Self-switchable radiative cooling

Static radiative cooling causes unwanted overcooling and consumes excessive energy in winter. Recently, two Science papers have explored self-switchable radiative cooling to tackle this issue and promote all-season energy saving in buildings. One fabricates thermochromic smart windows, and the other develops temperature-adaptive radiative coatings, both of which take advantage of the wide-modulation emissivity of vanadium dioxide. Static radiative cooling causes unwanted overcooling and consumes excessive energy in winter. Recently, two Science papers have explored self-switchable radiative cooling to tackle this issue and promote all-season energy saving in buildings. One fabricates thermochromic smart windows, and the other develops temperature-adaptive radiative coatings, both of which take advantage of the wide-modulation emissivity of vanadium dioxide. Radiative cooling (RC) enables passive, efficient, and sustainable heat dissipation by emitting thermal radiation through the atmospheric window (8–13 μm) into outer space. According to Stephan-Boltzmann law, all objects above 0 K spontaneously emit energy via electromagnetic waves, and the upper limit of thermal radiation power is proportional to the fourth power of their temperatures. Outer space, which is about 3 K, is an ideal, natural cold sink; thus, harvesting the coldness of outer space is of great significance to saving energy and alleviating the effects of global warming.1Zhai Y. Ma Y. David S.N. Zhao D. Lou R. Tan G. Yang R. Yin X. Scalable-manufactured randomized glass-polymer hybrid metamaterial for daytime radiative cooling.Science. 2017; 355: 1062-1066Google Scholar, 2Yin X. Yang R. Tan G. Fan S. Terrestrial radiative cooling: Using the cold universe as a renewable and sustainable energy source.Science. 2020; 370: 786-791Google Scholar, 3Mandal J. Fu Y. Overvig A.C. Jia M. Sun K. Shi N.N. Zhou H. Xiao X. Yu N. Yang Y. Hierarchically porous polymer coatings for highly efficient passive daytime radiative cooling.Science. 2018; 362: 315-319Google Scholar On the other hand, the atmosphere, mainly due to the carbon dioxide and water vapor, exhibits strong wavelength-selective emissivity/absorptivity, and its transmission band is called the atmospheric window, which happens to overlap with the principal spectrum of thermal radiation of objects on earth at about 300 K. The RC technology was first proposed about 60 years ago, but it was effective only during nighttime because the supply of solar energy far outweighs its cooling power during daytime.4Xi W. Liu Y. Zhao W. Hu R. Luo X. Colored radiative cooling: How to balance color display and radiative cooling performance.Int. J. Therm. Sci. 2021; 170: 107172Google Scholar In order to achieve daytime operation with subambient temperatures, it is essential to design a surface that marginally absorbs solar energy and efficiently radiates into outer space. Thanks to the advances in thermophotonics and micro/nano-fabrication techniques, daytime RC technology has achieved great breakthroughs over the last decade.2Yin X. Yang R. Tan G. Fan S. Terrestrial radiative cooling: Using the cold universe as a renewable and sustainable energy source.Science. 2020; 370: 786-791Google Scholar Various daytime RC materials and structures have been designed and manufactured in many forms of films,1Zhai Y. Ma Y. David S.N. Zhao D. Lou R. Tan G. Yang R. Yin X. Scalable-manufactured randomized glass-polymer hybrid metamaterial for daytime radiative cooling.Science. 2017; 355: 1062-1066Google Scholar coatings,3Mandal J. Fu Y. Overvig A.C. Jia M. Sun K. Shi N.N. Zhou H. Xiao X. Yu N. Yang Y. Hierarchically porous polymer coatings for highly efficient passive daytime radiative cooling.Science. 2018; 362: 315-319Google Scholar wood,5Li T. Zhai Y. He S. Gan W. Wei Z. Heidarinejad M. Dalgo D. Mi R. Zhao X. Song J. et al.A radiative cooling structural material.Science. 2019; 364: 760-763Google Scholar and textiles6Zeng S. Pian S. Su M. Wang Z. Wu M. Liu X. Chen M. Xiang Y. Wu J. Zhang M. et al.Hierarchical-morphology metafabric for scalable passive daytime radiative cooling.Science. 2021; 373: 692-696Google Scholar and are widely used in personal thermal management,7Hu R. Liu Y. Shin S. Huang S. Ren X. Shu W. Cheng J. Tao G. Xu W. Chen R. et al.Emerging Materials and Strategies for Personal Thermal Management.Adv. Energy Mater. 2020; 10: 1903921Google Scholar building energy saving,3Mandal J. Fu Y. Overvig A.C. Jia M. Sun K. Shi N.N. Zhou H. Xiao X. Yu N. Yang Y. Hierarchically porous polymer coatings for highly efficient passive daytime radiative cooling.Science. 2018; 362: 315-319Google Scholar,5Li T. Zhai Y. He S. Gan W. Wei Z. Heidarinejad M. Dalgo D. Mi R. Zhao X. Song J. et al.A radiative cooling structural material.Science. 2019; 364: 760-763Google Scholar power plant cooling,2Yin X. Yang R. Tan G. Fan S. Terrestrial radiative cooling: Using the cold universe as a renewable and sustainable energy source.Science. 2020; 370: 786-791Google Scholar and thermoelectric generation,2Yin X. Yang R. Tan G. Fan S. Terrestrial radiative cooling: Using the cold universe as a renewable and sustainable energy source.Science. 2020; 370: 786-791Google Scholar to a name a few. For most of the current RC materials and structures, they can only act as a cooling mode no matter how environmental conditions (e.g., temperature and humidity) change. Take the RC technology for buildings, for example. The RC technology is welcome for energy savings in buildings in summer by cutting down the use of energy for space cooling; on the contrary, it acts adversely in winter because overcooling of buildings leads to skyrocketing heating costs, which is exacerbated in cold regions. One may propose to use cover or shelter to inactivate the RC function, which is rather unrealistic from a practical viewpoint. The essence of such problems lies in the non-adjustable optical properties of existing RC materials and structures. Therefore, it is essential to develop self-switchable RC materials and structures to dynamically adapt themselves to ambient temperatures, especially for energy savings in buildings during summer and winter. Recently in Science, Wang and colleagues from Nanyang Technological University, University of Wyoming, and Huazhong University of Science and Technology proposed a practical solution to the self-switchable RC in smart windows to promote energy savings in buildings in both summer and winter. An ideal smart window must have high reflectance in the near-infrared (NIR) band (i.e., 1.0–2.5 μm) and high emissivity in the long-wave infrared (LWIR) band (i.e., 5–25 μm) during summer, whereas it must have high transmittance in the NIR band and low emissivity in the LWIR band during winter. Such a multispectral engineering blocks the solar energy in the NIR band and boosts RC in summer and vice versa in winter, as shown in Figure 1. To achieve this, they fabricated a self-adaptive, RC regulated thermochromic (RCRT) smart window using a solution process that consisted of a stack of vanadium dioxide (VO2)/dielectric spacer/low-emissivity layers.8Wang S. Jiang T. Meng Y. Yang R. Tan G. Long Y. Scalable thermochromic smart windows with passive radiative cooling regulation.Science. 2021; 374: 1501-1504Google Scholar Such a multilayer stacking forms a Fabry-Perot resonator, which enables a low LWIR emissivity at low temperatures and a large LWIR emissivity at high temperatures owing to the metal-to-insulator transition of VO2. The core mechanism for implementing such self-switchable RC lies in tungsten doped vanadium oxide (WxV1−xO2); its phase transition temperature is shifted down to 20°C, depending on the fraction (x) of the tungsten element (as opposed to ∼68°C for VO2). As a result, the LWIR emissivity of the RCRT smart window is 0.21 below 20°C while it abruptly increases to 0.61 above 20°C, which shows a great potential for wide-modulation emissivity by temperature. Furthermore, the range of modulated LWIR emissivity can be tailored by adjusting the structure parameters of resonators to maximize energy savings in distinct climate zones. According to the whole-building energy simulations in seven climate zones, the RCRT window shows energy savings up to 324.6 MJ m−2 when benchmarked by a commercial low-emissivity glass. In the same issue of Science, Tang and colleagues from University of California developed a temperature-adaptive radiative coating (TARC) to implement thermal regulation. As shown in Figure 1, the TARC can spontaneously switch its emissivity in the atmospheric window from 0.20 to 0.90 when its temperature changes across ∼22°C, which can be applied for roof coatings to reduce energy consumption in residential buildings.9Tang K. Dong K. Li J. Gordon M.P. Reichertz F.G. Kim H. Rho Y. Wang Q. Lin C.-Y. Grigoropoulos C.P. et al.Temperature-adaptive radiative coating for all-season household thermal regulation.Science. 2021; 374: 1504-1509Google Scholar This adaptive thermal switching is also based on the metal-insulator transition of WxV1−xO2. The TARC consists of a two-dimensional array of lithographically patterned thin WxV1−xO2 blocks that is embedded in a BaF2 dielectric layer sitting on top of an Ag film. When ambient temperature is lower than the phase transition temperature of the used WxV1−xO2, it becomes an insulating state; thus the TARC has a low emissivity in the atmospheric window. Conversely, the WxV1−xO2 becomes a metallic state, thereby augmenting the emissivity of the TARC by means of Fabry-Perot resonance. Outdoor field tests demonstrate that the thermal performance of the TARC is superior to regular roof coatings in terms of energy efficiency from an all-season perspective. According to numerical simulations considering specific environment and climate, the advantage of TARC is more significant in climate zones with wide temperature variations. The authors mapped space-conditioning source energy savings for 15 typical US climate zones and demonstrated a great potential for energy savings through the year when the TARC technology was adopted. These two recent important progresses on self-switchable RC in terms of materials, mechanisms, and performance for energy savings in buildings are intriguing and inspiring. One fabricates a thermochromic smart window with tunable emissivity, and the other develops a flexible, radiative coating with temperature adaptation to dynamically regulate their RC performance to reduce the electricity use for cooling and heating in buildings throughout the year. Both accomplishments focused on developing self-switchable RC materials by harnessing the thermochromic property of VO2 and shifting its high phase transition temperature to near room temperature by doping tungsten; the metal-insulator transition temperature can be lower by detwisting the V-V bonds in the monoclinic insulating phase of VO2.10Lee S. Hippalgaonkar K. Yang F. Hong J. Ko C. Suh J. Liu K. Wang K. Urban J.J. Zhang X. et al.Anomalously low electronic thermal conductivity in metallic vanadium dioxide.Science. 2017; 355: 371-374Google Scholar Note that the intrinsic emissivity of VO2 is high at low temperatures and low at high temperatures, which appears oppositely in these two studies. This is mainly due to the emergence of Fabry-Perot resonance induced by optically designed structures, which greatly enhances the LWIR emissivity when VO2 is in the metallic state at high temperatures. Despite recent breakthroughs, these developed self-switchable radiative cooling technologies are suffering from lack of reliability and scalability, high costs, and monotonous appearance. More efforts need to be devoted to improving switching capability to adapt to dramatically changing climates, enhancing reliability with longer service life, self-cleaning, and coloration for aesthetic applications. More adaptive designs should be proposed to broaden the scope and application of RC. We hope that this Preview can stimulate great interest and attention to further optimize emissivity spectrum for improving thermal performance and to provide reliable, scalable, cost-effective, and aesthetic solutions, thereby promoting energy savings and alleviating the effects of global warming. The authors acknowledge financial support from the National Natural Science Foundation of China (52161160332, 52076087), the Wuhan City Science and Technology Program (2020010601012197) and the National Research Foundation of Korea (2021M3D1A2049865). The authors declare no competing interests.