AP Solar Hot Water Collector - Solar Tubes
Solar tubes are the absorber of
the solar water heater. They absorb solar energy converting
it into heat for use in water heating. Solar tubes are also
referred to as evacuated tubes, as the space between the two
glass layers is evacuated to form a vacuum. Solar tubes have
already been used for years in Germany, Canada, China and
the UK. There are several types of solar tubes in use in the
solar industry. Apricus collectors use the most common
"twin-glass tube". This type of tube is chosen for its
reliability, performance and low manufacturing cost.
Each solar 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 solar 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 solar tube absorbs the radiation from the sun and converts it to heat, we don't won't to loose 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 solar 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 solar 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 solar 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. See picture below.
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 solar tube absorbs the radiation from the sun and converts it to heat, we don't won't to loose 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 solar 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 solar 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 solar 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. See picture below.
 The Getter is located at the bottom of the solar tube. |
Left Tube = Vacuum
Present |
Solar 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 solar water heater is dependent upon a number of factors, one important one being the level of solar radiation (insolation) in your region. To learn more about insolation and the average values for your area click here.
Solar 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
|
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Last modified:
08/10/15