[RE-wrenches] Solarex module match question

Wed Apr 28 23:15:41 PDT 2010

Hey Bill,

Two MPPT charge controls from different modules 
in parallel to one battery pack - do they settle 
into one voltage or do they sweep each other and 
constantly search?  Do they need to be identical 
controllers - what if they have different 
algorithms?

Jeff C.
Village Power Design

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>Allan,
>
>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.
>
>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.
>
>Bill.
>
>From: re-wrenches-bounces at lists.re-wrenches.org 
>[mailto:re-wrenches-bounces at lists.re-wrenches.org] 
>On Behalf Of Allan Sindelar
>Sent: Wednesday, April 28, 2010 12:20 PM
>To: RE-wrenches
>Subject: [RE-wrenches] Solarex module match question
>
>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
><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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-- 
~~~~~~~~~~~~~~~~~~~
Jeff Clearwater Lernould
Village Power Design

Solar Design Consultation for the Commercial Sector
http://www.villagepower.com
gosolar at villagepower.com

NABCEP Certified Solar PV Installer 
http://www.nabcep.org/

Voice: 831-427-2799
Fax: 413-825-0703
245 Dufour St
Santa Cruz, CA 95060
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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