Recent Progress in Daytime Radiative Cooling: Advanced Material Designs and Applications

Abstract

the terrestrial radiation with a temperature of ≈300 K at the Earth's surface is concentrated at wavelengths ranging from 2.5 to 50 µm. Meanwhile, the combined effects from all atmospheric components result in that the special atmospheric window between 8 and 13 µm that is highly transparent. Hence, most terrestrial areas can effectively radiate heat through the transparent atmospheric window to the cold universe to maintain a relatively stable temperature. To this end, radiative coolers should have high emissivity in the transparent atmospheric window (8-13 µm), which is transparent in this region and allows infrared (IR) light to pass through. In this regard, various materials and structures have been designed in the past few decades and have exhibited promising passive cooling performance at night. [8,9] However, during the daytime, the Sun heats the radiative coolers, which seriously compromises the cooling effect. To resolve this problem, the cooler should radiate more heat to the cold universe while reflecting sunlight to avoid solar heating. Fan and co-workers [10] first designed multilayer photonic materials and achieved daytime radiative cooling to a temperature below ambient conditions when subjected to direct sunlight. Since then, various materials have been demonstrated to realize subambient daytime radiative cooling and have shown great potential for practical applications. [11][12][13] Before some reviews have summarized these developments in radiative cooling, [14][15][16][17] but the limited net cooling power and instability of radiative cooling hinder its practical wide applications. In this review, by summarizing the state-of-the-art research and development in passive daytime radiative cooling (PDRC), we first propose three critical components for PDRC: 1) spectral design in the mid-IR range, 2) structure design for enhancing solar reflectance, and 3) thermal management. Second, we introduce various applications of PDRC, such as building cooling, solar cell cooling, water harvesting, clothing, and electricity generation (Figure 1). Finally, the remaining challenges and opportunities of PDRCs are also discussed. Fundamental Mechanisms of PDRC PDRC ConceptBased on Kirchhoff's law, an object with a temperature higher than 0 K continuously exchanges heat with other objects by absorbing Passive daytime radiative cooling (PDRC) is emerging as a promising cooling technology. Owing to the high, broadband solar reflectivity and high mid-infrared emissivity, daytime radiative cooling materials can achieve passive net cooling power under direct sunlight. The zero-energy-consumption characteristic enables PDRC to reduce negative environmental issues compared with conventional cooling systems. In this review, the development of advanced daytime radiative cooling designs is summarized, recent progress is highlighted, and potential correlated applications, such as building cooling, photovoltaic cooling, and electricity generation, are introduced. The remaining challenges and opportunities of PDRCs are also in...

Passive Daytime Radiative Cooling of Silica Aerogels

“…The results confirmed that the introduction of DMDMS can significantly increase the solar reflectance of the silica aerogels owing to their large pore size and silica particles. sThe results indicated that solar reflectance is the critical parameter to fulfill PDRC, as has also been confirmed in the literature for other radiative cooling materials [ 45 , 46 , 47 , 48 , 49 , 50 ]. Moreover, the IR emissivity spectra shown in Figure 6 b indicated that both MDA7-3 and MDA3-2 possessed high emissivity in the range of 8–13 µm.…”

A Continuous Gradient Colloidal Glass

“…Enhanced scattering of visible light is of great importance in fields such as thin reflective coatings and passive cooling applications. [44,45] Beside the use of high-n materials such as titania, recent studies have examined alternative approaches such as the variation of the particle morphology of low-n materials to improve the scattering performance. [21,46] An alternative/complementary approach is our optimized mesostructure.…”

Recent Advances in Material Engineering and Applications for Passive Daytime Radiative Cooling

“…Such fact of atmospheric long‐wave infrared (LWIR) transmission window is crucial to keep the earth in its viable temperature range. [ 24 ] The radiative sky cooling could be achieved via thermal radiation through the atmospheric transparency window, especially at night without any solar heating. [ 25 ] When under direct sunlight in the daytime, one could achieve PDRC if the heat absorbed inwards by the object is less than the energy radiated outwards, reaching temperatures below the ambient environment.…”

“…[14,15,16,17,[18][19][20][21] To possess the PDRC property, the candidate materials need to show extremely low absorptivity for wavelengths of solar irradiation (0.3-2.5 µm), while have high emissivity within the atmospheric window (8-13 µm). [22][23][24][25] Such requirement for the optical characteristics on the two very different ranges of wavelength presents a serious challenge for the PDRC material engineering. To meet such a challenge, rather complicated strategy of material engineering as well as sophisticated material combinations were commonly involved in earlier PDRC studies.…”