[neltnerb]

Any engine is limited to the carnot cycle; the themoelectric effect still requires a cool side and heat gets conducted along the two leads to equalize the temperature. That's where the power comes from, the temperature differential across the thermocouple wires, and you consume the temperature differential when you generate electricity.

Different types of engines have different details of where that energy gets lost, but ultimately they are devices that if left to run indefinitely would equilibrate in some way and the way that they idle tells you a fair bit about where the energy flows are.

Turbines are pretty impressively efficient, but something can always be better. They often utilize the low grade waste heat in steam heating though, so a lot of times the efficiency is extremely good when you include free heating or process heat (in a plant they might use that steam to heat another piece of equipment).