How much power, via battery, do I need to provide clean AC signal to Enphase microinverters.

bill32
bill32 Registered Users Posts: 2

I have installed a grid-tied system.  The Enphase inverters automatically shutdown when the clean utility line AC signal is lost, which is normal.  I am thinking about installing the minimum battery system, with an inverter to feed the Enphase inverters, so they will operate during a daytime power failure.   I don't find anything on-line with such info.  Need details as to exactly what equipment I need, and also if this is practical.

System details:  Two separate arrays, one with 40, 285 watt panels and another with 18, 285 watt panels.  All powered by Enphase M250 micro inverters.

Thanks,

Bill

billscobra@gmail.com

 


Comments

  • Photowhit
    Photowhit Solar Expert Posts: 6,002 ✭✭✭✭✭

    I think in general terms they are designed not to work this way.

    They have come out with some high voltage charge controllers such as the MorningStar TriStar MPPT 600V which can use your current strings to charge a battery bank. I thought Schneider had one as well, but didn't hunt much, not my end of the ocean...

    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    It can be done with an off grid True/Pure Sine Wave AC inverter (correct brand/model)--You connect the off grid AC inverter to a battery bank--Then connect the Enphase units to the output of the off grid inverter.

    There are several control methods... One is to get a designed for purpose off grid AC inverter that will "shift" the AC frequency from 60 Hz to 62/58 Hz -- That frequency shift with "knock off-line" The Enphase inverters. When battery bank needs charging again, the off grid inverter goes back to 60 Hz.

    A second method is to put a relay (mechanical or solid state) on the Enphase output to the off grid AC inverter. Use a voltage controller (or even modified dump/shut solar charge controller) to "turn off" The Enphase inverters when charging is not needed.

    A third method is to install a dump controller on the DC battery bank. When the battery bank is full, the solar dump controller turns on and send current to a resistor/heater bank--Dumping excess charging current from the Enphase (basically how a typical wind turbine charging system works).

    If you have a larger 48 volt battery bank... You would want a maximum Enphase array of 1,000 Watts per 100 AH @ 48 volt battery bank (or 1kW per 200 AH @ 24 volt, or 1kW per 400 AH @ 12 volt).

    In general, if you do not have frequent/long power outages (less than once per year, less than 2 weeks outages)--You might be better off with a genset and stored fuel (propane, fuel stabilized diesel or gasoline, etc.). I have a Honda eu2000i (1,000 Watt genset) that will run ~4-10 hours per gallon of gasoline. 20 gallons of stored fuel (siphoned from cars) will last me 10 days pretty easily.

    If you are still interested in battery backed power--You might decide what the minimum system you need (say 1,000 WH per day--lights, cell phone, laptop) power by battery bank + solar array) and use the genset just during the day for larger loads/drawing down the fridge/freezer.

    While it is possible to use your Enphase units for backup solar power--I would suggest not doing that unless you really like to tinker--It is a lot of work for something that is rarely used... And batteries have a typical life of 3-5 years (warm climate, typical "cheap" 6 volt @ 200 AH golf cart batteries) in this type of service. A "pickled genset" will be easier to use when needed for many folks.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • bill von novak
    bill von novak Solar Expert Posts: 891 ✭✭✭✭

    I have installed a grid-tied system.  The Enphase inverters automatically shutdown when the clean utility line AC signal is lost, which is normal.  I am thinking about installing the minimum battery system, with an inverter to feed the Enphase inverters, so they will operate during a daytime power failure.   I don't find anything on-line with such info.  Need details as to exactly what equipment I need, and also if this is practical.

    System details:  Two separate arrays, one with 40, 285 watt panels and another with 18, 285 watt panels.  All powered by Enphase M250 micro inverters.


    You can do this (it's called AC coupling) but for it to work you have to have a battery/inverter combo at least equal to the power of your GT inverter system.  In other words, if you have a 16kW system, your inverter/battery has to be able to accept 16kW of charge power to "balance" the system.  If not the voltage will very quickly run away.  If you are lucky the inverters will shut down before anything blows.

    With AGM batteries that means you would need around 48kwhr of storage, and you'd need at least three Radian inverters to be able to handle the 16kW.
  • zoneblue
    zoneblue Solar Expert Posts: 1,220 ✭✭✭✭
    Exactly. Its not a matter of the off grid inverter just providing a sine wave for the GTI to sync to, not at all.
    The GTI is designed to pump everything it has into the grid and/or house.  These hybrid systems as others have said are expensive, and most people will be better off spending the money on a wee Honda.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


  • WaterWheel
    WaterWheel Registered Users Posts: 375 ✭✭✭

    This topic is way above my knowledge level and I agree that a genset would be a more cost effective solution but since the system is split between 2 different arrays he could probably minimize the cost by breaking out only the smaller 18 panel array and run it through two (may be able to get away with only one)  Conext 80-600s to a 440 amp/hr 48v battery bank and then on to a Conext XW+ inverter.     Probably loose some efficiency dong this.     Since I don't know much about micro-inverters he may need to do as BB suggested to keep the Enphase inverters on the smaller array pushing power.  

    Leave the larger array hooked directly to the grid.

    Conext XW6848 with PDP, SCP, 80/600 controller, 60/150 controller and Conext battery monitor

    21 SW280 panels on Schletter ground mount

    48v Rolls 6CS 27P

  • jonr
    jonr Solar Expert Posts: 1,386 ✭✭✭✭
    edited June 2016 #7
    There are various algorithms used to detect the grid and it's not clear which one(s) the Enphase uses (even if they did disclose some information in a patent).  Maybe one could see it on a scope by using a small current sense resistor.

    In general, a GTI tries to disturb the grid's sinewave waveform and then detect if it is successful.  If so, it shuts down.  So simulating the grid with a non-GT inverter would be best done with a design that can (briefly) also absorb power from the lines.  Ie, one that provides active waveform correction.  No idea how one knows in advance which inverters do that well.

    AFAIK, a UL1741 GTI will shut down (or perhaps throttle back) before driving the output above 264VAC.

    I am available for custom hardware/firmware development