Pushing Radiative Cooling Technology to Real Applications
推动辐射冷却技术走向实际应用

Radiative cooling (RC) utilizes radiative heat transfer between the surface and the external environment, including heat gain from the sun and heat dissipation to the space, to achieve a certain heat balance. This technology is inspired by natural environmental processes. One such phenomenon is dew formation on surfaces during nighttime, facilitated by the atmospheric window. This atmospheric window allows the surface to radiate heat into space, thus lowering the surface temperature below the dew point of water and leading to condensation. Leveraging this natural effect, scientists have engineered radiative cooling surfaces capable of achieving subambient temperatures at night. ${}^{[1,2]}$${}^{[1,2]}$^([1,2]){ }^{[1,2]} However, sustaining these cooling effects during the daytime has been a significant challenge. A breakthrough occurred in 2014, when Fan’s group combined strong solar reflectance with high thermal emissivity within the atmospheric window through a photonic emitter. ${}^{[3]}$${}^{[3]}$^([3]){ }^{[3]} By effectively minimizing solar heat absorption, they demonstrated an RC surface that maintained temperatures below ambient air levels even at midday. This pioneering achievement has catalyzed extensive research and development in the field of daytime radiative cooling, opening new frontiers for radiative cooling across diverse applications.
辐射冷却（RC）利用表面与外部环境之间的辐射热传递，包括从太阳吸收的热量和向太空散发的热量，以实现一定的热平衡。这项技术受到自然环境过程启发。其中一种现象是夜间表面上的露水形成，这得益于大气窗口的作用。大气窗口允许表面向太空辐射热量，从而将表面温度降至水的露点以下，导致凝结。利用这种自然效应，科学家们设计出能够夜间实现低于环境温度的辐射冷却表面。然而，在白天持续这些冷却效果一直是一个重大挑战。2014 年，Fan 团队通过光子发射器，将强烈的太阳反射率与大气窗口内的高热发射率相结合，实现了突破。通过有效最小化太阳热吸收，他们展示了一种 RC 表面，即使在中午也能保持温度低于环境空气水平。 这项开创性成就推动了日间辐射冷却领域的广泛研究与发展，为辐射冷却在多种应用中的发展开辟了新的前沿。

In the evolution of radiative cooling technology, significant advancements have been achieved by modifying the micro- and nanostructures of materials to enhance their spectral selectivity, thereby improving cooling performance. ${}^{[4]}$${}^{[4]}$^([4]){ }^{[4]} However, this unidirectional cooling approach can lead to excessively low temperatures under cold conditions, diminishing human comfort or impairing the original functions of devices. Consequently, researchers have begun to explore radiative heat transfer materials with dual capabilities for both cooling and heating regulation. ${}^{[5]}$${}^{[5]}$^([5]){ }^{[5]} These innovative surface materials exhibit high solar reflectance and thermal emittance when cooling is required, demonstrating efficient RC effects; conversely, when heating is desired, the solar reflectance and thermal emittance decrease, thereby suppressing the RC effect and enhancing solar thermal conversion. The surface harvests solar heat through light absorption and preserves it by reducing heat loss via thermal radiation. This dynamic functionality marks a significant improvement of radiative heat management for various subjects in all-weather conditions (Figure 1).
在辐射冷却技术的发展过程中，通过改变材料的微纳结构来提高其光谱选择性，从而提升冷却性能，取得了显著进展。然而，这种单向冷却方法在低温条件下可能导致温度过低，降低人体舒适度或损害设备原有功能。因此，研究人员开始探索具有冷却和加热调节双重功能的辐射传热材料。这些新型表面材料在需要冷却时表现出高太阳反射率和热发射率，展现出高效的辐射冷却效果；相反，在需要加热时，太阳反射率和热发射率会降低，从而抑制辐射冷却效果并增强太阳能热转换。表面通过光吸收收集太阳热能，并通过减少热辐射损失来保存热量。这种动态功能显著改善了全天气条件下各种对象的辐射热管理（图 1）。

With further development of radiative cooling technologies, researchers have gradually shifted their focus from merely spec-
随着辐射冷却技术的进一步发展，研究人员逐渐将他们的焦点从仅仅 spec-
tral manipulation of materials to enhancing their applicability
材料的热操控以增强其适用性

DOI: 10.1002/adma. 202409738
DOI: 10.1002/adma.202409738

Figure 1. The schematic of outdoor radiative heat transfer.
图 1. 室外辐射传热示意图。
in real-world applications. This evolution inevitably increases the complexity of research. At this stage, the design of surface structures must not only consider optical performance but also evaluate the dynamic stimulating factors, long-term durability, manufacturing scalability, etc. Although substantial research has been conducted to address these issues, a comprehensive review of various solutions remains elusive. This review aims to elucidate the primary challenges encountered by structure design and fabrication for radiative cooling. It systematically compiles and evaluates the various solutions that have been employed, analyzing the characteristics and effectiveness of each approach. This work seeks to provide guidance for researchers dedicated to pushing radiative cooling technology for practical applications.
在实际应用中。这种发展不可避免地增加了研究的复杂性。在这个阶段，表面结构的设计不仅要考虑光学性能，还要评估动态刺激因素、长期耐久性、制造可扩展性等。尽管已经进行了大量研究来解决这些问题，但各种解决方案的全面综述仍然难以实现。本综述旨在阐明辐射冷却结构设计和制造过程中遇到的主要挑战。它系统地汇编和评估了所采用的各类解决方案，分析了每种方法的特征和有效性。这项工作旨在为致力于推动辐射冷却技术实际应用的科研人员提供指导。

A surface of a finite temperature will radiate heat through electromagnetic waves, of which the emission power density and spectrum depend on the surface temperature, surface material and morphology. Distinct from conduction and convection, radiative heat transfer does not require transfer media and can deliver heat far and fast. The intensity of thermal radiation $€$$€$€€ increases rapidly according to the Stefan-Boltzmann law ( $E=e\sigma T^4$$E=e\sigma T^4$E=e sigmaT^(4)E=e \sigma T^{4}, where $e,\sigma$$e,\sigma$e,sigmae, \sigma, and $T$$T$TT are the emissivity, Stefan-Boltzmann constant and temperature, respectively) and it shows nontrivial contribution to heat transfer at elevated temperatures. This mode of heat transfer in
一个有限温度的表面将通过电磁波辐射热量，其辐射功率密度和光谱取决于表面温度、表面材料和形态。与传导和对流不同，辐射传热不需要传递介质，并且可以快速远距离传递热量。热辐射的强度 $€$$€$€€ 根据斯特藩-玻尔兹曼定律迅速增加（ $E=e\sigma T^4$$E=e\sigma T^4$E=e sigmaT^(4)E=e \sigma T^{4} ，其中 $e,\sigma$$e,\sigma$e,sigmae, \sigma 、 $T$$T$TT 分别是发射率、斯特藩-玻尔兹曼常数和温度），在高温下对传热有显著贡献。这种传热方式在
an outdoor environment includes solar radiation, atmospheric radiation, and thermal radiation from terrestrial objects as shown in Figure 2a. The Sun, with a surface temperature of $\approx 5800\mathrm{K}$$\approx 5800\mathrm{K}$~~5800K\approx 5800 \mathrm{~K}, continuously emits solar radiation toward Earth due to ongoing nuclear fusion. The average solar radiation arriving at the top of the Earth’s atmosphere is about $1361\mathrm{W}{\mathrm{m}}^{-2}$$1361\mathrm{W}{\mathrm{m}}^{-2}$1361Wm^(-2)1361 \mathrm{~W} \mathrm{~m}^{-2}, which is referred as solar constant. Considering the diurnal rotation of the Earth around its axis, the daily average irradiance on the atmosphere is about $340\mathrm{W}{\mathrm{m}}^{-2}$$340\mathrm{W}{\mathrm{m}}^{-2}$340Wm^(-2)340 \mathrm{~W} \mathrm{~m}^{-2}. As solar light travels in the atmosphere, the atmospheric effects have several impacts on the solar irradiance on the Earth surface: 1) reduction in the power of the solar radiation due to absorption, scattering and reflection by the different gas components, dusts and clouds in the atmosphere; 2) variation in the solar spectrum due to selective absorption or scattering of some certain wavelengths, such as ultraviolet (UV) light, which is absorbed by the ozone layer; 3) the introduction of a diffuse or indirect component into the solar radiation; and 4) additional effects on the incident power, spectrum and directionality due to local variations in the atmosphere (such as water vapor, clouds, and pollution). The attenuation of solar irradiance highly relies on the optical path in the atmosphere, which is often normalized by the shortest possible path length under the normal incidence, referred as air mass (AM). The AM value is commonly set to $1/\cos \varphi$$1/\cos \varphi$1//cos phi1 / \cos \phi, where $\varphi$$\varphi$phi\phi is the zenith angle as an approximation that assumes the atmosphere is a flat horizontal layer and the altitude has negligible effect. To accurately calculate the air mass, one must consider uncertain factors such as atmospheric refractive index variation due to air pressure. Kasten and Young uses a spherical-shell approximation that presents a refined equation
一个户外环境包括太阳辐射、大气辐射和来自地球物体的热辐射，如图 2a 所示。太阳表面温度为 $\approx 5800\mathrm{K}$$\approx 5800\mathrm{K}$~~5800K\approx 5800 \mathrm{~K} ，由于持续的核聚变，它不断地向地球发射太阳辐射。到达地球大气层顶部的平均太阳辐射约为 $1361\mathrm{W}{\mathrm{m}}^{-2}$$1361\mathrm{W}{\mathrm{m}}^{-2}$1361Wm^(-2)1361 \mathrm{~W} \mathrm{~m}^{-2} ，这被称为太阳常数。考虑到地球围绕其轴的昼夜旋转，大气层的日平均辐照度约为 $340\mathrm{W}{\mathrm{m}}^{-2}$$340\mathrm{W}{\mathrm{m}}^{-2}$340Wm^(-2)340 \mathrm{~W} \mathrm{~m}^{-2} 。 当太阳光穿过大气层时，大气效应对地表的太阳辐照度有以下几个影响：1）由于大气中不同气体成分、尘埃和云层的吸收、散射和反射，导致太阳辐射的功率减弱；2）由于某些特定波长的选择性吸收或散射，如紫外线（UV）光被臭氧层吸收，导致太阳光谱的变化；3）太阳辐射中引入了漫射或间接成分；4）由于大气局部变化（如水汽、云层和污染）对入射功率、光谱和方向性的额外影响。太阳辐照度的衰减高度依赖于大气中的光程，通常用正常入射条件下的最短可能路径长度进行归一化，称为空气质量（AM）。AM 值通常设置为 3，其中 4 是天顶角，作为一个近似值，假设大气层是一个平坦的水平层，且海拔高度影响可以忽略不计。 要准确计算空气质量，必须考虑大气折射率因气压变化等不确定因素。Kasten 和 Young 使用球形壳近似，提出了一个考虑地球曲率的精化方程

Figure 2. a) Schematic of outdoor heat transfer between a surface and the environment. Reproduced with permission. ${}^{[7]}$${}^{[7]}$^([7]){ }^{[7]} Copyright 2022, Elsevier. b) The spectra of normalized solar irradiation (AM 1.5G), blackbody thermal irradiation ( 300 K ), and atmospheric transmittance. The ideal emittance (absorbance) spectra of coolers are plotted as dash lines.
图 2. a) 表面与环境之间室外热传递的示意图。经许可转载。 ${}^{[7]}$${}^{[7]}$^([7]){ }^{[7]} 版权所有 2022 年，Elsevier。b) 标准化太阳辐照（AM 1.5G）、黑体热辐射（300 K）和大气透射率的频谱。冷却器的理想发射率（吸收率）频谱以虚线表示。
that accounts for the curvature of the Earth as: $\mathrm{AM}=1/(\cos \varphi +$$\mathrm{AM}=1/(\cos \varphi +$AM=1//(cos phi+\mathrm{AM}=1 /(\cos \phi+ $0.50572(96.07995-\varphi {)}^{-1.6364}).{}^{[6]}$$\left.0.50572(96.07995-\varphi {)}^{-1.6364}\right).{}^{[6]}${: 0.50572(96.07995-phi)^(-1.6364)).^([6])\left.0.50572(96.07995-\phi)^{-1.6364}\right) .{ }^{[6]} AM 1.5, corresponding to a solar zenith angle of $\varphi ={48.2}^{\circ }$$\varphi ={48.2}^{\circ }$phi=48.2^(@)\phi=48.2^{\circ}, often used as the standard solar spectrum at the Earth surface for solar related applications, which has an intensity of $1000\mathrm{W}{\mathrm{m}}^{-2}$$1000\mathrm{W}{\mathrm{m}}^{-2}$1000Wm^(-2)1000 \mathrm{~W} \mathrm{~m}^{-2}. Figure 2b shows the AM 1.5G Spectrum, which consists of UV ( $0.3-0.38\mu \mathrm{m}$$0.3-0.38\mu \mathrm{m}$0.3-0.38mum0.3-0.38 ~ \mu \mathrm{~m} ), visible light ( $0.38-0.78\mu \mathrm{m}$$0.38-0.78\mu \mathrm{m}$0.38-0.78 mum0.38-0.78 \mu \mathrm{~m} ), and near-infrared (NIR) ( $0.78-3\mu \mathrm{m}$$0.78-3\mu \mathrm{m}$0.78-3mum0.78-3 \mu \mathrm{~m} ) radiation.
该方程考虑了地球的曲率，表示为： $\mathrm{AM}=1/(\cos \varphi +$$\mathrm{AM}=1/(\cos \varphi +$AM=1//(cos phi+\mathrm{AM}=1 /(\cos \phi+ $0.50572(96.07995-\varphi {)}^{-1.6364}).{}^{[6]}$$\left.0.50572(96.07995-\varphi {)}^{-1.6364}\right).{}^{[6]}${: 0.50572(96.07995-phi)^(-1.6364)).^([6])\left.0.50572(96.07995-\phi)^{-1.6364}\right) .{ }^{[6]} AM 1.5，对应太阳天顶角为 $\varphi ={48.2}^{\circ }$$\varphi ={48.2}^{\circ }$phi=48.2^(@)\phi=48.2^{\circ} ，常用于地表太阳能相关应用的标准太阳光谱，其强度为 $1000\mathrm{W}{\mathrm{m}}^{-2}$$1000\mathrm{W}{\mathrm{m}}^{-2}$1000Wm^(-2)1000 \mathrm{~W} \mathrm{~m}^{-2} 。图 2b 显示了 AM 1.5G 频谱，包括紫外线（ $0.3-0.38\mu \mathrm{m}$$0.3-0.38\mu \mathrm{m}$0.3-0.38mum0.3-0.38 ~ \mu \mathrm{~m} ）、可见光（ $0.38-0.78\mu \mathrm{m}$$0.38-0.78\mu \mathrm{m}$0.38-0.78 mum0.38-0.78 \mu \mathrm{~m} ）和近红外（NIR）（ $0.78-3\mu \mathrm{m}$$0.78-3\mu \mathrm{m}$0.78-3mum0.78-3 \mu \mathrm{~m} ）辐射。

In addition to solar radiation, outdoor environments also experience atmospheric radiation. Under clear sky conditions, atmospheric radiation originates from various gas molecules and aerosol particles, with significant contributions from water vapor, carbon dioxide, and ozone in the range of $0.3-50\mu \mathrm{m}{.}^{[7]}$$0.3-50\mu \mathrm{m}{.}^{[7]}$0.3-50 mum.^([7])0.3-50 \mu \mathrm{~m} .^{[7]} Furthermore, all objects emit thermal radiation based on their optical properties and temperature. ${}^{[8]}$${}^{[8]}$^([8]){ }^{[8]} Surfaces do not always emit or absorb thermal radiation perfectly, as electromagnetic radiation may be transmitted or reflected. In the ideal case of a black body, which perfectly absorbs and emits thermal radiation, its radiation is defined by Planck’s law and corresponds to the object’s surface temperature, as illustrated in Figure 2b at room temperature ( 300 K ). The emittance of a surface indicates the percentage of this emissive power relative to a black body.
除了太阳辐射，室外环境还会受到大气辐射的影响。在晴朗的天气条件下，大气辐射来源于各种气体分子和气溶胶颗粒，其中水蒸气、二氧化碳和臭氧在 $0.3-50\mu \mathrm{m}{.}^{[7]}$$0.3-50\mu \mathrm{m}{.}^{[7]}$0.3-50 mum.^([7])0.3-50 \mu \mathrm{~m} .^{[7]} 范围内有显著贡献。此外，所有物体都会根据其光学特性和温度发出热辐射。 ${}^{[8]}$${}^{[8]}$^([8]){ }^{[8]} 表面并不总是完美地发射或吸收热辐射，因为电磁辐射可能会被透射或反射。在理想情况下，一个完美吸收和发射热辐射的黑体，其辐射由普朗克定律定义，并对应于物体的表面温度，如图 2b 所示，在室温（300 K）下。表面的发射率表示其发射功率相对于黑体的百分比。

A critical aspect of radiative cooling is the concept of atmospheric windows, specific wavelength ranges in which the atmosphere is relatively transparent to thermal radiation. These windows allow thermal radiation from the Earth’s surface to pass directly into space, playing a significant role in the planet’s energy balance. Gas molecules can disrupt and weaken electromagnetic radiation propagating within certain bands, resulting from
辐射冷却的一个关键方面是大气窗口的概念，即大气对热辐射相对透明的特定波长范围。这些窗口允许地球表面的热辐射直接进入太空，在地球的能量平衡中发挥着重要作用。气体分子会干扰并削弱在特定波段内传播的电磁辐射，导致
their resonance absorption modes of vibration such as bending and stretching at certain frequencies. ${}^{[9]}{\mathrm{O}}_3$${}^{[9]}{\mathrm{O}}_3$^([9])O_(3){ }^{[9]} \mathrm{O}_{3} has a high infrared absorption band in the range of $9-10\mu \mathrm{m},{\mathrm{CO}}_2$$9-10\mu \mathrm{m},{\mathrm{CO}}_2$9-10 mum,CO_(2)9-10 \mu \mathrm{~m}, \mathrm{CO}_{2} has high absorption from 12.5 to $17.5\mu \mathrm{m}$$17.5\mu \mathrm{m}$17.5 mum17.5 \mu \mathrm{~m}, and water vapor shows high absorption when crossing over $16\mu \mathrm{m}$$16\mu \mathrm{m}$16 mum16 \mu \mathrm{~m}. From these three major contributors along with other gases, there are three atmospheric windows within $2.5-5,8-13$$2.5-5,8-13$2.5-5,8-132.5-5,8-13, and $16-22\mu \mathrm{m}{.}^{[10]}$$16-22\mu \mathrm{m}{.}^{[10]}$16-22 mum.^([10])16-22 \mu \mathrm{~m} .^{[10]} The most prominent atmospheric window is in the mid-infrared (MIR) range, approximately between 8 and $13\mu \mathrm{m}$$13\mu \mathrm{m}$13 mum13 \mu \mathrm{~m}. Within this range, absorption by atmospheric gases such as water vapor and carbon dioxide are minimal, enabling efficient transfer of heat from the ground to space. Coincidentally, those transparent windows overlap strongly with the peak of black body radiation at ground surface temperature. This phenomenon is crucial for the Earth’s ability to cool itself, as it facilitates the release of excess heat. Other atmospheric windows are also helpful in heat dissipation but with weaker radiative power (about $10\mathrm{\%}$$10\mathrm{\%}$10%10 \% of that in the major window ${}^{[7]}$${}^{[7]}$^([7]){ }^{[7]} ), and the 16$22\mu \mathrm{m}$$22\mu \mathrm{m}$22 mum22 \mu \mathrm{~m} window will disappear in a humid atmosphere because of infrared absorption by water vapor beyond $16\mu \mathrm{m}$$16\mu \mathrm{m}$16 mum16 \mu \mathrm{~m}.
它们的共振吸收模式在特定频率下的振动，如弯曲和拉伸。 ${}^{[9]}{\mathrm{O}}_3$${}^{[9]}{\mathrm{O}}_3$^([9])O_(3){ }^{[9]} \mathrm{O}_{3} 在 $9-10\mu \mathrm{m},{\mathrm{CO}}_2$$9-10\mu \mathrm{m},{\mathrm{CO}}_2$9-10 mum,CO_(2)9-10 \mu \mathrm{~m}, \mathrm{CO}_{2} 范围内具有高红外吸收带。 $17.5\mu \mathrm{m}$$17.5\mu \mathrm{m}$17.5 mum17.5 \mu \mathrm{~m} 在 12.5 到 $16\mu \mathrm{m}$$16\mu \mathrm{m}$16 mum16 \mu \mathrm{~m} 范围内有高吸收率，水蒸气在跨越 $2.5-5,8-13$$2.5-5,8-13$2.5-5,8-132.5-5,8-13 时表现出高吸收率。从这三个主要贡献者以及其他气体中， $16-22\mu \mathrm{m}{.}^{[10]}$$16-22\mu \mathrm{m}{.}^{[10]}$16-22 mum.^([10])16-22 \mu \mathrm{~m} .^{[10]} 存在三个大气窗口， $13\mu \mathrm{m}$$13\mu \mathrm{m}$13 mum13 \mu \mathrm{~m} 其中最显著的大气窗口位于中红外（MIR）范围，大约在 8 到 $10\mathrm{\%}$$10\mathrm{\%}$10%10 \% 之间。在这个范围内，大气气体如水蒸气和二氧化碳的吸收率极低，从而能够有效地将热量从地面传递到太空。巧合的是，这些透明窗口强烈地与地面温度下黑体辐射的峰值重叠。这种现象对于地球自我冷却的能力至关重要，因为它促进了多余热量的释放。其他大气窗口也有助于散热，但辐射能力较弱（约为主要窗口 ${}^{[7]}$${}^{[7]}$^([7]){ }^{[7]} 的 $22\mu \mathrm{m}$$22\mu \mathrm{m}$22 mum22 \mu \mathrm{~m} ），而 16 $16\mu \mathrm{m}$$16\mu \mathrm{m}$16 mum16 \mu \mathrm{~m} 窗口会在潮湿大气中消失，因为水蒸气在 #11# 范围外的红外吸收。

It is worth noting that atmosphere constituents and temperature show remarkable nonuniform vertical distributions. ${}^{[7]}$${}^{[7]}$^([7]){ }^{[7]} The densities of most gas species are very uniform below 10 km and gradually decrease at higher altitudes (except for Ozone that concentrates at around $30-40\mathrm{km}$$30-40\mathrm{km}$30-40km30-40 \mathrm{~km} ). Water vapor density is much higher than many other gases in the troposphere and it almost exponentially decreases with increasing altitude. As $99\mathrm{\%}$$99\mathrm{\%}$99%99 \% of the total water mass and aerosol are contained in the troposphere, the variation of water vapor amount in the troposphere will cause
值得注意的是，大气成分和温度表现出显著的垂直非均匀分布。 ${}^{[7]}$${}^{[7]}$^([7]){ }^{[7]} 大多数气体物种的密度在 10 公里以下非常均匀，并在更高海拔逐渐减少（臭氧除外，它集中在 $30-40\mathrm{km}$$30-40\mathrm{km}$30-40km30-40 \mathrm{~km} 左右）。水汽密度在对流层中远高于许多其他气体，并且几乎随着海拔升高呈指数级下降。由于 $99\mathrm{\%}$$99\mathrm{\%}$99%99 \% 总水质量和气溶胶都包含在对流层中，对流层中水汽含量的变化将导致