[Paper Review] Bulk Material Based Selective Infrared Emitter for Sub-Ambient Daytime Radiative Cooling

Yue Yang, Linshuang Long+4 more|arXiv (Cornell University)|Jul 6, 2019

Thermal Radiation and Cooling Technologies30 references4 citations

TL;DR

This paper proposes a bulk lithium fluoride (LiF) crystal with a silver backing as a selective infrared emitter for sub-ambient daytime radiative cooling. The structure achieves a solar absorptance of 4.7% and near-unity emissivity within the atmospheric window (8–13 μm), enabling a net cooling power of ~60 W/m² and a stagnation temperature 5 K below ambient under solar irradiance >900 W/m².

ABSTRACT

Through passively emitting excess heat to the outer space, radiative cooling has been demonstrated as an efficient way for energy saving applications. Selective surface with unity emittance only within the atmospheric window as well as zero absorption within the solar spectrum is sought to achieve the best sub-ambient radiative cooling performance during the daytime. In this work, we proposed a bulk radiative cooler consisting of a 1-mm-thick lithium fluoride crystal coated with silver backing, which exhibits a solar absorptance of 4.7% and nearly ideal infrared selectivity with high emission exactly within the atmospheric transmission band (i.e., 8-13 um). Excellent daytime cooling performance was demonstrated in an outdoor test with stagnation temperature below the ambient temperature by 5 K under solar irradiance above 900 W/m2 and a net cooling power of about 60 W/m2 when the cooler is in thermal equilibrium with the ambient. As a bulk material with the highest ultraviolet-visible-near infrared transmittance, lithium fluoride crystal has been widely employed as the optical windows and mirrors in various applications. The proposed simple selective infrared emitter based on lithium fluoride would open up an innovative way to radiatively cool optical systems.

Motivation & Objective

• To develop a simple, scalable, and efficient radiative cooling solution using bulk materials.
• To achieve high infrared selectivity within the atmospheric window (8–13 μm) while minimizing solar absorption.
• To demonstrate sub-ambient cooling performance under real-world daytime conditions with high solar irradiance.
• To leverage the high UV-Vis-NIR transmittance of lithium fluoride for optical applications.

Proposed method

• Utilization of a 1-mm-thick lithium fluoride (LiF) crystal as the primary optical material due to its high transparency in the solar spectrum and high infrared emissivity in the atmospheric window.
• Deposition of a silver back layer to enhance infrared emission and provide a reflective surface for directional emission.
• Design of the structure to achieve solar absorptance of 4.7% and near-unity emissivity in the 8–13 μm range.
• Optimization of the system for thermal equilibrium with ambient temperature during outdoor testing.
• Use of outdoor testing under solar irradiance >900 W/m² to validate real-world performance.
• Measurement of stagnation temperature and net cooling power to evaluate performance under natural conditions.

Experimental results

Research questions

• RQ1 Can a bulk material-based structure achieve high infrared selectivity and low solar absorption for effective daytime radiative cooling?
• RQ2 What is the maximum net cooling power achievable by a simple, scalable, and stable bulk emitter under high solar irradiance?
• RQ3 Can the LiF-silver system maintain a temperature below ambient during daytime operation?
• RQ4 How does the performance of the LiF-based emitter compare to other selective emitters in real-world conditions?
• RQ5 Can the high transmittance of LiF in the UV-Vis-NIR spectrum be leveraged to enable radiative cooling in optical systems?

Key findings

• The LiF-silver structure achieved a solar absorptance of 4.7%, minimizing unwanted solar heating.
• The system exhibited nearly ideal infrared selectivity with high emissivity precisely within the atmospheric transmission window (8–13 μm).
• Under solar irradiance exceeding 900 W/m², the cooler achieved a stagnation temperature 5 K below ambient air temperature.
• The net cooling power reached approximately 60 W/m² when the cooler was in thermal equilibrium with the ambient environment.
• Outdoor testing confirmed stable and effective sub-ambient cooling performance under real daylight conditions.
• The bulk LiF-silver system demonstrated potential for integration into optical systems requiring passive cooling.