[RE-wrenches] RE-wrenches Digest, Vol 2, Issue 944

William Korthof wkorthof at eesolar.com
Fri Sep 11 23:21:07 PDT 2009 

If the angle differs by more than a few degrees, there will be  
significant mismatch losses in a single string situation....
two parallel strings of equal size but different orientations (Fronius  
white paper) present a much smaller issue.


If the angle of modules in a single string varies significantly, say  
by 20 degrees, the maximum power point will be
compromised most of the time. If the string is mostly in one  
direction, the MPPT will tend toward one of three points:
1) when the majority of modules are brightly lit and the balance  
receiving much less intense light---a high I value to
capture the output from the lit panels, and V from all the lit  
panels.... the remaining less lit panels drop to 0 volts
(bypass diode) and don't contribute any power because the string  
current is too high

2) when the minority modules are brightly lit but the majority are  
more dimly lit, the MPPT will be close to the
max power volts from all modules, but the current drops to the lower  
value of the more dimly lit modules... again,
a significant part of the max power output of the array is lost.

3) when the sky is overcast or the sunlight intensity equally bright  
on all modules (once per day) , the combined
MPPT will end up landing close to the optimum output for the  
individual modules.

Power One inverters are great, but each PV string needs to have enough  
initial open circuit voltage to reliably
activate the inverter, which needs 200+ volts. 3 modules probably  
won't do that.

/wk




On Sep 11, 2009, at 5:03 PM, re-wrenches-request at lists.re-wrenches.org  
wrote:

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> Today's Topics:
>
>   1. Re: losses to competing angles in single strings (Wilco  
> Vercoelen)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Fri, 11 Sep 2009 20:03:29 -0400
> From: "Wilco Vercoelen" <w.vercoelen at dommelvalley.ca>
> To: "'RE-wrenches'" <re-wrenches at lists.re-wrenches.org>
> Subject: Re: [RE-wrenches] losses to competing angles in single
> 	strings
> Message-ID: <662021E7CD9644F088EF6FE609B1866E at LAPTOP>
> Content-Type: text/plain; charset="us-ascii"
>
> Dear wrenches,
>
>
>
> We would recommend the PowerOne inverters with two separate MPPT  
> Tables.
>
> The only thing, they have to raise the warranty to 10 years.
>
> Kind regards,
> Wilco Vercoelen
> President
>
> new-logo-email.gif
> Dommelvalley Green Power Inc.
> Solar Energy Solutions
> Utopia, Ontario, Canada
> <http://www.dommelvalley.ca/> http://www.dommelvalley.ca
> Tel. 705-423-9699
> Fax. 705-423-9880
> Toll Free 1-877-889-7657
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>
>
>  _____
>
> From: re-wrenches-bounces at lists.re-wrenches.org
> [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Peter
> Parrish
> Sent: September-11-09 19:53
> To: 'RE-wrenches'
> Subject: Re: [RE-wrenches] losses to competing angles in single  
> strings
>
>
>
> In principle, two strings with different orientations should not be  
> wired in
> parallel into the same inverter. However, when the mismatch is minor  
> (15
> degrees in pitch or 15 degrees in azimuth) there is practically no
> degradation. Fronius circulated a white paper for a while that  
> argued that
> their inverters did maximum power point tracking on the string with  
> the
> greatest irradiance and therefore did an optimum mppt  job on the  
> string
> with the greatest "available power". This part of the argument makes  
> sense.
> What I have never been clear about is how close to "optimum" this
> arrangement is (optimum being two separate monitors one for each  
> string).
>
>
>
> Fronius appears to be backing away from this white paper, as I was  
> told
> about six months ago from a member of their tech support staff that  
> they
> recommended two separate inverters.
>
>
>
> - 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
>
>
>
>
>  _____
>
> From: re-wrenches-bounces at lists.re-wrenches.org
> [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Nick  
> Vida
> Sent: Wednesday, September 09, 2009 6:38 PM
> To: wrenches
> Subject: [RE-wrenches] losses to competing angles in single strings
>
>
>
> hello wrenches.
>
> I was wondering if any of you have any data on what the losses would  
> be if
> you had your string split with 2 different angles, but at the same  
> azimuth.
> For instance, string of 11 with 8 facing 180 south at 30 degrees,  
> and 3
> facing south 180 at 45 degrees. I have heard that the voltages and  
> currents
> would differ and the IV curve would be inefficient as a string, but  
> I don't
> know how much of a loss would be expected.
>
> thanks in advance, and hopefully I didn't miss anything in the  
> archives.
>
> thanks,
>
> nick
>
>
>
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> End of RE-wrenches Digest, Vol 2, Issue 944
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