HI Allan 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. 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.
R. Walters [email protected] Solar Engineer On Apr 28, 2010, at 1:20 PM, Allan Sindelar wrote: > Calling the old-timer off-grid 12V graybeard Wrenches... > I am reposting this to the list in hopes of shakin' up a response, as nobody > tried to answer my request for help. > > 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 slightly 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? > > And a related question: the customer reminded me that he still had two > additional matching Solarex MSX77 modules stored away. If I could mount them > on the existing 12-module Wattsun tracker, and if 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? > > I'd appreciate any good advice. > Allan > > Original post: > > Wrenches, > We have an off-grid customer with a system that has been growing since 1996; > we have cared for it intermittently since 2000. > > 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. > > 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. > > 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. > > 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. > > 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. > > 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. > > > Thanks. > Allan > -- > Allan Sindelar > [email protected] > NABCEP Certified Photovoltaic Installer > EE98J Journeyman Electrician > Positive Energy, Inc. > 3201 Calle Marie > Santa Fe, New Mexico 87507 > 505 424-1112 > www.positiveenergysolar.com > _______________________________________________ > List sponsored by Home Power magazine > > List Address: [email protected] > > Options & settings: > http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org > > List-Archive: > http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org > > List rules & etiquette: > www.re-wrenches.org/etiquette.htm > > Check out participant bios: > www.members.re-wrenches.org >
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