Nonuniversality of heat engine efficiency at maximum power

TL;DR

This paper investigates the efficiency of a quantum dot heat engine at maximum power, revealing that the commonly assumed universal efficiency relation can break down depending on control constraints, especially beyond linear thermodynamics.

Contribution

It demonstrates that the universal efficiency relation at maximum power does not always hold for quantum dot engines under certain control constraints.

Findings

Efficiency deviations depend on control parameter constraints.

Universality breaks down beyond linear irreversible thermodynamics.

Quantum dot engines can deviate from Curzon-Ahlborn efficiency.

Abstract

We study the efficiency of a simple quantum dot heat engine at maximum power. In contrast to the quasi-statically operated Carnot engine whose efficiency reaches the theoretical maximum, recent research on more realistic engines operated in a finite time has revealed other classes of efficiencies such as the Curzon-Ahlborn efficiency maximizing the power. Such a power-maximizing efficiency has been argued to be always the half of the maximum efficiency up to the linear order near equilibrium under the tight-coupling condition between thermodynamic fluxes. We show, however, that this universality may break down for the quantum dot heat engine, depending on the constraint imposed on the engine control parameters (local optimization), even though the tight-coupling condition remains satisfied.It is shown that this deviation is critically related to the applicability of the linear irreversible thermodynamics.