Would this require a temperature gradient to function? My understanding is that a device that converts heat into power would violate thermodynamics.
Likewise the phonovoltaic would need to operate on a phonon spectrum that is shifted away from that of the material's own temperature, in order to not violate the laws of thermodynamics.
Much easier to replace the sun by some other source of EM-waves of suitable wave length and put it into a freezer, below the temperature of the cell.
How does the wave poking some electron to a higher state remember the temperature of its source?
The thermodynamic temperature of the radiation is connected to the entropy of its power spectrum. LEDs and especially lasers emit very low-entropy light, which can be focused and heat a surface up to very high temperatures. (Sunlight focused onto a surface cannot heat that surface above the temperature of the sun). Thermodynamically speaking, a low-entropy power source like a laser -- and whatever is driving it -- must have a very high exergy, which is equivalent to behaving like a high temperature heat source, even though it might feel cold to the touch.
Some more details here: https://en.wikipedia.org/wiki/Exergy#Quality_of_energy_types
Stored electricity's equivalence to a high-temperature heat source is one of the things that makes it so useful. It's intrinsically connected to why it takes a lot of low-grade heat to produce a small amount of electricity in the first place, and also why electric furnaces can produce such high temperatures. So while a battery can drive an LED that shines on a PV panel that generates power with everything feeling equally warm to the touch, a temperature gradient is still necessary; it's just been moved outside the boundaries of the system, to the process that distilled the entropy out of the energy that became the battery's stored charge. We can reversibly recreate this temperature gradient by driving a Carnot engine with that battery, instead of a laser.
The black-body spectrum of the source determines the amount of waves it outputs at each frequency, and in turn is determined by the temperature.
I looked for a good article, but didn't find anything. There was one relevant comment in the first article I skimmed that said "semiconductors do not behave remotely close to a blackbody": sounds authoritative but the comment was on stack-overflow so hard to judge its correctness!
Sorry.
isn't a coal burning steam engine converting heat into power? perhaps the violation applies if requiring efficiency in the conversion?
Carnot proved in 1824 that the maximum theoretical efficiency is purely a function of the hot and cold temperature (in Kelvin):
Eff = (T_hot - T_cold)/T_hot
So for instance a thermoelectric generator operating between 25 °C and 5 °C has a maximum efficiency of
Eff = (298 - 278) / 298 = 6.7%
> Eff = (298 - 278) / 298 = 6.7%
Does that mean a thermoelectric generator operating at a minimum of 0K has a maximum efficiency of
Eff = (N - 0) / N = 100%?
1600 times. Gas at room temp occupies 24 dm3/mole.
I'm probably missing the point entirely, but aren't we doing this already with Thermocouples as used in RTGs?