Fig. 4-2. Materia! Technology Trend 3. Introduction of a new alloy type, e.g., columbium-based B-66, can cause the most dramatic increase. 4.2 SOLAR CONCENTRATORS 4.2.1 High Concentration Ratio (Over 1000) The solar power generating systems require large solar concentrators with low mass per unit area. Concentration ratios of one thousand or more are required. Highest optical efficiency would be obtained with a rigid paraboloid; yet the structure required to provide accurate form despite thermal and gravity loads, aging and assembly inaccuracies is estimated to have a mass of at least 2.0 kg/m^ (0.41 Ibm/ft^). The baselined concentrator consists of a large number of individually steerable plastic film mirrors mounted on a relatively light framework. Active mirror control maintains focusing despite the disturbing forces mentioned above. Total concentrator mass for this type of system is estimated to be 0.29 kg/m^ (0.059 Ibm/ft^). The faceted concentrator is shown in Figure 4-3. Figure 4-4 shows a typical reflective facet. Metallized plastic film (baseline is aluminized Kapton) is tensioned to form a plane surface. The support system consists of three edge members with bridles tensioned by springs. The inherent flat facet is fitted with a two-axis servo drive which causes the sunlight reflected by the facet to enter the aperture of the cavity absorber. Fig. 4-3. Faceted Concentrator (Individual Steerable Facets Direct Solar Impages Onto Solar Array or Into Cavity Absorber) Fig. 4-4. Typical Reflective Facet The number of facets used influences the achievable solar concentration. The most efficient concentrator would of course be a paraboloid, consisting, in effect, of an infinite number of very small reflectors. With reflectors of a finite size the image of each reflector also increases in size. Since the sun has an apparent width of 0.53°, the light reflected by the facets must spread at least at this angle. A total angle of one degree was used in this analysis. Perfect reflectivity was assumed. Computer analysis of the multiple facet high concentration ratio solar concentrator are complete. Figure 4-5 identifies the variables in this analysis. Fig. 4-5. Variables in Solar Concentrator Analysis
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