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All-glass
evacuated solar collector tubes are the key component of
solar water heaters and solar collectors. 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withdrawn (evacuated) from the space between the two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.
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 vacuum 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 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 ever the vacuum is lost.
This makes it easy to determine whether or not a tube is
operating correctly.
All-glass
evacuated solar collector tubes, solar water heaters and
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.
All-glass evacuated solar collector 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.
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