CROSS-REFERENCE TO RELATED APPLICATIONS

[0001 ] This application claims the benefit of U.S. Provisional Patent Application

No. 61/755,699, filed January 23, 2013 and titled "SOLAR WAVEGUIDE

CONCENTRATOR" which application is incorporated herein by reference in its entirety.

TECHNICAL FIELD

[0002] The present invention relates to photovoltaic devices that convert incident light into electrical energy. More specifically, the present invention relates to a solar concentrator that captures sunlight over a large area and directs it to photovoltaic cells that convert that light into electricity.

BACKGROUND

Photovoltaic cells, which may also be referred to as solar cells or PV cells, are useful for converting incident light, such as sunlight, into electrical energy. However, because the semiconductor materials that are used in photovoltaic cells are somewhat costly and the corresponding construction of these cells is relatively expensive, it can be advantageous to utilize relatively inexpensive solar

concentrators, which are optical components with a large surface area, to collect sunlight. The concentrators then concentrate the light and direct it into a

photovoltaic cell with a smaller surface area. Transferring light to the photovoltaic cells at high conversion efficiencies can often lower the overall cost of solar energy produced by the system. Therefore, several types of solar concentrators have been developed to achieve higher conversion efficiency in photovoltaic cells.

One common method used for achieving higher photovoltaic efficiencies is to direct sunlight to mirrors that collect and focus light for use by a device that converts the light into electrical energy. Devices of this type often include systems for tracking the sun as it moves throughout the day in order to maximize their efficiencies. Other systems include the use of light coupled into waveguides via grating structures comprising periodic modulations that establish the phase matching condition for coupling an incoming light ray into a waveguide mode. Various waveguiding systems and devices are also available, such as systems that couple light into a waveguide by prismatic structures that protrude onto the surface of the waveguide and couple light at a shallow angle so that the light ray is trapped in the waveguide. Other systems can couple light using a wedge-type waveguide having a thickness that varies with distance. Systems that utilize solar concentrators have been described in the patent and technical literature. Examples include U.S. Pat. Publication. Nos. 2009/0126792 (Gruhlke et al.); 2011/0011449 (Morgan);

2011/0162712 (Tillen, et al.); 2012/0006382 (Dagli, et al.); and 2012/0140352 (Morgan); and U.S. Patent Nos 6,425,675 (Onishi, et al.); 6,639,733 (Minano, et al); 7,672,549 (Ghosh et al.); and 7,873,257 (Morgan).

[0005] While a number of solar concentrator systems have been developed, there is a continuing need to provide systems with improved conversion efficiencies that are relatively cost-effective.

SUMMARY

[0006] The present invention is directed to a photovoltaic system for converting incident sunlight into electrical energy. An embodiment of the invention includes a photovoltaic device with a waveguide layer adjacent to a light collector layer. The waveguide layer includes a first surface for receiving light, a second surface opposite the first surface for reflecting light, and at least one edge surface generally perpendicular to the first and second surfaces for emitting light. The light collector layer includes a first surface for receiving light, and a second surface opposite the first surface for emitting light. The second surface of the light collector layer includes a plurality of optical elements having three-dimensional tapered shapes with a surface area that is larger at their top than at their bottom, such as cones, prisms, and paraboloids, for example. In one particular example, the optical elements have a truncated paraboloidal shape, wherein the truncated area of each of the optical elements is in optical contact with the first surface of the waveguide layer. The chosen shape of these optical elements is generally designed to collect light from a broad range of angles. With such a system, at least one edge surface of the waveguide layer is optically coupled to at least one photovoltaic cell. The optical elements in the light collector layer of the photovoltaic system described above can be compound parabolic concentrators. Both the waveguide layer and the light collector layer include highly light transmissive materials. These materials can be organic (polymeric) or inorganic. The first surface of the waveguide layer is coated with a reflective material in areas that are not in contact with the truncated surfaces of the optical elements in the light collector layer. The second surface of the waveguide layer is coated with or attached to a suitable light reflective layer. In accordance with the present invention, the first surface of the waveguide layer can further include a plurality of exposed areas or openings (e.g., uncoated spots that can be round or differently shaped) arranged in a pattern aligned with the truncated surfaces of the optical elements of the light collector layer, wherein said pattern can be produced using a screen printing process or other suitable patterning processes.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The present invention will be further explained with reference to the

appended figures, wherein like structure is referred to by like numerals throughout the several views, and wherein:

[0009] Figure 1 is a schematic side view of a portion of a photovoltaic system of the invention;

[00010] Figure 2 is a schematic exploded side view of a photovoltaic system of the invention; and

[00011] Figure 3 is a schematic side view of the photovoltaic system of Figure 2 in its assembled state.

DETAILED DESCRIPTION

[00012] The embodiments of the present invention described below are not intended to be exhaustive or to limit the invention to the precise forms disclosed in the following detailed description. Rather the embodiments are chosen and described so that others skilled in the art may appreciate and understand the principles and practices of the present invention. All patents, pending patent applications, published patent applications, and technical articles cited throughout this specification are incorporated herein by reference in their respective entireties for all purposes.

[00013] Referring now to Figures 1 and 2, and more specifically to Figure 1, an

exemplary embodiment of a solar waveguide concentrator or photovoltaic system 10 of the invention is illustrated, which generally includes a waveguide layer 12 adjacent to a light collector layer 14. System 10 can be used for photovoltaic conversion of incident sunlight into electrical energy, as will be described in further detail below.

[00014] Waveguide layer 12 is provided with a first or top surface 28 and a second or bottom surface 34 that is opposite the first surface 28. The first surface 28 is provided for receiving light from the light collector layer 14, while the second surface 34 is provided for reflecting light. The waveguide layer 12 further includes at least one edge surface that is generally perpendicular to the first and second surfaces, wherein each edge surface emits light that reaches it. In this embodiment waveguide layer 12 includes a first edge surface 36 and a second edge surface 37. Edge surfaces 36 and 37 are coupled to photovoltaic cells 33 and 32, respectively. The waveguide layer 12 has a thickness 38 and a length 50, which dimensions can vary considerably, depending on the design and specifications of the photovoltaic system.

[00015] The waveguide layer 12 can be made of a solid material that is highly

transmissive to sunlight h order to minimize absorption losses and maximize the system performance. Exemplary materials for the waveguide layer include, but are not limited to, glass, poly (methyl methacrylate), polycarbonate, polycycloolefins and other transparent optical polymers. The first surface 28 of waveguide layer 12 is provided with a reflective layer or metal reflector 30 that can reflect light beams that impinge upon it and can also act as a heat sink for unreflected beams. In order to allow light from the light collector layer 14 to enter the waveguide layer 12 in a controlled manner that also limits the amount of light that can exit the waveguide layer, the reflective layer 30 is provided with a specific pattern of reflective areas (e.g., areas including a reflective material) and exposed areas (e.g., uncoated spots or areas) across the first surface 28 of the waveguide layer 12, wherein the uncoated or exposed areas are aligned with the optical elements 16 of the light collector layer 14. The size, number, spacing, and other features of the exposed areas on the first surface 28 of the waveguide layer 12 can be designed to optimize the light conversion efficiency of the system, as will be described in further detail below.

[00016] Light collector layer 14 generally includes multiple optical elements 16 that can be arranged in a grid pattern or other arrangement across its second surface. Each of the optical elements generally includes a three-dimensional tapered shape with a surface area that is larger at its top than at its bottom, such as a truncated cone, a truncated prism, or a truncated paraboloid, for example. In one particular example, the optical elements have a truncated paraboloidal shape, wherein the truncated area of each of the optical elements is in optical contact with the first surface of the waveguide layer. The chosen shape for these optical elements is generally designed to collect light from a broad range of angles.

[00017] In the particular embodiment of Figure 1 , the optical elements 16 are

compound parabolic concentrators having a truncated paraboloidal shape, although the relative shape and size of the illustrated optical elements 16 are intended to be exemplary. These elements can have a number of shapes other than the illustrated shape, such as other curvilinear shapes. Each of the optical elements 16 includes an input end 18, an output end 20, and a concentrator region 22 extending between the input and output ends 18, 20. The shapes and sizes of the features of the optical elements 16 are designed to optimize the concentration of light that enters the system and provided to the waveguide layer 12.

[00018] The quantity and types of elements provided for a particular light collector layer are generally selected to provide a desired concentrating power over a certain range of incident angles. For example, the concentrating power of optical elements 16 can be calculated as a ratio of an area 24 at the input end 18 to an area 26 at the output end 20. This power typically varies as a ratio in the range of approximately 2 to 20, although the ratio can be higher or lower than this range. That is, the optical elements 16 are relatively large at their top surface as compared to their bottom surface. In certain embodiments, concentration powers ranging from 2 to 10 will be desirable. In a particular example, the upper input area of an optical element 16 is approximately 9 mm ² and the area of the lower aperture of that same optical element is approximately 1 mm , thereby providing a concentration power of 9. [00019] The optical elements 16 can have shapes or geometries other than truncated paraboloids, such as truncated prisms, hemispheres, cones, hyperboloids, and combinations thereof, as well as other shapes. Such shapes and structures can produce different levels of concentration power and efficiencies as compared to the compound parabolic shapes discussed and illustrated herein. In some cases, however, features of the optical elements are selected to accommodate a certain material or structural limitation of the device.

[00020] Referring also to the exploded view of Figure 2, a light collector layer 14 is shown prior to its attachment to waveguide layer 12, wherein the waveguide layer 12 may be provided as a metalized sheet of acrylic or glass that is screen printed or otherwise processed to provide the patterned metalized layer 30. In this embodiment, the light collector layer 14 may further include a top layer 54 from which the optical elements 16 project or extend. The layer 54 is provided as a support member that can maintain the optical elements in a certain configuration for at least a period of time. For one example, the layer 54 is removable after the light collector layer 14 is placed in its desired location relative to the waveguide layer 12. In another example, the layer 54 is not removable and will remain permanently attached to the optical elements 16 after the layers are assembled. In a case where the layer 54 remains attached to the optical elements 16, it may be desirable for layer 54 to be highly transmissive to maximize the amount of light that reaches the input end 18 of the optical elements 16. It may also be desirable for input end 18 of at least one of the optical elements 16 to have a surface that is curved or shaped in such a way that it is not flat or planar, in order to further increase light coupling efficiency.

[00021] Figure 3 illustrates the layers of Figure 2 in an assembled state, in which an adhesive 56 can be positioned between the output ends of the optical elements 16 and the waveguide layer 12 to secure these layers to each other. As shown, the output ends of the optical elements 16 are aligned with the patterned "holes" or uncoated areas of the patterned metalized layer 30.

[00022] Referring again to Figure 1, and as is discussed above, the optical elements

16 of one embodiment of the invention have a paraboloidal shape that is truncated at the output end 20. This truncated portion has an area 26 that matches the uncoated spots or openings in layer 30 on the first surface of the waveguide layer 12. That is, the uncoated spots in the reflective layer 30 of waveguide layer 12 are aligned with the output ends 20 of the optical elements 16 and are positioned so that the elements 16 are in optical contact with the first surface of the waveguide layer 12.

[00023] The photovoltaic cells provided with embodiments of the invention can be relatively thin and long and can have approximately the same or a similar width as the waveguide thickness 38 and can have a length that is approximately the same as the waveguide length 50. Exemplary dimensions of these cells are 1 mm high x 50 mm long, or 2 mm high x 100 mm long, or 5 mm high x 500 mm long, although different dimensions are contemplated. The photovoltaic cells 32, 33 can optionally be optically coupled to the edge surface(s) of the waveguide layer 12 with optical adhesive and electrically wired to an electrical grid. Such thin and long cells can be cut (e.g., with a saw) from thin silicon wafers, for example. It is contemplated that the photovoltaic cells can be of any relatively high efficiency cell type, such as crystalline silicon, CdTe, CIGS, or multijunction III-V type cells. Due to the concentration of light, rear contact cells may be used in some embodiments to minimize the possibility of aperturing on the front of the cell.

[00024] With continued reference to Figure 1 , a number of exemplary paths of light within the waveguide layer 12 are shown, all of which are intended to represent incident light that first enters the relatively large input end 18 of an optical element 16 and that is concentrated down to a relatively small output end 20. The concentrated light can then enter the waveguide layer 12 through the uncoated spots or openings on the first surface of the waveguide layer that are in optical contact with the output end 20 of each optical element. One exemplary light path is illustrated with a first light ray 40, which is initially shown as entering one of the optical elements 16 at an arbitrary incident angle. The ray 40 is then directed toward the output end 20 of the optical element 16 until it enters the waveguide layer 12. As illustrated with the optical element 16 that is shown furthest to the right in the illustrated embodiment, a number of rays propagate from the output end 20 of this optical element and into the waveguide layer 12. One such exemplary light ray 42 is shown as entering the waveguide 12 from that output end 20, wherein it then travels through the thickness 38 of the waveguide layer 12 until it reaches the bottom surface 34 on the opposite side of the waveguide layer 12, and it is then reflected back into the waveguide layer. The reflected light ray then travels back up toward the reflective layer 30, wherein this particular ray impinges on the area between the uncoated spots or openings in the reflective layer (i.e., a reflective portion of the patterned layer). This light ray can continue to bounce between the surfaces of the waveguide layer until the light reaches a photovoltaic cell on one of the edge surfaces of the waveguide layer 12. At this point, the light impinging on the cell can be converted into electrical energy.

[00025] Another exemplary light ray 44 is illustrated as traveling through the

waveguide layer 12 until it reaches the reflective bottom surface 34 of the waveguide layer, and then it is reflected back (represented by arrow 46) toward the light collector layer 14. Because this light ray is reaching an uncoated spot or opening in the reflective layer 30, it can enter the optical element 16 through the area of the truncated surface 20. The arrow 48 represents this light ray as it exits or escapes through the optical element 16.

[00026] As described herein, reflective layers that are provided on the first and/or second surfaces of the waveguide layer can be provided in a number of ways. For example, the reflective layers can be provided as shiny, highly reflective metal coatings (i.e., metallic mirrors), as multilayer coatings that are otherwise known as Bragg mirrors, or as relatively low cost white reflectors of the type that are used in liquid crystal display backlights. When metallic mirrored surfaces are used, they may be produced in a sputtering machine, for example. Reflected light beams that contact these metallic mirrored surfaces can experience a small absorption and loss upon each reflection. Using Bragg mirrors or Bragg multilayer dielectric coatings, however, can provide coatings in which the light beams do not experience significant optical loss upon reflection, but are more difficult to coat onto substrates. White reflectors can be made of a relatively low cost polymeric material and can simply be placed adjacent to the reflective surface without the need to coat directly onto the surface of the waveguide.

[00027] Another exemplary embodiment of the invention includes a waveguide

surface with no reflective coatings. In such an embodiment, the waveguide is structured so that light rays can be reflected off its surfaces due to total internal reflection. For example, when light ray 42 is incident at an angle below a critical angle, then it can be reflected without loss. The critical angle is represented by the equation: 0 _c =Sin ^"1 (¾/¾), wherein nl is the refractive index outside the waveguide (e.g., air) and n2 is the refractive index of the waveguide 12. Thus, using a high refractive index material in constructing the waveguide and/or using cladding layers on both principal surfaces of the waveguide with a relatively low refractive index will increase the portion of light trapped in the waveguide using total internal reflection, thereby improving the light efficiency of the solar concentrator.

[00028] It is contemplated that the devices and systems of the invention are used in a stationary mode, such as through attachment to a roof on a building, for example, because the systems described herein have a relatively large acceptance angle for the capture of incident sunlight as the earth moves relative to the sun. However, it is contemplated that the devices and systems of the invention can be used in combination with a tracking system that follows the path of the sun. Such a tracking system can improve the efficiency of capture of sunlight, as is known in the art for increasing concentration and efficiency. The tracking can occur along one axis or two axes, depending on the latitude and longitude of the location of the photovoltaic device.

[00029] Certain features of the light collector layers of the type described herein will determine the acceptance angle of the system. That is, some systems can be adjusted and optimized for acceptance in one or more direction, which will lead to better performance in certain applications. For example, for a module which is to be mounted in a fixed orientation, polar aligned, at latitude tilt, a light collector layer with a broad acceptance angle across the horizontal axis will be relatively immune to the diurnal variation in illumination as the sun moves across the sky. This

"pseudotracking" can increase the annual output of a system by a significant amount (e.g., up to 30%).

[00030] The present invention has now been described with reference to several embodiments thereof. The entire disclosure of any patent or patent application identified herein is hereby incorporated by reference. The foregoing detailed description and examples have been given for clarity of understanding only. No unnecessary limitations are to be understood therefrom. It will be apparent to those skilled in the art that many changes can be made in the embodiments described without departing from the scope of the invention. Thus, the scope of the present invention should not be limited to the structures described herein, but only by the structures described by the language of the claims and the equivalents of those structures.