Cycling tames power fluctuations near optimum efficiency

TL;DR

This paper shows that cyclic, quasi-static heat engines can achieve near-Carnot efficiency at high power without large fluctuations, overcoming a fundamental thermodynamic trade-off.

Contribution

It introduces a design principle for heat engines that operate cyclically under quasi-static conditions to bypass the fluctuation-power trade-off.

Findings

Cyclic quasi-static engines can operate efficiently with low fluctuations.

The model demonstrates the fundamental difference between work in cyclic and quasi-static engines.

Experimental models confirm the theoretical predictions.

Abstract

According to the laws of thermodynamics, no heat engine can beat the efficiency of a Carnot cycle. This efficiency traditionally comes with vanishing power output and practical designs, optimized for power, generally achieve far less. Recently, various strategies to obtain Carnot's efficiency at large power were proposed. However, a thermodynamic uncertainty relation implies that steady-state heat engines can operate in this regime only at the cost of large fluctuations that render them immensely unreliable. Here, we demonstrate that this unfortunate trade-off can be overcome by designs operating cyclically under quasi-static conditions. The experimentally relevant yet exactly solvable model of an overdamped Brownian heat engine is used to illustrate the formal result. Our study highlights that work in cyclic heat engines and that in quasi-static ones are different stochastic processes.