U.S. patent application number 11/410682 was filed with the patent office on 2007-10-25 for wide angle solar concentrator.
This patent application is currently assigned to Applied Optical Materials. Invention is credited to David Schaafsma.
Application Number | 20070246040 11/410682 |
Document ID | / |
Family ID | 38618291 |
Filed Date | 2007-10-25 |
United States Patent
Application |
20070246040 |
Kind Code |
A1 |
Schaafsma; David |
October 25, 2007 |
Wide angle solar concentrator
Abstract
A non-imaging optical collecting and concentrating apparatus for
use in i.e., optical communications, passive lighting, and solar
power applications that is relatively immune from optical incidence
angle(s) and therefore does not need to track the movement of the
sun to efficiently collect and concentrate optical energy is
described. The apparatus includes a tubular support structure
having a source-facing entrance and an energy-outputting exit. An
interior surface of the structure includes a scattering, reflecting
and/or diffractive medium to direct incident energy toward an exit
of the tubular structure, such that the rays exiting the tube are
more collimated and substantially more parallel to the axis of the
tube. The collimated beam is then focused or directed by a lens or
similar optical element toward a point where the energy may be
collected by a detector, optical fiber, or other collection
means.
Inventors: |
Schaafsma; David;
(Fallbrook, CA) |
Correspondence
Address: |
David Schaafsma
744 Stewart Canyon Rd.
Fallbrook
CA
92028
US
|
Assignee: |
Applied Optical Materials
Fallbrook
CA
|
Family ID: |
38618291 |
Appl. No.: |
11/410682 |
Filed: |
April 25, 2006 |
Current U.S.
Class: |
126/698 |
Current CPC
Class: |
F24S 23/30 20180501;
G02B 19/0028 20130101; G02B 19/0042 20130101; Y02E 10/44 20130101;
F24S 23/00 20180501 |
Class at
Publication: |
126/698 |
International
Class: |
F24J 2/08 20060101
F24J002/08 |
Claims
1. An apparatus for collecting solar or other optical radiation
comprising: a tubular structure, defining an interior surface; and
a diffracting medium, disposed on the interior surface of the
tubular structure; such that light rays striking the interior of
the tubular structure are directed to a common point, said common
point being substantially an exit aperture of the tubular
structure, along ray paths which are substantially more aligned
with the axis of the tube than the paths along which the rays enter
the tube; a lens or functional equivalent optic, located at a point
past the exit aperture of the tube, relative to the direction of
propagation of rays; such that the rays exiting the tube will be
directed by the lens to a collection point, such collection point
being essentially the focal point of the lens or similar optic.
2. The apparatus of claim 1, where the diffractive surface of the
tube is made from a surface relief grating.
3. The optical apparatus of claim 2, wherein the surface relief
grating is designed to diffract principally in a direction away
from the specular reflection from the surface.
4. The optical apparatus of claim 3, where the surface grating uses
any combination or superposition of triangular, sinusoidal, step
width, or step height variations to achieve the desired diffraction
characteristic.
5. The optical apparatus of claim 1, where the diffractive surface
of the tube is made from a volume hologram or other periodic
refractive index structure.
6. The apparatus of claim 1, where the diffractive surface of the
tube is made from a photonic bandgap, moth-eye, or other
subwavelength, periodic, or quasi-periodic, diffractive or
preferentially scattering structure.
7. The optical apparatus of claim 1, where the tubular structure is
designed to preferentially reflect or diffract light through
geometric variations such as flared ends, tapered ends, or curved
sides.
8. The optical apparatus of claim 1, where the lens or focusing
optic is replaced or used in conjunction with a transparent tapered
structure located inside the tube to extract or guide light from
inside the tube.
9. The optical apparatus of claim 1, where a diffusing element,
diffraction grating, or similar device is used to more evenly
distribute light over the collection area.
10. A method of collecting solar or other optical energy comprising
the steps of: receiving the optical energy on a substantially
tubular structure having a diffractive surface for receiving the
optical energy; directing the optical energy by scattering,
reflecting, coherently reflecting, diffracting, or any combination
thereof, to a common point which is substantially an exit point of
the tubular structure, such that the rays exiting the tubular
structure are substantially more parallel to the axis of the tube
than the rays entering it; focusing or otherwise directing the rays
exiting the tubular structure using a lens or similar optical
instrument to a collecting point, said collecting point being
essentially a focal point of the lens; collecting the optical
energy into a collector positioned at the collecting point.
11. The method of claim 10, where the diffractive surface of the
tubular structure is made from a surface relief grating.
12. The method of claim 11, wherein the surface relief grating is
designed to diffract principally in a direction away from the
specular reflection from the surface.
13. The method of claim 12, where the surface grating uses any
combination or superposition of triangular, sinusoidal, step width,
or step height variations to achieve the desired diffraction
characteristic.
14. The method of claim 10, where the diffractive surface of the
tube is made from a volume hologram or other periodic refractive
index structure.
15. The method of claim 10, where the diffractive surface of the
tube is made from a photonic bandgap, moth-eye, or other
subwavelength, periodic, or quasi-periodic, diffractive or
preferentially scattering structure.
16. The method of claim 10, where the tubular structure is designed
to preferentially reflect or diffract light through geometric
variations such as flared ends, tapered ends, or curved sides.
17. The method of claim 10, where the diffractive surface of the
tube is made from a volume hologram or other periodic refractive
index structure.
18. The method of claim 10, where the lens or focusing optic is
replaced or used in conjunction with a transparent tapered
structure located inside the tube to extract or guide light from
inside the tube.
19. The method of claim 10, where a diffusing element, diffraction
grating, or similar device is used to more evenly distribute light
over the collection area.
20. An optical collector/concentrator comprising: a curved, tubular
means for supporting a scattering, reflective, or diffractive
surface or any combination thereof wherein said curved supporting
means defines an interior surface; and a means for preferentially
directing light rays, disposed upon the supporting means of the
tubular support structure; such that light rays striking the
interior surface of the tubular support means are directed to a
common point, said common point being substantially an exit
aperture of the tubular support means; a means for directing the
rays exiting the tubular support means to a collection point, said
collection point being essentially the focal point of the focusing
or directing means; such that optical energy directed to the
collection point may be collected by a collection device or other
means for collecting the optical energy.
Description
FIELD OF THE INVENTION
[0001] 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.
BACKGROUND OF THE INVENTION
[0002] 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.
[0003] 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.
[0004] 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).
[0005] 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.
[0006] 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.
[0007] 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 C.sub.1 and C.sub.2 with boundary K is governed by .intg. C 1
.times. n 1 .times. s 1 .times. d r + .intg. C 2 .times. n 2
.times. s 2 .times. d r + .intg. K .times. ( n 2 .times. s 2 - n 1
.times. s 1 ) .times. d r = 0 , ( 1 ) ##EQU1## 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 .intg. C 1 .times. n 1 .times. s 1
.times. d r + .intg. C 2 .times. n 2 .times. s 2 .times. d r = 0. (
2 ) ##EQU2## 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.sub.1d.sub.1 sin .alpha.=n.sub.2d.sub.2 sin .beta., (3) where
n.sub.1 and n.sub.2 are the refractive indices of the media on
either side of the system, d.sub.1 and d.sub.2 are the entrance and
exit aperture widths of the system, respectively, and .alpha. and
.beta. 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.
[0008] 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.
[0009] 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.
[0010] 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.
[0011] 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.
SUMMARY OF THE INVENTION
[0012] 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.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] A more complete understanding of the present invention may
be realized by reference to the accompanying drawing in which:
[0014] 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;
[0015] FIG. 2 shows the principle of operation of the device as the
beam solar and diffuse solar illumination change throughout the
day;
[0016] FIG. 3 shows the convention for positive and negative
diffraction used herein;
[0017] 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;
[0018] FIG. 5 shows a variant of the tube design using a tapered
waveguide to couple light from the tube; and
[0019] FIG. 6 illustrates the use of a diffusing element to produce
more uniform illumination.
DETAILED DESCRIPTION
[0020] 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.
[0021] 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 .alpha. with respect to the surface normal 28
are at least partially diffracted at a higher angle .beta.. This
diffractive effect is accumulated along the length of the tube, so
that the total angular spread .OMEGA..sub.in of rays 26 entering
the tube is greater than the angular spread .OMEGA..sub.out 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.
[0022] 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.
[0023] 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.
[0024] 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.
[0025] 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).
[0026] 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.
[0027] 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 .alpha..sub.R
with respect to the surface normal 104 will be totally internally
reflected at the opposite side of the taper provided that
.alpha..sub.R meets the condition that sin .times. .times. .alpha.
R .gtoreq. n 1 .times. sin .function. ( sin - 1 .times. 1 n 1 - 2
.times. .theta. ) , ( 4 ) ##EQU3## where n.sub.1 is the index of
refraction and .theta. 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
.alpha. z = .pi. 2 - .alpha. - .theta. . ( 5 ) ##EQU4## For rays
108 incident at angles .alpha. shallower than .alpha..sub.R, the
angle .beta. at which the ray 108 exits the opposite side of the
taper will be given by sin .times. .times. .beta. = n 1 .times. sin
.function. ( sin - 1 .times. sin .times. .times. .alpha. n 1 + 2
.times. .theta. ) . ( 6 ) ##EQU5##
[0028] As a numerical example, for a glass taper with n.sub.1=1.45
and .theta.=5 degrees, light incident at angles .alpha..sub.z less
than about 31 degrees (.alpha. greater than roughly 54 degrees)
will be totally internally reflected in the taper 100. At higher
incident angles .alpha..sub.z, light will transmit through the
taper, but will exit at a much shallower angle .beta..sub.z. In the
same taper as described above, a ray incident at .alpha..sub.z=32
degrees will emerge from the opposite side of the taper at
.beta..sub.z.apprxeq.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.
[0029] 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.
[0030] 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.
* * * * *