U.S. patent application number 10/623373 was filed with the patent office on 2004-05-06 for solar collector having an array of photovoltaic cells oriented to receive reflected light.
Invention is credited to Aylaian, Eric.
Application Number | 20040084077 10/623373 |
Document ID | / |
Family ID | 26934592 |
Filed Date | 2004-05-06 |
United States Patent
Application |
20040084077 |
Kind Code |
A1 |
Aylaian, Eric |
May 6, 2004 |
Solar collector having an array of photovoltaic cells oriented to
receive reflected light
Abstract
A solar collector (200) is provided having increased efficiency
and reduced cost over conventional collectors. The collector
includes an array (218) of substrates (202A, 220B) each having a
substantially planar surface with a photovoltaic cell (208) formed
thereon. Generally, the array (218) includes at least first and
second substrates (202A, 220B) oriented at an angle (.alpha.)
relative to each other, and to a direction of propagation of light
received on the surface of the first substrate such that light
reflected from the first substrate is reflected onto the surface of
the second substrate, thereby increasing efficiency of the
collector (200). The efficiency of the collector (200) varies
inversely with the angle between the first and second substrates
(202A, 220B) for angles between 140.degree. and 20.degree..
Preferably, the surfaces of the substrates (202A, 220B) are shaped
and oriented relative to one another to form part of a concave
inner surface of a polyhedron (242, 246) or geometric figure
defined by three or more planar sides.
Inventors: |
Aylaian, Eric; (Santa Clara,
CA) |
Correspondence
Address: |
DORSEY & WHITNEY LLP
INTELLECTUAL PROPERTY DEPARTMENT
4 EMBARCADERO CENTER
SUITE 3400
SAN FRANCISCO
CA
94111
US
|
Family ID: |
26934592 |
Appl. No.: |
10/623373 |
Filed: |
July 18, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10623373 |
Jul 18, 2003 |
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10241806 |
Sep 10, 2002 |
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10241806 |
Sep 10, 2002 |
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09953501 |
Sep 11, 2001 |
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6515217 |
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Current U.S.
Class: |
136/246 ;
136/244 |
Current CPC
Class: |
H01L 31/048 20130101;
H01L 2924/0002 20130101; H01L 31/0547 20141201; H01L 2924/00
20130101; H01L 31/041 20141201; H01L 31/043 20141201; H02S 40/425
20141201; H02S 40/42 20141201; Y02E 10/52 20130101; H01L 2924/0002
20130101 |
Class at
Publication: |
136/246 ;
136/244 |
International
Class: |
H01L 025/00 |
Claims
We claim:
1. A solar collector for converting light incident thereon into
electrical energy, the solar collector comprising: an array of a
plurality of substrates, each substrate having a photovoltaic cell
(PVC) formed on a first surface thereof, the array including at
least a first substrate and a second substrate; and wherein the
first surfaces of the first and second substrates are oriented at
an angle relative to each other, and the first substrate is
oriented to receive light on the first surface thereof such that
light reflected from the first substrate is reflected onto the
first surface of the second substrate, whereby an efficiency of the
solar collector is increased.
2. A solar collector according to claim 1, wherein the efficiency
of the solar collector varies inversely with the angle between the
first surfaces of the first and second substrates for angles
between 140.degree. and a predetermined minimum angle.
3. A solar collector according to claim 2, wherein the
predetermined minimum angle between the first surfaces of the first
and second substrates is greater than or equal to 20.degree..
4. A solar collector according to claim 1, wherein the first
surface of the second substrate is also oriented to receive light
thereon, and wherein the second substrate is oriented such that
light reflected from the second substrate is reflected onto the
first surface of the first substrate.
5. A solar collector according to claim 1, wherein the first
substrate has an edge proximal to an edge of the second substrate
second and to an apex of the angle between the first surfaces of
the first and second substrates.
6. A solar collector according to claim 1, wherein the first
surface of the first substrate is substantially absent an
anti-reflective coating.
7. A solar collector according to claim 1, wherein each of the
plurality of substrates in the array further comprises a second
surface, and wherein the first surface of each of the plurality of
substrates is substantially planar and substantially parallel to
the second surface.
8. A solar collector according to claim 1, wherein each of the
plurality of substrates comprises a single monolithic PVC formed on
the first surface thereof.
9. A solar collector according to claim 1, wherein the PVCs formed
on the first surfaces of the plurality of substrates comprise at
least two different types of PVCs selected from the group
consisting of: Silicon based PVCs; Gallium-Arsenide (GaAs) based
PVCs; Aluminum-Gallium-Arsenide (AlGaAs) based PVCs; Germanium (Ge)
based PVCs; and Gallium Indium-Phosphide (GaInP) based PVCs.
10. A solar collector according to claim 1, wherein the PVCs formed
on the first surfaces of the plurality of substrates include at
least one PVC comprising a multiple-junction PVC.
11. A solar collector according to claim 1, further comprising an
enclosure enclosing the array of the plurality of substrates, the
enclosure having inner walls with reflective surfaces to reflect at
least a portion of light incident thereon onto the first surfaces
of the plurality of substrates.
12. A solar collector for converting light incident thereon into
electrical energy, the solar collector comprising: an array of a
plurality of substrates, each substrate having a photovoltaic cell
(PVC) formed on a first surface thereof, the array including at
least a first substrate, a second substrate and a third substrate;
and wherein the first surfaces of the first, second and third
substrates are oriented at angles relative to each other and to a
direction of propagation of light incident on the solar collector,
such that light reflected from the first substrate is reflected
onto the first surfaces of at least one of the second and third
substrates, whereby an efficiency of the solar collector is
increased.
13. A solar collector according to claim 12, wherein each of the
first, second and third substrates comprise an edge proximal to an
edge of at least one other substrate.
14. A solar collector according to claim 13, wherein the first
surfaces of the plurality of substrates are shaped and oriented
relative to one another such that the first surfaces of the first,
second and third substrates form part of a concave inner surface of
a polyhedron.
15. A solar collector according to claim 14, wherein the first
surfaces of the first, second and third substrates form part of
first, second and third inner surfaces of an inverted three sided
pyramid.
16. A solar collector according to claim 15, wherein each of the
first, second and third inner substrates are shaped to form first,
second and third isosceles triangles.
17. A solar collector according to claim 14, wherein the array of
the plurality of substrates further comprises a fourth substrate
having a PVC formed on the first surface thereof, and wherein the
first surfaces of the first, second, third and fourth substrates
are oriented at angles relative to each other and to a light ray
incident on the first surface of the first substrate such that
light reflected from the first substrate is reflected onto the
first surface of the fourth substrate.
18. A solar collector according to claim 17, wherein the first
surfaces of the second, third and fourth substrates are also
oriented to receive light thereon, and the second, third and fourth
substrates are oriented such that light reflected from the second
substrate is reflected onto at least one of the first surfaces of
the first, third and fourth substrates, light reflected from the
third substrate is reflected onto at least one of the first
surfaces of the first, second and fourth substrates, and light
reflected from the fourth substrate is reflected onto at least one
of the first surfaces of the first, second and third
substrates.
19. A solar collector according to claim 17, wherein each of the
first, second, third and fourth substrates comprise an edge
proximal to an edge of at least one other substrate.
20. A solar collector according to claim 17, wherein the first
surfaces of the first, second, third and fourth substrates form
part of first, second, third and fourth inner surfaces of an
inverted four sided pyramid.
21. A solar collector according to claim 20, wherein each of the
first, second, third and fourth inner substrates are shaped to form
first, second, third and fourth isosceles triangles.
22. A solar collector according to claim 14, wherein the array of
the plurality of substrates further comprises fourth, fifth and
sixth substrates having a PVC formed on a first surface thereof,
and wherein the first surfaces of the first, second, third, fourth
and fifth substrates are oriented at angles relative to each other
substrates form at least part of inner side surfaces of an inverted
polyhedron having a pentagonal cross-section, and wherein the sixth
substrate forms an inner bottom surface thereof.
23. A solar collector according to claim 12, wherein each of the
plurality of substrates comprises a single monolithic PVC formed on
the first surface thereof.
24. A solar collector according to claim 12, wherein the PVCs
formed on the first surfaces of the plurality of substrates
comprise at least two different types of PVCs selected from the
group consisting of: Silicon based PVCs; Gallium-Arsenide (GaAs)
based PVCs; Aluminum-Gallium-Arsenide (AlGaAs) based PVCs;
Germanium (Ge) based PVCs; and Gallium Indium-Phosphide (GaInP)
based PVCs.
25. A solar collector according to claim 12, wherein the PVCs
formed on the first surfaces of the plurality of substrates include
at least one PVC comprising a multiple-junction PVC.
26. A solar collector for converting light incident thereon into
electrical energy, the solar collector comprising: an array of a
plurality of substrates, each substrate having a photovoltaic cell
(PVC) formed on a first surface thereof, the array including at
least a first substrate and a second substrate; an enclosure
enclosing the array of the plurality of substrates, the enclosure
including a top-wall with a concentrator through which light is
passed to at least the first substrate; and wherein the first
surfaces of the first and second substrates are oriented at an
angle relative to each other, and the first substrate is oriented
to receive light on the first surface thereof such that light
reflected from the first substrate is reflected onto the first
surface of the second substrate, whereby an efficiency of the solar
collector is increased.
27. A solar collector according to claim 26 wherein the enclosure
has inner walls with reflective surfaces to reflect at least a
portion of light incident thereon onto the first surfaces of the
plurality of substrates.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This is a continuation-in-part of application Ser. No.
10/241,806, filed Sep. 10, 2002, and entitled Solar Cell Having a
Three-Dimensional Array of Photovoltaic Cells Enclosed Within an
Enclosure Having Reflective Surfaces, which is a
continuation-in-part of application Ser. No. 09/953,501, filed Sep.
11, 2001, and entitled Solar Cell Having a Three-Dimensional Array
of Photovoltaic Cells Enclosed Within an Enclosure Having
Reflective Surfaces, now U.S. Pat. No. 6,515,217, both of which are
incorporated herein by reference in their entirety.
FIELD OF THE INVENTION
[0002] The present invention relates generally to solar collectors,
and more particularly to a solar collector having a
three-dimensional array of substrates oriented at angles relative
to each other and to light received on the solar collector to
provide improved efficiency, extended operating life and reduced
manufacturing cost.
BACKGROUND OF THE INVENTION
[0003] Solar or photovoltaic cells (PVCs) are semiconductor devices
having P-N junctions which directly convert radiant energy of
sunlight into electrical energy. Conversion of sunlight into
electrical energy involves three major processes: absorption of
sunlight into the semiconductor material; generation and separation
of positive and negative charges creating a voltage in the PVC; and
collection and transfer of the electrical charges through terminal
connected to the semiconductor material. PVCs are widely known and
commonly used in a variety applications, including providing
electrical energy for satellites and other space craft, marine
vessels, installations in areas not served by a grid of an electric
utility company, and portable consumer electronics devices such as
radios, tape/compact disc players and calculators.
[0004] Heretofore PVCs have not been widely used as a main or even
auxiliary source of power for residences and businesses having
access to conventional power sources, for example, through a power
grid of an electric utility company. There are several reasons for
this, the most important of which is cost. Electricity produced
from solar cells tends to be relatively expensive compared to that
available from conventional power sources such as hydroelectric,
oil-fired, coal fired and nuclear power plants.
[0005] Although the cost of installing, maintaining and repairing
solar electric generation arrays or systems is not insignificant,
the greatest cost associated with solar energy is the cost of the
manufacturing the PVCs. Referring to FIG. 1, prior art PVCs 20 are
typically formed on an ultra-pure silicon wafer or substrate 22
containing materials such as Indium Phosphide, Gallium Arsenide,
Germanium, and related materials, which in itself can cost from
about 300 hundred to about 5 thousand dollars apiece depending on
size. For example, an 8 inch diameter silicon commonly used in
manufacturing PVCs typically costs about 2.5 thousand dollars.
Furthermore, traditionally a large number of individual PVCs 20
were fabricated on a single substrate 22 by (i) depositing or
growing a doped layer of semiconductor material, such as silicon,
over the substrate 22 including a dopant of an opposite type; (ii)
patterning and etching the substrate 22 with the doped layer
thereon to form individual PVCs 20; (iii) depositing a metal layer
over the etched substrate 22; (iv) patterning and etching the metal
layer to form vias, contacts and lines interconnecting the
individual PVCs 20; and (v) inspecting and testing the finished
PVCs 20 to remove from an output circuit defective PVCs. The time,
equipment and skilled operators required to perform each of the
above steps makes the cost of solar electricity extremely
expensive, and impractical for just about any use for which an
alternative conventional energy source is available.
[0006] In an effort to reduce costs, some of the latest generations
of PVCs have been monolithic PVCs in which substantially the entire
surface of a substrate is taken up a by single large PVC, thereby
eliminating much of the time and costs associated with patterning
and etching the doped layer and the metal layer. However, this
approach has not been wholly successful, since unlike with a
substrate having numerous individual PVCs which can be individually
removed from the output circuit, a single defect at any point in
the monolithic PVC would render the entire substrate useless. In
practice, this has resulted in yields well below 40%, offsetting or
completely negating any cost savings realized with this
approach.
[0007] Yet another problem with prior art PVCs is their low
external quantum efficiency. By external quantum efficiency it is
meant the proportion of the available photons converted into
electrical energy. Power from the sun arrives at Earth in the form
of photons of light in a wide spectrum from approximately 120
nanometers to 20 micrometers. The total solar irradiance,
neglecting absorption in the atmosphere, is approximately 135
mW/cm.sup.2 (about 10,000 watts per square meter). Thus, a
significant amount of solar radiation is available, but is not
absorbed by today's commercially available PVCs. The challenge to
photovoltaic manufacturers has always been how to convert this
abundance of energy into electricity.
[0008] Inefficiency in converting available light into electrical
energy is particularly a problem for solar electric systems having
limited power generating capability. This is because usable solar
energy is available for only a fraction of a day, when it is
available the PVCs must generate energy to meet current demands and
generate sufficient energy to be stored for use when usable solar
energy is unavailable. Thus, conventional solar electric systems
must either have relatively large numbers of PVCs, which as
explained above are costly, or have a high degree of efficiency.
Unfortunately, prior art PVCs are often only from about 15 to 19%
efficient, and more typically from about 10 to 14% efficient.
[0009] Referring to FIG. 2 it is seen that a major reason for this
poor efficiency of conventional solar collectors comes from the
reflectance of photons from front and buried surfaces of PVCs
having substantially flat surfaces. External Quantum Efficiency is
reduced by the reflected photons, which either never enter the cell
(front surface reflection) or are reflected from the back surface
or metallization layer interfaces and exit the cell without being
absorbed. Thus, a significant or even a large proportion of the
light incident on a surface 24 of the PVC 20 is simply reflected
away again.
[0010] Some attempt has been made to overcome these problems
through the addition of anti-reflective coatings to or treatment of
the surface of the PVCs to reduce reflection. For example, U.S.
Pat. Nos. 5,080,725, and 5,081,049, describe methods of
manufacturing PVCs in which the surface of the PVCs are textured or
contoured to form features such as ridges, pyramids and grooves
which minimize surface reflection from the PVCs and maximizes
reflection of light from internal surfaces thereby trapping light
within the PVC and increasing the chances of light being
absorbed.
[0011] While an improvement over the prior art these shaping
approaches have not been wholly satisfactory for a number of
reasons. Fundamentally, although the reflection of light from the
surface of the PVCs is reduced it is not eliminated and
anti-reflective coatings or surface treatments do nothing provide
for the re-absorption of reflected light through secondary or
higher order reflections. Moreover, the addition of anti-reflective
coatings or contouring of the PVCs adds to the cost of the
manufacturing the PVCs making the already high cost of solar
electricity prohibitively expensive, and impractical for just about
any use for which an alternative conventional energy source is
available.
[0012] A more fundamental problem is due to quantum mechanical
properties of the semiconductor crystal of the PVCs. Conventional
PVCs are capable of utilizing or converting into electricity only a
narrow range of light wavelengths corresponding to a band-gap
energy of the p-n junction of the PVC, no matter how much light is
concentrated or incident thereon. For example, although solar
radiation includes wavelengths from 2.times.10-7 to 4.times.10-6
meters, silicon based PVCs having a band gap energy of about 1.1
electron volts (eV) are capable of utilizing only wavelengths from
about 0.3.times.10-6 to about 3.0.times.10-6 meters. Similarly,
gallium-arsenide (GaAs) based PVCs, aluminum-gallium-arsenide
(AlGaAs) based PVCs, and germanium (Ge) based PVCs have band gap
energies of 1.43, 1.7 and 0.5 eV respectively, and are therefore
sensitive to other wavelengths.
[0013] Accordingly, there is a need for a solar collector that is
inexpensive to fabricate, highly efficient in its utilization of
available solar radiation, and which has an extended operational
life.
[0014] The present invention provides a solution to these and other
problems, and offers other advantages over the prior art.
SUMMARY
[0015] It is an object of the present invention to provide a solar
collector having an array of photovoltaic cells with improved
efficiency, extended operating life and reduced manufacturing
cost.
[0016] According to one aspect of the present invention, the solar
collector includes a number of substrates arranged in a
two-dimensional array of, each substrate having a monolithic
photovoltaic cell (PVC) formed on a surface thereof for converting
light incident thereon into electrical energy. The PVCs may include
at least two different types of PVCs receptive to different
wavelengths of light and having different band gap energies. The
array of substrates are enclosed within an enclosure having a
top-wall with an anti-reflective coating through which light is
passed to the PVCs, and bottom and sidewalls having reflective
coatings to reflect at least a portion of light incident thereon
onto the PVCs. Preferably, the enclosure further includes end-walls
joining the top and bottom walls. Like the top-wall, the end-walls
also have anti-reflective coatings thereon and join the top-wall at
an angle selected to facilitate passage of light to the PVCs from a
light source inclined relative to a surface of the top-wall.
[0017] In one embodiment, the PVCs include at least two different
types of PVCs selected from a group consisting of silicon based
PVCs, gallium-arsenide (GaAs) based PVCs, aluminum-gallium-arsenide
(AlGaAs) based PVCs, and germanium (Ge) based PVCs. Preferably,
where the PVCs include GaAs, AlGaAs or Ge based PVCs, the PVCs
include a top passivation layer to filter damaging radiation.
[0018] In another embodiment, the solar collector further includes
a voltage output circuit or circuit electrically coupling all the
PVCs to a single voltage output from the solar collector.
Generally, the circuit has a number of voltage converters to match
voltages from the different types of PVCs to a common output
voltage. The circuit can couple the PVCs in parallel, in series or
in a combination of both. In one alternative embodiment, a number
of a particular type of PVCs may be connected in series with one
another and in parallel with a second number of a second type of
PVCs having a different band gap energy to provide a common output
voltage. For example, the solar collector can include 15 AlGaAs
based PVCs having a band gap energy of 1.7 electron volts (eV), 18
GaAs based PVCs having a band gap energy of 1.4 eV, and 23 silicon
based PVCs having a band gap energy of 1.1 eV to provide a common
output voltage of about 25 volts direct current (vdc).
[0019] In another aspect the present invention is directed to a
solar collector having a number of substrates arranged a
three-dimensional array. Each substrate has at least one PVC formed
on a surface thereof for converting light incident thereon into
electrical energy. The three-dimensional array includes a lower or
base-layer of substrates, and at least one elevated-tier of
substrates positioned above and separated from the base-layer of
substrates, so that at least a portion of the light passes between
the substrates of the elevated-tier and is absorbed by the
substrates of the base-layer.
[0020] In one embodiment, the elevated-tier includes substrates
having surfaces with the PVCs formed thereon oriented to receive at
least some of the light reflected from the substrates of the
base-layer. Preferably, the PVCs are monolithic PVCs, and include
at least two different types of monolithic PVCs selected from a
group consisting of silicon, GaAs, AlGaAs, and Ge based PVCs. More
preferably, the where the PVCs include GaAs, AlGaAs or Ge based
PVCs, these PVCs are oriented to receive only light reflected from
the substrates of the base-layer, thereby reducing their exposure
to damaging levels of short wavelength or ultraviolet radiation.
Optionally, the GaAs, AlGaAs and Ge based PVCs include a top
passivation layer to filter-out or further reduce their exposure to
damaging radiation.
[0021] In yet another aspect the present invention is directed to a
solar collector including a three-dimensional array of substrates
enclosed within an enclosure having a top-wall with an
anti-reflective coating through which light is passed to the PVCs,
and bottom and sidewalls having reflective coatings to reflect at
least a portion of light incident thereon onto the PVCs. As above,
each substrate has a PVC formed on a surface thereof, and the
three-dimensional array includes a base-layer of substrates, and at
least one elevated-tier of substrates positioned above and
separated from the base-layer of substrates, so that at least a
portion of the light passes between the substrates of the
elevated-tier and is absorbed by the substrates of the
base-layer.
[0022] In a preferred embodiment, the enclosure further includes
end-walls joining the top and bottom walls, and the substrates of
the elevated-tier are electrically coupled to and supported above
the base-layer by a ground conductor affixed at both ends thereof
to either the end-walls or the sidewalls of the enclosure. The
ground conductor can include one or more wires or straps, or a
stamped or extruded aluminum or similar metal carrier. Optionally,
the substrates of the elevated-tier can be further supported by
voltage conductors affixed the substrates and to the enclosure.
[0023] In one embodiment, the elevated-tier includes substrates
having surfaces with the PVCs formed thereon oriented to receive at
least a portion of light reflected from the substrates of the
base-layer and/or from the bottom-wall of the enclosure. It will be
understood that the solar collector can include multiple
elevated-tiers, each having substrates on a top portion thereof and
on a bottom portion thereof. The substrates on the top portion are
oriented to receive light directly through the top-wall of the
enclosure and light reflected from substrates on the bottom portion
of an overlying tier. The substrates on the bottom portion of the
elevated-tiers are oriented to receive light reflected from either
substrates on the top portion of an underlying tier, the bottom
layer of substrates, or the sidewalls and bottom-wall of the
enclosure. Preferably, the elevated-tiers are offset from one
another such that at some portion of the substrates of each
elevated-tier and the bottom layer receive at least some light
passed directly through the enclosure and onto the substrates.
[0024] In another embodiment, the PVCs include at least two
different types of monolithic PVCs selected from a group consisting
of silicon, GaAs, AlGaAs, and Ge based PVCs. Where the PVCs include
GaAs, AlGaAs or Ge based PVCs, these PVCs are oriented to receive
only light reflected from the substrates of the base-layer, thereby
reducing their exposure to damaging levels of short wavelength or
ultraviolet radiation. Optionally, the GaAs, AlGaAs and Ge based
PVCs include a top passivation layer to filter-out or further
reduce damaging radiation.
[0025] Generally, the solar collector further includes a circuit
electrically coupling the PVCs to a voltage output from the solar
collector, the circuit including a number of voltage converters to
match voltages from the different types of PVCs to a common output
voltage.
[0026] In yet another embodiment, the solar collector further
includes a cooling mechanism selected from the group consisting of:
(i) a number of vents in the enclosure to enable movement of air
therethrough; (ii) vents in the enclosure and a fan to facilitate
movement of air through the enclosure, the fan powered by at least
a part of the voltage output from the PVCs; and (iii) a heat
exchanger thermally coupled to at least some of the substrates
and/or the enclosure, the heat exchanger including one or more
passages or tubes through which a fluid is passed to cool the solar
collector. In one preferred version of this embodiment, the heat
exchanger is adapted to provide heat or heated water, in particular
potable water, to a residence or business.
[0027] In still another aspect the present invention is directed to
a solar collector including a three-dimensional array of substrates
each having a photovoltaic cell (PVC) formed on a surface thereof
for converting light incident thereon into electrical energy.
Generally, the three-dimensional array includes a base-layer of
substrates, and a first elevated-tier of substrates positioned
above and separated from the base-layer of substrates, and the
surfaces of the base-layer of substrates are oriented at an angle
relative to the light incident thereon to reflect light received
thereon to the substrates of the first elevated-tier of substrates.
Preferably, the surface of a first substrate of the first
elevated-tier of substrates is oriented at an acute angle relative
to the surfaces of the base-layer of substrates to receive light
reflected from the substrates of the base-layer, and to reflect
light onto the surface of a second substrate of the first
elevated-tier of substrates. Alternatively, a first substrate of
the first elevated-tier of substrates is oriented to reflect light
received from the substrates of the base-layer onto the surface of
a substrate in a second elevated-tier of substrates positioned
above and separated from the first elevated-tier of substrates.
[0028] In another aspect the present invention is directed to a
solar collector having an enclosure enclosing an array of
substrates, each substrate having a monolithic PVC formed on a
surface thereof for converting light incident thereon into
electrical energy, the enclosure including a top-wall with a
concentrator through which light is passed to a base-layer of
substrates.
[0029] According to another aspect of the present invention, the
solar collector has an array of substrates with photovoltaic cells,
the substrates oriented at angles relative to each other such that
light reflected from a first substrate is reflected onto the
surface of a second substrate, thereby improving efficiency and
reducing manufacturing cost.
[0030] In one embodiment, the solar collector includes an array of
a number of substrates, each substrate having a photovoltaic cell
(PVC) formed on a first or top surface thereof, the array including
at least a first substrate and a second substrate. The first
surfaces of the first and second substrates are oriented at an
angle relative to each other, and to a direction of propagation of
light received on the first surface of the first substrate such
that at least a portion of light reflected from the first substrate
is reflected onto the first surface of the second substrate,
thereby increasing efficiency of the solar collector. Generally,
the substrates further include second or lower surfaces, and the
first surface of each of the substrates is substantially planar and
substantially parallel to the second surface. In accordance with
the present invention, the first surface of each of the substrates
is substantially absent an anti-reflective coating.
[0031] The efficiency of the solar collector varies inversely with
the angle between the first surfaces of the first and second
substrates for angles between 140.degree. and a predetermined
minimum angle. Preferably, the predetermined minimum angle is
greater than or equal to 20.degree..
[0032] The PVCs formed on the first surfaces of the substrates can
include either or both monolithic PVCs and multiple-junction PVCs.
Generally, the PVCs are selected from the group consisting of
Silicon based PVCs, Gallium-Arsenide (GaAs) based PVCs,
Aluminum-Gallium-Arsenide (AlGaAs) based PVCs, Germanium (Ge) based
PVCs, and Gallium Indium-Phosphide (GaInP) based PVCs.
[0033] Optionally, the array of substrates is enclosed within an
enclosure having a top-wall through which light is passed to the
PVCs a bottom wall and side walls. Preferably, the inner surfaces
of all the walls are reflective surfaces. The reflective surfaces
of the inner walls reflect at least a portion of light incident
thereon onto the first surfaces of the substrates.
[0034] In another embodiment, the array further includes at least a
third substrate, and the first surfaces of the first, second and
third substrates are oriented at angles relative to each other and
to a direction of propagation of light incident on the solar
collector, such that light reflected from the first substrate is
reflected onto the first surfaces of at least one of the second and
third substrates. Generally, each of the first, second and third
substrates comprise an edge proximal to an edge of at least one
other substrate, and the first surfaces of the substrates are
shaped and oriented relative to one another to form part of a
concave inner surface of a polyhedron or geometric figure defined
by three or more planar sides.
[0035] In one version of this embodiment, the first surfaces of the
first, second and third substrates form part of first, second and
third inner surfaces of an inverted three sided pyramid.
Preferably, each of the first, second and third inner substrates
are shaped and sized to form first, second and third isosceles
triangles.
[0036] In yet another embodiment, the array further includes a
fourth substrate having a PVC formed on the first surface thereof,
and the first surfaces of the first, second, third and fourth
substrates are oriented at angles relative to each other and to a
direction of propagation of light incident on the first surface of
the first substrate such that light reflected from the first
substrate is reflected onto the first surface of the fourth
substrate. Preferably, the first surfaces of the second, third and
fourth substrates are also oriented to receive light thereon. More
preferably, the second, third and fourth substrates are oriented
such that light reflected from the second substrate is reflected
onto at least one of the first surfaces of the first, third and
fourth substrates. Light reflected from the third substrate is
reflected onto at least one of the first surfaces of the first,
second and fourth substrates. Light reflected from the fourth
substrate is reflected onto at least one of the first surfaces of
the first, second and third substrates.
[0037] In one version of this embodiment, the first surfaces of the
first, second, third and fourth substrates form part of first,
second, third and fourth inner surfaces of an inverted four sided
pyramid. Preferably, each of the first, second, third and fourth
inner substrates are shaped to form first, second, third and fourth
isosceles triangles.
[0038] In still another embodiment, the solar collector further
includes a plurality of substrates oriented at an angle relative to
each other, and an enclosure enclosing the array of substrates, the
enclosure including a top-wall with a concentrator through which
light is passed to at least the first substrate. Preferably, the
concentrator is a non-imaging concentrator that diffusely focuses
light on the first substrate.
[0039] Advantages of the solar collector of the present invention
include any one or all of the following:
[0040] (i) an improved efficiency of up to 3 times that of
similarly sized conventional solar collectors;
[0041] (ii) reduced size or `footprint` as compared to conventional
solar collectors with a similar power output, thereby simplifying
an installation process and enabling use of the inventive solar
collector in locations having a limited area available for a solar
cell;
[0042] (iii) extended operating life made possible by reducing
exposure of sensitive PVCs to damaging levels of short wavelength
or ultraviolet radiation, and by actively cooling the solar
collector to maintain the PVCs below a maximum desirable operating
temperature;
[0043] (iv) ability to use fluid from a heat exchanger used to cool
the solar collector to provide heat or heated water to a residence
or business;
[0044] (v) reduced manufacturing or fabrication cost made possible
by use of monolithic PVCs thereby eliminating the need to form and
interconnect multiple PVCs on a single substrate; and
[0045] (vi) reduced manufacturing time achieved by eliminating the
need to form and interconnect multiple PVCs on a single
substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0046] These and various other features and advantages of the
present invention will be apparent upon reading of the following
detailed description in conjunction with the accompanying drawings,
where:
[0047] FIG. 1 (Prior Art) is a plan view of a substrate having a
plurality of individual photovoltaic cells (PVCs) formed on a
surface thereof;
[0048] FIG. 2 (Prior Art) is a simplified block diagram of a
typical solar cell showing incident light striking active devices
on a top surface of the solar cell;
[0049] FIG. 3A is a plan view of a solar collector with a plurality
of substrates, each with a PVC formed thereon, enclosed within a
reflective enclosure and arranged in a three-dimensional array
having a single elevated-tier of substrates according to an
embodiment of the present invention;
[0050] FIG. 3B is a side view of the solar collector of FIG. 3A
showing the position of the substrates of the elevated-tier
relative to a base-layer, and the reflection of light among the
plurality of substrates according to an embodiment of the present
invention;
[0051] FIG. 4A is a perspective view of a solar collector with a
plurality of substrates, each with a PVC formed thereon, enclosed
within a reflective enclosure and arranged in a three-dimensional
array having multiple elevated-tier of substrates according to an
embodiment of the present invention;
[0052] FIG. 4B is a side view of the solar collector of FIG. 4A
showing the position of the substrates of the elevated-tiers
relative to a base-layer, and the reflection of light among the
plurality of substrates according to an embodiment of the present
invention;
[0053] FIG. 5 is a top view of an elevated-tier of substrates in a
solar collector showing use of a common ground wire(s) to support
and interconnect the substrates of the elevated-tier according to
an embodiment of the present invention;
[0054] FIG. 6 is a partial perspective view of an enclosure showing
an attachment of ground wires supporting elevated-tiers to the
enclosure according to an embodiment of the present invention;
[0055] FIG. 7 is a partial top view of a bottom layer of PVCs
showing orientation of wafers or substrates within a row and
interconnection of a common ground wire, and voltage output strips
according to an embodiment of the present invention;
[0056] FIG. 8 is a sectional side view of a PVC having a plurality
of layers, each sensitive to a different wavelength of light and
each having a different band gap energy to enhance collection of
incident light, which is particularly suitable for use in a solar
collector according to an embodiment of the present invention;
[0057] FIG. 9 is a graph of external quantum efficiency versus
wavelength for a triple junction PVC;
[0058] FIG. 10 is a simplified schematic diagram showing a scheme
for electrical connection of PVCs adapted to utilize different
wavelengths of light according to an embodiment of the present
invention;
[0059] FIG. 11A is a perspective view of an enclosure for a solar
collector according to an embodiment of the present invention;
[0060] FIG. 11B is a side view of the enclosure of FIG. 1A
according to an embodiment of the present invention;
[0061] FIG. 12 is a side view of an enclosure for a solar collector
showing cooling vents according to an embodiment of the present
invention;
[0062] FIG. 13 is a perspective view of an enclosure for a solar
collector showing a heat exchanger for cooling substrates of the
solar collector according to an embodiment of the present
invention;
[0063] FIG. 14 is a side view of an alternative embodiment of a
solar collector having a concentrator to increase efficiency
showing the orientation of the substrates of the elevated-tiers
relative to a base-layer, and the reflection of light among the
plurality of substrates according to the present invention
according the present invention;
[0064] FIG. 15 is a schematic perspective view of a solar collector
with a plurality of substrates enclosed within an enclosure, the
substrates having a PVCs formed thereon and oriented at angles
relative to each other such that light reflected from a first
substrate is reflected onto the surface of a second substrate
according to an embodiment of the present invention;
[0065] FIG. 16 is a schematic side view of the substrates of the
solar collector of FIG. 15 according to an embodiment of the
present invention;
[0066] FIG. 17 is a simplified block diagram of substrates of a
solar collector oriented at an angle relative to each other
according to an embodiment of the present invention showing
incident light reflecting from a surface of a first substrate on to
a second substrate;
[0067] FIG. 18 is a simplified block diagram of substrates of a
solar collector oriented at another angle relative to each other
according to an embodiment of the present invention showing
incident light reflecting from a surface of a first substrate on to
a second substrate;
[0068] FIG. 19 is a simplified block diagram of substrates of a
solar collector oriented at still another angle relative to each
other according to an embodiment of the present invention showing
incident light reflecting from a surface of a first substrate on to
a second substrate;
[0069] FIG. 20 is a perspective view of a solar collector with
three substrates shaped and oriented relative to one another to
form inner surfaces of an inverted three sided pyramid according to
an embodiment of the present invention;
[0070] FIG. 21A is a perspective view of a solar collector with
four substrates shaped and oriented relative to one another to form
inner surfaces of an inverted four sided pyramid according to an
embodiment of the present invention;
[0071] FIG. 21B is a side view of the solar collector of FIG. 21A
according to an embodiment of the present invention;
[0072] FIG. 21C is another perspective view of the solar collector
of FIG. 21A showing incident light reflecting from substrate to
substrate in the solar collector according to an embodiment of the
present invention;
[0073] FIG. 21D is a top view of the solar collector of FIG. 21C
showing incident light reflecting from substrate to substrate in
the solar collector according to an embodiment of the present
invention;
[0074] FIG. 21E is a plan view of a substrate of the solar
collector of FIGS. 21A-21D showing PVCs thereon according to an
embodiment of the present invention;
[0075] FIG. 22 is a perspective view of a solar collector with four
substrates shaped and oriented relative to one another to form
inner surfaces of an inverted four sided pyramid and further
including a concentration lens according to another embodiment of
the present invention;
[0076] FIG. 23A is a perspective view of a solar collector with a
number of substrates shaped and oriented relative to one another to
form inner surfaces of an inverted pentagon according to an
embodiment of the present invention;
[0077] FIG. 23B is a top view of the solar collector of FIG. 23A
according to an embodiment of the present invention;
[0078] FIG. 23C is a plan view of one side of the solar collector
of FIGS. 23A-23B showing three substrates making up the side and
the PVCs thereon according to an embodiment of the present
invention;
[0079] FIG. 24 is a plan view of a substrate of a solar collector
showing PVCs thereon according to an embodiment of the present
invention;
[0080] FIG. 25 is a graph of external quantum efficiency of the
PVCs on first and second substrates as a function of the angle
relative to a normal, top or bottom surface of the solar collector;
and
[0081] FIG. 26 is a normalized graph of external quantum efficiency
and power of a single PVC on one of the first and second substrates
as a function of the angle relative to a normal, top or bottom
surface of the solar collector.
DETAILED DESCRIPTION
[0082] The present invention is directed to an improved solar
collector having an array of substrates, each with at least one
photovoltaic cell (PVC) formed on a surface thereof for converting
light incident thereon into electrical energy.
[0083] A solar collector 100 according to the present invention
will now be described with reference to FIGS. 3A and 3B. FIG. 3A is
a plan view of a solar collector 100 including a number of wafers
or substrates 102, each with at least one PVC 104 formed on a
surface 106 thereof for converting light incident thereon into
electrical energy. The PVCs 104 can include a number of individual
discrete PVCs formed on a single substrate 102, or a single
monolithic PVC formed on a single substrate. Generally, the
substrates 102 are enclosed within a reflective enclosure or an
enclosure 108 according to an embodiment of the present invention.
For purposes of clarity, many of the details of solar collectors
100 that are widely known and are not relevant to the present
invention have been omitted. Referring to FIG. 3A, the substrates
102 of the solar collector 100 are ordered or arranged in a grid or
an array 110 including a lower-tier or a base-layer 112 of
substrates 102 electrically interconnected or coupled by a common
ground conductor and/or common voltage outputs (not shown in this
figure).
[0084] Preferably, the array 110 is a three-dimensional array
including at least one elevated-tier 114 of substrates 102
positioned above and separated from the base-layer of substrates,
such that at least a portion of the light passing between the
substrates of the elevated-tier is absorbed by the substrates of
the base-layer, thereby increasing the utilization of all light
falling on the solar collector and improving its' overall
efficiency. More preferably, referring to FIG. 3B, the enclosure
108 enclosing the array 110 of substrates 102, includes a top-wall
116 with an anti-reflective coating or surface 118 through which
light is passed to the PVCs 104, and a bottom-wall 120 and
sidewalls 122, 124, with reflective coatings or surfaces 126 to
reflect light incident thereon back to the PVCs.
[0085] Generally, the enclosure 108 further comprises end-walls
128, 130, joining the top-wall 116 and bottom-wall 120. The
end-walls 128, 130, also typically include anti-reflective coatings
or surfaces 118, and join the top-wall 116 at an angle selected to
facilitate passage of light to the PVCs 104 from a light source
(not shown) inclined relative to the surface of the top-wall.
Preferably, each of the end-walls 128, 130, form an angle of from
about 50 to about 75 degrees with the surface of the top-wall 116,
and an angle of from about 105 to about 130 degrees with the
bottom-wall 120. More preferably, the end-walls 128, 130, form an
angle of about 60 degrees with the top-wall 116, and an angle of
about 120 degrees with the bottom-wall 120. Angling of the
end-walls 128, 130, is particularly desirable to enable a solar
collector 100 located, installed or positioned in a substantially
horizontal position to catch the rays of the rising or setting
sun.
[0086] In one embodiment, shown in FIG. 3B, the elevated-tier 114
includes substrates 102 having surfaces 106 on which the PVCs 104
are formed oriented to receive light reflected from substrates of
the base-layer 112. As also shown, the substrates 102 of the
elevated-tier 114 can be suspended above the base-layer 112 by a
support 132, such as a cord, strip, wire or wires, fastened or
affixed to either the end-walls 128, 130, or sidewalls 122, 124, of
the enclosure 108. Alternatively, the support 132 can be affixed to
support pylons or structures (not shown) within the enclosure 108.
Preferably, the support 132 is a ground-conductor 134, for example
a metal strip, wire or wires, to which each of the substrates 102
are electrically coupled. More preferably, the substrates 102 of
the elevated-tier 114 are arranged in regularly spaced columns
extending from end-wall 128 to end-wall 130 of the enclosure 108
and in rows extending from sidewall 122 to sidewall 124, and the
support 132 includes a number of ground-conductors 134 extending
between the end-walls to support each column of substrates.
Generally, the ground-conductors 134 are joined and electrically
coupled to a bus-bar or ground strip (not shown in these figures)
bonded or otherwise affixed to an inner surface of the end-walls
128, 130.
[0087] In another embodiment, shown in FIGS. 4A and 4B, the solar
collector 100 includes multiple elevated-tiers 114 including an
upper or top elevated-tier 114A and a lower or bottom elevated-tier
114B. Each of the multiple elevated-tiers 114A, 114B, having upward
facing substrates 102A on a top half or portion thereof and
downward facing substrates 102B on a bottom half or portion
thereof. The upward facing substrates 102A on the top portions are
oriented to receive light directly through the top-wall 116 of the
enclosure 108 and, in the case of the bottom elevated-tier 114B,
also to receive light reflected from downward facing substrates
102B on the bottom portion of overlying top elevated-tier 114A. The
downward facing substrates 102B on the bottom portions of the
elevated-tiers 114A, 114B, are oriented to receive either light
reflected from upward facing substrates 102A on the top portion of
an underlying elevated-tier, light reflected from the bottom-layer
of substrates, or light reflected from the sidewalls 122, 124, and
bottom-wall 120 of the enclosure 108. Preferably, the
elevated-tiers 114A, 114B, are offset from one another such that at
least some portion of the substrates 102 of each elevated-tier and
of the bottom-layer 112 receive at least some light passed directly
through the top-wall 116 of the enclosure 108.
[0088] FIG. 5 is a top view of a portion or column of an
elevated-tier 114 of substrates 102 in a solar collector showing
use of a pair of ground wires 136 to support and interconnect the
substrates thereof according to an embodiment of the present
invention.
[0089] FIG. 6 is a partial perspective view of the enclosure 108
showing a ground strip 138 attached, bonded or otherwise affixed to
the inner surface of the end-walls 128, 130, and to which the air
of ground wires 136 supporting the substrates 102 of the
elevated-tiers 114 are physically and electrically coupled.
[0090] Generally, the substrates 102 of the base-layer 112 of the
array are also arranged in columns and/or rows. For example, in one
embodiment the substrates are arranged in a number of columns
extending from end-wall 128 to end-wall 130 of the enclosure 108.
FIG. 7 is a partial top view of such a column showing orientation
of alternating pairs of substrates 102 along large flats of the
1,1,1, crystal face. FIG. 7 also shows interconnection of the
substrates 102 by a common ground strip 140 and by voltage output
strips or wires 142 electrically coupling the substrates together
in parallel according to an embodiment of the present
invention.
[0091] In yet another embodiment, the PVCs 104 include a number of
different types of PVCs each sensitive to a different range of
wavelengths of light and each having different band gap energy.
Preferably, the PVCs 104 include at least two different types of
PVCs selected from a group consisting of silicon based PVCs,
gallium-arsenide (GaAs) based PVCs, aluminum-gallium-arsenide
(AlGaAs) based PVCs, and germanium (Ge) based PVCs. More
preferably, where the PVCs 104 include GaAs, AlGaAs or Ge based
PVCs, which can be damaged by exposure to high levels of short
wavelength or ultraviolet radiation, each substrate 102 has only a
single type of PVC formed thereon, and the substrates having GaAs,
AlGaAs or Ge based PVCs, are positioned and oriented within the
array 110 to receive substantially only light reflected from the
sidewalls 122, 124, bottom-wall 120, or other substrates, such
substrates of the base-layer 112 or upward facing substrates 102A
on the top portion of an underlying elevated-tier 114. Because the
reflected light is of lower overall intensity, and because certain
wavelengths of light are completely or substantially absorbed by
the surfaces which they first strike, the damaging radiation
reflected onto the GaAs, AlGaAs or Ge based PVCs is reduced.
Optionally, the GaAs, AlGaAs or Ge based PVCs 104 include a top
passivation layer of oxide or nitride to filter out or remove
damaging radiation further reducing the possibility or incidence of
damage.
[0092] In still another embodiment, shown in FIG. 8, the PVCs 104
include monolithically grown devices having multiple layers or
junctions, each sensitive to a different range of wavelengths of
light and each having different band gap energy. FIG. 8 illustrates
an example of a PVC 104 having a highly efficient triple junction
cell structure. Referring to FIG. 8, a top gallium Indium-Phosphide
(GaInP) layer absorbs short wavelength, high-energy photons, while
successive gallium-arsenide (GaAs) and Ge layers absorb longer
wavelength energy. As shown in the graph of FIG. 9, the total
efficiency of the cell is thus greatly improved. FIG. 9 is a graph
of external quantum efficiency versus wavelength for a triple
junction PVC. It has been found that triple junction PVCs 104 can
absorb photons in the range from 350 nanometers to 1800 nanometers,
with an efficiency of greater than 87% of the total available solar
spectral irradiance. Referring to FIG. 9, the external quantum
efficiency of the GaInP layer is represented by the line identified
by reference numeral 143; the external quantum efficiency of the
GaAs layer is represented by the line identified by reference
numeral 145; and the external quantum efficiency of the Ge layer is
represented by the line identified by reference numeral 147. The
line identified by reference numeral 149 represents the average
Spectral irradiance in mW/cm.sup.2.
[0093] FIG. 10 is a simplified schematic diagram showing a scheme
for electrical connection of PVCs 104 adapted to utilize different
wavelengths of light according to an embodiment of the present
invention. Referring to FIG. 10, in a preferred embodiment each of
the different types of PVCs 104, shown here as types 1, 2 and 3,
are connected to a different DC to DC voltage converter or
converter 144 within the enclosure 108 of the solar-collector 100.
Each of the different types of PVCs 104 are connected in parallel
to a single converter 144, and outputs of the converters are
connected in parallel to provide a common vdc output. The
converters 144 raise or lower the voltage provided from the
different types of PVCs 104 to match a voltage from another type of
PVCs or of the common output voltage. The number of converters 144
in the solar-collector 100 depends on the number of different types
of PVCs 104 contained therein, and generally is equal to or one
less than the number of different types of PVCs. The value of the
common output voltage can be chosen based power transport
efficiency, requirements of external elements, such as batteries
charged by the solar-collector or an inverter circuit for providing
alternating current (AC) to a business or residence, or to match
the output of one of the types of PVCs 104.
[0094] FIG. 11A is a perspective view of the enclosure 108 for the
solar collector 100 according to an embodiment of the present
invention. As shown the enclosure has a length (L) along the
end-walls 128, 130, much greater than a width (WT, WB) associated
with sidewalls 122, 124. The overall height (H) of the enclosure
108 is dependent on the number of elevated-tiers 114, the spacing
therebetween, and size of a cooling mechanism (not shown in this
figure) if any enclosed therein.
[0095] Preferably, the solar collector 100 is oriented so that the
sun travels in an arc across the width of the enclosure 108,
thereby maximizing the exposure of the substrates 102 with the PVCs
104 thereon to light passing through the end-walls 128, 130, when
the sun is at a relatively low inclinations or elevations. For
example, at or near sunrise and sunset. FIG. 11B is a side view of
the enclosure 108 of FIG. 11A illustrating the angles with which
the end-walls 128, 130, join the top-wall 116 and bottom-wall
120.
[0096] FIG. 12 is a side view of an enclosure 108 for the solar
collector 100 showing a sidewall 122 having cooling vents 146
formed therein according to an embodiment of the present invention.
Alternatively, the cooling vents 146 can be formed in the end-walls
128, 130, in the bottom-wall 120 or in a combination of the
sidewalls 122, 134, end-walls, and bottom-wall. The cooling vents
146 enable heated air that would otherwise be trapped inside the
enclosure 108 to escape. Preferably, both sidewalls 122, 124, have
cooling vents 146 formed therein to enable air to circulate and
pass through the enclosure 108. More preferably, the cooling vents
146 are sized, shaped and located to enable the substrates to be
maintained at or below a semiconductor junction temperature of
125.degree. F. This temperature is the maximum steady state
temperature that can be tolerated by PVCs 104 formed on silicon
substrates without resulting in diffusion or migration of dopant
materials out of the active layer, which can destroy or
detrimentally effect the operation of the PVC.
[0097] In one version of this embodiment, the cooling mechanism
includes, in addition to the cooling-vents 146, a number of fans
148, such as a box fan (only one of which is shown in phantom in
FIG. 12), located inside the enclosure 108 to facilitate movement
of air there through. Preferably, the fan 148 is electric and
powered by at least portion of the output of one or more of the
PVCs 104. The fan 148 may be directly wired to one or more PVCs 104
such that the output of the PVCs is solely dedicated to operating
the fan, or the fan may be wired to draw off a portion of the
output of the solar-collector 100. More preferably, the fan 148
draws power from the solar-collector 100 and is controlled by a
thermostat (not shown) so that it is operated only as necessary to
maintain the temperature in the enclosure 108 below a desired
maximum temperature, thereby increasing efficiency of the
solar-collector.
[0098] In another embodiment, the cooling mechanism consists of or
further includes a heat exchanger 150 built into or thermally
coupled to the bottom-wall 120 of the enclosure 108 for cooling
substrates 102 of the solar-collector 100. A perspective view of
this embodiment is shown in FIG. 13. Referring to FIG. 13, the heat
exchanger 150 generally includes one or more passages or tubes 152
through which a heat transfer fluid, such as a liquid or a gas, is
passed to cool the solar-collector 100. In one preferred version of
this embodiment, the heat transfer fluid is potable water that is
circulated through a large tank or reservoir (not shown) located
near the solar-collector 100 to provide heated water to a residence
or business. Alternatively, the heat transfer fluid can be
circulated through a second heat exchanger (not shown) over which
air is forced to provide heat to the residence or business.
[0099] In yet another alternative embodiment, the solar collector
104 can further include a concentrator 154, such as a lens, to
enhance collection of incident light, as shown in FIG. 14. In the
embodiment shown in FIG. 14, the top-wall 116 includes or has been
replaced by a Fresnel lens to focus or concentrate incident light
onto substrates 102 of the base-layer 112. A Fresnel is a lens
having a surface of stepped concentric circles, resulting in a
shape that is thinner and flatter than a conventional parabolic
lens of equivalent focal length.
[0100] In still another alternative embodiment, the surfaces of
PVCs 104 of the elevated-tiers 114 are positioned and oriented at
an angle relative to the surface of PVCs of the base-layer 112 to
produce multiple reflections of light reflected from the base
layer. In the example shown in FIG. 14 light reflected from the
base layer 112 is reflected to surfaces of PVCs 104A in a first
elevated-tier 114A positioned at an acute angle thereto, and from
the first elevated-tier to other PVCs with the same tier or to PVCs
104B in a second elevated tier 114B. This embodiment has the
advantages of enabling substantially all of the light reflecting
from the base-layer 112 to be reflected to the elevated-tiers 114
and be absorbed without providing a reflective surface on the
enclosure.
[0101] In a preferred version of the above embodiment, the types of
PVCs 104 in the base-layer 112 and elevated-tiers 114 include
multiple layer or junction devices, such as the triple junction PVC
104 shown in FIG. 8 above. More preferably, the PVCs 104 in the
elevated-tiers 114 include GaAs, AlGaAs, Ge or Si based PVCs, and
are positioned and oriented to receive substantially only reflected
light. As explained above, GaAs, AlGaAs, Ge or Si based PVCs have
high efficiencies at longer wavelengths, but can be damaged by
exposure to high levels of short wavelength or ultraviolet
radiation. Because the reflected light is of lower overall
intensity, and because certain wavelengths of light are completely
or substantially absorbed by the PVCs 104 which they first strike,
the damaging radiation reflected onto the GaAs, AlGaAs, Ge or Si
based PVCs is significantly reduced.
[0102] In yet another aspect of the present invention embodiment, a
solar collector is provided having an array of substrates with
photovoltaic cells, the substrates oriented at angles relative to
each other such that light reflected from a first substrate is
reflected onto the surface of a second substrate, thereby improving
efficiency of the solar collector.
[0103] A solar collector 200 according to this aspect of the
present invention will now be described with reference to FIGS. 15
to 26. For purposes of clarity, many of the details of solar
collectors 200 that are widely known and are not relevant to the
present invention have been omitted. FIG. 15 is a is a schematic
perspective view of a solar collector 200 including a number of
wafers or substrates 202A, 202B, enclosed within an enclosure 206
and each of the substrates including at least one PVC 208 formed on
a top surface 210 thereof for converting light incident thereon
into electrical energy. External electrical connection to the PVCs
208 is through a ground-conductor 212 electrically coupled, epoxied
(with an electrically conductive epoxy) or soldered to the
substrates 202A, 202B, and positive-conductors 214, 216,
electrically coupled or soldered to each of the PVCs. The PVCs 208
on the different substrates 202A, 202B, can be electrically
connected in series or in parallel depending on the voltage or
current produced by the PVCs and voltage or current required or
desired out of the solar collector 200.
[0104] In accordance with the present invention, the substrates
202A, 202B, are arranged in a three-dimensional array 218 having at
least first and second substrates 202A, 202B, oriented at an angle
relative to each other, such that at least a portion of the light
reflected from the first substrate 202 is reflected onto the second
substrate 202B and absorbed, thereby increasing the utilization of
all light falling on the solar collector and improving its' overall
efficiency. Preferably, both the first and the second substrates
202A, 202B, receive light thereon, and are oriented with respect to
each other and to a direction of propagation of light incident on
the solar collector 200 such that light reflected from either
substrate is reflected onto the other substrate. More preferably,
referring to FIG. 16, the angle .alpha. between the surfaces of the
first and second substrates is between about 140.degree. and a
predetermined minimum angle greater than or equal to about
20.degree.. Alternatively expressed, the first and the second
substrates 202A, 202B, are oriented relative to a normal or to a
top surface of the solar collector 200 to form angles .theta. of
between about 20.degree. and about 70.degree..
[0105] To maximize the incident light reflected between the
substrates 202A, 202B, the substrates are positioned such that the
edges nearest or forming the apex of the angle .alpha., are
proximal to each other and to the apex of the angle. Preferably,
the edges of the substrates 202A, 202B, are abutting or
adjoining.
[0106] The substrates 202A, 202B, of the array 218 can be held in
position above the base of the solar collector by a support (not
shown), such as a cord, strip, wire or wires, or stamped metal
plate(s) fastened or affixed to the sidewalls of the enclosure 206.
Alternatively, the substrates 202A, 202B, can be held in position
by support pylons or structures (not shown) below the substrates
within the enclosure 206.
[0107] The PVCs 208 can include a number of individual discrete
PVCs formed on a single substrate, as shown on substrate 202A, or a
single monolithic PVC formed on a single substrate, as shown on
substrate 202B. The PVCs 208 can include a number of different
types of PVCs each sensitive to a different range of wavelengths of
light and each having different band gap energy. For example, the
PVCs 208 include PVCs selected from a group consisting of silicon
based PVCs, gallium-arsenide (GaAs) based PVCs,
aluminum-gallium-arsenide (AlGaAs) based PVCs, and germanium (Ge)
based PVCs. Preferably, the PVCs 208 include monolithically grown
devices having multiple layers or junctions, each sensitive to a
different range of wavelengths of light and each having different
band gap energy. One example of such a multiple-junction PVC is a
triple junction PVC (not shown) that has a top gallium
Indium-Phosphide (GaInP) layer to absorb short wavelength,
high-energy photons, while successive gallium-arsenide (GaAs) and
Ge layers absorb longer wavelength, low-energy photons. Such PVCs
are commercially available, for example, from Spectrolab Inc. of
Sylmar, Calif.
[0108] Generally, the substrates 202A, 202B, are mounted on cooling
plates 220, if required. A heat exchanger 222 is built into or
thermally coupled to the cooling plates 220 of the enclosure 206
for cooling substrates 202A, 202B, of the solar-collector 200.
Preferably, the temperature and flow rate of a cooling fluid passed
through the heat exchanger 222 is selected to enable the substrates
to be maintained at or below a semiconductor junction temperature
of 125.degree. F. This temperature is the maximum steady state
temperature that can be tolerated by PVCs 208 formed on silicon
substrates without resulting in diffusion or migration of dopant
materials out of the active layer, which can destroy or
detrimentally effect the operation of the PVC.
[0109] The cooling plates 220 and the heat exchanger 222 can be
made on any suitable metallic, ceramic or polymeric material having
the necessary structural stability, and heat transfer
characteristics. Optionally, the cooling plates 220 are made of an
electrically conductive material and are electrically as well as
thermally coupled to the substrates 202A, 202B, and the
ground-conductor 212 is electrically coupled to the substrates
through the cooling plates.
[0110] In one embodiment, the enclosure 206 enclosing the array 218
of substrates 202A, 202B, is a reflective enclosure including a
top-wall 224 with an anti-reflective coating or surface (not shown)
on a top surface thereof through which light is passed to the PVCs
208, and a bottom-wall 228 and sidewalls 230, 232, 234, 236, with
reflective coatings or surfaces (not shown) to reflect light
incident thereon back to the PVCs. The top-wall 224 can also
include a reflective coating or surface on a lower surface thereof
to redirect light reflected from the substrates 202A, 202B, or the
bottom-wall 228 back on to the PVCs 208. Although not shown, the
sidewalls 230, 232, 234, 236, can join the top-wall 224 at an angle
selected to reflect light back to the substrates 202A, 202B.
Angling of the sidewalls 230, 232, 234, 236, is particularly
desirable to enable a solar collector 200 located, installed or
positioned in a substantially horizontal position, such as to catch
the rays of the rising or setting sun.
[0111] A number of embodiments of a solar collector 200 according
to the present invention having substrates of oriented at various
angles .alpha. with respect to one another will now be described
with reference to FIGS. 17, 18 and 19. FIGS. 17, 18 and 19 are
tracing models depicting reflections from one surface to the other.
For clarity only light incident on one surface and reflected to the
other is shown. It will be appreciated that since both substrates
202A, 202B, are oriented at equal angles with respect to the
direction of the incident light rays, the light incident on the
other surface and reflected therefrom will be the mirror image of
that shown.
[0112] Referring to FIG. 17 there is shown a simplified block
diagram of substrates 202A, 202B, of the solar collector 200
oriented at an angle of 120.degree. relative to each other or
30.degree. relative to the bottom-wall 228 of the solar collector.
In this embodiment, it appears there is little reflection of light
from substrate 202A to substrate 202B. However, it should be noted
that not all light rays incident on the first substrate 202A are
parallel or normal to the top of the solar collector 200. Rather,
some light rays will strike the first substrate 202A at a more
oblique angle and be reflected onto the second substrate 202B. The
light reflected onto the second substrate 202B, is absorbed by the
PVCs 208 and converted into electrical energy, thereby increasing
the external quantum efficiency of the solar cell 200.
[0113] FIG. 18 illustrates the substrates 202A, 202B, oriented an
angle of 90.degree. relative to each other or 45.degree. relative
to the bottom-wall 228 of the solar collector and the incident
light rays. When compared with the ray tracing of FIG. 17, this
model illustrates the increase in the intensity of light reflected
between the substrates 202A, 202B, for decreasing angles between
the substrates.
[0114] FIG. 19 illustrates the substrates 202A, 202B, oriented an
angle of 40.degree. relative to each other or 70.degree. relative
to the bottom-wall 228 of the solar collector and to the normal or
top of the solar collector 200. When compared with the ray tracing
of FIGS. 17 and 18, this model further illustrates the increase in
the intensity of light reflected between the substrates 202A, 202B,
for decreasing angles between the substrates. This model also
illustrates the occurrence of tertiary reflections in which light
reflected to the second substrate 202B and not absorbed thereby is
reflected back to the first substrate 202A where it can be
absorbed, thereby further increasing the external quantum
efficiency of the solar cell 200. Although not illustrated in the
preceding figure, it will be appreciated that tertiary or higher
order reflections can occur between any two substrates 202A, 202B,
and more particularly by those separated by angles of 45.degree. or
less.
[0115] In yet another embodiment, illustrated in FIGS. 20 to 23C,
the solar collector 200 can include a plurality of three or more
substrates are shaped and oriented relative to one another such
that the surfaces of the substrates form at least part of a concave
inner surface of a polyhedron.
[0116] For example, in FIG. 20 there is shown an array 218 of three
substrates 202A, 202B, 202C, shaped and oriented relative to one
another to form at least part of first, second and third inner
surfaces of an inverted three sided pyramid 242. In the embodiment
shown, because the three substrates 202A, 202B, 202C making up the
pyramid 242 are equilateral triangles, the angle .alpha. separating
any two adjoining substrates is 60.degree.. However, it will be
appreciated that each substrate 202A, 202B, 202C can further be
sized and shaped to form angles .theta. of between about 20.degree.
and about 70.degree. relative to the bottom-wall 228 or top-wall
226 of the enclosure 206 or to the normal of the solar collector
200. Moreover, although shown as a pyramid 242 with equilateral
sides this need not be the case in all embodiments. For example, in
applications where the solar collector 200 must be mounted such
that light will predominantly enter at an oblique angle to the
normal surface of the solar collector, it may be desirable that the
substrate 202A making up the side of the pyramid 242 receiving the
most direct incident light be larger than the other two substrates
202B, 202C.
[0117] In another embodiment, illustrated in FIGS. 21A through 21E,
the solar collector 200 can include four substrates 202A, 202B,
202C, 202D shaped and oriented relative to one another to form at
least part of first, second, third and fourth inner surfaces of an
inverted four sided pyramid 246. In the embodiment shown, because
the four substrates 202A, 202B, 202C, 202D, making up the pyramid
244 are equilateral triangles, the angle .alpha. separating any two
adjoining substrates is 90.degree.. However, as noted above each
substrate 202A, 202B, 202C, 202D, can further be sized and shaped
to form angles .theta. of between about 20.degree. and about
70.degree. relative to the bottom-wall 228 and top-wall 226 of the
enclosure 206 or to the normal of the solar collector 200.
[0118] FIG. 21C is another perspective view of the solar collector
200 of FIG. 21A showing an incident light ray 201 reflecting from
substrate to substrate in the solar collector 200. In this
embodiment, it appears the substrates 202A, 202B, 202C, 202D,
oriented an angle of 90.degree. relative to each other or
45.degree. or larger relative to the bottom-wall 228 of the solar
collector 200. This model also illustrates the occurrence of
secondary, tertiary and higher order reflections which further
increase the external quantum efficiency of the solar cell 200.
[0119] FIG. 21D is a top view of the solar collector 200 of FIG.
21C showing the incident light ray 201 reflecting from substrate
202 to other substrates in the solar collector.
[0120] FIG. 21E is a plan view of a substrate 202 of the solar
collector of FIGS. 21A-21D showing one or more PVCs 208 thereon,
and a schematic representation of the electrical connections
thereto. Although shown as a single monolithic PVC 208, it will be
appreciated that the substrate 202 can include any number of PVCs
sized and shaped as desired to conform to the substrate. For
example, the substrate 202 can include two identical PVCs 208
having a right triangular shape and adjoining along one leg
thereof. Also, it will be appreciated that where the substrate 202
includes multiple PVCs 208, the PVCs can be electrically connected
in parallel or in series with one another, as well as with PVCs on
other substrates, depending either on the voltage and/or current
produced or required.
[0121] FIG. 22 is a perspective view of another embodiment of the
solar collector 200 with four substrates 202A, 202B, 202C, 202D,
shaped and oriented relative to one another to form inner surfaces
of an inverted four sided pyramid 246 and further including a
concentration lens or concentrator 248 through which light is
passed to at least the first substrate 202A. Preferably, the
concentrator 248 is a lens, such as a one or two-sided convex or
concave lens or a Fresnel lens, adapted to enhance or concentrate
collection of incident light on one or substrate or substrates.
More preferably, the concentrator 248 is a non-imaging concentrator
that diffusely or non-diffusely focuses and/or concentrates light
on the substrates 202.
[0122] FIG. 23A is a perspective view of yet another embodiment of
the solar collector 200 with a number of substrates 202A, 202B,
202C, 202D, 202E, 202F shaped and oriented relative to one another
to form inner surfaces of a polyhedron having a pentagon or
pentagonal cross-section (hereinafter pentagon 250). Preferably,
the substrates 202A, 202B, 202C, 202D, 202E, are angled relative to
one another and to the bottom-wall 228 of the solar collector 200
to form an inverted pentagon 250 having a cross-sectional area that
decreases from top to bottom. FIG. 23B is a top view of the solar
collector 200 of FIG. 23A. Each substrate 202A, 202B, 202C, 202D,
202E, making up a side of the pentagon 250 is sized and shaped with
respect to each other and to a direction of propagation of light
incident on the solar collector 200 such that light reflected from
one substrate is reflected onto a substrate making up another side
of the pentagon. Preferably, the angle .alpha. between the surfaces
of the substrates 202A, 202B, 202C, 202D, 202E, is between about
60.degree. and about 80.degree.. More preferably, each substrate
202A, 202B, 202C, 202D, 202E, making up a side of the pentagon 250
is further sized and shaped to form angles .theta. of between about
20.degree. and about 70.degree. relative to the bottom-wall 228 or
top-wall 226 of the enclosure 206 or to the normal of the solar
collector 200. Moreover, although shown as a pentagon 250 with
equilateral sides this need not be the case in all embodiments.
[0123] FIG. 23C is a plan view of one side of the solar collector
200 of FIGS. 23A-23B showing three PVCs 208 or making up one
substrate or side of the inverted pentagon 250. Although shown as a
single substrate 202A having three PVCs 208 formed thereon, it will
be appreciated that each side of the pentagon 250 can also be made
up of a number of discrete substrates, for example three
substrates, each having one or more PVCs 208 thereon.
[0124] In one version of this embodiment, the solar collector 200
can have an inverted pentagon 250 further including a concentrator
248 as described above in connection with FIG. 22.
[0125] Finally, although described using the examples of three and
four sided inverted pyramids, and an inverted pentagon, it will be
appreciated that that the array 218 can include any number of
substrates 202 sized, shaped and oriented to form any geometric
figure or shape formed by planar surfaces that also form angles
.theta. of between about 20.degree. and about 70.degree. relative
to the bottom-wall 228 and top-wall 224 of the enclosure 206 or to
the normal of the solar collector 200.
EXAMPLES
[0126] The following examples made with reference to FIGS. 24
through 26 illustrate advantages of the solar collector 200
according to the present invention for a more efficient conversion
of the available photons or light into electrical energy. The
examples are provided to illustrate certain embodiments of the
present invention, and are not intended to limit the scope of the
invention in any way.
[0127] In this examples, the solar collector 200 included first and
second substrates 202A, 202B, sized, shaped and positioned
substantially as shown in FIGS. 15 and 16, and angled with respect
to each other at various angles including those shown in FIGS. 17
to 19. Each substrate 202A, 202B, included two triangular PVCs
208A, 208B, positioned as shown in FIG. 24. The substrates 202A,
202B, were electrically connected such that the power and
efficiency of each could be measured both independently and in
combination with the other.
[0128] FIG. 25 illustrates the results from a main experiment or
example with the efficiency as a function of an angle .theta.
relative to the normal or to a top surface of the solar collector
200 of bottom wall 228 for two different PVCs 208A on the first and
second substrates 202A, 202B. As shown by line labeled reference
number 252, the efficiency shows a gradual increase of from about
26.2% to about 35% for increasing angles .theta. of from 0.degree.
to about 70.degree. relative to the normal or to the top or bottom
surface 224, 228, of the solar collector 200. The average increase
in the efficiency is more than 9.4%.
[0129] FIG. 26 illustrates the increase in both normalized power
and efficiency as a function of an angle .theta. relative to the
normal or to the top or bottom surface 224, 228, of the solar
collector 200. In this experiment, light to emphasize the increase
power and efficiency due to reflected light, incident light was
shown on substrate 202A only, and blocked from substrate 202B. This
figure to shows that the power and efficiency increases rapidly for
both substrates 202, 202B, in the angular range shown between the
dashed lines as a result of absorbing most of the reflected light
after one reflection. The increase in power and efficiency for
substrate 202A, indicated by line labeled reference number 256, is
accounted for by secondary, tertiary or higher order reflections
back from the second substrate 202B back to the first substrate.
The increase in power and efficiency for substrate 202B is
indicated by line labeled reference number 254. Referring to FIG.
26, it is seen that a first onset of rapid increase in power and
efficiency is evident above angles of about 33.degree. as a result
of reflections. Above 60.degree., the second onset of rapid
increase in power and efficiency is evident as a result of
reflections. Both the efficiency and power increase with increasing
angle.
[0130] It believed that the efficiency increases as a function of
an increase in angle while the power decreases. Although, this
seems contradictory as the efficiency is dependent on the power
from by the expression:
Eff=P.sub.out/P.sub.in (1)
[0131] where P.sub.out is the power output of the solar collector
200 and P.sub.in is the power incident on the solar collector,
which in the above examples is about 1000 W/m2.
[0132] Another way to calculate the efficiency of the solar
collector 200 is by the expression:
Eff=I.sub.SCV.sub.SCFF/(Sun's Irradiance)(Area) (2)
[0133] where I.sub.SC is the current out of the solar collector
200; V.sub.SC is the voltage out of the solar collector, FF is the
fill factor, and Sun's Irradiance is the Sun's Irradiance or power
in W/m.sup.2 and area is the area of the solar collector. As is
known in the art, the fill factor on an I-V (current-voltage) curve
characterizing the output of a solar cell or module, the ratio of
the maximum power to the product of the open-circuit voltage and
the short-circuit current. The higher the fill factor (FF) the
"squarer" the shape of the I-V curve.
[0134] Hence, from Eq. (1), it would appear that as the power
decreases, so should the efficiency. However, referring to Eq. (2)
it is seen that because the product of I.sub.SCV.sub.SCFF decreases
at a slower rate than the cosine of the angle of the substrates of
the solar collector, and because the irradiance of the simulated
solar light is constant, the efficiency actually increases when as
the value in the numerator at a particular angle is divided by the
smaller number from the cosine of the area of that angle.
[0135] The foregoing description of specific embodiments and
examples of the invention have been presented for the purpose of
illustration and description, and although the invention has been
illustrated by certain of the preceding examples, it is not to be
construed as being limited thereby. They are not intended to be
exhaustive or to limit the invention to the precise forms
disclosed, and obviously many modifications, embodiments, and
variations are possible in light of the above teaching. It is
intended that the scope of the invention encompass the generic area
as herein disclosed, and by the claims appended hereto and their
equivalents.
* * * * *