Confused About AC vs. DC Coupling

digitalabacus

New Member
Joined
May 3, 2022
Messages
5
Hello all,

First time posting here, been lurking and learning.

I am a bit confused about when AC vs. DC coupling is better for a given setup. From what I have read so far, this coupling refers to the mode of energy ingress to the primary inverter from a battery backup or similar, either AC or DC. Makes sense so far. (side note: does anybody know what is being coupled in this terminology? Battery to primary inverter, battery to grid, grid to household via battery, etc?).

What I can't wrap my head around is why you would prefer one to another. AC holds the advantage for power transmission efficiency, sure, but trying to sync up transfer between AC sources sounds like a real bear when considering things like glitching and phase differences between the two AC waveforms. Given that just about every picture I have seen (a comparatively small sample size, sure) each has all of the various system components mounted right next to each other, it seems that transmission losses are minimal anyway. Why wouldn't you just DC couple and save the cost of the extra inverter?

This confusion is further compounded by items like the Sol-Ark 15k brochure (link) which shows both a DC battery input and a 24kW AC coupled input. My gut is telling me that this product is all about options so you can choose either or perhaps wire up the AC output of a generator or something.

So, in short, why would a given installation choose DC coupling vs. AC coupling?
 

wattmatters

Photon Sorcerer
Joined
Apr 16, 2021
Messages
1,513
So, in short, why would a given installation choose DC coupling vs. AC coupling?
There are probably a multitude of reasons no doubt.

In Australia (where vast majority of solar PV systems are grid tied), then the choice of an AC or DC coupled system can often be due to local regulatory restrictions on the size of inverter which is permitted to be connected to the grid.

e.g. in some areas (QLD for instance) there is a 10 kVA limit on inverter capacity which may be connected per phase, and the total inverter capacity includes any battery inverter.

So for instance if you had an existing 8 kW solar PV inverter then you would not be permitted to instal a Tesla Powerwall 2 as it has its own 5 kW inverter. You'd be limited to an AC coupled battery system with a maximum inverter capacity of 2 kW.

But with a hybrid DC coupled system you can have a 10 kVA inverter which manages both the solar PV and the battery in one, enabling the battery to supply high power demands and recharge more quickly as well.

Such regulatory limitations are not the same across Australia, in many areas battery inverter capacity is not included in the grid connection limits for small scale generation systems.

AC coupled batteries are convenient in a grid tied scenario as you can install a solar PV system now (which here are very financially beneficial) and add a battery later when/if it makes financial sense (they don't). Some have tried to "future proof" by installing a hybrid inverter in the expectation they will add a battery later. Unfortunately the experience is that after a couple of years there are no more compatible batteries available for that older hybrid inverter model and so the extra up front expense of the hybrid was not worth it.
 

digitalabacus

New Member
Joined
May 3, 2022
Messages
5
I see. So, if I am understanding correctly, it seems like the regulatory concern is mostly about ability to backfeed the grid? Does this imply that, in an AC coupled system, the AC coming off of the panel inverter(s) can "see" the grid AC and vice-versa, hence the regulatory regulatory involvement? And, if so, how is AC waveform distortion prevented? If I understand correctly, much effort is spent on "syncing" up the national electrical grid so all of the generation is in-phase (which is why it takes time to "restart" the electrical grid after a regional outage).

For my specific case, I am working backwards. I work from home as an engineer and, unfortunately, power outages are becoming more commonplace. Obviously, this will not work for business so my plan is to install an inverter and battery backup now and then add panels a bit later. Should satisfy the immediate need of not losing power while leaving options for installing panels a few months later.
 

SparkyJJO

I have that name for a reason.
Joined
Jan 31, 2022
Messages
292
Location
Ohio
I'm doing AC coupling because I already have grid tie microinverters. If I was DIYing my whole system, I would not do AC coupling.
 

wattmatters

Photon Sorcerer
Joined
Apr 16, 2021
Messages
1,513
So, if I am understanding correctly, it seems like the regulatory concern is mostly about ability to backfeed the grid?
Both DC and AC coupled system can feed the grid, if permitted.

To me the only difference between them is that AC coupled just means the battery has its own inverter. But happy to learn what others think.

Any inverter which is grid-tied has to go through a process of syncing to the grid's frequency. They are grid following devices. It's a normal part of the start up cycle. They will go through a process to monitor the grid and adjust their internal frequency until it matches and then it will connect.

When you are off grid (be it permanently or temporarily), then one of your inverters becomes the grid forming supply source, and anything else using or connecting to the AC supply has to sync to it.

For example, in grid tied set up with a solar PV system and a Tesla Powerwall 2 (which is an AC coupled battery), when the grid goes off the Powerwall's Gateway and inverter immediately takes over supplying power to the home and it becomes the grid forming supply source. The solar PV inverter then follows the grid forming inverter. The Powerwall then uses frequency control to manage the solar PV inverter's output.

When grid power returns, the Powerwall will take a bit of time to adjust its frequency so that it matches the grid before switching back to grid connection.

In a DC coupled system, the battery shares the same inverter as the PV array and grid connection, if any.
 

digitalabacus

New Member
Joined
May 3, 2022
Messages
5
I'm doing AC coupling because I already have grid tie microinverters. If I was DIYing my whole system, I would not do AC coupling.
Gotchya, so this is a particularly beneficial option if you already have existing infrastructure / a microgrid setup / a Tesla powerwall / etc. which already produces AC.

Any inverter which is grid-tied has to go through a process of syncing to the grid's frequency. They are grid following devices. It's a normal part of the start up cycle. They will go through a process to monitor the grid and adjust their internal frequency until it matches and then it will connect.

When you are off grid (be it permanently or temporarily), then one of your inverters becomes the grid forming supply source, and anything else using or connecting to the AC supply has to sync to it.
I see, I hadn't even considered that these would be "smart" on the AC following side. In hindsight, that makes obvious sense.

Thank you both for the feedback; this makes much more sense now. It seems like, for my particular use-case, the fact that I already have battery capacity in mind and am planning to use a monolithic hybrid inverter solution "from scratch" (e.g. minimal preexisting infrastructure) indicates that I should just use DC coupling (pending review of applicable capacity regulations, as described by @wattmatters).
 

wattmatters

Photon Sorcerer
Joined
Apr 16, 2021
Messages
1,513
Gotchya, so this is a particularly beneficial option if you already have existing infrastructure / a microgrid setup / a Tesla powerwall / etc. which already produces AC.
Yes. This is very often the case in Australia, where 25-30% of homes have a grid tied solar PV system but only a tiny fraction have a battery.

Of those much smaller number of homes with a battery, the most popular battery choice is a Tesla Powerwall 2. Here it is available as an independent retrofitted device. Expensive though, typically ~A$15,000 installed for 13.5 kWh of capacity, but that's about a typical cost for such battery installations here.
 

toms

Photon Sorcerer
Joined
Jul 24, 2020
Messages
1,052
Reasons why i use AC coupling (20kwh off grid)

- inexpensive second hand PV and GTI availability
- compatibility with batteries
- simplicity of system setup

I also have a 10kwh off grid system that is solely DC coupled, for different reasons. It really is dependent on your individual case.
 
Top