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  • Energy flow for PV systems with batteries

    Anyone could clear up/confirm the following?:
    - In stand-alone systems: Whenever the charging process is in BULK mode, PV power will feed the load demand and the excess will be used to recharge the batteries. If batteries are full and the load demand is less than PV production - current absorbed by battery, the inverter will shave the PV excess at its output.
    - In grid-tied with battery systems: Whenever the charging process is in BULK mode, ALL PV power will go to the batteries and any loads are fed from the grid. It is also optional to allow the grid to participate in charging the batteries. When the charging gets to the ABSORPTION MODE, batteries only use a certain amount of current and all the remaining goes to the loads, offsetting consumption from the grid. If PV production is less than the demand load, the remaining can be obtained either from the batteries or the grid.

  • #2
    You are making generalizations about multiple ways of combining off-grid and GTI operation in one system.
    Neither of your comments apply to pure (non-hybrid) systems and there are so many ways of putting together hybrid systems that you cannot generalize that way.

    In a pure off-grid system the inverter portion will only deliver the power requested by AC loads at any given time. If the load goes down the DC current into the inverter goes down to. How the CC reacts to that depends on the state of the battery and the PV power available.

    In a pure GTI system there are no batteries to charge and the GTI will always deliver to the combination of grid and local loads all the power that the panels are capable of producing. (Unless the GTI is underzied compared to panel output.)

    If you specify a particular set of components (make and model) that are combined into a hybrid system, we can explore the power flows in that particular system.
    SunnyBoy 3000 US, 18 BP Solar 175B panels.

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    • #3
      Seems like this comes up over and over.

      In a pure grid tie (no batteries) as much power as possible is converted to AC and fed to the grid. Loads run from the grid. So the local loads "take" some of the power that is generated by the grid tie system, but since the grid is essentially a perfect voltage source you don't see much difference (other than what your meter reads of course.)

      In a pure off-grid, the inverter always takes what it needs. "Excess" power goes to the battery. If there is more power than loads, then the array voltage rises. This moves the array off its peak power point and it effectively stops generating as much power.

      In a hybrid, the inverter works per whatever control law it is programmed to obey. A common one is "do not allow the battery voltage to rise above 50 volts; if it does, send more power to the grid until power returns to 50 volts. Loads are effectively fed from the grid.

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      • #4
        I fully understand the grid-tie and off-grid inverters operation. I would like to propose the following for a DC-coupled system connected to the grid and I would like to know if this is how it really works:
        - In the morning, when the battery is empty and as soon as the PV starts producing, the battery will absorb ALL of the PV production since it will be in the BULK charging phase and the loads will be fed from the grid. In other words PV production is totally dedicated to charging the battery until it reaches the ABSORPTION phase, and no share of the PV production is sent to the inverter during the BULK charging phase.. Is this true? If so, is it by design (e.g. Ohm's law) or is there something more complicated that involves the charger and the inverter?
        - Once the battery reaches the preset ABSORPTION voltage, the charger will keep the voltage constant and the current absorbed by the battery will reduce with time since its internal resistance will increase. The PV production minus the battery's need = power sent to the loads. IS this true?
        - The same as above goes for the FLOAT charging phase except of course for the fact that the voltage is reduced to a certain level. Is this true?

        I know that there could be many scenarios but is my analysis correct?

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        • #5
          btw this could also apply to AC coupled systems with hybrid inverters if programmed as such.

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          • #6
            Originally posted by lamagra View Post
            I fully understand the grid-tie and off-grid inverters operation. I would like to propose the following for a DC-coupled system connected to the grid and I would like to know if this is how it really works:
            - In the morning, when the battery is empty and as soon as the PV starts producing, the battery will absorb ALL of the PV production since it will be in the BULK charging phase and the loads will be fed from the grid. In other words PV production is totally dedicated to charging the battery until it reaches the ABSORPTION phase, and no share of the PV production is sent to the inverter during the BULK charging phase.. Is this true? If so, is it by design (e.g. Ohm's law) or is there something more complicated that involves the charger and the inverter?
            - Once the battery reaches the preset ABSORPTION voltage, the charger will keep the voltage constant and the current absorbed by the battery will reduce with time since its internal resistance will increase. The PV production minus the battery's need = power sent to the loads. IS this true?
            - The same as above goes for the FLOAT charging phase except of course for the fact that the voltage is reduced to a certain level. Is this true?

            I know that there could be many scenarios but is my analysis correct?
            I do not believe it works that way.

            As far as I know without a very sophisticated control & isolation system to direct power flow, some portion of the electricity generated from the pv system will flow to all the loads (including charging the battery) and if necessary to meet your demand additional power will come from the the grid.

            I do not think the battery will automatically get priority to receive 100% of the power from the pv system unless it has been wired that way.

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            • #7
              Originally posted by lamagra View Post
              I fully understand the grid-tie and off-grid inverters operation. I would like to propose the following for a DC-coupled system connected to the grid and I would like to know if this is how it really works:
              - In the morning, when the battery is empty and as soon as the PV starts producing, the battery will absorb ALL of the PV production since it will be in the BULK charging phase and the loads will be fed from the grid. In other words PV production is totally dedicated to charging the battery until it reaches the ABSORPTION phase, and no share of the PV production is sent to the inverter during the BULK charging phase.. Is this true? If so, is it by design (e.g. Ohm's law) or is there something more complicated that involves the charger and the inverter?
              - Once the battery reaches the preset ABSORPTION voltage, the charger will keep the voltage constant and the current absorbed by the battery will reduce with time since its internal resistance will increase. The PV production minus the battery's need = power sent to the loads. IS this true?
              - The same as above goes for the FLOAT charging phase except of course for the fact that the voltage is reduced to a certain level. Is this true?

              I know that there could be many scenarios but is my analysis correct?
              Well in a Grid tie bimodal system then the batteries generally are discharged in the morning unless the power was out, in which case it acts like an off grid system.
              I am most familiar with the Outback but most bimodal system have a controller like the Mate3 from outback with rules governing the operation. Ohms, law and the fact that there is a DC bus controls part of things and the computer controls other things. For example if at sunup (solar sun up for the system) the power just came on and batteries are low, the computer would have a pre-set charge current to send to the batteries. If say there is enough extra power, the inverter might be turned on and told to invert some amount, OR if there is not enough solar current, the inverter might be turned on and told to help charge....
              Further Outback has many modes like grid zero where it attempts to never feed power into the grid, useful in locations without net metering. Or someone could just get into it and change all the programming around.

              On an AC coupled system, with outback it works similarly though you do not have a CC and it basically only plays with the grid tie inverter when the grid is down, where it turns the grid tie inverter when the batteries are full and back on when they get down to a set point.
              OutBack FP1 w/ CS6P-250P http://bit.ly/1Sg5VNH

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              • #8
                Originally posted by lamagra View Post
                I fully understand the grid-tie and off-grid inverters operation. I would like to propose the following for a DC-coupled system connected to the grid and I would like to know if this is how it really works:
                - In the morning, when the battery is empty and as soon as the PV starts producing, the battery will absorb ALL of the PV production since it will be in the BULK charging phase and the loads will be fed from the grid.
                In a grid tied system the batteries will not be low. They will always be close to 100% charge.

                In an off grid system the batteries are driven by the charge controller (which is a separate device for solar, the inverter/charger for generator operation.) The power going to the batteries is power in - power to loads. If you want more power to the batteries, turn off some lights.

                In a scenario where the grid goes away for a few days and reconnects in the morning (unusual) then the full power of the charger in the inverter AND the available solar power is available to charge the battery. What actually happens depends on inverter settings.

                In other words PV production is totally dedicated to charging the battery until it reaches the ABSORPTION phase, and no share of the PV production is sent to the inverter during the BULK charging phase.. Is this true? If so, is it by design (e.g. Ohm's law) or is there something more complicated that involves the charger and the inverter?
                When power returns the inverter/charger AND the solar controller will start charging the batteries. You can disable either one if you want the batteries charged only by grid or solar. Whether you sell any power back is determined by the settings on the inverter.
                - Once the battery reaches the preset ABSORPTION voltage, the charger will keep the voltage constant and the current absorbed by the battery will reduce with time since its internal resistance will increase. The PV production minus the battery's need = power sent to the loads. IS this true?
                Depends. On the Outback GTFX series, when in sell mode, any power that is not needed to keep the batteries above the sell voltage goes to selling back to the grid. If you have a FlexNet DC, then there's an option called "Advanced Grid-Tie" that will prevent selling until a full charge cycle has been completed.

                However keep in mind that you are describing an unusual scenario - recovery from a power outage when there is solar available.

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                • #9
                  Let's add one more wrinkle to the picture:

                  If you have a hybrid system, either AC or DC coupled, using any combination of parts, there are several basic scenarios that may have led you to that solution. Each will involve a different optimal power flow and either different components or different settings to achieve that goal.

                  1. Hybrid for GTI use 100% of the time, but with the capacity to ride through a significant grid power outage.
                  1a. With solar to help supply power during the outage or to keep going indefinitely
                  1b. Without solar contribution at all for battery charging
                  2. Hybrid for use with grid power 100% of the time, but not allowing sell back into the grid. This system may sometimes be storing grid power or PV power at one time for use at a later time.
                  3. Hybrid for 100% off grid use but designed to interact with a local generator so that large loads can be started or driven by a combination of generator and inverter power. Probably incorporating autostart of the generator when loads are high or the batteries low but definitely allowing manual start of the generator before large loads are turned on.

                  There are probably one or two other basic scenarios and lots of variations that I have not covered, but this should give you the idea that when talking about power flows it is necessary to define the operating scenario as well as the parts involved.

                  Note that I am using the term Hybrid to designate any system that has PV and both battery powered inverter and grid tie inverter functionality. That requires batteries.
                  I am not specifying what mechanism is used to charge the batteries.
                  Last edited by inetdog; 09-29-2015, 07:39 PM.
                  SunnyBoy 3000 US, 18 BP Solar 175B panels.

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                  • #10
                    So in conclusion, can we say that there are many scenarios and that the energy flow in a grid-tied with battery system (both DC and AC coupled) depends on the programming of the inverter?

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                    • #11
                      Originally posted by lamagra View Post
                      So in conclusion, can we say that there are many scenarios and that the energy flow in a grid-tied with battery system (both DC and AC coupled) depends on the programming of the inverter?
                      Not entirely. In some systems the operational logic may be part of a separate AC charger and PV CC as well as the inverter. Not all hybrid systems use an integrated inverter-charger.
                      SunnyBoy 3000 US, 18 BP Solar 175B panels.

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