svetz
Works in theory! Practice? That's something else
Yesterday I postulated a competitor could build an AC battery that could be mixed into a Solar /Encharge home without having to do a lot of reverse engineering using UL-1741 (e.g., compatible with CA Rule 21).
What about DIY in that situation? Could we build our own using a UL 1741 compliant inverter?
Short answer: Not as far as I know currently unless you go the full AC coupling route.
Long Answer
The problem is a simple UL 1741 compliant inverter is a grid tied inverter - a current source. So once turned on it would immediate drain the battery as fast as it could invert pumping all the battery power into the grid.
You could get around this by controlling the inverter, say with an Arduino and some logic:
First is the insidious problem of surges since frequency shifting takes time. Just having a brain to frequency shift isn't enough, you need the hardware to handle it. For example, from the chart below you can see an Encharge 10 is only rated to handle up to 18 IQ7Xs, if you have more than that on the roof, they shut down when the grid is off because the Encharge 10 doesn't have the capability to handle more surge energy than they could provide.
Obviously as the number of AC coupled solutions show, the problems are not insurmountable. But it's probably beyond the skills of an average DIYer to build/size something and not worry about it catching fire.
The IQ8 inverters are designed for this, but as Ampster pointed out, may have a proprietary interface making it difficult for a DIYer. So, might not be able to just slap a battery on them as I'd like to do.
In the future I expect to see inexpensive small wattage (500 to 1,000W) third party inverters that are designed for this. Vendors that already have AC Coupled solutions have the tech and worked out the bugs... it's just that their existing line up is to large to mix and match to make scaling up easy. So when Outback or Fortress Power come out with a 500W unit where I can add my own LiFePO4, I'd definitely buy one, maybe two!
What about DIY in that situation? Could we build our own using a UL 1741 compliant inverter?
Short answer: Not as far as I know currently unless you go the full AC coupling route.
Long Answer
The problem is a simple UL 1741 compliant inverter is a grid tied inverter - a current source. So once turned on it would immediate drain the battery as fast as it could invert pumping all the battery power into the grid.
You could get around this by controlling the inverter, say with an Arduino and some logic:
- When the grid is up or the frequency is shifted (e.g., excess solar power), the inverter stays in charge only mode
- Anti-bounce logic for when the grid is down but the solar is providing some fraction of the power load (that is the inverter coming on could provide too much power for the house's load, which would cause the brain to frequency shift).
First is the insidious problem of surges since frequency shifting takes time. Just having a brain to frequency shift isn't enough, you need the hardware to handle it. For example, from the chart below you can see an Encharge 10 is only rated to handle up to 18 IQ7Xs, if you have more than that on the roof, they shut down when the grid is off because the Encharge 10 doesn't have the capability to handle more surge energy than they could provide.
Obviously as the number of AC coupled solutions show, the problems are not insurmountable. But it's probably beyond the skills of an average DIYer to build/size something and not worry about it catching fire.
The IQ8 inverters are designed for this, but as Ampster pointed out, may have a proprietary interface making it difficult for a DIYer. So, might not be able to just slap a battery on them as I'd like to do.
In the future I expect to see inexpensive small wattage (500 to 1,000W) third party inverters that are designed for this. Vendors that already have AC Coupled solutions have the tech and worked out the bugs... it's just that their existing line up is to large to mix and match to make scaling up easy. So when Outback or Fortress Power come out with a 500W unit where I can add my own LiFePO4, I'd definitely buy one, maybe two!
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