What the heck?

[ksouers]

It's been cloudy with drizzling rain all morning, the solar panels hardly putting out any power.
The clouds are starting to break up and I was watching the Status Panel for the Classic controller as the bank hasn't had a good charge in over a week of clouds and rain. As the clouds started breaking up I saw the watts spike to well over 200 watts output, at one point up to 240 watts. Now, I only have 170 rated watts of panel up. I know the MPPT does some twisting around of volts and amps, but how is it possible to get way over rated output?

I know Tom and Chris have mentioned seeing this happen but I thought it was just a percentage point or two. This was 140 percent of rated power and happened several times and lasted more than just a second or two, at times up to a full minute. It's not cold here, about 55 F. Is it really possible to get that much over rated output with MPPT?


Kevin

[Wolvenar]

It could likely be lensing effect of cloud edges ..

[ChrisOlson]

It's some kind of cloud effect - Ross explained it to me once.  The cells in the panels are cool from not being exposed to direct sunlight, and hence not making full power.  The sun comes out from under the cloud and you get a big momentary power spike from it, and yes it can go WAAAY over what the panels are rated at, for a short time.

I got my Classic pretty much maxed out so it hits the amp limiter at 94 amps when it does that.  But with some headroom so you don't run up against the limits of the controller, I'd say 140% is not out of line.
--
Chris

[MadScientist267]

Yep, I can confirm that this is pretty much it.

You'll notice it the most when there's a hole in the clouds about 4 or 5 "solar disks" in diameter and the sun is in the middle of it, with low ambient temps. 55 degrees is plenty cool for panels to put out some nice juice, and the extra reflection from the edges of the hole help "focus" the light on the panels. The result is PV thats operating in efficient temperature ranges with extra light hitting them.

I regularly complain about mine not putting out rated power, typically 45-50W and the panels are rated 60. Under the above type conditions though, I have seen as much as 72W for very short periods of time before the panels begin to warm up from the light directly hitting them.

Then, it's back to binness as usual. 

Steve

[rossw]

Wolv, can you clarify - because your statement is somewhat ambiguous - especially when added with your IRC comments - where you said you were getting "around twice the output you should".

When you say "140%", do you mean "you are getting 140% of your expected power" or "140% MORE than your expected power"?

That is to say, if your panels are supposed to be 100 watts, are you getting 140 watts (140% *OF* rated power) or 240 watts (140% *MORE* than rated power).

If the former - and you're getting about 40% more output (140% of rated output) then that's within the upper range I've observed here with "cloud edge" effect under reasonable conditions.

If you're getting about 140% more output, then it sure sounds like someone sold the wrong panels.
What are their physical dimensions, and what is their rated power?
If you work the watts per square metre and it comes out around 15-20% conversion efficiency, then they've sold you the wrong panels. If it comes out around 30-40%, then you have some bizzare local event going on, and you need to find it (and patent it!)

[ksouers]

Thanks for the replies everyone.

Wolv: I've seen that, ships "floating" in the air but they are really beyond the horizon. I thought it might have something to do with backscatter off the bottoms of the clouds. Then again maybe the controller was broken.

Chris: That certainly seems plausible, I'll go with it. I was afraid that maybe the controller was going wonky on me.

Steve: I get the same thing. Normally about 85-90% on a clear day with occasional excursions to 100% or slightly more.

Ross: 140% OF rated power, 40% more than expected (170 watts rated, 240 observed with most spikes around 210-215). I've observed about 3-5% more than rated before even with my Blue Sky controller, that's within reason given the rating is for a standard day and it's possible to briefly get uber-perfect conditions plus the space/time/amp/volt twisting that MPPT does. This is the first time I've noticed getting way over rated power, however briefly. Looking back I guess the spikes only lasted about 10-15 seconds or so, but when they happened it really caught me by surprise and seemed like much longer.

Thanks again, everyone.

Kevin

[rossw]

Ross: 140% OF rated power, 40% more than expected (170 watts rated, 240 observed with most spikes around 210-215). I've observed about 3-5% more than rated before even with my Blue Sky controller, that's within reason given the rating is for a standard day and it's possible to briefly get uber-perfect conditions plus the space/time/amp/volt twisting that MPPT does. This is the first time I've noticed getting way over rated power, however briefly. Looking back I guess the spikes only lasted about 10-15 seconds or so, but when they happened it really caught me by surprise and seemed like much longer.

I regularly see it here on cloudy but otherwise bright days. Ie, a day with bright sun, but scattered clouds.
There is a definate and measurable (but non-trivial) increase in solar radiation when the sun is shining down the "edge" of a cloud.

That was part of the justification for my pyranometer - I can actually see and measure the effect, and it's far more than a couple of percent.

Your observed 40% is certainly within the range of what I've seen here.

[boB]

Yep.  edge of cloud event.

Prismatic effect that can last around a minute or so if the cloud just stays
right where it is !   It's sure fun to watch !

boB