Low Solar Absorptance Research Articles

Temperature-adaptive metasurface radiative cooling device with excellent emittance and low solar absorptance for dynamic thermal regulation

Temperature-adaptive metasurface radiative cooling device with excellent emittance and low solar absorptance for dynamic thermal regulation

Towards Climate, Bioclimatism, and Building Performance—A Characterization of the Brazilian Territory from 2008 to 2022

Representative weather data are fundamental to characterizing a place and determining ideal design approaches. This is particularly important for large countries like Brazil, whose extension and geographical position contribute to defining diverse climatic conditions along the territory. In this context, this study intends to characterize the Brazilian territory based on a 15-year weather record (2008–2022), providing a climatic assessment based on a climatic and bioclimatic profile for the whole country. The climate analysis was focused on temperature, humidity, precipitation, and solar radiation, followed by a bioclimatic analysis guided by the Givoni chart and the natural ventilation potential assessment. In both situations, the results were analyzed using three resolutions: country-level, administrative division, and bioclimatic zones. This study also identified representative locations for the Brazilian bioclimatic zones for a building-centered analysis based on the thermal and energy performance of a single-family house with different envelope configurations. The results proved that most Brazilian territories increased above 0.4 °C in the dry bulb temperature and reduced relative humidity. The precipitation had the highest reduction, reaching more than 50% for some locations. The warmer and drier conditions impacted also the Köppen–Geiger classification, with an increase in the number of Semi-Arid and Arid locations. The bioclimatic study showed that ventilation is the primary strategy for the Brazilian territory, as confirmed by the natural ventilation potential results, followed by passive heating strategies during the year’s coldest months. Finally, building performance simulation underlined that, in colder climates, indoor thermal comfort conditions and air-conditioning demands are less affected by solar absorptance for constructions with low U-values, while in warmer climates, low solar absorptance with intermediary U-values is recommended.

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.

Intelligent Space Thermal Control Radiator Based on Phase Change Material with Partial Visible Transparency

Coating structures with dynamically adjustable infrared emissivity are crucial in spacecraft components to cope with the transient thermal environments of space. For a long time, thermochromic phase change materials have been widely used in applications requiring emissivity adjustment, and optimizing the range of adjustable infrared emissivity has always been at the forefront of research. However, reducing the absorption of solar radiation has significant implications for the practical application and thermal stability of spacecraft components in space environments. In this paper, we propose a multilayer film structure based on the phase change material VO2 combined with the materials ZnSe and ITO to achieve low solar radiation absorption and adjustable infrared emissivity for intelligent thermal radiators in space. Through finite element simulation analysis of the structure, we achieve a solar radiation absorption rate of 0.3 and an adjustable infrared emissivity of 0.49. According to Stefan–Boltzmann’s law, the structure exhibits strong radiative heat dissipation at high temperatures and weak energy dissipation at low temperatures to maintain the thermal stability of the device and ensure efficient operation. The intelligent thermal radiator operates based on the principles of Fabry–Perot resonance. Therefore, the multilayer structure based on the phase change material VO2 demonstrates excellent performance in both solar radiation absorption and adjustable infrared emissivity, showcasing its tremendous potential in the field of intelligent thermal control in aerospace.

Preparation and properties of pale‐colored and optically transparent fluoro‐containing polyimide films with low solar absorptivity and enhanced high‐temperature dimensional stability for potential applications in space environments

Preparation and properties of pale‐colored and optically transparent fluoro‐containing polyimide films with low solar absorptivity and enhanced high‐temperature dimensional stability for potential applications in space environments

Metasurface absorber based single junction thin film solar cell exceeding 30% efficiency.

In this article, we report, as per our knowledge, for the first time, a thin film single junction solar cell with a metasurface absorber layer directly incorporated. We have used an interconnected dual inverted split ring resonator pattern in the InAsP absorber layer. The structure eliminated patterns of conventional metals, such as silver, aluminum, and gold, from the active layer, a common drawback in conventional solar absorbers, hindering their direct integration into solar cells. Optical simulation results show a peak ideal short circuit current density of 76.23mA/cm2 for the meta-absorber structure under solar illumination. This current is the highest among previously reported absorbers based on Group IV materials and III-V compounds, overcoming the low solar absorption of such metasurfaces. The final proposed solar cell structure combines this meta-absorber layer with traditional efficiency enhancement methods namely anti-reflecting coating, textured back reflector, and transparent top electrode. This novel single junction structure shows a solar absorption efficiency of 97.86% and a power conversion efficiency of 30.87%, the highest for III-V solar cells. Our device proves the ability of metasurface absorber layers to produce high-efficiency solar cells and is expected to pave the way for integrating novel meta-devices into state-of-the-art photovoltaic devices, aiding the global transition towards clean energy sources.

Honeycomb-like porous copper with pleated surface for supporting phase change material with enhanced thermal conductivity and solar thermal storage performance

Paraffin wax (PW) is concerned in the field of thermal storage because of its high thermal storage density, but it generally has problems such as low thermal conductivity, poor shape stability, and low solar absorption, which limit its application in solar thermal storage and thermal management. In this study, a honeycomb-like 3D porous Cu skeleton with a pleated surface was prepared by the salt template method, and the phase change composites (PCCs) were prepared using this 3D thermal conductive skeleton to support PW. The honeycomb-like porous structure with pleated surface has positive effects on the shape stability and solar-thermal conversion of PW. The thermal conductivity, leakage resistance, and solar-thermal conversion efficiency of the PCCs were significantly improved. Among them, the thermal conductivity of PW@PSHCPC2 reached 3.3 Wm−1 K−1, which was 16.5 times higher than that of PW, and the solar-thermal conversion efficiency reached 89.2 %, and a stable current of 61.8 mA was generated in the solar thermal power generation experiment. These results show that these PCCs have great potential for applications in several fields such as solar thermal storage, waste heat recovery, solar power generation, and electronic thermal management.

Combination optimization, importance order of parameters and aging consequence prediction for thermal insulation coating with radiation characteristics

Thermal insulation coating (TIC) is beneficial for reducing building energy consumption. In this work, a joint optimization and analysis approach for multi-parameters of TIC was proposed to determine its optimization trend, importance order of parameter and available parameter combinations. Its radiation characteristics and thermal resistance were comprehensively considered. Based on the approach, TIC was evaluated from energy saving rate of cooling (ESRcool), heating (ESRheat), and total energy consumption (ESRtotal) in two climates: hot summer cold winter (Shanghai), and hot summer warm winter (Guangzhou). Results indicated that constantly positive ESRcool was only achieved by optimizing solar absorptivity, while constantly positive ESRheat can be obtained by optimizing different parameters. To guarantee positive ESRtotal, thermal resistance should exceed 0.11 m2·K·W−1 or solar absorptivity should be between 0.2 and 0.6 in Shanghai, while solar absorptivity should be between 0.1 and 0.3 in Guangzhou. For these two cities, the most important parameter was thermal resistance or solar absorptivity. The optimal TIC for Shanghai was “high thermal resistance + low solar absorptivity + low emissivity”, while high emissivity was suitable for Guangzhou. By utilizing the approach and results, energy-saving effect of TIC can be optimized during material design, and influence of material aging can be predicted.

Sustainable solar desalination through interfacial evaporation: Integration of chitosan aerogel-impregnated graphene nanoplatelets solar evaporator and phase change material

Solar energy offers several advantages for desalination, including affordability, eco-friendliness and sustainability. However, certain factors have influence on the evaporation rate and result in low yield. These factors include a low solar absorption rate, slow temperature rise, and insufficient heat trapping. To address these challenges, we integrated chitosan aerogel-impregnated with graphene nanoplatelets (CAGNPs) as a photothermal absorber (PTA) and soy wax (SWAX)-based thermal energy storage material (TESM) into spherical solar stills (SSS). This innovative combination enhances solar absorption rates and enables energy storage, thereby enabling all-day, all-weather freshwater production. Waste beverage aluminum cans were utilized to fill the phase change material (PCM) for storing thermal energy. Three identical SSSs were fabricated and their performance was examined from March to August 2023. This comprehensive evaluation demonstrated the effect of photothermal absorption and energy storage, elucidating their collective impact on system efficiency over this period. The test showed that the conventional SSS achieves a freshwater productivity of 1.6 L/m2 over 9 h. The integration of SWAX gives rise to 3.0 L/m2, significantly improving freshwater production compared to the conventional SSS. Moreover, the combined effect of CAGNP and SWAX into SSS results in substantial enhancement, yielding a freshwater productivity of 4.1 L/m2. The obtained exergy efficiencies for the conventional SSS, SSS-PCM, and SSS-CAGNP-SWAX were 1.21 %, 1.69 %, and 1.85 %, respectively. Furthermore, the use of SWAX facilitates the effective storage and release of thermal energy, enabling all-day, all-weather freshwater production.

High-entropy perovskite oxides for direct solar-driven thermochemical CO2 splitting

The global energy crisis and climate change have fueled interest in solar-driven thermochemical CO2 splitting as a potential solution. However, conventional materials like CeO2 encounter limitations attributable to their low solar absorptivity and CO yield (even at high reduction temperatures), exerting a detrimental influence on both solar energy utilization and process efficiency. This study proposes high-entropy A-site doped perovskites as a potential solution for efficient solar thermochemical CO2 splitting. The increased configurational entropy enhances the solar thermal CO2 splitting cycles by decreasing oxygen vacancy formation energy and lattice oxygen migration energy barrier. This is supported by experimental results, where Sm0.25Sr0.25Ca0.25La0.25MnO3 achieved a maximum CO yield of 764.76 μmol g−1 with low temperature difference between the reduction and oxidation steps, marking an important progress in solar thermal CO2 splitting cycles. The study demonstrates the potential of high-entropy perovskites for direct solar energy harvesting with high spectrum absorption coefficients and sustainable CO2 utilization.

Highly efficient flexible structured metasurface by roll-to-roll printing for diurnal radiative cooling

An ideal radiative cooler requires accurate spectral control capability to achieve efficient thermal emission in the atmospheric transparency window (8–13 μm), low solar absorption, good stability, scalability, and a simple structure for effective diurnal radiative cooling. Flexible cooling films made from polymer relying on polymer intrinsic absorbance represent a cost-effective solution but lack accuracy in spectral control. Here, we propose and demonstrate a metasurface concept enabled by periodically arranged three-dimensional (3D) trench-like structures in a thin layer of polymer for high-performance radiative cooling. The structured polymer metasurface radiative cooler is manufactured by a roll-to-roll printing method. It exhibits superior spectral breadth and selectivity, which offers outstanding omnidirectional absorption/emission (96.1%) in the atmospheric transparency window, low solar absorption (4.8%), and high stability. Impressive cooling power of 129.8 W m−2 and temperature deduction of 7 °C on a clear sky midday have been achieved, promising broad practical applications in energy saving and passive heat dispersion fields.

Design of VO2-based spacecraft smart radiator with low solar absorptance

Thermochromic film with the ability to self-adaptively switch emittance is important for spacecraft missions experiencing rapidly increasing demand. Current research on thermochromic films primarily focuses on optimizing emittance tunability, while the solar absorptance of these films remains high, limiting their practical application in spacecraft thermal control. In this work, a VO2-based spacecraft smart radiator with low solar absorptance and high emittance tunability is proposed. The smart radiator is a multilayer structured film composed of a switchable resonator (i.e., a superposed Fabry-Perot cavity) and a solar reflector (i.e., a Distributed Bragg Reflector). The performance of the smart radiator is optimized by combining the Rigorous Coupled Wave Analysis method and Genetic Algorithm, achieving a normal solar absorptance of 0.121 and emittance tunability of 0.825. The solar absorptance and emittance tunability of the smart radiator remain excellent even at large incident angles. The underlying mechanisms involved in the smart radiator are attributed to the combination of superposed Fabry-Perot resonance and multiple solar reflectance. In addition, we provide a numerical example using a geosynchronous orbit satellite to demonstrate that the regulation range of the net heat dissipation flux density can increase from 146.5–463.9 W/m2 by using Optical Solar Reflector to 0–494.6 W/m2 when using the proposed smart radiator within the permissible temperature range (263.15–318.15 K) of spacecraft. This VO2-based spacecraft smart radiator with outstanding properties which outperform the existing thermochromic films reveals the promising potential of thermochromic films for spacecraft intelligent thermal control applications.

Spacecraft smart radiation device with near-zero solar absorption based on cascaded photonic crystals

Smart radiation device (SRD) for spacecraft based on thermochromic material vanadium dioxide (VO2) attracts increasing interest due to its ability of self-adaptive emission regulation. However, the spacecraft SRD is constrained by the high solar absorption at a high temperature and low emission tunability in practical applications. In this paper, a SRD is proposed for spacecraft thermal control, which consists of a solar reflector based on cascaded photonic crystals and a thermal emitter (VO2/BaF2/Ag). By cascading four photonic crystals with photonic band gaps for different central wavelengths, the total reflection range of the SRD in the solar radiation band is broadened. It is found that the SRD can realize near-zero solar absorption and high emission modulation depth, simultaneously. Though optimizing the thicknesses of the VO2 and BaF2, the emission modulation depth can reach 0.69, and the solar absorption can reduce to 0.08. In addition, the physical mechanism is studied by the distribution of the electric field. It is proved that the near-zero solar absorption originates from the broadband total reflection of the cascaded photonic crystals. The phase transition properties of VO2 and the difference in inherent losses at high and low temperatures result in high emission modulation depth. Besides, the low solar absorption and high emission modulation depth performances of the SRD can remain excellent when the angle of incidence is small than 46° regardless of the transverse electric wave or transverse magnetic wave. This work not only provides a new idea for designing SRDs, but also is expected to accelerate the development of intelligent thermal control of spacecraft.

Flexible thin film optical solar reflectors with Ta2O5-based multimaterial coatings for space radiative cooling

Optical Solar Reflectors (OSRs) combine low solar radiation absorption (α) and high broadband infrared emissivity (ε) and are applied to the external surface of spacecraft for its thermal management. Bulk glass OSR tiles are the incumbent, but ultra-lightweight and thin-film flexible OSR coatings are raising considerable interest for both space and terrestrial radiative cooling applications. In this work, a genetic algorithm combined with a transfer matrix method is used for the design and optimization of multimaterial thin-film OSRs for broadband radiative cooling. The algorithm simultaneously optimizes the spectral performance of the OSR at two parts of the wavelength spectrum, solar (0.3–2.5 μm) and thermal infrared (2.5–30 μm). The designed optimized OSR structure consists of 18 alternating layers of three materials, SiN, SiO2, and Ta2O5, on top of an Al mirror backreflector, with a total thickness of only 2.088 μm. The optimized multilayer stack contributes distributed Bragg reflections that reduce the residual solar absorption below that of an uncoated Al mirror. The optimized OSR is demonstrated experimentally on a 150 mm (6 in.) Si wafer and on a flexible polyimide substrate using a production level reactive sputtering tool. The fabricated thin film OSR shows good thermal-optical property with α = 0.11 and ε = 0.75 and achieves a net cooling power of 150.1 W/m2 under conditions of one sun total solar irradiance in space. The ultrathin coating fabricated using hard inorganic materials facilitates its integration onto flexible foils and enables large-scale manufacture of low-cost OSRs for broadband radiative cooling applications.

Optical properties of aluminum oxide powder modified by nanoparticles and prospects for its use in solar power and space industry

Studies on the phase composition, diffuse reflectance spectra (ρλ) and solar absorptance (αs) and their changes after modification of aluminum oxide powder by nanoparticles of cerium dioxide, magnesium oxide, silicon dioxide, titanium dioxide and zinc oxide are presented. The diffuse reflectance spectra and solar absorptance αs of nanoparticles used for modification were studied.In its initial state, Al2O3 was found to exhibit high reflectivity. The obtained value of the solar absorptance, 0.074, was significantly less than that of the widely used white reflective powders – ZnO and TiO2. After modification by nanoparticles of wide-band compounds, MgO and SiO2, the reflectivity of Al2O3 powder changed slightly. Modification by SiO2 nanoparticles at a concentration of 1 wt % decreased the solar absorptance to 0.064. At small concentrations (0.1–1 wt. %) of nanoparticles of semiconductor compounds, CeO2, TiO2 and ZnO, the absorption edge of Al2O3 powder shifted to a longer wavelength region of the spectrum. At 10 wt % concentration of nanoparticles, the absorption edge of Al2O3 powder shifted to a value close to the absorption edge of the initial nanopowders.Such nano-micropowder systems can be used to develop new materials with improved optical properties for solar power and space technology applications, as well as other applications that need high reflectance and low solar absorptance. New data on shifts of the main absorption edge after modification by semiconductor nanoparticles can be useful in developing new materials for photocatalysis.

The coating with ZrO2-coated hollow glass microspheres: Low solar absorption and high microwave transmittance

Carbon fiber epoxy resin (CFRP) is gradually used as the space satellite antenna material, thermal control coatings (TCCs) are prepared on the surface to ensure the stable operation. However, the traditional TCCs have no wave transmission capability. The hollow glass microspheres (HGMs) were used to prepare the coating in our work, and ZrO2 was coated on its surface (Zr-HGM) to improve the solar reflectivity. The Zr-HGM powders were obtained by a homogeneous precipitation method with an appropriate reaction time of 4 h. By investigating the optical and dielectric properties, we found that the solar absorption of the coating with Zr-HGM was reduced by half compared to the coating with the original HGM. The microwave transmittances were all above 96.5% from 2 to 18 GHz. Our findings have proposed a preparation method for a new CFRP-based TCC, which will ensure the stable operation of the satellite antenna.

Evaluation of the thermal performance of EPS core panels: A multicriteria approach

To enhance the energy efficiency, it is crucial to develop construction systems that can effectively adapt to the diverse climates found in Brazil, particularly for application in social housing projects. This study aims to analyze the thermal viability of EPS core panels in Brazilian bioclimatic zones, which have been gaining prominence in the national market. For this, simulations were conducted using the EnergyPlus software to compare the performance of this material with that of masonry with ceramic blocks. The AHP-Gaussian method was employed to assist the decision-making process regarding the most viable system. The results indicated that EPS core panels with low solar absorptance in hot zones presented a 39.95% higher thermal load than masonry. On the other hand, in cold regions, EPS core panels were among the best solutions, reducing the thermal load by 50.39% in Curitiba when combined with high solar absorptance and shading of the openings. Consequently, through the multicriteria decision-making method, it was concluded that the use of EPS core panels in areas with high thermal amplitude offers advantages, while it should be avoided in hot regions. Thus, this study identified the technical feasibility of EPS core panels and masonry with ceramic blocks in Brazilian bioclimatic zones. It also offers guidance to professionals and public agents in the construction sector, helping them choose suitable materials for social housing. Additionally, this study presented innovation by demonstrating the benefits of employing multicriteria decision-making methods, expanding the possibilities for analyzing and selecting more energy-efficient construction systems.

Preparation of white ZrO2 coating with low solar absorptance on aluminum alloy by plasma electrolytic oxidation

The plasma electrolytic oxidation (PEO) coatings with the color of black or white have good prospects in thermal control application. However, it still remains a challenge for obtaining the PEO coatings on Al alloys to achieve high whiteness that in turn could give adequate low absorption thermal control characteristic. Herein, a bright white oxide coating mainly composed of ZrO2 was successfully prepared on 6061 aluminium alloy by PEO process from a fluorozirconate electrolytic solution. The microstructure and composition of the coating were studied by SEM, EDS, XRD and XPS. The thermal control properties were measured by UV-VIS-NIR spectrophotometer and infrared emissivity spectrometer, respectively. The stability of thermal control properties of the coating was also evaluated by simulated space environmental tests. The corrosion protection performances of the coating were investigated by potentiodynamic polarization and salt spray tests. Results show that the oxide coating is composed of t-ZrO2 as well as a little of m-ZrO2 and γ-Al2O3 with a porous microstructure. The coating exhibits low solar absorptance (αs) of 0.205 and high emissivity (ε) of 0.84, which is comparable for the typical space white paints. In addition, the coating owns excellent stability in thermal control properties under simulated space environments. The coating has a good durability in corrosive environment though it provides only slight improvement in corrosion resistance of 6061 aluminium alloy due to the presence of many larger micropores in the coating.

Effect of Sr doping and temperature on the optical properties of BaTiO3

Ceramics with low solar absorptivity and high infrared emissivity play an important role in radiative cooling applications. BaTiO3, for instance, is one promising radiative cooling ceramic, although it has been mainly explored only in electronics. In this work, the solar-reflective and infrared-radiative properties of Sr-doped BaTiO3 are studied systematically. Ba1-xSrxTiO3 (x = 0, 0.2, 0.25, 0.3, 0.35) ceramics synthesized by sol-gel method were investigated in terms of their microstructure, phase transition, and optical properties. Increasing infrared emissivity and solar absorptivity were observed with increasing Sr content. The optimized Ba1-xSrxTiO3 achieves solar absorptivity of 0.17 and infrared emissivity of 0.78. The lowest absorptivity-emissivity ratio of 0.24 was observed for Ba0.8Sr0.2TiO3. Meanwhile, the solar absorptivity and infrared emissivity exhibited temperature stabilization, even affront heat-induced phase transition. Finally, the coupling relation between band gap, Ti–O vibration, and optical properties of Ba1-xSrxTiO3 ceramics are clarified, which is promising to improve their radiative cooling performance.

Micro-structured polyethylene film as an optically selective and self-cleaning layer for enhancing durability of radiative coolers

Abstract Passive daytime radiative cooling (PDRC) as a zero-energy cooling technology that reflects most of sunlight and emits infrared thermal radiation to outer space, has attracted much attention. However, most PDRC materials suffer dust accumulation problem during long-term use, seriously detrimental to their cooling performance. Here, we demonstrate a micro-structured polyethylene film fabricated through a scalable hot embossing lithography (named HELPE), enables good superhydrophobic property and therefore excellent self-cleaning performance as a universal protective layer for most PDRC materials. Specifically, the precisely designed three-dimensional periodic micron columns on polyethylene film allow for high water droplet contact angle of 151°, and the intrinsic molecular bindings of polyethylene endow low solar absorption (A = 3.3 %) and high mid-infrared transmission (T = 82.3 %) for negligible optical impacts on underlying PDRC materials. Taking polyvinylidene fluoride (PVDF) radiative cooler as an example, when covered with the HELPE film the net cooling performance maintains unchanged (7.5 °C in daytime and 4.5 °C in nighttime) compared to that without HELPE film. After 12 days continuous outdoor experiment, none of obvious dust accumulation can be observed on the radiative cooler covered with HELPE film. Our work offers a universal pathway for most PDRC materials toward practical applications with minimal maintenance need.