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Description  |
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BACKGROUND OF THE INVENTION
This invention relates to the encapsulation of solar cell arrays and more
particularly to a solar cell enclosure and an improved method for
encapsulating a solar cell array within that enclosure.
Solar cells or photovoltaic devices are becoming of increasing importance
in the generation of electrical power. Individual solar cells are
connected in various series/parallel combinations to provide arrays having
several output voltage and current options. A module is then fabricated by
enclosing a solar cell array between a supportive backplate and an
optically transparent glass front plate. The volume between the back and
front plates and surrounding the solar cell array is filled with a potting
material. A seal between the front and back plates cooperates with the
potting material to provide a long-term moisture and contamination
barrier. The potting material provides a hydrodynamic cushion for the
glass cover plate and provides an adhesive seal with the glass plate, the
solar cell array, and the electrical interconnection to the array. The
potting material must provide a highly transmissive path for visible
light, and a low permeability to typical atmosphere contaminates. It must
be dimensionally stable, have a low shear strength, be of high thermal
conductivity, and high electrical resistivity. Silicone based polymers
that cure into a solid elastomer are often used for the potting material.
An alternative which exhibits the desirable properities is the partial
crosslinking of silicone monomers such as those utilized in commercial
silicone oils. This forms a gel-like material that exhibits excellent
cohesive and adhesive strengths. The uncured silicone gel has a
consistency similar to that of a medium weight motor oil, and this results
in handling problems concerning the containment within the enclosure prior
to complete cure. Heretofor, there has not been an effective procedure for
potting and enclosing solar cell arrays. Accordingly, it is an object of
this invention to provide a solar cell enclosure having a compatible seal
design. It is a further object of this invention to provide a solar cell
enclosure which has provisions for repairability. It is a still further
object of this invention to provide a method for enclosing solar cell
arrays. Such a method provides for temporarily sealing the edges of the
enclosure during the potting (curing) procedure. The process provides a
positive pressurization of the enclosure during the potting process by the
hydrostatic pressure of the potting material or by pressurized feeding of
the gel into the enclosure cavity. The process further provides the
flexibility of adding a second sealant around the panel perimeter prior to
the attachment of a spring loaded metal bezel. Still further, the process
permits the attainment of less voids in the potting material and imparts a
compressive loading on the potting material which in turn minimizes the
tendency of the potting material to separate or delaminate from the glass,
back plate, or the array surfaces. The enclosure and method provide a
highly transmissive, highly adhesive optical coupling of the solar cells
to the panel exterior.
SUMMARY OF THE INVENTION
In accordance with the invention a solar cell enclosure and a method for
assembling a solar cell module are provided. The solar cell module is
comprised of an array of interconnected solar cells sandwiched between a
metal pan backing and a glass cover. The interior of the enclosure is
filled with a silicone gel which surrounds the solar cell array and, in
the final structure, the edges of the metal pan and glass cover are sealed
together and held with a metal bezel.
In fabricating the module, the metal pan backing is placed on an assembly
fixture. The solar cell array is then placed on the pan and covered with a
front glass plate. Spacers are inserted between the metal pan and the
cover glass to provide a specified amount of separation. The fixture holds
the glass plate rigidly in place and also provides a temporary seal around
the edges of the enclosure. This temporary seal is provided with a filling
port through which potting material can be poured, and a vent port through
which air can escape from the enclosure. The fixture is shaped to permit a
desired amount of flexure of the pan as the enclosure is filled with the
potting material. As the enclosure is filled with potting material, the
fixture and the enclosure can be vibrated and subjected to a vacuum to
ensure that the potting material entirely fills the interior volume of the
enclosure, leaving no air filled voids. When the enclosure has been
satisfactorily filled with potting material, the assembly is placed in an
oven to cure the potting material. Following the curing of the potting
material, the supporting fixture can be removed and a permanent seal
affixed around the edges of the enclosure. This seal consists of a pliable
sealant material and a spring loaded bezel welded or riveted to the metal
pan. The metal bezel is so configured that it confines the sealant
material without rigidly constraining the glass cover, thus allowing for
thermal expansion. If a failure occurs that is internal to the solar cell
module, the rivets or spot welds which fasten the bezel to the pan can be
removed, the bezel pushed off, and the enclosure opened for repair.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects and advantages of the invention will become apparent upon
reading the following detailed description and upon reference to the
drawings, in which;
FIG. 1 is a partially cut-away perspective view of a finished solar cell
module.
FIG. 2 schematically shows a fixture used in the assembly of a solar cell
module.
FIG. 3 schematically shows in perspective a partially assembled module.
FIG. 4 shows in cross-section the details of the enclosure edge seal.
While the invention will be described in connection with a preferred
embodiment, it will be understood that it is not intended to limit the
invention to that embodiment. On the contrary, it is intended to cover all
alternatives, modifications, and equivalents as may be included within the
spirit and scope of the invention as defined by the appended claims.
DETAILED DESCRIPTION OF THE INVENTION
Turning first to FIG. 1, there is shown a completed solar cell module 10.
The module 10 is comprised of a back plate 12, the solar cell array 14,
and the cover glass 16. The edges of the enclosure are sealed by a bezel
18 which is riveted or spot welded to the back plate 12. The total
interior volume of the enclosure is filled with a potting material which
completely surrounds the solar cell array.
FIG. 2 shows a fixture 20 which is used in assembling the solar cell module
in accordance with the invention. The fixture 20 has a pan support 22 and
a glass support 24. The pan support 22 is shaped to receive the back plate
or pan 12 of the solar cell module. The surface of the pan support 22 is
contoured slightly to allow the back plate 12 to flex outwardly by about
120 mils at its center when the enclosure is filled with potting material.
The fixture then rigidly supports the pan in this position. The flexing
results from the hydrostatic pressure or weight of the potting material
itself. The pan support 22 can be, for example, molded from fiberglass to
the desired shape. The glass support 24 can be a rigid framework of ribs
26 bolted to and held in position by cross pieces 28. The glass support
24, which might be made from quarter-inch thick aluminum or other rigid
material, can be fastened at its edges to the pan support 22 by bolts 29.
Attached to the glass support 24 are side compression bars 30 and a bottom
compression bar 32. The compression bars 30, 32 are rigid aluminum pieces
faced with a deformable sealing material 34 such as soft sponge rubber.
Compression screws 36 allow the compression bars to be pulled up snugly
against the edges of the solar cell assembly to provide a temporary edge
seal. A similarly constructed compression bar 38 is provided at the top of
the glass support 24. Compression bar 38, however, has openings 40, 42
through the aluminum and sponge rubber for filling the enclosure with
potting material and for venting the air from within the enclosure,
respectively.
In fabricating a solar cell module 10, the backplate or pan 12 is placed in
position on the pan support 22. The backplate 12 can be formed from
stainless steel of about 10 mils thickness. The stainless steel provides
strength, good thermal conduction, corrosion resistance, and is readily
formable in the desired shape. Alternatively, the pan 12 could be formed,
for example, from aluminum or from some high temperature, thermally
conductive plastic. The interconnected array of solar cells 14 is placed
on the backplate and then is covered with a cover glass 16. The glass can
be about one-eighth inch in thickness and should have a low iron and iron
oxide content to insure high transparency in the solar spectrum. FIG. 3
schematically depicts the spatial relationship between the parts so far
assembled. The shaping of the stainless steel pan 12 with its edge lip 44
establishes the correct spacing between the pan, array, and glass. The
contoured surface 46 of the pan support 22 is exaggerated in this Figure
for illustrative purposes. Spacers 48 are positioned about the perimeter
of the pan 12 so as to provide a controlled spacing between the pan 12 and
the cover glass 16. These spacers 48 can be made of a plastic material and
can be about 6 mils in thickness. The glass support 24 is now placed on
top of the glass cover 16 and is bolted in place. The bolts push the ribs
26 firmly against the cover glass. Thus the pan support 22 and the glass
support 24 push the cover glass 16 against the pan lip 44. The pan and
glass are therefore separated only by the spacers 48. The side compression
bars 30 and the bottom compression bar 32 are then pulled up against the
edge of the glass and pan by the compression screws 36 to affect a
temporary seal around three edges of the enclosure. The fourth compression
bar 38 can then be screwed in place to seal the remaining edge of the
enclosure except for the filler opening 40 and vent opening 42. The
enclosure is now ready to be filled with a potting material. A number of
potting materials could be used; one that has been found to be
particularly appropriate is a partially cross-linked silicone oil such as
DC-Q3-6527, which is manufactured by the Dow Corning Corporation. The
potting material is poured through the filler hole 40 with the displaced
air escaping through vent hole 42. As the enclosure is filled, the
hydrostatic pressure of the potting material pushes against the metal pan
12 flexing the pan outwardly against the contoured surface 46 of the pan
support 22. To facilitate the total filling of the interior of the
enclosure and the elimination of voids or bubbles within the potting
material, the enclosure and fixture can be placed in a vacuum chamber. The
vacuum chamber (not shown) need only be a simple box large enough to
accommodate the fixture. It can be evacuated with a simple mechanical
roughing pump. Repeatedly evacuating the vacuum chamber, and adding more
potting material to the enclosure, as necessary, ensures the void-free
filling of the module 10. Eliminating voids is necessary to provide a high
integrity contamination barrier and to provide a good optical path. The
elimination of voids can also be facilitated by vibrating or shaking the
enclosure and fixture assembly. The potting material can then be cured.
This is done, for example, with the particular cross-linked silicone oil
by heating to approximately 75.degree. C. for about four hours. Such a
curing operation causes the potting material to become gel-like.
After the potting material has cured, the potting fixture assembly 20 can
be removed since it has now completed its function. The spacers 48 are
also removed, and the glass cover 16 and the lip 44 of the backplate 12
are pulled together. This step enhances the positive pressurization of the
enclosure and imparts a compressive loading on the potting material. This
in turn minimizes the tendency of the potting material to separate from
the glass, the metal, or the solar cell array. It also decreases the
available expansion volume and thus lessens the probability of
delamination. FIG. 4 shows the details of the permanent edge seal that now
completes the solar cell module. The pan lip 44 is shown to be in intimate
contact with the cover glass 16. A formed-in-place gasket material 50 is
spread around the edge of the module. This can be, for example, a pliable
polysulfide gasket material. Over this the bezel 18 is placed. The bezel
18 can be stainless steel or other rigid material. The top of the bezel 52
is so shaped as to provide a spring loaded action in contact with a cover
glass 16. The bezel thus confines the sealant material 50 and holds the
cover glass 16 in place without rigidly constraining the glass 16, thus
allowing for thermal expansion. The bezel 18 is pushed into place, flexing
top portion 52 against the cover glass 16 and then is riveted or spot
welded to the backplate 12. Should a failure ever occur within the module,
the rivets 54 could be removed to facilitate disassembly and repair of the
module. Another element of the edge seal is provided by the shaping of the
extreme edge 56 of the backplate 12. The edge 56 is shaped to form a
second spring loading action against the bezel 18 as the bezel is riveted
in place. Thus a combination of elements provides a long-term
moisture/contamination barrier of a disturbed nature. This consists of the
spring loading of the pan to bezel contact, the formed-in-place gasket 50,
the spring loading of the metal 52 to glass 16 seal, and finally the
barrier provided by the internal potting material itself.
Thus, it is apparent that there has been provided, in accordance with the
invention, a solar cell enclosure and a method for fabricating a solar
cell module that fully satisfies the objects and advantages set forth
above. While the invention hase been described in conjunction with
specific embodiments thereof, it is evident that many alternatives,
modifications, and variations will be apparent to those skilled in the art
in light of the foregoing description. Accordingly, it is intended to
embrace all such alternatives, modifications, and variations as fall
within the spirit and broad scope of the appended claims.
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