Characterization of radiative cooling materials

Radiative cooling is a renewable cooling mechanism with diverse applications. Many radiative cooling materials have been developed (e.g., fabrics, paints and building materials). It is, however, difficult to compare their properties because of a lack of standardized and transparent performance-evaluation systems. Here, we present procedures for assessing the optical and thermal properties of radiative cooling materials that can be applied to various forms such as films, coatings and fabrics. In the first procedure, hemispherical reflectance and transmittance spectra are collected by using two integrating sphere spectrometers for the solar spectrum (0.3-2.5 μm) and the IR spectrum (2.5-20 μm), respectively (2 h). We then describe how to build an outdoor performance-testing platform designed to accurately measure differences in temperature corresponding to the presence of different materials over extended periods. Thermal insulation and radiation shielding are fundamental requirements for this platform. Other environmental conditions are measured (e.g., humidity, sunlight, wind and external temperature). We also describe how to construct and use a smaller, indoor testing platform. Although it is challenging to fully replicate outdoor conditions, indoor testing still provides a standardized reference. Both platforms can be seamlessly integrated with a Proportional-Integral-Derivative temperature control system, enabling the simulation of applications in intricate thermal environments. Outdoor and indoor experiments typically take 7 and 1 d, respectively. Finally, we provide a simple MATLAB-based code for rapid theoretical performance evaluation (~10 min).