How to size strings- DC-DC optimizer required?

[ArkLightning]

Howdy folks,

Question about string size. So we're finally ready to pull the trigger on a solar array + storage. Settled on the Sol Ark 15K which has (3) MPPT inputs and max DC input of 19.5KW directly connected. Our utility allows maximum nameplate array of 25KW. System will be ground mount, no shade issues. Hybrid grid tied.

I was looking to use 450W 72 cell panel (Risen RSM144-7-450M for sake of argument) and running 13 in series or 5.85KW per MPPT. VoC is 46.43 and adjusted for our climate for the coldest day of the year (x 1.13) makes the math 52 VoC x 13 panels = 676V. Sol Ark has a limit of 500V so no bueno. Question is if I use a DC-DC optimizer, such as a Tigo TS4-xx-xx does this magically adjust voltage to =<500V? Is this is preferred method or less than ideal design? Is there programming on the Tigo to hard limit voltage to 500V or does the inverter communicate with the optimizers?

The remaining KW available (we want as close to 25KW solar array as possible) will be AC coupled and waiting for the Sol Ark micro inverters to hit the market Q1 '23. I would prefer a single inverter solution (mainly for cost reasons). Am I thinking right or myopic in my design? I also understand genny may not be supported when AC coupled but that's another topic for another day...

Battery wise I'm not 100%. Right now the SoK looks great on paper. Lithium Iron Phosphate seems to be the way to go and they certainly hit a nice price point.

Thanks in advanced

 

[jaggedben]

You'd have to consult the Tigo documentation for specifics but as I recall it will not limit string voltage like you want.

 

[ArkLightning]

You'd have to consult the Tigo documentation for specifics but as I recall it will not limit string voltage like you want.

I believe you're right. Tigo isn't a magic bullet to fix bad design. I used the Sol Ark Panel Sizing Tool ver 3.7 (link below) and picked Trina 455W panel. I hit the 'maxout my panel' button and it suggested a string of 7 panels & 2 strings per MPPT. VOC dropped to ~350 and each MPPT accepted 6730W nameplate.

Solar Panel Sizing 4.5

www.sol-ark.com

 

[Designer101]

I believe you're right. Tigo isn't a magic bullet to fix bad design. I used the Sol Ark Panel Sizing Tool ver 3.7 (link below) and picked Trina 455W panel. I hit the 'maxout my panel' button and it suggested a string of 7 panels & 2 strings per MPPT. VOC dropped to ~350 and each MPPT accepted 6730W nameplate.

Solar Panel Sizing 4.5

www.sol-ark.com

Tigo has many versions there is one that do optimizations well TS4-A-0

 

[ArkLightning]

Tigo has many versions there is one that do optimizations well TS4-A-0

Yes sir. If I was a ground mount design with inverter(s) located outside I would not need rapid shutdown. I'm placing the inverter inside the building so therefore I need DC optimizers (for the rapid shutdown functionality) for the 50' of cable running inside (to the utility room). I'm going with the TS4-A-O instead of the F-O (rapid shutdown only) since the cost difference is only $10-$15.ea and maybe they work...

The design I'm going with is 2 strings of 7 panels per MPPT input so 6300W per MPPT. That gives me 18.9KW of nameplate loaded on the inverter and I'm foregoing the micro inverters on the side. I'll use my small genny for back up and making the design more simple. Recently our utility lifted the restriction of 25KW (AC) connected to the grid so playing with the idea of doubling up the solar to 37.8KW... we're all electric and we use way more electrons in the winter than summer. I can't cut the cord since I need extra power in the winter but I'm getting close... open loop geo-thermal is an energy pig but much better than the propane alternative. I would hate the think how much battery I would need to cover 3 days of shade to power the geo in the winter

 

[Carultch]

Yes sir. If I was a ground mount design with inverter(s) located outside I would not need rapid shutdown. I'm placing the inverter inside the building so therefore I need DC optimizers (for the rapid shutdown functionality) for the 50' of cable running inside (to the utility room).

I'd like to make a counterpoint to this. Yes, rapid shutdown applies for systems "on or in" buildings, and if a part of the system is inside a building, it would require rapid shutdown in some form or another. However, if you have a system on a non-building, you don't necessarily need module-level rapid shutdown. or any rapid shutdown on your free-standing framework array. If the array is not on or in a building, all you'd need is rapid shutdown of the portion of the DC feeders that are inside of the building.

Module-level rapid shutdown very likely could be what is most practical with equipment currently available, but it isn't necessarily the only way to do it, in the situation you describe.

I'm going with the TS4-A-O instead of the F-O (rapid shutdown only)

The way the part numbers work:
TS4 is the product line

The middle letter refers to the mounting style. -A- is add-on that can go anywhere on a frame with a bottom lip. -R- is retrofit (the previous style that required corner-mounting). Lack of this letter = module-integrated.

The suffix refers to the functionality.
-F = rapid shutdown (F=fire safety) thru power-line-carrier signals. Simplest type that works for rapid shutdown, completely different signal type from all the others.
-S = rapid shutdown (S=safety) thru radio transmission. Also allows for monitoring.
-O = like -S, but with optimization (O) added. String sizing calculation is the same as it is for wild PV.
-L = (L=Long strings), like -O, but with provisions for extending the string length, beyond the maximum length for wild PV. I haven't explored how the string sizing calculation changes, since I've never worked with this variant.

For all of -S, -O, and -L, you'll need the CCA and TAP to set up the radio network that communicates the monitoring and shutdown.
For -F, you'll either need an inverter with a built-in transmitter, or a separately-installed external transmitter.

 

[tom baker]

Need you to update your occupation and relation to electrical industry. Send msg to any moderator.