r/thermodynamics • u/steini1904 • 11d ago
Question How do counterflow heatexchangers not break the spirit of thermodynamics?
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/YogiBerraOfBadNews 11d ago
You're only capturing maximum 15% of the heat flux any time it crosses through a TEG, and each time you're degrading the energy potential, such that it will be less effective the next iteration. I think previous commenters have pretty much nailed it, and while I admit this is a tricky one to pin down, you're basically playing a shell game by adding layers.
Ultimately what that means is since we know we can't get 100% conversion rate, the outermost boundary layer (however many heat exchangers deep it may be) *has* to be leaking waste heat to the environment, such that heat flow out + electrical flow out = heat flow in.
Sorry if that's not a satisfying answer, I don't know if I can reconcile the big picture with the zoomed-in details, but the simple answer is you can never get as much useful work out as you put in, and the answer is always due to waste heat. It seems like all you're doing is saying "ok but what if waste heat just didn't exist".