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<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Allan,<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'>My beard is pretty gray these days, so I’ll bite. Without
getting deep into exactly where these modules have degraded to, I think your
idea of adding the extra MSX77s is a good one, although it might be a little
overkill. Since the customer has them, I would use them.<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'>The other issue is that newer modules may have a better
temperature coefficient of voltage than the old Solarex modules. The SolarWorld
175 will not lose as much voltage at the same temperature. If you can split like
modules on their own MPP charge controllers, that would solve your problem—Not
sure that is an option for you.<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>
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<p class=MsoNormal><b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif";
color:windowtext'>From:</span></b><span style='font-size:10.0pt;font-family:
"Tahoma","sans-serif";color:windowtext'>
re-wrenches-bounces@lists.re-wrenches.org
[mailto:re-wrenches-bounces@lists.re-wrenches.org] <b>On Behalf Of </b>Allan
Sindelar<br>
<b>Sent:</b> Wednesday, April 28, 2010 12:20 PM<br>
<b>To:</b> RE-wrenches<br>
<b>Subject:</b> [RE-wrenches] Solarex module match question<o:p></o:p></span></p>
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<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Calling the old-timer off-grid 12V graybeard Wrenches...<br>
I am reposting this to the list in hopes of shakin' up a response, as nobody
tried to answer my request for help.<br>
<br>
In short: in theory, if you force an array to operate at a voltage above its
MPP, current drops off significantly on the string I/V curve. In this case, if
I add a fixed array of substantially larger current and <i>slightly</i> higher
Vmp (due to both what's currently available having a slightly higher module
Vmp, and 14 years of module degradation), will the new array force the old
array to operate at too high a Vmp and thus substantially reduce its
contribution to overall PV input?<br>
<br>
And a related question: the customer reminded me that he still had two
additional matching Solarex MSX77 modules stored away. <i>If</i> I could mount
them on the existing 12-module Wattsun tracker, and <i>if</i> I wired them as
two strings of seven modules (84Vnom), would that solve this issue, by boosting
string voltage enough to maintain current closer to Isc?<br>
<br>
I'd appreciate any good advice.<br>
Allan<br>
<br>
Original post:<br>
<br>
Wrenches,<br>
We have an off-grid customer with a system that has been growing since 1996; we
have cared for it intermittently since 2000.<br>
<br>
The customer has two tracked arrays currently. One has ten Astropower AP120s
for 1200 nominal watts, but that's not the topic today; it'll come up in 5
years or so. The other older array is 12 Solarex MSX77s, polycrystalline with
16.9Vmp, 21Voc (when new). The rest of the system is a SW4024, two Solar Boost
50s (one is currently failing) and two strings of 800 a/hr Rolls cells. <br>
<br>
The client wants to add 1,200 to 2,000 watts of additional PV. The problem is
that we have two sets of existing buried conductors between arrays and BOS and
don't want to dig up for more. Also, he wants to upgrade charge controllers,
and has had past failures of the SW inverter - in fact, he keeps a spare put
away for the next failure.<br>
<br>
I have suggested that by upgrading the inverter to a modern pure-sine 48V unit,
we can accomplish the array upgrade using the existing input conductors. I can
rewire the Solarexes from 6 24Vnom strings to two 72V strings and have plenty
of copper to add more array without voltage drop issues. (The AP120s can be
brought in as 2 60V strings to a separate FM60 or T80 or similar). This also
lets me work with existing 60 or 80A charge controllers, which I can't do at
24V.<br>
<br>
The customer likes this approach, so here's my question: What modules would you
suggest as having good compatibility with the existing Solarexes? I would
assume that any polycrystalline 72-cell module would be fairly close but not
ideal. I would especially expect the lower-rated ones within a particular size
category to be better choices, as they would have lower Vmp to more closely
match the existing. But would similar monocrystalline be close enough? Too high
a voltage in the dominant (new) array drags the voltage above MPP, which
reduces output considerably on the Solarexes. Also, I can't find the
temperature coefficient for the old modules, and I suspect that degradation
over 14 years has lowered the MPP of them anyway. <br>
<br>
I think of using 12 Kyocera KD135s Mitsubishi 125s or 130s; Solarworld's
discontinued SW175 (mono), Sharp 123s, or possibly some Suntech monos. But I'm
certainly open to advice and suggestions here.<br>
<br>
Also, while the existing arrays are on Wattsun DA trackers (customer's choice),
the new array will be pole-top. I'm not too worried about tracking/fixed
mismatch, as voltage quickly matches even with non-optimal orientation. But
I'll take advice here too.<br>
<br>
<br>
Thanks.<br>
Allan <o:p></o:p></p>
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<p class=MsoNormal>-- <br>
<br>
<o:p></o:p></p>
<p class=MsoNormal><span class=il><b>Allan</b></span><b> Sindelar</b><span
style='font-size:10.0pt'><br>
<a href="mailto:Allan@positiveenergysolar.com" target="_blank"><span class=il><u>Allan</u></span><u>@positiveenergysolar.com</u></a><br>
NABCEP Certified Photovoltaic Installer<br>
EE98J Journeyman Electrician<br>
<b>Positive Energy, Inc.</b><br>
3201 Calle Marie<br>
Santa Fe, New Mexico 87507<br>
<b>505 424-1112</b><br>
<a href="http://www.positiveenergysolar.com/" target="_blank"><u>www.positiveenergysolar.com</u></a></span><o:p></o:p></p>
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