two arrays [RE-wrenches]

Tue Apr 26 19:12:25 PDT 2005

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Bill,

Darryl mentions that his array is shaded in the morning.  That won't affect
the voltage will it?  I can't say that I've ever bothered to test the
voltage of a shaded array but I have observed a lot or arrays in cloudy
weather with what appeared to be voltages of what I would expect of the
array in full sun.

Travis Creswell
Ozark Energy Services, Inc.

-----Original Message-----
From: Bill Brooks [mailto:bill at brooksolar.com] 
Sent: Tuesday, April 26, 2005 6:42 PM
To: RE-wrenches at topica.com
Subject: RE: two arrays [RE-wrenches]

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Darryl,

Standard 12V modules are 36 cells. Very few modules were ever made with 33
cells (mostly Arco modules in the '80s). As you would expect, the 33-cell
module will have about 10% lower voltage than a typical 36-cell module and
with age even less. 6 of these old 33-cell modules in series should be fine
for way up in the cold country. Newer standard 36-cell modules might be a
different story.

You are correct, a good rule of thumb is that max power voltage is between
70-80% of Voc for standard crystalline silicon modules. Older modules will
be on the low end of that range. It is always good to have that ratio in
your back pocket when assessing whether a MPPT controller is actually
working. I find that most installers are far more trusting of MPPT functions
than they should be. Experience says these things can be messed up for many
reasons.

Bill.

-----Original Message-----
From: Darryl Thayer [mailto:daryl_solar at yahoo.com] 
Sent: Monday, April 25, 2005 10:37 PM
To: RE-wrenches at topica.com
Subject: Re: two arrays [RE-wrenches]

Hi folks
You have me scared, perhaps I have an unexpected
failure comming.  I did not check with Outback before
making these installations.  Don't do what I did, and
I will not do it either.  These modules are shaded in
the morning in the winter, so they do not have output
when the coldest temperature exist.  

The Modules are the polycrystaline, and they are a
discontinued older type, with the 33 cells? as opposed
to the more common today of 34 cells.  Also we have
not had a cold winter for several years.  Our design
temp is -10F I do not believe we got any colder last
year than once a -4F.  I know our record below exceeds
-30 but not once in the last 10 years.     

Using my old memory I think I remember seeing an MPPT
voltage of 105V to 110v? (perhaps even 115v) during a
cold clear days last year, which I think would be an
open circuit of 140 + volts I think.  So I must be
very close to the distruction limit. (people with a
memory is Voc typically 125% of MPP v,?)  I have been
watching this system very closly, but it is not mine
so it takes time from my day. (I am concerned about
battery life.)

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