Patent Application
20130192588
The present invention relates to a method for producing a 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 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 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 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 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.
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 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 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 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 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 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 flat plate collectors with T=100° C. up to 2.5 μm and in the case of CSP plants with T=250-400° C. up to 1.2 μm), in which the substantial part of the energy of the solar radiation is contained. The unusable heat radiation in the wavelength range of 4.0 to 50 μm which is emitted again should, in contrast, be kept as low as possible. In addition, the thermal emissivity ε should be thermally stable.
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 consisting of aluminium is known from DE 28 50 134 A1, 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, 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 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 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 thickness, even with identical substrate material, and hence lead to different absorption behaviour and to a different optical colour impression.
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 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.
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 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 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 method steps:
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 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 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 the day, i.e. at slightly differing temperatures, already lead to visible differences in the coating and hence lead to different layer thicknesses and correspondingly to non-uniform absorption behaviour.
By observing the Faraday Law, according to which the charge 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 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 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 for commercially producing porous oxidation layers without any problems. In the case of aluminium surfaces, by anodising them an Al2O3 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 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 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 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 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 pigmenting charge density defines the solar absorption coefficient α. As the inventors have surprisingly discovered, absorption coefficients α>90%, which provide optimum usability of the solar absorber components and hence maximum process efficiency when converting solar radiation energy into heat, are obtained with ratios of ρA/ρP=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 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 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, conductive oxide layer (TOO), here in particular indium tin oxide (ITO), fluorine tin oxide (FTO), aluminium zinc oxide (AZO) and antimony tin oxide (ATO) proved to be particularly suitable.
In addition, the substrate material can also be foil-like and 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 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 high degree of reflection in the infrared range and excellent thermal conductivity (thermal conductivity λCu=230-400 W/m*K; λAl=230-400 W/m*K).
If the substrate material is, for example, a cylindrical 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. 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° C. can be generated in the CSP receiver.
Of course, other methods, in particular vacuum methods, can also be used for depositing an adhesion-promoting layer on 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 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 microscopic scale, preferably by atomic force microscopy. For this purpose, a representative surface section of, for example, 10×10 μm2 or 50×50 μm2 is scanned.
According to the invention, the absorbing layer is 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 area charge density during anodisation ρA is directly proportional to the thermal emissivity ε. Empirically, the applicant has found the following relation: ε [%]=5*ρA [C/cm2]+εSubstrate[%], wherein ε stands for the emissivity of the anodised surface and εSubstrate stands for the emissivity of the substrate. Aluminium or copper qualify as substrate materials. Their emissivities can be taken as material constants from the literature (εAluminium=2.3% and εCopper=2.9%).
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 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. 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 and Sn.
Different strategies can be employed to electrolytically 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 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 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 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 the electrolyte, the absorbing layer is only, therefore, electrolytically produced on the circumferential section of the cylindrical solar absorber component immersed in the 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 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 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 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 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 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 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 Al2O3, SiO2, SiO2/SnO2, TiO2, 3-mercaptopropyltrimethoxysilane (MPTMS), cerium oxide, sodium silicate, or pyrolytic SnO2 or F:SnO2 (FTO or fluorine-doped tin oxide). This layer serves as a transparent, thin layer to stem losses by reflection and 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 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.
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 of the figures illustrating an exemplary embodiment.
a-c show a pore of the selectively absorbing coating in the unpigmented, pigmented and overpigmented state,
In
The absorber tube 10 in
If the steel tube 1 is a steel tube of lower surface quality 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 aluminium tube (e.g. AlMg3) 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.
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 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 temperatures of >520° C. can be achieved.
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 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.
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 μm, while long wave portions of 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
The outermost layer of the layer structure of the absorber tube from
In
After discontinuing anodisation after the predetermined charge quantity per unit area has been reached, the pores of 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 and Sn are suitable for pigmenting. As will be explained later in detail in connection with
In
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.
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 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 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 emissivity (ε [%]=5*ρA [C/cm2]+εAl [%]) of the finished coating.
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.
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. 120 μm2 with a 100 μm2 measuring field. With the aid of the empirically found relation between surface area charge density during anodisation ρA and the thermal emissivity ε (ρA [C/cm2]=(ε [%]−εA1 [%])/5), from this the required charge quantity per unit area in C/cm2 for direct-current anodising is calculated. The charge quantity for the pigmenting ρP is determined from ρA/ρP=0.65 to 0.8. In this connection, a charge quantity per unit area of, for example, 0.8 C/cm2 for ε=6.3% and εA1=2.3% results for anodising the adhesion-promoting layer.
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 (H3PO4-9% vol), wherein an Al2O3 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 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.
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 anodising voltage. The Al2O3 layer produced in the course of direct-current anodising corresponds to the one shown in
The thickness of the Al2O3 barrier layer forming in the course of direct-current anodising can be determined using the equation D=α*U, wherein α=1.2 to 1.4 nm/V and U=15 volts is the direct-current anodising voltage. The 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=β*U, wherein β=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
Alternatively, the Al2O3 layer or the copper oxide layer can 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.
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 following:
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/cm2 and 22.5 mA/cm2. Alternating-current pigmenting was 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 ρA for direct-current anodising and the charge quantity per unit area ρP for alternating-current pigmenting a value ρA/ρP=0.65 to 0.8 was chosen.
In
As the tests by the applicant on the whole show, comparatively low current densities in the range from 4-6 mA/cm2 are sufficient for both direct-current anodising and alternating-current pigmenting. Surface area charge densities of less than 1.2 C/cm2 during anodisation and of 0.8-0.95 C/cm2 during pigmenting result therefrom.
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 SiO2 layer is applied by means of dip coating. 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 approx. 0.5 mm/s. Then, the coating is tempered at 300-320° C.
If the anti-reflection layer is to be formed by an Al2O3 layer, a TiO2 layer, a 3-mercaptopropyltrimethoxysilane (MPTMS) layer, a cerium oxide layer, a sodium silicate layer or an SiO2/SnO2 layer, then here this can also be applied by dip coating.
In
In
In the coating device in
In
With the coating device in
| Number | Date | Country | Kind |
|---|---|---|---|
| 102010012573.3 | Mar 2010 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP11/54376 | 3/22/2011 | WO | 00 | 11/20/2012 |
