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Solar concentrators collect
light energy that reaches us directly from the sun as multitude of
rays. All this rays are concentrated in a line, the focus line of a
parable. In this line is placed a special tube, calle absorber tube,
the heart of this technology. The heat concentrated in the tube heat
the fluid that circulate inside of them, transforming the radiation in
thermal energy. The concentration ratio is about 82 times.
In
order to increase the efficiency, it is necessary to orientate the
parabolic trough in perpendicular direction of solar ray. The
solar collector is equiped with a hidraulic system that allow to
follows to the sun movement.
Figure 1. Parabolic trough collectors.
The main parts of a parabolic trough collector are:
1) Foundation.
Supports the collectors and fixes them to the floor so that the
structural assembly can support the loads for which is it designed,
usually of reinforced concrete. Are made according to the dimensions of
the collectors and the characterisitic structures, which are a function
of weight, wind loads and soil type.
2) Structure.
Its function is to stiffen the set of component parts. It is tipically made
of metal, but now are investigating other materials such as fiberglass,
plastics and even wood. It's important that the stucture is of quality
since any deformation along his life affect the concentration of light
and thus energy production.
Figure 2. Structure.
3) The parabolic trough reflector.
It is the concentrated part of the collector and consists of
reflecting the solar radiation incident on it and projecting it in a
concentrated form on the absorber tube. The reflectors used are mirrors
made of silver or aluminium applied on sheet metal, plastic or glass.
The mirrors ot be outdoors tend to get dirty so they must be cleaned,
which could reduce the yield, the main problem for cleaning the mirrors
are the delicate central tubes.
Figure 3. Reflector.
4) The absorber tube or receiver tube.
It is the responsible for converting concentrated sunlight into thermal
energy in the heat transfer fluid, mainly consists of two tubes, one
indoors of metal, coated with a special layer of black paint on the
basis of highly absorbent materials over 90% and low emissivity at high
temperatures and a transparent glass tube of high transmittance in the
solar interval. To join the two tubes should be used special seals
capable of withstanding the expansion. Also inside the tubes are
inserted 'Getters', which are responsible of absorbing the molecules of
substances that can penetrate between the metal tube and glass, to
maintain the vacuum.
Figure 4. Absorbent tube or receiver tube.
5) Transmission.
It is the solar tracking mechanism wich is responsible for changing the
position of the collector as the sun is moving, can be electric,
motor-reducer and hydraulic, the most common. Normally to lower cost
one single mechanism is responsible for moving twelve collectors in
serie.
Figure 5. Electrical transmission to the left and hydraulics to the right.
6)
The sun tracking system or 'suntracking system'.
This system is responsible for adjusting the position of the collector
so that the radiation received is maximum. To determine the position of the sub are use photocells
separated by a band of shadow, which in case of blur, produces a
tension that causes the motor turn or the pistons are moved
in desired direction. In addition to allowing the maximum use of
solar, the tracking system serves to get out of focus the mirror when
the captured energy is excessive
Figure 6. Solar sensor.
7) Connection between the collectors.
The collectors are connected in series in rows and these in turn are
attached parallel. These parts allow the fluid to circulate between the
modules, moving parts and circulation pipelines, fixed parts, etc. May
be of two types: ball joints or flexible pipelines.
Figure 7. Connecting joints between panels.
8) Heat transfer fluid.
They're in charge of absorbing solar energy in the tubes of the solar
field and transport it to the salt deposits. There are different types
of fluid used for this mission of them can highlight the
following:
- Water-steam.
Its advantages are: cheap, readily available, abundant, excellent heat
transfer medium, high specific heat, well-known properties and
behaviour, is not toxic and not flammable. Its disadvantages are that
it is aggressive, highly oxidizing, corrosion occurs, certain salts
precipitate producing scale, expands as it solidifies and pressure
increases strongly with temperature.
- Mixtures fo inorganic salts.
- Alkyl benzenes.
They are very stable, can withstand temperatures up to 300 ºC, do not
emit toxic gases, or corrosive and have a low freezing point between
-45 to -50 ºC.
- Mercury.
Very rarely used for reasons of toxicity and price, works up to
temperatures of 540 ºC, requires great vigilance to detect leakage of
vapors, as from 360 ºC requires pressure in the installation doing that
the costs are very high.
- Mixtures diphenyl and diphenyl oxide. It is the most common heat thermal fluid, becasue his high working
temperature (400ºC).
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