High performance sputtered Ni : SiO2 composite solar absorber surfaces

A solar selective absorber can be prepared by dispersing suitably sized metallic particles in an insulating host matrix. Absorption is assisted by controlling optical interference within the composite film. Graded index thin films of metallic nickel in quartz (Ni : SiO₂) were made by co-sputtering with metal volume fractions ranging from 10–90% from top (anti reflecting coating) to bottom (base layer) of the structure, to minimise optical interference peaks. The films are 100–170 nm thick with an additional 70 nm anti-reflection (AR) coating. Coatings of different thickness, metal volume fraction and compositional gradient were investigated. Substrates were Al and Cu and films were deposited on either the bare substrate or substrates coated with evaporated nickel. The influence of substrate choice on the optical properties was studied. Films with solar absorptance, α, in the range 0.90–0.96 and thermal emittance, ,=0.03–0.14 were achieved. The dependence of these properties on thickness, film composition and gradient and substrate were determined. A computer programme which calculates the solar absorptance and thermal emittance based on the assumptions of both Maxwell Garnett and Bruggeman theories for metal fill factors below and above 0.3, respectively, was used to design the structure of the composite films. The theoretical results are not presented here.     

Optimization of solar absorbing three-layer coatings

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.     

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.     

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.     

Solution-chemical derived nickel–alumina coatings for thermal solar absorbers

A promising novel solution-chemistry method to fabricate spectrally selective solar absorber coatings has been investigated. The selectively absorbing film consists of nickel nano-particles embedded in a dielectric matrix of alumina. Ejecting a precursor solution of nickel and aluminum onto an aluminum substrate using a spin-coating technique followed by a heat-treatment, generated the solar absorber samples. Smooth and homogeneous films with a nickel content of 0 to 80 vol.% were produced. The optimal coating had a nickel content of 65%, a thickness of 0.1 μm and a particle size of 10 nm. The absorbing layer attained a normal solar absorptance, _sol, of 0.83 and a normal thermal emittance, _therm, of 0.03. Adding an anti-reflection layer on top of the first absorbing layer further enhanced the performance of the absorber. The optimum anti-reflection coated sample reached a solar absorptance of 0.93 and a thermal emittance of 0.04.     

Computer simulation of the optical properties of high-temperature cermet solar selective coatings

A computer simulation is developed to calculate the solar absorptance and thermal emittance of various configurations of cermet solar selective coatings. Special attention has been paid to those material combinations, which are commonly used in high-temperature solar thermal applications. Moreover, other material combinations such as two-, three- and four-cermet-layer structures as solar selective coatings have been theoretically analyzed by computer simulation using three distinct physical models of Ping Sheng, Maxwell–Garnett and Bruggeman. The novel case of two-cermet-layer structure with different cermet components has also been investigated. The results were optimized by allowing the program to manipulate the metal volume fraction and thickness of each layer and the results compared to choose the best possible configuration. The calculated results are within the range of 0.91–0.97 for solar absorptance and 0.02–0.07 for thermal emittance at room temperature.     

Optical properties of higher and lower refractive index composites in solar selective coatings

The focus of attention in this study was the choice of material for optically solar selective coatings on the basis of their optical constants. A computer programme which calculates the optical constants, solar absorptance at air mass (AM)-2, α, and thermal emittance at 300 K, , of the 200-nm-thick selective coating on the assumption of both the Maxwell Garnett and Bruggeman theories for the metallic volume fraction below and above 0.3 respectively, was used to design the structure of the composite films. Two systems of composite thin films of metal and dielectric were investigated experimentally, fabricated by RF and DC sputter coater and were verified with computer simulations. One system consist of lower refractive index composites such as Ni : SiO₂ and the other of higher refractive index composites such as V : Al₂O₃ in the spectral range of 0.3–20 μm. These films were fabricated on infrared reflective substrates such as nickel plated copper or aluminium. Results of the copper substrates are being presented here. For comparison and verification, tungsten, cobalt and chromium based composites, having different refractive indices, were also investigated which validated the concept of the choice of material in selective coatings. It was observed that high refractive index composites have lower reflective properties by choosing suitable metallic volume fraction in dielectric and antireflection coating. The higher value of the imaginary part of refractive index, k, is responsible for higher absorption by a factor α_λ=4πk/λ. Solar absorptance of 0.98 and 0.96 was achieved by simulation and experimental findings with less than 0.05 thermal emittance for 200 nm thick composites of V : Al₂O₃. It results that higher values of both n and k of the material are more suitable in solar selective coatings.     

Spectral selectivity and accelerated life testing of Al-PbS solar selective surfaces

High absorptance and low thermal emittance selective surfaces have been formed by evaporating lead sulphide onto Al evaporated Al substrates. The dependence of selectivity, α/?, on thickness has been studied. The maximum selectivity was obtained for PbS thickness of 400 Å. The emissivity is found to increase with temperature. The coatings are stable up to a temperature of 240°C in air. Long term ultraviolet irradiation causes the photo-oxidation of PbS to PbSO₄ which results in the increase of emittance and a decrease in absorptance. When cycled between 0°C and 200°C, the coatings do not show any signs of degradation.     

Spectrally selective copper oxide films

Copper oxide coatings have been produced on aluminium substrates by the spray pyrolysis technique. Studies have been carried out to observe the variations of solar absorptance (α_S) and thermal emittance (ε_T) with different concentrations of the spraying solution for different coating thicknesses. The effect of baking temperature on the optical properties of the coatings has also been studied. Under optimum conditions, with a baking temperature of 350°C and a spray concentration of 0·005m, a solar absorptance of 0·90 with a corresponding thermal emittance of 0·15 is obtained for a film 1·35 μm thick. Structural studies carried out with X-ray diffraction and electron diffraction techniques have shown that the monovalent oxide CuO is the clear dominating constituent of the oxide layer with a small percentage of the divalent Cu₂O. The stagnation temperature measurements give a maximum stagnation temperature of 120·8°C for the selective surface whereas, under identical conditions, it is 106·8°C for a non-selective black surface.     

A feasibility study of integrally colored Al–Si as a solar selective absorber

The solar selective properties of integrally colored Al–Si alloy (11.6 wt% Si) have been investigated. Optical measurements showed a continuous decrease of reflectance, i.e. an increase of absorptance, with increasing film thickness. A maximum solar absorption of 0.85 was achieved for Si–Al₂O₃ coatings thicker than 13 μm but such thick aluminum oxide coatings have very high thermal emittance.The reflectance of the Si–Al₂O₃ coated aluminum could be understood from a four flux radiative transfer theory. Using this theory the optical performance of the coating as a solar absorber was modeled for different size and volume fractions of silicon particles and coating thicknesses. A solar absorptance of just 0.90 can be achieved from a 10 μm thick coating of about 0.3 volume fraction of silicon. For thinner coatings (1 μm) the solar absorptance was only 0.70 for the same volume fraction.     

Angular solar absorptance and incident angle modifier of selective absorbers for solar thermal collectors

The solar absorptance of absorbers for thermal solar collectors is usually characterized at near normal angle of incidence. The solar absorptance is however a function of the angle of the incident light on the absorbers. In this paper the angular solar absorptance of commercial nickel pigmented aluminum oxide and sputtered nickel/nickel oxide solar selective absorbers are reported. The solar absorptance was calculated from experimental total reflectance spectra in the wavelength range 300–2500 nm for angles of incidence between 5 and 80°. It was found that the solar absorptance at higher angles of incidence is lower for the sputtered nickel/nickel oxide than for the nickel pigmented aluminum oxide coating. This could be understood from theoretical calculations based on microstructure models of the two types of coatings. The nickel pigmented aluminum oxide with a double-layer structure of its coating has an enhanced higher angle solar absorptance due to thin film interference effects which can not be achieved from a graded-index thin film coatings as is the case for the sputtered nickel/nickel oxide absorber. When the absorbers were covered by glass, as is common for most solar collectors, a negligible difference in optical performance at the higher angles of incidence has been obtained. These results were consistent with a theoretical calculation by use of an incident angle modifier model.     

A new approach to low cost large area selective surfaces for photothermal conversion

In the present study, selective black paint coatings have been prepared by coating reflective metal particles with a layer of selective black material. the coated particles were mixed in a binder and applied easily as a thin layer onto aluminium or galvanized iron (G.I.) sheet. Three selective black materials, namely CuO, CuS and PbS + CuS have been deposited on zinc metal powder, the solar absorptance of the coatings is ±0.95 and the emittance is ±0.4. the thickness of the coatings was about 20 to 30 μm. the emissivity decreases as the thickness increases, while the solar absorptivity does not change appreciably. the improvement in the collector efficiency, which is the ratio of the temperature increase above the temperature of the standard panel to the temperature increase of the standard panel above the ambient temperature, is estimated to be around 11 per cent. the process is potentially a low cost one for large scale application in solar photothermal conversion.     

Spectrally selective Al/AlN/Al/AlN tandem solar absorber by inline reactive ac magnetron sputtering

Abstract

Spectrally selective Al/AlN/Al/AlN tandem solar absorbers were deposited onto soda lime glass substrates using inline ac magnetron sputtering in a reactive atmosphere containing argon and nitrogen. To achieve a reproducible and homogenous deposition process, the deposition of multilayer Al/AlN/Al/AlN films under different process conditions was investigated. Two main variables, ac power and the speed of substrate movement in the chamber, were varied in the ranges of 1–10 kW and 10·47–31·4 mm s^?1 respectively to obtain films with high absorption and low emittance. The effects of film thickness of different layers and deposition conditions on the optical performance of selective coatings were also studied. Structural features and surface morphology of the films were investigated by X-ray diffraction and field emission scanning electron microscopy analyses respectively. In the present study, tandem solar absorber films deposited onto glass substrate with optimised sputtering parameters can consistently achieve solar absorptance α of 0·864 and thermal emittance ? of 0·03 at 80°C.     

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.     

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.     

空心镍球在太阳能吸热涂层上的研究应用

将超细空心镍粉与丙烯酸树脂混合制成涂料,采用浸沾法制备太阳能选择性吸收涂层并测试涂层的光学性能.测试结果表明,涂层有很高的吸收率(0.98),可能由于丙烯酸树脂或者涂层过厚的原因,涂层有比较高的发射率.涂层的吸收率会随着镍粉粒径的增大而减小,发射率则随着涂层的厚度增大而增大,与镍粉粒径无关.     

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.     

Coloured paints based on iron oxide and silicon oxide coated flakes of aluminium as the pigment, for energy efficient paint: optical and thermal experiments

Energy efficient coloured paint coatings utilising flaky aluminium pigment with either single layer (Fe₂O₃) or double layer (Fe₂O₃ on SiO₂) interference coatings are optically and thermally characterised. Similar pigments with coatings on flaky dielectric particles and standard paints of similar colour are compared. Data presented includes hemispherical and specular reflectance spectra across visible and infra red wavelengths, thermal emittance from an emissometer and light spreading data from a photogoniometer. Solar absorptance, and colour show that, as theoretically predicted, and provided flaky metal pigments are used, a wide range of colours combined with a much lower solar absorptance than traditional paints of similar colour is achieved. Superior thermal performance to ordinary paints with similar colour is thus possible and is demonstrated for two such coloured layers via controlled heating-cooling studies under both an indoor lamp and outdoor clear sky solar illumination. It is seen that heat gains from the sun and associated cooling loads can be reduced by up to 50% for most colours, which is of some benefit for cars and metal roofs in hot climates.     

Spectral selectivity of black and green painted surfaces

Black, green and mixed paints were prepared from organically modified siloxane resin. The solar absorptance (a_s) of prepared black paint/metal coatings was 0.90 with corresponding thermal emittance (e_T) 0.20, whereas green paint coatings did not reach satisfactory solar absorptance (a_s<0.8O). To improve the absorptance of the green coating, the black paint was admixed into the green paint. Optical properties of the prepared coatings were determined by the help of Kubelka–Munk formalism.     

Characterization of Ti1−xAlxN coatings with selective IR reflectivity

Ti_1−xAl_xN thin films were deposited by reactive magnetron sputtering. The obtained different stoichiometries give rise to different optical properties as the films change from metallic to dielectric. In this work the IR reflectivity of these coatings is investigated taking into account different application fields for IR selective Ti_1−xAl_xN thin films.Low Al content coatings present high reflectivity, high absorptance and low thermal emittance. High Al compositions give raise to coatings with high absorptance and high thermal emittance.The composition of the coatings was evaluated combining electron energy loss spectroscopy (EELS) and energy dispersive spectroscopy. Scanning electron microscopy (SEM) revealed a columnar structure. Reflectance spectra for the visible and infrared spectral ranges were used to obtain the solar absorptance and thermal emittance values, used to calculate the equilibrium temperature of the coatings.The thermal stability in air from 300 to 600 °C was also evaluated.