Advice for Photovoltaic off-grid set up

[JaumeMI]

Hi everybody:

I'm not working with Photovoltaic in my day to day work, so I would be grateful if you could help me to validate my set up or give any advice you consider.

I'm planning to install an off-grid Photovoltaic system with LiFEPo4 batteries. The main blocks are these:

Solar capture:
6 x 24V solar panels connected as 2S3P to get 48V and 2700Wp.

Accumulation:
A pack 16S1P of 3,2V LiFePo4 50Ah batteries. Resulting on a global spec of: 50Ah, 2560Wh, 48V.
A BMS device to work on 48V and 100A charge/discharge currents.

Inverter:
Growatt 3500-80A (48V) with embedded MPPT charger to provide 3500W on output.

I would connect the panels to PV input, while the internal MPPT controls the charge of the battery pack (through "battery input") and the BMS manages the pack itself. Please, find a general scheme to illustrate it below.

My questions:
Will I burn down the house with this set up?
Would you add any additional protection?
Is in general a good balance for panels/accumulation/load?
Will the BMS and internal MPPT work well together?

The bill of materials links I would purchase for implementing the above set up:
LiFePo4 batteries
JK BMS
Growatt 3500-80A

Thanks in advance!

 

[MichaelK]

Please don't use acronyms that you haven't explained. What is "FV" supposed to mean?

What is the charging voltage range of your Growatt's MMPT? Just because a panel is labeled as "24V" does NOT mean that the output is 24V. 60-cell 24V panels output ~30V, and 72-cell panels output ~38V. Split panels have even higher voltages. You need to pay attention to both the Vmp for power production, and the Voc for the maximal voltage your unit can handle. What does the stickers on the backs of the panels say? What are your winter lows like? The Voc goes up as the winter temperature goes down.

If you have 3 parallel strings of panels you should be either fusing them or running them through a breakered combiner box. I like combiners, so I always recommend that.

 

[JaumeMI]

Yes, sorry for that, I didn't realize it makes no sense in English. "FV" stands for Photovoltaic. It's edited now.

As per Growatt referred doc, "MPPT ranges 120V~430V, 450Voc" (to be honest, I'm not understanding this as I was expecting something around 48V ).

OK, I meant "24V" for systems on that range. More precisely, Vmp=40,9V and Voc=49,6V.

Panels are 144 cells (Split).

"What does the stickers on the backs of the panels say?" Sadly, I can't provide an english link, but you will easily find the table with the rest of parameters here.

"What are your winter lows like?" I don't pretend you to make an exhaustive study of the yearly yield, but let me say it is in Spain, so it's pretty sunny. This are my average solar radiation per month in MJ/m2:

MADRID​

220​

307​

394​

516​

574​

645​

714​

636​

491​

330​

245​

206​

​

First is January... last is December.
Temperatures in winter may go under 0ºC at night, but rarely to -10ºC. In the summer we may reach 35ºC, rarely 40ºC.

About combiners, I'll have a look, that's new for me.

I hope it is a bit more clear now.

Thank you very much.

 

[MichaelK]

OK, here is a good example of why the details are very important. Your Growatt has a built-in MPPT charge controller, that has an operating range of between 120V and 430V. The cold weather maximum that can never be exceeded is 450V

If you wire two of your panels in series as you mention above, you are only going to get 40.9V + 40.9V = 81.8Vmp, which is NOT reaching the lower limit of what your controller is supposed to operate at. So, instead of 2S3P, you need to be wiring at least 3S2P. That would give you 40.9V + 40.9V + 40.9V =122.7Vmp, or just barely above minimum.

I'm not familar with your model so I can't tell you that operating that close to the minimum is going to work. Sometimes the panels might be a volt or so low, so what might happen is on a hot afternoon the voltage might drop below the 120V limit. Maybe, I don't know for sure.

What might be a better option for you is to get two additional panels and wire them 4S2P. That would raise your voltage to a solid 160V or so, so well above the lower limit, but nowhere near the upper limit. With a Voc of 49.6V four panels would put out 198Voc but would raise to about 222Voc at freezing (0C).

450W X 8 panels = 3600W, which appears to slightly exceed the limit of your unit, but I always include a fudgefactor to derate panel output. With an 85% derating, your 8 panels could honestly be expected to put out 3600W X 0.85= 3060W.

With only two parallel strings you will no longer be required to have a combiner, but they are still nice to have.

 

[gibbygoodtunes]

Can you not just run 1 string of 6S? 49.6 x 6 = 297.6Voc ( not accounting record lows) Does the growatt have multiple pv inputs? Is there a voc limit per pv input?