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Thread: Generator backup and PV, no batteries

  1. #1
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    Default Generator backup and PV, no batteries

    This question has come up twice in the past couple of days. Potential customers want PV for their house and a generator backup for when the grid goes down, and they want the generator to furnish the grid to the PV inverter so that it keeps running. I see a problem with backfeeding the generator in the case where the inverter output exceeds the load.

    The safe way to do this, of course, would be to put the inverter outside the transfer switch so that it shuts down when the grid goes down, but that's not what they want. They want their PV to keep running when the grid goes down, and they are not interested in spending the money to get a Sunny Island and batteries.

    Is there a safe and cost effective way to do this?

  2. #2
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    Default Re: Generator backup and PV, no batteries

    Depend on your definition of cost effective.

    An XW inverter will do this. With latest software you can connect a grid-tie inverter to its output.

  3. #3
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    Default Re: Generator backup and PV, no batteries

    Quote Originally Posted by RCinFLA View Post
    Depend on your definition of cost effective.

    An XW inverter will do this. With latest software you can connect a grid-tie inverter to its output.
    Aren't XW inverters battery inverters? These guys are not interested in batteries or extra inverters.

  4. #4
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    Default Re: Generator backup and PV, no batteries

    Quote Originally Posted by ggunn View Post
    This question has come up twice in the past couple of days. Potential customers want PV for their house and a generator backup for when the grid goes down, and they want the generator to furnish the grid to the PV inverter so that it keeps running. I see a problem with backfeeding the generator in the case where the inverter output exceeds the load.

    The safe way to do this, of course, would be to put the inverter outside the transfer switch so that it shuts down when the grid goes down, but that's not what they want. They want their PV to keep running when the grid goes down, and they are not interested in spending the money to get a Sunny Island and batteries.

    Is there a safe and cost effective way to do this?
    Probably not a problem. Most GT inverters will cut out when the voltage exceeds a certain level, this level is generally less than the max the voltage the generators can handle.

    I know several people around here who have run Enphase micro-inverters on generator power. Normally what happens is the generator just runs at idle to provide "grid" power/frequency and the micro-inverters handle the load, until the load exceeds the arrays power and then the generator ramps up to handle the rest.

    The micro-inverters cut out at about 264V, but there is enough variation between inverters that I suspect they drop out one at a time if the load is less than your array's output. It takes a few seconds for them to turn back on when the load increases, but the generator handles that.

    How other inverters handle running of generator will probably depend on the inverter. If the inverter scales back power out when line voltage goes up, you're good to go. If however if it produces full power until it hits max voltage and then shuts down, then it's not going to work very well. The inverter will keep turning on and off and likely something will fail.

    Also you'll need a generator that produces a very stable frequency. Many inverters will shut down if the frequency varies from normal by more than 0.5 hz (50 or 60hz depending on where you live)

  5. #5
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    Default Re: Generator backup and PV, no batteries

    I talked to SMA tech support. The guy I talked to said that he gets asked this question on average twice a week, and the answer is "don't do it". He said not to worry about the Sunny Boy, it's perfectly capable of protecting itself, but the control board of a generator is not so robust. He said that the line voltage may rise enough to shut off the Sunny Boy when the load consumption drops below the output of the SB, but the control board may very well be dead by the time that happens.

  6. #6

    Default Re: Generator backup and PV, no batteries

    Quote Originally Posted by ggunn View Post
    Aren't XW inverters battery inverters? These guys are not interested in batteries or extra inverters.
    It does both - it can grid-tie, be off-grid, or do some combination of both (either run from batteries until they are below a certain point and then use the grid, or act as a battery backup to the grid if it goes down). But, it doesn't do it as efficiently as a dedicated grid-tie inverter like the Sunny Boy. So you trade a lot of output over time for the additional ability if they don't want more than one inverter.

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    Default Re: Generator backup and PV, no batteries

    Quote Originally Posted by techntrek View Post
    It does both - it can grid-tie, be off-grid, or do some combination of both (either run from batteries until they are below a certain point and then use the grid, or act as a battery backup to the grid if it goes down). But, it doesn't do it as efficiently as a dedicated grid-tie inverter like the Sunny Boy. So you trade a lot of output over time for the additional ability if they don't want more than one inverter.
    But the question is, can it mediate between PV, grid, and generator without batteries? Can it keep the PV contributing to the load when the grid is down and still protect the generator from getting backfed? Or what if you installed one with a very small battery bank that would virtually always be too little to power the loads?

    Gensets are a lot cheaper than batteries. If there were a way to keep PV running in the event of an outage without having to shell out thousands for batteries and extra inverters, a lot of folks would be interested, especially in hurricane prone areas. A friend of mine who lives just outside the loop in Houston was without power for seven weeks after Hurricane Ike.

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    Default Re: Generator backup and PV, no batteries

    No, the Xantrex XW hybrid inverter needs the batteries... And batteries are still the expensive/weak point/need replacing over time too which drives up the $$/kWH price of the power produced.

    From my point of view, it is still a better solution (hybrid GT/Off Grid inverter) than a pure Off-Grid system. The ability to use grid for the 99% of the time the grid is up allows more energy to be used (over time; and more efficient/less battery losses) and less cycling stresses on the batteries--leading to a better $$/kWH price.

    However, at this point, there is no "cheap" solution for emergency power other than the genset (which can become very expensive to supply fuel for if there is a prolonged outage).

    Conservation so that you have the smallest possible loads and the smallest genset possible to run those loads can reduce fuel usage and storage needs by a whole lot.

    -Bill
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

  9. #9
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    Default Re: Generator backup and PV, no batteries

    Quote Originally Posted by BB. View Post
    No, the Xantrex XW hybrid inverter needs the batteries... And batteries are still the expensive/weak point/need replacing over time too which drives up the $$/kWH price of the power produced.

    From my point of view, it is still a better solution (hybrid GT/Off Grid inverter) than a pure Off-Grid system. The ability to use grid for the 99% of the time the grid is up allows more energy to be used (over time; and more efficient/less battery losses) and less cycling stresses on the batteries--leading to a better $$/kWH price.

    However, at this point, there is no "cheap" solution for emergency power other than the genset (which can become very expensive to supply fuel for if there is a prolonged outage).

    Conservation so that you have the smallest possible loads and the smallest genset possible to run those loads can reduce fuel usage and storage needs by a whole lot.
    Tha's what I thought and I agree, but what we are being asked to supply is PV to offset the grid charges when the grid is up (and sell back to the grid when production exceeds usage), and power the loads from a combination of PV and generator power when the grid is down without having to buy batteries and another inverter. What they are asking for is a way around anti-islanding by substituting generator power for grid power to the inverter. I don't know of a way to do that without putting the genset at risk if the power consumed by the load falls below the output of the inverter.

  10. #10
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    Default Re: Generator backup and PV, no batteries

    In general, I am not sure having solar "GT" supporting a genset is really the best use of technology and $$$...

    If you use solar GT to off set the generator, you are "unloading" the genset... And, as a very rough rule of thumb, unloading the genset below ~50% of rated load, you are saving very little fuel (and with diesels, you run the risk of "coking", "wet stack", "glazing the cylinder walls", etc.).

    My own two cents, would be to run a "Hybrid" (Grid Tied/Off Grid--if GT is supported by your utility) or the Xantrex/Schneider XW Hybrid inverter (with generator assist--possibly has the option for "grid assist" where Grid Tied is not allowed). And use set the system to load the genset >50% rated load while charging the battery bank.

    You only run the genset when needed and you run it in its fuel/mechanically efficient power band with the minimum run-time necessary to keep your batteries up (when loads exceed sunlight).

    Cycling batteries does wear them out faster--but with proper charging/cycling, you can still get many good years (or even a decade or more) with the batteries cycling.

    Without going into a whole bunch of assumption driven math--I would have to believe that running a "hybrid system" (solar PV + hybrid inverter + genset running >50% rated load to charge battery bank/support AC loads while charging) is going to be better/more cost efficient than trying to use GT solar (without battery bank) trying to "support" a generator by the time fuel costs and the 3,000-10,000 hour typical geset life.

    If we want to make some assumptions (generator size, fuel flow, costs,life, battery bank costs/life, efficiencies, amount of backup power needed per year based on primary genset vs primary hybrid inverter, etc.), we could try some simple $$$/kWH math and see what falls out.

    I would not be surprised if the costs all came out to a "wash" (similar $$$/kWH costs) for most of the various options (with one or two being way more costly).

    -Bill
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

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