Solar PV, One String or Two?

[Creakyride]

So, the technical drawing for my setup shows all 10 panels from two separate locations will be in one serial string. Each panel will have its own Tigo optimiser.
Am I correct in assuming that one string is going to be performing the same as if it would be with two parallel strings? Because of the Tigo optimisers?
This would leave the second MPP for a possible additional string at later stage.

The Inverter will be a SOLIS RHI-3.6K-48ES-5G 3.6KW HYBRID SOLAR INVERTER.

 

[binky]

So, the technical drawing for my setup shows all 10 panels from two separate locations will be in one serial string. Each panel will have its own Tigo optimiser.
Am I correct in assuming that one string is going to be performing the same as if it would be with two parallel strings? Because of the Tigo optimisers?
This would leave the second MPP for a possible additional string at later stage.

The Inverter will be a SOLIS RHI-3.6K-48ES-5G 3.6KW HYBRID SOLAR INVERTER.

Never a good idea, optimisers or not. As long as 5 panels meets the necessary start up voltage for the inverter, 2 strings is always better than one.

 

[Creakyride]

Thanks for your input Binky.
Can you elaborate on why is not a good idea?

 

[binky]

Thanks for your input Binky.
Can you elaborate on why is not a good idea?

The longer a string, the more the output can be affected by a single panel, albeit that is a fairly unlikely scenario. You have to remember how MPPTs work, Google it, they are trying to extract the maximum energy from the panels, the less panels in a string, the better that works. It's also good from a maintenance point of view.

Are your 2 roof spaces facing exactly the same way?

 

[Creakyride]

One roof is facing SE and it will hold 5 panels.
The other 5 panels will be facing SW, of which 2 panels will be on a garage roof and the remaining 3 panels will be on the wall. I wish there was more space for them on the roof.

 

[binky]

One roof is facing SE and it will hold 5 panels.
The other 5 panels will be facing SW, of which 2 panels will be on a garage roof and the remaining 3 panels will be on the wall. I wish there was more space for them on the roof.

SE and SW on the same string is a very bad idea indeed, it will work, but not well

 

[Creakyride]

Hmmm... I did ask about this, I was told that it would work because of the optimisers involved. This awkward and deflating... As a customer, I have no knowledge on PV installation so I have to trust the tech people in the design of the system for best performance, especially being a small one and complex.

I'll have to speak to the person and see if he can give me a reason why he chose a single string instead of two.

 

[binky]

Hmmm... I did ask about this, I was told that it would work because of the optimisers involved. This awkward and deflating... As a customer, I have no knowledge on PV installation so I have to trust the tech people in the design of the system for best performance, especially being a small one and complex.

I'll have to speak to the person and see if he can give me a reason why he chose a single string instead of two.

As I said, it will work, but there will be a constant conflict between the arrays which will confuse the MPPT. Think of it as trying to talk to 2 people at the same time over 1 telephone line.

 

[Creakyride]

Thanks Binky, that make sense, I will ask for change to 2 parallel strings.

 

[arwooldridge]

5 panels on two strings will not start up as quickly as 10 panels on one string. Optimisers should work ok on one string IMO. If there were no optimisers then two strings might be better grouping them by orientation.

 

[Creakyride]

5 panels on two strings will not start up as quickly as 10 panels on one string. Optimisers should work ok on one string IMO. If there were no optimisers then two strings might be better grouping them by orientation.

No, it's 5 panels on each string.
5 on facing SE and the other 5 facing SW.

The design has now been updated to two parallel strings.

 

[binky]

5 panels on two strings will not start up as quickly as 10 panels on one string. Optimisers should work ok on one string IMO. If there were no optimisers then two strings might be better grouping them by orientation.

Start up difference will be marginal, 5 panels will be around 150V, and Solis starts up at 90V from memory. The big difference is the different orientation of the panels will basically mean their power curves are out of synchronization, so the MPPT will be trying to maximize 2 power curves and the algorithms are designed to max 1. Hence overall power generation will be compromised. Have a search of YouTube videos on how MPPTs work, and you will see what I am getting at.

 

[Creakyride]

I understand that the 2 parallel strings on 2 MPPT and the 10 optimisers, one for each panel is the best configuration for what the system has to work with.

 

[binky]

I understand that the 2 parallel strings on 2 MPPT and the 10 optimisers, one for each panel is the best configuration for what the system has to work with.

Yep, but whether you need optimisers is debatable unless you have shading issues. Personally I would not bother with them unless you do have some shading.

 

[Creakyride]

Yes, on late afternoon the shared chimney will cast some shadow on some of the panels on the main roof facing SE and in the winter the tree from 2 properties away cast some shadows on the SW panels due to the sun being low in the horizon. I also wanted the optimisers just in case the neighbours decide to have a loft extension, as that will create shadowing. This was my decision for the Tigo optimisers, not the installer.

 

[binky]

Yes, on late afternoon the shared chimney will cast some shadow on some of the panels on the main roof facing SE and in the winter the tree from 2 properties away cast some shadows on the SW panels due to the sun being low in the horizon. I also wanted the optimisers just in case the neighbours decide to have a loft extension, as that will create shadowing. This was my decision for the Tigo optimisers, not the installer.

Fair enough.

 

[Creakyride]

Thanks again for your help, Binky.