How many Radians needed

Discussion about adding energy storage to grid-dependent inverters using OutBack Power technology
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How many Radians needed

Post by olegik » Thu Jul 30, 2020 8:32 am

Trying to figure out the minimum configuration for the following scenario:
- Rather large solar array on the ground - about 35kW size. All of that right now feeding directly into the grid. I do have the panels split up in 2 locations - 120 in one location and 22 in another. Both are connected at the subpanel and then fed into the main grid.
- 20kW propane generator that's connected to the main panel and has subpanel/shed load
- 32kW battery (can be split up) - not connected to anything yet.

What I would like:
- Charge batteries with extra from the PV
- Provide micro-grid when main grid is down
- Use generator as a last resort (first start batteries) and if batteries are depleted, start the generator.

I realize that Radian can deliver pretty much all that but for max 8kW (or more if stringed). But I don't want to purchase a bunch of them and spend a lot. So these are the Qs:
- Can I connect 1 Radian to this system and just have slower charging simply because the input will only be limited to 8kW?
- If not, what's the minimum config I can get to protect the batteries while charging them and not overloading them.

Appreciate any and all thoughts on this subject! :)

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Re: How many Radians needed

Post by raysun » Tue Aug 04, 2020 9:26 am

I'm not an AC coupled power user so can't speak to the PV production impact of hooking a standby battery storage/inverter system to the grid-coupled arrays, but as an off-grid user, can speak with some experience to the battery charging question.

The battery type, monoblock model, and C20 amp hour capacity aren't mentioned, but assuming Lead Acid of some sort, around 650AH maximum, the typical charging current would be in the 65 - 85A range for flooded lead acid. That is well within the GS8048 capability of 115A DC charging current. A sealed lead acid battery may accept a higher rate of charge that would exceed the charging amperage available, and the charging time extended, but a single Radian could eventually get the job done.

Given the size of the PV and backup generator, is the 8kW output from the Radian going to be enough to cover critical loads in a grid outage?

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Re: How many Radians needed

Post by pss » Tue Aug 04, 2020 11:46 am

I'm confused. Are you asking if you can install a Radian 8048a inverter, and a battery bank, then use the on board Radian charger fed from the grid tie to the Radian to charge the batteries? The answer to that question is yes you can. You of course will need to have the Radian tied into a load center to supply AC in the event of a power outage. And it would be run in backup mode.

The other question is would you use some of your panels and connect them to a charge controller and have the harvested PV charge the battery bank rather than the AC charger built into the Radian./ Then the Radian would still be in support mode and tied to a load. This is not as preferable because unless your Radian is also powering loads throughout the day, not just in backup mode, once the batteries were charged, the PV would not be harvested.

So strictly speaking, for backup use, in your case, charge with the Radian inverter charger and your non-grid tied AC source when PV is present and it would be no cost and no waste. But if running loads, then that is different.

Also, if the Radian is indoor in a climate controlled environment 80 degrees or below, it will run that 8k load pretty much without interruption. That would be pulling around 167 amps at 48 volts out of your battery bank. Using flooded lead acid batteries with a safe 50% discharge, a 40,000 watt 900 amp at C20 bank (like a bunch of Trojan renewables) can handle that, but even that is a 20,000 watt draw down to 50% and so at 8,000 watts, you are looking at 2.5 hours of battery life.

So your calculations really depend on how much load you want to power from the Radians and for what period of time. Then you need to determine the size and type of battery bank to do the job for the load and time. Then you determine how to charge this bank of batteries, from PV, inverter charger, a non-inverter charger, stand alone DC generator like from Polar power or your 20 kW generator.

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Re: How many Radians needed

Post by fcwlp » Tue Aug 04, 2020 4:52 pm

olegik wrote:
Thu Jul 30, 2020 8:32 am
- 32kW battery (can be split up) - not connected to anything yet.
Do not split your battery bank up! OutBack systems with more than one inverter are designed to run off of one battery bank.

For your present system, do the panels have micro-inverters or are they feeding into a typical grid-tie inverter. I have not done an AC coupled system but there are people here who have. There is an app note on AC Coupling to the Radian at ... radian.pdf.

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Re: How many Radians needed

Post by olegik » Tue Aug 11, 2020 12:00 pm

Appreciate all the replies!

I have several questions, really. To expand on batteries, it's actually 6 48v 14S batteries at this point. The batteries are Li-Ion, 45Ah cells in 2p configuration.
1) The panels are ground based and located about 250' away from the house. I have 125A fuses before the cables go through net meter and then into the grid
2) With the batteries mentioned above, do I need 6 Radians each connected to each battery or can I connect it in a different way?
3) Since panels are so far away, can Radian ("master") accept the main wire(s) and then figure out the master/slave or do I need to have them positioned at the subpanel out in the field where stringed microinverters all come together before going into the trench? That's the only place where I have them separate so I can distribute (if needed) them across Radians.
4) If that's the case, how do I connect it all back to the net meter?

I looked through user guide/documentation/YouTube videos that Outback has - and there is nothing there about these questions. All of the videos assume that incoming PV load is smaller than 8kW that Radian can support, so it all works well.

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Re: How many Radians needed

Post by clarkef » Sun Sep 06, 2020 8:26 pm

Coming to this thread a bit late.

So you have two GTI arrays, one with 22 panels and another with 120 or panels for a combined 35Kw of output. They are micro inverters, are fed underground to your panel where they are joined up to power loads and the grid. (Holy crap that’s a big array!)

Fcwip linked to the updated Radian app note on AC coupling, which has important details.

First point, you are going to need to rewire your existing GTI and change their termination point(s), by adding a critical loads panel so the GTI’s access and feed the critical loads, house loads, and the grid through the Radian. This allows the radian to provide the ‘grid’ signal from the radian - either as a pass-through or from the Radian(s) when the grid goes down.

Second: you can only AC couple 6KW of grid-tie per Radian. Basically this is to ensure that the GTI’s can’t push the radian around, ensures the backfeed through the GSLC doesn’t exceed the rated wiring and breakers, and that when the GTI trips off, that the radian(s) can pick up the load. If you want to use all 35K of grid tie, without other changes to break up their outputs, you would need 6 radian 8K’s. Otherwise you would need to carve up your GTI’s into separate outputs and land an appropriate number for the number of radians you want to use to the critical load panels. The others can be left on the main panel and will go offline when the grid drops.

Third, you should build one bank of batteries terminated at a common DC buss and feed all Radian’s from that common point (with appropriate wiring and protections, including a BMS with the appropriate safety controls for Li-Ion).

Fourth, the battery bank will AC charge via the radians from AC power provided by the GTI’s That are not concpsumed by load of the onboard Radian charger. If the batteries are full and the AC load drops, the radian(s) Will shift output frequency to drop some or all of the GTI output.

Hope that helps.

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