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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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    (Just found this one again - I do wish they wouldn't move things out of the Wiring Regs!)

    So, what appliances did you have running at 1.00 pm to make use of the electricity generated?

    Often just my baseload of various controls, gadgets, fridge etc. (typically 200-300W), sometimes the washing machine, but on a good day the majority is exported and so used by the neighbours (I'm paid generously for generating it after all).


    and yes, i can see what you mean about the overshoot on a restart vs  continuous, looks as if it could be as much as 10%.

    Indeed. I was aware of the reduced efficiency with temperature but was surprised the effect was so large. The theory of just-out-of-line clouds redirecting a little extra light towards the panels makes a lot of sense too - so perhaps a combination of the two might explain the 10%(ish).

    Thinking about it, the thermal effect should always lag the cloud passage, but the cloud diffusion effect would presumably happen just as much with an approaching cloud as with a departing one - so a peak should sometimes appear before a trough in that case. I'll look closer at the data to see if I can see any such effect….

    Thanks all - it's been enlightening!

       - Andy.

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  • 0

    (Just found this one again - I do wish they wouldn't move things out of the Wiring Regs!)

    So, what appliances did you have running at 1.00 pm to make use of the electricity generated?

    Often just my baseload of various controls, gadgets, fridge etc. (typically 200-300W), sometimes the washing machine, but on a good day the majority is exported and so used by the neighbours (I'm paid generously for generating it after all).


    and yes, i can see what you mean about the overshoot on a restart vs  continuous, looks as if it could be as much as 10%.

    Indeed. I was aware of the reduced efficiency with temperature but was surprised the effect was so large. The theory of just-out-of-line clouds redirecting a little extra light towards the panels makes a lot of sense too - so perhaps a combination of the two might explain the 10%(ish).

    Thinking about it, the thermal effect should always lag the cloud passage, but the cloud diffusion effect would presumably happen just as much with an approaching cloud as with a departing one - so a peak should sometimes appear before a trough in that case. I'll look closer at the data to see if I can see any such effect….

    Thanks all - it's been enlightening!

       - Andy.

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