With temperatures rising and heat waves disrupting lives around the world, cooling solutions are becoming ever more essential. This is a critical issue especially in developing countries, where summer heat can be extreme and is projected to intensify. But 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. 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. 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. They used a solution-based phase-inversion technique that 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.
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