life time analysis & accelerated aging...

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Life time analysis & accelerated aging tests

SFERA Summer School 16.5.2013

Florian Sutter, Johannes Wette, Arantxa Fernandez (Ciemat)

Contents

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Principles of degradation

Outdoor exposure testing

Accelerated aging testing

Conclusions and Outlook

Service life time modeling

Motivation

Chart 3

The cost of corrosion

Goals for improved optical materials

- >30 years lifetime

- >95% specular reflectance (φ = 7.5 mrad)

- specular reflectance loss <1% over life time

- low manufacturing cost < 27 US$/m²

100MW power plant, 40% annual capacity factor

Annual electricity production: 100 MW · 8760 h · 0.4 = 350,400 MWh

1% mirror reflectance loss ≙ 3,504 MWh per year

490,560 US$ / year loss due to degradation (estimated feed-in tariff 0.14 US$/kWh)

[C. Kennedy NREL]

Environmental material stresses during service life

Chart 4

Radiation

Temperature

Mechanical stresses

Environmental pollutants

AbrasionHumidity• ambient relative humidity• dew• rain

• airborne dust or sand• cleaning mechanism

• chemical reactions with dust or sand• pollutants from surrounding industry (e.g. coal plants)• proximity to sea

• wind loads• stresses due to mounting• total solar radiation

• especially UV-A, UV-B

• cyclic day/night temperature changes• frost

R R – O – O – H

R– HR

Photodegradation of paint films

Chart 5

[Zeus; C.Hare, Journal of Protective Coatings and Linings]

• Yellowing• Reduced mechanical strength • Reduced impact resistance• Small surface cracks

– H

UV radiation

+ •HInitiation: Photolisis

Propagation: Autooxidation + O2

Bond cleavage to form free radicals

Peroxy radical formation

– O – O – H +Peroxy radical attack of polymer chain to form hydroperoxide and free radical

– O• + •O – H Fragmentation of hydroperoxide

Termination: Embrittlement

R•R

R• R – O – O•

R – O – O• +

UV radiation

R•

R• •R R R + – Interchain cross linking

Thermal induced cracking and delamination

∆T > 0

Substrate αs

α1

α2

α2 < αS < α1

coating

coatingSubstratecoatingcoating

ET

1

Chart 6

Cracking and delamination

Chart 7

Corrosion of metals

Chart 8

Differential aeration

[U.R. Evans]

[L.Shreir]

0.3VCorrosion pit

M M2+ + 2 e-O2 + 2 H2O + 4e- 4 OH-

Corrosion of metals

Chart 9

Practical galvanic series 2)

Metal VAu +0.24Ag +0.15Cu +0.01X5CrNi18.8 -0.05Ti -0.11Sn -0.19Cr -0.29Pb -0.30Steel 8.8 -0.35Cd -0.52Al (99.5) -0.67Zn -0.80Mg -1.40

2) Potential in artificial sea water with pH 7,5based to the standard hydrogen electrode

Galvanic corrosion appears at potential differences of 0.05 – 0.1 V

Standard electrode potential 1)

Metal VAu +1.5Ag +0.8Cu +0.52Sn +0.15Pb -0.13Fe -0.36Cd -0.40Zn -0.76Cr -0.91Ti -1.21Al (99.5) -1.66Mg -2.36

1) 25 °C; 101,3 kPa; pH=0; effective ion activity= 1based to the standard hydrogen electrode

Galvanic corrosion

Galvanic corrosion of aluminum layer

Galvanic Corrosion

Chart 10

Possible degradation silvered glass mirrors

Chart 11

Glass 4mm

Silver

Low Pb protective lacquer

Pb free top coat

Copper Galvanic dissolution of Cu

UV-light

Embrittlement of lacquer

Delamination of paints

Corrosion of unprotected silver

Examples glass mirror corrosion in the field

Chart 12

Examples glass mirror corrosion in the field

Chart 13

Corrosion can appear <2 years in field

[NREL]

Corrosion of aluminum reflectors

Chart 14

SiO2 sol-gel

Al2O3

SiO2

TiO2

AlCorroded Al-layer

0.5mm

SiO2 - sol-gel protective coating

TiO2SiO2

Al (99.99% purity)

Al2O3

Al-Substrate

3μm

60nm95nm65nm

3μm

Nano-Composite

PVD-layer-system

Anodizing-layer

0.5mm

SiO2 - sol-gel protective coating

TiO2SiO2

Al (99.99% purity)

Al2O3

Al-Substrate

3μm

60nm95nm65nm

3μm

Nano-Composite

PVD-layer-system

Anodizing-layer

Chart 15

Corrosion of aluminum reflectors

O2O2

SiO2

Al

Al2O3

SiO2

SiO2

TiO2

Al

Defect

Al Al3+ + 3 e-

O2 + 2H2O + 4 e- 4OH-

e-

Al2O3

Al Al3+ + 3 e-

e-

Cathodic reaction

Al3+ + 3 H2O Al(OH)3+3H+

2H+ + 2 e- H2

H2-formation

Hydrolysis

O2 + 2H2O + 4 e- 4OH-

Anodic reaction

Crack due to H2-pressure

H2

Differential areation cell

Al-Substrate

Examples aluminum corrosion

Chart 16

PVD reflector in contact with NaCl-solution (50g/l), artificial defect

Chemical reactions dust – polymer coating

Chart 17

Lime rich dust Abu DhabiTransparent polymer coating

Contents

Chart 18


Outdoor Exposure Tests




Outdoor Exposure Testing

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Galvanic separation sample-rack

45° tilt angle

Facing towards equator

+ Covers all environmental stresses

+ Simple

- Requires partners- Sample degradation during handling / shipping

- No acceleration

[Q-lab]

- Various exposure sites needed

Abu Dhabi Tabernas PSACanary Islands Almería

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Outdoor exposure testing of aluminum prototype materials

Exposure program without cleaning of samples

Cleaning with pressurized demineralized water

Cleaning with soft tissue and demineralized water

No cleaning

Exposure program to test influence of cleaning on sample degradation

A B C D E F

S099

0A

S099

2

S099

0C

CanariasTabernasAlmeriaAbu Dhabi

-1,0

0,0

1,0

2,0

3,0

4,0

5,0

6,0

7,0

Spec

ular

refle

ctan

ce lo

ss [%

]Specular reflectance losses ∆ρ(660nm,15°,12.5mrad)

Chart 21

Exposure time: 4.6 - 6 months

16.6%12.7%11.6%6.4 %

Total loss per site

9 differently coated aluminum prototype materials

Reflectance – time dependence

Solar weighted hemispherical reflectance of Flabeg mirrors after outdoor exposure at NREL (Colorado), Arizona (APS), Florida (FLA) and accelerated exposure in Ci65 (1 sun / 60°C / 60%RH).

[C. Kennedy NREL]

Annual degradation rate ~ 0.2%/yr

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Contents

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- Increase temperature to accelerate water diffusion

- Lower wavelengths to accelerate photodegradation (>280nm)

- Increase humidity / condensation cycles / exposure to pollutants (Cl-, S2-, OH-, H+…)

- Extreme temperature cycling to induce cracking, delamination

- Increase mechanical abrasion of coatings

Methods to accelerate degradation:

Accelerated aging tests

Chart 25

IEC 62108 10.7a: Damp heat test 85/85

Chamber temperature: 85 ± 2°CHumidity: 85 ± 5 % relative humidityTesting time: 1000 hours

IEC 62108 10.7b: Damp heat test 65/85

Chamber temperature: 65 ± 2°C Humidity: 85 ± 5 % relative humidityTesting time: 2000 hours


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ISO 11507: UV+Water Test

Chamber temperature: 50 to 60°C Humidity: ambient to 100% relative humidityRadiation: lamp type II, UVA-340; 290-400 nm; peak emission at 340nm;

lamp power matches 1 sunCycle time: 8 hoursTesting time: >1000 hours


Chart 27

ISO 9227: Neutral salt spray test (NSS)

Chamber temperature: 35 ± 2 °C Humidity: constant 100% relative humiditySprayed solution: demineralized water + 50 g/l NaCl

(pH 6.5 – 7.2)Condensation rate: 1.5 ± 0.5 ml/h on a surface of 80 cm² Sample position: 20 ± 5° respect to vertical Testing time: 480 – 3500 hours


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ISO 9227: Copper accelerated salt spray test (CASS)

Chamber temperature: 50 ± 2 °C Humidity: constant 100% relative humiditySprayed solution: demineralized water + 50 g/l NaCl + 0.26 g/l CuCl2

(pH 3.1 – 3.3)Condensation rate: 1.5 ± 0.5 ml/h on a surface of 80 cm² Sample position: 20 ± 5° respect to vertical Testing time: 120 – 480 hours

CuCl2


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DIN 50018 / ISO 6988: Kesternich Test

Chamber temperature: ambient / 40 ± 3°C Humidity: ambient / 100% relative humidityInitial SO2 concentration: 0.33 or 0.67% of volume of testing chamberCycle time: 24 hoursTesting time: >20 cycles


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ISO 61215: Thermal Cycling

Chamber temperature: -40°C to +85°C Humidity: dryCycle duration: min. 2h 50min, max. 6h Recommended cycle number: >100


Chart 31

Thermal Cycling with humidity based on ISO 6270-2CH

Chamber temperature: -40°C to +85°C Humidity: ambient to 100% relative humidityCycle duration: 24 hRecommended cycle number: >20


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Humidity Freeze Test IEC 62108

Chamber temperature: -40°C to +65°C Humidity: ambient to 85% relative humidityPrecycling: 400 cyclesCycle duration: 24 hFreeze cycle number: 40Total testing time: ~2000h

400 precycles -40 to 65°C, dry 40 humidity-freeze cycles


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Abrasion testing

Available standards: MIL-STD 810G, ISO 11998, DIN ISO 9211-4

Simulation of windblown dust and sand particles

Simulation of cleaning cycles Scratching of coatings with controlled normal force

Accelerated aging of aluminum - Test comparison

Outdoors (5 months) Salt spray (NSS) Damp heat 85/85 UV+humidity

+ Realistic defect corrosion

- Unrealistic side effects

- Long exposure time >2000 h

Corrosion at coating defects

Corrosion due to diffusion

>600h 1000h 1000h

- Unrealistic cracking for some coatings- Long exposure time

+ Realistic corrosion due to diffusion- Unrealistic degradation due to high acceleration

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Accelerated aging of aluminum – Test comparison

70,0

72,0

74,0

76,0

78,0

80,0

82,0

84,0

86,0

0 500 1000 1500 2000 2500 3000

Hours

spec

ular

refle

ctan

ce [%

]NSSUV+humidityDamp HeatOutdoor 6 months

Chart 35

Typical testing program for reflectors

Chart 36

Contents

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Photodegradation of polymeric glazing materials

[NREL, Köhl et al. / Solar Energy 79 (2005) 618-623]

UV- light concentrator

nm

nmUV dtEtE

385

290

,)(

ttTk

EpUV dteEtEKt B

0

)(

0

E0 = 44.5 W/m²

K (s-1)3.69.6e-5

Chart 38

Life time prediction model for a PVD aluminum reflector

Chart 39

0 0.25 0.49 0.94 2.02 3.12 months

tkfc exp1

k = 6.4 ·10-3 months-0.5

k = 2.9 ·10-3 months-0.5

k = 1.1 ·10-3 months-0.5

k = 5.3 ·10-2 months-0.5

Life time prediction model for a PVD aluminum reflector

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tmonths

tktmonths

tkt

%057.0exp%04.0%1.44exp1 00

ρ0 = 86.5 for reflectors without sol gel

ρ0 = 83.5 for reflectors with sol gel

Initial specular reflectance Losses due to corrosion

average reflectance of corroded area

Losses due to abrasion

Correlation of Neutral Salt Spray test to outdoors

Only valid for sol-gel coated PVD aluminum reflectors!

10 years Florida ≙ 2200 h

10 years Golden ≙ 461 h

10 years Tabernas ≙ 67 h

Exposure time outdoors t [months]

Expo

sure

tim

e in

NSS

-test

t NSS

[h]

Chart 41

Chart 42


Further research required to

- Understand the degradation mechanisms of the several reflector types

- Improve existing accelerated aging tests (e.g. include reaction with dust)

- Define suited accelerated aging standard for CSP reflectors

- Correlate accelerated aging tests to reference outdoor sites for several materials

- Accelerated aging tests are a useful tool to compare materials under defined laboratory stresses

- Service life time prediction models are semi-empirical. Mechanistic models are aimed at but they are complex.

- At too high stresses unrealistic chemical processes may take place

- Comparison to Outdoor Exposure Tests recommended

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