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As global warming continues, people tend to switch on fans or air conditioners during hot weather. But the more we use electrical appliances, the more energy we consume and carbon dioxide we produce. With continuous breakthroughs in electricity-free cooling, City University of Hong Kong (CityUHK) recently developed a new infrared-transparent aerogel that boosts the cooling performance of subambient daytime radiative cooling materials by up to 5.2°C. The findings were published in Energy & Environmental Science.
Subambient daytime radiative cooling (SDRC) reflects solar radiation and emits thermal radiation into outer space, reducing surface temperature without consuming energy. But existing SDRC relies on a single exposed surface for both solar reflection and thermal emission, making deep subambient cooling vulnerable to parasitic heat gain.
Led by professor Edwin Tso Chi-yan, associate dean (internationalization and outreach) and chair professor of the School of Energy and Environment at CityUHK, the research team developed a white infrared-transparent aerogel (WITA) that spatially decouples solar reflection and thermal insulation. Using a simplified one-step solvent-exchange process, the team adopted ultra-high-molecular-weight PE (UHMWPE) as the substrate.
Compared with conventional high-density polyethylene, UHMWPE offers a higher entanglement density and stronger physical cross-linking, enabling the aerogel to maintain its stable three-dimensional porous structure. The resulting WITA exhibits high optical performance, achieving atmospheric-window transmittance of up to 94.5% and solar reflectance of 96.6%.
The team established a multiscale optical-thermal model that bridges the light-scattering and heat-transferring mechanisms of WITA, directly linking material formulations to spectral response and cooling performance.
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