunlight in every direction while they're at it.
No matter how you describe it, don't forget that this coating can be sprayed on like paint; it doesn't have to be made that way in the factory but can be applied out there in field. That makes this advance a really big deal.
Polymer coating cools down buildings
Sept 2018, phys.org
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