Treatment in a concentration system with solar mirrors

Abstract

Treatment in a concentration system with solar mirrors including a reflecting surface, the treatment consisting, after any possible removal of a pre-existing coating, possible preliminary cleaning and possible polishing of such reflecting surface, in applying thereon a self-cleaning coating made of a polymer for optics, by spray technique, wherein, while dispensing the polymer for optics, the reflecting surface is kept in a horizontal position and the spray follows a direction along a vertical axis perpendicular thereto, and consisting in a possible subsequent finishing whereby a surface wettability with a static contact angle (WCA) between 70 ° and 160°,, a difference in reflectance, with respect to the untreated reflecting surface, not exceeding 10%, nm a coating transmittance at least equal to 90 % for wavelengths between 380 nm and 700, a coating transmittance of at least 85% for wavelengths between 700 nm and 1500 nm, and a coating transmittance always higher than 50% for wavelengths between 1500 nm ad 2500 nm are obtained.

Description

[0001] TREATMENT IN A CONCENTRATION SYSTEM WITH SOLAR MIRRORS

[0002] The present invention relates to a treatment in a thermodynamic solar system (CSP ) , with concentration mirrors including a reflecting surface on which a self-cleaning coating is applied and related solar mirrors including a reflecting surface on which a self-cleaning coating is applied .

[0003] In particular, the invention relates to a treatment for solar mirrors allowing to reduce the consumption of washing water in ordinary maintenance procedures , and then the costs of cleaning and consequently of the electricity produced (LCOE ) in concentration systems .

[0004] Moreover, the invention relates to solar mirrors including a reflecting surface on which said selfcleaning coating is applied, by using the treatment the invention relates to .

[0005] In the present description, as well as in the claims enclosed thereto, some terms and expressions are considered to assume, unless otherwise explicitly indicated, the meaning expressed in the following definitions .

[0006] The term "front surface mirror" or "mirror having front surface" means a mirror having as external layer a reflecting layer with or without a thin protective layer .

[0007] The term "back surface mirror" or "mirror having back surface" means a mirror having as external layer a layer of metallized glass in the rear part and the reflecting layer placed inside said external layer . The term "reflecting surface" means a directly reflecting surface or a combination of layers , the whole set thereof results reflecting .

[0008] The term "wettability" means the capability of a liquid to maintain the contact with a surface, being possible to qualify the aforementioned as hydrophilic if the static contact angle (WCA) is lower than 40 ° and hydrophobic if it is higher than 70 ° .

[0009] The term "polymer with optical properties" means a family of transparent macromolecules with respect to the solar radiation . In this case : Thermoplastics , Thermosets , Elastomers , Amorphous elements characterized by transmittance are to be understood as such .

[0010] The term "solar mirror" means a mirror with a reflectance of wavelengths between 320 nm and 2500 nm at least equal to 85% .

[0011] It will be appreciated that said self-cleaning coating can be otherwise said self-cleaning layer .

[0012] The solar mirrors currently on the market , both of the back surface mirror (BSM) type or front surface mirror (FSM) type, used in the CSP systems , are subjected to soiling of different nature, depending on the installation geographical site, and require expensive cleaning procedures to restore the specular reflectance, thereupon the energy production effectiveness and its cost depend .

[0013] In fact , in the CSP systems the mirrors are used as optical concentrators , having reflecting surfaces used in the solar collectors to address and concentrate the sunlight on a receiving surface . The active concentrating element is known as "solar mirror" or concentration mirror and it is supported by a structure which maintains the shape thereof , by constituting the system reflector .

[0014] The quality of this reflector in the CSP systems influences directly the amount of radiation which can be transformed into power .

[0015] Soiling affects the optical properties of the solar mirrors , by reducing the efficiency in collecting the solar radiation and affects directly the overall yield of the solar plants , by determining a drop in efficiency by 0 . 3% per day, 3 . 5% per month and 60% per year

[0001] .

[0016] In order to try to keep profitable the efficiency of the CSP systems it is necessary to wash periodically the mirrors with the expenditure of considerable quantities of , preferably distilled, water, in relation to the huge sizes of the solar fields , by determining high costs for the maintenance of the plants .

[0017] The strategies for cleaning the mirrors aim at reducing the cost of the produced electricity and vary depending upon the plant size , the soiling type, the geographical site and the meteorological conditions , the manpower cost , the water cost and a whole series of factors related to optics ( level of reflectance to be guaranteed) , by contributing to the definition of the cost item LCOC (Levelized Cost of Cleaning) .

[0018] The cost estimated for the process of cleaning the solar mirrors in a 50-MW plant is between 0 . 2 and 5 USD / KWh about 70 , 000 USD / year . A solution to the problem of reducing the water and the washing cycles consists in developing selfcleaning solar mirrors : under self-cleaning mirror a mirror coated with a self-cleaning coating is meant , having wettability properties with photocatalytic hydrophilic character, by forming a film of water on the coating which, through exposure to UV rays , photodegrades dirt , or a hydrophobic character, wherein the water forms spherical drops rolling away bringing dirt therewith; in both cases the coating being so as to require a much lower water volume than the conventional mirrors for an effective washing .

[0019] The main difficulty met in coating a concentration mirror consists in preserving the high reflectance thereof in the whole solar range . The products used in other fields , such as for example the textile field, in fact , do not have the transparency required for the concentration mirrors and affect the optical properties of the mirror .

[0020] The limit of the hydrophilic / photocatalytic products existing in other technical fields based on titanium oxide is the non-transparency in the solar radiation range . Moreover, the sol-gel technology, usually used for manufacturing, has serious limits of scalability and compactness / durability of the coatings , since the procedures to dry up the films create an unwished porosity and lead to cracks in the coating .

[0021] An example is International patent application No . WO 2016 / 177 , 709 Al in the name of the company RIOGLASS, relating to the synthesis , through the SOL-GEL, of a hydrophilic photocatalytic self- cleaning TiO2-based layer, but which leads back to a process suitable only for coating glass , with all drawbacks of the SOL-GEL with respect to the large scale production, that is low density of the produced materials , need for thermal annealing, inhomogeneity on wide areas , dangerousness of reagents , cost of precursors .

[0022] The hydrophobic products present in literature, in turn, have the limit that to increase the static contact angle generally make use of fluorinated nanoparticles and in order to be transparent they have a perfluorinated matrix and then require expensive costs , even in terms of safety for the operators and of fluorination step to be added to the production lines of the mirrors .

[0023] In the scientific article, Scott R . , et al . "Low- cost anti-soiling coatings for CSP collector mirrors and heliostats . " High and Low Concentrator Systems for Solar Energy Applications IX. Vol . 9175 . SPIE, 2014 . , a self-cleaning layer is shown comprising nanoparticles made of fluorinated silica glued on a polymeric binder made of Teflon-AF, or polyurethanes or polysiloxanes deposited by spinning or by SAM technique ( self-assisted monolayer which provides the use of gas assisted spray) and exhibit superhydrophobic properties deriving from the exposure of silica NP with high surf ace / volume ratio (they incorporate air pockets which increase the contact angle ) . The superhydrophobic effect deriving from nano-structuring is notoriously inadequate for extreme environments such as deserts , since it is easily removed by the abrasive effects of sand blasts . The document shows that the binders alone do not have adequate superhydrophobic properties to be autonomously used as self-cleaning coatings , the nanoparticles alone do not adhere to the substrate and they have no suitable mechanical properties to form films resistant to the atmospheric agents , whereas the coating obtained with fluorinated nanoparticles glued on the binders shows superhydrophobic properties which satisfy the required standards . It is highlighted that the experimental tests were made in an indoor environment on mirrors having small sizes and of FSM type, therefore not directly scalable on larger surfaces for operating applications , which, in order to reach the intermediate scale ( 15x 20cm) , the selected deposition technique is the drawdown one and, it is highlighted that the article by G . G . Jang et al . ,

[0002] by comparing the performances in outdoor environment of the hydrophobic coatings with the hydrophilic ones produced with sol-gel methods , shows the functional limit of 160 days of the superhydrophobic coatings , due to the hardening of dirt (mechanical degradation) , which irreversibly compromises the reflectance of the treated mirrors .

[0024] The desired requirements for a self-cleaning coating for solar mirrors are : wettability very distant from the one of the mirror as such, and high transparency in the whole range of the solar radiation, so as to leave unaltered the optical properties of the mirror, and even applicability on large area and low costs of application .

[0025] Additional requirements such as : limitation of soiling, protective effect with respect to erosion and corrosion, that is the two main degradation mechanisms to which the mirrors are subjected, ease of application even on existing plants , and suitability to retrofitting of existing mirrors , represent a highly strategic goal in the field research .

[0026] Therefore, it is clear how important creating mirrors with a self-cleaning coating could be, by using a simple and cheap treatment , so as to reduce the cleaning and maintenance costs of mirrors exposed to atmospheric agents , and used, in particular in plants of concentration type but even in photovoltaic plants , in particular concentration plants , by detecting the need for providing a treatment which can be adapted to all types of mirrors and in case to other components of solar plants exposed to soiling, by making the treatment suitable even for the retrofitting of mirrors already in use .

[0027] The technical problem underlying the present invention is to provide a treatment for applying a self-cleaning coating to a mirror and a related mirror with said self-cleaning coating allowing to obviate the drawbacks mentioned with reference to the known art , involving high costs for cleaning and maintenance of the mirrors or complex and expensive coating processes .

[0028] Such problem is solved by a treatment as defined in the enclosed claim 1 .

[0029] The treatment can be applied in a concentration solar system with solar mirrors including a reflecting surface . The treatment consists , after any possible removal of a pre-existing coating, possible preliminary cleaning and possible polishing of such reflecting surface, in applying thereon a self-cleaning coating made of a polymer for optics , by spray technique .

[0030] Moreover, the treatment provides that while dispensing the polymer, the reflecting surface is kept in a horizontal position, and the spray follows a direction along a vertical axis perpendicular thereto, and a possible subsequent finishing is carried out until obtaining : a surface wettability with a WCA between 70 ° and 160 ° , a difference in reflectance with respect to the untreated reflecting surface not exceeding 10% , a coating transmittance at least equal to 90% for wavelengths between 380 nm and 700 nm, a coating transmittance of at least 85% for wavelengths between 700 nm and 1500 nm, and a coating transmittance at least higher than 50% for wavelengths between 1500 nm and 2500 nm .

[0031] The main advantage of the coating according to the present invention lies in providing a treatment , simple to be executed and cheap, allowing to save water, the frequency of washing ( and then personnel and means used in the periodic maintenance and cleaning of mirrors exposed to the atmospheric conditions ) and consequently a reduction in the cost of electricity produced with concentration technology .

[0032] An additional advantage lies in that said treatment can be adapted to all types of mirrors and, eventually, to other components of solar plants exposed to soiling, such as for example the photovoltaic panels , by making the treatment suitable also for the retrofitting of components already in use .

[0033] The present invention will be described hereinafter according to a preferred embodiment thereof , provided by way of example and not for limiting purposes with reference to the enclosed drawings wherein :

[0034] * figure 1 shows a section view of a structure of a BSM according to the state of art ;

[0035] * figure 2A shows a section view of a structure of a FSM according to the state of art ;

[0036] * figure 2B shows a section view of another structure of a FSM according to the state of art ; and

[0037] * figure 3 shows a flow chart of a treatment for applying a self-cleaning coating on a reflecting surface of a mirror according to the invention;

[0038] * figure 4A shows a graph of the trend of reflectance (R_C) and transmittance (T_C) of the self-cleaning coating on glass for optics , in particular Eagle XG®, and of the same glass without the self-cleaning coating (XG) ; and

[0039] * figure 4B shows a graph of the trend of reflectance of an untreated solar mirror ( SM) and of a solar mirror on which the self-cleaning coating is laid down with the treatment the invention relates to (C1_SM) and a solar mirror on which the self-cleaning coating has been laid down with a general air spray treatment , then not compliant with the treatment the invention relates to (C2_SM) .

[0040] With reference to the figures , a treatment applicable in a concentration solar system with solar mirrors including a reflecting surface, the treatment consisting in an application on said reflecting surface of a self-cleaning coating, hereinafter mentioned more simply as treatment , is designated as a whole with 1 .

[0041] The treatment is applicable in a concentration solar system with solar mirrors including a reflecting surface .

[0042] The treatment consists , after any possible removal of a pre-existing coating, possible preliminary cleaning and possible polishing of such reflecting surface, in applying thereon a self-cleaning coating made of a polymer for optics , by spray technique .

[0043] Preferably, the spray is a high air volume spray, not gas assisted .

[0044] In particular, the treatment provides that while dispensing the polymer for optics , the reflecting surface is kept in a horizontal position and the spray follows a direction along a vertical axis perpendicular thereto, and that a possible subsequent finishing is performed until obtaining a surface wettability with a static contact angle (WCA) between 70 ° and 160 ° and, a difference in reflectance with respect to the untreated reflecting surface not exceeding 10% and, for wavelengths between 380 nm and 700 nm a coating transmittance at least equal to 90% , for wavelengths between 700 nm and 1500 nm a coating transmittance of at least 85% and for wavelengths between 1500 nm and 2500 nm a coating transmittance always higher than 50% .

[0045] It will be appreciated that said self-cleaning coating is the coating of a reflecting surface exposed to the atmospheric agents and which protects the reflecting surface from corrosion, provides an optimum levelling and distension, a good quality of polishing; moreover, the self-cleaning coating is resistant to mechanical wear .

[0046] The purpose of the self-cleaning coating is to coat a reflecting surface, for example a mirror, to allow a cleaning of the mirror itself by exploiting the best interaction of the self-cleaning coating with water .

[0047] Preferably, said polymer for optics has hydrophobic or hydrophilic properties .

[0048] According to some embodiments , said treatment consists , after any possible removal of a preexisting coating, possible preliminary cleaning and polishing of such reflecting surface, in applying thereon a self-cleaning coating made of a polymer for optics , by spray technique .

[0049] According to other embodiments , said treatment consists , after any possible removal of a preexisting coating, in a possible preliminary cleaning and in a polishing of such reflecting surface, in applying thereon a self-cleaning coating made of a polymer for optics , by spray technique . Moreover, in this treatment example it is provided that a subsequent polishing is performed .

[0050] According to some embodiments , the treatment provides to lay down at least a self-cleaning layer, by spraying with a spray gun said polymer for optics , according to a spray technique, by positioning a spray gun at a distance from the reflecting surface between 10 cm and 50 cm, preferably at 30 cm . It will be appreciated that the correct spray procedure is one of the determining factors to define the quality of the reflecting surface on which the treatment , the invention relates to, is performed .

[0051] In fact , some solar mirrors treated by using the air spray technique at a smaller or larger distance than the optimal ones , illustrated above, and with a dustfree time higher than 20 minutes , result to be opaque due to a surface roughness of the self-cleaning coating also defined : "orange peel effect" .

[0052] Moreover, Figure 4A comprises a graph which reproduces the reflectance (R_C) and transmittance (T_C) data of the self-cleaning coating on glass for optics , in particular Eagle XG®, and of the same glass without the self-cleaning coating (XG) .

[0053] Figure 4B shows the trend of the reflectance of an untreated solar mirror ( SM) , of a solar mirror on which the self-cleaning coating is laid down on the solar mirror through the treatment the invention relates to (C1_SM) and a solar mirror on which the self-cleaning coating has been laid down through a general air spray treatment and thus not compliant with the treatment , the invention relates to, (C2_SM) .

[0054] It will be appreciated that in Figure 4A the trend of transmittance (T_C) between 2000 nm and 2500 nm of the self-cleaning coating alone, deposited on glass , although it meets the required requirements , is further improved when the coating is applied on a solar mirror according to the described treatment ( see curve C1_SM of Figure 4B) . At the same time if one compares the trend of the curve T_C with the trend of the curve C2_SM it is possible to note that the lowering in transmittance can be compared to the lowering in transmittance of the mirror on which the treatment the invention relates to was not used, therefore, the treatment the invention relates to succeeds in preserving the intrinsic features of reflectance and transmittance of the reflecting surface to be treated in the whole solar range .

[0055] According to other preferred embodiments said polymer for optics , is of thermosetting acrylic type, preferably with content of solid resin higher than 50% by weight of the acrylic polymer and, preferably, said dispensing occurs with a spray gun of HVLP type, which comprises preferably a nozzle with diameter between 0 . 3 m and 3 . 0 pm, preferably positioned at a distance between 10 cm and 50 cm, more preferably at 30 cm, from said reflecting surface, and preferably fed with a pressure not higher than 350 KPa .

[0056] It will be appreciated that in the state of art gas assisted deposition spray techniques (with propellants subject to stringent regulations ) by flame or powders are known, these techniques can be used in laboratory, but they are not scalable to surfaces having big sizes . In the present invention, instead, the spray technique thereto one refers is the one used, for example, for painting motor vehicles , which provides the use of a HVLP spray gun .

[0057] Alternatively, the deposition of the self-cleaning coating can take place by conventional painting techniques , such as for example with brush and / or roller .

[0058] Said polymer can be selected among transparent car paints belonging to the categories : Low Solid, Medium Solid, High Solid, Ultra High Solid . Preferably, the polymer for optics belongs to the category of the High Solid acrylics which comprises a base and a catalyst , for example it is a two- component acrylic resin with a thermosetting character, characterized by transparency in the interest range to preserve the reflecting properties of the solar mirror .

[0059] It will be appreciated that with 1 Kg of the preferred product it is possible to coat 2 . 5 m2of mirrors , by providing two depositions of the selfcleaning layer .

[0060] Preferably, a mirror on which said self-cleaning coating is applied has a wettability with a static contact angle (WCA) between 70 ° and 160 ° , more preferably it is equal to 110 ° .

[0061] Preferably, a mirror on which said self-cleaning coating is applied has a reflectance not lower than 10% of the uncoated mirror .

[0062] The treatment provides to detect a mirror architecture, corresponding to said reflecting surface, by identifying the architecture of front surface mirror (FSM) or back surface mirror (BSM) as mirror on which the coating must be applied .

[0063] The treatment can further provide to detect a type of mirror substrate . Under substrate a mirror layer is meant , on which said self-cleaning coating can be laid down .

[0064] Alternatively, the treatment can provide to detect a pre-existing coating present on the mirror . The pre-existing coating is a coating which can be removed to allow the deposition of the self-cleaning coating .

[0065] I f the mirror is of FSM type and comprises a selfcleaning coating as pre-existing coating of said mirror, said treatment comprises the removal of the pre-existing coating, the polishing, the application of the self-cleaning coating and the finishing .

[0066] Preferably, the removal of the pre-existing coating comprises the preparation of the reflecting surface to be coated .

[0067] I f the mirror is of FSM type and comprises a polymeric or oxide or nitride layer or a combination of the same as substrate of said mirror, the treatment comprises the application of the selfcleaning coating and the finishing .

[0068] I f the mirror is of BSM type and comprises a selfcleaning layer as pre-existing coating of said mirror, said treatment comprises the removal of the pre-existing coating, the polishing and the application of the self-cleaning coating .

[0069] I f the mirror is of BSM type it comprises a glassy layer as substrate, the treatment comprises the preliminary cleaning, the polishing, and the application of the self-cleaning coating .

[0070] It will be appreciated that if the mirror is of BSM type, preferably, the finishing is not required in case of performing the deposition of the selfcleaning coating compliant to the treatment the invention relates to and with average roughness of the coating limiting the diffuse reflectance of the coated mirror at minimum . Moreover, the finishing it is not advisable if the thickness of the metallic glass is lower than 1 mm in order not to induce useless mechanical stresses to the component .

[0071] Preferably, the reflectance in the architecture of mirror of BSM type comprising at least a selfcleaning layer is at least 94% .

[0072] Preferably, the types of substrate can be metals , glassy materials , polymers , oxides , nitrides , carbides and / or combinations of the latter, it is relevant for the purpose of the method, the exact detection of the type of substrate in order to implement profitably said method .

[0073] Preferably, the coating treatment further provides to arrange said polymer for optics thereby depositing it through a spray technique on the detected substrate .

[0074] In particular, in case of acrylic thermosetting polymer, it could be possible to mix a base and a catalyst according to the preferred proportion 2 : 1 , by weighing base and catalyst , by mixing them in the suitable proportions , by inserting said mixture in the tank of a spray gun and performing the deposition at the preferred temperature of 21 ° C on the solar mirror in periods of time compatible to the mixture crosslinking phenomenon .

[0075] Preferably, the drying time of acrylic thermosetting polymers is rapid, preferably lower than 15 minutes , and dust-free time is preferably not higher than 20 minutes .

[0076] According to some preferred embodiments , the polishing consists in removing the substrate with abrasive papers having suitable abrasiveness , for example P1500 paper and subsequently polishing the mirror with waxes , for example 3M 80345 - protective pink polish, restructuring and polishing for car body .

[0077] The abrasive paper can have an abrasiveness between P 600-P1500 according to FEPA classification .

[0078] The used waxes can be drying, and / or polishing oils for automotive finishes .

[0079] The preliminary cleaning can consist in removing the dust with antistatic cloths and subsequently removing oxide residues with an anti-silicone solvent and generally a solution of acetic acid .

[0080] Preferably, the anti-silicone diluent is IVAT - RDL .A111 anti-silicone degreasing diluent ivat acrylic paints .

[0081] According to some preferred embodiments of the invention, the finishing consists in the application of abrasive pastes and / or waxes with levelling effect with respect to possible imperfections of the process for painting and protecting the polymer, especially from the UV rays , and then for avoiding yellowing .

[0082] Preferably, the layers constituting the selfcleaning coating are at least two and can be laid down by following two directions perpendicular to each other .

[0083] Each one of the two perpendicular directions is preferably parallel to a respective additional axis . Said additional axes being perpendicular to each other and being perpendicular to said vertical axis .

[0084] The self-cleaning coating can be deposited by using the low-pressure spray technique . The technique consists in the deposition of a liquid solution contained in suitable tanks by low-pressure nebulizat ion, which guarantees optimum surface finishing with low dispersions of possible volatile chemical substances (VOC) .

[0085] Usually, the spray technique, preferably with high air volume and low pressure, is used for applying polymeric formulations , even consisting of composite materials in solvents of various nature and resulting viscosity, on supports of different nature (glasses , metals , laminates , etc . ) and sizes ( laboratory and / or prototype scale ) . It operates with compressed air or powered by a small turbine, which dispenses a great air flow through a tube to power the spray gun, which guarantees a transfer capacity higher than 65% and, in particular cases , even by 95% .

[0086] The type of gun, the pressure, the nozzles to be used, the coats to be applied, depend on the features of the product to be applied and on the type of starting reflecting surface .

[0087] According to some embodiments of the invention, the finishing consists in sanding with abrasive papers and / or pastes and water said self-cleaning coating .

[0088] The abrasive paper can have an abrasiveness between P1000 and P1500 according to FEPA classification .

[0089] The used abrasive pastes can be of the type used for the finishing of painting processes in the automotive field .

[0090] Once deposited, the self-cleaning coating can have a thickness between 150 nm and 3 pm, preferably between 500 nm and 1800 nm, more preferably 1500 nm . It will be appreciated that said treatment can be applied to other types of surfaces , in particular transparent but not reflecting surfaces and / or transparent surfaces absorbing the solar radiation, in particular the surfaces of the photovoltaic panels .

[0091] It will be further appreciated that said treatment can be applied to surfaces of solar plants different from the mirrors , but which require optical properties and maintenance due to soiling, such as for example, but not exclusively, the photovoltaic panels , the thermal panels , the glasses of the receiving tubes .

[0092] In another aspect of the invention, it also relates to a solar mirror used in a thermodynamic solar system, which comprises a reflecting surface obtained through the above-described treatment , comprising a self-cleaning coating laid down on said reflecting surface, made of a polymer for optics , so as to obtain a surface wettability with a static contact angle (WCA) between 70 ° and 160 ° and, a difference in reflectance with respect to the untreated mirror not exceeding 10% and, for wavelengths between 380 nm and 700 nm a coating transmittance at least equal to 90% , for wavelengths between 700 nm and 1500 nm a coating transmittance of at least 85% and for wavelengths between 1500 nm and 2500 nm a coating transmittance always higher than 50% .

[0093] It will be appreciated that said mirror, thanks to the application of said self-cleaning coating, even after repeated washing cycles ( for example 10 cycles ) and exposure to atmospheric agents , can have a wettability with a static contact angle (WCA) at least equal to 70 ° , a difference in reflectance with respect to the untreated mirror not exceeding 20% and, for wavelengths between 380 nm and 700 nm a coating transmittance at least equal to 80% .

[0094] In fact , one of the advantages provided by said selfcleaning coating is the capability of maintaining the properties of wettability, transparency and reflectance in the ranges admissible for operation of the mirrors , in particular those used in concentration systems , for operating periods of time of at least 4 years , without applying expensive procedures for replacing the component .

[0095] The main advantage of the treatment of a solar mirror with the self-cleaning coating is to provide a cheap method, simple to be executed, which can use commercial products in turn cheap and to provide a consolidated deposition method such as the low- pressure spray, by allowing to obtain a considerable economic saving in terms of maintenance costs and cleaning of the mirrors .

[0096] In fact , thanks to said self-cleaning coating, the costs linked to the washing water and consequently the producible electric current are reduced .

[0097] To the above-described treatment for coating a mirror with a self-cleaning coating a person skilled in the art , with the purpose of satisfying additional and contingent needs , could introduce several additional modifications and variants , however all within the protective scope of the present invention, as defined by the enclosed claims .

[0098] ★ ★ ★ [1] SARVER, Travis; AL-QARAGHULI , Ali; KAZMERSKI, Lawrence L. A comprehensive review of the impact of dust on the use of solar energy: History, investigations, results, literature, and mitigation approaches. Renewable and sustainable energy Reviews, 2013, 22: 698-733.

[0099] [2] Jang, Gyoung Gug, et al. "Transparent superhydrophilic and superhydrophobic nanoparticle textured coatings: Comparative study of anti-soiling performance." Nanoscale Advances 1.3 (2019) : 1249- 1260.

Claims

CLAIMS1 . Treatment in a concentration system with solar mirrors including a reflecting surface, the treatment consisting, after any possible removal of a pre-existing coating, possible preliminary cleaning and possible polishing of such reflecting surface, in applying thereon a self-cleaning coating made of a polymer for optics , by spray technique , wherein, while dispensing the polymer for optics , the reflecting surface is kept in a horizontal position and the spray follows a direction along a vertical axis perpendicular thereto, and consisting in a possible subsequent finishing until to obtain : a surface wettability with a static contact angle (WCA) between 70 ° and 160 ° , a difference in reflectance, with respect to the untreated reflecting surface, not exceeding 10% , a coating transmittance at least equal to 90% for wavelengths between 380 nm and 700 nm, a coating transmittance of at least 85% for wavelengths between 700 nm and 1500 nm, and a coating transmittance always higher than 50% for wavelengths between 1500 nm and 2500 nm .2 . The treatment according to claim 1 , wherein said polymer for optics is of thermosetting acrylic type with content of solid resin higher than 50% by weight of the acrylic polymer .3 . The treatment according to claim 2 , wherein said dispensing is performed with a spray gun of HVLP type, with a nozzle having a diameter between 0 . 3 m and 3 . 0 pm, positioned at a distance between 10 cm and 50 cm from said reflecting surface , and fed with a pressure not higher than 350 KPa .4 . The treatment according to claim 1 or 2 , providing to detect a mirror architecture, corresponding to said reflecting surface , of the front surface type (FSM) and a self-cleaning coating as pre-existing coating of said mirror, said treatment comprising said removal of said pre-existing coating, said polishing, said application of a self-cleaning coating and said finishing .5 . The treatment according to claim 1 or 2 , providing to detect a mirror architecture, corresponding to said reflecting surface , of the front surface type (FSM) and a polymeric or oxide or nitride layer as substrate of said mirror, said treatment comprising said application of a self-cleaning coating on said substrate and said finishing .6 . The treatment according to claim 1 or 2 , providing to detect a mirror architecture, corresponding to said reflecting surface, of the back surface type (BSM) and a self-cleaning coating as pre-existing coating of said mirror, said treatment comprising said removal of said pre-existing coating, said polishing, said application of the self-cleaning coating .7 . The treatment according to claim 1 or 2 , providing to detect a mirror architecture, corresponding to said reflecting surface, of the back surface type (BSM) and a glassy layer as substrate of said mirror, said treatment comprising said preliminary cleaning, said polishing, and said application of a selfcleaning coating on said substrate .8 . The treatment according to any one of the preceding claims , wherein said self-cleaning coating comprises at least two self-cleaning layers whichare laid down by following two directions perpendicular to each other, each one of the two directions developing parallelly to a respective additional axis .9 . Solar mirror used in a concentration solar system, said mirror comprising a reflecting surface comprising a treatment according to any one of the preceding claims , said mirror being characterized in that it comprises a self-cleaning coating laid down on said reflecting surface, made of a polymer for optics , whereby a surface wettability with a static contact angle (WCA) between 70 ° and 160 ° , a difference in reflectance , with respect to an untreated reflecting surface, not exceeding 10% , a coating transmittance at least equal to 90% for wavelengths between 380 nm and 700 nm, a percentage transmittance of the coating of at least 85% for wavelengths between 700 nm and 1500 nm, and a coating transmittance always higher than 50% for wavelengths comprised between 1500 nm and 2500 nm are obtained .

Images