Environmental Engineering Reference
In-Depth Information
describe how various debilitating factors reduce the work output of practical devices to values
below this limit. Thermodynamic analyses of this type provide guidance for improving the energy
efficiency of mechanical power production.
In the next five sections of this chapter we summarize the relevant principles of thermodynamics
as embodied in its two laws, including the concepts of energy, work, and heat, the definition of
useful thermodynamic functions, and their application to the steady flow of working fluids, such
as air and combustion gases. We then proceed to the special application of the combustion of fuels
and the various thermodynamic cycles that explain how common heat and combustion engines
function. A subsequent section treats separately the fuel cell, a more recent development that
operates on a different principle than heat engines, producing work directly in electrical form. The
fuel efficiencies of various power-producing cycles are then summarized. The chapter concludes
with a short discussion of the energy efficiency of synthetic fuel production.
3.2
THE FORMS OF ENERGY
The concept of energy, which originated with Aristotle, has a long history both in science and as
a colloquial term. It is a central concept in classical and quantum mechanics, where it appears
as a constant of the motion of mechanical systems. In the science of thermodynamics, energy
has a distinct definition that distinguishes it from heat, work, or power. In this section we define
thermodynamic energy as a quantity that is derived from an understanding of the physical and
chemical properties of matter.
3.2.1
The Mechanical Energy of Macroscopic Bodies
Newtonian mechanics identifies two forms of energy, the
kinetic energy
of a moving body and the
potential energy
of the field of force to which the body is subject. The kinetic energy
KE
is equal
to the product of the mass
M
of the body times one-half of the square of its velocity
V
,
1
2
MV
2
KE
≡
The potential energy
PE
of a body subject to a force
F
at a location
r
in space is equal to the
work done in moving the body to the location
r
from a reference position
r
ref
,
{
r
}
r
PE
≡−
F
{
r
}·
d
r
r
ref
While the kinetic energy has always a positive value with a zero minimum, the potential energy's
value is measured with respect to the reference value and may be positive or negative.
One consequence of Newton's laws of motion of a body in a force field is that the sum of the
kinetic and potential energies is a constant of the motion; that is, it is not a function of time. Calling
this sum the total energy
E
, we have
E
≡
KE
+
PE
=
constant
