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

robert ellison reellison at gmail.com
Thu Dec 11 06:43:21 PST 2008


The MPPT controllers have revolutionized how we can do off grid systems.
Wider panel choices and greatly reduced wire cost. Have you ever run or
remember running 600 watts (or more) of 12 volt panels 150' or so? It was
going to take 4 huge wires to counter the voltage drop or move the building
closer to the clearing!

Having to ability to drop a higher panel voltage to battery voltage is a
godsend.

Thanks for the MPPT controllers,

Bob Ellison



On Thu, Dec 11, 2008 at 7:55 AM, Wind-sun.com <windsun at wind-sun.com> wrote:

>  For larger off grid systems - pretty much anything over 600 watts - we
> have been recommending only MPPT type controllers, such as the Outback &
> Xantrex. Since they can match almost any panel size to any battery size, it
> has reduced our dependence on the often hard to get 12 volt panels.
>
>
> ..................................................................................................
> Northern Arizona Wind & Sun - Electricity From The Sun
> Solar Discussion Forum: http://www.wind-sun.com/ForumVB/
>
> ..................................................................................................
>
>  ----- Original Message -----
> *From:* Allan Sindelar <allan at positiveenergysolar.com>
>  *To:* 'RE-wrenches' <re-wrenches at lists.re-wrenches.org>
> *Sent:* Wednesday, December 10, 2008 1:16 PM
> *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* <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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