Vacuum Tube

Evacuated tubes are the absorber of the solar water heater. They absorb solar energy and convert it into heat for use in water heating. Each evacuated tube consists of two glass tubes made from extremely strong borosilicate glass. The outer tube is transparent allowing light rays to pass through with minimal reflection. The inner tube is coated with a special selective coating (Al-N/Al) which features excellent solar radiation absorption and minimal reflection properties. 

The top of the two tubes are fused together and the air contained in the space between the two layers of glass is pumped out while exposing the tube to high temperatures. This "evacuation" of the glass forms a vacuum, which is an important factor in the performance of the evacuated tubes. The vacuum in the evacuated glass tube is less than 5 x 10-3Pa. This can only be reached and maintained over a long period of time through a specialized evacuation process in production resulting in an almost total elimination of convection and conduction losses from the tube.