My trouble is with a simple steam turbine system. Like the one shown: My doodle of the system, including the condenser, but crossed out.

  1. Now in the systems I learned about, which mostly do volume-expansion-work, heat has to be given away through the condenser to bring the system back to its original state, which makes sense when the system is doing volume work. It has to be compressed in order to be able to work again.

My doubt is with the steam turbine system (shown above). If the heated steam (could be any gas, maybe?) is used to do work it will cool after losing energy. Then couldn't this cool gas be passed straight to the boiler or into contact with the heat source and used again to produce work?

  1. I know this is against the Kelvin-Planck statement but I can't tell why the gas needs to be cooled, i.e., needs to pass heat to a cooler or second source. I'm sure I'm missing something and would really appreciate help.

(Now I think its reasonable to assume ideal systems where heat is not lost through the pipes or the walls of any of the containers because that's what system 1 was like and it still had an explanation for why the second source was needed.)


1 Answer 1


The working gas becomes more rarefied every time it passes through the turbine. We could reheat it, but the power output would drop each cycle.

We don’t want an engine that trails off; we’d like to return everything to its original state every cycle, for indefinite consistent operation. So a compression step is needed, which steals a little work that the turbine produced, in essence.

This is consistent with the heat-engine efficiency-limitation laws that require any gained entropy to be dumped somewhere to the environment, which occurs through heat rejection when the compression is performed at some available, relatively low temperature.

  • $\begingroup$ Chemomechanics. I know you are not enthusiastic about the way Bronsted (Le Chatelier/Carnot, etc.) view the proper way of teaching thermodynamics but would you agree with me that if it were taught a la Bronsted a question like this would never even be asked? $\endgroup$
    – hyportnex
    Oct 5, 2023 at 22:05

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