Chapter 8

EXERGY: A MEASURE OF WORK POTENTIAL

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T

he increased awareness that the world’s energy
resources are limited has caused many countries to
reexamine their energy policies and take drastic mea-
sures in eliminating waste. It has also sparked interest in the
scientific community to take a closer look at the energy con-
version devices and to develop new techniques to better utilize
the existing limited resources. The first law of thermodynamics
deals with the quantityof energy and asserts that energy can-
not be created or destroyed. This law merely serves as a nec-
essary tool for the bookkeeping of energy during a process
and offers no challenges to the engineer. The second law,
however, deals with the qualityof energy. More specifically, it
is concerned with the degradation of energy during a process,
the entropy generation, and the lost opportunities to do work;
and it offers plenty of room for improvement.
The second law of thermodynamics has proved to be a
very powerful tool in the optimization of complex thermody-
namic systems. In this chapter, we examine the performance
of engineering devices in light of the second law of thermody-
namics. We start our discussions with the introduction of
exergy(also called availability), which is the maximum useful
work that could be obtained from the system at a given state
in a specified environment, and we continue with the reversi-
ble work,which is the maximum useful work that can be
obtained as a system undergoes a process between two
specified states. Next we discuss the irreversibility(also called
the exergy destructionor lost work), which is the wasted work
potential during a process as a result of irreversibilities, and
we define a second-law efficiency. We then develop the exergy
balancerelation and apply it to closed systems and control
volumes.

Objectives

The objectives of Chapter 8 are to:

• Examine the performance of engineering devices in light of
  the second law of thermodynamics.


• Define exergy, which is the maximum useful work that
  could be obtained from the system at a given state in a
  specified environment.


• Define reversible work, which is the maximum useful work
  that can be obtained as a system undergoes a process
  between two specified states.


• Define the exergy destruction, which is the wasted work
  potential during a process as a result of irreversibilities.


• Define the second-law efficiency.


• Develop the exergy balance relation.


• Apply exergy balance to closed systems and control
  volumes.