System Performance degradation

[ggunn]

Neil,thanks for your feedback! I was not aware the voc should not vary with conditions. I was under the impression that both the voc and isc will vary, both reaching their maximum when the sun is perpendicular to the panels... Is this what you're telling me?

No. Voc will not vary much (if any) with the angle of the sun if insolation is the only thing considered. However with heating effects you may very well see Voc be at its lowest when the sun is perpendicular to the modules. As cell temperature goes up, Voc goes down. See the nameplate temperature coefficient for Voc for how much; it's usually somewhere around -0.35% or -125mV per degree C for crystalline Si modules.

[ggunn]

You're right! Without any current flowing there will be no voltage drop. I should have worded differently... basically I concluded that there are minimal losses in the 12meter run.

Thanks

With no current flowing you will see no losses in voltage unless you have a serious problem with wiring insulation.

[niel]

Neil,thanks for your feedback! I was not aware the voc should not vary with conditions. I was under the impression that both the voc and isc will vary, both reaching their maximum when the sun is perpendicular to the panels... Is this what you're telling me?

yes, for pvs reach their voc with very little light to start and it is fairly steady from sun up to sun down without a load on it. there's a point though that the smallest amount of light like around sunrise and sunset that the voltage will drop below the voc and with that little bit of light solar is fairly useless, but once the voc is reached all that can be realized after that would be more current output capability. now when the pv gets loaded it will pull down the voltage and when it is at a point that it delivers its max power the voltage is noted and you see that as the vmp or volts max power. that max power point does vary a tad with solar intensity though.

i keep rambling here no doubt confusing you, but know that the voc is pretty steady throughout the day and that is without a load on it as it stands for voltage open circuit.

[inetdog]

yes, for pvs reach their voc with very little light to start and it is fairly steady from sun up to sun down without a load on it. there's a point though that the smallest amount of light like around sunrise and sunset that the voltage will drop below the voc and with that little bit of light solar is fairly useless, but once the voc is reached all that can be realized after that would be more current output capability. now when the pv gets loaded it will pull down the voltage and when it is at a point that it delivers its max power the voltage is noted and you see that as the vmp or volts max power. that max power point does vary a tad with solar intensity though.

i keep rambling here no doubt confusing you, but know that the voc is pretty steady throughout the day and that is without a load on it as it stands for voltage open circuit.

My guess is that the only reason that the open circuit voltage ever goes below the region of the rated Voc of the panel is that some current is being drawn by internal leakage or by the voltmeter you are using to make the measurement. Some of the steps in production of a solar cell (and just the semiconductor material itself) can cause leakage current between + and -, and the magnitude of that leakage is one of the things that cell manufacturers test for.

[vtmaps]

there's a point though that the smallest amount of light like around sunrise and sunset that the voltage will drop below the voc

are you trying to say that after sunset, and until sunrise, the volts may drop below Voc? I think that's generally true, but I've seen some pretty high voltages by moonlight in the winter. --vtMaps

[niel]

vtmaps,
i know i didn't explain it well.

moon volts are different than sun volts and you need a special moon volt meter to detect it.

[waynefromnscanada]

vtmaps,
i know i didn't explain it well.

moon volts are different than sun volts and you need a special moon volt meter to detect it.

I've been looking for one of those meters ever since my retailer told me years ago that my panels would charge the batteries with just moonlight, due to "the UV light given off by the moon". Thanks to his enlightening information, I now use special "Moon Blocker" SPF 30 to protect my skin when I expose myself on moonlit nights, but my panels still don't charge. Would this be a warranty issue?

[ggunn]

I've been looking for one of those meters ever since my retailer told me years ago that my panels would charge the batteries with just moonlight, due to "the UV light given off by the moon". Thanks to his enlightening information, I now use special "Moon Blocker" SPF 30 to protect my skin when I expose myself on moonlit nights, but my panels still don't charge. Would this be a warranty issue?

http://en.wikipedia.org/wiki/File:Go...Moontan_US.jpg

[inetdog]

http://en.wikipedia.org/wiki/File:Go...Moontan_US.jpg

The ultraviolet end of the spectrum is much greater proportionally once in a blue moon.

[Brianellul]

Hello Everyone

Update on my situation...

To eliminate the inverter MPPT failure probability (also it is the easiest one to eliminate!), my installer brought over another brand new SMA inverter and we just plugged it in, however as most of you said, the inverter was OK. Same behaviour with this inverter, just 1000W-1200W being generated/pumped out from a 2070W array.

I've also swapped two panels...
Panel 6 - 27.8 Voc / 5.83 Asc
Panel 7 - 32.4 Voc / 6.39 Asc

Panel 6 swapped position with panel 7, however still same Voc from the panels...

As was also suggested here and by my installer, I've aligned the panels to all point exactly in one directly (slightly off South). Although this does not explain why last year my output was more...

Now, we're looking at the panels and as suggested here, the panels seem out of spec! My installer is communicating with his supplier (IBC Solar) for a way forward. Unfortunately, It doesn't seem that straightforward/easy to just replace the panels!

Regards
Brian