DC or AC linked PV Panels

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Paul318
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DC or AC linked PV Panels

Post by Paul318 » Mon Apr 27, 2009 5:15 pm

My PV panels are 322 feet away from my main breaker box. I am planning a grid interactive system that uses a Flexmax 80,
2 GVFX3648 and 48 vdc batteries(of course). My PV panels will potentially be Suntech 270-24/vb. I will have 16 of them.
8 pairs in parallel. Where do i put the inverters, Flexmax 80, and the batteries? Near the main panel and be DC linked over
the 322 feet at about 70 vdc/ 50 amps? Or is there some way to be AC linked at the higher 240 vac which implies that the inverters, batteries, and charge controller are near the PV panels. The DC link requires only 2 cables (+ and -) plus a ground wire. The AC link requires 4 hot, 2 neutrals, 1 ground. It seems like a no-brainer, but the AC link adds 644 feet to the grid
power coming into the subpanel (322 from the grid to the PV location, 322 back to the house). The loss on this 644 feet of wire is power I have to pay for 24/7/365. Wouldn't it be better to be DC linked and only lose "free" power when the sun shines on the panels. I know I can get lower losses on the AC link with cheaper cables, but I don't think that will compensate for adding
644 feet to my expensive grid power coming into the house. Also, I already have brown-out problems when some appliances start up - adding 644 feet of wire will make this even worse. Has anybody tackled this problem?

Kent Osterberg
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Re: DC or AC linked PV Panels

Post by Kent Osterberg » Mon Apr 27, 2009 11:03 pm

Put your modules in series groups of three instead of two. Raising the voltage will reduce the current and greatly reduce the losses on the dc side. Keep the inverter and batteries close to the loads.

Paul318
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Re: DC or AC linked PV Panels

Post by Paul318 » Tue Apr 28, 2009 12:48 pm

Thanks for the quick reply. This is the way I wanted to go, and this is good confirmation.
I thought of putting 3 panels in series, but I originally rejected the idea because I did not
realize the the Flexware 80 had the ability to step down the higher voltage and still
transfer the power (which implies that the current is stepped up). I have no idea how
it accomplishes this, but a careful reading of the specs implies that it does.
With 3 panels in series, I calculate a 2% loss with 2 guage copper wire. This is acceptable.
Thanks again.

Paul318
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Re: DC or AC linked PV Panels

Post by Paul318 » Wed Apr 29, 2009 2:44 pm

The answer to all this is a wonderful feature of the Flexware 80 that is not emphasized enough. Buried deep
in the manual (page 80) we find out that the Flexware 80 is a buck type converter that steps down the voltage without
a significant loss of power (95% efficiency?). This means that you can transmit your DC power from the PV at higher
voltages (and thus lower amps) without doing something really complicated to convert to AC at the PV location.
I had assumed (incorrectly) that the charge controller used a simple voltage divider and that the power represented
by any excess voltage was simply thrown away as heat.
I am sure somebody made this point somewhere, but I can't find it.
By the way, my potential parts supplier tried really hard to convince me to convert to AC at the PV location. He
apparently did not understand this feature of the Flexware 80. Don't keep it a secret.
If I am off base here, will somebody let me know.

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tallgirl
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Re: DC or AC linked PV Panels

Post by tallgirl » Thu Apr 30, 2009 4:05 pm

It's not a secret. It's what all the "MMPT" discussion is about. I read the patents for the technology once and it's pretty trick stuff.

The decision to place the inverters can be more involved than just distance, Vdrop, and cost of wire. If the AC output was boosted at the inverters, then bucked at the main disconnect (a nice way to save Vdrop on the AC connection), you'd have to have two transformers, a place to keep them, extra disconnects, etc. AND transformers have their own losses, so keep that in mind.

The right decision very likely is to put the PV conductors in pipe out by the panels, then run them back to the charge controller and related gear. The array should have the highest possible Voc, limited only by the charge controller's input limitation and site weather. Compute the NEC required conductor size based on Isc, then look at the next few large conductors and recompute Vdrop and Ploss against an amortized cost of the wire as a power consuming item. That will tell you went to stop upsizing those conductors.
Julie in Texas

I ride bicycles. A lot.

Paul318
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Re: DC or AC linked PV Panels

Post by Paul318 » Mon May 04, 2009 12:48 pm

Good advice from both of you. I chose MITS PV-UD185MF5 solar panels; arranged in 6 strings
of 4 in series. This gives me a temp adj Voc of 138. The Voc varies from 117 to 138 in the
extreme temp range in my location. The Vmp at STC is 90v. This high voltage does cause
some reduced efficiency in the Flexmax 80, but it is still the way to go. I will be using
#2 guage, direct burial cable at about $1.33 per foot.
My overall losses are about 4% in the cable and about 6% in the Flexmax 80 (see page 62
of the manual).
I would still encourage Outback to be more explicit in their overview of the Flexmax 80
about the feature that allows higher voltages like this. I can now see that it is implied
by the description of MPPT; but, for newbees like me it didn't come clear until I got your
answers on this forum (and read the manual).

Paul318
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Re: DC or AC linked PV Panels

Post by Paul318 » Mon May 04, 2009 2:12 pm

Sorry, my mistake, that is #4 direct burial cable at 1.33/foot. #2 is 2.09/foot. See
how much you saved me? (644X(2.09-1.33)) = $489.

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tallgirl
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Re: DC or AC linked PV Panels

Post by tallgirl » Mon May 04, 2009 3:53 pm

Paul318 wrote:I would still encourage Outback to be more explicit in their overview of the Flexmax 80
about the feature that allows higher voltages like this. I can now see that it is implied
by the description of MPPT; but, for newbees like me it didn't come clear until I got your
answers on this forum (and read the manual).
This is one of the reasons to contact a knowledgeable person BEFORE spending money on solar equipment. There's still a lot of Wild Wild West with solar and I suspect people are buying things they don't need, or not buying things they do need (ignoring future planning and cash flow restrictions).

Anyhow, I'd still recommend you bury in pipe. PVC is relatively cheap and a lot easier than re-trenching if you ever need to.
Julie in Texas

I ride bicycles. A lot.

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