Is AC coupled even worth it?

[realpinochet]

I've been looking at the xwpro and radian systems which would be paired with a charge controller DC coupled...A traditional system. But, I ran across a article about AC coupling which from the way it read is where you have a string inverter that coverts PV DC to AC and then Injects that AC into a main panel. It seems like AC coupling wasn't meant to be paired with a battery and just merely offset electric grid draw in the daytime. The xwpro, solark, raidan etc was listed in the article as a battery inverter as opposed to a PV inverter. The reason I'm curious about AC coupling is the article said it's more efficient if your daytime load is high because you don't have to convert from DC to AC like a battery inverter. But, isn't that exactly what's a happening; you still have a inverter converting DC from the panels into AC. What's so much more efficient than having the DC from the charge controller being converted to AC by the battery inverter as it arrives in on the bus? Since I run a 4ton AC in the summer constantly I was thinking of looking into this AC couple way of doing things. Like I said it appears like string inverters don't really have a way to attach batteries so in my case I'd have to do something called AC/DC coupling? Schneider / Radian shows a way of doing it where you plumb in the output of the AC string inverter into the AC output load side of the battery inverter thus injecting AC into the loads side. From reading about this it seems cumbersome and potentially dangerous as this solution requires phase shifting to tell the string inverter to shutoff if too much power is being produced and the batteries are full and this seems to be fickle and might blow batteries up.

https://www.solar-electric.com/lib/wind-sun/Conext-XW-Pro-SW-AC-Coupling-Guide.pdf

https://www.outbackpower.com/downloads/documents/appnotes/acc_freq_app_note.pdf

I found where another person just separated his loads he wanted AC coupled to a sub panel and put the string inverter there so he didn't have to AC couple his DC coupled setup. For those that have AC coupled by itself or with a traditional DC coupled solution is it worth it?

 

[Ampster]

For those that have AC coupled by itself or with a traditional DC coupled solution is it worth it?

Yes it is worth it in terms of flexibility.
AC coupling has some benefits but they can be unique to each user. Almost two years ago I installed a Grid Tie System using micro inverters because of the simplicity of wiring and that they natively solved the Rapid Shut Down requirements of my building authority. I knew I was going to install a hybrid with batteries so I could have power when the grid was down, and therefore knew I was going to have to AC couple to get the micros to work when the grid is down. My AC coupling decision was driven by my needs and desire to find a cost effect way to have a battery backup as well as a GTf system. Now that I had a basic system I added some DC coupled solar panels which requiredenough panels to meet the minimum voltage of the MPPT controller. I also recently acquired some used solar panels that I can put on a barn 100 feet away from my hybrid. Stringing DC that far will be a pain but since I have a sub panel in the barn I can add micro inverters to those panels and add capacity in increments.

Ninety Nine percent of the time my two systems work on their own even though the AC coupled system passes through my hybrid inverter on the way to the grid. In the mornings the AC coupled inverters can help support battery charging. On the few ocassions when the grid is down the AC coupled solar can also charge the batteries and serve the loads at the house.

 

[BentleyJ]

AC Coupling is generally used only in situations where there is a grid-tied system already in place and the user wishes to add a grid forming inverter which will accomplish 2 things. 1) Provide back when the grid is down which also serves as the source or power to keep grid-tie inverters active and producing. 2) Disconnect from the grid for safety per regulations.

In cases where there is no system in place already it doesn't really make sense to go with an AC coupled system + Battery Inverter system. I suppose one could make the case that going from PV directly to AC is slightly more efficient than PV to MPPT charge controller to batteries to AC inverter but considering the cost and complexity of a combination AC and DC coupled system it has to be special circumstances that justify that configuration such as what Ampster just posted while this was being written.

 

[Ampster]

it has to be special circumstances that justify that configuration such as what Ampster just posted while this was being written.

I agree, the simplicity of a DC coupled system has its advantages.

 

[realpinochet]

Yes it is worth it in terms of flexibility.
AC coupling has some benefits but they can be unique to each user. Almost two years ago I installed a Grid Tie System using micro inverters because of the simplicity of wiring and that they natively solved the Rapid Shut Down requirements of my building authority. I knew I was going to install a hybrid with batteries so I could have power when the grid was down, and therefore knew I was going to have to AC couple to get the micros to work when the grid is down. My AC coupling decision was driven by my needs and desire to find a cost effect way to have a battery backup as well as a GTf system. Now that I had a basic system I added some DC coupled solar panels which requiredenough panels to meet the minimum voltage of the MPPT controller. I also recently acquired some used solar panels that I can put on a barn 100 feet away from my hybrid. Stringing DC that far will be a pain but since I have a sub panel in the barn I can add micro inverters to those panels and add capacity in increments.

Ninety Nine percent of the time my two systems work on their own even though the AC coupled system passes through my hybrid inverter on the way to the grid. In the mornings the AC coupled inverters can help support battery charging. On the few ocassions when the grid is down the AC coupled solar can also charge the batteries and serve the loads at the house.

I could see me putting panels on a outbuilding that has a sub panel and using this method. Do you have a schematic of your overall setup?

Specifically you stated this:

"AC coupled system passes through my hybrid inverter on the way to the grid"

You say passes through the hybrid, can you elaborate on how this is connected. I thought it would be placed on the AC output side of the hybrid that provides the load side.

 

[Ampster]

Do you have a schematic of your overall setup?

No, but it just involves a breaker for the GT solar in the subpanel. If you want the hybrid (grid forming ) inverter to AC couple and allow the GT micros to function when the grid is down, you have to feed the subpanel from the AC out port of the hybrid inverter. When the grid is up the grid passes through the hybrid and the flow is bidirectional in and out of the hybrid depending on loads and production.

 

[realpinochet]

No, but it just involves a breaker for the GT solar in the subpanel. If you want the hybrid (grid forming ) inverter to AC couple and allow the GT micros to function when the grid is down, you have to feed the subpanel from the AC out port of the hybrid inverter. When the grid is up the grid passes through the hybrid and the flow is bidirectional in and out of the hybrid depending on loads and production.

Ok, that's why I wanted to know; I thought you were saying it stayed up during a grid outage. It's kinda what I was thinking about doing by contemplating putting some constant draws in a sub panel and AC coupling that panel. What inverter did you use, what's a decent one that's not too expensive? I've been looking at a sunny boy and fronius.

 

[Ampster]

I thought you were saying it stayed up during a grid outage.

The AC coupled micros can stay up as long as there are enough loads on that sub panel. Otherwise the hybrid (grid forming) inverter will raise the AC frequency to cause the GT (micro) inverter(s) to shut down.

Its kinda what I was thinking about doing by contemplating putting some constant draws in a sub panel and AC coupling that panel. What inverter did you use, what's a decent one that's not too expensive? I've been looking at a sunny boy and fronius.

I have used both an Outback Skybox and a SolArk. The Skybox was expensive when I bought it five years ago and the SolArk is considered expensive. I needed a UL listed inverter so I went for tier 1.

 

[realpinochet]

The AC coupled micros can stay up as long as there are enough loads on that sub panel. Otherwise the hybrid (grid forming) inverter will raise the AC frequency to cause the GT (micro) inverter(s) to shut down.

I have used both an Outback Skybox and a SolArk. The Skybox was expensive when I bought it five years ago and the SolArk is considered expensive. I needed a UL listed inverter so I went for tier 1.

I'll most likely use outback radian or xwpro but I don't think I'll use micro inverters and might look for a string inverter ..that what they are called right? I hear sunny boy are good but they are so expensive.

 

[ChrisG]

No, but it just involves a breaker for the GT solar in the subpanel.

I wish this was true in my area. Only connection can be at the meter, nothing allowed into panels with their buy all/sell all approach.

 

[realpinochet]

I wish this was true in my area. Only connection can be at the meter, nothing allowed into panels with their buy all/sell all approach.

Wait what. Buy all / sell all...are you saying you have to buy your electricity from them and then sell back all that your panels produce. You can't self consume what you make?

 

[Ampster]

I wish this was true in my area. Only connection can be at the meter, nothing allowed into panels with their buy all/sell all approach.

That is unusual that the power company has any say in any of the connections beyond the meter. They do care about the generating equipment (GT inverter) but it is usually the building inspector that rules on how the GT inverters are connected but that is usually about breaker size and bus bar size. When you say the connection has to, "be at the meter." do you mean at the nearest electrical panel beyond the meter? In my jurisdiction I am not allowed into the meter section of the main service panel. What is your jurisdiction?

 

[ChrisG]

All PV must be done before the consumption meter in my area. Here is the compliance high level document. It really sucks.

 

[realpinochet]

All PV must be done before the consumption meter in my area. Here is the compliance high level document. It really sucks. View attachment 143034

That's some commie bullshit..why even bother? Can't you just put a traditional DC coupled inverter with traditional panels up..attach that to the main panel and tell it to not sell / backfeed. I don't think they'd know.

 

[ChrisG]

Wait what. Buy all / sell all...are you saying you have to buy your electricity from them and then sell back all that your panels produce. You can't self consume what you make?

Correct. They buy all at .02 kwh and sell back at .09kwh for what you produce. Normal consumption is .11kwh.

 

[realpinochet]

Correct. They buy all at .02 kwh and sell back at .09kwh for what you produce. Normal consumption is .11kwh.

You can't even use what you produce. If you had a garden would you have to turn over your produce to? Sorry but that's just so Anti American. I don't see how they get away with it.

 

[ChrisG]

That's some commie bullshit..why even bother? Can't you just put a traditional DC coupled inverter with traditional panels up..attach that to the main panel and tell it to not sell / backfeed. I don't think they'd know.

That’s the only thing we can do, DC coupled. Big argument going on in the city now where the utility is saying grid tied or off-grid still needs to go through a production meter.

 

[realpinochet]

That’s the only thing we can do, DC coupled. Big argument going on in the city now where the utility is saying grid tied or off-grid still needs to go through a production meter.

I'm surprised we don't see more class actions on this commie crap. It's like when the cities said you couldn't put up a tv antenna...forcing you to get cable. The FCC had to finally come out and say they'd sue the cities for this.

 

[Hedges]

The reason I'm curious about AC coupling is the article said it's more efficient if your daytime load is high because you don't have to convert from DC to AC like a battery inverter. But, isn't that exactly what's a happening; you still have a inverter converting DC from the panels into AC. What's so much more efficient than having the DC from the charge controller being converted to AC by the battery inverter as it arrives in on the bus?

Conversion efficiency from PV panel output (operating at maximum power point of algorithm and array configuration work well) is generally 95% to 99%. So PV --> AC loads can be quite efficient.

In comparison, I'm seeing posting where people expected low 90's round trip and were getting low 80's instead. I suspect due to power factor. The numbers I'm quoting for GT PV are from data sheet, and if used off-grid with poor PF loads, there is likely loss in the grid-forming inverter.

If you were purely off-grid I think only a modest portion of power would go that path, unless you had continuously variable loads with PID loop having them consume all surplus power. Likely easier to get PV direct pumps to do that.

Grid-tied, you get that efficiency from PV to loads + grid backfeed.

Off-grid, the unused power is either sent to battery (efficiency losses round trip) or curtailed (0% efficiency).

I'd like good variable loads, responding to frequency shift before the PV inverters curtail due to frequency. This does not appear to be an off-the shelf product. I'm thinking VFD with PID reading frequency and driving analog control voltage. But I need PF corrected VFD, because they tend to upset my Sunny Boys.

 

[Ampster]

All PV must be done before the consumption meter in my area. Here is the compliance high level document.

That looks like what we call a municipal utilty in which case the city is the utility and the building authority.