Key Takeaways & Executive Findings
- •• Inspired by human skin structure, an anisotropic synergistically performed insulation-radiation-evaporation cooler is developed by leveraging a dual-alignment structure both internal and external to the hydrogel. • The coordinated thermal and water transport through multiscale engineering contributed to high-power synergistic passive cooling in the day and water self-regeneration at night. • The cooler achieved an impressive cooling power of 311 W m−2 and an average sub-ambient cooling temperature of ~8.2 °C under direct sunlight. • The synergistic design provides new insights toward high-power, sustainable, and all-weather passive cooling applications.
Abstract
Hygroscopic hydrogel is a promising evaporative-cooling material for high-power passive daytime cooling with water self-regeneration. However, undesired solar and environmental heating makes it a challenge to maintain sub-ambient daytime cooling. While different strategies have been developed to mitigate heat gains, they inevitably sacrifice the evaporation and water regeneration due to highly coupled thermal and vapor transport. Here, an anisotropic synergistically performed insulation-radiation-evaporation (ASPIRE) cooler is developed by leveraging a dual-alignment structure both internal and external to the hydrogel for coordinated thermal and water transport. The ASPIRE cooler achieves an impressive average sub-ambient cooling temperature of ~8.2 °C and a remarkable peak cooling power of 311 W m−2 under direct sunlight. Further examining the cooling mechanism reveals that the ASPIRE cooler reduces the solar and environmental heat gains without comprising the evaporation. Moreover, self-sustained multi-day cooling is possible with water self-regeneration at night under both clear and cloudy days. The synergistic design provides new insights toward high-power, sustainable, and all-weather passive cooling applications.
1. Introduction
The demand for effective and sustainable cooling technologies has grown significantly in recent years driven by the increasing need for energy-efficient solutions and the challenges presented by global warming [1, 2]. Passive cooling strategies leverage various heat transfer mechanisms, including radiation, conduction, and convection, to remove heat from space and maintain comfortable temperatures without reliance on electricity [3–8]. They have emerged as promising alternatives to traditional air-conditioning systems, offering the potential to reduce energy consumption and environmental impact [9–17]. Evaporative cooling is a simple passive cooling technique that exploits the high vaporization enthalpy of water to achieve effective cooling [18–20]. Hygroscopic hydrogels made from hydrophilic polymers and hygroscopic salts are promising candidates for evaporative cooling because of the high cooling power arising from the evaporation of water stored inside the hydrophilic network [21, 22]. The hygroscopic salts can also absorb moisture from the surrounding environment to achieve water self-regeneration. However, this moisture absorption inevitably lowers the evaporative-cooling power, which is further negated by the heat absorbed from the environment and solar radiation when exposed under the sunlight. Therefore, it is challenging to achieve daytime sub-ambient cooling with hygroscopic hydrogel [23].
Recent attempts have been made to combine radiative cooling with evaporative-cooling hydrogels to mitigate solar heating [24–31]. Radiative cooling is an effective way to reduce the solar heat gain by emitting long-wavelength infrared (LWIR) to the cold outer space through the atmospheric transparent window (8–13 μm) while effectively reflecting the solar irradiation (0.3–2.5 μm) [32–41]. Radiative-cooling films made from cellulose acetate, poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) and poly (vinylidene fluoride-co-hexafluoropropene) (P(VDF-HFP)) have been placed at the top surface of hygroscopic hydrogels to construct bilayer coolers, reducing the energy absorption from sunlight [25, 27, 29]. While the top layer enhanced solar reflectivity to mitigate solar heat gain, the porous, thin film could not properly isolate the environmental heat from the hydrogel, leading to conductive heat transfer from the hot environment to the cool hydrogel surface. Such heat gains not only reduced the overall cooling power but also compromised the cooling time by fast depleting waters in the hydrogel, negatively affecting the sub-ambient cooling performance.
To inhibit the environmental heating, a thermally insulated evaporating strategy has been developed by integrating an aerogel on top of the hydrogel to form a bilay...
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Xiuli Dong, Kit-Ying Chan, Xuemin Yin, Yu Zhang, Xiaomeng Zhao, Yunfei Yang, Zhenyu Wang, Xi Shen (2025). Anisotropic Hygroscopic Hydrogels with Synergistic Insulation-Radiation-Evaporation for High-Power and Self-Sustained Passive Daytime Cooling. Nano-Micro Letters. https://doi.org/10.1007/s40820-025-01766-5
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Frequently Asked Questions
What is the ASPIRE cooler?
The ASPIRE cooler is an anisotropic synergistically performed insulation-radiation-evaporation cooler developed by leveraging a dual-alignment structure both internal and external to the hydrogel for coordinated thermal and water transport.
What cooling performance does the ASPIRE cooler achieve?
The ASPIRE cooler achieves an impressive average sub-ambient cooling temperature of ~8.2 °C and a remarkable peak cooling power of 311 W m−2 under direct sunlight.
How does the ASPIRE cooler achieve water self-regeneration?
The hygroscopic salts in the hydrogel absorb moisture from the surrounding environment at night, enabling water self-regeneration for self-sustained multi-day cooling.
What are the key mechanisms of the ASPIRE cooler?
The ASPIRE cooler reduces solar and environmental heat gains without compromising evaporation, through synergistic insulation, radiation, and evaporation mechanisms.
What are the potential applications of the ASPIRE cooler?
The ASPIRE cooler provides new insights toward high-power, sustainable, and all-weather passive cooling applications, such as building cooling, outdoor equipment, and thermal management.
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