buffer battery size

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NMNeil
NMNeil Registered Users Posts: 5 ✭✭
in Solar Beginners Corner #1
My very first post.
To begin, I am well aware that my 'project' is totally uneconomic, as is my existing small solar set up, but that's not my concern as I'm a devout tinkerer.
My plan is to run my 120 volt inverter mini split A/C from solar, but not in the normal way so I need help calculating the battery bank capacity and possibly other parts of the system.
I live in SE New Mexico and my roof orientation is due south so I can harvest the maximum amount of energy, but I only want to power it when the sun is shining. The house is on a massive 3' thick concrete pad so this will be used as thermal storage to store the 'cold' during the day to keep the house cooler at night.
The absolute maximum the A/C will draw is 1200 watts, but normal running will be around 800 watts, and according to the manufacturer it's soft start so no surge current and it will work happily with a modified square wave inverter.
I understand that I can't run the inverter directly from the panels as we do get the occasional cloud which may shade the panels for a few minutes, so I will need a battery bank as a buffer. The only thing is that I can't work out how much capacity it will need to keep the A/C running for about 15 minutes when the panels output has dropped to about 10% of maximum. (Just a guess number)
My thoughts are for 2Kw of panels, MPPT charge controller and a 3Kw modified sine wave inverter. I would prefer to stay with FLA batteries.
Can anyone help?


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  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    #2
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    1200w load for 15minutes is 300 watt-hours.  A MSW inverter may run the load, but likely not particularly efficiently, so add (eg) another 100wh.  If we keep cycling to say >60% state of charge (40%discharge), we need 400÷.4=~1000wh storage, so something in the 100ah@12v range.  40% discharge rate is quite high, so an AGM type battery capable of that fast a discharge rate would be better in this application.

    The charge controller would need output to control the AC, so it's turned off if battery SOC drops too far before the cloud passes.  This does potentially cycle the battery many times/day.

    Another strategy might be to turn the AC off if pv power < AC load for say 1 min (to allow for orderly cycle completion), and optionally turn on if/when a high-ish temp is hit, and off again when battery hits (eg) 60% SOC.  This effectively uses thermal mass storage to cover short cloud events rather than frequent battery storage deep cycling.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    edited November 2018 #3
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    Get a high end charge controller with programmable relay output, to control a well pump Contactor or inverter enable control line  
    Set the cut-off voltage to Float -0.1V 
    Set cut-in to Float +1.0V     (may have to adjust these a bit for your individual situation)
    won't run batteries down overnight, and you should be able to get by with a small battery.
    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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  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    #4
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    Due to the unpredictability of weather, it would probably be impossible to calculate with 100% accuracy a battery bank capacity that would work. The cycling of the batteries has the potential to leave them in a partial state of charge at times which needs to be replenished, lead acid batteries require long periods of absorption to complete the charging cycle, it would be a hit and miss situation at best, unless some  controls are added to prioritize the available PV to favor the charging needs first. The need to prevent the cooling unit from running without available sun intensity is obviously important to prevent the batteries being unnecessary discharged,  something like this 
    https://www.merkasol.com/SUN-SWITCH-Light-Sensor could be used.to disable/enable the unit allowing the batteries to receive what is available should they need.


    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
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  • NMNeil
    NMNeil Registered Users Posts: 5 ✭✭
    #5
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    Thank you all. A great deal of information to digest
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  • bill von novak
    bill von novak Solar Expert Posts: 891 ✭✭✭✭
    #6
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    NMNeil said:
    Can anyone help?


    To add what other people have said, I'd also recommend a battery charger that kicks on and charges your battery to 100%, say at midnight.  If you are using lead acid, having the battery regulary be at close to 100% charge is important for battery life.

    I would also add that there are now mini splits that will work directly off solar.

    https://www.minisplitwarehouse.com/product/ymgi-12000-btu-220v-solar-assisted-mini-split-heat-pump-air-conditioner-wmms-12ks-v2b56pv-mc295
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