Air-cooling buildings uses large amounts of energy, which is costly and can contribute to climate change. Passive radiative cooling materials that absorb energy and emit infrared radiation can be a promising, energy-saving alternative. While this works well at night, it is more challenging during the day due to solar absorption. Suitable radiative cooling materials for the daytime, thus, need to reflect sunlight well.

Yu Fu, Nanjing Forestry University, Jiangsu, China, Kai Zhang, University of Göttingen, Germany, and colleagues have developed an aerogel based on cross-linked cellulose nanocrystals (CNCs) that can serve as a thermal insulator and a daytime passive radiative cooler at the same time. Aerogels are ultralight materials in which a porous solid is filled with a gas. The team prepared CNCs via a hydrolysis process and then crosslinked them using methyltrimethoxysilane (MTMS), which reacts with surface OH groups. The resulting dispersion was subjected to a freeze casting process to create the desired aerogel.

The aerogel can be molded into different shapes, and according to researchers, its fabrication can be scaled up. The team found that the prepared aerogel coolers combine high solar reflectivity, high emissivity, and low thermal conduction. They tested the radiative cooling properties outdoors in summer. They found that during the day, the aerogel coolers showed a subambient temperature drop of ca. 9.2 °C even under the peak sunlight intensity. Under challenging humid conditions, it still reached a drop of ca. 7.4 °C. During the night, cooling to ca. 5.3 °C below ambient temperature was achieved.

• Dynamically Tunable All-Weather Daytime Cellulose Aerogel Radiative Supercooler for Energy-Saving Building,
  Chenyang Cai, Zechang Wei, Chunxiang Ding, Bianjing Sun, Wenbo Chen, Christoph Gerhard, Evgeny Nimerovsky, Yu Fu, Kai Zhang,
  Nano Lett. 2022.
  https://doi.org/10.1021/acs.nanolett.2c00844