Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

This equation can be expressed as the exergy of an isolated system during a

process always decreases or, in the limiting case of a reversible process,

remains constant.In other words, it neverincreases and exergy is destroyed

during an actual process. This is known as the decrease of exergy princi-

ple.For an isolated system, the decrease in exergy equals exergy destroyed.

Exergy Destruction

Irreversibilities such as friction, mixing, chemical reactions, heat transfer

through a finite temperature difference, unrestrained expansion, nonquasi-

equilibrium compression or expansion always generate entropy,and any-

thing that generates entropy always destroys exergy.The exergy destroyed

is proportional to the entropy generated, as can be seen from Eq. 8–31, and

is expressed as

(8–33)

Note that exergy destroyed is a positive quantityfor any actual process and

becomes zerofor a reversible process. Exergy destroyed represents the lost

work potential and is also called the irreversibilityor lost work.

Equations 8–32 and 8–33 for the decrease of exergy and the exergy destruc-

tion are applicable to any kind of systemundergoing any kind of processsince

any system and its surroundings can be enclosed by a sufficiently large arbi-

trary boundary across which there is no heat, work, and mass transfer, and

thus any system and its surroundings constitute an isolated system.

No actual process is truly reversible, and thus some exergy is destroyed

during a process. Therefore, the exergy of the universe, which can be con-

sidered to be an isolated system, is continuously decreasing. The more irre-

versible a process is, the larger the exergy destruction during that process.

No exergy is destroyed during a reversible process (Xdestroyed,rev
0).

The decrease of exergy principle does not imply that the exergy of a sys-

tem cannot increase. The exergy change of a system canbe positive or neg-

ative during a process (Fig. 8–31), but exergy destroyed cannot be negative.

The decrease of exergy principle can be summarized as follows:

(8–34)

This relation serves as an alternative criterion to determine whether a

process is reversible, irreversible, or impossible.

8–7 ■ EXERGY BALANCE: CLOSED SYSTEMS

The nature of exergy is opposite to that of entropy in that exergy can be

destroyed,but it cannot be created. Therefore, the exergy changeof a sys-

tem during a process is less than the exergy transferby an amount equal to

the exergy destroyedduring the process within the system boundaries. Then

the decrease of exergy principlecan be expressed as (Fig. 8–32)

°

Total

exergy

entering

¢°

Total

exergy

leaving

¢°

Total

exergy

destroyed

¢
°

Change in the

total exergy

of the system

¢

Xdestroyed •

70 ¬Irreversible process


 0 ¬Reversible process

60 ¬Impossible process

Xdestroyed
T 0 Sgen 0

444 | Thermodynamics

SurroundingsSurroundings

SYSTEMSYSTEM

∆Xsyssys = = –2 kJ2 kJ

Xdestdest = 1 kJ = 1 kJ

Q

FIGURE 8–31

The exergy change of a system can be
negative, but the exergy destruction
cannot.

SEE TUTORIAL CH. 8, SEC. 7 ON THE DVD.

INTERACTIVE

TUTORIAL