3.8 The Exergy Balance

The Combined Law


The exergy concept is a fairly modern formalization of work done in the late 1800s by J. Willard Gibbs, Hermann Helmholtz, and Louis Gouy. These works were not formalized into the exergy concept until 1953 by Zoran Rant. Because of the fairly modern timeline, the exergy concept is usually not taught in the mainstream first course in thermodynamics. Students may be exposed to exergy analysis in follow-on courses, but these are generally technical electives. The bottom line is that not everyone who is using this textbook may be familiar with exergy. This section of the text lays out the history of exergy, the development of the exergy balance, and the application of the exergy balance.

The exergy concept is very helpful in identifying irreversibilities within a system or device. It is much more intuitive than the second law concept of entropy generation. Exergy units are the same as energy units, making it fairly easy to put in perspective compared to entropy generation. This sounds like a second-law-bashing. However, the second law is still integral to the overall exergy concept since the exergy destruction that occurs in a system is a function of the entropy generation.

The easiest way to think about exergy is to view it as the maximum work potential compared to a reference state (called the dead state). Fluid states removed from the dead state have the ability to deliver work through some hypothetical reversible device. Work-delivering devices themselves, can deliver the maximum amount of work when they are reversible.

As you study Section 3.8.2, pay close attention to Example 3.11. This example builds on previous examples that used the conservation of energy and entropy balance to analyze a steam turbine. Example 3.11 is particularly enlightening as it shows the major sources of irreversibility within the turbine via an exergy analysis.

The more engineers that are trained in the ways of exergy analysis, the better off we will be as a population. The practice of exergy conservation will ensure that the available energy within a resource is used wisely and sparingly, fully understanding that some day, the resource may vanish.