Paving the way to alternatives to high-energy modes of cooling, like air conditioners, researchers now present a polymer that can cool down surfaces by reflecting sunlight and heat back into the sky. The white polymer reflects more than 96% of sunlight, they say, and thus stays cooler than the ambient air. Its use on myriad surfaces could greatly reduce high-energy costs associated with keeping people and places cool in hot climates, including in less developed nations where providing steady electricity is challenging. What's more, the PDRC polymer can be dyed and applied like a paint, making it a promising option for the paint industry. Cooling structures is a costly necessity in many parts of the world. Current methods such as air conditioners use large amounts of energy and require the use of coolants that can have a harmful effect on the atmosphere. An alternative approach is passive daytime radiative cooling, or PDRC, whereby a surface is cooled by reflecting sunlight and radiating heat back into space as long-wave infrared energy. While research has yielded other PDRC designs, their widespread use is limited due to limited applicability, susceptibility to degradation and being too costly or difficult to manufacture at larger scales. Jyotirmoy Mandal and colleagues present a new method for fabricating a porous polymer coating, which can be applied like a paint to irregular surfaces like large roofs. It has radiative cooling properties that equal or surpass current state-of-the-art PDRC designs, the authors say. Mandal et al. used a phase-inversion-based method to create a poly(vinylidene fluoride-co-hexafluoropropylene) (P(VdF-HFP)HP) polymer filled with finely tuned air-filled voids, which greatly improves the PDRC properties. In tests beneath the hot sun of Phoenix, Arizona, the authors demonstrate the polymer allows for sub-ambient temperature reductions of approximately 6° Celsius and a cooling power of nearly 96 Watts per meter squared.