This latter figure may appear conservative but spall zones of this ratio of damage to particle diameter were encountered on the Apollo windows. Although there is a difference in materials, it is possible that the material chosen for the concentrators may become embrittled with age and suffer a similar type of damage to the Apollo windows. To estimate the damage rate an omnidirectional meteoroid flux model was used. The model provides the cumulative flux corresponding to meteoroid mass, which was reduced to yield a total damaged area per unit area and time, using the criteria given previously. The estimated damage is 2.05 x 10-6 meter^ per meter2-day (2.05 x 10-6 foot^ per foot^-day). This is a maximum figure since it assumes no two hits in the same place. Since this represents only 2.25% area damage in 30 years, meteoroids appear to pose no threat to the optical qualities of the solar concentrator. However, the specular reflectance of metallized films may be significantly degraded by the proton flux. A possible explanation for this damage may be as follows: low energy protons are stopped within the metal layer and form hydrogen after gathering an electron. Hydrogen accumulation causes small "bubbles" to form in the metal, so that the surface is no longer planar. Some tests at relatively high exposure rates (to shorten the test period) were run by Boeing in connection with project ABLE (orbital reflectors for ground illumination). At a flux corresponding to 900 times the geosynchronous proton flux, reflectivity decreased to only 0.59 from an original value of 0.92 in a period of 3.25 days, which may correspond to only eight years of orbital exposure. There was some indication of a dose rate effect, so that the actual correspondence period may be much longer. However, it is evident that radiation damage may be quite severe for conventional metallized films. On the other hand, the ECHO satellites flew in intense regions of the Van Allen radiation belts with apparently little degradation. 4.2.2 Low Concentration Ratio (Under 10) Individual cell concentrators were investigated. A promising concentrator type is the "compound parabolic concentrator" (CPC) of Dr. Roland Winston of the Enrico Fermi Institude (1). In a three dimensional (as opposed to linear form) such concentrators have relatively little surface area compared to their solar capture area. They can accept sunlight at rather large off-axis angles. Figure 4-9, from (1) shows the basic geometric construction. Fig. 4-9. Compound Parabolic Concentrator The dashed line inclined to the left through angle 0 is the axis of the parabola which forms the right hand surface. The left hand parabolic surface has as its axis a line inclined to the right by the angle 0; hence the term "compound parabolic." All rays are captured which are within 0 of the central axis. Reflections are generally accomplished with shallow grazing angles which yield high reflectivity. Of course when the Sun is off axis a CPC will lose a small amount of output due to the inclination of its inlet aperture. Due to the relatively small grazing angles involved a reflectivity factor of 90% was assumed. Off axis capability (0max) was related to solar concentration ratio (C) geometric) by: Derivation of optimum values of C to yield minimum power cost is defined in Section 5. 4.3 STRUCTURE In the SPS, large electric currents have to be carried considerable distance. In order to minimize mass, members carrying these currents must also be primary structure and carry physical loads. Typical
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