2 strings or 4?

[Niko22]

Hi,

I’m trying to find out if I should have 2 strings or 4. I will have 12 panels on an east facing roof and 12 on the west.

I will be using Longi 430w panels and a Sunsynk 8.8kw inverter.

The DataSheet for the inverter states a maximum DC voltage of 500v and the datasheet for the panels states a Voc of 39v (12x39=469v) so I’m within the limits if I use 2 strings.

However 469v takes me outside the MPPT tracking range.

When calculating the voltage from the array for the MPPT should I be using Voc or Vmp? If I use the Vmp of 33v x 12 it gives me 396v which is nicely inside the range.

I have been advised by one person to use 4 strings because it’s safer for the inverter but I have also beefed advised to use 2 strings because the higher voltages help with efficiency.

Thanks

 

[rodrick]

You need to also make a temperature adjustment into your voc you may go over voltage on cold days 12s may be to much

 

[BentleyJ]

2 strings is the best choice but you have to perform the low temp Voc calculation. What is the temp coefficient listed on the PV datasheet? Usually its around -0.35% per degree C. What is the lowest winter temps expected for your location?

 

[Niko22]

Thank you very much. Exactly the sort of advice I was looking for. I will take a look for the coefficient and report back.

 

[Niko22]

2 strings is the best choice but you have to perform the low temp Voc calculation. What is the temp coefficient listed on the PV datasheet? Usually its around -0.35% per degree C. What is the lowest winter temps expected for your location?

Based In the UK. -10 Celsius is about as low as it ever gets when the sun is out

 

[BentleyJ]

So -10C represents a 35 degree deviation from STC panel spec. conditions.
35 x -0.35% = -12.25%

That would represent about a 57Voc increase at that temp. A string of 12 panels would exceed the max limit of the charge controller input.

 

[Niko22]

So -10C represents a 35 degree deviation from STC panel spec. conditions.
35 x -0.35% = -12.25%

That would represent about a 57Voc increase at that temp. A string of 12 panels would exceed the max limit of the charge controller input.

Thank you. Looks like I'll have to have 4 strings and take the hit on inverter efficiency.

 

[Niko22]

Is there anything that can be fitted to regulate or limit the voltage?

 

[BentleyJ]

Thank you. Looks like I'll have to have 4 strings and take the hit on inverter efficiency.

The panels on the East facing roof will probably lag a bit due to low voltage. The West array should produce close to the same output with 2 strings vs. 1, that's what the MPPT charge controller is supposed to do. Use whatever combination of voltage and current is available to maximize production.

 

[Niko22]

The panels on the East facing roof will probably lag a bit due to low voltage. The West array should produce close to the same output with 2 strings vs. 1, that's what the MPPT charge controller is supposed to do. Use whatever combination of voltage and current is available to maximize production.

What is meant by "lag" in the PV industry?

Also what's the issue with east compared to west? I thought they're both equal as they're both 90 degrees from south

Thanks

 

[BentleyJ]

What is meant by "lag" in the PV industry?

My term not an industry term. Simply saying that with fewer panels in series the voltage will take longer to reach the start up threshold in the morning and fall below the threshold earlier in the afternoon. But both of those conditions are marginal so the overall difference will not be much compared to if it was 12S.

Also what's the issue with east compared to west? I thought they're both equal as they're both 90 degrees from south

Perhaps this is true for your geographic location. I'm in the SW US.

 

[Niko22]

My term not an industry term. Simply saying that with fewer panels in series the voltage will take longer to reach the start up threshold in the morning and fall below the threshold earlier in the afternoon. But both of those conditions are marginal so the overall difference will not be much compared to if it was 12S.

Perhaps this is true for your geographic location. I'm in the SW US.

I hadn't even considered start up voltages. Thanks for pointing that out to me.

 

[zanydroid]

East vs West will have different implications for Time of use rates/duck curve

Afternoon is more valuable

If you have battery it matters a lot less, but it might still come into play if you want to try min maxing the battery size by diversifying when your generation happens. Still probably not.

 

[Niko22]

East vs West will have different implications for Time of use rates/duck curve

Afternoon is more valuable

If you have battery it matters a lot less, but it might still come into play if you want to try min maxing the battery size by diversifying when your generation happens. Still probably not.

Yes, starting off with one 5kwh battery as it's all I can afford for now. Hopefully I'll add 2 more soon.

 

[Niko22]

Am I right to think that the 2 MPPT’s run independently of each other? Or do the strings for each MPPT need to be of equal size? I understand that strings on the same MPPT need to be equal.

I’m now considering adding 4 extra panels on my east roof. Unfortunately I’m space limited on my west roof to 12 panels.

Is it possible and will it work well if I have 2 strings of 6 on the west and 2 strings of 8 on the east?

I have already learnt a lot from this forum and it’s members and potentially avoided some costly mistakes.

Thanks again

 

[Niko22]

So -10C represents a 35 degree deviation from STC panel spec. conditions.
35 x -0.35% = -12.25%

That would represent about a 57Voc increase at that temp. A string of 12 panels would exceed the max limit of the charge controller input.

I found the actual coefficient. It’s -0.23% but the nominal temp is 45 degrees so still way over the Voc max

 

[SeaGal]

I found the actual coefficient. It’s -0.23% but the nominal temp is 45 degrees so still way over the Voc max

agree - I just did some calcs for another UK user in East Anglia, min temp -15C, so 40 x 0.25(ish) is +10% on VoC.

You say, 4 strings, but does the Sunsynk support more than 2 strings?

 

[Niko22]

agree - I just did some calcs for another UK user in East Anglia, min temp -15C, so 40 x 0.25(ish) is +10% on VoC.

You say, 4 strings, but does the Sunsynk support more than 2 strings?

Yes 2+2 which I assume is 2 strings into into one MPPT and 2 into the other

 

[SeaGal]

which model?

 

[Niko22]

which model?

8.8kw ECCO