Patent Application
20160003498
The present invention relates to selective solar absorbers which can be used at high temperature and under air in solar applications, such as those for the production of electricity in solar power stations or for the production of heat or hot water in dwellings.
The present invention relates to selective solar absorbers which use selective surface coatings which have to operate at high temperatures in corrosive environments, for example comprising oxygen, and to a process for manufacture of such solar absorbers.
Selective solar absorbers are much used in the solar thermal field, where they make it possible to absorb visible solar radiation and to re-emit very little infrared radiation.
As described in the report by C. E. Kennedy, entitled “Review of mid-to-high-temperature solar selective absorber material”, published in July 2002 by the National Renewable Energy Laboratory under the reference NREL/TP-520-31267 (2002), several types of selective coatings have been studied and developed for use of solar energy as source of renewable energy for various applications. The selective coating must absorb the maximum of solar radiation in the range of wavelengths between 0.3 μm and 2.5 μm approximately while re-emitting the least possible thermal infrared radiation, the wavelengths of the infrared band being greater than 2.5 μm. From an optical viewpoint, this is expressed by a minimum, indeed even zero, reflectivity, subsequently denoted by the letter R, over the solar spectrum and by a maximum, indeed even total, that is to say equal to 1, reflectivity in the infrared region, a transmission, subsequently denoted by the letter T, of zero being taken into account. The absorption, denoted by the letter α, and the thermal infrared emissivity, denoted by the letter ε, are then calculated from the respective relationships described below.
α=1-R (1)
ε=1-R (2)
On studying the spectrum of solar radiation, standardized in the band of the wavelengths between 0.3 μm and 2.5 μm approximately, and the standardized infrared emission of a black body at different temperatures, for example 300° C., 400° C. and 500° C., it is noticed that the increase in the temperature results not only in the spectrum of the black body becoming closer to solar radiation but also in an increase in the infrared intensity emitted. The solar absorbers must thus exhibit good optical selectivities and be resistant to high temperatures.
In a known way, as described in the report by C. E. Kennedy already cited or in the paper by N. Selvakumar et al., ‘Review of physical vapor deposited (PVD) spectrally selective coatings for mid- and high-temperature solar thermal applications’, Solar Energy Materials & Solar Cells, 98 (2012), 1-23, there exist different types of selective coatings predominantly composed of a stack of thin layers. The stacks can be composed simultaneously of a sequence of metal, semi-conducting and dielectric layers, which are homogeneous or composite, such as inclusion of metal particles in a dielectric matrix, for example, on a metal or non-metal substrate. The different successive layers of the coating must exhibit optical properties and have highly specific geometric dimensions for the optimization of the optical selectivity between the absorption and the infrared emissivity.
An alternative route to the stacks of thin layers is the structuring of a material in the plane in order to form a photonic crystal. A photonic crystal is par excellence the ideal means for the control of optical mode(s) in a structure. The implementations of structures of this type are mainly theoretical and limited to laboratory R&D demonstrations, as described in the paper by Eden Rephaeli et al., “Tungsten black absorber for solar light with wide angular operation range”, Applied Physics Letters, 92, 211107—2008.
Whatever the optical selectivity performance, an important criterion for making use of a selective solar absorber, more particularly in a concentrated solar power (CSP) station, is the stability maximum temperature at which it can be used. In particular, the preparation of selective absorbers which are stable at high temperature, that is to say at temperatures of greater than 400° C., and which operate in an air-comprising environment constitutes a major economic and technical challenge for concentrated solar power (CSP) stations.
The current thin-layer solar absorbers are unfortunately stable in the open air only at moderate temperature, that is to say at temperatures of less than 400° C. This temperature limitation on the operation of absorbers in the free air is related to the absence or to the design of an oxidation-resistant or oxygen-diffusion barrier which is not efficient enough to protect the selective coating with regard to an oxidizing atmosphere.
Consequently, solar absorbers are made use of under vacuum in CSP plants in order to protect them from oxidation and to allow them to operate at high temperatures, which generates an excess cost for these plants.
The technical problem is to design and manufacture absorbers, stable at high temperature, which can operate in air-comprising environments and more generally corrosive environments in which other corrosive reactions may take place, for example originating from attack by salts, acid rain and any other corrosive particles.
To this end, the subject-matter of the invention is a selective solar thermal absorber capable of operating at high temperatures in a corrosive environment.
According to the invention, a selective solar thermal absorber capable of operating at high temperatures in a corrosive environment comprises, successively stacked:
substrate composed of a first thickness of a first material;
a selective solar coating of a second thickness of a second material, configured in order to absorb a large part of the solar radiation while re-emitting as little as possible of thermal infrared radiation at the high temperatures when it is not corroded; and
a corrosion-resistant barrier layer of a third thickness of a third material;
which absorber is noteworthy in that:
the corrosion-resistant barrier layer is a thick passivation layer which is thermally stable, which has a low optical refractive index and which is optically transparent to solar radiation, the thickness being adjusted as a function of the operating temperature and of the effectiveness of the third material in order to prevent the diffusion of constituent components of the corrosive environment.
According to specific embodiments, the selective solar thermal absorber comprises one or more of the following characteristics, taken alone or in combination:
the selective solar coating exhibits a low reflectivity over the solar spectrum and a high reflectivity in the infrared in order to ensure a high absorption of the solar radiation and a low infrared emissivity, a cut-off wavelength X between an absorption region and an infrared emissivity region depending on the high operating temperature desired;
the cut-off wavelength λc is between 1 μm and 3 μm;
the refractive index of the corrosion-resistant barrier layer is less than or equal to 2 over the range of the wavelengths between 0.3 μm and 10 μm and the third thickness is adjusted so that its transmittance T is greater than or equal to 90%, preferably greater than or equal to 92% and more preferably still greater than or equal to 95% or 97%;
the corrosion-resistant barrier layer is transparent to the wavelengths of the solar spectrum between 0.3 μm and 2.5 μm and is transparent to the infrared radiation for wavelengths between 2.5 μm and at least 10 μm;
the refractive thickness of the corrosion-resistant barrier layer is greater than or equal to 0.5 μpm, for example equal to 1 μm, 2.5 μm or 5 μm, the minimum third thickness required depending on the third material, on the corrosive environment and on the operating temperature desired;
the third material from which the passivation layer is prepared is included in the group of the oxides, nitrides, oxynitrides and complex oxides:
the oxides are included in the group formed by alumina (Al2O3), silicon oxide (SiO2), zirconium oxide (ZrO2) and boron trioxide (B2O3),
the nitride is silicon nitride (Si3N4),
the oxynitrides are included in the group formed by silicon oxynitride (SiON) and aluminium oxynitride (AlON),
the complex oxides are included in the group formed by spinel (MgAl2O4), calcite (CaCO3) and mullite (3Al2O3:2SiO2);
the corrosion-resistant passivation layer is composed either of a single material forming a homogeneous corrosion-resistant barrier or of a composite material formed by a mixture of at least two materials;
the compound or compounds of the corrosion-resistant barrier layer is or are in stoichiometric proportion;
the corrosion-resistant barrier layer is SiO2 or SiOx with x other than 2, or Al2O3 or AlxOy with x/y other than 2/3;
either the second material of the selective coating is a monolayer and the material of the substrate is chosen so that the bilayer assembly formed by the substrate and the single layer of the coating exhibits the desired selectivity property, or the second material of the selective coating is a multilayer material of different materials preferably having a number of layers of greater than or equal to 3;
the selective coating consists of a stack, starting from the substrate, of thin first, second and third layers of tungsten (W), aluminium nitride (AlN) and titanium/aluminium nitride (TiAlN) and of a fourth layer of aluminium nitride (AlN);
the thicknesses of the first, second, third and fourth layers are respectively equal to 200 nm, 50 nm, 25 nm and 70 nm.
Another subject-matter of the invention is a process for the manufacture of a selective solar thermal absorber capable of operating at high temperatures in a corrosive environment.
Advantageously, this process comprises, carried out successively, the stages consisting in:
providing a substrate composed of a first thickness of a first material; then
depositing, on the substrate, a selective solar coating of a second thickness of a second material, configured in order to absorb a large part of the solar radiation while re-emitting as little as possible of thermal infrared radiation at the high temperatures when it is not corroded; then
depositing a corrosion-resistant barrier layer of a third thickness of a third material;
which process is noteworthy in that:
the corrosion-resistant barrier layer is a thick passivation layer which is thermally stable, which has a low optical refractive index and which is optically transparent to solar radiation,
and the thickness is adjusted as a function of the operating temperature and of the effectiveness of the third material in order to prevent the diffusion of constituent components of the corrosive environment.
According to a specific embodiment, in the process for the manufacture of a selective solar thermal absorber, the deposition stages are carried out using one or more deposition techniques, taken alone or in combination, included in the group of the physical vapour deposition (PVD) techniques, chemical vapour deposition techniques, jet deposition techniques and electrodeposition.
A better understanding of the invention will be obtained on reading the description of several embodiments which will follow, which embodiments are given solely by way of examples and are made with reference to the drawings, in which:
According to
The substrate 4 is composed of a first thickness of a first material.
The selective solar coating 6 is composed of a second thickness of a second material, configured in order to absorb a large part of the solar radiation while re-emitting as little as possible of thermal infrared radiation at the high temperatures when it is not corroded.
High temperatures are temperatures greater than or equal to 400° C., preferably greater than or equal to 450° C.
The corrosion-resistant barrier layer 8, composed of a third thickness of a third material, is a thick passivation layer. The third material is thermally stable, has a low optical refractive index and is non-absorbent ideally in order to render the barrier layer optically transparent to solar radiation, even when it is thick. The third layer is adjusted as a function of the operating temperature and of the effectiveness of the third material in order to prevent the diffusion of constituent components of the corrosive environment.
Thus, the effectiveness of the corrosion-resistant function depends not only on the intrinsic properties of the material (resistance to oxidation and/or to the diffusion of oxygen, for example) but also on the thickness of the protective layer, and thus the durability of the selective coating can be improved by a judicious choice of the thickness of the protective layer as a function of the operating temperature targeted.
The subject-matter of the present invention is thus the use of a thick corrosion-resistant layer which takes advantage of the influence of the geometric factor which is the thickness of the layer on the resistance to corrosion. In particular, the diffusion of oxygen up to the selective coating by any component capable of being the vector thereof (air, gas, water, steam, and the like) can be eliminated or limited in order to protect the selective coating from oxidation.
It is thus possible to produce a selective absorber with an effective thick protective layer for use at high temperatures under oxidizing atmospheres, in particular in the air.
According to the invention, the subject-matter is thus the preparation of a thick corrosion-resistant passivation layer which has a low optical refractive index and is non-absorbent ideally on a selective coating exhibiting the characteristics of a solar absorber.
The low refractive index and the transparency of the passivation layer make it possible to have a low interface reflectivity with the surrounding environment, for example ambient air, and to provide maximum transmission of the solar radiation towards the selective coating.
Under these conditions, the optical characteristic, which is the reflectivity of the complete absorber, can be approximately compared to the product of the transmission of the passivation layer and of the reflectivity of the selective coating.
By being freed from any geometric constraint related to the optical requirements for the passivation layer, the thickness of the passivation layer is henceforth set only as a function of the targeted operating temperature of the selective coating and of its effectiveness against the diffusion of oxygen and/or of other predetermined corrosive components.
The selective solar coating 6 exhibits a low reflectivity over the solar spectrum and a high reflectivity in the infrared in order to provide a high absorption of the solar radiation and a low infrared emissivity. The cut-off wavelength λc which separates the absorption region and the infrared emissivity region depends on the high operating temperature desired. For an ideal or theoretical selective absorber, the cut-off wavelength characterizes the vertical transition point between the absorption and the emissivity. For a real absorber, there is instead observed a transition region with a certain width between the absorption region and the emissivity region. The cut-off wavelength for a real absorber corresponds to the wavelength for which the reflectivity is equal to half the difference in the mean reflectivities over the absorption and emissivity ranges.
The cut-off wavelength λc is between 1 μm and 3 μm.
The refractive index of the corrosion-resistant barrier layer is less than or equal to 2, preferably less than or equal to 1.8, over the range of the wavelengths between 0.3 μm and 10 μm, and beyond preferably up to 15 μm.
The corrosion-resistant barrier layer is transparent to the wavelengths of the solar spectrum between 0.3 μm and 2.5 μm and transparent to infrared radiation for wavelengths between 2.5 μm and at least 10 μm. Preferably, the third thickness is adjusted so that its transmittance, denoted by T, is greater than at least 90%, preferably by increasing values greater than 92%, 95% or 97%.
The thickness of the corrosion-resistant barrier layer is greater than or equal to 0.5 μm for example equal to 1 μm, 2.5 μm, 5 μm or 10 μm, the minimum third thickness required depending on the third material, on the corrosive environment and on the operating temperature desired.
The third material from which the passivation layer is prepared is included in the group of the oxides, nitrides, oxynitrides and complex oxides.
The oxides are included in the group formed by alumina (Al2O3), silicon oxide (SiO2), zirconium oxide (ZrO2) and boron trioxide (B2O3).
A nitride is, for example, silicon nitride (Si3N4).
The oxynitrides are included in the group formed by silicon oxynitride (SiON) and aluminium oxynitride (AlON).
The complex oxides are included in the group formed by spinel (MgAl2O4), calcite (CaCO3) and mullite (3Al2O3:2SiO2).
The corrosion-resistant passivation layer is composed either of a single material forming a homogeneous corrosion-resistant barrier or of a composite material formed by a mixture of at least two materials, such as, for example, a mixture of oxides Al2O3/SiO2 or Al2O3/ZrO2, or of a multilayer, for example a layer of Al2O3 above a layer of SiO2.
The corrosion-resistant barrier layer is, for example, SiO2 or SiOx with x other than 2, or Al2O3 or AlxOy with x/y other than 2/3.
The base structure of the oxygen-resistant passivation at high temperatures thus differs according to whether or not the corrosion-resistant layer is an oxide. In the first case, an oxygen diffusion barrier is essentially concerned. Oxides generally constitute an oxygen diffusion barrier which protects and/or slows down oxidation of the adjacent layers. Alumina is an oxide known to act as barrier to the effective surface oxidation of the components where it is encountered in appropriate proportion. In the second case, it is a matter of producing a “firm” oxygen barrier layer with a material known for its high resistance to oxidation. This is the case, for example, with silicon nitride and with oxynitrides. In the latter case, it is agreed that the screen layer and also its oxidation product should meet the optical criteria mentioned above relating to the refractive index, which has to be low over the entire wavelength range between 0.3 μm and 15 μm, and the absorption, which has to be low, between 0.3 μm and 10 μm.
In a first configuration, the second material of the selective coating is a monolayer and the material of the substrate is chosen so that the bilayer assembly formed by the substrate and the single layer of the coating exhibits the desired selectivity property.
In a second configuration, the second material of the selective coating is a multilayer material of different materials preferably having a number of layers of greater than or equal to 3.
According to
The substrate 104 is in this instance an AISI 310 stainless steel substrate. It should be pointed out here that the nature of the substrate 104 does not matter very much in the overall approach and that the stacks provided for the solar thermal absorbers 2 and 102 can be produced on all types of substrates of pure or alloyed materials.
The selective coating 106 consists of a stack of thin layers of tungsten (W), of aluminium nitride (AlN), of titanium/aluminium nitride (TiAlN) and of a final layer of aluminium nitride (AlN). A successive stack of 200 nm of W, of 50 nm of AlN, of 25 nm of TiAln and of 70 nm of AlN was chosen for this selective coating.
The layer of tungsten (W) acts as infrared reflector, while the AlN (50 nm)/TiAln (25 nm)/AlN (70 nm) stack ensures that good selectivity is obtained between the absorption of the solar radiation and the infrared emission. The final AlN layer also provides an antireflective role which makes it possible to maximize as much as possible the absorption in the selective coating.
The corrosion-resistant barrier 108 is a passivation layer for which the third material chosen is in this instance alumina. Alumina is an oxide known to act as barrier to the effective surface oxidation of the components where it is encountered in appropriate proportion. It has a low refractive index with a mean value of approximately 1.75 and remains transparent.
The reflectivity curve 152 of the coating 106 formed by the stack W/AlN/TiAlN/AlN is compared with the reflectivity curves 154, 156, 158 and 160 of the same coating surmounted by respectively 500 nm, 1000 nm, 5 μm and 10 μm of alumina (Al2O3) in the corresponding
The selective coating consists of a stack, starting from the substrate, of first, second and third thin layers of tungsten (W), of aluminium nitride (AlN) and of titanium/aluminium nitride (TiAlN) and of a fourth layer of aluminium nitride (AlN).
According to
In a first stage 204, a substrate composed of a first thickness of a first material is provided.
Then, in a second stage 206, a selective solar coating of a second thickness of a second material is deposited on the substrate. The second material is configured in order to absorb a large part of the solar radiation while re-emitting as little as possible of thermal infrared radiation at the high temperatures when it is not corroded.
Subsequently, in a third stage 208, a corrosion-resistant barrier layer of a third thickness of a third material is deposited on the selective solar coating. The corrosion-resistant barrier layer is a thick passivation layer which is thermally stable, which has a low optical refractive index and which is optically transparent to solar radiation and the third thickness is adjusted as a function of the operating temperature and of the effectiveness of the third material in order to prevent the diffusion of constituent components of the corrosive environment.
The deposition stages are carried out using one or more deposition techniques, taken alone or in combination, included in the group of the physical vapour deposition (PVD) techniques, chemical vapour deposition techniques, jet deposition techniques and electrodeposition. The physical vapour deposition (PVD) techniques come in the form of evaporation, plasma PVD, E-beam and EB-PVD.
Chemical vapour deposition techniques come in the form of CVD, PECVD, LPCVD, and the like. Jet deposition techniques include spraying or painting techniques. These various techniques can be combined in order to take advantage of their respective characteristics for a controlled deposition of the thin layers of the selective coating, for example, and a rapid deposition of the passivation layer with greater thicknesses.
A specific example of the implementation of the invention has been described comprising a stack of W/AlN/TiAlN/AlN layers as selective coating. It is possible to envisage preparing this coating with other materials and with a different number of layers. In particular, the selective coating can be composed at least of two layers (including the substrate). The advantage of a coating of more than 2 layers, if this coating is correctly proportioned, is a better optical selectivity. This is because more degrees of freedom of control of the selectivity are available with several layers. The present inventive proposal is also applicable to solar absorbers comprising “photonic crystals”.
It is also possible to envisage a two-fold or multiple corrosive-resistant barrier according to the oxygen sources and/or the corrosive components. The effectiveness of the oxidation-resistant barrier is not the same for a material according to whether the oxygen is contributed by gas or a liquid. Thus, it is possible to have a two-layer system with one oxygen barrier layer for liquid sources, such as, for example, water vapour, and another for gas sources, such as, for example, air. It should be pointed out that, even if the structure provided was designed to increase the resistance to oxidation in particular, the design principle remains applicable for increasing the resistance of the thermal absorbers faced with other damage, such as corrosion by salts or acid rain in particular.
A key point of the present invention is to take advantage of a low refractive index of the passivation layer in order to ensure a low reflectivity of the solar radiation at the interface with the surrounding environment. In order to further minimize this reflectivity, it is possible to terminate or to surmount the thick corrosion-resistant layer with a sequence of thin layers corresponding generally to the same optical criteria as those of the said thick corrosion-resistant layer. This can be carried out, for example, by the deposition of a sequence of thin layers of different oxides or of a porous layer at the surface, which layer would have a lower effective index than the real passivation material.
In a way complementary to a protective function against corrosion in general and oxidation in particular, the thick passivation layer provides an effective thermal barrier function of the solar absorber and constitutes a complete alternative to the conventional vacuum system using a glass tube in order to provide not only the optical transmission and corrosion resistance functions but also the functions of limiting thermal losses.
It should be remembered that, in concentrated solar power stations, the rise in operating temperature is one of the main levers for increasing the output of the plants. Unfortunately, this rise in temperature is handicapped by several technological challenges, including the temperature resistance of the solar absorbers. Many available absorbing coatings, whether or not commercially available, do not withstand high temperatures (that is to say, temperatures greater than 400° C.) in air. As has already been indicated, the employment of absorbers used under vacuum in a glass protective tube, as in parabolic solar plants, represents a major capital cost. However, in order to replace the solar absorbers used under vacuum with the help of a glass tube, the following functions of the glass tube have to be carried out concurrently:
(a) providing maximum transmission of the solar radiation towards the absorber, in order for it to be converted into heat;
(b) removing the contact between the absorber and the oxygen or any corrosive component, in order to retain the thermal stability of the absorber;
(c) limiting convective thermal losses with the atmosphere by virtue of the low thermal conductivity;
(d) absorbing the energy at large wavelengths in the infrared and resulting in a greenhouse effect by absorption of the infrared radiation emitted by the absorber, thus making it possible to partially limit radiative thermal losses and to save in temperature rise time.
As has been seen, the first two functions (a) and (b) are provided by the thick passivation layer. The optical and geometric properties of the passivation layer have been adjusted in order to provide good transmission of the solar radiation towards the absorber while contributing corrosion-resistant protection. Furthermore, the optical selectivity is preserved overall by the use of the thick passivation layer according to the invention.
It should be remembered that different materials have been identified for preparing the passivation layer. The following are distinguished among these:
oxides: alumina (Al2O3), silica (SiO2), zirconia (ZrO2), boron trioxide (B2O3), and the like,
nitrides: silicon nitride (Si3N4), and the like,
oxynitrides: silicon oxynitride (SiON), aluminium oxynitride (AlON), and the like,
complex oxides: spinel (MgAl2O4), calcite (CaCO3), and the like.
Oxides or oxide compounds are also known for their low thermal conductivity. More particularly and as described in the paper by F. Cipri et al., “Electromagnetic and Mechanical Properties of Silica-Aluminosilicates Plasma Sprayed Composite Coatings”, Journal of Thermal Spray Technology, Volume 16(5-6), mid-December 2007, 831, various oxide compounds are used as thermal barriers in heat engines.
By way of example and non-exhaustively, thermal barriers based on mullite (3Al2O3:2SiO2), on alumina (Al2O3) and on yttrium-stabilized zirconium oxide are reported in Patents WO 99/048837 and US 2004/0028941 A1.
The corrosion-resistant protective layers which are not oxides, such as oxynitrides and silicon nitride, will, on the oxidizing contact with air, also form a thermal barrier layer. This results, in the event of partial oxidation of these layers, in a bilayer structure which is also advantageous both for the oxidation-resistant protective and thermal barrier functions.
The thick protective layer of our absorber is thus indirectly a thermal barrier for limiting convective losses with the atmosphere. The functions of protection against oxidation and of thermal barrier are interdependent. Schematically, the thick protective layer is equivalent to the vacuum.
In the case of the use of a glass tube, the greenhouse effect is brought about by the absorption by the glazing or the protective tube of a portion of the infrared radiation emitted by the solar absorber. This is because the glasses or oxides are transparent in the visible and a large part of the infrared but absorb at high wavelengths typically above 10 μm for silicon oxide and for alumina. The result of this is that a portion of the radiative thermal energy emitted by the absorber is reabsorbed by the glass tube, bringing about a faster rise in temperature than in the absence of this effect.
In our case of the use of a thick passivation layer, this effect is also produced as the selective coating is embedded under the passivation and protective layer. Consequently, the thick passivation layer according to the invention makes it possible to provide the four functions (a), (b), (c) and (d) of the protective tube-vacuum system described above.
The selective solar absorber according to the invention as described above can also be made use of in a vacuum system in order to relax the constraints on the system for maintaining the vacuum and/or to have a system resistant to oxidation which can be protected against other sources of corrosion and/or damage (glazing resistant to corrosion by salt, for example).
| Number | Date | Country | Kind |
|---|---|---|---|
| 14 56486 | Jul 2014 | FR | national |
