Tecnologías de enfriamiento radiante diurno: el reto de la caracterización de los materiales y de la evaluación de su rendimiento de enfriamiento

Passive radiative cooling is the ability of materials to reduce their temperature by radiating thermal energy in the infrared range of the electromagnetic spectrum where the atmosphere is transparent. This phenomenon has been known since ancient times and, in the current situation of vulnerability of our planet, it represents a promising mechanism to reduce the contribution of buildings and infrastructures to global warming. The latest advances in nanotechnology have made possible to obtain materials that reach surface temperatures below ambient, even during the day and under solar irradiation. These innovative daytime radiative cooling (DRC) materials are characterized by having a high infrared emissivity, which favors the radiation of thermal energy and, at the same time, a high solar reflectance that prevents overheating. This paper analyzes the different types of DRC materials developed in recent years and some of the metrological challenges to be addressed for their implementation in real applications. In relation to the reliable characterization of materials, it is necessary to obtain the total hemispherical emissivity of the material, at all wavelengths and in all directions, from the near-normal emissivity measured by the usual equipment. In addition, reliable field measurements must be obtained by calibrating commercially available portable equipment using model DRC systems with well-defined properties. The main challenges related to the evaluation of the cooling performance of DRC systems are the need to define adequate figures of merit and the need to establish homogeneous test conditions, in order to be able to compare the behavior of different materials and of each type of material in different locations. Finally, some projects in the field of DRC technologies that have the participation of Spanish research groups are pointed out.