gidtie battery backup

SolarLurker
SolarLurker Solar Expert Posts: 122 ✭✭
I am in the process of having a Gridtie system installed. I choose SMA inverters so that I could use SMA's sunny islands when I am ready to add batteries.

Since I have have my back yard all dug up I want to have a generator installed.

The generator sales person has never worked with PV before, so he is a little unclear about how to tie it all together.

My solar guy has never really worked with gridtie battery with generator.

So what do I need to know to cordinate all of this? Does the generator guy install the system as usual?

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: gidtie battery backup

    Generally, the Generator and the generator controller (solar?) need to talk the same 2 wire or X wire format. Who's controller will do the weekly cycling, warm up, cool down, etc...

    Also, the genset needs to match the load (battery charger, etc.). For a generator dedicated to backup/charging of a battery bank, I like to ensure that the AC charge controller that is selected will operate correctly with the genset.

    Your guys need to exchange specifications to insure that both Power (watts) and KVAR (Volts*Amps) ratings are met. Also, personally, I like to see the battery charger AC load to be around 50-60% or so of the genset's rated output.

    This will ensure that the generator is giving you the best kWatt*Hours per gallon of fuel ratio. Most gensets, if operated at less than 50% electrical loading will output less power, but still consume around 50% fuel (i.e., 50% fuel flow with a 10% electrical load will consume ~5x as much fuel to recharge your battery bank).

    Also, if this is a diesel genset, they usually like to operate at upwards of 75% load for long life (low loading can glaze cylinder walls, coke / carbon up the combustion chamber/exhaust valves, etc.)....

    If you are using this genset to perform other tasks (water pumping, running a shop, etc.)--then those loads should be taken into account too.

    It is possible that you may end up with two generator requirements--a pretty small one to charge your battery bank (at the 5-13% rated C/20 capacity) and a second larger genset for other loads (pumping, shop, A/C, etc.) for best life and fuel efficiency (plus, it does give you a backup genset too).

    Sure would be interesting if you could get the solar guy and the genset guys to talk and workout an optimum solution for you.

    I am sure that others here can give you more/better advise.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • SolarLurker
    SolarLurker Solar Expert Posts: 122 ✭✭
    Re:

    The Generator will be a kohler propane 15res. The SMA Sunny Islands will be used for battery charging.

    My intent was that if power went out I would be able to use the battery bank. should the PV array fail to keep the batteries charged, than the generator would run, powering the house and charging the batteries untill batteries we refilled. Once, the batteries were recharged the generator would shut off.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: gidtie battery backup

    Have you gotten any further information based on what was discussed in this thread?:

    Kohler no longer warrented for off-grid

    How big of battery bank/battery charger set up are you planning on... a 15 kW genset is no small animal.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • SolarLurker
    SolarLurker Solar Expert Posts: 122 ✭✭
    Re: gidtie battery backup

    Thanks Bill

    I did read that previous thread, however my system will not be an offgrid system. As such, I posted my question in the Grid Tie forum. It is not uncommon for us to have a severe winter storm, knocking out power for a few days. I do not want to have to have a generator for and extended time. Especially since out night time loads are very small.

    Also, while I realize it is nit picking, just because something is not warrantied for someting does not mean that it is incapable. My jeep wrangler rubicon was not warrantied for off-road use, however you sould see some of the stuff it did off-road.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: gidtie battery backup

    Its OK--I don't have a dog in this hunt--your choice.

    My question was really more concerning the size of the genset... If you are charging a battery bank for limited night-time power usage--a 15kW genset is a pretty big guy for the job and could end up using 2-5x as much fuel as you really require (if, for the most part, your loads are small, your bank is small, and your AC battery charger is not large either).

    A reason I am a big stickler on fuel is--for many of us, storing fuel is the single biggest expense/danger/aging fuel/etc. for living through extended outages.

    From this data sheet (PDF download):
    [FONT=Fixedsys]
    LP Gas m3/hr. (cfh) kg/hr. (lb./hr.)
    ====================================
    100%    2.4   (85)   4.5   (10.0)
    75%     1.8   (65)   3.4    (7.6)
    50%     1.4   (51)   2.6    (5.9)
    25%     1.0   (37)   1.9    (4.3)
    ====================================
    LP gas conversion factors: 
    8.58 ft3  = 1 lb
    0.535 m3  = 1 kg
    36.39 ft3 = 1 gal[/FONT]
    

    This big guy takes (roughly) 1-2 gallons per hour of propane (0kW-15kW of load).

    Or, I can get a Honda eu2000i (2kW peak, 1,600 watts continuous). Will run on 1.1 gallons of gasoline for 4-15+ hours...

    If you have an 8+ kW battery charger--no problem.... If you have an 800 watt AC battery charger, you should be looking at a much smaller genset.

    For me, 20 gallons of gasoline (need to replace once per year) will last me 10+ days... (run 1/2 day to keep freezer/fridge cool, a few lights, radio in the evening).

    20 gallons of propane will not even last me 1 day using the 15kW genset...

    But for me, my loads are small (fridge, freezer, a few lights, maybe a central heat)--so a small 1,600 watt genset is more than enough.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • SolarLurker
    SolarLurker Solar Expert Posts: 122 ✭✭
    Re: gidtie battery backup

    I understand what your are saying about the size, I would have prefered to go a bit smaller. However, I wanted a low RPM unit that was liquid cooled. The Kohler was the Smallest I could find. I am hoping that with the PV and battery bank the Generator will not need to run that much. Plus, if the battery bank is depleted, than the Generator will need to run the house and charge batteries at the same time. I am not sure how much of a load charging batteries will be.

    Also, from reading stories of the ICE storm in Watertown NY, I realize that have power in the middle of a storm could increase my popularity amoung friends and family, Increasing my electricity demand.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: gidtie battery backup

    I understand your desires... And your needs are different than mine (I have no ice storms--just a little shake and bake--earthquakes and drought).

    Even so--I would suggest that you also look at a second small unit (say a used RV generator)... Get a 3-5 kW unit and your fuel flow will be dramatically lower for much of the time (1-2 gallons per hour, 24-48 gallons of propane a day--if running 24 hours--hopefully your battery bank + solar will reduce that need -- don't know if you have propane or electric heat, etc.)...

    Anyway--I am cheap and tend to take it a bit too far at times. :roll:;)

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • SolarLurker
    SolarLurker Solar Expert Posts: 122 ✭✭
    Re: gidtie battery backup

    I appreciate your advice and ideas

    I really do not know much about PV and electricity. However, I would like to be self sufficient. Until recently I could not afford much in the way of Solar, however the new federal taxes credits and PA grant money has made it feasable.

    What I am really trying to figure out is, does the Generator guy install his normal transfer switch or does the generator feed into the sunny island?
  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    Re: gidtie battery backup

    The generator is connected to the Sunny Island and the gen does not need a transfer switch. The gen should have a 2 wire start: wires connected means generator should start, wires disconnected means gen stop.

    The SI acts as the transfer switch, you can program it to turn the generator on based on 2 scenarios: a battery state of charge threshold, or a load demand threshold. Load demand threshold can be programmed as, for example, 30A load for 1 minute then turn gen on.
    You can also program the SI to turn the gen on periodically to exercise it.

    I think you will need some sort of transfer switch at your grid connection to prevent you from pushing power into the grid when it's down. Your electrician should be able to guide you with that.