In a study published in Science, a research group led by Prof. LI Wei from the Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, in collaboration with Prof. FAN Shanhui's team from the Stanford University and Prof. Andrea Alu's team from the City University of New York, designed an angularly asymmetric and spectrally selective thermal emitter, achieving daytime subambient radiative cooling on vertical surfaces. Since the first demonstration of daytime radiative cooling in 2014, many types of radiative cooler have been developed over the past decade. These radiative coolers typically exhibited omnidirectional thermal radiation properties, making them suitable only for horizontal surfaces. While many research teams have attempted to control the spectral or angular thermal radiation in recent years, daytime subambient radiative cooling for vertical surfaces (which are ubiquitous in practical scenarios, such as walls, clothing, the sides of vehicles) remains a significant challenge.
To address this challenge, the research team utilized thermal photonics to achieve cross-band synergistic control of thermal radiation in both angle and spectrum. They designed an angularly asymmetric and spectrally selective thermal emitter (AS emitter) using a cross-scale symmetry-breaking structure.
The AS emitter consists of a sawtooth grating covered by an ultraviolet-visible reflective, IR transparent nanoporous polyethylene (nanoPE) film. The combination of the Ag layers and the nanoPE film results in strong reflection over the entire solar wavelength range. The SiN layers provide spectrally selective emissions due to its phonon polarization resonance and the outermost Ag layer is designed to reflect the thermal radiation of ground.
The research team pointed out that the period of sawtooth grating must be substantially larger than the thermal wavelength to enable angular asymmetric emission because of the constraints stemming from thermodynamics and reciprocity, as well as to support a quasi-continuous frequency coverage of light coupling.
The research team conducted continuous outdoor temperature measurements to demonstrate its radiative cooling performance. Over the entire day, the AS emitter maintained a steady-state temperature substantially below the ambient temperature. Even under peak sunlight, AS emitter still maintained a temperature of 2.5℃ below ambient temperature, corresponding to a temperature reduction of 4.3 and 8.9℃ compared to conventional high-performance radiative cooler and commercial white paint, respectively.
The research team also conducted additional experiments with all emitters facing a south-facing wall that is the hottest at noon. They redesigned the AS emitter with a tilt angle to reject the radiation from the ground and wall. The AS emitter shows a temperature reduction of 3.5°C and 4.6°C compared to the conventional high-performance radiative cooler and commercial white paint. These results demonstrate the cooling potential of AS emitter in practical scenarios.
In conclusion, this innovative work breaks through the limitation of conventional radiative coolers, which can only function on horizontal surfaces, achieving a dimensional leap in radiative cooling technology from horizontal surfaces to practical three-dimensional scenarios. It also opens up new possibilities in the field of energy and sustainability, offering new thermal management solutions for energy-efficient technologies in a variety of applications.
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