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CLEAR VISION SOUND STRATEGIES SOLID PERFORMANCE
The Advantages of Industrial,
Full Double-Conversion, DC-Coupled
Off-Grid PV Systems
John Ely
AMETEK SCI
CLEAR VISION SOUND STRATEGIES SOLID PERFORMANCE
www.ametek.com
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CLEAR VISION SOUND STRATEGIES SOLID PERFORMANCE
Market Split
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• Off-grid PV systems typically take a back
seat to their grid-tied big brothers
• 95-5% split of grid-tie to off grid
• Off-grid PV is driven by a different set of
socio-economic factors than grid-tie
applications
CLEAR VISION SOUND STRATEGIES SOLID PERFORMANCE
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CLEAR VISION SOUND STRATEGIES SOLID PERFORMANCE
What is Changing?
• China, India, Asia and many parts of Africa are
enacting major rural electrification programs
• More attention is being directed to stand-alone
PV
• Currently, designs rely on an AC-coupled grid-tie
technology that is reconfigured for an off-grid
application
• DC-coupled design is specifically designed for
stand alone power - better meets the needs
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CLEAR VISION SOUND STRATEGIES SOLID PERFORMANCE
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CLEAR VISION SOUND STRATEGIES SOLID PERFORMANCE
Why the Need for Solar?
• Most off-grid PV applications are remote,
isolated areas
• S.E. Asia, rural China and India are non-
forgiving landscapes
• India alone has over 80,000 villages w/o power
• In China they estimate over 400M w/o power
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CLEAR VISION SOUND STRATEGIES SOLID PERFORMANCE
What’s the Difference?
• AC-coupled systems are optimized for “payback”
economics
• DC system is load-centric, optimized for power
storage, support of the load and reliability.
• ROI for an AC system was designed to be
monetary,
• ROI off-grid is socio-political; dropping the load
or substandard reliability is not acceptable, as
most times this is a rural area’s only source of
power.
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CLEAR VISION SOUND STRATEGIES SOLID PERFORMANCE
AC-Coupled Grid-tie Design
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CLEAR VISION SOUND STRATEGIES SOLID PERFORMANCE
AC-Coupled Off-Grid
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CLEAR VISION SOUND STRATEGIES SOLID PERFORMANCE
DC-Coupled Off-grid
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~
=~
= GenSet
==
==
==
Battery
Monitor
INVERTERSTATIC
SWITCHCHARGER
PV
CONTROLLERS
==
PV
PV
PV
PV
PV
==
CLEAR VISION SOUND STRATEGIES SOLID PERFORMANCE
DC Coupled
• DC-coupled design is completely isolated
from the back-up AC power source, and in
turn, immune to transients and other “dirty”
power produced by generators and the
like.
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CLEAR VISION SOUND STRATEGIES SOLID PERFORMANCE
Why Gensets Matter
• Generators also have no surge capability,
meaning the surge protection with an AC-
coupled design is merely peak shaving
while paralleled with the generator.
• The more robust DC-coupled systems are
full double-conversion configurations with
complete isolation, fully rated for surge –
ensuring clean, reliable power at all times.
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CLEAR VISION SOUND STRATEGIES SOLID PERFORMANCE
Lower Current = Longer Life
• Typical AC-coupled designs run on a 24-
48VDC bus
• A small 10kW system draws 200ADC!
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CLEAR VISION SOUND STRATEGIES SOLID PERFORMANCE
Lower Current = Longer Life
• A DC-coupled system is designed for a
240VDC minimum
• In contrast, pulls just over 40ADC
• Substantial reduction in current flow
• This means less heat, less line loss,
shallower battery discharges (deeper
discharges reduce battery life), and less
susceptibility to heat-related electronics
failures. 14
CLEAR VISION SOUND STRATEGIES SOLID PERFORMANCE
Economics
• Most AC-coupled off-grid systems utilize
two or more inverters - one to act as a
rectifier, one to act as the inverter and
sometimes one to act as the PV battery
controller.
• This design suffers from many design
flaws, most notably, using equipment for
purposes outside their primary functional
scope.
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CLEAR VISION SOUND STRATEGIES SOLID PERFORMANCE
DC Economics
• A true DC-coupled system is designed to
be the primary source of power under any
external condition
• Downtime is almost unheard of
• DC-coupled system design also means
less fossil fuel consumption because of
more efficient movement of energy.
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CLEAR VISION SOUND STRATEGIES SOLID PERFORMANCE
Environmental Issues
• Many off-grid environments, particularly
those in developing third-world countries,
have higher ambient temperatures, (i.e. >
30OC.)
• Can also be high particulate contamination
of the air.
• The critical off-grid system component
most affected by high ambient
temperatures is the battery
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CLEAR VISION SOUND STRATEGIES SOLID PERFORMANCE
Environmental Issues
• Other internal inverter components, such
as DC bus filtering capacitors, may have
their service life shortened by high
ambient temperatures
• Special high-temp capacitors should be
selected.
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CLEAR VISION SOUND STRATEGIES SOLID PERFORMANCE
Environmental Issues
• Industrial inverter equipment is designed
to tolerate moderate amounts of non-
conductive dust
• Industrial inverters also can handle very
high ambient air temperatures of at least
40 C.
• May even have an option for 50 C!
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CLEAR VISION SOUND STRATEGIES SOLID PERFORMANCE
Environmental Issues
• Industrial inverter systems have built-in
design margins to preserve the operation
life in tougher environments.
• In addition, Industrial inverter equipment
will also have predictive parts replacement
programs to insure high MTBF over the
entire 30 year service life.
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CLEAR VISION SOUND STRATEGIES SOLID PERFORMANCE
Reliability
• The most important aspect of the DC-
coupled design is the overall reliability
• Industrial inverters are designed for a 30-
year life or more, most hybrid inverters are
designed for 7-10 year service life.
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CLEAR VISION SOUND STRATEGIES SOLID PERFORMANCE
MTBF
• Mean time between failure (MTBF) figures
for hybrid inverters range in the 100,000
hour area.
• An industrial, full-double conversion
inverter’s MTBF can range in the 275,000
hour range or higher!
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CLEAR VISION SOUND STRATEGIES SOLID PERFORMANCE
MTBF
• This calculation means even more when
you consider that most off-grid systems
are the primary sources for power
• Located in some of the world’s most
remote locations and unforgiving terrains
• Sending a technician to fix the problem
can cost 10s of thousands of dollars and
take weeks or even months to repair!
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CLEAR VISION SOUND STRATEGIES SOLID PERFORMANCE
AC-Coupled Final Thoughts
• AC coupled systems were designed to
augment the grid power
• Power agreements sell power back to the
utility
• Efficiency is vitally important - however
they are not designed to be the primary
source of power.
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CLEAR VISION SOUND STRATEGIES SOLID PERFORMANCE
DC-Coupled Final Thoughts
• The most important factor of an off-grid
system is reliability
• We can slice efficiency rates, but at the
end of the day, the DC-coupled system is
designed as the primary source of AC
power
• Should rarely I ever, drop the load
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CLEAR VISION SOUND STRATEGIES SOLID PERFORMANCE
The Advantages of Industrial,
Full Double-Conversion, DC-Coupled
Off-Grid PV Systems
John Ely
AMETEK SCI
CLEAR VISION SOUND STRATEGIES SOLID PERFORMANCE
www.ametek.com
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