The notion that a system must go to the dead state at the end of the
process to maximize the work output can be explained as follows: If the
system temperature at the final state is greater than (or less than) the tem-
perature of the environment it is in, we can always produce additional work
by running a heat engine between these two temperature levels. If the final
pressure is greater than (or less than) the pressure of the environment, we
can still obtain work by letting the system expand to the pressure of the
environment. If the final velocity of the system is not zero, we can catch
that extra kinetic energy by a turbine and convert it to rotating shaft work,
and so on. No work can be produced from a system that is initially at the
dead state. The atmosphere around us contains a tremendous amount of
energy. However, the atmosphere is in the dead state, and the energy it con-
tains has no work potential (Fig. 8–4).
Therefore, we conclude that a 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. This represents the useful
work potentialof the system at the specified state and is called exergy. It is
important to realize that exergy does not represent the amount of work that
a work-producing device will actually deliver upon installation. Rather, it
represents the upper limit on the amount of work a device can deliver with-
out violating any thermodynamic laws. There will always be a difference,
large or small, between exergy and the actual work delivered by a device.
This difference represents the room engineers have for improvement.
Note that the exergy of a system at a specified state depends on the condi-
tions of the environment (the dead state) as well as the properties of the sys-
tem. Therefore, exergy is a property of the system–environment combination
and not of the system alone. Altering the environment is another way of
increasing exergy, but it is definitely not an easy alternative.
The term availabilitywas made popular in the United States by the M.I.T.
School of Engineering in the 1940s. Today, an equivalent term, exergy,
introduced in Europe in the 1950s, has found global acceptance partly
because it is shorter, it rhymes with energy and entropy, and it can be
adapted without requiring translation. In this text the preferred term is
exergy.
Exergy (Work Potential) Associated
with Kinetic and Potential Energy
Kinetic energy is a form of mechanical energy, and thus it can be converted
to work entirely. Therefore, the work potentialor exergyof the kinetic energy
of a system is equal to the kinetic energy itself regardless of the temperature
and pressure of the environment. That is,
Exergy of kinetic energy: (8–1)
where Vis the velocity of the system relative to the environment.
xkekeV^2
2¬¬ 1 kJ>kg 2
Chapter 8 | 425HOT
POTATO 70 °C25 °C25 °C
EnvironmentImmediate
surroundingsFIGURE 8–3
The immediate surroundings of a hot
potato are simply the temperature
gradient zone of the air next to the
potato.FIGURE 8–4
The atmosphere contains a
tremendous amount of energy, but
no exergy.
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