mterman avatar image
mterman asked

3+3kW AC coupled system design


We are designing 3+3 kW system. Because of financial constriction and pressing need to upgrade the current system we are planning to build our new system in two phases.

First phase would be 1 x Victron Multiplus II 48/3000, 1 x Fronius Primo 3.0-1 with around 4kW of PV, 320Ah 48V traction battery system (24 * 2V batterys) and Cerbo GX for system monitoring.

The second phase should be implemented next year. We would then add another Victron Multiplus II 48/3000 in parallel with the first one and another Fronius Primo 3.0-1 with around 4kW of PV.

This is a offgrid system, most of the power usage is during the day.

There is a 2kW generator for backup.

In addition to that we would probably add a micro water turbine (PowerSpout), also AC Coupled trough Ginlong Solis Mini string inverter. We could get from 500 to 1000W form the PowerSpout. I know that this takes the system over the 1:1 rule. But water turbine would be used when there is no PV power (bad weather or night).

I don't have the exact question, just need the sanity check for this long term plan.

Thank you!

Multiplus-IIoffgridFroniusAC PV Coupling
2 |3000

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shaneyake avatar image shaneyake commented ·

A few things that I would think about.

Why do you want the Multiplus-ii 48/3000 over the 48/5000, not that much more but almost double the power?

Why do you want to do AC-Coupled with the Fronius Primo vs an MPPT, Like the RS 450/100?

Please please consider LFP batteries, I know the up front cost is higher but the life and useable energy is so much better.

Small Water Turbines hardly make sense, unless your property meets unique requirements, I would rather spend the money and get more PV in 99% of the cases. Also less hassle and less materinace on PV.

If you did DC PV for phase 1 then you could add AC-PV for second phase. Would also look into Enphase micro inverters if you have any shading requirements.

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mterman avatar image mterman shaneyake commented ·

Thank you for your comments.
For us the Multiplus II 48/5000 is almost double the price of 48/3000. And getting one 48/3000 now and then adding one later in parallel with the first one we will get double the power. Also I must add that the power of one 48/3000 would be more than enough for AC consumption. Adding another 48/3000 will give us more room to add more AC coupled PV inverters, and the system will have redundancy.

We want AC coupled Fronius Primo because all the loads are on AC, and mostly during the day. In that kind of setup, AC coupled system is more efficient.

I know that LFP batteries are better and have more usable energy/capacity but the price would be 3 times that of the traction batteries for the similar usable energy.

Small water turbine is mainly for the two winter months when the usable sun drops to 2-3 hours a day. There was a small DIY turbine at the property in the past, so part of the problem is already solved (water pipe and the cabling to the turbine site). This kind of turbine can cower most of the power needs during the winter, as there is less power consumption then. That said, the plan is to also cover the entire usable roof with PVs.

We will thing about mixing AC coupled with DC coupled PVs in the future.

Micro inverters sound good to me, but are not readily available here. And the shading requirements at the location are usually complete shading not partial so nothing can help with that ...

Thank you again!

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shaneyake avatar image shaneyake mterman commented ·

Really interesting, what country are you in?
I would mainly work in South Africa the US.
South Africa the difference in price is 17% more for 48/5000
The US the price difference is 20% more for 48/5000

Yes to some degree AC systems are more efficient but you are also charging batteries and that can be a substantial amount of energy and DC -> AC then AC -> DC is worse than DC -> DC. or DC -> DC -> AC You have also already paid for this inverter, might as well use it.

Any chance you could give the specs and price of the traction battery? I am interested to see why that is the case, I do a lot of the grid projects for work and have moved completely to LFP for all applications.

Oh that's sweet, if you have everything then do it. I would just put the turbine on some sort of relay and have the GX turn it off if SOC is close to 100% to prevent overcharge or system shutdown.

Yeah, if you don't have shading string inverters are cheaper, I only like micro inverters as we can add one per month as you get money.

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mterman avatar image mterman shaneyake commented ·

We are in Croatia, Europe.
I exaggerated a bit with double the price for 48/5000, it actually 60% more than 48/3000. But as I mentioned, with two 48/3000 versus one 48/5000 we would have redundancy.

I will thing about making DC + AC coupled system, half PV with string inverter and half with Victron MPPTs. My main reason for preferring completely AC coupled system is because in that case only one device (or two of the same in parallel) would be charging/discharging the batteries and have complete control of the batteries.

Batteries are from TAB, 4 PzS 320 L
Price is around 3800 USD (converted)

Turbine would be connected trough Solis string inverter that should work with Micro Grid frequency shifting. It doesn't support power output tapering, but it supports cut-off frequency. Also we wold add some kind of dump load (water heater).

I looked into the micro inverters for the exactly the same reason.
Thanks again!

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shaneyake avatar image shaneyake mterman commented ·
Oh cool, Thanks for the info.

When you have a GX it controls the whole system, so it doesn't matter if it is MPPT or Inverter, It acts as one system. So it will remain in complete control. Your system will need a battery monitor or SmartShunt anyways.

Oh cool, this sounds great.

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mterman avatar image mterman shaneyake commented ·

We are planning to add Cerbo GX to the system. There is already BMV-712 on location, it will also be used in a new system so monitoring and synchronizeing everything should not be a problem.

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shaneyake avatar image shaneyake commented ·

@mterman I just saw that my battery supplier sells their batteries in Europe.

Batteries that we use
10/8 is proably close to your 320ah
We have only had good things with these batteries, lasting 10+ years getting cycled to 40% SOC daily and 10% occasionally.

European Distributor

Might be worthwhile to get a price. In South Africa they are $3700, will probably be more but I don't know how much more.

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marekp avatar image marekp commented ·
Hi @mterman

If you want 2 MP-II to work properly in parallel, you better buy them at the same time.

They will have the same hardware inside.

I had a problem with two MP-IIs that were made more than a year apart.

They would not balance the load evenly.

I agree with @shaneyake. Go with LFP.

I got mine, made by Winston, from their Czech dealer GWL.

They are a bit more than Lead-acid but they can be safely discharged down to 20% SOC.

Lead-acit should not be discharged below 50% SOC if you want them to last.

If you compare the usable capacity to price ratio of PB i LFP, LFP wins hands down.

If you want longevity LTO is the choice but still expensive.

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mterman avatar image mterman marekp commented ·

@shaneyake I looked at freedom won batteries, problem with the 10/8 is the size and weight. The location of the system is a bit remote, and everything has to be carried by people or horses/mules ... Maybe we could get 2 x 5/4 and connect them in parallel. Do you have experience with adding more LFP batteries to the system after some time? In example getting one 5/4 now, and then add another next year?

@MarekP I read about the problem with synchronizing 2 MP-II that were manufactured too far apart. Plan is to buy other MP-II as soon as possible. I am trying to arrange everything so we could possibly buy them boat at the same time ...

I will look into the Winson batteries, I think that GWL will ship anywhere in EU.

There is also a possibility we could get Pylontech US2000B+ battery in Croatia, 3 of those would cost as same as 320ah traction batteries. And we could add more down the line.

Thank you both for commenting!

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marekp avatar image marekp mterman commented ·

Hi @mterman

Do you have experience with adding more LFP batteries to the system after some time?

I am adding second set of 16/200Ah Winstons to the 16 I installed year ago.

They will go in between the cells of the first set, so the new cell will be paired with the old.

This way I will not need second BMS.

I think that GWL will ship anywhere in EU

Check on their web site but I believe they ship anywhere in EU and even Canada.

There is also a possibility we could get Pylontech US2000B+ battery in Croatia

They are also good choice I hear. Do not have experience with them but my Victron installer offers them. They are easier to connect to Victron using CAN bus.

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shaneyake avatar image shaneyake mterman commented ·

Yes, you can add more modules over time. FreedomWon recommends you only connect batteries that are installed within 4 years of each other.

5/4 is great, we use it more as a UPS battery for our clients because can get expensive if you expand to 40kwh which is the size most of our residational system become.
You can link multiply 5/4, connects via CAN to GX, does not need a battery monitor as all that is done via CAN.

If you go the Winson route, I would install an Orion BMS Jr.2 with CAN, integrates really nicely into Victron system.

Or a REC bms, but I have never used this one, it just seems to have Victron support.

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