Evacuated vacuum tube

The tubes are made from low emissivity borosilicate glass (glass with a very low iron content that has superior durability and heat resistance ) with an all-glass seal and they employ AL/N on AL selective coating, which enables the use of the whole solar energy spectrum to generate heat; this produces greater thermal efficiency in bright sunshine but also produces high efficiency in overcast or diffuse sunlight conditions. Further, the tubes are evacuated and have a barium getter (vacuum indicator) which changes color from silver to white if a tube’s vacuum has been compromised.
An examination of the tubes shows that the outside is actually 2 layers of glass and a vacuum has been created between them. A good way to demonstrate this would be to fill an empty tube with very hot water and notice that it does not even get warm as you hold the tube in your hands. That is because of the “thermos effect” created by having a vacuum between the layers of glass.
Evacuated-tube collectors can achieve extremely high temperatures (170°F to 350°F), making them more appropriate for cooling applications and commercial and industrial application. The collectors are usually made of parallel rows of transparent glass tubes. Each tube contains a glass outer tube and metal absorber tube attached to a fin. The fin is covered with a coating that absorbs solar energy well, but which inhibits radiative heat loss. Air is removed, or evacuated, from the space between the two glass tubes to form a vacuum, which eliminates conductive and convective heat loss.