Ultrawhite BaSO₄ Paints and Films for Remarkable Daytime Subambient Radiative Cooling

Abstract

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 subambient cooling performance with both BaSO4 nanoparticle films and BaSO4 nanocomposite paints. BaSO4 has a high electron band gap for low solar absorptance and phonon resonance at 9 μm for high sky window emissivity. With an appropriate particle size and a broad particle size distribution, the BaSO4 nanoparticle film reaches an ultrahigh solar reflectance of 97.6% and a high sky window emissivity of 0.96. During field tests, the BaSO4 film stays more than 4.5 °C below ambient temperature or achieves an average cooling power of 117 W/m2. The BaSO4-acrylic paint is developed with a 60% volume concentration to enhance the reliability in outdoor applications, achieving a solar reflectance of 98.1% and a sky window emissivity of 0.95. Field tests indicate similar cooling performance to the BaSO4 films. Overall, our BaSO4-acrylic paint shows a standard figure of merit of 0.77, which is among the highest of radiative cooling solutions while providing great reliability, convenient paint form, ease of use, and compatibility with the commercial paint fabrication process.

Polymer coating with gradient‐dispersed dielectric nanoparticles for enhanced daytime radiative cooling

“…Therefore, materials with a high refractive index and large band gap are preferred for use as fillers to enhance Mie scattering and thus the overall solar reflectance. Various numerical and experimental studies have explored different strategies for optimizing such scattering effect based on changing key parameters such as particle size, volume fraction and coating thickness, for which different fillers (e.g., TiO 2 , BaSO 4 , Al 2 O 3 , CaCO 3 ) have been utilized to scatter the incident sunlight in a random‐dispersed and multiple‐sized manner 6,12–14,25 . However, when mixing the nanoparticles and liquid polymer solution, sedimentation always occurs due to the gravity effect and results in the size and density gradients 26–28 .…”

“…3,4 To maximize the cooling effect, the radiative cooler should be entitled with high emissivity within the atmospheric window and high solar reflectance simultaneously. Recently, photonic structures, 3,[5][6][7] metamaterials, [8][9][10][11] particle dispersed polymeric coatings, [12][13][14][15] and porous structures [16][17][18][19][20][21] have been investigated to simultaneously achieve high solar reflectance as well as high infrared radiation. Among these methods, polymeric coatings have shown the best potential for real-world large-scale applications for its low cost, ease in production and excellent long-term durability.…”

“…Various numerical and experimental studies have explored different strategies for optimizing such scattering effect based on changing key parameters such as particle size, volume fraction and coating thickness, for which different fillers (e.g., TiO 2 , BaSO 4 , Al 2 O 3 , CaCO 3 ) have been utilized to scatter the incident sunlight in a random-dispersed and multiple-sized manner. 6,[12][13][14]25 However, when mixing the nanoparticles and liquid polymer solution, sedimentation always occurs due to the gravity effect and results in the size and density gradients. [26][27][28] In other words, a heterogeneous distribution may be formed instead of an ideal homogeneous one during the formation of the polymeric nanocomposite coating.…”

Colorful surfaces for radiative cooling

“…In recently reported literature for radiative cooling, researchers employed selected commercially available paints as the control to compare with their proposed cooling materials and structures. [65][66][67][68] In order to reveal the potential of colored radiative cooling, we first discuss the spectral characteristics of selected commercial products to reveal the radiative cooling potential.…”

Highly‐Scattering Cellulose‐Based Films for Radiative Cooling

“…[ 5 , 6 , 7 , 8 , 9 , 10 ] For example, remarkable designs include microsphere‐based photonic random media, [ 11 ] thin‐film multi‐layer structures, [ 12 , 13 , 14 , 15 , 16 ] microsphere‐periodic arrays, [ 17 , 18 ] metal–dielectric nanophotonic structures, [ 19 ] double‐layer nanoparticle‐based coatings, [ 20 , 21 ] aerogels, [ 22 , 23 ] porous synthetic polymer‐based coatings, [ 24 , 25 ] and hybrid dielectric–polymer materials. [ 26 , 27 , 28 , 29 , 30 ] However, several of these technologies come at the cost of demanding engineering and fabrication processes, making difficult their large scale production and therefore their use in real applications. Furthermore, one would ideally exploit sustainable materials to produce such coatings and avoid the use of plastics or heavy metals, to have a meaningful positive impact on the environment.…”