hybrid grid tie performance

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mikeo
mikeo Solar Expert Posts: 386 ✭✭✭
in Grid Tie and Grid Interactive Systems #1
I just checked the performance today of my grid tie system. I think it fell about where I would expect it to, which is not as bad as some on this site has suggested in other posts for battery based grid tie systems. I have an kilowatt hour meter on my AC output and have my MX60 set to display in AC kilowatts the daily output which by my understanding derates by some amount of power of which I don't understand completely. Anyway the MX60 AC kilowatt output read 9.8 kWh's today, and my Kilowatt hour meter by estimating the fractions of the last digit came out to about 8.3 Kilowatt hours. 8.3/9.8 = about 85% efficient in converting the PV output to AC power. Does this sound about right by other peoples experiences? I am alway trying to figure out how to tweak the MX60 to provide maximum performance. I also designed and built a Basic Stamp micro controller that talks to the SW4024 serial port and turns it off when the PV current drops below 1 amp. I don't turn on the SW4024 in the mornings until the PV current is above 6 amps which gives the battery some time to bulk up each day. This saves me the idling power of the inverter for about 16 hours a day that otherwise would diminish the efficiency.
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  • boB
    boB Solar Expert Posts: 1,030 ✭✭✭✭
    #2
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    Re: hybrid grid tie performance

    >>I don't turn on the SW4024 in the mornings until the PV current is above 6 amps which gives the battery some time to bulk up each day. This saves me the idling power of the inverter for about 16 hours a day that otherwise would diminish the efficiency.

    This sounds like a good idea. The SW draws around 15 Watts or so, so you're about right on at 1 Amp.

    Also, inefficiencies will be lower when you lower the sell voltage to 'just above' what the batteries need to be floated, if you can adjust it to sell at that point.

    Find what the surface charge voltage of your batteries are, and then add a wee bit to that for your sell voltage. Also keep the Bulk/absorb time to a minimum to keep from running the meter forwards too much. I would personally do an absorb charge once in a while. Not sure exactly what the optimum "once in a while" is, but maybe once a week maybe ?
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #3
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    Re: hybrid grid tie performance
    mikeo wrote: »
    660 amp hour battery bank

    What voltage is your battery bank ?

    Are you off grid, or grid tie ? Do you run off your battery bank daily, or is it stand-by usage only?
    Powerfab top of pole PV mount | 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 ,

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  • mikeo
    mikeo Solar Expert Posts: 386 ✭✭✭
    #4
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    Re: hybrid grid tie performance

    I'm grid tied with battery backup and 24 volt battery's, 1800 watts of PV located in the Ozarks.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #5
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    Re: hybrid grid tie performance

    From what I understand--the lead acid battery losses are much more sever when you cycle them, recharge and start to generate gas--and, of course, when you are equalizing and most of the energy goes into making hydrogen and oxygen gas...

    If you have a pure grid tie system, you are certainly avoiding a ~10% loss for discharge and bulk charging, and probably almost the other 10% in absorb charging (where the batteries are starting to out gas) since, presumably, your bank is (hopefully) sitting at long term float most of the time (not charging, not out gassing).

    The above would be my guess based on the, rough estimate, that AGM's (which should not gas) are about 90% energy efficient and wet cells are roughly 80% efficient (leading to a guess that 10% goes to out gassing, and the other ~10% losses go to cycling).

    So, you should be left with ~95% charge controller efficiency and ~95% grid tie inverter efficiency... .95x.95=90% overall efficiency--probably a bit high--the 85% measured number is probably closer to reality (~92%*92% efficiency)...

    Running Grid Tied with battery backup (not using batteries at night for load shifting/etc.) is probably the most cost efficient method of operating your system... You get higher efficiency and longer battery life because you are not cycling the batteries every day (assuming you are taking good care of them--appropriate charge voltages and watch the water usage).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Solar Guppy
    Solar Guppy Solar Expert Posts: 1,989 ✭✭✭
    #6
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    Re: hybrid grid tie performance

    I have done untold hours running gridtie thru an GTFX3048 doing my charge controller design work ...

    One thing I have found is you can't lower the sell voltage much below the float level ( 54V ) or the FX will drop offline with even moderate irradiance changes ... the internal sell/tracking is very slow to respond to battery voltage changes. Maybe if one has an MX60 networked its better ... I had a small 100ah/48V bank for testing ( long since replaced )

    As far as efficiency, the FX is OK in the mid power range, drops off hard on the high end and runs exceedingly hot in the non-vented verison I own. The batterys, even floating agms will gulp down 10-20 watts on a small battery floating

    85% seems doable with an Outback branded controller and inverter and small battery bank, keep in mind the MX60 isn't a percision watt meter so that would effect your calculations

    The New Xantrex XW line is the most efficient hybird system around, the inverter is 95% peak for sell mode ... almost as good as the GT series.
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  • mikeo
    mikeo Solar Expert Posts: 386 ✭✭✭
    #7
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    Re: hybrid grid tie performance

    Thanks all! I will play around with the sell voltage and see what it does. So far the only effect I get seems to be playing with the inverter float voltage and the MX60 float voltage. If the MX60 goes to float mode then it starts to continuously sweep, current drops to 0 and then the inverter gets turned off. So I have the MX60 float set to 28 volts and the SW4024 float set to 26.8 volts(cool weather setting). This keeps the MX60 in MPPT mode and causes the battery voltage to bounce around 27-28 volt range during the day, average around 27.5. As I adjust the inverter float voltage down, it pulls the average battery voltage down and as the weather warms up I have to adjust the inverter float voltage down to around 26.0 to keep the MX60 from going into float mode at 28 volts. I have been keeping the sell voltage below the float voltage on the inverter.
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  • boB
    boB Solar Expert Posts: 1,030 ✭✭✭✭
    #8
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    Re: hybrid grid tie performance

    Sounds like you're doing the right things.

    The basic idea is to adjust the charge controller higher than the sell voltage so you create a sort of "Tug of War" between the two units... The CC is trying to bring the voltage up and the inverter is trying to bring the battery voltage down. This works well as long as the inverter can sell all the current that the CC has to offer.

    Sounds like you might have a bit of oscillation where the system sometimes stops selling and starts again and stops again and starts again, etc.
    Maybe a wider differential voltage between inverter and CC ??

    boB
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