Controlled directional scattering cavity for tubular absorbers

#This invention relates to radiant energy collection and more particularly to a cavity design for reducing losses inherent in certain radiant energy collectors.

#This invention relates to the subject matter of my copending patent application, Ser. No. 113,169; assigned to the same assignee and filed on even date herewith now abandoned, and superseded by continuation patent application Ser. No. 230,137, filed Jan. 30, 1981.

#It is well-established that radiation which is partially collimated with an angular divergence of .+-..theta. can be concentrated on a receiver without transmission loss by the maximum concentration of C.sub.max=sin .theta..sup.-1 through the use of nonimaging reflectors known generically in the art as compound parabolic concentrators. These concentrators are disclosed in my U.S. Pat. Nos. 3,923,381; 4,002,499; and 4,033,638; the disclosures of which are incorporated herein by reference. The compound parabolic concentrator (CPC) includes an energy receiver positioned between two trough-like sidewalls which reflect substantially all incident energy received over a predetermined included angle (.+-..theta.) onto the energy receiver. The aforementioned patents describe how the reflective sidewalls should be configured to achieve the maximal energy concentration. For instance, in U.S. Pat. No. 4,002,499, it is shown, for a tubular energy receiver, that the lowermost portions of the sidewalls form an involute of the shape of the energy receiver. It is important to note that the energy receiver shown in the '499 patent is the theoretical "optical design" receiver, and a practical design dictates that a larger-than-theoretical receiver shape be emplaced between the sidewalls to ensure that placement tolerances, minor wall malformations, etc. do not significantly hinder the energy concentrator. More recently, it has been shown desirable to encompass the receiver in a vacuum bottle-like structure wherein the outermost surface is concentric to the inner receiver; is transparent; and encloses a vacuum between itself and the inner surface of the receiver (which itself may be coated with an energy selective surface).

#FIG. 1 is a sectional drawing of the prior art detailing a retroreflective cavity.

#FIG. 2 is a chart which depicts the change in concentration of the system of FIG. 1 as the ratio of the distance to the cavity to the radius of the optical receiver is modified.

#FIG. 3 is a schematic drawing of a retroreflective cavity constructed in accordance with the invention.

#Referring to FIG. 3, receiver 30 is emplaced in a cavity 32 whose wall segments 34 have the specific property that radiation incident on a segment 34 from outside the angular subtense of the receiver (2.phi.) is scattered (or redirected) onto the receiver. For instance, light beam 36, when incident upon segment 34, will be redirected towards receiver 30 along the line indicated by arrow 38.

#The detailed construction of each scattering segment 34 is shown in FIG. 4. As aforestated, the angular subtense of receiver 30, when viewed from segment 34, is 2.phi.. It will be noted that segment 34 is actually the aperture of a V trough which has mirrored sides capable of reflecting incident energy.

#In order to determine the distance g which can separate aperture 34 from the nearest point of receiver 30, it is necessary to understand that it is a requisite of aperture 34 that it reflect all of the energy coming in from solid angles A and B and redirect it within the solid angle R. The second law of thermodynamics requires that the extent of solid angle R must be equal to or greater than the sum of the extents of solid angles A and B (based upon the assumption that this is a reflecting system). It can be shown that the following angular relationships exist: