Hi Peter,
First, a possible way for you to confirm some answers in regards to
the 2 arrays.
set up two arrays in your yard, in whichever orientation you want.
Then hook up into 1 and then 2 inverters and see the wattage difference.
This would allow you to do lots of metering.
About the MPP in battery based, I don't have any data about the XW-
MPPT unit.
With the Outback one I have watched my 3 for long periods of time and
I can say they don't fluctuate outside of normal tracking and sunlight
variations.
Have you checked out Photon Mag. They have been doing some great
testing of inverters. Expensive mag, but worth it.
all the best,
jay
peltz power
On Dec 29, 2008, at 10:07 AM, Peter Parrish wrote:
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.parr...@calsolareng.com
-----Original Message-----
From: re-wrenches-boun...@lists.re-wrenches.org
[mailto:re-wrenches-boun...@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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