Upsizing Solar Array

[grizzley]

Ok, it’s been a while and thugs have changed. I need after 9 years to upgrade my solar panels. Which brings some questions with it.

I have a 48v battery bank that is charged by two classic 200. One coming of a wind turbine and one off a solar array. The solar array needs to be upgraded (currently 2x KD215 panels no longer made). I am planning on using Axitec 330 watt panels. So here are a couple of questions:

Given that it is a 48v system would I be better to use 5 strings of 3 panels or should I rather do 4 strings of 4. I know both wound a little on the more than what is recommended (1.2 and 1.3 vs 1.1 recommended) but there is also some recommendations to have a higher incoming voltage the the charge controller. Any thoughts recommendations from your experience?

There is about 90 feet between the panels and the classic 200. I currently have a combiner for the two strings and run a pair from there to the controller. Plan is to continue to do this but since I have to upgrade the combiner is it given the current code required to use a disconnecting combiner or is a regular of MidNite’s combiners with the fuses sufficient?  If the latter should one use a Disco anyway and if so why? (I always like to learn) BTW we are off-grid.

Thank you

[ClassicCrazy]

The best place to start is to plug your numbers into the Classic sizing calculator .
You can try various strings and compare them.
And if you have more questions post a screen shot of the results for more feedback.
http://www.midnitesolar.com/sizingTool/

Larry

[mike90045]

after 2x battery voltage, internal controller losses become larger by a few %.  ( more work to downconvert 170V to 60v, than 110VDC to 60V )
But other things may influence your decision - longer run has more loss at lower voltage, or maybe you are at cable amp limit and don't want to buy $1500 of new wire

Everything is a tradeoff.  Striving for higher voltage "just because" is wrong

[grizzley]

Hi Larry hi Mike
Thank you for your comments. I am not sure what I should be looking for but Larry suggested to post screenshots of the classic sizing tool so I am doing this here. Given that I have two lead acid battery strings of 48v would you recommend to go with three or four in series?  I would have gone with three but that comment from years ago has stuck with me and since I did not understand it at the time I am asking here now in case this is something I should consider. Hope you can shed some light on it for me.

[ClassicCrazy]

As Mike said you want the lower voltage from PV to Classic because then Classic is more efficient charging the batteries - meaning it won't get as hot doing it's job.
Seems like you would be better off running two PV in series and have more strings. That of course depends on the size of the wire you have coming in and the voltage drop calculations.

Larry

[UpNorthMan]

I would run 3s - 5p. You won't have enough voltage to only run 2s. I believe that you need around 78 volts minimum to use your mppt cc and your panels are only about 33 vmp. If you run 4s - 4p, you are getting close to your hypevolt limit at your lowest temperature. And your cc will be running hotter.
As far as over paneling, I don't think it will be a problem. 
330 * 15=4950w
4950 * 75% expected performance = 3712w.
3712/58(charging volts) = 64a
The wire from your combiner to your cc needs to handle the 50a plus safety and loss margens.

I'm no expert by any means. But this is the direction that I would be heading tword.
Ed

[Vic]

grizzley,

Agree with UpNorth..,

With 48V FLAs  (assume that you are still running FLAs),  you would really want string Vmps to be above 85,  or so volts.  With 60 cell PVs (your target 330 W PVs are really 60 series cells,  not 120),  one really has no choice but to run strings of three for an efficient and cool-ish CC.

Your Cl 200 could take strings of four PVs,  but the string voltage  AND  the power from four strings of four PVs,  would seem to be a bit much.   IIRC,  you mentioned previously,  that the power system is in a conditioned space,   but still,   a 4X4 configuration would result in the CC probably running HOT, and resulting in the Turbo fan running for hours daily,   and so on.

The requirement for a Disco Combiner,   really would stem mostly from the Code used by any building Authority for your location,   and  your personal desire.  Generally,  using circuit breakers in the Combiner will be more convenient,  and useful,  vs fuses.    If running strings of four PVs would generally require running 300 V breakers,  however.  These are generally double wide,  using two spaces in the Combiner,   but,  still would prefer breakers,   if it were my system.  The Disco Combiners that we have looked at,  use combiner breakers as the disco switch   ...    that may  have changed.


A 90 foot PV wire run is not excessive.   As noted,  look at the drop in this cable run.  You might want to increase its diameter,  if possible,  or it might need to be changed to handle the total Isc of the combined PVs for safety and voltage drop  --  please check this.

FWIW,   Vic

[grizzley]

Hi Vic and UpNorth,

Your explanations make sense and were what I remembered but had heard some other stuff that made me wonder if things had changed.

In regards to the the combiner vs Disco. I agree on the combiner with the breakers. That was my first choice but had heard from someone on the other side of the country that they needed the disco so was not sure if the NEC now required it but could not find any info.

I will have the wire size double checked. I think I am good as they were oversized when the system was put in. For now I am waiting on the project until the snow mounts have receded a bit.

Thank you all for your great inputs and suggestions.