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  • Battery System Size and Price - NOT Off Grid

    A customer has great incentive to install a PV system. However, the utility will not allow them to interconnect as their service is from a "secondary network" style connection. This gets technical, but so far the utility won't budge.

    Not being able to send power into the grid is not really an issue as the building would always consume what is produced. However, serving load without utility back-up when the system is not producing or even "modulating" is a problem.

    My take is that a battery system would need to be employed. Batteries would need to be sized to "buffer" the output, which would feed load through a transfer switch. The transfer switch would route utility power to the load when the PV system was not producing. The load would have to be able to see short power interruptions without consequence - the controller could possibly provide a mechanical system a warning it's coming. This might be a air handling fan.The utilization voltage would be 480/3.

    Any ideas as to 1) sizing the batteries if I serve a 35kW load that runs 50% of the time 2) price difference (assuming $7/watt installed with standard grid-tie) and 3) % efficiency loss?

    Obviously the goal is to use everything produces without oversizing batteries required.

    Does the reduced efficiency also reduce the amount of batteries I need as the system can't put out near is rating?

    Thanks for discussing!

  • #2
    I don't even want to think of the size of a battery bank,
    that could feed a 3 phase 408V system.

    I'm just wild guessing here, I suppose your battery bank needs to be at the low end of your solar PV array voltage, so it can charge from it.
    So you have a 500VDC battery bank! And some sort of charge controller for it.
    And you run your factory that makes widigts from the inverter supplied AC . Batteries handle short events like a small cloud, but eventually at night, you have to switch to utility power. (there is not enough battery capacity to run a factory all night, not even in a submarine). But, you will have to pay the utility company "stand-by fees" just to run the wires to the factory and leave the meter hooked up. for a huge array, installed cost should be less than $5/w I have no idea what the battery cost would be.
    Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister

    Comment


    • #3
      I don't want to think about the batteries either but need to tell the customer their "options". I think batteries=bad most of the time. This is an existing building that already has utility service, there is no effort to be totally "off-grid" here.

      Is a 500V battery bank outrageous? Isn't that just over 40 batteries in series (I sure hope 1 battery doesn't go bad). Is a charger/inverter like this commercially available?

      Don't be put off by the 480/3V power, it's normal utilization in a commercial building. It doesn't change the amount of power used. I think you'll see this as an option with larger inverters.

      Your description of the operation is good, but it's only for a portion of load in the building (maybe some air handling fans), not the entire thing.

      Also, if a typical DC to AC derate of a grid tie system is .77, what do you think it is for a system with batteries?

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      • #4
        Originally posted by adoublee View Post
        Also, if a typical DC to AC derate of a grid tie system is .77, what do you think it is for a system with batteries?
        Solar PV DC to AC is 95% of harvest.

        Batteries on float take that down a couple of %

        Batteries on a daily cycle is only about 50%


        What you need is some sort of Grid Tie, that cannot feed back to the utility, but
        still keep the utility for clouds, rainy days and nighttime. I don't know if anyone makes
        that, or just put a one-way ratchet on the meter, so the utility never sees it go backwards, but at noon, consumption is zero.
        Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
        || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
        || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

        solar: http://tinyurl.com/LMR-Solar
        gen: http://tinyurl.com/LMR-Lister

        Comment

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