Method of producing a solar panel curved in two directions

Abstract

The invention relates to a method of producing a solar panel curved in two directions. A problem occurs when solar cells are laminated (attached) to a curved surface (such as thetransparentroof of a car) that is, at least locally, curved in two directions. Solar cells can bend in one direction (following a cylindrical surface), but to a much smaller degree in two directions. The invention solves this problem by subdividing the multitude of solar cells (100) in subgroups (302L, 302R, 304L, 304R, 306L, 306R, 308L, 308R), each subgroup associated with an area of the curved surface (202). By choosing these subgroups such, that almost no curvature occurs in one direction, the solar cells can be bend in the perpendicular direction. To optimize the efficiency further solar cells are used where anode and cathode are positioned at one side (the side opposite to the photosensitive side), enabling flexible foil to be used for the interconnection of the solar cells in a subgroup.

Claims

1. A method of producing a solar panel curved in two directions, the method comprising: providing a multitude of solar cells, the solar cells showing a photosensitive side and connections to anode and cathode on the side opposite to the photosensitive side; providing a curved surface, the curved surface transparent or translucent, the curved surface at least locally curved in two directions; providing two or more flexible foils of an insulating synthetic material with a conductive pattern thereon, the solar cells are grouped in subgroups, each subgroup associated with an area on the curved surface and one flexible foil; a solder step comprising, for each of the subgroups, soldering the solar cells of said subgroup to the flexible foil associated with said subgroup or bonding the solar cells of said subgroup to the flexible foil associated with said subgroup with a conductive adhesive; a positioning step comprising draping the two or more subgroups, each of the subgroups being soldered to or bonded to the flexible foil that is associated with said subgroup, on or in the curved surface; and a final lamination step comprising laminating the subgroups to the curved surface.

2. The method of claim 1, in which after the solder step and before the positioning step a first lamination step is inserted, the first lamination step comprising: for each and all subgroups laminating the solar cells and the associated flexible foil, the lamination performed at a temperature and a period that does not result in complete crosslinking; the positioning step comprising draping the laminated subgroups on or in the curved surface; and the final lamination step comprises laminating the first laminated subgroups to the curved surface, thereby completely crosslinking the lamination applied in the first lamination step.

3. The method of claim 1, in which after the solder step and before the positioning step a first lamination step is inserted, the first lamination step comprising: for each and all subgroups laminating the solar cells and the associated flexible foil, the lamination performed at a temperature and a period that results in complete crosslinking; the positioning step comprising draping the laminated subgroups on or in the curved surface; and the final lamination step comprises laminating the first laminated subgroups to the curved surface.

4. The method of claim 1, in which, during the final lamination step, between the subgroups and the curved surface, several layers of lamination material are placed and laminated thereto.

5. The method of claim 1, in which at least during performing the soldering step the two or more flexible foils are laying in one flat plane.

6. The method of claim 1, the method further comprising: before providing the multitude of solar cells estimating the mechanical stress occurring in the solar cells of each subgroup and the associated flexible foil resulting from the curvature of the curved surface at the location of the solar cells and, based on this estimate, resize the solar cells to such a size that the mechanical stress in each solar cell is less than the mechanical stress at which fracture occurs.

7. The method of claim 1, the method further comprising: before providing the multitude of solar cells estimating the mechanical stress occurring in the solar cells of each subgroup resulting from the curvature of the curved surface at the location of the solar cells and, based on this estimate, at least locally change the thickness of laminate material to such a value that the stresses occurring in the solar cells are below the stress where fracture of the solar cells occurs.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is now elucidated using figures, in which identical reference signs indicate corresponding features. To that end:

(2) FIG. 1A schematically shows a solar cell used in the invention, seen from the photosensitive side,

(3) FIG. 1B schematically shows a solar cell used in the invention, seen from the side opposite to the photosensitive side,

(4) FIG. 2 schematically shows one possible cut-through of a solar panel fabricated according to the invention.

(5) FIG. 3A schematically shows a planar view of solar panel, showing the areas forming each subgroup,

(6) FIG. 3B schematically shows a planar view of solar panel, showing the solar cells forming each subgroup,

DETAILED DESCRIPTION OF THE INVENTION

(7) FIG. 1A schematically shows a solar cell used in the invention, seen from the photosensitive side.

(8) FIG. 1A schematically shows a solar cell 100 used in the invention. The solar cell shows a photosensitive area 102 and an insulating edge portion 104. It is noted that this insulated portion (actually an insulating side wall) only occurs at so-called passivated cells. An advantage is that it enables the cells to be placed against each other. Cells without passivation on the sides must be separated to avoid shorts. Most commercially available cells are cut from a large wafer and do not show a passivation of the side walls, so they need to be spaced from each other to avoid electrical shorting.

(9) A typical thickness for a solar cell is in the order of 200 m, but for more flexible solar cells thinner cells, for example with a thickness of 150 m, are preferred.

(10) FIG. 1B schematically shows a solar cell used in the invention, seen from the side opposite to the photosensitive side.

(11) On an otherwise insulating surface 106 cutouts are provided that provide contact areas to anode 108-i and cathode 110-j.

(12) FIG. 2 schematically shows one possible cut-through of a solar panel fabricated according to the invention.

(13) A curved surface 202 in the form of, e.g., the glass roof of a car, is bonded with a cross-linked layer 204 of lamination material, for example EVA (Ethylene Vinyl Acetate) or a polyolefin to solar cells 102.sup.a. The photosensitive side is facing the glass.

(14) The flexible foil comprises a polyester or polyimide film 206 and copper tracks 208. The solar cell is soldered on a flexible foil, either by soldering the cells and the flexible foil with solder paste that is heated to, for example, 200 C. or more, or by using electrically conductive adhesive (typically a metal filled epoxy) that is cured at a temperature of, for example, less than 150 C.

(15) Solar cells 102.sup.b and 102.sup.c are here depicted as belonging to another subgroup (the flexible foilalthough not depicted for these cellsis not the same as the flexible foil of cell 102.sup.a.

(16) It is noted that between the flexible foil and the solar cells a further layer of material can be placed, either as a solder mask (or a mask for electrically conductive adhesive), or as an esthetic screen to obscure the copper tracks on the flexible foil, or for any other purpose. This need not be a transparent material. A lamination layer may be added on this layer (thereby completely encapsulating the flexible foil), but is not necessary.

(17) Also the flexible foil may comprise only one layer of insulating material as a carrier, such as polyimide or polyester, with a conductive pattern thereon, or it may comprise further layers with or without cut-outs to act as a solder mask, or for other purposes, the further layers either on the side of the solar cells or on the opposite side.

(18) The amount of lamination material (bonding material, encapsulant) between the solar cell may be the result of one lamination layer but may comprise several layer of lamination material.

(19) It is further noted that a bonding layer (a lamination layer) between curved surface (glass) and solar cells is necessary, but a lamination layer covering the flexible foil is not essential to the invention.

(20) FIG. 3A schematically shows a planar view of solar panel, showing the areas forming each subgroup.

(21) FIG. 3A shows a curved surface 300 that is divided in areas 302L, 302R, 304L, 304R, 306L, 306R and 308.

(22) The distribution of the curved surface is the result of an analysis of the maximum stresses occurring when flexible foil and solar cells are adhered (laminated) to the curved surface. These stresses can occur in any direction, but in many applications (such as a car roof) there is an axis of symmetry (here the x-axis) simplifying the problem. Empirically (or using another method, for example computer based) a division of areas is then found that result in acceptable stresses for both the solar cells and the foils.
It is noted that, although related, a stress problem in a solar cell need not result in a problem in the flexible foil, and vice versa: the size of a solar cell is often much more limited than the (maximum) size of a flexible foil. Therefore, at modest curvatures a problem may first occur in the flexible foil. However, at a large local curvature may result in a problem in the solar cells before a problem in the flexible foil occurs.

(23) FIG. 3B schematically shows a planar view of solar panel, showing the solar cells forming each subgroup.

(24) FIG. 3B can be thought to be derived from FIG. 3A. Here the solar cells are depicted that form the subgroups (borders of subgroups indicated by thicker lines). As can be seen also the orientation of the cells can be changed, even within a subgroup, as shown in for example group 304L.

(25) It is remarked that the definition of subgroup used here has no relation to the definition of group usually used in solar technology. The standard definition of group comprises a group of cells, typically being part of a string, and in state of the art panels each group is associated with an optimizer (a Maximum Power Point Tracker). The definition of subgroup according to the invention is used to denote cells that are attached to a (doubly) curved surface without exceeding predetermined stress levels. This implies that one subgroup may comprise more than one group, or one group may extend over more than one subgroup, or any combination thereof. It is thus possible that two subgroups comprise three groups or vice versa, while each group may be associated with its own optimizer.

(26) It is further remarked that typically the curved surface is a transparent or translucent curved surface, but it is also possible to use the method with a non-transparent curved surface (for example a metal curved surface) and that the photosensitive side of the solar cells are most removed from the curved surface. In that case the flexible foils need to be placed between the curved surface and the solar cells However, this is in most cases a less robust solution as the lamination material is softer and more prone to scratches and abrasion.

(27) It is also remarked that electrical connections between one flexible foil to another flexible foil, or to other PCB's or FBC's, can be made by soldering or bonding another flexible foil (preferably formed as a strip with several lines forming the electrical connections) on the first flexible foil. Also electrical connections via wires can be used.

(28) It is noted that, in the context of this invention, laminating may describe a total encapsulation, but may also describe bonding one part (for example the solar cells) to another (for example the curved surface) using a bonding or lamination material, such as of a polyolefin or one of its copolymers, such as EVA (ethylene vinyl acetate), or polyvinylbutyral (PVB). For example, after the final lamination step defined earlier, the flexible foil may or may not be covered with lamination material.

(29) An exemplary curing cycle is, for example, form a sandwich of a sheet of EVA, the solar cells, the flexible foil and another sheet of EVA layer in a vacuum oven that is heated to 140 C., evacuate for 3 minutes, while evacuated press on the sandwich with a pressure of approximately 1 atmosphere using a silicon membrane, cure in this condition for 17 minutes, remove pressure, ventilate for 30 seconds, open the oven and remove the sandwich.
This results in a fully cured (fully crosslinked) sandwich. A partial crosslinked sandwich is obtained by reducing the time for curing to 8 minutes.

(30) Laminating the before described sandwich to a curved surface, such as the glass roof of a solar car, is done by, for example: Place the glass roof, an uncured sheet of EVA and the previously made and cured sandwich comprising solar cells and flexible foil, in an evacuable bag, evacuate the bag (thereby applying a pressure of one atmosphere to the contents of the bag), heat the bag to a temperature of 100 C. during a time of 40 minutes, gradually heat the bag further to a temperature of 130 C. in a ramp-up time of 15 minutes, cure for 20 minutes at the temperature of 130 C. and cool down.
It is noted that experiments show no degradation of the already fully crosslinked sandwich.