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<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Peter and Nick,<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>I believe the original question related to two different
orientations in the same series string. Peter, your response was related to two
parallel strings of the same length run into the same inverter. These are two EXTREMELY
different scenarios. They have been discussed in detail many times on this
forum, but bad information continues to be circulated—predominantly by a
variety of inverter manufacturers.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Although it may be technically best to put every string, and even
every module, on a different inverter to provide best tracking for PV, the
realities of the IV curve must be understood. Since IV curve tracers are so
expensive, most people have not had the luxury of testing many difference
scenarios of arrays as a few of us have had.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>With crystalline silicon products, strings of the same length
(number of modules), that do not experience different shading, will have nearly
identical operating voltages regardless of orientation. There will be small
differences due to the temperature impact of irradiance. The operating
temperature will vary about 3C for every 100W/m^2. This will cause a slight
change in operating voltage. However, higher irradiance, which causes a slight
depression in voltage due to increased temperature, has a very weak increase in
voltage due to irradiance. These are essentially offsetting results, making the
max power voltage nearly identical for two identical strings in two different
orientations—therefore no problem exists.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>However, differential shading on two identical strings will
always have a significant impact—usually on the string with the most
shading—depending on whether the inverter can see both maximum power points or
not.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Back to the first question from Nick. Many people want to know
what happens if you split a single string between two orientations. The first
answer is don’t ever do it, but the second answer is found in the IV curve. The
irradiance will favor one face or the other at different times of the day and
year (as in your example). In all cases, the surface with the lowest irradiance
will govern the current flowing through the string—meaning the lowest current
will always prevail. For two surfaces close together (30 and 45 tilt and same
azimuth) the loss will be small—maybe 5-10% annually. If the difference is
dramatic—I have seen strings split between east and west faces at a 45 degree
tilt angles—the loss can be 80-90% annually. Nick, although you would be
breaking the first rule of array design by doing what you suggest, the result
will likely be tolerable for this specific example. I would only allow such a
situation when all other options of mounting and aesthetics have been
exhausted. For example, I would support what you suggest instead of jacking up
the 30-degree section of the array to 45-degrees if this is on a prominent roof
face that can be seen by the world. <o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Aesthetics needs to take precedence over performance for highly
visible applications when other options make the array look like an idiot
installed it. If we are going to get PV on as many roofs as possible, it has to
look good. Putting up huge billboards on the top of houses that scream “AN
IDIOT WHO LOVES SOLAR LIVES HERE”, will not help us achieve our goal. As much
as my technical side hates to admit it, sometimes performance has to take a
back seat to aesthetics—but only if there is no other way.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Bill.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<div>
<div style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0in 0in 0in'>
<p class=MsoNormal><b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><span
style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>
re-wrenches-bounces@lists.re-wrenches.org
[mailto:re-wrenches-bounces@lists.re-wrenches.org] <b>On Behalf Of </b>Peter
Parrish<br>
<b>Sent:</b> Friday, September 11, 2009 4:53 PM<br>
<b>To:</b> 'RE-wrenches'<br>
<b>Subject:</b> Re: [RE-wrenches] losses to competing angles in single strings<o:p></o:p></span></p>
</div>
</div>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial","sans-serif";
color:navy'>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).</span><o:p></o:p></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial","sans-serif";
color:navy'> </span><o:p></o:p></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial","sans-serif";
color:navy'>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.</span><o:p></o:p></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial","sans-serif";
color:navy'> </span><o:p></o:p></p>
<div>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial","sans-serif";
color:navy'>- Peter</span><o:p></o:p></p>
<p> <o:p></o:p></p>
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src="cid:image001.gif@01CA33F0.44F56160" align=left hspace=12 v:shapes="_x0000_s1026"><![endif]><span
style='font-size:9.0pt;color:navy'>Peter T. Parrish, Ph.D., President<br>
California Solar Engineering, Inc.<br>
820 Cynthia Ave., Los Angeles, CA 90065<br>
CA Lic. 854779, NABCEP Cert. 031806-26<br>
<a href="mailto:peter.parrish@calsolareng.com">peter.parrish@calsolareng.com</a>
<br>
Ph 323-258-8883, Mobile 323-839-6108, Fax
323-258-8885
</span><o:p></o:p></p>
<p> <o:p></o:p></p>
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<p class=MsoNormal><b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><span
style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>
re-wrenches-bounces@lists.re-wrenches.org
[mailto:re-wrenches-bounces@lists.re-wrenches.org] <b>On Behalf Of </b>Nick
Vida<br>
<b>Sent:</b> Wednesday, September 09, 2009 6:38 PM<br>
<b>To:</b> wrenches<br>
<b>Subject:</b> [RE-wrenches] losses to competing angles in single strings</span><o:p></o:p></p>
</div>
<p class=MsoNormal> <o:p></o:p></p>
<div>
<div>
<p class=MsoNormal style='margin-bottom:12.0pt'>hello wrenches.<br>
<br>
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.<br>
<br>
thanks in advance, and hopefully I didn’t miss anything in the archives.<br>
<br>
thanks,<br>
<br>
nick<o:p></o:p></p>
</div>
</div>
<p class=MsoNormal> <o:p></o:p></p>
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