The inner tube is also made of borosilicate glass, but coated with a special selective coating (AI-N/AI) 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 The air is withdrawn (evacuated) from the space between the two glass tubes to form a vacuum.

This eliminates conductive and convective heat loss.

In order to maintain the vacuum between the two glass layers, a barium getter is used. During manufacture this getter is exposed to high temperatures which cause the bottom of the evacuation tube to be coated with a pure layer of barium. This barium layer actively absorbs any CO, CO2, N2, O2, H2O and H2 passed out from the tube during storage and operation, thus helping to maintain the vacuum. The barium layer also provides a clear visual indicator of the vacuum status. The silver colour barium layer will turn white if ever the vacuum is lost. This makes it simple to determine whether or not a tube is operating correctly.

Unlike some other types of solar collectors, evacuation tube solar collectors still provide excellent results on cloudy days.

The reason the tubes are able to absorb the energy from infrared rays which are able to pass through clouds. Wind and low temperatures also have less effect on the function of evacuation tubes when compared with flat plate solar collectors due to the insulating properties of the vacuum. Glass evacuation 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 with flat plate collectors for a number of reasons:

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

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, thus reducing the amount of solar radiation that the collector is exposed to.

Evacuation tubes, however, are circular, and thus the amount of solar radiation striking the collector is relatively constant from morning to 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 perpendicular to their surface thus reducing reflection. The efficiency of a solar collector is dependent upon a number of factors, the main one being the level of solar radiation (insulation) in your region.