Off-Boresight PV Array Effects

[jack11]

The only power derating for off-boresight pointing of a PV array I am aware of is the cosine effect (as the array points away from the sun, the generated power should drop off with the cosine of this off-boresight pointing angle, on-boresight being pointed straight at the sun).

Does anyone know if there are any other effects that would contribute to this off-boresight power derating, such as some silicon-related effects, glass refraction-related effects, etc, arising from some other properties of the solar panels?

I am only talking about these effects due to off-boresight pointing of the PV array, not about the power-altering effects that exist all the time (ex. dirty panels, shading, temperature effects, etc).

[joestue]

no i don't believe there are any optical effects other than operation near the critical angle at which all the light will be reflected away from the glass

http://en.wikipedia.org/wiki/Total_internal_reflection
hmm, i don't think this even works backwards.

[jack11]

Thanks Joe, I forgot about the critical angle.

So it looks like at the 1st air-glass surface the refraction helps bend the off-boresight lightwave toward the normal. But the total reflection is not possible here because the refraction index of air is smaller than one of glass.

Then at the 2nd surface the angle of incidence onto silicon (if silicon is directly under the glass) is smaller than at the 1st surface, so the glass helps redirect the energy toward normal in this case.
If air is under the glass, then the light wave exits with the same angle as the incidence on the 1st surface, and strikes the silicon as if the glass was not there at all.

There should also be baseline reflection losses at both surfaces, but that's a part of the low overall panel efficiency.

I don't think the total internal reflection is possible in either case.
If it were, then the lightwave energy would get trapped in the glass, and none would get into silicon.

But I think, the PROPORTION between the refracted and reflected energy at both glass surfaces may change with the incidence angle (the greater the incidence angle, the greater the reflected energy in proportion to the refracted energy, the total amount of energy being the same).
That proportion would be an additional derating parameter for off-boresight panel operation.
This would be in addition to the simple cosine effect that is still present even if there is no glass and the light strikes silicon directly.
Someone good at optics could probably answer that better.