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.
Why a vacuum? As you would know if you have used a glass lined thermos flask, a vacuum is an excellent insulator. The vacuum eliminates a physical connection between the two glass layers of the tube which means there is nothing to transfer thermal energy so the heat cannot escape! 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 remains within a few degrees of the ambient air temperature. This means that evacuated tube water heaters can perform well even in cold weather, when flat plate collectors perform poorly in these conditions due to heat loss.
In order to maintain the vacuum between the two glass layers, a barium getter is used (the same as in television tubes). During manufacture of the evacuated tube this getter is exposed to high temperatures causing the bottom of the evacuated tube to be coated with a layer of pure barium. Called a "getter", this barium layer actively absorbs any CO, CO2, N2, O2, H2O and H2 out-gassed from the evacuated tube during it's lifetime and increases the longevity of the vacuum. This barium layer also provides a clear visual indication of the vacuum status; the silver-colored barium layer turns white if the vacuum is lost making it easy to identify.
Evacuated Tube
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Name
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Single target
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Double target
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Three target
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Material
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High borosilicate glass 3.3
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CTE
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3.3×10-6/k
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Thickness of tube
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1.6mm—2.5mm
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Absorptance
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≥0.92
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≥0.91
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≥0.95
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Emittance
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≤0.07
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≤0.05
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≤0.05
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Vacuum Tightness
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P≤3×10-2Pa
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Anti-mechanical
shock
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No damage under the shock of 30mm steel ball and the height of 450mm
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Quality experiment
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Mirror surface loss no less than 50% under the 350℃ in the 48 hours
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Pressurization
experiment
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Keep one minute under the 0.6MPa
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