FR3026323A1 - SYSTEM AND METHOD FOR ULTRASONIC CLEANING OF A SURROUNDED SURFACE COMPRISING A ULTRASONIC WAVE GENERATING DEVICE - Google Patents

Abstract

The main object of the invention is a system for cleaning (1) by ultrasound of a fouled surface (M), comprising a main structure (2), a reservoir (4) of cleaning liquid, and a device (5). ) an ultrasonic wave generator, comprising a plurality of piezoelectric transducers (6), the active faces of which are attached to a plate for transmitting the acoustic power to the cleaning liquid, a support structure (8) in which are located the transducers (6), at least one flexible cleaning element (9a, 9b) extending from at least one edge (8a, 8b) of the support structure (8) and delimiting with one or more closure elements (10a). , 10b), a cavity (11) for the flow of the cleaning liquid, the ultrasonic waves being emitted into the cavity (11) in contact with the cleaning liquid by the piezoelectric transducers (6), and a flow device cleaning liquid in the cavity (11).

Description

Field of the Invention The present invention relates to the general field of fouled surface cleaning systems, and more particularly to the field of surface cleaning systems. mirrors for solar power plants, and more specifically for such solar power plants comprising linear Fresnel mirror reflectors (or "CLFR" for "Compact Linear Fresnel Reflector"). The invention has applications in various fields, for example for cleaning fouled flat surfaces located on buildings such as habitats, industrial premises or greenhouses. Such surfaces may for example be made of polymer, aluminum or most often glass. It can for example be surfaces of photovoltaic panels. However, in a preferred manner, the invention applies to the field of concentrated solar thermal (or "CSP" area for "Concentrated Solar Power" English), and more particularly to the field of Fresnel linear collectors, or even to field of heliostats of tower plants.

The invention thus proposes a system for ultrasonic cleaning of a fouled surface comprising an ultrasonic wave generating device, a solar power station comprising such a cleaning system, and an associated cleaning method. STATE OF THE PRIOR ART Among the known technologies of the solar thermodynamic sector, there is the technology of solar thermal concentration (CSP) that uses mirrors to focus solar radiation on a small area. The CSP technology is more precisely to use solar radiation to heat a heat transfer fluid used as a hot source in a thermodynamic cycle. The concentration makes it possible to reach higher or lower temperatures, and thus to benefit from more or less significant thermodynamic conversion efficiencies. The techniques developed around the CSP technology are distinguished by their method of concentration of the solar rays, their method of transport, and possibly storage, of the heat, in particular by means of one or more heat-transfer fluids, and their conversion method. thermodynamics, for example by means of steam turbines, gas turbines, Stirling type engines, among others. Thus, four families of concentrating solar systems belonging to the CSP technology are typically encountered: linear focus cylindro-parabolic collectors, Fresnel linear collectors, central receiver tower plants, and movable focus parabolic collectors. . Fresnel parabolic collectors and linear collectors are linear concentration solar systems, while tower plants and parabolic collectors are point-focused solar systems. In addition, Fresnel tower plants and linear collectors constitute concentrating solar systems with fixed receivers, while parabolic collectors and cylindro-parabolic collectors constitute concentrating solar systems with mobile receivers.

A key element of such solar concentrating systems is the field of primary reflectors. For example, in the case of Fresnel linear collectors (CLFR technology), the field of primary reflectors, following the path of the sun from east to west during a day, receives direct light rays and reflects them in the direction fixed receiver. The concentration factor for such linear collectors of Fresnel is typically of the order of 50, which means that the solar flux at the receiver is equal to about 50 times the solar flux that impacts the ground. The functions of the linear Fresnel mirror reflectors are to reflect, focus and orient the incident solar flux. The reflection of the mirrors must therefore be specular. By "specular reflection" is meant the amount of light reflected in a precise direction of incidence. In particular, the specularly reflected ray of light is the ray which lies at the symmetry of the incident ray with respect to the normal of the reflecting surface. On the other hand, the term "diffuse reflection" refers to the amount of reflected light other than specularly reflected light. Thus, the total reflection of a mirror corresponds to the sum of specular reflection and diffuse reflection. However, when these Fresnel mirrors are dirty, their total reflection decreases slightly but especially their specular reflection decreases considerably, thus impacting the primary functions of such mirrors. Indeed, a large diffuse reflection certainly provides a high reflectivity, but it does not ensure the function of orientation and concentration towards the precise target that of the receiver. These features and associated cleaning needs, discussed above for Fresnel linear collectors, are found for the four families of CSP technology. Indeed, for all this technology, the reflectors have the function of reflecting and concentrating all the incident direct solar radiation to the receiver, located a few meters or tens of meters from the reflectors. They are composed of mirrors assembled to a structure mounted on rotation systems to follow the course of the sun during the day. All families of CSP technology therefore use similar mirrors, subject to the same environmental conditions and sustainability requirements.

Also, all the technologies resulting from the CSP technology require a cleaning of the mirrors for their good functioning. In particular, of the three families of existing materials for the design of mirrors, namely polymer, aluminum and glass, glass mirrors represent more than 80% of the mirrors installed on thermodynamic solar power plants. It is for this type of mirror that cleaning issues are the most critical for manufacturers who have implemented, or planning to install, a concentrated solar thermal plant. In the case of the CLFR technology for Fresnel linear collectors, the specificities of Fresnel must be taken into account when developing mirror cleaning solutions. In particular, the reflector lines have a significant length, for example of the order of 100 to 300 m, or even up to 900 m. In addition, about 10 to 20 lines of reflectors are connected in parallel, illuminating the same receiver. The Fresnel mirrors are also slightly curved (the radii of curvature are between about 15 and 30 m), contrary for example to the mirrors of the cylindro-parabolic collectors. In addition, the Fresnel mirrors of the same line form a non-continuous surface, since there is a space between reflectors every 6 to 10 m because the reflectors are supported by metal structures fixed to the ground on which they are placed. in rotation. Solutions for cleaning the mirrors of solar power plants, and in particular solar power plant mirrors comprising linear Fresnel mirror reflectors, have already been envisaged in the prior art. In general, these solutions must be able to answer the problems posed by the cleaning of the mirrors. In particular, the mirrors get dirty, this decreases their reflectivity and therefore the thermal and electrical power generated by a solar power plant. Soils can come from many sources, for example aerosol particles, inorganic matter from soil erosion (agricultural land nearby), organic matter (such as pollen, bacteria, algae, lichen, droppings). ), or sand and / or dust from a desert environment. A review of the possible sources of fouling, their effect and the possibilities of cleaning has been carried out in the book "A comprehensive review of the impact of dust on the solar energy: History, investigations, results, literature, and mitigation at pproaches, T. Sa rver et al, Renewable and Sustainable Energy Reviews, Volume 22, June 2013, pages 698-733. In the CSP technology, the loss of reflectivity is particularly sensitive to surface dust deposits since only the specular reflectivity is exploited. The presence of dust on a mirror tends to scatter the reflected light, the fouling problem has consequences much stronger than in the case of a photovoltaic system on productivity, namely the thermal power generated, a CSP plant since it directly impacts the exploitable solar resource.

As mentioned above, glass mirrors are the most used in concentrating solar power plants. Such mirrors consist of a glass substrate with a thickness of about 2 to 5 mm, on which is deposited a thin layer of reflective silver protected by a metal layer on the rear face. A multi-layer paint system is applied to the back of the mirror to protect the metal layers against corrosion. Cleaning a mirror therefore amounts to cleaning glass generally, or even a hard layer deposited on the surface for aluminum and polymer mirrors. Concentrating solar power plants (CSPs) are generally installed in areas of the world where sunshine is strong, but most often dry and arid areas where clogging of mirrors can be rapid. In these areas, the "natural" cleaning of the mirrors by the rain is not very present, although there may be the presence of dew in the morning on the mirrors. Thus, the regular cleaning of the mirrors is required to maintain the performance of the solar field, at lower cost and with less water consumption which is a scarce resource in the areas adapted to the CSP technology. It should be noted that the reflectivity losses can range from less than 1% per week to more than 10%, depending in particular on the location, the presence of wind or a sandstorm, for example. The most used cleaning method since the 1980s, on solar fields is to spray, under pressure or not, distilled water on the mirrors and brush them. It is nevertheless not entirely satisfactory, in particular because of the need for large quantities of water that it involves and because of the risk of abrasion in the time in contact with the brushes. In addition, preventive techniques have also been studied to consider surface treatments to make the mirrors hydrophobic or self-cleaning. However, these techniques require the development of surface treatments that do not alter the reflectivity of mirrors and that withstand weather conditions such as temperature variations, abrasion or ultraviolet radiation over time. In addition, other technologies for cleaning solar power plant mirrors have been described in the prior art. They are presented below.

Thus, patent application US 2012/0152281 A1, international application WO 2010/106195 A1 and international application WO 2011/106665 A2 for example describe cleaning techniques used on solar fields of cylindroparabolic type, using a vehicle. , truck or tractor type, which carries a cleaning arm and carries a large amount of water. With such systems, the cleaning arm cleans the mirror by water spraying, coupled or not with mechanical brushing. However, these systems have only been developed for parabolic troughs. They are also very water-consuming, as it is generally not recovered and / or recycled. In addition, when brushes are used in addition to the spraying of distilled water, the cleaning is improved but the risk of breakage is important. The vehicles used are also very heavy because they have to carry a lot of water, which requires a preparation of the ground adapted to the repeated rolling of heavy vehicles. However, this soil preparation can be difficult or impossible, for example in the desert. In addition, it is also very expensive. Regular cleaning with such vehicles causes ruts to appear on the ground. On the other hand, when they move, the vehicles raise dust that can be redeposited on the mirrors and dirty them again. In addition, these solutions require the presence of an operator. Also known is the cleaning system "PARIS" (PARabolic trough cleaning System), developed by the company SENER Aerospace, which provides for the use of an autonomous machine without driver, and lighter than the vehicles described above. Cleaning is also done by brushing with water spray, and water consumption is important. The systems described above, using vehicles with remote arms for cleaning, are not adapted to the solar field of Fresnel type mirrors, precisely because of the use of remote arms. Indeed, a solar field of Fresnel mirrors has several lines of parallel mirrors which are placed under the receiver, the total width of these lines exceeding 10 m and up to 20 m. A vehicle, for example a truck, traveling on the edge of the solar field would have to unfold a cleaning arm of a very large length to be able to clean the mirrors, which seems difficult to achieve. In addition, the receiver is supported in height by feet, or beams, with in most cases reinforcement cables anchored to the ground, which hinders access from the sides of the solar field at the periphery of the latter and hinders also access between two rows of mirrors.

Other technologies have appeared in the prior art for the specific cleaning of Fresnel solar fields. Thus, the international application WO 2011/113555 A2, in connection with the system "HECTOR" (HEliostats Cleaning Team Oriented Robot) developed by the company SENER Aerospace, describes an autonomous energy robot for rolling on a mirror for cleaning, guided by an optical system and carrying only the amount of water needed to clean a line of mirrors. Cleaning is done by brushing and spraying water. This system is expensive and complex because it requires the management of a fleet of small robots with a transport system, each robot to be filled with water and recharged with energy at the end of Fresnel reflector lines. In addition, the guidance by optical system involves a reliable control control, maintenance of the optical system and carries the risk of failure, for example in case of fall of the robot. US Patent 8,240,320 B2 still describes a guided robot on the reflector structure, cleaning the mirrors by brushing with water. This system has the disadvantage of using a brush and thus generating a mechanical contact between the cleaning means and the mirror. This system thus induces a long-term risk of abrasion of the mirror because of the friction of the brush. The risk is even amplified if, during operation, it happens that the water supply of the brush is faulty. In this case, dry brushing of the dusted mirror quickly leads to its degradation.

Finally, the utility model CN 202 336 465 U describes the use of air-blast cleaning, self-supporting on the mirror. Air-blast cleaning, however, consumes a lot of energy, compresses the air, and is inefficient for the dust adhered to the mirror. Furthermore, the prior art has also considered using ultrasonic waves to perform the cleaning of solar power plant mirrors. In particular, work was carried out for Sandia Laboratories in a publication entitled "Preliminary Design Report: New for Heliosat Reflector Cleaning Systems", J. Schumacher et al, December 1979, Schumacher and Associates, Inc. In this work, he is imagined to use a robot that moves on the surface of the mirrors with ultrasonic inductors placed a short distance from the surface of the mirrors, the idea being that the ultrasonic waves can reduce the adhesion forces between the dust and the surface of the mirror. Associated with a blow by compressed air to drive out the dust made mobile by the action of the ultrasonic waves, this technique can thus lead to the cleaning of this surface. Nevertheless, this technique is not entirely satisfactory. Indeed, it induces a heating of the ultrasound inductor system and a difficulty in transmitting the waves into the air. Indeed, air is a poor medium for the propagation of ultrasonic waves. Moreover, even when the action of the ultrasonic waves propagated in the air is sufficient to destabilize the fouling layer on the surface of the mirror, the fact of blowing the dust by an inclined air jet causes major drawbacks: the blown dust can be redeposited on the surface of the mirrors near the cleaned mirrors; the fact of generating compressed air requires a lot of energy and seems not very compatible with having a mobile robot not too heavy and able to roll on the mirrors. Another technique using ultrasonic waves was developed by Cranfield University and presented at the SolarPACES conference in September 2013, entitled "A Comparison of Polymer Film and Glass Collectors for Concentrating Solar Power", C. Sa nsom et al. The idea is to project water on the surface of the mirror, and create bubbles in an injection nozzle by an ultrasonic inductor placed at the outlet of the nozzle. The bubbles thus created and projected on the surface of the mirror with the water must thus cavitate and create an impact on the surface much greater than that of a simple jet of water. It is therefore not really an ultrasonic cleaning, but a water jet cleaning with effect of bubbles created by ultrasound. This technique has the disadvantage of consuming a lot of water, treating a small mirror surface, and requiring a lot of energy for the compression of water. In addition, this laboratory technique is very difficult to transpose on a cleaning system of a solar field. In addition, another ultrasonic cleaning technique was described in Gérald Mazue's thesis for the cleaning of boat hulls, entitled "Characterization of high power ultrasonic activity at low frequency in the presence of disturbed hydrodynamics", supported December 21, 2012 at the University of Franche Comté. The idea is to clean the hulls of boats by an ultrasound system immersed and positioned at the end of an articulated arm. This technique is not suitable for cleaning mirrors of solar power plants and does not consider moving a blade of water for cleaning. SUMMARY OF THE INVENTION There is therefore a need to provide an alternative solution to allow the cleaning of fouled surfaces, and in particular the cleaning of the surfaces of the mirrors of solar power plants, in particular solar power plants comprising linear Fresnel mirror reflectors. More specifically, there is a need to design a mirror cleaning system to effectively maintain uniform and high reflectivity on the mirror field, to minimize or eliminate water consumption, and not to damage mirrors. by abrasion. Such a cleaning system must also be provided to be energy efficient, and not to break the mirrors (no mechanical contact between the cleaning means and the surface of the mirrors) and not to damage the surrounding terrain. Similarly, it must be low manufacturing cost, and able to tolerate climatic conditions, such as rain or wind, while being able to work day and night. The object of the invention is to remedy at least partially the needs mentioned above and the drawbacks relating to the embodiments of the prior art. The invention thus has, according to one of its aspects, an ultrasonic cleaning system for a fouled surface, characterized in that it comprises: a main chassis structure comprising means for moving the transmission system; cleaning on the fouled surface, - a reservoir of cleaning liquid carried by the main structure, - an ultrasonic wave generating device for cleaning the fouled surface carried by the main structure and comprising: - a plurality of piezoelectric transducers electric, capable of generating ultrasonic waves, the active faces of which are attached to at least one transmission plate for transmitting the acoustic power to the cleaning liquid, - a support structure, secured to the main structure, in which the piezoelectric transducers, - at least one flexible cleaning element, in particular at least one flexible cleaning element or proximal, located closest to the cleaning fluid reservoir, extending from at least one edge of the support structure and defining, with one or more closure elements extending from the other edge or edges of the support structure, a cavity, thus substantially closed, flow of the cleaning liquid from the cleaning liquid reservoir, the ultrasonic waves being emitted into the cavity in contact with the cleaning liquid by the piezoelectric transducers, flow of the cleaning liquid into the cavity, taken from the cleaning liquid reservoir. Thanks to the invention, it can be possible to obtain a high cleaning efficiency of dirty surfaces through the use of ultrasound. The cleaning can be done without mechanical contact between the cleaning means formed by the ultrasound and the fouled surface, thus avoiding any risk of degradation of the fouled surface. In addition, the cleaning system according to the invention may require only very low energy consumption (lack of air) and thus have a significant autonomy. The ultrasonic etched dirt from the fouled surface is moreover confined in the cavity (absence of projection), before being optionally evacuated at the passage between two adjacent fouled surfaces. The cleaning system can also be used during the night so as not to interfere with the operation of a solar power plant, for example. The need for labor is also very small, if not almost non-existent. Finally, the consumption of cleaning liquid, especially water, is very small in comparison with the solutions of the prior art, for example from 0.15 L / m2 to less than 0.03 L / m2. The cleaning system according to the invention may further comprise one or more of the following characteristics taken separately or in any possible technical combinations. Advantageously, the fouled surface is never in contact with the transmission plate so as not to damage it. Preferably, the ultrasonic wave generating device may comprise at least two flexible cleaning elements extending substantially parallel from at least two opposite edges of the support structure and delimiting, with one or more closure elements extending from the one or more other edges of the support structure, the flow cavity of the cleaning liquid. The cleaning system is moreover advantageously configured so that the volume of the flow cavity of the cleaning liquid is completely filled with cleaning liquid in operation, in order to put in contact the transmission plate, from which are emitted the ultrasonic waves, with the cleaning liquid. The means for moving the cleaning system on the fouled surface may comprise wheels.

The cleaning liquid reservoir may include water. The water may for example be demineralized water at a temperature of less than or equal to 60 ° C., in particular between 40 and 60 ° C. The cleaning system may further comprise means for supplying power to the device generating ultrasonic waves, in particular a battery and / or solar panels, embedded on the main structure. In particular, the power supply means may comprise a battery, possibly recharged by photovoltaic cells solar panels that can cover the upper surface of the cleaning system, including the tank. The photovoltaic cells can make it possible to recharge the battery out of operation of the system but also to bring a little energy during the operation during the day. It can also be a portable generator. The transmission plate may be metallic, in particular made of stainless steel.

The flexible cleaning element or elements may be made of elastic material, in particular rubber, thus forming cleaning squeegees. The flexible cleaning element or elements may have a height, between the transmission plate and the fouled surface, of between 2 and 20 mm, in particular of about 10 mm.

The piezoelectric transducers may for example have an electric power of about 75 W and a frequency of between 20 and 28 kHz. In addition, the invention also has, according to another of its aspects, a solar power plant, characterized in that it comprises a plurality of solar reflectors and a cleaning system as defined above, the system being able to allow cleaning mirrors clogged with solar reflectors. The solar power plant may in particular correspond to a linear Fresnel collector, comprising a plurality of parallel lines of reflectors of the linear Fresnel mirror type. In another aspect, the subject of the invention is also a method of ultrasonic cleaning of a fouled surface, characterized in that it is implemented by means of a cleaning system. as defined above, and in that it comprises the step of moving the cleaning system on the fouled surface. The cleaning system can be used to allow cleaning of at least two consecutive fouled surfaces spaced from one another by a vacuum. In addition, the cleaning liquid present in the flow cavity of the cleaning liquid and loaded with dirt can be discharged into said void during the passage of the cleaning system from one fouled surface to another.

In particular, the cleaning system can be used for cleaning a line of several reflectors of a solar power plant and the vacuum can correspond to the space formed between two consecutive reflectors. In addition, the speed of movement of the cleaning system can be reduced near said vacuum, just before reaching said vacuum and just after crossing said vacuum, to amplify the cleaning of fouled surfaces near said vacuum. Indeed, when the cleaning system arrives for example at the space between two reflectors of a solar power plant, at least one flexible cleaning element can be found in this space so that the cleaning liquid can be at least partly lost by flow in this space. In this case, the optimal cleaning may require a longer cleaning time near this space. The cleaning system, the solar power station and the cleaning method according to the invention may comprise any of the features set forth in the description, taken alone or in any technically possible combination with other features. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understood on reading the following detailed description of an example of non-limiting implementation thereof, and the examination of the figures, diagrammatic and partial. of the accompanying drawing, in which: - Figure 1 shows, in perspective and in part, an example of a cleaning system according to the invention, placed on the dirty surface of a reflector mirror type CLFR, - the FIG. 2 illustrates, in perspective and in part, a detail of embodiment of the ultrasonic wave generating device of the cleaning system of FIG. 1; FIG. 3 illustrates, in perspective and partially, another exploded representation of a detail; of the ultrasonic wave generator device of the cleaning system of FIG. 1, FIG. 4A represents, in section, an example of a piezoelectric transducer used in the ultrasonic wave generator device 1, with the presence of a transmission plate; and FIG. 4B shows, in perspective, the example of a piezoelectric transducer of FIG. 4A. In all of these figures, identical references may designate identical or similar elements. In addition, the different parts shown in the figures are not necessarily in a uniform scale, to make the figures more readable.

DETAILED DESCRIPTION OF A PARTICULAR EMBODIMENT With reference to FIGS. 1 to 4B, an exemplary embodiment of an ultrasound cleaning system 1 of contaminated surfaces M according to the invention is presented below. In this example, it is considered that the fouled surface is constituted by a mirror M of a reflector of the CLFR type, that is to say a linear Fresnel mirror reflector, a Fresnel solar power plant. This mirror M comprises for example a surface to be cleaned consisting of glass. Figure 1 thus shows, in perspective and partially, the cleaning system 1 according to the invention placed on the mirror surface of the reflector type CLFR. In this FIG. 1, the photovoltaic panels 17 have not been represented for the sake of clarity. Furthermore, FIG. 2 illustrates, in perspective and in part, a detail of embodiment of the ultrasonic wave generating device 5 of the cleaning system 1 of FIG. 1. In this FIG. 2, only the closure element 10b and the squeegee cleaning 9a have been shown, and not the cleaning squeegee 9b and the closure member 10a. FIG. 3 illustrates, in perspective and partially, another representation of the device 5 generating ultrasonic waves of the cleaning system 1 of FIG.

Finally, FIGS. 4A and 4B show an example of a piezoelectric transducer 6 used in the ultrasonic wave generating device 5 of the cleaning system 1 of FIG. 1. More precisely, FIG. 4A represents, in section, such an example of a piezo transducer. 6 with the presence of the transmission plate 7, and FIG. 4B represents, in perspective, this example of a piezoelectric transducer 6. According to the invention, the cleaning system 1, or cleaning robot 1, comprises a frame 2 comprising means 3 for moving the cleaning system 1 in the form of wheels on the surface of the mirror M. The function of the chassis 2 is to support all the components of the cleaning robot 1. The movement of the Wheels 3 on the surface of the mirror M can be automated, in particular with a guide provided mechanically, for example by rollers, or optically. FIG. 3 illustrates, by means of the arrow F, the movement of the cleaning robot 1 on the surface of the mirror M.

In addition, on this main chassis structure 2, are located a tank 4 of cleaning water and a battery 12, possibly rechargeable by photovoltaic panels 17 (visible only in Figures 2 and 3 for the sake of clarity of Figure 1 ) placed on the outer surface of the tank 4, for supplying electrical power to an ultrasonic wave generating device for cleaning the surface of the mirror M. This battery 12 can also, more generally, be used to the power supply of all the electrical components of the cleaning robot 1. This ultrasonic wave generator device 5 is carried by the main structure 2, and comprises a plurality of piezoelectric transducers 6, which are capable of generating ultrasonic waves . The active faces 6a of the piezoelectric transducers are fixed to a metal transmission plate 7, for example made of stainless steel, for transmitting the acoustic power to the cleaning water. This makes it possible not to directly contact the cleaning water, the soiling of the mirror M and the cavitation bubbles, with the ceramic materials of the piezoelectric transducers 6 so as not to damage them. The attachment of piezoelectric ceramic materials to a metal surface is well known to those skilled in the field of ultrasonic cleaning baths. In addition, the ultrasonic wave generating device 5 comprises a support structure 8, secured to the main structure 2, in which the piezoelectric transducers 6 are located. In order to allow the cleaning of the mirror M, two flexible cleaning squeegees 9a are provided. and 9b extend from the two opposite longitudinal edges 8a and 8b of the support structure 8, in parallel fashion. These cleaning squeegees 9a, 9b are for example similar to squeegees of the wiper type or industrial squeegees of supermarket washers. They may in particular be in the form of striated strips of rubber, with a larger bulge for positioning. The cleaning squeegee 9a is optional. Indeed, the cleaning squeegee 9a could, if necessary, cause a risk of abrasion of the surface of the mirror M.

As a result, the front or distal cleaning blade 9a (the furthest away from the tank 4) can optionally be replaced by a foam roller capable of ensuring a correct seal with a preferred bearing with respect to friction and therefore less damage. potential. The cleaning squeegee 9a can also according to a variant not shown do not equip the cleaning system according to the invention. Indeed, the mere presence of the cleaning squeegee 9b and the two closure elements 10a, 10b will create a "tidal bore", that is to say a wave of water pushed forward, during the movement of the system. It is then possible, during the embodiment, to position the ultrasonic wave generating device 5 sufficiently close to the cleaning scraper 9b in order to have the transducers 6 always in a portion of water pushed by the cleaning scraper 9b. These cleaning squeegees 9a, 9b, and especially the rear or proximal cleaning squeegee 9b (the closest to the tank 4), have a very important function on the quality of the cleaning. Indeed, if they do not ensure a proper seal, the level of cleaning water can drop, replacing the water with air.

This would then have the effect of increasing the impedance at the active surface formed by the transmission plate 7, irreparably degrading and short-term piezoelectric transducers 6. In addition, the water would stagnate on the mirror M and then would foul it again, thus rendering the cleaning useless.

In addition, the other edges of the support structure 8, namely the two opposite transverse edges, are closed by two closure elements 10a and 10b. In this way, the two closure elements 10a, 10b and the two cleaning scrapers 9a, 9b together define a closed cavity 11 for flushing the cleaning water, coming from the tank 4.

Thus, the volume of the cavity 11 is entirely occupied by the cleaning water, so that the transmission plate 7 is in contact with the cleaning water. The ultrasonic waves are then emitted by the piezoelectric transducers 6 in contact with the cleaning water. The cavity 11 being closed or delimited by the cleaning scrapers 9a, 9b, or even limited by the cleaning scraper 9b, and the closure elements 10a, 10b, any water loss can be avoided or at least limited. Advantageously, this then makes it possible to form a "water cushion" or a "water slide" which moves along the surface of the mirror M and in which the ultrasonic waves are emitted in order to eliminate the dirt located on the mirror M. The cleaning squeegees 9a, 9b allow the uniform maintenance of this blade of water and its movement along the mirror M at the same time as the cleaning robot 1. The length L of the cleaning squeegees 9a, 9b , shown in Figure 3, may for example be of the order of 1 m. The height H of the cleaning squeegees 9a, 9b, also shown in FIG. 3, is optimally close to 10 mm. It also defines the thickness of the water slide.

This thickness of the water layer, or the distance between the transmission plate 7 and the surface of the mirror M, must be sufficient to avoid any accidental contact between the active surface of the transmission plate 7 and the mirror M, which would be However, this thickness must also be chosen to guarantee the density of the cavitation bubbles necessary for efficient ultrasonic cleaning.

Furthermore, for optimum cleaning of the surface of the mirror M, it is preferable to provide that the ultrasonic wave generator device 5 has a shape matching the shape of the mirror M. In particular, the transmission plate 7 may be provided flat or curved depending on the shape of the mirror M. It may in particular follow the same radius of curvature as the mirror, for example between 15 and 30 m. In this way, it may be possible to ensure that the piezoelectric transducers 6 are constantly at the same distance from the mirror M. The typical size of the piezoelectric transducers 6, with a power of about 75 W and a frequency of 20 or 28 kHz, is for example 70 mm in diameter. FIGS. 4A and 4B are specific to an example of such a piezoelectric transducer 6. This comprises a clamping screw 13, a metal counter-mass 14, a ceramic / copper stack 15 comprising a ceramic layer 15b between two copper electrode layers 15a and 15c, and a metal diffusion cone 16. The diffusion cone 16 is fixed on the metal transmission plate 7 via an adhesive means 6a, in particular an adhesive. In order to be able to produce the ultrasound generator device 5 of the cleaning robot 1, and the piezoelectric transducers 6, many systems capable of producing ultrasound are already known. In general, a device producing ultrasound is commonly called ultrasonic transducer or converter. Transducer technology can be based on pneumatic generators, such as whistles or sirens, electrodynamics, such as speakers, or electric. In the latter case, magnetostrictive or piezoelectric materials are used to convert electrical energy into ultrasonic mechanical energy. The most used materials today are piezoelectric materials. These materials have the advantage over other systems of having good performance and being available in very diverse geometries. The disadvantage of these materials, however, is that they are heat sensitive and may stop working when their Curie temperature is reached, at about 60 to 70 ° C. Also, in the cleaning system 1 according to the invention, the water present under the ultrasonic wave generating device 5 may optionally be used to allow cooling thereof. The water can then flow from the reservoir to the cavity 11 by flowing on the outer surface of the transducers 6 to cool them. It is also possible to integrate the counter-mass 14 of the generator inside the water tank, thereby creating a heat sink that can cool the stack 15 which generates the heat. Moreover, these materials are most often in the form of a disk or a ring on whose faces are deposited two metallized electrodes.

When a voltage is applied to these two electrodes, the material expands or compresses depending on the orientation of the voltage with respect to the polarization of the ceramic. To provide an ultrasonic wave, the principle consists in providing the ultrasound systems with an electrical voltage whose frequency is equal to their resonance frequency. An electrical generator must be used to transform the mains voltage (220 V - 50/60 Hz) into an AC voltage at the resonance frequency of the ultrasound system (eg 20 kHz - 1000 V). A permanent control of the resonance conditions is necessary to correct the deviations between the frequency supplied by the electronic generator and the ultrasonic resonance frequency, in order to avoid a bad transmission of the power. The sounds are emitted by bodies animated by a vibratory movement and propagate in the form of mechanical waves likely to undergo reflections, refractions and interferences. Ultrasound is propagated at a rate that will depend on the nature of the medium, regardless of the wave frequency. The propagation of sounds can only be done in the material. The medium traversed undergoes successive compression and relaxation phenomena. The materials have some resistance to the passage of ultrasound. This resistance, called impedance, will be a function of the modulus of elasticity and the density of the medium considered. The impedance is different from one material to another and the boundary between two bodies is an interface. The moving sound wave in a given medium is characterized by its frequency and wavelength. The energy developed by ultrasonic cavitation combines several effects, producing the development of forces near the surface to be cleaned which induce the appearance of violent liquid micro-jets directed towards the surface to be cleaned, also producing cleaning in the pores and crevices of the surface, and also producing the dispersion of contaminants in the fluid under the influence of turbulent movements caused by the ultrasonic field. Ultrasonic cleaning has many advantages over conventional methods. Indeed, the microscopic size of the cavitation bubbles allows the cleaning of parts with irregular and complex surfaces, without brushing, and also induces the elimination of painful manipulations. To obtain good cleaning results, it is necessary to take into account several parameters, such as the ultrasonic powers and frequencies generating the phenomenon of cavitation in the liquid, the temperature of the bath, the composition of the bath, and the duration of the ultrasonic treatment. The physical principles of ultrasonic cleaning are discussed below. Ultrasound, when emitted in a fluid such as water, whose physical properties allow it to cavitate easily, generate different hydrodynamic phenomena.

The first consists in the generation and implosion of cavitation bubbles under the effect of the ultrasonic field, provided that it exceeds the threshold of acoustic power required. These bubbles, imploding near the walls to be cleaned, induce the generation of powerful micro-jets of fluid able to take off the cemented dust. These cavitation bubbles, like all forms of entities generating interfaces between different states media, need preexisting nuclei to appear. However, the dust and dirt present on the surface of the mirrors are dotted with crevices in which bubbles of gas are nestled. This allows to say that a strong cavitational activity will be concentrated at the level of the dirty surface of the mirrors. The second physical phenomenon involved consists of an acoustic current induced by the viscous second-order dissipation of acoustic energy in the fluid. This phenomenon, although unable to take off cemented dirt, nevertheless makes it possible to generate convective currents stirring the medium. This makes it possible to prevent the film of dirt previously peeled from being redeposited on the mirrors by sedimentation.

Finally, the third phenomenon is due to the micro current linked to the interaction between the cavitation bubbles and the acoustic field. This effect induces a displacement of the cavitation bubbles in the acoustic field. It then follows the generation of a strong shear at the liquid / gas interface of the wall of the bubble. Shear is also beneficial for cleaning the surface of mirrors. This phenomenon is indeed used in spiral heat exchangers to prevent and treat fouling. Furthermore, in general, the ultrasonic power used is of the order of 10 to 20 W / liter of bath for low ultrasonic frequencies, between 20 and 40 kHz. The ultrasonic frequency influences the size of the cavitation bubbles and the type of effect generated by their implosion. The high ultrasonic frequencies, ie between 400 kHz and 10 MHz, are in fact recognized for releasing radical entities of the OH type and are used in sonochemistry, especially in the treatment of effluents. Ultrasound thus makes it possible to degrade polluting organic molecules such as phenols. Low frequencies generate significant mechanical effects that can erode metal surfaces or loosen layers of cemented grime or grease. Thus, for degreasing applications and other applications requiring the stripping of the surfaces to be cleaned, the ultrasonic waves are generated with frequencies of the order of 20 to 40 kHz. Knowing that the more the frequency increases, and the more the size of the bubbles decreases, the frequencies around 20 kHz are rather used to take off the dirt on the surface whereas the frequencies around 40 kHz, or even around 100 kHz, allow to strip even in the anfractuosities and the pores of the pieces. At low frequency, around 20 kHz, the effects of the acoustic current are lower than at high frequency, around 1 MHz. However, since the latter has only one effect playing a secondary role in cleaning the mirrors, this disadvantage is minimal. Finally, high ultrasound frequencies, especially around 1 MHz, generate highly directive or even focused acoustic waves. Indeed, the characteristic wavelength dimensions for these frequencies are of the order of one millimeter. Therefore, a transducer a few centimeters in diameter emits several sources of waves that, by a game of destructive and constructive interference, will focus at a specific point to diverge behind this point. At low frequencies, this phenomenon is much less pronounced because the dimensions of the transducer are of the order of magnitude of the emitted wavelength. Therefore, the acoustic field is very directional, irradiating a larger area than the transducer. Also, preferably, the best frequency of the ultrasonic waves is between 20 and 28 kHz. Indeed, the mirrors having a relatively smooth surface state, there is no need for higher frequencies. Despite a lower acoustic current at these frequencies, stirring in the medium will be largely sufficient to prevent further deposition of dirt provided you choose the dimensions of the ultrasonic tray properly. In addition, it is generally accepted that for low frequency cleaning applications, the best efficiency is obtained for bath temperatures of the order of 40 to 60 ° C. In general, the addition of a detergent in the bath water, which if it is of the surfactant type decreases the surface tension of the water and also promotes cavitation, is important in order to chemically attack the soiling and to encourage their removal by the implosion of cavitation bubbles. However, care must be taken that the product does not attack the part to be cleaned.

The time required for cleaning depends on the bath, its temperature, type and degree of soiling. It can vary from a few seconds to several tens of minutes. Preferably, the cleaning system 1 uses deionized water at a temperature close to ambient temperature or slightly higher when the reservoir comprises water between 40 and 60 ° C. However, the temperature of the demineralised water should preferably not exceed 60 to 70 ° C because the ceramic materials of the type PZT (Titano-Lead Zirconate) that can be used in the piezoelectric inductors for ultrasound have a Curie temperature of 60 at 70 ° C. Beyond such a temperature, the piezoelectric effect is lost. Furthermore, the cleaning system 1 is preferably designed not to use detergent during cleaning, because the rear face of the mirrors, typically a silver coating and paint layers, is sensitive to chemical degradation by cleaning products . In particular, the mirrors are qualified in aging only in contact with demineralised water. In the method of using the cleaning robot 1, the fouling species of the mirror M are peeled off by the effect of the ultrasonic waves. They are then suspended in the water layer and moved at the same time as the cleaning robot 1. The water blade is preferably discharged, or drained, between two reflectors. However, alternatively, one can also imagine a system of circulation of water in the blade, with suction, filtration and return of water into the water layer. This would then require putting the cleaning robot 1 on a pump and using a filter, but the benefit in water consumption would be substantial since it could be recycled.

With regard to the electrical power supplied to the piezoelectric transducers 6, there are several aspects to be taken into account. First, it is necessary to provide a sufficient power density to exceed the cavitation threshold below which the cavitation is not transient, so not implosive, which means non-cleaning. However, it is not necessary to achieve a power density which induces a degradation of the surface of the mirror M. Then, the transducers 6 have mechanical conversion efficiencies of the electric energy brought from 50 to 60% maximum. Therefore, if it is desired to put for example seven transducers 6 operating at an acoustic power of 30 W over a length of 1 m, a transducer 6 every 3 cm, it will provide an electrical power of 420 W.

Regarding also the speed of advance of the cleaning robot 1 on the mirror M, it can be between 0.1 and 0.5 m / s. Cavitation is indeed not or only slightly affected by the flow velocities. Only the acoustic current is impacted. Also, the fact of cleaning the mirror M by ultrasound does not have an impact on the speed of the cleaning robot 1. Thus, advantageously, the invention makes it possible to clean mirrors M of solar power plants by means of ultrasound in a thin layer of water, generated temporarily between the transmission plate 7 and the surface of the mirror M, between the two cleaning blades 9a, 9b driven in motion. This combines the beneficial effect of water and ultrasound, the water to limit the power of ultrasound because it is an excellent medium for propagation. The unstuck dusts remain in the volume of the water layer for a time corresponding to the movement on the mirror M of a reflector, the water slide being then drained into the inter-reflector space so as to recreate a blade of water. clean water for the next reflector to clean. Of course, the invention is not limited to the embodiment which has just been described. Various modifications may be made by the skilled person. The phrase "with one" should be understood as being synonymous with "having at least one" unless the contrary is specified.

Claims (16)

REVENDICATIONS1. Ultrasonic cleaning system (1) for a fouled surface (M), characterized in that it comprises: - a main frame structure (2) comprising means (3) for moving the cleaning system (1) on the fouled surface (M), - a reservoir (4) of cleaning liquid carried by the main structure (2), - an ultrasonic wave generating device (5) for cleaning the fouled surface (M), carried by the main structure (2) and comprising: - a plurality of piezoelectric transducers (6), able to generate ultrasonic waves, whose active faces (6a) are fixed to at least one transmission plate (7) for the transmission from the acoustic power to the cleaning liquid, - a support structure (8), secured to the main structure (2), in which the piezoelectric transducers (6) are located, - at least one flexible cleaning element (9a, 9b) extending from at least one edge (8a, 8b) of e the support structure (8) and delimiting, with one or more closure elements (10a, 10b) extending from the other edge or edges of the support structure (8), a cavity (11), thus substantially closed, d flow of the cleaning liquid from the cleaning liquid reservoir (4), the ultrasonic waves being emitted into the cavity (11) in contact with the cleaning liquid by the piezoelectric transducers (6), - a flow device cleaning liquid in the cavity (11) taken from the reservoir (4) of cleaning liquid. 2. System according to claim 1, characterized in that the device (5) generating ultrasonic waves comprises at least two flexible cleaning elements (9a, 9b) extending substantially parallel from at least two opposite edges (8a, 8b). ) of the support structure (8) and defining, with one or more closure elements (10a, 10b) extending from the other edge (s) of the support structure (8), the cavity (11) for the flow of liquid from cleaning. 3. System according to claim 1 or 2, characterized in that it is configured so that the volume of the cavity (11) flow of the cleaning liquid is completely filled with cleaning liquid in operation, in order to implement contact the transmission plate (7), from which the ultrasonic waves are emitted, with the cleaning liquid. 4. System according to one of the preceding claims, characterized in that the displacement means (3) of the cleaning system (1) on the fouled surface (M) comprise wheels. 5. System according to any one of the preceding claims, characterized in that the reservoir (4) of cleaning liquid comprises water. 6. System according to claim 5, characterized in that the water is demineralized water at a temperature of less than or equal to 60 ° C, especially between 40 and 60 ° C. 7. System according to any one of the preceding claims, characterized in that it comprises energy supply means (12) of the device (5) generating ultrasonic waves, in particular a battery (12) and / or solar panels embedded on the main structure (2). 8. System according to any one of the preceding claims, characterized in that the transmission plate (7) is metallic, in particular made of stainless steel. 25 9. System according to any one of the preceding claims, characterized in that the or the flexible cleaning elements (9a, 9b) are made of elastic material, in particular rubber, thus forming cleaning scrapers. 10. System according to any one of the preceding claims, characterized in that the or the flexible cleaning elements (9a, 9b) have a height (H) between the transmission plate (7) and the fouled surface (M). , between 2 and 20 mm, in particular about 10 mm. 11. System according to any one of the preceding claims, characterized in that the piezoelectric transducers (6) have an electrical power of about 75 W and a frequency between 20 and 28 kHz. 12. Solar power station, characterized in that it comprises a plurality of solar reflectors and a cleaning system (1) according to any one of the preceding claims, the system (1) being adapted to allow the cleaning of mirrors (M) fouled solar reflectors. 13. Solar power plant according to claim 12, characterized in that it corresponds to a linear Fresnel collector, comprising a plurality of parallel lines of reflectors of the linear Fresnel mirror type. 14. A method of ultrasonic cleaning of a fouled surface (M), characterized in that it is implemented via a cleaning system (1) according to any one of claims 1 to 11, and in that it comprises the step of moving the cleaning system (1) on the fouled surface (M). Method according to claim 14, characterized in that the cleaning system (1) is used to allow the cleaning of at least two fouled surfaces (M) consecutive, spaced from one another by a vacuum, and in that the cleaning liquid present in the cleaning liquid flow cavity (11) and loaded with dirt is discharged into said vacuum when the cleaning system (1) from a fouled surface (M) passes from the other. 16. The method of claim 15, characterized in that the speed of movement of the cleaning system (1) is reduced in the vicinity of said vacuum, just before reaching said vacuum and just after crossing said vacuum, to amplify the cleaning of fouled surfaces. (M) near said vacuum.