shading effects

[diogenes]

What percentage of power is lost by shading a corner of Canadian solar 310P panels? the bottom?

[Mike90250]

A shaded cell limits current in the entire string. Bypass diodes alleviate this some, at the cost of loosing voltage . I don;t know how the CS panels are built, but if they had 3 bypass diodes, and rated for 300W @ 30V, the bypassed section where ONE cell was shaded, would reduce the voltage by 1/3, loosing 1/3 of the total wattage. for a couple of series panels, the system would still function, but the active bypass diode will eventually fry (not made to withstand this mode forever, folks are always trying to find how to repair blown diodes) and the panel will be trash.

[sensij]

Continuing the thought... if by "bottom" you mean the short side of the panel, in a string, you could cause the entire panel to be bypassed by shading just those 6 cells across. With panel level optimization or microinverters, as long as the shading across that edge was uniform, the panel would continue to produce at a level roughly proportional to the amount of shade free area.

[PNjunction]

With NO diodes blown, and just some simple debris is covering shading just ONE of the cells, then the overall output falls dramatically to make it nearly unusable - or so degraded that there is no good way to predict the loss. Only a thin-film panel loss from shading is somewhat predictable, but we've gone down that road in other threads.

In the case of my Sharp panel that had a blown bypass diode inside the panel's junction box, voltage was just fine, but the current output was cut in half, still under full illumination.

Thus, when I was expecting 4.5A normally, I got only 2.25A from it. Voltage stayed the same near 18v ocv. Thus, if I wasn't watching a current meter which tipped me off that something was wrong, and only relied on voltage, I would have never known. When scratching my head, the small snap I heard came to mind, and lo and behold - blown diode from a hasty errant connection. Note that this was noted with a heavily discharged battery, and NOT when the battery was in absorb, when current flow is naturally limited. I hadn't reached absorb, and knew that something was wrong.

Essentially, in the 80w panel, the bypass diodes were arranged so that the panel was really like TWO 40W panels in parallel.

BUT here's the kicker - if one of the bypass diodes is blown, and you are still under full illumination, then the current is cut in half - one half is missing more or less. Part of the reason for using bypass diodes to make this single panel act like two smaller panels, is indeed to protect the cells when there is shadowing on them. Instead of the shadowed cell receiving the panel's full reverse-bias current, it only has to deal with half the panel's reverse bias current - this protects the shaded cell from frying from the reverse bias.

When DIY'ers don't split their panel in two electrically with bypass diodes, then shaded cells receive the full brunt of the reverse bias current, thus damaging their cells faster than the typical commercial counterpart.

On a side note, if you aren't really shaded, but just cloudy conditions occur where shadows from everything else just start to disappear, then you can see only about 10% of the rated output. Once edge-shadows from objects start to appear again, then current rises rapidly.

To reiterate just to keep things separate - if no diodes are blown, then the OVERALL output is greatly reduced when any part of the panel is even slightly shaded. Don't think that the standard commercial panel with bypass diodes will merely cut the current in half on the part of the panel that is protected by it's own bypass diode with a little shading - it wont - the full output will be greatly reduced.

Moral - don't rely on trying to account for shading. Shading is bad, be it from a powerline, nearby object, long winter shadows from grass blades, wet leaves, snow, you name it. Get that thing fully illuminated and checked once in awhile for cleanliness.

[diogenes]

First, thank you all for the great input. The shading is from a house and occurs starting about 4 PM as the sun goes down. The house is about 50 feet from the panel edge and the house is two story, maybe 30 feet tall. The panels are mounted so the long side is horizontal.

Sorry, I did not mean to give the impression that the shading was constant.

[PNjunction]

Ah, no problem. At 4pm in most places, you are far outside the solar-insolation window for even a fully illuminated panel to be totally effective. And due to that, shading during this time has a very low reverse bias on the shaded cells - so no worries.