Processing of New Solar Absorbers in Steel Design Based on Partially Cold Roll Bonded Hybrid Semi-Finished Parts

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®).

• Periodical: Key Engineering Materials (Volumes 504-506)
• Pages: 137-142
• DOI: https://doi.org/10.4028/www.scientific.net/KEM.504-506.137
• Online since: February 2012
• Authors: Frank Steinbach, Lotta Koch, Michael Hermann, Jörg Witulski, A. Erman Tekkaya
• Keywords: Cold Roll Bonding, FracTherm^®, Hybrid Material, Sheet Hydroforming, Solar Absorber

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