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: [email protected]
[mailto:[email protected]]
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:[email protected]>[email protected]
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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