WO2022128446A1

WO2022128446A1 - Reference bifacial photovoltaic device

Info

Publication number: WO2022128446A1
Application number: PCT/EP2021/083559
Authority: WO
Inventors: Antoine GUERIN DE MONTGAREUIL
Original assignee: Commissariat A L'energie Atomique Et Aux Energies Alternatives
Priority date: 2020-12-17
Filing date: 2021-11-30
Publication date: 2022-06-23
Also published as: FR3118359A1; FR3118359B1

Abstract

The invention relates to a reference bifacial photovoltaic device (11) comprising at least: - a first reference bifacial cell (21) occluded on the rear face thereof by a first mask (51), and connected to a first closed electric circuit (31) comprising a first resistor (310); - a first temperature sensor (61) placed against the occluded face of the first cell; - a second reference bifacial cell (22) occluded on the front surface thereof by a second mask (52), and connected to a second closed electric circuit (32) comprising a second resistor (320); - a second temperature sensor (62) placed against the occluded face of said second cell; - voltage measuring means capable of measuring the short-circuit voltage VIcc1 of the first reference cell (21) and the short-circuit voltage VIcc2 of the second reference cell (22).

Description

DESCRIPTION Title of the invention: Reference bifacial photovoltaic device TECHNICAL FIELD OF THE INVENTION [0001] The invention lies in the field of photovoltaic devices. The invention lies more specifically in the field of reference photovoltaic devices making it possible to evaluate and monitor the performance of photovoltaic devices, and in particular to measure the illumination and/or the photogenerated current as well as the temperature of photovoltaic devices. [0002] More specifically, the invention relates to a reference bifacial photovoltaic device making it possible to evaluate and monitor the performance of bifacial photovoltaic devices. STATE OF THE ART [0003] The invention lies in the field of photovoltaic devices, defined as being devices grouping together several photovoltaic cells (or several thin-film cells), each cell being made of a photovoltaic material. The cells can be assembled into one or more photovoltaic modules and a photovoltaic device can thus comprise one or more photovoltaic modules. In the field of the invention, it is known that a photovoltaic material is characterized by its spectral response, which defines for each wavelength received the ratio between the number of electrons created and the number of photons received. , and which depends on the wavelength of the light received (or of the photons received). Similarly, it is known that a photovoltaic device can be characterized by its electrical characteristics: the photogenerated current, generally determined by the short-circuit current I _cc ; the open circuit voltage V _co ; the voltage-current curve making it possible to determine the maximum power point and as well as the current I _Pmax , the voltage V _Pmax and the power P _Pmax at this maximum power point. By power, it is well understood by those skilled in the art that the electrical power is meant. [0006] The energy yield of a photovoltaic device is defined as being the ratio between the power at the point of maximum power and the power light received by the photovoltaic material. In addition, the peak power of a photovoltaic device refers to its power at the maximum power point under standard test (or reference) conditions, i.e. when it receives spectral illumination of an intensity light and of a determined spectrum, in particular defined by standard IEC 60904-3, for a total light power of 1000 W/m ² and under a device temperature of 25°C: the values of the short-circuit current are also defined. circuit, open circuit voltage as well as current and voltage at the maximum power point under the same standard test conditions. It should be noted that we will speak of photogenerated current, although generally it is the short-circuit current I _cc . [0008] For a given illumination, the photogenerated current depends on the temperature of the material but this dependence is low: the temperature coefficient of the photogenerated current or of the short-circuit current I _cc is less than one per thousand per Kelvin at ambient temperatures for common photovoltaic materials. On the other hand, for a given illumination, the open circuit voltage V _co and the power at the maximum power point P _Pmax depend more strongly on the temperature: we speak of the temperature coefficient of the open circuit voltage and of the temperature coefficient of the power at the maximum power point. [0009] But the photogenerated current is also linked to the illumination received by the photovoltaic device, and this depends on many parameters: environmental parameters, layout parameters (orientation, inclination, mounting system) of the photovoltaic device. [0010] Monitoring the performance of a photovoltaic device in operation is the comparison between the electrical power that it delivers and the electrical power that it would deliver if, subject to the same environmental conditions, it operated constantly at its maximum power point. . [0011] Thus, the production of a photovoltaic device depends mainly on the illumination received and more precisely, on the current photogenerated by this illumination and on the temperature of the photovoltaic cells. To evaluate or follow as precisely as possible the electricity production of a photovoltaic device, whether in factory characterization of a photovoltaic device or in design or in operation of a solar installation, we therefore seek to know: - the photogenerated current (or the short-circuit current I _cc ) by the illumination received or, by default and indirectly, the value of this illumination averaged over the entire surface of the device; and - the temperature of the photovoltaic device. [0012] The temperature of a photovoltaic device is defined as being the average temperature of all the cells making up said photovoltaic device. In practice, a point representative of the average temperature of all the cells is determined, and the temperature is measured at this point. In certain formulas known in the field of the invention, a junction temperature of the cells is taken. [0013] The power of the photovoltaic device is then determined, generally using a model expressing this power as a function of the photogenerated current (or failing that, of the measurement of the illumination received) and of its temperature. [0014] The illumination can be measured by a radiometer. Radiometers (pyranometers, pyrheliometers) are sensors that measure heat flux (energy per unit area) by transforming the photons received into a rise in temperature of a black body. But radiometers are not suitable for precisely monitoring the performance of a photovoltaic device because they do not make it possible to determine the photogenerated current with precision. In addition, radiometers do not make it possible to accurately determine the operating temperature of the photovoltaic material, which must therefore be measured independently. [0015] To evaluate or monitor the performance and the electrical production of a photovoltaic device, it is known to use a reference photovoltaic device (which can be abbreviated as “reference device”). A reference device can be used to determine the photogenerated current (or the short-circuit current I _cc ) or, failing that, the illumination, or even measure the temperature of the photovoltaic materials. [0016] A reference photovoltaic device consists of cells, modules (or even mini-modules), the material of which is the same as that for which the performance is to be monitored, preferably the spectral response of which is also substantially the same. same, and of which we have precisely determined the characteristics under standard test conditions as well as temperature behavior. The reference device must also be in the same arrangements (orientation, inclination, mounting system) and in the same environment as the device whose performance is to be assessed or monitored. The use of a reference photovoltaic device is particularly interesting in the design of a future solar installation, in particular to determine the photovoltaic potential of each photovoltaic device of the solar installation in a given place, that is to i.e. the electrical power that the photovoltaic device would be capable of supplying in this place, if it were constantly maintained at its maximum power point. Indeed, in this case, it is not possible to have temperature sensors and/or measure the photogenerated current (or the short-circuit current I _cc ) directly on a photovoltaic device, since it is not yet installed and/ or in operation. The electrical power can then be determined based on the determination of the short-circuit current and the temperature of the reference photovoltaic device, and advantageously by using a model making it possible to determine, from the value of the short-circuit current and temperature, the values of open circuit voltage, current, voltage and power at the maximum power point. [0018] However, a reference photovoltaic device can also and advantageously be used to monitor the performance and electrical production of a photovoltaic device in operation, also based on the determination of the short-circuit current and the temperature of the device. reference photovoltaic, and advantageously by using a model making it possible to determine, from the value of the short-circuit current and the temperature, the values of the open-circuit voltage, of the current, of the voltage and of the power at the maximum power point. [0019] Historically, photovoltaic devices have been designed to exploit the illumination received on a single face, which is the face most exposed to light (or “front face”). We then speak of monofacial photovoltaic devices. [0020] In some cases, a photovoltaic device is likely to receive a non-negligible illumination on its face least exposed to light (or "rear face". Typically, this is the case of a north-vertical bifacial photovoltaic device). south, which can receive in total, depending on the location, more illumination on its two faces than a monofacial photovoltaic device optimally arranged with an orientation to the south.Thus, certain photovoltaic devices are designed to produce the photovoltaic effect at from the photons received on their two faces. They are called bifacial (or “double-sided” in English) in that they include cells capable of capturing and using the radiation on both faces. knows reference photovoltaic devices which are monofacial.These are devices comprising one, two, or even more than two cells of the same material with the same spectral response as the photovoltaic device whose we seek to evaluate or monitor performance. [0022] Certain reference monofacial photovoltaic devices make it possible to measure the temperature of the cell by placing a temperature sensor against the rear face of the cell. In this case, the sensor is encapsulated with the cell. Whether or not it is integrated into the reference device, the temperature sensor can be a thermocouple, a thermistor, a resistive probe. [0023] Some reference monofacial photovoltaic devices enclose in a casing arranged under the rear face a resistive system allowing the determination of the short-circuit current, with the same drawback as described above (temperature not representative). [0024] Furthermore, such a configuration is not suitable for producing a reference bifacial photovoltaic device. Indeed, a reference bifacial photovoltaic device, to be representative of a bifacial photovoltaic device, must comprise cells capable of capturing radiation on both faces. Thus, if we position a temperature sensor on the rear face, we see that it will be on one of the faces capable of capturing the radiation. There are two major drawbacks to this: - the temperature sensor at least partially shades the cell and thus reduces the amount of light reaching the cell, the photogenerated current is then not representative of the illumination that the cell would receive without shading ; and - By being placed on one of the faces of a bifacial photovoltaic cell, the temperature sensor is exposed to the sun, and its temperature is therefore not representative of the temperature of the cell; and if the probe is hidden to prevent it from being exposed to the sun, then the cell is even less representative of a bifacial cell without shading. It is possible to use two reference monofacial photovoltaic devices, one for each face, assuming that the current photogenerated is the sum of the currents photogenerated on the front and rear faces. However, each monofacial reference device comprises a monofacial cell which is of a different nature from a bifacial cell, and in particular does not have the same spectral response as a bifacial cell. Furthermore, as indicated above, the rear face, provided with a temperature sensor, does not make it possible to measure a temperature precisely representative of the photovoltaic cell. [0026] There is also a reference photovoltaic device presented as being bifacial, marketed by the company Rera Solutions. However, these are two monofacial photovoltaic cells, grouped together within the same device. A disadvantage of such a reference device is the same as that described above, namely that the two monofacial photovoltaic cells are of a different nature from a bifacial cell, in particular do not have the same spectral response as a bifacial cell. and not the same electrical behavior. They do not have the same thermal behavior as a bifacial device, insofar as there is a space between the two cells. As a result, such a reference photovoltaic device is not suitable for representing the bifacial photovoltaic device the performance of which is sought to be monitored. [0027] It is understood from the documents of the state of the art that it is not easy to measure the temperature of a bifacial photovoltaic cell without the measurement itself bringing a measurement bias, and that it is not easy to have a bifacial photovoltaic cell representative of a bifacial photovoltaic device. Thus, to date there is therefore no reference bifacial photovoltaic device allowing both to measure the illumination received and to obtain a realistic estimate of both the photogenerated current and the operating temperature of a cell. of a bifacial photovoltaic module or device. The invention aims to overcome the aforementioned drawbacks of the prior art. [0029] More particularly, it aims to have a reference bifacial photovoltaic device making it possible to evaluate or monitor the performance of one or more bifacial photovoltaic devices. In particular, the invention aims to have a bifacial reference photovoltaic device which makes it possible to accurately determine the short-circuit current and the temperature of a bifacial device subjected to bifacial illumination, and whose said measurements are representative of one or more bifacial photovoltaic devices. The invention advantageously aims to measure the ambient temperature around such a bifacial reference photovoltaic device. DESCRIPTION OF THE INVENTION [0031] A first object of the invention making it possible to remedy these drawbacks is a reference bifacial photovoltaic device comprising at least: - a first reference bifacial photovoltaic cell concealed on its rear face, respectively its front face , by a first cover, and connected to a first electric circuit, said first electric circuit being a closed circuit comprising a first resistor of known value R ₁ ; - a first temperature sensor placed against the concealed face of said first cell; a second reference bifacial photovoltaic cell concealed on its front face, respectively its rear face, by a second cover, and connected to a second electrical circuit, said second electrical circuit being a closed circuit comprising a second resistor of known value R ₂ ; - a second temperature sensor placed against the concealed face of said second cell; the reference bifacial photovoltaic cells being arranged substantially in the same plane; the covers and the electrical circuits being included in said device, and said device further comprising: - voltage measuring means, capable of measuring at least: - a first voltage V ₁ across the terminals of the first resistor, said first voltage V ₁ corresponding to the voltage of the short-circuit current V _Icc1 of the first reference cell; and -- a second voltage V ₂ across the terminals of the second resistor, said second voltage V ₂ corresponding to the voltage of the short-circuit current V _Icc2 of the second reference cell. The invention consists of a reference bifacial photovoltaic device comprising at least two short-circuited bifacial photovoltaic cells, shielded on two different faces (one cell shielded on the rear face and another shielded on the front face), each cell being equipped with a temperature sensor on its hidden side. [0033] A mask designates any means (film, layer, plate, sheet, etc.) suitable for maximizing reflections and for limiting transmissions on one side (hidden side) of a bifacial photovoltaic cell while limiting reflections on the other. face (unobstructed face). It generally comprises a light face which maximizes reflections and limits transmissions (for example a white face) and a dark face which limits reflections (for example a black face). The cache must also modify the thermal behavior of the photovoltaic cell as little as possible. It must be as thin as possible, while allowing the face to be concealed. This can be a film of a hundred microns to a few millimeters in thickness, for example a PVF (PolyVinyl Fluoride) film, in particular a PVF film distributed under the Tedlar® brand. [0035] A reference bifacial photovoltaic cell concealed by a cover may be referred to as a “concealed cell”. Similarly, a reference bifacial photovoltaic cell not obscured by a mask can be designated as an “unobscured cell”. The device according to the invention can be in a single part (all the cells being on the same support for example) or in several parts (for example in several modules or mini-modules). The photovoltaic cells of the same device are coplanar, so as to be able to characterize the illumination on a given plane. [0037] The bifacial cells of the reference device must be representative of the photovoltaic cells making up the bifacial photovoltaic devices whose performance is sought to be evaluated or monitored. Furthermore, all the bifacial cells of a reference device are made of the same material and have the same characteristics, and preferably they are all identical bifacial cells. [0039] Preferably, all or part of the bifacial cells of a reference device are chosen so that, when they are associated (that is to say when a cell is short-circuited while the other is in open circuit and/or when a cell is blanked on the front face while the other cell is blanked on the rear face), their respective front and rear faces are of the same nature, in particular if the manufacturing technology of the cells bifacial creates bifacial cells with two faces of different nature. Two cells thus associated are generally two adjacent cells. [0040] For short, a cell is said to be "short-circuited" when its terminals are connected to a closed circuit by/on a shunt (or shunt resistor) and a cell is said to be "open circuit" when its terminals are connected to an open circuit, i.e. the cell terminals are not connected to each other. [0041] Throughout the present description, for the sake of simplification, a reference bifacial photovoltaic cell can be designated by “reference cell” or even “cell”. Likewise, a reference bifacial photovoltaic device can be designated by “reference device”, or even “device”. In addition, a bifacial photovoltaic device whose performance is sought to be monitored or evaluated can be referred to as a “bifacial photovoltaic device”. Also for simplification, we can speak of a bifacial photovoltaic device, although it is obvious that a reference device can monitor or evaluate the performance of several bifacial photovoltaic devices. Obviously, for a bifacial device, the terms “front face” and “rear face” have a little less meaning than for a monofacial device. Also, by convention, we will designate by "front face" or "front plate" the face or the plate exposed most directly to the radiation and by "rear face" or "rear plate" the face or the plate exposed the least directly to the radiation. . More generally, "front" or "front" refers to the side most directly exposed to radiation, and "back" or "behind" refers to the side less directly exposed to radiation. According to the invention, an electrical circuit “connected to” a cell means that it is connected to the terminals of said cell. The reference bifacial photovoltaic device according to the invention may also comprise one or more of the following characteristics taken in isolation or in all possible technical combinations. According to one embodiment, at least one closed electrical circuit capable of measuring a short-circuit voltage comprises a jumper placed between the resistance of said closed electrical circuit and the reference cell connected to said closed electrical circuit. According to one embodiment, the device further comprises at least one non-occulted reference bifacial photovoltaic cell, each non-occulted cell being connected to an electrical circuit configured so that the cell is short-circuited or open-circuited; the reference bifacial photovoltaic cells being arranged substantially in the same plane. According to one embodiment, at least one unobstructed reference bifacial photovoltaic cell is short-circuited, said cell being connected to a closed electrical circuit comprising a resistance of known value R ₃ ; the voltage measuring means being further able to measure at least one voltage V ₃ at the terminals of the resistance of said closed circuit, said at least one voltage corresponding to the voltage of the short-circuit current V _Icc3 of said at least one cell not concealed in short-circuit. According to an embodiment that can be combined with the previous mode, at least one unobstructed reference bifacial photovoltaic cell is in open circuit, said cell being connected to an open electrical circuit; the voltage measuring means being further able to measure at least one voltage V ₄ across the terminals of said open electrical circuit, said at least one voltage corresponding to the open circuit voltage V _co4 of said at least one reference cell not obscured in open circuit. According to one embodiment, each reference bifacial photovoltaic cell comprises a front plate forming the front face and a rear plate forming the rear face, and is encapsulated in an encapsulation layer. The Reference bifacial photovoltaic cells can be encapsulated separately or together. According to a particular embodiment, at least one cover is integrated against (or on) the front plate and/or against (or on) the rear plate. According to an advantageous embodiment, at least one cache is encapsulated in the encapsulation layer of the cell. This makes it possible to counterbalance the extra thickness of the cache by reducing the thickness of the encapsulation layer at the place where the cache is encapsulated. This is a means of limiting the impact of the cache on the thermal behavior of the photovoltaic cell. Preferably, all caches are encapsulated in the encapsulation layer. A second object of the invention is a photovoltaic system comprising at least one bifacial photovoltaic device and at least one reference bifacial photovoltaic device according to the first object of the invention. According to one embodiment, at least one reference bifacial photovoltaic device is arranged next to the bifacial photovoltaic device. According to an embodiment that can be combined with the previous mode, at least one reference bifacial photovoltaic device is placed within the bifacial photovoltaic device, between several bifacial photovoltaic cells and/or at the level of at least one edge of the bifacial photovoltaic device. The reference bifacial photovoltaic device and the photovoltaic system according to the invention may include any of the characteristics listed above, taken in isolation or according to all technically possible combinations with other characteristics. A third object is a method for determining at least one temperature T of a reference device according to the first object of the invention, said method comprising the following steps: - a step for measuring the voltage of the current short-circuit V _Isc1 of the first reference cell; - a step of measuring the voltage of the short-circuit current V _Isc2 of the second reference cell; - a step of measuring the temperature T ₁ of the first reference cell by the first temperature sensor; - a step of measuring the temperature T ₂ of the second reference cell by the second temperature sensor; - a step for calculating the temperature T by the formula:

According to one embodiment, the method further comprises: - a step of determining an ambient temperature T _a close to the reference device by the formula:

According to an embodiment which can be complementary to the previous mode, the method further comprises: - a step of determining a temperature coefficient k by the formula:

A fourth object is a method for determining a short-circuit current of at least one short-circuited reference cell, connected to a closed electrical circuit comprising a resistance of known value R, of a device reference according to the first object of the invention, said method comprising: - a step of measuring a voltage of the short-circuit current V _Icc across the terminals of said resistor; then - a step of determining the short-circuit current I _cc of said cell from the voltage of the short-circuit current V _Icc measured, using Ohm's law:

According to one embodiment, the steps of measuring a voltage of the short-circuit current V _Icc and of determining the short-circuit current I _cc are carried out for each of the first and second reference cells, so as in determining the short-circuit current I _cc1 of the first reference cell and the short-circuit current I _cc2 of the second reference cell, the short-circuit current equivalent to an unobstructed reference cell being determined by adding the short-circuit current I _cc1 of said first cell and the short-circuit current I _cc2 of said second cell. A fifth object is a method for determining at least one temperature T of a reference device according to the first object of the invention, said method comprising: - a step of measuring the temperature T ₁ of the first reference cell by the first temperature sensor; - a step of measuring the temperature T ₂ of the second reference cell by the second temperature sensor; - a step for calculating the temperature T by the formula:

According to one embodiment, the above method further comprises a prior step of determining the constants (b, c, d, e, f) of the formula:

said preliminary step comprising the following sub-steps: - a sub-step of measuring an open circuit voltage V _co at the terminals of a non-occulted reference bifacial photovoltaic cell in open circuit; - a sub-step of measuring a voltage of the short-circuit current V _Icc at the terminals of the resistance of a closed electrical circuit connected to a reference bifacial photovoltaic cell not concealed in short-circuit; - a sub-step for determining the short-circuit current I _cc from the voltage of the short-circuit current V _Icc measured, using Ohm's law:

- a sub-step for determining a temperature T using the formula:

where: V _co is the measured open circuit voltage; I _sc is the determined short-circuit current; V _coSTC is the open circuit voltage under standard test conditions; I _ccSTC is the short-circuit current under standard test conditions; and ^ and β are coefficients, V _coSTC, I _ccSTC , α and β being determined beforehand; said sub-steps being repeated for several temperatures T, T ₁ and T ₂ determined under the same conditions so as to determine the constants (b, c, d, e, f). BRIEF DESCRIPTION OF FIGURES [0063] Other characteristics, details and advantages of the invention will become apparent on reading the description given with reference to the appended drawings given by way of example and which represent, respectively: [0064] [Fig.1] represents a first embodiment of a reference bifacial photovoltaic device according to the invention. [0065] [Fig.2] represents the second embodiment of a reference bifacial photovoltaic device according to the invention. [0066] [Fig.3] represents a third embodiment of a reference bifacial photovoltaic device according to the invention. represents a fourth embodiment of a reference bifacial photovoltaic device according to the invention. represents a fifth embodiment of a reference bifacial photovoltaic device according to the invention. represents an example of integration of a reference bifacial photovoltaic device according to the invention in a bifacial photovoltaic device. In all of these figures, identical references can designate identical or similar elements. [0071] In addition, the different parts shown in the figures are not necessarily shown on a uniform scale, to make the figures more readable. DETAILED DESCRIPTION OF THE INVENTION FIGS. 1 to 5 represent several embodiments of a reference bifacial photovoltaic device in accordance with the invention (hereinafter referred to as the “reference device”). In all of Figures 1 to 5, the reference bifacial photovoltaic cells are encapsulated in an encapsulation layer 40 transparent to light, for example polyethylene-vinyl acetate (EVA), or polyvinylbutyral (PVB ) or any other suitable material known in the field of the invention, in order to protect said cells from the external environment, while at the same time disturbing the reception of the light radiation to a minimum. At least part of conductors forming the circuits described below is also encapsulated in the encapsulation layer. The cells and encapsulation layer assembly is inserted between a front plate 41 forming the front face AV and a rear plate 42 forming the rear face AR. The front and rear plates are of suitable dimensions to cover the encapsulation layer. The reference device being bifacial, the front and rear plates are both transparent (for example made of bi-glass or any other suitable material known in the field of the invention). Note that in Figures 1 to 5, the cells are encapsulated together, but this is not limiting, each cell can be encapsulated individually, with its own front plate and its own rear plate. The cells/encapsulation layer/front and rear plates assembly is manufactured and assembled by one of the methods known in the field of the invention, typically by a hot (or even cold) and vacuum lamination process. , forming a structure. The covers on the reference bifacial photovoltaic cells can be integrated into this structure. A cover can thus be assembled on (or on) the rear plate of a cell concealed on the rear face and/or on (or on) the front plate of a cell concealed on the front face. Each cache is preferably encapsulated in the encapsulation layer. A temperature sensor placed against the concealed face of a reference bifacial photovoltaic cell can also be encapsulated in the encapsulation layer. It can in particular be arranged between the cover and the corresponding cell. The reference cells must be as representative as possible of the cells of the bifacial photovoltaic device whose performance is to be evaluated or monitored (hereinafter referred to as “bifacial photovoltaic device”). As indicated previously, for simplification, we will speak of a bifacial photovoltaic device although it is obvious that a reference device can monitor or evaluate the performance of several bifacial photovoltaic devices. The bifacial cells of the reference device are made of the same material as the cells making up the bifacial photovoltaic devices, and preferably with a close or equivalent spectral response. [0081] Furthermore, the materials of the encapsulation layer and of the front and rear plates are preferably chosen to be identical to those of the bifacial photovoltaic devices whose performance is to be evaluated or monitored. Thus, bifacial cells can be chosen from the production line of cells used for bifacial photovoltaic devices whose performance is sought to be evaluated or monitored and/or selected with the same selection criteria as the cells of photovoltaic devices. bifacial, for example the same power and/or the same current at the maximum power point. It should be noted that the layout of the cells in the device is not necessarily as shown in Figures 1 to 5. The main thing is to have cells arranged next to each other, according to the same plane. , preferably encapsulated together, but the arrangement need not be as regular as shown. Furthermore, these are not necessarily cells of the same size as the photovoltaic cell to be monitored or evaluated. It is however important that all the cells of the same reference device are made of the same material and have the same characteristics. It can be identical cells. [0084] Furthermore, the open and closed electrical circuits connected to the terminals of the cells are not necessarily arranged in the lower peripheral part of the reference device. They can be arranged on another peripheral part of the device. It is essential that they are not arranged on any of the front and rear faces of the cells. By shortcut in the present description, we can write that the electrical circuits are connected to the cells, obviously meaning that they are connected to the terminals of said cells. [0086] The voltage signals can generally be offset far from the cells. On the other hand, the current measurements are generally made by measuring the voltage at the terminals, therefore close to the cells, avoiding unnecessary cable lengths which create parasitic voltage drops. Finally, although this is not shown in the figures, the reference bifacial photovoltaic device comprises a processing unit able to perform at least the operations described later. The different voltages delivered by the reference bifacial photovoltaic device according to the invention can be processed conventionally at the level of the device by supplying 4-20 mA currents or by including them in registers of the Modbus type, with or without mathematical processing. prior. FIG. 1 represents a first reference device 11 comprising: - a first and a second reference bifacial photovoltaic cell 21, 22 (hereinafter referred to as “reference cells”); - a first and a second electrical circuit 31, 32, the first electrical circuit 31 being connected to the first bifacial photovoltaic cell 21 and the second electrical circuit 32 being connected to the second bifacial photovoltaic cell 22. The electrical circuits are arranged at the border (or periphery) of the reference cells. Each of these reference cells 21, 22 is concealed on one side, that is to say assembled with a cover 51, 52 on one side. The covers can be in the form of Tedlar® film with a black face and a white face. In the device shown, a first cover 51 is placed behind the first cell 21, the black face of the cover being oriented towards the front face AV of the device and the white face towards the rear face AR of the device, so as to avoid the reflections on the front face of the first cell 21 and return the light as much as possible to the rear face of said cell. A second mask 52 is arranged in front of the second cell 22, the white face of the mask being oriented towards the front face AV of the device and the black face towards the rear face AR of the device, so as to avoid reflections on the rear face of the second cell 22 and return the light to the front face of said cell as much as possible. Thus, the first cell 21 combined with the first cover 51 makes it possible to measure the short-circuit current and the temperature generated by the illumination on the front face only, whereas the second cell 22 combined with the second cover 52 makes it possible to measure the short-circuit current and the temperature generated by the illumination on the rear face only. This makes it possible to calculate the current of short-circuit which would be generated by illumination on both sides by adding the short-circuit currents of the two cells 21 and 22 and to calculate the temperature of a cell which would be illuminated by both sides by combining the temperatures of the two cells 21 and 22. Each reference cell is connected to an electrical circuit which is a circuit closed by/on a shunt, that is to say a shunt resistor. The first electrical circuit 31 includes a first resistor 310 of known value R ₁ and the second electrical circuit 32 includes a second resistor 320 of known value R ₂ . Voltage measurement means make it possible to measure a first voltage V ₁ across the terminals of the first resistor 310 and a second voltage V ₂ across the terminals of the second resistor 320 (measuring means not shown). The first voltage V ₁ measured corresponds to the voltage of the short-circuit current or short-circuit voltage V _Icc1 of the first cell 21 (more precisely the short-circuit voltage corresponding to the illumination received by the front face, c 'that is to say V _cc1AV ) and the second voltage V ₂ measured corresponds to the voltage of the short-circuit current or short-circuit voltage V _Icc2 of the second cell 22 (more precisely the short-circuit voltage corresponding to the illumination received by the rear face, ie V _cc2AR ). It is possible to determine the short-circuit current I _cc1 of the first cell (more precisely the short-circuit current corresponding to the illumination received by the front face, that is to say I _cc1AV ) and the current of short-circuit I _cc2 of the second cell (more precisely the short-circuit current corresponding to the illumination received by the rear face, i.e. I _cc2AR ) according to Ohm's law, i.e.: [0097] [ Math.1]

The equivalent short-circuit current of a non-occulted reference cell can be determined by adding the short-circuit current I _cc1 of the first cell and the short-circuit current I _cc2 of the second cell. The first and second cells are kept close to their short-circuit state, and the values R ₁ and R ₂ of the resistors 310, 320 must be known. with precision and must be suitably chosen, in a manner known to those skilled in the art. A resistance value defined is generally chosen so that, under the maximum illumination considered, the voltage across the terminals of the shunt makes it possible to remain in the rectilinear part of the current-voltage curve, ie 50 mV for example. For example, if the current is 10 A under maximum illumination, a suitable resistance value is 5 mΩ. [0101] In addition, the reference device 11 comprises a temperature sensor on each of the concealed faces of said first and second cells: a first sensor 61 is placed against the (hidden) rear face of the first cell 21, and a second sensor 62 is arranged against the (hidden) front face of the second cell 22. Although this is not shown, each sensor is connected, in a manner known to those skilled in the art, by wires to the measuring devices, the number of wires depends on the type of sensor and the mounting chosen. The temperature sensors can be chosen from: a thermocouple, a thermistor, a resistive probe (for example based on platinum, copper, nickel alloys or metal oxides, etc.). The determination of the temperature of the bifacial cell (temperature which would be obtained without obscuring the bifacial cells) can be carried out as follows. It is considered that the light energy arriving on the white face of the shield is reflected by said white face and does not contribute to the temperature rise of the cell. It is also considered that the thermal conductivity of the concealed face is very little modified by the positioning of the cover. Let T ₁ and T ₂ be the respective temperatures of the two blanked cells 21, 22 and V _Icc1 and V _Icc2 the voltages measured at their terminals (more precisely at the terminals of the shunts). It can be considered as a first approximation that the light energy E ₁ received by the first cell 21 is proportional to the short-circuit current I _cc1 and therefore to the voltage measured at the terminals of the short-circuited cells V _Icc1 , c i.e.: [0108] [Math.2]

k' being a first constant. [0109] We can then write, if T _a denotes the ambient temperature, that the rise in temperature of the cells is proportional to the light energy E ₁ received, i.e.: [0110] [Math.3]

k'' being a second constant. [0111] By combining these two formulas, we can write: [0112] [Math.4]

k being the product of the first and second constants and being the constant or thermal coefficient that will be sought. [0113] Or: [0114] [Math.5]

[0115] The same is true for the light energy E ₂ received by the second cell 22, and its temperature T ₂ , considering that the thermal coefficient k is the same, so that: [0116] [Math.6 ]

By combining these two formulas, and by measuring with the temperature sensors T ₁ and T ₂ , the values of the ambient temperature T _a and of the thermal coefficient k can be calculated. [0118] [Math.7]

[0119] [Math.8]

[0120] Similarly, one can deduce the temperature T of a bifacial cell (not occulted) receiving the sum of the illuminations of the two occulted cells, by the formula: [0121] [Math.9]

where V _Isc is the short-circuit voltage of the bifacial (non-occulted) cell. [0122] And so, using the expressions for the ambient temperature T _a and the thermal coefficient k, we deduce the temperature T of the non-occulted bifacial cell: [0123] [Math.10]

It can therefore be seen that the value of the temperature T of the bifacial cell can be determined (which would be obtained without obscuring the bifacial cells) from the measurements of the short-circuit voltages and the temperatures of the two bifacial cells concealed. It is also possible to determine the value of the ambient temperature T _a and that of the thermal coefficient k. The temperature T thus determined corresponds to the temperature of the reference bifacial cell, representative of the bifacial photovoltaic cells whose performance is sought to be determined. This thus makes it possible to determine a temperature of bifacial photovoltaic cells, without the disadvantages of the state of the art. [0126] FIG. 2 represents a second reference device 12 in accordance with the invention which differs from the first device 11 in that it further comprises a reference bifacial photovoltaic cell not occulted in short-circuit, designated third cell 23, which is connected to a closed electrical circuit, designated third circuit 33, comprising a shunt, that is to say a third resistor 330 in shunt, of known value R ₃ . The voltage measuring means also make it possible to measure a voltage (third voltage V ₃ ) across the terminals of the third resistor 330 (means of measurement not shown). The third voltage V ₃ measured corresponds to the voltage of the short-circuit current or short-circuit voltage V _Icc3 corresponding to the illumination received by the two faces of the third cell 23. This makes it possible to determine the current of short-circuit corresponding to the illumination received by the two faces of the non-occulted bifacial cell and therefore to check whether this short-circuit current is indeed the sum of the two short-circuit currents of the two obscured cells 21, 22. In otherwise, this can make it possible to detect, for example, the presence of debris, plants, dirt, shading, etc., on at least one of the three cells, and in this case, the short-circuit current to be taken into account is the highest. large value between the short-circuit of the bifacial cell and the sum of the short-circuit currents of the two occulted cells. [0129] FIG. 3 represents a third reference device 13 in accordance with the invention which differs from the first device 11 in that it comprises a reference bifacial photovoltaic cell not occulted in open circuit, designated fourth cell 24, which is connected to an open electrical circuit, designated fourth circuit 34. This third reference device does not include a short-circuited non-occulted cell. The voltage measuring means also make it possible to measure a voltage (fourth voltage V ₄ ) across the terminals of the fourth electrical circuit 34 (measuring means not shown). This fourth voltage V ₄ corresponds to the open circuit voltage V _co4 corresponding to the illumination received by the two faces of the fourth cell 24. This embodiment makes it possible to obtain a more precise formula giving the temperature T d a bifacial cell (which would be obtained without obscuring the bifacial cells) from the measurements of the temperatures T ₁ , T ₂ of the two occulted bifacial cells, and which is the following second degree polynomial function: [0132] [Math. 11]

where b, c, d, e, f are constants, which can be determined as follows. As indicated in the introduction to this description, it is known that the open circuit voltage V _co and the power at the maximum power point P _Pmax depend on the temperature: more precisely, they are considered to be affine functions of the temperature and one speaks of the temperature coefficient of the open circuit voltage and of the temperature coefficient of the power at the maximum power point. Furthermore, these temperature coefficients themselves vary with the short-circuit current. [0134] There is a mathematical formula commonly used in the literature expressing the open circuit voltage of a photovoltaic device as a function of its short-circuit current and its temperature: [0135] [Math.12]

where: - V _co is the open circuit voltage under the measurement conditions; - V _coSTC is the open circuit voltage under standard test conditions; - T is the temperature of the device (of the cells); - I _sc is the short-circuit current determined under the measurement conditions; - I _ccSTC is the short-circuit current under standard test conditions; - α and β are coefficients obtained by calibrating the device; β is generally referred to as the “open circuit voltage temperature coefficient”; it is considered that these coefficients are already known. An example of calibration is as follows: under flash test, the cell is subjected to a series of levels of illumination and temperatures, for example those defined in Table 2 of standard ICE 61853-1, then under each condition, the open-circuit voltage V _co , the short-circuit current I _cc and the temperature T are measured. Assuming that the short-circuit current under the standard test conditions I _ccSTC is known, a table is drawn up with the variables I _cc , T, V _co , ln(I _cc /I _ccSTC ), T-25, then, by linear regression, the coefficients of the formula [Math.12] are calculated. It is recalled that the standard test conditions, defined in particular by the IEC 60904 standard, correspond to illumination according to the AM1.5 spectral distribution, with a normal luminous power (or irradiance) of 1000 W/m ² and under a device temperature of 25°C. It is also possible to measure the voltages (short circuit and open circuit) under normal test conditions, then deduce the temperature T therefrom under the measurement conditions. By determining several temperatures T under the same conditions as the temperatures T ₁ and T ₂ , we see that we can determine the constants b, c, d, e, f of the polynomial function [Math.11]. When it is desired to carry out such a determination, by comparing temperatures obtained according to two different methods and with data from different cells, it is important that said cells (here cells 21, 22, 24) be the closest possible. [0141] This condition is less important when comparing short-circuit currents as in the embodiment of FIG. 3. [0142] It should be noted that by knowing the temperature and the short-circuit current d In a bifacial cell, and whatever the method, it is possible to determine the illumination as well as the electrical power of the reference device, and thereby the electrical power of the bifacial photovoltaic device of which it is representative. [0143] The illuminance E of a cell can be determined using the formula: [0144] [Math.13]

E _STC being the illumination under standard test conditions, ie 1000W/m ² . [0145] More precisely, by knowing the temperature T, the result can be refined using the formula: [0146] [Math.14]

where a is the short-circuit current coefficient, which can be obtained by calibrating the device. The electrical power can be determined by using a model making it possible to calculate, from the value of the short-circuit current and the temperature, the values of the open-circuit voltage, of the current, of the voltage and power at the maximum power point. This may be, for example, the MotherPV model described in particular in the publication "Description of MotherPV, the new method developed at INES / CEA for the assessment of the energy production of photovoltaic modules", Guérin de Montgareuil, Antoine, 22 ^nd European Photovoltaic Solar Energy Conference, Milano, 2007, Milano, Italy, 2007” and in the publication “A new tool for the MotherPV method: modeling of the irradiance coefficient of photovoltaic modules”, Guérin de Montgareuil, Antoine, Sicot, Lionel, Martin, Jean -Luc, Mezzasalma, Frédéric, Merten, Jens 24th European Photovoltaic Solar Energy Conference, Hamburg, 2009, Hamburg, Germany, 2009. [0148] This is applicable to all embodiments, and more broadly to any reference device entering into the scope of the invention. FIG. 4 represents a fourth reference device 14 in accordance with the invention which combines the second and third devices. Thus, the fourth reference device 14 comprises two blanked cells 21, 22 in short-circuit, a non-blanked cell 23 in short-circuit and a non-blanked cell 24 in open circuit. By having a cell that is not short-circuited, this makes it possible, as for the device of FIG. 2, to obtain redundancy of the short-circuit current values (in addition to the advantage of the device of the figure 3). More precisely, this makes it possible to determine the short-circuit current corresponding to the illumination received by the two faces of the non-occulted bifacial cell and therefore to check whether this short-circuit current is indeed the sum of the two short-circuit currents. circuit of the two occulted cells. Otherwise, this can make it possible to detect, for example, the presence of debris, plants, dirt, shading, etc., on at least one of the three cells, and in this case, the short-circuit current to be taken into account is the greater value between the short-circuit of the bifacial cell and the sum of the short-circuit currents of the two occulted cells. FIG. 5 partially represents a fifth reference device 15 in accordance with the invention which differs from the first device in that the first electrical circuit 31 further comprises a removable jumper 315. Such a removable jumper thus arranged on one of the conductors of the first circuit makes it possible to determine with better precision the value of the short-circuit current of the first cell of reference 21 under standard test conditions. It is recalled that the principle of the jumper is to remain closed as long as it is not necessary to make this measurement, it is removed and the circuit is thus opened to allow this measurement to be carried out by putting in place a well-defined shunt . In particular, it can be used by accredited metrology laboratories to carry out such measurements, which for greater accuracy directly measure the short-circuit current without passing through the voltage across the terminals of the shunt. Laboratories therefore remove the jumper and instead put their own shunts for current measurement. Thus, it is possible to more accurately measure the value of the short-circuit current I _ccSTC of a short-circuited cell under standard test conditions. This jumper characteristic in an electrical circuit closed by/on a shunt can be added to one or more closed circuits of one or more of the devices described according to the embodiments described previously, more generally to one or more closed circuits of a reference device according to the invention. The reference device according to the invention is ideally placed as close as possible to a bifacial photovoltaic device whose performance is to be monitored, and in the same arrangements (orientation, inclination, mounting system), as well as in the same environment as said device, when the latter is already in place, of course. When the bifacial photovoltaic device or the photovoltaic installation is not yet installed, the device according to the invention is preferably placed in the future place of installation, or at least in a representative place. In the case of a bifacial photovoltaic device already installed and/or already existing or being designed, it may be advantageous to integrate the device according to the invention within said device itself in order to monitor its performance. . The cells of the reference device according to the invention can thus be integrated during the manufacture of the bifacial photovoltaic device. It is possible, for example, to integrate cells 20 of a reference device 10 into one of the free spaces of a bifacial photovoltaic device 100, for example between four cells if the technology lends itself to it and/or cells 20' of a reference device 10' at the edge of a module of the bifacial photovoltaic device 100, as shown in FIG. 6 which represents a photovoltaic system 1. The circuits are also represented under the references 30 or 30' as being closed circuits. This configuration is obviously not limiting. In addition, it is possible to add in particular a bifacial cell not occulted in short-circuit and/or a bifacial cell not occulted in open circuit. The cells thus inserted are not necessarily cells of the same size as the cells of the bifacial photovoltaic device, as can be seen in FIG. 6. Any reference device according to any embodiment previously described can thus be integrated into a bifacial photovoltaic device, depending on the space available, or be placed next to it. The different modes presented can be combined with each other, according to all the technically possible combinations. Furthermore, the present invention is not limited to the embodiments described above but extends to any embodiment falling within the scope of the claims.

Claims

1. Reference bifacial photovoltaic device (11, 12, 13, 14, 15) comprising at least: - a first reference bifacial photovoltaic cell (21) concealed on its rear face, respectively its front face, by a first cover ( 51), and connected to a first electrical circuit (31), said first electrical circuit being a closed circuit comprising a first resistor (310) of known value R ₁ ; - a first temperature sensor (61) placed against the concealed face of said first cell; - a second reference bifacial photovoltaic cell (22) concealed on its front face, respectively its rear face, by a second cover (52), and connected to a second electrical circuit (32), said second electrical circuit being a closed circuit comprising a second resistor (320) of known value R ₂ ; - a second temperature sensor (62) arranged against the concealed face of said second cell; the reference bifacial photovoltaic cells being arranged substantially in the same plane; the covers and the electrical circuits being included in said device, and said device further comprising: - voltage measuring means, able to measure at least: - a first voltage V ₁ across the terminals of the first resistor (310), said first voltage V ₁ corresponding to the short-circuit current voltage V _Icc1 of the first reference cell (21); and -- a second voltage V ₂ across the terminals of the second resistor (320), said second voltage V ₂ corresponding to the voltage of the short-circuit current V _Icc2 of the second reference cell (22). 2. Device (15) according to claim 1, at least one closed electrical circuit (31) capable of measuring a short-circuit voltage comprising a jumper (315) disposed between the resistor (310) of said closed electrical circuit and the reference (21) connected to said closed electrical circuit. 3. Device (12, 13, 14) according to one of claims 1 or 2, further comprising at least one unobstructed reference bifacial photovoltaic cell (23, 24), each unobstructed cell being connected to an electrical circuit ( 33, 34) configured so that the cell is short-circuited or open-circuited; all the reference bifacial photovoltaic cells being arranged substantially in the same plane. 4. Device (12, 14) according to claim 3, at least one unobstructed reference bifacial photovoltaic cell (23) being short-circuited, said cell being connected to a closed electrical circuit (33) comprising a resistor (330) of known value R ₃ ; the voltage measuring means further being capable of measuring at least one voltage V ₃ across the terminals of the resistor (330) of said closed circuit, said at least one voltage corresponding to the voltage of the short-circuit current V _Icc3 of said at least one non-occulted short-circuited cell. 5. Device (13, 14) according to one of claims 3 or 4, at least one unobstructed reference bifacial photovoltaic cell (24) being in open circuit, said unobstructed cell being connected to an electrical circuit (34) open ; the voltage measuring means being further able to measure at least one voltage V ₄ across the terminals of said open electrical circuit (34), said at least one voltage corresponding to the open circuit voltage V _co4 of said at least one reference cell not blocked in open circuit. 6. Device (11, 12, 13, 14, 15) according to one of the preceding claims, each reference bifacial photovoltaic cell comprising a front plate (41) forming the front face (AV) and a rear plate (42) forming the rear face (AR) and being encapsulated in an encapsulation layer (40), the reference bifacial photovoltaic cells being able to be encapsulated separately or together. 7. Device according to claim 6, at least one mask being integrated against (or at) the front plate and/or against (or at) the rear plate of a concealed reference cell, and preferably being encapsulated in the layer of encapsulation (40) of said cell. 8. Bifacial photovoltaic system (1) comprising: - at least one bifacial photovoltaic device (100) and - at least one reference bifacial photovoltaic device (10, 10') chosen according to any one of claims 1 to 7. 9. Bifacial photovoltaic system (1) according to claim 8, at least one reference bifacial photovoltaic device being arranged next to the bifacial photovoltaic device. 10. Bifacial photovoltaic system (1) according to claim 8 or 9, the cells (20, 20') of at least one reference bifacial photovoltaic device (10, 10') being arranged within the bifacial photovoltaic device (100) , between several bifacial photovoltaic cells and/or at the level of at least one edge of said bifacial photovoltaic device. 11. Method for determining at least one temperature T of a reference device (11, 12, 13, 14, 15) defined according to one of claims 1 to 7, said method comprising the following steps: - a step measuring the voltage of the short-circuit current V _Isc1 of the first reference cell (21); - a step of measuring the voltage of the short-circuit current V _Isc2 of the second reference cell (22); - a step of measuring the temperature T ₁ of the first reference cell (21) by the first temperature sensor (61); - a step of measuring the temperature T ₂ of the second reference cell (22) by the second temperature sensor (62); - a step for calculating the temperature T by the formula:

12. Method according to claim 11, further comprising: - a step of determining an ambient temperature T _a close to the reference device by the formula:

13. Method according to one of claims 11 or 12, further comprising: - a step of determining a temperature coefficient k by the formula:

14. Method for determining a short-circuit current of at least one short-circuited reference cell, connected to a closed electric circuit comprising a resistance of known value R, of a reference device (11, 12 , 13, 14, 15) defined according to one of claims 1 to 7, said method comprising: - a step of measuring a voltage of the short-circuit current V _Icc across the terminals of said resistor; then - a step of determining the short-circuit current I _cc of said cell from the voltage of the short-circuit current V _Icc measured, using Ohm's law:

15. Method according to claim 14, the steps of measuring a voltage of the short-circuit current V _Icc and of determining the short-circuit current I _cc being carried out for each of the first and second reference cells (21, 22 ), so as to determine the short-circuit current I _cc1 of the first reference cell (21) and the short-circuit current I _cc2 of the second reference cell (22), the short-circuit current equivalent to an unobstructed reference cell being determined by adding the short-circuit current I _cc1 of said first cell and the short-circuit current I _cc2 of said second cell. 16. Method for determining at least one temperature T of a reference device (11, 12, 13, 14, 15) defined according to one of claims 1 to 7, said method comprising: - a step of measuring the temperature T ₁ of the first reference cell (21) by the first temperature sensor (61); - a step of measuring the temperature T ₂ of the second reference cell (22) by the second temperature sensor (62); - a step for calculating the temperature T by the formula:

17. Method according to claim 16, the reference device (13, 14) being chosen according to claim 4 in combination with claim 5, said method comprising a prior step of determining the constants (b, c, d, e, f ) of the formula

said preliminary step comprising the following sub-steps: - a sub-step of measuring an open circuit voltage V _co at the terminals of a non-occulted reference bifacial photovoltaic cell in open circuit (24); - a sub-step of measuring a voltage of the short-circuit current V _Icc across the resistor terminals of a closed electrical circuit (33) connected to a reference bifacial photovoltaic cell not concealed in short-circuit (23) ; - a sub-step for determining the short-circuit current I _cc from the voltage of the short-circuit current V _Icc measured, using Ohm's law:

- a sub-step for determining a temperature T using the formula:

where: V _co is the measured open circuit voltage; I _sc is the determined short-circuit current; V _coSTC is the open circuit voltage under standard test conditions; I _ccSTC is the short-circuit current under standard test conditions; and ^ and ^ are coefficients, V _coSTC, I _ccSTC , ^ and ^ being determined beforehand; said sub-steps being repeated for several temperatures T, T ₁ and T ₂ determined under the same conditions so as to determine the constants (b, c, d, e, f).