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The hypothetical Carnot cycle consists of an ideal gas subjected to two isothermal and two adiabatic processes. Since the internal energy of an ideal gas depends only on its temperature, which is the same before and after the completion of the Carnot cycle, there is no change in its internal energy. Hence, using the first law of thermodynamics, the total heat exchanged by the ideal gas equals the total work done. Thus, we can quantify the efficiency of the Carnot cycle via the heat exchanged...
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Converting work to heat is an irreversible process, and the purpose of a heat engine is to reverse the effect partially. Heat engines aim to increase the efficiency of the reversal, that is, maximize the work retrieved from heat. If the efficiency of a heat engine were 100%, it would imply reversing the process completely without introducing any other effect. Thus, it would violate the second law of thermodynamics.
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Achieving the classical Carnot efficiency in a strongly coupled quantum heat engine.

Y Y Xu1, B Chen1, J Liu2

  • 1Faculty of Science, Kunming University of Science and Technology, Kunming 650500, China.

Physical Review. E
|March 18, 2018
PubMed
Summary

Classical Carnot efficiency is achieved in strongly coupled quantum heat engines. This is possible when heat released during a nonequilibrium process is balanced by absorbed heat, overcoming previous efficiency limitations.

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Area of Science:

  • Quantum Thermodynamics
  • Statistical Mechanics
  • Condensed Matter Physics

Background:

  • Quantum heat engines coupled to heat baths typically exhibit efficiencies below the classical Carnot limit.
  • Understanding quantum thermodynamic cycles is crucial for developing efficient quantum technologies.

Purpose of the Study:

  • To demonstrate that classical Carnot efficiency can be achieved in strongly coupled quantum heat engines.
  • To identify the conditions and restrictions for achieving Carnot cycles and efficiency in the strong coupling regime.

Main Methods:

  • Development of a model-independent framework for quantum thermodynamics in strong coupling.
  • Analysis of the first law of quantum thermodynamics under strong coupling conditions.
  • Investigation of a paradigmatic two-level system model interacting strongly with a heat bath.

Main Results:

  • Classical Carnot efficiency is achievable in strongly coupled quantum heat engines.
  • Achievement of Carnot efficiency relies on balancing released and absorbed heat in nonequilibrium processes.
  • Identified two key restrictions: equal entropies at hot/cold temperatures and equal initial/final state entropies in nonequilibrium processes.
  • Derived conditions for positive work, including a novel entropy overlap requirement.

Conclusions:

  • Strong coupling does not inherently prevent reaching classical Carnot efficiency in quantum heat engines.
  • Specific thermodynamic pathways and control schemes are necessary to overcome restrictions and achieve optimal efficiency.
  • The findings offer insights into the fundamental limits and practical implementation of quantum heat engines.