Solar Absorptance Αs Research Articles

Optical properties of aluminum oxide powder modified by nanoparticles and prospects for its use in solar power and space industry

Studies on the phase composition, diffuse reflectance spectra (ρλ) and solar absorptance (αs) and their changes after modification of aluminum oxide powder by nanoparticles of cerium dioxide, magnesium oxide, silicon dioxide, titanium dioxide and zinc oxide are presented. The diffuse reflectance spectra and solar absorptance αs of nanoparticles used for modification were studied.In its initial state, Al2O3 was found to exhibit high reflectivity. The obtained value of the solar absorptance, 0.074, was significantly less than that of the widely used white reflective powders – ZnO and TiO2. After modification by nanoparticles of wide-band compounds, MgO and SiO2, the reflectivity of Al2O3 powder changed slightly. Modification by SiO2 nanoparticles at a concentration of 1 wt % decreased the solar absorptance to 0.064. At small concentrations (0.1–1 wt. %) of nanoparticles of semiconductor compounds, CeO2, TiO2 and ZnO, the absorption edge of Al2O3 powder shifted to a longer wavelength region of the spectrum. At 10 wt % concentration of nanoparticles, the absorption edge of Al2O3 powder shifted to a value close to the absorption edge of the initial nanopowders.Such nano-micropowder systems can be used to develop new materials with improved optical properties for solar power and space technology applications, as well as other applications that need high reflectance and low solar absorptance. New data on shifts of the main absorption edge after modification by semiconductor nanoparticles can be useful in developing new materials for photocatalysis.

New spectrally selective coatings for CSP linear receivers operating in air at high temperature

AbstractAn attractive candidate to reduce dependence on fossil fuels and replace them by renewable sources is represented by concentrated solar power (CSP) technology, whose wide deployment is still hindered by its high investment costs. In CSP linear collector plants, a key component with high technological content may be identified in receiver tubes, that are traversed by heat transfer fluid and located on focal line on which solar radiation is concentrated by mirrors. The proposed work aims at developing spectrally selective coatings for receiver tubes able to overcome state‐of‐the‐art technological limitation in terms of both vacuum encapsulation and maximum operative temperature, currently limited at 550°C for molten salts CSP plants. To this aim, a multilayer structure was designed and developed for coating a receiver tube to be employed in open‐air environment at temperature of 600°C. The multilayer coating is based on tungsten chromium titanium infrared reflector and includes a solar absorber layer developed with two alternative methods of reactive sputtering and thermal oxidation respectively, finally covered by antireflective and barrier bi‐layer of aluminum oxide and silica. Thermal stability was demonstrated of the new developed coatings, since only small variations were measured of the optical parameters after more than 2000 h of aging in open air conditions at temperature of 600°C (on average less than 5%), with final measured solar absorptance αs of 73% and 87% and thermal emissivity εth at 600°C of 18% and 27% for aged multilayer structures including the absorber layer obtained through thermal oxidation and sputtering process, respectively. These results pave the way for further improvement in terms of both optical performance and operative temperature increase, allowing to enhance efficiency and lower costs, thus contributing to ease to deployment of CSP technology.

NiCr–MgF2 spectrally selective solar absorber with ultra-high solar absorptance and low thermal emittance

NiCr–MgF2 spectrally selective solar absorber, which consists of the tandem films of Au, high-metal-volume-fraction (HMVF) NiCr–MgF2, low-metal-volume-fraction (LMVF) NiCr–MgF2 and MgF2, is designed and fabricated for ultra-high solar absorptance and low thermal emittance. The performance of Au/NiCr–MgF2 (HMVF)/NiCr–MgF2 (LMVF)/MgF2 is simulated using the effective medium theory and film matrix method, and the optimized NiCr–MgF2 spectrally selective solar absorber has a normal solar absorptance (αsn) of 0.981 and a normal thermal emittance (εn) of 0.042 at 25 °C. The tandem films of Au/NiCr–MgF2 (HMVF)/NiCr–MgF2(LMVF)/MgF2 are fabricated by magnetron sputtering and measured by UV-VIS-IR spectrometer, and it has an ultra-high near normal solar absorptance of αsn = 0.976 and a low near normal thermal emittance of εn = 0.045 at 25 °C, which approaches very closely to the simulated limit. Furthermore, NiCr–MgF2 spectrally selective solar absorber allows a wide incident angle (θ) with its angular solar absorptance αs(θ)≥0.957 and angular thermal emittance ε(θ)≤0.054 if θ ≤ 50°. Such spectrally selective solar absorber is an outstanding candidate for solar thermal applications.

INFLUENCE OF SUBSTRATE HEATING AND BIAS POTENTIAL ON THE Ti Al C N COATINGS OPTICAL CHARACTERISTICS

Ti–Al–C–N coatings were produced by reactive magnetron deposition at different substrate temperatures Ts (220, 340 and 440 °C) and bias voltages Ubias (–90, –150 and –200 V). Using the energy dispersive X-ray spectroscopy method, it was found that the increase of the bias voltage led to a growth of argon atomic concentration and the (Al + Ti) / (Ti + N) ratio and to a decrease of the trace oxygen concentration in Ti–Al–C–N coatings. The growth of Ts promoted a decrease in the oxygen concentration. By means of scanning electron microscopy, a change in the type of the microstructure (columnar, granular and mixed columnar-granular) of coatings by varying Ts and Ubias was found. Electrophysical measurements showed the change of the film resistivity (1982–3169 μΩ · cm) when the deposition conditions were varied. The solar absorptance αs was varied from 0.24 to 0.54, the emittance ε was varied from 0.33 to 0.52, and the αs / ε ratio was varied from 0.60 to 1.44 by changing Ts and Ubias. The obtained results indicate the opportunity to vary the Ti–Al–C–N films electrophysical and optical characteristics by choosing optimal substrate heating temperature and bias voltage.

Preparation and characterization of absorbing tubes with spectrally selective coatings using economical methods for low- to mid-temperature solar thermal collectors

Absorbing surfaces with spectrally selective coatings having both high solar absorptance (αS) and low thermal emittance (εT) are used in solar collectors. Selective absorbing surfaces can be made of specific materials or combinations of them with various methods. The common methods of fabrication are executed on the basis of the surface coating, whereas the solar absorptance depends on the black coated layer and the thermal emittance on the undercoating layer. In this study, absorbing surfaces from various materials were coated using electrodeposition, chemical conversion (such as the chromate conversion), anodizing and painting techniques in accordance to specified formulas and the existing operating conditions. Materials such as stainless steel 316, copper and aluminum have been deployed as substrates of the samples. The samples of the selective absorbing surface were considered as absorber–reflector tandems. According to the experimental results from the performed optical characterization of the fabricated samples, it can be deduced that most of them had high solar absorptance αS more than 0.80 (up to 0.940) and low thermal emittance εT (100°C) less than 0.12 (as low as 0.085). In addition, most samples had high photothermal conversion efficiency (η) and selectivity factor (αS/εT). Also, the morphology of surfaces and particular elements identification of the coatings were studied and evaluated by scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS) detector. Moreover, the cross-section image was used to determine the approximate thickness of the films formed in some of the prepared samples.

High-pressure cold gas dynamic (CGD)-sprayed alumina-reinforced aluminum coatings for potential application as space construction material

Alumina–aluminum MMC coatings were deposited by cold gas dynamic spraying onto aluminum alloy (Al 7570) substrates. The microstructure, phase composition, thermal conductivity, microhardness, and thermal optical properties (solar absorptance αs and thermal emittance ε) of four types of coating were determined on as-sprayed samples and samples subjected to atomic oxygen (AO) plasma, intended to simulate conditions in outer space environment. In addition, time-of-flight (ToF) secondary ion mass spectrometry (SIMS) was applied to determine the responses of the coatings to oxidation by AO plasma. The results indicate that the very low thermal conductivity (λ), low solar absorptance (αs), comparatively high infrared emittance (ε) and oxidation stability of the investigated coatings may provide adequate protection as environmental control coatings of space-bound materials against deleterious effects of space environment within the Low Earth Orbit (LEO) and beyond.

Optical Characterization and Modeling of Coatings Intended as High Temperature Solar Selective Absorbers

SiCNH thin coatings synthesized by Plasma Enhanced Chemical Vapor Deposition (PECVD) are considered as high temperature ceramic materials for solar selective multilayer absorbers. As indicated by ellipsometry and spectrophotometry measurements, their optical characteristics make them suitable as solar absorbing and antireflective coatings in multilayer stacks and/or ceramic- metal composites (cermets). These measurements gave access to spectral values of complex refractive index N = n + ik, reflectance R and transmittance T of the coatings. R and T were also modeled from N, based on Fresnel equations, and a good agreement was found between simulated and measured reflectance in the visible region. A transfer matrix method was then applied to predict the optical response (R, T) of virtual multilayer stacks including SiCNH materials. W-SiCNH cermets were also considered as absorptive layers in these stacks and their refractive indices were calculated using Bruggeman effective medium approximation theory. Solar absorptance αS and thermal emittance ɛ(T) were estimated from simulated reflectance. Layer thicknesses and cermet composition were optimized using CODE software and a MATLAB in-house code, so as to maximize solar-heat conversion efficiency which relied on high αS (up to 95%) and low ɛ (down to 6% at 500°C). The corresponding global yield which could be attained in typical Fresnel and central tower solar plants using these selective coatings was also calculated and compared to current technology.

Design of Nanostructured Selective Surfaces for Solar to Thermal Energy Conversion

ABSTRACTThe solar absorptance αs of nanostructured selective surface (NSS) for solar thermal energy is improved. The NSS are prepared by AC electrochemical impregnation of metal inclusions (MI) into porous anodized aluminum oxide (AAO). The dependence of the NSS performance with composition depth profile and MI is studied by numeric simulations based in a gradient index model and effective medium theory. The results are compared with experimental NSS prepared varying three control parameters and MI (Ni, Cu, Ag). The αs is improved to > 85% (keeping thermal emittance εT relatively low) for Ni MI, mainly by increasing MI content. Increasing AAO thickness or MI molecular weight (for a given experimental composition profile) also improves the performance. For Ag the αs was further improved to 90%.

Surface and Bulk Degradation of Teflon® FEP Retrieved from the Hubble Space Telescope Solar Arrays

Teflon® fluorinated ethylene propylene (FEP) is used on the Hubble Space Telescope (HST) as the outer layer of multilayer insulation (MLI) blankets. During space shuttle servicing missions (SM) to HST through thickness cracks were observed in the FEP layers. Material brought to Earth for investigation showed signs of severe degradation. During servicing mission 3B, the 4th servicing mission to HST in March 2002, the second pair of European Space Agency ESA built solar arrays was retrieved and flown back to Earth after 8.25 years in space. Samples of the MLI thermal control material were taken from the solar array drive arm (SADA) and investigated for surface and bulk degradation. The MLI was enveloped around the SADA and thus allowed the examination of FEP degradation in dependence of the orientation of space exposed areas with respect to the Sun. Therefore micrographs of FEP surfaces and fractographs of through thickness cracks were taken and the solar absorptance αS and the normal emittance εN was measured on the entire MLI. Differential scanning calorimetrical analyses as well as electron spectroscopy for chemical analyses were conducted to analyze chemical changes of the bulk polymer and the surface layer, respectively. Tensile properties and mass losses of the space exposed material were evaluated as well. Results of the investigated thermal control material are compared with samples exposed on the ground to thermal cycling, soft X-rays and vacuum ultra-violet radiation as well as to previously reported results on space-exposed FEP. The paper thus gives further insight into the mechanism and processes contributing to the on orbit degradation of Teflon® FEP and correlates the presented data with exposure levels and exposure conditions.

Optical degradation of polydimethylsiloxane under 150 keV proton exposure

AbstractThe optical degradation of polydimethylsiloxane (PDMS) was investigated under exposure of 150 keV protons to reveal the damage behavior of organic silicone in space. The exposure was performed in a ground‐based simulation for space protons. The experimental results show that the optical degradation mainly occurs in the visible region. With increasing proton fluence, the absorption band shifts toward longer wavelengths (red‐shift) and the solar absorptance αs ascends. The analysis of Fourier transform infrared spectroscopy indicates that under the exposure, the degradation of the PDMS occurs to form hexamethylcyclotrisiloxane, the amount of which increases with increasing the proton fluence. This could cause an increase in delocalization degree of the σ electrons, resulting in the red‐shift of absorption bands. Also, the energy for valence electrons to be excited decreases and the n→π* jump is intensified, increasing the probability of optical absorption. As a result, the solar absorptance αs of the PDMS increases under exposure of 150 keV protons. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Optical Absorption Properties of Nanotitanium Dioxide Doped ZnO/Silicone Thermal Control Coating

As a kind of white paint, ZnO/silicone thermal control coatings are used commonly in space. In order to implement more effective thermal control in space, nanotitanium dioxide was doped to the coatings. In this paper, different percent of nanotitanium dioxide was appended to the coatings, and thermal control principle was discussed. Optical properties of these coatings were discussed by spectrophotometry and infrared reflectometer. The experimental results show that solar absorptance (αs) of nanotitanium dioxide doped ZnO/silicone thermal control coating is lower in all wavelength region than the coatings without doping, especially in visible region and infrared region. The total solar absorptance of 1% nanotitanium dioxide doped coating is 0.09, which decreases by 36% than the undoped coating..

Electron beam evaporated rigid optical solar reflectors for space applications

Rigid optical solar reflectors (OSRs) for space applications were developed by using the electron beam evaporation technique. The component layers of the full stack were optimised for their electrical and optical properties. Thin films of Ta2O5 show absorption in the ultraviolet–visible range at high substrate temperatures, which can be eliminated by deposition at an oxygen partial pressure of >4 × 10−5 torr. Under optimised deposition conditions, indium–tin oxide (ITO) films have a sheet resistance Rsh of <0·95 kΩ/□ and a transmittance of <95% in the visible region. The electrical and optical properties of the OSR stacks were investigated as a function of various deposition parameters, such as substrate temperature, thickness and deposition rate. It was found that the functional properties of the OSRs developed with the electron beam technique are equivalent to those of the rigid OSRs deposited by the sputtering technique. A solar absorptance αs value of <0·047 and a sheet resistance Rsh value of <3 kΩ/□ were obtained for the OSR deposited under the optimised conditions. The samples were found to be environmentally durable.

Development of Spectral Selective Multilayer Film for a Variable Emittance Device and Its Radiation Properties Measurements

A smart radiation device (SRD) that is a variable emittance radiator has been developed as a thermal control material for spacecraft. The SRD has the unique feature of large variation of the total hemispherical emittance eH near room temperature. The eH of the SRD changes depending on its temperature. However, there is a drawback of a large solar absorptance αS. It is too large to use as a thermal control material for spacecraft. In order to reduce the large αS, spectral selective multilayer film was developed. This multilayer film reflects solar radiation and transmits far-infrared radiation to maintain the variation in the eH of the SRD. This paper presents thermal radiative properties of the SRD with spectral selective multilayer film. The multilayer film was designed by using a genetic algorithm (GA). The designed multilayer film was evaporated on the surface of the SRD by the electron beam evaporation method. The experimental results of αS and eH of the SRD with the multilayer film agreed well with calculated results.

A Method for Calculating Thermal Radiation Properties of Multilayer Films from Optical Constants

A calculation method for the incident angle dependence of the solar absorptance αS and the temperature dependence of the total hemispherical emittance eH of multilayer films is proposed. The method is based on calculation of αS and eH from optical constants in the wavelength region from 0.25 to 100 μm for thin polymer films and deposited metal. In this paper we provide values of αS in the incident angle region from 0 to 90° and eH in the temperature range from 173.15 to 373.15 K for two-layer samples of aluminum-deposited polyimide film. The results obtained for αS and eH by the present method are compared with experimental results measured by both spectroscopic and calorimetric methods. The calculated results of αS and eH agree well with the experimental results.

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 αs = 0.94 (AM2) and normal thermal emittance e = 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.

Solar absorptance and emittance properties of several solar coatings

Solar absorptance αs and total hemispherical emittance εt,H properties of two potential solar selective coating systems are reported. The first coating system studied is a semiconductor-pigmented paint which consists of a high-temperature silicone binder mixed with small particles of Ge, Si, or PbS. Although most of the paints have αs values above 0.90, all the paints suffer from high εt,H values (∠0.70 to 0.90), which are shown to be due to the high emittance of the silicone binder itself. The second system studied is electroplated films of ’’black nickel’’ and a proprietary ’’black chrome.’’ Both αs and εt,H values have been obtained as a function of the electroplating time and the substrate surface roughness. Increasing the plating time initially increases both αs and εt,H; however, for long plating times, there is no improvement in αs, while εt,H continues to increase. By increasing the surface roughness of nickel-plated substrates, the solar absorptance can be increased while maintaining approximately the same total hemispherical emittance. The coatings show minor decreases in solar absorptance at angles of incidence up to 75° from normal (e.g., about 25% decrease at 75°).