Bingo!
How is the gain of the evacuated tube panels?
1. The concentration of radiation to increase the T ° fluid during periods of low sunlight. No need for that tube, a stainless steel or polished aluminum, folded in the shape does the same job for much less. The tin can also
as was well noted Cuicui, or a cast-coated foil
2. improved insulation tube through the vacuum to keep warm in cold weather. can be replaced by tempered glass Plexiglas, less conductive, and may also be evacuated. indeed, plexi is supported by each edge of the reflector, it can not be deformed. There remains the question of the sealing device that should be feasible and it is conceivable to provide the panel with a vacuum pump jack.
The self of such panels would be technically more affordable than flat panels.