Dump batteries to grid at night?

[Shale MacGregor]

Hola fellow photon pharmers.

I am in the process of expanding my storage capacity closer to 200kWh to help handle cloudy days, and am considering adding some more panels to over panel to keep my usage as clean as possible.

If I do this then there will be sunny days (weekends primarily) that I am not using the full capacity of my system and I will know this at least 12 hours in advance.

Most systems don't support this so I am trying to figure out the best way to dump power from batteries back to the grid for the benefit of the environment and the slight (50%) credit buildup on account in case I need to use the grid sometimes.

I already have the ability to produce 240v AC at 25KW from batteries, is there a good way to take that and frequency match it to the grid so I can push it back up the line? It doesn't have to be at the full rate but would need to be able to do so at least at 5KW to get enough dumped.

Thanks for any input!

 

[Hedges]

There are hybrid/AIO/battery inverters that can drive power various ways. The trick is how to configure them, and if they will do this easily.
Probably (expensive) Sol-Ark, maybe some of the cheap imports.

An (expensive) inverter like my Sunny Island, if connected to a 48V battery which is at higher voltage than Sunny Island thinks it should be, will export to grid when so configured.

There are some GT PV inverters with modes besides MPPT. I have older SMA Sunny Boy 5000US which can be set for "turbine" (e.g. hydro) that seek constant voltage. If you were to rectify AC from your free-running inverter and feed DC input, should be able to process it. Use transformer isolation because PV inverter either grounds one lead or connects PV input to utility grid AC. Use a precharge resistor so inverter capacitor bank doesn't look like a dead short to your AC source.

Since induction motors draw power based on slipping below the RPM frequency would indicate, I wonder if a motor powered by your system, spinning an induction motor connected to grid, would also work? Fairly large motor, 5kW is in the 5 ~ 10 HP range. I think some wind turbines operate this way - just and induction motor with blades attached, and grid power brings them up to speed?

But feeding some grid-tie inverter seems the cleanest and safest small-scale approach.

 

[Shale MacGregor]

Helpful as always! I will take some time to look through these options. The motor option makes me curious. I could choose a motor that outputs a higher voltage so when connecting it to the grid it feeds it back rather than draws it, but how does it match the grid frequency?

I lack some of the knowledge to wrap my head around this stuff without specifically researching it so I will do more digging.

 

[Shale MacGregor]

I should also clarify, without replacing a lot of things or running new lines, I already have the AC from the 25KW of non grid tie inverters fed back 200 feet to where it could tie into the grid, but the batteries are located 200 feet away.

 

[Hedges]

Not motor voltage, just an induction motor suitable for the voltage. Some are good for 208V to 250V.
To spin it, your inverter has to power up a large motor, probably a starting surge issue. A large brush-type motor should start more easily.

I once got a 3-phase motor/generator welder. I "jump started" it with 24V of car batteries feeding the welding cables, then connected two legs to 240VAC and it ran. Maybe with enough DC it would have backfed the grid. But it had resistive elements as well I think, something very lossy.

Your inverter could feed a VFD to spin a 3-phase motor, as the mechanical source.

Some VFD are bidirectional, e.g. elevator going down backfeeds the grid. But no anti-islanding?

No guarantee any of this will work, or efficiently!

A grid-tie inverter meant for high-voltage AC input from a wind turbine would be ideal. Or one for 48V, which could be connected to your battery.

Ah, 200'. So look for a high voltage wind turbine setup. "Windy Boy", for instance, or other brands. Be sure to precharge (soft start) the DC from rectified AC.

 

[Hedges]

Easiest though to switch PV to GT PV inverter and backfeed from that rather than batteries. Simple, just run enough wire and set up a switch.

 

[Shale MacGregor]

Easiest though to switch PV to GT PV inverter and backfeed from that rather than batteries. Simple, just run enough wire and set up a switch.

I was considering that but will need to change out all inverters, but it might be the best option if I really want to do that.

I currently have non GT inverters with a total of 25KW output and 30KW pv input and was trying to bridge the gap another way. From all of the options you gave my guess is that I should put in one smaller GT inverter without mppt that can feed the grid to keep batteries at a lower voltage and then set some trigger to only do it on certain days.

Thanks again!

 

[Hedges]

Not necessarily change out, but maybe switch PV.
Get a few GT PV inverters, AC side goes to utility grid. DC to a transfer switch, I'm thinking of four PV breakers, two facing one way and two the other, ganged with a pin through handles. One direction connects PV to off-grid inverters, other direction to GT PV inverters.

Of course what you really want is a do-it-all system that monitors grid power flow an can drive either way.
My Sunny Island can, but doesn't have the control built in. People have tried altering settings over RS-485 to make it charge vs. discharge from grid.