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SUSTAINABILITY FOR THE PV INDUSTRY: FIELD SERVICE
TECHNICAL WHITE PAPER
Authored By: Brian Lydic, Standards & Technology Integration Engineer
Fronius USA LLC
lydic.brian@fronius.com
EXECUTIVE SUMMARY
/ When customers invest in a PV system, they expect 25 to 30
years of energy production with minimal service and
maintenance requirements. Depending on several factors, the
system may or may not provide the financial performance that
was originally expected. One factor that can lead to significant
differences in the financial performance of a system is the design
of the selected inverter.
/ Levelized cost of energy (LCOE) is a typical metric used to
evaluate an energy system’s financial performance. Often when
performing this calculation, a very detailed look at inverter
replacement costs is not taken. However, there could be widely
ranging differences in replacement or repair costs depending on
the type of inverter chosen.
/ It is essential the PV industry to take a realistic look at lifetime
costs and give customers up-front information regarding costs
incurred after the system is commissioned. Unless the lifetime
RELIABILITY AND SERVICE LIFE
costs are explained clearly and truthfully to customers when the
sale is made, a backlash will occur once they are surprised by
inverter replacement costs. Since the industry has grown so
rapidly in recent years, the majority of PV systems in the US are
less than 5 years old, with typical standard inverter warranties
being 5 to 10 years in length. Thus, the majority of inverters
installed in the field are still under warranty and the industry has
not needed to address large numbers of inverter replacements or
repairs due to end of lifetime, though this will
PRODUCT FAILURE RATE OVER TIME
/ Though the lifetime of PV systems can be 30 years or longer,
power electronics often have lifetimes on the order of 15 years
or less. Real-life data will be needed to get an accurate idea of
product lifetime for all types of inverters. Fronius inverters
are designed for a service life of 20 years.
/ Mean time between failures (MTBF), which is sometimes
misinterpreted to reflect lifetime, is simply a calculated
reliability criterion that is only applicable during the normal
become commonplace as more and more PV systems age.
/ Fronius examined the true costs associated with replacing or
repairing inverters 15 to 20 years from now as the major
differing factor in PV system cost of ownership. Fronius compared
three systems with three different types of inverters. The
SnapINverter is referring to Fronius’ generation of field-serviceable inverters. By estimating the total costs to maintain the PV
system over its 25 to 30 year lifetime and comparing to the orig-
useful life of the product. The typical “bathtub curve” (pictured
on the right) of failure rate over time shows the three phases
of a product’s life.
MTBF REFERS TO THE FAILURE RATE DURING THE
“INTRINSIC FAILURES” PERIOD, AND THUS CANNOT BE
CONSTRUED TO GIVE A PICTURE OF WHEN WEAR-OUT
COULD OCCUR.
/ failure rate= 1/MTBF
inal purchase price, we can get an extended cost factor that shows
how much more a system will cost over its lifetime based on the
type of inverter chosen. For example, if the original system cost
is $10,000 and the extended cost factor is 1.10, then the total cost
of the system over its lifetime is $10,000 x 1.10 = $11,000. Based
on the estimates given in the examples for three types of inverters, the following cost factors are derived:
/ A highly reliable product with high MTBF could reach
wear-out in a relatively short time. Conversely, a product with
low MTBF could reach wear-out after a much longer time.
/ In addition, it is important to note that an installation with
many high-MTBF components could see more failures in its
lifetime than one with a single lower-MTBF component. MTBF
is expressed as the hours or years of total product operation
before one failure occurs.
Inverter Type Extended Cost Factor
Traditional String Inverter 1.19
Micro-inverter 1.26
SnapInverter 1.05
/ For example, take a 7.5 kW PV system with 30 microinverters that have a calculated MTBF of 500 years. The
calculated time to failure for this system would be: (500 yr)/
(30 units)=16.7 yr/unit.
DATA CONCLUSION
/ While not all inverter manufacturers design their product with
an eye towards field-serviceability, this is one area that will have
a large impact on customer-borne costs when it comes to the
inverter’s end of life. Designing and specifying field-serviceable
inverters will minimize costs to bring the PV system back online
at the end of inverter lifetime and allow the PV industry to
enjoy a sustainable future.
RELIABILITY AND SERVICE LIFE, CONT.
PRODUCT FAILURE RATE OVER TIME
/ This means that if the useful life of the inverters was beyond 16.7 years, there would be at least one failure within that time.
Though the system would continue to operate at a lower capacity, the owner would generally prefer to have the system fixed and
a service trip would be required, possibly under warranty. Comparably, a single string inverter used on this 7.5 kW system would
only need an MTBF of 16.7 years to match the reliability of the micro-inverter system. A more realistic MTBF for string inverters
is 100 to 200 years, indicating significantly higher reliability in terms of number of failures per system. Of course, MTBF
expectations should be tempered with real-life data.
CODE UPDATES
/ NFPA 70, the National Electrical Code, is updated and
published every three years. Though many parts of the Code
are well-established, new technology, research and improved
safety measures can all prompt changes in any given revision
cycle, even in something as time-tested as building wiring. In
a newer area such as solar photovoltaic systems, much work
is done every year to sort out all the changes necessary for
this burgeoning technology. Section 690 was added to the
CHANGES IN TECHNOLOGY
/ Being a growth industry, the technology landscape of
photovoltaics and solar electronics is always changing. While
transformer-based inverters dominated the technology landscape for a long time, transformerless designs are now becoming commonplace.
/ Module-level electronics, whether retrofitted or module-integrated, are another recent area of advancement. As advances
Code in the 1984 edition.
/ Throughout the last several revision cycles, some rather
sweeping changes have been made to the requirements for
PV installations. DC arc-fault circuit interruption, or
instance,was introduced for building-mounted systems in
2011. A rapid shutdown system to control conductor voltage
within ten feet of an array was introduced in 2014. As more
grow in this regard, significant changes to PV system design
can result. As such, the products available on the market 15
years from now may very well look quite different from today.
Field-proven advancements in technology can also help drive
code changes, so module-level control being mandated in the
future is a distinct possibility.
and more PV systems are installed throughout the country, it
is quite likely more attention will be paid to section 690 and
result in more changes in requirements in future Code
editions. Throughout a thirty-year life of a PV system,
the opportunity for one or more significant Code changes
becomes quite possible.
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FIELD SERVICEABILITY: DEFINING THE
EXAMPLES
/ Though it is impossible to know exactly what will happen
in the future, some reasonable assumptions can be made in
order to get a range of costs that could be expected over a 30year timeframe. Fronius examined how the serviceability of
the inverter affects long term costs in three different cases by
SNAPINVERTER
/ There are two ways in which field-serviceability of an
inverter can affect system financial performance. First, system
downtime can by minimized when failures occur. For either
a failure during the warranty period or an end-of-life failure,
the ability to troubleshoot and fix an inverter in one trip for
the technician reduces the time that the system stays idle,
waiting to be repaired. It is conceivable that if a data
assuming a 7.5 kW residential PV system and calculating the
total cost of ownership (TCO) for each case. Since the up-front
cost is the most visible to the end customer, TCO is compared
to the initial system price to give a simple metric to compare
the three cases – the extended system cost factor. The three
cases examined are:
1. A TRADITIONAL STRING INVERTER INSTALLATION
monitoring system detects an inverter failure, a technician
could arrive on-site within 24 to 48 hours with a stock of
replacement parts. The repair to the inverter can be done in
minimal time in one trip and the system will resume full
power production.
/ Compare this to a typical scenario where a technician makes
a first trip to troubleshoot and determine the exact failure,
then orders a replacement. String inverters are often replaced
WHERE THE INVERTER MUST BE REPLACED AT THE END
OF ITS SERVICE LIFE
2. A MICRO-INVERTER INSTALLATION WHERE THE
INVERTERS MUST BE REPLACED AT THE END OF THEIR
SERVICE LIFE
3. A SNAPINVERTER INSTALLATION WHERE THE
ORIGINAL INVERTER CAN BE SERVICED WITH NEW PARTS
in whole, with wait times of one week typical for the
replacement unit to arrive before the technician visits the site
again. Altogether, this added wait time means about 4.5 times
more downtime compared to the Snapinverter.
/ In the case of micro-inverters, let’s assume that all 30
inverters in a 7.5 kW system fail over a 5-year period near
the end of their useful life. This would mean that 6 inverters
fail per year or an average of one every two months. Let’s
/ In order to derive the calculations, some assumptions had
to be made. In all cases, it is assumed that the inverter reaches
its end of life outside the warranty period but within the PV
system lifetime, such that the inverter must be repaired or
replaced at cost to the system owner in order to maintain
energy output. This allows for a 15 to 20 year useful life of
the power electronics. A $600 catch-all for other system
assume that a technician arrives with two replacement
inverters a week after every second inverter failure. This would
mean that half the system is down for about 67 days and the
other half is down for 7 days. This means the whole system
is down for an average of 37 days, amounting to 18.5 times
more downtime than the Snapinverter.
/ The second and most dramatic way in which fieldserviceability affects financial performance is by reducing the
service and maintenance is included.
/ The site is assumed to be a 50-mile, 1-hour drive (each way)
for the company servicing the inverter, with a $3.50/gal fuel
price and 15 miles/gal vehicle efficiency. Other assumptions
specific to each case are denoted within the example. Technician labor is valued at $50.30 per hour.[1] Engineering labor
is valued at $100 per hour. All values are in today’s dollars.
For these examples, the “cost” of downtime is not factored in.
total costs incurred when the inverter must be repaired or
replaced at the end of life. These affects are demonstrated in
the following examples.
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