This ratio has an upper limit that depends on whether the concentration is a three dimensional concentrator such as a paraboloid or a two dimensional concentrator such as a cylindrical parabolic concentrator. As the concentrator ratio increases, the receiver temperature increases and imposes an increasing requirement for precision in optical quality and positioning of the optical system. Figure 7.3 Possible Concentrator Collector Configurations: (a)Plane Receiver with Plane Reflectors (b)Parabolic Concentrator (c)Fresnel Reflector (d)Array of Heliostats [Duffie,1980] Figure 7.4 shows the relationship between the concentration ratio and the temperature of receiver operation for ground based systems. The shaded range corresponds to collection efficiencies of 40-60 % and represents the usual range of operations. The maximum concentration ratio is limited by the optics and for circular concentrators is less than 104. Since the collectors focus light on the receiver, accurate pointing of the mirror with respect to the sun to the order of 0.1° is required. This requires some kind of pointing mechanism. Previous proposals have put forward combinations of gimbals and actuators. It is also possible to use the satellites attitude control system to point the collectors but this can have a negative impact on other requirements of the satellite. A number of proposed Solar Dynamic Systems are shown in Figure 7.5 (a, b and c). These show the physical dominance of the solar concentrator as well as the large size of the radiators. Receivers [Eguchi K,1992] The receiver is located near the focal point of the concentrator. The solar energy is used to heat up the working fluid which can either by liquid or gaseous. Because the satellite will be in eclipse for at least part of its orbit (unless it is in sun-synchronous orbit), energy needs to be stored to provide the required energy during the dark periods of this orbit. One method of doing this is to surround the receiver with canisters containing an eutectic salt mixture and use its heat of fusion as the energy storage method. Thus when an eclipse occurs the salt mixture changes state releasing heat and providing power. This greatly increases the mass and hence cost of the receiver.
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