In a pioneering breakthrough that could revolutionize sustainable cooling technologies, researchers from The Hong Kong Polytechnic University and Jiangnan University have developed an advanced passive cooling system capable of delivering high-power daytime cooling without external energy input. Led by Professor Xi Shen, the team’s study, recently published in Nano-Micro Letters, introduces the first-of-its-kind anisotropic synergistically performed insulation-radiation-evaporation (ASPIRE) cooler. This innovative device harnesses the unique properties of anisotropic hygroscopic hydrogels, combining meticulous structural engineering with natural passive cooling mechanisms to achieve unprecedented performance under direct sunlight.
The fundamental impetus behind the ASPIRE cooler’s design stems from the urgent need for energy-efficient alternatives to conventional cooling systems, which typically rely heavily on electricity consumption and contribute significantly to environmental degradation. Unlike active cooling technologies, passive cooling materials and devices employ ambient conditions and intrinsic physical phenomena to facilitate heat dissipation without energy input. However, prevailing passive coolers often struggle with limited cooling power and lack self-sustaining mechanisms, especially under variable weather conditions. The ASPIRE cooler addresses these challenges head-on by synergistically integrating thermal insulation, radiative cooling, and evaporative cooling processes within a single, multifunctional material.
At the heart of the ASPIRE cooler lies an ingenious dual-alignment structure inspired by the multilayered architecture of human skin. This biomimetic approach involves configuring the internal and external layers of the hydrogel with different anisotropic alignments tailored for specific functions. Internally, hydrophilic polyvinyl alcohol (PVA) networks are vertically aligned to optimize directional water transport. This alignment facilitates rapid and controlled movement of water molecules, crucial for sustained evaporation cycles. Externally, a vertically aligned hydrophobic aerogel layer serves as an exceptional thermal barrier, minimizing conductive heat gain while enhancing radiative cooling by efficiently emitting long-wave infrared radiation through the atmospheric window.
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What distinguishes the ASPIRE cooler from previous iterations is the multiscale engineering strategy employed at molecular and nanoscale levels. By engineering molecular crosslinking densities within the PVA hydrogel and precisely structuring the cell walls at the nanoscale, the research team achieved a delicate balance between thermal resistance and water permeability. This coordination suppresses heat influx through conduction and convection, while simultaneously maintaining low resistance for water vapor diffusion. Such modulation is pivotal for upholding a high radiative cooling rate and continuous evaporative cooling, rendering the system effective through varying environmental conditions including both clear skies and overcast scenarios.
The synergistic mechanism underlying the cooler’s operation capitalizes on the complementary effects of insulation, radiation, and evaporation. Thermal insulation substantially limits incoming solar radiation and heat conduction from the surroundings, mitigating temperature elevation at the cooler surface. Concurrently, radiative cooling exploits the material’s ability to emit mid-IR radiation within the atmospheric transparency window (8–13 μm), effectively dissipating heat into outer space. Evaporative cooling further augments this effect by harnessing the endothermic nature of water evaporation, which absorbs latent heat from the cooler’s surface. The orchestrated interplay between these three cooling modes culminates in a remarkable net cooling power of 311 W·m⁻² and achieves sub-ambient temperature differentials averaging approximately 8.2 °C under intense solar irradiation.
Beyond its unmatched cooling performance, the ASPIRE cooler exhibits an impressive capacity for water self-regeneration. By leveraging the moisture absorbed during nighttime dew and ambient humidity, the hydrogel matrix replenishes its water content autonomously. This capability ensures uninterrupted cooling functionality over consecutive days without external water refilling, significantly enhancing convenience and practical applicability. The self-sustaining nature of the system positions it as a promising candidate for deployment in regions where water scarcity and energy limitations present critical constraints for cooling technologies.
A further remarkable aspect of this technology is its robustness under diverse climatic conditions. Experimental validations under both clear and cloudy skies demonstrate consistent cooling efficiencies, underscoring the cooler’s adaptability. This all-weather performance is particularly meaningful for geographic areas with fluctuating meteorological patterns, where reliance on solar radiation intensity alone may hamper other cooling devices. The ASPIRE cooler’s architecture empowers it to maintain a reliable cooling effect even during reduced sunlight exposure, an achievement that broadens its operational envelope significantly.
The potential scalability of the ASPIRE cooler is another highlight emphasized by the research. The synthesis approach employs relatively simple and inexpensive materials, paired with production methods conducive to large-scale manufacturing. Such attributes bode well for the material’s integration into real-world scenarios—ranging from building envelopes and roofing materials to portable cooling devices and outdoor equipment. The scalable fabrication technique combined with the cooler’s modular design allows adaptations to various sizes and applications, potentially spurring widespread industry adoption.
Looking forward, the research team envisions several avenues for further refinement and innovation. Ongoing studies aim to optimize hydrogel formulation and aerogel integration to boost durability, mechanical resilience, and overall cooling efficiency. Complementary investigations are also directed at incorporating novel nanomaterials and dynamic modulation capabilities, such as smart responsiveness to environmental stimuli. Furthermore, pairing the ASPIRE cooler with photovoltaic cells or other renewable energy systems could unlock multifunctional platforms offering simultaneous cooling and energy harvesting solutions.
The theoretical insights unravelled through this research furnish a deep understanding of the interplay between thermal phenomena and mass transport within hierarchically structured materials. By elucidating the role of anisotropy, molecular crosslinking, and microstructural design in orchestrating synergistic cooling effects, this study paves the way for next-generation passive cooling materials. Such advances have far-reaching implications for tackling global challenges related to energy consumption, urban heat islands, and climate change mitigation.
In conclusion, the ASPIRE cooler represents a paradigm shift in the field of passive daytime cooling technologies. Its biomimetic design, multiscale engineering, and synergistic cooling mechanism collectively establish a new benchmark for sustainable thermal management. By achieving superior cooling power with autonomous water regeneration under practical environmental conditions, it offers a viable, eco-friendly alternative to conventional cooling systems. As this transformative technology advances toward commercialization, it holds the promise to redefine cooling strategies in residential, commercial, and industrial sectors worldwide.
The achievements of this study are a testament to the power of interdisciplinary research and innovative material science, blending principles of polymer chemistry, thermal physics, and nanotechnology. The research team’s commitment to environmental stewardship and practical impact propels the ASPIRE cooler into a spotlight of global significance. With continued development and broader adoption, this technology could become a cornerstone of future sustainable infrastructure, contributing meaningfully to reducing humanity’s carbon footprint and enhancing quality of life amid rising global temperatures.
Subject of Research: Anisotropic hygroscopic hydrogels for passive daytime cooling
Article Title: Anisotropic Hygroscopic Hydrogels with Synergistic Insulation-Radiation-Evaporation for High-Power and Self-Sustained Passive Daytime Cooling
News Publication Date: 29-Apr-2025
Web References: DOI: 10.1007/s40820-025-01766-5
Image Credits: Xiuli Dong, Kit-Ying Chan, Xuemin Yin, Yu Zhang, Xiaomeng Zhao, Yunfei Yang, Zhenyu Wang, Xi Shen
Keywords
Hydrogels, Evaporation, Passive Cooling, Radiative Cooling, Thermal Insulation, Anisotropic Materials, Polyvinyl Alcohol (PVA), Aerogels, Sustainable Cooling, Biomimicry, Multiscale Engineering, Environmental Technology
Tags: advanced cooling device designanisotropic hygroscopic hydrogelsdual-alignment structure in materialsenergy-efficient cooling systemsenvironmental impact of cooling systemshigh-power daytime coolinginsulation-radiation-evaporation coolingmultifunctional cooling materialspassive cooling performancepassive cooling technologiesself-sustained cooling mechanismssustainable cooling innovations