<html><head></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; ">HI Allan<div><br></div><div>I would strongly encourage separate MPPT controllers for the different module types. Even if you get an acceptable voltage match under some conditions, it will never be optimal for all conditions.<div>I'd even consider using the venerable C40 with the old modules, as they have less to gain from MPPT. I was recently disappointed to see a 10 yr old array show almost no benefit from being put onto an MPPT controller (MX60 @ 100v vs. C40 @ 24v) . Likewise, I would expect very little benefit in your case with the aging poly Solarex modules. <div><br class="webkit-block-placeholder"></div><div>
<span class="Apple-style-span" style="border-collapse: separate; border-spacing: 0px 0px; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: auto; -khtml-text-decorations-in-effect: none; text-indent: 0px; -apple-text-size-adjust: auto; text-transform: none; orphans: 2; white-space: normal; widows: 2; word-spacing: 0px; "><div>R. Walters</div><div><a href="mailto:ray@solarray.com">ray@solarray.com</a></div><div>Solar Engineer</div><div><br class="khtml-block-placeholder"></div><div><br class="khtml-block-placeholder"></div><br class="Apple-interchange-newline"></span>
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<br><div><div>On Apr 28, 2010, at 1:20 PM, Allan Sindelar wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite">
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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
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<div class="Section1"><p class="MsoNormal" style=""><span class="il"><b style="">Allan</b></span><b style=""> Sindelar</b><span style="font-size: 10pt;"><br>
<a href="mailto:Allan@positiveenergysolar.com" target="_blank"><span class="il">Allan</span>@positiveenergysolar.com</a><br>
NABCEP Certified Photovoltaic Installer<br>
EE98J Journeyman Electrician<br>
<b style="">Positive Energy, Inc.</b><br>
3201 Calle Marie<br>
Santa Fe, New Mexico 87507<br>
<b style="">505 424-1112</b><br>
<a href="http://www.positiveenergysolar.com/" target="_blank">www.positiveenergysolar.com</a></span></p>
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