Ambesh Dixit

Indian Institute of Technology Jodhpur

Solar Selective CoatingsImportantance in CSP Technology

SCHOTT Solar Inc.

Parabolic Trough: an example

Radiation from the Sun transformed into thermal energyUsed for Heating air or water/fluid media

SCHOTT Solar Inc.

Presentation flowSolar thermal applicationsA bit about receiver tube and its designSpectral selectivitySelective absorbers with examplesMechanisms for solar spectral selectivitySolar absorber design constraints

Physical process (RF/DC magnetorn sputtering)Chemical process (Sol-gel process)

Surface engineering for enhanced solar absorptionConclusions

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 )

bellow to compen-sate expansion

cover tube withanti-reflective coating

selective absorber coating on steel,

getter to maintainvacuum

evacuatedannulus

glass-to-metal-seal

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.

0,4 0,60,8 1 2 4 6 8 10 200,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,0Absorber coating

= 0,95 = 0,13 @ 400°C

BB, 400°C(norm.)

AM 1.5 (norm.)

refle

ctan

ce

wave length / µm

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

Selective Absorber with Multilayer CERMET for High Temperatures

steel

AR-coating

cermet

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 ~ 0.9Thermal emittance ~ 0.1

Material Absorptance

() Emittance ()

Break down temparature

(°C)

Comments

Black siliconpaint

0.86-0.94 0.83-0.89 350 Slicone binder

Black silicon

paint 0.9 0.5 Stable at

hightemperature

Black copper

over copper 0.85-0.9 0.08-0.12 450 Patinates

with moisture

Black chorome over nickel

0.92-0.94 0.07-0.12 450 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

ÞPossible 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 = Transmissivityr = Reflectivityag = Absorptivity

1 g

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

As a designer for solar absorbers:

Vapour depositionThermal evaporatione-beam evaporation

Chemical vapour deposition

Physical vapour depositionMolecular beam epitaxyRF/DC magnetron sputteringPulse laser deposition (PLD)

Thin film Coating Process

Physical ChemicalElectrodeposition

Chemical depositionSprayingSol-gelMetal organic

deposition (MOD)

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

Low pressure coating processes in which the coating flux is produced by a physical process.

There are two main types:EvaporationSputtering

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 sputteringMetal oxide such as aluminum oxide, copper oxide, iron oxide etc

Metal nitrides such aluminum nitrides, titanium nitrides etceasy 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 tempe- rature applications

Systems of choice- Aluminum nitride (AlN) based cermets coatings using

RF/DC sputteringStable at high temperature (> 500 0C), radiation resist, high absorptance and low emittance

20 30 40 50 60 70 80

Inte

nsity

(ar

b. u

nits

)

2 (degree)

Glass substrate AlN/Glass (DC sputtered)

800 1600 2400 3200 4000

% R

(ar

b. u

nits

)

Wavenumber (cm-1)

AlN/glassDC sputtered

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 materials-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 processEasy to fabricate, state at intermediate temperature,

high absorptance and low emittance

20 30 40 50 60 70 80

Inte

nsi

ty (

arb

. u

nits

)

2 (degree)

Cr2O

3

(From Dip Coating)

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 by

ConclusionsSolar 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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