Design, Simulation, Fabrication, and Characterization of Photonic Radiative Coolers

  • Afra S. Alketbi

Student thesis: Master's Thesis

Abstract

Cooling is a significant end-use of energy globally and a major driver of peak electricity demand. Air conditioning contributes to over 60% of UAE's total electricity consumption during summer. Therefore, passive cooling using selective emittance materials attracted huge attention in the past few years. Such photonic materials demonstrate a great potential for radiative cooling applications due to their ability to reflect the majority of incident solar radiation while maintaining strong emission of longer wave radiation. The work in this thesis focuses on the design, optimization, fabrication, and experimental characterization of photonic multilayer coatings with radiative cooling properties. We begin with the experimental realization of the first effective one-dimensional (1D) radiative cooler proposed at Stanford University, where they demonstrated surface temperatures below ambient when exposed to direct sunlight. When fabricating the structure, temperature measurement of the stack's surface under direct sunlight shows that the structure is capable of 20°C reduction in temperature when compared to a surface without the multilayered thin film coating. The analytical method represented in the transfer matrix method along with FDTD simulations were used to investigate the optical performance of the fabricated multilayer films in the visible to the midinfrared range. Comparable emittance behavior was obtained to that of the published results, which validated the simulation tools. The performance of this multilayered stack was compared to another design of multilayer coating composed of four oxides with defined thicknesses using numerical optimization method. The optical performance of the newly designed stack was predicted by the simulation tools and experimentally characterized after fabrication. The results showed a superior performance to the multilayer stack developed at Stanford University. Additionally, a novel silicon carbide-based photonic coating with a distributed Bragg's reflector (DBR) is proposed in this thesis. Theoretically, this novel design shows a broadband absorption in the infrared range and sufficient reflection for the incident solar radiation provided by a DBR consisting of porous silicon dioxide and amorphous silicon films. The absorptance/emittance of the as-fabricated silicon carbide films demonstrates promising ultra-broadband absorption in the mid-infrared range for thicknesses above one micron. The addition of the DBR has proven to reduce the absorption in the visible range with no effect seen in the IR range. Multiple DBRs must be used in order to enhance the performance of the cooler. Finally, this design demonstrates a great potential of radiative cooling in hot and harsh environment, thus enabling new technological possibilities for energy efficiency.
Date of AwardMay 2017
Original languageAmerican English
SupervisorTJ Zhang (Supervisor)

Keywords

  • Photonic Radiative Coolers
  • Electricity Consumption
  • Photonic Multilayer Coatings
  • Energy efficiency
  • Cooler performance
  • Distributed
  • Bragg’s Reflector (DBR)
  • Solar Radiation.

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