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

mick abraham mick at abrahamsolar.com
Wed Dec 10 12:43:55 PST 2008


Call me "old school", but the only modules I keep in stock are 36 cell. I would consider 72 cell modules for stock but otherwise I "just say no" to all of the screwball cell counts that are out there. It's not that I refuse to sell anything else...but if I go goofy on the cell count I just order those modules as I need them.

"Why go goofy?", I ask myself. My occasional "string inverter" sale can often be accommodated with 36 cell modules...ditto for pumps...and of course a 36 cell module fits right in to battery charging setups regardless of whether the solar array is wired "classic" or at elevated voltage. Cost per watt for my preferred 36's is about the same as everything else, so again I ask: "Why go goofy?"

The main thing that causes me now to consider 72 cell modules for stock is the Enphase M175 inverter. My preferred 36 cell modules are a little too big on amperage to work well in pairs with the Enphase 175. If anyone has a surplus batch or used batch of "Enphase appropriate" modules with 72 cell count, contact me off list for a possible cash sale.

Mick Abraham, Proprietor 
www.abrahamsolar.com 
Voice: 970-731-4675

---------- Original Message -----------
From: "Allan Sindelar" <allan at positiveenergysolar.com> 
To: "'RE-wrenches'" <re-wrenches at lists.re-wrenches.org> 
Sent: Wed, 10 Dec 2008 13:16:26 -0700 
Subject: [RE-wrenches] Module Voltage Question for Off-Grid Designers

> 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
------- End of Original Message -------
 
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