1. Solar Selective Coatings Importantance in CSP Technology
Ambesh Dixit
Indian Institute of Technology Jodhpur
SCHOTT Solar Inc.

2. Parabolic Trough: an example
SCHOTT Solar Inc.
Radiation from the Sun transformed into thermal energy
Used for Heating air or water/fluid media

3. 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

4. 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)

5. 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 )

6. 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.

7. 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
W-Al2O3 Multilayer Cermet coating
steel
AR-coating
cermet
SCHOTT Solar Inc.

8. 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

9. 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

10. Material
Absorptance
()
Emittance
()
Break down temparature
(°C)
Comments
Black silicon
paint
350
Slicone binder
0.9
0.5
Stable at
high
temperature
Black copper over copper
450
Patinates with moisture
Black chorome over nickel
Stable at high temperatures
Jan F. Kreider et al Solar Design (1989)

11. As a designer for solar absorbers:
A serious look into solar irradiance &
Black body 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 infra-
red wavelength.
Different names:
Bandpass reflection filters
Black infrared mirrors
Spectrally selective absorbers/coatings
t = Transmissivity
r = Reflectivity
ag = Absorptivity

12. 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.

13. 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

14. Thin film Coating Process
Physical Chemical
Vapour deposition
Thermal evaporation
e-beam evaporation
Chemical vapour deposition
Physical vapour deposition
Molecular beam epitaxy
RF/DC magnetron sputtering
Pulse laser deposition (PLD)
Electrodeposition
Chemical deposition
Spraying
Sol-gel
Metal organic deposition (MOD)

15. Advantages Disadvantages
Low pressure coating processes in which the coating flux is produced by a physical process.
There are two main types:
Evaporation
Sputtering
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 1-5mm

16. RF/DC magnetron sputtering process
Physical:
RF/DC magnetron sputtering process
Main sputtering processes:
DC diode sputtering
(for conducting targets)
RF sputtering
(for insulating targets)
When energetic ions strike a surface, material is ejected by the transfer of momentum from the ion to the target atoms (akin to billiard ball collisions at the atomic scale). This can be conveniently achieved in a low pressure glow discharge of an inert gas such as argon.
In such a process the target material is made the cathode and is raised to a potential of several hundred volts. Electrons leaving the cathode stream out into the gas phase where they can impact with argon atoms, ionizing them. The positively charged argon is then accelerated to the cathode where it impacts and sputters away material.
The sputtering yields of different elements for given impact conditions do not vary very much so target alloy compositions can be maintained in the coating except in cases where there are large differences in the atomic weights of alloy constituents.

17. Mostly used for low deposition temperatures
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.

18. Numerous materials: Systems of choice-
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 emittance

20. Chemical: Sol-gel process Disadvantages Advantages
Film quality is not comparable with physical process
Heat treatment is necessary to develop the desired material stoichiometry and properties
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

21. Numerous materials- Systems of choice-
Our Target: High solar absorptance (~ 0.95 or more) and low emittance (~0.05 or less) for moderate temperature applications
Systems of choice-
Chromium oxide (Cr2O3) based cermets coatings using solution process
Easy to fabricate, state at intermediate temperature, high absorptance and low emittance

22. Surface engineering by
AR Coating with High Solar Transmittance
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
Only glass:
 = 92%
With AR-coating :
 > 96%

23. 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.

24. Acknowledgement Prof. Rajiv Shekhar (a driving force)
Dr. Laltu Chandra
Mr. Ritesh Patel
Funding agency- MNRE

25. Thank you
&
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