What is Evacuated Tube?

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 the solar industry. Ejai collectors use the most common “twin-glass tube”. These types of tube are chosen for its reliability, performance and low manufacturing cost.

Each evacuated tubes 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 special selective coating (AI-N/AI) 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 temperature. 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 150°C/304°F, the outer tube is cold to touch. This means the evacuate 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 temperature 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 colored 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 Getter is located at the bottom of the evacuated tube Left Tube = Vacuum Present
Right Tube = Faulty

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.

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



Products
Split Series
Solar Collector
Balcony Series
Integrated Pressurization
Integrated Non-Pressurization
Pre-Heated System
Evacuated Tube
Accessories
Solar Basics
How does solar help the environment?
What is Evacuated Tube?
What is Heat Pipe?
What is Insolation?
What is insolation in my area?
How much energy can solar collector Produce?
Installation
Ejai Installation Guide


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