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Research Looks at Coating for Heat, Cooling

Wednesday, October 23, 2019

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Researchers at Columbia Engineering, out of Columbia University in the City of New York, have developed porous polymer coatings that they say enable “inexpensive and scalable ways to control light and heat in buildings.”

According to the university, the researchers took advantage of the optical “switchability” of PPCs in solar wavelengths to regulate solar heating and daylighting, and then extended the concept to thermal infrared wavelengths to modulate heat radiated by objects.

“Our work shows that by wetting PPCs with common liquids like alcohols or water, we can reversibly switch their optical transmittance in the solar and thermal wavelengths,” said Jyotirmoy Mandal, lead author of the study and a former PhD student in the lab of Yuan Yang, assistant professor of materials science and engineering.

Columbia Engineering

Researchers at Columbia Engineering, out of Columbia University in the City of New York, have developed porous polymer coatings that they say enable “inexpensive and scalable ways to control light and heat in buildings.”

“By putting such PPCs in hollow plastic or glass panels, we can make building envelopes that can regulate indoor temperatures and light.”

The Work

The team says its design is similar to smart windows, but unlike previous fluoropolymer coatings, which are static, these designs can switch between heating and cooling modes, making them more useful for places like New York, which sees a wide range of temperatures.

But, in fact, the work actually began by accident, when Mandal noticed a few drops of alcohol spilled on a white fluoropolymer PPC and turned it transparent.

“What we saw was the same mechanism that causes paper to turn translucent when wetted, but at a near-optimal level,” said Mandal. “The physics of this has been previously explored, but the drastic switching we saw led us to explore this particular case, and how it can be used.”

A porous material like paper appears white because the air in the pores has a different refractive index (~1) to that of the porous material (~1.5), causing them to scatter and reflect light. When wetted by water, which has a refractive index (~1.33) closer to the material, scattering is reduced and more light goes through, making it translucent.

Transmission increases when the refractive indices are closely matched. The researchers discovered that their fluoropolymer (~1.4) and typical alcohols (~1.38) have very close refractive indexes.

Because of the near-perfect refractive-index matching of alcohols and the fluoropolymer, the team could change the solar transmittance of their PPCs by ~74%; for the visible part of sunlight, the change was ~80%.

Although the switching is slower than in typical smart windows, the transmittance changes are considerably higher, making PPCs attractive for controlling daylight in buildings, according to the research.

The idea in practice was to have white roofs during the summer and black roofs during the winter, for instance.

The researchers put panels containing PPCs on toy houses with black roofs. One panel was dry and reflective, while the other was wet and translucent, showing the black roof underneath. Under sunlight on a summer noon, the white roof became cooler than the ambient air by 5 degrees, while the black one became hotter, by 38 degrees.

“Given the scalability and performance of the PPC-based designs, we are hopeful that their applications will be wide-spread,” said Yang, “in particular, we are excited by their potential applications on building facades.”

   

Tagged categories: Coatings Technology; Cool roof coatings; NA; North America; Research; Research and development

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