Selection of nano-particulate material for improved passive cooling skylight performance

The passive radiative cooling (PRC) system cools objects without requiring external energy. Scientific interest has been growing in the use of thermal radiation via the atmospheric window (8-13μm) where the sky is transparent. This is most effective for nocturnal applications, but it is possible to obtain temperatures lower than the ambient under direct sunlight, thus allowing diurnal applications, using advancements in nanotechnology/metamaterials. With the aim to increase the cooling power, our research analyses the optimal properties and use of nanoparticles (NPs) based high solar irradiation-reflecting layer on the top surface of a skylight window as designed and tested at Åbo Akademi University (ÅAU). In this paper, the main purpose is to identify low-cost NPs (or mixes of these) for better performance during the daytime of PRC skylights. This
involves distinct NPs materials produced through different procedures. It is expected that NP-based approaches can easily handle large surfaces, which is important for widespread use in PRC for building thermal control (or in vehicles as well). The strong reflection of solar radiation is certainly a prerequisite; the
work of others has shown that NPs can actually tune absorption and emission in the atmospheric window by phonon resonance properties. The analysis presented here suggests that TiO2 and SiO2 NPs are most suitable for use with the ÅAU skylight. Furthermore, using a thermal imager, heat flux measurements were carried out experimentally on window glasses with and without NPs deposited on them. This established a relationship between absorptivity/emissivity) for longwave radiation at two different temperatures.