Vacuum tube collectors

Evacuated tube collectors usually consist of two nested glass tubes, with the inner tube being selectively coated. Heat loss is largely prevented by the vacuum. They have an additional heat-insulating effect due to the vacuum.

In addition, mirrored surfaces (integrated or external) can increase the degree of utilisation. These so-called concentrating collectors reach higher temperatures with the same absorber surface, because more solar radiation hits the same absorber surface due to the specifically positioned curved mirrors. However, this advantage cannot be used with diffuse radiation, as only directional rays can be concentrated. Depending on the shape of the mirrors, a distinction is made between CPC collectors (compound parabolic concentrator) and OPC collectors (optimized parabolic collector).

There are also products available on the market whose tubes can be adapted to the solar radiation by turning them so that the high efficiency of the collector is ensured even with a flat or steep angle of inclination.

Figure 7: Design of a vacuum tube collector (Source: RaBoe/Floh1992; http://de.wikipedia.org/w/index.php?title=Datei:Vakuumroehrenkollektor_aufbau.png&filetimestamp=20090214125700)