Methodolgy Table 3. The exergy efficiency of the

Methodolgy

Exergy
is a measure of the maximum capacity of a system to perform useful work as it
proceeds to a specified final state in equilibrium with its surroundings.
Exergy is not conserved conserved as energy in most cases but instead gets
destroyed in the system. Exergy destruction is actually the irreversibility
that causes a loss in performance. Therefore, an exergy analysis assessing the
magnitude of exergy destruction identifies the location, the magnitude and the
source of thermodynamic inefficiencies in a thermal system.

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Mass,
energy, and exergy balances for any control volume at steady state with
negligible potential and kinetic energy changes can be expressed, respectively,
by

??i=??e

Q-W=
??ehe-??ih

Xheat
– W= ??e?e-??i?i + I

where
the net exergy transfer by heat (Xheat) at temperature T is given by

Xheat
= ?(1-To/T)Q

and
the specific exergy is given by

?
= h-ho –To(s-so)

Then
the total exergy rate associated with a fluid stream becomes

X=??
= ?h-ho
–To(s-so)

For a steady state operation, and choosing each component in Fig. 1 as a control volume, the exergy destruction rate and the exergy
efficiency are defined as shown in Table 3. The exergy efficiency of the power
cycle may be defined in several ways, however, the used definition

will not only allow the irreversibility of heat transfer to the steam
in the boiler to be included, but also the exergy destruction associated with
fuel combustion and exergy lost with exhaust gases

from the furnace 17.

Note that the fuel specific exergy is calculated as:, where   , is the exergy factor
based on the lower heating value 18. In addition, the pump input power was calculated as ,
where , is
the combined

pump/motor
efficiency.

Table 3:
Given Parameters for the Power Plant

Points

T (K)

P (MPa)

? (kJ/kg)

h (kJ/kg)

s (kJ/kg K)

1

618.55

2.4231

17.80

3118.1

6.8419

2

547.85

1.3244

14.92

2986.9

6.8835

3

463.65

0.5690

16.40

2831.4

6.9511

4

394.35

0.2060

13.96

2707.7

7.1173

5

360.45

0.0628

6.39

2655.2

7.5169

6

343.15

0.0272

204.90

2626.9

7.8193

7

339.95

0.0272

204.90

279.66

0.91588

8

341.15

0.0270

226.00

278.82

0.9134

9

337.60

1.3734

226.00

285.79

0.9299

10

356.15

0.0245

21.10

269.81

0.8868

11

362.45

0.0536

226.00

347.61

1.1111

12

390.15

0.0687

13.96

374.09

1.1848

13

428.15

0.1815

226.00

491.08

1.4954

14

430.15

0.6867

275.00

653.88 1

1.8922

15

436.15

12.2630

275.00

669.49

1.8991

16

461.45

0.6671

32.70

688.52

1.9725

17

466.15

10.7910

275.00

804.43

2.2056

18

494.15

2.3544

17.80

821.28

2.2626

19

793.15

10.3010

275.00

950.46

2.5124

20

793.15

9.1233

275.00

3436.3

6.7168

Input air

298.15

0.1013

23900

424.54

3.8814

Output air

318.15

0.1013

23900

444.68

3.9468

x

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