Guidance for combining radiative cooling and evaporative cooling: A heat and mass transfer analysis

Radiative cooling technology has been demonstrated its effectiveness in various sectors. Rational combine of radiative cooling and evaporative cooling can greatly enhance its passive cooling performance. However, the present combinations generally lack appropriate thermodynamic design guidance, and the performance limitations for different cases are unclear. In this work, we analyzed the heat and mass transfer characteristics for different combination cases, and suggested their optimal application scenarios correspondingly. The current combinations can be divided to three types, namely, the monolayer structure, the infrared-emissive bilayer structure, and the infrared-transparent bilayer structure. Specifically, the monolayer hydrogel structure with low evaporation resistance can always produce larger cooling power, and the two bilayer structures can achieve lower cooling temperatures during the day and part of the night due to their lesser solar absorption and parasitic heat gain. Meanwhile, we proposed an optimized above-ambient cooling structure and a sub-ambient cooling structure, which can generate an extra cooling temperature reduction of 2 °C and 3 °C, respectively. Moreover, the multifaceted effect of hygroscopic salt on hydrogel evaporation was quantitatively analyzed. This work provides the design guidelines for combining radiative cooling with evaporative cooling from a thermodynamic perspective.