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John O | October 2018

Engineers use polymer coating on building exteriors to enhance passive daytime radiative cooling

By Josh Perry, Editor


Engineers from Columbia University (N.Y.) have developed a high-performance exterior passive daytime radiative cooling (PDRC) polymer coating with nanoscale air voids that can be applied like paint on rooftops, building exteriors, vehicles and more.


When exposed to the sky, the porous polymer PDRC coating reflects sunlight and emits heat to attain significantly cooler temperatures than typical building materials or even the ambient air. (Columbia University)


According to a report from the school, the researchers used a solution-based, phase-inversion technique to make the polymer porous. The air voids scatter and reflect sunlight (due to the difference in the refractive index between the voids and the polymer), turning the polymer white and avoiding solar heating.


This is an important enhancement on white paint, which typically includes pigments that absorb UV light and can’t reflect longer solar wavelengths. The coating is also a passive solution, requiring no energy consumption of its own and could reduce the costs associated with air conditioning.


The researchers built on earlier work that demonstrated materials such as acrylic, silicone, and PET are excellent thermal radiators. The challenges were making the transparent polymers reflect sunlight and determining how to deploy them.


“They decided to use phase-inversion because it is a simple, solution-based method for making light-scattering air-voids in polymers,” the article explained. “Polymers and solvents are already used in paints, and the Columbia Engineering method essentially replaces the pigments in white paint with air voids that reflect all wavelengths of sunlight, from UV to infrared.”


The coating was tested in a variety of environments with it producing 6°C better cooling in the Arizona desert and 3°C better cooling in the jungle of Bangladesh. Colored coatings with enhanced cooling capabilities were also developed.


The research was recently published in Science. The abstract stated:


“Passive daytime radiative cooling (PDRC) involves spontaneously cooling a surface by reflecting sunlight and radiating heat to the cold outer space. Current PDRC designs are promising alternatives to electrical cooling, but are either inefficient or have limited applicability.


“We present a simple, inexpensive and scalable phase-inversion-based method for fabricating hierarchically porous poly(vinylidene fluoride-co-hexafluoropropene) (P(VdF-HFP)HP) coatings with excellent PDRC capability.


“High, substrate-independent hemispherical solar reflectances (0.96 ± 0.03) and long-wave infrared (LWIR) emittances (0.97 ± 0.02) allow for sub-ambient temperature drops of ~6°C and cooling powers of ~96 W m−2 under solar intensities of 890 and 750 W m−2 respectively.


“The performance equals or surpasses those of state-of-the-art PDRC designs, while the technique offers a paint-like simplicity.”


Learn more about this coating in the video below:

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