Evacuated Tubes
Evacuated tubes are the absorber of the solar water heater. They absorb solar energy converting it into heat for use in water heating. Evacuated tubes have been in use in Germany, Canada, China and the UK, for many years. There are several types of evacuated tubes in use in the solar industry, the most common systems being “twin-glass tube”; which uses a sealed heat-pipe on each tube to extract heat from the absorber. A liquid is vaporised while in contact with the heated absorber, heat is recovered at the top of the tube while the vapour condenses, and condensate returns by gravity to the absorber. This type of tube is chosen for its reliability, performance and low manufacturing cost.
Evacuated tubes consist of two extremely strong borosilicate glass tubes. The outer tube is transparent allowing light rays to pass through with minimal reflection. The inner tube is coated with a special selective coating, which features excellent solar radiation absorption and minimal reflection properties. The top of the two tubes are fused together and the air contained in the space between the two layers of glass is pumped out while exposing the tube to high temperatures. This "evacuation" of the gasses forms a vacuum, which is an important factor in the performance of the evacuated tubes, vacuum being an outstanding insulator, there is no heat loss.  Graphic of showing heat pipe process. copyright Apricus Evacuated tubes are aligned in parallel; the angle of mounting depends upon the latitude of your location. Efficiency is dependent upon a number of factors one important one being the level of evacuated radiation (isolation) in your region. |
