Surpassing the Carnot Efficiency by extracting imperfect work

Abstract

A suitable way of quantifying work for microscopic quantum systems has been constantly debated in the field of quantum thermodynamics. One natural approach is to measure the average increase in energy of an ancillary system, called the battery, after a work extraction protocol. The quality of energy extracted is usually argued to be good by quantifying higher moments of the energy distribution, or by restricting the amount of entropy to be low. This limits the amount of heat contribution to the energy extracted, but does not completely prevent it. We show that the definition of "work" is crucial. If one allows for a definition of work that tolerates a non-negligible entropy increase in the battery, then a small scale heat engine can possibly exceed the Carnot efficiency. This can be achieved even when one of the heat baths is finite in size.

8 pages text (4 figures) + 12 pages appendix (1 figure)

References in corpus (8)

• A single-atom heat engine
• Fluctuation theorems: Work is not an observable
• Description of quantum coherence in thermodynamic processes requires constraints beyond free energy
• Single ion heat engine with maximum efficiency at maximum power
• The second law, Maxwell's daemon and work derivable from quantum heat engines
• The unlikely Carnot efficiency
• Quantum heat machines equivalence and work extraction beyond Markovianity, and strong coupling via heat exchangers
• From single-shot to general work extraction with bounded fluctuations in work

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