Abstract:
An improved window system for a solar receiver provides a high level of impedance to thermal re-radiation while minimizing Fresnel losses. The window system is characterized by a bundled array of optically transmissive members. In further aspects, a solar receiver employing the window system and a method for manufacture are provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the priority benefit under 35 U.S.C. § 119(e) of U.S. provisional patent application No. 61/107,889 filed Oct. 23, 2008. The aforementioned provisional application is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    This disclosure relates generally to concentrating solar thermal energy systems and, more specifically, to the solar receiver portion of a solar concentrator such as a solar receiver system employing a parabolic dish or mirror field surrounding a central tower. 
         [0003]    Concentrated solar energy, created by a focusing mirror system, has been used to heat working fluids for power conversion or high temperature process heat applications. Temperatures in the range of 700 to 1100 degrees Celsius are commonly achieved within the solar receiver portion of the system. 
         [0004]    Typically, the solar receiver has an aperture to receive the focused solar power and an absorber within the receiver cavity which is cooled by a fluid. The fluid may be the working fluid of an engine cycle, such as a Stirling, Brayton, or 
         [0005]    Rankine cycle engine. Alternatively, the fluid may be employed for a high-temperature process, such as a thermochemical conversion, energy transport, or thermal energy storage. Generally, the efficiency of the solar receiver is a strong function of the aperture size and the cavity temperature, as re-radiation from the absorber elements to the environment represents a dominant loss. 
         [0006]    Window or cover glass has been employed to reduce this re-radiation by impeding a fraction of the infrared energy while transmitting the majority of the solar spectrum. This is commonly referred to as the “greenhouse” effect. The gains due to the window&#39;s infrared absorption and its reduction of free convection losses are somewhat offset by the solar reflection from the window surfaces. Most window materials appropriate for solar receivers have an index of refraction of about 1.4 to 1.6. Even for the most transmissive window materials, the index of refraction change between air and the window results in a reflection loss of roughly 4% per interface or a total of 8% for the two sides of the window. These losses are generally referred to as Fresnel losses. 
         [0007]    The present disclosure is directed to an improved window system for a solar receiver which provides a high level of impedance to the thermal re-radiation while minimizing the Fresnel losses. In further aspects, the present disclosure is directed to a solar receiver employing the same and to a method of manufacture. 
       DESCRIPTION OF PRIOR ART 
       [0008]    Prior art methods of concentrating solar power generation use optics to deliver heat to a solar receiver for conversion into electricity by a heat engine.  FIG. 1  illustrates a prior art solar receiver system which includes a parabolic reflector  4  is mounted on a structure  2  to reflect and concentrate solar rays  1  onto a solar receiver  3 .  FIG. 2  provides a general view of the solar receiver  3 , which includes an aperture  5  for admitting solar energy into a cavity having a front cavity portion  6 , and an absorber  7 . The acceptance angle of the front cavity portion  6  is set to avoid direct irradiation of the reflected sun light. The absorber  7  is configured to absorb power and transfer that power to a working fluid. The absorber  7  may be tubular, of plate-fin construction, or an open matrix, such as honeycomb, standing pins, or porous foam. The fluid may be air, helium, hydrogen, or any number of fluids used in engine cycles, thermo-chemical reactions, or thermal storage applications. 
         [0009]    An optical window may be placed in the aperture  5  to help retain heat in the cavity of the receiver. The window material transmits the majority of the solar spectrum, but absorbs a large fraction of the infrared energy radiating from the cavity defined by the cavity portions  6  and  7 .  FIG. 3  shows a flat disk window  8  as generally known in the art. The flat window, though simple, reflects a portion  10  of the incident energy due to the so-called Fresnel loss, associated with the mismatched indices of refraction between the window and air.  FIG. 4  illustrates the Fresnel reflection from a flat window  8 . The principal ray  1  intersects the window surface, wherein the transmitted portion  9  passes through the window, while the reflected fraction  10  is redirected at an equal-but-opposite angle from the plane of the surface at the point of intersection. As best seen in  FIG. 4 , since there are two surfaces, i.e., the inward facing surface and the outward facing surface of the window  8 , there are two Fresnel reflection rays  10 . For typical window materials, this reflection represents about 8% of the total incident energy. 
         [0010]      FIG. 5  shows an alternative prior art solar receiver  3   a  having a concave window  11 . If the window concavity projects deep into the cavity, the Fresnel reflections are redirected elsewhere in the cavity, but are not lost as would be the case for the flat window. Not all concaved windows function efficiently in this manner. As an example, the Fresnel reflection component from a hemispherical concaved window will direct the majority of its reflected energy onto the front or proximal cavity portion  6 , rather than the distal cavity portion containing the absorber surface  7 . Therefore, it can be deduced that only a deep-domed window will efficiently capture the energy in the Fresnel reflections. A large, dome-shaped window device, suitable for power generation, is known to be expensive, particularly for high temperature solar receiver applications where quartz (fused silica) or Sapphire (aluminum oxide) are required. 
       SUMMARY 
       [0011]    In one aspect a window system for a solar receiver of a type having a solar energy receiving chamber, a solar energy receiving aperture defining an opening to the solar energy receiving chamber, and a solar energy absorber received within the solar energy receiving chamber is provided. The window system includes a plurality of optically transmissive members formed of an optically transmissive material and the plurality of optically transmissive members are attached together to form a bundled array. 
         [0012]    In another aspect, a solar receiver is provided. The solar receiver includes a cavity, an aperture for receiving light entering the cavity, a solar absorber disposed within the cavity, and a plurality of optically transmissive members formed of an optically transmissive material attached together to form a bundled array. The bundled array is disposed on the solar receiver at the aperture. 
         [0013]    In yet another aspect, a method for manufacturing a solar receiver includes forming a solar receiver of a type having a cavity, an aperture for receiving light entering the cavity, and a solar absorber disposed within the cavity, girding a plurality of optically transmissive members to form a bundled array, each of the optically transmissive members formed of an optically transmissive material, and attaching the bundled array to the solar receiver at the aperture. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating the preferred embodiments and are not to be construed as limiting the invention. 
           [0015]      FIG. 1  illustrates a prior art solar receiver apparatus. 
           [0016]      FIG. 2  provides a general view of the solar receiver portion appearing in  FIG. 1 . 
           [0017]      FIG. 3  shows a prior art solar receiver having a flat disc window. 
           [0018]      FIG. 4  illustrates the Fresnel reflection from a prior art solar receiver having a flat window. 
           [0019]      FIG. 5  shows an alternative prior art solar receiver having a concave window. 
           [0020]      FIG. 6A  shows a window system in accordance with an exemplary embodiment of the present invention. 
           [0021]      FIG. 6B  is a fragmentary, cross-sectional view of the tube bundle  12 , taken generally along the lines  6 B- 6 B appearing in  FIG. 6A . 
           [0022]      FIG. 6C  is an enlarged view of the region  6 C appearing in  FIG. 6A . 
           [0023]      FIG. 7  illustrates the general features of the exemplary cylindrical tubes  14 . 
           [0024]      FIG. 8A  shows a fragmentary, cross-sectional view of a bundled array of tubes secured via an alternative. 
           [0025]      FIG. 8B  shows a fragmentary, top view of the bundled array of tubes appearing in  FIG. 8A . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0026]    Referring now to  FIGS. 6A-6C , wherein like reference numerals refer to like or analogous components throughout the several views, there appears a window system  12  in accordance with an exemplary embodiment of this disclosure, which may advantageously be configured to capture Fresnel reflections and which may also be produced without excessive cost. The window  12  includes a packing of optically transmissive members  14  formed of an optically transmissive material. The optically transmissive members  14  may be of solid or hollow construction, and in the depicted preferred embodiment are elongate, straight tubes  14 . While the tubes  14  shown in the depicted exemplary embodiment are circular in cross-sectional shape, it will be recognized that tubes  14  of any geometrical configuration, e.g., polygonal in cross-section, may also be employed. As used herein, the term optically transmissive is intended to refer to a material that transmits a significant portion of solar radiation incident thereon. 
         [0027]    In alternative embodiments, the members  14  may be solid rods formed of an optically transmissive material, and may likewise have circular, polygonal, or other cross-sectional shape. Although the depicted preferred embodiment will be described herein primarily by way of reference to the preferred embodiment employing hollow or tubular optically transmissive members  14 , it will be recognized that the disclosure herein is equally applicable to window systems employing solid rods as the optically transmissive members. 
         [0028]    A band clamp  13  or similar tension device extends about the periphery, banding the array of tubes  14  together. Alternatively, the tubes  14  may be keyed or bonded together. The tubes  14  may be made from an optically transmissive material, including without limitation, quartz, borosilicate glass (e.g., PYREX®), glass, sapphire, metal oxide, or the like. Because the optically transmissive members  14  have poor thermal communication with their neighbors, those located on the outer perimeter, outside of the solar irradiated aperture, may be relatively cool. Therefore, the oversized bundle as shown in  FIGS. 6A and 6C  may be clamped with a tension spring mechanism. As best seen in  FIG. 6B , in the depicted preferred embodiment, the tubes  14  are bundled in a hexagonal, close-packed configuration. 
         [0029]      FIG. 7  illustrates the general features of a preferred embodiment herein employing cylindrical tubes  14 . The tube bundle is located nominally at the plane  21  of the aperture  5  of the solar received  3 , i.e., so that the outward, light-receiving face of the tube bundle array  12  is generally aligned with the plane  21  of the aperture  5 . The bundle  12  may be secured to the solar receiver body  3  using one or more mechanical fasteners as would be understood by persons skilled in the art, such as one or more brackets, clamps, clips, snap fit fasteners, clips, dogs, pawls, a bezel, or other attachment or fitment means. 
         [0030]    The tube bundle may, optionally, extend above the absorber plane  20 , which defines the boundary between the proximal or front cavity portion  6  and the distal portion of the receiver cavity containing the absorber surface  7 . However, this is typically not necessary to achieve good performance. The only portion of the tube window system that is subject to the Fresnel loss is the tube end, normal to the axial dimension of the cylinder. Though this generally represents a very small fraction of the incident energy, this loss may be reduced by choosing thin-walled tubes or by rounding, thinning, sharpening, or chamfering the tube ends. In a bundled array  12  composed of tubes  14 , a portion of the infrared radiation  17  emanating from the absorber  7  above the plane  20  of the absorber  7 , may pass directly through the tube bore; however, this fraction drops as the aspect ratio (length to diameter) of the tubes  14  increases. 
         [0031]    The radiation  17  from the absorber  7  is emitted in all directions. Thus, the so-called view factor through the tubes  14  diminishes with increasing tube aspect ratio. A higher aspect ratio tube serves as an effective radiation barrier, as the absorbed energy has a long conduction path to the front or outward-facing end of the tube, where it is exposed to cooler ambient air. In preferred embodiments, the length to diameter ratio, L/D, of the tubes  14  may be about 3 or greater to insure a high intersection of the cavity radiation  17 , although other aspect ratios are contemplated. While there is no constraint on the diameter of the tubes  14 , tubes having a diameter in the range of about 25 to about 50 millimeters (about 1 to about 2 inches) may advantageously be employed. The window system described herein may also function as barrier to cavity convection losses, impeding the transfer of buoyancy-driven air out of the cavity. 
         [0032]    In an alternative exemplary embodiment, the widow system herein may still employ a band clamp  13  encircling the bundled array of tubes  14  (see  FIGS. 6A and 6C ), but may further include features on the tubes to prevent relative movement or sliding of adjacent tubes  14 . In the depicted embodiment of 
         [0033]      FIGS. 8A and 8B , the tube bundle includes two types of tubes, namely, straight walled tubes  30  and non-straight walled tubes  31 , which have a flange, flare, or like protrusion  32  at the tube ends. It will be recognized that the protrusion  32  may be relatively small or slight. In addition, although the depicted preferred embodiment shows tubes  31  having a flange feature  32  at both ends, it will be recognized that window systems having tubes  31  with the flange feature  32  on only one end or the other are also contemplated. Alternatively, the flare or flange feature  32  need not be continuous, but may be a segmented flare or flange or may be a protrusion or other key-like feature. When bundled in a close packed hexagonal array, the flare  32  abuts the ends of the adjacent tubes  30  and prevents the straight tubes  30  from slipping. 
         [0034]    In still further embodiments, as noted above, the tubes  14  illustrated in the depicted preferred embodiment may be replaced with solid, optically transmissive rods. Such rods may be straight walled, or may be a combination of straight-walled rods and rods having a flange, flare, or like protrusion at one or both ends, as detailed above by way of reference to  FIGS. 8A and 8B . One or both ends of such rods may be flat, or, may be rounded (e.g., hemispherical or otherwise rounded), tapered, etc. 
         [0035]    In one exemplary embodiment, a window system in accordance with this disclosure may be positioned at the aperture of a solar receiver and may include a bundle of one or more tubes made from quartz, borosilicate glass (e.g., PYREX®), glass, or sapphire, or other optically transmissive materials. 
         [0036]    In another exemplary embodiment, the widow system may include a bundle of tubes, wherein the diameter of the bundle is significantly larger than the aperture of the solar receiver, permitting the use of a clamping mechanism with the purpose of binding the array of tubes into a planer module, i.e., having a generally planar light-receiving face. 
         [0037]    In yet another exemplary embodiment, the window system may further comprise or contain a clamping device for providing clamping support to the bundle of tubes. The clamping device may contain a metal spring and may be located within the cooler outer region of the bundle. 
         [0038]    In still another exemplary embodiment a window system as described in may employ tubes wherein one or both ends of one or more of the tubes have closed, e.g., hemispherical or otherwise rounded, for example, ends such as typically used in the closed end of a test-tube. 
         [0039]    The invention has been described with reference to the preferred embodiments. Modifications and alterations will occur to others upon a reading and understanding of the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.