8–1 ■ EXERGY: WORK POTENTIAL OF ENERGY
When a new energy source, such as a geothermal well, is discovered, the
first thing the explorers do is estimate the amount of energy contained in the
source. This information alone, however, is of little value in deciding
whether to build a power plant on that site. What we really need to know is
the work potentialof the source—that is, the amount of energy we can
extract as useful work. The rest of the energy is eventually discarded as
waste energy and is not worthy of our consideration. Thus, it would be very
desirable to have a property to enable us to determine the useful work
potential of a given amount of energy at some specified state. This property
is exergy,which is also called the availabilityor available energy.
The work potential of the energy contained in a system at a specified state
is simply the maximum useful work that can be obtained from the system.
You will recall that the work done during a process depends on the initial
state, the final state, and the process path. That is,In an exergy analysis, the initial stateis specified, and thus it is not a vari-
able. The work output is maximized when the process between two specified
states is executed in a reversible manner,as shown in Chap. 7. Therefore, all
the irreversibilities are disregarded in determining the work potential.
Finally, the system must be in the dead stateat the end of the process to
maximize the work output.
A system is said to be in the dead statewhen it is in thermodynamic equi-
librium with the environment it is in (Fig. 8–1). At the dead state, a system is
at the temperature and pressure of its environment (in thermal and mechanical
equilibrium); it has no kinetic or potential energy relative to the environment
(zero velocity and zero elevation above a reference level); and it does not
react with the environment (chemically inert). Also, there are no unbalanced
magnetic, electrical, and surface tension effects between the system and its
surroundings, if these are relevant to the situation at hand. The properties of
a system at the dead state are denoted by subscript zero, for example,P 0 ,T 0 ,
h 0 ,u 0 , and s 0. Unless specified otherwise, the dead-state temperature and
pressure are taken to be T 0 25°C (77°F) and P 0 1 atm (101.325 kPa or
14.7 psia). A system has zero exergy at the dead state (Fig. 8–2).
Distinction should be made between the surroundings, immediate sur-
roundings,and the environment. By definition,surroundingsare everything
outside the system boundaries. The immediate surroundingsrefer to the
portion of the surroundings that is affected by the process, and environment
refers to the region beyond the immediate surroundings whose properties
are not affected by the process at any point. Therefore, any irreversibilities
during a process occur within the system and its immediate surroundings,
and the environment is free of any irreversibilities. When analyzing the
cooling of a hot baked potato in a room at 25°C, for example, the warm air
that surrounds the potato is the immediate surroundings, and the remaining
part of the room air at 25°C is the environment. Note that the temperature of
the immediate surroundings changes from the temperature of the potato at
the boundary to the environment temperature of 25°C (Fig. 8–3).Workf 1 initial state, process path, final state 2424 | Thermodynamics
AIR
25 °C
101 kPa
V = 0
z = 0T 0 = 25°C
P 0 = 101 kPaFIGURE 8–1
A system that is in equilibrium with its
environment is said to be at the dead
state.
FIGURE 8–2
At the dead state, the useful work
potential (exergy) of a system is zero.
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TUTORIAL