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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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    I'm not too sure what this is showing - there are 2 colours of blue blocks and a yellow arc - is one of these power output, and why is it solid, and not a line, does this represent the short duration blackouts.


    In any case it is certainly true that solar cells are more efficient in terms of photons to electrons, when they are cooler, as the rinsing temperature increases the vibrational scattering of the charge carriers, lowering the effective mobility of the electrons and holes. It is not really worth cooling cells in the UK, as the losses would be ~ a fraction of a % per degree, but in hotter places or very large area set-ups it can become worthwhile to add fans and air ducting to keep the back cooler.  In the UK leaving an air gap between panel and building is usually good enough

    Mike
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  • 0
    I'm not too sure what this is showing - there are 2 colours of blue blocks and a yellow arc - is one of these power output, and why is it solid, and not a line, does this represent the short duration blackouts.


    In any case it is certainly true that solar cells are more efficient in terms of photons to electrons, when they are cooler, as the rinsing temperature increases the vibrational scattering of the charge carriers, lowering the effective mobility of the electrons and holes. It is not really worth cooling cells in the UK, as the losses would be ~ a fraction of a % per degree, but in hotter places or very large area set-ups it can become worthwhile to add fans and air ducting to keep the back cooler.  In the UK leaving an air gap between panel and building is usually good enough

    Mike
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