Stainless steel for evacuated-tube collectors.
In contrast to flat-plate collectors, which can use the structure of the collector frame for bracing and support, evacuated-tube collectors rely strictly on the rack for structural support. Since stainless steel is stronger than aluminum, the thickness and size of an evacuated-tube rack tend to be thinner and smaller than those for a flat-plate collector. Racks for evacuated tubes require significantly more bracing-the manifold could easily be pushed sideways since the collector itself lacks the rigidity of a flat-plate collector. Diagonal bracing is required for evacuated-tube tilt racks.