Abstract:
A non-imaging solar concentrator having a primary concentrator and a turntable. The primary concentrator is mounted to the turntable such that it is rotatable about a turntable axis and a secondary axis that is orthogonal to the turntable axis. Rotation of the primary concentrator about the turntable and secondary axes permits the primary concentrator to be positioned anywhere within the visible sky, even when a solar offset angle of 90 degrees is not used.

Description:
This invention was made with Government support under contract number F29601-98-C-0031 awarded by the Air Force. The Government has certain rights in this invention. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to solar concentrators and more particularly to a solar concentrator that employs a turntable to permit the solar concentrators to be rotated about two orthogonal axes so as to reduce both cost and weight while providing improved performance. 
     BACKGROUND OF THE INVENTION 
     BACKGROUND ART 
     Space-based satellites and solar orbit transfer vehicles frequently collect solar energy with solar concentrators to generate electrical energy and/or propulsive power. It is known in the art to employ imaging concentrators having a single reflective surface for this purpose. The known imaging concentrators typically are of very high optic quality, producing a highly focused beam of energy. These imaging concentrators, however, are relatively heavy and difficult to manufacture. Consequently, their incorporation into a satellite or vehicle platform tends to be costly, consuming both financial and payload capacity. 
     One alternative to imaging concentrators is a non-imaging concentrator, which employs both a primary concentrator and a secondary concentrator. The primary concentrator collects ambient light and provides a primary light beam that includes a focused portion and an unfocused portion. The secondary concentrator is situated across from the primary concentrator and includes a frusto-conical reflective surface. The secondary concentrator is configured such that the focused portion of primary light beam is transmitted through a hole in the secondary concentrator while the unfocused portion of the primary light beam is reflected back to the primary concentrator. The known non-imaging concentrators are typically lighter in weight, relatively easier to manufacture and less costly than similarly sized imaging concentrators, and as such, there use is more common. Several drawbacks, however, are known to exist. 
     The known arrangements typically employ a stationary boom having a gimbal mount for supporting the primary concentrator. The gimbal mount between the boom and the primary concentrator permits the primary concentrator to be rotated relative to the secondary concentrator. The gimbal mount, however, is rather costly and heavy. 
     Another drawback relates to the amount of sky that is visible to the primary concentrator and the type of primary concentrator that is used. Typically, the primary concentrator is constructed with either a 90° solar offset angle or a 70° solar offset angle. The 90° solar offset angle permits 100% of the visible sky to be viewed but is relatively costly. The 70° solar offset angle permits 85% of the visible sky to be viewed but is relatively less expensive. 
     Accordingly, an improved solar concentrator is needed which is relatively lighter in weight, less expensive, and which permits 100% of the visible sky to be viewed even with a primary concentrator having a 70° solar offset angle. 
     SUMMARY OF THE INVENTION 
     In one preferred form, the present invention provides a non-imaging solar concentrator having a primary concentrator, a turntable, a first support structure, a second support structure and a drive mechanism. The primary concentrator is configured to concentrate ambient light into a primary beam. The turntable has a central aperture, which is sized to permit the primary beam to be transmitted therethrough, and a rotatable portion that is rotatable about a turntable axis. The first support structure couples the primary concentrator to a first side of the rotatable portion of the turntable and the second support structure is coupled to a second side of the rotatable portion of the turntable opposite the first side. The secondary concentrator coupled to the second support structure and includes a frusto-conical reflective surface and a beam aperture. The beam aperture is sized to permit a focused portion of the primary beam to be transmitted therethrough, while the frusto-conical reflective surface is configured to reflect an unfocused portion of the primary beam back to the primary concentrator. The drive mechanism is coupled to the rotatable portion of the turntable and is operable for rotating the rotatable portion of the turntable about the turntable axis. 
     In another preferred form, the present invention provides a method for positioning a non-imaging solar concentrator about a structure having a first axis, the non-imaging solar concentrator having a primary concentrator and a secondary concentrator, the primary concentrator being operable for concentrating ambient light into a primary beam, the secondary concentrator having a frusto-conical reflective surface and a beam aperture, the beam aperture being sized to permit a focused portion of the primary beam to be transmitted therethrough, the frusto-conical reflective surface being configured to reflect an unfocused portion of the primary beam back to the primary concentrator, the method including the steps of: providing a turntable having a rotatable portion and a central aperture formed therethrough, the rotatable portion having a rotational axis, the turntable being coupled to the structure such that the rotational axis is perpendicular to the first axis; coupling the primary solar concentrator to a first side of the rotatable portion of the turntable; coupling the secondary solar concentrator to a second side of the rotatable portion of the turntable; and selectively rotating the primary and secondary concentrators about the turntable axis and the turntable about the first axis to position the non-imaging solar concentrator in a predetermined orientation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims, taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a perspective view of a rotating solar concentrator constructed in accordance with the teachings of the present invention and shown in operative association with a solar orbit transfer vehicle having a direct gain solar thermal engine; 
     FIG. 1A is a view similar to that of FIG. 1 but illustrating the rotating solar concentrator in operative association with a solar orbit transfer vehicle having a bimodal thermal storage engine; 
     FIG. 2 is an enlarged perspective view of the rotating solar concentrator of FIG. 1; 
     FIG. 3 is an enlarged portion of FIG. 2 illustrating the turntable, the drive mechanism, the secondary concentrator and the direct gain solar thermal engine in greater detail; and 
     FIG. 4 is a schematic illustration of the rotating solar concentrator. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to FIG. 1 of the drawings, a non-imaging rotating solar concentrator constructed in accordance with the teachings of the present invention is generally indicated by reference numeral  10 . In the particular embodiment illustrated, the solar concentrator  10  is shown in operative association with a solar orbit transfer vehicle  12 . Those skilled in the art will understand, however, that the illustration of the rotating solar concentrator  10  in association with the solar orbit transfer vehicle  12  is merely exemplary and not intended to limit the scope of the present invention in any manner. As such, it is contemplated that the rotating solar concentrator  10  may be used in conjunction with other spaced-based devices, such as satellites, for the production of electrical energy and/or propulsive power. 
     The solar orbit transfer vehicle  12  is conventional in its construction and operation and as such, need not be discussed in detail. Briefly, the solar orbit transfer vehicle  12  is illustrated to include means for generating electrical energy, such as an array of thermionic converters  20 , a propellant tank  22  for storing a propulsive substance  24 , such as hydrogen or ammonia, and a direct gain solar thermal engine  26  having at least one propulsion nozzle  28  that is used to propel the solar orbit transfer vehicle  12  in a selected direction. Thermal energy received by the direct gain solar thermal engine  26  is employed to directly heat the propulsive substance  24  to produce a highly energetic vapor that is subsequently expanded in the propulsion nozzle  28  to develop propulsive power. Similarly, thermal energy received by the direct gain solar thermal engine  26  may additionally or alternatively be employed to operate the array of thermionic converters  20  to develop electrical energy. Alternatively, the solar orbit transfer vehicle  12  may include a bimodal thermal storage engine  26   a  as illustrated in FIG. 1A which stores thermal energy in a receiver-absorber-converter  26   b . The receiver-absbrber-converter  26   b  is selectively controllable to release the thermal energy to heat the propulsive substance  24  and/or the array of thermionic converters  20 . 
     With additional reference to FIGS. 2 and 3, the rotating solar concentrator  10  is illustrated to include a turntable  30 , a drive mechanism  32 , a primary concentrator  34 , a first support structure  36 , a second support structure  38 , a secondary concentrator  40  and a tracking controller  42 . The turntable  30  includes a rotating portion  50 , a central aperture  52  and, in the particular embodiment illustrated, a nonrotating portion  54  which supports the rotating portion  50  for rotation about a turntable axis  56 . The drive mechanism  32  includes a movable portion  60  that is coupled to the rotating portion  50  and a non-movable portion  62  that couples the non-rotating portion  54  and the movable portion  60 . In the particular embodiment illustrated, the nonmovable portion  62  includes a drive motor  62   a  that is fixedly coupled to the non-rotating portion  54  of the turntable  30  and the movable portion  60  includes a gear  60   a  that is coupled for rotation with an output shaft (not shown) of the drive motor  62   a  and meshingly engaged with a plurality of gear teeth  64  formed into the perimeter of the rotating portion  50  of the turntable  30 . Those skilled in the art will appreciate that any suitable drive mechanism may be utilized, including those employing belts and/or friction rollers. The non-rotating portion  54  of the turntable  30  is fixedly coupled to the propellant tank  22  of the solar orbit transfer vehicle  12  such that the turntable axis  56  is orthogonal to the axis  22   a  of the propellant tank  22 . 
     The primary concentrator  34  may be a spline radial panel or a Fresnel reflector, but is preferably of an inflatable design, wherein a mounting structure  70  having an inflatable torus  72  is employed to shape and support a reflective member  74 . The primary concentrator  34  is illustrated to have a solar offset angle of about 70°, but other solar offset angles may be employed. The first support structure  36  fixedly couples the primary concentrator  34  to the rotating portion  50  of the turntable  30 . The first support structure  36  may be formed from rigid materials, or may be of an inflatable design that inflates prior to or concurrently with the inflation of the primary concentrator  34 . 
     The second support structure  38  is coupled to a second side of the rotating portion  50  of the turntable  30  opposite the side to which the first support structure  36  is mounted. In the example provided, the second support structure  38  is formed from a network of rigid members  38   a  and serves as the mount for both the direct gain solar thermal engine  26  and the secondary concentrator  40 . The secondary concentrator  40  is coupled to the direct gain solar thermal engine  26  and includes a frusto-conical reflective surface  80  and a beam aperture  82 . With additional reference to FIG. 4, during the operation of the solar concentrator  10 , the primary concentrator  34  collects ambient light and produces a concentrated beam of light  90  that is transmitted through the central aperture  52  in the turntable  30  and received by the secondary concentrator  40 . A focused portion  90   a  of the beam of light  90  passes through the beam aperture  82  in the secondary concentrator  40  and is received by the direct gain solar thermal engine  26 . An unfocused portion  90   b  of the beam of light  90  is reflected by the frusto-donical reflected surface  80  back to the primary concentrator  34 . 
     The tracking controller  42  is coupled to the solar orbit transfer vehicle  12 , the drive mechanism  32  and at least one sensor  94  (FIG. 1) that permits the tracking controller  42  to determine the location of a source of ambient light (i.e., the sun  96 ). The tracking controller  42  is employed to orient the primary concentrator  34  relative to the source of ambient light so as to produce a desired amount of propulsive power and/or electrical energy. In this regard, the tracking controller  42  orients the primary concentrator  34  relative to the ambient light source by selectively rotating the primary concentrator  34  about the turntable axis  56  (via the drive mechanism  32 ) and the tank axis  22   a  (via the at least one propellant nozzle  28 ). Since the turntable axis  56  and the tank axis  22   a  are orthogonal to one another, 100% of the visible sky is viewable by the primary concentrator  34 . 
     While the invention has been described in the specification and illustrated in the drawings with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention as defined in the claims. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment illustrated by the drawings and described in the specification as the best mode presently contemplated for carrying out this invention, but that the invention will include any embodiments falling within the foregoing description and the appended claims.