Papers by Keyword: Solar Absorber

Abstract: In present work, tin-pigmented alumina (Sn-Al₂O₃) solar absorber on the aluminium substrate was successfully prepared by anodic anodization and further characterized by different methods. The phase, morphology, reflectance (R) and thermal conductivity of the Sn-Al₂O₃ solar absorber were measured by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) equipped with energy dispersive X-ray (EDX) analyzer, and Ultraviolet-visible-near infrared spectrophotometer in the wavelength of 300-2500 nm. The solar absorptance (α) was calculated based on the relationship of the spectral reflectance, R(λ), and the solar spectral irradiance of AM 1.5, I_s(λ), in the wavelength interval of 300-2500 nm. As the results, the surface color of the Sn-Al₂O₃ film was dark-black color. The XRD pattern of Sn-Al₂O₃ films was indexed as aluminium and tin phases. The chemical composition of the Sn-Al₂O₃ films composed of tin (Sn), aluminum (Al) and oxygen (O) elements. The average thickness of the produced films was 18.9 μm. It was found that Sn-Al₂O₃ films showed the low R (0.09) and high α (0.93) values for the whole wavelength 300-2500 nm, corresponding to theoretical properties of the solar absorber. Therefore, it can be concluded that the Sn-Al₂O₃ film on aluminium substrate can be applied to be the solar absorber in solar collector due to high α, which is similar to the commercial solar absorbers.

Abstract: In this research, a Ni-Al solar absorber was successfully prepared by the flame spray technique with Ni-5 wt.%Al particles as a starting material. The Ni-5 wt.%Al particles were melted and sprayed onto the outer surface of a stainless steel 316L tube in order to form a Ni-Al composite coating. The phase, morphology and reflectance (R) spectrum of the Ni-Al solar absorber were characterized by X-ray Diffraction (XRD), a Scanning electron microscope (SEM) equipped with an energy dispersive X-ray (EDX) analyzer and an Ultraviolet-visible-near infrared spectrophotometer at the wavelength 300-2500 nm. The results revealed that the surface of the Ni-Al solar absorber was rough, and its cross section was overlapped layer by layer. The Ni-Al solar absorber was composed of Nickel (Ni) and aluminum (Al) phases. NiO and Al₂O₃ phases were also found on the surface. The chemical composition of the Ni-Al solar absorber was Ni (72.94 wt.%), Al (11.76 wt.%) and O (15.29 wt.%). The solar absorptance (α) of this solar absorber was 0.77. This demonstrates that a Ni-Al composite coating can be applied as a solar absorber material for solar collector at high operating temperatures.

Abstract: One of the ways to increase efficiency of the solar absorber is modification of absorption area in the system of pyramidal cavities, where the incident radiation is absorbed by multiple reflections. The paper deals with the technology of production of the flow solar absorber with a structured surface in a single technological operation using hydroforming. Two laser-welded sheets are inserted into the forming chamber. Then the space between them is pressurized with hydraulic fluid which causes their form to the desired shape. The first part of this article describes hydroforming device. The second section deals with theoretical simulation of forming a different structured surface in the programming environment of ANSYS. As follows practical test of hydroforming is performed. Austenitic stainless steel X5CrNi18-10 was used as a material for the production of samples.

Abstract: The conventional manufacturing of solar absorbers has the disadvantage that only simple pipe branch channel geometries are possible. For this reason, a new approach of manufacturing solar absorbers is presented with which it is possible to design the channel geometry as a quasi-fractal structure (FracTherm®, developed by Fraunhofer Institute for Solar Energy Systems) and which reduces the pressure drop and the required energy for the pump. To manufacture these new absorbers, a fast, nearly continuous production process was developed which consists of partial cold roll bonding and subsequent hydroforming, similar to symmetric hydroforming of sheet metal pairs. Various channel geometries, which are developed by Fraunhofer ISE, within the hybrid sheet metal, can be applied. Therefore, three technologies are combined to manufacture a new type of solar absorber. A special look is taken at cold roll bonding. By this process, the material for the new solar absorbers (copper-steel-copper/copper-steel-copper hybrid sheet material) is produced using a suitable release agent to avoid the bonding process along the subsequent water channels. To establish manufacturing guidelines, the characterization of the material is necessary. A peel test like ISO 11339 is used to determine the strength of the layers of the hybrid material. At least the forming characteristics of the hybrid material are dominated by the basic material, in this case steel. A further look is given to the hydroforming of the material. Different cross-section geometries have been tested by a simplified experiment to find out the best geometry for the absorber channels, and have been compared with simulation results, too. Hence, for a crack-free forming operation the choice of the die radius and corner radius is decisive as they are within the same range. To show the feasibility of the hydroforming process and validate the process chain, a small demonstrator was manufactured. Finally it is possible to manufacture a steel based solar absorber with a quasi-fractal structure by combining partial cold roll bonding and hydroforming. All these results are used as guidelines for the production of a new absorber with quasi-fractal structure (FracTherm®).

Abstract: This paper presents a new method for sputtering solar selective absorber coatings. Radio frequency magnetron reactive sputtering created stainless steel/stainless steel nitride (SS/SS-N) cermet (ceramic-metal composite) thin films under varied nitrogen gas flow ratios. In this present study, a stainless steel material is used for one-target sputtering to manufacture a solar selective absorber film having good solar absorptance of 0.91 and low thermal emittance of 0.06. This sputtering process shows considerable and stable usefulness due to the anti-oxidation and single target. Optical constants n and k have been determined by spectroscopic ellipsometry for cermet composite in a reactive gas flow from 0 percent to 50 percent. Based on the nitrogen content of the film, we used a different oscillator to analyze the film. When the nitrogen gas flow reaches 10 percent, the sputtered film has a significant transformation from metal to cermet properties. When the nitrogen gas flow goes above 17.5 percent, then the film transforms to a dielectric film which can be an anti-reflection layer. A solar absorptance of 0.92 was accepted as the theoretical prediction of optimal selective solar-absorbing three-layer processes which shows that the experimental results agree well with the theoretical prediction.

Abstract: Line shaped electron beam was used for the recrystallization of nanocrystalline silicon layer that had been deposited on the low cost borosilicate glass-substrate in this paper. Polycrystalline silicon films of a 20μm thickness, which are the base for a solar cell absorber, have been investigated. Tungstendisilicide (WSi2) was formed at the tungsten/silicon interface as well as grain boundaries of the silicon. WSi2 improved the wetting and adhesion of the silicon melt. The surface morphology of the film was strongly influenced by the recrystallization energy density applied. Low energy density resulted in non wetted WSi2/W areas due to the reaction between the silicon melt and the tungsten. With the increased energy, the capping layer become smooth and continuous due to the pinholes becomes fewer and smaller. Excess of the energy density led to larger voids in the capping layer, more WSi2/Si eutectic crystallites, a thinner tungsten layer, and a thicker tungstendisilicide layer.