Applied thermodynamics - Entropic and exergy balances

The second principle of thermodynamics teaches us that entropy is an extensive quantity which, on the one hand, accompanies all heat transfers and, on the other hand, appears spontaneously (is created) in systems operating with irreversibilities due to all gradients of intensive quantities. As the dynamics of all systems are precisely linked to these gradients, irreversibilities are present everywhere and are necessary to achieve energy transfers in limited times, i.e. to bring non-negligible powers into play. This creates a dilemma for the industrial system designer, i.e. the engineer. Irreversibility, i.e. the creation of entropy, which provides power, is the cause of a spontaneous transformation (degradation) of so-called "noble" energies (mechanical, electrical, etc.) into thermal energy, which generally degrades the efficiency of the system under consideration. So, in order to optimize an energy system, the engineer must be able to measure the impact of irreversibilities on system operation. There are two ways of doing this.

For the "thermodynamic system" under consideration (generally a fluid), industrial energy installations and their components are almost always open systems. Analysis of the irreversibilities developed in the system can then be based on a study of entropy balances, i.e. entropy flows through the system, in steady state or transient mode. As such balances show entropy creation or entropy production, among the various possible solutions the one which, for the same power, gives the lowest entropy production, should be preferred.

This first method is increasingly being replaced by the exergy method. The main advantage of this method is that it provides information on both the quantitative and qualitative aspects of energy transfer. It therefore covers the concepts of both the first and second principles of thermodynamics, whereas entropy analyses only take into account aspects related to the second principle.

The aim of this article is to lay the foundations for the application of both methods. However, the focus will be on exergy analyses.

It is important to point out that understanding the article is clearly conditioned by mastery of the concepts established in the preceding article. . In fact, these two articles are linked.