Focusing Type Concentrators - Deepakkumar Yadav

Focusing Type Concentrators

Various types of focusing type concentrators are :

1. Parabolic Trough Reflector :

This type is line focusing type collector. The cross-section of a parabolic through reflector is shown in Figure A. In this type of collector, the solar radiations falling on the area of the parabolic reflector are concentrated at the focus of parabola.
When the reflector is manufactured in the form of a trough with parabolic cross-section, the solar radiations gets focused along a line.
Mostly cylindrical parabolic concentrators are used in which the absorber is placed dong the focus axis. In this the collector pipe is used as an absorber with a selective coating.
Parabolic trough reflectors are usually made of highly polished or silvered glass or of a film of aluminized plastic on a firm base. Instead of the reflector having a continuous form, the reflector may be made of a large number of flat mirror strips on the parabolic firm base.
A parabolic trough or parabolic cylinder collector is shown in Figure B. In order that the solar radiations are always focused on a line with respect to changes in sun’s elevation by the parabolic reflector, either the trough or the collector pipe is rotated continuously about the axis of absorber.
The orientation of the trough type collectors is kept in the east-west or north-south directions.

Figure A

Figure B

2. Mirror Mirror Strip Reflector Type Concentrating Collector :

It is a line focusing type collector as shown in Figure C. It has the plane or slightly curved (concave) mirror strips mounted on a flat base.
The individual mirrors are placed at such angles that the reflected solar radiations fall on the same focal line where the absorber pipe is placed.
The collector pipe is strips rotated so that the reflected rays on the absorber remain focused with respect to changes in sun’s elevation.

Figure C

3. Fresnel Lens Concentrating Collector :

This is also a line focusing type solar collector. A Fresnel lens refracting type concentrating collector is shown in Figure D. In this collector a Fresnel lens which consists of fine, linear grooves on the surface of refracting material of optical quality on one side and flat on the other side is used.
The angle of each groove is so designed that the optical behaviour of the Fresnel lens is similar to spherical lens.
The solar radiations which fall normal to the lens are refracted by the lens and are focused on a line where the absorber (receiver) tube is placed to absorb solar radiations.
A Fresnel lens collector is generally made of acrylic plastic sheets having overall dimensions of 4.7 m in length and 0.05 m in width.

Figure D

4. Paraboloid Dish Collector :

It is a point focusing type collector. In these collectors, the receiver is placed at the focal point of the concentrator as shown in Figure E.
A typical paraboloid dish collector has a dish of 6 m diameter. It is made of about 200 curved mirror segments forming a paraboloidal surface.
The solar beam radiations are focused at a point where the receiver (absorber) is placed.
The absorber is a cavity made of zirconium-copper alloy with a black chrome selective coating.
This requires two axis, tracking. It can have concentration ratio ranging from 10 to few thousand. It can yield temperatures upto 3000°C.

Figure E