Common cooling methods such as air conditioners are expensive, consume significant amounts of energy, require ready access to electricity, and often require coolants that deplete ozone or have a strong greenhouse effect. 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. PDRC is most effective if a surface has a high solar reflectance that minimizes solar heat gain and a high thermal emittance that maximizes radiative heat loss to the sky.
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. Up to now, white paints, which are inexpensive and easy to apply, have been the benchmark for PDRC. White paints, however, usually have pigments that absorb UV light, and do not reflect longer solar wavelengths very well, so their performance is only modest at best.
A new high-performance exterior PDRC polymer coating with nano-to-microscale air voids was developed that acts as a spontaneous air cooler and can be fabricated, dyed, and applied like paint on rooftops, buildings, water tanks, vehicles, even spacecraft — anything that can be painted. A solution-based phase-inversion technique gives the polymer a porous, foam-like structure. 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.
Phase-inversion is a simple, solution-based method for making light-scattering air voids in polymers. Polymers and solvents are already used in paints; the new method essentially replaces the pigments in white paint with air voids that reflect all wavelengths of sunlight, from UV to infrared.
The polymer coating's high solar reflectance and high thermal emittance keep it significantly cooler than its environment under widely different skies, e.g. by 6 °C in the warm, arid desert in Arizona and 3 °C in the foggy, tropical environment of Bangladesh.
Colored polymer coatings with cooling capabilities were made by adding dyes. Environmental and operational issues such as recyclability, bio-compatibility, and high-temperature operability were taken into consideration. The technique can be generalized to a range of polymers to achieve these functionalities.