As a result of the past and expected future development of global warming, a simultaneous population growth as well as industrial growth, energy-free cooling solutions will play an increasingly important role. Technologies such as radiative cooling offer a sustainable and energy-free solution by using the wavelength regions of the atmosphere that are transparent to electromagnetic radiation, the so-called atmospheric window (8 - 13 μm), to emit thermal radiation into colder space.
Radiative cooling is a ubiquitous process in which a surface facing the sky loses heat through thermal radiation. The largest object that gives off heat by radiative cooling is the Earth itself. The cooling effect can be seen, among other things, on clear mornings by the formation of frost and dew. By using the principle of operation of radiative cooling, cooling can be achieved without external energy input and CO2 can be saved at the same time.
By radiating heat through the atmosphere to the universe at extremely low temperatures (about -270 °C) rather than to the immediate environment, terrestrial surfaces produce cooling below ambient temperature. If a material in the mid-infrared range (preferably between 8-13 μm) emits more radiation than it absorbs through the sun and the atmosphere, cooling occurs even during the day under full solar radiation.
The cooling performance here depends strongly on the physical properties of the material and the surrounding weather conditions. Spectral analysis can be used to determine the absorption pattern of materials over a wavelength range of 0.25 - about 25 μm, and cooling performance can be simulated based on this data. The atmosphere is predominantly composed of nitrogen (N2), oxygen (O2) and noble gases. In addition, there are aerosols, i.e. solid and liquid suspended particles in a gaseous envelope, as well as trace gases such as water vapor, carbon dioxide (CO2), methane (CH4), ozone (O3), which influence the transparency of the atmosphere and thus the possibility of heat dissipation to cold space. The calculation model can be used to vary settings such as solar irradiance, cloud density or humidity, so that clear statements can be made about the theoretical cooling potential of materials under different external weather influences.
