Interestingly, while the Carnot Limit is derived using heat engines, it applies to any "device" that extracts energy from a temperature difference. If it were not so, then one could place, for example, a thermoelectric device between two energy reserves at different temperatures and use it to power a Carnot heat pump to drive the temperature difference higher, or to sustain the temperature difference and extract work. This is a perpetual motion machine, violating the thermodynamic laws of which the Carnot Limit is a manifestation. (It is a general result, obtained from hypothetical devices.)

One can sort of "feel" this result by imagining the voltage pushing a cloud of electrons around a circuit, producing shaft work in an electric motor, and/or heat if there is any electrical resistance.