U.S. patent number RE29,833 [Application Number 05/823,119] was granted by the patent office on 1978-11-14 for tubular solar cell devices.
This patent grant is currently assigned to Mobil Tyco Solar Energy Corporation. Invention is credited to Abraham I. Mlavsky.
United States Patent |
RE29,833 |
Mlavsky |
November 14, 1978 |
Tubular solar cell devices
Abstract
Tubular solar cells are provided which can be coupled together
in series and parallel arrays to form an integrated structure.
Solar energy concentrators are combined with the solar cells to
maximize their power output. The solar cells may be cooled by
circulating a heat exchange fluid through the interior of the solar
cells and the heat captured by such fluid may be utilized, for
example, to provide hot water for a heating system. The coolant
circulating system of the solar cells also may be integrated with a
solar thermal device so as to form a two-stage heating system,
whereby the coolant is preheated as it cools the solar cells and
then is heated further by the solar thermal device.
Inventors: |
Mlavsky; Abraham I. (Lincoln,
MA) |
Assignee: |
Mobil Tyco Solar Energy
Corporation (Waltham, MA)
|
Family
ID: |
24070392 |
Appl.
No.: |
05/823,119 |
Filed: |
August 9, 1977 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
519920 |
Nov 1, 1974 |
03976508 |
Aug 24, 1976 |
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Current U.S.
Class: |
136/246; 126/651;
136/255; 136/257; 257/714; 257/773; 60/641.13; 136/248; 136/259;
136/256 |
Current CPC
Class: |
F24S
23/70 (20180501); H01L 31/035281 (20130101); F24S
30/40 (20180501); H01L 2924/0002 (20130101); Y02E
10/47 (20130101); Y02E 10/52 (20130101); Y02E
10/544 (20130101); H01L 2924/0002 (20130101); H01L
2924/00 (20130101) |
Current International
Class: |
F24J
2/10 (20060101); F24J 2/06 (20060101); H01L
31/00 (20060101); H01L 031/06 (); F24J
003/02 () |
Field of
Search: |
;136/89PC,89SJ,89CC,89HY
;250/211R,211J,212 ;357/20,30,15 ;126/270,271 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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657,485 |
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Sep 1951 |
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GB |
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1,154,043 |
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Jun 1969 |
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GB |
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Primary Examiner: Mack; John H.
Assistant Examiner: Weisstuch; Aaron
Attorney, Agent or Firm: Schiller & Pandiscio
Claims
I claim:
1. A solar cell unit comprising a tubular semiconductor body having
an outer radiation-receiving region of a first conductivity type
and an inner region of a second opposite conductivity type
separated by a P-N or N-P junction, and .[.electrically conductive
contacts.]. .Iadd.first and second electrodes respectively carried
on opposite sides of said junction .Iaddend.for coupling said outer
and inner regions to an external circuit .Iadd., said first
electrode comprising a plurality of contacts electrically-connected
to one another and to said outer radiation-receiving region.
.Iaddend.
2. A solar cell unit according to claim 1 wherein said outer and
inner regions are substantially concentric with one another.
3. A solar cell unit according to claim 1 wherein said body is
generally cylindrical.
4. A solar cell unit according to claim 1 wherein said body has
outer and inner surfaces and said contacts are attached to said
.[.surfaces..]. .Iadd.outer surface. .Iaddend.
5. A solar cell unit according to claim 1 wherein said outer and
inner regions have P-type and N-type conductivities
respectively.
6. A solar cell unit according to claim 1 wherein said outer and
inner regions have N-type and P-type conductivities
respectively.
7. A solar cell unit according to claim 1 wherein said body is made
of silicon.
8. A solar cell unit according to claim 1 further including a
radiation filter surrounding and spaced from said outer
radiation-receiving region.
9. A solar cell unit according to claim 8 wherein said radiation
filter is a self-supporting tubular element, and further including
means holding said tubular element fixed with respect to said
tubular semiconductor body.
10. A solar cell unit according to claim 1 wherein .[.one of.].
said contacts .[.comprises.]. .Iadd.comprise .Iaddend.a grid of
.Iadd.conductively connected .Iaddend.electrical conductors bonded
to the outer surface of said body and said outer
radiation-receiving region comprises portions of said outer surface
between said conductors.
11. A solar cell unit according to claim 10 wherein said grid
includes a first conductor at one end of said body and a second
conductor at the other end of said body, and at least one other
conductor extending between said first and second conductors.
12. A solar cell unit according to claim 1 further including
radiation-reflecting means positioned adjacent to said body for
directing received radiation onto said radiation-receiving
region.
13. A solar cell unit comprising a .Iadd.self-supporting
.Iaddend.tubular electrically semiconductive body having a
radiation-receiving outer surface and an inner surface, said body
also having annular regions of N-type conductivity and P-type
conductivity that are separated by a P-N junction, one of said
regions being contiguous with said outer surface and the other of
said regions being contiguous with said inner surface, said P-N
junction extending generally parallel with and lying close to said
outer surface, and contact means .Iadd.forming a single electrode
.Iaddend.on .Iadd.each of .Iaddend.said outer and inner surfaces
.Iadd.respectively .Iaddend.for coupling said cell to an external
circuit.Iadd., said electrode on said outer surface comprising a
plurality of contacts electrically connected to one another and to
said outer surface. .Iaddend.
14. A solar cell unit according to claim 13 .[.wherin.].
.Iadd.wherein .Iaddend.said .[.contact means.]. .Iadd.electrode
.Iaddend.on said outer surface comprises a plurality of strips
.Iadd.electrically-connected to one another .Iaddend.each .Iadd.of
said strips being .Iaddend.formed of a layer of electrically
conductive material bonded to said outer surface.
15. A solar cell unit according to claim 13 wherein said body is
made of silicon.
16. A solar cell array comprising a plurality of solar cell units
.Iadd.electrically-connected to one another.Iaddend., each unit
comprising a tubular electrically semiconductive body having an
outer radiation-receiving region of one conductivity type and an
inner region of another conductivity type separated by a P-N or N-P
junction, and .[.electrically conductive contacts.]. .Iadd.first
and second electrodes electrically .Iaddend.connected to said outer
and inner regions .Iadd.respectively .Iaddend.of .Iadd.each of
.Iaddend.said bodies for coupling said solar cell units to an
exterior circuit.Iadd., said first electrode comprising a plurality
of contacts electrically-connected to one another and to said outer
radiation-receiving region. .Iaddend.
17. A solar cell array according to claim 16 wherein at least some
of the .[.contacts.]. .Iadd.electrodes .Iaddend.of said bodies are
interconnected so that at least some of said solar cell units are
connected electrically in parallel.
18. A solar cell array according to claim 16 wherein at least some
of the .[.contacts.]. .Iadd.electrodes .Iaddend.of said bodies are
interconnected so that at least some of said solar cell units are
connected electrically in series.
19. A solar cell array according to claim 16 further including
means for circulating a fluid coolant through said hollow
bodies.
20. A solar cell array comprising a plurality of solar cell units,
each of said units comprising a hollow electrically semiconductive
body having an outer radiation-receiving region of one conductivity
type and an inner region of another conductivity type separated by
a P-N junction, at least some of said units being disposed so that
their hollow bodies are disposed end-to-end, means mechanically
interconnecting said end-to-end bodies, and electrically conductive
contacts connected to said outer and inner regions of said bodies
for coupling said solar cell units to an external circuit.
21. A solar cell array according to claim 20 further including a
coupling member disposed between and connecting the end of one body
with the adjacent end of another body.
22. A solar cell unit according to claim 21 wherein said coupling
member provides an electrical connection between a contact on said
one body and a contact on said another body.
23. A solar cell array according to claim 20 wherein said
interconnecting means comprises an elongate support member disposed
within and extending lengthwise of the bodies of said at least some
units, and means extending radially of said support member for
preventing movement of the bodies of said at least some units
radially of said support member.
24. A solar cell array according to claim 23 wherein said radially
extending means has openings to permit a fluid to flow lengthwise
within and between the bodies of said at least some units.
25. A solar cell array according to claim 24 wherein said radially
extending means are mounted on said support member.
26. A tubular photovoltaic semiconductor barrier device comprising
a tubular semiconductor body, a surface on the outer side of said
body forming a radiation-receiving region, means forming a
photovoltaic junction between said surface and said body, and
.[.ohmic contacts.]. .Iadd.first and second electrodes
electrically-connected .Iaddend.on .Iadd.respective
.Iaddend.opposite sides of said junction for coupling said device
to an external circuit .Iadd.said first electrode comprising a
plurality of ohmic contacts electrically-connected together and to
said radiation-receiving region. .Iaddend.
27. A device according to claim 26 wherein said semiconductor body
is of a first conductivity type, said means comprises a
semiconductor of a second conductivity type, and said photovoltaic
junction is a P-N or N-P junction.
28. A device according to claim 26 wherein said means comprises a
conductive metal or metal oxide layer and said photovoltaic
junction is a surface barrier junction.
29. A device according to claim 26 wherein said body is formed of a
first semiconductor, said means comprises a second semiconductor,
and said photovoltaic junction is a hetero-junction.
30. A device according to claim 26 wherein said semiconductor body
has an inner surface, and .[.further including a first grid-like
electrically-conductive contact.]. .Iadd.said ohmic contacts form a
grid .Iaddend.attached to the surface on the outer side of said
body and .[.a second.]. .Iadd.said second electrode comprises an
.Iaddend.electrically-conductive contact attached to the inner
surface of said body.
31. A photovoltaic device comprising a tubular semiconductive body
having radially spaced outer and inner surfaces, an outer
radiation-receiving region of a first conductivity type extending
inwardly of said body from said outer surface and an inner region
of a second conductivity type material extending radially outward
away from said inner surface, with said outer and inner regions
forming a rectifying junction which is generally parallel and close
to said outer surface and is capable of generating a current in
response to radiant energy passing .[.thorugh.]. .Iadd.through
.Iaddend.said outer surface into said outer region, and first and
second electrodes carried by said body and connected to said outer
and inner regions respectively for coupling said device to an
external circuit .Iadd.wherein said first electrode comprises a
plurality of contacts electrically-connected to one another and to
said outer region. .Iaddend.
32. A device according to claim 31 further including means for
circulating a fluid coolant lengthwise through the interior space
defined by said inner surface.
33. A photovoltaic device comprising a tubular body which is made
of silicon and has an outer radiation-receiving region of a first
conductivity type silicon and an inner region of a second
conductivity type silicon, with said outer and inner regions
forming a semiconductor rectifying junction which is capable of
generating a current in response to radiant energy impinging upon
said outer region, and first and second electrodes connected to
said .[.first.]. .Iadd.outer .Iaddend.and .[.second.]. .Iadd.inner
.Iaddend.regions .Iadd.respectively .Iaddend.for coupling said
device into an external circuit, .Iadd.wherein said first electrode
comprises a plurality of contacts electrically-connected to one
another and to said outer region. .Iaddend.
34. A solar cell array comprising a plurality of tubular solar cell
units, each unit comprising a tubular semiconductor body having a
radiation-receiving outer surface and a rectifying junction closely
adjacent to and generally parallel to said outer surface, at least
two of said solar cell units being disposed so that the said
tubular bodies thereof are aligned end-to-end, means mechanically
interconnecting said at least two solar cell units so that the said
tubular bodies thereof are retained in end-to-end alignment,
electrically conductive .[.contacts.]. .Iadd.electrodes
.Iaddend.connected to said bodies at .Iadd.respective
.Iaddend.opposite sides of the said rectifying junctions
.[.thereof.]., .Iadd.one of said electrodes including a plurality
of contacts electrically-connected to one another and to said
radiation receiving outer surface .Iaddend.and means attached to
the .[.contacts.]. .Iadd.electrodes .Iaddend.of said at least two
solar cell units for electrically interconnecting said at least two
solar cell units.
35. A solar cell array according to claim 34 further including
means for circulating a heat transfer fluid through the tubular
bodies of said at least two solar cell units. .Iadd. 36. A device
according to claim 31 wherein at least said first electrode
comprises a plurality of conductors conductively connected to each
other and overlying said outer surface. .Iaddend..Iadd. 37. A
device according to claim 36 wherein said second electrode
comprises a continuous conductive film overlying said inner
surface. .Iaddend.
Description
This invention relates to apparatus for converting solar energy
into electrical energy and more particularly to improved solar
cells and solar cell arrays.
PRIOR ART
It is well known that radiation of an appropriate wavelength
falling on a p-n junction of a semiconductor body serves as a
source of external energy to generate hole-electron pairs in that
body. Because of the potential difference which exists at a p-n
junction, holes and electrons move across the junction in opposite
directions and thereby give rise to flow of an electric current
that is capable of delivering power to an external circuit.
Accordingly, it is presently common practice to provide an array of
solar cells to generate electrical energy from solar radiation.
Most solar cells are made of silicon but cells made of other
materials, e.g., cadmium sulfide and gallium arsenide, have also
been developed and tested. Silicon is a favored material since it
has a band gap of approximately 1.1 electron volts and thus
responds quite favorably to electromagnetic energy having a
wavelength in the visible and ultraviolet regions of the
spectrum.
At the state of the art prior to this invention, solar cells are
most commonly fabricated as separate physical entities with light
gathering surface areas in the order of 4-6 cm.sup.2. For this
reason it is standard practice for power generating applications to
mount the cells in a flat array on a supporting substrate or panel
so that their light gathering surfaces provide an approximation of
a single large light gathering surface. Also since each cell itself
generates only a small amount of power (a silicon solar cell has an
open circuit voltage of about 0.52 volt), the required voltage
and/or current is realized by interconnecting the cells of the
array in a series and/or parallel matrix.
Another method is to fabricate integrated solar cell panels wherein
one region of semiconductivity of each cell is formed by a portion
of a continuous body of semiconductor material. Such integrated
panels may be used singly but more commonly are connected in series
to obtain the desired current.
A number of problems have been encountered in the manufacture of
solar cell panels using individual solar cells. Among the more
significant problems and limitations are relatively low packing
density due to consumption of space by cell interconnections, poor
current collecting efficiency, heating up of the cells due to
absorption of radiation of wavelengths greater than about 1.1
micron, energy loss due to reflection of incident solar radiation
from the light gathering surfaces of the solar cells, eclipsing of
portions of the cells by the cell interconnecting means, physical
damage to cells and cell interconnections due to thermal cycling or
physical stress, and high cost of manufacture. Some of the
approaches proposed to overcome such problems are set forth, for
example in U.S. Pat. Nos. 3,359,137, 3,575,721, 3,116,171,
3,150,999, 3,778,312, 3,502,507, 3,489,615, 3,378,407, 3,819,417,
3,546,542, 3,811,954, 3,457,427, 3,459.597, 3,411,050, 3,175,929,
3,361,594, 3,615,853, 3,682,708, 3,089,070, and 3,574,925, and the
references cited therein. Certain of the foregoing problems are
quite more important than others. For example, it is known that the
power of a solar cell increases with increasing intensity of the
impinging radiation as long as the temperature remains constant,
but decreases again with increasing temperature. Also common
solders used for interconnections are commonly of the soft variety
which cannot withstand elevated temperatures and also tend to
absorb infrared radiation which, as is well known, has a heating
effect. Hence, it is also essential or at least desirable to
provide some means for cooling the solar cells and also to protect
the panel from infrared radiation. It also is important to maximize
the amount of ultraviolet radiation absorbed by the solar cells so
as to maximize their electrical power output. It also is important
from the standpoint of providing a reliable power source to have an
array which can withstand thermal expansion and contraction and
mechanical stress of components. For space application, it also is
essential to provide solar panels with a high power-to-weight
ratio. The latter is also important if solar cells are to compete
with other means of generating electricity for terrestrial use.
Although silicon is an abundant material, the cost of silicon in
the purity required for cell manufacture is quite high and its
production consumes large amounts of electricity. Hence, it is
desirable from the standpoint of cost and to reduce resistive
losses (which have the effect of decreasing conversion efficiency)
to produce solar panels wherein the bulk of the semiconductor
material in each cell is minimized.
Unfortunately, certain of the foregoing problems and limitations
can be overcome only with difficulty or at relatively great
expense.
SUMMARY OF THE INVENTION
Accordingly, a primary object of this invention is to provide a
solar cell of new and unique configuration which substantially
avoids or overcomes a number of the problems encountered in the
manufacture and use of solar cells made according to prior art
techniques. A further object is to provide solar cell arrays which
comprise a plurality of such cells electrically interconnected in a
series and/or parallel matrix. More specific objects of the
invention are to provide solar cells and arrays thereof which have
a modular form, can be easily cooled, have structural integrity,
can be made by existing techniques, and are capable of withstanding
changes in dimensions due to thermal cycling. A further object is
to provide a solar cell unit wherein current leakage is minimized
by the use of a geometry which minimizes the ratio of exposed
active surface area to exposed junction region area. Still another
object is to provide a solar cell module which can be integrated
with a solar thermal system. Yet another object is to provide solar
cell modules which can be easily and efficiently interconnected
physically and electrically. Another important object is to provide
solar cells and solar cell arrays of the type described in
combination with radiant energy concentrators for maximizing the
intensity of radiation received by such cells and also for
distributing the concentration of such radiation.
This invention provides a solar cell which comprises a tubular
structure with a P-N junction formed close to its outer light
gathering surface. To collect current from the solar cell, a first
electrode comprising a grid of conductors is provided on and forms
an ohmic contact with the outer surface of the tubular structure,
and a second electrode in the form of a layer of conductive
material is provided on and forms an ohmic contact with its inner
surface. Each tubular structure may comprise one or more
photovoltaic cells and a number of such structures may be
physically attached end-to-end with appropriate means provided for
series and/or parallel electrical connection of cells. Because of
the tubular structure, a fluid coolant may be circulated through
the interior of each tube so as to provide cooling by direct
conduction of heat.
Still other objects of the invention are set forth or rendered
obvious by the following detailed description of the invention
which should be considered together with the accompanying drawings,
wherein like numbers refer to like parts and:
FIG. 1 is a perspective view with a portion broken away of a
preferred form of a cylindrical solar cell constructed in
accordance with this invention;
FIG. 2 is a longitudinal sectional view of an array of photo cells
of the type shown in FIG. 1;
FIGS. 2A, 2B and 2C are enlarged sectional views of certain
components of the array of FIG. 2;
FIG. 3 is a longitudinal sectional view of a second form of solar
cell array;
FIG. 4 is a view similar to FIG. 3 of a solar cell array with a
central support;
FIG. 4A is a fragmentary sectional view showing an alternative form
of solar cell array with a cylindrical mandrel support;
FIGS. 5-8 are fragmentary sectional views on an enlarged scale
showing different methods of interconnecting tubular solar cells in
an array;
FIG. 9 is a view like FIG. 3 of a parallel-connected array;
FIG. 10 is a perspective view showing several solar cell arrays
combined with solar energy concentrators; and
FIG. 11 is a cross-section of a Schottky-barrier solar cell.
DESCRIPTION OF THE INVENTION
The present invention is predicated on the use of
semiconductor-grade silicon (or other suitable semiconductor
material as hereinafter described) in tubular form. As is already
known to persons skilled in the art, silicon and other
semiconductor materials may be grown as hollow, i.e., tubular,
substantially mono-crystalline bodies with cylindrical, rectangular
or other cross-sectional shapes by the processes described and
claimed in U.S. Pat. Nos. 3,471,266 and 3,591,348 issued to Harold
E. LaBelle, Jr. on 10/7/69 and 7/6/71 respectively (see also U.S.
Pat. No. 3,826,625 issued 7/30/74 to J. S. Bailey). By controlling
the growth environment and using a high purity melt, it is possible
to grown tubular bodies with a purity suitable for semiconductor
purposes. Also by introducing suitable
conductivity-type-determining impurities, i.e., dopants, to the
melt it is possible to produce tubular bodies by the aforesaid
processes which have a P- or N-type conductivity and a
predetermined resistivity. The addition of a dopant to a melt from
which a crystal is grown is conventional, for example, with
Czochralski-type processes and also is exemplified by U.S. Pat.
Nos. 3,129,061, 3,162,507 and 3,394,994.
In the preferred mode of practicing this invention, a tubular body
of one type conductivity is provided initially, and such body is
then treated to provide one or more zones of opposite type
conductivity so that a P-N junction is created between such zone or
zones and the adjacent portion or portions of the hollow body. The
zone of opposite type conductivity may be formed in various ways
known to persons skilled in the art, e.g., by diffusion or ion
implantation of dopants or by epitaxial deposition of opposite type
conductivity material. Preferably, the opposite type conductivity
zone is formed at the outer surface of the hollow body, preferably
by diffusing a suitable dopant into such surface. Thus if the
hollow body is a P-type semiconductor, a suitable N-type dopant is
diffused into it to create an N-type conductivity zone. Similarly,
if the hollow body is an N-type semiconductor, a suitable P-type
dopant is diffused into it to create a P-type conductivity zone.
The choice of dopant used depends on the material of which the
hollow body is composed and also its conductivity type. Thus, for
example, boron may be diffused into N-type silicon to produce a
zone of P-type conductivity while phosphorus may be diffused into
P-type silicon to produce a zone of N-type conductivity. The
several types of dopants used for modifying the conductivity of
silicon and how such conductivity-modifying impurities may be
diffused into a silicon body are well known (see, for example, U.S.
Pat. Nos. 3,162,507; 3,811,954; 3,089,070; 3,015,590; and
3,546,542). The types of dopants required to modify the
conductivity type of other materials, e.g., gallium arsenide,
cadmium telluride, etc., also are well known to persons skilled in
the art. In accordance with prior art knowledge, the concentration
of dopants in the P and N regions of the tubular structures is
controlled to obtain the desired resistivity of the P- and N-type
regions. Preferably, the resistivity of such regions is held to
less than about 100 ohm-cm and for best conversion efficiency is
between about 0.001 to about 10 ohm-cm. In order to improve the
efficiency of collecting the photoelectrically produced carriers,
the depth of the P-N junction from the outer surface is made small,
preferably in the order of 1/2 micron. After the P-N junction is
formed, the hollow body is provided with ohmic contacts or
electrodes for its P- and N-type zones whereby the resulting solar
cell unit may be connected to an exterior circuit. Additionally,
the hollow body may be coated with some sort of anti-reflection or
interference film to reduce reflection losses or to block
absorption of infrared radiation. In the case of a silicon solar
cell for terrestrial use, it is preferred that the hollow body by
N-type silicon and the P-type zone be doped with boron and created
at its outer surface, since the reflectivity of boron-doped silicon
is only about 5% as contrasted with the normal reflectivity of 35%
for undoped silicon. In contrast, if an N-type zone is produced by
diffusing phosphorus into the outer surface of a P-type silicon
body, the reflectivity of that surface will be reduced only a minor
amount. However it appears that P on N cells are less resistant to
radiation deterioration than N on P cells. Hence, for space
applications, it may be preferred to employ N on P cells with an
anti-reflection coating or else a P on N cell with an interference
film or filter that narrows the wavelength of the incident
radiation according to the spectrum of the solar radiations in
space and the spectral response of the solar cell.
Referring now to FIG. 1, the illustrated solar cell comprises a
cylindrical silicon tube 2 of N-type conductivity which has been
subjected to diffusion of boron into its outer surface to form an
outer p-conductivity type region 4 and a P-N junction 6. The inner
surface of the cylindrical tube is provided with a first electrode
in the form of an adherent metal conductive film 8 which forms an
ohmic contact with the tube. The film 8 covers the entire inner
surface of the tube and consists of a selected metal or metal alloy
having relatively high conductivity, e.g., gold, nickel, aluminum,
copper or the like, as disclosed in U.S. Pat. Nos. 2,984,775,
3,046,324 and 3,005,862. The outer surface is provided with a
second electrode in the form of a grid consisting of a plurality of
circumferentially extending conductors 10 which are connected
together by one or more longitudinally-extending conductors 12. The
opposite ends of the outer surface of the hollow tube are provided
with two circumferentially-extending terminal conductors 14 and 16
which intercept the longitudinally-extending conductors 12. The
spacing of the circumferentially-extending conductors 10 and the
longitudinally-extending conductors 12 is such as to leave
relatively large areas 18 of the outer surface of the tube exposed
to solar radiation. Preferably, but not necessarily, the conductors
12, 14 and 16 are made wider than the circumferentially-extending
conductors 10 since they carry a greater current than any of the
latter. These conductors are made of an adherent metal film like
the inner electrode 8 and form ohmic contacts with the outer
surface of the tube. The several conductors 10, 12, 14 and 16 and
the film 8 may be applied by any of several suitable techniques
well known in the art, e.g., by evaporation deposition.
As is obvious to a person skilled in the art, the unit of FIG. 1
constitutes a discrete photovoltaic solar cell. When the unit is
connected by its inner and outer electrodes into an exterior
circuit and the exposed portions 18 of the outer surface of the
unit are exposed to solar radiation, electron-hole pairs are
generated in the tube with the result that current will flow
through the exterior circuit via the inner and outer electrodes.
The open circuit potential of the unit is approximately 0.52 volt.
The same results will be produced if the solar cell unit is made by
providing a tube made of P-type conductivity and treating its outer
surface to provide regions of N-type conductivity with an
intervening P-N junction.
A plurality of P on N or N on P units as shown in FIG. 1 may be
combined to form a solar cell array, with the individual solar
cells being interconnected electrically either in series or in
parallel according to the output voltage and the output current
desired. Preferably, but not necessarily, the several units are
mechanically connected end-to-end to form an integrated
structure.
FIG. 2 illustrates a solar cell array comprising three of the units
shown in FIG. 1 interconnected electrically in series with one
another. As seen in FIG. 2, the three units are disposed end-to-end
with the central unit 20B mechanically coupled to the two end units
20A and 20B by means of two like coupling members 22 which
preferably are made of an electrically insulating material such as
plastic, but also may be made of an electrically conducting
material which is provided with an insulating coating. As seen best
in FIG. 2A, the coupling members 22 are made of electrically
insulating material and comprise an annular portion 24 formed with
cylindrical flanges 26 and 28 at its inner and outer edges
respectively. The inner flange extends within and engages the inner
surface of one solar cell unit while the outer flange surrounds and
engages the outer surface of the adjacent solar cell unit.
Interposed between the annular portion of the coupling member and
the solar unit which is surrounded by the outer flange 28 is a ring
30 which also is made of insulating material and has a width in its
radial direction which is sufficient to span the entire end surface
of the solar unit which it engages, thereby preventing short
circuiting across the P-N junction of the solar unit. Each of the
coupling members is provided with a plating 32 of conductive
material on the inner surface of its outer flange, the adjacent
surface of its annular portion, and the inner, end and outer
surfaces of its inner flange. The inner and outer flanges of each
coupling member make a tight fit with the two solar cell units
between which it is disposed, with the result that a direct ohmic
contact is made between the outer conductor 16 of unit 20A and the
inner electrode 8 of unit 20B, and similarly between the outer
conductor 16 of unit 20B and the inner electrode of unit 20C. A
first end member 36 is attached to the free end of unit 20A and
another end member 38 is attached to the free end of the third unit
20C. As seen in FIG. 2B, end member 36 comprises a circular ring
portion which engages the end surface of unit 2A and a cylindrical
ring portion which engages the inner surface of the same unit. End
member 36 is provided with a conductive metal coating 40 on the
exposed annular end surface of its ring portion and also on the
inner, end and outer surfaces of its flange portion. The opposite
end member 38 is formed as a cylindrical sleeve with a circular
groove 42 in one end to receive the free end of the third solar
cell unit 20C. The outer surface of this end member is provided
with a conductive metal coating 44 which extends around the outer
part of its inner end surface and along the outer side of groove
42, as shown in FIG. 2C. End members 36 and 38 make a tight fit
with solar cell units 20A and 20C so that their conductive metal
layers 40 and 44 make direct ohmic contacts with the inner
electrode of unit 2A and the outer conductor 16 of unit 20C.
The above-described array is coupled to an exterior circuit (not
shown) by means of terminal leads 46 and 48 which are conductively
secured to the conductive coatings on end members 36 and 38. As
will be obvious to a person skilled in the art, the terminal lead
46 is connected to the N side of unit 20A while the other
conductive lead 48 is connected to the P side of unit 20C.
Furthermore, the P side of unit 20A is electrically connected to
the N side of unit 20B, while the P side of unit 20B is connected
to the N side of unit 20C. As a consequence, the three units are
connected electrically in series with the result that the open
circuit voltage of the array is equal to the sum of the voltages
generated by the three solar cell units, i.e. about 1.56 volts.
The three units 20A-20C of FIG. 2 may be held fixed in end-to-end
relation in several ways. One way is to connect the coupling
members 22, 36 and 38 to the solar cell units by means of a
conductive cement located where electrical coupling is desired.
Another approach is to encircle the coupling members and the ends
of tubes 2 with mechanical clamping rings, e.g., split rings with
screw means for drawing the ends of the rings together so as to
radially compress the tubes and coupling members together. Still
another method is to provide means for axially compressing the
tubes together. A fourth approach is to force fit the coupling
members to the hollow tubes. Still other techniques obvious to
persons skilled in the art may be used to hold the assembled solar
cell units together so as to form an integrated structure.
Preferably the mode of holding a number of tubular solar cell units
assembled end-to-end so as to form a sturdy structure in such as to
permit a coolant to be circulated through the interior of the
units. Three such modes are illustrated in FIGS. 3, 4 and 4A. FIG.
3 also illustrates how P on N and N on P cells may be combined in
one array.
Turning now to FIG. 3, there is shown an array of tubular solar
cell units 50A, B, C and D which are like the solar cell units of
FIG. 2 except that units 50B and 50D are N on P cells whereas units
50A and 50C are P on N cells. Thus, units 50A and 50C are like the
solar cell unit of FIG. 1 while unit 50B comprises a tube of P-type
silicon with the outer surface treated to provide a cylindrical
N-type region separated from the interior portion of the tube by a
P-N junction which is the reverse of the junction 6. The end cells
50A and 50D are provided with end members 36 and 38 as described
above while four coupling members 52A-D are located between the
mutually confronting ends of successive tubes. Coupling members 52
are made of electrically insulating material and are in the form of
cylindrical sleeves with a groove in each end face to accommodate a
tube end. Coupling members 52A and C are provided with a conductive
metal coating (represented by the heavy line 54) in FIG. 3 which
covers its outer surface and extends around the outer portion of
each of its end edges and along the outer side of each of its end
grooves. Thus the end conductors 16 and 14 of units 50A and 50B and
the corresponding conductors of units 50C and 50D engage and make a
direct ohmic contact with the conductive coating 54 on coupling
members 52A and C. Coupling members 52B and 52D are like members
52A and 52C except that each is provided with a conductive metal
coating (represented by heavy line 56) which covers its inner
surface and extends around the inner portion of each of its end
edges and along the inner side of each of its end grooves. Thus,
the inner electrodes 8 of units 50B and 50C engage and make an
ohmic contact with the conductive coating on coupling member 52B
and a similar contact is made by the conductive coating on coupling
member 52D with the inner electrodes of units 50C and 50D. As a
consequence, the several P-N junctions are connected in series so
that the open circuit potential of the array is the sum of the open
circuit potentials of the individual cells. The several solar cell
units 50A-50 may be secured together in the same manner as the
units of FIG. 2 and may be cooled by passing a suitable fluid
through the several units via the openings provided by members 36,
38 and 52A-D.
FIG. 3 also illustrates how a radiation filter may be combined with
a solar cell or solar cell array constructed in accordance with
this invention. In this case, the radiation filter is formed as a
cylindrical tube 58 which is slipped over the several units and is
secured, e.g., by mechanical means or by bonding with a suitable
cement, to at least the two end members 36 and 38 so as to hold the
array together. For this modification the end member 36 is modified
as shown in dotted lines so as to provide a surface for engaging
tube 58. As an optional feature, the filter tube 58 may also be
secured to the coupling members 52. The tube 58 is made of a
suitable material, e.g., a selected glass, which is transparent to
radiation with a wavelength which will produce electron-hole pairs
and thereby produce the desired photovoltaic effect but will pass
little or no infrared radiation. Thus, in the case of silicon, the
filter is made preferably of a material which will block radiation
of wavelengths greater than about 1.2 microns.
FIG. 4 shows a solar cell array like that of FIG. 2 wherein the
several units are mounted on a central support. In this case, three
like units 20A-C are separated by coupling members 60A and 60B
which are similar to coupling members 22 except that their inner
diameters are sized so that they make a snug sliding fit with a
center support rod or mandrel 62. Additionally, each of the
coupling members 60 is provided with one or more apertures 64 so as
to permit a coolant to pass from the interior of one solar cell
unit to the next solar cell unit. The coupling members 60 may be
plated like the coupling members 22, in which case insulating
spacer rings like those shown at 30 in FIG. 2 may be introduced
between each coupling member and the adjacent solar cell unit which
is embraced by the outer flange of the coupling member.
Alternatively, the coupling members may be plated with a conductive
metal film which covers the inner, end and outer surfaces of the
outer flange and extends to and covers the outer surface of the
inner flange, as represented by the heavy line 66 in FIG. 4. In
such case, a circular spacer 30A made of electrical insulating
material is interposed between each coupling member and the solar
cell unit which fits over the inner flange of the coupling member
so as to prevent short circuiting of the P/N junction by the metal
film 66. In this way, each coupling member provides an ohmic
connection between the end conductor 16 of one unit and the inner
electrode 8 of the adjacent unit. The opposite ends of the array
are fitted with end members 68 and 70. The end member 68 is
essentially a cylindrical plug with a reduced diameter axial
extension 72 at one end and a peripheral flange 74 at the other
end. The circumferential surface of the flange 74 is coated with a
conductive metal film represented by heavy line 75 which extends to
and covers the circumferential surface of that portion of the plug
which fits within the unit 20A, whereby an ohmic contact is made to
the inner electrode 8 of that unit. The end member 68 is provided
with an axial bore 76 and one end of the center support 62 is
provided with a reduced diameter section which fits within the
inner end of bore 76. Additionally, the member 68 is provided with
one or more radially-extending passageways 78 which intersect the
axial bore 76. A non-conductive spacer 80 is interposed between the
flange 74 of end member 68 and the adjacent end surface of unit
20A, so as to prevent short circuiting of the P/N junction. The
member 68 is secured in place by bonding it and the spacer 80 to
the unit 20A and/or by bonding its extension 72 to the central
support 62. The opposite end member 70 is also formed with an axial
extension 82 and a central bore 84. One or more radially-extending
ports 86 are provided which intersect bore 84, and the adjacent end
of center support 62 has a reduced diameter section which fits
within the axial bore 84. The end member 70 is formed with a
cylindrical flange 87 which is sized to fit over and engage the
adjacent end of unit 20C. The inner surface of the flange of end
member 70 is coated with a conductive metal film represented by
heavy line 89 which extends around the edge surface of that flange
and covers the cylindrical outer surface of the same flange,
whereby an ohmic contact is made to the end conductor 16 of unit
20C. End member 70 is bonded to the unit 20C and/or to the center
support 62. As a result, the several units and the center support
62 form an integrated structure. Terminal leads 46 and 48 may be
coupled to the conductive metal films on end members 68 and 70 as
shown, whereby the illustrated array may be connected onto an
exterior circuit (not shown). The above-described array offers the
advantage that the end members 68 and 70 not only are used to form
a sturdy mechanical assembly but also function as means for
circulating a coolant fluid through the interior of the array. A
coolant may be introduced, for example, through the axial bore 76
and radial ports 78 and removed via radial ports 86 and axial bore
84, with the coolant passing from one unit to the other via the
passageways 64 of coupling members 60A and B.
FIG. 4A shows a further modification of the invention. In this
case, the center support 62 extends through an end member 88 which
is similar to end member 68 except that it lacks the reduced
diameter extension 72. An O-type seal 90 is located in a groove
surrounding the axial bore in end member 88 and tightly engages the
central support 62. The outer end of the center support 62 is
threaded as shown at 92 to receive a nut 94 which cooperates with
the central support to urge the end member 88 against it in a
direction to compress the spacer 80 between it and the end of the
solar cell unit 20A. The central support 62 is provided with a
blind axial bore 96 and one or more radial ports 98 which intersect
bore 96. At the opposite end of the array, an end member 100 is
employed which is similar to end member 38 and has a conductive
coating 101 like coating 44. A second nut 94 at the adjacent end of
support 62 urges end member 100 against the end of solar cell unit
20C. Hence, the several solar cell units are held together by the
axial compression exerted on end members 88 and 100 by coaction of
nuts 94 and center support 62. A coolant may be introduced into one
end of the array via axial bore 96 and ports 98 and is withdrawn
from the other end of the array by the corresponding ports and
axial bore in the opposite end of center support 62. The use of a
center support 62 with end members as shown in FIGS. 4 and 4A is
advantageous regardless of whether the solar cell array comprises P
on N or N on P cells or a combination of P on N and N on P
cells.
FIG. 5 shows one alternative method of electrically and
mechanically coupling together two solar cell units of the type
shown in FIG. 1. In this case a non-conductive coupling member 104
is employed which is in the form of a cylindrical annulus having an
inner diameter sized to make a close sliding fit with the center
support 62. The coupling member 104 is provided with passageways 64
as shown for permitting a coolant to flow from one solar cell unit
to the other. The outer surface of the coupling member 104 is
provided with a rib 106 which fits between and forms two oppositely
disposed shoulders for engaging the two solar cell units. In this
case each of the solar cell units 20A and 20B is modified so that
at one end its inner electrode 8 terminates a short distance from
its end edge, while at the other end the metal film which forms the
electrode is extended around the end edge and up over the outer
surface of the hollow tube so as to form a tab as shown at 107.
However, between the extended portion of the inner electrode 8 and
the end and outer surfaces of the tube 2, a thin layer of
insulating material 108 is provided so as to prevent short
circuiting of the P/N junction. By way of example, if the tubes of
solar cell units 20A and 20B are made of silicon, the insulating
material 108 may be a film or layer of silicon dioxide (SiO.sub.2).
In this case also, the end conductor 16 is spaced from the end edge
of the tube 16 so that a gap exists between it and the extended
portion 107 of the inner electrode. The adjacent ends of the two
tubes are fitted over the coupling member 104 so that they abut the
shoulders formed by its rib 106, and a suitable non-conductive
cement or adhesive may be applied between the rib and the adjacent
end surfaces of the two tubes as shown at 110 so as to bond the two
tubes to the coupling member 104. Thereafter, a direct electrical
connection is made between the inner electrode 8 of the tube 20A
and the other conductor 14 of unit 20B by means of one or more
conductive straps 112 which are secured to tab 107 of unit 20A and
conductor 14 of unit 20B by soldering or by a conductive cement or
by other suitable means known to persons skilled in the art. To
allow for expansion and contraction due to temperature variations,
the cement 110 may be omitted and the conductive strap 112 may be
formed with a bowed portion as shown in phantom at 114, whereby
endwise movement of one tube relative to coupling member 104 and
the other tube may be compensated for by flexing of the bowed
portion 114.
FIG. 6 shows still another way of providing electrical connections
between two adjacent tubular units. In this case the coupling
member 116 is similar to coupling member 104 except that its outer
rib 117 is bevelled as shown. The outer surface of coupling member
116 is provided with a coating of a conductive metal as shown at
118 which is soldered to and makes an ohmic contact with the inner
electrode 8 of the unit 20B. The other unit 20A has its inner
surface bonded to coupling member 116 by a non-conductive cement as
shown at 119. The end conductor 16 of unit 20A is coupled to the
metal film 118 on coupling member 116 by one or more flexible
conductive wire straps 120. If desired, the straps 120 may be
replaced by a flexible conductive cylinder with one large enough to
surround and engage the end conductor 16 of unit 20A and the other
end small enough to surround coupling member 116 and be
conductively bonded to the metal film 118.
FIG. 7 shows an arrangement wherein the central support 62 extends
through spacer elements 122 which are similar to coupling members
104 and 116 except that they do not extend between two solar cell
units. Preferably, but not necessarily, the spacer elements 122 are
bonded to units 20A and 20B and preferably are sized to make a
close but sliding fit with center support 62. Spacers 122 are
provided with passageways 64 to permit flow of coolant as
previously described. Interposed between and connecting the two
solar cell units is an accordian-type bellows 124. One end of the
bellows has a cylindrical extension 126 which fits over and is
bonded to the end conductor 16 of unit 20A. The other end of the
bellows has a cylindrical extension 128 which fits within and is
bonded to the inner electrode 8 of unit 20B. Preferably, bellows
124 is made entirely of a conductive metal or metal alloy;
alternatively, it may be made of a non-conductive material but
plated with a conductive metal so that a direct electrical
connection is made between the conductor 16 of unit 20A and the
inner electrode 8 of unit 20B. The cylindrical portions 126 and 128
are preferably soldered but may be bonded by a conductive cement to
units 20A and 20B so that a good ohmic contact is assured.
This modification offers the advantage that the bellows 124 allows
one or both of the coupled units 20A and 20B to shift lengthwise to
compensate for shock or temperature-induced expansion or
contraction without rupturing the connections between the coupled
unit.
FIG. 8 shows a modification of the invention which is like that of
FIG. 7 except that the bellows 124 is replaced with a bowed
flexible sleeve 130 which has a cylindrical end section 132 which
is bonded to end conductor 16 of unit 20A and a smaller cylindrical
end section 134 which is bonded to the inner electrode 8 of unit
20B. The sleeve 130 may be made of a conductive material or of an
insulating material with conductive surface coatings so as to
provide a direct electrical path between end conductor 16 of unit
20A and inner electrode 8 of unit 20B. If desired, insulating
spacers 136 may be bonded to the confronting end surfaces of units
20A and 20B as shown in FIGS. 7 and 8 so as to prevent portions of
the bellows 124 and sleeve 130 from making electrical contact with
those end surfaces; in this way short circuiting of the P/N
junctions by the elements 124 and 130 is avoided in the event the
units 20A and 20B are moved toward one another. The embodiments of
FIGS. 7 and 8 offer the advantage that the units 20A and 20B are
free to move lengthwise of the central support 62 to a limited
extent, thereby preventing rupture of the electrical connections
between them when the units are subjected to shock or vibrations or
when they contract or expand due to changes in temperature. The
different ways of coupling together adjacent solar cell units shown
in FIGS. 5-8 may be employed in arrays where the opposite ends of
the center support 62 are connected to end members as shown in
FIGS. 4 and 4A, or otherwise.
A further advantage of the use of center support 62 is that it may
be used as a common conductor or bus for the inner electrodes 8 of
several solar cell units where it is desired to electrically
connect the several units in parallel. Thus, as shown in FIG. 9,
three tubular solar cell units 20A-20C are connected end-to-end by
means of coupling members 140 which are shaped generally like the
coupling elements 52A and C of FIG. 3 and have corresponding
conductive coatings 54. However, the inner diameters of coupling
member 140 are sized so that their inner surfaces tightly grip
center support 62, and passageways 64 are provided to allow a
coolant to be passed through the several units. Additionally, the
inner surface of each coupling member is coated with a conductive
metal film which, as represented by the heavy line 142, extends
around the inner portions of its opposite end surfaces and along
the inner sides of its two grooves. The metal films 54 are bonded
to the end conductor 16 of one unit and the opposite end conductor
14 of the adjacent unit, while the metal films 142 are bonded to
the inner electrodes 8 of the corresponding units and tightly grip
the center support 62. The latter is made of an electrically
conductive material or else has an electrically conductive coating
so that it will serve to electrically connect the inner electrodes
8 of the three units to end member 68A. The latter is like end
member 68 except that it is made of a conductive material. End
member 70 is made the same as the correspondingly numbered element
in FIG. 4. Terminal leads 46 and 48 are bonded to the end member 68
and the conductive metal film 89 of member 70 respectively. As a
consequence, the three cells are connected in parallel with one
another so that when the array is connected to an exterior circuit,
the total current output will be the sum of the currents generated
by the individual solar cell units.
FIG. 10 illustrates how tubular solar cells as provided by the
present invention may be combined with solar energy concentrators,
the solar cells acting as energy receivers. The embodiments of FIG.
10 comprises four solar cell arrays or batteries 150 like the one
shown in FIG. 4 and, for convenience of illustration, only three of
the arrays are combined with concentrators. Each concentrator 152
is affixed to a support plate 151 and comprises flat opposite end
walls 154 and 156, opposite side walls 158 and 160 which are
parabolically curved in cross-section, and a bottom wall 162 which
is circularly curved in cross-section. The open upper end of each
concentrator forms and entrance pupil with a width d.sub.1.
Each concentrator is made so that the inner surfaces of its end,
side and bottom walls are capable of functioning as reflectors of
solar radiation. Thus, for example, each concentrator may be made
of sheet metal with a mirror surface, e.g. aluminum, or may be made
of a plastic with a reflective metal film deposited on its inner
surfaces. The junction of the side walls 153 and 160 with bottom
wall 162 forms an exit pupil with a width d.sub.2. The curved
bottom wall 126 forms a chamber to receive the associated solar
cell array 150 which is centered in the chamber. The radius of
curvature of the bottom wall is great enough to provide a space
between it and the associated array which is large enough to permit
its inner surface to receive and reflect a substantial portion of
whatever radiation passes into the exit pupil. Preferably but not
necessarily, the outer diameter of the tubes which form each solar
cell array is about one-half of the width d.sub.2. Preferably but
not necessarily the width of the entrance and exit pupils are set
so as to provide a ratio of d.sub.1 d.sub.2 equal to 1/sin
.theta.max, where .theta.max is the angle formed between the center
axis of the concentrator and a line extending from one edge of the
entrance pupil to the opposite edge of the exit pupil. The
concentrator accepts radiation (diffused or collimated) over an
angle of 2.theta.max and concentrates it all in the exit pupil.
This type of concentrator is described in a preprint of an article
by Roland Winston, "Solar Concentrators of a Novel Design",
scheduled for publication in the October 1974 issue of Solar Energy
Journal. /d.sub.2
The opposite ends of each array extend through insulating sleeves
164 mounted in the opposite end walls of the associated
concentrator and conduits 166 and 167 are attached to the end
members 68 and 70. The conduits 166 and 167 are connected to header
pipes 168 and 169 respectively. The latter are connected to
conduits 171 and 173 whereby coolant is fed into one end of each
array and fed out of the opposite end of each array. The coolant
circulating system is preferably of the closed loop type comprising
an exterior heat exchanger shown schematically as box 170 and a
pump 172 for circulating the coolant through the solar cell arrays
and the heat exchanger. With such a circulating system, the coolant
absorbs heat from the solar arrays and is relieved of heat in the
heat exchanger. For terrestrial installations, the heat exchanger
may be replaced by a refrigeration plant or a large reservoir of
coolant which is adapted to give up the heat recovered from the
solar cells by radiative cooling or by heat exchange with a solid
or fluid medium, e.g., stones, water, air, etc.
As an alternative measure, the coolant circulating system may be
arranged so that coolant circulates through the several arrays in
series instead of in parallel. However, a parallel cooling system
as shown in FIG. 10 is preferred since it enables all of the arrays
to be maintained at substantially the same temperature.
Still referring to FIG. 10, the three cells in each array are
connected in series in the manner shown in FIG. 4, but the four
arrays are connected in parallel, whereby a series parallel matrix
is formed. The parallel connections are provided by (a) connecting
together the coupling members 68 with conductive straps 174 which
are bonded to and make ohmic contacts with the metal films 75 of
the coupling members, and (b) connecting together the coupling
members 70 with conductive straps 176 which are similarly secured
to the metal films 89. Terminal leads 46 and 48, similarly
connected to one of the coupling members 68 and 70, are provided to
connect the solar cell matrix to an exterior circuit. The latter
may comprise a power consuming load such as, for example, a d.c.
motor, an electric heater or electric lights, or a power storage
means such as a rechargeable storage battery. In the embodiment of
FIG. 10, some of the solar radiation entering the entrance pupil of
a concentrator may pass directly through the exit pupil and be
received by the associated solar cells either directly or after
reflection from the bottom wall 162. The remainder of the radiation
entering the entrance pupil is reflected by the end or side walls
of the concentrator into the exit pupil where it strikes the solar
cells either directly or after reflection from bottom wall 162. The
latter wall functions to direct radiation onto the bottom half of
the solar cell array so that each solar cell is irradiated
substantially uniformly over its entire circumference. This has the
dual effect of maximizing the current output and avoiding local hot
spots. Simultaneously, the circulating coolant removes any heat
generated in the solar cells by absorption of infrared radiation or
by resistive losses, whereby the solar cell arrays are maintained
at an even temperature. The coolant employed and the rate at which
it is circulated are selected so as to maintain the solar cells at
a temperature which will enable the cells to operate with a
satisfactory conversion efficiency. Also the coolant must be a
non-conductor of electricity since otherwise it might cause
short-circuiting of the cells. By way of example but not
limitation, the coolant may be de-ionized water, fluorinated
hydrocarbon, a silicone oil, Freon, air or nitrogen.
It is to be understood that the tubular solar cells and arrays may
be combined with other forms of solar energy concentrators. Thus
the concentrator may take the form of a simple trough-like
reflector which has a parabolic cross-section, with a tubular solar
cell or reflector extending lengthwise of the trough substantially
coaxially with the focus of the parabola. Furthermore, a
transparent cover may be mounted over the concentrator(s) to
provide protection from rain, dust, etc.
While the invention as herein described preferably takes the forms
of silicon P-N junction solar cells, it is not limited to devices
made of silicon or to devices with homo-junctions. Instead the
tubular cells may be made of other semiconductor materials and
comprise a hetero-junction or a surface barrier junction (e.g., a
Schottky-barrier) in place of a homo-junction. Furthermore, the
semiconductor material need not be substantially monocrystalline
since photovoltaic devices are known which comprise polycrystalline
semiconductor materials, e.g., cadmium telluride. Thus, for
example, tubular solar cells may be made which essentially comprise
gallium arsenide P-N junctions, gallium arsenide phosphide P-N
junctions, .[.cadimium.]. .Iadd.cadmium .Iaddend.telluride P-N
junctions, cadmium/sulfide/copper sulfide and gallium
arsenide/gallium phosphide hetero-junctions. Similarly, for
example, the tubular solar cells may be surface barrier devices
which comprise metal or metal oxide/semiconductor junctions, e.g.,
solar cells using gold on N-type silicon, aluminum on P-type
silicon, tin oxide on N-type silicon, chromium on P-type silicon,
and indium oxide on cadmium telluride. The aforesaid P-N and
hetero-junctions may be made by providing a tubular body of one
junction material and forming a layer of the other junction
material at or on the inner or outer surface of such body by
methods well known in the art, e.g., by diffusing an
opposite-conductivity-type dopant into the outer surface in the
case of a homo-junction or epitaxially growing a thin layer of the
other junction material on the outer surface of the tubular body in
the case of a hetero-junction. Similarly, solar cells with surface
barrier junctions may be made by depositing a metal or metal oxide
barrier material on the outer surface of a tubular semiconductor
body by vacuum deposition, sputtering, .[.electorless.].
.Iadd.electroless .Iaddend.plating or other suitable technique. An
essential requirement of the barrier material is that it have a
suitable optical transmission capability so that the device will
exhibit a photovoltaic behavior.
FIG. 11 illustrates a cross-section of a tubular Schottky-barrier
solar cell which comprises a tubular body 180 of P-type silicon, an
aluminium ohmic contact layer 182 on its inner surface, and a
layered Schottky barrier on its outer surface which is made
according to the teachings of W. A. Anderson et al., An 8%
Efficient Layered Schottky-Barrier Solar Cell, Journal of Applied
Physics, Vol. 45, No. 9, pp. 3913-3915, September 1974. The layered
barrier consists of a chromium barrier layer 184, a copper
conductive layer 186, a chromium oxidation layer 188 over the
copper layer, an aluminum ohmic contact or current collector 190,
and a silicon monoxide anti-reflection coating 192. The ohmic
contact is represented as several discrete sections since it is
fabricated as a grid, preferably a grid with sections corresponding
to conductors 10, 12 and 14 and 16 of the cell unit shown in FIG.
1, whereby a plurality of relatively large areas of the chromium
barrier layer 184 (corresponding to areas 18 of FIG. 1) are exposed
for stimulation by solar radiation. The silicon monoxide
.[.laayer.]. .Iadd.layer .Iaddend. is applied over the copper layer
in the spaces between the discrete sections of the grid-like outer
contact 190. Preferably the ohmic contact 182 covers most if not
all of the inner surface of the silicon body and has a thickness of
about 1 micron or less. The thickness of chromium layer 184, copper
layer 186, chromium oxidation layer 188, ohmic contact 190 and the
SiO coating 192 are preferably made with thicknesses of
44A.degree., 58A.degree., 23A.degree., 1000A.degree., and
690A.degree. respectively.
Obviously, a plurality of tubular hetero-junction and surface
barrier junction solar cells may be arranged to form arrays and be
combined with concentrators in the various ways illustrated in
FIGS. 2-10.
It is to be appreciated also that the tubular solar cells need not
be cylindrical, but instead, for example, they may have an
elliptical, square, rectangular, or other cross-sectional
configuration. The essential requirement of the invention is that
the solar cell unit comprise a tubular semiconductor body adapted
to exhibit a photovoltaic behavior. In this connection, it is to be
noted further that the term "photovoltaic semiconductor barrier
device" is intended to encompass devices which have a
homo-junction, a hetero-junction, or a surface barrier junction,
and also that the term "surface barrier junction" includes
metal/semiconductor barrier devices and metal oxide/semiconductor
barrier devices, and notably Schottky barrier devices.
The following specific example illustrates a preferred mode of
practicing the invention.
EXAMPLE
A cylindrical substantially monocrystalline P-type silicon tube is
grown according to the method described in U.S. Pat. No. 3,591,348.
The tube is made with a length of about 6 inches, an outside
diameter of 0.50 inch and wall thickness of about 0.01 inch. The
interior surface is plated with a 0.001 inch layer of nickel and
phosphorus is diffused into the outer surface of the tube to a
depth of about 0.5 micron to form an N-type outer region with a
distinct P-N junction. Then aluminum is vacuum deposited onto the
outer surface of the tube in the form of a grid consisting of a
plurality of longitudinally- and circumferentially-extending
conductors. The aluminum grid is formed with a thickness of about
4.0 microns. The inner and outer conductors are connected to a
measuring circuit and the device irradiated by sunlight. The device
exhibits an open circuit voltage of about 0.5 volts and a
conversion efficiency of about 10%.
The advantages of the invention are numerous. The tubular structure
renders the cell units self-supporting even with tubes of
relatively small wall thicknesses (e.g., 1/2 inch silicon tubes
with a wall thickness of 100-200 microns), thereby obviating the
need for a supporting tubular substrate. The absence of a
supporting substrate reduces weight, cost and also facilitates
mechanical and electrical interconnection of two or more cells.
Furthermore, the tubular cells may be connected electrically in
parallel or in series, and by means of inexpensive reflectors such
as a parabolic reflector it is possible to achieve energy
concentration ratios of 10 or more. By connecting a plurality of
tubular cells in series, it is possible to obtain a high electrical
power output at a moderate current level and at a voltage level
suitable for charging conventional batteries, thereby obviating the
need for heavy conductors on the cells. It is to be appreciated
also that to achieve a reasonable voltage output from a
photovoltaic array, individual cells must be connected in series.
In the case of planar cells, a rectangular heat exchanger is
required for cooling purposes if solar concentration is used, but
the heat exchanger typically must be electrically insulated from
the solar cells. The intervening insulating layer reduces the rate
at which heat can be conducted away from the solar cells and also
tends to complicate the heat exchanger design. The instant
invention facilitates cooling since the coolant is in direct
contact with the inner surface of the tubular solar cell. Hence, no
auxiliary heat exchanger structure need be mounted immediately
adjacent to the solar cell unit. A further advantage is that
whatever portion of the received solar energy is absorbed by the
solar cells as heat may be recovered by the coolant. Hence, the
coolant for the solar cell arrays may advantageously be coupled to
solar thermal devices of the type which are designed to heat a
fluid by solar energy and to use the heated fluid as a heat supply
or, if it is steam, to drive a turbine and thereby an electrical
generator. More specifically, the exit coolant from the solar cells
may be used as the entry heat absorber for a solar thermal device.
As a result of the preheating of the fluid by its transit through
the solar cells, less heating is required to be accomplished in the
solar thermal device to produce a selected fluid temperature, e.g.,
200.degree. F. and therefore, the solar thermal device can be
corresponding reduced in size from what it would have to be if the
fluid was not preheated in the solar cells.
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