Anisotropic Hygroscopic Hydrogels with Synergistic Insulation-Radiation-Evaporation for High-Power and Self-Sustained Passive Daytime Cooling

Abstract

Hygroscopic hydrogel is a promising evaporative-cooling material for high-power passive daytime cooling with water self-regeneration. However, undesired solar and environmental heating makes it a challenge to maintain sub-ambient daytime cooling. While different strategies have been developed to mitigate heat gains, they inevitably sacrifice the evaporation and water regeneration due to highly coupled thermal and vapor transport. Here, an anisotropic synergistically performed insulation-radiation-evaporation (ASPIRE) cooler is developed by leveraging a dual-alignment structure both internal and external to the hydrogel for coordinated thermal and water transport. The ASPIRE cooler achieves an impressive average sub-ambient cooling temperature of ~ 8.2 °C and a remarkable peak cooling power of 311 W m⁻² under direct sunlight. Further examining the cooling mechanism reveals that the ASPIRE cooler reduces the solar and environmental heat gains without comprising the evaporation. Moreover, self-sustained multi-day cooling is possible with water self-regeneration at night under both clear and cloudy days. The synergistic design provides new insights toward high-power, sustainable, and all-weather passive cooling applications.