Solar Absorber Coatings Research Articles

Solar Selective Coatings Based on Nickel Oxide Obtained via Spray Pyrolysis

The paper presents the optimization process for obtaining NiO thin layers on copper substrate for solar absorber coatings, using an inexpensive and up-scalable technique: spray pyrolysis deposition (SPD). Efficient selective coatings must present a high absorption coefficient of the incident solar irradiation, and low emission of heat. The solar selective coatings design involves tailoring the surface properties for superior optical properties. The deposition parameters were varied for maximizing the solar absorbance and minimizing the thermal emittance. The film morphology was controlled using copolymers of the maleic anhydride as additives into the precursors' solution. The structural and surface properties of the films were investigated by X-ray diffraction and atomic force microscopy, respectively. The Cu/CuO(x)/NiO solar absorber shows good values for the solar absorptance (alpha(s) = 0.95) and thermal emittance (epsilon(T) = 0.05) compared with the ones obtained by other methods employed in literature and new additives are recommended in tailoring the surface of solar selective coatings.

Preparation and thermal stability on non-vacuum high temperature solar selective absorbing coatings

Spectrally selective TiAl/TiAlN/TiAlON/TiAlO coating was deposited on stainless steel and copper substrates using a multi-arc ion plating system. The structure, morphology, optical reflectance and electrical resistivity were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), spectrophotometer and four-point probe meter, respectively. The results show that the coating exhibits high absorptance (−0.9) and low emittance (0.09–0.19). The coating remains stable in air up to 650°C for 1h. These properties are of extraordinary interest in solar thermal power generations and energy saving buildings.

Nanostructured black cobalt coatings for solar absorbers

AbstractNanocrystalline black cobalt electrically deposited onto a steel substrate from aqueous solution was investigated. The influence of electrolyte composition and operating parameters on the appearance and optical properties of the coat was studied. The deposition conditions that ensure the highest solar absorptance were optimized. The chemical composition of fabricated thin films before and after annealing at 400 °C was determined by energy dispersive X‐ray analysis (EDS) and XPS technique. The crystal structure analysis showed that the bulk composition of the films was mainly cobalt oxide. The surface analysis reveals that the topmost surface layers of the films are made of different cobalt compounds confirming the multivalence state of Co on the surface with an oxidation state of ≥ + 2. Scanning electron microscope (SEM) observation indicated that the surface morphology was changed from dendritic structure to lamellar at higher current density. The black cobalt film showed soft magnetic characteristicsand excellent optical properties to transform solar energy into thermal energy. Copyright © 2008 John Wiley & Sons, Ltd.

Optical, thermal and structural characteristics of carbon nanoparticles embedded in ZnO and NiO as selective solar absorbers

Selective solar absorber coatings of carbon embedded in ZnO and NiO matrices on aluminium substrates have been fabricated by a sol–gel technique. Spectrophotometry was used to determine the solar absorptance and the thermal emittance of the composite coatings. The surface morphology of the samples was studied by scanning electron microscopy (SEM). Cross-sectional high-resolution transmission electron microscopy (X-HRTEM) was used to study the fine structure of the samples. Chemical composition analysis was done by energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS). The crystal structure of ZnO and NiO samples was also investigated with an X-ray diffraction (XRD) technique. Samples were subjected to an accelerated ageing test for 95 h, with condensation at relative humidity of 95% and at a climate chamber temperature of 45 °C. The thermal emittances of the samples were 6% for the ZnO and 4% for the NiO matrix materials. The solar absorptances were 71% and 84% for ZnO and NiO samples, respectively. The SEM revealed a smooth featureless surface for both C–ZnO and C–NiO samples. Some C–NiO samples showed dentritic features. X-HRTEM, EDS and EELS studies revealed a nanometric grain size for both types of samples. The C–ZnO and C–NiO coatings contained amorphous carbon embedded in nanocrystalline ZnO and NiO matrices, respectively. Selected area electron diffraction (SAED) showed that a small amount of Ni grains of 30 nm diameter also existed in the NiO matrix. The accelerated ageing tests produced performance criterion (PC) values of 0.15 and 0.054 at 95 h for the C–ZnO and C–NiO samples, respectively. Based on these results, C–NiO samples proved to have better solar selectivity behaviour than the C–ZnO counterparts.

Preparation of a TiMEMO nanocomposite by the sol–gel method and its application in coloured thickness insensitive spectrally selective (TISS) coatings

An organic–inorganic nanocomposite was prepared via sol–gel processing from 3-(trimethoxysilyl)propyl methacrylate (MAPTMS) and titanium(IV) isopropoxide (TIP) precursors (TiMEMO) in the form of a viscous resin, and used as a binder for the preparation of coloured thickness insensitive spectrally selective (TISS) paints and corresponding solar absorber coatings. The spectral selectivity of TiMEMO-based TISS paints was optimized by varying the concentrations of binder and different pigments: black, coloured (red, green and blue) and aluminium flakes, the latter imparting low thermal emittance, which was correlated to the presence of titanium in the TiMEMO sol–gel host. The formation and the ensuing structure of the sol–gel TiMEMO hybrid was studied in detail and the nanocomposite structure of the TiMEMO binder formed was assessed from infrared and 29Si NMR measurements, which confirmed the formation of Ti–O–Si linkages established after the hydrolysed precursors condensed into a compliant resinous material. XRD measurements provided additional information about the existence of small coherent domains of silsesquioxane units in the sol–gel host. The abrasion resistance of the non-pigmented TiMEMO binder deposited in thin film form on a PMMA substrate was assessed by the Taber test, and its hardness compared with other resin binders which have been used for making TISS paint coatings. The surface properties of the non-pigmented TiMEMO binder and the ensuing TISS paint coatings were determined from contact angle measurements. The results showed that the water contact angles of non-pigmented TiMEMO binder increased from 70° to 125–135° for the corresponding pigmented TISS paint coatings, inferring the influence of surface roughness on surface energy in the presence of pigments. SEM measurements revealed a striking similarity in the surface morphology of the TISS paint coatings with some other surfaces exhibiting the Lotus effect.

Optical and structural properties of pulsed laser deposited Ti:Al 2O 3 thin films

Spectrally selective solar absorber coatings using metal–dielectric composites offer a high degree of flexibility since their solar selectivity can be optimized by the proper choices of constituents, coating thickness, metal particle concentration, size, shape, and orientation. In this article, Ti:Al 2O 3 composite films of various Ti contents were prepared by the pulsed laser deposition technique, and their structural and optical properties were investigated by scanning electron microscope, X-ray photoelectron spectroscope, and X-ray diffraction methods. The results demonstrate promising solar spectrally selective behavior indicating Ti:Al 2O 3 composite as an excellent choice for solar thermal applications.

Nickel electrolytic colouring of anodic alumina for selective solar absorbing films

The influence both of the waveform of the pigmentation current and of the type of electrolyte on the quality of nickel-pigmented aluminium solar-absorbing coatings was investigated. Alternating current (AC) and rectified AC current with half-wave and full-wave rectification, respectively, were applied. As a pigmentation agent, Watts and citrate electrolytes on the nickel (Ni 2+) basis were used. Thickness, weight, colour shade of the deposited coatings were recorded before and composition and microstructure ones after stripping procedure. The stripping procedure of anodic alumina was carried out in the chrome/phosphoric acid mixture. It was confirmed that the negative half wave of anodising current is responsible for nickel deposition. Morphology study revealed that the positive half wave causes softening/dissolution of the structure deposit. Complexing agent of the nickel-based electrolyte for pigmentation of anodic alumina causes high-degree arrangement of the nickel deposit at presented conditions.

Solar selective absorber functionality of carbon nanoparticles embedded in SiO2, ZnO and NiO matrices

AbstractIt is possible to design an optical material, through control of the substrate and coatings, to absorb a large fraction of the solar spectrum and to emit very little radiation energy in the near and far infra‐red wavelength range. This requires a coating structure that transmits in the ultraviolet and visible, but reflects in the near and far infrared, with a sharp drop in transmission near 2 (m. In this paper, we report on selective solar absorber coatings of carbon nano‐particles embedded in SiO2, ZnO and NiO matrices, fabricated by a sol‐gel technique. The coatings were tested on aluminium substrates. UV‐VIS and FTIR spectroscopies were used to determine the spectral response of the composite coatings, from which the solar absorptance and thermal emittance were determined. We show that the NiO matrix samples have the best solar selective behaviour, and suggest a theoretical evaluation framework for future comparison of such materials. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

Microstructure and spectral selectivity of Mo–Al 2O 3 solar selective absorbing coatings after annealing

Mo, Al 2O 3 single layer, Mo–Al 2O 3 granular cermet layer, Mo/Al 2O 3 tandem and an optimized Mo–Al 2O 3 multilayer coating with a double cermet layer configuration were deposited on stainless steel substrates by magnetron sputtering technique. The samples were annealed in vacuum at different temperatures ranging from 350 °C to 1000 °C for 2–5 h to evaluate their thermal stability. The spectral absorbance and thermal emissivity for the multi-layer selective coatings in the region of 1.3–25 μm were 0.91–0.93 and 0.19–0.27, respectively, depending on heat treatment temperature. Increasing annealing temperature has more obvious influence on the rise of emissivity than the drop of spectral absorbance. The microstructure, surface morphology, composition distribution and diffusion for various films before and after high temperature aging were investigated employing scanning electronic microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. Fe 2O 3, as a diffusion barrier between Mo layer and SS substrate, effectively holds back the presence of Mo 2C and Fe 2MoC complex phases. Al 2O 3 is a stable phase even at 1000 °C for 3 h. However, defects such as widened boundaries, cracks and holes, which could appear in Al 2O 3 layer when heated at higher temperature than 650 °C, will provide the paths of Mo diffusion. Two diffusion modes of Mo, including inner self-congregation of Mo inside the cermet layer and Mo infiltration through the Al 2O 3 layer in Mo/Al 2O 3 tandem at 800 °C for 5 h, were observed after annealing.

Low Cost Selective Solar Absorber Coatings: Characteristics of Carbon-In-Silica Synthesized with Sol-Gel Technique

Carbon-silica composite films have been fabricated using sol-gel synthesis of silica-carbon precursor composites, followed by carbonization in an inert atmosphere. Four categories of samples were studied. These were the tetraethyl orthosilicate-only (TEOS-only), methyl trimethoxysilane (MTES), acetic acid anhydride (Ac2O) and soot (SOOT) samples. The carbon-silica films, spin-coated on aluminium and steel substrates, have been investigated for selective solar absorber functionality. Optical measurements were performed on these samples to determine solar absorptance and thermal emittance of each. The morphology of the films was studied by electron microscopy. Electron energy-loss spectroscopy (EELS) mapping was used to determine the carbon distribution. An optimum performance of 0.88 for solar absorptance and 0.41 for thermal emittance has been achieved. The sol-gel technique produced films with very flat surfaces and uniform thicknesses in the 1 μm range. The fine structure showed homogeneous mixing of the carbon and silica in the TEOS-only samples while the separate additions of MTES and Ac2O resulted in segregation of composition of the silica and carbon at nano-scale. The addition of 20 wt% MTES or 15 wt% Ac2O to the TEOS-only sols also helped to reduce the cracks in the TEOS-only samples. The addition of soot in place of the carbon precursor did not yield a net advantage.

Durability tests of solution-chemically derived spectrally selective absorbers

A promising novel solution-chemical method to fabricate spectrally selective solar absorber coatings has been developed. The objective was to create highly efficient, flexible, inexpensive and durable selective absorbers for solar thermal applications using simple techniques. The selectively absorbing film consists of a composite with nickel nano-particles embedded in a dielectric matrix of alumina. The anti-reflection (AR) material should have the following properties: the proper refractive index, low thermal emittance, dense, flexible and long-term stable. The AR materials tested were silica, alumina and mixtures of silica–titania. The refractive indexes of the above-mentioned materials range from 1.4 (silica) to about 2.1 (50/50 molar ratio silica/titania mixture). Besides increasing the normal solar absorptance, α sol , it is equally important that the AR layer is long-term stable in order to create a successful solar selective coating. The anti-reflection coatings were synthesized using different solution-chemical methods and deposited on the absorbing layer by spin coating. Prepared samples were subjected to an accelerated lifetime test. In the test procedure the temperature of the environment was set to 40 °C and the relative humidity to 95%. Samples made with alumina as anti-reflection coatings failed the aging test. All other materials, silica and silica–titania mixtures proved to be very resilient. Samples that were coated with these anti-reflection materials showed no visible degradation of the sample surface even after 600 h of testing. Single-layer absorbers without an AR layer typically attain a normal solar absorptance of 0.80 and a normal thermal emittance of 0.03. Of the samples made with durable films a 70/30 silica/titania mixture showed the greatest increase of the α sol value, 0.91, while the thermal emittance remained unaltered.

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.

New method to optimize a solar absorber graded film profile

Normalized dc conductivity was used to analyze the inhomogeneously graded depth profile structure of deposits from dc magnetron reactive sputtering in a roll coater. The graded coating was obtained by moving the substrate through the deposition zone with a non-uniform oxidation along the zone mainly due to the asymmetrical position of the oxygen inlet to the right of the target. With this arrangement, the composition of the deposited film was gradually changed from metal to metal oxide as the substrate moved from left to right through the zone. The profile control therefore relied on the non-uniform oxidation along the sputtering zone. The study shows that the normalized dc conductivity of stationary samples in this roll coater offers a simple and effective method to optimize the graded composition in spectrally selective solar absorber coatings. A solar absorptance of 0.91 with thermal emittance of 0.05 at 100 °C was achieved for a single graded film without antireflection treatment.

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

Abstract 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.

Prospects of chemically deposited CoS-CU2S coatings for solar control applications

Good quality thin films of Cu2S were successfully deposited on CoS-precoated glass substrates by the chemical bath deposition process using triethanolamine (TEA) and aqueous ammonia NH3(aq) as complexing agents and Copper II nitrate and thiourea as precursors. The thin films of Cu2S deposited on CoS-precoated glass substrates from chemical baths and annealed at 100oC were found to have desirable solar control characteristics superior to commercial tinted glass and magnetron sputtered multilayer metallic solar control coatings. These include: transmission spectra in the visible region similar to photopic vision spectra, with an integrated transmittance in the range of about 23-28% and an integrated reflectance of about 10-15%, coupled with a range of colours in transmitted and reflected daylight. Moreover, the coating of the glass substrates with a thin layer of CoS prior to Cu2S film deposition was found to improve the mechanical stability of the Cu2S thin films, offering the choice of different shades of colours for the same integrated transmittance in the visible region in solar control applications. The possibility therefore exists for multiple-dip deposition of Cu2S films to enhance the film thickness, enabling the realisation of solar absorber coatings. Key words: CoS – Cu2S Thin Films, Solar Control Coatings, Chemical Deposition (Global Journal of Pure and Applied Sciences: 2003 9(4): 591-598)

Solar Selective Absorber Coating Composed of Aluminum and Nitrogen with High Performance Induced by Surface Roughness

Pure Al film with surface roughness, incompletely nitrided Al film (Al–N film), and transparent AlN film are deposited in turn on a glass substrate by an rf sputtering method. The Al film as the infrared reflector with optimized surface roughness is prepared by a method of two-step deposition of an Al layer at two different substrate temperatures. By coating with an Al–N film, the Al film reduces its diffuse reflectance while maintaining its characteristic low emittance. It is also confirmed that the top layer of the AlN film can perform the role of antireflector coating even if the surface is rough. As a result, an optimized AlN/Al–N/Al film with surface roughness on the glass substrate realizes the high performance of a solar selective absorber, e.g., high solar absorptance of 0.92 and low normal emittance of 0.06 at 373 K.

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, α sol, of 0.83 and a normal thermal emittance, ε 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.

Solar absorber coatings based on CoCuMnOx spinels prepared via the sol–gel process: structural and optical properties

Abstract Titanium-doped and undoped CuCoMnOx spinel films were deposited on Al substrates from sols which were made from the following: Co-acetate, Cu-chloride and Mn-acetate (Ti:CoCuMnOx-I); and Co-acetate, Cu-nitrate and Mn-acetate (CoCuMnOx-II). The precursors’ ratio Co:Cu:Mn was equal to 1:3:3. The solar absorptance (αs) and the thermal emittance (eT) of the films, which were annealed at 450°C for 15 or 30 min, were determined from the corresponding diffuse reflectance spectra in the 0.32–20 μm range. The results show that the CoCuMnOx-II films with SiOx protective over-coatings exhibited values of αs=0.85–0.91 and eT below 0.036 after just a single dipping/annealing cycle. The structure of the films was studied with X-ray diffraction (XRD), infrared (IR) absorbance and near-grazing incidence angle (NGIA) reflection-absorption spectroscopy. Our results suggested that the films have a spinel structure with the composition CoCuMnOx. The stability of the films was tested by soaking them in boiling water for 2 h. NGIA IR spectra of the treated films confirmed the formation of the hydrated mixed oxide (Mn-, Co-, Cu-) phases. To improve the stability of the films two kinds of protective over-coatings were tested: one over-coating was based on polysiloxane resin and the other on high-density silica (T-resin). Films that were resistant to boiling water were obtained by applying the high-density silica protective over-coating, which was cured at 140°C for 30 min.

Deposition of PVD solar absorber coatings for high-efficiency thermal collectors

In this contribution, we present results of a research project for the development of new layered and graded black solar absorbers by magnetron sputtering techniques to be included as selective surfaces in very high-efficiency thermal solar collectors. Solar absorber based in layered and graded cermet coating of Mo–Al 2O 3 was deposited by DC reactive magnetron sputtering. The ceramic and metallic components of the cermets were deposited in a sub-layer system consisting of alternating metallic and oxide sub-layers. In a way to change the metal volume fraction, the thickness of the sub-layers is changed. The effect of metallic fraction and the number of sub-layers in optical properties of the films were studied and good results were obtained. The optimum film selectivity achieved was a solar absorptance of 94% and a thermal emmissivity of 5%. The magnetron sputtering technique has demonstrated that it is a potential method to produce high reproducible spectrally selective coatings with graded structure since it is possible to control in an atomic level the addition of metal phases to the dielectric matrix. These composite coatings are attractive candidates for use in photothermal solar energy conversion.