With the development of societies, cooling demand from buildings, electronic devices and power plants is huge and growing up. However, conventional cooling methods like air conditioning are high energy consumption. Radiative cooling and evaporative cooling with passive and low carbon footprints are considered to be promising strategies to retard energy scarcity and global warming.

Recently, a new study published in Science Advances, researchers from Nanjing University, Stanford University, and Changchun Institute of Optics, Fine Mechanics and Physics (CIOMP) of the Chinese Academy of Sciences (CAS) demonstrated a tandem radiative/evaporative (TRE) cooler for passive daytime cooling.

The TRE cooler consisted of two parts, a hydrogel and a cellulose acetate fibrous network. A hydrogel with moisture adsorption/desorption capacities for water harvesting and evaporation. And a cellulose acetate fibrous network was covered on the top of the hydrogel, which has spectrum properties of high solar reflectivity and high mid-infrared emissivity for radiative cooling.

The working principle of TRE cooler is that, at nighttime, the radiative cooling of the top cellulose acetate fibrous network layer can assist hydrogel harvesting water from the atmosphere and store the water in the hydrogel. During the daytime, the top layer reflects most of the sunlight and strongly emits infrared radiation, and the stored water evaporates for cooling which enhance the cooling power.

The advantages of TRE cooler are also outstanding. By combining the two cooling mechanism, the TRE cooler can achieve higher cooling power. In addition, unlike conventional single radiative cooler whose cooling performance is easily affected by tweather such as cloud and fog, the TRE cooler still shown good cooling performance on cloudy day due to the evaporative cooling part. Moreover, The ability to harvest water from the atmosphere shows advantage in ruducing the demand of external continuous water supply especially in water scare regions.

This study demonstrated a novel system for combining radiative cooling, evaporative cooling, and atmospheric water harvesting togther. Its outstanding performance shows portential for many cooling applications, and provides a new way for daytime passive cooling.