18kPV AC coupled vs DC

[cowduck]

I am getting a new system installed with the 18kPV without batteries. 30 400w REC panels.

Thinking if I should go DC coupled with Tigo Optimizers OR AC coupled with the enphase IQ8.

• If I go with the enphase microinverters, and there is a grid outage during the day with sun, will the inverter still stay powered on import solar?
• I understand with the DC coupled system, the inverter will stay on and produce solar during a grid outage.

 

[Ampster]

It is not AC coupled or DC coupled connection that determines whether the inverter stays on during a grid outage. It is the type of inverter that determines whether that inverter stays on. Grid Tie inverters typically do not stay on. However Enphase IQ8 micros can stay on with the addition of additional equipment.

 

[cowduck]

Do you know which additional equipment is needed?

 

[Ampster]

Not exactly. but from reading posts here and on other forums, it is several thousand dollars. That makes a hybrid inverter competive. Before you decide on some components watch some of Wills videos and peruse the forums to decide what your goals are. Then list the functions you want to satisfy those goals and then start looking at equipment that satisfies your goals. I don't know the rate structure in Texas but I do know the grid there is being stressed and there may be a good reason for risk management to consider batteries to get through the predicted power outages. My own system was designed to reduce my annual bill as low as possible and provide three days of backup power. With rates in California the batteries allow me to shift loads so I do not have to pay the high rates for peak time usage.

 

[Quattrohead]

I would not consider a system without some battery involved.

 

[zanydroid]

You need two or three Enphase boxes, including their automatic transfer switch/autotransformer and the gateway brain box. You need to use at least 50% IQ8 microinverters (preferably 100%). And for that you get a pretty flaky sunlight backup solution, that Enphase probably uses to upsell you to a battery because of the limited usefulness.

18kpv may not be able to grid form with IQ8. Folks have had trouble with IQ8 and other top end hybrids. I haven’t seen any posts from people with this equipment combo, so you would be a Guinea pig.

You should just go with tigo or plain RSD since that’s what people have the most experience with on the 18kpv. Being a pioneer requires bravery and expertise

Regarding battery less 18kpv, I’m not sure but you can check the manual or tag EG4/Luxpower support on that mega thread. I would hope you get that option for $6k… though again note that it is not a common config on this forum so you may get burned

 

[cowduck]

Got it. I’ll go the Tigo route.

Waiting for battery prices to drop a bit.

 

[pvgirl]

The 18kpv supposedly can be configured to provide backup power just using DC connected solar, without a battery. I have not heard any experiences with this setup. So I have no idea how well it works. Additionally if you have rapid shut down, or optimizers, without a battery as soon as the system loses solar power due to clouds, night, or overload, there is no way for the inverter power the circuits needed to send the signal to the panel modules, to allow them to start outputting power, until grid power returns. I don't think ac coupled backup will work on the 18kpv without a battery.

 

[zanydroid]

What kind of shading do you have?

 

[zanydroid]

there is no way for the inverter power the circuits needed to send the signal to the panel modules

Is this a known limitation with 18kpv? I thought it works on some inverters (SunSpec RSD is specced to facilitate it).

EDIT: it probably requires specific transmitter/RSD combos. I doubt TIGO CCA with optimizers will work batteryless

I don't think ac coupled backup will work on the 18kpv without a battery.

The only battery less microinverter backup I can think of is using Enphase as described above.

Well apart from those weird unlisted microinverters that have anti islanding disabled and are flipped to form grid.

 

[pvgirl]

Is this a known limitation with 18kpv? I thought it works on some inverters (SunSpec RSD is specced to facilitate it).

I'm not completely sure, I haven't seen this mentioned as a feature, so I wouldn't assume it's there. I've not seen any documentation that requires RSD modules that have the high power output allowed by the sunspec RSD spec. With just 0.6v at 10ma, per module, from standard SunSpec modules there's not much power work with.

 

[cowduck]

This is the specific optimizers I’ll be using.

Tigo TS4-A-O​

 

[zanydroid]

This is the specific optimizers I’ll be using.

Tigo TS4-A-O​

Those are activated via CCA and TAP which is an external transmitter. I don’t really expect an external transmitter to work, as compared to an integrated transmitter in the inverter.

So safest to assume that it won’t be able to start without some kind of battery. Maybe you can hack up a 12V or 24V small UPS/battery thing to keep it powered up in lieu of a full sized battery. It’s probably semi viable but I doubt you’ll get the vendor or installation contractor to tell you how to do this since it’s solidly a DIY science fair experiment

 

[n2aws]

I understand with the DC coupled system, the inverter will stay on and produce solar during a grid outage.

You mentioned tigo optimizers, and operating without batteries.

If the grid were to go down while PV power is available, things would continue to work.

The problem is what happens when the grid fails during no PV production. This is known as the 'dark start' problem.

Without batteries, there is no power source to keep the tigo CCA alive. So when the sun comes back out the panels will still be shutdown because they aren't receiving the power signal for the CCA.

There are several things you can do to address the dark start issue. If you add batteries, eg4 has a document about using the CCA to deal with dark starts. But if thats not an option..


Have an alternate power source available to power the CCA. A 12v battery.. a small solar panel with a 12-20v output, a small UPS and a wall wart plugged into it, etc. It'd just require a manual process to start the 18kpv without grid power or batteries, when using RSD.

What I did was wire the power input on the CCA with a 30a anderson powerpole. Get a 12v wallwart and put a 30a powerpole on that. During normal operation, plug the wallwart into an outlet served by the inverter (so that when RSD is triggered the outlet loses power and shuts the CCA off, complying with RSD requirements). Then, I added an anderson powerpole to the batteries on my electric gate opener.. (if you don't have one, you can just buy a small 12v 5ah battery, and keep it trickle charged). If the power goes out and I'm in a dark start scenario, I just disconnect the powerpole from the wallwart connection, and plug the CCA into the battery to get the PV arrays going again.

 

[zanydroid]

What I did was wire the power input on the CCA with a 30a anderson powerpole. Get a 12v wallwart and put a 30a powerpole on that. During normal operation, plug the wallwart into an outlet served by the inverter (so that when RSD is triggered the outlet loses power and shuts the CCA off, complying with RSD requirements). Then, I added an anderson powerpole to the batteries on my electric gate opener.. (if you don't have one, you can just buy a small 12v 5ah battery, and keep it trickle charged). If the power goes out and I'm in a dark start scenario, I just disconnect the powerpole from the wallwart connection, and plug the CCA into the battery to get the PV arrays going again.

I’ve thought about doing this during dark start, but what is the situation where it happens? Don’t you set a SoC reserve on the main battery?

Note also that AC coupling with no DC also has a dark start problem except you need the whole inverter to come up. Might need a generator or bigger “jump” battery.

 

[n2aws]

I’ve thought about doing this during dark start, but what is the situation where it happens? Don’t you set a SoC reserve on the main battery?

Note also that AC coupling with no DC also has a dark start problem except you need the whole inverter to come up. Might need a generator or bigger “jump” battery.

In my case, yes.. the likelihood is minimal. I just like to have plans a, b, and c.

However in the OPs case with no batteries, it'd be anytime the grid failed overnight and into the next day. In that case, the PV would be in RSD, and thus wouldn't produce power to start the inverter back up.