On measurement techniques for the spectral absorptance of glazing materials in the solar range

Advanced glazing materials may be thick, scattering, and inhomogeneous, and this poses problems for conventional absorptance measurement techniques. This paper describes a technique for accurately measuring the spectral absorptance of such materials. The measurement technique, in which the sample is mounted at the centre of an integrating sphere, was analysed and the errors that occur when applying the technique to large samples were analysed. The technique was used to measure absorptance of several materials, and the results are presented. For heterogeneous materials it is necessary to irradiate a relatively large area to get a representative sample. It was found that under certain conditions (depending on the sample absorptance), sample sizes as large as 30% of the sphere diameter can be used without incurring a prohibitively large error. This paper examines the effect of the finite absorptance of the rod supporting the sample, showing that it can be important unless certain precautions are taken.     

Solar radiation absorptance in tray solar water heating collectors with exposed evaporation surface

A relationship for determining effective solar radiation absorptance of the “water-blackened bottom” system in capacious tray water heating collectors with exposed evaporation surface is presented.     

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.     

The effect of the inner structure on the absorptance of cement based exterior coatings

The relationship between the inner structure and the surface radiative properties of cement based exterior coatings has been studied. The radiative properties of exterior building coatings have considerable effect on the energy conservation used to maintain thermal comfort, and the cement based exterior materials are commonly used as the exterior coatings of buildings. The surface photographs of various cement based samples taken by scanning electron microscopy (SEM) and formal and statistical analysis of the SEM photograph images made by VIDS III equipment are presented. The monochromatic absorptance between 0.3–0.8 μm wavelength of the same surfaces has been measured before. The monochromatic absorptance of white Portland cement surfaces decreases when the water/cement ratio of the mixture increases since the inner structure of this cement has more crystalline content and the crystallites increase with the lower water/cement ratio. However, in the case of added Portland cement the monochromatic absorptances remain constant regardless of the water/cement ratio because the mixing water has no effect on the inner structure of this type of cement.     

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.     

Compatibility of antireflective coatings on glass for solar applications with photocatalytic properties

Antireflective coatings for solar cover glasses and photocatalytic coatings for self-cleaning glasses are two types of nano-functionalized thin films, which are extensively investigated at present. Broadband antireflective coatings typically receive their function by the low refractive index of nano-porous silica (<1.4), whereas the most established photocatalytic coatings consist of high refractive index materials, such as titania (>2.0). Compatibility of these two functional materials was investigated using sol–gel dipcoating-technology. Therefore silica sol, used to prepare nano-porous antireflective coatings, was mixed with different amounts of titanium n-butoxide to achieve titania concentrations in the range from 1 to 50 wt%. The fired coatings on silica glass showed a decrease in solar transmittance and an increase in photocatalytic activity with increasing titania concentration. With respect to applications in solar energy conversion systems compatible SiO₂–TiO₂ materials of low titania fractions (10 wt%) were established, which combine minor loss in transmittance and exceptional high photocatalytic activity. In comparison to conventional nano-porous silica, the decrease in the solar transmittance was only 0.7%, whereas the degradation rate of stearic acid was 30 times faster.     

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.     

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.     

The effect of surface coatings on the solar radiation equilibrium skin temperature of an earth satellite

The skin temperature of an earth satellite is governed mainly by equilibrium between absorbed solar radiation and re-emitted infrared radiation. A polished metal surface will reflect a large portion of the incident solar radiation but will give a relatively high skin temperature because its infrared emissivity is low. A coating of white nonmetallic material will give a lower skin temperature, even though it absorbs a greater portion of the incident solar radiation, because its infrared emissivity is much greater.     

Effect of the front surface field on GaAs solar cell photocurrent

It is well established that the inclusion of a high–low junction at the back of a solar cell or the so-called back surface field (BSF), improves its performance. In the other hand, if the same structure is placed at the front of the cell as high–low junction emitter, front surface field (FSF), a notable amelioration is observed. In this work, a simulation of the influence of the FSF on GaAs solar cell spectral response and photocurrent is carried out. The model is based on a simple analytical approach. The most important role of an FSF layer is the enhancement of light-generated free carriers’ collection, and this property is primarily responsible for the increase of the short-wavelength quantum efficiency. This is achieved by the drastic reduction of the effective recombination at the emitter upper boundary. The effects of the FSF layer doping concentration and its thickness on the cell performance are discussed by using the computed results.     

Photothermal radiometry on nickel (pigmented aluminium oxide) selective solar absorbing surface coatings

Photothermal radiometry (PTR) is applied to characterize nickel-pigmented aluminium oxide solar absorbing coatings. A modulated laser beam is used to heat the solar samples. The subsequent emission of thermal radiation is measured as a function of modulated frequency in the range of 10 Hz to 10 kHz. A simple one-dimensional model is used to fit the experimental PTR results, allowing for the extraction of some thermal parameters for the solar absorbing coatings. Finally, comparison of the emissivity measured by traditional technique and the photothermal radiometry is made.     

Effect of the front surface field on crystalline silicon solar cell efficiency

The present paper reports on a simulation study carried out to determine and optimize the effect of the high–low junction emitter (n⁺-n) on thin silicon solar cell performance. The optimum conditions for the thickness and doping level of the front surface layer with a Gaussian profile were optimized using analytical solutions for a one dimensional model that takes on the theory relevant for highly doped regions into account. The photovoltaic parameters of silicon solar cells with front surface field layer (n⁺-n-p structure) and those of the conventional one (n-p structure) are compared. The results indicate that the most important role played by the front surface field layer is to enhance the collection of light-generated free carriers, which improves the efficiency of the short wavelength quantum. This is achieved by a drastic reduction in the effective recombination at the emitter upper boundary, a property primarily responsible for the decrease in the emitter dark current density. The findings also indicate that the solar cell maximum efficiency increase by about 2.38% when the surface doping level of the n⁺-region and its thickness are equal to 2.10²⁰ cm^?3 and 0.07 μm, respectively.     

Expressions for absorptance of the cover plates with/without an absorbing surface

Expressions are obtained for calculating the fraction of insolation absorbed by each plate of a cover system for solar thermal devices having n non-identical plates in the presence/absence of an absorbing surface below the cover system. The average angle of incidence, averaged over 277 clear days of 1974, is obtained for the absorbing surface and for a cover system having one, two or three plates.     

On the stability of optical properties of antireflection coatings for solar cells based on a silicon-silicon-oxide mixture

It is investigated how the storage duration influences the optical properties of antireflection coatings based on silicon-silicon-oxide mixture intended for silicon solar cells. It is shown that the spectral reflection coefficient changes by 2.7% for 22 years; i.e., the antireflection effect is retained. We draw the conclusion that coatings based on silicon-silicon-oxide mixture can greatly increase the lifetime of solar cells and a solar battery.     

Effect of selective surface degradation on the performance of solar water heating systems

Selective surface often degrade in the field. Their solar absorptivity α_s and thermal emittance change with time in service by some amount, say Δα_s and Δ, from their starting values. It is important to quantify the effect this degradation has on the annual fraction solar F_s. A given relative change in F_s can be caused by different combinations of Δα_s, and Δ. In this paper we use computer simulation of solar domestic hot water systems to graph these combinations, in a plot of Δα_s versus Δ, for relative changes in F_s, of 10% and 5%. The slope and intercepts of this plot, which is found to be linear, are studied for their dependence on a wide range of solar system parameters, such as geographical location, collector area, and set point temperature. We find that the slope, and - for starting values of F_s less than about 0.5 - the intercepts, are relatively insensitive to the system parameters. We show that this result is consistent with a simple model. For F_s 0.5, the intercepts rise sharply with F_s, in a way that is strongly (and to some extent, only) dependent on the geographical latitude of location. These results have direct application to projecting the useful service life of a selective surface.     

Optical properties of higher and lower refractive index composites in solar selective coatings

The focus of attention in this study was the choice of material for optically solar selective coatings on the basis of their optical constants. A computer programme which calculates the optical constants, solar absorptance at air mass (AM)-2, α, and thermal emittance at 300 K, , of the 200-nm-thick selective coating on the assumption of both the Maxwell Garnett and Bruggeman theories for the metallic volume fraction below and above 0.3 respectively, was used to design the structure of the composite films. Two systems of composite thin films of metal and dielectric were investigated experimentally, fabricated by RF and DC sputter coater and were verified with computer simulations. One system consist of lower refractive index composites such as Ni : SiO₂ and the other of higher refractive index composites such as V : Al₂O₃ in the spectral range of 0.3–20 μm. These films were fabricated on infrared reflective substrates such as nickel plated copper or aluminium. Results of the copper substrates are being presented here. For comparison and verification, tungsten, cobalt and chromium based composites, having different refractive indices, were also investigated which validated the concept of the choice of material in selective coatings. It was observed that high refractive index composites have lower reflective properties by choosing suitable metallic volume fraction in dielectric and antireflection coating. The higher value of the imaginary part of refractive index, k, is responsible for higher absorption by a factor α_λ=4πk/λ. Solar absorptance of 0.98 and 0.96 was achieved by simulation and experimental findings with less than 0.05 thermal emittance for 200 nm thick composites of V : Al₂O₃. It results that higher values of both n and k of the material are more suitable in solar selective coatings.     

Electron beam physical vapour deposited zirconia based ceramics for developments of solar surface coatings

Abstract

Infrared radiative properties of thermal barrier coatings have been investigated for developments of solar absorbing surface coatings. Various types of multilayer broadband optical filters were designed incorporating 7 wt-% yttria stabilised zirconia (7YSZ) and aluminium oxide (Al₂O₃). Electron beam physical vapour deposition method was employed to deposit 7YSZ thin films on quartz substrates to study their optical properties. The refractive index and thickness of 7YSZ films were estimated using spectroscopic ellipsometry technique, giving the index as 1·95 at 633 nm. Multilayer thermal barrier coating structures were fabricated by depositing alternate layers of 7YSZ and Al₂O₃ on quartz using electron beam physical vapour deposition. The spectral characteristics of multilayer 7YSZ–Al₂O₃ coatings were evaluated using a spectrometer in the visible and near infrared range. Preliminary results obtained from multilayer 7YSZ–Al₂O₃ coatings had exhibited 75% transmission over a wide band of 0·4–1·0 μm.