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Solar panels, basic question

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honeybadger:
Hi,
I had a thought about solar panels and what happens with the light energy when they are not in use.


There is some amount of energy per surface area from the Sun.

When it strikes the solar cell - some of it is reflected - some of it is absorbed as a heat energy - the rest is converted to the electricity.

So when the solar cell is producing an electric current it gets heated by the absorbed energy and by the ohmic losses (simplified).

Now unplug the solar cell.

What happens with the "electric" part of the incoming energy?

I guess there are only two options:
1. The "electric" part is reflected - which means the cell becomes more reflective when unloaded - I don't think so.
2. The "electric" part is absorbed as additional heat - which means the cell will be crazy hot. Because of the energy sum it will be hotter than fully loaded cell (short circuit with maximum ohmic loses).

I think the second option is the only possibility?

David Hess:
With no load attached, the P-N junction becomes forward biased and conducts like any other diode which is what limits the open circuit output voltage.  Under these conditions, power dissipation in the cell increases by the cell efficiency so perhaps 20%.

honeybadger:
Thanks, that makes sense.

Rick Law:

--- Quote from: honeybadger on January 06, 2020, 03:46:51 pm ---...
Now unplug the solar cell.

What happens with the "electric" part of the incoming energy?
...
2. The "electric" part is absorbed as additional heat - which means the cell will be crazy hot. Because of the energy sum it will be hotter than fully loaded cell (short circuit with maximum ohmic loses).
...

--- End quote ---

Even if the electricity continues to be generated, physics "almost" dictates it will be no more heat than "normal" defining normal as the heat that is generated by your typical black-tar roof tiles without solar panels on top.

("almost" because black-tar tile is not a perfect "black body", it does reflect some light)

The electrical energy the solar panel outputs is converted from the energy of the light falling on it.   Black-tar tile is rather "light absorbing."  It doesn't reflect much.  So black-tar tiles pretty much convert all the light to heat.  Even if the solar panel converts that energy into electrical to drive a heater, it would not be more energy than what fell onto it as light so the electrical heat energy would not be more than a direct black-tile converted heat.  It could be more concentrated (say electrical energy driving a high temperature coil heater coming out of a small nozzle), but total energy would be at best the same amount as the energy carried by the light coming in.  When the waste-energy is spread and dissipate over the entire area of the panel, you have at worst (hottest) same as black roof tiles.

Now, if you used to have white tile on your roof a lot more energy was reflected away, it would be cooler than black roof tiles.  So un-plugged solar panel under the sun would be hotter than the white tiled roof, but not much hotter than the black tiled roof if hotter at all.

unitedatoms:
Semiconductors are photosensitive and in return photoemitting. As @David Hess noted that PN junction turns open when unloaded. It may mean that panel is radiating back at some infrared or visible green.

So perhaps the unloaded panel should look different to bystander than when it is loaded.

If color change is in visible band, then it is possible to send subconscious pixelated messages to unsuspecting public staring at hills and roofs covered with panels.

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