Radiative Cooling Research Articles

Recent Advances in Fluorescence-Based Colored Passive Daytime Radiative Cooling for Heat Mitigation

Recent Advances in Fluorescence-Based Colored Passive Daytime Radiative Cooling for Heat Mitigation

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 717Wm-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.

Passive energy-saving design strategy and realization on high window-wall ratio buildings in subtropical regions

Indoor overheating in high window-wall ratio (WWR) buildings has drawn widespread attention, but there is limited research on it during winter. Similarly, the application of radiative cooling (RC) technology on high-WWR facades and the effectiveness of combining RC with phase change materials (PCMs) need exploration. In Guangzhou, typical rooms in commercial buildings were studied, revealing that high-WWR buildings experience greater indoor overheating in winter compared to summer. For a 100 % WWR, the total equivalent energy consumption (EEC) is 71.389 kWh·m−2 in winters, 7.8 % higher than summer. Using RC glass with high solar (diffuse) reflectivity (0.65) on south-facing windows reduced EEC by over 70 %. RC glass also provided significant cooling capacity during unoccupied periods, which is sufficient to meet daytime cooling needs. In the case of a 100 % WWR, the proportion of available cooling capacity (37.5 %) during unoccupied period exceeded daytime cooling EEC (24.9 %). Therefore, PCMs were adopted to store this cooling capacity and transfer it for release during the occupied period, which bring an additional improvement in energy-saving performance by 4.7 %. The combination of PCMs and RC technology further achieves building energy efficiency. This study offers insights for addressing winter indoor overheating in high-WWR buildings.

Supramolecularly Connected Armor-like Nanostructure Enables Mechanically Robust Radiative Cooling Materials.

Passive daytime radiative cooling (PDRC) is a promising practice to realize sustainable thermal management with no energy and resources consumption. However, there remains a challenge of simultaneously integrating desired solar reflectivity, environmental durability, and mechanical robustness for polymeric composites with nanophotonic structures. Herein, inspired by a classical armor shell of a pangolin, we adopt a generic design strategy that harnesses supramolecular bonds between the TiO2-decorated mica microplates and cellulose nanofibers to collectively produce strong interfacial interactions for fabricating interlayer nanostructured PDRC materials. Owing to the strong light scattering excited by hierarchical nanophotonic structures, the bioinspired film demonstrates a desired reflectivity (92%) and emissivity (91%) and an excellent temperature drop of 10 °C under direct sunlight. Notably, the film guarantees high strength (41.7 MPa), toughness (10.4 MJ m-3), and excellent environmental durability. This strategy provides possibilities in designing polymeric PDRC materials, further establishing a blueprint for other functional applications like soft robots, wearable devices, etc.

A systematic analysis of energy-saving potential of spectrum-selective radiative cooling materials in buildings

Passive cooling of buildings through radiative cooling (RC) is considered an effective method for reducing the energy consumption of air conditioning. In recent years, spectrum-selective RC materials (RCMs) have received extensive attention owing to their low solar absorptivity and high thermal emissivity in the atmospheric window. The energy-saving performance of RCMs is related to their own properties and local meteorological conditions, building type, building form, and energy use patterns. Although several studies have explored the performance of RCMs, most existing studies have considered a single building type and meteorological condition, and simple building forms that differ significantly from real buildings. Hence, the application analyses of RCMs to buildings are not sufficiently comprehensive. This study focused on the energy-saving potential of RCMs for building applications, and presented a systematic framework to analyze the energy-saving potential of RCMs under different conditions. DeST, which is embedded with a long-wave radiative heat transfer calculation module based on the equivalent sky temperature, was adopted as the simulation software. It can accurately calculate the radiative heat transfer in different wavelength bands, thus making the simulation results more accurate. Based on the calculation results of large-scale cases considering four major factors, namely climate, building type, construction year, and installation location of RCMs, the energy-saving performance of RCMs in different cases was analyzed, and the return on investment is performed, and more comprehensive suggestions were provided for applying RCMs in future buildings.

Fluorescence‐Enabled Colored Bilayer Subambient Radiative Cooling Coatings

AbstractPassive daytime radiative cooling has emerged as a promising green technology for the thermal management of buildings, vehicles, textiles, and electronics. Typically, both high solar reflectance and high thermal emissivity are prerequisites to achieve sufficient daytime cooling. However, colored radiative cooling materials are facing the dilemma of introducing visible light absorption, leading to challenges in balancing cooling and aesthetic demands. Here, three colored bilayer radiative cooling coatings, each comprised of a white base layer and a colored top layer with fluorescence enhancement are fabricated. Three phosphors (Sr2Si5N8:Eu2+, Y3Al5O12:Ce3+, and (Ba,Sr)SiO4:Eu2+) are employed with respective photoluminescence quantum yields (PLQYs) of 81%, 95.8%, and 91.0% as the colored pigment in the top layer. To mitigate the contradiction between coloration and solar reflectance, SiO2 microspheres are introduced into the top layer and utilize their Mie‐resonance‐based multiple scattering to increase the photoluminescent (PL) properties of the phosphors, which jointly boosts the effective solar reflectance (ESR) of the top layer. As a result, the three bilayer coatings exhibit soft colors while achieving subambient cooling with temperature drops of up to 1.5 °C. This fluorescence‐enhancement strategy may pave the way for preparing highly efficient radiative cooling coatings with tunable colors.

Development of a Janus radiative cooler using a versatile fabrication process

Radiative cooling is a passive heat dissipation technique that emits thermal radiation to outer space through the atmosphere window. A radiative cooler has one side facing the sky, and the other side extracts heat from hot targets. Excellent cooling performance of a cooler can be archived via broadband absorption on one side and wavelength-selective emission on the other side like a Janus structure. However, Janus radiative coolers are usually constructed with complicated structures that take tremendous effort and costly facility. This work thus proposes an alternative Janus radiative cooler, which can be fabricated using a versatile and cost-effective fabrication technique, the electrophoretic deposition. The same setup and procedure can deposit chitosan and carbon black thin films on each side of a metallic substrate to achieve dual functionality. Five measurement units were constructed, and fabricated samples were placed on the top surface of each. The cooler successfully lowered the heater temperature inside the unit. Temperature reduction of 5.6 °C and 0.2 °C can be achieved respectively during daytime and nighttime with the proposed cooler compared to the case without thin film depositions. The repeatability of results was validated, and numerical modeling was conducted to exhibit thermal fields. The proposed cooler is promising in diminishing energy demand as well as carbon emissions while taking cost-effectiveness, feasibility, and simplicity into consideration.

Model development and preliminary study of the concentrated-radiative cooling based thermoelectric generator

Photovoltaic (PV) cells, widely adopted in solar energy applications, achieve a maximum efficiency of only 30 % and remain inactive during nighttime. In contrast, solar-thermal power generation has surpassed 50 % efficiency. To address these limitations, we introduce the Solar- Radiative cooling -Thermoelectric system (Solar-RC-TE), integrating solar energy, radiative cooling (RC), and thermoelectric power generation. This system enhances energy density, reduces consumption, lowers costs, and minimizes environmental impact while enabling nighttime power generation. For the optimization of the Solar-RC-TE model to further improve power generation efficiency, we employ the comprehensive three-dimensional methodology of COMSOL, considering material properties, structural configurations, and environmental factors. Research findings reveal that a 710 mm × 710 mm radiative cooling film can generate 386.12 mV of voltage on the thermoelectric generator (TEG). Environmental factors, such as increased solar irradiance, lead to temperature elevation, affecting efficiency. Overheating and higher wind speeds also contribute to decreased TEG efficiency. Regarding materials, improving the thermoelectric figure of merit (ZT) and film emissivity positively impact power generation efficiency. Elevating the film emissivity from 0.7 to 0.89 significantly enhances performance by 1.2 %. These outcomes provide a tangible and practical solution for addressing energy crises and environmental concerns.

Molecular‐Engineered Wool for Sustainable All‐Weather Radiative Cooling Textiles

AbstractPersonal thermal regulation through passive radiative cooling is an effective strategy to achieve sustainable development. Although recent advances have introduced various cooling textile designs, the extensive use of materials unsuitable for common textiles, alongside coating or compositing technologies, not only compromises the wearability and safety of these textiles but also impacts green sustainability. Herein, it is shown that wool is a promising natural material for transferring mid‐infrared human body radiation, but the high absorption in the solar spectrum prohibits wool from achieving satisfactory daytime radiative cooling. Thus the natural wool impregnates zinc ions into the protein chains to form a zinc‐ion woolen (wool‐Zn) fabric. The wool‐Zn fabric can provide humans with a cooling sensation both indoors and daytime outdoors, showing a temperature reduction of 2 °C on human skin indoors and a reduction of 7 °C on artificial skin under direct sunlight, respectively, when covered by wool‐Zn fabrics compared to when covered with cotton. Moreover, the wool‐Zn fabric shows desirable antibacterial properties and durability against washing and abrasion, while maintaining desirable wearability. As a result, the wool‐Zn fabric shows great potential for commercial application in personal thermoregulation and provides an effective avenue toward sustainable energy and environment.

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.

Efficient daytime radiative cooling films originating from controllable multi-scattering effect

Passive radiative cooling technology has received extensive attention as an environmentally friendly cooling solution. However, existing radiative coolers often suffer from complexities in preparation and high costs, limiting their practical applications. Here, a novel porous structure with randomly distributed microspheres is fabricated in a low-cost and simple method known as solvent volatilization-induced phase separation, employing polyvinylidene fluoride (PVDF) and monodispersed SiO2 microspheres. Utilizing the regulation of parameters such as the size of the pores and microspheres, controllable multi-scattering of light by the microstructures is achieved. The porous PVDF/SiO2 film, with an average solar reflectance of 93.8% and an average atmospheric window emissivity of 0.958, is self-supporting and exhibits excellent flexibility and mechanical strength. When applied to closed and windowed models, the porous PVDF/SiO2 film effectively lowers the internal temperatures by an average of 7.0 °C and 4.6 °C in summer, respectively, and by an average of 6.7 °C and 4.4 °C in spring. This work provides a viable strategy for achieving efficient radiative cooling and improving building energy management in a simple, cost-effective, and scalable manner.

Experimental performance characteristics of the daytime radiative cooler at various surface orientations

The daytime radiative cooling technology is becoming crucial with several prospective applications, either as a standalone or in conjunction with conventional cooling systems; however, the dilute cooling flux is a main drawback. The cooling flux can be enhanced by proper surface orientation (direction or azimuth and tilting or inclination), but it has not been studied extensively. Hence, this study describes the radiative cooler's angular and directional-dependent performance. The angular and directional dependent study is essential to emphasize the further development in the radiative coatings' suitable macro-scale geometry. An optimized polymer-particle composite structured daytime radiative cooler has been developed and tested with maximum temperature drop from the ambient as an objective function. The north-facing radiative cooler starts to perform better up to a 30° tilt angle; after that, its performance tends to decrease. The south-facing surfaces have reduced performance due to higher solar flux exposure as well as reduced sky exposure. A flat surface receives the maximum sky exposure since its horizontal placement does not interfere with the earth's objects. A maximum of 10.5 °C temperature drop and an average maximum of 5.5 °C are observed. North-facing tilt up to 30° is recommended in the northern hemisphere and vice-versa for best performance.

Experimental Study on Energy-Free Superhydrophobic Radiative Cooling Versatile Film with Enhanced Environmental Tolerance.

Clean, energy-free methods of cooling are an effective way to respond to the global energy crisis. To date, cooling materials using passive daytime radiative cooling (RC) technology have been applied in the fields of energy-efficient buildings, solar photovoltaic cooling, and insulating textiles. However, RC materials frequently suffer from comprehensive damage to their microstructure, resulting in the loss of their initial cooling effect in complex outdoor environments. Here, a superhydrophobic daytime passive RC porous film with environmental tolerance (SRCP film) was fabricated, which integrated strong solar reflectivity (approximately 90%), mid-infrared emissivity (approximately 0.97), and superhydrophobicity (water contact angle (WCA) of 160° and sliding angle of 3°). This study revealed that SRCP film had an average reflectivity of 14.3% higher than SiO2 particles in the 0.3-2.5 μm wavelength region, achieving a cooling effect of 13.2 °C in ambient conditions with a solar irradiance of 946 W·m-2 and a relative humidity of 74% due to the synergistic effect of effective solar reflection and thermal infrared emission. In addition, empirical results showed that the attained films possessed outstanding environmental tolerance, maintaining high WCA (156°), stable cooling effect (8.3 °C), and low SiO2 loss (less than 5.1%) after 30 consecutive days of UV irradiation and 14 days of corrosion with acidic and alkaline solutions. More importantly, this work could be flexibly prepared by various methods without the use of any fluorine-containing reagents, which greatly widens the practical application scope.

MHD Simulation in Galactic Center Region with Radiative Cooling and Heating

We investigate the role of magnetic field on the gas dynamics in a galactic bulge region by three-dimensional simulations with radiative cooling and heating. While a high-temperature corona with T > 106 K is formed in the halo regions, the temperature near the midplane is ≲104 K following the thermal equilibrium curve determined by the radiative cooling and heating. Although the thermal energy of the interstellar gas is lost by radiative cooling, the saturation level of the magnetic field strength does not significantly depend on the radiative cooling and heating. The magnetic field strength is amplified to 10 μG on average and reaches several hundred microgauss locally. We find the formation of magnetically dominated regions at midlatitudes in the case with the radiative cooling and heating, which is not seen in the case without radiative effect. The vertical thickness of the midlatitude regions is 50–150 pc at the radial location of 0.4–0.8 kpc from the Galactic center, which is comparable to the observed vertical distribution of neutral atomic gas. When we take the average of different components of energy density integrated over the galactic bulge region, the magnetic energy is comparable to the thermal energy. We conclude that the magnetic field plays a substantial role in controlling the dynamical and thermal properties of the galactic bulge region.

Controllable preparation of wollastonite-based composite coating for highly-efficient passive daytime radiative cooling in building

The building cooling has aroused widespread attention due to its significant contribution to both global warming and energy usage. To combat global warming and reduce energy consumption, an environmentally friendly beneficial alternative to electric cooling for buildings is passive daytime radiative cooling (PDRC). However, many of the reported radiative coolers still suffer from the challenge of complex preparation processes and high costs, making it difficult to scale up for building applications. Herein, the utilization of abundant and low-cost natural resources (i.e., cellulosic biomass and minerals) is strategized to produce radiative cooling material and incorporate this material into building cooling systems to boost global carbon neutrality. Guided by this concept, a hydrophobic SiO2/EC composite (HSEC) coating is designed by embedding nano-sized wollastonite-based SiO2 particles into porous ethyl cellulose (EC) polymer, and subsequently through polydimethylsiloxane (PDMS) dispersion spray treatment. Benefiting from the sufficient scattering by the nano-SiO2 particles and randomly dispersed light-scattering air voids, the resulting cooling coating shows a high solar reflectivity of 95.1 %. More importantly, the molecular vibration of the chemical in HSEC coating results in a high atmospheric window emissivity of 92.5 %. In the field test, the HSEC coating remained approximately 4.2 °C below the ambient temperature under intense solar irradiation of 412.5 W⋅m−2. This work not only offers a promising strategy for realizing the high-value utilization of wollastonite resources in building materials, but also provides a controllable approach for large-scale preparation of highly efficient radiative cooling materials to alleviate building cooling demands.

Sub-ambient radiative cooling with thermally insulating polyethylene terephthalate aerogels recycled from plastic waste

We demonstrate that the aerogels produced from the polyethylene terephthalate (PET) fibers, recycled from plastic bottle wastes, can function both as a radiative cooler and a thermal insulator. An object under and in contact with the recycled PET aerogel is thermally insulated from the surroundings and, at the same time, cooled below ambient temperature via radiative cooling. These dual capabilities are tested under the warm sky of Singapore with more than 420 W/m2 downwelling. With a 0.48 cm thick recycled PET aerogel, a heat transfer coefficient of 1.67 W/m2·K is measured when the object underneath is cooled by 2 °C from ambient temperature. Even under a 4.5 cm thick recycled PET aerogel, completely opaque to the thermal emission and having a heat transfer coefficient less than 1 W/m2·K, a sub-ambient radiative cooling is observed. Our measured heat transfer coefficient is lower than that of other porous polymer materials or aerogels reported to date as a radiative cooling substrate. Our results demonstrate that the recycled PET aerogels are a multi-functional material, integrating radiative cooling and thermal insulation in a single substrate. Produced by processing waste materials, the recycled PET aerogels hold promise to be a highly scalable and sustainable thermal management solution, with a positive impact on the environment.

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.

Annual Energy-Saving Smart Windows with Actively Controllable Passive Radiative Cooling and Multimode Heating Regulation.

Smart windows with radiative heat management capability using the sun and outer space as zero-energy thermodynamic resources have gained prominence, demonstrating a minimum carbon footprint. However, realizing on-demand thermal management throughout all seasons while reducing fossil energy consumption remains a formidable challenge. Herein, an energy-efficient smart window that enables actively tunable passive radiative cooling (PRC) and multimode heating regulation is demonstrated by integrating the emission-enhanced polymer-dispersed liquid crystal (SiO2@PRC PDLC) film and a low-emission layer deposited with carbon nanotubes. Specifically, this device can achieve a temperature close to the chamber interior ambient under solar irradiance of 700W m-2, as well as a temperature drop of 2.3°C at sunlight of 500W m-2, whose multistage PRC efficiency can be rapidly adjusted by a moderate voltage. Meanwhile, synchronous cooperation of passive radiative heating (PRH), solar heating (SH), and electric heating (EH) endows this smart window with the capability to handle complicated heating situations during cold weather. Energy simulation reveals the substantial superiority of this device in energy savings compared with single-layer SiO2@PRC PDLC, normal glass, and commercial low-E glass when applied in different climate zones. This work provides a feasible pathway for year-round thermal management, presenting a huge potential in energy-saving applications.

Radiative loss and ion-neutral collisional effects in astrophysical plasmas.

In this paper, we study the role of radiative cooling (RC) in a two-fluid model consisting of coupled neutrals and charged particles. We first analyse the linearized two-fluid equations where we include radiative losses in the energy equation for the charged particles. In a one-dimensional geometry for parallel propagation and in the limiting cases of weak and strong coupling, it can be shown analytically that the instability conditions for the thermal mode and the sound waves, the isobaric and isentropic criteria, respectively, remain unchanged with respect to one-fluid radiative plasmas. For the parameters considered in this paper, representative for the solar corona, the RC produces growth of the thermal mode and damping of the sound waves. In the weak coupling limit, the growth of the thermal instability and the damping of the sound waves is as derived in Field (Field 1965 Astrophys. J. 142, 531(doi:10.1086/148317)) using the charged fluid properties. When neutrals are included and are sufficiently coupled to the charges, the thermal mode growth rate and the wave damping both reduce by the same factor, which depends on the ionization fraction only. For a heating function that is constant in time, we find that the growth of the thermal mode and the damping of the sound waves are slightly larger. The numerical calculation of the eigenvalues of the general system of equations in a three-dimensional geometry confirm the analytic results. We then run two-dimensional fully nonlinear simulations that give consistent results: a higher ionization fraction or lower coupling will increase the growth rate. The magnetic field contribution is negligible in the linear phase. Ionization-recombination effects might play an important role because the RC produces a large range of temperatures in the system. In the numerical simulation, after the first condensation phase, when the minimum temperature is reached, the fraction of neutrals increases four orders of magnitude because of the recombination. This article is part of the theme issue 'Partially ionized plasma of the solar atmosphere: recent advances and future pathways'.

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.