Key Takeaways & Executive Findings
- •• Subambient daytime radiative cooling of vertical surfaces is achieved by a sawtooth grating with a period significantly greater than the thermal wavelength. • Adjusting the grating period and aspect ratio allows the cooler to be adapted to various inclined surfaces. • Angular asymmetry and spectral selectivity effectively address the problem of thermal radiation directionality, evading ground-generated thermal radiation interference. • This work enables intelligent temperature regulation of inclined surfaces, advancing smart thermal interaction systems.
Abstract
During the daytime, conventional radiative coolers disregard the directionality of thermal radiation, thereby overlooking the upward radiation from the ground. This upward radiation enhances the outward thermal radiation, leading to a substantial reduction in the subambient daytime radiative cooling performance. Conversely, radiative coolers featuring angular asymmetry and spectral selectivity effectively resolve the problem of thermal radiation directionality, successfully evading the interference caused by the ground-generated thermal radiation. This cooler overcomes the limitations posed by the angle of incident light, making it suitable for subambient daytime radiative cooling of vertical surfaces. Furthermore, by adjusting the structure of the cooler, the angular range of thermal radiation can be modulated, enabling the application of radiative cooling technology for intelligent temperature regulation of various inclined surfaces encountered in daily life. This innovative work makes a significant contribution to the development of subambient smart thermal interaction systems and opens up new possibilities for the practical application of radiative cooling technology.
1. Introduction
The progress of human society hinges on the utilization of energy. However, the consumption of non-renewable energy has led to an increase in greenhouse gas emissions [1]. The ensuing issue of global warming has compelled a sharp rise in demand for cooling solutions, imposing a huge burden on the environment. In light of current development needs, cooling technology is no longer confined to temperature regulation. It is evolving toward achieving high-performance, integrating advanced intelligence, and giving priority to environmental sustainability. In this context, radiative cooling technology has emerged as a promising passive cooling strategy [1]. This technology harnesses a natural phenomenon of energy transfer called photon heat flow to dissipate energy and entropy into outer space. By doing so, it achieves cooling effects without the need for additional energy input [2]. This revolutionary approach is expected to facilitate the integration and development of various disciplines, including materials science, energy development, and intelligent systems. It holds the potential to usher in a new era of zero-emission cooling solutions worldwide.
Manipulating materials and microstructures allows for control of light across a wide spectral range and is thus considered a highly effective method for modulating subambient daytime radiative cooling. The current research primarily focuses on the direct impact of solar radiation on the surfaces of radiators that are oriented toward the sky, facilitating the outward emission of infrared heat. Based on this principle, various photonic metasurfaces [3], such as periodic arrays and multilayer film structures, have been designed to maximize thermal emission. However, a significant gap in the current research is the neglect of upward radiation from the ground, especially during daylight hours when surfaces quickly heat up due to solar exposure, significantly enhancing outward thermal radiation. Additionally, fluctuating weather conditions and mobile targets create dynamic temperature fluctuations, posing challenges for achieving high-efficiency subambient daytime radiative cooling. In essence, the current limitation of radiative cooling technologies in subambient environments is the lack of "vectorization," which requires not only consideration of cooling performance but also directional control. To break this directional deadlock, previous thermal radiators with photonic metasurfaces exhibited central symmetry, resulting in angularly symmetric thermal radiation. Zhou et al. have introduced a novel approach by utilizing periodically tilted wedge-shaped cavities [4], which introduces "direction" to microstructures and breaks the symmetry of radiative coolers. This innovative technique successfully manipulates the direction of thermal radiation, paving the way for more advanced and efficient radiative cooling solutions.
Imagine a scenario where the direction of thermal radiation could be extended to the normal direction of vertical surfaces, which would have a profoundly positive impact on energy efficiency and practical applications.
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Fan Liu, Qichong Zhang (2025). Angle-Selective Photonics for Smart Subambient Radiative Cooling. Nano-Micro Letters. https://doi.org/10.1007/s40820-025-01698-0
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Frequently Asked Questions
What is the main innovation of this radiative cooling technology?
The main innovation is the use of a sawtooth grating with a period significantly greater than the thermal wavelength to achieve angular asymmetry and spectral selectivity, enabling subambient daytime radiative cooling of vertical surfaces by effectively avoiding ground-generated thermal radiation interference.
How does the sawtooth grating enable cooling of vertical surfaces?
The sawtooth grating introduces angular asymmetry in thermal radiation, allowing the cooler to emit heat preferentially toward the sky while suppressing upward radiation from the ground. This overcomes the limitations of conventional radiative coolers that are only effective on horizontal surfaces facing the sky.
Can the radiative cooler be adapted to different inclined surfaces?
Yes, by adjusting the grating period and aspect ratio, the angular range of thermal radiation can be modulated, making the cooler adaptable to various inclined surfaces encountered in daily life.
What are the potential applications of this technology?
This technology can be applied for intelligent temperature regulation of vertical and inclined surfaces, such as building facades, vehicle surfaces, and other structures, contributing to energy-efficient and environmentally sustainable cooling solutions.
What is the significance of this work for radiative cooling research?
This work addresses the critical issue of thermal radiation directionality, which has been overlooked in conventional radiative coolers. By introducing angular asymmetry and spectral selectivity, it opens new possibilities for practical applications of radiative cooling technology in real-world scenarios.
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