Effective Array String design

[mnlara]

Which string design is more effective, single string of 12 modules or 2 strings of 6 modules? Using SolarWord SW250 Mono modules and SMA SB3000-US(240V) inverter (Min. DC voltage / start voltage = 200/228 V.) I'm leaning more on the single string...can you please explain why what you choose is more effective layout versus the other?.

 

[ron]

As long as you don't exceed the maximum voltage to the inverter with the deratings (low temperature), go with the single sring, which will likely give you a onger period of time during the day, as the voltage varies, to keep the inverter on, within its acceptable DC voltage range.

 

[mnlara]

I assumed that Voltage goes down when the temperature is high. But even when with the record high temperature still puts up above the 200V startup voltage, is 2 string OK? or is the single string design better? probably in terms of installation cost?

 

[ggunn]

As long as you don't exceed the maximum voltage to the inverter with the deratings (low temperature), go with the single sring, which will likely give you a onger period of time during the day, as the voltage varies, to keep the inverter on, within its acceptable DC voltage range.

The voltage varies during the day only due to temperature change, and not by very much (on the order of .3% per degree C).

 

[ggunn]

Which string design is more effective, single string of 12 modules or 2 strings of 6 modules? Using SolarWord SW250 Mono modules and SMA SB3000-US(240V) inverter (Min. DC voltage / start voltage = 200/228 V.) I'm leaning more on the single string...can you please explain why what you choose is more effective layout versus the other?.

Have you run it through Sunny Design? I would be surprised if both 6 and 12 module strings are acceptable.

 

[mnlara]

Have you run it through Sunny Design? I would be surprised if both 6 and 12 module strings are acceptable.

I run them both and they are both acceptable, but question is which layout is cost effective or efficient?

 

[ggunn]

I run them both and they are both acceptable, but question is which layout is cost effective or efficient?

I usually opt for as many modules in a string as I can get. Most inverters are a little more efficient with higher DC voltages, and you have fewer home runs to wire up if the same number of modules is connected in longer strings. Also, higher DC voltages mean lower currents; if there is a significant distance between your array and your inverter, you have less voltage drop to contend with. Plus, with more modules per string your array is less prone to dropping below the threshold voltage of the inverter when the weather gets hot.

I can't think of any advantages of shorter strings.

 

[ron]

I assumed that Voltage goes down when the temperature is high. But even when with the record high temperature still puts up above the 200V startup voltage, is 2 string OK? or is the single string design better? probably in terms of installation cost?

The design needs to include that voltage goes up when temeprature goes down. You do not want to exceed the maximum permited input voltage to the inverter. That is why there is design criteria mentioned in the informational note below section 690.7 (A)

 

[jaggedben]

I can't think of any advantages of shorter strings.

Maybe if you have one half of the array that is going to get significantly more shade than the other, it might make sense to split the array into two strings so that when one is shaded the other can still produce at closer to max power.

 

[PWDickerson]

If the extreme minimum temperature in your area is less than 23 degrees F, you will not be able to string 12 SW250's together because the voltage will exceed 500V at temperatures below 23F. 500 volts is the max DC voltage for this inverter. If your temps will never get this low, than 1 string of 12 is definitely the way to go. Most inverters have a max DC voltage of 600, but not this one.

The Vmp of 6 modules in series on a hot day (about 160V at 90F) will be well under the minimum peak power tracking range for the inverter (200-400v), so this is a bad choice.

Check out the Blue Oak string sizing web site. I use it all the time. You have to remember that your module voltage will degrade a little over the years, so you want to make sure that after a certain amount of degredation, you still have enough DC voltage to be in the inverters MPPT range. I wouldn't go less than 9 modules for the equipment you specified.

Check out the SolarABC's website for extreme minimum temps where the system will be located. they have a solar reference map that is very handy.

 

[mnlara]

Even if both 2 or 1 strings are acceptable and that record low was only 25 deg F, I think the 2 string array is more efficient when there are shading (shading on one panel affects all the panel on that string leaving the other string unaffected). Assuning that temperature doesn't go up high to lower the voltage. This I believe the word balancing comes in to play. Em i right?

 

[ron]

Maybe if you have one half of the array that is going to get significantly more shade than the other, it might make sense to split the array into two strings so that when one is shaded the other can still produce at closer to max power.

The SB3000 only has one MPP tracker, so even if you split it into two strings, you are essentially terminating them to the same terminals in the inverter, as the inverter will not compensate for partial shading in the way an inverter with 2 MPPT's would. Granted, if you had two strings, even with one tracker, if a panel was fully shaded, making it an open circuit for that part of the paralleled string, you would still have the other string. But again, you need enough voltage to turn the inverter on and keep it on for the longest time during the day.

If the earlier post regarding the deration for low temperature is correct and you exceed the 500V rating of the inverter, than you have no choice but to split into two strings.

 

[ggunn]

The SB3000 only has one MPP tracker, so even if you split it into two strings, you are essentially terminating them to the same terminals in the inverter, as the inverter will not compensate for partial shading in the way an inverter with 2 MPPT's would. Granted, if you had two strings, even with one tracker, if a panel was fully shaded, making it an open circuit for that part of the paralleled string, you would still have the other string. But again, you need enough voltage to turn the inverter on and keep it on for the longest time during the day.

If the earlier post regarding the deration for low temperature is correct and you exceed the 500V rating of the inverter, than you have no choice but to split into two strings.

What he said. When you run the Sunny Design tool you should make sure that you are using the correct climate information, and you should read the notes when it tells you that the configuration is conditionally acceptable. The most important number is the low temperature adjusted Voc, because that's the one which will damage the inverter, void the warranty, and all sorts of mean nasty things if you exceed it. I use the record low for the location as the cap because generally the coldest it will get will be just before dawn, and when the first light hits the array and the inverter is off, the voltage will jump up to Voc and the cells will still be at whatever the low temp was.

Here's another thing to consider: many inverters and charge controllers have a "black box" recorder in them which keeps track of the highest DC voltage it has been exposed to. If you turn in an inverter to the manufacturer for warranty work, no matter what the reason, if they look at that and see that you have *ever* exceeded their DC voltage limit, they may void the warranty. Don't take chances with DC overvoltage.

 

[jaggedben]

The SB3000 only has one MPP tracker, so even if you split it into two strings, you are essentially terminating them to the same terminals in the inverter, as the inverter will not compensate for partial shading in the way an inverter with 2 MPPT's would.

You will still get essentially full production out of the unshaded string. It will still be better than having the same amount of shading on a one string system. (All else being equal, which in this example it seems it is not.)

 

[Zee]

I run them both and they are both acceptable, but question is which layout is cost effective or efficient?

I do not know how 6 in a string can be acceptable to SUNNY DESIGN, REGARDLESS of your ambient Temperature. I am hmy: confused.

6 x 31.1 Vmpp per module = 186.6 V!! That is less than the minimum of 200V for the Inverter. And that is at room Temperature, which it never will be. And the V will go way down as modules will be 40 deg F hotter than even ambient T.
I don't think this system would even turn on.

On the other end: 12 modules x 37.8 Voc = 453.6 V.
MAx V allowable for this specific Inverter is only 500 V!! Not 600. as PW Dickerson very aptly pointed out.
That means that 500 / 453.6 = 1.10 cold T adjustment factor is the highest you can go.
Accordingly, the coldest T allowable is 32 F. RECORD lowest cold.
(per NEC Table 690.7)

If it can ever get colder, that is a problem. As GGUNN pointed out: it will void your warranty.
You need a SMA HFUS if you expect colder ambient Temp.'s. That'll allow 600 Vdc. max.

I don't see 6 as acceptable ever.
Nor 12, as good design, unless you are in Coastal CA, Florida, etc........

 

[Zee]

You will still get essentially full production out of the unshaded string. It will still be better than having the same amount of shading on a one string system. (All else being equal, which in this example it seems it is not.)

What JAGGEDBEN said. Splitting an array into strings can greatly increase ourput under shady consitions. IF shade is isolated to one string.

 

[SolarPro]

Check out the Blue Oak string sizing web site. I use it all the time. You have to remember that your module voltage will degrade a little over the years, so you want to make sure that after a certain amount of degredation, you still have enough DC voltage to be in the inverters MPPT range. I wouldn't go less than 9 modules for the equipment you specified.

FWIW: the string sizing tool PWDickerson is talking about is located here:

http://www.pvselect.com/

And the SolarABCs reference temperature map is here:

http://www.solarabcs.org/about/publications/reports/expedited-permit/map/index.html

Very handy design resources....