the fluid to absorb the heat within reasonable sized cavities. For example, consider a 3 ft diameter by 3 ft long cavity with a 1 ft aperture at the focal point of a 36 ft diameter dish with 1000 ft2 of area. Using only the interior surface of this cavity, the available heat transfer area is about 40 ft2 . By also hanging a spiral of tubing on a 2 ft diameter in the cavity with 2D spacing, the total area becomes about 70 ft2 . A heat transfer fluid must have a film coefficient of more than 50 BTU/ft2 °F to transfer about 55 kW while keeping the temperature difference between the exit fluid and the cavity to less than 50°F. This is not normally achieved by superheated steam, but is achievable by subcooled water and is within a factor of 2 of helium heat transfer capability. Therefore, in some cases, the cavity temperature must be more than 50°F above the fluid and/or the effective heat transfer area must be enlarged. This area requires further study for the poorer heat transfer fluids. The concentration ratio that is achieved depends upon the slope error of the sunlight in relation to cavity aperture. This slope error is made up of 2 major components: the surface error and the pointing inaccuracy. Figure 8 indicates this relationship. A concentration ratio of 1000 is achieved if the total slope error is 0.32 degrees. To achieve a higher concentration ratio a more perfectly contoured surface and/or better pointing accuracy must be achieved. This relationship is a crucial aspect of the economics of this system. If a sufficiently accurate surface can be achieved with inexpensive mass production fabrication techniques, the economics of this collector is enhanced. Initial results using 0.050 inch thick backsilvered glass on glass foam substrate achieved a surface error of less than 0.05 degrees. The approach used, which is amenable to mass production, can apparently be relaxed somewhat so that when coupled with expected pointing accuracies, the total slope error will be within the required 0.32 degrees.
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