1.1 Parabolic body

The stainless steel body is designed by four parabolic shaped ribs (two L-shaped and two T — shaped) which are screwed with two side plates and a trough sheet. Thus the corpus is built of seven components hold together by numerous screws. This results in a long assembly time and high expenditure of material. Since the components are individually fabricated and made of high — grade steel, the production costs are high. In addition the inaccuracy of each single component con­tributes to the total inaccuracy of the system, which in generally results in a decrease of the effi­ciency. The prototype collector at the test facility of the SIJ is shown in Fig. 2.

As a requirement for a high efficiency, the main function of the body is to guarantee the shape ac­curacy of the collector. Although the mentioned ribs were given the parabolic shape by a CNC — based bending machine, the ribs partially lost the given shaped because of material tensions. For this reason the body is supported by two tensioning and counteracting ropes as can be seen in Fig. 2. Nevertheless the shape is far of the ideal parabolic shape, especially at its borders. The DLR determined the irregularity with a photogrammetry, and figured that only about 90 % of the total collector area can be considered to be usable (see Fig. 3).

Planned optimization: The aperture area will not vary from the prototype. But in order to achieve a better shape accuracy of the body, it will be constructed of less individual parts. A deep drawing process promises good results with regard to shape accuracy and cost reduction in a series produc­tion. The body may consist of different materials like stainless steel, aluminium or even plastics. Last alternative has to be considered properly, due to the presumed linear thermal expansions. Be-

cause of the deep drawing process it is necessary to provide the side plates with a draft of 4° to en­sure the save removal of the component from the deep drawing tool. This draft also allows the stacking of the troughs for transport, thus providing the option of transporting high numbers of col­lector bodies in narrow space.

Nevertheless, the body of the first new prototypes within the optimization phase will be fabricated by rolling and welding, since the costs for a fabrication tool for deep drawing are too high for the aspired numbers of prototypes. But the rolled and welded design will be of a shape that allows deep drawing at a later series production.