Optimal panel array? Shading issue

[Guest]

Can someone draw a diagram of the circuit of one panel with the diode in place so I can understand whats happening in shaded panels?

[ButchDeal]

You will have to look a the specs for your particular modules. There are different ways to striping the cells internally.

Can you expand on this . I have standard 250 watt 38Volt max type rigid panels . What I don't follow is the operation of the diode discussed here . Normally a diode stops current flowing in one direction. So what possible effect could shading have on that? All current must flow through all the cells of the panel at all times but if a panel is shaded then that cell does not add to the accumulated voltage by as much. Current would still be flowing at the full string rate wouldn't it?

[Mike90250]

Before you installed the panels, did you get HOA approval ? If so, I don't see how an approved system can be forced to be made less efficient,
Also, some states have rules prohibiting HOA from requiring "adjustments" to solar arrays if it decreases system efficiency by 10% or more.

Now if you just self-installed with no sign-off or approval, you may be on your own.

[Micho]

Before you installed the panels, did you get HOA approval ?

I didn't; didn't think my neighbors would mind (they did, thus the low level frames). My mistake.

Also, some states have rules prohibiting HOA from requiring "adjustments" to solar arrays if it decreases system efficiency by 10% or more.

This is Mexico; harvesting of solar power for self-consumption was recently allowed by the government.

[bcroe]

Bypass diodes are added to a PV panel, so if some cells are shaded and can't deliver the current of the other cells in series, the
full current from unshaded cells can go around the shaded cells. This forces the voltage from the shaded cells to near zero, in
effect shorting them out. The string will continue to deliver full current, but at reduced voltage due to the disabled cells. The
reduced voltage isn't a problem unless another string of unshaded cells is wired in parallel, and continues to operate at full
voltage. Then the shaded string will attempt to match that voltage and in the process slide up its curve to where very little
current can be delivered.

Typical panels of 60 or 72 cells are divided into 3 groups of cells. While the groups are directly in series, each group has a
bypass diode wired in parallel with it; frequently 3 groups, 3 diodes. Normally all cells simply operate in series and the bypass
diode doesn't conduct: it is reverse biased. As the current capability of a group decreases, continuing current from the rest of
the cells forces the voltage of the disabled cells to zero and slightly beyond, when the bypass diode starts conducting.
Bruce Roe

[inetdog]

The reduced voltage isn't a problem unless another string of unshaded cells is wired in parallel, and continues to operate at full
voltage. Then the shaded string will attempt to match that voltage and in the process slide up its curve to where very little
current can be delivered.
Bruce Roe

Or the MPPT input may pull the voltage down until the shaded string is at just below its Imp and in the process pulling the unshaded string down below its Vmp, operating at above Imp but less than maximum power.
Which way it falls out depends on the effective voltage difference between the strings, on a percentage basis, and the design of the MPPT algorithm, in particular whether is sweeps from high current down or from high voltage down.
SMA among others advertise that their units actually sweep over the entire voltage current curve, which allows them to realize that there is more than one local maximum in the power curve and choose the one with the highest total output. It works!

[Micho]

I agree; this is my best option.

I am going to rotate the panels 90 degrees on the vertical axis and lay them side-by-side and raised away from the shade as much as possible; the angle should remain the same. Hopefully I'll manage to reutilize most of the original aluminum base/framing.

Will report back in a few weeks.

This is me reporting back. I rotated the panels. Here's a BEFORE picture:



Picture was taken an hour or so before noontime. The shadow is very visible.

And here's is the AFTER picture.



No shading whatsover. I did some research and couldn't find anything that would confirm that there would be a difference in power generation between portrait vs landscape panel orientation

That said, I'm on the northern hemisphere; it's Dec 2nd and there's good chance I may see some shading come December 24th (solstice). I'm yet to report if there's any significant improvement on the power generated.

---------------------------

Once on the roof I did notice that panels are very dusty and that there's some shadow from my TV antenna on the panel array I have at the other end of the roof. I'm wondering if I should do something about this or if the generation losses are minimal.




Thanks for your input.

[solarix]

The dust is not worth doing anything about but do move the antenna to somewhere else...

[Mike90250]

The dust is not worth doing anything about but do move the antenna to somewhere else...

Ditto, The antenna shade is going to hurt a lot.

[Micho]

The dust is not worth doing anything about but do move the antenna to somewhere else...

So yesterday I repositioned the TV antenna and washed the panels with a soft bristle long handle brush... my totally unscientific estimates put the power generation improvement in the 25 to 30% range (that is after rotating panels, relocating antenna and cleaning the panels).