Persistent sub-ambient cooling of vertical surfaces via a unique selective radiative-evaporative cooler

Daytime sub-ambient radiative cooling offers a passive means of reducing surface temperatures below ambient levels under direct sunlight. Existing hybrid systems that combine radiative and evaporative cooling rely exclusively on broadband radiative coolers, which are effective only for sky-facing horizontal surfaces. However, their performance deteriorates sharply in vertical orientations, such as building façades and vehicle exteriors, where angular limitations and environmental heat gains significantly diminish cooling efficiency. Here, we present a robust approach that achieves persistent sub-ambient daytime cooling on vertical surfaces by integrating a spectrally selective thermal emitter with a self-hygroscopic hydrogel. The selective emitter minimizes radiative heat influx from the ground and surroundings while protecting the hydrogel, which absorbs atmospheric moisture at night and drives evaporative cooling during the day. Under direct solar irradiation of 900 W m^-2, our system achieves temperature reductions of up to 6.1 °C below ambient temperatures. This durable and orientation-tolerant cooling strategy provides a practical pathway to extend passive cooling technologies from horizontal to vertical and other non-traditional surfaces.