Optical and Thermal Properties of Cool White Coatings based on Ceramic Hollow Microspheres
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Optical and Thermal Properties of Cool White Coatings based on Ceramic Hollow Microspheres


Worldwide energy consumption associated with global warming, rapid urbanization, and heat island effect has surged in recent years, calling for building thermal management to provide energy savings and increase thermal comfort. Conventional air conditioning accounts for more than half of building energy, and alternative cooling technologies, including passive cooling, are needed. Development of advanced materials with engineered optical and thermal properties enables passive radiative cooling and significantly reduces the energy required to cool a surface. This doctoral research focuses on the use of ceramic hollow microspheres in radiative cooling devices and the study of their optical and thermal properties.While solid and hollow microsphere composites have received significant attention as solar reflectors or selective emitters, the driving mechanisms and the fundamental determinants for their optical properties and material selection criteria are relatively unknown. In this dissertation, the solar reflectivity of solid and hollow microspheres with varying diameters are studied. Based on Mie theory and finite-difference time-domain (FDTD) simulations, hollow microspheres with a thinner shell are more effective in scattering the light compared to solid microspheres, leading to a higher solar reflectivity. The effects of refractive index and extinction coefficient values are studied, and it explains how the high reflectivity of hollow microspheres is attributed to a combination of strong backscattering and limited absorption. Six ceramic materials with high refractive index and low extinction coefficient are studied using FDTD simulations and yttrium oxide (Y2O3) exhibits highest solar reflectivity among them. The FDTD simulation results indicate hollow microspheres enable low-refractive-index materials such as silicon oxide (SiO2) to achieve high solar reflectivity. SiO2 has a low extinction coefficient in the solar region, which is beneficial for high reflectivity. In this dissertation, techno-economically viable and scalable white coatings are presented by integrating hollow glass microspheres into a polydimethylsiloxane (PDMS) binder. The main chemical composition of hollow glass microspheres in this study is SiO2. The combination of high solar reflectivity from hollow glass microspheres and infrared emissivity of the binder material leads to a significant temperature reduction when coated on the surface of building materials. The white coating is shown to be superior to commercially available white paints and demonstrates considerable cooling energy savings on common commercial and residential building models. Although white coatings based on PDMS and hollow glass microspheres exhibit high solar reflectivity, the inherent absorption of PDMS as a binder material limits its reflectivity to the visible wavelengths and is not effective in the near-infrared wavelength region. To improve near-infrared reflection, hollow glass microspheres are integrated within potassium bromide (KBr), which inherently has a lower near-infrared absorption compared to PDMS. The use of KBr binder material and spray coating method makes the white coating more scalable and improves solar reflectivity while maintaining a high infrared emissivity. The coating can be further improved by utilizing a different hollow microsphere material. Numerical simulation results demonstrate that Y2O3 hollow microspheres are promising to achieve near-unity solar reflectivity with low thicknesses and are shown to outperform hollow glass microspheres. Therefore, white coatings using Y2O3 hollow microspheres are prepared. The thermal analysis shows white coatings provide a surface temperature reduction up to 8°C lower than ambient air and a cooling power of 111 W/m2, assuming a constant ambient temperature of 27°C and a solar intensity of 1000 W/m2. The ultrahigh solar reflectivity of white coatings exceeds most of the reported values of pigment/polymer composite systems without expensive metallic reflectors. Y2O3 hollow microspheres provide a promising solution for cooling applications requiring near-unity solar reflectivity. Although white coatings have been demonstrated as promising materials for radiative cooling, energy and cost savings of white coating in buildings have not been fully evaluated. In this dissertation, the energy and cost savings are analyzed for ideal white coatings, with unity solar reflectivity and infrared emissivity, and polymer coatings based on hollow glass microspheres and PDMS binder material, with a solar reflectivity of 0.92 and an infrared emissivity of 0.9, for midrise apartment models in various climate zones across the United States. Ideal white coatings and polymer coatings achieve annual cooling energy savings of 0.7-6.6 kWh/m2 and 0.6-5.8 kWh/m2, respectively, in various climate zones for midrise apartments. Both the positive and negative effects of white coatings are summarized including cooling energy savings, cooling peak load reduction, heating energy increase, carbon emission savings, utility cost savings, and thermal comfort benefits in buildings. Additionally, the statistical relationships between critical performance metrics and general weather and location parameters are examined using the analysis of variables (ANOVA) coupled with linear regression. The findings in this dissertation will guide the material selection for ultrahigh solar reflectivity and fabrication of future white coatings for thermal management.

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