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CHAPTER 4
EXERGY ANALYSIS, ENTROPY
GENERATION MINIMIZATION,
AND CONSTRUCTAL THEORY
Adrian Bejan
Department of Mechanical Engineering and Materials Science
Duke University
Durham, North Carolina
1 INTRODUCTION
118
2 EXERGY ANALYSIS
120
3 ENTROPY GENERATION
MINIMIZATION
124
4 CRYOGENICS
126
5 HEAT TRANSFER
127
6 STORAGE SYSTEMS
129
7 SOLAR ENERGY
CONVERSION
130
8 POWER PLANTS
131
9 CONSTRUCTAL THEORY
133
REFERENCES
143
1
INTRODUCTION
In this chapter, we review three methods that account for much of the newer work in engi-
neering thermodynamics and thermal design and optimization. The method of exergy analysis
rests on thermodynamics alone. The first law, the second law, and the environment are used
simultaneously to determine (1) the theoretical operating conditions of the system in the
reversible limit and (2) the entropy generated (or exergy destroyed) by the actual system,
that is, the departure from the reversible limit. The focus is on analysis. Applied to the
system components individually, exergy analysis shows us quantitatively how much each
component contributes to the overall irreversibility of the system.1,2
Entropy generation minimization (EGM) is a method of modeling and optimization. The
entropy generated by the system is first developed as a function of the physical characteristics
of the system (dimensions, materials, shapes, constraints). An important preliminary step is
the construction of a system model that incorporates not only the traditional building blocks
of engineering thermodynamics (systems, laws, cycles, processes, interactions), but also the
fundamental principles of fluid mechanics, heat transfer, mass transfer and other transport
phenomena. This combination makes the model ‘‘realistic’’ by accounting for the inherent
irreversibility of the actual device. Finally, the minimum entropy generation design (Sgen,min)
is determined for the model, and the approach of any other design (Sgen) to the limit of
realistic ideality represented by Sgen,min is monitored in terms of the entropy g