GB2229736A

GB2229736A - Corrosion protection of module frame of photovoltaic generator

Info

Publication number: GB2229736A
Application number: GB9005079A
Authority: GB
Inventors: Vincent Coutrot; Chou Isidore Ah
Original assignee: AGENCE FSE MAITRISE EN; POLYNESIE FRANCAISE; Commissariat a lEnergie Atomique CEA
Current assignee: AGENCE FSE MAITRISE EN; POLYNESIE FRANCAISE; Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 1989-03-17
Filing date: 1990-03-07
Publication date: 1990-10-03
Anticipated expiration: 2010-03-07
Also published as: GB2229736B; GB9005079D0; NZ232873A; AU5144390A; FR2644633A1; FR2644633B1; AU633057B2

Abstract

A photovoltaic generator supplies load 6 with electrical energy, the photovoltaic generator comprising at least one module having a group of photovoltaic cells (18) connected in series, so that the positive terminal of the group is raised to a positive potential compared with the negative terminal thereof when the cells receive sunlight on one side, as well as two plates between which the group is located, the plate located on the side of the cells being transparent to light, whilst a metal frame (28) is provided for sealing the periphery of the two plates, and the frame is raised to a potential higher than, equal to or slightly lower than the potential to which is raised the positive terminal by connection to a point (A, B) of the electrical circuit to slowdown corrosion. Application is to supplying electrical energy to buildings. <IMAGE>

Description

Photovoltaic generator.

DESCRIPTION The present invention relates to a photovoltaic generator making it possible to directly transform sunlight into electrical energy. The invention more particularly applies to the electric power supply to buildings, water pumping stations, radio beacons, etc., preferably in very sunny areas.

Throughout the present text the term "connected" means "electrically connected".

A photovoltaic generator has at least one photovoltaic module.

It can have several of these connected in series and/or in parallel. Fig. 1 diagrammatically shows an example of a known photovoltaic generator 1 having a plurality of groups 2 of photovoltaic modules 4 connected in series, the actual groups being connected in parallel. Said generator is intended to supply electrical energy use means or loads 6 (e.g. domestic equipment and lamps). These use means 6 are mounted between the positive and negative terminals of the generator modules 4.

In known manner, means 8 for accumulating the electrical energy or power produced by the generator (e.g. a battery of accumulators) can be connected between the positive and negative terminals of the group of modules, the positive or negative terminal of means 8 being connected to the positive or negative terminal of the group of modules.

Moreover, the generator 1 can be provided with a regulating group 9 having a first regulating means 10 and a second regulating means 12, the positive terminal of the accumulation means 8 being connected to the positive terminal of the group of modules via the first means 10 and a terminal 14 of the use means via the second means 12. In known manner, the function of the first regulating means 10 is to interrupt the charge of the accumulating means 8 when the maximum charge of the latter is reached, whilst the function of the second regulating means 12 is to stop the discharge of the accumulation means 8 (i.e.

cut off the use), when the charge of said accumulation means 8 reaches its lower limit.

It should be noted that the first regulating means 10 is of the "series regulator" type. As a variant (fig. 1A), it is possible to use a first regulating means of the "parallel regul atop' type and which carries the reference 10a in fig. 1A and which has the same function as means 10 in fig. 1, but which is connected between the positive and negative terminals of the group of modules.

Finally, the generator 1 can be equipped with non-return diodes 6 in the same or a smaller number with respect to the groups 2 of modules. The anode of each diode 16 is connected to the positive terminal of the associated group 2, whilst all the cathodes of the diodes are connected to the same point, which then forms the positive terminal of the group of modules.

Each photovoltaic module 4 (figs. 2 and 3) can be of the type comprising a group of photovoltaic cells 18 connected in series via electrical connections 20, so that the group of cells has a negative terminal and a positive terminal, two plates 22,24 between which is placed the group of interconnected cells and means 26 for tightly sealing the periphery of the two plates, said sealing means having on the said periphery a metal frame 28.

Plates 22 and 24 are protective plates for protecting the cells and their interconnections both mechanically and against corrosion.

Each of the cells has a side to be exposed to sunlight and the cells are placed between the plates in such a way that all the sides are turned towards one of the plates carrying reference 22. This plate is transparent to sunlight and can be made from a transparent, electrically insulating material such as glass.

The other plate 24 can be made from an electrically insulating material such as glass or a metallic material.

In addition, the cells and their interconnections are encapsulated in a light-transparent, electrically insulating material 30, the thus encapsulated assembly being positioned between the plates.

The frame 28, which is e.g. made from aluminium, also permits the fixing of module 4 to a not shown support. A sealing joint or gasket 32 can be placed on the periphery of the group of plates. This joint 32 is then held in place by frame 28.

The positive and negative terminals of the group of cells pass out of the module 4, e.g. by the edge of the latter, through electrically insulating, tight passages 33 (fig. 2).

when the cells receive sunlight, the positive terminal of the group of cells is raised to a positive potential compared with the negative terminal thereof.

It has been found that certain modules of the type described with reference to figs. 2 and 3 have sealing defects. Thus, moisture, accompanied by salt in areas close to the ocean, penetrate between the plates 22 and 24. This can lead to corrosion of the cells and their interconnections.

It has been found that this corrosion increases with a rise in the difference between the potential of the cells or the interconnections thereof and the metal frame 28. An electrolysis phenomenon occurs, whose intensity is a rising function of the value of said potential difference and which leads to a migration of metal ions from the cells or their interconnections to the frame 28.

Thus, certain modules suffer from serious corrosion at their cells or interconnections and after a few years such corrosion can lead to the said modules ceasing to operate.

Thus, module 4 described relative to figs. 2 and 3 is of the type whose cells are made from a crystalline semiconductor material, such as crystalline silicon.

As a variant, each of the modules 4 of the generator shown in fig. 1 can have photovoltaic cells made from an amorphous semiconductor material, such as amorphous silicon. A module of this type is diagrammatically shown in section in fig. 3A and comprises amorphous silicon photovoltaic cells 18 mounted in series between two plates 22 and 24. Plate 22 is made from an electrically insulating material, which is transparent to solar radiation, e.g. glass or a transparent plastics material.

Plate 24 can be electrically conductive or electrically insulating and is e.g. made from glass, plastic or metal. The inner face of plate 22 is provided with transparent electrodes 20a respectively associated with the cells. These cells are produced from an amorphous silicon deposit on said inner face provided with electrodes 20a. For each cell, the face thereof facing the electrode 20a is exposed to solar radiation. The other face is coated with a metallic coating 20b which, as a result of a window made in the amorphous silicon, is in contact with the electrode 20a of the adjacent cell. Therefore the cells are interconnected.

As shown in fig. 3A, an electrically insulating encapsulating material 30 fills the space between plate 24 and the layers 20b and, as hereinbefore, a gasket or seal 32 is placed on the periphery of the two plates and is held in place by a metal frame 28.

The not shown positive and negative terminals of the group of cells are still located outside the module.

Such a module is also subject to deterioration by electrolysis.

Following moisture penetration between plates 22 and 24, the metallic layers 20b can oxidize and become detached, whilst the amorphous silicon can also oxidize.

The object of the present invention is to slow down the deterioration of modules of the type described relative to figs.

2,3 and 3A, by reducing to a maximum the electrolysis undergone by their photovoltaic cells or by the interconnections of the latter.

More specifically, the present invention relates to a photovoltaic generator having at least one photovoltaic module and which serves to supply with electrical energy use means or loads, each photovoltaic module having a group of series-connected photovoltaic cells, in such a way that the group of cells has a negative terminal and a positive terminal, which is raised to a positive potential compared with the negative terminal when the cells receive on one side sunlight, an electric circuit being provided between the positive terminal of the module and a terminal of the use means for supplying the latter with electrical power, two plates between which is located the group, the plate located on said side of the cells being transparent to sunlight and means for sealing the periphery of the two plates, said sealing means comprising a metal frame on said periphery, characterized in that the frame is connected to a point of the electric circuit and is raised to a potential higher than, equal to or slightly lower than the potential to which is raised the positive terminal of the group of cells.

The expression "potential slightly below the potential of the positive terminal" is understood to mean a potential at least equal to approximately 80% of the potential of the positive terminal. Thus, in the present invention, the metal frame is raised to a potential at least equal to approximately 80% of the potential to which is raised the positive terminal of the group of cells.

For example, for a module permitting its positive terminal to be raised to a nominal potential of + 24V with respect to its negative terminal, its frame is raised to a potential at least equal to approximately 19V compared with its negative terminal, in accordance with the present invention.

Thus, there is a reduction to the magnitude of the electrolysis phenomenon affecting cells and their interconnections. This electrolysis is even reversed when the frame is raised to a potential above that of the positive terminal of the module.

Thus, there is a migration of the metal ions from the frame to the group of cells and their interconnections, which is much less serious than the reverse phenomenon.

The connection between the frame and the point can incorporate an electrical resistor able to limit the intensity of an electric current flowing in said connection when the negative terminal of the group of cells is accidentally connected to the frame.

Thus, the connection of the frame to a point of said circuit can create confusion in the mind of the user accustomed to seeing the negative terminals connected to earth or ground, so that there is a risk of an accident (short-circuit, fire, etc.).

The use of the electrical resistor (preferably with a high value chosen as a function of the electrical installation of which the photovoltaic generator forms part) makes it possible to avoid such an accident or at least reduce the seriousness thereof.

In a constructional variant, the resistor is replaced by a fuse able to interrupt the connection between the frame and the point if the intensity of an electric current liable to flow in said connection exceeds a predetermined value.

In a particular embodiment of the invention, the connection between the frame and said point can incorporate a d.c. voltage supply able to raise the frame to a potential above the maximum of the potentials of the points of said circuit. Said point can be the positive terminal of the photovoltaic module.

As a variant, the generator according to the invention has also at least one non-return diode, whose anode is connected to the positive terminal of the photovoltaic module and which forms part of said circuit, said point being located in the portion of the circuit extending from the anode of the diode to said terminal of the use means. According to another variant, said point is said terminal of the use means.

when the generator according to the invention comprises means for accumulating the electrical energy produced, said circuit has a bypass to the positive terminal of the accumulation means and the point can belong to said bypass. The point can be the positive terminal of the accumulation means.

Finally, in the case where the generator according to the invention has at least one other photovoltaic module coupled to said module, the frame of each other module is connected to the frame of said module.

The invention is described in greater detail hereinafter relative to non-limitative embodiments and the attached drawings, wherein show: Figs. 1 and 1A diagrammatic views (partial in the case of fig.

1A) of known photovoltaic generators which have already been described.

Fig. 2 a diagrammatic view of a photovoltaic module forming part of these generators and already described.

Fig. 3 a partial, diagrammatic sectional view of said module and already described.

Fig. 3A a partial, diagrammatic sectional view of a variant of said module and already described.

Fig. 4 diagrammatically a generator with a series regulator according to the invention and having a photovoltaic module, as well as a variant of said generator.

Fig. 4A diagrammatically and partially a generator comparable to that of fig. 4, but with a parallel regulator.

Figs. 5 to 7 diagrammatically other variants of the generator.

Fig. 8 diagrammatically a generator according to the invention and having a plurality of photovoltaic modules.

Fig. 4 diagrammatically shows a photovoltaic generator according to the invention and simply having a photovoltaic module 4 of the type shown in figs. 2 and 3 or of the type shown in fig.

3A and which is therefore provided with a metal frame 28. The use means or loads 6 are connected between the positive and negative terminals of the module. As has already been stated, the generator can have electrical energy accumulation means 8, whose positive and negative terminals are respectively connected to the positive and negative terminals of module 4, a regulating group having a first regulating means 10 connected between the positive terminal of the module and the positive terminal of the accumulation means and a second regulating means 12 connected between the positive terminal of the accumulation means and a terminal 14 of the use means 6 and a non-return diode 16 connected between the positive terminal of the module and the first means 10 and whose anode is connected to the positive terminal of the module.

It should be noted that the first regulating means is of the series regulator type. Obviously, means 10 could be replaced by a first regulating means 10a (fig. 4A) of the parallel regulator type connected between the cathode of diode 16 and the negative terminal of the module.

Thus, the installation has on the one hand a circuit or positive branch extending from the positive terminal of the module to terminal 14 of the use means 6, with a bypass to the positive terminal of the accumulation means 8 and on the other hand a circuit or negative branch extending from the negative terminal of the module to the other terminal 34 of the use means 6 with a bypass to the negative terminal of the accumulation means 8.

According to the invention, an electrical connection is formed between frame 28 and a random point of the positive branch of the installation. This point can be located on the positive terminal of module 4 (point A - case I in fig. 4) or between diode 6 and terminal 14, or between diode 16 and the positive terminal of the accumulation means 8 (point B - case II in fig.

4), or at the positive terminal of the accumulation means 8 (point C - case III in fig. 5), or at the terminal 14 of the use means (point D - case IV of fig. 5).

For reasons indicated hereinbefore, the connection between frame 28 and point A or B or C or D can have, as shown in fig. 6, an electric resistor 36 or a fuse 37.

In another embodiment, said connection between frame 28 and point A or B or C or D can have a d.c. voltage supply 38 (fig.

7),whose output 40 is connected to the metal frame 28 and whose positive and negative supply terminals are respectively connected to the negative terminal of the module and to said point A,B,C or D. Supply 38 is such that its output 40 and therefore frame 28 is at a potential above the maximum of the potentials of the points of the positive branch of the installation (compared with the negative terminal of the module). In a purely indicative and non-limitative manner, the potential of output 4 is equal to twice that of the positive terminal of the module.

FIg. 8 diagrammatically shows a generator according to the invention and having a plurality of photovoltaic modules. It is e.g. the generator shown in fig. 1, or that shown in fig. 1A, but which is modified in the manner indicated hereinafter.

The corresponding installation also has a positive branch between the positive terminals of the modules 4 placed at one end of groups 2 and connected to the diode 16 to the positive terminal of the accumulation means 8 and to the terminal 14 of the use means 6, as well as a negative branch passing from the negative terminals of the modules 4 located at the other end of the groups 2 to the negative terminal of the accumulation means 8 and to the other terminal 34 of the use means 6.

This generator is modified in the following way. The frames 28 of all the modules (which can obviously be of the type shown in figs. 2 or 3 or the type shown in fig. 3A) are connected to one another and the electrical connection is formed between frame 28 of one 4a of the modules and a random point of the positive branch of the installation: point A (positive terminal of one of the modules, e.g. module 4a), or point B (point between one of the diodes 11 and terminal 14 of means 6 or the positive terminal of means 8), or point C (positive terminal of means 8), or point D (terminal 14).

As hereinbefore, said connection can have a resistor, or a fuse, or a d.c. voltage supply (with appropriate values). In the case of using a voltage supply, the output of the latter is at a potential above the maximum of the potentials of the points of the positive branch of the installation (compared with the negative terminal of the group of modules), e.g. a potential equal to twice the potential of the positive terminal of the group of modules. The positive and negative supply terminals of said voltage supply are respectively connected to the negative terminal of the group of modules and to said point A or B or C or D.

when a resistor is used, its value is based on a compromise.

It must be as low as possible in order to polarize all the metal frames of the modules (or the metal frame of the module when the generator only has a single module) and must be sufficiently high to ensure that it is not destroyed in the case of an accidental connection between the negative terminal of the generator and the metal frames of the modules (or the metal frame of the module when the generator only has one module). In an informative, but non-limitative manner, for a 24V installation, it is possible to use a resistor for an electric power of 1W of approximately 3 to 10 kOhm.

Claims

1. Photovoltaic generator having at least one photovoltaic module and which serves to supply with electrical energy use means (6) or loads, each photovoltaic module (4) having a group of series-connected photovoltaic cells (18), in such a way that the group of cells has a negative terminal and a positive terminal, which is raised to a positive potential compared with the negative terminal when the cells receive on one side sunlight, an electric circuit being provided between the positive terminal of the module (4) and a terminal (14) of the use means (6) for supplying the latter with electrical power, two plates (22,24) between which is located the group, the plate (22) located on said side of the cells being transparent to sunlight and means (26) for sealing the periphery of the two plates, said sealing means comprising a metal frame (28) on said peri phery, characterized in that the frame (28) is connected to a point (A,B,C,D) of the electric circuit and is raised to a potential higher than, equal to or slightly lower than the potential to which is raised the positive terminal of the group of cells.

2. Generator according to claim 1, characterized in that the connection between the frame (28) and the point (A,B,C,D) has an electrical resistor (36) able to limit the intensity of an electric current flowing in said connection when the negative terminal of the group of cells is accidentally connected to the frame.

3. Generator according to claim 1, characterized in that the connection between the frame (28) and point (A,B,C,D) has a fuse (37) able to interrupt the connection between the frame and the point if the intensity of an electric current liable to flow in said connection exceeds a predetermined value.

4. Generator according to claim 1, characterized in that the connection between the frame (28) and point (A,B,C,D) has a d.c. voltage supply (38) able to raise the frame (28) to a potential above the maximum of the potentials of the points of said circuit.

5. Generator according to any one of the claims 1 to 4, chara cterized in that said point is the positive terminal (A) of the photovoltaic module (4).

6. Generator according to any one of the claims 1 to 4, char acterized in that said generator also comprises at least one non-return diode (16), whose anode is connected to the positive terminal of the photovoltaic module (4) and which forms part of said circuit, said point (B) being located in the portion of said circuit extending from the anode of the diode to said terminal (14) of the use means (6).

7. Generator according to any one of the claims 1 to 4, chara cterized in that said point (D) is the terminal (14) of use means (6).

8. Generator according to any one of the claims 1 to 7, char acterized in that said generator comprises means (8) for accumulating the electrical energy produced, said circuit having a bypass to the positive terminal of the accumulation means, said point belonging to said bypass.

9. Generator according to claim 8, characterized in that said point (C) is the positive terminal of the accumulation means (8).

10. Generator according to any one of the claims 1 to 8, char acterized in that it incorporates at least one other photo voltaic module coupled to said module (4) and in that the frame of each other module is connected to the frame of said module.

11. Generator according to any one of the claims 1 to 10, char acterized in that the two plates (22,24) between which is located the assembly are made from glass.

12. A photovoltaic generator substantially as herein described with reference to figs 4 to 8 of the accompanying drawings.