[RE-wrenches] Actual losses from dissimilar orientationsonsingle inverter

Sun Sep 21 09:56:58 PDT 2008

The link doesn't work Doug. If it is the same article that they published
about 2.5 years ago, I was unimpressed (actually the one I read was not long
on field measurements, so we may be talking about two different articles).

What is important to think about is the possible reason for the white paper:
the Sharp JH-3600 inverter was being discontinued and there were no other
commercially available inverters with multiple independent inputs (the only
sure way to deal with sub-arrays with different orientations, different
module numbers, etc.).

Intuition tells us that two sub-arrays differing in 3 degrees of tilt and 12
degrees of azimuth probably won't cause a problem, as long as the two
sub-arrays are other identical (number and type of PV modules). Twice that,
I couldn't say. I should think that a "worst case" calculation could be made
fairly easily, but that is not what we are looking for.

I am sure the problem can be simulated using an appropriate circuit
simulator and model for the PV modules. Is there a college-level course out
there whose teacher would be willing to make this an assignment/project?

Sky, are the results of your studies written up in a way that you could
share them with the rest of us? I for one would gladly read anything you
could send my way, off line.

 - 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 Doug Pratt
Sent: Saturday, September 20, 2008 5:37 PM
To: 'RE-wrenches'
Subject: Re: [RE-wrenches] Actual losses from dissimilar
orientationsonsingle inverter

There's a really good white paper posted on the Fronius site. Reaction to
Non-Optimal Conditions.
http://www3.fronius.com/worldwide/usa.solarelectronics/downloads/fronius_ig_
reaction_to_non_optimal_conditions.pdf 

They went out and tried all these experiments. Great stuff! Every techie
should read this. It's eye-opening.

Cheers,
Doug Pratt

-----Original Message-----
From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Sky Sims
Sent: Friday, September 12, 2008 8:27 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] Actual losses from dissimilar orientations
onsingle inverter

August makes a good point. There really isn't much solid info on the
topic of dissimilar orientations.

In the experiments I've conducted using different orientations and tilts
on the same strings the production almost exactly matches the average
projected production across the orientations and tilt. 

In regard to using mix matched modules in parallel strings I haven't
seen any noticeable difference in performance either. Of course this is
an advanced technique and requires an awareness of string voltages and
amperages in order to avoid excessive power clipping. The biggest
problem with using different manufacturers panels with each other
appears to be more bureaucratically driven rather than performance
based.

What I mean to say is that because the manufacturers are unlikely to
provide a written endorsement allowing you to use their panels in the
same string with someone else's, you may have trouble with a bureaucrat
requiring that endorsement.

There is a lot of rumor floating around about how important it is to use
the same panels and the same orientations and tilts. And I agree it is
important, but there may be circumstances that over ride those rules. 

I've had many systems where for esthetic or available space reasons a
panel or 2 in a string of modules was put at an entirely different tilt
and orientation. I've played quite a bit with it and haven't noticed any
reduction in performance that would warrant the elimination of what
might be 10 to 30% of the string or perhaps even the entire string.
However with inverters like enphase coming to market it becomes easier
to ensure that every panel in a string is at the same orientation and
tilt.

I always recommend the best tilt and orientation possible but sites and
circumstances coupled with other details specific to the site sometimes
force us to go with less than optimal string placement practices. A
healthy well rounded understanding is important and I don't feel that
the industry has done enough to create that understanding yet. Best
practices have certainly been promulgated but the context has not been
fully explained or explored yet.

Inverter reps and installation reps and panel reps have made many many
representations based on their best guesses and in some cases what they
have seen first hand. We the implementers are the guinea pigs, get used
to it. Thankfully the kinks are getting worked out.

Perhaps in another couple decades when things settle down we'll have
properly documented studies of mix matched panels, orientations and
tilts. Of course by then things will be so completely different that it
wont matter any way.

My 2 cents,
Sky Sims
Ecological Systems
http://www.ecologicalsystems.biz
220 County Road 522
Manalapan, NJ 07726
732-462-3858  toll free)866-759-7652  fax)732-462-3962

-----Original Message-----
From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Darryl
Thayer
Sent: Friday, September 12, 2008 12:16 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] Actual losses from dissimilar orientations
onsingle inverter

Hi all
There are a couple of principals here
1) NEVER NEVER PUT TWO STRINGS OF DIFFERENT ORIENTATION IN SERIES!!
2)The currents are dependent upon irradiance, which varies greatly.
3) The voltage and the Vmp is rather close for varying irradiance, and
therefore if forced non coincident orientation in parallel is not so
great.  In that the currents will add and both strings will be near MPP.
4) of course the caveats that parallel strings must always be of the
same modules and same count.
Darryl 

--- On Fri, 9/12/08, August Goers <august at luminalt.com> wrote:

> From: August Goers <august at luminalt.com>
> Subject: [RE-wrenches] Actual losses from dissimilar orientations on
single inverter
> To: "'RE-wrenches'" <re-wrenches at lists.re-wrenches.org>
> Date: Friday, September 12, 2008, 9:38 AM
> Wrenches  -
> 
> The correspondence below brings up a question I've been
> pondering... What
> are the actual losses due to differing string orientations
> on a single
> inverter? Are there significant differences between the
> major inverter
> manufacturers? What if we have a very steep roof facing
> East - West, would
> we be better off with two inverters? 
> 
> The well known Fronius white paper
>
(http://www.fronius-usa.com/worldwide/usa.solarelectronics/downloads/fro
nius
> _ig_reaction_to_non_optimal_conditions.pdf) seems to
> indicate that losses
> will be in the ~1% range. I took a SMA class a couple years
> back and they
> indicated that the losses were probably closer to the 3%
> range, maybe even
> more. Maybe things have changed by now, it would be great
> if some
> manufacturer reps would chime in.
> 
> Does anyone have any data or solid info on this issue?
> 
> Looking forward to hearing the latest.
> 
> Best,
> 
> August
> 
> August Goers
> 
> Luminalt Energy Corporation
> O:  415.564.7652
> M:  415.559.1525
> F:   650.244.9167
> www.luminalt.com
> august at luminalt.com
> 
> -----Original Message-----
> From: re-wrenches-bounces at lists.re-wrenches.org
> [mailto:re-wrenches-bounces at lists.re-wrenches.org] On
> Behalf Of William
> Miller
> Sent: Thursday, September 11, 2008 4:34 PM
> To: RE-wrenches
> Subject: Re: [RE-wrenches] trouble shooting system with Sun
> Tie
> 
> Dana:
> 
> The failure mode of the original ST inverters were a
> failure in the MPPT 
> algorithm.  The inverter would sweep the impedance too high
> until the 
> voltage would collapse.  The inverter would then take
> several minutes until 
> it allowed the PV voltage to rise again.  This would repeat
> throughout the 
> day resulting in a significant decrease in overall output. 
> You could 
> witness this over any given half hour period.
> 
> Regarding arrays in dissimilar orientation:  The loss due
> to differing 
> orientations should be about 1%, IF all modules in each
> string are facing 
> the same direction.  This is because the different IV
> curves are of a 
> similar shape when superimposed on each other.  The ST
> inverter might be 
> less able to average these curves and more susceptible to
> collapse.
> 
> Did you check the array fuses?
> 
> William Miller
> 
> 
> At 03:23 PM 9/11/2008, you wrote:
> >Hi all,
> >
> >I was called out this week to trouble shoot a system I
> didn't install.
> >It's a five year old 1.44kW system with the
> following: Sun Tie ST 1500
> >inverter and 12 Astro Power modules. It only produced
> 634kWh last
> >year. With our sun here, the shading, orientation,
> equipment, etc. it
> >should have produced around 1123kWh.
> >
> >I've heard a lot about Sun Tie failures, but
> don't know the details.
> >What are the common failure modes? Is it likely that a
> failure of the
> >Sun Tie would leave it operating, but only producing
> half what it
> >should?
> >
> >When I showed up the system was producing 950W on a
> relatively sunny
> >day - seemed pretty reasonable. After restarting the
> inverter it only
> >produced around 150W. Is the MPPT very, very slow to
> find that sweet
> >spot, or what was going on there?
> >
> >The system is comprised of three strings of 4 modules
> each. When I was
> >there all three strings were producing about the same
> current, so that
> >leads me to believe it's not a module failure.
> >
> >To make things more fun, the modules were installed at
> two different
> >orientations. One string is at 38d slope, 30d east of
> true south,
> >while the other two are at 36d slope, 75k east of true
> south. This
> >certainly has a detrimental impact on the performance,
> but it doesn't
> >seem to me that it could account for all of that energy
> loss.
> >
> >Thoughts?
> >
> >Thanks a lot!
> >
> >Dana
> 
> 
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