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nicolas-g asked

Advise requested on asymmetric & variable PV configuration

Hello,

I’m preparing a new installation, and I have a few things I need to validate. I’ve spend numerous hours studying many sources, but I can’t find a definite answer. I also lack the required experience, which I hope you might have.

  • Use case: installation which follows me, in a sense that it can be used both @ home and while travelling (camper/expedition-truck).
  • Description: The installation would have 4 PV-arrays, which are asymmetric and not always all available.
    • Array1: 200Wp on top of truck cabin
    • Array 2A 600Wp + Array 2B 1200Wp: foldable stack on the roof of box at the rear of the truck, when folded (driving) only 2A is exposed, when unfolded (stationary) 2B is also exposed
    • Array3 2400 WP on top of my house which is only plugged in when the truck is home.
  • To validate 1: an installation with asymmetric & variable PV arrays can work well
  • To validate 2: to work well, each array needs its own MPPT to make the best of it, if & when it is available
  • To validate 3: multiple/different MPPT's will only work well in a controlled way when they can be synchronized (e.g. charge cycles), which means having plenty of VE.direct ports (when using right size/smaller/cheaper MPPT’s). Octo GX is the most cost-efficient solution which can handle it.
  • Question: A 100/20 48V MPPT (array 1 & 2 A) gets in trouble with Max PV voltage at -5°C and below, also with PV panels with rather low voltage. Is this a theoretical problem or a real problem? For example: @ -15°C for the panel to get at max V, the sun is warming it up to 5°C and the problem disappears. Or is the only safe options: A: to move to a 150/35, accepting it will be overkill 95% of the time, B:to disconnect it when it's freezing.

Obviously there are many more components in the system, but I focused on the part where my questions are, not to take more time than necessary from you.

Many thanks for your advice & opinions

Nicolas

PS: Added 2 simple schematics to try to clarify

Update 4/9/20: I was in touch with a Victron Partner in Belgium which has build similar and bigger things before. He validated/confirmed all assumptions. Hope this can help somebody else too. Success with your projects!

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Matthias Lange - DE avatar image Matthias Lange - DE ♦ commented ·

What battery voltage do you have?

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redmcc avatar image
redmcc answered ·
  • 1: Yes - asymmetric & variable PV arrays can work well
  • 2: Correct to work (really) well, each array should get its own MPPT (counting 2A and 2B as separate arrays); however, there are string combinations that would still work well for these as a single array)
  • 3: Not really. In most cases, multiple/different MPPT's can still work well as long as the MPPT output voltages into your BMS are the same (eg 48V). Charge cycles isn't an issue that I'm aware of.
  • Excellent Question!: Exceeding Voc(max) is generally a bad thing. Its not always clear what will happen, but the Voc(max) is recorded for the life of many MPPTs and is used to declare the warrenty void. It might still work, or you might need to purchase a new MPPT in a years time...
  • When actually at max charging, an MPPT is specifically designed to be at Vpp (ie quite a bit less than Voc.
  • If your solar angles are all optimum, its cold, the battery is full or nearly full, and the system isn't shaded and the system can theoretically exceed Voc, then it normally will (eg. when the battery is full or nearly full). In the real world, batteries are normally not full at dawn when it is really cold. Depending on the price difference between the MPPTs , then a blanket over a portion of one panel on those really cold days when you have a full battery might do the job!
  • My recommendation would be to spend the extra to get the slightly larger MPPT. :)


Just my thoughts :)

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nicolas-g avatar image nicolas-g commented ·

Thanks!

The cover part of a panel part idea in winter crossed my mind too, but then like you say having to tape up part of the panels in winter vs cost difference between an 100/20/48 and a 150/35 ... would maybe be a bit greedy I guess ;)

regarding parallel MPPTs, I read quite a lot about 1 MMPT taking the lead, and the rest going into float mode when not synchronizing charge cycles, or the fact you can't control who's boss with the new bluetooth sync so the one with the PV in the shade could become boss. Also with some of them not active at some points in time, it just feels safer to be in "hard" control. Anyway I need a CAN bus capable brains to talk with the non-victron BMS of the Battery too. I would like to use a 96Ah 45V battery module, which can drop to 40V, so not a standard 48V battery, CAN bus required so the BMS of the battery can talk to the Victron brains. Its a 2nd hand BMW i3 module. It's about 1/3 to 1/4 of the cost of new Lithium batteries for similar capacity, and well for a trial/first set-up it is expensive enough as such.

the inverter would be a multiplus-II 48 5000VA, this seems cost/power a (heavy) good start, basically it allows at one hand to run household devices in the truck, more cost/effective than the specialized camping/boating stuff. And at the other hand to deliver an amount of power which "worth-your-while" in a house. But it also requires some solid battery to deliver the power, and frankly honest investing in 4x100Ah new lithium batteries springs my current budget.

Hope this clarifies & thanks again!

Nicolas

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nicolas-g avatar image nicolas-g commented ·

Thanks!

The cover part of a panel part idea in winter crossed my mind too, but then like you say having to tape up part of the panels in winter vs cost difference between an 100/20/48 and a 150/35 ... would maybe be a bit greedy I guess ;)

regarding parallel MPPTs, I read quite a lot about 1 MMPT taking the lead, and the rest going into a battery is full mode when not synchronizing charge cycles, or the fact you can't control who's boss with the new bluetooth sync so the one with the PV in the shade could become boss. Also with some of them not active at some points in time, it just feels safer to be in "hard" control. Anyway I need a CAN bus capable brains to talk with the non-victron BMS of the Battery too. I would like to use a 96Ah 45V battery module, which can drop to 40V, so not a standard 48V battery, CAN bus required so the BMS of the battery can talk to the Victron brains. Its a 2nd hand BMW i3 module. It's about 1/3 to 1/4 of the cost of new Lithium batteries for similar capacity, and well for a trial/first set-up it is expensive enough as such.

the inverter would be a multiplus-II 48 5000VA, this seems cost/power a (heavy) good start, basically it allows at one hand to run household devices in the truck, more cost/effective than the specialized camping/boating stuff. And at the other hand to deliver an amount of power which "worth-your-while" in a house. But it also requires some solid battery to deliver the power, and frankly honest investing in 4x100Ah new lithium batteries springs my current budget.

Hope this clarifies & thanks again!

Nicolas

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klim8skeptic avatar image
klim8skeptic answered ·

Reality check.

*"Array1: 200Wp on top of truck cabin " Finding a 200w panel that will charge a 48v battery will be challenging.

*"Array3 2400 WP on top of my house which is only plugged in when the truck is home " A grid tie system would make more sense. Use power in house 24/7/365, plug in multiplus to mains, to use in truck.

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nicolas-g avatar image nicolas-g commented ·

Thanks for your input

The 200wp would in real life be 60wp 4s1p or something like that. i’m aware I need more voltage in the pv array than 48v

the reason why I put the core in the truck is dual:

- it can follow me, if i go fo a 3 week holiday in my truck, the system comes along and does not stay home unused. Ok the biggest array is disconnected but for the truck by itself the remaining arrays should be ample and if not can be extended

- if the truck has no sizeable standalone electricity power system, it needs other sources. Which also cost money and are of no use when I’m not travelling. Which is 90%+ of the time


hope this clarifies

Nicolas

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klim8skeptic avatar image klim8skeptic ♦ nicolas-g commented ·

You still have not validated array 3.

Stealth camper/truck.


How much power would you use per day? (solar panels/mppt)

Hot much power would you use between dusk til dawn? (battery storage)

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nicolas-g avatar image nicolas-g klim8skeptic ♦ commented ·

Array 3 is a larger array, on the roof of my house, which is connected to the truck when it’s home (90% or more of the time). 230v line runs from the truck to the house to feed some ‘ big ‘ consumers. Think kitchen, laundry dryer...

does that explain?

the fancier option: a setup to provide power when electricity is most expensive, as in Belgium we are moving to a model where price is linked to demand. So electricity will be expensive when everybody wants it. Eg 18h to 20h when people are cooking it would be more expensive.

on the power usage, to be frankly honest, I would rather create a solid modular base and upgrade when getting real life based insights. 4kWh battery and 4kW inverter and >1kWp PV sounds like a solid starting point to get real life based insights from.

in Flemish the engineers saying is ‘to measure is to know’ ;)


nice truck btw, mine below, 12t with about 5t payload as such not so much worried by weight of the electric power set-up

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