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

[Starlight Solar, Yuma, AZ]

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

larry at starlightsolar.com
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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