AC Coupled mini-grid (micro-grid)

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  • gww1gww1 Solar Expert Posts: 963 ✭✭
    Re: AC Coupled mini-grid (micro-grid)
    For a budget system, and really for most systems, there isn't a compelling reason to use AC coupling at all. Much simpler and more efficient to go the classic DC route: panels to charge controller to battery bank to inverter.

    If you had really heavy daytime loads that stressed your inverter to start and run, would ac coupling be more efficiant and possibly remove stresses from your current systems?

    You know like as an add on without adding batteries and such to a system already there.

    Thanks
    gww
  • BB.BB. Super Moderators, Administrators Posts: 32,005 admin
    Re: AC Coupled mini-grid (micro-grid)

    Generally, the Off Grid Inverter needs to be equal to, or larger than, the GT inverter.

    And, if you are OK with the Off Grid Inverter "collapsing" if the sun goes behind a cloud...

    Also remember that the typical Off Grid Inverter will supply upwards of 2x rated power for surge load. And a GT inverter has no surge capabilities.

    In more detail, a GT Inverter typically outputs a Power Factor of 1.0 ... Meaning that the Off Grid inverter needs to still supply current (VA) for inductive and non-PF corrected loads.

    I don't really see a GT inverter as providing power that would limit stress/size of the OG inverter system.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: AC Coupled mini-grid (micro-grid)
    gww1 wrote: »
    If you had really heavy daytime loads that stressed your inverter to start and run, would ac coupling be more efficiant and possibly remove stresses from your current systems?

    You know like as an add on without adding batteries and such to a system already there.

    Thanks
    gww

    Depending on the exact system, yes AC coupled GTI could work like "gen support" on the fancier inverters. But as with all of them you have to be able to control that input (which does not work with all OG inverters) and the solar provided power is only there when the sun shines so it's not quite as handy as having either actual generator support or large battery bank & array.
  • gww1gww1 Solar Expert Posts: 963 ✭✭
    Re: AC Coupled mini-grid (micro-grid)

    I was just trying to figure a postion that tallgirl posted on the outback forum. Basically someone had out grew their system and was wanting to expand. She pointed out that ac coupling was much easier to control if it was added at a smaller copacity then the dc system. Say 6000 watt dc system and adding a 3000 watt ac couple. The dc part would shut down first helping to control the grid tie part.

    I keep getting hung on the imposible, like trying to suplimenting the grid running on good days my 3 ton ac and getting heat to supliment propane heat on good days in the winter.

    I can't cover my total ac use, don't want to buy new ac, and don't think my inverter will start/run the old ac.

    Just learning about this incase I win the lottery or just go crazy and want to try something.

    Can't buy enough batteries so just may want to supliment on good days with out them.

    Thanks
    gww
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: AC Coupled mini-grid (micro-grid)

    What Tallgirl was talking about is that the back-feed of excess power from the AC coupled GTI will push the batteries up causing the DC charging system to "back down" as it sees the batteries are "full"; in essence the AC coupled charging will take priority.

    The trouble is in the practical application where that power is only available as the sun shines because unlike the (balanced) DC side of things you can't store up the capacity to use when needed. In other words adding 1kW AC coupled is not the same as adding 1kW DC with storage capacity because the AC supplies only that 1kW as the sun shines whereas the DC could store up 1kW * hours of sun and have that capacity available anytime.

    Yes; it all costs money. You can't drive a Ferrari for the price of a Fiat. :roll:
  • gww1gww1 Solar Expert Posts: 963 ✭✭
    Re: AC Coupled mini-grid (micro-grid)
    Yes; it all costs money. You can't drive a Ferrari for the price of a Fiat.

    How true but if your inverter won't run the ac at all and you can't cover all the power it uses, you might be able to suppliment what you do use for 4 hours and it might be the cheapest way to get that till your ac goes out and you get some minisplits. Or not.
    Thanks
    gww
  • stephendvstephendv Solar Expert Posts: 1,571 ✭✭
    Re: AC Coupled mini-grid (micro-grid)
    NorthGuy wrote: »
    May not be that inefficient. Sunny Boys claim to be 98% efficient, compare to MPPT+Inverter at 85%, so you get much better utilizaton of solar when used directly. When going to batteries, you get (Sunny Boy + Sunny Island) efficuiency at 90%, compared to MPPT at 95%. Depending on the usage pattern, SMA system may turn out to be more efficient.

    Yes, the AC coupling is more efficient for direct loads, but for a "budget" system based on year-round use, you'd be looking for the best efficiencies in winter, not summer. Also, a GTI is considerable more expensive than a top of the line MPPT controller.
    I threw some numbers together to compare the efficiencies of the two systems. Attachment not found.
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: AC Coupled mini-grid (micro-grid)
    maskani wrote: »
    Hello all, I'm reading all of your post here, they are so useful and I thanks to all. I think Dr. Strenglov said true about sunny webbox, sunny webbox is necessary for programming inverters but after that it can be removed.

    So, I have a question, is it possible to replace Sunny Island with a Ups and V/f Controller? I want to setup a not expensive system, with a 350w microinverter, and 200 or 300 watt UPS, maybe I can design a V/f controller, too. SMA is so good, but it's so expensive. we can't find customer with this prices. we must build a system with lower power and not expensive. if we can build this system, we can sell more and more pv system, and expanding usage of renewable energy in the world.
    One important feature of the Sunny Boy/Sunny Island pairing is the SI's ability to slightly change the frequency of the microgrid when the batteries are fully charged and the loads on the AC side are low or off, coupled with the SB's feature where it responds to this frequency change by throttling back its output. This is important to keeping the batteries from getting overcharged.

    But it looks like the 5000W SI might be overkill for what you want; Outback has a number of smaller off-grid systems which might be better for your purposes. As I said in the other thread, your odds of succeeding by cobbling together equipment which is not designed for your application are, IMO, very low. I suspect that designing a one-off solution likewise isn't going to be a viable path for you, either.

    Forums like this one are frequently asked how a grid tied inverter can be fooled into thinking the grid is up when it's not. It's just not easy or practical to do.
  • NorthGuyNorthGuy Solar Expert Posts: 1,913 ✭✭
    Re: AC Coupled mini-grid (micro-grid)
    stephendv wrote: »
    Yes, the AC coupling is more efficient for direct loads, but for a "budget" system based on year-round use, you'd be looking for the best efficiencies in winter, not summer. Also, a GTI is considerable more expensive than a top of the line MPPT controller.
    I threw some numbers together to compare the efficiencies of the two systems.

    Interesting graph. AC coupling seems to win where it is 60+ % of energy going direct to the loads. It sure is true in winter (when efficiuency is at a premium).

    If you use transformerless Sunny Boy (even more expensive), which is 98% efficent, you get better results.

    It's hard to tell if AC coupling is inherently more expensive, or SMA is more expensive compared to others.
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