Abstract:
A solar receiver for a solar thermal system that is formed with ytterbium hexaboride (YbB6) is provided. A solar thermal system having a solar receiver of this kind is also provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to PCT Application No. PCT/EP2012/062702, having a filing date of Jun. 29, 2012, the entire contents of which are hereby incorporated by reference. 
     
    
     FIELD OF TECHNOLOGY 
       [0002]    The invention relates to a solar receiver for a solar thermal energy system, and to a solar thermal energy system. 
       BACKGROUND 
       [0003]    In solar thermal energy systems, sunlight is collected by means of reflectors and focused onto a solar receiver. The solar receivers comprise an absorber material that absorbs the focused sunlight and, on account of the solar energy absorbed in the process, heats a heat carrier in the interior of the solar receiver. 
         [0004]    However, it is not just the heat carrier that is heated in solar thermal energy systems. In addition, the solar receiver together with absorber material itself is also heated, with the result that the solar receiver emits thermal radiation to the surroundings. This emission of thermal radiation reduces the energetic efficiency of a solar receiver and thus the efficiency of a solar thermal energy system. The emission of thermal radiation by the absorber to the surroundings is therefore intended to be minimized. 
         [0005]    Solar receivers are known which are operated at such low temperatures that the thermal radiation emitted by the absorber and the radiation of the sunlight are spectrally separated to a sufficient extent. In this regard, from the sun radiation in the wavelength range of 0.3 μm to 2.5 μm usually impinges on the surface of the earth. The solar receiver, by contrast, during operation is at a temperature of typically 350° C. or 400° C., and therefore emits thermal radiation approximately in the wavelength range of 2 to 20 μm or 1.5 to 20 μm. In such cases, the absorber can be embodied as a spectrally selective absorber with regard to the absorber material, i.e. the material forms a good absorber at wavelengths of the solar radiation and a poor emitter at wavelengths of the thermal radiation. 
         [0006]    At higher operating temperatures of solar receivers, however, the spectra of incident sunlight and thermal radiation of the absorber are not separated to a sufficient extent. Consequently, known absorber materials hitherto have emitted an undesirably large amount of thermal energy at long wavelengths and/or have absorbed an undesirably small amount of radiation energy in the spectral range of sunlight. With regard to the absorber material alone, solar receivers consequently have an excessively low energy yield or do not operate with sufficiently low losses. 
         [0007]    Therefore, it has been necessary hitherto to provide absorber materials with layer structures of optically thin materials which influence the emission and/or absorption of radiation by means of interference effects. In this regard, it is known, for example, to provide solar receivers with Bragg mirrors. However, even in the case of solar receivers having a layer structure, the spectral emission and/or absorption profile cannot be adjusted arbitrarily, but rather is limited by the degree of adjustment of the optical properties of the individual layer materials and by the number of layers. 
         [0008]    As an alternative to such layer structures, known solar receivers have an individual cermet layer for utilizing interference effects. Such a cermet layer, that is to say a layer composed of a ceramic-metal or ceramic-metal oxide composite substance, in this case has islands of metal or metal oxide in a ceramic layer. Electromagnetic radiation that penetrates into the cermet layer is scattered at said islands. In this case, the radiation scattered by different islands can interfere in a suitable manner given an expedient choice of the average spacing of said islands. 
         [0009]    Therefore, a need exists for an improved solar receiver for solar thermal energy systems. In particular, the solar receiver is intended to have a higher energy yield. Moreover, a need exists for an improved solar thermal energy system having an improved energy yield in comparison with the prior art. 
       SUMMARY 
       [0010]    An aspect relates to a solar receiver comprising the features specified in claim  1  and by means of a solar thermal energy system comprising the features specified in claim  12 . Some specific embodiments are evident from the associated dependent claims, the following description and the drawing. 
         [0011]    The solar receiver for a solar thermal energy system is formed with ytterbium hexaboride (YbB 6 ). In this case, ytterbium hexaboride expediently forms an absorber material of the solar receiver. The solar receiver can be embodied with particularly low losses and with high energy yield compared with solar receivers in accordance with the prior art. 
         [0012]    In this regard, an increased energy yield of solar receivers in solar thermal energy systems can be achieved precisely if the solar receiver can be operated with a higher operating temperature. This is because the resultant higher temperature of the heat carrier gives rise to a significantly higher energy yield in power generation. 
         [0013]    A particularly high energy yield of a solar receiver requires operating temperatures of the solar receiver of 500° C. or more. However, absorber materials known hitherto have emissivity edges at 2.0 μm to 2.5 μm. In other words, the emission of thermal radiation of the absorber material is sufficiently efficiently suppressed only at wavelengths beyond 2 μm. However, since the emission of thermal radiation is dependent on temperature, known absorber materials can be operated efficiently only up to a temperature of 450° C. This is because at higher temperatures, even at shorter wavelengths compared with the spectral position of the abovementioned emissivity edges, so much thermal radiation occurs that the energy losses caused as a result prevent an efficient embodiment of solar receivers. 
         [0014]    Ytterbium hexaboride, by contrast, has an emissivity edge at a wavelength that is significantly less than 2 μm. In this regard, ytterbium hexaboride has a sharp emissivity edge at 1.5 μm, wherein the reflectivity of ytterbium hexaboride is 0.8 or more at wavelengths of greater than 2 μm and 0.2 or less at wavelengths of less than 1.5 μm. Consequently, with regard to its spectral emission and absorption properties, ytterbium hexaboride is significantly better adapted to the requirement of high operating temperatures of solar receivers. Accordingly, a solar receiver comprising ytterbium hexaboride can be operated at significantly higher operating temperatures, in particular at temperatures of greater than/equal to 500° C. and preferably at temperatures of greater than/equal to 600° C., with regard to the emission and absorption balance and, consequently, with a high energy yield. Moreover, ytterbium hexaboride, even at the abovementioned operating temperatures, has a significantly higher thermal stability. In other words, ytterbium hexaboride forms a spectrally significantly improved selective absorber compared with known selective absorbers at high absorber temperatures. 
         [0015]    In one embodiment, the solar receiver has a layer structure or a layer, in particular formed with cermet. For example, the solar receiver can have a Bragg mirror, wherein the Bragg mirror is part of a layer structure as mentioned above. Such a layer structure having a Bragg mirror makes it possible to further reduce the emissivity, that is to say the thermal emission, at wavelengths beyond the emissivity edge. Accordingly, the energy yield of the solar receiver is further increased in this development. In another embodiment, a layer structure composed of exactly two layers is present instead of a Bragg mirror. In this case, the geometry and material parameters of said layers, in particular their thicknesses and refractive indices, are expediently chosen in such a way that the emissivity of the absorber is further increased. 
         [0016]    In yet another embodiment of the solar receiver, an individual layer is present instead of or in addition to the layer structure. In particular, the layer is a cermet layer, i.e. a layer comprising a ceramic-metal or ceramic-metal oxide composite substance. Expediently, the cermet layer has islands formed with metal in a ceramic layer and/or islands formed with ceramic in a metal or metal oxide layer. In this development, the radiation that penetrates into the cermet layer is scattered at the islands of the cermet layer. Expediently, the average spacing and also the distribution of the spacings of the islands from one another are chosen in such a way that the emissivity of the absorber is reduced at wavelengths of the thermal radiation. 
         [0017]    Moreover, in this case at least one part of the ytterbium hexaboride of the solar receiver forms at least one part of the layer or of the layer structure. For instance, said part is situated within the layer structure and forms, in particular, the innermost layer of the layer structure. Further, said part is surrounded fully circumferentially by the layer or layer structure. In this way, the layer structure can be embodied as a spectral filter which further reduces the emissivity at wavelengths of the occurring thermal radiation of the solar receiver. In principle, the wavelength range in which the emissivity is further reduced can in this case be chosen freely by means of the parameters of the layer structure in a manner known per se. Precisely in the case of the solution, however, the layer or layer structure can be embodied particularly advantageously, in particular particularly simply, for instance with particularly few or not perfectly precisely embodied layers, since the emissivity spectrum of ytterbium hexaboride is already particularly suitably adapted to the spectrum of an ideal selective absorber. Consequently, the technical requirements made of the embodiment of the layer system or of the layer are significantly reduced in comparison with the prior art. In particular, the layer structure can be embodied with a reduced number of layers or with a thinner layer such as, for instance, a cermet layer, with the result that the production costs are significantly reduced. Furthermore, the reduced requirements made of the embodiment of the layer or layer structure means that a significantly wider selection of materials can be used for embodying the layer or layer structure. Therefore, the layer structure can be suitably adapted to further technical or economic requirements. In particular, an adaptation with regard to the thermal stability or with regard to reduced production costs can be effected. 
         [0018]    Advantageously, the solar receiver is formed, and in particular coated, with non-oxidizing or weakly and/or slowly oxidizing material and/or the solar receiver has a coating with or composed of non-oxidizing or weakly and/or slowly oxidizing material. Particularly expediently and as an alternative or in addition to the above-described solar receivers having a layer structure, at least one part of the layer structure of the solar receiver, at least one outer layer of the layer structure, is formed with or composed of non-oxidizing or weakly and/or slowly oxidizing material, such as ytterbium hexaboride. In this regard, ytterbium hexaboride scarcely oxidizes at temperatures of at least 700° C. in oxygen. In comparison therewith, previously known coatings or layers of absorber material in solar receivers react greatly with oxygen at customary operating temperatures. In respect of that, hitherto the absorber material of the receiver has had to be encapsulated and evacuated to high vacuum. Typically, the vacuum must be maintained to a high degree for a service life of approximately 20 years. However, if a coating or layer and/or layer structure of the absorber material is formed with non-oxidizing or weakly and/or slowly oxidizing material, then an encapsulation and evacuation of the absorber material can be dispensed with. 
         [0019]    Precisely on account of the significantly expanded selection possibilities for materials for embodying the layer or layer structure as explained above, it is also possible to use suitable non-oxidizing or weakly and/or slowly oxidizing materials for forming the solar receiver. In this context, weakly and/or slowly oxidizing materials denote such materials which are stable in oxygen over a period of at least one, preferably at least five and in particular at least 20, years at a temperature of 500° C., preferably 600° C., and ideally 700° C. 
         [0020]    For instance, the solar receiver has a chamber that is transparent in the visible spectral range, wherein the ytterbium hexaboride is arranged at least partly and/or completely in the chamber. Convection losses and thus efficiency losses can be significantly reduced in this way. In this case, in a suitable manner, the transparent chamber is embodied as at least partly cylindrical and/or tubular. In an expedient manner, the chamber is formed with glass in a manner known per se. 
         [0021]    Advantageously, the chamber that is transparent in the visible spectral range is at least partly gas-evacuated. Convection losses and thus efficiency losses of a solar receiver and of a solar thermal energy system formed with the solar receiver can be significantly reduced in this way. In this case, “at least partly gas-evacuated” means in this context that at least part of the chamber is gas-evacuated and/or evacuated of at least one component of a gas or a gas mixture, in particular oxygen-evacuated. In the last-mentioned development, faster and/or more strongly oxidizing materials can also be used in a manner known per se. 
         [0022]    The solar thermal energy system comprises a solar receiver such as has been described above. Consequently, the solar thermal energy system can be embodied with a high energy yield compared with the prior art. 
         [0023]    For example, the solar thermal energy system is designed for the operation of the solar receiver at temperatures of greater than/equal to 500° C., and in particular at temperatures of greater than/equal to 600° C., and particularly advantageously at temperatures of greater than/equal to 700° C. Expediently, collector areas, degree of focusing and geometrical dimensions of the solar receiver in the solar thermal energy system are designed for attaining one or more of the abovementioned temperatures, for instance in the manner of one or more operating temperatures provided. 
         [0024]    The suitability of ytterbium hexaboride as a selective absorber is explained in greater detail below with reference to the drawing. 
       BRIEF DESCRIPTION 
       [0025]    The aspects defined above and further aspects are apparent from the examples of embodiment to be described hereinafter and are explained with reference to the examples of embodiments. The invention will be described in more detail hereinafter with reference to examples of embodiment but to which the invention is not limited, wherein: 
         [0026]      FIG. 1  shows a reflectivity spectrum of ytterbium hexaboride (YbB 6 ), (taken with modification from “Incandescent lamp with filament consisting of a hexaboride of a rare earth material”, E. Kauer, U.S. Pat. No. 3,399,321). 
     
    
     DETAILED DESCRIPTION 
       [0027]    The reflectivity R of ytterbium hexaboride is shown as a solid curve C as a function of the wavelength λ of electromagnetic radiation in  FIG. 1 . The indication of the reflectivity R is indicated in percent on the vertical axis in  FIG. 1 . As illustrated in  FIG. 1 , at wavelengths λ (abscissa axis, plotted in μm, illustrated by the indication “[μm]” in the drawing) of less than 1.5 μm, ytterbium hexaboride has a low reflectivity R (R is less than/equal to 40% in the visible spectral range and less than/equal to 20% for wavelengths of less than 1.5 μm and greater than/equal to 1 μm). 
         [0028]    The reflectivity R is related to the emissivity E in a manner known per se by means of the relationship (1): 
         [0000]        R= 1− E    (1).
 
         [0029]    Consequently, ytterbium hexaboride has a high emissivity (emissivity greater than/equal to 60%) at wavelengths λ of less than 1.5 μm. The absorbance of ytterbium hexaboride is a variable identical to the emissivity of ytterbium hexaboride. Consequently, ytterbium hexaboride has a high absorbance at wavelengths of less than 1.5 μm. Ytterbium hexaboride therefore forms an efficient absorber in the wavelength range mentioned. 
         [0030]    As can be gathered from the drawing, for wavelengths of greater than/equal to 1.5 μm, the reflectivity (R≈80% for wavelengths of greater than/equal to 2 μm) is high and the emissivity is thus extremely low. Accordingly, ytterbium hexaboride simultaneously forms a poor, not very efficient emitter for radiation in the range of wavelengths λ of greater than/equal to 1.5 μm and in particular of wavelengths λ of greater than/equal to 2 μm, that is to say for radiation in the range of the thermal radiation. 
         [0031]    The spectral ranges of high and low reflectivity R and thus the spectral ranges of low and high emissivity and absorbance of ytterbium hexaboride, as illustrated in  FIG. 1 , are linked by a spectrally sharp emissivity edge S situated at wavelengths λ of around 1.5 μm. In the region of the emissivity edge S, the emissivity and absorbance of ytterbium hexaboride depend very greatly on the wavelength λ.