10.07.2015   Particle Image Velocimetry (PIV)

General methodology for durability assessment

The methodology adopted by Task 27 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.

Initial risk analysis

The initial risk analysis is performed with the aim of obtaining (a) a checklist of potential failure modes of the component and associated with those risks and critical component and material properties, degradation processes and stress factors, (b) a framework for the selection of test methods to verify performance and service life requirements, (c) a frame­work for describing previous test results for a specific component and its materials or a similar component and materials used in the component and classifying their relevance to

the actual application, and (d) a framework for compiling and integrating all data on avail­able component and material properties.

The programme of work in the initial step of service life assessment is structured into the following activities: a) Specify from an end-user point of view the expected function of the component and its materials, its performance and its service life requirement, and the in­tended in-use environments; b) Identify important functional properties defining the per­formance of the component and its materials, relevant test methods and requirements for qualification of the component with respect to performance; c) Identify potential failure modes and degradation mechanisms, relevant durability or life tests and requirements for qualification of the component and its materials as regards durability.

Table 1 Specification of end-user and product requirements for the booster reflectors stud­ied in IEA SHCP Task 27

Function and gen-

General requirements for

In-use conditions and severity of envi-

eral requirements

long-term performance dur­ing design service time

ronmental stress

Efficiently reflect solar radiation to increase the solar gain of a flat plate solar collector

Loss in material performance should not result in reduction of the solar system perform­ance with more than 5%, in relative sense, during the material service life.

Material service life should exceed 25 years

The reflector is exposed to open air condi­tions involving climatic stress of UV irradia­tion, high temperature, high humidity and moister, and the effect of icing.

It may be exposed to corrosion promoting air pollutants and acid rain.

It may also be subjected to mechanical loads from hail and wind, stress from mechanical fixing and due to its own weight Soiling agents, e. g. from birds, may effect performance as well as cleaning agents as required to maintain performance

Table 2 Specification of critical functional properties of booster reflectors and requirements set up by the IEA SHCP Task 27 group

Critical functional

Test method for determining functional

Requirement for functional

properties

properties

capability and long-term per-

formance

Reflectance (specu­lar, A pspec, and dif­fuse, Pdif)

ASTM E903-96 „Standard test method for solar Absorptance, Reflectance, and Trans­mittance of Materials Using Integrating Spheres“

PC = 0.35-A pspec +(0.1/C)-Apdif < 0.05

with concentration ratio C=1.5

Adhesion between coating and sub­strate

Visual assessment

ISO 4624:2002 „Pull-off test for adhesion“ ISO 2409:1992 „Paints and varnishes — Cross cut test“

No blistering Adhesion > 1 MPa Degree 0 or 1

The first activity specifies in general terms the function of the component and service life requirement from an end-user and product point of view, and from that identifies the most important functional properties of the component and its materials. In Table 1 and Table 2 results are shown from the analysis made by the Task 27 group on booster reflectors. 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 the performance requirements in terms of those functional properties. If the performance requirements are not fulfilled, the

particular component is regarded as having failed. Performance requirements can be for­mulated on the basis of optical properties, mechanical strength, aesthetic values or other criteria related to the performance of the component and its materials.

Potential failure modes and important degradation processes should be identified after failures have been defined in terms of minimum performance levels. In general, there exist many kind of failure modes for a particular component and even the different parts of the component and the different damage mechanisms, which may lead to the same kind of failure, may sometimes be quite numerous. In Table 3 an example from the Task 27 work on booster reflectors is presented.

Table 3 Potential failure modes and associated degradation mechanisms, degradation in­dicators and critical factors of environmental stress for booster reflectors identified by the IEA SHCP Task 27 group^_ ________________________________________________________________________

Failure/Damage mode / Degradation mechanism

Degradation indicator

Critical factors of environmental stress

Unacceptable loss in reflector performance

PC= 0.35Aps+(0.1/1.5)APd) < 0.05

Degradation of the protec­tive layer

Reflectance spectroscopy, visual inspection, TIS, FTIR,

Film thickness measure­ment

High humidity, high temperature, air pollutants (acid rain), UV irradiation, hail, wind

Corrosion of the reflecting layer

Reflectance spectroscopy, visual inspection, TIS

High humidity, high temperature, air pollutants (acid rain), and impacts from other materials in contact with reflect­ing layer

Surface abrasion

Visual inspection, TIS

Sand, dust, cleaning, icing, hail, touch­ing, scratching

Surface soiling

Reflectance spectroscopy, visual inspection, TIS

Microorganisms, wind, dust, pollutants, birds, etc

Degradation of the sub­strate

Visual inspection, FTIR mechanical testing

High humidity, high temperature, air pollutants (acid rain), UV irradiation, and impacts from other materials in contact with reflecting layer

Loss of adhesion of pro­tective coating

Visual inspection, Cross­cut testing

High humidity, high temperature, air pollutants (acid rain), and UV irradia­tion

Loss of adhesion of reflec­tor from substrate

Visual inspection, Cross­cut testing

High humidity, high temperature, air pollutants (acid rain), and UV irradia­tion

Fault tree analysis is a tool, which provides a logical structure relating failure to various damage modes and underlying chemical or physical changes. It has been used for the static solar materials studied in Task 27 to better understand observed loss in performance and associated degradations mechanisms of the different materials studied. In Figure 1 and Figure 2 are shown examples on how the different failure modes and associated deg-

radation mechanisms can be represented for booster reflectors and antireflective glazing materials.

A.

B1

Degradation of protective coating on reflector

Insufficient coating of reflective

layer at production

Increase

Increase

C1

of

of surface

Corrosion of reflective layer

absorp-

rough-

tion and

ness

scatter-

ing

Loss of reflector performance

Figure 1 Representation of failure modes and associated degradation mechanisms for booster reflectors from the IEA SHCP Task 27 study

The risk associated with each potential failure/damage is taken as the point of departure to judge whether a particular failure mode needs to be further evaluated or not. Risks may be estimated jointly by an expert group adopting the methodology of FMEA (Failure Modes and Efffect Analysis) [2,3]. In Table 4 the result of a risk analysis made by the Task 27 group on booster reflectors is presented.