Thermodynamic cycle and entropy

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SUMMARY

Thermodynamic cycles cannot achieve 100% efficiency due to the inherent nature of entropy and irreversible processes. While reversible processes theoretically exhibit zero change in entropy, real-world applications always involve some degree of irreversibility, resulting in heat rejection. This means that not all input energy is converted into work, as some energy is inevitably lost as waste heat. Consequently, even in idealized scenarios, a cycle composed entirely of reversible processes cannot exist without some entropy production.

PREREQUISITES
  • Understanding of the laws of thermodynamics
  • Familiarity with the concept of entropy in thermodynamic systems
  • Knowledge of reversible and irreversible processes
  • Ability to interpret t-s (temperature-entropy) diagrams
NEXT STEPS
  • Explore the Second Law of Thermodynamics in detail
  • Study real-world applications of thermodynamic cycles, such as Carnot and Rankine cycles
  • Learn how to analyze thermodynamic efficiency using the Carnot efficiency formula
  • Investigate the role of heat exchangers in minimizing entropy generation
USEFUL FOR

Students and professionals in mechanical engineering, thermodynamics researchers, and anyone interested in the principles of energy conversion and efficiency in thermal systems.

EvilKermit
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Alright, I'm confused about why thermodynamic cycles can never have 100% efficiency. There reversible processes right? And a reversible process has zero change in entropy, so then why can't they achieve 100% efficiency. I thought entropy was the measure of energy that cannot be used. How would you measure efficiency if the system increases in entropy?
 
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In a thermodynamic cycle, not all of the energy inputted is converted into work. Some heat is always rejected. Not all processes are 100% reversible.
 
...so why no try buiding a cycle with all reversible processes, so no heat is rejected? You'll find you can't. Real processes that can be combined into real thermodynamic cycles will always result in a cycle with some entropy.

Try drawing a cycle with no heat rejection on a t-s diagram and see if you can find real processes (devices) that do what the t-s diagram suggests.
 
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