High-performance daytime passive cooling: A tree-inspired approach utilizing integrated radiative and evaporative cooling

Radiative cooling technology, a passive method that enables objects to reach temperatures below the environment without using electricity, has gained attention as a potential substitute for traditional cooling systems. However, according to Planck's law of blackbody radiation, the theoretical maximum net cooling power for an object at around 300 K is limited to 150 W m-2 which presents a challenge to its commercialization. The study presents a system that combines radiative and evaporative cooling, built from treated wood and topped with a composite layer of hydrophobic aluminum oxide nanoparticles and cellulose microfibers. This design achieves an average cooling power of up to 980 W m-2 within a 30-min test. The cooler has an average emissivity of 91.67% within the atmospheric window and reflects 98.55% of incoming sunlight. This cooling system exhibits strong radiative cooling during daylight hours while maintaining efficient evaporative cooling, substantially boosting passive cooling effectiveness throughout the day. It achieves an average air temperature reduction of 7.9 C compared to the air temperature during the day, with a peak difference of up to 19.7 C. Even under cloudy conditions with lower solar radiation, this cooler consistently maintains an average air temperature difference of 5.5 C from the surrounding environment, with a maximum difference of up to 10.1 C. This study provides valuable insights for designing highly efficient, scalable passive cooling systems that optimize daytime cooling performance by simultaneously harnessing infrared radiation and water evaporation.