Abstract:
A solar concentrator having a concentrator element for collecting input light, a reflective component with a plurality of incremental steps for receiving the light and also for redirecting the light, and a waveguide including a plurality of incremental portions enabling collection and concentration of the light.

Description:
This invention is directed to a solar concentrator for producing electrical, thermal and radiative energy. More particularly, the invention is directed to a solar concentrator using a combination of refractive and reflective optics to concentrate and aggregate sunlight from a plurality of concentrator systems. 
     BACKGROUND OF THE INVENTION 
     Solar collectors have long been developed for the collection and concentration of sunlight. Increasing the energy density of ambient sunlight enables more efficient conversion to useful forms of energy. Numerous geometries and systems have been developed, but the mediocre performance and high costs of such systems do not permit widespread use. In order to achieve adequate performance and manufacturability, improvements in solar energy collectors are needed. 
     SUMMARY OF THE INVENTION 
     A concentrator system includes a combination of optical elements comprising a concentrating element, such as a refractive and/or reflective component, a reflective and/or refractive element to redirect sunlight into a light waveguide which is constructed with a plurality of stepped reflective surfaces for efficient aggregation and concentration into a receiver unit (thermal and/or photovoltaic) and other conventional energy conversion systems. The control of the geometry of the reflective surfaces along with the aspect ratio of the light waveguide enables ready manipulation, collection and concentration of sunlight preferably onto a contiguous area for a variety of commercial applications, including solar cell devices, light pipe applications, heat exchangers, fuel production systems, spectrum splitters and other secondary manipulation of the light for various optical applications. 
     These and other objects, advantages and applications of the invention, together with the organization and manner of operation thereof, will become apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a solar energy concentrator generally constructed in accordance with an embodiment of the invention; 
         FIG. 2  illustrates a cross-sectional view of one embodiment of a light waveguide shown schematically in  FIG. 1 ; 
         FIG. 3  illustrates another cross-sectional view of a linear embodiment of a light waveguide shown schematically in  FIG. 1 ; 
         FIG. 4  illustrates another cross-sectional view of a rotational embodiment of a light waveguide shown schematically in  FIG. 1 ; 
         FIG. 5A  shows a first edge shape of a reflecting element of a waveguide;  FIG. 5B  shows a second edge shape for a reflecting element of a waveguide;  FIG. 5C  shows a first separate element for redirecting light as part of a stepped waveguide;  FIG. 5D  shows a second separate element for redirecting light as part of a stepped waveguide;  FIG. 5E  shows a system with plural light pipes coupled to a stepped waveguide and  FIG. 5F  shows a waveguide with embedded redirecting components; 
         FIG. 6  shows a curved concentrating element and curved reflector coupled to a waveguide; 
         FIG. 7  shows a curved concentrating element and two planar reflectors coupled to a waveguide; 
         FIG. 8A  shows a closed optical element coupled to a waveguide and  FIG. 8B  shows an enlarged view of a portion of  FIG. 8A  at the juncture of the optical element and waveguide; 
         FIG. 9A  shows another closed optical element coupled to a waveguide and  FIG. 9B  shows an enlarged view of a portion of  FIG. 9A  at the juncture of the optical element and the waveguide; 
         FIG. 10A  shows another closed optical element coupled to a waveguide and  FIG. 10B  shows an enlarged view of a portion of  FIG. 10A  at a juncture of the optical element and the waveguide; 
         FIG. 11A  shows a further closed element coupled to a waveguide and  FIG. 11B  shows an enlarged view of portion of  FIG. 11A  at a juncture of the optical element and the waveguide; and 
         FIG. 12  shows ray tracing results for the optical systems of FIGS.  2  and  6 - 11 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A solar energy concentrator system constructed in accordance with a preferred embodiment of the invention is indicated schematically at  10  in  FIG. 1 . The solar energy concentrator system  10 , includes an optical concentrating element  12  which can be any conventional optical concentrator, such as an objective lens, a Fresnel lens, and/or a reflective surface element, such as a parabolic or compound shaped reflector. This optical concentrating element  12  acts on input light  14  to concentrate the light  14  to a small focal area  16 . In the preferred embodiment, the small focal area  16  is disposed within reflective component  18 , or other conventional optical redirecting element which causes total internal reflection. The reflective component  18  redirects the concentrated light  20  into a waveguide  22 . The waveguide  22  is constructed to cause internal reflection of the light  20  which propagates along the waveguide  22  in accordance with Snell&#39;s law wherein total internal reflection occurs when the angle of the light  20  incident on surface  24  of the waveguide  22  is greater than the critical angle, Ø c :
 
Ø c =sin(η waveguide /η cladding )
 
     Where Ø c =critical angle for total internal reflection, 
     η waveguide =refractive index of waveguide material 
     η cladding =refractive index of a cladding layer or the index at the ambient/waveguide interface. 
     A receiver  26  is disposed at the end of the waveguide  22  and receives the light  20  for processing into useful energy or other optical applications. 
     In a preferred form of the concentrator system  10  shown in  FIG. 2 , the incident light  14  is concentrated or focused in a first step using the element  12  described hereinbefore. The concentrated light  20  is further processed by associating sections of the concentrator system  10  with reflector/waveguide sections  28 . Each of the reflector/waveguide sections  28  comprises a reflective section  32  which receives the concentrated light  20  and redirects light  30  within the associated waveguide section  28  with the light  30  undergoing total internal reflection (TIR) along the length of the entire waveguide  22 . A plurality of the reflector/waveguide sections  28  comprise the waveguide  22  and forms a stepped form of waveguide construction. 
     The cross-section of the various reflector/waveguide sections  28  provides a basic building block for various configurations of the concentrator system  10 . One exemplary commercial embodiment is shown in  FIG. 3  with an aspect ratio A/B, an area concentration factor or energy density ΔØ which is proportional to A/B where A is the length of the waveguide  22  and B is the largest thickness (see  FIGS. 2 and 3 ). In a most preferred embodiment, the thickness B is comprised of a plurality of incremental step heights, C, which provide a clear light pathway for TIR light from each of the reflector/waveguide sections  32 . 
       FIG. 4  illustrates another example of the concentrator system  10  in the form of a rotationally (or axially) symmetric geometry having a concentrator system  10 ′ and the concentrating element  12  in association with the reflector/waveguide sections  28  of the waveguide  22 . This rotationally symmetric form of the concentrator system  10 ′ (or the system  10 ), which can be any portion of a full circle, enables three dimensional radial convergence of the incident light  14  resulting in ΔØ being proportional to (A/B) 2  thereby substantially enhancing collection and concentrator efficiency. In a most preferable embodiment of  FIG. 4  two axis solar tracking is used as opposed to the single axis tracking for the embodiment of  FIG. 3 . 
     In addition to the linear and rotational embodiments of  FIGS. 3 and 4 , the concentrator system  10 ′ can be disposed both above and/or below the waveguide  22  relative to the direction of the incident light  14 . In such embodiments, some of the light  14  will pass through the waveguide  22  and be redirected back to the waveguide  22  by the concentrator system  10 ′. These forms of systems enable light recycling and thus improve end efficiency and the use of the reflective systems for concentration, described herein, show an increased efficiency for concentration of light relative to conventional refractive system. 
     In other embodiments, the reflective elements  18  can be angularly adjusted with respect to the waveguide  22  in order to cause TIR. The reflective element  18  can be an integral part of the waveguide  22  with a variety of angular profiles (see  FIGS. 5A and 5B ). The element  18  also can be separate elements  38  and  39  (see  FIGS. 5C and 5D ). In addition, the reflective element  18  and the associated waveguide  22  can also take the form of complex light collector pipes  42  and light redirecting components  43  as shown in  FIGS. 5E and 5F , respectively. 
     The above described forms of the concentrator system  10  and  10 ′ provide concentrated light  20  to a contiguous area as opposed to a nodal area, thereby allowing delivery of concentrated solar energy to a variety of downstream receivers  26 , such as a solar cell, a light pipe for further processing, a heat exchanger, a secondary concentrator and a light spectrum splitter. 
     In yet another series of embodiments shown in  FIGS. 6-11B , a variety of optical components can be used in combination to further and substantially enhance both the concentration and collection efficiency.  FIG. 6  in a most preferred embodiment shows a curved concentrating element  50  directing light  52  onto a curved reflector  54  which passes the light  52  into the waveguide  22 .  FIG. 7  in another most preferred embodiment shows another curved concentrating element  56  which directs the light  52  off a reflector  58  having two planar surfaces  59  and  60  which redirect the light  52  by TIR into the waveguide  22 .  FIG. 8A  shows a partially closed optical element  64  which redirects the light  52  at interface  66 , reflects the light  52  off curved reflector  68  focusing the light  52  onto interface  70  between a bottom reflective surface  72  of the optical element  64 . As best seen in the enlarged view of  FIG. 8B , the waveguide  22  has a substantially complementary angular match to the reflective surface  72 . 
     In  FIG. 9A  in another most preferred embodiment is a similar system as in  FIG. 8A , but the optical element  65  is closed and coupled to an extension waveguide  74  (a form of light pipe) which collects the light  52  and transmits it into the waveguide  22  (as best seen in  FIG. 9B ). 
     In  FIG. 10A  an optical element  76  is closed with the input light  52  reflected by TIR from reflective surface  77  with a particular angular cross section best shown in  FIG. 10B  which enables collection of the light from TIR and coupling with the waveguide  22  from reflection off surfaces  80  and  82 . 
     In  FIG. 11A  an optical element  82  cooperates with another reflector  84  to direct the light  52  into the waveguide  22  from the two different optical sources  82  and  84 , thereby further ensuring collection of all the light incident on surface  86  of the optical element  82 . In this embodiment the optical elements  82  and  84  perform the role of both concentrating elements and reflecting elements. 
     The concentration of light achieved by the concentrator system  10  being a function of the aspect ratio A/B leads to a highly compact concentrator system  10 . The device can aggregate light from a relatively wide area and concentrate it to a relatively small receiver that has a contiguous area while remaining highly compact. This simplifies production by reducing the volume of material required, allowing for multiple units to be made from a single mold and reducing assembly complexity. 
       FIG. 12  shows the results of ray tracings performed on the designs depicted in FIGS.  2  and  6 - 11 . Each design demonstrates a particular performance in terms of its ability to concentrate light in the linear dimension, as shown by the ratio of A/B. The data is for light having an input cone half angle of +−1 degree, an output cone half angle of +−20 degrees, an initial refractive index of n=1, and a final refractive index of n=1.5. The theoretical maximum allowable concentration of light with those input parameters is 30× in the linear dimension, whereas  FIG. 9  for example achieves a concentration factor of 25×. Since the concentration factor in the linear dimension is proportional to the aspect ratio A/B, the design shown in  FIG. 9  can deliver a concentrator that is 250 millimeters long (A) while only 10 millimeters in thickness (B); or a concentrator that is 500 millimeters long (A) while only 20 millimeters in thickness (B). This represents a highly compact concentrator system  10  that can effectively aggregate concentrated light from a relatively wide area and deliver it to a single receiver. 
     The foregoing description of embodiments of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the present invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the present invention. The embodiments were chosen and described in order to explain the principles of the present invention and its practical application to enable one skilled in the art to utilize the present invention in various embodiments, and with various modifications, as are suited to the particular use contemplated.