Prioritize solar power production/consumption

[ExpertNoob]

I have:

• 4000W worth of solar panels (total).
• Just finished building a 16 kWh LiFePO4 battery using 16 320Ah A-grade cells (2P 8S, 24 V) and JK-BMS (limited to 120A charge/discharge, hot weather).

Main goal : lower the bills !

Secondary: water supply/grid not 100% reliable, domestic well uses an electric pump.

Intended use strategy : cycle of 2 modes.

1. Battery charge, connected to the grid: send PV power to a 1500W inverter (feeds domestic load), any leftovers are used to charge the battery. This continues until battery is full (95% SOC).
2. Battery is full, disconnect the grid: battery+PV feed into a 3000W inverter (with good surge capacity), this powers the house until battery is drained (5% SOC).

What do I need to :

• Prioritize feeding the inverter up to a given power (1500W, +15W auto-consumption), and leave the rest to the battery in production mode (first mode).
• Prioritize using PV power in consumption mode (second mode), example with 1500W PV available: 2000W load -> pull 500W from the battery; 800W load -> leave the battery alone (or bettter : after 10s send 700W to the battery, its BMS will manage further details).

?

Tnanks !

 

[kmin]

Why would you play with two inverters, charge controller and grid disconnection hardware, when one hybrid inverter can do all what you need?

 

[ExpertNoob]

Why would you play with two inverters, charge controller and grid disconnection hardware, when one hybrid inverter can do all what you need?

I was thinking a lesser inverter has lower auto-consumption, that's all.
You know of a hybrid inverter that can prioritize inverter (domestic load, capped at 1500W) over charger (battery) ?
Also, I'm really not interested in a charger, I only need an MPPT : my strategy forbids using the grid to charge the battery (lower the bills first & foremost !).

 

[kmin]

I'm not sure if I follow you completely... Not interested in a charger but you have a battery...
Anyway. Normal grid tie hybrid inverter could be set like this:
-PV production: priority domestic loads, secondary battery charging, no export to grid(optional).
-In case domestic load is bigger than PV production, take rest from battery if charged, otherwise from grid.
-Never charge battery from grid, never export from battery to grid.

It's not a either/or logic, all the sources are always available.

But I noticed, your battery is only 24V, I don't know one to recommend for that low voltage. Anyway I'm sure you cand find one.

 

[ExpertNoob]

Reading through many inverter manuals, I noticed "charger" is used to describe the function of using secondary AC source (usually the grid) to charge the battery. When an MPPT is present (hybrid inverters), it's referred to as such (MPPT), example : the popular MultiPlus/Quattro line from Victron.

Never mind the voltage (I can easily arrange my cells as a 16S string).
What you describe gets me 90% there (capping AC output to a given power - 1.5kW - can wait, it's at least achievable by choosing a 1500W inverter).
I didn't look into grid-tie options because they stop in the absence of a grid (my consumption mode is off grid), but I can revise my strategy..
Can you recommend a 48V grid-tie hybrid inverter that does what you describe ?
Many thanks.

 

[hwy17]

If you're in France probably Victron equipment is your best option.

 

[kmin]

Even if you don't "officially" connect it to grid, I would choose inverter that respects european standard (CEI0-21)for grid tie. Certified cinese inverters are actually not so bad at all. Goodwe, Sofar, Growatt, ZCS, Solis.... I personally haven't been using Victron, but looks like they have solution for almost whatever situation.

If you have 4000 kWp panels, you just pick inverter 3+kW and you are good to go, no separate charge controllers MPPT, inverters etc.
Biggest advantage is coming from battery topping.
Let's say you have batteries charged to 60%, solar output 1kW and you have domestic load at lunch time 3kW. So you get 1 from solar and 2 from batteries. Nothing from grid.
After an hour you go back to work and you have 40% battery, 2kW from solar and 0.5kW of domestic loads. So you charge your battery with 1.5kW.

You just can't get that efficiency from your "Intended use strategy : cycle of 2 modes." Perhaps half of it.
May be you can with an Aliexpress off-grid all in one inverter, Voltronic type, but I don't know them, the whole sector is not regulated and looks chaotic to me. But for sure you can find help from forums, there are lot of guys playing with Voltronics . That way you could get the backup feature, which is not possible with grid tie inverters.

 

[ExpertNoob]

I agree with you (more and more) about an MPPT being superfluous in the presence of a decent/well configured BMS (simple charger would be enough). I will even test a simple DC-DC buck converter at some point (but as a newbie, I'm getting ahead of myself for now).
Never saw a test of the tracking feature of an MPPT, don't know how helpful it really is.. (such a test is not easy to carry out, given PV variability).
Lots of common points between MPPT & BMS, but saturation current can't beat BMS's direct access to cell-grained data, so MPPT goes.
MPPTs are designed for black-box batteries ; when you have a DIY battery and actually have access to the cell details, it's not really justified.
Not having access to BMS data/conf. in vendor batteries was my point of entry into the DIY world !

I disagree with your statement about efficiency : both of our strategies have identical absolute (theoretical) energetic balances because they are based on the same (eternal) principle : use battery as a Watt reservoir. They differ in use modes only, but 2-1+3-2 Watts (yours) is the same as 1+2+1+1+1 -1-2-1 Watts (mine). But how you store/pull in/from the battery makes a world of difference (physics).
In practice, mine is better long term because your constant store/pull approach is harsher on the battery than my separate-mode use (store only / pull only), and maybe delays for switching battery mode (store/pull) once a week vs. several a day.
[ Mine also caters to the grid reliability issue by capping consumption during production mode, giving the battery a better chance to fill up more steadily (so in case of black-out we have some to consume) : yours would send everything to the load during bad weather, leaving me vulnerable in case of an after-the-rain black-out ]
Also : my modes use 2 sources instead of 3 and are thus more efficient in theory (yours mixes grid, PV & battery all the time) : laws of thermodynamics, man, no way around them !

 

[ExpertNoob]

If you're in France probably Victron equipment is your best option.

I read the Victron docs several times (not so big) and can confidently declare that they do not cover the described use.
No inverter I examined even offers a notion of power capping.
Here is the conf. doc for the Multiplus II (a joke). The EasySolar doc will send you back to this.
But you can read the "Operation" section (half a page) that describes the device as a state-machine : it's a dummy on/off robot.
That doesn't mean that using the communication capabilities & relays you can't program an external Raspberry PI to implement a sophisticated algo. But then port activation signals are mostly battery voltage/alarm based, not very useful for what I describe.

Actually Victron is pretty bad, unless you invest into a shunt, GX system, a few cables, .. and a programmer ! They do offer the possibility..

Here is a much better alternative (priority management built-in) :
Growatt SPF 3000 ES Solar Inventer documentation, page 9 of the PDF you have the configurable priority modes : SBU (Solar, Battery, Utility) (+ setting 14 : battery charging mode on the next page). One single conditional : sign of (load - solar). This covers what kmin is describing.

 

[hwy17]

No inverter I examined even offers a notion of power capping.

Power capping is not something an inverter can conceptually do. If the loads exceed the inverter's output it can only shutdown, there is no way to throttle the load.

 

[kmin]

I agree with you (more and more) about an MPPT being superfluous in the presence of a decent/well configured BMS (simple charger would be enough). I will even test a simple DC-DC buck converter at some point (but as a newbie, I'm getting ahead of myself for now).
Never saw a test of the tracking feature of an MPPT, don't know how helpful it really is.. (such a test is not easy to carry out, given PV variability).
Lots of common points between MPPT & BMS, but saturation current can't beat BMS's direct access to cell-grained data, so MPPT goes.
MPPTs are designed for black-box batteries ; when you have a DIY battery and actually have access to the cell details, it's not really justified.
Not having access to BMS data/conf. in vendor batteries was my point of entry into the DIY world !

I disagree with your statement about efficiency : both of our strategies have identical absolute (theoretical) energetic balances because they are based on the same (eternal) principle : use battery as a Watt reservoir. They differ in use modes only, but 2-1+3-2 Watts (yours) is the same as 1+2+1+1+1 -1-2-1 Watts (mine). But how you store/pull in/from the battery makes a world of difference (physics).
In practice, mine is better long term because your constant store/pull approach is harsher on the battery than my separate-mode use (store only / pull only), and maybe delays for switching battery mode (store/pull) once a week vs. several a day.
[ Mine also caters to the grid reliability issue by capping consumption during production mode, giving the battery a better chance to fill up more steadily (so in case of black-out we have some to consume) : yours would send everything to the load during bad weather, leaving me vulnerable in case of an after-the-rain black-out ]
Also : my modes use 2 sources instead of 3 and are thus more efficient in theory (yours mixes grid, PV & battery all the time) : laws of thermodynamics, man, no way around them !

I don't follow you now. What you mean with MPPT? Sounds like some mysterious element in the play ... No, it is not. You can create your own with programmable dc-dc converter and ten lines of code: Control your output to keep PV voltage at Vmpp. Many MPPT controllers don't even do frequent tracking, because it's not necessary. Vmpp of your string is almost constant, few volts of temperature variation. It really needs tracking only in case of shading.

Maybe your "Intended use strategy : cycle of 2 modes." works for you, but what you descripted "1.do something until battery is full, then 2. do something different" is no sense to me. Why battery should be 95% charged at any point? It's a buffer to balance the difference of PV output and your load at any time.

 

[ExpertNoob]

Power capping is not something an inverter can conceptually do. If the loads exceed the inverter's output it can only shutdown, there is no way to throttle the load.

I mean to enable charger if output exceeds a given value for 10s. Doesn't seem too hard to do.
Actually I only need this as a parameter in the mode selection setting (battery first, load first, ..), but it looks like vendors are offering everything and the stock markets voltage to configure this, but never anything related to isolated output power (which is right there on the display !!)
Maybe you mean to exactly balance the power distribution between load & battery ? I am no electrician, but this is the first thing a PowerStream offers to set. If it wasn't for the proprietay connections (can be managed) & most importantly the limited power, PowerStream would be the answer to all of my prayers. I'm starting to consider buying a used one to start the investigation.. (use multiple units)

 

[ExpertNoob]

I don't follow you now.

Agreed. You're also reading too fast
How am I making a big deal out of the MPPT when I say "MPPT goes, useless, not sure tracking is useful" ?? (what you said).
"A buffer to balance the difference of PV output and your load". Yes, a "Watt reservoir" as I said. Agreed.
I also said a thing or two about grid reliability being a concern (preserving a minimal SOC). You can read again.