However, plastics are widely used in comparable applications (e. g. in buildings as doors and windows) and even in the highly sophisticated automobile industry under increased requirements concerning structural stability and quality of surface.

Despite the lack of flexibility, a trough was favoured to a frame design for cost reasons. In any case, it is first of all necessary to find the appropriate way of production: injection moulding or deep-drawing. Apparently, injection moulding offers much more freedom of design, however, the economically favourable solution counts. A commercially interesting alternative to the existing aluminium troughs means production costs of less than 25 € per trough (2 m2 collector, ready for assembly).

For that reason, a detailed cost simulation was carried out regarding several approaches of injection moulding. As was expected, caused by the necessary high investments, injection moulding is interesting only above 60.000 pieces p. a., yet provides opportunities to reduce the cost to about 20 € per trough. At the time of the project, such a high number of collectors of one single type was not produced by any of the big producers of solar — thermal collectors. Hence, for cost reasons, a deep-drawn trough design was selected, that leads to only moderate investments of tooling. The cost limit can be met using this production technology.

The following design work, as shown in figure 7 and figure 8, was accompanied by detailed material selection. Typical solar-thermal collector operation conditions have to be considered, such as

► life-time of at least 20 years,

► all-year unprotected weather exposure,

► temperature changes from -20 °C up to +80 °C,

► UV-exposure,

► salt water atmosphere, etc.

Thermoplastics such as ASA, especially when combined with polycarbonate and glass fibres, appear well suited and are already used in relevant applications, however, they are relatively expensive (figure 6).

Luran® S, produced by BASF AG, Ludwigshafen/D, for example is a thermoplastic based on ASA and is widely used in applications comparable to solar-thermal collectors:

► automotive construction (figure 6; also commercial and agricultural vehicles),

► electrical engineering (TV antennae parts, cable connection housings, weatherproof protective housings),

► sports and other outdoor uses (sailboats, surfboards, snowboards).

PFLEIDERER DACHZIEGEL GmbH, Winnenden/D, is even using this material for roof — integrated PV-systems as shown in figure 9.