Solar Absorptance Values Research Articles

Radiation properties modeling for plasma-sprayed-alumina-coated rough surfaces for spacecrafts

Spacecraft thermal control materials (TCMs) play a vital role in the entire service life of a spacecraft [NASA Preferred Reliability Practices: Spacecraft Thermal Control Coatings Design and Application (Practice No. PD-ED-1239), 1995, pp. 1–6]. Most of the conventional TCMs degrade in the harmful space environment [R.D. Karam, Satellite Thermal Control for Systems Engineers, AIAA, Virginia, USA, 1998, pp. 147–163]. In the previous study, plasma sprayed alumina (PSA) coating was established as a new and better TCM for spacecrafts, in view of its stability and reliability compared to the traditional TCMs [R.M. Li, S.C. Joshi, H.W. Ng, Presented in 2nd International Conference on Materials for Advanced Technologies & IUMRS, Singapore, 2003, pp. 1–4]. During the investigation, the surface roughness of PSA was found important, because the roughness affects the radiative heat exchange between the surface and its surroundings. Parameters such as root-mean-square roughness cannot properly evaluate surface roughness effects on radiative properties of opaque surfaces [M.F. Modest, Radiative Heat Transfer, 2nd ed., Academic Press/Elsevier Science, USA, 2003, pp. 90–92]. Some models have been developed earlier to predict the effects, such as Davies’ model [H. Davies, Proceedings of IEEE vol. 101, part IV, 1954, pp. 209–214], Tang and Buckius's statistical geometric optics model [K. Tang, R.O. Buckius, Int. J. Heat Mass Transfer 44 (2001) 4059–4073]. However, they are valid only in their own specific situations. In this paper, an energy absorption geometry model was developed and applied to investigate the roughness effects with the help of 2D surface profile of PSA coated substrate scanned at micron level. This model predicts effective normal solar absorptance ( α ne) and effective hemispherical infrared emittance ( ɛ he) of a rough PSA surface. These values, if used in the heat transfer analysis of an equivalent, smooth and optically flat surface, lead to the prediction of the same rate of heat exchange and temperature as that of for the rough PSA surface. The model was validated through comparison between a smooth and a rough PSA coated surfaces. Even though not tested for other types of materials, the model formulation is generic and can be used to incorporate the rough surface effects for other types of thermal coatings, provided the baseline values of normal solar absorptance ( α n) and hemispherical infrared emittance ( ɛ h) are available for a generic surface of the same material.

Optimization of solar absorbing three-layer coatings

Abstract The optimization of spectrally selective solar-absorbing three-layer coatings was studied from both theoretical and experimental points of view. The base layer consists of a cermet (ceramic–metal composite) material, where small metal clusters are embedded in the dielectric surrounding. The results show considerable enhancement of the solar absorption due to the metal nanoparticles. Theoretical treatment of the optical properties of the cermet was carried out using effective medium theory. The experimentally determined optical constants of calibration samples were used to calculate the film reflectance of single or multiple layers. Theoretically optimized three-layer coatings show solar absorptance value as high as 0.97 and moderate reflectance at 2.5 μm giving low thermal emittance. Experimental optimization was carried out through control of the coatings layer by layer. The metal content and the film thickness are two important parameters for cermet films. Only thickness is sensitive to the middle and top layer. The optical properties are matched in such a way that the base layer functions as main absorber, the top layer reduces the reflectance from the front surface, and the middle layer links the base and the top layers. An experimentally optimized solar absorptance of 0.97 with a thermal emittance of 0.05 was obtained, which shows that the optimization procedure is reliable.

Electrochemical deposition of black nickel solar absorber coatings on stainless steel AISI316L for thermal solar cells

Abstract We report the electrochemical deposition of nanostructured nickel-based solar absorber coatings on stainless steel AISI type 316L. A sol–gel silica-based antireflection coating, from TEOS, was also applied to the solar surface by the dip-coating method. We report our initial results and analyze the influence of the stainless steel substrate on the final total reflectance properties of the solar absorber. The relation between surface morphology, observed by SEM and AFM, the composition of the electrodeposited surfaces analyzed by X-ray powder diffraction and the study of different electrodeposition conditions and silica sol–gel coatings is described. The best solar absorptance and thermal emittance values obtained on stainless steel substrates were 0.91and 0.1, respectively.

Evaluation of Thermal Control Coatings Degradation in Simulated Geo-Space Environment

This paper describes a study carried out to evaluate solar absorptance and infrared emissivity degradation of many different thermal control materials (white paints, second-surface mirrors and aluminized films) exposed to geostationnary orbit. For this purpose, a long-term test of about 3 months in constant vacuum was performed reproducing a 3-year space environment exposure including ultraviolet and charged particle effects on North/South panels of a geostationary three-axis stabilized spacecraft. Reflectance spectra were measured in situ in the solar range (250-2500 nm) and the corresponding solar absorptance values were calculated. The test methodology and the obtained degradations of many materials are discussed. Instead of a constant increase with irradiations, oscillations of solar absorptance were obtained for some materials depending on the test sequence. The most important degradations were found in white silicone paints whereas second surface mirrors and aluminium paints remained quite stable.

Zr–ZrO2 cermet solar coatings designed by modelling calculations and deposited by dc magnetron sputtering

High solar performance Zr–ZrO2 cermet solar coatings were designed using a numerical computer model and deposited experimentally. The layer thickness and Zr metal volume fraction for the Zr–ZrO2 cermet solar selective coatings on a Zr or Al reflector with a surface ZrO2 or Al2O3 anti-reflection layer were optimized to achieve maximum photo-thermal conversion efficiency at 80°C under concentration factors of 1–20 using the downhill simplex method in multi-dimensions in the numerical calculation. The dielectric function and the complex refractive index of Zr–ZrO2 cermet materials were calculated using Sheng's approximation. Optimization calculations show that Al2O3/Zr–ZrO2/Al solar coatings with two cermet layers and three cermet layers have nearly identical solar absorptance, emittance and photo-thermal conversion efficiency that are much better than those for films with one cermet layer. The optimized Al2O3/Zr–ZrO2/Al solar coating film with two cermet layers has a high solar absorptance value of 0.97 and low hemispherical emittance value of 0.05 at 80°C for a concentration factor of 2. The Al2O3/Zr–ZrO2/Al solar selective coatings with two cermet layers were deposited using dc magnetron sputtering technology. During the deposition of Zr–ZrO2 cermet layer, a Zr metallic target was run in a gas mixture of argon and oxygen. By control of oxygen flow rate the different metal volume fractions in the cermet layers were achieved using dc reactive sputtering. A solar absorptance of 0.96 and normal emittance of 0.05 at 80°C were achieved.

Titanium–aluminum–nitride coatings for satellite temperature control

Intense solar irradiation, radiative cooling to outer space, and internal heat generation determine the equilibrium temperature of a spacecraft. The balance between the solar absorption and thermal emittance of the surface is therefore crucial, in particular for autonomous parts directly exposed to the solar radiation and thermally insulated from the main thermal mass of the spacecraft. The material composition but also the coating thickness are found to influence the equilibrium temperature of an object in space. In this paper we report on a systematic search for a suitable composition and thickness of Ti x Al y N z alloy coatings prepared by reactive, unbalanced magnetron sputtering from targets consisting of differently sized titanium and aluminum sectors. The films were deposited on glass, glassy carbon, aluminum sheet metal, and on sputtered aluminum and Ti x Al (1− x) films on glass. The stoichiometry and sheet resistance of the films was determined with Rutherford backscattering and four-point probe measurements respectively. Reflectance spectra for the visible and infrared spectral ranges were used to obtain average solar absorptance and thermal emittance values used in model calculations of the equilibrium temperature. Neglecting internal heat contributions, the lowest calculated equilibrium temperature in orbit around the Earth, 32.5°C, was obtained for a 505-nm-thick Ti 0.14Al 0.47N 0.40-film.

Degradation study of black cobalt coating

Black cobalt coatings were electrodeposited on 304 stainless steel substrates. In order to study the thermal stability of the films, they were subjected to several cycles of heating in air at a fixed temperature of 400°C. The phase compositions of fresh samples and of heat treated films were identified using X-ray diffraction. Values of solar absorptance were calculated from the measured reflectance values before and after heat treatment. The studies revealed that the fresh samples of black cobalt were composed of (002) reflection phase (5–727 JCPDS), which suggests that the cobalt grows epitaxially along the [002] direction of the γ002 phase of the substrate. The high absorptance of fresh samples and their deep black appearance can be explained in terms of the multiple reflection model, owing to the intrinsic absorption and roughness of the surface. This has a direct bearing on the electrodeposition method, because it is also possible to prepare cobalt with metallic appearance. The thermal oxidation behaviour and morphological changes of the black cobalt were identified as the main causes of the elevated temperature degradation of the coatings.

Studies on white anodizing on aluminum alloy for space applications

A process of white anodizing in an electrolyte system consisting of sulfuric acid, lactic acid, glycerol and sodium molybdate was studied for space applications. The influence of anodic film thickness and various operating parameters, viz., electrolyte formulation, operating temperature, applied current density, on the optical properties of the coating has been investigated to optimize the process. The coatings were characterized by atomic absorption spectroscopic analysis, thickness and microhardness evaluation. The space worthiness of the coating has been evaluated by humidity, thermal cycling, thermo-vacuum performance tests and measurement of optical properties. The anodic film developed herein provides solar absorptance value as low as 0.16, and infrared (IR) emittance of the order of 0.80. These results indicate that the process developed is suitable for thermal control applications in space environment.

Correlation of predicted and observed optical properties of multilayer thermal control coatings

Thermal control coatings on spacecraft will be increasingly important as spacecraft grow smaller and more compact. New thermal control coatings will be needed to meet the demanding requirements of next generation spacecraft. Computer programs are now available to design optical coatings, and one such program was used to design several thermal control coatings consisting of alternating layers of WO3 and SiO2. The coatings were subsequently manufactured with electron beam evaporation and characterized with both optical and thermal techniques. Optical data were collected in both the visible region of the spectrum and the infrared. Solar absorptance values were predicted in the range of 0.177–0.196 and were observed in the range of 0.155–0.228. Infrared emittance values were predicted in the range of 0.074–0.083 and were observed optically in the range 0.048–0.093 and calorimetrically in the range of 0.069–0.100.

Stainless-steel–AlN cermet selective surfaces deposited by direct current magnetron sputtering technology

Stainless-steel–AlN cermet selective surfaces were deposited by a direct-current (DC) magnetron sputtering technology. The aluminum nitride ceramic and stainless-steel (SS) components in the cermet were deposited by simultaneously running both SS and Al metal targets in a gas mixture of argon and nitrogen. Nitrogen partial pressure was set sufficiently high to ensure that a nearly pure AlN ceramic sublayer was deposited by DC reactive sputtering. Because of the excellent nitriding resistance of stainless steel and the very low nitrogen partial pressure, a nearly pure SS sublayer was deposited by DC sputtering. By substrate rotation, a multi-sublayer system, consisting of alternating SS and AlN sublayers, was deposited. By controlling the thickness of SS and AlN sublayers this multi-sublayer system can be considered as a macrohomogeneous cermet layer. Following this procedure, the SS–AlN cermet selective surfaces with a double cermet layer structure were deposited. The deposited films have the following structure: a low metal volume fraction cermet layer is placed on a high metal volume fraction cermet layer which in turn is placed on an aluminum metal infrared reflection layer. The top surface layer consists of an aluminum nitride anti-reflection layer. A solar absorptance of 0.93–0.96 and normal emittance of 0.03–0.04 at room temperature have been achieved for the SS–AlN cermet selective surfaces. After vacuum baking at 500°C for 1 h there is only a slight change in optical reflectance in the wavelength range of 0.3–2.0 μm for the SS–AlN cermet selective surfaces. However, the values of solar absorptance and emittance change insignificantly, around 0.005.

Characterization of TSSS paint coatings for solar collectors by FTIR spectroscopy

The background to the spectral selectivity of thickness sensitive spectrally selective (TSSS) paint coatings is discussed from the chemical point of view. This means considering the solar absorptance ( a s) and thermal emittance ( e T) of the TSSS paint coatings as a function of paint film thicknesses in terms of the degree of flocculation of the pigment dispersion and from the viewpoint of the relevant intermolecular interactions which influence the ability of the pigment particles to become distributed uniformly in the paint, thus increasing its colour strength. We investigated eight TSSS paint coatings based on the standard formulation of Solariselect ® paint [Sol. Energy Mater. 18 (1989) 97], which is currently used for solar collector panels. The influence of carbon soot (C - Degussa FW2), inorganic pigment (F - Ferro 6331) and fumed silica thixotropic agent (Aerosil - Degussa) in TSSS paint coatings on the degree of their flocculation was determined from the infrared hemispherical reflectivity R ( d) λ = 2.5 which was measured from the paint film thickness. The absorption coefficient for diffuse radiation ( K) and the scattering coefficient ( S) were determined by fitting the experimentally determined R ( d) λ = 2.5 curves to a modified Kubelka-Munk function derived by Körte and Otto [Appl. Spectrosc. 43 (1988) 38]. Correlations between the type of black pigment (F and C) and elementary particle size and surface activity of fumed silica (Aerosil 380, 200 and 972) were established and compared with the experimentally determined solar absorptance values ( a s) of the corresponding TSSS paint coatings. Since fumed silica in spite of its small content (<1% by weight) in the paint dispersion, decreases the gloss of the paint, the correlation between the RMS heights of irregularities (σ) and RMS slopes ( m) of irregularities derived from the infrared reflectivity measurements according to the procedure by Bennett and Porteus [J. Opt. Soc. Am. 51 (1961) 123] were determined. This revealed that the σ values are inversely proportional to the elementary particle size of the fumed silica used (Aerosil 380, 200, 972) in the TSSS paint coatings. FTIR spectra of Solariselect ® paints, the corresponding TSSS paint coatings and model dispersions prepared from the resin binder (phenoxy resin) and various types of Aerosils were measured, and the influence of the composition of the paint dispersions on the frequency shifts of -OH vibrations and intensity changes of skeletal modes of the resin binder were established. The changes of infrared bands observed revealed that resin binder molecules become more strongly bonded when fumed silica is present in dispersions. The correlation among the strength of the interactions in a dispersion, the degree of flocculation and σ values are given and discussed.

NICKEL-COBALT SELECTIVE COATINGS BY CONVERSION PROCESS

ABSTRACT Black nickel-cobalt coatings have been found to be selective in the solar spectral region and to be more stable than the widely used conventional selective black nickel (NiS-ZnS) coatings. Solar selective black nickel-cobalt coatings formed on zincated aluminium, zinc electroplated aluminium and galvanised iron substrates by conversion process is described. Optical and thermal properties of the coatings are also reported. These coatings show an integrated solar absorptance ( α ) value in the range of 0.90 to 0.94 and a room temperature emittance (e ) in the range of 0.07 to 0.12. The effect of various deposition parameters and substrate preparation on the optical properties and microstructure has been reported. The plating parameters of various coatings have been determined, which yield the optimum optical and thermal properties of the final selective coatings. The thermal stability of the coatings as a function of annealing temperature and time, and the effect of thermal treatment on the optical...

Calorimetric measurements of thermal control surfaces at geosynchronous orbit

Calorimeter data for up to 18,000 equivalent sun-hours are presented for a.number of satellite thermal control surfaces. This has been an ongoing study by Aerojet ElectroSystems. Solar absorptance values as a function of time were collected at geosynchronou s orbit on samples of zinc orthotitanate paint, silver-alumina-silica, silvered Teflon, aluminized Teflon, silica cloth, and clean second-surface mirrors. Over 6000 equivalent sunhours of calorimeter data were also collected on precontaminated second-surface mirror samples (also on-orbit). A useful collection of solar absorptance degradation equations is the result. These equations are of major significance in predicting satellite useful life. Nomenclature t - time T = temperature am - maximum solar absorptance a0 = initial solar absorptance as = solar absorptance £ = emissivity r = time constant

Maxwell-Garnett theory analysis of thick PbSAl selective absorbers

Previous measurements of the solar absorptance and thermal emittance values of PbSAl tandem solar selective surfaces have resulted in α ϵ ratios significantly higher than predicted for thick (>200 nm) PbS layers, assuming optical constants equal to those of the bulk material. We report calculations of photothermal conversion efficiencies for PbsAl tandem absorbers using the Maxwell-Garnett theory to determine α and ϵ values for porous PbS films. We have also prepared samples with PbS thicknesses up to 550 nm and have determined their α and ϵ values using reflectance measurements. Thermal efficiencies in the range of 0.55 to 0.86 were obtained at 300 K and are consistent with values expected for films with moderate (0.15–0.35) void fractions.

Solution grown [formula omitted] composite selective surfaces

Spectrally selective ( PbS) 1−x-( CdS) x composite coatings have been developed using the solution growth technique. Optical, structural, composition and depth profile studies have been carried out. Composition and film thickness have been optimized to get solar absorptance (α s) and total emittance (ϵ 100) values of 0.90 and 0.10, respectively.

Titanium-based spectrally selective surfaces for solar thermal systems

A study of spectrally selective surfaces based on anodic oxide films on titanium is presented. These surfaces have low values of solar absorptance, ≈ 0.77, due to the non-ideal optical properties of the anodic TiO 2 for antireflection of titanium. A simple chemical etching process is described which gives a textured surface with dimensions similar to the wavelengths of solar radiation, leading to spectral selectivity. The performance of this dark etched surface can be further improved by anodising and optimum absorbers have been produced with α s = 0.935 and ϵ th (400 K) = 0.23 . The surface texturing effects a significant improvement in α s at oblique incidence.

Black chrome solar selective coatings optimized for high temperature applications

A statistical design approach has been used on a specified set of plating parameters to optimize the thermal stability of electrodeposited black chrome solar coatings. Plating parameters studied included the bath composition (chromic acid, acetic acid, trivalent chromium and iron), current density, plating time, substrate and bath temperature. The optimization procedure consisted of maximizing the solar absorptance values after thermal aging at high temperatures (up to 450°C in air). The estimated functional dependence of the thermally aged solar absorptance values on the four bath constituents was determined in the region of most stable coatings. This relationship was used to specify a range of bath compositions that provide coatings with optimum thermal stability. It was also found that chloride ion contamination of the black chrome plating solution significantly degraded the thermal stability of plated coatings. Fortunately, the chloride ions could be effectively removed through the addition of silver oxide, which precipitates silver chloride. When the plating variables were properly controlled, electrodeposited black chrome coatings were obtained which should maintain solar absorptance values of 0.96 or higher during years of operation at 300°C in air.

Microstructural and mechanical property evaluation of black-chrome coated solar collectors

Concentrating solar collector assemblies of the substrate steel surface electroplated with a nickel coating followed by a thin black chrome solar absorber overlayer have been studied for the role and optimization of plating parameters such as; plating time, current density, plating bath temperature, chromic acid concentration, role of chromonyx addition agent, anode-to-cathode ratio of the plating arrangement, current efficiency of electroplating, and role of substrate finish; and over-growths were evaluated in each case for solar absorptance values. Within the range studied (3–36 min), 18 min of plating time is seen to give the highest absorptance value (96%). Current density variation, within the limits of this study (21.6–54 A/dm 2) shows that an optimum absorptance (93.2%) is attained with a current density of 43.2 A/dm 2. Variations in bath temperature of plating (0–30°C) yields a maximum in absorptance (93.2%) in coatings done at 10°C. The optimum value in solar absorptance (97.9%) is obtained at 375.95 g chromic acid/ l (50 oz/gal) addition to the plating bath, within the range, 225.57–413.54 g/ l (30–55 oz/gal), studied. Chromonyx addition agent, suggested for the plating bath, is seen to optimize at 30% of the bath volume in the range (21–33% of the volume) studied. Anode-to-cathode proportions of the plating arrangement are seen to yield a maximum absorptance (98.4%) at a ratio of 2 within the ratios evaluated (1–6 anode/cathode ratios). Current efficiencies of plating show a much higher value (92%) for nickel electroplating than for black-chrome coating (0.20%) for platings done at 10°C, 43.2 A/dm 2, and an electrode spacing of 1.27 cm. Finally, the role of substrate-surface finish, evaluated through coating black-chrome onto steel and nickel or nickel sheets show absorptance values to be quite similar in both cases, (93.0%) for black-chrome on steel plus nickel arrangement and (93.1%) for black chrome plated onto nickel sheets, at 10°C for a period of 3 min, and were extremely close in value for all bath temperatures (0–30°C) evaluated. Scanning electron microscopy and replication electron transmission microscopy done on plated and plated and ion-milled surfaces of solar collector growths did not yield conclusive data, but high magnification studies of the surfaces with the scanning electron microscope indicate an alteration of growth characteristics with variations in deposition temperatures. Finally, the strength of adhesion of the plated layers onto substrate surfaces was measured through tensile testing at ambient temperature. Values attained for adhesion strength (in excess of 100 kg/cm 2 in each case) indicate that the electroplated nickel and black-chrome layers are well attached to their respective substrates and possible failure of these coatings is unlikely to be due to insufficient adhesion.

Optical and electrical properties of ion-beam-textured Kapton and Teflon

An electron bombardment argon ion source was used to ion etch polyimide (Kapton) and fluorinated ethylene propylene, FEP (Teflon). Samples of polyimide and FEP were exposed to 0.5–1.0-keV Ar ions at ion current densities of 1.0–1.8 mA/cm2 for various exposure times. Changes in the optical and electrical properties of the samples were used to characterize the exposure. Spectral reflectance and transmitance measurements were made between 0.33 and 2.6 μm using an integrating sphere after each exposure. From these measurements, values of solar absorptance were obtained. Total emittance measurements were also recorded for some samples. Surface resitivity was used to determine changes in the electrical conductivity of the etched samples. A scanning electron microscope was used to record surface structure after exposure. Presented in the paper are spectral optical data, resistivity measurements, calculated absorptance and emittance measurements, along with photomicrographs of the surface for the various exposures to Ar ions.

High temperature; stable, spectrally selective solar absorbers for thermochemical hydrogen production

The thermochemical reduction of water by reactions similar to the Mark I process requires a processing temperature of 730°C. The efficient utilization of solar photothermal energy conversion, in distributed collector systems, to attain this temperature will require the use of suitable spectrally selective surfaces which are stable at the operating temperature. A coating system with demonstrated high temperature capability has been developed at the Optical Sciences Center under NSF/RANN sponsorship. A silicon thin film absorber is deposited by chemical vapor deposition (CVD) on a silver thin film reflector. This optical stack is fabricated at temperatures in excess of 800°C, and the CVD technology is amenable to large scale production in a flow-through system. At 500°C the present Si-Ag system has typical solar absorptance and total normal emittance values of 0.75 and 0.06, respectively. Samples were fabricated which maintained their high spectral selectivity after 2000 thermal cycles between 150°C and 450°C, and after 100 h at 600°C. Further process studies now underway indicate that the solar absorptance can be improved to better than 0.85 by employing a Si-Ge multilayer absorber and that the operating range of the stacks can potentially be raised to the 800–900°C range by employing a refractory metal thin film reflector.