CYLINDRICAL PARABOLIC CONCENTRATING COLLECTOR
The cylindrical parabolic collector (CPC) is also referred to a parabolic trough or a Linear parabolic collector is shown on previews figure 4.29(c). The basic elements making up a conventional collector are the absorber tube located at the focal axis through which the liquid to be heated flows, the concentric transparent cover, the reflector, and the support structure. Element and together constitute the receiver, while element and constitute the concentrator.
The collector are available over a wide range of aperture areas from about 1 to 60 cm2 and with widths ranging from 1 to 6m. Concentration ratios range from 10 to 80, and rim 70 to 120, as shown in figure.
The absorber tube is usually made of stainless steel or copper and has a diameter of 2.5 to 5cm. It is coated with a heat resistant black point and is generally surrounded by a concentric glass cover with an annular gap of 1 or 2cm. In the case of high performance collectors, the absorber tube is coated with a selective surface and the space between the tube and the glass cover is evacuated. In some small collectors, the concentric cover is replaced by a glass or plastic sheet covering the whole aperture area of the collector. Such an arrangement helps in protecting the reflection surface from the weather.
The liquid heated in the collector depends upon the temperature required. Usually organic heat transfer liquids (referred to as thermic fluids) are used. Because of their low thermal conductivities, these liquids yield low heat transfer coefficients. Augmentative devices in the form of twisted taps or central plugs (Which create annular passages) are therefore used to increase the value of the heat transfer fluid.
The reflecting surface is generally curved back silvered glass. It is fixed on light-weight structure usually made of aluminium sections. The proper design of this supporting structure and of the system for its movement is important, since it influences the shape and orientation of the reflecting surface. Some of the factors to be considered in designing the structure are that it should not distort significantly due to its own weight and it should be able to withstand wind loads.
Compared to flat-plate collectors, there are very few manufactures of concentrating collectors all over the world. The volume of production is also low. In India, many experimental collectors have been built and tested. However , commercial manufacture has not yet begun.
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