IL274513B1 - Composite material for a solar collector - Google Patents

Description

WO 2019/145166 PCT/EP2019/050676 ALANOD GmbH & Co. KG, Egerstraße 12, D-58256 Ennepetal "Composite material for a solar collector" The present invention relates to a composite material for a solar collector comprising a carrier consisting of aluminium, having an intermediate layer situated on one side on the carrier, and having an optically effective multilayer system applied on the intermediate layer and comprising at least two dielectric and/or oxidic layers, namely an upper layer and an underlying further layer – hereinafter: "light-absorbing layer" – having the primary function of light absorption, in particular in the solar spectral range, wherein the upper layer is a dielectric layer having a refractive index n ≤ 2.0, and wherein a metallic layer is arranged directly on the intermediate layer below the at least two dielectric and/or oxidic layers. As is known, solar collectors are used for generating energy from solar radiation. The spectrum of the sun can be described in wide ranges by that of the so-called "black body" at a temperature of approximately 5800 K. The spectrum which however actually arrives on the Earth’s surface is however characterized by a multiplicity of absorptions in the atmosphere, wherein the loss of intensity that occurs here is dependent on the angle of incidence of the solar rays, that is to say on the position on the terrestrial globe. A normalized indication that was therefore introduced is the normalized, so-called "air mass coefficient AM", which expresses the actual path of the radiation through the atmosphere as a ratio to the shortest possible path of the radiation through the atmosphere – that is to say in the case of perpendicular incidence. An air mass coefficient AM = WO 2019/145166 PCT/EP2019/050676 - 2 - 1.5 corresponds here to the conditions for the solar spectrum in central Europe. As a result of the incident solar spectrum, temperatures of up to 100ºC are typically reached during normal operation in the case of flat-plate collectors. If the layer were an ideal "black body", then it would emit a spectrum having a distribution similar to that of the incident light, but the maximum of said distribution is in the thermal radiation range. This means that part of the thermal energy previously obtained by absorption and heating-up is lost again in the form of thermal radiation. Generally, in the case of an object on which a radiation impinges, said radiation is split into a reflected, an absorbed and a transmitted portion, which are determined by the reflectance (reflectivity) R, the absorptance (absorptivity) A and the transmittance (transmissivity) T of the object. In the case of a radiation-nontransmissive substrate, here the transmission is equal to 0, such that only the sum of absorption and reflection is of relevance, i.e. r+A = 100 percent. The quantification of absorption and reflection in the range of 250 nm to 2500 nm is based here on the standard EN 410:2011 "Glass in building: Determination of luminous and solar characteristics of glazing", according to which it is possible to determine a total reflectance from which, in the absence of transmission, the absorptance results as a complement to 1 or to 1percent. In this case, for determining the solar-weighted hemispherical total reflectance, the solar spectrum AM 1.5 according to ASTM G173-03 is used as incident radiation.

WO 2019/145166 PCT/EP2019/050676 - 3 - In order to prevent thermal losses as a result of emission, applications of absorber composite materials of the type mentioned in the introduction require a maximum absorptance in the solar wavelength range (3nm to 2500 nm, in particular in the range of 360 nm to 1800 nm) and a maximum reflectance in the thermal radiation range (above approximately 2500 nm, in particular above 3000 nm). An ideal of such a selective absorber, which however does not occur in nature, is accordingly considered by those skilled in the art to be an absorber which absorbs 100 percent of the solar spectrum below a specific jump wavelength of 2500 nm (2.5 µm), for example, and reflects 100 percent of the solar spectrum above said wavelength. The absorption and thus also the thermal emittance would thus be equal to zero. Technical absorber materials, with regard to the assessment of their quality, are thus measured inter alia by the extent to which their absorption-reflection characteristic approximates to this ideal step response function. They are deemed to be all the better, the steeper the rise in reflection at the transition from the solar wavelength range to the thermal radiation range and the higher the reflectivity level present in the latter range. For quantitatively determining the thermal emittance in the spectral range of 2500 nm to 19000 nm (4000 cm-1 to 526 cm-1), recourse is had to the specifications in the standard EN 12898 2001-4 "Glass in building – Determination of the emissivity". EP 2 336 811 B1 discloses a composite material similar to the type mentioned in the introduction, in which the refinement procedure for the aluminium carrier consists WO 2019/145166 PCT/EP2019/050676 - 4 - in particular of two different processes, both of which can be operated continuously, specifically of producing an intermediate layer consisting of aluminium oxide in a wet-chemical process, which is referred to in summary as anodization and comprises electrolytic brightening and anodic oxidation, and of applying the optically effective multilayer system in vacuo. The layers of the optical multilayer system here are generally dielectric layers, wherein the use of oxidic layers such as, for example, aluminium oxide or titanium oxide as topmost layer and silicon dioxide as central layer represents a preferred special case. In the case of this composite material, the intermediate layer has a thickness of not more than 30 nm, wherein the lower light-absorbing layer is applied directly on the intermediate layer and the intermediate layer is situated directly on the carrier. EP 1 217 394 A1 discloses a composite material of the type mentioned in the introduction which comprises a carrier consisting of aluminium, an intermediate layer situated on one side on the carrier, and an optically effective multilayer system applied on the intermediate layer. In this case, the intermediate layer preferably consists of anodically oxidized or electrolytically brightened and anodically oxidized aluminium formed from the carrier material. The optically effective multilayer system consists of three layers, wherein the upper two layers are dielectric and/or oxidic layers, and the bottommost layer is a metallic layer applied on the intermediate layer, which metallic layer has no transmission and has an exclusively reflective effect. In this case, provision is made for the topmost layer of the optical multilayer system to be a dielectric layer, preferably an oxidic, fluoridic or nitridic layer having the chemical composition MeOa, MeFb, MeNc, having a refractive index n < 1.8 and for the central WO 2019/145166 PCT/EP2019/050676 - 5 - layer of the optical multilayer system to be a chromium-oxidic layer having the chemical composition CrOz, and for the bottommost layer of the optical multilayer system to consist of gold, silver, copper, chromium, aluminium and/or molybdenum, wherein the indices a, b, c and z denote a stoichiometric or non-stoichiometric ratio in the oxides, fluorides or nitrides. Particularly the layer consisting of gold, silver, copper, chromium, aluminium and/or molybdenum can consist of a plurality of partial layers and have a preferred thickness in the range of up to 500 nm. One composite material of this type, in which the bottommost layer of the optical multilayer system is formed from a double layer consisting of an upper chromium ply and a lower aluminium ply, is commercially available under the tradename Mirotherm® on the filing date of this application. EP 1 217 394 A1 describes the fact that in the case of solar collectors, a distinction is drawn between low- temperature collectors having operating temperatures of up to 100ºC and high-temperature collectors having operating temperatures of above 100ºC, wherein in the case of so-called tower installations that serve for providing process heat, the absorber temperature can be up to 1200ºC. A characteristic variable that is often specified for a solar collector is the so-called equilibrium, rest or stagnation temperature, which should be understood to mean the maximum theoretically possible use temperature of the collector at which the material is in thermal equilibrium with the surroundings. If the solar collector is running in normal operation, for example, the heat transfer medium circulates through it and the energy taken up by absorption is carried away. The operating temperature established here may lie e.g. in WO 2019/145166 PCT/EP2019/050676 - 6 - the range of from less than 100ºC up to 120ºC. However, if the passage of the heat transfer medium having a cooling effect on the collector is halted – for example in a desired manner because the required consumer temperature has been reached, or in an undesired manner because a heat transfer pump fails – then energy is no longer carried away continuously and the temperature of the solar collector rises. In the case of an incident solar power of approximately 1 kW/m, as is characteristic for Europe on a sunny day, said temperature can reach values of 220ºC to 250ºC, for example. In this case, part of the liquid coolant remains in the line. Every collector has to be designed for this stagnation temperature since, in these temperature ranges higher than the operating temperature, there is the risk otherwise for the heat transfer liquids used, such as water-glycol mixtures, for example, and also for the substrates and coatings of the collector, that decompositions, a partial evaporation, outgassing, undesired pressure increases, but at least a relatively short-term decrease in the performance of the collector element will occur. The stagnation temperature thus determines what thermal requirements the materials used for absorbers must withstand, which can result in high material costs. Therefore, there is a technical need to keep the stagnation temperatures of solar collectors low. In order to achieve this, WO 2012/069718 A1 provides a multi-layered material comprising a layer having a variable surface morphology, which varies reversibly depending on temperature and which has a surface roughness parameter that is greater than a first roughness value if the temperature is lower than a threshold temperature, and that is less than a second WO 2019/145166 PCT/EP2019/050676 - 7 - roughness value if the temperature is higher than the threshold temperature, wherein the second roughness value is less than the first roughness value, and wherein the layer having the variable surface morphology is coated with an absorbing layer. What can be achieved with this multi-layered material is that the stagnation temperature does not rise above 180ºC since the different roughnesses below and above the threshold temperature are also associated with a different absorption behaviour. However, producing such layers having a variable surface morphology has proved to be complicated on an industrial scale. US 9,671,137 B2 describes an alternative technical solution in this regard. Here, too, a different behaviour below and above a threshold temperature is exploited in the composite material, but this behaviour is brought about by a specific combination of thermochromically acting vanadium oxides in a selective layer. In the case of this oxide combination, below a threshold temperature in the wavelength range of 6 µm to 10 µm there is a high transmittance of more than percent, and above the threshold temperature there is a low transmittance in the range of between 20 and percent. Above the threshold temperature here in comparison with a conventionally used vanadium oxide having the formula VO2, a lower reflectivity and thus a higher emission are achieved, which has the effect of lowering the stagnation temperature. The present invention is based on the object of providing a composite material of the type described in the introduction having a particular suitability for absorbers with operating temperatures in the low-temperature range, with comparatively little technological outlay, in particular whilst avoiding the WO 2019/145166 PCT/EP2019/050676 - 8 - use of layer constituents which have temperature-dictated abrupt changes in properties. This is achieved according to the invention by virtue of the fact that the intermediate layer and the optically effective multilayer system are selected with regard to their optical properties and the layers are dimensioned in such a way that given a solar absorption of at least 92 percent, not more than 20 percent to percent of the solar radiation in a wavelength range of 1500 nm to 5000 nm is reflected and 25 percent to percent of the solar radiation in a wavelength range of 5000 nm to 10000 nm is reflected, wherein in the wavelength range of 5000 nm to 10000 nm across the entire wavelength range, a continuous rise in a wavelength-dependent reflectance of the composite material with the wavelength takes place. In a preferred embodiment of the invention, provision can be made for the intermediate layer and the optically effective multilayer system to be selected with regard to their optical properties, in particular with regard to the refraction and extinction of the layers, and the layers are dimensioned in such a way that at least 30 percent, preferably at least 40 percent, of incident radiation in a wavelength range of greater than/equal to 5200 nm is reflected and at least percent, preferably at least 60 percent, of incident radiation in a wavelength range of greater than/equal to 6500 nm is reflected. The invention breaks through the mindset, routine in the art, that the quality of an absorber composite material is all the better, the more the absorption-reflection characteristic thereof approximates to the ideal step-response function of a "black body" related only to a specific operating temperature, the thermal WO 2019/145166 PCT/EP2019/050676 - 9 - dynamics established after a collector stoppage being taken into account. The approach according to the invention is based on the fact that the emission maximum of a "black body" changes with temperature, in particular upon transition from an operating temperature to a stagnation temperature, and the wavelength-dependent absorption-reflection characteristic is designed taking account both of the operating temperature and of the stagnation temperature. In this regard, by way of example, the emission maximum of a "black body" at a temperature of 80ºC, which may describe a customary operating temperature in the low- temperature range, is at a wavelength of approximately 8200 nm, while said maximum at a temperature of 200ºC, which constitutes a customary stagnation temperature of conventional absorber materials of this type, is at only 6124 nm. While the customary absorber materials have reflectance values of more than 90 percent at this wavelength, according to the invention the reflectance is significantly lower at this wavelength, e.g. amounting to approximately only half of the aforementioned value. That means that the thermal emission at this temperature is very much higher, with the result that the equilibrium between solar absorption and emission upon interruption of carrying away the useful heat is already established at comparatively lower temperatures. According to the invention, this can advantageously lead to a reduction of the stagnation temperature of a flat-plate collector by 40 K or more, which, with regard to heat transfer liquids, substrates and coatings of the collector, significantly minimizes the risk of decompositions, evaporation, outgassing, WO 2019/145166 PCT/EP2019/050676 - 10 - undesired pressure increases, decrease in the performance of the collector element, etc., that is to say provides a remedy in regard to all the disadvantageous phenomena associated with high rest temperatures as mentioned in the introduction. In this case, the use of materials which are based on a temperature-dictated abrupt change in properties can advantageously be dispensed with. In this case, the optical multilayer system according to the invention is advantageously able to be applied in a technologically proven manner. In this regard, the at least two dielectric and/or oxidic layers of the optical multilayer system, the metallic layer and/or the intermediate layer can be sputtering layers, in particular layers produced by reactive sputtering, CVD or PECVD layers or layers produced by evaporation, in particular by electron bombardment or from thermal sources, such that they are layers advantageously applied in vacuum succession in a continuous method. The intermediate layer on the carrier can also be a layer which consists of aluminium oxide and which is preferably formed from anodically oxidized or electrolytically brightened and anodically oxidized carrier material. In this case, the light-absorbing layer can contain a titanium-aluminium mixed oxide and/or a titanium-aluminium mixed nitride and/or a titanium-aluminium mixed oxynitride having the chemical composition TiAlqOxNy, wherein the indices q, x and y respectively denote a stoichiometric or non-stoichiometric ratio. Provision can advantageously also be made for the light-absorbing layer of the optical multilayer system to contain chromium oxide having the chemical composition CrOz and/or chromium nitride having the WO 2019/145166 PCT/EP2019/050676 - 11 - chemical composition CrNv and/or chromium carbide CrCp and/or chromium oxynitride having the chemical composition CrOzNv and/or chromium oxycarbide CrOzCp and/or chromium oxycarbonitride CrOzCpNv, wherein the indices z, v and p respectively denote a stoichiometric or non-stoichiometric ratio. The upper layer can preferably be a silicon-oxidic layer having the chemical composition SiOw, wherein the index w once again denotes a stoichiometric or non- stoichiometric ratio in the oxidic composition. This layer has an antireflective effect and has a high transmissivity, thus resulting in an increase in the proportion of the radiation values in the solar range that are absorbable in the light-absorbing layer. The abovementioned methods advantageously make it possible here not just to set the chemical composition of the layers to specific, discrete values with regard to the indices p, q, v, w, x, y and z, but rather to vary the stoichiometric or non-stoichiometric ratio fluidly in each case with specific limits. The stoichiometric or non-stoichiometric ratios p, q, v, x, y, z here can lie in the range 0 < p and/or q and/or v and/or x and/or y and/or z < 3 and/or 1 ≤ w ≤ 2. As a result, by way of example, the refractive index of the reflection-reducing topmost layer, which also brings about an increase in the values for the mechanical loading capacity (measured according to DIN ISO 9211-4:2008-06), and the absorptance of the light-absorbing layer can be set in a targeted manner, wherein the absorptivity decreases for example as the value of the indices x and/or z increases. The respective proportions of the titanium-aluminium mixed oxide, nitride and/or oxynitride or the proportions of the corresponding chromium compounds in the light-absorbing layer can also be controlled in this way.

WO 2019/145166 PCT/EP2019/050676 - 12 - The wavelength-dependent absorption-reflection characteristic that has changed according to the invention by comparison with the prior art provides a reduction of the reflectance in comparison with conventional absorber materials even at the operating temperature. However, the slightly lower efficiency that results brings about merely a reduction of the annual yield of thermal energy of at most approximately percent, said reduction being significantly underproportional in relation to the advantages achieved, which reveals a synergistic effect of the invention in a cost-benefit analysis. As far as the setting of the wavelength-dependent absorption-reflection characteristic according to the invention is concerned, in this respect in a preferred embodiment of the invention provision can be made for the metallic layer to be semi-transparent at least in a wavelength range of 300 nm to 2500 nm, in particular in the range of 360 nm to 1800 nm, i.e. in particular to have a transmittance in the range of 30 percent to percent, preferably of 40 percent to 60 percent, wherein the metallic layer can consist in particular of a non-noble metal, in particular of a transition metal, which can preferably consist of titanium or of chromium or of a rare earth metal, or of an alloy thereof. In this case, the metallic layer can have just a very small thickness in the range of 3 nm to 14 nm, in particular in the range of 5 nm to 12 nm, and to be configured in particular in monolayer fashion. Further advantageous embodiments of the invention are contained in the dependent claims and in the detailed description below.

WO 2019/145166 PCT/EP2019/050676 - 13 - The invention will be explained in greater detail on the basis of exemplary embodiments illustrated by the accompanying drawings. Here in the figures: Figure 1 shows a diagrammatic illustration of the wavelength-dependent relative intensity of a solar-weighted hemispherical total reflectance and of a "black body" and also the wavelength-dependent reflectivity of a selective absorber material considered routinely in the art to be ideal, Figure 2 shows an illustration similar to that in Figure 1, wherein the wavelength-dependent reflectivities of three commercially available composite materials are indicated as comparative examples instead of the wavelength-dependent reflectivity of the selective absorber material considered to be ideal, Figure 3 shows an illustration similar to that in Figures 1 and 2 which shows the simulated wavelength-dependent reflectivity of a commercially available composite material from Figure 2 in comparison with a typical embodiment, illustrated in an idealized manner, of a composite material according to the invention, Figure 4 shows an illustration of the dependence of the spectral energy density of a WO 2019/145166 PCT/EP2019/050676 - 14 - "black body" on wavelength and temperature, Figure 5 shows an illustration of the layer construction of a composite material according to the invention, Figure 6 shows the result of a depth profile analysis in a diagrammatic illustration of element atom concentration versus a layer removal time for one preferred embodiment of a composite material according to the invention, Figure 7 shows a diagrammatic illustration of the reflection spectrum, measured in the solar spectral range, of two preferred embodiments of a composite material according to the invention, Figure 8 shows a diagrammatic illustration of the reflection spectrum, measured in the infrared spectral range, of the two preferred embodiments of a composite material according to the invention, Figure 9 shows a diagrammatic illustration of the typical profile of the emittance of the two preferred embodiments of a composite material according to the invention as a function of temperature, Figure 10 shows a diagrammatic illustration of the temperature-dependent reflectance of the composite material according to the invention before and after ageing.

WO 2019/145166 PCT/EP2019/050676 - 15 - With regard to the description hereinafter, it is expressly emphasized that the invention is not restricted to the exemplary embodiments, nor to all or a plurality of features of feature combinations described, rather each individual partial feature of each exemplary embodiment including when detached from all other partial features described in association therewith may have an inventive significance by itself and also in combination with arbitrary features of another exemplary embodiment. In the various figures of the drawing, the same parts are also always provided with the same reference signs, and so as a rule they are also described only once in each case. The diagrammatic illustration of the wavelength-dependent relative intensity of the solar spectrum and of the "black body" in Figure 1 firstly reveals on the left-hand side – in the solar range – the standardized solar spectrum AM1.5 according to ASTM G173-03, which has already been explained in the introduction. The right-hand side of the diagram shows the spectral distribution density of the "black body" BB at a temperature of 100ºC (normalized). Furthermore, Figure 1 shows the wavelength-dependent reflectivity of a selective absorber material IA considered routinely in the art to be ideal, which material absorbs almost 100 per cent of the solar spectrum in a range below a specific jump wavelength SL of 2500 nm (2.5 µm), for example, and reflects 100 percent of the solar spectrum in a range above said wavelength. The invention breaks away from this ideal picture of an absorber. Figure 2 reveals, for three composite materials from the applicant that are commercially available on the filing date of this application, how the wavelength- WO 2019/145166 PCT/EP2019/050676 - 16 - dependent reflectivity R (total reflection in percent) really approximates to the selective absorption-reflection characteristic considered to be ideal. Said materials are offered under the tradenames Eta plus®, mirotherm® and mirosol® TS on the filing date of this application and are selective absorber layer systems for solar collectors. On the filing date of this application, the layer systems of eta plus® and mirotherm® are vapour-deposited in a PVD method continuously in a so-called "air-to-air" process. Absorptances of up to 95% in conjunction with low emission ε of a maximum of 5% are achieved in this case. This is illustrated in particular by the curve profiles ε (λ) starting from a wavelength λ of approximately 4.0 µm, the curves each reaching a plateau where they then change only little. The respective jump wavelength SL of the reflection curves lies in a range λ of approximately 1.2 µm to 1.8 µm (1200 nm to 1800 nm). On the filing date of this application, the material mirosol® TS is a system comprising a selectively absorbing lacquer applied in a so-called "coil coating" method (roll-to-roll fabrication). With the aim of high protection of the surface, this selective lacquer is advantageously hydrophobic vis-à-vis contaminants and insensitive to fingerprints, but brings about a local reflection minimum at approximately 10 µm. The absorber composite materials mentioned above find application in various types of solar-thermal collectors. Here copper or aluminium pipes as heat conductors are welded for the most part on the rear side of the absorbers. In this regard, laser welding has become established as optimal connection technology. The composite materials mentioned have proved worthwhile in practice, having achieved a saving WO 2019/145166 PCT/EP2019/050676 - 17 - of CO2 produced per year in the millions of tonnes range. Figure 3 shows, from the wavelength-dependent reflectivities R illustrated in Figure 2, only that of the abovementioned material mirotherm® in comparison with a typical embodiment, illustrated in an idealized manner, of a composite material V according to the invention. It can be seen that the jump wavelength SL of the composite material V according to the invention is shifted into a longer-wavelength range by comparison with the material mentioned above. Moreover, it can be seen that the composite material V according to the invention – and as will also be explained subsequently, in particular with reference to Figures 5 to 8 – in particular its intermediate layer situated on the carrier 1, and also the optically effective multilayer system 3 applied thereon, are selected with regard to their optical properties and the layers 2, 4, 5, 6 are dimensioned in such a way that a solar absorptance α of at least 92 percent is present. Not more than 20 percent to 35 percent of the incident radiation is reflected in a wavelength range λ of 1500 nm to 5000 nm and 25 percent to 90 percent of the incident radiation is reflected in a wavelength range λ of 5000 nm to 10000 nm. In this case, a continuous, in particular almost linear, rise in reflectance R with wavelength λ can be observed in this range. In a preferred embodiment, the intermediate layer 2 and the optically effective multilayer system 3 of the composite material V according to the invention are selected with regard to their optical properties, in particular with regard to the refraction and extinction of the layers 2, 4, 5, 6, and the layers 2, 4, 5, 6 are WO 2019/145166 PCT/EP2019/050676 - 18 - dimensioned in such a way that at least 30 percent, preferably at least 40 percent, of the incident radiation is reflected in a wavelength range λ of above 5200 nm and at least 55 percent, preferably at least percent, of incident radiation is reflected in a wavelength range λ of above 6500 nm, as is likewise illustrated by the exemplary illustration in Figure 3. In a preferred embodiment of the invention, in the wavelength range λ of 4200 nm to 6600 nm, preferably in the range of 5000 nm to 6200 nm, there is an average rise in reflectance versus wavelength ΔR/Δλ in the range of 21 percent per µm to 25 percent per µm. In the wavelength range λ of 6200 nm to 10000 nm, preferably in the range of 6600 nm to 9000 nm, said average rise in reflectance versus wavelength ΔR/Δλ can preferably lie in the range of 5 percent per µm to percent per µm. In contrast to the known absorber materials, in the structural design of which – as shown in Figure 1 – reference is made to the emission of the "black body" at a standardized temperature of 100ºC, the invention, in its original absorption-emission characteristic, takes account of the temperature-dependent shift in the emission maximum of the "black body" versus wavelength λ. In this case, the illustration of the dependence of the spectral energy density of a "black body" on wavelength λ (on the abscissa) and on temperature T (as a parameter of the curves) in Figure 4 illustrates how the radiation maximum of the "black body" in the range of 273 K to 453 K shifts to smaller wavelengths λ at temperature T increases, wherein the absolute value of the radiation maximum rises. This is illustrated by the dotted line "max". For defining the absorption-emission characteristic according to the invention, each WO 2019/145166 PCT/EP2019/050676 - 19 - absorber temperature T can thereby be assigned a wavelength λ at which a respective maximum emission takes place, and vice versa. In this case, the composite material V according to the invention is designed such that if the absorber temperature T rises upon interruption of the carrying away of the useful heat, the thermal emission, that is to say the emittance ε, also increases at the same time. In this way, the composite material V according to the invention advantageously fulfils an autoregulatory function in that at higher temperature as a result of the greater emission automatically a greater cooling is also established because the epsilon value increases continuously in the relevant temperature range, in particular in the range of 80ºC to 200ºC. In this case, at a temperature of 30ºC, for example, the value of the emittance ε is 20%, that is to say less than 25%, as a result of which a temperature of the maximum emission of less than 50ºC is taken into account. At 80ºC, the emission maximum of the "black body" is at a wavelength λ of approximately 8200 nm. At this wavelength, the reflection of the abovementioned material mirotherm® is approximately 95%, and that of the composite material V according to the invention is for example approximately 80%, both curves having the same solar light absorption α. If an absorber surface temperature of 200ºC is taken as a basis, then the emission maximum shifts to a wavelength of approximately 6124 nm. While the reflection of the mirotherm® material mentioned above remains practically unchanged there – that is to say that the ε value is virtually not influenced at all in that case – the reflection of the composite material V according to the invention decreases to approximately 55%. As a result, WO 2019/145166 PCT/EP2019/050676 - 20 - an absorber surface emits significantly more at this temperature, which leads to a temperature reduction, that is to say to a lower stagnation temperature. In this case, the difference present in the wavelength range λ above the jump wavelength SL of the mirotherm® material mentioned above is marked by a hatched area F in Figure 3. The radiation portion which is known to be reflected in this area F is absorbed according to the invention. Figure 5 shows that the composite material V according to the invention consists in each case of an, in particular deformable, strip-shaped carrier 1 composed of aluminium wherein an intermediate layer 2 is situated on one side of the carrier 1 and an optically effective multilayer system 3 is in turn applied on the intermediate layer 2. The composite material V according to the invention can preferably be configured as a coil having a width of up to 1600 mm, preferably of 1250 mm, and having a thickness D of approximately 0.1 mm to 1.5 mm, preferably of approximately 0.2 mm to 0.8 mm. In this case, the carrier 1 can preferably have a thickness Dof approximately 0.1 mm to 0.7 mm. The aluminium of the carrier 1 can have in particular a purity higher than 99.0 percent, thereby promoting the thermal conductivity. The intermediate layer 2 on the carrier 1 is a layer which consists of aluminium oxide and which can preferably be formed from anodically oxidized or electrolytically brightened and anodically oxidized carrier material. It can have in particular a thickness D2 in the range of 100 nm to 250 nm, preferably in the range of 130 nm to 180 nm.

WO 2019/145166 PCT/EP2019/050676 - 21 - The multilayer system 3 according to the invention comprises two individual layers 4, 5, wherein a metallic layer 6 likewise associated with the optical multilayer system 3 is arranged directly on the intermediate layer 2 below said two layers 4, 5. The upper layer 4 of the optical multilayer system 3 is a dielectric layer having a refractive index n ≤ 2.and can be in particular a silicon-oxidic layer having the chemical composition SiOw. The lower layer 5 is a light-absorbing layer that preferably contains a titanium-aluminium mixed oxide and/or a titanium-aluminium mixed nitride and/or a titanium-aluminium mixed oxynitride having the chemical composition TiAlqOxNy. Said layer 5, alternatively or additionally and/or in a separate partial layer, can also contain chromium oxide having the chemical composition CrOz and/or chromium nitride having the chemical composition CrNv and/or chromium oxynitride having the chemical composition CrOzNv. The indices q, v, x, y, z here in each case denote a stoichiometric or non-stoichiometric ratio of the oxidized or nitrided substance to the oxygen in the oxides and/or in the oxynitride and/or of the aluminium to the titanium. The stoichiometric or non-stoichiometric ratios can preferably lie in the range 0 < q and/or v and/or x and/or y and/or z < 3, while the stoichiometric or non-stoichiometric ratio w can assume values in the range 1 ≤ w ≤ 2. By virtue of the fact that, according to the invention, the two layers 4, 5 of the optical multilayer system can be sputtering layers, in particular layers produced by reactive sputtering, CVD or PECVD layers or layers produced by evaporation, in particular by electron bombardment or from thermal sources, it is possible to set the ratios q, v, w, x, y, z without gradation (that is to say also to non-stoichiometric values of the WO 2019/145166 PCT/EP2019/050676 - 22 - indices), as a result of which the respective layer properties can be varied and the layers can also be configured as gradient layers having indices q, v, w, x, y, z increasing and/or decreasing over the layer thickness. With regard to a sufficient efficiency for the function of reducing reflection, by way of example, an upper limit value of the layer thickness D4 of the upper layer 4 of the optical multilayer system 3 is 500 nm. A preferred range of the thickness D4 extends from 60 nm to 250 nm, and a particularly preferred range from 1nm to 150 nm. An optimum value for the light-absorbing layer 5 of the optical multilayer system 3 according to the invention from the standpoints mentioned is a minimum thickness D5 of greater than/equal to 100 nm, a maximum of 1 µm, in particular a value in the range of 150 nm to 500 nm, particularly preferable in the range of 200 nm to 300 nm. According to the invention, the metallic layer 6 is semi-transparent at least in a wavelength range of 300 nm to 2500 nm, in particular in the range of 360 nm to 1800 nm, i.e. it has in particular a transmittance in the range of 30 percent to 80 percent, preferably of percent to 60 percent. It can preferably consist of a non-noble metal, in particular of a transition metal, such as particularly preferably of titanium or of chromium or of a rare earth metal, or of an alloy thereof, and have a thickness D6 in the range of 3 nm to 14 nm, in particular in the range of 5 nm to 12 nm, wherein it is configured in particular in monolayer fashion. The metallic layer 6 can also be a sputtering layer or a layer produced by evaporation, in particular WO 2019/145166 PCT/EP2019/050676 - 23 - a layer produced by electron bombardment or from thermal sources. Figure 6 represents the result of an analysis that was carried out by means of photoelectron spectroscopy (XPS). For continuous layer removal, argon sputtering ions accelerated by a 4 keV source, for example, can be used here, as with the use of the device "Quantum 2000" used for the analysis from PHI Physical Electronics, which is based on excitation with Al-Kα-radiation. In Figure 6, the removal times (sputtering times) indicated in minutes are assigned to a particularly preferred constitution of the various layers 1, 2, 4, and 6 of the composite material according to the invention with regard to their composition and thickness ratios, and the way in which the respective layer 8 or the optical multilayer system 3 results therefrom is also indicated. Firstly, the upper layer 4 of the optical multilayer system 3 can be seen on the left in Figure 6, said layer being a silicon-oxidic layer and having a negligibly small proportion of carbon at the surface. The thickness D4 of the layer is approximately 120 nm. The ratio of the atomic concentrations of silicon (Si) to oxygen (O) is approximately 33.3 percent to 66.percent in the overall chemical composition, whereby the presence of a silicon dioxide layer (SiO2) having a stoichiometric composition is illustrated. The index w of the silicon-oxidic layer having the chemical composition SiOw is thus 2. Further towards the right in Figure 6 there follows the light-absorbing layer 5, in which the oxygen content (O) continuously decreases the chromium content (Cr) continuously increases. The thickness D5 of the layer is approximately 268 mm. The proportion of further WO 2019/145166 PCT/EP2019/050676 - 24 - elements is negligibly small taking account of the abovementioned indeterminacies at the layer boundaries. Indicated prominently in the curve profile here are the instants of the sputtering times, which are designated by I, II and III in the graph. At the instant I there is a ratio of the atomic concentrations of chromium (Cr) to oxygen (O) of approximately 33.3 percent to 66.6 percent in the overall chemical composition, whereby the presence of a chromium-oxidic layer (CrO2) having an over-stoichiometric composition relative to the trivalent chromium is illustrated at this point. The index z of the chromium-oxidic layer having the general chemical composition CrOz is approximately here. At the instant II there is a ratio of the atomic concentrations of chromium (Cr) to oxygen (O) of approximately 40.0 percent to 60.0 percent in the overall chemical composition, whereby the presence of a chromium-oxidic layer (Cr2O3) having a stoichiometric composition in relation to the trivalent chromium is illustrated at this point. The index z of the chromium- oxidic layer having the general chemical composition CrOz is approximately 1.5 here. With increasing depth, the Cr-O ratio of the layer 5 then becomes under-stoichiometric, wherein at the instant III of the removal it was possible to determine a ratio of the atomic concentrations of chromium (Cr) to oxygen (O) of approximately 50.0 percent to 50.0 percent in the overall chemical composition, which indicates the presence of a chromium-oxidic layer having the composition CrO at this point. The index z of the chromium-oxidic layer having the general chemical composition CrOz is approximately 1.0 here and becomes less than 1.0 with increasing layer depth. Since the instant II was already reached approximately after percent of the time that was required overall for removing the light-absorbing layer 5 by sputtering, to the extent of approximately 90 percent of its depth the WO 2019/145166 PCT/EP2019/050676 - 25 - layer 5 thus consists of Cr-O compounds having an under-stoichiometric composition relative to the trivalent chromium. In the metallic layer 6 consisting of chromium, the curve should theoretically climb to 100 atom percent chromium and the oxygen content should fall to 0 atom percent. Although this is not the case owing to the abovementioned indeterminacies and the small layer thickness D6 of 3 to 6 nm, the chromium peak IV occurring in this region of the sputtering time and the oxygen minimum assigned to said chromium peak are sufficiently significative for the demonstration of the metallic chromium layer 6. The intermediate layer 2 illustrated in Figure 6 is – as already mentioned – a layer consisting of aluminium oxide (Eloxal) on the aluminium carrier 1, which is expressed by an average relation of the atomic concentrations of 33.3 percent aluminium (Al) to 66.6 percent oxygen (O), wherein the intermediate layer then finally transitions to the pure aluminium of the carrier 1. The layer has a thickness D2 of approximately 173 nm. The composite material V according to the invention is in contrast to the prior art, where the known metallic layer is not semi-transparent, but rather reflective. As a result, the known intermediate layer and the known carrier have no optical functions. According to the invention, instead, advantageously the optical multilayer system 3 is optically active, and so are the intermediate layer 2 and the carrier 1. The optically effective multilayer system 3 according to the invention acts as a light trap according to the invention, wherein in the wavelength range λ in which WO 2019/145166 PCT/EP2019/050676 - 26 - conventional absorber materials have already undergone the jump SL in reflectivity ε that takes place at the transition towards higher wavelengths λ, according to the invention a comparatively high absorption α still takes place, as is expressed by the area F in Figure 3. Examination results that were obtained with the composite material V according to the invention constructed in accordance with Figure 6 are visualized by way of example by Figures 7 to 10. As already mentioned, the latter firstly involve diagrammatic illustrations of the wavelength-dependent reflectance R for the solar and infrared range of two preferred embodiments of a composite material V, (Figures 7 and 8) ("position 1" and "position 2"). The results in each case represent average values of measurements carried out at at least three different locations of the samples. In order to determine the solar absorption, the abovementioned standard being complied with, a UV-Vis spectrometer "Lambda 950" from Perkin Elmer was used, and a traceable Spectralon standard was used as a reference for solar absorbers. In order to determine the thermal emission, the abovementioned standard being complied with, an FT-IR spectrometer "Spektrum 400" with a gold-coated integrating sphere from PIKE was used, and the reference for solar absorbers was a traceable gold mirror. The use of certified reference samples in these measurements ensures a high reproducibility here. In this case, it should be noted especially with respect to Figure 8 that the reciprocal of the wavelength λ is plotted on the abscissa there on account of the device-internal Fourier transformation, wherein the larger numerical values on the left WO 2019/145166 PCT/EP2019/050676 - 27 - indicated in cm-1 correspond to smaller wavelength values λ than the larger values on the right indicated in cm-1; by way of example, the value of 2500 cm-1 corresponds to a wavelength of 4 µm and the value of 1000 cm-1 corresponds to a wavelength of 10 µm. Advantageously for the purposes of the illustration, the abscissa axis is hyperbolically compressed by these substantive facts in the longer-wave IR range, where the changes in the reflectance R are only small. The experimental results coincide – almost with congruence – with the idealized curve illustration in Figure 3. Moreover, by way of example, the reflectance R of a composite material V according to the invention as a function of the temperature is illustrated first in Figure 9 as a typical profile of the two preferred embodiments and second in Figure 10 before and after ageing. In relation to Figures 9 and 10, table 1 below shows the individual results gained from the measured infrared reflection spectrum of a composite material V according to the invention. In this case, "position 1" denotes a composite material 7 according to the invention in which the chromium of the metallic layer 6 had a thickness D6 of 5.7 nm, while this thickness Dwas 10 to 12 nm for "position 2". The measurements were carried out at room temperature (25ºC), wherein the values between 30ºC and 80ºC relevant to the performance of an absorber and the values thereabove relevant to the stagnation were calculated in accordance with Planck’s radiation law. In this case, the values for 30ºC correspond to the graphical illustration in Figure 9. 35 WO 2019/145166 PCT/EP2019/050676 - 28 - Table 1: Emittance ε in percent as a function of temperature given an absorptance α in the range of 93.6% to 94.7% Position 30ºC 80ºC 100ºC 150ºC 200ºC 250ºC 23.0 26.3 29.4 34.0 38.5 42.2 22.6 25.1 27.9 32.0 36.0 39. In this case, the following formulae are of importance for the emittance ε and the absorptance α: and In these formulae, R(λ) is the wavelength-dependent reflectance, IBB(λ) is the wavelength-dependent intensity of the radiation of a "black body" and IAM1.5(λ) is the intensity of the solar spectrum, which by its nature is likewise wavelength-dependent – and, as described above, is standardized according to ASTM G173-03. The composite material V according to the invention was loaded under two temperature regimes, specifically in the first place for 300 hours at 200ºC and in the second place for 72 hours at 250ºC. After the loadings, the reflection spectra were measured and the alpha and epsilon values were calculated therefrom.

WO 2019/145166 PCT/EP2019/050676 - 29 - Table 2 below contains in this regard the values of the absorption α before and after the temperature loading and also the change Δα that occurred during the temperature loading. The values before the temperature loading ("position 1": α = 93.8%; "position 2": α 94.4%) correspond here to the graphical illustration in Figure 7. Table 2: Solar absorptance α before and after temperature loading Alpha in percent Delta-alpha in percent Position Before 300 h @ 200ºC h @ 250ºC 300 h @ 200ºC h @ 250ºC 93.8 93.5 93.2 -0.3 -0.2 94.5 94.9 94.5 0.3 0. Tables 3 and 4 represent the reflection values ε established after the temperature loading from the values contained in table 1 and correspond to the graph in Figure 10 for "position 2" therein. Table 3: Emittance ε in percent after 300 hours at 200ºC Position 30ºC 80ºC 100ºC 150ºC 200ºC 250ºC 1t 22.5 25.4 28.3 32.7 37.0 41.2 21.7 24.0 26.7 30.6 34.5 38. Table 4: Emittance ε in percent after 72 hours at 250ºC Position 30ºC 80ºC 100ºC 150ºC 200ºC 250ºC 20.9 23.6 26.5 30.7 35.1 39.2 22.2 24.3 26.9 30.6 34.4 38.

WO 2019/145166 PCT/EP2019/050676 - 30 - Tables 1 to 4 and the corresponding graphs reveal that advantageously the optical properties of the composite material V according to the invention barely changed on account of the temperature loading. Table 5 shows contains the stagnation temperatures Tst resulting from a comparative model calculation with the measured values and the efficiencies η of a typical collector comprising a composite material V according to the invention in comparison with the abovementioned material mirotherm®. Table 5: Stagnation temperatures Tst and efficiencies η in comparison (simulation) Collector exit temperature mirotherm® Invention Efficiency η 35ºC 0.77 0.50ºC 0.71 0.75ºC 0.60 0. Tst = 210ºC Tst = 163ºC It is readily evident from this that an advantageously large reduction of the stagnation temperature Ts of 57ºC by the composite material V according to the invention is accompanied on the other hand by an only very small – underproportional – decrease in performance of a maximum of just 0.04 points at a collector exit temperature of 75ºC. The present invention is not restricted to the exemplary embodiments illustrated, but rather encompasses all means and measures that have an identical effect within the meaning of the invention. Furthermore, the invention is not restricted to the combination of features defined in Claim 1, but rather 30 WO 2019/145166 PCT/EP2019/050676 - 31 - can also be defined by any desired other combination of specific features of all individual features disclosed overall. This means that, in principle, practically 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 should be understood merely as a first attempt at formulating an invention.

WO 2019/145166 PCT/EP2019/050676 - 32 - Reference signs Carrier Intermediate layer Optical multilayer system Upper layer of 5 Light-absorbing layer of 6 Metallic layer A Incident and emergent ray in 5 (Figure 7) AM 1.5 Standardized solar spectrum – Figures 1, B1 Reflection ray of 6/2 (Figure 7) B2 Reflection ray of 2/1 (Figure 7) C1 Ray formed from superposition of A and B(Figure 7) C2 Ray formed from superposition of C1 and B(Figure 7) BB Curve: "black body" – Figures 1, D (Total) thickness of V D1 Thickness of D2 Thickness of D4 Thickness of D5 Thickness of D6 Thickness of DI Destructive interference (Figure 7) KI Constructive interference (Figure 7) n2 Refractive index of n5 Refractive index of n6 Refractive index of IA Curve of ideal absorber – Figure R Reflectance SL Jump wavelength T Temperature V Composite material ε Emittance

Claims (15)

1./ Patent Claims 1. Composite material (V) comprising a carrier (1) comprising of aluminium, having an intermediate layer (2) situated on one side of the carrier (1) and consisting of aluminium oxide, and having an optically effective multilayer system (3) applied on the intermediate layer (2) and comprising at least two dielectric and/or oxidic layers (4, 5), namely an upper layer (4) and an underlying light-absorbing layer (5), wherein the upper layer (4) is a dielectric layer having a refractive index n ≤ 2.0, and wherein a metallic layer (6) is arranged directly on the intermediate layer (2) below the at least two dielectric and/or oxidic layers (4, 5), wherein the intermediate layer (2) and the optically effective multilayer system (3) are selected with regard to their optical properties and the layers (2, 4, 5, 6) are dimensioned in such a way that given a solar absorption (α) of at least 92 percent, not more than 20 percent to percent of the solar radiation in a wavelength range (λ) of 1500 nm to 5000 nm is reflected and 25 percent to 90 percent of the solar radiation in a wavelength range (λ) of 5000 nm to 10000 nm is reflected, wherein in the wavelength range (λ) of 5000 nm to 10000 nm across the entire wavelength range (λ), a continuous rise in a wavelength-dependent reflectance (R) of the composite material (V) with the wavelength (λ) takes place, wherein the metallic layer (6) has a thickness (D6) in the range of 3 nm to 14 nm and is semi-transparent at least in a wavelength range of 300 nm 2500 nm, i.e. has a transmittance in the range of 30 percent to 80 percent, wherein the intermediate layer (2) on the carrier (1) has a thickness (D2) in the range of 100 nm to 250 nm, and wherein the light-absorbing layer (5) of the optical multilayer system (3) has a thickness (D5) of more than 50 nm and a maximum of µm.

2. Composite material (V) according to Claim 1, characterized in that the intermediate layer (2) and the optically effective multilayer system (3) are selected with regard to their optical properties, in particular with regard to the refraction and extinction of the layers (2, 4, 5, 6), and the layers (2, 4, 5, 6) are dimensioned in such a way that at least 30 percent, preferably at least 40 percent, of incident radiation in a wavelength range (λ) of greater than/equal to 5200 nm is reflected and at least 55 percent, preferably at least 60 274513/ percent, of incident radiation in a wavelength range (λ) of greater than/equal to 6500 nm is reflected.

3. Composite material (V) according to Claim 1 or 2, characterized in that the metallic layer (6) is semi-transparent in the range of 360 nm to 1800 nm, i.e. in particular has a transmittance in the range of 30 percent to 80 percent, preferably of 40 percent to percent.

4. Composite material (V) according to any of Claims 1 to 3, characterized in that the metallic layer (6) consists of a non-noble metal, in particular of a transition metal, such as preferably of titanium or of chromium or of a rare earth metal, or of an alloy thereof.

5. Composite material (V) according to any of Claims 1 to 4, characterized in that the metallic layer (6) has a thickness (D6) in the range 5 nm to 12 nm, and is configured in particular in monolayer fashion.

6. Composite material (V) according to any of Claims 1 to 5, characterized in that the intermediate layer (2) on the carrier (1) is a layer which consists of aluminium oxide and which is preferably formed from anodically oxidized or electrolytically brightened and anodically oxidized carrier material.

7. Composite material (V) according to any of Claims 1 to 6, characterized in that the intermediate layer (2) has a thickness (D2) in the range of 130 nm to 180 nm.

8. Composite material (V) according to any of Claims 1 to 7, characterized in that the refractive index (n2) of the intermediate layer (2) in the solar wavelength range and, if appropriate, also in a wavelength range (λ) of 1500 nm to 5000 nm is less than the refractive index (n6) of the metallic layer (6), wherein the materials of the intermediate layer (2) and of the metallic layer (6) are preferably selected in such a way that a critical angle of total internal reflection of the solar radiation at the intermediate layer (2) lies in the range of 35º to 63º. 274513/

9. Composite material (V) according to any of Claims 1 to 8, characterized by an average rise in reflectance versus wavelength (ΔR/Δλ) in the wavelength range (λ) of 4200 nm to 6600 nm, preferably in the range of 5000 nm to 6200 nm, in the range of 21 percent per µm to 25 percent per µm.

10. Composite material (V) according to any of Claims 1 to 9, characterized by an average rise in reflectance versus wavelength (ΔR/Δλ) in the wavelength range (λ) of 6200 nm to 10000 nm, preferably in the range of 6600 nm to 9000 nm of 5 percent per µm to percent per µm.

11. Composite material (V) according to any of Claims 1 to 10, characterized in that the light-absorbing layer (5) of the optical multilayer system (3) contains a titanium-aluminium mixed oxide and/or a titanium-aluminium mixed nitride and/or a titanium-aluminium mixed oxynitride having the chemical composition TiAlqOxNy, wherein the indices q, x and y respectively denote a stoichiometric or non-stoichiometric ratio, or in that the light-absorbing layer (5) of the optical multilayer system (3) contains chromium oxide having the chemical composition CrOz and/or chromium nitride having the chemical composition CrOzNv and/or chromium oxycarbide CrOzCp and/or chromium oxycarbonitride CrOzCpNv, wherein the indices z, v and p respectively denote a stoichiometric or non-stoichiometric ratio, wherein the stoichiometric or non-stoichiometric ratios p, q, v, x, y, z lie in the range of 0 < p and/or q and/or v and/or x and/or z <3, wherein the light-absorbing layer (5) of the optical multilayer system (3) preferably has a thickness (D5) in the range of 70 nm to 350 nm, particularly preferably in the range 200 nm to 270 nm.

12. Composite material (V) according to any of Claims 1 to 11, characterized in that the upper layer (4) of the optical multilayer system (3) is a silicon-oxidic layer having the chemical composition SiOw, wherein the index w denotes a stoichiometric or non-stoichiometric ratio that lies in the range 1 ≤ w ≤ 2. 274513/

13. Composite material (V) according to any of Claims 1 to 12, characterized in that the at least two dielectric and/or oxidic layers (4, 5) of the optical multilayer system (3), the metallic layer (6) and/or the intermediate layer (2) are sputtering layers, CVD or PECVD layers or layers produced by evaporation, wherein in particular the intermediate layer (2) and/or the optical multilayer system (3) consist(s) of layers applied in vacuum succession in a continuous method.

14. Composite material (V) according to any of Claims 1 to 13, characterized in that the upper layer (4) of the optical multilayer system (3) has a thickness (D4) of a maximum of 500 nm, wherein said thickness (D4) lies in particular in the range of 60 nm to 250 nm, preferably in the range of 100 nm to 150 nm.

15. Composite material (V) according to any of Claims 1 to 14, preferably according to any of Claims 11 or 12, characterized in that layers (2, 4, 5) of the optically effective multilayer system (3), preferably at least the light-absorbing layer (5), are/is configured as gradient layer(s) in which in particular at least one of the indices q, v, w, x, y or z and/or the proportion – preferably specified in atom percent – of a non-indexed element contained in the respective layer (2, 4, 5) change(s) continuously.