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

Note that heat transfer through a finite temperature difference is irreversible,

and some entropy is generated as a result. The entropy generation is always

accompanied by exergy destruction, as illustrated in Fig. 8–27. Also note

that heat transfer Qat a location at temperature Tis always accompanied by

entropy transferin the amount of Q/Tand exergy transferin the amount of

(1 T 0 /T)Q.

Exergy Transfer by Work, W

Exergy is the useful work potential, and the exergy transfer by work can

simply be expressed as

Exergy transfer by work: (8–26)

where Wsurr
P 0 (V 2 V 1 ),P 0 is atmospheric pressure, and V 1 and V 2 are the

initial and final volumes of the system. Therefore, the exergy transfer with

work such as shaft work and electrical work is equal to the work Witself. In

the case of a system that involves boundary work, such as a piston–cylinder

device, the work done to push the atmospheric air out of the way during

expansion cannot be transferred, and thus it must be subtracted. Also, during

a compression process, part of the work is done by the atmospheric air, and

thus we need to supply less useful work from an external source.

To clarify this point further, consider a vertical cylinder fitted with a

weightless and frictionless piston (Fig. 8–28). The cylinder is filled with a

gas that is maintained at the atmospheric pressure P 0 at all times. Heat is

now transferred to the system and the gas in the cylinder expands. As a

result, the piston rises and boundary work is done. However, this work can-

not be used for any useful purpose since it is just enough to push the atmo-

spheric air aside. (If we connect the piston to an external load to extract

some useful work, the pressure in the cylinder will have to rise above P 0 to

beat the resistance offered by the load.) When the gas is cooled, the piston

moves down, compressing the gas. Again, no work is needed from an exter-

nal source to accomplish this compression process. Thus we conclude that

the work done by or against the atmosphere is not available for any useful

purpose, and should be excluded from available work.

Exergy Transfer by Mass, m

Mass contains exergyas well as energy and entropy, and the exergy, energy,

and entropy contents of a system are proportional to mass. Also, the rates of

exergy, entropy, and energy transport into or out of a system are proportional

to the mass flow rate. Mass flow is a mechanism to transport exergy, entropy,

and energy into or out of a system. When mass in the amount of menters

or leaves a system, exergy in the amount of mc, where c
(hh 0 )

T 0 (ss 0 ) V^2 /2 gz, accompanies it. That is,

Exergy transfer by mass: (8–27)

Therefore, the exergy of a system increases by mcwhen mass in the

amount of menters, and decreases by the same amount when the same

amount of mass at the same state leaves the system (Fig. 8–29).

Xmass
mc

Xwork
e

WWsurr 1 for boundary work 2

W¬ 1 for other forms of work 2

442 | Thermodynamics

MEDIUM 1 MEDIUM 2

Wall

Heat QQ
transfer

T 1

T 2

Entropy
transfer

Entropy

generated

Q

T 1

Q

T 2

Exergy
transfer

Exergy

destroyed

1 – T^0 (Q

( T 1

1 – T^0 (Q

( T 2

FIGURE 8–27

The transfer and destruction of exergy

during a heat transfer process through

a finite temperature difference.

Weightless

piston

P 0

Heat

P 0

FIGURE 8–28

There is no useful work transfer

associated with boundary work when

the pressure of the system is

maintained constant at atmospheric

pressure.