[RE-wrenches] Inverter with two strings of different orientation

[Peter Parrish]

Thanks Bob, these are really great reference articles. The equation for Vmp
had four terms, three of which are temperature dependent. So I think we put
that issue to bed. The reason that this issue is so important for us is
this: if Vmp is temperature dependent, then there must be some loss when
combining two strings together with different tilts/azimuths. How much, I
don't know. Specifically, I have a client for whom we are designing a system
with two strings with different tilts/azimuths (otherwise identical)
18deg/East and 15deg/South. The questions is, "Go with one inverter and wire
the strings in parallel, or go with two inverters one for each string."
Since the cost for two inverters is considerably more than one inverter of
twice the capacity, I would like to go with the single inverter, if the
losses are in the 1-2% range.

I am also very skeptical about MMP tracking algorithms. I documented the
"saw-toothing" on the Fronius IG-4000 input (July 2006), and we still have
an outstanding malfunction with a Xantrex XW6048 with an output power
oscillation.

Right now I will be telling our clients that "using a single inverter --
with two or more subarrays of widely differing orientations (see above) --
is not recommended, because there may be significant losses (significantly
more than 1-2%)". 

- Peter

Peter T. Parrish, Ph.D., President
California Solar Engineering, Inc.
820 Cynthia Ave., Los Angeles, CA 90065
Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885
CA Lic. 854779, NABCEP Cert. 031806-26
peter.parrish at calsolareng.com 

-----Original Message-----
From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of boB Gudgel
Sent: Sunday, December 28, 2008 5:07 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] Inverter with two strings of different
orientation

Peter Parrish wrote:
> Bill,
>
> I looked into this several years ago, and I was able to get some digitized
> I-V data. I can't find that now (but I will). The data supports my claim. 
>   
Vmp does change with irradiance.  It just changes  very little. Depends 
on your definition of "change" I guess.
Negligible I would say... That is, assuming a higher irradiance than 
"dark" of course.

I can see where some confusion might come from from irradiance and PV 
parameter variation...  As the irradiance
changes, the ratio of Vmp to Voc changes.  Lower irradiance normally 
makes the Vmp/Voc rise somewhat but only
because the Voc is changing.

Now, put two or more modules in series, then shade one of them and 
that's something entirely different.


Here's a good paper with some good data on this subject.    I also see 
Bill Brooks contributed to this article,

http://photovoltaics.sandia.gov/docs/PDF/King%20SAND.pdf   (the %20  is 
a space)

And for temperatue relations, here's another good one:

http://photovoltaics.sandia.gov/docs/PDF/IEDFB5~1.pdf

boB


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