High absorption, low reflection

Evacuated tubes minimize the re-radiation of infrared energy from the collectors, allowing them to reach considerably higher temperatures than most flat-plate collectors. For this reason they can perform well in colder conditions. A further advantage this design has over the flat-plate type is that the constant profile of the round tube means that the collector is always perpendicular to the sun's rays and therefore the energy absorbed is approximately constant over the course of a day.

 

Borosilicate Glass

An evacuated tube collector consists of rows of glass tubes connected to a header pipe. Each tube has the air removed from it to eliminate heat loss through convection and radiation. This double wall vacuum tube is made of two concentric transparent borosilicate glass tubes. Borosilicate glass was invented by Corning in 1915 and is marketed under the trade name Pyrex used in glass cookware. This material has a very high thermal expansion coefficient able to withstand large temperature fluctuations. Extremely strong, it is able to resist impact from hail up to 25mm in diameter. Borosilicate has natural low-emissivity (Low-E) properties. Metal oxides and borosolicates are often used on glass in Low-E windows to reflect thermal radiation back into the interior, and allow solar radiation into the room.

 

Selective Coating

The inner glass tube is coated with a selective coating of AL-N-AL (Aluminum Nitride). The selective coating has a high absorptivity rate, meaning the amount of solar radiation transformed to heat, and a low emissivity rate, meaning the relative amount of heat loss by heat emission (long-wave radiation). Such coatings make it possible to absorb and transform a large part of the short-wave solar radiation into heat, and, simultaneously, to reduce the emission of the long-wave heat radiation from the absorber itself significantly.

 

Barium Getter

To prevent any remaining gases from remaining in a free state in the evacuated tube, vacuum tubes are constructed with "getters", which are metals that oxidize quickly, the most common being barium. This is the same technology used in the manufacture of television picture tubes. Once the double wall tube is evacuated and sealed, the getter is heated to a high temperature causing the material to evaporate, absorbing and reacting with any residual gases and leaving a silver-colored metallic layer on the inside bottom of the tube. If a tube develops a crack and loses its vacuum this layer turns a white color when it reacts with oxygen, turning to barium oxide.