07.si thermo 8e chap 8 lecture m

7/23/2019 07.SI Thermo 8e Chap 8 Lecture m

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EXERGY

Copyright 2015 The McGraw-Hill Education. Peri!!ion re"uired #or reproduction or di!play.


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Objectives

E$aine the per#orance o# engineering de%ice! in light

o# the !econd law o# therodynaic!. &e#ine exergy' which i! the a$iu u!e#ul wor( that

could )e o)tained #ro the !y!te at a gi%en !tate in a

!peci#ied en%ironent.

&e#ine reversible work' which i! the a$iu u!e#ul

wor( that can )e o)tained a! a !y!te undergoe! a

proce!! )etween two !peci#ied !tate!.

&e#ine the e$ergy de!truction' which i! the wa!ted wor(

potential during a proce!! a! a re!ult o# irre%er!i)ilitie!.

&e#ine the second-law efficiency. &e%elop the e$ergy )alance relation.

*pply e$ergy )alance to clo!ed !y!te! and control

%olue!.


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EXERGY: WORK POTENTIAL OF ENERGY

The u!e#ul wor( potential o# a

gi%en aount o# energy at !oe!peci#ied !tate i! called exergy,

which i! al!o called the availability

or available energy.

* !y!te i! !aid to )e in the dead!tatewhen it i! in therodynaic

e"uili)riu with the en%ironent it

i! in.


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* system delivers the maximum possible work as it undergoes a reversible process

from the specified initial state to the state of its environment, that is, the dead state.

Thi! repre!ent! the useful work potential o# the !y!te at the !peci#ied !tate and i!

called exergy.

E$ergy repre!ent! the upper liit on the aount o# wor( a de%ice can deli%er without

%iolating any therodynaic law!.


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Exergy (Wor Pote!ti"#$ Associ"te%

&it' Ki!etic "!% Pote!ti"# E!ergy

Exergy of kinetic energy:

Exergy of potential energy:

The e$ergie! o# (inetic and

potential energie! are e"ual to

the!el%e!' and they are entirely

a%aila)le #or wor(.


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REER)I*LE WORK AN+ IRREER)I*ILITY

*! a clo!ed

!y!te e$pand!'

!oe wor( need!

to )e done to pu!h

the ato!phericair out o# the way

W!urr.

or con!tant-%olue

!y!te!' the total

actual and u!e#ul

wor(! are identicalWu=W.

Reversib#e &or Wrev:The maximum amount of

useful work that can be produced or the

minimum work that needs to be supplied! as asystem undergoes a process between the

specified initial and final states"

The di##erence )etween

re%er!i)le wor( and

actual u!e#ul wor( i! theirre%er!i)ility.


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T'e R"te o, Irreversibi#ity o, "

-e"t E!gi!e


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Irreversibi#ity %.ri!g t'e

/oo#i!g o, "! Iro! *#oc


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)E/ON+0LAW EFFI/IEN/Y


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General de#inition o# e$ergy e##iciency4


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)eco!%0L"& E,,icie!cy o, Resist"!ce -e"ters


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EXERGY /-ANGE OF A )Y)TE1

Exergy o, " Fixe% 1"ss: No!,#o&

(or /#ose% )yste2$ Exergy

E$ergy o# a clo!ed !y!te


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Clo!ed !y!te

e$ergy per unit

a!!

E$ergy

change o#

a clo!ed

!y!te

hen the propertie! o# a !y!te are

not uni#or' the e$ergy o# the !y!te i!


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Exergy o, " F#o& )tre"2: F#o& (or )tre"2$ Exergy

E$ergy o# #low energy

low

e$ergy

E$ergy change o# #low


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Wor Pote!ti"# o,

/o23resse% Air i! " T"!


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Exergy /'"!ge +.ri!g "

/o23ressio! Process


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EXERGY TRAN)FER *Y

-EAT4 WORK4 AN+ 1A))

Exergy by -e"t Tr"!s,er4 QE$ergy tran!#er

)y heat

hen teperature i!

not con!tant


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Exergy Tr"!s,er by Wor4 W

Exergy Tr"!s,er by 1"ss4 m


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T-E +E/REA)E OF EXERGY PRIN/IPLE

AN+ EXERGY +E)TR5/TION

The exergy of an isolated system during a process always decreases or, in

the limiting case of a reversible process, remains constant" 6n other word!' it

never increa!e! and exergy is destroyed during an actual proce!!. Thi! i!

(nown a! the%ecre"se o, exergy 3ri!ci3#e.


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Exergy +estr.ctio!

E$ergy de!troyed i! apositive #uantity #or

any actual proce!! and )ecoe! $ero #or a

re%er!i)le proce!!.

E$ergy de!troyed repre!ent! the lo!t wor(

potential and i! al!o called the irreversibility

or lost work"

Can the e$ergy change

o# a !y!te during a

proce!! )e negati%e7

Con!ider heat tran!#er #ro a !y!te to

it! !urrounding!. How do you copare

e$ergy change! o# the !y!te and the

!urrounding!7


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EXERGY *ALAN/E: /LO)E+ )Y)TE1)

The e$ergy change o#

a !y!te during a

proce!! i! e"ual to thedi##erence )etween the

net e$ergy tran!#er

through the !y!te

)oundary and the

e$ergy de!troyed

within the !y!te)oundarie! a! a re!ult

o# irre%er!i)ilitie!.


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The heat tran!#er to

a !y!te and wor(

done )y the !y!te

are ta(en to )epo!iti%e "uantitie!.

%ki! the heat tran!#er through the )oundary at teperature Tkat location k"

E$ergy

de!troyed

out!ide !y!te

)oundarie! can

)e accounted #or

)y writing an

e$ergy )alance

on the e$tended

!y!te that

include! the

!y!te and it!

iediate

!urrounding!.

E$ergy

)alance #or

a clo!ed

!y!te

when heat

tran!#er i! tothe !y!te

and the

wor( i! #ro

the !y!te.


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Ge!er"# Exergy *"#"!ce ,or /#ose% )yste2s


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Exergy +estr.ctio! %.ri!g -e"t /o!%.ctio!


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The e$ergy )alance applied on the extended

system !y!te 8 iediate !urrounding!

who!e )oundary i! at the en%ironent

teperature o# T0gi%e!

Exergy +estr.ctio! +.ri!g Ex3"!sio! o, )te"2

*lternati%e ethod o# e$ergy

de!truction calculation4


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+ro33i!g " -ot Iro! *#oc i!to W"ter


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Wor Pote!ti"# o, -e"t Tr"!s,er *et&ee! T&o T"!s


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EXERGY *ALAN/E: /ONTROL OL51E)

The rate o# e$ergy change within the

control %olue during a proce!! i!

e"ual to the rate o# net e$ergy tran!#er

through the control %olue )oundary

)y heat' wor(' and a!! #low inu! the

rate o# e$ergy de!truction within the)oundarie! o# the control %olue.

E * # , )t % F# ) t


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Exergy *"#"!ce ,or )te"%y0F#o& )yste2sMo!t control %olue! encountered in practice !uch a! tur)ine!' copre!!or!' no99le!'

di##u!er!' heat e$changer!' pipe!' and duct! operate !teadily' and thu! they e$perience

no change! in their a!!' energy' entropy' and e$ergy content! a! well a! their %olue!.

There#ore' d&C:;dt =0 and d'C:;dt =0 #or !uch !y!te!.


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Reversib#e Wor

The e$ergy de!troyed i! 9ero only #or a re%er!i)le proce!!' and

re%er!i)le wor( repre!ent! the a$iu wor( output #or wor(-

producing de%ice! !uch a! tur)ine! and the iniu wor( input #orwor(-con!uing de%ice! !uch a! copre!!or!.

The e$ergy )alance relation! pre!ented a)o%e can )e u!ed to

deterine the re%er!i)le wor( Wre%)y !etting the e$ergy de!troyed

e"ual to 9ero. The wor( W in that ca!e )ecoe! the re%er!i)le wor(.

)eco!% L"& E,,icie!cy o, )te"%y F#o& +evices


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)eco!%0L"& E,,icie!cy o, )te"%y0F#o& +evicesThe second-law efficiency o# %ariou! !teady-#low de%ice! can )e deterined #ro it!

general de#inition' 66< E$ergy reco%ered;E$ergy e$pended. hen the change! in

(inetic and potential energie! are negligi)le and the de%ice! are adia)atic4

Heate$changer

Tur)ine

Copre!!or

Mi$ing

cha)er

)eco!% #"& "!"#ysis o, " ste"2 t.rbi!e


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)eco!%0#"& "!"#ysis o, " ste"2 t.rbi!e


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Exergy +estroye% +.ri!g 1ixi!g

o, F#.i% )tre"2s130 (=;in

< 20C

200 (Pa 0C

10C

150C

/' i / % Ai )t ) t


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/'"rgi!g " /o23resse% Air )tor"ge )yste2


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).22"ry

E$ergy4 or( potential o# energy E$ergy wor( potential a!!ociated with (inetic and potential energy

>e%er!i)le wor( and irre%er!i)ility

?econd-law e##iciency

E$ergy change o# a !y!te E$ergy o# a #i$ed a!!4 @on#low or clo!ed !y!te e$ergy

E$ergy o# a #low !trea4 low or !trea e$ergy

E$ergy tran!#er )y heat' wor(' and a!! The decrea!e o# e$ergy principle and e$ergy de!truction

E$ergy )alance4 Clo!ed !y!te!

E$ergy )alance4 Control %olue! E$ergy )alance #or !teady-#low !y!te!

>e%er!i)le wor( ?econd-law e##iciency o# !teady-#low de%ice!


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07.si thermo 8e chap 8 lecture m

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