Solar Evacuated Glass Tube

Descriptions of Solar Evacuated Glass Tube
A evacuated tube or vacuum tube, known as a main component of evacuated tube solar collector, is mainly made of double-walled glass, namely inner tube, outer tube.
Two glass tubes are closed on one side and sealed with each other on the other side so that an evacuated space is formed.
The inner tube is coated with aluminium nitrite, a special selective material, which serves to absorb radiation falling on it.
To maintain the vacuum between the two glass tubes during manufacture, a barium getter (see below) is used at the bottom of the glass tube. High temperature produced during manufacture will cause tube bottom to be coated with a barium layer, which provides a visual indicator of the vacuum status. The silver coloured barium layer will turn white if the vacuum is lost. Based upon this, we can easily determine whether or not a tube is operating correctly. See right picture below.
Main Advantages of Solar Evacuated Glass Tube
-Good hail resistance: the rigid borosilicate glass outer tube can withstand impact from hail up to 30mm in diameter
-The superior insulating properties of the vacuum enable it to absorb the radiations from the sun, even it works on cloudy days since the tubes are able to absorb the energy from infrared rays which can pass through clouds.
-Superior absorption: the round profile of tube enables the rays to reach the collector directly and therefore the energy can be constantly absorbed; the inner tube is coated with a special selective coating featuring excellent soar heat absorption and minimal heat reflection properties.
Main Applications of Solar Evacuated Glass Tube
As an indispensable component of Glass Evacuated Solar Collector, it mainly serves to provide heat for domestic use.
Technical Specification of Glass Evacuated Tube
Material: borosilicate glass 3.3 of ISO3585:1991standard; Transmittance of borosilicate glass: τ≥0.92. Absorbance of solar selective absorptive coating: α≥0.92; Hemispherical emittance:ε≤0.08
Start-up temperature: ≤25℃.
Start-up time: ≤2 minutes under normal sunshine.
Low temperature resistance: Max -30℃
Heat impact resistance: over 3 times’ alternative impacts of cold water below 25℃ and hot water over 90℃.
Pressure resistance: Max 0.8 Mpa.
Hail resistance: max ¢ 25mm hailstone
Absorber Area : the total of the 360% absorber surfaces of the inner coated vacuum tube
Appearance standard: the selective absorptive coatings should be even and intact. The permissible tolerance of borosilicate glad tube’s diameter should follow by ISO4803:1978. The camber of the borosilicate glass tube should be less than 0.3%.
The translation the borosilicate glass tube, a distance of 40-45mm away from flange end, should be round. The ratio of the longest to the shortest radial dimensions of the borosilicate glass tube should be no more than 1.02.
Technical Parameters (TYY-AA series vacuum tubes)

Model	Glass vacuum tubes
Glass Material	High-quality borosilicate glass 33
Thermal Expansion	3.3x10-6 ℃
Glass Tube Diameter	¢47mm / 58mm
Inner Glass tube diameter	37mm / 47mm
Thickness of Vacuum	¢5mm
Length	1500mm ,1800 mm
Glass Wall thickness	1.6 mm / 1.8mm
Single Tube weight	1.35kg ; 2.0kg
High Vacuum, long-term stability	5×10-3 Pa
Absorber Material	Aluminum
Absorptive Coating	Aluminum Nitride
Absorption Coefficient	α 0.90-0.93
Wind Resistance	30m/s
Emittance	<8% (80℃)
Heat Loss	<0.8W/ ( m2℃ )
Maximum Strength	0.8MPa
Freezing Tolerance	-30 ℃
Hail Resistance	¢ 30mm hail
Insolation Temperature	250 ℃
Start-up Temperature	≤25 ℃
Life Time (years)	15 years