Solar panel string design

[mnlara]

Can you mix or add solar panels of different azimuth or orientation in one string?

 

[SolarPro]

Not without module-level power electronics. Sounds like a job for microinverters or dc-to-dc optimizers.

It is generally accepted that you can parallel relatively unshaded source circuits that have different orientations?say, one string on an east-facing roof and another string on a west-facing roof?on a string inverter with a single MPPT input. But if you have modules with different orientations within a single source circuit, the resulting I-V curve just gets too screwy. The resulting energy losses over time will be unacceptable.

 

[G._S._Ohm]

And if you can supply values for these components
http://www.google.com/search?q="solar+cell+equivalent+circuit"&hl=en&safe=off&client=safari&tbo=u&rls=en&tbm=isch&source=univ&sa=X&ei=dD_GUMDhK6W10QHthYGYBw&ved=0CC4QsAQ&biw=1039&bih=749
for 1 kw and less per sq. meter of panel, you can figure out how bad/good the series circuit will be.

 

[mnlara]

And if you can supply values for these components
http://www.google.com/search?q="solar+cell+equivalent+circuit"&hl=en&safe=off&client=safari&tbo=u&rls=en&tbm=isch&source=univ&sa=X&ei=dD_GUMDhK6W10QHthYGYBw&ved=0CC4QsAQ&biw=1039&bih=749
for 1 kw and less per sq. meter of panel, you can figure out how bad/good the series circuit will be.

So what if there's no shading issues, can you still add a panel of different orientation on a series string? ei; one south-east panel orientation added to the 7 panel south west oriented string? The design calls for 3 strings of 8 panels connected to a single inverter.

 

[ggunn]

So what if there's no shading issues, can you still add a panel of different orientation on a series string? ei; one south-east panel orientation added to the 7 panel south west oriented string? The design calls for 3 strings of 8 panels connected to a single inverter.

I wouldn't do it. The current from a module is proportional to the sunlight striking it, and the current in the string is limited by the lowest current producing module. In the afternoon when the SW facing modules would be at their peak production the SE facing module would restrict all their output to what it is producing.

 

[mnlara]

Thanks You.

Thanks You.

I wouldn't do it. The current from a module is proportional to the sunlight striking it, and the current in the string is limited by the lowest current producing module. In the afternoon when the SW facing modules would be at their peak production the SE facing module would restrict all their output to what it is producing.

That was my initial understanding. Em i right that you can still add a string of different orientation in parallel to other strings?

 

[G._S._Ohm]

Perhaps you could use two isolating diodes, also called current-steering diodes.

Each + array output goes to an anode and the + combo output is taken off of the diode cathodes which are tied together. Whichever array puts more voltage into the load supplies the load current and no current will be taken up by the shaded array.

For more efficiency use Schottky diodes.

 

[jaggedben]

So what if there's no shading issues, can you still add a panel of different orientation on a series string?

Having panels with different orientations is a shading issue. Or at least that's how you should think of it. Because the differently oriented panels will receive different amounts of sun exposure, just like if you have some panels shaded.

 

[ggunn]

That was my initial understanding. Em i right that you can still add a string of different orientation in parallel to other strings?

Yes, you can. It won't result in the optimum output for the entire array unless you use an inverter that has two separate MPPT channels (they exist), but the negative effect won't be as pronounced as it would if a string were split between modules with different orientations.

 

[Zee]

Fronius doesn't highlight the fact that their inverters are the only ones I know of (apart from multiple MPPT ones) that can handle "split orientations" or "split angles" - AS LONG AS ALL PANELS WITHIN A STRING ARE FACING THE SAME ANGLE AND ORIENTATION.

"Splitting" a string between orientations or angles is one of the worst things you can do in terms of power loss. Ok, you can burn a house down, but as far as deliberate design, it is a terribly counterproductive move.

 

[Zee]

For example:
in your case,
installing 3 @ 7 panels would be much better than
3 @ 8 requiring you to split a string of 8 between two orientations!
(to reiterate, Even then you will need multi MPPT or a Fronius - as Jben and GGunn, and others, have pointed out.)

 

[PWDickerson]

I am familiar with the Power-One dual MPPT inverters, but I was not aware of any design feature of the Fronius inverters that would allow them to optimize two different strings operating at different voltages. How do they do that?

 

[ggunn]

I am familiar with the Power-One dual MPPT inverters, but I was not aware of any design feature of the Fronius inverters that would allow them to optimize two different strings operating at different voltages. How do they do that?

Different orientations may operate at different voltages when separate MPPT channels load them differently based on their available current output. Open circuit voltage is virtually the same as long as there is sunlight irrespective of orientation. I don't know of anything Fronius does that enables differently oriented strings in parallel to be optimally loaded for maximum output if it's not done with multiple MPPT channels, or even how it could be done if the source circuits are paralleled before encountering any active circuitry.

 

[Garrison]

I am familiar with the Power-One dual MPPT inverters, but I was not aware of any design feature of the Fronius inverters that would allow them to optimize two different strings operating at different voltages. How do they do that?

Fronius claims that the net loss resulting from strings at different orientations is about 1% using their inverter. This is because the voltage of strings at different orientations will be very similar assuming no string is shaded, and their inverter tracks the highest producing string, which results in maximum power from all strings. Note, this is not the case with all inverters, and only true with the IG on low sloped roofs.

Below is a link to their white paper on this topic:

http://www.fronius.com/cps/rde/xbcr...to_non_optimal_conditions_207252_snapshot.pdf

 

[Zee]

I am familiar with the Power-One dual MPPT inverters, but I was not aware of any design feature of the Fronius inverters that would allow them to optimize two different strings operating at different voltages. How do they do that?

I have yet to find more than a couple installers that are familiar with Fronius' "quasi-multiple MPPT" feature!
I am too lazy now to find the exact link but they have a white paper on this topic at their website. I have employed this design on split orientations for many many years..
Many systems in operation for many years, no complaints.
Can save you from micro-inv.s or two or three ridiculously small inv.s!

Again if , say 7 panels go West then 7 must go East etc.....

Take home message: less than 1% loss.
Why? Their MPPT algorithm finds and locks on to the higher string output and optimizes that. Sma will do terribly, as all other inv.s - they track the lower producing string...while Fronius rides the peak string.

as for power-one...I want to like them...but cannot given my single installation and immediate failure of their product.

 

[Garrison]

Apparently SMA no longer discourages this practice with their inverters. The article linked below suggests that since peak string performance will be at different times, the inverter size can be smaller. From an economic perspective this can increase your rate of return for a given array size, which can make up for some of the power loss [they say].

http://www.smainverted.com/files/2012/12/Article_Polystring_East-West-Orientation-TEN122510.pdf

 

[G._S._Ohm]

Their MPPT algorithm finds and locks on to the higher string output and optimizes that.

So it is sort of like a wired-or circuit using diodes.