Heat-shedding with photonic structures: radiative cooling and its potential

Radiative cooling, which is based on radiative heat exchange between the universe and Earth, can provide a passive and renewable route to reducing energy consumption. Radiative cooling was historically limited to nighttime applications owing to a lack of solar reflectivity. However, recent advances in photonics have facilitated the realization of multi-spectral features, such as near-unity solar spectrum reflection and atmospheric transparent window thermal emission for daytime radiative cooling. This review highlights recent progress and continued efforts in photonic radiators for daytime radiative cooling. First, we provide an overview of the fundamentals of passive radiative cooling, using the universe as a heat sink. Then, we assess advances in radiators, from traditional applications of buildings and textiles, to newly pioneered fields such as vehicles, wearables, solar cells, thermoelectric generators, and radiative condensers. Each application includes the requirements in terms of materials and optical/mechanical/thermal designs. Next, we discuss self-adaptive radiative thermostats for considering seasonal climate variation. Subsequently, challenges from fundamental limitations in passive radiative cooling and emerging issues with technical evolution are discussed, along with potential solution strategies.