Integrating a PowerWall into a Fossil-fuel Backed up Low-Energy Home

SageBrush · May 21, 2015

I own a Prius, aka a back-up generator

I've been wondering whether a PowerWall does not simplify the design of an off-grid, PV based, passivHaus spec'd home. The specifics of how often the generator is used depends on location and home demand, but I would appreciate advice and comments regarding the design and operation of an integrated energy system for a home of this type.

As an example of such a home:
Average 6 kWh a day electricity including hot water and AC, not including any *EV
PV capacity set to 30% over average daily consumption
300 'sunshine days' a year
Two 7kWh PowerWalls, to supply 2-3 days of moderated demand
The Prius can output 6-10 kW DC by tapping the high voltage battery output.

I imagine the schematic to look something like this:

Reasonable ?
Any functional aspects I am overlooking (e.g., overcharging protection, energy flow etc) ?

Why use a Prius, you ask ?
205V, 30 Amp DC output
Quiet
Pollution controlled
Perhaps higher efficiency than competing generators
I have one

nwdiver · May 21, 2015

Unless something new is released there are no inverters I'm aware of that use ~350vdc AND can operate off-grid with our a second 48v inverter 'mimicking' the grid. Solaredge might have something but I haven't seen the details...

SageBrush · May 21, 2015

Can the problem be solved by adding buck/boost converters ?
I'm also unclear what is needed (or if it is even possible) to arrange energy flow so that the home is supplied first and then the PW takes up the excess.
I also haven't incorporated a sink for excess power, although I imagine using a reserve hot water tank for that purpose.

miimura · May 21, 2015

If you want to connect your Prius HV battery to the bus same DC bus as the panels, PowerWall and inverter, you will need a DC/DC Converter just like the PowerWall has inside and it will have to talk to the system just like the PowerWall. We don't have enough information about that system yet to know how to integrate it. However, if you get something like a Prius Plug-Out kit, then you could use the Prius just like any other backup generator. It is well known how to integrate that in a number of different ways. You could also use a traditional On/Off-Grid system with 48VDC nominal batteries and inverter and connect the Prius Plug-Out to that on the inverter generator input. That would also be an easy integration job.

nwdiver · May 21, 2015

SageBrush said:
Can the problem be solved by adding buck/boost converters ?
I'm also unclear what is needed (or if it is even possible) to arrange energy flow so that the home is supplied first and then the PW takes up the excess.
I also haven't incorporated a sink for excess power, although I imagine using a reserve hot water tank for that purpose.

No doubt there are a lot of options... but none are commercially available yet. EVERY ~300vdc inverter I'm aware of is UL1741 and cannot island itself off-grid. You CAN mimic a mini-grid with an AC-coupled inverter but those use 48vdc.

Should be no need for a 'sink for excess power'... any MPPT charger can simply raise the input voltage to prevent overcharging the batteries. Excess energy is just never converted from sunlight.

AAAAANNNNDDD..... still nothing on how this is supposed to work OFF-GRID :cursing:

schueppert · May 21, 2015

Is there actually any way to tap the high voltage battery on a Prius? All the kits I have seen tap the 12v battery, meaning lower voltage and much less power

miimura · May 21, 2015

schueppert said:
Is there actually any way to tap the high voltage battery on a Prius? All the kits I have seen tap the 12v battery, meaning lower voltage and much less power

See my link above for the Prius Plug-Out kit. It uses a 5kVA inverter that was originally designed for datacenter UPS duty that happens to use a battery pack voltage similar to the Prius.

schueppert · May 21, 2015

Kinda cool. You can't do that on a Tesla!

SageBrush · May 21, 2015

nwdiver said:
No doubt there are a lot of options... but none are commercially available yet. EVERY ~300vdc inverter I'm aware of is UL1741 and cannot island itself off-grid. You CAN mimic a mini-grid with an AC-coupled inverter but those use 48vdc.

Should be no need for a 'sink for excess power'... any MPPT charger can simply raise the input voltage to prevent overcharging the batteries. Excess energy is just never converted from sunlight.
AAAAANNNNDDD..... still nothing on how this is supposed to work OFF-GRID :cursing:

Thanks -- very helpful.

In regard to my excess power comment, I should clarify that I mean what to do with PV power not being used to power home devices or fill up the PW. My thought was to heat water at that time.

- - - Updated - - -

Miimura,
Thanks for the link. I was imagining a DC tap, with the energy then either flowing into the PW or into a main inverter that also handles the PW and PV.
NWdiver has pointed out at least one current obstacle in a post above.

- - - Updated - - -

nwdiver said:
No doubt there are a lot of options... but none are commercially available yet. EVERY ~300vdc inverter I'm aware of is UL1741 and cannot island itself off-grid.

I was thinking about this a bit more.

The 7kWh PW is supposed to allow time-shifting in an on-grid setup with PV. This would seem to imply that the inverter 'sees' a grid compatible signal but does not use it if PV or PW energy is available. Can that situation be safely mimicked to trick the UL1741 circuitry ?

nwdiver · May 22, 2015

SageBrush said:
The 7kWh PW is supposed to allow time-shifting in an on-grid setup with PV. This would seem to imply that the inverter 'sees' a grid compatible signal but does not use it if PV or PW energy is available. Can that situation be safely mimicked to trick the UL1741 circuitry ?

From the diagram that Tesla posted the PW is on the DC side of the inverter... you can't mimic the grid from the DC side... it appears to be providing power to the inverter instead of the PV modules. The PW so far as has been released, is DC coupled. To go off-grid with a grid-tie inverter you need to be AC coupled like this...

SageBrush · May 22, 2015

nwdiver said:
From the diagram that Tesla posted the PW is on the DC side of the inverter... you can't mimic the grid from the DC side... it appears to be providing power to the inverter instead of the PV modules. The PW so far as has been released, is DC coupled. To go off-grid with a grid-tie inverter you need to be AC coupled like this...

Wow.

Would it be possible to define each component in terms of its functional use(s) ?

nwdiver · May 22, 2015

SageBrush said:
Wow.

Would it be possible to define each component in terms of its functional use(s) ?

Don't get lost in the weeds; The important take-away here is that an anti-islanding grid-tie inverter can't work unless it 'sees' 240vac. AND an off-grid inverter needs to isolate itself from the grid to work off-grid. How the PW intends to do this remains to be seen...

schueppert · May 22, 2015

"AND an off-grid inverter needs to isolate itself from the grid to work off-grid. How the PW intends to do this remains to be seen..."

Sure, it remains to be seen. It looks like SolarEdge will demonstrate it at Intersolar next month. But it's not exactly rocket science is it? (Though of course Elon can do that too if necessary!) Don't you just need to add a relay that isolates the home when the grid is down?

nwdiver · May 22, 2015

schueppert said:
Sure, it remains to be seen. It looks like SolarEdge will demonstrate it at Intersolar next month. But it's not exactly rocket science is it? (Though of course Elon can do that too if necessary!) Don't you just need to add a relay that isolates the home when the grid is down?

That's certainly one requirement but grid-tie inverters cannot regulate their output (below their capacity). If there's 8kW being produced that's what they invert into AC. They work in parallel AC coupled because the off-grid AC coupled inverter will increase freq to 60.6Hz when the batteries are charged and trip the grid-tie off-line... it's not ideal but it works. The AC coupled off-grid inverter does its best to 'act' like the grid... buffering the output of the grid-tie inverter with batteries. It can be hard on the whole system once the batteries are charged since the grid-tie will cycle on/off every ~5 minutes.

schueppert · May 22, 2015

Good point, so ability to regulate PV production is also a requirement , since you otherwise may not have anyplace to put it. But again, the SolarEdge inverters can certainly already do this (because they can just turn off optimizers as needed) and I suspect most other grid-tie inverters with MPPT can just set the MPPT so as to reduce the power. Grid-tie inverters have not supported mini-grid in the past because the market opportunity was small, not because it's hard to do.

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