U.S. patent application number 10/722738 was filed with the patent office on 2009-11-05 for integrating sphere photovoltaic receiver (powersphere) for laser light to electric power conversion.
Invention is credited to Ugur Ortabasi.
Application Number | 20090272424 10/722738 |
Document ID | / |
Family ID | 41256314 |
Filed Date | 2009-11-05 |
United States Patent
Application |
20090272424 |
Kind Code |
A1 |
Ortabasi; Ugur |
November 5, 2009 |
INTEGRATING SPHERE PHOTOVOLTAIC RECEIVER (POWERSPHERE) FOR LASER
LIGHT TO ELECTRIC POWER CONVERSION
Abstract
A photovoltaic module for converting laser radiation from a
laser emitting light at a wavelength to electrical power is
provided. The module comprises: (a) a housing having a cavity of
generally optimized closed shape inside the housing, the cavity
having an internal surface area A.sub.s and including an opening
for admitting the laser radiation into the cavity, the opening
having an entrance aperture area A.sub.i that is substantially
smaller than A.sub.s; and (b) a plurality of photovoltaic cells
within the cavity, the photovoltaic cells having an energy bandgap
to respond to the wavelength and generate the electrical power.
Inventors: |
Ortabasi; Ugur; (Encinitas,
CA) |
Correspondence
Address: |
JOHN P. DE LUCA
17420 RYEFIELD CT.
DICKERSON
MD
20842
US
|
Family ID: |
41256314 |
Appl. No.: |
10/722738 |
Filed: |
November 25, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10151640 |
May 17, 2002 |
6689949 |
|
|
10722738 |
|
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Current U.S.
Class: |
136/246 |
Current CPC
Class: |
H01L 31/0543 20141201;
H01L 31/0547 20141201; H01L 31/0549 20141201; Y02E 10/52
20130101 |
Class at
Publication: |
136/246 |
International
Class: |
H01L 31/055 20060101
H01L031/055 |
Claims
1. A photovoltaic cavity converter module for admitting therein
concentrated radiation produced by a laser emitting a highly
collimated power beam of coherent light having a selected
wavelength and energy, and converting the admitted laser radiation
at a high efficiency of about 60 to about 70% into electrical
power, said module comprising: (a) a housing having a cavity of
generally optimized closed shape inside said housing, said cavity
having a light input aperture or opening of a selected diameter
with a total aperture area of A.sub.i for the cavity, said aperture
for admitting incident radiation thereon produced by the laser into
the cavity, the cavity having a total internal surface area A.sub.s
and wherein the total aperture area of the opening being in a ratio
of up to about 0.01 of the total internal surface area of the
cavity, such that the aperture allows only a relatively small
portion of the radiation admitted into the cavity to escape out of
the cavity, thereby trapping in the cavity the incident radiation
admitted therein in an amount proportional to the ratio of the
total internal surface area to the total entrance aperture area to
thereby define the total energy trapped in the cavity, the beam
produced by the laser having a diameter greater than the aperture
area; (b) a concentrator exterior of the housing for intercepting
and concentrating the laser radiation to a selected beam diameter
smaller than the diameter of the aperture and for directing the
radiation into the housing through the light input aperture to
thereby capture the energy of the collimated laser beam; and (c) a
plurality of photovoltaic cells within said cavity, said
photovoltaic cells having an appropriate energy bandgap maximally
responsive to said wavelength for generating said electrical
power.
2. The photovoltaic module of claim 1 wherein each said
photovoltaic cell is a single junction cell having a receiving
surface on which said laser radiation is incident.
3. The photovoltaic module of claim 2 wherein each photovoltaic
cell is provided with a back surface mirror for reflecting photons
not absorbed by a photovoltaic cell on which said photons are
incident.
4. The photovoltaic module of claim 2 wherein said photovoltaic
cells have a given quantum efficiency selected to optimize the
conversion of said wavelength of said laser.
5. The photovoltaic module of claim 1 wherein the concentrator
comprises a primary concentrator and a secondary concentrator, the
primary concentrator further including a Cassegranian comprising a
parabolic concentrator for prefocusing the laser radiation, and a
hyperbolic concentrator for receiving pre-focused laser radiation
from the parabolic concentrator and directing the beam into said
opening.
6. The photovoltaic module of claim 5 wherein said secondary
concentrator has a mirrored inner surface.
7. The photovoltaic module of claim 6 wherein said secondary
concentrator is a non-imaging, compound parabolic of hollow
design.
8. The photovoltaic module of claim 6 wherein said secondary
concentrator has a Bezier optimized contour to provide a
combination of maximum acceptance angle at optimal concentration,
and minimum height.
9. The photovoltaic module of claim 5 wherein said secondary
concentrator is dielectric and further includes an integral
extractor rod for guiding said light towards the center of said
cavity and then to emit photons near uniformly in all directions to
provide good angular isotropy of said photons.
10. The photovoltaic module of claim 1 wherein the energy of the
beam in coherent form enters the sphere where said energy scatters
such that the probability of escape through the aperture is reduced
in accordance with the ratio, and wherein the overall concentration
of the module is at least 20.
11. The photovoltaic module of claim 1 wherein said photovoltaic
cells have an optimized energy bandgap to respond to said
wavelength.
12. The photovoltaic module of claim 11 wherein said photovoltaic
cells have a peak of quantum efficiency response matching said
wavelength.
13. The photovoltaic module of claim 1 wherein (a) at least some of
the plurality of photo voltaic cells within said cavity, have
different energy bandgaps so that their spectral responses span
different wavelength ranges; and (b) at least one wavelength filter
associated with each photo voltaic cell, said wavelength filter
comprising at least one of a Rugate filter and stack interference
filters, providing selective transmission and reflection of
incident radiation.
14. The photovoltaic module of claim 13 wherein said photo voltaic
cells are multi-junction cells.
15. In combination, a photovoltaic module and a concentrator system
external of the photovoltaic module for admitting therein coherent
radiation produced by a laser emitting coherent light at a selected
wavelength and initial diameter, and converting the admitted
radiation into electrical power, wherein: (a) said module
comprises: (1) a housing having a cavity of generally optimized
closed shape inside said housing, said cavity having a total
internal surface area A.sub.s and including an opening having a
selected diameter smaller than the diameter of the light, and
having a total aperture area A.sub.i for admitting said light into
said cavity, said opening having an entrance aperture area A.sub.i
of in a ratio of up to about 0.01 of the total internal surface
area of the cavity, such that the aperture allows only a relatively
small portion of the light admitted into the cavity to escape out
of the cavity, thereby trapping in the cavity the light radiation
admitted therein in an amount proportional to the ratio of the
total internals surface area to the entrance aperture area, the
initial diameter of the light being greater than the diameter of
the opening; (2) a plurality of photovoltaic cells within said
cavity, said photovoltaic cells having selected appropriate bandgap
energy responsive to said wavelength to generate said electrical
power; (b) said reflecting concentrator comprises: (1) a primary
concentrator for intercepting and concentrating said light from the
selected diameter to a diameter smaller that the initial diameter,
and (2) a secondary concentrator coupled to the for receiving said
concentrated light from the primary concentrator and further
concentrating said light from said primary concentrator to a
diameter less than the diameter of the aperture and injecting the
light into the housing through the aperture; and.
16. The combination of claim 15 wherein said concentrator comprises
a reflecting Cassegranian concentrator.
17. The combination of claim 16 wherein said Cassegranian
concentrator comprises a parabolic concentrator and a hyperbolic
concentrator.
18. The combination of claim 15 wherein each said photovoltaic cell
is a single junction cell having a receiving surface on which said
laser radiation is incident.
19. The combination of claim 18 wherein each photovoltaic cell is
provided with a back surface mirror for reflecting photons not
absorbed by a photovoltaic cell on which said photons are
incident.
20. The combination of claim 18 wherein said photovoltaic cells
have a given quantum efficiency selected to optimize the conversion
of said wavelength of said laser.
21. The combination of claim 15 wherein the energy of the beam in
coherent form enters the cavity where said energy scatters such
that the probability of escape through the aperture is reduced in
accordance with the ratio, and wherein the overall concentration of
the combination is at least 20.
22. The combination of claim 15 wherein said secondary concentrator
includes inner surfaces that are mirrored.
23. The combination of claim 22 wherein said secondary concentrator
is a non-imaging, compound parabolic of hollow design.
24. The combination of claim 22 wherein said secondary concentrator
has a Bezier optimized contour to provide a combination of maximal
acceptance angle, maximal concentration, and minimal height.
25. The combination of claim 21 wherein said secondary concentrator
is dielectric and further includes an integral extractor rod
extending into the housing for guiding said light towards the
center of said cavity and to emit photons near uniformly in all
directions to provide good angular isotropy of said photons.
26. (canceled)
27. The combination of claim 15 wherein said photovoltaic cells
have an optimized energy bandgap to respond to said wavelength.
28. The combination of claim 27 wherein said photovoltaic cells
have a peak of quantum efficiency response matching said
wavelength.
29. The combination of claim 15 further including means for
transferring waste heat from said photovoltaic module to a back
surface of said primary concentrator for radiation into the
surrounding environment.
30. The combination of claim 15 further including (a) a plurality
of photo voltaic cells within said cavity, at least some of said
cells each having different energy bandgaps so that their spectral
responses span a least a portion of the spectrum of the incident
radiation; and (b) at least one wavelength filter associated with
each cell, said at least one wavelength filter comprising Rugate
filters and a combination of Rugate filters and stack interference
filters, thereby providing selective transmission or reflection of
incident radiation.
31. (canceled)
32. (canceled)
33. (canceled)
34. A photovoltaic cavity converter module for admitting therein
radiation produced by a laser emitting a power beam in the form
highly collimated coherent light having a selected energy,
wavelength and relatively large diameter, and converting the
admitted radiation at a high efficiency of at least 60% into
electrical power, said module comprising: a housing having a cavity
of generally optimized closed shape inside said housing and having
a total internal surface area A.sub.s, said cavity having a light
input aperture of a selected diameter smaller than the relatively
large diameter of the beam for admitting said beam, the aperture
having a total aperture area of A.sub.i for the cavity; a
concentrator external of the housing for intercepting the laser and
reducing the diameter of the beam to a diameter smaller than the
aperture for concentrating the laser radiation and for directing
the energy of the light contained in the relatively large diameter
power beam produced by the laser into the small diameter of the
aperture; the total aperture area of the aperture being in a ratio
of up to about 0.01 of the total internal surface area of the
cavity, such that the aperture allows only a relatively small
portion of the light admitted into the cavity to escape out of the
cavity, thereby trapping in the cavity the light admitted therein
in an amount proportional to the ratio of the total internal
surface area to the total entrance aperture area to thereby define
the total energy trapped in the cavity; and a plurality of
photovoltaic cells within said cavity, said photovoltaic cells
having an appropriate energy bandgap maximally responsive to the
wavelength of the light for generating said electrical power; and
wherein the cavity converter module exhibits an overall
concentration of at least 20.
35. The photovoltaic cavity converter module of claim 34 wherein
the concentrator comprises a first concentrator stage for
intercepting and reducing the diameter of the beam to a first
smaller diameter, and a second concentrator stage coupled to the
aperture for intercepting the beam of smaller diameter and further
reducing the diameter to less than the diameter of the aperture and
for injecting the light into the housing.
36. The photovoltaic cavity converter module of claim 34 wherein
the second concentrator stage further includes an extractor located
within the cavity for distributing the light inside the cavity such
that the energy of the beam in coherent form enters the cavity
where said energy scatters such that the probability of escape
through the aperture is reduced in accordance with the ratio.
37. The photovoltaic cavity converter module of claim 35 wherein
the extractor comprises a rod extending from the aperture to the
center of the housing for guiding the light towards the center of
the cavity and for emitting the light substantially uniformly in
all directions within the cavity.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] The present application is a continuation-in-part
application of Ser. No. 10/151,640, filed May 17, 2002, and
recently allowed, the contents of which are incorporated herein by
reference.
TECHNICAL FIELD
[0002] The present invention is related generally to laser power
beaming, employing photovoltaic cells, and, more particularly, to a
novel photovoltaic module for converting a directed laser beam into
electrical power.
BACKGROUND ART
[0003] State-of-the-art single junction solar arrays as well as
concentrators using single junction solar cells utilize only a
limited portion of the available solar spectrum, thereby wasting
the remainder of available energy outside of their limited spectral
response. The limitation is caused mainly by two basic "photon
loss" mechanisms within the cells, namely, (1) loss by longer
wavelengths and (2) loss by excess energy of photons. In the former
case, photons with energy smaller than the "energy bandgap" or
"forbidden gap" E.sub.g (direct bandgap semiconductor) or
E.sub.g-E.sub.Phonon (indirect bandgap semiconductors where
E.sub.phonon is the phonon quantum of energy) cannot contribute to
the creation of electron-hole pairs. In the latter case, in the
spectrum range of interest, one photon generates only one
electron-hole pair. The rest of the energy larger than the bandgap
is dissipated as heat. Photons with energy hv.gtoreq.E.sub.g thus
can only use a portion of E.sub.g of their energy for generation of
electron-hole pairs. The excess energy raises the temperature of
the solar cell and degrades its performance. Thus, even high
quality cells with excellent quantum efficiencies, such as GaAs,
exhibit relatively modest conversion efficiencies since they cannot
respond to more than a relatively small portion of the incident
spectrum.
[0004] One way of circumventing this limitation is the use of two
or more different bandgap cells that are stacked, or monolithically
grown, in a vertical manner. Such a multi-junction (MJ) system with
appropriately chosen bandgaps can span a significantly greater
portion of the incident solar spectrum than achievable with
single-junction cell systems. Such multi-junction solar cells are
well-known. For example, three-junction cells have been devised
that can control a relatively larger portion of the solar spectrum,
and are further described below. Because of their potential for
very high efficiencies, MJ cells have enjoyed increased interest
over the last two decades.
[0005] At a NCPV (National Center for Photovoltaics) meeting in
Denver, Colo. on Apr. 16-19, 2000, it was reported that
triple-junction GaInP2/GaAs/Ge concentrator cells developed by NREL
(National Research Energy Laboratory) and Spectrolab have achieved
32.3% at 47 suns and 29% at 300 suns (AM1.5, 25C), with an obvious
drop of 3.3% (absolute) or 10.2% (relative), indicating one of the
many limitations of MJ concentrator systems at higher
concentrations. It should be kept in mind that the above-mentioned
encouraging achievement with a pulsed solar simulator does not
represent a real life situation. Under actual operating conditions,
the MJ concentrator system performance can drop more than 12 to 15%
(absolute) against the bare cell performance and defeat the use of
high efficiency MJ cells. Some of the major concentration-related
performance losses in MJ cells are caused by the following
shortcomings: absorption of light in the top cells, chromatic
aberrations caused by the concentrator optics, flux non-uniformity
on the cells, limited heat removal from the top cells, current
limitation in the cells, series resistance, shadowing losses due to
finger contacts on the cells, and limited acceptance angle for
photon incidence on the cells. Most of these limiting factors apply
to all conventional concentrator types based on a variety of cells.
MJ cells, however, are more vulnerable to most of these
performance-limiting factors.
[0006] The relative deterioration of MJ cells becomes worse as the
number of junctions increases. Several authors in the field have
predicted that for vertically stacked or monolithically-grown
systems, limited improvements are expected beyond triple-junction
cells. A recent press release by Boeing (Spectrolab) on Aug. 15,
2001, confirmed that a triple junction cell developed by Spectrolab
and NREL has reached a conversion efficiency of 34% (a world record
at that time) at 400.times.. That appears to be very much the limit
of three-junction cells. Four-junction cells are predicted to be
able to reach upper 30% and lower 40% efficiencies. Theoretical
studies have shown that to achieve this kind of efficiency level, a
four-junction cell system requires a 1 eV bandgap III-V cell that
meets all requirements including: optical, thermal, and electronic
issues involved. In spite of extensive efforts, this material
remains elusive.
[0007] Another shortcoming of the monolithic MJ cells lies in the
limitation of complementary bandgap cell materials with matching
lattices. In vertically-grown MJ cells, all the adjacent
"sub-cells" must have matching or slightly mis-matching lattices
for proper performance. Thus, even the best bandgap matched
sub-cell cannot result in a multi-junction cell if their lattices
mis-match. This requirement narrows down significantly the
available set of sub-cells that could be used.
[0008] These apparent limitations represent a formidable bottleneck
in the development of high and very high efficiency (and therefore
cost-competitive) concentrator systems in the near future.
According to analytical studies, ideal four bandgap cell systems
utilizing a new 1 eV material can improve the solar to electricity
conversion efficiency over 48% at 500 suns. Even at a cost of
$250/Watt for such a system, the effective cell system cost for a
500.times. flux concentrator can be as low as $0.50/Watt. At this
cost level, the concentrators would be ahead of the long range
goals of the Department of Energy for PV flat plate technology
(installed system cost of $1.00/Watt to $1.50/Watt by the year
2030), if the balance of concentrator system could be built for
$0.50/m.sup.2 to $1.00/m.sup.2. Thus, very high cell and system
efficiencies are paramount to achieve the long term cost goals for
photovoltaics in general.
[0009] In the late 1990s, NASA and JPL scientists proposed an
alternative technique, called "Rainbow", to circumvent the problems
of vertical MJ systems and improve the performance of multi bandgap
cell systems. Their method is to split the solar spectrum into
several frequency bands and focus each frequency band onto separate
cells with corresponding energy bandgaps. The Rainbow multi-bandgap
system represents a combination of solar cells, concentrators, and
beam splitters. The use of separate discrete cells offers the
widest possible scope of semiconductor choices. Based on data for
"real" cells and optical components, Rainbow was expected in 1997
to convert over 40% of incident solar energy to electricity at the
system level.
[0010] To the knowledge of the present inventor, this concept has
never come to a closure, presumably due to extreme difficulties
encountered with the associated optics. In addition, this space
system would only have a concentration ratio of a maximum of
20.times., i.e., much lower than the 500.times. or more to reduce
the effective cell cost dramatically. A thorough literature search
has shown that in the past, the very promising method of spectral
splitting and simultaneous use of discrete solar cells with
different bandgaps has never reached its potential capacity and the
technology was never exploited fully. The parent application to the
present application represents a straight-forward approach to
achieve break-through performance levels and with it to rapidly
lower the cost of solar energy to competitive levels.
[0011] To address the large demand for noise-free and safe power
transmission, without the use of electrical wiring, several new
technologies are being introduced. The two major approaches are:
(1) microwave and millimeter wave beaming and (2) optical fiber
light transmission in conjunction with optically powered, sensors,
transducers and data communications equipment. At the receiving
end, microwaves and millimeter waves are converted into electricity
via highly tuned phased array antennas. In the case of optical
fiber power transmission, the conversion of light into electricity
happens via a photovoltaic power converter, which is basically a
slightly modified solar cell.
[0012] The conversion of beamed microwaves and millimeter waves
into electric power is highly efficient. However, concerns with the
potential hazardous impact of high intensity beams and the strong
beam divergence limit the area of applicability of such
power-beaming technologies to high altitudes and space. Optical
fiber power transmission is distance- and power-limited due to
optical absorption in the fiber and light input/output coupling
losses. Most of the reported fiber optics power transfer
applications are limited to local area networks (<<1 km) of
power levels less than 1 watt and for the most a few microwatts.
Thus, there is a need for a power beaming technology that can
provide a wireless electric power source ranging from 1 watt to
tens of kWatts and can be beamed from, say, 10 meters to several
kilometers and beyond. Such high laser power levels are now
available, due to emerging laser technologies such as chemical
oxygen-iodine lasers (COIL) that are scalable up to 40 kW at a
wavelength of 1.315 microns.
[0013] More recently, proposals have been made to convert coherent
light to electricity. Such applications have been termed "Laser
Power Beaming" (LPB). LPB technology uses the properties of
coherent light to transfer power between two locations without the
need of any material or man-made medium. Thus, LPB is extremely
fast and weightless. Over the last decade, total energy
efficiencies for some lasers have improved significantly (40% and
up) and reliable operation of high power lasers over long periods
of time has been demonstrated in real life applications. The most
efficient method of converting beamed laser power into electricity
at the receiving end is the use of photovoltaic (PV) cells. As a
result of recent research and development efforts on solar PV cell
technology, solar-to-electricity conversion efficiencies as high as
36% has been achieved at 500.times.AM1.5 suns or about 50
W/cm.sup.2. Efficiencies for monochromatic light, as it is the case
with LPB, are expected to be much higher. Research efforts in the
field of thermo-photovoltaics (TPV) made it possible to develop new
photovoltaic materials that are responsive in the near infrared
range of the electromagnetic spectrum, that would, for example,
operate at 40 to 45% efficiency at 1.315 wave length of the COIL
lasers mentioned above. Such a TPV cell, for example,
GaInAsSb/AlGaAs, can be used effectively with the COIL lasers
mentioned above.
[0014] As an aside, it is important to note that in the past,
integrating sphere systems have been used to measure, control, and
monitor laser and laser diodes. However, to the knowledge of the
inventor, the PowerSphere approach disclosed and claimed herein is
the first disclosure that teaches how the integrating sphere
concept can be exploited to convert beamed laser energy into
electric power.
DISCLOSURE OF INVENTION
[0015] In accordance with the present invention, a photovoltaic
module, or PowerSphere, for converting coherent laser radiation
from a laser emitting light at a wavelength into electrical power
is provided. The module comprises: [0016] (a) a housing having a
cavity of generally optimized closed shape inside the housing, the
cavity having an internal surface area A.sub.s and including an
opening for admitting the laser radiation into the cavity, the
opening having an entrance aperture area A.sub.i that is
substantially smaller than A.sub.s; and [0017] (b) a plurality of
photovoltaic cells within the cavity, the photovoltaic cells having
a bandgap energy to respond to the wavelength and generate
electrical power.
[0018] Further in accordance with the present invention, a
combination of a reflecting concentrator and the photovoltaic
module is provided. The reflecting concentrator comprises: [0019]
(a) a primary concentrator for intercepting and concentrating the
laser radiation, and [0020] (b) a secondary concentrator for
receiving the concentrating said laser radiation from the primary
concentrator and further concentrating the laser radiation.
[0021] The photovoltaic module is positioned for receiving the
further concentrated laser radiation from the secondary
concentrator.
[0022] The PowerSphere of the present invention has the potential
to yield laser-to-electricity conversion efficiencies from 60% to
70%.
[0023] Other objects, features, and advantages of the present
invention will become apparent upon consideration of the following
detailed description and accompanying drawings, in which like
reference designations represent like features throughout the
FIGURES.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] The drawings referred to in this description should be
understood as not being drawn to scale except if specifically
noted.
[0025] FIG. 1 is a cross-section view, in schematic, depicting the
basic principles of the multi-bandgap Photovoltaic Cavity Converter
(PVCC);
[0026] FIG. 2 is a view similar to that of FIG. 1, depicting the
escape probability of a photon representing a discrete frequency
band from the PVCC;
[0027] FIG. 3 is a cross-section view, similar to that of FIG. 1,
depicting the principles of the single bandgap integrating sphere
photovoltaic receiver (Power-Sphere) operation with a dielectric
light injector;
[0028] FIG. 3a is an enlargement of a portion of FIG. 3;
[0029] FIG. 4 is a schematic diagram, depicting a Cassegranian
concentrator, coupled to the PowerSphere, to accommodate beam
broadening for long range power beaming applications for both space
and terrestrial use;
[0030] FIG. 5 is a schematic diagram, depicting a deployed space
vehicle equipped with a combination of laser and solar power
modules;
[0031] FIG. 6 is a schematic diagram, depicting a sequence of a
continuous flight drone powered by laser beaming; and
[0032] FIG. 7 is a schematic diagram, depicting the use of the
PowerSphere as a source of DC power, free of electromagnetic
interference (EMI).
BEST MODES FOR CARRYING OUT THE INVENTION
[0033] Reference is now made in detail to a specific embodiment of
the present invention, which illustrates the best mode presently
contemplated by the inventor for practicing the invention.
Alternative embodiments are also briefly described as
applicable.
[0034] The embodiments herein are directed to an integrating sphere
photovoltaic (PV) receiver, or module, for converting laser light
into electric power. The receiver is called a "PowerSphere".
[0035] The basic concept of the PowerSphere is based on a
photovoltaic cavity converter (PVCC) module that has been designed
for a concentration in the range of 500 to over 1000 suns and a
power output range of a few kilowatts to 50 kW.sub.e when combined
with a primary dish and a secondary concentrator. That PVCC module
is disclosed and claimed in the above-identified parent
application, now U.S. Pat. No. ______. The PVCC module herein is
expected to find use in, for example, DOE's Concentrating Solar
Power (CSP) program to develop systems in the 1 to 5 kW.sub.e and
10 to 30 kW.sub.e size ranges based on reflective optics. A typical
power range is about 30 to 50 kiloWatts. Connecting a plurality of
such modules together in a power plant permits power generation up
to several hundred mega Watts.
[0036] The PVCC module is based on advanced single junction cells,
including III-V cells, for example, manufactured by EMCORE
Photovoltaic (Albuquerque, N.M.). The PVCC module is based on
reflective optics, and is capable of delivering power in the range
of 0.5 to 3 kW.sub.e at concentrations in the range of 100 to
500.times. when optically coupled to the exit aperture of the
second reflective stage (CPC) currently located at HFSF. According
to NREL specifications, this second reflective stage provides an
average flux density of 20,000 AM1.5 suns at its exit aperture. The
overall targeted module conversion efficiency for near- and midterm
is to exceed 33% to 45%, respectively.
[0037] The PVCC module is a light-trapping cavity equipped with
internal solar cells of different energy bandgaps. A unique system
of Rugate filters is applied to the cells to "split" the solar
spectrum by the method of selective energy extraction (spectral
screening). This novel conversion device actually defocuses to a
certain extent the pre-focused solar flux entering the cavity in a
controllable manner by determining the diameter of the sphere.
[0038] FIG. 1 illustrates the principles of the Photovoltaic Cavity
Converter (PVCC). In FIG. 1, the PVCC 10 comprises a housing 12
having an internal cavity 14 that is generally spherical, but may
be some other optimized closed shape. By an "optimized closed
shape" is meant, for example, a generally spherical shape, a
generally ellipsoidal shape, or a generally conical shape. In
general, any shape that is closed upon itself is useful. However, a
generally spherical shape is preferably employed. The closed shape
is optimized to promote an efficiency that is as high as is
possible ("optimized") in the collection of photons.
[0039] The cavity 14 contains therein a plurality of solar cells
16, grouped into voltage-matched cell strings of different energy
bandgaps. Simultaneous spectral splitting occurs by means of
selective transmission and/or reflection of the photons by matching
(conjugated) Rugate filters 17 associated with the cells 16.
Alternatively, a combination of Rugate filters and stack
interference filters may be used as filters 17. In an exemplary
embodiment, there are four groups of solar cells, denoted 16a, 16b,
16c, 16d, although it will be appreciated by those skilled in this
art that less than four groups or more than four groups of solar
cells may be employed. In a preferred embodiment for PVCC, four or
more cell types are employed, which, when properly selected, is
expected to result in higher efficiencies.
[0040] Each group of solar cells 16 is responsive to a different
portion of the solar spectrum 18. Examples of suitable solar cells
that are responsive to different portions of the solar spectrum are
discussed below.
[0041] The light 18 entering into the spherical cavity 14 is first
pre-focused by a primary concentrator (dish) (not shown in FIG. 1)
and then by a second-stage, or secondary, concentrator 20 that has
its inner surfaces 22 mirrored. An example of such a second-stage
concentrator is disclosed in U.S. Pat. No. 6,057,505, issued May 2,
2000, to the present inventor. The second-stage concentrator 20 has
a Bezier optimized contour to provide a combination of maximum
acceptance angle, highest concentration, and minimum height.
[0042] After passing through the second-stage concentrator 20, the
light 18 then enters the spherical cavity 14 through a small
entrance aperture 24 (similar to an integrating sphere) and is
defocused to the desired flux concentration by the choice of the
diameter of the sphere 12. The escape probability of the trapped
photons can easily be kept below a few percent by making the
aperture 24 small enough as compared to the surface area of the
interior wall 26. The highly reflective interior surface 26 of the
sphere 12 is lined with discrete single junction cells, including
III-V solar cells 16 of different energy bandgaps and/or IV solar
cells, such as Si and/or Ge. Other types of cells are also
permissible if they meet the performance criteria.
[0043] Photons, once trapped by the cavity 14, undergo several
bounces from the cells 16 and cavity wall 26 until they are either
(1) absorbed to generate waste heat or (2) transmitted into the
appropriate cells to generate electron-hole pairs with a high
probability or (3) escape back to space through the aperture 24.
The probability of escaping through the aperture 24 is dependent to
a first approximation upon the ratio A.sub.i/A.sub.s, where A.sub.i
is the diameter of the aperture and A.sub.s is the diameter of the
sphere 12. A small A.sub.i and a large A.sub.s means a small escape
probability. Preferably, the ratio of A.sub.i:A.sub.s is less than
0.01.
[0044] As shown in FIG. 1, beam 18 is depicted as comprising
photons at four different wavelengths .lamda..sub.1, .lamda..sub.2,
.lamda..sub.3, and .lamda..sub.4. Each of the solar cells 16a, 16b,
16c, and 16d are each responsive to a different wavelength. In this
example, .lamda..sub.1 is associated with solar cell 16a,
.lamda..sub.2 is associated with solar cell 16b, etc.
[0045] For example, diffusely-reflected .lamda..sub.4 photons,
denoted at 118', are reflected from the cavity wall 26. A
.lamda..sub.3 photon, denoted 118c, enters a matching
.DELTA..lamda..sub.3 solar cell 16c. As another example,
.lamda..sub.1 photon, denoted 118a, is rejected by a
.DELTA..lamda..sub.2 solar cell 16b, but is absorbed by solar cell
16a.
A. Photon Capture by the Spherical Cavity (Photon Escape
Probability)
[0046] The highly concentrated beam (photons 18 in FIG. 1) from the
secondary concentrator 20 is injected into the spherical cavity 14
and is trapped within the boundaries of the cavity wall 26. (In
actuality, the beam 18 becomes divergent or defocuses after
entering the cavity 14 at aperture 24.) The escape probability of a
trapped photon representing a frequency band is, to a first degree,
proportional to the ratio of entrance aperture area (A.sub.i) to
the total interior surface area (A.sub.s) of the sphere. FIG. 2
illustrates the escape probability of a photon representing a
discrete frequency band.
[0047] The incoming flux of photons is represented by 218a and the
outgoing flux by 218b. The entrance aperture 24 has area A.sub.i.
The photon 218a can enter a solar cell 16a or be reflected off its
surface and enter another solar cell 16b, or the photon can be
reflected off the surface of the first solar cell 16a and in turn
reflected off the interior surface 26 or reflected back through the
entrance aperture 24. A Rugate filter 17, for example, 17a, is
shown associated with each solar cell 16, for example, 16a. The
Rugate filter 17 may be formed directly on top of the solar cells
16 or deposited on a fused glass cover and may be cemented to the
cell or spaced apart from the solar cells.
[0048] Using the integrating sphere radiance equation, it can be
shown that for a given frequency band the escape probability for a
photon within that band is given to a first order by:
Q.sub.out/Q.sub.in=A.sub.i/A.sub.s{r(1-f)/1-(1-f)},
where Q.sub.out/Q.sub.in is the ratio of the outgoing flux 118b to
the incoming flux 118a, f=(A.sub.i+A.sub.e)/A.sub.s, A.sub.s=total
interior surface area of the sphere 14, A.sub.i=area of the
entrance aperture 24, and A.sub.e=total surface area of the cells
16 in the given frequency band, for example, cell 16a. This
simplified equation assumes that the cells 16 (i.e., cells 16a,
16b, 16c, 16d) with bandgaps outside the frequency band have the
same reflectance as the highly reflective sphere coating 26a
(discussed in greater detail below). Now consider a sphere 14 of 10
cm diameter with an entrance aperture of 1.4 cm in diameter. The
interior of the sphere is 20% occupied with cells of a given
bandgap. All cells 16 are coated with Rugate filters 17 with
complete (100%) transmission and reflection characteristics,
respectively. The calculated escape probability becomes then about
1.2%. This probability can be reduced by choosing a larger diameter
for the sphere 14. This, however, will lower the overall
concentration ratio (see below) and increase the absorption of the
photons by the uncovered cavity wall.
B. The Choice of Diffuse Reflector
[0049] The quality of the diffuse reflector 26a on the wall 26 of
the cavity 14, shown in FIG. 1, is important. It is directly
related to reflection losses, flux uniformity, and sphere
multiplier factor M. The stability of the reflector under high
levels of flux and possible, accidental temperature runaways is
also very important as these may change the reflectance.
[0050] Preliminary evaluation of reflector materials 26a resulted
in the selection of "space-grade" SPECTRALON that combines very
high-reflectance with an extremely lambertian reflectance profile.
It also has excellent low outgassing features. SPECTRALON, which is
manufactured by Labsphere (North Sutton, N.H.), is a thermoplastic
resin with special pigments added that can be machined into a wide
variety of shapes for the construction of optical components. The
material is chemically inert and is thermally stable up to
400.degree. C. Further details of this material are disclosed and
claimed in U.S. Pat. No. 5,763,519, entitled "Diffusively
Reflecting Sintered Fluorinated Long-Chain Addition Polymers Doped
with Pigments for Color Standard Use", and issued to A. W.
Springsteen on Jun. 9, 1998, incorporated herein by reference.
[0051] SPECTRALON reflectance material gives the highest diffuse
reflectance of any known material or coating over UV-VIS-NIR
(ultraviolet-visible-near infrared) region of the spectrum. The
reflectance is generally >99% over a range from 400 to 1500 nm
and >95% from 250 to 2500 nm. The material is also highly
lambertian at wavelengths from 250 to 10,600 nm. A "lambertian"
surface is a perfectly diffusing surface having the property that
the intensity of light emanating in a given direction is
proportional to the cosine of the angle of the normal to the
surface (lambertian cosine law). A material such as SPECTRALON
obeying this law is said to be an isotropic diffuser that has the
same radiance in all directions. "Highest diffuse reflectance"
means the known highest reflectance of 99.1% over the solar
spectrum. Another suitable reflectance material for use as the
coating 26a in the practice of the present invention is barium
sulfate.
C. Integrating Sphere Photovoltaic Receiver
[0052] In accordance with the present invention, since the
integrating sphere PV receiver, or PowerSphere, is intended for a
single wavelength laser source, then only one type of solar cell,
sensitive to that wavelength, need be used. For the same reason,
Rugate, or other spectral filters, are unnecessary.
[0053] The laser-to-electricity conversion approach, or
PowerSphere, is depicted in FIG. 3. As with the PVCC 10, the
PowerSphere 310 includes an optimized closed shape, preferably, a
spherical cavity 314 with a small port 324 for insertion of light
318 and an array of photovoltaic cells 316 that almost completely
cover the interior wall 326 of the cavity. A secondary, dielectric
concentrator, or light injector/booster, 320 with an extractor rod
336 serves the same function (i.e., boosting and injecting
pre-focused laser beam into the sphere 310) as the secondary
concentrator 20 discussed above. However, the extractor rod 336
evenly distributes the light in all directions inside the cavity
314. As with the secondary concentrator 20, the inner surfaces 322
of the secondary concentrator 320 may be mirrored. Preferably, the
secondary concentrator 320 is a non-imaging, compound parabolic of
hollow design, as disclosed in the above-referenced U.S. Pat. No.
6,057,505.
[0054] The individual cells 316 are interconnected with each other
in a certain fashion (in parallel and in series) to achieve the
voltage and current levels for the required electrical power
output. The laser light 318 enters the cavity 314 via the
dielectric concentrator 320. As shown in FIG. 3, the E-rod 336 fits
exactly the light entry port 324 of the sphere 310. The function of
the E-rod 336 is first to guide the light 318 towards the center of
the cavity 314 and then to emit the photons 318 uniformly in all
directions. This angular isotropy is required to achieve the best
possible flux uniformity before the photons 318 experience their
first reflection at the cavity wall 326. Strong non-uniformities in
the flux distribution have a degrading effect on the performance of
the array of the photovoltaic cells 316.
[0055] Dielectric, non-imaging, secondary concentrators based on
total internal reflection are abbreviated as DTIR. This type of
refractory secondary was introduced in the mid-1970s to enhance the
performance of reflective, non-imaging, two-stage concentrators for
larger acceptance angle at a given concentration ratio and to
reduce the focal distance of the two-stage concentrator system. In
addition, they proved to have higher throughput than refractive
systems.
[0056] An E-rod 336 works by multiple reflections of light rays at
increasingly increasing angles along its length, causing each ray
to eventually fail TIR and refract out of the E-rod into a lower
index medium. In the case of the PowerSphere 310, the low index
medium is air and the isotropic distribution of the rays are
achieved by facets in a certain pattern on the surface of the E-rod
336. The impact of the use of the E-rod 336 is the elimination of
the lambertian material 26a (e.g., SPECTRALON). In the PowerSphere
design, the photons 318 are distributed evenly and the photovoltaic
cells 316 see a uniform flux to start with.
[0057] The photovoltaic cells 316 are carefully selected with
regard to their quantum efficiency to optimize the conversion of
the monochromatic light 318 of the chosen laser. The best
conversion is achieved when the laser wavelength coincides with the
peak of the quantum efficiency response. For laser power beaming
applications, the cell design must take into consideration the very
high flux concentrations that are involved. The required cell
design features for laser power beaming are achieved by proper
doping of the cells 316 and by enhancing the top metal contacts
(grid fingers) (not shown) of the cell in order to mitigate the
rapidly growing series resistance as the flux density increases. As
is explained below, the PowerSphere concept allows much wider and
denser grid fingers than the conventional flat plate PV receivers
without the associated "shadowing" losses. Further, an
anti-reflective coating 316a may be formed on the front surface of
the photovoltaic cells 316, as shown in FIG. 3a. Thus, in
principle, the PowerSphere can reach higher efficiencies than
possible with the flat plate PV receivers at high flux
densities.
[0058] The photovoltaic cells 316 may be provided with a thin
mirrored back surface 316b for reflecting photons not absorbed by
the bulk of the cell. Such reflected photons, however, are absorbed
by another PV cell 316 inside the cavity 314. Thus, the photon
utilization factor is improved.
[0059] The PowerSphere is a novel technology that aims at the
highest laser-to-electric power conversion efficiency for power
beaming applications. Modified versions of the PowerSphere concept
are applicable to both space and terrestrial applications. For
example, NASA, the Air Force, and others are intensively exploring
utilization of laser power beaming in space. All these applications
require photovoltaic laser beam receivers that are highly efficient
and reliable. FIG. 4 shows the principles of a lightweight
Cassegranian/PowerSphere for space applications, although such a
configuration could also be suitably employed, with minor
modifications, for terrestrial applications.
[0060] Specifically, as shown in FIG. 4, a laser power beam 318 is
intercepted by a reflecting concentrator 400, e.g., a Cassegranian
concentrator, comprising a primary concentrator 434 that is
preferably parabolic in shape and also serves as a heat radiator. A
secondary concentrator 434' that is preferably hyperbolic in shape
is located at the focus of the primary concentrator 434, and
directs the laser light 318 into the light injector/booster 320,
which can be either refractive or reflective. The light
injector/booster 320 serves essentially the same function as the
secondary concentrator 20, as described above. The secondary
concentrator 434' is suspended above the primary concentrator 434
by telescopic arm 456. The light 318 then enters the integrating
sphere photovoltaic receiver 310 through the injector/booster 320,
as described above.
[0061] A loop heat pipe system designed for zero gravity space
environment, shown at 450 and described in greater detail in
above-referenced U.S. Pat. No. ______ [D-2K042], removes waste heat
from the sphere 310 and transfers it to the back surface 434a of
the primary concentrator 434. The rejected heat 452 is radiated
into space (or the surrounding environment) by the primary
concentrator 434. The primary concentrator 434, a parabolic dish,
that intercepts the somewhat broadened laser beam, is made of
highly conductive carbon fiber composite for excellent surface
thermal diffusivity, stiffness and lightweight. A non-limiting
example of such a highly conductive carbon fiber is K1100,
available, for example, from AMOCO. The back-surface 434a of the
primary concentrator 434 is highly emissive to facilitate good
radiative waste heat rejection into space or surrounding
environment.
[0062] FIG. 5 illustrates how a space vehicle 500 can be equipped
with both a laser PV module 310 and a solar PV module 10 if the
missions power demand requires two independent power sources. The
two modules 10, 310, are deployed from the space vehicle 500 by
deployment booms 502. At least the laser PV module 310 has
rotational capability about axes A and B to optimize interception
of the laser power beam 318. In this latter connection, the back
surface of the secondary concentrator 434' (facing the laser beam
318 or sun's rays 18) may be provided with centering means (not
shown) to align the concentrator 434 to incoming radiation 18, 318.
An example of one such means is disclosed in U.S. Pat. No.
4,330,204, entitled "Self-Aligning Laser Communicator Utilizing
Reciprocal Tracking" and issued to Richard A. Dye on May 18, 1982,
the contents of which are incorporated herein by reference. In
essence, a quadrature detector, composed of four equal segments,
receives radiation. So long as all four segments generate the same
current, no further alignment is necessary. An imbalance in current
is indicative of mis-alignment (off-center position) of the beam,
and the information can be used to rotate the PV module 310 about
axes A and/or B to bring the concentrator 434 into alignment.
[0063] It is also possible to construct a "dual purpose" PVCC that
can convert both direct solar radiation and a directed laser beam
efficiently into electricity. Such a PVCC would find application in
certain space missions where the satellite must fly through an
eclipse during which no solar radiation is available. However, a
laser based on a space platform at a suitable distance and position
can provide the power during the eclipse period. Such a dual
purpose PVCC may contain a combination of multi-junction cells and
single junction cells, such as InGaP/GaAs (multi-junction), silicon
(single junction), and InGaAsP/InGaAs (multi-junction). The dual
purpose PVCC would essentially combine the solar cells 16a-16d with
filters 17 of FIG. 1 and the photovoltaic cells 316 of FIG. 3 in a
single cavity 14 or 314. The solar cells 16a-16d would be selected
to span at least a portion of the solar spectrum.
[0064] There is also a growing need for terrestrial power beaming
with lasers for a multitude of applications where electric power
cables cannot be used or are not practical and also the use of
batteries is limited. Examples include: energy transfer to rotating
systems or flying unmanned drones (see FIG. 6), potentially
explosive surroundings, facilities for radioactive and other
hazardous materials, remote robotics, power supply to switches,
remote sensor applications with high power demand, high power
electronic systems (e.g. telemetry equipment as shown in FIG. 7)
susceptible to electromagnetic interference (EMI), unmanned surface
maritime vessels, off-shore oil exploration, etc.
[0065] In FIG. 6, an unmanned drone 600, such as used for border
surveillance, is shown moving across the page. The drone 600 is
fitted with an integrated sphere photovoltaic receiver 310, such as
described above. A ground-based laser network, comprising a
plurality of lasers 602a, 602b, 602c emitting laser radiation 318
that is receivable by the PV receiver 310, is established over an
area to be patrolled by the drone 600. For improved reception of
the laser power beam 318, the PV receiver 310 could be mounted on
gimbals (not shown), which would permit the drone 600 to track the
laser power beam 318 as the drone moves from a first laser 602a to
a second laser 602b to a third laser 602c. It will be appreciated
that there is a beam transfer range 606 as the drone "passes off"
from one laser, e.g., 602a, to another laser, e.g., 602b.
[0066] For an unmanned drone 600 flying at an altitude of 35,000
feet, the distances between each two beam transfer range (604a,
604b, 604c) would be about 13.4 miles, if the traveling power beam
318 is allowed to sweep an angular distance from -45.degree. to
+45.degree. around a point (zenith) directly overhead on a given
laser beam source 602 in the laser network that covers a specified
area. In this configuration, the distances between neighboring
laser sources 602 would be also about 13.4 miles.
[0067] In FIG. 7, a missile 700 is equipped with the PowerSphere
310 of the present invention, which provides DC power along line
702. The PowerSphere 310 receives light 318 from a high power laser
704. The DC power provides a source of power to telemetry equipment
706 in the missile 700, where, for example, the equipment is
undergoing long duration ground testing and it is required that the
power be free of electro-magnetic interference.
[0068] High power lasers have been under intense investigation as a
directed energy source. The U.S. Government and industry have a
long standing interest in developing high-power lasers for a
variety of applications including materials processing, isotope
separation, nuclear fusion, long range sensing and long range
communications and other defense activities. Key development goals
are high brightness and high efficiency. Laser technologies that
are in an advanced stage of development include chemical
oxygen-iodine laser (COIL), photolithic iodine lasers, carbon
dioxide lasers, diode pumped solid-state lasers, and high power
semiconductor diode lasers. High power semiconductor diode Laser
technology has some definite advantages over other types of lasers
because of their extremely small size and efficiency (potentially
up to 70%). Chemical oxygen-iodine lasers have also been proven to
be scalable up to 40 KW with improved efficiency.
[0069] In the past, several semiconductor cell materials have been
studied by NASA in conjunction with PV converters for laser power
beaming. The most rigorously studied PV materials are Si, GaAs,
InP, Ge, and certain III-V cells, including InGaP, InGaAs, and
InGaAsP. All these cells have reached a mature technology state and
are commercially available. Silicon cells, although not the most
efficient of them all, are the best initial choice because of their
reliability, availability, and low cost.
[0070] Over the last decade "bandgap engineering" studies have
opened up the possibilities to a new era of bandgap "tunable"
semiconductors. Some examples are recently discovered
indium/gallium alloys and quantum-well systems. Once these
technologies are fully developed, it will become possible to
closely match a given laser frequency with the quantum efficiency
peak of a tunable PV cell. This is particularly important for the
near-infrared range where COIL lasers with good atmospheric
penetration can be matched with the recently developed photovoltaic
GaAlInAsSb alloys with bandgaps ranging from 0.52 to 0.55 eV.
D. Expected Laser-to-Electric Power Efficiencies with
PowerSphere:
[0071] Some fiber optic driven photovoltaic power converters are
already on the market. These devices are quite efficient (about
40%) but their power capability is very low (about a few mW). The
Power/Sphere system disclosed herein, however, involves a high
power laser/PV converter that can generate electric power ranging
from few watts to tens of kilowatts. As mentioned above, such high
power concepts have been mostly of interest to NASA and DOD, and
the bulk of the available data comes from these sources.
[0072] Theoretical modeling by NASA indicates that by tuning the
wavelength of a laser to 840 nm, a power beam system based on GaAs
can achieve quantum conversion efficiencies approaching 60%. The
highest literature cell efficiency reported under selective
illumination is 59% for the AlGaAs/GaAs hetero-junction cell, at
laser input intensities up to 54 W/cm.sup.2. The integrating sphere
receiver of the present invention is expected ultimately to reach
efficiencies approaching 70%. This improvement is due to minimized
series resistance at very high flux densities and the photon
recycling process in the cavity, as explained below.
E. PowerSphere vs. Flat Plate PV Receivers:
[0073] There are fundamental differences between the operational
principles of PowerSphere 310 and flat plate PV receivers for
concentrator systems. These are briefly highlighted below: [0074]
1. Photon utilization:
[0075] Incident photons (laser beam) on a flat plate PV receiver
either enter the solar cells or are reflected from the active cell
surface and from the top surface metallization (grid fingers,
bus-bar, etc.). Photons striking the non-active areas between the
cells are either absorbed or reflected. Reflected and absorbed
photons are lost for the conversion process and can no longer
contribute to the photocurrent. These losses are substantial in the
case of high flux densities such as the laser power beaming require
(50 Watts/cm.sup.2 or higher).
[0076] In contrast to flat plate receivers, the PowerSphere 310
shown in FIG. 3 traps almost 99% of the photons 318 that enter the
cavity 314. Reflected photons 318b return back into the cavity 314
and are recycled. A high reflectivity material (not shown), such as
discussed above, e.g., SPECTRALON, covering the non-active areas of
the interior cavity wall 326 (much like coating 26a in FIG. 1
above), also reflects photons 318 striking the areas between the PV
cells 316. This photon recycling mechanism leads to a higher photon
utilization factor and consequently to higher efficiencies not
obtainable with flat PV counterparts. [0077] 2. Series
Resistance:
[0078] At flux densities of about 50 to 100 W/cm.sup.2, the series
resistance of photovoltaic cells becomes the predominant loss
mechanism and drives down the conversion efficiency. There are
several components that contribute to the overall series
resistance. For the sake of brevity, we mention here only three key
components that are relevant in this comparison. The three key
components are the series resistances of: (1) the metal grid path,
(2) the bus-bar path, and (3) the emitter path. The grid resistance
and bus-bar resistance are linearly dependent on the width and
thickness of the respective metallization. Cells with wider grid-
and bus bar units have lower series resistance. Emitter resistance
is proportional to the distance between the fingers. The closer the
fingers are, the lower is the emitter resistance. Thus, by making
the fingers and bus-bar wider and the distance between the fingers
smaller, the series resistance can be reduced. However, the
photocells designed for flat panel PV receivers for concentrator
systems are limited to relatively small finger widths and bus-bar
widths and large distances between the fingers. This is because of
the shading factor F=W/d, where W is the average finger and bus-bar
width and d is the distance between the fingers. As seen from this
definition, a wider metallization and a smaller finger distance
increases the shading factor as a result of increased reflective
losses.
[0079] An advantage of the PowerSphere 310 is that this effect is
mitigated because of the photon recycling process explained above.
Thus, the photovoltaic cells 316 for the PowerSphere 310 can have a
much wider metallization width and closer fingers. This capability
pushes the downturn of conversion efficiency towards much higher
flux levels without the penalty of shadowing losses. [0080] 3. Flux
Uniformity:
[0081] Flux uniformity across a high flux PV receiver is of outmost
importance. A non-uniform flux that is impinging on a string of
cells that are connected in series may force a cell into reverse
bias if this cell receives less light than the neighboring cells in
the string. The bias reversal occurs when the current in the string
exceeds the short circuit current of that cell in question. This
reverse bias condition increases the resistive power dissipation in
the cell and causes the temperature to rise, thus forming a "hot
spot". At high flux levels, such a hot spot very likely destroys
the converter as a whole. The laser power beam profile at a flat
plate receiver is naturally not uniform across the surface of the
cell array. The flux density is high in the center and declines
rapidly towards the edges. This is a major concern for flat plate
PV receivers. On the other hand, the PowerSphere 310 of the present
invention is equipped with a dielectric light injector 336 (as
shown in FIG. 3), which eliminates this flux non-uniformity
problem, since the extractor 336 of the dielectric light injector
320 distributes the light evenly in all directions as the light 318
travels towards the its tip. The result is that in the PowerSphere
310, the flux density across the cells 316 is substantially uniform
for the entire cavity 314. [0082] 4. Operation in Pulse Mode:
[0083] The duty cycle of some lasers (with short pulse duration)
may be much shorter than the carrier lifetime in the particular PV
cells used. Although an average power output will be realized from
the array as a whole, the cells must have a metal grid system that
can handle the peak photo-current to minimize the resistive losses
that are proportional to the peak current squared. The problem in
the case of flat plate PV receivers is that high metallization
coverage leads to excess shadowing losses. As discussed above, the
PowerSphere 310 allows the use of highly enhanced metallization
with minimal grid coverage losses. Hence, a PowerSphere system is
more suitable to operate in pulse mode than a flat plate PV
receiver. [0084] 5. Additional Features of the PowerSphere for
Space Applications:
[0085] A key advantage of PowerSphere design for space applications
is that the PV cells 316 are located inside the cavity 314 and are
not exposed to the space environment. Assume a sphere 310 having a
housing 312 made of beryllium or lightweight carbon composite with
a thin exterior metal cladding. Such a conductive spherical
structure in space provides highly improved space hardening for the
PV cells 316. These hardening features protect the cells 316
against: (a) radiation damage by charged particles (particularly
for Van Allen Belt-crossing missions), (b) space charging and power
losses (Faraday Cage effect provides electro-statically shielding
of the interior), (c) atomic oxygen, (d) space debris and meteor
showers, (e) UV, (f) solar flares and magnetic storms, etc. As a
result of these effective space-hardening features, some of the
redundant, oversized beginning of life (BOL) array can be
eliminated and the specific power [$/Watt] can be improved.
[0086] For terrestrial applications, the PowerSphere 310 offers
extreme robustness against damaging environmental conditions,
including sand storms, hail, acid rain, and salt spray.
[0087] The PowerSphere 310 of the present invention has the
potential to yield laser-to-electricity conversion efficiencies
from 60% to 70%. Thus, excluding any atmospheric losses, a finely
tuned laser/PowerSphere system has the near-term potential to reach
an electricity-to-electricity conversion efficiency in the order 24
to 2%, assuming that the electricity-to-laser energy conversion
efficiency is 40%. If the primary energy source is a 50% efficient
solar PVCC concentrator, such as disclosed and claimed in the
parent patent (U.S. Pat. No. ______), then the overall
solar-to-electricity conversion efficiency via
solar-PVCC/laser-PowerSphere systems becomes 12% to 14%.
INDUSTRIAL APPLICABILITY
[0088] The concentrating photovoltaic module is expected to find
increasing use in space and terrestrial-based photovoltaic power
systems for converting laser radiation to electricity.
[0089] Thus, there has been disclosed a photovoltaic module for
converting laser light to electrical power. It will be readily
apparent to those skilled in this art that various changes and
modifications of an obvious nature may be made, and all such
changes and modifications are considered to fall within the scope
of the present invention, as defined by the appended claims.
* * * * *