(picture frame thickness) which is bent to the curved surface of the foamed glass substrate and bonded with epoxy. The radius of curvature associated with a dish with a diameter greater than 30 ft is gentle enough to allow bending of this thickness of glass without breakage. A number of individual pieces make up the dish surface. This type of surface on a lightweight foamed glass substrate produces a strong, lightweight, mass producible mirror which has high performance (high reflectivity and accurate surface) and long life. The overall slope error can be as great as 0.32° to achieve a concentration ratio of 1000 required for a collector efficiency of 70% at a 815°C cavity temperature. This total slope error includes the pointing error of the dish and the slope error of the surface. The very accurate mirror surface achieved (< 0.1 degree error) will allow a larger margin for the pointing. This may introduce some design margin to reduce costs below current estimates. The current estimates for the entire col- lector are in the range of $10.70/ft2 to $15.85/ft2 (1974 dollars). These costs are based on a collector which is capable of tracking in winds up to 20 mph. The maximum survival wind capability in a stowed position was 100 mph. The Honeywel1-Black and Veatch team estimated a 16 ft diameter dish to cost $11.63/ft2 (excluding cavity) (Ref. 1). A breakdown of this estimate is shown in Table 4. Excluding the mirrored surface (0.020 inch Alzak surface), the dish cost is $9.88/ft2 . This is in good agreement, but on the low side of the previous estimate in Table 3. Although the estimate is for a 16 ft diameter dish, Honeywell feels it is representative of units which are as large as 23 ft in diameter. This design was based on a 1/4 degree rms uncertainty in the angular position of the mirror normal at any point. The dish was designed for a pressurized water (2000 psi) receiver which is at the focal point and protrudes through a 6 inch slot in the collector surface. The cavity receiver cost ($0.82/ft2) was excluded so that there could be a direct comparison to the earlier estimate. The value of $11.63/ ft2 compares favorably to the $10.70 to $15.85/ft2 range suggested by the JPL study.
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