KA/LO/sv 100068WO 18. September 2012 Method and device for producing a highly selectively absorbing coating on a solar absorber component and solar absorber having such a coating The present invention relates to a method for producing a 5 selectively absorbing coating on a solar absorber component and to a solar absorber component produced according to this method. With increasingly scarcer supplies of fossil fuels an 10 increasingly greater importance is being attached to the use of solar energy when industrial nations are securing their energy supplies. The energy from solar radiation can be directly converted 15 into electrical energy by means of photovoltaics. A further use of solar energy entails operating conventional solar thermal flat plate collectors for water heating and solar thermal power plants (Concentrating Solar Power, CSP). In these power plants, focussing reflector surfaces concentrate 20 the incident sunlight on absorber surfaces which are then in contact with a heat transfer medium, for example thermal oil or superheated steam, and heat it. The steam which is heated up further in the absorber or the steam produced in a further heat exchanger fed by the heated thermal oil thereafter, in a 25 way which is known, drives a turbine and a generator connected to the turbine to generate electricity. Heat stores enable electricity to be generated virtually independent of the particular time of the day. KA/LO/sv 100068WO 18. September 2012 - 2 So-called parabolic trough power plants form a special type of solar thermal power plants. These comprise a plurality of parabolic trough collectors, which in turn consist of troughs up to 400 metres long consisting of mirror segments formed 5 parabolically in cross-section, in the caustic curve of which vacuum-insulated absorber tubes, so-called receivers, are arranged which as a result of focussing the solar radiation are exposed to up to 80 times the radiation intensity. So that an optimum position relative to the sun can always be 10 occupied, the troughs can be repositioned according to the diurnal variations of the sun. In addition to the optical precision of the mirrors, the receivers, which convert the solar radiation into heat and 15 which in each case are about four metres long and are insulated vacuum-tight by a glass envelope, play a key role with regard to the efficiency of parabolic trough power plants. The receivers comprise a casing tube consisting of a coated, highly transparent and robust borosilicate glass and 20 a steel absorber tube which is enclosed by the casing tube and may absorb as much solar radiation as possible and emit little in the way of heat radiation. Due to the different coefficients of thermal expansion of steel and the casing tube, the casing tube has to be held by steel bellows. A 25 stable highly selective coating of the absorber tube surface is crucial for maximum solar radiation absorption and minimal emission. This must be able to absorb radiation in the wavelength range of 0.3 to 2.5 micrometres (dependent on the operating temperature of the absorber surface; in the case of 30 flat plate collectors with T=100'C up to 2.5 pm and in the case of CSP plants with T=250-400 0 C up to 1.2 pm), in which the substantial part of the energy of the solar radiation is KA/LO/sv 100068Wo 18. September 2012 - 3 contained. The unusable heat radiation in the wavelength range of 4.0 to 50 pm which is emitted again should, in contrast, be kept as low as possible. In addition, the thermal emissivity c should be thermally stable. 5 Manufacturing methods are known from practice for absorber components as components for example of solar flat plate collectors in various designs. Thus, a method for producing a solar energy absorber in the form of a plate-like element 10 consisting of aluminium is known from DE 28 50 134 Al, in which on one side of the plate-like element a fine-pored aluminium layer is produced by anodic oxidation, which in a second method step is pigmented into the pores by electrolytic deposition of a metal, for example nickel, 15 cobalt, copper, iron, tin, silver or zinc. By anodising the aluminium surface with subsequent deposition of metal pigments in the pores a high absorption is obtained in the wavelength range of 0.3 to 2.5 micrometres with comparatively low heat emission. In addition, the solar energy absorber is 20 effectively protected against corrosion. However, it has proved to be a disadvantage that the layer thickness which is to be set precisely for absorption in the visible spectral range can only be obtained under precisely 25 constantly maintained ambient conditions, namely constant temperature, constant pressure and constant electrolyte concentration. Temperature fluctuations, which can always occur when production is carried out under industrial conditions, lead to visible fluctuations in the layer 30 thickness, even with identical substrate material, and hence lead to different absorption behaviour and to a different optical colour impression. KA/LO/sv 100068WO 18. September 2012 - 4 In addition to the previously mentioned electrolytic production of absorbing layers on solar absorber components, such layers can also be produced in a vacuum using known coating methods, such as for example physical vapour 5 deposition (PVD). However, such methods require a very high technical effort and outlay, which with absorber tubes having a length of several metres or with plate products in general no longer permits economic manufacture on an industrial scale. 10 Taking this as the starting point, the object of the invention is to specify a method for producing a selectively absorbing coating on a solar absorber component, by means of which it is possible to produce highly selectively absorbing 15 layers on metallic surfaces of different geometry, also in particular on tubular components, on plate products, as well as on coil products, on a commercial scale with high reproducibility and maximum absorption capacity and hence optimum usability. The investment costs associated with 20 implementing the method should be low. The object is achieved according to the invention with a method for producing a selectively absorbing coating on a solar absorber component, which comprises the following 25 method steps: - Providing a substrate having a metallic surface, - Determining the inner surface of the metallic surface, - Determining the charge quantity per unit area 30 required for producing the absorbing coating according to the inner surface, KA/LO/sv 100068WO 18. September 2012 -5 - Electrolytically producing the absorbing layer in a first step by direct-current anodising the metallic surface of the substrate, forming a porous oxide layer, and in a second step by alternating-current 5 pigmenting the pores of the oxide layer, wherein direct-current anodising and alternating-current pigmenting are carried out until the charge quantity per unit area determined for the respective step from the inner surface is reached, 10 wherein the ratio between the charge quantity per unit area PA for the direct-current anodising and the charge quantity per unit area pp for the alternating-current pigmenting PA/PP is = 0.65 to 0.8. 15 The particular advantage of the method according to the invention is that the selectively absorbing layers can be produced with high accuracy and the highest degree of reproducibility on the metallic substrate surface. By determining the charge quantity per unit area required for 20 producing the absorbing coating according to the previously determined inner surface of the metallic substrate surface, it is ensured that fluctuations in the ambient conditions which are not to be fully suppressed, such as e.g. air temperature and pressure as well as the temperature of the 25 electrolyte and the ion concentration therein, do not have an adverse effect on the coating outcome. Thus, investigations by the applicant in preparation for the invention showed that the production of the selectively absorbing layer on completely identical substrate surfaces at different times of 30 the day, i.e. at slightly differing temperatures, already lead to visible differences in the coating and hence lead to KA/LO/sv 100068WO 18. September 2012 -6 different layer thicknesses and correspondingly to non uniform absorption behaviour. By observing the Faraday Law, according to which the charge 5 quantity and electrolytic conversion of material and hence layer production are strictly proportional in relation to one another, a uniform and reproducible layer growth is ensured which is independent of external ambient parameters. Here, the required charge quantity is determined according to the 10 inner surface, which according to the microscopic composition of the substrate can differ from its macroscopically determinable surface. According to the invention, the absorbing layer is 15 electrolytically produced in a first step by direct-current anodising the metallic surface of the substrate, forming a porous oxide layer. Such anodising methods ("eloxadizing") have been used on an industrial scale for years, in particular with aluminium surfaces, and are hence suitable 20 for commercially producing porous oxidation layers without any problems. In the case of aluminium surfaces, by anodising them an A1 2 0 3 layer is, for example, produced. In the case of copper, this is a copper oxide layer. In a second step, according to the invention, alternating-current pigmenting of 25 the pores of the oxide layer is carried out, wherein direct current anodising and alternating-current pigmenting are carried out until the charge quantity per unit area determined for the respective step from the inner surface is reached and subsequently are discontinued. By means of this 30 two-stage electrolytic process, a reproducible absorber layer of high quality is produced on the substrate surface as a so called "cermet" layer (ceramic metal) having excellent KA/LO/sv 100068WO 18. September 2012 absorption properties with comparatively little technical effort and outlay. The alternating-current voltage forming the basis of the alternating current can in particular flow sinusoidally, rectangularly or asymmetrically over time. It 5 is also possible to apply a direct-current component to the alternating current. The frequency is also not fixed. In particular, the alternating-current pigmenting can therefore also be carried out with the power frequency of 50 Hz. Finally, the ratio from anodising charge density and 10 pigmenting charge density defines the solar absorption coefficient a. As the inventors have surprisingly discovered, absorption coefficients (x > 90%, which provide optimum usability of the solar absorber components and hence maximum process efficiency when converting solar radiation energy 15 into heat, are obtained with ratios of PA/PP = 0.65 to 0.8. According to a first embodiment of the method according to the invention, the substrate can be a metallic component, in particular a plate-like or tubular component. Equally, the 20 substrate can be formed as an aluminium cushion absorber produced in the roll-bond process. Furthermore, it is possible to produce coil products in the roll-to-roll process. As a matter of fact, non-metallic substrate materials, for example plastics, which have a metallic 25 surface can also be used. Established processes are e.g. electroplated plastic coating or plasma coating. Glass substrates can also be highly selectively coated using the method according to the invention. In tests carried out by the applicant, glass surfaces coated with a transparent, 30 conductive oxide layer (TCO), here in particular indium tin oxide (ITO), fluorine tin oxide (FTO), aluminium zinc oxide KA/LO/sv 100068Wo 18. September 2012 (AZO) and antimony tin oxide (ATO) proved to be particularly suitable. In addition, the substrate material can also be foil-like and 5 in particular can be formed as an aluminium foil. In order to ensure that the selectively absorbing coating adheres securely to the substrate base, provision is made, according to a further embodiment of the method according to 10 the invention, for the substrate to be provided with an adhesion-promoting layer. Different adhesion-promoting layers can be applied according to the composition of the substrate material. Preferably, an aluminium or copper layer is used as the adhesion-promoting layer, since these elements have a 15 high degree of reflection in the infrared range and excellent thermal conductivity (thermal conductivity Xcu = 230-400 W/m*K; XA1 = 230-400 W/m*K). If the substrate material is, for example, a cylindrical 20 component, in particular a steel tube, then the adhesion promoting layer can also be applied onto the steel tube by pulling over an aluminium tube. For this purpose, a thin walled aluminium tube is used, the inner diameter of which is slightly less than the outer diameter of the steel tube. 25 When heated it can then be pulled over the steel tube and after cooling forms a very firm bond with the steel tube. Instead of an aluminium tube, a copper tube can also be used to pull over the steel tube. The copper tube has the advantage that in later use temperatures > 520 0 C can be 30 generated in the CSP receiver. Of course, other methods, in particular vacuum methods, can also be used for depositing an adhesion-promoting layer on KA/LO/sv 100068WO 18. September 2012 -9 the substrate material. Equally, components consisting of aluminium or copper solid material can also be used. According to the teaching of the invention, the charge 5 quantity required for the electrolytic production of an absorbing layer on the solar absorber component is determined according to the inner surface of the substrate. The inner surface, i.e. the microscopic surface of the substrate, can, for example, be determined by mechanical scanning on a 10 microscopic scale, preferably by atomic force microscopy. For this purpose, a representative surface section of, for example, 1Ox10 pm 2 or 50x50 pm 2 is scanned. According to the invention, the absorbing layer is 15 electrolytically produced in a first step by direct-current anodising the substrate surface, wherein direct-current anodising is carried out until the charge quantity per unit area determined from the inner surface is reached and subsequently is correspondingly discontinued. The surface 20 area charge density during anodisation PA is directly proportional to the thermal emissivity e. Empirically, the applicant has found the following relation: 6 [%] = 5 * PA [C/cm 2 ] + CSubstrate[%], wherein stands for the emissivity of the anodised surface and Esubstrate stands for the emissivity of 25 the substrate. Aluminium or copper qualify as substrate materials. Their emissivities can be taken as material constants from the literature (A1iiiuml = 2.3% und ccopper 2.9%). 30 In a second step, according to the invention, alternating current pigmenting of the pores of the oxide layer is carried out, wherein direct-current anodising and alternating-current KA/LO/sv 100068WO 18. September 2012 -10 pigmenting are carried out until the charge quantity per unit area determined for the respective step from the inner surface is reached and subsequently are discontinued. Various metals can be used for pigmenting the anodised layer. 5 Preferably, alternating-current pigmenting is carried out using a metal from the group consisting of Ni, C, Al, Mg, Ca, Ti, V, Cr, Mn, Fe, Co, Cu, Zn, Ag und Sn. Different strategies can be employed to electrolytically 10 produce the absorbing layer according to the geometry of the solar absorber component to be coated. Roll-bond absorbers with a non-plane surface (due to the inflated channels) can be coated just the same as plate products or foils. According to a particularly advantageous embodiment of the method, the 15 solar absorber component is cylindrically formed, wherein the absorbing layer is electrolytically produced with the solar absorber component positioned standing in a container filled with an electrolyte, wherein the cylindrical solar absorber component is arranged coaxially to a surrounding cylindrical 20 counter electrode. Alternatively, a cylindrically formed solar absorber component can be arranged lying in a container filled with an electrolyte, wherein the cylindrical solar absorber component 25 is arranged essentially coaxially to a gutter-shaped counter electrode with an essentially cylindrical inner surface. Here, the cylindrical solar absorber component with respect to its circumference can be fully or partly immersed in the electrolyte. In the case where it is only partly immersed in 30 the electrolyte, the absorbing layer is only, therefore, electrolytically produced on the circumferential section of the cylindrical solar absorber component immersed in the KA/LO/sv 100068WO 18. September 2012 -11 electrolyte. If the cylindrical solar absorber component is provided as a receiver (or as an absorber tube in a receiver tube) in a parabolic trough collector, then it shall be understood that the coated circumferential section is 5 arranged facing the trough-shaped mirror segment and the non coated section is arranged facing the sun. Coil products, in particular made of aluminium or copper, can be highly selectively coated by means of roll-to-roll 10 processes. To this end, the coil material, according to one configuration, is drawn successively through four dip tanks. In the first dip tank the material is cleaned, in the second a porous oxide layer is formed by means of direct-current anodising, in the third tank the coil material is cleaned of 15 anodising electrolytes and in the last tank alternating current pigmenting is carried out. According to a first alternative, the coil material is connected to the earth potential, while the counter 20 electrodes are connected to a corresponding potential (direct-current voltage for anodising, alternating-current voltage for pigmenting). According to a second alternative, the anodising direct-current voltage is applied to the coil material, while pigmenting is carried out with a total 25 potential of pigmenting alternating-current voltage and anodising voltage. Tests by the applicant proved that both were achievable without any problems. According to a further embodiment of the invention, a 30 transparent anti-reflection layer can finally be applied onto the electrolytically produced absorbing layer. This can, for example, be formed from a material of the group consisting of KA/LO/sv 100068WO 18. September 2012 -12 A1 2 0 3 , SiO 2 , SiO 2 /SnO 2 , TiO 2 , 3-mercaptopropyltrimethoxysilane (MPTMS), cerium oxide, sodium silicate, or pyrolytic SnO 2 or F:SnO 2 (FTO or fluorine-doped tin oxide) . This layer serves as a transparent, thin layer to stem losses by reflection and 5 additionally to provide protection against atmospheric moisture and atmospheric pollution. Degradation of the layer is also thereby prevented by protecting the embedded metal particles from oxidation or "hydroxidation". Furthermore, the surface roughness is reduced, which makes cleaning the 10 surfaces easier. A further aspect of the present invention relates to a solar absorber component produced according to a method according to any one of Claims 1-18. 15 Reference is made to the foregoing with regard to the advantages of this solar absorber component. The invention is explained in more detail below with the aid 20 of the figures illustrating an exemplary embodiment. Fig. 1 shows an absorber tube provided with a selectively absorbing coating for a parabolic trough collector in cross-section, 25 Fig. 2 shows the highly selectively absorbing coating of the absorber tube from Fig. 1 in a schematised detailed view, Figs. 3 a-c show a pore of the selectively absorbing coating 30 in the unpigmented, pigmented and overpigmented state, KA/LO/sv 100068Wo 18. September 2012 -13 Fig. 4 shows the reflectivity of the selectively absorbing coating in per cent, Fig. 5 shows the current characteristic curve of 5 direct-current anodising for producing the selectively absorbing coating under charge control, Fig. 6 shows the current characteristic curve of 10 alternating-current pigmenting of the coating produced by anodising under charge control, Fig. 7 shows a device for electrolytically coating an absorber tube in a first embodiment, 15 Fig. 8 shows a device for electrolytically coating an absorber tube in a second embodiment, Fig. 9 shows a device for electrolytically coating an 20 absorber tube in a third embodiment and Fig. 10 shows a device for applying a selectively absorbing coating on a coil product by means of a roll-to-roll process. 25 In Figure 1, an absorber tube 10 provided with a selectively absorbing coating for a parabolic trough collector is shown in a cross-sectional view. Here, the respective thickness of the individual layers relative to the thickness of the 30 substrate material is not represented to scale for the sake of clarity. KA/LO/sv 100068WO 18. September 2012 -14 The absorber tube 10 in Fig. 1 comprises a steel tube 1 which is provided with an adhesion-promoting layer 2 on its outer surface. This is preferably an aluminium or copper layer which preferably is electrolytically deposited on the steel 5 surface. Since the adhesion-promoting layer has to be completely non-porous, here a minimum thickness of the electrolytically deposited layer of 8 - 10 pm is preferred. If the steel tube 1 is a steel tube of lower surface quality 10 with a porous surface, then it makes sense to firstly electrolytically close the pores on the surface by electrolytic deposition of a laterally growing metal layer (e.g. nickel) and subsequently electrolytically deposit the aluminium adhesion-promoting layer. Alternatively, a thin 15 aluminium tube (e.g. AlMg 3 ) or a copper tube can be pulled over the steel tube. Here, the aluminium or the copper tube, the outer diameter of which is slightly less than the outer diameter of the steel tube, is heated and pulled over the steel tube. After cooling, a very solid material bond forms. 20 In addition to electrolytically depositing an adhesion promoting layer, it is also possible to apply this layer in a vacuum process, for example PVD. Here, care has to be taken that the adhesion-promoting layer is absolutely free of 25 pores. Furthermore, it goes without saying, of course, that instead of a steel tube, aluminium or copper tubes can also be used. The advantage of copper tubes is that with them surface 30 temperatures of > 520'C can be achieved. KA/LO/sv 100068WO 18. September 2012 -15 An embodiment of a solar absorber component, in which the substrate is plate-like, is not illustrated. A foil-like substrate, in particular in the form of an aluminium or copper foil, is also not illustrated. This foil can have a 5 typical thickness of 0.05 and 0.2 mm. As tests by the applicant have shown, even household aluminium foils can be selectively coated using the method according to the invention. 10 The selectively absorbing coating 3 is applied onto the adhesion-promoting layer 2. Due to the selectivity of the absorption properties of the present coating 3, the main portions of the solar radiation are heavily absorbed in the wavelength range of 0.3 - 2.5 pm, while long wave portions of 15 the solar radiation are reflected. The selectively absorbing coating 3 is a pigmented anodised layer which is produced on the adhesion-promoting layer 2 in a two-stage process. The microscopic structure of the layer is explained further below in connection with Figures 2 and 3. 20 The outermost layer of the layer structure of the absorber tube from Fig. 1 is formed by an anti-reflection layer 4 which as a transparent, thin layer is to minimise losses by reflection and, at the same time, provides protection against 25 atmospheric moisture and atmospheric pollution and prevents degradation of the highly selective coating. A1 2 0 3 , SiO 2 /SnO 2 , TiO 2 , 3-mercaptopropyltrimethoxysilane (MPTMS), cerium oxide, sodium silicate or pyrolytic SnO 2 or F:SnO 2 (FTO or fluorine doped tin oxide) can be used as materials for the anti 30 reflection layer. Particularly preferably, an SiO 2 layer is used as the anti-reflection layer, since the refraction index of quartz glass (nsio2 ~ 1.55) is better adapted to the KA/LO/sv 100068WO 18. September 2012 -16 refraction index of the air surrounding the absorber tube in use (nAir 1) than is the case, for example, with A1 2 0 3 (nA1203 2 1.76), so that with an SiO 2 anti-reflection layer comparatively low losses by reflection occur. 5 In Fig. 2, now the anodised and pigmented selectively absorbing coating of the absorber tube of Fig. 1 is illustrated in sections in schematised form. Again, for the sake of clarity, this is not shown to scale. During direct 10 current anodising of the underlying aluminium adhesion promoting layer 2, initially a thin A1 2 0 3 barrier layer 23 forms which functions as an anti-diffusion layer between the adhesion-promoting layer 2 and the highly selectively absorbing layer 3. Here, the thickness of the barrier layer 15 23 is about 25 nm. With continued direct-current anodising, a porous layer 3a, having a thickness of approx. 300 - 500 nm, forms above the barrier layer 23. In the present exemplary embodiment, the diameter of the pores 3b can be indicated as 50 to 80 nm. This layer structure can be precisely reproduced 20 independent from the varying ambient conditions by keeping the charge quantity constant during direct-current anodising. After discontinuing anodisation after the predetermined charge quantity per unit area has been reached, the pores of 25 the oxide layer are pigmented in an alternating-current pigmenting step, wherein again the charge quantity determined according to the inner surface of the substrate is kept constant. The metals Ni, C, Al, Mg, Ca, Ti, V, Cr, Mn, Fe, Co, Cu, Zn, Ag und Sn are suitable for pigmenting. As will be 30 explained later in detail in connection with Fig. 6, it is important not to pigment the pores 3b of the porous oxide layer 3a in such a way that the metal pigment 3c escapes from KA/LO/sv 100068Wo 18. September 2012 -17 the pores 3b and forms a closed metal film in the monolayer area above the porous oxide layer. In Fig. 4, the reflectivity of the selectively absorbing 5 coating according to Fig. 2 is plotted according to the wavelength in nanometres. What are particularly noticeable are a very low reflectivity of the coating of < 5% below a wavelength of approx. 800 nm and a reflectivity in the region of constantly 90 % from a wavelength of 7,000 nm. 10 The complete operation cycle of the production of a selectively absorbing layer on a solar absorber tube is described below by means of a specific exemplary embodiment. 15 Initially, an aluminium adhesion-promoting layer is deposited on a cylindrical steel substrate. After cleaning with isopropanol, the aluminium adhesion-promoting layer is etched or pickled in sodium hydroxide solution or conventional aluminium pickling is carried out. By that means, on the one 20 hand, surface impurities are removed and, on the other hand, the barrier oxide and the natural oxide on the aluminium surface are removed. It is important that after pickling or etching only aluminium, silicon or magnesium are present on the surface, since otherwise the anodising and the pigmenting 25 and hence the coating results will not be perfect. By means of a subsequent chemical or electrolytic bright plating bath, the emissivity of the Al substrate can be lowered still further, which in turn results in a lower total 30 emissivity (E [%) = 5 * PA [C/cm 2 ) + &A1 [%]) of the finished coating. KA/LO/sv 100068Wo 18. September 2012 -18 In the following anodising process, an oxide of the same thickness can be formed in all places, into the pores of which the metal particles can be incorporated. In this way, an outstanding coating quality can be guaranteed. 5 In a subsequent step, the inner surface is determined by means of atomic force microscopy (AFM). The inner surface of the aluminium adhesion-promoting layer e.g. is, with an average surface roughness of Ra = 56 nm, typically approx. 10 120 pm 2 with a 100 pm 2 measuring field. With the aid of the empirically found relation between surface area charge density during anodisation PA and the thermal emissivity 6 (PA [C/cm 2 ] = (s [%] - A1 [%])/5), from this the required charge quantity per unit area in C/cm 2 for direct-current 15 anodising is calculated. The charge quantity for the pigmenting pp is determined from PA/PP = 0.65 to 0.8. In this connection, a charge quantity per unit area of, for example, 0.8 C/cm 2 for e = 6.3 % und CA = 2.3 % results for anodising the adhesion-promoting layer. 20 Then, direct-current anodising of the aluminium adhesion promoting layer is carried out. In the present exemplary embodiment, this is carried out with a direct-current voltage of 15 volts in phosphoric acid (H 3
PO
4 - 9 % vol), wherein an 25 A1 2 0 3 layer is produced on the aluminium surface. The process is stopped after the calculated charge quantity per unit area has been reached. As a result of controlling the charge quantity, fluctuations in the temperature of the electrolyte and fluctuations in the electrolyte concentration, which lead 30 to considerable fluctuations in current, can be compensated. With a purely time-dependent layer growth, such fluctuations would result in coating results which are not reproducible. KA/LO/sv 100068WO 18. September 2012 -19 With pure current control, the applied voltage would be varied with a change in temperature, which in turn would lead to a different pore distribution in the surface. The pore sizes and intervals are crucially dependent on the applied 5 anodising voltage. The A1 2 0 3 layer produced in the course of direct-current anodising corresponds to the one shown in Fig. 2 in schematic form. The pores 3b, illustrated in Fig. 2 in a lateral sectional view, in the plan view form a hexagonal lattice. 10 The thickness of the A1 2 0 3 barrier layer forming in the course of direct-current anodising can be determined using the equation D = c * U, wherein a = 1.2 to 1.4 nm/V and U = 15 volts is the direct-current anodising voltage. The 15 thickness of the barrier layer is according to this approx. 18 to 21 nm. The distance between the pores of the selectively absorbing layer can be calculated using the formula D = P*U, wherein B 20 = 2.5 nm/V and U = 15 volts again corresponds to the anodising voltage. According to the above formula, the distance between the pores is therefore approx. 37.5 nm. In Fig. 5, the current characteristic curve for direct 25 current anodising of the aluminium adhesion-promoting layer is illustrated. Here, the anodising current is plotted against time. As can be identified in Fig. 5, the current initially drops exponentially from a value of approx. 315 mA to a value of approx. 50 mA. The barrier layer is formed in 30 this phase. Following this, the current slowly increases again to a value of approx. 60 mA up to a processing time of approx. 430 s. In this phase, field magnifications occur on KA/LO/sv 100068Wo 18. September 2012 -20 the irregularly thick oxide layers in the places with a thin barrier oxide. The depressions in the barrier layer form and serve as seeds for pore growth. In the third phase, beginning at a processing time of approx. 430 s until the process is 5 discontinued after the calculated charge quantity per unit area has been reached, with the previously mentioned direct current voltage value, the electrolyte composition and concentration a stable growth of pores with a diameter of approx. 50 to 80 nm occurs. At a processing time of approx. 10 2000 s, the calculated charge quantity per unit area is reached, so that anodisation is stopped, as is illustrated in Fig. 5 by the abrupt drop in the anodising current. Alternatively, the A1 2 0 3 layer or the copper oxide layer can 15 also be produced in the acids (chromic acid, sulphuric acid) known from anodic oxidation technology, wherein the pore sizes and distributions deviate from the present exemplary embodiment. 20 After direct-current anodising of the aluminium adhesion promoting layer has been carried out, it is pigmented in an alternating-current pigmenting step. In the present exemplary embodiment, this is carried out using a nickel counter electrode and a pigmenting solution composed of the 25 following: Nickel sulphate hexahydrate 30 g/l Magnesium sulphate 20 g/l Ammonium sulphate 20 g/l 30 Boric acid 20 g/l. KA/LO/sv 100068WO 18. September 2012 -21 Several series of experiments were carried out with alternating-current voltages from 5 to 12 V, in particular 7.5 V, wherein the alternating-current densities were between 6.5 mA/cm 2 und 22.5 mA/cm 2 . Alternating-current pigmenting was 5 again carried out until the charge quantity per unit area calculated from the inner surface of the adhesion-promoting layer was reached and then automatically terminated. For the relationship between the charge quantity per unit area PA for direct-current anodising and the charge quantity per unit 10 area pp for alternating-current pigmenting a value PA/PP = 0.65 to 0.8 was chosen. In Fig. 6, the current characteristic curve of alternating current pigmenting is plotted as the course of the current 15 over time. As can be identified in Fig. 6, the current drops as the process time progresses and at approx. 60 s crosses a local minimum ("delta peak"). Then, the current increases again. As tests by the applicant have shown, in the phase where the current flow is fading away the pores of the 20 anodised layer are filled with the pigment in a regular fashion. A regularly filled pore is illustrated in Fig. 3b. As soon as the delta peak has been crossed, overpigmenting of the pores occurs in such a way that the pigment escapes from the completely filled pore and begins to form a closed metal 25 film in the monolayer area above the porous anodised layer. This is schematically shown in Fig. 3c. In order to avoid overpigmenting, which would be accompanied by a drop in absorption in the visual spectral range, the alternating current pigmenting is always, therefore, conducted in the 30 monotonically decreasing area of the current characteristic curve according to Fig. 6. Pigmenting is correspondingly KA/LO/sv 100068WO 18. September 2012 -22 discontinued when the calculated charge quantity per unit area has been reached before the delta peak has been reached. As the tests by the applicant on the whole show, 5 comparatively low current densities in the range from 4 - 6 mA/cm 2 are sufficient for both direct-current anodising and alternating-current pigmenting. Surface area charge densities of less than 1.2 C/cm 2 during anodisation and of 0.8-0.95 C/cm 2 during pigmenting result therefrom. 10 After the anodised coating has been pigmented, an anti reflection layer has still to be applied which is also given a protective function against external influences. Preferably, an SiO 2 layer is applied by means of dip coating. 15 The application of the anti-reflection layer by dip coating on the selectively absorbing layer of the absorber tube is carried out, in the present exemplary embodiment, in a tetraethyl orthosilicate solution (TEOS) with a concentration of 105 g/l (solvent isopropyl alcohol) at a dip speed of 20 approx. 0.5 mm/s. Then, the coating is tempered at 300-320 0 C. If the anti-reflection layer is to be formed by an A1 2 0 3 layer, a TiO 2 layer, a 3-mercaptopropyltrimethoxysilane (MPTMS) layer, a cerium oxide layer, a sodium silicate layer 25 or an SiO 2 /SnO 2 layer, then here this can also be applied by dip coating. In Figs. 7 und 8, two different embodiments of a device for producing a selectively absorbing coating on the surface of 30 an absorber tube 10 are illustrated. With the device according to Fig. 7, the cylindrical absorber tube 10 is positioned standing in a dip tank 30 filled with an KA/LO/sv 100068WO 18. September 2012 -23 electrolyte 20 and coaxially to a surrounding counter electrode 40, so that between the absorber tube 10 and the counter electrode 40 when the voltage source 50 is activated (this can be a direct-current voltage source or an 5 alternating-current voltage source depending on the method step carried out) an E-field which is constant over the entire circumference of the absorber tube 10 forms. The advantage of this coating device is that it coats the absorber tube 10 very uniformly over its entire 10 circumference. However, in operation this device requires corresponding hoisting gear for handling the absorber tubes which are often several metres long. In Figs. 8 und 9, two alternative embodiments of a coating 15 device to Fig. 7 are illustrated. In the coating device according to Fig. 8, the surface of the absorber tube 10 is coated in the lying position in an elongated, comparatively flat tank 30'. Here, the absorber tube is surrounded by a gutter-shaped counter electrode 40' having an essentially 20 cylindrical inner surface. As illustrated, the absorber 10 is only partly immersed in the electrolyte 20, so that coating with the selectively absorbing layer with respect to the circumference only occurs on a part of the surface of the absorber tube 10. By choosing the immersion depth, the coated 25 circumference section can be correspondingly set. Of course, the coated circumference section, when the absorber tube 10 is used later in a parabolic trough collector, must be facing the mirror segments, while the uncoated or non-uniformly coated circumference section has to be facing the sun. The 30 advantage of this device is that the absorber tubes can be fed into and taken out of the tank 30' easily. KA/LO/sv 100068WO 18. September 2012 -24 In the coating device in Fig. 9, the surface of the absorber tube 10 is coated in the lying position again in an elongated, flat tank 30''. Here, in contrast to the device in Fig. 8, the absorber tube is surrounded by a cylindrical 5 counter electrode 40'' which is open on the upper side in the longitudinal direction. Furthermore, in contrast to the device in Fig. 8, the absorber tube 10 is fully immersed in the electrolyte 20. As with the device in Fig. 7, an all-over coating is correspondingly carried out with the selectively 10 absorbing layer here. In Fig. 10, the coating of an aluminium or copper surface of a coil product by means of a roll-to-roll process on a corresponding coil coating line 100 is illustrated. To this 15 end, the aluminium or copper strip C wound up into a coil steel strips with a corresponding surface coating can also be used - is drawn successively through six dip tanks 110-160. In the first tank 110, the strip material C is etched and in the following rinsing tank 120 it is subsequently rinsed. In 20 the third tank 130, a porous oxide layer is formed on the strip surface by means of direct-current anodising with preferably UAnod. = approx. 15 VDc and in the subsequent rinsing tank 140 cleaned of the anodising electrolyte. In the fifth tank 150, the alternating-current pigmenting is then 25 carried out with preferably Upi,. = approx. 7.5 VAc and in the rinsing tank 160 arranged behind it the cleaning by the pigmenting electrolyte. With the coating method carried out with the device in Fig. 10, the aluminium or copper strip C, i.e. the coil product, is, as illustrated, connected to the 30 earth potential, while the counter electrodes 131, 151 are connected to a correspondingly inverse potential. KA/LO/sv 100068Wo 18. September 2012 -25 With the coating device in Fig. 11, the coating of the aluminium or copper surface of the strip material C* is also carried out by means of a roll-to-roll process. To this end, the coil material C* is drawn successively through six dip 5 tanks 210-260. In the first tank 210, the material is etched and in the rinsing tank 220 it is subsequently rinsed. In the third tank 230, a porous oxide layer is formed on the strip surface by means of direct-current anodising and is subsequently cleaned in the subsequent fourth tank 240 by the 10 anodising electrolyte. In the fifth tank 250, the alternating-current pigmenting is carried out. In the subsequent sixth tank 260 again, the cleaning is carried out by the pigmenting electrolyte. In the process, as illustrated in Fig. 11, the anodising direct-current voltage of 15 preferably UAnod. = approx. 15 VDC is applied to the coil material C*, wherein the pigmenting is carried out with a superimposition of the anodising direct-current voltage with the alternating-current voltage of preferably Ui,. = approx. 7.5 VAC for the pigmenting. 20 KA/LO/sv 100068WO 18. September 2012