Advice needed for optimal MPPT/Array setup

[TQPF]

Hi,

I need some advice regarding panel configuration for 3 x 250W panels connected to a Tommatech 3KVA (re-branded Axpert Ex 3K-24) inverter with MPPT charger. I was previously running 4 x 150W panels in series but a friend kindly gifted me 3 new panels on a nice automatic tracking system that was surplus to his needs.

The inverter's MPPT specs are as follows:

Rated Power    1000W
Operating Voltage    30-80V
Max VOC    102V

The 3 panels specs are as follows:

Pmax (W)    250
Voc (V)    37.18
Vmp (V)    29.13
Imp (A)    8.6

The guys that set it up configured the panels in parallel which I understand gives me the same voltage as 1 panel but increased amps. This is at the lowest range of the operating voltage for the MPPT charger.

My question is - would it not be better if they were either in series - which would take it over the operating voltage slightly & also a little over the max VOC or maybe have 2 in parallel connected to 1 in series - this would result in being within all the ranges but having a significantly increased voltage.

Am I correct in understanding that an increased voltage is better for MPPT charges and the surplus will be converted to increased amps??

I'm a newbie but to be honest - I don't have confidence in the guys that configured the panel array as they don't deal with MPPT chargers normally. I would really appreciate advice on the optimal configuration for my setup as we are completely off-grid.

[Chris Hobson]

Good  friend! Might even be your best friend.

Have a look here . Midnite solar sizing tool. Accurately determine the  temperature compensation of your array.

I would be extremely cautious of exceeding the MPPT's max voltage.

Three panels is awkward. Three in series the voltage is too high, three in parallel is too low. Ideally try and find one more compatible panel and have two strings of 2 panels.

How big is your battery bank? Do you need the three extra panels?

[TQPF]

Thanks for the reply Chris. Our temperatures here are about 35 in the summer (20-25 in spring/autumn) & it can go to 0 or slightly subzero for a about a week in the winter. I have 2 x 200Ah 12v gel batteries in parallel.  I accepted the new panels from my friend because a) they are 3 brand new panels whereas mine are over a year old & b - they are mounted on a tracking system. I was going to sell the 4 x 150W panels.

Maybe I shouldn't have accepteed the gift! The problem is that the tracking system holds 3 panels only. Maybe I need to see if it can be modified to carry another.

Decisions!

[TQPF]

Sorry Chris - I also wanted to check again - if I can't add a fourth panel - is it possible to have 2 in parallel connected to 1 in series which would result in about 60V VMP or 74V VOC which would be within my MPPT chargers ranges?? Or am I going down the wrong path here?

 

[Chris Hobson]

47 minutes ago, TQPF said:

Sorry Chris - I also wanted to check again - if I can't add a fourth panel - is it possible to have 2 in parallel connected to 1 in series which would result in about 60V VMP or 74V VOC which would be within my MPPT chargers ranges?? Or am I going down the wrong path here?

 

Having never actually tested it I cannot answer definitively, Folk who seem to know say that connecting strings of panels in series will be dragged down to the voltage of the lowest string. So a string composed of 2 panels in series connected to a single panel in parallel will behave basically like two panels in parallel.

http://solarpanelsvenue.com/mixing-solar-panels/

I wonder whether one should perhaps look at an independent SCC that could handle your three panels in series so graciously bestowed upon you.

[Guest]

11 minutes ago, TQPF said:

... would it not be better if they were either in series ...

Yes it is. With MPPT's the higher the volts the better they operate.

Just don't exceed the manufacturers maximum voltages spec - unless you know exactly what you are doing. 

You may need another panel. MAX VOC of 102v
= 2 panels in series (37.2 x 2) = 74.4v.
= 2 sets of series connected panels connected in parallel. (74.4v @ 17.2amps)
= Max of 1000w - with cloud effect at max amps and volts 1279w

[___]

25 minutes ago, Chris Hobson said:

connecting strings of panels in series will be dragged down to the voltage of the lowest string.

It could even be dangerous. If you have a high voltage across an PV cell and it is shaded, you can overcome the reverse breakdown voltage of the cell and permanently damage it. Other than that, yes, PV panel is a constant current device, the voltage is dragged down and the current stays the same, so it will be equivalent to just placing two panels in parallel.

Cable will be cheaper than a new MPPT. Put in a thick enough cable (or double up) and put two panels in series. Buck converters don't necessarily like large conversion ratios, I was told way back when I installed my first setup to aim for twice the battery absorption voltage, ie about 60V. It's not really true that "higher is better", there is a ballpark you want to be in.

[TQPF]

Plonkster - do you mean just use 2 panels in series - without the third? Or get a fourth & put 2 sets of series connected panels connected in parallel as Chris suggested?

[___]

Oh dear, my teacher always did say read the question properly!

I see roughly three options.

1. Get a fourth panel and make it 2 x 2.

2. Get another MPPT that can take all three in series.

3. Put 2 panels on the existing in-inverter MPPT and get a small external one for the 3rd panel.

Option 2 will be the most expensive. An extra panel will be around 2.2k, a BlueSolar 100/15 (smallest MPPT in the blue range) is about the same price, and at this level I really wouldn't bother trying to go cheaper. For option 2, you're looking at something that can do 35A or more, and then you're talking 4k or more.

Option 1 gives you an extra 250W for the same price. So do that :-)

If no spare cash at the moment, get cheap PWM controller and put the extra panel on that in the mean time. Or put panel in storage for later :-)

[TQPF]

Thanks for the clarification - I'm going to the guys that made the tracking system stand to see if it can be modified to take a fourth panel.

Thanks again for everyone's suggestions.

[Chris Hobson]

11 hours ago, plonkster said:

It could even be dangerous.

That was the part that was bothering me - but I have no practical experience. Know the recommendation is not to differ by more than 10% on either voltage or amps. No sure what happens when you ignore recommendations as we are so wanton to do.

[Guest]

1 hour ago, plonkster said:

BlueSolar 100/15 (smallest MPPT in the blue range

Nope, it is a 75/10 or 75/15 - +-R1.1k.

[Chris Hobson]

8 minutes ago, The Terrible Triplett said:

Nope, it is a 75/10 or 75/15 - +-R1.1k.

... But that is not going to handle 3 panels in series.

[Guest]

3 minutes ago, Chris Hobson said:

... But that is not going to handle 3 panels in series.

Nope, it is for 1 panel as per Plonks suggestion No 2. Cheaper than the 100/15.

[___]

1 hour ago, Chris Hobson said:

no practical experience

Me neither, just researched it some time ago and noted that like all diodes, PV cells (which are just diodes too, especially when in the dark) also have a reverse breakdown. It's usually an order higher than the forward voltage, and it is mitigated somewhat by the protection diodes inside the module, but if you are sufficiently dumb-arsed about it you can actually put the smoke out of a cell :-)

21 minutes ago, The Terrible Triplett said:

Nope, it is a 75/10 or 75/15 - +-R1.1k.

75/* models are no longer being manufactured far as I know? And we're talking three times 250W, or 750W total, that's at least a 35A controller. So you're looking at the 150/35, and that controller is 4.5k. There isn't even really anything not-blue that is cheaper, unless you want to try a WRND (my advice... don't).

Something else to remember, the smaller controllers only go up to 24V (which is sufficient for this application). If you want something a little future proof, the 150/35 is the best option, smallest controller that can also handle 48V.

1 hour ago, TQPF said:

guys that made the tracking system

Didn't know there was a tracking system involved. With the present price of PV it is usually better to spend the money on more panels. Modifications to the frame, or perhaps other parameters such as wind shear and things like that, might well then tip the balance towards option 3: Put the third panel on a separate smaller MPPT, and in that case the 100/15 is perfect (it scales up to 415W at 24V, tested it myself).

[Guest]

 

1 hour ago, TQPF said:

I'm going to the guys that made the tracking system stand to see if it can be modified to take a fourth panel.

Best plan!

 

13 minutes ago, plonkster said:

75/* models are no longer being manufactured far as I know?

Here is the list for SA.

[___]

16 minutes ago, The Terrible Triplett said:

Here is the list for SA.

I know it is still on the price lists... it's an older design and I expect it to eventually disappear, as it does nothing that the 100/15 doesn't do, and the price difference is small (R250).

Sonop has them both, special on the 75/15 at the moment. It is indeed cheaper than an extra panel, for some reason I had it in my head that it costs closer to 1.8k vat inc.

Still beter if you can get an extra panel. Roughly the same money for more output.

Edit: If you decide to get the additional external small MPPT... it would be really really cool if you can add some monitoring. That would be the perfect setup to test the Axpert MPPT against the blue one, it would literally be the same type of panel pointing in the exact same direction as the others. Of course I'm rooting for my champion, but in the larger scheme of things, it would give us an idea how bad the "reset" effect is (where it restarts from zero on load changes), since we already know the blue chargers try hard to minimise that.

[Guest]

18 minutes ago, plonkster said:

That would be the perfect setup to test the Axpert MPPT against the blue one,

It is close to being done, Axpert vs Victron on the same site, same loads, same batts, same BMV with data in Emon.

Thank you Andre from Victron for a loan unit. 

[___]

12 minutes ago, The Terrible Triplett said:

Thank you Andre from Victron for a loan unit. 

Have you met Andre? That guy is the definition of a "Hemelbesem" (so tall his hair acts as a broom sweeping the heavens), or as we said in school, 'n "Sateliet spietkop" (satellite speed-cop), As hy vandag struikel val hy eers oormôre :-P

I have a spare panel at my place... if someone were to loan me an Axpert... :-P

[DownTime]

2 hours ago, plonkster said:

Sonop has them both, special on the 75/15 at the moment. It is indeed cheaper than an extra panel, for some reason I had it in my head that it costs closer to 1.8k vat inc.

SegenSolar is cheaper than Sonop and it's not on special, the only issue you have to register as a re seller or installer before you can buy from them.