Tracking concentrators provide
high delivery temperatures but require accurate tracking device and fine
surface accuracies and hence are expensive. However, for medium temperature
operation, less expensive concentrators have been designed, without the
tracking required.
The
description of some of these concentrators is given below:
Classification of Non-Tracking Concentrators
Broad classification of the solar concentrators involves tracking and non-tracking types
Tracking Type Concentrators are mainly of two type. For having a descriptive knowledge of those type, please follow these links.
Flat receiver with booster mirror
Figure shows a flat receiver
with plane at the edges to reflect additional radiation into the receiver. Mirrors
are also called booster mirrors. The concentration ration of these
concentrators is relatively low, with a maximum value less than four. As the
solar incidence angle increases, the mirrors become less effective. For a
single collector, booster mirrors can be used on all the four sides. When the
sun angle exceeds the semi angle of booster mirrors, the mirror actually starts
casting shadow on the absorber. In case of an array of collectors, booster
mirrors can be used only on two sides. The efficiency of a boosted flat plate system
can be increased if the angle of the flat mirrors can be changed several times
during the year. The advantage of such a system is that it makes use of the
diffuse radiation in addition to the beam radiation. The attainable temperature
and collection efficiency will be higher than that of a flat plate collector of
the same collection area.
Tabor-Zeimer circular cylinder
Figure shows such a
concentrator. It is a very simple cylindrical optical system which consists of
an inflated plastic cylinder with a triangular pipe receiver. The cylinder has
a clear portion on the top to permit radiation to enter and fall on its rear
portion which is aluminized to act as a mirror. The incident radiation is
reflected by the mirror and is focussed on the absorber near the bottom of the
cylinder.
A concentration of about 3 can be
achieved without tracking. It can be placed along East-West axis and requires
only seasonal tracking. The concentrator uses, in addition to beam component,
some diffuse radiation. The delivery temperatures and collection efficiencies
are higher than that possible with an ordinary flat plate collector.
Compound parabolic concentrator
This concentrators is a
non-imaging one and belongs to a family of concentrator which has highest
possible concentration permissible by thermodynamic limit for a given
acceptance angle. Further, it has a large acceptance angle and needs to be
intermittently turned towards the sun.
The first design of a compound
parabolic concentrator (CPC) was found independently by Winston (1965) and
Baranov (1966). It consists of two parabolic segments, oriented such that focus
of one is located at the bottom end point of the other and vice versa
(Fig.8.14). The axes of the parabolic segments subtend an angle, equal to
acceptance angle, with the CPC axis, and the slope of the reflector surfaces at
he aperture plane are parallel to the CPC axis. The receiver is a flat surface
parallel to the aperture joining two foci of the reflecting surfaces. Rays incident in the central region of the
aperture undergo no reflection whereas those near the edges undergo one or more
reflections. The number of reflections depend on the incident angle, collector
depth and concentration ratio (Rabl, 1976). To reduce cost of the unit, the CPC
can be truncated in height to half, without any significant change in
concentration.
Extensive investigations on this
concentrator have led to several modified designs of the ideal CPC. The salient
modifications can be listed as follows:
- The use of receiver shapes such as fins, circular pipes for better optical thermal performance.
- Truncation of CPC height to reduce the physical size and cost.
- Asymmetric orientation of source and aperture to deliver seasonal varying outputs.
- Design of CPC as a second stage concentrator.
In view of the above modification the reflecting surface of all resulting concentrators may not be parabolic, but still belong to nonimaging group of concentrators.
The CPC can be used in a non tracking mode for concentration ratios of about 6. However for higher ratios the reflector surface area becomes very large and hence cannot be used.
V-Trough
Figure shows schematically
such a concentrator. It consists of highly reflecting side walls which reflect
solar rays to a receiver plate placed at the base of the trough. The trough is
aligned in East-West direction. So as to avoid diurnal tracking these
concentrators provide higher concentration (of the order of 3 in straight wall
case) than flat plate collectors with booster mirrors, because in the latter
case the acceptance angle is very large and so the concentration is low.
Different combinations of depth to base-width ratios and cone angle are
possible for optimum performance depending on the frequency of seasonal tilt adjustments. The average number of reflections in a
V-trough is more or less the same as that in a CPC. For low concentrations,
performance of both is comparable. For high concentrations, these appear
impractical. The performance of these can be improved by using more than one
mirror element in each side wall at suitable angle thus resulting in polygonal
troughs.
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