Sometimes it's like you looked at knives and bread for years. Sliced bread seemed so obvious in retrospect.
(Not a physicist, summarizing my best)
So, a material 'radiates' photons according to its color (duh). Some photons go through our atmosphere. Some get captured by greenhouse gases and thus 'retain heat'.
They made a material that was mostly ("nearly entirely") reflective to normal sunlight and to 'normal' IR wavelengths we see in the atmosphere
, but of a heat-radiating color that the atmosphere doesn't capture very well. So, what happens is that the ambient heat doesn't enter the material very easily, but it radiates heat in a wavelength that the air doesn't them capture.
The net result? A material that's actually cooler than the air around it.
Apologies for the poor size of the graph. The blue (I think, I'm colorblind) line is the temperature of the film. The black line is the temperature of the air.
Passive radiative cooling below ambient air temperature under direct sunlight
When exposed to direct sunlight exceeding 850 watts per square metre on a rooftop, the photonic radiative cooler cools to 4.9 degrees Celsius below ambient air temperature, and has a cooling power of 40.1 watts per square metre at ambient air temperature. These results demonstrate that a tailored, photonic approach can fundamentally enable new technological possibilities for energy efficiency. Further, the cold darkness of the Universe can be used as a renewable thermodynamic resource, even during the hottest hours of the day.
So, my commentary. I think
my description is correct. And, if I'm figuring correctly, if they can learn how to make the 'bottom' black (so that it absorbs all incoming photons) and keep the upside 'reflective', something like this could go onto the surface of buildings and would actually siphon heat from the inside of a building and shoot it into space?