ED 107 521

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Corliss, William R.Direct Conversion of Energy.Atomic Energy Commission, Washington, D. C. Office ofInformation Services.6441p.

USAEC Technical Information Center, P.O. Box 62, OakRidge, TN 37830 (cost to the general public for 1-4.copies, $0.25 ea., 5-99, $0.20 ea., 100 or mare,$0.15 ea.)

MF-$0.76 HC-$1.95 PLUS POSTAGEElectricity; Electronics; *Energy; Nuclear Physics;Physics; *Scientific Research; *ThermodynamicsAEC; Atomic Energy Commission

ABSTRACTThis publication is one of a series of information

booklets for the general public published by the United States AtomicEnergy Commission. Direct energy conversion Involves energytransformation Without moving parts. The concepts of direct anddynamic energy conversion plus the laws governing energy conversionare investigated. Among the topics discussed are: Thermoelectricity;Thermionic Conversion; Magnetohydrodynamic Conversion; ChemicalBatteries; The Fuel Cell; Solar Cells; Nuclear Batteries;Ferroelectric Conversion and Thermomagnetic Conversion. Five problemsrelated to the reading material are included. Alist of suggestedreferences concludes this report. A complete set of these bookletsmay be obtained by school and public libraries without charge.(BI).

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U S ATOMIC EN ER COMMISSIONOf flee of Information Services

Nuclear energy is playing a vital role inthe life of every man, woman, and child in theUnited States today. In the years ahead it willaffect increasingly all the peoples of the earth.It is essential that all Americans gain anunderstanding of this vital force if they are todischarge thoughtfully their responsibilities ascitizens and if they are to realize fully themyriad benefits that nuclear energy offersthem.

The United States Atomic Energy Com-mission provides this booklet to help youachieve such understanding.

3

Direct Conversion of Enemy

by William R. Corliss

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

United States Atomic Energy CommissionOffice of Information Services

Library of Congress Catalog Card Number: 64.617941964

ii

ABOUT THE AUTHOR

WILLIAM R. R. CORLISS is an atomicenergy consultant and writer with 12years of industrial experience in-cluding service as Director of Ad-vanced Programs for the MartinCompany's Nuclear Division. Mr.Corliss has B.S. and M.S. Degreesin Physics from Rensselaer Poly-technic Institute and the Universityof Colorado, respectively. He hastaught at those two institutions andat the University of Wisconsin. Heis the author of Propulsion Systemsfor Space Flight (McGraw-Hill 1960),Space Probes and Planetary Explo-ration (Van Nostrand 1965), Alys-ieriesof the Universe (Crowell 1967), Scientific Satellites (GPO1967), and coauthor of Radioisotopic Power Generation (Prentice-Hall 1964), as well as numerous articles and papers for technicaljournals and conferences. In this series he has written NeutronActivation Analysis, Power Reactors in Small Packages, SNAPNuclear Reactor Power in Space, Computers, Nuclear Propulsionfor Space, Space Radiation, and was coauthor of Power from Ra-dioisotopes.

Direct Conversion of Energy

v

By William R. Corliss

INTRODUCTION

A flashlight battery supplies electricity without movingmechanical parts. It converts the chemical energy of itscontents 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 itsintimate relation with magnetism spawned the mightyturboelectric energy converterssteam and hydroelectricturbineswhich power modern civilization.

We have improved upon Volta's batteries and have cometo rely on them as portable, usually small, power sources,but only recently has the challenge of nuclear power andspace 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 longperiods of time. Nuclear power plants using direct conver-sion techniques hold promise of surpassing conventionalpower sources in these attributes. In addition, the hiller-

6 1

ently silent operation of direct conversion power plants isan important advantage for many military applications.

The Atomic Energy Commission, the Department of De-fense, and the National Aeronautics and Space Administra-tion collectively sponsor tens of millions of dollars worthof research and development in the area of direct conver-sion each year. In particular, the Atomic Energy Commis-sion supports more than a dozen research and developmentprograms in thermoelectric and thermionic energy conver-sion in industry and at the Los Alamos Scientific Labora-tory, and other direct conversion research at Argonne Na-tional Laboratory and Brookhaven National Laboratory.Reactor and radioisotopic power plants utilizing direct con-version are being produced under the AEC's SNAP* pro-gram. Some of these units are presently in use poweringsatellites, Arctic and Antarctic weather stations, and navi-gational buoys.

Further applications are now being studied, but the costof direct conversion appears too great to permit its generaluse for electric power in the near future. Direct techniqueswill be used first where their special advantages outweighhigher cost.

*Systems for Nuclear Auxiliary Power.

2

DIRECT VERSUS DYNAMIC ENERGY CONVERSION

Dominance of Dynamic Conversion

We live in a world of motion. A main task of the engineeris to find better and more efficient ways of transformingthe energy locked in the sun's rays or in fuels, such as coaland the uranium nucleus, into energy of motion. Almost allthe world's energy is now transformed by rotating or re-ciprocating machines. We couple the energy of explodinggasoline and air to our automobile's wheels by a recipro-cating engine. The turbogenerator at a hydroelectric plantextracts energy from falling water and turns it into elec-tricity. Such rotating or reciprocating machines are calleddynamic converters.

A New Level of Sophistication: Direct Conversion

A revolution is in the making. We know now that we canforce the heat-and-electricity-carrying electrons residingin matter to do our bidding without the use of shafts andpistons. This is a leading accomplishment of modern tech-nology: energy transformation without moving parts. It iscalled direct conversion.

The thermoelements shown above the turbogenerator inFigure 1 illustrate the contrast between direct and dynamic

3

DIRECT VERSUS DYNAMIC CONVERSION

5"

SNAP 3 LESS THAN 5 WATTS

SNAP 2 3000 WATTS

ALTERNATOR ROTOR ALTERNATOR STATOR 7'

MERCURY JETBOOSTER PUMP . r_

TURBINE NA PUMPROTORS DIFFUSER

NaK PUMPROTOR

MERCURYCENTRIFUGAL PUMP

MERCURYTHRUST BEARING

MERCURY BEARING

24Wsr / MERCURY BEARING

rFigure 1 Direct conversion devices, such as the spokelike lead tel-luride thermoelectric elements inside the SNAP 3 radioisotope gener-ator shown above (courtesy Martin Company), convert heat into elec-tricity without moving parts. in contrast, the SNAP 2 dynamic convertershown below SNAP 3 (courtesy Thompson Ramo Wooldridge, Inc.) in-cludes a high-speed turbine, an electric generator, and pumps to pro-duce electricity from heat. (NaK is a liquid mixture of sodium andpotassium.)

conversion. The thermoelements convert heat into elec-tricity directly, without any of the intervening machineryseen in the turbogenerator.

4

Why is Direct Conversion Desirable?

There are places where energy conversion equipmentmust run for years without maintenance or breakdown.Also, -there are situations where the ultimate in reliabilityis required, such as on scientific satellites and particularlyon manned space flights. Direct conversion equipment seemsto offer greater reliability than dynamic conversion equip-ment for these purposes.

We should recognize that our belief in the superiority ofdirect conversion is based more on intuition than proof. Itis true that direct converters will never throw piston rodsor run out of lubricant. Yet, some satellite power failureshave been caused by the degradation of solar cells underthe bombardment of solar protons. The other types of-directconversion devices described in the following pages mayalso break down in ways as yet unknown. Still, today'sknowledge gives us hope that direct conversion will bemore reliable and trustworthy than dynamic conversion.Direct conversion equipment is begii.ning to be adopted forsmall power plants, producing less than 500 watts, designedto operate for long periods of time in outer space and underthe ocean. Some day, large central-station power plantsmay use direct conversion to improve their efficiencies and

How is Energy Transformed?

What. is energy and how do we change it? Energy is afundamental concept of science involving the capacity fordoing work. Ktnitic or mechanical energy is the most obvi-ous form of energy. It is defined as

E = 1/2 inv2

where E = energy (expressed in joules)= mass of the moving object (in kilograms)

v = velocity (in meters per second)

Energy can also be stored in chemical and nuclear sub-stances or in the water behind a dam. In these quiescentstates it is called pohnhat energy. If the potential energyin a substance is abundant and easily released, the energy-rich substance is called a fuel.

5

ENERGY CONVERSION MATRIX

TO FROM -11I CHEMICAL NUCLEAR THERMAL KINETICmELECTROMAGNErICouticAu

ELECTROMAGNETIC

Chenniuminescence(fireflies)

GAMMA reactions(Co" source)

A-bomb

Thermal radiation(hot iron)

Accelerating char(cyclotron)

Phosphor'

CHEMICAL

Photo,ynthesis(plants)

Photochemistry(pht tographic film)

Radiation catalysis(hydrazine plant)

lonizatiOn(cloud chamber)

(wate r /steam)Dissociaonti

Dissociationby ratholysis

.,

NUCLEARGamma -neutron

reactions(Be' % ) - Iles n)

Unknown Unknown Unknown

THERMALSolar absorber

(hot sidewalk)Combustion

dire)Fission

(fuel element)Fusion

Friction(brake shoes)

imam RadiometerSolar cell

MuscleRadioactivity

(alpha particles)Atsimb

Thermal expansion(turbines)

Internal combustion(engines)

. . ...,

ELECTRICAL

Photoelectricity(light meter)

Radio antennaSolar cell'

Fuel cellBatteries

Nuclear battery'

Thermoelectricity'TherimonicsThermomagnettsmFerroelectricity

MIIDConventional

generator

GRAVITATIONAL Unknown Unknown Unknown Unknown Rising objects(rockets)

Described In this booklet. f ltagnetohydrodynarnicr-

0

The Energy Conversion Matrix

Forms of energy are interchangeable. When gasoline isburned in an automobile engine, potential energy is firstturned into heat. A portion of this heat, say 25%, is thenconverted into mechanical motion. The remainder of theheat is wasted and must be removed from the engine.

A multitude of processes and devices have been foundwhich make these transformations from one form of energyto another. Many of these are listed in the blocks in Figure2. Asterisks refer to direct conversion processes, the sub-ject matter of this booklet.

6

-',1116101TATIONAL

lectromagneticradintion(TV transmitter)

lectrolumtnesceoct

Unknown

Electrolysis(productionof aluminum)

Battery chargingUnknown

Unknown Unknown

Resistance - heating(electric stove) Unknown

otorslectrostriction(sonar transmitter)

FallIn4 objects

Unknown

Unknown

Figure 2 To find how one form ofenergy is converted into another, startat the proper column and move downuntil the column intersects with the de-sired row. The box at the intersectionwill give typical conversion processesand examples.

To demonstrate how this diagram is to be read, let ilsuse it to trace the energy transformations involved in anautomobile engine. We begin with sunlight because all coaland oil deposits (the fossil fuels) received their initialcharge of energy in the form of, sunlight.

The first conversion, therefore, is from electromagneticenergy to chemical energy via photosynthesis in livingthings. We trace the transformation by moving down thecolumn marked Electromagnetic Energy until it intersectsthe horizontal row labeled Chemical Energy. There we seephotosynthesis listed in the block. The next conversion isfrom chemical energy to thermal/energy via combustion.

ki 7

We trace this by moving down the Chemical Energy columnto the Thermal Energy row; combustion is listed in the ap-propriate block. The third and final conversion takes placewhen thermal energy is transformed into mechanical en-ergy via the internal combustion engine.

By the repeated use of the Energy Conversion Matrix inthis way, we can chart any energy transformation.

Continue the automobile example by going throughthe matrix twice more showing how mechanicalenergy is converted into stored chemical energyin the car's battery.

If 1 gram of gasoline (about a tablespoonful) yields48,000 joules of thermal energy when burned withair, how fast can it make a 1000 kilogram car go?Assume the car starts from rest and its engine is25% 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 al-ways copse rued together in any transformation. This sum-wary of experience has been made into a keystone of sci-ence: the Law of Conservation of Energy and Mass. Itstates that the total amount of mass and energy cannot bealtered. This law applies to everything we do, from drivinga nail to launching a space probe. While the conscience ofthe scientist insists that he continually recheck the truth ofthis law, it remains a bulwark of science.

The Law of Conservation of Energy and Mass is alsocalled the First Law of Thermodynamics. It is related tothe Second Law of Thermodynamics, which also governs en-ergy transformations. The Second Law says, in effect, thatsome energy will unavoidably be lost in all heat engines.The first two laws of thermodynamics have been para-phrased 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 conservedindependently, and for many practical purposes we stillconsider them so conserved. But Einstein united the twowith the famous equation

E = mc2

where E = energy (in joules)in = mass (in kilograms)c = speed of light (300,000,000 meters per second)

Notice the resemblance to the kinetic energy equation shownearlier. Energy cannot appear without the disappearance ofmass. When energy is locked up in a fuel, it is stored asmass. In the gasoline combustion problem, 1 gram of gaso-line was burned with air to give 48,000 joules of energy.Einstein's equation says that in this case mass disappearedin the amount

E 4.8 x 104ni -

c2 9 x 101-- 5.3 x 10-13 kilogram(half a billionth of a gram)

But, when an H-bomb is exploded, grams and even kilo-grams of mass are converted to energy.

In direct conversion processes we do not need to worryabout these mass changes, but at each point we must makesure that all energy is accounted for. Forexample, in outerspace all energy released from fuels (even-food) must ulti-mately be radiated away to empty space. Otherwise the ve-hicle 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 Thermody-namics guides us here. Experience has shown that heat can-not be transformed into another form of energy with 100%

efficiency. We can't explain Nature's idiosyncrasies, but wehave to live with them. So, we accept the fact that every en-gine that starts out with heat must ultimately waste some of

that energy (Figure 3).9

HEAT IN

SOURCEittACIVR,

*OMER

ENERGYCONVERTER

3=0141111

ELECTRICITYOUT

WASTE HEAT OUT

FLUID PIPE

"IMMM

MIESSURE`VOLUME DIAGRAM

HEAT IN

WASTE MEAT OUT

ENERGY OUT

GAS VOLUM

A TYPICALHEAT ENGINE

Figure 3 A typical heal engineshowing heal input, useful poweroutput, and the unavoidable wasteheal that must be rejected to the.environment. A pressurevolumediagram is shown underneath for aclosed gas-turbine cycle. Circlednumbers correspond. The energyproduced is represented by theshaded area. Similar diagrams canbe made for all heal engines as anaid in studying their performance.

Direct conversion devices are no exception. Consequently,every thermoelectric element or thermionic converter willhave to provide for the disposition of waste heat. The de-signer will try, however, to make the engine efficiency highso that the waste heat will be small. Figure 4 shows theextensive waste heat radiator on a SNAP 50 power plantplanned for deep space missions.

$

10

Figure 4 Model of SNAP 50 power plant plannedfor deep space missions showing extensive wasteheat radiator. The system will provide 300 to1000 kilowatts of electrical power.

; "),

Carnot Efficiency

In 1824 Sadi Carnot, a young French engineer, conceivedof an idealized heat engine. This ideal engine had an effi-ciency given by

e 1Tc Th TcTh Th

where e = the so-called Carnot efficiency (no units))z,N,Tc = the temperature of the waste heat reservoir (in yt

degrees Kelvin, °K*)Th = the temperature of the heat source (in °K),

Unhappily, Tc cannot be made zero (and e therefore madeequal to 1, which is 100% efficiency). Physicists have shownabsolute zero to be unattainable, although they have ap-proached to within a hundredth of a degree in the laboratory.

Waste heat, since it must be rejected to the surroundingatmosphere, outer space, or water (rivers, the ocean, etc.),must be rejected at Tc greater than 300°K. The reason forthis is that these physical reservoirs have average tem-peratures around 300°K (about 80°F) themselves. The factthat Tc must be 300°K or more is a basic limitation on theCarnot efficiency. The loss in efficiency with increased Tcexplains why a jet plane has a harder job taking off on a hotday.

One way to improve the Carnot efficiency, which is themaximum efficiency for any heat engine, is to raise Th ashigh as possible without melting the engine. For a coal-fired electrical power plant, Th = 600°K and Tc = 300°K, sothat

300e = 1 - -= 0.5 = 50%00

The actual efficiency is somewhat less than this idealvalue because some power is diverte.1 to pumps and other

The KO% in temperature scale starts with zero at absolute zeroinstead of at the freezing point of water. Therefore, °K = °C + 273;°K = % (°F 460).

"P 11

equipment and to unavoidable heat losses. Later on, weshall see that magnetohydrodynamic (MHD) generators holdprospects for increasing Th by hundreds of degrees.

Everything that has been said .i.bout the Second Law ofThermodynamics (You can't even break even) applies toheat engines, where we begin with thermal energy. Supposeinstead that we start with kinetic or chemical energy andconvert 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 weshall discuss. Thus, we circumvent the Carnot efficiencylimitation by using processes to which it does not apply.

Nirvo'Z'.,:fil.MC2:- Some space !Amer plants contemplate using thew.n space cabin heat (Th = 300°K) to drive a heat en-. gine which rejects its waste heat to the liquid -

hydrogen rocket fuel stored at 20°K. Whatwould be the Carnot efficiency of this engine?

THERMOELECTRICITY

After 140 Years: Seebeck Makes Good

The oldest direct conversion heat engine is the thermo-couple. Take two different materials (typically, two dis-similar metal wires), join them, and heat the junction. Avoltage, or electromotive force, can be measured acrossthe unheated terminals. T. J. Seebeck first noticed this ef-fect in 1821 in his laboratory in Berlin, but, because of amistaken interpretation of what was involved, he did notseek any practical application for it. Only recently has anyreal progress been made in using his discovery for powerproduction. To use the analogy of A. F. Joffe, the Russianpioneer in this field, thermoelectricity lay undisturbed forover a hundred years like Sleeping Beauty. The Prince thatawoke her was the semiconductor.

As long as inefficient metal wires were used, textbookwriters were correct in asserting that thermoelectricitycould never be used for power production. The secret of12

tf

practical thermoelectricity is therefore the creation ofbetter thermoelectric materials. (Creation is the right wordsince the best materials for the purpose do not exist innature.) To perform this alchemy, we first have to under-stand the Seebeck effect.

Electrons and Holes

Let's examine the latticework of atoms that make up anysolid material. In electrical insulators all the atoms' outerelectrons* are held tightly by valence bonds to the neigh-boring atoms. In contrast, any metal has many relativelyloose electrons which can wander freely through its lattice-work. This is what makes metals good conductors.

WASTE MCAT OUT

e0 9 0.out

p SENDCONTACTOR

CIACTIMITS

TOAD.

COLOJUNCTION

0 ee90 0e IttC111.3

e 0 9som

CONDUCTOR

HOT JUNCTION

TA

LAT IN',

Simplified Sketch of AtomiclatticeTHERMOELECTRICITY

Figure 5 Top: thermoelec-Inc couple made from p- and

HOLE ELVTRON - n-type semiconductors. Bottom:the imp; ity atoms (I) are dif-ferent in each leg and contrib-ute an excess or deficiency ofvalence electrons. Heal drives

= aro both holes and electrons toward11w cold Junction.

Figure 3 suggests the latticework of a semiconductor. Itis called a semiconductor because its conductivity falls farshort of that of the metals. The few electrons available forcarrying electricity are supplied by the deliberately intro-

MO.,

*Termed valence or conduction electrons, these are responsiblefur chemical properties, bonds with other atoms, and the conduc-tion of electricity.

13

duced impurity atoms, which have more than enough elec-trons to satisfy the valence-bond requirements of the neigh-boring atoms. Without the impurities, we would have aninsulator. 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 inFigure 5. Here the impurity atom does not have enoughvalence electrons to satisfy the valence-bond needs of thesurrounding lattice atoms. The lattice has been short-changed and is, in effect, full of positive holes. Strangelyenough, these holes can wander through the material justlike positive charges.

The electron-hole model does not have the precision thephysicist likes, but it helps us to visualize semiconductorbehavior.

The Seebeck effect is demonstrated when pieces of p- andn-type material are joined as shown in Figure 5. -Heat atthe hot junction drives the loose electrons and holes towardthe cold junction. Think of the holes and electrons as gasesbeing driven through the latticework by the temperaturedifference. A positive and a negative terminal are thus pro-duced, giving us a source of power. The larger the tem-perature difference, the bigger the voltage difference. Notethat just one thermocouple leg can produce a voltage acrossits length, but couples made from p and n legs are superior.

Practical Thermoelectric Power Generators

The first nuclear-heated thermoelectric generator wasbuilt in 19M by the Atomic Energy Commission's MoundLaboratory in Miamisburg, Ohio. It used metal-wire ther-mocouples. In contrast, the SNAP 3 series thermocouplesshown in Figure 1 are thick lead telluride (PbTe) semicon-ductor cylinders about two inches long. In contrast to thethermocouple wires' efficiency of less than 1%, SNAP 3series generators have overall efficiencies exceeding 5%.This value is still low compared to the 35-40% obtained ina modern steam power plant, but SNAP 3 generators canoperate unattended in remote localities where steam plantswould be totally unacceptable.

Look again at the thermoelements in Figure 1 and theschematic, Figure 5. Underlying the apparent simplicity of14

the thermoelectric generator are extensive development ef-forts. 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 thermo-electric elements to the so-called hot shoe (metal plate) atthe bottom. The joint has to b strong, must withstand hightemperatures, and must have 'low electrical resistance. Wesee also that the elements are encased in mica sleeves toprevent chemical disturbance of the delicate balance of im-purities in the semiconductor by the surrounding gases. Afurther complication is the extreme fragility of the ele-ments, and this has yet t be overcome.

Nuclear thermoelectric generators that provide smallamounts of electrical power have already been launched intospace aboard Department of Defense satellites (Figure 12),installed on land stations in both polar regions, and placedunder the ocean.* Propane-fueled thermoelectric genera-tors, such as shown in Figure 6, are now on the market for

GENERALPURPOSEGENERATOR

Figure 6 Commerciallyavailable thermoelectricgenerators using propanefuel can provide more thanenough electrical power tooperate a portable TV set.

"er Courtesy Minnesota Mining teManufau,:t

truing Company.

use in camping equipment, in ocean buoys, and in remotespots where only a few watts of electricity are needed. TheRussians have long manufactured a kerosene lamp withthe rmoelements placed in its stack for generating power inwilderness areas.

For the present the role of thermoelectric power appearsto be one of special uses such as those just mentioned.When higher efficiencies are attained, thermoelectric powermay, one day, supplant dynamic conversion equipment incertain low-power applications regardless of location.

*See the companion Understanding the Atom booklet, Power fromRadioisotopes.

15

THERMIONIC CONVERSION

"Boiling" Electrons Out of Metals

Like the thermoelectric element, the thermionic conver-ter is a heat engine. In its simplest form it consists of twoclosely spaced metallic plates and resembles the diode radiotube. Whereas thermoelectric elements depend on heat todrive electrons and holes through semiconductors to an ex-ternal electricity-using device or load, the salient featureof the thermionic diode is thermionicenzission,* or, sim-ply, the boiling-off of electrons from a hot metal surface.The thermionic converter shown in Figure 7powers a smallmotor when heated by a torch.

Metals, as we have already seen, havean abundance of loosely bound conductionelectrons roaming the atomic latticework.These electrons are easily moved byelectric fields while within the metal; butit takes considerably more energy to boilthem out of the metal into free space.Work has to be done against the electricfields set up by the surface layer ofatoms, which have unattached valencebonds on the side facing empty space.

The energy required to completely de-tach an electron from the surface is called

Figure 7 Vacuum type the the metal's work function. In the case ofrm-ionic converge+ In opet atton. tungsten, for example, the work function

Courtesy General Electric Company. is about 4.5 electron voltst of energy.As we raise the temperature of a metal, the conduction

electrons in the metal also get hotter and move with greatervelocity. We may think of some of the electrons in a metalas forming a kind of electron gas. Some electrons will gainsuch high speeds that they can escape the metal surface.

*Discovered by Thomas Edison in 1883.tAn electron volt is equal to the kinetic energy acquired by an

electron accelerated through a potential difference of 1 volt. It isequal to 1.6 x 10-19 joule.

16

r..

This happens when their kinetic energy exceeds the metal'swork function.

Now that we have found a way to force electrons out ofthe metal, we would like to make them do useful electricalwork. To do this we have to push the electrons across thegap between the plates as well as create a voltage differ-ence to go with the hoped-for current flow.

Reducing the Space Charge

The emitted or boiled-off electrons between the conver-ter plates (Figure 8) form a amid of negative charges thatwill repel subsequently emitted electrons back to the emit-ter plate unless counteraction is taken. To circumventthese space cha) ge effects, we fill the space between theplates with a gas containing positively charged particles.These mix with the electrons and neutralize their charge.The mixture of positively and negatively charged particlesis called a plasma.

The presence of the plasma makes the gas a good con-ductor. The emitted electrons can now move easily acrossit to the collector where, to continue the gas analogy, theycondense on the cooler surface.

THERMIONIC CONVERSION

0 b

----Figure 8 Thermionic tonVerterit, ,inciy ,hilAttp:.plate types or 'cylindrical- -types., .iht'ejliiictripalconverter (a)- is. an eiperimentat.tYpe for 'uttimatc-use in nuclear reactors.

c#twfuY La jarseocknOc

Ca 4117

Result: A Plasma ThermocoupleUnless a voltage difference exists across the plates, no

external work can be done. In the thermocouple the voltagedifference was caused by the different electrical propertiesof the p and n semiconductors. Both the emitter and collec-tor in the thermionic converter are good metallic conduc-tors rather than semiconductors, so a different tack mustbe taken.

The key is the use of an emitter and a collector with dif-ferent work functions. If it takes 4.5 electron volts to forcean electron from a tungsten surface and if it regains only3.5 electron volts when it condenses on a collector with alower work function, then a voltage drop of 1 volt exists be-tween the emitter and collector.

To summarize, then, the thermionic emission of elec-trons creates the potentiality of current flow. The differ-ence in work functions makes the thermionic converter apower producer.

There is an interesting comparison that helps describethis phenomenon. Consider the emitter to be the ocean sur-face and the collector a mountain lake. The atmosphericheat engine vaporizes ocean water and carries it to thecooler mountain elevations, where it condenses as rainwhich collects in lakes. The lake water as it runs backtoward sea level then can be made to drive a hydroelectricplant with the gravitational energy it has gained in thetransit. The thermionic converter is similar in behavior:hot emitter (corresponding to the 0.n-heated ocean); coolercollector (lake); electron gas (water); different electricalvoltages (gravity). Without gravity the river would not flow,and the production of electricity would be impossible.

Thermionic Power in Outer SpaceThermionic converters for use in outer space may be

heated by the sun, by decaying radioisotopes, or by a fis-sion reactor. Thermionic converters can also be made intoconcentric cylindrical shells (Figure 8a) and wrappedaround the uranium fuel elements in nuclear reactors. Thewaste heat in this case would be carried out of the reactorto a separate radiator* by a stream of liquid metal. Since

*In outer space, waste heat must be radiated away. The rate at%%Each heat is radiated is proportional to the fourth power of T,(Stefan-Boltzmann law).

18 6-43

the rnut,nic converters can operate at much higher tempera-tures than thermoelectric couples or dynamic power plants,the radiator temperature, Tc, will also be higher. Conse-quently, space power plants using thermionic converterswill have small radiators. Once thermionic converters aredeveloped which have high reliability and long life, they willprovide the basis for a new series of lighter, more efficientspace power plants.

MAGNETOHYDRODYNAMIC CONVERSION

Big Word, Simple Concept

Magnetohydrodynamic (MHD) conversion is very unlikethermoelectric or thermionic conversion. The MHD gener-ators use high-velocity electrically conducting gases toproduce power and are generically closer to dynamic con-version concepts. The only concept they carry forwar Ifromthe preceding conversion ideas is that of the p/asma, theelectrically conducting gas. Yet they ar! commonly clas-sified as dto Lc/ because they replace the rotating turbogen-erator of the dynamic systems with a stationary pipe orduct.

a 00 CONVDMONAL GOKRAToot

Figure 9 In the duel (a), the electron:, in Ha hot plasma MMT10 the right under 171,110CMC .force P irr lin magnetic pelt( 8. Theelectrons collected b% the 710114101111 ardc 01 Ike drat (roc C10 Pled to Iluload. In a WIPe in the UP PPIIIII0 Co! a COM ['Mom( gale? afar (b) the clic--Irons are forced to the right by the Miliplelle

CoI

19

In the conventional dynamic generator, an electromotiveforce is created in a wire that cuts through magnetic linesof force, as shown in Figure 9b. It may be helpful to visual-ize the conduction electrons as leaving one end of the wireand moving to the other under the inflnence of the magneticfield.

The force on the electrons in the wire is given by

F = gyp

where F = the force (in newtons')q = the charge on the electron (1.6 x 10-19 coulomb)v = the wire's velocity (in meters per second)B = the magnetic field strength (in webers* per

square meter)

The surge of electrons along the length of the wire setsup a voltage difference across the ends of the wire. A gen-erator uses this difference to convert the kinetic energy ofthe moving wire or armature into electrical energy. Thewire is kept spinning by the shaft which is connected to aturbine driven of steam or water.

Let us try to eliminate the moving part, the generatorarmature. What we need is a moving conductor that has noshaft, no bearings, no wearing parts. The substance thatmeets these requirements is the plasma. Examine Figure9a. The MHD generator substitutes a moving, conducting gasfor the wires. Under the influence of an external magneticfield, the conduction electrons move through the plasma toone side of the duct which carries electrical power away tothe load.

The MHD generator gets its energy from an expanding,hot gas; but, unlike the turbogenerator, the heat engine andgenerator are unitedrfn the static duct. The gradualwideningof the duct shown in Figure 9a reflects the lower pressure,cooler plasma at the duct's end. Some of the plasma'sthermal energy content has been tapped off by the duct'selectrodes as electrical power.

*The newton and the weber are mks (meter-kilogram-second)units.

20

The Fourth State of Matter

Plasma Scan be created by temperatures over 2000°K. Atthis temperature many high-velocity gas atoms collide withenough energy to knock electrons off each other and thusbecome ionized. The material thus created, shown as aglowing gas in Figure 10, does not behave consistently asany of the three fa:ailiar states of matter: solid, liquid, orgas. Plasma has been called a foul lit stoic of mane) . Sincewe have difficulty in containing such high temperatures onearth, we adopt the strategy of seeding. In this technique

nr

Figure 10 Glowing plasma in experimenlal device alGeneral Aloniic's Join: Jay Ilopkhzs Laboralory. SanDiego. T-shaped plasma gun provides data for researchin thermonuclear fusion.

Courtesy Texas Atomic Energy Research Foundation.

gases that are ordinarily difficult to ionize, like helium,are made conducting ,by adding a fraction of a perceat of analkali metal such as potassium. Alkali metal atoms haveloosely bound outer electrons and quickly become ionizedat temperatures well below 2000°K.

A heliumpotassium mixture is a good enough conductorfor use in an MHD generator. In this plasma the electronsmove rapidly under the influence of the applied fields,though not as well as in metals. The positive ions move inthe opposite direction from the electrons, but the electronsare much lighter and move thousands of times faster thuscarrying the bulk of the electrical current.

21

MHD Power Prospects

The MHD duct is not a complete power plant in itself be-cause, after leaving the duct, the stream of gas must becompressed, heated, and returned to the duct. Very hightemperature materials and components must be developedfor this kind of service. Moreover, while the duct is simplein concept, it must operate at very high temperatures in thepresence of the corrosive alkali metals. This presents uswith difficult materials problems. When the problems aresolved, probably within the next decade, MHD power plantsshould be able to provide reliable power with high efficiency.They may then serve in large space power plants, and, mostimportant, they may provide cheaper electricity for generaluse through their higher temperatures and greater effi-ciencies.

CHEMICAL BATTERIES

Electricity from the Chemical Bond

If you vigorously knead a lemon to free the juices and thenstick a strip of zinc in one end and a copper strip in theother, you can measure a voltage across the strips. Elec-trons will flow through the load without the inconvenience ofhaving to supply heat. You have made yourself a chemicalbattery.

The chemical battery was the iirst direct conversion de-vice. Two hundred years ago it was the scientists' only con-tinuous source of electricity.

Since the chemical battery does not need heat for its op-eration, it is logical to ask what makes the current flow.Where does the energy come from?

The battery has no semiconductors, but, like the thermo-electric couple and the thermionic diode, it uses dissimilarmaterials for its electrodes. A conducting fluid or solid isalso present to provide for the passage of current betweenthe electrodes. In the example of the lemon, the copper andzinc are the dissimilar electrodes, and the lemon juice isthe conducting fluid or acc II uLtic that supplies positive andnegative ions. The battery derives its energy from its com-plement of chemical fuel. The voltage difference arises22

because of the different strengths of the chemical bonds.The chemical bond is basically an electrostatic one; someatoms have stronger electrical affinities than others.

Chemical Reactions Used in Batteries and Fuel Cells

Consider the following chemical reactions of commonbatteries together with some fuel cell reactions which willbe discussed further in the next section.

-Batton Reactant, Fuel CO Reactions-

Pb 1;:Pb02 + 2142SO4 c 2Pb.04 17 2140..Fe NiO44i:1%40 + Ri0 At+.2110}02,.Pb +-AO vft'llbo 3-24_

2euprct-26.213r1+:Biy._2Hv+ 2H20 ;03a664-pehY-

Pb12:---:P1) +12

In principle all these reactions are the same as thosegoing on inside the lemon, although each type of cell pro-duces a slightly different voltage because of the varyingchemical affinities of the atoms and molecules involved.There are literally hundreds of materials which can be usedfor electrolytes and electrodes.

No heat needs to be added as the electrostatic chemicalbonds are broken and remade in a battery to generate elec-trical power. The chemical reaction energy is transferredto the electrical load wEh almost 100% efficiency. TheCarnot cycle is no limitation here; only "cold" electro-static forces are in action. The reactions cannot go on for-ever, however, because the battery supplies the energy con-verter with a very limited supply of fuel. Eventually thefuel is consumed and the voltage drops to zero. This defi-ciency is remedied by the fuel cell in which fuel is suppliedcontinuously.

An Old Standby in Outer Space

Almost every satellite and space vehicle has a chemicalbattery aboard. It is not there so muchfor continuous powerproduction but as a rechargeable electrical accumulator orreservoir to provide electricity during peak loads. The bat-tery is also 'needed to store energy for use during the pe-riods when solar cells are in the earth's shadow and there-fore inoperative. In this capacity the dependable old batteryserves the most modern science very well indeed.

236 40

fw%

THE FUEL CELL:

A CONTINUOUSLY FUELED BATTERY

Potential Fuels

The battery has a very close relative, the fuel cell. Un-like the battery the fuel cell has a continuous supply of fuel.

The hydrogenoxygen cell of Figure 11 is typical of allfuel cells. It essentially burns hydrogen and oxygen to formwater. If the hydrogen and oxygen can be supplied continu-

Figure 11 This diagram shows how a hydrogenoxygen fuel cellworks. The chemical battery work:, in lire same way, except that thechemicals a, c different and apc not contmuously supplied pore ontsuletlrc cell. The water produced by 1lre 11-0 cell shown can be used fordrinking on spaceships.

ously and the excess water drained off, we can greatly ex-tend the life of the battery. The fuel cell accomplishes this.Fueled elcchical cell would be more descriptive since thephysical principles are identical with those of the battery.

Perhaps the most challenging task contemplated for thefuel cell is to bring about the consumption of raw or slightlyprocessed coal, gas, and oil fuels with atmospheric oxygen.If fuel cells can be made to use these abundant fuels, thenthe high natural conversion efficiency of the fuel cells willmake them economically superior to the lower efficiencysteam-electric plants now in commercial service.

24

So far we have dwelt on the fuel cell as a cold energy con-version device that is nol limited by the Carnot efficiency.A variation on this theme is possible. Take a hydrogeniodide (HI) cell, and heat the HI to 2000°K. Some of the HImolecules will collide at high velocities and dissociate intohydrogen and iodine: 2111,..--- H2 + 12; the higher the tempera-ture, the more the dissociation. By separating the hydrogenand iodine gases and returning them for recycling to thefuel cell where they are recombined, we have eliminatedthe fuel supply problem and created a )egenoallec fuelcell. We have, however, also reintroduced the heat engineand the Carnot cycle efficiency. The thermally regenerativefuel cell is a true heat engine using a dissociating gas asthe working fluid.

Scheme for Project Apollo

Most of the impetus for developing the fuel cell as a prac-tical device comes from the space program. The cell hasadmirable properties for space missions that are less thana few months in duration. It is a clean, quiet, vibrationlesssource of energy. Like the battery it has a high electricaloverload capacity for supplying power peaks and is easilycontrolled. It can even provide potable water for a crew ifthe Bacon H-0 cell is used. For short missions wherelarge fuel supplies are not needed, it is also among thelightest power plants available.

These compelling advantages have led the National Aero-nautics and Space Administration to choose the fuel cell forsome of the first manned space ventures. Project Apollo,the manned lunar landing mission, is the most notable ex-ample. Here the fuel cell will be not only an energy source,but also part of the ecological cycle which keeps the crewalive.

A manned space vehicle requil es an average of 2electrical kilowatts. A nuclear reactor thermo-electric plant having a mass of 1000 kilograms,including shielding, can supply this power for

...:;,T.rn'L 10,000 hours. The basic fuel cell has a mass of 50kilograms and consumes 1/2 kilogram of chemicalsper hour. The chemical containers weigh 25 kilo-grams. What is the longest mission where thetotal weight of the fuel cell will be less than theweight of the nuclear power plant?

25

SOLAR CELLS

Photons as Energy Carriers

All our fossil fuels, such as coal and oil, owe their exist-tence to the solar energy stream that has engulfed the earthfor billions of years. The power in this stream amounts toabout 1400 watts per square meter at the earth, nearlyenough to supply- an average home if all the energy wereconverted to electricity. The problem is to get the sun'srays to yield up their energy with high efficiency.

The sun's visible surface has a temperature around6000°K. Any object heated to this temperature will radiatevisible light mostly in the yellow-green portion of the spec-trum (5500 AI). Our energy conversion device should betuned to this wavelength.

The energy packets arriving from the sun are calledphotons. They travel, of course, at the speed of light, andeach carries an amount of energy given by

heE = hf =X

where E = energy (in joules)h = Planck's constant (6.62 x 10-34 joule-second)f = the light's frequency (in cycles per second = c /a)c = the velocity of light (300,000,000 meters per

second)A = the wavelength (in meters)

Using the fact that an angstrom unit is 1010 meter, the en-ergy of a 5500 A photon could be calculated as

E= - he - 6.62 x 10-34 x 3.00 x 108hfX 5.50 x 10-1

= 3.61 x 10 -19 joule = 2.2 electron volts

Comparing this result, 2.2 electron volts, with the ener-gies required to cause atomic ionization or molecular dis-

*An angstrom unit (A) is a unit of distance measurement equalto 10-10 meter.

26

sociation (an electron volt or so), we see that it is in theright range to actuate direct conversion devices based onsuch phenomena.

Harnessing the Sun's Energy

Historically, the sun's energy has most often been usedby concentrating it with a lens or mirror and then convert-ing it to heat. We could' o this and run a heat engine, but amore direct avenue is open.

About a decade ago it was found that the junction betweenp and n semiconductors would generate electricity if illumi-nated. This discovery led to the development of the solar

LS WOWS-4 i-.

THE SOLAR CELL

rigure it The photograph shows the solar cell in useon a satellite. The spherical, radioisotope, thermoelectricgenerator at the bottom of the satellite is used to supple-ment the solar cells. In the solar cell, holeelectron pairsare created by solar photons in the vicinity of a pn junc-tion. Courtesy U. S. Air Force and National Aeronautics and Space AdministratIon.

cell, a thin, lopsided sandwich of silicon semiconductors.As shown in Figure 12, the top semiconductor layer exposedto the sun is extremely thin, only 2.5 microns. Solar pho-tons can readily penetrate this layer and reach the junctionseparating it from the thick main body of the solar cell.

Whenever p- and n-type semiconductors are sandwichedtogether a voltage difference is created across the junction.The separated holes and electrons in the two semiconductorregions establish this electric field across the junction.Unfortunately, there are usually no current carriers in theimmediate vicinity of the junction so that no power is pro-duced.

The absorption of solar photons in the vicinity of the junc-tion will create current carriers, as the photons' energy istransformed into the potential energy of the hole-electronpairs. These pairs would quickly recombine and give uptheir newly acquired potential energy if the electric fieldexisting across the junction did not whisk them away to anexternal load.

The solar cell produces electricity when hole-electronpairs are formed. Any other phenomenon that creates suchpairs will also generate electricity. The source of energyis irrelevant so long as the current carriers are formednear the junction, Thus, particles emitted by radioactiveatoms can also produce electricity from solar cells, al-though too much bombardment by such particles can dam-age the cell's atomic structure and reduce its output.

The solar cell is not a heat engine. Yet it loses enoughenergy so that the sun's energy is converted at less than15',"(, efficiency. Losses commonly occur because of the re-combination of the hole-electron pairs before they can pro-duce current, the absorption of photons too far from thejunction, and the reflection of incident photons from the topsurface of the cell. Despite these losses solar cells arenow the mainstay of nonpropulsive space power.

NUCLEAR BATTERIES

Energy from Nuclear Particles

As we have seen, solar cells are able to convert the ki-netic energy of charged nuclear particles directly, into elec-tricity, but a simpler and more straightforward way of doingthis exists. This involves direct use of the flow of chargedparticles as current.

The nuclea; balk; v shown in Figure 13 performs thistrick. A central rod is coated with an electron-emitting ra-dioisotope (a beta-emitter; say, strontium-90). The high-velocity electrons emitted by the radioisotope cross the gapbetween the cylinders and are collected by a simple metallicsleeve and sent to the load. Simple, but why don't space

28

charge effects prevent the electronsfrom crossing the gap as they do inthe the rmioni c converter? The answerlies in the tact that the nuclear elec-trons have a million times more ki-netic energy than those boiled off thethe rmionic converter's emitter sur-face. Consequently, they are too pow-erful to be stopped by any space chargein the narrow gap.

Nuclear batteries are simple andrugged. They generate only microam-peres of current at 10,000 to 100,000volts.

Figure 13 The nuclear batterydepends upon the emission ofcharged particles from a surfacecoated with a radioisotope. Theparticles are collected on anothersurface.

Double Conversion

, 'NSW. ATOR'' '

AYE)! OFBE TA. EMITTING::

AO1615010k

VACUUM

In the earlier description of the energy conversion ma-trix, we saw that we could go through the energy transfor-mation process repeatedly until we obtained the kind of en-ergy we wanted. This is exemplified in a type of nuclearbattery which uses the so-called double conversion ap-proach. First, the high-velocity nuclear particles are ab-sorbed in a phosphor which emits visible light. The photonsthus produced are then absorbed in a group of strategicallyplaced solar cells, which deliver electrical power to theload. Although efficiency is lost at each energy transforma-tion, the double conversion technique still ends up with anoverall efficiency of from 1 to 5%, an acceptable value forpower supplies in the watt and milliwatt ranges.

29

ADVANCED CONCEPTS

Ferroelectric and thermomagnetic conversion are subtleconcepts which depend upon the gross alteration of a mate-rial's physical properties by the application of heat. De-vices employing such concepts are true heat engines. In-stead of the gaseous and electronic working fluids used inthe other direct conversion concepts, the ferroelectric andthermomagnetic concepts employ patterns of atoms andmolecules that are actually rearranged periodically by heat.

Ferroelectric Conversion

Ferroelectric conversion makes use of the peculiar prop-erties of dielectric* materials. Barium titanate, for exam-ple, 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 dielec-tric behavior thoroughly in this booklet; suffice it to say thatin this process heat is absorbed in a realignment of mole-cules within the barium titanate latticework.

If we now place a slab of barium titanate between the twoplates of an electrical condenser and charge the condenser,as shown in Figure 14, we have a unique way of convertingheat into electricity directly. When the barium titanate isheated above its Curie poinit of 120°C, the condenser'scapacitance is radically reduced as the dielectric constantfalls. The condenser is forced to discharge and move elec-trons through an external circuit consisting of the load andthe original source of charge. Useful electrical energy isdelivered during this step. Figure 14 shows the processschematically and mathematically. When the dielectric iscooled, waste heat is given up by the barium titanate, andthe cycle is complete.

*Dielectric materials are nonconductors such as are those usedbetween theplates of a condenser to increase its electrical capacity.

tThe Curie point is the temperature at which a material's crys-talline structure radically changes and becomes less orderly.

30 r,7;.L:td'

FERROELECTRIC ENERGY CONVERSION

141 CIRCUIT

WASTE HEAT OUT

1111.1111Mgai

SWITCH 2

TORO

SWITCH 'I

HEAT IN\ EUNUM

TtTANATECOUCTRIC

TTERY

Tol CYCLE DIACRUR

vvon:stn[Rn utroRmArioN c,< c.

Y =

Figure 14 tia tei lot It( ti it t,nttt1h1 ts 1callt on It eh it al a -pa, :101 illit)st (WU t 11 tluaigtd 11S (mitrenitre t'. It helt heal 18

410.104,I, ruput Nun, 1110111, 1 0110g, 1 1st 8, Mill (lit COpill 11111 to Ol(ldt

Nt hat wag: flu load. (FC LA: 0 Jtc tleh I t lost tl, a 2 011(11.

(101;elt Os( t lid; Aft s 11 ant ballt r I It: :ha: gt at tollagt ri Hill: Ca-

i:, If , Ct 0 .111 81111( op, 11.. that added, capaci It change., boatelan gt I enauns «»Islanl, su t ollagt changt s 11 um l'i la

tit It t lost I opt H. imult use dr s: ha: (nigh Mall and

"WI( It) ( ha) g( the at t ollagt ettpat tit C,. 0 .tii an achesop( H. ih la 1 t I« h d, cam( hangi s 51 OM C, to Ct, rhingi )enn/J)/N

t On\ Ian!, \ (1 / (diagA changes t) C l'(.; 1.h TIIhS

hut: 4;1 suppled 10 on: butlert eat It 18 hi. hut )4ift debit 10

load and built t t t e St It is 1:., Net cut gt t ont e: led is (lien L.- hi,

dirt: l en: t in (LI

Thermomagnetic Conversion

The aaiog- of ferroelectricity is ferromagnetism. Aconverter employing similar principles to those in ferro-electricity can be made using an electrical huhu !mire witha ferromagnetic core, When the temperature of the ferro-magnetic material is raised above its Curie point, its mag-netic pt t tuttlbllllt drops quickly, causing the magnetic fieldto collapse partially. Energy may be delivered to an exter-nal load during this change. Instead of energy being storedin an electrostatic field, it is stored in a magnetic field,

Ferroclectricity and ferromagnetism are very similar. Theequations describing these phenomena are almost identical ewept:hat capacitance is replaced by its magnetic analog, inductance,

roll so on.

31

Ferroelectric and thermomagnetic conversion both rep-resent a class of energy transformati,iis which involve in-ternal molecular or crystalline rearrangements of solids.There is no change of phase as in a steam engine, but theenergy changes are there nevertheless. In thermodynamicssuch internal geometrical changes are called second-ordertransitions, as opposed to the first-order transitions ob-served with heat engines using two-phase working fluidslike water/steam.

On the Frontier

Other potential energy conversion schemes are being in-vestigated by scientists and engineers. Those listed in theEnergy Conversion Matrix (Figure 2) only scratch thesurface.

In particular, we are just learning how to manipulatephotons. There are photochemical, photoelectric, and evenphotomechanical transformations. These have hardly beentapped.

Consider the reaction when an electron and its antimatterequivalent, the positron, meet. They mutually annihilateeach other in a burst of energy! This energy will be har-nessed someday.

What energy conversion device are we going to use tocompletely convert mass into energy? The energy require-ments for interstellar exploration are so great that thesevoyages will be impossible unless a new device is foundthat can completely transform mass into energy.

Then again, we haven't the faintest idea of how to controlgravitational energy, but we may learn.

The panorama is endless.

A 1 ,000,000-kilogram spaceship takes off for Alpha:4;!-4 Centauri, our nearest star, 4.3 light years away.

If it accelerates to nine-tenths the velocity of light,what is its kinetic energy? flow much fuel masswill have to be completely converted to energy toacquire this velocity?

32

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 11. Corliss, Sue/conics. 18: 58 (August1960). A review of all nuclear space power plants.

Fuel Cells for Space Vehicles. M.G. Del Duca...Is/roam/tics, 5. 36 (March 1960).Fuel Cells. E. Gorin and II. L. Becht, Chemical Engineering Progress,55; 51

(August 1959).Thermionic Converters. KarFG. liernqvist. 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 Revieu. 20. 373

(September 1959). Recommended for advanced students.The Prospects of MIII) 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 advancedstudents.

Improved Solar Cells Planned for IMP-D. R. D. Ilibben, Maxima Week & SpaceTechnology, 83: 53 (July 26. 1965).

Thin-film Solar Cells Boost Output Ratio. P. J. Klass, Aviation Week & SluiceTechnology,83: 67 (November 29, 1965).

Books

Direct Conversion of Heat to Electricity, Joseph Kaye and John A. Welsh,John Wiley & Sons. Inc.. MN/ York 10016, 1960, 387 pp.. $11.50. Recom-mended for advanced students.

Selected Papers on Neu Techniques for Energy Conversion. Sumner N. Levine.(Ed.). Dover Publications, Inc., New York 10014, 1961, 444 pp., $3.00. A re-printing of many classical papers on direct conversion.

Energy Coin crs:on for Space Pouer, Nathan W. Snyder. (Ed.), Academic Press.Inc., New York 10003. 1961. 779 pp.. $8.50. A collection of American RocketSociety papers.

Alan and Eneigy. Alfred Ilene 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 avail-able 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.Consennition of Energy, 0313. 27 minutes, $150.Photo-Electric Effect, 0417, 28 minutes, $220.

33

ANSWERS TO PROBLEMS

First, mechanical energy drives the car's elec-tric generator. Second, the electrical energy isconverted into chemical energy when the batteryis recharged.

From the kinetic energy equation we get

v = INIT. I13.

Since the engine is 25% efficient, the energy avail-able to propel the car is 48,000 x 0.25 or 12,000joules. So

v = N/24,000/1,000 = 2i = 4.9 meters per second

300 - 20 140.93 = 93%

300 15

The crossover point, t, in hours is found by equat-ing the nuclear power plant mass and that of thefuel cell with its associated fuel. The equation is

1000 = 50 + 25 + 1/2t t = 1850 hours = 77 days

E = 1/2 mv2 _ 106(0.9 x 3 x 108)22 3.6 x 1022 joules

The ship will use the same amount of energy todecelerate at its destination. Note that this calcu-lation assumes a perfect efficiency in convertingthe energy of matter annihilation into the kineticenergy of the space ship. The mass consumed is

2E 3 6 x 102

4.0 x 105 kg

almost half the spaceship mass.

34

The U. S. Atomic Energy Commission publishes this series ofinformation booklets for the general public. The booklets are listed onthe facing page by subject category.

Members of the general public may purchase booklets at thefollowing rates:

1-4 booklets SO 25 each5-99 booklets SO 20 each100 or more booklets SO 15 each

Schools and public libraries may obtain a complete set of thebooklets without charge. These requests must be made on school orlibrary stationery. Orders for booklets should be sent to:

USAECTechnical Information CenterP. 0. Box 62Oak Ridge, TN 37830

7:r.U.-.S,GOVERNMENY-PRINYINGOFFICEfte73-747197j12. 1

Chemistry

IB-303 The Atomic Fingerprint: Neu-tron Activation Analysis

18.301 The Chemistry of the NobleGases

!B+302 Cryogenics: The UncommonCold

IB-304 Nuclear ClocksIB-306 Radioisotopes in Industry18-307 Rare Earths: The Fraternal Fif-

teen18.308 Synthetic Transuranium E*1-

ments

Biology

IB-101 Animals in Atomic Research18.102 Atoms in AgricultureIB-105 Thr. Genetic Effects of Radia-

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EnergyIB-106 Radioisotopes and Life Pro-

cessesRadioisotopes in MedicineYour Body and Radiations

IB-107IB-109

The Environment

IB-201IB-20218.414

The Atom c;nd the OceanAtoms, Nature, and ManNature's Invisible Rays

General Interest

IB009 Atomic Energy and Your WorldIB-010 Atomic PioneersBook 1: From.

Ancient Greece to the 19thCentury

18.01 1 Atomic PioneersBook 2: Fromthe Mid-19th to the Early 20thCentury

I B-012 Atomic PioneersBook 3: Fromthe Late 19th to the Mid-20thCentury

18.002 A Bibliography of Basic Bookson Atomic Energy

IB004 Computers

4

.1B-008 Electricity and ManI B-005 Index to AEC Information

BookletsIB-310 Lost Worlds: Nuclear Science

and ArcheologyIB-309 The Mysterious Box: Nuclear

Science and Art1B-006 Nuclear Terms: A GlossaryIB-013 Secrets of the Past: Nuclear

Energy Applications in Art andArchaeology

IB-017 Teleoperators: Man's MachinePartners

IB-014, Worlds Within Worlds: The015, a Story of Nuclear Energy Vol-016 umes 1, 2, and 3

Physics

B-401 Accelerators8.402 Atomic Particle Detection8.403 Controlled Nuclear Fusion8.404 Direct Conversion of Energy8.410 The Electron8.405 The Elusive Neutrino8.416 Inner Space: The Structure of

the Atom8.406 LasersB -407 Microstructure of Matter8.415 The Mystery of Matter8.411 Power from Radioisotopes8.413 SpectroscopyB -412 Space Radiation

Nuclear Reactors

8.501 Atomic Fuel8.502 Atomic Power Safety8.513 Breeder Reactors8.503 The First Reactor8.505 Nuclear Power Plants8.507 Nuclear ReactorsB -510 Nuclear Reactors for Space

PowerB-508 Radioactive WastesB-511 Sources of Nuclear FuelB-512 Thorium and the Third Fuel