I know that heat engines (heat to kinetic) are limited by Carnot cycle and that kinetic energy to electric energy conversion via standard generator reaches over 90%.
However I would like to know wheter Carnot cycle limits every kind of thermal to kinetic/electric energy like thermocuoples? If not then what is the limit of such conversion?
Also what is the highest possible efficiency of Carnot cycle?
Best Answer
Yes, since any conversion of heat into "work" is limited by Carnot, as you said:
The maximum efficiency of a Carnot cycle is given by:
$$ \eta_{max} = 1- \frac{T_c}{T_h} $$ Where $T_c$ is the temperature of the cold sink and $T_h$ is the absolute temperature of the hot source.
This is not limited to Carnot since it is not a heat engine (converting heat to work). Photo voltaic solar cells are also not heat engines since they convert light not heat.
This paper is titled "Nanoscale Heat Engine Beyond the Carnot Limit" However it is a theoretical paper and the system has not been built, it may not be possible to construct as described or behave as described, or a new nanoscale Carnot description may be needed.
If some process were to beat the Carnot limit it would be possible to reduce global entropy. This would violate the second law of thermodynamics and is believed not to be possible, Stephen Hawking and Leonard Suskind "invented new physics" just to ensure that black holes would not violate this principle. See the black hole war.