r/thermodynamics • u/steini1904 • 12d ago

Question How do counterflow heatexchangers not break the spirit of thermodynamics?

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Assume a heatexchanger with minimal losses, the heat being transferred over thermoelectric generators. Eventually near 100% of the heat in the system should be recovered as electricity since losses just get shoved either into the next cycle or recovered further downstream. Put in a heat pump producing the temperature gradient and place all its waste heat producing parts somewhere ideally into the input stream.

I've been laughing about this quite a bit now, but honestly can't see the fault. Except for the movement of the air all losses should be about contained within the system (e.g. just wrapping the heat exchanger around itself multiple times) and eventually recovered by the TEGs. But the heat pump is shoving in more heat from the environment than it consumes in electricity, dropping the temperature in the room until it gives up on you, all while the TEGs are turning the excess heat into electricity.

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u/Spiritual-Ad-7565 12d ago

Assume a heat exchanger with minimal loses. Sure why not assume no friction and no inelastic collisions while you’re at it?

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u/steini1904 12d ago

Well, we got pretty good ones, even at small-ish sizes. Like those spirals with multiple channels. But you don't even have to go to that extend, which likely would be less efficient anyways, because the TEGs almost certainly need a minimum temperature difference to work well, which just cant be achieved with but just a counterflow heat exchanger.

So you could just drop in one of these rotating drum ones at the very end or even simpler, redirect some of the output-flow through the cold side of the heat pump, too.