two arrays [RE-wrenches]

Travis Creswell, Ozark Solar ozsolar at ipa.net
Tue Apr 26 19:03:48 PDT 2005


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Bill,

My bad on that one.  There it is plain and simple.  That shows how long it
been since I've opened the code book.  But then again, we weren't talking
about code or MPPT Inverters.

I think we are paying the customer a disservice by using the published
derate with out at least trying understand what the module will do in
various temps.  If one just takes a little time to do it right you might
find that you could safely squeeze one more module in a string.


Travis

-----Original Message-----
From: Bill Brooks [mailto:bill at brooksolar.com] 
Sent: Tuesday, April 26, 2005 4:40 PM
To: RE-wrenches at topica.com
Subject: RE: two arrays [RE-wrenches]

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Travis,

The 125% number Darryl mentioned has to do with the ratio of Voc/Vpmax.
Normally I would express it the other way around--Vpmax is 70-80% of Voc at
any condition. This is a good rule of thumb since you can walk up to any
array, test the Voc and know the range that Vpmax should be in once the
inverter believes it has reached max power. Without this knowledge, it is
almost impossible to know if an inverter is max power tracking properly.

The conservative way to estimate highest voltage is to use the coldest
recorded temperature and the NEC table 690.7. This table covers large
temperature ranges so manufacturer's temperature coefficients are better to
use, but sometimes impossible to find. An aging array will have lower specs
and many modules are 5% below Voc at STC rating out of the box.

Bill.

-----Original Message-----
From: Travis Creswell, Ozark Solar [mailto:ozsolar at ipa.net] 
Sent: Tuesday, April 26, 2005 2:23 PM
To: RE-wrenches at topica.com
Subject: RE: two arrays [RE-wrenches]

Whoa ....

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

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