Concentrating Collectors

CONCENTRATING COLLECTORS

INTRODUTION

Concentration of solar radiation is achieved using a reflecting arrangement of mirrors or a reflecting arrangement of lenses. The optical system directs the solar radiation on to an absorber of smaller area which is usually surrounded by a transparent cover. Because of optical system, certain losses (in addition to those which occur while the radiation is transmitted through the cover) are introduced. These include reflection or adsorption losses in the mirrors or lenses, and losses due to geometrical inperfections in the optical system. The combined effect of all such losses is indicated through the introduction of a term is called “optical efficiency”. The introduction of more optical losses is compensated for by the fact that the flux incident on the absorber surface is concentrated on a smaller area. As a result, the thermal loss terms do not dominate to the same extent as in a flat-plate collector and the collector efficiency is usually higher.

It has been noted earlier that some of the attractive features of a flat-plate collector are simplify of design and ease of maintenance. The same cannot be said of a concentrating collector. Because of the presence of an optical system, a concentrating collector usually has to follow or “track” the sun so that the beam radiation is directed on to the absorber surface. The method of tracking adopted and the precision with which it has to be done varies considerably. In collector giving a low degree of concentration, it is often adequate to make one or two adjustment of the collector orientation everyday. These can be made manually. On the other hand, with collector giving a high degree of concentration, it is necessary to make continuous adjustment of the collector orientation. The need for some form of tracking introduces a certain amount of complexity in the design. Maintenance requirements are also increased. All these factors add to the cost. An added disadvantages is the fact that much of the diffuse radiation is lost because it does not focussed.

In the last few years, significant advances have been made in the development of concentrating collectors and a number of types have been commercialised abroad. Almost all of them are line focussing Cylindrical Parabolic Collectors, and yield temperature upto 400