Cleaning solid-state elements by means of acoustic wave transmission

Abstract

The invention relates to a method for cleaning solid-state elements (8), such as grains of sand or dust particles, that may be found on a surface of a body (5), the cleaning method using at least one transducer (11) acoustically coupled to the body and comprising the steps of: - generating an electrical signal; - applying the electrical signal across the terminals of the transducer; - thereby producing an acoustic wave that propagates through the body, the acoustic wave being a surface wave or a Lamb wave and being such that, under the effect of the acoustic wave, the solid-state elements are moved over the surface of the body in order to be cleared from this surface.

Description

[0001]Cleaning of solid-state elements by emission of acoustic waves The invention relates to the field of cleaning surfaces on which solid-state elements may be found, such as grains of sand, dust particles, etc. These surfaces are, for example, the external faces of the protective plates of photovoltaic panels. BACKGROUND OF THE INVENTION The efficiency of a particular photovoltaic panel is the essential parameter which, combined with its cost and robustness, determines the competitiveness of said panel on the global market for photovoltaic panels. However, it is known that photovoltaic panels lose their efficiency every day due to the accumulation of solid-state elements, which stick to the external face of the protective plate (also called "protective glass", or simply "glass") of the photovoltaic panels and which obstruct the light.These solid-state elements include, for example, grains of sand, dust particles, soil, insects, bird droppings, etc. This effect, present on all photovoltaic panels, is particularly critical when they are installed in hot and arid areas where the quantities of sand and dust likely to be deposited on the photovoltaic panels are significant, where nighttime condensation has the effect of creating strong adhesion of sand and dust, and where the lack of precipitation prevents the natural cleaning of the photovoltaic panels. Thus, photovoltaic panels installed in hot and arid areas lose an average of 1% of their energy production efficiency every day (loss of 2% on the 2nd day, 3% on the 3rd day, and so on) until the day of cleaning.There is an additional risk with bird droppings, which completely obstruct the protective plate on a very localized surface and thus risk causing the destruction of the photovoltaic cell located underneath. Cleaning photovoltaic panels today requires on-site operators to manually clean the panels. Organizing operator travel is complex when the photovoltaic panels are positioned in isolated areas. Furthermore, in the case of large photovoltaic farms, a large number of operators is required. These operators use a significant amount of water to clean the panels. Manual cleaning therefore leads to high operating costs, and also raises the problem of guaranteeing these panels in the event of damage by the operators in charge of cleaning.Prior art solutions, automatic or semi-automatic, are known, which have been developed to overcome the difficulties just mentioned. Among these solutions, we find in particular a solution based on the use of robots that move on the external face of the protective plate of the photovoltaic panels. The robots are equipped with brushes and clean the external face using these brushes. This solution has a certain number of disadvantages. First of all, the robots include batteries and wearing parts that require regular maintenance as well as complex parts which therefore have a limited lifespan, much lower than the warranty period of the photovoltaic panel, which is typically 25 years. These robots are also relatively expensive, which is problematic because, as we saw earlier, the operating cost of the panel is a determining criterion for its attractiveness.In addition, brushes can, in the long term, damage the anti-reflective treatments that have been applied to the external face of the panel's protective plate. Furthermore, the presence of mechanical parts subject to wear makes regular maintenance of these robots necessary. In addition, robots cannot cover the entire solar panel farm simultaneously, so that, despite their use, sand and dust necessarily reduce the productivity of the panels. The problems just mentioned are found at least partially on the surfaces of other types of bodies, and for example on the surfaces of optical devices: cameras, viewfinders, etc.OBJECT OF THE INVENTION The object of the invention is to effectively clean a surface of a body, on which there may be elements in the solid state such as grains of sand or particles of dust, in an automatic, simple, inexpensive manner, without requiring an artificial supply of water or maintenance operations, and by limiting the deterioration of the surface.SUMMARY OF THE INVENTION In order to achieve this aim, a method is proposed for cleaning solid-state elements, such as grains of sand or dust particles, which may be located on a surface of a body, the cleaning method using at least one transducer acoustically coupled with the body, and comprising the steps of: - generating an electrical signal; - applying the electrical signal between terminals of the transducer; - thus producing an acoustic wave propagating in the body, the acoustic wave being a surface wave or a Lamb wave, and being such that, under the effect of the acoustic wave, the solid-state elements are displaced on the surface of the body to be removed from this surface. The cleaning method according to the invention therefore uses surface acoustic waves or Lamb waves, to move and remove from a surface solid-state elements which pollute said surface.The cleaning method is particularly advantageous. It is implemented automatically, without any manual operation, and without artificial water supply (which is a big advantage for cleaning photovoltaic panels located in arid areas). The cleaning method does not use a brush, which significantly reduces long-term deterioration of the body surface. The cleaning method does not require any maintenance operation. The components used to generate the acoustic wave (electronics, transducers) are inexpensive and very simple so that their use does not reduce the overall service life or reliability of the equipment comprising the body whose surface is cleaned. In addition, a cleaning method as previously described is proposed, in which a frequency of the acoustic wave is between 1 MHz and 100 MHz.Further provided is a cleaning method as previously described, wherein the solid-state elements are moved and discharged directly via a force generated by the acoustic wave and transmitted by contact between the surface of the body and the solid-state elements. Further provided is a cleaning method as previously described, wherein the acoustic wave propagates in the body in a propagation direction and in a first direction, and wherein, under the effect of the acoustic wave, the solid-state elements move on the surface of the body in said propagation direction but in a second direction opposite to the first direction.Further provided is a cleaning method as previously described, wherein, when elements in the liquid state are present on the surface of the body, the acoustic wave induces non-linear acoustic phenomena of acoustic streaming and / or radiation pressure, under the effect of which the elements in the liquid state are displaced.Further provided is a cleaning method as previously described, using at least a first transducer and at least a second transducer, the cleaning method comprising the steps of: - generating a first electrical signal, applying the first electrical signal between terminals of the first transducer, and thereby producing a first acoustic wave to directly move and evacuate the elements in the solid state; - generating a second electrical signal, applying the second electrical signal between terminals of the second transducer, and thereby producing a second acoustic wave to move the elements in the liquid state.Further provided is a cleaning method as previously described, wherein the body is inclined and comprises a first end and a second end, the first end being lower than the second end due to the inclination of the body, the first transducer being positioned on a first side of the body comprising the first end, and the second transducer being positioned on a second side of the body comprising the second end.A cleaning method as previously described is further proposed, comprising the steps, carried out each day during which the cleaning method is implemented, of: - generating and applying the first electrical signal between the terminals of the first transducer during a first predefined period included in the day of said day; - generating and applying the second electrical signal between the terminals of the second transducer during a second predefined period included in the night or morning of said day. A cleaning method as previously described is further proposed, using at least one row comprising several transducers acoustically coupled with the body, and comprising the step of applying phase-synchronized electrical signals to the terminals of said transducers.A cleaning method as previously described is further provided, using several rows each comprising at least one transducer acoustically coupled with the body, the cleaning method comprising the step of applying, for each row successively and according to a predefined sequence, electrical signals to the terminals of the transducer(s) of said row. A cleaning method as previously described is further provided, the rows extending successively along a length or a width of the body to form a series of rows, the predefined sequence consisting, for each row of the series of rows, of applying the electrical signals between the terminals of the transducer(s) of said row and then, after a certain duration, of applying the electrical signals between the terminals of the transducer(s) of a following row of the series of rows.Further provided is a cleaning method as previously described, wherein the certain duration is a predefined duration which has been determined during a test phase, the predefined duration being a duration sufficient to remove a quantity of the elements in the solid state, greater than a first predefined threshold, from a portion of the surface of the body extending between said row and the following row or between said row and a previous row of the series of rows.A cleaning method is further provided as previously described, using at least a third transducer and at least a fourth transducer acoustically coupled with the body, and comprising the steps of: - generating a third electrical signal; - applying the third electrical signal between terminals of the third transducer; - thereby producing a third acoustic wave propagating in the body between the third transducer and the fourth transducer; - acquiring a fourth electrical signal produced by the fourth transducer when it receives the third acoustic wave; - analyzing the fourth electrical signal to detect a presence and / or to evaluate a quantity, of the elements in the solid state and / or of the elements in the liquid state, between the third transducer and the fourth transducer.A cleaning method is further provided as previously described, comprising the step of analyzing the fourth electrical signal to evaluate the quantity of elements in the liquid state between the third transducer and the fourth transducer, the second electrical signal being generated and applied only if said quantity of elements in the liquid state is greater than a second predefined threshold. A cleaning method is further provided as previously described, comprising the step of analyzing the fourth electrical signal to evaluate the quantity of elements in the solid state between said third transducer and said fourth transducer, the certain duration being such that, at the end of this certain duration, the quantity of elements in the solid state between said third transducer and said fourth transducer is less than a third predefined threshold.We further propose an electrical control system arranged to implement the cleaning method as previously described. We further propose an electrical control system as previously described, comprising at least one ASIC. We further propose a cleaning device comprising: - at least one transducer arranged to be acoustically coupled with a body; - an electrical control system as previously described. We further propose a photovoltaic panel comprising a protective plate and a cleaning device as previously described, the protective plate being the body with which the at least one transducer is acoustically coupled. The invention will be better understood in the light of the following description of particular non-limiting embodiments of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Reference will be made to the appended drawings, among which: [Fig.1] Figure 1 is a perspective view of a photovoltaic panel, on which the transducers of a cleaning device according to a first embodiment are visible; [Fig. 2] Figure 2 is a figure similar to Figure 1, on which the different stacked layers of the panel are visible; [Fig. 3] Figure 3 represents the electrical control system and the transducers; [Fig. 4] Figure 4 illustrates the movement of the elements in the solid state under the direct effect of the acoustic wave; [Fig. 5] Figure 5 illustrates the movement of the elements in the solid state via the movement of the elements in the liquid state; [Fig. 6] Figure 6 illustrates the combined day / night (or morning) operation of the cleaning device; [Fig. 7] Figure 7 is a figure similar to Figure 1, with a cleaning device according to a second embodiment; [Fig.8] Figure 8 is a figure similar to Figure 1, with a cleaning device according to a third embodiment. DETAILED DESCRIPTION OF THE INVENTION With reference to Figures 1 and 2, a photovoltaic panel 1 is carried by two feet 2a, 2b which extend vertically from a first base 3a placed on the ground, and by two feet 2c, 2d which extend vertically from a second base 3b also placed on the ground. The two feet 2a, 2c have the same length. The two feet 2b, 2d have the same length, which is shorter than that of the two feet 2a, 2b, so that the photovoltaic panel 1 is inclined. This inclination makes it possible to optimize the orientation of the photovoltaic panel 1 relative to the direction of the sun's rays, and therefore to maximize the efficiency of the photovoltaic panel 1. The photovoltaic panel 1 comprises several stacked layers, and a frame 4 (or chassis) forming side walls of the photovoltaic panel 1.The stacked layers have been assembled, for example via a lamination process. The frame 4 makes it possible to improve the rigidity and reinforce the mechanical strength of the photovoltaic panel 1, and to prevent any penetration of water between the stacked layers. The stacked layers notably comprise a protective plate 5, a layer 6 comprising the photovoltaic cells, and a rigid layer 7 (as well as, possibly, intermediate layers which are not shown here). The frame 4 is for example made of aluminum. The protective plate 5 is for example made of glass (tempered, laminated, etc.), or of another transparent or translucent material. The plate 5 is given the name “protective plate”, but, as we saw earlier, it is sometimes also called “protective glass”, or “glass”.Generally speaking, the plate 5 in question here is the plate forming the outer upper layer of the photovoltaic panel 1, that is to say the upper plate which separates the exterior from the interior of the photovoltaic panel 1. Solid state elements 8 may be found on the outer face 9 of the protective plate 5, and for example grains of sand, particles of dust, earth, insects (mosquitoes for example), bird droppings, etc. In particular, in arid areas, sand and dust are deposited significantly on the panel 1. The photovoltaic panel 1 comprises a cleaning device, which makes it possible to clean the outer face 9 of the protective plate 5.With reference to Figure 3, the cleaning device 10 comprises at least one transducer 11, in this case a plurality of transducers 11, which are acoustically coupled with the protective plate 5 of the photovoltaic panel 1, as well as an electrical control system 12. All the transducers 11 are here identical. The transducers 11 are here, and in a non-limiting manner, interdigital comb transducers, with thin layers. Each transducer 11 here comprises a piezoelectric thin layer, a first electrode 14a and a second electrode 14b. The electrodes have been deposited on the piezoelectric layer which itself has been deposited on the external face 9 of the protective plate 5. The first electrode 14a forms a first comb and the second electrode 14b forms a second comb. Each comb comprises a base and a row of fingers extending parallel to each other from the base. The first and second combs are interdigitated.The resonant frequency of the transducer 11 depends on the spacing between the fingers. The electrical control system 12, for its part, comprises a master electrical module 15 and a plurality of elementary electrical modules 16. The master electrical module 15 is connected to all the elementary electrical modules 16. The master electrical module 15 is integrated into a housing 17 which is here positioned under the rigid layer 7 by being fixed thereto. A different positioning of the housing 17 is of course conceivable. The components of the master elementary module 15 can also be integrated into the plate 5 and, conversely, the elementary electrical modules 16 could be positioned in housings. The plate 5 is therefore “instrumented” to implement the invention.The master electrical module 15 is arranged at least to power the elementary electrical modules 16 from electrical energy produced by the photovoltaic panel 1, and to control the elementary electrical modules 16. An input voltage Ve (continuous), produced by the photovoltaic cells of the layer 6, is applied to the input of the master electrical module 15. The master electrical module 15 here comprises a DC / DC converter 18 and produces from the input voltage Ve an output voltage Vs adapted to the operation of the elementary electrical modules 16. The master electrical module 15 comprises a processing component 19 (for example a microcontroller, a processor, an FPGA (for Field Programmable Gate Arrays), an ASIC (for Application Specific Integrated Circuit), etc.) which produces control signals Sc to control the elementary electrical modules 16. The master electrical module 15 also comprises a clock component 20.The master electrical module 15 thus transmits the control signals Sc and a clock signal CLK to the elementary electrical modules 16, the role of which will be specified below. Each elementary electrical module 16 comprises a processing component, in this case an ASIC 22, and is associated with a transducer 11. Each ASIC 22 here comprises a power conversion unit 23, an impedance matching unit 24 and an amplification unit 25. The power conversion unit 23 is powered by the output voltage Vs produced by the master electrical module 15, and generates an electrical signal Se (alternating, and for example square or sinusoidal) at the resonant frequency of the transducer 11. The impedance matching unit 24 makes it possible to optimize the power transfer between the elementary electrical module 16 and the transducer 11 by limiting power reflections.The amplification unit 25 amplifies the electrical signal Se before it is applied between the terminals of the transducer 11. The electrical signal Se, after amplification, is such that the transducer 11 generates a surface acoustic wave or a Lamb wave having a frequency and an amplitude allowing, as will be seen, to move elements in the solid state and elements in the liquid state. By "terminals of a transducer", we mean here the ends of the bases of the combs of the transducer. By "applying an electrical signal between terminals of the transducer", we mean either applying a voltage between the terminals (the signal considered then being a voltage), or circulating a current between the terminals (the signal considered then being a current). The cleaning device 10 consumes on average, over a day, less than 1% of the total electrical energy produced by the panel 1 (or even less than 0.5%).The operation of the cleaning device 10 is now explained. We are first interested in a single transducer 11, with reference to Figures 4 and 5. Solid-state elements 8 (here grains of sand) are present on a surface of the external face 9 of the protective plate 5. The master electrical module 15 transmits a control signal Sc to the elementary electrical module 16 associated with said transducer 11, to activate it. Upon receiving the control signal Sc, the elementary electrical module 16 generates the electrical signal Se, and applies the electrical signal Se between the terminals of the transducer 11. The transducer 11 thus produces an acoustic wave which propagates in the protective plate 5. The acoustic wave is a surface wave (for example a Rayleigh wave or a Love wave), or a Lamb wave.The (fundamental) frequency of the acoustic wave is here between 1 MHz and 100 MHz, advantageously between 10 MHz and 40 MHz, and preferably between 18 MHz and 22 MHz. If the thickness of the protective plate 5 is greater than the wavelength of the acoustic wave, the latter is a surface acoustic wave. Otherwise, it is a Lamb wave. The acoustic wave is such that, under the effect of the acoustic wave, the solid-state elements are displaced on the surface of the external face 9 of the protective plate 5 to be evacuated from this surface. The displacement of the solid-state elements can be achieved in two ways. When only solid state elements 8 are present on the surface, as is the case in Figure 4, the solid state elements 8 are displaced and evacuated directly via a force produced by the acoustic wave and transmitted by contact between the surface and the solid state elements 8.The left-hand drawing of Figure 4 shows the transducer 11 and the solid-state elements 8, prior to the generation of the electrical signal Se and the application of the electrical signal Se between the terminals of the transducer 11. The central drawing of Figure 4 shows the transducer 11 and the solid-state elements 8, while the electrical signal Se is generated and applied between the terminals of the transducer 11. The acoustic wave propagates in the protective plate 5 in a propagation direction and in a first direction S1. Under the effect of the acoustic wave, the solid-state elements 8 move on the surface of the external face 9 of the protective plate 5 in said propagation direction but in a second direction S2 opposite to the first direction S1.This movement of the solid-state elements 8, in a direction opposite to the direction of propagation of the acoustic wave, is a phenomenon specific to the interactions between surface acoustic or Lamb waves propagating in a body, and solid-state elements present on a surface of said body. This phenomenon was observed during all the tests carried out during the development of the cleaning device described here. The tests were notably carried out with acoustic wave frequencies equal to 17.6 MHz and 19.6 MHz, with grains of sand (grain size typically between 100 μm and 2 mm) and plastic particles (for example polyamide 11; particle diameter typically equal to 30 μm). We see, in the right-hand drawing of figure 4, that the solid state elements 8 have been evacuated (in less than 10 s typically) from the surface located "in front of" the transducer (i.e. from the surface external to the transducer and located on the finger side).When elements in the liquid state 27, such as water drops, are present on the external face 9 of the protective plate, the acoustic wave induces non-linear acoustic phenomena of acoustic streaming and / or radiation pressure, under the effect of which the elements in the liquid state 27 are displaced. The solid state elements 8 are then displaced and evacuated by being carried by the liquid state elements 27. The left-hand drawing of Figure 5 shows the transducer 11, the solid state elements 8 and the liquid state elements 27, prior to the generation of the electrical signal Se and the application of the electrical signal Se between the terminals of the transducer 11. The central drawing of Figure 5 shows the transducer 11, the solid state elements 8 and the liquid state elements 27, while the electrical signal Se is generated and applied between the terminals of the transducer 11.This time, under the effect of the acoustic wave, the liquid state elements 27 move on the surface of the external face 9 of the protective plate 5 in the direction of propagation of the acoustic wave, and in the same direction S, and drive the solid state elements in this direction and in this direction S. We see, in the right-hand drawing of Figure 5, that the solid state elements 8 have been carried away by the liquid state elements 27 and evacuated from the surface located in front of the transducer. It is important to note that this effect cannot be obtained in a regime of drop sliding, but only in a forced regime of displacement induced by the acoustic waves (because otherwise the drops remain blocked on the impurities). We now return to Figure 1.The transducers of the cleaning device 10 according to a first embodiment comprise at least one first transducer 11a, in this case several first transducers 11a, and at least one second transducer 11b, in this case several second transducers 11b. The first transducers 11a here form a first row 30 of first transducers 11a (here, "first transducers 11a" means all the transducers of the first row 30). The second transducers 11b here form a second row 31 of second transducers 11b (here, "second transducers 11b" means all the transducers of the second row 31). The rows 30, 31 extend parallel to the widths of the protective plate 5 (and therefore of the photovoltaic panel 1).The protective plate 5 comprises a first end 32 and a second end 33, the first end 32 being lower than the second end 33 due to the inclination of the photovoltaic panel 1. The first row 30 of first transducers 11a is positioned on a first side of the protective plate 5 comprising the first end 32, and the second row 31 of second transducers 11b is positioned on a second side of the protective plate 5 comprising the second end 33. For each first transducer 11a of the first row 30, the electrical control system 12 generates a first electrical signal, applies the first electrical signal between the terminals of the first transducer 11a, and thus produces a first acoustic wave 34a to directly move and evacuate the solid-state elements 8.Similarly, for each second transducer 11b of the second row 31, the electrical control system 12 generates a second electrical signal, applies the second electrical signal between terminals of the second transducer 11b, and thus produces a second acoustic wave 34b to move the elements in the liquid state 27. The first transducers 11a are advantageously used when only elements in the solid state 8 are a priori likely to be present on the external face 9 of the protective plate 5, that is to say during the day. The second transducers 11b are advantageously used when elements in the liquid state 27 are a priori likely to be present on the external face 9 of the protective plate 5, that is to say, mainly at night, or in the morning. These elements in the liquid state are mainly drops of water, and more precisely drops of condensation due to dew or drops of rain.Thus, each day during which the cleaning device 10 is activated, the electrical control system 12 implements the cleaning method, and generates and applies the first electrical signals between the terminals of the first transducers 11a during a first predefined period included in the day of said day (for example between 9 a.m. and 7 p.m.). The electrical control system 12 generates and applies the second electrical signals between the terminals of the second transducers 11b during a second predefined period included in the night or morning of said day (for example between 5 a.m. and 7 a.m.). Thus, during the day, the vibrations induced by the acoustic waves detach and move the solid-state elements 8 towards the bottom of the panel 1. At night (or in the morning), these same vibrations move the condensation drops via non-linear acoustic effects which clean the surface of the panel 1.Dry cleaning is therefore combined during the day with cleaning via the movement of condensation drops at night. Note that here, as the electrical control system 12 is powered by the photovoltaic cells of the photovoltaic panel 1, operation of the second row 31 in the morning will be preferred. However, it would be possible to equip the master electrical module 15 with a battery that recharges during the day, so as to operate the cleaning device 10 at night. The combined mode is extremely advantageous. With reference to Figure 6, during the day (left drawing), when there are no (a priori) elements in the liquid state 27 on the external face 9 of the protective plate 5, the first acoustic waves 34a produced by the first transducers 11a propagate towards the upper part of the photovoltaic panel 1, which has the effect of moving the elements in the solid state 8 downwards.This force, which is added to gravity, makes it possible to efficiently move and evacuate the solid-state elements from the surface located above the first row 30 of first transducers 11a. At night or in the morning (right drawing), when liquid-state elements 27 are present, the second acoustic waves 34b produced by the second transducers 11b propagate towards the lower part of the photovoltaic panel 1, which has the effect of moving the liquid-state elements 27 downwards. Again, this force, which is added to gravity, makes it possible to efficiently move the liquid-state elements 27 downwards, and therefore to efficiently evacuate the solid-state elements 8 from the surface of the external face 9 of the protective plate 5. The use of these combined effects is extremely clever. Indeed, as we saw earlier, the cementing process, resulting from condensation, usually poses a problem.A strong prejudice of the person skilled in the art is therefore overcome by taking advantage of condensation. It should be noted that the solid-state elements 8 are not necessarily completely removed from the entire external face 9; it is possible that a small number of solid-state elements 8 remain on the protective plate 5, in particular on surfaces very close to the edges and / or the transducers. These residual elements have a minimal impact on the efficiency of the panel 1. Advantageously, the electrical control system 12 applies first electrical signals, which are synchronized in phase, to the terminals of the first transducers 11a. This makes it possible to increase the amplitude of the first acoustic waves (at constant amplitude of the first electrical signals). Similarly, the electrical control system 12 applies second electrical signals, which are synchronized in phase, to the terminals of the second transducers 11b.To produce the first phase-synchronized electrical signals (respectively, the second phase-synchronized electrical signals), the elementary electrical modules 16 associated with the first transducers 11a (respectively, associated with the second transducers 11b), use the clock signal CLK transmitted by the master electrical module 15, and are therefore all synchronized to the same clock. Thus, in the first embodiment of the cleaning device, the bottom row of transducers (first row 30) is used to move and evacuate the solid-state elements 8, and the top row of transducers (second row 31) is used to move the liquid-state elements 27 and therefore, at the same time, the solid-state elements 8. The photovoltaic panel 1 comprises a first end (corresponding to the first end 32 of the plate 5) and a second end (corresponding to the second end 33 of the plate 5).Advantageously, the upper face of the photovoltaic panel 1 is flat and has no discontinuity, at least at the first end of the photovoltaic panel 1 (lower end). The photovoltaic panel 1, and in any case its lower part, is therefore designed in particular to avoid there being a rim. Here, the frame 4 comprises an upper surface 36 which extends over a contour of the protective plate 5. The upper surface 36 of the frame 4 and the external face 9 of the protective plate 5 are coplanar, at least at the first end of the photovoltaic panel 1. This smooth surface allows the elements in the solid state 8 (and also the elements in the liquid state 27) to be evacuated without being retained on the photovoltaic panel 1. It is noted that other arrangements are possible to obtain a perfectly flat surface. For example, it could be envisaged not to use a frame 4.It could also be envisaged that the protective plate 5 covers the upper surface 36 of the frame 4 and is fixed thereto (by gluing for example). In these two cases, the external face 9 of the protective plate 5 then extends over the entire upper face of the photovoltaic panel 1. A second embodiment of the cleaning device is now described with reference to FIG. 7. The cleaning device comprises several rows, each comprising at least one transducer acoustically coupled with the protective plate 5. Here, more precisely, the cleaning device comprises six rows of transducers. The six rows comprise three first rows 30a, 30b, 30c of first transducers 11a (which each produce first acoustic waves propagating upwards), and three second rows 31a, 31b, 31c of transducers (which each produce second acoustic waves propagating downwards).Therefore, here, by "first transducers 11a", we mean all the transducers of the first rows 30a, 30b, 30c, and by "second transducers 11b", all the transducers of the second rows 31a, 31b, 31c. The first rows 30a, 30b, 30c and the second rows 31a, 31b, 31c are positioned alternately on the photovoltaic panel (i.e. each row adjacent to a first row is a second row, and vice versa). The highest row on the panel 1 is the second row 31a, which is followed, going downwards, by the first row 30a, which is itself followed by the second row 31b, etc. The first rows 30a, 30b, 30c extend successively along the length of the protective plate 5 to form a first series of rows 38. The second rows 31a, 31b, 31c extend successively along the length of the protective plate 5 to form a second series of rows 39.We are first interested in the first rows 30a, 30b, 30c. The electrical control system 12 applies, for each first row 30a, 30b, 30c successively and according to a first predefined sequence, first electrical signals to the terminals of the first transducers 11a of said first row. For each first row, the first predefined sequence consists of applying the first electrical signals between the terminals of the first transducers 11a of said first row and then, after a certain duration, applying the first electrical signals between the terminals of the first transducers of the next first row of the first series of rows 38. The first rows are thus activated successively, one after the other. The first transducers 11a thus interfere constructively. For example, the first row 30a, the highest on the photovoltaic panel 1, is activated first.Then, after a certain duration, the first row 30b is activated. Then, after a certain duration, the first row 30c is activated. If it is necessary to perform several cycles, the next row of the first row 30c can be defined as the first row 30a. The certain durations can be different depending on the rows. Each certain duration, which is therefore the duration separating the activation of a first row from the activation of the next first row, is for example a predefined duration that was determined during a test phase. By "test phase" is meant here any phase carried out prior to the commissioning of the photovoltaic panel 1 (or at least of the cleaning device): the test phase may have been carried out in the laboratory, in the factory, or even on site. Previously acquired knowledge on the typical dynamics of solid-state elements 8 is thus used.The predefined duration is typically equal to 3 s; it may vary according to the rows. For each first row, the predefined duration is a duration sufficient to evacuate a quantity of the solid-state elements 8, greater than a first predefined threshold, from a portion of the surface of the external face 9 extending between said first row and the first preceding row. For example, the first row 30b is therefore activated, and the solid-state elements 8 are waited for to be evacuated from the surface separating the first row 30a from the first row 30b, before activating the first row 30c. To define the predefined duration associated with the first row 30a, it is considered for example that the position of the first preceding row corresponds to the second end 33 of the protective plate 5. The first predefined threshold is for example equal to 50%.After the certain duration, it is therefore estimated that the cleaning device, by the action of each first row, has made it possible to evacuate at least 50% of the solid-state elements located between said first row and the first preceding row. As has been seen, the first rows 30 are preferably activated during the day. At night, or in the morning, the second rows 31 are activated. The electrical control system 12 likewise applies, for each second row 31 successively and according to a second predefined sequence, second electrical signals to the terminals of the second transducers 11b of said second row 31.For each second row 31, the second predefined sequence consists of applying the second electrical signals between the terminals of the second transducers 11b of said second row and then, after a certain duration, applying the second electrical signals between the terminals of the second transducers 11b of the next second row of the second series of rows. The second rows 31 are thus activated successively, one after the other. The second row 31a, the highest on the photovoltaic panel 1, is activated first. Then, after a certain duration, the second row 31b is activated. Then, after a certain duration, the second row 31c is activated. If it is necessary to make several cycles, it can be defined that the next row of the second row 31c is the second row 31a. The certain durations may be different depending on the rows. The certain durations may be different from those defined for the first rows.For each second row, the predefined duration is a duration sufficient to evacuate a quantity of the elements in the solid state (via the elements in the liquid state), greater than a first predefined threshold, from a portion of the external surface extending between said second row and the second following row (and not the previous one, this time). To define the predefined duration associated with the second row 31c, it is considered that the position of the following row corresponds to the first end 32 of the protective plate 5 (end corresponding to the first end 32 of the panel 1). The first predefined threshold is for example again equal to 50%. After the certain duration, it is therefore estimated that the cleaning device, by the action of each second row, has made it possible to evacuate at least 50% of the elements in the solid state located between said second row and the second following row. The rows of transducers can of course be arranged differently.Thus, in a third embodiment of the cleaning device 10, visible in Figure 8, it can be seen that the first three rows 30a, 30b, 30c are positioned on the lower part of the panel 1, while the second three rows 31a, 31b, 31c are positioned on the upper part of the panel 1. The first predefined sequence for the first series of rows 38, and the second predefined sequence for the second series of rows 39, can be the same as those which have just been described. It has therefore been seen that the generated acoustic waves can be used to directly move the elements in the solid state 8, and also the elements in the liquid state 27 in order to evacuate the elements in the solid state 8. The acoustic waves can also be used to detect the presence of these elements, and possibly to evaluate the quantity of these elements present on the external face 9 of the protective plate 5.It is thus possible to determine, on the one hand, the state of pollution of the panel 1 by the solid-state elements 8, and on the other hand, the quantity of condensation drops present on the panel 1. For this purpose, at least one emission transducer and at least one reception transducer are used. The emission transducer emits an acoustic wave which, after having traveled through the protective plate 5 along a path of predefined length, is received by the reception transducer. Analysis of the electrical signal produced by the reception transducer makes it possible to detect and possibly evaluate the quantity of the elements. The cleaning device 10 therefore comprises at least a third transducer 11c (emission) and at least a fourth transducer 11d (reception) acoustically coupled with the protective plate 5.The elementary electrical module 16 associated with the third transducer 11c generates a third electrical signal and applies the third electrical signal between the terminals of the third transducer 11c. A third acoustic (detection) wave is thus produced and propagates in the protective plate 5 between the third transducer 11c and the fourth transducer 11d. The elementary electrical module 16 associated with the fourth transducer 11d acquires a fourth electrical signal produced by the fourth transducer 11d when the latter receives the third acoustic wave. The elementary electrical module 16 associated with the fourth transducer 11d thus analyzes the fourth electrical signal to detect a presence and / or to evaluate a quantity, of the elements in the solid state 8 and / or of the elements in the liquid state 27, between the third transducer 11c and the fourth transducer 11d.The analysis of the fourth electrical signal consists, for example, of comparing its amplitude with one or more predefined thresholds, of carrying out a phase measurement, of detecting a zero crossing time, of analyzing particular lobes, etc. Of course, the third transducer(s) 11c may be one or more of the first transducers 11a previously described, or one or more of the second transducers 11b previously described. Similarly, the fourth transducer(s) 11d may be one or more of the first transducers 11a previously described, or one or more of the second transducers 11b previously described. This is not necessary, and it would be perfectly possible to have transducers 11c, 11d dedicated to detection.The ASICs 22 of the elementary electrical modules 16 associated with the fourth transducers 11d (and, here, in this case, the ASICs 22 of all the elementary electrical modules 16) each comprise a detection module 40 (visible in FIG. 3), which acquires the fourth electrical signal and performs the analysis to detect the presence and / or to evaluate the quantity of the elements in the solid and / or liquid state. We return to FIG. 6. We see in the right-hand drawing a first transducer which is also a third transducer 11c. We see a second transducer which is also a fourth transducer 11d. The third transducer 11c and the fourth transducer 11d are for example used in the following manner. At night (or in the morning), the master electrical module 15 controls the emission, by the elementary electrical module 16 associated with the third transducer 11c, of a third acoustic (detection) wave.The third transducer 11c generates the third acoustic wave, which is received by the fourth transducer 11d. The elementary electrical module 16 associated with the fourth transducer 11d analyzes the fourth electrical signal generated by the fourth transducer 11d to evaluate the quantity of elements in the liquid state 27 between the third transducer 11c and the fourth transducer 11d, and transmits a detection signal Sd (visible in FIG. 3) to the master electrical module 15. The master electrical module 15 then controls the elementary electrical modules 16 associated with the second transducers 11b of the second row 31, so that these generate and apply the second electrical signals to the terminals of the second transducers 11b of the second row 31 only if the quantity of elements in the liquid state 27 is greater than a second predefined threshold.The quantity of elements in the liquid state 27 is for example a number of drops on a rectangular surface 41, the length of which is the distance between the third transducer 11c and the fourth transducer 11d, and the width of which is the width of these transducers. The second predefined threshold is for example equal to 100 drops. This ensures that the second transducers 11d are activated only if there are enough drops on the external face 9 of the protective plate 5. We now return to Figure 7. We see a second transducer of the second row 31a which is also a third transducer 11c. We see a first transducer of the first row 30a which is a fourth transducer 11d. We also see a first transducer of the first row 30b which is a fourth transducer 11d. We also see a first transducer of the first row 30c which is a fourth transducer 11d.As seen earlier, the first electrical signals are applied to the terminals of the first transducers 11a of each first row 30, for each first row 30 successively and according to a predefined sequence. The predefined sequence consists, for each first row, in applying the first electrical signals between the terminals of the first transducers of said first row and then, after a certain duration, in applying the first electrical signals between the terminals of the first transducers of the following row. For each first row 30 (i.e. 30a, 30b or 30c), when said first row is activated, the master electrical module 15 here regularly or continuously controls the emission of a third acoustic wave by the third transducer 11c. The fourth transducer 11d of said first row 30 receives the third acoustic wave.The elementary electrical module 16 associated with said fourth transducer 11d analyzes the fourth electrical signal to evaluate the quantity of solid-state elements 8 between the third transducer 11c and said fourth transducer 11d. The certain duration is such that, at the end of this certain duration, the quantity of solid-state elements 8 between the third transducer and said fourth transducer is less than a third predefined threshold. For example, the first row 30c is of interest. The quantity of solid-state elements 8 is, for example, a number of particles on a rectangular surface 42, the length of which is the distance between the third transducer 11c and the fourth transducer 11d of said first row 30c, and the width of which is the width of these transducers. The third predefined threshold is, for example, equal to 1000 particles. This operation is repeated successively for each first row 30.For each first row 30, it is thus ensured, before moving on to the next first row, that, under the effect of the first acoustic waves emitted by said first row, the surface of the external face 9 of the protective plate 5 located above said first row is sufficiently cleaned. It could also be envisaged, for example, in the configuration of FIG. 7, to have a third transducer 11c on the second row 31a and a fourth transducer 11d on the first row 30a (or vice versa), a third transducer 11c on the second row 31b and a fourth transducer 11d on the first row 30b (or vice versa), a third transducer 11c on the second row 31c and a fourth transducer 11d on the first row 30c (or vice versa). This configuration makes it possible to have, for each pair of rows 31a, 30a; 31b, 30b; 31c, 30c, a third transducer and a fourth transducer opposite and very close to each other.This configuration makes it possible to detect very efficiently and very precisely the presence and / or the quantity of elements in the solid state and / or elements in the liquid state between two adjacent rows. Of course, in the case of Figure 1 as well as in that of Figure 7 or Figure 8, the acoustic waves used to move the elements in the solid state 8 or the elements in the liquid state 27 can also be used to carry out the detection. In the case of Figure 1, for example, a first transducer 11a could be used in transmission (it therefore plays the role of a third transducer 11c), and a second transducer 11b in reception (it therefore plays the role of a fourth transducer 11d). During the day, a first acoustic wave is emitted to evacuate the elements in the solid state. Then, a third acoustic wave is emitted to detect whether the cleaning has been effective. If this is the case, cleaning is stopped.Otherwise, a first acoustic wave is emitted again. A third acoustic wave can also be emitted for detection purposes regularly, until a significant presence of the solid-state elements 8 is detected. A first acoustic wave is then emitted for cleaning. It is therefore understood that the first waves can be third waves, and vice versa. Similarly, the second waves can be third waves, and vice versa. The first and second electrical signals can be, but not necessarily, identical. The first electrical signals and / or the second electrical signals can also be identical to the third electrical signals, but not necessarily. The level of the third electrical signals, when used for detection only, can in particular be lower than the level of the first and second electrical signals, used to move the solid 8 and liquid 27 elements.Of course, the invention is not limited to the embodiments described but encompasses any variant falling within the scope of the invention as defined by the claims. The cleaning method is not necessarily implemented to clean the external face of a protective plate of a photovoltaic panel, but can be used to clean a surface of another type of body. The invention can be implemented for any body in which surface acoustic waves or Lamb waves can propagate. The bodies in question include, for example, surfaces of optical devices: cameras, viewfinders, etc. The cleaning method is particularly interesting in the case of equipment which comprises at least one component which captures light rays for its operation, and a transparent or translucent body through which these light rays pass before being captured by said component(s).The cleaning method is then used to clean an internal or external surface of said body. In the case of a photovoltaic panel, the components in question are the photovoltaic cells. In the case of a camera or viewfinder, the components are the photosensitive sensors. The transducer(s) used may be different from those described here. Any type of transducer capable of producing surface acoustic waves or Lamb waves may be used. In the case of cleaning a surface of a piezoelectric body, the electrodes of the transducers could be applied directly to the body. An angle transducer technology could be used. A transducer and a shoe are used, positioned between the transducer and the surface of a body to be cleaned. The shoe tilts the axis of the transducer so that said axis forms a certain angle with the normal to the surface to be cleaned.A bulk acoustic wave propagates in the shoe. The certain angle is defined (Snell-Descartes law) so that the bulk acoustic wave transforms into a surface wave at the interface between the shoe and the surface to be cleaned. Other systems for transforming bulk acoustic waves into surface acoustic waves could be used. The transducers are not necessarily positioned on the surface of the face to be cleaned; they could be positioned on a surface of a face opposite the said face to be cleaned (in the case of a plate-shaped body in particular). The architecture of the electrical control system could of course be different from that described here. For example, one could have a single electrical module for the entire panel.This single electrical module would be connected to all the transducers and would perform all the functions described here: generation of electrical signals, analysis of signals for detection, etc. This single electrical module could, for example, include an ASIC (or an FPGA) and power amplifiers. It would also be possible to have a single elementary electrical module per row of transducers. The master module and / or the elementary modules, or the single electrical module, could also, instead of an ASIC or an FPGA, include another type of processing component, and for example a "general-purpose" processor, a processor specialized in signal processing (or DSP, for Digital Signal Processor), a microcontroller. The master module and / or the elementary modules, or the single electrical module, then include one or more memories (and in particular one or more non-volatile memories), connected to or integrated into the processing component.At least one of these memories forms a computer-readable recording medium, on which is recorded at least one computer program comprising instructions that cause the processing component to execute at least some of the steps of the cleaning process. It is also possible, in a system of several photovoltaic panels, to have only one panel that detects the presence of elements in the liquid or solid state, and which transmits to the other panels information relating to this detection. The transmission can be made by any means of communication, wired or wireless. The arrangement of the transducers could be different from those described here. The cleaning device can operate with a single transducer. Each row of transducers may comprise only one transducer. It is possible to have a single series of rows.In each series of rows comprising several rows, said rows could extend successively along a width and not a length of the body. In a row of transducers, all the transducers are not necessarily oriented in the same way. We could also have, in a row of transducers, a transducer dedicated to the detection of the presence of elements in the solid or liquid state. In a row of transducers, the transducers could be connected together in parallel (or some in series, and others in parallel). The transducers can be grouped without forming rows. The transducers carrying out the detection could be positioned differently from the other transducers. The acoustic wave for detection could thus be emitted across the width of the body. The use of transducers to carry out the detection is not mandatory.Cleaning activation could be performed automatically, for example at regular intervals (e.g. daily), without prior detection.

Claims

CLAIMS 1. A method of cleaning solid-state elements (8), such as grains of sand or dust particles, which may be located on a surface of a body (5), the cleaning method using at least one transducer (11) acoustically coupled with the body, and comprising the steps of: - generating an electrical signal (Se); - applying the electrical signal between terminals of the transducer; - thereby producing an acoustic wave (34a, 34b) propagating in the body, the acoustic wave being a surface wave or a Lamb wave, and being such that, under the effect of the acoustic wave, the solid-state elements are moved on the surface of the body to be removed from this surface.

2. A cleaning method according to claim 1, wherein a frequency of the acoustic wave is between 1 MHz and 100 MHz. 3.A cleaning method according to any preceding claim, wherein the solid-state elements (8) are moved and evacuated directly via a force produced by the acoustic wave and transmitted by contact between the surface of the body and the solid-state elements.

4. A cleaning method according to claim 3, wherein the acoustic wave propagates in the body in a propagation direction and in a first direction (S1), and wherein, under the effect of the acoustic wave, the solid-state elements move on the surface of the body in said propagation direction but in a second direction (S2) opposite to the first direction.

5. Cleaning method according to one of the preceding claims, in which, when elements in the liquid state (27) are present on the surface of the body, the acoustic wave induces non-linear acoustic phenomena of acoustic streaming and / or radiation pressure, under the effect of which the elements in the liquid state are displaced. 6.A cleaning method according to claims 3 and 5, using at least one first transducer (11a) and at least one second transducer (11b), the cleaning method comprising the steps of: - generating a first electrical signal, applying the first electrical signal between terminals of the first transducer, and thereby producing a first acoustic wave for directly moving and evacuating the solid-state elements (8); - generating a second electrical signal, applying the second electrical signal between terminals of the second transducer, and thereby producing a second acoustic wave for moving the liquid-state elements (27). 7.A cleaning method according to claim 6, wherein the body (5) is inclined and comprises a first end (32) and a second end (33), the first end being lower than the second end due to the inclination of the body, the first transducer being positioned on a first side of the body comprising the first end, and the second transducer being positioned on a second side of the body comprising the second end.

8. A cleaning method according to claim 6 or 7, comprising the steps, carried out each day during. of which the cleaning method is implemented, of: - generating and applying the first electrical signal between the terminals of the first transducer (11a) during a first predefined period included in the day of said day; - generating and applying the second electrical signal between the terminals of the second transducer (11b) during a second predefined period included in the night or the morning of said day.

9. Cleaning method according to one of the preceding claims, using at least one row (30, 31) comprising several transducers acoustically coupled with the body, and comprising the step of applying phase-synchronized electrical signals to the terminals of said transducers. 10.

11. A cleaning method according to claim 10, the rows extending successively along a length or a width of the body to form a series of rows (38, 39), the predefined sequence consisting, for each row of the series of rows, of applying the electrical signals between the terminals of the transducer(s) of said row and then, after a certain duration, applying the electrical signals between the terminals of the transducer(s) of a following row of the series of rows.

12. A cleaning method according to claim 11, wherein the certain duration is a predefined duration which has been determined during a test phase, the predefined duration being a duration sufficient to remove a quantity of the elements in the solid state, greater than a first predefined threshold, from a portion of the surface of the body extending between said row and the following row or between said row and a preceding row of the series of rows. 13.Cleaning method according to one of the preceding claims, using at least one third transducer (11c) and at least one fourth transducer (11d) acoustically coupled with the body, and comprising the steps of: - generating a third electrical signal; - applying the third electrical signal between terminals of the third transducer; - thereby producing a third acoustic wave propagating in the body between the third transducer and the fourth transducer; - acquiring a fourth electrical signal produced by the fourth transducer when the latter receives the third acoustic wave; - analyzing the fourth electrical signal to detect a presence and / or to evaluate a quantity, of the elements in the solid state (8) and / or of the elements in the liquid state (27), between the third transducer and the fourth transducer.

14. A cleaning method according to claims 6 and 13, comprising the step of analyzing the fourth electrical signal to evaluate the quantity of elements in the liquid state between the third transducer and the fourth transducer, the second electrical signal being generated and applied only if said quantity of elements in the liquid state is greater than a second predefined threshold.

15. A cleaning method according to claims 11 and 13, comprising the step of analyzing the fourth electrical signal to evaluate the quantity of elements in the solid state between said third transducer and said fourth transducer, the certain duration being such that, at the end of this certain duration, the quantity of elements in the solid state between said third transducer and said fourth transducer is less than a third predefined threshold.

16. An electrical control system (12) arranged to implement the cleaning method according to one of the preceding claims.

17. Electrical control system according to claim 16, comprising at least one ASIC (22).

18. Cleaning device (10) comprising: - at least one transducer (11) arranged to be acoustically coupled with a body; - an electrical control system (12) according to one of claims 16 or 17.

19. Photovoltaic panel (1) comprising a protective plate (5) and a cleaning device (10) according to the. claim 18, the protective plate (5) being the body with which the at least one transducer is acoustically coupled.

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