Battery and array voltage 24 vs 48

RaydogRaydog Registered Users Posts: 3
Raydog (newbie) here with questions about voltage both in the battery
bank and the pv array. I want to design a new system . We (wife and self
plus sheep, goats horses etc) are in South Central Washington state.
Lots of sun and wind. The energy budget is 4.5 kw hours per day (2 freezers+).
Lots of space. The questions: What are the postive and negative aspects
of 24 volt vs 48 volt battery banks? And similarly, with each ( 24/48 ) what
is best pv array voltage to run? I have been lurking for a bit, and probably
know just enough to get in to deep doodoo. Love the site and REALLY
appreciate the expertise of all of you
Thanks a bunch

Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,019 admin
    Re: Battery and array voltage 24 vs 48

    Welcome Raydog,

    Basically, you are talking about a large system... So you will want a full sized MPPT (Maximum Power Point Tracking) Solar Charge Controller (Outback & Xantrex are two major brands to look at).

    Regarding bank voltage--unless you have a specific device that needs a lot of DC voltage (12, 24 or 48 volts)--your best bet for a larger system is to pick the highest voltage that makes sense... And 48 volts is usually it.

    Comes down to current, voltage, and power:

    Power=Voltage*Current=Watts

    If you can keep the maximum current to 100 amps or less--the size of the wiring, fuses/breakers, switches, etc. are kept to a cost effective size.

    P=48v*100a=4,800 watts
    P=12v*400a=4,800 watts

    Both are the same (and the total size/cost of batteries) will remain the same...

    However, there are some devices that can run at 12-48 volts (nominal), but the same current... For example, the solar charge controller max. out at 60-80 amps... To charge a battery bank:

    60amps*15volts (max 12v battery voltage)=~900 watts maximum
    60amps*60volts (max 48v battery voltage)=~3,600 watts maximum

    The same controller can handle 4x the number of solar panels at the higher voltage. So, it costs 1/4 the amount for the "charge controller" used in larger systems.

    If you look at the costs of TSW inverters, there is not much of a difference in price between a 24vdc and 48vdc unit (48 volt inverters will probably have higher wattage ratings at the same price vs lower voltage units--although you might be limited to fewer vendor choices at 48v).

    Also, personally, I prefer a series string battery bank at 48 volts, vs a parallel string of batteries at 12 volts--although our host (Windsun) has not seen any real difference in battery life between series vs parallel connected batteries (assuming proper installation and maintenance).

    Regarding PV array voltage--If you use a MPPT Controller (typically 140 VDC operational maximum input voltage)--they higher PV array voltage means you can install the array farther from the battery/controller shed--higher PV voltage means lower PV current, smaller gauge wires and/or lower voltage drop back to the battery shed... There is a little extra losses in the Charge Controllers (couple percent?) when running at the higher PV input voltage--but the less loss, and lower wiring costs, probably make up for much of the difference).

    There is a configuration issue with certain "high voltage" solar panels with high Vmp (Vmp ~48+ volt) where one panel is not enough voltage to charge the battery bank (need ~62+ volts) and two panels in series is higher than the controller maximum of 140-150 VDC (Voc open circuit voltage) on very cold days. So--before laying out the money, make sure your configurations work on paper first.

    The "high voltage" panels are fine--but they are designed to work with Grid Tied Inverters which have Vpv input of ~200-600vdc... Not with battery charge controllers and 48 volt battery banks (this will work on 12 and 24 volt battery banks).

    Back to the other question... If you want 4.5 kWhr of AC power per day to run your freezer (135 kWhrs per month). Just as a rough sizing of solar panels... Use the PV Watts program. Enter in 1 kW, and 0.52 for solar panel derating factor (assume flooded cell batteries and using AC inverter... the rest, defaults. Lets assume that you want to size the solar array to run this load 9 months of the year--and you will use a generator (when required) to help during the winter/bad weather.

    Tossing the lowest 3 months (~34-36kWh per 1kw of solar panels), we get 52kWhrs per month in February.

    1kW of solar panels * 125kWhr/52kWhr = 2.4 kW of solar panels for 9 month no-genset operation

    The above numbers are a bit on the conservative--but are a pretty solid starting point.

    More questions?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Battery and array voltage 24 vs 48
    The questions: What are the postive and negative aspects of 24 volt vs 48 volt battery banks? And similarly, with each ( 24/48 ) what is best pv array voltage to run?
    Raydog,

    48 V systems are usually “better”:

    Everything else being equal, 48 V inverters are typically more efficient than 24 V models. Everything else being equal, a solar charge controller configured for a 48 V battery bank can handle twice the array power of a system configured for 24 V. 48 V battery banks allow for fewer battery strings for the same stored energy content

    Assuming an MPPT controller with DC-DC step-down and a 48 V battery system, the array should usually be configured for 60 V (nominal). PV array voltage drops as the array heats up, and there’s voltage drop in the wiring and in the controller. A 48 V array will often have trouble charging a 48 V battery bank in the summer, when array temperature can reach 35 C above ambient. The winter Voc of a 72 V array might exceed the charge controller’s input voltage limit.

    NEC Article 690.7 ("Maximum Voltage") uses PV module specs and local record low temperature data to estimate cold temperature corrected array Voc. There are plenty of modules available with which to configure a "60 V" array that'll work very well over a broad range of temperatures.

    Assuming an MPPT controller with DC-DC step-down and a 24 V battery system, the array should be configured for 36 V (nominal), 48 V (nominal), or 60 V (nominal). The controller will operate at higher efficiency with a 36 V array, but the 60 V array will allow for smaller size wire between the array and the controller.

    Another concern is array mounting style. An array mounted on and parallel to a roof and with under 6” of clearance may operate at ~35 C above ambient. An array mounted on a frame on a flat roof will operate at ~30 C above ambient. A pole-mounted array with plenty of clearance to the ground will operate at ~25 C above ambient. The 10 C range translates into ~4% to 5% mid-day operating efficiency.

    HTH,
    Jim / crewzer
  • n3qikn3qik Solar Expert Posts: 741 ✭✭
    Re: Battery and array voltage 24 vs 48

    Is there a reason you need/want to be off-grid. At 4.5 KW+ daily, you are looking a big $$ in batteries and backup generator.
  • RaydogRaydog Registered Users Posts: 3
    Re: Battery and array voltage 24 vs 48

    Neil and Bill
    Thanks very much for the comments. I ave a much better idea of what
    I am looking for now.
    Ken-Nearest power pole is 1 1/2 miles away. There is a small 12 volt
    system now, but it is tired (batteries about gone, mismatched panels).
    I am really just camping now. I have a 6000 watt genset for the well and
    the battery system, and plan on making changes there -sometime.
    All comments welcome.

    Ray
  • BB.BB. Super Moderators, Administrators Posts: 32,019 admin
    Re: Battery and array voltage 24 vs 48

    It might be worth at least getting a quote from the power company on what they would charge to string a line and price per month (kWhrs+whatever charges)...

    If you have an idea of how much power you would want/need to use (setup a works shop with lots of big, power hungry, motors, welders, etc... or just a simple little home using less than 200 kWhrs per month...

    Basically, just run a costing model assuming 20 year payback:

    • (Total cost of lines + power used over 20 years) / (# of kWhrs used over 20 years) = $ / kWhr...

    Run one with minimal kWhrs per 20 years, and another with heavy power use over 20 years...

    Generally, grid power is around $0.10-$0.30 per kWhr. Off grid power is around $1-$2 per kWhr... See where the "power line power" costs fall in.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Battery and array voltage 24 vs 48
    Raydog wrote: »
    Neil and Bill
    Thanks very much for the comments. I ave a much better idea of what
    ...................................

    Ray


    sorry, but i can't take credit for comments i did not make. i believe it is jim that you should be thanking.
  • RaydogRaydog Registered Users Posts: 3
    Re: Battery and array voltage 24 vs 48

    Neil
    my apologies and thanks to Jim- typing and thinking at the same time can be
    difficult for some of us

    sez ray
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