Periodic Reporting for period 4 - VISIRday (VISible to far-IR optical tuning: passive DAYtime cooling by hierarchical structures and hybrid materials)
Periodo di rendicontazione: 2021-09-01 al 2022-07-31
Cooling is a crucial technology of our modern life. A device capable of efficient sub-ambient daytime cooling without the need for an external power input would considerably lower our overall energy consumption. Nature offers a chance to off-load heat directly into the cold outer space via the so-called "sky window": a wavelength range from 8 – 13 µm, where our atmosphere is transparent. Inspired by examples from nature, namely the white beetle and the Saharan silver ant, this project aims to develop materials that selectively emit thermal energy into this sky window wavelength range, and scatter and reflect radiation of all other wavelengths, particularly visible and near-infrared (300 nm – 2.5 µm) irradiated by the sun. Based on this fundamental work, VISIRday strives to provide concepts for functional paints and fibers - advanced materials that enable passive daytime cooling as a new green energy technology.
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.
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.
Major results beyond the state-of-the-art have been achieved in four topics:
1) Contributions addressing gradient materials and structures. This comprises our theoretical work to outline an angle-dependent filter to reflect or transmit light in the solar range,( Nanoscale Advances, 2020, 2, 249-255) our work to make colloidal crystals with a gradient along the x-y-direction,( Adv. Mater., 2021, 33, e2101948) and our work on realizing gradients along the z-direction.(under review)
2) The development of an indoor cooling setup.(Cell Rep Phys Sci, 2022, 3, 100986) Field tests with all the drawbacks regarding seasons and reproducibility are still being accepted to characterize the cooling performance. The outline of our indoor passive cooling setup is needed to make the various materials and devices comparable to one another.
3) Chitosan and chitin are bio-derived polymers that offer great potential owing to their low solar absorption and high mid-infrared emission. Our work to transform chitosan to chitin renders this polymer much more durable for outdoor applications.(manuscript ready)
4) Corner reflectors can serve as broadband light reflectors without the need for a metallic layer. Key is the right combination of the angular slope and the refractive index to allow for total internal reflection. Nevertheless, owing to the propagation of the evanescent field, a complete back-reflection is not possible by such structures.( ACS Omega, 2022, 7, 23353)
1) Contributions addressing gradient materials and structures. This comprises our theoretical work to outline an angle-dependent filter to reflect or transmit light in the solar range,( Nanoscale Advances, 2020, 2, 249-255) our work to make colloidal crystals with a gradient along the x-y-direction,( Adv. Mater., 2021, 33, e2101948) and our work on realizing gradients along the z-direction.(under review)
2) The development of an indoor cooling setup.(Cell Rep Phys Sci, 2022, 3, 100986) Field tests with all the drawbacks regarding seasons and reproducibility are still being accepted to characterize the cooling performance. The outline of our indoor passive cooling setup is needed to make the various materials and devices comparable to one another.
3) Chitosan and chitin are bio-derived polymers that offer great potential owing to their low solar absorption and high mid-infrared emission. Our work to transform chitosan to chitin renders this polymer much more durable for outdoor applications.(manuscript ready)
4) Corner reflectors can serve as broadband light reflectors without the need for a metallic layer. Key is the right combination of the angular slope and the refractive index to allow for total internal reflection. Nevertheless, owing to the propagation of the evanescent field, a complete back-reflection is not possible by such structures.( ACS Omega, 2022, 7, 23353)