Modern Engineering Thermodynamics

320 CHAPTER 10: Availability Analysis
CRITICAL THINKING
Is some of the energy with a system “unavailable”? Does the magnitude of the available energy within a system depend on
the accessible technology? What about nuclear energy, is it “available” to do useful work? If not, could future technologies
make it available?
Ground
A
B
C
−h C
More available
energy than B
h A
Less available
h B
energy than A
Unavailable energy
FIGURE 10.1
A system’s available and unavailable energy. The
potential energy of weight A has more “available”
potential energy to do useful work relative to the
ground than does weight B. However, the potential
energy of weight C is negative relative to the
ground and therefore is “unavailable” to do useful
work relative to the ground.
combustion engine can convert only about 20–30% of the chemical energy in its
fuel into useful output work. Therefore, electrical energy is more valuable to our
society than chemical fuel (notice that we have not taken into account how the
electrical energy is generated). Being more valuable, it costs more because it has a
higher “quality” of work “availability.”
Much of the world around us today is driven by various energy conversion technologies.
Automobiles, power plants, manufacturing facilities, and computers are
all important elements in our society today. Since we know that energy is conserved
in all processes, energy would seem to be an inexhaustible resource to be
used over and over again, powering our technological needs. However, though
energy is conserved, it is degraded through use to a form that is not useful in technological
systems. For example, the kinetic energy of a bouncing rubber ball is
continuously degraded by internal friction produced by the deformation of the
ball during impact. This causes the height to which the ball rebounds after each
impact to progressively decrease. Using the second law of thermodynamics, we
can pinpoint where energy degradation occurs within an engineering system; and
by redesigning the system to minimize the energy degradation, we can improve
the overall energy conversion performance of the system.
With increasing material costs and decreasing resources, it is apparent that the
efficient use of energy resources will be of primary importance to engineers in the
future. In engineering design, the concept of pure energy in somewhat misleading.
What a designer needs to know is how much of the energy present in a given system can
be used for a particular process. That is, of the total energy contained within a system, how much is available to do
useful work? Therefore, it is important for engineers to understand how to determine the consequence (i.e., the
amount of useful energy available) of the energy within a given system. The manipulation of the amount of energy
present is the subject of the first law of thermodynamics. The second law of thermodynamics tells us where the
irreversibilities within the system are located (through the entropy production term). If we combine these two
laws and define a new thermodynamic property (called availability) that is a measure of the useful energy, we can
then identify where energy is being degraded or lost within a system and decide how to modify the system to
reduce these losses.
This chapter provides a basic introduction to the availability property and the availability balance. Open and
closed system availability balances are developed, and a new system efficiency based on the second law of thermodynamics
is developed. Examples are presented illustrating the use of this material for power plants, refrigeration
systems, heat pumps, internal combustion engines, and heat exchangers. A summary at the end of the
chapter reiterates the main concepts and equations developed in the chapter.
Useful work is associated with a quantity called the potential of a conservative force. The following section on fields
and forces provides a background on the mathematical nature of fields, potentials, and conservative forces.
10.2 FUN WITH SCALAR, VECTOR, AND CONSERVATIVE FIELDS
10.2.1 Scalar and Vector Fields
A scalar (or vector) function is a mathematical expression defined at each point in a region of space whose resultant
value is a scalar (or vector) quantity that depends only on the point where the expression was evaluated and
not on the coordinate system used. The region of space over which a scalar function is defined is called the scalar
field of the scalar function, and the region of space over which a vector function is defined is called the vector
field of the vector function. For example, the scalar pressure, temperature, and density functions at each point
within an object define the pressure, temperature, and density scalar fields of the object. Similarly, the velocity
vector at each point within a flowing fluid defines the velocity vector field of the fluid.