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Can clouds improve PV output?

AJJewsbury
I've been playing with a slightly home-brew version of an open energy monitor, and been looking a some graphs of my minute-by-minute PV output and noticed something odd.


On a nice clear sunny day I get a nice bell-shaped curve for PV output peaking at solar midday (my panels face almost exactly due south) with a peak output a little below their peak rating. All as expected. When it's cloudy, output is of course much reduced, again as expected. What I've noticed is that when the sun returns after a cloud passes it seems that the PV output is for a short while higher than what would have been expected had there been continuous sunshine - not for long - a matter of minutes - before it returns to normal.


My guess is that this is due to the panels getting hot in the sun - which  drops their efficiency. When a cloud passes they get a chance to cool down for a but - so their efficiency goes up for a while until they've heated up again. Does that make sense?


A couple of graphs (from a few days apart this month) (PV is yellow, ignore the blue, that's just my consumption).


Firstly from a pretty clear sunny day (just a few clouds first thing in the morning) - peak is well below the 2000W line.
f620f26d5e1fd3d8ff017864ce84e3f3-huge-pvsunny.png


and then from a day with a few more broken clouds (unfortunately at a different scale - but after each cloud the output seems to be higher than the normal trend - at mid-day the spikes are much closer to the 2000W line than on a clear day) -
693782b11c6d89b9c59940095bf3bfdd-huge-pvsunnyish.png
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    Yes, clouds can increase PV output but only under certain relatively rare conditions.

    Consider a largely cloudy sky, but with a relatively small gap in the cloud cover that allows direct sunlight to fall on your array.

    The output power can exceed that produced under a clear sky. because the PV modules are exposed to direct sun AND also to the diffuse daylight from the clouds.


    In MOST conditions, PV modules are exposed to EITHER direct sun, or to diffuse daylight from white clouds, but under the conditions described above, they can be exposed to direct sun AND to diffuse daylight.


    In America, this fact is recognised by the NEC, which requires conductors connected to PV arrays to be sized in excess of that size suggested by the full load current. The phenomena is known as "solar lensing"


    If you have a light meter, this can be demonstrated as follows.

    Take a large piece of tracing paper and cut a hole in the middle.

    Support this exposed to direct sun, such that a large shadow is cast by the tracing paper, but with a bright spot of direct sun in the middle.

    Measure the light level in the bright spot, this will be the total of the direct sun through the hole and the diffuse light via the tracing paper. The reading will be greater than that from direct sun with the paper removed.
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  • 0
    Yes, clouds can increase PV output but only under certain relatively rare conditions.

    Consider a largely cloudy sky, but with a relatively small gap in the cloud cover that allows direct sunlight to fall on your array.

    The output power can exceed that produced under a clear sky. because the PV modules are exposed to direct sun AND also to the diffuse daylight from the clouds.


    In MOST conditions, PV modules are exposed to EITHER direct sun, or to diffuse daylight from white clouds, but under the conditions described above, they can be exposed to direct sun AND to diffuse daylight.


    In America, this fact is recognised by the NEC, which requires conductors connected to PV arrays to be sized in excess of that size suggested by the full load current. The phenomena is known as "solar lensing"


    If you have a light meter, this can be demonstrated as follows.

    Take a large piece of tracing paper and cut a hole in the middle.

    Support this exposed to direct sun, such that a large shadow is cast by the tracing paper, but with a bright spot of direct sun in the middle.

    Measure the light level in the bright spot, this will be the total of the direct sun through the hole and the diffuse light via the tracing paper. The reading will be greater than that from direct sun with the paper removed.
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