[bobsy]

As someone with poor circulaton to the extremities I fear I would be walking around in the dark holding the flash light with cold hands.

Its a really nice idea though. Its stories like this that make me wish i paid a bit more attention in science when i was younger.

[koudelka]

If your extremities' temperature is cooler than the ambient temperature, maybe all you need is a version with the heat-collection plates flipped?

[schiffern]

If your extremities' temperature is cooler than the ambient temperature, how did you do that? Heat pumps in your arms? ;)

It would be trivial to put some circuitry in there to rectify the voltage so it works in either direction. No separate version required.

(changing ambient temperature is the obvious answer to my joke, btw)

[chinpokomon]

Although others that have responded to this thread have far more experience, I believe Peltiers are optimized to work in one direction. That is to say there is a hot and cold side, so I'm not sure it is as simple as adding an inverter. Not that you'd likely be using your hands as a heat sync anyway.

Yes, I realize this was meant as a joke, but I'm still curious if current can be reversed by just changing the gradient. Does it still work but maybe it is less efficient? Is there no difference at all?

[schiffern]

Ah, good point. I hadn't considered that. It looks like you are correct! You might sandwich two Seebeck-Peltier devices and switch between them, though.

https://en.wikipedia.org/wiki/Thermoelectric_effect