Comment by UniverseHacker

[Hard question!]

Each photon, no. But the distribution of the spectrum of a black body follows a pattern that depends on the temperature (and other stuff).

<bold>The important part is that the PV panel not only absorbs photons, it also emits photons. Usually the emission is much much much smaller than the light it absorbs.</bold>

If you assume that the PV panel is another black body and the sun is another black body, then the energy must flow from the hot one to the cold one.

Real objects are not perfect black bodies, so you have emission and absorcion coefficients that makes the calculation harder.

For LEDs, bioluminescence, lasers, and other sources, the calculation is harder because the distribution of light in the spectrum is different, and they usually don't match. You must consider work, entropy, and more technical stuff. But the oversimplified version is that the energy also flows from the "hot" to the "cold" one. You need a gradient of temperature or something equivalent to make the energy flow.

PS1: You can use a LED as a light detector, like a photodiode. It's somewhat the inverse effect of using a PV panel as a lamp.

PS2: I know that <bold></bold> doesn't work in HN, but that it's the important paragraph.

Yes, indirectly- this is how infrared thermometers can measure temperature at a distance. If there is no temperature differential then both surfaces will be radiating the same number of photons at one another, and therefore radiate at least as much energy as they absorb.

For a “non thermal” source, the thermodynamics aren’t changed, you are still radiating the energy you put into it, and a sufficiently “hot” absorber will be unable to use the energy. The thermodynamics are the same, you just have more control over the frequency and direction of the radiation.