As temperatures rise during hot weather, keeping buildings and other structures cool can require large amounts of energy.
asserted
keeping → rise → energy
Air conditioners are a common way to deal with the heat, but they consume electricity and need access to cooling systems and, in some cases, coolants that can affect the environment.
asserted
that → deal → environment
Researchers at Columbia Engineering have created a polymer coating that can be applied like paint and uses microscopic air voids instead of white pigments.
asserted
that → create → pigments
According to the University's engineering department, the coating can reflect sunlight while also releasing heat towards the sky, allowing the surface to cool below the surrounding air temperature.
uncertain
surface → accord → temperature
The researchers tested the coating in different environments.
asserted
researchers → test → environments
Under the warm, dry conditions of the Arizona desert, the coated surface became about 6°C cooler than the ambient air.
asserted
surface → coat → air
On the ther hand, in a foggy, tropical environment in Bangladesh, it achieved cooling of about 3°C below the surrounding air.
asserted
it → achieve → air
The work is based on a process called passive daytime radiative cooling (PDRC).
asserted
work → base → process
It allows a surface to cool without using an external energy source by reflecting sunlight and radiating heat towards the colder sky.
asserted
surface → allow → sky
The study, titled 'Hierarchically Porous Polymer Coatings for Highly Efficient Passive Daytime Radiative Cooling', was published in journal Science.
asserted
study → title → Science
How coating works
For PDRC to work effectively, a surface needs to do two things.
asserted
surface → work → things
It must reflect a large amount of sunlight so that it does not gain too much heat, and it must have high thermal emittance so that it can release heat towards the sky.
asserted
it → reflect → sky
The Columbia Engineering team designed the polymer coating to achieve both.
asserted
team → design → both
The researchers used a solution-based process known as phase-inversion to create a porous, foam-like structure inside the polymer.
asserted
researchers → use → polymer
The structure contains air voids ranging from the nano to microscale.
asserted
structure → contain → microscale
These tiny spaces help the material scatter and reflect sunlight because air and the surrounding polymer have different refractive indices.
asserted
air → help → indices
This approach also changes how the polymer looks.
asserted
polymer → change → ?
The polymers used by the researchers can normally be transparent, but the air voids make the material appear white.
asserted
material → use → researchers
The researchers therefore used the air voids in place of the pigments normally used to make white paint.
asserted
researchers → use → paint
The coating has a solar reflectance of more than 96% and a thermal emittance of about 97%.
asserted
coating → have → %
According to the researchers, these properties allowed it to remain significantly cooler than its surroundings under different sky conditions.
uncertain
it → accord → conditions
Air voids replace white pigments
The researchers built on earlier work showing that simple polymers such as acrylic, silicone and PET can be effective heat radiators and could be used for PDRC.
uncertain
polymers → replace → PDRC
A transparent material does not reflect enough sunlight for effective daytime radiative cooling.
asserted
material → reflect → cooling
Existing approaches could use reflectors such as silver mirrors, but the researchers wanted a method that would be easier to make and apply.
uncertain
that → use → method
They chose phase-inversion because it is a simple, solution-based method for producing light-scattering air voids inside polymers.
asserted
it → choose → polymers
Since polymers and solvents are already used in paints, the process could produce a coating that is close to paint in how it can be applied.
uncertain
it → use → paint
The researchers said the air voids can reflect sunlight across wavelengths ranging from ultraviolet to infrared.
asserted
voids → say → infrared
This allows the coating to avoid absorbing much of the incoming solar energy while its thermal properties help it release heat towards the sky.
asserted
it → allow → sky
“This simple but fundamental modification yields exceptional reflectance and emittance that equal or surpass those of state-of-the-art PDRC designs, but with a convenience that is almost paint-like,” lead author Jyotirmoy Mandal said.
asserted
Mandal → yield → convenience
Impact in desert and tropical conditions
The researchers tested the material under different environmental conditions to see whether its cooling ability could work beyond a single climate.
uncertain
ability → test → climate
In the warm, arid desert environment in Arizona, the coating cooled to about 6°C below the ambient temperature.
asserted
coating → cool → temperature
In Bangladesh's foggy, tropical environment, it cooled to about 3°C below ambient temperature.
asserted
it → cool → temperature
The difference was important because PDRC can be affected by atmospheric conditions.
asserted
PDRC → affect → conditions
“The fact that cooling is achieved in both desert and tropical climates, without any thermal protection or shielding, demonstrates the utility of our design wherever cooling is required,” Yang said.
asserted
Yang → achieve → design
The coating was designed as an exterior material.
asserted
coating → design → material
The researchers said it could be applied to rooftops and buildings, as well as water tanks, vehicles and spacecraft.
uncertain
it → say → rooftops
In principle, it can be used on surfaces that can be painted.
asserted
that → use → surfaces
The team also worked on color limitation that can affect cooling.
asserted
that → work → cooling
Conventional cooling surfaces often rely on white because white materials can reflect sunlight, but exterior surfaces are not always required or preferred to be white.
asserted
surfaces → cool → sunlight
Researchers made coloured versions
The Columbia team added dyes to create coloured versions of the polymer coating while retaining cooling capabilities.
asserted
team → make → capabilities
…and 25 more, not listed.
The Aporia
