Passive daytime cooling has tremendously developed during the past 5.5 years. Many groups are now active in this important field of sustainable energy research, and various materials and concepts have been introduced to promote passive cooling technologies. We worked on several material-based aspects of passive cooling structures, which comprised the synthesis of defined nanoparticles of SiC, SiO2, or TiO2. Often these particles alone do not suffice to yield passive cooling performances but are added as scatterers in polymer matrices. We developed robust techniques to measure the absorption, transmission, scattering, and reflection of various thin films across the entire spectral range of interest (300 nm - 20 µm) to fully understand our material's optical performances. This rigorous characterization is indispensable for the designed development of optimized structures. In addition to the thin film composition, which has been optimized by many groups, the thin film thickness is another important parameter to control. We demonstrated that an optimum layer thickness exists, which balances solar radiance absorption and mid-infrared emission. Furthermore, it is crucially important to address the solar radiance range and thermal emission range separately. In many cases, metallic back-reflectors have been used to prevent solar absorption, and these have proven to perform efficiently. However, from the viewpoint of recyclability, multilayered, metallic structures are less desirable. Consequently, we also explored pathways to porous materials based on freeze casting, solvent exchange, or fiber processing. In addition to diffuse scattering, exploiting structural colors based on photonic crystals or glasses is also a promising route to prevent light from propagating through a material. Even micro-structured objects, such as dielectric corner reflectors, are suitable to reflect light in a broadband fashion under certain boundary conditions. Mirroring the efforts in preventing light absorption, we also worked on improving the thermal emission at room temperature. For this, plasmonic resonators and metal-insulator-metal structures are suitable for a better understanding of the light-matter interaction. We addressed such structure-property relationships not only from an experimental point of view but also employed theory and simulations for a complete understanding.
For passive cooling applications, one needs to decide between selective and broadband emitters, which lead to different optimum performances with respect to temperature reduction and power emission, respectively. A range of polymers exists that is well-suited as passive cooling materials, either by the polymer itself, e.g. PDMS or chitin, or via mixing it with emissive additives such as PE or PEG. Especially chitosan and its acetylated counterpart, chitin was of great interest to us owing to its biocompatibility. We developed a route to increasing its resistance to water and measured its passive cooling capabilities. When quantifying the cooling performance of distinct materials, the field is still lacking a rigorous and quantitative approach to assess this admittedly most relevant parameter. Field tests with all their deficiencies regarding the variability of environmental conditions such as temperature, humidity, solar radiance, a cloudy overcast, wind speed, etc., are still standard for performance characterization. We outlined a novel measurement setup, which allows for reproducible performance characterization. It is based on an absorptive aluminum dome at 77 K, a solar simulator, and a temperature-controlled sample stage. These parts are not capable of reproducing all properties of a field test experiment quantitatively, particularly the atmosphere, but it is the first step towards a standardized test method to allow for a comparison between materials from different groups. Such a test method will be strongly needed for future improvements in passive cooling materials and processes.
We mainly resorted to peer-reviewed journal publications to disseminate our results. Wherever appropriate, we accompanied the publication with a press release by the University of Bayreuth. Owing to the Corona pandemic traveling and conference presentations have been greatly hampered. Nevertheless, all co-workers participated in at least one (on-site or online) conference to present our results to a wider audience.
Our results are of fundamental nature and have not been exploited for commercial use up to now.
