Atmospheric Window Research Articles

All-fiber 2–6 [formula omitted] coherent supercontinuum source based on chalcogenide fibers pumped by an amplified mid-IR soliton laser

We numerically and experimentally investigate an all-fiber configuration of coherent mid-infrared supercontinuum generation in the femtosecond pumping regime. In particular, we demonstrate the advantageous combination of various dispersion-engineered chalcogenide (Ge-Se-Te glass) fibers with a turn-key amplified femtosecond fiber laser delivering 175-fs soliton pulses around 3.4 μm and developed from the erbium-doped fluoride fiber technology. Our results show that compact and coherent octave-spanning fiber supercontinuum sources centered on the 2–5 μm infrared atmospheric window can be obtained with a maximum average power of 50 mW at 25 MHz repetition rate.

Enhanced performance of diurnal radiative cooling for solar cells based on a grating-textured PDMS photonic structure

The extra heat generated by solar cells during the daytime will inevitably erode its efficiency and shorten working lifetime. In this work, a grating-textured polydimethylsiloxane (PDMS) photonic structure is proposed to lower the working temperature and enhance the efficiency through radiative cooling. A comprehensive study is carried out to enhance the infrared thermal radiation in the atmospheric window. It can effectively reflect the waste solar irradiance of 128.95 W/m2 at 300 K. In addition, the photonic cooler features angle and polarization-independent properties, which is desirable in the application of solar panels. As a case study of encapsulated solar cells, it can ideally realize a temperature drop of 11.47 K through thermal analysis, achieving an efficiency enhancement of 1.39%. The improvement in other types of solar cells is also carried out. Therefore, this cooler paves a new avenue to explore thermal management of solar energy harvesting and radiative vapor condenser systems.

A flexible PDMS@ZrO2 film for highly efficient passive radiative cooling

Passive daytime radiative cooling (PDRC) technology can make objects cooling down without energy consumption by radiating out heat through the atmospheric transparent window (ATW, 8–13 μm) and simultaneously reflect incident sunlight to avoid heated. In this work, a flexible radiative cooling film (FRCF) was fabricated by embedding zirconium dioxide (ZrO2) particles into polydimethylsiloxane (PDMS). It exhibited a high emissivity over 0.95 in ATW and a high reflectance beyond 0.92 in range of visible light. It achieved better cooling performances and yields an average temperature drop of 16.1℃ compared to the air around the film under the solar intensity of 735 W/m2. Because of its good cooling performance, great flexibility and acid resistance, this FRCF will have a wide application of radiative cooling in buildings and equipment.

Global Evaluation of the Fidelity of Clouds in the ECMWF Integrated Forecast System

AbstractWeather forecasting centers mainly assimilate infrared sounder data in clear‐conditions or in channels with their main sensitivity to the atmosphere well above the cloud tops. Sometimes channels with stronger cloud sensitivity are used in overcast conditions, but currently no cloud information is used from infrared sounders, and all‐sky assimilation approaches are still under development. However, cloudy radiances could already be used for validating the quality of clouds in forecasts. We illustrate this by comparing the brightness temperatures observed (obs) with AIRS (Atmospheric Infrared Sounder) to those calculated (cal) based on the clouds specified in the ECMWF (European Centre for Medium Range Weather Forecasting) Integrated Forecast System (IFS). Our analysis is based on a 12 hr ingest of AIRS data into the ECMWF assimilation system. We show that the standard deviation of (obs‐cal) using the 1,231 cm−1 atmospheric window channel is a metric of the fidelity of the clouds in the IFS. The global standard deviation of 5 K after accounting for likely space/time interpolation errors, appears to be dominated by clouds in the IFS which are not seen in the AIRS data, and vice versa. Our metric capitalizes on the unique sensitivity of infrared sounders to clouds for the routine monitoring of the fidelity of clouds in weather forecasts.

Dynamically Tunable Subambient Daytime Radiative Cooling Metafabric with Janus Wettability

AbstractIncorporating zero‐energy‐input cooling technology into personal thermal management (PTM) systems is a promising solution for preventing heat‐related illnesses while reducing energy consumption. Although concepts for passive radiative cooling materials are proposed, achieving subambient cooling performance while providing good wearing comfort remains a challenge. Here, a moisture‐wicking nonwoven metafabric is reported that assembles radiative cooling and evaporative heat dissipation to achieve high‐performance thermal and moisture comfort management. This metafabric demonstrates excellent spectral‐selectivity (sunlight reflection of ≈92%, atmospheric window thermal emissivity of ≈97%) and Janus wettability through large‐scale electrospinning and hierarchical design, and also inherits superior elasticity, air/moisture permeability of nonwoven fabric. Subambient temperature drops of ≈6.5 °C (≈750 W m−2solar intensity) for stand‐alone metafabric are observed. Thanks to the moisture‐wicking effect (water evaporation rate of 0.31 g h−1and water transport index of 1220%) of metafabric that enables fast evaporation of sweat, a maximum generation of 1 mL h−1of sweat can cool the skin, thus reducing the excessive sweating risk after intense exercise. Additionally, the cooling performance of metafabric can be regulated by applying various strains (0–100%). The cost‐efficiency and good wearability of metafabric provide an innovative way to sustainable energy, smart textiles, and thermal wet comfort applications.

Outdoor Thermal Performance of Photovoltaic Devices with Enhanced Daytime Radiative Cooling Glass

The increase in the operating temperature decreases the efficiency and the lifetime of photovoltaic (PV) systems. Radiative cooling effect offers a promising solution to passively reduce the operating temperature of PV modules using the atmospheric window (AW). Glass is a well‐known material used as front cover of PV modules. It presents a fairly good emissivity in the AW although a low‐absorption resonance at 9–10 μm decreases its overall emissivity, hampering its radiative cooling capacity. This effect can be reduced by tailoring the glass surface. Herein, glass samples are microstructured and characterized obtaining a decrease in reflectance and a substantial increase of the emissivity in the AW. Next, the glass samples are laminated with solar cells and installed outdoors without any convection barrier and monitored in open‐circuit configuration for almost half a year. The days are sorted by three meteorological conditions: sunny, partly cloudy, and cloudy days, which correspond to a decreasing availability of the AW. The average temperature difference between two samples is 0.8, 0.7, and 0.1 °C for the sunny, partly cloudy, and cloudy days, respectively. The maximum temperature difference obtained is 2.0 °C. No correlation between temperature difference and wind speed is observed.

Passive daytime radiative cooling with thermal energy storage using phase change n-octadecane/SiO2 nanobeads

In this study, a simple, facile, and high-performance passive daytime radiative cooling (PDRC) coating was developed by employing phase change n-octadecane/SiO2 (P–SiO2) nanobeads (NBs) for dual thermal management of both daytime radiative cooling and thermal heat energy storage. Monodisperse P–SiO2 NBs were synthesized via emulsion polymerization and were reversibly melted and crystalized at the phase change temperature of n-octadecane, which was used as a core phase change material (PCM). PDRC coating was fabricated by simple spray coating of P–SiO2 NBs/polymer solution on a glass substrate. Owing to the presence of the light-scattering air voids in the porous core-shell structure and the vibronic absorption of Si–O bonding in SiO2 NBs, the PDRC coating exhibited high reflectivity of 91.8% in the solar spectrum and high emissivity of 95.5% in the atmospheric window. We systematically investigated the influence of PCMs on thermal behavior by monitoring the PDRC coating under heating and cooling processes using infrared thermal imaging. We confirmed that the PCMs in the PDRC coating can effectively reduce the temperature of the coating by storing the thermal energy via the phase change process. During the daytime, the PDRC coating exhibited a temperature drop of 9.0 °C with average solar irradiation of 894 W/m2, which indicates that the designed PDRC coating is highly effective for radiative cooling.

Frequency-Selective Surface Based on Negative-Group-Delay Bismuth–Mica Medium

Negative group delay may be observed in dispersive media with anomalous dispersion in a certain frequency range. The fact that an outgoing wave packet precedes an incoming one does not violate the causality principle but is only a consequence of a waveform reshaping. This effect is observed in media such as photonic crystals, hyperbolic and epsilon-near-zero metamaterials, undersized waveguides, subwavelength apertures, side-by-side prisms, and resonant circuits at various frequencies. The current work is devoted to the design of a simple negative-group-delay medium with tunable properties in the THz frequency range. This medium consists of a bismuth-based frequency-selective surface on a dielectric substrate and may be tuned both statically and dynamically. While a geometry variation defines a main form of an effective permittivity dispersion and group delay/group velocity spectra, an external voltage allows one to adjust them with high precision. For the configuration proposed in this work, all frequency regions with noticeable change in group delay/group velocity lie within atmospheric transparency windows, which are to be used in 6G communications. This medium may be applied to THz photonics for a tunable phase-shift compensation, dispersion management in systems of THz signal modulation, and for encoding in next-generation wireless communication systems.

Scalable nanofibrous silk fibroin textile with excellent Mie scattering and high sweat evaporation ability for highly efficient passive personal thermal management

Passive personal thermal management based on radiative cooling textiles is emerging as a facile, cost-effective, and energy-efficient way for outdoor human body thermal comfort. Existing passive radiative cooling textiles are mainly built from synthetic polymers that are usually nonrenewable and non-degradable, and lack sweat management function. Herein, we report the fabrication of a biopolymer-based, nanofibrous textile that integrates passive radiative cooling and sweat transportation functions via a scalable electrospinning technique. The optimized control over the nanofibers’ diameter enables the 200 µm thick film with high reflectance in both the ultraviolet (UV) range (92%) and the entire solar spectrum (95%), and the abundant chemical bonds of silk fibroin allow a high emissivity of 95% in the atmospheric window. Consequently, the nanostructured textile can achieve a temperature of ∼3.8 °C below the ambient temperature during the daytime and ∼6.4 °C at night. More importantly, the use of hygroscopic silk fibroin offers additional functions of sweat absorption and evaporation. Outdoor sweat evaporation experiment demonstrates a temperature reduction of ∼5.5 °C for nanofibrous silk fibroin textile, compared with the traditional nonhygroscopic textile. Owing to its excellent combination of biodegradability, superior cooling ability, and high sweat transportation capacity, the scalable silk nanofabric would be an effective textile for passive personal thermal management.

Preparation of Flexible Wavelength-Selective Metasurface for Infrared Radiation Regulation

Perpetual advancements in modern detection techniques have augmented the requirement of infrared camouflage; however, its development is impeded by multiband compatible regulation and curved application targets. Here, a flexible wavelength-selective metasurface based on two metal-dielectric-metal resonators is experimentally demonstrated for infrared radiation regulation with thermal management utilizing magnetic polariton. Low emissivity in atmosphere windows (infrared stealth) and high emissivity in the wavelength of 5-8 μm nonatmospheric window (radiative cooling) are simultaneously achieved. In comparison with conventional hard substrates, it is for the first time the composite wavelength-length metasurface is successfully prepared directly on a flexible polyimide film via applying polyimide double-sided tapes and S1805/LOR5A bilayer stack lift-off technology. Not only does this method successfully overcome the debonding problem of photoresist on the flexible substrate, but it also solves the bulging problem of the substrate as well as the limitation of high temperature. Besides, the temperature and infrared radiation distributions of flexible wavelength-selective metasurfaces with different curvatures are first investigated. The compared results reveal that the metasurface with larger curvature has a better infrared camouflage performance. Furthermore, the cycle stability of the flexible metasurface is tested, and the results show that the infrared radiation regulation is stable after 30 cycles with essentially no change. This study provides a guideline for preparing flexible composite metasurfaces and avoids the trouble of replacing the metal/dielectric material of the initial structure with a flexible material to improve the structure for application to curved surfaces, thus broadening implications in enhancing the effective bonding of metasurfaces to target surfaces.

Significant Substrate Effects on Electromagnetic Scattering by Particles in the Infrared Atmospheric Window

Particle-dispersed coatings emerged as a promising approach to regulate the apparent radiative properties of underlying substrates in various applications, including but not limited to radiative cooling, thermal management, and infrared stealth. However, most research efforts in this field overlooked the dependent scattering mechanisms between the particles and the substrate, which can impact the optical properties of the particles. In this study, we explored the particle-substrate interactions within the atmospheric radiative window of 8–14 µm. Using the T-matrix method, we calculated the scattering and absorption efficiencies of a dielectric/metallic particle situated above a metallic/dielectric substrate, considering the different gap sizes. Near the small gaps (<0.5a with a the sphere radius), we found that the strong local fields induced by the interaction between the induced and image charges largely enhanced the absorption and scattering efficiencies of the particles. With the increasing gap sizes, the absorption and scattering efficiencies presented a significant oscillation with a period of about 4.5a, which was attributed to the interference (standing wave) between the scattered fields from the sphere and the reflected fields from the substrate. Our findings identify a crucial role of the particle–substrate interactions in the infrared properties of particles, which may guide a comprehensive insight on the apparent radiative properties of the particle composite coatings.

Scalable Bio-Skin-Inspired Radiative Cooling Metafabric for Breaking Trade-Off between Optical Properties and Application Requirements

Passive daytime radiative cooling (PDRC) provides a zero-energy cooling technology to reduce the global fossil energy consumption and has already attracted tremendous interest. However, breaking the trade-off between the pursuit of ultrahigh dual-band (solar and atmospheric window) optical properties and the compatibility of multiple functional requirements by application is still a big challenge for PDRC. By introducing the photon slab-porous effect with strong sunlight backward scattering and inspired by human skin (epidermis and dermis) with recorded medical infrared emittance and multi-functions, we proposed an efficient dual-band optical property design strategy for PDRC. Through a simple and scalable dip dyeing process, the fabricated bio-skin-inspired PDRC metafabric exhibited superior dual-band optical properties, while both the solar reflectance and atmospheric window emittance can reach 97%. Outdoor tests demonstrated that the bio-PDRC metafabric achieved a maximum sub-ambient temperature drop of 12.6 °C in daytime. A human wearing a hat made of bio-PDRC metafabric can be 16.6 °C cooler than the one wearing a common hat. The bio-PDRC metafabric also exhibited superior performance of breathability, waterproofness, flexibility, strength, and durability to fulfill the multiple demands of personal thermal management, vents, and car covers.

Thermal gradients integrated on-chip by passive radiative cooling of silicon nitride nanomechanical resonators

Radiative coupling of objects with outer space, through the atmospheric spectral transparency window, allows passive cooling below ambient temperature. Such passively created cold sinks can be used for converting ambient heat into other useful forms of energy. Using this approach, conversion of ambient heat to electricity has been demonstrated in macroscopic settings, where the radiatively cooled heat sink is a macroscopic object coupled with ambient heat through a heterogeneously attached, standalone, thermoelectric generator. In contrast, miniaturization and on-chip integration of such heat engines has not been demonstrated, although it could be useful in portable remote technological applications. Such integration would first require that both the radiatively cooled heat sink and the hot (ambient temperature) side of the engine are monolithically integrated on a single chip. This first step is the focus of our work: we demonstrate the creation of a local thermal gradient on-chip by radiative cooling of a 90 nm thick freestanding silicon nitride nanomechanical resonator integrated on a silicon substrate that remains at ambient temperature. The reduction in temperature of the thin film is inferred by tracking its mechanical resonance frequency, under high vacuum, using an optical fiber interferometer. Experiments were conducted on 15 different days during fall and summer months, under various solar illumination, resulting in successful radiative cooling of the membrane in each case. Maximum temperature drops of 9.3 K and 7.1 K are demonstrated during the day and night, respectively, in close correspondence with our heat transfer model. Future improvements to the experimental setup could improve the temperature reduction to 48 K for the same membrane, while emissivity engineering potentially yields a maximum theoretical cooling of 67 K with an ideal emitter.

Infrared Electrochromic Devices Based on Thin Metal Films

AbstractTunable emissivity technology is promising for the dynamic regulation of infrared radiation. Herein, infrared electrochromic devices based on thin metal films that operate via a novel hydrogen‐induced metal–insulator transition are demonstrated. The use of thin magnesium–nickel (MgxNi) alloy films as both a variable emissivity material and top conductive electrode simplifies the device structure and ensures that large changes in emissivity can be achieved. The constructed sandwich‐structured electrochromic devices also have polyethyleneimine (PEI) as a middle proton‐conducting electrolyte layer and hydrogen tungsten bronze (HxWO3)/indium tin oxide (ITO) as a bottom ion‐storage layer. Upon application of a voltage of ±2.6 V, the emissivity of the MgxNi/Pd/PEI/HxWO3/ITO device can be reversibly regulated, with emissivity changes of 0.48 and 0.43 in the 3–5 and 7.5–14 µm atmospheric windows, respectively. Under open‐circuit conditions, the high‐emissivity state of the device can be stably maintained for 3 h. The emissivity change is affected by the composition and thickness of the MgxNi film and the device failure mechanism involves the breakage and oxidation of this film after cycling. Corresponding flexible devices that exhibit electrochromism in the visible region have great potential for adaptive thermal camouflage, smart thermal management, and dynamic information displays.

Enhanced passive radiative cooling coating with Y2O3 for thermal management of building

Passive daytime radiative cooling (PDRC), a green cooling technology with energy-free consumption, has demonstrated great potential in solving global warming and other problems. Dielectric particles embedded in polymers are considered efficient radiative cooling strategies devoted to solving the problems of the complex preparation process and low cooling efficiency. However, the high ultraviolet ray (UV) absorption or low refractive index of dielectric materials results in their low solar scattering. Herein, we propose a simple and efficient method to prepare a radiative cooling coating consisting of yttrium oxide (Y2O3) and polydimethylsiloxane (PDMS). The PDMS/Y2O3 radiative cooling film was fabricated according to the theoretically optimized particle size, volume fraction, and coating thickness. The result indicated that the randomly distributed Y2O3 nanoparticles were employed in PDMS to enhance the scattering efficiency of sunlight. The solar reflectance and “atmospheric window” emissivity of the PDMS/Y2O3 film was about 94.52% and 93.4%. Outdoor cooling tests exhibited that the PDMS/Y2O3 hybrid film could achieve sub-ambient radiative cooling of 7.4 °C under solar irradiation of 1132 W/m2. In comparison, commercial white paint was 1.8 °C higher than ambient temperature. Consequently, the film is promising for large-scale applications in green buildings, cold chain logistics, photovoltaic fields, modern agricultural production, and other fields.

Transparent, anti-corrosion and high broadband emission coating for zero energy consumption cooling technology

Daytime radiative cooling as passive cooling technology with zero consumption of energy and zero emission of green gas, has recently attracted tremendous interest by reflecting sunlight and radiating heat to the ultracold outer space. Some progress has been made, while it still remains big challenge in transparent and radiation cooling double characteristics radiative coolers. Here, we propose a spectrally selective coating based on tricyclodecane dimethanol diacrylate (DCPDA) monomer. Acid- and basic-proof coating are fabricated with simple and scalable blade and spray coating methods at room temperature. The coated glass displays a transmissivity around 90% in visible wavelengths from 400 nm to 800 nm and a thermal emissivity above 95% over atmospheric window (8–13 μm). In clear summer in Yichang city with no wind, outdoor cooling tests demonstrated that realizes sub-ambient cooling of 18.6 °C during midday when applied on the aluminum (Al) sheets substrate. Featured with scalability, durability, and superior cooling performance make the film shows promising potentials for transparent radiative cooling applications.

Incorporation form-stable phase change material with passive radiative cooling emitter for thermal regulation

Passive radiative cooling (RC) can remove heat without any external energy. Nonetheless, it would bring redundant heating energy consumption under low temperature. In this work, dynamic radiative cooling material consisting selective super-hydrophobic radiative cooling emitter (SRCE) and phase change material (PCM) was proposed. PCM (dodecanol) was vacuum-impregnated into porous nickel foam (PCM@NF), and then stitched with SRCE (PCM@NF@SRCE). PCM@NF had the phase change temperature near the comfort temperature for human body (∼25 ℃) and large latent heat. Moreover, the SRCE has high solar reflectivity (0.93) and superb selective emission property (emissivity in atmospheric window: 0.83, emissivity outside atmospheric window: 0.49). Besides, the SRCE exhibited excellent super-hydrophobicity (162.2°), mechanical robustness and UV resistance. During outdoor measurement, the PCM@NF@SRCE achieved better sub-ambient cooling under hot weather while kept higher temperature in cold weather compared with bare SRCE. In addition, the energy saving simulation verified the energy saving performance of the PCM@NF@SRCE in both hot and cold weather. The integration of PCM and SRCE shows promising application in various climates.

Superhydrophobic nanoparticle mixture coating for highly efficient all-day radiative cooling

Radiative cooling without energy consumption is an ideal environmentally friendly cooling method. Despite the remarkable progress made so far, the majority of radiative cooling materials still suffer from high costs and vulnerability to outdoor contamination challenges. Especially in humid environments, dew attaches to the surface of radiators at night, leading to decreased performance. Here, we present a new radiative cooling structure that consists of a superhydrophobic SiO2 nanosphere top layer, a polyvinyl fluoride (PVF) middle layer, and a metallic Ag bottom layer. The SiO2 nanosphere layer not only induces efficient light scattering and helps to obtain ultrahigh solar reflectivity (94.2%) but also provides a low-surface-energy nanoscale textured surface for self-cleaning, with a maximum surface contact angle of 154°. Meanwhile, infrared phonon-polarization resonances, such as SiO2′s Si-O-Si bond and PVF’s -C-F group, result in a high average thermal emissivity (>90%) in the atmospheric transparency window (ATW, 8–13 μm). Outdoor experiments in humid Xiamen showed promising sub-ambient cooling of ∼ 3.3 °C for surface cooling and ∼ 2.4 °C for space cooling in direct sunlight. Additionally, the tilt experiment demonstrated that the cooling performance is further enhanced by a tilted radiator surface on highly humid nights. More importantly, the superhydrophobic self-cleaning properties of this coating prevented outdoor contamination or wetting of its surface, making it a promising option for building cooling.

A novel aqueous scalable eco-friendly paint for passive daytime radiative cooling in sub-tropical climates

Passive daytime radiative cooling (PDRC) is an emerging technology that dissipates heat from terrestrial objects to the cold universe directly via the atmospheric transparency window. Along with the advancement of PDRC materials, fabrication methods by precision lithography or phase change chemistry are no longer a preferred option due to the concomitant hazardous organic solvents. Instead, it is desirable to develop radiative coolers by aqueous processing-based and cost-effective methods so as to promote their mass production and applications. Herein, we report a new scalable polymer-based aqueous paint without volatile organic compounds (VOCs) emission. By using the simple mechanical agitation-based synthetic method with mixing polymer emulsion as the matrix and glass beads as the functional additive, the cooling paint exhibits an average infrared emissivity of 92% and a solar reflectivity of 94%. A detailed field experiment was conducted in sub-tropical climates to verify the practical cooling performance of the developed cooling paint, and the experimental results showed that the achievable sub-ambient cooling temperature was up to 6 °C under high humidity conditions without any solar shading or convection cover. Meanwhile, in-depth evaluation on the cooling power of the developed cooling paints in a variety of cities was conducted, which shows its wide application potential in building roof across varying climate regions.

Flexible Radiative Cooling Textiles Based on Composite Nanoporous Fibers for Personal Thermal Management.

Passive radiative cooling textiles can reflect sunlight and dissipate heat directly to the outside space without any energy input. However, radiative cooling textiles with high performance, large scalability, cost effectiveness, and high biodegradability are still uncommon. Herein, we exploit a porous fiber-based radiative cooling textile (PRCT) via nonsolvent-induced phase separation and scalable roll-to-roll electrospinning technology. Nanopores are introduced into single fibers, and the pore size can be accurately optimized by managing the relative humidity of the spinning environment. The anti-ultraviolet radiation and superhydrophobicity of textiles were improved by the introduction of core-shell silica microspheres. An optimized PRCT yields a strong solar reflectivity of 98.8% and atmospheric window emissivity of 97%, which results in a sub-ambient temperature drop of 4.5 °C, with the solar intensity over 960 W·m-2 and 5.5 °C at night. For personal thermal management, it is demonstrated that the PRCT can obtain a temperature drop of 7.1 °C compared to the bare skin under direct sunlight. Given the excellent optical and cooling properties, flexibility, and self-cleaning property, PRCT was demonstrated to be a potential candidate for commercial applications in multifarious complex scenarios to afford a style for global decarbonization.