Tesla Powerwall

[ggunn]

Which, as I understand it, is not being manufactured and there are no plans to go forward with it. See this amusing article from down under. Also, looks like the inverter won't support off-grid solutions as there is no way to match charging current to demand when the demand is less than 100%. At least, that's how I read it.

You understand correctly; the PowerWall 2 AC is the only Tesla solution now. Offgrid operation can be done with a StorEdge inverter and a PowerWall 2 DC, but it ain't easy and SolarEdge doesn't really want to help. Another designer here in my office made it work for one of our customers but I don't know the details; I just know that he was pretty stressed there for a while.

SolarEdge is understandably unhappy with Tesla; their StorEdge inverter is now obsolete.

 

[gadfly56]

Tesla's solution would support off grid systems, albeit perhaps not optimally. To work decently well, the solar system just can't be too large per Powerwall.
An advantage of their design is that it does not matter what type of solar a homeowner already has or wants.

Which is pretty much what I said. So, you have to have battery capacity that allows a discharge rate higher than incoming solar can make up, and you have to be sure that your demand at all times is higher than the solar recharge rate???

 

[ggunn]

Which is pretty much what I said. So, you have to have battery capacity that allows a discharge rate higher than incoming solar can make up, and you have to be sure that your demand at all times is higher than the solar recharge rate???

The StorEdge inverter was designed to coexist with the DC PowerWall and manage that stuff, but both of those pieces of gear are now gone the way of the dodo. The AC PowerWall is a self contained battery backup system; it doesn't interact directly with PV at all.

 

[jaggedben]

Which is pretty much what I said. So, you have to have battery capacity that allows a discharge rate higher than incoming solar can make up, and you have to be sure that your demand at all times is higher than the solar recharge rate???

The 'at all times' part is incorrect. You just miss out on potential solar production if solar production is higher than battery+load demand, because the solar gets kicked offline. If the solar system isn't too big then the 'shoulders' of the daily solar production curve should be enough to charge the battery for the daily energy need.

 

[wwhitney]

Also, looks like the inverter won't support off-grid solutions as there is no way to match charging current to demand when the demand is less than 100%.

Per https://www.tesla.com/powerwall off-grid is supported. The page is light on technical details, but presumably that means that the Powerwall 2 AC plus Energy Gateway have a mechanism to tell the solar panels to shut off when off grid, the battery is full, and instantaneous demand is less than solar power output. I'm just trying to figure out what mechanisms it supports, I was hoping a certified installer might be on the group here and could tell us what the technical manuals say.

Cheers, Wayne

 

[gadfly56]

Per https://www.tesla.com/powerwall off-grid is supported. The page is light on technical details, but presumably that means that the Powerwall 2 AC plus Energy Gateway have a mechanism to tell the solar panels to shut off when off grid, the battery is full, and instantaneous demand is less than solar power output. I'm just trying to figure out what mechanisms it supports, I was hoping a certified installer might be on the group here and could tell us what the technical manuals say.

Cheers, Wayne

I believe it's the inverter that has to shut off; the panels are producing power whenever the sun shines. As I understand the article I linked to, you only have "bang-bang" control for the PowerWall 2 AC inverter. It can't modulate its output to trickle charge the batteries, but the Solar Edge product was supposed to be able to hook this all up, but only the DC PowerWall.

 

[wwhitney]

I believe it's the inverter that has to shut off; the panels are producing power whenever the sun shines. As I understand the article I linked to, you only have "bang-bang" control for the PowerWall 2 AC inverter. It can't modulate its output to trickle charge the batteries, but the Solar Edge product was supposed to be able to hook this all up, but only the DC PowerWall.

My understanding is that in off-grid conditions when the battery isn't full, the Powerwall inverter produces an AC waveform within the sync window of the solar grid tie inverters, which then produce power during the day. Depending on whether the local demand exceeds the solar production or not, the batteries get discharged or charged.

If the batteries get full, it is desirable to shut down the grid tie inverter, but still operate the Powerwall inverter, so there is still power. To do this, the Powerwall inverter must have some communications method with the grid tie inverter. The crudest method is for the Powerwall inverter to shift its AC waveform so it is no longer within the sync window of the grid tie inverter. For example, if the grid tie inverter only syncs when the AC frequency is between 59.8 Hz and 60.2 Hz (made up numbers, not sure what is typical), then the Powerwall inverter could shift its output frequency to 60.3 Hz, causing the grid tie inverters to trip off line.

I'm wondering if the Powerwall inverter supports any other methods of managing this condition. Two possibilities come to mind: support for controlling a dump load, which is turned on when the batteries get full and is large enough to cause the local demand to exceed the solar power production; or support for smoother throttling of the solar power production than just binary on-off.

Cheers, Wayne

 

[GoldDigger]

My understanding is that in off-grid conditions when the battery isn't full, the Powerwall inverter produces an AC waveform within the sync window of the solar grid tie inverters, which then produce power during the day. Depending on whether the local demand exceeds the solar production or not, the batteries get discharged or charged.

If the batteries get full, it is desirable to shut down the grid tie inverter, but still operate the Powerwall inverter, so there is still power. To do this, the Powerwall inverter must have some communications method with the grid tie inverter. The crudest method is for the Powerwall inverter to shift its AC waveform so it is no longer within the sync window of the grid tie inverter. For example, if the grid tie inverter only syncs when the AC frequency is between 59.8 Hz and 60.2 Hz (made up numbers, not sure what is typical), then the Powerwall inverter could shift its output frequency to 60.3 Hz, causing the grid tie inverters to trip off line.

I'm wondering if the Powerwall inverter supports any other methods of managing this condition. Two possibilities come to mind: support for controlling a dump load, which is turned on when the batteries get full and is large enough to cause the local demand to exceed the solar power production; or support for smoother throttling of the solar power production than just binary on-off.

Cheers, Wayne

One way or another, this GTI shutdown requires that the Powerwall sense its own output power and trigger a shutdown when that approaches zero. Whether or not the battery is full or not simply determines whether the Powerwall can use the battery charging circuitry as an additional load to bleed off the "surplus" power from the GTI and maintain the net output from its own inverter.

Sent from my XT1585 using Tapatalk

 

[wwhitney]

One way or another, this GTI shutdown requires that the Powerwall sense its own output power and trigger a shutdown when that approaches zero. Whether or not the battery is full or not simply determines whether the Powerwall can use the battery charging circuitry as an additional load to bleed off the "surplus" power from the GTI and maintain the net output from its own inverter.

Good point, I was assuming that when the battery isn't full, it could always accept the full output of the solar grid tie inverter. But with a big solar installation and only one Powerwall, this may not be the case. That explains the earlier comments about the ratio of solar system size to number of Powerwalls, which I hadn't fully understood.

Cheers, Wayne

 

[GManov]

Tesla Powerwall

Ok, here is what I see as Tesla's sales hype. If you loose utility power then your solar system goes off line. Tesla is selling the Powerwall in combination with a data controlled contactor and power monitor that will take the solar inverter and sub panel off the grid and supply it with an UPS (Powerwall) to simulate the utility. It tricks the solar inverter in thinking that power is restored and then provides output in a stand-alone system mode. A great marketing tool if one is in an area with lots of short power outages. A single Powerwall will support 5kW for about 2.5 hours.
The drawback I see to this is 690.10 (A) reads that the inverter can supply current levels less than the calculated load. So low that one only need to supply current equal or greater than the largest single load. There is no load management provided and appears to require that the end user adjust the load, turning off circuits, to enable the Powerwall to not go into overload and self preservation mode. This might not be a problem during the day but after dark and with no output from the solar system the Powerwall could be easily overloaded. I also see that this appears to be contrary to 702.4(B) that requires an optional standby system to be sized in accordance with load calculations.
Am I missing something here?

 

[jaggedben]

...
Am I missing something here?

Not really.

The discrepancy between 690.10 (A) and 702.4(B) is an interesting point, especially since Tesla is really not selling a solar inverter. I suspect they will 'get away' with invoking 690.10 with a lot of AHJs but they are on thin ice with that in my opinion.

There are two ways to deal with avoiding overload and complying with 702.4(B) (the two of which amount to the same thing).

1) Only put as much load on the backed-up subpanel as the system can handle. Start with the refrigerator and enough efficient lighting circuits to walk around the house, plus an outlet or two for your cell-phone, etc.
2) Add more Powerwall units, if money and space is no object. This is probably the biggest design advantage of Tesla's product, it is easily scalable.

 

[MAC702]

I called down to Phoenix once and asked what the design ambient temp was for them. Guy said, 104F.

I'll bet he misunderstood the question. 104 ºF is the very common temperature used for equipment ratings, such as duty cycles. But when used as such, it is not the maximum operating temperature, but just means you will have a less-than-published duty cycle at higher ambient temps.

 

[gadfly56]

I'll bet he misunderstood the question. 104 ºF is the very common temperature used for equipment ratings, such as duty cycles. But when used as such, it is not the maximum operating temperature, but just means you will have a less-than-published duty cycle at higher ambient temps.

He was wrong. It's 107°F. See here.