Evacuated Tubes vs. Flat Pannel Collectors
– Why Evacuated Tubes
All Glass evacuated tubes (see figure 1 below) are the key component of our Solar
Collectors
and Solar Water Heaters.
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,
which features excellent solar heat absorption and minimal heat reflection properties.
The air is evacuated from the space between the two glass tubes to form a vacuum,
which
eliminates conductive and convective heat loss. This is why the tubes are able
to
absorb the
energy from infrared rays which can pass through clouds. Wind and low temperatures
also
have less of 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 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 tubes exposure to, and
subsequent
absorption of solar radiation is
therefore maximized. See figure 2 below.
 
Figure 1 Figure 2
As can be seen in figure 3, our solar collector has a curve ( blue line) which is quite
different than
a flat panel collector ( green line). This is due to the cylindrical absorber
area, which passively tracks
the sun throughout the day. At 40-50 o there is no light lost between the tubes, and no
tube overlap, and reflection of neighbouring tube, hence a
peak in relative performance.
This is ideal, as during this period (mid morning through mid afternoon) solar isolation
levels are quite high. The peak at 70 o provided by the ET-reflect is of little benefit as
this angle corresponds to early morning or late afternoon when solar insolation levels
are very low. The flat plate collector's IAM values drop away from 1 as the angle
increases, and as such solar conversion efficiency is only at peak levels at midday.
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To understand how the tubes passively track the sun through
out the day, refer
to the diagram
to
the left.
When looking at the tubes from above
(0o) each tube's surface is clearly visible, and therefore exposed to the maximum amount
of sunlight. At this angle
however some light is lost between
the tubes, and therefore because
this
is used as a reference point
for
the IAM value of 1, when the
gaps close up, the IAM value
with actually increase (a
greater
% of light shining on the
collector
is actually being absorbed).
When the sun reaches an angle
of 40o which correlates to 2h 40
min before
or after midday, the solar tubes are
still fully visible
with no gaps between,
and no overlap.
It is at this point that
the pure
IAM values reach their peak.
The tubes are exposed to
all the sunlight shining towards them,
and all the tubes are still perpendicular to the sun. |
As the angle increases, the tubes start to overlap, shading each other. They are still
facing the sun, but the actual surface area of absorber exposed to the sunlight is
reduced. Only a small amount of sunlight falls beyond 40o (early morning and late
afternoon), and so this decrease in surface area has minimal influence on the total
daily energy output of the collector.
The IAM effect increases the heat output of the Tri Solar solar collector by as much as
25% when compared to a flat collector surface of the same absorber area and
performance variables. It is very important to consider IAM effect when calculating the
heat output of Tri Solar solar collectors and indeed when comparing to other solar
collectors.
Figure 3
When comparing peak efficiency levels, it may seem that there is little difference
between
flat plate and evacuated tubes. In fact, the efficiency of flat plate may be
higher, but this is under minimal heat loss conditions. When averaged over a year,
the evacuated tube solar collector has a clear advantage.
The key points are as follows:
1) Due to the cylindrical shape of the evacuated tube, the solar tubes are able to
passively track the sun throughout the day. Flat plate collectors only provide peak
energy output at midday when the sun is perpendicular to the collector's surface.
2) Air is evacuated from the tube to form a vacuum. This greatly reduces conductive
and convective heat loss from the interior of the tube. As a result, wind and cold temperatures have less effect on the efficiency of the evacuated tube collector.
3) Evacuated tube solar collectors can be used under cold temperatures when flat
plate collectors have limited or no heat output.
4) Evacuated tubes are strong, long lasting, and, should one be broken, inexpensive
and easy to replace. If a flat plate collector panel is damaged, the whole panel must be
replaced.
5) Due to the high efficiency absorption of solar radiation even during overcast
conditions, combined with excellent insulative properties of the solar tube, solar tube collectors can
heat water all the year around (backup from gas and electricity is still required).
6) Due to the various advantages of evacuated tube collectors over flat plate collectors,
a smaller collector can be used to provide the same heating performance. For example,
a standard household of 4-5 people would usually require a 250L / 66gallon water
storage tank. Depending on your location, only 30 evacuated tubes would be required
to provide
all summer hot water needs and a large percentage in other seasons.
7) Flat plate solar collectors can produce similar heat output as evacuated tube collectors, but generally only under hot, sunny conditions. When averaged over an entire year, the
heat output of evacuated tube collector per net m2 of absorber area, is about 25% to
40%, which is greater than that of flat plate collector.
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