Ultrawhite Paint for Passive Cooling

The use of air-conditioning to cool buildings uses large amounts of energy, and thus, contributes to climate change. Passive cooling technologies could help to significantly reduce the environmental impact of cooling indoor spaces. Radiative cooling, for example, can reduce temperatures by reflecting solar light and emitting thermal radiation in the so-called atmospheric transparency window. This means the deep sky is used as a heatsink by emitting wavelengths that are easy to transmit through the atmosphere. This effect can allow passively cooled surfaces to reach below-ambient temperatures, even in direct sunlight. However, creating high-performance radiative cooling paints that combine solar reflectance and thermal emission is challenging.

Xiulin Ruan, Purdue University, West Lafayette, IN, USA, and colleagues have developed BaSO₄ nanoparticle films and ultrawhite BaSO₄-containing acrylic paints that can be used for daytime sub-ambient cooling. The team prepared a BaSO₄ particle film with a thickness of 150 μm on a silicon wafer using commercially available purchased BaSO₄ particles with sizes of about 400 nm. The same nanoparticles were used to create a BaSO₄–acrylic nanocomposite paint by mixing and ultrasonicating dimethylformamide (DMF) and the nanoparticles and then adding acrylic resin.

Using the BaSO₄nanoparticle films, the researchers achieved a high solar reflectance of 97.6 % and maintained surface temperatures more than 4.5 °C below ambient temperatures during the day in a field test. A comparable white commercial paint, in contrast, led to an increase of 6.8 °C above the ambient temperature in the early afternoon. According to the team, the interfaces between the BaSO₄ nanoparticles and the air increase scattering, and thus, the overall solar reflectance. Acrylic paint with a 60 % volume concentration of BaSO₄ achieved a solar reflectance of 98.1 % and a similarly high cooling performance as the nanoparticle films.