Vacuum Tube (LQ-03)
Each evacuated tube consist 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 layer of glass is pumped out while exposing the tube to high temperatures, This "evacuation " of the gasses form a vacuum, which is an important factor in the performance of the evacueted tubes
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Why a vacuum? As you would know if you have used a glas lined thermos flask, a vacuum is an excellent insulator, This is important because once the evacuated tube absorbs the radiation from the sun and converts it to heat, we don't want to lose it! The vacuum helps to achieve this, The insulation properties are so good that while the inside of the tube may be 150º C / 304oF, the outer tube is cold to touch, This means that evacuated tube water heaters can perform well even in cold weather when flat plate collectors perform poorly due to heat loss (during high Delta-Tconditions)
In order to maintain the vacuum between the two glass layer, a barium getter is used (the same as in television tubes ) Durin manufacture of the evacuated tube this getter is exposed to high temperatures which causes the bottom of the evacuated tube to be coated with a pure layer of barium, This barium layer actively absorbs any CO, CO2, N2, O2, H2O and H2 out-gassed from the evacuated tube during storage and operation thus helping to maintaining the vacuum, The barium layer also provides a clear visual indicator of the vacuum status, The silver coloured barium layer will turn white if the vacuum is ever lost, This makes it easy to determine wheather or not a tube is in good condition,
Evacuated tube basic specifications
Length(nominal)  1500mm/1800mm
Outer tube diameter  58mm
Inner tube diameter  47mm
Glass thickness  1.6mm
Thermal expansion  3.3*10-6º C
Material  Borosilicate Glass3.3
Absorptive Coating  Graded Al-N/Al
Absorptance  >92%(AM1.5)
Emittance  <8%(80º C)
Vacuum  P<5*10-3Pa
Stagnation Temperature  >200º C
Heat Loss  <0.8W/(m2º C)
Maximum Strength  0.8MPa