output too high

buck-canuck · February 2011

Greetings all. My system has been performing great since installing this past summer. Today it's spiking way past normal. I have 8-180 w mono panels on a tracker and hooked to a Xantrex mppt 60 controller. It is sunny right now with a few thin clouds going by. Here is the possible problem. When a thin cloud goes by the sun, my wattage is spiking up to as high as 1763w and 55a. It holds there for a few seconds and then drops back down to 800 to 900 w. After the cloud goes by it returns to normal ranging from 1350 to 1445 w and 51w. Do I have a ground problem or something else? Is this possibly normal and I just haven't noticed it before? I'm thinking that because of the panels getting bright sun and then less and then bright again, is the controller just trying to catch up? Any help is appreciated. Thanks.

Cariboocoot · February 2011

Re: output too high

No problem at all: you are describing the classic Edge Of Cloud Event, where the cloud actually acts as a lens and concentrates the sun's rays briefly causing unusually high output. Do not fear it: it won't harm anything. And it's fun to watch your panels produce at 110%.

buck-canuck · February 2011

Re: output too high

That takes a load off. That's great. Thanks for the quick response.

Ralph Day · February 2011

Re: output too high

Edge of cloud events are talked about a lot on RE forums. I thought the final word was...your panels cool somewhat under cloud resulting in higher efficiency (under less light), then the cloud edge clears the sun and the cooler more efficient cells provide a boost/surge of power...because they are acting more efficiently.

I see this happen often, but the surge is not a problem, unless perhaps you have fully charged AGM batteries and the surge causes a little outgassing to occur.

Output from cold panels (-20C or so) can be above rated capacity by 10% or slightly more in my system. 2.1kw outputting 2.3 or so. When the temp is sustained low the output stays up there, with edge of cloud events the surge is momentary, not sustained.

Ralph

BB. · February 2011

Re: output too high

Actually, from what little I understand, it is a bit more complex...

The output voltage is proportional to temperature (as temperature increases, the output voltage falls). The output current is proportional to amount of sunlight (and mildly positive temperature coefficient for output--as temperature rises, the output current slightly increases, much less than the temperature effect on voltage).

For a PWM controller--More or less, the increase in current output is the result of edge of clouds "focusing light onto the panel (more sun, more current).

With a MPPT type charge controller (such as Buck-Canuk has), you have the increased current from edge of cloud effect; Plus, you have the increased voltage from a cooler panel (no sun heating while covered by clouds--panels will quickly warm up again and voltage will fall, and Pmp=Vmp*Imp will fall too). So, you have a mixture of two effects with MPPT controllers.

In the case of a PWM controller--they cannot "control" the extra current/power--and they could (in theory) shut down from overheating/over current.

A MPPT type controller is a digitally controlled switching power supply (down converter/Buck Mode power supply typically). These controllers have the ability to limit their output current to a "safe" level (i.e., the XW 60 amp MPPT charge controller will never output more than 60 amps, no matter how much energy is available).

As Marc/Cariboocoot says, just enjoy the extra output.

-Bill

Dave Angelini · February 2011

Re: output too high

Ralph Day wrote: »

Edge of cloud events are talked about a lot on RE forums. I thought the final word was...your panels cool somewhat under cloud resulting in higher efficiency (under less light), then the cloud edge clears the sun and the cooler more efficient cells provide a boost/surge of power...because they are acting more efficiently.

I see this happen often, but the surge is not a problem, unless perhaps you have fully charged AGM batteries and the surge causes a little outgassing to occur.

Output from cold panels (-20C or so) can be above rated capacity by 10% or slightly more in my system. 2.1kw outputting 2.3 or so. When the temp is sustained low the output stays up there, with edge of cloud events the surge is momentary, not sustained.

Ralph

I just changed out a neighbors AGM's for the second time for this problem of spikey voltage peaks when in absorption. Keep a load going if you must have these batteries.

RandomJoe · February 2011

Re: output too high

I always had the impression that as a cloud passed over the sun there was a brief "extra bright" moment just at the edges. I also just figured it was my imagination! Wasn't until I put solar panels on the roof and actually saw the same effect I realized it truly did happen! :cool:

icarus · February 2011

Re: output too high

Edge of cloud events are even more exaggerated with reflection off of snow or water. I have seen some amazing spikes on a cold winter day with puffy cumulous clouds. I used to joke that I would see double numbers over the summer harvest. Probably not double, but early morning, ~ -20 when the ice fog curls into cloud and the panels get exposed and I am pegging the Rogue! 510 watts out of 400 watts of panel. (Probably is higher but the Rogue limits itself to 30 amps output.

t

mike95490 · February 2011

Re: output too high

Here's a thread here, that has some photos of the intensified edges taken from an airplane.
http://forum.solar-electric.com/showthread.php?p=25841&highlight=lensing#post25841

output too high

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