The glass evacuated solar collector tubes are the key component of solar heating each tube consists of 2 glass tubes. The outer tube is made of extremely strong transparent borosilicate glass that is able to resist impact from hail up to 25mm in diameter.
The inner tube is also made of borosilicate glass, but coated with a special coating (Al-N/Al) which features excellent solar heat absorption and minimal heat reflection properties. The air is withdrawn (evacuated) from the space between the two glass tubes to form a vacuum, which eliminates conductive and convective heat loss.
Can the evacuated solar tubes work on a cloudy day?: Yes the tubes are round and the amount of solar radiation striking the collector is relatively constant from mid morning to mid afternoon. This feature maximizes the total amount of solar radiation the collector is exposed to each day, Furthermore, the sun is always striking the tubes at angle which is perpendicular to their surface thus reducing reflection and making it effective at most times.