Evacuated tubes are the most improtant part of the solar water heater which works as an absorber. It absorbs solar energy and converting it into heat to transfor the heat into water.There are several types of evacuated tubes in use in the solar industry. It designs and the twin-glass tube".which is similar to a conventional Dewar flask.

Each evacuated tube consists of two borosilicate glass . the inner tube is coated with a sputtered solar selective coating (Al-N/Al) , this coated inner tube is closed at one end and sealed at the other end to the outer tube.the annular space between the outer tube and inner tube is evacuated to virtualy eliminate heat loss by conduction and convection. And the outer tube is transparent allowing light rays to pass through with minimal reflection. This "evacuation" of the gasses forms a vacuum, which is an important factor in the performance of the evacuated tubes.

Basic information of the vacuum tube:
1.Why a vacuum? As you would know if you have used a glass lined thermos flask, a vacuum is an excellent insulator. This is important because once the evacuated tube absorbs the radiation from the sun and converts it to heat, we don’t want to lose it!! The vacuum helps to achieve this. The insulation properties are so good that while the inside of the tube may be 150oC / 304oF , the outer tube is cold to touch. This means that evacuated tube water heaters can perform well even in cold weather when flat plate collectors perform poorly due to heat loss (during high Delta-T conditions).
2.In order to maintain the vacuum between the two glass layers, a barium getter is used. During manufacture of the evacuated tube of 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 out-gassed from the evacuated 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 coloured barium layer will turn white if the vacuum is ever lost. This makes it easy to determine whether or not a tube is in good condition. See picture below.



The difference between the ordinary vacuum tube and the Three-high vacuum tube is the coating of its inner tank ,the first coating of the three-high vauum is aluminium ,the second is stainless and the last is copper, while the ordinary vacuum tube is aluminum.

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 efficiency of a evacuated water heater is dependent upon a number of factors, one important one being the level of evacuated radiation (insolation) in your region.