Passive Sub-Ambient Cooling from a Transparent Evaporation-Insulation Bilayer

Hydrogel-based evaporative cooling, which mimics perspiration of mammals, provides great potential to reduce energy consumption for thermal regulation. Yet, environmental heat gain presents a critical challenge to this cooling strategy at sub-ambient temperatures. Herein, inspired by the fur layer of desert animals, we propose an evaporation-insulation cooling design based on hydrogels and aerogels. Our transparent bilayer structure relies on water evaporation from hydrogels through highly porous aerogels that are of ultralow thermal conductivity and minimize heating from the surroundings. Consequently, this bilayer design allows for optimization between the temperature drop from the ambient and the effective cooling time, which we demonstrated through both modeling and experiments. We show that our hydrogel-aerogel structure can extend the lifetime of the cooling package by 400% compared with the conventional single-layer design. We further provide design guidelines for various cooling applications, including thermal regulation of food, pharmaceuticals, and buildings.