Power flow and priorities in solar hybrid system

lamagra wrote: »

I tried looking all over the internet for an answer on the power flow in DC-coupled grid-tie systems with battery back-up. All sources claim that if PV production > load then the surplus will go to charge the battery. I'd like to know why it is like that and not the opposite, meaning why isn't the battery primarily fed and the surplus sent to the loads. From what I understand, the charge controller comes directly after the PV array and is not considered a load, but rather the batteries are.

They are both loads. The battery doesn't take any power if it is fully charged though; it just floats.

The CC basically just dictates the voltage at which the batteries should be set depending on their state of charge. I think it does matter how the power flows in that system since according to most sources, PV primarily feeds the loads and any excess is used to recharge the batteries.

The loads pull as much power as they need. If the loads are too high, and PV cannot supply the load at the battery's float voltage, then the voltage goes down and down. The inverter always takes the same power, but the batteries reduce their current draw as the voltage drops. So the battery "suffers" until either PV increases or load declines. Then the voltage goes back up, the battery takes more current and charges.

OK, I'll try to explain how I, as a non-expert, understand the setup for my XW6048, grid-tied setup.  I also, as a non-expert, think of the batteries as a sort of "bathtub" that can be filled or emptied.

From the PV panels it goes to the charge controllers (2).  They then send the DC energy directly to the battery, 48V, which as is probably basic knowledge the XW6048 can't operate without, although it would be nice if it could have a "grid-tie" only mode and operate that way too.

Then, from the battery, even if the battery is depleted, the DC energy will flow to the inverter.  The inverter will then change the DC into AC, where the energy then goes either to the subpanel loads, which are setup to be powered by the inverter in case of an outage, or back-fed to the grid, where I'll get a credit for that energy sent to the electric company.

Now, it doesn't have to be that way if I change the settings on the controller.

I can set it so that it won't take energy from the battery until the battery is at a certain voltage.  I've thought that was supposed to be around 52.5V to 53.5V, which then allows the battery to be charged, with the DC from the PV panels, before any energy was depleted from the batteries.  So in that case, even though my subpanel is a load, that load is being supplied as a pass-through from the grid, AC, as opposed to being from the PV panel, battery, DC to inverter, AC out of inverter to the panel.

I can set it to force the battery to supply AC through the inverter out to either the grid or the subpanel.  And during that time, if the subpanel loads are less than the inverter is pushing out, the excess goes back out to the grid, with the credit to me for that energy I've supplied to the electric company.

I believe that no matter what though, when the grid goes down, it will pull from the batteries, through the inverter, to supply the subpanel loads, until the low-battery-cutoff is reached.  And that value can be changed to different values so the batteries wouldn't be harmed.  After it reaches the low-battery-cutoff, it is my understanding that everything shuts off, and the subpanel is then also left without any possibility of being supplied with AC electricity until things return to normal.

lamagra said:

right but the AC coupled case is fairly easy to understand. What I'm talking about is the DC coupled case: how is power distributed between the inverter and the battery? why is it that the loads are primarily fed through the inverter and any excess/surplus is used to recharge the battery? why is it not the other way around?

Because inverters cannot live with 3/4 of the power they need.  They will shut down.  Batteries can live with 3/4 (or 1/4, or 1/100) of their maximum charge rate.

Where did Lamagra go? This was starting to get fun.  I was going to try explaining with Spinal Tap references but I guess he is gone.
All things Spinal Tap are much easier to grasp. Oh well back to watching a Black bear and her cubs. Kind of late for cubs....

SkiDoo55 said:

My XW 4024 and XW Mppt 60-150 networked together, Enhanced grid setting, sell on perform as he is asking.
...
I don't have inverter charger on which would recharge battery when grid returned, I rely on solar PV to recharge them.


Dave Angelini said:
Where did Lamagra go? This was starting to get fun.  I was going to try explaining with Spinal Tap references but I guess he is gone.
All things Spinal Tap are much easier to grasp. Oh well back to watching a Black bear and her cubs. Kind of late for cubs....

In the U.S., for tax purposes, it is essential to have it set to recharge from solar PV, or you will lose that portion of your 30% tax credit for the batteries for your PV system that is recharged from the grid. 

For Dave, I will only set it to "11" if it is a cloudy day and I need the solar panels to produce more energy than they receive from the sun.

I got too busy for this now but I would go to the U-tubes on Schneider's XW and SW. The best one for this is the basic inverter video engineering wise. They go into DC coupling really well and compare the AC coupling.
https://www.youtube.com/watch?v=lMGxfKJ4dNE&list=PLxuEvT_BmJp7Pul9uYQKpPEjzFSIz2LjF


Nigel Tufnel: The numbers all go to eleven. Look, right across the board, eleven, eleven, eleven and...
Marty DiBergi: Oh, I see. And most amps go up to ten?
Nigel Tufnel: Exactly.
Marty DiBergi: Does that mean it's louder? Is it any louder?
Nigel Tufnel: Well, it's one louder, isn't it? It's not ten. You see, most blokes, you know, will be playing at ten. You're on ten here, all the way up, all the way up, all the way up, you're on ten on your guitar. Where can you go from there? Where?
Marty DiBergi: I don't know.
Nigel Tufnel: Nowhere. Exactly. What we do is, if we need that extra push over the cliff, you know what we do?
Marty DiBergi: Put it up to eleven.
Nigel Tufnel: Eleven. Exactly. One louder.
Marty DiBergi: Why don't you just make ten louder and make ten be the top number and make that a little louder?
Nigel Tufnel: [pause] These go to eleven.