Hemispherical Emissivity Research Articles

Reticulated porous volumetric solar receiver designs guided by normal absorptance and hemispherical volumetric emittance investigations

Here, we designed volumetric receivers from the angle of their normal absorptance and hemispherical volumetric emittance. We showed that the optimal receiver with the reticulated porous structure to date should have 95% porosity, 0.4mm average pore diameter and 8mm length. This receiver has both high normal absorptance and low hemispherical volumetric emittance because these geometric parameters are designed to achieve the minimum optical thickness that is sufficient to achieve the largest possible normal absorptance. In addition, its high porosity and small pore size achievable using replication technique contribute to high specific surface area and resultant high volumetric convective heat transfer coefficient. Our results demonstrated that the thermal efficiency can be potentially further enhanced by integrating textures comparable to the wavelength within the millimeter-scale pores to tune radiation as well as convection properties.

Low temperature thermal radiative properties of gold coated metals

In order to assess the effect of gold deposition on thermal radiative properties of metallic materials, we measured total hemispherical emissivity and absorptivity of Au layers with a minimal thickness of 1 µm in dependence on the temperature TR of thermal radiation (from 20 K up to 320 K). Technically pure copper or titanium alloy (Ti–6Al–4V) was used as substrate while three types of Au layers were deposited by a sputtering process or a galvanic method. Emissivities among samples with Au layers ranged from 1.8% to 3.9% at room temperature. Galvanic deposition of pure Au lowered the original emissivity of Ti–6Al–4V alloy at 300 K from 15.8% to 3.4%, whereas the same deposition on mechanically polished Cu resulted in increase of emissivity by a factor of three, up to 3.2%. Absorptivity of each sample with Cu or Au surface was lower than its emissivity and showed a weaker dependence on TR.

The impact of the oxidation on the optical properties of TaC

A major challenge in the development of high-efficiency high-temperature solar receivers for concentrated solar power plants is the development of high-temperature materials with an ideal spectral selectivity (high solar absorptivity and low hemispherical total emissivity). TaC could be an interesting candidate due to its very good spectral selectivity, which could be increased by surface treatments and microstructuring. However, Ta2O5 presents the opposite selectivity. Therefore, we investigated the oxidation of TaC in air and in helium at 1100K and determined the impact of this oxidation on the optical and thermal properties using reflectivity and bidirectional reflectance distribution function (BRDF) measurements and laser flash analysis (LFA), respectively. We observed that the oxidation in air was too severe. Additionally, the samples treated in helium were sufficiently oxidized by the residual oxygen to lose their spectral selectivity. Therefore, despite its promising optical properties, TaC cannot be used for high-temperature applications in air or even in helium.

Optical characterization of TiAlNx/TiAlNy/Al2O3 tandem solar selective absorber coatings

Solar selective absorber coatings, consisting in a TiAlNx / TiAlNy tandem absorber with low (metallic-like) and high (semiconductor-like) nitrogen content, and an Al2O3 antireflective coating, were deposited on stainless steel. A full optical characterization was carried out on these coatings. Reflectance spectra were measured by spectrophotometry in the UV to mid-IR range (0.25−25µm), allowing for the calculation of total solar absorptance and thermal emittance. An absorptance of 0.93 and an emittance of 0.22 at 550°C were obtained for the best coating. The temperature dependence of reflectance was investigated in the infrared region and results demonstrated the validity of the classical room temperature measurement approximation to calculate thermal emittance at high temperature for these coatings. The total hemispherical emittance of the coatings was calculated from hemispherical directional reflectance spectra measured at different angles, showing that the near normal hemispherical emittance is a good approximation of the total hemispherical emittance. The total solar absorptance was also directly measured under natural solar irradiation using a unique solar facility.

Total Hemispherical Emissivity of SS 316L with Simulated Very High Temperature Reactor Surface Conditions

Type 316L stainless steel (SS 316L) is a candidate material for the reactor core barrel and selected internal components for high (and very high) temperature gas reactors. An apparatus constructed in accordance with the standard ASTM C835-06 was used for measuring total hemispherical emissivity of this material for the following surface conditions: (1) “as-received” from the manufacturer, (2) sandblasted with alumina beads, (3) sandblasted and coated with IG-11 nuclear-grade graphite powder, and (4) oxidized in air at 973 K for different durations. The emissivity of the as-received samples increased from 0.25 at 436 K to 0.36 at 1166 K. Sandblasting with 60-grit–sized alumina beads increased the emissivity from 0.32 to 0.44 in the temperature range from 561 to 1095 K. The emissivity continued to increase with sandblasting with 120- and 220-grit alumina beads, despite decrease in surface area associated with the more finely sized alumina beads. The coating of IG-11 graphite powder further increased the emissivity of the sandblasted surfaces. Following a similar trend, the IG-11–coated surfaces sandblasted by 120- and 220-grit alumina had an emissivity from 0.42 at 540 K to 0.57 at 1075 K. Electron micrographs showed more deposition of IG-11 powder on the 120- and 220-grit sandblasted surfaces. Oxidation in air at 973 K for 5 min also increased the emissivity of SS 316 L. Oxidations for 10 and 15 min provided an additional increase, but it was not as significant. Analysis indicates that spallation of oxide layer occurred between 10 and 15 min oxidation. This is consistent with studies on the time variation of total normal emissivity of SS 316L for oxidation at similar temperature.

Collimated thermal radiation transfer via half Maxwell's fish-eye lens for thermophotovoltaics

Thermophotovoltaics (TPV) convert heat into electricity by capturing thermal radiation with a photovoltaic (PV) cell, ideally at efficiencies of 50% or more. However, excess heating of the PV cell from close proximity to the emitter substantially reduces the system efficiency. In this work, we theoretically develop and numerically demonstrate an approach to fundamentally improving TPV systems that allow for a much greater separation of an emitter and a receiver. Thus, we solve the excess heating dilemma, required for achieving theoretically high efficiencies. It consists of a spherically graded index lens known as Maxwell's Fish-Eye (MFE) structure, capable of collimating hemispherical emission into a much narrower range of angles, close to the normal direction. To fully characterize the power radiation profile of the MFE, we perform finite-difference time-domain simulations for a quarter MFE and then map it onto a Gaussian beam approximation. The modeled beam properties are subsequently used to study a half MFE. In an optimized half MFE design, 90% of all thermal photons reach a receiver at a distance of 100 λ; by comparison, only 15.6% of a blackbody emitter reach a receiver in the same geometry. It is also shown that the emission achieved by a half MFE can lead to a photon recycling rate above 95% for below bandgap photons at an emitter-receiver separation of 100 λ. By applying a half MFE, the absolute TPV efficiency can be improved from 5.74% to 37.15%, which represents a significant step forward in realizing high-efficiency TPV systems.

High spectral selectivity for solar absorbers using a monolayer transparent conductive oxide coated on a metal substrate

A spectrally selective absorber composed of a monolayer transparent conductive oxide (TCO) coated on a metal substrate is investigated for use in solar systems operating at temperatures higher (>973 K) than the operation temperature of conventional systems (∼ 673 K). This method is different from the currently used solar-selective coating technologies, such as those using multilayered and cermet materials. The spectral selective absorption property can be attributed to the inherent optical property of TCO owing to the plasma frequency and interferences between the substrates. Since spectral selectivity can be achieved using monolayered materials, the effect of atomic diffusion occurring at each layer boundary in a multilayer or cermet coatings under high-temperature conditions can be reduced. In addition, since this property is attributed to the inherent property of TCO, the precise control of the layer thickness can be omitted if the layer is sufficiently thick (>0.5 μm). The optimum TCO properties, namely, carrier density and mobility, required for solar-selective absorbers are analyzed to determine the cutoff wavelength and emittance in the infrared range. A solar absorptance of 0.95 and hemispherical emittance of 0.10 at 973 K are needed for achieving the optimum TCO properties, i.e., a carrier density of 5.5 × 1020 cm−3 and mobility of 90 cm2 V−1 s−1 are required. Optical simulations indicate that the spectrally selective absorption weakly depends on the incident angle and film thickness. The thermal stability of the fabricated absorber treated at temperatures up to 973 K for 10 h is verified in vacuum by introducing a SiO2 interlayer, which plays an important role as a diffusion barrier.

Emissivity of Elgiloy and pure niobium at high temperature for the Solar Orbiter mission

The science payload of the Solar Orbiter mission that will be launched in 2018 comprises the RPW (Radio and Plasma Waves) experiment that will perform in situ and remote-sensing measurements (magnetic and electric fields). Some parts of this instrument will be made of Elgiloy for the antennas and must be protected from the intense solar radiation by a heat shield made in niobium. Emissivity of both materials is important to know in order to correctly model before launch the heat transfer in the instrumentation. Hemispherical emissivity results in the two following wavelength ranges 0.6–2.8 and 0.6–40 μm are presented at temperatures up to 1500 K for these two materials according to their surface roughness particularly for the niobium samples and at room temperature in the wavelength range 0.25–25 μm. New original data are obtained for both the materials at high temperature.

Measurement of spectral emissivity and constant pressure heat capacity of liquid platinum with an electrostatic levitator

Spectral emissivity and constant heat capacity of molten platinum at its melting temperature were measured. A sample was levitated in an electrostatic levitator and the radiation intensity from the molten sample was measured by spectrometers over a wide wavelength range. The spectrometers were calibrated with a blackbody radiation furnace and the spectral hemispherical emissivity was obtained. The obtained emissivity showed a negative wavelength dependence, which can be explained by Drude model. The total hemispherical emissivity of platinum at the melting temperature calculated by integrating the spectral hemispherical emissivity was found to be 0.25. Also, the constant pressure heat capacity was calculated to be 38.8J·mol−1·K−1.

High-Accuracy Emissivity Data on the Coatings Nextel 811-21, Herberts 1534, Aeroglaze Z306 and Acktar Fractal Black

The Physikalisch-Technische Bundesanstalt determined the directional spectral emissivities of several widely used black coatings: Nextel 811-21, Herberts 1534, Aeroglaze Z306 and Acktar Fractal Black. These are and were often applied in different industrial and scientific applications. The measurements are taken angularly resolved over a range from 10{^{circ }} to 70{^{circ }}. They cover the temperature range typical for the application of the respective coating and a wide wavelength range from 4~upmu hbox {m} to 100~upmu hbox {m}. The respective directional total emissivities and hemispherical total emissivities are given as well. The measurements were taken under vacuum at the reduced background calibration facility to achieve low uncertainties and avoid atmospheric interferences. Additionally, some measurements were taken with the emissivity measurement setup in air.

A Lookup Table-Based Method for Estimating Sea Surface Hemispherical Broadband Emissivity Values (8–13.5 μm)

Sea surface hemispherical broadband emissivity (BBE, 8–13.5 μm) is a vital parameter for calculating surface radiation budgets. Such data are currently unavailable. This paper proposes a lookup table-based method for retrieving sea surface hemispherical BBE values. The physicallybased sea surface emissivity model of Wu and Smith, together with the optimal refractive index, were used to generate hemispherical BBE values under wind speeds ranging from zero to 50 m/s. A lookup table of hemispherical BBE values as a function of wind speed was established and used to retrieve sea surface hemispherical BBE values under foam-free conditions. The accuracy of the estimates of hemispherical BBE was 0.003, given a wind speed of zero. The foam effect was explicitly considered. After incorporating the foam effect, hemispherical BBE was expressed as a linear function of the hemispherical BBE values of sea water and foam, weighted by the fraction of foam coverage. With this method, we have produced an hourly sea surface hemispherical BBE product with a resolution of 10 km and global coverage that covers the period from 2003 to 2005, using wind speed data from Modern-Era Retrospective analysis for Research and Applications (MERRA)-2.

Computation of total hemispherical emissivity from directional spectral models

Kirchoff’s law and experimentally obtained data and/or theory (such as from Drude’s model) for optical constants have been used in a modified geometric optics approximation (GOA) to obtain directional spectral emissivity. Then this spectral emissivity is used to obtain the total hemispherical emissivity by integration over direction and spectral range as weighted by the Planck distribution. The computations account for surface dependencies on temperature, composition, and roughness. The computed results for both the spectral directional and the total hemispherical emissivity are compared with the available data for platinum, stainless steel 304, and tungsten surfaces.

Apparent Emissivity of Combustion Soot Aggregate Coating at High Temperature

Soot aggregates frequently occur during combustion or pyrolysis of fuels. The radiative properties of soot aggregates at high temperature are important for understanding soot characteristics and evaluating heat transfer in combustion systems. However, few data for soot radiative properties at high temperature were available. This work experimentally investigated the apparent emissivity of the soot aggregate coating at high temperature using spectral and total hemispherical measurements. The soot aggregate coatings were formed on nickel substrates by a paraffin flame. The surface and inner morphology of the coatings were characterized by scanning electron microscope (SEM). The thickness of the coating was 30.16 μm so the contribution of the smooth nickel substrate to the apparent radiation from the coating could be neglected. The total hemispherical emissivity of the coating on the nickel substrate was measured using the steady-state calorimetric method at different temperatures. The spectral directional emissivity of the coating was measured for the wavelength of 0.38–16.0 μm at the room temperature. The measurements show that the total hemispherical emissivity decreases from 0.895 to 0.746 as the temperature increases from 438 K to 1052 K. The total hemispherical emissivity of the coating deposited on the nickel substrate is much larger than those of the nickel substrate and a nickel oxidization film. The measured spectral emissivity of the coating at the room temperature was used to theoretically calculate the total hemispherical emissivity at different temperatures by integration with respect to wavelength. The measured and calculated total hemispherical emissivities were similar, but their changes relative to temperature were completely opposite. This difference is due to the fact that the spectral emissivity of the coating is a function of temperature. The present results provide useful reference data for analyzing radiative heat transfer at high temperature of soot aggregates in combustion processes.

Atmospheric short-chain-chlorinated paraffins in Melbourne, Australia – first extensive Southern Hemisphere observations

Environmental contextThis study presents the first comprehensive set of ambient atmospheric concentrations of short-chain-chlorinated paraffins in the Southern Hemisphere. The data show a seasonal cycle with a summer maximum and a winter minimum. The seasonal cycle is consistent with temperature dependence of the vapour pressure of the short-chain-chlorinated paraffins resulting in partitioning between the atmosphere and other reservoirs with a secondary modulation by soil moisture. AbstractThe first extensive measurements of short-chain chlorinated paraffins (SCCPs) in the atmosphere of the Southern Hemisphere are presented. The analytical and sampling methodologies used in this Australian study were verified by systematic testing along with two inter-comparisons with Northern Hemisphere laboratories with established SCCP programs. In the ambient atmosphere of Melbourne, Australia, in 2013–14, there was a clear seasonal cycle in SCCP monthly averaged concentrations, these ranging from 28.4ng m–3 in summer to 1.8ng m–3 in winter. Air temperature was the factor most closely related to the seasonal cycle in SCCPs in Melbourne. The average SCCP concentrations observed indoors were less than those observed outdoors. Atmospheric concentrations of SCCPs in Melbourne are more than two orders of magnitude higher than concentrations in the background atmosphere. Surprisingly, the SCCP concentrations in Melbourne are similar to those observed in cities in Japan, South Korea and the United Kingdom, and less than those observed in China. Direct transport of SCCPs in the atmosphere from the Northern Hemisphere emissions to Melbourne is ruled out. Instead elevated concentrations in the Melbourne air-shed are most likely a result of the long-term import of SCCPs as industrial chemicals and within manufactured materials from the Northern Hemisphere so that the use of SCCPs in Melbourne and their consequent release to the environment has produced environmental reservoirs of SCCPs in Melbourne that are comparable with those in some Northern Hemisphere cities. The increase in SCCP concentrations from winter to summer is consistent with the temperature dependence of partitioning of SCCPs between the atmosphere and other reservoirs. Insufficient information exists on SCCP use and its presence in soils and sediments in Australia to indicate whether the atmospheric presence of SCCPs in Melbourne is a legacy issue due to its import and use as a metal cutting agent in past decades or due to ongoing imports of manufactured materials containing SCCPs today.

Measurements of the Temperature-Dependent Total Hemispherical Emissivity Using an Electrostatic Levitation Facility

Among the three fundamental processes of heat transfer (conduction, convection, and radiation), radiation is the most dominant at high temperatures. The total hemispherical emissivity is an important property that determines the amount of heat loss by radiation. Unfortunately, the emissivity, especially its temperature dependence \((\varepsilon (T))\), is unknown for most materials. Here, we demonstrate the feasibility of measuring \(\varepsilon (T)\) using an electrostatic levitation (ESL) technique that allows such measurements to be made on levitated solid and liquid samples in a contamination-free, high-vacuum environment. The \(\varepsilon (T)\) for solid Ni and liquid \(\hbox {Zr}_{60}\hbox {Al}_{10}\hbox {Cu}_{18}\hbox {Ni}_{9}\hbox {Co}_{3}\) from these measurements is consistent with the existing literature data.

Fractional integration and nonlinear deterministic trends in the analysis of time series data

ABSTRACTThis article examines the interaction between fractional integration and nonlinear structures by using for the latter the Chebyshev polynomials in time that can be taken as an alternative, less abrupt way of modelling breaks in time series data. A Lagrange multiplier test, developed for testing the order of integration in the context of nonlinear deterministic trends, is implemented in three well-known and previously studied time series data: the Nile river data, the temperatures in the Northern hemisphere and CO2 emissions in the US. The results suggest that the second and especially the third time series display nonlinear behaviour still with fractional degrees of differentiation.

Total hemispherical emittance of low temperature curing thermochromic coatings

Thermal control device based on thermochromis plays an important role in spacecrafts thermal management by automatically adjusting its radiative properities in response to large temperature gradients. The thermal management ability was determined by the device surface radiation performance. During the sun shadow, it is required to dissipate heat as little as possible from spacecraft, which is a low emittance device. While, it desired the device emittance is large enough to meet the heat dissipation requirement facing with the direct sunlight. As a type thermochromic material, La1 - x Sr x MnO3 (LSMO) has obtained broad attention because it showed a considerable superiority in saving energy consumption in space thermal management. It can hold the heat at low temperature due to its low emittance behavior, whereas dissipate the excess heat at high temperature for its high emittance properties. In this work, we prepared the La1 - x Sr x MnO3 ( x= 0125, 0.175, and 0.2) nanoparticles by sol-gel method from the precursors such as lanthanum nitrate hexahydrate, strontium nitrate and manganese nitrate solution (50%). LSMO nanoparticles were dispersed homogeneously in a solution of resins. The mixture then was spin-coated on the clean Al substrates. After standing for 10 h, the coating was cured at 120°C. The structure, morphology, and composition analysis of LSMO nanoparticles or coatings were investigated by X-ray diffraction technique, scanning electron microscopy, and the energy dispersive X-ray spectroscopy, respectively. The total hemispherical emittance of coating was measured by calortmetric method, which test sample was suspended in the center of a liquid nitrogen thermostat. According to the input power, sample temperature, and thermostat temperature, the emittance can be indirectly obtained. Two resins, which is the acrylic resin and the solution of terpineol and ethyl cellulose respectively, was considered to investigated the effect of reslin type on the emittance of coatings. The result of X-ray diffraction shows a single perovskite structure for LSMO nanoparticles at different Sr doped level. A fined LSMO nanoparticle with 50–300 nm, which is detected using scanning electron microscopy, can be obtained by control sol-gel process. The composition analysis results show a comparable stoichiometric ratio. The two resins coating show a different surface morphology, which is more compact in acrylic resin than in the terpineol. Experimental results show that the variation amplitude of coating emittance is 0.45 within the temperature range of 173–370 K. For the acrylic resin coating, the emittance changes from 0.42 at 173 K to 0.85 at 370 K, and it rises from 0.39 at 173 K to 0.83 at 370 K in terpineol resin coating.

Numerical and experimental determination of the temperature dependence of the spectral and integral emissivities of quartz ceramics of various porosity

The optical parameters of quartz ceramics (absorption and scattering indices) are determined by solving the inverse problem of radiation transfer based on the total-reflection spectra of two layers with different thicknesses measured in the wavelength range of 0.5–18 μm. The spectra of hemispherical emissivities and integral thermal emissivities are calculated in the temperature range of 20–1400°C. The influence of porosity on the optical parameters and emissivities is analyzed in the range of 7–11%.

Chemical cycling and deposition of atmospheric mercury in polar regions: review of recent measurements and comparison with models

Abstract. Mercury (Hg) is a worldwide contaminant that can cause adverse health effects to wildlife and humans. While atmospheric modeling traces the link from emissions to deposition of Hg onto environmental surfaces, large uncertainties arise from our incomplete understanding of atmospheric processes (oxidation pathways, deposition, and re-emission). Atmospheric Hg reactivity is exacerbated in high latitudes and there is still much to be learned from polar regions in terms of atmospheric processes. This paper provides a synthesis of the atmospheric Hg monitoring data available in recent years (2011–2015) in the Arctic and in Antarctica along with a comparison of these observations with numerical simulations using four cutting-edge global models. The cycle of atmospheric Hg in the Arctic and in Antarctica presents both similarities and differences. Coastal sites in the two regions are both influenced by springtime atmospheric Hg depletion events and by summertime snowpack re-emission and oceanic evasion of Hg. The cycle of atmospheric Hg differs between the two regions primarily because of their different geography. While Arctic sites are significantly influenced by northern hemispheric Hg emissions especially in winter, coastal Antarctic sites are significantly influenced by the reactivity observed on the East Antarctic ice sheet due to katabatic winds. Based on the comparison of multi-model simulations with observations, this paper discusses whether the processes that affect atmospheric Hg seasonality and interannual variability are appropriately represented in the models and identifies research gaps in our understanding of the atmospheric Hg cycling in high latitudes.

Spectral emissivity and constant pressure heat capacity of molten nickel and rhodium measured by spectrometers combined with an electrostatic levitator

Spectral emissivity and constant heat capacities of molten nickel and rhodium at their melting temperatures were measured using containerless techniques. Samples were levitated in an electrostatic levitator and the radiation intensities from the molten samples were measured with spectrometers over a wide wavelength range. The spectrometers were calibrated with a blackbody radiation furnace and the spectral hemispherical emissivity was calculated. The total hemispherical emissivity of nickel at the melting temperature calculated by integrating the spectral hemispherical emissivity was found to be 0.21, and the constant pressure heat capacity of molten nickel at the melting temperature was calculated to be 39.9J·mol−1·K−1. The total hemispherical emissivity and the constant pressure heat capacity of rhodium at the melting temperature were measured to be 0.23 and 41.8J·mol−1·K−1, respectively.