[RE-wrenches] Solarex module match question

Jeff Clearwater jeffc at villagepower.com
Wed Apr 28 23:39:23 PDT 2010


Hey Bill and All,

Scratch that last question.  I do know that MPPT 
controllers are just going to reference battery 
voltage which will be pretty rock solid and the 
MPPTs act fast - and so no hysteresis - I do 
wonder though if there is a slight interaction or 
any chance of cycling.  I've been doing grid-tie 
too long - I'm losing my feel for off-grid!

One question though - do manufacturer's release 
efficiency curves for controllers?   Let's say 
you have two Sanyo HIT 200s (55.8 Vmp, 68.7 Voc) 
modules feeding a TriStar MPPT 45Amp into a 12 or 
24 Volt Battery.  The Tristar can handle 150 Voc 
- so is it better to wire the modules in parallel 
or series?  In other words, is the controller 
more efficient converting 112 Vmp into 12 or 24 
volts nominal or converting 56 Vmp at twice the 
amperage?

Best,

Jeff Clearwater
Village Power Design




>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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-- 
~~~~~~~~~~~~~~~~~~~
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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