Fraunhofer Institute for Solar Energy Systems, Heidenhofstr. 2, 79110 Freiburg, Germany
Tel.: +49 7 61 / 45 88 – 54 09, Fax: +49 7 61 / 45 88 – 94 09
michael. hermann@,ise. fraunhofer. de
Aluminium becomes more and more interesting as a solar absorber material. One possibility to produce an absorber entirely made of aluminium is given by roll-bond technology which is well established for large-scale series production of e. g. evaporators for refrigerators. This technology offers the possibility to build solar absorbers with high efficiency, since the channel design can be varied without additional costs. At Fraunhofer ISE a computer algorithm called FracTherm® was developed which is capable of generating a fractal-like, multiply branched channel design on a given area (also non-rectangular), similar to natural hydraulic networks. The aim of this bionic approach was to obtain a high efficiency by means of a uniform flow distribution as well as a low pressure drop. A small solar collector with FracTherm® roll-bond absorber (0.59 m x 1.0 m) had already been built and compared with absorbers featuring serial (meander absorber) and parallel (harp absorber) channel arrangements, respectively, within the research work of a doctoral thesis. Since the test absorber was relatively small, the focus of current research and development is on investigations of larger absorbers with sizes typical for solar collectors. This work, which is still at the beginning, will be done together with industry partners within the European project BIONICOL. This paper describes the challenges coming up with the development of large aluminium roll-bond absorbers with FracTherm® channel design including issues such as inside and outside corrosion, application of a selective coating, the connection of several roll-bond absorbers, fluid flow investigations and stagnation behaviour.
Keywords: FracTherm®, roll-bond, bionic, aluminium absorber
State-of-the-art solar collectors with fin-and-tube absorbers mostly feature serial (serpentine or meander absorber) or parallel (harp absorber) channel arrangements. However, serial arrangements lead to a high pressure drop due to the channel length, whereas the parallel arrangements can feature non-uniform flow distributions (depending on diameters and lengths of riser and header tubes) . The bionic so-called FracTherm® approach developed at Fraunhofer ISE (Fig. 1) allows creating fractallike, multiply branched channel structures on a given area which lead to a low pressure drop as well as a uniform flow distribution. The resulting complex structures are not suitable for being built as a fin- and-tube construction, but they can easily be realized as aluminium roll-bond absorbers. Small
FracTherm® test absorbers (0.59 m x 1.0 m) had already been designed, built and compared with meander and harp absorbers within the frame of a doctoral thesis. These investigations had already been published in detail in ,  and . The thermal efficiency had been very high for all test absorbers. It could have been shown that the flow distribution of the FracTherm® absorber was much more uniform and the pressure drop for high volume flows was up to 8 % less than in the roll-bond harp absorber. However, both the results of the experiments carried out so far and the construction of the collector are not directly scalable to a standard size collector (1.0 m x 2.0 m). Fig. 2 shows the small test absorber investigated earlier and a standard size collector with a possible FracTherm® channel structure. The questions arising with the development of a large aluminium absorber with FracTherm® channel design are to be answered within the European research project BIONICOL.
Fig. 1. The FracTherm® algorithm
Fig. 2. Test absorber (left) and standard size absorber to be developed (right)