Sunny Boy upgrade question [RE-wrenches]

[John Berdner]

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Allan:
 
Adding another full string of the other module type is the right way to
go and is probably the best you can do given the module supply
situation.
The higher currents of the larger modules should add in parallel
without too much mismatch loss as long as they are in their own string.
 
The different MPP voltages will cause the inverter to operate at an MPP
between the MPP of each of the two parallel strings.
It will not be optimal for either string but, as long as the number of
modules in series is the same, it will be close to optimal for both.
You can estimate losses to a reasonable degree by of accuracy looking
at the voltage differential between the 2 strings, e.g. 7 Vdc (1.0 Vdc
per module).
Assume the inverter will operate at an MPP half way between the two
sub-arrays, i.e. 3.5 Vdc (0.5 Vdc per module) low for the new modules
and 3.5 Vdc (0.5 Vdc per module) high for the older modules
You can then look at the power curve for each module and estimate the
losses by moving 0.5 Vdc per module towards Isc for the new modules and
0.5 Vdc towards Voc for the older modules.  
This should give you a ratio of "optimum theoretical" output to the
"probable" output for each series string in the new configuration.
You can then use this ratio and the historic system performance to
estimate the future performance of the system including the mismatch
losses.
In actuality the inverter will operate at an MPP point that is not
exactly half way between the two optimal points.
IMHO, this is a second order effect and is likely insignificant when
compared to the half way assumption.
 
For a mental exercise you can more accurately estimate the mismatch
losses through mathematical approximation.
Starting with the half way between (50%) assumption calculate the sum
of the power of the two strings.
Repeat for 40%, and 60% to see if the sum of power goes up or down in
one way or the other.
If the power was higher at 40% then try 35% and 45%, etc. etc.
You should be able to find the theoretical optimal MPP of the two
dissimilar strings.
This MPP then becomes the basis for your ratio to estimate future
performance of the combined array with mismatch loss.
 
I hope this was clearer than mud and is somewhat useful.
 
Best Regards,
 
John Berdner 
 
>>>

Esteemed Wrenches,
How would you solve this? Two years ago we installed a batteryless
system
with a Sunny Boy 1800 and seven BP SX140s, with racks in place for a
second
7-module array in the future, when the client could afford it. (This is
the
client that was featured in the RE Financing article in HP103:97, who
financed his system through the Permaculture Credit Union.)

He's ready to add the second row of seven modules now, and SX140s are
no
longer available. We can get 160s and 170s, I think. I would expect
that as
long as the MPP voltage is closely aligned (it's close but not perfect,
34V
vs. 35 at mpp), and the cell type is similar (it is, polycrystalline),
it
should be OK to mix outputs, as the two strings are in parallel and
combine
prior to the inverter.

Any thoughts on this are welcome.

Allan at Positive Energy





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