Allan, I haven't tried this in the field but I believe you'd be pulling
the string voltages to roughly that of a 10.67-module string. So your
11-module string voltage is down by 3.3%, 10-module string voltage is up
by 6.7%. Your 11-module string performance is down by about 1V/module
(perhaps power reduced from ideal by 2%) and your 10-module performance
is up by about 2V/module (perhaps power reduced from ideal by 4%) for a
total power loss of about 3%. When you get on site and see consistent
irradiance, you could use some jumpers to bypass modules and better
quantify the loss by comparing the performance of a 32-module array with
a 30-module array.
With this many modules in series, and given that most manufacturers
don't specify voltage tolerance any tighter than 10%, it's probably not
a big deal. If you are bored on a spare afternoon, you could sort the
modules so your higher-voltage modules are on the 10-module strings...
fun, right?
Dave
On 2012/6/26 11:38, Allan Sindelar wrote:
Wrenches,
We have been hired to serve as independent consultants on a large Sunny
Boy/Sunny Island off grid project. The array and the battery bank (of
unknown type and capacity at this time) have proven inadequate for the
loads and the owners want outside review of the proposed upgrade, which
we have agreed to provide.
In reviewing project information, I have learned that each Sunny Boy
inverter is serving 32 260W modules in three parallel strings. Two of
the strings have eleven modules and one has ten. This is to us a
fundamental violation of good design practice, such that we have never
tried it to learn the results. That's my question: what will be the
output of this array of mismatched strings?
We figure that the inverter will find a MPPT "sweet spot" that is pulled
down below the MPP of the two eleven-module strings, to where the
ten-module string's contribution is close enough to its MPP to maximize
the output of the three strings together. This would result in slightly
more than 30 modules' worth of wattage output from the 32-module array.
We have not yet been to the installation or seen photos of it. My verbal
understanding is that the 96 modules are on eight pole-top racks. It
would be fun to figure out the array wiring on this one.
Has anyone encountered this mismatch situation? What did you learn?
Thank you,
Allan
--
*Allan Sindelar*
[email protected]_ <mailto:[email protected]>
NABCEP Certified Photovoltaic Installer
NABCEP Certified Technical Sales Professional
New Mexico EE98J Journeyman Electrician
Founder and Chief Technology Officer
*Positive Energy, Inc.*
3209 Richards Lane (note new address)
Santa Fe, New Mexico 87507
*505 424-1112*
_www.positiveenergysolar.com_ <http://www.positiveenergysolar.com/>
*
*
_______________________________________________
List sponsored by Home Power magazine
List Address: [email protected]
Options& settings:
http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org
List rules& etiquette:
www.re-wrenches.org/etiquette.htm
Check out participant bios:
www.members.re-wrenches.org
_______________________________________________
List sponsored by Home Power magazine
List Address: [email protected]
Options & settings:
http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org
List rules & etiquette:
www.re-wrenches.org/etiquette.htm
Check out participant bios:
www.members.re-wrenches.org
