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

8–5 ■ EXERGY TRANSFER BY HEAT, WORK,

AND MASS

Exergy, like energy, can be transferred to or from a system in three forms:

heat, work,and mass flow.Exergy transfer is recognized at the system

boundary as exergy crosses it, and it represents the exergy gained or lost by

a system during a process. The only two forms of exergy interactions asso-

ciated with a fixed mass or closed system are heat transferand work.

Exergy by Heat Transfer, Q

Recall from Chap. 6 that the work potential of the energy transferred from

a heat source at temperature Tis the maximum work that can be obtained

from that energy in an environment at temperature T 0 and is equivalent to

the work produced by a Carnot heat engine operating between the source

and the environment. Therefore, the Carnot efficiency hc
 1 T 0 /Trep-

resents the fraction of energy of a heat source at temperature Tthat can be

converted to work (Fig. 8–26). For example, only 70 percent of the energy

transferred from a heat source at T
1000 K can be converted to work in

an environment at T 0 
300 K.

440 | Thermodynamics

The properties of the refrigerant at the inlet and the exit states are

Inlet state:

Exit state:

The exergy change of the refrigerant during this compression process is

determined directly from Eq. 8–23 to be

Therefore, the exergy of the refrigerant increases during compression by

38.0 kJ/kg.

The exergy change of a system in a specified environment represents the

reversible work in that environment, which is the minimum work input

required for work-consuming devices such as compressors. Therefore, the

increase in exergy of the refrigerant is equal to the minimum work that

needs to be supplied to the compressor:

Discussion Note that if the compressed refrigerant at 0.8 MPa and 50°C

were to be expanded to 0.14 MPa and 10°C in a turbine in the same envi-

ronment in a reversible manner, 38.0 kJ/kg of work would be produced.

win,min
c 2 c 1 
38.0 kJ/kg

38.0 kJ/kg

1 286.69246.36 2 kJ>kg 1 293 K 231 0.98020.9724 2 kJ>kg#K 4

 1 h 2 h 12 T 01 s 2 s 12

¢c
c 2 c 1 
 1 h 2 h 12 T 01 s 2 s 12 

V^22 V^21

2

Q^0

g 1 z 2 z 12 Q

0

P 2 
0.8 MPa

T 2 
50°C

f¬

h 2 
286.69 kJ>kg

s 2 
0.9802 kJ>kg#K

P 1 
0.14 MPa

T 1 
10°C

f¬

h 1 
246.36 kJ>kg

s 1 
0.9724 kJ>kg#K

SEE TUTORIAL CH. 8, SEC. 5 ON THE DVD.

INTERACTIVE

TUTORIAL