Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F24—HEATING; RANGES; VENTILATING
  • F24S—SOLAR HEAT COLLECTORS; SOLAR HEAT SYSTEMS
  • F24S10/00—Solar heat collectors using working fluids
  • F24S10/40—Solar heat collectors using working fluids in absorbing elements surrounded by transparent enclosures, e.g. evacuated solar collectors
  • F24S10/45—Solar heat collectors using working fluids in absorbing elements surrounded by transparent enclosures, e.g. evacuated solar collectors the enclosure being cylindrical
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F24—HEATING; RANGES; VENTILATING
  • F24S—SOLAR HEAT COLLECTORS; SOLAR HEAT SYSTEMS
  • F24S20/00—Solar heat collectors specially adapted for particular uses or environments
  • F24S20/20—Solar heat collectors for receiving concentrated solar energy, e.g. receivers for solar power plants
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F24—HEATING; RANGES; VENTILATING
  • F24S—SOLAR HEAT COLLECTORS; SOLAR HEAT SYSTEMS
  • F24S70/00—Details of absorbing elements
  • F24S70/20—Details of absorbing elements characterised by absorbing coatings; characterised by surface treatment for increasing absorption
  • F24S70/225—Details of absorbing elements characterised by absorbing coatings; characterised by surface treatment for increasing absorption for spectrally selective absorption
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F24—HEATING; RANGES; VENTILATING
  • F24S—SOLAR HEAT COLLECTORS; SOLAR HEAT SYSTEMS
  • F24S80/00—Details, accessories or component parts of solar heat collectors not provided for in groups F24S10/00-F24S70/00
  • F24S80/50—Elements for transmitting incoming solar rays and preventing outgoing heat radiation; Transparent coverings
  • F24S80/52—Elements for transmitting incoming solar rays and preventing outgoing heat radiation; Transparent coverings characterised by the material
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E10/00—Energy generation through renewable energy sources
  • Y02E10/40—Solar thermal energy, e.g. solar towers
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E10/00—Energy generation through renewable energy sources
  • Y02E10/40—Solar thermal energy, e.g. solar towers
  • Y02E10/44—Heat exchange systems
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10S126/00—Stoves and furnaces
  • Y10S126/907—Absorber coating

Definitions

• the invention relates to a solar absorber with an absorber body, which has a side facing a concentrator and a side facing away from the concentrator and which is provided with a selective absorption layer which absorbs the spectrum of solar radiation below a cut-off wavelength and suppressed the emission of the absorber body above the cut-off wavelength.
• the invention also relates to solar absorbers according to the preambles of claims 13, 17 and 22.
• the solar absorber is the part of a solar collector, where the solar radiation is converted into heat. At the absorber occur in addition to optical and thermal losses, since it assumes a higher temperature than the environment by converting the solar radiation into heat.
• the absorbers In concentrating collectors, such as parabolic trough collectors, Fresnel collectors, and solar towers, the absorbers are typically heated to a few hundred C ° C.
• the radiation losses which according to Planck's radiation formula are proportional to the fourth power of the temperature, dominate the convective losses in the case of vacuum-isolated absorbers.
• the technical problem lies in the contradictory desire that the absorber on the one hand to absorb the solar radiation well, on the other hand, but the emission of bad radiation.
• the spectral absorption and emissivity of a body are equal.
• This problem can be solved by an optical selective coating having a cut-off wavelength below which the absorptivity is large and above which the emitted natural radiation is low.
• Such an absorption layer makes use of the fact that the wavelength range for the radiation to be absorbed and the emitted emitted radiation of the absorber to be suppressed are different.
• absorber body which have an optically selective coating. These coatings are characterized by the fact that on the one hand they absorb the short-wave radiation emitted by the sun (maximum at 0.5 ⁇ m) well, but on the other hand the long-wave radiation emitted by the absorber (maximum at 3.5 to 5 ⁇ m) to the environment is poor emit.
• the selective absorption layers have a low spectral reflectance and a high absorptivity in the short-wave solar spectral range, while in the long-wave spectral range the spectral reflectivity is as high as possible, which means low emissivity.
• the ideal cut-off wavelength at which the transition from low to high reflectivity occurs is primarily dependent on the operating temperature of the absorption layer. Therefore, various selective layers are being developed for flat plate collectors, evacuated tube collectors and parabolic trough collectors.
• the cut-off wavelength of the ideal optically selective absorption layer When designing the optimum cut-off wavelength of the ideal optically selective absorption layer, only the temperature of the absorber has hitherto been considered. Depending on the temperature, the cut-off wavelength is adapted to a standard solar spectrum. The ideal cutoff wavelength is also dependent on the concentration of solar radiation. For concentrating solar collectors, the concentration factor is often not homogeneously distributed over the absorber surface. So is only about half of a parabolic trough collector of the absorber tube hit by concentrating solar radiation, while the other half is illuminated by non-concentrated solar radiation. On the side facing the concentrator, the radiation is typically concentrated by factors of 10 to 100 times. In the case of a selective absorption layer which is homogeneous over the circumference of the absorber body, the thermal losses due to radiation are therefore not minimized.
• the object of the invention is to minimize the radiation losses altogether in the case of a concentrating solar absorber, namely both on the side facing the concentrator and on the side facing away from the concentrator in order to increase the gain of trapped heat.
• the solutions provide an absorption layer on the absorber body, a coating on the cladding tube and at least one element between absorber tube and cladding tube, which will be described in detail below. It is also possible to combine the solutions together, as is the subject of claim 22.
• the absorption layer of the absorber body on the side facing the concentrator has a cutoff wavelength which is greater than the cutoff wavelength on the side facing away from the concentrator.
• the invention is based on the recognition that the two opposite sides of the absorber body are exposed to different radiation densities and thus have different absorption behavior and emission behavior.
• the cut-off wavelength On the side facing the concentrator, the cut-off wavelength z. B. 2.350 nm. Due to the different cut-off wavelengths on both sides, it is achieved that the cut-off wavelength is tuned to the respective spectral density, so that the gain of the incident radiation is optimized as a function of the spectral density. Thus, a very high yield (gain) of radiation can be achieved.
• a concentrator is to be understood as any concentrating optic.
• Fresnel lenses, mirrors, lens systems or collection optics are concentrators.
• the coating is divided into two areas with different absorption layers.
• the subdivision can be refined by providing more than two regions of stepwise varying cut-off wavelength.
• Each of the areas is adapted to the local radiation intensity.
• the invention is particularly applicable to parabolic trough collectors having an elongate absorber tube which is supported by a heat transfer medium, e.g. As oil is flowed through, with an elongated parabolic mirror concentrates the solar radiation on the absorber tube. Also in other types of solar absorbers receiving concentrated solar radiation, the invention is applicable. It is suitable for all cases in which the radiation intensity at different surface areas of the absorber body is different.
• the absorber body is preferably a tube, in particular a steel tube or a porous ceramic body.
• the thickness of the absorption layer is preferably less than 10 ⁇ m, in particular less than 200 nm.
• the absorber layer is preferably subdivided into two regions with different cut-off wavelengths. Here, the one area extends over the entire side facing away from the concentrator and the second area extends over the entire side of the absorber body facing the concentrator.
• a region is understood to mean a surface that extends on the one hand over the pipe length and on the other hand over a circular arc. Several areas are adjacent to each other in the circumferential direction of the tube.
• the absorber layer may be subdivided into regions of continuously varying cut-off wavelength.
• the transition from one area to an adjacent area can be configured continuously. Even within the ranges, the cut-off wavelength can change continuously.
• the setting of different cut-off wavelengths in the individual regions can be achieved by a different thickness of the absorber layer or of individual partial layers of the overall layer system.
• layer thickness changes by 5% to 20%, preferably by 10% to 20% are preferred.
• the layer thickness decrease can be carried out running around the entire pipe circumference, which manufacturing technology, for example. can be achieved by varying the rotational speed of the tube during the coating process.
• the absorber layer is applied on the side facing away from the concentrator by 5 to 20% thinner than on the side facing the concentrator. This shifts the cut-off wavelength, in which the absorber layer of absorption and Reflection passes to shorter wavelengths. The emissivity for high temperatures is thereby reduced. At the same time, the solar absorption decreases as a larger part of the solar spectrum is reflected.
• the absorber layer may be composed of a three-layer system consisting of a metallic mirror layer, a cermet layer and a dielectric anti-reflection layer.
• the thickness variation for changing Grenzwellenlä ⁇ ge is preferably carried out in the cermet layer.
• the absorption coefficient By reducing the cermet layer thickness, the absorption coefficient also decreases in the region of the visible spectrum.
• By increasing the thickness of the anti-reflection layer this can be partially compensated without affecting the cut-off wavelength. Therefore, it makes sense to increase the thickness of the antireflection coating when reducing the cermet thickness.
• a variation of the layer thickness can also be carried out by performing a non-uniform rotation of the tube in individual or in all coating processes.
• the absorber layer may have different compositions in the individual regions.
• the different composition is preferably achieved by metallic filler that is present in different proportions in the individual areas.
• This metallic filler may preferably be present in the cermet layer.
• chromium, aluminum, copper and / or molybdenum are preferred.
• the proportions of the filling material in the cermet layer are in the individual ranges between 20% and 60%.
• the concentrator has a high reflection in the entire wavelength range between the visible and the infrared range.
• the thickness and / or composition of the cermet layer and / or the antireflection coating can influence the proportion of the absorbed radiation in the visible or in the near infrared range.
• the solar absorber has an absorber tube and a cladding tube which surrounds the absorber tube, wherein in combination with a concentrator the solar absorber has a side facing the concentrator and a side facing away from the concentrator. It is provided according to the invention that at least one region of the cladding tube is provided on the side facing away from the concentrator with a coating reflecting the thermal radiation. This coating can be partially transparent in the wavelength range of solar radiation (low-e coating) or reflective (metallic mirror).
• This coating can extend over a maximum of half the circumference of the cladding tube.
• the coating is preferably mounted on the inside of the cladding tube.
• the coating is provided in two lateral edge regions, between which no coating is arranged.
• This makes it possible to use the proportion of direct solar radiation incident in the section between the edge regions.
• the proportion that comes directly from the sun and the absorber directly, that means without deflection by the concentrator, makes in conventional parabolic trough collectors about 1% of the total detectable radiation.
• the lateral edge areas are those areas in which neither direct radiation from the sun nor reflected radiation from the concentrator impinges. It therefore makes sense to provide a corresponding coating in these edge regions in order to reduce heat losses in this region.
• a complete coverage by means of a transparent or metallic coating of the entire region of the cladding tube, which faces away from the concentrator, may be useful if the operating temperature of the solar absorber is high. At lower operating temperatures, the coating in the peripheral areas is preferred.
• Partial transparency is understood as meaning a transmission of preferably 30% to 70%, the transmission relating to the wavelength range of the solar radiation in the range from 300 nm to 2500 nm.
• the preferably metallic coating is highly reflective, which is understood to mean a reflectance of> 70%.
• the coating is preferably made of a high reflectance material.
• the material used for this coating is preferably aluminum or silver.
• the invention is between the cladding tube and the absorber tube on the side facing away from the concentrator arranged at least one element of a material reflecting in the IR region.
• This element may be a metallic foil, a fabric or a sheet.
• the arrangement of one or more such elements can be selected according to the coating of the cladding tube.
• Absorber tube extend. But it can also be provided two elements which are arranged in lateral edge regions, according to the configuration of the coating of the cladding tube in the edge regions.
• the element or elements may have raised edges, thereby providing an additional concentration effect.
• the erected edges preferably have an angle ⁇ 45 ° with respect to the axis of the concentrator.
• the element or elements are preferably made of aluminum or stainless steel. To increase the reflection in the infrared wavelength range, the element may additionally be coated with silver.
• Fig. 1 is a schematic representation of a solar absorber
• Fig. 3 is a diagram of the spectral radiance
• Fig. 6 is a partial cross-section through that shown in Fig. 5
• FIGS. 7-11 Solar absorber with absorber tube and cladding in section according to various embodiments.
• Fig. 1 shows the absorber body 10, which is an elongated absorber tube 30, the interior 11 is flowed through by a heat transfer medium.
• the incident solar radiation 12 is concentrated or focused by a concentrator 13 in the form of a parabolic mirror on the absorber body 10.
• the absorber body 10 absorbs the solar radiation and converts these into heat, which is transferred to the heat transfer medium.
• the heat transfer medium is thereby heated.
• Fig. 2 shows the distribution of the radiation density 14 over the circumference of the absorber body in polar coordinates.
• the radiation density On the concentrator 13 facing side 36, the radiation density is high.
• the radiation density On the side facing away from the concentrator 38, the radiation density is equal to the radiation density of the incident solar radiation, which could be denoted by the value "1".
• Fig. 3 the spectral distribution of the radiation density of the incident solar radiation is indicated by the curve denoted by 20.
• 21 is a curve indicating the spectral radiance of a black body at a temperature of 500 0 C.
• the two curves 20 and 21 each have a maximum and fall from there to both sides. Both curves are separated, but overlap in one foot area.
• Denoted at 22 is the ideal cut-off wavelength that passes through the intersection of the solar spectrum 20 and the blackbody spectrum 21. Below this wavelength, the ideal optically selective absorber completely absorbs the solar radiation while emitting only minor radiation losses. These radiation losses consist of the area which is below the cut-off wavelength of 1350 nm below the curve 21. Above the cut-off wavelength 22, the degree of absorption - and thus the emissivity search - equal to zero. This means that the absorption body 10 does not radiate heat, while it loses only slightly solar irradiation by reflection. This radiation loss is proportional to the area under the curve 20 for wavelengths ⁇ > greater than 1350 nm. FIG.
• FIG. 4 shows the solar spectrum 20a concentrated by the factor 50 and the curve 21a the spectrum of a black body at 500 ° C.
• the scale of the graph of FIG. 4 has been changed with respect to FIG.
• the invention takes advantage of this fact by the absorber body 10, which is designed as an absorber tube 30, according to FIG. 5 different absorber layers 17, 18 has.
• the absorber layer 17 in the region 40 is located on the side facing the concentrator 13 and the absorber layer 18 in the region 45 on the side 38 facing away from the concentrator.
• the absorber layers 17 and 18 are thin layers in the nanometer range. They consist of materials as described, for example, in WO 97/26488. In particular, they are Ti-NO-based interference absorber layers which cause different color effects as a function of the layer thickness.
• the absorption layers 17, 18 can basically consist of the same base material, the individual regions differing by different layer thicknesses. By interference effect for the areas different cut-off wavelengths 22.
• the thickness of the absorption layer should be smaller than 10 microns and in particular smaller than 1000 nm, most preferably smaller than 100 nm.
• the absorber layer is homogeneous applied.
• the solar absorber as a whole has a high absorption capacity over all ranges of different radiation density and provides a high gain of incident radiation.
• the absorber layers 17 and 18 in FIG. 5 can also both have the same layer structure.
• the layers are preferably produced by a thin-film technique in which several layers are applied successively to a substrate.
• a mirror layer 25 a cermet layer 26 and a dielectric antireflection coating 27 can be deposited on a steel tube 31 (see Fig. 6).
• the mirror layer 25 reflects electromagnetic radiation in the infrared and visible wavelength range.
• the thickness and / or composition of Cermet and anti-reflection layer 26, 27 influences the proportion of absorbed radiation.
• a filler is contained, which usually constitutes 20% to 60% of the cermet layer 26.
• filler in particular chromium, aluminum, copper or molybdenum in question.
• the optical properties of the absorber layer are varied by varying the layer thickness and / or composition in regions 17 and 18.
• the absorber layer 18 is made thinner by 5 to 20% than on the absorber layer 17 facing the concentrator.
• the thickness is preferably not changed.
• the change of the absorber layer is achieved solely by varying the cermet and anti-reflection coatings.
• the absorber layer 18 facing away from the concentrator 13 comprises at most half the pipe circumference.
• the decrease in the layer thickness can be achieved, for example, by varying the rotational speed of the tube during the coating process.
• a further embodiment which has an absorber tube 30 and a cladding tube 32, wherein on the inside of the cladding tube 32, a coating 60 is applied.
• the preferably metallic coating 60 is located in the region 55 of the cladding tube, which faces away from the concentrator, not shown. In the region 50 of the cladding tube 32, which faces the concentrator, no coating is provided on the cladding tube 32.
• Fig. 8 a modified embodiment is shown, which differs from the embodiment shown in Fig. 7 in that the coating 60 is applied only in an edge region 56, 57, on which neither the direct sunlight, by the Arrows is marked, still hits the reflected radiation from the concentrator.
• an opaque or partially transparent coating is provided, so that in this region 58 the solar radiation is transmitted and can impinge on the absorber tube 30.
• the partial transparency of the metallic coating in the region 58 is present in the wavelength range of solar radiation 3000 nm to 2500 nm.
• the proportion of the transmitted radiation of the total incident radiation is in the spectrum of sunlight preferably about 30% to 70%.
• the coating is highly reflective, i. the percentage of reflected radiation is more than 70%.
• an area of the cladding tube 20 can be made uncoated.
• the uncoated area forms as it were a window for the incident on the opposite side 38 of the cladding tube 32 solar radiation. This ensures that this solar radiation can be used to generate heat.
• an element 70 is arranged, which consists of a reflecting material in the infrared region.
• This element 70 extends over half of the circumference and is located in the region which faces away from the concentrator.
• the element 70 consists of a metallic foil, which is preferably made of aluminum or stainless steel. To increase the reflectivity in the IR wavelength range can also be used as a film coated with silver materials.
• the metallic foil is thermally coupled to the absorber tube or to the cladding tube. This prevents the metallic foil from heating up locally and consequently changing its shape or position in the annular gap as a result of thermal expansion. In operation, the absorber tube 30 is heated so that it expands.
• the metal foil itself is expansible and movable, so that differences in length between the absorber tube 30 and the metal foil are compensated.
• FIG. 11 shows a further embodiment which, in the space between the absorber tube 30 and the cladding tube 32, has two elements 71, 72 which are arranged in the edge regions 76 and 77, which are not affected by the radiation reflected by the concentrator direct solar radiation are taken.
• edges 78 are provided at the edges, which form an angle of 45 ° with the concentrator axis (not shown).
• Cladding tube the concentrator facing side facing away from the concentrator side

Landscapes

• Engineering & Computer Science (AREA)
• Physics & Mathematics (AREA)
• Life Sciences & Earth Sciences (AREA)
• Sustainable Development (AREA)
• Sustainable Energy (AREA)
• Thermal Sciences (AREA)
• Chemical & Material Sciences (AREA)
• Combustion & Propulsion (AREA)
• Mechanical Engineering (AREA)
• General Engineering & Computer Science (AREA)
• Spectroscopy & Molecular Physics (AREA)
• Optical Elements Other Than Lenses (AREA)

Applications Claiming Priority (2)

Publications (1)

Family

ID=35427271

Family Applications (1)

Country Status (7)

Families Citing this family (24)

Citations (1)

Family Cites Families (56)

• 2004
  • 2004-08-05 DE DE102004038233A patent/DE102004038233A1/de not_active Ceased
• 2005
  • 2005-08-05 US US11/573,117 patent/US7607428B2/en not_active Expired - Fee Related
  • 2005-08-05 MX MX2007001448A patent/MX2007001448A/es active IP Right Grant
  • 2005-08-05 EP EP05779044A patent/EP1776550A1/de not_active Withdrawn
  • 2005-08-05 CN CN2005800263947A patent/CN101023305B/zh not_active Expired - Fee Related
  • 2005-08-05 WO PCT/EP2005/008522 patent/WO2006015815A1/de active Application Filing
• 2007
  • 2007-02-01 IL IL181111A patent/IL181111A/en not_active IP Right Cessation

Patent Citations (1)

Also Published As

Similar Documents

Legal Events