The focal length is considered to be 0.6 diameter for a reasonable design. Such a large focal length reduces the surface curvature so that simple glass bending techniques can be used. 3.1.2 Collector Cost The parabolic dish economics were reviewed from several sources including preliminary estimates made by JPL. Since such devices are only in the very preliminary stages of development, there is a poor understanding of exactly what the commercial cost is of an acceptable long life product. Initial estimates at JPL indicated that a 36 foot diameter parabolic dish would cost as little as $75/m 2 ($7/ft2 ). More recent estimates by JPL conclude that a more reasonable collector cost would be from $10/ft2 to $15/ft2 (1974 dollars) with expectations in the lower end of this range. A breakdown by major subsystems is shown in Table 3. The structural area includes reflector support, gimbal, pedestal, counter weight and quadripod. Drives, bearings and controls are in the mechanical area, while the other category mainly includes the foundation, site work, safety, overhead and profit. The costs in Table 2 exclude the mirror surface as well as the cavity receiver. The receiver heat exchanger is included as part of the Brayton engine. Previously, $2/ft2 was added for a mirror surface based on backsilvered thin glass attached to a stretched aluminum substrate. Fabrication and testing of this surface proved that image distortion due to thermal expansion would prevent this surface from being within 0.1 degree slope error. Work is underway now using a substrate material (foamed glass) that has the same coefficient of expansion as the silvered glass. The optical properties of the combination seem excellent (<0.1 degree slope error). Our preliminary cost estimate is still $2/ft2 for the installed surface. The dish reflection surface is made of thin backsilvered glass
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