(420b) Computational Method for Optimization of Micro-Structured Materials for Passive Cooling Applications

Over $372 billion is spent on electricity for cooling annually. Passive cooling, which requires no electricity, can be used to dramatically reduce energy usage for refrigeration and other cooling requirements. Engineered materials, structured at the micro-scale, are designed to emit more radiant energy than they absorb so they can passively cool the bodies they are attached to. Optimization is performed by iterative simulation of harmonic Maxwell’s equations implemented in a research finite-element-method (FEM) code. Particular analyses of several structures compare results from this implementation to other simulations used by the passive cooling research community. [2019 Gao et al.]

Solar radiation impinging on the Earth’s atmosphere is approximately the spectrum of a 5800 K blackbody, spanning wavelengths from 250 nm to 2.5 um. Objects at Earth’s ambient temperature (roughly 300 K) radiate at longer wavelengths in the mid- to long-wavelength IR spectrum - roughly 3 to 30 um wavelength range. Several metamaterial structures have been proposed that radiate more energy than the blackbody level by preferentially radiating in the 8-13 um window. These are nano-layered films or glass bead composite films or structured surfaces utilizing resonance from localized surface plasmon modes – often referred to as Frohlich resonance - in the 8-13 um atmospheric window. If thermal emission can be increased, energy saved from cooling films operating 24 hours/day could rival or exceed that produced by solar PV panels of the same area.

Kirchoff’s law of thermal radiation states that at steady-state and for a given wavelength, a material’s surface emissivity and absorptivity are the same. This law enables study of emissivity because absorbence in a thin and structured film can be modeled simply through the extinction coefficient attributed to it’s bulk material. Electromagnetic and energy equations are coupled so planewave-light absorbed in the material is converted to thermal energy, resulting in an increased temperature of the structure driving heat flow through the panel. Analysis of the energy balance results in wavelength-specific values of absorptivity.

Results of a material property study of photonic crystal-structured UV-curable adhesive (PCSUVA) for passive cooling provides dielectric properties and structure geometry to create simulations of thin film and truncated cone photonic structures. Simulation results are compared to those obtained using commercial software. Sensitivity analysis of absorptivity to material properties and structure geometry shows how the metamaterial may be tuned to improve it’s efficacy in the application of passive cooling.

[2019 Gao et al.] Gao, M.; Han, X.; Chan, F.; Zhou, W.; Liu, P.; Shan, Y.; Chen, Y.; Li, J.; Zhang, R.; Wang, S.; Zhang, Q.; Zheng, W. Approach to fabricating high-performance cooler with near-ideal emissive spectrum for above-ambient air temperature radiative cooling. Solar Energy Materials and Solar Cells 2019, 200.