Evaluation of 12-Year-Old PV Power Plant in Hot-Dry Desert Climate:

Potential Use of Field Failure Metrics for Financial Risk Calculation

J. Mallineni, B. Knisely, K. Yedidi, S. Tatapudi, J. Kuitche and G. TamizhMani

Arizona State University, Photovoltaic Reliability Laboratory (ASU-PRL)

ABSTRACT

This paper provides a metric definition for the safety

failures, reliability failures and degradation loss for the PV modules.

These metrics are then used in real power plant evaluations to

calculate the distribution among these three metrics which in turn

could objectively be used to perform financial risk calculations. The

results obtained on 2352 and 1280 modules in two of the evaluated

power plants, aged 12 and 4 years, in a hot-dry desert climate are

analyzed using these defined metrics. The results indicate that the

mean and median degradations, respectively, are 0.95 and 0.96

%/year for the 12-year old, and 0.96%/year and 1%/year for the 4-

year old power plants. The distribution between safety failures,

reliability failures and durability loss is determined to be 7%, 42%

and 51%, respectively for the 12 year old power plant.

Index Terms – reliability metrics, degradation rate, O&M, hot dry

desert, durability

I. INTRODUCTION

Higher safety failure, reliability failure and

performance degradation rates of PV modules in power plants

will have serious financial impacts due to reduced energy

production than expected, increased O&M costs, safety risks and

increased warranty claim rates. These failure and performance

degradation rates are dependent on the climatic condition of the

site where the power plant is located. An extensive literature

review and analysis performed by NREL on nearly 2000

publications indicates that the module degradation rate can be as

high as 4%/year, but the median and average degradation rates are

calculated to be 0.5%/year and 0.8%/year, respectively [1]. These

degradation rates have been reported for a wide and highly

diversified range of global climatic conditions. Arizona State

University Photovoltaic Reliability Laboratory (ASU-PRL) has

been investigating a large number of power plants with several

thousands of modules for a single climatic condition: hot-dry

desert climate. In our previous investigation on about 1,900

modules installed in Tempe, Arizona (a hot-dry desert climate) we

reported a degradation rate ranging between 0.6%/year and

2.5%/year depending on the module model/manufacturer, design

and number of years operating in the field [2]. The major

degradation modes experienced in “hot-dry” climates have been

determined to be solder bond deterioration and encapsulant

discoloration [3].

Standard of measurement by which quality of a product

can be assessed is called the “Definition of Metrics.” This paper

provides a metric definition for the safety failures, reliability

failures and degradation loss for the PV modules. These metrics

are then used in real power plant evaluations to calculate the

distribution among these three metrics which in turn could

objectively be used to perform financial risk calculations. The

results obtained on two PV power plants, aged 4 and 12 years, in a

hot-dry desert climate are analyzed using these defined metrics.

II. METHODOLOGY

Metric definition for the terms safety failures,

reliability failures or durability loss for PV modules is

currently not consistent within the industry and even the

minor/cosmetic visual defects are often considered/confused

as/with the term failures. The metrics for the definition of

safety failures, reliability failures and durability loss need to be

clearly established so a consistent industry wide financial

model can be developed and established. All visual defects are

not necessarily failures, and all the failures may not be

detected through visual inspection. Figure 1 (see the last page)

provides metrics for these terms and Figure 2 (see the last

page) shows how these terms and metrics are applied in the

streamlined power plant evaluation approach developed and

executed at ASU-PRL. Assuming the conventional 20/20

warranty (20% degradation over 20 years), as per the metric

definition shown in Figure 1, all the modules which are

degrading at a rate higher than 1%/year, excluding any safety

failed modules, are considered as reliability failed modules.

Similarly, as per the metric definition shown in Figure 1, all the

modules which are degrading at a rate lower than 1%/year,

excluding safety failed modules, are considered to have

durability issues. These metric definitions have been applied on

the results obtained from the experiments shown in Figure 2.

All I-V measurements were taken at an irradiance of

above 850W/m2. These measured curves were normalized to

STC using the temperature co-efficients obtained from the

baseline I-V curves. Each and every module was inspected for

defects using visual, IR and diode checking tools. A diode

checker instrument was used to detect failed diodes and broken

interconnects on a PV module. The diode checker has two

parts, a transmitter and a receiver. The transmitter is connected

to the string terminals at the combiner box and the receiver

was placed on the module busbars to detect the signal

transmitted. The STC power outputs of all the individual

strings in the power plant were first determined. Then, 2 Best,

2 Median and 2 Worst strings were selected. The I-V curves of

all the individual modules in each of these six strings were

then obtained. Again, from these 6 selected strings 3 Best, 3

Median and 3 Worst modules were selected for an in depth

analysis. The degradation rate (%/year) for the five

performance parameters (Isc, Imax, Voc, Vmax, FF and Pmax)

were calculated for the corresponding modules. In the case of

2 best strings, 6 best, 6 median and 6 worst modules were

analyzed using the box plots of Minitab software. The primary

parameter responsible for the cause of power degradation is

identified from the graph by choosing the median of the five

parameters (%/year) aligning close to the median of the Pmax

degradation (%/year). This paper presents results obtained

from two PV power plants having the following characteristics:

12 years old plant (site 3): Glass/polymer frameless

modules (Model G), 1-axis tracking, 2352 modules and

located in Glendale/Arizona (site condition: hot-dry

climate)

4 years old plant (site 4c): Glass/polymer framed

modules (Model H), 1-axis tracking, 1280 modules and

located in Mesa/Arizona (site condition: hot-dry

climate)

III. RESULTS AND DISCUSSION

a) Safety Failures As defined in Figure 1, all the modules with safety issues

are called safety failed modules. The safety failures of the modules

(Model G) shown in Figure 3 of the 12-year old power plant were

detected using just three non-destructive techniques shown in

Figure 2: visual inspection, bypass diode checker, and infrared

imager. The safety failed modules are: hotspot issues leading to

backsheet burns (37 out of 2352); ribbon-ribbon solder bond

failure with backsheet burns (86 out of 2352); failed diodes with

no backsheet burns (26 out of 2352); backsheet delamination (10

out of 2352) and other safety failures (3 out of 2352). All these

safety failed 162 modules out of 2352 modules (7%) qualify for

the safety returns and for the highest risk premium rate calculation.

No safety failures were found in the 4-year old power plant.

b) Reliability Failures and Durability Loss As defined in Figure 1, all modules degrading at higher

than 1% per year, excluding the safety failures already identified

in Figure 3, are called reliability failed modules. Similarly, all

modules degrading at less than 1% per year, excluding the safety

failures already identified in Figure 3, are called durability loss

modules. These definitions were applied on the performance

results obtained as per the two-step experiments shown in Figure

2. First, the performance results (I-V curves) were obtained on all

the strings in the power plant. Second, the performance results

were obtained on all the modules of the selected best, worst and

best strings. Based on these two steps, 285 modules were

identified for the Model-G plant and the distribution between

reliability failed modules and durability loss modules is shown in

Figure 4.

Figure 4 Distribution between reliability failure and durability

loss for Model-G plant (based on I-V curves of 285 modules

sourced as per metric definition shown in Figure 1 and performed

experiments per Figure 2)

All the reliability failed modules (45%) qualify for the

warranty claims proportional to the rate of degradation and for a

risk premium rate calculation proportional to the rate of

degradation. All the modules with durability issues (55%) do not

qualify for the warranty claims and for the risk premium rate at

all. The durability issues are attributed only to the material

issues, and the reliability issues are primarily attributed to the

design and/or manufacturing issues. The primary failure and

degradation modes for this power plant are solder bond breakage

and fatigue, respectively. A statistical analysis (FMECA) on

these failures modes is presented elsewhere [4]. Similar to

Model-G plant, 94 modules were identified for the Model-H

plant and the distribution between reliability failed modules and

durability loss modules is shown in Figure 5.

Figure 3: Mapping of safety failures for site 3 power plant

(Model G). Color coding indicates different types of safety

failures of the modules.

Figure 5 Distribution between reliability failure and durability loss

for Model-H plant (based on I-V curves of 94 modules sourced as

per metric definition shown in Figure 1 and performed

experiments shown in Figure 2; safety failure is 0%)

c) Distribution between Safety Failures, Reliability Failures

and Durability Loss The distribution between safety failures, reliability

failures and durability loss is shown in Figure 6. The safety failure

rate (7%) is determined based on the entire population of 2352

modules. As shown in Figure 6, the remaining 93% of the modules

is distributed between the reliability failures (93% x 0.45 = 42%)

and durability loss (93% x 0.55 = 51%). Based on the objective

data presented in Figures 3 and Figure 4 (for Model-G) and

distribution presented in Figure 6, the financial risk calculations

can now be performed in conjunction with the degradation rate

histograms presented later (section f).

Figure 6: Distribution of safety failures, reliability failures and

durability losses (Model G; 12 years of field exposure)

d) Model-G Performance Degradation Parameter The best modules from the best string were analyzed to

find out the I-V parameters (Isc, Voc, FF, Vmax and Imax)

responsible for the power degradation and the results are shown in

Figure 7. The median of the Pmax degradation rate is close to the

median of the Vmax degradation rate (which in turn influences the

FF degradation). The results obtained on the best modules from

the best, worst and median strings is shown in Figure 8. These

results again indicate that the Pmax degradation is primarily

caused by Vmax degradation. The Vmax contribution to the power

drop could be associated with series resistance increase which is

attributed to the solder bond degradation. Since the fresh module

data for this specific model was not available, another model of

the same manufacturer with closest nameplate rating has been

assumed as the resistance of the fresh modules. The series

resistance of the fielded modules is calculated to be increased by

about 21%. These modules are made of two strings and each

string has one ribbon-ribbon solder bond. For the worst

modules, as shown in Figures 9, the power loss is primarily

dictated by the string loss due to ribbon-ribbon solder failure. If

one ribbon-ribbon solder bond fails, the power loss is primarily

attributed to the voltage loss, and if both ribbon-ribbon solder

bonds fail, the power loss is attributed to both voltage and

current loss. Photograph shown in Figure 10 indicates the

failure of both ribbon-ribbon solder bonds. Based on the

degradation of Vmax and FF, the primary degradation mode is

attributed to the solder bond degradation, and the primary

failure mode is attributed to ribbon-ribbon solder bond failure

with backskin burning.

Figure 7 Plot of annual degradation rate of various I-V

parameters for the best modules in the best string (Model-G)

Figure 8 Summary plot of annual degradation rate of various I-

V parameters for best modules in best, median and worst strings

Figure 9 Summary plot of annual degradation rate of various

I-V parameters for worst modules in best, median and worst

strings

Figure 10 Failure of both ribbon-ribbon solder bonds of the

module (leading to backsheet burn and 100% power loss )

e) Model-H Performance Degradation Parameter

The results of the best modules of Model-H shown in Figure 11

indicate that FF, Voc and Imax are the main contributors for

Pmax degradation. The observed degradation is suspected to be

due to the Staebler-Wronski effect in this c-Si/a-Si heterojunction

technology. The broken Si-H bonds in the a-Si layer creates trap

centers which would lead to a drop in Voc [5, 6]. The Imax drop

is attributed to the decrease in shunt resistance which in turn is

attributed to the generation of trap centers in the band gap.

Figure 11 Plot of annual degradation rate of various I-V

parameters for the best modules in the best string (Model-H)

f) Degradation Rates

Based on the I-V curves of 285 modules for Model-G (and 94

modules for Model-H) sourced as per metric definition shown in

Figure 1 and performed experiments per Figure 2 (see Section b

for details), the histograms shown in Error! Reference source

not found.12 and Figure 13 were generated for Model-G and

Model-H, respectively. The mean and median annual degradation

rates are 0.95% and 0.96%, respectively, for Model-G, and 0.96%

and 1.00%, respectively, for Model-H. For Model-G, based on the

safety failure rate presented in Figure 3, the distribution presented

in Figure 6 and the degradation rate histogram presented in Figure

12, the financial risk calculations (revenue loss or O&M risk rate

calculations) can be performed. As an example, for Model-G , 7%

of the modules which failed in safety (Figure 3) should be

included in the 100% risk rate category, 51% of the modules

which do meet the warranty limit (Figure 6 and left-side of the

dotted blue vertical bar in Figure 12) should be included in the

0% risk rate category, and finally, 42% of the modules which

do not meet the warranty limit (Figure 6 and right-side of the

dotted blue vertical bar of Figure 12) should be included in the

0%-to-100% risk rate category proportional to the number of

modules in each degradation bin of the histogram shown in

Figure 12 (right-side of the dotted blue vertical bar).

Figure 12: Histogram of power degradation rate (%/year) for

Model-G Modules (285 modules selected based on Figure 1 and

Figure 2 approaches; safety failed modules not included)

Figure 13 Histogram of power degradation rate (%/year) for

Model-H Modules (94 modules selected based on Figure 1 and

Figure 2 approaches; safety failed modules, if any, not

included)

g) Visual Inspection and Diode Check Results

Since the Model-H power plant is only 4 years old, no visual or

diode failures were observed. A Pareto chart on the defects

(cosmetic or failure) observed, through visual inspection and

diode check, for the 12-year old Model-G is shown in Figure

14. The dominant defects like encapsulant delamination and

encapsulant yellowing appear to be just cosmetic as there was

no Isc degradation found for the best modules of Model G.

Ribbon-ribbon solder bond failure (see Figure 10), hotspots

leading to burnthrough backsheet (see Figure 15), diode failures

(not shown here) and backsheet delamination around module

corners of these frameless modules (not shown here) are the

four safety failures observed in this power plant. Based on the

PmaxFFVmaxVocImaxIsc

1.0

0.5

0.0

-0.5

-1.0

-1.5

-2.0

De

gra

da

tio

n R

ate

(%

/y

ea

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Model-H

Field Age = 4 years

Best String- Best Modules(2 Strings; 6 Modules)

degradation of Vmax and FF, the primary degradation mode is

attributed to the solder bond degradation.

Figure 14 Pareto chart of the defects of Model-G (12-year old)

In the entire power plant, 26 diodes were found to be failed (open

circuited). Out of these failed diodes, only 19% of them failed

randomly throughout the power plant and the other 81% of them

failed in just two strings, 14-4 and 14-5 in Figure 3 (red squares).

As shown in Figure 3, strings 14-4 and 14-5 are located next to

one of the array’s single-axis tracking motors. The reasons for

these concentrated diode failures only in these two strings are not

known. It is known that the moving shadows can cause diode

failures [7]. The possible reasons for these concentrated failures

which occurred only in two strings near the tracking motor may

be speculated as follows: The repair or other maintenance

personnel of the power plant could have parked and moved their

vehicles at that particular location. This could cause a moving

shadow issue. During shaded state the diodes were forward biased

and triggered, and will reach a high temperature. After a shading

occurrence and while the diode is still at a high temperature, the

diode goes into a normal mode where it sees an operating voltage

of 18 cells or roughly 10V. This induces a reverse leakage current

that can exceed the diode reverse current rating at that temperature

(thermal runaway) with the destruction of the diode most likely in

the open circuit mode (8).

Figure 15 Backskin burning of hotspot cell along the cell

interconnect

IV. CONCLUSION A metric definition for the safety failures, reliability failures and degradation loss for the PV modules is provided. These metrics are then used in real power plant evaluations to calculate the distribution among these three metrics. For the 12-year old plant, the results indicate that the mean and median degradations are 0.95 and 0.96 %/year, respectively. The distribution between safety failures, reliability failures and durability loss is determined to be 7%, 42% and 51%, respectively. These metric based field results could potentially be used for the financial risk calculation.

ACKNOWLEDGEMENT The funding and technical support of Salt River Project (SRP) is gratefully acknowledged. This work was partly funded by the DOE/SERIIUS project.

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Figure 1: Metric definitions for safety failures, reliability failures and degradation loss

Figure 2: Systematic evaluation of PV modules in power plants using the metric definitions provided in figure 1