U.S. patent application number 14/174096 was filed with the patent office on 2014-10-30 for pulsed stimulated emission luminescent photovoltaic solar concentrator.
This patent application is currently assigned to MORGAN SOLAR INC.. The applicant listed for this patent is MORGAN SOLAR INC.. Invention is credited to Pascal DUFOUR, John Paul MORGAN.
Application Number | 20140319377 14/174096 |
Document ID | / |
Family ID | 51788480 |
Filed Date | 2014-10-30 |
United States Patent
Application |
20140319377 |
Kind Code |
A1 |
MORGAN; John Paul ; et
al. |
October 30, 2014 |
PULSED STIMULATED EMISSION LUMINESCENT PHOTOVOLTAIC SOLAR
CONCENTRATOR
Abstract
A solar concentrator comprising: a light-transmissive sheet
including: a plurality of luminescent particles capable of becoming
excited by absorbing light within at least a first spectrum of
absorption frequencies and, once excited, capable of being
stimulated to emit light having a spectrum within at least a first
spectrum of emission frequencies; and a first light-guide; and a
light source for generating a pulsed probe light having a spectrum,
at least a portion of which is within at least the first spectrum
of emission frequencies, for stimulating at least one of the
excited luminescent particles having absorbed light within the
first spectrum of absorption frequencies such that when the probe
light traveling in a first direction of travel stimulates the
excited luminescent, the excited luminescent particles emit emitted
light having a spectrum within the first spectrum of emission
frequencies in the first direction of travel of the probe
light.
Inventors: |
MORGAN; John Paul; (Toronto,
CA) ; DUFOUR; Pascal; (Toronto, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
MORGAN SOLAR INC. |
Toronto |
|
CA |
|
|
Assignee: |
MORGAN SOLAR INC.
Toronto
CA
|
Family ID: |
51788480 |
Appl. No.: |
14/174096 |
Filed: |
February 6, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13452588 |
Apr 20, 2012 |
|
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14174096 |
|
|
|
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61477265 |
Apr 20, 2011 |
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Current U.S.
Class: |
250/459.1 |
Current CPC
Class: |
Y02E 10/40 20130101;
H01L 31/167 20130101; H01L 31/0547 20141201; H01L 31/055 20130101;
H01L 31/02322 20130101; G02B 6/0003 20130101; F24S 23/12 20180501;
G02B 6/4298 20130101; Y02E 10/52 20130101 |
Class at
Publication: |
250/459.1 |
International
Class: |
G02B 6/10 20060101
G02B006/10 |
Claims
1-19. (canceled)
20. A method of concentrating light, comprising: (i) exposing at
least one light-transmissive sheet, having a plurality of
luminescent particles and at least one light-guide, to light,
causing the luminescent particles to become excited by absorbing
light within at least a first spectrum of absorption frequencies,
the excited luminescent particles capable of being stimulated to
emit emitted light having a spectrum within at least a first
spectrum of emission frequencies; (ii) stimulating the excited
luminescent particles via pulsed probe light having a spectrum, at
least a portion of which is within at least the first spectrum of
emission frequencies, and traveling in a first direction, to cause
the excited luminescent particles to emit emitted light having a
spectrum within the first spectrum of emission frequencies in the
first direction of travel of the pulsed probe light; (iii)
concentrating and converging the emitted light and the pulsed probe
light via total internal reflection in the at least one
light-transmissive sheet toward a light collection area.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a divisional application of U.S.
patent application Ser. No. 13/452,588, filed Apr. 20, 2012,
entitled "Pulsed Stimulated Emission Luminescent Photovoltaic Solar
Concentrator". Through the'588 application, the present application
claims priority to U.S. Provisional Patent Application No.
61/477,265, filed Apr. 20, 2011, entitled "Pulsed Stimulated
Emission Luminescent Light-Guide Solar Concentrators". The entirety
of both of these applications is incorporated herein by
reference.
FIELD
[0002] The present invention relates to luminescent solar
concentrators.
BACKGROUND
[0003] The harvesting of solar energy is a field with a
multiplicity of different technologies for converting sunlight to
electricity. To date, none of the technologies has become
sufficiently inexpensive to displace traditional means of
generating electricity, and as a result solar energy remains a
marginal contributor to global power needs. The main cost driver in
solar power systems is the high cost of the photovoltaic (PV)
cells, which are the semiconductor junctions that convert light
into electricity.
[0004] One of the many avenues being investigated for reducing the
cost of electricity produced by solar power is called "Concentrated
Photovoltaics", or CPV. The basic idea behind CPV is to use some
sort of optic, generally a Fresnel lens or another focusing optic,
to concentrate sunlight onto tiny, high-efficiency PV cells. The PV
cells employed are compound semi-conductor cells with multiple
junctions in a stack and electrically connected in series. The most
typical conventional cells for CPV are three junction cells using
indium gallium phosphide, indium gallium arsenide, and germanium
cells all arranged in an electrical series connection. Each of
these cells converts a portion of the solar spectrum into
electricity. CPV systems are very energetically productive but they
have a major downside in that they require trackers to orient them
to face the sun at all times in order for their optics to function.
This need for trackers makes these systems practical for use in
solar farms, where large post-mounted trackers are mounted on the
ground. Trackers are impractical, however, for systems intended for
building integration and roof mounting (which represents a massive
portion of the solar market). CPV systems use high sunlight
concentration, as high as 2000 suns, meaning that only a tiny
amount of photovoltaic material would be required as compared with
a conventional non-concentrated PV system.
[0005] Another approach to concentration is the use of luminescent
solar concentrators. These devices consist of a sheet of glass that
contains either a layer of luminescent particles or has luminescent
particles impregnated throughout the glass. Luminescent particles
typically absorb light over a wide band of frequencies and emit
light at lower frequencies over a narrower band. Examples of
luminescent particles are organic dyes, laser dyes and
nano-crystals.
[0006] When these luminescent particles emit light, the light
emitted travels in a random direction. Because this light is
emitted evenly in every direction from inside the glass, any
emitted radiation which strikes the top or bottom faces of the
glass sheet, and which has an angle of incidence with respect to
the surface normal of the glass sheet greater than the critical
angle for total internal reflection, will be trapped within the
glass sheet by total internal reflection. (If the glass has an
index of 1.5 and the surrounding media is air then the critical
angle is approximately 41.8 degrees.)
[0007] In fact, the only light which will not become trapped within
the glass is any light that is emitted within one of two cones of
emission centered on the normal of the top and bottom glass
surfaces and with base angles of 83.6 degrees in the foregoing
example. The critical angle is given by Snell's law:
n.sub.1 sin .theta..sub.1=n.sub.2 sin .theta..sub.2
[0008] Light thus trapped will travel in all directions within the
glass to the four edges of the glass where it can be harvested for
energy production by photovoltaic cells. Because the frequency of
the emitted light is relatively narrow, it is possible to use
single junction cells in this instance in a very efficient manner,
provided the single junction cells have a band-gap closely matched
to the energy of the emitted photons. In principal, infinite
concentration could be achieved in this manner except there are two
fundamental limitations: absorption within the glass and
re-absorption by the luminescent particles. The first, absorption
within the glass itself, limits the practical optical path length
and thus the size of the glass sheet and the concentration.
Re-absorption and emission also limit the practical concentration.
To date the best-predicted concentration by this means is on the
order of 150 suns. This is far lower that the concentrations
achievable by CPV as noted above. Thus cost savings in a
luminescent concentration system achieved by not having a tracker
are greatly overwhelmed by the extra cost of requiring several
times more photovoltaic cell material. Thus, luminescent
concentration systems are not in widespread commercial use and
improvements in this technology are desirable, given its inherent
advantages noted above.
SUMMARY
[0009] It is thus an object of the present invention to provide an
improved luminescent solar concentrator as compared with at least
some of those of the prior art.
[0010] In one aspect, a solar concentrator is provided. The solar
concentrator comprises: a light-transmissive sheet including: a
plurality of luminescent particles capable of becoming excited by
absorbing light within at least a first spectrum of absorption
frequencies and, once excited, capable of being stimulated to emit
light having a spectrum within at least a first spectrum of
emission frequencies and a first light-guide. Also included in the
concentrator is at least one light source for generating a pulsed
probe light having a spectrum, at least a portion of which is
within at least the first spectrum of emission frequencies, for
stimulating at least one of the excited luminescent particles
having absorbed light within the first spectrum of absorption
frequencies such that when the probe light traveling in a first
direction of travel stimulates the excited luminescent particles,
the excited luminescent particles emit emitted light having a
spectrum within the first spectrum of emission frequencies in the
first direction of travel of the probe light. The first light-guide
is for assisting in guiding the emitted light and the probe light
via total internal reflection. The emitted light and the pump light
are concentrated and converging, within the solar concentrator,
toward a light collection area.
[0011] In some embodiments, the pulsed probe light is generated by
turning each of the at least one light source on and off. In some
embodiments, each of the at least one light source is a constant
light source, and the pulsed probe light is generated by shuttering
each constant light source.
[0012] In some embodiments, the solar concentrator is generally of
a shape selected from the group consisting of a circular disk, an
elliptical disk, a section of an elliptical disk, a plurality of
sections of elliptical disks forming a reflecting edge of many
reflecting facets. If the shape is a circular disk, the circular
disk has a focal point, and the at least one light source and the
light collection area are substantially at the focal point. If the
shape is an elliptical disk, the elliptical disk has two foci, and
the at least one light source is at one of the foci and the light
collection area is at the other of the foci. If the shape is a
section of an elliptical disk, the section of the elliptical disk
has two foci on an edge thereof, and the at least light source is
at one of the foci and the light collection area is at the other of
the foci. If the shape is a plurality of sections of elliptical
disks, each of the plurality of sections of elliptical disks have
two foci in common with each of the other plurality of sections of
elliptical disks, and the at least one light source is at one of
the common foci and the light collection area is at the other of
the common foci.
[0013] In some embodiments, the solar concentrator is disk-shaped
and comprises a first parabolic portion facing a second parabolic
portion, the first parabolic portion having a first focal point,
the second parabolic portion having a second focal point. The at
least one light source is at the first focal point and the light
collection area is at the second focal point.
[0014] In some embodiments, the solar concentrator further includes
at least one secondary optic adjacent at least one of the light
collection area and the at least one light source. In some
embodiments, the at least one secondary optic is made of a
different material than adjacent materials.
[0015] In some embodiments, the solar concentrator further includes
a first reflector positioned at an edge of the light-guide so as to
reflect light toward the light collection area.
[0016] In some embodiments, the solar concentrator further includes
a second light-guide optically coupled to the sheet, the second
light-guide for guiding light received from the sheet to the light
collection area. The sheet and the second light-guide are stacked
one upon the other and separated from one another by a first
material having a lower index of refraction than that of both the
sheet and the second light-guide. A macroscopic direction of travel
of light within the sheet and a macroscopic direction of travel of
light within the second light-guide is generally opposite one
another.
[0017] In some embodiments, the sheet includes a transparent
substrate and the luminescent particles are a luminescent dye
impregnated in the substrate.
[0018] In some embodiments, the luminescent particles are within a
luminescent layer adjacent to and optically coupled with the first
light-guide.
[0019] In some embodiments, light is reflected from an edge of the
light-guide via total internal reflection toward the light
collection area.
[0020] In some embodiments, the solar concentrator further includes
a fiber optic for collecting light from the light collection area
and re-inserting the light into the concentrator, such that the at
least one light source is the fiber optic.
[0021] In another aspect, a photovoltaic solar concentrator is
provided which includes a solar concentrator and at least one
photovoltaic cell disposed at the light collection area of the
solar concentrator. In some embodiments of the photovoltaic solar
concentrator, the at least one light source and the at least one
photovoltaic cell are disposed on a single circuit board. In some
embodiments of the photovoltaic solar concentrator, the at least
one light source is powered by energy from the at least one
photovoltaic cell.
[0022] In an additional aspect, a solar concentrator module is
provided. The solar concentrator module includes at least two solar
concentrators, each concentrator being adjacent to and optically
coupled with the other concentrators. The luminescent particles of
each of the concentrators are capable of becoming excited by
absorbing light within a spectrum of absorption that includes, at
least in part, different frequencies from the other concentrators
and capable of being stimulated to emit light of at least one
frequency within a spectrum of emission frequencies that is, at
least in part, different from the other concentrators. Also
included is at least one photovoltaic cell disposed at each of the
light collection areas of the concentrators.
[0023] In some embodiments of the solar concentrator module, the at
least two solar concentrators are separated from one another by a
second material having a lower index of refraction than that of the
at least two solar concentrators.
[0024] In another aspect, a solar energy collector assembly is
provided, comprising a plurality of solar concentrators. At least
two light sources of the concentrators are in optical communication
with a central light source such that the at least two light
sources emit probe light generated by the central light source.
[0025] In yet another aspect, a method of concentrating light is
provided. The method includes: (i) exposing at least one
light-transmissive sheet, having a plurality of luminescent
particles and at least one light-guide, to light, causing the
luminescent particles to become excited by absorbing light within
at least a first spectrum of absorption frequencies, the excited
luminescent particles capable of being stimulated to emit emitted
light having a spectrum within at least a first spectrum of
emission frequencies; (ii) stimulating the excited luminescent
particles via pulsed probe light, the pulsed probe light having a
spectrum, a portion of which is within at least the first spectrum
of emission frequencies, and traveling in a first direction, to
cause the excited luminescent particles to emit emitted light
having a spectrum within the first spectrum of emission frequencies
in the first direction of travel of the probe light; and (iii)
concentrating and converging the emitted light and the probe light
via total internal reflection in the at least one light-guide
toward a light collection area.
[0026] Embodiments of the present invention each have at least one
of the above-mentioned object and/or aspects, but do not
necessarily have all of them. It should be understood that some
aspects of the present invention that have resulted from attempting
to attain the above-mentioned object may not satisfy this object
and/or may satisfy other objects not specifically recited
herein.
[0027] Additional and/or alternative features, aspects, and
advantages of embodiments of the present invention will become
apparent from the following description, the accompanying drawings,
and the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] For a better understanding of the present invention, as well
as other aspects and further features thereof, reference is made to
the following description which is to be used in conjunction with
the accompanying drawings, where:
[0029] FIGS. 1A-1E schematically illustrate the stimulated emission
process in a luminescent particle;
[0030] FIG. 2A is a cross sectional view of a first embodiment of a
pulsed stimulated emission luminescent light-guide solar
concentrator;
[0031] FIG. 2B is a cross sectional views a second embodiment of a
pulsed stimulated emission luminescent light-guide solar
concentrator;
[0032] FIG. 3 is an Absorption Emission Spectrum of a typical
dye;
[0033] FIG. 4 shows optical power profiles of exemplary continuous
and pulsed light sources;
[0034] FIG. 5A is a cross sectional view of a circular disk-shaped
embodiment of a pulsed stimulated emission luminescent light-guide
solar concentrator;
[0035] FIG. 5B is a perspective view of the circular disk-shaped
embodiment of a pulsed stimulated emission luminescent light-guide
solar concentrator of FIG. 5A;
[0036] FIG. 5C is a plan view of the circular disk-shaped
embodiment of a pulsed stimulated emission luminescent light-guide
solar concentrator of FIG. 5A;
[0037] FIG. 6A is a cross sectional view of another embodiment of a
circular disk-shaped embodiment of a pulsed stimulated emission
luminescent light-guide solar concentrator;
[0038] FIG. 6B is a perspective view of the circular disk-shaped
embodiment of a pulsed stimulated emission luminescent light-guide
solar concentrator of FIG. 6A;
[0039] FIG. 6C is a plan view of the circular disk-shaped
embodiment of a pulsed stimulated emission luminescent light-guide
solar concentrator of FIG. 6A;
[0040] FIG. 6D shows a portion of a cross section of another
embodiment of another embodiment of a circular disk-shaped pulsed
stimulated emission luminescent light-guide solar concentrator;
[0041] FIG. 7A is a perspective view of an embodiment of an
elliptical disk-shaped pulsed stimulated emission luminescent
light-guide solar concentrator;
[0042] FIG. 7B is a plan view of the elliptical disk-shaped pulsed
stimulated emission luminescent light-guide solar concentrator of
FIG. 7A;
[0043] FIG. 8A is a perspective view of an embodiment of a half
elliptical disk-shaped pulsed stimulated emission luminescent
light-guide solar concentrator;
[0044] FIG. 8B is a plan view of the half elliptical disk-shaped
pulsed stimulated emission luminescent light-guide solar
concentrator of FIG. 8A;
[0045] FIG. 9A is a perspective view of an embodiment of an
elliptical section disk-shaped pulsed stimulated emission
luminescent light-guide solar concentrator;
[0046] FIG. 9B is a plan view of the elliptical section disk-shaped
pulsed stimulated emission luminescent light-guide solar
concentrator of FIG. 9A;
[0047] FIG. 10A is a plan view of an embodiment of a pulsed
stimulated emission luminescent light-guide solar concentrator
having multiple reflecting facets;
[0048] FIG. 10B is a plan view of an embodiment of a pulsed
stimulated emission luminescent light-guide solar concentrator
having many small reflecting facets which forms a peripheral
reflective edge that appears to be generally planar;
[0049] FIG. 10C is a plan view of a solar array comprising a
plurality of tightly packed pulsed stimulated emission luminescent
light-guide solar concentrators of the type shown in FIG. 10B;
[0050] FIG. 10D is a plan view of an embodiment of a square
disk-shaped pulsed stimulated emission luminescent light-guide
solar concentrator;
[0051] FIG. 11A is a cross sectional view of an embodiment of a
bi-layer pulsed stimulated emission luminescent light-guide solar
concentrator;
[0052] FIG. 11B is a perspective view of the bi-layer pulsed
stimulated emission luminescent light-guide solar concentrator of
FIG. 11A;
[0053] FIG. 11C is a plan view of the bi-layer pulsed stimulated
emission luminescent light-guide solar concentrator of FIG. 11A
showing an outward moving wavefront;
[0054] FIG. 11D is an illustrative graph of captured power and
intensity of an outward moving wavefront of the bi-layer pulsed
stimulated emission luminescent light-guide solar concentrator of
FIG. 11A;
[0055] FIG. 11E is a plan view of the bi-layer pulsed stimulated
emission luminescent light-guide solar concentrator of FIG. 11A
showing an inward moving wavefront;
[0056] FIG. 11F is an illustrative graph of captured power and
intensity of an inward moving wavefront of the bi-layer pulsed
stimulated emission luminescent light-guide solar concentrator of
FIG. 11A;
[0057] FIG. 12A is a cross sectional view of another embodiment of
a bi-layer pulsed stimulated emission luminescent light-guide solar
concentrator with a secondary light-guide;
[0058] FIG. 12B is a detailed cross sectional view of a
semi-circular optic of the bi-layer pulsed stimulated emission
luminescent light-guide solar concentrator of FIG. 12A;
[0059] FIG. 12C is a detailed cross sectional view of the secondary
optical element of the bi-layer pulsed stimulated emission
luminescent light-guide solar concentrator of FIG. 12A;
[0060] FIG. 13A is a cross sectional view of an embodiment of an
inverted bi-layer pulsed stimulated emission luminescent
light-guide solar concentrator that allows for mounting of a solar
energy collector and a light source on the same circuit board;
[0061] FIG. 13B is a detailed cross sectional view of a
semi-circular optic of the inverted bi-layer pulsed stimulated
emission luminescent light-guide solar concentrator of FIG.
13A;
[0062] FIG. 13C is a detailed cross sectional view of the secondary
optical element of the inverted bi-layer pulsed stimulated emission
luminescent light-guide solar concentrator of FIG. 13A;
[0063] FIG. 14A is a cross sectional view of another embodiment of
a bi-layer pulsed stimulated emission luminescent light-guide solar
concentrator that allows for mounting of a solar energy collector
and a light source on the same circuit board;
[0064] FIG. 14B is a detailed cross sectional view of a
semi-circular optic of the bi-layer pulsed stimulated emission
luminescent light-guide solar concentrator of FIG. 14A;
[0065] FIG. 14C is a detailed cross sectional view of the central
portion of bi-layer pulsed stimulated emission luminescent
light-guide solar concentrator of FIG. 14A;
[0066] FIG. 15A is a cross sectional view of an embodiment of
circular disk-shaped bi-layer stimulated emission luminescent light
guide solar concentrator having a thin luminescent sheet;
[0067] FIG. 15B is a detailed cross sectional view of a
semi-circular optic of the bi-layer pulsed stimulated emission
luminescent light-guide solar concentrator of FIG. 15A;
[0068] FIG. 16A is a perspective view of a stimulated emission
solar concentration assembly;
[0069] FIG. 16B is a plan view of the stimulated emission solar
concentration assembly of FIG. 16A;
[0070] FIG. 16C is a cross sectional view of the stimulated
emission solar concentration assembly of FIG. 16A;
[0071] FIG. 17A is a perspective view of the stimulated emission
solar concentration assembly of FIG. 16A with a central light
source;
[0072] FIG. 17B is a detailed cross sectional view of the emitting
end of an optical fiber which conducts light from the central light
source;
[0073] FIG. 18A is an illustrative graph of absorbed solar power
and power of stimulated emission as a function of radius;
[0074] FIG. 18B is an illustrative graph of the probability of
stimulated emission as a function of radius;
[0075] FIG. 19 is an illustrative graph of absorption and emission
spectra of a single luminescent material;
[0076] FIG. 20 is an illustrative graph of a set of absorption and
emission spectra of three luminescent materials;
[0077] FIGS. 21A and 21B are a cross sectional views of an
embodiment of a multi-layer pulsed stimulated emission luminescent
light guide solar concentrator;
[0078] FIG. 22A is a perspective view of an embodiment of a
pie-shaped three-layer pulsed stimulated emission luminescent light
guide solar concentrator where the light sources and solar energy
collectors are vertically aligned;
[0079] FIG. 22B is a perspective view of an embodiment of a
pie-shaped three-layer pulsed stimulated emission luminescent light
guide solar concentrator similar to that of FIG. 22A where the
light sources and solar energy collectors are vertically
misaligned;
[0080] FIG. 22C is another perspective view of the pie-shaped
three-layer pulsed stimulated emission luminescent light guide
solar concentrator of FIG. 22B;
[0081] FIG. 22D is a perspective view of the light sources and
solar energy collectors of FIG. 22B mounted on a single
substrate;
[0082] FIG. 23A is a plan view of a solar panel assembly;
[0083] FIG. 23B is a perspective view of the flexible solar panel
assembly of FIG. 23A;
[0084] FIG. 24A is a plan view of an embodiment of a pulsed
stimulated emission luminescent light guide solar concentrator
having two compound parabolic concentrators; and
[0085] FIG. 24B is a plan view of an array of pulsed stimulated
emission luminescent light guide solar concentrators of the type
shown in FIG. 24A connected at their distal ends.
DETAILED DESCRIPTION
[0086] In stimulated emission luminescent solar concentrators, a
luminescent sheet is pumped by sunlight and is probed by a light
source, such as a laser, a diode or other light source, to
stimulate emission of photons which can be harvested by
photovoltaic cells. FIGS. 1A-1E illustrate the stimulated emission
process in a luminescent particle showing the energy states of
electrons during the process. FIG. 1A shows an incident photon 10
from sunlight 116 being absorbed by a luminescent particle 26
resulting in an electron 12 of the luminescent particle 26 being
excited from the ground state 14 (E.sub.0) into a higher energy
state 16 (E.sub.H). FIG. 1B shows the electron 12 decaying to a
lower energy state 18 (E.sub.L), by releasing some of the energy
gained from the absorbed photon 10 as light or heat 11. The lower
energy state (E.sub.L) 18 is herein referred to as the luminescent
state 18 (also known in the art as a metastable excited state). An
electron 12 left alone in the luminescent state 18 for a long
enough period of time, will spontaneously decay back to the ground
state 14 with the spontaneous emission of a photon in a random
direction. The frequency of the spontaneously emitted photon,
called the luminescent frequency, is lower than the frequency of
the originally absorbed photon 10 as the emitted photon has less
energy than the absorbed photon 10. FIG. 1C shows a probe photon 22
having a frequency equal to the luminescent frequency incident on
the luminescent particle 26 having an electron 12 in the
luminescent state 18. As shown in FIG. 1D, this probe photon 22
will perturb (stimulate) the electron 12 in the luminescent state
18 and cause it to decay to the ground state 14, with the
stimulated emission of a photon 24. The photon 24 emitted by
stimulated emission has the same frequency, phase, and direction of
travel as the probe photon or stimulating photon 22.
[0087] FIG. 1E is a schematic illustration of the stimulated
emission process in luminescent particles. Sunlight 116 strikes a
cluster of luminescent particles 26 (single or several molecules).
The sunlight 116 acts as a pump light exciting electrons 12 of the
luminescent particles as described above. A passing probe beam 28
having a frequency equal to the luminescent frequency stimulates
the decay of the excited electrons 12 causing the luminescent
particles 26 to emit a beam 30 parallel to the probe beam 28 and
with the same frequency as the probe beam 28. The probe beam 28 and
the beam 30 created by stimulated emission continue to propagate in
the same direction.
[0088] The luminescent state 18 described above is only one example
of an excited energy state of a luminescent particle, and in fact
the stimulated emission process can be more complicated. The
process may involve continuous or pseudo-continuous energy bands
instead of discrete states, in which case, the spontaneous emission
has a luminescent spectrum instead of luminescent frequency, and
the probe beam from a light source such as a diode would have the
same spectrum as the luminescent spectrum. In other words, the
light in question (probe or emitted), has frequencies varying over
a narrow band and defining a spectrum, rather than being at a
particular, precise frequency.
[0089] Multiple energy states of the luminescent system can be
involved in the stimulated emission process. Multiple photons can
be involved in the excitation step, and there can be multiple decay
steps prior to the luminescent emission step. The present
disclosure is intended to cover any luminescent system,
irrespective of the number of energy states. Additionally, some
luminescent particles contain multiple luminescent particles with
different absorption and emission spectra where the emission from
one particle is absorbed by another particle. This disclosure is
intended to cover those luminescent systems as well.
[0090] It should be noted that the word "dye" in the present
specification refers to a luminescent material, including, but not
limited to organic and inorganic dyes, doped glasses and crystals
(e.g. Nd.sup.3+ in Yttrium aluminium garnet (YAG) or glass;
titanium in sapphire), quantum dots, and nano-crystals. In should
also be noted that luminescence in the present application is used
to refer in short-form to photoluminescence.
[0091] The probability that an excited dye molecule will decay via
stimulated emission is given by the ratio of the rate of stimulated
emission to the total relaxation rate:
p.sub.stim=R.sub.stim/(R.sub.stim+R.sub.sp)
where p.sub.stim is the probability of stimulated emission,
R.sub.stim is the rate of stimulated emission, and R.sub.sp is the
rate of spontaneous emission. The rate of stimulated emission is
given by:
R.sub.stim=.sigma..sub.eI/hv
where .sigma..sub.e is the stimulated emission cross-section at the
probe wavelength, I is the intensity of the incident light, h is
Planck's constant, and v is the frequency of the light. The rate of
spontaneous emission, R.sub.sp, is given by the inverse of the
luminescent state lifetime .tau..sub.sp, as in
R.sub.sp=1/.tau..sub.sp.
[0092] Dye molecules decay through non-radiative as well as
radiative mechanisms, resulting in a luminescence quantum yield
(QY) less than unity. The non-radiative mechanism can be a
probabilistic splitting between pathways from a high energy singlet
state during the initial relaxation, with some probability QY that
the radiative path was taken, and probability 1-QY that the
non-radiative path was taken. Alternatively, the non-radiative
mechanism can be a relaxation process from the luminescent state
that competes with the radiative path, characterized by a
non-radiative relaxation rate R.sub.nr, with the quantum yield
given by QY=R.sub.sp/(R.sub.sp+R.sub.nr).
[0093] In the first case, with a non-radiative path from the highly
excited state, the probability of stimulated emission is replaced
with:
p.sub.stim=QY*R.sub.stim/(R.sub.stim+R.sub.sp).
In the second case, the probability of stimulated emission is
replaced with:
p.sub.stim=R.sub.stim/(R.sub.stim+R.sub.sp+R.sub.nr).
[0094] Dyes are chosen so that they have a high stimulated emission
cross section and high quantum yield.
[0095] A first embodiment of a pulsed stimulated emission
luminescent light-guide solar concentrator 100 is shown in FIG. 2A.
The solar concentrator 100 includes a light-guide 132, a
luminescent layer 124 and a pulsed light source 126.
[0096] The light-guide 132 is planar and can be made of a light
transmissive material. Non-limiting examples of light-transmissive
materials include optical polyvinyl chloride (PVC), Polymethyl
methacrylate) (PMMA), silicones, Cyclo-Olefin Polymers (COP),
Cyclo-Olefin Copolymers (COC), glasses, epoxy-based materials,
urethane materials, other co-polymer materials and other polymeric
materials. The light-guide 132 is bounded by a first surface 107, a
second surface 104, a coupling surface 108 and an exit surface 106.
The light source 126 is optically coupled to the light-guide 132
via the coupling surface 108.
[0097] The luminescent layer 124 is a thin sheet or film of
light-transmissive material containing luminescent particles 130.
The luminescent layer 124 is optically coupled to the first surface
107 of the light-guide 132. Non-limiting examples of
light-transmissive materials include optical polyvinyl chloride
(PVC), Poly(methyl methacrylate) (PMMA), silicones, Cyclo-Olefin
Polymers (COP), Cyclo-Olefin Copolymers (COC), glasses, epoxy-based
materials, urethane materials, other co-polymer materials and other
polymeric materials. The light-transmissive material of the
luminescent layer 124 has a luminescent dye (containing luminescent
particles 130) impregnated, evenly or unevenly, throughout the
layer 124. Exemplary uneven distributions of luminescent particles
in a layer include a distribution with a concentration gradient,
for example, a gradient with the concentration of luminescent
particles increasing (or decreasing) from the end near the light
source 126 towards the end near the exit surface 106. Alternately,
or additionally, the concentration gradient of luminescent
particles in the luminescent layer 124 can also vary in a direction
perpendicular to the first surface 107. The luminescent layer 124
can be created by dissolving the luminescent dye in solution and
applying it directly, for example, as a thin film, on the first
surface 107 of the light-guide 132. The thickness of the
luminescent layer 124 in a direction perpendicular to the first
surface 107 of the light-guide 132 can be varied in different
regions of the first surface 107, for example, by applying a
different number of layers of the luminescent dye solution on the
different regions.
[0098] The solar concentrator 100 has a light collection surface
102, which in this embodiment is the first surface of the
luminescent layer 124. Sunlight 116 incident on the light
collection surface 102 enters the solar concentrator 100 and the
photons having wavelengths within the absorption spectrum or
spectra of the luminescent particles 130 in the luminescent layer
124 are absorbed by the luminescent particles 130. The incident
sunlight 116 acts as a pump light exciting the luminescent
particles 130 to a luminescent state and creating a population
inversion in the luminescent layer 124. An electron of the
luminescent particle 130 in a luminescent state can drop to the
ground state, either spontaneously or when stimulated by a passing
photon. If the excited electron spontaneously drops to the ground
state, the luminescent particle emits a photon in a random
direction. If the excited electron is stimulated by a passing
photon to drop to the ground state, a photon will be emitted that
has the same wavelength and is in phase with the stimulating
photon, that is, with the probe light 134.
[0099] The difference in wavelength between the peak of the
absorption and emission profiles is called the Stokes shift. FIG. 3
shows the Stokes shift between the absorption and emission spectra
of a luminescent material. A luminescent material with a large
Stokes shift and minimal overlap between the absorption and
emission spectra can be used to prevent the reabsorption of emitted
photons by the luminescent particles. Non-limiting examples of
luminescent materials with large Stokes shifts are doped
nanoparticles or quantum dots, such as Mn-doped ZnSe,
Eu.sup.3+-doped Gd.sub.2O.sub.3, and Te-doped CdSe.
[0100] Although there is generally at least some overlap between
the absorption and emission spectra of a luminescent material,
ideally less than 10% overlap is desired for the purposes of this
invention.
[0101] The probe light 134 (referred to as pump light in
International Application No. PCT/CA2010/000363 and Canadian Patent
Application No. 2,658,193) stimulates the emission of photons in
the luminescent layer 124. The probe light 134 is transmitted
through the main body 131 of the solar concentrator 100 via total
internal reflections (TIR) at the second surface 104 of the
light-guide 132 and at the light collection surface 102 of the
luminescent layer 124. The indices of refraction of the light-guide
132 and the luminescent layer 124 can be matched such that the
probe light 134 is coupled from the light-guide 132 to the
luminescent layer 124 through the first surface 107 of the
light-guide 132. A photon of the probe light 134 which enters the
luminescent layer 124 may cause an excited electron of an excited
luminescent particle 130 in the luminescent layer 124 to fall into
its ground state, thereby releasing a photon having the same
frequency, phase, and direction of travel as the stimulating photon
of the probe light 134.
[0102] A cross section of a second embodiment of a pulsed
stimulated emission luminescent light-guide solar concentrator 200
is shown in FIG. 2B. The solar concentrator 200 includes a
luminescent light-guide 232 and a pulsed light source 126.
[0103] In the embodiment of FIG. 2B, the entire volume of the main
body 231 is impregnated with luminescent particles 130. The
luminescent particles 130 can be suspended for example in a
substrate or matrix of light-transmissive material that forms the
main body 231. Non-limiting examples of light-transmissive
materials that can be used include optical polyvinyl chloride
(PVC), Poly(methyl methacrylate) (PMMA), silicones, Cyclo-Olefin
Polymers (COP), Cyclo-Olefin Copolymers (COC), epoxy-based
materials, urethane materials, other co-polymer materials, other
polymeric materials, or a solvent such as ethanol sealed in a glass
enclosure. In some embodiments, the luminescent particles 130
distributed through the entire volume of the main body 231 are
clusters of luminescent particles 130. The clusters may be of
substantially uniform sizes or of non-uniform sizes.
[0104] In this embodiment, the first surface of the light-guide 232
is the light collection surface 102. Sunlight 116 is therefore
received by the light-guide 232 directly through the light
collection surface 102. The incident sunlight 116 acts as a pump
light. The photons of the incident sunlight 116 are absorbed by the
luminescent particles 130 in the light-guide 232 and the
luminescent particles become excited to a luminescent state. The
pulsed probe light 134, which is optically coupled into the
light-guide 232 through the coupling surface 108, is transmitted
through the light-guide 232 via multiple total internal reflections
at the second surface 104 and at the first surface 107 (which is
also the light collection surface 102). When a photon of the probe
light 134 stimulates an excited luminescent particle 130, an
excited electron of the excited luminescent particle 130 will drop
to the ground energy state emitting a photon having the same
frequency, phase and direction of travel as photons of the probe
light 134.
[0105] The solar concentrator 100, 200 can be a part of a solar
concentrator module in which a solar energy collector 128 is placed
in optical communication with the exit surface 106 in a light
collection area 127 of the solar concentrator 100, 200 to harvest
the solar energy concentrated by the solar concentrator 100, 200,
as shown in FIGS. 2A and 2B. The solar energy collector 128 can be,
for example, a multi-junction photovoltaic cell or a silicon
photovoltaic cell, which can be adapted for receiving pulsed light.
The solar energy collector 128 can comprise a waveguide (not
shown), such as an optical fibre, to guide the concentrated light
towards a light harvesting device such as a central photovoltaic
cell or solar thermal energy collector.
[0106] In the embodiments of FIGS. 2A and 2B, the light source 126
generates a divergent probe light beam 134 that is pulsed on and
off. The probe light 134 floods the main body 131, 231 of the solar
concentrator 100, 200. The light source 126 can be, for example, a
laser diode that emits a wavelength of light within the emission
spectrum of the dye. The light source 126 can pulsed by powering
the light source 126 on and off, by using a timed shutter (i.e.,
opening and closing a shutter) or by any other means known in the
art. The pulsed laser diode can be coupled to a divergent optical
element (not shown). Other narrow band light sources that can be
used to generate a pulsed beam of light include LEDs and compact
incandescent light sources coupled to narrow band transmission
filters. The use of LEDs may be advantageous because they can
provide longevity and reduced power consumption.
[0107] An excited luminescent particle 130 illuminated by the
pulsed probe light 134 can undergo spontaneous emission to emit
photons having the same frequency, phase and direction of travel as
the probe light 134. The excited luminescent particle 130 can also
emit photons by spontaneous emission which will have the same
frequency as photons of the probe light 134. Photons resulting from
stimulated emission will be transmitted through the light-guide
132, 232 towards a light collection area 127 where a solar energy
collector 128 can be placed. Photons resulting from spontaneous
emission are emitted in a random direction, with some fraction of
the photons (those with angles of incidence smaller than the
critical angle for TIR) escaping out of the solar concentrator 100,
200.
[0108] As described with respect to the embodiments of FIGS. 2A and
2B, the absorbed sunlight 116 causes electrons of the luminescent
particles 130 to be excited to a luminescent state, therefore the
sunlight 116 is referred to as the pump light. A pulsing probe
light source 126 emits a probe light 134 in short pulses, at least
some of which light is incident on the luminescent particles 130.
The probe light 134 is guided within the main body 131, 231 of the
solar concentrator 100, 200 via total internal reflection. The
duration of each pulse of the probe light 134 is shorter than the
time period between pulses. The probe light 134 can stimulate
excited electrons of the luminescent particles 130 to decay to the
ground state emitting a photon that travels parallel to the
direction of the probe light 134 beam. Light is emitted by the
excited luminescent particles 130 in pulses that correspond with
the pulses of the probe light 134. The intensity of the probe light
134 gradually increases as emitted light (from stimulated emission)
is added to the beam, forming an augmented light beam 138, which is
an intensified light beam. Furthermore, light emitted upstream
(i.e., towards the light source 126) is superimposed on the probe
light 134 beam and can stimulate emission downstream (i.e., towards
the exit surface 106). As shown in the cross sectional views of
FIGS. 2A and 2B, the intensity of the probe light 134 increases
inside the solar concentrator 100, 200; the light intensity is
lowest near the coupling surface 108 and highest at the exit
surface 106. The beam of augmented light 138 exits the light-guide
132, 232 at the exit surface 106 toward the light collection area
127 for harvesting.
[0109] The main body 131, 231 of the solar concentrator 100, 200
can be a thin sheet fabricated by spin coating of a polymer and can
have an overall thickness in the range of 0.1 to 0.3 mm. A thin
solar concentrator 100, 200 is desirable, because it causes the
intensity of light output from the solar concentrator 100, 200 to
increase, which improves the efficiency of the luminescent
system.
[0110] Concentrating light in pulses onto a photovoltaic cell
(rather than continuously) may be advantageous because it can
reduce carrier recombination at the cell level. With reference to
the cell, a higher current increases the cell voltage
logarithmically, as given by:
V.varies. ln(l)
[0111] For a single junction cell, the voltage increases by
.about.60 mV for every decade increase in current. An advantage of
the pulsed probe light is that it causes the stimulated emission
rate to increase thereby increasing the quantum yield. A factor of
10 increase in intensity, increases the rate of stimulated emission
by a factor of 10. FIG. 4 shows the power (P) of a light source as
a function of time (t) comparing the time progression of a
continuous light source and a pulsed light source. The dotted line
shows the time progression of an exemplary continuous probe light
source, and the solid line shows the time progression of an
exemplary pulsed light source. In this example, a pulsed light
source is provided with a 10% duty cycle. The on-time t.sub.ON of
the pulsed probe having a duration of approximately 30 ns, and the
off-time t.sub.OFF having a duration of approximately 270 ns. The
power of the pulsed probe light is approximately 1 W, while the
power of the continuous probe light is approximately 0.1 W. Since
the power of the pulsed light-source is higher, so is its
intensity. This graph is a non-limiting example of two light
sources. It should be understood that light sources of greater of
less power, and having different duty cycles may be employed in any
of the embodiments described of this application.
[0112] A light source 126 that generates a pulsed probe light
having the characteristics shown in FIG. 5 can be used in the solar
energy concentrator 100, 200. In general, a higher intensity pulsed
probe light 134 stimulates emission of photons by excited
luminescent particles 130 more efficiently than a lower intensity
continuous probe light over an extended period of time due to the
reduction of reabsorption at the photovoltaic cell level and the
increase in probability that a photon of the probe light 134 will
impinge upon an excited luminescent particle 130. Therefore, having
a pulsed, high intensity light source can increase the overall
efficiency of the solar concentrator 100, 200 without increasing
the energy consumed by the light source 126. When a high intensity
pulse of light is delivered to a photovoltaic cell, the
photovoltaic cell will typically produce a pulse of DC power.
[0113] The embodiments described below are generally similar in
material and design to the solar concentrator of FIGS. 2A and 2B,
and any elements not described in relation to these embodiments can
be found in the description above, wherein like reference numerals
designate like parts.
[0114] FIGS. 5A-5C show a circular disk-shaped pulsed stimulated
emission luminescent light-guide solar concentrator 500. In this
embodiment, the pulsed light source 126 can be a point source
positioned at the center 537 of the circular disk shaped
light-guide 532. Coupled to the first surface 107 of the
light-guide 532 there is a thin circular disk-shaped luminescent
layer 524. While the solar concentrator 500 shown in FIG. 5A has a
luminescent layer similar to that of FIG. 2A, the whole main body
231 can be impregnated with luminescent particles 130 as in FIG.
2B. The solar concentrator 500 is generally planar and thin. The
probe light 134 from the pulsed light source 126 is transmitted
through the main body 531 of the solar concentrator 500 and
stimulates the excited luminescent particles 130 such that they
emit photons which add to the augmented light 138. The power of the
augmented light 138 therefore increases from the center 537 of the
solar concentrator 500 towards its peripheral edge 542 where one or
more solar energy collectors 528 can be positioned to harvest the
light.
[0115] The light rays in FIGS. 5A-5C are only shown as being
straight for ease of illustration. As would be understood by one of
skill in the art on reading this specification, the light rays
would follow the jagged path due to reflections on the light
collection surface 102 and the second surface 104 as described
above with reference to FIGS. 2A and 2B.
[0116] While the light source 126 can be pulsed on and off by any
means described above, one way to shutter the light source 126
would be to provide a reflector (not shown), such as a mirror, that
continuously rotates around the light source 126 and reflects light
generated by the light source 126 to produce a narrow beam of light
that sweeps 360 degrees around the light source 126 as the
reflector is rotated. The reflector has a curved reflective surface
that faces the light source 126. The curved reflective surface may,
for example, be a parabolic section with the focal point of the
parabola at the light source 126. The area of the solar
concentrator 500 not illuminated at a given time by the beam of
light is therefore in the shadow of the reflector. The probe light
134 would therefore appear to be pulsed at any given location in
the main body 531 of the solar concentrator 500.
[0117] Another embodiment of a circular disk-shaped pulsed
stimulated emission luminescent light-guide solar concentrator 600
is shown in FIGS. 6A-6C. This embodiment is similar to that of
FIGS. 5A-5C except that the peripheral edge 542 has a reflective
surface 644 that reflects and refocuses the augmented light 138
back towards a light collection area in the vicinity of the center
537 of the disk, where a solar energy collector 126 can be disposed
to harvest the augmented light 146 transmitted thereto. The
reflective surface 644 can, for example, be a mirror coating made
of metal, such as aluminum or silver, a dielectric or any other
suitable material known in the art. The augmented light 146 will
have a higher intensity near the center 537 than the probe light
134 initially launched into the main body 531 by the light source
126 due to the addition of emitted light collected along the way
(i.e., a cascade effect). Stimulated emission by the probe light
134 occurs as light travels outward towards the peripheral edge
542, and also as the augmented light 146 is refocused and converges
towards the center 537 of the disk.
[0118] Where at least one solar energy collector 126, such as a
photovoltaic cell, is coincident (or nearly coincident) with the
light source 126, this device can convert the solar energy
converging on the center 537 of the disk into electricity. This
electricity would be able to power the light source and would also
deliver electric current for use elsewhere. Alternatively, a
portion of the concentrated light could be redirected back into the
light-guide 532 with an optical element such as an optical fiber
and used as the probe light source. If the pump light source
(sunlight in this embodiment) is removed from the system, then the
solar concentrator 600 ceases to function immediately and the
device will cease to operate until sunlight or another pump light
source is applied.
[0119] An LED or laser, is almost identical to a PV cell, having
the same overall structure of materials, therefore, in one
embodiment the same device can be used to collect and emit probe
light. An electrical pulse can be sent to the semiconductor device
creating light. When the pulse of light is emitted into the solar
concentrator 600, the current would be stopped while the light
propagates through the solar concentrator 600. When the light
converges onto the semiconductor device, it becomes electrical
current. This embodiment requires actively switching the
connections on the semiconductor from a power source, to a
load.
[0120] A portion of a cross-section of a circular disk-shaped
pulsed stimulated emission luminescent light-guide solar
concentrator 600D is shown in FIG. 6D. This embodiment is generally
similar to that of FIGS. 6A-6C, however the light-guide 632 may be
injection molded, embossed or machined in such a way that it
contains annular triangular gaps 643 at the center of the disk
where the light source 126 and the solar energy collector 128 are
located. The annular triangular gaps 643 are concavities formed at
the bottom surface 104 of the light guide, and each annular
triangular gap 643 includes one reflective surface 645 for
reflecting light from the light source 126 into the main body 531
of the solar concentrator 600D, and one output surface 647 for
accepting reflected and augmented light 146, such that it is
transmitted to the solar energy collector 128.
[0121] A pulsed stimulated emission luminescent light-guide solar
concentrator 700 having the shape of an elliptical disk with two
foci is shown in FIGS. 7A and 7B. In this embodiment the pulsed
probe light source 126 is positioned at a first focal point 748 and
the solar energy collector 128 is positioned at a second focus 754
of an elliptical disk-shaped light-guide 732. The light-guide 732
can contain luminescent particles within its main body 731, as in
FIG. 2B, or there may be a luminescent layer (not shown) optically
coupled to the first surface 107 of the light guide 732, as in FIG.
2A. The peripheral edge 742 of the light-guide 732 has a reflective
surface 744 as in the embodiment of FIGS. 6A-6C. The probe light
134 spreads out from the first focal point 748, is reflected by the
reflective surface 744 and converges to the second focal point 754
where a solar energy collector 128 can be placed. The probe light
134 stimulates emission as it is transmitted towards the peripheral
reflective edge, and as it converges towards the second focal point
754. Therefore, the pulses of light 146 have highest intensity at
the light collection area and can deliver more power thereto than
was used to power the pulsed light source 126. The pulsed light
source 126 and the solar energy collector 128 can be placed on a
single circuit board for convenience if they are relatively close
together. (The ellipse can be altered to increase or decrease the
spacing distance between the light source 126 and the solar energy
collector 128.)
[0122] A pulsed stimulated emission luminescent light-guide solar
concentrator 800 having the shape of half an elliptical disk is
shown in FIGS. 8A and 8B. This embodiment is very similar to the
solar concentrator 700 described above and shown in FIGS. 7A and
7B, except that this embodiment only employs half an elliptical
disk and as such the light-guide 832 has a planar edge 858 along
which the two foci 748, 754 of the ellipse are located. The light
source 126 is optically coupled to the planar edge 858 at the first
focal point 748 and a solar energy collector 128 can be positioned
at the light collection area located at the second focal point 754.
The solar concentrator 800 also has a semi-elliptical peripheral
edge 842 with a reflective surface 844 similar to the reflective
surface 744 of the elliptical solar concentrator 700. The light
source 126 and solar energy collector 128 may be located on a
single circuit board 859 edge mounted to the planar edge 858. In
some applications, edge mounting the circuit board 859 may be more
convenient than mounting it at or near the center of the
light-guide 732 as in the embodiments shown in FIGS. 7A and 7B. In
the present embodiment, the light-guide 832 can contain luminescent
particles within its main body 831, as in FIG. 2B, or there may be
a luminescent layer (not shown) optically coupled to its first
surface, as in FIG. 2A. A portion of the concentrated light
collected in the light collection area at the second focal point
754 could be redirected back into the light-guide 732 through the
first focal point 748 with an optical element such as an optical
fiber 839 and used as the probe light source.
[0123] A pulsed stimulated emission luminescent light-guide solar
concentrator 900 having the shape of a section of an elliptical
disk (herein referred to as "pie-shaped"), is shown in FIGS. 9A and
9B. This embodiment is another variant of the solar concentrator
700 of FIGS. 7A and 7B and is quite similar to the solar
concentrator 800 of FIGS. 8A and 8B. The solar concentrator 900 has
a main body 931 having a shape less than half an ellipse. In the
embodiment illustrated in FIGS. 9A and 9B, the ellipse is nearly
circular such that the two foci 748, 754 are close together and the
light source 126 and the solar energy collector 128 can be mounted
on a single circuit board more easily and reduce the dead space
between the light source 126 and the light collection area. This
pie-shaped design can be useful for tiling a plurality of solar
concentrators 900, as described below with respect to FIGS. 23A and
23B.
[0124] In any of the above elliptical or partial elliptical
embodiments, instead of having a single pulsed probe light source
126 positioned at the first focal point 748, there can be two or
more pulsed probe light sources 126 in the vicinity of the first
focal point 748, and an equal number of solar energy collectors 128
in the vicinity of the second focal point 754, where the light from
each pulsing probe light source is focused onto each solar energy
collector.
[0125] Alternatively, each of the focal points 748, 754 can have
both a pulsed light source 126 and a solar energy collector 128
located in its vicinity. Pulsed probe light 134 emitted by the
light source 126 at the first focal point 748 is augmented and
convergent upon the solar energy collector at the second focal
point 754. Similarly, Pulsed probe light 134 emitted by the light
source 126 at the second focal point 754 is augmented and
convergent upon the solar energy collector at the first focal point
748. The light sources can have the same or different wavelengths
to excite one or more types of luminescent particles
respectively.
[0126] Additionally, in embodiments where there are two or more
pulsed probe light sources 126, the light sources 126 can alternate
or otherwise be pulsed "on" at different times to produce a series
of augmented light pulses converging to the solar energy collector
at different times.
[0127] The period between the probe light pulses can be controlled
using a feedback system, particularly in embodiments where the
light source 126 and the solar energy collector 128 are mounted on
a single circuit board. Such a feedback system can measure and take
into account the power arriving at the solar energy collector
128.
[0128] A pulsed stimulated emission luminescent light-guide solar
concentrator 1000A that is pie-like in shape is shown in FIG. 10A.
This embodiment is the same as the embodiment of FIGS. 9A-9B except
that in this embodiment, the peripheral edge 1074 comprises a
plurality of reflecting facets 1070. The reflecting facets 1070 are
curved and the curve can be, for example, portions of an ellipse, a
circle, a parabola, or a non-analytical curve. Five elliptically
curved reflecting facets 1070 sharing two common foci 748, 754 are
shown in FIG. 10A; however this is shown merely for illustrative
purposes, and the number of reflecting facets 1070 may be increased
or decreased in other embodiments. If the peripheral edge 1074
comprises many small reflecting facets 1074, the peripheral edge
1074 can be substantially planar; the smaller the facets 1070, the
more planar the peripheral edge 1074 can be made, as shown in FIG.
10B. As in the embodiments of FIGS. 8A, 8B, 9A and 9B, the
light-guide 1032 of solar concentrator 1000A has a planar edge 958
along which the two foci 748, 754 of an ellipse are located. The
pulsed light source 126 is optically coupled to the planar edge 958
at the first focal point 748 and a solar energy collector 128 can
be positioned at the second focal point 754. Pulsed probe light 134
is emitted by the light source 126 and propagates towards the
peripheral edge 1074 where it is reflected and redirected towards
the second focal point 754 by the reflective facets 1070. The
reflective facets 1070 can have a mirror coating like that of the
reflective surface 744. The advantage of the design in FIG. 10B is
that is allows for very close packing, as shown in FIG. 10C.
[0129] A pulsed stimulated emission luminescent light-guide solar
concentrator 1000D in the shape of a square disk is shown in FIG.
10D. In this embodiment, the peripheral edges 1074 of the
light-guide 1033 have a plurality of reflective facets 1070. As
described above, the reflective facets 1070 are curved and the
curve can be, for example, portions of a curve, such as an ellipse,
a circle, a parabola, or a non-analytical curve. An advantage of
using portions of curves rather than an entirely curved peripheral
reflective edge to refocus the light, is that the solar
concentrator can be made into a shape that is easier to stack or
tile, for example, a square as shown in FIG. 10D. In the embodiment
of FIG. 10D, each of the reflective facets 1070 reflects light from
the pulsed light source 126 near the center of the disk back
towards a light collection area also near the center of the disk,
where a solar energy collector 128 can be placed. The reflected
augmented rays 146 are shown at a slight angle for clarity, but in
fact the reflected augmented rays 146 would overlap the probe light
134 coming from the pulsed light source 126 at the center of the
sphere. The idea of breaking up a smooth curved reflective surface
644, 744, 844, 944 into a series of small curved reflective facets
1070 to form a substantially flat peripheral edge can apply to any
of the embodiments described herein.
[0130] It is possible to devise systems where the main body absorbs
incoming sunlight 116, but is not exposed to excessive
concentration. All of the embodiments described above absorb
incident light 116 and concentrate light 134, 138, 146 within the
main body 131, 231, 531, 731, 831, 931 of the solar concentrator.
FIGS. 11A and 11B show a bi-layer pulsed stimulated emission
luminescent light-guide solar concentrator 1100, having the shape
of a circular disk. The bi-layer solar concentrator 1100 includes a
luminescent sheet 1111 (similar to the main body 531 of the solar
concentrator 500 of FIGS. 5A-5C) and a secondary light-guide 1184
adjacent to the luminescent sheet 1111.
[0131] The luminescent sheet 1111 has a disk-shaped primary
light-guide 1132 and a thin disk-shaped luminescent layer 124. As
in the embodiments of FIGS. 2A, 5A, 6A, the primary light-guide
1132 has a first surface 1107 and a second reflective surface 1104.
The luminescent layer 124 is optically coupled to the first surface
1107 of the primary light-guide layer 1132. Alternatively, the
luminescent sheet 1111 can comprise a primary light-guide layer
1132 that has luminescent particles impregnated within its main
body, as in FIG. 2B. In the embodiment illustrated in FIG. 11A, the
luminescent layer 124 has a circular or annular light collection
surface 1102. Sunlight 116 incident on the light collection surface
1102 enters the luminescent sheet 1111 and is absorbed, at least in
part, by the luminescent particles 130 in the luminescent sheet
1111. The secondary light-guide 1184 has a first reflective surface
1151 and a second reflective surface 1153.
[0132] The bi-layer solar concentrator 1100 has a reflective
surface 1144 on its peripheral edge 1142. The reflective surface
1144 can, for example, be a mirror coating made of metal, such as
aluminum or silver, a dielectric or any other suitable material
known in the art. Incident sunlight 116 enters the luminescent
sheet 1111 via the light collection surface 1102 and excites
luminescent particles 130 contained in the luminescent sheet 1111.
The pulsed probe light 126, positioned at the center of the primary
luminescent solar concentrator 1111, simulates the excited
luminescent particles 130 such that they emit photons having a
frequency or frequencies within the emission spectrum or spectra of
the luminescent particles. The emitted photons travel in phase with
the probe light 134 towards the peripheral edge of 1142, where it
is reflected by the reflective surface 1144.
[0133] The secondary light-guide 1184 is optically coupled to the
primary luminescent solar concentrator 1111 along the peripheral
edge 1142. At least the upper portion of the reflective surface
1144, adjacent to the luminescent sheet 1111 is disposed at an
oblique angle to the plane of the luminescent sheet 1111 and the
secondary light-guide 1184 such that the light 146 augmented in the
luminescent sheet 1111 is reflected by the reflective surface 1144
into the secondary light-guide 1184. The augmented light 146
propagates in the secondary light-guide 1184 towards the exit
surfaces or surfaces 1106 of the secondary light-guide 1184. The
bi-layer solar concentrator 1100 illustrated in FIGS. 11A and 11B
has four intersecting exit surfaces 1106 which form a square recess
1188 at the center 1137 of the bi-layer solar concentrator 1100.
Four solar energy collectors 1186 can be optically coupled to the
exit surfaces 1106 of the secondary light-guide 1184 to harvest the
solar energy.
[0134] Light is guided within the luminescent sheet 1111 via TIR on
the light collection surface 1102 and the second reflective surface
1104, and within the secondary light-guide 1184 via TIR on the
first surface 1151 and the second surface 1153. A gap 1190 filled
with a material of lower refractive index than the
light-transmissive material(s) of the luminescent sheet 1111 and
the secondary light-guide 1184 is provided to facilitate TIR on the
second reflective surface 1104 of the luminescent sheet 1111 and
the first reflective surface 1151 of the secondary light-guide
1184. The gap can be filled by air, silicone or any other suitable
optical encapsulant, bonding or cladding material. This gap 1190
does not extend all the way to the peripheral reflective edge 1142
in order to allow the augmented light 146 guided by the luminescent
sheet 1111 to be optically coupled into the secondary light-guide
1184.
[0135] Referring to FIG. 11C, a wavefront of light 1194 is shown
expanding outwards through the luminescent sheet 1111. The graph in
FIG. 11D shows the captured power and the intensity as the
wavefront moves out from the center towards the peripheral edge
1142 of the solar concentrator 1100. The captured power increases
as the wavefront 1194 of light expands over the whole disk. The
intensity is shown to drop slightly over the same distance.
Depending on the particular luminescent dye and the amount of
available sunlight, the intensity can drop, increase, or stay
constant. FIG. 11E shows the same wavefront 1194, having been
reflected by the reflective surface 1144 on the peripheral edge
1142 of the solar concentrator 1100, converging in the secondary
light-guide 1184 towards the light collection area at the center of
the disk where the solar energy collectors 1186 are located. The
graph in FIG. 11F shows the captured power and the intensity as the
wavefront moves inward towards the exit surfaces 1106 of the
secondary light-guide 1184. The captured power drops off slightly
as the light converges towards the center of the disk. This is due
to scattering mechanisms and absorption in the bulk. No new
sunlight is being captured in the secondary light-guide 1184. The
intensity on the other hand increases dramatically as light
converges towards the center of the disk. In the embodiment shown
in FIGS. 11A and 11B, the solar energy collectors 1186 are
positioned at a point of high intensity, where the light is very
concentrated.
[0136] One advantage of using bi-layer stimulated emission
luminescent solar concentrators is that the highest concentration
only occurs in the secondary light-guide 1184, away from the
luminescent dye. The secondary light-guide 1184 can be made of a
resilient light-transmissive material such as glass, and the
concentrations achievable could be in the range of 1000-3000 suns,
although, theoretically in ideal conditions, concentrations up to
10000 suns could be achieved. Such super high concentrations might
damage dyes, but using a bi-layer design prevents the dye from
experiencing the high flux associated with high concentration.
[0137] While the bi-layer solar concentrator 1100 described with
reference to FIGS. 11A-11F was circular disk-shaped, bi-layer solar
concentrators can be elliptical, pie-shaped or any other suitable
shape.
[0138] The arrangement and means of coupling light between the
luminescent sheet 1111 and the secondary light-guide 1184 and means
of coupling light between the secondary-light guide 1184 and a
solar energy collector 128 of bi-layer pulsed stimulated emission
luminescent light-guide solar concentrators is the subject of FIGS.
12A-15B. The embodiments described below (FIGS. 12A through 15B)
show cross sectional views of circular disk-shaped solar
concentrators, but the concepts are equally applicable to any of
the above described shapes.
[0139] The bi-layer solar concentrator 1200 of FIG. 12A is
generally similar to the bi-layer solar concentrator 1100 of FIGS.
11A-11F. In this embodiment, the luminescent sheet 1111 and the
secondary light-guide 1184 are joined by a semi-circular optic
1295. The exit surface 1297 of the luminescent sheet 1111 and the
input surface 1287 of the secondary light-guide 1184 are optically
coupled to the semi-circular optic 1295. An external reflector 1296
is placed over (i.e., covers the peripherally exposed surface of)
the semi-circular optic 1295, although the peripheral surface of
the semi-circular optic 1295 could alternatively be coated with a
reflective material. Augmented light 146 (which includes photons
from the light source 126 and photons from stimulated emission) is
transmitted through the luminescent sheet 1111, and is emitted from
the exit surface 1297, such that it is transmitted into the
semi-circular optic 1295, as shown in FIG. 12B. Some of the
augmented light 146 is redirected within the semi-circular optic
1295 via total internal reflection (such as at 1298). Some light
exits the semi-circle optic 1295 and is reflected by the external
reflector 1296 (such as at 1229). In this manner the augmented
light 146 is redirected into the secondary light-guide 1184, where
it is transmitted via TIR and converges towards the light
collection area at or near the center of the disk.
[0140] In this embodiment, the secondary light-guide 1211 has an
output surface 1217 that is annular. The bi-layer solar
concentrator 1200 therefore has a circular recess at its center,
bounded by the luminescent sheet 1111 and the output surface 1217
of the secondary light-guide 1211, and continuous with the gap
1290. A secondary optical element 1211 is positioned within the
circular recess, a detailed view of which is shown in FIG. 12C. The
secondary optical element 1211 has an annular input surface 1213
which is optically coupled to the annular output surface 1217 of
the secondary light-guide 1184. The secondary optical element
guides the augmented light 146 received through the annular input
surface 1213 toward a light collection area where a solar energy
collector 128 can be placed to harvest the solar energy.
[0141] The secondary optical element has a curved surface 1215,
which redirects the augmented light 146 towards the light
collection area where a solar energy collector 128 can be placed.
The curved surface can redirect the light 146 via total internal
reflection, or by means of a curved mirror insert 1219. The curved
mirror insert has at least one reflective surface 1230 that has the
same curvature as the curved surface 1215 of the secondary optical
element 1211. Alternatively, the secondary optical element 1211 can
have a mirror coating on its curved surface 1215.
[0142] In the solar concentrator module shown in FIG. 12A, the
solar energy collector 128 lays substantially parallel to the light
collection surface 102 of the solar energy collector 1200. The
solar energy collector 128, which can be a photovoltaic cell, can
be mounted on a circuit board 1221 and is optically coupled to the
exit surface 1223 of the secondary optical element 1211 by means of
optical bonding agent 1225. A reflective coating 1227 can be
applied to the surface of the circuit board 1221 in order to
prevent losses due to absorption by elements other than the solar
energy collector 128. A bypass diode 1229, typical of concentrator
cells, is shown attached to the backside of the circuit board
1221.
[0143] FIGS. 13A-13C shows the same bi-layer solar concentrator
module as FIGS. 12A-12C, only in an inverted orientation, such that
sunlight 116 enters the solar concentrator 1200 through the second
surface 1153 of the secondary light-guide 1184. Because the
bi-layer solar concentrator 1200 is substantially a disk of
light-transmissive material such as glass with a light collecting
system at its center, light passes through the solar concentrator
1200 largely undisturbed and can be concentrated onto the solar
energy collector 128 as described with reference to FIGS. 12A-12C.
This embodiment allows for the solar energy collector 128 and the
light source 126 to be mounted on the same circuit board.
Antireflection coatings can be applied to the second surface 1104
of the luminescent sheet 1111 and to the first reflective surface
1151 of the light-guide 1184 to reduce Fresnel losses at those
interfaces. The solar concentrators described herein and above are
bi-facial, i.e., they can be made to function in either
orientation.
[0144] FIGS. 14A-14C shows the same solar concentrator module as
FIG. 12A-12C, except that the secondary optical element 1121 is in
an inverted position such that the solar energy collector 128 is
disposed in the gap 1190. The solar energy collector 128 and light
source 126 can therefore both be mounted on the same circuit board
1441.
[0145] The bi-layer pulsed stimulated emission luminescent
light-guide solar concentrator 1500, shown in FIGS. 15A and 15B, is
similar to the bi-layer solar concentrator 1300 of FIGS. 13A-13C,
but has an inverted orientation such that incoming sunlight 116
enters the system through the second surface 1153 of the secondary
light-guide 1148. In this embodiment, the luminescent sheet 1111 is
very thin compared to the secondary light-guide layer 1148. A thin
luminescent sheet can have two advantages: (1) a relatively high
flux can be maintained in the luminescent sheet 1111 to keep the
probability of stimulated emission high; and (2) the need for the
external reflector 1296 can be eliminated. Light entering the
semi-circular optic 1295 will TIR and couple into the secondary
light-guide 1184.
[0146] As array of pulsed stimulated emission luminescent
light-guide solar concentrators can be interconnected to form a
stimulated emission solar concentration assembly 1600 as shown in
FIGS. 16A-16C. Any of the embodiments of a solar concentrator of
the present invention may be employed in this manner, though some
shapes are more tileable than others.
[0147] The solar concentration assembly 1600 can be arranged such
that the solar concentrators 1601 are spaced apart from each other,
as shown in FIGS. 16A and 16B. In this embodiment a scattering
reflector sheet 1661 is placed bellow the array of solar
concentrators 1601, such that any incoming light 218 that misses or
passes through the solar concentrators 1601 can be scattered and
reflected by the scattering reflector sheet 1661 and potentially
absorbed by one of the solar concentrators 1601. The bifacial
nature of the solar concentrators 1601 (which may be any of the
solar concentrators described above) can therefore be used to
achieve higher concentration with fewer optical components.
[0148] In a similar embodiment, the solar concentration assembly
can be made from an array of square, pie or pie-like shaped solar
concentrators, such as those described in FIGS. 9A-10D. An
advantage of these shapes is that they are tileable such that
compact solar concentration assemblies can be made.
[0149] A stimulated emission solar concentration assembly 1700 with
a central pulsed light source 1755 is shown in FIG. 17A. This
embodiment is generally similar to that of FIGS. 16A-16C. The
central pulsed light source 1755 can be used to generate a probe
light 134 for a plurality of solar concentrators 1701 of the solar
concentration assembly 1700. The light source 1755 feeds light into
optical fibers 1757 that deliver the light to the plurality of
solar concentrators 1701 of the solar concentration assembly 1700.
FIG. 17B shows how the end 1759 of the optical fiber 1757 can be
structured with an inverted triangle shape which can serve as a
side emitter. Light delivered by an optical fiber can be used in
the exact same way as light from a diode, and has the advantage of
requiring only one, central light source rather than several, which
may reduce costs and be less prone to failure.
[0150] A finite element model was created to model the collection
of energy in a stimulated emission luminescent light-guide solar
concentrator in the shape of a circular disk. A dye system is
modeled using a phosphorescent dye Pt-(TPBP) (a platinum-porphyrin
derivative), with absorption maxima at 430 nm and 615 nm, and an
emission peak at 772 nm. The circular solar concentrator is divided
into a series of annular rings, with a radial light source
introduced at the center to act as the probe. A sheet thickness of
100 mm is used to maintain a high probe light intensity. In each
ring the absorbed solar power is determined along with the
probability of stimulated emission and reabsorption.
P.sub.n=P.sub.n-1+P.sub.solar,np.sub.stim-P.sub.abs,n
[0151] P.sub.n--power in ring n
[0152] P.sub.solar,n--absorbed solar power in ring n
[0153] P.sub.stim--probability of stimulated emission
[0154] P.sub.abs,n--power lost to reabsorption in ring n
[0155] This first-order model gives a lower bound to the power that
can be extracted from the luminescent solar concentrator as it
overestimates losses. The model considers photons that are absorbed
by the dye to be irretrievably lost, while reabsorption actually
leads to an excited dye molecule that once again relaxes via
spontaneous or stimulated emission. There also exists an
overestimate of lost energy to spontaneous emission, which occurs
with probability 1-p.sub.stim. Spontaneously emitted photons that
are emitted at large angles relative to the solar concentrator are
lost from the system, however those photons that remain in the
solar concentrator will eventually be reabsorbed, whereupon there
is again a chance to undergo stimulated emission.
[0156] FIG. 18A shows the cumulative absorbed solar power and power
stimulated in the probe mode as a function of the radial distance,
moving from the inner radius of 3.0 mm, out to the edge of the disk
at 10 cm. FIG. 18B shows the probability of stimulated emission as
a function of the radial distance with an initial probe intensity
of 530 kW/cm2. The probability of stimulated emission falls with
increasing distance from the center as the area illuminated by the
probe light (from original probe and stimulated emission) increases
with radius.
[0157] Different dies and geometries can be used in order to reduce
the requirements for initial probe intensity power and to improve
the sunlight absorption efficiency. However, the first order models
demonstrate that it is possible to achieve a net gain in power by
capturing sunlight in this way.
[0158] Higher efficiency can be gained for stimulated emission
luminescent light guide solar concentrators if multiple luminescent
materials are used, each of which reacts to a different portion of
the solar spectrum. For example, FIG. 19 shows a single luminescent
material with an absorption spectrum 1900 and an emission spectrum
1902. FIG. 20 shows three distinct sets of absorption spectra 2000,
2004, 2008 and corresponding emission spectra 2002, 2006, 2010,
(each denoted by a different line style), representing the
luminescence characteristics of three different luminescent
materials. A first luminescent material has the absorption spectrum
2000 and emission spectrum 2002, a second luminescent material has
the absorption spectrum 2004 and emission spectrum 2006, and a
third luminescent material has the absorption spectrum 2008 and
emission spectrum 2010. The first luminescent material has a peak
absorption at approximately 375 nm, the second material has a peak
absorption at approximately 475 nanometers and the third material
has a peak absorption at approximately 575 nanometers. These
wavelengths and absorption emission curves are used for
illustrative purposes and are meant to be non-limiting. In fact,
any number of luminescent materials or dyes could be employed with
absorption and emission spectra in the ultraviolet band, the
visible band, the near infrared band or the infrared band or any
combination thereof. For clarity, in the subsequent examples, three
luminescent materials will be used and they will be referred to as
short wavelength, medium wavelength, and long wavelength materials
to denote that each material acts on a different, but related,
portion of the spectrum. For example, these portions of the
spectrum could be blue, green, and red but other portions of the
spectrum are equally applicable.
[0159] FIGS. 21A and 21B show a multi-layer pulsed stimulated
emission luminescent light-guide solar concentrator 2100. Multiple
layers of different luminescent material can be combined in a
single device as shown. Three solar concentration layers, a short
wavelength luminescent layer 2112, a medium wavelength luminescent
layer 2114, and a long wavelength luminescent layer 2116 are
stacked, with cladding layers 2118 separating them. Each layer is a
luminescent solar concentrator, with a pulsed light source 2120,
2122, 2124 and a light collection area where a solar energy
collector 2126, 2128, 2130 can be placed. Each of the light sources
2120, 2122, 2124, are adapted to emit the spectrum of light
required to stimulate emission in the corresponding luminescent
layers 2112, 2114, 2116. The solar energy collectors 2126, 2128,
2130 are selected to efficiently convert the portion of the spectra
being emitted in their corresponding layer. The solar energy
collectors 2126, 2128, 2130 and the pulsed light sources 2120,
2122, 2124 can be mounted onto substrates 2132 and 2134. The
cladding 2118 has a lower index of refraction than the solar
concentration layers 2112, 2114, 2116 it separates and prevents the
intensified probe light 2136, 2138, and 2140, from leaving one
layer and entering another. The cladding 2118 could be made, for
example, out of fluorinated ethylene propylene or another low index
material. The layers could be made from glass or polymers.
[0160] A three-layer, pie-shaped stimulated emission luminescent
solar concentrator 2200 is shown in FIGS. 22A-22C. The solar
concentrator 2200 has an elliptical peripheral edge 2242 with a
reflective surface 2244 for reflecting augmented light 146, such
that the reflected light converges towards the a solar energy
collector.
[0161] The solar concentration layers are a short wavelength
luminescent layer 2212, a medium wavelength luminescent layer 2214,
and a long wavelength luminescent layer 2216. Three pulsed light
sources 2220, 2222, 2224 and three solar energy collectors 2226,
2228, 2230 can either be aligned vertically, as shown in FIG. 22A,
so that each is perfectly at the focal point of the ellipse (as
shown in FIG. 22A) or they can be misaligned slightly (as shown in
FIGS. 22B and 22C). Moving both the light emitting device and solar
energy collectors slightly away from the focal points can provide
more space for wiring and mounting to circuit board 2246. A single
substrate 2246 can be used to mount all the light emitting devices
and photovoltaic cells as shown in FIG. 22D.
[0162] A solar panel assembly 2300 comprising a plurality of
pie-shaped pulsed stimulated emission luminescent light-guide solar
concentrators 2301 is shown in FIGS. 23A and 23B. Solar
concentrators 2301, are arranged into an array and packed tightly
to form a panel.
[0163] A pulsed stimulated emission luminescent light-guide solar
concentrator 2400 similar to that of FIGS. 7A and 7B, but having a
compound, planar disk shape is shown in FIG. 24A. The disk shape of
the solar concentrator 2400 comprises a first parabolic portion
2470 facing a second parabolic portion 2472. The two parabolic
portions 2470, 2472 may be joined by a rectangular portion 2474 as
shown in FIG. 24A, but need not have such a rectangular portion.
The first parabolic portion 2470 has a first focal point 2448 at
which a puled light source 126 is placed. The second parabolic
portion 2472 has a second focal point 2454 where a solar energy
collector 128 can be placed. The solar concentrator 2400 has a
peripheral reflective edge 2442 that can reflect light via TIR
without the need for a reflective coating. Probe light 134 spreads
out from the first focal point 2448 is augmented by stimulated
emission, is reflected via TIR by the peripheral reflective edge
and the augmented light 146 converges on the second focal point
2454.
[0164] An array of solar concentrators 2400, which are connected at
their distal ends, is shown in FIG. 24B. A stimulated emission
solar concentration assembly comprising such an array of solar
concentrators 2400 would facilitate the use of a single circuit
board for mounting the light source 126 of one solar concentrator
2400 and the solar energy collector 128 of a neighbouring solar
concentrator 2400. Similar embodiments could be made, having a
bi-layer or a multi-layer design.
[0165] Modifications and improvements to the above-described
embodiments of the present invention may become apparent to those
skilled in the art. The foregoing description is intended to be
exemplary rather than limiting. The scope of the present invention
is therefore intended to be limited solely by the scope of the
appended claims.
* * * * *