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

450 | Thermodynamics

That is, if the process between states 1 and 2 were executed in a reversible

manner, the system would deliver 9.6 kJ of useful work.

(c) The total exergy destroyed during this process can be determined from

the exergy balance applied on the extended system (system + immediate

surroundings) whose boundary is at the environment temperature of T 0 (so

that there is no exergy transfer accompanying heat transfer to or from the

environment),

Net exergy transfer Exergy Change

by heat, work, and mass destruction in exergy

where Wu,outis the useful boundary work delivered as the system expands. By

writing an energy balance on the system, the total boundary work done dur-

ing the process is determined to be

Net energy transfer Change in internal, kinetic,

by heat, work, and mass potential, etc., energies

This is the total boundary work done by the system, including the work done

against the atmosphere to push the atmospheric air out of the way during

the expansion process. The useful work is the difference between the two:

Substituting, the exergy destroyed is determined to be

That is, 4.3 kJ of work potential is wasted during this process. In other

words, an additional 4.3 kJ of energy could have beenconverted to work

during this process, but was not.

The exergy destroyed could also be determined from

which is the same result obtained before.

4.3 kJ

1 298 K2e10.05 kg 231 7.28107.1246 2 kJ>kg#K 4 

2 kJ

298 K

f

Xdestroyed
T 0 Sgen
T 0 cm 1 s 2 s 12 

Qsurr

T 0

d

Xdestroyed
X 1 X 2 Wu,out
35.025.45.3
4.3 kJ

5.3 kJ

8.8 kJ 1 100 kPa 21 0.05 kg 231 0.95990.25799 2 m^3 >kg4a

1 kJ

1 kPa#m^3

b

Wu
WWsurr
Wb,outP 01 V 2 V 12 
Wb,outP 0 m 1 v 2 v 12

8.8 kJ

 1 2 kJ 2  1 0.05 kg 21 2577.12793.7 2 kJ>kg

Wb,out
Qout¢U
Qoutm 1 u 2 u 12

QoutWb,out
¢U

EinEout¬
¬ ¢Esystem

Xdestroyed
X 1 X 2 Wu,out

Xwork,outXheat,out Xdestroyed
X 2 X 1

XinXout¬ Xdestroyed
¢Xsystem

0

¡

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