Patent Publication Number: US-2012042932-A1

Title: Solar collector

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     The present invention claims priority U.S. patent application Ser. No. 12/982,703, entitled “Solar Collector Having A Spaced Frame Support Structure With A Multiplicity Of Linear Struts,” filed Dec. 30, 2010, which is incorporated herein by reference in its entirety for all purposes and claims priority to U.S. Provisional Application No. 61/362,591, entitled “Optimized Solar Collector,” filed Jul. 8, 2010, which is incorporated herein in its entirety for all purposes. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to solar technologies. More specifically, the present invention relates to various collector, reflector and support structure designs for use in concentrating photovoltaic systems. 
     BACKGROUND OF THE INVENTION 
     Typically, the most expensive component of a photovoltaic (PV) solar collection system is the photovoltaic cell. To help conserve photovoltaic material, concentrating photovoltaic (CPV) systems use minors or lenses to concentrate solar radiation on a smaller cell area. Since the material used to make the optical concentrator is less expensive than the material used to make the cells, CPV systems are thought to be more cost-effective than conventional PV systems. 
     One of the design challenges for any CPV system is the need to balance multiple priorities. For one, a CPV system requires a support structure that arranges the optical concentrators and the photovoltaic cells such that incoming sunlight is efficiently converted into electricity. This support structure should also accommodate a tracking system and provide for the adequate dissipation of heat. Another consideration is the cost of manufacturing, installing and repairing the CPV system. Existing CPV designs address these issues in a wide variety of ways. Although existing CPV systems work well, there are continuing efforts to improve the performance, efficiency and reliability of CPV systems. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention relates to a photovoltaic solar energy collector suitable for use in a solar energy collection system that includes a stand and a tracking system. The solar collector includes multiple reflector panels that each extend in a longitudinal direction, one or more solar receivers, and a space frame support structure. In this aspect of the present invention, there is a gap between two of the reflector panels. At least one solar receiver is positioned above the gap between the reflector panels. The reflector panels are arranged to direct incident sunlight to the solar receiver(s). The space frame support structure physically supports the reflector panels and the solar receiver(s). The space frame support structure also includes multiple linear struts that are connected at nodes to form a repeating geometric pattern. At least two of these struts slant downward and outward from the solar receiver(s) and extend through the gap between the reflector panels to help support the solar receiver(s). 
     In another aspect of the present invention, a photovoltaic solar energy collector will be described. The solar collector includes at least one reflector panel that extends in a longitudinal direction, one or more solar receivers and a space frame support structure. The space frame support structure physically supports the reflector panel(s) and the receiver(s). The space frame support structure includes multiple linear struts that are connected at nodes to form a repeating geometric pattern. The repeating geometric pattern repeats along the longitudinal axis. The longitudinal period of the repeating geometric pattern does not equal the length of any reflector panel or solar receiver in the collector. 
     In another aspect of the present invention, a photovoltaic solar energy collection system will be described. The solar energy collection system includes a solar energy collector, a stand and a tracking system. The solar collector includes one or more reflector panels, one or more solar receivers and a space frame support structure. The space frame support structure includes multiple linear struts that are connected at nodes to form a repeating geometric pattern. The stand pivotally supports the collector for pivotal movement around a pivot axis. The tracking system causes the collector to pivot around the pivot axis to track movements of the sun along at least one axis. In various embodiments, the range of motion around the pivot axis is at least 150 degrees, 160 degrees and/or 170 degrees. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention and the advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which: 
         FIGS. 1A and 1B  are diagrammatic perspective and cross-sectional views of a solar collector according to a particular embodiment of the present invention. 
         FIG. 2A  is a diagrammatic side view of a reflector panel with a convex shape according to a particular embodiment of the present invention. 
         FIG. 2B  is a diagrammatic side view of a reflector made of multiple reflector panels according to a particular embodiment of the present invention. 
         FIG. 2C  is a diagrammatic side view of the reflector panel illustrated in  FIG. 2A . 
         FIGS. 3A-3D  are diagrammatic cross-sectional views of solar receivers and reflector panels according to various embodiments of the present invention. 
         FIGS. 4A and 4B  are diagrammatic perspective and cross-sectional views of a collector unit according to a particular embodiment of the present invention. 
         FIGS. 5A-5C  and  6 A- 6 C are diagrammatic cross-sectional views of various collectors that are formed from different arrangements of the collector units illustrated in  FIGS. 4A and 4B . 
         FIGS. 7A-7D  are diagrammatic cross-sectional and perspective views of a solar collector with a space frame support structure in accordance with various embodiments of the present invention. 
         FIGS. 8A and 8B  are diagrammatic perspective views of a connector suitable for use in a space frame support structure according to a particular embodiment of the present invention. 
         FIGS. 9A and 9B  are diagrammatic cross-sectional and perspective views of a solar collector with a space frame support structure in accordance with another embodiment of the present invention. 
         FIGS. 10A and 10B  are diagrammatic cross-sectional and perspective views of a reflector panel according to a particular embodiment of the present invention. 
         FIG. 11  is a diagrammatic perspective view of multiple solar collectors that are arranged to form a solar collector row in accordance with a particular embodiment of the present invention. 
         FIG. 12  is a diagrammatic cross-sectional view of a solar collector that is suitable for pivoting around an axis in accordance with a particular embodiment of the present invention. 
         FIGS. 13A and 13B  are diagrammatic cross-sectional and perspective views of a solar collector with a support cable in accordance with a particular embodiment of the present invention. 
     
    
    
     In the drawings, like reference numerals are sometimes used to designate like structural elements. It should also be appreciated that the depictions in the figures are diagrammatic and not to scale. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention relates generally to concentrating photovoltaic systems. The assignee for the present application, Skyline Solar, Inc., has received multiple patents related to such technologies, such as U.S. Pat. No. 7,709,730, entitled “Dual Trough Concentrating Solar Photovoltaic Module,” filed Apr. 10, 2008, which is hereby incorporated by reference in its entirety for all purposes and is hereinafter referred to as the &#39;730 patent. 
     The &#39;730 patent describes various solar collector designs that involve a trough-shaped reflector that directs incident sunlight to a string of photovoltaic cells. The described designs work well for many applications. During the course of installing, manufacturing and operating solar energy collection systems, however, the assignee has identified various areas in which the designs could be further improved. For example, ordinary wear and tear can form gaps in the reflector. This can skew the reflection of light by the reflector and may reduce the collection of solar energy. It would also be desirable to develop an improved support structure for the collector that is resilient, lightweight and cost-effective. The present application contemplates a wide variety of design concepts relating to collectors, support structures, reflector panels, power plants and tracking systems that address these and other concerns. 
     Initially, with reference to  FIGS. 1A and 1B , a concentrating photovoltaic solar collector  100  according to a particular embodiment of the present invention will be described. The solar collector includes multiple solar receivers  102  and reflector panels  104  that each have a compound curvature. The reflector panels  104  are arranged in one or more rows that extend along the longitudinal axis  106 . Each row includes multiple, adjacent reflector panels that cooperate to form a reflector  108  with a substantially continuous reflective surface. The solar receivers  102  each have at least one photovoltaic cell  118  and are arranged to form a string of photovoltaic cells  118 . A space frame support structure  110  physically supports the reflector panels  104  and the solar receivers  102 . A tracking system causes the collector to pivot to track the movements of the sun.  FIG. 1B  is a diagrammatic cross-sectional view of the solar collector  100  illustrated in  FIG. 1A . 
     Foundation settling, differential thermal expansion, and mechanical tolerances in the manufacturing and installation of a reflector can cause undesirable gaps to open up between the reflector panels  104  in the reflector. If the reflective surface becomes non-continuous, then the flux line that forms on the string of photovoltaic cells  118  may include gaps and become non-continuous as well. As a result, the exposure of the photovoltaic cells to concentrated sunlight will be less uniform, which can substantially reduce the cell strings&#39; efficiency. 
     Various embodiments of the present invention relate to reflector panels  104  that are designed to address this problem. More specifically, each reflector panel  104  incorporates two different curvatures along two different axes. Along a plane defined by an x axis  114  and a y axis  112 , the reflector panel  104  has a concave shape. Along another axis (e.g., the longitudinal axis  106 , which goes into the page in  FIG. 1B  and is perpendicular to the x-y plane), the reflector panel has a convex shape. The concave shape is arranged to direct incident light  116  to the solar receivers  102 , as shown by the arrows in  FIG. 1B . The convex shape causes the light that is reflected by the panel  104  to be reflected in a wider arc along the longitudinal axis  106 . That is, the convex shape of each panel  104  forms a wider flux line (as measured along the longitudinal axis  106 ) than would be the case if the panel were flat along the longitudinal axis. As a result, gaps in the flux line that are formed by gaps in the reflective surface are covered up or washed away. This can produce a more continuous, uniform flux line, which contributes to greater cell string efficiency. 
     Another notable feature of the collector  100  illustrated in  FIGS. 1A and 1B  is the space frame support structure  110  that supports the solar receivers and reflector panels. The space frame support structure includes multiple linear struts  120  that are connected with nodes  122  to form repeating geometric shapes (e.g., triangles, pyramids, tetrahedrons, etc.) This arrangement offers various advantages. For one, the large apertures in between the struts  120  allow the free flow of air and reduce wind load on the collector. The use of repeating components, such as the nodes  122  and the struts  120 , can streamline the manufacturing, installation and repair of the space frame. The longitudinal period of the repeating geometric pattern does not have to correspond to the longitudinal length of either a receiver or reflector panel, allowing flexibility in receiver and reflector panel design. 
     Additionally, the space frame support structure  110  in  FIGS. 1A and 1B  is designed to operate as a unitary structure. That is, struts that support different components of the collector are interlocked with the rest of the space frame and are therefore more stable and resistant to stress and bending. In the illustrated embodiment, for example, the support for the solar receivers is not a single, isolated column or pedestal. Instead, it is arranged in the form of multiple receiver support struts  124  that are connected to nodes  122  that in turn connect with and help support other struts and components, such as the reflector panels  104 . The struts are arranged to firmly anchor important components of the collector within the support structure, reduce or eliminate bending, and form multiple paths through which mechanical loads may be dispersed. 
     The struts  120 / 124  and nodes  122  of the space frame structure may take a variety of forms. By way of example, the struts  120 / 124  may be cylinders, tubes, pipes, roll formed steel sections, hot rolled steel sections, etc. Many of the struts may have similar circumferences and may be formed using the same or similar manufacturing processes. The struts  120 / 124  may be connected with one another at the nodes  122  in any suitable manner, including the use of welding, metal clinching, adhesive, rivets, bolts and nuts, fasteners, etc. In some embodiments, the nodes  122  include connectors that are formed from extruded aluminum, sheet metal, forged steel spheres or other materials. 
     Preferably, the space frame support structure is designed to be compatible with many different types of reflector panels and solar receivers. In the illustrated embodiment, for example, the struts  120  underneath the reflector panels  104  form a stable base platform for physically supporting the reflector panels. This base platform is not arranged to be compatible only with a reflector panel of a highly specific length and curvature. That is, the base platform is not necessarily form fitted to the dimensions of a particular reflector panel. Instead, with minimal modifications (e.g., the drilling of holes, the securing of fasteners, etc.), the base platform can be configured to accommodate reflector panels of varying lengths, shapes and/or curvatures as long as the reflector panels, when lined up along the length of the collector, generally fit the dimensions of the space frame support structure. 
     The reflector panels  104  and solar receivers  102  may be arranged in a wide variety of ways, depending on the needs of a particular application. In the illustrated embodiment, for example, the reflector panels  104  form a U-like shape. The solar receivers  102  are arranged in two adjacent rows that are positioned higher and over a region that is between the reflectors. The two rows of solar receivers  102  support strings of photovoltaic cells  118  that face away from one another and towards the reflectors  104 . When sunlight is incident on one of the longitudinally extended reflectors  104 , a longitudinally extended flux line is formed on the corresponding string of photovoltaic cells  118 . In another embodiment, the solar receivers  102  are positioned at the periphery of the collector  100 , and the reflector panels  104 , rather than directing incident light inward towards the center of the collector  100 , instead direct light outward towards the periphery of the collector  100 . 
     The reflector panels  104  may be made from any suitable reflective material. For example, metalized glass and aluminum work well as materials for the reflector panels  104 . Each solar receiver  102  may include or be attached with a wide variety of components, such as a heat sink, fins, base plates, etc. In some embodiments, the heat sink and/or the solar receiver include a fluid conduit. The sunlight that is reflected onto the solar receivers  102  may be used to heat a liquid that is flowing through the fluid conduit. A wide variety of possible solar receiver, heat sink and fin designs are described in U.S. Pat. No. 7,820,906, entitled “Photovoltaic Receiver”, filed May 20, 2008, and U.S. patent application Ser. No. 12/340,379, entitled “Solar Receiver,” filed Dec. 19, 2008, which are hereby incorporated in their entirety for all purposes. 
     Referring next to  FIG. 2A and 2C , a reflector panel  200  with a compound curvature according to a particular embodiment of the present invention will be described. The reflector panel has a convex shape in one direction and a concave shape in another.  FIGS. 2A and 2C  are diagrammatic side views of the reflector panel  200  that show its convex shape. The convex shape is defined in part by a distance d, which measures the amount of maximum displacement of the convex shape relative to a flat reference surface, and a longitudinal length l. While the reflector panel in  FIGS. 2A and 2C  is shown as having a single convex bow, the reflector panel may also include multiple convex regions. In some embodiments, these convex regions may be interspersed with concave and/or flat regions. When properly utilized in a suitable collector, the convex shape of the reflector panel  200  may substantially improve the efficiency and performance of the collector. 
     As discussed earlier in connection with  FIG. 1A , the present invention contemplates collector designs where multiple reflector panels  200  are arranged together along a longitudinal axis  106  to form a reflector  202  with a curved reflective surface. The reflective surface receives incident sunlight and directs it to a string of one or more photovoltaic cells. As a result, a flux line (e.g., a strip or band of concentrated illumination formed by the reflected light) is formed on the string of cells. The shape and dimensions of the flux line are defined in large measure by the shape of the reflector panel  200 . 
     When the reflector panels are flat along the longitudinal axis, the flux line will tend to mirror the continuity of the reflective surface. That is, if the reflective surface is continuous, then the flux line tends to be continuous. However, if the reflective surface has gaps, then the flux line will also tend to have gaps (e.g., portions of the cell face within the periphery of the flux line that do not receive light from the reflective surface.) 
     Since the movement of the sun throughout the day causes light to be reflected by the reflector panels at a variety of angles, the effect of any gap between the reflector panels may appear almost anywhere along the length of the cell string. That is, a gap between two reflector panels could prevent a central portion of one of the photovoltaic cells from being illuminated by reflected light. This can substantially reduce cell efficiency, particularly when the cells are electrically connected in series. Unfortunately, it is not uncommon for gaps to develop between the reflective panels that make up the reflective surface. The gaps may arise due to errors in manufacturing, installation, operation or constant thermal expansion and contraction. 
     The convex shape of the reflector panels  200  helps to eliminate gaps in the flux line.  FIGS. 2A and 2C  illustrate how incoming sunlight  204  is reflected when it strikes the convex curvature of the reflector panel  200 . The reflected light tends to spread or fan out from the reflector panel  200 . Depending on the direction of the incoming sunlight, the reflected light may spread out on both sides of the reflector panel (e.g., as in  FIG. 2A , where the incoming light is perpendicular to the longitudinal axis  106 ) or more to one side (e.g., as in  FIG. 2C , where the incoming light is coming in at an angle.) Generally, the reflected light forms a wider flux line (along the longitudinal axis) then would be the case if the reflector panel  200  were flat. 
     Referring next to  FIG. 2B , a reflector  202  made of the reflective panels  200  illustrated in  FIGS. 2A and 2C  will be described. The reflective panels  200  are arranged adjacent to one another along a longitudinal axis  106 . Collectively, the reflective panels  200 , each with length  1 , form a reflector  202  with a length L. The reflector includes multiple convex shapes, where each reflective panel  200  forms one of the concave shapes. When multiple reflector panels are arranged together in this manner, the aforementioned spreading out of the reflected light washes out or eliminates gaps in the flux line that would be normally be caused by gaps between the reflector panels. As a result of the washing out effect, the flux line on the photovoltaic cells is more continuous and uniform. 
     Various designs involve a reflector  202  where at least two or more adjacent reflector panels  200  of the reflector  202  have the same concave shape (e.g., as seen in  FIG. 2B .) In some embodiments, the convex shape of each reflector panel  200  is also substantially symmetrical and the axes of symmetry  206  of the convex shapes of the reflector panels  200  run substantially parallel to one another. This arrangement promotes the spreading of light on both sides of each reflector panel  200 , which can be particularly effective in washing out the effects of gaps between the reflector panels  200 . 
     It should also be appreciated that the amount of convex curvature in each reflector panel  200  of a reflector  202  need not be the same. By way of example, the end reflector panels  200  shown in  FIG. 2B  may have a greater amount of convex curvature than the other reflector panels, since there may be a larger gap between these reflector panels and reflector panels on a longitudinally adjacent solar collector than the gap between adjacent reflector panels on the same solar collector. 
     The convex curvature of the reflector panel  200  may be different near the lower edge of a reflector panel  200  (e.g., the displacement d may be greater or lower) than it is near the upper edge of the reflector panel  200 . The variation in convex curvature may be inversely related to the distance variation between the reflector panel and receiver. In particular the lower edge of the reflector panel may be closer to the receiver than the upper edge of the reflector panel. The convex curvature may thus be larger near the lower edge of the reflector panel and smaller near the upper edge of the reflector panel. This variation in convex curvature across the reflector panel may result in a longitudinally more uniform flux line. 
     It should be noted that in the figures, the degree of convex curvature is exaggerated for the purpose of clarity. Generally, the concavity of the reflector panel  200  is significantly greater than the convexity of the reflector panel (e.g., the concave radius of curvature may be 20 or more times less than the convex radius of curvature.) Note that a smaller radius of curvature corresponds to more curvature or a higher degree of concavity or convexity. Various embodiments involve a reflector panel with a convex radius of curvature for the reflector panel  200  that is approximately between 50 and 70 m, although the degree of curvature may be lower or higher for other implementations. In particular reflector panels  200  having a longer length in the longitudinal direction may generally have a larger convex radius of curvature. 
     Referring next to  FIGS. 3A-3D , solar receiver and reflector arrangements according to various embodiments of the present invention will be described.  FIG. 3A  is a diagrammatic cross-sectional view of an arrangement  300  including a reflector panel  302  and a solar receiver  304 . From this vantage point, the reflector panel  302  has a concave shape. The solar receiver includes one or more photovoltaic cells  306 . The reflector panel  302 , whose optical aperture has a width w, is arranged to direct incident sunlight to the solar receiver. 
     This arrangement  300  offers various advantages. For one, the solar receiver is positioned outside of the optical aperture  308  of the reflector panel  302 . In the illustrated embodiment, for example, the solar receiver  304  (and any associated support structure) does not directly overlie the reflector panel  302  and does not shade the reflector panel  302  during the normal operation of the solar collector. This lack of shadowing helps maximize the energy output of the photovoltaic cells  306  by increasing the amount of light that is reflected to the cells. 
     Also, the arrangement  300  allows the reflector panel  302  to be positioned relatively close to the solar receiver  304 . If the reflected light has to traverse a shorter distance between the reflector and the solar receiver, less precision is required from the collector and the tracking system. By way of example, assume that f avg  is the average distance between all the points on the surface of the reflector panel  302  and the photovoltaic cell  306 . Various collector designs arrange the solar receiver  304  and the reflector panel  302  such that f avg  is approximately between 0.5 m and 1.5 m, although larger or smaller values of f avg  may be used. 
     The size and shape of the reflector panel  302  may be adjusted so that light is concentrated on the photovoltaic cell  306  to the desired degree. The optical concentration factor, which may be defined as the ratio of the width Iv, of the reflector panel  302  to the height of the photovoltaic cell h, relates to the average increase in the sunlight intensity as compared to the intensity of the incoming sunlight  315 . Since the photovoltaic cell  306  is typically the most expensive component of a collector, if a larger reflective surface can be used to focus light on a smaller photovoltaic area, there may be substantial cost savings. In various embodiments of the present invention, the reflector panels and solar receivers of the collector are arranged to have an optical concentration factor of approximately between 5 and 50. 
     The efficiency of the solar collector may also be improved by controlling the width w s  of the solar receiver. If the receiver width w s  along the x axis is reduced relative to the width w of the optical aperture, a greater proportion of the total collector area can be taken up by the reflectors, which in turn results in the direction of more light to the photovoltaic cells of the collector. Some implementations of the present invention contemplate a receiver width that is less than approximately 10%, 15% or 20% of the optical aperture width w. 
     Referring next to  FIG. 3B , a variation on the solar receiver and reflector arrangement of  FIG. 3A  according to another embodiment of the present invention will be described. The concave reflective surface of  FIG. 3A , instead of being formed by a single reflector panel, is instead formed by multiple reflector panels  310   a/   310   b/   310   c  that are arranged side by side. That is, the reflector panels  310   a/   310   b/   310   c  form strips that extend substantially parallel to one another down the longitudinal length of the collector. For some applications, it is more cost-effective to manufacture a larger amount of smaller reflector panels as opposed to a smaller amount of larger ones. 
     Although three separate reflector panels are shown, it should be appreciated that the concave reflective surface may be formed from almost any number of reflector panels. The reflector panels may be attached in any suitable manner. In the illustrated embodiment, for example, the edges of the reflector panels  310   a/   310   b/   310   c  overlap one another and can be secured together using any suitable means, such as a fastener, a bolt, an adhesive, a latch, welding, etc. Various implementations involve multiple reflective panels that are each curved such that they cooperate to form a single reflective surface with a concave and/or parabolic shape. In other embodiments, each reflective panel  310   a/   310   b/   310   c  is substantially flat in the x-y plane, but are angled such that they collectively approximate a single concave or parabolic curve. They may have a slight convex curvature along the longitudinal axis. 
     Referring next to  FIGS. 3C and 3D , a solar receiver and reflector arrangement  320  according to another embodiment of the present invention will be described. In  FIGS. 3A-3B , the solar receivers  304  were generally positioned higher than the highest edge of the associated reflector panel  302 . By contrast,  FIG. 3C  illustrates a solar receiver  304  that is positioned at a height than is in between the heights of the lower and upper edges  312  and  314  of the reflector panel. Since the solar receiver  304  is more centrally located relative to the reflector panel  302 , the angles of incidence (in a plane defined by the x axis  114  and the y axis  112 ) of reflected light on the cell face may be smaller than in other designs where the solar receiver  304  is positioned particularly low or high relative to the reflector panel  302 . A reduced angle of incidence may cause less of the light to be reflected off the face of the cell, which helps increase the collection of solar energy. In the illustrated embodiment, the face of the photovoltaic cell  306  on the solar receiver  304  is substantially perpendicular to the optical aperture  308  of the reflector panel  302 , although in other embodiments it may be tilted. 
     Referring now to  FIG. 3D , possible arrangements of a reflector panel and a solar receiver according to various embodiments of the present invention are described.  FIG. 3D  illustrates a parabolic curve  322 , which represents the shape of one or more possible reflector panels along a plane defined by a y axis  112  and a x axis  114 . The focus  324  indicates a location of a photovoltaic cell where these possible reflective panels direct incident sunlight. A first portion  326  of the parabolic curve approximates the shape and the position of the reflector panel in  FIG. 3A . A second portion  328  of the parabolic curve approximates the shape and position of the reflector panel in  FIG. 3C . Any number of reflector panels can be imagined that have reflective surfaces that conform substantially with the parabolic curve, which is arranged to direct substantially all incoming incident sunlight to the focus  324 . A review of  FIG. 3D  indicates that the degree of curvature of a reflector panel may be affected by whether the solar receiver is positioned high, low or centrally relative to those solar receivers. It should be appreciated that  FIG. 3D  is applicable only to some of the embodiments contemplated by the present invention and that the present invention contemplates other embodiments in which the reflector panels have other curvatures, arrangements and/or shapes. 
     Referring next to  FIGS. 4A and 4B , a collector unit  400  according to a particular embodiment of the present invention will be described. The collector unit  400  may represent a stand alone structure, or it may be understood as a basic building block for many other designs. That is, multiple collector units  400  may be arranged together to form a wide variety of collector designs, some of which are described in  FIGS. 5A-5C  and  FIGS. 6A-6C .  FIG. 4A  illustrates a diagrammatic perspective view of a collector unit  400  that includes one or more solar receivers  402  and one or more reflector panels  404 .  FIG. 4B  illustrates a cross-sectional view of the collector unit  400 , which may also have any of the features described in connection with  FIGS. 2A-2C  and  3 A- 3 D. The collector unit  400  is arranged such that the reflector panels  404  direct incident sunlight  410  to the solar receivers  402 . The reflected light forms a flux line on the receivers  402  that extends longitudinally down the length of the collector unit. 
     As discussed previously with respect to other embodiments, the design of the collector unit  400  offers several advantages. Because the reflective surface of the reflector panels  404  is positioned in close proximity to the solar receivers  402 , the light need not traverse a long distance between the reflector panels  404  and the solar receivers  402 , which improves mechanical and tracking tolerances for the collector. Additionally, the solar receivers  402  are positioned over a region just outside a lower edge  406  of the reflective surface. As a result, the solar receiver  402  (and any associated support structures) do not shade the reflective surface and do not prevent incident sunlight from reaching the reflective surface. 
     Some collector units involve multiple solar receivers and reflector panels that are arranged in rows that run parallel to one another along a longitudinal axis. In the illustrated embodiment, for example, multiple longitudinally adjacent reflector panels  404  cooperate to form a reflector  408  with a substantially continuous reflective surface. The solar receivers  402  are also arranged longitudinally adjacent to one another to form a solar receiver row that extends in the longitudinal direction  106 . The length of the solar receiver row is generally substantially similar to that of the length L of the reflector. In various implementations, however, the solar receiver row may be somewhat longer or shorter than the reflector. It should be appreciated that although the figure illustrates ten solar receivers  402  in the solar receiver row and seven reflector panels  404  forming the reflector, almost any suitable number or combination of solar receivers and reflector panels may be used. 
     Multiple collector units  400  may be arranged in a wide variety of ways to form different types of solar collectors. Some examples are provided in  FIGS. 5A-5C .  FIG. 5A  illustrates a solar collector  500  made of two collector units  400   a  and  400   b  in which the first and second reflectors  502   a/   502   b  are arranged with their backsides to one another in a tent-like arrangement. In the illustrated embodiment, the first and second reflectors  502   a/   502   b  are arranged substantially symmetrically around the center of the collector. A reflective surface on each reflector  502   a/   502   b  has a parabolic or otherwise curved shape. The first and second reflectors  502   a/   502   b  are arranged to form an A-like shape, i.e. the inner edges  503   a/   503   b  of the reflectors  502   a/   502   b  are positioned adjacent to one another at the middle of the collector, the reflectors  502   a/   502   b  curve inward relative to one another, and the outer edges  506   a/   506   b  of the reflectors  502   a/   502   b  are positioned at the periphery of the collector  500  and at a lower height than the inner edges  503   a/   503   b  of the reflectors  502   a/   502   b.  The first and second solar receivers  504   a/   504   b  are positioned above a peripheral region outside the outer edges  506   a/   506   b  of the first and second reflectors  502   a/   502   b,  respectively, and are physically supported by receiver support structures. An advantage of this implementation is the location of the solar receivers  504   a/   504   b  at the periphery of the collector  500 , which allows easy access to the receivers for installation, cleaning and repair. Additionally, the solar receivers  502   a/   502   b  are somewhat removed from other collector components, which may facilitate heat dissipation. 
       FIG. 5B  illustrates a different way of arranging two collector units  400   a/   400   b.  In the illustrated embodiment, the first and second reflectors  522   a/   522   b,  which may have a curved or parabolic shape, are substantially symmetrically arranged and curve outward to form a trough- or U-like shape. That is, the inner edges  523   a/   523   b  of the reflectors  522   a/   522   b  are positioned closer to the middle of the collector  520 , while the outer edges  526   a/   526   b  of the reflectors  522   a/   522   b  are positioned at the periphery of the collector  520 . The outer edges  526   a/   526   b  are positioned higher than the inner edges  523   a/   523   b,  which results in the formation of a U-type design. Two solar receivers  524   a/   524   b,  whose respective photovoltaic cells face away from one another, are positioned over a central region between the inner edges  523   a/   523   b  of the reflectors  522   a/   522   b.  An advantage of this implementation is that the two solar receivers  524   a/   524   b  are positioned at a single location, which means that fewer structures are required to physically support them then if they were separated across the width of the collector  520 . 
       FIG. 5C  illustrates a collector  540  in which the collector units  400   a/   400   b  are staggered at different heights. Rather than being arranged side-by-side, the second solar receiver  544   b  is positioned over and shades the first solar receiver  544   a.  To maintain its proper alignment with the second solar receiver  544   b,  the second reflector  542   b  is positioned higher than the first reflector  542   a.  That is, the collector  540  is generally symmetrical between the two collector units  400   a/   400   b,  except that the collector units are offset along the vertical y axis  112 . An advantage of this design is that the solar receivers  544   a/   544   b  take up less space along the x axis  114 . A review of  FIG. 5C  indicates that the pairs of solar receivers  524   a/   524   b  and  544   a/   544   b  of  FIGS. 5B and 5C  take up a distance G and G′, respectively, which extends along the x axis  114 . Because the two solar receivers  544   a/   544   b  in  FIG. 5C  are stacked over one another, the distance G′ of the collector  540  in  FIG. 5C  is substantially less than the distance G of the collector  520  in  FIG. 5B . As a result, the ratio of the reflective area of the collector to the total area of the collector is increased, which means that for the same amount of real estate, more light is directed to the photovoltaic cells of the collector  540  in  FIG. 5C . 
     Referring next to  FIGS. 6A-6C , extended solar collectors in accordance with various embodiments of the present invention will be described. Each extended solar collector involves multiple collector units that are coupled with a common base. The common base allows a larger number of reflectors and solar receivers to be part of a single collector structure that can be pivoted to track the movements of the sun. 
       FIG. 6A  relates to an extended collector  600  that is an extended version of the A-type collector illustrated in  FIG. 5A . That is, in addition to the reflectors  502   a/   502   b  and solar receivers  504   a/   504   b  illustrated in  FIG. 5A , the extended collector includes at least a third reflector  502   c  and a fourth reflector  502   d.  The third and fourth reflectors  502   c/   502   d  are positioned outside the outer edges of the first and second reflector panels  502   a/   502   b,  respectively, and are oriented in a similar manner (e.g., curving inwards towards a central region of the collector, with the inner edges being higher than the outer edges.) Each reflector has a reflective surface and an opposing backside surface. The reflective surface of each reflector is arranged to direct incident light to one of the solar receivers. The first and second solar receivers  504   a/   504   b  are positioned near the inner edges and on the backsides of the third and fourth solar receivers  502   c/   502   d,  respectively. As a result, it is not necessary to use a separate structure to support the solar receivers  504   c/   504   d,  since the reflectors  502   c/   502   d  can be used to hold the solar receivers in position. In the illustrated embodiment, there are two more reflectors and two more solar receivers in the collector beyond the ones described above, that essentially repeat the aforementioned receiver-reflector arrangement (e.g., each reflector is arranged to direct sunlight to a solar receiver that is on the backside of the next reflector, except for the solar receivers at the far ends of the collector  600 .) It should be appreciated that fewer or more reflectors and solar receivers may be used. All of the reflectors and receivers are mounted on a common base structure  602  that can be pivoted along at least one axis to track incident sunlight. 
       FIG. 6B  relates to an extended collector  620  that is an extended version of the U-type collector  520  illustrated in  FIG. 5B . That is, in addition to the reflectors  522   a/   522   b  and solar receivers  524   a/   524   b  illustrated in  FIG. 5B , the extended collector  520  includes at least a third reflector  522   c  and a fourth reflector  522   d.  The third and fourth reflectors  522   c/   522   d  are positioned outside the outer edges of the first and second reflectors  522   a/   522   b,  respectively, and are oriented in a similar manner (e.g., curving outwards away from a central region of the collector, with the outer edges being higher than the inner edges.) Additional third and fourth solar receivers  524   c/   524   d  are attached with the backsides of the first and second reflectors  522   a/   522   b.  The third and fourth reflectors  522   c/   522   d  are arranged to direct incident light into the third and fourth solar receivers  524   c/   524   d,  respectively. The solar receivers  524   a - 524   d  and reflectors  522   a - 522   d  are mounted on a common base structure  604  that is arranged to pivot in order to track movements of the sun. Additional reflector panels and solar receivers may be attached with the common base structure  604  in the manner of the third and fourth solar receivers  524   c/   524   d  and the third and fourth reflectors  522   c/   522   d  to further expand the reflective area and power generating capacity of the solar collector  620 . 
       FIG. 6C  illustrates an extended collector according to another embodiment of the present invention. The features of the collector  640  are almost identical to those of the collector  600  in  FIG. 6A , except that at least some of the reflectors  502   a - 502   d  are at different heights. In the illustrated embodiment, for example, the collector  640  is symmetrical along a bisecting axis  608  and the common base structure  606  supports at least some of the reflectors on each side of the bisecting axis  608  such they are offset from one another along the vertical y axis  112 . 
     It should be appreciated that  FIGS. 5A-5C  and  6 A- 6 C provide only a few of the possible arrangements of reflectors and solar receivers that are contemplated in the present invention. Although most of the figures involve substantially symmetrical collector designs, in some embodiments the designs may instead be asymmetrical. The reflector panels and solar receivers of each solar collector may or may not have substantially identical dimensions and/or shapes in the same collector. The common base structures  602 / 604 / 606  of  FIGS. 6A-6C  are drawn diagrammatically as platforms with flat surfaces, but it should be appreciated that a base structure may take a wide variety of forms, including a space frame support structure, multiple separate support structures, etc. 
     Referring next to  FIGS. 7A-7C , a space frame support structure  702  for use in a solar collector  700  according to a particular embodiment of the present invention will be described.  FIG. 7A  illustrates a diagrammatic cross-sectional view of an U-type collector  700  (e.g., as described in  FIGS. 1A-1B  and  5 A), which includes a space frame support structure  702  that physically supports the solar receivers  706   a/   706   b  and the reflector panels  704   a/   704   b.    FIGS. 7B and 7C  illustrate diagrammatic perspective views of the collector  700 , with some of the reflector panels removed to reveal more of the underlying space frame. The space frame support structure  702  includes multiple struts that are connected at nodes. The struts  708  include one or more receiver support struts  714   a/   714   b/   714   c  and one or more reflector support struts  712   a/   712   b.    
     The space frame support structure  702  is arranged to disperse loads originating at the solar receivers  706   a/   706   b  and the reflector panels  704   a/   704   b.  In the illustrated embodiment, for example, the receiver support struts  714   a/   714   b/   714   c  are linear pipes, tubes, or cylinders that extend diagonally out from a location immediately underneath the solar receivers to one or more nodes within the space frame. As a result, loads originating at the solar receivers are largely converted into compressive or tensile forces on one or more of the underlying struts, which are better tolerated. Additionally, the structures that are supporting the receiver and reflector panels are not physically isolated from the rest of the rest of the space frame support structure, which may render them more vulnerable to bending or damage. Instead, components such as the receiver support structures are integrated into the overall space frame support structure such that the loads from the solar receivers are carried not only by the structures that are in direct contact with them, but also by the rest of the space frame support structure. The space frame support structure  702  may be viewed as a unitary structure, since it supports and distributes the loads from both the solar receivers  706   a/   706   b  and the reflector panels  704   a/   704   b  in an integrated or unitary fashion. 
     Various implementations involve a space frame support structure  702  that is made of linear struts that are connected at nodes to form multiple geometric shapes, such as pyramids, tetrahedrons, etc. These shapes help to disperse stresses from carried components through multiple paths within the space frame. The space frame support structure has large apertures  716  and internal spaces that allow the passage of air, which substantially reduces wind load on the collector  700 . The large apertures  716  also facilitate cleaning of and access to the space frame support structure  702  and are less likely to catch debris. 
     Generally, the space frame support structure  702  is arranged to minimize or eliminate any shading of the reflector panels  704   a/   704   b.  In some embodiments, no portion of the solar receivers, the space frame support structure or any part of the solar collector shades the reflector panels  704   a/   704   b  during the normal operation of the solar collector. 
     The space frame support structure may include multiple, longitudinally extended longerons that form a base framework for the rest of the space frame support structure. The longerons may be continuous tubes substantially equal in length to the collector length. The tube cross-section may be square, rectangular, circular, or some other shape. Struts are attached to nodes on the longerons to create an integrated, web-like, highly resilient structure. (The term “longeron” may be understood in the present application as any suitable type of linear member that extends in a longitudinal direction.) For example,  FIGS. 7B-7C  illustrate a space frame support structure  702  that includes an upper longeron  718 , side longerons  720  and lower longerons  722 . Each longeron is a linear member that extends along a longitudinal axis  106 . The upper longeron  718  is arranged to physically support one or more solar receivers  706   a/   706   b.  The side longerons  720  are positioned near the outer edges  724  of the reflector panels  704   a/   704   b  and may indirectly help to support the panels. The lower longerons  722  are arranged at a bottom portion of the space frame support structure  702 . All of the longerons extend substantially parallel to one another along the longitudinal axis  106 . 
     Each of the longerons have multiple nodes that are separated by gaps and are distributed along the length of the longeron. The nodes are connecting points for multiple additional struts that can cause forces to be dispersed through multiple paths. For example, the upper longeron  718  has multiple upper nodes  726  along its length. In some embodiments, each upper node  726  is adjacent to and directly underlies one or more solar receivers  706   a/   706   b,  although this is not a requirement. Multiple receiver support struts  714   a/   714   b/   714   c  extend diagonally downward from each upper node  726  to help support the solar receivers  706   a/   706   b.  While the upper longeron  718  may experience some bending force, the receiver support struts  714  are subject primarily to tensile or compressive loads. Force that is applied to the solar receivers  706   a/   706   b  is dispersed through multiple, suitably angled receiver support struts  714   a/   714   b/   714   c,  rather than a single support member. 
     The solar receivers  706   a/   706   b  may be attached at multiple points along the upper longeron  718 . The attachment points may be located at regular intervals along the longeron. These attachment points need not correspond with space frame nodes. If the longeron is formed from a rectangular tube, the attachment to the longeron may include a U-shaped channel that fits over the longeron. Both the longeron and the U-shaped may include holes. The U-shaped channel may be secured to the longeron by aligning these holes and inserting a fastener through the holes. 
     In some designs, the receiver support struts  714  fan out from the upper nodes  726  and connect to various nodes deeper within the space frame support structure. By way of example, in  FIG. 7A , one receiver support strut  714   b  extends straight down and directly underneath the solar receivers  706   a/   706   b  and is coupled to a central node  728  in the space frame support structure. Two other receiver support struts  714   a/   714   c  extend diagonally downward to lower nodes  730   a/   730   b,  respectively, which are coupled to the lower longerons  722  at the bottom of the collector  700 . In the illustrated embodiment, the lower nodes  730   a/   730   b  are positioned below the reflector panels. While  FIG. 7A  illustrates three receiver support struts  714   a/   714   b/   714   c  extending out an upper node  726 , it should be appreciated that there may also be more or fewer receiver support struts extending out of the upper node or any single node. 
     There are also multiple side nodes  732  that are separated by gaps and are distributed along the length of each side longeron  720 . Multiple reflector support struts  712   a/   712   b  extend diagonally out from each side node  732  to help physically support the reflector panels. One of these struts  712   a,  which extends towards and is coupled to the side longeron  720 , is coupled to the central node  728 . Another is coupled to the lower node  730   b  and also extends to the side longeron  720 . The two reflector support struts  712   a/   712   b  are attached to the side longeron  720  via the same side node  732 . 
     The reflector support struts  712   a/   712   b  cooperate to form a base upon which the reflector panels  704   a/   704   b  may be positioned. Various implementations involve reflector support struts  712   a/   712   b  that extend in a direction substantially perpendicular to the upper and side longerons  718 / 720 . To further facilitate the mounting of reflector panels  704   a/   704   b  to the space frame support structure  702 , one or more stringers  734  may be positioned over the aforementioned receiver support struts  712   a.  In the illustrated embodiment, for example, the stringers  734  overlie and extend perpendicular to the receiver support struts  712   a.  Each stringer  734  includes one or more attachment points for attaching to an overlying reflector panel. The stringers  734  provide additional stiffness to facilitate more accurate alignment of the reflector panel. Together with the reflector support struts  712   a/   712   b,  the stringers  734  also help to distribute the load of the reflector panels more evenly over the space frame. Generally, stringers  734  facilitate the mounting of different sized reflector panels, since they may be substantially invariant along the longitudinal direction, attachment points between the stringers and reflector panels may be installed at any location. In some embodiments, stringers are not used and the reflector panel is attached with a different support member (e.g., a receiver support strut  712   a. ) 
     The struts and nodes may take a wide variety of forms, depending on the needs of a particular application. They may be made of almost any suitably resilient material, such as aluminum or steel. For example, tubular aluminum extruded members, roll formed steel sections and hot rolled steel sections work well as struts in the space frame support network. The nodes may be understood as connecting points for multiple struts. Struts can be connected at nodes without a connector (e.g., by form fitting or crushing the struts together, bending the struts around one another, fabricating through holes in one strut for the insertion of another strut, or any other suitable technique.) Welding, metal clinching, adhesive, rivets and bolts may be used to connect struts, connectors and/or nodes of the space frame support structure. In some embodiments, each node includes a separate connector that helps secure the incoming struts. The connector may take any suitable form, such as a metal sphere, a hub, etc. In some implementations, the connector is arranged such that the long axes of the incoming struts are arranged to meet substantially at a single point within the node and/or connector. Struts may be attached to the connector using various mechanisms, including pins, fasteners, bolts, etc. 
     In the illustrated embodiment, the gap G between the reflector panels  704   a/   704   b  is utilized to provide extra structural support for the solar receivers  706   a/   706   b.  By widening this gap appropriately, room can be made for multiple receiver support structures  714   a - 714   c  that help to stabilize the position of the solar receivers. In any case, a large portion of the gap G is covered by the solar receivers  706   a/   706   b,  and thus cannot be used for the reflection of sunlight. In some prior art concentrating photovoltaic systems, this gap between the reflector panels  704   a/   704   b  is either non-existent or not wide enough to accommodate a substantial support framework. 
     The length of the receiver support struts  714   a - 714   c  may be adjusted to help make the overall collector more symmetrical around its pivot axis  736 . That is, the upward extension of the receiver support struts  714   a - 714   c  helps to balance out the lateral extension of the reflector panels  704   a/   704   b.  Thus, in various embodiments, the height h to width w ratio of the collector  700  (as measured along a plane defined by the x axis  114  and the y axis  112 ) is greater than approximately 0.4. A more symmetrical arrangement helps reduce the buildup of stresses on a particular portion of the space frame support structure, even when it is rotated to track movements of the sun. 
     Referring next to  FIG. 7D , a solar collector according to another embodiment of the present invention will be described.  FIG. 7D  is a diagrammatic cross-sectional view of a solar collector  740  that includes a space frame support structure  742  with a different arrangement of struts, nodes and longerons than what was shown in  FIGS. 7A-7C . The space frame support structure  742  includes side longerons  752 , a single upper longeron  754  and a single lower longeron  750 . The upper longeron  754 , which is attached to one or more solar receivers  744   a/   744   b  through upper node  762 , is supported by two receiver support struts  748   a/   748   b  that extend diagonally down from the upper longeron  754  and are coupled to two central nodes  756   a/   756   b.  Two additional struts  758   a/   758   b  connect the central nodes  756   a/   756   b  with the single lower longeron  750  to form a diamond shape between the upper longeron  754  and the lower longeron  750 . In the illustrated embodiment, the collector  740  is symmetrical along a bisecting plane  759 , and both the upper and lower longerons  754 / 750  are on this bisecting plane  759 . Multiple reflector support struts  760   a/   760   b  are coupled to and extend from the central and lower nodes  756   a/   756   b/   750  to each side longeron  752 , where they are attached to the side longeron  752  via the same side node  762 . Similar to the space frame support structure  702  illustrated in  FIG. 7A , the interconnections and structures illustrated in  FIG. 7D  may be repeated down the length of the longitudinally extended collector  740  (e.g., there are multiple upper nodes and side nodes arranged along the longitudinal lengths of the upper and side longerons, which may each have the same interconnections as described above.) A notable feature of this space frame design is that any node is limited to a relatively low number of connecting struts (e.g., in the case of  FIG. 7D , only four.) For some applications, this feature is desirable, since it makes the node somewhat easier to handle, install and repair. 
     It should be appreciated that there is almost an infinite number of ways to arrange the longerons, struts and nodes of the space frame support structure.  FIGS. 7A-7D  should be understood as merely exemplary and should not be interpreted as limiting the range of space frame support structures that are contemplated in the present application. 
     The use of struts and nodes also makes the space frame support structure relatively easy to ship and assemble. By way of example, individual struts and nodes may be first compactly stored in a shipping container so that they can be assembled almost entirely on-site. Alternatively, portions of the space frame support structure (e.g., interconnected nodes and struts that form a plane of the space frame support structure) may be preassembled prior to shipping. They may be preassembled in planar trusses that can be assembled in the field. Some portions of the space frame support structure (e.g., a planar combination of struts and nodes) may be arranged to be collapsible for shipment and fold out in the field. In particular the planar trusses may be collapsible such that the struts are allowed to pivot about the nodes in such a way as for the planar parts of the truss to collapse into a smaller volume. In another approach, the main body of the space frame supporting structure may be preassembled at the factory, and peripheral components are added on site. 
     Referring next to  FIG. 8A , a connector for use at a node of the space frame support structure according to a particular embodiment of the present invention will be described.  FIG. 8A  illustrates a connector  800  that includes a body  802  and multiple connector fins  806 . The body  802  is arranged to accept a longeron  810 , such as one of the longerons described in connection with  FIGS. 7A-7D . Each connector fin  806  is a solid sheet or planar surface that extends out of the body  802 . Preferably, the body  802  and fins  806  have simple geometries (e.g., cylinders, planes, holes, slots, etc.) so that they are cost-effective to manufacture. 
     The body  802  of the connector  800  includes a feature for engaging a longeron  810  In the illustrated embodiment, for example, the body  802  is a hollow cylinder with open ends that is arranged to accept a longeron  810 . The body  802  may be secured to the longeron  810  using any suitable means, including a pin  814 , a bolt, fastener, a latch, adhesive, welding, etc. If a pin is used, the pin may be stepped and the hole diameter in the strut and connector appropriately sized so that each pin step has substantially equal engagement with the hole in the strut or connector. 
     Each fin  806  is arranged to securely engage one or more additional struts  808 . This may be performed in a variety of ways. By way of example, each strut  808  may be a metal tube, bar, rod or cylinder whose end has a slot  812 . The edge of the fin  806  of the connector  800  is arranged to slide into the slot  812  of the strut  808 . By lining up alignment holes  804  in the fin  806  and the end of the strut  808  and extending a pin  816  through the holes and the slot  812 , the strut  808  can be secured to the fin  806 . Preferably, the struts  808  contact the connect  800  at substantially different angles to avoid mechanical interference. In some embodiments, a connector  800  and its connected struts  808  are arranged such that the long axes of the incoming connecting struts  808  meet at substantially the same point, which may or may not be in the connector  800 . 
     Referring next to  FIGS. 9A-9B , a solar collector  900  with a space frame support structure  902  according to another embodiment of the present invention will be described.  FIG. 9A  is a diagrammatic cross-sectional view of a space frame support structure  902  suitable for use in A-type solar collector, such as the one described in connection with  FIG. 5A .  FIG. 9B  is a diagrammatic perspective view of the space frame space structure  902  illustrated in  FIG. 9A . In the illustrated embodiment, the space frame support structure  902  is substantially symmetrical along a bisecting plane  904 , although this is not a requirement. The solar collector  900  is arranged to be rotated around pivot axis  921  to track movements of the sun. 
     The solar collector includes an upper longeron  910 , two side longerons  912  and two lower longerons  914  that extend parallel along the longitudinal axis  106  of the solar collector  900 . The upper longeron  910 , in contrast to the upper longeron  718  of the space frame support structure  702  of  FIGS. 7A-7C , does not have attachment sites for solar receivers. Multiple upper nodes  920  are separated by gaps and arranged along the length of the upper longeron  910 . Multiple support struts  922   a/   922   b/   922   c  fan downward from each upper node  920 . Each of these struts  922   a/   922   b/   922   c  are connected to one of the lower nodes  924   a/   924   b/   924   c.  Reflector support struts  926   a/   926   b  also extend from the upper node  920  and help physically support and underlie the reflector panels  908   a/   908   b.    
     The solar receivers  906   a/   906   b  are coupled to various attachment sites that are on each side longeron  912 . The side longeron  912  is supported by receiver support struts  928   a/   928   b  that are positioned at the periphery of the solar collector  900 . In the illustrated embodiment, for example, each solar receiver is physically supported by a first receiver support strut  928   a  and a second receiver support strut  928   b.  The first receiver support strut extends upward from a lower node  924   c  to the side node  930 . The second receiver support strut extends upward from a peripheral node  932  to the side node  930 . 
     The above arrangement of nodes and support struts are repeated at various points along the length of the longerons and the space frame support structure  902 . That is, there are multiple upper nodes  920 , which are separated by gaps, along the length of the upper longeron  910 . Support structures  922   a - 922   c  fan downward from each of these upper nodes  920  in the manner described above. Similarly, there are side nodes  930 , separated by gaps, along the length of each side longeron  912 . Receiver support struts  928   a/   928   b  extend diagonally downward from each of these side nodes  930  to corresponding lower and peripheral nodes  924   c/   932 . As a result, a lattice of interlocking struts and nodes is formed that helps to prevent too much stress from building up on a narrow portion of the support structure. 
     Referring next to  FIGS. 10A and 10B , a reflector panel  1000  suitable for coupling to the space frame support structure according to a particular embodiment of the present invention will be described.  FIG. 10A  is a diagrammatic side view of the reflector panel  1000 .  FIG. 10B  is a diagrammatic perspective view of the reflector panel  1000 . The reflector panel  1000  may be understood as an enlarged view of one of the reflector panels illustrated in the previous figures. By way of example, the length l of the reflector panel  1000  may correspond with the length l of the reflector panel  200  illustrated in  FIG. 2A . The width w r  of the reflector panel  1000  may correspond with the width w r  of the reflector panel  300  illustrated in  FIG. 3A . As previously discussed, the reflector panel  1000  may have a compound curvature e.g., a convex curvature in a plane including the longitudinal axis  106 , and a concave curvature along a plane defined by the x and y axes  112 / 114 . The reflector panel has a reflective frontside  1002  and a backside  1003 . 
     The backside  1003  of the reflector panel  1000  includes attachment features  1004  for coupling the panel to the space frame support structure. In various embodiments, for example, the attachment features are used to secure the reflector panel  1000  to the stringers  734  illustrated in  FIG. 7B . In still other embodiments, the attachment features are used to secure the reflector panel  1000  to a reflector support strut or other support member. The attachment features may use any suitable means to securely fasten the backside  1003  of the reflector panel  1000  to the underlying support structure. By way of example, the attachment features  1004  may involve adhesive, glue pads, holes, fasteners or threaded screw holes. 
     The reflector panel  1000  may have a variety of different compositions and dimensions, depending on the needs of a particular application. Any reflective material, such as metalized glass, aluminum, etc, may be used to form the reflector panel. The reflective panel  1000  may be rectangular, curved, parabolic, flat, and/or arranged in the form of rectangular sheets or longer strips (e.g., as shown in  FIG. 3B ). In various embodiments, all of the reflector panels of a particular solar collector have the same dimensions and curvatures, although in other embodiments the panels may differ in their shape and size. 
     Referring next to  FIG. 11 , a solar collector row  1100  according to a particular embodiment of the present invention will be described.  FIG. 11  is a diagrammatic perspective view of a solar collector row  1100  that includes multiple solar collectors  1102  that have been arranged adjacent to one another along a longitudinal axis  106 . Multiple mounting posts  1106  form a stand, which physically support the solar collector row for pivotal motion  1104 , are positioned at the ends and at various points along the length of the collector row. The illustrated collectors may be any of the collectors described in the present application. 
     Preferably, the solar collectors  1102  are coupled together such that they pivot together in tandem to track movements of the sun. Coupling devices (not shown) are positioned underneath adjacent collectors  1102  to help link the collectors  1102  together. Some embodiments of the collector include a space frame support structure with short tube assemblies at the longitudinal ends of the collector, which are each arranged to be connected with a coupling device. Each coupling device is in turn supported by one of the mounting posts  1106 , which physically supports the solar collector row  1100 . When torque is applied to one of the collectors, the torque is transferred through the coupling devices may rotate the entire solar collector row  1100 . Various implementations of this approach are described in U.S. patent application Ser. No. 12/846,620, entitled “Manufacturable Dual Trough Solar Collector,” filed Jul. 29, 2010, which is incorporated herein in its entirety for all purposes. 
     The present invention also contemplates a power generation plant that includes multiple solar collectors  1102  and solar collector rows  1100 . The solar collector rows  1100  may be arranged in any suitable manner (e.g., in an array, side by side in a parallel formation, etc.) In some embodiments, multiple solar collectors  1102  are positioned on a common carousel type platform that can be rotated to track movements of the sun. All the collectors on the carousel rotation axis may rotate about a common rotation axis. The rotation axis may be substantially vertical. In various embodiments, the collector longitudinal axes may lie in a substantially horizontal plane, or the collector longitudinal axes may have a fixed, oblique tilt angle relative to the rotation axis. Alternatively, the collectors may be rotated about two axes. Such approaches are described in greater detail in U.S. patent application Ser. No. 12/642,704, entitled “High Ground Cover Ratio Solar Collection System,” filed Dec. 18, 2009, which is hereby incorporated in its entirety for all purposes. 
     Referring next to  FIG. 12 , the tracking and pivoting of a solar collector or solar collector row according to a particular embodiment of the present invention will be described.  FIG. 12  is a diagrammatic end view of solar collectors  1102  that are at the ends of two of the solar collector rows  1100  illustrated in  FIG. 11 . The two solar collector rows  1100  are arranged in parallel and extend in a longitudinal direction (i.e., into the page). A mounting post  1204  physically supports each solar collector for pivotal movement around a pivot axis  1206  that also extends in the longitudinal direction. The mounting posts  1204  may be anchored into the ground or a common base  1210  (e.g., a roof top, car park cover, etc.) A tracking system is arranged to pivot the solar collector to track the movements of the sun, such that the incoming sunlight  1212  is substantially incident on the optical aperture of the collector  1102 . 
     Various designs involve a pivot axis  1206  that substantially passes through the center of gravity of each collector in a solar collector row  1100 . That is, the weight of the various components of the collector  1102  are distributed evenly around the pivot. As a result, less force is required to rotate the collector  1102 . The location of the center of gravity of the collector  1102  depends on the weights of the various components. By way of example, it may be located along a bisecting plane  1216  of the collector and/or between the reflector panels  1214   a/   1214   b.  Some embodiments involve adding weights to the bottom of the collector  1102  to push the center of gravity lower. An economical and simple way to do so is to fill some of the lower struts or longerons with solid material, such as gravel, cement, sand, earth or steel balls. 
     The solar collector  1102  is arranged to accommodate a wide range of motion around the pivot axis. In some embodiments, for example, the tracking system, stand and solar collector  1102  are arranged to pivot the solar collector at least 170 degrees, or at least 160 degrees, or at least 150 degrees, while keeping the pivot axis  1206  substantially at the center of gravity of the collector. Some implementations involve a pivot range of at least 120 or 140 degrees. The pivot range can be increased by pushing the pivot axis  1206  further from the center of gravity. The pivot range may be adjusted to be lower or higher, depending on the solar insolation characteristics of a particular solar power plant site. 
     Referring next to  FIGS. 13A-13B , a solar collector  1300  with a support cable  1306  according to a particular embodiment of the present invention will be described.  FIG. 13A  is a diagrammatic cross-sectional view of the solar collector  1300 , which may be understood as the A-type collector  500  illustrated in  FIG. 5A . A support cable  1306  extends across the span of the collector  1300  to couple together the solar receivers  1302   a/   1302   b  and/or the receiver support struts  1308  on either side of the collector.  FIG. 13B  is a diagrammatic perspective view of the solar collector illustrated in  FIG. 13A . 
     The support cable  1306  is arranged to provide additional support for the solar receivers  1302   a/   1302   b  and their associated support structures. Preferably, the support cable  1306  has a small diameter so that it only minimally shades the underlying reflector panels  1304   a/   1304   b.  For example, a diameter of less than approximately 3 mm works well for various applications. Moreover, the effect of any shading by the cable  1306  on the reflector panel can be further reduced if the reflective panel has a convex curvature. That is, even when portions of the reflective panel are rendered unable to reflect light, there may be few or no breaks in the flux line formed by the reflective panel, for the reasons previously discussed in connection with  FIGS. 2A-2C . 
     The support cable  1306  may be made of any suitably resilient material, such as a metal wire. It may be attached directly to opposing solar receivers  1302   a/   1302   b,  to a portion of the support structure for the solar receivers, are some other suitable portion of the collector  1300 . Various embodiments of the present invention involves a support cable  1306  that extends diagonally multiple times across the span of the collector (e.g., as seen in  FIGS. 13B ), rather than directly across (e.g., in a direction that is perpendicular to the longitudinal axis  106  of the collector.) 
     Although only a few embodiments of the invention have been described in detail, it should be appreciated that the invention may be implemented in many other forms without departing from the spirit or scope of the invention. In the foregoing description, components in one figure can be modified or replaced based on corresponding elements in another figure. For example, the features of the reflector panel  1000  illustrated  FIG. 10  may be included in any figure that references a reflector panel, including but not limited to  FIGS. 1A ,  2 A- 2 C and  3 A- 3 D. It should also be noted that the axes may be understood as having a consistent meaning across all the figures. That is, the x, y, and z axes are perpendicular to one another. Additionally, it should be noted that the axes may be used to understand the design of various embodiments that are based on more than one figure. For example,  FIG. 2B  illustrates a line of convex shaped reflector panels that extend along a longitudinal axis.  FIG. 1B  illustrates a relatively zoomed out view of a collector in which the panels are illustrated in less detail. Therefore, the present invention also contemplates a particular embodiment where the features of the reflector panels of  FIG. 2B  are included in the reflector panels of  FIG. 1A . In understanding this embodiment, the longitudinal axes  106  of  FIGS. 1A and 2B  may be used as a common reference point to understand how the reflector panels of  FIG. 2B  are used in the collector of  FIG. 1A . Additionally, the specification and claims sometimes refer to “the normal operation of the solar collector.” This generally refers to a mode in which the collector is tracking the movement of the sun (e.g., as shown in  FIG. 12 ). However, it should be appreciated that the collector does not necessarily always track the sun during normal operation. For example, in some implementations the collector does not track the sun in the early morning. Therefore, the present embodiments should be considered as illustrative and not restrictive and the invention is not limited to the details given herein, but may be modified within the scope and equivalents of the appended claims.