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Cloudy Day Paradox

March 19th, 2012 · 6 Comments

On cloudy or partly cloudy days, like today, I generate less electricity from my solar panels. Of course, that makes sense.

The paradox is that on a partly cloudy day like today, although the power may fluctuate up and down as the clouds cover and then uncover the sun, the peak power is higher than on a clear sunny day. The most power I see on a sunny day is about 6.5 KW. This morning the power is peaking at about 7.5 KW.

It’s pretty clear to me what is happening. When the sun breaks through the clouds and shines directly on the solar panels, it is also reflecting off the nearby white clouds, increasing the total amount of sunlight that hits the panels. Similar to how you can get a worse sunburn on snow, or on the beach. Reflected sunlight combines with direct sunlight to increase the total insolation.

It doesn’t make up for the losses when the sun is covered, but it does help to mitigate it.

Tags: Computers, Tech & Science

6 responses so far ↓

  • 1 Richard // Mar 19, 2012 at 12:06 pm

    This is a well known phenomenon in solar circles. I’ve heard it explained as being caused by the water droplets in the edge of the clouds acting like lenses and focusing more sunlight. It usually happens just as the sun peeks out from or disappears behind a cloud.

    I just checked my controller. It shows a high watts today of 963. That’s essentially impossible with only 800 watts of solar panels, but the focusing of the light makes them put out much more power than normal. On a normal clear sunny day my high is usually in the 750 range.

  • 2 Daryl // Mar 19, 2012 at 4:08 pm

    I’m skeptical of the “droplets acting as lenses” theory. To me it makes more sense that it is light being reflected from the clouds.

    I would think that droplets acting as lenses should split the colors apart, just as they do when they create a rainbow. I would expect to see some colors around the edges of the clouds if that was happening. But they look completely white, as they would with reflections.

    But I’m open to seeing some data or experiments.

  • 3 Daryl // Mar 19, 2012 at 7:18 pm

    I’ve done a little research on this, with a good source here.

    The effect seems to be mainly caused by scattering: “a physical process by which a particle in the path of an electromagnetic wave continuously abstracts energy from the incident wave and re-radiates that energy”.

    One interesting result of this is the blue sky. With small particles, such as air molecules, the longer wavelengths are scattered mostly forward, while the shorter wavelengths are scattered in all directions. Red and yellow light have a longer wavelength, and so get scattered mostly forward. That’s why the sun looks yellow through the atmosphere. But blue light has a shorter wavelength and so gets scattered in all directions. That’s why the sky looks blue.

    With water droplets making up the clouds which are huge compared to air molecules, most of the light is scattered in all directions, which is why clouds look white. So clouds in the sky cause more light to fall on the solar panels. It isn’t focusing, and strictly speaking it isn’t reflection, it’s scattering.

    There could be other principles at work here too, but this seems pretty reasonable to me.

  • 4 Richard // Mar 20, 2012 at 10:24 am

    Like I said, I heard it was “droplets acting as lenses.” I didn’t say I believed it.

    I figured you would did a lot deeper into this. It’s really interesting.

    In scanning the document you referenced I have come to the conclusion that the cause of the phenomenon is much more complicated and involved than anyone would think. People like simple answers and explanations, but often such simple explanations don’t exist.

    The reason more energy strikes the panel is because some of the light from behind the cloud is bent from its original path and directed toward the panel. This is not due to the prismatic effect of the water droplets, but rather due to the light being knocked about by the water molecules in the cloud. The scientific term for this is “scattering”. So, the effect is caused by the water droplets scattering the light, not focusing it.

    Interesting. This raises some other questions, but I’d rather discus them with you face to face instead of trying to write it all out on a blog. Maybe the next time we get together.

  • 5 Dale // Mar 20, 2012 at 1:04 pm

    I’m not at all sure about this but I think it’s aliens causing it.

  • 6 Don // Mar 22, 2012 at 7:08 am

    Gremlins…