Quantum, cyclic and particle-exchange heat engines

TL;DR

This paper compares quantum, cyclic, and particle-exchange heat engines, highlighting that their thermodynamic behavior differences are due to the type of heat transfer mechanism rather than quantum effects.

Contribution

It demonstrates that classical and semiclassical heat engines share the same thermodynamic mechanism as the quantum three-level amplifier, challenging the notion that quantum nature causes differences.

Findings

Particle exchange mechanism is key to reversibility in various heat engines.

Differences from Carnot cycle are due to engine type, not quantum effects.

Classical engines can operate via the same thermodynamic principles as quantum engines.

Abstract

Differences between the thermodynamic behavior of the three-level amplifier (a quantum heat engine based on a thermally pumped laser) and the classical Carnot cycle are usually attributed to the essentially quantum or discrete nature of the former. Here we provide examples of a number of classical and semiclassical heat engines, such as thermionic, thermoelectric and photovoltaic devices, which all utilize the same thermodynamic mechanism for achieving reversibility as the three-level amplifier, namely isentropic (but non-isothermal) particle transfer between hot and cold reservoirs. This mechanism is distinct from the isothermal heat transfer required to achieve reversibility in cyclic engines such as the Carnot, Otto or Brayton cycles. We point out that some of the qualitative differences previously uncovered between the three-level amplifier and the Carnot cycle may be attributed to the fact that they are not the same 'type' of heat engine, rather than to the quantum nature of the three-level amplifier per se.