The development of high performance,low cost solar-selective absorbers

In this paper the first results of a project concerned with the development of low cost transition metal nitride selective absorbers with a high solar absorptance and a low thermal emittance are presented. These absorbers are intended for application at operating temperatures up to 200 °C in high performance solar collectors.We found that reactively d.c.-sputtered chromium nitride films on a copper substrate exhibit a low emittance (about 0.04) and a moderate solar absorptance (about 0.80). The relatively high production cost and capital investment associated with sputtering can be avoided by producing such films by (electro)chemical deposition. Encouraging results have been obtained with the nitridation of electrodeposited chromium films.The drawback of the moderate solar absorptance can be overcome by depositing the nitride films onto electrodeposited rough copper base layers. We found that the addition of small carbon particles (india ink pigment) to the copper deposition bath gives rise to layers with a suitable surface roughness. This surface roughness causes the solar absorptance to increase to 0.90. However, because of the small characteristic dimensions (about 0.5–2 μm) of the roughness, the low thermal emittance is hardly affected.     

Solution‐Processed All‐Ceramic Plasmonic Metamaterials for Efficient Solar–Thermal Conversion over 100–727 °C

Low‐cost and large‐area solar–thermal absorbers with superior spectral selectivity and excellent thermal stability are vital for efficient and large‐scale solar–thermal conversion applications, such as space heating, desalination, ice mitigation, photothermal catalysis, and concentrating solar power. Few state‐of‐the‐art selective absorbers are qualified for both low‐ ( < 200  ° C) and high‐temperature ( > 600  ° C) applications due to insufficient spectral selectivity or thermal stability over a wide temperature range. Here, a high‐performance plasmonic metamaterial selective absorber is developed by facile solution‐based processes via assembling an ultrathin ( ≈ 120 nm) titanium nitride (TiN) nanoparticle film on a TiN mirror. Enabled by the synergetic in‐plane plasmon and out‐of‐plane Fabry–Pérot resonances, the all‐ceramic plasmonic metamaterial simultaneously achieves high, full‐spectrum solar absorption (95%), low mid‐IR emission (3% at 100  ° C), and excellent stability over a temperature range of 100–727  ° C, even outperforming most vacuum‐deposited absorbers at their specific operating temperatures. The competitive performance of the solution‐processed absorber is accompanied by a significant cost reduction compared with vacuum‐deposited absorbers. All these merits render it a cost‐effective, universal solution to offering high efficiency (89–93%) for both low‐ and high‐temperature solar–thermal applications.     

Spectrally selective absorber coating from transition metal complex for efficient photothermal conversion

Copper–manganese oxide (CuMnO _x ) thin films are proposed as efficient and thermally stable selective solar absorbers. The coatings were deposited on aluminum, stainless steel, and glass substrates by dip-coating method from the alcoholic solution of the Cu and Mn nitrate. An organic filmogen was introduced in order to get better adherence with the substrate hence to get uniform films even for larger substrates. The coated films were dried and subsequently heat-treated at 500 °C. X-ray diffraction spectra of the annealed film showed the formation of pure Cu–Mn oxide spinel structure (Cu_1.5Mn_1.5O₄) in the film. FTIR spectra show complete removal of the organic species after thermal treatment at 500 °C. The solar absorptance and thermal emittance were calculated from the hemispherical reflectance spectra in the UV/Vis/NIR and IR range, respectively. The maximum visible absorptance with minimizing the infrared thermal emittance was optimized by controlling the thickness of the films, choosing substrates, and introducing a SiO₂ overlayer.     

Influence of solvents on properties of solar selective coatings obtained by spray pyrolysis

Solar selective coatings for solar thermal flat-plate collectors consisting of crystalline copper oxides and amorphous nickel oxide composites were obtained by robotic spray pyrolyzed deposition. The parameters were optimized for increased spectral selectivity (S): high solar absorptance and low thermal emittance. The coatings were deposited using nickel and copper acetate, dissolved in mixed solvents with various water: ethanol ratios. The coatings’ properties were characterized in terms of crystalline composition (XRD), surface morphology (AFM, contact angle) and optical properties (solar absorptance, thermal emittance and spectral selectivity). Considering the precursor solutions composition (solvent, wetting behaviour), the growth processes were modelled for two different systems: predominant hydrophilic and predominant hydrophobic. The high selectivity values (S > 30) of the optimized composite coatings were explained based on two parallel mechanisms: intrinsic absorption and multiple reflections generated when absorbers with controlled roughness are deposited.     

Fabrication and optimisation of highly efficient cermet-based spectrally selective coatings for high operating temperature

In terms of both high photo-thermal efficiency and high stability, multi-layer structures based on metal-dielectric composites (cermet) can be considered the most attractive selective solar absorbers for receiver tubes operating at medium-high temperatures in the field of solar thermodynamic plants. The double cermet layer approach represents a very simple fabrication method and can give high performances in terms of high solar absorptance and low hemispherical emittance. Optimised solar coatings based on cermet layers were fabricated in our laboratories by sputtering technique following the double layer approach. The joined employment of ellipsometric measurements and optical simulation is proposed as an effective method to optimise and fabricate coatings showing the best performances at a fixed operating temperature of the receiver tube. Interesting results concerning an optimised spectrally selective coating are shown. Solar absorptance higher than 0.94 and hemispherical emittance at 580 °C lower than 0.13 were obtained.     

Hochselektive Absorberschichten für thermische Sonnenkollektoren

High‐selective absorber coatings for solar thermal collectors Highly selective absorber coatings are necessary for the effective operation of state‐of‐the‐art solar thermal collectors. The thin film gradient optical coating with its spectrally selective characteristics achieves high solar absorptance combined with low thermal emittance. Such complex multi‐layer systems are produced in modular vacuum coating processes. Industrial air‐to‐air coating lines allow the continuous coating of metal bands in a pass‐through process and provide absorber coatings which meet highest demands for efficiency, durability and esthetics.     

Solar selective black molybdenum coatings by chemical conversion

Solar selectivity, with emphasis on a high solar absorptance and a low thermal emittance, was measured for black molybdenum films prepared by a chemical conversion process. The best values of the solar absorptance (0.88) and the thermal emittance (0.20) were achieved by optimizing the preparation parameters, e.g. the bath composition, the deposition temperature and the immersion time. The films deposited onto galvanized steel sheet and zinc-plated mild steel sheet show high adherence and good thermal stability at elevated temperatures (250 °C).     

Angular solar absorptance of absorbers used in solar thermal collectors

The optical characterization of solar absorbers for thermal solar collectors is usually performed by measurement of the spectral reflectance at near-normal angle of incidence and calculation of the solar absorptance from the measured reflectance. The solar absorptance is, however, a function of the angle of incidence of the light impinging on the absorber. The total reflectance of two types of commercial solar-selective absorbers, nickel-pigmented anodized aluminum, and sputtered nickel nickel oxide coated aluminum are measured at angles of incidence from 5 to 80 in the wavelength range 300 2500 nm by use of an integrating sphere. From these measurements the angular integrated solar absorptance is determined. Experimental data are compared with theoretical calculations, and it is found that optical thin-film interference effects can explain the significant difference in solar absorptance at higher angles for the two types of absorbers.     

Stability of coatings of low atomic number under irradiation

A detailed study of the structural, thermal and optical properties of selective black nickel coatings on galvanized iron was carried out. The spectral reflectances of the coatings in the visible and infrared regions were measured with an integrating sphere reflectometer. Deposition parameters were optimized to achieve high solar absorptance (α = 0.94) and low emittance (ε = 0.09 at 100°C). Temperature cycling and humidity tests established long-term durability of the coatings for solar thermal conversion applications.     

Absorbeurs selectifs de l'energie solaire elabores a partir d'acier inoxydable : Influence de la composition chimique et de l'etat metallurgique de l'alliage sur les proprietes optiques

Efficient photothermal converters exhibit high solar absorptance and low thermal emittance. An original method has allowed spectrally selective coatings to be obtained by chemical conversion of stainless steel. These coatings exhibit high selectivity (absorptance higher than 95 % and emittance lower than 20 %) and excellent durability.The dependance of the optical properties of these coatings on the chemical composition and metallurgical state of the substrate is studied. Low emittance appeared to be closely related to the corrosion resistance of the metallic surface in the treatment bath. Owing to this fact, convenient conditions can be found to produce selective coating for each alloy.     

Optical properties and solar selectivity of flash-evaporated copper sulphide films

The optical properties of copper sulphide films deposited onto glass and aluminium substrates using the flash evaporation technique were investigated. The films are stable and show good adhesive properties. A study of the optical properties of the films revealed that the films possess solar-selective properties. A film 7000 Å thick deposited onto aluminium showed a solar absorptance α_s of 0.92 with a thermal emittance ε_T (at temperature T = 100°C) of 0.20. The thermal emittance of the films increased linearly from 0.20 to 0.23 as the temperature was increased from 100 to 250°C. The films do not suffer from any UV degradation and can withstand temperatures as high as 150 °C for more than 250 h. Films coated on glass also show low values (about 0.20) of thermal emittance, thereby indicating the presence of intrinsic selective properties. The films on glass also possess excellent conducting properties and a film 2500 Å thick on glass has p-type conductivity with a sheet resistance of 25 Ω/□. X-ray and transmission electron microscopy studies revealed that the films were amorphous in nature. Auger electron spectroscopy studies carried out on the copper sulphide films indicated a slight excess of copper.     

Optical and topographical properties of selective black chrome

The optical and surface topographical properties of black chrome deposited onto differently treated substrates were investigated to study the effect of substrate surface roughness. Coatings with a solar absorptance α_s of 0.94 and a thermal emittance ε of 0.14 at 100 °C were obtained on steel substrates with dull nickel overlayers and anodically etched nickel overlayers. A polarimetric technique was employed to study the angular dependence of the reflectance of the coatings.     

Sputtering deposition and characterization of tandem absorbers for photo-thermal systems operating at mid temperature

In this paper the first results of a work concerned with the development of TiAlON selective solar absorbers with high solar absorptance and low thermal emittance are presented. These absorbers are thought for solar collectors application at operational temperature of about 300 °C. Solar absorber tandems, consisting of an absorbing TiAlON compound sputtered on a TiC buffer layer deposited on a metal (Cu) substrate, have been produced in two ways: by changing the nitrogen flux percentage in (Ar + N₂ + O₂) mixture at fixed thickness (300 nm) and by varying the absorber thickness at fixed nitrogen percentage (5%) in the sputtering gas. The optical response in the UV-IR range is discussed. The tuning of the nitrogen flux allows to change the metal elements ratio (Ti/Al) in the oxynitride layer, varying the reflectance intensity both in the IR and visible range. The thickness variation permits to shift the cut-off wavelength of the transition from short wavelength high absorptance to long wavelength low emittance. The tuning of these two parameters (nitrogen flux and absorber thickness) enables to control the optical response of the samples to make them suitable for photo-thermal device application.     

等离子喷涂法制备黑铬太阳能选择性吸收涂层

采用等离子喷涂法制备了黑铬太阳能选择性吸收涂层,采用XRD、SEM等测试方法对涂层的物相、微观结构、太阳能吸收性能进行了表征。对涂层进行了打磨,并在涂层表面制备SnO2选择性透过薄膜。研究表明,采用等离子喷涂方法制备的黑铬涂层吸收率为0.93,发射率为0.88,经打磨处理后,发射率降至0.76。添加SnO2薄膜后,涂层吸收率变化小,发射率降至0.50。热震实验表明该涂层具备良好的抗热震性能。     

Optimization and microstructural analysis of cobalt-sulfide-pigmented aluminum oxide

Porous anodic films formed on aluminum in a phosphoric acid electrolyte were used to develop selective solar coatings by double precipitation of cobalt acetate and ammonium sulfide indide the pores. The black coatings containing cobalt sulfide were then sealed in hot water and their optical, mechanical and thermal degradation properties were tested. Multiple linear regression routines were used to optimize the absorptance and emittance values and the coating formation conditions were accordingly adjusted. After optimization, the coatings were characterized by scanning and transmission electron microscopy, and thermal stability tests were performed to determine the long-term quality of the coatings. The coatings were found to have good thermal stability up to 150°C and high adhesion strength as well as absorptances of up to 0.965 at emittances ranging from 0.15 to 0.25.     

Quasicrystal films: numerical optimization as a solar selective absorber

Selective absorbers for solar thermal energy applications have to show high solar absorptance _s and low thermal emittance ε_h. Stability against oxidation and diffusion is indispensable, especially at high absorber temperatures. The new class of quasicrystalline materials seems to have favourable properties regarding stability. With a genetic algorithm a thin film stack based on dielectric and quasicrystal films was optimized as a selective absorber. A sandwich system dielectric/quasicrystal/dielectric on copper has highly selective properties: _s = 0.86 and ε_h (400 °C) = 0.051. Even better results can be achieved, at least in theory, by the use of a cermet. The optical constants of cermets with the quasicrystalline material as the metal were calculated with the Bruggeman theory. A system of a cermet film and an additional antireflective coating on copper shows _s = 0.92 and ε_h (400 °C) = 0.048.     

Assembling Mesoscale‐Structured Organic Interfaces in Perovskite Photovoltaics

Mesoscale‐structured materials offer broad opportunities in extremely diverse applications owing to their high surface areas, tunable surface energy, and large pore volume. These benefits may improve the performance of materials in terms of carrier density, charge transport, and stability. Although metal oxides–based mesoscale‐structured materials, such as TiO₂, predominantly hold the record efficiency in perovskite solar cells, high temperatures (above 400 °C) and limited materials choices still challenge the community. A novel route to fabricate organic‐based mesoscale‐structured interfaces (OMI) for perovskite solar cells using a low‐temperature and green solvent–based process is presented here. The efficient infiltration of organic porous structures based on crystalline nanoparticles allows engineering efficient “n‐i‐p” and “p‐i‐n” perovskite solar cells with enhanced thermal stability, good performance, and excellent lateral homogeneity. The results show that this method is universal for multiple organic electronic materials, which opens the door to transform a wide variety of organic‐based semiconductors into scalable n‐ or p‐type porous interfaces for diverse advanced applications.     

Textured black brass spectrally selective coating

A textured black brass selective coating has been reported in this paper. These textured black brass selective coatings have been obtained by using chemical conversion process. They exhibit initial solar absorptance ()0.96 and thermal emittance ₁₀₀0.17 and are reproducible. These have been annealed up to 250°C in air and at 150°C in vacuum (10⁻³ Torr) possessing solar absorptance ()0.90. TEM, XRD and FTIR analyses have also been reported which suggests that the coating consists of multi-phases.     

Structure,optical properties and thermal stability of Al2O3-WC nanocomposite ceramic spectrally selective solar absorbers

Traditional metal-dielectric composite coating has found important application in spectrally selective solar absorbers. However, fine metal particles can easily diffuse, congregate, or be oxidized at high temperature, which causes deterioration in the optical properties. In this work, we report a new spectrally selective solar absorber coating, composed of low Al₂O₃ ceramic volume fraction (Al₂O₃(L)-WC) layer, high Al₂O₃ ceramic volume fraction (Al₂O₃(H)-WC layer) and Al₂O₃ antireflection layer. The features of our work are: 1) compared with the metal-dielectric composites concept, Al₂O₃-WC nanocomposite ceramic successfully achieves the all-ceramic concept, which exhibits a high solar absorptance of 0.94 and a low thermal emittance of 0.08, 2) Al₂O₃ and WC act as filler material and host material, respectively, which are different from traditional concept, 3) Al₂O₃-WC nanocomposite ceramic solar absorber coating exhibits good thermal stability at 600 °C. In addition, the solar absorber coating is successfully modelled by a commercial optical simulation programme, the result of which agrees with the experimental results.     

Spray-pyrolysed cobalt black as a high temperature selective absorber

Cobalt oxide coatings with an integrated solar absorptance of 0.93 and a hemispherical emittance at 100°C of 0.14 were prepared by spray pyrolysis on stainless steel substrates kept at 300°C. The coatings are strongly adherent and stable up to temperatures of about 600°C. Auger electron spectroscopy, X-ray photoelectron spectroscopy and X-ray and electron diffraction investigations revealed the coatings to be composed of an upper layer of Co₃O₄ and subsequent layers of CoO down to the substrate. The absorption mechanism is primarily intrinsic. Lower substrate temperatures (about 150°C) can be used for preparing coatings from equimolar aqueous solutions of cobaltous acetate and thiourea. Such coatings, containing both cobalt oxide and cobalt sulphide, have a comparable absorptance but a higher emittance and are stable only up to about 300°C.