Dixit Indian Institute of Technology Jodhpur Solar Selective Coatings Importantance in CSP Technology SCHOTT Solar Inc Parabolic Trough an example Radiation from the Sun transformed into thermal energy ID: 187857
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Slide1
Ambesh
DixitIndian Institute of Technology Jodhpur
Solar Selective CoatingsImportantance in CSP Technology
SCHOTT Solar Inc.Slide2
Parabolic Trough: an example
Radiation from the Sun transformed into thermal energy
Used for Heating air or water/fluid media
SCHOTT Solar Inc.Slide3
Presentation flow
Solar
thermal
applications
A bit about receiver tube and its design
Spectral selectivity
Selective absorbers with examples
M
echanisms for solar spectral selectivity
Solar absorber design constraints
Physical process (RF/DC magnetorn sputtering)
Chemical process (Sol-gel process)
Surface engineering for enhanced solar absorption
Conclusions
Slide4
Temperature ranges for solar thermal applications
Low
temperature (< 100
0C)
Water heating and swimming pools
Medium temperature (< 350
0
C)
Space heating or cooling and water desalination
High temperature (> 350
0
C)
Mechanical energy production
and catalytic dissociation of water, CSP (concentrating solar power ~ 500
0
C
or more)Slide5
Receiver is
an important
Component in Parabolic Trough Collectors
A receiver should comply with
Low
thermal losses
(
vacuum, absorber with low thermal
emittance
)
High solar
absorptance
( efficient absorber, highly transmitting outer glass tube
)Slide6
For power
plant with a life span of more than 20 years is required to Match the long operational sustainability.Keep maintenance costs low during operation.
During operation receivers are mechanically and thermally stressed.
Most important issues
are:
Durability
of glass-to-metal seal
Stability
of vacuum
(
low hydrogen permeation, appropriate getter)
Durability
of absorber coating
(
only small degradation of efficiency acceptable)
Abrasion
resistance of anti-reflective
glass coating.Slide7
Performance data:
Temperature
stable up to 500 °C Solar absorptance >= 95 %
Thermal emittance <= 10% at 400°C
Material:
Polished
low-carbon steel as substrate material
W-Al
2
O
3
Multilayer Cermet
coating
Selective Absorber with Multilayer
CERMET for
High Temperatures
steel
AR-coating
cermet
SCHOTT Solar Inc.Slide8
Spectral selective surface:
Non-selective surfacesModerate selective surfaces
Selective surfacesPerformance quantification:
Solar absorptance:
Absorbed fraction of incoming radiation
Thermal emittance:
Emitted fraction of absorbed energy through infrared radiation
Selective absorbers can accomplish this requirement by having
(i) high solar absroptivity and
(ii) high thermal reflectivity simultaneouslySlide9
Different mechanisms for solar spectral selectivity
(i) Semiconductor with suitable band gaps (ii) Optical interference effect of a multilayer stack of thin films
(iii) Materials, which are black for solar wavelengths but transparent for heat like metal-ceramic nanocomposites (called CERMET) (iv) Metallic surface with designed roughness
Multiple reflections of the light inside surface groves -> enhanced
solar absorption
Examples:
Black chrome
Black zince, cobalt, nickel
Copper oxide, iron oxide, aluminum oxide
Electroplating Technique
Solar absorption ~ 0.9
Thermal
emittance
~ 0.1Slide10
Material
Absorptance
(
)
Emittance
(
)
Break down temparature
(
°
C)
Comments
Black silicon
paint
0.86-0.94
0.83-0.89
350
Slicone binder
Black silicon
paint
0.9
0.5
Stable at
high
temperature
Black copper over copper
0.85-0.9
0.08-0.12
450
Patinates with moisture
Black chorome over nickel
0.92-0.94
0.07-0.12450Stable at high temperatures
Jan F.
Kreider
et al
Solar Design (
1989
)Slide11
As a designer for solar absorbers:
A serious look into solar irradiance &
Black body radiation @ 300 0C:
BB radiation 2 mm – 30
m
m
No overlap between these two curves
Possible to prepare surfaces that
may absorb the soalr wavelengths
and emitt poorly at thermal infra-
red wavelength.
Different names:
Bandpass reflection filters
Black infrared mirrors
Spectrally selective absorbers/coatings
t
=
Transmissivity
r
= Reflectivity
a
g
=
AbsorptivitySlide12
Number of choices to fabricate solar selective coatings
Combination of various mechanisms to control and improve the optical property of an absorber layer such as Textured surface with required spectral selectivity, graded cermet or double cerment structure
Equiped with an anti-reflectition layer may exhibit enhanced spectral selectivity
Such structures may result in good solar absorptance ~ 0.98 and poor thermal emittance ~ 0.02 or less, yet these structures are complicated and thickness sensitive.
As a designer for solar absorbers:Slide13
Solutions:
Improve the selectivity of cermet based absrobers in single layer geometry surface roughness on the absorber/air interface (laser structuring)
Easy thin film process such as sol-gel for quick fabrication of thin films and tunability
using stable colloidal suspensiions of nano-powders for cermat composites
As a designer for solar absorbers:Slide14
Vapour
deposition Thermal evaporation e-beam evaporationChemical vapour depositionPhysical vapour deposition
Molecular beam epitaxy RF/DC magnetron sputtering Pulse laser deposition (PLD)
Thin film Coating Process
Physical Chemical
Electrodeposition
Chemical deposition
Spraying
Sol-gel
Metal organic deposition (MOD)
Slide15
Advantages
Excellent process control
Low deposition temperature
Dense, adherent coatings
Elemental, alloy and compound coatings possible
Disadvantages
Vacuum processes with high capital cost
Limited component size treatable
Relatively low coating rates
In both cases the source material is a solid (metal or ceramic).
A reactive gas may be used in the deposition chamber to deposit compound coatings from an elemental source or maintain the
stoichiometry
of coatings from compound sources.
Typical coating thicknesses range from 1-5
m
m
Low
pressure coating processes in which the coating flux is produced by a physical process.
There
are two main types
:
Evaporation
SputteringSlide16
Physical:
RF/DC magnetron sputtering process
Main sputtering processes:
DC diode sputtering
(for conducting targets)
RF sputtering
(for insulating targets) Slide17
Mostly
used for low deposition temperatures. No post deposition heat treatment required. Fine thickness control. Easy to dope with noble metals.
The coating rate scales with the electrical power used to sustain the discharge.
The coating rate also depends on the plasma density, so techniques to increase this (e.g. by confining the electrons close to the target using magnets) will increase the coating rate.
However, as much as 95% of the power is dissipated as heat in the target so good cooling is essential.
Materials may be deposited using sputtering
Metal
oxide such as aluminum oxide, copper oxide, iron oxide etc
Metal nitrides such aluminum nitrides, titanium nitrides etc
easy
to dope simultaneously during growth.Slide18
Numerous materials:
Our Target: High solar absorptance (~ 0.95 or more) and low emittance (~0.05 or less) for high tempe- rature applications
Systems of choice- Aluminum nitride (AlN) based cermets coatings using RF/DC sputtering Stable at high temperature (> 500 0C), radiation resist, high absorptance
and low emittanceSlide19Slide20
Chemical:
Sol-gel process
Advantages
Low temperature treatment
Easy synthesis process
Can coat complex shapes uniformly
Hard particles can be incorporated to increase hardness
Can coat most metals and insulators
Disadvantages
Film quality is not comparable
with physical process
Heat treatment is necessary to develop the desired material
stoichiometry
and p
roperties Slide21
Numerous materials-Our Target: High solar
absorptance (~ 0.95 or more) and low emittance (~0.05 or less) for moderate temperature applicationsSystems of choice- Chromium oxide (Cr2O
3) based cermets coatings using solution process Easy to fabricate, state at intermediate temperature, high absorptance and low emittanceSlide22
Sol-Gel
coating for borosilicate glass based on alcoholic dilutions with SiO2 nano- particles for improved abrasion resistance
Solar transmittance of > 0,96 achieved Challenges in production: - homogenous and stable coating of long glass
tubes
- automated high precision solar transmittance test for long glass tubes
AR Coating with High
Solar Transmittance
Only glass
:
=
92%
With AR-coating
:
>
96%
Surface engineering bySlide23
Conclusions
Solar selective coatings are important for numerous solar thermal applications.
Stable high temperature solar selective coatings are essential to realize CSP applications.Nitrides based CERMET coatings may be promising candidates for CSP applications
, where temperature may go beyond 500 0C.
Sol-gel process
may be explored for development of
oxide based CERMET coatings.
Surface engineering
may enhance the solar absorption beyond the material’s intrinsic limit enhancing multiple reflection assisting absorption by reducing bulk reflection.Slide24
Acknowledgement
Prof. Rajiv Shekhar
(a driving force)Dr. Laltu Chandra
Mr. Ritesh
Patel
Funding
agency- MNRESlide25
Thank you
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