UC Irvine

Radiative coolers have been in existence to facilitate additional surface cooling. Many conventional radiative coolers that are currently in existence require either complicated cooler structures with complex fabrication processes or expensive reflector materials to further enhance the cooling mechanism. To investigate a more practical radiative cooling solution from material level, this work presents a thermal radiative cooling device in the form of coating based on hollow microspheres. This study focuses on the effects of an inorganic binder material that is combined with hollow microspheres. The hollow microspheres used in this study are silicon dioxide (SiO2) and yttrium oxide (Y2O3). The binder of interest is potassium bromide (KBr), which helps the microspheres bind together to form a coating. This binder is compared against another type of binder: Polydimethylsiloxane (PDMS) – an organic binder. To validate the effects of the inorganic binder material, indoor and outdoor experimental measurements are conducted. Indoor measurements present the optical properties in the UV-Vis-NIR regions, also known as the solar region, and a segment of mid-infrared (MIR) region, also known as the atmospheric window, in the electromagnetic wave spectrum. To characterize the optical properties, UV-Vis-NIR and FTIR spectrometers are used. UV-Vis-NIR spectrometer is used to measure the reflectivity in its spectral region, and FTIR spectrometer is used to characterize the emissivity in the mid-infrared region. For outdoor experiments, an IR thermography technique and thermocouples are involved to measure the effects of the SiO2 hollow microsphere-based coating on the substrate throughout a day-and-night cycle. This research also incorporates the principle of diffusion theory that studies the optical properties of the coating at varying thicknesses. The measurement outcome indicates the solar reflectivity to be over 0.95 and MIR emissivity be 0.86. This cooler design can achieve approximately 10 °C sub-ambient cooling during midday time due to the high solar reflectivity and mid-infrared emissivity. This study, at its core, will provide an insight into the relationship between thermal and optical properties of the coating at the interface, broadening the possibilities of developing more effective means of radiative cooling technique.