Solar concentrators may be classified as (i) tracking type and (ii) nontracking type. Tracking may be continuous or intermittent. It may be of one-axis or two-axes design. The Sun may be followed by moving either the focusing part or the receiver or both. Solar concentrators may also be classified on the basis of optical components. They may be (i) reflecting or refracting type, (ii) imaging or nonimaging type, and (iii) line focusing or point focusing type.
There are a number of methods by which the flux of radiation on receivers can be increased. A few of them are described below:
A cylindrical parabolic trough is a conventional optical imaging device used as a solar concentrator. It consists of a cylindrical parabolic reflector and a metal tube receiver at a focal plane (Figure 2.19). The receiver is selective/blackened at the outside surface. It is covered by a cylindrical glass tube. It is rotated about one axis to track the Sun’s diurnal motion. The working fluid flows through the absorber tube, gets heated and thus carries away the thermal energy.
The aperture diameter, rim angle, absorber size and shape may be used to define the solar concentrator. The absorber tube may be made of mild steel/copper. Depending on the temperature requirement different heat transfer liquids may be used. Reflectors may be of anodised aluminium sheet, aluminised Mylar or curved silvered glass. Since it is difficult to curve a very large glass, mirror strips are sometimes used in the shape of a parabolic cylinder. The reflecting part having high reflectance is fixed on a lightweight structure. A cylindrical parabolic trough may be oriented in any of three directions: East-West, North-South or polar. The concentration ratio for a cylindrical absorber varies from 5 to 30. Such solar concentrators have been in use for many years.