thermal components

Solar Energy Materials & Solar Cells 84 (2004) 255274

The applicability of accelerated life testingfor assessment of service life of solar

thermal components

B. Carlssona,*, K. M .ollera, M. K .ohlb, M. Heckb, S. Brunoldc,U. Freic, J.-C. Marechald, G. Jorgensene

aSP Swedish National Testing and Research Institute, P.O. Box 857, SE-501 15, Bor (as, SwedenbFraunhofer Institute for Solar Energy Systems (ISE), Heidenhofstr. 2, D-79110 Freiburg, Germanyc Institut f .ur Solartechnik (SPF), Hochschule Rapperswil HSR, CH-8640 Rapperswil, Switzerland

dCSTB Centre Scientific et Technique du Batiment, F-38400 Saint-Martin DHeres, FranceeNational Renewable Energy Laboratory (NREL), Golden, CO 80401-3393, USA

Received 31 October 2003; accepted 16 January 2004

Available online 2 June 2004

Abstract

To achieve successful commercialisation of new advanced windows and solar fa@adecomponents for buildings, the durability of these need to be demonstrated prior to installation

by use of reliable and well-accepted test methods. In Task 27 Performance of Solar Facade

Components of the IEA Solar Heating and Cooling Programme work has therefore been

undertaken with the objective to develop a general methodology for durability test procedures

and service lifetime prediction methods adaptable to the wide variety of advanced optical

materials and components used in energy efcient solar thermal and buildings applications. As

the result of this work a general methodology has been developed. The proposed methodology

includes three steps: (a) initial risk analysis of potential failure modes, (b) screening testing/

analysis for service life prediction and microclimate characterisation, and (c) service life

prediction involving mathematical modelling and life testing.

The applicability of the working scheme to be employed in the development of durability

test procedures has been analysed for selective solar absorber surfaces and polymeric glazing

materials in at plate solar collectors. The examples show the great applicability of the general

methodology for accelerated life testing. This will allow much shorter development cycle times

ARTICLE IN PRESS

*Corresponding author.

E-mail address: [email protected] (B. Carlsson).

0927-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.solmat.2004.01.046

for new products and will allow improvements to be identied and readily incorporated in new

products prior to market introduction.

r 2004 Elsevier B.V. All rights reserved.

Keywords: Solar thermal materials; Durability; Service life prediction; Accelerated testing; Selective solar

absorber surface; Polymeric glazing material

1. Introduction

To achieve successful and sustainable commercialisation, building products mustmeet three important criteria, namely minimum cost, sufcient performance, anddemonstrated durability.Durability assessment directly addresses all three segments of this triad. First, it

permits analysis of life cycle costs by providing estimates of service lifetime, O&Mcosts, and realistic warranties. Understanding how performance parameters areaffected by environmental stresses (for example by failure analysis) allows improvedproducts to be devised. Finally, mitigation of known causes of failure directly resultsin increased product longevity. Thus, accurate assessment of durability is of para-mount importance to assuring the success of solar thermal and building products.The IEA Solar Heating and Cooling Programme, Task 27 on the Performance of

Solar Facade Components started at the beginning of year 2000 with the objectivesof developing and applying appropriate methods for assessment of durability,reliability and environmental impact of advanced components for solar buildingfacades.For the work on durability there are two main objectives. The rst was to develop

a general framework for durability test procedures and service lifetime predictionmethods that are applicable to a wide variety of advanced optical materials andcomponents used in energy efcient solar thermal and buildings applications. Thesecond is to apply the appropriate durability test tools to specic materials/components to allow prediction of service lifetime and to generate proposals forinternational standards.This paper presents a general methodology to meet the rst objective. A thorough

description of the methodology can be found in the Final Report of IEA SolarHeating and Cooling Task 27 Project B1 on Durability Assessment MethodologyDevelopment [1]. The methodology is also a part of the forthcoming bookPerformance and Durability Assessment of Optical Materials for Solar ThermalSystems [2].

2. General methodology

Many efforts have been made to develop systematic approaches to service lifeprediction of components, parts of components and materials so that all essentialaspects of the problem will be taken into consideration, see e.g. Refs. [38].

ARTICLE IN PRESSB. Carlsson et al. / Solar Energy Materials & Solar Cells 84 (2004) 255274256

In one such methodology, which was the focus in IEA Task 27, a predictive failuremodes and effect analysis (FMEA) serves as the starting point for service lifeprediction from accelerated life test results as is illustrated schematically in Fig. 1.The analysis is made on the component level.The diagram in Fig. 1 is based on a similar scheme that was developed for the

purpose of accelerated life testing of selective solar absorber surfaces in a joint casestudy of Task 10 of the IEA Solar Heating and Cooling Program [8].

PENALTY is the level at which an assessment is made of the economic effects of acomponent failure. Based on this assumption, it is possible to set a reliability levelthat must be maintained for a given number of years.

FAILURE is the level at which performance requirements are determined. If therequirements are not fullled, the particular component or part of component isregarded as having failed. Performance requirements can be formulated on the basisof optical properties, mechanical strength, aesthetic values or other criteria related tothe performance of the component and its materials.

DAMAGE describes the stage of failure analysis at which various types of damage,each capable of resulting in failure, can be identied.

CHANGE is related to the change in the material composition or structure thatcan give rise to the damage of the type previously identied.

EFFECTIVE STRESS is the level at which various factors in the microclimate,capable of being signicant for the durability of the component and its materials can

ARTICLE IN PRESS

A. Initial risk analysis of

potential failuremodes

B. Qualification testing andscreening testing/analysis for service life prediction

C. Service lifeprediction

PENALTY

Performance requirement

FAILURE

Qualification testing(envi- Mathematical modelling (rate of degrada-

DAMAGE Screening testing (acceler-ated aging at elevated levels

tion process in terms of effective levels of deg-

of the degradation factors) Analysis of material change

radation factors) Life testing

CHANGE mechanisms)

Assessment of expected service life

Reasonability of assessment/validation

EFFECTIVESTRESS

Microclimate characteriza- tion(influence of degradation

factors at materials level)

LOADS

(minimum material property)

ronmental resistance testing)

(identification of degradation

Fig. 1. Failure mode analysis for planning of accelerated tests for service life prediction.

B. Carlsson et al. / Solar Energy Materials & Solar Cells 84 (2004) 255274 257

be identied. An important point here is that it is possible to make quantitativecharacterisation.

LOADS is the level that describes the macro-environmental conditions (climatic,chemical, mechanical), and which is therefore a starting point for description of themicroclimate or effective stress as above.Each step in the scheme on the left hand side of Fig. 1 may be related to the

subsequent step by an appropriate deterministic or statistical relationship. Therelation should dene the expected results of all the various activities involved inaccelerated life testing, as indicated on the right hand side of Fig. 1.

3. Initial risk analysis of potential failure modes

The rst step in the scheme illustrated in Fig. 1 is an analysis of potential failuremodes with the aim of obtaining (a) a checklist of potential failure modes of thecomponent and associated with those risks and critical component and materialproperties, degradation processes and stress factors, (b) a framework for the selectionof test methods to verify performance and service life requirements, (c) a frameworkfor describing previous test results for a specic component and its materials or asimilar component and materials used in the component and classifying their relevanceto the actual application, and (d) a framework for compiling and integrating all dataon available component and material properties and material degradation technology.From a practical point of view, but also from an economic viewpoint, an

assessment of durability or service life has to be limited in its scope and focused onthe most critical failure modes. An important part of the initial step in such anassessment is therefore estimating the risk associated with each of the potentialfailure modes of the component.The programme of work in the initial step of service life assessment may be

structured into the following activities [9]: (a) Specify from an end-user point of viewthe expected function of the component and its materials, its performance and itsservice life requirement, and the intended in-use environments; (b) Identifyimportant functional properties dening the performance of the component andits materials, relevant test methods and requirements for qualication of thecomponent with respect to performance; (c) Identify potential failure modes anddegradation mechanisms, relevant durability or life tests and requirements forqualication of the component and its materials as regards durability. Whenidentifying potential failure modes, it is important to distinguish between (1) failuresinitiated by the short-term inuence of environmental stress, the latter representingevents of high environmental loads on the component and its materials, (2) failuresinitiated by the long-term inuence of environmental stress, the latter causingmaterial degradation so that the performance and sometimes also the environmentalresistance of the component and its materials gradually decrease, and (d) estimationof the risk associated with each failure mode identied.The result of the initial risk analysis of potential failure modes may be documented

as shown in Tables 1ac using information from a case on accelerated life testing


performed in Task 10 of the IEA Solar Heating and Cooling Programme [8]; see alsoFig. 2.The rst activity species in general terms the function of the component and

service life requirement from an end-user and product point of view; see Table 1a,and from that identies the most important functional properties of the componentand its materials; see Table 1b.How important the function of the component is from an end-user and product

point of view needs to be taken into consideration when formulating theperformance requirements in terms of those functional properties. If theperformance requirements are not fullled, the particular component is regardedas having failed. Performance requirements can be formulated on the basis of opticalproperties, mechanical strength, aesthetic values or other criteria related to theperformance of the component and its materials. Dening performance requirementshould be accompanied by an assessment of the economic effects of a componentfailure. Based on this, it is possible to dene a service life requirement or set areliability level that must be maintained for a given number of years.Potential failure modes and important degradation processes should be identied

after failures have been dened in terms of minimum performance levels. In general,there exist many kinds of failure modes for a particular component and even thedifferent parts of the component and the different damage mechanisms, which maylead to the same kind of failure, may sometimes, be quite numerous.The objective of analysis is to identify potential failure/ damage modes and

mechanisms that may lead to material degradation and the development of damage, andassociated critical factors of environmental stress or degradation factors; see Table 1c.The risk or risk number associated with each potential failure/damage mode

identied can be estimated by use of the well-known methodology of FMEA; see e.g.Ref. [10]. The risk number is estimated by use of the following factors: Severity (S),Probability of occurrence (PO), and Probability of escaping detection (PD). The risknumber RPN is the product of all these factors, i.e. RPN=S PO PD. In the riskassessment of potential failure modes, the relevance of durability and life data foundin the literature for the component and its materials must be taken into account. Therisk assessment should most advantageously be performed by a group of experts.The estimated risk number is taken as the point of departure to judge whether a

particular failure mode needs to be further evaluated or not. The estimated risknumber may also be used to determine what kind of testing is needed for quali-cation of a particular component and its materials; see the example in Table 1c.

4. Screening testing and analysis for service life prediction

4.1. Screening testing by accelerated ageing

Screening testing is thereafter conducted with the purpose of qualitativelyassessing the importance of the different degradation mechanisms and degradationfactors identied in the initial risk analysis of potential life-limiting processes.

ARTICLE IN PRESSB. Carlsson et al. / Solar Energy Materials & Solar Cells 84 (2004) 255274 259

ARTICLE IN PRESS

Table 1

Example of result from an initial risk analysis of potential failure modes based on information taken from

the IEA Task 10 case study on selective solar absorber surfaces

Function and general

requirements

General requirements

for long-term

performance during

design service time

In-use conditions

and severity of

environmental stress

(a) Specification of end-user and product requirements on component

Efciently convert

solar radiation

into thermal energy

Suppress heat losses in

the form of thermal

radiation

Loss in optical

performance

should not result in

reduction of the

solar system energy

performance (solar

fraction) with more

than 5%, in

relative sense,

during a design

service time of

25 years

Behind glazing in

contact with air

Casing of collector

exchange air with the

ambient, meaning that

airborne pollutants

will enter collector

If the collector is not

rain tight the humidity

level of air in

the collector may

become high

Maximum

temperature 200C

Critical functional

properties

Test method

for determining

functional property

Requirement for

functional capability and

long-term performance

b) Specification of functional properties and requirements on component and its materials

Solar absorptance (a)Thermal emittance (e)Adhesion (ad)

ISO CD 12592.2

ISO CD 12592.2

ISO 4624

Functional capability

a>0.92eo0.15ad>0.5MPa

Long-term

performance

Da+0.25De r0.05

Failure/damage mode/

degradation process

Degradation

indicator

Critical factors of

environmental stress/

degradation factors

Estimated risk of

failure/damage mode

from FMEA

(c) Potential failure modes, critical factors of environmental stress/degradation factors, and risk assessment

by failure modes effect analysis (FMEA)

Unacceptable loss in

optical performance

PCa S PO PD Risk

RPN

High temperature

oxidation of

metallic nickel

Reection spectrum

Vis-IR

High temperature 7 2 8 112

Electrochemical corrosion

of metallic nickel

Reection spectrum

Vis-IR

High humidity, sulphur

dioxide (atmospheric

corrosivity)

7 5 5b 175

B. Carlsson et al. / Solar Energy Materials & Solar Cells 84 (2004) 255274260

ARTICLE IN PRESS

Table 1 (Continued)

Failure/damage mode/

degradation process

Degradation

indicator

Critical factors of

environmental stress/

degradation factors

Estimated risk of

failure/damage mode

from FMEA

Hydratization of

aluminium oxide

Reection

spectrum IR

Condensed water,

temperature

7 7 4b 196

FMEA involves estimation of Severity (S), Probability of occurrence (PO), and Probability of escaping

detection (PD) of a particular failure/damage mode. The risk number RPN is the product of all these

factors, i.e. RPN=S PO PD.aPC = Performance criteria = Da+0.25De.bPD value resulted from possible failure of the glazing.

Fig. 2. Principal components of the selective solar absorber system studied in IEA Task 10 for use in single

glazed at plate solar collectors to be installed in domestic hot water (DHW) systems [8]. (a) Component/

material: Selective solar absorber coating of anodised aluminim pigmented with small metallic nickle

particles; (b) Cross section of at-plate collector; (c) Application: Use in single-glazed at-plate solar

collector for Domestic Hot Water system.


When selecting the most suitable test methods for screening testing, it is importantto select those with test conditions representing the most critical combination ofdegradation factors; see example in Table 2.

4.2. Analysis of material change during ageing

Using articially aged samples from the screening testing, changes in the keyfunctional properties or the selected degradation indicators are analysed with respectto associated material changes. This is made in order to identify the predominantdegradation mechanisms of the materials in the component. When the predominantdegradation mechanisms have been identied also the predominant degradationfactors and the critical service conditions determining the service life will be known.Screening testing and analysis of material change associated with deterioration in

performance during ageing should therefore be performed in parallel. Suitabletechniques for analysis of material changes due to ageing may vary considerably; seee.g. Ref. [11].In Table 3 an example from the IEA absorber surface case study is shown that

demonstrates how different techniques for analysing material changes resulting fromageing can be used to get information on what material degradation mechanisms arecontributing to deterioration in performance.

ARTICLE IN PRESS

Table 2

Programme for screening testing in the IEA case study on solar absorber surfaces

Possible degradation

mechanism

Critical periods of high

environmental stress

Suitable accelerated test

methods and range of

degradation factors

High temperature oxidation of

metallic Ni particles

Stagnation conditions of solar

collector at high levels of solar

irradiation (no withdrawal of

heat from the collector)

Constant load high

temperature exposure tests in

the range of 200500C

Electrochemical corrosion of

metallic Ni particles at high

humidity levels and in the

presence of sulphur dioxide

Under starting-up and under

non-operating conditions of

the solar collector when the

outdoor humidity level is high

Exposure tests at constant high

air humidity (7595% RH),

constant temperature

(2050C), and in the presence

of sulphur dioxide (01 ppm)

Hydratization of aluminium

oxide and electrochemical

corrosion of metallic Ni

particles by the action of

condensed water

Under humidity conditions

involving condensation of

water on the absorber surface

Exposure tests under constant

condensation (sample surface

cooled 5C below surrounding

air which is kept at 95% RH)

and temperature conditions

ranging from 1090C


4.3. Microclimate characterisation for service life prediction

In order to be able to predict expected service life of the component and itsmaterials from the results of accelerated ageing tests, the degradation factors underservice conditions need to be assessed by measurements. In Table 4 measurementtechniques that were used in the IEA Task 10 absorber case study previouslyreviewed are given as an example of what factors were needed to take intoconsideration in this study. It is of extreme importance to characterize the serviceconditions in terms relevant for the most important degradation mechanismsidentied for the materials of the component but also in terms relevant for andconvertible into the test conditions for the environmental resistance tests to be usedfor accelerated life testing.

ARTICLE IN PRESS

Table 3

Results from the IEA Task 10 solar absorber case study in the analysis of material change upon durability

testing

Degradation

mechanism

Techniques for analysis

of material changes

Results

High temperature

oxidation of metallic Ni

particles

UV-VIS-NIR reectance

spectroscopy

AES depth proling

SEM-EDX

XRD

Reduction of absorption in solar range

corresponding to reduction in metal

concentration

Formation of Ni oxides

Small changes in surface morphology

Formation of NiO

Electrochemical


particles at high

humidity levels and in

the presence of sulphur

dioxide

UV-VIS-NIR reectance

spectroscopy

FTIR reectance

spectroscopy

AES depth proling

SEM-EDX

Reduction of absorption in solar range

corresponding to reduction in metal

concentration

Formation of sulphate

Increase in surface concentration of Ni

accompanied with sulphur at the

surface

Surface morphology affected and

detection of sulphur

Hydratization of

aluminium oxides and

electrochemical

corrosion of the

metallic particles by the

action of condensed

water

UV-VIS-NIR

reectance spectroscopy

FTIR reectance

spectroscopy

AES depth proling

SEM-EDX

Some changes hard to explain

Formation of hydrated forms of

aluminium oxide leading to increased

thermal emittance

Change in surface structure

Considerable change in surface

morphology

The following analytical techniques were used ultraviolet-visible-near infrared-reectance spectroscopy

(UV-VIS-NIR reectance spectroscopy), Auger electron spectroscopy depth proling (AES depth

proling), Fourier transform infrared reectance spectroscopy (FTIR reectance spectroscopy), X-ray

diffraction (XRD), scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX).


If only the dose of a particular environmental stress is important then thedistribution or frequency function of a degradation factor is of interest. In the IEAabsorber case study, only the distribution in the absorber temperature was neededfor predicting the service life limited by high temperature degradation; see Fig. 3a. Incase of service life prediction considering degradation caused by the action of highhumidity and condensed water, both air humidityabove a threshold of 99%andsurface temperature were taken into account; see Fig. 3b. In case of service life

ARTICLE IN PRESS

Table 4

Techniques that were used in the IEA Task 10 absorber case study for measurment of degradation factors

in solar collectors operating under service conditions

Degradation mechanism Degradation factors/

measurement variables

Sensors

High temperature oxidation

of metallic Ni particles

Temperature: Surface temperature

of absorber plate

Pt sensors in holders

screwed directly on the

absorber plate. To

accomplish a good thermal

contact heat sink compound

was used

Electrochemical corrosion

of metallic Ni particles at

high humidity levels and in

the presence of sulphur

dioxide

Atmospheric corrosivity:

Measurement of corrosion mass

loss rate of standard metal

specimens

Air pollutants: Measurement of

sulphur dioxide concentration

inside and outside of the solar

collector

Metal coupons of carbon

steel, zinc and copper and

evaluation of corrosion

mass loss according to ISO

9226

Exposed metal coupons

analyzed in respect of the

sulphate content of the

corrosion products by EDX

UVuorescence

instrument for direct

measurement of sulphur

dioxide concentration in the

air outside and inside of the

solar collector





condensed water

Humidity: Measurement of air

humidity in the air gap between the

absorber plate and glazing cover of

the collector

Time of condensation:

Measurement of specular

reectance of absorber surface

Surface humidity: Measured

relative air humidities converted to

relative humidity on surface by use

of measured surface temperatures

Capacitance humidity

sensors carefully shielded

from solar radiation and

thermal radiation of the

ambient

Special designed reectance

mode condensation sensor


prediction considering electrochemical corrosion of the metallic nickel particles theyearly corrosion of zinc in the air gap between the absorber plate and the transparentglazing was determined.

ARTICLE IN PRESS

Fig. 3. Results from measurement of microclimate for the absorber in the IEA absorber surface case study

(a) Absorber temperature frequency function for one year. For 1 month of the year the collector was under

stagnation conditions (upper diagram). (b) Absorber temperature frequency function when RHX99% ofthat year (lower diagram).


5. Service life prediction from results of accelerated testing

5.1. Mathematical modelling

To perform an accelerated test, D; means that the level of at least one stress factor,X ; causing degradation is kept at a higher level relative to the situation in service.Consequently this means the time to failure in the accelerated test, tf ;D; will beshorter relative to service life, ts: The ratio between the latter and the former iscommonly referred to as the acceleration factor, A: If the applied stress is constant intime also for service conditions, the acceleration factor a can be expressed as

A tS=tf ;D aX gXD=gXS; 1

where the expression gXD=gXs is called the time transformation function or theacceleration factor function. Examples of time transformation functions can befound in e.g. reports by Martin [12] and by Carlsson et al. [1,13]. The timetransformation functions, aX ; used in the absorber case study are shown in Table 5.

5.2. Accelerated life testing and assessment of expected service life

Accelerated life testing means to quantitatively assess the sensitivity to the variousdegradation factors on the overall deterioration of the performance of thecomponent and its materials in terms of the mathematical models set up tocharacterize the different degradation mechanisms identied. Life testing thereforerequires conducting a series of tests.From the accelerated life test results the parameters of the assumed model for

degradation are determined and the service life then estimated by extrapolation toservice conditions. If the service conditions vary, effective mean values of stress needto be assessed from measured service stress data [1,8]; see also Fig. 3 and Table 6.As a result, it may be possible to express the importance of different degradation

mechanisms in terms of expected service life values, see example from the absorbersurface case study in Table 5.

5.3. Reasonability assessment and validation

By use of accelerated life testing, potential degradation mechanisms limiting theservice life of a component may be identied. However, it is important to point outthat it is only the service life determined by the material degradation mechanismsobserved in the accelerated tests at relative high levels of stress that can be assessed.Life-limiting degradation mechanisms may exist that cannot be identied by way ofaccelerated life testing because the knowledge and experience in what may causedegradation of a particular material in a component may be too limited.The best approach in validating an estimated service life from accelerated testing,

therefore, is to use the results from the accelerated life tests to predict expectedchange in material properties or component performance versus service time andthen by long-term service tests check whether the predicted change in performance


with time is actually observed or not. In 6.1 an example of such a check forpolymeric glazing materials are presented.The results of validation tests therefore can be used to revise a predicted service life

and form the starting point also for improving the component tested with respect toenvironmental resistance, if so required. It should be remembered that the mainobjective of accelerated life testing is to try to identify those failures which may lead

ARTICLE IN PRESS

Table 5

Estimated service life of the nickel-pigmented anodised aluminium absorber surface in the IEA Task 10

absorber case study

Degradation mechanism Time transformation

function

Estimated service life with

PC=Da+De o 0.05a(years)



aT=exp[(Ea/R) (1/TD1/Ts)]Ea=activation energy

R=general gas law constant

TD=temperature of test

Ts=effective mean temperature

at service

>105

Electrochemical corrosion of

metallic Ni particles at high

humidity levels and in the

presence of sulphur dioxide

aCo=tM,s/tM,DtM,s=time to reach a certainextent of corrosion of reference

metal in service

tM,D=time needed to reach thesame extent of corrosion of

reference metal in test D

(zinc used as reference metal)

12

(The coating is assumed to

be installed in a non-

airtight highly ventilated

collector)

34


be installed in an airtight

collector with controlled

ventilation)





condensed water

a1T ;H tH expEH;T=R

T1H;eff T1D

9


be installed in an non-

airtight highly ventilated

collector)

TH,eff= effective mean

temperature of the absorber

surface when the relative

humidity in the air gap is equal

to or higher than 99%

tH=the time fraction of the year,time-of-wetness, during which

the relative humidity in the air

gap is equal to or higher

than 99%

EH,T=Arrhenius activation

energy

Values are given for the different degradation mechanisms and assuming that the different degradation

mechanisms are acting alone. How the effective mean values of stress were calculated are shown in Table 6.aPC=0.05 corresponds to a decrease in the solar system performance of 5%.


to an unacceptable short service life of a component. In terms of service life, the mainquestion is most often, whether it is likely or not, that the service life is above acertain critical value.In order to validate the predicted service life data from accelerated life testing in

the IEA absorber case study the actual service degradation in optical performance ofthe nickel pigmented anodized aluminium absorber coating was investigated [14].Samples from the coating taken from collectors used in solar DHW systems for timeperiods of 10 years or more were analysed for that purpose. It could be concluded

ARTICLE IN PRESS

Table 6

Denitions of effective mean values of different stresses used in the IEA Task 10 absorber case study

Degradation mechanism Effective mean value of stress



Effective mean temperature of stress at service TEFF,s was

dened as

expEa=R 1=TEFF;s

R Tmax

Tminexp Ea=R 1=T f T dT

Ea=activation energy

R=general gas law constant

f (T)=frequency function for absorber temperature

distribution during one year; see Fig. 3a

Tmin=minimum temperature during the year (K)

Tmax=maximum temperature during the year (K)

Electrochemical corrosion of metallic

Ni particles at high humidity levels and

in the presence of sulphur dioxide

Effective atmospheric corrosivity in the air-gap between

absorber plate and transparent glazing was determined by

use of metallic zinc as reference material. Zinc coupons

were exposed in the collector for one year and the metallic

mass loss was determined to 0.3 g/m2/year

Hydratization of aluminium oxide and

electrochemical corrosion of metallic Ni

particles by the action of condensed

water

Effective mean temperature of the absorber surface when

the relative humidity in the air gap is equal to or higher

than 99%, i.e. TH,eff, was dened as

expEH;T =R T1H;eff R TH;max

TH;minexpEH;T =R T1H

fHTH dTH

EH,T=Arrhenius activation energy

fH(TH)=the yearly based frequency function for the service

temperature of the absorber surface in a solar collector

when the relative humidity level exceeds 99%, being the

time fraction of a year when the service temperature is in

the interval T to T+dT and the relative humidity level

exceeds 99%; see Fig. 3b

TH,max=the maximum service temperature of the absorber

surface in a collector, when the relative humidity level

exceeds 99% (K)

TH,min=equal to 273K, as below this temperature ice is

formed on the surface of absorber


that the agreement between degradation data determined for the absorber samplesfrom the DHW systems and that from accelerated life testing from the Task 10 studywas astonishingly good both from a quantitative and a qualitative point of view; seeTable 7 and Fig. 4. For the absorber coating in a properly designed solar collector,the service life seems good enough. For the absorber coating in a non air tight solarcollector, probably because of glazing failures, the humidity level is raised to suchhigh levels that the service life is reduced to an unacceptable level.

6. Adoption of the general methodology for durability and service life assessment topolymeric glazing materials

Polymeric glazing materials offer signicant potential for cost savings both asdirect substitutes for glass cover plates in traditional collector systems and as anintegral part of all-polymeric systems. However, glazing materials should have hightransmittance over the entire solar spectrum and must be able to resist long term (25years) exposure to service conditions under solar ultraviolet (UV) light and elevatedtemperature loads sometimes exceeding 80C under shorter periods of stagnationconditions of a solar collector. They must retain mechanical integrity (for example,impact resistance and exural rigidity) under these harsh environmental stresses.In the IEA Working Group Materials in Solar Thermal Collectors, Polyvinyl

cloride (PVC) and UV stabilized Polycarbonate (PC) glazing materials were studied.For validating the applicability of the general methodology on durability assessmentdeveloped by IEA Task 27, it was therefore decided to make use of the results fromthis previous IEA polymeric glazing study [1]. This study is also reviewed in theforthcoming book Performance and Durability Assessment of Optical Materials forSolar Thermal Systems [15].

ARTICLE IN PRESS

Table 7

Some general characteristics of six solar DHW systems from which nickel-pigmented anodised aluminium

absorber samples were analysed after long period of service. More details about the systems can be found

in Ref. [14]

DHW-system and

location

Age (years) Notes

DK 1 (Denmark) 12

DK 2 (Denmark) 11 One n replaced due to frost burst

DK 3 (Denmark) 10

DK 4 (Denmark) 10 One n replaced due to frost burst

DK 1 (Switzerland) 15 One collector leak has been repaired

DK 2 (Switzerland) 15 Plastic cover has been replaced once and a

collector leak repaired


6.1. Initial risk analysis of potential failure modes of polymeric glazing materials

and screening testing

One of the most critical material properties related to the use of polymericmaterials as solar collector covers is the maximum recommended service temperaturefor the material or in this case the closely related deection temperature of the

ARTICLE IN PRESS

Fig. 4. Comparison between the estimated service life of an absorber coating from actual service exposure

and from the results of accelerated life testing [14].


material under mechanical load. This requirement, set by the maximum covertemperature observed when the collector is under stagnation conditions, narrowsdown the number of possible polymeric materials considerably. The PC glazingmaterial (APEC 9353 UV-stabilized polycarbonate) meets this requirement but thePVC material (Duroglas) does not, because its glass transition takes place alreadyaround 80C. The PVC material also does not meet the requirement on minimumservice temperature.The mechanical properties of a polymeric glazing material may also be critical

when used as a collector cover. Especially as an effect of ageing the mechanicalproperties of the polymeric materials may deteriorate to a level at which failure mayoccur induced by e.g. wind or snow loads and because of impacts from ying objectslike hail.To summarize the analysis on the critical functional properties of the two

polymeric glazing materials, it is evident that the PVC glazing material does notqualify for use as a solar collector material. However, the PC material was qualiedat this stage of evaluation.With respect to durability, two kinds of failures were considered, one related to a

gradual decrease in the optical performance of the cover material and one related toinsufcient mechanical strength of the material leading to either breakage or plasticdeformation of the cover.Reduction in the optical performance of a PC glazing may be due to yellowing.

This kind of chemical degradation of a PC material, due to photooxidation orthermal oxidation, may also result in deterioration in the mechanical properties ofthe material, which may result in a breakage of the cover when subjected to a highmechanical load. It was believed that photooxidation and thermal oxidation resultingin an unacceptable optical performance were the most important and needed to befurther studied by ageing testing. Failures resulting from an insufcient mechanicalperformance caused by ageing are believed to be of less probability of occurrence.The optical performance of a PVC glazing material may be deteriorated by

dehydrochlorination causing yellowing. As the lowest as well as the highest expectedcover temperature for the considered application are outside of the recommendedservice temperature range of PVC, mechanical failures due to that most likely willoccur. But, it was also believed that dehydrochlorination and photooxidationresulting in an unacceptable optical performance are important and need to befurther studied by ageing testing. Failures resulting from an insufcient mechanicalperformance caused by ageing were believed to be of second order interest.From the screening tests it could be concluded that the dominating mechanism of

degradation of the PC glazing material was photooxidation and in the case of thePVC material photochemical dehydrochlorination. The two most dominatingdegradation factors were thus temperature and UV-radiation.

6.2. Service life prediction from accelerated ageing

Because only the degradation mechanism of photooxidation seemed to contributesignicantly to the service life of the PC glazing, the time-transformation function as


shown in Eq. (2) was used for modeling of accelerated test data. The same timetransformation was used to model the degradation of the PVC glazing.

aPO IP expEa=RTEFF;D=IP expEa=RTEFF;s; 2

where aPO is the acceleration factor for photooxidation, I the intensity ofphotoactive light, p the material dependent constant which value has to bedetermined by accelerated testing, Ea the activation energy, R the general gas lawconstant, T the temperature [K], s the index for service; D the index for test, EFF theeffective mean value.By conducting a series of accelerated tests at elevated levels of surface temperature

and UV-light intensity, the parameters p and Ea in Eq. (2) could be determined forthe two glazing materials by Jorgensen et.al. [16]. By measuring glazing surfacetemperartures and UV-light intensities during outdoor exposure, it was then possiblefor them to predict expected degradation in optical performance by making use ofthe results from accelerated ageing. As shown in Fig. 5 the agreement betweenpredicted and actually measured degradation in optical performance was quite good.

7. Conclusions

Durability assessment, accelerated life testing, and service lifetime prediction areall very important elements in the development of solar products for achieving

ARTICLE IN PRESS

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 50 100 150 200 250 300 350 400Time of Exposure (days)

Chan

ge in

Tra

nsm

ittan

ceMeasured; PC; COPredicted; PC; COMeasured; PVC; COPredicted; PVC; COMeasured; PC; AZPredicted; PC; AZMeasured; PVC; AZPredicted; PVC; AZ

Predicted; PC; CO

Predicted; PC; AZ

Predicted; PVC; CO

Predicted; PVC; AZ

Fig. 5. Measured vs. predicted change in hemispherical transmittance between 400500nm for the PC and

PVC glazing materials during outdoor exposure at test sites in Colorado (CO) and Arizona (AZ) [16]. The

service life with PC=change in global transmittance o0.05 for the PVC glazing material (Duroglas) wasestimated to be around 1

2of a year at the test site in Arizona and 3

4of a year for the test site in Colorado.

The service life with PC=change in global transmittance o0.05 for the PC glazing material (APEC 9353UV-stabilized polycarbonate) was estimated to 5 years at the test site in Arizona and to 6 years at the test

site in Colorado.


successful and sustainable commercialization. It has been shown that the systematicapproach to service lifetime prediction described in this paper may be a valuable toolwhen applied to specic products in their special applications and stress conditions.The relation between materials performance and system performance constitute

the starting point in dening performance requirements and criteria and requireknowledge on measurement of material performance properties and on systemperformance testing and simulation. The environmental stress conditions for theproducts need to be properly characterized and measured to serve as a base forservice life prediction. To identify and characterize lifetime limiting degradationprocesses with respect to changes in materials performance and properties,accelerated aging tests are performed for screening the relevant stress parameterlevels and for life testing. Mathematical models are employed for analyzing theaccelerated test data and the environmental stress conditions for estimation ofservice life.Service life prediction at an early stage of product development will enable a fast

improvement of the product quality. Materials or designs can be chosen withsufciently durability or protection against environmental stresses. This will reduceleading times for new products.

References

[1] B. Carlsson, K. M .oller, M. K .ohl, M. Heck, S. Brunold, J-M. Marechal, G. Jorgensen, General

methodology of accelerated testing for assessment of service life of solar thermal components, Final

Report of IEA Solar Heating and Cooling Task 27: Solar Facade Components - Performance,

durability and sustainability of advanced windows and solar components for building envelopes,

Project B1: Durability assessment methodology development, December 2004, Report to be ordered

from SP Swedish National Testing and Research Institute, P.O.Box 857, SE-50115 Bor(as, Sweden.

[2] B. Carlsson, General methodology, in: A. W. Czanderna (Ed.), Performance and Durability

Assessment of Optical Materials for Solar Thermal Systems, Elsevier Science, Amsterdam, 2004,

in press.

[3] G.B. Gaines, R.E. Thomas, G.C. Derringer, C.W. Kistler, D.M. Brigg, D.C. Carmichael,

Methodology for designing accelerated aging tests for predicting life of photovoltaic arrays, Batelle

Colombus Laboratories, Final Report ERDA/JPL-954328-77/1, 1977.

[4] C. Sj .ostr .om, Overview of methodologies for prediction of service life, in: L. Masters (Ed.), Problems

in Service Life Prediction, NATO ASI Series E No. 95, Martin Nijhoff Publishers, Dorderchk, 1985.

[5] CIB W 80/RILEM 71-PSL, Final Report Materials and Structures, Vol. 19, No. 114, 1986.

[6] ISO 15686 Building and constructed assetsService life planningPart 1: General principles, ISO

International Standardization Organization Geneva, Switzerland, 2000.

[7] ISO 15686 Building and constructed assetsService life planningPart 2: Service life prediction

procedures, ISO International Standardization Organization Geneva, Switzerland, 2001.

[8] B. Carlsson, U. Frei, M. K .ohl, K. M .oller, Accelerated life testing of solar energy materialscase

study of some selective solar absorber coatings for DHW systems, International Energy Agency,

Solar Heating and Cooling Programme Task X: Solar Materials Research and Development,

Technical Report, SP- Report 1994:13, 1994.

[9] B. Carlsson, Methods for service life prediction, Handbook in Life Time Technology, Swedish

Defence Material Administration , 1993-06-21 (in Swedish) (Chapter 4,5).

[10] R.E. Mc Dermott, R.J. Mikulak, M.R. Beauregard, R. Mikylak, The Basics of FMEA, Productivity

Inc., 1996, ISBN: 0527763209.


[11] K. M .oller, Analytical techniques for studying solar materials degradation processes, in: A.W.

Czanderna (Ed.), Performance and Durability Assessment of Optical Materials for Solar Thermal

Systems, Elsevier Science, Amsterdam, 2004, in press.

[12] J.W. Martin, Time transformation functions commonly used in life testing analysis, Durability of

Building Materials 1 (1982) 175.

[13] B. Carlsson, Mathematical models for service life prediction, Survey of service life prediction methods

for materials in solar heating and cooling, International Energy Agency, Solar Heating and Cooling

Programme Task X: Solar Energy Materials Research and Development, Technical Report, Swedish

Council for Building Research Document D16, 1988.

[14] B. Carlsson, K. M .oller, U. Frei, S. Brunold, M. K .ohl, Comparison between predicted and actually

observed in-service degradation of a nickel pigmented anodized aluminium absorber coating for solar

DHW systems, Sol Energy Mater. Sol. Cells 6 (2000) 223.

[15] G. Jorgensen, S. Brunold, B. Carlsson, M. Heck, M. K .ohl, K. M .oller, Case study on polymeric

glazing, in: A. W. Czanderna (Ed.), Performance and Durability Assessment of Optical Materials for

Solar Thermal Systems, Elsevier Science, Amsterdam, 2004, in press.

[16] G. Jorgensen, C. Bingham, J. Netter, R. Goggin, A. Lewandowski, in: D. R. Bauer, J. W. Martin

(Eds.), Service Life Prediction of Organic Coatings, A Systems Approach; ACS Symposium Series

722, American Chemical Society, Oxford University Press, Washington DC, 1999, pp. 170185.


The applicability of accelerated life testing for assessment of service life of solar thermal componentsIntroductionGeneral methodologyInitial risk analysis of potential failure modesScreening testing and analysis for service life predictionScreening testing by accelerated ageingAnalysis of material change during ageingMicroclimate characterisation for service life prediction

Service life prediction from results of accelerated testingMathematical modellingAccelerated life testing and assessment of expected service lifeReasonability assessment and validation

Adoption of the general methodology for durability and service life assessment to polymeric glazing materialsInitial risk analysis of potential failure modes of polymeric glazing materials and screening testingService life prediction from accelerated ageing

ConclusionsReferences

thermal components

Documents

energy efcient solar

iea solar heating

solar energy systems

plate solar collectors

durability assessment

estimates of service

anddemonstrated durability

failure analysis