Patent Application
20140261390
The present disclosure relates generally to solar selective coatings, and, more particularly, to solar selective coatings for use in components of a solar tower system.
The present disclosure relates to a solar receiver with a wavelength selective coating comprising a first diffusion barrier layer, a metallic infrared (IR) reflective layer, a solar absorptive layer, an anti-reflective layer, and/or a hard coat protective layer.
Selective absorber coatings, which are characterized by a high solar absorption coefficient and low thermal emission, can be used in solar thermal energy applications to convert captured solar radiation into usable heat. Thin layer systems based on CERMET (ceramic/metal mixture) can be used. Such layered systems can be produced by vapor deposition or sputtering.
The layer system, starting from the substrate surface and progressing to the exterior of the coating, may include any of the following layers: a metallic IR reflective layer, a solar absorptive layer, an anti-reflective layer and a hard coat protective layer. The metallic IR reflective layer can include a metal that is highly reflective in the infrared range such as a noble metal or a refractory metal silicide. The solar absorptive layer may include a CERMET.
The CERMET may include a metal, such as platinum, nickel, palladium, tungsten, chromium or molybdenum, which is embedded in an oxide, such as Al2O3 or SiO3. The anti-reflective layer may include a pure oxide, for example SiO2 or Al2O3.
Additionally, the solar selective coating may include an adhesive layer in order to provide good adhesion of the coating to the substrate. In some embodiments, the substrate may include a carbon steel, a low alloy steel, a high alloy steel, a stainless steel or a superalloy.
Operating temperatures greater than 600° C. may accelerate the diffusion processes within the absorptive layer and through the layers of the solar selective coating. These diffusion processes act negatively on the performance of the entire system. At extremely high temperatures, elements from the substrate may diffuse into the absorber coating which may cause a change in the layer's properties. For example, iron, manganese, molybdenum, chromium, or nickel may diffuse into the layer system.
In embodiments of the disclosed subject matter, more efficient selective coatings are provided that combine relatively high solar absorbance (e.g., greater than about 0.96) and relatively low thermal emittance (e.g., less than about 0.07 at 700° C.), and that are thermally stable above 600° C., ideally in outdoor conditions. This may allow for an increase in the solar fields operating efficiencies at operating temperatures of about 600° C. or greater.
Some embodiments relate to a solar selective coating which may include the following layers in sequence: a first diffusion barrier layer, which includes at least one diffusion barrier material; a metallic IR reflective layer; a solar absorptive layer; and an anti-reflective layer. In one or more embodiments, the solar selective coating may have an absorptivity of at least 95% with respect to the AM 1.5 spectrum at long term operating temperatures of at least 600° C. The solar absorptive layer may have a thickness of between approximately 80 nm and 120 nm. The diffusion barrier material can include at least one selected from SiOx, SiN, TiO2, TiOx, a metal/AlOx CERMET and a metal/SiOx CERMET. The solar selective coating may further include a second diffusion barrier layer adjacent to the first diffusion barrier layer. One of the first and second diffusion barrier layers can include at least one selected from SiOx, SiN, TiO2 and TiOx, while the other of the first and second diffusion barrier layers can include at least one selected from a metal/AlOx CERMET and a metal/SiOx CERMET.
In some embodiments, the solar selective coating may further include a natural oxide layer of a substrate. The substrate includes at least one of a carbon steel, a low alloy steel, a high alloy steel, a stainless steel, and a superalloy.
The IR reflective layer may include at least one of a noble metal and a refractory metal silicide. The solar absorptive layer may be a CERMET layer. The ceramic portion of the CERMET may include at least one of an aluminum oxide or a silicon oxide and the metal portion of the CERMET may include at least one of Pt, Ni, Pd, W, Cr or Mo. In some embodiments, the solar selective coating may further include a third diffusion barrier layer between the IR reflective layer and the solar absorptive layer. The third diffusion barrier layer may include at least one selected from SiOx, SiN, TiO2 and TiOx. The solar selective coating may further include a hard coat protective layer. In some embodiments, the solar absorptive layer is a thick hard coat protective layer, and the solar absorptive layer may have a thickness greater than 120 nm.
Some embodiments relate to a coated metal article which may include a metal layer comprising a carbon steel, a low alloy steel, a high alloy steel, a stainless steel, or a superalloy. A solar selective coating can be provided over a surface of said metal layer. The solar selective coating may include: (a) a first diffusion barrier layer, including at least one diffusion barrier material; (b) a metallic IR reflective layer; (c) a solar absorptive layer; (d) an anti-reflective layer; and (e) a hard coat protective layer. The solar selective coating can have an absorptivity of at least 95% with respect to the AM 1.5 spectrum at long term operating temperatures of at least 600° C. The metal layer can form a conduit and the solar selective coating is provided over an external surface of the conduit. The external surface of the metal layer can be a polished surface.
In some embodiments, the coated metal article includes a portion of a solar receiver.
Some embodiments relate to a solar thermal energy system including the abovementioned coated metal article.
Objects and advantages of embodiments of the disclosed subject matter will become apparent from the following description when considered in conjunction with the accompanying drawings.
Embodiments will hereinafter be described with reference to the accompanying drawings, which have not necessarily been drawn to scale. Where applicable, some features may not be illustrated to assist in the illustration and description of underlying features. Throughout the figures, like reference numerals denote like elements.
Insolation can be used by a solar thermal system to generate solar steam and/or for heating a fluid, such as a molten salt or a gas, which may subsequently be used in the production of electricity. Referring to
The receiver in each solar tower can include one or more fluid conduits or pipes configured to convey a working fluid or heat transfer fluid at high temperatures and/or pressures. For example, the pipes can be configured to convey pressurized water and/or pressurized steam at temperatures in excess of 290° C. and pressures in excess of 160 bar. Referring to
When pipes 202 are constructed from metal, the native surface of the metal may be at least partially reflective to the solar radiation, thereby reducing the efficiency by which heat energy of the insolation is transferred to the fluid flowing through the pipes 202. The metal pipes 202 can thus be treated or painted to maximize or at least improve the solar absorption and lower thermal emission of the pipes 202. However, high-temperature operation of the solar thermal system (for example, at temperatures in excess of 600° C.) and environmental exposure (for example, to a desert atmosphere where the solar thermal system is located) may adversely affect the outer layers of the metal surface of the pipes 202, including any coating applied thereto.
In an embodiment, the metal article is a pipe 202 of a receiver 200 in a solar thermal system. For example, one or more of the coatings/treatments described herein may be applied to at least a portion of the exterior surface of pipe 202, as shown in
Pipe 202 has a metal wall 314 separating an interior volume 311 of pipe 202 from the external environment. Water and/or steam (or other heat transfer or working fluid), which may be preheated and/or pressurized, flows through the pipe interior volume. An exterior surface side 316 of the metal wall 314 can receive reflected insolation from the field of heliostats, so as to heat the metal wall 314 and thereby the flowing water and/or steam.
The substrates to which the coating is applied may be selected from one of carbon steel, a low alloy steel, a high alloy steel, a stainless steel, and a superalloy. The substrate may be planar, curved or tubular and may be employed as solar absorber tubes (e.g., pipe 202) for solar receivers.
The exterior surface side 316 of the pipe's metal wall 314 can optionally be pre-treated prior to application of any other layers. For example, the surface 316 can be subjected to grit-blasting or polishing. Predominantly thin layer systems based on CERMET (ceramic-metal mixture) are used, which are produced by various deposition methods (e.g., CVD, PVD, electron-beam deposition, etc. . . . ) or sputtering. The one or more coatings applied to the exterior surface 316 can improve absorption of solar insolation and/or protect the metal surface.
In some embodiments, the substrate exterior surface 316 may be pre-treated. For example, the pre-treating may include polishing or grit-blasting the substrate surface. After pre-treating the surface may be cleaned to remove any residue from the surface of the substrate. The substrate may then undergo heat treatment wherein a natural oxide layer may be formed on the substrate surface. The heat treatment may occur at temperatures of about 400° C., 500° C., 600° C., 650° C., 700° C. or 750° C. The natural oxide layer may aid in preventing the diffusion of the substrate into the solar selective coating.
In some embodiments, the layers of the solar selective absorber coating can be applied by at least any one of various suitable methods, such as but not limited to, a physical vapor deposition (PVD) method, a chemical vapor deposition (CVD) method, an electron beam (e-beam) method, and sputtering methods. The solar selective coating may be applied on the substrate by itself or in combination with one or more surface treatments. For example, the metal article may be provided with a substrate surface treatment such as, but not limited to, grit blasting or polishing.
There are a number of available processes which can be used to deposit coatings. The most common occur under vacuum and are classified as physical vapor deposition (PVD) and chemical vapor deposition (CVD). In PVD processes, the thin film condenses directly into the solid phase from the vapor. CVD relates to techniques where the growing film differs substantially in composition and properties from the components of the vapor phase.
Planar magnetron sputtering is a vacuum process used to deposit thin films. The process provides a plate of material of which the coating is to be made (called the target) and uses powerful magnetron magnets arranged behind the target to create a magnetic trap for charged particles, in particular the electrons, in front of the target. When the magnetron drive power supplies are turned and the target is held at a negative voltage (e.g., ˜−300V or more), across a low-pressure gas (e.g., argon at about 5 millitorr) a “plasma” is created. The plasma consists of electrons and gas ions in a high-energy state. Argon ions (or other positively charged particles) are attracted to the target surface at high speed. When the ions impact the target, atoms are knocked out of the target surface with enough energy to travel to and subsequently bond with the substrate. This process is referred to as sputtering. The sputtered atoms from the target are not negatively or positively charged, so they can travel straight out of the magnetic trap. In addition, the target surface also releases electrons, which are retained in the magnetic trap where their energy is used to produce more argon ions (or other positively charged particles). This means that the ions which are attracted to the target surface are constantly replenished, so that the magnetron can operate continually. The magnetic field vastly improves the deposition rate by maintaining a higher density of ions, which makes the electron/gas molecule collision process much more efficient.
PVD may be classified based on the methods used to produce the vapor and the energy involved in the deposition and growth of the film. In some examples, the method may include evaporation and/or sputtering.
In designing effective solar selective coatings, the thickness of the layers should be considered. For example, the solar selective coating can be applied to the external surface (or at least a portion thereof) of a pipe assembly of one or more pipes (e.g., pipe 202). For example, the coating can be provided at a thickness of between 450 nm-600 nm.
Solar selective coatings according to one or more embodiments of the disclosed subject matter can exhibit one or more of the following features:
In embodiments shown in
In embodiments shown in
In some embodiments, the solar selective coating may include a thick film layer as a diffusion barrier layer. A thick film layer may be used instead of the combination of the first diffusion barrier layer 321 and the second diffusion barrier layer 322. The thick film diffusion barrier layer may include a SiC/SiN, an enamel, a ceramic-like mixture of Al2O3 and SiO2, a thick metal layer (e.g., nickel), or a diamond hard coating. The thickness of the thick film diffusion barrier layer may be greater than 100 nm.
The embodiments of
At extremely high temperatures (e.g., between approximately 500° C. and 600° C., or higher, which may occur in solar thermal energy systems) elements from the substrate may diffuse into the solar selective coating, which may cause a change in the coating properties. For example, iron, manganese, molybdenum, chromium, or nickel may diffuse into the layer system. In order to prevent diffusion between the substrate and the absorber coating and its accompanying negative effects, at least one diffusion barrier layer may be provided. The diffusion barrier layers prevent or reduce transport and diffusion processes which may include transport from the substrate as well as gas diffusion through the substrate in solar selective coatings.
A first diffusion barrier layer 321 may include at least one of SiOx, SiN, TiO2, TiOx, a metal/AlOx CERMET and a metal/SiOx CERMET. The first diffusion barrier layer 321 may have a thickness of between 50 and 100 nm. In some embodiments, the first diffusion barrier layer 321 may have a thickness of between 50 and 80 nm.
The solar selective coating may include a second diffusion barrier layer 322. The second diffusion barrier layer 322 may be adjacent to the first diffusion barrier layer 321. The second diffusion barrier layer 322 may have a thickness of between 60 and 120 nm. In some embodiments, the second diffusion barrier layer may have a thickness of between 70 and 100 nm. In some embodiments, one of the first and second diffusion barrier layers may include at least one selected from SiOx, SiN, TiO2 and TiOx, and the other of the first and second diffusion barrier layers includes at least one of a metal/AlOx CERMET and a metal/SiOx CERMET.
The metallic IR reflective layer 223 usually includes a metal that is highly reflective in the infrared range, such as silver, platinum, nickel, palladium, tungsten, chromium or molybdenum. The IR reflective materials may include silicides, borides, carbides, and other suitable compounds of the refractory metals above. IR reflective layer 323 may also include at least one noble metal selected from the group consisting of platinum, palladium, silver, rhodium, ruthenium, indium, gold, and osmium.
CERMETs are highly solar absorbing metal-dielectric composites containing fine metal particles in a dielectric or ceramic matrix, or a porous oxide impregnated with metal. As such, CERMETs may be used as a solar absorptive layer. The solar absorptive layer 324 can include a metal, such as Pt, Ni, Pd, W, Cr or Mo, which is embedded in an oxide, such as Al2O3, SiO2.
The anti-reflective layer 325 may include a pure oxide, such as SiO2 or Al2O3. An anti-reflection coating (AR) coating is a dielectric coating applied to an optical surface to reduce the optical reflectivity of that surface in a certain wavelength range. Such properties may be achieved by introducing one or more additional optical interfaces so that the reflected waves from all the different interfaces largely cancel each other by destructive interference. In the simplest case, an antireflection coating designed for normal incidence (i.e., perpendicular to the incident surface) uses a single quarter-wave layer of a material, the refractive index of which is close to the geometric mean value of the refractive indices of the two adjacent media. By obtaining two reflections of equal magnitude from the two interfaces, the reflections cancel each other by destructive interference.
Reflection can be minimized when n1=√{square root over (nons)}, where n1 is the refractive index of the thin layer, and no and ns are the indices of the two media. Such AR coatings can reduce the reflection for ordinary glass from about 4 percent per surface to around 2 percent. Practical AR coatings rely on an intermediate layer not only for its direct reduction of reflection coefficient, but also to use the interference effect of a thin layer. If the layer's thickness is controlled precisely such that it is exactly one-quarter of the wavelength of the light (i.e., a quarter-wave coating), the reflections from the front and back sides of the thin layer will destructively interfere and cancel each other. This may significantly reduce the reflection from the surface such that most of the light is transmitted through.
Refractory metal oxide compounds (e.g., HfO2, Ta2O3, TiO2Y2O3, and ZrO2) can be used as the materials in the AR coating and absorbing layers because of their indices of refraction, their chemical, mechanical, and thermal stabilities, and their relatively high melting points. Refractory metal or metalloid oxides (e.g., SiO2, MgO, Al2O3, and Ta2O5), fluorides (e.g., AlF2, MgF2, and YF3), nitrides (e.g., TiN, TaN), and oxynitride (e.g., SiOXNY and AlOXN) compounds can also be used for AR coatings because of their low indices of refraction, and can also be used as a high-index of refraction material in both AR coating and absorbing layers.
In some embodiments, refractory and noble metals are used as an AR coating for their high melting points. Refractory transition metals are those possessing high melting points and boiling points.
Hard coatings can be used for applications where high temperature stability and excellent wear resistance are required. Coatings of a few microns thickness may be used. A hard coat protective layer, e.g., layer 326 as shown in
In embodiments, an article of manufacture can include a heat transfer member having a receiving surface, which has an absorptivity of at least 95% with respect to the AM 1.5 spectrum that is maintainable at temperatures of 600° C. for at least 1000 hours. The article can include a solar receiver and/or a heat transfer member that is part of a solar receiver. The heat transfer member can include a surface coating, e.g., a solar selective coating on the heat transfer member that defines properties of the receiving surface thereof.
The solar selective coating was prepared using the components listed in Table 1, each layer of the coating was added in the order listed in the table.
The solar selective coating was applied to a stainless steel substrate (Super 304H) which had been polished prior to the coating application. The substrate was cut into small samples and heated to a temperature of 650° C. for 30 minutes in order to form a native oxide layer. Each of the layers was applied to the substrate using a sputtering technique. The coated substrate was stored at 650° C. for 1720 hours.
The solar selective coating of Example 1 produced an absorptivity of ˜95% with respect to the AM 1.5 spectrum and an emissivity of 36.7% at 650° C. It was also shown that there was practically no decrease in reflectivity in the IR-range and the solar absorptive layer remained stable with no diffusion of the substrate into the solar absorptive layer.
The solar selective coating was prepared using the components listed in Table 2, each layer of the coating was added in the order listed in the table.
The solar selective coating was applied to a stainless steel substrate (Super 304H) which had been polished prior to the coating application. The substrate was cut into small samples and heated to a temperature of 650° C. for 30 minutes in order to form a native oxide layer. Each of the layers was applied to the substrate using a sputtering technique. The coated substrate was stored at 650° C. for 2000 hours.
The solar selective coating of Example 2 produced an absorptivity of ˜95% with respect to the AM 1.5 spectrum and an emissivity of 30% at 650° C. It was also shown that after 2000 hours at 650° C. there was a slight decrease in reflectivity in the IR-range and the solar absorptive layer remained stable with no diffusion of the substrate into the solar absorptive layer.
Although particular formulations have been discussed herein, other formulations can also be employed. Furthermore, the foregoing descriptions apply, in some cases, to examples generated in a laboratory, but these examples can be extended to production techniques. For example, where quantities and techniques apply to the laboratory examples, they should not be understood as limiting. In addition, although certain materials, chemicals, or components have been described herein, other materials, chemicals (elemental or compositions), or components are also possible according to one or more contemplated embodiments.
Features of the disclosed embodiments may be combined, rearranged, omitted, etc., within the scope of the present disclosure to produce additional embodiments. Furthermore, certain features may sometimes be used to advantage without a corresponding use of other features.
It is, thus, apparent that there is provided, in accordance with the present disclosure, high temperature radiation selective coatings and related apparatus. Many alternatives, modifications, and variations are enabled by the present disclosure. While specific embodiments have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles. Accordingly, Applicants intend to embrace all such alternatives, modifications, equivalents, and variations that are within the spirit and scope of the present invention.
The present application claims the benefit of U.S. Provisional Application No. 61/779,773, filed Mar. 13, 2013, which is hereby incorporated by reference herein in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61779773 | Mar 2013 | US |
