Measurement of spectra of normal incidence absorptance of surfaces in life space

The thermal and radiative environment of human life space is characterized strongly by solar radiation and by the radiation from surrounding walls as well as by air temperature and humidity. To consider the environmental radiation quantitatively, absorptance spectra and total emittance of the surfaces should be measured properly. In this work we measure spectra of normal incidence absorptance of such surfaces in a near‐ultraviolet through infrared region of wavelength of 0.30–11 μm. On the basis of measured spectra we evaluate solar absorptance of the surfaces to respond to the needs of radiation characteristics of the surfaces in the fields of architecture, life science, and solar engineering. Objective surfaces of the measurement are surfaces of indoor and outdoor environments: cloths, indoor wall materials, painted surfaces, ceramics, bricks, outdoor wall materials, road materials, leaves, etc. We also observe the transition of the absorptance spectra of a water–wet cement surface in a drying process. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20356     

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.     

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.     

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.     

Thermal radiative properties of a smooth air-water interface

Thermal radiative properties for an air-water interface were calculated by considering the interface to be smooth and the water to be at uniform temperature. Monochromatic hemispherical and total normal reflectance, monochromatic and total normal transmittance, Planck mean and Rosseland mean absorption coefficients were evaluated. The wavelength interval covered by the calculations was between 0.2 and 200 μ. Other thermal radiative properties, such as emittance and absorptance, were related to the above properties, and a comparison with available results shows favorable agreement. The use of these properties in the area of remote sensing and global energy balance makes the data essential for identifying the most effective region of the spectrum and for explaining some of the naturally occurring processes where radiative transfer plays an important role.     

Calorimetric measurement of absorptance and emittance of the Sydney University evacuated collector

Simple calorimetric techniques have been developed for determining the absorptance and emittance of individual evacuated tubular collectors incorporating a selective surface, and the efficiency, η_o, of evacuated collectors in various mirror systems. The absorptance and efficiency measurements are made in natural sunlight without the use of a solarimeter by establishing an absorptance standard based on Nextel black paint. Calibration of solarimeters using the established absorptance standard is discussed. Emittance measurements are made by measuring radiative heat losses from the absorber tube of a collector.Absorptance and emittance measurements for a number of Sydney University evacuated collectors gave values of absorptance α = (0.92 ± 0.01) and emittance = 0.05 at 120°C for the selective surface utilized. Efficiency (η_o) measurements for Sydney University collectors in two simple mirror systems are also reported.     

High-temperature optical properties and stability of AlxOy–AlNx–Al solar selective absorbing surface prepared by DC magnetron reactive sputtering

Al_xO_y–AlN_x–Al selective absorbing surface was prepared by DC magnetron reactive sputtering with aluminum alloy (LY13)¹ in air and argon. The studies were carried out to access the high-temperature (400°C–600°C) optical properties and stability of the coatings. The coatings were found to withstand heating at 600°C for 30 min in 4.5×10⁻³ Pa vacuum with absorptance 0.94 and emittance 0.07 after annealing. After heating at 450°C for 10 h, the specimen still had good performance whose absorptance and emittance was 0.93 and 0.07, respectively. Auger electron spectroscopy was used to analyse the structure of the solar selective surface before and after annealing.     

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.     

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.     

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.     

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.     

Infrared radiative heat transfer in nongray nonisothermal gases

In the present paper, both nongray and nonisothermal behaviors of an infrared emittingabsorbing gas have been taken into account in radiative transfer analyses through the use of the nonisothermal band absorptance. Consideration is given specifically to a simple system consisting of a radiating medium bounded by two infinite parallel black surfaces of different temperatures. Solutions are presented for the cases of radiative equilibrium and combined conduction and radiation. Results based on different methods of evaluating the nonisothermal band absorptance are also compared among themselves. Differences in several fundamental features are exhibited in the nongray nonisothermal solutions as compared to those with nongray but isothermal properties.     

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 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.     

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.     

Solar absorptance of black cobalt and black cobalt–silver films and its relation with roughness coefficient

Cobalt oxide and cobalt oxide–silver selective surfaces were prepared in a galvanostatic mode by means an electrodeposition bath, using different deposition times, on stainless steel substrates. Later, the roughness factor of the selective surfaces, were determined in order to establish one relation with the electrodeposition time and the solar absorptance of the black cobalt and black cobalt–silver coatings.     

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.     

The effect of substrate surface preparation on the optical properties of a black chrome solar absorber coating

In this study, black chrome (ChromeOnyx) was electrodeposited onto mechanically roughened, mechanically polished and electrochemically polished copper plates in order to investigate the influence of substrate surface preparation on the optical properties of the coating. The spectral reflectances of the uncoated and coated surfaces were measured. In addition, the total normal emittance of the coated surfaces and the directional solar absorptance of the coated roughened surfaces are reported. The spectral reflectance of black chrome in the visible spectrum was found to be unaffected by the substrate surface preparations considered in this study. However, the near infrared reflectance was dependent upon the surface preparation. Black chrome on electropolished and mechanically polished substrates has higher near-infrared absorptance than black chrome on fine ground (rough) substrates. Scanning electron micrographs revealed that the average size of the surface particles of the coating was largest for the mechanically polished and smallest for the electrochemically polished substrates, with that of the mechanically roughened substrates falling in between. The results suggest that the variations in near-infrared absorptance are a consequence of differences in the structure and composition of the films resulting from substrate preparation.     

Thermal stability and cross-linking studies of diisocyanate cured solar absorptance low-emittance paint coatings prepared via coil-coating process

Thickness-sensitive spectrally-selective (TSSS) paint coatings were prepared by using FeMnCuO_x pigment in combinations with phenoxy and silicone resins. The spectral selectivity expressed as a ratio between solar absorptance (a_s) and thermal emittance (e_T) was 0.92/0.14 for the first and 0.87/0.18 for the second type of the paint. The surface of phenoxy resin based coatings is characterized by pigment particles protruding from the surface, while silicone based paints exhibit a much smoother surface. Abrasion resistance and adhesion of both types of coatings were enhanced by cross linking the resin binder with diisocyanate hardener. Cured coatings withstand temperatures up to 135°C. Cross-linking and degradation mechanisms of both types of coatings were studied by using FT-IR spectroscopy and thermogravimetric measurements, cycling and ageing tests.