This document contains 20 figures from Chapter 7 describing concepts related to exergy, a measure of work potential. Exergy is defined as the useful work possible during a reversible process bringing a system to equilibrium with its environment. The figures address topics like the dead state, work potential of temperature gradients, reversible versus irreversible work, exergy transfers through heat, work and mass flows, and exergy balances for systems. Irreversibility and exergy destruction are also discussed.

1. CHAPTER
7
Exergy: A Measure
of Work Potential

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FIGURE 7-1
A system that is in equilibrium
with its environment is said to
be at the dead state.
7-1

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FIGURE 7-3
The immediate
surroundings of a
hot potato are
simply the
temperature
gradient zone of the
air next to the
potato.
7-2

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FIGURE 7-8
As a closed system expands, some
work needs to be done to push the
atmospheric air out of the way
(Wsurr).
7-3

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FIGURE 7-10
The difference
between reversible
work and actual useful
work is the
irreversibility.
7-4

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FIGURE 7-15
Two heat engines
that have the
same thermal
efficiency, but
different
maximum
thermal
efficiencies.
7-5

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FIGURE 7-16
Second-law efficiency
is a measure of the
performance of a
device relative to its
performance under
reversible conditions.
7-6

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FIGURE 7-20
The exergy of a
specified mass at a
specified state is the
useful work that
can be produced as
the mass undergoes
a reversible process
to the state of the
environment.
7-7

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FIGURE 7-21
The exergy of a cold
medium is also a
positive quantity
since work can be
produced by
transferring heat to
it.
7-8

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FIGURE 7-22
The exergy of flow energy is
the useful work that would
be delivered by an
imaginary piston in the flow
section.
7-9

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FIGURE 7-23
The energy and exergy
contents of (a) a fixed
mass and (b) a fluid
stream.
7-10

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FIGURE 7-26
The Carnot efficiency
η c = 1 - T0 /T represents
the fraction of the
energy of a heat source
at temperature T that
can be converted to
work in an
environment at
temperature T0 .
7-11

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FIGURE 7-27
The transfer and
destruction of
exergy during a heat
transfer process
through a finite
temperature
difference.
7-12

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FIGURE 7-29
Mass contains
energy, entropy, and
exergy, and thus
mass flow into or
out of a system is
accompanied by
energy, entropy, and
exergy transfer.
7-13

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FIGURE 7-30
The isolated system
considered in the
development of the decrease
of exergy principle.
7-14

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FIGURE 7-32
Mechanisms of
exergy transfer for
a general system.
7-15

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FIGURE 7-33
Exergy balance for a closed
system when the direction
of heat transfer is taken to
be to the system and the
direction of work from the
system.
7-16

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FIGURE 7-34
Exergy destroyed
outside system
boundaries can be
accounted for by
writing an exergy
balance on the
extended system that
includes the system and
its immediate
surroundings.
7-17

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FIGURE 7-42
Exergy is transferred into or
out of a control volume by
mass as well as heat and work
transfer.
7-18

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FIGURE 7-49
The irreversibility associated with a
student studying and watching a movie
on television, each for two hours.
7-19

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FIGURE 7-50
A poetic
expression of
exergy and
exergy
destruction.
7-20