WO1991004580A1 - Photovoltaic solar systems with dispersive concentrators - Google Patents

Photovoltaic solar systems with dispersive concentrators Download PDF

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Publication number
WO1991004580A1
WO1991004580A1 PCT/US1990/004439 US9004439W WO9104580A1 WO 1991004580 A1 WO1991004580 A1 WO 1991004580A1 US 9004439 W US9004439 W US 9004439W WO 9104580 A1 WO9104580 A1 WO 9104580A1
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WIPO (PCT)
Prior art keywords
radiation
dispersive
concentrator
photovoltaic
component
Prior art date
Application number
PCT/US1990/004439
Other languages
French (fr)
Inventor
Melvin S. Cook
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Holobeam, Inc.
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Publication date
Application filed by Holobeam, Inc. filed Critical Holobeam, Inc.
Publication of WO1991004580A1 publication Critical patent/WO1991004580A1/en

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Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/04—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices
    • H01L31/054—Optical elements directly associated or integrated with the PV cell, e.g. light-reflecting means or light-concentrating means
    • H01L31/0543—Optical elements directly associated or integrated with the PV cell, e.g. light-reflecting means or light-concentrating means comprising light concentrating means of the refractive type, e.g. lenses
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F24—HEATING; RANGES; VENTILATING
    • F24S—SOLAR HEAT COLLECTORS; SOLAR HEAT SYSTEMS
    • F24S23/00—Arrangements for concentrating solar-rays for solar heat collectors
    • F24S23/10—Prisms
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/40—Solar thermal energy, e.g. solar towers
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/52—PV systems with concentrators

Definitions

  • the invention relates to solar systems which are characterized by novel dispersive concentrators and multijunction photovoltaic cells, and in which the dispersive concentrators incorporate multiple pris ⁇ matic elements and also focusing means, with the re ⁇ fractive indices of the prismatic elements and the focusing means being different from each other in the wavelength range of interest.
  • the concentration ratio of radiation incident on a photovoltaic cell in a concen ⁇ trator solar system is the ratio of the mean power density of that radiation to the power density of the direct solar radiation incident on the system.
  • Multijunction photovoltaic cells incorporate at least two component photovoltaic cells. They con ⁇ vert radiation to electricity, with shorter wavelength radiation being converted in component cells with larger energy bandgap values, and with longer wave- length radiation being converted in component cells with smaller energy bandgap values. Since the largest energy losses in photovoltaic cells arise from the thermalization of photogenerated current carriers and from photons passing through the cells, such multi- junction cells may exhibit high conversion efficien ⁇ cies.
  • a multijunction cell with four ideal component (single-junction) cells operated at 300°K and at a concentration ratio of 500 has a poten ⁇ tial conversion efficiency of approximately 45% for AMO solar radiation.
  • the potential conversion effi- ciency of multijunction cells increases with the num ⁇ ber of component cells differing in their energy band- gap values. It will be assumed herein that the multi- junction cells utilized in solar systems according to the present invention include at least two optically parallel component photovoltaic cells that differ in their energy bandgap values.
  • the component cells of a multijunction pho ⁇ tovoltaic cell may be optically in series, i.e., stacked, or may be in parallel.
  • Each of these alter- native approaches has certain advantages and disadvan ⁇ tages relative to the other.
  • Optical systems for stacked cells are easier to implement, but it is sub ⁇ stantially more difficult to build stacked cells than single-junction cells for operation in parallel.
  • parallel component cells when parallel component cells are used, the radiation must be dispersed and distributed to the proper cells, and it has been substantially more difficult to implement the optics for dispersive systems than for non-dispersive systems. Disclosure of the Invention
  • the present invention relates to solar sys ⁇ tems incorporating novel dispersive concentrators and multijunction photovoltaic cells with at least two optically parallel component photovoltaic cells dif- fering in their energy bandgap values.
  • the dispersive concentrators disclosed herein incorporate multiple, i.e., a plurality of, prismatic elements that do not all have constant cross-sections or cross-sections identical with those of other of the prismatic ele- ments, and that are optically in parallel to incident direct solar radiation, but are not necessarily geo- metrically parallel to each other.
  • focusing surfaces are formed on the sides of these dispersive concentrators from which radiation exits.
  • the refractive indices of the focusing surfaces and the prismatic elements may be equal in part, but not all, of the wavelength range of interest.
  • the solar systems dis ⁇ closed herein produce electricity from direct solar radiation.
  • a solar system for the production of elec ⁇ tricity from solar radiation incorporates a dispersive concentrator according to the invention and a multi- junction photovoltaic cell including at least two optically parallel component photovoltaic cells dif ⁇ fering in their energy bandgap values.
  • a dispersive concentrator according to the invention and a multi- junction photovoltaic cell including at least two optically parallel component photovoltaic cells dif ⁇ fering in their energy bandgap values.
  • series-parallel component cell arrangements are considered examples of parallel component cell arrangements, as the radiation incident thereon is dispersed before being distributed to the cells located in parallel.
  • the dispersive concentrator has a first portion and a second portion.
  • Direct solar radiation typically enters the dispersive concentrator through entrance surfaces oriented normal (perpendicular) to the mean direction of propagation of such radiation.
  • the first portion of the concentrator incorporates multiple prismatic elements located optically in par ⁇ allel to the incident direct solar radiation. The radiation then passes from the prismatic elements into the second portion of the dispersive concentrator and leaves the latter via focusing surfaces, provided on the second portion, that deviate the exiting radiation into converging beams. These beams ultimately impinge on the multijunction cell.
  • a "prismatic element” is an element having a first surface and a second surface which are inclined to each other at a non-zero angle. Radiation enters the prismatic elements through their first surfaces, and leaves via their second surfaces. Cross-sections of the prismatic elements include an ⁇ gles between the first and second surfaces, herein termed prism angles. The prism angles vary between different prismatic elements and may vary along par ⁇ ticular individual prismatic elements.
  • An "incidence section” is defined to be a section of a prismatic element lying in a plane containing the mean direction of propagation of direct solar radiation passing through the prismatic element.
  • Such radiation enters the incidence section through a first side thereof and is refracted at a second side thereof on leaving the prismatic element and passing into the second portion of the dispersive concentrator.
  • the first and the second sides of the incidence section are inclined with respect to each other at a non-zero angle, herein termed the "angle of inclination".
  • the angles of inclination are established by the orientations of the prismatic elements with respect to the incident direct solar radiation.
  • the first sides of the prismatic elements and their incidence sections are oriented normal (perpendicular) to the mean direc- tion of incidence on the dispersive concentrator of the direct solar radiation entering the dispersive concentrator (in other embodiments of the present invention this may not be the case) .
  • the law of refraction states that when radiation propagates across, and is refracted at, an interface, the re ⁇ fracted ray lies in the plane of incidence of the incident radiation, and the ratio, n, of the sine of the angle of refraction Q 2 to the sine of the angle of incidence Q 1 for radiation of a given wavelength is equal to the ratio of the index of refraction n 1 of the medium in which the incident ray propagates to the index of refraction n 2 of the medium in which the re ⁇ fracted ray propagates.
  • the "plane of incidence” is the plane that contains both the incident ray and the refracted ray as well as the normal to the interface surface at the point of incidence. Incidence sections lie in planes of incidence.
  • the mean direc ⁇ tion of propagation of incident direct solar radiation is normal to the entrance surfaces of the dispersive concentrator.
  • the cross-sections of the pris ⁇ matic elements are incidence sections, and the prism angles equal the angles of inclination of the inci ⁇ dence sections.
  • ⁇ Q 2 [ ⁇ n]-[(sin Q ⁇ "2 - n 2 ] '1/z . (3)
  • Equation (3) describes how ⁇ Q 2 varies as a function of n, ⁇ n and Q., for radiation of a given wavelength re ⁇ fracted at the second sides of incidence sections. Snell's law further states that the refracted ray lies in the plane of incidence. In the preferred embodi ⁇ ment, the value of Q 1 for the mean direction of the direct solar radiation in the prismatic elements equals the value of the prism angle. •
  • the magnitude of the refractive index of a medium typically varies inversely with wavelength in the absence of anomalous dispersion, i.e., the refrac ⁇ tive index is larger for shorter wavelength radiation than for longer wavelength radiation.
  • the dispersive concentrator disclosed herein is a single or unitary optical element that can effi ⁇ ciently disperse and distribute incident direct solar radiation onto the component cells of a multijunction photovoltaic cell incorporating at least two component photovoltaic cells differing in their energy bandgap values.
  • radiation passes from the prismatic elements into the second portion of the concentrator via the first side of the second portion and is refracted at the interface.
  • Such radiation is deviated in its direction of propagation at this in ⁇ terface if n n (the refractive index of the prismatic elements) differs from n 2 (the refractive index of the second portion of the dispersive concentrator) .
  • n n.,/n 2
  • n varies as a function of radiation wave ⁇ length, and thus the radiation in the second portion of the concentrator has been dispersed.
  • n varies with wavelength over the wavelength range of interest, i.e., for radiation of wavelengths which the component photovoltaic cells of the multi- junction cell efficiently convert to electricity.
  • the focusing surfaces of the dispersive concentrator deviate the exiting radiation and trans- form it into beams that converge onto the optically parallel component photovoltaic cells of the multi- junction photovoltaic cell.
  • the dis ⁇ persive concentrator transmits converging beams of dispersed radiation onto the component cells.
  • the focusing surfaces formed on the exit 5 side of the dispersive concentrator may be thought of as constituting one side of a convergent lens, e.g., a circular (radial) Fresnel lens or a lens with a spher ⁇ ical focusing surface, the other side of this lens being a non-free plane surface embedded in the second ° portion of the dispersive concentrator.
  • the lens is formed of the same material as the remain ⁇ der of the second portion of the dispersive concentra ⁇ tor (although in certain cases the materials may dif ⁇ fer) .
  • the angle of incidence Q 1 ' at a particular point on the focusing surfaces, of radiation of a particular wavelength coming from a particular incidence section depends on the angle of inclination of that incidence section as well as on the values of n 1 and n 2 for radiation of that wavelength.
  • Q.,• is thus a varying function, F of location on the focusing surfaces and of radiation wavelength.
  • the variation of n 2 with radiation wave ⁇ length results in a further dispersion of the radia- tion exiting the focusing surfaces, and this second dispersion is also a varying function, F 2 , of location on the focusing surfaces and of radiation wavelength.
  • the cross-sections of at least some of the prismatic elements vary in order that the angles of inclination of the incidence sections lying within these prismatic elements can be adjusted to help prop ⁇ erly distribute radiation onto the component photovol ⁇ taic cells.
  • the prismatic ele- ments may in certain embodiments depart from being geometrically parallel to each other.
  • the component cells face towards the disper ⁇ sive concentrator so that the radiation coming from the latter is incident on the cells, either directly or indirectly via secondary focusing means, e.g., reflectors placed around the component cells.
  • This radiation is distributed to the component cells such that shorter wavelength radiation is incident on com ⁇ ponent cells with larger energy bandgap values and longer wavelength radiation is incident on component cells with smaller energy bandgap values.
  • the disper ⁇ sion of the radiation coming from the dispersive con ⁇ centrator must result in such a distribution.
  • the ability to adjust F., by varying the angles of inclina ⁇ tion of incidence sections helps to adjust the total dispersion in order to obtain the desired distribution of radiation onto the component cells.
  • Obtaining a desired distribution is further assisted by selection of materials for the prismatic elements and the second portion of the dispersive concentrator having suitable refractive indices and by design of the focusing sur ⁇ faces.
  • the dispersive concentrator should be as transparent as possible in order to minimize optical absorptions. Materials such as, e.g., transparent glasses and/or plastics, can be used to form the dis ⁇ persive concentrator.
  • the prismatic elements can be individually manufactured or, alternatively, manufac ⁇ tured as part of a continuous sheet containing many prismatic elements. The manufacturing process can involve molding or extrusion processes, for example. Ultraviolet or other stabilizers may be incorporated in the materials or coated on surfaces in order to inhibit changes in the materials used, and anti-re ⁇ flection coatings can be used to minimize reflections. Cooling means such as are well-known in the art may be employed to control the temperatures of the component photovoltaic cells since the conversion efficiency of a photovoltaic cell since is inversely related to the cell temperature.
  • first and second portions of the dispersive concentrator may be formed of different materials.
  • Fresnel's formulae express the amount of radiation reflected and refracted at an interface between materials of differ ⁇ ent refractive indices.
  • the amount of incident radia ⁇ tion that is reflected at such an interface is a func ⁇ tion of the refractive indices and the angles of inci ⁇ dence and refraction.
  • Such reflections reduce the efficiency of the dispersive concentrator. If the refractive indices are close in value, therefore, reflections at the interfaces are minimized.
  • Direct solar radiation viewed in the atmo ⁇ sphere appears to originate in a solid angle of ap- proximately 30 arc minutes subtended by the solar disk. This results in angular spreads in the propaga ⁇ tion directions around mean directions of propagation of radiations of each particular wavelength entering and passing through the dispersive concentrator. Manufacturing errors and material inhomogeneities further affect such angular spreads.
  • radiation of a particular wavelength incident on the cells may be distributed onto more than one component cell. Such spreading reduces the potential conversion efficiency of the multijunction cell since some radiations of particular wavelengths will be incident on other than the optimum component cell.
  • a photovoltaic solar system typically in ⁇ cludes power conditioning means to convert the low- voltage direct current output of photovoltaic cells to a higher voltage alternating current.
  • Such equipment is well-known in the art.
  • a direction is defined as that direction along the sur ⁇ faces of the component cells along which component cells of different energy bandgap values are distrib ⁇ uted. Component cells with larger energy bandgap values have smaller "A" coordinates than component cells with smaller energy bandgap values, and vice versa.
  • a "B” direction is defined as that direction which is orthogonal to the "A" direction and parallel to the faces of the component cells.
  • the multijunction photovoltaic cell in- eludes four component cells: a germanium component cell, a silicon component cell, a gallium arsenide component cell, and an aluminum gallium arsenide com ⁇ ponent cell.
  • the optical axes of the focusing surfac ⁇ es are assumed in this example to lie along a common line, hereinafter termed the optical axis of the fo ⁇ cusing surfaces.
  • the component cells are square, measuring 0.3 cm by 0.3 cm, and are arranged with respect to the optical axis of the focusing surfaces of the dispersive concentrator such that the juncture between the silicon component cell and the gallium arsenide component cell is located on the optical axis of the focusing surfaces.
  • this junc ⁇ ture is located at "A" equal to zero.
  • the silicon component cell thus has positive “A” coordinates and the gallium arsenide component cell has negative “A” coordinates.
  • the germanium component cell has larger “A” coordinates than the silicon component cell, and the gallium arsenide component cell has larger A coor ⁇ dinates than the aluminum gallium arsenide component cell.
  • the juncture between the silicon component cell and the gallium arsenide cell is located 35 cm from the focusing surfaces.
  • the focusing surfaces are designed to focus incident parallel radiation of wave ⁇ length 0.881 microns onto a point on the optical axis that is located 35 cm from the dispersive concentra- tor.
  • the dispersive concentrator is square and mea ⁇ sures 20 cm by 20 cm.
  • the prismatic elements have right triangular cross-sections, and thus the first and third sides of the incidence sections of the pris ⁇ matic elements are orthogonal to each other.
  • the angles of inclination of the second sides of the incidence sec ⁇ tions, i.e., the prism angles, of the prismatic ele ⁇ ments vary with location on the focusing surfaces along the direction orthogonal to the direction of linear extension of the prismatic elements.
  • an "X" direction at the disper ⁇ sive concentrator as a direction which runs along the first sides of the prismatic elements and is both parallel to the "A" direction at the multijunction cell and perpendicular to the optical axis of the focusing surfaces
  • the first sides of the incidence sections of the prismatic elements are parallel to (or in the aggregate constitute) a planar surface of the dispersive concentrator and are oriented orthogonal to the mean direction at which 10 direct solar radiation enters the dispersive concen ⁇ trator.
  • the first sides of the incidence sections where the solar radiation enters are 0.2 cm in length.
  • Secondary focusing reflectors are located adjacent to the cells to help direct the radiation coming from the I 5 dispersive concentrator onto the component cells.
  • the refractive index of the material of which the first portion of the dispersive concentrator, i.e., the prismatic ele ⁇ ments, is composed has the values 1.506 at a wave- 0 length of 0.75 microns, 1.500 at 0.881 microns, and 1.494 at 1.12 microns.
  • the refractive index of the material of which the second portion of the dispersive concentrator is composed has the values 1.502 at 0.75 microns, 1.500 at 0.881 microns, and 1.498 at 1.12 5 microns.
  • the refractive indices of the first and second portions of the dispersive con ⁇ centrator have a common value, and thus radiation of that wavelength is not deviated in propagation direc ⁇ tion on passing from the first into the second portion 0 of the dispersive concentrator.
  • the system according to this particular example of the preferred embodiment has a potential peak electrical output of approximately 14 watts.
  • Fig. 1 is a schematic representation of a partial section through a dispersive concentrator and a 4-cell multijunction photovoltaic cell of a solar system in accordance with one embodiment of the pres ⁇ ent invention, where the prismatic elements have cross-sections of right triangular configuration, and diagrammatically shows the mean propagation paths through the dispersive concentrator and onto the multijunction cell of radiation of three different wavelengths;
  • Fig. 1 is a schematic representation of a partial section through a dispersive concentrator and a 4-cell multijunction photovoltaic cell of a solar system in accordance with one embodiment of the pres ⁇ ent invention, where the prismatic elements have cross-sections of right triangular configuration, and diagrammatically shows the mean propagation paths through the dispersive concentrator and onto the multijunction cell of radiation of three different wavelengths;
  • Fig. 1 is a schematic representation of a partial section through a dispersive concentrator and a 4-cell multijunction photovoltaic cell of a solar system in accord
  • FIG. 2 is a graphical representation of the variation of the indices of refraction of the prismat ⁇ ic elements and the focusing surfaces of a dispersive concentrator shown as a function of radiation wave ⁇ length for a particular example of the present inven- tion where there is a common value of the refractive indices at a wavelength within the wavelength range of interest;
  • Fig. 3 is a schematic fragmentary section through a solar system according to the preferred embodiment of the present invention incorporating a dispersive concentrator such as is shown in Fig. 1 and a multijunction photovoltaic cell composed of four parallel component cells, and shows diagrammatically the mean propagation paths of radiation of four dif- ferent wavelengths through the system; and
  • Fig. 4 is a graphical representation of the conversion efficiency of multijunction photovoltaic cells as a function of the number of ideal component cells at different cell temperatures.
  • Fig. 1 is a schematic representation of a partial section of a solar system 1 showing a part of a dispersive concentrator 2 and a 4-cell multijunction photovoltaic cell 3. Fig. 1 is not drawn to scale.
  • the dispersive concentrator 2 is composed of a first portion 4 and a second portion 5.
  • the first portion 4 has a planar surface 6 at one side thereof and incorporates a plurality of prismatic elements 7 of right triangular cross-sections located optically in parallel with each other relative to the mean di ⁇ rection of incident solar radiation R.
  • the first portion 4 of the concentrator can be formed using various techniques; for example, each prismatic ele- ment 7 may be separately formed and attached to the second portion 5 of the dispersive concentrator, or a continuous sheet of prismatic elements can be formed first and then attached as a unit to the second por ⁇ tion of the concentrator.
  • Each of the prismatic elements 7 in this embodiment of the invention has incidence sections 8 having right triangular shapes.
  • incidence sec ⁇ tions are also cross-sections of the prismatic ele ⁇ ments in this embodiment, since the mean direction of incidence of the direct solar radiation R is normal (orthogonal) to the planar entrance surface 6.
  • Each incidence section 8 has a straight first side 9, a straight second side 11 oriented at an angle of incli ⁇ nation 10 to the first side 9, and a straight third side 12 orthogonal to the first side 9.
  • the angles of inclination 10 vary among at least some different incidence sections 8.
  • third sides 12 are orthogonal to first sides 9 in the embodiment shown in Fig. 1, the third sides of the incidence sections may be inclined at other than right angles to the first sides in alternative embodiments of the present inven ⁇ tion.
  • Second sides 11 and third sides 12 meet at apexes 13.
  • the planar surface 6 of the first portion 4 of the disper ⁇ sive concentrator 2 is defined by or parallel to the first sides 9 of the incidence sections 8 and is ori ⁇ ented normal to the mean direction of incidence of direct solar radiation R.
  • the second portion 5 of the dispersive con ⁇ centrator 2 has a zig-zag or sawtooth-shaped first side 14 which is in full engagement with the second sides 11 and third sides 12 of the incidence sections 8.
  • Focusing surfaces 15 are formed on the second portion 5 of the dispersive concentrator 2 at a second side thereof opposite its first side 14. In the em- bodiment shown in Fig. 1, the focusing surfaces 15 have a circular (radial) symmetry in order to focus, i.e., to form and distribute, radiation onto the multijunction cell 3.
  • the focusing surfaces 15 are provided with a plurality of steps 16 (only one step 16 is shown in Fig. 1) and have a plurality of surface facets 23 (portions of only two surface facets 23 are shown in Fig. 1) .
  • the material of which the first portion 4 of the dispersive concentrator 2 is composed has over most or all of the radiation wavelength range of in ⁇ terest an index of refraction, n.,, that differs in magnitude from the index of refraction, n 2 , of the second portion 5 of the dispersive concentrator 2.
  • n. an index of refraction
  • n 1 and n 2 are equal to each other for radiation 18, i.e., for that wavelength n. j /n-, is unity, so that radiation 18 is not deviated in its direction of propagation as it enters the second portion 5. (It should be understood, how ⁇ ever, that this condition is not universal; in some embodiments of the present invention, n 1 and n 2 do not have a common value in the radiation wavelength range of interest.)
  • n 1 and n 2 do not have a common value in the radiation wavelength range of interest.
  • the propagation direction of radiation 17 passing through a prismatic element or incidence section 8 is deviated towards the normal 22 to the second side 11 of that incidence section on being refracted at that second side 11, and the propagation direction of radi- ation 19 is deviated away from the normal 22 on being refracted at that second side 11, i.e., radiations 17 and 19 undergo angular deviations that are opposite in rotational sense as they enter the second portion 5 of the dispersive concentrator 2.
  • radia ⁇ tion 17 is deviated through an angle of deviation 20 from the direction of propagation of radiation 18 towards the perpendicular or normal 22 to second side 11, and radiation 19 is deviated through an angle of deviation 21 from the direction of propagation of radiation 18 away from the perpendicular or normal 22 to second side 11.
  • Radiations 17, 18 and 19 are incident on focusing surface 15 at different angles of incidence due to the difference in angles of deviation 20 and 21. These differences in the angles of incidence and the differences in the indices of refraction of the second portion 5 of the dispersive concentrator 2 for radiations 17, 18 and 19 mean that the latter are deviated through different angles on leaving the fo ⁇ cusing surface 15. (Although it is not so shown in Fig.
  • radi ⁇ ations 17, 18 and 19 may be considered as being inci ⁇ dent on the same point on the focusing surface 15 with, however, different angles of incidence.
  • radiation 17 is devi- ated in propagation direction to become radiation 37
  • radiation 18, which was incident on the surface 15 at angle 24 is deviated in propagation direction to become radiation 38
  • radiation 19 is deviated in propagation direction to become radiation 39.
  • all the focusing sur ⁇ faces 15 have a common optical axis 26.
  • Radiation 38 is incident on the multijunction photovoltaic cell 3 at the juncture 32 between component cells 28 and 29 thereof, at an angle 25 to the common optical axis 26.
  • n,-sin(angle 10) n 2 'sin(angle 10 + angle 20) .
  • the prism angle of the cross-section of the prismatic element for the radiation crossing the lat ⁇ ter to the point on the focusing surface in question is equal to angle 10 at that location.
  • That com ⁇ promise choice can be used in determining the prism angle of the prismatic element cross-section in ques ⁇ tion or, alternatively, the dimension f of component cell 29 can be selected so that radiation 39 impinges on juncture 33. Repeating the procedure for different regions of the dispersive concentrator then permits the full design of the prismatic elements to be achieved. It will be noted that if the mean direction of radiation 38 impinges on juncture 32, and if the mean direction of propagation of radiation 37 impinges on juncture 31, then the mean direction of propagation of radiation with wavelengths lying between that of radiations 37 and 38 will impinge on component cell 28, i.e., such radiation will be distributed onto component cell 28. Similar considerations apply to radiations of other wavelengths.
  • the sequence of prism angle magnitudes in one sense or the other along the "X" direction i.e., whether the prism angle of any given prismatic element is greater or smaller than or equal to the prism angle of the next adjacent prismatic element as viewed in the "X" direction, need not be such as is schematical ⁇ ly indicated in Fig. 1 or described earlier herein as characterizing one particular example of the preferred embodiment of the present invention. Rather, the sequence can be selected as desired depending on the particular extent of dispersion and focusing sought to be achieved by means of the dispersive concentrator.
  • making such a choice of prism angle may also involve adjustment of the values of the refractive indices by varying material compositions or may in ⁇ volve deviations of the prismatic elements from being geometrically parallel to each other.
  • compo- nent cell 30 has a larger energy bandgap value than does component cell 29
  • component cell 29 has a larger energy bandgap value than does component cell 28
  • component cell 28 has a larger energy bandgap value than does component cell 27.
  • the distribution of radiation involves bringing onto component cell 27 radiation of longer wavelengths than the wavelengths of the radiation impinging on component cell 28, bringing onto component cell 23 radiation of longer wavelengths than the wavelengths of the radiation impinging on component cell 29, and bringing onto component cell 29 radiation of longer wavelengths than the wavelengths of the radiation impinging on compo ⁇ nent cell 30.
  • Secondary reflectors 34 are used in the embodiment shown in Fig. 1 to help bring radiation coming from the focusing surfaces to proper incidence onto the multijunction photovoltaic cell 3.
  • a graphical representation is shown of the variations in the refractive index 41 of the prismatic elements and the refractive index 42 of the focusing surfaces of a dispersive concentrator according to the present invention as a function of radiation wavelength for a particular example where they have a common value at wavelength 43.
  • refractive index 41 is less than refractive index 42
  • refractive index 41 is greater than refractive index 42.
  • a fragmentary diagrammatic sec ⁇ tion of a solar system 1 according to the preferred embodiment of the present invention is shown.
  • Direct solar radiation R (shown in only two locations for the sake of simplicity) is incident on the dispersive concentrator 2 at the surface 6 thereof, as shown also in Fig. 1.
  • the dispersive concentrator disperses radiation R to form radiations 47, 48, 49 and 50, each of different wavelength, which it focuses onto multi- junction cell 3 at the component cells 27, 28, 29 and 30 thereof, respectively.
  • the wavelength of radiation 47 is greater than that of radiation 48 "
  • the wavelength of radiation 48 is greater than that of radiation 49
  • the wavelength of radiation 49 is greater than that of radiation 50.
  • the arrangement ensures, as previously mentioned, that radiation of longer wavelength is incident on a compo- nent photovoltaic cell of smaller energy bandgap value and that radiation of shorter wavelength is incident on a component cell of larger energy bandgap value.
  • Power conditioning means electrical connec ⁇ tions, system pointing means, cooling equipment means and arrangements, and supporting structure means are not shown in Fig. 3, but such means and arrangements are well-known in the art.
  • the types of electrical connections used to connect the component cells to the power conditioning means and to each other depend on whether or not the individual component cells are electrically connected in series, in parallel or in series-parallel.
  • component cells are electrical ⁇ ly connected in series, it is well-known in the art that the current carriers photogenerated in the indi- vidual cells should, preferably, be equal in magnitude to each other in order to avoid energy losses. Pro ⁇ ducing such an equality can be assisted by varying the positions of the component cells or by adjusting sec ⁇ ondary focusing reflectors 34, for example.
  • FIG. 4 is a graphical representation of the conversion efficiency of multijunction photovoltaic cells as a function of the number of ideal component cells. As shown in Fig. 4, conversion efficiency tends to increase as the number of component cells (with well-distributed values of energy bandgap) in ⁇ creases. It is also shown in Fig. 4 that an increase in the temperatures of the component cells of the multijunction cell results in a decrease in conversion efficiency. The applicability of these relationships to the advantageous implementation of the present invention will be readily apparent.
  • the present invention is applicable for the generation of electricity from direct solar radiation.

Abstract

A photovoltaic solar system (1) for the production of electricity from direct solar radiation includes a dispersive concentrator (2) incorporating in a first portion (4) thereof a plurality of optically parallel prismatic elements (7) which vary in cross-section or index of refraction, and incorporating in a second portion (5) thereof focusing means (15). The radiation (R) passing from the prismatic elements (7) into the second portion of the dispersive concentrator is subjected to an initial dispersion, and that dispersed radiation is further dispersed (37, 38, 39 or 47, 48, 49, 50), and also focused and concentrated, on leaving the focusing means. The system also incorporates a multijunction photovoltaic cell (3) with at least two optically parallel component photovoltaic cells (27, 28, 29, 30) differing in their energy bandgap values and positioned to receive, either directly or via secondary focusing means (34), the dispersed and focused radiation coming from the dispersive concentrator. The dispersed radiation incident on the component photovoltaic cells is distributed such that longer wavelength radiation is incident on those cells with smaller energy bandgap values and that shorter wavelength radiation is incident on those cells with larger energy bandgap values.

Description

PHOTOVOLTAIC SOLAR SYSTEMS WITH
DISPERSIVE CONCENTRATORS
Technical Field
The invention relates to solar systems which are characterized by novel dispersive concentrators and multijunction photovoltaic cells, and in which the dispersive concentrators incorporate multiple pris¬ matic elements and also focusing means, with the re¬ fractive indices of the prismatic elements and the focusing means being different from each other in the wavelength range of interest.
Background Art As used herein, the "concentration ratio" of radiation incident on a photovoltaic cell in a concen¬ trator solar system is the ratio of the mean power density of that radiation to the power density of the direct solar radiation incident on the system.
The direct solar radiation incident on a system is that portion of the incident solar radiation that has not been scattered by the atmosphere. Multijunction photovoltaic cells incorporate at least two component photovoltaic cells. They con¬ vert radiation to electricity, with shorter wavelength radiation being converted in component cells with larger energy bandgap values, and with longer wave- length radiation being converted in component cells with smaller energy bandgap values. Since the largest energy losses in photovoltaic cells arise from the thermalization of photogenerated current carriers and from photons passing through the cells, such multi- junction cells may exhibit high conversion efficien¬ cies. For example, a multijunction cell with four ideal component (single-junction) cells operated at 300°K and at a concentration ratio of 500 has a poten¬ tial conversion efficiency of approximately 45% for AMO solar radiation. The potential conversion effi- ciency of multijunction cells increases with the num¬ ber of component cells differing in their energy band- gap values. It will be assumed herein that the multi- junction cells utilized in solar systems according to the present invention include at least two optically parallel component photovoltaic cells that differ in their energy bandgap values.
The component cells of a multijunction pho¬ tovoltaic cell may be optically in series, i.e., stacked, or may be in parallel. Each of these alter- native approaches has certain advantages and disadvan¬ tages relative to the other. Optical systems for stacked cells are easier to implement, but it is sub¬ stantially more difficult to build stacked cells than single-junction cells for operation in parallel. On the other hand, when parallel component cells are used, the radiation must be dispersed and distributed to the proper cells, and it has been substantially more difficult to implement the optics for dispersive systems than for non-dispersive systems. Disclosure of the Invention
The present invention relates to solar sys¬ tems incorporating novel dispersive concentrators and multijunction photovoltaic cells with at least two optically parallel component photovoltaic cells dif- fering in their energy bandgap values. The dispersive concentrators disclosed herein incorporate multiple, i.e., a plurality of, prismatic elements that do not all have constant cross-sections or cross-sections identical with those of other of the prismatic ele- ments, and that are optically in parallel to incident direct solar radiation, but are not necessarily geo- metrically parallel to each other. On the sides of these dispersive concentrators from which radiation exits, focusing surfaces are formed. The refractive indices of the focusing surfaces and the prismatic elements may be equal in part, but not all, of the wavelength range of interest. The solar systems dis¬ closed herein produce electricity from direct solar radiation.
It is an object of the present invention to provide solar systems incorporating novel dispersive concentrators and multijunction photovoltaic cells including at least two optically parallel component photovoltaic cells differing from each other in their energy bandgap values, for the low-cost production of electricity from solar radiation.
Briefly, in accordance with the basic prin¬ ciples of the present invention and in its preferred embodiment, a solar system for the production of elec¬ tricity from solar radiation incorporates a dispersive concentrator according to the invention and a multi- junction photovoltaic cell including at least two optically parallel component photovoltaic cells dif¬ fering in their energy bandgap values. (For purposes of the present discussion, series-parallel component cell arrangements are considered examples of parallel component cell arrangements, as the radiation incident thereon is dispersed before being distributed to the cells located in parallel.)
The dispersive concentrator has a first portion and a second portion. Direct solar radiation typically enters the dispersive concentrator through entrance surfaces oriented normal (perpendicular) to the mean direction of propagation of such radiation. The first portion of the concentrator incorporates multiple prismatic elements located optically in par¬ allel to the incident direct solar radiation. The radiation then passes from the prismatic elements into the second portion of the dispersive concentrator and leaves the latter via focusing surfaces, provided on the second portion, that deviate the exiting radiation into converging beams. These beams ultimately impinge on the multijunction cell.
As used herein, a "prismatic element" is an element having a first surface and a second surface which are inclined to each other at a non-zero angle. Radiation enters the prismatic elements through their first surfaces, and leaves via their second surfaces. Cross-sections of the prismatic elements include an¬ gles between the first and second surfaces, herein termed prism angles. The prism angles vary between different prismatic elements and may vary along par¬ ticular individual prismatic elements. An "incidence section" is defined to be a section of a prismatic element lying in a plane containing the mean direction of propagation of direct solar radiation passing through the prismatic element. Such radiation enters the incidence section through a first side thereof and is refracted at a second side thereof on leaving the prismatic element and passing into the second portion of the dispersive concentrator. The first and the second sides of the incidence section are inclined with respect to each other at a non-zero angle, herein termed the "angle of inclination". The angles of inclination are established by the orientations of the prismatic elements with respect to the incident direct solar radiation.
In the preferred embodiment of the system according to the present invention, the first sides of the prismatic elements and their incidence sections are oriented normal (perpendicular) to the mean direc- tion of incidence on the dispersive concentrator of the direct solar radiation entering the dispersive concentrator (in other embodiments of the present invention this may not be the case) .
The law of refraction, usually called Snell's law, states that when radiation propagates across, and is refracted at, an interface, the re¬ fracted ray lies in the plane of incidence of the incident radiation, and the ratio, n, of the sine of the angle of refraction Q2 to the sine of the angle of incidence Q1 for radiation of a given wavelength is equal to the ratio of the index of refraction n1 of the medium in which the incident ray propagates to the index of refraction n2 of the medium in which the re¬ fracted ray propagates. The "plane of incidence" is the plane that contains both the incident ray and the refracted ray as well as the normal to the interface surface at the point of incidence. Incidence sections lie in planes of incidence.
In the preferred embodiment, the mean direc¬ tion of propagation of incident direct solar radiation is normal to the entrance surfaces of the dispersive concentrator. Thus, the cross-sections of the pris¬ matic elements are incidence sections, and the prism angles equal the angles of inclination of the inci¬ dence sections. At the second sides of the incidence sec¬ tions, Snell's law becomes: n^sin Q1 = n2-sin Q2 , (1) or: n-sin Q., = sin Q2 , (2) where n = n,,/n2. Taking increments of n and Q2, i.e., Δn and ΔQ2, respectively, while holding Qt constant, then yields:
ΔQ2 = [Δn]-[(sin Q^"2 - n2]'1/z . (3)
Equation (3) describes how ΔQ2 varies as a function of n, Δn and Q., for radiation of a given wavelength re¬ fracted at the second sides of incidence sections. Snell's law further states that the refracted ray lies in the plane of incidence. In the preferred embodi¬ ment, the value of Q1 for the mean direction of the direct solar radiation in the prismatic elements equals the value of the prism angle. •
The magnitude of the refractive index of a medium typically varies inversely with wavelength in the absence of anomalous dispersion, i.e., the refrac¬ tive index is larger for shorter wavelength radiation than for longer wavelength radiation.
The dispersive concentrator disclosed herein is a single or unitary optical element that can effi¬ ciently disperse and distribute incident direct solar radiation onto the component cells of a multijunction photovoltaic cell incorporating at least two component photovoltaic cells differing in their energy bandgap values. In the concentrator, radiation passes from the prismatic elements into the second portion of the concentrator via the first side of the second portion and is refracted at the interface. Such radiation is deviated in its direction of propagation at this in¬ terface if nn (the refractive index of the prismatic elements) differs from n2 (the refractive index of the second portion of the dispersive concentrator) . Since n = n.,/n2, n varies as a function of radiation wave¬ length, and thus the radiation in the second portion of the concentrator has been dispersed. For the pur¬ poses of the following description it will be assumed that n varies with wavelength over the wavelength range of interest, i.e., for radiation of wavelengths which the component photovoltaic cells of the multi- junction cell efficiently convert to electricity. The focusing surfaces of the dispersive concentrator deviate the exiting radiation and trans- form it into beams that converge onto the optically parallel component photovoltaic cells of the multi- junction photovoltaic cell. In other words, the dis¬ persive concentrator transmits converging beams of dispersed radiation onto the component cells.
The focusing surfaces formed on the exit 5 side of the dispersive concentrator may be thought of as constituting one side of a convergent lens, e.g., a circular (radial) Fresnel lens or a lens with a spher¬ ical focusing surface, the other side of this lens being a non-free plane surface embedded in the second ° portion of the dispersive concentrator. Preferably, the lens is formed of the same material as the remain¬ der of the second portion of the dispersive concentra¬ tor (although in certain cases the materials may dif¬ fer) . 5 When n1 is not equal to n2, the angle of incidence Q1' , at a particular point on the focusing surfaces, of radiation of a particular wavelength coming from a particular incidence section depends on the angle of inclination of that incidence section as well as on the values of n1 and n2 for radiation of that wavelength. Q.,• is thus a varying function, F of location on the focusing surfaces and of radiation wavelength. The variation of n2 with radiation wave¬ length results in a further dispersion of the radia- tion exiting the focusing surfaces, and this second dispersion is also a varying function, F2, of location on the focusing surfaces and of radiation wavelength.
In the dispersive concentrators of the pres¬ ent invention, the cross-sections of at least some of the prismatic elements vary in order that the angles of inclination of the incidence sections lying within these prismatic elements can be adjusted to help prop¬ erly distribute radiation onto the component photovol¬ taic cells. For a similar reason, the prismatic ele- ments may in certain embodiments depart from being geometrically parallel to each other. The component cells face towards the disper¬ sive concentrator so that the radiation coming from the latter is incident on the cells, either directly or indirectly via secondary focusing means, e.g., reflectors placed around the component cells. This radiation is distributed to the component cells such that shorter wavelength radiation is incident on com¬ ponent cells with larger energy bandgap values and longer wavelength radiation is incident on component cells with smaller energy bandgap values. The disper¬ sion of the radiation coming from the dispersive con¬ centrator must result in such a distribution. The ability to adjust F., by varying the angles of inclina¬ tion of incidence sections helps to adjust the total dispersion in order to obtain the desired distribution of radiation onto the component cells. Obtaining a desired distribution is further assisted by selection of materials for the prismatic elements and the second portion of the dispersive concentrator having suitable refractive indices and by design of the focusing sur¬ faces. It is also possible to help in producing a desirable distribution of dispersed radiation onto the component cells by having prismatic elements that are not geometrically parallel to each other. The dispersive concentrator should be as transparent as possible in order to minimize optical absorptions. Materials such as, e.g., transparent glasses and/or plastics, can be used to form the dis¬ persive concentrator. The prismatic elements can be individually manufactured or, alternatively, manufac¬ tured as part of a continuous sheet containing many prismatic elements. The manufacturing process can involve molding or extrusion processes, for example. Ultraviolet or other stabilizers may be incorporated in the materials or coated on surfaces in order to inhibit changes in the materials used, and anti-re¬ flection coatings can be used to minimize reflections. Cooling means such as are well-known in the art may be employed to control the temperatures of the component photovoltaic cells since the conversion efficiency of a photovoltaic cell since is inversely related to the cell temperature.
It should be noted that different sections of the first and second portions of the dispersive concentrator may be formed of different materials.
This can help produce a desirable distribution of the dispersed radiation on the component cells.
It is well-known in the art that Fresnel's formulae express the amount of radiation reflected and refracted at an interface between materials of differ¬ ent refractive indices. The amount of incident radia¬ tion that is reflected at such an interface is a func¬ tion of the refractive indices and the angles of inci¬ dence and refraction. Such reflections reduce the efficiency of the dispersive concentrator. If the refractive indices are close in value, therefore, reflections at the interfaces are minimized.
Direct solar radiation viewed in the atmo¬ sphere appears to originate in a solid angle of ap- proximately 30 arc minutes subtended by the solar disk. This results in angular spreads in the propaga¬ tion directions around mean directions of propagation of radiations of each particular wavelength entering and passing through the dispersive concentrator. Manufacturing errors and material inhomogeneities further affect such angular spreads. As a conse¬ quence, radiation of a particular wavelength incident on the cells may be distributed onto more than one component cell. Such spreading reduces the potential conversion efficiency of the multijunction cell since some radiations of particular wavelengths will be incident on other than the optimum component cell.
A photovoltaic solar system typically in¬ cludes power conditioning means to convert the low- voltage direct current output of photovoltaic cells to a higher voltage alternating current. Such equipment is well-known in the art.
It will be understood that the system must be oriented with respect to the sun such that the dispersed radiation is properly focused onto the com¬ ponent cells. Means to point the system and to adjust cell locations are well-known in the art and may in¬ clude, e.g., clock-type mechanisms, devices that mea¬ sure and optimize cell output, and sensors. An "A" direction is defined as that direction along the sur¬ faces of the component cells along which component cells of different energy bandgap values are distrib¬ uted. Component cells with larger energy bandgap values have smaller "A" coordinates than component cells with smaller energy bandgap values, and vice versa. A "B" direction is defined as that direction which is orthogonal to the "A" direction and parallel to the faces of the component cells. If the system is ideally oriented, movements of the sun with respect to the system will, to first order, move the radiation incident on the component cells only along the "B" direction, and thus system tracking of the sun need only move the radiation on the cells aiong the "B" direction. With such an orientation, the tracking system need not make corrective adjustments as accu¬ rately or as rapidly as would be the case where, for example, movements of the sun require tracking only in the "A" direction. Pointing errors resulting in not focusing onto optimum "B" locations are less serious than pointing errors resulting in not focusing onto optimum "A" locations, particularly if secondary fo- cusing means are employed. The peak electrical power output from a system occurs with the ideal orientation with respect to the sun. Any deviation from the ideal orientation will reduce the power output of the sys- tern. Thus, a zero point in the rate of change of the power output with such deviation exists at the maximum power output orientation. An electronic control for a pointing system can make use of such a zero point. If focusing surfaces that focus to a line are used, then the system should be oriented so that movements of the sun with respect to the system move the radiation in the "B" direction.
In a particular example of the preferred embodiment, the multijunction photovoltaic cell in- eludes four component cells: a germanium component cell, a silicon component cell, a gallium arsenide component cell, and an aluminum gallium arsenide com¬ ponent cell. The optical axes of the focusing surfac¬ es are assumed in this example to lie along a common line, hereinafter termed the optical axis of the fo¬ cusing surfaces. The component cells are square, measuring 0.3 cm by 0.3 cm, and are arranged with respect to the optical axis of the focusing surfaces of the dispersive concentrator such that the juncture between the silicon component cell and the gallium arsenide component cell is located on the optical axis of the focusing surfaces. By definition, this junc¬ ture is located at "A" equal to zero. The silicon component cell thus has positive "A" coordinates and the gallium arsenide component cell has negative "A" coordinates. The germanium component cell has larger "A" coordinates than the silicon component cell, and the gallium arsenide component cell has larger A coor¬ dinates than the aluminum gallium arsenide component cell. The juncture between the silicon component cell and the gallium arsenide cell is located 35 cm from the focusing surfaces. The focusing surfaces are designed to focus incident parallel radiation of wave¬ length 0.881 microns onto a point on the optical axis that is located 35 cm from the dispersive concentra- tor. The dispersive concentrator is square and mea¬ sures 20 cm by 20 cm. The prismatic elements have right triangular cross-sections, and thus the first and third sides of the incidence sections of the pris¬ matic elements are orthogonal to each other. In the particular example, the angles of inclination of the second sides of the incidence sec¬ tions, i.e., the prism angles, of the prismatic ele¬ ments vary with location on the focusing surfaces along the direction orthogonal to the direction of linear extension of the prismatic elements. Defining for such an embodiment an "X" direction at the disper¬ sive concentrator as a direction which runs along the first sides of the prismatic elements and is both parallel to the "A" direction at the multijunction cell and perpendicular to the optical axis of the focusing surfaces, the prism angle of the prismatic element at X = 0, i.e., at the intersection of the "X" direction with the optical axis, is 60 degrees, the prism angle of the prismatic element at X = 5 cm is 58 degrees, the prism angle of the prismatic element at X = 10 cm is 52 degrees, the prism angle of the prismat¬ ic element at X = -5 cm is 63 degrees, and the prism angle of the prismatic element at X = -10 cm is 64 degrees. These differences in the angles of inclina- tion or prism angles result in variation of the cross- sections of the prismatic elements in the "X" direc¬ tion. It is, of course, possible to achieve varia¬ tions in the cross-sections of the prismatic elements either by varying the prism angle within one or more of the individual primatic elements in a direction orthogonal to the "X" direction, or by varying the prism angles both in and orthogonal to the "X" direc¬ tion, if such is deemed desirable in order to achieve the sought for distribution of radiation on the compo¬ nent cells. 5 In the particular example, the first sides of the incidence sections of the prismatic elements are parallel to (or in the aggregate constitute) a planar surface of the dispersive concentrator and are oriented orthogonal to the mean direction at which 10 direct solar radiation enters the dispersive concen¬ trator. The first sides of the incidence sections where the solar radiation enters are 0.2 cm in length. Secondary focusing reflectors are located adjacent to the cells to help direct the radiation coming from the I5 dispersive concentrator onto the component cells.
In this particular example, the refractive index of the material of which the first portion of the dispersive concentrator, i.e., the prismatic ele¬ ments, is composed has the values 1.506 at a wave- 0 length of 0.75 microns, 1.500 at 0.881 microns, and 1.494 at 1.12 microns. The refractive index of the material of which the second portion of the dispersive concentrator is composed has the values 1.502 at 0.75 microns, 1.500 at 0.881 microns, and 1.498 at 1.12 5 microns. At 0.881 microns, the refractive indices of the first and second portions of the dispersive con¬ centrator have a common value, and thus radiation of that wavelength is not deviated in propagation direc¬ tion on passing from the first into the second portion 0 of the dispersive concentrator.
In this particular example, furthermore, approximately 85% of the direct solar radiation inci¬ dent on the dispersive concentrator reaches the cells, and the mean concentration ratio at the component 5 cells is approximately 560. If the direct solar radi¬ ation has an AMO spectrum, then the potential conver- sion efficiency of the system for radiation reaching the component cells is approximately 40%, assuming that the cells operate at a temperature of 300° K. Under such conditions, the system according to this particular example of the preferred embodiment has a potential peak electrical output of approximately 14 watts.
It will be understood by those skilled in the art that supporting structures must be used in systems to support the dispersive concentrator and the component cells, and, indeed, ribs or other supportive structures can be formed on the dispersive concentra¬ tor.
Brief Description of the Drawings Further objects, features and advantages of the present invention will become apparent upon con¬ sideration of the following detailed description thereof when read in conjunction with the accompanying drawings, in which: Fig. 1 is a schematic representation of a partial section through a dispersive concentrator and a 4-cell multijunction photovoltaic cell of a solar system in accordance with one embodiment of the pres¬ ent invention, where the prismatic elements have cross-sections of right triangular configuration, and diagrammatically shows the mean propagation paths through the dispersive concentrator and onto the multijunction cell of radiation of three different wavelengths; Fig. 2 is a graphical representation of the variation of the indices of refraction of the prismat¬ ic elements and the focusing surfaces of a dispersive concentrator shown as a function of radiation wave¬ length for a particular example of the present inven- tion where there is a common value of the refractive indices at a wavelength within the wavelength range of interest;
Fig. 3 is a schematic fragmentary section through a solar system according to the preferred embodiment of the present invention incorporating a dispersive concentrator such as is shown in Fig. 1 and a multijunction photovoltaic cell composed of four parallel component cells, and shows diagrammatically the mean propagation paths of radiation of four dif- ferent wavelengths through the system; and
Fig. 4 is a graphical representation of the conversion efficiency of multijunction photovoltaic cells as a function of the number of ideal component cells at different cell temperatures. Modes for Carrying Out the Invention
Referring now to the drawings in greater detail, Fig. 1 is a schematic representation of a partial section of a solar system 1 showing a part of a dispersive concentrator 2 and a 4-cell multijunction photovoltaic cell 3. Fig. 1 is not drawn to scale.
The dispersive concentrator 2 is composed of a first portion 4 and a second portion 5. The first portion 4 has a planar surface 6 at one side thereof and incorporates a plurality of prismatic elements 7 of right triangular cross-sections located optically in parallel with each other relative to the mean di¬ rection of incident solar radiation R. The first portion 4 of the concentrator can be formed using various techniques; for example, each prismatic ele- ment 7 may be separately formed and attached to the second portion 5 of the dispersive concentrator, or a continuous sheet of prismatic elements can be formed first and then attached as a unit to the second por¬ tion of the concentrator. Each of the prismatic elements 7 in this embodiment of the invention has incidence sections 8 having right triangular shapes. These incidence sec¬ tions are also cross-sections of the prismatic ele¬ ments in this embodiment, since the mean direction of incidence of the direct solar radiation R is normal (orthogonal) to the planar entrance surface 6. Each incidence section 8 has a straight first side 9, a straight second side 11 oriented at an angle of incli¬ nation 10 to the first side 9, and a straight third side 12 orthogonal to the first side 9. The angles of inclination 10 vary among at least some different incidence sections 8. While third sides 12 are orthogonal to first sides 9 in the embodiment shown in Fig. 1, the third sides of the incidence sections may be inclined at other than right angles to the first sides in alternative embodiments of the present inven¬ tion. Second sides 11 and third sides 12 meet at apexes 13. In the embodiment shown in Fig. 1, the planar surface 6 of the first portion 4 of the disper¬ sive concentrator 2 is defined by or parallel to the first sides 9 of the incidence sections 8 and is ori¬ ented normal to the mean direction of incidence of direct solar radiation R.
The second portion 5 of the dispersive con¬ centrator 2 has a zig-zag or sawtooth-shaped first side 14 which is in full engagement with the second sides 11 and third sides 12 of the incidence sections 8. Focusing surfaces 15 are formed on the second portion 5 of the dispersive concentrator 2 at a second side thereof opposite its first side 14. In the em- bodiment shown in Fig. 1, the focusing surfaces 15 have a circular (radial) symmetry in order to focus, i.e., to form and distribute, radiation onto the multijunction cell 3. The focusing surfaces 15 are provided with a plurality of steps 16 (only one step 16 is shown in Fig. 1) and have a plurality of surface facets 23 (portions of only two surface facets 23 are shown in Fig. 1) .
For reasons that have been indicated previ¬ ously, i.e., to produce a first dispersion of the direct solar radiation R entering the dispersive con¬ centrator, the material of which the first portion 4 of the dispersive concentrator 2 is composed has over most or all of the radiation wavelength range of in¬ terest an index of refraction, n.,, that differs in magnitude from the index of refraction, n2, of the second portion 5 of the dispersive concentrator 2. At the second sides 11 of the incidence sections 8, radi¬ ations 17, 18 and 19, each differing in wavelength from each of the others, are dispersed as they pass into the second portion 5 of the dispersive concentra¬ tor 2.
In Fig. 1, it is assumed that n1 and n2 are equal to each other for radiation 18, i.e., for that wavelength n.j/n-, is unity, so that radiation 18 is not deviated in its direction of propagation as it enters the second portion 5. (It should be understood, how¬ ever, that this condition is not universal; in some embodiments of the present invention, n1 and n2 do not have a common value in the radiation wavelength range of interest.) At the same time it is assumed that for radiation 17, the wavelength of which is longer than that of radiation 18, n1 is greater than n2, and that for radiation 19, the wavelength of which is shorter than that of radiation 18, n2 is greater than n,. Thus, the propagation direction of radiation 17 passing through a prismatic element or incidence section 8 is deviated towards the normal 22 to the second side 11 of that incidence section on being refracted at that second side 11, and the propagation direction of radi- ation 19 is deviated away from the normal 22 on being refracted at that second side 11, i.e., radiations 17 and 19 undergo angular deviations that are opposite in rotational sense as they enter the second portion 5 of the dispersive concentrator 2. In particular, radia¬ tion 17 is deviated through an angle of deviation 20 from the direction of propagation of radiation 18 towards the perpendicular or normal 22 to second side 11, and radiation 19 is deviated through an angle of deviation 21 from the direction of propagation of radiation 18 away from the perpendicular or normal 22 to second side 11.
Radiations 17, 18 and 19 are incident on focusing surface 15 at different angles of incidence due to the difference in angles of deviation 20 and 21. These differences in the angles of incidence and the differences in the indices of refraction of the second portion 5 of the dispersive concentrator 2 for radiations 17, 18 and 19 mean that the latter are deviated through different angles on leaving the fo¬ cusing surface 15. (Although it is not so shown in Fig. 1, for the purposes of the present analysis radi¬ ations 17, 18 and 19 may be considered as being inci¬ dent on the same point on the focusing surface 15 with, however, different angles of incidence.) On leaving the focusing surface 15, radiation 17 is devi- ated in propagation direction to become radiation 37, radiation 18, which was incident on the surface 15 at angle 24, is deviated in propagation direction to become radiation 38, and radiation 19 is deviated in propagation direction to become radiation 39. As shown in Fig. 1, all the focusing sur¬ faces 15 have a common optical axis 26. Radiation 38 is incident on the multijunction photovoltaic cell 3 at the juncture 32 between component cells 28 and 29 thereof, at an angle 25 to the common optical axis 26. Thus, radiation 38 has been deviated from the mean direction of propagation of the incident solar radia- tion R, i.e., from the direction of propagation of radiation 18 through the second portion 5 of the dis¬ persive concentrator, through an angle equal to angle 25 on leaving the focusing surface. Juncture 32 is located a distance D from the focusing surface mea¬ sured along optical axis 26, and the point where radi¬ ation 38 leaves the focusing surface in Fig. 1 is a distance H from the optical axis. Then: angle 25 = tan"1(H/D} , and: n2 sin Q^ = sin [Q-,• + tan"1{H/D}] , which yields Q1' where Q1' is the angle of incidence 24 of the radiation 18 on the focusing surface 15. Radi¬ ation 37, on the other hand, is deviated such that it impinges on the multijunction cell 3 at the juncture 31 between component cells 27 and 28, i.e., the de¬ sired distribution of radiation on the multijunction photovoltaic cell 3 is such that the mean location at which radiation with the wavelength of radiation 37 impinges on the multijunction cell is preferably at juncture 31 at an angle 35 to the optical axis 26. If the height of component cell 28 measured in the "X", direction, i.e., the distance between the junctures 31 and 32, is k, then: angle 35 = tan"1{ (H-k)/D) , and: n^sinfQ,' - angle 20) = sinfQ^ - angle 20 + angle 35), which yields angle 20, the decrease of the angle of incidence of radiation 17 as compared to the angle of incidence of radiation 18 at that same point on the focusing surface 15. Thus, the angle of incidence of radiation 17 on focusing surface 15 is [Q,,* - angle 20]. Knowing n1 and n2 for radiation 37 and angle 20, it is possible to solve the following equation for the required angle of inclination 10 of the particular co¬
incidence section through which the radiation in ques¬ tion propagates: n,-sin(angle 10) = n2'sin(angle 10 + angle 20) . Thus, the prism angle of the cross-section of the prismatic element for the radiation crossing the lat¬ ter to the point on the focusing surface in question is equal to angle 10 at that location. From this the significance of the use of variations in the cross- sections of the prismatic elements, both as between different prismatic elements and for particular indi¬ vidual prismatic elements, for producing desirable distributions of radiation on the component cells, will be readily appreciated.
Likewise, radiation 39 impinges on multi- junction cell 3 at juncture 33 between component cells 29 and 30, and assuming the height of the component cell 29 measured in the "X" direction, i.e., the dis¬ tance between the junctions 32 and 33, is f (which may be equal to or different from k) , then a similar cal- culation for values of the angles 36 and 21 yields the required angle of inclination of the same incidence section required to achieve that result. If the two calculated angle of inclination values differ, a com¬ promise choice can be made for the angle of inclina- tion of the relevant incidence section which achieves the most desirable distribution of both radiations 37 and 39 on multijunction cell 3 in terms of the produc¬ tion of electrical output from the celis. That com¬ promise choice can be used in determining the prism angle of the prismatic element cross-section in ques¬ tion or, alternatively, the dimension f of component cell 29 can be selected so that radiation 39 impinges on juncture 33. Repeating the procedure for different regions of the dispersive concentrator then permits the full design of the prismatic elements to be achieved. It will be noted that if the mean direction of radiation 38 impinges on juncture 32, and if the mean direction of propagation of radiation 37 impinges on juncture 31, then the mean direction of propagation of radiation with wavelengths lying between that of radiations 37 and 38 will impinge on component cell 28, i.e., such radiation will be distributed onto component cell 28. Similar considerations apply to radiations of other wavelengths. It should also be noted, in this regard, that the sequence of prism angle magnitudes in one sense or the other along the "X" direction, i.e., whether the prism angle of any given prismatic element is greater or smaller than or equal to the prism angle of the next adjacent prismatic element as viewed in the "X" direction, need not be such as is schematical¬ ly indicated in Fig. 1 or described earlier herein as characterizing one particular example of the preferred embodiment of the present invention. Rather, the sequence can be selected as desired depending on the particular extent of dispersion and focusing sought to be achieved by means of the dispersive concentrator. In some embodiments of the present inven¬ tion, making such a choice of prism angle may also involve adjustment of the values of the refractive indices by varying material compositions or may in¬ volve deviations of the prismatic elements from being geometrically parallel to each other.
In multijunction photovoltaic cell 3, compo- nent cell 30 has a larger energy bandgap value than does component cell 29, component cell 29 has a larger energy bandgap value than does component cell 28, and component cell 28 has a larger energy bandgap value than does component cell 27. The distribution of radiation involves bringing onto component cell 27 radiation of longer wavelengths than the wavelengths of the radiation impinging on component cell 28, bringing onto component cell 23 radiation of longer wavelengths than the wavelengths of the radiation impinging on component cell 29, and bringing onto component cell 29 radiation of longer wavelengths than the wavelengths of the radiation impinging on compo¬ nent cell 30. Secondary reflectors 34 are used in the embodiment shown in Fig. 1 to help bring radiation coming from the focusing surfaces to proper incidence onto the multijunction photovoltaic cell 3.
In Fig. 2, a graphical representation is shown of the variations in the refractive index 41 of the prismatic elements and the refractive index 42 of the focusing surfaces of a dispersive concentrator according to the present invention as a function of radiation wavelength for a particular example where they have a common value at wavelength 43. For wave¬ lengths less than wavelength 43, refractive index 41 is less than refractive index 42, and for wavelengths greater than wavelength 43, refractive index 41 is greater than refractive index 42.
In Fig. 3, a fragmentary diagrammatic sec¬ tion of a solar system 1 according to the preferred embodiment of the present invention is shown. Direct solar radiation R (shown in only two locations for the sake of simplicity) is incident on the dispersive concentrator 2 at the surface 6 thereof, as shown also in Fig. 1. The dispersive concentrator disperses radiation R to form radiations 47, 48, 49 and 50, each of different wavelength, which it focuses onto multi- junction cell 3 at the component cells 27, 28, 29 and 30 thereof, respectively. As shown, the wavelength of radiation 47 is greater than that of radiation 48", the wavelength of radiation 48 is greater than that of radiation 49, and the wavelength of radiation 49 is greater than that of radiation 50. Thus, since the respective energy bandgap values of the component cells 27, 28, 29 and 30 are such that the energy band- gap value of component cell 30 is greater than that of component cell 29, the energy bandgap value of compo- nent cell 29 is greater than that of component cell 28, and the energy bandgap value of component cell 28 is greater than that of component cell 27, the arrangement ensures, as previously mentioned, that radiation of longer wavelength is incident on a compo- nent photovoltaic cell of smaller energy bandgap value and that radiation of shorter wavelength is incident on a component cell of larger energy bandgap value.
Power conditioning means, electrical connec¬ tions, system pointing means, cooling equipment means and arrangements, and supporting structure means are not shown in Fig. 3, but such means and arrangements are well-known in the art. The types of electrical connections used to connect the component cells to the power conditioning means and to each other depend on whether or not the individual component cells are electrically connected in series, in parallel or in series-parallel. When component cells are electrical¬ ly connected in series, it is well-known in the art that the current carriers photogenerated in the indi- vidual cells should, preferably, be equal in magnitude to each other in order to avoid energy losses. Pro¬ ducing such an equality can be assisted by varying the positions of the component cells or by adjusting sec¬ ondary focusing reflectors 34, for example. Fig. 4 is a graphical representation of the conversion efficiency of multijunction photovoltaic cells as a function of the number of ideal component cells. As shown in Fig. 4, conversion efficiency tends to increase as the number of component cells (with well-distributed values of energy bandgap) in¬ creases. It is also shown in Fig. 4 that an increase in the temperatures of the component cells of the multijunction cell results in a decrease in conversion efficiency. The applicability of these relationships to the advantageous implementation of the present invention will be readily apparent.
Although the invention has been described with reference to particular embodiments thereof, it is to be understood that these embodiments are merely illustrative of the application of the principles of the invention. Numerous modifications may be made therein and other arrangements may be devised without departing from the spirit and scope of the invention as defined by the hereto appended claims. For exam¬ ple, prismatic elements having isosceles triangular cross-sections may be used. Also, the focusing sur¬ faces in some embodiments of the present invention may not have a common optical axis if that is deemed de¬ sirable for a better distribution of radiation on the component cells of the multijunction cells of the respective systems. Still further, the radiation dispersion effect achieved by varying the prism angles of the prismatic elements can be equivalently achieved by varying the indices of refraction of the prismatic elements. Industrial Applicability
The present invention is applicable for the generation of electricity from direct solar radiation.

Claims

Claims
1. A photovoltaic solar system for the production of electricity from direct solar radiation, which system is characterized by: (a) a dispersive concentrator positioned to receive direct solar radiation and to transmit dispersed and focused radiation deriving from said direct solar radiation, said dispersive concentrator including (i) a first portion composed of trans¬ parent material and comprising a plurality of prismatic elements optically in parallel with each other with respect to the mean direction of said direct solar radiation, and
(ii) a second portion composed of transparent material and disposed contiguous to and having a first side thereof in full engagement with said prismatic elements of said first portion, said second portion hav¬ ing focusing surfaces formed on a second side thereof, wherein
(iii) said first portion and said sec¬ ond portion have indices of refraction which differ from each other for at least some radiation wavelengths in a radiation wave¬ length range of interest,
(iv) each of said prismatic elements serves to effect a wavelength-dependent dis- persion of said direct solar radiation pass¬ ing through said first portion into said second portion to form an initially dis¬ persed radiation, and
(v) said focusing surfaces serve to further disperse said initially dispersed radiation and to focus said further dis¬ persed radiation; and
(b) at least one multijunction photo¬ voltaic cell capable of converting incident radiation to electricity and disposed at a location spaced from and facing towards said second portion of said disper¬ sive concentrator such that said further dispersed radiation focused by said focusing surfaces is inci¬ dent on said multijunction photovoltaic cell, (i) said at least one multijunction photovoltaic cell including at least two component photovoltaic cells positioned op¬ tically in parallel with respect to said focused further dispersed radiation coming from said dispersive concentrator, and
(ii) different ones of said component photovoltaic cells having respective differ¬ ent energy bandgap values and being located so that different selected wavelength compo- nents of said further dispersed radiation are incident on said different ones of said component photovoltaic cells, with said in¬ cidence being in an inverse relationship of the magnitude of the wavelengths of said different selected wavelength components of said further dispersed radiation to the mag¬ nitude of the respective different energy bandgap values of said different ones of said component photovoltaic cells.
2. A photovoltaic solar system as claimed in claim 1 wherein secondary focusing means are associated with said multijunction photovoltaic cell.
3. A photovoltaic solar system as claimed in claim 2 wherein power conditioning means are electrically connected to said component photovoltaic cells.
4. A photovoltaic solar system as claimed in any one of claims 1, 2 and 3 wherein cooling means are provid¬ ed to control the temperatures of said component pho¬ tovoltaic cells.
5. A photovoltaic solar system as claimed in claim 1 wherein said first and second portions of said disper¬ sive concentrator are composed of different transpar¬ ent materials.
6. A photovoltaic solar system as claimed in claim 1 wherein the section of said second portion of said dispersive concentrator which is contiguous to said first portion is composed of a different transparent material than the section of said second portion which defines said focusing surfaces.
7. A photovoltaic solar system as claimed in claim 1 wherein said focusing surfaces constitute a Fresnel lens.
8. A photovoltaic solar system as claimed in claim 1 wherein at least some of said prismatic elements have cross-sections that differ from the cross-sections of at least some of the other prismatic elements.
9. A photovoltaic solar system as claimed in any one of claims 1, 7 and 8 wherein said prismatic elements have right triangular cross-sections. ιo. A photovoltaic solar system as claimed in any one of claims 1, 7 and 8 wherein each of said prismatic elements has a respective prism angle the magnitude of which is between about 52 degrees and about 64 de¬ grees. li. A photovoltaic solar system as claimed in claim 9 wherein said prismatic elements have respective prism angles the magnitudes of which are between about 52 degrees and about 64 degrees. 12. A photovoltaic solar system as claimed in claim 1 wherein at least some of said prismatic elements have indices of refraction for at least one of said radia- tion wavelengths in said radiation wavelength range of interest that differ from the indices of refraction of at least some of the other prismatic elements for said at least one radiation wavelength.
PCT/US1990/004439 1989-09-21 1990-08-08 Photovoltaic solar systems with dispersive concentrators WO1991004580A1 (en)

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ES2046105A2 (en) * 1992-02-18 1994-01-16 Fraunhofer Ges Forschung Solar energy converter simultaneously gaining electrical and thermal power - uses successive heat sinks distributed to suit energy band levels
US5977478A (en) * 1996-12-05 1999-11-02 Toyota Jidosha Kabushiki Kaisha Solar module
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US6469241B1 (en) * 2001-06-21 2002-10-22 The Aerospace Corporation High concentration spectrum splitting solar collector
WO2005060009A1 (en) * 2003-12-18 2005-06-30 The University Of Sydney A beam splitter
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CN102227820A (en) * 2008-11-27 2011-10-26 日本电石工业株式会社 Optical layer for light-adjusting type solar power-generating module, light-adjusting type solar power-generating module and light-adjusting type solar power-generating panel
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ES2046105A2 (en) * 1992-02-18 1994-01-16 Fraunhofer Ges Forschung Solar energy converter simultaneously gaining electrical and thermal power - uses successive heat sinks distributed to suit energy band levels
US5977478A (en) * 1996-12-05 1999-11-02 Toyota Jidosha Kabushiki Kaisha Solar module
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US6469241B1 (en) * 2001-06-21 2002-10-22 The Aerospace Corporation High concentration spectrum splitting solar collector
WO2005060009A1 (en) * 2003-12-18 2005-06-30 The University Of Sydney A beam splitter
WO2008024201A3 (en) * 2006-08-22 2008-06-12 Eastman Kodak Co Optically enhanced multi-spectral detector structure
WO2008024201A2 (en) * 2006-08-22 2008-02-28 Eastman Kodak Company Optically enhanced multi-spectral detector structure
EP2351096A4 (en) * 2008-10-17 2017-02-15 Massachusetts Institute of Technology Ultra-high efficiency multi-junction solar cells using polychromatic diffractive concentrators
JP2010114349A (en) * 2008-11-10 2010-05-20 Konica Minolta Holdings Inc Hybrid power generating device
CN102227820A (en) * 2008-11-27 2011-10-26 日本电石工业株式会社 Optical layer for light-adjusting type solar power-generating module, light-adjusting type solar power-generating module and light-adjusting type solar power-generating panel
WO2012107607A1 (en) * 2011-02-11 2012-08-16 Caselles Fornes Jaime Wavelength classification and radiation-intensity regulating system
RU2520803C2 (en) * 2012-05-15 2014-06-27 Государственное научное учреждение Всероссийский научно-исследовательский институт электрификации сельского хозяйства Российской академии сельскохозяйственных наук (ГНУ ВИЭСХ Россельхозакадемии) Solar module with concentrator and method of its production
RU2684685C1 (en) * 2018-05-14 2019-04-11 федеральное государственное бюджетное образовательное учреждение высшего образования "Национальный исследовательский университет "МЭИ" (ФГБОУ ВО "НИУ "МЭИ") Photoelectric module
RU2700655C1 (en) * 2019-04-19 2019-09-18 Федеральное государственное бюджетное научное учреждение "Федеральный научный агроинженерный центр ВИМ" (ФГБНУ ФНАЦ ВИМ) Solar module with concentrator

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