Positive ground with batteries? [RE-wrenches]

[John Berdner]

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Geoff:
 
In answer to your question:
 
The SI is presently configured for negative ground on the battery.
 
Yes, Ac coupling would work very will in this application since battery
ground and array ground polarity are decoupled and isolated.
 
You can order any of the SB's  with positive ground as special order.
If the loads are 120 only then you could use a single SI and one or
more 1800's without autoformer.
If you need 120/240 loads then you would add an autoformer (for a small
system).
 
For larger system you can run 2 SI's in split phase and then you can
use 240 Vac inverters.
The maximum pass through current for the SI's is 56 Amps continuous (70
amp thermal breaker).
This is the limiting factor for a grid tied system passing through the
isolation relay in the SI.
For larger arrays you there are other options with external xfer
relays.
 
Efficiency of the SI is about 95% peak at around 1200 Watts and around
93% at full power.
This is true in both charging and inverting mode.
 
The SB's vary, but the 3800 and 6000's are a little shy of 95 at full
power (all the test data is available on the CEC website). 
In a grid backup system the SB's deliver power directly to the gird in
one conversion.
It is only when the grid fails that you have a dual conversion.
Even then the dual conversion is only true of the power that is
surplus, i.e. PV power - load power >> battery
 
Hope this helps.
Let me know if you have any questions.
 
Best Regards,
 
John Berdner
 
 

Here's a unique one:
I am asked to design a grid tied system with battery back up using the
new
positive grounded Sunpower PV cells. (not the Sunpower modules, but
the
cells assembled in flat roof laminates from a company called solar
roofing
systems).

Question # 1 - any battery based inverters allow positive ground?

Question #2 - Assuming NO to #1, this looks like a great application
for AC
bus, with an appropriate battery free box tied to a sub panel served
by
either a Sunny Island or Outback system.

We have not yet done an Island, it looks like they require an
additional
transformer to serve/tie into 120/240. Advantage is modulation of SBs.
Outback is talking about this being a viable use of their product as
well,
with either a 5 minute countdown based charge control during off grid
operation or a Batt voltage controlled relay based modulation (both
seem
rough). Add to this the ability to use the Bergey Gridtek inverter and
we
are all very interested in an AC bus approach.

OK John B and Chris F (and others) lets lay out the pluses and minuses
of
each approach. Efficiency during normal grid operation? Scalability
and
capacity? Warranty coverage? Field experience/reliability/surprises?



For a brighter energy future,

Geoff Greenfield
NABCEP Certified Energy Practitioner

THIRD SUN SOLAR & WIND POWER Ltd.
340 West State Street
Athens, OH 45701

www.third-sun.com 
(740) 597-3111



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