Fabrication of SSAl2O3 Solar Selective Absorbers for HCEs of Solar Thermal Power Stations, by RF Spu

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Experimental and Theoretical Investigation of
Selective Solar Absorbers, Optical Properties and
Related Technologies using Computer Simulation
and Fabrication by RF Sputtering Technique
By Mohammad Reza Nejati
Supervisors Dr M. K. Asadi Dr.
D. Fathi Counselor Dr. M. M. Ardehali
Khajeh Nasir Toosi University of Technology (KNTU)
Mechanical Engineering Department, Energy Systems
Engineering Division
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Necessity of study-Goals-Concepts
Mechanisms of Spectral Selectivity in Solar
Absorbers
Method of Fabrication
Calculation of Absorptance and Emittance
Optical Simulation of Solar Selective Absorbers
and its Results
Results and suggestions
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Why should we consider the solar thermal energy ?

• Huge amount of solar insolation in Iran
• An inexhaustible source
• Free of charge
• Available everywhere
• Environment friendly
• Conservation of fossil fuel resources
• Reduction of CO2 emission
• Creation of job opportunities
• The high price and low efficiency of PV

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So, solar thermal power plant technologies are
important candidates for providing the clean and
renewable energy needed in the future.
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Kinds of Solar thermal power plants

• Parabolic Trough Systems
• Central Receiver Systems (CRS)
• Parabolic Dish Systems

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Kinds of Solar thermal power plants
Parabolic Dish Systems
CRS
Parabolic Trough Systems
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Parabolic Trough Power Plant Parabolic trough
field arrays of five SEGS plants at Kramer
Junction in The US-California (total capacity
354 MW)
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Heat Collection Element (HCE)
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The most critical part of solar collectors is
their absorber coatings . To improve the
performance of collectors, these absorber
surfaces should have the highest possible solar
absorptance And lowest possible thermal emittance
i.e. they should be Spectrally SELECTIVE.
The Goal of this research work is achieving to
the technologies and ideal structures for
fabrication of solar selective absorber with high
thermal and chemical stability and suitable
optical properties for using in solar thermal
power plants
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What is a solar selective absorber ?
Solar spectral irradiance and Black body
radiation spectrum for 3 different temperatures.
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As there is almost NO OVERLAP between the 2
spectrum, it is possible to design a surface
with high solar absorptance while keeping its
thermal emittance at low levels.
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The solar radiation distribution and the
blackbody spectra for the temperatures of 100,
300 and 400oC. The spectral optical reflectance
spectra of an ideal selective surface and a
Mo-Al2O3 cermet selective surface are also shown.
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Mechanisms of Spectral Selectivity in Solar
Absorbers
Simplest way using materials having intrinsic
solar selective properties Absorber-reflector
tandem by combining 2 surfaces(one is highly
Absorbing in solar spectrum and another highly
reflecting in infrared) 1-dark mirror
Multilayer absorber Semiconductor
Coatings Textured Surface Coatings Composite
Coatings Painted Coatingconsist of an absorbing
pigment dispersed in a binder material 2-heat
mirror
Thin highly solar absorber
Metal (highly IR reflector)
Solar transparent IR reflecting film
Thick absorbing surface
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Types of Solar Selective Absorbers
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Composite Coatings
These surfaces consisting of small metal
particles embedded in a dielectric matrix usually
oxide matrix known as Cermet (Ceramic Metal)
deposited on a highly IR reflecting metal
substrate These surfaces show a very good
selectivity.
The metal particles can be dispersed
homogeneously or graded in oxide matrix
VFVolume Fraction of metal particles in the
ceramic matrix
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Why composite absorbers?

• To have a solar absorptance higer than 0.9 , it
  has been proved that the optical constants of the
  film should be 0.2ltklt0.8 and 1.5ltnlt2.
• As there are no material with the above optical
  constants one have to use cermet composites
• -Absorbing Solar radiation by multiple resonant
  scattering phenomenon
• - The possibility of using various materials as
  ceramic and metalic components of the cermet.
• - Easy changing of optical properties just by
  changing the thickness and metal content of
  cermet.
• - Cermets are almost transparent for IR radiation
  then they have very little effect on thermal
  emittance

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Double Optimized cermet structure
Prof. Q.Zhang has proposed this structure in
1992, this structure shows better optical results
than the other structures
Anti Reflection Layer (transparent Dielectric)
Al2O3,AlN, SiO2, MgF2
LMVF Cermet SS-Al2O3, SS-AlN, W-Al2O3, Ni-Al2O3,
Cr-Cr2O3 Cu-CuO, Ni-NiOx, Mo-Al2O3
HMVF Cermet
IR-Reflector layer Au, Cr, Mo, Cu, Ag, Al, W
Substrate
HMVF High Metal Volume Fraction LMVF Low
Metal Volume Fraction
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Part 3- Fabrication Methods

• Electrochemical techniques

• Physical Vapor Deposition
• Sputtering
• Radio Frequency (RF)
  Sputtering
• Direct Current (DC)
  Sputtering
• DC magnetron sputtering
• RF magnetron Sputtering
• Vacuum evaporation
• Ion plating
• Electron beam evaporation

• Chemical Vapor Deposition (CVD)

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Advantagous of using RF sputtering techniques
- Higher rate of deposition - The possibility of
deposition from insulator materials - Better
mechanical adherent - Good control facilities
during the deposition - Less damaged and
defected films even in comparison with DC
obtains -It uses lower energy The better
Stechiometery compared to reactive
sputtering Environmentally friendly and no
chemical waste
Non-Reactive
Reactive
Sputtering from an Al target in O2 environment
Directly Sputtering from Al2O3 target
Al2O3 Film
?Al2O3 Film
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Fabrication of composite layer.
The thickness of each sublayer is about 3-5 nm
Cermet
Al2O3
SS
Using thicker sublayers has shown worse optical
properties
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Part 4- Calculation of Absorptance and Emittance
Direct methods -Emissometer -Calorimetric
emittance measurement -Willey Alphameter
Indirect methods Through the reflectance-waveleng
hts spectrum For absoptance UV/VIS/NIR
Spectrophotometer For emittance FT-IR
Spectrophotometer
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Calculation of emittance and absorptance from
reflectance curve
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Part 5 Simulation and optimization of optical
response of solar selective absorber
Matrix techniques for calculation of reflectance
of a multilayer
For an indivisual thin film deposited on
substrate
nT
l n1
n0
For multilayer Films
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Using 3 physical models to estimate the optical
constants of cermet layer by knowing those of
cermet components and VF
Separated-Grain Structure
Aggregate Structure
Assumed microstructures for extracting the
Effective Medium Theories
RUC for Maxwell-Garnet Ping Sheng Theory
RUC for Bruggeman theory
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Anti Reflection Layer Al2O3 50nm
LMVF Cermet 40.78nm VF0.185 SS-Al2O3
HMVF Cermet 50.92 nm VF0.387 SS-Al2O3
IR-Reflector layer Au 50nm
Substrate SS
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Effect of each layer in double cermet structure
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Effect of changing VF on Absorptance and emitance
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Effects of thickness of Anti reflection and IR
reflector layer on optical properties
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The effect of changing the IR reflector material
in optimized double cermet structure absorber
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The effect of changing the place of LMVF and HMVF
cermets on optical properties.
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Investigation the effect of increasing the
number of cermet layers from 2 to 7 layers.
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Sequence graded
Integral graded
Equal thickness for cermet ublayers ETAC
Graded in 2 homogeneous Cermets by optimization
Linear graded
Variable thicknesses for Cermet sublayer VTAC
Prime Graded Equal Thickness PGETAC
Prime graded
Prime Graded (PGSAC) (farooq Hutchines 2002)
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Investigation of the optimum grading level in
PGSAC
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Comapision of PGSAC absorbers with other
structures
Assumptions4-layers for integral graded , 4
distinct cermets for sequence graded (4 optimized
cermet)
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Comparision of one and double optimized cermet
structures
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Verification of Simulation
The optical reults of simulation were compared
with thoes of Mills and Zhang had obtained by
experimental and simulation work in 1992
Anti Ref. Al2O3
Cu-Al2O3
Cu-Al2O3
IR Ref Cu
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Results

• - Better chemical and mechanical stability for
  the thin films prepared by Sputtering
• Using RF Sputtering technique, allows that
  compound (insulator) targets also be sputtered
  and better stechiometery will be obtained
• Using spectrophotometers are preffered
• Double optimized cermet selective absorber is the
  best candid for high themp. applications
• The one optimized cermet absorber includes the
  Anti reflection has almost the same optical
  results in comparision with double cermet
  structures.
• The newest structure (PGSAC) has not optical
  properties better than double cermet structure
• The new defined structure PGETAC has better
  optical properties than ETAC

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• - Increasing the number of cermet
  sublayers(grading level) does not
• show better results and one should find the
  optimum grading level for
• each case
• In double cermet absorbers incresing the number
  of cermet layer
• does not lead to better optical results
• -In optimized double cermet structure the
  material of IR ref Is not so
• important
• - Increasing the thickness of IR ref. Layer does
  not lead to lower emittance

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Thank you
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