Ellipsometric and spectral measurements in the study of chemically coloured 304 stainless steel selective surfaces

Ellipsometric studies and reflectance measurements on five anodized coloured 304 stainless steel samples (Brown, Violet, Blue, Light Grey and Dark Grey) are reported. The optical properties, solar absorptance and thermal emittance of samples have been obtained. Measured values of absorptance lay between 0.75–0.87 together with a near normal emittance value 0.10 at 100°C. The total hemispherical emittance has been measured at 100 and 200°C for the sample having the best selective surface. This study shows that the blue coloured surface has the most favourable combination of radiation properties. The surface deteriorates at temperatures above 210°C. Oxidation is assumed to be the deterioration mechanism. The different samples exhibited differences in the ψ and Δ parameters as determined by ellipsometry both before and after heat treatments. When brown and violet samples were coated with a 125 nm layer of Si₃N₄ there was an improvement in optical properties and the samples remained stable up to a temperature of 300°C. These composite surfaces exhibited better selectivity than untreated blue coloured samples. The optical properties of blue samples on the other hand were not improved by coating with Si₃N₄.     

High-temperature optical properties and stability of AlxOy–AlNx–Al solar selective absorbing surface prepared by DC magnetron reactive sputtering

Al_xO_y–AlN_x–Al selective absorbing surface was prepared by DC magnetron reactive sputtering with aluminum alloy (LY13)¹ in air and argon. The studies were carried out to access the high-temperature (400°C–600°C) optical properties and stability of the coatings. The coatings were found to withstand heating at 600°C for 30 min in 4.5×10⁻³ Pa vacuum with absorptance 0.94 and emittance 0.07 after annealing. After heating at 450°C for 10 h, the specimen still had good performance whose absorptance and emittance was 0.93 and 0.07, respectively. Auger electron spectroscopy was used to analyse the structure of the solar selective surface before and after annealing.     

Selective black nickel coatings on zinc surfaces by chemical conversion

An electrochemical conversion technique has been developed to deposit selective black nickel coatings of solar absorptance 0.90–0.94 and thermal emittance (at 100°C) 0.08–0.15 on galvanized iron, zincated, and zinc electroplated aluminium surfaces. The effect of electrochemical conversion parameters on the microstructure, optical and thermal properties and durability of the coatings has been established.     

Selection of a black chrome bath for continuous tube-plating and the properties of the coatings deposited from it

The planning and design of a 50-kW experimental solar thermal power plant which was to have a selective coating on the absorber initiated an investigation into black chrome deposition in order to find the most suitable type of solution for a continuous plating process. Acetic acid-type and fluorosilic acid-type electrolytes were tested with a variety of additives and deposition parameters. The best results were achieved with an electrolyte containing fluorosilic acid as the catalyst. With this solution, coatings with optimum thermo-optical properties were formed in only 22 sec at current densities of 0.5 A cm⁻² and a temperature of 15°C. These yielded values of 0.97 for total absorptance (AM2 spectrum) and 0.08 (100°C) for the thermal emittance on a basis of light Watts-type nickel. Other nickel undercoatings which were investigated produced less satisfactory results.Both the thermal stability of the plating up to temperatures of 400°C and the dependence of its IR emittance on temperature were investigated. The tubes coated in the way described have a uniform, matt-black appearance and the thermo-optical properties are constant along their whole length and round their whole circumference.     

High efficiency Mo---Al2O3 cermet selective surfaces for high-temperature application

Highly efficient Mo---Al₂O₃ cermet solar absorbers have been designed with a numerical model and deposited experimentally. The typical film structure is an Al₂O₃ anti-reflection layer on a double Mo---Al₂O₃ cermet layer on a Mo or Cu metal thermal reflector. In numerical calculations of the thermal emittance at high temperature for these selective surfaces, the temperature dependencies of the complex refractive indices of the metal reflector and cermet in the infrared region have been considered, and the dielectric functions of the cermet materials are evaluated using Sheng's approximation. An optimization calculation yields a photothermal conversion efficiency as high as 0.914 at 350°C for a concentration factor of 26 for the film structure consisting of a double cermet layer on a Mo metal thermal reflector with an Al₂O₃ anti-reflection coating. The corresponding normal absorptance and hemispherical emittance at 350°C are 0.96 and 0.11, respectively. Mo---Al₂O₃ cermet selective surfaces using the double cermet layer structure were deposited by vacuum co-evaporation, and an absorptance of 0.955 and near normal emittance of 0.032 at room temperature have been achieved. An emittance of 0.08 at 350°C is estimated based upon room temperature experimental data for the film structure of a double cermet layer on a Cu metal thermal reflector with an Al₂O₃ anti-reflection coating.     

The impact of optical and thermal properties on the performance of flat plate solar collectors

The impact of the optical properties on the annual performance of flat plate collectors in a Swedish climate has been estimated with the MINSUN program. The collector parameters were determined with a theoretically based calculation program verified from laboratory measurements. The importance of changes in solar absorptance and thermal emittance of the absorber, the addition of a teflon film or a teflon honeycomb, antireflection treatment of the cover glazing and combinations of these improvements were investigated. The results show that several improvements can be achieved for solar thermal absorbers. A combined increase in absorptance from 0.95 to 0.97 and a decrease in emittance from 0.10 to 0.05 increase the annual performance with 6.7% at 50 °C operating temperature. The increase in performance by installing a teflon film as second glazing was estimated to 5.6% at 50 °C. If instead a teflon honeycomb is installed, a twice as high performance increase is obtained, 12.1%. Antireflection treatment of the cover glazing increases the annual output with 6.5% at 50 °C. A combination of absorber improvements together with a teflon honeycomb and an antireflection treated glazing results in a total increase of 24.6% at 50 °C. Including external booster reflectors increases the expected annual output at 50 °C to 19.9–29.4% depending on reflector material.     

Microstructural optimization and extended durability studies of low-cost rough graphite–aluminium solar absorber surfaces

Mechanically manufactured low-cost selective inhomogeneous rough graphite–aluminium (C/Al₂O₃/Al) solar absorber surfaces provide, at the current state of development, solar absorptance α=0.90 and thermal emittance =0.22. Based on a literature review, significantly lower emittance values could theoretically be achieved by utilization of surface plasmons in rough surfaces with an optimal graphite layer thickness, groove depth and sinusoidal surface profile periodicity. It is possible that an arbitrary form of roughening could produce equally good optical properties. In order to achieve lower emittance manufacturing parameters, the composition of silicon carbide grinding pad and the corresponding grinding pattern are required to be enhanced. The commercially available grinding pads used so far have not yielded optimal results. An antireflection coating could improve α to >0.90. Absorber samples were subjected to 383 days of temperature and irradiance cycling. In total, the samples were exposed to ultraviolet (UV) irradiation equivalent to 5–15 years of normal outdoor use. The results show that the samples are not sensitive to natural levels of UV irradiation or cycling-induced degradation of optical properties. A clear increment in absorptance was observed after the first 50 days of cycling. The elevated temperature of 130 °C is the probable cause for the increase. Reference samples indicated similar aging behaviour both after 4 years of natural exposure and after relatively short constant temperature tests at 120 and 180 °C.     

Calorimetric measurement of absorptance and emittance of the Sydney University evacuated collector

Simple calorimetric techniques have been developed for determining the absorptance and emittance of individual evacuated tubular collectors incorporating a selective surface, and the efficiency, η_o, of evacuated collectors in various mirror systems. The absorptance and efficiency measurements are made in natural sunlight without the use of a solarimeter by establishing an absorptance standard based on Nextel black paint. Calibration of solarimeters using the established absorptance standard is discussed. Emittance measurements are made by measuring radiative heat losses from the absorber tube of a collector.Absorptance and emittance measurements for a number of Sydney University evacuated collectors gave values of absorptance α = (0.92 ± 0.01) and emittance = 0.05 at 120°C for the selective surface utilized. Efficiency (η_o) measurements for Sydney University collectors in two simple mirror systems are also reported.     

High performance W–AlN cermet solar coatings designed by modelling calculations and deposited by DC magnetron sputtering

High solar performance W–AlN cermet solar coatings were designed using a numerical computer model and deposited experimentally. In the numerical calculations aluminium oxynitride (AlON) was used as ceramic component. The dielectric function and then complex refractive index of W–AlON cermet materials were calculated using the Sheng's approximation. The layer thickness and W metal volume fraction were optimised to achieve maximum photo-thermal conversion efficiency for W–AlON cermet solar coatings on an Al reflector with a surface AlON ceramic anti-reflection layer. Optimisation calculations show that the W–AlON cermet solar coatings with two 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 optimised calculated AlON/W–AlON/Al solar coating film with two cermet layers has a high solar absorptance of 0.953 and a low hemispherical emittance of 0.051 at 80°C for a concentration factor of 2. The AlN/W–AlN/Al solar selective coatings with two cermet layers were deposited using two metal target direct current magnetron sputtering technology. During the deposition of W–AlN cermet layer, both Al and W targets were run simultaneously in a gas mixture of argon and nitrogen. By substrate rotation a multi-sub-layer system consisting of alternating AlN ceramic and W metallic sub-layers was deposited that can be considered as a macro-homogeneous W–AlN cermet layer. A solar absorptance of 0.955 and nearly normal emittance of 0.056 at 80°C have been achieved for deposited W–AlN cermet solar coatings.     

Titanium–tin oxide protective films on a black cobalt photothermal absorber

We report the effect of covering an electrodeposited black cobalt absorber film with a Ti : Sn oxide film at various atomic ratios prepared by the sol–gel dip process. The resulting composite was characterized in its optical, structural and morphological properties. After thermal treatment at 400°C, the uncoated black cobalt film is oxidized and Co₃O₄ is formed. Samples covered with Ti : Sn films and thermally treated at the same temperature suffered lower oxidation maintaining in great extent the original metallic cobalt structure. The optical properties of the resulting material were affected by the presence of the Ti : Sn coating, and the best protective film obtained was a transparent Ti : Sn (8 : 2) atomic ratio oxide film, with a 210 nm thickness. This composite system exhibits an absorptance value of 0.91 and an emittance value of 0.34 after a 100 h, 400°C thermal treatment. A photothermal material composed of a layer of black cobalt and a protective oxide film coating seems then a promising solar energy absorber capable of withstanding high operating temperatures (400°C).     

Thermal degradation of black chrome coatings

Black Chrome coatings were prepared on Ni, Fe, Cu and stainless-steel substrates by an electroplating technique. The changes in the physical properties of the sample due to thermal degradation were characterized by Auger electron spectroscopy and spectrophotometry. The optimum black chrome coating was made on Ni substrates using newly synthesized electrolyte at a current density of 0.3–0.4 A/cm² for 20–30 s. Its optical properties were absorptance (α) of 0.90–0.96 and emittance () of 0.25–0.30. From the surface analysis, it was found that thermal degradation occurred primarily by oxidation of metallic Cr particles with an increase of the annealing temperature. It was also found that stainless steel #304 suffered less optical degradation of the black chrome coating by the selective oxidation process than other pure substrate materials for temperatures up to 450°C.     

Cr-N-O型选择性吸收涂层制备及热稳定性研究

采用反应磁控溅射方法制备SiO_x/Cr-N-O/Al选择性吸收涂层,该涂层太阳吸收比为95.9%、发射比3.8%、吸收发射比25.2。结合光学显微镜微观形貌分析、X射线衍射结构分析(XRD)、X射线光电子能谱成分分析(XPS)探讨涂层在250和400℃大气环境下热稳定性机理:250℃大气热处理后,涂层保持较高光谱选择性,表面形貌与物相结构未出现明显变化,SiO_x层氧化程度增大导致太阳吸收比升高,金属Cr和金属Al相互扩散导致发射比升高;400℃大气热处理后,涂层光谱选择性降低,表面出现微米级孔洞,XRD及XPS结果表明Cr-N-O吸收层被氧化,导致太阳吸收比降低,金属Cr和金属Al相互扩散导致发射比明显升高。     

The progress and prospect of middle/high temperature evacuated tubular solar collector

The all-glass evacuated solar collection tubes, incorporating the dc sputtered double layer metal-aluminium nitride cermet selective surface, have been mass-produced by TurboSun in large quantities under license to the University of Sydney since 1995. A solar absorptance of 0.94–0.95 and emittance of 0.04–0.05 at room temperature has been achieved for the SS-AIN cermet solar coatings. These solar tubes are stable at 330–400°C. These M-AIN cermet tubes have widespread application for solar hot water and steam heaters, as well as the demonstration test units for solar thermal electricity. In China, the production of solar water heaters using all-glass evacuated solar heat collection tubes has rapidly increased since 1995. The experimental results show that the solar selective coatings incorporating dc sputtered tungsten and dc reactively sputtered aluminium nitride components in a cermet should be stable at 500°C in vacuum. It would be possible to produce solar collector tubes for solar thermal electricity application with superior solar performance at a much lower cost.     

A comparative study of the optical properties of nickel pigmented alumina films of different thicknesses exposed to elevated temperature and humidity

Commercial solar absorbers of nickel pigmented anodized aluminium are composed of an inner nickel pigmented sublayer of about 0.3 μm thickness and a 0.4–0.5 μm thick top layer of plain alumina. Thermal emittance can be reduced from 0.17 to 0.12 if the top layer is made thinner, to be about 0.1 μm. The solar absorptance is 0.96 as for the thicker coating. In this study degradation is analysed for samples with thin or thick alumina top layer after exposure to elevated temperature, 300–500°C, or humidity. The results from these tests show that a thinner aluminium oxide top layer has the same durability as a thicker top layer. The implication of making commercial nickel pigmented anodized aluminium with an oxide half as thick as today is a reduction of the anodization time to about half the time and lower manufacturing costs.     

The use of moderate vacuum environments as a means of increasing the collection efficiencies and operating temperatures of flat-plate solar collectors

It is shown that evacuating a flat-plate solar collector to a pressure 1–25 torr results in elimination of the natural convection heat loss from the absorber for absorber-to-cover spacings up to 15 cm. This mode of heat transfer then reduces to pure conduction through the air space between the absorber and the cover. The effect of this reduction on the total upward heat loss from the collector is considered for a variety of collector operating conditions and is shown to be especially pronounced for collectors employing wavelength-selective surfaces (high absorptance for solar radiation, but low emittance for the energy re-radiated by the absorber). Computer simulations of collector performance for the Dallas, Texas area indicate that the combination of a moderate vacuum and a selective surface (α = 0·90, = 0·15) can increase daily energy collection as much as 278 per cent over that obtained with a non-vacuum collector using a flat-black (α = = 0·95) surface and can make it possible to operate at a temperature of 150°C with a daily energy collection efficiency of more than 40 per cent. The theoretical predictions are supported by the results of twelve experiments with a no-load solar tester. At an absorber-to-cover spacing of 7·5 cm, the steady-state temperature of a moderately selective absorber (α = 0·75, = 0·3) was increased from 115°C at atmospheric pressure to 179°C at a pressure of 25 torr.     

Spectral selectivity of composite enamel coatings on 321 stainless steel

Composite porcelain-enamel coatings consisting of spinel-type transition-metal-oxide pigments embedded in a borosilicate-glass matrix were deposited on 321 stainless steel using screen-printing techniques and characterized using total reflectance over the spectral range of 200 nm to 25 μm. The composite coatings exhibited moderate spectral selectivity and excellent thermal stability at 450 °C. Solar absorptance (α) ranged from 0.82 to 0.90 and calculated thermal emittance at 500 °C (ε) ranged from 0.46 to 0.79. Both depended on coating thickness and pigment volume concentrations. As an example, the highest solar selectivity was measured in a 0.9 μm coating with α and ε (500 °C) being 0.82 and 0.46, respectively.     

Spectrally selective NbAlN/NbAlON/Si3N4 tandem absorber for high-temperature solar applications

A new spectrally selective NbAlN/NbAlON/Si₃N₄ tandem absorber was deposited on copper substrates using a reactive direct current magnetron sputtering system. A high solar absorptance (0.956) and a low emittance (0.07) were achieved by gradually decreasing the refractive index from the substrate to the surface. The tandem absorber was characterized using solar spectrum reflectometer and emissometer, X-ray photoelectron spectroscopy, phase-modulated spectroscopic ellipsometry, atomic force microscopy and micro-Raman spectroscopy techniques. In order to study the thermal stability of the tandem absorbers, they were subjected to heat treatment (in air and vacuum) at different durations and temperatures. The tandem absorber deposited on copper substrate exhibited high solar selectivity in the order of 13–15 even after heat treatment in air up to 500 °C for 2 h. These tandem absorbers also exhibited high thermal stability (450 °C) in air for longer durations (116 h). The onset of oxidation for the tandem absorber deposited on silicon substrates was 650 °C, indicating a high oxidation resistance. The results of the present study indicate the importance of NbAlN/NbAlON/Si₃N₄ tandem absorber for high-temperature solar selective applications.     

Characterisation of high-temperature-resistant spectrally selective paints for solar absorbers

The spectrally selective paint coatings were prepared from organically modified siloxane resin and inorganic pigment (FeMnCuO_x-P320). To optimise the low-emittance properties, different thicknesses of paints were applied on high-reflecting Al foil by a draw bar coater. For all paints, optical and thermal properties were determined as well as their adhesion resistance. Pigment to volume concentration ratio was 20% and for thicknesses of about 1.7–2.0 g/m², the solar absorptance for these samples were a_s=0.90–0.92 with corresponding thermal emittance of e_T=0.20–0.25. Temperature stability of these samples was followed by FT-IR spectroscopy at 300°C. The obtained results indicated good temperature stability of prepared paint coatings.     

Studies on anodic oxide coating with low absorptance and high emittance on aluminum alloy 2024

Anodization of AA 2024 in sulfuric acid bath containing glycerol, lactic acid and ammonium metavenadate has been studied to develop white anodic oxide coating. Investigation on the influence of various operating parameters — coating thickness, current density and ammonium metavenadate concentration on the optical properties was carried out to optimize the process. Infrared, atomic absorption spectroscopic techniques and scanning electron micrograph were used to characterize the coating. The obtained oxide coating provides a ratio of solar absorptance (α) to infrared emittance (), as low as 0.2. The optical properties and hardness values measured under optimum experimental conditions support its use as a thermal control coating.     

Characterization of black nickel solar absorber coatings electroplated in a nickel chlorine aqueous solution

Black nickel coatings electroplated in a nickel- and sodium chlorine aqueous solution have been prepared and parameters in the process have been optimized to achieve optimal solar selectivity. The best result is a solar absorptance of 0.96 and a thermal emittance of 0.10, as also has been obtained previously for the same type of coating ¹H(¹⁵N, αγ)¹²C nuclear resonance reaction (NRR) have contributed with new information about surface chemical composition, morphology and atomic composition in depth profile. These investigations reveal that the coating is porous and contains mainly metallic nickel at the substrate–coating interface and mainly nickel hydroxide at the front surface.The stability of the coating has been tested regarding high-temperature and condensation at high humidity. It has been found that the solar absorptance changes initially, during the first hour of exposure at high temperature, but will then stabilize. Condensation causes a more severe attack on the coating by cracking it. This is contradictory to the results from previous tests at high temperature and humidity in which the black nickel coating was found to be resistant