From Chitosan to Chitin: Bio‐Inspired Thin Films for Passive Daytime Radiative Cooling

Abstract

Passive radiative daytime cooling is an emerging technology contributing to carbon‐neutral heat management. Optically engineered materials with distinct absorption and emission properties in the solar and mid‐infrared range are at the heart of this technology. Owing to their low emissive power of about 100 W m−2 during daytime, substantial areas need to be covered with passive cooling materials or coatings to achieve a sizeable effect on global warming. Consequently, biocompatible materials are urgently needed to develop suitable coatings with no adverse environmental impact. It is shown how chitosan films with different thicknesses can be produced from slightly acidic aqueous solutions. The conversion to their insoluble form chitin in the solid state is demonstrated and the conversion is monitored with infrared (IR) and NMR spectroscopy. In combination with a reflective backing material, the films show below‐ambient temperature cooling capabilities with a suitable emissivity in the mid‐IR region and low solar absorption of 3.1–6.9%, depending on the film thickness. This work highlights the potential of chitosan and chitin as widely available biocompatible polymers for passive radiative cooling applications.

Recent advances in dynamic dual mode systems for daytime radiative cooling and solar heating

“…51 This is signicant challenge when combined to emit strongly and selectively in the atmospheric window. To achieve efficient passive radiative cooling, [52][53][54] specic optical properties are required to maximize radiative heat transfer toward outer space in the atmospheric window while minimizing the absorption of solar heat within the 0.3 to 2.5 mm range.…”

“…51 This is signicant challenge when combined to emit strongly and selectively in the atmospheric window. To achieve efficient passive radiative cooling, [52][53][54] specic optical properties are required to maximize radiative heat transfer toward outer space in the atmospheric window while minimizing the absorption of solar heat within the 0.3 to 2.5 mm range. where the symbol l represents the wavelength, I solar (l) is the solar radiation intensity of the standard AM1.5 spectrum, and r(l) refers to the solar reectance of the sample.…”

A Perspective for Radiative Cooling Materials

“…Thin films made of materials like silicon dioxide (SiO 2 ) or silicon nitride (Si 3 N 4 ) can be used to enhance RC. , These films are deposited on a substrate and can modify the surface’s emissivity and thermal properties. Thin films are commonly used for RC applications due to their unique optical properties and versatility.…”

Spectral Emissivity Profiles for Radiative Cooling

“…Cellulose nanofibers are held responsible from high backscattering of solar irradiance and emission in mid-infrared wavelengths. After the demonstration of the potential of cellulose as a material for radiative cooling, many other works that propose cellulose-based radiative coolers were demonstrated. − Many other types of bioinspired structures and materials were also reported up to this date, − including studies based on near-field radiative transfer. , Finally, the radiative cooling capability of natural silk opens the way for realizing radiative cooling phenomenon in textiles. − Successful demonstrations of radiative cooling with textiles considerably enlarges the application area and its potential contribution to energy consumption in general.…”