Patent Publication Number: US-2010108124-A1

Title: Solar energy concentrator

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority to U.S. Provisional Application No. 61/102,306, filed on Oct. 2, 2008, the entire contents of which are incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to solar panels used to generate electrical or thermal power. More specifically the present invention relates to concentrator solar panels utilizing photovoltaic cells to generate electrical power. 
     BACKGROUND OF THE INVENTION 
     Concentrators for solar energy have been in use for many years. These devices are used to focus the sun&#39;s energy into a small area to raise the power level being concentrated on a photovoltaic converter to generate electrical power directly, or on a fluid line to heat water to make steam to drive a turbine to generate electrical power. 
     One difficulty with these concentrators has been that they are generally large and bulky and are not suitable for residential applications or other locations where the aesthetics of the installation are of importance. Additionally they are very susceptible to environmental damage due to wind and other elements. 
     In a common implementation a refractive or reflective lens is used to focus the energy on a small photovoltaic device. An example of a refractive device  100  is presented in  FIG. 1  and shows a refractive lens  104  concentrating solar illumination  108  on a photovoltaic cell  112 . This simple device concentrates light in a manner similar to the child&#39;s experiment wherein sunlight passing through a magnifying glass is focused onto a sheet of paper, thus setting it alight. Arrays of these are ganged together to generate greater amounts of power. An example of a reflective solar concentrator device  200  previously disclosed in FIG. 3 of U.S. Pat. No. 4,177,083, the contents whereof are incorporated by reference, is presented in  FIG. 2 . The optical principle is identical to that of a Cassegrain telescope first made known in the seventeenth century, with an energy conversion device replacing the eyepiece. Specifically, solar illumination  204  enters the device  200  and is reflected off of a main reflector  208  to a sub-reflector  212 . The sub-reflector  212  reflects the illumination  204  to a photovoltaic cell  216 . It suffers from the deficiency that the sub-reflector  212  blocks a substantial portion of the aperture of the main reflector  208  and thus decreases the ability of the device to concentrate light. 
     Stepped wave guides have long been known in the art. In U.S. Pat. No. 5,202,950, Arego et al, and U.S. Pat. No. 5,050,946, Hathaway et al, the contents of which are incorporated herein by reference in their entirety, one inventor of the present invention discloses a faceted light pipe and a light pipe system suitable to backlight a transmissive liquid crystal display from a single side light. In Arego et al, FIG. 8 depicts one embodiment of the light pipe further described in column 6, line 53, to column 7, line 58.  FIG. 3  of this document repeats FIG. 8 previously referenced. In  FIG. 3  the front surface portion  304  and the rear surface portion  308  of the light pipe  320  are substantially parallel. As stated in Arego et al these surfaces are specular surfaces so as to avoid diffuse reflections or refraction that make control of the light path more difficult. The light facet  312  is oriented at an angle α of 135° from the parallel rear surface portion  308 . The light facets  312  are optionally coated with a reflective material  316 , stated to be aluminum. The light facet  312  is designed to perform an angle transformation on light propagating in TIR mode within the light pipe  320  to allow that light to exit the light pipe  320  in order to provide illumination for the LCD. 
     Faceted light pipes like those disclosed by Arego et al. have also been described in solar applications in U.S. Pub. No. 2009/0064993 to Ghosh et al. (Banyan). However, there remains a need for an improved system that can yield higher efficiency and be practically manufactured at a reasonable cost. 
     The cost advantages of a solar concentrator can best be realized if the concentration ratio is high. Highly efficient photovoltaic converters (PVC) can efficiently convert a flux density equivalent to many hundreds of suns. Concentration ratios approaching 1000:1 and higher are considered desirable. The concentration goal is best determined after consideration of the technical and cost constraints a solar concentrator system must satisfy. 
     SUMMARY OF THE INVENTION 
     A light concentrator in the form of a relatively thin, planar assembly takes sunlight in at an orientation normal to the planar surface and direct it via a plurality of small linear aspheric or spheric sections into a TIR (total internal reflection) light guide which collects and transports the sunlight from the linear aspheric sections to one edge of the light guide where it illuminates a solar photovoltaic converter or heats water or other medium. As is well known in the art of light guides, TIR is the most efficient method for transporting light within a wave-guide. The efficiency of reflection is nominally 100% with the only losses coming from the transmission efficiency of the optical material. Optionally the solar energy may undergo an additional stage of concentration, for example through the use of a Simple Parabolic Concentrator (SPC) or similar device. 
     The concentrator of the present invention can include a plurality of aspheric mirror sections in a first stage, or element of concentration in the system. (Refer to  FIG. 4  and associated text for an explanation of the following terminology regarding orientation used in this application.) Each aspheric mirror section concentrates light by illuminating a turn mirror that redirects the light down a wave guide (light pipe) that relies upon Total Internal Reflection (TIR) and geometric optics to contain the light within the wave guide. In one embodiment the aspheric section concentrates light only in the longitudinal axis of these aspheric sections. Additional concentration, or gain, may be had to this axis and to the perpendicular axis thus providing two additional stages of gain. 
     A concentrator with very high gain and a method of constructing a concentrator using plastic extrusion and aluminum or silver metallization to produce low cost, thin concentrators with very high gain is described. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a known refractive solar concentrator. 
         FIG. 2  depicts a known reflective solar concentrator based on Cassegrain optics. 
         FIG. 3  is a depiction of a wave-guide from the backlight assembly of a flat panel liquid crystal display. 
         FIG. 4  presents an isometric view of an exemplary embodiment of a set of principle elements in a solar concentrator after the present invention. 
         FIG. 5  presents a side view of an exemplary embodiment of a wave-guide after the present invention. 
         FIG. 6A  presents an isometric view of an exemplary embodiment of a mirror assembly after the present invention comprising a plurality of mirror sections. 
         FIG. 6B  presents an isometric view of a trough mirror section. 
         FIG. 7  presents an isometric view of a Simple Parabolic Concentrator (SPC). 
         FIG. 8A  is a depiction of a basic wave-guide based solar concentrator with turn mirrors on the wave-guide assembly and with a series of trough mirrors in fixed relationship thereto. 
         FIG. 8B  provides a detail of a turn mirror and its positioning on a segment of the wave-guide. 
         FIG. 9A  depicts the geometrical and optical relationship between the Wave-Guide Assembly and the Trough Mirror Assembly. 
         FIG. 9B  depicts details of a section of the assembly further depicting a set of ray traces that illustrate how light from the Trough Mirror Assembly enters the Wave-Guide Assembly and is converted to Total Internal Reflection. 
         FIG. 10A  depicts an alternative turn mirror arrangement wherein an air gap exists between an angled surface of a wave-guide assembly and the turn mirror. 
         FIG. 10B  depicts a functioning of the alternative mirror arrangement wherein the mode of reflection from the angled surface is Total Internal Reflection. 
         FIG. 10C  depicts a function of the alternative mirror arrangement wherein the mode of reflection is a first surface reflection from the mirror. 
         FIG. 10D  depicts a means for attaching the mirror of  FIG. 10B  to the Wave-Guide Assembly. 
         FIG. 11A  depicts an arrangement of the inventive assembly illustrating a shift in curvature. 
         FIG. 11B  depicts Section  1  of the mirror assembly of  FIG. 11A  located adjacent to the SPC. 
         FIG. 11C  depicts Section  2  of the mirror assembly of  FIG. 11A  located at the end opposite the SPC. 
         FIG. 12A  depicts an arrangement of the SPC in relation to the Wave-Guide Assembly and the Trough Mirror Assembly. 
         FIG. 12B  is a detailed depiction of the arrangement of  FIG. 12A . 
         FIG. 12C  depicts a difference in material of the SPC and the wave-guide. 
         FIG. 12D  depicts one method of attaching the SPC to the Wave Guide Assembly. 
         FIG. 13A  presents an overhead view illustrating a ray trace showing the movement of solar energy down the Wave-Guide and into the SPC. 
         FIG. 13B  is a side view of a ray trace illustrating the TIR motion of solar energy through the wave guide. 
         FIG. 14  presents an annotated isometric view of a solar concentrator with key elements identified for reference in tables. 
         FIG. 15A  presents an exploded isometric view of an array of solar concentrators and their photovoltaic cells. 
         FIG. 15B  depicts the assembled solar energy concentrator assembly. 
         FIG. 15C  depicts an exploded view of a weather housing for a solar energy concentrator assembly. 
         FIG. 15D  depicts the assembled weatherproof solar energy concentrator. 
         FIG. 16  depicts an alternative solar energy concentrator unit in which the SPC is illuminated underneath by a trough minor. 
         FIG. 17  depicts the areas where the reflection from the trough minor is not captured. 
         FIG. 18  illustrates the location of turn minors relative to the trough mirrors. 
         FIG. 19A  depicts a side view illustrating the location of turn mirrors and the change of materials. 
         FIG. 19B  provides an isometric illustration of the positions of the materials. 
         FIG. 20  provides an isometric illustration of an alternative embodiment in which the mirror under the SPC is a compound mirror. 
         FIG. 21  presents an isometric view of a compound mirror segment. 
         FIG. 22  provides an isometric view of ray traces from the compound mirror into the SPC. 
         FIG. 23  provides an isometric view of an embodiment without an SPC in which all mirrors on the mirror assembly are compound mirrors. 
         FIG. 24  presents an isometric view of a section of a solar concentrator with exemplary ray traces. 
         FIG. 25  illustrates the focus of the collected light within the SPC onto the photovoltaic cell as viewed from above the device. 
         FIG. 26  provides an alternative in which the SPC is present and recessed into the assembly and all mirrors on the mirror assembly are compound mirrors. 
         FIG. 27  depicts an exemplary solar concentrator assembly utilizing asymmetric mirrors and entry facets along the solar axis where the opposing surfaces of the wave-guide assembly are parallel. 
         FIG. 28  presents a detailed drawing of a concentrator mirror segment and its relationship to the wave-guide segment. 
         FIG. 29  depicts an exemplary solar concentrator assembly utilizing asymmetric mirrors and entry facets along the solar axis where the opposing surfaces of the wave-guide assembly are converging. 
         FIG. 30  depicts an exemplary solar concentrator assembly where the width along the transverse axis is tapered. 
         FIG. 31A  depicts an exemplary wave-guide assembly fabricated by extrusion. 
         FIG. 31B  depicts a cross section of an exemplary component manufactured by extrusion. 
         FIG. 32A  presents an isometric view of the principle components of an exemplary solar concentrator assembly. 
         FIG. 32B  presents an isometric view of the assembled components of the exemplary solar concentrator assembly. 
         FIG. 32C  presents a detailed view of a section of the mirror assembly affixed to the wave-guide assembly. 
         FIG. 32D  provides an alternative view of selected components of a wave-guide assembly. 
         FIG. 33  presents a side view of an exemplary solar concentrator assembly utilizing elements such as holographic optical elements or diffractive optical elements as a concentrating component. 
         FIG. 34A  presents a ray trace view of an exemplary solar concentrator assembly. 
         FIG. 34B  presents details of how rays entering a rise facet refract because of the change in refractive index of the wave-guide material. 
         FIG. 34C  presents an exemplary rise facet with a complex three-segment structure. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     In a pending provisional patent application 61/102,306 the inventors of this invention disclose many aspects of the design and fabrication of solar energy concentrators and components thereof, the contents whereof are incorporated into this application by reference in its entirety. 
       FIG. 4  depicts an embodiment of the solar concentrator  400  disclosed in this application. The concentrator  400  includes a mirror assembly  404  to collect and concentrate solar radiation and to direct it to a set of turn mirrors  408  affixed to a stepped wave-guide  412 . The turn mirrors  408  are arrayed so as to receive the solar radiation from the mirror assembly  404  and to reflect the solar radiation into the stepped wave-guide  412  at least partially using TIR between a plurality of parallel surfaces. The stepped wave-guide  412  captures the light redirected by the turn mirrors  408  so that the light propagates in TIR mode to a target. A Simple Parabolic Concentrator (SPC)  416  is optionally installed at the end of the propagation path of the stepped wave-guide  412  to further concentrate the captured light. Finally, a photovoltaic converter (PVC) can be affixed to the stepped wave-guide  412  or to the optional SPC  416  at the PVC mounting position  420  to convert the concentrated solar radiation to electrical energy. 
     As shown in  FIG. 4 , three axes of the system are defined. The longitudinal axis is the long axis of the solar concentrator  400 . The transverse axis is the axis across the surface of the solar concentrator  400  orthogonal to the longitudinal axis. The solar axis is the axis orthogonal to the longitudinal and transverse axes and therefore orthogonal to the upper and lower surfaces of the stepped wave-guide  412 . 
       FIG. 5  depicts the side view of a stepped wave-guide  500  according to one embodiment of this invention. The stepped upper surface  504  and planar lower surface  512  are substantially parallel to each other. The stepped upper surface  504  includes a series of surfaces parallel to one another. The height of the step between each section is that which is needed to accommodate a turn mirror  508 , preferably 1 mm, but the step height may be more or less depending on the particular design. As used herein, the term “upper” indicates that it is the side of the waveguide that faces the sun while the term “lower” indicates the side that is closest to the mirror assembly. Light propagates down the stepped wave-guide  500  in the direction of propagation indicated by arrow A to the exit end  516  of the stepped wave-guide  500 . 
     The wave-guide  500  may be fabricated from an optical quality acrylic polymer material such as poly(methyl methylacrilate) (PMMA), commercially available as Plexiglas™ and in many other forms. Alternatively, it may be formed from a crown glass material such as Schott BK7. Both have refractive indices of approximately 1.50, which is the nominal refractive index used for optical materials in this application unless otherwise noted. 
       FIG. 6A  is an isometric view of a mirror assembly  600 . A typical mirror assembly  600  comprises a plurality of mirror segments  604 , each of which may be designed and constructed to a different optical curvature. 
     Mirror assemblies may be fabricated in several ways. For example, they may be formed by injection molding or casting PMMA to the required optical shape and then evaporating aluminum or silver onto the resultant surface. Alternatively the surface can be coated by a set of dielectric thin-film coatings. 
       FIG. 6B  is an isometric view of a trough mirror segment  604 . The mirror curvature is oriented along the longitudinal axis and the non-curved mirror surface is oriented along the transverse axis. The radius of curvature of the mirror may vary across a single mirror segment. The focus of a trough mirror assembly will be a line coincident with the width of the assembly in the transverse direction. It is also possible that the curvature in one section of the mirror differs from the curvature in another section of the mirror. 
       FIG. 7  is an isometric view of a Simple Parabolic Concentrator (SPC)  700 . The SPC  700  is well known in the field of optics. The SPC  700  is able to collect light focused at or near infinity (collimated light) or light focused at some point beyond the SPC  700  and bring it to a relatively narrow point over a very short distance. 
     The SPC  700  may be fabricated from a crown glass material such as Schott BK7 to provide necessary heat handling capacity at the point where the solar energy achieves its highest level of concentration. At the required concentration levels the residual absorption losses from solar radiation in an SPC  700  fabricated from PMMA may cause the SPC  700  to change shape over time and therefore change its optical properties. 
       FIG. 8A  depicts a first embodiment of the invention. The first embodiment comprises a solar concentrator  800  including a stepped wave guide  804  with turn mirrors  808  affixed thereto at approximately 45°, aspheric trough mirrors  812  with curvature on the longitudinal axis and no curvature on the transverse axis, and other incidental hardware suitable to assemble the unit. The trough mirrors  812  are designed and arrayed to focus solar energy in a line onto the turn mirrors  808 , which are in turn designed so as to redirect the solar radiation to an angle at which it propagates within the wave-guide  804  by Total Internal Reflection (TIR) between the upper and lower surfaces of the wave-guide  804 . Optionally the assembly includes a second level concentrator such as a Simple Parabolic Concentrator (SPC)  816  affixed to the wave-guide assembly  804  to further concentrate the solar radiation. 
       FIG. 8B  depicts a side view of the wave-guide  804  noting the position of one of the turn mirrors  808  on the wave-guide  804 . Other than the turn mirrors  808 , the surfaces of the wave-guide  804  can be subjected to optical polishing sufficient to avoid significant surface scattering that might interfere with the TIR propagation but are otherwise uncoated. The wave-guide may optionally be coated with a thin-film coating to improve passage of light through the wave-guide to the mirror assembly without altering its TIR properties. Techniques for thin film coating are well known. 
     The description of the curvature of a mirror is based on the Surface Formula: 
     
       
         
           
             z 
             = 
             
               
                 
                   cr 
                   2 
                 
                 
                   1 
                   + 
                   
                     
                       1 
                       - 
                       
                         
                           ( 
                           
                             1 
                             + 
                             k 
                           
                           ) 
                         
                          
                         
                           c 
                           2 
                         
                          
                         
                           r 
                           2 
                         
                       
                     
                   
                 
               
               + 
               
                 
                   ∑ 
                   
                     i 
                     = 
                     1 
                   
                   12 
                 
                  
                 
                   
                     α 
                     i 
                   
                    
                   
                     r 
                     i 
                   
                 
               
             
           
         
       
     
     wherein
 
z=height y-axis (dependent variable)
 
r=horizontal×axis (independent variable)
 
c=radius of curvature
 
k=conic constant
 
α i =higher order constant
 
i=power of r i  associated with corresponding higher order constant
 
     Optical CAD design and analysis software programs such as Zemax™, ASAP and Code V™ may receive data in this or similar formats and, through computational analysis, provide a detailed understanding of the important performance factors of an optical description such as optical efficiency (throughput), aberration, coma etc. Zemax™ may also calculate a solution based on programmed constraints. 
     In the present application (redirection and focusing of solar radiation) solar radiation is a distributed source subtended approximately 0.5° over a great distance, thus rendering the illumination effectively collimated. Calculations in Zemax™ and subsequent experimentation confirm that the effective focal distance of a reflective curved mirror is equal to c/2 or half the radius of curvature when the illumination is sufficiently collimated. In some instances the actual focal distance may be slightly shorter than the effective focal distance to overcome the effects of aberration. 
       FIGS. 9A and 9B  depict the opto-mechanical relationship between the aspheric trough mirrors  812  and the turn mirrors  808 .  FIG. 9A  provides an overview of the concentrator assembly  800 . The principle elements can be arrayed as shown.  FIG. 9B  provides a detailed exposition (Detail A from  FIG. 9A ) of the relative positions and optical function of the aspheric trough mirrors  812  and the turn mirrors  808  when illuminated by solar energy. The ray trace examples show how the surface of the mirror segment is designed so that the reflected solar energy (rays A, B) is focused in a line upon the turn mirror  808  for that segment. 
     The turn mirror  808  may be formed by placing reflective optical coatings such as silver, aluminum or a dielectric mirror on the angled surface. Deposition techniques suitable for this are well known in the state of the art. For example, the entire upper surface of the wave guide  804  may be coated with silver. The silver at the turn mirror  808  may be subsequently covered with a photo resist material using exposure and development processes similar to those used to fabricate printed circuit boards, thus allowing the silver material on the surfaces not covered by photo resist material to be removed by chemical means. 
       FIG. 10A  depicts a wave-guide  804  including a variation on the turn mirror wherein an air gap separates the turn mirror  820  from the surface of the wave-guide, the turn mirror  820  otherwise being substantially parallel to the corresponding angled surface of the wave-guide  800 . This variation takes advantage of the improved efficiency of a TIR reflection (100%) over first surface reflection from a highly efficient mirror (nominally 92-95% maximum). The gap between the mirror  820  and the angled surface of the wave-guide  804  need exceed only a few wavelengths of light for the mirror  820  not to interfere in the TIR reflection. A larger gap would normally be used for ease of manufacturing. Some solar radiation will satisfy the conditions for TIR and reflect off the angled surface of the wave-guide  804  adjacent to the turn mirror  820  while other solar radiation will not satisfy the conditions for TIR and will therefore pass through the angle surface and reflect off the turn mirror  820 .  FIG. 10B  is a detailed drawing of a ray that reflects from the angle surface in TIR mode. The angle of incidence exceeds the critical angle relative to normal to the surface and thus satisfies the conditions for TIR. The critical angle for a beam of light within a material with a refractive index of 1.5 at its interface to air is 41.8° relative to the normal to the surface.  FIG. 10C  depicts a ray that is not reflected in TIR mode because it does not exceed the critical angle relative to the normal to the surface. In this mode the external minor  820  reflects the light and returns it to the wave-guide  804 .  FIG. 10D  depicts a design to enable mounting of the turn mirror  820  onto the wave-guide  804 . The wave-guide  804  is modified with slots  824  into which a flange formed on the turn mirror  820  can be inserted. Thus, the turn mirror  820  can be mounted at a predetermined distance from the angled surface  832  of the wave-guide  804 . Other mechanical means of attachment are well known in the art. Additionally, such a turn mirror  820  can be mounted at a predetermined distance from an angled surface of the SPC  816 . 
       FIG. 11A  illustrates that in the first embodiment the series of trough mirrors  812  depicted will not be identical. The optical path from each trough mirror  812  to its corresponding turn mirror  808  is slightly different because the thickness of the wave-guide  804  increases in the direction of the SPC  816 . To achieve optimal focus on the turn mirror  808  it is therefore highly preferable that each trough mirror cross section has a slightly different shape.  FIG. 11B  and  FIG. 11C  are representative of the curvature relationship between two of the trough mirrors  812 .  FIG. 11B  corresponds to the trough mirror  812  of Section  1  from  FIG. 11A  that is closest to the SPC  816  while  FIG. 11C  corresponds to the trough mirror  812  of Section  2  from  FIG. 11A  that is at the opposite end from the SPC  816 . The radius of curvature depicted on  FIG. 11C  is significantly less than the radius of curvature of  FIG. 11B , and the intervening mirrors  812  follow this same general relationship with each successive mirror  812  having a slightly smaller radius depending on its specific order in the sequence leading away from the SPC  816 . 
       FIG. 12A  depicts an exemplary position of the SPC  816  in relation to the wave-guide  804 .  FIG. 12B  is an enlarged view of a section of  FIG. 12A  and calls out the materials used in the fabrication of the wave-guide  804  and the SPC  816 .  FIG. 12C  depicts the materials to indicate that the SPC  816  and the wave-guide  804  need not be fabricated from the same type of material. For example, the wave-guide  804 , noted as Material  1 , may be fabricated from an optical quality acrylic polymer material such as poly (methyl methylacrilate) (PMMA), commercially available as Plexiglas, while Material  2  (the SPC  816 ) may be fabricated from a crown glass material such as BK7 to provide necessary heat handling capacity at the point where the solar energy achieves its highest level of concentration. Note that the effective indices of refraction of the two cited materials are similar at approximately 1.49 and 1.51 respectively, thus simplifying the optical mating of the materials. An optical system using two or more different materials with coefficients of thermal expansion (C TE ) that vary significantly must be designed so as to assure that the components are in the correct optical arrangement when the device is at nominal operating temperature. 
       FIG. 12D  presents one example of a mounting mechanism that mates the wave-guide  804  to the second level concentrator (SPC  816 ) along the transverse axis of the wave-guide  804 . This method of mounting avoids interfering in the TIR method of propagation because in the first embodiment TIR predominantly takes place in the solar axis direction within the wave-guide  804  and incidentally in the transverse axis direction of the wave-guide  804 . Mated mounting flanges  840  are formed on either side of the SPC  816  and the wave guide  804  and they are then attached to one another by fastening means  844  such as a common screw. 
       FIG. 13A  depicts a partial ray trace A along the longitudinal axis within the wave-guide  804  that passes into the SPC  816  as viewed from the solar axis. The light does not TIR substantially between the lateral surfaces of the wave-guide  804  and SPC  816  assemblies until it enters the SPC  816 .  FIG. 13B  depicts a partial ray trace as seen from the transverse axis of the system. The light propagates down the longitudinal axis by TIR between the upper and lower surfaces of the wave-guide  804  and the SPC  816 . 
     In a series of simulations of the first embodiment exemplary, parameters for an implementation were defined. A series of wave-guide segments (WG 1 -WG 6 ) of an exemplary wave-guide assembly  900  and a series of trough mirror segments (MS 1  to MS 6 ) of an exemplary mirror assembly  904  are identified in  FIG. 14  for cross-reference to various tables describing these features in the present application. The exemplary wave-guide  900  shown in  FIG. 14  has an SPC  908  on an exit end thereof. 
     The mirror assembly  904  for a simulation implementing the first embodiment is presented in the following data table. 
     
       
         
           
               
               
               
               
            
               
                   
                   
               
               
                   
                   
                 Radii of curvature 
                   
               
               
                   
                 Dimensions (mm) 
                 (mm) 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Mirror 
                 Longi- 
                 Trans- 
                 Trans- 
                 Longi- 
                 Conic 
                 α i   
               
               
                 Segment 
                 tudinal 
                 verse 
                 verse 
                 tudinal 
                 Constant 
                 values 
               
               
                   
               
               
                 MS 1 
                 60 
                 25 
                 ∞ 
                 74.379 
                 −0.7860 
                 All = 0 
               
               
                 MS 2 
                 60 
                 (Arbitrary 
                 ∞ 
                 73.035 
                 −0.8230 
                 All = 0 
               
               
                 MS 3 
                 60 
                 but affects 
                 ∞ 
                 71.691 
                 −0.8127 
                 All = 0 
               
               
                 MS 4 
                 60 
                 SPC size) 
                 ∞ 
                 70.348 
                 −0.9000 
                 All = 0 
               
               
                 MS 5 
                 60 
                   
                 ∞ 
                 69.000 
                 −0.9400 
                 All = 0 
               
               
                 MS 6 
                 60 
                   
                 ∞ 
                 67.666 
                 −0.9806 
                 All = 0 
               
               
                   
               
            
           
         
       
     
     All mirror segments in this example have identical external dimension of 60 millimeters (mm) longitudinally by 25 mm in the transverse direction. The 25 mm transverse dimension is selected to insure the dimensions of the SPC are reasonable. Subsequent data on the corresponding wave-guide segment will show that the mirror position over MS  1  is 5 mm further from the center of the mirror than the corresponding position over MS  6  and that is reflected in the change to the radius of curvature across the mirror assembly  904 . The center of each mirror segment in the mirror assembly is nominally 33.5 mm from the bottom of the wave-guide assembly  900 . Use of added components α i  in the surface formula to correct for aberration proved unnecessary in this series of simulations. However, it is foreseeable that some of these components may be needed in alternative implementations according to this invention. 
     The wave-guide assembly  904  for this implementation is described in the following table. 
     
       
         
           
               
               
               
               
               
             
               
                   
                   
               
               
                   
                   
                 Segment 
                   
                 Thickness 
               
               
                   
                 Segment 
                 longitudinal 
                 Transverse 
                 (solar axis) 
               
               
                   
                 Number 
                 length (mm) 
                 width (mm) 
                 (mm) 
               
               
                   
                   
               
             
            
               
                   
                 WG 1 
                 30 
                 25 (match 
                 6 
               
               
                   
                 WG 2 
                 60 
                 mirror 
                 5 
               
               
                   
                 WG 3 
                 60 
                 assembly) 
                 4 
               
               
                   
                 WG 4 
                 60 
                   
                 3 
               
               
                   
                 WG 5 
                 60 
                   
                 2 
               
               
                   
                 WG 6 
                 60 
                   
                 1 
               
               
                   
                   
               
            
           
         
       
     
     The thickness along the solar axis increases by 1 mm at each turn mirror. In order to be fully efficient in capturing the reflected solar radiation, the trough mirrors have a width in the transverse direction that matches the width of the wave-guide in the transverse direction. 
     The spatial relationship between the turn mirrors and the mirror segments of the mirror assembly  904  places the center of each mirror underneath the corresponding turn mirror. The spatial distance from the center of each mirror to the bottom of the wave-guide assembly  900  in this example is fixed at 33.5 mm. In an alternative embodiment, the distance from the center of each mirror segment of the mirror assembly to the corresponding turn mirror could be constant. 
     Therefore, the distance from the center of each mirror segment to the corresponding turn mirror differs by 1 mm from the adjacent mirror-pairs. The following table describes the information. Note that the center of the angled surface of each turn mirror is midway between the adjacent stepped surfaces. 
     
       
         
           
               
               
               
               
             
               
                   
               
               
                   
                 Distance to 
                 Distance to 
                   
               
               
                   
                 Bottom of 
                 Center of Turn 
                 Nominal Focal 
               
               
                 Mirror Segment 
                 Wave-guide 
                 Mirror 
                 Distance 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 MS 1 
                 33.5 
                 39.0 
                 37.2 
               
               
                 MS 2 
                   
                 38.0 
                 36.5 
               
               
                 MS 3 
                   
                 37.0 
                 35.9 
               
               
                 MS 4 
                   
                 36.0 
                 35.2 
               
               
                 MS 5 
                   
                 35.0 
                 34.5 
               
               
                 MS 6 
                   
                 34.0 
                 33.83 
               
               
                   
               
            
           
         
       
     
     The thickness of the wave-guide with its higher refractive index is not optically significant. The difference between the actual distance to the turn mirror and the nominal focal distance (defined as the radius of curvature divided by 2 when the light is collimated) is an alternative solution to the issue of aberration. 
     The specification for the SPC  908  is determined by specific performance criteria required for its performance. Thus, it is beneficial for the SPC  908  to collect all the light exiting the wave-guide or at least a high percentage. The shape of the SPC  908  is a parabola truncated at its vertex so that the focal point of the parabola is located at the mid-point of the exit aperture. The formula for a parabola with vertex located at the intersection of the x and y coordinate systems is 
         y=x   2 /(4 f ) 
     where f is the focal distance, x is the axis of input/output and y is the axis of the length. 
     The exit aperture X out  is calculated from the preceding formula to be 2f wide. The input aperture is defined as required to capture the output of the wave-guide assembly  900 , that being 25 mm in the present example. The geometrical concentration of an SPC  908  is defined as X in =concentration×X out . Given a desired concentration ratio of 20 and the preceding 25 mm value for X in  the resultant value for X out  is 1.25 mm. From these numbers and the formula for a parabola the length required to achieve that input aperture and concentration ratio is 250 mm. The calculated values are summarized in the following table. 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 Parameter 
                 Value 
               
               
                   
                   
               
             
            
               
                   
                 X IN  (Input aperture) 
                  25 mm 
               
               
                   
                 X OUT  (Output aperture) 
                 1.25 mm  
               
               
                   
                 Length (longitudinal) 
                 250 mm 
               
               
                   
                 Geometrical concentration 
                 20 
               
               
                   
                   
               
            
           
         
       
     
       FIG. 15A  depicts a blow up isometric view of an exemplary solar energy concentrator assembly  1000  formed by an array of solar energy concentrator devices. The boundaries between each individual wave-guide  1004  are not shown. The elements are aligned so that the photovoltaic converters  1008  are mounted at the end of the SPC  1012 .  FIG. 15B  depicts an isometric view of an assembled solar energy concentrator assembly  1000  comprising the individual elements depicted in  FIG. 9A . In  FIG. 15B , the wave-guides  1004  are attached to the trough mirrors  1016  by connecting plates  1020 .  FIG. 15C  depicts a blow up isometric view of a plurality of solar energy concentrator assemblies  1000  and its associated components to protect it from the elements. The plurality of solar energy concentrator assemblies is inserted into a weather cover frame  1024  and a transparent weather cover  1028  is attached thereto.  FIG. 15D  presents the weatherproof solar energy assembly  1100  based on the components of  FIG. 15A  and  FIG. 15C . 
     In a preferred implementation the entire array is mechanically steered so that the solar energy illuminating the device is approximately normal to the stepped surface of the light guide. Such steering systems are well known in the art. 
       FIG. 16  depicts a second embodiment of a solar concentrator  1200  of the present invention. In this embodiment the relationship between the wave-guide  1204 , the simple parabolic concentrator  1216  and the trough mirror assembly  1212  is modified so that the transition from the wave-guide assembly  1204  to the simple parabolic concentrator  1216  occurs in the area over the trough mirror assembly  1212 . The 250 mm SPC length noted in the first embodiment is significant compared to the overall 360 mm length of the other components in the system. This approach sacrifices some efficiency in one sense, as discussed below with respect to  FIG. 17 , but permits a decrease in the overall length of the full assembly. In an installation where the total size is constrained, such as a rooftop, this modest sacrifice of efficiency may result in greater total output due to the decreased footprint of the individual components of the system. Stated otherwise, the collecting area in the second embodiment may cover a higher percentage of the roof than in the first embodiment, and thus more of the solar energy impinging on the roof may be captured. The trough mirror assembly  1212  is otherwise unchanged from the first embodiment. 
       FIG. 17  presents a top view depicting the areas (shaded) adjacent to the SPC  1216  where the assembly of  FIG. 10  cannot collect and capture solar energy. Solar energy passing through the shaded areas  1220  is reflected back and not collected as this light is not focused on a turn mirror. However, this arrangement still permits the capture of more solar energy than in embodiment 1 which has no trough mirrors  1212  under the SPC  1216 . 
       FIG. 18  presents an isometric projection of the same features noted in  FIG. 17  that depicts the spatial relationship between the trough mirrors  1212 , the turn mirrors  1208  and the SPC  1216 . The SPC  1216  in this case is depicted as approximately the length of a mirror segment on the trough mirror assembly  1212 . A turn mirror  1208  and corresponding thickness change comprise modified features of the SPC  1216 . 
       FIG. 19A  provides a side view of the SPC  1216  and a section of the wave-guide  1204  with the two materials noted. Material  1  in the wave-guide section  1204  may be any suitable optical material including PMMA. Material  2  in the SPC section  1216  may be any suitable material including a crown glass material such as BK7 selected for its heat handling capacity. Material  1  and material  2  may in fact be the same material and the entire assembly may be made as a single piece without need for a seam. Turn mirrors  1208  may be formed in either of the sections of the assembly, as noted. Attachment of the SPC  1216  to the wave-guide  1204  may be made using conventional methods such as utilizing an adhesive layer to match the two. Alternatively the flange method of  FIG. 12D  may be used.  FIG. 19B  presents an isometric view of the assembly of  FIGS. 16-19A  including trough mirrors  1212  for additional clarity. 
     Other features of this second embodiment do not differ significantly from the first embodiment. In an example of the second embodiment, the specification for the mirror assembly and its individual mirror segment descriptions may be unchanged from the exemplary data presented for the first embodiment. The turn mirror arrangement presented in  FIGS. 9A and 9B  may be applied in this embodiment without change except as previously noted. The spacing between turn mirrors remains unchanged in a modeled example from the 60 mm previously presented. The alternative turn mirror arrangement of  FIGS. 10A ,  10 B,  10 C and  10 D may be applied without change. The magnification considerations related to the curvature of the trough mirrors are also unchanged from  FIGS. 11A ,  11 B and  11 C. The ray trace and TIR consideration depicted in  FIGS. 13A and 13B  are unchanged in this embodiment. The assemblies can be ganged together in a manner similar to the method previously shown in  FIGS. 15A ,  15 B,  15 C and  15 D. 
     The table of descriptive data for an exemplary mirror assembly according to the first embodiment applies to this example of the second embodiment. The following tables describe an implementation of the wave-guide and SPC based on an SPC with a concentration ratio of approximately 5. The data for the wave-guide follows. Note that Segment  1  is completely replaced by the recessed SPC as well as part of Segment  2 . 
                                                                 Thickness           Segment   Segment length   Transverse   (solar axis)           Number   (mm)   width (mm)   (mm)                                                    1   Not applicable                                     2   27.5   25   5           3   60       4           4   60       3           5   60       2           6   60       1                        
The data for the SPC follows.
 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 Parameter 
                 Value 
               
               
                   
                   
               
             
            
               
                   
                 X IN  (Input aperture) 
                 25 mm  
               
               
                   
                 X OUT  (Output aperture) 
                 5 mm 
               
               
                   
                 Length (longitudinal) 
                 62.5 mm   
               
               
                   
                 Turn mirror position 
                 30 mm  
               
               
                   
                 (from output aperture) 
               
               
                   
                 Turn Mirror width 
                 17 mm  
               
               
                   
                 (SPC width) 
               
               
                   
                 Thickness (at input aperture) 
                 5 mm 
               
               
                   
                 Thickness (at exit aperture) 
                 6 mm 
               
               
                   
                 Geometrical concentration 
                 5 
               
               
                   
                   
               
            
           
         
       
     
     Note that a low concentration ratio does not necessarily indicate a lower output as a larger PVC may be used. The issue is primarily one of cost. 
       FIG. 20  depicts a third embodiment of a solar concentrator  1300  of the present invention. In this embodiment the SPC  1316  is again substantially retracted to the extent of the underlying mirror assembly  1312 . However, the issue of efficiency loss described above for the second embodiment is substantially eliminated though a modification to the mirror assembly  1312 . The mirror section  1324  of the mirror assembly  1312  underlying the recessed SPC  1316  may be constructed with spherical, parabolic, or any other suitable curvature along the transverse direction while retaining the aspheric shape along the longitudinal axis. The focal point of the curved mirror section  1324  along the transverse axis is located approximately at the photovoltaic cell (to be mounted at the PVC mounting point  1332 ) while the focal point of the mirror cross section is a line coincident with the turn mirror  1308  corresponding to that mirror section  1324 . The sections  1328  of the mirror assembly  1312  underlying the full width wave-guide  1304  may be retained as trough mirrors. The sections with gain along the transverse axis are referred to as compound mirrors. 
       FIG. 21  presents an exemplary compound mirror  1336  as discussed above. The mirror  1336  is designed with curvature on the longitudinal axis as is the case in the trough mirror  1328  and also has curvature on the transverse axis. It is also possible that the curvature in one section of the mirror differs from the curvature in another section of the mirror. 
       FIG. 22  depicts a ray trace of a compound mirror segment  1336  underneath a SPC  1316 . Incident solar energy at the extremes along the central longitudinal axis (rays B, D) pass through the SPC  1316  to the mirror  1336  and then are reflected onto the turn mirror  1308 . Incident solar energy at the extremes of the transverse axis (rays A, C) do not pass through the SPC  1316  but are reflected onto the turn mirrors  1308  with some level of gain since the transverse axis of the compound mirror  1336  is wider than the turn mirror  1308  on the SPC  1316 . The focal point of the transverse section of the compound mirror  1336  is the photovoltaic converter to be mounted at the PVC mounting point  1332  at the end of the SPC  1316  as previously shown in  FIG. 15A . Alternatively a focal point may be chosen that ensures that all the solar radiation thus reflected enters the SPC with angles suitable for TIR without substantial regard for the PVC mounting point (not shown). 
     Other features of this third embodiment do not differ significantly from the first embodiment. The turn mirror arrangement presented in  FIGS. 9A and 9B  may be applied in this embodiment without change except as previously noted. The alternative turn mirror arrangement of  FIGS. 10A ,  10 B,  10 C and  10 D may be applied without change. The magnification considerations related to the curvature of the trough mirrors are also unchanged from  FIGS. 11A ,  11 B and  11 C. The ray trace and TIR consideration depicted in  FIGS. 13A and 13B  are unchanged in this embodiment. The assemblies can be ganged together in a manner similar to the method previously shown in  FIGS. 15A ,  15 B,  15 C and  15 D. 
     The following tables present data for an implementation of this embodiment. The mirror assembly is defined by the following data for use with the surface formula presented elsewhere in this document. 
     
       
         
           
               
               
               
               
            
               
                   
                   
               
               
                   
                   
                 Radii of curvature 
                   
               
               
                   
                 Dimensions (mm) 
                 (mm) 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Mirror 
                 Longi- 
                 Trans- 
                 Trans- 
                 Longi- 
                 Conic 
                 α i   
               
               
                 Segment 
                 tudinal 
                 verse 
                 verse 
                 tudinal 
                 Constant 
                 values 
               
               
                   
               
               
                 MS1 
                 60 
                 25 
                 130-170 
                 74.379 
                 −0.7860 
                 All = 0 
               
               
                 MS 2 
                 60 
                 (Arbitrary 
                 ∞ 
                 73.035 
                 −0.8230 
                 All = 0 
               
               
                 MS 3 
                 60 
                 but affects 
                 ∞ 
                 71.691 
                 −0.8127 
                 All = 0 
               
               
                 MS 4 
                 60 
                 SPC size) 
                 ∞ 
                 70.348 
                 −0.9000 
                 All = 0 
               
               
                 MS 5 
                 60 
                   
                 ∞ 
                 69.000 
                 −0.9400 
                 All = 0 
               
               
                 MS 6 
                 60 
                   
                 ∞ 
                 67.666 
                 −0.9806 
                 All = 0 
               
               
                   
               
            
           
         
       
     
     The curve for Mirror Segment  1  on the transverse axis is spherical. Although the α i  values are 0 in this implementation it is anticipated that other implementations may require use of these constants in order to define a fully optimal mirror surface definition, The wave-guide assembly and SPC are identical to that of the second embodiment. 
       FIG. 23  presents a fourth embodiment of a solar concentrator  1400  of the present invention. In this embodiment the significant change is that all mirrors in the mirror assembly  1412  are now compound mirrors  1436 . The compound mirrors  1436  have an aspheric curvature along the longitudinal axis and a spherical curvature along the transverse axis. The focal points of the mirrors  1436  along the longitudinal axis are the turn mirrors  1408  located above each mirror  1436 . The focal point of all mirrors  1436  on the transverse axis is now the photovoltaic converter to be mounted directly at the PVC mounting point  1432  at the end of the wave-guide  1404 . This focus simplifies the construction of a system although it requires additional care in the choice of materials for the wave-guide  1404  because of thermal heating that may take place at the focal point. If thermal heating becomes an issue the entire wave-guide  1404  may be manufactured from crown glass rather than PMMA. 
       FIG. 24  presents a ray trace depicting the interaction of a compound minor segment  1436  with the corresponding wave-guide  1404  and turn mirror segment  1408 . All minor segments  1436  have curvature along the transverse axis with the degree of curvature related to its distance from the photovoltaic converter. Accordingly, incident solar energy at the extremes along the central longitudinal axis (rays B, D) and at the extremes of the transverse axis (rays A, C) pass through the wave-guide  1404  to the mirror  1412  and then are reflected as reflected rays A&#39;-D′ onto the turn minor  1408   
     The following table presents data on an implementation of this embodiment. 
     
       
         
           
               
               
               
               
            
               
                   
                   
               
               
                   
                   
                 Radii of curvature 
                   
               
               
                   
                 Dimensions (mm) 
                 (mm) 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Mirror 
                 Longi- 
                 Trans- 
                 Trans- 
                 Longi- 
                 Conic 
                 α i   
               
               
                 Segment 
                 tudinal 
                 verse 
                 verse 
                 tudinal 
                 Constant 
                 values 
               
               
                   
               
               
                 1 
                 60 
                 25 
                 130-170 
                 74.379 
                 −0.7860 
                 All = 0 
               
               
                 2 
                 60 
                   
                 240-280 
                 73.035 
                 −0.8230 
                 All = 0 
               
               
                 3 
                 60 
                   
                 260-380 
                 71.691 
                 −0.8127 
                 All = 0 
               
               
                 4 
                 60 
                   
                 420-480 
                 70.348 
                 −0.9000 
                 All = 0 
               
               
                 5 
                 60 
                   
                 450-530 
                 69.000 
                 −0.9400 
                 All = 0 
               
               
                 6 
                 60 
                   
                 460-610 
                 67.666 
                 −0.9806 
                 All = 0 
               
               
                   
               
            
           
         
       
     
     Although the α i  values are all zero in this example it is anticipated that some values may be other than zero in other implementations. 
     The following table presents information relating to the wave-guide assembly. 
     
       
         
           
               
               
               
               
             
               
                   
               
               
                 Segment 
                 Segment length 
                 Transverse 
                 Thickness (solar 
               
               
                 Number 
                 (mm) 
                 width (mm) 
                 axis) (mm) 
               
               
                   
               
             
            
               
                 1 
                 30 
                 25 (may be 
                 6 
               
               
                 2 
                 60 
                 narrower or 
                 5 
               
               
                 3 
                 60 
                 tapered 
                 4 
               
               
                 4 
                 60 
                 depending on 
                 3 
               
               
                 5 
                 60 
                 exact light 
                 2 
               
               
                 6 
                 60 
                 path) 
                 1 
               
               
                   
               
            
           
         
       
     
     In an alternative implementation of the fourth embodiment an SPC  1416  may be attached at the point shown for the mounting point for the PVC  1432  (PVC not shown).  FIG. 25  provides a ray trace of the concentrated light within the SPC  1416  as viewed from above. This enables further concentration of the light with a relatively short SPC  1416  as the light is already substantially concentrated by the transverse curvature of the mirrors  1436 . One implementation from simulation is presented. 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 Parameter 
                 Value 
               
               
                   
                   
               
             
            
               
                   
                 X IN  (Input aperture) 
                  10 mm 
               
               
                   
                 X OUT  (Output aperture) 
                  0.5 mm 
               
               
                   
                 Length (longitudinal) 
                 100 mm 
               
               
                   
                 Thickness 
                  6 mm 
               
               
                   
                 Geometrical concentration 
                 20 
               
               
                   
                   
               
            
           
         
       
     
       FIG. 26  depicts an additional alternative implementation of the fourth embodiment that includes the SPC  1416  implementation of embodiment 2 and embodiment 3 that includes a turn mirror  1408  on the SPC  1416 . 
     Other features of this fourth embodiment do not differ from the first embodiment. The turn mirror arrangements presented in  FIGS. 9A and 9B  may be applied in this embodiment without change. The alternative turn mirror arrangement of  FIGS. 10A ,  10 B,  10 C and  10 D may be applied without change. The assemblies can be ganged together in a manner similar to the method previously shown in  FIGS. 15A ,  15 B,  15 C and  15 D. In an alternative implementation of this embodiment the mirror assembly may possess additional mirror segments along the transverse axis (not shown) that interact with a single wave-guide or multiple wave-guides wherein each longitudinal mirror/wave-guide set interacts with a separate SPC. An array after this implementation would be similar to  FIG. 15B . 
     In a fifth embodiment a solarlight concentrator  2000  that includes an acrylic, PMMA or other transparent light guide  2040  configured with “stair step” features on one side where the step rise becomes the input rise facet  2120  for collecting light from a concentrating reflector  2080  positioned to direct light from a ray direction principally parallel to the face of the rise facet  2120  and principally parallel to the sun rays in to the guide  2040  is shown in  FIG. 27 . 
     The first surface, or top view in  FIG. 27  is parallel to all the “run facets  2160 .” Therefore with each additional step added to add additional parabolic sections  2080 , the thickness of the light guide  2040  is increased by the thickness of the step rise height (See  FIG. 28 ). The thickness of the light wave-guide  2040  over the first parabolic section  2080  at the thin end of the wave-guide  2040  only needs to be thick enough for structural support of that section since it does not conduct light toward the output. Light passing through the wave-guide  2040  from the top, or sun side, to the parabolic section  2080  passes through unobstructed like a window pane. The only losses here are due to Fresnel reflections from the upper and lower surfaces of wave-guide  2040  and absorption within the guide  2040 . Fresnel losses can be minimized with anti-reflection thin film coatings or “moth eye” type structures such as those produced by Fresnel Optics, Inc. 
     The incoming solar radiation then passes through the air cavity and is reflected from the specular parabolic reflector section  2080  to pass through the focus A  2180  and enter the light guide  2040  through the rise facet  2120 . This transition from air to the acrylic light guide  2040  refracts the light rays according to Snell&#39;s Law to the extent that all rays entering are within the TIR (total internal reflection) angle of greater than about 41 degrees relative to the normal to the light wave-guide  2040  upper and lower surfaces and thereby except for absorption and scattering of the light guide material, propagate in TIR losslessly (e.g., without loss) down the light wave-guide  2040  to the exit end  2200  where it illuminates PVC  2220 . 
     In the exemplary section  3000  shown in  FIG. 28 , the curve  3080  is a parabolic section conforming to the function: 
         Y= 2 X/ (4 a ) 
     where X and Y are measured from the Vertex as shown and a=distance from the vertex to the focus as shown as reference character  3240  in  FIG. 28 . 
     The parabolic section is truncated when the Y value equals the focus height plus ½ of the rise height  3120 . The rise height  3120  is chosen to minimize thickness of the wave-guide  2040  while being large enough to etendue match (i.e., capture) the solid angular extent of the sun (approximately 0.5 degrees full angle) integrated over the run length ( 3160  in  FIG. 28 ) to the 90 degree exit angular extent of the rays passing through the focus of the parabolic section  2080  and entering the wave-guide  2040  plus some additional height to allow for reflection surface tolerances and sun tracking tolerance which both have the effect of tilting the incoming sun rays off normal axis. 
     It should be noted that many other aspheric and nonaspheric curve functions for the mirror surface which that serve to direct the incoming sun&#39;s rays to the entrance aperture of the rise  2120  around the focus are also functional and may offer additional advantages and improvements in efficiency. For instance, the focal point could be shifted vertically as the parabolic curve is generated to more evenly distribute the power over the rise facet  2120  to prevent localized heating. Additionally the rise facet  2120  could be a series of linear sections of varying angle to better accommodate the varying input angles from the parabolic sections  2080 . 
     An important reason for using an air interface to the rise facet  2120  for the incoming rays is that additional refraction occurs to the ray direction inside the wave-guide  2040  as it enters bending the ray toward the longitudinal axis of the wave-guide  2040  and more into the TIR range. In one embodiment of the invention all rays over the 0 to 90 degree range from the parabolic surface of the parabolic sections  2080  will enter the guide  2040  within TIR. 
     In one embodiment the parabolic segment and the vertical section of the parabolic sections  2080  from the vertex to the focus can be also made from an acrylic wall and molded as part of the light guide  2040 . Since the cross sections can be constant, the product can be manufactured by Acrylic plastic extrusion in the direction out of the page. The part can be reflectorized (made into a reflector) on the bottom by evaporating aluminum, silver, or other specular material or by thin film interference coatings on the outer surface or the inner surface of the parabolic section completing the product. Methods and materials for carrying out acrylic extrusion, injection molding and casting may be found at 
     http://www.plexiglas.com/acrylicresin/technicaldata/extrusion;
 
http://www.plexiglas.com/acrylicresin/technicaldata/injectionmolding; and
 
http://www.acryliccasting.com/id14.html, respectively, each of which is incorporated herein by reference in its entirety.
 
     Additionally and instead of metal specular coating to reflectorize the parabolic sections  2080 , thin film interference filters may be applied by JDS Uniphase, Inc.&#39;s custom optics division that can effectively reflect only light wavelengths into the guide  2040  that are appropriate for the particular photovoltaic devices being used. Thin films and thin film techniques include those described in “Thin Film Custom Optics” available at http://www.jdsu.com/product-literature/thinfilmco_br_co_ae.pdf (incorporated herein by reference in its entirety) and “JDSU Interference Filter Handbook, Second Edition” (incorporated herein by reference in its entirety). Other conventional reflectorizing techniques such as those described in “Thin Films for Optical Systems”, François R. Flory, published by CRC Press, 1995, ISBN 0824796330, 9780824796334 (incorporated herein by reference in its entirety) may also be used. The film can be designed to pass unwanted wavelengths, such as infra-red and ultra-violet, through the back and out to reduce heating and long term damage to the PVC and wave-guide. 
     Another embodiment introduces a stair-stepped taper to the light guide  4000  as shown in  FIG. 29 . Such a light guide  4000  can include a wave-guide  4040 , parabolic sections  4080  having a rise  4120  and a run  4160 , and a PVC  4220  positioned at an exit end  4200  of the wave-guide  4040 . This serves to make the system thinner and by varying the angle of taper, additional gain can be achieved thereby adding an additional optical gain element to the system. The taper can be allowed to vary by stage from one end to the other in order to minimize the TIR limitation of elements more distant from the output end of the light guide  4000  as additional concentration from the tapered sections increases the solid angle extent as the cross sectional area of the guide  4000  is reduced preserving the etendue of the system. 
     While this embodiment adds an additional stage of gain for the system and produces a thinner product, a gain limitation is imposed when the increasing TIR angle which results from the more steep taper causes the ray angle to exceed the TIR angle and light begins to escape from the guide  4000 . 
     An additional gain stage can be added by tapering the width of the wave-guide  5000  as shown in the plan view of the extrusion in  FIG. 30 . 
     The untapered wave-guide section  5040  may be of one piece with tapered wave-guide section  5260 . The particular taper in this region  5260  can be any of various optical concentration functions such as straight linear taper or Compound Parabolic Concentrator (CPC) functions to achieve the desired output uniformity and concentration level. In these cases the chosen function can be maximally efficient if the rays do not exceed the TIR limit of the light guide material as in the case above for the tapered guide  5000 . More aggressive concentration is possible however by coating the side walls of the guide  5000  in the concentration region with a specular reflector such as the one used in the parabolic segments  2080  shown in  FIG. 27 . Light exits the tapered section at exit point  5200  where it illuminates SPC  5220 . A description of exemplary coatings may be found at 
     http://www.kruschwitz.com/HR&#39;s.htm; http://www.goldstone-group.com/en/products_view.asp?pid=23; http://www.okjvc.com/products.asp?id=21 (incorporated herein by reference in its entirety). 
     Additionally, a reflectorized end cap (not shown) can be added to the open parabolic ends of the third stage region to further reduce collection losses and maximize efficiency. Additionally, a reflectorized end cap (not shown) can be added to each of the open parabolic section ends along the entire system to lower the sun tracking tolerance in the axis perpendicular to the concentration direction of the parabolic sections. 
     In a preferred embodiment, the concentrator is constructed using an Acrylic resin such as Plexiglas V825UVA5A manufactured by Altuglas International or other optically clear material which is injection moldable, injection-compression moldable, or extrusion moldable. 
     In the preferred embodiment the Acrylic material is formed using a linear extrusion molding process such as that described by Altuglas International&#39;s Plexiglas Acrylic Molding Resin Technical Data in Extrusion on their web site: www.plexiglas.com/acryolicresin/technicaldata/extrusion, or IAPD magazine, August/September 2003 (incorporated herein by reference in its entirety). The extrusion direction of the components  6200  is out of the page as viewed in  FIG. 31B . 
     The extruded lengths are crosscut to width using a common plastic saw and the cut sides are polished or fly cut to an optical finish suitable for total internal reflection. The more fine the side polish, the more efficient the reflection would be and therefore selected to meet cost and performance tradeoffs of a particular application. Plastic fabrication that may be used to make the light concentrator of the invention may be found at http://website.lineone.net/mike.bissett/advice.htm (incorporated herein by reference in its entirety). 
     An isometric rendering of an extruded wave-guide  6000  is shown in  FIG. 31A . Also shown at the left end is a photovoltaic cell assembly  6040  positioned to receive the concentrated sunlight for conversion to electricity. The photocell  6040  may be either attached or unattached and fixed in position relative to the concentrating light guide  6000  by external mechanical means. 
     In the limit the maximum concentration ratio of this system is simply the surface area of the plan view divided by the surface area of the output edge region, also known as the geometrical concentration ratio. 
     Another assembly method and construction is shown in  FIGS. 32A ,  32 B and  32 C. The concentrating light guide  7000  is produced as two pieces comprising a top  7040  and bottom  7080  sections with a PVC  7120  affixed in close proximity to an exit point. These sections are then seamed together as shown in  FIG. 32C  by glue, mechanical interlocks or alignment features and/or glued, or connected using any of various mechanical fastening means using mechanical features molded into each respective part. 
     In this embodiment, as all surfaces are exposed prior to assembly, secondary coatings, machine finishing, or polishing are possible if necessary to produce the surface finish of feature secondary machining required on any of the surface sections. 
     As shown in  FIG. 32D , another advantage of this construction is that the upper wave-guide  7040  may be made of optical grade Acrylic, glass or other transparent high index material, while the lower parabolic reflector section or sections  7080  can be made of another material such as aluminum or other lower cost plastic since light does not have to propagate through the material. Aluminum fabrication methods and materials that may be used to make the light concentrator of the invention may be found at 
     http://www.bonlalum.com/Login/SlsMfg/extrusion_process.jsp (incorporated herein by reference in its entirety). 
     Another advantage of this construction is that material wall sections in cross section can be designed such that there is less variation in the extremes of wall thickness and thereby allow for more uniform extrusion processing which produces a more geometrically accurate part. 
     Another advantage of this construction is that, due to the open exposure of all surfaces, manufacturing means other than extrusion may be used such as injection molding, compression-injection molding, casting, die casting, or secondary machining of various material stocks are feasible. 
     Another advantage of this construction is that the specular reflection surfaces may be accessed for secondary operation coatings, polishing, or insertion and attachment of high efficiency specular reflection films such as 3M Vikuiti™ ESR (Enhanced Spectral Reflector) film on the parabolic surfaces as well as secondary sputtering or evaporation of materials such as silver or aluminum to make the specular surfaces. 
     In another embodiment of the system a HOE (holographic optical element) can be used to perform the function of the parabolic segment, but would be easier to manufacture and can make the system thinner yet. These holographic or diffractive elements can be generated from real parabolic elements using holographic recording techniques well known in the industry or computer generated holographic elements that are available in the industry from such suppliers as Zebra Imaging or Fusion Optix. A description of the holographic techniques that may be used is provided in http://www.fou.uib.no/fd/1996/h/404001/kap02.htm (incorporated herein by reference in its entirety). 
     In another embodiment,  FIG. 33  shows a light guide  8000  including the HOEs (holographic optical elements) or DOEs (diffractive optical elements)  8080 . The light guide  8000  also includes run facets  8120  and a PVC can be placed at the exit end  8200  of the light guide  8000 . Alternatively, the HOEs or DOEs shown in  FIG. 33  may be replaced by reflective Fresnel lenses performing a cylindrical concentration function as in the previous examples. 
     Additionally, HOEs can provide functionality similar to the thin film interference coatings described above and selectively pass or reflect certain wavelengths as appropriate to the PV devices and light guide materials. 
     Another embodiment of a light guide  9000  using an alternative rise facet profile on wave-guide  9020  and compound segmented parabolic concentrating reflector  9040  is shown in  FIGS. 34A ,  34 B, and  34 C. 
     In other embodiments, the light after being refracted into the rise facet  9080  (e.g., a face) has an included ray extent of from 0 degrees to +42.16 degrees as a result of the 0 degrees to 90 degrees ray extent in air of the incident light from the parabolic reflector  9040  as shown in  FIGS. 34A and 34B . 
     There are two efficiency loss effects with this configuration, which can be improved by the embodiment to follow, but can be traded off against the cost of implementation versus the performance needs of the application. 
     First, as the rays entering the rise facet  9080  become progressively parallel to the facet surface, the Fresnel reflection losses become increasingly significant and get to 100% in the limit thereby limiting the efficiency of the system significantly depending on the light guide material and any facet coating used. 
     Second, since the ray extent after entering the facet  9080  ranges from 0 degrees, which is ideal for propagation down the light guide, to +42.16 degrees which is near the TIR critical angle and fine for the system unless an additional gain stage is applied as described prior by tapering the light guide as shown in  FIG. 29 . In this case, non-zero ray angles are progressively increased due to the additional concentration and after a few reflections exceed the critical angle and escape the light guide  9000  thereby reducing efficiency. 
       FIG. 34C  shows an embodiment that addresses this feature of the concentrating element. The rise facet  9120  is reconfigured into three segments (e.g., third, second and first faces) A, B, and C as shown in  FIG. 34C . The A segment is angled at 25.5 degrees from the run facet  9160  and serves as a TIR angled mirror to the light rays from facet B. Facet B is angled at 45 degrees as shown and accepts light from a lower parabolic section whose shape is configured so that its focus is at the center of facet B. Facet C is at 90 degrees as shown with respect to the run facet  9160  and accepts light from an upper parabolic section configured so that its focus is at the center of facet C. 
     This serves to redistribute the ray extent within the light guide  9000  to a more symmetrical distribution around the ideal 0 degree, or parallel to the run facet run direction with a lower angle max of −22.5 degrees and an upper angle max of &lt;+28.33 degrees which allow for more efficient propagation of light down the guide  9000  and additional gain without exceeding the TIR critical angle of the material and significantly reduces Fresnel losses on the rise facet segments  9120 . 
     This approach is not limited to the particular angles and segment configuration shown above, but is exemplary of other design points which trade cost and performance to the particular application. For instance, more than 3 rise facets and more than 3 parabolic sections can be utilized to further reduce losses. Additionally continuous surface functions rather than sections could be contrived to further reduce losses.