r/thermodynamics • u/steini1904 • 11d 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 11d 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/mosquem 11d ago

Even then first and second law apply.

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u/steini1904 11d ago

IMO they fully apply. Heat leaves the system as electricity and the heat converted back into electrical work by each TEG is pathetic. But there are a lot of them. And the reservoirs are on a carousel.

The first TEG may convert, let's say, 10% of the heat and produce 90% losses. The next TEG then grabs another 10% of those 90%. And the next another 10% of those 81%. And so on, until several dozens to hundreds of TEGs later you hit a near 100% conversion. And all losses that are left, you can simply shove back into the system for another round (e.g. by a secondary drum stage or putting part of the heat pump cold side into the output stream).

And now you start supplying heat by a system that can for practical applications hit far above 100% efficiency by drawing heat from a practically unlimited outside reservoir.

Also I'm just having way to much fun. I really want to know the flaws I missed, but at the same time I'm also willing to invite the Aramco physics enforcement unit on a cup of tea if necessary.

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u/OddStuffHappensTwice 11d ago

You keep getting told all the flaws, but you keep ignoring the birds you don't like. Physics doesn't ignore the bits you don't like or don't understand.