PV string sizing: Cutting it too close?

[HX_Guy]

Another question here, hope you guys don't mind and can give me some constructive criticism.
Ideally I would like to setup my system in 5 strings of 10 panels, however using this PV String Sizing Tool it looks like the system would be at the Inverter's lower limit of operation and if the temperature goes above 45.6º (114º F) which is often does, would the system shut down?



The installer suggested a 4 string x 12 panel setup, but I said I would prefer the 5 x 10, which he said we could do as well.
With the 4 x 12 setup, the numbers are much more within range.



I'm pretty sure I know what the right answer is but figured I'd confirm here. Thanks for any input.

[silversaver]

I am surprise your installer spot Min DC Voltage on your inverter 230V. You have to calculate the high temp operating voltage base on panel's Temp Coefcient (Pmax):0.46 %/degree C. It looks like your extreme high temp operating voltage is right at boarder line 230V. You can take the chance. I have recently hit the same problem with power loss over 35C outside temp, and my installer have no idea at all about high temp operating voltage but using VoC to calculate string sizing......

The inverter will not shut down, but you will see power loss.

[HX_Guy]

To be fair, the installer actually suggested a 4 x 12 setup, which would give a high temp voltage of 276.2, well within range. That's probably the setup I have to go with, but I don't like it. Maybe I can look at some other inverters and see if they allow for lower operating voltages than 230V, though I think that's pretty standard?

[silversaver]

I don't think you can find other inverter give you the lower value. I was wonder how their MPPT tracking on multi roofs. The MIX technology.

[HX_Guy]

Well, there are other inverters that require a lower min. voltage, such as the SMA SB5000TL, which only needs 175v. The problem is that it can only handle 24 panels maximum, 8 per string in 3 strings, so the voltage is 184v at high temp, barely within the limit.

There's also the Power One PVI 4.2 which goes down to 140v min but I would need 3 inverters for the size system I want.

I'm going to keep looking at other options but it sounds like the easiest and cheapest is the Fronius with a 4 string x 12 panel setup.

[silversaver]

Well, there are other inverters that require a lower min. voltage, such as the SMA SB5000TL, which only needs 175v. The problem is that it can only handle 24 panels maximum, 8 per string in 3 strings, so the voltage is 184v at high temp, barely within the limit.

There's also the Power One PVI 4.2 which goes down to 140v min but I would need 3 inverters for the size system I want.

I'm going to keep looking at other options but it sounds like the easiest and cheapest is the Fronius with a 4 string x 12 panel setup.

I told you already. There's none at that capacity unless you use 2 inverters

[HX_Guy]

I can't even seem to find a combination that would work with 2 inverters, seems it would take 3 inverters. Or I need different panels. But that's not really an option, I'm not looking to spend additional money so I have to make it work as best as possible with what I've been quoted.

Edit: Seems that the SMA inverter I mentioned earlier would work, the voltage up to 128º F is still within range. But now I wonder when they ask for these temps, are they figuring in that they will actually get higher than the maximum outdoor temp?

Edit 2: I looked at different panels too and surprisingly, the CentroSolar are actually some of the best in terms of the voltage per panel. Canadian Solar, LG, and Hanwha all had lower voltage for the same size string (so you would need more panels per string to have enough voltage). The best by far were SunPower which you could do some wicked setups like 5 strings of 8 panels and be way above the needed voltage (332v at high temp and 230v is the minimum needed) or 4 strings of 9.

[Ian S]

With its single MPPT, does the Fronius present any limitations with respect to your somewhat complex layout?

[HX_Guy]

What kind of limitations are you thinking Ian?

[Ian S]

What kind of limitations are you thinking Ian?

You have some different orientations, right? I saw this pdf at the Fronius site. Not sure if any of that affects your layout in a major way. I understood multiple MPPT's were preferred for different orientations.

[HX_Guy]

Funny, I was literally just ready the exact same PDF.

I have looked at some inverters with dual MPPTs but it would be quite the complicated setup and would require multiple inverters as all the dual MPPT inverters seem to be pretty small in size. I really can't come up with an ideal setup where I could use less than 12 panels per string really.

Inverter #1: SB5000 Dual MPPT
String 1: 12 panels
String 2: 12 panels

Inverter #2: SB5000 Dual MPPT
String 1: 12 panels
String 2: 12 panels

That would actually give me a slightly smaller system at 11.75kW but the advantage would probably be the best setup for the orientations and shading.

Of course I wouldn't have to go with dual MPPT inverters if I used two of them, I could have a small inverter for the 12-14 panels facing east and then an inverter for the south facing panels (3 strings of 11). That may be the best setup as it would get the voltage to where it needs to be but how much would two inverters cost (a Fronius 3.8 and 7.5)?

[silversaver]

You have some different orientations, right? I saw this pdf at the Fronius site. Not sure if any of that affects your layout in a major way. I understood multiple MPPT's were preferred for different orientations.

I read the spec and feature twice and still no clue about how their MPPT works......... It seems to be able to track multi roofs/orientation with single MPPT or ???

http://www.fronius.com/cps/rde/xbcr/SID-B9D11845-67AC7142/fronius_usa/IG_Plus_Brochure_v1.09_178881_snapshot.pdf

[HX_Guy]

This goes into detail on how their MPPT works and how it handles shade:

https://www.fronius.com/cps/rde/xbcr/SID-04C2F324-090C83A9/fronius_usa/PDF_fronius_ig_reaction_to_non_optimal_conditions_207252_snapshot.pdf

Says"The FRONIUS IG inverter’s was designed to track the “strongest”, or least affected, string. The IG will find the optimum operating point of the unshaded (or least shaded) string and operate there."

[HX_Guy]

I read this in another Fronius PDF and it's a great point.

"When designing your systems, keep this question in mind: 15 years from now, on a hot day, will you still achieve at least 230Vmpp from the aging modules to produce the needed voltage to stay within the Maximum Power Point range?"

I'm going to talk to my installer about upping the number of panels to 13 in 4 strings which even at 118º, each string would be at over 300V, well above the minimum of 230V. There will be more shade but the system will also be bigger (13.0kW) so hopefully that will make more power when not shaded so we don't need the banked kWh from the winter months.

[+3 Golfer]

I read this in another Fronius PDF and it's a great point.

I'm going to talk to my installer about upping the number of panels to 13 in 4 strings which even at 118º, each string would be at over 300V, well above the minimum of 230V. There will be more shade but the system will also be bigger (13.0kW) so hopefully that will make more power when not shaded so we don't need the banked kWh from the winter months.

LOL, there you go again bigger, bigger, bigger not smarter, smarter, smarter (IMO).

I would go with 2 dual MPPT inverters which will be more efficient than the Fronius. The slightly smaller system size will be offset to an extent by a likely 1-2% gain in efficiency with the SMA inverters and you solve the voltage issue with 4 less panels (cost savings). Also, have you looked at the ABB PVI-5000 or PVI-6000 inverters (formerly Power One inverters) which will also likely work.