Redirecting optics for concentration and illumination systems

An optical system having an optical waveguide for collecting light, a receiver for receiving the light, and redirecting optics for transferring the light from the optical waveguide to the receiver. The optical system can be used for concentrating light such as in solar applications. The optical system can also be used for diffusing light in illumination applications by replacing the receiver with a light source such that the light flows in the reverse of the concentration system.

FIELD OF THE INVENTION

The invention is directed to the field of optics for light collection and delivery. Applications include concentration of sunlight onto photovoltaic or thermal receivers, and diffusion of light for illumination applications.

BACKGROUND AND SUMMARY OF THE INVENTION

Edge collectors or optical waveguides are used for collection and concentration of light; in particular, sunlight. An edge collector or optical waveguide is defined for this application as an optical device that receives light from a top surface, and delivers the concentrated energy to the edge of the device.FIG. 1ashows a simple schematic of the cross section of an optical waveguide10.FIG. 1bis a 3D representation of the same optical waveguide10.

In practice, these types of optical waveguides10are generally of the type described in U.S. Pat. Nos. 7,664,350 and 7,672,549. Other types of optical waveguides include luminescent solar concentrators, or dye luminescent solar concentrators.FIG. 1cshows an optical system of the former type. Input light20falls on multiple concentrating units40across the aperture, and the waveguide10collects the concentrated light from all the units and delivers it to an edge30of waveguide10.

However, there are many advantages to having a secondary set of optics50(seeFIGS. 2 and 3a) at the edge30to redirect the light20in a favorable manner. InFIG. 2, the light20is delivered to the edge30of the waveguide10and is redirected approximately 90 degrees towards a receiver60placed parallel to the base of the waveguide10. The invention articulated herein describes a variety of methods to design these secondary redirecting optics50. The invention helps make the optical waveguide10more useful. Key commercial criteria for the optical waveguide concentrating systems include compactness, efficiency, level of concentration, and manufacturability. Different methods for the redirection impact these criteria in different ways.

It should also be noted that the applications for this optical waveguide10or device are several. The light energy can be delivered to a variety of receivers.FIGS. 3ato3dshow some examples of receivers60, including further concentrating or diffusing optics such as lenses, compound parabolic concentrating optics, photovoltaic cells, or heat exchangers which will be described in more detail hereinafter.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In a preferred embodiment, as shown inFIG. 4a, light from the waveguide10travels roughly perpendicular to the plane of collection. The redirecting optics50redirects this light20to a receiver60so as to change the angle of propagation of the cone of the light20.

In a preferred embodiment, the optical waveguide10redirects the light20substantially perpendicular to its angle of propagation in the waveguide10, as shown inFIG. 4b. This enables the receiver60to lie parallel to the plane of collection, as inFIG. 4c.FIG. 4dshows how the waveguide10and receivers60interfere with one another if the optical waveguide10does not have redirecting optics associated therewith. The “optics layer” (the redirecting optics50) and “receiver layer” (the receiver60) thereby form horizontal slab sections that can be easily assembled and mated together. The two layers can form a laminate construction that is sturdy and durable. Connections between receivers60can be more easily made; for example, electrical interconnects between photovoltaic cell receivers. As the receiver60(e.g., photovoltaic cell) heats up, having a separate receiver layer enables more effective heat transfer off a back plane heat sink.

Each of the following sections outlines a specific problem being addressed by the redirecting optics50and some preferred methods of solving that problem.

1. Redirector as Same or Separate Part

The redirecting optics50can be constructed to be a feature of the same manufactured part as the waveguide optics10, as shown inFIG. 5a. It can also be a separate part in another embodiment, as shown inFIG. 5b.

Being made in the same part has advantages for the waveguide10—less manufacturing steps, no alignment required between parts, and potentially greater efficiency from no losses between interfaces.

Being made as a separate part for the components of the waveguide10and the redirecting optics60can help preserve or enhance the level of light concentration. The interface70can act as a totally internally reflecting (TIR) surface and thus better contain the light20within the waveguide10.

2. Mirror Image

FIG. 6ashows an optical waveguide10and redirecting optics50as previously discussed, with the collection area of input light20and the height of the optics components noted.

InFIG. 6b, the combination of the waveguide10optics, redirecting optics50, and the receiver60is mirrored about a central axis. The receiver60can then be constructed as one contiguous part. The collection area delivering light to a single receiver60is thus doubled while the height of the total construction is not changed—therefore the construction is twice as compact for a given area of receiver60.

Additionally, the entire mirrored waveguide and redirecting combination can be manufactured in one part, thereby simplifying manufacturing.

3. Secondary Concentration

The optical waveguide10delivers a certain level of concentration, shown as C1in the embodiment ofFIG. 7a, defined by the collection area A1divided by the area of the edge A2. The redirecting optic50receives the light20at this level of concentration and may change the level of concentration upon delivery to the receiver60.

The secondary concentration caused by the redirecting optic50is shown as C2and defined as the ratio of A2, the input area for the redirecting optic50, and A3, the output area for the redirecting optic50. The final level of concentration is shown as Cfinal and defined as A1/A3.

InFIG. 7a, the redirecting optic50increases the level of concentration from the optical waveguide10. Cfinal is therefore greater than C1. When concentration factor is a critical parameter, such as in reducing the area of photovoltaic cell material in order to reduce costs of solar panels, secondary concentration from the redirecting optic50can be beneficial.

In the embodiment ofFIG. 7b, the redirecting optic50reduces the level of concentration from the optical waveguide10(Cfinal<C1). In the particular embodiment ofFIG. 7b, a simple flat facet80is disposed at an appropriately shallow angle with respect to the horizontal base of the optical waveguide10, or the edge collector, and can deliver a reduction in concentration. InFIG. 7cthis embodiment provides a further example. Even though the optical waveguide10delivered an appreciable level of concentration, the redirecting optic80reduced the final concentration factor to about 2×. This can be beneficial when, for example, it is desired to reduce solar cell material by a factor of two, and also to have large areas of contiguous solar cell, all the while having the overall optical system be highly compact.

4. Limitations of Total Internal Reflection

Total internal reflection (TIR) can be employed to reflect the light20for redirection. It is superior to a reflective coating as it is nearly lossless, while a coating will absorb some of the energy falling on it. It is also cheaper as it removes the extra manufacturing step and material cost of a reflective coating.

However, total internal reflection takes place only for light rays providing angles of incidence larger than the critical angle formed by the refractive indices of the optic and the surrounding material.

In the embodiment ofFIG. 8ais shown a flat redirecting optic50at the TIR limit. The angle theta formed between the light ray90and the normal to the interface is just larger than the critical angle.FIGS. 14a-cshow what happens when the angle of the light ray90is smaller than the critical angle—the ray is refracted through the redirecting optic50surface and is not delivered to the receiver60. A requirement to employ TIR thus places a constraint on how much additional secondary concentration can occur while preserving efficiency.

In another embodiment reflective coatings may be employed to concentrate the light further than the TIR limit allows.FIG. 9shows this embodiment. In this case, the angle of incidence is similar to that inFIG. 8bwhere the light90was essentially lost. Here, the light ray90is instead reflected down towards the receiver60. A reflector100thus allows the redirecting optics50to have a form which provides a steeper incline with respect to the optical waveguide10which can enhance secondary concentration without losing light rays through the interface. (However, as described in section4hereinafter, the tradeoff on efficiency is that a reflector absorbs some of the energy falling on it).

A curved section such as a parabolic section110inFIG. 10acan be more effective than a flat surface at delivering secondary concentration. A curve, such as a parabola, is a focusing optical surface and therefore can take a light cone from the optical waveguide10and perform a secondary level of concentration on it, delivered to a receiver60.

However, the curve110can become steep enough to have the incident angle of light90exceed the critical angle for total internal reflection, as shown inFIG. 10b. Two solutions are possible. First, a reflective coating120may be applied to the section of the curve110that would otherwise see light leaking out, as shown inFIG. 10c. Second, the parabola110can be truncated at the TIR limit, and a flat facet130be used at the very same angle as the end of the curve, such that TIR is always achieved along the redirecting optic. This feature is shown inFIG. 10d.

In yet another embodiment when the optical waveguide10and redirecting optic60are mirrored, some leaking of light140from the redirecting optic50surface may be tolerated, since it is collected by the opposite redirecting surface and delivered to the receiver60, as shown inFIG. 11a. This can increase the secondary concentration achievable because the redirecting optic50surface can be placed at a steeper angle and therefore shrink the required area of the receiver60.

In another embodiment a cascade of facets160are possible to take advantage of this effect, with the steeper ones of the facets160located closer to the bottom surface, as shown inFIG. 11b. Each facet160can be placed at the appropriate angle to maximize both concentration and efficiency given the range of angles of light rays incident upon it. Alternatively, a curved surface170that approximates the cascade of facets160may be applied to achieve the same, as shown in the embodiment ofFIG. 11c. The redirecting optic50can also be comprised of multiple curves, such as parabolic section170and an arc180, as shown in11c.

8. Cladding on Base

In another embodiment if air190is used between the base of the optics50and the top of the receiver60, then light210exiting the redirecting optic50will refract to increase the cone angles, as shown inFIG. 12a.

A cladding material200may be applied to the base of the waveguide-redirecting optic construction, as shown inFIG. 12b. The cladding200is of a lower refractive index than the medium of the redirecting optic50. This allows the cone of light210at the receiver60to be narrower as the refraction off the base of the optic system is mitigated (the rays210will refract by larger angles traveling from an optic to air which has the lowest possible refractive index, versus travelling from an optic to another material). Mitigating the cone angles out of the redirecting optic50can help preserve the level of concentration achieved by the overall optical system.

Cladding will also provide an efficiency advantage. Fresnel reflection occurs at interfaces of different refractive indices, with losses being larger for greater differences in index. Having air between the optic and the receiver160will result in the greatest Fresnel reflection losses.

Cladding can also provide structural and reliability advantages. It can encapsulate a sensitive material that needs environmental protection, like a photovoltaic cell. It can also decouple stresses between the optic and the receiver160, for example as a result of differing rates of expansion under temperature increases.

9. Angled Light Guide

In an alternate embodiment the direction of propagation of the light cone in the waveguide10need not be exactly perpendicular to the input light, as shown inFIG. 13a. In a variation the waveguide220may be constructed at an angle to the horizontal, as shown inFIG. 13b.

An angled form of the waveguide10is advantageous for the redirecting optic50, since it allows for greater secondary concentration to remain with the TIR limit. The reason is because light has a smaller angle of required redirection.FIG. 13bshows this advantage—the light ray90can be reflected back in via total internal reflection over a larger angle than as shown inFIG. 13a.

FIG. 13cshows the change in angle of facets230required when an angled waveguide220combines with the cladding200on the base. In order to deal with extreme rays reflecting off the focal area25, the final facet24(“Facet2”) needs to occur at a shallower angle than the previous facet23(“Facet1”), otherwise light rays235reflected off the focal area will refract through the Facet2and escape upwards.

10. Redirecting Light Pipes

An alternative embodiment involves increasing the aspect ratio in order to win greater secondary concentration.FIG. 14ashows the use of a curved light pipe260to redirect the light265. The aspect ratio of the optical waveguide10—redirecting optic260is made significantly larger.FIG. 14bshows the same but achieving secondary concentration by tailoring the curved surfaces of the redirecting optics, or the pipe feature type of redirecting optics260to taper.FIG. 14cshows a similar approach but with flat facets for the redirecting optics260instead of curved sections.

The pipe feature260can in principle achieve greater secondary concentration than the redirecting optics discussed thus far, because it preserves the level of concentration given by the optical waveguide10, orients the light cone to face the receiver60directly, and then can achieve the maximal level of secondary concentration. Previous redirecting optics faced total internal reflection constraints that prevented achieving the maximum allowable levels of secondary concentration.

However, previous redirecting optics260do retain the compactness of the optical waveguide. Hence the following tradeoffs are seen:TIR non-pipe approaches that achieve highest compactness and highest efficiency but not highest concentrationReflector non-pipe approaches that achieve highest compactness and highest concentration but not highest efficiencyTIR pipe approaches that achieve highest concentration and highest efficiency but not highest compactness
11. Base Glass

Another embodiment also increases the aspect ratio somewhat in order to win greater concentration.FIG. 15ashows a design already discussed—the redirecting optics50with the cladding layer200.FIG. 15bshows how a sheet of optical material like glass270can be placed at the base of the waveguide-redirecting optic construction. The additional height enables greater secondary concentration as the light275has a greater distance to travel, and the angles of the edge rays are such that greater travel shrinks the final focal area.

The base glass270can also act as a mechanical and environmental barrier, protecting the receiver60(e.g., solar cell) from dirt and moisture that may enter the voids in the optical components, and from mechanical stresses from thermal and other expansion and contraction.

12. Supporting Optics for Central Redirection

Since the overall waveguide-redirecting optic construction is designed to efficiently collect light20from the top surface, the light20falling on the central region of the construction (above the redirecting optic) must also be collected for optimal efficiency.

FIG. 16ashows an embodiment with the complete waveguide10—redirecting optic50combination, including supporting optics for the central redirection. It is designed so that input light20falling across the entire front surface of the optical waveguide10is delivered to the receiver60.

FIG. 16bshows a close-up of the central redirecting region. A light ray280from the optical waveguide10is depicted, with a path to the receiver60as has been discussed previously.

The question is how light rays20from the central redirecting50region can make their way to the receiver60. Three approaches are possible, preferably in combination. First, the design of sections of the optical waveguide10may be modified to accommodate the different angled surfaces on the redirecting optic50. InFIG. 16c, the system through which the light ray280travels has larger lens290and facet300features to accommodate the tapering of the redirecting optic surface310towards the receiver60. The light ray280therefore travels through a different pathway than the rays20from other embodiments described before in the optical waveguide10system.

Second, features can be placed in the top element to direct the light280to an appropriate place on the redirecting optic50such that it is delivered to the receiver60. InFIG. 16d, the tooth feature320on the base of the top element redirects the light ray280such that it intersects with the redirecting optic surface310in the region directly above the receiver60. Without that tooth feature320, the light ray280would not hit the receiver60, limiting efficiency, or the receiver60would have to be wider to accommodate the ray280, limiting concentration.

Third, rays sufficiently near the center of the system of optical components are allowed to pass through with no change in direction. They undergo some refraction upon hitting the redirecting surface optic310, but they are sufficiently near the center of the system so as to ensure they end up on the receiver60.

Options two and three above require that the entire redirecting optic310be constructed with no reflector—i.e., pure TIR is employed. A reflective coating on the redirecting optic310would block incoming rays from the central redirecting region, limiting efficiency.

FIGS. 16a-ecombine many elements discussed in the previous sections in a preferred embodiment:From Section 1 the redirecting optic310is made as one part with the optical waveguide10, for simplified manufacturing.From Section 2 the optical waveguide10—redirecting optic50is mirrored about a central axis, increasing collection area and compactness of the overall device.From Section 3 the optical waveguide10achieves secondary concentration such that Cfinal>C1, reducing the receiver60area and thereby receiver cost.From Section 4 the redirecting optic310employs total internal reflection to maximize efficiencyFrom Section 6 the redirecting optic310employs a parabolic curve to increase secondary concentration, and then truncates the curve with a flat facet attached to it as the TIR limit is reached. (The parabolic curve may be approximated by several flat facets to achieve a comparable result.)From Section 7 the redirecting optic310allows “TIR leaking”—some rays are allowed to breach the critical angle and refract over to the mirror image surface, where they are nevertheless collected by the receiver60. This allows the redirecting surface310to have steeper angles enabling further secondary concentration.From Section 8 cladding200is used between the redirecting optic and the receiver, to enhance concentration, efficiency, structural support, and reliabilityFrom Section 9 the optical waveguide10is an angled waveguide—i.e., the waveguide10is not perfectly perpendicular to the input light20. This allows the redirecting optic310to have steeper angles which increases secondary concentration achievable.From Section 12 supporting optics290,300,320are constructed in the optical waveguide10in the central redirecting region above the redirecting optic310. These supporting optics ensure that light incident on the central region is delivered to the receiver60through the redirecting optic310, maximizing efficiency and concentration. As has been illustrated above, the redirecting optic50can be comprised of any combination of a parabolic surface, an elliptical surface, a hyperbolic surface, an arc, a flat reflective surface, a tailored shape reflective surface, a total internal reflecting surface, a component parabolic concentrator optic, a light pipe, and a refractive component.

The previous sections described ways to design various embodiments of the redirecting optics50. The following describe two alternative implementations for the redirecting optics50. All of the descriptions in the previous elements apply to the following two implementations.

Linearly Symmetrical Optics Versus Axially Symmetrical Optics

Since the design lies in the cross-section, the optical components may be rendered in a linear extrusion as shown inFIG. 17a, or a rotational extrusion as shown inFIG. 17b. All the elements described in this application are applicable to either extrusion.

However, axially symmetric optics in rotational extrusion face an additional challenge.FIG. 17cshows a familiar case—a light ray20from the optical waveguide320is redirected downwards towards a central receiver60. However, in the rotational extrusion, the light ray20would have to be in perfect alignment with the radius of the disc in order to strike the redirecting optic330for redirection as designed (note that the tip of the redirecting optic330comes down to a point in the rotational extrusion). InFIG. 17d, a light ray340that is slightly off center misses the redirecting optic330and does not arrive at the receiver60.

InFIG. 17e, this is ameliorated by shifting the axis of rotation away from the tip of the redirecting optic330. Thus the light ray340faces not a point but a wall350. The distance of the axis of rotation from the tip of the optic360can be tuned so that substantially all of the waveguide light20is captured, maximizing efficiency. The disadvantage is weaker secondary concentration as compared to the linear extrusion of the same cross-sectional design.

FIG. 17fshows an embodiment in another approach to solving the problem. The redirecting optic tip360is treated as the axis of rotation. However, the redirecting region50has vertical slits380sliced into it at periodic intervals about the axis. These slits380may be filled with air or a low index cladding material, or the walls of the redirecting region50may be coated with a reflector. A ray of light340that is off center will hit one of the walls380and be reflected back in towards the center. After one or several reflections of the “guide wall” the ray340will finally interact with the redirecting surface of the redirecting optic370, and be redirected downwards to the receiver60. This approach can preserve the secondary concentration achievable in a linear extrusion of the same cross-section. If no reflective coating is used, i.e., total internal reflection is the sole mechanism, then this approach maximizes efficiency as well.

Optical Path in Reverse for Light Diffusion or Illumination

In yet another embodiment the optics described in this application are so far for light collection and concentration. However, the optical system in reverse is an effective diffuser of light.

InFIG. 18, light400enters the device from a light source290where the receiver60has so far been—the central region at the base. The light400is redirected into the optical waveguide10via the redirecting optics50, and the optical waveguide10takes the light travelling substantially horizontal and diffuses it to output rays410that are substantially normal to the surface of the waveguide10. The top surface of the waveguide10may be tuned with lenses or other optical elements to emit light at any range of angles. Thus the device is a highly compact and efficient light diffuser. Applications include but are not limited to LED optics, luminaires, spotlights, and automotive headlights and taillights.