Abstract:
In one instance, a solar collector uses vacuum to maintain or help maintain its components in an assembled position and to insulate or help insulate a plurality of thermal conduits. Curved members, edge members, and end caps combine to form an interior space and to provide a system with substantially no bending forces. An absorption-and-tensioning member traverses the interior space and keeps the curved members from collapsing when placed under partial or complete vacuum and helps absorb thermal energy. The edge members have seats for receiving the curved members that help firmly press the curved members into the seats to form a desired seal. Other solar collector systems, features, and methods are disclosed.

Description:
RELATED APPLICATION 
       [0001]    The present invention claims the benefit, under 35 USC §119(e), of the filing of U.S. Provisional Patent Application Ser. No. 61/237,950 entitled “Solar Collectors and Methods,” filed Aug. 28, 2009, which is incorporated herein by reference for all purposes. 
     
    
     BACKGROUND 
       [0002]    The present invention relates generally to solar collectors for capturing solar thermal energy, and more particularly, to improved solar collectors and methods. 
         [0003]    Today there is an increased push to develop clean, renewable energy sources. One such effort involves the use of solar thermal collectors, or solar collectors. Solar collectors are intended to collect thermal energy by absorbing sunlight. Some solar collectors involve focusing solar energy using solar parabolic collectors, solar troughs, and solar towers. Other solar collectors use un-concentrated sunlight. Two commonly encountered solar collectors include flat plate collectors and vacuum tube collectors. These two types are used often for space heating and for domestic hot water production. 
       BRIEF SUMMARY 
       [0004]    Shortcomings with aspects of conventional solar collectors are addressed by the present invention as shown and described in a variety of illustrative, non-limiting embodiments herein. According to one illustrative, non-limiting embodiment, a solar collector for collecting thermal energy includes a first curved member, a second curved member, a plurality of edge members, and a plurality of end caps. The first curved member has a first end and a second end, a first longitudinal edge and a second longitudinal edge. The first curved member has a lateral cross section with an arc defined by a radius R 1 . The second curved member has a first end and a second end, a first longitudinal edge and a second longitudinal edge. The second curved member has a lateral cross section with an arc defined by a radius R 2 . One end cap of the plurality of end caps is associated with the first end of the first curved member and the first end of the second curved member and one end cap is associated with the second end of the first curved member and the second end of the second curved member. The solar collector further includes an internal absorption-and-tensioning member and a plurality of thermal conduits coupled to the absorption-and-tensioning member. Each edge members of the plurality of edge members has a first seat, a tension connection portion, and a second seat. The first seat and second seat each have a force-absorbing surface and a shelf surface with an angle β formed between the force-absorbing surface and the shelf surface. The angle β is less than ninety degrees)(90°). At least a portion of the internal absorption-and-tensioning member is coupled to the tension connection portion of the plurality of edge members. The first seat of at least one of the plurality of edge members receives the first longitudinal edge and at least one of the plurality of edge members receives the second longitudinal edge. 
         [0005]    According to another illustrative, non-limiting embodiment, a method of manufacturing a solar collector includes the steps of forming a first curved member, forming a second curved member, forming a plurality of edge members, and forming a plurality of end caps. The first curved member is formed with a first end and a second end, a first longitudinal edge and a second longitudinal edge. The first curved member has a lateral cross section with an arc defined by a radius R 1 . The second curved member is formed with a first end and a second end, a first longitudinal edge and a second longitudinal edge. The second curved member has a lateral cross section with an arc defined by a radius R 2 . One end cap is coupled to the first end of the first curved member and the first end of the second curved member and another end cap is coupled to the second end of the first curved member and the second end of the second curved member. The method further includes forming an internal absorption-and-tensioning member and coupling a plurality of thermal conduits to the absorption-and-tensioning member. Each edge member of the plurality of edge member has a first seat, a tension connection portion, and a second seat. The first seat and second seat each have a force-absorbing surface and a shelf surface with an angle β formed between the force-absorbing surface and the shelf surface. The angle β is less than 90°. The method further includes coupling at least a portion of the internal absorption-and-tensioning member to the tension connection portion of the plurality of edge members. The first seat of at least one of the plurality of edge members receives the first longitudinal edge and at least one of the plurality of edge members receives the second longitudinal edge. 
         [0006]    According to another illustrative, non-limiting embodiment, a method for assembling a solar collector includes the steps of providing a first curved member, second curved member, plurality of edge members, and plurality of end caps. The first curved member has a first end and a second end, a first longitudinal edge and a second longitudinal edge. The first curved member also has a lateral cross section with an arc defined by a radius R 1 . The second curved member has a first end and a second end, a first longitudinal edge and a second longitudinal edge. The second curved member has a lateral cross section with an arc defined by a radius R 2 . One end cap of the plurality of end caps is associated with the first end of the first curved member and the first end of the second curved member and one end cap is associated with the second end of the first curved member and the second end of the second curved member. The method further includes providing an internal absorption-and-tensioning member having a plurality of thermal conduits coupled to the absorption-and-tensioning member. Each edge member has a first seat, a tension connection portion, and a second seat. The first seat and second seat each have a force-absorbing surface and a shelf surface with an angle β formed between the force-absorbing surface and a shelf surface. The angle β is less than 90°. At least a portion of the internal absorption-and-tensioning member is coupled to the tension connection portion of the plurality of edge members. The method further includes disposing the first longitudinal edge of the first curved member into the first seat of at least one of the plurality of edge members and disposing the second longitudinal edge of the first curved member into the first seat of at least one of the plurality of edge members. The method also includes disposing the first longitudinal edge of the second curved member into the second seat of at least one of the plurality of edge members and disposing the second longitudinal edge of the second curved member into the second seat of at least one of the plurality of edge members. With these steps, an interior space is formed. The method also includes evacuating the interior space to form a sealed interior space. 
         [0007]    According to another illustrative, non-limiting embodiment, a method of manufacturing a solar collector includes the steps of: forming a first curved member, forming a second curved member, forming a plurality of edge members, and forming a plurality of end caps. The method further includes associating the first curved member, second curved member, plurality of edge members, and plurality of end caps to form an interior space. The method also includes evacuating the interior space at least partially to maintain the interior space without fasteners. 
         [0008]    Other features and advantages of the illustrative embodiments will become apparent with reference to the drawings and the detailed description that follow. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    A more complete understanding of the present invention may be obtained by reference to the following Detailed Description when taken in conjunction with the accompanying Drawings wherein: 
           [0010]      FIG. 1  is a schematic, perspective view of an illustrative, non-limiting embodiment of a solar collector; 
           [0011]      FIG. 2  is a schematic, cross-sectional view of the illustrative solar collector of  FIG. 1  taken along line  2 - 2 ; 
           [0012]      FIG. 3  is a schematic, plan view of an illustrative, non-limiting embodiment of a first curved member; 
           [0013]      FIG. 4  is a schematic, lateral cross-sectional view of the illustrative embodiment of the first curved member of  FIG. 3 ; 
           [0014]      FIG. 5  is a schematic, plan view of an illustrative, non-limiting embodiment of a second curved member; 
           [0015]      FIG. 6  is a schematic, lateral cross-sectional view of the illustrative embodiment of the second curved member of  FIG. 5 ; 
           [0016]      FIG. 7  is a schematic, cross-sectional view of an illustrative, non-limiting embodiment of an edge member; 
           [0017]      FIG. 8  is a schematic, plan view of an illustrative, non-limiting embodiment of a solar collector; 
           [0018]      FIG. 9  is a schematic, side view of the solar collector of  FIG. 8 ; 
           [0019]      FIG. 10  is a schematic, perspective view of an illustrative first spherical segment member and a portion of an illustrative first curved member; 
           [0020]      FIG. 11  is a schematic, plan view of the first spherical segment member and first curved member of  FIG. 10 ; 
           [0021]      FIG. 12  is a schematic, side view of the first spherical segment member and first curved member of  FIGS. 10-11 ; 
           [0022]      FIG. 13  is a schematic, end view of the first spherical segment member and first curved member of  FIGS. 10-12 ; and 
           [0023]      FIG. 14  is a schematic, perspective view of the first curved member, first spherical segment member, and of a second spherical segment member. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings that form a part hereof, and in which is shown, by way of illustration, specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is understood that other embodiments may be utilized and that logical structural, mechanical, electrical, and chemical changes may be made without departing from the spirit or scope of the invention. To avoid detail not necessary to enable those skilled in the art to practice the invention, the description may omit certain information known to those skilled in the art. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims. 
         [0025]    Referring now to  FIGS. 1-7 , and initially to  FIGS. 1 and 2 , an illustrative, non-limiting embodiment of a solar collector  100  is presented. The solar collector  100  includes a first curved member  102 , a second curved member  104 , a plurality of edge members  106 , and a plurality of end caps  108 . When assembled, the first curved member  102 , second curved member  104 , plurality of edge members  106 , and end caps  108  form an interior space  110 . An internal absorption-and-tensioning member  112  traverses the interior space  110  and holds a first edge member  114  and a second edge member  116  of the plurality of edge members  106  in relative position. 
         [0026]    The solar collector  100  may include a vacuum port (not explicitly shown) to which a vacuum source  111  may be coupled to remove some or all of the fluid within the interior space  110 . As the fluid is removed from the interior space  110 , a complete seal is established to create a sealed interior space. The assembled solar collector  100  may be maintained using the vacuum or partial vacuum. Moreover, in some illustrative embodiments, the solar collector  100  may be assembled without any fasteners being required. It will be apparent that the solar collector  100  may be relatively easy to assemble. Furthermore, the solar collector may be less expensive to manufacture than conventional solar collectors. The solar collector  100  may be transported to a desired location in an unassembled state (a more compact form) and then assembled at the desired site. 
         [0027]    The solar collector  100  may reflect less, or absorb more, sunlight than a tube collector because of the relatively flat nature of the first curved member  102  of the solar collector  100 . The first curved member  102  may be flatter in some embodiments than the second curved member  104  in order to minimize reflection losses. The larger curve radius, however, may mean a higher compression stress on the material of the first curved member  102  in such instances. Light conducting pyramids and other devices may be added to the exterior of the first curved member  102 . 
         [0028]    The internal absorption-and-tensioning member  112  includes an absorber  118  and a tension member  120 . A plurality of thermal tubes, or tubes  122 , are coupled to the absorber  118  or formed integrally with the absorber  118 . A space may be formed between the absorber  118  and the tension member  120 . The space may be used to thermally isolate or partially isolate the absorber  118  and tension member  120  from each other. As will be explained in more detail below, the internal absorption-and-tensioning member  112 , and in particular the tension member  120 , holds tension between the first edge member  114  and second edge member  116  when a load is placed on the first curved member  102  and second curved member  104 . The tension member  120  has a first end  124  and a second end  126 . Without the internal absorption-and-tensioning member  112 , and in particular the tension member  120 , the curved members  102 ,  104  would typically collapse when placed under pressure. The tension member  120  may be made of a number of different materials capable of handling the tensile forces involved, including organic fiber material (e.g., a Kevlar® material), carbon fiber material, and resistant spring steel. The thickness of the tension member  120  will depend on the material and the forces to be carried. The tension member  120  or a portion of the tension member  120  may be made from a material, such as a Kevlar® material, that has minimal thermal expansion characteristics. 
         [0029]    The absorber  118  may be placed underneath the tension member  120  (i.e., further from the sun than the tension member  120 ) or, as shown, may be placed above the tension member  120 . Placing the absorber  118  above the tension member  120  facilitates capture of sunlight that passes through the first curved member  102 . The absorber  118  may include a high-selective coating (not explicitly shown) to facilitate maximum absorption of sun radiation and to minimize the emission of thermal radiation. The selective coating may have a high absorption coefficient between the near infrared and the near ultraviolet range and a low emission coefficient in the mid-infrared range. The selective material coating may be applied to the lower surface of the absorber  118 , both sides of the tension member  120 , the inner surface of the second curved member  104 , or any other surfaces desired so as to decrease the emission and absorption of infrared radiation produced by hot surfaces. 
         [0030]    The plurality of tubes  122  that are coupled or formed integrally to the absorber  118  carry a working fluid. The thermal energy absorbed by the absorber  118  is delivered by way of thermal conduction to the plurality of tubes  122 . The plurality of tubes  122  may directly absorb thermal energy as well. The working fluid within the plurality of tubes  122  absorbs thermal energy and may thereby transport thermal energy for storage or direct utilization. The working fluid may be any fluid suitable for receiving thermal energy, such as water, Glyco/water mixture, hydrocarbon oils, silicones, refrigerants, Dowtherm® fluid, etc. The plurality of tubes  122  may be formed from a highly conductive material, such as copper or aluminum. In other embodiments, the tubes  122  need not be highly conductive but may have a high surface area. 
         [0031]    The absorber  118  and tubes  122  may be formed separately or integrally. In the latter embodiment, for example, the absorber  118  and tubes  122  may be extruded as an integral element. In another illustrative, non-limiting embodiment, the tubes  122  may be bonded to the absorber  118 . In another illustrative, non-limiting embodiment, re-enforced plastics may be used for the absorber  118  and tubes  122 . The tubes  122  may take numerous shapes and configurations, and in one illustrative, non-limiting embodiment, are formed as channels formed within the absorber  118 . 
         [0032]    Referring now primarily to  FIGS. 1-4 , the first curved member  102  will be described in more detail. The first curved member  102  has a first longitudinal edge  128  and a second longitudinal edge  130 . The first curved member  102  also has a first end  132  and a second end  134 . The ends  132 ,  134  have a lateral dimension D 1 . The longitudinal edges  128 ,  130  have a longitudinal dimension D 2 . The first curved member  102  has an exterior surface  136  and an interior surface  138 . The first curved member  102  is curved to form an arc defined by a radius R 1 . The first curved member  102  has an arc angle γ. The first curved member  102  is formed from a translucent material and preferably is formed from glass. Depending on the specific application and material, the thickness of the first curved member  102  will vary. In one illustrative, non-limiting embodiment using glass, the first curved member  102  has a thickness between 3 mm and 10 mm. As used herein, “lateral” means in the general direction of the lateral dimension D 1  (or shorter dimension) and “longitudinal” means in the general direction of the longitudinal dimension D 2  (or longer dimension) for the embodiment shown in  FIG. 3 . 
         [0033]    Similarly, as shown in  FIGS. 1-2  and  5 - 6 , the second curved member  104  has a first longitudinal edge  140  and a second longitudinal edge  142 . The second curved member  104  also has a first end  144  and a second end  146 . The second curved member  104  has an exterior surface  148  and an interior surface  150 . The curve of the second curved member  104  is defined by an arc having a radius R 2  and an arc angle δ. The curvature of the first curved member  102  and second curved member  104  may be the same (R 1 =R 2 ), the first curved member  102  may be flatter (R 1 &gt;R 2 ), or the first curved member  102  may be more curved (R 1 &lt;R 2 ). The second curved member  104  may be formed from numerous materials, such as a ceramic material, a low-grade glass, or metal, but glass is preferred. The curvature of the first curved member  102  and second curved member  104  is such that the orthogonal pressure force of the surrounding air on the exterior surfaces  136 ,  148  generates a substantially tangential force or a pure tangential force. Thus, the material from which the curved members  102 ,  104  are formed need only be stressed for compression. 
         [0034]    Referring now primarily to  FIG. 7  a lateral cross-section of the first edge member  114  of the plurality of edge members  106  is presented. The first edge member  114  has an edge member body  152  that is formed with a first seat  154  and a second seat  156 . The first seat  154  is sized and configured to receive the first longitudinal edge  128  (or second longitudinal edge  130 ) of the first curved member  102 . Similarly, the second seat  156  is sized and configured to receive a longitudinal edge (e.g., the first longitudinal edge  140  or second longitudinal edge  142 ) of the second curved member  104 . 
         [0035]    The first seat  154  includes a force-absorbing surface  158  and a shelf surface  160 . The angle formed between the force-absorbing surface  158  and the shelf surface  160  is an angle β that is preferably less than 90°. Angle α is 90° from the shelf surface  160  in  FIG. 7  and β is less than 90°. Angle β may be, for example, any angle between 90° and 70°, e.g., 90°, 89°, 88°, 87°, 86°, 85°, 75° or less. While integral angles are given as example, it should be understood that any angle over the range may be used, e.g., 88.743°. The end of the curved members  102 ,  104  may also be formed with an angle β so that the end sits perfectly within the seat, e.g., the first seat  154 . 
         [0036]    The edge body member  152  may be formed with a first protrusion  157  formed as a protrusion from a portion of the force-absorbing surface  158 . The first protrusion  157  together with the force-absorbing surface  158  and shelf surface  160  forms a slot for receiving and maintaining the first longitudinal edge  128  (or second longitudinal edge  130 ) of the first curved member  102 . As noted, the force-absorbing surface  158  and shelf surface  160  form an angle β that is less than 90° to help push the first end  132  into the first seat  154 . The first protrusion  157  may be spaced with respect to the exterior surface  136  of the first curved member  102  or may have minimal clearance. The first protrusion  157  may help prevent the first curved member  102  from sliding out when a bending force is applied to a flange portion  162  or when first curved member  102  otherwise is urged to move away from the shelf surface  160 . 
         [0037]    With respect to the first curved member  102 , the interior surface  138  near the first longitudinal edge  128  (or second longitudinal edge  130 ) is placed in the first seat  154  and, in particular, the interior surface  138  is placed on the shelf surface  160  and the first longitudinal edge  128  (or second longitudinal edge  130 ) is placed near to or adjacent the force-absorbing surface  158 . Because angle β is less than 90°, as the vacuum is applied to the interior space  110 , the first curved member  102  is pulled more tightly into the first seat  154  thereby improving the sealing characteristic. A ductile material may be used to form the first seat  154  or may be added to the first seat  154  to further increase the sealing ability of the first seat  154 . 
         [0038]    The second seat  156  is analogous to the first seat  154 . A second protrusion  159  is analogous to the first protrusion  157 . The operation of the second seat  156  with respect to the second curved member  104  is also analogous to the first seat  154 . In addition, while the first edge member  114  is described above, it should be understood that the second edge member  116  is analogous to the first edge member  114 . 
         [0039]    The first edge member  114  also may include the flange portion  162 , or mount, formed as part of the edge member body  152 . The flange portion  162  may further include an aperture  164 . The flange portion  162  and the aperture  164  may be used to attach the first edge member  114  to a mounting structure, e.g., a frame or pedestal, to hold the solar collector  100  at a desired site. A portion of the first edge member  114 , e.g., a flange portion  162 , may further include a tension connection portion  166  for coupling to the internal absorption-and-tensioning member  112 , tension member  120 , or absorber  118 . As used herein, “or” does not require mutual exclusivity. The edge members, e.g., the first edge member  114 , may be formed on all the edges or a portion of the edges. 
         [0040]    As shown in  FIG. 1 , the plurality of end caps  108  provides a seal to the lateral ends and may be coupled to the plurality of edge members  106 . A plurality of reinforcement members  168  may be included to further strengthen the plurality of end caps  108 . The end caps  108  may be any device for providing a seal on the lateral ends as shown. For example, the end caps  108  may be sufficiently strong vertical members or more preferably may be formed as spherical segment members as will be described further below in connection with  FIGS. 8-14 . The end caps  108  may be formed integrally with the curved members  102 ,  104  or may be coupled. As used herein, the term “coupled” includes coupling via a separate object and includes direct coupling. The term “coupled” also encompasses two or more components that are continuous with one another by virtue of each of the components being formed from the same piece of material or integrally formed. Also, the term “coupled” may include chemical (such as via a chemical bond), mechanical (such as with fasteners), thermal, or electrical coupling. 
         [0041]    In operation, the components for forming solar collector  100  may be transported to a desired location. The internal absorption-and-tensioning member  112 , if not already coupled to the first edge member  114  and second edge member  116 , may be so coupled. The coupling may occur by any known technique, such as welding, mechanical fasteners, mating notches, gluing or other technique. The first curved member  102  may be placed within the first seat  154  and the second curved member  104  may be placed in the second seat  156  and maintained in position. The end caps  108  may be coupled if not already formed integrally with or otherwise coupled to the curved members  102 ,  104 . At this point, the interior space  110  has been formed and the vacuum source  111  may be used to evacuate the interior space  110  completely or partially. 
         [0042]    As the interior space  110  is evacuated, the curved members  102 ,  104  are pulled tightly into the seats  154 ,  156  and a sealed interior space is formed. The sealing of the interior space  110  in this way may hold all of these components in their respective place without requiring any fasteners. In other embodiments, fasteners may be used to augment or initially hold these major components. 
         [0043]    As the interior space  110  is evacuated, the first curved member  102  and second curved member  104  tend to be urged towards each other and want to flatten out. This action places the internal absorption-and-tensioning member  112  into tension. The internal absorption-and-tensioning member  112  prevents the solar collector  100  from collapsing. As the pressure differential develops between the evacuated interior space  110  and the ambient outside pressure, the solar collector  100  transforms compression forces that develop into tangential forces. 
         [0044]    The first curved member  102  and second curved member  104  may be readily manufactured, or produced, in large quantities. If the curved members  102 ,  104  are formed as glass sheets, the curved members  102 ,  104  may be produced by rolling glass sheets into an arc shaped cross-sectional form. The rolled glass sheets are formed with a desired longitudinal length. In this manner, long collectors may be produced. In some situations, e.g., solar power farms, it may be desirable to have a large quantity of long, relatively slender solar collectors  100 . For example, with respect to  FIGS. 3 and 5 , and given the longitudinal dimension D 2  and lateral dimension D 1 , the solar collectors  100  may have an aspect ratio (longer dimension/shorter dimension) ranging from 1 to 100, and more typically 4 to 20. The lateral dimension may be any size, but in one illustrative, non-limiting embodiment is in the range of 1 to 10 meters. 
         [0045]    The tension member  120  carries tension and prevents the first curved member  102  and second curved member  104  from collapsing. As such, the tension member  120  may face fairly high stresses. For example, with reference to  FIGS. 3-6 , in an illustrative, non-limiting example in which the width, D 1 , is 1 meter, the arc angle γ is 45°, the arc angle δ is 60°, the stress carried by the tension member  120  would be greater than 200 kN/m 2 . If spring steel, which has a resistance to stress greater than 2500 N/mm 2 , is used in this example for the tension member  120 , the tension member  120  would theoretically need only be 0.1 mm in thickness. It should be apparent that numerous materials and parameters may be selected for the tension member  120  to carry the design loads for a particular solar collector. 
         [0046]    Referring now primarily to  FIGS. 8 and 9 , another illustrative, non-limiting embodiment of a solar collector  200  is presented. The solar collector  200  has a first curved member  202  and a second curved member  204 . Analogous to the solar collector  100  of  FIGS. 1-7 , the first curved member  202  and the second curved member  204  together with a plurality of end caps define an interior space  210 . An internal absorption-and-tensioning member  212  transverses the interior space  210 . 
         [0047]    A plurality of end caps  208  includes a first spherical segment member  272 , a second spherical segment member  274 , a third spherical segment member  276  and a fourth spherical member  278 . These spherical segment members  272 ,  274 ,  276 ,  278  are formed as sections of a sphere, or ball. The first and second spherical segment members  272  and  274  are segments of a sphere having a radius equivalent to the curvature of the radius R 1  of the first curved member  202 . Similarly, the third and fourth spherical segment members  276  and  278  are formed as sections of the surface of a sphere with a radius R 2  equivalent to the curvature of the second curved member  204 . In this way, the spherical segment members  272 ,  274 ,  276 ,  278  preferably only produce tangential loads that are delivered to the curved members  202 ,  204 . As such, bending forces and other forces may be substantially or altogether avoided. 
         [0048]    Referring now primarily to  FIGS. 10-14 , one illustrative, non-limiting embodiment of a first curved member  302 , an associated first spherical segment member  372 , and a second spherical segment member  374  are presented. Referring initially to  FIG. 10 , the curvature of the first curved member  302  and the first spherical segment member  372  may be described by visualizing a sphere  380  having a radius R 1 . The external surface of the sphere  380  matches the curvature of the interior surface of the first curved member  302  as shown well in  FIGS. 10 and 13 . 
         [0049]    The first spherical segment member  372  is shaped as a spherical segment that matches the sphere  380  as shown best in  FIGS. 10 and 12 . A lower edge  382  of the first spherical segment member  372  is defined by a footprint radius r 1  that substantially equals one-half a distance D 1  between a first longitudinal edge  328  and a second longitudinal edge  330 . The first spherical segment member  372  may be formed integrally with the first curved member  302  or the first spherical segment member  372  may be coupled to the first curved member  302 . As seen clearly in  FIGS. 12 and 13 , the first spherical segment member  272  conforms to the radius R 1  of the sphere  380  in cross-sections taken in two orthogonal planes, i.e., a longitudinal cross-section as suggested by  FIG. 12  and a lateral cross-section as suggested by  FIG. 13 . 
         [0050]    The second spherical segment member  374  is formed analogously to the first spherical segment member  372 . The completed first curved member  302  and the spherical segment members  372 ,  374  are shown in  FIG. 14 . It should be understood that a third and fourth spherical segment members on a second curved member, e.g., the second curved member  104  in  FIGS. 1-7 , would be formed in analogous fashion with a sphere matching the curvature or radius R 2  of the second curved member. 
         [0051]    In an alternative embodiment, the internal absorption-and-tensioning member, e.g., internal absorption-and-tensioning member  112  of  FIG. 2 , may include a tension member and absorber formed as an integral unit. In other words, the absorber and tubes may carry the tension of the tensile forces or separate components formed as an integral piece. In one illustrative, non-limiting embodiment, the internal absorption-and-tensioning member is formed from a material that is tension resistant and has a low thermal transfer coefficient, such as from a Kevlar® material, carbon composite, Invar steel (steel allow with low thermal expansion coefficient), etc. 
         [0052]    In another illustrative, non-limiting embodiment, the interior space, e.g., interior space  110  of  FIG. 1 , of the solar collector need not be completely evacuated for a particular application. In such instances, the second curved member may be readily modified to use less expensive materials or may include additional thermal insulation applied to the exterior of the second curved member. Moreover, if the pressure differential (between the exterior and the interior space) is not great, the tension member may simply be metal cables or fibers added to the absorber or simply the absorber itself. In another alternative embodiment, the absorber is placed below the tension member and the tension member is made of a transparent material. 
         [0053]    Although the present invention and its advantages have been disclosed in the context of certain illustrative, non-limiting embodiments, it should be understood that various changes, substitutions, permutations, and alterations can be made without departing from the scope of the invention as defined by the appended claims. It will be appreciated that any feature that is described in a connection with any one embodiment may also be applicable to any other embodiment.