High Thermal Emittance Research Articles

Probing the spectrally selective property of NbB2‐based tandem absorber coating for concentrated solar power application

AbstractSolar selective coating with good thermal stability is the primary requirement for a concentrated solar power (CSP) plant to function with better photothermal efficiency. In recent years, ultra‐high‐temperature ceramic‐based coatings have been explored as potential materials for solar selective coatings. In this context, NbB2/Nb(BN)/Al2O3 tandem absorber coating was designed to be fabricated on a stainless‐steel substrate by the radio frequency magnetron sputtering of spark plasma sintered ceramic target. In the bulk form, the NbB2 ceramic exhibits high solar absorptance (α = 0.756) and thermal emissivity (ε = 0.43), whereas the amorphous single NbB2 coating exhibits α/ε = 0.716/0.13. Reactive sputtering of NbB2 in nitrogen produced a semi‐transparent coating with an optical bandgap of ∼2.80 eV and was used as the secondary absorber layer. Raman and X‐ray photoelectron spectroscopy analyses reveal mild oxygen incorporation in the absorber layers. The developed SS/NbB2/Nb(BNO)/Al2O3 tandem absorber exhibits a good solar absorptance of 0.950 and a moderate thermal emissivity of 0.15 at room temperature. The coatings exhibited good thermal stability when heated in vacuum for 5 h up to 700°C, and the selectivity (α/ε) remains above 6. The present work shows the possibility of exploring NbB2‐based tandem absorber coatings for CSP applications.

Thermally stable Ni@SiO2 core-shell nanoparticles for high-temperature solar selective absorber

Improving the thermal stability of solar absorbers persists to be the challenge for solar-thermal technologies operated at high temperatures, such as concentrated solar power trough system. Herein, we report a rationally designed solar selective absorber based on Ni@SiO2 core–shell nanoparticles with significantly improved thermal stability at temperatures up to 700 °C. Ni nanoparticles were encapsulated into dense ceramic SiO2 nanoshells, which effectively prevented the formation of Ni-alloys and oxides by inhibiting diffusion of both inclusion Ni and metallic atoms from the substrate at high temperatures. Besides, the spectral absorption properties of the Ni@SiO2 solar absorber can be optimized by tuning the sizes of Ni cores and absorbing structures. As a result, high solar absorptance (∼0.913) but low thermal emittance (∼0.085) have been achieved for a double-layer Ni@SiO2 absorber with Ni cores of 40 nm and 80 nm. The excellent thermal stability and high absorption selectivity of the Ni@SiO2 solar absorber indicate its great potential for high-temperature solar-thermal conversion applications.

Bioinspired Radiative Cooling Structure with Randomly Stacked Fibers for Efficient All-Day Passive Cooling.

Without the help of compression-based cooling systems, natural creatures have to make use of other things to decrease their body temperature to survive under thermally harsh conditions. This work finds that the silkworm cocoon of Bombyx mori protects pupae from the rapid temperature fluctuations via the randomly stacked silk fibers, which possess high solar reflectance and thermal emittance for thermal regulation. Inspired by this microstructure, the melt-blown polypropylene (MB-PP) with randomly stacked fibers is fabricated by a large-scale melt-blown fabrication method. For enhancing the thermal emittance of MB-PP, the surface-modified MB-PP (SMB-PP) is obtained by constructing the poly(dimethylsiloxane) film on the MB-PP. As the reason for its high solar reflectance (∼95%) and thermal emittance (∼0.82), the SMB-PP displays subambient temperature drops of 4 °C in the daytime and 5 °C in the nighttime, respectively. Moreover, building energy simulation indicates that the SMB-PP could save ∼132 GJ (∼58.1% of the baseline energy consumption) for 1 year in the contiguous United States. Overall, the bioinspired structures offer a novel pathway out of cooling buildings, showing great promising application prospects in zero-energy buildings.

Reflective and transparent cellulose-based passive radiative coolers

Radiative cooling passively removes heat from objects via emission of thermal radiation to cold space. Suitable radiative cooling materials absorb infrared light while they avoid solar heating by either reflecting or transmitting solar radiation, depending on the application. Here, we demonstrate a reflective radiative cooler and a transparent radiative cooler solely based on cellulose derivatives manufactured via electrospinning and casting, respectively. By modifying the microstructure of cellulose materials, we control the solar light interaction from highly reflective (> 90%, porous structure) to highly transparent (≈ 90%, homogenous structure). Both cellulose materials show high thermal emissivity and minimal solar absorption, making them suitable for daytime radiative cooling. Used as coatings on silicon samples exposed to sun light at daytime, the reflective and transparent cellulose coolers could passively reduce sample temperatures by up to 15 °C and 5 °C, respectively.

Development of ZrB2-Based Single Layer Absorber Coating and Molten Salt Corrosion of Bulk ZrB2–SiC Ceramic for Concentrated Solar Power Application

This study has dual objectives, one to develop single layer ZrB2-SiC-based ceramic absorber coating on stainless substrate, and another aim is to explore the degradation of ZrB2- SiC bulk ceramic in molten solar salt. In particular, we report the molten salt corrosion behavior of earlier developed ZrB2-SiC ceramic composite at 400 °C for 100 h. The corrosion rate in solar salt was determined to be 0.73 ± 0.09 mg h-1/cm2by gravimetric analysis. Binary and ternary oxide phases were formed due to electrochemical corrosion. The hot-pressed bulk ZrB2-SiC ceramic exhibits a high solar absorptance of 0.848 and also a high thermal emissivity of 0.66. A one-layer solar selective absorber coating without antireflection layer was developed using the ceramic disk as a sputtering target. When deposited using radio frequency (RF) magnetron sputtering technique on stainless steel substrate, a solar absorptance of 0.769 and thermal emissivity of 0.15 was obtained for an amorphous coating of thickness ~90 nm, without antireflection coating. The composition, determined by electron probe microanalyzer and X-ray photoelectron spectroscopy techniques, revealed nonstoichiometric phases in the layer with a relatively higher concentration of the lighter elements. The implication of the present work for concentrated solar power application is discussed. © 2021 American Chemical Society. All rights reserved.

Investigation into ethanol effects on combustion and particle number emissions in a spark-ignition to compression-ignition (SICI) engine

Spark assistance along with oxygenated components addition is a promising method to achieve stable compression ignition, high thermal efficiency and low particle emission. To this end, ethanol blended with non-oxygenated gasoline was fueled to an engine working with spark-ignition to compression-ignition (SICI) mode under air dilution and exhaust-diluted conditions. The effects of ethanol addition on engine performance including combustion characteristics, fuel economy, particle number (PN) emissions, were studied in two categories: changing research octane number (RON) by varying ethanol content and maintaining RON by changing fuel type. The results showed that ethanol addition by splash blending suppressed knock tendency, and the knock intensity could be lowered by up to 65–75% with increasing ethanol content. However, when maintaining the same RON, the ethanol-gasoline blend exhibited higher knock intensity than pure gasoline due to synergistic effects between ethanol and aromatics on auto-ignition. Compared to pure spark ignition with high-RON gasolines, using ethanol-gasoline blends under SICI could reduce the minimum fuel consumption rate by up to 25 g/(kW·h). To characterize the high-efficiency cycles under SICI, two dimensionless parameters were proposed by considering the ratios of heat release amount and duration between the flame propagation stage and auto-ignition stage. The two parameters showed good exponential correlation. As for emissions, blending ethanol could basically reduce PN emissions under SICI mode except for the cases with significant increase in nucleation particles, such as those with high knock intensity under stoichiometric condition and poor combustion quality under heavily exhaust-diluted conditions. The total PN reduction by blending ethanol is mainly due to the decrease of accumulation mode particles, during the stage of flame propagation rather than auto-ignition. Blending ethanol into non-oxygenated gasoline will directly increase unburned hydrocarbons and nitrogen oxides due to the low auto-ignition propensity of ethanol under stoichiometric or moderately lean conditions according to the temperature-pressure trajectory. Therefore, a dedicated combustion system with higher compression ratio and lean-boosted mixture is required to enhance ethanol's reactivity and achieve better fuel economy along with low PN emission for diluted SICI combustion.

Experimental investigation about the adoption of high reflectance materials on the envelope cladding on a scaled street canyon

In the last years innovative building envelope materials were studied in order to mitigate the urban heat island phenomenon in cities. Among them, cool materials represent a valid solution to achieve this goal. These materials are characterized by high solar reflectance and high thermal emittance. Another way to reduce the urban heat island effect is the adoption of retroreflective materials on the building façades, in order to reduce the amount of solar radiation entrapped within the urban fabric. The retroreflective materials have a particular surface conformation that allows to reflect the solar radiation back in the same direction of the incident radiation. In this case, the temperature of the surfaces inside an urban canyon should have lower values compared with the case with common construction materials. Consequently, also the air temperature inside the urban canyon has low values with significant advantages on outdoor thermal comfort and on building thermal energy demands. In this work the solar reflectance directional dependence was investigated with a Goniophotometer. Furthermore, experimental measurements of retroreflective materials effects on a scaled urban canyon were performed. It was found that the albedo of the RR material increases with the incident angle of the light beam from 38.2% to 42.3% with an angle of 8° and 60° respectively. An increase of the reflected radiation to the sky in the case of the use RR materials despite of a commercial Lambertian paint with same albedo at 8° of incident light beam was evaluated. In particular, the measurement brings to assess a maximum average percentage canyon albedo difference of 2.03%.

Multifunctional Membrane for Thermal Management Applications.

Space cooling and heating consume a large proportion of global energy, so passive thermal management materials (i.e., without energy input), especially dual-mode materials including cooling and heating bifunctions, are becoming more and more attractive in many areas. Herein, a function-switchable Janus membrane between cooling and heating consisting of a multilayer structure of polyvinylidene fluoride nanofiber/zinc oxide nanosheet/carbon nanotube/Ag nanowire/polydimethylsiloxane was fabricated for comprehensive thermal management applications. In the cooling mode, the high thermal radiation emissivity (89.2%) and sunlight reflectivity (90.6%) of the Janus membrane resulted in huge temperature drops of 8.2-12.6, 9.0-14.0, and 10.9 °C for a substrate, a closed space, and a semiclosed space, respectively. When switching to the heating mode, temperature rises of 3.8-4.6, 4.0-4.8, and 12.5 °C for the substrate, closed space, and semiclosed space, respectively, were achieved owing to the high thermal radiation reflectivity (89.5%) and sunlight absorptivity (74.1%) of the membrane. Besides, the Janus membrane has outstanding comprehensive properties of the membrane, including infrared camouflaging/disguising, electromagnetic shielding (53.1 dB), solvent tolerance, waterproof properties, and high flexibility, which endow the membrane with promising application prospects.

Bio-inspired structure using random, three-dimensional pores in the polymeric matrix for daytime radiative cooling

Inspired by the structural white of natural fibers, this work here demonstrated a novel passive radiative cooling design that builds random, three-dimensional pores in polymer matrix via the phase separation-based method. The pore size and structure are controlled by the compatibility between the shape-supporting matrix and other soluble polymers, where the semi-crystalline polypropylene (PP) was used as the shape-supporting matrix that is insoluble in any organic solvent at room temperature, and the soluble polymers were styrene-butadiene-styrene block copolymer (SBS) and styrene-ethylene-propylene-styrene block copolymer (SEPS). The experimental results revealed that those macroporous PP sheets have high solar reflectance (~97%) and tunable thermal emissivity (0.81-0.67). The outdoor thermal measurement also shows that those macroporous PP film-covered devices displayed a temperature (44.0 °C), much lower than those of air (50.0 °C) and neat-PP-covered device (60.4 °C). The complete simulation of a building thermal behavior evaluation shows that the macroporous PP roof-covered buildings exhibit a higher cooling effect than the neat PP roof-covered building in summer. Therefore, the required cooling power of the macroporous PP roof-covered buildings saved about 200 W compared with the neat PP roof-covered building on a typical summer day. • The bio-inspired design of the random pore structure for radiative cooling. • Investigate the effect of the pores' size and shape on the optical properties. • Estimate the heat insulation effect of building covered by the structured material.

GRB 130310A: very high peak energy and thermal emission

The special GRB 130310A was observed by Fermi Gamma-Ray Burst Monitor and Large Area Telescope, with T 90∼ 2.4 s. With a combination of a Band function and a blackbody (BB) function, the time-resolved spectral analysis of GRB 130310A confirmed that there is a sub-dominate thermal component in the early period (e.g., slice T 0 + [4.03 – 4.14] s) spectrum with BB temperature (kT) being ∼7∼5 keV, which can be interpreted as photosphere emission. The precursor of GRB 130310A can be fitted well with a BB component with kT ∼ 45 keV, which is higher than that of the main burst. It suggests that the radiation of GRB 130310A is in transition from thermal to non-thermal. Such a transition is an indication of the change in jet composition from a fireball to a Poynting-flux-dominated jet. A very high peak energy is obtained in the first time bin, with the peak energy Ep of the Band component for Band+BB and Band model being ∼8.5∼5.2 MeV and ∼11.1∼7.4 MeV, respectively. Afterwards, the Ep drops to ∼ 1 MeV. The Ep evolution patterns with respect to the pulses in the GRB 130310A light curves show a hard-to-soft evolution. The interpretation of the high peak energy Ep within the photosphere and internal shock model is difficult. It also suggests that at least for some bursts, the Band component must invoke a non-thermal origin in the optically thin region of a GRB outflow. Assuming the redshift is z ∼ 0.1 ∼ 8, the radius of the jet base r 0 ∼ 109 cm to allow (1 + σ 15) > 1 in line with the calculation results of the magnetization parameter at ∼1015 cm (σ 15). However, the value of (1 + σ 15) is ≃ 1 in the zone z around 3 for r 0 ∼ 109 cm, suggesting the non-excluded possibility that the origin is from ICMART with a low value. The photosphere-internal shock seems capable of interpreting the high peak energy, which requires electron Lorentz factor γe ∼ 60 and εe ∼ 0.06.

Theoretical insight into the solar thermal absorption property of ultra-high temperature ceramics TMB2 (TM = Ti, Zr, and Hf)

TMB2 (TM = Ti, Zr, and Hf) ultra-high temperature ceramics have experimentally been proved as efficient sunlight absorbers operating at elevated temperatures. However, few have been known on the origination and factors that control the absorption performance. In this manuscript, the electronic structure and optical properties of TMB2 (TM = Ti, Zr, and Hf) were investigated by first principles calculations. Quasiparticle correction is introduced in the calculation of electronic structure. The theoretical dielectric function and optical constants are in good agreement with experiments. The reflectance spectra of TMB2 demonstrate they are typical spectrally selective materials. The spectral selectivity of these borides at high temperature is weakened when effect of temperature is considered. The anisotropy in solar absorption of TMB2 is rather weak. Discussion on the solar absorbance and thermal emittance reveals that plasma frequency acts on the solar absorbance and high temperature thermal emittance contradictorily. Our theoretical results demonstrate that it is feasible to tune the spectral selectivity of diborides by regulating the plasma frequency through cation doping.

Construction of a ternary channel efficient passive cooling composites with solar-reflective, thermoemissive, and thermoconductive properties

Current passive radiative cooling is a promising alternative to electrical cooling systems, but it is still difficult to be used in cooling devices with an internal heat source. Hence, we here present a ternary channel efficient passive cooling composites by embedding hexagonal boron nitride (h-BN) into high-density polyethylene (HDPE). Like the traditional passive radiative cooling materials, the h-BN/HDPE composite possesses a high solar reflectance (~87%) that minimizes solar heat gain, and a high thermal emittance (~0.71) that maximizes radiative heat loss, but the composites also have a high thermal conductivity (~1.47 W/(m·K)) that maximizes the internal heat conduction. Therefore, the combination of the optical and thermal properties allows for the LED's temperature drops of 5 °C, compared with the bare LED, under the average solar intensity of 1100 W/m2. The notably cooling effect provides the guidance for the future application of the ternary channel efficient passive cooling composites in cooling the devices with internal heat.

Solar selective reflector materials: Another option for enhancing the efficiency of the high-temperature solar receivers/reactors

The cavity wall is an important part of a cavity receiver in determining the receiver efficiency. Using solar selective reflector (SSR) materials with low solar absorptivity and high thermal emissivity for the cavity wall design is one efficient way to improve the receiver efficiency. In this work, we present a systematic study of the optical and high-temperature stability performances of six different SSR materials: one refractory ceramic fiber based substrate material (Fiberfrax 140) and five metallic oxide coatings which are prepared by mixing metallic oxide powders of alumina, magnesium oxide and titanium dioxide with commercial inorganic adhesives. The thermal stability was studied by heating up and keeping the six candidate materials in atmospheric conditions at a temperature of 1250 °C for 200 h. The spectrum of hemispherical reflectance in the spectrum band 0.25–25 μm was measured for analyzing the optical performance of the candidate materials. The obtained results show that all the six materials studied have good solar selective reflection characteristics, i.e, low solar absorptivity and relatively high thermal emissivity. Especially, the alumina coated substrate material shows excellent performances both for thermal stability and solar selective reflection. The solar reflectivity can reach 94.6%.

Dynamic thermal radiation modulators via mechanically tunable surface emissivity

Inspired by cephalopods, we design a hybrid structure comprised of a rigid film with a low thermal emissivity and a substrate with a high thermal emissivity combined with a stretchable heater. The film’s topography is characterized by distributed strain-dependent micro-cracks, enabling the exposure of the substrate to be tuned by the applied strain. Thus, the effective surface thermal emissivity originating from the combination of the film and the exposed substrate can be instantaneously and reversibly modulated simply via mechanical means. The system exhibits various pronounced advantages, including ease of fabrication, low working temperature, broad emissivity modulation range, observing angle independence, excellent reversibility, instantaneous response, high strain sensitivity, feasibility for patterning and multiplexing, and autonomous actuation. Additionally, the system demonstrates intriguing thermographic-based applications in finger motion sensing, information encryption, multiplexing display, and thermal camouflage. Therefore, this work can facilitate the invention of next-generation thermal modulators with autonomous, on-demand, and board-range control.

Living cool: An approach for Architectural “COOL ROOF” to Decrease the Electricity Consumption in Iraq

One of the most important problems in Iraq is the insufficient and inefficiency of the electrical power generating station, that is causing many problems in the availability of electricity in public and private buildings. This problem is leading to use privet power generators, causing air pollution and highly energy consumption. The second exploratory symposium “the reality of power in Iraq and its impact on the society” which had been held on May 2011 in Iraq proposed a large number of recommendations, a standout amongst these proposals was: urge the utilization of the thermal insulation in the building’s roofs to reduce the energy consumption. This study will prompt to the new idea of “living cool” in the Iraqi houses and different types of buildings throughout the day without the electricity’s power using by utilizing the new thermal insulation technology. This study eventually examines the idea of the “COOL ROOF” innovation to Iraqi’s houses and public buildings. The problem of the energy consumption might be the critical issue over the world generally, what’s more done Iraq specially. The transmission of the energy through top roofs might be more than (50%) starting with those general transmission energies through the other surfaces. Roofs with “high solar reflection”, high daylight reflection and high thermal emittance “high heat radiant” are remaining unflappable throughout the day. Previous researches approved that “the roof surfaces with low thermal emittances properties and high solar reflection might also remain cool in the sun and throughout the day. Use this type of roofs (cool roof) will reduce cooling electricity power in the different types of buildings in Iraq. Cool roofs can also reduce the citywide encompassing air temperature over summer by decreasing those gigantic number for air-conditioning equipment, abating ozone framing furthermore, eventually expanding human comfort demands by using the nature and minimum requirements to reach human comfortable zones. This research has been used practical details to apply the cool roof technology in Iraq.

A highly controlled fabrication of porous anodic aluminium oxide surface with versatile features by spatial thermo-anodization

Abstract Thermo-anodization technology has been considered as an effective means to improve the thermal, physical and chemical properties for metal alloy. In this work, we achieve a porous black layer on the surface of aluminium alloy through an environmentally friendly anodic oxidation process, with a high thermal emittance (0.96) and a high solar absorptivity (0.921). In addition, the black thermo-anodized coating layer shows a unique anti-corrosion property under UV irradiation. A hybrid hydrophobic surface has been facilitated through treating the thermo-anodized porous layer with silane. Moreover, an anti-icing feature can be realised that can effectively delay the freezing of water droplet on the surface of AA2024 aluminium alloy. As such, the specific anodic process of coating provides a simple method for improving the solar absorptivity and infrared emittance of aluminium alloys, enabling broad applications in aerospace engineering.

Experimental study on in-flame soot formation and soot emission characteristics of gasoline/hydrogenated catalytic biodiesel blends

Gasoline compression ignition engine is one of the most promising low-temperature combustion strategies using gasoline as fuel due to its high thermal efficiency and low emission characteristics. While the ignition difficulty under low loads and knocking combustion under high loads limit its practical applications. To solve these problems low reactive gasoline fuel blending with high reactive hydrogenated catalytic biodiesel was proposed. However, soot characteristics of large hydrocarbons are not yet known. In order to understand the relationship between the in-flame soot formation and soot emission for these blending fuels and to find an optimum ratio for GCI engines. Fundamental study on in-flame soot formation in a constant volume combustion chamber is studied using a diffused background-illumination extinction method. The results show that the in-flame soot mass and soot combustion duration was found to be reduced with increasing the ratio of gasoline. Lower ratios of hydrogenated catalytic biodiesel blends are tested in a gasoline compression ignition engine and the experimental results reveal that the increase of hydrogenated catalytic biodiesel ratio increased the total number of soot particle emission. The total number of particle emission showed a positive relationship with soot formation. But there is a weak relationship between the particle size distribution and in-flame soot formation under the same working loads. The results also demonstrated that the particle diameter was mainly depending on working conditions rather than soot formation. The blend ratio of hydrogenated catalytic biodiesel and the injection strategies were mainly contributed to the concentration of particle size.

The role of nanostructure morphology of nickel-infused alumina on solar-thermal energy conversion

Solar-thermal energy conversion can be useful in many applications, including water desalination, and thermal energy storage. In this regard, using spectrally-selective solar absorbers is vital due to their high solar absorptance and low thermal emittance. While selective absorbers can be created using a wide range of nanomaterials, the underlying geometry may control the overall performance of solar-thermal energy conversion. With different geometries, it is possible to obtain a wide range of optical responses ranging from broadband to selective absorption of light. In this study, we focus on the role of nanostructure morphology of nickel-infused alumina (Ni/NPA) based spectrally-selective solar absorbers. This study demonstrates the use of the design of experiments to analyze the effect of various geometric factors on the resulting optical response of Ni/NPA in the context of solar-thermal energy conversion. We show how this approach can provide a unique insight into the role of various geometric factors on the solar absorptance and thermal emittance of Ni/NPA-based absorbers, and demonstrate how it can guide the development of spectrally-selective materials. We believe a similar approach can be useful in the development of other optical materials for different applications.

Semiconductor quantum plasmons for high frequency thermal emission

AbstractPlasmons in heavily doped semiconductor layers are optically active excitations with sharp resonances in the 5–15 μm wavelength region set by the doping level and the effective mass. Here, we demonstrate that volume plasmons can form in doped layers of widths of hundreds of nanometers, without the need of potential barrier for electronic confinement. Their strong interaction with light makes them perfect absorbers and therefore suitable for incandescent emission. Moreover, by injecting microwave current in the doped layer, we can modulate the temperature of the electron gas. We have fabricated devices for high frequency thermal emission and measured incandescent emission up to 50 MHz, limited by the cutoff of our detector. The frequency-dependent thermal emission is very well reproduced by our theoretical model that let us envision a frequency cutoff in the tens of GHz.

Preparation and characterization of nanocomposite of Co:CuO by radio-frequency sputtering for solar selective absorber application

In this work, CuO thin films with different cobalt percentages were deposited, at room temperature on glass and copper substrates, using radio-frequency sputtering under 30% oxygen partial pressure. The effect of Co content on structural, electrical, morphological and optical properties of CuO films was investigated in detail. The X-ray diffraction measurements revealed a polycrystalline monoclinic structure for all sputtered films. Raman analyses showed a peak broadening and shift due to the quantum confinement effect in the copper oxide nanoparticles. Adding cobalt leads to the decrease of the crystallite size and the band gap energy. Electrical measurements showed that the film conductivity increases with the Co content. All films exhibited good solar selectivity (>12) with high solar absorptance (>92%) and low thermal emittance (<7%). The obtained Co:CuO thin films could be suitable as potential candidates for selective absorber applications in CSP technology.