The FracTherm® program is to be improved especially with a focus on practical aspects concerning both production and operation of the collector. This concerns the algorithm itself as well as the implementation of additional features. The further program development can increase the number of possible geometries and thus the flexibility in designing absorbers. An example for an improvement is the possibility to use different bifurcation geometries, which can have an important mechanical and hydraulic impact on the resulting roll-bond absorbers.

2.1. Adaptation of the roll-bond process for the production of solar absorbers Nowadays roll-bonding is basically used — and optimized — for the production of evaporators for refrigerators. However, the requirements for a solar absorber are different. Therefore it is necessary to find out the restrictions of roll-bonding such as the maximum width and height of channels as well as the minimum radius of bifurcations depending on the internal pressure. It has to be checked whether common dimensions of roll-bond channels can be changed in order to gain more flexibility in design and reduce the pressure drop. The effect of different bifurcation geometries has an important impact on mechanical and hydraulic behaviour of the absorber. The minimum radius of roll-bond structures is one important limitation, because it has an influence on the minimum distance between channels. Currently the use of circular constructions is common in roll-bond panels. Mattheck has shown that notch stresses of circular notches can be reduced dramatically by shape optimization based on load — adaptive growth which was observed in nature (trees, bones). In [5] Mattheck shows that a significant reduction of notch stresses can be obtained even with the pure graphic “method of tensile triangles”. It is very likely that the mechanical stability — and thus the flexibility in design — can be increased if roll-bond geometries are changed from circular to optimized shapes. Different bifurcation geometries are shown in Fig. 3; a), b) and c) are already implemented in the FracTherm® program, d) is not yet (the shape in the figure is generated manually). The different geometries are to be investigated experimentally within the project.