WO2011014688A2

WO2011014688A2 - Solar concentrator for use with a bi-facial cell

Info

Publication number: WO2011014688A2
Application number: PCT/US2010/043761
Authority: WO
Inventors: Roland Winston; Weiya Zhang
Original assignee: The Regents Of The University Of California
Priority date: 2009-07-30
Filing date: 2010-07-29
Publication date: 2011-02-03
Also published as: AP2012006141A0; WO2011014688A3

Abstract

An apparatus is disclosed including: a trough shaped concentrator extending along a longitudinal axis, tapering in a direction transverse the longitudinal axis from a relatively wide top to a relatively narrow bottom, and surrounding an interior region. The trough shaped concentrator includes a light admitting surface located at the wide top; and a trough shaped light reflecting surface which concentrates light admitted through the light admitting surface from a source to a concentration region within the interior region. In some embodiments, the apparatus also includes a bifacial photovoltaic cell located in the concentration region to receive a first portion of concentrated light on a first face and to receive a second portion of concentrated light at the second face.

Description

SOLAR CONCENTRATOR FOR USE WITH A BI-FACIAL CELL CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The application claims benefit of U.S. Provisional Patent Application Ser. No. 61/230,060 filed July 30, 2009, the entire contents of which are incorporated herein by reference.

BACKGROUND

[0002] This disclosure relates to devices for the transmission of radiation, especially of light. In particular, it is related to a non-focusing reflector for the concentration of radiation such as sunlight at a desired region over a wide range of angles of incidence. [0003] A number of systems for passive or non-tracking reflecting concentration of solar energy have been produced in the past. Among such systems are those shown in U.S. Pat. Nos. 5,537,991; 3,957,041; 4,002,499; 4,003,638; 4,230,095; 4,387,961;

4,359,265; 5,289,356; and 6,467,916 all of which are incorporated here by reference as if set forth fully. It is appropriate to refer to the reflectors as light-transmission devices because it is immaterial whether the reflectors are concentrating radiation from a large solid angle of incidence (e.g. concentrating solar light onto a solar cell) or broadcasting radiation from a relatively small source to a relatively large solid angle (e.g. collecting light from an LED chip to form a beam).

[0004] Concentration of radiation is possible only if the projected solid angle of the radiation is increased. This requirement is the direct consequence of the law of conservation of the etendue, which is the phase space of radiation. Solar concentrators which achieve high concentration must track the sun; that is, they must continuously reorient in order to compensate for the apparent movement of the sun in an earth center (Ptolemaic) coordinate system. Reflectors, in contrast, are fixed in position for most lighting purposes. For tracking collectors the direction to the center of the sun is stationary with respect to their aperture. Such concentrators can achieve very high concentrations of about 45,000 in air. Even higher concentrations have been achieved inside transparent media.

[0005] Tracking, however, is technically demanding because solar collectors are commonly fairly large and designing these systems for orientational mobility may add significantly to their cost. Moreover, the absorber, which incorporates some heat transfer fluid as well as piping, also may need to be mobile. This is the motivation to study the concentration which can be achieved with stationary, non-tracking devices. The same principles apply when it is desired to deliver light or other radiant energy from a small source to a relatively large solid angle.

SUMMARY

[0006] The inventors have realized that a concentrator assembly featuring a bifacial photovoltaic cell may be used, e.g., to collect solar energy to produce electricity. Embodiments of the concentrator assembly feature a wide acceptance angle, allowing for use in non-tracking applications.

[0007] In one aspect, an apparatus is disclosed including: a trough shaped concentrator extending along a longitudinal axis, tapering in a direction transverse the longitudinal axis from a relatively wide top to a relatively narrow bottom, and surrounding an interior region. The trough shaped concentrator includes a light admitting surface located at the wide top; and a trough shaped light reflecting surface which concentrates light admitted through the light admitting surface from a source to a concentration region within the interior region. In some embodiments, the apparatus also includes a bi-facial photovoltaic cell located in the concentration region to receive a first portion of

concentrated light on a first face and to receive a second portion of concentrated light at the second face.

[0008] In some embodiments, the trough shape concentrator is substantially symmetric about an optic plane which extends in a first direction from the wide top to the narrow bottom and in a second direction along the longitudinal axis. In some embodiments, the optic plane divides the interior region into a first region having a first sidewall including a first portion of the reflecting surface and a second region having a second sidewall including a second portion of the reflecting surface. In some embodiments, light admitted through the light admitting surface and incident upon the first sidewall is concentrated onto the first face of the cell. In some embodiments, light admitted through the light admitting surface and incident upon the second sidewall is concentrated onto the second face of the cell. [0009] In some embodiments, the cell is located proximal to the narrow bottom of the concentrator and substantially within the optic plane such that the first face of the cell faces towards the first region and the second face of the cell faces towards the second region. [0010] In some embodiments, the cell: extends in the optic plane in a first direction along the longitudinal axis, extends in the optic plane in a second direction from a bottom edge located at the narrow bottom towards a top edge located in the interior region.

[0011] In some embodiments, a first portion of the reflecting surface lies substantially along a parabolic cylindrical surface extending in the longitudinal direction and having a focal line extending along the top edge of the cell.

[0012] In some embodiments, a second portion of the reflecting surface located proximal the narrow bottom lies substantially along a circular cylindrical surface having a center line extending along the top edge of the cell.

[0013] In some embodiments, the internal region includes a refractive material. In some embodiments, the refractive material includes dielectric material. In some embodiments, the concentrator is composed of a solid dielectric material.

[0014] In some embodiments, at least a portion of the light reflecting surface includes an interface between the refractive material in the interior region and a material outside the interior region having a differing index of refraction, and where a portion of light reflected from reflecting surface and concentrated at the concentration region is reflected due to total internal reflection.

[0015] In some embodiments, the material outside the interior region having a differing index of refraction has an index of refraction which is less that the index of refraction of the refractive material in the interior region. [0016] In some embodiments, the material outside the interior region includes a fluid having an index of refraction of about n=l . [0017] In some embodiments, the refractive material in the interior region includes a fluid. In some embodiments, the refractive material includes a material selected from the list consisting of: water, oil, mineral oil.

[0018] Some embodiments include a shell surrounding the interior region, the shell including the light admitting and light reflecting surfaces. Some embodiments include a circulator for circulating the fluid through the interior region. In some embodiments, the circulator is adapted to remove heat from the interior region.

[0019] In some embodiments, the light reflecting surface includes a metalized surface portion. In some embodiments, the light reflecting surface includes a metalized surface portion located proximal the bottom narrow end of the trough shaped concentrator. In some embodiments, a non-metalized portion of the reflecting surface operates by total internal reflection.

[0020] In some embodiments, the concentrator has a lateral acceptance angle of about 10 degrees or more, 12 degrees or more, or 15 degrees or more. In some

embodiments, the concentrator has a longitudinal acceptance angle of about 40 degrees or more, about 50 degrees or more, about 60 degrees or more, or about 65 degrees or more.

[0021] In some embodiments, the concentrator has an optical efficiency of about 80% or more, about 85% or more, or about 90% or more.

[0022] In some embodiments, the concentrator has a geometric concentration ratio in the range of about 3 to about 6. In some embodiments the concentrator has a geometric concentration ratio of about 3 or more, about 5 or more, or about 10 or more.

[0023] In some embodiments, at least about 85% of the light concentrated on each face of the bi-facial cell is incident at angles less than about 60% from normal to the face. In some embodiments, at least about 80% of the light concentrated on each face of the bi- facial cell is incident at angles less than about 60% from normal to the face.

[0024] In some embodiments, the source is the sun.

[0025] Some embodiments include the source. In some embodiments, the source includes a light emitting diode, an organic light emitting diode, a laser, or a lamp. [0026] In some embodiments, the bi-facial photovoltaic cell is an integrated bifacial photovoltaic cell.

[0027] In some embodiments, the bi-facial photovoltaic cell includes a plurality of mono-facial photovoltaic cells. In some embodiments, the bi-facial photovoltaic cell includes a fist and a second mono-facial photovoltaic cell each having a respective active face, where the first and second mono-facial photovoltaic cells are attached to each other such that the active faces face outward and away from each other.

[0028] In another aspect, a method is disclosed including: providing a trough shaped concentrator extending along a longitudinal axis, tapering in a direction transverse the longitudinal axis from a relatively wide top to a relatively narrow bottom, and surrounding an interior region. The trough shaped concentrator includes a light admitting surface located at the wide top; and a trough shaped light reflecting surface. The method further includes providing a bi-facial photovoltaic cell located in the concentration region; using the reflective surface, concentrating light admitted through the light admitting surface from a source to a concentration region within the interior region; using the cell, receiving a first portion of concentrated light on a first face of the cell and receiving a second portion of concentrated light at the second face; and generating electrical power in response to the concentrated light received by the first and second faces of the cell.

[0029] Various embodiments may include any of the above described features, either alone, or in combination.

BRIEF DESCRIPTION OF DRAWINGS

[0030] The accompanying drawings, are not intended to be drawn to scale. In the drawings, each identical or nearly identical component that is illustrated in various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every drawing. In the drawings:

[0031] Fig. 1 is an illustration of the solar geometry;

[0032] Fig. 2a is a perspective view of a concentrator assembly;

[0033] Fig. 2b is a cross sectional view of a concentrator assembly; [0034] Fig. 3 is a ray trace diagram of a concentrator assembly;

[0035] Fig. 4 is a chart of the performance characteristics of the concentrator assembly of Fig. 2;

[0036] Fig. 5 is a plot of optical efficiency vs. lateral (e.g. north-south) pointing angle of the concentrator assembly of Fig. 2;

[0037] Fig. 6 is a plot of optical efficiency vs. longitudinal (e.g. east west) pointing angle of the concentrator assembly of Fig. 2;

[0038] Fig. 7 is a schematic illustration of a system for generating electrical power featuring a concentrator assembly; and [0039] Fig. 8 is a schematic illustration of a circulation system featuring a concentrator assembly.

[0040] Figs. 9A-9F illustrate a method of designing a concentrator.

[0041] Figs. 10A- 1OC show a listing of a SciLab script for an exemplary design method. DETAILED DESCRIPTION

[0042] From a thermodynamic viewpoint, the solar geometry may be described in direction cosines, which are the momenta of the light rays. Referring to Fig. 1, the sun over the year occupies a band 101 inside a unit radius circle 102 which is ± sin 23.5 deg.. This band is Vi the area of the circle, from which follows, e.g. as described in detail U.S. Patent No. 6, 467,916 to Roland Winston (incorporated by reference above), that the maximum theoretical concentration for a trough shaped concentrating reflector using no seasonal adjustments is 2.

[0043] In some embodiments, this limit may be increased. In cases where the target (solar cell) is immersed in a refractive material having an index of refraction (n), this limit is multiplied by n squared because the momentum of a light ray is actually the index of refraction x direction cosine. Then for n ~ 1.5 (typical of glass or PMMA) and restricting the cell irradiance to 60 deg., the maximum concentration becomes ~3. In some embodiments featuring a low concentration design which can be switched seasonally between 2 positions/year (summer and winter) the limits are multiplied by 2 (4 - 6 concentration). For example, embodiments described herein may feature a concentration of about 5 or more. [0044] Referring to Fig. 2a and 2b, concentrator assembly 200 includes a trough shaped concentrator 201 which extends along a longitudinal axis A. Bi-facial cell 205 runs along the bottom of the trough shaped concentrator 201. Concentrator 201 concentrates light onto bi-facial cell 205 which generates electrical power in response to the concentrated light. For simplicity, in the examples described herein, it will be assumed that assembly 200 is used as a solar collector placed outdoors with the longitudinal axis running along an east-west direction. However, it is to be understood that assembly 200 may be used to concentrate light from any source, and arranged in any suitable orientation.

[0045] Bi-facial solar cell 205 is a bi-facial photovoltaic cell having two opposing active faces 206a and 206b. Each face receives incident light and converts the light energy to electrical energy. In the embodiment shown, cell 205 is an integrated bi-facial solar cell. However, in some embodiments, two photovoltaic cells having single active faces may be attached together to form a bi-facial cell. In some embodiments, the cell may include an object featuring one or more surfaces coated with a thin film photovoltaic material.

[0046] Fig. 2b shows a cross section of assembly 200 taken in a plane

perpendicular to longitudinal axis A. Concentrator 201 is symmetric about meridional plane which extends in the vertical direction and along longitudinal axis A. Concentrator 200 tapers in the vertical direction from a wide top to a narrow bottom, where cell 205 is located. Concentrator 200 has a top light admitting surface 210, a reflecting surface 212, and an interior region 214. Reflecting surface 212 can be considered to be made up of two sidewalls 212a and 212b located on opposite sides of optical plane OP. Cell 205 is positioned in the optic plane OP and oriented such that active faces 206a and 206b face sidewalls 212a and 212b, respectively. Cell 205 may have any suitable dimensions. In the example shown, the dimensions of cell 205 are lmm x 50 micron x 100mm.

[0047] Concentrator 201 may be made of refractive material. In some

embodiments, e.g., as shown in Fig. 2, concentrator 201 may be formed as a solid element made of a refractive dielectric material. Such embodiments are advantageous in that cell 205 will be surrounded by the solid material and thereby protected (e.g. protected from rain when assembly 200 is positioned outdoors). Any suitable refractive material may be used including glass, plastic, quartz, etc. In one embodiment PMMA is used. [0048] In other embodiments concentrator 201 is formed as an outer shell surrounding interior region 214. Region 214 may be evacuated, or filed with a fluid, e.g. a transparent refractive fluid. The fluid may be, for example, a liquid (e.g. water or mineral oil), gas, gel, or a mixture thereof. The shell may be formed of any suitable material, e.g. plastic, glass, quartz, etc. In some embodiments, the fluid may have an index of refraction which is substantially the same as that of the shell. In other embodiments, the indices of refraction may differ.

[0049] Exemplary dimensions for concentrator 201 are shown in Fig. 2b, however, it is to be understood that other suitable dimensions may be used.

[0050] Fig. 3 shows a ray trace diagram illustrating the operation of concentrator assembly 200. As shown, rays 301 are incident on light admitting surface 210 at the maximum acceptance angle of concentrator 201 (in this example 15 degrees). Rays 301 pass through light admitting surface 210, are refracted. Rays 301 pass through interior region 214 and are incident on sidewall 212a of reflecting surface 212. The rays are reflected and concentrated to face 206a of cell 205 at a concentration region at the narrow bottom of concentrator 201. A symmetric set of rays (not shown) are concentrated from sidewall 212b onto face 206a of cell 205. Accordingly, light is concentrated onto both active faces 206a and 206b of cell 205.

[0051] As shown, reflecting surface 212 is an interface between refractive material in interior region 214 and the exterior to assembly 200 (e.g. an air/glass or air/plastic interface). In some such embodiments, the material outside the interior region has a differing index of refraction which is less that the index of refraction of the refractive material in the interior region. In some embodiments, the material outside the interior region includes a fluid (e.g. air) having an index of refraction of about n=l.

[0052] Reflective surface 212 operates by total internal reflection at this interface, except in region 302, where the conditions for total internal reflection are not met. In region 302, reflecting surface 212 is metalized to provide reflection. In other embodiments, the entirety of reflective surface 212 may be metalized or otherwise formed as a mirror.

[0053] In some embodiments, the shape of reflecting surface 212 may be determined using edge ray techniques as follows. As shown in Fig. 3, rays 301 represent the edge rays for concentrator 201. In other words, these are the rays incident with the maximum lateral (i.e. north-south) acceptance angle of the concentrator, or θ=15 degrees in this case. A first portion of the side wall 212a of the concentrator, indicated as section AB, is taken to be a Cartesian oval which focuses the edge rays to point O', the top edge of the cell 205. Section AB thus belongs to a parabola having a focus at O'. A second portion of side wall 212a, indicated as section BO, is taken to be part of a circle with O' being the center and having a radius of the length of segment OO' (i.e. the height of cell 205).

Sidewall 212b is constructed by noticing that the concentrator 201 is symmetric about the optical plane OP.

[0054] Accordingly, in some embodiments, a portion of reflecting surface 212 located near the narrow bottom of concentrator 201 is formed as a section of a circular cylindrical surface centered on a line running in the longitudinal direction along the top edge of cell 205. Another portion (e.g. the remainder) of reflecting surface 212 is formed as a section of a parabolic cylindrical surface whose focal line is a line running in the longitudinal direction along the top edge of cell 205. [0055] Figs. 10A- 1OC includes an exemplary algorithm script written in the well known scientific computing environment (available at "http:// www.scilab.org") for designing a concentrator of the type described above. As will be understood by those skilled in the art, this particular exemplary algorithm may be modified or extended based on particular design requirements. [0056] Figs. 9A-9F illustrate an alternate method for designing concentrator 201 which accommodates for the shape of cell 205 to ensuring concentration at or near the thermodynamic limit. (It is to be understood that FIGs. 9A-9F are illustrative only and not drawn to scale.) Referring to Fig. 9A is a first step a string segment 901 is constructed which wraps around the outer surface of cell 205 with one end anchored to the cell (e.g., as shown, anchored to the center of the bottom surface of the cell). Note that while string segment 901 is indicated with a bold line, for accurate results a string construct with vanishing thickness should be used.

[0057] Referring to Fig. 9B, while holding the length of string segment 901 constant and the string taught, the string segment is unwrapped (e.g., as shown starting from an end located at the bottom center surface of the cell). As string segment 901 is unwrapped, its free end travels along a path (indicated with a dotted line) which defines the shape of the reflective surface of concentrator 201.

[0058] Referring to Fig. 9C, line 902 is constructed along the wave front of an edge ray (the ray incident with the maximum lateral acceptance angle of the concentrator). Once string segment 901 is tangent to the surface of cell 205 (i.e., as shown, it extends horizontally), a second string segment 905 is constructed extending from the end point 903 to intersect line 902 normally (i.e. at right angles to the line). Accordingly, segments 901 and 905 form a string 907 arranged to extend from cell 205 around point 903 to an end 909 as if a push pin has been inserted into the page at point 903. [0059] Referring to Fig. 9D, end 909 of string is allowed to slide along line 902 while maintaining the normal orientation of segment 905 to line 902. Thus, as end 903 slides with the string kept taught, point 903 is adjusted to maintain the normal orientation of segment 905 to line 902. The path of point 903, continues to trace out the reflective surface of concentrator 201. [0060] Referring to Fig. 9E, as the slope of the reflective surface defined by the movement of point 903 increases towards infinity, the process is stopped, and one half of the reflective surface of concentrator 201 has been defined. The remainder of the surface may be obtained by reflection about axis O, as shown in Fig. 9F. Alternatively, the process may be ended before the slope approaches infinity to produce a truncated design (e.g., in cases where material costs, weight, or other considerations preclude full extension of the reflective surface).

[0061] Not wishing to be bound by theory, using the framework of the Hottel string method familiar from thermodynamics (see, e.g., Hoyt C. Hottel, Radiant-Heat

Transmission 1954, Chapter 4 in William H. McAdams (ed.), Heat Transmission, 3rd ed. McGraw-Hill), it can be shown that the above described string-based design method will result in a concentrator design which operates at the thermodynamic limit. As will be apparent to one skilled in the art, the method may be easily adapted to cells having other cross sectional shapes, including square shapes, round shapes, irregular shapes, etc.

Further, the design method may be used to obtain three dimensional concentrators by rotation of the two dimensional solution about the optic axis.

[0062] The design method described above may be implemented as software on a computer. The output of the design method software may be, e.g., a data file, an image, a print out, etc. The output of the design method software may control one or more automated fabrications tools to fabricate a concentrator. The designs may be evaluated and/or optimized using optical design tools, e.g., including ray tracing applications and other optical design applications know in the art.

[0063] Fig. 4 is a chart of exemplary performance characteristics for the concentrator assembly show in Fig. 2. Note in particular that the wide lateral (e.g. north- south) acceptance angle of +/- 15 degrees and longitudinal (e.g. east- west) acceptance angle of +/- 65 degrees. As shown in Figs. 5 and 6, the optical efficiency of concentrator 201 may be 85% or greater for light incident at angles less than the acceptance angles.

[0064] These acceptance angles are more than sufficient to allow concentrator assembly 201 to operate efficiently as a non-tracking solar collector during the entire year using only 2 positions (e.g. summer/winter). Referring to Fig. 1, the first position corresponds to the top half of band 101, while the second position corresponds to the lower half of band 101.

[0065] It is also notable that, because a bi-facial cell is used, the generated power is increased by a factor equal to twice the geometric concentration factor. For example, as shown in Fig. 4, for a geometric concentration factor of 5, the generated power is increased by a factor of 10.

[0066] While the above characteristics are given for light directly incident from the source (e.g. the sun), concentrator 201 also performs well in concentrating diffuse incident light (e.g. light from the sun diffusely scattered by the atmosphere). For a concentrator with an angular acceptance of 15 degrees, which occupies half the band 101 in Fig. 1 for two positions a year (summer and winter), the usual formula for diffuse fraction would be Sin(15°)~0.25. However, using a dielectric concentrator, the etendue is enhanced, e.g., by a factor of -1.15. See, e.g., Equation 5.5 and the accompanying discussion in Nonimaging Optics by Roland Winston, Juan C. Minano, Pablo Benitez, with contributions by Narkis Shatz and John C. Bortz, Academic Press, 2004. (ISBN 0-12-759751-4).

Therefore the total diffuse capture fraction may be -0.3. For example, for an average diffuse fraction of 20% the concentrator 201 will collect 6% more energy than high concentration designs.

[0067] Although one example of performance characteristics for concentrator assembly 201 is provided above, in various embodiments it may have other characteristics. For example, in some embodiments, concentrator 212 has a lateral (e.g. north-south) acceptance angle of about 10 degrees or more, 12 degrees or more, or 15 degrees or more.

[0068] In some embodiments, concentrator 201 has a longitudinal (e.g. east-west) acceptance angle of about 40 degrees or more, about 50 degrees or more, about 60 degrees or more, or about 65 degrees or more. [0069] In some embodiments, concentrator 201 has an optical efficiency (e.g. for light incident at angles within the acceptance angles of the concentrator) of about 80% or more, about 85% or more, or about 90% or more.

[0070] In some embodiments, concentrator 201 has a geometric concentration ratio (e.g. for light incident at angles within the acceptance angles of the concentrator) in the range of about 3 to about 6. In some embodiments the concentrator has a geometric concentration ratio of about 3 or more, about 5 or more, or about 10 or more.

[0071] In some embodiments, at least about 85% of the light concentrated on each face 206a and 206b of the bi- facial cell 205 is incident at angles less than about 60% from normal to the face. In some embodiments, at least about 80% of the light concentrated on each face of the bi- facial cell is incident at angles less than about 60% from normal to the face.

[0072] Fig. 7 shows a system 700 for generating electrical power featuring concentrator assembly 200 (shown in a top down view). Bi-facial cell 205 is connected in an electrical circuit with load 701. Bi-facial cell 205 generates an electrical current in response to incident light concentrated onto the cell by concentrator 201, thereby powering load 801. In some embodiments, more than one concentrator assembly 200 may be used. These concentrator assemblies 200 may be connected in any suitable fashion (e.g. in series or in parallel) to generate electrical power. In general, concentrator assembly 200 may be used with any known techniques for generating power using photovoltaic cells.

[0073] Fig. 8 shows a system 800 featuring concentrator assembly 200 (shown in a top down view) having a fluid filled interior region. Circulator 801 is in fluid

communication with concentrator assembly 200 through tubes 802. As indicated by small arrows, circulator 801 circulates fluid through concentrator assembly 200, thereby providing cooling. In some embodiments, circulator 801 may include a heat exchanger which removes heat from the circulating fluid, as indicated by the broad arrow. In some embodiments this heat may be used to generate electrical energy, generate steam power, provide home heating, etc.

[0074] Although the specific examples described above have dealt with concentrating light to a bi-facial photovoltaic cell, it is to be understood that the devices and techniques described herein may by applied to concentrate light to any other suitable light receiving element including a mono-facial photovoltaic cell, a photodiode, a laser gain medium, a photographic medium, a digital imaging sensor, a digital light processor or a MEMs device. [0075] In some embodiments, the cell may include an object featuring one or more surfaces coated with a thin film photovoltaic material or one or more thin film photovoltaic cell. In some embodiments, the thin film material includes one or more of: amorphous silicon (e.g., a-Si or a-Si:H), protocrystalline sillicon, nanocrystalline (nc-Si or nc-Si:H) and black silicon. Any other suitable photovoltaic materials may be used, e.g., cadmium telluride, copper indium gallium (di)selenide (CIGS), cadmium sulphide, organic photovoltaic material, etc. The thin films may be fabricated using any suitable technique including chemical vapor deposition, roll to roll processing, spray printing techniques, etc. The thin films may include one ore more semiconductor junctions including p-n junctions, p-i-n junction, heterojunctions, etc. [0076] Although the specific examples described above have dealt concentrating radiation from a relatively large solid angle of incidence onto a relatively small target (e.g. concentrating solar light onto a solar cell), it will be understood that they may equally well be applied to broadcasting radiation from a relatively small source to a relatively large solid angle (e.g. collecting light from an LED chip to form a beam or sheet of light). The small source may, for example, include a light emitting diode, an organic light emitting diode, a laser, or a lamp.

[0077] One or more or any part thereof of the techniques described herein can be implemented in computer hardware or software, or a combination of both. The methods can be implemented in computer programs using standard programming techniques following the method and figures described herein. Program code is applied to input data to perform the functions described herein and generate output information. The output information is applied to one or more output devices such as a display monitor. Each program may be implemented in a high level procedural or object oriented programming language to communicate with a computer system. However, the programs can be implemented in assembly or machine language, if desired. In any case, the language can be a compiled or interpreted language. Moreover, the program can run on dedicated integrated circuits preprogrammed for that purpose.

[0078] Each such computer program is preferably stored on a storage medium or device (e.g., ROM or magnetic diskette) readable by a general or special purpose programmable computer, for configuring and operating the computer when the storage media or device is read by the computer to perform the procedures described herein. The computer program can also reside in cache or main memory during program execution. The analysis method can also be implemented as a computer-readable storage medium, configured with a computer program, where the storage medium so configured causes a computer to operate in a specific and predefined manner to perform the functions described herein.

[0079] A number of embodiments of the invention have been described.

Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. [0080] As used herein the term "light" and related terms (e.g. "optical") are to be understood to include electromagnetic radiation both within and outside of the visible spectrum, including, for example, ultraviolet and infrared radiation.

[0081] Having thus described several aspects of at least one embodiment of this invention, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description and drawings are by way of example only.

Claims

What is claimed is:

1. An apparatus comprising:

a trough shaped concentrator extending along a longitudinal axis, tapering in a direction transverse the longitudinal axis from a relatively wide top to a relatively narrow bottom, and surrounding an interior region, said trough shaped concentrator comprising:

a light admitting surface located at the wide top; and

a trough shaped light reflecting surface which concentrates light admitted through the light admitting surface from a source to a concentration region within the interior region; and

a bifacial photovoltaic cell located in the concentration region to receive a first portion of concentrated light on a first face and to receive a second portion of concentrated light at the second face.

2. The apparatus of claim 1, wherein:

the trough shape concentrator is substantially symmetric about an optic plane which extends in a first direction from the wide top to the narrow bottom and in a second direction along the longitudinal axis;

the optic plane divides the interior region into a first region having a first sidewall comprising a first portion of the reflecting surface and a second region having a second sidewall comprising a second portion of the reflecting surface;

light admitted through the light admitting surface and incident upon the first sidewall is concentrated onto the first face of the cell; and

light admitted through the light admitting surface and incident upon the second sidewall is concentrated onto the second face of the cell.

3. The apparatus of claim 2, wherein the cell is located proximal to the narrow bottom of the concentrator and substantially within the optic plane such that the first face of the cell faces towards the first region and the second face of the cell faces towards the second region.

4. The apparatus of claim 3, wherein the cell:

extends in the optic plane in a first direction along the longitudinal axis, extends in the optic plane in a second direction from a bottom edge located at the narrow bottom towards a top edge located in the interior region.

5. The apparatus of claim 4, wherein a first portion of the reflecting surface lies substantially along a parabolic cylindrical surface extending in the longitudinal direction and having a focal line extending along the top edge of the cell.

6. The apparatus of claim 5, wherein a second portion of the reflecting surface located proximal the narrow bottom lies substantially along a circular cylindrical surface having a center line extending along the top edge of the cell.

7. The apparatus of claim 2, wherein the internal region comprises a refractive material.

8. The apparatus of claim 7, wherein the refractive material comprises dielectric material.

9. The apparatus of claim 7, wherein the concentrator is composed of a solid dielectric material.

10. The apparatus of claim 7, wherein at least a portion of the light reflecting surface comprises an interface between the refractive material in the interior region and a material outside the interior region having a differing index of refraction, and wherein a portion of light reflected from reflecting surface and concentrated at the concentration region is reflected due to total internal reflection.

11. The apparatus of claim 10, wherein the material outside the internior region having a differing index of refraction has an index of refraction which is less than the index of refraction of the refractive material in the interior region.

12. The apparatus of claim 11 , wherein the material outside the interior region comprises a fluid having an index of refraction of about n=l .

13. The apparatus of claim 12, wherein the refractive material in the interior region comprises a fluid.

14. The apparatus of claim 13, wherein the refractive material comprises a material selected from the list consisting of: water, oil, mineral oil.

15. The apparatus of claim 14, comprising a shell surrounding the interior region, said shell comprising the light admitting and light reflecting surfaces.

16. The apparatus of claim 15, further comprising a circulator for circulating the fluid through the interior region.

17. The apparatus of claim 16, wherein the circulator is adapted to remove heat from the interior region.

18. The apparatus of claim 2, wherein the light reflecting surface comprises a metalized surface portion.

19. The apparatus of claim 18, wherein the light reflecting surface comprises a metalized surface portion located proximal the bottom narrow end of the trough shaped concentrator.

20. The apparatus of claim 18, wherein a non-metalized portion of the reflecting surface operates by total internal reflection.

21. The apparatus of claim 2, wherein the concentrator has a lateral acceptance angle of about 10 degrees or more.

22. The apparatus of claim 21, wherein the concentrator has a lateral acceptance angle of about 15 degrees or more.

23. The apparatus of claim 22, wherein the concentrator has a longitudinal acceptance angle of about 40 degrees or more.

24. The apparatus of claim 23, wherein the concentrator has a longitudinal acceptance angle of about 65 degrees or more.

25. The apparatus of claim 23, wherein the concentrator has an optical efficiency of about 80% or more.

26. The apparatus of claim 25, wherein the concentrator has an optical efficiency of about 85% or more.

27. The apparatus of claim 25, wherein the concentrator has an optical efficiency of about 90% or more.

28. The apparatus of claim 23, wherein the concentrator has a geometric concentration ratio in the range of about 3 to about 6.

29. The apparatus of claim 23, wherein the concentrator has a geometric concentration ratio of about 10 or more.

30. The apparatus of claim 2, wherein at least about 85% of the light concentrated on each face of the bifacial cell is incident at angles less than about 60% from normal to the face.

31. The apparatus of claim 2, wherein the source is the sun.

32. The apparatus of claim 2, further comprising the source.

33. The apparatus of claim 32, wherein the source comprises at least one chosen from the group consisting of: a light emitting diode; a laser; a lamp.

34. The apparatus of claim 2, wherein the bifacial photovoltaic cell is an integrated bifacial photovoltaic cell.

35. The apparatus of claim 2, wherein the bifacial photovoltaic cell comprises a plurality of mono-facial photovoltaic cells.

36. The apparatus of claim 2, wherein the bifacial photovoltaic cell comprises a fist and a second mono-facial photovoltaic cell each having a respective active face, wherein the first and second mono-facial photovoltaic cells are attached to each other such that the active faces face outward and away from each other.

37. A method comprising:

providing a trough shaped concentrator extending along a longitudinal axis, tapering in a direction transverse the longitudinal axis from a relatively wide top to a relatively narrow bottom, and surrounding an interior region, said trough shaped concentrator comprising:

a light admitting surface located at the wide top; and

a trough shaped light reflecting surface;

providing a bifacial photovoltaic cell located in the concentration region using the reflective surface, concentrating light admitted through the light admitting surface from a source to a concentration region within the interior region;

using the cell, receiving a first portion of concentrated light on a first face of the cell and receiving a second portion of concentrated light at the second face; and generating electrical power in response to the concentrated light received by the first and second faces of the cell.