Abstract:
The invention relates to a solar collector element having an absorber part and a tube for a heat transfer liquid connected thereto on a first side. The absorber part consisting of a composite material having a metallic substrate and an optically active coating on a second side of the substrate. The coating is a multilayer system having three layers. The top layer is a dielectric layer, preferably an oxide, fluoride or nitride layer of chemical composition MeO z , MeF r , MeN s , having a refractive index n&lt;1.8. The middle layer is a chromium oxide layer of chemical composition CrO x . The bottom layer is gold, silver, copper, chromium, aluminium and/or molybdenum. The indices x, z, r and s indicate a stoichiometric or non-stoichiometric ratio in the oxides, fluorides or nitrides.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a solar collector element having an absorber part and having a tube for a heat-transfer liquid. The absorber part is a composite material having a metallic substrate with an optically active coating on a second side of the absorber part opposite the tube.  
           [0002]    It is known to use solar collectors to obtain energy from solar radiation. The solar radiation is converted into heat at an absorber part, which is in the form of a plate, for example, of a solar collector and heats a heat-transfer liquid contained in the collector. For the heat-transfer liquid, there is a circuit system composed of tubes that enables the heat which has been taken up to be released again to a consumer, such as for example to a heat exchanger, in which service water can be heated, or to heat a swimming pool.  
           [0003]    In general, when radiation impinges on an object—as is the case with the coated surface of an absorber part—it is split into a reflected fraction, an absorbed fraction and a transmitted fraction, which are determined by the reflectivity (reflectance), the absorptivity (absorptance) and the transmissivity (transmittance) of the object. Reflectance, absorptance and transmittance are optical properties which, depending on the wavelength of incident radiation (e.g. in the ultraviolet region, in the region of visible light, in the infrared region and in the region of thermal radiation) can adopt different values for the same material. Kirchhoff&#39;s law, according to which the absorptivity, in each case at a defined temperature and wavelength, has a constant ratio to the emittance, is known to apply to the absorptance. Therefore, Wien&#39;s displacement law and Planck&#39;s radiation law as well as the Stefan-Boltzmann law are of importance for the absorptance, describing defined relationships between radiation intensity, spectral distribution density, wavelength and temperature of a black body. Calculations should take account of the fact that the black body per se does not exist, and real substances each deviate in a characteristic way from the ideal distribution. To ensure highly effective utilization of energy, absorber parts are required to have a maximum absorptivity in the solar wavelength region (approximately 300 to approximately 2500 nm) and a maximum reflectivity in the thermal radiation region (above approximately 2500 nm).  
           [0004]    Absorbers for flat collectors, in which solar collector elements of the type described above with coated absorber parts that satisfy this demand for selective absorption to a high level, are known under the name Tinox. In these collectors, the material of the absorber parts comprises a copper strip substrate to which a layer of titanium oxynitride has been applied, followed by a covering layer of silicon dioxide. The tube which is connected to an absorber part likewise consists of copper and is soldered to the absorber part.  
           [0005]    In connection with solar collectors a distinction is drawn between low-temperature collectors, with operating temperatures of up to 100° C., and high-temperature collectors, with operating temperatures of over 100° C. In the case of tower installations, which are used to provide process heat, the absorber temperature may be up to 1200° C. The so-called steady temperature, which is to be understood as meaning the theoretically possible maximum temperature of use of a collector at which the material is in thermal equilibrium with the environment, is often referred to as a characteristic variable for a solar collector. Steady temperatures in the low-temperature range are characteristic of the Tinox absorbers described. In higher temperature ranges, for the known solar collector element with the Tinox coating or with other coatings which are known to be used, such as black paints or pigmented plastics, there is a risk of decomposition of the layer, of gases being evolved, but also of the capacity of the collector element falling at least for a relatively short time, for example on account of bleaching of the black layer.  
           [0006]    With regard to the joining of the absorber part and tube, it should be noted that this jointing only has to ensure the required strength, but also has to provide a sufficiently high heat transfer. In connection therewith, it should be ensured that, in the event of any desired changes in the absorber part/tube combination of materials (no longer Cu/Cu), problems do not arise with regard to finding a suitable joining technique.  
           [0007]    The present invention is based on the object of providing a solar collector element of the type described in the introduction which, on the one hand, leads to a high light absorption and a high reflectivity in the solar radiation region and, on the other hand, which has improved use characteristics in particular under high thermal load operating conditions, and has a longer service life, in combination with a production method that involves minimum possible capital outlay. Furthermore, the invention is also intended to allow an optimum solution to the problem with regard to ensuring a strong mechanical joint and good heat transfer between the absorber part and the tube. Finally, the solar collector element is to be distinguished by the possibility of steady temperatures within the range of use of high-temperature collectors and also by high long-term chemical stability.  
         SUMMARY OF THE INVENTION  
         [0008]    According to the invention, this is achieved by the fact that the coating comprises a multilayer system which is composed of three layers, of which the top layer is a dielectric layer, preferably an oxide, fluoride or nitride layer of chemical composition MeO z , MeF r , MeN s , having a refractive index n&lt;1.8, and of which the middle layer is a chromium oxide layer of chemical composition CrO x , and of which the bottom layer consists of gold, silver, copper, chromium, aluminium and/or molybdenum. The indices x, z, r and s indicating a stoichiometric or non-stoichiometric ratio in the oxides, fluorides or nitrides.  
           [0009]    The optical multilayer system which is present according to the invention can firstly be applied advantageously, since there is no need for environmentally hazardous, in some cases toxic, salt solutions during production. For example, the metallic layer of the optical multilayer system may be a sputtered layer or a layer which is produced by vaporization, in particular by electron bombardment or from thermal sources. The two upper layers of the optical multilayer system may likewise be sputtered layers, in particular layers produced by reactive sputtering, CVD or PECVD layers or layers produced by vaporization, in particular by electron bombardment or from thermal sources, so that the entire optical multilayer system comprises layers which are applied in vacuum order, in particular in a continuous process.  
           [0010]    The top layer may alternatively be a silicon oxide layer of chemical composition SiOy, where the index y once again indicates a stoichiometric or non-stoichiometric ratio in the oxide composition.  
           [0011]    In addition to having a high long-term thermal and chemical stability, the solar collector element according to the invention is also distinguished, on account of the ease of processing, in particular deforming, the composite material from which the absorber part is produced. This is primarily achieved on account of the metallic substrate, which may be made from copper or preferably from aluminium, by a production method which involves little complexity and by a high thermal conductivity. The latter property is particularly important since it allows rapid, highly efficient transfer of the heat taken up as a result of the light absorption to the heat-transfer liquid.  
           [0012]    The above processes for applying the layers of the system also advantageously enable the chemical composition MeO z , MeF r , MeN s  of the top layer and the chemical composition CrO x  of the chromium oxide layer, with regard to the indices x, y, z, r and s, to be not only set at defined, discrete values but also allows a stoichiometric or non-stoichiometric ratio between the oxidized substance and the oxygen to be varied continuously within defined limits. In this way it is possible to specifically set, by way of example, the refractive index of the reflection-reducing top layer (which is also responsible for increasing the mechanical load-bearing capacity (DIN 58196, part 5)) and the absorptivity of the chromium oxide layer (the absorptance decreasing as the value of the index x rises).  
           [0013]    According to the invention, it is in this way possible to set a total light reflectivity, determined in accordance with DIN 5036, part 3, on the side of the optical multilayer system to a preferred level of less than 5%. In addition to a high resistance to ageing, it is also possible to ensure a high thermal stability, in such a manner that under a thermal load of 430° C./100 hours, only changes of less than 7%, and preferably of less than 4%, in the reflectivity occur. Moreover, in the event of a thermal load of this nature, there is advantageously also no evolution of gases.  
           [0014]    The composite material which is used for the absorber part according to the invention therefore, on account of its synergistic combination of properties  
           [0015]    of the substrate layer, for example its excellent deformability, by means of which it withstands stresses produced in the production process of the solar collector element according to the invention during the shaping processes which are to be performed without problems, for example its high thermal conductivity and—in particular in the case of aluminium as substrate material—the capacity for a surface patterning which in the light wavelength region additionally promotes adsorption and is then followed by the other layers in relief, and moreover with a reflectance in the solar radiation region which reinforces the action of the metallic layer of the optical three-layer system;  
           [0016]    of the metallic layer which, on account of its constituents, which have a high reflectivity and therefore a low emission in the thermal radiation region, takes account of the fact that, according to the Lambert-Bouguer law, the radiation characteristic is absorbed exponentially as the penetration depth grows, and for most inorganic substances is available as a store or thermal energy which can be passed on at even a very low depth (less than approximately 1 μm);  
           [0017]    of the chromium oxide layer, with its high selectivity of the absorptivity (peak values over 90% in the wavelength region from approximately 300 to 2500 nm, minimum values below 15% in the wavelength region&gt;approx. 2500 nm) and its capacity for modification (index x) which has already been explained, and  
           [0018]    of the top, in particular silicon oxide, layer, the advantages of which have to some extent already been pointed out above and which, in addition to its antireflective action, also has a high transmittance and, as a result, increases the proportion of the radiation values in the solar region which can be absorbed by the chromium oxide layer;  
           [0019]    is eminently suitable for the production of the solar collector element according to the invention. Therefore, by using the solar collector element according to the invention, not only is it possible to produce low-temperature collectors with an operating temperature of up to 100° C., but also it is possible to produce high-temperature collectors, in which steady temperatures of over 250° C. are possible.  
           [0020]    Beneath the optical multilayer system, it is also possible to provide an intermediate layer on the substrate, the intermediate layer being responsible firstly for mechanical and corrosion-inhibiting protection for the substrate and secondly for high adhesion for the optical multilayer system. A lower layer may be applied to the substrate—likewise as a protection layer—on the side which is remote or opposite from the side to which the optical multilayer system is applied. In the case of an aluminium substrate, both layers may consist of aluminium oxide, which can be produced from the anodically oxidized or electrolytically brightened and anodically oxidized substrate material. In addition, a further layer, with a reflection-increasing action, can be applied to the substrate or to the lower layer.  
           [0021]    Preferably, in the case of an aluminium substrate material of the absorber part and a copper tube, these two parts can advantageously be joined to one another (even if a layer consisting of aluminium oxide is present on the substrate) by laser welding. This leads to a material-to-material bond as a result of melted and resolidified aluminium and as a result of migration of the aluminium into the copper. By way of example, radiation from a CO 2  or Nd-YAG laser with sufficient power can be used for the welding.  
           [0022]    Irrespective of the combination of materials used for the absorber part/tube, the use of laser welding has advantages over soldering. One advantage is that there are no unnecessary metallic interfaces (no solder). Another advantage is that it is possible to achieve a higher mechanical strength of the joint and, therefore, a greater resistance to vibrations and impacts. A further advantage is that the permissible operating temperature (steady temperature) of the absorber part according to the invention is increased. Still another advantage is that a greater operational reliability is ensured.  
           [0023]    In particular, the tube and the absorber part may be joined where they are in abutment with one another by spot-welded seams which run on both sides of the tube and are produced by a pulse welding process. When determining the laser power and pulse frequency, it should be noted that the weld spot dimensions are primarily dependent on the thermal conductivity, with surface temperature, irradiation time, thickness of the absorber part and type of material representing factors which influence one another. There is a proportional relationship between the fusion depth and the mean laser power.  
           [0024]    Further advantageous embodiments of the invention are given in the subclaims and in the detailed description which follows. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]    The invention is explained in more detail on the basis of an exemplary embodiment which is illustrated by the appended drawing, in which:  
         [0026]    [0026]FIG. 1 shows an outline sectional illustration through an absorber part of a solar collector element according to the invention;  
         [0027]    [0027]FIG. 2 shows a perspective view of an area of an embodiment of a solar collector element according to the invention; and  
         [0028]    [0028]FIG. 3 shows a plan view of an embodiment of a solar collector element according to the invention with a meandering tube path. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]    Throughout the various figures of the drawing, identical parts are always provided with identical reference symbols and are therefore generally also in each case only described once.  
         [0030]    The absorber part (reference symbol  10  in FIG. 2) of the solar collector element (reference symbol E in FIG. 2) according to the invention consists of a composite material with a highly selective absorptivity and reflectivity in the solar wavelength region and in the thermal radiation region. This composite material for its part comprises a strip-like substrate  1 , which in particular can be deformed and consists of aluminium, an intermediate layer  2  which is applied to the substrate  1  on a side A, and an optically active multilayer system  3  which is applied to the intermediate layer  2 .  
         [0031]    The substrate  1  may preferably have an in particular regular rolled structure of grooves which run substantially parallel to one another in a preferred direction. A structure of this type, if these grooves are oriented parallel to the north/south direction, enables the absorptance of a solar collector element E, according to the invention, to attain a level which is as far as possible independent of the particular angle of the sun, which changes over the course of the day.  
         [0032]    A total light reflectivity, determined according to DIN 5036, part 3, on side A of the optical multilayer system  3  may preferably be less than 5%.  
         [0033]    The composite material may preferably be processed in the form of a coil with a width of up to 1600 mm, preferably of 1250 mm, and with a thickness D of approximately 0.1 to 1.5 mm, preferably of approximately 0.2 to 0.8 mm, it being possible to produce the solar collector element E according to the invention from this coil in a simple manner by stamping out a plate-like absorber part  10  and joining it to a tube (reference symbol  11  in FIG. 2). The substrate  1  of the composite material may preferably have a thickness D 1  of approximately 0.1 to 0.7 mm.  
         [0034]    The aluminium of the substrate  1  may in particular be more than 99.0% pure, which promotes a high thermal conductivity.  
         [0035]    (The intermediate layer  2  consists of anodically oxidized or electrolytically brightened and anodically oxidized aluminium, which is applied to the substrate material.  
         [0036]    The multilayer system  3  comprises three individual layers  4 ,  5 ,  6 , an upper and middle layers  4 ,  5  being oxide layers and a bottom layer  6  being a metallic layer applied to the intermediate layer  2 . The top layer  4  of the optical multilayer system  3  is, preferably, a silicon oxide layer of chemical composition SiO y . The middle layer  5  is a chromium oxide layer of chemical composition CrO x , and the bottom layer  6  consists of gold, silver, copper, chromium, aluminium and/or molybdenum.  
         [0037]    The indices x, y indicate a stoichiometric or non-stoichiometric ratio of the oxidized substance to the oxygen in the oxides. The stoichiometric or non-stoichiometric ratio x may preferably lie in the range 0&lt;×&lt;3, while the stoichiometric or non-stoichiometric ratio y may adopt values in the range 1≦y≦2.  
         [0038]    The fact that the two upper layers  4 ,  5  of the optical multilayer system  3  may be sputtered layers, in particular layers produced by reactive sputtering, CVD or PECVD layers or layers produced by vaporization, in particular by electron bombardment or from thermal sources, means that it is possible to adjust the ratios x, y continuously (i.e. to set them to non-stoichiometric values of the indices), with the result that the layer properties can in each case be varied.  
         [0039]    The top layer  4  of the optical multilayer system  3  may advantageously have a thickness D 4  of more than 3 nm. At this thickness D 4 , the layer is already sufficiently efficient, yet the outlay on time, material and energy is low. An upper limit for the layer thickness D 4 , in view of these aspects, is approximately 500 nm. An optimum value for the middle layer  5  of the optical multilayer system  3 , in view of the abovementioned aspects, is a minimum thickness D 5  of more than 10 nm and a maximum thickness D 5  of approximately 1 μm. The corresponding value for the bottom layer  6  is a thickness D 6  of at least 3 nm, at most approximately 500 nm.  
         [0040]    With a view to achieving high efficiency, the bottom layer  6  of the optical multilayer system  3  should preferably be more than 99.5% pure. As has already been mentioned, the layer may be a sputtered layer or a layer which is produced by vaporization, in particular by electron bombardment or from thermal sources, so that the entire optical multilayer system  3  advantageously comprises layers  4 ,  5 ,  6  which are applied in vacuum order in a continuous process.  
         [0041]    A lower layer  7  which—like the intermediate layer  2 —consists of anodically oxidized or electrolytically brightened and anodically oxidized aluminium, is applied to that side B of the strip-like substrate  1  which is remote from the optical multilayer system  3 . The intermediate layer  2  and the lower layer  7  may advantageously be produced simultaneously by wet-chemical means, in which case the pores in the aluminium oxide layer can be as far as possible closed off by hot-sealing during the final phase of the wet-chemical process sequence, resulting in a surface with long-term stability. Therefore, the lower layer  7 —like the intermediate layer  2 —offers mechanical and corrosion-inhibiting protection to the substrate  1 .  
         [0042]    According to the invention it is possible, in particular, for the layer structure to be assembled in such a manner that the total light reflectivity, determined in accordance with DIN 5036, part 3, on side A of the optical multilayer system  3 , under a thermal load of 430° C./100 hours, undergoes changes of less than 7%, preferably of less than 4%.  
         [0043]    [0043]FIG. 2 illustrates the overall structure of a solar collector element E according to the invention. The drawing diagrammatically depicts the absorber part  10  and the tube  11  (for a heat-transfer liquid) as parts of the solar collector element E. The absorber part  10  consists of the composite material having the substrate  1  consisting of aluminium and the multilayer system  3  built up from three layers  4 ,  5 ,  6 , as has been explained above. The absorber part  10 , which can be produced at low cost and in an environmentally friendly manner, results in high light absorption and dissipation of heat to the tube  11 , while, under collector operating conditions which involve high thermal loads, it is possible to ensure a comparably long service life.  
         [0044]    The nature of the joint between the absorber part  10  and the tube  11  (which consist in particular of copper) is produced by means of a laser welding process, in particular in the form of a pulse welding process, also contributes to the latter effect. Laser welding is a fusion welding process, i.e. the parts which are to be joined are melted under the action of the laser radiation. A particular feature is the high power density and, when using pulse welding, the rapid cooling associated with the short duration of action. Since the laser welding of the absorber part  10  to the tube  11  is preferably carried out without filler, the material-to-material bond which is formed between the two parts which are to be joined consists only of the respective materials of the absorber part  10  and of the tube  11 ; on account of the lower melting point of aluminium, drop-shaped solidified small molten balls  12  predominantly comprising aluminium are formed on the absorber part  10 , and the aluminium has diffused into the copper of the tube  11 . The small molten balls  12  are responsible for bridging any gap or air cushion which may be present between absorber part  10  and tube  11 . To produce an optimum joint, the power density of the laser during welding, taking account of the criteria listed above, should not exceed 10 7  W/cm 2 , preferably 10 6  W/cm 2 . The total energy for a weld spot should be active for a time of up to approximately 10 ms, preferably distributed over the course of time. As well as the criteria which have already been listed, the actual spatial and temporal intensity distribution at the location of action should also be taken into account (spiking, hot spots).  
         [0045]    In particular, the tube  11  and the absorber part  10  may—as illustrated in FIG. 2—be joined where they are in contact with one another by weld seams which run on both sides of the tube  11  and are formed from welds spots (small molten balls  12 ) which are spaced apart from one another (distance a) and are in particular arranged at regular intervals.  
         [0046]    Since, when a solar collector is operating, the heat transfer from the absorber part  10  to the tube  11  takes place predominantly at the weld spots, the size of the small molten balls  12  and the distance a between the small molten balls  12  are the decisive factors in determining the efficiency of the collector. On the other hand, the heat resistance of the absorber part  10 , in its plane of extent, limits the efficiency of the collector. This heat resistance of the absorber part  10  is determined substantially by the thermal conductivity of the composite material, primarily that of the substrate  1 , on the one hand, and by the thickness D of the absorber part  10 , on the other hand. The optimum distance a between the small molten balls  12 , for a predetermined composite material of the absorber part  10  and a fixed size (diameter d) of the small molten balls  12 , therefore depends on the thickness D of the absorber part. In the case of a substrate  1  made from aluminium, a thickness D of the absorber part of approximately 0.3 to 0.8 mm, and a diameter d of the small molten balls  12  of approximately 0.2 to 3.2 mm, this optimum distance a (center-to-center distance of the small molten balls  12 ) is approximately 0.5 to 2.5 mm. The greater the thickness D of the absorber part  10 , the shorter the distance a between the weld spots has to be.  
         [0047]    The present invention is not restricted to the exemplary embodiment which has been described, but rather encompasses all means and measures which achieve the same effect within the context of the invention. For example, it is also possible for the bottom layer  6  of the optical multilayer system  3  to comprise a plurality of partial layers of gold, silver, copper, chromium, aluminium and/or molybdenum arranged above one another.  
         [0048]    As has already been mentioned, the top layer may alternatively also consist of fluorides or nitrides. As is known, copper is also eminently suitable as the substrate material, although aluminium, for approximately the same heat transfer properties, achieves a higher strength without it being necessary to provide beads.  
         [0049]    With aluminium strip as substrate  1 , there is a wide range of different rolled surfaces available, in particular surfaces with a grooved structure, which, when used as absorber composite material, advantageously minimize and homogenise the extent to which the absorptance is dependent on the angle of the sun, given a suitable orientation.  
         [0050]    Furthermore, the person skilled in the art can supplement the invention by means of additional advantageous measures without departing from the scope of the invention. For example, the tube  11  may in particular be laid in straight form or, as illustrated in FIG. 3, in meandering form on the absorber part  10 . If the tube is laid in meandering form, the welding can be restricted to straight sections I of tube, while curved sections K of tube are not welded.  
         [0051]    Furthermore, the invention is not restricted to the combination of features defined in claim  1 , but rather may also be defined by any other desired combination of specific features of all the individual features disclosed. This means that in principle virtually any individual feature of claim  1  can be omitted or replaced by at least one individual feature disclosed elsewhere in the application. In this respect, claim  1  is only to be understood as an initial attempt at putting an invention into words.