Power flow and priorities in solar hybrid system

lamagralamagra Registered Users Posts: 8
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. 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. What I'd like to know is how they came up with that statement. Could it be something between the CC and the Inverter or maybe something related to basic electric circuits theory that I must have missed?

Comments

  • BB.BB. Super Moderators, Administrators Posts: 31,443 admin
    Welcome to the forum Imagra.

    I am not quite sure I understand your set of questions... First are we talking about a Hybrid AC Inverter type system... AC in / AC out with a hybrid inverter that is capable of both running GT (feeding power back to the grid) and off grid when utility power has failed (feeding AC power to protected loads).

    Hybrid inverter PV systems are pretty complex setups. But, the more or less standard setup has the MPPT solar charge controller setup to output maximum power from the solar array to the battery bank.

    The Hybrid inverter (when on grid) will act like the battery charger. If the batteries are "full", the hybrid inverter sends the "excess" charging energy out its AC connections. Whether that goes to the loads or is fed back to the AC mains (and turning your meter backwards) depends on the size of the loads (if loads>pv power, then some AC power is used to help support the loads; if PV power>loads, the excess power is feed back to the utility).

    If you are not allowed to feed power backwards to the utility, then the MPPT solar charge controller has to be in control, and simply sets the proper absorb/float voltages for the battery bank. When the loads > PV power, the battery supplies energy to support the loads. When PV power > loads, the PV power is split between charging the battery bank and supporting the loads.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • bill von novakbill von novak Solar Expert Posts: 891 ✭✭✭✭
    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.
  • lamagralamagra Registered Users Posts: 8
    What I mean is, for a hybrid/dual-mode dc-coupled system, how is it that if PV production > demand load then the loads are given priority and fed from the solar power and the remaining/surplus is used to charge the batteries? Why is it not the other way around whereby batteries are primarily charged and in case of any surplus then it is fed to the loads?
  • BB.BB. Super Moderators, Administrators Posts: 31,443 admin
    Technically, it is because the "Loads" are what you want to be reliably powered.

    If you made battery charging the priority "load", then the system would load shed (turn of loads) to maintain battery charging current/power. For most of us that is not what we would want a system to do (turn off loads if clouds pass in front of the sun).

    The batteries "buffer" the solar energy (recharge when there is more sun, and supply energy when there is less sun/more loads).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • lamagralamagra Registered Users Posts: 8
    I understand that but what I'm asking about is HOW is this achieved? How is a load or the battery set as a "priority"?
  • BB.BB. Super Moderators, Administrators Posts: 31,443 admin
    A "Hybrid Inverter" that uses the AC utility grid as a "dump load" is setup to have the battery as a priority, and excess energy is fed back into the home/utility grid... But notice here, that the utility itself is keeping the AC loads powered.

    What you are asking for is done in solar (and wind, water) systems--And that is where the charging source dumps 100% of the available energy to the battery bank, and a "dump controller" takes any excess power (say you have 14.5 volts as the battery charging voltage) and the dump controller turns on and sends excess power (if the battery voltage goes over 14.5 volts) to a "dump load" -- Typically a water heater element, but some people run well pumps or other loads that can be cycled without issue (i.e., filling a water tank with an on/off flow is acceptable--But power lights in the home that flicker on/off depending on the battery charging needs is not).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • lamagralamagra Registered Users Posts: 8
    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?
  • sub3marathonmansub3marathonman Solar Expert Posts: 300 ✭✭✭
    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.
  • BB.BB. Super Moderators, Administrators Posts: 31,443 admin
    Because AC loads cannot be supplied with variable voltage/current/frequency without causing damage to the loads and/or making them "useless" for the user (blinking lights, TV that turns on/off while watching, damaging a refrigerator motor, etc.).

    A batter can accept variable current (charging) or supply variable current/fixed voltage (while discharging).

    Let me ask a question of you--Why would you want to have the battery as "primary" and the loads as secondary in a power system?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • bill von novakbill von novak Solar Expert Posts: 891 ✭✭✭✭
    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.
  • Raj174Raj174 Solar Expert Posts: 787 ✭✭✭✭
    I believe the loads make themselves a priority by design, otherwise you would just have a battery charger.
    Just my opinion.
    3600W PV, MNE175DR-TR, MN Classic 150, Outback Radian GS4048A, Mate3, 51.2V 360AH LiFePO4, Kohler Pro 5.2E.
  • Dave AngeliniDave Angelini Solar Expert Posts: 5,807 ✭✭✭✭✭
    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....
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • SkiDoo55SkiDoo55 Solar Expert Posts: 414 ✭✭✭
    My XW 4024 and XW Mppt 60-150 networked together, Enhanced grid setting, sell on perform as he is asking.  Battery is given priority charging and surplus goes to the loads (house load/grid) the inverter makes that happen. It follows the settings from the charge controller for what it can draw. If during the day the PV input falls off the inverter will reduce the output to load as a function of battery voltage. If it goes below what the charger setting is trying to maintain it will go to pass thru and draw from the grid to support the loads. It's follow setting is @0.5 vdc below the charger setting, so it will slowly draw from battery.  With no PV input my DC draw is @18W, al long as grid is available.  If grid failure it will switch to Inverting as stated until low batt cutout voltage reached, then it will stop inverting and wait for grid to come back or PV input to return to bring batteries up. I don't have inverter charger on which would recharge battery when grid returned, I rely on solar PV to recharge them.
    GT3.8 w/4600W Trina 230W, TX5000 w/5000W ET-250W, XW4024 w/1500W ET-250W, 4 L16, 5500W Gen. (never had to use) Yet!!
  • sub3marathonmansub3marathonman Solar Expert Posts: 300 ✭✭✭
    edited September 2015 #15
    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.

  • lamagralamagra Registered Users Posts: 8
    edited September 2015 #16
    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....
    I'm here now so you can give it a try with spinal taps :) please keep in mind that what I'm interested in is the DC coupling case and how the energy flow is distributed between the batteries and the loads (thru the GRID TIE inverter).
  • Dave AngeliniDave Angelini Solar Expert Posts: 5,807 ✭✭✭✭✭
    edited September 2015 #17
    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.



    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.

    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

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