Ac and DC coupling on a single offgrid system.

Hi All,
I am seeking your thoughts on an idea. Is it possible to set up an off grid system to handle both ac and dc coulping. For example, during the day when running heavy load, a current sensing relay is used to activate the GT when the heavy load is turned on and the drawing amps pass a certain point. Thus the GT then supply the power to this load. While on the other side of the system, the cc are used to charged the batteries and run the other loads. Because the GT would only be activated with the relay, is a system like this practical? what are your thoughts if any?
thanks in advance

Comments

  • bill von novakbill von novak Solar Expert Posts: 870 ✭✭✭✭
    Re: Ac and DC coupling on a single offgrid system.
    surveyor1 wrote: »
    Hi All,
    I am seeking your thoughts on an idea. Is it possible to set up an off grid system to handle both ac and dc coulping. For example, during the day when running heavy load, a current sensing relay is used to activate the GT when the heavy load is turned on and the drawing amps pass a certain point. Thus the GT then supply the power to this load. While on the other side of the system, the cc are used to charged the batteries and run the other loads. Because the GT would only be activated with the relay, is a system like this practical? what are your thoughts if any?
    thanks in advance

    You could do that but - why? A good charge controller + hybrid inverter is close to the efficiency of a dedicated GT inverter.
  • BB.BB. Super Moderators, Administrators Posts: 29,719 admin
    Re: Ac and DC coupling on a single offgrid system.

    In theory--A GT Inverter connected to an Off Grid inverter that does frequency shifting based on battery state of charge--If the OG inverter decides the battery bank needs charging, shift to 60 Hz and GT inverter turns on 5 minutes later. OG Inverter decides battery bank is full--Shift to ~+/- 1.0 Hz to knock GT inverter off line.

    Would work and regulate. "Bang Bang" controller design--Not great for batteries but should work.

    Or as Bill von Novak said--Just get an MPPT controller and pick Vmp-array voltage between ~100 VDC and 400 VDC (there are several controller choices that can cover different Vmp-array ranges). Better for battery charging/regulation.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • stephendvstephendv Solar Expert Posts: 1,571 ✭✭
    Re: Ac and DC coupling on a single offgrid system.

    Have to side with what Bill von Novak said: Why would you want to do this?

    When designing my system, I had already decided on the sunny island as an inverter which has full support (and was designed for AC coupling)- but doing the maths on efficiency and economics- there really wasn't a reason to use AC coupling for a pure off-grid home. DC through a good MPPT controller is both cheaper and more efficient when it counts.
  • LongboatLongboat Registered Users Posts: 11 ✭✭
    By the way: Off-grid system. Conext SW4024, MPPT 60-150 (2), Combox, SCP, 3000w Renesola panels. Trojan l-16s. Adding 3000w of AC coupled panels and (eventually) LFP batteries. Off-grid for 30 years. AC couple is the only way to add load capacity to my existing system without replacing the inverter altogether - it is an affordable solution with m215s and new panels. Considering a relay cut-off or relay load dump load as well to protect the batteries. Any thoughts on which is best practice - and firing the relay - with the CCs or? Thanks, Bill.
  • bill von novakbill von novak Solar Expert Posts: 870 ✭✭✭✭
    Longboat said:
    By the way: Off-grid system. Conext SW4024, MPPT 60-150 (2), Combox, SCP, 3000w Renesola panels. Trojan l-16s. Adding 3000w of AC coupled panels and (eventually) LFP batteries. Off-grid for 30 years. AC couple is the only way to add load capacity to my existing system without replacing the inverter altogether - it is an affordable solution with m215s and new panels. Considering a relay cut-off or relay load dump load as well to protect the batteries. Any thoughts on which is best practice - and firing the relay - with the CCs or? Thanks, Bill.
    I'd recommend setting up several strings of inverters (i.e. four AC strings) each with its own breaker and relay.  Then build a controller that turns off the strings sequentially as the battery voltage approaches maximum.  The biggest problem for many AC coupled system is the massive transient you get when all the AC coupled generation drops at the same time, via the relay.  By using four relays the transient is cut by 75%.  You also get more use out of your array as you approach maximum voltage, since instead of none of the array generating, a fraction of the array will be generating.

    For extra credit you could use an intelligently controlled AC diversion load to heat water with the excess power.
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