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

[Starlight Solar, Yuma, AZ]

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

larry at 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.parrish at calsolareng.com
>
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