Spectrally Selective Inorganic-Based Multilayer Emitter for Daytime Radiative Cooling

Bionic dual-scale structured films for efficient passive radiative cooling accompanied by robust durability

Passive radiative cooling (PRC), as an energy-free cooling approach, is ingeniously harnessed for certain natural organisms to withstand extreme high-temperature climates, which has inspired numerous bionic designs. However, it is a great challenge to enhance the durability of the designed materials in practical scenarios while inheriting the natural biological principles. We demonstrate bionic dual-scale structured (BDSS) films for efficient passive radiative cooling accompanied by robust durability after discovering the excellent thermoregulatory properties of the inner surface of Hawaiian scallop shell. We found that the inner surface of the shell consists of large-scale triangular ridges scattered with small-scale terrace steps. This dual-scale structure can enhance the reflectivity of sunlight by efficient Mie scattering and increase the emissivity in the mid-infrared range by lengthening the propagation of photons, thereby decreasing the surface temperature. Underpinned by this finding, we developed a BDSS film that features a strong solar spectrum reflectivity of 0.95 and a high mid-infrared emissivity of 0.98, achieving a sub-ambient cooling of 10.8 °C under direct sunlight. Additionally, the designed films possess robust durability including excellent self-cleaning, flexibility, mechanical strength, chemical stability, and anti-ultraviolet radiation, which is promising for thermal thermoregulation in various harsh scenarios.

Anti-UV Passive Radiative Cooling Chiral Nematic Liquid Crystal Films for Thermal Management

Passive radiative cooling (PRC) can spontaneously dissipate heat to outer space through atmospheric transparent windows, providing a promising path to meet sustainable development goals. However, achieving simultaneously high transparency, color-customizable, and thermal management of PRC anti ultraviolet (anti-UV) films remains a challenge. Herein, a simple strategy is proposed to utilize liquid crystalline polymer, with high mid-infrared emissive, forming customizable structural color film by molecular self-assembly and polymerization-induced pitch gradient, which guarantees the balance of transparency in visible spectrum and sunlight reflection, rendering anti-UV colored window for thermal management. By performing tests, temperature fall of 5.4 and 7.9 °C are demonstrated at noon with solar intensity of 717 W m-2 and night, respectively. Vivid red-, green-, blue-structured colors, and colorless films are designed and implemented to suppress the solar input and control the effective visible light transmissivity considering the efficiency function of human vision. In addition, temperature rise of 11.1 °C is achieved by applying an alternating current field on the PRC film. This study provides a new perspective on the thermal management and aesthetic functionalities of smart windows and wearables.

Synthesis, Optical Performance Characterization, and Durability of Electrospun PTFE-PEO Materials for Space Applications

Passive heat management is crucial in space, especially for extended missions involving protection from sunlight. Thermal coatings with desirable optical properties can drastically reduce the power consumed by active cooling systems, thereby reserving more resources for other critical systems onboard. Specifically, materials with wavelength-dependent reflectance and emittance are desirable for managing incident sunlight and self-cooling by thermal emission. This study demonstrates the use of polymer nanofibers, specifically poly(tetrafluoroethylene) (PTFE), for passive temperature control in space applications. This study describes the electrospinning fabrication process to create nanofibers and how process parameters can be varied to control the fiber geometry. We combine poly(tetrafluoroethylene) (PTFE) and poly(ethylene oxide) (PEO) polymers to fabricate highly reflective thermal control materials by electrospinning. To understand the role of material and fiber geometry, we measure spectral reflectance, absorptance, and transmittance using spectrophotometers interfaced with integrating spheres. We control the materials' fiber geometry and solar reflectance by modifying the solution properties, flow rate, rotating collector speed, and fabrication time. With 220-1560 μm thick electrospun nanofiber materials, we demonstrate an average solar reflectance of 94.73-99.75%, with values approaching 99.9% for thicker samples, which is among the highest for space applications. Meanwhile, a thermal emittance of 81.4% was observed at 300 K for a 3360 μm thick sample. The durability of these samples was also tested under ultraviolet light and atomic oxygen. Compared to the state-of-the-art materials, the electrospun PTFE-PEO fibers present a new paradigm for passive thermal management in space applications.

A Smart Window with Passive Radiative Cooling and Switchable Near-Infrared Light Transmittance via Molecular Engineering

Smart windows with synergetic light modulation have heightened demands for applications in smart cars and novel buildings. However, improving the on-demand energy-saving efficiency is quite challenging due to the difficulty of modulating sunlight with a broad bandwidth in an energy-saving way. Herein, a smart window with switchable near-infrared light transmittance and passive radiative cooling is prepared via a monomer design strategy and photoinduced polymerization. The effects of hydrogen bonds and fluorine groups in acrylate monomers on the electro-optical properties as well as microstructures of polymer-dispersed liquid crystal films have been systematically studied. Some films show a high contrast ratio of 90.4 or a low threshold voltage (Vth) of 2.0 V, which can be roll-to-roll processed in a large area. Besides, the film has a superior indoor temperature regulation ability due to its passive radiative cooling and controllable near-infrared light transmittance properties. Its radiative cooling efficiency is calculated to be 142.69 W/m2 and NIR transmittance could be switched to below 10%. The introduction of a carboxylic monomer and fluorinated monomer into the system endows the film with a highly efficient temperature management capability. The film has great potential for applications in fields such as flexible smart windows, camouflage materials, and so on.

A multi-scale layered helical structure composite using the co-dispersion of cellulose nanocrystals and the micro-nano Al sheets and its efficient near-infrared stealth performance

To design flexible functional materials with high efficiency, light weight, less metal consumption, stable structure for the thermal infrared stealth materials is a great challenge. We hypothesized that the use of crystal materials with different sizes to design composites with a chiral layered helical structure, the layered structures can repeatedly reflect infrared ray. Here, we reported the novel multi-scale layered helical chiral structure composite by self-assembly using the co-dispersion of cellulose nanocrystals (CNC) and micro-nano Al sheets. A new stable interlocking supermolecular structure is formed between the positively charged metal sheet and the negatively charged CNC photonic crystals. Metal sheets and CNC organic crystals were hybridized at the molecular level and form the Pickering-like CNC-Al co-dispersion system. The metal sheets in CNC chiral helical layered structure greatly improve its near-infrared reflection and stealth camouflage. Surprisingly, the CNC/Al composite on the heated glass substrate enabled the temperature drop 23 °C, and made its emissivity in the range of 7-14 μm significantly reduce. The synergetic effect of the Al sheets and the CNCs helical structure greatly improved the thermal infrared reflection and heat insulation properties. It is expected to provide a chiral layered material for the infrared stealth, and pattern camouflage fields.

Development of Microparticle Implanted PVDF-HF Polymer Coating on Building Material for Daytime Radiative Cooling

A passive cooling method with great potential to lower space-cooling costs, counteract the urban heat island effect, and slow down worldwide warming is radiant cooling. The solutions available frequently require complex layered structures, costly products, or a reflective layer of metal to accomplish daytime radiative cooling, which restricts their applications in many avenues. Furthermore, single-layer paints have been used in attempts to accomplish passive daytime radiative cooling, but these usually require a compact coating or only exhibit limited cooling in daytime. In our study, we investigated and evaluated in daytime the surrounding cooling outcome with aid of one layer coating composed of BaSO4/TiO2 microparticles in various concentrations implanted in the PVDF-HF polymers on a concrete substrate. The 30% BaSO4/TiO2 microparticle in the PVDF-HF coating shows less solar absorbance and excessive emissivity. The value of solar reflectance is improved by employing micro-pores in the structure of PVDF polymers without noticeable effect on thermal emissivity. The 30% BaSO4/TiO2/PVDF coating is accountable for the hydrophobicity and proportionate solar reflection in the UV band, resulting in efficient solar reflectivity of about 95.0%, with emissivity of 95.1% and hydrophobicity exhibiting a 117.1° water contact angle. Also, the developed coating could cool to about 5.1 °C and 3.9 °C below the surrounding temperature beneath the average solar irradiance of 900 W/m-2. Finally, the results demonstrate that the 30% BaSO4/TiO2/PVDF-HF microparticle coating illustrates a typical figure of merit of 0.60 and is also capable of delivering outstanding dependability and harmony with the manufacturing process.

Machine Learning Aided Design and Optimization of Thermal Metamaterials

Artificial Intelligence (AI) has advanced material research that were previously intractable, for example, the machine learning (ML) has been able to predict some unprecedented thermal properties. In this review, we first elucidate the methodologies underpinning discriminative and generative models, as well as the paradigm of optimization approaches. Then, we present a series of case studies showcasing the application of machine learning in thermal metamaterial design. Finally, we give a brief discussion on the challenges and opportunities in this fast developing field. In particular, this review provides: (1) Optimization of thermal metamaterials using optimization algorithms to achieve specific target properties. (2) Integration of discriminative models with optimization algorithms to enhance computational efficiency. (3) Generative models for the structural design and optimization of thermal metamaterials.

Materials in Radiative Cooling Technologies

Radiative cooling (RC) is a carbon-neutral cooling technology that utilizes thermal radiation to dissipate heat from the Earth's surface to the cold outer space. Research in the field of RC has garnered increasing interest from both academia and industry due to its potential to drive sustainable economic and environmental benefits to human society by reducing energy consumption and greenhouse gas emissions from conventional cooling systems. Materials innovation is the key to fully exploit the potential of RC. This review aims to elucidate the materials development with a focus on the design strategy including their intrinsic properties, structural formations, and performance improvement. The main types of RC materials, i.e., static-homogeneous, static-composite, dynamic, and multifunctional materials, are systematically overviewed. Future trends, possible challenges, and potential solutions are presented with perspectives in the concluding part, aiming to provide a roadmap for the future development of advanced RC materials.

One-dimensional photonic crystal with tilted termination and its angular filtering properties for radiative cooling

Photonic crystals are known for their band-gap structures. Due to their band-gaps, they can act as filters in both temporal and spatial domains. However, in most cases, due to their physical symmetry, their angular responses are symmetrical. Here, a structure based on a 1D photonic crystal is introduced and analyzed, which has an asymmetric angular selectivity. The structure is analyzed using the plane wave expansion method. The properties of the structure are expressed and verified by a commercial full-wave simulator software. Based on the analysis and its results, some simple design rules are derived. By using the extracted rules and some approximations, the potential of the structure to be used in radiative coolers, which are not completely toward the sky, is introduced. It is shown that if the structure is used as windows in buildings, it can save up to tens of watts per square meter in energy consumption for air conditioning. Finally, the whole structure including the radiative cooler is simulated, and the results support the calculations and approximations.

Porous Structure of Polymer Films Optimized by Rationally Tuning Phase Separation for Passive All-Day Radiative Cooling

Passive all-day radiative cooling (PARC) films with porous structures prepared via nonsolvent-induced phase separation (NIPS) have attracted considerable attention owing to their cost-effectiveness and wide applicability. The PARC performances of the films correlate with their porous structures. However, the porous structure formed using the NIPS process cannot be finely regulated. In this study, we prepared polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) films with porous structures optimized by rationally tuning the phase separation, which was achieved by adjusting the proportions of two good solvents with varying solubility parameters. The optimized PVDF-HFP film with a hierarchically porous structure exhibited a high solar reflectance of 97.7% and an infrared emissivity of 96.7%. The film with excellent durability achieved an average subambient cooling temperature of approximately 5.4 °C under a solar irradiance of 945 W·m-2 as well as a temperature of 11.2 °C at nighttime, thus demonstrating all-day radiative cooling. The results indicate that the proposed films present a promising platform for large-scale applications in green building cooling and achieving carbon neutrality.

Smart building block with colored radiative cooling devices and quantum dot light emitting diodes

In this study, we design a smart building block with quantum-dot light-emitting diode (QLED) and colored radiative cooling devices. A smart light-emitting building block is fabricated using a bottom-inverted QLED that emits green light, an insulating layer, and a top radiative cooling structure that emits mid-infrared light. The heat generated during QLED operation is measured and analyzed to investigate the correlation between heat and QLED degradation. The top cooling part is designed to have no impact on the QLED's performance and utilizes Ag-polydimethylsiloxane as a visible-light reflector and mid-infrared absorber/emitter. For the colored cooling part, white radiative cooling paint is used instead of Ag-polydimethylsiloxane to improve cooling performance, and red and yellow paints are employed to realize vivid red and yellow colors, respectively. We demonstrate a smart imitation house system with a smart light-emitting building block as the roof and analyze the cooling of the heat generated during QLED operation. A maximum cooling effect of up to 9.6 °C is observed compared to the imitation house system without the smart light-emitting building block, effectively dissipating heat generated during QLED operation. The smart light-emitting building block presented in this study opens new avenues in the fields of lighting and cooling systems.

Radiative Cooling for Energy Sustainability: From Fundamentals to Fabrication Methods Toward Commercialization

Radiative cooling, a technology that lowers the temperature of terrestrial objects by dissipating heat into outer space, presents a promising ecologically-benign solution for sustainable cooling. Recent years witness substantial progress in radiative cooling technologies, bringing them closer to commercialization. This comprehensive review provides a structured overview of radiative cooling technologies, encompassing essential principles, fabrication techniques, and practical applications, with the goal of guiding researchers toward successful commercialization. The review begins by introducing the fundamentals of radiative cooling and the associated design strategies to achieve it. Then, various fabrication methods utilized for the realization of radiative cooling devices are thoroughly discussed. This discussion includes detailed assessments of scalability, fabrication costs, and performance considerations, encompassing both structural designs and fabrication techniques. Building upon these insights, potential fabrication approaches suitable for practical applications and commercialization are proposed. Further, the recent efforts made toward the practical applications of radiative cooling technology, including its visual appearance, switching capability, and compatibility are examined. By encompassing a broad range of topics, from fundamental principles to fabrication and applications, this review aims to bridge the gap between theoretical research and real-world implementation, fostering the advancement and widespread adoption of radiative cooling technology.

Vivid Colored Cooling Structure Managing Full Solar Spectrum via Near-Infrared Reflection and Photoluminescence

Colored radiative cooling (CRC) offers an attractive alternative for surface and space cooling, while preserving the aesthetics of an object. However, there has been no study on the CRC using phosphors in regard to vivid coloration, sophisticated performance investigation, retention of properties, functionality, and structural flexibility all at once. Thus, to manage the entire solar spectrum, a colored cooling structure comprising a near-infrared (NIR)-reflective bottom layer and a top colored layer with a phosphor-embedded polymer matrix is proposed. The structure is paintable, vividly colored, hydrophobic, and ultraviolet (UV) and water resistant. In the daytime outdoor measurement, the structure with red, orange, and yellow colors exhibited lower temperature than a control group using commercial white paint by 4.7 °C, 7.2 °C, and 7.4 °C, respectively. After precise theoretical and experimental time-tracing temperature validation, the CRC performance enhancement from NIR reflection and photoluminescence effects was thoroughly analyzed, and a temperature reduction of up to 16.1 °C was achieved for the orange-colored structure. Furthermore, experiments of hydrophobicity infusion and exposure to UV and deionized water verified the durability of the colored cooling structure. In addition, flexible-film-type colored cooling structures were demonstrated using different bottom reflective layers, such as a silver thin film and porous aluminum oxide particle-embedded poly(vinylidene fluoride-co-hexafluoropropylene), suggesting the potential applicability of these colored cooling structures for vivid-colored, functional, and durable CRC.

Development of High-Performance Flexible Radiative Cooling Film Using PDMS/TiO2 Microparticles

Radiative cooling, which cools an object below its surrounding temperature without any energy consumption, is one of the most promising techniques for zero-energy systems. In principle, the radiative cooling technique reflects incident solar energy and emits its thermal radiation energy into outer space. To achieve maximized cooling performance, it is crucial to attain high spectral reflectance in the solar spectrum (0.3-2.5 μm) and high spectral emittance in the atmospheric window (8-13 μm). Despite the development of various radiative cooling techniques such as photonic crystals and metamaterials, applying the cooling technology in practical applications remains challenging due to its low flexibility and complicated manufacturing processes. Here, we develop a high-performance radiative cooling film using PDMS/TiO2 microparticles. Specifically, the design parameters such as microparticle diameter, microparticle volume fraction, and film thickness are considered through optical analysis. Additionally, we propose a novel fabrication process using low viscosity silicone oil for practical fabrication. The fabricated film accomplishes 67.1 W/m2 of cooling power, and we also analyze the cooling performance difference depending on the fabrication process based on the measurement and optical calculation results.

Self-Cleaning and Spectral Selective Membrane for Sustainable Radiative Cooling

Radiative cooling materials have attracted great attention due to their superiority in energy-free cooling, especially for outdoor applications. However, outdoor cooling performance is threatened by surface pollution. Herein, we demonstrate a ternary compound system, including polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), boron nitride nanosheets (BNNS), and hydrophobic silicon dioxide (SiO2), to synchronously achieve self-cooling and self-cleaning properties through biomimetically building a lotus-like papillomatous structure. The optimized membrane has a high infrared emissivity of 0.93, a sunlight reflectivity of 97.2%, and a water contact angle of 150.5°and not only efficiently cools the object to a suitable temperature but also protects the membrane from polluting and keeps cooling for a long time. The result shows that the membrane can cool a nonfebrile object by 30.5 and 1.7 °C for noon and night, respectively, and the noon and night-time temperature drops are 10.8 and 13.5 °C for the self-heating object, compared to the bare state. Meanwhile, the membrane always keeps self-cleaning if slurry is splashed onto its surface or it is exposed to slurry. Importantly, the integration of superhydrophobic and radiative cooling properties ensures that the membrane has permanent cooling performance by protecting it from being contaminated, which is significant for outdoor applications.

A solution-processed radiative cooling glass

Passive daytime radiative cooling materials could reduce the energy needed for building cooling up to 60% by reflecting sunlight and emitting long-wave infrared (LWIR) radiation into the cold Universe (~3 kelvin). However, developing passive cooling structures that are both practical to manufacture and apply while also displaying long-term environmental stability is challenging. We developed a randomized photonic composite consisting of a microporous glass framework that features selective LWIR emission along with relatively high solar reflectance and aluminum oxide particles that strongly scatter sunlight and prevent densification of the porous structure during manufacturing. This microporous glass coating enables a temperature drop of ~3.5° and 4°C even under high-humidity conditions (up to 80%) during midday and nighttime, respectively. This radiative "cooling glass" coating maintains high solar reflectance even when exposed to harsh conditions, including water, ultraviolet radiation, soiling, and high temperatures.

Advances in photothermal regulation strategies: from efficient solar heating to daytime passive cooling

Photothermal regulation concerning solar harvesting and repelling has recently attracted significant interest due to the fast-growing research focus in the areas of solar heating for evaporation, photocatalysis, motion, and electricity generation, as well as passive cooling for cooling textiles and smart buildings. The parallel development of photothermal regulation strategies through both material and system designs has further improved the overall solar utilization efficiency for heating/cooling. In this review, we will review the latest progress in photothermal regulation, including solar heating and passive cooling, and their manipulating strategies. The underlying mechanisms and criteria of highly efficient photothermal regulation in terms of optical absorption/reflection, thermal conversion, transfer, and emission properties corresponding to the extensive catalog of nanostructured materials are discussed. The rational material and structural designs with spectral selectivity for improving the photothermal regulation performance are then highlighted. We finally present the recent significant developments of applications of photothermal regulation in clean energy and environmental areas and give a brief perspective on the current challenges and future development of controlled solar energy utilization.

Bioinspired Switchable Passive Daytime Radiative Cooling Coatings

Passive daytime radiative cooling (PDRC) relies on simultaneous reflection of sunlight and radiation toward cold outer space. Current designs of PDRC coatings have demonstrated potential as eco-friendly alternatives to traditional electrical air conditioning (AC). While many features of PDRC have been individually optimized in different studies, for practical impact, it is essential for a system to demonstrate excellence in all essential aspects, like the materials that nature has created. We propose a bioinspired PDRC structure templated by bicontinuous interfacially jammed emulsion gels (bijels) that possesses excellent cooling, thinness, tunability, scalability, and mechanical robustness. The unique bicontinuous disordered structure captures key features of Cyphochilus beetle scales, enabling a thin (130 μm) bijel PDRC coating to achieve high solar reflectance (≳0.97) and high longwave-infrared (LWIR) emissivity (≳0.93), resulting in a subambient temperature drop of ∼5.6 °C under direct sunlight. We further demonstrate switchable cooling inspired by the exoskeleton of the Hercules beetle and mechanical robustness in analogy to spongy bone structures.

Passive Daytime Cooling Foils for Everyone: A Scalable Lamination Process Based on Upcycling Aluminum-Coated Chips Bags

The increasing energy demand for space cooling and environmental pollution caused by post-consumer plastic waste are two of the most challenging issues today. Passive daytime cooling, which dissipates heat to outer space without external energy input, has emerged recently as a sustainable technique for space cooling. In this work, a plastic waste-based passive daytime cooling foil is reported to alleviate both issues simultaneously. The mirror-like aluminum-plastic laminate (APL) waste exhibits a satisfactory solar reflectance of 85.7%. Combining the APL waste with a laminated pouch foil reveals a remarkably simple but effective plastic waste-based cooling foil with a high emissivity of 0.87 in the atmospheric window, resulting in a compelling daytime cooling performance. The sustainable aluminum-plastic laminate waste-based cooling foil is flexible, easily scalable, low-cost, and fabricated with a common laminator. This makes the fabrication of passive cooling materials possible even for nonexperts, which will help to provide advanced sun shelters and comfortable temperatures to a wider community.

Photonic structures in radiative cooling

Radiative cooling is a passive cooling technology without any energy consumption, compared to conventional cooling technologies that require power sources and dump waste heat into the surroundings. For decades, many radiative cooling studies have been introduced but its applications are mostly restricted to nighttime use only. Recently, the emergence of photonic technologies to achieves daytime radiative cooling overcome the performance limitations. For example, broadband and selective emissions in mid-IR and high reflectance in the solar spectral range have already been demonstrated. This review article discusses the fundamentals of thermodynamic heat transfer that motivates radiative cooling. Several photonic structures such as multilayer, periodical, random; derived from nature, and associated design procedures were thoroughly discussed. Photonic integration with new functionality significantly enhances the efficiency of radiative cooling technologies such as colored, transparent, and switchable radiative cooling applications has been developed. The commercial applications such as reducing cooling loads in vehicles, increasing the power generation of solar cells, generating electricity, saving water, and personal thermal regulation are also summarized. Lastly, perspectives on radiative cooling and emerging issues with potential solution strategies are discussed.

Daytime radiative cooling multilayer films designed by a machine learning method and genetic algorithm

Recently, there has been growing interest and attention towards daytime radiative cooling. This cooling technology is considered a potentially significant alternative to traditional cooling methods because of its neither energy consumption nor harmful gas emission during operation. In this paper, a daytime radiative cooling emitter (DRCE) consisting of polydimethylsiloxane, silicon dioxide, and aluminum nitride from top to bottom on a silver-silicon substrate was designed by a machine learning method (MLM) and genetic algorithm to achieve daytime radiative cooling. The optimal DRCE had 94.43% average total hemispherical emissivity in the atmospheric window wavelength band and 98.25% average total hemispherical reflectivity in the solar radiation wavelength band. When the ambient temperature was 30°C, and the power of solar radiation was about 900W/m 2, the net cooling power of the optimal DRCE could achieve 140.38W/m 2. The steady-state temperature of that could be approximately 9.08°C lower than the ambient temperature. This paper provides a general research strategy for MLM-driven design of DRCE.

Emerging Materials and Strategies for Passive Daytime Radiative Cooling

In recent decades, the growing demands for energy saving and accompanying heat mitigation concerns, together with the vital goal for carbon neutrality, have drawn human attention to the zero-energy-consumption cooling technique. Recent breakthroughs in passive daytime radiative cooling (PDRC) might be a potent approach to combat the energy crisis and environmental challenges by directly dissipating ambient heat from the Earth to the cold outer space instead of only moving the heat across the Earth's surface. Despite significant progress in cooling mechanisms, materials design, and application exploration, PDRC faces potential functionalization, durability, and commercialization challenges. Herein, emerging materials and rational strategies for PDRC devices are reviewed. First, the fundamental physics and thermodynamic concepts of PDRC are examined, followed by a discussion on several categories of PDRC devices developed to date according to their implementation mechanism and material properties. Emerging strategies for performance enhancement and specific functions of PDRC are discussed in detail. Potential applications and possible directions for designing next-generation high-efficiency PDRC are also discussed. It is hoped that this review will contribute to exciting advances in PDRC and aid its potential applications in various fields.

Colloidal inorganic nano- and microparticles for passive daytime radiative cooling

Compared to traditional cooling systems, radiative cooling (RC) is a promising cooling strategy in terms of reducing energy consumption enormously and avoiding severe environmental issues. Radiative cooling materials (RCMs) reduce the temperature of objects without using an external energy supply by dissipating thermal energy via infrared (IR) radiation into the cold outer space through the atmospheric window. Therefore, RC has a great potential for various applications, such as energy-saving buildings, vehicles, water harvesting, solar cells, and personal thermal management. Herein, we review the recent progress in the applications of inorganic nanoparticles (NPs) and microparticles (MPs) as RCMs and provide insights for further development of RC technology. Particle-based RCMs have tremendous potential owing to the ease of engineering their optical and physical properties, as well as processibility for facile, inexpensive, and large area deposition. The optical and physical properties of inorganic NPs and MPs can be tuned easily by changing their size, shape, composition, and crystals structures. This feature allows particle-based RCMs to fulfill requirements pertaining to passive daytime radiative cooling (PDRC), which requires high reflectivity in the solar spectrum and high emissivity within the atmospheric window. By adjusting the structures and compositions of colloidal inorganic particles, they can be utilized to design a thermal radiator with a selective emission spectrum at wavelengths of 8-13 μm, which is preferable for PDRC. In addition, colloidal particles can exhibit high reflectivity in the solar spectrum through Mie-scattering, which can be further engineered by modifying the compositions and structures of colloidal particles. Recent advances in PDRC that utilize inorganic NPs and MPs are summarized and discussed together with various materials, structural designs, and optical properties. Subsequently, we discuss the integration of functional NPs to achieve functional RCMs. We describe various approaches to the design of colored RCMs including structural colors, plasmonics, and luminescent wavelength conversion. In addition, we further describe experimental approaches to realize self-adaptive RC by incorporating phase-change materials and to fabricate multifunctional RC devices by using a combination of functional NPs and MPs.

Selective spectral absorption of nanofibers for color-preserving daytime radiative cooling

Passive radiative cooling is a promising solution for cooling objects without consuming energy. However, chemical colors absorb visible light and generate heat, posing a challenge in the design of a colored sub-ambient daytime radiative cooler (CSDRC) in a simple and scalable way. Herein, we used nanofibers (NF) to achieve selective spectral absorption of the daytime radiative cooler through a dope-dyeing electrospinning technique. This approach allows for the selective absorption of desired colors in the visible spectrum, while the nanofiber structure provides strong visible and near-infrared light scattering to minimize solar heating. We selected cellulose acetate (CA) with mid-infrared emittance characteristics for efficient sky cooling. Our design enabled the CA NF CSDRC to exhibit an ultra-high NIR reflectance of 99%, a high MIR emittance of 95%, and vibrant colors. These unique optical properties resulted in a reduction of the maximum ambient temperature by 3.2 °C and a cooling power of ≈40 W m^-2 at a solar intensity of 700 W m^-2. Additionally, the flexibility and deformability of the colored nanofiber cooler make it suitable for thermal management in various practical applications. Our work provides a simple and scalable solution for designing colored passive radiative cooling materials.

From Chitosan to Chitin: Bio-Inspired Thin Films for Passive Daytime Radiative Cooling

Passive radiative daytime cooling is an emerging technology contributing to carbon-neutral heat management. Optically engineered materials with distinct absorption and emission properties in the solar and mid-infrared range are at the heart of this technology. Owing to their low emissive power of about 100 W m^-2 during daytime, substantial areas need to be covered with passive cooling materials or coatings to achieve a sizeable effect on global warming. Consequently, biocompatible materials are urgently needed to develop suitable coatings with no adverse environmental impact. It is shown how chitosan films with different thicknesses can be produced from slightly acidic aqueous solutions. The conversion to their insoluble form chitin in the solid state is demonstrated and the conversion is monitored with infrared (IR) and NMR spectroscopy. In combination with a reflective backing material, the films show below-ambient temperature cooling capabilities with a suitable emissivity in the mid-IR region and low solar absorption of 3.1-6.9%, depending on the film thickness. This work highlights the potential of chitosan and chitin as widely available biocompatible polymers for passive radiative cooling applications.

Dual-Mode Porous Polymeric Films with Coral-like Hierarchical Structure for All-Day Radiative Cooling and Heating

Passive radiative cooling (PRC) and passive radiative heating (PRH) have drawn increasing attention as green and sustainable cooling and heating approaches, respectively. Existing material designs for PRC/PRH are usually static and unsuitable for dynamic seasonal and weather changes. Herein, we demonstrate an all-day dual-mode film fabricated by decorating MXene nanosheets on porous poly(vinylidene fluoride) with abundant coral-like hierarchical structures obtained via phase inversion. The cooling side of the dual-mode film exhibits high solar reflectivity (96.7%) and high infrared emissivity (96.1%). Consequently, daytime subambient radiative cooling of 9.8 °C is achieved with a theoretical cooling power of 107.5 W/m² and nighttime subambient cooling of 11.7 °C is achieved with a theoretical cooling power of 140.7 W/m². Meanwhile, the heating side of the dual-mode film exhibits low infrared emissivity (11.6%) and high solar absorptivity (75.7%), contributing to a PRH capability of 8.1 °C, and excellent active solar and Joule heating as effective compensation for PRH. The dual-mode film could be easily switched between cooling and heating modes by flipping it to adapt to dynamic cooling and heating scenarios, which is important for alleviating the energy crisis and reducing greenhouse emissions.

Janus Interface Engineering Boosting Visibly Transparent Radiative Cooling for Energy Saving

Visibly transparent radiative cooling (VTRC) shows great potential in energy-saving buildings or car glasses for lighting and cooling. How to balance the lighting and cooling performance is of significance to VTRC. In addition, the thermal radiative performance on the inner side should also be determined for cooling. Here, we designed a Janus VTRC coating consisting of a thermal emitter, PDMS, and a transparent near-infrared reflector, TiO₂/Ag/TiO₂. On the outer side, the visible transmittance T̅_vis = 0.70, while the solar reflectance R̅_solar = 0.40, and the thermal emittance in the atmospheric window ε̅_LWIR = 0.94 can be achieved experimentally. On the inner side, the thermal emittance ε̅_IR can be 0.90 or 0.01 depending on the substrate (glass or near-infrared reflector), which acts as the radiative conductor or barrier for energy saving in hot or cold internal situations. Compared with glass, the designed PDMS/NIR/glass achieves an average temperature drop of 14.6 °C experimentally. The energy-saving calculation based on seven cities in China shows that the VTRC coating can save 34-44% of the annual cooling energy consumption. This Janus visibly transparent radiative cooling technology with internal and external regulation provides a potential strategy for energy saving under the requirement of simultaneous lighting and cooling.

Transparent planar solar absorber for winter thermal management

Indoor heating during winters accounts for a significant portion of energy consumed by buildings in regions of cold climate. Development of transparent coatings for windows that efficiently harvest solar energy can play a major role in reducing energy consumption and fuel costs incurred for winter heating. In recent years, there has been a great research effort towards designing transparent solar absorber coatings using nanophotonic structures. The potential of coatings based on planar multilayer structures, however, has received very little attention. In this work we investigate the performance of planar multilayer thin films using low-cost materials for design of transparent solar absorber window coatings. Our study led to the proposal of two planar multilayer designs. Simulation results predict that an increase in surface temperature by 27 K and 25 K, while maintaining mean visible transmittance of over 50% is possible using these designs. These results illustrate the great promise planar multilayer structures hold for winter thermal management of buildings.

Daytime Sub-Ambient Radiative Cooling with Vivid Structural Colors Mediated by Coupled Nanocavities

Daytime radiative cooling is a promising passive cooling technology for combating global warming. Existing daytime radiative coolers usually show whitish colors due to their high broadband solar reflectivity, which is not suitable for aesthetic demands and effective display. It is challenging to produce high-cooling performance materials with vivid colors because colors are often produced by the absorption of visible light, decreasing net cooling power. In this work, we design a series of colorful multilayered radiative coolers (CMRCs) consisting of an optimized selective emitter for cooling and coupled nanocavities for structural coloration, which can successfully delicately balance the trade-off between the chromaticity and cooling performance. By judiciously designing the geometric parameters and manipulating the coupling effect inside the coupled nanocavities, our coolers show sub-ambient cooling performance and a larger color gamut (occupying 17.7% sRGB area) than reported ones. We further fabricate CMRCs and demonstrate that they have temperature drops of 3.4-4.4 °C on average based on outdoor experiments. These CMRCs are promising in thermal management of electronic/optoelectronic devices and outdoor facilities.

Superhydrophobic Porous Coating of Polymer Composite for Scalable and Durable Daytime Radiative Cooling

Passive daytime radiative cooling (PDRC) technology provides an eco-friendly cooling strategy by reflecting sunlight reaching the surface and radiating heat underneath to the outer space through the atmospheric transparency window. However, PDRC materials face challenges in cooling performance degradation caused by outdoor contamination and requirements of easy fabrication approaches for scale-up and high cooling efficiency. Herein, a polymer composite coating of polystyrene, polydimethylsiloxane and poly(ethyl cyanoacrylate) (PS/PDMS/PECA) with superhydrophobicity and radiative cooling performance was fabricated and demonstrated to have sustained radiative cooling capability, utilizing the superhydrophobic self-cleaning property to maintain the optical properties of the coating surface. The prepared coating is hierarchically porous which exhibits an average solar reflectance of 96% with an average emissivity of 95% and superhydrophobicity with a contact angle of 160°. The coating realized a subambient radiative cooling of 12.9 °C in sealed air and 7.5 °C in open air. The self-cleaning property of the PS/PDMS/PECA coating helped sustain the cooling capacity for long-term outdoor applications. Moreover, the coating exhibited chemical resistance, UV resistance, and mechanical durability, which has promising applications in wider fields.

Green-Manufactured and Recyclable Coatings for Subambient Daytime Radiative Cooling

Passive daytime radiative cooling, which reflects sunlight and simultaneously emits heat into space to cool surfaces without energy input, is a promising strategy for energy conservation. Integrating radiative cooling with building systems can tremendously alleviate electrical cooling, but manufacturing high-efficient and eco-friendly coatings remains an urgent and challenging task. Here, we present a simple and scale-up strategy for fabricating ultrawhite coatings consisting of porous ethyl cellulose matrix-random BaSO₄ nanoparticles utilizing green solvents. With the synergistic effect of the ideal intrinsic properties of the materials and the strong Mie scattering of the porous structure, the ultrawhite coating possesses a record solar reflectance of 98.6% and a thermal emittance of 98.1%, resulting in a subambient temperature drop of over 2.5 °C under a solar intensity of ∼920 W m^-2. Better yet, our coatings can be conveniently brushed, rolled, or sprayed onto various types of substrates, with excellent durability, self-cleaning, and cost-effectiveness, paving an attractive and viable pathway for large-scale applications in practical buildings.

A tailored indoor setup for reproducible passive daytime cooling characterization

Passive daytime cooling materials can lower global energy consumption owing to their autonomous cooling capability. Although a significant number of passive cooling materials have been developed recently, their performance characterization is still challenging. Field tests experience high variability due to uncontrollable changes in environmental conditions. Here, we design an indoor setup to characterize the performance of passive cooling materials reproducibly and independently of weather and season. Outdoor measurement conditions are approximated using a liquid-nitrogen-cooled aluminum dome, a solar simulator, and a wavelength-selective inverse sky-window filter. In contrast to outdoor measurements, the results of various reference materials show remarkable precision and repeatability. Additionally, the impact of solar light intensity and temperature on the passive cooling performance can be experimentally investigated. Our setup is a first step in the development of a standardized test method to bring accuracy, reproducibility, and comparability to the emerging field of passive cooling materials.

Iridescent Daytime Radiative Cooling with No Absorption Peaks in the Visible Range

Coatings for passive radiative cooling applications must be highly reflected in the solar spectrum, and thus can hardly support any coloration without losing their functionality. In this work, a colorful daytime radiative cooling surface based on structural coloration is reported. A designed radiative cooler with a bioinspired array of truncated SiO₂ microcones is manufactured via a self-assembly method and reactive ion etching. Complemented with a silver reflector, the radiative cooler exhibits broadband iridescent coloration due to the scattering induced by the truncated microcone array while maintaining an average reflectance of 95% in the solar spectrum and a high thermal emissivity (ε) of 0.95, owing to the reduced impedance mismatch provided by the patterned surface at infrared wavelengths, reaching an estimated cooling power of ≈143 W m^-2 at an ambient temperature of 25 °C and a measured average temperature drop of 7.1 °C under direct sunlight. This strong cooling performance is attributed to its bioinspired surface pattern, which promotes both the aesthetics and cooling capacity of the daytime radiative cooler.

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

Passive daytime radiative cooling (PDRC) is emerging as a promising cooling technology. Owing to the high, broadband solar reflectivity and high mid-infrared emissivity, daytime radiative cooling materials can achieve passive net cooling power under direct sunlight. The zero-energy-consumption characteristic enables PDRC to reduce negative environmental issues compared with conventional cooling systems. In this review, the development of advanced daytime radiative cooling designs is summarized, recent progress is highlighted, and potential correlated applications, such as building cooling, photovoltaic cooling, and electricity generation, are introduced. The remaining challenges and opportunities of PDRCs are also indicated. It is expected that this review provides an overall picture of recent PDRC progress and inspires future research regarding the fundamental understanding and practical applications of PDRC.

Passive and Dynamic Phase-Change-Based Radiative Cooling in Outdoor Weather

Radiative cooling has attracted considerable attention due to its tremendous potential in exploiting the cold reservoir of deep sky. However, overcooling always occurs in the conventional static radiative coolers because they operate only in the cooling mode in both hot and cold. Therefore, a dynamic radiative cooler based on phase change materials is highly desired. Nevertheless, the practical outdoor phase-change-based dynamic radiative cooling has not yet been experimentally demonstrated. To satisfy the stringent requirement of the phase-change-based radiative cooler in outdoor weather conditions, we engineered the phase-change material (VO₂) to possess the room-temperature phase-transition capability for typical weather conditions. Second, the reconfigurable cavity consists of the lossless spacer to ensure the magnitude of thermal modulation and suppress the solar absorption simultaneously. Third, the practical selective-filtering method is devised to shield the solar irradiance while permitting the thermal emission. Our experiment demonstrates that these materials and photonic measures can work together to realize the dynamic radiative cooling in actual weather conditions, which shows a self-adaptive switch between the ON-cooling state in hot daytime and the OFF-cooling state in cold nighttime. The study pushes the radiative cooler toward multifunctionality and provides beneficial guidance for the phase-change-based intelligent thermal control.

A Solution-Processed Inorganic Emitter with High Spectral Selectivity for Efficient Subambient Radiative Cooling in Hot Humid Climates

Daytime radiative cooling provides an eco-friendly solution to space cooling with zero energy consumption. Despite significant advances, most state-of-the-art radiative coolers show broadband infrared emission with low spectral selectivity, which limits their cooling temperatures, especially in hot humid regions. Here, an all-inorganic narrowband emitter comprising a solution-derived SiO_x N_y layer sandwiched between a reflective substrate and a self-assembly monolayer of SiO₂ microspheres is reported. It shows a high and diffusive solar reflectance (96.4%) and strong infrared-selective emittance (94.6%) with superior spectral selectivity (1.46). Remarkable subambient cooling of up to 5 °C in autumn and 2.5 °C in summer are achieved under high humidity without any solar shading or convection cover at noontime in a subtropical coastal city, Hong Kong. Owing to the all-inorganic hydrophobic structure, the emitter shows outstanding resistance to ultraviolet and water in long-term durability tests. The scalable-solution-based fabrication renders this stable high-performance emitter promising for large-scale deployment in various climates.

Nanometer-optical-coating-based visibly tinted films with 24-hour sub-atmospheric passive cooling

Colored films absorb solar radiation at specific visible wavelengths, and they consequently heat up above atmospheric temperatures when exposed to sunlight. In this Letter, we report nanometer-thick TiN-based multilayers of light cyan, magenta, and yellow colors that can provide 24 h sub-atmospheric cooling when covered with high-emissivity polymers. Outdoor experiments have demonstrated that these visibly tinted films retain sub-atmospheric temperatures during daytime and nighttime. All fabricated films generated almost color-independent cooling powers and even surpassed commercial white paint at TiN thicknesses <5nm. Our work thus highlights the potential of multispectral selective absorbers as esthetic passive coolers.

Ultra-thin and near-unity selective emitter for efficient cooling

For the efficient radiative cooling of objects, coolers should emit heat within atmospheric transparent window and block heat absorption from the surrounding environments. Thus, selective emitters enable highly efficient cooling via engineered photonic structures such as metamaterials and multi-stacking structures. However, these structures require sophisticated fabrication processes and large quantities of materials, which can restrict mass-production. This study introduces an ultra-thin (∼1 μm) and near-unity selective emitter (UNSE) within the atmospheric window, which can be fabricated using simple and affordable process. The combination of infrared (IR) lossy layers and high index lossless layer enhances the resonance in the structure thus, the emissivity in long wavelength IR region increases to near-unity within a thickness of ∼1 μm.

Scalable and Flexible Electrospun Film for Daytime Subambient Radiative Cooling

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.

Efficient Thin Polymer Coating as a Selective Thermal Emitter for Passive Daytime Radiative Cooling

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.

Spectrally Selective Nanoparticle Mixture Coating for Passive Daytime Radiative Cooling

Passive daytime radiative cooling, which is a process that removes excess heat to cold space as an infinite heat sink, is an emerging technology for applications that require thermal control. Among the different structures of radiative coolers, multilayer- and photonic-structured radiative coolers that are composed of inorganic layers still need to be simple to fabricate. Herein, we describe the fabrication of a nanoparticle-mixture-based radiative cooler that exhibits highly selective infrared emission and low solar absorption. Al₂O₃, SiO₂, and Si₃N₄ nanoparticles exhibit intrinsic absorption in parts of the atmospheric transparency window; facile one-step spin coating of a mixture of these nanoparticles generates a surface with selective infrared emission, which can provide a more powerful cooling effect compared to broadband emitters. The nanoparticle-based radiative cooler exhibits an extremely low solar absorption of 4% and a highly selective emissivity of 88.7% within the atmospheric transparency window owing to the synergy of the optical properties of the material. The nanoparticle mixture radiative cooler produces subambient cooling of 2.8 °C for surface cooling and 1.0 °C for space cooling, whereas the Ag film exhibits an above-ambient cooling of 1.1 °C for surface cooling and 3.4 °C for space cooling under direct sunlight.

Flexible Daytime Radiative Cooling Enhanced by Enabling Three-Phase Composites with Scattering Interfaces between Silica Microspheres and Hierarchical Porous Coatings

Daytime radiative cooling has attracted considerable attention recently due to its tremendous potential for passively exploiting the coldness of the universe as clean and renewable energy. Many advanced materials with novel photonic micro/nanostructures have already been developed to enable highly efficient daytime radiative coolers, among which the flexible hierarchical porous coatings (HPCs) are a more distinguished category. However, it is still hard to precisely control the size distribution of the randomized pores within the HPCs, usually resulting in a deficient solar reflection at the near-infrared optical regime under diverse fabrication conditions of the coatings. We report here a three-phase (i.e., air pore-phase, microsphere-phase, and polymer-phase) self-assembled hybrid porous composite coating, which dramatically increases the average solar reflectance and yields remarkable temperature drops of ∼10 and ∼ 30 °C compared to the ambient circumstance and black paint, respectively, according to the rooftop measurements. Mie theory and Monte Carlo simulations reveal the origin of the low reflectivity of as-prepared two-phase porous HPCs, and the optical cooling improvement of the three-phase porous composite coatings is attributed to the newly generated interfaces possessing the high scattering efficiency between the hierarchical pores and silica microspheres hybridized with appropriate mass fractions. As a result, the hybrid porous composite approach enhances the whole performance of the coatings, which provides a promising alternative to the flexible daytime radiative cooler.

Multifunctional Membrane for Thermal Management Applications

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.

Cross-Linked Porous Polymeric Coating without a Metal-Reflective Layer for Sub-Ambient Radiative Cooling

Passive daytime radiative cooling provides cooling without energy input. This method is eco-friendly, which is beneficial, considering the increasing problems of global warming and urban heat islands. A poly(vinylidene fluoride) (PVDF) and polyurethane acrylate (PUA) matte white coating was prepared via photo-initiated free-radical polymerization. The porous polymeric coating without a metal-reflective layer exhibited an average emissivity of 0.9333 in the atmospheric window and an average solar reflectance of 0.9336 in the direct AM1.5 solar spectrum (888 W m^-2 in the 0.3-2.5 μm region). The radiative cooling power of the fabricated radiative cooler with a thickness of 518 μm was 94.2 W m^-2. Furthermore, the radiative cooler demonstrated radiative cooling performance during both daytime and nighttime in Seoul, Korea, and Chiang Mai, Thailand. The PVDF/PUA matte white coating without a silver reflector can prevent solar absorption caused by the oxidation of silver and reduce the light pollution caused by the metallic film because of the antiglare surface of the matte coating.

Daytime radiative cooler using porous TiO2: new approach

Passive daytime radiative cooling remains a topic of intense interest that has gained great attention recently, mainly due to its important applications, such as electronic heat dissipation, solar cells, and photo-thermal technologies. In order to achieve the daytime radiative cooling for thermal management applications, a new smart shield should be designed to have both a low absorptivity in the solar range of 0.3-2.5 µm and high emissivity in the atmospheric transparency windows of 8-13 µm and 16-26 µm. In this work, we propose and analyze a new, inexpensive, and highly scalable double-layer coating to reach an efficient passive daytime radiative cooling. Double-layer coatings consisting of a top TiO₂ porous adsorbent layer and a bottom providing excellent transmittance in the solar spectrum are achieved properly by borosilicate-crown glass (BK7). We have shown that the proposed device preserves an average absorptivity/emissivity well below 7% in the entire solar spectrum and almost 95% in the atmospheric transparency windows. Besides, our design allows for sub-ambient temperature drops of 45° and diurnal cooling power output of 133W/m², even in the case of solar irradiance up to 1000W/m². The cooling performance persists under high humidity, even when taking the non-radiative heat exchange progress into account. The results revealed that the proposed design can be easily applied to a large area and promotes a significant step towards achieving large scale application in solar cells and related systems.

High-Performance Daytime Radiative Cooler and Near-Ideal Selective Emitter Enabled by Transparent Sapphire Substrate

Daytime radiative cooling serving as a method to pump heat from objects on Earth to cold outer space is an attractive cooling option that does not require any energy input. Among radiative cooler structures, the multilayer- or photonic-structured radiative cooler, composed of inorganic materials, remains one of the most complicated structures to fabricate. In this study, transparent sapphire-substrate-based radiative coolers comprising a simple structure and selective emitter-like optical characteristics are proposed. Utilizing the intrinsic optical properties of the sapphire substrate and adopting additional IR emissive layers, such as those composed of silicon nitride thin film or aluminum oxide nanoparticles, high-performance radiative coolers can be fabricated with a low mean absorptivity (3-4%) at 0.3-2.5 µm and a high mean emissivity of over 90% at 8-13 µm. Experiments show that the fabricated radiative coolers reach temperature drops of ≈10 °C in the daytime. From the theoretical calculations of radiative cooling performance, the sapphire-substrate-based radiative coolers demonstrate a net cooling power as high as 100 Wm-2.