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  Glass Evacuated Tubes
Glass evacuated tubes are the key component of solar collectors systems. Each evacuated tube consists of two glass tubes.

The outer tube is made of extremely strong transparent borosilicate glass, that is able to resist impact from hail up to 25mm in diameter.

The inner tube is also made of borosilicate glass, but coated with a special selective coating (Al-N/Al) which features excellent solar heat absorption and minimal heat reflection properties.
   
 

The air is withdrawn (evacuated) from the space between the two glass tubes to form a vacuum, which eliminates conductive and convective heat loss.

Unlike some other types of solar collectors, evacuated tube solar collectors still provide excellent results on cloudy days. This is because the tubes are able to absorb the energy from infrared rays which can pass through clouds. Wind and low temperatures also have less of an effect on the function of evacuated tubes when compared to flat plate solar collectors due to the insulating properties of the vacuum.

In order to maintain the vacuum between the two glass layers, a barium getter is used (the same as in television tubes). During manufacture 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 outgassed from the 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 ever the vacuum is lost.

Glass 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 shape of the tubes provides superior absorption when compared to flat plate collectors for a number of reasons:

   
 
  1. As the tube is round, the sun's rays are always striking the tubes surface at right angles,thus minimizing reflection.
   
 
2. If the collector surface is flat, the amount of solar radiation striking the collector surface is only at its maximum at midday when the sun is directly above the collector. In the morning or afternoon the sun's rays strike the collector's surface at an angle, and thus the amount of solar radiation that the collector is exposed to is reduced.
Evacuated tubes, however, are round, and thus the amount of solar radiation striking the collector is relatively constant from mid morning to mid afternoon. This feature maximizes the total amount of solar radiation the collector is exposed to each day. Furthermore, the sun is always striking the tubes at an angle which is perpendicular to their surface thus reducing reflection
   
 
   
 

The inorganic heat pipe evacuated tube consists of the glass evacuated tube described above. In addition a copper heat pipe is installed within the tube. The copper heat pipe transfers from within the evacuated tube to its tip (condenser) which is plugged into the header pipe contained in the manifold. As water runs through the header pipe, heat is transferred from the copper heat pipe to the water.The diagram to the left shows the glass evacuated tube, copper heat pipe, and contoured aluminium heat transfer fins. The heat pipe is simply inserted into the glass tube, sandwiched between the two aluminium fins. The fins are molded to maximize contact area with both the heat pipe and the inside surface of the evacuated tube. This fin design greatly improves heat transfer to the copper heat pipe, and ultimately the water in the manifold.The heat pipe transfers heat to the manifold by a very simple method. The copper heat pipe is hollow and contains a patented inorganic non-toxic heat-transfer liquid. The hollow centre of the heat pipe is a vacuum, so that at even at temperatures of around 25-30oC the compound will vaporize.

   
  When heated the vapor rises to the tip (condenser) of the heat pipe where the heat is transferred to the water flowing through the manifold. The loss of heat causes the vapor to condense and flow back down the heat pipe where the process is once again repeated. Heat pipes using inorganic heat transfer compounds exhibit heat transfer performance that is up to 3,000 times that of silver
   
  Using inorganic heat pipes, heat is very efficiently transferred from the glass evacuated tube to the water. Heat pipes are widely used in many applications including air conditioning and computers (CPU cooling), as they provide reliable long term performance.
   
 

The collectors that we offer use heat pipes from a reputable manufacturer using a US patented inorganic heat transfer compound. This compound is nontoxic if ingested and does not irritate either eyes or skin. Even so, during normal use, the heat pipe is never in direct contact with the liquid circulating through the header pipe.

Inorganic heat pipes have a vacuum level of 4 x 10-6 Pa, which allows the heat transfer compound to vapourise at low temperatures of 20-25°C.

   
 
Length
1500 mm
Outer tube diameter
47 mm
Inner tube diameter
37 mm
Glass thickness
1.6 mm
Thermal expansion
3.3x10-6 oC
Material
Borosilicate Glass 3.3
Absorptive Coating
Graded Al-N/Al
Absorptance
93%
Emittance
7% (100oC)
Vacuum
P<0.005 Pa
Stagnation Temperature
>200oC
Heat Loss
<0.8W/ ( m2oC )
Tube Life
>15 years
Warranty
6 years
   
   
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