From Flat to Structured: Geometry-Driven Dual-Mode Radiative Emitters for Simultaneous Hotspot Cooling and Water Harvesting

Abstract

Radiative cooling has been developed for two largely separate functions: passive thermal management and atmospheric water harvesting (AWH). Combining them on a single surface under an imposed heat load is intrinsically difficult, as hotspot cooling raises the emitter temperature, whereas condensation requires sustained sub-dew-point regions. Here we establish, to our knowledge, a previously unreported geometry-driven co-functional regime in which a localized hotspot is coupled to an extended planar or pyramidal radiative emitter. Lateral heat spreading creates a warm, dry zone near the hotspot while preserving cooler peripheral regions for dew formation, allowing the same passive platform to reject heat and harvest water simultaneously. We combine angle-resolved optical characterization, raytracing, coupled heat and mass transfer modelling, and outdoor experiments. At a hotspot heat flux of 1021 W m-2, the largest planar emitter and 45° pyramidal emitter retain dew fluxes of 9.3 and 7.9 g m-2 h-1, respectively, while lowering the near-hotspot temperature by more than 45 °C relative to a non-concentrated control. At 1500 W m-2, the same architectures keep the hotspot below 20 °C, whereas the control approaches 85 °C. Finally, an experiment-calibrated global assessment further identifies where this regime is climatically accessible. These results provide a general framework for the passive co-management of localized heat and atmospheric water.