Allan,
Because many MPPT controllers convert to any battery voltage, new
installations allow you to use just about any solar module. When I
install any higher voltage module on RV's (12 volt battery), I design
series strings up to the max design voltage to minimize voltage drop.
No more 6 gauge wire runs. Now I can install Sanyo's or even Kaneka's
and charge 12 volt batteries with up to 97% conversion efficiency.
If you are trying to add to an existing array it may be harder to
match up your Vmp for parallel strings. I would not be worried about a
few volts. The voltages will average out somewhere in the middle. For
instance, I have customers in Mexico that have very old systems (think
Arco). They want to keep the PV modules they have and add more while
upgrading to MPPT. I have done this by re-stringing the old modules in
series and try to get close to the published Vmp of the new modules.
Some power will be lost in this parallel setup as it is difficult to
estimate the Vmp of a 20 year old module. I have done this for several
customers and so far it has worked well.
Kindest Regards,
Larry Crutcher
Starlight Solar
11279 S. Glenwood Ave #11114
Yuma, AZ 85367
(928) 941-1660
[EMAIL PROTECTED]
www.starlightsolar.com
Retail Store: 2998 Shari, Yuma, AZ
Renewable Energy Products, Service and Installation
On Dec 10, 2008, at 1:16 PM, Allan Sindelar wrote:
Wrenches,
Here's an issue I haven't seen directly addressed, and I would like
to know what the rest of you are doing.
It's getting harder to get 24V nominal modules. Because the bulk of
industry growth is high-voltage grid-tied applications, modules no
longer have to be a standard voltage. Just pick the number of
modules to best fit the chosen inverter's string voltage range.
But for off-grid, the odd cell counts and resulting "nontraditional"
DC input voltages challenge the traditional rules of off-grid
design. I may be fighting a losing battle here, as the industry
changes so rapidly and fundamentally. I have been trying to stay
with 72-cell modules for offgrid, as this most readily combines with
existing systems with 36-cell modules (12V) and 72-cell modules (24V).
For example, I will use the new Canadian Solar 170-200W modules.
They are 60-cell modules, with a Vmppt of around 28 volts - too low
to charge 24V batteries with a standard charge controller. We can
use them in any new systems with an MX60 or similar voltage-
converting MPPT controller. Two or three in series would charge a
24V battery; 3 in series would charge 48V. Four in series would
violate Code, as low-temp voltage would easily exceed 150V.
Unless 60-cell modules remain a standard in the future, any future
array additions would have to be on their own charge controller, in
order to match a different I-V curve and MPP voltages into the same
battery bank. Is this prohibitive? No, it just runs counter to the
longstanding standards of off-grid design that allow modules to be
added in the future: these modules will not add well to existing
systems, and will not easily allow additional dissimilar modules to
be added later.
I had this same objection to using Day4 modules, although they were
better at 16Vnom. Three made up a standard 48V array, so strings
could be combined with two-module strings of similar 24Vnom modules.
Who else is trying to stay with 24V modules? Anyone still using 12V
modules in off-grid (residential-scale, not little apps) designs?
Who has a crystal ball and knows what modules will be like in 20
years, or even two years? Wat are the rest of you doing?
Thanks, as usual.
Allan
Allan Sindelar
allan_(at)_positiveenergysolar.com
NABCEP certified solar PV installer
Positive Energy, Inc.
3225A Richards Lane
Santa Fe NM 87507
505 424-1112
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