Simulations of micro-sphere/shell 2D silica photonic crystals for radiative cooling.

G L Whitworth,J Jaramillo-Fernandez,J A Pariente,A Blanco,C M Sotomayor-Torres,C Lopez,P D Garcia

doi:10.1364/oe.420989

Abstract

Passive daytime radiative cooling has recently become an attractive approach to address the global energy demand associated with modern refrigeration technologies. One technique to increase the radiative cooling performance is to engineer the surface of a polar dielectric material to enhance its emittance at wavelengths in the atmospheric infrared transparency window (8-13 µm) by outcoupling surface-phonon polaritons (SPhPs) into free-space. Here we present a theoretical investigation of new surface morphologies based upon self-assembled silica photonic crystals (PCs) using an in-house built rigorous coupled-wave analysis (RCWA) code. Simulations predict that silica micro-sphere PCs can reach up to 73 K below ambient temperature, when solar absorption and conductive/convective losses can be neglected. Micro-shell structures are studied to explore the direct outcoupling of the SPhP, resulting in near-unity emittance between 8 and 10 µm. Additionally, the effect of material composition is explored by simulating soda-lime glass micro-shells, which, in turn, exhibit a temperature reduction of 61 K below ambient temperature. The RCWA code was compared to FTIR measurements of silica micro-spheres, self-assembled on microscope slides.

Highlights

The global demand for cooling technologies represents an escalating problem in society, accounting for about 15% of global energy consumption and 10% of greenhouse gas emissions
Two main types of material designs are studied: selective emitters, which have extremely high emittance in the atmospheric IR transparency window (8–13 μm) but close to zero over all other wavelengths, and broadband emitters which radiate over the entire infrared spectral region (>2.5 μm) [13,14]
While selective emitters are the preferred choice for cooling surfaces below the ambient temperature due to their lower absorbed incoming radiation, broadband emitters drop the requirement for limiting atmospheric absorption in favour of high spectral emittance in a broad wavelength range

Summary

Introduction

The global demand for cooling technologies represents an escalating problem in society, accounting for about 15% of global energy consumption and 10% of greenhouse gas emissions. In order to achieve efficient radiative cooling on Earth, surfaces with high emittance in the atmospheric transparency window that neither absorb the incoming solar radiation nor thermal radiation from the ambient atmosphere must be engineered.

Results

Conclusion