U.S. patent application number 14/180341 was filed with the patent office on 2015-08-13 for metamaterial enhanced thermophotovoltaic converter.
This patent application is currently assigned to Palo Alto Research Center Incorporated. The applicant listed for this patent is Palo Alto Research Center Incorporated. Invention is credited to Bernard D. Casse.
Application Number | 20150228836 14/180341 |
Document ID | / |
Family ID | 52446259 |
Filed Date | 2015-08-13 |
United States Patent
Application |
20150228836 |
Kind Code |
A1 |
Casse; Bernard D. |
August 13, 2015 |
Metamaterial Enhanced Thermophotovoltaic Converter
Abstract
A thermophotovoltaic (TPV) converter includes a
spectrally-selective metamaterial emitter and an associated
"matched" photovoltaic (PV) cell. The PV cell has an optimal
conversion spectrum (i.e., the wavelength range of in-band photons
efficiently converted into electricity). The metamaterial emitter
is fabricated with bull's eye (circular target-shaped) structures
made up of concentric circular ridges that are set at a fixed
grating period roughly equal to the associated optimal conversion
spectrum. When the emitter is heated to a high temperature (i.e.,
above 1000.degree. K), thermally excited surface plasmons generated
on the concentric circular ridges produce a highly-directional
radiant energy beam having a peak emission wavelength that is
roughly equal to the fixed grating period and is directed onto the
PV cell. The metamaterial emitter is optionally provided with
multiple bull's eye structures in a multiplexed (overlapping)
pattern and with different grating periods to produce a broad
emission spectrum overlapping the optimal conversion spectrum.
Inventors: |
Casse; Bernard D.;
(Saratoga, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Palo Alto Research Center Incorporated |
Palo Alto |
CA |
US |
|
|
Assignee: |
Palo Alto Research Center
Incorporated
Palo Alto
CA
|
Family ID: |
52446259 |
Appl. No.: |
14/180341 |
Filed: |
February 13, 2014 |
Current U.S.
Class: |
136/253 ;
438/56 |
Current CPC
Class: |
H01L 31/0549 20141201;
H01L 31/02325 20130101; B82Y 30/00 20130101; Y02E 10/52 20130101;
B82Y 40/00 20130101; G02B 5/008 20130101; H02S 30/10 20141201; B82Y
20/00 20130101; H02S 10/30 20141201; H02S 40/22 20141201; H01L
31/02322 20130101 |
International
Class: |
H01L 31/08 20060101
H01L031/08; H01L 31/0232 20060101 H01L031/0232 |
Claims
1. A thermophotovoltaic (TPV) converter comprising: a
spectrally-selective metamaterial emitter including one or more
bull's eye structures disposed on a solid base substrate, each of
said one or more bull's eye structures including a plurality of
concentric circular ridge structures separated by intervening
circular grooves such that each adjacent pair of ridge structures
is separated by a fixed grating period; and a photovoltaic (PV)
cell fixedly disposed to receive radiant energy generated by said
spectrally-selective metamaterial emitter, said PV cell including
means for converting in-band photons at a higher efficiency than
out-of-band photons, said in-band photons having associated first
wavelengths within an optimal conversion wavelength range, and said
out-of-band photons having associated second wavelengths outside of
said optimal conversion wavelength range, wherein said one or more
bull's eye structures are configured such that, when the
metamaterial emitter is heated to a temperature above 1000.degree.
K, radiant energy is emitted from said one or more bull's eye
structures having a peak emission wavelength that is within said
optimal wavelength range, whereby said spectrally-selective
metamaterial emitter is matched to the PV cell.
2. The TPV converter of claim 1, wherein the optimal wavelength
range of said PV cell is 1 microns to 3 microns, and wherein said
one or more bull's eye structures are configured such that the
fixed grating period separating the concentric circular ridge
structures of each of said one or more bull's eye structures is in
the range of 10 nanometers and 5.0 microns.
3. The TPV converter of claim 2, wherein the PV cell comprises a
low-bandgap PV cell and the optimal wavelength range of said PV
cell is 1.0 microns to 2.0 microns, and wherein said one or more
bull's eye structures are configured such that the fixed grating
period is in the range of 0.5 microns and 3.0 microns.
4. The TPV converter of claim 3, wherein said spectrally-selective
metamaterial emitter consists entirely of metal.
5. The TPV converter of claim 1, wherein said one or more bull's
eye structure comprises a plurality of bull's eye structures
disposed in sets on said base substrate, each said set comprising a
first bull's eye structure including a first group of concentric
circular ridge structures having a first fixed grating period, and
a second bull's eye structure including a second group of
concentric circular ridge structures having a second fixed grating
period, and wherein the second fixed grating period is larger than
the first fixed grating period.
6. The TPV converter of claim 1, wherein said one or more bull's
eye structure comprises a first bull's eye structure and a second
bull's eve structure disposed on said base substrate, said first
bull's eye structure including a first group of concentric circular
ridge structures, and said second bull's eye structure including a
second group of concentric circular ridge structures, and wherein
the first and second bull's eye structures are multiplexed such
that at least some of the circular ridge structures of the first
group intersect at least some of the circular ridge structures of
second group.
7. The TPV converter of claim 6, wherein said first group of
concentric circular ridge structures of said first bull's eye
structure have a first fixed grating period, and said second group
of concentric circular ridge structures of said second bull's eye
structure have a second fixed grating period, and wherein the
second fixed grating period is larger than the first fixed grating
period.
8. A TPV converter comprising: a metamaterial emitter including: a
box-like enclosure at least partially formed by a peripheral wall
including spaced-apart first and second wall portions respectively
having first and second inward-facing surfaces that faces an
interior cavity of the enclosure, and respectively having first and
second outward-facing surfaces that face away from the interior
cavity, at least one first bull's eye structure disposed on the
first outward-facing surface of the first wall portion and
including a plurality of first concentric circular ridge structures
disposed in an associated first fixed grating period such that,
when the metamaterial emitter is heated to a temperature above
1000.degree. K, a first radiant energy beam is emitted from said
one or more first bull's eye structures having a peak emission
wavelength that is roughly equal to said first fixed grating
period, and at least one second bull's eye structure disposed on
the second outward-facing surface of the second wall portion and
including a plurality of second concentric circular ridge
structures disposed in an associated second fixed grating period
such that, when the metamaterial emitter is heated to said
temperature above 1000.degree. K, a second radiant energy beam is
emitted from said one or more second bull's eye structures having a
peak emission wavelength that is roughly equal to said second fixed
grating period; and first and second photovoltaic (PV) cells
fixedly disposed adjacent to the spectrally-selective metamaterial
emitter such that said first PV cell is positioned to receive said
first radiant energy beam emitted from said first bull's eye
structure, and such that said second PV cell is positioned to
receive said second radiant energy beam emitted from said second
bull's eye structure.
9. The TPV converter of claim 3, wherein said box-like enclosure
comprises an all-metal structure.
10. The TPV converter of claim 9, wherein said box-like enclosure
comprises one or more refractory metals.
11. The TPV converter of claim 9, wherein said box-like enclosure
comprises an inlet end and outlet end, wherein said peripheral wall
includes first and second peripheral wall portions disposed in an
opposing spaced-apart relationship and respectively extending
between of said inlet and outlet ends of said box-like enclosure
such that an inlet opening is defined between respective first end
portions of said first and second peripheral wall portions, and an
outlet opening is defined between respective second end portions of
said first and second peripheral wall portions, and wherein the at
least one bull's eye structure includes a first bull's eye
structure disposed on the first outward-facing surface of said
first peripheral wall portion, and a second bull's eye structure
disposed on the second outward-facing surface of said second
peripheral wall portion.
12. The TPV converter of claim 9, wherein said box-like enclosure
further comprises first and second compound parabolic trough
structures respectively integrally connected to the first end
portions of said first and second peripheral wall portions.
13. The TPV converter of claim 12, wherein said box-like enclosure
further comprises first and second funnel-shaped outlet structures
respectively integrally connected to the second end portions of
said first and second peripheral wall portions.
14. The TPV converter of claim 13, wherein the at least one bull's
eye structure includes a first array of multiplexed bull's eye
structures disposed on the first outward-facing surface of said
first peripheral wall portion, and a second array of multiplexed
bull's eye structures disposed on the second outward-facing surface
said second peripheral wall portion.
15. The TPV converter of claim 14, wherein the first and second
peripheral wall portions, the first and second compound parabolic
trough structures and the first and second funnel-shaped outlet
structures comprise a single refractory metal.
16. A method for producing a thermophotovoltaic (TPV) converter
including a spectrally-selective metamaterial emitter and a
photovoltaic (PV) cell, said PV cell including means for
efficiently converting into electricity radiant energy that is
directed onto said PV cell and has an associated optimal conversion
spectrum, the method comprising: determining the associated optimal
conversion spectrum of the PV cell; and fabricating the
spectrally-selective metamaterial emitter such that the
spectrally-selective metamaterial emitter includes one or more
bull's eye structures, each of said one or more bull's eye
structures including a plurality of concentric circular ridge
structures separated by a fixed grating period that is within the
determined associated optimal conversion spectrum; and fixedly
mounting the PV cell relative to the metamaterial emitter such
that, when the spectrally-selective metamaterial emitter is
subsequently heated to a temperature above 1000.degree. K, radiant
energy emitted from said one or more bull's eye structures is
directed onto the PV cell.
17. The method of claim 16, wherein fabricating the metamaterial
emitter comprises: utilizing photolithography to generate a
patterned mask on a planar surface of a solid metal substrate such
that the patterned mask includes a plurality of concentric circular
resist structures having said fixed grating period; utilizing the
mask to form said plurality of concentric circular ridge structures
on the planar surface; and removing said mask from the planar
surface of the solid metal substrate, thereby forming said one or
more bull's eye structure including said plurality of concentric
circular ridge structures having said fixed grating period.
18. The method of claim 17, wherein said solid metal substrate
comprises a first refractory metal, and wherein utilizing the mask
to form a plurality of concentric circular ridge structures
comprises one of: depositing a second refractory metal into the
intervening concentric circular slots of said mask; and etching
said solid metal substrate through said intervening concentric
circular slots of said mask.
19. The method of claim 16, wherein fabricating the metamaterial
emitter comprises forming said one or more bull's eye structures
with said fixed grating period in the range of 0.5 microns and 5.0
microns.
20. The method of claim 19, wherein the PV cell comprises a
low-bandgap PV cell having said optimal wavelength range of 1.0
microns to 2.0 microns, and wherein fabricating the metamaterial
emitter comprises forming a plurality of said bull's eye structures
having associated said fixed grating periods in the range of 0.5
microns and 3.0 microns.
Description
FIELD OF THE INVENTION
[0001] This invention relates to apparatus and methods for
generating electricity using thermophotovoltaic energy
conversion.
BACKGROUND OF THE INVENTION
[0002] Thermophotovoltaic (TPV) energy conversion involves the
conversion of heat to electricity, and has been identified as a
promising technology since the 1960's. A basic TPV system includes
a thermal emitter and a photovoltaic diode receiver. The thermal
emitter is typically a piece of solid material or a specially
engineered structure that generates thermal emission when heated to
a high temperature (i.e., typically in a range from about
1200.degree. K to about 1500.degree. K). Thermal emission is the
spontaneous radiation (emission) of photons due to thermal motion
of charges in the thermal emitter material. For normal TPV system
operating temperatures, the radiated photons are mostly at near
infrared and infrared frequencies. The photovoltaic diode receiver
includes a photovoltaic (PV) cell positioned to absorb some of
these radiated photons, and is constructed to convert the absorbed
photons into free charge carriers (i.e., electricity) in the manner
typically associated with conventional solar cells. In effect, a
solar energy system is a type of TPV energy conversion system where
the sun acts as the thermal emitter. However, the present invention
is directed to "earth-bound" TPV energy conversion systems in which
the thermal emitter is solid structure that is heated from an
external source (e.g., by concentrated sunlight or other heat
generator).
[0003] Although TPV energy conversion is promising in theory,
practical conventional TPV systems have achieved far lower
efficiencies than theoretically predicted. A TPV system can be
modeled as a heat engine in which the hot body (i.e., the heated
thermal emitter) is described as a blackbody radiation source
having a black body temperature T.sub.BB, and the relatively cold
PV receiver has a temperature T.sub.PV, whereby the theoretical
thermodynamic efficiency limit is given by the Carnot cycle
.eta..sub.Carnot=(T.sub.BB-T.sub.PV)/T.sub.BB. For a thermal
emitter temperature T.sub.BB equal to 1500.degree. K and a PV
receiver temperature T.sub.PV equal to 300.degree. K, a theoretical
efficiency.eta..sub.Carnot equals 0.8 (80%), which exceeds the
Shockley-Queisser limit (i.e., the maximum theoretical efficiency
of a solar cell using a p-n junction to collect power). In reality,
however, the efficiencies of conventional TPV systems are reported
to be below 10%. This is believed to stem from a mismatch between
the spectrum of the thermal emitter and the PV cell.
[0004] One reason for the lower realized efficiencies of
conventional TPV systems is related to carrier thermalization at
high temperatures caused by a mismatch between the emitted photons
and the PV cells. Thermal radiation from the thermal emitter (hot
body) of a TPV system has a spectral power density given by
Planck's law, and the peak wavelength .lamda..sub.max is given by
Wien's law (.lamda..sub.max.about.(2898/T.sub.BB) .mu.m). For
high-temperature emitters (1100.degree.
K.ltoreq.T.sub.BB.ltoreq.1500.degree. K), the peak wavelength
.lamda..sub.max is in the range of 1.9 to 2.6 .mu.m, which requires
the TPV system to utilize PV cells having low bandgap
semiconductors (i.e., around 0.5-0.8 eV). Using such low bandgap PV
cells requires the use of emitter materials having bandgaps closer
to 0.5 eV (.about.2.5 .mu.m) in order to obtain a larger fraction
of in-band photons at reasonable emitter temperatures (i.e.,
1100-1500.degree. K). If emitter materials having bandgaps below
0.5 eV are used, the PV cell performance suffers from high carrier
thermalization at the elevated temperatures required in TPV
systems.
[0005] What is needed is a thermophotovoltaic (TPV) converter that
overcomes the problems set forth above and converts heat energy to
electricity with a much higher efficiency than achieved using
conventional TPV approaches. What is particularly needed is a TPV
converter that achieves efficiencies of at least 25% with a wide
variety of different PV cell types.
SUMMARY OF THE INVENTION
[0006] The present invention is directed to a TPV converter that
utilizes a spectrally-selective metamaterial emitter and an
associated PV cell to convert heat energy (e.g., concentrated
sunlight or heat from combustion process) into electrical energy
(electricity) with efficiencies that are much higher than those
achieved using conventional TPV approaches. The
spectrally-selective metamaterial emitter includes a novel bull's
eye (circular target-shaped) structure that converts heat energy
into a highly directional, narrow bandwidth radiant energy beam
having a peak emission wavelength that is roughly equal to a fixed
grating period separating concentric circular ridge structures that
form the bull's eye structure. By fabricating a
spectrally-selective metamaterial emitter including bull's eye
structures having a fixed grating period that roughly equals (or
overlaps) the EQE curve (optimal conversion spectrum) of the
associated ("matched") PV cell, the TPV converter achieves an
efficiency of 25% or higher at elevated operating temperatures of
more than 1400.degree. K. Moreover, because the peak emission
wavelength of the radiant energy beam is easily changed by way of
changing the fixed grating period, the spectrally-selective
metamaterial emitter is effectively "tunable" (adjustable) to match
the optimal conversion spectrums of different PV cell types.
[0007] According to a generalized embodiment of the present
invention, the TPV converter utilizes a spectrally-selective
metamaterial emitter having one or more bull's eye structures
disposed on a planar surface of a plate-like (solid) base
substrate, and an associated PV cell disposed in a fixed
relationship to receive the radiant energy beam emitted from the
bull's eye structure(s). Each bull's eye structure includes
multiple concentric circular ridge structures that extend from the
planar surface of the base substrate and are separated by
intervening circular grooves, where each adjacent pair of ridge
structures is separated by a fixed grating period. The associated
PV cell includes one or more semiconducting materials that convert
light to electricity by capturing (absorbing) "in-band" photons
(i.e., photons having certain energies/wavelengths that are
conducive to capture by the PV cell's particular semiconducting
material composition) in way that generates electron-hole pairs,
which are then separated for extraction. The "in-band" photons
associated with the PV cell are photons that have
energies/wavelengths within the PV cell's optimal conversion
spectrum (i.e., as determined by the PV cell's External Quantum
Efficiency (EQE) curve), which in part depends on the associated PV
cell's composition). The associated PV cell's efficiency (i.e., the
ratio of photons absorbed/converted over the total number of
incident photons) is directly related to the percentage of incident
photons that are "in-band" photons (i.e., photons having energies
within its optimal conversion spectrum). That is, each PV cell
converts "in-band" photons at a higher efficiency than
"out-of-band" photons (i.e., photons having energies outside the
optimal conversion spectrum). According to an aspect of the present
invention, the bull's eye structure is configured to "match" the
EQE curve of the associated PV cell by fabricating the bull's eye
structure with a fixed grating period that is roughly equal to the
wavelength of an "in-band" photon that is within the optimal
absorption spectrum of the associated PV cell. Accordingly, when
the metamaterial emitter is heated to a temperature above
1000.degree. K, a radiant energy beam emitted from the bull's eye
structure has a peak emission wavelength that is within the
associated PV cell's optimal conversion spectrum, whereby the
spectrally-selective metamaterial emitter is "matched" to the
associated PV cell in the sense that a high percentage of the
photons forming the radiant energy beam are absorbable/convertible
by the associated PV cell. Further, when the metamaterial emitter
is heated to a temperature above 1400.degree. K, the overall
efficiency of the TPV emitter is increased to 25% or higher.
[0008] In accordance with an embodiment of the present invention, a
TPV converter utilizing a low-bandgap PV cell having "in-band"
photons that are in the optimal conversion wavelength range of 0.5
microns to 3 microns. In order to generate/emit a high percentage
of in-band photons for such low-bandgap PV cells, the metamaterial
emitter is fabricated to include bull's eye structures having fixed
grating periods in the range of 10 nanometers and 5 microns. In a
presently preferred embodiment, a metamaterial emitter is produced
with bull's eye structures having a grating period in the range of
0.5 to 3.0 microns (e.g., 1.5 microns) in order to match the
specific absorption curve of a selected low-bandgap (e.g., GaSb) PV
cell having a cell-specific optimal conversion wavelength range of
1.0 to 2.0 microns. By determining the optimal wavelength range of
a PV cell and then fabricating a "matching" spectrally-selective
metamaterial emitter (i.e., including bull's eye structures having
one or more fixed grating periods that are within the determined
associated optimal wavelength range), the present invention
facilitates the production of high-efficiency TPV converters that
utilize a wide range of PV cell types, thereby facilitating a
strategic selection of the associated PV cell type (e.g., based on
low-cost or high-performance) without sacrificing converter
efficiency.
[0009] According to an aspect of the present invention, the entire
metamaterial thermal emitter (i.e., both the base substrate and the
bull's eye" structure) are entirely formed using one or more
metals. This all-metal construction is critical for withstanding
high optimal operating temperatures (i.e., 1000 to 1500.degree. K)
without delamination (which can occur when one or more dielectric
are used), and because metallic surfaces support surface plasmons
(or spoof surface plasmons) that are required to facilitate the
efficient emission of the highly directional, narrow band radiant
energy and having the desired peak emission wavelength. In a
specific embodiment, both the base substrate and the bull's eye"
structure are entirely formed using one or more refractory metals
(e.g., Rhenium, Tantalum or Tungsten), or metal alloys, because
these metals are able to withstand the higher operating
temperatures (i.e., approaching 1500.degree. K) without melting or
deforming. In a presently preferred embodiment, both the base
substrate and the bull's eye" structure are entirely formed using
Rhenium or a Rhenium alloy because the ability of this metal/alloy
to withstand high temperatures is well known from their use in
high-performance jet and rocket engines.
[0010] According to a specific embodiment of the present invention,
the metamaterial thermal emitter of a TPV converter is configured
to include multiple bull's eye structures disposed in an array of
the base substrate in order to produce radiant energy beam with a
broad spectral bandwidth, in order to increase the number of
in-band photons that are efficiently converted by the associated PV
cell into electricity. In a specific embodiment, the bull's eye
structures are disposed in sets of two or more, with each bull's
eye structure of each set having a corresponding fixed grating
period between adjacent concentric circular ridges that is
different from the fixed grating periods of the other bull's eye
structures of that set. In an exemplary embodiment, the associated
PV cell is a low-bandgap PV cell having a bandgap of approximately
1.72 microns, and each set includes three bull's eye structures
respectively having concentric circular ridges that are formed with
corresponding fixed grating periods that are equal to 1.3 microns,
1.5 microns and 1.7 microns, respectively. The different bull's eye
structures of each set emit a mix of photons having the various
wavelengths determined by the different grating periods, thereby
producing a broadened emission spectrum that increases the number
of in-band photons for conversion by the associated PV cell.
[0011] In another specific embodiment, a metamaterial emitter of a
TPV converter is configured to include an array of bull's eye
structures arranged in a multiplexed (overlapping) pattern (i.e.,
such that at least some of the concentric circular ridge structures
of each bull's eye structure intersect at least some of the
concentric circular ridge structures of an adjacent bull's eye
structure, thereby concentrating the emitted radiant energy to
increase spectral bandwidth. Further, by disposing the bull's eye
structures in sets having different fixed grating periods, as
described above, the metamaterial emitter both concentrates and
combines adjacent narrowband spectra to produce a high energy
emission with a broader overall spectrum that can be used, for
example, to maximize the number of in-band photons converted by a
target PV cell, thereby maximizing the PV cell's output power
density.
[0012] According to an exemplary practical embodiment of the
present invention, the metamaterial emitter of a TPV converter
includes a box-like enclosure formed by a peripheral wall, with one
or more bull's eye structures disposed as described above on at
least two outward facing surfaces of the peripheral wall (i.e.,
such that at least two radiant energy beams are emitted in at least
two (e.g., upward and downward) directions from the metamaterial
emitter), and the TPV converter further includes at least two PV
cells positioned to receive the at least two radiant energy beams.
The peripheral wall of the emitter's box-like enclosure surrounds a
substantially rectangular interior cavity and includes an inlet
opening through which heat energy (e.g., concentrated sunlight) is
supplied into the cavity during operation, and an outlet opening
through which waste heat is allowed to exit the cavity. Each bull's
eye structure is configured to generate a radiant energy beam that
is matched to the associated PV cell positioned to receive the
radiant energy beam. In one embodiment, the box-like enclosure is
an all-metal structure to facilitate the required high operating
temperatures (i.e., 1000 to 1500.degree. K) without delamination
(which can occur with metal/dielectric structures). In a specific
embodiment, the all-metal box-like enclosure is formed entirely
from refractory metals (e.g., Rhenium, Tantalum or Tungsten) to
further enhance the enclosure's operational lifetime. In another
specific embodiment, at least one bull's eye structure is disposed
on the outward-facing surfaces of two opposing (e.g., upper and
lower) peripheral wall portions, thereby generating two radiant
energy beams that are directed in different directions from the
metamaterial emitter. This arrangement provides optimal energy beam
generation because the flat/planar wall surfaces facilitate
cost-effective fabrication of the bull's eye structures (i.e.,
using existing photolithographic fabrication techniques), and the
rectangular-shaped interior cavity defined between the two opposing
flat peripheral wall portions facilitates efficient transfer of
heat energy (e.g., by allowing concentrated sunlight to reflect
between the opposing interior surfaces as it propagates along the
interior cavity).
[0013] In yet another specific embodiment optimized for converting
concentrated solar energy into infrared emissions, the all-metal
box-like enclosure is configured to channel solar energy into the
interior cavity defined between the two opposing peripheral wall
portions in a manner that maximizes the transfer of heat energy to
the peripheral wall portions, which in turn maximizes the amount of
radiant energy emitted in first and second radiant energy beam
respectively emitted from the bull's eye structures formed on the
respective outward-facing surfaces to two associated PV cells
fixedly positioned to receive the first and second radiant energy
beams, respectively. First, a compound parabolic trough is formed
by corresponding metal structures that are respectively integrally
connected to corresponding front end portions of the opposing
peripheral wall portions, wherein the compound parabolic trough is
operably shaped to channel concentrated sunlight through the inlet
opening into the interior cavity such that it reflects between the
inside surfaces of the two opposing peripheral wall portions. In
addition, a funnel-shaped outlet is formed by corresponding metal
structures respectively integrally connected to the rear end
portions of the peripheral wall portions that releases waste heat
from interior cavity through the outlet opening in a manner that
enhances energy transfer to the bull's eye structures. To maximize
the amount of emitted radiant energy, multiple multiplexed bull's
eye structures are formed in arrays as described above on the
outward-facing surfaces of the peripheral wall portions. To
minimize thermal cycling stresses and to maximize the operating
lifetime of the metamaterial emitter, the entire all-metal box-like
enclosure (i.e., including the peripheral wall portions, the
compound parabolic trough structures, and the funnel-shaped outlet
structures) are constructed using a refractory metal (e.g.,
Rhenium, Tantalum or Tungsten) or a refractory metal alloy (e.g.,
Rhenium alloy).
[0014] In yet another embodiment of the present invention, a TPV
converter is produced by determining an optimal conversion spectrum
(wavelength range) associated with in-band photons converted by a
PV cell into electricity, and then fabricating a
spectrally-selective metamaterial emitter by generating a patterned
mask on a planar surface of a refractory metal substrate by way of
photolithography such that the patterned mask includes concentric
circular resist structures having a fixed grating period that is
roughly equal to the predetermined optimal wavelength, then
utilizing the mask to form concentric circular refractory metal
ridge structures on the planar surface having the fixed grating
period. In alternative embodiments, the concentric circular ridge
structures are formed either using an additive process (e.g., where
refractory metal, which can be the same or different from the base
substrate, is deposited by way of sputtering or other technique
into slots formed in the mask) or a subtractive process (e.g.,
where the base substrate is dry etched through the mask slots,
whereby the concentric ridge structures comprise the same
refractory metal as the base substrate). To generate multiplexed
bull's eye structures, the mask is formed with concentric circular
resist structures disposed in the desired multiplexed arrangement.
After forming the concentric circular ridge structures, the mask is
removed to expose the intervening concentric circular grooves
separating the ridges, thereby completing fabrication of one or
more bull's eye structures that generates radiant energy whose peak
emission wavelength is "matched" to the optimal conversion spectrum
of the PV cell. Once fabrication of the spectrally-selective
metamaterial emitter is completed, the PV cell is fixedly mounted
over the one or more bull's eye structures, thereby completing
production of the TPV converter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] These and other features, aspects and advantages of the
present invention will become better understood with regard to the
following description, appended claims, and accompanying drawings,
where:
[0016] FIG. 1 is a perspective view showing a partial TPV converter
according to an embodiment of the present invention;
[0017] FIG. 2 is a perspective view showing a TPV converter
according to a second embodiment of the present invention;
[0018] FIGS. 3(A), 3(B) and 3(C) are cross-sectional side views
taken along section lines 3A-3A, 3B-3B and 3C-3C, respectively, of
FIG. 2 showing bull's eye structures with different fixed grating
periods;
[0019] FIG. 4 is a perspective view showing a TPV converter
according to a third embodiment of the present invention;
[0020] FIG. 5 is a perspective view showing a TPV converter
according to a fourth embodiment of the present invention;
[0021] FIGS. 6(A) and 6(B) are perspective and cross-sectional side
views showing a TPV converter according to a fifth embodiment of
the present invention;
[0022] FIG. 7 is a flow diagram showing a generalized method for
producing a TPV converter according to another embodiment of the
present invention; and
[0023] FIGS. 8(A), 8(B), 8(C), 8(D), 8(E) and 8(F) are simplified
cross-sections illustrating a method for fabricating bull's eye
structures according to a specific embodiment of the method of FIG.
7.
DETAILED DESCRIPTION OF THE DRAWINGS
[0024] The present invention relates to an improvement in TPV
(heat-to-electricity) converters. The following description is
presented to enable one of ordinary skill in the art to make and
use the invention as provided in the context of a particular
application and its requirements. As used herein, directional terms
such as "upper", "upward", "lower", "downward", "over", "under",
"front" and "rear", are intended to provide relative positions for
purposes of description, and are not intended to designate an
absolute frame of reference. In addition, the phrases "integrally
formed" and "integrally connected" are used herein to describe the
connective relationship between two portions of a single fabricated
or machined structure, and are distinguished from the terms
"connected" or "coupled" (without the modifier "integrally"), which
indicates two separate structures that are joined by way of, for
example, adhesive, fastener, clip, or movable joint. Various
modifications to the preferred embodiment will be apparent to those
with skill in the art, and the general principles defined herein
may be applied to other embodiments. Therefore, the present
invention is not intended to be limited to the particular
embodiments shown and described, but is to be accorded the widest
scope consistent with the principles and novel features herein
disclosed.
[0025] FIG. 1 is a perspective top view showing a generalized TPV
converter 200 comprising a simplified spectrally-selective
metamaterial emitter 100 that converts heat energy E.sub.S into a
highly directional radiant energy beam E.sub.R, and an associated
PV cell 210 that is fixedly disposed to receive photons transmitted
in radiant energy beam E.sub.R.
[0026] Spectrally-selective metamaterial emitter 100 generally
includes a base substrate 111 and a novel bull's eye (circular
target-shaped) structure 120 that is integrally formed on base
substrate 111.
[0027] Base substrate 111 is a solid (wall-like) plate having a
planar lower (first) surface 112 and an opposing planar upper
(second) surface 113 on which bull's eye structure 120 is
integrally formed. During operation, lower surface 112 faces a
source of heat energy E.sub.S, and upper surface 113 faces away
from the heat energy source. Base substrate 111 is preferably
entirely constructed from metal, and more preferably is entirely
constructed using one or more refractory metals (e.g., Rhenium,
Tantalum, or Tungsten), or a refractory metal alloy (e.g., Rhenium
alloy). In an exemplary practical exemplary embodiment (e.g., when
used as part of TPV system 200), base substrate 111 has a thickness
T on the order of more than a wavelength of emitted radiant energy
E.sub.R (described below), but may have any arbitrary thickness
outside of this constraint.
[0028] Bull's eye structure 120 includes concentric circular ridge
structures 121-1, 121-2 and 121-3 that are integrally formed on
upper surface 113 of base substrate 111 (i.e., either formed from
the same material as base substrate 111 by a subtractive process
such as etching or milling, or formed by an additive process such
as sputtering that effectively melds (fuses) the added material to
the base substrate material). Ridge structures 121-1, 121-2 and
121-3 are respectively separated by intervening circular grooves
122-1, 122-2 and 122-3 that extend into (but not through) base
substrate 111 such that each adjacent pair of ridge structures is
separated by a fixed grating period (pitch distance) .LAMBDA.. For
example, ridge structures 121-1 and 121-2 are separated by circular
groove 122-1 such that the distance between an outside edge of
ridge structure 121-1 and and outside edge of ridge structure 121-2
is equal to the grating period .LAMBDA.. Similarly, ridge
structures 121-2 and 121-3 are separated by circular groove 122-2
such that the distance between an outside edge of ridge structure
121-2 and and outside edge of ridge structure 121-3 is equal to the
same grating period .LAMBDA. as that separating ridge structures
121-1 and 121-2. Ridge structures 121-1 to 121-3 comprise metal
that may be different from the material that forms base substrate
111, but preferably both the ridge structures and the base
substrate comprise the same metal material to avoid thermal
mismatch issues.
[0029] According to an aspect of the present invention, bull's eye
structure 120 is configured such that, when heat energy E.sub.S is
applied to lower surface 112 and is sufficient to heat base
substrate 111 to a temperature above 1000.degree. K, radiant energy
E.sub.R is emitted from upper surface 113 having a peak emission
wavelength .lamda..sub.peak that is roughly equal to (i.e., within
25% of) fixed grating period .LAMBDA.. A relationship between the
specific geometric dimensions associated with bull's eye structure
120 and the characteristics of emitted radiant energy beam E.sub.R
are explained in additional detail in co-owned and co-pending U.S.
patent application Ser. No. ______, entitled "SPECTRALLY-SELECTIVE
METAMATERIAL EMITTER" [Atty Docket No. 20131311US01 (XCP-186-1)],
which is incorporated herein by reference in its entirety.
[0030] According to another aspect of the present invention,
emitted radiant energy beam E.sub.R is highly directional (i.e.,
90% of the emitted radiant energy is within 0.5.degree. of
perpendicular (angle .theta.) to the planar outward-facing surface
113), narrow band (i.e., the full-width at half maximum of the
emitted radiant energy is within 10% of peak emission wavelength
.lamda..sub.peak), and peak emission wavelength that is roughly
equal to the fixed grating period .LAMBDA. separating each adjacent
pair of concentric circular ridge structures (i.e., the peak
emission wavelength is within 25% of the grating period .LAMBDA.).
Accordingly, metamaterial emitter 100 is selectively "tunable"
(adjustable) by way of adjusting the fixed grating period .LAMBDA.
separating the concentric circular ridge structures 121-1 to 121-3.
For example, if a first metamaterial emitter is found to generate
radiant energy whose peak wavelength is non-optimal (e.g., too low)
for a particular PV cell, then a second metamaterial emitter with
an appropriately adjusted (e.g., larger) fixed grating period can
be fabricated that generates a higher peak wavelength, thereby
effectively "tuning" the radiant energy to the optimal peak
wavelength.
[0031] Although emitter 100 can include grating periods of almost
any size, in the exemplary practical embodiment depicted in FIG. 1
(e.g., when used as part of TPV system 200 including a low-bandgap
PV cell 210), ridge structures typically has a grating period
.LAMBDA. in the range of 0.5 microns and 5.0 microns, which
corresponds with the absorption curves of most commercially
available PV cells. In a particularly preferred embodiment (e.g.,
when PV cell 210 is a GaSb PV cell having an optimal wavelength
range of said PV cell is 1.0 microns to 2.0 microns), emitter 100
is produced with a grating period .LAMBDA. in the range of 0.5
microns and 3.0 microns.
[0032] According to another aspect of the present invention,
metamaterial emitter 100 effectively functions as a narrowband
filter element (spectral control element) that only passes in-band
photons to an associated target (e.g., photons having wavelengths
within the EQE curve of a target PV cell), and is capable of
preventing out-of-band photons from reaching the target. This
filtering function is illustrated in FIG. 1, where the broadband
characteristics of the photons associated with heat energy E.sub.S
are identified using S-parameter S11 to indicate out-of-band
photons, and S-parameter S12 to indicate in-band photons. Because
bull's eye structure 120 is "tuned" to the EQE curve of PV cell
210, metamaterial emitter 100 effectively "passes" in-band photons
S12 to PV cell 120. Conversely, the all-metal structure of
metamaterial emitter 100 forms a type of barrier between heat
energy E.sub.s and PV cell 210 that effectively "blocks" (i.e.,
prevents out-of-band photons S12 from reaching the PV cell. Note
that this filtering functionality would not be possible if base
substrate 111 were not a solid sheet (i.e., if the substrate
included holes that allowed both S11 and S12 to pass through). By
passing only in-band photons to associated PV cell 210, the present
invention greatly increases the efficiency of TPV converter 200
over conventional approaches.
[0033] FIG. 2 is a simplified perspective view showing a TPV
converter 200A including a metamaterial emitter 100A having
multiple bull's eye structures 120A-11 to 120A-13 and 120A-21 to
120A-23 that are formed on a "target-facing" surface 113A of a base
substrate 111A in a manner similar to that described above. FIGS.
3(A) to 3(C) are cross-sectional views taken along section lines
3A-3A, 3B-3B, and 3C-3C of FIG. 2.
[0034] Metamaterial emitter 100A is characterized in that it
utilizes multiple bull's eye structures arranged in sets of three,
where each bull's eye structure of each set has a different fixed
grating period to effectively broaden a total radiant energy beam
E.sub.R-TOTAL emitted by the metamaterial emitter 100A. Referring
to FIG. 2, the multiple bull's eye structures are arranged in two
sets 120A-1 and 120A-2, where each set includes three bull's eye
structures (i.e., set 120A-1 includes bull's eye structures 120A-11
to 120A-13, and set 120A-2 includes bull's eye structures 120A-21
to 120A-23). Each set 120A-1 and 120A-2 includes one bull's eye
structure having grating period .LAMBDA.1, one bull's eye structure
having grating period .LAMBDA.2, and one bull's eye structure
having grating period .LAMBDA.3. Specifically, as indicated in FIG.
3(A), set 120A-1 includes structure 120A-11 having concentric
circular ridge structures spaced at a fixed grating period
.LAMBDA.1 (e.g., the distance between adjacent structures 121A-111
and 121A-112 is equal to grating period .LAMBDA.1). FIG. 3(A) also
represents bull's eye structure 210A-22 of set 120A-2, which
includes ridge structures having fixed grating period .LAMBDA.1
formed in the same manner depicted by adjacent structures 121A-111
and 121A-112. Similarly, FIG. 3(B) shows that both structure
120A-12 of set 120A-1 and structure 210A-22 of set 120A-2 have
grating period .LAMBDA.2 (e.g., as depicted by adjacent structures
121A-121 and 121A-122, which are separated by grating period
.LAMBDA.2), and FIG. 3(C) shows that both structure 120A-13 of set
120A-1 and structure 210A-23 of set 120A-2 have grating period
.LAMBDA.3 (e.g., as depicted by adjacent structures 121A-131 and
121A-132, which are separated by grating period .LAMBDA.3).
[0035] The benefit of forming metamaterial emitter 100A with three
different grating periods is that this approach can be used to
selectively broaden the overall spectrum of the total radiant
energy beam E.sub.R-TOTAL emitted by metamaterial emitter 100A to
associated PV cell 210A. That is, because the radiant energy
generated by a particular bull's eye structure is related to its
fixed grating period, a broadened the total radiant energy beam
E.sub.R-TOTAL is generated by emitter 100A (shown in FIG. 2) by
utilizing three different grating periods .LAMBDA.1, .LAMBDA.2 and
.LAMBDA.3. For example, assume fixed grating period .LAMBDA.3 is
larger than fixed grating period .LAMBDA.2, and fixed grating
period .LAMBDA.2 is larger than fixed grating period .LAMBDA.1. As
indicated in FIGS. 3(A) to 3(C), these different grating periods
generate component radiant energy beams to having different peak
emission wavelengths. That is, because fixed grating period
.LAMBDA.3 is greater than fixed grating period .LAMBDA.2, component
radiant energy beams E.sub.R3 emitted from bull's eye structures
120-13 and 120-23 have a peak emission wavelength .lamda..sub.peak3
that is greater than a peak emission wavelength .lamda..sub.peak2
of radiant energy E.sub.R2 generated by bull's eye structures
120-12 and 120-22. Similarly, the peak emission wavelength
.lamda..sub.peak2 of radiant energy beam E.sub.R2 is greater than a
peak emission wavelength .lamda..sub.peak1 of radiant energy
E.sub.R1generated by bull's eye structures 120-11 and 120-21).
Referring again to FIG. 2, the total radiant energy beam
E.sub.R-TOTAL is a combination of component radiant energy beams
E.sub.R1, E.sub.R2 and E.sub.R3, and the effect of combining the
adjacent narrowband spectra of component beams E.sub.R1, E.sub.R2
and E.sub.R3 is to broaden the overall spectrum of total radiant
energy beam E.sub.R-TOTAL. This approach can be used, for example,
to provide more in-band photons to associated PV cell 210A, and
consequently to increase the output power density of TPV converter
200A.
[0036] The approach set forth above with reference to FIGS. 2 and
3(A) to 3(C) is extendible to any number of fixed grating periods
in order to selectively broaden the overall spectrum of a total
radiant energy beam. That is, although the approach is described
with reference to six bull's eye structures disposed in two sets of
three that utilize three different grating periods, it is
understood that the approach is extendable to any number of bull's
eye structures disposed in any number of sets of two or more bull's
eye structures. For example, a metamaterial emitter may include
only two grating periods to facilitate the emission of a relatively
narrow emission spectrum, or a spectrum having two separated "peak"
emission wavelengths. Alternatively, the use of a large number of
grating periods facilitates the emission of a relatively broad
emission spectrum. It is also possible to fabricate a metamaterial
emitter that in which all bull's eye structures have a unique fixed
grating period (i.e., no two bull's eye structures have the same
grating period). Unless otherwise specified, the appended claims
are intended to cover all of the above-mentioned combinations of
different grating periods.
[0037] FIG. 4 is a perspective view showing a TPV converter 200B
having a metamaterial emitter 100B and a PV cell 210B, where
metamaterial emitter 100B includes multiple bull's eye structures
formed on a "target-facing" surface 113B of a base substrate 111B
in a manner similar to that described above. Metamaterial emitter
100B differs from the previous embodiments in that the multiple
bull's eye structures are formed in a "multiplexed" (overlapping)
pattern (i.e., such that at least some of the circular ridge
structures of one bull's eye structure intersect at least some of
the circular ridge structures of at least one adjacent bull's eye
structure). For example, referring to the upper left corner of FIG.
4, bull's eye structure 120B-11 includes a first group of
concentric circular ridge structures 121B-11, and adjacent bull's
eye structure 120B-12 includes a second group of concentric
circular ridge structures 121-12. Bull's eye structures 120B-11 and
120B-12 form a multiplexed pattern in that at least some of
circular ridge structures 121B-11 of bull's eye structure 120B-11
intersect (overlap) at least some of circular ridge structures
121B-12 of bull's eye structure 120B-12. This multiplex pattern
serves to concentrate radiant energy E.sub.R-TOTAL emitted by
metamaterial emitter 100B, which can be used, for example, to
provide more in-band photons to PV cell 210B, and consequently to
increase the output power density of TPV converter 200B.
[0038] According to a presently preferred embodiment, in addition
to the multiplexed pattern, metamaterial emitter 100B is also
fabricated to employ the multiple-grating-period approach described
above with reference to FIG. 2 (i.e., such that at least one bull's
eye structure has a fixed grating period that is different (e.g.,
larger) than the fixed grating period of another bull's eye
structure). By way of example, the various multiplexed bull's eye
structures of metamaterial emitter 100B are shown as being arranged
in three sets: a first set 120B-1 including bull's eye structures
120B-11, 120B-12 and 120B-13, a second set 120B-2 including bull's
eye structures 120B-21, 120B-22 and 120B-23, and a third set 120B-3
including bull's eye structures 120B-31, 120B-32 and 120B-33. Each
set includes one bull's eye structure having a common first grating
period (i.e., bull's eye structures 120B-11, 120B-21 and 120B-31
are fabricated using the same grating period in the manner
described above with reference to FIG. 3(A)), one bull's eye
structure having a second grating period (i.e., bull's eye
structures 120B-12, 120B-22 and 120B-32 are fabricated in the
manner described above with reference to FIG. 3(B)), and one bull's
eye structure having a third grating period (i.e., bull's eye
structures 120B-13, 120B-23 and 120B-33 are fabricated in the
manner described above with reference to FIG. 3(B)). With this
arrangement, metamaterial emitter 100B generates total radiant
energy E.sub.R-TOTAL that both concentrates and combines adjacent
narrowband spectra to produce a high energy emission with a broader
overall spectrum that maximizes the number of in-band photons
supplied to PV cell 210B, thereby maximizing the output power
density of PV cell 210B.
[0039] FIG. 5 is a perspective view showing a TPV converter 200C
including a metamaterial emitter 100C and two PV cells 210C-1 and
210C-2. In this embodiment, the "base substrate" of TPV converter
200C is formed as part of a box-like enclosure 110C that
facilitates achieving optimal high operating temperatures (i.e.,
1000.degree. K to 1500.degree. K). Box-like enclosure 110C includes
a peripheral wall 111C having an upper (first) peripheral wall
portion 111C-1 and a lower (second) peripheral wall portion 111C-2
that are connected by respective side wall portions in an opposing
spaced-apart (e.g., parallel) relationship such that a
substantially rectangular interior cavity 114C is defined between
wall portions 111C-1 and 111C-2. Peripheral wall portions 111C-1
and 111C-2 respectively include inward-facing surface portions
112C-1 and 112C-2 that face an interior cavity 114C, and
outward-facing surfaces 113C-1 and 113C-2 that face away from
interior cavity 114C (i.e., upward and downward, respectively, from
box-like enclosure 110C). Peripheral wall portions 111C-1 and
111C-2 extend between an inlet end 110C-1 and an outlet end 110C-2
of box-like enclosure 110C such that an inlet opening 115C is
defined between respective front end portions 111C-1F and 111C-2F
of peripheral wall portions 111C-1 and 111C-2, and an outlet
opening 116C is defined between respective rear end portions
111C-1R and 111C-2R of peripheral wall portions 111C-1 and 111C-2.
During operation, "source" heat energy E.sub.S is supplied into the
interior cavity 114C through inlet opening 115C, and "waste" heat
energy is evacuated through outlet opening 116C.
[0040] Metamaterial emitter 100C includes two bull's eye structures
120C-1 and 120C-2 formed in the manner described above that are
disposed on outward facing surfaces 113C-1 and 113C-2,
respectively. Specifically, bull's eye structure 1200-1 is disposed
on upward-facing surface 113C-1 of upper peripheral wall portion
111C-1 and includes concentric circular ridge structures 121C-1
separated by intervening circular grooves 122C-1 and separated by a
fixed grating period .LAMBDA.1, and bull's eye structure 120C-2 is
disposed on downward-facing surface 113C-2 of lower peripheral wall
portion 111C-2 and includes concentric circular ridge structures
121C-2 separated by intervening circular grooves 122C-2 and
separated by a fixed grating period .LAMBDA.2. In one specific
embodiment, PV cells 210C-1 and 210C-2 are essentially identical
(i.e., having the same spectral response), so fixed grating period
.LAMBDA.2 is the same as fixed grating period .LAMBDA.1. In an
alternative embodiment, PV cells 210C-1 and 210C-2 have different
spectral responses, so fixed grating period .LAMBDA.2 is different
from (e.g., larger or smaller than) fixed grating period .LAMBDA.1.
With this arrangement, when heat energy E.sub.S is supplied into
the interior cavity 114C and is sufficient to heat peripheral wall
111C to a temperature above 1000.degree. K, radiant energy E.sub.R1
is emitted upward from box-like enclosure 110C having a peak
emission wavelength that is roughly equal to the fixed grating
period .LAMBDA.1 for conversion to electricity by PV cell 210C-1.
At the same time, heat energy E.sub.S causes bull's eye structure
120C-2 to emit radiant energy E.sub.R2 downward from box-like
enclosure 110C to PV cell 210C-2, where radiant energy E.sub.R2 has
a peak emission wavelength that is roughly equal to the fixed
grating period .LAMBDA.2.
[0041] The box-like enclosure arrangement of metamaterial emitter
100C provides optimal energy beam generation because flat/planar
peripheral wall portions 1110-1 and 111C-2 facilitate
cost-effective fabrication of the bull's eye structures thereon
(e.g., using existing photolithographic fabrication techniques),
and because rectangular-shaped interior cavity 1140 facilitates the
efficient transfer of heat energy over the "base substrate" formed
by peripheral wall portions 111C-1 and 111C-2. In one embodiment,
the rectangular box-like arrangement facilitates the transfer of
heat energy in the form of concentrated sunlight that reflects
between the opposing upper and lower interior surfaces 112C-1 and
112C-2, thereby heating peripheral wall portions 111C-1 and 111C-2,
and allowing associated waste heat to be removed from interior
cavity 114C through outlet opening 116C. Moreover, because
electricity is being generated by two PV cells 210C-1 and 210C-2
instead of just one (as in the generalized embodiments set forth
above), this arrangement allows TPV converter 200C to provide
increased electricity generation over a single PV cell approach. In
yet another embodiment (not shown), the gist of this approach is
further expanded to employ additional bull's eye structures and
additional PV cells (e.g., disposed along the sides of box-like
enclosure 110C, or along multiple surfaces of a non-rectangular
box-shaped enclosure) to potentially further increase electricity
generation.
[0042] According to an aspect of the invention, box-like enclosure
110C is constructed as an all-metal structure (e.g., constructed
from a single metal block or by welding or otherwise securing four
metal plates together). The all-metal structure facilitates
achieving the required high operating temperatures (i.e., 1000 to
1500.degree. K) over a suitable operating lifetime of metamaterial
emitter 100C. In a specific embodiment, the all-metal box-like
enclosure 110C is formed entirely using one or more refractory
metals (e.g., Rhenium, Tantalum or Tungsten) or refractory metal
alloys to further enhance the enclosure's operational lifetime.
[0043] FIGS. 6(A) and 6(B) are perspective and cross-sectional side
views showing a TPV converter 200D including a metamaterial emitter
100D and two PV cells 210D-1 and 210D-2 arranged in accordance with
another embodiment. In this embodiment, metamaterial emitter 100D
includes an all-metal box-like enclosure 110D that is configured
for use in a solar tower power harvesting system such as that
described in co-owned and co-pending U.S. patent application Ser.
No. ______, entitled "Solar Tower Power Harvesting System With
Metamaterial Enhanced Solar Thermophotovoltaic Converter (MESTC)"
[Atty Docket No. 20131311US03 (XCP-186-3)], which is incorporated
herein by reference in its entirety.
[0044] Metamaterial emitter 100D is similar to that described above
with reference to FIG. 5 in that box-like enclosure 110D includes
opposing upper (first) and lower (second) peripheral wall portions
111D-1 and 111D-2 that are connected by respective side wall
portions in an opposing spaced-apart (e.g., parallel) relationship,
and such that arrays 120D-1 and 120D-2 of bull's eye structures are
respectively formed on outward-facing surfaces 113D-1 and 113D-2 of
wall portions 111D-1 and 111D-2. Bull's eye structure arrays 120D-1
and 120D-2 are implemented using any of the various arrangements
described above, but preferably include a multiplexed arrangement
such as that shown in FIG. 4 to maximize the amount of energy
transmitted in emitted radiant energy beams E.sub.R1 and
E.sub.R2.
[0045] Metamaterial emitter 100D differs from previous embodiments
in that it includes a compound parabolic trough 117D disposed at
the inlet end of box-like enclosure 110D. As indicated in FIG.
6(B), the compound parabolic trough includes an upper (first)
compound parabolic trough structure 117D-1 integrally connected to
a front end portion 111D-1F of upper peripheral wall portion
111D-1, and a lower (second) compound parabolic trough structure
117D-2 integrally connected to a front end portion 111D-2F of lower
peripheral wall portion 111D-2. As indicated by the dashed-line
arrows in FIG. 6(B), compound parabolic trough structures 117D-1
and 117D-2 are operably shaped to channel concentrated sunlight
E.sub.S through the inlet opening 115D into interior cavity 114D
between peripheral wall portions 111D-1 and 111D-2 such that the
sunlight reflects between the inside surfaces 112D-212 and 112D-222
of peripheral wall portions 111D-1 and 111D-2 in a manner that
maximizes the transfer of heat energy to bull's eye structure
arrays 120D-1 and 120D-2, which in turn maximizes the amount of
radiant energy emitted in beams E.sub.R1 and E.sub.R2 respectively
emitted from bull's eye structure arrays 120D-1 and 120D-2 to PV
cells 210D-1 and 210D-2, respectively.
[0046] Metamaterial emitter 100D also differs from previous
embodiments in that it includes a funnel-shaped outlet 117D
disposed at the outlet end of box-like enclosure 110D that serves
to control the release of "waste" heat from interior cavity 114D.
As indicated in FIG. 6(B), the funnel-shaped outlet includes an
upper (first) funnel-shaped outlet structure 118D-1 integrally
connected to a rear end portion 111D-1R of upper peripheral wall
portion 111D-1, and a lower (second) funnel-shaped outlet structure
118D-2 integrally connected to a rear end portion 111D-2R of lower
peripheral wall portion 111D-2. As indicated by the dashed-line
arrows in FIG. 6(B), funnel-shaped outlet structures 118D-1 and
118D-2 are operably shaped to channel "waste" heat energy E.sub.W
from interior cavity 114D through outlet opening 116D at a rate
that optimizes energy transfer to bull's eye structure arrays
120D-1 and 120D-2.
[0047] For reasons similar to those set forth above (e.g., to
minimize thermal cycling stresses and to maximize the operating
lifetime) the entirety of all-metal box-like enclosure 110D (i.e.,
including peripheral wall portions 111D-1 and 111D-2, compound
parabolic trough structures 117D-1 and 117D-2, and funnel-shaped
outlet structures 118D-1 and 118D-2) is constructed using metal,
and more preferably using a single refractory metal (e.g., Rhenium,
a Rhenium alloy, Tantalum or Tungsten).
[0048] FIG. 7 is a flow diagram showing a generalized method for
producing a TPV converter according to another embodiment of the
present invention. The production method applies to any of the TPV
converters described with reference to FIGS. 1 to 6(B), that is,
including a spectrally-selective metamaterial emitter having one or
more bull's eye structures and an associated PV cell.
[0049] Referring to the upper portion of FIG. 7 (block 401), the
production method begins by determining the optimal conversion
spectrum (e.g., from a previously generated EQE curve, or through
testing) of the associated PV cell. As explained above, the optimal
conversion spectrum identifies "in-band" photon wavelengths that
are most efficiently converted by the associated PV cell into
electricity, and varies by PV cell type.
[0050] As indicated in block 702 (FIG. 7), the production process
then involves fabricating a spectrally-selective metamaterial
emitter with bull's eye structures having concentric circular ridge
structures separated by a fixed grating period that is roughly
equal to the PV cell's optimal conversion spectrum. In an exemplary
embodiment, this portion of the production process is carried out
as set forth with reference to FIGS. 8(A) to 8(F), which illustrate
fabricating a spectrally-selective metamaterial emitter using
photolithographic processing techniques.
[0051] FIGS. 8(A) to 8(D) illustrate the use of photolithography to
generate a patterned mask on a planar upper surface 113 of a solid
base substrate 111 (shown in FIG. 8(A). According to a preferred
embodiment, base substrate 111 comprises one of the refractory
metals (or alloys thereof) that are mentioned above. FIG. 8(B)
shows the deposition of a photoresist 210 that forms a photoresist
layer 215 on upper surface 113 of base substrate 111. FIG. 8(C)
depicts the subsequent use of a reticle 220 to expose photoresist
layer 215 using known techniques, but modified in that reticle
includes an aperture pattern having a "mirror" shaped pattern
corresponding to a "negative" of the desired bull's eye structures
(i.e., including concentric circular apertures 222 set at a grating
period in the range of 10 nanometers to 5 microns), whereby light
225 passing through apertures 222 develop corresponding concentric
circular portions of photoresist layer 215. FIG. 8(D) show the
subsequent removal of undeveloped photoresist (i.e., using a
suitable etchant 229), thereby completing photoresist mask 230
including a plurality of concentric circular resist structures 231
having a fixed grating period .LAMBDA. in the range of 0.5 to 5
microns, wherein each adjacent pair of concentric circular resist
structures 231 are separated by an intervening concentric circular
slot 232.
[0052] FIG. 8(E) depicts utilizing mask 230 to form a bull's eye
structure on upper surface 113 according to a specific "additive"
embodiment of the present invention. In this case, a refractory
metal 240 (which can either be the same refractory metal as forming
base substrate 111 or a different refractory metal) is deposited
(e.g., by way of sputter deposition) over mask 230 and exposed
portions of upper surface 113 that are exposed between concentric
circular resist structures 231, thereby forming ridge structures
121 having the fixed grating period .LAMBDA. between adjacent pairs
of resist structures 231. In an alternative "subtractive"
embodiment (not shown), mask 230 is utilized to form bull's eye
structures by dry etching exposed portions of the base substrate
through the mask slots, thereby forming the concentric grooves
between corresponding circular ridge structures 121-2 that have a
composition identical to that of the base substrate.
[0053] FIG. 8(F) depicts the subsequent removal of the photoresist
mask using a suitable etchant 250 mask 230 from the planar upper
surface 113, thereby completing the fabrication of metamaterial
emitter 100 including a bull's eye structure 120 disposed on base
substrate 111, where bull's eye structure 120 includes concentric
circular ridge structures 121-2 separated by intervening circular
grooves 122 and spaced at a grating period .LAMBDA. in the range of
0.5 to 5 microns.
[0054] Returning to FIG. 7, once the bull's eye structure
fabrication process described with reference to FIGS. 8(A) to 8(F)
is completed, final construction of the metamaterial emitter is
completed, and then (as indicated in block 403), the PV cell (or PV
cells) are operably secured in a fixed relationship relative to the
metamaterial emitter such that radiant energy generated by the
bull's eye structures is directed onto the PV cell(s) for
conversion into electricity. Final construction of the metamaterial
emitter may include, for example, connecting the metal plate (base
substrate) on which the bull's eye structures are formed into a
box-like enclosure similar to those described above. Securing the
PV cells may involve, for example, mounting the PV cells directly
to the box-like enclosure, mounting the PV cells onto a
water-cooled platform disposed adjacent to the metamaterial
emitter, or at a suitable distance from the metamaterial emitter
that minimizes heat damage of the PV cell.
[0055] Although the fabrication methodology described above with
reference to FIGS. 8(A) to 8(F) depict the formation of a
metamaterial emitter having a single bull's eye structure, it is
understood that the method is expandable using known techniques to
generate multiple bull's eye structures, such as the bull's eye
structure sets depicted in FIGS. 3 and 4. In the case of the
multiplexed bull's eye structures shown in FIG. 4, a reticle is
produced in which a concentric circular pattern is repeated in an
overlapping manner to form apertures having a multiplexed
arrangement, and then the reticle is used to generate a patterned
mask including concentric circular resist structures disposed in
the multiplexed arrangement, and then using the process described
above to form multiple pluralities of overlapping concentric
circular metal ridge structures in the multiplexed arrangement.
[0056] Although the present invention has been described with
respect to certain specific embodiments, it will be clear to those
skilled in the art that the inventive features of the present
invention are applicable to other embodiments as well, all of which
are intended to fall within the scope of the present invention. For
example, although the TPV converters described herein include
metamaterial emitters having all-metal structures to maximize
operational lifetime, TPV converters utilizing less robust
metamaterial emitters (e.g., having metal bull's eyes structures
formed on dielectric base substrates or all-semiconductor
platforms) may be also used.
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