I have pondered this problem often. Here's my thoughts: If you could
disconnect the two orientations and monitor them simultaneously, in
the morning the easterly array would have a higher Vmp while the
westerly array would necessarily be lower. This relationship would
change throughout the day with an equilibrium being when the sun is
equidistant between the two arrays. (I'm just stating the obvious here)
My question is will the lower Vmp array pull down the voltage of the
higher? I notice this effect with dissimilar solar panels and MPPT
controllers that many DIY RV customers have. It was stated that you
may have a 2-5% loss. I can't imagine that the losses would be that
low averaged for the day.
If the strings were split equally accross the two orientations, wont
the Vmp track throughout the day and give the most production
possible? Of course the inverter threshold voltage will not be met for
as many hours daily but how much will this arrangement loose?
Kindest Regards,
Larry Crutcher
Starlight Solar
11279 S. Glenwood Ave #11114
Yuma, AZ 85367
(928) 941-1660
la...@starlightsolar.com
www.starlightsolar.com
Retail Store: 2998 Shari, Yuma, AZ
Renewable Energy Products, Service and Installation
On Dec 24, 2008, at 12:41 PM, Peter Parrish wrote:
I don’t think this topic has been dealt with in this forum.
Ocassionally we want to use a single inverter with two or more
strings, each string being loaded by a single sub-array. A sub-array
being defined as having a single uniform tilt angle and azimuth. All
modules are identical. And each string/sub-array contains the same
number of modules.
The question is: “How different can the orientation be for the
different sub-arrays?
Let me pose a concrete example: Xantrex GT5.0, two strings of BP
Solar SX3190s. One string has an orientation of 15 deg tilt and 188
degrees azimuth; the other has 23 degrees tilt and 139 degrees
azimuth. For reasons that are difficult to explain, none of the
orientation angles can be changed, at least for the purposes of this
discussion. To visualize what I am talking about: the tilts are
fairly close (8 degrees) and the azimuths differ by about 45 degrees.
How do I calculate the combined performance? How much power does one
give up compared to using separate inverters for each sub-array/
string? Are there rules of thumb as to what works and doesn’t? Do
conventional inverters have algorithms that can navigate the non-
ideal I-V characteristic under load? I know there was a tech
bulletin from Fronius a while ago: I read it but didn’t come away
with any definitive answer. I also had first hand knowledge with
MPPT algorithm problems with the Fronius inverters during that time
period, which caused me to discount the tech bulletin.
I am quoting a system as we speak, so my inquiry is not hypothetical
in nature. Any help would be appreciated.
- 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
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