[RE-wrenches] Solarex module match question

Bill Brooks billbrooks7 at yahoo.com
Wed Apr 28 22:31:13 PDT 2010


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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