Title of Invention

A SOLAR SELECTIVE MATERIAL FOR AN ABSORBER

Abstract The present invention provides a solar absorptive material for a solar selective surface of an absorber of solar radiation. The solar absorptive material comprises a dispersed metallic material and a receiving boundary through which the solar radiation is received. Further, the solar absorptive material comprises a first region and a second region. The first region being located at a position closer to the receiving boundary than the second region and the first region has an average volume fraction of the dispersed metallic material that is larger than that of the second region.
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A SOLAR ABSORPTIVE MATERIAL FOR A SOLAR SELECTIVE SURFACE
COATING
Field of the Invention
The present invention broadly relates to a solar
absorptive material for a solar selective surface coating.
Background of the Invention
In order to increase the efficiency of solar
absorbers, solar selective coatings are applied to the
solar absorbers. Such coatings increase absorbance of
solar radiation in a spectral range in which the solar
radiation has high intensity and reduce loss of energy in
the infrared spectral range.
Solar selective coatings for solar absorbers
typically comprise a metallic layer on an absorber body, a
solar absorptive coating and a top layer. Alternatively,
the solar absorptive coating may be positioned directly on
a metallic absorber body. The absorptive layer typically
comprises a metallic component and a non-metallic
component, such as a dielectric component. Typically the
metallic material forms islands in the dielectric material
so that a Cermet material is formed. The top layer has a
metal concentration that is lower than that of the solar
absorptive layer or is free of metal. Such a solar
selective coating absorbs solar radiation while the
emission of infrared radiation is reduced compared with,
for example, a uniform metallic coating.
Solar absorptive coatings typically have a non-
uniform metal volume fraction. Figure 1 show plots of
metal volume fraction versus depth from an outer boundary
for an exemplary selection of such solar absorptive
coatings. The figure shows a plot 10 for a coating having

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a graded metal volume fraction, a multilayer structure 12,
and a combination of graded and multilayer profiles 14.
For example, a solar energy reflector array may be
used to collect sunlight which is then focused onto the
absorber coated with the solar selective surface coating.
The collected sunlight heats the absorber and the solar
selective coating locally to relative high temperatures
such as 350°C. In order to increase the lifetime of the
solar selective surface coating at such high temperatures,
and to reduce thermal losses of the absorber, the absorber
may be positioned in an evacuated housing.
It is known that the conversion efficiency of the
energy from the collected sunlight is better at even
higher temperatures, but further increase of the
temperature may have a substantial negative impact on the
lifetime of the solar selective coatings.
The positioning of the absorber with solar selective
surface coating in air, which would be advantageous for
some applications, can cause even more problems.
There is a need for technological advancement.
Summary of the Invention
The present invention provides in a first aspect a
solar absorptive material for a solar selective surface of
an absorber of solar radiation, the solar absorptive
material comprising:
a dispersed metallic material,
a receiving boundary through which the solar
radiation is received,
a first region and a second region, the first region
being located at a position closer to the receiving
boundary than the second region, and the first region
having an average volume fraction of the dispersed

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metallic material that is larger than that of the second
region.
Such a solar absorptive material typically forms a
part of a solar selective surface coating of an absorber
body and typically is sandwiched between a surface layer
having a lower average metal volume fraction and a
reflective bottom layer (or the absorber body itself)
having a higher average metal volume fraction or being
metallic.
Because the solar absorptive material has a larger
average metal volume concentration in the first region
which is closer to the receiving boundary than the second
region, the solar absorptive material typically has an
increased lifetime at elevated temperatures compared with
conventional solar absorptive materials. Experiments have
shown that embodiments of the solar absorptive material
have reduced molecular diffusion which increases the
lifetime. Further, conventional solar absorptive materials
have a higher concentration of the metallic material at or
near the interface between the solar absorptive material
and the bottom layer or the absorber body. The solar
absorptive material of the present invention may have a
reduced concentration of metallic material near the
interface which typically improves the adhesion of the
solar absorptive material at elevated temperatures.
For example, the solar absorptive material may
comprise a third region positioned at a distance below the
receiving boundary that is further than that of the second
region and having an average metal volume fraction that is
larger than that of the second region.
Alternatively or additionally, the solar absorptive
material may comprise a fourth region positioned between

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the first region and the receiving boundary, the first
region having an average metal volume fraction larger than
that of the fourth region.
At least one region of the solar absorptive material
may comprise a dispersion of at least two material
components and at least one of the material components
comprises itself a composition of more than one material.
At least one of the material components typically
comprises the metallic material and at least one other
material. Alternatively, each of the material components
may comprise the metallic material. In another variation
at least one of the material components may comprise a
dielectric material.
Further in at least one of the regions the metallic
material component may be dispersed in a dielectric
material or in another metallic material.
In one specific embodiment the average metal volume
fraction of at least one of the regions is selected so
that diffusion of metallic material from the or each
adjacent region is substantially inhibited for a
temperature range of 300°C to 500°C.
In one embodiment of the present invention the solar
absorptive material comprises a higher concentration of
the dispersed metallic material at or near the receiving
boundary than at or near an interface between the solar
absorptive material and the bottom layer or the absorber
body.
At least one of the regions may be a layer. In one
embodiment, the absorptive material comprises a multi-
layered structure. Alternatively or additionally, the
metallic volume fraction of at least one of the regions
may decreases in a direction from the receiving boundary

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into the material in a gradual manner or may be
substantially uniform.
In one particularly advantageous embodiment of the
present invention the absorptive material comprises a
multilayered structure having a large number of very thin
layers. Each layer may have a thickness of typically more
than 20 to 200 Angstrom. This embodiment has the advantage
that the layer thicknesses are small compared to the
wavelength of the solar light. Consequently possible
interference effects due to reflections at layer
interfaces are not problematic.
The dispersed metallic material may comprise any
suitable material, such as any suitable metallic material
including metal alloys, metal nitrides and transition
metals, but typically comprises a silicide material such
as a salicide material. Because of the temperature
stability of the silicide material, the high temperature
suitability of the solar selective material is further
improved. The first component typically has a resistivity
of less than 50 Ωcm, typically less than 10 Ωcm.
The silicide material may comprise any suitable
metallic material including for example a titanium
silicide, a tungsten silicide, a cobalt silicide.
A person skilled in the art will appreciate that the
solar absorptive material according to embodiments of the
present invention has a range of advantages. The concept
of having at least one region which has an average metal
volume fraction that is larger than that of another region
and that is positioned closer to the receiving boundary
than the other region offers significant flexibility for
design optimisation and facilitates fabrication. For
example, the solar absorptive material may comprise multi-

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layered or multi-region structures having layers or
regions with different average metal volume fractions and
may be designed having optimised optical properties (i.e.
optical material constants). In addition, relatively thick
structures may be fabricated and typically controlling of
particular parameters is less critical as the metal volume
fraction may increase and also decrease during fabrication
of the structures.
Further, adhesion of a layer or region may be
optimised by selecting an average a metal volume fraction
for the layer or region of the solar absorptive material
in accordance with the first aspect of the present
invention.
In addition, positioning of a layer or region having
a lower average metal volume fraction than immediately
adjacent layers or regions typically reduces inter-
diffusion of metallic material at elevated temperatures
and consequently increases the lifetime of the solar
selective material. Further, relatively thick structures
may be fabricated.
The present invention provides in a second aspect a
solar absorptive material for a solar selective surface of
an absorber of solar radiation, the solar absorptive
material comprising a silicide material for absorptive
solar radiation.
The silicide material typically is a salicide
material.
The present invention provides in a third aspect an
absorber having a solar selective surface coating
comprising the solar absorptive material according to the

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first aspect of the present invention, wherein the
absorptive material is sandwiched between an outer layer
and a metallic region.
The absorptive material typically is sandwiched
between an outer layer and a bottom layer positioned over
an absorber body. Alternatively, the absorptive material
may be positioned directly on the absorber body. The outer
layer typically has a lower metallic volume fraction that
the solar absorptive layer (or is non-metallic) . The
bottom typically has a higher average metal volume
fraction than the absorptive material or is metallic. The
absorber body typically is also metallic.
The present invention provides in a fourth aspect a
solar absorptive material for a solar selective surface of
an absorber of solar radiation, the solar absorptive
material comprising:
at least three layers having a dispersed metallic
material, an intermediate one of the at least three layers
having an average volume fraction of the dispersed
metallic material that is lower than that of the adjacent
layers,
wherein the average metal volume fraction of the
intermediate layer is selected so that the diffusion of
metallic material from the adjacent layers is
substantially inhibited for a temperature range of 300°C
to 500°C.
The invention will be more fully understood from the
following description of specific embodiments of the
invention. The description is provided with reference to
the accompanying drawings.
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Brief Description of the Drawings
Figure 1 shows a plot illustrating a depth profile
of a relative material component concentration for a solar
absorptive material (prior art),
Figure 2 shows a plot illustrating a depth profile
of a relative material component concentration for a solar
absorptive material according to a fist specific
embodiment of the present invention,
Figure 3 shows a plot illustrating a depth profile
of a relative material component concentration for a solar
absorptive material according to a second specific
embodiment of the present invention,
Figure 4 shows a plot illustrating a depth profile
of a relative material component concentration for a solar
absorptive material according to a third specific
embodiment of the present invention and
Figure 5 shows schematically a side-view of a solar
absorber having a solar selective coating according to
another specific embodiment of the present invention.
Detailed Description of Specific Embodiments
Referring to Figs. 2-4, solar absorptive materials
according to specific embodiments of the present invention
are now described. Figs 2-4 show depth profiles of
relative material concentration which typically is closely
related to the electrical conductivity. Consequently, the
relative material concentrations could also be represented
by depth profiles showing the specific electrical
conductivity as a function of depth.
Fig. 2 shows a plot 20 illustrating a depth profile
through the solar absorptive material according to the
first specific embodiment of the present invention. In
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this embodiment the solar absorptive material comprises a
dispersion of two components. A first component is
metallic and typically has a relatively high specific
absorption coefficient for visible solar radiation where
the solar radiation is of relatively high intensity. A
second component is in this example dielectric and at
least in part transmissive for visible solar radiation but
reflective for infrared radiation emitted by the absorber.
The absorptive material has an outer boundary for
receiving the solar radiation and typically is applied to,
or forms the part of, a solar selective surface coating on
an absorber body. For example, the solar absorptive
material may be positioned directly on a metallic absorber
body, or may be positioned on a metallic bottom layer over
the absorber body and may be covered by a top layer having
a low average metal volume fraction (or being
substantially free of metals) .
The effects of the distribution of the first and
second components is shown in plot 20, which identifies
regions of the absorptive material having a different
relative material component distribution. The plot 2 0
shows a depth profile through the solar absorptive
material comprising regions 22, 24, 26, 28 and 29. The
plot schematically shows the metal volume fraction through
the entire thickness of the material.
Region 22 has a relatively small average metal
volume fraction and is positioned just below the surface.
Below region 22 is region 24 which has a slightly higher
average metal volume fraction, region 26 has a lower
average metal volume fraction and regions 28 and 2 9 have
higher average metal volume fractions.
Fig. 3 shows plot 3 0 which illustrates a depth
profile through a solar absorptive material according to a
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second specific embodiment of the present invention. In
this embodiment the solar absorptive material comprises
regions 32, 34, 36, 38 and 39. Regions 34, 36 and 38 have
a graded metal volume fraction. Region 32, positioned
just below the outer boundary surface (or top surface if
the absorptive material is not coated) , has a relatively-
small average metal volume fraction. Region 3 6 has a
graded metal volume fraction that decreases in a direction
from the surface through the region. Region 38 has a
metal volume fraction that increases in a direction from
the surface through the region. Region 3 9 has a uniform
metal volume fraction.
Fig. 4 shows a plot 40 illustrating a depth profile
through a solar absorptive material according to a third
specific embodiment of the present invention. In this
embodiment the depth profile includes regions with
substantially constant metal volume fraction regions and
also regions with a graded metal volume fraction. The
material comprises regions 41, 42, 43, 44, 45, 46, 47, 48
and 49. The region 41 is positioned just below the
surface and has a relatively small, uniform metal volume
fraction. The region 42 has a metal volume fraction that
increases in a graded manner from the interface to
region 41 to the interface of the region 43. The
region 45, between two adjacent higher metal volume
fraction regions, can be thought of an insulating region
because it can reduce inter-diffusion of metal atoms. The
regions 44, 45 and 46 have a uniform metal volume fraction
which differ from one another and the region 47 has a
metal volume fraction that gradually increases from the
interface to the region 46 to the interface to the region
48. Region 48 has a uniform metal volume fraction.
A person skilled in the art will appreciate that in
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each embodiment the average metal volume fraction of the
material typically is closely related to the optical
properties of the material in the visible solar energy
range. If the average metal volume fraction in a first
region of the material is larger than in a second region,
the absorbance of the first region typically is also
larger than that of the second region. For example,
regions 29, 39 and 49 have a relatively high absorbance.
In each embodiment the materials comprise first
regions which are positioned closer to the top surface
than subsequent regions which have a average metal volume
fraction which is larger than that of at least one of the
subsequent regions. For example, the region 24 has a
average metal volume fraction that is larger than that of
the region 26, the region 36 has a average metal volume
fraction that is larger than that of the region 38, the
regions 43 and 44 have a average metal volume fraction
that is larger than that of region 45 and region 46 has a
average metal volume fraction that is larger than that of
the region 48.
If the metal volume fraction is decreasing in a
direction away from the surface into the material, as in
the above described examples, the integrity of the solar
absorptive material at elevated temperatures is improved.
In each embodiment the regions may have any
thickness, but typically the regions, which may be layers,
have thicknesses of more than 20 Angstrom.
In each embodiment the material that comprises the
regions has different or varying metal volume fractions
dependent on the relative concentrations and combination
of the first and second components. Regions with
predetermined absorption properties can be fabricated by
mixing the two components and then applying the mixed
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material to an absorber to form a layer of a multi-layered
structure. Alternatively, the regions or layers may, for
example, be prepared by physical or chemical vapour
deposition methods such as ac or dc sputtering. Two
sputter sources may be used, one for first component (for
example a metallic component) and the other for the second
component (for example a dielectric component). The
relative deposition rate of the two components determines
the relative metal volume fraction and therefore other
characteristics such as the conductivity of the deposited
layer.
Alternatively, each layer may be formed by
depositing very thin layers of the first or second
component, for example dielectric and metal sub-layers
each having a thickness of only a few Angstrom or less.
As the sub-layers are extremely thin, they together have
physical properties which correspond to an average of the
metal volume fraction throughout the layer composed by the
sub-layers.
Furthermore, the optimisation of the absorptive
material is not limited to combining only two components.
The metallic component may comprise any metal,
metallic alloy, metallic nitrite, nitrites of transition
metals, or any other materials. In this embodiment,
however, the solar absorptive material comprises a
silicide material such as a salicide material.
Particularly suitable is a titanium silicide material, a
tungsten silicide material, or cobalt silicide material,
but a salicide material that comprises any other suitable
metal may also be used.
Figure 5 schematically shows a solar absorber 50
having an absorber body 52 which is coated with a solar
selective material 54 including the above-described
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absorptive material.
In a further embodiment of the present invention,
the use of layers or graded sections of the absorbing
material, where the metal volume fraction decreases in a
direction from the top surface (receiving boundary) into
the material, allows the absorptive material to be
optimised for improved efficiency and other
characteristics such as stability in air, adhesion of the
layers, less oxidation, stress relaxation or reduction,
improved resistance of diffusion (particularly heavy
atoms) or permeability. For example a layer or layers
having an increased average metal volume fraction compared
with adjacent layers may be positioned at predetermined
depth from the surface in a manner such that the
efficiency of the solar selective material is improved.
In a further embodiment, the use of one or more
regions where the metal volume fraction decreases in a
direction from the top surface, allows for a larger number
of regions with more variance of properties, such as metal
volume fraction or optical constants, between the regions.
This flexibility leads to improved optimisation of the
material. For example the material can be optimised for
interference effects over a wider wavelength range. From
a manufacturing point of view there is more flexibility.
For example, when using sputtering techniques the present
invention allows for the sputtering to be less sensitive
to deposition parameters compared to conventional
techniques.
In a further embodiment, the larger number of
regions due to the partial reversing of the depth profile
leads to an absorber with more depth of absorbing material
compared to one comprising only two regions or layers.
In a further embodiment, the top region, which may
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be a dielectric, extends further into the material.
Although the invention has been described with
reference to particular examples; it will be appreciated
by those skilled in the art that the invention may be
embodied in many other forms. For example, even though the
embodiments illustrated in Figs. 2-4 show multi-layered
structures, it will be appreciated that the absorptive
material according to the present invention may also
comprise only a single layer having a graded metal volume
fraction that decreases from the surface through the
thickness of the material.
Further, it will be appreciated by a person skilled
in the art that the solar absorptive material may comprise
more than one type of the first component (for example
more than one type of a metallic material) and/or more
than one type of the second component (for example more
than one type of a dielectric material). The metal volume
fraction may be substantially uniform throughout a layer
or region and a variation of the conductivity and the
absorbance properties of that layer can be achieved by
using different types of metallic components throughout
the layer which have different electrical conducting and
absorptive properties. Alternatively, the metal volume
fraction may vary throughout a layer or region but the
conductivity and the absorbance properties of that layer
may be substantially uniform throughout the layer or
region.
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The Claims:
1. A solar absorptive material for a solar selective
surface of an absorber of solar radiation, the solar
absorptive material comprising:
a dispersed metallic material,
a receiving boundary through which the solar
radiation is received,
a first, a second and a third region, the first
region being located at a position closer to the receiving
boundary than the second region and having an average
volume fraction of the dispersed metallic material that is
larger than that of the second region, the third region
being positioned at a distance below the receiving
boundary that is further than that of the second region
and having an average metal volume fraction that is larger
than that of the second region.
2. The solar absorptive material of claim 1 comprising
a fourth region positioned between the first region and
the receiving boundary, the first region having an average
metal volume fraction that is larger than that of the
fourth region.
3. The solar absorptive material of claim 1 or 2
wherein at least one region comprises a dispersion of at
least two material components and at least one of the
material components comprises itself a composition of more
than one material.
4. The solar absorptive material of claim 3 wherein at
least one of the material components comprises the
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metallic material and at least one other material.
5. The solar absorptive material of claim 3 or 4
wherein each of the material components comprises the
metallic material.
6. The solar absorptive material of claims 3 or 4
wherein at least one of the material components comprises
a dielectric material.
7. The solar absorptive material of any one of the
preceding claims wherein in at least one of the regions
the metallic material component is dispersed in a
dielectric material.
8. The solar absorptive material of any one of the
preceding claims in which one of the regions has a lower
average metal volume fraction than an adjacent region and
wherein the average metal volume fraction is selected so
that the diffusion of metallic material from the adjacent
region is substantially inhibited for a temperature range
of 300°C to 500°C.
9. The solar absorptive material of any one of the
preceding claims comprising a silicide material.
10. The solar absorptive material of claim 9 comprising
a salicide material.
11. The solar absorptive material of any one of the
preceding claims wherein at least one region is a layer.
12. The solar absorptive material of any one of the
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preceding claims wherein the metallic volume fraction of
at least one of the regions decreases in a direction form
the receiving boundary into the material in a gradual
manner.
13. The solar absorptive material of any one of claims 1
to 11 wherein at least one of the regions has a
substantially uniform metal volume fraction.
14. A solar absorptive material for a solar selective
surface of an absorber for solar radiation, the solar
absorptive material comprising a silicide material.
15. The absorber of claim 14 wherein the silicide
material is a salicide material.
16. An absorber having a solar selective surface coating
comprising the solar absorptive material of any one of
claims 1 to 12, wherein the absorptive material is
sandwiched between an outer layer and a metallic region.
17. A solar absorptive material for a solar selective
surface of an absorber of solar radiation, the solar
absorptive material comprising:
at least three layers having a dispersed metallic
material, an intermediate one of the at least three layers
having an average volume fraction of the dispersed
metallic material that is lower than that of the adjacent
layers,
wherein the average metal volume fraction of the
intermediate layer is selected so that the diffusion of
metallic material from the adjacent layers is
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substantially inhibited for a temperature range of 300°C
to 500°C.
Amended Sheet
IPEA/AU

The present
invention provides a solar
absorptive material for a solar
selective surface of an absorber of
solar radiation. The solar absorptive
material comprises a dispersed
metallic material and a receiving
boundary through which the solar
radiation is received. Further, the
solar absorptive material comprises
a first region and a second region.
The first region being located at
a position closer to the receiving
boundary than the second region
and the first region has an average
volume fraction of the dispersed
metallic material that is larger than
that of the second region.

Documents:

http://ipindiaonline.gov.in/patentsearch/GrantedSearch/viewdoc.aspx?id=iBkix5zHZ4Xd4yvO4w6kXw==&loc=wDBSZCsAt7zoiVrqcFJsRw==


Patent Number 279985
Indian Patent Application Number 3393/KOLNP/2007
PG Journal Number 06/2017
Publication Date 10-Feb-2017
Grant Date 06-Feb-2017
Date of Filing 11-Sep-2007
Name of Patentee THE UNIVERSITY OF SYDNEY
Applicant Address PARRAMATTA ROAD, SYDNEY, NEW SOUTH WALES
Inventors:
# Inventor's Name Inventor's Address
1 HANG LINGXIA C/O THE UNIVERSITY OF SYDNEY, PARRAMATTA ROAD,, SYDNEY, NSW 2006
2 YIN YONGBAI 66 CHELMSFORD AVENUE,, EPPING, NEW SOUTH WALES 2121
3 MILLS DAVID 15 THOMAS AVENUE,, ROSEVILLE, NEW SOUTH WALES 2069
PCT International Classification Number F24J 2/48
PCT International Application Number PCT/AU2006/000288
PCT International Filing date 2006-03-03
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 2005901000 2005-03-03 Australia