Pitfalls of exergy analysis

TL;DR

This paper critically examines the limitations of exergy analysis and entropy production minimization in thermodynamic optimization, especially in non-isothermal steady-state systems like fuel cells, highlighting cases where these methods may be misleading.

Contribution

It demonstrates, through non-equilibrium thermodynamics, specific scenarios where traditional exergy analysis and entropy minimization are not applicable or can be misleading.

Findings

Exergy analysis assumptions are invalid in certain steady-state fuel cell scenarios.

Entropy production minimization may not always lead to optimal thermodynamic performance.

Caution is needed when applying classical thermodynamic optimization methods to complex systems.

Abstract

The well-known Gouy-Stodola theorem states that a device produces maximum useful power when working reversibly, that is with no entropy production inside the device. This statement then leads to a method of thermodynamic optimization based on entropy production minimization. Exergy destruction (difference between exergy of fuel and exhausts) is also given by entropy production inside the device. Therefore, assessing efficiency of a device by exergy analysis is also based on the Gouy-Stodola theorem. However, assumptions that had led to the Gouy-Stodola theorem are not satisfied in several optimization scenarios, e.g. non-isothermal steady-state fuel cells, where both entropy production minimization and exergy analysis should be used with caution. We demonstrate, using non-equilibrium thermodynamics, a few cases where entropy production minimization and exergy analysis should not be applied.