Evacuated glass tube
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Basic specifications
Length | : | 1500mm/1800mm |
Outer tube | : | Ø58mm |
Inner tube | : | Ø47mm |
Glass thickness | : | 1.6mm |
Thermal expansion | : | 3.3x10-6 oC |
Material | : | Borosilicate Glass 3.3 |
Absorptive coating | : | Graded Al-N/Al |
Emittance | : | <8% (80oC) |
Absorptance | : | >92% (AM1.5) |
Vacuum | : | P<5x10-3 Pa |
Stagnation temperature | : | >200oC |
Heat loss | : | <0.8W/ ( m2oC ) |
Max strength | : | 0.8MPa |
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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 gasses forms a vacuum, which is an important factor in the performance of the evacuated tubes.
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Why a vacuum? As you would know if you have used a glass 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 150oC / 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.
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In order to maintain the vacuum between the two glass layers, a barium getter is used. During manufacture of the evacuated tube this getter is exposed to high temperatures which cause 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.
