Page 239 - The Mechatronics Handbook

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Exergy Balance
Exergy provides an alternative to entropy for applying the second law. When exergy concepts are combined
with principles of engineering economy, the result is known as thermoeconomics. Thermoeconomics allows
the real cost sources to be identified: capital investment costs, operating and maintenance costs, and the
costs associated with the destruction and loss of exergy. Optimization of systems can be achieved by a
careful consideration of such cost sources. From this perspective thermoeconomics is exergy-aided cost
minimization. Discussions of exergy analysis and thermoeconomics are provided by Moran (1989), Bejan
et al. (1996), Moran and Tsatsaronis (2000), and Moran and Shapiro (2000). In this section salient aspects
are presented.
Defining Exergy
An opportunity for doing work exists whenever two systems at different states are placed in communication
because, in principle, work can be developed as the two are allowed to come into equilibrium. When one
of the two systems is a suitably idealized system called an environment and the other is some system of
interest, exergy is the maximum theoretical useful work (shaft work or electrical work) obtainable as the
system of interest and environment interact to equilibrium, heat transfer occurring with the environment
only. (Alternatively, exergy is the minimum theoretical useful work required to form a quantity of matter
from substances present in the environment and bring the matter to a specified state.) Exergy is a measure
of the departure of the state of the system from that of the environment, and is therefore an attribute of
the system and environment together. Once the environment is specified, however, a value can be assigned
to exergy in terms of property values for the system only, so exergy can be regarded as an extensive property
of the system. Exergy can be destroyed and, like entropy, generally is not conserved.
Models with various levels of specificity are employed for describing the environment used to evaluate
exergy. Models of the environment typically refer to some portion of a system’s surroundings, the intensive
properties of each phase of which are uniform and do not change significantly as a result of any process
under consideration. The environment is regarded as composed of common substances existing in abun-
dance within the Earth’s atmosphere, oceans, and crust. The substances are in their stable forms as they
exist naturally, and there is no possibility of developing work from interactions—physical or chemical—
between parts of the environment. Although the intensive properties of the environment are assumed to
be unchanging, the extensive properties can change as a result of interactions with other systems. Kinetic
and potential energies are evaluated relative to coordinates in the environment, all parts of which are
considered to be at rest with respect to one another. For computational ease, the temperature T 0 and
pressure p 0 of the environment are often taken as typical ambient values, such as 1 atm and 25°C (77°F).
However, these properties may be specified differently depending on the application.
When a system is in equilibrium with the environment, the state of the system is called the dead state.
At the dead state, the conditions of mechanical, thermal, and chemical equilibrium between the system
and the environment are satisfied: the pressure, temperature, and chemical potentials of the system equal
those of the environment, respectively. In addition, the system has no motion or elevation relative to
coordinates in the environment. Under these conditions, there is no possibility of a spontaneous change
within the system or the environment, nor can there be an interaction between them. The value of exergy
is zero. Another type of equilibrium between the system and environment can be identified. This is a
restricted form of equilibrium where only the conditions of mechanical and thermal equilibrium must
be satisfied. This state of the system is called the restricted dead state. At the restricted dead state, the
fixed quantity of matter under consideration is imagined to be sealed in an envelope impervious to mass
flow, at zero velocity and elevation relative to coordinates in the environment, and at the temperature
T 0 and pressure p 0 .
Exergy Transfer and Exergy Destruction
Exergy can be transferred by three means: exergy transfer associated with work, exergy transfer associated
with heat transfer, and exergy transfer associated with the matter entering and exiting a control volume.
All such exergy transfers are evaluated relative to the environment used to define exergy. Exergy also is

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