Daytime Radiative Cooling Coating Based on the Y2O3/TiO2 Microparticle-Embedded PDMS Polymer on Energy-Saving Buildings

Designing Nanoporous Polymer Films for High-Performance Passive Daytime Radiative Cooling

Energy-free passive daytime radiative cooling (PDRC) technology makes it an attractive solution to both the building energy crisis and global warming. Spectrally selective porous polymers have great potential for practical PDRC applications owing to their cooling performance and scalability. A fundamental understanding of the relationship between the cooling performance and pore properties is crucial for guiding future structural designs of high-performance PDRC materials. However, one of the key challenges is achieving uniform nanopores and tailorable pore morphologies in the PDRC coating films. Here we demonstrate a strategy to use advanced metal-organic framework (MOF) nanocrystals as a sacrificial template creating a nanoporous poly(vinylidene fluoride) (PVDF) coating film with uniform-sized nanopores for highly daytime passive radiative cooling. The experimental evidence indicates that nanopores around 400 nm in size, comparable to the wavelength within the ultraviolet and visible spectra, along with an appropriate porosity of 37%, contribute to excellent solar reflectance (94.9 ± 0.8%) and high long-wave infrared emission (92.8 ± 1.4%) in the resulting porous PVDF films. This leads to subambient cooling of ≈9.5 °C and a promising net cooling power of 137 W/m2 at midday under solar intensities of ∼1275 and ∼1320 W/m2. The performance equals or exceeds that of state-of-the-art polymeric PDRC designs, and this general strategy of tailing nanostructures is expected to open a new avenue toward high-performance radiative cooling materials for PDRC applications.

Superhydrophobic Composite Coatings Can Achieve Durability and Efficient Radiative Cooling of Energy-Saving Buildings

Passive daytime radiative cooling (PDRC) technology has received a great deal of attention in the field of energy efficiency and environmental protection as a sustainable technology and a large-scale and promising solution to mitigate the environmental impact of global warming. In this study, we prepared PDRC material by combining FEP with modified Al2O3 particles and using the method of spray combined with phase separation. The synergistic effect of the formed surface micronanostructures, combined with the molecular vibration of FEP and the phonon polarization resonance of Al2O3, further improves the optical performance of the PDRC coating. The PDRC coating has an average reflectivity of 0.96 in the solar spectral band (0.3-2.5 μm) and an average emissivity of 0.963 in the atmospheric window band ((8-13 μm). In addition, the PDRC coating had good hydrophobicity, and its water contact angle (WAC) reached 159.3°. Under direct sunlight conditions, PDRC materials have a good temperature drop (4.9 °C) compared to ambient temperatures and radiative cooling power (81.2 W/m2). The prepared coating maintains superhydrophobicity and excellent cooling performance when soaked in solutions of different pH values and UV radiation, which was of great significance for sustainable applications. Our work provides a form of long-term cooling that can be effectively implemented in green and energy-efficient buildings.

Durable Self-Cleaning Radiative Cooling Coatings for Building Energy Efficiency

Passive daytime radiative cooling (PDRC) is an energy-saving technology without an additional energy supply or environmental pollution. At present, most PDRC coatings for buildings are only aiming at high solar reflectivity (RS) and high mid-infrared emissivity (EMIR) while ignoring practicalities such as adhesion strength, scalability, and durability. In this work, modified calcined kaolin/(ethylene trifluorochloroethylene copolymer-polydimethylsiloxane) (MK/(FEVE-PDMS)) coating with super practicability is prepared by using MK as a filler, FEVE as an adhesive, and PDMS as a hydrophobic modifier. The RS and EMIR of the coating are 92.5 and 94.6%, respectively. The MK/(FEVE-PDMS) coating exhibits superhydrophobicity, with an advancing contact angle (ACA) of 160.2° and a hysteresis contact angle of 7.3°. At an average solar irradiance of 742.78 W m-2, the coating achieved a temperature drop of 13.12 °C (shielded with PE film) and 3.09 °C (without shielding), respectively, relative to the environment. The coating adheres firmly to the substrate with an adhesion strength of class 2. The superhydrophobicity of the coating provides excellent durability and ease of repair, which can resist UV aging and mechanical damage. The durable superhydrophobicity gives the coating long-term stability in PDRC performance. Additionally, the cheap raw materials and the preparation process, consistent with the production of existing paints, show excellent scalability. Moreover, the energy consumption simulation results show that the energy saving ratio of the coating is more than 10% in the densely populated Yangtze River Delta and Pearl River Delta. The durable self-cleaning radiative coating developed in this work has potential application prospects in areas where the demand for cooling in summer is large and the demand for heating in winter is small.

Construction of a Robust Radiative Cooling Emitter for Efficient Food Storage and Transportation

Nowadays, food safety is still facing great challenges. During storage and transportation, perishable goods have to be kept at a low temperature. However, the current logistics still lack enough preservation ability to maintain a low temperature in the whole. Hence, considering the temperature fluctuation in logistics, in this work, the passive radiative cooling (RC) technology was applied to package to enhance the temperature control capability in food storage and transportation. The RC emitter with selective infrared emission property was fabricated by a facile coating method, and Al2O3 was added to improve the wear resistance. The sunlight reflectance and infrared emittance within atmospheric conditions could reach up to 0.92 and 0.84, respectively. After abrasion, the sunlight reflection only decreased by 0.01, and the infrared emission showed a negligible change, revealing excellent wear resistance. During outdoor measurement, the box assembled by RC emitters (RC box) was proved to achieve temperature drops of ∼9 and ∼4 °C compared with the corrugated box and foam box, respectively. Besides, the fruits stored in the RC box exhibited a lower decay rate. Additionally, after printing with patterns to meet the aesthetic requirements, the RC emitter could also maintain the cooling ability. Given the superior optical properties, wear resistance, and cooling capability, the emitter has great potential for obtaining a better temperature control ability in food storage and transportation.

Calcium-Salt-Enhanced Fiber Membrane with High Infrared Emission and Hydrophilicity for Efficient Passive Cooling

Radiative cooling fabrics have gained significant attention for their ability to enhance comfort without consuming extra energy. Nevertheless, sweat accumulation on the skin and diminishing cooling efficiency usually exist in the reported polymer cooling membranes. Herein, we report a universal method to obtain a calcium (Ca)-salt-enhanced fiber membrane with high infrared emission and hydrophilicity for efficient passive cooling and flame retardancy. The modification by Ca salts (including CaSiO3, CaSO3, and CaHPO4) with strong infrared emission results in an improvement in hygrothermal management ability, especially for moisture absorption and perspiration regulation in hot and humid environments. As an example, the CaSiO3@PMMA fiber membrane exhibits exceptional reflectivity in the solar spectrum (∼94.5%), high emittance in the atmospheric window (∼96.7%), and superhydrophilicity with a contact angle of 31°. Under direct sunlight, the CaSiO3@PMMA membrane exhibits an obvious temperature drop of 11.7 °C and moisture management achieves an additional cooling of 8.9 °C, as further confirmed by the ability to reduce the rate of ice melting. Additionally, the composite membrane provides notable flame retardancy and UV resistance. This work paves a new path in developing new materials with perspiration management and flame retardancy for zero energy consumption cooling in hot and humid environments.

Anti-Environmental Aging Passive Daytime Radiative Cooling

Passive daytime radiative cooling technology presents a sustainable solution for combating global warming and accompanying extreme weather, with great potential for diverse applications. The key characteristics of this cooling technology are the ability to reflect most sunlight and radiate heat through the atmospheric transparency window. However, the required high solar reflectance is easily affected by environmental aging, rendering the cooling ineffective. In recent years, significant advancements have been made in understanding the failure mechanisms, design strategies, and manufacturing technologies of daytime radiative cooling. Herein, a critical review on anti-environmental aging passive daytime radiative cooling with the goal of advancing their commercial applications is presented. It is first introduced the optical mechanisms and optimization principles of radiative cooling, which serve as a basis for further endowing environmental durability. Then the environmental aging conditions of passive daytime radiative cooling, mainly focusing on UV exposure, thermal aging, surface contamination and chemical corrosion are discussed. Furthermore, the developments of anti-environmental aging passive daytime radiative cooling materials, including design strategies, fabrication techniques, structures, and performances, are reviewed and classified for the first time. Last but not the least, the remaining open challenges and the insights are presented for the further promotion of the commercialization progress.

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.

Diatomite-Based Recyclable and Green Coating for Efficient Radiative Cooling

Radiative cooling is a promising strategy to address energy challenges arising from global warming. Nevertheless, integrating optimal cooling performance with commercial applications is a considerable challenge. Here, we demonstrate a scalable and straightforward approach for fabricating green radiative cooling coating consisting of methyl cellulose matrix-random diatomites with water as a solvent. Because of the efficient scattering of the porous morphology of diatomite and the inherent absorption properties of both diatomite and cellulose, the aqueous coating exhibits an excellent solar reflectance of 94% in the range of 0.25-2.5 μm and a thermal emissivity of 0.9 in the range of 8-14 µm. During exposure to direct sunlight at noon, the obtained coating achieved a maximum subambient temperature drop of 6.1 °C on sunny days and 2.5 °C on cloudy days. Furthermore, diatomite is a naturally sourced material that requires minimal pre-processing, and our coatings can be prepared free from harmful organic compounds. Combined with cost-effectiveness and environmental friendliness, it offers a viable path for the commercial application of radiative cooling.

Tailoring High-Entropy Oxides as Emerging Radiative Materials for Daytime Passive Cooling

In the framework of intense research about high-entropy materials and their applications in energy-oriented technologies, in the present work, we discuss the potential applicability of selected oxides and of the alloys they form at different concentrations for daytime radiative cooling implementation. In particular, by combining density functional theory and the finite difference method, we provide an unbiased, scattering-free description of structural, electronic, and dynamic features of the best candidates, showing the required strong radiative properties for passive cooling while offering the benefits of affordability and compatibility with commercial coating fabrication processes.

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.

Corrosion-Resistant Hydrophobic Thermal Barrier Composite Coating on Metal Strip: A New Dimension to Steel Strips for Roofing Segment

This study demonstrates a cost-effective, thin, multifunctional composite coating system with outstanding thermal insulation for thermal management and heat shield applications, such as roofs, as well as outstanding resistance to corrosion. The hydrophobic multifunctional epoxy composite coating systems were designed with surface-modified fillers to impart both reduced heat conduction and high infrared reflectance in a thin coating with a 65-100 μm dry film thickness (DFT). With a judicial combination of hollow microspheres (HMS) activated and modified with silica (sHMS) and stearic acid-modified TiO2 (sMO), the developed composite coating attained the highest thermal insulation property with a temperature drop of 21-31 °C at different distances below the coated panel, which is superior to the values of temperature drop reported earlier. The high solar reflectance of the composite coating in the near-infrared (NIR) region exceeds 72% with a low thermal conductivity of 0.178 W m-1 K-1. After 720 h of exposure in a 3.5 wt % NaCl solution, the composite coating revealed a corrosion protection efficiency of 99%. The work demonstrates that high solar reflectivity and low thermal conductivity must be active simultaneously to achieve superior thermal shielding in a thin coating on a metal. A careful selection of fillers and appropriate surface modifications ensures hydrophobicity and proper distribution of the fillers in the coating for a high barrier effect to prevent environmental deterioration. With these superior performance parameters, the developed composite coatings make an essential contribution to energy sustainability and the protection against environmental degradation.

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.

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.