Glass evacuated tube Technique specification
Material: borosilicate glass according to ISO3585:1991;transmittance of borosilicate glass:τ≥0.92. Absorbance of solar selective absorptive coating:α≥0.94;Hemispherical emittance:ε≤0.08;Getter according to GB/T9505-1998.
Start-up temperature: no more than 25℃.
Start-up time: no longer than 2 minutes under normal sunshine.
Low temperature resistance: no breaks at -30℃
Heat impact resistance: no damages after over 3 times’ alternative impacts of cold water below 25℃ and hot water over 90℃.
Pressure resistance: can withstand 0.6 Mpa.
Hail resistance: no damage by the impact of ¢ 25mm hail.
Appearance standard: the color of the selective absorptive coatings should be even, and the coatings should have no wrinkles and peelings. The allowed tolerance of borosilicate glad tube’s diameter should be in accordance with ISO4803:1978. camber of the borosilicate glass tube should be no more than 0.3%.
The translation the borosilicate glass tube, which is 40-60mm 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 data
Model Glass vacuum tube
Glass material High-quality borosilicate glass 33
Thermal expansion 3.3x10-6 oC
Glass tube diameter ¢47mm
Inner Glass tube diameter 37mm
Thickness of vacuum ¢5mm
Length 1500mm or 1800 mm
Glass Wall thickness 1.6 mm
Single tube weight 1.5kg
High vacuum, long-term stability 5×10-3 Pa
Absorber material Aluminum
Absorptive Coating Aluminum Nitride
Absorption coefficient α≥0.94
Wind resistance 30m/s
Emittance <8% (80oC)
Heat Loss <0.8W/ ( m2oC )
Maximum Strength 0.6MPa
Freezing tolerance -30 oC
Hail resistance ¢ 25mm hail
Insolation temperature 250 oC
Start-up temperature ≤25 oC
Warranty 5 years

Glass evacuated tubes are the key component of the Solar heat pipe evacuated tube solar collectors. Each evacuated tube consists of two glass tubes. The outer tube is made of extremely strong transparent borosilicate glass, that is able to resist impact from hail up to 25mm in diameter. The inner tube is also made of borosilicate glass, but coated with a special selective coating (Al-N/Al) which features excellent solar heat absorption and minimal heat reflection properties. The air is withdrawn (evacuated) from the space between the two glass tubes to form a vacuum, which eliminates conductive and convective heat loss.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 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 outgassed from the 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 ever the vacuum is lost. This makes it easy to determine whether or not a tube is operating correctly. See picture below.
The Getter is located at the bottom of the outer tube. See right. Left Tube = Good --- Right Tube = FaultyIn the left tube two "arms" of the getter are visible. The right tube has had the inner tube and getter removed.

Unlike other types of solar collectors, evacuated tube solar collectors still provide excellent results on cloudy days. This is because the tubes are able to absorb the energy from infrared rays which can pass through clouds. Wind and low temperatures also have less of an effect on the function of evacuated tubes when compared to flat plate solar collectors due to the insulating properties of the vacuum.
Glass evacuated tubes are aligned in parallel, the angle of mounting depends upon the latitude of your location. In a North South orientation the tubes can passively track heat from the sun all day. In an East West orientation they can track the sun all year round. The shape of the tubes provides superior absorption when compared to flat plate collectors for a number of reasons:

1. As the tube is round, the sun's rays are always striking the tubes surface at right angles, thus minimizing reflection.

2. If the collector surface is flat, the amount of solar radiation striking the collector surface is only at its maximum at midday when the sun is directly above the collector. In the morning or afternoon the sun's rays strike the collector's surface at an angle, and thus the amount of solar radiation that the collector is exposed to is reduced. Evacuated tubes, however, are round, and thus the amount of solar radiation striking the collector is relatively constant from mid morning to mid afternoon. This feature maximizes the total amount of solar radiation the collector is exposed to each day. Furthermore, the sun is always striking the tubes at an angle which is perpendicular to their surface thus reducing reflection. Any sunlight that is reflected off the glass surface or that passes between the tubes is reflected back to the underside of the tube by the reflective backing panel. The tubes exposure to, and subsequent absorption of solar radiation is therefore maximized. See diagram below.

The efficiency of a solar collector is dependent upon a number of factors, the main one being the level of solar radiation (insolation) in your region.
Heat Pipe Evacuated Tube System
The heat pipe evacuated tube consists of the glass evacuated tube described above. In addition a copper heat pipe is installed within the tube. The copper heat pipe transmits heat to its tip which is pl ed into the collector's heat transfer manifold. As water runs through the manifold heat is transferred from the copper heat pipe to the water. The heat pipe is a very efficient means of of transferring heat from within the evacuated tube to the water. The following diagram shows both the glass evacuated tubes and the copper heat pipe. The heat pipe is simply inserted into the glass tube, held in place with high quality silicon based glue. There is no need for a air tight seal between the glass tube and heat pipe, in fact air must be able to move in and out of the tube as the air inside heats up and cools down.

The heat pipe transfers the heat to the manifold by a very simple method. The copper heat pipe is hollow and contains acetone. The hollow centre of the heat pipe is a vacuum, so that at even at temperatures of around 40oC the acetone will vaporize (boil). The vapor rises to the tip (condenser) of the heat pipe where the heat is transferred to the water flowing through the manifold. The loss of heat causes the vapor to condense and flow back down the heat pipe where the process is once again repeated. This method of heat transfer is thousands of times more efficient than a solid copper rod. Heat is therefore very efficiently transferred from the glass evacuated tube to the water.