Exergy
transfer
by work:

Exergy
transfer
by mass:

The exergy of an isolated system during a process always
decreases or, in the limiting case of a reversible process,
remains constant. This is known as the decrease of exergy
principleand is expressed as

Exergy balance for any systemundergoing any processcan
be expressed as

General:

Net exergy transfer Exergy Change

by heat, work, and mass destruction in exergy

General,
rate form: Rate of net exergy transfer Rate of exergy Rate of change
by heat, work, and mass destruction in exergy

General,
unit-mass basis:

1 xinxout 2 xdestroyed
¢xsystem

X

#

inX

#

out^ ^ X

#

destroyed^ 
^ dXsystem>dt^

XinXout Xdestroyed
¢Xsystem

¢Xisolated
 1 X 2 X 12 isolated 0

Xmass
mc

Xwork
e

WWsurr 1 for boundary work 2

W 1 for other forms of work 2

470 | Thermodynamics

where

For a reversible process,the exergy destruction term Xdestroyed

drops out. Taking the positive direction of heat transfer to be to

the system and the positive direction of work transfer to be

from the system, the general exergy balance relations can be

expressed more explicitly as

¬a

in

m

#

ca

out

m

#

cX

#

destroyed

dXCV

dt

(^) aa 1 
T 0
Tk
bQ

kaW

P 0
dVCV
dt
b
¬a
in
mca
out
mcXdestroyed
X 2 X 1
(^) aa 1 
T 0
Tk
bQk 3 WP 01 V 2 V 124
X

mass
m

c
X

work
W

useful
X

heat
^11 T 0 >T^2 Q

REFERENCES AND SUGGESTED READINGS
1.J. E. Ahern. The Exergy Method of Energy Systems
Analysis.New York: John Wiley & Sons, 1980.
2.A. Bejan. Advanced Engineering Thermodynamics.2nd
ed. New York: Wiley Interscience, 1997.
3.A. Bejan. Entropy Generation through Heat and Fluid
Flow.New York: John Wiley & Sons, 1982.
4.Y. A. Çengel. “A Unified and Intuitive Approach to
Teaching Thermodynamics.” ASME International
Congress and Exposition, Atlanta, Georgia, November
17–22, 1996.
5.M. S. Moran and H. N. Shapiro. Fundamentals of
Engineering Thermodynamics.3rd ed. New York: John
Wiley & Sons, 1996.
6.K. Wark and D. E. Richards. Thermodynamics.6th ed.
New York: McGraw-Hill, 1999.
PROBLEMS
Exergy, Irreversibility, Reversible Work,
and Second-Law Efficiency
8–1C How does reversible work differ from useful work?
8–2C Under what conditions does the reversible work equal
irreversibility for a process?
8–3C What final state will maximize the work output of a
device?
8–4C Is the exergy of a system different in different
environments?
8–5C How does useful work differ from actual work? For
what kind of systems are these two identical?
Problems designated by a “C” are concept questions, and students
are encouraged to answer them all. Problems designated by an “E”
are in English units, and the SI users can ignore them. Problems
with a CD-EES icon are solved using EES, and complete solutions
together with parametric studies are included on the enclosed DVD.
Problems with a computer-EES icon are comprehensive in nature,
and are intended to be solved with a computer, preferably using the
EES software that accompanies this text.
⎭⎪⎬⎪⎫ ⎭⎪⎬⎪⎫ ⎭⎪⎬⎪⎫
⎭⎪⎬⎪⎫ ⎭⎪⎬⎪⎫ ⎭⎪⎬⎪⎫