High Solar Reflectance Research Articles

Switchable Surface Coating for Bifunctional Passive Radiative Cooling and Solar Heating

AbstractThe ability to achieve dual‐mode thermal regulation for switchable heating and cooling on a single platform has thus far been challenged by the availability of suitable materials. The materials need to possess both high solar reflectance and high transmittance, necessitating large and small thicknesses in the same coating layer, respectively (i.e., the thickness constraint). Herein, for the first time, a single‐layer coating made in a facile one‐step process is reported, which exhibits rapid switch between high solar reflection (≈96.6%) and high solar transmission (≈86.6%). In the dry state, high solar reflectance and infrared (IR) emittance (>96% from 8 to 13 µm) enable passive radiative cooling, resulting in all‐day near/sub‐ambient temperatures in the demanding weather conditions of the tropical climate. Upon wetting, high transparency in the broadband range (0.3–2.5 µm) allows solar heating, leading to switchable thermal regulation. Such unprecedented performances are achieved through a unique hierarchical porous structure comprising of vertically aligned microscale pores in nanoscale pore matrix. This structure breaks the thickness constraint and broadens its applicability, in particular for seasonal areas with large temperature variation throughout the day.

Aerogel‐Functionalized Thermoplastic Polyurethane as Waterproof, Breathable Freestanding Films and Coatings for Passive Daytime Radiative Cooling

Passive daytime radiative cooling (PDRC) is an emerging sustainable technology that can spontaneously radiate heat to outer space through an atmospheric transparency window to achieve self‐cooling. PDRC has attracted considerable attention and shows great potential for personal thermal management (PTM). However, PDRC polymers are limited to polyethylene, polyvinylidene fluoride, and their derivatives. In this study, a series of polymer films based on thermoplastic polyurethane (TPU) and their composite films with silica aerogels (aerogel‐functionalized TPU (AFTPU)) are prepared using a simple and scalable non‐solvent‐phase‐separation strategy. The TPU and AFTPU films are freestanding, mechanically strong, show high solar reflection up to 94%, and emit strongly in the atmospheric transparency window, thereby achieving subambient cooling of 10.0 and 7.7 °C on a hot summer day for the TPU and AFTPU film (10 wt%), respectively. The AFTPU films can be used as waterproof and moisture permeable coatings for traditional textiles, such as cotton, polyester, and nylon, and the highest temperature drop of 17.6 °C is achieved with respect to pristine nylon fabric, in which both the cooling performance and waterproof properties are highly desirable for the PTM applications. This study opens up a promising route for designing common polymers for highly efficient PDRC.

A low-cost sustainable coating: Improving passive daytime radiative cooling performance using the spectral band complementarity method

As a passive cooling method without extra energy expenditure, the passive daytime radiative cooling (PDRC) technology has the potential for wide range of applications. Manufacturing PDRC materials with low-cost and high solar band reflectivity are still facing challenges for their commercialization. In the present study, we used the spectral band complementarity method to realize high reflectivity in the sunlight band and excellent cooling performance of the PDRC coating, with a simple, inexpensive, and scalable preparation process. PDRC coating with a solar reflectance of 97.6% was demonstrated by properly designed BaSO4, CaCO3, and SiO2 particles. During the outdoor test, the average daytime temperature of PDRC coating was 8.3 °C lower than the air temperature in the cavity, and 5.5 °C lower than that of commercial white paints. Under the thermal equilibrium condition, the theoretical radiative cooling power of PDRC coating at nighttime and daytime can reach 119.3 W/m2 and 94.3 W/m2, respectively. The assessment results indicate that the PDRC coating has the potential for large-scale commercial production, with a low-cost (approximately $0.5/m2) and simple manufacturing process. This study can provide new ideas for the design and preparation of high-performance low-cost PDRC materials.

Radiative Cooling and Solar Heating Janus Films for Personal Thermal Management

Hot and cold seasonal temperature fluctuations pose a serious public health threat. Radiative thermal management has been shown to be an effective method for personal thermal management. However, the currently available materials cannot maintain human thermal comfort against the hot and cold seasonal temperature fluctuations, such as heating in cold weather or cooling in hot weather. Here, a Janus film that integrates the two opposite requirements of heating and cooling into one functional dual-mode film is fabricated. In cooling mode, the Al backing and embedded silicon dioxide (SiO2) microparticle can achieve a high solar reflectivity (∼0.85) and high IR emissivity (∼0.95) to induce a temperature drop of ∼2 °C. In contrast, the embedded carbon nanotubes (CNTs) can improve solar absorption (∼0.95) and induce a temperature increase of ∼7 °C. Owing to its radiative cooling and solar heating capability and compatibility with large-scale production, this Janus film is promising to bring new insights into the design of the next-generation functional textiles.

Commercial‐Like Self‐Cleaning Colored ZrO2‐Based Bilayer Coating for Remarkable Daytime Sub‐Ambient Radiative Cooling

AbstractPassive daytime radiative cooling (PDRC) is a low‐carbon energy technology to early achieve the “carbon neutrality” goal. Developing the commercial‐like colored PDRC coatings with sub‐ambient cooling performance is still a big challenge. Herein, a colored bilayer ZrO2‐PVDF PDRC coating with sub‐ambient cooling performance, self‐cleaning, and good weather resistance property is first reported. The colored bilayer coating is composed of a thick ultra‐white ZrO2‐PVDF reflective layer (Rsolar = 0.984) and a thin colorful layer. The obtained red bilayer ZrO2‐PVDF PDRC coating has a high solar reflectance (Rsolar = 0.950), which could realize the maximum temperature drop of 3.0 °C, and the calculated theoretical cooling power is as high as 131.66 W m−2, showing a good sub‐ambient cooling performance. Furthermore, they show a good self‐cleaning property. Meanwhile, the accelerated aging tests prove that they have a good weather resistance. This work provides a low‐cost, simple, and large‐scale method to prepare the commercial‐like colored PDRC coatings with the sub‐ambient cooling performance, showing a great practical application value.

Subambient daytime cooling enabled by hierarchically architected all-inorganic metapaper with enhanced thermal dissipation

Daytime radiative cooling shows great promise for cooling terrestrial objects without energy consumption. Intensive research has yielded numerous material candidates, such as nanophotonic structures and polymer-dielectric composites, however, challenges exist, ranging from the cost of nanofabrication for nanophotonic structures to environmental threats of micro and nanoplastics during polymer degradation under sunlight exposure. Moreover, the effect of a material’s thermal conductivity on heat dissipation has been overlooked in the past. Herein, we report the synthesis and study of a hierarchical hydroxyapatite inorganic radiative cooling (HIRC) metapaper to simultaneously achieve efficient radiative cooling and enhanced thermal dissipation to accelerate heat release. We demonstrate that the HIRC metapaper is featured with high solar reflectance (0.99) and high mid-infrared thermal emittance (0.90) and it yields a subambient temperature drop of 5.1 °C under solar irradiance of 950 W m−2 and a peak radiative cooling power of 104 W m−2 under the solar intensity of 910 W m−2 without polyethylene windshields. Moreover, the thermal conductivity of the metapaper exceeds that of polymer composites, thus enhancing the thermal dissipation from the underlying space. Furthermore, the hydroxyapatite’s biocompatibility eliminates the concern of micro and nanoplastics release into the environment.

Scalable and waterborne titanium-dioxide-free thermochromic coatings for self-adaptive passive radiative cooling and heating

Titanium dioxide (TiO2) is the most widely used white pigment for coatings, plastics, papers, and cosmetics, but its high price, high-pollution production process, and potential cancer risks have led to scientists and engineers attempting to find alternatives. However, little progress has been made thus far. Herein, we report the fabrication of TiO2-free coatings by mixing common solvent-based or waterborne resins and hollow glass beads of specific sizes. The TiO2-free white coatings exhibit much higher solar reflectance (∼0.96) and longwave infrared near-normal emittance (∼0.94) than many TiO2-based coatings, thus realizing highly efficient subambient cooling of ∼7.8°C at night and ∼7.1°C under direct sunlight. Specifically, the scalable and self-adaptive subambient radiative cooling and above-ambient solar heating coatings (ΔTcooling-heating = 9.5°C) are demonstrated when reversible thermochromic microcapsules are added, which significantly expands the scope of year-round energy-saving applications.

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 SiOx Ny 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.

Flexible composite film with artificial opal photonic crystals for efficient all-day passive radiative cooling.

All-day passive radiative cooling has recently attracted broader attention for its potential as a viable energy technology. Although tremendous progress has been achieved, the design and fabrication of low-cost high-efficiency radiators for all-day passive radiative cooling remains a challenge. Herein, we report a new type of flexible composite radiator film with built-in artificial opal-like structures for all-day passive radiative cooling. Using artificial opal structure concepts, the proposed polydimethylsiloxane (PDMS) radiator film with embedded polystyrene (PS) microsphere photonic crystals exhibits a sufficiently high solar reflectance of ∼92.7% when in a direct sunlight region, and a thermal emittance of ∼93.6% within the atmospheric window. Without the need for traditional reflectors like silver or aluminum foils, this composite film realizes subambient temperature reduction of ∼4.8 °C in direct sunlight and ∼8.5 °C during the night. This work provides a new fabrication approach for the low-cost production of structural polymer films for high performance and potential real word applications.

Superhydrophobic waste paper-based aerogel as a thermal insulating cooler for building

Daytime subambient radiative cooling is a most promising alternative to electricity-free building cooling. However, optical performance degradation arising from surface contamination and inevitable parasitic heat gain still pose unprecedented challenges to cool building at subambient temperatures. This paper proposed a superhydrophobic cellulose aerogel cooler (SHB-CAC) as building envelope by integrating self-cleaning capacity, passive daytime radiative cooling and thermal insulation to reduce environmental heat gain. The SHB-CAC demonstrates high solar reflectance (93%) and long-wave infrared emittance (91%), accomplishing a temperature drop of 8.5 °C lower than the ambient under sunlight of 800 W/m2 in the outdoor experiment. Notably, the SHB-CAC possesses low thermal conductivity (28 mW/(m∙K)) that suppresses parasitic heat gain from warmer surrounding and reduces cooling energy consumption. The self-cleaning property resulting from superhydrophobicity protects SHB-CAC from water wetting and dust contamination but also maintains its good surface radiation capacities under different humidity environment. A building energy simulation was conducted and results showed that 43.4% of cooling energy on average could be saved compared to the building baseline consumption, if SHB-CAC was widely used in China, indicating that the strategy of optical and thermal conductivity management of cooler has the potential to reduce the energy consumption of buildings.

Color-preserving daytime passive radiative cooling based on Fe3+-doped Y2Ce2O7

State-of-the-art materials for daytime passive radiative cooling (DPRC) often utilize a combination of solar reflector and infrared emitter in different structures, or even in expensive nanofabricated photonic structures, which limits their practical applications. In this study, pure Y2Ce2O7 is prepared by Sol-Gel method to realize DPRC. More attracting, the aesthetic performance of Y2Ce2O7 is modified by doping Fe3+ ions to regulate its color from ivory white to darker ones, realizing color-preserving DPRC. The high solar reflection and strong thermal infrared emission of a series of Y2Ce2-xFexO7+δ are caused by the high bandgap within solar spectrum and lattice strain and distortion of various bonds (e.g., Y-O, Ce-O), respectively. When using Y2Ce2-xFexO7+δ as a coating on a building in a hot season, the aesthetic of the building becomes better due to the darker color and the cooling performance is weaker with x increasing, indicating a tradeoff between energy saving and human aesthetics. According to the simulated results, the Y2Ce2O7 and Y2Ce1.9Fe0.1O7 coatings save 50.0% and 21.9% energy consumption compared with similar-colored coatings, respectively. Without expensive nanofabrication process, this series of Y2Ce2-xFexO7+δ provide new promising selections to develop buildings with the harmony of energy saving and human aesthetics.

Fabrication of Hydrophobic Multilayered Fabric for Passive Daytime Radiative Cooling

AbstractPassive daytime radiative cooling (PDRC) has attracted great attention recently due to its high potential for reducing global energy consumption. However, PDRC materials are easily contaminated in practical applications, which will seriously attenuate their long‐term cooling performance. In this work, a multilayered PDRC fabric that is composed of polydimethylsiloxane (PDMS), polymethyl methacrylate (PMMA), and cotton is shown. This multilayered fabric displays a high solar reflectivity (0.94) and an appropriate atmospheric window emissivity (0.79). A practical application test demonstrates the glass covered by this PDRC fabric can lower the temperature up to 7.8 °C under a solar intensity of ≈495 W m−2. Meanwhile, the multilayered PDRC fabric exhibits self‐cleaning property due to its high water contact angle of 118°. Rolling water can effectively remove the contaminants on the fabric surface. It is believed that this multilayered fabric is a promising material to alleviate the energy crisis and reduce greenhouse gas emissions.

Integration of Janus Wettability and Heat Conduction in Hierarchically Designed Textiles for All-Day Personal Radiative Cooling.

Personal cooling textiles are a promising energy-free pathway for confronting serious heat-related public health threats and improving industrial worker productivity. Current cooling strategies mainly focus on passive daytime radiation, and there is a lack of research on all-day cooling methods which utilize synergistic radiative, conductive, and evaporative heat dissipation. Herein, we demonstrate a hierarchical polyurethane/silicon nitride fibrous membrane with Janus wettability fabricated via a scalable electrospinning method followed by single-side hydrophilic plasma treatment. High angular-dependent solar reflectance (91%) and human body infrared emittance (93%) allow for a temperature drop of ∼21.9 °C under direct sunlight and ∼2.8 °C at night compared with traditional cotton. The innovative integration of Janus wettability and heat conduction in hierarchically designed textiles ensures a minimum sweat consumption of 0.5 mL h-1, avoiding harmfully excessive perspiration. The excellent all-day cooling performance of this hierarchical textile presents great advantages for smart textile, energy-saving, and personal cooling applications.

Optimization of random silica-polymethylpentene (TPX) radiative coolers towards substantial cooling capacity

In the context of global warming, radiative coolers with high solar reflectance and strong emissivity in the atmospheric window can cool the substrate as well as the ambient air. Silica at its nano or micro-scale being randomly dispersed into a uniform transparent polymer can form scalable radiative coolers for large-scale application. Promising cooling performance has been reported for silica-polymers compared with conventional cooling materials, but their performance can be largely influenced by various fabrication parameters. So far, how fabrication parameters influence the emissivity and the cooling performance has not been experimentally demonstrated and the cooling capacity of silica-polymers reported was not substantial compared to other superior radiative coolers. In this work, random silica-polymer has been optimized experimentally. Lab measurement and experimental testing of six fabricated silica-polymers under subtropical and desert climates indicated that due to the complexity of the thermo-radiative balance, high emissivity and strong selectivity are both indispensable in the production of high cooling power. If combined with superior reflectors with higher solar reflectance and especially the emissivity in 8–13 μm enhancing the heat dissipation ability, substantial cooling capacity can be achieved: under the harsh desert climate with average peak solar radiation over 1100 Wm-2, the combination presented sub-ambient temperature of maximum 4.7 °C when air temperature reached its peak and the maximum daytime and night-time sub-ambient temperatures were 12.5 °C and 15.9 °C respectively.

Optically selective PDMS/AIN coatings as a passive daytime radiative cooling design

Passive radiative cooling is a natural process in which an object or surface emits thermal radiation into the cold sky to prevent it from heating up. Daytime passive cooling technology requires a selective design to reflect incoming solar radiation and simultaneously emit thermal radiation to the cold universe through the sky window 8–13 µm. In this study, we propose multilayer design of cooling device consisting of bilayer of polydimethylsiloxane foil (PDMC) and aluminum nitride (AIN) both deposed on silver coated glass. The spectral radiative properties of the proposed device are examined using the transfer matrix method. Numerical results show that our design acts as infrared optical window, as it has a high solar reflection with an average of over 96 % and strong thermal emissivity in atmospheric transparency window with an average of over 90 %. In absence of parasitic heat exchange, high cooling power of 107 W/m2 at ambient temperature with an effective temperature drop of 45 K below the ambient are obtained. The cooling effect persists even in the presence of parasitic heat exchange with temperature reduction of 17 K below the ambient. To assess the validity of this study, the performance of the multilayer design is compared with an experimental black emitter and the results indicate that our design has good properties in terms of temperature reduction. In the light of our findings, we can state that our design can fundamentally enable new methods for advancing the applications of exploiting radiative cooling.

The effect of various nickel oxide ratios on the color and reflectivity of PY53 nickel-titanate yellow pigment

Complex inorganic nickel-titanate PY53 yellow pigments with a high thermal solar reflectance were synthesized. Four different compositions were made with increasing nickel oxide (NiO) ratios, with Sb2 O3 and TiO2 as the fixed components. Mixtures were prepared by adding excess NiO of 0/2/4/6 wt%. Excess 6 wt% NiO gives a maximum near infrared solar reflectance of 55.2%. The reflectance (%R) analysis of the pigments was measured with a reflection spectrophotometer. Pigment powders were used as reflective material in a polymer matrix. The chemical composition of the pigments was determined using an X-ray Fluorescence Spectrometer (XRF). The crystal structure of the pigments was analyzed using X-ray Diffraction (XRD) measurements. The distribution and particle structure of the particles were determined using a scanning electron microscope (SEM). The particle size distribution (PSD) was characterized by measuring with a laser diffraction particle size analyzer. The PY53 yellow pigment colors were measured using the CIE L*a*b* color index. The PY53 yellow pigment was milled, using a Rosetti mill, into the same particle sizes to decrease the effects of the milling process on its properties. The yellowness (*b) increased as the NiO ratio increased in all applications. Excess 6 wt% NiO gives a maximum yellowness value polymer matrix 50.8%. The newly synthesized PY53 yellow pigment can be used in construction, for enamel coatings with its high reflectivity performance, for comfortable clothing, and for durable properties in plastic products.

Near-infrared solar reflectance and chromaticity properties of novel green ceramic pigment Cr-doped Y3Al5O12

Y3Al5O12 as substrates accompanied by Cr3+ as dopant ions were prepared as new NIR reflective ceramic pigments. The structure verification of Cr3+ substituted Y3Al5O12 was conducted by FT-IR and XRD, and its particle morphology showed a combination of baseball-like and granular shapes via FE-SEM. The doped samples exhibited high NIR solar reflectance values ranging from 91.47% to 75.96%, which is inversely proportional to the Cr3+ concentration. The variation of the near-infrared reflectance was elucidated from the aspect of the carrier concentration and electron traps. The green tone of the Cr-doped Y3Al5O12 ceramic powders has concerned with its absorption peak in the UV–Vis region, originating from the energy level transition of 4A2g→4T1g and 4A2g→4T2g of Cr3+. Also, the new ceramic pigments possessed the characteristics of acid and alkali resistance (Δ E∗ ​< ​3), and high-temperature (1000 ​°C) resistance. The Cr-doped Y3Al5O12 pigments have the virtue of high NIR reflectance, pleasant green color and high tolerability, and are promising to become the colorant for blanks and glazes, which can make up for the inadequacy of a single material.

Scalable aqueous processing-based radiative cooling coatings for heat dissipation applications

Passive daytime radiative cooling (PDRC) is an electricity-free cooling technology that has received great interest for its’ environmental benefits. Current organic polymer PDRC materials often require harmful organic solvents for dispersion and inorganic dielectric PDRC materials require costly vacuum processing or have poor flexibility and water-repellence. In addition, most PDRC applications focus on refrigeration at sub-ambient temperatures using a selective emittance spectrum. Here a scalable aqueous-based processing method utilizing commercially available hollow dielectric microspheres is reported to create dielectric PDRC coatings with broadband emittance spectra for the heat dissipation of outdoor devices used above-ambient temperatures. A high solar reflectance and thermal infrared emittance of 0.93 at a thickness of 500 μm can be achieved due to the multi-interface reflectivity between the dielectric material and air in the hollow stacked microspheres. The max temperature (∼ 46 °C) of the outdoor device at a heat density of 5000 W m−2 can drop 20 °C at a solar intensity of ∼ 925 W m−2 compared to an Al metal wall (∼ 66 °C), which is only 4 °C higher than the ambient temperature of 42 °C. More importantly, the aqueous paint does not require any volatile organic compounds and has great stability, water repellence, and excellent flexibility.

Facile fabrication of EVA cellular material with hydrophobic surface, high solar reflectance and low thermal conductivity via chemical foaming

Recently, cellular materials grasp researchers’ attention for the high solar reflectance performance. However, studies on the construction of hydrophobic surface and solar reflectance based on the closed cell structure obtained by azodicarbonamide (AC) foaming are still insufficient. In this work, a series of ethylene-vinyl acetate copolymer (EVA) cellular materials with different foaming ratios were fabricated by chemical foaming. By increasing the weight content of foaming agent AC from 0 to 12 phr, the apparent density of samples reduced from 0.935 to 0.045 g/cm3, while the closed cell size expanded from 0 to 169.49 μm. In contrast with pristine EVA resin, the water contact angle on the internal surface of EVA-AC12 (the sample with AC weight content of 12 phr) enhanced to 145°. Simultaneously, EVA-AC12 possessed the high near infrared reflectance of 80.5% and low thermal conductivity of 0.04 W/m·K, resulting in an excellent passive cooling effect of 18.6 °C than pristine EVA resin. Moreover, the as-prepared EVA cellular material has great mechanical properties. In summary, the EVA soft cellular material with a hydrophobic surface fabricated in this work can be used as self-cleaning passive cooling material.

Double skin polystyrene- aluminium radiation reflector roofs in arid environments for passive cooling - A case study in Sohar, Sultanate of Oman

Global warming increases the indoor temperature and energy consumption of residential buildings in arid regions. No doubt, development of effective passive cooling techniques are crucial to reduce the room air temperature as well as the energy consumption of buildings. The present study analyzes merits of a polystyrene foam - aluminium radiation reflector double skin system to be used as a passive cool roof in an arid region like Oman and assess the effect of air ventilation as a thermal barrier between the double skin layers. The present study incorporates a radiation reflective layer, dynamic air ventilation, and thermal insulation configurations. The study was performed in 3 stages for 0, 20 and 40 cm air ventilation between polystyrene-aluminium radiation reflector layers. It was confirmed by field experiments that a significant decrease in indoor and roof slab temperatures were observed for polystyrene-aluminium radiation reflector passive cool room in comparison to a traditional bare roof. The experimental studies reveal that comparing with a traditional roof, the room air temperature of double skin polystyrene-aluminium radiation reflector was reduced by 2.42 °C (6.7%), 3.69 °C (10.4%) and 4.7 °C (12.8%) for 0, 0.2 m and 0.4 m air ventilation between the polystyrene foam and aluminium reflector, respectively. The corresponding reduction in roof surface temperature was 6.91 °C (16.8%), 8.1 °C (24.5%) and 11.11 °C (31.9%) respectively. It was observed that the indoor air and roof surface temperature of the passive cool roof reduced as the air ventilation increased from 0 to 0.4 m. This proves that dynamic nature of air ventilation has a major role in improving the natural convective heat flux and thereby increase in thermal resistance of double skin roof. Also, Aluminium radiation reflector of the double-skin roof has high solar reflectance (albedo) which results in reduced short wave and long wave radiative heat flux to the ceiling in comparison with the conventional roof. Integration of polystyrene foam thermal insulator, air ventilation and aluminium radiation reflector in the passive cool roof system reduces conduction, convection, and radiation heat transfers, respectively between the roof and ceiling. It is conclusively proven that the double-skin polystyrene-aluminum radiation reflector passive cool roof with air ventilation can appreciably lower the room air and roof slab temperatures during hot sunny days.