Double-layer nanoparticle-based coatings for efficient terrestrial radiative cooling

One passive cooling approach is pumping energy to outer space through thermal radiation. Such a radiative cooling mechanism widely exists in nature and is important to maintain the temperature of the earth. However, natural materials generally have poor radiative cooling efficiency. To better utilize the radiative cooling for thermal management applications, the surface should be designed to have a high reflectivity in the solar spectrum and high emissivity in the "sky window" region (8-13 mu m in wavelength). In this work, we propose and demonstrate a highly scalable nanoparticle-based double-layer coating to achieve such selective radiative properties. Double-layer coatings consisting of a top reflective layer with high solar albedo and a bottom emissive layer are achieved by properly designed TiO2, SiO2, and SiC nanoparticles. These coatings were fabricated on both low- and high-emissivity substrates and their spectral radiative properties were characterized. The coating composed of TiO2 and SiO2 on a reflective substrate has excellent selective emission property for radiative cooling purpose. Under dry air conditions and assuming non-radiative heat transfer coefficient h(c)=4 W/m(2) K, TiO2+SiO2 and TiO2+SiC can theoretically achieve about 17 degrees C below ambient at night and 5 degrees C below ambient under direct solar radiation (AM1.5). On-site measurements have also been conducted. Under direct solar irradiation, significant temperature reduction was observed for both aluminum and black substrate after the coating was applied. At nighttime, radiative cooling effect can cool the surface to a few degrees below ambient temperature. Although the theoretical cooling under dry weather condition is not observed, the experiment results can be well explained by theoretical calculations with the consideration of high humidity and non-radiative heat transfer. This nanoparticle-based approach can be easily applied to large area, which is a significant step of achieving large scale application of the radiative cooling technology.