svetz
Works in theory! Practice? That's something else
TL;DR: Now that we've solved (?) Incrementally adding DC batteries, how can you add AC batteries from different manufactures or a DIY AC battery?
Since an AC battery has its own inverter/charger you don't have any of the internal resistance problems you do with DC batteries.
Instead, the problem is trickier. Basically, it's the same problems you see with AC Coupling.
In order for an AC battery to work, it must be AC coupled to the roof array's inverter. This is a problem for grid-tied inverters as they act as charge sources rather than a normal inverter which is a voltage source. A voltage source is what most people use every day and are familiar with (e.g., batteries, a wall socket, an off-grid inverter). A device connected to it draws amps and the source tries to hold the voltage steady. A current source is the opposite of that, it varies the voltage in order to push the available power as current. So if your house's grid power is 122V, a grid-tied inverter might up the voltage to 125v or so in order to push its current through the house and out onto the grid. The grid acts like a huge sponge so can absorb what the panels are cranking out. If the panels aren't able to supply enough current the voltage drops back allowing the grid to make up the difference. When the grid goes out grid-tied inverters shut down, otherwise, they'd supply power to the grid (a legal issue).
An AC Coupling solution allows storage to work with a grid-tied system. Basically, it's a smart inverter (aka hybrid inverter) that can control the power from grid-tied inverter either by shifting the frequency to request the inverter to throttle back output, shut them off altogether, or some other means. The inverter also needs to be able to rapidly handle surges from the solar panels, e.g., the solar is pumping 5 kW into the house to power the AC and the AC shuts off. Generally, this is done by keeping a large battery bank that the surge can flow into.
So, what's the actual problem?
The problem might be that there can only be one "brain", and that whatever you pick in the beginning you're stuck with. Let's look at an example...
Imagine you install a Tesla or Outback hybrid inverter and setup the AC coupling solution. Great!
But next year you want to come back and add another 10 kWh. If you had an Outback you could just add more batteries to the DC side and all done.
But what do you do if you had a Tesla or IQ8? Are you always locked into Tesla or Enphase which are controlled by the Gateway or Envoy controllers? That is, the brain tells the AC battery's inverter what to do; but that brain can only work with batteries it knows how to talk to.
Or, is there a way to add your own DIY AC Battery? It seems like you'd need a synchronous inverter that also operated as a matching voltage source rather than a current source (which removes all existing grid-tied inverters) or one that was smart enough that it could play nicely with other charge sources.
Anyway, that's the boat I'm in. I'll have Enphase IQ8s and Envoy, but would like to be able to add a DIY AC battery in the future.
WARNING/UPDATE
Enphase does use frequency shifting from the grid to throttle its microinverters. But as that's relatively slow the IQ8s use high-speed IP over powerline for throttling when offgrid; so much of the "hypothetical" discussion in this thread is incorrect.
Since an AC battery has its own inverter/charger you don't have any of the internal resistance problems you do with DC batteries.
Instead, the problem is trickier. Basically, it's the same problems you see with AC Coupling.
In order for an AC battery to work, it must be AC coupled to the roof array's inverter. This is a problem for grid-tied inverters as they act as charge sources rather than a normal inverter which is a voltage source. A voltage source is what most people use every day and are familiar with (e.g., batteries, a wall socket, an off-grid inverter). A device connected to it draws amps and the source tries to hold the voltage steady. A current source is the opposite of that, it varies the voltage in order to push the available power as current. So if your house's grid power is 122V, a grid-tied inverter might up the voltage to 125v or so in order to push its current through the house and out onto the grid. The grid acts like a huge sponge so can absorb what the panels are cranking out. If the panels aren't able to supply enough current the voltage drops back allowing the grid to make up the difference. When the grid goes out grid-tied inverters shut down, otherwise, they'd supply power to the grid (a legal issue).
An AC Coupling solution allows storage to work with a grid-tied system. Basically, it's a smart inverter (aka hybrid inverter) that can control the power from grid-tied inverter either by shifting the frequency to request the inverter to throttle back output, shut them off altogether, or some other means. The inverter also needs to be able to rapidly handle surges from the solar panels, e.g., the solar is pumping 5 kW into the house to power the AC and the AC shuts off. Generally, this is done by keeping a large battery bank that the surge can flow into.
So, what's the actual problem?
The problem might be that there can only be one "brain", and that whatever you pick in the beginning you're stuck with. Let's look at an example...
Imagine you install a Tesla or Outback hybrid inverter and setup the AC coupling solution. Great!
But next year you want to come back and add another 10 kWh. If you had an Outback you could just add more batteries to the DC side and all done.
But what do you do if you had a Tesla or IQ8? Are you always locked into Tesla or Enphase which are controlled by the Gateway or Envoy controllers? That is, the brain tells the AC battery's inverter what to do; but that brain can only work with batteries it knows how to talk to.
Or, is there a way to add your own DIY AC Battery? It seems like you'd need a synchronous inverter that also operated as a matching voltage source rather than a current source (which removes all existing grid-tied inverters) or one that was smart enough that it could play nicely with other charge sources.
Anyway, that's the boat I'm in. I'll have Enphase IQ8s and Envoy, but would like to be able to add a DIY AC battery in the future.
WARNING/UPDATE
Enphase does use frequency shifting from the grid to throttle its microinverters. But as that's relatively slow the IQ8s use high-speed IP over powerline for throttling when offgrid; so much of the "hypothetical" discussion in this thread is incorrect.
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