Numerical Evaluation of Angle-Dependent IR-Transparent Radiative Cooling Performance for Asymmetric Periodic Structures

TL;DR

This paper emphasizes the importance of evaluating the angular distribution of IR transparency in designing passive radiative cooling structures, showing that single-angle asymmetry at normal incidence is insufficient for predicting real-world cooling performance.

Contribution

It introduces an angle-resolved electromagnetic modeling approach to accurately assess the angular dependence of IR transparency in asymmetric periodic structures for radiative cooling.

Findings

Normal-incidence asymmetry does not predict oblique-angle performance.

Angular integration shows marginal or negative cooling effects.

Full angular modeling is essential for accurate design of IR-transparent radiative coolers.

Abstract

Infrared (IR)-transparent passive radiative cooling (PRC) enables non-contact thermal management by regulating radiative heat exchange without direct attachment to the cooling object. While asymmetric IR transmission at a specific incidence angle -- typically normal incidence -- is often emphasized, we show that such single-angle asymmetry is neither sufficient nor predictive of practical cooling performance. In this work, we demonstrate that effective non-contact PRC requires angularly distributed asymmetric IR transparency evaluated through hemispherical integration over emission directions, rather than asymmetry at a single incidence angle. To quantify this effect, an angle-resolved full-wave electromagnetic (EM) model with Bloch periodic boundary conditions and Floquet mode decomposition is employed to compute wavelength- and angle-dependent bidirectional reflection and transmission of periodic PRC structures. The resulting EM response is coupled to an energy-balance-based thermal model to predict the transient temperature evolution of the cooling object. By comparing models that account for the full angular distribution with normal-incidence-only approximations, we show that pronounced asymmetric transmission at normal incidence is generally not preserved at oblique angles. As a result, angular integration yields only marginal cooling or may even result in net heating, whereas normal-incidence-based models can substantially overestimate cooling performance. These results establish angularly distributed asymmetric transparency as a key EM design principle for IR-transparent PRC and wide-angle asymmetric metasurfaces.