Passive isothermal film with self-switchable radiative cooling-driven water sorption layer for arid climate applications

Reducing substantial energy demand of active heating, ventilation, and air conditioning in arid climates is of paramount importance. Here, we develop a millimeter-scale passive isothermal film that maintains temperature near 25 °C without relying on energy consumption solely through natural phenomena. A radiative cooling unit comprising polydimethylsiloxane (PDMS) with diffraction grating and fused SiO₂/Ti/Ag film facilitates radiative cooling during daylight hours. Net thermal energy is stored through water desorption of porous materials (latent cooling) and dissolution of ammonium nitrate in water (endothermic reaction). At nighttime, thermal emission is suppressed by the destructive interference of PDMS diffraction, and thermochemical reverse reaction occurs to sustain temperature in response to heat loss from the ambient environment. It reduces cooling and heating loads by approximately −1.1 and 0.3 kW m⁻², respectively, throughout a full day. Our results indicate the potential of designing a passive system suitable for human habitation without an active system.