Soiling sensor for photovoltaic surface

Abstract

The present invention relates to a device (1) for monitoring the cleanliness of a surface (2), comprising an electrical power source (3) configured to be connected to the surface (2) in order to apply voltage to the surface (2) at a plurality of points, a system for measuring the voltage and the current (4), a meteorological station (6) configured to acquire environmental data of the surface (2), the monitoring device (1) comprising a control unit (5) configured to: - operate the power source (3) according to a power supply profile; - determine, on the basis of the measured current and voltage, an insulation impedance induced by the surface (2); - determine a level of soiling of the surface (2) with respect to a "clean" initial state of the surface (2) on the basis of the impedance measurement and the environmental data obtained from a meteorological station (6).

Description

[0001] DESCRIPTION

[0002] Dirt sensor for photovoltaic surfaces

[0003] TECHNICAL FIELD

[0004] This disclosure relates to a soiling sensor establishing a soiling level of a surface relative to an initial state of the surface and a method of calibrating such a sensor.

[0005] STATE OF THE ART

[0006] It is common to have to assess the cleanliness of a surface in order to plan its maintenance, particularly in the case of solar power plants. These installations, which can be classified according to whether they are photovoltaic or thermal, are in fact particularly sensitive to problems of fouling or soiling of their surfaces. For example, a so-called "tower" solar power plant, in which the radiation is concentrated towards a fixed focus placed high up, generally uses a minimum of several hundred heliostats, which represents tens of thousands of square meters of mirrors whose soiling, by reducing their reflective capacity, directly impacts the overall efficiency of the installation. In a power plant using photovoltaic panels, the problem is the same and is even greater when the plant is located in a desert region.Dust is then responsible for losses ranging from 2 to 10% of total yield.

[0007] It is therefore necessary to regularly clean a large quantity of surfaces, which involves significant additional costs to control. To plan cleaning operations, it is necessary to measure the level of soiling. To do this, it is common to measure the short-circuit current of a clean cell and that of a dirty cell and to compare the two after correction for temperature and illumination; this ratio gives the level of soiling.

[0008] This measurement consists of physically putting the positive and negative polarities or terminals, i.e. an anode and a cathode of a reference module or cell in contact and measuring the current at these terminals, which is directly proportional to the illumination. By calculating the ratio between the short-circuit current values ​​of a clean reference module and the short-circuit current of a dirty module, a level of soiling can be deduced, as soiling generates a loss of illumination received by the solar cells and therefore a proportional loss of associated short-circuit current.

[0009] However, this measurement does not distinguish between losses due to natural aging of the cell and those due to soiling and, above all, requires frequent intervention and sometimes even cleaning of all the cells.

[0010] There is therefore a need for a sensor that continuously calculates a level of soiling expressing the loss of efficiency compared to a clean initial state of a surface. This sensor is ideally low cost, simple and eliminates the need for human intervention. It provides qualitative information on soiling, i.e. whether the module is becoming dirty or not and, after calibration and application of a model, provides quantitative information.

[0011] EXPOSED

[0012] An aim of the invention is to remedy the aforementioned drawbacks by proposing a monitoring device continuously measuring the insulation resistance of the monitored surface in order to deduce therefrom a level of soiling defined with reference to an initial "clean" state and the state at the time of measurement based on the fact that a dirty surface conducts electricity better on the surface than a clean surface.

[0013] To this end, according to a first aspect of the invention, there is provided a device for monitoring the cleanliness of a surface, comprising an electrical power source configured to be connected to the surface in order to energize the surface at two or more points, a system for measuring the voltage and current resulting from the energizing of the surface, a weather station configured to acquire environmental data from the surface, the monitoring device comprising a control unit configured to

[0014] - control the power source according to a power profile7

[0015] - determine from the measured current and voltage an insulation impedance induced by the surface,

[0016] - determine a level of soiling of the surface in relation to an initial “clean” state of the surface from the impedance measurement and environmental data from a weather station. The device according to the invention is advantageously supplemented by the following characteristics, taken independently or in one of their technically possible combinations:

[0017] - the control unit and the voltage and current measuring system are configured to determine the level of soiling of the surface from measurements of alternating voltage and current at variable frequencies by deducing from these measurements an electrical capacitance induced by the surface, or from measurements of direct voltage and current, by deducing from these measurements an electrical resistance induced by the state of the surface,

[0018] - a conductive frame frames the monitored surface, the power source being adapted to be connected to the frame on one side and to positive and negative polarities of the surface on the other, the voltage and current measuring system measuring the voltage and current flowing in the frame;

[0019] - the power source is configured to be connected to several of the points on the monitored surface, and the voltage and current measuring system measures the voltage and current flowing in the frame;

[0020] - environmental data measured by the weather station includes particle quantity, particle mass, humidity, illumination level, ambient temperature, surface temperature and dew point, rain gauge, wind speed, spectrometer, date and time;

[0021] - the voltage and current measuring system comprises a voltage divider bridge and / or a current measuring resistor;

[0022] - the control unit comprises a communication system configured to emit a warning message when the level of soiling exceeds a predetermined threshold;

[0023] - the voltage applied to the surface by the power source is between 1000 volts and 10,000 volts.

[0024] The invention also relates to a method for calibrating a monitoring device comprising the following steps:

[0025] - arrange a reference surface equipped with the monitoring device in an environment in which environmental data is monitored;

[0026] - acquire measurements of an insulation resistance of the reference surface and a level of soiling between states of the reference surface at a succession of predetermined times and an initial “clean” state, the environmental data varying during the succession of times;

[0027] - establish, from the measurements acquired, a model linking the level of soiling to the insulation impedance and environmental data.

[0028] The process steps may be supplemented by the following features, taken independently or in one of their technically possible combinations:

[0029] - the acquisition step includes the acquisition of measurements of volume, covered surface area and mass of the particles deposited on the reference surface during the succession of instants as well as the electrical impedance of the reference surface during the succession of instants, the model being established by taking these measurements into account,

[0030] - the model is established by performing a regression on the acquired measurements using a machine learning method.

[0031] DESCRIPTION OF FIGURES

[0032] Other characteristics, aims and advantages will emerge from the following description, which is purely illustrative and not limiting, and which must be read in conjunction with the attached drawings in which:

[0033] Figure 1 schematically represents a device for monitoring a surface according to one embodiment of the invention;

[0034] Figure 2 schematically represents a detail of an embodiment of the surface monitoring device;

[0035] Figure 3 is a flowchart illustrating steps of a method for calibrating a monitoring device according to one embodiment of the invention.

[0036] Throughout the figures, similar elements have identical references.

[0037] DETAILED DESCRIPTION

[0038] Referring to Figure 1, a monitoring device 1 according to the invention is installed to monitor the cleanliness of a surface 2, for example the surface 2 of a solar module. The monitoring device 1 comprises an electrical power source 3, connected to the surface 2 so as to energize it. For example, such a power source 3 may be a battery connected to a voltage boost converter, the converter being connected by conductive cables at two points close to the surface. Typically, the voltage is raised from between 3.7V to 5V up to 1500V to 20,000V. Such converters have the advantage of being very inexpensive. The surface 2 is also connected to a voltage and current measuring system 4, comprising for example a voltage divider bridge and a current measuring resistor, in other words a current shunt. The high voltage applied to the surface facilitates the measurement of the current.

[0039] A control unit 5 is configured to control the power source 3 according to a specific power profile. The power profile applies, for example, an increasing and then decreasing voltage so that the voltage and current measuring system 4 measures the current accurately, this being particularly low. The voltage and current measurements make it possible to calculate the insulation impedance of the surface 2, i.e. an insulation resistance of the surface 2 when the voltage of the power profile is direct current, or an electrical capacitance in the case where the voltage of the power profile is alternating current. In the direct current case, the insulation resistance varies according to the amount of dirt present on the surface 2, the resistance decreasing and the leakage current increasing when dirt particles are deposited on the surface 2.Throughout the present description, dirt and fouling are understood to mean the presence in contact with the surface 2 of any particle deposited by the environment, whether pollen, earth, sand, etc. Conversely, the term "clean" or initial state, which is the state of the surface 2 used as a reference, is understood to mean a state where the surface 2 is new and contains a zero or negligible quantity of particles.

[0040] The control unit 5 uses an algorithm for determining the soiling of the surface 2. Since many environmental data affect the insulation resistance of the surface 2, in particular humidity, the determination algorithm takes into account the measurements from a weather station 6. The weather station 6 comprises a set of environmental sensors, among which may be a particle detector, a hygrometer, a brightness sensor and a thermometer measuring an ambient temperature, a thermometer measuring the temperature of the surface 2, a dew point sensor, a rain gauge, an anemometer, a spectrometer, a clock. Advantageously, the weather station 6 comprises several particle sensors configured to detect particles of different sizes and the mass of these particles.

[0041] From these measurements and the insulation resistance, a model makes it possible to determine a level of soiling, which corresponds to a ratio between a state of the surface 2 called current, that is to say at the time of calculation by the control unit 5, the time being defined by an internal clock of the control unit 5, and an initial state of the surface 2 which preferably corresponds to the time of commissioning of the monitoring device 1, the time when the surface 2 was clean, free of any particles and was therefore, in the case of a solar module, at its maximum efficiency. When the soiling ratio exceeds a predetermined threshold value, a message can be sent by a communication unit 51 of the control unit 5 to a remote management platform, which can then initiate maintenance operations in order to restore the full efficiency of the solar modules.

[0042] The power profile applied by the power source 3 to the surface 2 can be modified by the control unit 5 according to the weather conditions determined by the measurements of the weather station 6 in order to accurately extract the insulation parameters, in particular the insulation impedance.

[0043] The accuracy of the soiling level determined by the model can be further improved when the control unit 5 controls a power supply profile in which the applied voltage is alternating. Such an alternating power supply profile can be used in succession with the continuous power supply profile described previously. The insulation impedance of the surface 2 is then deduced from the measurements of the alternating voltage and current, provided by the measuring system 4 and represents an electrical capacitance of the surface 2. The determination algorithm uses in this case a model determining from the electrical capacitance and the data from the weather station 6 a soiling level relative to the initial state. Indeed, the particles on the surface 2 form a “parasitic” capacitance, of variable value depending on the granulometry of the particles present.

[0044] Preferably, in the case where measurements based on alternating current and voltage are used, the frequency of the power supply profile is around 1 Hz to 100 kHz. This allows for additional precision in the measurements because the dirt particles induce a different parasitic capacitance depending on their typologies, capacitance which is measured in this value range.

[0045] Both voltage supply profiles, direct or alternating, can be used independently or one after the other.

[0046] Advantageously, the surface 2 is framed by a conductive frame 7, for example made of aluminum, which is in contact with the entire periphery of the surface 2. The power source 3 and the voltage and current measuring system 4 are also connected to the frame 7 on one side and to the positive and negative polarities on the other.

[0047] Insulation resistance is defined as the resistance value between the module frame and the polarities. This depends on material constants (e.g., glass resistivity, etc.), the module size, and also the surface condition of the module. Insulation resistance can also be the surface resistance, which is measured by measuring probes, typically two or more probes placed on the surface 2. These surface probes can be of any type, for example, an electrically conductive adhesive tape made of aluminum from the RAJA™ brand or copper from the MANUTAN™ brand, a copper plate, etc.

[0048] The same definition can be applied to insulation capacity which is measured in the case of alternating currents and voltages.

[0049] The surface probes can be placed or glued to the surface 2 and can be placed at equal distances from the frame 7. Preferably, their locations are no longer changed during the measurements, in order to avoid parasitic effects, in particular created by the resistances between the edge of the surface 2 and the frame 7.

[0050] The distance between the probes may be significantly less than the distance to frame 7.

[0051] The invention also relates, with reference to Figure 2, to a method for calibrating a monitoring device 1 according to one of the embodiments above. A first step (S1) consists of placing a reference surface 2 in a controlled environment, for example a closed volume in a laboratory. By reference surface 2, it is meant that this surface 2 has characteristics of area, thickness and material similar to the surface 2 that it is desired to monitor subsequently in real conditions of use. The controlled environment is preferably closed and has characteristics of humidity, illumination and temperature measured continuously. Advantageously, systems make it possible to vary these characteristics precisely, in order to subject the surface 2 to the widest possible range of characteristics during an acquisition step (S2).During the acquisition period, during a succession of instants constituting the measurement intervals, the insulation impedance, the resistance or the capacitance, of the surface 2 is therefore measured for different values ​​of humidity, temperature and illumination, the values ​​preferably varying one by one so that the measurements cover as many situations as possible. At each instant when the insulation impedance is measured, the parameters necessary for calculating the soiling ratio are also measured, which can be defined for example as follows in the case where a surface 2 is a photovoltaic module:.

[0052] B —Ea -I- e -c*RHi+D * ekT1 * ( fl * PMI 2.5 + b * PMI 10 + c * PMI 25 + d * PMI 50 + e * PMI 100)

[0053] = AB —Ea -I- e -c*RHï+D * ekT2 * ( a * PM2 2.5 + b * PM2 10 + c * PM2 25 + d * PM2 50 + e * PM2 100)

[0054] Where SR Ris° is the soiling ratio;

[0055] Rs°i led | e leakage current between the two points of application of the voltage on the dirty module;

[0056] R™ t the reference leakage current of the photovoltaic module between the two points of application of the voltage on the reference module, in amperes, measured under controlled conditions;

[0057] RH2 is the humidity at surface 2 of the reference module, which can be expressed from the ambient humidity and temperature of a module;

[0058] RH1 is the surface humidity 2 of the dirty module, which can be expressed from the ambient humidity and the temperature of a module, expressed without a unit as a percentage; T2 temperature of the clean module in degrees Celsius (°C);

[0059] T1 dirty module temperature in degrees Celsius (°C);

[0060] V2 voltage applied between the two points of the own module in volts (V);

[0061] V1 voltage applied between the two points of the dirty module in volts (V);

[0062] V1 and V2 can be used as nonlinear parameters of R iso ;

[0063] A, B, C, D empirical coefficients estimated in the laboratory, without unit;

[0064] PM particle size value that can be categorized, e.g. less than 2.5pm, between 2.5-1 Opm etc. and a, b, c, d, e) laboratory estimated regression coefficients, unitless.

[0065] All the measurements acquired are then used to establish (S3) a model or an approximation of the law linking the level of soiling to the insulation impedance and to the environmental data.

[0066] The accuracy of the model is considerably improved when the acquisition step (S2) also includes the acquisition of measurements of volume, covered area and mass of particles deposited on the reference surface 2 during the succession of instants. Even more advantageously, the particles are chosen to be similar or identical to those which will be deposited in real conditions on the surface 2.The electrical insulation capacity of the reference surface 2, which is modified by the particles present, is also measured during the succession of instants and at a voltage and current frequency defined in the given range of values ​​and which can be modified, the established model then linking the measurement of the capacity, the measurement of the insulation resistance and the measurements of the environmental data at a specific instant to the soiling ratio between the specific instant and a reference instant, typically an initial state which can be established in the controlled environment from the reference surface 2.

[0067] Of course, the methods described above are also applicable to the case where the reference surface 2 is framed by a conductive frame 7.

[0068] The model can be established by regression, using any suitable methods, particularly machine learning methods, for example using an artificial neural network.

[0069] The device according to the invention is simple, inexpensive and allows continuous monitoring of the cleanliness of a surface without requiring human intervention. The detection method using insulation impedance is particularly suitable for monitoring the cleanliness of photovoltaic cells, since the measurement is independent of losses due to the natural aging of the module.

Claims

CLAIMS 1. Device (1) for monitoring the cleanliness of a surface (2), comprising an electrical power source (3) configured to be connected to the surface (2) in order to energize the surface (2) at two or more points, a system for measuring the voltage and current (4) resulting from the energizing of the surface (2), a weather station (6) configured to acquire environmental data from the surface (2), the monitoring device (1) comprising a control unit (5) configured to - control the power source (3) according to a power profile7 - determine from the measured current and voltage an insulation impedance induced by the surface (2), - determine a level of soiling of the surface (2) compared to an initial “clean” state of the surface (2) from the impedance measurement and environmental data from a weather station (6).

2. Monitoring device according to claim 1, wherein the control unit (5) and the voltage and current measuring system (4) are configured to determine the level of soiling of the surface (2) from measurements of alternating voltage and current at variable frequencies by deducing from these measurements an electrical capacitance induced by the surface (2), or from measurements of direct voltage and current, by deducing from these measurements an electrical resistance induced by the state of the surface (2).

3. Monitoring device according to one of the preceding claims, a conductive frame (7) framing the monitored surface (2), the power source (3) being adapted to be connected to the frame (7) on one side and to positive and negative polarities of the surface (2) on the other, the voltage and current measuring system (4) measuring the voltage and current flowing in the frame (7).

4. Monitoring device according to claim 3, the power source (3) being configured to be connected to several of the points of the monitored surface (2), and the voltage and current measuring system (4) measuring the voltage and current flowing in the frame (7).

5. Monitoring device according to one of claims 1 to 4, wherein the environmental data measured by the weather station (6) comprise a quantity of particles, the mass of the particles, a humidity, a level of illumination, an ambient temperature, a temperature of the surface (2) and a dew point, rain gauge, wind speed, spectrometer, date and time.

6. Monitoring device according to one of claims 1 to 5, wherein the voltage and current measuring system (4) comprises a voltage divider bridge and / or a current measuring resistor.

7. Monitoring device according to one of claims 1 to 6, wherein the control unit (5) comprises a communication system (51) configured to emit a warning message when the level of soiling exceeds a predetermined threshold.

8. Monitoring device according to one of claims 1 to 7, wherein the voltage applied to the surface (2) by the power source (3) is between 1000 volts and 10,000 volts.

9. Method for calibrating a monitoring device according to one of claims 1 to 8, comprising the following steps: - arranging (S1) a reference surface (2) provided with the monitoring device (1) in an environment in which the environmental data are controlled; - acquiring (S2) measurements of an insulation resistance of the reference surface (2) and of a level of soiling between states of the reference surface (2) at a succession of predetermined times and an initial “clean” state, the environmental data varying during the succession of times; - establish (S3), from the acquired measurements, a model linking the level of soiling to the insulation impedance and environmental data.

10. Calibration method according to claim 9, in which the acquisition step (S2) comprises the acquisition of volume measurements, of covered surface and mass of the particles deposited on the reference surface (2) during the succession of instants as well as the electrical impedance of the reference surface (2) during the succession of instants, the model being established (S3) taking these measurements into account.

11. Calibration method according to one of claims 8 or 10, in which the model is established (S3) by carrying out a regression on the acquired measurements using a machine learning method.

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