Designing an offgrid AC coupled PV system

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Stijn asked Jun 23, '23

Designing an offgrid AC coupled PV system

I would like some input on the following design, just to make sure I understood everything correctly.

BYD Battery-Box Premium LVS 24.0

Adhering to the minimum battery size: 1,5kWp AC-tied PV Power = 4,8 kWh battery

So, 24 kWh/4,8kWh = 5

So, max installed PV AC is 5*1,5kWp = 7500 Wp max

Fronius Primo 8.2-1

max 8200W, which is smaller than 7500 Wp solar installed, so I will not use the Fronius to its full capacity.

Victron Quattro 48/15000/200-100/100

Following the 1.0 rule (max PV Power (min of installed solar or installed grid-tie inverter) =< VA rating of inverter/charger.)

So, 7500 Wp installed AC tied solar =< 15000VA. So, this is ok as well.

Luxor ECO LINE P72/330W

7500 Wp/330 Wp = 22,7 => 22 panels connected to the Fronius.

Is this correct or am I missing something?

Thanks,

Stijn

offgrid AC PV Coupling

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Stijn commented · Jun 24, 2023 at 10:19 PM

https://www.victronenergy.com/live/battery_compatibility:byd_b-box

Just found this, more reading to do.

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Stijn commented · Jul 02, 2023 at 03:42 PM

Hi all,

Thanks for all your comments, much appreciated!

However, thanks to this thread...

https://community.victronenergy.com/questions/211981/cascading-installations-or-supersizing-off-grid-sy.html?childToView=215449#comment-215449

I decided not to buy the BYD but to go for EVE LFPs.

They are 304Ah each, with a max voltage of 3,65V.

Usually, conventionally, people use 16S for a 48V system. I am considering going for 18S however.

The Victron Quattro has a max DC input voltage of 66V, 66/18=3,66. The Victron pages say to bulk charge them to 3,55, so I have some reserve. And after that absorption charge at 3,375V.

https://www.victronenergy.com/media/pg/Manual_BlueSolar_100-30__100-50/en/configuration-and-settings.html

EVE 304AH datesheet.pdf

Any reason this is ill-advised?

Thanks

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eve-304ah-datesheet.pdf (3.4 MiB)

Modifications Space

Node Red Space

Answer 1 · 2023-06-25T07:38:47Z

Marc Heymans answered · Jun 25, 2023 at 07:38 AM

This will work, however, I recommend a DC coupled component too. In the event that your batteries go flat, and the inverter shuts down, it wont be able to restart from the AC coupled part. Having the two different charging sources also makes for more accurate charging for whatever GX device you choose to use

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seb71 commented · Jun 25, 2023 at 08:17 AM

I recommend a DC coupled component too.

That's a must for an off grid system.

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seb71 seb71 commented · Jun 25, 2023 at 10:42 AM

Also having a large (24kWh) battery probably means you do use quite a lot of energy during the night.

It is more efficient to charge the battery with MPPTs.

So not just a small MPPT, but a large(r) one might be a good option. Put 1/3 or 1/2 of the PV panels on it. Or something like this, depending on your day/night energy usage.

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rvicev commented · Jun 25, 2023 at 09:02 AM

Not per sé.

A DC coupled component is the Victron recommended way and offers the advantage that it´s automatic (once the sun comes back). But there are other ways (e.g. based on SoC, or via a small generator / mobile powerstation) to kick-start the MultiPlus and thus get the AC coupled part going, once the sun comes back; these do however require some manual intervention.

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Answer 2 · 2023-06-25T09:19:04Z

bipedalprimate answered · Jun 25, 2023 at 09:19 AM

For an Off Grid system, I would start with how many kwh per day you need and how many days do you need it for before you can reliably recharge at your latitude in middle of winter.

Secondly, what is the maximum demand in kw (not kwh) that your loads will require and for how long - this can help determine the inverter size.

The Primo 8 may be too large - you can over provision a smaller Primo with panels above its rated output. Look at max MPPT input power in the specification sheets.

If you are Off Grid, why do you want a Quattro? A Quattro has 2 AC-In ports which you will not be using - I think.

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Answer 3 · 2023-06-25T10:00:40Z

solarmax-canarias answered · Jun 25, 2023 at 10:00 AM

With off-grid i recommend you to think about doing it with an RS450/200 instead of AC-Coupling, if you don´t have high constant consumption by heat pumps or similiar. The thing is that you will always have issues with the frequency shitfting with lights, clocks, etc. If its an industrial or agricultural installation would not matter, but always add an MPPT charger for more stability and better SOC once you reach battery idle - and also for cold-start. I´ve done a lot installations in the same size, and just added an 100/20, witch is suitable for 48V. An classical Blue-/ Smartsolar has near to 99% of efficiency, so maybe you combine a smaller AC-Inverter (Primo 5.0) for 16 panels and the rest you do it with an 150/45 with 6 panels (3+3).

Good luck :-)

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Stijn commented · Jun 27, 2023 at 09:15 AM

Hey guys,

Thanks for the answers.

I omitted something, which is that I already have 2 MPPT controllers with panels...8*330Wp on a Victron MPPT 150/45 and 3*175Wp on a Victron MPPT 100/20. I might add to that.

I am indeed off grid and do not need the Quattro but it is cheaper here then the MultiPlus 2 15kVA.

I want so much power and storage because I plan to charge an electric car with it.

I am still reading up on the BYD-Victron page, which basically states that I need 28kWh BYD for a Quattro 15kVA.

Thank you!

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solarmax-canarias Stijn commented · Jul 02, 2023 at 07:20 AM

Hello, in case of car charging you have a lot of consumption, I guess 7,4 kW, there an AC Coupling makes sense, always with an small DC charger too ;-)

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