Is AC coupled even worth it?

[ChrisG]

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

20 minutes down the road from me is Duke Energy and those consumers don’t have to put up with this shit. They allow ‘normal’ grid tie like you are talking about. All my
Solar aspirations are done at my grid
Connected cabin where there is more flexibility.

 

[Ampster]

I'm surprised we don't see more class actions on this commie crap

Class actions don't work against government entities. Theoretically these are democratically electected entities.

 

[BentleyJ]

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.

If you are considering the XW Pro for AC coupling it should work quite well for you. I'm using the XW+ 6848 which shares many of the same features. If you are planning on using the GT inverters with a grid interconnect agreement and you are on Time of Use billing, Peak Load Shaving may be a feature you would plan on using. If this is the case, the XW Pro has a specific quirk that you should know about if its relevant to your use case.

 

[realpinochet]

If you are considering the XW Pro for AC coupling it should work quite well for you. I'm using the XW+ 6848 which shares many of the same features. If you are planning on using the GT inverters with a grid interconnect agreement and you are on Time of Use billing, Peak Load Shaving may be a feature you would plan on using. If this is the case, the XW Pro has a specific quirk that you should know about if its relevant to your use case.

I don't plan on selling anything back, I don't plan on the local or federal gov having anything to do with my solar install at all. The PV inverter was going to inject AC right on the output bus to the panel along with the xw or into it's own sub panel. Are you saying there's zero way to not back feed into the grid with the PV inverter? As long as my meter doesn't roll backwards and I'm not backfeeding to hurt a lineman I don't see them caring.

 

[400bird]

20 minutes down the road from me is Duke Energy and those consumers don’t have to put up with this shit. They allow ‘normal’ grid tie like you are talking about. All my
Solar aspirations are done at my grid
Connected cabin where there is more flexibility.

What state are you in? Those are rough regulations.

 

[ChrisG]

What state are you in? Those are rough regulations.

North Carolina

 

[400bird]

That's a surprise, NC doesn't often get accused of being left wing communist...
Almost like it's capitalism at work; if they can make money from your power, the company will. Even if that means passing regulations and making excuses to make it look legit.

Communism would be allowing you to make power but forcing you to share it (possibly equally) with the neighbors.


To the OP, are you looking to be grid tied with an AC coupled Schneider?
There's a weak link in that exact combo: the Schneider won't charge your battery from excess PV when the grid is present. If there's no grid input, then the Schneider will suck up 6 or 6.8kw from the AC coupled solar.
I've got my Schneider set to work AC coupled and grid tied, but it required an external controller. I use a raspberry pi, another member here uses a small industrial PLC.

Depending on your usage profile, DC coupled or a mix of AC and DC, might be a better fit.
Do you use significantly more energy while the sun is out? If so, AC coupling is more efficient as the daytime power it's converted down to lower DC voltage, then back up to higher AC voltage.
But, if you're looking to power the heaters and house overnight, DC coupled is likely better as it can charge the battery more efficiently.

 

[BentleyJ]

I don't plan on selling anything back, I don't plan on the local or federal gov having anything to do with my solar install at all. The PV inverter was going to inject AC right on the output bus to the panel along with the xw or into it's own sub panel. Are you saying there's zero way to not back feed into the grid with the PV inverter? As long as my meter doesn't roll backwards and I'm not backfeeding to hurt a lineman I don't see them caring.

This is where the details get a little complicated. A GT inverter is an MPPT device that will produce as much AC power as it can at any given moment based on solar conditions and what the PV's can deliver. GT inverters are also UL1741 (SA) compliant which means they only operate between specified voltage and frequency ranges which are fairly narrow. (The SA designation includes some ride through provisions but that is off topic)
When a GT inverter is AC coupled with a grid forming, battery inverter at the AC Output, the power from the GT inverter has to go somewhere. Somewhere being the loads on the subpanel with any excess power Backfeeding into the Battery Inverter. There are 2 possible states of operation.
1) Grid is functional and connected to the AC Input of the Battery Inverter and the Inverter has qualified the power and Closed the transfer relay.
2) Grid is down completely or out of tolerance in voltage or frequency and the Inverter has disqualified power and Opened the transfer relay.

In case 1, excess production from the GT inverters goes out to the grid (more likely your neighbors house) Your meter doesn't register the credit and the excess power is lost. Worst case scenario the meter detects backfeed and sends a message to the utility if its one of the newer electronic communicating meter. Operating without an approval could get their attention.
In case 2, excess production from the GT inverters goes through the grid forming Inverter built in AC charger to the batteries. When the batteries hit full voltage setting the Inverter ramps up frequency and the GT inverters will begin to curtail their output and eventually shut down at 60.5Hz.

Sol-Ark has a nice feature where AC Coupling can be done via the Generator Input on the Inverter thus it is better suited to regulating the AC Coupled inverters. I have no idea how sophisticated the Sol-Ark firmware is but its unlikely that you could achieve true 0 export because of the lag between sensing exported power and reaction as well as rather course increments of curtailment with GT inverters.

 

[toms]

I’m off-grid, and use AC coupling as where I live people discard grid-tie systems frequently, so it is very cheap power for me.

Typical cost for a <5 year old 5kw SMA system (Including all racking and cabling), is around $500.

Another advantage if you have outbuildings or remote PV is the cabling is inexpensive.

As far as efficiency, I doubt there is a significant difference.

 

[realpinochet]

That's a surprise, NC doesn't often get accused of being left wing communist...
Almost like it's capitalism at work; if they can make money from your power, the company will. Even if that means passing regulations and making excuses to make it look legit.

Communism would be allowing you to make power but forcing you to share it (possibly equally) with the neighbors.


To the OP, are you looking to be grid tied with an AC coupled Schneider?
There's a weak link in that exact combo: the Schneider won't charge your battery from excess PV when the grid is present. If there's no grid input, then the Schneider will suck up 6 or 6.8kw from the AC coupled solar.
I've got my Schneider set to work AC coupled and grid tied, but it required an external controller. I use a raspberry pi, another member here uses a small industrial PLC.

Depending on your usage profile, DC coupled or a mix of AC and DC, might be a better fit.
Do you use significantly more energy while the sun is out? If so, AC coupling is more efficient as the daytime power it's converted down to lower DC voltage, then back up to higher AC voltage.
But, if you're looking to power the heaters and house overnight, DC coupled is likely better as it can charge the battery more efficiently.

I have not decided on a inverter yet, I was looking at the xwpro or radian 8048. I plan on having a traditional system as in two inverters with two charge controllers and a rack of batteries with a ground mounted solar array. I will have grid connection to the inverters for grid assist and charging of batteries when there's no sun. But, in Texas we run the AC all day in the summer. The article said PV AC is more efficient in this case. So, I was trying to find out how to add a little PV AC into the mix for assisting with running the conditioner and a few other things and do it correctly. I would be AC/DC coupled I think. I do not want to send anything back to the grid. And, should the grid be down I'd still like to be able to use the PV AC and the incoming DC to the charge controllers. As stated it appears the way to achieve this is to couple it to the AC output side of the battery inverters and injecting AC right along that bus into the loads panel. I'm assuming the AC output from the xwpro keeps the PV inverter from shutting down...a false grid? It shouldn't back feed like this right?

Is this correct?

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

 

[realpinochet]

This is where the details get a little complicated. A GT inverter is an MPPT device that will produce as much AC power as it can at any given moment based on solar conditions and what the PV's can deliver. GT inverters are also UL1741 (SA) compliant which means they only operate between specified voltage and frequency ranges which are fairly narrow. (The SA designation includes some ride through provisions but that is off topic)
When a GT inverter is AC coupled with a grid forming, battery inverter at the AC Output, the power from the GT inverter has to go somewhere. Somewhere being the loads on the subpanel with any excess power Backfeeding into the Battery Inverter. There are 2 possible states of operation.
1) Grid is functional and connected to the AC Input of the Battery Inverter and the Inverter has qualified the power and Closed the transfer relay.
2) Grid is down completely or out of tolerance in voltage or frequency and the Inverter has disqualified power and Opened the transfer relay.

In case 1, excess production from the GT inverters goes out to the grid (more likely your neighbors house) Your meter doesn't register the credit and the excess power is lost. Worst case scenario the meter detects backfeed and sends a message to the utility if its one of the newer electronic communicating meter. Operating without an approval could get their attention.
In case 2, excess production from the GT inverters goes through the grid forming Inverter built in AC charger to the batteries. When the batteries hit full voltage setting the Inverter ramps up frequency and the GT inverters will begin to curtail their output and eventually shut down at 60.5Hz.

Sol-Ark has a nice feature where AC Coupling can be done via the Generator Input on the Inverter thus it is better suited to regulating the AC Coupled inverters. I have no idea how sophisticated the Sol-Ark firmware is but its unlikely that you could achieve true 0 export because of the lag between sensing exported power and reaction as well as rather course increments of curtailment with GT inverters.

This is one of my main concerns, the article said what you just stated. The PV AC has to go somewhere and it's to the loads or battery. I understand now how it makes it to the battery..you said it back feeds through the bettery inverters. So, the AC is back fed and converted to DC and pushed into the battery? What if the battery is full? What if the air conditioner is on and pulling a decent load and then goes off..all that incoming AC has to go somewhere. The article said that's where phase shifting was suppose to come in and shut the pv inverter off or throttle it down. The article said this could take some time and might be too long if the batteries are full and you risk blowing up batteries.

In case 2, that means the batter inverters has to be able to read he state of charge for the batteries ..right? If this is true I'm assuming you can't use batteries that don't have the proper communication. I don't think EG4 or Trophy batteries communicate with xwpro or radian.

 

[400bird]

I have not decided on a inverter yet, I was looking at the xwpro or radian 8048. I plan on having a traditional system as in two inverters with two charge controllers and a rack of batteries with a ground mounted solar array. I will have grid connection to the inverters for grid assist and charging of batteries when there's no sun.

Are you planning to turn off/open the grid connection other than those times when production is low? If not, and you don't want/can't backfeed the grid, then either you'll need to program something to speak Modbus and command the AC charging or the XW isn't the correct choice.

But, in Texas we run the AC all day in the summer.

It's hot here too, those 100+ degree days all summer really suck down a lot of juice!

The article said PV AC is more efficient in this case.

Yup, using more power when the sun is up makes AC coupled more efficient. It won't help on those days when the AC runs all night, but that's what the battery and DC panels are for.

So, I was trying to find out how to add a little PV AC into the mix for assisting with running the conditioner and a few other things and do it correctly. I would be AC/DC coupled I think.

That ratio is a hard number to nail down.
What does your AC consume?
What are your base loads?
Have you decided on a total PV or have a rough idea?

I do not want to send anything back to the grid. And, should the grid be down I'd still like to be able to use the PV AC and the incoming DC to the charge controllers.

I've got a net metering agreement, but I can run the AC and most everything in the house on the XW when the grid goes down.

As stated it appears the way to achieve this is to couple it to the AC output side of the battery inverters and injecting AC right along that bus into the loads panel. I'm assuming the AC output from the xwpro keeps the PV inverter from shutting down...a false grid?

Correct

It shouldn't back feed like this right?

The XW (and Radian) has relays that will disconnect from the grid if the grid goes out of spec (high voltage, low voltage, high frequency, low frequency)

Is this correct?

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

View attachment 143059

Yes that's correct. You'd then add a charge controller and PV connected to the battery.

I really like the next diagram down in that sheet, it gives an example of current flow when off grid.

This is one of my main concerns, the article said what you just stated. The PV AC has to go somewhere and it's to the loads or battery. I understand now how it makes it to the battery..you said it back feeds through the bettery inverters. So, the AC is back fed and converted to DC and pushed into the battery?

Yup, the inverter turns into a charger and the home loads stay on, powered by PV.

What if the battery is full? What if the air conditioner is on and pulling a decent load and then goes off..all that incoming AC has to go somewhere. The article said that's where phase shifting was suppose to come in and shut the pv inverter off or throttle it down. The article said this could take some time and might be too long if the batteries are full and you risk blowing up batteries.

You don't run the battery up to 100%

If you stop at 90 or 95% that should give you enough wiggle room to absorb the excess PV while the AC coupled inverter ramps down.

In case 2, that means the batter inverters has to be able to read he state of charge for the batteries ..right? If this is true I'm assuming you can't use batteries that don't have the proper communication. I don't think EG4 or Trophy batteries communicate with xwpro or radian.

EG4 things their batteries do, I haven't seen success stories here, just complaints. But that's all anyone comes here for when EG4 or Signature Solar is involved.

You can also use battery voltage to regulate how the XW handles charging/discharging.
That's what I am doing, but my batteries are not Lifepo4, NMC has a much more linear voltage to SOC relationship.
You could set it to stop charging when AC coupled at 3.4 volts per cell.

 

[realpinochet]

Are you planning to turn off/open the grid connection other than those times when production is low? If not, and you don't want/can't backfeed the grid, then either you'll need to program something to speak Modbus and command the AC charging or the XW isn't the correct choice.

It's hot here too, those 100+ degree days all summer really suck down a lot of juice!

Yup, using more power when the sun is up makes AC coupled more efficient. It won't help on those days when the AC runs all night, but that's what the battery and DC panels are for.

That ratio is a hard number to nail down.
What does your AC consume?
What are your base loads?
Have you decided on a total PV or have a rough idea?

I've got a net metering agreement, but I can run the AC and most everything in the house on the XW when the grid goes down.

Correct

The XW (and Radian) has relays that will disconnect from the grid if the grid goes out of spec (high voltage, low voltage, high frequency, low frequency)

Yes that's correct. You'd then add a charge controller and PV connected to the battery.

I really like the next diagram down in that sheet, it gives an example of current flow when off grid.
View attachment 143074


Yup, the inverter turns into a charger and the home loads stay on, powered by PV.

You don't run the battery up to 100%

If you stop at 90 or 95% that should give you enough wiggle room to absorb the excess PV while the AC coupled inverter ramps down.

EG4 things their batteries do, I haven't seen success stories here, just complaints. But that's all anyone comes here for when EG4 or Signature Solar is involved.

You can also use battery voltage to regulate how the XW handles charging/discharging.
That's what I am doing, but my batteries are not Lifepo4, NMC has a much more linear voltage to SOC relationship.
You could set it to stop charging when AC coupled at 3.4 volts per cell.

Thanks for this input. I don't have a loads audit yet, I've been looking at the following to achieve this on a overall big picture level and per circuit level. This should allow me to clamp onto the mains and a individual circuit. The AC is a typical Train 4ton unit. I have a killowatt but don't have a way of looking at the large appliances. What do you think of the item below? *Another question...when the battery inverter phase shifts to shutdown the PV inverter does the change in frequency and voltage have a effect on other things connected in the panel such as computers?

IoTaWatt™ Open WiFi Electric Power Monitor

Electric Power Monitor. Residential and Commercial. Single, split and three-phase. Any voltage. Local data storage and analysis and also upload to your cloud service. Economical. Ships worldwide, in use in dozens of countries on four continents.

iotawatt.com

 

[Ampster]

Are you saying there's zero way to not back feed into the grid with the PV inverter?

I do know that with my Enphase micros there is a way to limit export to zero but CT placement is critical when also used with a hybrid inverter.

As long as my meter doesn't roll backwards and I'm not backfeeding to hurt a lineman I don't see them caring.

Oh they care, but I agree with you that there is nothing that I know of that prevents us from generating our own electricity in most cases. It may require a building permit as would any new circuit

 

[400bird]

Thanks for this input. I don't have a loads audit yet, I've been looking at the following to achieve this on a overall big picture level and per circuit level. This should allow me to clamp onto the mains and a individual circuit. The AC is a typical Train 4ton unit. I have a killowatt but don't have a way of looking at the large appliances. What do you think of the item below?

I've heard good things about the Iotawatt. It would do a great job of providing that load and usage info.

*Another question...when the battery inverter phase shifts to shutdown the PV inverter does the change in frequency and voltage have a effect on other things connected in the panel such as computers?

Most of them don't care. If you have clocks on AC power, they might loose time. But many electrical devices are designed with a wide frequency range in mind (50-60 hz)

 

[BentleyJ]

There are few discussions regarding the IoTaWatt, try doing a search on this Forum. I'm using Emporia Vue 2 mostly due to price vs. features and accuracy. IoTaWatt costs a bit more but other users like it because it doesn't depend on a 3rd party webserver.

The frequency shift is only up to 60.5Hz. I haven't seen any problems with our appliances. Others forum members have reported their clocks gaining or loosing time based on inverters with poor frequency control. Keep in mind the AC coupling frequency shift is generally a short term issue. Once the GT inverters shut down, your batteries have to carry 100% of the load thus the voltage drops and the frequency returns to normal.

 

[BentleyJ]

Also, you mentioned a 4 ton AC condenser. A soft starter is highly recommended. I'm using a HyperEngineering brand on a 4 ton, 2 stage, 18 SEER Goodman. Cuts inrush to a small "blip" on the Emporia 1 second energy monitor graph. There are other brands that have been discussed on this forum.

 

[Darren Orange]

I've debated this with many people. The short answer actually is no AC coupled is fairly dumb. Consider these many factors.
1.) Solar Panels are DC by default....
2.) Batteries are DC by default....
3.) If you want MOST things actually are DC not AC(not that most of us would be you could run most things without ever converting to AC)
4.) There are massive losses in being AC Coupled(which I think is really the main reason)

I've tried to tell people to use the grid as backup power via a transfer switch or a all in one.

The only reason we have these AC coupled systems mostly is because of the grid and selling power back to the grid. I don't think it is wise to design a system today that uses the grid, there are some benefits but those are quickly going down or will soon be manipulated to no longer be a benefit.

 

[Ampster]

I don't think it is wise to design a system today that uses the grid, there are some benefits but those are quickly going down or will soon be manipulated to no longer be a benefit.

Until then, I will continue to use the grid as a resource. It is an efficient seasonal battery to complement my 42 kWh pack and hybrid grid interactive system. I do agree with your view of the future and feel fortunate to have locked in some benefits before they were eroded.

 

[Darren Orange]

Until then, I will continue to use the grid as a resource. It is an efficient seasonal battery to complement my 42 kWh pack and hybrid grid interactive system. I do agree with your view of the future and feel fortunate to have locked in some benefits before they were eroded.

Currently my area supports hourly rate power. It's never a benefit for me to push power to the grid in these situations. I almost always charge my house batteries at night some even if I don't need it because often the power is either very cheap so why not, or its even free or negative, meaning I get paid for taking power from the grid. I expect these benefits to go away at some point but yea the ~20% loss treating keeping things as AC coupled for solar to a house battery to me is very wasteful in that case.