Elements of Energy Conversion

412 Pages·1967·8.984 MB·English

by  Charles R. Russell (Auth.)

ELEMENTS OF
ENERGY CONVERSION
BY
CHARLES R. RUSSELL
PERGAMON PRESS
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Copyright © 1967
Pergamon Press Inc.
First edition 1967
Library of Congress Catalog Card No. 66-17812
2562/67
Dedicated to
DOLORES RUSSELL
for patience and
encouragement
P R E F A CE
THE subject of energy conversion recently has been extended from
conventional heat power engineering (steam and internal combus-
tion engines) to include many energy conversion and storage
principles. Information is now scattered under several titles
through many publications including voluminous government
reports. Many of these are difficult to obtain and use. This book
has been written, therefore, to bring together this information and
to present it in terms of the fundamental thermodynamics that
apply to energy conversion by any process. Emphasis is given to
the development of the theory of heat engines because these are
and will remain most important power sources. Descriptive
material is then presented to provide elementary information on
all important energy conversion devices.
Many individuals and organizations contributed to the prepara-
tion of this text. The manuscript was edited by Martha Davis.
Helpful suggestions were received from Mr. O. P. Prachar and
others. Several industrial organizations generously have provided
illustrations and data from current programs. The author is
indebted particularly to the U.S. Air Force for permission to use
extensive material from their publication Energy Conversion Sys-
tems Reference Handbook, prepared for them by Electro-Optical
Systems, Inc.
However, any opinions that may be expressed or implied in the
material presented in this publication are those of the author and
are not necessarily the opinions of any other organization. In
addition, whereas the author has endeavoured to make all content
as up-to-date and as factual as possible, the author is neither res-
ponsible nor accepts responsibility for the safety of personnel
following procedures described herein or for the losses and dama-
ges that may arise as a result of errors and omissions in the presen-
ix
X PREFACE
ted material. Neither does the author represent that the perfor-
mance of any work or effort in accordance with the material or
techniques referred to will produce the results herein described.
A conscientious effort has been made to provide adequate refe-
rences to original documents. These original works should be
consulted whenever their content is involved.
CHAPTER 1
ENERGY
FORMS OF ENERGY
Energy is capacity for doing work. Power is the rate of doing
the work. Energy is a scalar quantity expressed in terms of a force
acting through a distance. Typical units for energy and work are
foot-pounds, horsepower-hours, ergs, joules or kilowatt-hours.
There are many forms of energy. Some of these are listed in
Table 1-1 under the categories of potential, kinetic and electro-
magnetic. In addition there are heat and work. These are energy
in the process of transfer from one body to another. After the
transfer, the energy is designated again according to its nature
as, for example, heat transferred may become thermal energy and
work done may appear as mechanical energy.
Potential energy results from position or configuration. An
elevated mass and a wound spring possess external potential
energy relative to their normal states. This energy is equal just
to the minimum work required to elevate the mass or to wind the
spring. Fuels for an engine and the chemicals in a battery possess
internal potential energy — chemical energy —as the result of
forces between atoms and molecules. A nuclear fuel has internal
potential energy associated with forces between subatomic par-
ticles.
Kinetic energy is associated with a moving mass and equals the
minimum work required to produce the motion. The kinetic
energy of a translating or vibrating body is classified as external
kinetic energy. A material also possesses internal kinetic energy
by virtue of motions of its molecules and motions within these
molecules. These random motions are identified with thermal
1
2 ELEMENTS OF ENERGY CONVERSION
TABLE 1-1 FORMS OF ENERGY
Potential energy
(1) Mechanical (Position)
(2) Chemical
(3) Nuclear
Kinetic energy
(1) Mechanical (Velocity)
(2) Free Particles
(3) Thermal
Electromagnetic energy
(1) Radiant
(2) Electrical
(3) Magnetic
energy. A flowing gas has both internal random and externa
directed kinetic energy.
Beta- and alpha-radiations emitted by radioisotopes possess
large amounts of kinetic energy. These are high velocity electrons
and helium nuclei respectively. In the fissioning of an atom of
nuclear fuel, much of the energy appears initially as the kinetic
energy of the two fission products released at high velocity. When
these and other emissions are absorbed within a fuel element,
their energy is transformed into thermal energy. Thus the energy
released in radioactive decay and in nuclear fissioning appears as
thermal energy although originating mostly as the energy of free
particles.
Electromagnetic radiations extend from long radio-frequency
waves to X-rays. Thermal radiation is intermediate, with longer
wavelengths than visible light. An electromagnetic wave has
electric and magnetic components normal to each other. An elec-
tric current is visualized as a stream of electrons flowing through
a conductor under the force of a potential difference.
ENERGY 3
ENERGY CHANGES
Energy can be converted in form, but the total amount of
energy in an isolated system remains unchanged. This is the basic
principle that energy cannot be created or destroyed. Frequent
use is made of this in energy conversion calculations accounting
for the unchanging total amount of energy at each step in a
process. For example, the potential chemical energy of a fuel plus
the thermal energy of the fuel and air for combustion in an engine
can be accounted for as the sum of the work delivered, the thermal
energy released to the surroundings and the chemical energy
remaining in the combustion products.
Although the total amount of energy remains unchanged, there
is a great difference in the quality of different forms of energy.
Potential mechanical energy can in theory be transformed com-
pletely into work or other forms of energy. This is true of all energy
in forms that are completely directed such as electrical, mechan-
ical and even chemical energy. The term "free energy" designates
all forms of directed energy. The undirected random motions
associated with thermal energy, however, are not completely
available for conversion into directed energy. The part of thermal
energy that is available for conversion at some elevated tempera-
ture depends upon this temperature and also upon the tempera-
ture at which the remainder of the energy is rejected. The energy
may be rejected in the condenser of a steam turbine or in the
exhaust products from an internal combustion engine. If the heat
rejection temperature is postulated to be absolute zero, all the
thermal energy in theory then could be transformed into directed
energy, since molecular motions decrease with temperature and
approach zero as the temperature approaches absolute zero.
At practical temperatures for exhausting heat from a thermal
energy conversion device, there is a limiting theoretical maximum
efficiency. Only a fraction of this efficiency is realized in actual
engines. These efficiency limitations apply whenever thermal
energy becomes an intermediate energy form. Although chemical
potential energy can be converted directly into electrical energy
in a battery or fuel cell at very high efficiency, the combustion of
4 ELEMENTS OF ENERGY CONVERSION
the fuel to produce thermal energy for a heat engine again imposes
the efficiency limitation defined by the temperatures of heat addi-
tion and rejection. A quantitative description of these energy
conversion processes is provided by the science of thermodynam-
ics and its specialized branches such as thermochemistry.
CONVERSION METHODS
Many methods for energy conversion are known. A few are
listed in Table 1 -2. All of these have been studied as part of current
pover programs. Devices invented a century ago but put aside
1900 °F
Generator -v Turbine
Heat
Reactor -XN exchanger -,
1350 °F
&J3F
Pump
FIG. 1-1 Mercury turbine power system (NASA).
Regenerator
Radiator Compressor Turbine Generator
FIG. 1-2 Closed-cycle gas turbine power system (NASA).
ENERGY 5
for lack of a requirement or for some technical reason are now
under active development. Direct energy conversion devices hav-
ing no moving mechanical parts are of special interest because of
their potential reliability and freedom from vibration and inertial
forces. These include electrochemical combustion in fuel cells,
FIG. 1-3 Hydraulic turbine and generator in 1903 (Allis-Chalmers).
direct radiant energy conversion in solar cells, and thermal energy
conversion in thermoelectric and thermionic generators.
Dynamic energy conversion devices are illustrated by the
turbine and reciprocating internal combustion engine. The
dynamic heat engines (Figs. 1-1 and 1-2) convert thermal energy
into work through expansion of a fluid to exert a force against
a turbine blade or piston. A gas can expand in a nozzle to produce