Need help Designing 10KW S PV System on Paper

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  • emccemcc Solar Expert Posts: 44
    Re: Need help Designing 10KW S PV System on Paper

    Please clear my confusion. If I have array of 5600 Watt, will a 5000 Watt Inverter Charger would be fine or I need a bigger Inverter/charger?
  • NorthGuyNorthGuy Solar Expert Posts: 1,925 ✭✭
    Re: Need help Designing 10KW S PV System on Paper
    emcc wrote: »
    Please clear my confusion. If I have array of 5600 Watt, will a 5000 Watt Inverter Charger would be fine or I need a bigger Inverter/charger?

    The inverter size is not directly related to the size of the solar array. It only determines how much load you can put on the inverter. Or, for charging, how much charging current you can get for your batteries.
  • BB.BB. Super Moderators, Administrators Posts: 31,419 admin
    Re: Need help Designing 10KW S PV System on Paper

    Think of the battery bank as the "heart" of your system... For a start, look at the battery to inverter (ignoring the solar array) and design the output power system. Then look at the battery bank to solar array connection (and ignore the loads/AC Inverter) and design the Solar Charging system. Then do the same thing with the AC Backup Power system (again ignoring loads).

    The one issue to look at both loads + charging together, is to understand your "base loads" during the day (when the solar power is charging). We recommend that the solar charging rate be around 5% to 13% of the battery bank's AmpHour capacity. If you have significant steady state loads during the day, then you should add to the solar array AmpHours out to account for the base load reducing battery charging current.

    For example, say you have a 100 AH battery bank (at 20 Hour battery discharge rating). And you want a 10 amp rate of charge (10%). But you have a 5 amp load during the day. So, 10 amp charge - 5 amp load = 5 amp to battery or 5% rate of charge--Lower than your initial charging requirement. So, in this example, you would want a 15 amp charging current from the solar array to give you a 10% rate charge to the battery bank during the day.

    Similar for your genset/backup AC charging. If your genset will be supplying charging current to the battery bank and sustaining AC inverter loads too--You have to add the bank charging current+AC load currents together for AC charger's output current requirement. If you run AC loads directly from the genset, then the AC charger can be smaller, but the backup genset still needs to power both the Battery Charger and the normal AC loads too.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • emccemcc Solar Expert Posts: 44
    Re: Need help Designing 10KW S PV System on Paper

    Ok, like if I have a 1000Ah Battery of C20 rating, then the chargin current would be 50A. Right? Now this is for a 100% Discharge. At 50% DOD, it can recharged with in 10 hours?

    So, if during that 10Hours, my total load is another 50A , so I would require a System capable of producing 100A during day?
  • BB.BB. Super Moderators, Administrators Posts: 31,419 admin
    Re: Need help Designing 10KW S PV System on Paper
    emcc wrote: »
    Ok, like if I have a 1000Ah Battery of C20 rating, then the chargin current would be 50A. Right? Now this is for a 100% Discharge. At 50% DOD, it can recharged with in 10 hours?

    Yes, from a math based point of view... Normally, I would suggest adding another 2-6 hours of "absorb time" (where the battery is held at "absorb voltage" for another 2-6 hours to fully recharge the battery bank).

    Normally, we would suggest you don't cycle a battery bank lower than 50% for longer battery life. If you are using forklift type batteries, they do cycle the batteries deeper--But have very big chargers and 16 hours of charging time to bring the batteries back to full charge.
    So, if during that 10Hours, my total load is another 50A , so I would require a System capable of producing 100A during day?

    To be clear, if you have a 50 amp "steady state load" (say your computer server farm), and you want a C/20 rate of charge (another 50 amps), then, yes, at a minimum you would need 100 amps from your charging source to get 50 amps to the battery bank itself.

    Note--You still have to account for the base load too (i.e., 50 amps * 24 hour per day = 1,200 AH per day), so if you were "pure solar", your array would need to be much larger to recharge the bank + support the 50 amp * 24 hour per day base load. This may also make the battery bank requirement larger too (i.e., a battery bank has a maximum charge current--typically around C/8 -- So to recharge the battery bank in X hours of sun per day, you need a larger array which would push for a larger battery bank to safely absorb the charging current).

    Of course, if you have grid and/or generator power to support charging when the sun is not up--Then you can extend your charging time and (possibly) keep the battery bank on the smaller side.

    We are getting a bit into the difference between a "typical" off grid home where a person uses power at night and recharges during the day. You have (I presume) a mixed use system where you both use power and generate (via solar) during the day, and use power during the night too (backup power, waiting for grid to come back up). You have to look at each operating mode of your system and determine the requirements/not to exceed numbers.

    Solar without loads
    Solar with loads
    Solar without loads + backup generator
    Solar + loads + backup generator
    Solar without loads + backup generator + AC grid
    etc...

    You might try designing the system with Grid + Battery Bank as your "base system" requirement (i.e., design the system to operate purely as a 8 hour afternoon power outage + 16 hours of grid per day). Then start hanging on the AC Generator (backup power for X days of no power. Then, lastly, figure out how much solar array you want to add (perhaps in stages) to reduce fuel usage, then reduce AC Grid usage--While not exceeding the charging current + loads rating(s) of your system.

    It makes your calculations/design requirements a bit more complex (issues of maximum charging current, adding loads+solar, etc.). Given that you still have utility power most of the day, you can at least recharge outside of "daylight" hours when needed/is cost effective.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • emccemcc Solar Expert Posts: 44
    Re: Need help Designing 10KW S PV System on Paper

    Thank you for detailed information. But I guess things will bemore easy in practical than only calculations. Once we start the work after exact Load figure, we will start the solar system. Till then a 50KVA Genset would do the work.
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