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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 The air is withdrawn (evacuated) from the space between the two glass tubes to form a vacuum, which eliminates conductive and convective heat loss.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.

Unlike some 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. 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. To learn more about insolation and the average values for your area click here.
Evacuated Tube Basic Specifications:
Length 1500mm/1800mm
Outer tube diameter 47mm/58mm
Inner tube diameter 37mm/47mm
Glass thickness 1.6mm
Thermal expansion 3.3x10-6 oC
Material Borosilicate Glass 3.3
Absorptive Coating Graded Al-N/Al
Absorptance >92% (AM1.5)
Emittance <8% (80oC)
Vacuum P<5x10-3Pa
Stagnation Temperature >200oC
Heat Loss <0.8W/ ( m2oC )
Maximum Strength 0.8MPa
Warranty 5 years