Curvilinear prismatic film which eliminates glare and reduces front-surface reflections for solar panels and other surfaces

This invention is a novel transparent curvilinear prismatic film which eliminates glare and reduces front-surface reflections when applied to solar panels and other surfaces with a transparent adhesive. The new prismatic film comprises an exposed top surface with substantially triangular prisms following a curvilinear path across the top surface and a bottom surface which is substantially flat and smooth. The bottom surface is configured to enable a transparent adhesive bond to the solar panel or other surface beneath the prismatic film to eliminate glare and to reduce front-surface reflections from this solar panel or other surface. The curvilinear prismatic film is configured to be transparent, thin, lightweight, and inexpensive.

CROSS REFERENCE TO RELATED APPLICATIONS

A provisional patent application was previously filed with the U.S. Patent and Trademark Office by the inventor disclosing key elements of the present invention. Application No. 63/064,499, entitled “Curvilinear Prismatic Film Which Eliminates Glare and Reduces Front-Surface Reflections for Solar Collectors and Other Surfaces,” was filed Aug. 12, 2020. The inventor claims the filing date of this provisional application for the key elements disclosed in this provisional application.

BACKGROUND OF THE INVENTION

This invention is a novel transparent curvilinear prismatic film which eliminates glare and reduces front-surface reflections when applied to solar panels and other surfaces with a transparent adhesive. The new prismatic film comprises an exposed top surface with substantially triangular prisms following a curvilinear path across the top surface and a bottom surface which is substantially flat and smooth. The bottom surface is configured to enable a transparent adhesive bond to the solar panel or other surface beneath the prismatic film to eliminate glare from this solar panel or other surface. The curvilinear prismatic film is configured to be transparent, thin, lightweight, and inexpensive.

The new prismatic film achieves mitigation of glare by using curvilinear prisms. The curvature of such prisms scatters the light reflected from such prisms in a variety of azimuthal directions, thereby preventing glare which is due to reflection from flat surfaces oriented in the same direction. The triangular prism shape minimizes front-surface reflection losses by reducing the incidence angle of incident rays and by redirecting front-surface reflected rays into the adjacent prism rather than outward.

In this patent application, a solar panel is defined as an assembly of electrically interconnected photovoltaic cells integrated into a common structure. For space applications, such cells are generally multi junction photovoltaic devices which are typically covered on their exposed surface with individual thin ceria-doped glass windows, one per cell. For ground applications, such cells are generally single-junction silicon photovoltaic devices which are typically covered with one large glass window per panel. For space applications, the present invention will typically replace all of the individual cell glass windows with one curvilinear prismatic window attached with a transparent adhesive over the full panel area. For ground applications, the present invention will typically be attached with a transparent adhesive to the conventional glass window over the full panel area. In the above-referenced provisional patent application, the inventor used the term “solar collector” rather than “solar panel” but the intended meaning is the same. Solar panel seems to be the more widely accepted term.

When applied to a solar panel, the new curvilinear prismatic sheet reduces the reflection loss from the exposed surface of the glass window which is typically the outermost surface of most solar panels. The reflection loss is reduced by the present invention for all incidence angles of sunlight, thereby leading to greater collection of solar energy by the panel and a correspondingly greater electrical power output by the panel. Therefore, the value of the solar panel is enhanced. This enhancement is greatest for non-sun-tracking solar panels which experience the largest variation in incidence angles for solar rays through the day and through the year. The enhancement is greater for larger solar incidence angles.

The reduction in glare is important for ground-based solar arrays which are being deployed in increasing numbers around the world. The glare problem is severe when such arrays are near roadways or airports or occupied buildings. The present invention may be applied in the field to resolve the glare problem after the solar array has been installed, or in the factory to preclude the glare problem after installation.

In space applications, the present invention can not only be used for solar panels but also for other surfaces on spacecraft, to eliminate glare and to minimize front-surface reflection losses. When applied to a solar panel in space, the reduction in reflection losses leads to greater energy collection and greater value for the solar panel, while also saving weight and cost compared to the individual cell glass covers typically used. The reduction in glare is also important, especially for spacecraft in low-earth orbit such as the constellation of thousands of Starlink® spacecraft now being deployed by Space X to provide global internet coverage. The glare from such spacecraft is causing huge problems for earth-based telescopes, as widely reported in news articles over the past two years. If the glare is from surfaces other than solar panels, the present invention can be applied to such surfaces to minimize glare.

For ground-based applications, the new curvilinear prismatic film can be produced by low-cost, roll-to-roll embossing of thermally softened thermoplastic material such as acrylic, polycarbonate, thermoplastic polyurethane, or thermoplastic fluoropolymer such as ETFE or ECTFE. This process is well known for making prismatic road sign sheeting and similar products.

For ground-based applications, the curvilinear prisms are configured to allow rainwater or wash water to run downward and off the prismatic film, thereby minimizing dirt buildup on the exposed surfaces. For space-based applications, the curvilinear prisms are configured to allow cleaning liquids to run downward and off the prismatic film, thereby minimizing dirt retention on the exposed surfaces prior to launch into space.

For space-based applications, the new curvilinear prismatic film can be produced by casting a space-qualified silicone polymer against disposable molding tools made by low-cost, roll-to-roll embossing of thermoplastic material. This process has been previously developed and proven for making space Fresnel lenses by the inventor under NASA funding.

For curvilinear prisms with 45 degree tilt angles on both faces of each prism, the curvilinear prismatic film produced by low-cost, roll-to-roll embossing of thermoplastic material can be used directly as the prismatic window for terrestrial solar panels or used indirectly as the molding tool for the prismatic window for space solar panels, since both the embossed thermoplastic material and the material molded against the embossed thermoplastic material have essentially the same geometric configuration. This dual-use design is out preferred prism geometry.

In summary, the present invention solves the glare problem for solar panels and other surfaces both on the ground and in space. Furthermore, the present invention reduces front-surface reflections thereby enhancing the performance of solar panels on the ground and in space.

BRIEF SUMMARY OF THE INVENTION

This invention is a novel transparent curvilinear prismatic film which eliminates glare and minimizes front-surface reflections from solar panels or other surfaces to which the new film is bonded with a transparent adhesive.

The invention is a thin polymer film with triangular prisms on the exposed upper surface with such prisms following curvilinear paths. The bottom surface of the polymer film is smooth and flat to facilitate bonding to the underlying solar panel or other surface using a transparent adhesive.

The prismatic structure of the invention minimizes front-surface reflections and thereby enhances the performance and value of a solar panel to which it is applied. Such a solar panel may be on the ground or in space.

The curvilinear prismatic structure of the invention eliminates glare from the solar panel or other surface to which it is applied.

The curvilinear prismatic film of the present invention is configured for mass production by continuous roll-to-roll embossing of thermoplastic material, which may be used directly for ground-based applications or used as disposable molding tools for space-based applications. The invention solves major glare problems for both ground-based and space-based solar arrays and other surfaces.

The present invention can also be applied to space solar arrays to not only eliminate glare but to also improve performance by minimizing front-surface reflections using the optical effects shown previously inFIGS. 2, 3, 5, and 6.

For space applications, the curvilinear prismatic film can be protected from space environmental effects, including atomic oxygen, ultraviolet radiation, and charged particle radiation with a thin protective coating. Such coatings have been developed and proven in space for other applications, including silicone Fresnel lenses for NASA applications.

DETAILED DESCRIPTION AND BEST MODE OF IMPLEMENTATION

The present invention is a transparent curvilinear prismatic film which eliminates glare and reduces front-surface reflections for underlying solar panels and other surfaces.FIG. 1shows in isometric view the presently preferred embodiment of the invention. The invention includes a top surface comprising many curvilinear prisms1each following a curved path across the top surface of the film. The prisms2are triangular in shape when viewed in cross section. The curvilinear prisms1are oriented to facilitate runoff of liquids, including rain or cleaning liquid, using gravity to enable the liquid to run downward easily in the valleys between prisms2. The shape of the prisms2is triangular in cross section with typical angles for the two sides of the triangle of about 45 degrees relative to the plane of the prismatic film. The typical angles used to define the sun position relative to the prismatic film are azimuth (az) and elevation (el) as defined inFIG. 1.

The transparent prismatic film is configured to be thin, lightweight, low-cost, and easily mass-produced. The curvilinear prisms1with triangular cross-sectional shape2are shown in an enlarged manner inFIG. 1to make them visible. In actuality, the prisms2will be extremely small, on the order of 100 micrometers wide by 50 micrometers tall. Such small prisms make the film very thin and minimize material content and therefore cost. The curvilinear prismatic film is configured to be easily mass-produced by a high-speed, low-cost, roll-to-roll embossing process, which uses patterned and polished rollers to impart the prismatic pattern on the top surface and a smooth, flat back surface onto thermally softened polymer. The best thermoplastic polymers for ground applications are acrylic, polycarbonate, thermoplastic polyurethane, and fluoropolymers such as ETFE and ECTFE. Many companies make these thermoplastic materials in huge quantities and sell them at commodity prices.

For space applications, the curvilinear prismatic film will be made of a transparent space-qualified material such as silicone. Dow Corning makes one such silicone known as Sylgard 93-500. Other firms make competing space-qualified silicones. To produce the curvilinear prismatic film for space applications, a two-stage process will be used. First, a high-speed, low-cost, roll-to-roll embossing process will be used to make the “negative” pattern for the desired curvilinear prismatic film. The embossed product will be made of one of the common thermoplastics such as acrylic. This embossed product will be used as a disposable molding tool to make the final silicone film to be applied to the surface of the spacecraft. The inventor has perfected this process to produce silicone Fresnel lenses for NASA and other space customers. The final silicone product can be made as a stand-alone film prior to bonding it to the spacecraft surface with a transparent adhesive, or it can be molded directly against the spacecraft surface to save the thickness and mass of a transparent adhesive.

The new curvilinear prismatic film offers two distinct benefits for both ground and space applications:1. Significant reduction in front-surface reflections2. Elimination of glare.

FIGS. 2 and 3show in cross-sectional view how the prisms2minimize front-surface reflections for incident rays4at both high lateral incidence angles and small lateral incidence angles, respectively. For high lateral incidence angles, the prisms2present a more normal surface to the incident rays4than a non-prismatic surface would experience. This minimizes front-surface reflection losses which are known by those of ordinary skill in the art of optics to be much lower for more normal incidence angles than for more grazing incidence angles. The incident rays4are refracted by the surface of the prism2to become refracted rays5. Some of rays5go directly to the bottom surface3of the film, while others are first totally internally reflected (TIR) by the opposing side of prism2before proceeding to bottom surface3. The overall optical effect of the prisms2is to deliver the incident rays4very efficiently to the back surface3of the curvilinear prismatic film.

Similarly, for incident rays4arriving at the prismatic surface with small incidence angles,FIG. 3shows that these rays4are also delivered very efficiently to back surface3by a number of different phenomena. Some of the rays5enter the prismatic film and are refracted before proceeding to the back surface3. Others of the rays6are reflected by the front surface of the prism2and proceed to the adjacent neighboring prisms2. At the neighboring prisms2, these front-surface reflected rays6enter the neighboring prisms2and proceed to the back surface3after refraction and in some cases total internal reflection (TIR) from the opposite side of the neighboring prisms2. The overall optical effect of the prisms2is to deliver the incident rays4very efficiently to the back surface3of the curvilinear prismatic film.

The avoidance of glare from the new curvilinear prismatic film is due to the curvature of the prisms1as they run across the top surface of the film, as shown in a top view of the film inFIG. 4. For large lateral incidence angles of sunlight onto the curvilinear prismatic film, some rays will be reflected by the front surface of the prisms beyond the neighboring prisms2. For such a situation, incident rays4are approximately parallel to each other until they encounter the curved surface of the curvilinear prism1. Front-surface reflected rays7then depart in a wide variety of circumferential angles which eliminates glare for a distant observer.

FIGS. 5 and 6quantify the remarkable improvement in net transmittance of incident rays4into the curvilinear prismatic film compared to conventional flat surfaces made of the same transparent material. The curvilinear path assumed for the prisms inFIGS. 5 and 6is a cosine curve with a minimum slope of −45 degrees and a maximum slope of +45 degrees. The difference betweenFIG. 5andFIG. 6is the assumed refractive index of the polymer, namely 1.40 forFIG. 5and 1.49 forFIG. 6. The lower index corresponds to materials like silicone and ETFE. The higher index corresponds to materials like acrylic and thermoplastic polyurethane. Regardless of the value of the refractive index of the polymer, the improvement in net transmittance of incident rays4into the polymer and onto the bottom surface3is remarkable, for all solar azimuth and solar elevation angles. When the smooth bottom surface3of the new curvilinear prismatic film is bonded to a solar panel with a transparent adhesive, the solar panel performance will correspondingly improve with the increase in net transmittance. The solar panel will therefore produce more usable energy output and provide greater value to the owner.

FIGS. 7 and 8show the results of further analysis by the inventor for alternate curvilinear curve shapes corresponding to a cosine curve with a minimum slope of −30 degrees and a maximum slope of +30 degrees. These results for two assumed values of refractive index, namely 1.40 forFIG. 7and 1.49 forFIG. 8. The gain in performance compared to a conventional cover glass is substantial for all solar azimuth and elevation angles, but is not quite as high as for the slightly larger slope cosine paths corresponding toFIGS. 5 and 6. The presently preferred path is therefore the path corresponding toFIGS. 5 and 6. Still larger slope cosine paths could be chosen but drainage of liquid might be impeded. Liquids may be used to clean space prismatic windows prior to launch, and such liquids need to easily drain. Rain needs to easily drain from ground-based prismatic windows.

For comparison to the present invention,FIG. 9shows similar results for linear prisms. Linear prisms suffer from a weak spot when both solar elevation and solar azimuth angles approach zero. At this condition, the transmittance into the prismatic structure is zero, just the same as for a conventional glass window, as shown inFIG. 9. Similarly, linear prisms do not eliminate glare. The present invention therefore provides substantial benefits over linear prisms and other prismatic structures.

FIG. 10provides geometrical details of one preferred embodiment of the present invention. The prisms are small and have an included angle of 90 degrees at each apex. The repeating curvilinear path is a cosine curve with minimum and maximum slopes of −45 and +45 degrees, respectively. The prismatic paths are offset vertically by the maximum width of each prism. The prism width varies continuously over the cosine path from a maximum value of about 0.0100 cm where the slope is zero to about 0.0071 cm where the slope is a minimum of maximum value. The base thickness is shown as 0.0100 cm, but this is just a typical value which can vary widely depending on the material and application.

Perhaps more important than the reduced front-surface reflections provided by the new curvilinear prismatic film is the minimization of glare provided by the invention.FIG. 11shows photographs of severe glare problems caused by ground-based solar panels (FIG. 11-A) and by space-based surfaces of low earth orbit (LEO) spacecraft (FIG. 11-B). The new curvilinear prismatic film can solve both of these problems in a simple, low-cost, low-mass manner. The preferred embodiments of ways to apply the invention to both ground-based and space-based surfaces to minimize glare is shown inFIG. 12. For ground-based solar panels, which typically have a glass or plastic front window surface, the glare can be severe for certain times of day and days of the year. The curvilinear prismatic film can be applied as shown schematically inFIG. 12-A to solve this glare problem. The film is attached to the glass or plastic window surface using a transparent adhesive to bond the back surface of the film3to the window. The transparent adhesive can be selected from a pressure sensitive adhesive (PSA), a liquid adhesive, or a spray-on adhesive. To promote adhesion, the window surface and the film surface can be treated with primers or other surface treatments.

For space-based applications, the curvilinear prismatic film can be applied to spacecraft surfaces that would otherwise be plagued with glare, as shown inFIG. 12-B. For space applications, the curvilinear prismatic film will need to be made of a space-qualified material like silicone, such as Dow Corning Sylgard 93-500. One preferred embodiment method of applying such a silicone film to a spacecraft surface is to mold it in place in the factory where the spacecraft is manufactured. The disposable molding tools can be made by the low-cost, high-speed, roll-to-roll embossing process using a thermoplastic material such as acrylic. This method results in the thinnest and lowest mass curvilinear prismatic film for the spacecraft surface to be protected from glare. An alternate method is to make the silicone curvilinear prismatic film as a stand-alone product and apply it to the spacecraft surface with a transparent adhesive, as shown inFIG. 12-B. The transparent adhesive can be the same silicone material used to make the film, and the surfaces to be bonded together may be treated with primer to promote aggressive adhesion.

The discussion and description of preferred embodiments in previous paragraphs is meant to be exemplary but not exclusionary. Those of ordinary skill in the art of optics will recognize that may other prismatic geometries, dimensions, materials, and methods of manufacture can be used to accomplish the basic improvements in reflection reduction and glare elimination by using the fundamental new approach of curvilinear prismatic films disclosed for the present invention. All of these variations fall within the scope and spirit of the present invention.