Schottky diode / inverter with 2 MPPT

[man]

Hi
I have a system of 3.45 kW with 20 photovoltaic panels (LG 235 M1C-C2 ) and two strings; one oriented to the East and other one to West (10 panels E and 10 panels W). The slope of my roof is + / - 25º. I have a Sunny Boy 3300 with one MPPT. I consider a hypothesis to change to another one that has two MPPT (but I have I have many doubts about this). I started my production only 30 days ago.
Someone told me that is possible to use in each string, a Schottky diode of at least 10A and 600V to isolate the two strings together (diode in series) to improve my system with my Sunny boy (in this way one string (in the shade) not absorb the energy that the other one is producing).
I read that my panel have 3 bypass diodes, but I think that Schottky diode have another function (blocking diode).
I would like to know more about this.
Thanks in advance

 

[wstevew]

I have a 3.84kWp 16 panel 8 east/8west 22.5 degrees. Sunnyboy. Sb4000tl. Until 2 weeks ago wired into a single mppt. Now dual mppt. No real difference this time of year. Maybe when temperatures vary there will be. With even strings and low roof angle it may not make much difference but I took the safe option and had a dual tracker even though the 4000 is over rated for the panels so will lose some of the inverter efficiency.

 

[wstevew]

Also the Sb4000tl shows the voltages of both strings. Usually within 1 to 2v of each other. But current varies. I can't say. If it will be the same at the summer solstice as I have only had the system for just over a month.

Hopefully your strings are. Wired in parallel (4 dc wires to the inverter) and you won't need a diode.

 

[SolarCity]

I'd ask for a Power One 3.0 instead, assuming that the voltages/current are ok.

One MPPT and two orientations is a very, very bad idea.

 

[man]

Thanks for your advice.
How do I know if the voltages/current are correct?

 

[wstevew]

I assume he means use the datasheet for your panels to check your strings are ok for the power one 3.0 from the inverter specifications.

E.g if you have 10 panels in series at 30v you have a string voltage of 300v.

 

[SolarCity]

Exactly that. You need to be sure that the inverter will cope with the string voltages and the overall current. The Power One has a good range so there should be no issues.

 

[man]

I'd ask for a Power One 3.0 instead, assuming that the voltages/current are ok.

One MPPT and two orientations is a very, very bad idea.

Do you think that with one MPPT and E/W split (eg. with Sunnyboy 3300) The panels in the shade will absorb energy from the other side (that are in the sun)?

Regards
Man (from Portugal)

 

[SolarCity]

Possibly, unless your diode arrangement removes this possibility - not sure if that is actually possible.

What will definitely happen, however, is that your MPP will be a lot lower than it should be.

 

[man]

Today I received my first invoice. My production last month was 156 kWh (22/11 to 22/12, North of Portugal). I will try to increase my production ...
A very good year 2012
Man(uel)

 

[man]

What will definitely happen, however, is that your MPP will be a lot lower than it should be.

Thank you for your information.
This paper is about "Fronius" inverter. This inverter with one Mppt is different from the others (eg. SunnyBoy3300)?
http://www.fronius.com/cps/rde/xbcr...st_oriented_PV_systems_EN_224003_snapshot.pdf

Regards
Man(uel)

 

[wstevew]

That paper I believe is based on 15 degree roof pitch so almost flat and the important part all strings are identical build.

Bottom line is where string voltages differ there will be losses using 1 mppt.

They admit there are some losses using identical strings on a 15 degree pitch and that it kind of evens up the losses with 2 inverters.

Key points are identical strings and the low roof pitch.

 

[Zach Halley]

Hi Man,

You should be wary about a paper published by a company that mitigates the shortcomings of their own products. You will notice there is no reference to the current (which is what really varies with the irradiance). The voltage of the panels stays roughly the same from about 200W/sqm irradiance up, whereas the current varies much more linearly with increasing irradiance. You will also see they are comparing 2 strings one 1 inverter with 2 strings on 2 inverters (all Fronius inverters) as opposed to 2 strings on a single inverter with multiple MPPTs