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diff --git a/old/66033-0.txt b/old/66033-0.txt deleted file mode 100644 index 6ca4874..0000000 --- a/old/66033-0.txt +++ /dev/null @@ -1,1966 +0,0 @@ -The Project Gutenberg eBook of Direct Conversion of Energy, by -William R. Corliss - -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 -www.gutenberg.org. 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. - -Title: Direct Conversion of Energy - -Author: William R. Corliss - -Release Date: August 11, 2021 [eBook #66033] - -Language: English - -Produced by: Stephen Hutcheson and the Online Distributed Proofreading - Team at https://www.pgdp.net - -*** START OF THE PROJECT GUTENBERG EBOOK DIRECT CONVERSION OF -ENERGY *** - - - - - - Direct Conversion of Energy - - - [Illustration: uncaptioned] - - By William R. Corliss - - - U.S. ATOMIC ENERGY COMMISSION - Division of Technical Information - - - _ONE OF A SERIES ON - UNDERSTANDING THE ATOM_ - - UNITED STATES ATOMIC ENERGY COMMISSION - _Dr. Glenn T. Seaborg, Chairman_ - _James T. Ramey_ - _Dr. Gerald F. Tape_ - _Wilfrid E. Johnson_ - - -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. - -The United States Atomic Energy Commission provides this booklet to help -you achieve such understanding. - - {Edward J. Brunenkant} - Edward J. Brunenkant - Director - Division of Technical Information - - - - - CONTENTS - - - INTRODUCTION 1 - DIRECT VERSUS DYNAMIC ENERGY CONVERSION 3 - LAWS GOVERNING ENERGY CONVERSION 8 - THERMOELECTRICITY 12 - THERMIONIC CONVERSION 16 - MAGNETOHYDRODYNAMIC CONVERSION 19 - CHEMICAL BATTERIES 22 - THE FUEL CELL: A CONTINUOUSLY FUELED BATTERY 24 - SOLAR CELLS 26 - NUCLEAR BATTERIES 28 - ADVANCED CONCEPTS 30 - SUGGESTED REFERENCES 33 - ANSWERS TO PROBLEMS 34 - - -Library of Congress Catalog Card Number: 64-61794 - - -ABOUT THE AUTHOR - - [Illustration: William R. Corliss] - -WILLIAM R. CORLISS 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 -_Propulsion Systems for Space Flight_ (McGraw-Hill 1960), _Space Probes -and Planetary Exploration_ (Van Nostrand 1965), _Mysteries of the -Universe_ (Crowell 1967), _Scientific Satellites_ (GPO 1967), and -coauthor of _Radioisotopic Power Generation_ (Prentice-Hall 1964), as -well as numerous articles and papers for technical journals and -conferences. In this series he has written _Neutron Activation -Analysis_, _Power Reactors in Small Packages_, _SNAP—Nuclear Reactor -Power in Space_, _Computers_, _Nuclear Propulsion for Space_, _Space -Radiation_, and was coauthor of _Power from Radioisotopes_. - - - - - INTRODUCTION - - -A flashlight battery supplies electricity without moving mechanical -parts. It converts the chemical energy of its contents _directly_ into -electrical energy. - -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. - -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. - -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 silent operation of direct conversion power plants is an -important advantage for many military applications. - -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[1] program. Some of these units are -presently in use powering satellites, Arctic and Antarctic weather -stations, and navigational buoys. - -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. - - - - - DIRECT VERSUS DYNAMIC ENERGY CONVERSION - - -Dominance of Dynamic Conversion - -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 _dynamic_ converters. - - -A New Level of Sophistication: Direct Conversion - -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 _direct_ conversion. - -The thermoelements shown above the turbogenerator in Figure 1 illustrate -the contrast between direct and dynamic conversion. The thermoelements -convert heat into electricity directly, without any of the intervening -machinery seen in the turbogenerator. - - [Illustration: Figure 1 _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.)_] - - DIRECT VERSUS DYNAMIC CONVERSION - SNAP 3 LESS THAN 5 WATTS - 5″ - SNAP 2 3000 WATTS - 24″ - ALTERNATOR ROTOR - ALTERNATOR STATOR - TURBINE ROTORS - NaK PUMP DIFFUSER - NaK PUMP ROTOR - MERCURY JET BOOSTER PUMP - MERCURY CENTRIFUGAL PUMP - MERCURY THRUST BEARING - MERCURY BEARING - MERCURY BEARING - - -Why is Direct Conversion Desirable? - -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. - -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. - - -How is Energy Transformed? - -What is energy and how do we change it? Energy is a fundamental concept -of science involving the capacity for doing work. _Kinetic_ or -mechanical energy is the most obvious form of energy. It is defined as - - E = ½ mv² - where - E = energy (expressed in joules) - m = mass of the moving object (in kilograms) - v = velocity (in meters per second) - -Energy can also be stored in chemical and nuclear substances or in the -water behind a dam. In these quiescent states it is called _potential_ -energy. If the potential energy in a substance is abundant and easily -released, the energy-rich substance is called a _fuel_. - - -ENERGY CONVERSION MATRIX - - [Illustration: Figure 2 _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._] - - FROM⇒ ELECTROMAGNETIC CHEMICAL NUCLEAR THERMAL KINETIC ELECTRICAL GRAVITATIONAL - (MECHANICAL) - TO⇓ - - ELECTROMAGNETIC Chemiluminescence Gamma reactions Thermal radiation Accelerating Electromagnetic Unknown - (fireflies) (Co⁵⁸ source) (hot iron) charge radiation[2] - A-bomb (cyclotron) (TV transmitter) - Phosphor[2] Electroluminescence - CHEMICAL Photosynthesis Radiation Boiling Dissociation by Electrolysis Unknown - (plants) catalysis (water/steam) radiolysis (production of - Photochemistry (hydrazine plant) Dissociation aluminum) - (photographic Ionization Battery charging - film) (cloud chamber) - NUCLEAR Gamma-neutron Unknown Unknown Unknown Unknown Unknown - reactions - (Be⁹+γ → Be⁸+n) - THERMAL Solar absorber Combustion Fission Friction Resistance-heating Unknown - (hot sidewalk) (fire) (fuel element) (brake shoes) (electric stove) - Fusion - KINETIC Radiometer Solar Muscle Radioactivity Thermal expansion Motors Falling objects - cell[2] (alpha particles) (turbines) Electrostriction - A-bomb Internal combustion (sonar transmitter) - (engines) - ELECTRICAL Photoelectricity Fuel cell[2] Nuclear Thermoelectricity[2] MHD[2][3] Unknown - (light meter) Batteries[2] battery[2] Thermionics[2] Conventional - Radio antenna Thermomagnetism[2] generator - Solar cell[2] Ferroelectricity[2] - GRAVITATIONAL Unknown Unknown Unknown Unknown Rising objects Unknown - (rockets) - - -The Energy Conversion Matrix - -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. - -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 Figure 2. Asterisks refer to direct conversion -processes, the subject matter of this booklet. - -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 _fossil fuels_) -received their initial charge of energy in the form of sunlight. - -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. 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. - -By the repeated use of the Energy Conversion Matrix in this way, we can -chart any energy transformation. - - - Problem 1 - - 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. - - - Problem 2 - - 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. - - Answers to problems are on page 34. - - - - - LAWS GOVERNING ENERGY CONVERSION - - -The Big Picture: Thermodynamics - -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. - -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. - - -You Can’t Win - -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 - - E = mc² - - where - E = energy (in joules) - m = mass (in kilograms) - c = speed of light - (300,000,000 meters per second) - -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 - - m = E/c² = (4.8 × 10⁴)/(9 × 10¹⁶) = 5.3 × 10⁻¹³ kilogram - (half a billionth of a gram) - -But, when an H-bomb is exploded, grams and even kilograms of mass are -converted to energy. - -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. - - -You Can’t Even Break Even - -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 (Figure -3). - - [Illustration: Figure 3 _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._] - - A TYPICAL HEAT ENGINE - HEAT IN - HEAT SOURCE - REACTOR, BOILER - ELECTRICITY OUT - ENERGY CONVERTER - PUMP - FLUID PIPE - RADIATOR - WASTE HEAT OUT - PRESSURE-VOLUME DIAGRAM - HEAT IN - ENERGY OUT - GAS PRESSURE - WASTE HEAT OUT - GAS VOLUME - -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. Figure -4 shows the extensive waste heat radiator on a SNAP 50 power plant -planned for deep space missions. - - [Illustration: Figure 4 _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._] - - -Carnot Efficiency - -In 1824 Sadi Carnot, a young French engineer, conceived of an idealized -heat engine. This ideal engine had an efficiency given by - - e = 1 - T_c/T_h = (T_h - T_c)/T_h - - where - e = the so-called Carnot efficiency (no units) - T_c = the temperature of the waste heat reservoir (in degrees - Kelvin, °K[4]) - T_h = the temperature of the heat source (in °K) - -Unhappily, T_c 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. - -Waste heat, since it must be rejected to the surrounding atmosphere, -outer space, or water (rivers, the ocean, etc.), must be rejected at T_c -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_c must be 300°K or more is a basic -limitation on the Carnot efficiency. The loss in efficiency with -increased T_c explains why a jet plane has a harder job taking off on a -hot day. - -One way to improve the Carnot efficiency, which is the maximum -efficiency for any heat engine, is to raise T_h as high as possible -without melting the engine. For a coal-fired electrical power plant, T_h -= 600°K and T_c = 300°K, so that - - e = 1 - 300/600 = 0.5 = 50% - -The actual efficiency is somewhat less than this ideal value because -some power is diverted to pumps and other equipment and to unavoidable -heat losses. Later on, we shall see that magnetohydrodynamic (MHD) -generators hold prospects for increasing T_h by hundreds of degrees. - -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. - - - Problem 3 - - Some space power plants contemplate using the space cabin heat (T_h = - 300°K) to drive a heat engine which rejects its waste heat to the - liquid-hydrogen rocket fuel stored at T_c = 20°K. What would be the - Carnot efficiency of this engine? - - - - - THERMOELECTRICITY - - -After 140 Years: Seebeck Makes Good - -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. - -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 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. - - -Electrons and Holes - -Let’s examine the latticework of atoms that make up any solid material. -In electrical insulators all the atoms’ outer electrons[5] 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. - - -THERMOELECTRICITY - - [Illustration: Figure 5 _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._] - - T_c WASTE HEAT OUT - ELECTRONS - LOAD - COLD JUNCTION - HOLES - ELECTRONS - _p_ SEMICONDUCTOR - _n_ SEMICONDUCTOR - HOT JUNCTION - T_h HEAT IN - Simplified Sketch of Atomic Lattice - HOLE - ELECTRON - VALENCE BONDS - SEMICONDUCTOR LATTICES - I = Impurity atom - -Figure 5 suggests the latticework of a _semiconductor_. 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 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 _n_-type semiconductor. The _n_ is for the extra -_negative_ electrons. - -A _p_- or _positive_-type semiconductor is also included in Figure 5. -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 _positive holes_. -Strangely enough, these holes can wander through the material just like -positive charges. - -The electron-hole model does not have the precision the physicist likes, -but it helps us to visualize semiconductor behavior. - -The Seebeck effect is demonstrated when pieces of _p_- and _n_-type -material are joined as shown in Figure 5. 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 _leg_ can produce a voltage across its length, but -_couples_ made from _p_ and _n_ legs are superior. - - -Practical Thermoelectric Power Generators - -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 Figure 1 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. - -Look again at the thermoelements in Figure 1 and the schematic, Figure -5. Underlying the apparent simplicity of the thermoelectric generator -are extensive development efforts. The Figure 1 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 _hot shoe_ (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. - -Nuclear thermoelectric generators that provide small amounts of -electrical power have already been launched into space aboard Department -of Defense satellites (Figure 12), installed on land stations in both -polar regions, and placed under the ocean.[6] Propane-fueled -thermoelectric generators, such as shown in Figure 6, 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. - - [Illustration: Figure 6 GENERAL PURPOSE GENERATOR - _Commercially available thermoelectric generators using propane fuel - can provide more than enough electrical power to operate a portable - TV set._ - Courtesy Minnesota Mining & Manufacturing Company.] - -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. - - - - - THERMIONIC CONVERSION - - -“Boiling” Electrons Out of Metals - -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 _load_, the -salient feature of the thermionic diode is _thermionic emission_,[7] or, -simply, the boiling-off of electrons from a hot metal surface. The -thermionic converter shown in Figure 7 powers a small motor when heated -by a torch. - -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. - -The energy required to completely detach an electron from the surface is -called the metal’s _work function_. In the case of tungsten, for -example, the work function is about 4.5 electron volts[8] of energy. - - [Illustration: Figure 7 _Vacuum type thermionic converter in - operation._ - Courtesy General Electric Company.] - -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 _electron gas_. -Some electrons will gain such high speeds that they can escape the metal -surface. This happens when their kinetic energy exceeds the metal’s work -function. - -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. - - -Reducing the Space Charge - -The emitted or boiled-off electrons between the converter plates (Figure -8) form a cloud of negative charges that will repel subsequently emitted -electrons back to the emitter plate unless counteraction is taken. To -circumvent these _space charge_ 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 _plasma_. - -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. - - [Illustration: Figure 8 THERMIONIC CONVERSION - _Thermionic converters may be flat-plate types or cylindrical types. - The cylindrical converter (a) is an experimental type for ultimate - use in nuclear reactors._ - Courtesy Los Alamos Scientific Laboratory.] - - a - INSULATOR - COOLED COLLECTOR - INCANDESCENT URANIUM - FUEL ELEMENT - CESIUM PLASMA - CIRCULATING COOLANT - VACUUM INSULATOR - CESIUM POOL - b - WASTE HEAT OUT - LOAD - ELECTRONS - LOW WORK FUNCTION COLLECTOR - T_c - CESIUM ION - PLASMA FILLED GAP - BOILED OFF ELECTRONS - HIGH WORK FUNCTION EMITTER - T_c - HEAT IN - - -Result: A Plasma Thermocouple - -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 _p_ and _n_ semiconductors. -Both the emitter and collector in the thermionic converter are good -metallic conductors rather than semiconductors, so a different tack must -be taken. - -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. - -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. - -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. - - -Thermionic Power in Outer Space - -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 (Figure -8a) 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[9] by a stream of liquid metal. Since thermionic -converters can operate at much higher temperatures than thermoelectric -couples or dynamic power plants, the radiator temperature, T_c, 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. - - - - - MAGNETOHYDRODYNAMIC CONVERSION - - -Big Word, Simple Concept - -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 _plasma_, the electrically -conducting gas. Yet they are commonly classified as _direct_ because -they replace the rotating turbogenerator of the dynamic systems with a -stationary pipe or _duct_. - - [Illustration: Figure 9 _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._] - - a - MHD Duct - HOT PLASMA IN - COOL GAS OUT TO RADIATOR - Magnetic Field - LOAD - ELECTRONS - b - CONVENTIONAL GENERATOR - SHAFT - LOAD - Magnetic Field - ARMATURE WIRE - ELECTRONS - -In the conventional dynamic generator, an electromotive force is created -in a wire that cuts through magnetic lines of force, as shown in Figure -9b. 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. - -The force on the electrons in the wire is given by - - F = qvB - - where - F = the force (in newtons[10]) - q = the charge on the electron (1.6 × 10⁻¹⁹ coulomb) - v = the wire’s velocity (in meters per second) - B = the magnetic field strength (in webers per square meter[10]) - -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. - -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 Figure 9a. 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. - -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 Figure 9a -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. - - -The Fourth State of Matter - -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 Figure 10, does not behave -consistently as any of the three familiar states of matter: solid, -liquid, or gas. Plasma has been called a _fourth state of matter_. Since -we have difficulty in containing such high temperatures on earth, we -adopt the strategy of _seeding_. 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. - - [Illustration: Figure 10 _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._ - Courtesy Texas Atomic Energy Research Foundation.] - -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. - - -MHD Power Prospects - -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. - - - - - CHEMICAL BATTERIES - - -Electricity from the Chemical Bond - -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. - -The chemical battery was the first direct conversion device. Two hundred -years ago it was the scientists’ only continuous source of electricity. - -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? - -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 _electrolyte_ that supplies positive -and negative ions. The battery derives its energy from its complement of -chemical fuel. The voltage difference arises 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. - - -Chemical Reactions Used in Batteries and Fuel Cells - -Consider the following chemical reactions of common batteries together -with some fuel cell reactions which will be discussed further in the -next section. - - Battery Reactions - Pb + PbO₂ + 2H₂SO₄ ⇔ 2PbSO₄ + 2H₂O - Fe + NiO₂ ⇔ FeO + NiO - Zn + AgO + H₂O ⇔ Ag + Zn(OH)₂ - Pb + Ag₂O ⇔ PbO + 2Ag - Fuel Cell Reactions - 2LiH ⇔ 2Li + H₂ - 2CuBr₂ ⇔ 2CuBr + Br₂ - 2H₂ + O₂ ⇔ 2H₂O (Bacon cell) - PbI₂ ⇔ Pb + I₂ - -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. - -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. - - -An Old Standby in Outer Space - -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. - - - - - THE FUEL CELL: A CONTINUOUSLY FUELED BATTERY - - -Potential Fuels - -The battery has a very close relative, the fuel cell. Unlike the battery -the fuel cell has a continuous supply of fuel. - - [Illustration: Figure 11 _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._] - - ANODE H₂ IN - CATHODE O₂ IN - ELECTRONS - LOAD - KOH ELECTROLYTE - K⁺ ION - OH⁻ ION - NEGATIVE ION FLOW - 40H⁻ + 2H₂ ⇒ 4H₂O + 4e - O₂ + 2H₂O + 4e ⇒ 40H⁻ - -The hydrogen-oxygen cell of Figure 11 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 _electrical_ cell would be more descriptive since the -physical principles are identical with those of the battery. - -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. - -So far we have dwelt on the fuel cell as a cold energy conversion device -that is _not_ 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 _regenerative_ 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. - - -Scheme for Project Apollo - -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. - -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. - - - Problem 4 - - 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? - - - - - SOLAR CELLS - - -Photons as Energy Carriers - -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. - -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[11]). Our energy conversion -device should be tuned to this wavelength. - -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 - - E = hf = hc/λ - - where - E = energy (in joules) - h = Planck’s constant (6.62 × 10⁻³⁴ joule-second) - f = the light’s frequency (in cycles per second = c/λ) - c = the velocity of light (300,000,000 meters per second) - λ = the wavelength (in meters) - -Using the fact that an angstrom unit is 10⁻¹⁰ meter, the energy of a -5500 A photon could be calculated as - - E = hf = hc/λ = (6.62 × 10⁻³⁴ × 3.00 × 10⁸)/(5.50 × 10⁻⁷) - = 3.61 × 10⁻¹⁹ joule = 2.2 electron volts - -Comparing this result, 2.2 electron volts, with the energies required to -cause atomic ionization or molecular dissociation (an electron volt or -so), we see that it is in the right range to actuate direct conversion -devices based on such phenomena. - - -Harnessing the Sun’s Energy - -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. - -About a decade ago it was found that the junction between _p_ and _n_ -semiconductors would generate electricity if illuminated. This discovery -led to the development of the _solar cell_, a thin, lopsided sandwich of -silicon semiconductors. As shown in Figure 12, 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. - - [Illustration: Figure 12 THE SOLAR CELL - _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._ - Courtesy U. S. Air Force and National Aeronautics and Space - Administration.] - - _p_ SILICON - _n_ SILICON - ELECTRON-MOLE PAIRS - JUNCTION - PHOTONS FROM SUN OR RADIOISOTOPE - ELECTRONS - ENERGY OUT - -Whenever _p-_ and _n-_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. - -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. - -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. - -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. - - - - - NUCLEAR BATTERIES - - -Energy from Nuclear Particles - -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. - -The _nuclear battery_ shown in Figure 13 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 -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. - -Nuclear batteries are simple and rugged. They generate only microamperes -of current at 10,000 to 100,000 volts. - - [Illustration: Figure 13 A NUCLEAR BATTERY - _The nuclear battery depends upon the emission of charged particles - from a surface coated with a radioisotope. The particles are - collected on another surface._] - - ENERGY OUT - INSULATOR - LAYER OF BETA-EMITTING RADIOISOTOPE - VACUUM - - -Double Conversion - -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 _double conversion_ -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. - - - - - ADVANCED CONCEPTS - - -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. - - -Ferroelectric Conversion - -Ferroelectric conversion makes use of the peculiar properties of -_dielectric_[12] 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. - -If we now place a slab of barium titanate between the two plates of an -electrical condenser and charge the condenser, as shown in Figure 14, we -have a unique way of converting heat into electricity directly. When the -barium titanate is heated above its _Curie point_[13] 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. Figure -14 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. - - [Illustration: Figure 14 FERROELECTRIC ENERGY CONVERSION - _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: ① Switch #1 closed, #2 open. - Condenser charges from battery to charge Q₂ at voltage V₁ with - capacity C₁. ② All switches open. Heat added, capacity changes from - C₁ to C₂, charge remains constant, so voltage changes from V₁ to V₂. - ③ Switch #2 closed, #1 open. Condenser discharges through load and - battery to charge Q₁ at voltage V₁ with capacity C₂. ④ 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._] - - (a) CIRCUIT - HEAT IN - BARIUM TITANATE DIELECTRIC - WASTE HEAT OUT - SWITCH #2 - LOAD - SWITCH #1 - BATTERY - (b) CYCLE DIAGRAM - charge - volts - Q₂, Q₁, E₁, E₂, V₀ V₁ V₂ - GENERAL INFORMATION: - C₂ < C₁ - V = Q/C - - -Thermomagnetic Conversion - -The _analog_[14] of ferroelectricity is ferromagnetism. A converter -employing similar principles to those in ferroelectricity can be made -using an electrical _inductance_ with a ferromagnetic core. When the -temperature of the ferromagnetic material is raised above its Curie -point, its magnetic _permeability_ 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. - -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 _second-order_ transitions, -as opposed to the _first-order_ transitions observed with heat engines -using two-phase working fluids like water/steam. - - -On the Frontier - -Other potential energy conversion schemes are being investigated by -scientists and engineers. Those listed in the Energy Conversion Matrix -(Figure 2) only scratch the surface. - -In particular, we are just learning how to manipulate photons. There are -photochemical, photoelectric, and even photomechanical transformations. -These have hardly been tapped. - -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. - -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. - -Then again, we haven’t the faintest idea of how to control gravitational -energy, but we may learn. - -The panorama is endless. - - - Problem 5 - - 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? - - - - - SUGGESTED REFERENCES - - -Articles - -Fuel Cells, Leonard G. Austin, _Scientific American_, 201: 72 (October - 1959). A survey of the different types. - -Nuclear Power in Outer Space, William R. Corliss, _Nucleonics_, 18: 58 - (August 1960). A review of all nuclear space power plants. - -Fuel Cells for Space Vehicles, M. G. Del Duca, _Astronautics_, 5: 36 - (March 1960). - -Fuel Cells, E. Gorin and H. L. Recht, _Chemical Engineering Progress_, - 55: 51 (August 1959). - -Thermionic Converters, Karl G. Hernqvist, _Nucleonics_, 17: 49 (July - 1959). - -The Revival of Thermoelectricity, Abram F. Joffe, _Scientific American_, - 199: 31 (November 1958). Excellent historical and technical - review. - -The Photovoltaic Effect and Its Utilization, P. Rappaport, _RCA Review_, - 20: 373 (September 1959). Recommended for advanced students. - -The Prospects of MHD Power Generation, Leo Steg and George W. Sutton, - _Astronautics_, 5: 22 (August 1960). - -Conversion of Heat to Electricity by Thermionic Emission, Volney C. - Wilson, _Journal of Applied Physics_, 30: 475 (April 1959). - Recommended for advanced students. - -Improved Solar Cells Planned for IMP-D, R. D. Hibben, _Aviation Week & - Space Technology_, 83: 53 (July 26, 1965). - -Thin-film Solar Cells Boost Output Ratio, P. J. Klass, _Aviation Week & - Space Technology_, 83: 67 (November 29, 1965). - - -Books - -_Direct Conversion of Heat to Electricity_, Joseph Kaye and John A. - Welsh, John Wiley & Sons, Inc., New York 10016, 1960, 387 pp., - $11.50. Recommended for advanced students. - -_Selected Papers on New Techniques for Energy Conversion_, Sumner N. - Levine, (Ed.), Dover Publications, Inc., New York 10014, 1961, - 444 pp., $3.00. A reprinting of many classical papers on direct - conversion. - -_Energy Conversion for Space Power_, Nathan W. Snyder, (Ed.), Academic - Press, Inc., New York 10003, 1961, 779 pp., $8.50. A collection of - American Rocket Society papers. - -_Man and Energy_, 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. - - -Motion Pictures - -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. - - _Energy and Work_, 0311, 29 minutes, $150. - _Mechanical Energy and Thermal Energy_, 0312, 27 minutes, $120. - _Conservation of Energy_, 0313, 27 minutes, $150. - _Photo-Electric Effect_, 0417, 28 minutes, $220. - - - - - ANSWERS TO PROBLEMS - - -First, mechanical energy drives the car’s electric generator. Second, -the electrical energy is converted into chemical energy when the battery -is recharged. - - - * * * * * * * - -From the kinetic energy equation we get - - v = √(2 E/m) - -Since the engine is 25% efficient, the energy available to propel the -car is 48,000 × 0.25 or 12,000 joules. So - - v = √(24,000/1,000) = 2√6 = 4.9 meters per second - - - * * * * * * * - - e = (300 - 20)/300 = 14/15 = 0.93 = 93% - -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 - - 1000 = 50 + 25 + ½t - t = 1850 hours = 77 days - - - * * * * * * * - - E = ½ mv² = (10⁶(0.9 × 3 × 10⁸)²)/2 = 3.6 × 10²² joules - -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 - - m = E/c² = (3.6 × 10²²)/(9 × 10¹⁶) = 4.0 × 10⁵ kg - -almost half the spaceship mass. - - - - - Footnotes - - -[1]Systems for Nuclear Auxiliary Power. - -[2]Described in this booklet. - -[3]Magnetohydrodynamics. - -[4]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). - -[5]Termed _valence_ or _conduction_ electrons, these are responsible for - chemical properties, bonds with other atoms, and the conduction of - electricity. - -[6]See the companion Understanding the Atom booklet, _Power from - Radioisotopes_. - -[7]Discovered by Thomas Edison in 1883. - -[8]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. - -[9]In outer space, waste heat must be radiated away. The rate at which - heat is radiated is proportional to the fourth power of T_c - (Stefan-Boltzmann law). - -[10]The newton and the weber are mks (meter-kilogram-second) units. - -[11]An angstrom unit (A) is a unit of distance measurement equal to - 10⁻¹⁰ meter. - -[12]Dielectric materials are nonconductors such as are those used - between the plates of a condenser to increase its electrical - capacity. - -[13]The Curie point is the temperature at which a material’s crystalline - structure radically changes and becomes less orderly. - -[14]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. - - -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. 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