New paint formulation could help buildings rely less on air conditioning
From Perdue News Services
April 15, 2021 -- WEST LAFAYETTE, Ind. —
In an effort to curb global warming, Purdue University engineers have created
the whitest paint yet. Coating buildings with this paint may one day cool them
off enough to reduce the need for air conditioning, the researchers say.
In October, the team created an ultra-white paint that pushed limits on how
white paint can be. Now they’ve outdone that. The newer paint not only is
whiter but also can keep surfaces cooler than the formulation that the
researchers had previously demonstrated.
“If you were to use this paint to cover
a roof area of about 1,000 square feet, we estimate that you could get a cooling
power of 10 kilowatts. That’s more powerful than the central air conditioners
used by most houses,” said Xiulin Ruan, a Purdue professor of mechanical
engineering.
The researchers believe that this white
may be the closest equivalent of the blackest black, “Vantablack,” which absorbs up to 99.9% of visible
light. The new whitest paint formulation reflects up to 98.1% of sunlight –
compared with the 95.5% of sunlight reflected by the researchers’ previous
ultra-white paint – and sends infrared heat away from a surface at the same
time.
Typical commercial white paint gets
warmer rather than cooler. Paints on the market that are designed to reject
heat reflect only 80%-90% of sunlight and can’t make surfaces cooler than their
surroundings.
The team’s research paper showing how
the paint works publishes Thursday (April 15) as the cover of the journal ACS Applied
Materials & Interfaces.
What makes the whitest paint so white
Two features give the paint its extreme
whiteness. One is the paint’s very high concentration of a chemical compound
called barium sulfate, which is also used to make photo paper and cosmetics
white.
“We looked at various commercial
products, basically anything that’s white,” said Xiangyu Li, a postdoctoral
researcher at the Massachusetts Institute of Technology, who worked on this
project as a Purdue Ph.D. student in Ruan’s lab. “We found that using barium
sulfate, you can theoretically make things really, really reflective, which
means that they’re really, really white.”
The second feature is that the barium
sulfate particles are all different sizes in the paint. How much each particle
scatters light depends on its size, so a wider range of particle sizes allows
the paint to scatter more of the light spectrum from the sun.
“A high concentration of particles that
are also different sizes gives the paint the broadest spectral scattering,
which contributes to the highest reflectance,” said Joseph Peoples, a Purdue
Ph.D. student in mechanical engineering.
There is a little bit of room to make
the paint whiter, but not much without compromising the paint.
“Although a higher particle
concentration is better for making something white, you can’t increase the
concentration too much. The higher the concentration, the easier it is for the
paint to break or peel off,” Li said.
How the whitest paint is also the
coolest
The paint’s whiteness also means that
the paint is the coolest on record. Using high-accuracy temperature reading
equipment called thermocouples, the researchers demonstrated outdoors that the
paint can keep surfaces 19 degrees Fahrenheit cooler than their ambient
surroundings at night. It can also cool surfaces 8 degrees Fahrenheit below
their surroundings under strong sunlight during noon hours.
The paint's solar reflectance is so
effective, it even worked in the middle of winter. During an outdoor test
with an ambient temperature of 43 degrees Fahrenheit, the paint still managed
to lower the sample temperature by 18 degrees Fahrenheit.
This white paint is the result of six years
of research building on attempts going back to the 1970s to develop radiative
cooling paint as a feasible alternative to traditional air conditioners.
Ruan’s lab had considered over 100
different materials, narrowed them down to 10 and tested about 50 different
formulations for each material. Their previous ultra-white paint was a
formulation made of calcium carbonate, an earth-abundant compound commonly
found in rocks and seashells.
The researchers showed in their study
that like commercial paint, their barium sulfate-based paint can potentially
handle outdoor conditions. The technique that the researchers used to create
the paint also is compatible with the commercial paint fabrication process.
Patent applications for this paint formulation
have been filed through the Purdue
Research Foundation Office of Technology Commercialization. This
research was supported by the Cooling Technologies Research Center at Purdue
University and the Air Force Office of Scientific Research through the Defense
University Research Instrumentation Program (Grant No.427 FA9550-17-1-0368).
The research was performed at Purdue’s FLEX Lab and Ray W. Herrick
Laboratories and the Birck
Nanotechnology Center of Purdue’s Discovery Park.
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