Posted by Anthony Matonak on July 21, 2004, 3:28 pm
You could think of it this way. Typical PV panels are around 12%
efficient at converting sunlight into electricity. This means
that the other 88% is converted into heat. Clearly they are much
better at making heat than electricity.
Posted by Fred B. McGalliard on July 21, 2004, 5:59 pm
I thought around 30% was reflected? This would leave about 50% as heat.
Posted by Rod Speed on July 21, 2004, 6:37 pm
Nope, that would only be true if they were a perfect
black body absorber of all energy that falls on them.
In practice quite a bit is just reflected off them.
But then so is say black roofing material.
Posted by nicksanspam on July 22, 2004, 2:24 pm
Well, maybe 16 and 84%, eg 0.84x800 = 672 W/m^2 in AM2 sun.
Buth they are better than that. Rich Komp says the silicon is almost
transparent to IR, and the aluminum contact beneath is shiny, so PVs
are a selective surface.
Posted by Fred B. McGalliard on July 22, 2004, 5:01 pm
I think that has a lot to do with whether there is an antireflective coating
on the surface or not. Silicon is very shiny in the optical, (and I think
the reflectivity is a function of doping), where most of the energy comes
in, so, especially if the sun is at a substantial angle to the array, a lot
of the energy is reflected from the top surface. What gets in and is not
absorbed or reflected from the top surface metalization (around 10-20% I
think for typical cells) passes through the active cell and what isn't
absorbed there then passes through the rest of the bulk silicon used as a
handle. The back side metalization would be under all this heavily doped
handle. Very thin cells would be very different from the currently more
common thick cells. I am not sure about the ones that are epi deposited on
material other than silicon. If they are thrown down on a metal contact they
are probably very reflective.