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.

中文翻译：

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辐射冷却和蒸发冷却相结合的指南：传热传质分析


辐射冷却技术已在各个领域证明了其有效性。辐射冷却和蒸发冷却的合理结合可以大大增强其被动冷却性能。然而，目前的组合通常缺乏适当的热力学设计指导，并且不同情况下的性能限制尚不清楚。在这项工作中，我们分析了不同组合情况下的传热传质特性，并相应地提出了它们的最佳应用场景。目前的组合可分为三种类型，即单层结构、红外-发射双层结构和红外-透明双层结构。具体来说，具有低蒸发阻力的单层水凝胶结构总是可以产生更大的冷却功率，而两个双层结构由于太阳吸收和寄生热增益较小，可以在白天和部分夜晚实现较低的冷却温度。同时，我们提出了一种优化的高于环境温度的冷却结构和一种低于环境温度的冷却结构，可以分别产生 2 °C 和 3 °C 的额外冷却温度降低。此外，定量分析了吸湿盐对水凝胶蒸发的多方面影响。这项工作从热力学角度为将辐射冷却与蒸发冷却相结合提供了设计指南。