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Lecture Notes on Thermodynamics 2008Chapter 8 Exergy

Prof. Man Y. Kim, Autumn 2008, ⓒmanykim@chonbuk.ac.kr, Aerospace Engineering, Chonbuk National University, Korea

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Exergy : Work Potential• Work=f ( initial state, path, final state)

• Exergy of Kinetic and Potential Energies

To maximize the work, reversible process and final dead states are necessary

dead state dead state dead state

Kinetic Energy :

2 2kex ke V Potential Energy : pex pe gz

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Reversible Work and Irreversibility• Total Energy = Exergy + Unavailable Energy

• Surrounding Work : surr 0 2 1W P V V• Useful Work : surr 0 2 1uW W W W P V V• Reversible Work : revW

If the final state is dead state, reversible work becomes exergy

• Irreversibility : rev,out ,outuI W W ,in rev,inuI W Wor for energy production devices : rev uW W for energy consuming devices : rev uW W

• Second Law Efficiency,

th

IIth,rev

for heat engine

IIrev

COPCOP

for refrigerator and heat pump

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Exergy Change• Nonflowing (Closed System) Exergy

in out systemE E d Q WE dUHere,

, 00 0 b usefulPd P P d P dW W P dV V + V V

0

0 001HE HE

T TW Q Q Q Q T dS

T TQ W T dS

, , 0 0total useful HE b usefulW W W dU P d T dSV

, 0 0 0 0 0total usefulW U U P T S SV V

• Flow Exergy 0 0flowx Pv P v P P v

2

0 0 0 2V

h h T s s gz

2

0 0 0 0 0 02flowing fluid nonflowing fluid flow

Vx x x u u P v v T s s gz P P v

2

0 0 0 0 0 2mV

X U U P T S S mgzV V

2

0 0 0 0 0 2V

u u P v v T s s gz

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Exergy Transfer by Heat, Work, and Mass• Exergy Transfer by Heat

01heat

TX Q

T

• Exergy Transfer by Work

surr

work

W WX

Wfor boundary workfor other forms of work

• Exergy Transfer by Mass

massX m

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Decrease of Exergy Principle• Decrease of Exergy Principle 2 1 0isolated isolated

X X X

• Exergy Destruction 0 0destroyed genX T S• Exergy Balance for a Closed System

• Exergy Balance for a Control Volume , , 2 1heat work mass in mass out destroyed CV

X X X X X X X

• Reversible Work 1 2revW m

in out destroyed system heat work destroyed systemX X X X X X X X

0

0 2 1 0 2 11 k genk

TQ W P T S X X

TV V

0

0 01 system systemk gen

k

d dXTQ W P T S

T dt dt

V

00 2 1 0 2 11 k gen

k in out

TQ W P m m T S X X

TV V

00 01 CV CV

k genk in out

T d dXQ W P m m T S

T dt dtV

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Homework #8Solve the Problems 8–1C, 2C, 3C, 4C, 5C, 6C, 7C, 8C, 9C, 10C, 11C, 13C, 14C