[RE-wrenches] Solarex module match question

[toddcory at finestplanet.com]

UNLESS you are moving that power any distance. I have a 20+ year old rack of 12, Solarex MSX-60's traveling 200' to the FM60. Wired in two strings of 6, allowed me to drop the wire size considerably!

Todd

On Thursday, April 29, 2010 1:44pm, "R Ray Walters" <ray at solarray.com> said:

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
[mailto:ray at solarray.com] ray at solarray.com
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, asnobody tried to answer my request for help.

In short: in theory, if you force an array to operate at a voltageabove its MPP, current drops off significantly on the string I/V curve.In this case, if I add a fixed array of substantially larger currentand slightly higher Vmp (due to both what's currently availablehaving a slightly higher module Vmp, and 14 years of moduledegradation), will the new array force the old array to operate at toohigh a Vmp and thus substantially reduce its contribution to overall PVinput?

And a related question: the customer reminded me that he still had twoadditional matching Solarex MSX77 modules stored away. If Icould mount them on the existing 12-module Wattsun tracker, and ifI wired them as two strings of seven modules (84Vnom), would that solvethis issue, by boosting string voltage enough to maintain currentcloser 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 since1996; we have cared for it intermittently since 2000.

The customer has two tracked arrays currently. One has ten AstropowerAP120s for 1200 nominal watts, but that's not the topic today; it'llcome 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 systemis a SW4024, two Solar Boost 50s (one is currently failing) and twostrings of 800 a/hr Rolls cells. 

The client wants to add 1,200 to 2,000 watts of additional PV. Theproblem is that we have two sets of existing buried conductors betweenarrays and BOS and don't want to dig up for more. Also, he wants toupgrade charge controllers, and has had past failures of the SWinverter - in fact, he keeps a spare put away for the next failure.

I have suggested that by upgrading the inverter to a modern pure-sine48V unit, we can accomplish the array upgrade using the existing inputconductors. I can rewire the Solarexes from 6 24Vnom strings to two 72Vstrings and have plenty of copper to add more array without voltagedrop issues. (The AP120s can be brought in as 2 60V strings to aseparate FM60 or T80 or similar). This also lets me work with existing60 or 80A charge controllers, which I can't do at 24V.

The customer likes this approach, so here's my question: What moduleswould you suggest as having good compatibility with the existingSolarexes? I would assume that any polycrystalline 72-cell module wouldbe fairly close but not ideal. I would especially expect thelower-rated ones within a particular size category to be betterchoices, as they would have lower Vmp to more closely match theexisting. But would similar monocrystalline be close enough? Too high avoltage in the dominant (new) array drags the voltage above MPP, whichreduces output considerably on the Solarexes. Also, I can't find thetemperature coefficient for the old modules, and I suspect thatdegradation 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 someSuntech monos. But I'm certainly open to advice and suggestions here.

Also, while the existing arrays are on Wattsun DA trackers (customer'schoice), the new array will be pole-top. I'm not too worried abouttracking/fixed mismatch, as voltage quickly matches even withnon-optimal orientation. But I'll take advice here too.


Thanks.
Allan
-- 

 

Allan Sindelar
[mailto:Allan at positiveenergysolar.com] Allan at positiveenergysolar.com
NABCEP Certified Photovoltaic Installer
EE98J Journeyman Electrician
Positive Energy, Inc.
3201 Calle Marie
Santa Fe, New Mexico 87507
505 424-1112
[http://www.positiveenergysolar.com/] www.positiveenergysolar.com_______________________________________________
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