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margin-left:2em; } -span.attr { font-size:80%; font-family:sans-serif; } -span.pn { display:inline-block; width:4.7em; text-align:left; margin-left:0; text-indent:0; } -</style> -</head> -<body> -<p style='text-align:center; font-size:1.2em; font-weight:bold'>The Project Gutenberg eBook of Direct Conversion of Energy, by William R. Corliss</p> -<div style='display:block; margin:1em 0'> -This eBook is for the use of anyone anywhere in the United States and -most other parts of the world at no cost and with almost no restrictions -whatsoever. You may copy it, give it away or re-use it under the terms -of the Project Gutenberg License included with this eBook or online -at <a href="https://www.gutenberg.org">www.gutenberg.org</a>. If you -are not located in the United States, you will have to check the laws of the -country where you are located before using this eBook. -</div> - -<p style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Title: Direct Conversion of Energy</p> - <p style='display:block; margin-top:1em; margin-bottom:0; margin-left:2em; text-indent:-2em'>Author: William R. Corliss</p> -<p style='display:block; text-indent:0; margin:1em 0'>Release Date: August 11, 2021 [eBook #66033]</p> -<p style='display:block; text-indent:0; margin:1em 0'>Language: English</p> - <p style='display:block; margin-top:1em; margin-bottom:0; margin-left:2em; text-indent:-2em; text-align:left'>Produced by: Stephen Hutcheson and the Online Distributed Proofreading Team at https://www.pgdp.net </p> -<div style='margin-top:2em; margin-bottom:4em'>*** START OF THE PROJECT GUTENBERG EBOOK DIRECT CONVERSION OF ENERGY ***</div> -<div id="cover" class="img"> -<img id="coverpage" src="images/cover.jpg" alt="Direct Conversion of Energy" width="1000" height="1557" /> -</div> -<div class="box"> -<h1><span class="smallest">Direct Conversion of Energy</span></h1> -<div class="img"> -<img src="images/p01.jpg" id="ncfig1" alt="uncaptioned" width="600" height="296" /> -</div> -<p class="center">By William R. Corliss</p> -<p class="tbcenter"><span class="ss smaller">U.S. ATOMIC ENERGY COMMISSION -<br />Division of Technical Information</span></p> -<p class="tbcenter"><i class="ss"><span class="smaller">ONE OF A SERIES ON</span> -<br />UNDERSTANDING THE ATOM</i></p> -</div> -<div class="pb" id="Page_i">i</div> -<dl class="undent"><dt><span class="ssn smaller">UNITED STATES ATOMIC ENERGY COMMISSION</span></dt> -<dt><i>Dr. Glenn T. Seaborg, Chairman</i></dt> -<dt><i>James T. Ramey</i></dt> -<dt><i>Dr. Gerald F. Tape</i></dt> -<dt><i>Wilfrid E. Johnson</i></dt></dl> -<p class="tb">Nuclear energy -is playing a vital role -in the life of -every man, woman, and child -in the United States today. -In the years ahead -it will affect increasingly -all the peoples of the earth. -It is essential -that all Americans -gain an understanding -of this vital force if -they are to discharge thoughtfully -their responsibilities as citizens -and if they are to realize fully -the myriad benefits -that nuclear energy -offers them.</p> -<p>The United States -Atomic Energy Commission -provides this booklet -to help you achieve -such understanding.</p> -<p class="jr1"><img class="inline" src="images/sig.jpg" alt="Edward J. Brunenkant" width="300" height="99" /> -<br />Edward J. Brunenkant -<br />Director -<br />Division of Technical Information</p> -<h2 id="toc" class="center">CONTENTS</h2> -<dl class="toc"> -<dt><a href="#c1">INTRODUCTION</a> 1</dt> -<dt><a href="#c2">DIRECT VERSUS DYNAMIC ENERGY CONVERSION</a> 3</dt> -<dt><a href="#c3">LAWS GOVERNING ENERGY CONVERSION</a> 8</dt> -<dt><a href="#c4">THERMOELECTRICITY</a> 12</dt> -<dt><a href="#c5">THERMIONIC CONVERSION</a> 16</dt> -<dt><a href="#c6">MAGNETOHYDRODYNAMIC CONVERSION</a> 19</dt> -<dt><a href="#c7">CHEMICAL BATTERIES</a> 22</dt> -<dt><a href="#c8">THE FUEL CELL: A CONTINUOUSLY FUELED BATTERY</a> 24</dt> -<dt><a href="#c9">SOLAR CELLS</a> 26</dt> -<dt><a href="#c10">NUCLEAR BATTERIES</a> 28</dt> -<dt><a href="#c11">ADVANCED CONCEPTS</a> 30</dt> -<dt><a href="#c12">SUGGESTED REFERENCES</a> 33</dt> -<dt><a href="#c13">ANSWERS TO PROBLEMS</a> 34</dt> -</dl> -<p class="tb"><span class="smaller">Library of Congress Catalog Card Number: 64-61794</span></p> -<div class="pb" id="Page_ii">ii</div> -<h3>ABOUT THE AUTHOR</h3> -<div class="img"> -<img src="images/p02.jpg" id="ncfig2" alt="William R. Corliss" width="433" height="599" /> -</div> -<p><span class="ss">WILLIAM R. CORLISS</span> is an atomic -energy consultant and writer with 12 -years of industrial experience including -service as Director of Advanced -Programs for the Martin -Company’s Nuclear Division. Mr. -Corliss has B.S. and M.S. Degrees -in Physics from Rensselaer Polytechnic -Institute and the University -of Colorado, respectively. He has -taught at those two institutions and -at the University of Wisconsin. He -is the author of <i>Propulsion Systems -for Space Flight</i> (McGraw-Hill 1960), -<i>Space Probes and Planetary Exploration</i> -(Van Nostrand 1965), <i>Mysteries -of the Universe</i> (Crowell 1967), <i>Scientific Satellites</i> (GPO -1967), and coauthor of <i>Radioisotopic Power Generation</i> (Prentice-Hall -1964), as well as numerous articles and papers for technical -journals and conferences. In this series he has written <i>Neutron -Activation Analysis</i>, <i>Power Reactors in Small Packages</i>, <i>SNAP—Nuclear -Reactor Power in Space</i>, <i>Computers</i>, <i>Nuclear Propulsion -for Space</i>, <i>Space Radiation</i>, and was coauthor of <i>Power from Radioisotopes</i>.</p> -<div class="pb" id="Page_1">1</div> -<h2 id="c1"><span class="small">INTRODUCTION</span></h2> -<p>A flashlight battery supplies electricity without moving -mechanical parts. It converts the chemical energy of its -contents <i>directly</i> into electrical energy.</p> -<p>Early direct conversion devices such as Volta’s battery, -developed in 1795, gave the scientists Ampere, Oersted, -and Faraday their first experimental supplies of electricity. -The lessons they learned about electrical energy and its -intimate relation with magnetism spawned the mighty -turboelectric energy converters—steam and hydroelectric -turbines—which power modern civilization.</p> -<p>We have improved upon Volta’s batteries and have come -to rely on them as portable, usually small, power sources, -but only recently has the challenge of nuclear power and -space exploration focused our attention on new methods of -direct conversion.</p> -<p>To supply power for use in outer space and also at remote -sites on earth, we need power sources that are reliable, -light in weight, and capable of unattended Operation for long -periods of time. Nuclear power plants using direct conversion -techniques hold promise of surpassing conventional -power sources in these attributes. In addition, the inherently -<span class="pb" id="Page_2">2</span> -silent operation of direct conversion power plants is -an important advantage for many military applications.</p> -<p>The Atomic Energy Commission, the Department of Defense, -and the National Aeronautics and Space Administration -collectively sponsor tens of millions of dollars worth -of research and development in the area of direct conversion -each year. In particular, the Atomic Energy Commission -supports more than a dozen research and development -programs in thermoelectric and thermionic energy conversion -in industry and at the Los Alamos Scientific Laboratory, -and other direct conversion research at Argonne National -Laboratory and Brookhaven National Laboratory. -Reactor and radioisotopic power plants utilizing direct conversion -are being produced under the AEC’s SNAP<a class="fn" id="fr_1" href="#fn_1">[1]</a> program. -Some of these units are presently in use powering -satellites, Arctic and Antarctic weather stations, and navigational -buoys.</p> -<p>Further applications are now being studied, but the cost -of direct conversion appears too great to permit its general -use for electric power in the near future. Direct techniques -will be used first where their special advantages outweigh -higher cost.</p> -<div class="pb" id="Page_3">3</div> -<h2 id="c2"><span class="small">DIRECT VERSUS DYNAMIC ENERGY CONVERSION</span></h2> -<h3>Dominance of Dynamic Conversion</h3> -<p>We live in a world of motion. A main task of the engineer -is to find better and more efficient ways of transforming -the energy locked in the sun’s rays or in fuels, such as coal -and the uranium nucleus, into energy of motion. Almost all -the world’s energy is now transformed by rotating or reciprocating -machines. We couple the energy of exploding -gasoline and air to our automobile’s wheels by a reciprocating -engine. The turbogenerator at a hydroelectric plant -extracts energy from falling water and turns it into electricity. -Such rotating or reciprocating machines are called -<i>dynamic</i> converters.</p> -<h3>A New Level of Sophistication: Direct Conversion</h3> -<p>A revolution is in the making. We know now that we can -force the heat-and-electricity-carrying electrons residing -in matter to do our bidding without the use of shafts and -pistons. This is a leading accomplishment of modern technology: -energy transformation without moving parts. It is -called <i>direct</i> conversion.</p> -<p>The thermoelements shown above the turbogenerator in -<a href="#fig1">Figure 1</a> illustrate the contrast between direct and dynamic -<span class="pb" id="Page_4">4</span> -conversion. The thermoelements convert heat into electricity -directly, without any of the intervening machinery -seen in the turbogenerator.</p> -<div class="img" id="fig1"> -<img src="images/p03.jpg" alt="" width="800" height="1039" /> -<p class="pcap"><b>Figure 1</b> <i>Direct conversion devices, such as the spokelike lead telluride -thermoelectric elements inside the SNAP 3 radioisotope generator -shown above (courtesy Martin Company), convert heat into electricity -without moving parts. In contrast, the SNAP 2 dynamic converter -shown below SNAP 3 (courtesy Thompson Ramo Wooldridge, Inc.) includes -a high-speed turbine, an electric generator, and pumps to produce -electricity from heat. (NaK is a liquid mixture of sodium and -potassium.)</i></p> -</div> -<dl class="undent pcap"><dt>DIRECT VERSUS DYNAMIC CONVERSION</dt> -<dd>SNAP 3 LESS THAN 5 WATTS</dd> -<dd class="t">5″</dd> -<dd>SNAP 2 3000 WATTS</dd> -<dd class="t">24″</dd> -<dd class="t">ALTERNATOR ROTOR</dd> -<dd class="t">ALTERNATOR STATOR</dd> -<dd class="t">TURBINE ROTORS</dd> -<dd class="t">NaK PUMP DIFFUSER</dd> -<dd class="t">NaK PUMP ROTOR</dd> -<dd class="t">MERCURY JET BOOSTER PUMP</dd> -<dd class="t">MERCURY CENTRIFUGAL PUMP</dd> -<dd class="t">MERCURY THRUST BEARING</dd> -<dd class="t">MERCURY BEARING</dd> -<dd class="t">MERCURY BEARING</dd></dl> -<div class="pb" id="Page_5">5</div> -<h3>Why is Direct Conversion Desirable?</h3> -<p>There are places where energy conversion equipment -must run for years without maintenance or breakdown. -Also, there are situations where the ultimate in reliability -is required, such as on scientific satellites and particularly -on manned space flights. Direct conversion equipment seems -to offer greater reliability than dynamic conversion equipment -for these purposes.</p> -<p>We should recognize that our belief in the superiority of -direct conversion is based more on intuition than proof. It -is true that direct converters will never throw piston rods -or run out of lubricant. Yet, some satellite power failures -have been caused by the degradation of solar cells under -the bombardment of solar protons. The other types of direct -conversion devices described in the following pages may -also break down in ways as yet unknown. Still, today’s -knowledge gives us hope that direct conversion will be -more reliable and trustworthy than dynamic conversion. -Direct conversion equipment is beginning to be adopted for -small power plants, producing less than 500 watts, designed -to operate for long periods of time in outer space and under -the ocean. Some day, large central-station power plants -may use direct conversion to improve their efficiencies and -reliabilities.</p> -<h3>How is Energy Transformed?</h3> -<p>What is energy and how do we change it? Energy is a -fundamental concept of science involving the capacity for -doing work. <i>Kinetic</i> or mechanical energy is the most obvious -form of energy. It is defined as</p> -<div class="verse"> -<p class="lc">E = ½ mv²</p> -<p class="t0">where</p> -<p class="t">E = energy (expressed in joules)</p> -<p class="t">m = mass of the moving object (in kilograms)</p> -<p class="t">v = velocity (in meters per second)</p> -</div> -<p>Energy can also be stored in chemical and nuclear substances -or in the water behind a dam. In these quiescent -states it is called <i>potential</i> energy. If the potential energy -in a substance is abundant and easily released, the energy-rich -substance is called a <i>fuel</i>.</p> -<div class="pb" id="Page_6">6</div> -<h3>ENERGY CONVERSION MATRIX</h3> -<div class="img" id="fig2"> -<img src="images/p04.jpg" alt="" width="1200" height="878" /> -<p class="pcap"><b>Figure 2</b> <i>To find how one form of -energy is converted into another, start -at the proper column and move down -until the column intersects with the desired -row. The box at the intersection -will give typical conversion processes -and examples.</i></p> -</div> -<table class="center"> -<tr class="th"><th>FROM⇒ </th><th>ELECTROMAGNETIC </th><th>CHEMICAL </th><th>NUCLEAR </th><th>THERMAL </th><th>KINETIC<br />(MECHANICAL) </th><th>ELECTRICAL </th><th>GRAVITATIONAL</th></tr> -<tr class="th"><th>TO⇓</th></tr> -<tr><td class="l"><b>ELECTROMAGNETIC</b> </td><td class="l"> </td><td class="l">Chemiluminescence<br />(fireflies) </td><td class="l">Gamma reactions<br />(Co⁵⁸ source)<br />A-bomb </td><td class="l">Thermal radiation<br />(hot iron) </td><td class="l">Accelerating charge<br />(cyclotron)<br />Phosphor<a class="fn" id="fr_2" href="#fn_2">[2]</a> </td><td class="l">Electromagnetic radiation<a class="fn" href="#fn_2">[2]</a><br />(TV transmitter)<br />Electroluminescence </td><td class="l">Unknown</td></tr> -<tr><td class="l"><b>CHEMICAL</b> </td><td class="l">Photosynthesis<br />(plants)<br />Photochemistry<br />(photographic film) </td><td class="l"> </td><td class="l">Radiation catalysis<br />(hydrazine plant)<br />Ionization<br />(cloud chamber) </td><td class="l">Boiling<br />(water/steam)<br />Dissociation </td><td class="l">Dissociation by radiolysis </td><td class="l">Electrolysis<br />(production of aluminum)<br />Battery charging </td><td class="l">Unknown</td></tr> -<tr><td class="l"><b>NUCLEAR</b> </td><td class="l">Gamma-neutron reactions<br />(Be⁹+γ → Be⁸+n) </td><td class="l">Unknown </td><td class="l"> </td><td class="l">Unknown </td><td class="l">Unknown </td><td class="l">Unknown </td><td class="l">Unknown</td></tr> -<tr><td class="l"><b>THERMAL</b> </td><td class="l">Solar absorber<br />(hot sidewalk) </td><td class="l">Combustion<br />(fire) </td><td class="l">Fission<br />(fuel element)<br />Fusion </td><td class="l"> </td><td class="l">Friction<br />(brake shoes) </td><td class="l">Resistance-heating<br />(electric stove) </td><td class="l">Unknown</td></tr> -<tr><td class="l"><b>KINETIC</b> </td><td class="l">Radiometer Solar cell<a class="fn" href="#fn_2">[2]</a> </td><td class="l">Muscle </td><td class="l">Radioactivity<br />(alpha particles)<br />A-bomb </td><td class="l">Thermal expansion<br />(turbines)<br />Internal combustion<br />(engines) </td><td class="l"> </td><td class="l">Motors<br />Electrostriction<br />(sonar transmitter) </td><td class="l">Falling objects</td></tr> -<tr><td class="l"><b>ELECTRICAL</b> </td><td class="l">Photoelectricity<br />(light meter)<br />Radio antenna<br />Solar cell<a class="fn" href="#fn_2">[2]</a> </td><td class="l">Fuel cell<a class="fn" href="#fn_2">[2]</a><br />Batteries<a class="fn" href="#fn_2">[2]</a> </td><td class="l">Nuclear battery<a class="fn" href="#fn_2">[2]</a> </td><td class="l">Thermoelectricity<a class="fn" href="#fn_2">[2]</a><br />Thermionics<a class="fn" href="#fn_2">[2]</a><br />Thermomagnetism<a class="fn" href="#fn_2">[2]</a><br />Ferroelectricity<a class="fn" href="#fn_2">[2]</a> </td><td class="l">MHD<a class="fn" href="#fn_2">[2]</a><a class="fn" id="fr_3" href="#fn_3">[3]</a><br />Conventional generator </td><td class="l"> </td><td class="l">Unknown</td></tr> -<tr><td class="l"><b>GRAVITATIONAL</b> </td><td class="l">Unknown </td><td class="l">Unknown </td><td class="l">Unknown </td><td class="l">Unknown </td><td class="l">Rising objects<br />(rockets) </td><td class="l">Unknown</td></tr> -</table> -<h3>The Energy Conversion Matrix</h3> -<p>Forms of energy are interchangeable. When gasoline is -burned in an automobile engine, potential energy is first -turned into heat. A portion of this heat, say 25%, is then -converted into mechanical motion. The remainder of the -heat is wasted and must be removed from the engine.</p> -<p>A multitude of processes and devices have been found -which make these transformations from one form of energy -to another. Many of these are listed in the blocks in <a href="#fig2">Figure 2</a>. -Asterisks refer to direct conversion processes, the subject -matter of this booklet.</p> -<div class="pb" id="Page_7">7</div> -<p>To demonstrate how this diagram is to be read, let us -use it to trace the energy transformations involved in an -automobile engine. We begin with sunlight because all coal -and oil deposits (the <i>fossil fuels</i>) received their initial -charge of energy in the form of sunlight.</p> -<p>The first conversion, therefore, is from electromagnetic -energy to chemical energy via photosynthesis in living -things. We trace the transformation by moving down the -column marked Electromagnetic Energy until it intersects -the horizontal row labeled Chemical Energy. There we see -photosynthesis listed in the block. The next conversion is -from chemical energy to thermal energy via combustion. -<span class="pb" id="Page_8">8</span> -We trace this by moving down the Chemical Energy column -to the Thermal Energy row; combustion is listed in the appropriate -block. The third and final conversion takes place -when thermal energy is transformed into mechanical energy -via the internal combustion engine.</p> -<p>By the repeated use of the Energy Conversion Matrix in -this way, we can chart any energy transformation.</p> -<h4>Problem <span class="larger">1</span></h4> -<blockquote> -<p>Continue the automobile example by going through -the matrix twice more showing how mechanical -energy is converted into stored chemical energy -in the car’s battery.</p> -</blockquote> -<h4>Problem <span class="larger">2</span></h4> -<blockquote> -<p>If 1 gram of gasoline (about a tablespoonful) yields -48,000 joules of thermal energy when burned with -air, how fast can it make a 1000 kilogram car go? -Assume the car starts from rest and its engine is -25% efficient.</p> -<p>Answers to problems are on <a href="#Page_34">page 34</a>.</p> -</blockquote> -<h2 id="c3"><span class="small">LAWS GOVERNING ENERGY CONVERSION</span></h2> -<h3>The Big Picture: Thermodynamics</h3> -<p>To the best of our knowledge, energy and mass are always -conserved together in any transformation. This summary -of experience has been made into a keystone of science: -the Law of Conservation of Energy and Mass. It -states that the total amount of mass and energy cannot be -altered. This law applies to everything we do, from driving -a nail to launching a space probe. While the conscience of -the scientist insists that he continually recheck the truth of -this law, it remains a bulwark of science.</p> -<p>The Law of Conservation of Energy and Mass is also -called the First Law of Thermodynamics. It is related to -the Second Law of Thermodynamics, which also governs energy -transformations. The Second Law says, in effect, that -some energy will unavoidably be lost in all heat engines. -The first two laws of thermodynamics have been paraphrased -as (1) You can’t win; (2) You can’t even break even. -Let us look at them further.</p> -<div class="pb" id="Page_9">9</div> -<h3>You Can’t Win</h3> -<p>We used to think that energy and mass were conserved -independently, and for many practical purposes we still -consider them so conserved. But Einstein united the two -with the famous equation</p> -<div class="verse"> -<p class="lc">E = mc²</p> -</div> -<div class="verse"> -<p class="t0">where</p> -<p class="t">E = energy (in joules)</p> -<p class="t">m = mass (in kilograms)</p> -<p class="t">c = speed of light</p> -<p class="t3">(300,000,000 meters per second)</p> -</div> -<p>Notice the resemblance to the kinetic energy equation shown -earlier. Energy cannot appear without the disappearance of -mass. When energy is locked up in a fuel, it is stored as -mass. In the gasoline combustion problem, 1 gram of gasoline -was burned with air to give 48,000 joules of energy. -Einstein’s equation says that in this case mass disappeared -in the amount</p> -<div class="verse"> -<p class="lc">m = E/c² = (4.8 × 10⁴)/(9 × 10¹⁶) = 5.3 × 10⁻¹³ kilogram</p> -<p class="t2">(half a billionth of a gram)</p> -</div> -<p>But, when an H-bomb is exploded, grams and even kilograms -of mass are converted to energy.</p> -<p>In direct conversion processes we do not need to worry -about these mass changes, but at each point we must make -sure that all energy is accounted for. For example, in outer -space all energy released from fuels (even food) must ultimately -be radiated away to empty Space. Otherwise the vehicle -temperature will keep rising until the Spaceship melts.</p> -<h3>You Can’t Even Break Even</h3> -<p>Any engineer is annoyed by having to throw energy away. -Why is energy ever wasted? The Second Law of Thermodynamics -guides us here. Experience has shown that heat cannot -be transformed into another form of energy with 100% -efficiency. We can’t explain Nature’s idiosyncracies, but we -have to live with them. So, we accept the fact that every engine -that starts out with heat must ultimately waste some of -that energy (<a href="#fig3">Figure 3</a>).</p> -<div class="pb" id="Page_10">10</div> -<div class="img" id="fig3"> -<img src="images/p05.jpg" alt="" width="600" height="688" /> -<p class="pcap"><b>Figure 3</b> <i>A typical heat engine -showing heat input, useful power -output, and the unavoidable waste -heat that must be rejected to the -environment. A pressure-volume -diagram is shown underneath for a -closed gas-turbine cycle. Circled -numbers correspond. The energy -produced is represented by the -shaded area. Similar diagrams can -be made for all heat engines as an -aid in studying their performance.</i></p> -</div> -<dl class="undent pcap"><dt>A TYPICAL HEAT ENGINE</dt> -<dd>HEAT IN</dd> -<dd>HEAT SOURCE</dd> -<dd class="t">REACTOR, BOILER</dd> -<dd>ELECTRICITY OUT</dd> -<dd>ENERGY CONVERTER</dd> -<dd>PUMP</dd> -<dd>FLUID PIPE</dd> -<dd>RADIATOR</dd> -<dd>WASTE HEAT OUT</dd> -<dt>PRESSURE-VOLUME DIAGRAM</dt> -<dd>HEAT IN</dd> -<dd>ENERGY OUT</dd> -<dd>GAS PRESSURE</dd> -<dd>WASTE HEAT OUT</dd> -<dd>GAS VOLUME</dd></dl> -<p>Direct conversion devices are no exception. Consequently, -every thermoelectric element or thermionic converter will -have to provide for the disposition of waste heat. The designer -will try, however, to make the engine efficiency high -so that the waste heat will be small. <a href="#fig4">Figure 4</a> shows the -extensive waste heat radiator on a SNAP 50 power plant -planned for deep space missions.</p> -<div class="img" id="fig4"> -<img src="images/p05a.jpg" alt="" width="800" height="478" /> -<p class="pcap"><b>Figure 4</b> <i>Model of SNAP 50 power plant planned -for deep space missions showing extensive waste -heat radiator. The system will provide 300 to -1000 kilowatts of electrical power.</i></p> -</div> -<div class="pb" id="Page_11">11</div> -<h3>Carnot Efficiency</h3> -<p>In 1824 Sadi Carnot, a young French engineer, conceived -of an idealized heat engine. This ideal engine had an efficiency -given by</p> -<div class="verse"> -<p class="lc">e = 1 - T<sub>c</sub>/T<sub>h</sub> = (T<sub>h</sub> - T<sub>c</sub>)/T<sub>h</sub></p> -</div> -<div class="verse"> -<p class="t0">where</p> -<p class="t">e = the so-called Carnot efficiency (no units)</p> -<p class="t">T<sub>c</sub> = the temperature of the waste heat reservoir (in degrees Kelvin, °K<a class="fn" id="fr_4" href="#fn_4">[4]</a>)</p> -<p class="t">T<sub>h</sub> = the temperature of the heat source (in °K)</p> -</div> -<p>Unhappily, T<sub>c</sub> cannot be made zero (and e therefore made -equal to 1, which is 100% efficiency). Physicists have shown -absolute zero to be unattainable, although they have approached -to within a hundredth of a degree in the laboratory.</p> -<p>Waste heat, since it must be rejected to the surrounding -atmosphere, outer space, or water (rivers, the ocean, etc.), -must be rejected at T<sub>c</sub> greater than 300°K. The reason for -this is that these physical reservoirs have average temperatures -around 300°K (about 80°F) themselves. The fact -that T<sub>c</sub> must be 300°K or more is a basic limitation on the -Carnot efficiency. The loss in efficiency with increased T<sub>c</sub> -explains why a jet plane has a harder job taking off on a hot -day.</p> -<p>One way to improve the Carnot efficiency, which is the -maximum efficiency for any heat engine, is to raise T<sub>h</sub> as -high as possible without melting the engine. For a coal-fired -electrical power plant, T<sub>h</sub> = 600°K and T<sub>c</sub> = 300°K, so -that</p> -<p class="center">e = 1 - 300/600 = 0.5 = 50%</p> -<p>The actual efficiency is somewhat less than this ideal -value because some power is diverted to pumps and other -<span class="pb" id="Page_12">12</span> -equipment and to unavoidable heat losses. Later on, we -shall see that magnetohydrodynamic (MHD) generators hold -prospects for increasing T<sub>h</sub> by hundreds of degrees.</p> -<p>Everything that has been said about the Second Law of -Thermodynamics (You can’t even break even) applies to -heat engines, where we begin with thermal energy. Suppose -instead that we start with kinetic or chemical energy and -convert it into electricity without turning it into heat first. -We can then escape the Carnot efficiency strait jacket. -Chemical batteries perform this trick. So do fuel cells, -solar cells, and many other direct conversion devices we -shall discuss. Thus, we circumvent the Carnot efficiency -limitation by using processes to which it does not apply.</p> -<h4>Problem <span class="larger">3</span></h4> -<blockquote> -<p>Some space power plants contemplate using the -space cabin heat (T<sub>h</sub> = 300°K) to drive a heat engine -which rejects its waste heat to the liquid-hydrogen -rocket fuel stored at T<sub>c</sub> = 20°K. What -would be the Carnot efficiency of this engine?</p> -</blockquote> -<h2 id="c4"><span class="small">THERMOELECTRICITY</span></h2> -<h3>After 140 Years: Seebeck Makes Good</h3> -<p>The oldest direct conversion heat engine is the thermocouple. -Take two different materials (typically, two dissimilar -metal wires), join them, and heat the junction. A -voltage, or electromotive force, can be measured across -the unheated terminals. T. J. Seebeck first noticed this effect -in 1821 in his laboratory in Berlin, but, because of a -mistaken interpretation of what was involved, he did not -seek any practical application for it. Only recently has any -real progress been made in using his discovery for power -production. To use the analogy of A. F. Joffe, the Russian -pioneer in this field, thermoelectricity lay undisturbed for -over a hundred years like Sleeping Beauty. The Prince that -awoke her was the semiconductor.</p> -<p>As long as inefficient metal wires were used, textbook -writers were correct in asserting that thermoelectricity -could never be used for power production. The secret of -<span class="pb" id="Page_13">13</span> -practical thermoelectricity is therefore the creation of -better thermoelectric materials. (Creation is the right word -since the best materials for the purpose do not exist in -nature.) To perform this alchemy, we first have to understand -the Seebeck effect.</p> -<h3>Electrons and Holes</h3> -<p>Let’s examine the latticework of atoms that make up any -solid material. In electrical insulators all the atoms’ outer -electrons<a class="fn" id="fr_5" href="#fn_5">[5]</a> are held tightly by valence bonds to the neighboring -atoms. In contrast, any metal has many relatively -loose electrons which can wander freely through its latticework. -This is what makes metals good conductors.</p> -<h3>THERMOELECTRICITY</h3> -<div class="img" id="fig5"> -<img src="images/p06.jpg" alt="" width="579" height="800" /> -<p class="pcap"><b>Figure 5</b> <i>Thermoelectric -couple made from p- and -n-type semiconductors. -The impurity atoms (I) are different -in each leg and contribute -an excess or deficiency of -valence electrons. Heat drives -both holes and electrons toward -the cold junction.</i></p> -</div> -<dl class="undent pcap"><dd>T<sub>c</sub> WASTE HEAT OUT</dd> -<dd>ELECTRONS</dd> -<dd>LOAD</dd> -<dd>COLD JUNCTION</dd> -<dd>HOLES</dd> -<dd>ELECTRONS</dd> -<dd><i>p</i> SEMICONDUCTOR</dd> -<dd><i>n</i> SEMICONDUCTOR</dd> -<dd>HOT JUNCTION</dd> -<dd>T<sub>h</sub> HEAT IN</dd> -<dt>Simplified Sketch of Atomic Lattice</dt> -<dd>HOLE</dd> -<dd>ELECTRON</dd> -<dd>VALENCE BONDS</dd> -<dd>SEMICONDUCTOR LATTICES</dd> -<dd>I = Impurity atom</dd></dl> -<p><a href="#fig5">Figure 5</a> suggests the latticework of a <i>semiconductor</i>. It -is called a semiconductor because its conductivity falls far -short of that of the metals. The few electrons available for -carrying electricity are supplied by the deliberately introduced -<span class="pb" id="Page_14">14</span> -impurity atoms, which have more than enough electrons -to satisfy the valence-bond requirements of the neighboring -atoms. Without the impurities, we would have an -insulator. With them, we have an <i>n</i>-type semiconductor. -The <i>n</i> is for the extra <i>negative</i> electrons.</p> -<p>A <i>p</i>- or <i>positive</i>-type semiconductor is also included in -<a href="#fig5">Figure 5</a>. Here the impurity atom does not have enough -valence electrons to satisfy the valence-bond needs of the -surrounding lattice atoms. The lattice has been short-changed -and is, in effect, full of <i>positive holes</i>. Strangely -enough, these holes can wander through the material just -like positive charges.</p> -<p>The electron-hole model does not have the precision the -physicist likes, but it helps us to visualize semiconductor -behavior.</p> -<p>The Seebeck effect is demonstrated when pieces of <i>p</i>- and -<i>n</i>-type material are joined as shown in <a href="#fig5">Figure 5</a>. Heat at -the hot junction drives the loose electrons and holes toward -the cold junction. Think of the holes and electrons as gases -being driven through the latticework by the temperature -difference. A positive and a negative terminal are thus produced, -giving us a source of power. The larger the temperature -difference, the bigger the voltage difference. Note -that just one thermocouple <i>leg</i> can produce a voltage across -its length, but <i>couples</i> made from <i>p</i> and <i>n</i> legs are superior.</p> -<h3>Practical Thermoelectric Power Generators</h3> -<p>The first nuclear-heated thermoelectric generator was -built in 1954 by the Atomic Energy Commission’s Mound -Laboratory in Miamisburg, Ohio. It used metal-wire thermocouples. -In contrast, the SNAP 3 series thermocouples -shown in <a href="#fig1">Figure 1</a> are thick lead telluride (PbTe) semiconductor -cylinders about two inches long. In contrast to the -thermocouple wires’ efficiency of less than 1%, SNAP 3 -series generators have overall efficiencies exceeding 5%. -This value is still low compared to the 35-40% obtained in -a modern steam power plant, but SNAP 3 generators can -operate unattended in remote localities where steam plants -would be totally unacceptable.</p> -<p>Look again at the thermoelements in <a href="#fig1">Figure 1</a> and the -schematic, <a href="#fig5">Figure 5</a>. Underlying the apparent simplicity of -<span class="pb" id="Page_15">15</span> -the thermoelectric generator are extensive development efforts. -The <a href="#fig1">Figure 1</a> thermoelectric couple, for example, -shows the fruits of thousands of experimental brazing tests. -It turns out to be uncommonly difficult to fasten thermoelectric -elements to the so-called <i>hot shoe</i> (metal plate) at -the bottom. The joint has to be strong, must withstand high -temperatures, and must have low electrical resistance. We -see also that the elements are encased in mica sleeves to -prevent chemical disturbance of the delicate balance of impurities -in the semiconductor by the surrounding gases. A -further complication is the extreme fragility of the elements, -and this has yet to be overcome.</p> -<p>Nuclear thermoelectric generators that provide small -amounts of electrical power have already been launched into -space aboard Department of Defense satellites (<a href="#fig12">Figure 12</a>), -installed on land stations in both polar regions, and placed -under the ocean.<a class="fn" id="fr_6" href="#fn_6">[6]</a> Propane-fueled thermoelectric generators, -such as shown in <a href="#fig6">Figure 6</a>, are now on the market for -use in camping equipment, in ocean buoys, and in remote -spots where only a few watts of electricity are needed. The -Russians have long manufactured a kerosene lamp with -thermoelements placed in its stack for generating power in -wilderness areas.</p> -<div class="img" id="fig6"> -<img src="images/p07.jpg" alt="" width="800" height="591" /> -<p class="pcap"><b>Figure 6</b> <b>GENERAL PURPOSE GENERATOR</b> -<br /><i>Commercially -available thermoelectric -generators using propane -fuel can provide more than -enough electrical power to -operate a portable TV set.</i> -<span class="jr">Courtesy Minnesota Mining & Manufacturing Company.</span></p> -</div> -<p>For the present the role of thermoelectric power appears -to be one of special uses such as those just mentioned. -When higher efficiencies are attained, thermoelectric power -may, one day, supplant dynamic conversion equipment in -certain low-power applications regardless of location.</p> -<div class="pb" id="Page_16">16</div> -<h2 id="c5"><span class="small">THERMIONIC CONVERSION</span></h2> -<h3>“Boiling” Electrons Out of Metals</h3> -<p>Like the thermoelectric element, the thermionic converter -is a heat engine. In its simplest form it consists of two -closely spaced metallic plates and resembles the diode radio -tube. Whereas thermoelectric elements depend on heat to -drive electrons and holes through semiconductors to an external -electricity-using device or <i>load</i>, the salient feature -of the thermionic diode is <i>thermionic -emission</i>,<a class="fn" id="fr_7" href="#fn_7">[7]</a> or, simply, -the boiling-off of electrons from a hot metal surface. -The thermionic converter shown in <a href="#fig7">Figure 7</a> powers a small -motor when heated by a torch.</p> -<p>Metals, as we have already seen, have -an abundance of loosely bound conduction -electrons roaming the atomic latticework. -These electrons are easily moved by -electric fields while within the metal; but -it takes considerably more energy to boil -them out of the metal into free space. -Work has to be done against the electric -fields set up by the surface layer of -atoms, which have unattached valence -bonds on the side facing empty space.</p> -<p>The energy required to completely detach -an electron from the surface is called -the metal’s <i>work function</i>. In the case of -tungsten, for example, the work function -is about 4.5 electron volts<a class="fn" id="fr_8" href="#fn_8">[8]</a> of energy.</p> -<div class="img" id="fig7"> -<img src="images/p08.jpg" alt="" width="503" height="600" /> -<p class="pcap"><b>Figure 7</b> <i>Vacuum type thermionic -converter in operation.</i> -<span class="jr">Courtesy General Electric Company.</span></p> -</div> -<p>As we raise the temperature of a metal, the conduction -electrons in the metal also get hotter and move with greater -velocity. We may think of some of the electrons in a metal -as forming a kind of <i>electron gas</i>. Some electrons will gain -such high speeds that they can escape the metal surface. -<span class="pb" id="Page_17">17</span> -This happens when their kinetic energy exceeds the metal’s -work function.</p> -<p>Now that we have found a way to force electrons out of -the metal, we would like to make them do useful electrical -work. To do this we have to push the electrons across the -gap between the plates as well as create a voltage difference -to go with the hoped-for current flow.</p> -<h3>Reducing the Space Charge</h3> -<p>The emitted or boiled-off electrons between the converter -plates (<a href="#fig8">Figure 8</a>) form a cloud of negative charges that -will repel subsequently emitted electrons back to the emitter -plate unless counteraction is taken. To circumvent -these <i>space charge</i> effects, we fill the space between the -plates with a gas containing positively charged particles. -These mix with the electrons and neutralize their charge. -The mixture of positively and negatively charged particles -is called a <i>plasma</i>.</p> -<p>The presence of the plasma makes the gas a good conductor. -The emitted electrons can now move easily across -it to the collector where, to continue the gas analogy, they -condense on the cooler surface.</p> -<div class="img" id="fig8"> -<img src="images/p08a.jpg" alt="" width="800" height="495" /> -<p class="pcap"><b>Figure 8 THERMIONIC CONVERSION</b> -<br /><i>Thermionic converters may be flat-plate -types or cylindrical types. The cylindrical -converter (a) is an experimental type for ultimate -use in nuclear reactors.</i> -<span class="jr">Courtesy Los Alamos Scientific Laboratory.</span></p> -</div> -<dl class="undent pcap"><dt>a</dt> -<dd>INSULATOR</dd> -<dd>COOLED COLLECTOR</dd> -<dd>INCANDESCENT URANIUM</dd> -<dd>FUEL ELEMENT</dd> -<dd>CESIUM PLASMA</dd> -<dd>CIRCULATING COOLANT</dd> -<dd>VACUUM INSULATOR</dd> -<dd>CESIUM POOL</dd> -<dt>b</dt> -<dd>WASTE HEAT OUT</dd> -<dd>LOAD</dd> -<dd>ELECTRONS</dd> -<dd>LOW WORK FUNCTION COLLECTOR</dd> -<dd>T<sub>c</sub></dd> -<dd>CESIUM ION</dd> -<dd>PLASMA FILLED GAP</dd> -<dd>BOILED OFF ELECTRONS</dd> -<dd>HIGH WORK FUNCTION EMITTER</dd> -<dd>T<sub>c</sub></dd> -<dd>HEAT IN</dd></dl> -<div class="pb" id="Page_18">18</div> -<h3>Result: A Plasma Thermocouple</h3> -<p>Unless a voltage difference exists across the plates, no -external work can be done. In the thermocouple the voltage -difference was caused by the different electrical properties -of the <i>p</i> and <i>n</i> semiconductors. Both the emitter and collector -in the thermionic converter are good metallic conductors -rather than semiconductors, so a different tack must -be taken.</p> -<p>The key is the use of an emitter and a collector with different -work functions. If it takes 4.5 electron volts to force -an electron from a tungsten surface and if it regains only -3.5 electron volts when it condenses on a collector with a -lower work function, then a voltage drop of 1 volt exists between -the emitter and collector.</p> -<p>To summarize, then, the thermionic emission of electrons -creates the potentiality of current flow. The difference -in work functions makes the thermionic converter a -power producer.</p> -<p>There is an interesting comparison that helps describe -this phenomenon. Consider the emitter to be the ocean surface -and the collector a mountain lake. The atmospheric -heat engine vaporizes ocean water and carries it to the -cooler mountain elevations, where it condenses as rain -which collects in lakes. The lake water as it runs back -toward sea level then can be made to drive a hydroelectric -plant with the gravitational energy it has gained in the -transit. The thermionic converter is similar in behavior: -hot emitter (corresponding to the sun-heated ocean); cooler -collector (lake); electron gas (water); different electrical -voltages (gravity). Without gravity the river would not flow, -and the production of electricity would be impossible.</p> -<h3>Thermionic Power in Outer Space</h3> -<p>Thermionic converters for use in outer space may be -heated by the sun, by decaying radioisotopes, or by a fission -reactor. Thermionic converters can also be made into -concentric cylindrical shells (<a href="#fig8">Figure 8</a>a) and wrapped -around the uranium fuel elements in nuclear reactors. The -waste heat in this case would be carried out of the reactor -to a separate radiator<a class="fn" id="fr_9" href="#fn_9">[9]</a> by a stream of liquid metal. Since -<span class="pb" id="Page_19">19</span> -thermionic converters can operate at much higher temperatures -than thermoelectric couples or dynamic power plants, -the radiator temperature, T<sub>c</sub>, will also be higher. Consequently, -space power plants using thermionic converters -will have small radiators. Once thermionic converters are -developed which have high reliability and long life, they will -provide the basis for a new series of lighter, more efficient -space power plants.</p> -<h2 id="c6"><span class="small">MAGNETOHYDRODYNAMIC CONVERSION</span></h2> -<h3>Big Word, Simple Concept</h3> -<p>Magnetohydrodynamic (MHD) conversion is very unlike -thermoelectric or thermionic conversion. The MHD generators -use high-velocity electrically conducting gases to -produce power and are generically closer to dynamic conversion -concepts. The only concept they carry forward from -the preceding conversion ideas is that of the <i>plasma</i>, the -electrically conducting gas. Yet they are commonly classified -as <i>direct</i> because they replace the rotating turbogenerator -of the dynamic systems with a stationary pipe or -<i>duct</i>.</p> -<div class="img" id="fig9"> -<img src="images/p09.jpg" alt="" width="1200" height="533" /> -<p class="pcap"><b>Figure 9</b> <i>In the MHD duct (a), the electrons in the hot plasma move -to the right under influence of force F in the magnetic field B. The -electrons collected by the right-hand side of the duct are carried to the -load. In a wire in the armature of a conventional generator (b) the -electrons are forced to the right by the magnetic field.</i></p> -</div> -<dl class="undent pcap"><dt>a</dt> -<dd>MHD Duct</dd> -<dd>HOT PLASMA IN</dd> -<dd>COOL GAS OUT TO RADIATOR</dd> -<dd>Magnetic Field</dd> -<dd>LOAD</dd> -<dd>ELECTRONS</dd> -<dt>b</dt> -<dd>CONVENTIONAL GENERATOR</dd> -<dd>SHAFT</dd> -<dd>LOAD</dd> -<dd>Magnetic Field</dd> -<dd>ARMATURE WIRE</dd> -<dd>ELECTRONS</dd></dl> -<div class="pb" id="Page_20">20</div> -<p>In the conventional dynamic generator, an electromotive -force is created in a wire that cuts through magnetic lines -of force, as shown in <a href="#fig9">Figure 9</a>b. It may be helpful to visualize -the conduction electrons as leaving one end of the wire -and moving to the other under the influence of the magnetic -field.</p> -<p>The force on the electrons in the wire is given by</p> -<div class="verse"> -<p class="lc">F = qvB</p> -</div> -<div class="verse"> -<p class="t0">where</p> -<p class="t">F = the force (in newtons<a class="fn" id="fr_10" href="#fn_10">[10]</a>)</p> -<p class="t">q = the charge on the electron (1.6 × 10⁻¹⁹ coulomb)</p> -<p class="t">v = the wire’s velocity (in meters per second)</p> -<p class="t">B = the magnetic field strength (in webers per square meter<a class="fn" href="#fn_10">[10]</a>)</p> -</div> -<p>The surge of electrons along the length of the wire sets -up a voltage difference across the ends of the wire. A generator -uses this difference to convert the kinetic energy of -the moving wire or armature into electrical energy. The -wire is kept spinning by the shaft which is connected to a -turbine driven by steam or water.</p> -<p>Let us try to eliminate the moving part, the generator -armature. What we need is a moving conductor that has no -shaft, no bearings, no wearing parts. The substance that -meets these requirements is the plasma. Examine <a href="#fig9">Figure 9</a>a. -The MHD generator substitutes a moving, conducting gas -for the wires. Under the influence of an external magnetic -field, the conduction electrons move through the plasma to -one side of the duct which carries electrical power away to -the load.</p> -<p>The MHD generator gets its energy from an expanding, -hot gas; but, unlike the turbogenerator, the heat engine and -generator are united in the static duct. The gradual widening -of the duct shown in <a href="#fig9">Figure 9</a>a reflects the lower pressure, -cooler plasma at the duct’s end. Some of the plasma’s -thermal energy content has been tapped off by the duct’s -electrodes as electrical power.</p> -<div class="pb" id="Page_21">21</div> -<h3>The Fourth State of Matter</h3> -<p>Plasma can be created by temperatures over 2000°K. At -this temperature many high-velocity gas atoms collide with -enough energy to knock electrons off each other and thus -become ionized. The material thus created, shown as a -glowing gas in <a href="#fig10">Figure 10</a>, does not behave consistently as -any of the three familiar states of matter: solid, liquid, or -gas. Plasma has been called a <i>fourth state of matter</i>. Since -we have difficulty in containing such high temperatures on -earth, we adopt the strategy of <i>seeding</i>. In this technique -gases that are ordinarily difficult to ionize, like helium, -are made conducting by adding a fraction of a percent of an -alkali metal such as potassium. Alkali metal atoms have -loosely bound outer electrons and quickly become ionized -at temperatures well below 2000°K.</p> -<div class="img" id="fig10"> -<img src="images/p10.jpg" alt="" width="800" height="542" /> -<p class="pcap"><b>Figure 10</b> <i>Glowing plasma in experimental device at -General Atomic’s John Jay Hopkins Laboratory, San -Diego. T-shaped plasma gun provides data for research -in thermonuclear fusion.</i> -<span class="jr">Courtesy Texas Atomic Energy Research Foundation.</span></p> -</div> -<p>A helium-potassium mixture is a good enough conductor -for use in an MHD generator. In this plasma the electrons -move rapidly under the influence of the applied fields, -though not as well as in metals. The positive ions move in -the opposite direction from the electrons, but the electrons -are much lighter and move thousands of times faster thus -carrying the bulk of the electrical current.</p> -<div class="pb" id="Page_22">22</div> -<h3>MHD Power Prospects</h3> -<p>The MHD duct is not a complete power plant in itself because, -after leaving the duct, the stream of gas must be -compressed, heated, and returned to the duct. Very high -temperature materials and components must be developed -for this kind of service. Moreover, while the duct is simple -in concept, it must operate at very high temperatures in the -presence of the corrosive alkali metals. This presents us -with difficult materials problems. When the problems are -solved, probably within the next decade, MHD power plants -should be able to provide reliable power with high efficiency. -They may then serve in large space power plants, and, most -important, they may provide cheaper electricity for general -use through their higher temperatures and greater efficiencies.</p> -<h2 id="c7"><span class="small">CHEMICAL BATTERIES</span></h2> -<h3>Electricity from the Chemical Bond</h3> -<p>If you vigorously knead a lemon to free the juices and then -stick a strip of zinc in one end and a copper strip in the -other, you can measure a voltage across the strips. Electrons -will flow through the load without the inconvenience of -having to supply heat. You have made yourself a chemical -battery.</p> -<p>The chemical battery was the first direct conversion device. -Two hundred years ago it was the scientists’ only continuous -source of electricity.</p> -<p>Since the chemical battery does not need heat for its operation, -it is logical to ask what makes the current flow. -Where does the energy come from?</p> -<p>The battery has no semiconductors, but, like the thermoelectric -couple and the thermionic diode, it uses dissimilar -materials for its electrodes. A conducting fluid or solid is -also present to provide for the passage of current between -the electrodes. In the example of the lemon, the copper and -zinc are the dissimilar electrodes, and the lemon juice is -the conducting fluid or <i>electrolyte</i> that supplies positive and -negative ions. The battery derives its energy from its complement -of chemical fuel. The voltage difference arises -<span class="pb" id="Page_23">23</span> -because of the different strengths of the chemical bonds. -The chemical bond is basically an electrostatic one; some -atoms have stronger electrical affinities than others.</p> -<h3>Chemical Reactions Used in Batteries and Fuel Cells</h3> -<p>Consider the following chemical reactions of common -batteries together with some fuel cell reactions which will -be discussed further in the next section.</p> -<dl class="undent"><dt><span class="ss">Battery Reactions</span></dt> -<dd>Pb + PbO₂ + 2H₂SO₄ ⇔ 2PbSO₄ + 2H₂O</dd> -<dd>Fe + NiO₂ ⇔ FeO + NiO</dd> -<dd>Zn + AgO + H₂O ⇔ Ag + Zn(OH)₂</dd> -<dd>Pb + Ag₂O ⇔ PbO + 2Ag</dd> -<dt><span class="ss">Fuel Cell Reactions</span></dt> -<dd>2LiH ⇔ 2Li + H₂</dd> -<dd>2CuBr₂ ⇔ 2CuBr + Br₂</dd> -<dd>2H₂ + O₂ ⇔ 2H₂O (Bacon cell)</dd> -<dd>PbI₂ ⇔ Pb + I₂</dd></dl> -<p>In principle all these reactions are the same as those -going on inside the lemon, although each type of cell produces -a slightly different voltage because of the varying -chemical affinities of the atoms and molecules involved. -There are literally hundreds of materials which can be used -for electrolytes and electrodes.</p> -<p>No heat needs to be added as the electrostatic chemical -bonds are broken and remade in a battery to generate electrical -power. The chemical reaction energy is transferred -to the electrical load with almost 100% efficiency. The -Carnot cycle is no limitation here; only “cold” electrostatic -forces are in action. The reactions cannot go on forever, -however, because the battery supplies the energy converter -with a very limited supply of fuel. Eventually the -fuel is consumed and the voltage drops to zero. This deficiency -is remedied by the fuel cell in which fuel is supplied -continuously.</p> -<h3>An Old Standby in Outer Space</h3> -<p>Almost every satellite and space vehicle has a chemical -battery aboard. It is not there so much for continuous power -production but as a rechargeable electrical accumulator or -reservoir to provide electricity during peak loads. The battery -is also needed to store energy for use during the periods -when solar cells are in the earth’s shadow and therefore -inoperative. In this capacity the dependable old battery -serves the most modern science very well indeed.</p> -<div class="pb" id="Page_24">24</div> -<h2 id="c8"><span class="small">THE FUEL CELL: A CONTINUOUSLY FUELED BATTERY</span></h2> -<h3>Potential Fuels</h3> -<p>The battery has a very close relative, the fuel cell. Unlike -the battery the fuel cell has a continuous supply of fuel.</p> -<div class="img" id="fig11"> -<img src="images/p12.jpg" alt="" width="800" height="559" /> -<p class="pcap"><b>Figure 11</b> <i>This diagram shows how a hydrogen-oxygen fuel cell -works. The chemical battery works in the same way, except that the -chemicals are different and are not continuously supplied from outside -the cell. The water produced by the H-O cell shown can be used for -drinking on spaceships.</i></p> -</div> -<dl class="undent pcap"><dt>ANODE H₂ IN</dt> -<dt>CATHODE O₂ IN</dt> -<dt>ELECTRONS</dt> -<dt>LOAD</dt> -<dt>KOH ELECTROLYTE</dt> -<dt>K⁺ ION</dt> -<dt>OH⁻ ION</dt> -<dt>NEGATIVE ION FLOW</dt> -<dt>40H⁻ + 2H₂ ⇒ 4H₂O + 4e</dt> -<dt>O₂ + 2H₂O + 4e ⇒ 40H⁻</dt></dl> -<p>The hydrogen-oxygen cell of <a href="#fig11">Figure 11</a> is typical of all -fuel cells. It essentially burns hydrogen and oxygen to form -water. If the hydrogen and oxygen can be supplied continuously -and the excess water drained off, we can greatly extend -the life of the battery. The fuel cell accomplishes this. -Fueled <i>electrical</i> cell would be more descriptive since the -physical principles are identical with those of the battery.</p> -<p>Perhaps the most challenging task contemplated for the -fuel cell is to bring about the consumption of raw or slightly -processed coal, gas, and oil fuels with atmospheric oxygen. -If fuel cells can be made to use these abundant fuels, then -the high natural conversion efficiency of the fuel cells will -make them economically superior to the lower efficiency -steam-electric plants now in commercial service.</p> -<div class="pb" id="Page_25">25</div> -<p>So far we have dwelt on the fuel cell as a cold energy conversion -device that is <i>not</i> limited by the Carnot efficiency. -A variation on this theme is possible. Take a hydrogen -iodide (HI) cell, and heat the HI to 2000°K. Some of the HI -molecules will collide at high velocities and dissociate into -hydrogen and iodine: 2HI = H₂ + I₂; the higher the temperature, -the more the dissociation. By separating the hydrogen -and iodine gases and returning them for recycling to the -fuel cell where they are recombined, we have eliminated -the fuel supply problem and created a <i>regenerative</i> fuel -cell. We have, however, also reintroduced the heat engine -and the Carnot cycle efficiency. The thermally regenerative -fuel cell is a true heat engine using a dissociating gas as -the working fluid.</p> -<h3>Scheme for Project Apollo</h3> -<p>Most of the impetus for developing the fuel cell as a practical -device comes from the space program. The cell has -admirable properties for space missions that are less than -a few months in duration. It is a clean, quiet, vibrationless -source of energy. Like the battery it has a high electrical -overload capacity for supplying power peaks and is easily -controlled. It can even provide potable water for a crew if -the Bacon H - O cell is used. For short missions where -large fuel supplies are not needed, it is also among the -lightest power plants available.</p> -<p>These compelling advantages have led the National Aeronautics -and Space Administration to choose the fuel cell for -some of the first manned space ventures. Project Apollo, -the manned lunar landing mission, is the most notable example. -Here the fuel cell will be not only an energy source, -but also part of the ecological cycle which keeps the crew -alive.</p> -<h4>Problem <span class="larger">4</span></h4> -<blockquote> -<p>A manned space vehicle requires an average of 2 -electrical kilowatts. A nuclear reactor thermoelectric -plant having a mass of 1000 kilograms, -including shielding, can supply this power for -10,000 hours. The basic fuel cell has a mass of 50 -kilograms and consumes ½ kilogram of chemicals -per hour. The chemical containers weigh 25 kilograms. -What is the longest mission where the -total weight of the fuel cell will be less than the -weight of the nuclear power plant?</p> -</blockquote> -<div class="pb" id="Page_26">26</div> -<h2 id="c9"><span class="small">SOLAR CELLS</span></h2> -<h3>Photons as Energy Carriers</h3> -<p>All our fossil fuels, such as coal and oil, owe their existence -to the solar energy stream that has engulfed the earth -for billions of years. The power in this stream amounts to -about 1400 watts per square meter at the earth, nearly -enough to supply an average home if all the energy were -converted to electricity. The problem is to get the sun’s -rays to yield up their energy with high efficiency.</p> -<p>The sun’s visible surface has a temperature around -6000°K. Any object heated to this temperature will radiate -visible light mostly in the yellow-green portion of the spectrum -(5500 A<a class="fn" id="fr_11" href="#fn_11">[11]</a>). Our energy conversion device should be -tuned to this wavelength.</p> -<p>The energy packets arriving from the sun are called -photons. They travel, of course, at the speed of light, and -each carries an amount of energy given by</p> -<div class="verse"> -<p class="lc">E = hf = hc/λ</p> -</div> -<div class="verse"> -<p class="t0">where</p> -<p class="t">E = energy (in joules)</p> -<p class="t">h = Planck’s constant (6.62 × 10⁻³⁴ joule-second)</p> -<p class="t">f = the light’s frequency (in cycles per second = c/λ)</p> -<p class="t">c = the velocity of light (300,000,000 meters per second)</p> -<p class="t">λ = the wavelength (in meters)</p> -</div> -<p>Using the fact that an angstrom unit is 10⁻¹⁰ meter, the energy -of a 5500 A photon could be calculated as</p> -<div class="verse"> -<p class="lc">E = hf = hc/λ = (6.62 × 10⁻³⁴ × 3.00 × 10⁸)/(5.50 × 10⁻⁷)</p> -<p class="lc">= 3.61 × 10⁻¹⁹ joule = 2.2 electron volts</p> -</div> -<p>Comparing this result, 2.2 electron volts, with the energies -required to cause atomic ionization or molecular dissociation -<span class="pb" id="Page_27">27</span> -(an electron volt or so), we see that it is in the -right range to actuate direct conversion devices based on -such phenomena.</p> -<h3>Harnessing the Sun’s Energy</h3> -<p>Historically, the sun’s energy has most often been used -by concentrating it with a lens or mirror and then converting -it to heat. We could do this and run a heat engine, but a -more direct avenue is open.</p> -<p>About a decade ago it was found that the junction between -<i>p</i> and <i>n</i> semiconductors would generate electricity if illuminated. -This discovery led to the development of the <i>solar -cell</i>, a thin, lopsided sandwich of silicon semiconductors. -As shown in <a href="#fig12">Figure 12</a>, the top semiconductor layer exposed -to the sun is extremely thin, only 2.5 microns. Solar photons -can readily penetrate this layer and reach the junction -separating it from the thick main body of the solar cell.</p> -<div class="img" id="fig12"> -<img src="images/p13.jpg" alt="" width="1200" height="481" /> -<p class="pcap"><b>Figure 12 THE SOLAR CELL</b> -<br /><i>The photograph shows the solar cell in use -on a satellite. The spherical, radioisotope, thermoelectric -generator at the bottom of the satellite is used to supplement -the solar cells. In the solar cell, hole-electron pairs -are created by solar photons in the vicinity of a p-n junction.</i> -<span class="jr">Courtesy U. S. Air Force and National Aeronautics and Space Administration.</span></p> -</div> -<dl class="undent pcap"><dt><i>p</i> SILICON</dt> -<dt><i>n</i> SILICON</dt> -<dt>ELECTRON-MOLE PAIRS</dt> -<dt>JUNCTION</dt> -<dt>PHOTONS FROM SUN OR RADIOISOTOPE</dt> -<dt>ELECTRONS</dt> -<dt>ENERGY OUT</dt></dl> -<p>Whenever <i>p-</i> and <i>n-</i>type semiconductors are sandwiched -together a voltage difference is created across the junction. -The separated holes and electrons in the two semiconductor -regions establish this electric field across the junction. -Unfortunately, there are usually no current carriers in the -immediate vicinity of the junction so that no power is produced.</p> -<div class="pb" id="Page_28">28</div> -<p>The absorption of solar photons in the vicinity of the junction -will create current carriers, as the photons’ energy is -transformed into the potential energy of the hole-electron -pairs. These pairs would quickly recombine and give up -their newly acquired potential energy if the electric field -existing across the junction did not whisk them away to an -external load.</p> -<p>The solar cell produces electricity when hole-electron -pairs are formed. Any other phenomenon that creates such -pairs will also generate electricity. The source of energy -is irrelevant so long as the current carriers are formed -near the junction. Thus, particles emitted by radioactive -atoms can also produce electricity from solar cells, although -too much bombardment by such particles can damage -the cell’s atomic structure and reduce its output.</p> -<p>The solar cell is not a heat engine. Yet it loses enough -energy so that the sun’s energy is converted at less than -15% efficiency. Losses commonly occur because of the recombination -of the hole-electron pairs before they can produce -current, the absorption of photons too far from the -junction, and the reflection of incident photons from the top -surface of the cell. Despite these losses solar cells are -now the mainstay of nonpropulsive space power.</p> -<h2 id="c10"><span class="small">NUCLEAR BATTERIES</span></h2> -<h3>Energy from Nuclear Particles</h3> -<p>As we have seen, solar cells are able to convert the kinetic -energy of charged nuclear particles directly into electricity, -but a simpler and more straightforward way of doing -this exists. This involves direct use of the flow of charged -particles as current.</p> -<p>The <i>nuclear battery</i> shown in <a href="#fig13">Figure 13</a> performs this -trick. A central rod is coated with an electron-emitting radioisotope -(a beta-emitter; say, strontium-90). The high-velocity -electrons emitted by the radioisotope cross the gap -between the cylinders and are collected by a simple metallic -sleeve and sent to the load. Simple, but why don’t space -<span class="pb" id="Page_29">29</span> -charge effects prevent the electrons -from crossing the gap as they do in -the thermionic converter? The answer -lies in the fact that the nuclear electrons -have a million times more kinetic -energy than those boiled off the -thermionic converter’s emitter surface. -Consequently, they are too powerful -to be stopped by any space charge -in the narrow gap.</p> -<p>Nuclear batteries are simple and -rugged. They generate only microamperes -of current at 10,000 to 100,000 -volts.</p> -<div class="img" id="fig13"> -<img src="images/p14.jpg" alt="" width="486" height="800" /> -<p class="pcap"><b>Figure 13 A NUCLEAR BATTERY</b> -<br /><i>The nuclear battery -depends upon the emission of -charged particles from a surface -coated with a radioisotope. The -particles are collected on another -surface.</i></p> -</div> -<dl class="undent pcap"><dt>ENERGY OUT</dt> -<dt>INSULATOR</dt> -<dt>LAYER OF BETA-EMITTING RADIOISOTOPE</dt> -<dt>VACUUM</dt></dl> -<h3>Double Conversion</h3> -<p>In the earlier description of the energy conversion matrix, -we saw that we could go through the energy transformation -process repeatedly until we obtained the kind of energy -we wanted. This is exemplified in a type of nuclear -battery which uses the so-called <i>double conversion</i> approach. -First, the high-velocity nuclear particles are absorbed -in a phosphor which emits visible light. The photons -thus produced are then absorbed in a group of strategically -placed solar cells, which deliver electrical power to the -load. Although efficiency is lost at each energy transformation, -the double conversion technique still ends up with an -overall efficiency of from 1 to 5%, an acceptable value for -power supplies in the watt and milliwatt ranges.</p> -<div class="pb" id="Page_30">30</div> -<h2 id="c11"><span class="small">ADVANCED CONCEPTS</span></h2> -<p>Ferroelectric and thermomagnetic conversion are subtle -concepts which depend upon the gross alteration of a material’s -physical properties by the application of heat. Devices -employing such concepts are true heat engines. Instead -of the gaseous and electronic working fluids used in -the other direct conversion concepts, the ferroelectric and -thermomagnetic concepts employ patterns of atoms and -molecules that are actually rearranged periodically by heat.</p> -<h3>Ferroelectric Conversion</h3> -<p>Ferroelectric conversion makes use of the peculiar properties -of <i>dielectric</i><a class="fn" id="fr_12" href="#fn_12">[12]</a> materials. Barium titanate, for example, -has good dielectric properties at low temperatures, -but, when its temperature is raised to more than 120°C, -the properties get worse rapidly. We cannot discuss dielectric -behavior thoroughly in this booklet; suffice it to say that -in this process heat is absorbed in a realignment of molecules -within the barium titanate latticework.</p> -<p>If we now place a slab of barium titanate between the two -plates of an electrical condenser and charge the condenser, -as shown in <a href="#fig14">Figure 14</a>, we have a unique way of converting -heat into electricity directly. When the barium titanate is -heated above its <i>Curie point</i><a class="fn" id="fr_13" href="#fn_13">[13]</a> of 120°C, the condenser’s -capacitance is radically reduced as the dielectric constant -falls. The condenser is forced to discharge and move electrons -through an external circuit consisting of the load and -the original source of charge. Useful electrical energy is -delivered during this step. <a href="#fig14">Figure 14</a> shows the process -schematically and mathematically. When the dielectric is -cooled, waste heat is given up by the barium titanate, and -the cycle is complete.</p> -<div class="pb" id="Page_31">31</div> -<div class="img" id="fig14"> -<img src="images/p15.jpg" alt="" width="1000" height="440" /> -<p class="pcap"><b>Figure 14 FERROELECTRIC ENERGY CONVERSION</b> -<br /><i>The ferroelectric converter is really an electrical capacitor -whose capacitance is changed by temperature. When heat is -added, capacitance drops, voltage rises, and the capacitor is made to -discharge through the load. CYCLE: <b>①</b> Switch #1 closed, #2 open. -Condenser charges from battery to charge Q₂ at voltage V₁ with capacity -C₁. <b>②</b> All switches open. Heat added, capacity changes from -C₁ to C₂, charge remains constant, so voltage changes from V₁ to V₂. -<b>③</b> Switch #2 closed, #1 open. Condenser discharges through load and -battery to charge Q₁ at voltage V₁ with capacity C₂. <b>④</b> All switches -open. Heat rejected, capacity changes from C₂ to C₁, charge remains -constant, so voltage changes from V₁ to V₀. CYCLE THEN REPEATS. -Energy supplied from battery each cycle is E₁. Energy delivered to -load and battery each cycle is E₂. Net energy converted is then E₂ - E₁, -the difference in the shaded areas.</i></p> -</div> -<dl class="undent pcap"><dt>(a) CIRCUIT</dt> -<dd>HEAT IN</dd> -<dd>BARIUM TITANATE DIELECTRIC</dd> -<dd>WASTE HEAT OUT</dd> -<dd>SWITCH #2</dd> -<dd>LOAD</dd> -<dd>SWITCH #1</dd> -<dd>BATTERY</dd> -<dt>(b) CYCLE DIAGRAM</dt> -<dd>charge</dd> -<dd>volts</dd> -<dd>Q₂, Q₁, E₁, E₂, V₀ V₁ V₂</dd> -<dd>GENERAL INFORMATION:</dd> -<dd class="t">C₂ < C₁</dd> -<dd class="t">V = Q/C</dd></dl> -<h3>Thermomagnetic Conversion</h3> -<p>The <i>analog</i><a class="fn" id="fr_14" href="#fn_14">[14]</a> of ferroelectricity is ferromagnetism. A -converter employing similar principles to those in ferroelectricity -can be made using an electrical <i>inductance</i> with -a ferromagnetic core. When the temperature of the ferromagnetic -material is raised above its Curie point, its magnetic -<i>permeability</i> drops quickly, causing the magnetic field -to collapse partially. Energy may be delivered to an external -load during this change. Instead of energy being stored -in an electrostatic field, it is stored in a magnetic field.</p> -<div class="pb" id="Page_32">32</div> -<p>Ferroelectric and thermomagnetic conversion both represent -a class of energy transformations which involve internal -molecular or crystalline rearrangements of solids. -There is no change of phase as in a steam engine, but the -energy changes are there nevertheless. In thermodynamics -such internal geometrical changes are called <i>second-order</i> -transitions, as opposed to the <i>first-order</i> transitions observed -with heat engines using two-phase working fluids -like water/steam.</p> -<h3>On the Frontier</h3> -<p>Other potential energy conversion schemes are being investigated -by scientists and engineers. Those listed in the -Energy Conversion Matrix (<a href="#fig2">Figure 2</a>) only scratch the -surface.</p> -<p>In particular, we are just learning how to manipulate -photons. There are photochemical, photoelectric, and even -photomechanical transformations. These have hardly been -tapped.</p> -<p>Consider the reaction when an electron and its antimatter -equivalent, the positron, meet. They mutually annihilate -each other in a burst of energy! This energy will be harnessed -someday.</p> -<p>What energy conversion device are we going to use to -completely convert mass into energy? The energy requirements -for interstellar exploration are so great that these -voyages will be impossible unless a new device is found -that can completely transform mass into energy.</p> -<p>Then again, we haven’t the faintest idea of how to control -gravitational energy, but we may learn.</p> -<p>The panorama is endless.</p> -<h4>Problem <span class="larger">5</span></h4> -<blockquote> -<p>A 1,000,000-kilogram spaceship takes off for Alpha -Centauri, our nearest star, 4.3 light years away. -If it accelerates to nine-tenths the velocity of light, -what is its kinetic energy? How much fuel mass -will have to be completely converted to energy to -acquire this velocity?</p> -</blockquote> -<div class="pb" id="Page_33">33</div> -<h2 id="c12"><span class="small">SUGGESTED REFERENCES</span></h2> -<h3>Articles</h3> -<p class="revint">Fuel Cells, Leonard G. Austin, <i>Scientific American</i>, 201: 72 (October 1959). -A survey of the different types.</p> -<p class="revint">Nuclear Power in Outer Space, William R. Corliss, <i>Nucleonics</i>, 18: 58 (August -1960). A review of all nuclear space power plants.</p> -<p class="revint">Fuel Cells for Space Vehicles, M. G. Del Duca, <i>Astronautics</i>, 5: 36 (March 1960).</p> -<p class="revint">Fuel Cells, E. Gorin and H. L. Recht, <i>Chemical Engineering Progress</i>, 55: 51 -(August 1959).</p> -<p class="revint">Thermionic Converters, Karl G. Hernqvist, <i>Nucleonics</i>, 17: 49 (July 1959).</p> -<p class="revint">The Revival of Thermoelectricity, Abram F. Joffe, <i>Scientific American</i>, 199: 31 -(November 1958). Excellent historical and technical review.</p> -<p class="revint">The Photovoltaic Effect and Its Utilization, P. Rappaport, <i>RCA Review</i>, 20: 373 -(September 1959). Recommended for advanced students.</p> -<p class="revint">The Prospects of MHD Power Generation, Leo Steg and George W. Sutton, -<i>Astronautics</i>, 5: 22 (August 1960).</p> -<p class="revint">Conversion of Heat to Electricity by Thermionic Emission, Volney C. Wilson, -<i>Journal of Applied Physics</i>, 30: 475 (April 1959). Recommended for advanced -students.</p> -<p class="revint">Improved Solar Cells Planned for IMP-D, R. D. Hibben, <i>Aviation Week & Space -Technology</i>, 83: 53 (July 26, 1965).</p> -<p class="revint">Thin-film Solar Cells Boost Output Ratio, P. J. Klass, <i>Aviation Week & Space -Technology</i>, 83: 67 (November 29, 1965).</p> -<h3>Books</h3> -<p class="revint"><i>Direct Conversion of Heat to Electricity</i>, Joseph Kaye and John A. Welsh, -John Wiley & Sons, Inc., New York 10016, 1960, 387 pp., $11.50. Recommended -for advanced students.</p> -<p class="revint"><i>Selected Papers on New Techniques for Energy Conversion</i>, Sumner N. Levine, -(Ed.), Dover Publications, Inc., New York 10014, 1961, 444 pp., $3.00. A reprinting -of many classical papers on direct conversion.</p> -<p class="revint"><i>Energy Conversion for Space Power</i>, Nathan W. Snyder, (Ed.), Academic Press, -Inc., New York 10003, 1961, 779 pp., $8.50. A collection of American Rocket -Society papers.</p> -<p class="revint"><i>Man and Energy</i>, Alfred Rene Ubbelohde, George Braziller, New York 10016, -1955, 247 pp., $5.00 (hardback); $1.25 (paperback), from Penguin Books, Inc., -Baltimore, Maryland 21211. A popular treatment of energy and power.</p> -<h3>Motion Pictures</h3> -<p>The following films are produced by Educational Services, Inc., and are available -from Modern Learning Aids, A Division of Modern Talking Picture Service, -Inc., 3 East 54th St., New York 22, New York.</p> -<div class="verse"> -<p class="t0"><i>Energy and Work</i>, 0311, 29 minutes, $150.</p> -<p class="t0"><i>Mechanical Energy and Thermal Energy</i>, 0312, 27 minutes, $120.</p> -<p class="t0"><i>Conservation of Energy</i>, 0313, 27 minutes, $150.</p> -<p class="t0"><i>Photo-Electric Effect</i>, 0417, 28 minutes, $220.</p> -</div> -<div class="pb" id="Page_34">34</div> -<h2 id="c13"><span class="small">ANSWERS TO PROBLEMS</span></h2> -<p>First, mechanical energy drives the car’s electric -generator. Second, the electrical energy is -converted into chemical energy when the battery -is recharged.</p> -<hr class="dwide" /> -<p>From the kinetic energy equation we get</p> -<p class="center">v = √(2 E/m)</p> -<p>Since the engine is 25% efficient, the energy available -to propel the car is 48,000 × 0.25 or 12,000 -joules. So</p> -<p class="center">v = √(24,000/1,000) = 2√6 = 4.9 meters per second</p> -<hr class="dwide" /> -<p class="center">e = (300 - 20)/300 = 14/15 = 0.93 = 93%</p> -<p>The crossover point, t, in hours is found by equating -the nuclear power plant mass and that of the -fuel cell with its associated fuel. The equation is</p> -<p class="center">1000 = 50 + 25 + ½t -t = 1850 hours = 77 days</p> -<hr class="dwide" /> -<p class="center">E = ½ mv² = (10⁶(0.9 × 3 × 10⁸)²)/2 = 3.6 × 10²² joules</p> -<p>The ship will use the same amount of energy to -decelerate at its destination. Note that this calculation -assumes a perfect efficiency in converting -the energy of matter annihilation into the kinetic -energy of the space ship. The mass consumed is</p> -<p class="center">m = E/c² = (3.6 × 10²²)/(9 × 10¹⁶) = 4.0 × 10⁵ kg</p> -<p>almost half the spaceship mass.</p> -<div class="pb" id="Page_35">35</div> -<h2 id="c14"><span class="small">Footnotes</span></h2> -<div class="fnblock"><div class="fndef"><a class="fn" id="fn_1" href="#fr_1">[1]</a>Systems for Nuclear Auxiliary Power. -</div><div class="fndef"><a class="fn" id="fn_2" href="#fr_2">[2]</a>Described in this booklet. -</div><div class="fndef"><a class="fn" id="fn_3" href="#fr_3">[3]</a>Magnetohydrodynamics. -</div><div class="fndef"><a class="fn" id="fn_4" href="#fr_4">[4]</a>The Kelvin temperature scale starts with zero at absolute zero instead of at the freezing point of water. Therefore, °K = °C + 273; °K = ⁵/₉ (°F + 460). -</div><div class="fndef"><a class="fn" id="fn_5" href="#fr_5">[5]</a>Termed <i>valence</i> or <i>conduction</i> electrons, -these are responsible -for chemical properties, bonds with other atoms, and the conduction -of electricity. -</div><div class="fndef"><a class="fn" id="fn_6" href="#fr_6">[6]</a>See the companion Understanding the Atom booklet, <i>Power from -Radioisotopes</i>. -</div><div class="fndef"><a class="fn" id="fn_7" href="#fr_7">[7]</a>Discovered by Thomas Edison in 1883. -</div><div class="fndef"><a class="fn" id="fn_8" href="#fr_8">[8]</a>An electron -volt is equal to the kinetic energy acquired by an -electron accelerated through a potential difference of 1 volt. It is -equal to 1.6 × 10⁻¹⁹ joule. -</div><div class="fndef"><a class="fn" id="fn_9" href="#fr_9">[9]</a>In outer space, waste heat must be radiated away. The rate at -which heat is radiated is proportional to the fourth power of T<sub>c</sub> -(Stefan-Boltzmann law). -</div><div class="fndef"><a class="fn" id="fn_10" href="#fr_10">[10]</a>The newton and the weber are mks (meter-kilogram-second) units. -</div><div class="fndef"><a class="fn" id="fn_11" href="#fr_11">[11]</a>An angstrom unit (A) is a unit of distance measurement equal -to 10⁻¹⁰ meter. -</div><div class="fndef"><a class="fn" id="fn_12" href="#fr_12">[12]</a>Dielectric materials are nonconductors such as are those used -between the plates of a condenser to increase its electrical capacity. -</div><div class="fndef"><a class="fn" id="fn_13" href="#fr_13">[13]</a>The Curie point is the temperature at which a material’s crystalline -structure radically changes and becomes less orderly. -</div><div class="fndef"><a class="fn" id="fn_14" href="#fr_14">[14]</a>Ferroelectricity and ferromagnetism are very similar. The -equations describing these phenomena are almost identical except -that capacitance is replaced by its magnetic analog, inductance, -and so on. -</div> -</div> -<hr class="dwide" /> -<p class="tb">This booklet is one of the “Understanding the Atom” -Series. Comments are invited on this booklet and others -in the series; please send them to the Division of Technical -Information, U. S. Atomic Energy Commission, Washington, -D. C. 20545.</p> -<p>Published as part of the AEC’s educational assistance -program, the series includes these titles:</p> -<div class="verse"> -<p class="t0"><i>Accelerators</i></p> -<p class="t0"><i>Animals in Atomic Research</i></p> -<p class="t0"><i>Atomic Fuel</i></p> -<p class="t0"><i>Atomic Power Safety</i></p> -<p class="t0"><i>Atoms at the Science Fair</i></p> -<p class="t0"><i>Atoms in Agriculture</i></p> -<p class="t0"><i>Atoms, Nature, and Man</i></p> -<p class="t0"><i>Careers in Atomic Energy</i></p> -<p class="t0"><i>Computers</i></p> -<p class="t0"><i>Controlled Nuclear Fusion</i></p> -<p class="t0"><i>Cryogenics, The Uncommon Cold</i></p> -<p class="t0"><i>Direct Conversion of Energy</i></p> -<p class="t0"><i>Fallout From Nuclear Tests</i></p> -<p class="t0"><i>Food Preservation by Irradiation</i></p> -<p class="t0"><i>Genetic Effects of Radiation</i></p> -<p class="t0"><i>Index to the UAS Series</i></p> -<p class="t0"><i>Lasers</i></p> -<p class="t0"><i>Microstructure of Matter</i></p> -<p class="t0"><i>Neutron Activation Analysis</i></p> -<p class="t0"><i>Nondestructive Testing</i></p> -<p class="t0"><i>Nuclear Clocks</i></p> -<p class="t0"><i>Nuclear Energy for Desalting</i></p> -<p class="t0"><i>Nuclear Power and Merchant Shipping</i></p> -<p class="t0"><i>Nuclear Power Plants</i></p> -<p class="t0"><i>Nuclear Propulsion for Space</i></p> -<p class="t0"><i>Nuclear Reactors</i></p> -<p class="t0"><i>Nuclear Terms, A Brief Glossary</i></p> -<p class="t0"><i>Our Atomic World</i></p> -<p class="t0"><i>Plowshare</i></p> -<p class="t0"><i>Plutonium</i></p> -<p class="t0"><i>Power from Radioisotopes</i></p> -<p class="t0"><i>Power Reactors in Small Packages</i></p> -<p class="t0"><i>Radioactive Wastes</i></p> -<p class="t0"><i>Radioisotopes and Life Processes</i></p> -<p class="t0"><i>Radioisotopes in Industry</i></p> -<p class="t0"><i>Radioisotopes in Medicine</i></p> -<p class="t0"><i>Rare Earths</i></p> -<p class="t0"><i>Reading Resources in Atomic Energy</i></p> -<p class="t0"><i>Research Reactors</i></p> -<p class="t0"><i>SNAP, Nuclear Space Reactors</i></p> -<p class="t0"><i>Sources of Nuclear Fuel</i></p> -<p class="t0"><i>Synthetic Transuranium Elements</i></p> -<p class="t0"><i>The Atom and the Ocean</i></p> -<p class="t0"><i>The Chemistry of the Noble Gases</i></p> -<p class="t0"><i>The First Reactor</i></p> -<p class="t0"><i>Whole Body Counters</i></p> -<p class="t0"><i>Your Body and Radiation</i></p> -</div> -<p>A single copy of any one booklet, or of no more than three -different booklets, may be obtained free by writing to:</p> -<p class="center"><span class="ss">USAEC, P. 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Requests -should be made on school or library letterheads and indicate -the proposed use.</p> -<p>Students and teachers who need other material on specific -aspects of nuclear science, or references to other -reading material, may also write to the Oak Ridge address. -Requests should state the topic of interest exactly, and the -use intended.</p> -<p>In all requests, include “Zip Code” in return address.</p> -<p class="center smallest">Printed in the United States of America -<br />USAEC Division of Technical Information Extension, Oak Ridge, Tennessee -<br />May 1968</p> -<hr class="dwide" /> -<h2 id="trnotes">Transcriber’s Notes</h2> -<ul> -<li>Silently corrected a few typos.</li> -<li>Modified some image references to reflect the pageless flowable eBook format.</li> -<li>Retained publication information from the printed edition: this eBook is public-domain in the country of publication.</li> -<li>In the text versions only, text in italics is delimited by _underscores_.</li> -</ul> -<div style='display:block; margin-top:4em'>*** END OF THE PROJECT GUTENBERG EBOOK DIRECT CONVERSION OF ENERGY ***</div> -<div style='text-align:left'> - -<div style='display:block; margin:1em 0'> -Updated editions will replace the previous one—the old editions will -be renamed. -</div> - -<div style='display:block; margin:1em 0'> -Creating the works from print editions not protected by U.S. copyright -law means that no one owns a United States copyright in these works, -so the Foundation (and you!) can copy and distribute it in the United -States without permission and without paying copyright -royalties. 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