[RE-wrenches] Module Voltage Question for Off-Grid Designers

[Allan Sindelar]

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 <mailto: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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