Download Thermodynamics Chapter 7

7
CHAPTER
Çengel
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Thermodynamics
Exergy:
A Measure of
Work Potential
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7-1
Irreversibility is the Difference Between
Reversible Work and Actual Useful Work
(fig. 7-9)
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Irreversible Heat Transfer Can be Made
Reversible by a Reversible Heat Engine
(Fig. 7-12)
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Thermodynamics
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Second Law of Efficiency
The second law of efficiency is a measure of the performance of a device
relative to its performance under reversible conditions
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Thermodynamics
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The Second-Law Efficiency of
All Reversible Devices is 100%
(Fig. 7-16)
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Thermodynamics
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The Work Potential or Exergy of Potential
Energy Equals the Potential Energy Itself
(Fig. 7-18)
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Thermodynamics
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The Exergy of a Specified Mass
The exergy of a specified mass at a specified state is the useful work that can be
produced as it undergoes a reversible process to the state of the environment
(Fig. 7-19)
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Thermodynamics
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The Exergy of a Cold Medium
The exergy of a cold medium is also a positive quantity since work can be
produced by transferring heat to it
(Fig. 7-20)
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Thermodynamics
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The Exergy of Flow of Work
The exergy of flow of work is the useful work that would be deliverd by an
imaginary piston in the flow section
(Fig. 7-21)
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Thermodynamics
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The Exergy of Enthalpy
The exergy of enthalpy is the sum of the exergies of the internal energy
and flow energy
(Fig. 7-22)
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Thermodynamics
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The Energy and Exergy contents of (a)
a Fixed Mass and (b) a Fluid System
(Fig. 7-23)
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Thermodynamics
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The Transfer and Destruction of
Exergy During Heat Transfer
The transfer and destruction of exergy during a heat transfer process
through a finite temperature difference
(Fig. 7-27)
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Thermodynamics
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Mechanisms of Exergy Transfer for
a General System
(Fig. 7-32)
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Thermodynamics
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Exergy Transferrence
Exergy is transferred into or out of a control volume by mass
as well as by heat and work transfer
(Fig. 7-42)
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Thermodynamics
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Chapter Summary
Thermodynamics
• The energy content of the universe is constant,
just as its mass content is. Yet at times of crisis
we are bombarded with speeches and articles on
how to "conserve" energy. As engineers, we know
that energy is already conserved. What is not
conserved is exergy, which is the useful work
potential of the energy. Once the exergy is wasted,
it can never be recovered. When we use energy (to
heat our homes for example), we are not
destroying any energy; we are merely converting
it to a less useful form, a form of less exergy.
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Çengel
Boles
Chapter Summary
Thermodynamics
• The useful work potential of a system at the
specified state is called exergy. Exergy is a
property and is associated with the state of the
system and the environment. A system that is in
equilibrium with its surroundings has zero exergy
and is said to be at the dead state. The exergy of
the thermal energy of thermal reservoirs is
equivalent to the work output of a Carnot heat
engine operating between the reservoir and the
environment.
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Chapter Summary
Thermodynamics
• Reversible work Wrev is defined as the maximum
amount of useful work that can be produced (or
the minimum work that needs to be supplied) as a
system undergoes a process between the
specified initial and final states. This is the useful
work output (or input) obtained when the process
between the initial and final states is executed in a
totally reversible manner.
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Chapter Summary
Thermodynamics
• The difference between the reversible work Wrev
and the useful work Wu is due to the
irreversibilities present during the process and is
called the irreversibility I. It is equivalent to the
exergy destroyed and is expressed as
I = Xdestroyed = ToSgen = Wrev,out - Wu,out = Wu,in - Wrev,in
where Sgen is the entropy generated during the
process. For a totally reversible process, the
useful and reversible work terms are identical and
thus irreversibility is zero.
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Chapter Summary
Çengel
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Thermodynamics
• Exergy destroyed represents the lost work
potential and is also called the wasted work or lost
work.
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7-19
Çengel
Boles
Thermodynamics
Third Edition
Chapter Summary
• The second-law efficiency is a measure of the
performance of a device relative to the
performance under reversible conditions for the
same end states and is given by
for heat engines and other work-producing
devices and
for refrigerators, heat pumps, and other workconsuming devices.
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7-20
Çengel
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Chapter Summary
• In general, the second-law efficiency is expressed
as
Thermodynamics
=
Exergy recovered
Exergy supplied
=1-
Exergy destroyed
Exergy supplied
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Chapter Summary
• The exergy of various forms of energy are
Çengel
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Thermodynamics
Third Edition
Exergy of kinetic energy:
V2
xke = ke =
2
Exergy of potential energy:
xpe = pe = gz
Exergy of internal energy:
xu = (u - uo) + Po(v - vo) - To(s - so)
Exergy of flow energy:
xpv = Pv - Pov = (P - Po)v
Exergy of enthalpy:
xh = (h - ho) - To(s - so)
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Chapter Summary
• The exergies of a fixed mass (nonflow exergy) and
of a flow stream are expressed as
Çengel
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Thermodynamics
Nonflow exergy:
Flow exergy:
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Çengel
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Chapter Summary
• The exergy change of a fixed mass or fluid stream
as it undergoes a process from state 1 to state 2 is
given by
Thermodynamics
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7-24
Chapter Summary
• Exergy can be transferred by heat, work, and
mass flow, and exergy transfer accompanied by
heat, work, and mass transfer are given by
Çengel
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Thermodynamics
Exergy transfer by heat:
Exergy transfer by work:
W - Wsurr (for boundary work)
Xwork =
W
(for other forms of work
Exergy transfer by mass:
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Çengel
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Chapter Summary
Thermodynamics
• The exergy of an isolated system during a process
always decreases or, in the limiting case of a
reversible process, remains constant. This is
known as the decrease of exergy principle and is
expressed as
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Chapter Summary
• Exergy balance for any system undergoing any
process can be expressed as
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General:
Thermodynamics
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7-27
Chapter Summary
• Exergy balance for any system undergoing any
process can be expressed as
Çengel
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General, rate form:
Thermodynamics
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7-28
Chapter Summary
• Exergy balance for any system undergoing any
process can be expressed as
Çengel
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Thermodynamics
Third Edition
General, unit-mass basis:
(xin - xout) - xdestroyed =
where
xsystem
To .
Xheat = 1 Q
T
.
.
Xwork = Wuseful
.
.
.
.
Xmass = m
Xsystem - dXsystem / dt
For a reversible process, the exergy destruction term
Xdestroyed drops out.
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Çengel
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Chapter Summary
Thermodynamics
• Taking the positive direction of heat transfer to be
to the system and the positive direction of work
transfer to be from the system, the general exergy
balance relations can be expressed more explicitly
as
where the subscripts are i = inlet, e = exit, 1 =
initial state, and 2 = final state of the system.
Third Edition
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