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mterman avatar image
mterman asked

3+3kW AC coupled system design

Hello,

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-IIoffgridAC PV CouplingFronius
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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 commented ·

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

Batteries that we use
https://www.freedomwon.co.za/storage/2021/07/lite-home-and-business-spec-sheet-design-1.pdf
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
http://www.esergetica.it/customized-solution/?lang=en

http://www.esergetica.it/energy-storage-3-reasons-to-choose-them/?lang=en

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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