Non-tracking Concentrators Classification and Working Principles

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. 

1. One axis tracking concentrators

2. Two-axes tracking Concentrators  

 Non Tracking Solar concentrators are mainly of 4 types. These are described below: 

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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