WO2008147937A1

WO2008147937A1 - Redirection of light incident on a solar cell module

Info

Publication number: WO2008147937A1
Application number: PCT/US2008/064623
Authority: WO
Inventors: Bernhard Piwczyk
Original assignee: American Solar Technologies, Inc.
Priority date: 2007-05-23
Filing date: 2008-05-23
Publication date: 2008-12-04
Also published as: TW200915589A

Abstract

A structure and methodology for disposing a light scattering structure in solar cell modules. The light scattering structure is disposed in spaces between multiple solar cells and redirects incident light from the spaces onto the solar cells, thus concentrating light onto the solar cells.

Description

REDIRECTION OF LIGHT INCIDENT ON A SOLAR CELL MODULE

TECHNICAL FIELD

The present invention relates to a method and apparatus to use scattering and reflection to redirect light incident on a solar cell module. More specifically, it relates to a polymer film incorporating certain surface structures or particles embedded within the film to cause incident light to be scattered preferentially at angles greater than the critical angle of θ from the normal to the surface of the film, wherein θ is the angle at which total internal reflection of light occurs in the transparent medium covering the solar cell module.

BACKGROUND OF THE INVENTION

Redirection of light has been accomplished in the prior art by reflective grooves embossed in plastic and over-coated with a metallic thin film. Diffractive surfaces have also been used to serve the same purpose. Another methodology uses a so-called Lambertian light scattering surface. This methodology is essentially a white surface using finely dispersed particles, of TiO2 or AI2O3, for instance, to scatter light impinging on the surface. In the case of a solar cell module, having a front glass cover of a given thickness, any light scattered at an angle smaller than the critical angle θ, which in glass is about 42 degrees to a normal to the surface, is lost for conversion into electrical power because it exits the front glass surface, but any light scattered at a larger angle will be redirected toward an adjacent solar cell by total internal reflection.

When a radiator or reflector has a luminance independent of the viewing (or illuminating) angle, it is said to be perfectly diffuse. If it is plane, its apparent area, and therefore its intensity, will vary with cos θ, where θ is the angle between the normal to the surface and the direction of viewing. Such a reflector is said to obey Lambert's law: Where (I) is the intensity of the light scattered at a given angle, and Io is the intensity of the incident light.

Lambert's law applies if the surface scatters light equally in all directions. Certain surfaces can be constructed that do not obey Lambert's law. This is the case for projection screens that are coated with small spheres of glass. Here a much larger proportion of light is reflected in the direction of the incident light than at greater viewing angles. Such a screen does not obey Lambert's law and can be referred to as Non-Lambertian. It is predictable that preferred scattering can occur if (i) particles of certain optical properties, due to the shapes, surface morphology or refractive index of the particles, can be incorporated in the surface, (ii) these particles by themselves have reflective properties that are directionally preferential and (iii) they can be oriented so that they tend to reflect or scatter light at angles greater than the critical angle of the transparent medium through which the light travels. That is to say, that, if these particles are embedded in a transparent polymer layer and if these particles have directionally preferential reflective properties, Lambert's law can be violated. A detailed discussion of the physics involved is given by L. Levi, Applied Optics, Vol. 1 , p. 335-342, John Wiley & Sons, 1980, which is incorporated herein by reference.

Another methodology to produce preferred scattering employs a reflective surface that has preferential reflective properties. Such a surface can be generated by crystallizing certain chemicals or salts on a surface. The specific chemistry is based on the shape or form of the crystal that is formed so that the facets of the crystal tend to reflect light in an angle larger than a designated angle with respect to the normal to the surface. This is mainly a function of the crystal morphology of a given chemical or salt.

In addition, the orientation of the facets of a given crystal can be influenced by special seeding techniques. The surface formed by such crystallization can be used directly after over-coating with a thin reflective coating on the surface or the surface can be replicated by Ni plating and further replication in a polymer film with application of a reflective coating.

Another related methodology is the incorporation of small, even micron sized, bubbles in an optically clear polymer film. Spherical bubbles in themselves will impart significant scattering properties to an optically clear polymer film. Moreover, these bubbles can be made to depart from a spherical shape by the film extrusion process or other means, thus imparting optical properties in the film that further deviate from Lambert's law.

Yet another methodology is the incorporation of asymmetric or platelet type light reflecting particles into the polymer film. These particles when given suitable electrical or magnetic properties can be oriented within the film, by means of electrostatic or magnetic forces during the film formation process, to impart anisotropic light scattering properties to the film. A similar effect can also be achieved if particles are incorporated in the polymer film in a random orientation and the polymer film is then extruded or blown. During the extrusion or blowing process, the platelet particles will be oriented in a preferential way. The flat or large surfaces areas of the particles will be preferentially oriented within the film thereby imparting reflective properties not conforming to Lambert's Law. The film or foil resulting from this process may be provided with a reflective coating on the side opposite to the incident light so that light is reflected/scatteredpreferentiallytoward the front glass cover of a solar module.

The above methodologies present a significant improvement over a Lambertian diffuser/reflector without the use of expensive and complex manufacturing techniques as required for a diffractive foil while offering similar advantages to diffractive foils. Further development of a methodology resulting in optimal preferential scattering/reflection is to be expected. This is subject to detailed research and re-crystallization techniques and polymer film fabrication.

As shown in FIG. 1 , solar cell modules 10 comprise multiple solar cells 12 with spaces 14 between adjacent solar cells 12. It is a purpose of the present invention to decrease the cost per watt of the electricity produced by solar cell modules by causing incident light 16 striking the solar cell module 10 in a space 14 between solar cells to be redirected to a solar cell 12. In the case of a solar cell module having a front glass cover of a given thickness, any light scattered at an angle smaller than the critical angle θ, which is about 42 degrees, to a normal to the surface, is lost for conversion into electrical power because it exits the front glass cover, but any light scattered at a larger angle will be redirected toward an adjacent solar cell by total internal reflection. The critical angle θ in a first transparent medium is dependent on the refractive index of the first medium and the refractive index of the second transparent medium forming a boundary with the first medium.

SUMMARY OF THE INVENTION

The present invention relates to a structure and methodology for disposing a light scattering structure in solar cell modules. The light scattering structure is disposed in spaces between multiple solar cells and redirects incident light from the spaces onto the solar cells, thus concentrating light onto the solar cells.

The light scattering structure comprises a polymeric film having (i) a light scattering surface or light scattering structures within the film and may also be provided with (ii) a light reflecting coating layer disposed over the back of the film. In a preferred embodiment, the light scattering surface comprises a three-dimensional pattern selected to scatter light preferentially at angles greater than the critical angle to a normal to the surface, wherein the critical angle θ is the angle at which total internal reflection of light occurs in the transparent medium in which the light travels. In another embodiment, the film contains light scattering structures within the body of the film to scatter light preferentially at angles greater than the critical angle θ to a normal to the surface, wherein the critical angle θ is the angle at which total internal reflection of light occurs in the transparent medium in which the light travels.

A preferred film or foil structure, which is to have a front glass cover, is a polymer film from 13 - 625 μm (0.5 - 25 mils) in thickness and transparent in the solar spectrum from 400-1000 nm incorporating a light scattering surface designed to scatter light preferentially at angles greater than about 42 degrees with respect to the normal to the surface of the film and having a thin light reflective coating over the back of the film or foil. In another embodiment, the film or foil incorporates particles, preferably from 0.1 - 800 μm in diameter, of certain shape and or optical properties to cause light to scatter preferentially at angles greater than about 42 degrees with respect to the normal to its surface. In the latter case, a light reflecting coating is deposited on the side of the film or foil away from the light incident side of the foil or film and the polymer film is light transparent.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will be better understood by reading the following detailed description, taken together with the drawings wherein preferred embodiments are shown as follows:

FIG. 1 is a schematic (top) view of a solar cell module incorporating a preferred embodiment of the present invention; and

FIG. 2 is a schematic (side) view of a solar cell module incorporating a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of the present invention comprises a light scattering film or foil for responding to incident radiation with a substrate having a light scattering surface and a light reflecting layer such as aluminum or silver, or a dielectric coating comprised of either a single, or, preferably multiple, layers. In embodiments employing metallic layers, an insulation layer of silicon dioxide, aluminum oxide, magnesium fluoride, polymer, or other electrically nonconductive material may be disposed over the metal coating layer to prevent corrosion of the metallic coating by way of oxidation and reactions involving moisture, and to prevent the flow of electrical current within the solar module along undesired pathways. The light scattering film or foil comprises a three-dimensional pattern selected to scatter incident light radiation with substantial efficiency preferentially at angles with respect to the surface normal greater than the critical angle θ for the transparent medium covering the film or foil. The scattering film or foil can be formed by many three dimensional surface or topographic shapes at various in-plane orientations and having different heights. These features may have random or ordered orientations.

In another preferred embodiment of the present invention, a light scattering film or foil for responding to incident radiation comprises a substrate having an essentially flat surface but incorporating small particles within the thickness of the film or foil that have optical properties resulting in the scattering of incident light with substantial efficiency preferentially at angles with respect to the surface normal greater than the critical angle θ for the transparent medium covering of the film or foil. The film or foil surface opposite to the light incident side of the film or foil may be coated with a light reflecting coating. The reflective coating may comprise a metallic layer such as aluminum or silver, or a dielectric coating comprised of either a single, or, preferably multiple, layers. In embodiments employing metallic layers, an insulation layer of silicon dioxide, aluminum oxide, magnesium fluoride, polymer, or other electrically nonconductive material may be disposed over the metal coating layer to prevent corrosion of the metallic coating by way of oxidation and reactions involving moisture, and to prevent the flow of electrical current within the solar module along undesired pathways.

In an application of a light scattering film or foil to solar cell modules, as shown in FIG. 1 , the light scattering film or foil is disposed in spaces 14 between multiple solar cells 12 and redirects light 16 incident on the spaces 14 onto the solar cells 12, thus concentrating solar radiation onto the cells 12. Accordingly, a solar cell module 10 of a preferred embodiment, as shown in FIG. 2, comprises a support structure having a planar surface 20 and a plurality of solar cells 12 overlying the planar surface, the cells having front 22 and back surfaces 24 with the back surfaces 24 facing the planar surface 20, the cells being spaced from one another, with predetermined areas 14 of the planar surface free of solar cells. The solar cell module further includes a transparent cover member 28, in this embodiment glass, overlying and spaced from the solar cells 12, having front surface 30 disposed toward incident radiation, and a light scattering optical film or foil 32 overlying the predetermined areas 14 of the planar surface 20. The light scattering film or foil 32 is incorporated within coating layers 34 disposed over the light scattering film or foil 32. The light scattering film or foil 32 comprises a relief pattern selected to scatter incident radiation preferentially with substantial efficiency at angles 40 larger than about 42 degrees with respect to the surface normal, the critical angle 8 for total internal reflection at the boundary of the air and the front surface 30 of the glass cover 28. The refraction index of the coating layer 34 is chosen such that when compared to the refractive index of the glass cover number 28, light is allowed to pass through, and not be reflected at, the boundary of the glass cover number 28 and the coating layer 34.

With the present approach, much of the incident radiation incident on the spaces between the solar cells is redirected from the spaces onto the solar cells, thus increasing the overall power production of the solar cells. Other advantages of the present approach include ease of fabrication, low cost of fabrication, ease of use, wide angle of acceptance of the light scattering and light redirecting element, and reduction of the necessity of mechanical tracking of the sun by continuous adjustment of the solar module to maintain the effectiveness of the light scattering element over substantial variations in the angle of incidence of solar radiation during passage of the sun during the day.

While the principles of the invention have been described herein, it is to be understood by those skilled in the art that this description is made only by way of example and not as a limitation as to the scope of the invention. Other embodiments are contemplated within the scope of the present invention in addition to the exemplary embodiments shown and described herein. Modifications and substitutions by one of ordinary skill in the art are considered to be within the scope of the present invention.

Claims

CLAIMSWhat is claimed is:

1. A non-Lambertian light scattering structure for concentrating light onto solar cells within a solar cell module comprising a polymeric film, the polymeric film comprising a light scattering surface or light scattering structures within the film, whereby incident light striking a space between solar cells is redirected onto solar cells by internal reflection, thereby concentrating the light.

2. The structure of claim 1 , the light scattering surface comprising a three- dimensional pattern selected to preferentially scatter light at angles greater than the critical angle to a normal to a film surface, wherein the critical angle is the angle at which total internal reflection of light occurs at a front or outer surface of a transparent cover member of a solar module.

3. The structure of claim 2, further comprising a light reflective coating on a back side of the polymer film, and wherein: the polymer film is between 13 and 625 μm in thickness; the polymer film is transparent to light of wavelengths between 400 and 1000 nm; and the light scattering polymer film scatters light preferentially at angles greater than about 42 degrees with respect to the normal to the film surface.

4. The structure of claim 1 , wherein the light scattering structures within the film are selected to preferentially scatter light at angles greater than the critical angle to a normal to a film surface, wherein the critical angle is the angle at which total internal reflection of light occurs at a front or outer surface of a transparent cover member of the solar module.

5. The structure of claim 1 , wherein the light scattering structures within the film are particles between 0.1 and 800 μm in diameter and of a shape or optical property to cause light to scatter preferentially at angles greater than about 42 degrees with respect to a normal to the film surface.

6. The structure of claim 1 , further comprising a light reflective coating on a back side of the polymer film.

7. The structure of claim 1 , the light reflective coating comprising an aluminum or silver layer, the structure further comprising an insulation layer on the reflective coating layer to prevent corrosion of the reflective coating and the conduction of electrical current within the reflective coating layer.

8. The structure of claim 7, wherein the insulation layer is made of one taken from the group consisting of silicon dioxide, aluminum oxide, magnesium fluoride, and/or a polymer.

9. A solar cell module comprising: a support structure having a planar surface; a plurality of solar cells overlying the planar surface, the solar cells comprising a cell back surface with the cell back surface facing the planar surface, and the solar cells spaced apart from one another thereby defining predetermined areas free of solar cells; a transparent cover member overlying and spaced from the solar cells having a cover front surface disposed toward incident light; and a light scattering film or foil overlying the predetermined areas, the light scattering film or foil incorporating particles selected to scatter incident light preferentially at angles larger than about 42 degrees with respect to a surface normal whereby incident light striking a space between solar cells is redirected onto solar cells by internal reflection, thereby concentrating the light.

10. The solar cell module of claim 9 wherein the transparent cover member is glass.

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11. The solar cell module of claim 9 further comprising two coating layers, the light scattering film or foil and solar cells being disposed within the coating layers, the coating layer selected to have a refractive index that does not permit reflection at the coating layer / transparent cover boundary.

12. The solar cell module of claim 9, the light scattering film or foil further comprising chemical crystals selected to reflect light at angles larger than about 42 degrees with respect to a surface normal.

13. The solar cell module of claim 9 comprising a light scattering film, the light scattering film comprising: optically clear polymer film; and bubbles within the optically clear polymer film, thereby imparting non- Lambertian properties to the light scattering film.

14. The solar cell module of claim 9 comprising a light scattering film, the light scattering film comprising: optically clear polymer film; and asymmetric light reflecting particles or platelets within the optically clear polymer film such that a major surface of the particles or platelets are preferentially oriented to impart non-Lambertian properties to the light scattering film.

15. A method of making a non-Lambertian light scattering structure for concentrating light onto solar cells within a solar cell module comprising the steps of: providing a reflective surface; and crystallizing chemicals within a film, the chemicals selected so that crystal facets reflect light in an angle larger than a designated angle with respect to a normal to the reflective surface.

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16. The method of claim 15 further comprising the step of seeding the crystallizing chemicals to influence the desired orientation of the crystal facets.

17. The method of claim 15 further comprising the step of over-coating the reflective surface having the crystallized chemicals with a second reflective coating, and applying a polymer coating to the second reflective surface, thereby producing a polymer film or foil incorporating a non-Lambertian reflective surface.

18. The method of claim 15 further comprising the steps of: nickel plating the reflective surface having the crystallized chemicals to make a negative; and applying a polymer coating to the negative; and removing the polymer coating in the form of a film or foil; and applying a reflective coating to the polymer film or foil to make a copy.

19. A method of making a non-Lambertian light scattering structure for concentrating light onto solar cells within a solar cell module comprising the steps of: providing an optically clear polymer film; incorporating spherical bubbles within the polymer film; and extruding the polymer film to distort the bubbles sufficiently to impart non-Lambertian properties to the film.

20. A method of making a non-Lambertian light structure for concentrating light onto solar cells within a solar cell module comprising the steps of: providing an optically clear polymer film; incorporating asymmetric or platelet light reflecting particles within the polymer film; and applying electrostatic or magnetic forces to the polymer film to preferentially orient the particles to impart anisotropic light scattering properties to the film.

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21. A method of making a non-Lambertian light structure for concentrating light onto solar cells within a solar cell module comprising the steps of: providing an optically clear polymer film; incorporating asymmetric or platelet light reflecting particles within the polymer film; and extruding or blowing the polymer film to orient the particles in a preferential way.

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