This white paint keeps surfaces cooler than surroundings, even under
direct sunlight

Date:
October 21, 2020
Source:
Cell Press
Summary:
Scientists have developed a white paint that cools below the
temperature of its ambient surroundings even under direct
sunlight. Their research demonstrates a radiative cooling
technology that could be used in commercial paints, that could be
less expensive to manufacture, and that passively reflects 95.5%
of sunlight that reaches its surface back into outer space.



FULL STORY ========================================================================== Scientists have developed a white paint that cools below the temperature
of its ambient surroundings even under direct sunlight. Their research, published October 21 in the journal Cell Reports Physical Science,
demonstrates a radiative cooling technology that could be used in
commercial paints, that could be less expensive to manufacture, and
that passively reflects 95.5% of sunlight that reaches its surface
back into outer space. In contrast, commercial "heat rejecting paints" currently on the market only reflect 80%-90% of solar irradiation and
cannot achieve below-ambient temperatures.


========================================================================== During the summer months and in regions with warm climates, most buildings
rely on conventional air conditioning systems to transfer heat from the
inside environment to the outdoors. These systems require energy, emit
excess heat that transforms cities into "heat islands," and contribute
to the climate crisis. But while scientists have sought to develop
radiative cooling paints since the 1970s, previously developed paints
have not been capable of reflecting enough sunlight to function as viable, commercializable alternatives to traditional air conditioners.

"It is a persistent task to develop a below-ambient radiative cooling
solution that offers a convenient single-layer particle-matrix paint
form and high reliability," says Xiulin Ruan, a professor at the School
of Mechanical Engineering at Purdue University in Indiana and an author
of the study. "This is critical to the wide application of radiative
cooling and to alleviate the global warming effect." To develop a
commercially applicable radiative cooling paint, Ruan and colleagues
used calcium carbonate fillers, an earth-abundant compound, instead
of standard titanium dioxide particles, since the fillers have large
band gaps (energy differences between the valence electron band and the
bottom of the conduction electron band) that help minimize the amount
of ultraviolet light the paint absorbs. The researchers also leverage a
high particle concentration of 60%, which boosts sunlight scattering, as
well as a broad particle size distribution instead of a single particle
size for efficient broadband scattering.

To demonstrate how well these modifications enhanced the paint's
radiative cooling abilities, the researchers performed cooling tests in
West Lafayette, Indiana over a two-day period. The paint sample remained
10 degrees C below ambient temperature at night and at least 1.7 degrees
C below the temperature of the ambient surroundings when the Sun was at
its zenith. (The cooling power was shown to exceed 37W/m2 under direct
sun.) Ruan and his team then performed a second test in which part of a
pattern was painted with the novel paint while another part was painted
using a commercial white paint of the same thickness.

An infrared camera revealed that the calcium carbonate-based acrylic
paint was able to maintain a lower temperature under direct sunlight
than its commercial counterpart.

Ruan expects that the technology may benefit a wide range of industries, including residential and commercial buildings, data centers,
warehouses, food storage, automobile, outdoor electrical equipment,
military infrastructures, and utility vehicles. The paint may be applied directly to buildings to reducing cooling costs. Since the paint lacks
metallic components, telecommunication companies may use it to prevent
outdoor equipment from overheating, an important step toward enabling
a 5G network.

"This paint may even be used to combat climate change since it rejects
sunlight and radiates heat into space," says Ruan.

Next, the researchers plan to perform long-term reliability studies
to test the paint's resistance to ultraviolet light exposure, dust,
surface adhesion, water, and detergent in order to ensure its function
as a commercial product.

"Our paint is compatible with the manufacturing process of commercial
paint, and the cost may be comparable or even lower," says Ruan. "The
key is to ensure the reliability of the paint so that it is viable in
long-term outdoor applications." This work 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.


========================================================================== Story Source: Materials provided by Cell_Press. Note: Content may be
edited for style and length.


========================================================================== Journal Reference:
1. Li et al. Full Daytime Sub-ambient Radiative Cooling in
Commercial-like
Paints with High Figure of Merit. Cell Reports Physical Science,
2020 DOI: 10.1016/j.xcrp.2020.100221 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/10/201021112358.htm

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