U.S. patent application number 14/203602 was filed with the patent office on 2014-09-18 for high temperature radiation-selective coating and related apparatus.
The applicant listed for this patent is BRIGHTSOURCE INDUSTRIES (ISRAEL) LTD.. Invention is credited to Ophir CHERNIN, Andreas GEORG, Wolfgang GRAF, Christina HILDEBRANDT, Thomas KROYER.
Application Number | 20140261390 14/203602 |
Document ID | / |
Family ID | 51501607 |
Filed Date | 2014-09-18 |
United States Patent
Application |
20140261390 |
Kind Code |
A1 |
CHERNIN; Ophir ; et
al. |
September 18, 2014 |
HIGH TEMPERATURE RADIATION-SELECTIVE COATING AND RELATED
APPARATUS
Abstract
A solar receiver includes a wavelength selective coating
comprising a first diffusion barrier layer, a metallic 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 to convert
captured solar radiation into usable heat. In embodiments, more
efficient selective coatings are provided that combine relatively
high solar absorbance (e.g., greater than about 0.96) with
relatively low thermal emittance (e.g., less than about 0.07 at
700.degree. C.), and that are thermally stable above 600.degree.
C., for example, in outdoor conditions. The use of such coatings in
a solar field may allow for an increase in the operating
efficiencies thereof at operating temperatures of about 600.degree.
C. or greater.
Inventors: |
CHERNIN; Ophir; (Beit
Shemesh, IL) ; HILDEBRANDT; Christina; (Freiburg,
DE) ; GEORG; Andreas; (Wildtal, DE) ; KROYER;
Thomas; (Freiburg, DE) ; GRAF; Wolfgang;
(Eschbach, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
BRIGHTSOURCE INDUSTRIES (ISRAEL) LTD. |
JERUSALEM |
|
IL |
|
|
Family ID: |
51501607 |
Appl. No.: |
14/203602 |
Filed: |
March 11, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61779773 |
Mar 13, 2013 |
|
|
|
Current U.S.
Class: |
126/710 |
Current CPC
Class: |
F24S 70/30 20180501;
F24S 20/20 20180501; F24S 70/225 20180501; Y02E 10/40 20130101 |
Class at
Publication: |
126/710 |
International
Class: |
F24J 2/46 20060101
F24J002/46; F24J 2/48 20060101 F24J002/48 |
Claims
1. A solar selective coating for use on a solar central tower, the
coating comprising, in sequence: a first diffusion barrier layer; a
metallic IR reflective layer; a solar absorptive layer; and an
anti-reflective layer; wherein said first diffusion barrier layer
comprises at least one diffusion barrier material, and the solar
selective coating has an absorptivity of at least 95% with respect
to the AM 1.5 spectrum at long term operating temperatures of at
least 600.degree. C.
2. The solar selective coating of claim 1, wherein the solar
absorptive layer has a thickness of between approximately 80 nm and
120 nm.
3. The solar selective coating of claim 1, wherein the diffusion
barrier material includes at least one selected from SiOx, SiN,
TiO.sub.2, TiOx, a metal/AlOx CERMET and a metal/SiOx CERMET.
4. The solar selective coating of claim 1, further comprising a
second diffusion barrier layer adjacent to the first diffusion
barrier layer.
5. The solar selective coating of claim 4, wherein one of the first
and second diffusion barrier layers includes at least one selected
from SiOx, SiN, TiO.sub.2 and TiOx.
6. The solar selective coating of claim 5, wherein the other of the
first and second diffusion barrier layers includes at least one
selected from a metal/AlOx CERMET and a metal/SiOx CERMET.
7. The solar selective coating of claim 1, further comprising a
natural oxide layer of a substrate on which the solar selective
coating is disposed.
8. The solar selective coating of claim 7, wherein the substrate
includes at least one of a carbon steel, a low alloy steel, a high
alloy steel, a stainless steel, and a superalloy.
9. The solar selective coating of claim 1, wherein the IR
reflective layer includes at least one of a noble metal and a
refractory metal silicide.
10. The solar selective coating of claim 1, wherein the solar
absorptive layer is a CERMET layer.
11. The solar selective coating of claim 10, wherein the ceramic
portion of the CERMET includes at least one of an aluminum oxide or
a silicon oxide and the metal portion of the CERMET includes at
least one of Pt, Ni, Pd, W, Cr or Mo.
12. The solar selective coating of claim 4, further comprising a
third diffusion barrier layer between the IR reflective layer and
the solar absorptive layer.
13. The solar selective coating of claim 12, wherein the third
diffusion barrier layer includes at least one selected from SiOx,
SiN, TiO.sub.2 and TiOx.
14. The solar selective coating of claim 1, further comprising a
hard coat protective layer.
15. The solar selective coating of claim 1, wherein the solar
absorptive layer is a thick hard coat protective layer.
16. The solar selective coating of claim 1, wherein the solar
absorptive layer has a thickness greater than 120 nm.
17. A coated metal article for use in a solar central tower, the
coated metal article comprising: a metal layer comprising carbon
steel, low alloy steel, high alloy steel, stainless steel, or a
superalloy; and a solar selective coating provided over a surface
of said metal layer, the solar selective coating comprising: a
first diffusion barrier layer; a metallic IR reflective layer; a
solar absorptive layer; an anti-reflective layer; and a hard coat
protective layer, wherein said diffusion barrier layer comprises at
least one diffusion barrier material, and the solar selective
coating has an absorptivity of at least 95% with respect to the AM
1.5 spectrum at long term operating temperatures of at least
600.degree. C.
18. The coated metal article of claim 17, wherein said metal layer
forms a conduit and the solar selective coating is provided over an
external surface of the conduit.
19. The coated metal article of claim 18, wherein the external
surface of the metal layer is a polished surface.
20. The coated metal article of claim 17, wherein the coated metal
article comprises a portion of a solar receiver of the solar
central tower.
21. The coated metal article of claim 17, wherein the solar
selective coating has an emissivity of less than about 0.07 at
700.degree. C.
22. A solar thermal energy system comprising: a coated metal
article; and at least one heliostat that reflects insolation onto
the coated metal article, wherein the coated metal article
comprises: a metal layer comprising carbon steel, low alloy steel,
high alloy steel, stainless steel, or a superalloy; and a solar
selective coating provided over a surface of said metal layer, the
solar selective coating comprising: a first diffusion barrier
layer; a metallic IR reflective layer; a solar absorptive layer; an
anti-reflective layer; and a hard coat protective layer, wherein
said diffusion barrier layer comprises at least one diffusion
barrier material, and the solar selective coating has an
absorptivity of at least 95% with respect to the AM 1.5 spectrum at
long term operating temperatures of at least 600.degree. C.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] 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.
FIELD
[0002] 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.
SUMMARY
[0003] 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.
[0004] 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.
[0005] 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.
[0006] The CERMET may include a metal, such as platinum, nickel,
palladium, tungsten, chromium or molybdenum, which is embedded in
an oxide, such as Al.sub.2O.sub.3 or SiO.sub.3. The anti-reflective
layer may include a pure oxide, for example SiO.sub.2 or
Al.sub.2O.sub.3.
[0007] 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.
[0008] Operating temperatures greater than 600.degree. 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.
[0009] 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.degree. C.), and that are thermally stable above 600.degree.
C., ideally in outdoor conditions. This may allow for an increase
in the solar fields operating efficiencies at operating
temperatures of about 600.degree. C. or greater.
[0010] 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.degree. 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,
TiO.sub.2, 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, TiO.sub.2 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.
[0011] 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.
[0012] 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, TiO.sub.2 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.
[0013] 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.degree. 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.
[0014] In some embodiments, the coated metal article includes a
portion of a solar receiver.
[0015] Some embodiments relate to a solar thermal energy system
including the abovementioned coated metal article.
[0016] 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.
BRIEF DESCRIPTION OF DRAWINGS
[0017] 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.
[0018] FIG. 1A is a simplified diagram illustrating an elevation
view of a solar thermal system with a single solar tower, according
to embodiments of the disclosed subject matter.
[0019] FIG. 1B is a simplified diagram illustrating an elevation
view of a solar thermal system with multiple solar towers,
according to embodiments of the disclosed subject matter.
[0020] FIG. 2A is a simplified diagram illustrating a top view of
pipes in a receiver of a solar tower, according to embodiments of
the disclosed subject matter.
[0021] FIG. 2B is a simplified diagram illustrating an isometric
view of the receiver pipes of FIG. 2A, according to embodiments of
the disclosed subject matter.
[0022] FIG. 3A is a simplified diagram illustrating a
cross-sectional view of one of the receiver pipes of FIG. 2A,
according to embodiments of the disclosed subject matter.
[0023] FIGS. 3B-3C are simplified diagrams illustrating
cross-sectional views of surface sections of the receiver pipe of
FIG. 3A with different coatings, according to embodiments of the
disclosed subject matter.
DETAILED DESCRIPTION
[0024] 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 FIG. 1A, a solar thermal system employing
a single solar tower is shown. The system can include a solar tower
100, which has a target 102 that receives reflected insolation 110
from a solar field 104, which at least partially surrounds the
solar tower 100. The solar tower 100 can have a height of, for
example, at least 25 m. The target 102 can be a solar energy
receiver system, which can include, for example, an insolation
receiving surface of one or more solar receivers configured to
transmit heat energy of the insolation to a working fluid or heat
transfer fluid flowing therethrough. The target 102 may include one
or more separate solar receivers (e.g., an evaporating solar
receiver and a superheating solar receiver) arranged at the same or
different heights or positions. The solar field 104 can include a
plurality of heliostats 106, each of which is configured to direct
insolation at the target 102 in the solar tower 100. Heliostats 106
within the solar field can adjust their orientation to track the
sun 108 as it moves across the sky, thereby continuing to reflect
insolation onto one or more aiming points associated with the
target 102. The solar field 104 can include, for example, over
50,000 heliostats deployed in over an area of approximately 4
km.sup.2.
[0025] FIG. 1B shows a "multi-tower" version of a solar thermal
system. Each tower can have a respective target, which may include
one or more solar receivers. The first solar tower 100A has a
target 102A thereon and is at least partially surrounded by solar
field 104 for receiving reflected insolation therefrom. Similarly,
a second solar tower 100B has a target 102B thereon and is at least
partially surrounded by solar field 104 for receiving reflected
insolation therefrom. For example, the solar receiver in one of the
towers may be configured to produce steam from insolation (i.e., an
evaporating solar receiver) while the solar receiver in another one
of the towers may be configured to superheat the steam using
insolation (i.e., a superheating solar receiver). In another
example, one or more of the solar towers may have both an
evaporating solar receiver and a superheating solar receiver. A
limited number of components have been illustrated in FIGS. 1A-1B
for clarity and discussion. It should be appreciated that actual
embodiments of a solar thermal system can include, for example,
optical elements, control systems, sensors, pipelines, generators,
and/or turbines.
[0026] 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.degree.
C. and pressures in excess of 160 bar. Referring to FIGS. 2A-2B, an
exemplary configuration of a portion 200 of a solar receiver is
shown. Pipes 202 of the receiver portion 200 can be arranged in a
single row following a particular geometric configuration, for
example, in the shape of a circle, hexagon, or rectangle (as shown
in FIG. 2A), or in any other suitable configuration. At least a
portion of the exterior surface of each pipe 202 can be arranged to
receive insolation reflected by heliostats in the solar field onto
the receiver. The solar insolation can heat pipes 202 and thereby
heat the fluid flowing therethrough for use in producing
electricity or in other applications.
[0027] 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.degree. 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.
[0028] 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 FIGS.
3A-3C. FIGS. 3B-3C show a close-up cross-sectional view 312 of pipe
202 of FIG. 3A, illustrating solar selective absorber coatings with
(as shown in FIG. 3B) and without a top hard coat protective layer
(as shown in FIG. 3C) applied to the wall of pipe 202. It is noted
that the layers illustrated in figures have not been drawn to
scale. Rather, the relative sizes of the layers have been
exaggerated for illustration purposes.
[0029] 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.
[0030] 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.
[0031] 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.
[0032] 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.degree. C., 500.degree. C., 600.degree. C., 650.degree. C.,
700.degree. C. or 750.degree. C. The natural oxide layer may aid in
preventing the diffusion of the substrate into the solar selective
coating.
[0033] 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.
[0034] 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.
[0035] 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.,
.about.-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.
[0036] 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.
[0037] 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.
[0038] Solar selective coatings according to one or more
embodiments of the disclosed subject matter can exhibit one or more
of the following features: [0039] (1) the solar selective coating
has an absorptivity with respect to the AM 1.5 spectrum of greater
than 95% at operating temperatures which may exceed 600.degree. C.;
[0040] (2) the coating applied to a metal article (e.g., carbon
steel, low alloy steel, high alloy steel, stainless steel,
superalloy) has sufficient thermal durability (i.e., does not
ablate over time) to withstand high temperatures (e.g., at least
550.degree. C., 600.degree. C., 650.degree. C., or higher) over a
sustained period of time (i.e., hundreds or thousands of
consecutive hours under accelerated exposure conditions, for
example, at least 1000 hours); and [0041] (3) the solar selective
coating is applied to the metal article at a thickness of between
approximately 450 nm-600 nm.
[0042] In embodiments shown in FIG. 3B, the solar selective
absorber coating 320 is composed of the following layers in
sequence from the outer surface 316 of the pipe 202 toward the
exterior: a first diffusion barrier layer 321, a second diffusion
barrier layer 322, a metallic IR reflective layer 323, a solar
absorptive layer 324, an anti-reflective layer 325, and a hard coat
protective layer 326. Predominantly thin layer systems based on
CERMET (ceramic-metal mixture) can be used, which are produced by
vapor deposition or sputtering. The first and/or second diffusion
barrier layer may be a thin film layer. Thin film layers may be
described as those layers which have a thickness of less than 100
nm. The layers mentioned in this example may have compositions as
described hereinbelow.
[0043] In embodiments shown in FIG. 3C, the solar selective
absorber coating 330 is composed of the following layers in
sequence from the outer surface 316 of the pipe 202 toward the
exterior: a first diffusion barrier layer 321, a second diffusion
barrier layer 322, a metallic IR reflective layer 323, a solar
absorptive layer 324, and an anti-reflective layer 325. In this
embodiment, the solar absorptive layer may act as a thick hard coat
layer, thereby providing protection to the coating as well as the
substrate.
[0044] 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 Al.sub.2O.sub.3 and SiO.sub.2, 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.
[0045] The embodiments of FIGS. 3B-3C, or variations thereof, may
also modified to include a third barrier diffusion layer (not
shown) between the IR reflection layer (e.g., 323) and the solar
absorptive layer (e.g., 324). The third diffusion barrier layer may
be one of SiOx, SiN, TiO.sub.2, TiOx, a metal/AlOx CERMET and a
metal/SiOx CERMET.
[0046] At extremely high temperatures (e.g., between approximately
500.degree. C. and 600.degree. 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.
[0047] A first diffusion barrier layer 321 may include at least one
of SiOx, SiN, TiO.sub.2, 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.
[0048] 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, TiO.sub.2 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.
[0049] 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.
[0050] 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 Al.sub.2O.sub.3,
SiO.sub.2.
[0051] The anti-reflective layer 325 may include a pure oxide, such
as SiO.sub.2 or Al.sub.2O.sub.3. 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.
[0052] Reflection can be minimized when n.sub.1= {square root over
(n.sub.on.sub.s)}, where n.sub.1 is the refractive index of the
thin layer, and n.sub.o and n.sub.s 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.
[0053] Refractory metal oxide compounds (e.g., HfO.sub.2,
Ta.sub.2O.sub.3, TiO.sub.2Y.sub.2O.sub.3, and ZrO.sub.2) 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., SiO.sub.2, MgO,
Al.sub.2O.sub.3, and Ta.sub.2O.sub.5), fluorides (e.g., AlF.sub.2,
MgF.sub.2, and YF.sub.3), nitrides (e.g., TiN, TaN), and oxynitride
(e.g., SiO.sub.XN.sub.Y and AlO.sub.XN) 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.
[0054] 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.
[0055] 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 FIG. 3B, may include
oxides, nitrides, carbides, borides or carbon. In some examples,
the hard coat protective layer may include ZrN, TiN, AlTiN,
CrN.
[0056] 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.degree. 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.
Example 1
[0057] 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.
TABLE-US-00001 TABLE 1 Layer Component Layer Thickness (nm)
1.sup.st Diffusion Barrier Layer SiN 60 nm 2.sup.nd Diffusion
Barrier Layer AlOx--Pt 88.5 nm IR reflective layer Pt 240 nm
3.sup.rd Diffusion Barrier Layer SiN 20 nm Solar Absorptive Layer
AlOx--Pt 112 nm Anti-Reflective Layer SiOx 80 nm
[0058] 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.degree. 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.degree. C. for 1720 hours.
[0059] The solar selective coating of Example 1 produced an
absorptivity of .about.95% with respect to the AM 1.5 spectrum and
an emissivity of 36.7% at 650.degree. 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.
Example 2
[0060] 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.
TABLE-US-00002 TABLE 2 Layer Component Layer Thickness (nm) 1st
Diffusion Barrier Layer SiN 80 nm IR reflective layer Pt 240 nm 3rd
Diffusion Barrier Layer SiOx 20 nm Solar Absorptive Layer AlOx--Pt
80 nm Anti-Reflective Layer SiOx 100 nm
[0061] 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.degree. 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.degree. C. for 2000 hours.
[0062] The solar selective coating of Example 2 produced an
absorptivity of .about.95% with respect to the AM 1.5 spectrum and
an emissivity of 30% at 650.degree. C. It was also shown that after
2000 hours at 650.degree. 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.
[0063] 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.
[0064] 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.
[0065] 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.
* * * * *