Design Challenge (Was Sunny Island Experience?) [RE-wrenches]

Jeff Clearwater clrwater at earthlink.net
Sat Feb 4 12:21:00 PST 2006


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Hi Good Colleagues,

Thanks for the feedback Jeff and Travis - good to hear AC coupling is 
out there and being used successfully in the US!

OKAY DESIGN MINDS - Here's my design challenge - input most 
appreciated!  Consolation prizes available after the contest (just 
kidding).

I'm designing a multi-residence off-grid system with 12 KW PV, 30 KW 
(120 A) Diesel Genset, 12-15 KWs of inverter loads with about 24 KW 
peak, a 100 KWH battery pack and the following long distances between 
components:

Picture an equilateral triangle with roughly 600 foot sides. Point 1 
is Loads, Point 2 is  Generator, Point 3 is PV array site.  This is 
pretty close to the actual situation on the ground (as unfortunate as 
that might be). Close enough for easy discussions.  Power Shed has 
yet to be located:  Generator site is practically set in stone. 
There are also some loads past the Genset side of the triangle.

In the future we will also have wind and hydro coming in from about 
2000' away from the PV side of the triangle.  Generator is in and 
can't be moved, neither can the house complex, and the PV site is the 
only one non-shaded so it's a given too.

PROPOSED SYSTEM #1:  4 Sunny Islands (2 sets of 2 stacked split 
phase) in power shed located near the generator shed.  This allows us 
to feed the long distance array to the system at high voltage - 
either high voltage DC or 240 VAC depending on where we located the 
SB.  It also allows us to not have a ditch and conduit along the 
PV<>Power Shed side of the triangle as the PV can feed into the AC 
line at the house.  It also allows us to to feed Windy Boys and Hydro 
Boys at high voltage in the future to any point on the system.

MAJOR DISADVANTAGE:  Unfortunately the present US version of the SI 
only allows 2 SIs stacked in split phase with no parallel stacking 
for bigger systems.  So that means we'd have two sets of the 
batteries, two sets of AC, 3-wire outputs at 56 amps each instead of 
of one at 120.  And the load peaks would have to always be 
distributed amongst the outputs to handle the peaks and overall 
loads.  It also means that any expansion faces the same hassle.

PROPOSED SYSTEM #2:  8 Stack of Outbacks located near the array.

MAJOR DISADVANTAGES:   A 1200' line and ditching would have to be 
added from gen site to array/power shed to house to feed the 
inverters from the generator with major sized wire as all power would 
have to be fed through inverters. - 120 Amps of 240 now having to go 
1200'  (over 4% drop using 750 MCM)  when there are already existing 
AC wire runs are from Generator to house.  So all the power would 
have to run around two sides of the triangle to get to the load and 
the existing 600' of 3/0 Al 3 wire goes unused. .  Also another AC 
line would have to be run back to feed the loads that I didn't 
mention that are on the other side of the generator from the house. 
Getting into massive AC wire runs here just because we are limited to 
the MX60s voltage for charging from the array and hence need to keep 
the  power shed close to the array.  Future wind and hydro inputs 
will work if we use a HV/LV scheme (240-600 V Alternator rectified to 
48VDC at power shed) as they are closer to the array.  All future PV 
or other input expansion would have to come to this power shed, 
however - not be able to tie in anywhere on the AC "grid" as in the 
SI/SB system.

SO I SEE THIS SITUATION AS THE PERFECT ILLUSTRATION OF THE NEED AND 
LIMITIATIONS IN THE INDUSTRY.  I do alot of small village power 
systems.  AC coupling is the cat's pajamas,  IF you have the european 
SI!

On the SI side, the limitation of no parallel stacking is a MAJOR 
problem.  Kind of eliminates the whole point of AC coupling for 
larger systems - the flexibility to grow systems and feed them 
anywhere on the AC side with high voltage.  On the Outback side no AC 
coupling means that array and generator have to be close to the power 
shed and all future inputs are relegated to DC (which for wind and 
hydro is not too bad as you can do them high voltage AC and then 
rectify and transformer down).   But in many situations (not this 
one) having to always come back to one point instead of being to tie 
into the AC grid anywhere on the property will always be a major 
limitation.

SO QUESTIONS AND POSSIBLE SOLUTIONS:

1) I could try to do a SB running into an Outback stack located near 
the gen shed.  But we are talking 12 KWs of PV here - with no 
software talking - this could get hairy.  And I'd have to run control 
wire between SB and Outback for the relay.

2)  Any charge controls out there that go higher voltage than the 
MX60s 72 volt nominal?  If I run the calcs of trying to run even the 
72 volt (figure 84 volt input?) 600 feet so I can have my power shed 
at my genset, we are looking at over 750mcm wire!  not going there. 
What we need is a 500 VDC MPPT charge control that feeds into 48!

3)  I can play with locating the power shed at various places within 
the triangle . . . . . but all the problems are still there - just 
trading off different sets of problems for other sets.

4)  According to SMA an updated version of the SI is planned - 
hopefully with increased parallel stacking (and hopefully throttling 
of inverters like the Outbacks).  According to rumors, Outback is 
looking at AC coupling.  In the meantime my client is burning through 
THOUSANDS of dollars of diesel running that 30 KW genset to just to 
cook breakfast.  The race is on.  In the meantime, what do I do?!!!!!

Well if you read this far, this post should raise some discussion! 
Let's be gentle on both SMA and Outback now folks!  It's great we can 
even be having this discussion thanks to the great innovation from 
both companies!

Have at if if you will!

Thanks!

Jeff Clearwater
Village Power Design








>Hi Jeff,
>
>About 6 months ago we set up an off grid system with two SI's coupled with
>three SB3800's and about 9 kW of Sanyo HIT's spread evenly amongst the
>SB3800's.    It's all set up as 120/240 with no transformers.
>
>No problems after we upgraded the software.  Everything talks and gets along
>fine.
>
>Travis Creswell
>Ozark Energy Services
>
>-----Original Message-----
>From: Jeff Clearwater [mailto:clrwater at earthlink.net]
>Sent: Friday, February 03, 2006 4:37 PM
>To: RE-wrenches at topica.com
>Subject: Sunny Island Experience? [RE-wrenches]
>
>Hi Folks,
>
>Anyone running SBs into SIs with AC coupling?  What's your experience been?
>
>I talked to SMA a few times about running WBs and HBs  into SIs, but
>was advised that that would be smoother when SI version 2 is released
>- but they didn't know when that would be.  Anyone try that yet?
>
>Apparently version 2 may have true multimastering as well - where
>you can have different SIs on different points on the AC network,
>have SBs feeding in and not have the two SIs confuse or compete with
>each other.  The present SIs don't support that.
>
>Thanks in advance!
>
>Jeff C.
>Village Power Design
>--
>~~~~~~~~~~~~~
>Jeff Clearwater
>Village Power Design Associates
>Sustainable Energy & Water Solutions for Home & Village
>http://www.villagepower.com
>gosolar at villagepower.com
>
>530-470-9166
>877-SOLARVillage
>877-765-2784
>425 Nimrod St.
>Nevada City, CA 95959
>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~`~


-- 
~~~~~~~~~~~~~
Jeff Clearwater
Village Power Design Associates
Sustainable Energy & Water Solutions for Home & Village
http://www.villagepower.com
gosolar at villagepower.com

530-470-9166
877-SOLARVillage
877-765-2784
425 Nimrod St.
Nevada City, CA 95959
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~`~

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