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.     

Optical Characterization Of Industrially Sputtered Nickel–Nickel Oxide Solar Selective Surface

Tandem absorbers are often used in the design of solar absorbers for photo thermal conversion. They consist of a thin coating, selectively absorbing in the wavelength range of the solar spectrum, on a metal substrate. The optical performance of a tandem absorber depends on the optical constants and thickness of the absorbing coating and also on the reflectivity of the underlying metal. A very high solar absorptance is achieved when the coating has a non-uniform composition in the sense that the refractive index is highest closest to the metal substrate and then gradually decreases towards the front surface. This type of composition suppresses coating interference and gives a low front surface reflection if the refractive index at the front surface is low. We report on optical analysis of a solar absorber with a graded index coating of sputtered nickel–nickel oxide deposited on aluminium. The optical constants have been determined from reflectance, transmittance and ellipsometry data by fitting the data to a two-layer model of the coating. The optical constants of the two layers can be regarded as effective optical constants for the lower and upper part of the graded index coating respectively. It is found that the effective refractive index of the top layer is somewhat lower than for the base layer. The extinction coefficient is higher in the lower part of the coating. Both effective refractive index and extinction coefficient of the base layer increase monotonically with increasing wavelength as for metallic materials.     

Advanced procedure for the assessment of the lifetime of solar absorber coatings

New solar absorber coatings are developed and used in advanced collector designs with improved efficiency. The operation temperature and stagnation temperature as the main durability load for the absorbers were increasing during the past due to these innovations. Especially the highly selective new coatings have to suffer by these stronger loads. The service life estimation procedures developed in the framework of research activities of the IEA Solar Heating and Cooling Programme (Task 10 and Working Group Materials in Solar–Thermal Collectors) were based on load profiles for less-advanced absorbers and collectors and did not take into account the impact of the optical properties of the absorber coatings on the stagnation temperature of the collectors, which is the main reason for temperature degradation. A new procedure was developed, which allows testing depending on the optical properties (Solar absorptance and thermal emittance) of the absorbers.     

Optimization design of Ti0.5Al0.5N/Ti0.25Al0.75N/AlN coating used for solar selective applications

A new solar selective coating was optimization designed in our paper. Ti_0.5Al_0.5N and Ti_0.25Al_0.75N coatings were chosen as absorber layers and AlN coating was chosen as anti-reflection layer. The phase structure and optical constants of these coatings were studied by X-ray diffraction and spectroscopic ellipsometry, respectively. The Ti_0.5Al_0.5N coating has a cubic NaCl-type structure and exhibits metallic character. However, the Ti_0.25Al_0.75N coating consists of cubic NaCl-type structure and hexagonal AlN structure and exhibits semiconducting property. AlN coating is expected as a dielectric layer. The solar selective coating was optimized using a computer program named TFCalc and verified by experimental results. The absorptance and low emittance of the optimized solar selective coating were 0.945 and 0.04 (82 °C), respectively.     

The real utility ranges of the solar selective coatings

The efficiency of the solar selective coatings (η_sel) with various combinations of the optical properties (solar absorptance and thermal emittance), and their impact on the performance of solar thermal systems of different concentration ratios (CRs) have been analyzed. The stagnation temperatures of the selective coatings have been measured using stagnation temperature measurement (STM) chamber. From the results of the simulation study, it is recommended that selective solar absorber coatings should be used only in systems with CR=1 (solar flat plate collectors) and its use in systems with high CR values (parabolic collector) is an additional expenditure with reduced efficiency. The dependence of the performance of the system with CR=1 on the low emittance of the absorber coating has also been experimentally confirmed by the stagnation temperatures recorded for different selective coatings measured in an STM chamber, a system with unit CR. For systems with CR=1, a simple and new parameter, specific area ratio (SAR) has been proposed which further pinpoints the exact solar system wherein the selective coatings are to be used.     

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.     

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.     

The Electrolytic-Coloured,Anodized Aluminium Selective Absorber Coating and its Optical Properties

This paper describes the technology and optical properties of the selective absorber coating prepared on industrial pure aluminium or aluminium alloys by the anodizing and colouring method. The optical properties of the coating are solar absorptance α_s = 0.94 (AM2) and normal thermal emittance ε = 0.10; the coating was stable during the ageing test. Scanning electron micrographs of surface morphology and fracture section as well as Auger depth profiling data of the coating are given. Based on this information, a coating structure model of Ni particle pigmented A1₂O₃ film is proposed in which the Ni concentration has a graded distribution. Bruggeman effective medium theory and multilayer matrix technique are used to calculate the spectral reflectance of the coating in the wavelength range 0.3 to 20 μm. The result shows that the theoretical value is approximately in agreement with the experimental data. The effect of morphology on the optical properties of coatings is also discussed.     

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.     

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 constants of sputtered Ni/NiO solar absorber film—depth-profiled characterization

An optical analysis of a solar absorber with a graded index coating of nickel–nickel oxide deposited in a continuous process for reactive DC magnetron sputtering is reported. The optical constants were determined by a set of 14 samples deposited under the same sputtering conditions as in the continuous process representing the composition at 14 different depths in the continuously graded index coating solar absorber. The optical constants of the 14 layers were derived from optical measurements of reflectance and transmittance in the wavelength range 300–2500 nm. The optical constants were used to construct a model for the graded refractive index in the solar-absorbing coating. It was found that the graded index was not linear in depth profile and hence could be modelled with four layers: two metallic layers in the base closest to the substrate, a dielectric layer at the top of the coating and one intermediate layer representing the change from metallic to dielectric properties in the middle part of the coating.     

Computations of the optical properties of metal/insulator-composites for solar selective absorbers

Solar selective absorbers are very useful for photo thermal energy conversion. The absorbers normally consist of thin films (mostly composite), sandwiched between the antireflection layer and (base layer on) a metallic substrate, selectively absorbing in the solar spectrum and reflecting in the thermal spectrum. The optical performance of the absorbers depends on the thin film design, thickness, surface roughness and optical constants of the constituents. The reflectivity of the underlying metal and porosity of the antireflection coating plays important roles in the selectivity behavior of the coatings. Computer simulations, applying effective medium theories, have been used to investigate the simplest possible design for composite solar selective coatings. A very high solar absorption is achieved when the coating has a non-uniform composition in the sense that the refractive index is highest closest to the metal substrate and then gradually decreases towards the air interface. The destructive interference created in the visible spectrum has increased the solar absorption to 98%. This paper also addresses the optical performance of several metals/dielectric composites like Sm, Ru, Tm, Ti, Re, W, V, Tb, Er in alumina or quartz on the basis of their refractive indices. The antireflection coating porosity and surface roughness has been analyzed to achieve maximum solar absorption without increasing the thermal emittance. Antireflection layer porosity is a function of dielectric refractive index and has nominal effect on the performance of the coating. While, up to the roughness of 1×10⁻⁷ m RMS, the solar absorption increases and for higher roughness, the thermal emittance increases only.     

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

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.     

Stable selective coating “black nickel” for solar collector surfaces

Electrodeposited black nickel coatings plated from chloride baths on metallic substrates have high solar absorptance coefficients (> 0,92) and low thermal emittances (< 0,15). The black nickel selective coating consisted of two layers of different porosity with non-overlapping pores. This coating is very stable under thermal implication and humid conditions.     

Optimisation of metal sputtered and electroplated substrates for solar selective coatings

This study describes the performance of selective coatings to have maximum solar absorptance and minimum thermal emittance, in relation to substrate preparation. Aluminium and copper substrates, covered with sputtered or electroplated metal base layer, have been used to see the influence of different types of substrates for solar selective coatings. The effect of the base layer material, thickness, deposition process and deposition condition, on the optical performance of selective coatings has been analysed. Nickel was electroplated and nickel and vanadium were sputtered as a base layer on the Al and Cu substrates. A comparison of plated and sputtered nickel substrates for Ni:SiO₂ and V:Al₂O₃ composite solar selective coatings is presented. Theoretical results using computer simulation for solar selective composites on various substrates, and the effect of the base layer thickness on these substrates are compared with experimental results. The effects of the base layer thickness for cobalt and tungsten are also included. The sputtered base layers selective coatings produced higher absorptance along with higher emittance and electroplated base layer coatings resulted in comparatively lower absorptance and lower emittance. Hundred nanometre metal sputtered base layer is optimised for solar selective coatings.     

Optimization of an industrial DC magnetron sputtering process for graded composition solar thermal absorbing layer

Optimization of an industrial DC magnetron sputtering process for thin graded index coatings for solar thermal absorbers is reported. The optimization concerned the main processing parameters: sputtering power, argon flow, oxygen flow, and system set-up for graded control. The purpose of the optimization was to achieve a surface with efficient solar-thermal energy conversion based on the concept graded index coating, using a metal-dielectric composite coating of nickel–nickel oxide with a continuous change in composition through the film depth profile. It was found that the optimization of the materials composition could be controlled by one parameter related to the sputtering process, the relative oxygen flow RO, defined as the ratio of applied oxygen flow to the critical oxygen flow. For optimized sputtering conditions a solar absorptance of 0.92 was obtained for a single graded index coating on aluminum for RO value of 0.8. From the materials characterization it was found that this gave a graded index coating of two thick sub-layers, a top layer of nano-sized nickel oxide grains and a base layer of nano-size metallic nickel grains with a very thin interface of a mixture of nickel and nickel oxide that was almost amorphous.     

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.     

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.     

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.     

High performance selective surfaces from stainless steel for the photothermal conversion of solar energy

Chemical and electrochemical working processes for the preparation of selective absorbers on stainless steels are described. These methods have allowed conversion coatings to be obtained which exhibit high solar absorptance (_s 0.95) and low thermal emittance (₂₀ 0.20) and excellent durability. The relationships between the chemical treatment conditions and the optical properties of the coatings are discussed. The surface analysis (SIMS, ESCA) shows that the coating is heterogeneous and presents, with depth, varying proportions of metallic elements in various oxidation states (oxides, sulphides and metals).