Spectral Hemispherical Reflectance Research Articles

Photothermal performance of three chromia-forming refractory alloys for high-temperature solar absorber applications

Spectral hemispherical emissivity is a crucial material characteristic that determines radiation heat transfer. In high-temperature solar thermal applications, it affects not only the efficiency of the solar energy absorption but also the heat losses caused by thermal radiation and the radiative heat transfer within the receiver. Due to the limitations of the working temperature of existing solar absorber coatings, the spectral hemispherical emissivity of the oxidized surface is a key performance indicator for evaluating the potential of a candidate refractory alloy for high-temperature (> 1000 °C) solar receiver/reactor designs. In this work, we systematically studied the photothermal performances of the oxidized surfaces of three widely used high-performance commercial chromia-forming alloys (Haynes 230, Hastelloy X, and SS 253MA) by analyzing the spectral hemispherical reflectance in the band 0.25–25 μm. The stability of the optical properties of the formed oxide layers have also been studied by exposing the three alloys at 1150 °C in air for three different exposure periods (10 h, 100 h, and 200 h). The results show that the solar absorptivity of all the samples is in the range of 0.800–0.855, with SS 253MA showing the best performances in offering both high and stable solar absorptivity in the range of 0.837–0.855. The evaluation of the photothermal performances suggest the potential of these three alloys in solar-thermal applications.

Novel ceramic fibre - Zirconium diboride composites for solar receivers in concentrating solar power systems

Ultra-high temperature ceramic matrix composites (UHTCMCs) are a new class of composites which combine boride phases with carbon fibres to achieve new structural and functional properties and were originally designed for aerospace applications. For the first time, this work investigates the optical response of UHTCMCs in view of their possible application as high temperature solar absorbers in Concentrating Solar Power systems. Carbon fibre-reinforced ZrB2 pellets with different fibre type and arrangement were manufactured and characterized from the point of view of optical properties and surface microstructure. The composites had high fractions of ZrB2 (∼40 vol%) and carbon fibres (up to 60 vol%). Spectral hemispherical reflectance in the range 0.3–16 μm wavelength was assessed and compared to pure boride and graphite phases. Irrespective of the fibre architecture, the solar absorptance significantly increased with respect to the pure boride, reaching or even overpassing the solar absorptance of silicon carbide. Thermal emittance, calculated from 800 K to 1500 K temperature, increased as well in comparison to the pure boride, but always remained lower than that of SiC. These optical properties, combined with structural damage tolerance and light weight, suggest that UHTCMCs are of great interest as high temperature solar absorbers, overpassing the weaknesses of current solar receivers, as well as those of the most promising alternative materials to date.

Development of a multivariate spectral emissivity model for an advanced high strength steel alloy through factorial design-of-experiments

Variations in the spectral emissivity of advanced high strength steels (AHSS) during intercritical annealing leads to errors in pyrometry measurements, which, in turn, cause thermal excursions that impact the mechanical properties of the steel. This paper presents an empirical approach for modelling the spectral emissivity of advanced high strength steel. Samples of two dual-phase steel (DP980) alloys, having Si/Mn ratios of 0.04 and 0.23, are heated within a galvanizing simulator in atmospheres of 95%/5% N2/H2 and dew points of 10°C and −30°C. The spectral hemispherical reflectance of the annealed samples was measured with an FTIR spectrometer. The variation of the spectral emissivity with dew point, alloy composition, pre-annealed surface state, and wavelength is analyzed using full factorial designs. The significant main and interaction effects vary across the spectral range, with the ratio of alloy components and pre-annealed surface state dominating at shorter and longer wavelengths, respectively. The predicted spectral emissivity values obtained from the model fitted for a three-channel pyrometer shows good agreement with the measurements. This study shows response surface methods (RSM) to be a viable approach for developing spectral emissivity models for pyrometry applications.

Optical characterization and modeling of nanoporous gold absorbers fabricated by thin-film dealloying

This work studies the optical reflectance of nanoporous gold (NPG) thin films of varying pore volume fraction (PVF) synthesized by chemical dealloying of Ag-Au alloy precursors. The fabricated samples are characterized by scanning electron microscopy, and spectral hemispherical reflectance is measured with an integrating sphere. The effective isotropic optical constants of NPG with varying PVF are modeled for the wavelength range from 0.4 to 1.6 μm using the Bruggeman effective medium theory. As the thickness of the NPG thin films is more than ten times larger than the effective penetration depth, the spectral reflectance is simply modeled with the Fresnel coefficients at the interface of air and semi-infinite NPG with different incident angles and polarizations. Consistent with the modeling results, the optical measurement data shows that the spectral normal reflectance of NPG significantly decreases with larger PVF values in the near-infrared regime. On the other hand, the reflectance increases greatly only within visible range at larger oblique angles for transverse-electric polarized waves compared to transverse-magnetic waves. Moreover, the NPG samples demonstrate good thermal stability from room temperature up to 100 °C with little changes in the temperature-dependent spectral hemispherical reflectance.

Uncertainty Study of Reflectance Measurements for Concentrating Solar Reflectors

The solar reflector is one of the main components of concentrated solar thermal systems. Therefore, accurate knowledge of its solar-weighted, near-specular reflectance is highly important. Currently, this parameter cannot be properly measured with a single commercial instrument. There is a great interest in having a suitable procedure that can guarantee the accuracy of reflector quality analysis, which already led to the publication of an international measurement guideline (title “Parameters and method to evaluate reflectance properties of reflector materials for concentrating solar power technology”). Still, more research work is needed to improve the state of the art. At present, both the specular reflectance and the spectral hemispherical reflectance are measured by using commercial portable reflectometers and spectrophotometers, respectively, to gain enough information. This article concentrates on the evaluation and calculation of the type-B (nonstatistical) uncertainties associated with these employed instruments and, therefore, leads to a more accurate definition of the measurement uncertainty. Considering type-B uncertainty, the expanded uncertainties of measurements for most of the reflector types are $U_{\mathrm {B,ref}} = 0.006$ for monochromatic specular reflectance and $U_{\mathrm {B,spec}} = 0.016$ for solar-weighted hemispherical reflectance.

Solar mirror qualification setup, the key instrument in a new strategy for evaluating off-normal near-specular solar-reflectance.

Solar reflectance is one of the most important parameters characterizing the mirrors used to redirect the solar radiation toward a receiver in concentrating solar power plants. In Solar Power and Chemical Energy Systems (SolarPACES) Task III, an international group of experts is drafting the reflectance guidelines and debating the hard task of measuring off-normal near-specular solar-reflectance. Because of the lack of suitable commercial instruments, several research institutes are studying and setting up special equipment. Since 2012, the Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA) proposed an articulated strategy based on modeling the hemispherical spectral reflectance with the equivalent model algorithm and the ratio of near-specular to hemispherical reflectance with the total integral scattering equation; in the latter, the roughness is replaced by the heuristic parameter σ φ , nonphysically dependent on the acceptance angle. The solar mirror qualification experimental setup was conceived for measuring σ φ . Recently, SolarPACES approved a new project to support the working group for improving the proposed instruments. The paper summarizes the ENEA strategy and presents the latest version of the instrument, SMQ2, discussing its several important new features: self-calibration, setup simplification, straightforward applicability to high specular mirrors, measurement-time cutoff, and acceptance-angle range extended up to 50 mrad.

Impacts of coal dust from an active mine on the spectral reflectance of Arctic surface snow in Svalbard, Norway

AbstractLight‐absorbing particles (LAPs) in snow such as dust and black carbon influence the radiative forcing at the Earth's surface, which has major implications for global climate models. LAPs also significantly influence the melting of glaciers, sea ice, and seasonal snow. Here we present an in situ study of surface snow near an active coal mine in the Norwegian Arctic. We couple measurements of spectral hemispherical directional reflectance factor (HDRF) with measurements of LAPs characterized in two ways, as refractory black carbon using a Single Particle Soot Photometer and the total light absorption of LAPs measured with the Light Absorption Heating Method. The Snow Ice and Aerosol Radiation model was constrained by LAP measurements. Results were compared to observed spectral albedo measurements. Modeled and observed albedos were similar at the cleaner and more remote sites. However, the modeled spectral albedos do not fully account for the low spectral albedo measured next to the mine. LAP measurements also showed a large variation in particle sizes (tenths to tens of microns) related to transport distance of the particles from the mine. Here we find that LAPs from coal dust reduce the spectral HDRF by up to 84% next to the mine and 55% 0.5 km downwind of the mine. The coupling of extreme LAP observations (1 ng g−1 to 4863 ng g−1) with HDRF measurements from 350 to 2500 nm has facilitated the development of spectral band pairs, which could be used in the future to remotely assess LAPs in Arctic snow.

Compositional dependence of optical properties of zirconium, hafnium and tantalum carbides for solar absorber applications

Besides ultra-refractoriness and favorable mechanical and chemical characteristics, carbides of early transition metals show intrinsic spectral selectivity, making them appealing for high-temperature solar absorber applications. However these kinds of ceramics can be produced using many processing methods resulting in different compositions, density and surface finishing. Thus the present work reports on the systematic study of microstructural, mechanical and optical properties of dense zirconium, hafnium and tantalum carbides as a function of the sintering method (high pressure or pressureless), implying use of 10 or 20vol% of MoSi2 as sintering aid. The spectral hemispherical reflectance of Zr-, Hf- and Ta-carbides has been measured in the 0.25–16.5μm wavelength range and correlated to the surface microstructure and roughness. Room and high temperature fracture strength has been measured as well.

Correction of ocean hemispherical spectral reflectivity for longwave irradiance computations

This study demonstrates that upward infrared irradiances have negative modeling biases when the ocean hemispherical spectral reflectivity is used. The biases increase with increasing air temperature and with decreasing water vapor amount. Spectral biases in the surface upward longwave irradiance from 4μm to 80μm are between −0.4 and 0Wm−2μm−1, while longwave broadband biases are between −2 and −1Wm−2. The negative biases stem from surface-reflected component because an irradiance radiative transfer model ignores the correlation between the downward radiance and directional reflectivity. Therefore, a positive correction factor to the hemispherical spectral reflectivity for the irradiance radiative transfer model is needed. A simple parameterization using an anisotropic factor for downward radiances is developed to correct reflectivity for various atmospheric conditions.

Investigating performance prediction and optimization of spectral solar reflectance of cool painted layers

The air temperatures are generally found to be higher in the urban areas in comparison with the suburban or rural areas. One of the countermeasures to mitigate this urban heat island is to coat the surfaces of buildings with paint that has higher solar reflectance; the method is commonly referred to as “cool painting”. It is important to evaluate the solar reflectance of the paints used for such paintings. Though solar reflectance can be measured by a spectrophotometer with an integrating sphere, accurate numerical prediction of reflective properties of the painted layer including arbitrary pigments can contribute to further development of the cool paint material. The effect of diffuse reflection in the painted layer is more significant than that of specular reflection on the painted surface. It is also important to evaluate the scattering properties of pigments, which are related to the reflection performance. For evaluating the scattering properties of a pigment in a medium, anisotropic scattering should be considered. In this study, we applied the radiation element method for predicting the solar reflectance of a painted layer. The hemispherical spectral reflectance of a painted layer, including TiO2, ZnO or Al2O3, was evaluated. Our results showed that spectral reflectance of a painted layer with arbitrary values of particle sizes, volume fraction of pigment, and coating thickness can be calculated by this method. Further, the validity of this numerical method was evaluated by comparison with the reflective property of the actual painted layer by means of spectrophotometry.

Tantalum diboride-based ceramics for bulk solar absorbers

Ultra-high temperature ceramics (UHTCs) are the ideal material for extreme conditions thanks to very high melting points, high refractoriness and good thermo-mechanical properties at high temperatures. This paper reports, for the first time to the best of our knowledge, on the microstructural and optical characterization of several tantalum diboride (TaB2) samples with density values from 67% to full density. Pristine samples have been investigated at room temperature by means of SEM, XRD and spectral hemispherical reflectance measurements. Thermal emittance in the temperature range 1050–1800K has been measured. Structural, compositional and optical properties after high temperature exposure have been characterized as well and property changes have been explained. The obtained results favorably compare TaB2 over conventional solar absorbers for novel solar plants operating at higher temperatures.

Hemispherical-directional reflectance factor measurements of snow on the Greenland Ice Sheet during the Radiation, Snow Characteristics and Albedo at Summit (RASCALS) campaign

During the summer of 2010 a snow measurement campaign was carried out at Greenland Environmental Observatory, on Greenland Ice Sheet. Broadband snow albedo (bihemispherical reflectance), spectral hemispherical conical reflectance factor, temperature, density and other physical parameters were measured. Especially, the Hemispherical Directional Reflectance Factor (HDRF) of 46 snow samples was measured using the Finnish Geodetic Institute Field Goniospectrometer (FIGIFIGO). Additionally, linear polarization (Stokes parameters I, Q, and U), or Muller matrix elements R11, R12, and R13, were measured from some samples. Albedo was also calculated from the HDRF and these values were compared to those from the albedometer. Values were found to be within 3% of each other, which is within the accuracy limits of the instruments. We also compared our results to previous measurements at the Summit (by Bourgeois et al. during 2005 and Carmagnola et al. during 2011) and found our results to be in agreement with their measurements. Compared to previous studies, our measurements have full solar spectrum spectral coverage and linear polarization.

Infrared emitting nanostructures for highly efficient microhotplates

A highly emissive Si-based microhotplate based on self-organizing nanostructures is presented. The silicon was structured by a self-masking deep reactive ion etching process resulting in needle-like non-periodical microstructures. Evaporated platinum settles in a kind of glancing angle deposition as well-defined nanocrystals on the silicon microstructures. Finite-difference time-domain simulation allowed the evaluation of the ideal platinum thickness for maximized infrared absorption and emission. We measured the hemispherical spectral transmittance and reflectivity of the fabricated surfaces and found the hemispherical spectral absorbance to be up to 0.97 in the investigated wavelength range. To demonstrate the advantages of these micro-nano-structures, we present the fabrication and characterization of a thermal infrared hotplate-emitter. With integrated Pt-on-Si-needles, the emitter shows a 2.6 times higher IR emission without wavelength-dependent interference patterns as compared to an uncoated Si-based emitter at the same membrane temperature.

Optical properties of boride ultrahigh-temperature ceramics for solar thermal absorbers

It is a known rule that the efficiency of thermodynamic solar plants increases with the working temperature. At present the main limit in temperature upscaling is the absorber capability to withstand high temperatures. The ideal solar absorber works at high temperatures and has both a low thermal emissivity and a high absorptivity in the solar spectral range. The present work reports on the preparation and optical characterization of hafnium and zirconium diboride ultra-high-temperature ceramics for novel solar absorbers operating at high temperature. Spectral hemispherical reflectance from the ultraviolet to the mid-infrared wavelength region and high- temperature hemispherical emittance reveal their potential for high-temperature solar applications. Boride samples are compared to silicon carbide (SiC), a material already used in solar furnaces.

Mid-infrared absorption properties of green wood

There is a lack of quantitative data on the penetration depth and the amount of energy absorbed by green wood under infrared (IR) radiation. This lack of knowledge is a potential barrier to the development of IR heating as an alternative to soaking as a means of warming logs prior to peeling in the manufacture of plywood. Experimental measurements of normal hemispherical spectral reflectance and transmittance over the range 550–5,500 cm−1 wavenumbers on four wood species, beech, birch, Douglas-fir and spruce have brought new knowledge on mid-infrared absorption properties of green wood and removed some uncertainties. For instance, it is not possible to deliver energy deeper than up to 0.3 mm below the wood surface because 70–90 % of all incident IR radiation on the wood surface is absorbed in this layer. Some wood features, such as surface quality, the presence of knots and of free water in wood (the latter two having a more significant effect) influence the amount of energy absorbed. These results illustrate that IR radiation can heat the surface layers, but then heat penetrates deeper into the inside layers of wood by conduction.

Caractérisation radiative de liquides volatils semi-transparents dans le domaine infrarouge

The LNE has developed a technique to measure infrared radiative properties of volatile liquids. LNE uses a homemade reflectometer for the measurement of the spectral directional hemispherical reflectance from 0.8 μm to 14 μm and a Fourier transform spectrometer for the measurement of the regular spectral transmittance from 1 μm to 25 μm. For volatile liquids, a tight cell with two infrared windows was constructed. Optical models were developed to calculate the spectral complex refractive index of the liquid from reflectance and transmittance results. Results were validated for water by comparison to data found in literature.

Radiative Properties of Expanded Polystyrene Foams

Expanded polystyrene foams are one of the most widely used materials for a building’s thermal insulation. Owing to their very low density, a substantial proportion of the heat transfer is due to thermal radiation propagating through their porous structure. In order to envisage an optimization of their thermal performances, an accurate modeling of their radiative behavior is required. However, the previous studies on this subject used several drastic simplifications regarding their radiative behavior (optically thick material) or their porous morphology (homogeneous cellular material, dodecahedral cells). In this study, we propose a more accurate model based on a detailed representation of their complex morphology allowing us to predict their entire monochromatic radiative properties. We investigated the influence of the different structural parameters on these properties. We checked the validity of our model by comparing the spectral hemispherical reflectance and transmittance measured on slabs of foam samples with values predicted by our model. A good accordance was found globally.

A Monte Carlo study of the chlorophyll fluorescence emission and its effect on the leaf spectral reflectance and transmittance under various conditions

Spectral hemispherical reflectance R(lambda) and transmittance T(lambda) are affected by chlorophyll (Chl) fluorescence which may complicate the evaluation of optical parameters of leaves. Measured Chl a fluorescence spectral emission F(lambda) is itself affected by several distortion effects on the leaf level (fluorescence reabsorption, secondary fluorescence, inner filter, surface and subsurface reflections etc.). In this work we propose a Monte Carlo photon transport (MCPT) model capable for treating a variety of optical distortion effects on the leaf level. In the forward mode the model decouples R(lambda), T(lambda) and their fluorescence contributions FR(T)(lambda). To obtain the absorption and scattering spectra of the leaf, utilized in the forward modeling, we have suggested an inversion procedure employing the experimental R(lambda), T(lambda). The attention was paid on the correction of the leaf absorption and scattering spectra caused by the optical effects on the sample level including Chl fluorescence contribution to measured R(lambda), T(lambda).

Study of annealing time on sol–gel indium tin oxide films on glass

Indium and tin salt-based precursors maintaining In:Sn atomic ratio as 90:10 were utilized for the development of sol–gel dip coated indium tin oxide films (ITO) on SiO 2 coated (∼ 200 nm thickness) soda lime silica glass substrate. The gel films were initially cured in air at ∼ 450 °C to obtain oxide films of physical thickness ∼ 250 nm. These were then annealed in 95% Ar–5% H 2 atmosphere at ∼ 500 °C. The annealing time was varied from 0.5 h to 5 h. Variation of annealing time did not show any considerable change of transmittance in the visible region. Thermal emissivity ( ε d, 0.67–0.79) of the films were evaluated from their hemispherical spectral reflectance. These passed through a minima with increasing annealing time as the reflectivity of the films in the mid-IR passed through a maxima. The microstructure of the films revealed systematic growth of the ITO grains. XRD and XPS studies revealed the presence of both In and Sn metals in addition to the metal oxides. The energy dispersive X-ray (EDX) analysis showed little lowering of tin content in the films with increasing annealing time.

Emissivity Measurements on Metallic Surfaces with Various Degrees of Roughness: A Comparison of Laser Polarimetry and Integrating Sphere Reflectometry

Both integrating sphere reflectometry (ISR) as well as laser polarimetry have their advantages and limitations in their ability to determine the normal spectral emissivity of metallic samples. Laser polarimetry has been used for years to obtain normal spectral emissivity measurements on pulse-heated materials. The method is based on the Fresnel equations, which describe reflection and refraction at an ideally smooth interface between two isotropic media. However, polarimetry is frequently used with surfaces that clearly deviate from this ideal condition. Questions arise with respect to the applicability of the simple Fresnel equations to non-specular surfaces. On the other hand, reflectometry utilizing integrating spheres provides a measurement of the hemispherical spectral reflectance, from which the normal spectral emissivity can be derived. ISR provides data on spectral-normal-hemispherical reflectance and, hence, normal spectral emissivity for a variety of surfaces. However, the resulting errors are minimal when both the sample and the reference have a similar bidirectional reflectance distribution function (BRDF). In an effort to explore the limits of polarimetry in terms of surface roughness, room temperature measurements on the same samples with various degrees of roughness were performed using both ISR and a laser polarimeter. In this paper the two methods are briefly described and the results of the comparison are discussed.