Passive Sub-Ambient Cooling with an Evaporation-Insulation Bilayer

Passive evaporative cooling, which does not rely on electricity, shows promise to alleviate the ever-increasing global cooling needs but has been largely limited by environmental heating. Once sub-ambient temperatures are reached, the environmental heat gain accelerates the dry-out of evaporative materials, which is also detrimental to the cooling performance. Here, we address this critical challenge with an evaporation-insulation cooling design based on hydrogels and aerogels, inspired by the fur layer on the sweating skins of desert animals. Our transparent bilayer structure relies on water evaporation from hydrogels through highly porous aerogels for heat removal. At the same time, the low thermal conductivity of aerogels minimizes the parasitic heat gain. We establish a comprehensive model for heat and mass transfer in the system, which shows good agreement with our experimental data. We demonstrate that the addition of the porous insulation layer allows for optimization between the temperature drop from the ambient and the effective cooling time. Our bilayer structure can extend the lifetime of the cooling package by 400% compared to the conventional single-layer design. We further identify the relationship between the performance metrics and material properties of the insulation layer and provide design guidelines for various cooling applications including thermal regulation of food, pharmaceuticals, and buildings.