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CHAPTER
7
Exergy: A Measure of Work Potential
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7-1
FIGURE 7-1A system that is in equilibrium with its environment is said to be at the dead state.
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7-2
FIGURE 7-3The immediate surroundings of a hot potato are simply the temperature gradient zone of the air next to the potato.
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7-3
FIGURE 7-8As a closed system expands, some work needs to be done to push the atmospheric air out of the way (Wsurr).
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7-4
FIGURE 7-10The difference between reversible work and actual useful work is the irreversibility.
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7-5
FIGURE 7-15Two heat engines that have the same thermal efficiency, but different maximum thermal efficiencies.
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7-6
FIGURE 7-16Second-law efficiency is a measure of the performance of a device relative to its performance under reversible conditions.
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7-7
FIGURE 7-20The 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.
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7-8
FIGURE 7-21The exergy of a cold medium is also a positive quantity since work can be produced by transferring heat to it.
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7-9
FIGURE 7-22The exergy of flow energy is the useful work that would be delivered by an imaginary piston in the flow section.
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7-10
FIGURE 7-23The energy and exergy contents of (a) a fixed mass and (b) a fluid stream.
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7-11
FIGURE 7-26The Carnot efficiency 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 .
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7-12
FIGURE 7-27The transfer and destruction of exergy during a heat transfer process through a finite temperature difference.
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7-13
FIGURE 7-29Mass contains energy, entropy, and exergy, and thus mass flow into or out of a system is accompanied by energy, entropy, and exergy transfer.
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7-14
FIGURE 7-30The isolated system considered in the development of the decrease of exergy principle.
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7-15
FIGURE 7-32Mechanisms of exergy transfer for a general system.
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7-16
FIGURE 7-33Exergy 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.
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7-17
FIGURE 7-34Exergy 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.
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FIGURE 7-42Exergy is transferred into or out of a control volume by mass as well as heat and work transfer.
7-18
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7-19
FIGURE 7-49The irreversibility associated with a student studying and watching a movie on television, each for two hours.
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7-20
FIGURE 7-50A poetic expression of exergy and exergy destruction.
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