[RE-wrenches]

Bill Brooks bill at brooksolar.com
Thu Apr 21 16:30:51 PDT 2005


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Lee,

Given that you have some old VLX modules, I would put all 6 in series and
place that in parallel with the pairs of new 165s. That may sound crazy, but
if took an IV curve of those old modules, I think you would find that their
peak power voltage has reduced significantly. 3 in series is a big mistake.
It would not have enough voltage to match well at all with the new 165s.
Alternatively, you can put 5 of the VLXs in series and take the sixth one
home to mount on your wall. With an old module like the VLX, the IV curve
has become so soft that you will be near max power when operating at 60% of
open circuit voltage.

The max power tracker will not pick the average as you suggest. You will be
creating a very pronounced double-hump IV curve and the MPPT will likely
choose the higher voltage 165 curve to track and operate your VLX-53s at
nearly open circuit--try it for your fun to see.

Bill.


-----Original Message-----
From: Lee Tavenner [mailto:solplex at montana.com] 
Sent: Thursday, April 21, 2005 3:34 PM
To: RE-wrenches at topica.com
Subject: [RE-wrenches]

Hi Rick, Allan, Darryl, Kent, Geoff, and Wrenches.

Thanks for your suggestions on the two arrays project. Still looking 
for solution with fewer negatives.   The new array is 230 feet on the 
other side of the old array from the house, and the #4 copper from the 
house to the old array is buried, so I would prefer a 1-controller 
solution that doesn't require 2 more wires from the old array to the 
house.

How about wiring the 6 VLX 53s with 3 each in two series strings, and 
the two strings in parallel?  Then wire 2 of the 165s in series and 2 
more of the 165s in series, and parallel these two 2-165 strings.    
This results in a 36-volt nominal 6.2 amp old array and a 48-volt 
nominal 9.7 amp new array.  Then wire these two arrays in parallel.   
Net result--nominal 42 volt array with MPPT around 60.  In parallel the 
voltages go to the average and the amperages add.  Total power should 
be the sum of all 10 panels.  Would there be any mismatch losses?  If 
so, how would you calculate or  estimate these losses?  Any reasons I 
should not do this?  Thanks for you input.

  Lee Tavenner
Solar Plexus

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