Peak PV watt output question

[IdlePhaedrus]

Hi All,

I have a 3kw solar array, and I have noticed a strange thing. On partly cloudy days the array generates a peak wattage that is much higher than on a normal sunny day.

On a normal sunny day in CT currently the peak wattage is always less than the peak wattage on a partly cloudy day. For example on a sunny day the peak wattage will be around 2.6kw but on a partly cloudy day it might hit 3.6kw.

This has nothing to do with the average kwh generated during the course of the whole day, on a sunny day that is always better.

I have Googled, and others have noticed the same, but I cannot find anything about why it actually happens.

Does anyone care to speculate as to the reason this happens?

Regards
Phaedrus

 

[Sinbad]

Cloud edge effect

 

[IdlePhaedrus]

Thanks Sinbad, that is exactly what I was looking for.

Clearly my Google search terms were not sufficiently explicit.

I shall now do some reading.

Much appreciated.

Cloud edge effect

 

[itareanlnotani]

You also want to look at the temperature coefficient for the panels aka Pmax.
Cooler panels give higher voltage.

Panels are STC rated at 25C, for every degree above that, you decrease in percentage by the panels PMax value.
If its lower than 25c you'll get an improvement.

Explained a little better here - http://www.civicsolar.com/resource/how-heat-affects-solar-panel-efficiency
and here http://www.solarquotes.com.au/blog/...specification-part-1-power-temperature-specs/

I have covered this in the long thread about panels.

You may want to consider lightly misting panels in summer to cool them down. Does actually improve efficiency.
I'd also suggest a clean once in a while (every few months is fine) with a hose if the rain isn't getting rid of dust.

 

[Arthur]

It's why I've pointed out in several threads over the past few years that you need to make sure your cabling and MPPTs can handle up to 30% more wattage than the nominal rating.

As the cloud edge transitions the sun, the rays are refracted, causing a concentration and thus a higher energy burst than normal direct sunlight.

 

[IdlePhaedrus]

Thanks Arthur, am pretty sure the system as a whole is up to the challenge it was put in by two Germans with 60 years combined experience in the industry (I am a complete novice and reading up on what an MPPT is tends to make me cross-eyed :wtf.

http://forums.energymatters.com.au/solar-wind-gear/topic1507.html

There is an interesting discussion there about what the actual cause of the peak is, refraction, reflection, scattered light and a few other things to boot. On the whole, other articles and discussions mostly err towards the refraction side of things though.

It's why I've pointed out in several threads over the past few years that you need to make sure your cabling and MPPTs can handle up to 30% more wattage than the nominal rating.

As the cloud edge transitions the sun, the rays are refracted, causing a concentration and thus a higher energy burst than normal direct sunlight.

 

[IdlePhaedrus]

Thanks Isheed, I will give that a read.

You also want to look at the temperature coefficient for the panels aka Pmax.
Cooler panels give higher voltage.

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