Extraction of ergotropy: free energy bound and application to open cycle engines

TL;DR

This paper investigates the maximum energy extractable from a system interacting with a thermal bath, establishing fundamental bounds on ergotropy, and applies these insights to design and optimize open-cycle microscopic heat engines.

Contribution

It introduces a fundamental bound on ergotropy extraction from environments based on non-equilibrium free energy differences and applies this to optimize open-cycle microscopic engines.

Findings

Bound on ergotropy extraction expressed in terms of free energy difference.

Numerical analysis of ergotropy saturation for finite-dimensional systems.

Design and optimization of open-cycle heat engines for dimensions 2 and 3.

Abstract

The second law of thermodynamics uses change in free energy of macroscopic systems to set a bound on performed work. Ergotropy plays a similar role in microscopic scenarios, and is defined as the maximum amount of energy that can be extracted from a system by a unitary operation. In this analysis, we quantify how much ergotropy can be induced on a system as a result of system's interaction with a thermal bath, with a perspective of using it as a source of work performed by microscopic machines. We provide the fundamental bound on the amount of ergotropy which can be extracted from environment in this way. The bound is expressed in terms of the non-equilibrium free energy difference and can be saturated in the limit of infinite dimension of the system's Hamiltonian. The ergotropy extraction process leading to this saturation is numerically analyzed for finite dimensional systems. Furthermore, we apply the idea of extraction of ergotropy from environment in a design of a new class of stroke heat engines, which we label open-cycle engines. Efficiency and work production of these machines can be completely optimized for systems of dimensions 2 and 3, and numerical analysis is provided for higher dimensions.