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A Self-Assembled Two-Dimensional Thermo-Functional Material for Radiative Cooling

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A Self-Assembled Two-Dimensional Thermo-Functional Material for Radiative Cooling A self-assembled two-dimensional thermo-functional material for radiative cooling. J. Jaramillo-Fern´andez,1, ∗ G. L. Whitworth,1 J. A. Pariente,2 A. Blanco,2 P. D. Garc´ıa,1 C. L´opez,2 and C. M. Sotomayor-Torres1, 3 1Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Spain 2Materials Science Factory, Instituto de Ciencia de Materiales de Madrid (ICMM), c/Sor Juana In´esde la Cruz 3, 28049 Madrid, Spain 3ICREA - Instituci´oCatalana de Recerca i Estudis Avan¸cats,08010 Barcelona, Spain The regulation of temperature at the macro and mi- fect balance between the incoming and outgoing energy. This croscale is a major energy-consuming process of hu- energy exchange between the Earth, the Sun and the outer mankind. Modern cooling systems account for 15 % of space takes place through the Earth's atmosphere, enabled by the global energy consumption and are responsible for its radiative properties. To illustrate this, the atmospheric 10 % of greenhouse gas emissions. Due to global warm- transmission and the solar irradiance (1.5 A.M spectral refer- ing, a ten-fold growth in the demand of cooling tech- ence [5]) are plotted in Fig. 1a. It shows how the Sun's irra- nologies is expected in the next 30 years [1, 2], thus diance is filtered through the optical transmittance window of linking global warming and cooling needs through a the atmosphere, principally at visible and near-infrared wave- worrying negative feedback loop. Here, we propose an lengths (0:3 µm to 2:5 µm). This radiation is then absorbed by inexpensive solution to this global challenge based on the Earth's surface, rising its temperature. A portion of this a single-layer of silica microspheres self-assembled on a energy runs the climate's heat engine while the rest is radiated soda-lime glass substrate. This two-dimensional (2D) back to outer space as infrared thermal energy in the range be- crystal acts as a visibly translucent thermal blackbody tween 10 to 13 µm (IR window) in amounts proportional to T 4, for above-ambient daytime radiative cooling and can thus being a process very sensitive to the temperature. Inter- be used to passively improve the thermal performance estingly, the peak emission of a blackbody at average ambient of devices that undergo critical heating during oper- temperature (Ta= 300 K e.g the Earth) perfectly coincides with ation. The temperature of a crystalline silicon wafer this IR window. This opto-thermal mechanism keeps the tem- was found to be 14 K lower during daytime when cov- perature of our planet stable, making use of naturally occurring ered with our thermal emitter, reaching an average green non-toxic materials, such as sand, primarily made of sil- temperature difference of 19 K when the structure was ica (SiO2). In fact, the sand in deserts plays a major role in backed with a silver layer. In comparison, the soda- this process by reflecting sunlight and radiating thermal en- lime glass used as a reference in our measurements ergy to the outer cold space. The energy radiated is directly lowered the temperature of the silicon wafer by just proportional to its emissivity, which in turn depends strongly 5 K. The cooling power density of this rather sim- on the geometry, size and surface of sand grains. A comparison ple radiative cooler under direct sunlight was found of the emissivity of granular silica for two ranges of particle to be up to 350 W=m2 when applied to hot surfaces sizes [7] and bulk silica is shown in Fig. 1c. Bulk silica ex- with relative temperatures of 55 K above the ambi- hibits emissivity dips at around 8 µm and 20 µm, arising from ent. This is crucial to radiatively cool down electronic the interaction of optical phonons and electromagnetic radia- devices, such as solar cells, where an increase in tem- tion, so-called bulk phonon-polariton resonances, which result perature has drastic effects on performance. Our 2D in high reflectivity (low emissivity) spectral regions, something thermo-functional material includes a single layer of rather detrimental for radiative cooling. In contrast, granular spheres emitting long-wave radiation through the in- silica exhibits high broadband emissivity over the full infrared frared atmospheric window over a broad wavelength spectrum, which increases with decreasing particle size until it range, thus providing effective radiative cooling using nearly reaches unity for diameters below 75 µm. Such emissivity the outer space as a heat sink at 3 K. response closely resembles that of an ideal infrared blackbody (cyan continuous line in Fig.1b), revealing the enhanced radia- All modern cooling technologies are overwhelmingly energy- tive cooling properties of sand grains as compared to their bulk consuming. Air conditioners and refrigerators together account counterpart. for almost a quarter of the total electricity used in the United Earth's efficient temperature-regulation mechanism relies States [3]. Thus, it is not surprising that the latest Interna- on this cooling principle and its understanding opens up an tional Energy Agency report [2] identifies cooling as one of incredibly promising possibility of using microstructuring of the main drivers of global electricity demand over the coming polar materials to achieve energy-efficient heat removal. For decades, aggravated in particular by global warming. This re- instance, attempts to exploit this concept for terrestrial ther- sults in a negative feedback mechanism as cooling technologies mal management have been done in the past [9{12], yet it was already account for 15 % of the global electricity consumption not until very recently that passive cooling under direct sun- arXiv:1910.10594v2 [physics.app-ph] 30 Oct 2019 and 10 % of greenhouse gas emissions [1], figures predicted to light was achieved for a silicon absorber using a visibly trans- triple by 2050 [2]. In addition, the efficiency of non-fossil alter- parent broadband thermal blackbody [13]. Daytime cooling is natives such as solar photovoltaics [4] or nuclear power often challenging because even a small amount of visible light ab- relies on their operation temperature. Efficient heat removal sorption by the structure results in solar heat gain, which is in such systems will translate into increased performance and detrimental for radiative cooling. The daytime cooling solution global energy savings. Modern cooling approaches thus require proposed by Zhu et al. consisted of a periodic silica photonic a fundamental rethinking in terms of environmental and eco- structure fabricated by photo-lithography Since then, other de- nomical sustainability, to reduce their impact on the global signs including photo-lithography-based photonic microstruc- energy use. tured surfaces [14{17], polar mutilayered structures [14, 18], Radiative cooling is a passive thermodynamic mechanism coatings [19, 20] and polar-polymer metamaterials on metal through which the temperature of the Earth is kept stable [8]. mirrors have been proposed [21{24], achieving radiative cool- Under ordinary conditions (neglecting anthropogenic forcings) ing power densities higher than 90 W/m2. Very recently, using our planet is in radiative equilibrium, a state of nearly per- a glass-polymer hybrid 80 µm-thick structure composed of ran- 2 a) Optical window IR window 100 1.0 /nm) 80 2 60 Atmosphere Tramsmission 0.1 40 Solar Atmospheric 20 Solar irradiance Transmission (%) Transmission A.M. 1.5 0.01 0 Irradiance (W/m 1.0 b) 100 80 TerrestrialTerrestrial blackbody blackbody irradiance at 300K irradiance at 300K 0.1 60 40 Ideal broadbandIdeal broadband 300 K BB 20 emitteremitter 0.01 Emissivity (%) 0 -20 0.001 Irradiance (W/m2/nm) 0.5 1.0 2.0 4.0 8.0 16.0 Wavelength (µm) c) d) 100 Grained SiO 2 500 ) - 2 - + - + + 80 - - + 400 - + - + Broadband 60 + 300 emitter 40 0-75µm 200 Emissivity (%) 75-250µm 20 100 0 Cooling Power (W/m 0 4 8 12 16 20 24 -60 -40 -20 0 20 Bulk SiO2 ∆ Wavelength (µm) Ta-r (K) Figure 1: Ideal thermal infrared blackbody for radiative cooling. (a) The solar irradiance A.M 1.5 (reference spectrum at mid-latitudes with an air mass coefficient of 1.5) [5] and the atmosphere transmission calculated for Barcelona with MODTRAN [6] (b) Spectral response of an ideal broadband radiator (continuous cyan line), reflecting or transmitting visible and near-IR light up to 2:5 µm and re-emitting as a blackbody in the infrared spectral range. The terrestrial blackbody irradiance (at 300K) is also plotted for comparison. For the sake of clarity, the top color-bar relates the color-code used for the solar irradiance, the atmospheric transmittance and the terrestrial blackbody irradiance spectra to a schematic representation of the Earth's temperature-regulation mechanism: radiative cooling. The Sun's irradiance is filtered through the optical transmittance window of the atmosphere and is then absorbed by the Earth's surface, rising its temperature. A portion of this energy runs the climate's heat engine while the rest is radiated back to outer space as infrared thermal energy through the IR atmospheric window. (c) Emissivity spectra of bulk (black line) and granular (red and blue lines) silica, extracted from reflectivity data of the ASTER library for particle sizes from 75-250 µm and 0-75 µm, respectively.[7] (d) Net radiative cooling power density calculated for a perfect broadband radiator considering relative temperatures between the ambient, Ta, and the radiator surface, Tr, from -60 to 30 K. domly positioned silica spheres encapsulated by a transparent steady state operation temperature. We also measured the polymer, a 93 W=m2 cooling power density was achieved un- average radiative cooling power above-ambient temperature to der direct sunlight [21]. In this work, we present a radiative be between 160 W=m2 to 350 W=m2 for values of relative tem- cooler for above-ambient daytime passive cooling based on a peratures ∆Ta−r ranging from -25 to -50 K, with Tr being the self-assembled two-dimensional array of silica microspheres on operating temperature of the cooler.
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