Why not put smaller solar panels beneath much larger lenses?

[AskNott]

I saw a TV feature once on a solar power field. To combat the loss through dust, they had a robot to go down the rows every few days to wash the panels. It seems to me that having to also wash rows of lenses would be more complicated.

[Stranger_On_A_Train]

@iamthewalrus_3

Do you have a cite for this? I don’t claim otherwise, but it surprises me that clouds actually scatter light so fully that lenses cease to work at all.

Under full cloud conditions, the incoming polarized light is essentially randomly scattered, hence why there are no shadows and colors are muted. This is expected because the incoming photons don’t have a direct path through the opaque clouds. The ray trace on a parabolic mirror or double convex or Fresnel lens will only focus light coming in from a specific orientation.

Plastic fresnel lenses could be used to provide a cheap focusing method, but they don’t reduce the total footprint required and would be difficult to keep clean, and as otherwise noted would required continuous rotation to maintain correct orientation to the light source. What might make sense are film lenses on top of the solar cell which separate the spectra and project chroma onto different substrates optimized for that frequency, but that is just a marginal improvement.

Stranger

[Napier]

@iamthewalrus_3

Do you have a cite for this? I don’t claim otherwise, but it surprises me that clouds actually scatter light so fully that lenses cease to work at all.

And I realize that the focusing power of a lens probably doesn’t drop to actual 0, but if it gets to < 20% or so, I’d consider it close enough.

If the sun doesn’t cast a shadow because of clouds, a lens can’t concentrate its light either.

If you don’t see the sun in the sky it’s because the lens in your eye no longer works to focus it to a small spot.

[scr4]

@iamthewalrus_3

Do you have a cite for this? I don’t claim otherwise, but it surprises me that clouds actually scatter light so fully that lenses cease to work at all.

Laws of thermodynamics require that any passive optical system (lens, mirror, etc) cannot increase the apparent surface brightness of an object. (If it could, it would allow heat to flow from a lower temperature surface to a higher temperature surface.)

An optical system can increase the apparent size of an object. So if the sky is clear, a lens can preserve the apparent surface brightness of the sun, and make it look bigger (as seen from the focal point). This is what is happening when a lens is used to focus light onto a small spot.

But if the sky is cloudy, a lens will simply enlarge one patch of cloud and make it look bigger, without increasing its apparent surface brightness. So, the enlarged view of the cloud looks no brighter than the bare cloudy sky. I.e. the amount of light that falls on the focal point of the lens is the same as without the lens.

You can easily experiment by taking a magnifying glass and looking at the ceiling through it (or a cloudy sky - never the sun!). The view through the magnifying glass will not be any brighter than what’s behind it.