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Elements of Energy Conversion PDF

412 Pages·1967·8.984 MB·English
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ELEMENTS OF ENERGY CONVERSION BY CHARLES R. RUSSELL PERGAMON PRESS OXFORD · LONDON · EDINBURGH · NEW YORK TORONTO · SYDNEY · PARIS · BRAUNSCHWEIG Pergamon Press Ltd., Headington Hill Hall, Oxford 4 & 5 Fitzroy Square, London W.l Pergamon Press (Scotland) Ltd., 2 & 3 Teviot Place, Edinburgh 1 Pergamon Press Inc., 44-01 21st Street, Long Island City, New York 11101 Pergamon of Canada, Ltd., 6 Adelaide Street East, Toronto, Ontario Pergamon Press (Aust.) Pty. Ltd., 20-22 Margaret Street, Sydney, N.S.W. Pergamon Press S.A.R.L., 24 rue des Écoles, Paris 5e Vieweg & Sohn GmbH, Burgplatz 1, Braunschweig 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

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