Spectral reflectance properties of black chrome for use as a solar selective coating

The NASA-Lewis Research Center has determined that a widely available, commercially electroplated decorative finish known as black chrome has desirable solar selective properties. Black chrome electroplated coating has high absorbence in the solar spectrum and low emissivity in the 250°F blackbody thermal spectrum.The discovery of the solar selective properties of black chrome adds another high-efficiency coating to the older, previously known coatings. Additionally, the black chrome is significant as a solar selective coating because the current extensive use of black chrome in the electroplating industry as a durable, decorative finish makes black chrome widely available and potentially lower cost as a solar selective coating.The spectral reflectance properties of a commercially prepared black chrome on steel have been measured. Values are presented for reflectance of the black chrome. These are compared with the reflectance of black paint (Nextel) and with two available samples of black nickel which had been prepared for solar selective properties.The reflectance of black chrome, the two black nickels, and black paint integrated over the solar spectrum for air mass 2 were 0·132, 0·123, 0·133 and 0·033, respectively.The reflectance of the black chrome, two black nickels, and black paint integrated over the blackbody spectrum for 250°F from 3 to 15 μm are 0·912, 0·934, 0·891 and 0·033, respectively. These reflectance measurements indicate absorptivity-to-emissivity ( ) values of 9·8, 13·8, 8·0 and 1·00, respectively.     

太阳能选择性吸收涂层

综述了国内外选择性涂层的研究进展与现状,介绍了涂层的制备方法和分类,分析了太阳能选择性吸收涂层的工作机理,对太阳能选择性吸收涂层的发展趋势做了展望。     

Technical and economical comparison between solar water heaters using electrodeposited chrome selective coating and selective paints

In the present paper, the advantages of using electrodeposited chrome selective surfaces, or other selective paints that are widely used today in the solar water heater industry, are examined. The performances of the solar water heaters are evaluated by the f-chart method (Beckman et al., Solar Heating Design by the f-chart Method (1977), Ref. [1]). The cost of each type of solar heater is calculated, and the optimal coating is determined by the Chang method. The results have shown that the use of chrome selective coatings is economically sound only for the northern regions of Greece. Consequently, the viability of a selective surface industry will depend mainly on it's exports to northern Europe countries.     

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

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

Experimental analysis and evaluation of a vacuum enclosed concentrated solar thermoelectric generator coupled with a spectrally selective absorber coating

Significant research in the past decade has been focused on quantitatively and qualitatively validating potential of solar thermoelectric modules to harness electricity. In the present study, we have experimentally analysed steady-state temperature variation of a spectrally selective solar absorber coating (α?=?0.954, ε?=?0.13) with variation in solar irradiation flux (concentration ratios?=?39, 50 and 65) using Fresnel lens and vacuum enclosure pressure (200?mbar to 900?mbar in steps of 100?mbar). It is observed that the experimental results so obtained go hand in hand with a COMSOL simulation model of the set-up. Further, we have carried out performance analysis of a solar thermoelectric generator (STEG) set-up enclosed in vacuum conditions equipped with Fresnel lens and absorber set-up coupled to Bi₂Te₃ thermoelectric module array electrically connected in series. The results depict a maximum power output of 0.91?W and a peak efficiency of 2.21% at a hot-side temperature of 642?K.     

选择性吸收黑铬涂层及其应用

从世界范围来看，主流的太阳热水器总体上可分为真空管式和平板式两大类，因为客观上它们各有其优劣势，所以二者将在消费市场上共存将是不争的事实。真空管式太阳热水器在我国家用市场有优势；而平板式太阳集热器以其优异的性价比和承压好被选为系统工程和。太阳能与建筑相结合。方案的理想产品，目前它在国外太阳能市场占有很大优势。     

Optimization of a solution-chemically derived solar absorbing spectrally selective surface

A previously studied solution chemically derived two layer absorber has been improved by adding a third layer. Simulations done with a thin film program showed that the optimized three layer selective absorber should be composed of an 80% nickel–20% alumina film of 100 nm at the base, a 40% nickel–60% alumina film of 60 nm in the middle and finally a 100% silica film of 85 nm at the top. Experimental results confirmed the computer simulated three layer structure. Prepared absorber consisting of the theoretically found optimized layer structure achieved a solar absorptance, α_sol, of 0.97 and a thermal emittance, ε_therm, of 0.05.     

Low-cost black Cu–Ni alloy coatings for solar selective applications

The development of the Cu–Ni alloy coating as a selective surface for solar energy use is reported. The coatings were deposited by electrodeposition using Hull cell. Effects of electrolyte concentration and operating parameters on the appearance and optical properties of the coating were studied. Deposition parameters were optimized to achieve high solar absorptance alpha α=0.94 and low emittance epsilon ε=0.08. The elemental composition and morphology were evaluated by SEM and energy dispersive X-ray diffraction analysis (EDAX). The coating may be promising for solar collector panels used in domestic water heaters.     

Corrosion protection of Sunselect, a spectrally selective solar absorber coating, by (3-mercaptopropyl)trimethoxysilane

The formation of a protective layer from (3-mercaptopropyl)trimethoxysilane (MPTMS) on commercial Sunselect, cermet-based spectrally selective coating (Alanod, DE), was studied by non-electrochemical (infrared reflection–absorption—IR RA, X-ray photoelectron spectroscopy—XPS), electrochemical (cyclic voltammetry (CV) in the presence of a redox probe (Cd⁺²), and potentiodynamic (PD)) techniques. By simple immersion and subsequent dip coating of the Sunselect substrate in the MPTMS sol, the hydrolyzed MPTMS precursor was adsorbed on the substrate, forming a protective layer imparting corrosion stability to Sunselect in a salt spray chamber for at least 20 days, outperforming any other sol–gel coating used so far for the corrosion protection of Sunselect. This was attributed to the penetration of MPTMS into the porous cermet structure through the upper antireflective Sn-oxide layer, as shown from XPS depth profile. Detailed analysis of the hydrolysis and condensation reactions of the MPTMS precursor by ²⁹Si NMR and IR spectroscopic techniques is reported.The most important finding was the observation that the applied MPTMS layer did not affect the spectral selectivity, as inferred from the solar absorptance increase of 1% and thermal emittance increase of not more than 2%.     

Testing of a new solar coating for solar water heating applications

A novel and affordable solar selective coating exhibiting higher solar absorption efficiency compared to the commercial black paint coating used in most ordinary solar water heating systems (SWHSs) has been developed. The coating is fabricated by embedding a metallic particle composed of a nickel-aluminium (NiAl) alloy into the black paint. The optical behaviour of several percentages of the NiAl alloy in the coating is studied using UV-Vis and IR spectroscopies. The chemical composition of the coating was characterized using XRD and thermo-gravimetric analysis (TGA) for both the black and alloy-containing paint. The results allowed deducing that the optimum composition to consider for further testing was 6% NiAl alloy by mass. The applicability of the coating in a real thermosyphonic SWHS was evaluated throughout the year, spanning both hot and cold seasons. It is found that the new coating shows better performance compared to the untreated black paint by an average of 5 °C over a period of 1 year. The corrosion resistance of the coating was investigated using electrochemical polarization and weight-loss measurements in the corrosive medium of 3% NaCl in 0.50 M HCl. Higher inhibition efficiency of corrosion was found for the alloy-containing paint compared to the untreated paint by more than 12%. Finally, Scanning Electron Microscopy (SEM) was used to explore the morphology of the modified coating surface, and compared to the untreated surface.     

The optical characteristics of black chrome solar selective films coated by the pulse current electrolysis method

The application of the pulse current method for black chrome electrocrystallization has been investigated. Plating parameters to optimize the optical properties of the solar selective film included the bath composition, current density, plating time, duty cycle and substrate. The bath composition was 250–300 g/ℓ of chromic acid, 10–15 g/ℓ of propionic acid and 0.5 g/ℓ of a proprietary additive. It has been observed that the black chrome coatings exhibited reasonable optical properties for commercialization when the plating parameters were properly controlled; absorptance (α) was 0.944 and 0.94, and emittance () was 0.084 and 0.15 for nickel and cooper substrates, respectively. Thermal stability of the black chrome coatings was also studied by aging at 300°C and 450°C in air for 24 h. This study implies that the pulse current electrolysis method could enhance the optical properties of black chrome solar selective coatings for practical solar applications.     

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.     

中高温太阳能选择性吸收涂层

从太阳能选择性吸收薄膜的原理出发,对膜系结构、材料选择、表面形貌等方面进行了论述,提出了制约当前太阳能选择吸收涂层发展的因素,并对今后的发展进行了展望。     

Silicon oxynitride multilayers as spectrally selective material for passive radiative cooling applications

Silicon oxynitride has optical properties that makes it attractive as radiative cooling material. Because of the interface contributions, multilayered configurations provide a very promising way to enhance the device performances. Three silicon oxynitride multilayers deposited onto aluminum-coated glass substrate were prepared and tested. A broadening of the absorption peak within the atmospheric window was observed. Radiative cooling parameters _S2^H and ν^H were deduced from infrared reflection measurements. They correspond to temperature drops of 52, 48 and 56°C (from ambient temperature of 27°C), and cooling powers of 125, 118 and 119 Wm⁻². Temperature drops and cooling power remain competitive for practical uses, even after non radiative exchange processes are taken into account.     

Antireflection of glazings for solar energy applications

Several antireflection treatments such as Teflon dipping, dipping in silica saturated fluosilic acid, have previously been tested. The films made with the fluosilicic acid showed an increased solar transmittance of up to 5%. Outdoor testing for more than 7 years indicated an excellent long-term stability. The problem with this process is the involved chemicals, which are too harmful to permit a large-scale production in Sweden. Therefore, a dip-coating process has been investigated, which uses silica sols. Two sols were investigated, one polydisperse and one monodisperse. The highest attained solar transmittance was achieved with the monodisperse sol. It is believed that the increase in the solar transmittance is connected to the size of the silica particles, the larger the smallest particles the higher the solar transmittance. A decrease by up to 5.2% in the solar reflectance has been noted. A problem is the mechanical properties and especially the adhesion tends to be low. It is, however, possible to improve these properties by baking the film at approximately 550°C during 30 min. The solar transmittance decreases when baking the film since it collapses partially, but the mechanical properties seemed to improve. During baking the temperature is close to that of commercial tempering. It should therefore be possible to bake and temper at the same time. The deposited film does not introduce any absorption. It is concluded that this process should be suitable for large-scale production, since the involved chemicals are harmless and the process is fairly rapid. Films were made at withdrawal rates of up to 1 cm/s.     

High temperature; stable,spectrally selective solar absorbers for thermochemical hydrogen production

The thermochemical reduction of water by reactions similar to the Mark I process requires a processing temperature of 730°C. The efficient utilization of solar photothermal energy conversion, in distributed collector systems, to attain this temperature will require the use of suitable spectrally selective surfaces which are stable at the operating temperature.A coating system with demonstrated high temperature capability has been developed at the Optical Sciences Center under NSF/RANN sponsorship. A silicon thin film absorber is deposited by chemical vapor deposition (CVD) on a silver thin film reflector. This optical stack is fabricated at temperatures in excess of 800°C, and the CVD technology is amenable to large scale production in a flow-through system.At 500°C the present Si-Ag system has typical solar absorptance and total normal emittance values of 0.75 and 0.06, respectively. Samples were fabricated which maintained their high spectral selectivity after 2000 thermal cycles between 150°C and 450°C, and after 100 h at 600°C. Further process studies now underway indicate that the solar absorptance can be improved to better than 0.85 by employing a Si-Ge multilayer absorber and that the operating range of the stacks can potentially be raised to the 800–900°C range by employing a refractory metal thin film reflector.     

Maximally concentrating collectors for solar energy applications

The performance characteristics of a solar energy collector can be described by a function relating the concentration to the direction of incident radiation. A strict bound on the integral of this function is established. This is used to define a realistic class of collectors with maximal concentration which can serve as a basis for design and optimality studies.     

Scratch-resistant zeolite anti-reflective coating on glass for solar applications

Sol-gel SiO₂ anti-reflection (AR) coating on solar glass is known to increase the current output by a few percent, but its mechanical durability is of concern. To improve its strength, the amorphous SiO₂ may be replaced by zeolite, which is a microporous aluminosilicate crystalline material. Scratch-resistant AR coating has been prepared by the dip coating of a composition which contains aggregated zeolite nanoparticles as the structure provider and zeolite precursors as the reactive binder. AR coating with better than 5H pencil hardness can be made with a composition containing BEA zeolite. Nanoindenter measurements show that this BEA zeolite films have 1.5 GPa indent hardness and 35 GPa elastic modulus. Furthermore, the freshly prepared AR coating shows a self-cleaning effect with a water contact angle (WCA) close to zero. It can be made super-hydrophobic with larger than 130° WCA after modifying with hexamethyl-disilazane (HMDS), upon which the pencil hardness further increases to 6H.