Hierarchically Hollow Microfibers as a Scalable and Effective Thermal Insulating Cooler for Buildings.

Daytime passive radiative cooling is a promising electricity-free pathway for cooling terrestrial buildings. Current research interest in this cooling strategy mainly lies in tailoring the optical spectra of materials for strong thermal emission and high solar reflection. However, environmental heat gain poses a crucial challenge to building cooling at subambient temperatures. Herein, we devise a scalable thermal insulating cooler (TIC) consisting of hierarchically hollow microfibers as the building envelope that simultaneously achieves passive daytime radiative cooling and thermal insulation to reduce environmental heat gain. The TIC demonstrates efficient solar reflection (94%) and long-wave infrared emission (94%), yielding a temperature drop of about 9 °C under sunlight of 900 W/m. Notably, the thermal conductivity of the TIC is lower than that of air, thus preventing heat flow from external environments to indoor space in the summer, an additional benefit that does not sacrifice the radiative cooling performance. A building energy simulation shows that 48.5% of cooling energy could be saved if the TIC is widely deployed in China.