Polymer coating cools buildings

With rising temperatures and heat waves disrupting lives around the world, cooling solutions are becoming more and more essential. This is a critical problem, especially in developing countries, where the summer heat can be extreme and should intensify. But common cooling methods, such as air conditioners, are expensive, consume large amounts of energy, require rapid access to electricity, and often require cooling liquids that deplete ozone or have a strong greenhouse effect. .

An alternative to these energy-intensive cooling methods is daytime radiative cooling (CRBD), a phenomenon where a surface spontaneously cools by reflecting sunlight and diffusing heat into the colder atmosphere. The PDRC is more efficient if a surface has a high solar reflectance (R) that minimizes solar heat gain and a high thermal emittance (ε) that maximizes radiative heat loss to the sky. If R and ε are high enough, a clear heat loss can occur, even under the sun.

Developing practical PDRC designs has been difficult: many recent design proposals are complex or expensive and can not be widely applied or applied to roofs and buildings of different shapes and textures. Until now, white paints, inexpensive and easy to apply, have been the benchmark for PDRC. White paints, however, usually have pigments that absorb UV rays and do not reflect very well the longer solar wavelengths, so their performance is only modest at best.

Columbia Engineering researchers have invented a high-performance exterior PDRC polymer coating with nano to microscopic air gaps that acts as a spontaneous air cooler and can be fabricated, dyed and applied like paint on roofs, buildings, water tanks, vehicles. even a spaceship – 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 diffuse and reflect sunlight, due to the difference in refractive index between the air voids and the surrounding polymer. The polymer becomes white and thus avoids solar heating, while its intrinsic emittance effectively makes it lose heat in the sky. The study is published online today. Science.

The team – Yuan Yang, Assistant Professor in Materials Science and Engineering; Nanfang Yu, associate professor of applied physics; and Jyotirmoy Mandal, lead author of the study and PhD student in Yang's group (all of the Departments of Applied Physics and Applied Mathematics) – based on previous work that demonstrated that simple plastics and polymers, including the Acrylic, silicone and PET radiators and could be used for PDRC. The challenges were how to ensure that these normally transparent polymers reflect sunlight without using silver mirrors as reflectors and how to make them easily deployable.

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

"This simple but fundamental modification provides exceptional R & E, equivalent to or better than advanced PDRC models, but with a convenience that almost looks like a painting," says Mandal.

The researchers found that the high solar reflectance (R> 96%) and the high thermal emittance (ε ~ ​​97%) of their polymer coating kept it significantly cooler than its environment in very different skies, eg. by 6 C in the arid and hot desert of Arizona and 3 C in the foggy tropical environment of Bangladesh. "The fact that cooling is achieved in desert and tropical climates, without thermal protection or shielding, demonstrates the utility of our design wherever cooling is needed," notes Yang.

The team has also created colored polymer coatings with cooling capabilities by adding dyes. "Achieving a superior balance between color and cooling performance compared to current paints is one of the most important aspects of our work," Yu notes. "For exterior siding, the choice of color is often subjective and paint manufacturers have been trying for decades to make colored coatings, such as those used for roofs. "

The group took into account environmental and operational issues, such as recyclability, biocompatibility and high temperature operation, and showed that their technique can be generalized to a range of polymers to achieve these features. "Polymers are a surprisingly diverse class of materials, and since this technique is generic, desirable additional properties can be easily incorporated into our PDRC coatings, if suitable polymers are available," adds Mandal.

"Nature offers many ways of heating and cooling, some of which are extremely well known and widely studied and others poorly known.Hydrothermal cooling, using the sky as heat sink, belongs to the latter group and its potential has been Only a few years ago, materials scientists strangely neglected these methods, "says Uppsala University physics professor Claes-Göran Granqvist, a pioneer in the field of radiative cooling. "The publication of Mandal et al. Emphasizes the importance of radiative cooling and represents an important step forward in demonstrating that hierarchically porous polymer coatings, which can be prepared cheaply and conveniently, provide excellent cooling even in direct sunlight. "

Yang, Yu and Mandal are refining their design in terms of applicability, while exploring possibilities such as the use of completely biocompatible polymers and solvents. They are in discussion with the industry about the next steps.

"The time has come to develop promising solutions for a sustainable humanity," said Yang. "This year, we have experienced record heat waves and record temperatures in North America, Europe, Asia and Australia – the climate challenge – and we are very excited to be working on this new technology that is addressing it."

Yu added that he had the habit of thinking that white was the most inaccessible color: "When I studied watercolor years ago, white paints were the more expensive have now shown that white is actually the most feasible color.It can be manufactured using only correctly dimensioned voids incorporated into a transparent medium.

The study entitled "Hierarchically porous polymer coatings for highly efficient daytime passive cooling".

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New engineering equipment can cool roofs, structures without energy consumption

More information:
J. Mandal el al., "Hierarchically Porous Polymer Coatings for Highly Effective Daytime Passive Radiative Cooling" Science (2018). science.sciencemag.org/lookup/… 1126 / science.aat9513