Radiative Cooling Nanofabric for Personal Thermal Management

Radiative Cooling of Outdoor Electronic Devices by Polyimide Fiber Films Fabricated via an Electrospinning-Electrospray Process

Radiative cooling materials have attracted extensive research interest due to their unique characteristics of zero energy consumption and minimized carbon footprint. However, the design and manufacture of radiative cooling materials for outdoor electronic devices remain to explore. In this study, a silica/fluorinated polyimide fiber film was particularly designed and fabricated for the radiative cooling of outdoor electronic devices. The film was prepared by an electrospinning-electrospray process with hexafluoroisopropylidene, diaminobiphenyl, and silica as raw materials. Composed of polyimide nanomicrometer fibers and polyimide-silica bead-like particles, the prepared film delivered an average solar reflectivity of 94.29% and an average mid infrared emissivity of 93.12%. Especially, the film exhibited high emissivity in the band of 6-16 μm, where the emission of electronic devices is relatively concentrated. Under direct sunlight, the radiative cooling film achieved a temperature 30.6 and 9.4 °C cooler than the commercial polyimide films and commercial high reflective coatings, respectively. The film also cooled a smartphone in high load operation under sunlight by 3.6-4.1 °C. The findings of this work provide new insight into the research and development of radiative cooling materials for outdoor electronic devices.

Fundamental concepts, design rules and potentials in radiative cooling

Amidst the escalating environmental concerns driven by global warming and the detrimental impacts of extreme climates, energy consumption and greenhouse gas emissions associated with refrigeration have reached unprecedented levels. Radiative cooling, as an emerging renewable cooling technology, has been positioned as a pivotal strategy in the fight against global warming. This review examines the theoretical model of radiative cooling emitters and complex practical environment. We first investigate the thermodynamic interactions between environmental factors and the cooling surface, followed by an examination of innovative modulation techniques such as asymmetric/non-reciprocal radiative heat transfer mechanisms. Additionally, we summarize the latest advancements in structural design and simulation methodologies for radiative cooling materials at the device level. We then delve into potential applications of radiative cooling materials in various scenarios including energy-efficient construction, personal thermal management, photovoltaic cooling, and dynamic PDRC materials with seasonal adaptability. In conclusion, we provide a comprehensive overview of this technology's strengths and current challenges to inspire further research and application development in radiative cooling technology with a focus on contributing towards energy conservation objectives and promoting a sustainable society.

Auto-Deposited Microparticle Composite Coating for Low-Cost and Efficient Daytime Radiative Cooling

Radiative cooling is an excellent strategy for mitigating global warming, by enhancing heat fluxes away from the Earth, thus balancing the Earth's heat flow. However, for randomly particle-dispersed radiative cooling materials, the particle content as high as 94-96 wt % or 60 vol %, far exceeds the critical pigment percentage (40-50%) of traditional coatings, preventing its large-scale application. Here, inspired by particle deposition under gravity in solution, we demonstrate an auto-deposited SiO2 composite radiative cooling coating (ADRC) which reduces the amounts of particles required and lowers costs. This particle density gradient structure enhances the local volume fraction of particles, thereby the coating exhibits high solar reflectance (93%) and infrared emissivity (89%), contributing to a daytime subambient temperature drop of 12 °C. The cooling energy-saving potential of buildings in China utilizing the ADRC as roofs ranges from 6.42 to 13.52%. This low-cost and scalable radiative cooler is expected to reduce energy consumption and carbon emissions, addressing global warming issues.

Gradient Porous and Carbon Black-Integrated Cellulose Acetate Aerogel for Scalable Radiative Cooling

Passive temperature controls like passive daytime radiative cooling (PDRC)-heating (PDRH), and thermal insulation are essential to meet the growing demand for energy-efficient thermal solutions. When combined with advanced functions like electromagnetic interference shielding, these technologies can significantly enhance scalability. However, existing approaches using single thin films or uniform porous materials face inherent limitations in optimizing versatile functions, while lightweight, insulating aerogels can extend their multifunctionality by manipulating pores and fillers. Herein, carbon black (CB)-containing cellulose acetate (CA)aerogels (CB/CA aerogel) featuring gradient pores and bilayer structures are devised to implement switchable PDRC-PDRH and broadband spectral emissivity from infrared to microwave, along with high thermal insulation and elasticity. Using stirring and freeze-drying methods, the CA aerogels show low thermal conductivity (≈0.034 W mK-1) and demonstrate broad-spectrum functionality, with over 95.7% solar reflectivity and 93% long-wavelength infrared (LWIR) emittance, achieving a 12.25 °C temperature reduction in outdoor conditions. Furthermore, the CB/CA aerogels enhance LWIR emittance to 98.7% and provide broadband spectral emissivity with 39.4% microwave absorption. This study offers a viable solution to simultaneously control radiative cooling and microwave absorption across a broad spectrum in porous media.

Advanced Bioinspired Personal Thermoregulation Textiles for Outdoor Radiative Cooling

Radiative cooling textiles designed to reflect incoming sunlight and enhance mid-infrared (MIR) emissivity show great potential for ensuring personal thermal comfort. Thus, these textiles are gaining prominence as a means of combating the heat stress induced by global warming. Nonetheless, integrating radiative cooling effects into scalable textile materials for personal thermoregulation remains a formidable challenge. To achieve optimal cooling performance, textiles must exhibit finely tuned optical properties and spectral selectivity. In this study, a radiative cooling smart textile was devised by drawing inspiration from the structure of greater flamingo (Phoenicopterus roseus) feathers, which have effective thermoregulatory properties. Specifically, a nanoporous nonwoven material was fabricated from polyacrylonitrile and alumina particles and integrated with a cellulosic cotton knit fabric through an efficient electrospinning and hot pressing process to produce smart textile metafabric (PAC@T) with superior optical properties and wearer comfort. PAC@T exhibited an average fiber diameter of 501.6 nm and pore size of 857.6 nm, resulting in a solar reflectance of 95 ± 1.2% and an MIR emissivity of 91.8 ± 0.98%. It also demonstrated an enhanced water vapor transmission rate (5.5 kg/m2/24 h), water vapor evaporation rate (334 ± 2.2 mg/h), and significant radiative cooling performance, leading to temperatures 6.1 °C cooler than those achieved by a traditional knitted textile. Thus, PAC@T offers several distinct advantages, namely superior cooling efficiency, long-term durability, and energy-free operation. In addition, it is formed from accessible raw materials via a potentially scalable process that is likely to have substantial applications in industrial generation of smart textiles for personal thermoregulation.

Revolutionizing Sports with Nanotechnology: Better Protection and Stronger Support

Modern sports activities have increasingly benefited from the development of nanotechnology, which is extensively applied in various sports events and associated activities and facilities. Nanotechnology deals with materials with nanoscale size, providing unique properties and functions compared with their bulk counterparts. Nanotechnology can not only provide better training feedback by tracking the athlete's physiological signals as well as performance details but also protect humans with nanomaterial-functionalized sports fabrics, equipment, and medicine. Nanotechnology has significantly advanced sports in various aspects, thereby leading to a rising research interest in this interdisciplinary field. This article highlights several representative nanotechnologies applied in sports such as nanomaterials in wearable sensors, personal heat management devices, functional sports fabrics, and sports medicine and discusses the principles, current challenges, as well as future opportunities.

High-Performance Radiative Cooling Sunscreen

Radiative cooling is a zero-energy-consumption cooling technology that shows great potential for outdoor human thermal management. To keep human skin comfortable in hot days, we herein develop a radiative cooling (RC) sunscreen that exhibits a low ultraviolet (UV) transmissivity (4.86%), a high solar reflectivity (90.19%), and a high mid-infrared emissivity (92.09%) to effectively provide both UV protection and skin cooling. As a result, the RC sunscreen exhibits a high cooling performance for decreasing the human skin temperature by 2.3-6.1 °C more than commercial sunscreens and 4.2-6.0 °C more than bare skin in a variety of outdoor scenarios in summer (e.g., low-humidity sunny days, high-humidity sunny days, and high-humidity cloudy days). In addition, the RC sunscreen also shows a good UV stability (12 h, 125 W), a high water resistance (106°), a long working life (30 days), and a good biocompatibility, thereby exhibiting promising commercial potentials in the sunscreen market.

Tough and thermal insulating cellulose-based aerogel fiber via long yarn-assisted interfacial polyelectrolyte complexation spinning

Cellulose-based aerogel fibers are recognized as a promising candidate for wearable thermal insulation textiles due to their high porosity, extremely low thermal conductivity, and environmental friendliness. Unfortunately, their practical application in textiles is severely limited by their brittleness. Herein, a novel "long yarn-assisted interfacial polyelectrolyte complexation (YAIPC) spinning" technique is proposed to fabricate cellulose-based aerogel fibers with a unique core-shell structure. The as-prepared core-shell aerogel fibers show excellent thermal insulation performance (34.3 mW m-1 K-1) and robust mechanical strength (∼100 MPa, 31.5 MJ m-3), providing great potential as wearable thermal insulating materials. Accordingly, our research would open a new avenue for designing and constructing wearable aerogel fibers and textiles.

Scalable, Waterproof, Breathable, and Flexible Polyolefin-Elastomer/Polyethylene Glycol@Zinc Oxide Microfibrous Fabrics for Daytime Radiative Cooling Clothing

In the face of escalating global temperatures, the demand for innovative passive cooling technologies that are both low-cost and environmentally sustainable is more critical than ever. However, traditional cooling fabrics face challenges in achieving wearing comfort while maintaining breathability and durability. Herein, a novel fluffy microfibrous fabric utilizing polyolefin-elastomer and polypropylene with embedded zinc oxide nanoparticles is fabricated through melt-blown technology. The results reveal that the prepared samples demonstrate exceptional daytime radiative cooling properties that present a 12.5 °C cooling capacity under 1083 W/m2 solar radiation, highlighted by their ability to reflect up to 90.8% of solar radiation and their significantly enhanced thermal emissivity. Moreover, key findings include that the samples have robust mechanical strength, high elastic performance, and excellent antifouling capabilities, alongside superior cooling performance, which will provide an opportunity to explore the development of cooling garments for outdoor environments and contribute substantially to sustainable cooling solutions.

The Influence of Background Materials on the Radiative Cooling Performance of Semi-Transparent and Opaque Textiles: A Theoretical and Experimental Analysis

This paper investigates the theoretical and experimental cooling performance of textile materials utilizing radiative cooling technology. By applying Kirchhoff's law, the emissivity of surfaces is determined, revealing that materials with high transmission values can achieve comparable cooling performance to those with high reflection values. Notably, materials exhibiting moderate reflectance and transmittance in the solar range tend to absorb minimal solar radiation, thus offering high theoretical cooling performance. However, practical applications like building envelopes or clothing present challenges due to the impact of background radiation on overall cooling capacity. Despite their intrinsic cooling properties, a significant portion of solar radiation is transmitted, complicating matters as the background can significantly affect overall cooling performance. This study provides a solution that accounts for the influence of background materials. Based on spectral data, various background materials and their impact on different semi-transparent comparison materials can be considered, and cooling performance can be simulated. This enables the simulation of cooling performance for various application scenarios and facilitates comparisons between transparent, semi-transparent, and opaque textile materials.

Emerging Trends of Nanofibrous Piezoelectric and Triboelectric Applications: Mechanisms, Electroactive Materials, and Designed Architectures

Over the past few decades, significant progress in piezo-/triboelectric nanogenerators (PTEGs) has led to the development of cutting-edge wearable technologies. Nanofibers with good designability, controllable morphologies, large specific areas, and unique physicochemical properties provide a promising platform for PTEGs for various advanced applications. However, the further development of nanofiber-based PTEGs is limited by technical difficulties, ranging from materials design to device integration. Herein, the current developments in PTEGs based on electrospun nanofibers are systematically reviewed. This review begins with the mechanisms of PTEGs and the advantages of nanofibers and nanodevices, including high breathability, waterproofness, scalability, and thermal-moisture comfort. In terms of materials and structural design, novel electroactive nanofibers and structure assemblies based on 1D micro/nanostructures, 2D bionic structures, and 3D multilayered structures are discussed. Subsequently, nanofibrous PTEGs in applications such as energy harvesters, personalized medicine, personal protective equipment, and human-machine interactions are summarized. Nanofiber-based PTEGs still face many challenges such as energy efficiency, material durability, device stability, and device integration. Finally, the research gap between research and practical applications of PTEGs is discussed, and emerging trends are proposed, providing some ideas for the development of intelligent wearables.

Hierarchical porous dual-mode thermal management fabrics achieved by regulating solar and body radiations

Personal thermal management (PTM) of fabrics is vital for human health; the ever-changing location of the human body poses a big challenge for fabrics to maintain a favorable metabolic temperature. Herein, a dual-mode thermal management fabric is designed to achieve both cooling and heating functions by regulating simultaneously solar and body radiations. The cooling or heating mode can be exchanged by flipping the fabric without an external energy supply. The passive cooling side consists of an electrospun polyacrylonitrile (PAN) fabric with a hierarchical porous structure, exhibiting high sunlight reflectance (91.42%) and an ∼14 °C temperature decrease under direct sunlight irradiation. The co-existence of nanoscale and microscale pores is proven to be essential for improved cooling performances. The other heating side, coated with an MXene layer, shows high photothermal conversion efficiency (37.5%) and outstanding heating capability outdoors. Furthermore, the contrary mid-infrared emissivity of the two sides (high emissivity of the cooling side while low emissivity of the heating side) leads to the dual-mode passive regulation of body thermal energy. Besides, this fabric demonstrates satisfactory wearability and excellent stability. Our work proposes an energy-saving and cost-effective approach for PTM fabrics potentially suitable for various scenarios (e.g., indoors/outdoors, summer/winter, low/high latitude areas).

Wettability Gradient-Induced Diode: MXene-Engineered Membrane for Passive-Evaporative Cooling

Thermoregulatory textiles, leveraging high-emissivity structural materials, have arisen as a promising candidate for personal cooling management; however, their advancement has been hindered by the underperformed water moisture transportation capacity, which impacts on their thermophysiological comfort. Herein, we designed a wettability-gradient-induced-diode (WGID) membrane achieving by MXene-engineered electrospun technology, which could facilitate heat dissipation and moisture-wicking transportation. As a result, the obtained WGID membrane could obtain a cooling temperature of 1.5 °C in the "dry" state, and 7.1 °C in the "wet" state, which was ascribed to its high emissivity of 96.40% in the MIR range, superior thermal conductivity of 0.3349 W m-1 K-1 (based on radiation- and conduction-controlled mechanisms), and unidirectional moisture transportation property. The proposed design offers an approach for meticulously engineering electrospun membranes with enhanced heat dissipation and moisture transportation, thereby paving the way for developing more efficient and comfortable thermoregulatory textiles in a high-humidity microenvironment.

Advanced Cooling Textiles: Mechanisms, Applications, and Perspectives

High-temperature environments pose significant risks to human health and safety. The body's natural ability to regulate temperature becomes overwhelmed under extreme heat, leading to heat stroke, dehydration, and even death. Therefore, the development of effective personal thermal-moisture management systems is crucial for maintaining human well-being. In recent years, significant advancements have been witnessed in the field of textile-based cooling systems, which utilize innovative materials and strategies to achieve effective cooling under different environments. This review aims to provide an overview of the current progress in textile-based personal cooling systems, mainly focusing on the classification, mechanisms, and fabrication techniques. Furthermore, the challenges and potential application scenarios are highlighted, providing valuable insights for further advancements and the eventual industrialization of personal cooling textiles.

Switchable radiative cooling and solar heating for sustainable thermal management

Radiative thermal management technologies that utilize thermal radiation from nano/microstructure for cooling and heating have gained significant attention in sustainable energy research. Passive radiative cooling and solar heating operate continuously, which may lead to additional heating or cooling energy consumption due to undesired cooling or heating during cold nighttime/winters or hot daytime/summers. To overcome the limitation, recent studies have focused on developing radiative thermal management technologies that can toggle radiative cooling on and off or possess switchable dual cooling and heating modes to realize sustainable and efficient thermal management. This review will explore the fundamental concepts of radiative thermal management and its switching mechanisms, utilizing novel systems composed of various materials and nano/microstructures. Additionally, we will delve into the potential future research directions in radiative thermal management technologies.

Advances in Biocarbon and Soft Material Assembly for Enthalpy Storage: Fundamentals, Mechanisms, and Multimodal Applications

High-value-added biomass materials like biocarbon are being actively pursued integrating them with soft materials in a broad range of advanced renewable energy technologies owing to their advantages, such as lightweight, relatively low-cost, diverse structural engineering applications, and high energy storage potential. Consequently, the hybrid integration of soft and biomass-derived materials shall store energy to mitigate intermittency issues, primarily through enthalpy storage during phase change. This paper introduces the recent advances in the development of natural biomaterial-derived carbon materials in soft material assembly and its applications in multidirectional renewable energy storage. Various emerging biocarbon materials (biochar, carbon fiber, graphene, nanoporous carbon nanosheets (2D), and carbon aerogel) with intrinsic structures and engineered designs for enhanced enthalpy storage and multimodal applications are discussed. The fundamental design approaches, working mechanisms, and feature applications, such as including thermal management and electromagnetic interference shielding, sensors, flexible electronics and transparent nanopaper, and environmental applications of biocarbon-based soft material composites are highlighted. Furthermore, the challenges and potential opportunities of biocarbon-based composites are identified, and prospects in biomaterial-based soft materials composites are presented.

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.

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.

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.

Personal Cooling Garments: A Review

Thermal comfort is of critical importance to people during hot weather or harsh working conditions to reduce heat stress. Therefore, personal cooling garments (PCGs) is a promising technology that provides a sustainable solution to provide direct thermal regulation on the human body, while at the same time, effectively reduces energy consumption on whole-building cooling. This paper summarizes the current status of PCGs, and depending on the requirement of electric power supply, we divide the PCGs into two categories with systematic instruction on the cooling materials, working principles, and state-of-the-art research progress. Additionally, the application fields of different cooling strategies are presented. Current problems hindering the improvement of PCGs, and further development recommendations are highlighted, in the hope of fostering and widening the prospect of PCGs.