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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


www.ametek.com

1

http://www.ametek.com/


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


3


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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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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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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AC-Coupled Grid-tie Design

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AC-Coupled Off-Grid

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DC-Coupled Off-grid

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~

=~

= GenSet

==

==

==

Battery

Monitor

INVERTERSTATIC

SWITCHCHARGER

PV

CONTROLLERS

==

PV

PV

PV

PV

PV

==


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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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.

12


Lower Current = Longer Life

• Typical AC-coupled designs run on a 24-

48VDC bus

• A small 10kW system draws 200ADC!

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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


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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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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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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• 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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• 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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• 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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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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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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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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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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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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The Advantages of Industrial,

Full Double-Conversion, DC-Coupled

Off-Grid PV Systems

John Ely

AMETEK SCI


www.ametek.com

26

http://www.ametek.com/

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