A gradient atmospheric model reveals enhanced radiative cooling potential and demonstrates the advantages of broadband emitters

TL;DR

This paper introduces a gradient atmospheric model for passive radiative cooling, showing it predicts higher cooling power than uniform models and finds broadband emitters outperform wavelength-selective ones in practical applications.

Contribution

The study develops a more accurate atmospheric model considering altitude variations and compares broadband and selective emitters, highlighting the advantages of broadband emitters for scalable cooling solutions.

Findings

Gradient model estimates 10-40% higher cooling power than uniform models.

Broadband emitters outperform wavelength-selective emitters in practical scenarios.

Large-scale cooling should focus on low-cost, broadband surfaces with minimal solar absorption.

Abstract

Passive radiative cooling toward the sky is a developing technology for adaptation in hot climates. Previous calculations of cooling performance have generally used uniform atmospheric models that assume a single sky temperature and atmospheric transmittance spectrum. Here, we introduce a gradient atmospheric model that accounts for altitude-dependent temperature and gas composition, revealing that uniform models underestimate cooling power by 10 - 40%. Using our improved model, we systematically compared broadband emitters (BEs) and wavelength-selective emitters (SEs) for sky-facing radiative cooling at various locations on Earth. We find that the differences in cooling power between the two types of emitters in the sub-ambient temperature range are generally small, even under ideal conditions. Furthermore, in practice, BEs actually have superior performance than realistic SEs, because they have fewer design degrees of freedom and thus can be engineered to have lower solar absorption. Our analysis suggests that large-scale deployment of sky-facing passive radiative cooling technologies should prioritize the development of scalable, low-cost surfaces with minimal solar absorption, rather than focusing on achieving selective thermal emission.