two arrays [RE-wrenches]

Travis Creswell, Ozark Solar ozsolar at ipa.net
Tue Apr 26 14:22:36 PDT 2005


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Whoa ....

What does the 125% derate have to do with voltage?  That's for amps, isn't
it?

To determine the Open circuit voltage of the array I use the manufacturers
published derate for voltage temperature coefficient.  It might take some
digging but they do publish it for both amps and volts.  Amps go up very
slightly with heat (pretty much no need to factor it in, IMO) and voltage
goes down with heat.  It's linear, so the same factor applies for below 25c
as above 25c.

In the field when I'm in a hurry I use .05 v per degree C as a rule of
thumb.  For example a temp of -15 C (5f) that's 40 degrees under STC x .05
volt/c and get 2 volts to add to the OCV per module in the string.

Travis Creswell
Ozark Energy Services, Inc. 


-----Original Message-----
From: Darryl Thayer [mailto:daryl_solar at yahoo.com] 
Sent: Tuesday, April 26, 2005 12:37 AM
To: RE-wrenches at topica.com
Subject: Re: two arrays [RE-wrenches]

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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.)

--- David Katz <david at aeesolar.com> wrote:
> Hi Jeff,
> I have ad an idea for a long time about a device
> that shorts one module 
> in a series at a preset low temperature. 
> David Katz

> Jeff Wongstrom wrote:
> >Hello Daryl,
> >
> >I just spoke with an Outback tech that was ADAMANT
> that I not design a
> >system with an MX-60 and a 72Vnom array in
> Montana(low temps similar to
> >Minnesota).  He said that with cold temps the MX60
> would see greater than
> >140Voc and that the MX60 could NOT handle this.
> >
> >Multiplying the Voc of a 72Vnom Sharp crystalline
> array by the NEC 1.25
> >gives Voc of greater than 160Voc.   I am not as
> concerned with code as I am
> >with having MX-60's fail prematurely.
> >
> >I was wondering if the MX-60 72Vnom arrays are
> crystalline and what kind of
> >low temps they have experienced the past three
> years?
> >
> >
> >Jeff Wongstrom
> >NABCEP certified solar installer
> >Thirsty Lake Solar
> >406-889-5324 



		
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