Minimum battery to retain magic smoke

[okaygolombruler]

Hello, newbie here with what is starting to look like a 'stump the chumps' question. :/

Demand/Usage: projected 27MWH/year, rising to 35MWH/year as we finish electrification and move to EVs. Peak power (less EV chargers) ~15kw.

I'm looking to install my system in two phases.

Phase 1, this year: 30-35kw of panels + the remaining charge controllers, inverters, bus bars/panels, BoS, ... needed to sell back to the grid.

Phase 2: (future work) battery bank sized according to our evolving usage and self-sufficiency targets.

I understand that after Phase 1 we may not have adequate power to run loads during a power outage.

I don't want to spend a bunch of money on battery tech I don't want (LFP) up front when the tech I want (VRFB) is coming down the pike. (Worst case, I end up having to install LFP batts later.)

So far every vendor I've talked to has said "you can't run DC-coupled / string inverters without a battery", and I can't figure out why. (other than not wanting support calls / wanting to sell batteries...)

In AC-coupled, microinverters convert DC to AC and push it towards your mains without any kind of battery. No grid, no power, have a nice day.

In a batteryless DC-coupled system, MPPT/charge controllers convert DC to lower-voltage DC and feed it to inverters which convert DC to AC and push it towards the mains without any kind of battery. No grid, no power, have a nice day.

1) What am I missing here?

2) Is there a minimum amount of battery required for a DC-coupled system like this to work safely?

3) If there is a minimum, how is it calculated? (max inverter draw? max charge controller output? max power slew rate on the DC bus? max AC load?)

I could understand wanting to 'smooth out' the power available to the inverters (say from a passing cloud bank) but I feel like I'm missing a core concept here. Any ideas?

 

[BentleyJ]

Battery storage capacity is determined by a few factors. The following is not necessarily in order of importance.

1) 30MWH per year is a big system and will require multiple inverters in parallel even if you were to use higher output commercial equipment in the 30kW and higher range its more than one inverter. In most cases, depending on Brand, inverters that have battery-less operation capability is usually limited to 1 inverter. With multiple inverters batteries are required.

2) Battery storage is usually calculated based on overnight and/or multiple days of desired or required energy needs to meet as goal such as keeping a business open during an outage of X days or be able to charge an EV enough to commute to a job Etc.

3) Recharge rates of typical LFP batteries is generally limited to 0.5C or lower for longest life span. With a large system such as proposed here perhaps a high voltage battery (400V) such is used with commercial inverters is called for. A 48V, 1000Ah LFP battery would only be able to absorb about 300A (0.3C) which is 16.5kW. Even at 500A, which is hard on the cells, is only 27.5kW. A 36MWH system is going to produce 100kWh per day.

4) How much of daily PV production is used immediately vs. how much needs to be stored for later use or otherwise is wasted.

 

[rogerheflin]

Hello, newbie here with what is starting to look like a 'stump the chumps' question. :/

Demand/Usage: projected 27MWH/year, rising to 35MWH/year as we finish electrification and move to EVs. Peak power (less EV chargers) ~15kw.

I'm looking to install my system in two phases.

Phase 1, this year: 30-35kw of panels + the remaining charge controllers, inverters, bus bars/panels, BoS, ... needed to sell back to the grid.

Phase 2: (future work) battery bank sized according to our evolving usage and self-sufficiency targets.

I understand that after Phase 1 we may not have adequate power to run loads during a power outage.

I don't want to spend a bunch of money on battery tech I don't want (LFP) up front when the tech I want (VRFB) is coming down the pike. (Worst case, I end up having to install LFP batts later.)

So far every vendor I've talked to has said "you can't run DC-coupled / string inverters without a battery", and I can't figure out why. (other than not wanting support calls / wanting to sell batteries...)


In a batteryless DC-coupled system, MPPT/charge controllers convert DC to lower-voltage DC and feed it to inverters which convert DC to AC and push it towards the mains without any kind of battery. No grid, no power, have a nice day.

1) What am I missing here?

2) Is there a minimum amount of battery required for a DC-coupled system like this to work safely?

3) If there is a minimum, how is it calculated? (max inverter draw? max charge controller output? max power slew rate on the DC bus? max AC load?)

I could understand wanting to 'smooth out' the power available to the inverters (say from a passing cloud bank) but I feel like I'm missing a core concept here. Any ideas?

DC coupled without a battery will ONLY be able to feed back to grid. Most(all?) of the inverters are unstable without a battery on the load terminals(the terminals that are what can run loads when the grid is out) when the solar is coming and going. Ie early in the morning and late in the evening and any cloudy times there will be a lot of blips. When I was working on my battery (battery breakers turned off-about 10 minutes before I bypassed it) just as the sun came out it had enough outages to cause my router to lose all of its saved settings, so it was not good for anything that was connected to the load terminals.

My system does around 15MwH a year and I have 31.5kwh of batteries, 10.3kw bifacial panels and a 12kw inverter (my peak power is around 12kw). You would need 2 of my inverters and probably at least double the batteries. The inverter I have can take significantly more panels than I have. My batteries can sustain my inverters full charge rate 12kw, and can also supply my inverter for its full rate (including surge).

It needs to be sized for max charge rate OR max expected inverter rates, whichever is the limiting factor. Typically charge is .5c and discharge is 1c so typically charge is going to be the limiting factor if you have a lot of panels.

 

[okaygolombruler]

Battery storage capacity is determined by a few factors. The following is not necessarily in order of importance.

1) 30MWH per year is a big system and will require multiple inverters in parallel even if you were to use higher output commercial equipment in the 30kW and higher range its more than one inverter. In most cases, depending on Brand, inverters that have battery-less operation capability is usually limited to 1 inverter. With multiple inverters batteries are required.

2) Battery storage is usually calculated based on overnight and/or multiple days of desired or required energy needs to meet as goal such as keeping a business open during an outage of X days or be able to charge an EV enough to commute to a job Etc.

3) Recharge rates of typical LFP batteries is generally limited to 0.5C or lower for longest life span. With a large system such as proposed here perhaps a high voltage battery (400V) such is used with commercial inverters is called for. A 48V, 1000Ah LFP battery would only be able to absorb about 300A (0.3C) which is 16.5kW. Even at 500A, which is hard on the cells, is only 27.5kW. A 36MWH system is going to produce 100kWh per day.

4) How much of daily PV production is used immediately vs. how much needs to be stored for later use or otherwise is wasted.

1) Yes, the bar-napkin math suggests something like 4 MNS Barcelona and 5 Rosies. That'$ another rea$on I'd like to defer the battery for $ome time....

2) Yes, but for phase 2. Phase 1 is minimum viable battery.

3) I thought the battery would only pull as much as the BMS wanted to charge it? I would expect the rest to go to the grid, or panel shutdown if there was nowhere else to sink it. Do I misunderstand? Or is the idea that the battery bank should be able to sink the full output of the chargers at 0.5C so

4) It will depend on the time of year, far and away the heatpumps are the largest potential draw.

 

[Brucey]

1) Yes, the bar-napkin math suggests something like 4 MNS Barcelona and 5 Rosies. That'$ another rea$on I'd like to defer the battery for $ome time....

2) Yes, but for phase 2. Phase 1 is minimum viable battery.

3) I thought the battery would only pull as much as the BMS wanted to charge it? I would expect the rest to go to the grid, or panel shutdown if there was nowhere else to sink it. Do I misunderstand? Or is the idea that the battery bank should be able to sink the full output of the chargers at 0.5C so

4) It will depend on the time of year, far and away the heatpumps are the largest potential draw.

Jusy an fyi, Midnite Rosie doesnt currently officially support sellback to grid. They may submit for ul1741sb listing at some point.

 

[Brucey]

I don't see why you need 5 rosies if your peak demand is 15kW. Two Rosies should be sufficient for that.

 

[okaygolombruler]

DC coupled without a battery will ONLY be able to feed back to grid. Most(all?) of the inverters are unstable without a battery on the load terminals(the terminals that are what can run loads when the grid is out) when the solar is coming and going. Ie early in the morning and late in the evening and any cloudy times there will be a lot of blips. When I was working on my battery (battery breakers turned off-about 10 minutes before I bypassed it) just as the sun came out it had enough outages to cause my router to lose all of its saved settings, so it was not good for anything that was connected to the load terminals.

That's acceptable for Phase 1. I just keep getting "It Can't Happen Without Batterie$$" with no explanation.

My system does around 15MwH a year and I have 31.5kwh of batteries, 10.3kw bifacial panels and a 12kw inverter (my peak power is around 12kw). You would need 2 of my inverters and probably at least double the batteries. The inverter I have can take significantly more panels than I have. My batteries can sustain my inverters full charge rate 12kw, and can also supply my inverter for its full rate (including surge).

It needs to be sized for max charge rate OR max expected inverter rates, whichever is the limiting factor. Typically charge is .5c and discharge is 1c so typically charge is going to be the limiting factor if you have a lot of panels.

IIRC it was <= 4 Barcelonas and 5 Rosies. The perfect-world math had the Barcs maxed at about 700A @51V on the DC bus and running at max continuous load, each of the 5 Rosies would kick out a total of 150A@240VAC. Figuring 25% losses all-in, that's about 27kw peak AC power. At 15KW peak load, about 12kw going back to the grid. (Less than that, b/c peak load here is winter, but that's the top-line number.)

So, to confirm, you're saying that my minimum battery array must...

1. produce 700A @ 51VDC (the most Rosie could consume when grid-up)?
2. produce 388A@51VDC (the most my loads could consume when grid-down)?
3. consume 700A @ 51VDC (the most the Barc could produce if no load, grid down?)
4. Consume (700-388=)312A @ 51VDC (the most the Barc could produce above my peak load?)
5. ...something completely different?

 

[okaygolombruler]

I don't see why you need 5 rosies if your peak demand is 15kW. Two Rosies should be sufficient for that.

I would like to invert and push the rest back to the grid, for net-metering purposes.

 

[Brucey]

I would like to invert and push the rest back to the grid, for net-metering purposes.

The rosie is not a hybrid. It's an off grid unit that can handle grid support etc but selling back to grid isnt officially supported.

 

[okaygolombruler]

The rosie is not a hybrid. It's an off grid unit that can handle grid support etc but selling back to grid isnt officially supported.

Got it, thanks for the heads-up! For the sake of the battery sizing arithmetic, let's assume it's a fictional "Wosie" which is identical to a Rosie except it supports hybrid operation.

 

[Brucey]

Got it, thanks for the heads-up! For the sake of the battery sizing arithmetic, let's assume it's a fictional "Wosie" which is identical to a Rosie except it supports hybrid operation.

So for my Rosie, I budget 150A at 48V for 7kW output. And then it can surge to 15kW for up to 60 seconds. 300A. I use an mnedc250 breaker. Midnite specifies 3/0 cable for the Rosie. Temporarily using 1/0 but runs are very short, will replace with 4/0 once my temco crimper arrives.

Barcelona would be 200A on the bus for loads or charging. So in the case of a single rosie and a single barcelona the bus bar would need to be rated for 300A minimum. I like the victron lynx power ins and distributors keeps things clean and tidy, rated for 1000A but prob want to keep it around 800A max.

 

[okaygolombruler]

So for my Rosie, I budget 150A at 48V for 7kW output. And then it can surge to 15kW for up to 60 seconds. 300A. I use an mnedc250 breaker. Midnite specifies 3/0 cable for the Rosie. Temporarily using 1/0 but runs are very short, will replace with 4/0 once my temco crimper arrives.

Barcelona would be 200A on the bus for loads or charging. So in the case of a single rosie and a single barcelona the bus bar would need to be rated for 300A minimum. I like the victron lynx power ins and distributors keeps things clean and tidy, rated for 1000A but prob want to keep it around 800A max.

OK, so 4 Barcs in would be 800A sourced, so that would work with the Lynx you mentioned. I'll keep that in mind; I understand some of the blue boxes now have US certs, though I need to read up on it. The question though, is do I need a battery to smooth out power dips when feeding the Wosies while the grid is up? Or can I just pass it through to the grid in Phase 1 (no outage backup) and add storage in phase 2?