Need help with grid backup system

petectpetect Posts: 6Registered Users ✭✭

Hi ALL. I’ve been struggling with designing a PV system, and hope that someone here can offer some advice that might point me in the right direction.

 I’m looking to have a 3 – 4K system.   I currently have (10) Trina 320 W modules.

 MY GOALS:
System operating similar to Xanterx “utility backup / Outback “minigrid “ mode. Draw power from the grid to supplement pv, with NO SELLBACK

I need the main charge controller & inverter to be dependable & fairly flexible
I would like to have a small battery bank – mostly to help with appliance “start” surges.

CHARGE CONTROLLERS: MPPT Charge controllers(s) to handle modules.

 24V battery. The modules. will be 50 – 60 away so copper is a consideration. I may add (possibly different) modules later, or need to split them unbalanced strings, so the ability to handle unbalanced loads is a BIG plus.

Easily varied charge rates is a must.

 INVERTERS: Ability to do “utility backup – NO SELL BACK IS A MUST.

Dependable Pure sine wave is needed. Might add a “Chinese wonder” later for noncritical loads. I can easily separate house circuits.

It seems like Optimizers used with a higher voltage inverter ( Fronius ?) could make setting up the module strings easier, and reduce wiring costs,  but would that work in the “mini grid” mode. What does that do to the battery bank – charging and inverter dc inputs of 300+V from the modules and 24V from the batteries.

 BATTERIES: To get things started I will probably use a couple 12V deep cycle batteries that I have. Want to switch to another type later – possibly LIPo4.

Min. record daytime temp here is about 0 deg. F.

Inverter(s) brand / model recommendations please. Fronius seems to make some interesting units – I wonder about service as they are non-us made.

 It seems the more I dig into this, the deeper in the hole I get.

BTW  I don’t expect any savings from this system to pay for itself. It’s gust something I really want to do. And of course keeping costs down is a must.

Thanks in advance.
Pete


Comments

  • littleharbor2littleharbor2 Posts: 814Solar Expert ✭✭✭✭
    petect said:


    CHARGE CONTROLLERS: MPPT Charge controllers(s) to handle modules.

     24V battery. The modules. will be 50 – 60 away so copper is a consideration. I may add (possibly different) modules later, or need to split them unbalanced strings, so the ability to handle unbalanced loads is a BIG plus.


    I see Outback is coming out with a 300 volt charge controller. This would allow you to run your panels in just two strings. saving you the cost of combiners and fusing. You will also save on copper wiring at such high voltage.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

  • mike95490mike95490 Posts: 7,562Solar Expert ✭✭✭✭
    Schneider (XW series gear) & Morningstar both have 600V PV string charge controllers, which in a 48V system, would need only 1 controller. 

    Why try to do this with only 24V ?
    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 ,

  • petectpetect Posts: 6Registered Users ✭✭
    Hi Guys
    Thanks for the suggestions.
    Mike   Where I live it's not uncommon to have power outages lasting for a few hours to a few days - or more. Without fail we have several days of clear skies after each weather event.  I mainly want the batteries for startup surges (fridge, freezer, oil burner). I can use a gasoline powered generator for my well.
     I have access to a couple 12V deep cycle marine batteries that I can use to get started. But I want to switch to LiFOP4 as soon as possible. The cost of even 50ah of LiFOP4  at 24V is very high, 48V isn't a possibility.
    Thanks again
    Pete
  • mike95490mike95490 Posts: 7,562Solar Expert ✭✭✭✭
    i'd suggest doing it with Eight, 6V 200ah golf cart batteries to learn the ropes with (at 48V)

    Common mistake - assuming a 48V bank costs 2x what a 24V bank cost.  
    Only the voltage doubles - not the cost
     Instead of 50ah @ 24V, you buy 25ah @ 48V.  That's mostly a pack re-arrangement.  Same watt hours in either bank.
    Some incremental increase for the extra BMS gear, but I hope over the next 2 years, the BMS systems and charging options will settle down so each install can use presets, and not rely on the owner/installer caculating some wrong setting that frys the bank in the first week.
    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 ,

  • petectpetect Posts: 6Registered Users ✭✭
    Mike  Thanks for the suggestions. I've been messing with LAB's, including sealed and AGMs for a lot of years. I've never done 48V, but lots of 24V stuff. I talked with a couple of guys who have been using LiFePO4 for a couple of years and (other than the cost) they were both pretty enthusiastic about that type of battery. I DO appreciate that LA can't be beat for the bang for your buck, but I'm ready to leave behind the maintenance rituals LA require,

    Adding to the cost of LiFePO4, it seems that some CC manufacturers say they need 100ah minimum. of battery. I can't get  an answer as to why, and what would happen if I were to use a smaller bank. In the mean time, I can get a couple 12V marine deep cycle batteries to tide me over, but unless things change, I still like the idea of LiFePO4.
    Thanks again
    Pete
  • mike95490mike95490 Posts: 7,562Solar Expert ✭✭✭✭
    some CC manufacturers say they need 100ah minimum. of battery. I can't get  an answer as to why, and what would happen if I were to use a smaller bank
    Odd.  i've seen ah minimums listed for Inverters, to help deal with the AC ripple, but not for any charge controller.
    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 ,

  • petectpetect Posts: 6Registered Users ✭✭
    Hi Mike      I'm sure you're right. I've been doing a lot of reading lately as I try to decide on components for BOS. The minimum ah I mentioned is probably for inverters. BTW    I see you are using NiFe. VERY COOL! Are they restored Edison, China made?
    Thanks again, and have a Happy New Year
    Pete




  • EstragonEstragon Posts: 2,047Registered Users ✭✭✭✭
    I wonder if there may be a higher risk of a too high charge rate with a too small lithium bank. Could be a problem with some sealed LA too, but maybe more so with lithium?
    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
  • mike95490mike95490 Posts: 7,562Solar Expert ✭✭✭✭
    Estragon said:
    I wonder if there may be a higher risk of a too high charge rate with a too small lithium bank. Could be a problem with some sealed LA too, but maybe more so with lithium?
    There is, but you can usually (in most higher end charge controllers) set a max amps limit for charging. This allows you to over panel a system to get more harvest in cloudy weather and less generator runtime

    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 ,

  • karrakkarrak Posts: 271Solar Expert ✭✭✭✭
    The upfront costs of LFP (LiFePO4) are high but when you calculate the cost per kWh of power cycled through them over their lifetime they work out cheaper. This is only true if you are using them on a regular or full time basis. For backup power it is probably still cheaper to use LA batteries.


    As far as I can see there are three different options if you want to change to Lithium Ion batteries
    1. Get the individual cells, design and put together a BMS which can be connected to the charge controller and the inverter to switch them off or disconnect the battery from the charge controller and inverter if there is a problem with one of the cell voltage going out of its safe operating zone . I would pick LFP cells if you are doing this as they are the safest of the lithium ion battery technologies and should last the longest.
    2. Get a battery with inbuilt BMS which will keep the battery balanced and will disconnect the battery from the outside world if there is a fault that could damage the battery.
    3. Get a highly integrated inverter/charge controller and battery package like a LG RESU10 and Schneider inverter/charge controller or Tesla Powerwall 2 with compatible inverter/charge controller.
    The third options is the easiest and requires the least technical expertise to set up. Interestingly these highly integrated systems can be quite cost competitive with larger DIY systems.
    The LFP batteries in the second option work out at around twice the price of just buying the cells by themselves.
    With the first option you are responsible for setting up the BMS, battery safety and commissioning the battery correctly.

    For long lifespan and safety you want to limit the charge current of LFP batteries to around 0.5C and the discharge current to around 1C with peak short term current to less than 2C.

    We have been very happy with the performance of our LFP battery which is nearly five years old now.

    Simon


    Off-Grid with LFP (LiFePO4) battery, battery Installed April 2013
    32x90Ah Winston cells 4p8s (24V), 4kW Latronics Inverter, 1160W of Solar Panels, homemade MPPT controller
    Homemade BMS https://github.com/simat/BatteryMonitor
     

  • petectpetect Posts: 6Registered Users ✭✭
    THANKS GUYS!!   I really appreciate your advice. You're helping to flatten my learning curve, and probably reducing the chance of me making a costly mistake.
    I'll be back
    Pete 
  • jonrjonr Posts: 930Solar Expert ✭✭✭✭
    From what I can tell, the Tesla Powerwall 2 is quite limited in control-ability and only works in a grid-tied/AC design.  It can charge from excess solar (AC only), but as soon as it's full, it's going to allow excess solar to flow to the grid.   
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