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Radiative cooling without energy consumption and environmental pollution holds great promise as the next-generation cooling technology. To date, daytime radiative cooling performance is still slightly low, especially in humid areas. In this work, we demonstrated that nanoporous polyethylene (Nano PE) film can improve solar reflectivity from 96% to 99%, thus boosting radiative cooling performance. Moreover, the experimental results in humid areas indicate that Nano PE films can improve radiative cooling performance by ∼76% in a clear day and 120% in a day with few clouds. Additionally, compared with ordinary PE films, thin Nano PE films have significantly higher weather fastness and mechanical strength. More importantly, nano PE films can scatter part of visible light, thus suppressing the generation of light pollution in practical applications. Lastly, the modeling results reveal that with Nano PE films, more than 95% of China's areas can achieve daytime cooling performance. Our work can boost the development of radiative cooling technology with a very low cost.

Global warming has become one of the major environmental problems facing mankind in the 21st century. The existing refrigeration technology of buildings, like air conditioning, consumes a lot of energy. Passive daytime radiative cooling technology works without consuming energy, nor emitting carbon dioxide and other greenhouse gases. This review summarizes the development of daytime passive radiative cooling technology from the basic principles, structure and materials of radiative coolers; analyses and evaluates the various existing radiative coolers. The core of radiative cooling lies in the combination of multi-scale micro/nano structures. The cooler reflects sunlight thus preventing the building from being heated up; while allows the building to radiate its own heat out thus being cooled down; meanwhile maintains the temperature difference by the heat insulation effect of the porous structure in the film. The common challenges and potential solutions for the commercialization of radiative cooling technologies are analyzed, which may promote the applications of the technology in the near future.

Decreasing energy consumption is critical to sustainable development. Because temperature regulation for human comfort consumes vast amounts of energy, substantial research efforts are currently directed towards developing passive personal thermal management techniques that cool the human body without any energy consumption1–9. Although various cooling textile designs have been proposed previously, textile-based daytime radiative cooling to a temperature below ambient has not been realized6–13. Silk, a natural protein fabric produced by moth caterpillars, is famous for its shimmering appearance and its cooling and comforting sensation on skin14–17. It has been recently recognized that silk, with its optical properties derived from its hierarchical microstructure, may represent a promising starting point for exploring daytime radiative cooling18–21. However, the intrinsic absorption of protein in the ultraviolet region prevents natural silk from achieving net cooling under sunlight. Here we explore the nanoprocessing of silk through a molecular bonding design and scalable coupling reagent-assisted dip-coating method, and demonstrate that nanoprocessed silk can achieve subambient daytime radiative cooling. Under direct sunlight (peak solar irradiance >900 W m–2) we observed a temperature of ~3.5 °C below ambient (for an ambient temperature of ~35 °C) for stand-alone nanoprocessed silks. We also observed a temperature reduction of 8 °C for a simulated skin when coated with nanoprocessed silk, compared with natural silk. This subambient daytime radiative cooling of nanoprocessed silk was achieved without compromising its wearability and comfort. This strategy of tailoring natural fabrics through scalable nanoprocessing techniques opens up new pathways to realizing thermoregulatory materials and provides an innovative way to sustainable energy. Processing silk through a molecular bonding design and scalable coupling reagent-assisted dip-coating method can lead to subambient daytime radiative cooling.

Daylighting structures, including solar cells and building windows, utilize sunlight whilst suffering from undesired solar heat and outdoor dust contamination. A radiative cooling system that is transparent to sunlight and has a superhydrophobic surface would cool and clean the daylighting structures in a sustainable manner. However, the majority of the current daytime radiative cooling systems were designed to fully reflect the incident sunlight to maximize the cooling power. In this work, we optimized both the sunlight transmission and infrared thermal irradiation by modeling the size-dependent scattering and absorption of light by SiO2 spheres embedded in a polymer matrix, we found that the use of nanospheres (20 nm) enabled both high sunlight transmittance (> 90%) and infrared emissivity (~0.85). This theoretical prediction was confirmed by experimental measurements of a solution-processed nanocomposite film. When coated on a solar cell, the as-prepared film not only preserved the power conversion efficiency of the cell (14.71%, uncoated cell has an efficiency of 14.79%) but also radiatively cooled the cell by up to 5 °C under direct sunlight. This reduction of the operating temperature of the solar cell further enhanced its electrical power output, evidenced by an increase in the equilibrium temperature of the LED load by about 14 °C. The nanoscale textured surface formed by the nanospheres further led to superhydrophobicity and thus excellent self-cleaning performance (efficient removal of dust by wind and/or water droplets).

Application of the passive approach as a non-mechanical method is an effective technique to tackle high energy consumption as well as diminishing the destructive effects of buildings on the environment. Using passive cooling can lead to peak load reduction as well as peak load offset, diminishing the interior temperature fluctuation, maintaining indoor air temperature in a comfortable range which consequently reduces fossil fuel usage as well as decrease the greenhouse gas emission. In this study, using passive approach from the economic point of view was discussed. The competition between cost saving (owing to applying passive methods) and added cost (owing to the initial investment, maintenance and operation cost) can specify the usefulness and effectiveness of using passive techniques. In this regard, special points such as building function, ventilation requirements, sensible and latent heat gains, the ability of any technique to save energy, geographical location, user financial ability, initial investment, maintenance and operation cost, regional climate data should be taken into account. Literature review affirmed that owing to applying passive approaches in building, energy consumption is diminished by 8%–70% (using external shading), 37% (utilizing cool colored paints roof), 25% (Creating green space), 7.88% (construction of the prismatic building), 32%–100% (using vegetation-based wall), 50% (using PCM-base wall), 33% (incorporation of insulation into the wall), 10%–20% (building equipped with solar chimney), 25% (using radiative cooling system). From the economic viewpoint, it was found that through finding optimal passive approaches, life-cycle cost saving reached 52%.

Radiative cooling in textiles is one of the important factors enabling cooling of the human body for thermal comfort. In particular, under an intense sunlight environment such as that experienced with outdoor exercise and sports activities, high near-infrared (NIR) reflectance to block sunlight energy influx along with high IR transmittance in textiles for substantial thermal emission from the human body would be highly desirable. This investigation demonstrates that a nanoscale geometric control of textile structure alone, instead of complicated introduction of specialty polymer materials and composites, can enable such desirable NIR and IR optical properties in textiles. A diameter-dependent Mie scattering event in fibers and associated optical and thermal behavior were simulated in relation to a nonwoven, nanomesh textile. As an example, a nanomesh structure made of PVDF (polyvinylidene fluoride) electrospun fibers with ∼600 nm average diameter was examined, which exhibited a significant radiative cooling performance with over 90% solar and NIR reflectance to profoundly block the sunlight energy influx as well as ∼50% IR transmittance for human body radiative heat dissipation. An extraordinary cooling effect, as much as 12 °C, was obtained on a simulated skin compared to the normal textile fabric materials. Such a powerful radiative cooling performance together with IR transmitting capability by the nanomesh textile offers a way to efficiently manage sunlight radiation energy to make persons, devices, and transport vehicles cooler and help to save energy in an outdoor sunlight environment as well as indoor conditions.

Radiative cooling is an electricity-free method for solar cells to dissipate waste heat, which has drawn much attention in recent years. In this paper, a periodic pyramid-textured polydimethylsiloxane (PDMS) film is proposed for radiative cooling of encapsulated commercial silicon solar cells. Spectral tests show that the proposed pyramid-textured PDMS film emits like a blackbody and simultaneously has an average transmittance of >0.9 in the silicon absorption spectrum from 0.3 µm to 1.1 µm. Outdoor cooling tests demonstrate that the encapsulated commercial silicon solar cells with the proposed textured PDMS film can be cooled by over 2 °C. Due to its good optical and flexible properties, the textured PDMS film has great potential for applications in the photovoltaic industry.

The temperature of outdoor structures, such as automobiles, buildings, and clothing, can be tuned by designing photonic properties. However, particular challenges arise when considering the temperature of an object itself rather than the enclosure in these outdoor structures. We present a double-side photonic thermal (DSPT) system. In the DSPT system, the tunable range of photonic thermal load for heating and cooling functions is calculated by designing the absorption spectra of both sides to adapt to different temperature conditions. These include the proper photonic design of not only the side facing outward but also the inner side and more complex temperature conditions of the object, enclosures, and atmosphere. According to the DSPT mechanisms, we developed a Janus material that can achieve the opposite functions (cooling and heating) with one film by simply flipping the sides of the Janus material, which does not require any additional energy input. The Janus material is designed and fabricated by common materials and a simple multilayer structure, which is attractive for large-scale fabrication. The thermal experiment proved the Janus multilayer could achieve a high temperature in the heating mode and a low temperature in the cooling mode, and the range of the tunable temperature would be wider with stronger sun radiation. The Janus material can passively achieve more efficient temperature control in enclosures while offering both side photonic design comparable to conventional radiative coolers and heaters.

As a passive, efficient, and renewable powerless cooling method, radiation cooling has received widespread attention in the field of energy conservation. Radiators with high reflectivity in the solar radiation band can achieve daytime radiation cooling, which can better meet people’s needs. In this work, zeolites were used as a functional material for radiative cooling due to its high solar reflectivity, high mid-infrared emissivity, and low heat conductivity. The influence of zeolite spectral properties and thermal insulation performance on their cooling effect were systematically studied. The cooling test showed that the ZSM-5-470 zeolite-based coating achieved a coating temperature (T) decrease of ~ 21 °C compared to a pure aluminum plate and a decrease of ~ 4 °C compared to a TiO2 coating under direct sunlight. However, the inside temperature (Tin) of ZSM-5-470 coating reached at most ~ 3.2 °C lower than TiO2 coating. This may suggest that the spectral properties of zeolites have more obvious effect on the daytime radiative cooling than their thermal conductivities. Zeolites are convenient to manufacture for mass production, which makes them the potential daytime cooling materials to achieve economic and environmental friendly cooling.

Radiative cooling passively removes heat from objects via emission of thermal radiation to cold space. Suitable radiative cooling materials absorb infrared light while they avoid solar heating by either reflecting or transmitting solar radiation, depending on the application. Here, we demonstrate a reflective radiative cooler and a transparent radiative cooler solely based on cellulose derivatives manufactured via electrospinning and casting, respectively. By modifying the microstructure of cellulose materials, we control the solar light interaction from highly reflective (> 90%, porous structure) to highly transparent (approximate to 90%, homogenous structure). Both cellulose materials show high thermal emissivity and minimal solar absorption, making them suitable for daytime radiative cooling. Used as coatings on silicon samples exposed to sun light at daytime, the reflective and transparent cellulose coolers could passively reduce sample temperatures by up to 15 degrees C and 5 degrees C, respectively. Funding Agencies|Linkoping University

Daytime radiative cooling materials reflect solar light and dissipate heat directly to outer space without any energy consumption, and thus, have attracted much attention due to the potential applications in many fields. Recently, elaborately designed photonic crystal and metamaterials have been reported for daytime subambient radiative cooling. However, such materials and structures have the drawbacks of complex shapes, inflexibility, high cost, and limitation in scaling up. It is also extremely difficult to apply such materials to buildings, vehicles, and other objects having complex surfaces. Here, a scalable and flexible hybrid film for daytime subambient radiative cooling was fabricated by a facile electrospinning method. The hybrid film consists of poly(vinylidene fluoride)/alumina (PVDF/Al2O3) fibers with diameters of 0.5-2.5 μm. Owing to the efficient scattering by fibers and Al2O3 nanoparticles, the hybrid film exhibits an extremely high average solar reflectance of 0.97. A high average atmospheric window emittance of 0.95 is simultaneously achieved due to the molecular vibrations of PVDF and the phonon polariton resonance of Al2O3 nanoparticles. The composite film delivers an average net radiative cooling power of 82.7 W/m2, and a temperature drop of up to 4.0 °C under direct sunlight. The hybrid film exhibits remarkable radiative cooling performance under different weather conditions including sunny, cloudy, overcast, and rainy. It can be used not only for cooling buildings and vehicles but also for delaying the melting of glaciers. This work demonstrates a promising method for scale-up production of the radiative cooling film with high performance.

Daytime passive radiative cooling is a promising electricity-free pathway for cooling terrestrial buildings. Current research interest in this cooling strategy mainly lies in tailoring the optical spectra of materials for strong thermal emission and high solar reflection. However, environmental heat gain poses a crucial challenge to building cooling at subambient temperatures. Herein, we devise a scalable thermal insulating cooler (TIC) consisting of hierarchically hollow microfibers as the building envelope that simultaneously achieves passive daytime radiative cooling and thermal insulation to reduce environmental heat gain. The TIC demonstrates efficient solar reflection (94%) and long-wave infrared emission (94%), yielding a temperature drop of about 9 °C under sunlight of 900 W/m2. Notably, the thermal conductivity of the TIC is lower than that of air, thus preventing heat flow from external environments to indoor space in the summer, an additional benefit that does not sacrifice the radiative cooling performance. A building energy simulation shows that 48.5% of cooling energy could be saved if the TIC is widely deployed in China.

Radiative cooling to subambient temperatures can be efficiently achieved through spectrally selective emission, which until now has only been realized by using complex nanoengineered structures. Here, a simple dip-coated planar polymer emitter derived from polysilazane, which exhibits strong selective emissivity in the atmospheric transparency window of 8-13 μm, is demonstrated. The 5 μm thin silicon oxycarbonitride coating has an emissivity of 0.86 in this spectral range because of alignment of the frequencies of bond vibrations arising from the polymer. Furthermore, atmospheric heat absorption is suppressed due to its low emissivity outside the atmospheric transparency window. The reported structure with the highly transparent polymer and underlying silver mirror reflects 97% of the incoming solar irradiation. A temperature reduction of 6.8 °C below ambient temperature was achieved by the structure under direct sunlight, yielding a cooling power of 93.7 W m-2. The structural simplicity, durability, easy applicability, and high selectivity make polysilazane a unique emitter for efficient prospective passive daytime radiative cooling structures.

Since the breakthrough of daytime radiative sky cooling, more and more effort has been devoted to this promising technology. But the low cooling performance places great restrictions on the application of this technology. In this work, various strategies are used to further improve the radiative cooling performance. The theoretical analysis reveals that the parameters corresponding to different strategies have great impact on the cooling performance. The experiments with individual strategy indicate that nano polyethylene wind cover strategy can achieve daytime temperature drop of about 5.6 K, improving by about 143.4% compared to that without wind cover (2.3 K). And ordinary polyethylene wind cover strategy and tilt strategy can also achieve daytime temperature drop of about 3.2 K and 4.3 K, improving by about 78.3% and 39.1%, respectively. But inverted pyramid strategy reduces daytime cooling performance by about 33%, due to the higher solar intensity on the radiative cooler. Subsequently, experiments with combined strategy show that the combination of nano polyethylene wind cover strategy and tilt strategy can enable radiative cooler achieve the higher all-day cooling performance, comparable to the results in arid areas. Further discussion on other strategies indicates that polyethylene aerogel has great potential in further improving the cooling performance.

Space cooling and heating consume a large proportion of global energy, so passive thermal management materials (i.e., without energy input), especially dual-mode materials including cooling and heating bifunctions, are becoming more and more attractive in many areas. Herein, a function-switchable Janus membrane between cooling and heating consisting of a multilayer structure of polyvinylidene fluoride nanofiber/zinc oxide nanosheet/carbon nanotube/Ag nanowire/polydimethylsiloxane was fabricated for comprehensive thermal management applications. In the cooling mode, the high thermal radiation emissivity (89.2%) and sunlight reflectivity (90.6%) of the Janus membrane resulted in huge temperature drops of 8.2-12.6, 9.0-14.0, and 10.9 °C for a substrate, a closed space, and a semiclosed space, respectively. When switching to the heating mode, temperature rises of 3.8-4.6, 4.0-4.8, and 12.5 °C for the substrate, closed space, and semiclosed space, respectively, were achieved owing to the high thermal radiation reflectivity (89.5%) and sunlight absorptivity (74.1%) of the membrane. Besides, the Janus membrane has outstanding comprehensive properties of the membrane, including infrared camouflaging/disguising, electromagnetic shielding (53.1 dB), solvent tolerance, waterproof properties, and high flexibility, which endow the membrane with promising application prospects.

A transparent radiative cooling (T-RC) film with low transmittance in solar spectra and selectively high emissivity in the atmospheric window (8–13 μm) is applied on roof glazing for building energy saving. To evaluate the performance of the T-RC film, two identical model boxes (1.0 m × 0.6 m × 1.2 m, L × W × H) were constructed and the inside air temperatures were measured in August in Ningbo, China. Results show that the maximum temperature difference between the two model boxes with and without the T-RC film was 21.6 °C during the experiment. A whole building model was built in EnergyPlus for the model box. With a good agreement achieved between the calculation results and the measured temperature data, the experimentally validated EnergyPlus model was then extended to an 815.1 m2 exhibition building with roof glazing to analyze the annual air conditioning (AC) energy consumption. The results show that by incorporating both the T-RC film's cooling benefit in summer and heating penalty in winter, the annual AC energy consumption of the exhibition building can be reduced by 40.9–63.4%, varying with different climate conditions.

Specific control on the mid-infrared (mid-IR) emission properties is attracting increasing attention for thermal camouflage and passive cooling applications. Metal-dielectric-metal (MDM) structures are well known to support strong magnetic polariton resonances in the optical and near-infrared range. We extend the current understanding of such an MDM structure by specifically designing Au disc arrays on top of ZnS-Au-Si substrates and pushing their resonances to the mid-IR regime. Therefore, we combine fabrication via lift-off photolithography with the finite element method and an inductance-capacitance model. With this combination of techniques, we demonstrate that the magnetic polariton resonance of the first order strongly depends on the individual disc diameter. Furthermore, the fabrication of multiple discs within one unit cell allows a linear combination of the fundamental resonances to conceive broadband absorptance. Quite importantly, even in mixed resonator cases, the absorptance spectra can be fully described by a superposition of the individual disc properties. Our contribution provides rational guidance to deterministically design mid-IR emitting materials with specific narrow- or broadband properties.

Advanced materials with tailored thermal emissivity underpin the performance of many technologies, including thermal detection, spacecraft components, and camouflage platforms. The thermal emissivity of materials can be modified through surface treatment such as coating and surface nanostructure. Dynamically controlling the thermal radiation of objects is of great importance, which endows materials with smart functions integrated with sensing, reacting, and even adapting and with potential for practical applications such as personal thermal management, infrared camouflage, radiative cooling, wireless communication technology, etc. This article reviews the working principles, material systems under various stimulation, and applications based on the recent progress in dynamic infrared regulation, including temperature, humidity, electricity, mechanical strain, and the above stimuli-triggered reconfigurable systems. Phase transition, ion or electron intercalation, refractive index variation, and topographic change can lead to energy level variation or change in the path of light, resulting in dynamic thermal emissivity. It is expected that new materials such as graphene and various nanostructures such as cavities and gratings have been developed rapidly in recent years, giving the possibility of low-energy-input control of flexible dynamic emissivity materials.

Passive radiative cooling is a promising direction in energy conservation and environmental protection. One of the ways to increase the efficiency of radiative cooling systems is the use of multi-layer coatings. In recent years, several novel materials with high emissivity have been proposed, which allow the creation of radiators that provide an average daily cooling power of approximately 100 W/m2 during daytime. Based on the developed mathematical model, the optical properties of the multi-layer coating for the radiative cooling system were evaluated by the Transfer Matrix Method and the effectiveness of radiative cooling was determined due to the use of the multi-layer coating in the climatic conditions of Syria (Latakia). The results of modeling the atmospheric transmittance in the summer months in Syria (Latakia) are presented. The developed mathematical model, methods for modeling atmospheric transmittance, calculating solar radiation, and evaluating the optical properties of multi-layer coating, allow determining an effective multi-layer coating for radiative cooling systems in any climatic conditions

Photovoltaics has played a significant and increasingly important role in renewable energy harvesting. However, it only works during the daytime when the sun is accessible. In this paper, we propos...

Summary Radiative cooling can reject significantly more waste heat than convection and conduction at high temperatures by sending it directly into space. As a passive and compact cooling mechanism, radiative cooling is lightweight and does not consume energy. These qualities are promising for thermal management in outdoor systems generating low grade heat, such as concentrating photovoltaics (CPV) and thermophotovoltaics (TPV). In this work, we first simulate radiative cooling for a wide range of working conditions, including heat loads from 6 to 100 W with different CPV cooling designs. We then demonstrate a CPV system integrated with radiative coolers, achieving a 5°C to 36°C temperature drop and an 8% to 27% relative increase of open-circuit voltage for a GaSb solar cell, under a heat load of above 6 W with different cooling designs. We show that the temperature drops from radiative cooling may significantly improve CPV system lifetimes.

Electricity demand leveling has actively been pursued in order to reduce power supply costs and reduce carbon dioxide emissions. In addition, large-scale customers have attracted great interest because of the expectation of contract power reduction. Focusing on the air conditioning equipment whose seasonal factors greatly change their demand, the peak cut operation has been considered for customers who have a large proportion of the load on the air conditioning equipment. The thermal radiative cooling/heating system is applied as the air conditioning equipment, and power control is performed according to the electricity demand of the customer. The comfort is evaluated by simulating the change in room temperature at that time. As a result, it is shown that the proposed demand response method can perform peak cut operation while ensuring comfort.

The test results of the prototype of the secondary-emission probe operating in the mode of self-oscillatory sweep intended for diagnosing nonequilibrium plasma in the presence of epithermal group of electrons are given. Earlier, the method of self-oscillatory probe was tested using an aluminum water-cooled contact surface with formation of a nanoscale thick high-emission oxide coating. The structure of the described probe is peculiar owing to application of refractory tantalum, which is capable of working in the mode of radiation cooling, which simplifies the structure and increases the reliability of the probe. It is shown that, owing to secondary-emission characteristics, oxide-coated tantalum allows under plasma loading obtaining a pronounced N-type current–voltage characteristic with a large excess in the electron-emission current component against the ion saturation current and satisfies all requirements for realization of the method of self-oscillatory probe.

In this study, multifunctional light-emitting and passive radiative cooling (LEPC) materials and devices are designed by embedding chemically designed perovskite nanocrystals (NCs) into the radiative polymer layer. Lead halide perovskite NCs are chosen as the light-emitting material, owing to their high photon radiation rate and low phonon generation. To integrate the perovskite NCs into the radiative polymer layers, a surface passivation is achieved by coating the NCs with silica. The silica shell synergistically improves the chemical stability and cooling efficiency. Both outdoor experimental and simulation results demonstrate that the fabricated LEPC devices show better cooling performance than conventional cooling devices. The LEPC devices are easily patterned by utilizing pixelating, assembling, and simple cutting or drawing techniques with the LEPC materials. This study also demonstrates the potential applications of these materials as components of smart building systems, in smart window displays, or for anticounterfeiting cooling systems, thus proving the practicality of these multifunctional LEPC devices.

Discussions of countering global warming with solar geoengineering assume that warming owing to rising greenhouse-gas concentrations can be compensated by artificially reducing the amount of sunlight Earth absorbs. However, solar geoengineering may not be fail-safe to prevent global warming because CO(2) can directly affect cloud cover: It reduces cloud cover by modulating the longwave radiative cooling within the atmosphere. This effect is not mitigated by solar geoengineering. Here, we use idealized high-resolution simulations of clouds to show that, even under a sustained solar geoengineering scenario with initially only modest warming, subtropical stratocumulus clouds gradually thin and may eventually break up into scattered cumulus clouds, at concentrations exceeding 1,700 parts per million (ppm). Because stratocumulus clouds cover large swaths of subtropical oceans and cool Earth by reflecting incident sunlight, their loss would trigger strong (about 5 K) global warming. Thus, the results highlight that, at least in this extreme and idealized scenario, solar geoengineering may not suffice to counter greenhouse-gas-driven global warming.

Daytime radiative cooling devices consisting of various materials are used to transmit and radiate heat energy from a target body outward through the atmosphere and into cold outer space, without using external energy and in the presence of sunlight. In this study, a daytime radiative cooling thin film structure is designed to achieve high emission in the atmospheric transparency window (8–13 µm) and a high reflection of solar radiation. Simple and chemically stable inorganic materials of silicon dioxide, silicon nitride, aluminum oxide, and silver are selected for the initial design of the structure, initially having four layers. The needle optimization technique is used to enhance the initial design to acquire a final structural design of 10 layers of the selected materials. The average emissivity of the cooler in the range of 8–13 µm was numerically found to be 94.5% and the average reflectance for the solar spectrum was determined to be 96.3% according to validated calculations. Lastly, a high net cooling performance of 125 W/m2 is theoretically generated by the structure when it is equal to the ambient temperature and a temperature reduction of 8 °C is achieved from the ambient temperature according to validated calculations.

Optimized thermal emitters using optical resonances have been attracting increased attention for diverse applications, such as infrared (IR) sensing, thermal imaging, gas sensing, thermophotovoltaics, thermal camouflage, and radiative cooling. Depending on the applications, the recently developed IR devices have been tailored to achieve not only spectrally engineered emission but also spatially resolved emission using various nanometallic structures, metamaterials, and multistacking layers, which accompany high structural complexity and prohibitive production cost. Herein, this article presents a simple and affordable approach to obtain spatially and spectrally selective hybrid thermal emitters (HTEs) based on spoof surface plasmons of microscaled Ag grooves manifested in encapsulation polymer layers. Theoretical analyses found that the polymer hybrid plasmonics allows diverse emission tuning within the long-wave IR (LWIR; 8-14 μm) region as follows: (1) spatially selective emission peaks only exist in the interface of Ag grooves and IR-transparent layers and (2) near-unity spectrally selective emission is obtained by refining inherent emissivity of a thin IR-opaque layer. Also, parametric studies computationally optimized the structural parameters for spatially and spectrally selective HTEs. Using the optimized parameters, the authors fabricated two HTEs and demonstrated the intriguing emission features in terms of infrared data encoding/decoding and radiative cooling, respectively. These successful demonstrations open up the applicability of HTEs for tailoring IR emission in a spatially and spectrally selective manner.

Solar-driven interfacial evaporation has received increasing research attention for clean water generation, but freshwater harvesting often occurs only during daytime. Herein, we report a strategy for all-day freshwater harvesting through combining solar-driven interfacial desalination and dew water generation. Through spray-coating carbon nanotubes onto a flexible substrate that has high-infrared emittance in the atmosphere window, a dual functional film was prepared to efficiently absorb solar energy for desalination at daytime and passively cool down the surface temperature to collect dew water at night. By integrating the dual functional film within a three-stage membrane distillation device, the home-made system achieved a drinkable freshwater collection efficiency of 71.1% when desalinating seawater under 1 sun illumination and achieved a dew water collection rate of 0.1 L m-2 day-1 at night. The readily available low-cost raw materials, simple fabrication process of the dual functional films, and the resultant all-day freshwater collection systems make the combined solar-thermal interfacial desalination and dew water collection a promising solution to alleviate the freshwater shortage in many underdeveloped regions, arid areas, and islands.

Passive daytime radiative cooling represents one of the boldest answers to tackle the future cooling needs of the built environment and to mitigate urban heat island effects. Recent developments in the field targeted sub-ambience with several successful examples. On the other side, heating demands may get exacerbated unless effective countermeasures against overcooling are identified, especially in wintertime or heating-dominated climates. This review aims at collecting state-of-the-art technologies and techniques to dynamically control the heat transfer to and from the radiative emitter and ultimately modulate its cooling capacity. Potential solutions are selected from different applicative fields, including spacecraft thermal control, thermal camouflage and electronics. Environmentally-responsive solutions are analyzed in depth given their perfect match with radiative cooling design requirements. Among them, VO2-tuned Fabry-Perot resonators are given particular emphasis, owing to their proven applicability. Active solutions are presented for completeness, but in less detail. Underlying principles, structural composition and experimental/simulated results are detailed and discussed to identify prominent pathways towards technically and economically effective integration in the built environment.

Radiative coolers spectrally tailored at mid-infrared wavelengths (5–30 μm) achieve passive and efficient heat dissipation, thereby working together with existing conduction and convection channels...

Effective daytime radiative cooling materials with high solar reflectance and strong infrared emittance can greatly reduce the cooling demand of buildings without any energy consumption, which is a...

Radiative coolers that can passively cool objects by radiating heat into the outer space have recently received much attention. However, the ultimate limits of their performance as well as their ideal spectral design are still unknown. We present the fundamental lower bound of the temperature of a radiatively cooled object on earth surfaces under general conditions, including non-radiative heat transfer, and the upper bound of the net radiative power density of a radiative cooler as a function of temperature. We derive the ideal spectral emissivities that can realize such bounds and, contrary to common belief, find that the ideal emission window is different from 8 to 13 um and forms disjointed sets of wavelengths, whose width diminishes at lower temperatures. We show that ideal radiative coolers with perfect thermal insulation against conduction and convection have a steady-state temperature of 243.6 K in summer and 180.5 K in winter, much below previously measured values. We also provide the ideal emission window for a single-band emitter and show that this window should be much narrower than that of previous designs if the objective is to build a radiative freezer that can operate in summer. We provide a general guideline for designing spectral emissivity to achieve the maximum temperature drop or the maximum net radiative power density.

International audience; Field assisted sintering (FAST) processesallow a direct transmission of the heating energy to the specimen (through the electric, magnetic fields or the electrical current). FAST allows higher heating rates, faster sintering response and a better control of the sintered microstructures. However, FAST suffers high heating instability in direct heating configurations which generally takes the form of a hot spot. The origin of these hot spots is well known and is correlatedto the convective/radiative cooling at the specimen surfaces and the thermal dissipation in the specimen. Nevertheless, the impact of these cooling fluxes evolves with the sample dimensions, thermal insulation, heating rate and hybrid heating conditions and there is not clear quantification of the relative importance of these fluxes in regards totheprevious citedheating conditions.In this work we developafinite element (FE) tool whichcan easily explore the heating stability of an “Equivalent ThermalCavity” (ETC). We illustrate the ETCconceptby the case study of the microwave sintering of zirconia.We show that the dominant heat transfer is radiative, but the convective fluxes have a high importance for the temperatures homogenization, in particular in the case of a hybrid heating configuration.

Recent development in spectral selective metamaterials makes daytime radiative cooling possible by engineering the material surface to achieve high reflectance in the solar irradiation waveband (0.3–2.5 μm) but high emissivity in the 8–13 μm atmospheric window. In this paper, a spectral selective polymer-based metamaterial was applied on top of a shallow aluminum box that contained with an enclosed body of water for daytime radiative cooling. The performance of such a system was compared to a similar bare Aluminum alloys plate; and a similar one with an Aluminum foil covered plate. The water temperatures were continuously measured for the three cases in a 72-hour period. The results showed that the metamaterial can effectively achieve daytime radiative cooling effect and maintain a consistent water temperature below the ambient air temperature for 2–14 °C. In contrast, the water temperatures in the aluminum foil covered plate and the bare aluminum alloy plate were above the ambient for a 10 °C and 15 °C max during the daytime. The radiative cooling power of the metamaterial water plate was also computed to be of 15–100 W/m2 during the daytime. Finally, the validated radiative cooler model was numerically studied for serving as “free cooling” source for a typical single-house building. Properly integrated into a radiant floor hydronic cooling system, the radiative cooler was found to be able to compensate >60% of annual cooling electricity consumptions. The study shed insights on the potential of applying daytime radiative cooling component and systems in buildings as a sustainable design measure to significantly reduce peak cooling load and annual cooling energy consumptions.

Temperature is a fundamental parameter for all forms of lives. Natural evolution has resulted in organisms which have excellent thermoregulation capabilities in extreme climates. Bioinspired materials that mimic biological solution for thermoregulation have proven promising for passive radiative cooling. However, scalable production of artificial photonic radiators with complex structures, outstanding properties, high throughput, and low cost is still challenging. Herein, we design and demonstrate biologically inspired photonic materials for passive radiative cooling, after discovery of longicorn beetles’ excellent thermoregulatory function with their dual-scale fluffs. The natural fluffs exhibit a finely structured triangular cross-section with two thermoregulatory effects which effectively reflects sunlight and emits thermal radiation, thereby decreasing the beetles’ body temperature. Inspired by the finding, a photonic film consisting of a micropyramid-arrayed polymer matrix with random ceramic particles is fabricated with high throughput. The film reflects ∼95% of solar irradiance and exhibits an infrared emissivity >0.96. The effective cooling power is found to be ∼90.8 W⋅m(−2) and a temperature decrease of up to 5.1 °C is recorded under direct sunlight. Additionally, the film exhibits hydrophobicity, superior flexibility, and strong mechanical strength, which is promising for thermal management in various electronic devices and wearable products. Our work paves the way for designing and fabrication of high-performance thermal regulation materials.

Improving building energy efficiency is one of the most important challenges towards the mitigation of climate change concerns. Buildings use significant amount of energy for cooling loads. Development of new night-time and day-time radiative cooling modalities by roofs is essential for reducing the energy consumption during the summer months. If a surface is desired to be kept cool while exposed to the sun, it should have (i) the maximum reflection of solar energy at visible wavelength range, and (ii) the maximum radiative emission from the surface at atmospheric radiation bands (8–13 μm wavelength range). In this study, reinforced concrete panels and three different types of plant-covered roof layers were investigated for their potential use for passive cooling applications, including moss, cactus and green leaves. Fourier transform infrared spectroscopy (FTIR) measurements were conducted to determine the absorbance of different samples at infrared wavelengths. In addition, reverse heat leak method was used to determine the effective conductivity values (R-values). The power of cooling parameter of each sample was determined first, and after that a coupled radiation and conduction heat transfer analysis was carried out to evaluate their insulation potential. It was demonstrated that moss is a better candidate to be used as a radiative cooling material, and it is a better insulator than the other tested materials.

Despite impressive advances in rain forecasts over the past decades, our understanding of rain formation on a microphysical scale is still poor. Droplet growth initially occurs through diffusion and, for sufficiently large radii, through the collision of droplets. However, there is no consensus on the mechanism to bridge the condensation coalescence bottleneck. We extend the analysis of prior methods by including radiatively enhanced diffusional growth (RAD) to a Markovian turbulence parameterization. This addition increases the diffusional growth efficiency by allowing for emission and absorption of thermal radiation. Specifically, we quantify an upper estimate for the radiative effect by focusing on droplets close to the cloud boundary. The strength of this simple model is that it determines growth-rate dependencies on a number of parameters, like updraft speed and the radiative effect, in a deterministic way. Realistic calculations with a cloud-resolving model are sensitive to parameter changes, which may cause completely different cloud realizations and thus it requires considerable computational power to obtain statistically significant results. The simulations suggest that the addition of radiative cooling can lead to a doubling of the droplet size standard deviation. However, the magnitude of the increase depends strongly on the broadening established by turbulence, due to an increase in the maximum droplet size, which accelerates the production of drizzle. Furthermore, the broadening caused by the combination of turbulence and thermal radiation is largest for small updrafts and the impact of radiation increases with time until it becomes dominant for slow synoptic updrafts.

Radiative cooling has proven to be a powerful strategy for sustainable thermal management. Nanophotonic structures enabling broadband reflection lead to minimization of sunlight absorption, which has brought nighttime-limited radiative cooling into daytime applications. However, this broadband reflection strategy in turn restricts the accessible colorization of radiative coolers to white or neutral, consequently hindering their practical applications, particularly for aesthetic purposes. With a few exceptions, selective absorption at a specific visible wavelength has been the most prevalent paradigm for colorization of radiative coolers. However, this absorption-based colorization inevitably makes the radiative cooler prone to heating, thus decreasing the cooling efficiency. Here, we demonstrate an undiscovered usage of opals for advancing color-preserved daytime radiative coolers. Opals, which have served mainly as Bragg reflective color pigments thus far, can be considered an effective homogeneous medium in the mid-infrared region. Thus, opals can also be envisioned as reflectively colorful metamaterials capable of radiative cooling even under the direct summer sun. Together with the soft fluidity of colloidal suspensions, opals can serve as platforms for easy-to-craft, large-scale, and colorful radiative coolers with minimal solar absorption.

Daytime radiative cooling under sunlight can be achieved by reflecting solar irradiation and emitting infrared thermal radiation in the transparency regions of earth atmosphere. For the sake of aes...

As an entirely passive cooling strategy, radiative cooling (RC) is an appealing approach to draw heat from the terrestrial surface to outer space. RC can be easily and effectively realized during the nighttime with the absence of sunlight, which coincidently mismatches the operation time of solar collectors. Therefore, it is of possibility and significance to unite the RC device and solar collector to extend the working period and function. In the present work, a novel combined photo-thermal and radiative cooling (PT-RC) module was proposed, experimentally and numerically investigated. By applying separated heating and cooling components, the PT-RC module demonstrated a daytime stagnation temperature of 159.8 °C and a nighttime one of 0.6 °C through a consecutive 24 h experimental duration. By contrast, a conventional photo-thermal (PT) module as a reference showed the highest panel temperature of 153.5 °C during the daytime and the lowest panel temperature of 7.9 °C in the nighttime. Further modeling was conducted to evaluate the thermal performance of the proposed PT-RC module compared with stand-alone PT module and spectrally coupled PT-RC module under different working conditions. This dual-function collector may contribute a potential solution for sustainable energy technology in Zero Energy Building applications.

Cost reduction and enhanced cooling performance are strongly demanded for daytime passive radiative cooling due to its attractive cooling strategy that does not require any energy input. Its potential application varies widely from air conditioning systems for buildings, photovoltaic cells, electronic device cooling and automobiles. However, recently proposed daytime passive radiative coolers are based on photonic structures which are high in cost. A relatively cheap metal oxide material, TiO2, which lowers the cost but is highly emissive in the mid-infrared range has been used, also improving the cooling performance of the photonic daytime passive radiative cooler. An optimized TiO2–SiO2 alternating multi-layered photonic daytime radiative cooler with average emissivity of 0.84 within 8–13 μm while reflecting 94% of incident solar energy is developed. Its net cooling power is estimated to be 136.3 W/m2 at ambient air temperature of 27 °C which shows an improvement of 90 W/m2 compared to that of the HfO2-SiO2 photonic radiative cooler. Last, a field test has been conducted in Hong Kong's subtropical climate (i.e. relative humidity = 60–70%) to investigate its feasibility, and with the help of solar shading, successfully demonstrated temperature reduction of 7.2 °C with a net cooling power of 14.3 W/m2 under direct sunlight.

The radiative cooling is a natural phenomenon, which allows rejecting the heat energy stocked by earthly objects by radiation outward, through the atmosphere-window (8–13 µm). In this work, a combination of the solar heating and the radiative cooling technologies by using a photovoltaic-thermal collector (PVT) has been investigated. A numerical modeling was evaluated by using the TRNSYS16 simulation software. The thermal performances of the collector were investigated under two climates and for different water flow rates. The monthly performances of two Moroccans cities were also conducted. The numerical results obtained show that the variation of the panel temperature in comparison with that of air allows to estimate cooling powers between 50 and 70 W/m2 during the night. Likewise, the daily variation allows exploiting heating powers between 100 and 250 W/m2. The results also indicate that the combination of radiative cooling and solar heating technologies by using (PVT) save the energy for air conditioning and heating in the both Moroccans cities (Marrakech and Ouarzazate).

The energy consumption of outdoor structures, such as automobiles and buildings, has significantly contributed to the problem of energy shortage and global warming. Photonic thermal management plays a crucial role in the thermal behavior of outdoor energy consumption by the functions of solar heating and radiative cooling. Here, we provide a conceptive design of self-adaptive photonic thermal management by tunable radiative cooling and solar heating, which are determined by the absorption spectra of the atmospheric window range and solar energy. These absorption spectra can be tuned by a large cross resonator and a small cross resonator independently, while the compound metasurface of the large and small cross resonators can realize self-adaptive radiative cooling and solar heating in one system. The result shows that the compound metasurface can remain cool at 35 °C and remain warm at 25 °C. These results lead to better designs by photonic thermal management for an outdoor structure.

Taking the frigid outer space as the heat sink, a terrestrial body can cool itself to a sub-ambient temperature via radiative sky cooling (RC) scheme. However, poor seasonal and regional adaptabilities and low cooling power density of the RC technology confine its effective applications only in hot seasons and regions with RC-friendly ambient conditions. Similarly, a solar thermal collector cannot work at night and is of little value when heat is needless. To tackle these challenges, a hybrid solar photothermic and radiative cooling (PT-RC) collector equipped with a rotatable absorber/emitter panel is proposed and experimentally investigated. This dual-mode collector can flexibly switch between PT and RC to match specific energy demands in different scenarios. The daily solar thermal efficiency of the PT-RC system at zero-reduced temperature reached 50.4%, a typical level of flat-plate solar water heating systems. The PT-RC system could not provide cooling energy during most daytime hours. However, the system could easily achieve nighttime cooling, with the cooling power of the collector decreasing from about 60 to 50 W/m2 as the circulated water being continuously cooled down from around 20 to 12 °C. The average nocturnal cooling power of the PT-RC collector ranged from 30.5 to 57.5 W/m2 in a multi-night RC test. This rotatable PT-RC collector offers a new strategy to flexibly deliver heat and coldness in a renewable and environmental-friendly manner and shows the potential of smart thermal management in buildings, vehicles, agriculture, etc.

Objects can radiate emission of heat to outer empty space (3 K) through an atmospheric window (8–13 μm), resulting in a possibility for radiative cooling. Multilayer film stacking designs and compl...

Summary Adaptive control of broadband light is essential for diverse applications including building energy management and light modulation. Here, we present porous polymer coatings (PPCs), whose optical transmittance changes upon reversible wetting with common liquids, as a platform for optical management from solar to thermal wavelengths. In the solar wavelengths, reduction in optical scattering upon wetting changes PPCs from reflective to transparent. For poly(vinylidene fluoride-co-hexafluoropropene) PPCs, this corresponds to solar and visible transmittance changes of up to 0.74 and 0.80, respectively. For infrared (IR) transparent polyethylene PPCs, wetting causes an “icehouse-to-greenhouse” transition where solar transparency rises but thermal IR transparency falls. These performances are either unprecedented or rival or surpass those of notable optical switching (e.g., electrochromic and thermochromic) paradigms, making PPCs promising for large-scale optical and thermal management. Specifically, switchable sub-ambient radiative cooling (by 3.2°C) and above-ambient solar heating (by 21.4°C), color-neutral daylighting, and thermal camouflage are demonstrated.

Photon-to-cooling phenomenon relies on the atmospheric transparency window to dissipate heat from the earth into outer space, which is an energy-saving cooling technique. This work demonstrates a highly effective aluminized Polymethylpentene (PMP) thin-film thermal structure. The emissivity of aluminized PMP thin films matches well to the atmospheric transparency window so as to minimize parasitic heat losses. This photon-to-cooling structure yields a temperature drop of 8.5 K in comparison to the ambient temperature and a corresponding radiative cooling power of 193 W/m2 during a one-day cycle. The easy-to-manufacture feature of an aluminized PMP thin film makes it a practically scalable radiative cooling method.