WO2016156592A1 - Device and method for regenerating the performance of a photovoltaic module - Google Patents

Abstract

The invention relates to a device (300) for regenerating the performance of a photovoltaic module (100), including: a heating element equipped with an electrical resistor (310) and configured to heat photovoltaic cells belonging to the module, when the photovoltaic cells are exposed to solar radiation, using the electrical power generated by the photovoltaic cells; a switch (320) capable of switching between a first position, in which the photovoltaic cells are connected to the electrical resistor (310), and a second position, in which the photovoltaic cells are connected to an external electrical circuit (200); and, a circuit (330) for controlling the switch (320), configured to measure an amount of solar radiation received by at least one reference photovoltaic cell and to switch the switch to the second position when the amount of received solar radiation reaches a threshold value.

Description

 DEVICE AND METHOD FOR PERFORMANCE REGENERATION

 A PHOTOVOLTAIC MODULE

TECHNICAL AREA

The present invention relates to a device and a method for regenerating the performance of a photovoltaic module, for example after degradation under illumination of the conversion efficiency of the photovoltaic cells.

STATE OF THE ART

Photovoltaic cells which contain boron and oxygen atoms can suffer from degradation of the efficiency under illumination. This phenomenon, called "LID" (for "Light Induced Degradation" in English), occurs during the first times of exposure of cells to solar radiation. It is linked to the formation, during this exhibition, of complexes that associate a boron atom (B) and an oxygen atom (O). The boron-oxygen complexes act as recombination centers of the free charge carriers, and therefore decrease the lifetime of the carriers and the photovoltaic conversion efficiency of the cells (ratio of the electrical power output to the received light power). The degradation can reach 7 to 8% of the conversion efficiency before exposure. The p-type monocrystalline silicon substrates, doped with boron and crystallized according to the "Czochralsky" (Cz) technique, are particularly sensitive to the degradation of the efficiency under illumination because of the high concentrations of oxygen, the latter being mainly derived from the crucible. silica used for crystallization. However, this degradation has also been demonstrated in n-type Cz-type silicon substrates, compensated with boron, as well as in multicrystalline silicon substrates (when they contain boron).

The document WO2007 / 107351 aims to solve this problem of decreasing the yield of a photovoltaic cell, by adding a so-called stabilization step to the manufacturing process of the cell. This step consists in generating excess minority charge carriers in the silicon substrate, while heating the substrate at a temperature between 50 ° C and 230 ° C. The charge carriers are generated either by illuminating the substrate, for example by means of a halogen lamp, or by polarizing it by means of an external voltage source.

We then observe that the performance of the photovoltaic cell, and in particular its performance, degrade initially and then regenerate at their initial level. The cell then has stable performance under normal operating conditions. The cell is called "regenerated" since it has undergone, first, a degradation of its performance and then a regeneration.

The stabilization step can also take place at the module scale after it has been finalized, ie after the photovoltaic cells have been encapsulated and electrically interconnected, but before the module is delivered to the final consumer. For example, the module is energized to generate excess minority charge carriers and stored in a chamber heated at 140 ° C for approximately one hour.

Whether it is carried out before or after the encapsulation of the photovoltaic cells, the stabilization step of the document WO2007 / 107351 constitutes an additional step in the method of manufacturing a photovoltaic module. This therefore requires the provision of means, human and material, specifically for this manufacturing step, for example a halogen lamp, an oven or a room for storing and heating the photovoltaic modules. In addition, the manipulation of the modules to store them in the heated room leads to a risk of breakage of the modules. It follows that the solution proposed by the document WO2007 / 107351 for combating the reduction of the conversion efficiency of the cells is not economically viable. SUMMARY OF THE INVENTION

There is therefore a need to provide a device for regenerating the performance of a photovoltaic module, to perform a regeneration process at lower cost while avoiding additional manipulation of the modules.

According to the invention, this need is satisfied by providing in this regeneration device:

 a heating element configured to heat photovoltaic cells belonging to the module, when the photovoltaic cells are exposed to solar radiation, using the energy coming from the solar radiation;

 a switch capable of switching between a first position, in which the heating element is activated, and a second position, in which the heating element is deactivated; and

 a switch control circuit configured to measure a quantity of solar radiation received by at least one reference photovoltaic cell and to switch the switch to the second position when the quantity of solar radiation received reaches a threshold value.

By providing a heating element capable of raising the temperature of the photovoltaic cells of the module using solar energy, the performance of the module can be regenerated rapidly, not during its manufacture, but during its use. This avoids providing a full step in the manufacturing process of the module, and thus generate significant additional costs. There is no need to move and store the modules in a heated room specially designed for this purpose. The risk of breakage of the modules is therefore limited. The switch and its control circuit make it possible to control the energy charge produced by the module for this purpose of regeneration, this charge being directly proportional to the number of photons absorbed and converted. Thus, we monopolize the photovoltaic module only the time to obtain a satisfactory regeneration of its performance and solar energy can then be used exclusively for the production of electricity. Indeed, the control circuit deactivates the heating element when the amount of solar radiation received by at least one reference cell reaches a threshold value. This threshold value corresponds to a maximum amount of radiation allowing regeneration of the reference cell, that is to say an improvement of the yield up to an expected yield. The expected yield is preferably the initial yield of the cell at the end of its manufacture. The threshold value is determined beforehand by experimentation for each production line of photovoltaic cells, depending on the level of regeneration expected.

Rather than talking about "amount of radiation received", we can speak of "received solar energy" (radiation being defined as a transfer of energy). In other words, thanks to the regeneration device according to the invention, the performance regeneration takes place on the installation site of the modules, for example on the roof or in a photovoltaic solar power station, and autonomously. The exposure to solar radiation of a photovoltaic module equipped with the regeneration device makes it possible to create excess minority charge carriers in the cells, in addition to heating the cells. The regeneration device can therefore remedy the degradation under illumination photovoltaic conversion efficiency (phenomenon "LID") in photovoltaic cells containing boron and oxygen. However, unlike restoration methods of the prior art, no artificial light source is needed. Natural light from the sun advantageously replaces the halogen lamp used in the method of restoring WO2007 / 107351. In addition, it is now possible to regenerate the output several times in the life of the photovoltaic module, without having to uninstall the module to submit it to this source of artificial light.

The regeneration device described above finds other applications than the regeneration of the yield under illumination. It also makes it possible to fight against potential-induced degradation ("Potential Induced Degradation" in English, PID), a phenomenon related to the ionic migration of recombinant elements (for example sodium) through the encapsulation material of the cells. especially from the glass front to the cell surface. This migration is favored in the presence of moisture in the encapsulating material. The regeneration device is effective against the PID phenomenon because it reduces the humidity level in the module by heating it, in other words by "drying" it. In addition, the heating element comprises an electrical resistance. The switch is then configured to connect the photovoltaic cells of the module to the electrical resistance in the first position, so that the electrical resistance is supplied by the photovoltaic cells when exposed to solar radiation, and to connect the photovoltaic cells of the module. to an external electrical circuit in the second position.

Electrical resistance is a simple and economical way to heat the photovoltaic cells of the module. In addition, since this resistor is supplied with electrical energy directly by the module, it is not necessary to provide an additional source of energy. The regeneration device according to the invention is therefore autonomous and easy to use.

According to a development of the regeneration device, the heating element further comprises a thermal insulation box provided with at least one ventilation flap. The switch is then configured to close the venting flap in the first position and open the venting flap in the second position.

The so-called reference photovoltaic cell, by which the amount of solar radiation received can be measured, can be one of the cells of the photovoltaic module, or an additional photovoltaic cell forming part of the regeneration device and subjected to the same solar radiation as the cells. of the module. It is also possible to use several photovoltaic cells of the module as reference cells, or even all the photovoltaic cells of the module. The device according to the invention may also have one or more of the following characteristics, considered individually or in any technically possible combination:

 the control circuit comprises:

 a sensor configured to measure the current supplied by said at least one photovoltaic cell exposed to solar radiation;

 a current-frequency converter configured to convert the current supplied by said at least one photovoltaic cell into a pulse train of variable frequency; and

 a pulse counter, receiving as input the pulse train coming from the converter and configured to switch the switch in the second position when the number of pulses counted reaches a number of reference pulses corresponding to the threshold value of the amount of solar radiation;

 the pulse counter of the control circuit is provided with a reset input; and

 the regeneration device comprises a device for limiting the temperature of the photovoltaic cells.

Another aspect of the invention relates to a photovoltaic module comprising photovoltaic cells and a regeneration device according to the invention.

When the photovoltaic cells of the module are coated with an encapsulating material and arranged between a rear protection plate and a front protection plate, exposed to solar radiation, the electrical resistance can be coated with the encapsulating material and arranged next to photovoltaic cells, advantageously between the photovoltaic cells and the rear protection plate.

Alternatively, the electrical resistance can be contained in the rear protection plate or cover the rear face of the module. The rear face of the module is opposite to the front face, which is exposed to solar radiation The invention also aims at a process of regeneration of the performances, which is easy to implement and economically viable. This process comprises the following steps:

 - exposing photovoltaic cells to solar radiation;

 - Heating the photovoltaic cells during exposure of photovoltaic cells to solar radiation, by means of an electrical resistance supplied with electrical energy by the photovoltaic cells;

measuring a quantity of solar radiation received by at least one reference photovoltaic cell; and

 - Disconnect the electrical resistance of the photovoltaic cells and connect the photovoltaic cells to an external electrical circuit when the amount of solar radiation received reaches a threshold value.

BRIEF DESCRIPTION OF THE FIGURES

Other features and advantages of the invention will emerge clearly from the description which is given below, as an indication and in no way limiting, with reference to the appended figures, among which:

- Figure 1 is an abacus giving the amount of energy required to regenerate the performance of a photovoltaic cell operating at its maximum operating point, depending on the temperature and irradiance of the cell;

 - Figure 2 is an abacus giving the amount of energy required to regenerate the performance of a photovoltaic cell in open circuit, depending on the temperature and irradiance of the cell;

 FIG. 3 diagrammatically represents a first embodiment of a regeneration device according to the invention, coupled to a photovoltaic module;

FIG. 4 schematically represents a second embodiment of a regeneration device according to the invention, coupled to a photovoltaic module; - Figure 5 is a cross-sectional view of a photovoltaic module in which is integrated a heating element;

 FIG. 6 represents current-voltage characteristics of a commercial photovoltaic module, for several irradiance values; and

 - Figure 7 shows a passive heating element coupled to a photovoltaic module, according to a third embodiment of the regeneration device according to the invention.

DETAILED DESCRIPTION OF AT LEAST ONE EMBODIMENT

To achieve a complete regeneration of the performance of a photovoltaic module, a certain amount of energy must be provided to the photovoltaic cells of the module. A study conducted by the inventors of the present application has shown that this amount depends on many parameters, in particular the operating temperature and the irradiance of the cells. Irradiance, also called energy irradiance, refers to the amount of power, or energy flux, received per unit area. It is expressed in W / m ² .

FIG. 1 is an example of an abacus representing the energy E necessary for a complete regeneration of the performances of a photovoltaic cell, as a function of the operating temperature and the average irradiance of this cell. For each photovoltaic cell technology, this abacus can be obtained by experimentation, on the one hand by measuring the electrical power generated by the cell, when it is subjected to given levels of irradiance and temperature, and on the other hand following the evolution of the conversion efficiency of the cell. The energy value E for this irradiance / temperature pair is the energy absorbed by the cell until the efficiency returns to its initial value (ie immediately after manufacture). The photovoltaic cell corresponding to the abacus of FIG. 1 is a monocrystalline silicon cell with a homojunction whose surface is equal to 1 cm ² . It comprises a monocrystalline siliconized silicon substrate (with a resistivity of between 0.5 and 100 Ω.cm), a passivation layer (eg SiN) on the front face of the substrate, and a aluminum plate producing a repulsive field on the back of the substrate (BSF, "Back Surface Field").

On the chart of Figure 1, the cell operates at its maximum power, commonly called "MPP" (for "Maximum Power Point"). It is found that the energy E is significant, but decreases as the operating temperature of the cell increases. Performance regeneration is therefore a thermally activated process, i.e. that occurs faster at high temperatures. The influence of irradiance on energy E is low, compared to that of temperature.

The study was also able to show that the energy needed for a complete regeneration of the cell's performance depended on the concentration of minority charge carriers in the silicon substrate (which explains why the energy E depends on the level of irradiance - the photon radiation received by the cell creates electron-hole pairs). However, this concentration of minority charge carriers, sometimes called "injection level", is strongly influenced by the current operating point of the cell. In FIG. 2, the same type of energy abacus is shown for comparison, but when the photovoltaic cell is in open circuit. Under these conditions, the cell produces no current and the voltage at its terminals is maximum (open circuit voltage). The energy E always depends as much on the irradiance. On the other hand, the energy E decreases much faster by increasing the temperature, compared to the case where the module is at its maximum power (Fig.1). It is therefore preferable, to regenerate the performance of a photovoltaic module, to bias this module to a voltage close to the open circuit voltage, hereinafter noted Uoc. Based on the results of this study, the inventors have developed a regeneration device operating with solar energy, for heating the photovoltaic cells of a module to a target temperature and to stop the heating when the amount of radiation received by one or more cells reference photovoltaics reaches a threshold value corresponding to the target temperature. This threshold value is preferably a solar energy value received per unit of active surface (ie area of the reference cells) and is therefore expressed in Wh / m ² . It is advantageously calculated from an abacus of the energy E (the energy required to obtain a complete regeneration of the performances of a cell) to the operating voltage of the reference cell: for example that of FIG. voltage is equal to the open circuit voltage or another abacus of the same type when the voltage is lower than the open circuit voltage. This other abacus can also be derived from that of Figures 1 and 2 (for example by assuming that the variation between the two operating points is linear). For example, when using a single reference cell, the threshold value is equal to the energy value E obtained on the chart for a point of ordinate equal to the target temperature and of abscissa equal to the irradiance (solar) average which is subjected to the reference cell, divided by the surface of the cell used to build the abacus (here 1 cm ² ). The cell used to construct the abacus is preferably identical to the reference cell. Average irradiance depends on weather conditions during regeneration of performance. The target temperature is preferably greater than 70 ° C. FIG. 3 represents a first embodiment of this regeneration device, coupled to a photovoltaic module 100. By module, is meant a set of photovoltaic cells interconnected electrically and encapsulated in order to protect them from the environment, in particular from the oxygen and moisture. The photovoltaic module 100 is itself coupled to an external electrical circuit 200, for example that of a photovoltaic installation. Other photovoltaic modules can be connected to the electrical circuit 200, thus forming a chain of photovoltaic modules. At the end of the chain, there is usually an inverter capable of transforming the direct current of the photovoltaic modules into an alternating current.

The regeneration device 300 comprises a heating element 310 for heating the photovoltaic cells of the module 100, a switch 320 for activating and deactivating the heating element 310, and a circuit for 330 command of the switch 320.

The heating element 310 comprises, in this embodiment, a heating electric heating element powered by the photovoltaic cells of the module 100, when the switch 320 electrically connects the module to the resistor. Indeed, the switch 320 can switch, on the order of the control circuit 330, between a first position (see Fig.3; position (1)) in which the module 100 is connected to the heating resistor, and a second position ( see Fig.3; position (2)), in which the module is disconnected from the heating resistor and connected to the electrical circuit 200. In other words, the heating element 310 is active when the switch 320 is in the first position and inactive when the switch is in the second position. On the other hand, in the second position, the photovoltaic module 100 supplies power to the electrical circuit 200 of the photovoltaic installation, which of course corresponds to a conventional use of the module.

The control circuit 330 decides from when the heating resistor 310 can be disconnected from the photovoltaic module 100, by switching the switch 320 from the first position to the second position, that is to say when to terminate the process regeneration of module performance.

As illustrated in FIG. 3, the control circuit 330 preferably comprises a current sensor 332 making it possible to measure the electric current I produced by the module 100 when it is exposed to solar radiation and that it supplies power to the power supply. heating resistor 310 (switch 320 in position (1)). The current sensor 332 is for example formed of a measurement shunt connected in series with the heating resistor 310.

The circuit 330 further comprises a current-frequency converter 333 connected to the terminals of the current sensor 332 and a pulse counter 334, one of whose inputs is connected to the output of the converter 333.

In particular, the converter 333 captures the voltage V at the terminals of the sensor of current 332 and output a pulse train 335 of variable frequency F. This pulse train 335 constitutes a periodic signal whose frequency (or the period, noted 1 / F in Figure 3) depends on the voltage V. The pulses are for example of rectangular shape.

Since the voltage V across the sensor 332 is an image of the current I produced by the module 100 at a given instant t, the frequency F of the pulses is representative of the level of irradiance of the module at this time. Thus, by integrating through the pulse counter 334 the level of irradiance over time (ie from the moment when the switch 320 is in position (1) until time t), a value of the solar energy received per unit of active area. The active surface is the surface of the reference cells used to measure the level of solar irradiance, ie the surface of all the cells of the module in this embodiment. In other words, the number N of pulses recorded by the counter 334 is equivalent to a value of solar energy per unit area, or amount of solar radiation, received by the photovoltaic module 100.

The pulse counter 334 is further configured to compare the number of pulses N to a number of reference pulses NREF. Thus, in this embodiment of the control circuit 330, the number of reference pulses NREF corresponds to the threshold value of solar energy (per unit area) required to regenerate (at least in part) the performance of the module. The threshold value of solar energy has previously been described in connection with FIG. 2, in the case of a complete regeneration of the performances (for example, energy E divided by the surface of the cell).

When the number of pulses N of the counter 334 reaches the value NREF, it means that the module has received enough solar energy to recover an acceptable level of performance and that the regeneration process can be stopped. The counter 334 then controls the switching of the switch in the position (2).

In a variant of the control circuit not shown in the figures, it is possible to consider only one cell of the module to measure the amount of solar radiation received, rather than using all of its cells. In this case, another reference value NREF can be chosen or, if the same reference value is retained, the value of the measurement shunt is adapted.

The solar radiation threshold value (from which NREF derives) depends in fact on the current generated by the reference cell (s). With a single reference cell, it is adjusted according to the characteristics of this cell, and in particular its photon efficiency (the photon yield allows, from the irradiance, to go back to the generated current). When using several reference cells connected electrically in series, which is generally the case in a photovoltaic module, the characteristics of the cell that generates the least current to determine the threshold value will be taken into account. When using several reference cells connected electrically in parallel, the currents add up. In this case, to be representative of the regeneration, the threshold value can be adapted to the number of cells connected in parallel (or to the number of parallel connected cell chains), for example by multiplying the threshold value determined for a cell by the number of cells connected in parallel. It is therefore the characteristics of all these cells that will be taken into account. More generally, in order to set the threshold value, all the cells which will contribute to the current used to measure the quantity of radiation received will be pre-characterized.

The pulse counter 334 is preferably equipped with a reset input RST allowing the number of N pulses to be reset. This reset can be controlled manually, for example during the maintenance of the module. , or automatically, for example after a certain period of exposure to solar radiation. Resetting the pulse counter 334 makes it possible to carry out several regeneration processes during the lifetime of the photovoltaic module. This is particularly advantageous for combating LID and PID phenomena, since these phenomena may reappear after healing of the module. FIG. 4 shows a second embodiment 400 of the regeneration device, and in particular another configuration of the control circuit of the switch. The control circuit 430 of FIG. 4 differs from the control circuit 330 of FIG. 3 in that a reference photovoltaic cell 431, distinct from the cells belonging to the photovoltaic module 100, is used to measure the level of irradiance of the module. . It is preferably identical to the photovoltaic cells of the module 100. Outside this control circuit 430, the regeneration device 400 is identical to the regeneration device 300 of FIG. 3. In FIG. 4, the heating resistor and the switch have the same references (respectively 310 and 320) as in FIG.

The reference photovoltaic cell 431 is disposed near the cells of the module 100, so that it is subjected to the same solar radiation, and therefore the same level of irradiance as the module. A current sensor 432, similar or identical to the current sensor 332 of FIG. 3, measures the electric current produced by the reference cell 431. Preferably, the current sensor 432 is a measurement shunt connected in series with the reference cell 431. The voltage V across the shunt is then proportional to the current of the cell 431. Like the control circuit 330 (FIG. 3), the control circuit 430 of FIG. 4 comprises a current-frequency converter 433 and a pulse counter 434. The converter 433 and the pulse counter 434 operate in the same way as the converter 332 and the counter 333 of FIG. 3. Since the reference cell 431 shows the same level of irradiance as the photovoltaic module 100, the received energy per unit area of the cell 431 is equal the energy received per unit area of the module. The reference cell 431 can therefore be validly used to estimate the amount of radiation received by the module.

This embodiment of the regeneration device is particularly advantageous because it makes it easier to deport the control electronics of the switch 320. In addition, in this embodiment, the circuit for control 430 can be shared between several photovoltaic modules 100, each being equipped with a heating resistor 310 and a switch 320. The switches 320 are then controlled simultaneously by this single control circuit 430.

As soon as the switch 320 of the control device switches to the first position (Figs.3-4), the temperature of the photovoltaic cells increases gradually. Starting from the ambient temperature, that is to say the temperature outside the module (for example 25 ° C), it increases until reaching an equilibrium temperature, which depends on the meteorological conditions, in particular the temperature. level of sunshine and wind speed. If this equilibrium temperature is lower than the target temperature at which it is desired to regenerate the performance of the module, it means that the electrical energy supplied by the module to the heating resistor 310 is not sufficient to heat the cells to the temperature. target. This scenario is likely to arise for photovoltaic modules of small surface area (and therefore low power), where the thermal dissipation by edge effects is preponderant (due to a higher ratio perimeter / area). Rather than reconsidering the target temperature value, the heating resistor 310 of the regeneration device equipping this module may be powered by other photovoltaic modules, an external electrical generator or the electrical network to which the photovoltaic installation is connected.

In a chain of photovoltaic modules where each module is equipped with a regeneration device, the regeneration of the performances can take place on each module, one after the other, by using the photovoltaic energy produced by the whole of the module. chain. A single switch is then in the first position at each instant of the process of regeneration of the photovoltaic module chain. When weather conditions are favorable, or when the electrical energy supplied to the heating resistor is larger than necessary, the equilibrium temperature may exceed the target temperature. To avoid degradation of the module (in particular of the cell encapsulation material photovoltaic) under the effect of too high a temperature, the regeneration device advantageously comprises a device for limiting the temperature. This limitation device is preferably set to the target temperature. It is for example formed of a bimetallic thermal protector wired in series with the heating resistor 310. Bonded on the back of the module or integrated with the laminate which composes the module, this component opens the electrical circuit of the heating resistor 310 when it reaches a predefined temperature threshold, for example 120 ° C (temperature from which ethylene-vinyl acetate (EVA), usual used as encapsulation material, begins to degrade) and closes the circuit again after that the temperature has decreased below the threshold.

Alternatively, the heating resistor and the temperature control device may form a single component, for example a self-regulating cable, i.e. a cable whose electrical resistance increases with temperature. This type of cable comprises an insulating matrix in which electrically conductive particles are dispersed. At low temperatures, the particles are sufficiently close to each other to form conducting paths, which produce heat by the Joule effect. As the temperature of the cable increases, the particles disperse, thus reducing the number of conductive paths. In other words, the cable self-regulates in temperature. A self-regulating polymer sheet operates on the same principle as a self-regulating cable and can therefore be used to simultaneously form the heating resistor and the regulating device.

The heating resistor 310 of the regeneration device of FIGS. 3 and 4 may consist of a metal foil (for example of aluminum), a fabric comprising textile and metal fibers (for example the fabrics marketed under the trademark "Devifoil" by the company "Danfoss Electric Heating Systems") or a heating blanket (for example that marketed by the company "INSULFLEX"). It is then pressed against the rear face of the module. The heating resistor 310 may also be formed of a conductive polymeric coating (for example polyethylene dioxythiofen, PEDT) deposited on the rear face of the module, for example by immersing the module in an aqueous dispersion. It covers, preferably, the entire back of the module. By thus transferring the heating element to the rear face of the module, the regeneration device can equip existing photovoltaic modules. However, in an alternative embodiment, the heating element is integrated in the photovoltaic module during its manufacture.

FIG. 5 schematically represents a photovoltaic module 500 comprising a batch of interconnected photovoltaic cells 510 and a heating element 310, each being encapsulated with encapsulation material 520. The encapsulation material 520 is transparent to solar radiation and aims at protecting the cells 510 photovoltaic oxygen and moisture. The photovoltaic cells 510 (surrounded by the encapsulating material 520) are arranged facing the heating element 310 between a front plate 530 made of a transparent material, for example glass, and a back plate 540 (or "backsheet" in English). ). The front plate 530, subjected to solar radiation, protects the photovoltaic cells from bad weather (rain, hail ...), while the back plate plays the role of mechanical support. In addition, the backplate 540 contributes to the protection of the cells against oxygen and moisture, as well as the encapsulation material 520 and the front plate 530. It can be made of glass or formed of a laminate of several layers, in particular polymers, for example of TPT type (PVF / PET / PVF, PVDF / PET / PVDF) or TPE (PVF / PET / EVA).

In order not to reduce the amount of photons absorbed by the photovoltaic cells, the heating element 310 can be placed between the photovoltaic cells 510 and the rear plate 540. Preferably, the heating element 310 is arranged parallel to the photovoltaic cells 510 and its dimensions are substantially identical to those of the set of photovoltaic cells 510, so that the heating effect on the cells is homogeneous. When integrated in the module, the heating element 310 is preferably formed of an electrically resistive ribbon comprising a multitude of metal wires, intermingled, woven or otherwise arranged parallel to each other.

The photovoltaic module 500 advantageously comprises an insulation sheet electrical 550, for example polyethylene terephthalate (PET), disposed between the photovoltaic cells 510 and the heating element 310. This sheet 550 reinforces the electrical insulation produced by the encapsulation material 520, for example ethylene vinyl acetate (EVA), between the photovoltaic cells 510 and the heating element 310. It is particularly useful when the thickness of the encapsulation material 520 between the cells 510 and the heating element 310 (thickness D2 on Figure 5) is weak.

Rather than placing the heating element 310 in the encapsulating material 520 of the photovoltaic cells, the heating element can be inserted within the back plate 540 of the module 500, when it consists of a laminate of several layers. . The heating element may be in the form of a ribbon of electrical wires, as previously described, or consist of a metal foil, for example aluminum (Al). Some commercially available backplates already include aluminum foil to prevent water vapor from entering the module, such as TAT backplanes (PVF / AI / EVA), PAP (PEN / AI / PET) or TPAT (PVF / PET / AI / PVF). It is therefore sufficient to apply tension on this aluminum foil to generate heat by Joule effect.

Assuming that all of the energy dissipated by the heater 310 serves to raise the temperature at the cells 510, i.e. neglecting heat losses between the encapsulating material 520 and the outside of the module 500, the thermal losses between the encapsulation material 520 and the backplate 540, as well as the influence of the insulation sheet 550, the energy Q that the heating element 310 must generate to reach a variation of temperature ΔΤ is written:

Q = p _e XC _p x V _e X AT

where V _e is the volume of the encapsulation material 520, p _e its density and C _p its specific heat capacity.

The volume V _e of the encapsulation material 520 is equal to: V _e = (l + D2 + D3) XL * l

where L is the length of the module, / is its width, and where D1, D2 and D3 are the thicknesses of the encapsulating material 520, respectively between the backplate 540 and the electrical insulation sheet 550, between the sheet of electrical insulation 550 and the photovoltaic cells 510, and between the photovoltaic cells 510 and the front plate 530. (see FIG.

The dimensions of the photovoltaic module are for example the following: L = 150 cm, I = 100 cm and D1 = D2 = D3 = 0.04 cm; an encapsulant volume equal to:

V _e = 0.12 X 15000 = 1800 cm ³

The most common encapsulation material is EVA with the following characteristics:

p _e = 0.98 g / cm ³ ; and

C _P = 2.5 J / g / K.

Thus, to generate an increase of 50 ° C in the temperature of the photovoltaic cells 510 in the module shown diagrammatically in FIG. 5, the heating element 310 must release a thermal energy Q equal to:

Q = 0.98 X 2.5 X 1800 X 50 = 220 kd = 61.2 Wh

Depending on the electrical power provided by the photovoltaic module, the rise in temperature will be more or less quickly. Indeed, the power supplied by the module is the same as that dissipated by the heating resistor, because the heating resistor 310 is connected in parallel with the module 100 (see Figs.3-4 - the possible measurement shunt 332 connected in series with heating resistor 310 being of negligible value). For example, if the power of the module is 120 W, more than half an hour will be needed to go from an ambient temperature of 25 ° C to the target temperature of 75 ° C. Preferably, the energy supplied by the module is counted (via the pulse counter) as energy useful for the regeneration process, after expiration of this heating time. The electrical power developed by a photovoltaic module depends on its intrinsic characteristics and the level of irradiance, but also on the way it is polarized.

As indicated above, it is preferable for the module to be biased to a voltage as close as possible to the open circuit voltage Uoc, so that the energy required for the regeneration is contained. However, the polarization of the photovoltaic module 100 is effected by means of the heating resistor 310 in the embodiments of FIGS. 3 and 4.

FIG. 6 represents, by way of example, current-voltage characteristics (IV) of a commercial photovoltaic module (module marketed by the company "Canadian Solar" under the reference "Quartech CS6P-255P"), for different values of Irradiance:

C1 curve: 400 W / m ²

- curve C2: 600 W / m ²

curve C3: 800 W / m ² ; and

curve C4: 1000 W / m ² .

 To these characteristics l-V, two lines corresponding to two values of the heating resistor were superimposed: R1 = 4.5 Ω and R2 = 12 Ω.

Considering any of the characteristics IV of the module (the observation is true for all the curves C1 to C4), for example the curve C1, it is found that the higher the resistance (R2> R1), the higher the operating voltage U of the module (ie the abscissa of the intersection bridge A1 / A2 between the line R1 / R2 and the curve C1) is close to the open circuit voltage (Uoc = 36 V for an irradiance of 400 W / m ² ). By cons, the current I from the module (ordinate) weakens when increasing the resistance value. However, the module must be able to produce enough current to supply the heating resistor 310 and reach the target temperature. The polarization of the module by the heating resistor therefore results from a compromise between the voltage (the highest possible to minimize the energy required for regeneration) and the current (strong enough to reduce the heating time and / or maintain the temperature ). The point of intersection between the line of the heating resistor 310 (R1 or R2) and the selected characteristic IV (here C1) of the module corresponds to the following equations:

 U

 '= Έ

 I = / ({/) where f is a function describing the selected l-V characteristic. Determining the optimum resistance value, that is to say the one offering the best compromise between regeneration and heating, therefore consists in solving these equations, while also giving as input parameter a given value of heating gradient, by example 1 ° C / minute. This last criterion, in relation to the mass capacity of the encapsulant, provides an additional condition on the power of the module, and therefore on the current I and on the voltage U. If there are several solutions to this system of equations, one will choose the one whose operating voltage U is the highest.

In the module example above, a resistance of about 12 Ω at 25 ° C (R2 right) responds to the different equations and is therefore the best compromise between voltage and current (given the targeted heating gradient).

To obtain such a resistance value, electrical wire or conductive tape may be used. The term "ribbon" refers to a continuous, monoblock conductive strip (whose cross section is rectangular) or a sheet composed of several wires (whose section is a disc) that can be isolated from each other. Two examples of 12 Ω electrical heating resistance formed of a ribbon are given below. The first example of a heating resistor consists of a ribbon comprising 100 copper wires alloyed with zinc (CuZns) (electrical resistivity: pc × nns = 1.80 × ¹⁰ -8 μm) .The wires each have a length of 150 cm (ie a total length of wire 150 m), a disc section of approximately 0.0019 cm ² and are spaced from each other by a distance equal to 0.9 cm. The wires are distributed over the entire surface of the module - which measures 164.5 cm long by 98.6 cm wide (measures taken at the glass backplate) - and oriented lengthwise. At the temperature of 25 ° C, the corresponding values of voltage U and current I, for an irradiance of 1000 W / m ² and 400 W / m ² , are given in Table 1 below. They come from the resolution of the equations above. They are also visible on the characteristics IV of FIG.

Table 1 Under the effect of temperature, the resistance value R (initially determined for a temperature of 25 ° C) increases, because the total length of wire increases (by 14 cm in the present case) (coefficient of expansion of the copper-zinc alloy: Acuzns = 1, 8.10 ⁵ / ° C). The variation of resistance is accompanied by a modification of the voltage U at the terminals of the module and the current I coming from the module. The new values of the resistance R, the voltage U and the current I at the target temperature of 75 ° C. were calculated from the following equations:

U? 5 ° C =? 25 ° C * (1 +? AT)

R _7SOC = R _{2 SO} * (i + τΔΓ)

 , _ ^ 75 °

 z ° c- n

 75 ° C

where U75 ° C, 175 ° C and R75 ° C are respectively the values of voltage, current and resistance at 75 ° C, U25 ° C and R25 ° C are respectively the values of voltage and resistance at 25 ° C (see Table 1), a is the temperature coefficient in voltage (a = -0.34% / ° C for module CS6P-255P), τ is the thermal coefficient (τ = 3.93E-03 / ° K, this coefficient indicates the dependence the resistivity of the material as a function of temperature) and ΔΤ is the temperature gradient (here, 50 ° C).

They are listed in Table 2 below: Irradiance (W / m ² ) Resistance R (Ω) Voltage U (V) Current I (A)

1000 14.3 28.7 2

 400 14.3 25.8 1, 8

 Table 2

The second example of a heating resistor (P "25 ° c = 12 Ω) is formed of two 24 Ω resistors connected in parallel. Each 24 Ω resistor consists of a ribbon comprising 15 wires of a nickel-chromium alloy (Ni80 / Cr20) (electrical resistivity: pisncr = 1, 08.10 ^"6 μm) .The wires each have a length of 90 cm (one total length of wire 27 m), a section of approximately 0.0061 cm ² and spaced from each other by a distance of 5 cm The wires are distributed over the entire surface of the module and oriented in the direction At 25 ° C, the values of voltage U and current I are the same as those of Table 1 above, but at 75 ° C the new values of resistance, voltage and current, shown in Table 3 below, differ from the first example: the nickel-chromium alloy expands less than copper under the influence of temperature (coefficient of expansion of Ni80 / Cr20 = 1, 4.10 - ⁵ / ° C).

 Table 3

The regeneration device has heretofore been described in connection with an active heating element, ie which needs to be supplied with energy (in this case photovoltaic) to produce heat, and more particularly a resistive type heating element. However, other types of heating element can be envisaged, for example infrared radiative heating which also requires being powered by a source of electrical energy. It is also possible to use the rear face of the module a reservoir comprising chemicals which, by reacting between them, give off heat. For example, the "Crosse &Blackwell" food warming system is based on the reaction of magnesium and salt water. FIG. 7 represents a third embodiment 700 of a device for regenerating the performance of a module 100, in which the heating element 710 is of the confinement type. The electrical resistance is replaced by a thermal insulation box pressed against the back of the module.

This heating element can be qualified as passive in the sense that it prevents the heat generated by the photovoltaic module from escaping from the module by natural convection (without ventilation) or forced (with ventilation) on the rear face of the module. On the contrary, an "active" heating element such as electrical resistance generates additional heat, but does not act on the heat dissipation of the module.

In this embodiment, the increase in temperature is therefore solely due to the fact that at least a portion of the energy of the photons absorbed by the semiconductor material is converted into thermal energy (referred to as "thermalization of photons The other part being converted into electrical energy by photovoltaic effect. When an absorbed photon does not have enough energy to generate an electron-hole pair (following the forbidden band of the material), all of its energy is dissipated in the form of heat.

The isolation box 710 comprises at least one ventilation flap 71 1, and advantageously several ventilation flaps 71 1 to create a stream of air at the back of the module. The shutters 71 1 are for example 3 in number in Figure 7. In the closed position, the flaps 71 1 occupy openings in the wall of the box 710. Each flap 71 1 is controlled opening or closing by an actuator 720 for example a rotating electromagnet. When the flaps 71 1 are closed, the air can no longer circulate on the back of the module 100 and the temperature of the module increases until it reaches or exceeds the target temperature. Because performance regeneration is thermally activated, it occurs much faster at high temperatures. On the contrary, when the flaps 71 1 are open, the module 100 is ventilated. Its temperature reaches a much lower equilibrium threshold than that reached when the shutters 71 1 are closed. In these conditions, the photovoltaic module can be used effectively for power generation (the electrical power of the module tends to decrease with temperature).

The actuators 720 are equivalent to the switches 320 of Figures 3 and 4, because in a first position (called "closed"), the heating element 710 is activated (it heats the photovoltaic cells), and in a second position ("open" ), the heating element 710 is deactivated (the rear face of the module is ventilated).

The actuators 720 can be controlled by one of the control circuits 330 and 430 described with reference to FIGS. 3 and 4. In other words, the flaps 71 1 open as soon as the module 100 (FIG. or the additional photovoltaic cell 331 (FIG. 4) has received sufficient solar energy.

This embodiment of the heating element can be combined with that of FIGS. 3 and 4 to reach the target temperature more quickly or to reach a higher target temperature. The heating element then comprises an electrical resistance, preferably integrated in the module (for example in the encapsulant or in the back plate), and a thermal insulation box. The regeneration device then comprises two switches, one for the opening / closing of the electrical circuit connecting the resistance to the module, the other for the opening / closing of the thermal insulation box.

With the regeneration device 700 of FIG. 7, the photovoltaic cells of the module do not need to be polarized at a voltage strictly lower than the open circuit voltage. Indeed, since here the current generated by the cells is not used for their heating, we can polarize the cells at their open circuit voltage, thus minimizing the solar energy necessary for their regeneration. We will then rely on the chart in Figure 2 to determine the threshold value of solar radiation.

Many variations and modifications of the regeneration device described above will be apparent to those skilled in the art. The device of FIGS. 3, 4 and 7 is particularly applicable to all types of modules and to all types of cells. photovoltaic: based on silicon (monocrystalline, multicrystalline, amorphous), based on copper, indium, gallium and selenium (CIGS), organic cells ... The regeneration device can respond to other issues that the Under-illumination degradation (LID), which affects only silicon-based cells containing boron and oxygen.

Claims

1. Device (300, 400) for regenerating the performance of a photovoltaic module (100) comprising:

 a heating element provided with an electrical resistance (310) and configured to heat photovoltaic cells belonging to the module, when the photovoltaic cells are exposed to solar radiation, by using the electrical energy produced by the photovoltaic cells;

 a switch (320) capable of switching between a first position, in which the photovoltaic cells are connected to the electrical resistance (310), and a second position, in which the photovoltaic cells are connected to an external electrical circuit (200); and

a control circuit (330,430) of the switch (320) configured to measure a quantity of solar radiation received by at least one reference photovoltaic cell and to switch the switch to the second position when the amount of solar radiation received reaches a value threshold.

2. Device according to claim 1, wherein the heating element (710) further comprises a thermal insulation box provided with at least one ventilation flap (71 1), the switch (720) being further configured to close the ventilation flap in the first position and open the ventilation flap in the second position.

3. Device according to one of claims 1 and 2, wherein said at least one reference photovoltaic cell belongs to the photovoltaic module (100).

4. Device according to claim 3, wherein the switch (320) switches to the second position when the amount of solar radiation received by all the photovoltaic cells of the module (100) reaches the threshold value.

5. Device according to one of claims 1 and 2, comprising an additional photovoltaic cell (431) subjected to solar radiation and forming said at least one reference photovoltaic cell.

Apparatus according to any one of claims 1 to 5, wherein the control circuit (330, 430) comprises:

 a sensor (332, 432) configured to measure the current (I,) supplied by said at least one photovoltaic cell exposed to solar radiation;

 - a current-frequency converter (333, 433) configured to convert the current (I,) supplied by said at least one photovoltaic cell into a pulse train (335) of variable frequency; and

 a pulse counter (334, 434), receiving as input the pulse train from the converter (333, 433) and configured to switch the switch (320) to the second position when the number of pulses counted reaches a number of reference pulses (NREF) corresponding to the threshold value of the amount of solar radiation.

The device of claim 6, wherein the pulse counter (334, 434) is provided with a reset input (RST).

8. Device according to any one of claims 1 to 7, comprising a device for limiting the temperature of the photovoltaic cells.

9. Photovoltaic module (500) comprising photovoltaic cells (510), characterized in that it comprises a device (300, 400, 700) of performance regeneration according to any one of claims 1 to 8.

The module (500) according to claim 9, wherein the photovoltaic cells (510) are encapsulated with encapsulating material (520) and disposed between a rear guard plate (540) and a front guard plate (530). ), exposed to solar radiation, and wherein the electrical resistance (310) is coated with the encapsulating material (520) and arranged facing the photovoltaic cells (510).

1 1. Module according to claim 10, comprising an electrical insulation sheet (550) disposed between the photovoltaic cells (510) and the heating element (310).

Module according to claim 9, in which the photovoltaic cells are arranged between a rear protection plate (540) and a front protection plate (530), exposed to solar radiation, and in which the electrical resistance (310) is contained. in the rear guard plate (540).

13. Module according to claim 9, comprising a front face, exposed to solar radiation, and a rear face, opposite to the front face, and wherein the electrical resistance (310) covers the rear face.

14. A method of regenerating the performance of a photovoltaic module (100) comprising photovoltaic cells, comprising the following steps:

 - exposing photovoltaic cells to solar radiation;

 - Heating the photovoltaic cells during the exposure of photovoltaic cells to solar radiation, by means of an electrical resistance (310) supplied with electrical energy by the photovoltaic cells;

 measuring a quantity of solar radiation received by at least one reference photovoltaic cell; and

 - Disconnect the electrical resistance (310) of the photovoltaic cells and connect the photovoltaic cells to an external electrical circuit (200) when the amount of solar radiation received reaches a threshold value.