Thermionic Generators: Low Efficiency Explained

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The low efficiency of thermionic and thermoelectric generators is primarily due to the low potential difference generated for a given temperature difference, compounded by issues like current leakage. In thermoelectric devices, the need for low impedance to maximize current results in close junction placements, which leads to significant heat loss through conduction. Thermionic devices face challenges as they radiate considerable heat; insulating them sufficiently would cause the collecting electrode to heat up and act as an emitter, negating efficiency gains. Overall, both types of generators struggle with heat management and voltage limitations, impacting their overall performance. Understanding these factors is crucial for improving generator efficiency.
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Why is the efficiency of generators working on thermionic emmission and the generators working on the seebeck effect (thermoelectric) so low?
 
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I believe it is related to the fact that the potential difference is low for a given temperature difference, and perhaps current leakage also limits the current.

You might wish to look at this paper - http://www.humphrey.id.au/papers/JAP2005.pdf
 
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I alway thought it was heat loss. In thermoelectric devices, since the voltage is so low, you need low impedance to get lots of current. To get low impedance, you have to place the junctions close together and the material connecting the junctions wastes a lot of heat by conduction between the juncitons.

With thermionic devices, they radiate lots of heat. If you insulated them enough to catch most of the heat, the collecting electrode would get hot and become an emitter, itself, counteracting the emitting electrode.---I think.
 
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