Radiation-selective absorber coating and absorber tube with radiation-selective absorber coating

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Radiation-selective absorber coating and absorber tube with radiation-selective absorber coating

SCHOTT Solar AG

Radiation-selective absorber coating and absorber tube with radiation-selective absorber coating

The invention relates to a radiation-selective absorber coating in particular for absorber tubes of parabolic trough collectors, comprising a layer which is reflective in the infrared range, at least one absorption layer arranged above the reflective layer, and comprising an antireflection layer arranged above the absorption layer, the reflective layer being arranged on at least two barrier layers and the first barrier layer of the at least two barrier layers consisting of a thermally produced oxide. The invention also relates to an absorber tube with such a radiation-selective coating, and to a method for operating a parabolic trough collector using such absorber tubes.

Customary absorber coatings consist of a layer which is reflective in the infrared range and is applied on a substrate, in particular a metal tube, a cermet layer having a high absorptance in the range of the solar spectrum and a covering layer applied on the cermet layer, which covering layer is referred to as an antireflection layer and, owing to the high refractive index of the cermet layer, is provided for reducing the surface reflection on the cermet layer. A fundamental endeavour is to achieve an energy yield that is as high as possible. The energy yield is dependent, inter alia, on the coefficients of the absorptance a and the emissivity ε, a high absorptance (a > 95%) and a low emissivity (ε < 10%) of the absorber coating always being striven for.

Furthermore, the efficiency of the collector is determined by the temperature at which it is operated. From this standpoint, a temperature that is as high as possible is desired. Contrary to this, the durability of -2- the layer system ofthe absorber coating decreases, however, with increasing operating temperature on account of ageing and/or diffusion processes, as a result of which, for example, the absorption property of the cermet layer and the reflection property of the layer which is reflected in the infrared range can decline significantly.

In Michael Lanxner and Zvi Elgat in SPIE Vol. 1272 Optical Materials Technology for Energy Efficiency and Solar Energy Conversion IX (1990), pages 240 to 249, entitled “Solar selective absorber coating for high service temperatures, produced by plasma sputtering”, an absorber coating applied on a steel substrate and comprising an antireflection layer composed of SiO2, a cermet layer and a layer of molybdenum which is reflective in the infrared range is described, a diffusion barrier consisting of AI2O3 being arranged between the layer which is reflective in the infrared range and the substrate. Within such a layer system, stresses exist which have an adhesion-reducing effect or lead to destruction and lumping of the layer system. DE 10 2004 010 689 B3 discloses an absorber comprising a radiation-selective absorber coating having a metal substrate, a diffusion barrier layer, a metallic reflection layer, a cermet layer and an antireflection layer. The diffusion barrier layer is an oxide layer consisting of oxidized components of the metal substrate.

Molybdenum is usually used for the layers which are reflective in the infrared range. However, the reflection properties of a molybdenum layer are not optimal, and so it is desirable to use better reflective materials.

The operating temperature of known absorber tubes is 300-40CTC in a vacuum. For the reasons above, what is fundamentally striven for is to increase the operating temperature further, but without impairing, for -3- example, the absorption properties of the cermet layer and the reflection properties of the layer which is reflective in the infrared range.

Such endeavours are summarized in C.E. Kennedy, "Review of Mid- to High-Temperature Solar Selective Absorber Materials", Technical Report of the National Renewable Energy Laboratory, July 2002 edition. This discloses a layer construction composed of a ZrOxNy or ZrCxNy absorption layer and a layer of Ag or Al which is reflective in the IR range, said layer construction having an improved thermal stability in air by virtue of the introduction of an AI2O3 diffusion barrier layer. It was furthermore ascertained that the thermal stability of the infrared reflection layer under reduced pressure can be improved by the introduction of a diffusion barrier layer below this layer. For this barrier layer, Cr2O3, AI2O3 or SiO2 are proposed as the layer material. The hope is to achieve stability of the silver reflection layer up to 500°C.

However, this does not end the striving for more durable layers in conjunction with improved absorptance and emissivity.

Therefore, DE 10 2006 056 536 A1 describes a radiation-selective absorber coating comprising at least two barrier layers, a layer which is reflective in the IR range and is arranged thereon, an absorption layer arranged above the reflective layer, and comprising an antireflection layer arranged above the absorption layer, which have a high solar absorptance and a low thermal emissivity. Although the adhesion of this absorber coating is sufficient, it is still in need of improvement.

In this context, it is an object of the invention to provide an absorber coating whose individual layers have very good adhesion, such that the absorber coating is intrinsically stable. It is furthermore an object of the invention to provide an absorber tube having such a coating and a -4- method for operating parabolic trough collectors in which such absorber tubes are used.

These objects are achieved by means of the independent patent claims, namely by virtue of the fact that the second barrier layer, which is arranged above the first barrier layer, consists of cermet material composed of at least one element from the group aluminium oxide, silicon oxide, nickel oxide, chromium oxide, and at least one element from the group molybdenum, nickel, tungsten, vanadium.

Preferably, the second barrier layer consists of cermet material composed of aluminium oxide and molybdenum.

The oxides can also be produced substoichiometrically. In the case of aluminium and molybdenum, an AlxOy/AI/Mo cermet arises where x can assume the values 1 to 2 and y can assume the values 2 to 3.

This cermet layer preferably has a thickness of 5 nm to 200 nm, particularly preferably a thickness of 10 nm to 50 nm. With thicknesses greater than 200 nm, the mechanical stresses in the layer system become so great that the layers above the adhesion-enhancing layer (25) flake off or can be stripped away with very low force in a tape test. With thicknesses which are less than 5 nm, the barrier effect, that is to say the function of the layer as a diffusion barrier layer, is no longer provided.

The cermet layer composed of aluminium oxide and molybdenum preferably has a molybdenum filling factor of 20% to 70%, particularly preferably of 30% to 50%. With a filling factor higher than 70%, the metallic proportion in the layer is too high and the barrier effect is cancelled. With a filling factor lower than 20%, problems are manifested in the adhesion of the IR-reflective layer on the barrier layer (24b). -5-

Preferably, the cermet layer as second barrier layer has a constant filling factor.

The screening of the layer which is reflective in the IR range with respect to the substrate by a two-layered barrier, in which the first barrier layer consists of a thermal oxide, e.g. contains chromium oxide and/or iron oxide, consists of e.g. chromium iron oxide, more effectively prevents diffusion, in particular thermally governed diffusion, of the substrate material, in particular of iron, from the steel absorber tube into the layer which is reflective in the IR range, and hence increases the long-term thermal stability of the coating.

By virture of the second barrier layer being formed from the cermet material described, in particular composed of aluminium oxide and molybdenum, the adhesion of the barrier layer is significantly improved by comparison with those in the prior art. By virtue of the arrangement of at least one adhesion-enhancing layer, which is preferably present, between the first and second barrier layers, the adhesion of the overall layer can be significantly improved further.

This adhesion-enhancing layer between the barrier layers comprises molybdenum. Preferably, it consists of molybdenum. This adhesionenhancing layer preferably has a thickness of 2 nm to 40 nm, particularly preferably of 5 nm to 20 nm.

The molybdenum has no optical function in this position in the layer stack. This adhesion-enhancing layer is inactive.

Preferably, the thickness of the first barrier layer of the at least two barrier layers is between 20 nm and 100 nm. With thicknesses of less than 20 nm, the barrier effect is not satisfactory, depending on the composition of the adjoining layer. With thicknesses of greater -6- than 100 nm, thermal stresses occur which, under certain circumstances, could lead to layer detachment, A third barrier layer can be arranged between the layer which is reflective in the IR range and the absorption layer, which preferably consists of cermet material, the cermet layer preferably being embodied as a gradient layer, said third barrier layer preferably consisting of an AlxOy compound, where x can assume the values 1 or 2 and y can assume the values 1,2 or 3. It preferably has thicknesses of 10 nm to 50 nm.

The embedding of the layer which is reflective in the IR range between barrier layers and the associated formation of a sandwich has the advantage that it is also impossible for any material from the layer which is reflective in the infrared range to diffuse into the overlying absorption layer and in this way to impair the absorption properties of the absorption layer. The substantial suppression of diffusion within the layer system, in particular into or from the layer which is reflective in the IR range, and into the absorption layer, can thus be ensured.

In this way, it is possible to achieve a high absorption where a > 95.5% and a low emissivity where ε < 9 % at an operating temperature of 400°C under reduced pressure. These properties remain unchanged even after accelerated ageing at 590°C over a period of 3000 hours. The efficiency of a collector comprising an absorber tube provided with this coating can thereby be improved equally from two standpoints: the improved selectivity ratio α/ε > 0.95/0.1 means a higher yield of the radiation energy, and an increased operating temperature enables more efficient conversion into electrical energy, only the long lifetime of such a coating ensuring the economic operation of a corresponding parabolic trough collector comprising absorber tubes coated in this way. -Ί -

Specifically, the high temperature resistance of the absorber coating permits the use of inexpensive heat carrier media. The high thermal stability of the absorber coating allows operating temperatures for the absorber tubes of > 450°C up to 550°C.

It is advantageously possible to use a heat carrier medium having a boiling point of < 110°C, in particular water. At such high operating temperatures, water vapour arises, which can be introduced directly into steam turbines. Additional heat exchangers for the transfer of the heat from the oil used heretofore to water are no longer required, and so, from this standpoint, parabolic trough collectors comprising absorber tubes with an absorber coating according to the invention can be operated very economically viably. A further advantage is that the flow speed of the heat carrier liquid through the absorber tubes can be reduced since a higher operating temperature is permissible without disadvantages for the absorber tube coating. In this way, it is possible to save energy for operating the pumps of a parabolic trough collector.

The embedding of the layer which is reflective in the infrared range between barrier layers has the further advantage that for said layer it is possible to use materials such as silver, copper, platinum, or gold, which, although they diffuse more readily, have the crucial advantage over molybdenum that they reflect significantly better in the infrared range, such that an emissivity ε < 10% can be achieved.

Preferably, the layer which is reflective in the IR range comprises gold, silver, platinum or copper or consists of gold, silver, platinum or copper.

The thickness of the layer which is reflective in the infrared range is preferably 50 nm to 250 nm, depending on the material. Within this - 8- thickness range, a layer thickness of 100 nm to 150 nm is preferred if, in particular, copper or silver is used. Particularly when silver is used, preference may also be given to layer thicknesses in the range of 60 nm to 150 nm, preferably 80 nm to 150 nm. 110 nm ± 10 nm is especially preferred. In other cases, layer thicknesses of 50 nm to 100 nm, in particular 50 to 80 nm, are also appropriate.

These small layer thicknesses for the layer which is reflective in the infrared range are possible because the materials gold, silver, platinum and copper have a significantly higher reflectivity and, as a result of the packing between two barrier layers, cannot diffuse away into other layers or are not impaired in terms of their positive properties as a result of the indiffusion of other, disturbing elements.

The higher cost of the noble metals Au, Ag and Pt can be compensated for, in some instances even overcompensated, by the significantly smaller layer thickness by comparison with the known layer thicknesses for the layer which is reflective in the infrared range.

The thickness of the absorption layer is preferably 60 nm to 180 nm, particularly preferably 80 nm to 150 nm. The absorption layer is preferably a cermet layer composed of aluminium oxide with molybdenum or composed of zirconium oxide with molybdenum.

Instead of a homogeneous absorption layer, it is also possible to provide a plurality of absorption layers having different compositions, in particular with decreasing metal proportion, or a gradually variable absorption layer. This cermet layer is preferably a gradient layer, which is understood to mean a layer in which the metal proportion within the layer increases or decreases continuously, and also in steps in practice. -9-

The layer thickness of the antireflection layer situated on the absorption layer is preferably 60 to 120 nm, preferably 70 nm to 110 nm. This layer preferably consists of silicon oxide or aluminium oxide.

An absorber tube, in particular for parabolic trough collectors, comprising a steel tube, on the outer side of which is arranged a radiation-selective absorber coating comprising at least one layer which is reflective in the infrared range, comprising at least one absorption layer, in particular composed of cermet material, arranged above the reflective layer, and comprising an anti reflection layer applied on the absorption layer, the layer which is reflective in the infrared range being arranged on at least two barrier layers and the first barrier layer of the at least two barrier layers consisting of a thermally produced oxide, is characterized in that the second barrier layer of the at least two barrier layers consists of cermet material composed of aluminium oxide and molybdenum.

The absorber tube preferably has radiation-selective absorber coatings in the embodiments outlined as preferred for the absorber coating.

Preferably, the cermet layer composed of aluminium oxide and molybdenum has a molybdenum filling factor of 20% to 70% and preferably of 30% to 50%.

Preferably, an adhesion-enhancing layer comprising molybdenum, preferably consisting of molybdenum, is arranged on the absorber tube between the first and second barrier layers.

With the absorber coating according to the invention and the absorber tube according to the invention, a method for operating a parabolic trough collector with absorber tubes through which a heat carrier medium is passed can be carried out, wherein absorber tubes are used

I - 10- with a radiation-selective absorber coating having at least one layer which is reflective in the infrared range, at least one absorption layer, in particular composed of cermet material, arranged above the reflective layer, and an antireflection layer arranged above the absorption layer, at least two barrier layers being arranged between the absorber tube and the reflective layer, of which barrier layers the first barrier layer, which faces the absorber tube, consists of a thermally produced oxide and a second barrier layer, which is arranged above the first barrier layer, consists of cermet material composed of aluminium oxide and molybdenum.

Heat carrier liquids having a boiling point of < 110°C, in particular water, can be used as heat carrier liquid that is passed through the absorber tubes. However, it is also possible to use heat carrier liquids having a higher boiling point.

In accordance with a further embodiment, the method for operating a parabolic trough collector provides for the operating temperature of the absorber tubes to be set to 450°C to 550°C, in particular to 480°C to 520°C.

The method for operating a parabolic trough collector is preferably operated with absorber tubes having radiation-selective absorber coatings in the embodiments outlined as preferred for the absorber coating.

Exemplary embodiments of the invention are illustrated in greater detail below with reference to the drawings.

In the figures:

Figure 1 shows a parabolic trough collector, and - 11 -

Figure 2 shows a section through an absorber tube in accordance with one embodiment of the invention.

Figure 1 illustrates a parabolic trough collector 10, which has an elongated parabolic reflector 11 with a parabolic profile. The parabolic reflector 11 is held by a support structure 12. Along the focal line of the parabolic reflector 11 there extends an absorber tube 13, which is fixed to supports 14 connected to the parabolic trough collector. The parabolic reflector 11 forms a unit with the supports 14 and the absorber tube 13, which unit is pivoted about the axis of the absorber tube 13 and thereby tracked uniaxially to the position of the sun S. The parallel solar radiation incident from the sun S is focused by the parabolic reflector 11 onto the absorber tube 13. A heat carrier medium, in particular water, flows through the absorber tube 13, the latter being heated by the solar radiation absorbed. At the outlet end of the absorber tube, the heat transfer medium can be withdrawn and fed to an energy consumer or converter.

Figure 2 schematically illustrates a section through an absorber tube 13. The absorber tube 13 has a steel tube 1, through which the heat carrier medium 2 flows and which forms the substrate for the absorber coating 20 applied on the outer side of the tube 1. The layer thicknesses of the individual layers of the absorber coating 20 are depicted as enlarged for simple illustration and with approximately equal thicknesses.

The absorber coating 20 has, from the inside outwards, a first barrier or diffusion barrier layer 24a composed of chromium iron oxide applied on the steel tube 1 by means of thermal oxidation. On this, an adhesionenhancing layer composed of molybdenum is applied. On this, between a second barrier layer 24b composed of a cermet material composed of - 12- aluminium oxide and molybdenum and a third barrier layer 24c, there is embedded a layer 21 which is reflective in the infrared range and is composed of gold, silver, platinum or copper. A cermet layer 22 is applied on the third barrier layer 24c, and the layer system terminates towards the outside with an antireflection layer 23.

The absorber tube in accordance with the embodiment of Figure 2 is coated by the method described below.

The steel tube 1, preferably a stainless steel tube, is polished and then cleaned. A surface roughness Ra of < 0.2 pm is preferably achieved during polishing. The stainless steel tube is subsequently oxidized thermally at a temperature of > 400°C for approximately half an hour to 2 hours, in particular at 500°C for approximately 1 hour. In the process, an oxide layer having a thickness of 15 nm to 50 nm, preferably 30 nm ± 10 nm, arises as the first barrier layer 24a.

Subsequently, the steel tube is introduced into a vacuum coating installation and the installation is evacuated. After a pressure of less than 5 x 10'4 mbar, preferably 1 x 10'4 mbar, has been attained, the subsequent layers are applied by means of physical vapour deposition (PVD), in particular by means of cathode sputtering. For this purpose, the steel tube is led in rotating fashion past sputtering sources, i.e. past targets consisting of the coating substances, for example Al, Si, Ag and Mo.

In the first deposition step, the adhesion-enhancing layer composed of Mo is applied by the steel tube being led in rotating fashion past the target. The layer thickness is 5 nm to 20 nm.

In the second deposition step, the second barrier layer 24b is applied in the form of a multilayer, by the tube being led through in rotating fashion - 13- between sources arranged opposite one another. A chamber pressure (argon) of between 10’2 mbar and 10‘3 mbar, preferably 4 x 10 '3 mbar to 7 x 10'3 mbar, is set in this case. Oxygen is fed to the aluminium target in order to produce aluminium oxide. The preferred layer thickness of this second barrier layer is 10 nm to 50 nm, and very particularly preferably 30 nm ± 10 nm. The molybdenum filling factor has a value of between 20% and 70% in a constant fashion over the layer thickness.

In the subsequent third deposition step, the layer 21 which is reflective in the infrared is applied, by gold, silver, platinum or copper, preferably silver, being deposited with a thickness of 60 nm to 150 nm, particularly preferably of 110 nm ±10 nm, on the second barrier layer 24b.

In the fourth deposition step, the third barrier layer 24c is applied in the form of a further SiOx- or AlxOy layer, by silicon or aluminium being sputtered and deposited reactively with supply of oxygen. The preferred layer thickness of this third barrier layer is at most 50 nm, particularly preferably 10 nm ± 5 nm. However, this barrier layer can also be completely dispensed with, since it has been found that, given a suitable composition of the absorption layer 22 applied on the reflection layer 21, diffusion does not have to be inhibited by an additional barrier.

In the fifth deposition step, the absorption layer, i.e. here the cermet layer, 22, is applied by simultaneous evaporation/sputtering of aluminium and molybdenum from one common crucible or from two separate targets. In this case, oxygen is introduced simultaneously into the sputtering region of the aluminium target in order to deposit aluminium oxide besides molybdenum.

In this case, in the fifth deposition step, the composition can be set differently and varied in the course of the layer deposition by - 14- appropriate selection of the operating parameters (sputtering rates and amount of oxygen). Particularly when separate targets are used, the deposition of the molybdenum proportion can thus be configured variably relative to the deposition of the aluminium oxide proportion in the absorption layer 22. In other words, the molybdenum proportion of the absorption layer 22 is configured as a gradient, in which case it is preferably lowered during application of the absorption layer 22. The cermet layer is thus composed of a plurality of thin individual layers which consist of aluminium oxide and molybdenum in an alternating sequence, the thickness of the molybdenum layers decreasing in the outward direction. On the inside, the molybdenum proportion is preferably 25% by volume to 70% by volume, particularly preferably 40 ± 15% by volume, and decreases in the outward direction to 10% by volume to 30% by volume, particularly preferably 20 + 10% by volume.

Oxygen is preferably added substoichiometrically in relation to the aluminium proportion deposited, such that a non-oxidized aluminium proportion remains in the absorption layer 22. This is then available as redox potential or oxygen getter, such that there is no formation of molybdenum oxide. The non-oxidized aluminium proportion in the absorption layer 22 is preferably less than 10% by volume, particularly preferably between 0 and 5% by volume, based on the overall composition of the absorption layer. The non-oxidized aluminium proportion can likewise be varied within the absorption layer by altering the operating parameters of evaporation rate and amount of oxygen.

Overall, the absorption layer 22 is preferably applied with a thickness of 60 nm to 180 nm, particularly preferably with a thickness of 80 nm to 150 nm, especially preferably with 120 ± 30 nm.

In the sixth deposition step, the antireflection layer 23 is applied in the form of an SiO2 layer, by depositing it by means of physical vapour - 15- deposition of silicon with supply of oxygen. The preferred thickness of the antireflection layer 23 thus deposited is 70 nm to 110 nm, particularly preferably 90 ± 10 nm.

An absorber tube produced in this way was heated at 550°C for 250 h in a vacuum heating apparatus. The pressure in the vacuum chamber was less than 1 χ 10'4 mbar during this heating period. After 250 h the heating was switched off. After the sample had been cooled to below 100°C, the vacuum chamber was ventilated and the sample was removed. The sample was subsequently analysed spectrometrically, in the course of which it was possible to determine an integral solar absorptance of 95.5 % ± 0.5% for an AM 1.5 direct solar spectrum and the wavelength range of 350-2500 nm. The thermal emissivity for a substrate temperature of 400°C was determined to be 8% ± 2%.

Alongside samples coated according to the method described above, both samples having no second barrier layer below the metallic refection layer (21) and samples having a pure silicon oxide layer or a pure AI2O3 layer as a second barrier layer instead of a cermet layer according to the invention were coated. After coating, pull-off tests using adhesive film strips (“adhesive tape test”) were carried out on the coatings. In the case of the samples without a second barrier layer and in the case of the samples having a pure silicon oxide layer as second barrier layer, detachment of the coating was ascertained at pull-off values of < 10 N. In the case of samples having a pure AI2O3 layer, it was possible to detect adhesion up to approximately 20 N but within 24 h cracking was manifested on account of high inherent stresses in the coating. In the case of samples produced according to the method described above, it was possible to carry out pull-off tests up to 40 N without layer detachment. These experiments were repeated with the same result after ageing of the samples for 10 h at 590°C. - 16-

The absorption coating according to the invention therefore has not only the other required properties such as high solar absorptance and low thermal emissivity but also a good adhesion of the individual layers among one another.

In particular the adhesion of the barrier layers is significantly improved by comparison with the prior art. - 17- of reference symbols

Steel tube Heat carrier liquid Parabolic trough collector Parabolic reflector Support structure Absorber tube Support

Radiation-selective absorber coating Layer which is reflective in the infrared range Absorption layer

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