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
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

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.

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

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

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

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

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).

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.

Carnot Efficiency

In 1824 Sadi Carnot, a young French engineer, conceived of an idealized heat engine. This ideal engine had an efficiency given by

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

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.

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.

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.

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.