selective coatings

Selective absorbers

Dr. Ángel Morales

Optical Coatings Technology Lab.

Unit of Solar Concentrating Systems

Plataforma Solar de Almería PSA

CIEMAT

Solar absorbers

• Solar absorbers are the responsible to

collect solar radiation.

• Solar radiation produces absorber heating

that is transferred to the substrate.

• Collected energy is exhausted by heat

transfer fluid.

• Thermal losses: conduction-convection

with air and IR emission.

Thermal losses

• Conduction when a material is in direct

contact with other.

• Convection when a material is in contact

with a moving fluid.

• Radiation when a material is at higher

temperature than surroundings.

Absorber thermal losses

• Absorber conduction-convection losses

are reduced:

– Glass covers

– Vacuum between glass and absorber.

• Absorber IR losses????

– Selective absorbers

Solar and IR radiation

• Absorber collects solar radiation and some

energy is emitted as infrared radiation.

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0 2 4 6 8 10Wavelength (microns)

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rma

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5673 K

Optical parameters

Solar radiation

Reflectance (R)

Transmittance (T)

Absorption (a)

1 aTR

Spectral selectivity

• Spectral selectivity:

– Materials that present different behavior in

two broad spectral ranges.

• Examples:

– Glass

– Transparent conductor oxides (TCO)

– Solar selective absorbers

Glass selectivity

• Glass is transparent to solar radiation and

it absorbs far IR radiation

Transparent conductor oxide

• TCO is transparent to visible radiation and

it reflects mid and far IR

Optical properties

1 aTR

1 TR

a Kirchoff’s law

1aR R1a

1R R1

Selective absorber

m

m

m

mhem

s

dGAM

GdAMR

a

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3,0

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5.1

5.1)1(

m

mbb

m

mbbhem

T

dTi

dTiR

25

3,0

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3,0

),(

),()1(

Solar absorptance Thermal emittance

Solar absortance and thermal

emittance

Absorption edge location

Maximum solar absorptance (absorption edge position) vs. absorber

temperature and a fixed thermal emittance.

Tª(ºC) =0,01 =0,05 =0,10 =0,15

5 99,9(4,81) 99,9(6,27) 99,9(7,41) 99,9(8,15)

100 99,7(3,87) 99,9(5,06) 99,9(5,96) 99,9(6,55)

200 99,0(3,05) 99,9(3,99) 99,9(4,70) 99,9(5,17)

300 98,8(2,52) 99,2(3,30) 99,7(3,88) 99,9(4,26)

400 96,8(2,14) 98,8(2,80) 99,2(3,30) 99,4(3,63)

500 95,3(1,87) 98,5(2,44) 98,9(2,87) 99,0(3,16)

Absorptance vs. emittance

• Increasing solar absorptance produces higher thermal

emittance.

• For non concentrating systems, solar absorptance is

more critical than thermal emittance. As temperature

increases, emittance contribution is higher.

– Collection area equal than emission area.

• For high temperature applications, increasing

concentration makes solar absorptance to be more

critical than thermal emittance.

• Collection area higher than emission area.

Absorption edge location

Solar absorptance and thermal emittance (400ºC) for two absorbers with different

absorption edge position and slope.

Solar selective materials

• There is no any material with good solar

selectivity:

– Metals have low emittance and low solar

absorptance.

– Semiconductors and metal oxides have

moderate absortance and high thermal

emittance.

– It is necessary to combine two materials to

obtain good solar selectivity.

Selective absorber types

• Absorber/metal tandem

• Absorber/metal tandem with AR layer

• Optical trapping

• Metal/dielectric composite materials

(Cermets)

• High performance selective absorbers

Infrared reflectors

• Metals such as Cu, Al, Ag, W, Mo:

– Good IR reflectance.

– Low thermal stability in air.

• Noble metals such as Au, Pt:

– Au RIR,=0,98, Pt RIR=0,93.

– Good IR reflectance.

– High thermal stability in air.

– Very thin layers are needed (50 nm)

Metals emissivity

200 300 400 500 600 700 800 900 1000 1100 0

0.01

0.02

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HEMISPHÉRICAL TOTAL EMISSIVITY

FOR SEVERAL METALS

TEMPERATURE (K)

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Ag Cu Al Mo Au Pt

Absorber /IR reflector tandem

• Simplest solar absorber: metal oxide layer

over an infrared reflective metal layer.

• Low absorptance (<0.90) due to high

reflectance losses on oxide layer.

2. Absorber layer

3. Infrared reflector

Metallic substrate

2. Absorber layer

3. Infrared reflector

Metallic substrate

Antireflective layers

1. Antireflective coating

2. Absorber layer

3. Infrared reflector

Metallic substrate

1. Antireflective coating

2. Absorber layer

3. Infrared reflector

Metallic substrate

• AR layers: refractive index between air and metal oxide

layer.

• Solar absorptance increases from 0.90 to 0.96

• Thickness:

Optical trapping

• Increasing average roughness of substrate, solar

absorptance and thermal emittance are higher.

• Actually, it is not used because thermal emittance is too

high and coatings adhesion and durability are poorer.

d

<d d

Metal/dielectric composites (Cermets)

• Cermet: metal particles dispersed in a dielectric matrix.

• High solar absorptance and low thermal emittance.

• Two types: homogenous or graded.

Metal particles Dielectric matrix

GRADED ABSORBER CERMET

Metal particles Dielectric matrix

HOMOGENEOUS ABSORBER CERMET

Solar radiation (optical trapping)

IR radiation (no optical trapping)

LMVF/HMVF cermets

• Increasing metal volume fraction:

– Higher absortance

– Higher reflectance

– Higher emittance

• It is necessary to optimize metal volume

fraction, in both cermet layers, to obtain

best optical properties.

High performance absorbers

• Absorbing layer composed of low and high metal volume

fraction (LMVF/HMVF cermets).

• Diffusion barrier to avoid infrared reflector diffusion into

metal substrate.

2. Absorber layer

3. Infrared reflector

Metallic substrate

1. Antireflective coating

3. HMVF cermet

4. Infrared reflector

Metallic substrate

5. Difussion barrier

2. LMVF cermet

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Wavelength (nm)

Effect of absorbing layer

thickness

AR

absorber

IR reflector

d1 (a=0.937)

d2 (a=0.941) d2>d1

absorber

Effect of AR layer thickness

d1 (a=0.941)

d2 (a=0.947)

d3 (a=0.945) d3>d2>d1

AR (d1,d2,d3)

absorber

IR reflector

Selective absorber types

• Different absorber for each application:

– flat collectors and low temperature vacuum

systems T<150ºC).

– CPC systems for industrial heat (250-350ºC)

– CPC for electricity production (400-550ºC)

– Central receivers (500-100ºC).

Low temperature absorber

Cu or Al

SiO2 AR layer

TiNOx

SnO2 AR layer

• a=0.95, (100C)=0.08

• Stable in air at 200ºC

CPC systems for industrial heat

• Black Chrome: a=0.95, 300ºC=0.13

• Thermal stability: 250ºC

Medium temperature absorbers

High temperature absorbers

• Black paints: high solar absorptance and thermal

emittance

• Solar selectivity is reduced. There is no any selective

absorber for this application.

Durability

• Degradation agents:

– Temperature

– Humidity and condensation

• Degradation modes:

– Oxidation of metallic particles in cermets

and/or IR reflectors.

– Peeling, cracking, adhesion problems.

Durability tests

• Low temperature (IEA task X test): – 600h condensation + 40ºC

– 300h at 250ºC

– PC = -a + 0.25x

• Medium temperature (no test): – 300h temperature 50-100ºC higher than operation

– 600h condensation

• High temperature in vacuum (no test): • xh? temperature 50-100ºC higher than operation

Production methods

• Chemical methods: – Electrodeposition

– Anodic oxidation

– Chemical vapor deposition (CVD)

– Spray pyrolysis

– Sol-gel / Dip-coating

• Physical methods: – Evaporation

– Sputtering

Commercial production methods

• Electrodeposition:

– Black chrome.

• Sputtering:

– All commercial coatings are produced by

sputtering (flat collectors and CPC).

– Graded and homogeneous cermets.

• Sol-gel / dip-coating:

– Spinels (mixed metal oxide coatings).

Electrodeposition

• Advantages:

– Low technology requirements

– Very cheap

– Easy scalability for different sizes and shapes

• Disadvantages:

– Chrome is quite poisonous and contaminant.

– No further production new plants

Sputtering

• Advantages: – Any material can be produced.

– Very good reproducibility and homogeneity.

– Price????.

• Disadvantages: – High technology requirements.

– Not easy scalability for different sizes and shapes

with the same machine.

– Price???.

Sputtering

• In a vacuum chamber, a noble gas is introduced.

• It is ionized and accelerated against the target,

using an electric field.

• Material is removed from the target and it is

deposited on the substrate.

• Coating thickness and properties are controlled

by substrate temperature, time and power of

deposition, controlled atmosphere…

Dip-coating technology

• Three steps:

• Precursor solution

preparation (sol-gel or

inorganic metal salts)

• Sample withdrawal at

constant speed

• Thermal treatment to remove

organics and obtain dense

films Tª

1

-

v

cte precursor

solution

2-

3-

heating

system

sample

precursor

solution

preparation

engine

Sol-gel materials

• Metal oxides:

– SiO2, TiO2, Al2O3, …

• Mixed metal oxides:

– SiO2+TiO2, SiO2 +Al2O3

• Cermets:

– SiO2(Me), TiO2(Me)

Dip-coating materials

• Metal oxides:

– Co, Cu, Al, Mn, Al, …

– Mixed metal oxides

• Cermets:

– Al2O3(Pt), Al2O3(Au), TiO2(Au)

• Metals:

– Pt, Au, Pd

• Metal sulphides

– PbS, CdS, CuS

Advantages and disadvantages

• Advantages: – Low technology requirements.

– Very cheap (low cost materials, energy).

– Easy scalability for different sizes and shapes.

– Precursor solutions last years.

• Disadvantages: – Materials produced are limited.

– Production time for multilayer coatings.

– Bankability.

Layer deposition system

• Coating large tubes vertically

implies high height industrial

installations to withdraw tube from

the precursor solution. So, solution

containers must to be in the

ground.

• It is possible to avoid this problem

using a mobile pot patented by

CIEMAT.

• It consists of a cylindrical container

with a centered hole and a tight O-

ring to avoid solution losses.

Coating is performed moving down

the pot along the tube.

v

Low temperature absorber

Aluminum

Absorbing layer

SiO2 AR layer

MnCuOx/SiO2 layer

a=0.954, (100C)=0.045

Stable in air at 350ºC

Industrial heat applications

• a=0.96, (300C)=0.16

• Stable in air at 400ºC

Stainless steel

Absorbing layer

SiO2 AR layer

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CPC (electricity production)

• a=0.96, (400ºC)=0.087

• Stable in air at 500ºC

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Stainless steel

Absorbing layer

SiO2 AR layer

Difussion barrier

Pt reflector

Difussion barrier

CPC (electricity production)

Stainless steel

Absorbing layer

SiO2 AR layer

Difussion barrier

Pt reflector

Difussion barrier

CPC (electricity production)

Silicon dioxide

Diffusion barrier

Diffusion barrier

Platinum reflector

Absorbing layer

AR layer

Central receivers

• Platinum absorber components are stable

at temperatures higher than 1000ºC.

• Thermal durability depends on interlayer

diffusion.

• Increasing platinum and diffusion barriers

thicknesses thermal stability is improved.

• At present, we are testing 700ºC stable

selective absorbers for tube receivers.