  Radiative cooling is a passive cooling technology that offers great promises
to reduce space cooling cost, combat the urban island effect and alleviate the
global warming. To achieve passive daytime radiative cooling, current
state-of-the-art solutions often utilize complicated multilayer structures or a
reflective metal layer, limiting their applications in many fields. Attempts
have been made to achieve passive daytime radiative cooling with single-layer
paints, but they often require a thick coating or show partial daytime cooling.
In this work, we experimentally demonstrate remarkable full daytime sub-ambient
cooling performance with both BaSO4 nanoparticle films and BaSO4 nanocomposite
paints. BaSO4 has a high electron bandgap for low solar absorptance and phonon
resonance at 9 um for high sky window emissivity. With an appropriate particle
size and a broad particle size distribution, BaSO4 nanoparticle film reaches an
ultra-high solar reflectance of 97.6% and high sky window emissivity of 0.96.
During field tests, BaSO4 film stays more than 4.5C below ambient temperature
or achieves average cooling power of 117 W/m2. BaSO4-acrylic paint is developed
with 60% volume concentration to enhance the reliability in outdoor
applications, achieving solar reflectance of 98.1% and sky window emissivity of
0.95. Field tests indicate similar cooling performance to the BaSO4 films.
Overall, our BaSO4-acrylic paint shows standard figure of merit of 0.77 which
is among the highest of radiative cooling solutions, while providing great
reliability, the convenient paint form, ease of use and the compatibility with
commercial paint fabrication process.
