Cool paint. Literally.

NSF Science 360 News gazes at a new, foamy polymer paint that stays so cool in the sunlight, it could be used to replace air conditioning:

An alternative to these energy-intensive cooling methods is passive daytime radiative cooling (PDRC), a phenomenon where a surface spontaneously cools by reflecting sunlight and radiating heat to the colder atmosphere. Developing practical PDRC designs has been challenging: many recent design proposals are complex or costly, and cannot be widely implemented or applied on rooftops and buildings, which have different shapes and textures.

But now, National Science Foundation-funded researchers have invented a high-performance exterior PDRC polymer coating with nano-to-microscale air voids that acts as a spontaneous air cooler and can be fabricated, dyed and applied like paint on rooftops, buildings, water tanks, vehicles and even spacecraft — anything that can be painted.

The air voids in the porous polymer scatter and reflect sunlight, due to the difference in the refractive index between the air voids and the surrounding polymer. The polymer turns white and thus avoids solar heating, while its intrinsic emittance causes it to efficiently lose heat to the sky.

At the link, there’s a really nice thermal photo of the cool polymer at work on a hot afternoon.