Solar Selective Coatings
Importantance in CSP Technology
Ambesh Dixit
SCHOTT Solar Inc.
Indian Institute of Technology Jodhpur
Parabolic Trough:
an example
SCHOTT Solar Inc.
Radiation from the Sun transformed into thermal energy
Used for Heating air or water/fluid media
Presentation flow
Solar thermal applications
A bit about receiver tube and its design
Spectral selectivity
Selective absorbers with examples
Mechanisms 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
Temperature ranges for solar thermal
applications
Low temperature (< 100 0C)
Water heating and swimming pools
Medium temperature (< 350 0C)
Space heating or cooling and water desalination
High temperature (> 350 0C)
Mechanical energy production and catalytic
dissociation of water, CSP (concentrating solar power ~ 500 0C
or more)
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 )
evacuated
annulus
getter to maintain
vacuum
cover tube with
anti-reflective coating
selective absorber
coating on steel,
glass-to-metalseal
bellow to compensate expansion
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.
Selective Absorber with Multilayer CERMET
for High Temperatures
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
Absorber coating
1,0
W-Al2O3 Multilayer Cermet coating
0,9
 = 0,95
 = 0,13 @ 400°C
0,8
AR-coating
cermet
reflectance
0,7
0,6
0,5
0,4
AM 1.5
(norm.)
0,3
BB, 400°C
(norm.)
0,2
0,1
steel
0,0
0,4 0,6 0,8 1
2
4
6 8 10
wave length / µm
20
SCHOTT Solar Inc.
Spectral selective surface:
Non-selective surfaces
Moderate selective surfaces
Selective surfaces
Performance 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 simultaneously
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 ~ 0.1
Material
Absorptance
()
Emittance
()
Break down
temparature
Comments
(°C)
Black silicon
paint
0.86-0.94
0.83-0.89
Black silicon
0.9
0.5
350
Stable at
high
temperature
paint
Black copper 0.85-0.9
over copper
0.08-0.12
Black
chorome
over nickel
0.07-0.12
0.92-0.94
Slicone
binder
450
Patinates
with moisture
450
Stable at high
temperatures
Jan F. Kreider et al Solar Design (1989)
As a designer for solar absorbers:
A serious look into solar irradiance &
Black body radiation @ 300 0C:
BB radiation 2 mm – 30 mm
No overlap between these two curves
Possible to prepare surfaces that
may absorb the soalr wavelengths
and emitt poorly at thermal infrared wavelength.
1  t  r  g
Different names:
t = Transmissivity
r = Reflectivity
g = Absorptivity
Bandpass reflection filters
Black infrared mirrors
Spectrally selective absorbers/coatings
As a designer for solar absorbers:
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:
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
Thin film Coating Process
Physical
Vapour deposition
Thermal evaporation
e-beam evaporation
Chemical vapour deposition
Chemical
Electrodeposition
Chemical deposition
Spraying
Sol-gel
Physical vapour deposition
Metal organic
Molecular beam epitaxy
deposition (MOD)
RF/DC magnetron sputtering
Pulse laser deposition (PLD)
Low pressure coating processes in which the coating flux is produced by a physical
process.
There are two main types:
Evaporation
Sputtering
Advantages
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
1-5mm
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
Physical:
RF/DC magnetron sputtering process
Main sputtering
processes:
DC diode sputtering
(for conducting targets)
RF sputtering
(for insulating targets)
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.
Numerous materials:
Our Target: High solar absorptance (~ 0.95 or more) and
low emittance (~0.05 or less) for high temperature applications
Systems of choiceAluminum nitride (AlN) based cermets coatings using
RF/DC sputtering
Stable at high temperature (> 500 0C), radiation resist,
high absorptance and low emittance
20 30 40 50 60 70 80
2  (degree)
% R (arb. units)
Intensity (arb. units)
Glass substrate
AlN/Glass (DC sputtered)
AlN/glass
DC sputtered
800 1600 2400 3200 4000
-1
Wavenumber (cm )
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 properties
Numerous materialsOur Target: High solar absorptance (~ 0.95 or more) and
low emittance (~0.05 or less) for moderate
temperature applications
Systems of choice-
Intensity (arb. units)
Chromium oxide (Cr2O3) based cermets coatings using
solution process
Easy to fabricate, state at intermediate temperature,
high absorptance and low emittance
Cr O
2
3
(From Dip Coating)
20 30 40 50 60 70 80
2 (degree)
Surface engineering by
AR Coating with High Solar Transmittance
Sol-Gel coating for borosilicate glass based on alcoholic dilutions with SiO2 nanoparticles 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
Only glass:
t = 92%
With AR-coating :
t > 96%
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.
Acknowledgement
Prof. Rajiv Shekhar (a driving force)
Dr. Laltu Chandra
Mr. Ritesh Patel
Funding agency- MNRE
Thank you
&
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