SolarCity Photovoltaic Modules with 35 Year Useful Life...SolarCity defines 35 year Useful Life as...
Transcript of SolarCity Photovoltaic Modules with 35 Year Useful Life...SolarCity defines 35 year Useful Life as...
SolarCity Photovoltaic Modules with 35 Year Useful Life
Andreas Meisel1, Alex Mayer1, Sam Beyene1,
Jon Hewlett1, Karen Natoli Maxwell1, Nate Coleman1,
Frederic Dross2, Chris Bordonaro3, Jenya Meydbray2,
Elizabeth Mayo2
1) SolarCity, 161 Mitchell Blvd, Suite 104, San Rafael, CA 94903 2) DNV GL, 1360 Fifth Street, Berkeley, CA 94710 3) DNV GL – Renewables Advisory, 155 Grand Ave, Oakland, CA 94612
SolarCity Photovoltaic Modules with 35 Year Useful Life
Andreas Meisel1, Alex Mayer1, Sam Beyene1, Jon Hewlett1, Karen Natoli Maxwell1, Nate Coleman1,
Frederic Dross2, Chris Bordonaro3, Jenya Meydbray2, Elizabeth Mayo2
1) SolarCity, 161 Mitchell Blvd, Suite 104, San Rafael, CA 94903 2) DNV GL, 1360 Fifth Street, Berkeley, CA 94710
3) DNV GL – Renewables Advisory, 155 Grand Ave, Oakland, CA 94612
Table of Contents SolarCity Photovoltaic Modules with 35 Year Useful Life ............................................................................. 1
1 Executive Summary ............................................................................................................................... 3
2 Useful Life – Introduction ..................................................................................................................... 3
3 SolarCity Total Quality Control Program ............................................................................................... 5
3.1 Rigorous Supplier Selection and Three-Level Oversight Process .................................................. 5
3.2 Stringent Module Quality Specifications ...................................................................................... 5
3.3 Effective Prevention of Quality Deviations ................................................................................... 6
3.4 Constant Refinements and Total Integration ............................................................................... 6
3.5 World-Class Team behind the Scenes ........................................................................................... 7
4 Useful Life Extrapolation from Accelerated Testing ............................................................................. 7
4.1 Ongoing Reliability Testing – Overview ........................................................................................ 7
4.2 ORT data – Thermal Cycling .......................................................................................................... 8
4.3 ORT data – Damp Heat, Humidity Freeze, and Dynamic Mechanical Load Testing ................... 10
4.4 PQP Testing – Testing Beyond Standard Qualification Tests ...................................................... 13
4.5 Product Qualification Program Testing – Overview .................................................................... 14
4.6 Product Qualification Program Testing – Extended Thermal Cycling ......................................... 14
4.7 Product Qualification Program Testing – Extended Damp Heat ................................................. 17
4.8 Product Qualification Program Testing – PID Testing ................................................................. 18
4.9 Product Qualification Program Testing – Extended Humidity Freeze and UV Test .................... 19
5 Next Steps – Tests with Improved Correlation to Real Life ................................................................ 19
6 Useful Life Extrapolation Based on Degradation of Fielded Modules ................................................ 20
7 Conclusion ........................................................................................................................................... 21
8 References .......................................................................................................................................... 21
3 SolarCity Photovoltaic Modules with 35 Year Useful Life
Executive Summary
SolarCity believes that the Useful Life of the photo-
voltaic (PV) modules that are being installed on its
residential and commercial systems is 35 years or
longer. Per this definition, 95 % of all modules in-
stalled are expected to have an annual average deg-
radation rate of less than ~0.5 % and produce at
least 80 % of their power after 35 years of service.
Experimental data from accelerated stress tests ac-
cording to the industry standard IEC 61215, which
were performed by DNV GL (the leading US certified
3rd
party), demonstrates that the median power deg-
radation of modules supplied by seven key SolarCity
approved module manufacturers for all tests and all
module suppliers combined is as low as -1.1 % and
as much as 35 % lower than for a comparable indus-
try-wide selection of non-SolarCity modules meas-
ured at DNV GL. Furthermore, data from DNV GL
demonstrates that after extending accelerated test-
ing to more than 3x beyond the conditions of IEC
61215, the modules produced for SolarCity show
only 1 to 2 % median degradation and outperform
non-SolarCity modules, which are typically warrant-
ed for 25 years.
The reason for this advantage is SolarCity’s imple-
mentation of a stringent and industry-leading Total
Quality Program, which adopted its features from
the Automotive Industry and was implemented by
SolarCity in early 2014. Following this program, So-
larCity strategically chooses to engage with a select
group of Tier-1 suppliers only. In order to be quali-
fied as a SolarCity supplier, manufacturers need to
have effective Quality Assurance programs and re-
fined manufacturing processes in place, and steady
product and manufacturing quality must be demon-
strated. Rigorous tests need to be passed on an on-
going basis, performed by a qualified 3rd
party lab.
Furthermore, we require that factory controls and
in-line testing are in place to ensure quality is sus-
tained over time and deviations are rapidly detected,
so the deployment of faulty products in the field is
prevented. Additional work is underway to demon-
strate that the degradation rate from SolarCity mod-
ules in the field is lower than industry-standard. Last-
ly, the development and implementation of state-of-
the-art accelerated testing methods will enable So-
larCity to probe degradation modes that are not de-
tectable with the current industry-standard suite of
testing and to more reliably predict real-life perfor-
mance in the field.
The most comprehensive meta-study of Field Degra-
dation rates to date, where more than 11,000 annual
degradation rates have been aggregated and ana-
lyzed, observed a near-linear degradation behavior
for the majority of crystalline-Silicon (Si) modules
and established a median degradation rate for Si
modules of around 0.5 % per year [1, 2]. The data in
this study was analyzed and filtered by DNV GL ana-
lysts, and the annual median degradation rate for
crystalline-Si modules was confirmed to be ~0.50 %
per year, while the corresponding value determined
for systems is 0.77 % per year [3].
The data presented in the following supports the
assumption that SolarCity’s PV modules, as a result
of its Total Quality Program and advancements in
Materials Science, manufacturing, and quality con-
trol, perform at least similar, if not better than the
median of all crystalline-Si modules observed in the
study above. Therefore, an annual module degrada-
tion rate of 0.5-0.6% per year is a realistic assump-
tion, which warrants a postulation of Useful Life of
35 years with a power output of 80 to 82.5 % there-
after.
1 Useful Life – Introduction
SolarCity defines 35 year Useful Life as 95% of mod-
ules producing at least 80% of their power after 35
years in their use environment [4]. ‘Use environ-
ment’ is defined as all geographic and meteorologi-
cal conditions that the PV modules will experience
during their lifetime. Site environmental conditions,
installation, and handling are included in use-
environment considerations. This definition of Useful
Life postulates a higher threshold for the remaining
power output than the value of 70 % that has been
assumed elsewhere [5].
Industry analysists have been getting more comfort-
able with the idea of Useful Life beyond 30 years [6].
The Useful Life of a PV module is determined by
4 SolarCity Photovoltaic Modules with 35 Year Useful Life
wear-out failures, which occur at the end of the
working lifetime of the module. SolarCity defines the
end of a PV module’s Useful Life if a safety problem
occurs or if the module power drops below 80 % of
the initial power rating. Long-term studies that have
investigated wear-out failures [7] found that the
predominant End-of-Life failures led to a median
power loss of only 10 % (between 0 % and 20 %),
and that nearly all of these PV modules were still
functional and met the manufacturer’s power war-
ranty. Another literature meta-study summarizing
~400 reports on degradation rates of silicon modules
confirms that modules are usually observed to de-
grade slowly in the field [8]. The degradation most
often is dominated by a gradual loss of short-circuit
current, which is mostly associated with discolora-
tion and/or delamination of the encapsulant materi-
al. In other words, the most critical module failures
have been observed to occur relatively fast, whereas
modules that do not show early failures are likely to
reach the wear-out portion of the ‘bathtub’ product
reliability curve, where the power declines in a grad-
ual and slow manner rather than showing abrupt
failure [9, 10].
There are numerous examples of installations that
have delivered stable performance for well over
25 years [11]. In 1984, Sweden’s first grid-connected
photovoltaic system was built in Stockholm. Since its
installation, the 2.1 kW system has been continuous-
ly and reliably producing energy – with less than 3 %
change since the system was installed 31 years ago.
Another system installed in 1984 is at Kyocera’s Sa-
kura Solar Energy Center near Tokyo. The 43 kW
array continues to generate a stable amount of elec-
tricity today 32 years later [12].
Given the drastic advancements in terms of Materi-
als Science, manufacturing processes, quality control
and standards, and theoretical understanding over
the last 30+ years, it is considered reasonable to as-
sume that the quality and reliability of modules fab-
ricated over the last few years can have Useful Life
well beyond 35 years, provided that adequate quali-
ty assurance measures, such as SolarCity’s Qualifica-
tion Program, have indeed been implemented.
The most comprehensive study of Field Degradation
rates to date, where more than 11,000 annual deg-
radation rates have been aggregated and analyzed,
established a median degradation rate for crystal-
line-Silicon (Si) modules of consistently around 0.5 %
per year (Figure 1) [1-3]. The data presented in the
following supports the assumption that SolarCity’s
PV systems, as a result of its Total Quality Program
and industry-wide advancements in Materials Sci-
ence, manufacturing, and quality control, perform at
least similar, if not better than the median of all
crystalline-Si systems observed in the study above.
Therefore, an annual module degradation rate of
0.5 % per year is a realistic assumption, which war-
rants a postulation of Useful Life of 35 years with a
power output of 82.5 % thereafter.
Figure 1 [Taken from Ref 1]: Histograms of all data, high
quality data and the median per study and system pre-
sented as the normalized frequency (a). Cumulative distri-
bution functions for high-quality x-Si systems and modules
(b). The median is indicated by a dashed horizontal line,
0.5 %/year and 1%/year degradation are indicated as a
dashed and dash-dotted line, respectively. The number of
data points for the respective subsets is given in parenthe-
ses.
Modules, all (1552)Systems, all (385)Modules, median (61)Systems, median (71)
Cu
mu
lati
ve p
rob
abili
ty
Degradation rate (%/year)
No
rmal
ized
Fre
qu
ency
(a)
(b)
5 SolarCity Photovoltaic Modules with 35 Year Useful Life
2 SolarCity Total Quality Control Program
2.1 Rigorous Supplier Selection and Three-Level Oversight Process
SolarCity has taken on an industry thought-
leadership position in the field of Quality and Relia-
bility (Q&R) by developing and implementing a Total
Quality Control Program, which is unique in its depth
for the solar industry (Table 1). Its features were
adopted from the Automotive Industry, and it was
implemented by SolarCity in early 2014.
Table 1: Comparison of Quality and Reliability practices for
typical solar installers and SolarCity, respectively.
The Total Quality Program starts with a stringent
selection process to establish its product suppliers.
SolarCity chooses strategically to only engage with a
select group of Tier-1 suppliers that have effective
Quality Assurance programs and refined manufac-
turing processes with well-controlled Bills of Materi-
als (BOM), which are thoroughly tested to rigorous
standards. Tier 1 manufacturers are required to in-
vest heavily in R&D, use highly automated manufac-
turing techniques and have at least five years history
of producing solar panels. By exclusively selecting
strategic Tier1 suppliers, SolarCity demonstrates its
unconditional commitment to not compromise qual-
ity and reliability in the pursuit of ever lower cost
targets, which is in contrast to numerous competi-
tors who have been plagued by serious quality prob-
lems. For example, significant defect rates of PV
modules were detected during audits of 50 Chinese
factories between 2012 and 2013 [13]. SolarCity has
been working relentlessly with suppliers to ensure
that SolarCity products are free of such defects.
SolarCity’s commitment to quality is reflected in the
fact that Q&R requirements are directly embedded
into and enforceable through the Master Procure-
ment Agreements (MPA) for its product suppliers. In
order to be qualified as a SolarCity supplier, suppli-
ers are contractually required to subscribe to a well-
documented three-level process: (1) Initial vendor
qualification, which requires demonstrating the ca-
pability to manufacture the products according to
well defined specifications and quality requirements,
to pass an onsite factory audit, and to pass reliability
testing through a chosen 3rd
party lab (DNV GL); (2)
Continuous Production Oversight, which ensures
consistent production of goods of high quality by
means of regular BOM inspections and factory audits
through SolarCity as well as 3rd
party auditors; (3)
Ongoing Quality Assurance and Testing to ensure
compliance with the Initial Qualifications and all
quality criteria.
The Total Quality Program was first implemented for
PV modules and has since been extended to all key
components of the entire photovoltaic system. It is
now also in place for inverters, which often are con-
sidered the central part of the PV system, as well as
for pre-installed and field-made PV connectors,
which are another essential system component. Ad-
ditionally, the program has been implemented for
battery & storage and gateways. This three-pronged
Quality Program that is in place for all relevant sys-
tem components ensures that SolarCity products are
designed for long-term reliability and consistent per-
formance over the entire system life exceeding
35 years.
2.2 Stringent Module Quality Specifica-tions
SolarCity has developed a rigorous testing program
of all the components in the PV system, which goes
well beyond the common practices in the solar in-
dustry space. The Module Quality Specification,
which is part of the initial vendor qualification, was
developed with input from over half a dozen Tier-1
module suppliers and standardizes well-defined
module quality requirements across all vendors to
Conventional Method SolarCity Method
Choose suppliers offering lowest cost No supplier qualification Buy off the shelf
☺ Tier-1 suppliers only
☺ Rigorous supplier qualification
☺ Design to spec
Implicit Quality/Reliability
☺ Automotive-based Q&R programs:
For all key system components
☺ Define/control/validate
☺ Gates with sign-off, involvement
No feedback loops No learning from past experiences
☺ Constant feedback between
Installers, O&M, Engineering
☺ Analyze field and warranty data
No Quality philosophy ☺ Company-wide Quality philosophy
☺ Industry thought-leadership in Q&R
6 SolarCity Photovoltaic Modules with 35 Year Useful Life
ensure consistency and highest quality in SolarCity’s
products.
Product Qualification Program (PQP) Testing and
Ongoing Reliability Testing (ORT) are implemented in
parallel to ensure that performance parameters
achieved on the first set of modules qualified are
reliably sustained over time. PQP and ORT testing
are performed for every single BOM variation per
module supplier. The extended testing conditions of
the Product Qualification significantly exceed indus-
try common test standards. Besides extended test
durations and exposure conditions for well-
established tests, SolarCity also requires an addi-
tional salt-mist corrosion certification for materials,
as well as reliable PID resistance under most aggres-
sive test conditions for up to 600 hours. Further-
more, in order to obtain product certification, mod-
ule suppliers are required to not only meet UL (Un-
derwriter Laboratories) PV module standards, but at
the same time also IEC (International Electrotech-
nical Commission) standards. This leads to additional
quality enhancements, since IEC certifications in-
volve performance testing, whereas UL certification
is limited to safety-related requirements only.
2.3 Effective Prevention of Quality Devi-ations
This distinctive Quality and Reliability (Q&R) program
has led module suppliers to improve their product
quality when they produce SolarCity modules, which
over time has raised the quality of products deliv-
ered to SolarCity and helped to push excellence in
the entire Solar Industry. The clearly defined system
of controls and tests guarantees that quality is en-
sured from the beginning and sustained over time,
while new quality deviations are rapidly detected.
Monthly ORT testing reveals unforeseen quality
problems. Once problems are detected, we have a
systematic plan in place to implement corrective
actions.
Under this program, SolarCity has implemented the
requirement to perform an end-of-line (EOL) Electro-
luminescence (EL) inspection on 100 % of all mod-
ules fabricated in order to detect defects such as
cracks and micro-cracks across all suppliers. Follow-
ing input from literature and industry collaborations,
guidelines for allowable crack types and size of inac-
tive areas were developed and implemented in or-
der to prevent long-term power loss and risk of
hotspot formation.
Another feature of the Quality Program is that in-
tended changes to the approved BOM require prior
notification and approval by SolarCity, and strict re-
test requirements are in place for any BOM modifi-
cation. SolarCity enforces defined change manage-
ment procedures on each supplier. This feature has
proven successful in many instances. In one case, it
was detected that a supplier had modified the BOM
components without any notification. SolarCity is-
sued a Corrective Action Request (CAR) requiring the
supplier to perform adequate qualification testing
for this modification and any changes thereafter.
Another supplier was found to have modified BOM
components without notification, and the consecu-
tive Corrective Action Request and vendor manage-
ment plan resulted in the implementation of a Glob-
al Change Management program at the supplier in
order to improve oversight and maintain quality.
SolarCity also performs regular factory audits
through internal personnel or independent third-
party auditors to detect issues regarding inconsistent
quality management or quality escapes. As an exam-
ple, inspections at two supplier factories discovered
and corrected a decrease in quality standards before
the product would have been deployed at large scale
in the field. Similarly, factory audits resulted in the
request of improvements to four modules suppliers
to correct deviations with respect to product and
manufacturing quality.
In summary, SolarCity has an unparalleled system in
place to protect quality and prevent the deployment
of faulty modules in the field.
2.4 Constant Refinements and Total In-tegration
The Quality Program is constantly evolving and ex-
panding. Test conditions for initial product qualifica-
tion and ongoing reliability testing are refined on a
regular basis to ensure best possible correlation with
real life performance. As a consequence, the tests
7 SolarCity Photovoltaic Modules with 35 Year Useful Life
are becoming more effective and more efficient at
the same time.
The latest revision of SolarCity’s Module Product
Qualification and Ongoing Reliability Test procedures
has been refined and optimized in close collabora-
tion with DNV GL, and DNV GL decided to use it as a
new test standard and implement the program
across all suppliers.
In order to guarantee highest possible quality on a
system level, the Quality Program has been extend-
ed and implemented for key components of the PV
system, such as inverters and connectors. Several
key features of SolarCity’s inverter test program
have also been adopted by DNV GL and implement-
ed in their standard test program.
2.5 World-Class Team behind the Scenes
SolarCity’s Quality team consists of industry-wide
recognized experts on technician-, engineer- and
PhD-level, with extensive experience in relevant in-
dustries such as Solar and Automotive and with
background in fields such as Engineering, Physics,
Chemistry, Materials Science, and Quality Assurance.
The team has repeatedly been recognized by manu-
facturers for their ability to avoid or detect quality
aberrations, as well as to rapidly resolve those by
advising and guiding manufacturers in terms of pro-
cess optimizations and/or materials selection.
3 Useful Life Extrapolation from Accelerated Testing
In the following section, it is demonstrated that So-
larCity’s Total Quality Program is succeeding, and as
a result of the strategic supplier engagement, cou-
pled with rigorous quality requirements, SolarCity
modules show improved performance and projected
lifetime versus other Tier-1 modules tested under
identical conditions in similar timeframes. The sup-
porting data has been generated by accelerated test-
ing within the framework of SolarCity’s PQP and ORT
programs, which was performed by DNV GL PVEL (PV
Evolution Labs), a world-renowned, independent
certified third-party testing laboratory.
3.1 Ongoing Reliability Testing – Over-view
A central part of SolarCity’s Total Quality Program is
Ongoing Reliability Testing. Module manufacturers
are required to submit a defined number of modules
every month, which have randomly been selected
from a typical manufacturing line under supervision,
for reliability testing by an independent third-party,
such as DNV GL. The testing is done according to the
IEC 61215 standard. The overall test duration is
about 16 weeks per batch. An overview of the re-
quired test procedures is listed in Table 2.
Table 2: Overview of ORT test conditions for monthly qual-
ity assurance.
Certifications according to standards such as
IEC 61215 have gained industry-wide acceptance
over the last 15 years. The stress tests defined in the
standards are short-duration accelerated tests per-
formed at stress levels higher than the operating
stress level, so the occurrences of failure modes can
be stimulated within reasonable timeframes. The
qualification tests constitute a minimum require-
ment on reliability testing and are a measure for the
ability of the module to withstand prolonged expo-
sure in real life use environments. It is widely ac-
cepted that these test procedures are appropriate to
identify infancy failures and product weaknesses.
While the tests prescribed in these standards are not
fully adequate to determine the exact working life-
time of a module, the stress conditions prescribed by
these standards are, however, derived from real-life
outdoor stresses. The climate chamber tests yield an
accepted indication of the longevity to be expected,
the quality of the materials, and the workmanship of
the products.
The ORT data shown in the following sections, was
obtained from DNV GL PVEL, SolarCity’s approved
independent third-party testing lab. It summarizes
# Test ORT
1 Initial characterization IV, EL, Visual
2 Thermal Cycling TC-200
3 Damp Heat DH-1000
4 Humidity Freeze TC50/HF10
5 Dynamic Load DML/TC50/HF10
8 SolarCity Photovoltaic Modules with 35 Year Useful Life
data from 10 monthly batches each consisting of two
modules per test condition from seven SolarCity ap-
proved module manufacturers.
Figure 2: Summary of ORT data obtained from DNV GL
PVEL for accelerated testing on modules from seven Solar-
City approved module manufacturers. Each symbol repre-
sents the average module power degradation for 10
monthly batches each consisting of two modules per test
condition: Thermal Cycling (‘TC’, 200 cycles; green circles),
Damp Heat (‘DH’, 1000 hours; red triangles), combined
Thermal cycling and Humidity Freeze (‘TC+HF’, 50 thermal
cycles followed by 10 humidity freeze cycles; grey dia-
monds), Dynamic Mechanical Load testing followed by
Thermal Cycling and Humidity Freeze (‘DML+TC+HF’, 1000
mechanical load cycles followed by 50 thermal cycles and
10 Humidity Freeze cycles; orange squares).
Figure 2 shows a summary of ORT data for acceler-
ated testing that was performed on modules sup-
plied by seven key SolarCity approved module manu-
facturers. Each symbol represents the average mod-
ule power degradation of 10 monthly batches each
consisting of two modules per test condition. The
median power degradation for all tests and all mod-
ule suppliers combined is as low as -1.1 % (± 0.1 %
standard error) and therefore significantly lower
than the pass criteria of -5 % of IEC 61215. The mod-
ules from all suppliers have a tight distribution
across all test conditions, indicating excellent pro-
cess control and product quality. While still well be-
low the allowed pass criteria, the modules of suppli-
er 7 showed a slightly larger degradation for Thermal
Cycling and mechanical load testing than SolarCity
considers satisfactory, and the supplier was request-
ed to implement a corrective action plan to improve
the reliability performance.
3.2 ORT data – Thermal Cycling
The industry-standard test to simulate thermal
stresses in PV modules as a result of changes of ex-
treme temperatures is Thermal Cycling (TC). PV
modules are fabricated using several materials in-
cluding silicon, metals, polymers, glass, etc. During
temperature changes, these materials expand and
contract according to their coefficient of thermal
expansion (CTE). Therefore, interfaces in the mod-
ules are mechanically stressed due to their differ-
ences in CTE every time a module heats up during
day-night cycles or, what is more, during cycles be-
tween cloud coverage and sunlight. For example,
copper-based ribbons, which electrically connect
neighboring cells in the module, are soldered to the
cells made of silicon, and due to a large difference in
the CTE of metal and silicon, temperature changes
can cause significant mechanical stress to these sol-
der joints. One of the main effects of Thermal Cy-
cling is to simulate the stress on the soldered con-
nections within the module. This may trigger fatigue
of the ribbons, interruption of the electric circuitry,
cell cracking, and power degradation. The modules
are placed in an environmental chamber and sub-
jected to extreme temperature swings from -40 ⁰C to
+85 ⁰C for 200 times, while maximum power current
is sourced into the panel whenever the temperature
exceeds 25 °C. Thermal Cycling is considered a key
accelerated test. Together with Damp Heat testing,
failures due to Thermal Cycling can account for more
than 70% of the total failures for c-Si modules after
accelerated testing.
There is no consensus on the acceleration factor of
this test due to the dependency on environmental
factors, so it is difficult to relate number of cycles to
years in the field. However, the interconnection fail-
ures seen after TC testing are among the most com-
mon failures that are observed in the field. For ex-
ample, long-term studies of modules in the field of
21 manufacturers have shown that of all failures
observed, the highest fraction was due to failed elec-
trical interconnects (as much as 36 %, see Figure 3)
[6].
-6
-5
-4
-3
-2
-1
0
Supplier 1 Supplier 2 Supplier 3 Supplier 4 Supplier 5 Supplier 6 Supplier 7
Po
wer
deg
rad
ati
on
[%
]
ORT Summary
TC
DH
TC+HF
DML+TC+HFIEC Pass Criteria
9 SolarCity Photovoltaic Modules with 35 Year Useful Life
Figure 3: Field study of PV module failures found for vari-
ous PV modules of 21 manufactures installed in the field
for 8 years. The rate is given relative to the total number
of failures. Approximately 2% of the entire fleet are pre-
dicted to fail after 11-12 years (do not meet the manufac-
turer's warranty). [Taken from Ref 5].
Similarly, a study on returns from a fleet of
>3 million modules from ~20 manufacturers [14]
highlights the significance of Thermal Cycling testing
and the fact that this test is suitable to reveal weak-
nesses of the electrical interconnections. The study
found that the majority (~66%) of modules with in-
fancy failures (returned after an average deployment
of 5 years), were returned because of problems with
electrical interconnections in the laminate (e.g.
breaks in the ribbons and solder bonds).
Figure 4 shows a statistical overview of Thermal Cy-
cling TC-200 ORT test data that was obtained from
DNV GL PVEL, SolarCity’s approved independent
third-party testing lab. The data is a statistically sig-
nificant overview of ~350 modules submitted for
ORT testing performed at DNV GL PVEL. It compares
TC data from ~70 modules fabricated for SolarCity
against data of ~280 Non-SolarCity modules.
Figure 4: Thermal Cycling data comparing the power change after 200 thermal cycles from -40 C to +85 C of ~70 modules
fabricated for SolarCity (‘SCTY’, green bars and line) against data of ~280 Non-SolarCity modules from an industry mix of mod-
ule makers (‘Non-SCTY’, red bars and line). The colored bars represent actual data points, while the lines are Gaussian fits.
0.7% better
10 SolarCity Photovoltaic Modules with 35 Year Useful Life
The data demonstrates that SolarCity modules show
significantly better performance after Thermal Cy-
cling stress testing than their Non-SolarCity counter-
parts. From the Gaussian fits to the actual data
points, it can be seen that the power degradation for
SolarCity modules peaks at -1.4 %, which is 0.7 %
better than the Non-SolarCity-type modules (-2.1 %),
corresponding to an improvement by almost
one sigma. A statistical hypothesis test confirmed
that the difference in means is statistically significant
(p < 0.001). In addition, SolarCity modules show a
35 % tighter distribution than the Non-SolarCity
modules, and with a maximum degradation of 4 %,
there are no outliers falling beyond the maximum
allowable 5 % threshold. In contrast, as much as 6 %
of all modules not made for SolarCity show degrada-
tion in excess of 5 %, and 3 percent degrade by as
much as 7 %.
The significantly improved reliability performance of
SolarCity modules is attributed to the strict require-
ments that SolarCity imposes on its modules suppli-
ers with respect to all processes that are related to
the electrical interconnections, since this aspect is
considered key to reliable long-term performance as
explained above. For example, SolarCity successfully
imposed 100 % end-of-line electroluminescence (EL)
inspection on all of its suppliers to reliably detect
problems related to soldering of electrical intercon-
nections. During factory audits, SolarCity’s experts
place key focus on inspecting all process steps relat-
ed to the interconnections, and great success has
been achieved in detecting and resolving problems
with these processes.
3.3 ORT data – Damp Heat, Humidity Freeze, and Dynamic Mechanical Load Testing
Another industry-standard test is the Damp Heat
1000 (DH-1000) test, which simulates the effects of
moisture and humidity effects. In this test, the prod-
uct is placed in an environmental chamber at 85 °C
and 85% relative humidity (RH) for 1000 hours. Simi-
lar to other tests within the standard certification
procedures, there is no consensus as to its accelera-
tion factor and the time of exposure in the field it
corresponds to, especially given that there is a
strong influence of the climate zones which the
modules are deployed in; rather, the test is consid-
ered appropriate to exclude short- and near-term
problems and indicate a nominal level of safety in
the field. The test is useful to evaluate the quality of
lamination, which protects the solar cells from hu-
midity ingress. In particular, Damp Heat is a stress
test to evaluate the quality of the encapsulant (mois-
ture protection) and test for any degradation due to
corrosion. Typically, PV module backsheets and en-
capsulants do allow water vapor to pass through,
which may cause stress on interfacial adhesion and
lead to delamination. However, for safe operation,
the interfaces in a PV module must remain adhered
during the entire product lifetime. The main failure
modes triggered by DH testing are backsheet and/or
encapsulant adhesion loss resulting in delamination
and junction box adhesion loss, both of which can
cause safety problems, and other modes are con-
tamination problems, material weaknesses, and
electrochemical corrosion.
In general, the failure rates for Damp Heat testing
appear to have declined during recent years. Nowa-
days, manufacturers have on-site environmental
chambers for the assessment of new products and
materials, which is very effective for failure preven-
tion. Additionally, advances in encapsulation materi-
als and the lamination process, as well as better
edge sealing methods led to an improved protection
against moisture ingress.
The graph in Figure 5 shows a statistical overview of
Damp Heat ORT test data (1000 hours at 85 °C / 85 %
relative humidity) that was obtained from DNV GL
PVEL. The data is a statistically significant overview
of more than 350 modules submitted for ORT testing
performed at DNV GL PVEL. It compares DH data
from ~70 modules supplied to SolarCity against data
of ~280 modules from modules that were not fabri-
cated for SolarCity.
For both SCTY- and Non-SCTY-type modules, the
degradation after Damp Heat testing is low and on-
ly -0.8 % and -0.6 %, respectively. This points to the
fact that the encapsulation materials and lamination
processes are well controlled. However, for all Solar-
11 SolarCity Photovoltaic Modules with 35 Year Useful Life
City modules the degradation stays below 3 % after
Damp Heat testing, whereas 6 % of Non-SolarCity
modules show degradation between 3 and 5 %. At
the same time, the SolarCity modules have a signifi-
cantly tighter distribution. The standard deviation
for Non-SolarCity modules is with 1.2 % twice as high
as for SolarCity modules (0.6 %), and a statistical
hypothesis test confirms that the difference is statis-
tically significant (p < 0.001). These observations
indicate a further improvement in process and quali-
ty control for the SolarCity modules and an effective
prevention of problems due to moisture ingress.
Figure 5: Damp Heat data comparing the power change
(bottom graph) after 1000 hours of Damp Heat at 85 C
and 85 % relative humidity (RH) of ~70 modules fabricated
for SolarCity (‘SCTY’, green bars and line) against data of
~280 Non-SolarCity modules from an industry mix of mod-
ule makers (‘Non-SCTY’, red bars and line). The colored
bars represent actual data points, while the lines are
Gaussian fits. The top graph shows the standard deviations
for both types of modules.
As described above, PV modules are not impermea-
ble to water vapor, which can lead to a weakening of
interfacial adhesion over time. When moisture pre-
sent inside of the laminate freezes, ice crystals may
cause additional damage to the interfaces in the
module and cause delamination. The Humidity
Freeze (HF) test is an environmental test designed to
determine the module's ability to withstand the ef-
fects of high temperatures combined with humidity,
followed by extremely low temperatures. PV mod-
ules are subjected to temperatures of 85°C and rela-
tive humidity of 85 % for 21 hours, which causes
partial saturation of the module with water. The
modules are then cooled down to -40 °C, which
causes the moisture to freeze. The modules are sub-
jected to 10 complete cycles in the closed climatic
chamber.
Figure 6: ORT data comparing the power change after
combined 50 cycles of Thermal Cycling and 10 Humidity
Freeze cycles (top), and Dynamic Mechanical Load testing
followed by 50 cycles of Thermal Cycling and 10 Humidity
Freeze cycles (bottom) of ~200 modules of SolarCity mod-
ule suppliers against data of ~200 modules from industry
mix of module makers that are not suppliers to SolarCity.
Symbols are actual data points, while the lines are Gaussi-
an fits with the following fit parameters: a is the height of
the curve's peak, b is the position of the center of the
peak, and c (Gaussian RMS width) controls the width of
the "bell".
20.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
5- 4- 3- 2- 1- 0 1
-0.8333 0.6068 67
-0.5644 1.204 281
Mean StDev N
P
sel
ud
om ll
a fo
noit
car
F
)%( noitadargeD rewo
S
elbairaV
YTCS-noN
YTC
H lamroN
YTCS-noN ,xamP atleD ,YTCS ,xamP atleD fo margotsi
0%
5%
10%
15%
-5 -4 -3 -2 -1 0 1 2
Fra
cti
on
of
all
mo
du
les
(%
)
Power degradation (%)
DH1000
SCTY
Non-SCTY
SCTY
Non-SCTY
Gaussian Fit Non-SCTY SCTY
a 0.1 0.1
b -0.3 -0.5
c 1.2 -0.7
0%
10%
20%
30%
40%
50%
-5 -4 -3 -2 -1 0 1 2
Fra
cti
on
of
all
mo
du
les
(%
)
Power degradation (%)
TC50-HF10
SCTY
Non-SCTY
SCTY
Non-SCTY
Gaussian Fit Non-SCTY SCTY
a 0.2 0.2
b -1.3 -1.1
c -1.2 0.9
0%
5%
10%
15%
20%
25%
30%
35%
40%
45%
-7 -6 -5 -4 -3 -2 -1 0 1 2
Fra
cti
on
of
all
mo
du
les
(%
)
Power degradation (%)
DML
SCTY
Non-SCTY
SCTY
Non-SCTY
Gaussian Fit Non-SCTY SCTY
a 0.1 0.4
b -3.2 0.0
c 4.0 1.2
12 SolarCity Photovoltaic Modules with 35 Year Useful Life
The main failure modes triggered by Humidity Freeze
testing are caused by thermo-mechanical stress due
to the different thermal coefficients of glass, Silicon,
and copper, and consist of delamination, junction
box detachment, and/or cell interconnect failures.
Further, high-temperature glass corrosion can occur
as a result of alkali removal from the glass surface.
The freezing of the moisture propagates the corro-
sion effect deeper into the glass. A porous silica ma-
terial may form as a result of glass corrosion, which
affects the transmission properties of the glass and
can cause reduced module power output.
From Figure 6 (top), it can be seen that for both
SCTY- and Non-SCTY-type modules, the degradation
after 10 cycles of Humidity Freeze testing is on-
ly -1.3 % and -1.1 %, respectively. This confirms that
the encapsulation materials and lamination process-
es are well controlled, resulting in a high-quality lam-
inate. The degradation in SCTY-type modules is 0.2 %
lower than in Non-SCTY-type modules, and similar to
Damp Heat testing, the distribution is tighter. The
data for SolarCity modules are closer to Gaussian,
while the non-SCTY data deviate from a Gaussian
shape and reflect an inhomogeneous distribution,
confirming the improved process and quality control
for the SolarCity-type modules that was suggested
from the Damp Heat results and indicating the effec-
tive prevention of problems due to moisture ingress
and laminate deficiencies.
Dynamic Mechanical Load (DML) testing will be dis-
cussed next. Modules in the field are subjected to
mechanical stress due to wind and snow loads. The
resulting deflections depend on glass thickness, en-
capsulant and backsheet properties, frame design, as
well as temperature and magnitude of the loads.
Cycling deflections may result in the formation of
cell cracks. In order to simulate the stress caused by
wind and snow loads, the modules are first subject-
ed to 1,000 Dynamic Mechanical Load cycles to test
for the formation of cracks. However, the output
power often stays unaffected unless the crack fully
penetrates the metallization on the rear side of the
cell. Therefore, the modules are also exposed to 50
Thermal Cycles, which can cause the propagation of
cracks that may have formed. Lastly, the modules
need to undergo ten Humidity Freeze cycles. The
high humidity followed by freezing temperatures
causes the cracks to propagate through the cell met-
allization. Failures that are seen after DML testing
are broken glass, cracked cells, and/or damaged
electrical interconnect ribbons.
The results from Dynamic Mechanical Load testing
on the 400 modules submitted for ORT testing to
DNV GL PVEL are plotted in the bottom graph of Fig-
ure 6. A clear advantage of SolarCity-type modules is
evident. After the DML stress testing, the distribu-
tion for the degradation of Non-SolarCity-type mod-
ules has a maximum at -3.2 %, whereas this value for
the SolarCity-type modules is at 0 %. Additionally,
the distribution is noticeably tighter for SCTY-type
modules with an RMS value of 1.2 compared to 4.0.
The excellent mechanical stability confirms the re-
sults from above and demonstrates the high quality
of SCTY-type modules and their integrity against
thermo-mechanical stresses.
In Figure 7, electroluminescence (EL) images of a
representative SCTY module supplier before DML
testing (bottom left), after 1,000 cycles of Dynamic
Mechanical Load testing (bottom center), and after
the completed DML test sequence DML/TC/HF (bot-
tom right) are shown. The data verifies the mechani-
cal integrity of SCTY-type modules. Even after this
aggressive test procedure, the power degradation is
as low as -0.8 %, demonstrating the improved me-
chanical stability of SolarCity’s patented Zep Solar
Panel Mounting System compared to the conven-
tional mounting system of Non-SolarCity modules.
13 SolarCity Photovoltaic Modules with 35 Year Useful Life
Figure 7: Electroluminescence images and corresponding power degradation of a representative SCTY module supplier before
DML testing (left), after 1,000 cycles of Dynamic Mechanical Load testing (center), and after the completed DML test sequence
of 1,000 Dynamic Mechanical Load cycles, 50 Thermal Cycles, and 10 Humidity Freeze cycles (right).
3.4 PQP Testing – Testing Beyond Stand-ard Qualification Tests
The ORT test sequence described above follows the
IEC 61215 test standard. It is generally understood
that these qualification tests are a minimum re-
quirement of reliability tests and appropriate to de-
tect infant mortality failures and anticipate short-
term reliability issues in the field. Passing these
standard tests demonstrate the ability of modules to
withstand prolonged exposure in general use envi-
ronments. However, there is a gap with respect to
long-term performance prediction, and there is
broad consensus in the solar community that the IEC
standards allow no conclusions to be made concern-
ing the actual lifetime expectancy for tested prod-
ucts. There is agreement that lifetime depends on
the design, the materials, the manufacturing quality,
and the use environment under which the product is
operated.
To address the gap between the standard IEC qualifi-
cation tests and long-term performance prediction,
several global standards development activities are
underway, which are primarily based on extending
the individual tests of IEC 61215. SolarCity follows
this methodology and implemented a Product Quali-
fication Program testing sequence that is based on
extended IEC 61215 tests, with an added electrically
biased Damp Heat test to evaluate Potential In-
duced Degradation (PID) and a sequence of dynamic
mechanical load testing followed by Thermal Cycling
and humidity freeze. While adding significant confi-
dence that the more demanding test procedures
allow to better predict the long-term reliability of
the tested modules, there is still no clear under-
standing of whether the expanded tests trigger real-
istic failures or instead failures that are not found
under realistic operating conditions. Therefore, So-
larCity has a program underway to evaluate the va-
lidity of the test results of these extended tests and
correlate these to the performance in the field under
real life conditions. SolarCity has a unique advantage
of having direct access to one of the largest net-
works of installed residential and commercial PV
systems in a large variety of use environments. Deg-
radation rates under real-life conditions will be eval-
Pre stress DML-1000 DML-1000 / TC50 / HF10
Pmax change
( rel. %)0 -0.9% -0.8%
14 SolarCity Photovoltaic Modules with 35 Year Useful Life
uated and failure modes observed will be correlated
to the ones seen in accelerated testing.
3.5 Product Qualification Program Test-ing – Overview
Besides ORT testing discussed above, a second key
part of SolarCity’s Total Quality Program is the Prod-
uct Qualification Program Testing. The tests are
based on extended IEC 61215 tests, with an added
electrically biased Damp Heat test to evaluate Po-
tential Induced Degradation (PID) as well as a se-
quence of dynamic mechanical load testing followed
by Thermal Cycling and humidity freeze testing.
While not yielding conclusive information about the
product life expectancy, the successful passing of
such tests still justify the assumption of longer life-
times. SolarCity believes that the extended upfront
testing required for product qualification in conjunc-
tion with the continuous Ongoing Reliability Testing
in regular intervals, constitute a quality program that
is industry-leading and appropriate to justify module
Useful Life of 35 years.
Table 3: Overview of PQP test conditions for qualification
of initial products or modified BOMs.
For initial product qualification and after any chang-
es to the Bill of Materials, module manufacturers are
required to submit a defined number of modules,
which have randomly been selected from a typical
manufacturing line under supervision, for reliability
testing by an independent third-party, such as DNV
GL PVEL. An overview of the required test proce-
dures is listed in Table 3.
3.6 Product Qualification Program Test-ing – Extended Thermal Cycling
As mentioned above, the tests that show the largest
effect on PV module performance and appearance
are Temperature Cycling tests as well as Damp Heat
testing. It has been shown that Thermal Cycling with
injected current is an appropriate test to reveal de-
sign weaknesses and identify early failures of cell
interconnect ribbons and solder bonds [15, 16].
However, it is generally understood that the 200
thermal cycles from IEC 61215 testing are not suffi-
cient to give confidence in a module lifetime of more
than 25 years [17, 18]. While there is evidence that
longer Thermal Cycling is a more adequate test to
ensure long-term reliability and reduce field failures,
there is no consensus as to how many cycles corre-
spond to what lifetime, especially given that there
may be significant variations with climate and use
conditions.
Studies on an extended number of modules have
shown that with an increasing number of thermal
cycles beyond the standard of 200 cycles, problems
with electrical interconnects between the cells can
occur, as is evident from dark areas in electrolumi-
nescence images. In general, the degree of damage
will get more severe and the output power will de-
crease with increasing number of cycles. The in-
creased number of cells with broken busbars can
lead to an inhomogeneous current distribution be-
tween the cells. This can lead to serious safety prob-
lems, since high temperatures or even hot spots and
arcing can occur.
In Figure 8, the relative output power after a con-
secutive sequence of 200, 400, 600, and 800 Ther-
mal Cycles is plotted for a representative mix of So-
larCity-type modules (green line with filled circles;
median and standard error) and compared against a
statistically significant mix of Non-SolarCity-type
modules (66 %ile: black line; 50 %ile: grey line;
33 %ile: light grey line) that have been measured at
DNV GL PVEL over time. The modules discussed here
are the collection of modules that were submitted
by module suppliers to DNV GL for PQP testing,
where ‘SolarCity-type’ modules were fabricated for
SolarCity, while ‘Non-SolarCity-type’ modules made
# Test PQP
1 Thermal Cycling TC-800
2 Damp Heat DH-3000
3 Humidity Freeze TC50/HF10 (3x)
4 UV Exposure 90 kWh
5Dynamic Mechanical Load /
Thermal Cycling/ Hum. Freeze
1000 cycles (1440 Pa) /
TC50 / HF10
6 PID testingBoth Polarities;
600 hours @ 85C/85%
15 SolarCity Photovoltaic Modules with 35 Year Useful Life
up the rest of the collection and are referred to as
‘Industry mix’. Additionally, data from a selection of
modules from a comprehensive literature study is
shown (red dashed lines) [14]. As can be seen from
the plot, the median degradation for a representa-
tive mix of modules from SolarCity suppliers after
this extensive stress test is as low as 2 %. The com-
parison with a large number of Non-SolarCity-type
modules demonstrates that SolarCity-type modules
perform at least as well as the very best modules in
industry and outperform the vast majority of their
industry counterparts. After 800 cycles, DNV GL
PVEL’s 66 percentile for the average degradation is
1 % higher.
Figure 8: Relative output power after a consecutive series of 200, 400, 600, and 800 Thermal Cycles from -40 °C to +85 °C for a
representative mix of SolarCity-type modules (green line with filled circles; median and standard error) and a statistically signifi-
cant mix of Non-SolarCity-type modules (66 %ile: black line; 50 %ile: grey line; 33 %ile: light grey line) that have been measured
at DNV GL PVEL over time. Additionally, data from a comprehensive literature study is shown (red dashed lines) [14].
This important result further strengthens the find-
ings from section 3.2. Even when subjected to one of
the most aggressive test procedures currently used
in industry, SolarCity-type modules are resilient to
metallization-related problems and do not show any
gridline interruptions or problems with the electrical
interconnects, which have been shown to be one of
the most prevalent failure modes that have been
observed in modules in the field [6].
Figure 9 visualizes this finding. Electroluminescence
images and corresponding power degradation of
modules from a representative SCTY supplier before
Thermal Cycling testing (left), after 400 Thermal cy-
cles (center), and after 800 Thermal cycles (right). It
can be seen that the electrical interconnects and
gridlines are intact, and the degradation after 800
cycles is as low as -1.3 %. For comparison, EL images
of modules from the literature [14] after 200 ther-
mal cycles (left), 400 thermal cycles (center), and
after 600 thermal cycles (right) are shown in Figure
10, and dark areas appearing after >400 cycles indi-
cate disconnection of busbars (red markers).
The insignificant degradation after extended TC is
credited to the strict requirements that SolarCity
16 SolarCity Photovoltaic Modules with 35 Year Useful Life
imposes on its modules suppliers with respect to all
processes that are related to the electrical intercon-
nections. As mentioned above, SolarCity successfully
imposed 100 % end-of-line electroluminescence (EL)
inspection on all of its suppliers to reliably detect
problems related to soldering of electrical intercon-
nections. During factory audits, SolarCity’s experts
place key focus on inspecting all process steps relat-
ed to the interconnections, and great success has
been achieved in detecting and helping resolve prob-
lems with these processes.
However, as discussed above, the correlation of de-
fects seen after such extended TC tests and failures
occurring in the field has not unambiguously been
proven, and it still a matter of debate whether these
extensive stress tests might overstress the modules
and trigger issues that would not occur in the same
way in the field.
Nonetheless, the fact that after accelerated TC test-
ing problems commonly affecting a large fraction of
modules after extended amounts of time in the field
are not observed, justifies the assumption that the
degradation rate of SolarCity modules will be at least
as low as the industry average for modules of 0.5 %
per year [1] if not better, and a Useful Life of
35 years yielding a power output of 82.5 % thereaf-
ter appears realistic.
Figure 9: Electroluminescence images and corresponding power degradation of modules from a representative SCTY supplier
before Thermal Cycling testing (left), after 400 Thermal cycles (center), and after 800 Thermal cycles (right).
Pre stress TC-400 TC-800
Pmax change
( rel. %)0 -0.9% -1.3%
17 SolarCity Photovoltaic Modules with 35 Year Useful Life
Figure 10: Electroluminescence images of a modules after 200 Thermal cycles (left), 400 Thermal cycles (center),
and after 600 Thermal cycles (right). Dark areas indicate disconnection of busbars (red markers) [14].
3.7 Product Qualification Program Test-ing – Extended Damp Heat
The second most common test to demonstrate long-
term PV module performance is extended Damp
Heat testing. Figure 11 shows the change of output
power after Damp Heat testing for 1000, 2000, and
3000 hours for a mix of SolarCity-type modules
(green line with filled circles; median and standard
error) and a statistically significant mix of Non-
SolarCity-type modules (66 %ile: black line; 50 %ile:
grey line; 33 %ile: light grey line) that have been
measured at DNV GL PVEL over time. The modules
discussed here are the collection of modules that
were submitted by module suppliers to DNV GL for
PQP testing, where ‘SolarCity-type’ modules were
fabricated for SolarCity, while ‘Non-SolarCity-type’
modules made up the rest of the collection and are
referred to as ‘Industry mix’. Additionally, data from
a selection of modules of a comprehensive literature
study is shown [14].
Even after exposure to 3000 hours of Damp Heat,
which is three times the time required by IEC 61215,
the SolarCity-type modules only show ~2 % median
power degradation.
The extension of Damp Heat testing to 2000 or even
3000 hours has become common practice in the at-
tempt to demonstrate greater durability of a particu-
lar module design, and SolarCity requires this test as
standard for the Product Qualification Program.
However, 3000 hours of Damp Heat is considered a
test to failure, and it has increasingly been reported
that the problems observed in this test are not rep-
resentative of failures occurring in the field [14]. It is
under debate whether the 3000 hour Damp Heat
test performed on a module with a breathable back-
sheet is useful for the prediction of service life in the
field. However, the fact that the SolarCity-type mod-
ules do not show significant degradation after this
extended test suggests excellent protection against
long-term effects caused by excess humidity and/or
heat. In order to address possible limitations of ul-
tra-long Damp Heat testing, SolarCity will perform
additional UV exposure tests, as described in sec-
tion 4.
18 SolarCity Photovoltaic Modules with 35 Year Useful Life
Figure 11: Power degradation after Damp Heat testing for 1000, 2000, and 3000 hours for a mix of SolarCity-type modules
(green line with filled circles; median and standard error) and a statistically significant mix of Non-SolarCity-type modules
(66 %ile: black line; 50 %ile: grey line; 33 %ile: light grey line) that have been measured at DNV GL PVEL over time. Additionally,
data from a comprehensive literature study is shown [14].
3.8 Product Qualification Program Test-ing – PID Testing
Another extended stress test that SolarCity requires
within the framework of its Product Qualification
Program is a test for Potential Induced Degradation
(PID). In the field, the voltage of modules that are
connected in series within strings commonly reaches
-600 V or -1000 V. Per US code, the module frames
need to be grounded, which causes a voltage differ-
ence between the grounded frames and the cells in
the module. This voltage can cause a migration of
mobile ions through the module either towards or
away from the cells. As a result, mobile positive so-
dium ions contained in the glass substrate can mi-
grate towards the cells. Crystalline defects known as
stacking faults with lengths of just a few microme-
ters permit the ingression of these sodium atoms,
which results in short circuits (shunts) that are symp-
tomatic of the Potential Induced degradation. The
effect is triggered by humidity, temperature, and
voltage. SolarCity implemented one of the most ag-
gressive procedures in the industry to test for PID.
The modules are subjected to 85 °C and 85 % RH,
while simultaneously biased with positive or nega-
tive 1000 V with respect to the module frame. The
degradation is tested after 100 and 600 hours expo-
sure, respectively. As a comparison, NREL’s ad-
vanced ‘Qualification Plus’ standard that was de-
signed to address the shortcomings of IEC 61215
only postulates a 96 hours long exposure to 60 °C
and 85 % RH and -1000 V.
Figure 12 shows a plot of the power degradation
after PID testing at for 100 and 600 hours for a mix
of SolarCity-type modules (green line with filled cir-
cles; median and standard error) and a statistically
significant mix of Non-SolarCity-type modules
(66 %ile: black line; 50 %ile: grey line; 33 %ile: light
19 SolarCity Photovoltaic Modules with 35 Year Useful Life
grey line) that have been measured at DNV GL PVEL
over time. The modules discussed here are the col-
lection of modules that were submitted by module
suppliers to DNV GL for PQP testing, where ‘SolarCi-
ty-type’ modules were fabricated for SolarCity, while
‘Non-SolarCity-type’ modules made up the rest of
the collection and are referred to as ‘Industry mix’.
Similar to the extended Thermal Cycling and Damp
Heat tests discussed above, SolarCity-type modules
outperform the majority of their counterparts in
industry. Even after 600 hours of aggressive PID ex-
posure at 85 °C and 85 % RH and applied bias
of -1000 V, the output power degrades less than 1 %
and is less than the degradation seen in the best
66 percentile of Non-SolarCity-type modules tested
at DNV GL PVEL. SolarCity’s modules are considered
‘PID-free’, even under the most aggressive operating
conditions currently tested for in the industry.
Figure 12: Power degradation after PID testing at 85 °C /
85%RH and -1000V for 100 and 600 hours for a mix of
SolarCity-type modules (green line with filled circles; me-
dian and standard error) and a statistically significant mix
of Non-SolarCity-type modules (66 %ile: black line; 50 %ile:
grey line; 33 %ile: light grey line) that have been measured
at DNV GL PVEL over time.
3.9 Product Qualification Program Test-ing – Extended Humidity Freeze and UV Test
Similar findings to the ones described in sections 3.6
to 3.8 were observed for the remaining tests of So-
larCity’s PQP program that are listed in Table 3.
A representative mix of SolarCity-type modules show
less than 2 % power degradation (not shown here)
after extended Humidity Freeze testing (3 times the
duration of IEC 61215 testing that is described in
section 3.3). This result confirms the findings from
extended Damp Heat testing and shows that the
encapsulation materials and lamination processes
are well controlled, resulting in a high-quality lami-
nate and an effective prevention of problems due to
moisture ingress and laminate deficiencies.
Similarly, the power degradation observed for a rep-
resentative mix of SolarCity-type modules is ~1 %
after exposure to UV radiation at 60 °C for a total
exposure of 90 kWh/m2 (not shown here), indicating
that these modules do not show any UV-induced
optical or mechanical degradation under the condi-
tions tested.
4 Next Steps – Tests with Im-proved Correlation to Real Life
As discussed above, there is no broad consensus as
to how well current extended accelerating test pro-
cedures mimic the stress conditions that modules
experience under real-life conditions and how the
failure modes triggered by these extreme stress tests
match defects seen in real modules. It is not clear
whether the extended conditions lead to overstress-
ing of the modules and to failure modes that are not
observed in the field under real life conditions. At
the same time, defects and failures that are seen in
modules in the field may not be detected by the cur-
rent accelerated test procedures.
For example, there has been substantial discussion
of a general need for longer UV exposure [19], given
that the UV exposure in current stress tests is orders
of magnitude weaker than the expected UV dosage
that modules experience during their life in the field.
The UV preconditioning test according to IEC 61215
with a UV dosage of 15 kWh/m2 between 280 and
385 nm only simulates about 18 days of AM1.5 ir-
radation at 1000 W/m2 [20]. Even for an assumed
typical outdoor day/night average of 250 W/m2 for
outdoor irradiation, the test still only simulates
~2.4 months of outdoor irradiation. Similarly, there
20 SolarCity Photovoltaic Modules with 35 Year Useful Life
are currently no standardized test procedures to
detect the formation of snail trails, which have be-
come a widespread phenomenon that is encoun-
tered by a large number of module makers and solar
farms across the world.
SolarCity considers itself a thought leader in the in-
dustry and is an active player in the development
and implementation of more advanced accelerated
testing procedures. These tests address some of the
serious shortcomings of current accelerated stress
tests and help generate new relevant and meaning-
ful results. SolarCity has been working with leading
institutions and industry partners such as NREL, DNV
GL PVEL, and DuPont to detect and address serious
and common failure modes occurring in modules
after exposure to real life use conditions in order to
ensure that its modules are robust against the
mechanisms causing these real life failures.
In view of this, novel tests have been developed and
implemented for detection of snail trails, hotspots,
advanced UV degradation of backsheets, and degra-
dation of mechanical properties of backsheets. So-
larCity also introduced a field degradation study
where modules are placed outdoor and character-
ized after six and twelve months to evaluate the
degradation rates under real-life conditions. Last,
more sensitive methods to allow for early detection
of defects related to increased series resistance have
been incorporated and are under evaluation.
All these tests have been implemented as an addi-
tion to SolarCity’s PQP product qualification test plan
in February 2016, and they are now a mandatory
requirement for product qualification for all SolarCi-
ty 3rd
party module suppliers.
The development of the advanced test procedures
was led by SolarCity engineers, in collaboration with
engineers from DNV GL PVEL. Approving of these
advanced test methods, DNV GL PVEL implemented
the same test procedures as a new standard for their
Product Qualification plan.
5 Useful Life Extrapolation Based on Degradation of Fielded Mod-ules
Accelerated testing is a valuable tool to uncover ear-
ly product failures and indicate reliable long-term
performance, but as mentioned above, it is challeng-
ing to obtain quantitative information about degra-
dation rates under realistic use conditions. A real-
world environment is a unique combination of dif-
ferent stressors, which no accelerated testing cham-
ber is able to accurately duplicate. Such stressors in
include, but are not limited to high and low temper-
atures, rain and moisture, UV irradiance, snow, salt
fog, and soiling. Therefore, outdoor testing under
realistic exposure conditions is the most appropriate
method to correlate indoor accelerated testing to
real-world long-term performance.
SolarCity is in the unique position to have access to
and gain insight from a very large fleet of modules
and systems. The company is currently working on
determining real-life module degradation rates
based on studies of modules that have been de-
ployed in the field in order to support the argument
of 35-Year Useful Life for their modules. The work is
based on a three-pronged approach:
1. Sample representative modules that have been
deployed in the field for several years and test
them in the laboratory to compare against
nameplate rating, following D. Jordan’s et al.
methodology [1,2]. As discussed in the refer-
enced publication, the accuracy of this method is
affected by the uncertainty of nameplate rating
and possible light-induced degradation (LID).
2. Estimate the annual degradation rate from sys-
tem performance data that is collected through
SolarCity (correct the data using weather infor-
mation). This method has a larger error due to
limited accuracy of online performance data, cli-
mate data, impact of soiling, etc.
3. Deploy modules in the field (in various climates)
after thorough characterization in the laboratory
and light exposure to eliminate the effects of LID,
and test them in the laboratory in regular inter-
21 SolarCity Photovoltaic Modules with 35 Year Useful Life
vals. This method will yield the best possible in-
formation on degradation rates, since the meas-
urements are performed under controlled labor-
atory conditions, and accurate information about
initial performance is available.
The goal of this study is to demonstrate that SolarCi-
ty modules deployed in the field under real-life con-
ditions show an annual module degradation rate of
no more than 0.5-0.6 % per year, which warrants a
postulation of Useful Life of 35 years with a power
output of 80 to 82.5 % thereafter.
This data will be presented in a follow-up publica-tion.
6 Conclusion
SolarCity presented data and supporting information
to support the claim that the Useful Life of the pho-
tovoltaic (PV) modules used in its residential and
commercial systems is 35 years or longer. Data from
accelerated stress testing according to and beyond
IEC 61215, which was performed by DNV GL,
demonstrates that power degradation of modules
supplied to SolarCity by external suppliers is as much
as 35 % lower than for a comparable industry-wide
selection of non-SolarCity modules measured at
DNV GL, which are typically warranted for 25 years.
The reason for this advantage of modules fabricated
for SolarCity is the implementation of a stringent and
industry-leading Total Quality Program, which
adopted its features from the Automotive Industry
and was implemented by SolarCity in early 2014. Per
contractual requirement in Master Purchasing
Agreements, SolarCity’s third-party suppliers need to
have effective Quality Assurance programs and re-
fined manufacturing processes in place, and steady
product and manufacturing quality must be demon-
strated. Rigorous tests need to be passed on an on-
going basis, performed by a qualified 3rd
party lab.
The data presented supports the assertion that So-
larCity’s PV modules, as a result of its comprehensive
Total Quality Program and industry-wide advance-
ments in material, manufacturing, and quality con-
trol, perform at least similar, if not better than the
median crystalline-Si modules observed in the larg-
est meta-study to date of more than 11,000 modules
[1,2]. Therefore, an annual module degradation rate
of 0.5-0.6 % per year is a realistic assumption, which
warrants a postulation of Useful Life of 35 years with
a power output of 80 to 82.5 % thereafter.
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