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  • Newb's ask foolish questions . . . I should know, I am a Newb!

    About a year ago I had a professionally installed grid-tie system added to my home. It's a standard grid-tie with no battery backup. I've not really given a lot of thought to how the system works -- but now I am wondering something and I can't seem to find the answer (or I've not hit on the right google search phrase), so here is a foolish question:

    When the grid goes down I know that the inverter can "sense" this and shuts off the flow of power to my breaker box (and thus, isolates the power from the grid -- I get that this is important to protect the lineman working to repair it), but where is the power that the panels are producing "going" (I mean, it's not like the sun stopped shining, so the PVC's are still making power)?

    I've not had any significant power outages since installing the system, but I am wondering if I should have investigated "Hybrid" systems that allow you to switch to off grid use during a blackout. So here is another foolish question (actually two): Is it too late to add the components to make it a hybrid system and would it be worth the expense?

  • #2
    Hello JSKoehler and welcome to Solar Panel Talk. Some more info would be needed, what type (make and model) inverter do you have now. I think you have answered your own question though, if you rarely have black outs why go to the added expense? When the grid goes down the inverter can see this and shuts down (goes into stand by mode) so you are not inverting any DC current to AC, some call this power shedding, the panels are still producing current but its not being inverted into anything your house can use.

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    • #3
      Thanks. Off hand, I am not sure of the make or model (Newb!) -- and yes, my feeling was that it would be a needless expense (thus guaranteeing a major ice storm will hit my region knocking out power for days).

      I get that the DC is not converted to AC when the grid is down, but where is the DC power going -- does it somehow dissipate safely. I guess I have to simplistic a view of electricity -- but in my mind I see the DC current flowing to the inverter and being "told" "Sorry but the grid is down, so I can't use you now." It seems to me that the DC power would have to be discharged somehow,right? Or have I got the wrong end of the stick?
      Last edited by JSKoehler; 11-15-2016, 01:49 AM.

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      • #4
        Originally posted by JSKoehler View Post
        I get that the DC is not converted to AC when the grid is down, but where is the DC power going -- does it somehow dissipate safely. I guess I have to simplistic a view of electricity -- but in my mind I see the DC current flowing to the inverter and being "told" "Sorry but the grid is down, so I can't use you now." It seems to me that the DC power would have to be discharged somehow,right? Or have I got the wrong end of the stick?
        Power is voltage times current.
        A PV module has a curve - it has a "max power point" and two spots where zero power is generated.
        The zero power generation are at maximum voltage (but zero current)
        And at maximum current (but zero voltage)
        In either case zero power is being generated (and sent to the inverter)
        So there is zero power that needs to be sent to the grid (or disipated as heat)

        My guess is that when there is a grid issue the modules go to near Voc - the inverter goes to an operating point that is at (or near) the highest possible voltage, but with very little current.
        Not much different than if you cut the wires to the modules. Sure there's a lot of voltage, but no current flowing, so now power. (and I say "or near" because it might be just very close to the Voc, but still have some tiny amount of current flowing. And all the power/current gets disiplated as heat by the inverter.)

        Hope that explains things.

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        • #5
          When the grid goes down the only power that the GTI is drawing from the panels is the very small amount needed to run its electronics to watch to see whether the grid has come back and maybe to report information to any network monitoring that is in place.

          The remainder of the potential power from the panel array is simply not produced in the first place. So there is no need for it to be dissipated anywhere.
          You could think of it as having a 300HP gasoline engine and letting it run at slow idle. You do not need to figure out where the additional 299 HP is going when the engine is idling; it is simply not being produced.
          SunnyBoy 3000 US, 18 BP Solar 175B panels.

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          • #6
            Nice to both explanations. Though I do appreciate the technicals foo1bar. Thanks to the OP for asking, also.

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            • #7
              Originally posted by JSKoehler View Post
              About a year ago I had a professionally installed grid-tie system added to my home. It's a standard grid-tie with no battery backup. I've not really given a lot of thought to how the system works -- but now I am wondering something and I can't seem to find the answer (or I've not hit on the right google search phrase), so here is a foolish question:

              When the grid goes down I know that the inverter can "sense" this and shuts off the flow of power to my breaker box (and thus, isolates the power from the grid -- I get that this is important to protect the lineman working to repair it), but where is the power that the panels are producing "going" (I mean, it's not like the sun stopped shining, so the PVC's are still making power)?

              I've not had any significant power outages since installing the system, but I am wondering if I should have investigated "Hybrid" systems that allow you to switch to off grid use during a blackout. So here is another foolish question (actually two): Is it too late to add the components to make it a hybrid system and would it be worth the expense?
              For you and your array, when the grid is down, there is effectively no power generated so there is nothing, to go, anywhere.

              When the grid is up and your array is producing electricity, about 15 -20 % or so of the solar energy your array intercepts winds up as electricity. The remaining 80 - 85 % winds up as heat that gets rejected to the environment. If the grid is down, about 100 % instead of, say 85 % of the intercepted solar energy winds up as rejected heat, just like a dumb, black slab sitting in the sun.
              Last edited by J.P.M.; 11-15-2016, 03:31 PM.

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              • #8
                I think what happens it the panel goes to Voc, and has a soft break over to allow the current to recirculate back internal to the cell. So
                it probably gets hotter disconnected than in operation, anybody ever check that? By loading it to a lower voltage we get a lot of the
                current instead, but taking too much current will bring the voltage down so less energy is taken. Bruce Roe

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                • #9
                  Originally posted by bcroe View Post
                  I think what happens it the panel goes to Voc, and has a soft break over to allow the current to recirculate back internal to the cell. So
                  it probably gets hotter disconnected than in operation, anybody ever check that? By loading it to a lower voltage we get a lot of the
                  current instead, but taking too much current will bring the voltage down so less energy is taken. Bruce Roe
                  Much more likely a charge distribution and voltage gradient is built up inside the semiconductor material such that the dislodged electrons simply do not make it across the junction to the other side, so no net accumulation of charge beyond that point and no increase in voltage.

                  The theory says that if you either leave a panel open circuited or short circuit it the light hitting the panel that is not reflected will all go to heat. Same equilibrium panel temp either way.
                  But if you put an MPP load on the panel some energy will be carries away electrically and the panel will end up cooler.
                  SunnyBoy 3000 US, 18 BP Solar 175B panels.

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                  • #10
                    I don't know the physics, but the model shows a forward biased diode. A silicon diode doesn't conduct much till
                    the voltage rises across it, which I see as what happens approaching Voc. Voc limits V with all the current breaking
                    over. Bruce Roe

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                    • #11
                      Like any switch, when you turn it off, no current flows, thus no power. Power does not go anywhere, it is not being used.
                      MSEE, PE

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                      • #12
                        Originally posted by bcroe View Post
                        I don't know the physics, but the model shows a forward biased diode. A silicon diode doesn't conduct much till
                        the voltage rises across it, which I see as what happens approaching Voc. Voc limits V with all the current breaking
                        over. Bruce Roe
                        The model shows a forward biased diode, but it does not visually represent the fact that electrons are given kinetic energy, and hence the ability to move against a voltage gradient, when the semiconductor layer is hit by photons.
                        Because that energy moves them across a junction at the same time as it moves them in the voltage potential, they are not able to simply fall back again.
                        SunnyBoy 3000 US, 18 BP Solar 175B panels.

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