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solar evacuated tube

What is solar evacuated tube
Solar vacuum evacuated tubes is the core component of vacuum tube solar water heater,
Being known as the "heart" of the evacuated tubes solar water heaters, the solar vacuum evacuated tubes quality will directly affect the life and performance of vacuum tubes solar water heaters. The rapid development of solar industries has also drive the development of solar vacuum evacuated tubes enterprises. One is the patented technology of the Solar vacuum evacuated tubes continuously renew .on the other hand, solar vacuum evacuated tubes business and production lines is increasing in recent years like mushroomed after raining.

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 already been used for years in Germany, Canada, China and the UK. There are several types of evacuated tubes in use in the solar industry. Newfriend collectors use the most common "twin-glass tube". This type of tube is chosen for its reliability, performance and low manufacturing cost.

Each evacuated tube consists of two glass tubes made from extremely strong borosilicate glass. The outer tube is transparent allowing light rays to pass through with minimal reflection. The inner tube is coated with a special selective coating (Al-N/Al) 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.
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).

In order to maintain the vacuum between the two glass layers, a barium getter is used (the same as in television tubes). During manufacture of the evacuated tube 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 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 the vacuum is ever lost. This makes it easy to determine whether or not a tube is in good condition.

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. To learn more about insolation and the average values for your area .

Evacuated Tube Basic Specifications

Length (nominal) 1500mm /1800mm
Outer tube diameter 58mm
Inner tube diameter 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<5x10-3 Pa
Stagnation Temperature >200oC
Heat Loss <0.8W/ ( m2oC )
Maximum Strength 0.8MPa

 As the heart of evacuated tubes solar water heater, Solar vacuum evacuated tubes will be changed as the environment constantly changes. Because of regional differences and differences of target consumers and architectural differences require a effective combination of solar water heater and building due to the changing circumstances.