SURFACE COATING AND METHOD FOR REDUCING THE TENDENCY TO SOILING OF AN OBJECT SURFACE

Abstract

The present invention relates to a surface coating and a method for reducing the susceptibility of an object's surface to soiling, particularly the surface of photovoltaic modules. The surface coating is formed of a layer or sequence of nanoporous layers (2) that has a hydrophobic surface and is hydrophilic internally. The surface coating can reduce the sensitivity of an object's surface to soiling even under extreme environmental conditions, such as those found in desert regions, for example.

Description

Technical application area

[0001] The present invention relates to a surface coating with reduced susceptibility to soiling, formed by a nanoporous layer or layer sequence, and to a method for reducing the susceptibility to soiling of an object surface, in particular the surface of a photovoltaic module. The surface coating and the method generally serve to reduce the soiling of surfaces caused by the adhesion of particles and dirt, especially on transparent or reflective surfaces such as those found in photovoltaic modules, solar thermal mirrors, window and architectural glass, greenhouses, or museum display cases.

[0002] The problem underlying the invention is explained below using photovoltaic modules as an example, but can easily be transferred to other applications where unwanted particle adhesion leads to impairments. These include, for example, process dusts in industrial production plants, such as allergens in food production, paint powder residues in paint production, or dust particles in pharmaceutical products.

[0003] Photovoltaic modules (PV modules) are installed outdoors. Their surfaces are therefore exposed to local weather conditions. These can vary considerably depending on the location, as ambient temperature, humidity, typical wind speeds, solar radiation, and air pollution can all fluctuate significantly. Dust and air pollution can be of natural origin, such as mineral dust in desert regions, or of artificial or human-induced origin, such as soot particles from road traffic or industrial emissions. Depending on the site conditions, the deposition of dust particles leads to surface soiling. This effect is also known as "soiling" in the case of PV modules.The deposits on the surfaces, in turn, lead to a shading of the incident sunlight through scattering and absorption processes, which directly translates into power losses in PV modules. These losses can be very high, especially in extreme climates such as desert regions, amounting to over 1% relative power loss per day. The adhesion of dust particles is strongly promoted by condensed moisture and the resulting capillary forces. Furthermore, the cyclical effects of moisture (dew) and drying can lead to the cementation of dust particles, resulting in permanent and difficult-to-remove soiling. State of the art

[0004] To reduce the soiling or susceptibility to soiling of surfaces under outdoor conditions, various methods are known, which can be classified into three categories. The first category utilizes natural cleaning effects from rain, wind, and / or snow. The second category employs active manual or automated cleaning, which can be carried out with water, compressed air, mechanically with brushes, or even with electric fields (EDS). The third category concerns passive soiling reduction through suitable surface coatings. The present invention can be classified in the third category. As such, it essentially enhances the effect of natural cleaning processes, such as cleaning by wind or precipitation. In the following, only this third category of functional coatings will be explicitly considered.

[0005] From a scientific perspective, there is a wide range of coatings that are potentially suitable as anti-dirt coatings and also exhibit transparent properties for use in PV modules. The focus here is on coatings with either (super)hydrophilic, (super)hydrophobic, or photocatalytic properties. Examples of such coatings can be found, for instance, in the publications by S. Nishimoto et al., "Bioinspired self-cleaning surfaces with superhydrophobicity, superoleophobicity, and superhydrophilicity" in: RSC Adv 3 (2013), No. 3, pp. 671-690, or by S. Banerjee et al., "Self-cleaning applications of TiO2 by photo-included hydrophilicity and photocatalysis" in: Applied Catalysis B: Environmental 176-177 (2015), pp. 396-428. Most of these layers are characterized by a micro- or nanoscopic surface roughness and the associated reduced contact area between contaminants (e.g.,Dust particles) and the surface achieve a reduction in adhesion. Furthermore, a self-cleaning effect is assumed due to the (super)hydrophilic or (super)hydrophobic character of the respective layer, often achieved through chemical functionalization. This is usually described analogously to the lotus effect, with water droplets beading up and carrying dirt with them, or dirt particles sliding off on a film of water. In photocatalytically active layers, exposure to light causes a chemical change in the surface, which, in addition to hydrophilizing the surface, can also lead to the breakdown of organic compounds, thus minimizing the adhesion of organic compounds.

[0006] Compared to the scientific attention given to self-cleaning glazing, only a small number of commercially available self-cleaning anti-dirt coatings with transparent properties exist. Examples are given in K. Midtal et al., "Self-cleaning glazing products: A state-of-the-art review and future research pathways" in: Solar Energy Materials and Solar Cells 109 (2013), pp. 126-141. However, no commercial coatings are currently known that exhibit sufficiently long-lasting self-cleaning properties under extreme outdoor conditions, such as those found in desert regions.

[0007] US patent 2013 / 199612 A1 discloses a hydrophobic substrate that exhibits a self-cleaning effect and low reflectivity on its surface. The substrate has a nanostructured surface coated with a hydrophobic layer. The hydrophobic layer provides the self-cleaning effect, while the nanostructure creates the low reflectivity.

[0008] A key component for the self-cleaning function of known hydrophobic, hydrophilic, and photocatalytic coatings is the sufficient presence of water, which is supplied, for example, by precipitation. However, water is only available in limited or even non-existent quantities, especially in desert regions. Without regular rainfall, the effectiveness of these coatings is severely limited, even if condensation of water droplets occurs in areas of high humidity. Furthermore, superhydrophobic and superhydrophilic coatings, in particular, quickly lose their properties due to stress factors and weather influences such as moisture, wind, or mechanical wear, e.g., from abrasion, meaning they do not lead to the desired reduction in soiling, especially in the field.Although "soiling" affects a large number of installed photovoltaic systems and is the subject of scientific research at a number of renowned institutes, in practice hardly any non-mechanical methods are used to keep PV modules free of dust in the field or to clean them.

[0009] From MA Bahattab et al., "Anti-soiling surfaces for PV applications prepared by sol-gel processing: Comparison of laboratory testing and outdoor exposure", in: Solar Energy Materials and Solar Cells 157 (2016), pp. 422-428, and W. Glaubitt et al., "Anti-soiling effect of porous SiO2 coatings prepared by sol-gel processing", in: Journal of Sol-Gel Science and Technology 59 (2011), No. 2, pp. 239-244, nanoporous silicon oxide-based coatings are known as surface coatings produced using sol-gel processes and exhibit nanoparticles with a bimodal distribution. These hydrophilic nanoporous coatings possess anti-reflective properties on glass surfaces and showed lower surface soiling in field tests compared to uncoated glass.

[0010] The object of the present invention is to provide a surface coating and a method for reducing the susceptibility to soiling of an object surface, which achieve their effect for a long time even under extreme environmental conditions such as in desert regions and are also suitable - when using appropriately optically transparent layer materials - for use in photovoltaics and solar thermal energy. Description of the invention

[0011] The problem is solved by the surface coating and the method according to claims 1 and 14. Advantageous embodiments of the surface coating and the method are the subject of the dependent claims or can be found in the following description and the exemplary embodiments.

[0012] The proposed surface coating with reduced susceptibility to soiling is formed by a nanoporous layer or layer sequence that has a hydrophobic surface and a hydrophilic interior, acting as a water storage layer with a water storage capacity of at least 60 × 10⁻⁶ l / m². The term nanoporous here refers specifically to pore sizes in the range ≤ 100 nm, enabling water absorption due to capillary action. The specified minimum water storage capacity, also referred to as minimum water storage volume, corresponds to a value of 60 nm (water column), since 1 l / m² = 1 mm (water column). In the proposed method for reducing the susceptibility to soiling of an object surface, such a surface coating is applied to the object surface.

[0013] For dust particles in the single- and double-digit micrometer size range, such as those typically found in desert climates, the capillary forces between these particles and the respective surface can be orders of magnitude greater than other forces like gravity, electrostatic, thermal, or van der Waals forces, especially under high humidity or during dew formation. Furthermore, the presence of water in combination with dust is a necessary condition for the formation of certain chemical bonds, such as those observed during cementation processes on the surfaces of PV modules in desert regions. The presence of water on the surface, even in the form of microscopic droplets and films such as those formed by condensation, is therefore considered the primary cause of contaminant adhesion to surfaces.Therefore, preventing the surface from becoming wetted with liquid water is a key approach to reducing soiling within the scope of the present invention. However, lateral transport of water droplets and films is only possible to a limited extent for surfaces that are only slightly inclined, such as those typically found on PV modules, and in desert regions with correspondingly low rainfall.

[0014] The proposed surface coating and associated process are based on the concept of vertical moisture transport across the coating's surface and the storage of liquid water in the coating's internal, hydrophilic regions, combined with thermal moisture management. The anti-adhesion effect is achieved through the hydrophobic surface of the nanoporous coating structure with its corresponding surface roughness. The hydrophilic properties of the coating's interior cause condensed water to bind and be stored within the coating, which thus acts as a water storage layer. This keeps the surface dry and free of condensed water droplets or films for extended periods, preventing the aforementioned cementation processes.The moisture storage layer (water storage layer) is emptied by the outgassing of water vapor under dry and warm ambient conditions. In PV modules, this occurs automatically when exposed to sunlight and is further supported by the typically elevated temperatures of PV modules, which can reach up to 80°C (at midday). The removal of water from the water storage layer of the surface coating can, of course, also be achieved through other heat sources, such as regular artificial heating. Potential optical losses due to changes in the refractive indices of water-filled pores in the nanoporous layer are accepted, as water absorption in this application generally occurs during the cool night hours without sunlight.

[0015] The nanoporous layer or layer sequence can, for example, consist of at least two superimposed layers, the upper layer being hydrophobic and the lower layer hydrophilic. Alternatively, the nanoporous layer can be a single layer exhibiting a gradual gradient of hydrophobic and hydrophilic properties from the surface to the interior. The hydrophobic / hydrophilic gradient is adjusted accordingly so that the nanoporous layer or layer sequence possesses sufficient internal water storage capacity to reduce susceptibility to fouling.

[0016] The water storage volume, and thus also the water storage capacity, is largely determined by the layer thickness and the porosity of the layer. The porosity of the nanoporous layer or layer sequence of the proposed surface coating is preferably in the range of 30–70%, and particularly preferably 50% ± 5%. The layer thickness required for a specific water storage capacity is obtained, for a porosity of X%, by multiplying the water column by a factor of 100 / X, and for a porosity of 50%, by multiplying the water column by a factor of 2.

[0017] When using photovoltaic modules, the design of the surface coating with regard to water storage capacity is based on the amount of dew that accumulates in the relevant local region during a single night. The following are exemplary value ranges (water column) for the water storage capacity for which the nanoporous layer or layer sequence of the proposed surface coating, particularly for photovoltaic modules, can be designed. • Low storage capacity: 60 nm - 1 µm • Medium storage capacity: 1 µm - 0.1 mm water column, based on a typical average dew amount of 0.1 mm for one night for optimized, planar, and insulated condenser surfaces at relevant locations (e.g., Jerusalem or the coastal region of Corsica) • High storage capacity: 0.1 mm - 0.37 mm • Very high storage capacity: 0.37 mm - 0.8 mm • Maximum storage capacity: 0.8 mm, based on the theoretically possible absolute maximum for a daily dew amount

[0018] The thickness of the hydrophilic region of the nanoporous layer or layer sequence is preferably at least 120 nm. The term "hydrophilic region" refers to the hydrophilic layer in a layer sequence consisting of a hydrophobic and a hydrophilic layer, and to the hydrophilic layer region in a layer with a gradual transition between hydrophobic and hydrophilic properties, whereby the hydrophilic layer or layer region is always located between the object surface, e.g., a glass pane, and the hydrophobic layer.

[0019] For use in photovoltaic modules or other objects requiring optical transmittance to the object surface, the nanoporous layer or layer sequence is formed from an optically transparent material. Preferably, the thickness of the nanoporous layer or layer sequence is selected such that it forms an antireflective coating for optical radiation on the object surface. A porosity of approximately 50% (50% ± 5%) is particularly advantageous for the antireflective effect while maintaining layer stability.

[0020] By establishing a gradient of wettability / hydrophobicity in the proposed surface coating, a dry surface of the coating, and thus of the object, is achieved. Particularly with microscopically small water droplets, such as those formed during condensation, the water is rapidly transported into the layer where it is then stored. Reducing the water coverage and residence time on the surface, in turn, reduces the primary mechanism of dirt particle adhesion through capillary action and simultaneously prevents subsequent cementation processes. The reduced adhesion forces (absent capillary action) allow cleaning mechanisms, especially natural cleaning by wind, to work significantly more effectively, thus reducing surface soiling.

[0021] In contrast to self-cleaning (super)hydrophobic and (super)hydrophilic surfaces, the water accumulation on the surface of the proposed surface coating is reduced because – especially under condensation conditions such as high humidity – most of the water is located within, rather than on, the coating. Furthermore, no extreme contact or wetting angles are required for its functionality.

[0022] Compared to other porous layers that lack a wettability / hydrophobicity gradient, the additional capillary forces directed towards the object surface lead to a significant reduction in the amount of water at the layer's surface. In contrast, purely hydrophobic porous layers are characterized by water droplets with a long residence time at the surface, while in purely hydrophilic porous layers, capillary forces can cause the water to rise within the layer's pores to the surface. Both scenarios offer the potential for the formation of small water capillaries between the surface and any contaminants, such as dust particles, due to prolonged wetting. These effects can be minimized by applying the wettability gradient proposed here.

[0023] The proposed surface coating can also be produced with anti-reflective properties using industrially proven methods and exhibits effective dirt-repellent, and in particular dust-repellent, properties.

[0024] The nanoporous layer or layer sequence of the proposed surface coating is preferably produced using a sol-gel process. The sol can be applied using various known techniques, such as dip coating, rotational coating, or spraying. Preferably, the nanoporous layer or layer sequence is composed of nanoparticles. To achieve a greater layer thickness with hydrophilic properties, microparticles can also be incorporated into the interior of the layer.

[0025] The water storage capacity can also be increased by using or incorporating materials, especially nano- and / or microparticles, into the nanoporous layer or layer sequence that can absorb water. This water absorption can occur, for example, in the form of: • Water of crystallization, coordination water, structural water (chemically bound water); e.g., using sodium sulfate, minerals of the zeolite group; • Adsorption on large internal and external surfaces; e.g., using aerogels and xerogels such as silica gel, as well as other silicates (e.g., montmorillonite, bentonite, palygorskite); • Absorption, e.g., in polymers and / or plastic materials such as polyamides, chitosan, starch. In this case, not only the pore volume is available for water absorption, but also a portion of the introduced materials.

[0026] Suitable layer materials include inorganic materials such as SiO2, Al2O3, TiO2, ZrO2, MgF2, ZnO2, or CaF2, from which a nanoporous layer can be formed to act as a water storage layer. To create the hydrophobic surface, compounds containing alkylsilanes, such as methylsilane (CH6Si), or fluorine-containing compounds, such as fluorosilane (H3FSi), can be applied to or infiltrated into the surface of such a layer. These layers exhibit optically transparent properties, making them suitable for applications on PV modules or other objects requiring a correspondingly optically transparent coating. If such optically transparent layers are not required, porous layers made of other materials, such as organic materials, can also be used to form the proposed surface coating.

[0027] It is also possible to delay the reaching of the dew point temperature on the surface of photovoltaic modules or other surface-coated objects located outdoors by implementing suitable measures. This reduces the volume of water that needs to be absorbed, thus lowering the required water storage capacity. One possibility is the use of microparticles with encapsulated phase change materials (PCMs) whose phase change temperature is close to the dew point (weather-dependent on ambient temperature and humidity), typically in the range of 5°C to 25°C. The PCMs, which are liquid at higher temperatures, begin to crystallize within the microparticles as soon as the temperatures approach the dew point and continue to fall. The released latent heat warms the surface, preventing the dew point from being reached or delaying its occurrence. This reduces the amount of dew that needs to be stored.Another possibility is the use of particles or particle coatings with low-E properties (low IR emissivity). These reduce radiative cooling, so that the surface cools down more slowly compared to a design without such particles or particle coatings, and the dew point is not reached or is reached at a later time. This also reduces the amount of dew that needs to be stored.

[0028] The proposed surface coating and process are applicable in all areas where the surface deposition of dust and / or dirt adversely affects the functionality of objects and should therefore be reduced. This includes the aforementioned process dusts in industrial production facilities, such as allergens in food production, paint powder residues in paint production, or dust particles from pharmaceutical products. Further examples are mentioned in the introductory section. Photovoltaics and solar thermal energy offer particularly high application potential, as solar modules and CSP systems are increasingly being installed in dusty environments. Brief description of the drawings

[0029] The proposed surface coating and the associated process are explained in more detail below using exemplary embodiments in conjunction with the drawings. These show: Fig. 1 shows a schematic representation of different variants of the layer structure of the proposed surface coating; and Fig. 2 shows a schematic representation of the operating principle of the proposed surface coating. Ways to implement the invention

[0030] Examples of the proposed surface coating structure are given below using the cover glass of a PV module as an application example. However, the application of the proposed surface coating is not limited to PV modules but can be transferred to any object by utilizing the same layer structure. A key component of the proposed surface coating is the separation of water and particles at the surface by a water transport layer, the storage of the water in a water storage layer, and the subsequent outgassing of the water. These processes can be achieved or enhanced by nanoporous layers with a corresponding pore volume in the water storage layer and by vertically varying wettability / hydrophobicity within the layer, from hydrophobic at the layer surface (atmospheric side) to hydrophilic in the interior or on the object side.

[0031] Nanoporous layer materials are used to efficiently transport liquid and gaseous water vertically, for example, based on oxide nanoparticles or nanocrystallites. The surface coating can be a single-layer system with a graduated wettability / hydrophobicity profile, ranging from hydrophobic at the surface (water transport layer) to hydrophilic in the interior of the layer or towards the object (water storage layer). Alternatively, the surface coating can be a multi-layer system consisting of at least two layers, in which the layer facing the atmosphere exhibits hydrophobic properties (water transport layer), while the layer facing the object is hydrophilic (water storage layer).

[0032] The thickness of the water storage layer is adapted to the expected water volumes for the application and target region. Large particles in the micrometer range (e.g., made of glass), whose spaces are then filled with the nanoporous layer material, allow for the creation of sufficiently large volumes to achieve adequate water storage capacity. The layer thickness is preferably optimized as an antireflective layer according to the desired optical properties, insofar as the function of the water storage layer with the necessary storage capacity permits. For this purpose, it can be designed as a single λ / 4 layer or, in a multi-layer system with different refractive indices, as two λ / 4 layers. In the application of photovoltaics, the wavelength λ is optimized for the type of solar cell used and the corresponding wavelength-dependent quantum yield. An antireflective function of multi-layer systems can, for example, be achieved by...The refractive index of each layer is optimized by varying its density due to different particle sizes or materials. The optically denser layer faces the object or glass substrate. For outdoor applications, the absorption / emission properties of the layer or layer stack are preferably optimized to minimize emission in the wavelength range of the atmospheric window (IR radiation), thereby preventing radiative cooling and resulting dew formation.

[0033] Figure 1 shows examples in sub-figures A to F of the schematic layer structure of the cover glass of a solar module with the various configurations of the nanoporous layer system mentioned above. Figure 1A shows a single-layer system consisting of a nanoporous layer 2 on a glass substrate 1, formed from oxide nanoparticles 3. The layer has a thickness of λ / 4 and is therefore designed as an antireflective layer. The layer is composed of monodisperse nanoparticles with a gradual transition from hydrophobic to hydrophilic properties, as indicated by the gradient triangle also shown.

[0034] Figure 1B shows another single-layer system with the indicated layer thickness of λ / 4, in which the nanoporous layer 2 is formed from oxide nanoparticles 3 with a bimodal size distribution. Here, too, the layer exhibits a gradual transition from hydrophobic to hydrophilic properties, as indicated by the corresponding gradient triangle.

[0035] Figure 1C shows an example of a multi-layer system consisting of a stack of two nanoporous layers. This surface coating is formed from a lower hydrophilic layer 5 and an upper hydrophobic layer 4, each composed of monodisperse nanoparticles 3 with the same refractive index n1. The layer sequence has a thickness of λ / 4 and thus functions as an anti-reflective coating.

[0036] Figure 1D shows another example of a layer stack consisting of a lower hydrophilic nanoporous layer 5 and an upper hydrophobic nanoporous layer 4. In this example, the two layers are formed from nanoparticles 2 of different sizes / densities with correspondingly different refractive indices n1 and n2, as indicated in the figure. Each of the two layers 4, 5 of the layer stack has a thickness of λ / 4.

[0037] To increase the water storage capacity of the interior of the hydrophilic layer or layer region, microparticles can be embedded within this layer or layer region. Figure 1E shows an example of a single-layer structure with a gradual transition from hydrophobic to hydrophilic properties (see gradient triangle). The microparticles 6, for example made of glass, applied to the glass substrate 1, between which the oxide nanoparticles 2 are then inserted, achieve a correspondingly larger storage volume. This can be achieved in the same way by forming a surface coating from a stack of layers consisting of a hydrophilic layer 5 and a hydrophobic layer 4, with the microparticles 6 then embedded in the hydrophilic layer 5. In this example, the two nanoporous layers also differ in particle size.Particle density of the nanoparticles 3 and correspondingly in their refractive index, as indicated in Figure 1F.

[0038] The functional properties of the layers as water transport or water storage layers can be extended for specific applications. For example, in one embodiment, the substrate 1 can be heated to minimize water saturation in the nanoporous layer 2 and restore the layer's proper function. This can be done cyclically or as needed. In another advantageous embodiment, photocatalytically active nanoparticles made of materials such as TiO2 or ZnO are used in the substrate-adjacent regions of the nanoporous layers. These can enhance the hydrophilic character of the substrate-adjacent layers, particularly when exposed to UV light. In yet another advantageous embodiment, the substrate can also be functionalized or plasma-cleaned before the coating is applied to ensure the hydrophilic character in this region.

[0039] The layer thicknesses of λ / 4 shown in Figure 1 can of course be replaced by a larger layer thickness d (>λ / 4) without the anti-reflective effect, if this is necessary to achieve a higher water storage capacity.

[0040] The functionality and operating principle of the proposed surface coating are illustrated again in Figure 2. Part 2A of the figure shows a glass substrate 1 with a nanoporous layer 2 made of nanoparticles 3, which exhibits a gradual transition from hydrophobic to hydrophilic properties from the surface to the substrate. This gradual transition is indicated by the gradient triangle in the figure. A microscopic water droplet 8, which strikes the surface of the coating, moves from the surface to the glass substrate, driven by gravity and capillary forces assisted by the hydrophobicity gradient. This is schematically illustrated by arrows in Figure 2A. Figure 2B shows the initial state with the microscopic water droplets 6 on the nanoporous layer 2, which do not wet the surface of the nanoporous layer due to its hydrophobic properties.After a short time, these water droplets 6 are transported into the interior of the layer and stored there, as indicated by the stored water 8 in Figure 2C.

[0041] This prevents cementation processes on the surface, which can lead to stubborn soiling. The water stored in the water storage layer is removed again through evaporation when exposed to sufficient heat. This can be achieved solely through solar radiation, particularly in photovoltaic or solar thermal applications. It is also possible to dry out the water storage layer again at regular or irregular intervals through targeted, repeated heating. Reference symbol list

[0042] 1 Glass substrate 2 Nanoporous layer 3 Nanoparticles 4 Upper hydrophobic layer 5 Lower hydrophilic layer 6 Microparticles 7 Microscopic water droplet 8 Trapped water

Claims

1. A surface coating with reduced susceptibility to contamination, in particular surface coating of a photovoltaic module, which is formed by a nanoporous layer or layer sequence (2) which has a hydrophobic surface, characterized in that the nanoporous layer or layer sequence (2) is designed in its interior hydrophilic as a water storage layer with a water storage capacity of at least 60*10-6 l / m2.

2. The surface coating according to claim 1, characterized in that the nanoporous layer sequence (2) is formed from at least two superimposed layers of which an upper layer (4) is hydrophobic and a lower layer (5) is hydrophilic.

3. The surface coating according to claim 1, characterized in that the nanoporous layer (2) has a gradual progression of the hydrophobic and hydrophilic properties starting from the surface towards the interior of the layer.

4. The surface coating according to any one of claims 1 to 3, characterized in that the nanoporous layer or layer sequence (2) is formed from an optically transparent material, particularly in the wavelength range between 300 and 1200 nm.

5. The surface coating according to claim 4, characterized in that the nanoporous layer or layer sequence (2) is formed as an anti-reflection coating for optical radiation.

6. The surface coating according to any one of claims 1 to 4, characterized in that the nanoporous layer or layer sequence (2) has a hydrophilic region having a thickness of at least 120 nm.

7. The surface coating according to any one of claims 1 to 6, characterized in that the nanoporous layer or layer sequence (2) has a porosity between 30 and 70 %.

8. The surface coating according to any one of claims 1 to 7, characterized in that the nanoporous layer or layer sequence (2) is a sol-gel layer or layer sequence.

9. The surface coating according to any one of claims 1 to 8, characterized in that the nanoporous layer or layer sequence (2) is formed from nanoparticles (3).

10. The surface coating according to any one of claims 1 to 9, characterized in that microparticles (6) are embedded in the interior of the nanoporous layer or layer sequence (2).

11. The surface coating according to any one of claims 1 to 10, characterized in that nano- and / or microparticles which can absorb water are embedded in the nanoporous layer or layer sequence (2).

12. The surface coating according to any one of claims 1 to 11, characterized in that microparticles with an encapsulated phase change material which function to prevent dropping below a dew point temperature on the surface of the nanoporous layer or layer sequence (2) by releasing heat are embedded in the nanoporous layer or layer sequence (2).

13. The surface coating according to any one of claims 1 to 11, characterized in that particles or particles with coatings which have a low IR emissivity are embedded in the nanoporous layer or layer sequence (2).

14. A method for reducing a susceptibility to contamination of an object surface, in particular the surface of photovoltaic modules, in which a surface coating according to one or more of the preceding claims is applied to the object surface.

15. The method according to claim 14, characterized in that the surface coating is heated repeatedly to remove water stored in the interior of the coating.