FIELD OF THE INVENTION
- BACKGROUND OF THE INVENTION
This invention relates generally to the fields of optics, solar energy, and lighting and in particular to an apparatus that efficiently collects and concentrates incident optical energy over a wide range of incident angles.
The efficient collection, concentration and distribution of solar energy remain some of the most significant, yet persistent problems of contemporary society. Its importance cannot be overstated. As fossil fuels continue to dwindle in supply and contribute to undesirable environmental effects, the importance of solar energy will only increase. Efforts to realize the potential(s) of solar energy—and in particular efforts directed toward the efficient collection and concentration of solar energy—are therefore of great significance.
The prior art has produced a variety of solar energy collectors and concentrators having a solar energy receiver upon which solar energy to be collected is directed. Most of these designs feature an imaging configuration, where an image of the sun is produced on the receiver by an optical instrument. Such a configuration will either allow the image of the sun to move relative to the receiver as the sun moves across the sky or will require the collector-receiver combination to track the sun during its daily motion.
A portion of the prior art has also been devoted to so-called “non-imaging” methods, where sunlight is collected and directed at a receiver in a way that does not produce an image of the sun on the receiver, but instead merely directs the sunlight in a random spatial pattern to the surface of the receiver. These methods typically are limited by the range of acceptance angle of the optical system (i.e. they cannot collect over a wide range of incident solar angles), or are not truly non-imaging (i.e. they produce a poor image which still moves on the receiver as the sun moves relative to the collector).
For example, one type of imaging solar collector is the familiar parabolic mirror which directs radiant energy incident thereon to a particular point or focus. Examples of this type of apparatus include U.S. Pat. No. 6,244,264 issued to R. Winston on Jun. 12, 2001, which describes a single-axis parabolic reflector which can be used to concentrate sunlight onto a long pipe or heating element. This configuration is a variant of earlier art such as the Solar Radiation Collector and Concentrator made from metallic aligned curved reflectors which are used to channel solar radiation to heat a cylindrical tube, described by V. J. Hockman in U.S. Pat. No. 3,964,464 (Jun. 22, 1976). A later variant is the combination of lens and reflector troughs as described by Habraken et al in U.S. Pat. No. 6,903,261 (Jun. 7, 2005). In these configurations, the reflectors described are aligned in a general east-west orientation so that concentrated solar radiation moves along the tube during the day and heat is captured without diurnal tracking mechanisms, though the image of the sun moves along the length of the cylindrical tube. A symmetrical conical reflective concentrator was described by Clegg in U.S. Pat. No. 4,325,612 on Apr. 20, 1982, while a large, multi-element parabolic reflector was disclosed by Dietrich et al in U.S. Pat. No. 4,583,520, granted Apr. 22, 1986, and an example of an ellipsoidal reflector concentrator can by found in U.S. Pat. No. 4,665,895, issued to Meier on May 19, 1987. Though these devices concentrate sunlight symmetrically in all directions, they must be repositioned throughout the day in order to track the sun.
Among the quasi-non-imaging solar concentrators are designs such as curved Fresnel lenses and gradient-index (GRIN) lenses. Fresnel lens designs have been described in the patents of O'Neill, such as U.S. Pat. No. 4,069,812 (Jan. 24, 1978), U.S. Pat. No. 4,545,366 (Oct. 8, 1985), and U.S. Pat. No. 6,111,190 (Aug. 29, 2000). GRIN lens designs can be found in the patents of Dempewolf (U.S. Pat. No. 5,936,777, Aug. 10, 1999) and Ortabasi (U.S. Pat. No. 6,057,505, May 2, 2000, and U.S. Pat. No. 6,252,155 B1, Jun. 26, 2001). For reasons described further below, the Fresnel lens and the GRIN lens must be considered imaging optical devices, so that a modified system based on these devices can be at best a quasi-non-imaging system, where poor solar images will move across the receiver surface as the sun moves.
The fundamental physical constraint on these types of optical collection systems is the conservation of optical throughput, known from the so-called Lagrange invariant of geometric optics, which can be derived from first principles. In mathematical terms, the conservation of optical path between two media C1 and C2 with boundary K is governed by
where n is the refractive index, and s is the ray vector. The throughput, or the product of angular acceptance and optical aperture, in a non-diffractive optical system is limited by the component with the smallest throughput, so that
This formulation is equivalent to the so-called Liouville form of non-imaging optics, wherein conservation of refractive and reflective systems is often expressed as
n 1 d 1 sin α=n 2 d 2 sin β, (3)
where n1 and n2 are the refractive indices of the media on either side of the system, d1 and d2 are the entrance and exit aperture widths of the system, respectively, and α and β are the angles over which the input and output beams are distributed. This derivation is based on the Liouville theorem, which applies to conformal transformations between three-dimensional spaces. Reflectors, lenses, Fresnel lenses, and similar optical instruments are all limited by this constraint.
Of critical importance in Eq. 1 is the surface K, which in refractive and reflective optics cannot alter the wavevector ns. In diffractive optics, the surface K can cause discontinuity in ns, thereby allowing a different conservation relationship. Diffractive optics provide the only means by which this constraint may be relaxed to allow larger angles and areas to be converted to smaller angles and areas, or a larger mode distribution to be condensed into a smaller distribution of degenerate modes.
Holographic or diffractive devices have been explored in the prior art, but always in planar configuration. Such examples include the patents of Afian et al (U.S. Pat No. 4,691,994, Sep. 8, 1987, and U.S. Pat. No. 4,863,224, Sep. 5, 1989), wherein a planar hologram is coupled to a prism to guide incident sunlight by both diffraction and total internal reflection. Riccobono et al (U.S. Pat No. 5,517,339, May 14, 1996) disclose a means for exposing planar transmission holograms for solar collection and the use (U.S. Pat. No. 5,491,569, Feb. 13, 1996) of planar holograms as window coverings to diffuse light into a room. A more recent invention is that of Rosenberg, (U.S. Pat. No. 6,274,860, Aug. 14, 2001) wherein an optical radiation concentrating device comprises a holographic planar concentrator including a planar, highly transparent plate and at least one multiplexed holographic optical surface mounted on a surface thereof. Solar collector devices can be mounted at the edges of the plate, or on the back surface of the plate where gaps in the diffractive surface are made.
While the devices of Afian et al and Rosenberg do make use of multiple diffraction events to steer a light ray, they are limited to planar formats and rely on partial transmission of the optical radiation through the hologram. Afian et al in particular limit their inventions to volume or three-dimensional transmission holograms. Rosenberg limits his invention to a planar device with a highly transmitting plate between sandwiched holograms. In the present invention, I disclose a concentrator which does not rely on transmission through a hologram in order to access the guided region where the light will be concentrated.
- SUMMARY OF THE INVENTION
Though a great deal of prior art exists in this area, there exists a continuing need for optical collecting and concentrating structures providing high efficiency, while eliminating the need to track the source of the optical energy. Such structures would represent a significant advance in the art. The present invention represents a fundamental departure from prior art at the level of basic physical principles as well as structural design of the system.
BRIEF DESCRIPTION OF THE DRAWINGS
I have developed, in accordance with the principles of the invention, an optical collecting and concentrating apparatus for use in, i.e., passive lighting, solar power, and optical communications applications. In sharp contrast to prior art devices, my inventive collector and concentrator is a non-imaging, non-planar, high acceptance angle device. Consequently it is relatively immune from solar (or other optical source) incidence angles and therefore does not need to track the movement of the sun to efficiently collect and concentrate solar energy.
A more complete understanding of the present invention may be realized by reference to the accompanying drawing in which:
FIG. 1 shows a perspective view of an optical collection and concentration system constructed according to the teachings of the present invention, using a diffractive tube and collecting lens;
FIG. 2 shows the principle of operation of the device as the beam solar and diffuse solar illumination change throughout the day;
FIG. 3 shows the convention for positive and negative diffraction used herein;
FIG. 4 shows a variant of the basic invention, where a flared profile is used at the output end of the tube to achieve higher collimation of the input with a shorter tube;
FIG. 5 shows a variant of the tube design using a tapered waveguide to couple light from the tube; and
FIG. 6 illustrates the use of a diffusing element to produce more uniform illumination.
FIG. 1 shows a perspective view of a passive optical collection system constructed according to the present invention. More specifically collector tube 10 includes an optically diffractive medium 12 disposed on the inner surface thereof. This medium is designed to partially scatter, reflect, or diffract light of various wavelengths at an angle greater than the incident angle at which the light strikes the surface. The tube may be hollow or filled with a transparent or partially transparent medium. The net effect is that the output of the tube at a clear aperture 14 opposite the opening 16 where sunlight or other optical radiation is incident, is a beam 18 consisting of light rays which are substantially oriented in a direction parallel to the central axis 20 of tube 10. This beam thereafter impinges on a lens or similar optical element 22, which is designed so as to focus the collimated or nearly collimated beam 18 onto an optical receiver or collecting device 24. The lens element 22 may be a conventional lens or related instrument, including but not limited to Fresnel lenses, reflectors, and diffractive optical elements. The collection device may be an optical detector, solar cell, optical fiber, or similar type of collecting, conducting, or converting instrument.
As described above, the interior of the tube 10 is a diffractive medium 12, such that rays 26 striking the interior of the tube at an angle α with respect to the surface normal 28 are at least partially diffracted at a higher angle β. This diffractive effect is accumulated along the length of the tube, so that the total angular spread Ωin of rays 26 entering the tube is greater than the angular spread Ωout of rays 30 exiting the tube. In optical terms, the diffractive interior of the tube translates a larger input numerical aperture to a smaller output numerical aperture.
As depicted in FIG. 2, the net result is that both beam (direct) and diffuse solar illumination will be collected with very high efficiency by a fixed collector/concentrator device. As the incident angle of the direct beam solar illumination (BSI) 40 changes throughout the day, the diffractive tube 10 will collimate the beam so that the output 42 is focused by the lens 22 to essentially the same collection point 24.
It will be apparent to skilled technicians that the diffraction grating 14 must be optimized to reduce negative diffraction, or diffraction of rays 60 in a direction proximal of the specular ray 62 with respect to the incident ray 64, as shown in FIG. 3. Equivalently, the desired diffractive effect is a positive one, where diffracted rays 66 are directed along an angle greater than the specularly reflected ray 62. Proper grating design to maximize positive diffraction will in many cases also have the effect of directing scattered light substantially more toward the output of the tube rather than the input.
Several grating design variations may also be used to optimize the multiple diffraction effect. In particular, the angular distribution of rays 24 striking the inner surface of the tube 10 near the entrance 12 will be slightly greater than the angular distribution of rays striking the inner surface of the tube farther down its length, due to the diffractive effect. In particular, it can be anticipated that the number of rays which strike the surface at near normal incidence will be successively diminished at points closer to the output of the tube 10. This means that while the grating must be designed to diffract efficiently over a wide range of angles, including near-normal incidence, at the entrance 12 of the tube 10, it can be designed for much higher efficiency at glancing incidence farther down the tube, closer to the exit 16.
Likewise, the length and width of the tube can be optimized for given materials and geometries. As is known from the technology of hollow waveguides, longer tubes will result in greater interaction of the light with the sides of the tube, or a greater number of reflections or diffractions and thus greater loss. At the same time, the multiple diffraction effect will require a certain number of diffraction events in order to confine a given percentage of incident beams into a cone of a given output angle (NA).
Several variations of this basic concept are also encompassed within the present invention, including taper profiles for the basic tubular shape, which may be parabolic, hyperbolic, exponential, or a general power series function. As illustrated in FIG. 4, some advantage may be gained by using a tube 80 with a flare 82 (e.g. of a axial profile described by an exponential function) at the output end. In this case, rays 84 impinging on the distal end of the tube 80 at angles greater than a certain minimum determined by the geometry of the flare 82 will be reflected or diffracted by the flare 82 at an angle more parallel to the tube axis 86 than such rays would be reflected or diffracted by a similar unflared or straight-sided tube. The effect of a flare 82 will thus be to more efficiently collimate the rays exiting the tube 80, at the expense of the width 88 of the exiting beam. Other advantageous variations may include a similar flare at the input of the tube.
Another variation is the use of a tapered waveguide 100 concentric to a concentrating tube 10, as shown in FIG. 5. The waveguiding properties of a tapered structure are known from basic optics; light rays 102 striking the taper at an angle αR with respect to the surface normal 104 will be totally internally reflected at the opposite side of the taper provided that αR meets the condition that
where n1 is the index of refraction and θ the opening half angle of the taper 100. It is important to note that this angle is measured relative to the surface normal 104 of the taper on the side from which the ray is incident; relative to the axis of the tube 106, the angle is
For rays 108 incident at angles α shallower than αR, the angle β at which the ray 108 exits the opposite side of the taper will be given by
As a numerical example, for a glass taper with n1=1.45 and θ=5 degrees, light incident at angles αz less than about 31 degrees (α greater than roughly 54 degrees) will be totally internally reflected in the taper 100. At higher incident angles αz, light will transmit through the taper, but will exit at a much shallower angle βz. In the same taper as described above, a ray incident at αz=32 degrees will emerge from the opposite side of the taper at βz≈9.7 degrees. The tapered waveguide may thus be used to both capture and guide light with lower loss than a metal clad waveguide (such as the tube itself) and may be used to ameliorate the angular translation effect of the grating tube 100. The taper may be terminated with a lens to focus the light, or may continue past the end of the tube to guide the light into another optical apparatus. Other modifications to taper 100 such as anti-reflection coatings, core-clad structures (as found in optical fibers for communications), and varying blunt, flat, convex, or concave ends on the taper may also be used.
A further modification of my invention uses a diffusing optic 120 to more evenly distribute the light incident at the collection point 22, as shown in FIG. 6. Since the most common distributions of light from a device such as the tubular structure described above into a circular aperture are typically biased toward the outer portions of the circle, such a diffusing optic 120 may be designed with radially increasing scattering or diffraction toward its outer circumference. Thus, at high solar incident angles (e.g. early or late day), rays 122 which are directed toward the outer portion of the diffuser 120 will be partially scattered toward the center of the collection point 22. At mid-day, or lower solar incidence angle, the broader distribution of rays 124 will result in less preferential scattering from the center of the diffuser 120. Additionally, elliptical bias may be introduced to compensate for east-west motion of the solar disc.
At this point, while I have discussed and described my invention using some specific examples, those skilled in the art will recognize that my teachings are not so limited. Accordingly, my invention should be only limited by the scope of the claims attached hereto.