Page 128 - Mechanical Engineers' Handbook (Volume 4)

Mechanical Engineers’ Handbook: Energy and Power, Volume 4, Third Edition.

Edited by Myer Kutz

Copyright  2006 by John Wiley & Sons, Inc.

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 6 STORAGE SYSTEMS 129
2 EXERGY ANALYSIS 120 7 SOLAR ENERGY
CONVERSION 130
3 ENTROPY GENERATION
MINIMIZATION 124 8 POWER PLANTS 131
4 CRYOGENICS 126 9 CONSTRUCTAL THEORY 133
5 HEAT TRANSFER 127 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 (S gen,min
) to the limit of
is determined for the model, and the approach of any other design (S gen
is monitored in terms of the entropy generation num-
realistic ideality represented by S gen,min
S /S 1.
ber N S
gen gen,min
and minimize it, the analyst does not need to rely on the concept of
To calculate S gen
exergy. The EGM method represents an important step beyond thermodynamics. It is a self-
standing method that combines thermodynamics, heat transfer, and fluid mechanics into a
3
117