[RE-wrenches] Two strings of Different Orientation

[Bill Brooks]

Peter,

The issue is the same as it was last year. The operating voltage of each
string will be nearly identical throughout the day if the strings are the
same length and not shaded. Put them on a single inverter.

The Fronius report was not discussing a special algorithm for dealing with
this situation. It was showing that operating voltage is nearly the same for
two different orientations. 

The only way to prove this with a simulation program is to run an hourly
simulation for each orientation and compare the Vmp voltage hour-by-hour. If
you can get to the dc voltage data in PV:WATTS it should tell you the same
story. Next you would have to calculate the difference in power that would
be delivered because the inverter MPPT will be dominated by the higher power
string (creating a 1 or 2% loss at worst case). I think it can be done with
free software, but it will take a lot of handwork and spreadsheets.

One day I-V curve tracers will be readily available, or inverters will be
able to do the same thing, and this issue will go away. DC and ac Optimizers
may also be able to provide dc voltage data that could also show this
phenomenon. 

My fear is that just as the Sharp 3-input was sold inaccurately on the
capability of tracking different orientations that optimizers will be sold
the same way neglecting the more accurate benefits of these devices. Maybe
it is a pipe dream, but I would like to think that designers, engineers, and
knowledgeable installers will one day be able to accurately understand the
irradiance and temperature impacts on I-V curves so that manufacturers will
be unable to use inaccurate information to sell their wares. Call me crazy.

Bill.

-----Original Message-----
From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Peter
Parrish
Sent: Thursday, May 20, 2010 8:58 AM
To: 'RE-wrenches'
Subject: [RE-wrenches] Two strings of Different Orientation

Abut once a year we encounter this problem and each time I reach out to
folks for guidance. Up until last year, I don't believe that there was a
readily accessible tool that accurately simulated the behavior of "two
strings of modules with different orientations". PVSyst is too expensive and
hard to master to solve this problem once a year. So here I go again,

I have a roof with an 18 deg pitch and nearly N-S axis (actually 355-175 deg
true). We want to put eight modules on each side of the roof. We can tilt
the modules on each side of the roof 15 or 25 degrees (or any angle in
between, if desired), producing a more southerly (but not equal) orientation
for each sub-array. The results are as follows:

Before (as is)
Sub-array1: pitch 18 deg, azimuth 85 deg
Sub-array2: pitch 18 deg, azimuth 265 deg

After/Case #1 (15 degree tilt-kit)
Sub-array1: pitch 23 deg, azimuth 126 deg (54 deg east of south)
Sub-array2: pitch 23 deg, azimuth 224 deg (44 deg west of south)

After/Case #2 (25 degree tilt-kit)
Sub-array1: pitch 30 deg, azimuth 141 deg (39 deg east of south)
Sub-array2: pitch 30 deg, azimuth 209 deg (29 deg west of south)

My question: Can either Case #1 or Case #2 be wired to a single inverter as
two parallel strings of eight modules each?

Has anyone seen a PVSyst or similar took simulate this two-string
situtation? Does anyone believe that the Fronius IG inverters have special
algorithms that accommodate this lack of idential orientation (their White
Paper notwithstanding)?

We have until Monday to figure this out. Any help would be appreciated.

- Peter

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

 


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