WO2008003005A2

WO2008003005A2 - Electro-optic steering assembly

Info

Publication number: WO2008003005A2
Application number: PCT/US2007/072290
Authority: WO
Inventors: Dwight P. Duston
Original assignee: Solbeam, Inc.
Priority date: 2006-06-27
Filing date: 2007-06-27
Publication date: 2008-01-03
Also published as: WO2008003005A3

Abstract

A light steering system including an electro-optic steering assembly operable (300) to provide controllable steering of light rays. In one example, the electro-optic steering assembly (300) includes first (304), second (306) and third (303) electrode layers. An electrically insulating layer (305) can be positioned between the first (304) and second (306) electrode layers. A layer of electro-optic material (301) is positioned between the second (306) and third (303) electrode layers. The first electrode layer (304) is positioned on a first substrate (302) and includes substantially parallel linear electrodes having longitudinal axes orientated in a first direction. The second electrode layer (306) includes substantially parallel linear electrodes having longitudinal axes orientated in a second direction orthogonal to the first direction. A layer of electro-optic material is not provided between the first (304) and second (306) electrode layers.

Description

ELECTRO-OPTIC STEERING ASSEMBLY

TECHNICAL FIELD [0001] This invention relates to steering light rays.

BACKGROUND

[0002] Steering light rays emanating from either a natural or an artificial source can be useful for various different applications. For example, steering solar rays to direct them toward a photovoltaic cell or to direct them toward a light focusing element, which then focuses the solar rays on a photovoltaic cell, can be useful in solar energy collection applications. Generally, a photovoltaic cell (or other device for capturing solar energy) is a device that captures solar radiation and converts the radiation into electric potential or current. A conventional photovoltaic cell is typically configured as a flat substrate supporting an absorbing layer that captures impinging solar radiation, and electrodes (or conducting layers) that serve to transport electrical charges created within the cell. [0003] A solar concentrator is a light focusing element that can be employed to increase the amount of sunlight, i.e., the solar flux, impinging on a photovoltaic cell. A solar energy collection assembly, or array, can be mounted on a moveable platform, in an attempt to keep the absorbing layer directed approximately normal to the solar rays as the sun tracks the sky over the course of a day. If a light focusing element, such as a lens or curved mirror, is included in the solar energy collection assembly to focus the solar rays toward the photovoltaic cells, the assembly's position can be adjusted in an attempt to keep the receiving surface of the light focusing element directed approximately normal to the solar rays. The platform can be moved manually or automatically by mechanical means, and various techniques can be employed to track the sun. [0004] So-called "electro-optic" materials can steer a light ray in one or more directions by altering the index of refraction within the material as the light passes through. An induced index of refraction can be created by applying an electric field to the material. The extent to which the light is steered as it passes through an electro-optic material can be dependent upon the strength of the applied electric field.

SUMMARY

[0005] The invention relates to steering light rays. In general, in one aspect, the invention features a light steering system including an electro-optic steering assembly operable to provide controllable steering of light rays. The electro-optic steering assembly includes first, second and third electrode layers. An electrically insulating layer is positioned between the first and second electrode layers. A layer of electro-optic material is positioned between the second and third electrode layers. The first electrode layer is positioned on a first substrate and includes substantially parallel linear electrodes having longitudinal axes orientated in a first direction. The second electrode layer includes substantially parallel linear electrodes having longitudinal axes orientated in a second direction orthogonal to the first direction. A layer of electro-optic material is not provided between the first and second electrode layers.

[0006] Implementations of the invention can include one or more of the following features. A refractive index gradient can be generated in the layer of electro-optic material by applying electric potential to the linear electrodes included in the first and second electrode layers. Light rays impinging on the electro-optic steering assembly can be controllably steered by controlling the refractive index gradient. The refractive index gradient is generated from a superposition of electric fields generated by the first and second electrode layers. The system can further include a light focusing element in optical communication with the electro-optic steering assembly and configured to receive and concentrate the light rays after having passed through the electro-optic steering assembly. The electro-optic steering assembly can be operable to steer impinging light rays, such that the light rays exit the assembly in a direction substantially normal to the light focusing element.

[0007] In one implementation, the light rays are solar rays impinging on the electro-optic steering assembly with both azimuthal and elevational angles of incidence, and the system further includes a photovoltaic device in optical communication with the light focusing element, wherein the light focusing element concentrates the solar rays on the photovoltaic device.

[0008] In one implementation, the linear electrodes are transparent and, for example, can be formed from indium tin oxide. The layer of electro-optic material can be a liquid crystal material, for example, a cholesteric liquid crystal or a nematic liquid crystal. Examples of electrically insulating layers include a plastic layer or a glass layer. In one example the electrically insulating layer is formed from multiple electrically insulating elements positioned at regions where the linear electrodes of the first electrode layer intersect the linear electrodes of the second electrode layer. In one example, the third electrode layer is contiguous across the second substrate. The system can further include a support configured to support the electro-optic steering assembly; the support can be movable or stationary.

[0009] In general, in another aspect, a method of steering light rays is provided.

Light rays are received onto a surface of an electro-optic steering assembly including a first electrode layer positioned on a first substrate, a second electrode layer, an electrically insulating layer positioned therebetween, a third electrode layer and a layer of electro- optic material positioned between the second and third electrode layers. The first electrode layer includes substantially parallel linear electrodes having longitudinal axes orientated in a first direction. The second electrode layer includes substantially parallel linear electrodes having longitudinal axes orientated in a second direction orthogonal to the first direction. The third electrode layer is positioned on a second substrate. A layer of electro-optic material is not provided between the first and second electrode layers. Voltages are selectively applied to at least one of the first or second electrode layers to selectively adjust a refractive index of the layer of electro-optic material, such that the light rays exit the electro-optic steering assembly at a predetermined angle. [0010] Implementations of the invention can include one or more of the following features. Selectively applying voltages to at least one of the first or second electrode layers can include applying multiple voltages to the plurality of linear electrodes providing an electric field varying in intensity across the electro-optic steering assembly. The method can further include selecting the predetermined angle such that the light rays impinge on a light focusing element in optical communication with the electro-optic steering assembly substantially normal to a receiving surface of the light focusing element. The light focusing element can be used to concentrate the light rays on a solar energy collector, for example, a photovoltaic device.

[0011] In general, in another aspect, the invention features a solar energy collection system including an electro-optic steering assembly, a light focusing element and a photovoltaic device. The electro-optic steering assembly is operable to provide controllable steering of solar rays and includes a first electrode layer positioned on a first substrate and having substantially parallel linear electrodes having longitudinal axes orientated in a first direction. The electro-optic steering assembly further includes a second electrode layer including substantially parallel linear electrodes having longitudinal axes orientated in a second direction orthogonal to the first direction. An electrically insulating layer is positioned between the first electrode layer and the second electrode layer. A third electrode layer is positioned on a second substrate. A layer of electro-optic material is positioned between the second electrode layer and the third electrode layer. A layer of electro-optic material is not provided between the first and second electrode layers. The light focusing element is in optical communication with the electro-optic steering assembly and positioned to receive and concentrate the solar rays after having passed through the electro-optic steering assembly. The photovoltaic device is in optical communication with the light focusing element. The light focusing element concentrates the solar rays on the photovoltaic device.

[0012] Implementations of the invention can include one or more of the following features. The layer of electro-optic material can be positioned between the electrode layers such that when separately controllable voltages are provided to at least some of the linear electrodes, a gradient electric field is provided within the layer of electro-optic material to cause the electro-optic material to have a refractive index gradient. The refractive index gradient can be controlled by varying the magnitude of the separately controllable voltages provided to at least some of the linear electrodes. The magnitude of the separately controllable voltages provided to the linear electrodes can be varied based on a position of the sun relative to the electro-optic steering assembly. The layer of electro-optic material can have a substantially uniform thickness. The electro-optic material can be a liquid crystal material.

[0013] Implementations of the invention can realize one or more of the following advantages. The assemblies and techniques described herein provide for effective steering of light rays impingement on an electro-optic prism in two directions, while requiring only a single layer of electro-optic material. Advantageously, the overall assembly can be thinner, lighter and less expensive to manufacture. Light ray steering in multiple directions can be achieved in an apparatus that does not require moving parts. Eliminating moving parts can reduce maintenance costs, reduce the failure rate and not require the same bulk and weight of a system including, for example, a tracking assembly (e.g., for solar tracking).

[0014] The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. DESCRIPTION OF DRAWINGS

[0015] FIG. 1 shows a schematic representation of a simplified solar energy collection assembly.

[0016] FIGS 2A-E show schematic representations of example solar energy collection assemblies including electro-optic prisms.

[0017] FIGS. 3A-B show schematic representations of an example electro-optic steering assembly.

[0018] FIGS. 4A-B show schematic representations of solar energy collection using an example electro-optic steering assembly.

[0019] FIG. 5 shows a schematic representation of a solar energy collection assembly. [0020] Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

[0021] Assemblies and techniques are described to steer light rays, including artificial or naturally occurring light. One application where steering light rays has beneficial effects is in the context of solar energy collection. For illustrative purposes, the assemblies and techniques shall be described in the context of solar rays, however, it should be understood that the assemblies and techniques can be applied in other contexts and to other light sources. The solar energy collection application described herein is but one implementation.

[0022] To reduce the cost of manufacturing photovoltaic systems, the amount of photovoltaic material required is preferably minimized. Concentrating captured solar rays onto a photovoltaic cell is one technique for maximizing solar energy collection efficiency, as more sunlight impinges on the photovoltaic cell than would otherwise impinge on its surface area. As described above, conventional solar concentrating arrays generally require adjusting the position of a solar energy collection assembly to track the position of the sun. The assemblies and techniques described herein to steer and concentrate light rays provide for configurations that minimize or eliminate physical adjustment, i.e., pointing, of the solar energy collection assembly. [0023] Referring to FIG. 1, a schematic drawing shows a point light source, i.e., the sun 110, which emits a broad spectrum of electromagnetic radiation (solar rays) 120. The sun 110 continuously travels relative to a terrestrial position, such as the location of a photovoltaic cell 170. A light focusing element 140 can receive the solar rays 120 and focus them toward the photovoltaic cell 170 (positioned along the optical axis 145 of the light focusing element 140), thereby concentrating the amount of solar radiation that would otherwise have impinged on the photovoltaic cell 170. In one example, the light focusing element 140 is a Fresnel lens. To be most effective, however, the solar rays 120 should impinge on a receiving surface 142 of the light focusing element 140 at an approximate 90° angle. That is, to obtain optimal focusing conditions, the point source lies at a point along the optical axis 145 of the light focusing element 140. The optical axis 145 of the light focusing element 140 is generally an axis of rotational symmetry about the light focusing element 140.

[0024] The optical axis 145 in most cases is the axis which, given a point light source at a point along the axis 145, would focus or image the light source with a minimum of spherical or chromatic aberrations or coma. If the solar rays 120 impinge on the light focusing element 140 at an angle, other than normal, a significant portion of the solar rays 120 can be refracted away from the absorbing, or active area, of the photovoltaic cell 170, dramatically decreasing the light intensity at the photovoltaic cell 170. The reduction in light intensity has a direct bearing on the overall efficiency of solar energy collection.

[0025] A light-steering mechanism 150 can steer incoming solar rays 120, such that solar rays 120 exiting the light- steering mechanism 150 are incident on the receiving surface 142 of the light focusing element 140 approximately normal to the receiving surface 142. The light focusing element 140 can thereby focus a maximum of the solar rays 120 on the photovoltaic cell 170.

[0026] In one implementation, the light- steering mechanism 150 includes an electro-optic material configured to direct solar light rays 120 that pass through the light- steering mechanism 150 by means of optical refraction and/or diffraction. The amount of solar light ray steering required, such that light impinges on the receiving surface 142 at normal incidence, depends on the refractive index of the electro-optic material and the size and shape of optical structures included in the light steering mechanism 150, which in turn can vary with an electric potential applied to the material.

[0027] Referring to FIG. 2A, in this implementation, the light-steering mechanism

150 is an electro-optic prism 202. The electro-optic prism 202 can include multiple, individual electrodes 210 on a first substrate 220 and a reference electrode (e.g., a ground electrode) 230 on a second substrate 240. An electro-optic material 250 of substantially uniform thickness is positioned between the electrodes 210 and 230. In one implementation, the electro-optic material 250 can be liquid crystal. In one implementation, the electrodes 210 and 230 are transparent electrodes, for example, formed of indium tin oxide.

[0028] Applying voltages to the electrodes 210 generates an electric field in the electro-optic material 250, causing polar molecules therein to rotate in the direction of the applied electric field. In some implementations, the reference electrode 230 is electrical ground. By controlling the voltages to the individual electrodes 210, a gradient in the refractive index ("index gradient") of the electro-optic material 250 can be created. The index gradient is controlled in accordance with the angle of incident solar rays 207, which can be in accordance with the position of the sun relative to the surface 205 of substrate 220. As the sun moves, i.e., the angle θ in FIG. 2A changes, the index gradient can be controllably modified, such that the incident solar rays 207 are steered from their angle of incidence θ so as to exit the bottom surface 242 of the substrate 240 substantially normal to a receiving surface 142 of the light focusing element 140. The solar rays 207 are therefore incident at an approximate 90° angle on the receiving surface 142 and can thereby properly focused toward the photovoltaic cell 170.

[0029] FIGS. 2B-D illustrate an implementation where solar rays 207 are steered throughout the course of a day by a light steering mechanism of the type described above. Light rays 207 can be steered such that they impinge on the light focusing element 140 substantially normal to the receiving surface 142, so that the solar rays 207 can be substantially focused to a photovoltaic 170. In FIG. 2B, solar rays 207 impinge on a receiving surface 205 of a first transparent substrate 220 at an angle θ with respect to the receiving surface 205 of the first substrate 220. In FIGS. 2B-D, the axis of angle θ is at the intersection of solar ray 207 and the receiving surface 205 of the substrate 200; θ = 0° when the solar ray 207 is parallel with the receiving surface 205 and increases to the incidence angle of the solar ray 207 when the solar ray 207 impinges non-parallel, as indicated in FIG. 2B. Such is the situation, for example, when the sun rises from the east, from the perspective of a stationary viewer in the northern hemisphere of the earth, looking south. A series of linear, patterned, transparent electrode strips 210a, 210b, 210c, 21Od, 21Oe, and 21Of can be formed on the substrate 220, such that the long axes of the electrodes are substantially parallel. An electric field can be applied to an electro-optic material 250 by applying voltages to the electrodes 210a-f, wherein the reference electrode 230, formed on the substrate 240, is electrical ground. [0030] An index gradient can be created in the electro-optic material 250 that bends the solar rays 207 an angle φ as shown in FIGS. 2B-D, by applying successively increasing or decreasing voltages to electrodes 210a, 210b, 210c, 21Od, 21Oe, and 21Of. The order of increasing or decreasing voltage applied to electrodes 210a-f can depend on the incidence angle of the solar rays 207, and how much refraction is necessary to bend the solar rays 207 to their target (i.e., the photovoltaic 170). In FIG. 2B, the order of increasing voltage applied to the electrodes 210a-f can increase in the order: 210a, 210b, 210c, 21Od, 21Oe, and 21Of for the incidence angle shown. In this implementation, the spatial gradient in the refractive index created in the material 250 is controllable from one side of the electro-optic material 250 (e.g., near electrode 210a) to the other (e.g., near electrode 21Of), due to the electric fields created between each of the electrodes 210a-f and the reference electrode 230.

[0031] The electric field gradient (and therefore the index gradient) is exemplified in FIG. 2B as arrows 252 between the electrodes 210a-f and the reference electrode 230. In this example, the strength of the electric field is indicated by the width of the arrow, where larger arrows indicate higher electric field. The magnitude of the electric field at each location (each arrow 252) can be governed by the voltage applied to electrodes 210a-f. The electro-optic prism 202 in FIG. 2A is the electro-optical analog of a conventional (e.g. , triangular glass or other optical material) prism. The solar rays 207 encountering the index gradient at an angle θ are refracted at an angle φ as shown in FIG. 2B; the magnitude of the index gradient can be controlled via the applied voltages to the electrodes 210a-f, such that the solar rays 207 impinge substantially normal on the surface of light focusing element 140.

[0032] As the sun moves to a position substantially normal to the surface of the substrate 220 (thereby increasing the angle θ to substantially 90°), as shown in FIG. 2C, the index gradient can gradually decrease in magnitude by applying appropriate voltages to the electrodes 210a-f. In this circumstance the solar rays 207 can propagate substantially free of angular steering, such that they impinge normal to the receiving surface 142 of the light focusing element 140.

[0033] FIG. 2D illustrates the reverse process as shown in FIG. 2B, which occurs as the sun continues its course across the sky. Now, the voltages applied to electrodes 210a-f can increase in the order: 21Of, 21Oe, 21Od, 210c, 210b, and 210a. This steers the solar rays 207 an angle φ and can cause the solar rays 207 to impinge substantially normal to the receiving surface 142 of light focusing element 140. [0034] FIGS. 2B-D illustrate how the electro-optic prism 202 can effectively capture solar radiation at a wide range of incidence angles (θ) without necessitating angular adjustment of the receiving surface 205 of the first substrate 220, or other optical components included within the electro-optic prism 202. By this virtue, referring back to FIG. 1, together, the light steering assembly 150, light focusing element 140, and photovoltaic 170 can remain stationary, yet still capture solar rays 120 throughout the day. This is unlike the conventional solar concentrating systems that necessitate physical movement of the components such that they are always facing the sun. [0035] Liquid crystal molecules have a long axis (usually substantially parallel to their polar axis) that may be set in a selected orientation, i.e., the orientation that the liquid crystal molecules will assume under zero applied electric field, by "brushing" one or more alignment layers (for example, a layer of polyimide). Applying an alignment layer aligns the long axes of the liquid crystal molecules near the adjoining surfaces of the liquid crystal layer (i.e., top and bottom of the liquid crystal layer) under zero external field conditions, and subsequently aligns the liquid crystal molecules throughout the volume of the material. The process of aligning the liquid crystal molecules throughout creates a single crystalline domain of the liquid crystal material 250, which is optically anisotropic in refractive index (i.e., birefringent). Birefringence is a is well known optical effect, and arises out of the difference in refractive index which parallel and perpendicular polarization components of light experience while traveling through the liquid crystal with respect to the long (or polar) axis of the molecules.

[0036] In the absence of an applied electric field, light traveling through the liquid crystal (for a given polarization) is primarily steered in a direction governed by the orientation of the liquid crystal molecules, which should be parallel with the alignment layer. Light polarized orthogonal to the liquid crystal director (generally the direction of the long axis of the liquid crystal molecules when they are aligned) experiences substantially no change in refractive index as it passes through the liquid crystal. In most cases, the preferred orientation of the director (when no field is applied) is perpendicular to the electric field, when created.

[0037] An embodiment of an electro-optic prism can include, for nematic liquid crystal, all or some of the elements in FIGS 2A-D. An embodiment of an electro-optic prism can include, for cholesteric liquid crystal, all or some of a substrate 253, electrodes 259, liquid crystal alignment layer 262, liquid crystal layer 265, liquid crystal alignment layer 268, electrode 271, and substrate 274. For electro-optic prisms using cholesteric liquid crystal, a second layer of orthogonally-aligned liquid crystal is not necessary to steer light in one direction (as is shown for the light steering mechanism 295 in FIG. 2E), but may be used in some situations, since an index gradient within a cholesteric liquid crystal layer can refract unpolarized light.

[0038] In one implementation, a solar energy collection assembly, such as that described in reference to FIGS 2A-D above, can use a portion of the collected solar energy for providing the voltages applied to the electro-optic material 250. [0039] Because optical switching speed is not a significant factor in solar steering applications, i.e., the speed at which the liquid crystal molecules align under the influence of the applied electric field, thicker layers of electro-optic material 250 as compared to layers used in other applications can be desirable, as a thicker layer allows for a greater optical phase delay, making larger angular deflections possible.

[0040] Dynamic electro-optic prisms and static prisms described herein can be of either a refractive or diffractive nature, depending on their design and construction, and the implementations described may include either prism type. A difference between the two is that a refractive prism steers light using structures (e.g., electrodes) of a relatively large size compared to the wavelength of light, while diffractive structures steer light using structures of a relatively comparable size to the wavelength of light. The behavior of refractive devices can be adequately described using Snell's law, while the wave nature of light and diffraction theory is used to describe the behavior of diffractive devices. [0041] Referring again to FIG. 2A, an electric field is created in the electro-optic material 250 when a voltage is applied to the electrodes 210, and the electrode 230 is a ground electrode. The electrodes 210 can be linear strips of transparent conducting material. The linear electrodes 210 can be formed using any convenient technique, for example, by photolithography, chemical etching, and the like. The ground electrode 230 can also be a transparent electrode, and in one implementation can be similarly constructed of linear strips of conducting material, or in another implementation, can be a contiguous planar material. In the latter case, the electrodes may be formed by techniques known by those skilled in the art of making planar transparent electrodes, such as by chemical vapor deposition (CVD), sputtering, spin-coating, and the like. In one implementation, the electrodes 210 and 230 are formed from indium tin oxide. [0042] When refraction of incident light rays 207 is desired, such as that shown in

FIG. 2 A, it is desirable to space the linear strips of transparent electrodes 210 a distance that minimizes diffraction of the light rays 207. Diffractive effects become more prominent when the spacing of a gradient approaches the wavelength of incident light. In one implementation, such as that shown for FIG. 2A, the spacing of the electrodes 210 is on the order of three to five microns apart, and the width of each electrode (e.g., each linear electrode 259 in FIG. 2E) can be of the same scale. The length of the electrodes 210 can extend to the boundaries of the substrate 220. In one implementation, a length of the electrodes 210 can be from six to thirty centimeters.

[0043] In certain implementations, a contiguous electrode, rather than strips of individual electrodes, can be used to create the index gradient in the electro-optic material. For example, a variable resistance electrode can be used, which is discussed further below. In this case, the index gradient can be formed by the potential drop from a first end to a second end when voltage is applied to the first end. The index gradient can be formed in a selected direction by applying the driving voltage to a selected end of the variable resistance electrode and grounding the other end. In this manner, sunlight from one direction can be refracted in a selected direction by applying the driving voltage to one end of the variable-resistance electrode. The end to which the driving voltage is applied is then reversed when light rays are incident from the opposite angle. [0044] In other implementations, a variable-thickness electrode can provide the index gradient. A variable-thickness electrode will produce a potential drop from one end to which the driving voltage is applied due to its increasing thickness. The variable- thickness electrode can be placed on a solar ray-receiving surface of a substrate and is substantially transparent. A variable-thickness electrode composed of transparent conducting material can be formed on a substrate by various means known to those skilled in the art, including CVD, dipping, or sputtering.

[0045] The sun's path across the sky, relative to a fixed terrestrial position or plane, is generally not a straight line, but rather an arc that incorporates both diurnal and elevational movement, with the later being more significantly pronounced on a seasonal timescale. The ability to steer light in two dimensions can increase the efficiency of a solar collection assembly (for example, those described above) by providing solar ray steering corrections that account for both diurnal and elevational trajectory. In order to steer light in two directions, a refractive index gradient can be formed in a layer of electro-optic material that mimics the effect of two, orthogonally-aligned electro-optic prisms 202 stacked atop one another. That is, rather than stack one electro-optic prism providing light ray steering in a first direction on top of a second electro-optic prism providing light ray steering in an orthogonal direction, a single electro-optic steering assembly can be used. Stacking two electro-optic prisms together doubles the amount of material that sunlight must pass through, which can result in absorption and reflection losses that decrease the efficiency of the overall device. Such deleterious effects can be minimized or avoided using the single electro-optic steering assembly described below. [0046] FIG. 3 A is one embodiment of an electro-optic steering assembly 300 designed for steering light rays in two dimensions, without requiring the stacking of two electro-optic prisms. The assembly 300 includes an electro-optic material layer 301 (e.g., composed of electro-optic material 250) in optical communication with a transparent, conducting reference electrode 303 (e.g., reference electrode 230). Also in optical communication with the electro-optic material layer 301 is a first electrode array 304 having linear electrodes aligned in a first direction and a second electrode array 306 having linear electrodes aligned in a second direction; between the two electrode arrays 304, 306 is a thin layer of transparent insulating material 305. The linear electrodes are configured the same as or similar to the electrodes 210 described above in reference to FIG. 2A. In the particular implementation shown, two layers of transparent substrate 302 (e.g., substrate 220 in FIG. 2A) form the top and bottom of the electro-optic steering assembly 300 and provide protection (for example, from weather) of the aforementioned electro-optic (301), electrode (303, 304, 306), and insulating (305) layers. [0047] The thin layer of transparent insulating material 305 can be, for example, constructed of glass or plastic. In one implementation, the insulating layer is continuous between the two electrode arrays 304 and 306. In another implementation, the insulating layer is formed from one or more elements that are only present in certain regions as between the two electrodes arrays 304 and 306. For example, in one implementation, the insulating layer is formed from multiple elements present at regions where the linear electrodes of the first electrode array 304 intersect with the linear electrodes of the second electrode array 306.

[0048] The electrodes in the electrode layers 304, 306 can be each individually addressable such that a first voltage gradient can be created in a first electrode layer (e.g. , electrode layer 304), and a second voltage gradient can be created in a second electrode layer (e.g., layer 306). Each voltage gradient created induces a refractive index gradient within the electro-optic material layer 301. The first voltage gradient can be entirely independent of the second voltage gradient, or the voltage gradients can add (if both are present) to create a superposition of electric fields, wherein a refractive index grating is similarly formed from this superposition. Two-dimensional light steering is afforded because each electrode layer 304, 306 contributes a refractive index grating that steers light in one of two orthogonal directions by virtue of the gradient superposition, which is further explained in FIG. 3B.

[0049] A focusing element, such as focusing element 142 in FIG. 1, is excluded from FIG. 3B for clarity, although it should be understood that a light focusing element can be used in conjunction with the electro-optic steering assembly 300, or other configurations thereof. Further, the light focusing element can be transmissive, as has been described with respect to FIG. 1, or reflective (not shown). In embodiments where the light focusing element is reflective, the elements described in the electro-optic steering assembly 300 can be deposited on a reflective surface, such as a mirror, preferably a mirror with a curved surface, which allows for focusing light rays to a point along its optical axis. The principle operation of the light steering assembly 300 remains the same for the embodiments where a reflective focusing element is used: a two- dimensional refractive index grating created in an electro-optic material layer (e.g. , layer 301) allows light to be steered prior to striking the reflective surface so that the light is focused on a chosen target.

[0050] FIG. 3B is a schematic top-view of select elements from the electro-optic steering assembly 300, namely, electro-optic material layer 301, and two orthogonally- aligned electrode layers 304 and 306. The electrode layer 304 includes individually addressable electrodes 304a-f, where the thickness of the lines representing the electrodes 304 in FIG. 3B signifies a magnitude of applied voltage, i.e., electrode 304f has the least voltage applied to it, and 304a holds the highest voltage. The lines indicating the electrodes 304, 306 in FIG. 3B do not indicate actual or relative width of the physical electrodes. Similarly, electrode layer 306 includes individually-addressable electrodes 306a-f with a voltage magnitude represented the width of the line representing the electrode.

[0051] The electrode layers 304 and 306 are shown in FIG. 3B having approximately the same voltage gradient, i.e., the applied voltage is the least on electrodes 304f and 306f and progressively increase towards 304a and 306a respectively, although the gradients are orthogonal. However, in other implementations, the voltage gradients are different as between the two electrode layers. At the intersection of orthogonal electrodes, e.g., electrode 304f and 306f, the electric fields generated by each electrode 304f, 306f can combine to create a superposition of fields in the vicinity of the intersection. This superposition of fields is experienced by the electro-optic material layer 301, and is indicated by the shaded gradient in FIG. 3B. Lighter shading indicates a lesser electric field within the electro-optic layer 301 than darker shading. [0052] FIG. 3B indicates a voltage gradient in the electro-optic material layer 301 that increases upon moving from the bottom right corner to the top left corner of the electro-optic material layer 301 as indicated by line 320. This gradient is created because the relative magnitude of the voltages applied to the individual electrodes 304, 306 increases linearly for each electrode layer 304, 306. Macroscopically, the gradient can be visualized by the dashed lines that run diagonally; for example gradient line 340b runs between the intersection of electrodes 304e and 306e. The gradient is weakest along line 340a because it lies at the intersection of the two electrodes 304f, 306f that have the weakest applied voltage. The dashed line 340c indicates a gradient that is midway between the weakest gradient line 340a, and the strongest gradient 34Od, where the two electrodes intersect having the highest applied voltage (304a, 306a). [0053] The superposition of electric fields experienced by the electro-optic material layer 301 translates to a refractive index gradient in the same direction (i.e., the direction indicated by the arrow 320). The magnitude of the applied voltages to the electrode layers 304, 306 in FIG. 3B are presented as linearly increasing from, for example, 304a to 304f. However, since each electrode 304a-f and 306a-f is individually addressable, the gradient corresponding to the superposition of the two electrode layers 304, 306 can be created in multiple patterns, and/or shapes. In practice, shape of the gradients may only be limited by the number of electrodes that can be formed as the grid shown in FIG. 3 A and 3B, the thickness of the electro-optic layer, and by the achievable change in refractive index of the electro-optic layer upon application of an external voltage.

[0054] The dynamic refractive index gradient patterns that can be produced within the electro-optic material layer 301 by the electrode layers 304 and 306 allow controllable steering of light rays incident at angles other than perpendicular to either electrode layer. FIGS. 4A and 4B illustrate light rays 405 from the sun 401a-e impinging on an electro- optic steering assembly, which, for simplicity is represented by an electro-optic layer 410; the electrode layers are not shown. The electro-optic steering assembly can be configured the same as or similar to the electro-optic steering assembly 300 described above in reference to FIGS. 3A-B, although other configurations are possible. [0055] FIG. 4 A illustrates positions of the sun 401a-e at various times throughout a day. For example, position 401a illustrates the sun rising in the east, and 40 Ie represents the sun setting in the west. The sun's course in FIG. 4A remains substantially within the x-z plane with respect to the terrestrial position of the electro-optic steering assembly 300 as indicated on the supplied coordinate system, and the solar rays emanating therefrom impinge on the electro-optic layer 410 substantially orthogonal to one of the electrode layers (e.g. layer 304 of the electro-optic steering assembly 300). Because in this circumstance there is no azimuthal component to the incidence angle of the sunlight, only one electrode layer is charged, creating a refractive index grating in the x-y plane (where the gradient is along the x-dimension), resulting in light being steered toward target 420 as shown. As the zenith angle φ of the sun changes, the refractive index gradient can be altered, such that impinging light is consistently steered toward the target 420. For example, when the sun is directly above the electro-optic steering assembly 300 ( i.e., position 401c), there may be no voltage applied to the electrode layers, as light steering may not be necessary to strike the target 420. As the sun progresses across the sky to positions 40 Id and 40 Ie, the gradient direction is reversed from that generated in the electro-optic layer 410 when the sun was at positions 401b and 401a.

[0056] FIG. 4B illustrates a situation where the sun's position includes both azimuthal θ and zenith φ components relative to the electro-optic steering assembly 300, i.e., the position of the sun includes does not travel strictly in the x-z plane as in FIG. 4A, but also includes a component in the j-direction. To steer the sun's rays 405 to the target 420 in this case, both electrode layers (e.g., electrode layers 304 and 306) generate a refractive index gradient; the sum, or superposition of these two gratings is illustrated by the hatched portion of the electro-optic layer 410 in FIG. 4B. By applying appropriate voltages to the respective electrodes 304a-f and 306a-f, a two-dimensional refractive index grating can be generated that steers light rays to a chosen target 420, despite the light source's (i.e., the sun's position) with respect to azimuthal angle θ and zenith angle φ.

[0057] In one implementation, the target 420 is a light focusing element. One example of a light focusing element is a Fresnel lens, although other light focusing elements can be used. The light focusing element is in optical communication with the electro-optic steering assembly and is positioned to receive and concentrate light rays after having passed through the electro-optic steering assembly. The electro-optic steering assembly is operable to substantially steer the light rays to the light focusing element at a predetermined angle, e.g., normal to the receiving surface of the light focusing element.

[0058] In one implementation, a solar energy collector, e.g. , a photovoltaic device, is in optical communication with the light focusing element; the light focusing element concentrates the light rays on the solar energy collector. For example, referring to FIG. 5, in one implementation, a solar collection assembly 500 includes an electro-optic steering assembly in optical communication with a light focusing element 502, which is in optical communication with a photovoltaic device 504. In the particular example shown, the electro-optic steering assembly is the electro-optic steering assembly 300 shown in FIG. 3 and described above. Accordingly, the various components of the assembly 300 are label similarly as in FIG. 3. However, it should be understood, that other configurations of the electro-optic steering assembly can be used. Solar rays impinging on the electro-optic steering assembly 300 at different angles of incidence can be effectively steered to impinge on the light focusing element 502 at an ideal angle, e.g., normal, such that a maximum amount of solar radiation can be concentrated on the photovoltaic device 504 in optical communication therewith.

[0059] A feedback mechanism can be used in conjunction with the electro-optic steering assemblies described herein that provide control over the voltages applied to the electrode layers (e.g., electrode layers 304, 306) such that the induced refractive index grating is formed to consistently steer incident light at a target. The feedback mechanism can include computer software and hardware that, for example, measures light intensity on the target and applies appropriate voltages to the individual electrodes within the electrode layers to keep the measured light intensity at a maximum value. Light intensity can be measured at the target by monitoring electrical current generated by the target, if the target is a photovoltaic device, or by photodiodes or similar optical components, for example.

[0060] A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosed embodiments. For example, the embodiments disclosed herein use a focusing element to aid in directing light to a target, e.g., a photovoltaic, however such use of a focusing element is not required, and in some embodiments may not be preferred. [0061] The embodiments of the solar collection assemblies disclosed herein have been described as being more or less stationary, i.e., being mounted on a rooftop or a stationary platform. There is, however, great utility for embodiments of the disclosed solar collection assemblies to be used on mobile platforms, such as automobiles. A tradeoff between solar panel efficiency (energy collection) and aerodynamics must often be made for solar-powered cars that require their solar panels to be pointed at the sun, especially when traveling north-south. An automobile fashioned with solar panels including variants of the electro-optic steering assemblies described above would allow the surface of the automobile to remain essentially maximized for aerodynamic performance while also capturing sunlight energy from multiple incidence angles. Accordingly, other embodiments are within the scope of the following claims.

Claims

WHAT IS CLAIMED IS:

1. A light steering system comprising: an electro-optic steering assembly operable to provide controllable steering of light rays, comprising: a first electrode layer positioned on a first substrate, the first electrode layer comprising a plurality of substantially parallel linear electrodes having longitudinal axes orientated in a first direction; a second electrode layer comprising a plurality of substantially parallel linear electrodes having longitudinal axes orientated in a second direction orthogonal to the first direction; an electrically insulating layer positioned between the first electrode layer and the second electrode layer; a third electrode layer positioned on a second substrate; and a layer of electro-optic material positioned between the second electrode layer and the third electrode layer; wherein a layer of electro-optic material is not provided between the first and second electrode layers.

2. The system of claim 1, wherein a refractive index gradient is generated in the layer of electro-optic material by applying electric potential to the plurality of linear electrodes included in the first and second electrode layers.

3. The system of claim 2, wherein light rays impinging on the electro-optic steering assembly can be controllably steered by controlling the refractive index gradient.

4. The system of claim 2, wherein the refractive index gradient is generated from a superposition of electric fields generated by the first and second electrode layers.

5. The system of claim 1, further comprising a light focusing element in optical communication with the electro-optic steering assembly and configured to receive and concentrate the light rays after having passed through the electro-optic steering assembly.

6. The system of claim 5, wherein the electro-optic steering assembly is operable to steer impinging light rays such that the light rays exit the assembly in a direction substantially normal to the light focusing element.

7. The system of claim 5, wherein the light rays are solar rays impinging on the electro-optic steering assembly with both azimuthal and elevational angles of incidence, the system further comprising: a photovoltaic device in optical communication with the light focusing element, wherein the light focusing element concentrates the solar rays on the photovoltaic device.

8. The system of claim 1, wherein the linear electrodes are transparent.

9. The system of claim 1, wherein the linear electrodes are formed from indium tin oxide.

10. The system of claim 1, wherein the layer of electro-optic material comprises a liquid crystal material.

11. The system of claim 10, wherein the liquid crystal is a cholesteric liquid crystal.

12. The system of claim 10, wherein the liquid crystal is a nematic liquid crystal.

13. The system of claim 1, wherein the electrically insulating layer comprises a plastic layer.

14. The system of claim 1, wherein the electrically insulating layer comprises a glass layer.

15. The system of claim 1, wherein the electrically insulating layer comprises a plurality of electrically insulating elements positioned at regions where the linear electrodes of the first electrode layer intersect the linear electrodes of the second electrode layer.

16. The system of claim 1, wherein the third electrode layer is contiguous across the second substrate.

17. The system of claim 1 , further comprising: a support configured to support the electro-optic steering assembly.

18. The system of claim 17, wherein the support is movable.

19. The system of claim 17, wherein the support is stationary.

20. A method of steering light rays, comprising:

(a) receiving light rays onto a surface of an electro-optic steering assembly comprising: a first electrode layer positioned on a first substrate, the first electrode layer comprising a plurality of substantially parallel linear electrodes having longitudinal axes orientated in a first direction; a second electrode layer comprising a plurality of substantially parallel linear electrodes having longitudinal axes orientated in a second direction orthogonal to the first direction; an electrically insulating layer positioned between the first electrode layer and the second electrode layer; a third electrode layer positioned on a second substrate; and a layer of electro-optic material positioned between the second electrode layer and the third electrode layer; wherein a layer of electro-optic material is not provided between the first and second electrode layers; and

(b) selectively applying voltages to at least one of the first or second electrode layers to selectively adjust a refractive index of the layer of electro-optic material such that the light rays exit the electro-optic steering assembly at a predetermined angle.

21. The method of claim 20, wherein selectively applying voltages to at least one of the first or second electrode layers comprises applying a plurality of voltages to the plurality of linear electrodes providing an electric field varying in intensity across the electro-optic steering assembly.

22. The method of claim 20, further comprising: selecting the predetermined angle such that the light rays impinge on a light focusing element in optical communication with the electro-optic steering assembly substantially normal to a receiving surface of the light focusing element.

23. The method of claim 22, further comprising: using the light focusing element to concentrate the light rays on a solar energy collector.

24. The method of claim 23, wherein the solar energy collector comprises a photovoltaic device.

25. A solar energy collection system, comprising:

(a) an electro-optic steering assembly operable to provide controllable steering of solar rays, comprising: a first electrode layer positioned on a first substrate, the first electrode layer comprising a plurality of substantially parallel linear electrodes having longitudinal axes orientated in a first direction; a second electrode layer comprising a plurality of substantially parallel linear electrodes having longitudinal axes orientated in a second direction orthogonal to the first direction; an electrically insulating layer positioned between the first electrode layer and the second electrode layer; a third electrode layer positioned on a second substrate; and a layer of electro-optic material positioned between the second electrode layer and the third electrode layer; wherein a layer of electro-optic material is not provided between the first and second electrode layers;

(b) a light focusing element in optical communication with the electro-optic steering assembly and positioned to receive and concentrate the solar rays after having passed through the electro-optic steering assembly; and

(c) a photovoltaic device in optical communication with the light focusing element, wherein the light focusing element concentrates the solar rays on the photovoltaic device.

26. The system of claim 25, wherein the layer of electro-optic material is positioned between the electrode layers such that when separately controllable voltages are provided to at least some of the linear electrodes, a gradient electric field is provided within the layer of electro-optic material to cause the electro-optic material to have a refractive index gradient and wherein the refractive index gradient can be controlled by varying the magnitude of the separately controllable voltages provided to at least some of the linear electrodes.

27. The system of claim 26, wherein the magnitude of the separately controllable voltages provided to the linear electrodes is varied based on a position of the sun relative to the electro-optic steering assembly.

28. The system of claim 25, wherein the layer of electro-optic material has a substantially uniform thickness.

29. The system of claim 25, wherein the electro-optic material comprises a liquid crystal material.