TYPES OF CONCENTRATING COLLECTORS
The different types of collector geometries are shown in figure 4.29.
The first type shown in figure 4.29(a) is a flat-plate collector with adjustable minors at the edges to reflect radiation on to the absorber plate, It is simple in design, has a concentration ratio a little above unity and is useful for giving temperatures about 20oC or 30 oC higher than those obtained from a flat-plate alone.
A compound parabolic concentrating collector (CPC) is shown in figure. The concentrator consists of curved segments which are parts of two parabolas. Like the first type, this collector is also non-imaging. The type concentration ratio is moderate and imaging. The concentration ration is moderate and generally ranges from 3 to 10. The main advantage of the compound parabolic collector is that it has a high acceptance angle and consequently requires only occasional tracking. In addition, its concentration ratio is equal to the maximum value possible for a given acceptance angle.
The next type collector (figure 4.29(c)) is a cylindrical parabolic collector in which the image is formed on the focal axis of the parabola. Many commercial versions of this type are now available.
Unlike the cylindrical parabolic collector in which the concentration has to rotate in order to track the sun, the type shown in figure 4.29(d) has a fixed concentrator and a moving receiver. The concentrator is an array of long, narrow, flat mirror strips fixed along the cylindrical surface. The strip produce a narrow line image which follows a circular path as the sun moves. This path is on the same circle on which the mirror strips are fixed. Thus, the receiver has to be moved along the circular path in order to track the sun.
Concentration is also achieved by using lenses. The most commonly used device is the Fresnel lens shown in figure 4.29(e). The one shown in the figure is a thin sheet, flat on one side with fine longitude grooves on the other. The angles of these grooves are such that radiation is brought to a line focus. The lens is usually made of extruded acrylic plastic sheets. Line focussing collectors like the one shown in figure 4.29(c,d,e). Usually have concentration rations between 10 to 80 and yield temperature between 150 to 400.
In order to achieve higher concentration ratios and temperatures it becomes necessary to have point focussing rather than line focussing. A sketch is shown in figure4.29(f). Such collectors can have concentration ratios ranging from 100 to a few thousand and have yielded temperatures upto 2000o. However, from the point of view of the mechanical design, there are limitations to the size of the concentrator and hence the amount of energy which can be collected by one dish. Commercial versions have been built with dish diameter up to 17m. In order to collect larger amounts of energy at one point, the central receiver concept has been adopted. In this case, beam radiation is reflected from a number of independently controlled mirrors called ‘Heliostats’ to a central receiver located at the top of a tower.
“Rabl” has show that for a given acceptance angle (2qa) the maximum possible concentration ratio of a two-dimensional (line-focus) concentrator is
For a three dimensional (point - focus) concentrator, has shown that,
The half-angle subtended by the earth is 0.267o. Substituting this value in equation 4.104 and 4.105, we see that the maximum value of concentration ratio for a line-focus concentration is 215 and for a point-focus concentrator, it is 46000. In actual system, the values of concentration ratio are much lower, since the acceptance angle needs to be greater than 0.267o for a number of reasons. These include tracking errors, imperfections in the reflecting or refracting component of the concentrator, mechanical misalignment between the concentrator and the receiver etc.
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