New Hybrid Build Planning Discussion

[babgvant]

We just bought a lot, and have started the planning phase for an all-electric passive house. All the design, sizing, build work will be handled by professionals, but I suspect that they won't have much experience doing what we want because there isn't much demand for a grid-tied-off-grid-capable housing around here. My goal is to better understand the options, devices, trade-offs, etc. around selecting from the massive pool of approaches to solving that problem.

The house is going to be on a 33x150 lot the Chicago suburbs and it will be grid-tied. We get power outages surprisingly often (out for 30-45m last night). I would like the system to seamlessly handle the off/on grid transition without any downtime. We have a NG generator at our current house and it takes ~30s to transition, so all the sensitive electronic devices have to have UPS, I have to reset a bunch of clocks and other state storing devices every time it happens. I would like to make that go away along with having the ability to be mostly self-sufficient.

We have two EV and the house will be all electric, I'm guessing we'll need at least 400A service. We haven't had that discussion with the architect yet to get their thoughts, so mostly a way to frame the discussion right now.

The roof of the house and detached garage will be flat and angled south to maximize PV potential. I suspect that the architect will want to use AC coupled panels because that's what people do around here. We have AC coupled now because it totally made sense for our house, but I'm not sure what the right answer is here given our goals and design freedom with the new house.

I suspect that the architect will want to use either Tesla or Enphase AIO battery stacks because it's what they will be familiar with and they are very consumer friendly. It's the "safe" choice. I'm not in love with this idea for a few reasons; cost, Tesla battery chemistry, load restrictions, and scaling complexity being the top few. It would be great to better understand the trade-offs around this approach vs. my current thinking.

My current thought is to use something like an EG4 18k (or Sol-Ark 15k) to handle the battery/PV/grid integration; sizing at one hybrid-inverter per 200A panel. Probably starting with ~30kW of PV and 30kWh of battery, and add more battery/PV as we figure out consumption. This isn't as "safe" because I'm probably going to have to push for it, and do a lot of education around the trade-offs with the architect to get them on-board.

Inital Questions:

1. If loads exceed the inverter capacity in a grid-tied hybrid system will they seamlessly pull additional power from the grid? Best scenario is charging both EV at the same time (~22kW) as normal household loads like the heat pumps, stove, cook tops, etc.
2. DC vs AC PV? I have a decent handle on the high level trade-offs around this choice. In our current setup it totally made sense to opt for micro-inverters, but I don't know much about the real world trade-offs of a DC system. I'm leaning toward DC because the conversion losses associated with round-tripping might be larger than potential shading concerns. The panels will be on a "unshaded" roof facing south, but it is semi-urban residential area so there is some potential that the trees on the other side of the alley from our garage may shade the panels on the garage a little.
3. Is the hybrid inverter sizing scheme sensible or should we be sizing larger?
4. Is a hybrid inverter setup the best way to do this? If not, what are the better options?
5. What other things should we be aware of? Very much at the I don't know what I don't know phase :D.

Thanks

 

[DIYrich]

If you have net metering, don't bother with batteries to handle the charging of the EV's.
Put the EG4 or Sol-Ark on the panel that powers the house.
DC PV is the way to go with an EG4 or Sol-Ark.
If you have more than 15kW of panels, consider AC coupling the excess, or parallel a 2nd inverter.
#1) Yes.
#2) DC. Use optimizers if you must.
#3) Sizing is fine. Unless you need 2 inverters for PV, I would try to stick with one. Maybe put just one charger on the Inverter.
#4) EG4/Sol-Ark is the way to go.
#5) Design your complete system now, even if you build it in sections. Then you will see that Enphase storage is so expensive, and not the way to go. The original Tesla Powerwalls were Li-Ion and a fire risk. If they haven't switched to LiFePO4, then don't put them in your house or attached garage.

 

[websolar]

I'v created two simple designs for you:

1) Sol-Ark 15k + 19kW PV: https://websolar.cloud/share/3b23b4...91c3ba453756c539cb04aea6935de65ff3f5818b29422
2) Enphase: https://websolar.cloud/share/b79c68...a7fdf98494eb6e4f3237508d07e9a4e4da8cf5199523f

You can interact with the designs by inputting your consumption and system cost.

Also, if you want to avoid shading problems, check out SolarEdge or microinverters.

 

[peufeu]

1. If loads exceed the inverter capacity in a grid-tied hybrid system will they seamlessly pull additional power from the grid? Best scenario is charging both EV at the same time (~22kW) as normal household loads like the heat pumps, stove, cook tops, etc.

It depends how you wire it.

These machines contain only one physical inverter, which is bidirectional and capable of handling all working modes:
- drawing from grid to charge the battery (if you program it to do so),
- injecting power into the grid (grid-tied mode) either to sell or to compensate the whole house's power use,
- acting as a stand-alone offgrid inverter in case of blackout.

Since this inverter is bidirectional, the AC side isn't really an "output", so I'll call it "AC port".
Then there is a bunch of internal relays for routing, to connect the inverter's AC port to various wire terminals. On my Solis there's two: grid and backup ; on SolArk/Deye there are three: grid, backup, and I don't remember what the third one does.
Here's a random schematic:



Notice two types of loads: "backed-up loads" which are connected to the inverter's backup port, and "Loads" which are connected to the grid along with the the inverter's grid port.

When the grid is present the inverter works in grid tied injection mode:
- Non-backup loads draw power both from the grid and the inverter's grid port. This is the normal way a grid-tied inverter works.
- Internal relays connect Inverter's backup port to grid port, so the backup loads are powered through these relay in "passthrough mode" as they call it.
So when grid is present
- there is no difference between grid and backup port, they're connected together.
- the answer to your question 1) is "yes", but for loads on the backup port, "up to the maximum current allowed in passthrough mode". It depends on the current ratings of the relays, terminals, etc.

Note backup load power goes through the "inverter" (as in: the big SolArk box) but not through the inverter (as in the electronics that convert DC to AC), only through relays. In this mode it is possible to start a huge motor with considerable inrush current on the backup port, since it's really connected to the grid port, and the inrush current spike comes from the grid.

When the grid is down
- Internal relays disconnect the grid port
- Non-backup loads are not powered
- Backup port is powered by the inverter from solar & battery

In this mode output power is limited by the inverter electronics, maximum battery current, etc, so even though the backup loads are still connected to the same backup port, you can expect a different maximum current/power rating. Also the grid isn't there to provide the huge inrush current spike for starting your big motor, so that only depends on the inverter itself.

Summary
- Check the maximum allowed current in passthrough mode. If you exceed the rating, then you need to move some loads from the backup port to the grid port: non-essential stuff, and loads that will restart whatever they were doing when the power comes back. Most washers, dryers, etc, do that.

- If you only plan for short blackouts (1 hour) then it's okay to put everything else on the backup port. However 15kW at 48V is a ton of amps, so make sure the batteries and cables can handle it...
- If you plan for extended blackouts then it is not okay. In winter during gray dull days your solar production will not be enough. In this case you need a special panel, wiring, sockets etc for critical loads, and if you expect a long blackout you flip a few breakers to cut off everything except the critical panel.

> DC vs AC PV?

DC.

Is a hybrid inverter setup the best way to do this? If not, what are the better options?

Yes

AC coupling depends on frequency shifting for regulating power, and this is much slower than the internal control loop in a DC based hybrid.

In other words with AC coupling, during a blackout, if you switch large loads the micro inverters may not react fast enough and you may get spurious disconnections and safety systems tripping, huge nuisance.

What other things should we be aware of? Very much at the I don't know what I don't know phase :D.

Don't assume anything, in particular don't assume the equipment can do what you want until you know it can.

Find someone who has done it and check how they did.

If someone utters the words "cloud", "app", or otherwise implies that using any non-local solution to configure and/or manage equipment that you will be counting on to power your house in a situation where there is no grid and no internet, then this person is a clown. "The cloud" is fine for data logging, who cares. But if you have to use a cloud app to change the settings because there's no front panel with display and buttons, then it's garbage.

The system will not do what you want out of the box. You will inevitably try to make it do what you want. This is much easier if it has open and documented communication protocols like modbus, HTTP, etc. So a good question to ask is how to integrate it with HomeAssistant or any other open source home automation package. If the answer is "well you buy our sleek cloud-based home automation interface which costs $1000" then nope.

Likewise if you don't have net metering, ask "how do I direct excess solar power to the water heater and EV charging?"

If you're feeling extra cruel, you can ask the installer how to modulate power on your airconditioning so it uses excess solar without drawing from the grid. That one will probably involve some coding, lol

This also applies to EV charging stations: if it doesn't have a simple and clean way to set charging power depending on solar production then nope. Don't assume it will work. You have two EVs so that will be two charging stations: for example if they both come with their own smartmeters then you get two extra smartmeters, and when your solar produces 5kW excess, they both measure it and they both decide to use 5kW which instead of sharing it, therefore, d'uh.

It is quite similar to doing a puzzle where none of the pieces were designed to fit together.

 

[Sleepingbeautyc]

DC vs AC PV? I have a decent handle on the high level trade-offs around this choice. In our current setup it totally made sense to opt for micro-inverters, but I don't know much about the real world trade-offs of a DC system. I'm leaning toward DC because the conversion losses associated with round-tripping might be larger than potential shading concerns. The panels will be on a "unshaded" roof facing south, but it is semi-urban residential area so there is some potential that the trees on the other side of the alley from our garage may shade the panels on the garage a little.
Thanks

I saw an interesting video on this. The guy does a test to see the reaction of his power output from the solar array when one panel is covered. He has a dc system. There is a negligible drop in energy. I have dc optimizers on my system that are not necessary for shading but they will be awesome if one panel goes dark.