Author Topic: MPPT-500 Panel configuration advice sought.  (Read 4482 times)

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Offline tomw

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MPPT-500 Panel configuration advice sought.
« on: March 16, 2012, 11:41:10 am »
Ok, not to put too fine a point on it I have just purchased a lightly used BZ MPPT-500 controller.

I have a 24 volt system.

I have 4X75 watt 21.7 Voc panels (SP75 Siemens)

I also have 4X130 watt 21.9 Voc panels (KC-130GT Kyocera)

I did not consider how I was going to split up my system to drive this controller.

I just figured I have 850 watts nominal so I knew I couldn't feed it all into one BZ.

The BZ has a "nominal" input limit of 100 volts and 500 watts.

I understand in cold climates you should use a factor of 1.25 times your Voc to cover high voltage from extreme cold. We get that kind of weather usually every winter at least for awhile! That puts me at or about 110 Voc with that 1.25 factor in deep cold.

BZ tech support says 520 watts the Kyoceras can provide is OK. They did not address the voltage question. Their advice was 3 panels in series.  That is OK but leaves me with an orphan nominal 12 volt panel whichever set I use.  They only seem to answer one question per email?

Anyway, assuming the 100 volts is a hard limit:

Any advice on how to utilise my existing panels to drive this?

I am of the opinion that most electronics can tolerate a small over voltage condition. I am concerned at both an over voltage condition and an over power condition simultaneously.

If I can get away with a 110 volt input with that 1.25 factor I would just feed all 4 130 watt panels into it.

If it could work as MPPT with 2 series panels paralleled at a nominal 24 volt and make MPPT work into 24 volt bank I could do that.

I either am asking the wrong questions or their tech support doesn't really want to say much about the voltage limits and only offer stock answers?

By orphaning a 130 or 75 watt panel I will probably not gain anything so I thought someone might have a solution to this.

Thanks.

Tom
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I thought that they were angels, but much to my surprise, We climbed aboard their starship and headed for the skies

Offline ChrisOlson

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Re: MPPT-500 Panel configuration advice sought.
« Reply #1 on: March 16, 2012, 10:20:31 pm »
I also have 4X130 watt 21.9 Voc panels (KC-130GT Kyocera)

I would do a series/parallel with these four panels (above).  You will not gain a lot (if anything) by running the panels at ~87 Voc vs 43.8 Voc.  And you would be over-voltage in cold weather.  You will gain more from the panels operating them at Vmp no matter what your bank voltage is, and the series/parallel configuration will work fine for that.

I'm guessing Vmp has to be somewhere around 34 volts with two of those panels in series.

Edit:  I looked those panels up and Vmpp is 17.6 volts.  Two in series is 35.2 volts.  That will work much better than hooked direct because if your bank is down to 24 volts, for example, you're "clamping" those panels 11.2 volts below their rated maximum power voltage.  The MPPT controller will "fix" that and them run at full throttle.  It will easily handle two series pairs paralleled.  The specs show Impp at 7.39 amps.  Two series pairs in parallel will feed 14.78 amps into the controller @ 35.2 volts, and the controller will finagle that around to ~21.6 amps output @ 24 volt nominal (minus a few percent loss in the controller at full output).
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Offline tomw

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Re: MPPT-500 Panel configuration advice sought.
« Reply #2 on: March 16, 2012, 11:44:49 pm »
Chris;

That is kind of what I would have tried first and the way you explain it it makes perfect sense.

Funny, I should know how to work the raw math but I "thought"you needed a large voltage difference to achieve MPPT? Time to quit thinking I guess.

Blades got here today in fine shape, well packed ,etc.

Thanks for the feedback.

Tom
Do NOT mistake me for any kind of "expert".

( ?° ?? ?°)


24 Trina 310 watt modules, SMA SunnyBoy 7.7 KW Grid Tie inverter.

I thought that they were angels, but much to my surprise, We climbed aboard their starship and headed for the skies

Offline ChrisOlson

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Re: MPPT-500 Panel configuration advice sought.
« Reply #3 on: March 17, 2012, 12:25:13 pm »
Funny, I should know how to work the raw math but I "thought"you needed a large voltage difference to achieve MPPT? Time to quit thinking I guess.

No.  MPPT controllers are actually more efficient (from what I have measured) if you don't have to make a large voltage differential.  It's not a lot.  But with the Classic 150 I experimented with running my panels at ~141 Voc vs ~70 Voc.  The amp input the controller is double at 70 Voc.  But I got big wire buried from the panels so that's no big deal.  The controller puts out a few extra watts running the panels at lower voltage.  I assume because the electronics in it are more efficient when it doesn't have to make such a big "step down" in voltage from 100+ down to 24 nominal.

The important thing with solar, from what I found, is to run the panels at Vmp instead of "clamp" voltage from the bank.  If you take s single 12 volt panel charging a 12 volt battery, the panel is probably designed for 17 point something Vmpp.  And it won't put out its full power running at 12 volts nominal.

When the bank is low is when you want the most power for bulk charging.  And that's where the panels perform the worst when they're direct hooked because they're further off from their Vmpp.  As the battery charges up, the panels' operating voltage gets closer to the Vmpp and they start to perform better, when you actually don't need them to perform better (with the battery almost charged up).

The MPPT controller "fixes" that allowing the panels to run at full blast at their peak power they can make, regardless of bank voltage.  Typically this will require wiring panels in series for higher voltage systems (24 or 48 volt).  But when you wire 12 volt panels in series the difference between bank and Vmpp gets even more dramatic, making the MPPT controller even more important than it is with a single panel on a 12 volt battery.

I got along fine with my old 12 volt system with the panels hooked direct.  And even got full rated power from them sometimes.  When I went to 24 volt the performance of the solar was pretty dismal compared to what the same panels had done on 12 volt before.  That's when I started playing with MPPT.  The MPPT woke them panels right up    :)
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Chris