Advice on System Design for DC Loads

solartek
solartek Solar Expert Posts: 69 ✭✭✭✭
I have a potential client who wants to power a dc load using solar and batteries. He has access to utility power but only wants to use that as a backup source to charge the batteries if necessary. The load draws around 19A at 12V and needs to run for about 10 hours a night. I will eventually size the battery bank and solar array based on his location, but for now I'm concentrating on possible design approaches.

I've attached a simple block diagram of the first design that came to mind. It uses a solar charge controller, an AC battery charger, a relay to control when the battery charger needs to be turned on and off, and a relay to disconnect the load in the event the batteries don't have sufficient charge to power the load for whatever reason.

I've since though of a second approach using an inverter/charger setup similar to what is done in grid-tied solar electric systems with battery backup. Think Xantrex XW or Outback FX. But in this case nothing would be hooked up to the AC output of the inverter. One advantage of this approach would be the possibility of selling excess solar energy back during the summer months since the solar array will be sized based on the minimum solar insolation for the location.

I've already made clear to the client that solar and batteries will never be the most economical way to handle his load, particularly since he has utility power available.

I would appreciate any advice on the potential merits of the two approaches or any other approach you may think of.

Scott.

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Advice on System Design for DC Loads

    For the Relay Logic... I would look at using a Battery Monitor like the Xantrex series which has a programmable output (turn AC on if <75%, turn AC off if >85% or whatever value you like).

    Much more accurate than any other voltage based model and would reduce the amount of AC power used (not wasted on last 15% charge, optimize to keep battery above 75% state of charge.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Solar Guppy
    Solar Guppy Solar Expert Posts: 1,989 ✭✭✭
    Re: Advice on System Design for DC Loads

    MorningStar also offers a smart relay module that is very configurable and can be networked with either the pwm or Mppt charger controllers.
  • halfcrazy
    halfcrazy Solar Expert Posts: 720 ✭✭✭
    Re: Advice on System Design for DC Loads

    I second morning starts relay driver it is a nice piece of hardware
  • solartek
    solartek Solar Expert Posts: 69 ✭✭✭✭
    Re: Advice on System Design for DC Loads

    Thanks for the replies. I've been leaning the Morningstar route for the charge controller and relay driver if I use that design approach.

    Anybody have any comments on the second design approach versus the first?

    Scott.
  • AntronX
    AntronX Solar Expert Posts: 462 ✭✭
    Re: Advice on System Design for DC Loads
    solartek wrote: »

    Anybody have any comments on the second design approach versus the first?

    Scott.

    For second design, you would have to go 24V or 48V system voltage, and use DC-DC converter to step down to 12V. Like this one for 24V: http://www.powerstream.com/dc1-extreme-25A.htm Or 48V > 12V (cheaper too) http://www.powerstream.com/dc48.htm

    I like second design at 48V if the customer would ever want to add more panels later and for power outage backup purpose. But this design is a lot more expensive.

    Edit: BTW, nice talking to you at Orlando RE Expo on Oct. 24.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Advice on System Design for DC Loads

    the morningstar mppt and many others do downconvert with the morningstar itself limited from up to 36v. the powerstreams are examples too of this downconverting, but who would pay that much $ to get so little in capability by going with the powerstreams?
  • Solar Guppy
    Solar Guppy Solar Expert Posts: 1,989 ✭✭✭
    Re: Advice on System Design for DC Loads

    The SunSaver Mppt from MorningStar ( 15Amp ) has a Maximum input spec of 75V.

    The New TriStar Mppt from MorningStar ( 45 and 60 Amps models ) is 150V, similar to the Outback, Xantrex Mppt units
  • solartek
    solartek Solar Expert Posts: 69 ✭✭✭✭
    Re: Advice on System Design for DC Loads

    I agree with Solar Guppy that for the first design approach the Morningstar TriStar MPPT charge controller and Relay Driver are a good combination. What about for the battery charger? I'm leaning toward one of the Iota DLS series.

    Scott.
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: Advice on System Design for DC Loads

    using the 2nd approach (Grid-tie inverter) it has an integral battery charger.

    Charger will need to be sized according to the battery bank, but I don't think the IOTA has a way to place in stand-by. You could build up a relay - use the morningstar MPPT controllers generator start, as a way to enable the IOTA.
    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 ,

  • solartek
    solartek Solar Expert Posts: 69 ✭✭✭✭
    Re: Advice on System Design for DC Loads
    mike90045 wrote: »
    Charger will need to be sized according to the battery bank, but I don't think the IOTA has a way to place in stand-by. You could build up a relay - use the morningstar MPPT controllers generator start, as a way to enable the IOTA.

    That's what the Morningstar relay driver and relays are for in the first design approach: to toggle on and off the battery charger and/or load based on battery SOC and load demands.

    Scott.