Understanding entropy through a discussion of exergy

Abstract

Entropy is an extremely important physical quantity in thermodynamics. However, students studying thermodynamics commonly find it difficult to understand entropy. In most thermodynamics textbooks, there is only a microscopic explanation of the physical meaning of entropy, with a lack of a macroscopic interpretation. This lack is far from sufficient for people who are more concerned about the macroscopic engineering applications of entropy. In this study, we comparatively analyse entropy and exergy to explain the macroscopic physical meaning of entropy based on the concept of exergy. That is, entropy is a measure of the unavailable energy of a system during reversible heat interaction with the environment. Based on this physical interpretation of entropy, we have answered three questions that students may raise when learning about the concept of entropy. In addition to theoretical derivation, we also try to use several examples from daily life to help readers better understand the macroscopic physical meaning of entropy. Finally, through a questionnaire survey, we learned about students’ evaluations and their understanding of an engineering thermodynamics course. We have also learned whether the students prefer entropy or exergy and the reasons for their preference, as well as what aspects regarding the contents and methods the students would prefer the lecturer to improve upon when teaching entropy and exergy. The results of this work can make it easier for students to understand the physical quantity of entropy. Additionally, the results of the questionnaire analysis can be of a certain reference value for the instructions of an engineering thermodynamics course.