NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Development of a Rating System for a 

Comparative Accelerated Test Standard

Sarah Kurtz, representing 

discussions with Task Group #6 

and seeking your input!

NREL PV Module Reliability 

Workshop

Feb. 26, 2013

NREL/PR-5200-58374
 NATIONAL RENEWABLE ENERGY LABORATORY

• Identify field failures that could be reduced by improved 

accelerated testing

• Analyze how to group types of accelerated tests to best 

correlate with field performance

• Propose how to structure a useful Rating System 

• Propose how to communicate the results of the Rating 

System 

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Objective: Develop a Useful Rating System
 NATIONAL RENEWABLE ENERGY LABORATORY 3

Need for Rating System

Task Groups develop accelerated tests to predict experience in the field

How do we communicate the results?

Rating System 

Task Group 2: Testing for Thermal and mechanical fatigue

Task Group 3: Testing for Humidity, temperature, and voltage

Task Group 4: Testing for Diodes, shading and reverse bias

Task Group 5: Testing for UV, temperature and humidity 

Task Group 7: Testing for Snow and Wind Loading
 NATIONAL RENEWABLE ENERGY LABORATORY 4

Types of Accelerated Tests – This work 

focuses on Comparative tests, even though 

we would prefer Lifetime testing

Qualification Comparative Lifetime

Purpose

Minimum 

design 

requirement

Comparison 

of products

Substantiatio

n of warranty

Quantification Pass/fail Relative Absolute

Mechanisms 

studied

Infant 

mortality Wear out Wear out

Climate or 

application

No 

differentiation Differentiated Differentiated
 NATIONAL RENEWABLE ENERGY LABORATORY 5

What failures are seen in the field?

Observation Sample size

Laminate internal electrical circuit 36% of failures (~2% of 

modules failed after 8 yr); glass 33%; j-box and cables 12%; 

cells 10%; encapsulant, backsheet 8%

21 manufacturers; ~60% 

of fleet of > 1.5 GW

16% of systems required replacement of some or all modules 

because of a variety of failures, with many showing breaks in 

the electrical circuitry

483 systems 

3% developed hot spot after < 7 years; 47% had non-working 

diodes 1232-module system

External wiring, shattered, failed ~70,000 modules 

Early degradation linked to optical transmission losses (through 

glass and encapsulant) and light-induced degradation; Later 

degradation from increased series resistance is more dramatic

204 modules from 20 

manufacturers

Encapsulant discoloration 66%; delamination 60%; corrosion 

26%; glass breakage 23%; j-box 20%; broken cells 15%* ~2000 reports

200 thermal cycles corresponded to ~10 y in the field ?

A.L. Rosenthal, M.G. Thomas, and S.J. Durand "A Ten Year Review of Performance of Photovoltaic Systems". Proc. 23rd IEEE PVSC, pp. 1289-1291.

 D. Degraaff, R. Lacerda, and Campeau "Degradation Mechanisms in Si Module Technologies Observed in the Field", PV Module Reliability Workshop, 2011

 K. Kato "PVRessQ!: a research activity on reliability of PV systems from a user's viewpoint in Japan". Proc. SPIE, San Diego

 K. Kato ""PVRessQ!" PV Module Failures Observed in the Field", PV Module Reliability Workshop, Golden, CO2012

 A. Skoczek, et al "The Results of Performance Measurements of Field-aged Crystalline Silicon Photovoltaic Modules", Prog. in PV, 17, 2009, pp. 227-240. 

D.C. Jordan, J.H. Wohlgemuth, and S.R. Kurtz "Technology and Climate Trends in PV Module Degradation". Proc. 27th Eu PVSEC, Frankfurt, Germany

J.H. Wohlgemuth, et al. "Using Accelerated Tests and Field Data to Predict Module Reliability and Lifetime". Proc. 23rd Eu PVSEC, Valencia, Spain, 4EP1.2. 

J.H. Wohlgemuth, D.W. Cunningham, A.M. Nguyen, and J. Miller "Long Term Reliability of PV Modules". Proc. 20th Eu PVSEC, Barcelona, Spain, pp. 1942. 
 NATIONAL RENEWABLE ENERGY LABORATORY

1. In response to:

• Broken interconnections, solder bonds, diodes

Add:

- Additional thermal cycling or mechanical stress, plus 

bypass diode/shading testing

2. In response to:

• Encapsulant discoloration and/or delamination

Add:

- Additional UV stress

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Rating System – First address wear out that 

is slipping past the qualification tests
 NATIONAL RENEWABLE ENERGY LABORATORY

Rating System – Additional testing

New Tests Will Require Additional Stress Targeted Meaning of Rating

Failure types, 

loosely grouped

Thermal 

cycling & 

diode

testing

UV High 

Temperature

High

humidity

Proposed 

labels ★ ★★★★★

Infant 

mortality - - - - Qualification test - - 

Interconnects, 

discoloration, 

delamination

✔ ✔ - - Hot-cold Better than 

qualification test

30 y in location/appl. w 

worst thermal cycling

Heat-induced 

failures ✔ ✔ ✔ - Hot-dry Better than qualification test

30 y in location/appl. w 

worst heat-induced 

degradation

Humidityinduced 

failures

- ✔ ✔ ✔ Hot-humid Better than 

qualification test

30 y for location/appl. 

w worst heat-induced 

degradation

The two primary extremes that have not yet been addressed are: 

Heat 

Humidity 

So add additional stress for these, indicated by ✔

Note: Wind is also a priority in some locations
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• Must be predictive 

• (correlate with field experience)

• Must be relevant

• (predict 10-40 y, not 1 y or 300 y)

• Must be communicated in useful ways 

• (both simple and detailed for different audiences)

• We’ll do our best and communicate uncertainty

• (when we don’t know, we’ll communicate that we guessed)

• Must be designed so we learn from the results

• (application of the standard will help improve standard)

• Must be cost and time effective

• (manufacturers must bring the product to market)

• Must define who is responsible/accountable

• (customers need confidence in information)

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Principles for creating tests/rating system
 NATIONAL RENEWABLE ENERGY LABORATORY

Rating System Proposal – Communicate four ways:

1. Nameplate:

2. Report: 

 

Pmax 205 W 

Durability rating: . 

Hot-cold 

★★★

Hot-dry ★

★

Hot-humid not rated 

Snow/wind 2400 Pa

Salt spray etc. 

A high level summary on the nameplate will allow 

researchers to correlate tested rating with field 

experience 20 y from now.

A detailed report 

can be used by 

engineers to more 

closely compare 

specific products

Test results

By Test Lab X

Standards

3. Interpretive maps: 

Publications/Guides

4. Climate charts that link climates with 

stresses (see next slide):
 NATIONAL RENEWABLE ENERGY LABORATORY

Use 

environment

“Hotdry”“Hot-humid” “Hot-cold” Snow load

Cfa/open rack ★ ★ ★★★★ 2400 Pa

Geneva/open 

rack

★★ ★ ★★★★★ 5600 Pa

Tropical/rooftop B A C n.a. 

Choose your 

favorite use 

environment

? ? ? 

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Climate charts – similar to the interpretative 

maps: define relationship between climate 

zones and stress testing needed in these. 

Communicate meaning of tests for all climate zones, locations, and applications

Chart can define:

• 25 years estimated service life

• retention of 80% power and safe operation of 90% of modules
 NATIONAL RENEWABLE ENERGY LABORATORY

Different module constructions will have different 

acceleration factors. Good science tells us that the test 

must vary with module construction, but manufacturers will 

complain if they have to bake longer or shake harder.

The stresses are applied in different combinations and 

different sequences. We need to simplify a complex 

problem! Can we simplify and still be meaningful?

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Other challenges
 NATIONAL RENEWABLE ENERGY LABORATORY

Conclusions 

• A Rating System is necessary for the 

success of the QA Task Force

• Building consensus on:

• Principles: tests must be meaningful/useful

• Assessing today’s most common wear out mechanisms 

and those expected in hotter and wetter climates defines 

our current opportunity to strengthen the standards

• Must find simple way of summarizing test results to 

standardize communication of a complicated picture

• Meaning of test results should be communicated in 

maps and publications

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