The concentrator shown is a mirror reflector having

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a receiver. The concentrator shown is a mirror reflector having the shape of a cylindrical parabola. It focuses the sunlight on to its axis, where it is absorbed on the surface of the absorber tube and transferred to the fluid flowing through it. A concentric glass cover around the absorber tube helps in reducing the convective and radiative losses to the surroundings. In order that the sun's rays should always be focused on to the absorber tube, the concentrator has to be rotated. This movement is called tracking. In the case of cylindrical parabolic concentrators, rotation abou t a single axis is generally required. Fluid temperature up to 400 o C can be achieved in cylindrical parabolic focusing collector systems. With paraboloid concentrators still higher temperatures up to 600 o C are possible with single point focus. These require two way tracking systems so as to face the sun. Sun will be in line with the focus and the vertex of the paraboloid all the time. Typical values of efficiency obtained from flat- plate collectors, evacuated tube collectors, line- focusing collectors and paraboloid dish collectors are shown in Fig. 3.7. The efficiency is plotted as a function of the operating temperature and as is to be expected, it decreases with increasing temperature. The data presented is useful for doing approximate calculations of thermal systems using solar collectors. Such calculations help in giving estimates of the collector area required for a given performance. Alternatively they can be used for obtaining a rough idea of the performance of a solar thermal system using a certain number of collectors of a specified type. Figure 3.6: Cylindrical parabolic and Paraboloid concentrating collectors
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20 3.12.2 Heat Storage-Sensible and Latent forms Solar energy being a time dependent energy resource must be stored and used in day to day basis. The energy storage must be considered, keeping in view, the solar process system, major components of which are solar collector, storage units, conversion devices, loads, auxiliary energy supplies and control systems. The performance of each of these components is related to that of the other. The dependence of collector performance on temperature makes the whole system performance sensitive to temperature. The optimum capacity of an energy storage system depends on the expected time dependence of solar radiation availability, the nature of load to be expected in the process, degree of reliability, the manner in which auxiliary energy is supplied, and economic analysis weighing the relative use of solar and auxiliary energy. Solar energy can be stored in two forms, sensible and latent. Sensible heat storage systems operate over a range of temperatures, while latent heat storage systems operate essentially at the temperature at which the phase change takes place. The solar thermal energy storage can be classified as follows.
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