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Produced by the NASA Center for Aerospace Information (CASI)
NASA CR 168272

Contract No. NAS -23052

ELECTRIC CONVERSION

IN SPACE ---

A CONCEPTUAL DESIGN STUDY

September 1983

Chronos Research :.abo-ateries. Inc.

A ^

("^^C O

_C

(NASA-CB-168272) PYRCELECIRIC l.'CNVERSICN IN N84- 145E`,

SPACE: A CONCEPTUAL IEFIGN STUCY Final

Report, 21 Sep. 1982 - Z- C FeF,. 1983 (Chronos

Research Labs. , Itic. ) 78 p HC A05/MF A01 Uncla_

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OF POOR QUALITY

SUMMARY

°yroelectric conversion is potentially a very lightweight

means of providing electrical power generation in space, This

Study evaluated two conceptualized systems approaches for the

direct conversion of heat (from sunlight) into electrical energy

using the pyroelectric effect of a new class of polar polymers.

Both of the approaches involved a,arge area thin sheets of plastic

which are thermally cycled by radiative input and output of

thermal energy. The systems studied are expected to eventually

achieve efficiencies of the order of 8% and may deliver as much

as one half kilowatt per -kilogram.

In addition to potentially very high specific power, the

pyroelectric conversion approaches outlined appear to .offer low

cast per watt in the form of an easily deployed, flexible,

strong, electrically 'self-healing", and high voltage sheet.

This study assessed several potential problems such as

plasma interactions and radiation degradation and suggests

approaches to overcome them. The fundamental technological issues

for space pyroelectric conversion are 1)demonstration of the

conversion cycle with the proposed class of polymers, 2)

achievement of improved dielectric strength of the material, 3)

demonstration of acceptable plasma power losses for low altitude,

and 4)establishment of reasonable lifetime for the pyroelectric

material in the space environment. Recommendations included

an experimental demonstration 'of the pyroelectric conversion

cycle followed by studies to improve the dielectric strength of

the polymer and basic studies to discover additional pyroelectric

materials.
r

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E.,

1, Report No. 2. Government Accession No. 3. Recipient's Catalog No.

NASA OR-168272

4. Title and Subtitle 6, Report Date

September 1983

PYROELECTRIC CONVERSION IN SPACE— --'---

6, Performing Organization Code

A CONCEPTUAL DESIGN STUDY

7, Authorp) 6, Performing Organisation Repot No,

F.B. Olsen

10, Work Unit No,

8, Performing Organisation Name and Address

Chronos Research Laboratories, Inc. 11, Contract or Grant No.

3025 Via de Caballo NAS_^ 052

Olivenhain, California 9202/,

13. Type of Report and Period Covered

Contractor Report

12. Sponsoring Agency Name and Address

National Aeronautics and Space Administration 14. Sponsoring Agency Cod.

Washington, D.C. 20546

16. Supplementary Notes

Project M.>.rsger, Bernard L. Sater, Advanced Energetics Program,

NASA Lewis Research Center, Cleveland Ohio

18, Abstract

Pyroelectric conversion in potentially a very lightweight means of providing

electrical power generation in space. This study evaluated two conceptualized

systems approaches for the direct conversion of heat (from sunlight) into

electrical energy using the pyroelectric effect of a new class of polar polymers.

Both of the approaches involved large area thin sheets of plastic which are

thermally cycled by radiative input and output of thermal onergy r : The systems

studied are expected to eventually achieve efficiencies of the order of 8%

and may deliver as much as one half kilowatt per kilogram.

In addition to potentially very high specific power, the pyroelectric conversion

approaches outlined appear to offer low cost per watt in the form of an easily

deployed, flexible, strong, electrically ' t eelf-healing", and high voltage sheet.

This study !Asessed several potential problems such as plasm interactions and

radiation degradation and suggests approaches to overcome them, T146 fundamental

technological issues for space pyroelectric conversion are 1) di -tration of

the conversion cycle with the proposed class of polymers, 2) act , ment of

improved dielectric strength of the material, 3) demonstration of acceptable

plasma power losses for low altitude, and 4) establishment of reasonable lifetime

for the pyroelectric material in the space environment. Recommendations include

an experimental demonstration of the pyroelectric conversion cycle followed by

studies to improve the dielectric strength of the polymer and basic studies to

discover additional pyroelectric materials.

17. Key Words ISunnttd by Author(s)) 18, Distribution Statement

Energy conversion Unclassified - unlimited Pyroelectric

Power source

18. Security Claself, (of this report) 20, Security Clauif, (of this page) 21. No. of Pages 22, Price'

Unclassified Unclassified 76

' For sale by the National Technical Information Service, Springfield, Virginia 22161

NASA-C•168 (Rev. 10-75)
ORIMMAI~ PAG: 19 

OF POOR QUALITY

FOREWORD

This final report was prepared by Chronos Research

Laboratories, Inc. for "'SA Lew is Research Center ( LeR C) in

AS

compliance with Contract N3-23052.

The principal results were developed during the period of

performance September 21, 1982 to September 20, 1983. A final

presentation of the study results was conducted on September 20,

1983 at NASA/LeRC.

The study was conducted at Chronos by R. B. Olsen. The NASA

project manager is B. L. Sater of LeRC Advanced Energetics

Program.

For further information contact:

B. L. Sater R. B. Olsen

NASA/LeRC Chronos Research Laboratories, Inc.

21000 P.rookpark Rd. 3025 Via de Caballo

Cleveland, Ohio 44135 Olivenhain, California 92024

(216) 433-4000 x5221 (619) 756=1447

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ACK NOW LED GMEtryS

Chroaos Research Laboratories, Inc. owes thanks to several

individuals for their comments and suggestions during the

preparation of this report. R.L. Anderson of American Control

Engineering outlined the approach for power extraction. J.C.

Hicks of the Naval Ocean System Center collaborated on the early

measurements of the electrical behavior of an important

copolymer. T. Mahefkey and P. Rahilly of the Aero Propulsion

Laboratory at Wright - Patterson AFB, J.E. McCoy of NASA Johnson,

and D. Rockey of J.P.L. were all very helpful and provided needed

insight regarding the near' earth space environment. Last

alphabetically, but not least, special thanks go to B.L. Sater of

NASA/LeRC. He first suggested the thermal storage concept

described here and gave numerous quality comments during the

course of this study.

iv
TABLE OF CONTENTS

1 INTRODUCTION

THEORY OF OPERATION 1

PYROELECTRIC CONVERSION CYCLE I

SPACE STRUCTURES "5

2 THERMAL, ELECTRICAL AND

DYNAMICAL ASPECTS 8

HEAT FLOW 8

POWER EXTRACTION 13

ELECTRODE RESISTANCE 20

POINTING DYNAMICS 20

3 LOSS MECHANISMS 21

PLASMA INTERACTIONS 21

Plasma conduction loss 21

Plasma arcing 25

Unresolved questions 25

PYROELECTRIC LOSS MECHANISMS 25

Ferroelectric

Bulk conduction

hysteresis

losses 25`l

26

Conduction

Improvements

loss mechanisms 28

31

EXTRACTOR POWER LOSSES 31

POINTING SYSTEM POWER REQUIREMENTS 35

4 PERFORMANCE 36

ENERGY DENSITY 36

DIELECTRIC STRENGTH 37

Current value 37

Expected limit 37

RADIATION EFFECTS 38

ELECTRODES 41

EFFICIENCY 42

POWER TO 14ASS 43

COST ESTIMATES 49

5 SPECIAL•CONSIDERATIONS

55

'AliIATION HARDENED DESIGNS 55

Heat flow for the "can"

hardened design 57

A second approach to hardening 57

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ORIGINAL PAGE IS

OF POOR QUALITY

TABLL Or" CO ►rT%NTS (Cunt . )

5 (cont.) THERMAL ENERGY STORAGE 59

ION PROPULSION 62

ORBIT RAISING 62

TECHNOLOGICAL BARRIERS 63

CONCLUSIONS 64

a

6 REFERENCES 65

VI.
10

11

12

ORIGINAL PAGE IS

T 13 M O F F i G u n w S OF FLOOR QUALITY

ZZ"-2.

Charge versus applied voltage for

various temperatures of 0.025 cm

thick Pb Nb (Zr ,Sn

0.99 0.02 0.68 0.25

Ti ) 0 (PZST). 2

0.07 0.98 3

An electrical energy production

cycle using PZST. 4

Illustration of a spacecraft

application of radiant energy

conversion using a thin pyroelectric film (8). 6

Collector and rotating pyroelectric sheet design (CS-2). 6

Temperature extremes of a segment

of a rotating cylinder. 8

Average temperature of the CS-2

sheet as a function of concentration

ratio of the collecting optics. 12

Simple version of a switched-mode

power converter. 14

Reference points for an extraction

cycle are given on a charge-voltage

diagram. 15

Temperature excursions during a

cycle (for the CS-2 design). 15

Voltage profile for an individual

segment of the pyroelectric disk. 17

Power extraction profile for an

individual segment of the pyroelectric disk. 17

Simplified circuit design which

can accomplish both power extraction a.nd pyroelectric generator

recharging. 19

vii

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ORIGINAL PAGE IS

OF POOR QUALITY

13 Kih voltage array test 6et-4p

in clamber (17 .13

` 14 Measured current leakage from

..SPS" to plasma ( 17) . 23

15 Plasma numberdensity vs. altitude

in equatorial orbit (18). 24

16 PIX flight results (18). 24

' 17 Constant power loss contour lines

{ for various values of resistivity

and electric field. 27

i

18 Diagram showing the deformation

' of ionic potential-energy well by

' an applied electric field. 30

19 Graphs showing the effect of

electric field strength on

conduction. 30

20 Charge versus voltage plot of a

pyroelectric cycle. 32

' 21 Rerharge portions of the cycle. 33

22 Dischargeg portions P o£ the cycle. Y 33

23 Estimated lifetime of a pyroelectric

sheet as a function or altitude. 40

24 Ionizing radiation shield. 56

25 Average temperature for the pyroelectric sheet for the "cane

shielded approach. 58

26 Thermal storage element incorporated

in a pyroelectric conversion

system. 61

viii
ORlUMAL PAGE 13

OF POOR QUALITY

LIST nF TABLES

I Power to mass parameters for the

the pyroelectric sheet and

supporting stru .,;%ure only, 45

II Power to mass including collectors. 47

III Power to mass including collectors

and power extractors. 48

IV Estimated cost components for

pyroelectric converters

(CS-2, 2%) , 50

V Estimated cost components for

pyroelectric converters

(CS-2, 8%). 51

VI Estimated cost components for

pyroelectric converters

(Cylinder,l.2%). 52

VII Estimated cost components for

pyroelectric converters

(Cyli,nder,4.7%). 53

VIII System and transportation (to GEO)

cost. 54

ix
ORIGINAL PAGF- IS

5 : t o n 1 OF POOR QUALITY

INTRODUCTION

The pyroelectric effect is the flow of charge to and from

thesurfaces of a material resulting from a change in temperature

(1). This effect may be used for the conversion of heat directly

into electrical energy. In spite of its appa-r ent lack of promise

in the early 1960s this a;.proach to energy conversion has been

revived by recent advances. Some researchers are now suggesting

that this type of conversion may be more efficient than

previously believed (2), (a). In addition, its the past year,

pyroelectric generators have been built which produce more than

four orders of magnitude greater power than the devices of twenty

years ago (4), (5), (6).

Some of the work in the early 1960s centered precisely on

pyroelectric conversion in space. Experiments at NASA by Beam,

Fry and Russel (7), (8) were not encouraging primarily because of

1) the particular choice wf thermal-electrical cycle and 2) the

lack of a pyroelectr^r_^ material with high performance

characteristics. In a theoretical study done by NASA in 1965

Margosian (9) summarized.

"1ith currently known materials, this generator is too

heavy to be of interest for electric propulsion. As a source

of auxiliary power it is comparable, at best, to solar cells

in weight. if materials with substantially better

characteristics than those considered are found, competitive

performance as a power source might be possible.

A new material has been discovered which is expected to

fulfill that promise.

The purpose of the present, work was to perform a

conceptualized design study of a space power system utilizing

such pyroelectric materials. -rni,-, would permit us to establish

pertinent operational characteristics and estimate performance

factors on a first order basis. Also this would allow us to

assess the technological barriers and potential problem areas

involved with pyroelectric conversion using this film.

} 

p

THEORY OF OPERATION

^' 

K{

t 11A pyroelectric converter is a form of heat engine which

directly transforms heat energy into electrical energy. The

thermodynamics of the pyroelectric converter are analogous to the

more familiar steam engine with pressure-volume Mechanical work r

replaced by voltage-charge electrical. output. Before describing a

pyroelectric conversion system for space power an introduction to

the basic energy producing thermodynamic cycle will be helpful. 

^t ^

1
ORIC, r!A,L PAGE 15

nrr POOR QUALITY

i

Fig. 1. Charge (Polarization) versus Applied Voltage (Electric

Field) for various temperatures of 0.025cm thick Pb Nb (Zr 0,99 0.02 0.6

Sn 0.25 1 0.07)0.98C3'

2
ORIONAL PAGE IS

OF POOR QUALITY

PYROELECTkIC CONVERSION CYCLE

Theelectrical energy production cycle may be described in

terms of the charge - volth;e behavior of a ferroelectric mater a'

(FE . Figure 1 illustrates, at various temperatures, the ch.ar7,,;

voltage (polarization versus applied electric field / behavior

u ferroelectric material which may be utilized for pyroelectric

conversion, Notice that the polarization is not a single valued

function of the applied electric field. The polarization depends

in a hysteretic way upon the applied field,

For any cyclic process an area on a charge-voltage diagram

represents an electrical work since

W = j Vdq (1)

where W=electrical work, V=voltageo and q = charge. The direction

of the path (clockwise or counterclockwise) determines whether

electrical energy is produced or dissipated. A familiar example

is the hysteretic heating of a ferroelectric element. Every time

that the voltage applied to a ferroelectric sample is cycled

isothermally, 'an amount of ,electrical energy (equal to the

hysteresis loop area) is dissipated as heat. In this case the

loop is cycled in the counterclockwise sense, as indicated by the

arrows in Figure 1.

pyroelectric generator exploits the fact that, by

revwving the isothermal constraint, it is possible to reverse the

3i,ection in which the loop is cycled. The loop may be cycled in

a clockwise sense, thus resulting in the production of ele'etrical

energy from heat.

Shown in Figure 2 is an overlay of the charge-voltage

characteristics, of a ferroelectric at two different temperatures.

Figure 2 also shows how clockwise cycling may be achieved when

operating between these two temperatures. Starting in the upper

right-nand corner at high voltage and low temperature, the FE is

discharged as it is heated. After reaching a high temperature,

the FE is further discharged by reducing the externally applied

voltage. Thus, an electrical cycle may be executed in a clockwise

manner -- opposite to the hysteretic loop direction -- in which

an amount of electrical energy,

.E j Vdq = Shaded area of Figure 2, (2)

may be produced.

m

The data shown in Figures 1 and 2 correspond to measure^;sents

on a ceramic of lead zirconate modified with tin and titanium

(PZST). The discovery of this material, with its unusually good

pyroelectric properties, allowed the recent upsurge in the

'L

development of pyroelectric conversion (4), (5), (6).

3
^4 r ' 7► 6' f , PAGE is

OF POOR OUA!»iTY

. 

Fig. 2. An electrical energy production cycle may be realized by

appropriately phased changes in temperature and applied electri

Meld. The shaded area is equal to electrical energy per cycle

which may be produced by Pb0.99Nb0.02 (Zr 0.681Sn0.25'Tio.07)0.980

d4
OF Poo "U AWY

,tow, however, an even more attractive pyroelectric material

is appearing. Japanese workers (10), (11),

(12), (13), (14) have reported the first

polymer shown to have a clear-cut ferroelectric to pyraelectric

transition. Such a transition is necessary for highly active

thermodynamic medium for use in pyroconversion. The name of the

copolymer is vinylidene fluoride-trifluoroethylene and it is

denoted as P(VDF-TrFE).

Thermodynamically, the copolymer is extremely impressive.

The copolymer has a transition similar to that shown for the

ceramic PZST in Figure 1 with a few important di£fereaces. One

d difference is that the transition (ferroelectric to paroel ectric)

occurs at 70 C rather than at 150 C. Much more importantly

though, the dielectric strength of the copolymer is presently

fw a n.L3C f iXg Limas. gLal"L than that of the ceramic. Since the

electrical output energy density of the pyroelectric converter 

scales as the product of the electric field times the

polarization, the higher dielectric strength of the copolymer

represents a very important improvement. The present d-ielectric

u strength of commercial capacitor grade homopolymer F(VDF) is yet

another five times greater (500 MV/m). As described later, it is

expected that the copolymer dielectric strength will, with proper

processing achieve comparable strength.

^• Before we discuss the space pyroelectric converter concept

it is helpful to estimate the electrical output energy density 

which is possible using this new copolymer material. Since 

electrical output energy density is equal to the product of

, polarization density times electric field (corresponding to the

r. w area in Figure 2) the scale of this density is, (for the

copolymer),

-6 6 3

5X10 C/cm X 5X10 V/cm = 25 Joule/cm

, M Actual cycles do not utilize the complete energy density

calculated this way and are smaller by a factor of three or four.

Thus we estimate that P(VDF-TrFE) may produce an impressive

<<, 4 3

` 7 J/cm per cycle once its dielectric strength is improved to

that of

per

 grade P(VDF).

SPACE STRUCTURES 

Two types of structures were analyzed in this study. The

ar, first approach is illustrated in Figure 3. In its simplest

versiin a pyroelectric converter can take the form of a rotating

cylinde A given segment of the cylinder will alternately rotate

'x into and out of the direct sunlight. As a result, the temperature

of the segment will rise (during the lighted time) and fall (due 

B to reradiation) each revolution. If an externally applied voltage 

is raised and lowered in the appropriate phase relation to the 

L temperature oscillation a substantial amount of electrical power

5 
ORIGINAL PAGE to

OF POOR QUAU'rY

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DIE

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SOLAR RA01ATION

SPIN

3*ADILiZE- 2 cps

rPAYLOAD

' y--+-'^ y

THERMAL

1

RADIATION

c Figure 3 Illustration of a spacecraft, 

application of radiant energy

;. conversion using a thin Tyrops electric film (8),

COLLECTOR COLLECTOR—P,OTATiNG

TOP VIEW PYROELECT&ZC SiEET

DESIGN 100 Kw

1 

4 40 MEmDIAMTER

DISK CONCENTI—EA'TED COLLECTORS

SUPILIGHT

;t

SIDE VIEV^ FRONT VIEW

COLLECTORS EEMOVED 

I SOM TER I C 

Fi g ure A Collector and rotating gyroelectric sheet design (CS-Z). 

"' 

6
7

OF ('OOR QUALITY

may be produced. Although this rotating cylinder design is

attractive because of its simplicity, a somewhat more complex

design may yield a greater power-to-mass.

The second Structure is comprised of a rotating

pyroelectricaliy active polymer sheet (in the shape of a disk)

and , a concentrating collector of sunlight. This collector plus

sheet structure is called the CS-2 structure and is shown in

Figure 4. (A precursor to this design involving a collector,

rotating mirror and sheet was called the CS-1 structure.)

In the next section various - working" aspects of the space

pyroelectric converter are examined. Later sections discuss power

loss mechanisms, and system performance. The final section

describes several special aspects of the pyroconverter which make

it a particularly attractive electrical p.ower generator.
C

p" PCOR QUALITY

O Yf1^'YMIU PAGe is

Section 2

THERMAL, ELECTRICAL AND DY:JdMICAL ASPECTS

HEAT FLOW

' We have calculated the radiant transfer of heat into and out

of the pyroelectric converter for the two configurations.

Consider first the simp le cylinder structure. The cylinder

rotates at an angular frequency omega and a given spot on the

outer surface receives heat input from the sun during the

daylight" half of the revolution. The input heat varies as a

sinusoidal function of the angle between the incoming light and

the surface (at the particular spot) on the cylinder. It was

assumed that the inner surface of the cylinder was totally

reflecting (i.e. emissivity equal to zero). Therefore, the

various segments of the cylinder were radiatively decoupled. This

assumption simplifies the calculation greatly. The resulting

input of heat to a'given segment during the "night" portion of

the rotation is zero.

The heat output is governed by the Stefan-Boltzmann equation

4

Q(output) = a Cr T (3)

-2 -4

where a =emissivity, T =5.67E-8 watt meter kelvin and 

T=absolute temperature in degrees kelvin.

Figure 5 shows the results of a numerical calculation of the

e temperature extremes experienced by a segment of the cylinder

surface as a function of the rotational frequency. The

assumptions were; emissivity equal to 0.9 (independent of

wavelength), volume specific heat of the cylinder surface equal

to 3.0' megaJoule per cubic meter degree, pyroelectric sheet

thickness equal to 10 micrometer, input solar power of 1300 watt

per square meter. Also, an important simplifying assumption was

used. The heat associated with the phase change (ferroelectric to

paraelectric) was ignored. While this heat may he large (about 40

megaJoule per cubic meter for a Carnot limited conversion with a

temperature swing of 50 degrees Kelvin at an input temperature of

400 Kelvin ) it is about four times smaller than the sensible

heat (150 megaJoule per cubic meter for a 50 degree Kelvin

temperature excursion).

In order to achieve a temperature swing of 58 degrees the

rotational frequency must be as slow as 0.13 Hertz. Notice that

the temperature swing is approximately inversely proportional to

the frequency over the illusf;,rated parameter range. This result

coupled with an output energy density (discussed in Section 3)

which is linear in the temperature swing will yield an output 

'F power which is roughly independent of the rotational frequency

over a restricted frequency rang

8

.r,
J

ORIGINAL PAGE I3

OC POOR QUALITY

330

320

310

W

300 I'

a

oc

W 290 [L.

LU

280

270 WW

,13 .2 .5 1 2

ROTATIONAL FREQUENCY

H Z)

Figure 5 Temperature extremes of a segment

of a rotating cylinder.
ORIGINAL PtJGE^ Eg

OF POOR QUALrry

It is possible to estimate the maximum efficiency which may

be obtained using this design (within the limiting assumption

that the heat associated with the phase transition may be

ignored). The calculated temperature swing for the 0.13 Hertz

case was 58 degrees Kelvin. This temperature swing may be

compared with the temperature swing which would occur if no

reradiation were allowed during the heating process. Such a

calculation yields a temperature swing of 110 degrees Kelvin, The

ratio of the two temperature swings (58/110) sets an upper limit,

within the simplifying assumptions,to the fraction of Carnot

efficiency which may be achieved with the simple cylinder design.

Thus one may estimate that the cylinder approach is limited to an

energy conversion efficiency of about 50 per cent of the Carnot

limit on the basis of heat transfer alone. This is not

necessarily an extreme disadvantage for this design since

efficiency alone is the not the prime paramet.er for a space power

system. What this does mean though t is that the cylinder des-ign

suffers , a penalty of almost a factor of two in power-to-weight

compared with concentrator pyroelectric designs.

The second configuration for 'which we studied the heat flow

was the CS-2 (concentrator and rotating pyroelectric sheet). In

this case a concentrav).r focuses heat onto a restricted area of a

rotating pyroelectric disk. The portion of the disk which is

within the wedged shaped concentration zone heats up rapidly. As

soon as that portion of the pyroelectric disk leaves the

concentrator zone it begins to cool slowly by reradiating. Due to

the concentrator geometry and the temperature of the heat source

(i.e. the sun at about 5000k) the heat up zone takes up only a

small angular wedge of the pyroelectric disk. The computer code

;. that we have written has among its variables the collector

concentration ratio,and the angular size of the heat up zone.

For example, for the conditions;

concentration ratio= 8

frequency = 1 Hz

wedge fraction = 0.1 (heatup angle .63 radian)

sheet thickness = 10 micrometer

the resulting temperatures for the pyroelectric sheet disk are,

low temperature = 300K = 27C

high temperature = 330K = 57C

temperature swing = 30K.

An estimate of the radiative heat transfer limit on the

conversion efficiency as was shown earlier for the cylinder case

yields (30/35) 86 per cent of the Carnot limit. Thus, only about

14% of the heat leaks away during the heating portion of the

cycle as compared with nearly 50% calculated for the simple

cylinder'design. 

10
ORIGINAL PAGE YS

OF POOR QUALITY

it is important to note that an advantage that the CS-2

design has over the simple cylinder is that the CS-2 approach has

an extra design parameter,the input power per unit area (the

concentration ratio combined with the concentration zone angular

size). This extra parameter allows the designer to set the

average temperature of the converter. If we refer back to figure

1 for the cylinder design notice that the average temperature is

essentially fixed (for a spacecraft at a given distance from the

sun). This flexibility of the CS-2 design allows the spacecraft

temperature to be set to optimize the output performance of a

given pyro-electric material (the pyroelectric material should

operate in a temperature range which spans the ferroelectric to

paroelectric transition).

Figure 6 shows the average temperature of the pyroelectric

sheet (in the CS-2 design) as a function of the concentration

ratio of the collection optics. The assumptions of the plot are

that the thickness of the sheet is 10 micrometer, t'he frequency

of the thermal cycle is one Hertz, and the wedge of concentrated

ine,oming light impinges upon one tenth of the area of the sheet .

The average temperature is very insensitive to the frequency of

the thermal cycle. The amplitude of the temperature swing which

is also shown in Figure 6 does, however, depend upon the

frequency of the thermal cycle. The amplitude of the temperature

oscillations is approximately proportional to the period of the

thermal cycle.

It is useful to note that the abscissa of the figure 6

corresponds to the input thermal power of the pyroelectric

converter. Thus the average temperature of the pyroelectric sheet

is determined once this input heating power is set. Conversely,

the input thermal power must be controlled so as to result in

the proper operating temperature for a given pyroelectric

material. As with all space power systems, higher temperatures

correspond -to higher thermal power per unit area of radiator.

One important difference between the simple cylinder and the

CS-2 design is that the radiated power per unit area for the CS-2

design is double that for the cylinder approach for any given

temperature. The reason for this is that the CS-2 pyroelectric

sheet can reradiate from both its front and back sides. The

cylinder, however, has only one surface which views deep space.

11 
2

THERMAL POWER (KW/M

0.3 0.5 1 2 4 8

700

600

Soo

400

LU

300

Uj

200

Uj

100

n

1 2 4 8 16 32 6

CONCENTRATION RATIO

Figure 6 Average temperature of the CS-2

sheet as a function of concentration

ratio of the collectin , cpt-.--s.

12
ORIGINAL PAGE 18

OF POOR QUALITY

POWER EXTRACTION

The key element of the power extractor is essentially a

switching power supply. The high efficiency of such power

supplies (developed by NASA) is well known (see for example (15)•

For our purposes, switched-mode power conversion may be thought

of as simply an efficient way to transform energy at one voltage

into energy at another voltage. As this efficient energy

transformation occurs, a transfer of charge takes place. The

charge moves from one place and voltage in the circuit to a new

place and voltage in the circuit. As the charge is transferred

the number of Coulombs changes so as to (approximately) conserve

energy.

A simple version of a switched-mode power convertor is

illustrated in figure 7. To the left is a capacitor (Cl)

initially charged to some high voltage. To the right is another

capacitor (C?,) initially uncharged. If the switch is momentarily

closed, then current begins to flow through the inductor. After

only a very short time (a time less than the resonant time- for

the capacitor - inductor pair) the switch is reopened. What has

happened? Energy has been passed from the capacitor to the

inductor. In the next moment energy passes from the inductor to

tnr capacitor (C2) as the current now flows though the diode. The

net result is that energy in the form of charge on capacitor (Cl)

has been transferred to energy stored on capacitor (C2). It turns

/Jut that the transfer may be done very efficiently. Note that

simply connecting the two capacitors directly together (without

an intermediate inductor) would have resulted in a loss of 50% of

the transferred energy. With this brief introduction to switchedmode power conversion in hand we next discuss the approach for

power extraction from the pyroelectric generator.

The reference points for an extraction cycle are given on a

charge-voltage diagram in figure 8. The temperature excursions

during a cycle are shown in figure 9. The temperature history in

figure 9 corresponds to that of a point within the sheet of a

concentrator type design. The rapid increase in the temperature

between points 0 and l is indicative of the concentrated' heat

input. Similarly, the much slower temperature decline between

points 2 and 3 indicates the slower process of reradiation of

heat out to deep space.

13
OrJr7fMA1: PAM N

O' MAR QUALITY

SWITCH

I Ll r% I I e% "r ^ M

fi

Figure 7 Simple version of a switched-mode

fj_ power converter.
ORIGINAL PAGE

OF POOR Q ALITY COLD

w

C^

oc

X

U

Figure ,8

Figure 9 Temperature excursions during a

cycle (for the CS-2 design).

15

LU

LU

a

92

W

F

,a

VOLTAGE

Reference points for an extraction

cycle are given on a charge-volta$e

diagram. -

TIME

2

1
^3 ► ' 1i^ 9^' NOR IS

t C ^ POOR QUALITY

For optimum p efficient y in the transfer of Power from a

pyroelectric generator to a useable power output bust there will

be several separate pyroelectric generator segments.

The voltage profile and power extraction profile for an

r- individual segment are shown in figures 10 and 11 respectively.

The voltage during power extraction will remain approximately

constant for the period of time between points 0 and 1 ai shown

k in figure 10. For a 10 micrometer thick pyroelectric sheet which

is stressed to 300 MV/m the extraction voltage would be 3000 V.

As power extraction contiauesr the voltage will drop to some

lower point marked by point.2. At the same time as this lowest

voltage is reachedo power becomes available from the next segment

for collection. Also *

at this point further power extraction from

the first section stops and a recharge cycle begins. The geometry

and number of these sections will be arranged so that one segment

is always at a high voltage potential and in an extraction mode

at the time that another section or sections must be brought up

to high voltage by adding charge. This time is indicated by 

points 3 and 4 in figure 10.

N^

^^ Y 
W

Q

C

F

t

i

Y

i^

L

ORIGINAL ME W

OF POOR QUALITY

Figure 10 Voltage profile foc an individual

segment of the pyroelectric disk.

rvTT Iwo n rvI..a`.

1 1 IVI C

Figure 11 Power extraction profile fear an

individual segment e` the pyruelectric disk.

17

a

U

dc

00

a
d OF Imo, irly QUALITY

A simplified circuit design which can accomplish both power

extraction and pyroel g ctric generator recharging is shown in

sib^:re 12. A central. bus (point A) is fed from the several

5 individual generators (only four axe shown), Whenever a

particular generator is at high voltage and ready to begin

extraction, the SCR switch is closed thereby transferring this

section's voltage onto the central bus. Because this transferred

voltage is higher than that of the previous segment (which has

become almost depleted), the previous contributor to the bus is

back biased thereby going off (i.e. opening that switch) until it

is again f ted on by the controlling circuit. This new section

then contributes output energy until it is also depleted and

replaced by the nk%xt in a repetitive cyclical manner. During the

balance of the cyc l e time, ^.e. when a segment is not

contributing power, the segment is recharged. Figure 10

illustrates the power required for this recharge cycle (from

point 2 to 4). This recharge is accomplished by high voltage,

high frequency switching regulator as depicted by FET 1 (field

effect transistor) and inductor L1 (the recharging diodes are not

shown ). This regulator is ~ programmed"to recharge each segment

utilizing power from the central bus. Energy efficiency during

recharging should be high, perhaps 95%. It is also important to

realize that the recharging energy will be only some fraction of

the output energy. Therefore the 5% recharging energy loss

applies only to a fraction of the output energy. The switching

regulator will be programmed in order to profile both the

a recharging voltage and coulombic transfer as depicted by figure

r 10 and 11.

x Finally, the extractor power on the central bus must be

.: conditioned in order to remove the high percentage of low

frequency ripple. This can be accomplished by using high

frequency switch regulators made up of components FFT 2, L2 and

C3. The output from this power conditioning and regulating

" zircuit will be slightly lower than the lowest voltage on the

central bus. Extraction could typically reduce the voltage on a

given segment down to approximately 500 volt- resulting in a

t final user bus voltage of about 400 volts. This output

conditioning could be done at 95 % efficiency.

e

t 18
FE

l.1

ORlCMAL Pr,0f.: IS

OF POOR QUALITY

l ^.gllrc 1^ J imp Iif led : ircu ic uCJ;.wn whit

can accomplish both power extra^-

tion and pyroelectric generat.D

recharging.
OF POOR QUALITY

ELECTRODE RESISTANCE

The pyroelectric converter is a high voltage device. This

 high voltage nature is an advantage ir. terms of current

U collection. Simple calculations have been made to determine the

sheet resistance of thin electrodes on the surfaces of the

pyroelectric sheet. The results indicate that sheet resistance

effects (Joule losses) will be negligible. For the conservative

C5-2 design the losses in the electrical lead mires were more

important than the sheet losses. The lead losse,g may be set to

any desired level simply by sizing. Wire sizes of the order of 10

to 12 gauge (about a millimeter in diameter) for aluminum and

,copper, respectively; correspond to power losses which are one

percent of the output power. These wire sizes are for the

s individual segments on the pyroelectric sheet. For a 100 kilowatt

C'S-2 design ((«onservative 2% efficient version), this yields the

mass of the conducting wires at 8 to 16 kilogram. Thus sheet

resistance is unimportant ( no need to lay down elaborate current

collecting grids) and the current leads may be very lightweight

without causing undue power losses.

POINTING DYNAMICS AND DRAG

The rigid body dynamics of a spacecraft in the near earth

a'nvironment is, of course, a well studied subject. An excellent

paper on thin subject is presented by Wildman ( 16). Not only has 

Wildman studied the drag and torque on a spacecraft but he has

also considered the effects of rotation. It turns out that the

spin reducing drag is a very small effect at least for rotational

a speeds similar to those which were used in the Echo spacecraft.

The dominant torque effect which we must contend with is the

} gravity gradient torque. This torque may, in principle, be

eliminated by arranging for all of the moments of inertia of the

spacecraft to be equal.

Apart from torque effects upon the spacecraft the most

important rigid body dynamic aspect is that of altitude reduction

from straightforward drag. Photovoltaic arrays orbiting at

altitudes of the order of 500 km will have decay rates of between

0.2 and 2 km/day. Since the pyroelectric systems will be lighter

per unit area they ' re decay rates will be even faster.

Fortunately, the orbital decay rates drop by a factor of 100 if

the altitude is increased by only 200km. Therefore, though the

very lowest altitudes do not appear to be-reasonable locations

for the , use of the pyroe 'lectrically powered spacecraft, this

 restriction upon the altitude is rather minor.

^R

r^

20
YqC., C ryC}']^i^

Section 3 ()i" M R QUALITY

LOSS MECHANISMS

PLASMA INTERACTIONS

Plasma conduction loss

The two •aspects of,electrical interaction between the high

voltage of the pyroelectric spacecraft and the space plasma are

the power loss and the threshold for the occurrence of arcing.

The existing information in this area has been obtained with an

eye toward photovoltaic applications. However, several of the

results of the photovoltaic studies can be applied with caution

to the pyroelectric converter.

We begin by considering the power loss due to the finite ,

conduction of the near earth plasma. Even though the *vacuum"

above 500 km is ultra high by laboratory standards, its current

carrying capacity may be substantial. An oversimplified model of

the conduction current might read as;

1/2

J=Ne(2eV/m) =NeS (4)

where J is the current density, N is the density of electrons, e

is the charge of an electron, V „,s the voltage of the spacecraft

surface relative to the plasma, m is the mass of the particle

(either electron or ion) and S indicates the speed of the

particles as they impact the spacecraft. For low earth orbit

(LEO) conditions, this equation yields 10 amp per square meter

for a voltage of +10,000V. The corresponding power loss is, of

course, tremendous at 100 kilowatt per square meter. Even for the

heavier oxygen ion current this simple equation yields a huge

power loss of 560 watt for a negative voltage of -10,000V.

McCoy (17) gives a good account of the many factors which

limit the current, and hence the power loss, to a much smaller

value than the previous simple model of equation (4). Among these

factors are collisions, magnetic field effects, orbit-limited

current collection (arising from particle angular momentum about

the collecting probe), and space-charge effects. The space-charge

effect is particularly important for large area spacecraft. When

a large area is maintained at a high potential relative to a

plasma, the charged entities rearrange themselves so as to screen

out the hi in electric field The screen region is normally

referred

g

to as the sheath. The entire difference in potential

between the high v oltage surface and the surrounding plasma

becomes concentrated within this sheath. The total ion current

collected is limited to the ions that cross the outer boundary,

of

the sheath as a result of their existing thermal or drift motions

`! in the undisturbed plasma. The thickness and therefore the area

21
OWGINRL PAGE 1S

OF POOR QUALITY

of the outer surface of the sheath can be calculated from spacecharge equations.

It turns out that the sheath thickness depends upon the

voltage o f

the surface; the higher the voltage, the thicker the

sheath and the larger the effective collecting area. For small

objects the growth in the sheath size with voltage can be quite

substantial. Fortunately, for large area surfaces, the same

growth in the thickness of the sheath results in only a small

t increase in the effective collecting area.

r The result of all of this is that the large power losses

F which one might expect by erroneous scaling o f

small probe plasma

experiments do not occur.

McCoy (17) studied the sheath in a large area laboratory

experiment ( Fig. 13). Figure 14 shows some of his measurements of

i current collection for a 10 square meter solar power satellite

(SPS) surface in fairly high density plasmas. From the figure one

can estimate the power loss to be between 0.1 and 10 watts per

R= square meter depending upon the plasma density. The plasma

density limits correspond the range of altitudes from LEO to the

near Van Allen belt as shown in figure 15. Stevens ( 18) also

" studied simulated solar cells at high voltage in plasmas of

approximately the same conditions and found power losses of about

0.1 watt per square meter for a negative - 1000V potential and a

loss of about 10 watts per square meter for a positive + 1000V

potential. Stevens ( 18) found a good correlation between his lab

data and the PIX spacecraft test data as shown in figure 16.

Freeman (19) estimated that the power losses for a photovoltaic

^." SPS configuration operating at tens of kilovolts will be 9 watts

per square meter in LEO and about 0.1 watt per square meter in

geostationary earth orbit (GEO).

In summary, the losses are expected to be in the range of

0.1 to 10 watts per square meter. The upper limit of that range

would present a problem for the pyroelectric converter since itT

would represent a loss of between 8% and 33% of the produced 

power for the CS-2 design (and could potentially extinguish the

cylinder design output).

22 
CURRENT LEAKAGE FOR LOW ALTITUDE SATELLITES

SOFT

994..ttt ORMNAL PAGE IS

OF POOR QUALITY

^*T

ftN t(vtl 1 40 FT

GTT1At1C

7110 LCVIL

LLTV

22

^i

i

000T

INT WCL p R

r 7,

Figure

1

13 High voltage array test set-up

in chamber (17).

1p

.•ew

10

ItOtl `^

11051

_

E

2

C

u 

• Y•l

.

s

0 100 1.000

SW11 A'E ►OTCNTIAL (VOLTS) CONSTANT NV

c :.gore iw Measured current leakage `r ,, m

=• SPS' to plasma (1 7 )

2^
^ra

}wi

Vf

x

' um

i

^r o.

i

ORIGINAL PAGE IS

OF POOR QUALITY

` 100. 1000 10000 100000 ,1000 000

ALnTUOE km

Figure 15 Plasma slumber density vs. altitude in equatorial orbit (18).

INTERACTIONS OF SPACE STRUCTURES WITH ENVIRONMENT

1x10"z PLAIN DISK SOLAR ARRAY

FLIGHT DATA

 GROUND TEST RESULTS

` <

1x10"3

'

LY1D' 5

PLAIN DISK SOLAR ARRAY ^

^ r- ARC

_ 1x10-4

^ f 1xi0'6 d,P

W

1X10-5 O R 1X10-7 V10:7

d 

1XIO"6

100 500 1000 100500 1000

Iz10"8-1N

#

11-1000 -100 •500 -1000 

A PP ED MllrjF- V

Figure 16 P'2X flight results (13),

24
25

ORIGINAjl: RACP'

a'lasma arcing

Arcing was observed in McCoy's (17) laboratory experiments

for voltages exceeding either +400V or -5000V. All of those arcs,

however, were observed to originate at insulating surfaces. Note

also that McCoy's experiments were performed under LEO type

plasma conditions

In Stevens' (18) measurements (also LEO conditions) arcing

was observed at -500 to -1000V (in this case from conductors)

while no arcing was reported for positive potentials up to

+1000V. This group estimated that the threshold-for arcing at GEO

will be at -5000 to -10004V.

Reiff (20) also does - not anticipate anomalous arcing in the

GEO environment ~ . And Purvis (21) made no mention of arcing for

the ATS-5 and 5 spacecraft even though they measured charging

potentials of up to 20 kilovolts for these vehicles id G,EO.

Unresolved que's t ions

If high voltage interactions with the plasma are important

then why can we not consider embedding the high voltage within }

the bulk of the pyroelectric material? This two sheet ..approach,

with an inner high voltage electrode and a pair of grounded"

outer electrodes, is worth a bit of consideration. Pinholes in

the sheet, for the dual sheet design, may give rise to plasma

losses which are comparable to an exposed sheet due to current

focusing which apparently occurs within the space-charge sheath.

How large must the unelectroded edge region of the

pyroelectric sheet be? Large insulating surfaces on spacecraft

appear to be invitations to arcing. On the other hand, small

capacitor - edges will necessarily invite surface conduction

losses.

PYROELECTRIC LOSS MECHANISMS

Ferroelectric Hysteresis

In the course of a pyroelectric conversion cycle, the

electric polarization is constantly changing (in response to

thermal and electric field changes). Though the polarization

changes are expected to be nearly reversible, in a thermodynamic

sense, the reversibility will certainly not be perfect. To the

extent that the polarization is not perfectly reversible, some

electrical energy (which might otherwise be generated) will be

lost. This irreversibility gives rise to the ferroelectric

hysteresis loss.

It is important to emphasize that the ferroelectric

hysteresis loss which accompanies a pyroelectric conversion cycle

is not the same as the energy associated with the standard

ferroelectric hysteresis loop.
ORIGINAL. PAOLP I#

OF POOR QUALITY

The extent to which ferroelectric hysteresis effects the

power production cycle in the copolymer is currently not hnuwn.

The only detailed measurements that have been made on the

hysteresis which is relevant to pyroelectric conversion are the

measurements of Olsen and Evans (22) on ceramic lead zirconate

modified with tin and titanium (PZST). The measurements on PZST

indicate that the hysteretic loss is small, about 8% to 16% of

the output electrical power for that ceramic. This result is only

suggestive of what the case might be for the copolymer,

One experiment which has been performed on the copolymer

gives a small hint as to the possible extent of hysteresis loss

during the pyroelectric cycle. While in the course of a standard

ferroelectric hysteresis study (wherein the electric displacement

is measured versus a cyclically applied electric field) we have

measured the displacement for small, unipolar, excursions of the

electric field. The ferroelectric hysteresis associated with

these unipolar-excursions is very small even for the case where

the minimum electric field approaches zero. Such isothermal

unipolar excursions, of course, do not address the question about,

the reversibility of the polarization under varying thermal

conditions. It is possible that the low voltage limit for the

repoling (polarization increase during cooling) process of the

pyroelectric conversion cycle may be very nearly equal the to

high voltage limit. Though we currently do not expect this to be

the case, only an experiment can tell for certain. Rather than a

ratas'trophic reduction of the predicted output energy density we

expect that the impact of the ferroelectric hysteresis loss upon

the converter to be minimal, perhaps amounting to 10% of the

output power. 

Bulk Conduction Losses

The power loss due to electrical conduction through the bulk

of the pyroelectric material is the Joule loss represented by

2

Joule loss = E / p (5)

2

where the Joule loss power (V /R in unnormalized terms) is stated

per unit volume of pyroelectric material, E is the electric

field, and p is the bulk resistivity. The equation looks very

simple but the resistivity is a function of both temperature and

electric field (the materials are not generally simple ohmics at

the high electric fields expected for the pyroelectric

converter). Calculation of the Joule loss is further complicated

by the fact that the temperature and field are continually

changing during the course of the conversion cycle.

To get some feel for the power loss numbers consider figure

17. Plotted in figure 17 are constant power loss contour lines

for various values of resistivity and electric f=ield. The power

Loss values are denoted next to each line. Note that the power

loss is greater for larger fields and smaller resistivities. also

26
14

tr TU

13

0

12

0

.j

^A ^

10

9

Uj

Wn

OF POOR QUALITY

5X10 3 WATT/M 3

5X104

SX110 5

5X 106

X

X

0 1 2 3 4

E FIELD (100mv/m)

Figure 17 Constant power loss contour lines

9J ,

I . for var i ous values of ressL.J.VLky -*L

and electric f ie ld.

27
ORIGINAL PAGE IS

OF POOR QUALITY

note that the solid contour lines are shown for each factor of

ten in the power loss. For comparison the projected output power

density points (shown as x in the figure) are included. The

dotted line connecting the output power points is, of course, not

a constant power line but rather a linear function of the

electric field (according to the approximation of Section 4)

using a temperature swing of 50K and an assumed frequency of one

Hertz.

Figure 17 is useful for picking target resistivities for

various operating fields. For example, let's say that we desire

+ the Joule losses to be less than one per cent of the output power

density. Once the power density is located on the figure, then

the desired resistivity is that which lies two contour lines

above.

11 7

The circle at a resistivity of 3x10 ohm-meter and 5x10 V/M

corresponds to the present value of resistivity and electric

r_ field strength. It is seen from figure 17 that if it is possible

jr achieve about the same resistivity at a 500 megavolt per

meter electric field then the Joule loss may be of the order of

• ten per cent. Actually, when the thermal cycle timing details are

taken into account, it is realized that for CS-2 approach (in

^. which the pyroelectric material spends only about one tenth of

the time at high fields) the Joule loss may be more on the order

of one percent.

Conduction Loss Mechanisms

F^ Recent studies on electrical conduction in FVDF by Oka and

Koizumi (23), Saito et al. (24) and by Olsen et al. (25) are F

cotisistent with the concept of ionic conduction. The origin of

the impurity ions in polymers is considered to be mainly

polymerization catalysts and decomposition products of the

E` polymer. Trace amounts of other ionic contaminants may be picked

up during the various synthesis steps. It has been suggested (26)

that Na, Al and Si are major contributors to the ionic

conductivity in a polyimide. Also, Kosaki et al. (27) concluded

that fluorine ions are the main charge carriers in PVDF, Osaki et

al. (28) pointed out that Na, Ca, Al, S, and C1 are present in

k PV.DF as trace impurities.

The following excerpt from Blythe (29) elucidates several 

important aspects of ionic conduction in polymers. 

The most definitive evidence for ionic conduction is the

detection of electrolysis products formed on discharge of the

ions as they arrive at the electrodes. Unfortunately, the very

low level of conductivity in polymers generally precludes such

- 9

-1

detection. Even at a conductivity of 10 mho m , and we must

bear in mind that many polymers exhibit conductivities several

orders of magnitude lower than this, 100 V applied across a

^^ 

28
OMIQPR ,I: ppj^c Iq

0-F P C-IR Q';ALL 'Y

2

specimen 100 mm in area and 1 mm thick would only produbo about

-11 3

10 m of gas at NTP per hour. We therefore have to rely on

rather more indirect means of elucidating the mechanism of

conduction.

When the conductivity in question is very low, there are

other experimental difficulties too. On application of a step

voltage :across a specimen, the initial current may be dominated

by a displacement current due to polarization of the material.

Since some dipole orientation - may be very stow to reach

equilibrium 'the displacement current can swamp a small

conduction current for a long time. Taking into account the drift

characteristics of a practical measurement system, we can

appreciate that in extreme cases a steady current reading will

never be obtained, and a direct measurement of conductivi4 will

not be possible.

Some physical insight into what a conductivity of around 10

-1

mho m or less entails may be gauged through the equation.

conductivity = q n p (6)

Where q charge, n= concentration and it drift mobility of the

carriers. We can tentatively use a mobility value of the order of

-9 2 -1 -1

10 m V s , as found for ionic carriers in hydrocarbon liquids

at room temperature. Although we would expect the true value for

solid polymers to be lower, it should not differ by more than a

factor of 100 or so for small ions. Then, assuming that the ions

carry a single electronic charge, substitution in `equation

19 -3

(6) gives a carrier concentration of 10 m . Comparing this

with the typical number density of molecular repeat units

28 -3

(nominal molecular weight=100) of about 10 m , we can see that

an exceedingly low carrier concentration is implied. This means

than degrees of ionic impurities which may be totally ignored in

the context of other properties may have a significant effect on

conductivity.

Thus impurity levels as small as one part per billion may be

important.

With the foregoing cautions regarding experimental

difficulties in mind, consider a particular method for obtaining

evidence for ionic conduction. A simple model of Mott and Gurney

(30) describes the dependence of current on applied voltage as

J = A sinh(eaE/2Kt) ( 7 )

where J is the current density, eaE/2 is the change in barrier

heights described in figure 1$, "a" is the distance between

neighboring potential wells, a is the charge of an electron, E

29
0

^ c

0

7

8

ORIGINAL. PACUr to

OF POOR QUALITY

l;d^rty;y 4`

s

F

A U'

4

i

Eioctn¢ Arid. Z

7

Figure 18 Diagram showing the deformation

c Of ionic p otential-energy well by

an applied electric field.

-0--'95 C

^o ~

,,44*0

WC

51 0-0-0-

---- Ohmb law

-- .4 tinh { y . E)

•v

+u OU SU IUU

t (MvM 1)

Figure 19 Graphs showing the effect of

electric field sz:L crLgt .gin

conduction.

1

30
OPICIMAL PAGE ig

OF POOk QUALITY

is an applied electric field, K is the Boltzmann constant, and

the constant A contains the information for the vibrational

attempt frequency as well as the concentration of the ions.

Figure 19 shows the fit of the data of Kosaki et a.. (31)

for a sample of polyvinyl chloride) to the equation for ionic

conduction. Notice that for this PVC case the actual conduction

at 67 degrees C and 100MV/m is approaching a value ten times

greater than the ohmic value which one might have projected from

the lower field data. We suggest that the ionic conduction

equation may be very useful in projecting conduction losses at

high fields (beyond the presently attainable electric field

strengths) once the moderate field data for the copolymer have

been obtained.

Improvements

Once the conduction of the pyroelectric material is

quantified at high electric field for various temperatures it may

become apparent that the corresponding Joule losses are

unacceptably high. If such turns out to be the case then steps

must be taken to improve the resistivity of the pyroelectric

material. The 'most obvious improvements may be those associated

with the polymer, synthesis. A careful analysis of the chemistry

of the synthesis process will, surely result in important

improvements in the insulating properties of the pyroelectric

materials produced.

To dates at least two post-synthesis techniques have been

successfully applied to "clean-up" partially the ionic impurities

in PVDF. One technique was electrodialysis in pure water (23).

Another technique was high temperature vacuum bake out (25). Both

techniques have shown at least one order of magnitude increase in

the resistivity of the polymer,

EXTRACTOR POWER LOSSES

There are three separable types of losses associated with

the solid state power extraction-conditioning system. The first

is associated with charge up portions of the thermodynamic cycle,

the second corresponds to the discharge (extraction) legs of the

y cycle and the third relates to conditioning the output for

[ consumption by the load. We will discuss only the first two

losses here since the third loss is determined by the

s requirements of an (as yet) unspecified load.

' In order to estimate the extractor power losses we must

first describe the electrical energy flows into and out of the

f pyroelectric material. Figure 20 shows the charge versus voltage

plot of the pyroelectric cycle. The charge up portion of the

cycle begins during the low voltage cool-down between points 2

and 3. During that portions an external source must transfer

{ electrical energy to the pyroelectric. The area A in figure 21 is

equal to the electrical energy which is delivered to the

31
3

ORIGINAL PAaV 11,

OF POOR QI1111. I T N

COLD 4

c^

e

V

2

O 

1

►R

VOLTAGE

Figure 20 Charge versus voltage plot ^f a i. pyrue.ectr:c cycle. f

k

S

r

32

r

r
Q

V

f

i

LLI

rLU

V

k

Q

♦

Z

V

VOLTAGE

Figure 21 Recharge portions of the cycle.

VOLTAGE

i

r Figure 22 Discharge portions of the cycle.
ORIGINAL PAGe fey

OF POOR QUALITY

pyroelectric during the first portion of the charge up. Further

charging of the pyroelectric: occurs during the cool isothermal

voltage ramp-up between points 3 and 4. once again an external

circuit must supply electrical energy, area B, to the

pyroelectric. The sum of the electrical energies required to

charge-up the pyroelectric, area A plus area B, is loosely

analogous to the mechanical compression energy which is necessary

in steam turbine cycles.

The electrical energy investment used to charge up the

system must necessarily be more than repaid during the remainder

of the cycle in order for the pyroelectric converter to be a net

producer of electrical energy.

Power output of the pyroelectric begins with the high

voltage warm-up which occurs between points 4 (or equivalently 0)

and 1. The electrical energy extracted from the pyroelectric to

the external circuit is indicated by the area C in figure 22.

^- Additional energy is extracted from the pyroelectric to the

xF external circuit in the hot isothermal voltage reduction from

point 1 to * point 2 and the energy transferred is represented by

the area D 4, n figure 22. The electrical energy output from the

pyroelectric during the discharge is equal to areas C plus D. The

net electrical output of the cycle is the difference of the

discharge energy (area C and D figure 22) minus the charge up

energy (area A and B figure 21). The discharge energy exceeds the

charge up energy because a portion of the thermal energy which 

entered the pyroelectric material duri.t t,; the high voltage warm up 

leg was converted to electrical energy. The net energy of the

process is the area shaded in figure 20 (which is identically the

same area as had been described earlier when the cycle was first

introduced).

What have we gained from this detailed description? First,

the electrical energy flows, into and out from the pyroelectric

material, have been explicitly shown. second, we may now more

r easily describe the electrical circuit losses associated with the

injection and extraction of the electrical energy to and from the

pyroelectric. As described earlier, the charge transfers may be

accomplished with an efficiency of 95% or greater by utilization

of switched-mode power supply techniques. The energy loss then

may be approximated as 5% of the energy which is circulated into

and out of the pyroelectric material during the cycle. If the

eventual cycle obtained has the appearance of the cycle in figure

4 then the total injection and extraction loss will be about 10%

compared with the net output energy. Measurements of pyroelectric

materials under actual power production conditions must be

obtained, however, before the number can be refined.

ti

34
.gw

i@

A!;

.i

Y

35

r

r

1J

t

r

a

7

a^

ORIGINAL PAGE 1!

OF POOR QUALITY

POINTING SYSTEM POWER REQUIREMENTS

Some power will undoubtedly be required in order to maintain

the pyroelectric converter pointed toward the Sun and properly

oriented with respect to the Earth. Two techniques for

maintaining the spacecraft orientation which appear attractive

are electric propulsion and controllable solar reflecting panels

mounted on long arms. The magnitudes of the powers required

appear to be small (less than one percent of the output power)

but should be studied further.
OR16INAE: PAGIE Ig

OF POOH QUALITY

Section 4

PERFORMANCE

ENERGY DENSITY

Recent measurements of the charge versus applied voltage

(hysteresis) loops of the pyroelectric copolymer have been made

at Chronos in collaboration with Dr. J. C. Hicks of the Naval

Ocean System Center (NOSC). Preliminary results yield a

g displacement maximum of about 3 microcoulombs per square cm (30

millicoulombs per square meter) at an electric field maximum of

about 500 kv per cm (50 megavolt per meter).

Satiiple preparation methods limit the dielectric strength

and, indirectly, the maximum displacement (the hysteresis loops

are not tn "hard" saturation).

No power production loop (actual power cycle) has been

:measured yet (the subject of a future experiment) but, the

polarization measurements versus temperature indicate that about

0.5 Joule per cubic centimeter-cycle (0 9 5 megaJoule per cubic

meter-cycle) of electrical energy should be obtainable (high

? temperature of 1000 and a low temperature of 40C) even under the 

present electric field limitations. 

The output energy density is obs.erved to be approximately a

linear function of the temperature swing within the temperature

band between 40 and 100 degrees Celsius.

If the dielectric strength of this material.can be increased

to 3.5 MV/cm (350 MV/M), which we have already achieved at 90C.

with the related homopolymer PVDF, then the energy density will

approach 5J/cc-cyle (5 megaJoule per cubic meter-cycle). This

value is within the range (2-7 megaJoule per cubic meter-cycle)

which we had earlier predicted. The extrapolation to the higher

fields is linear in the electric field. Though a high field

saturation of the energy density is actually anticipated, an

increase in the polarization (displacement) scale is also

expected. In the absence of the actual data, we have made the 

linear estimate.

We caution once again that no power cycles have yet been

measured and that the higher dielectric strength copolymer may

require substantial effort to obtain.

The crude working approximation for the energy density is;

• W = constant X T X E (3)

e s max

where T = the temperature swing, E = the maximum electric

s max

^a

.' 36 
ORIGINAL PAGE 13

OF POOR QUALITY

-1 -1 -2

field and the constant is 2.4 E- 4 (J V K M ).

i

DIELECTRIC STRENGTH

Current Value

P

The present limit of the dielectric strength of the

copolymer is about 500 kv/cm (50 megavolt per meter) by our

measurements. Yamada (32) has achieved about double this value in

some of his experiments even at elevated temperatures. The

current limitation"on the breakdown strength is likely due the

crude physical form of the films which we are able to make. Our

method of making the film is to press powders of the copolymer in

between two hot (200 C) metal platens. This technique is used

because the quantity of the copolymer is very limited.

1^

Rn

4wto Expected Limit

Contrast the presently low dielectric strength of the

copolymer with that of the homopolymer (PVDF). The manufacturers

of the homopolymer sheet state that its strength at room

temperature is equal to 5 megavolt/cm (500 megavolt/m). We have

measured the ferroelectric properties of the homopolymer at

temperatures up to 130 degrees Celsius and have subjected the

material to electrical fields in excess of 3.5 megavolt/cm at

 90C.' Under those conditions the homopolymer was found to degrade

(as measured by a decrease in the remaaent polarization

inducible) in several minutes. At somewhat reduced temperatures

and voltages,(70C and 2.0 megavolt/cm) this degradation was not

detected over the course of hours. These observations were made

only incidentally in an experiment which was not concerned with

w. the lifetime of the homopolymer.

Why does the homopolymer seem to have about an order of

magnitude greater dielectric strength than the copolymer? Most of

' the answer lies within the physical form of the experimental

films and not in the chemical nature of the material. We have 

applied our crude preparation procedures to powders of the

homopolymer and have also seen low dielectric strength (similar

to that measured with the copolymer). The lack of strength in the

crude preparations is attributed to l)greater thickness of the

film (it is well established that thinner films in general have

higher strength), 2)nonuniformity of thickness of the film, and

3)some impurities (including dust particles) are added to the

film during its preparation.

The basic advantage that the existing best-to-date

homopolymer has over the copolymer is the physical quality of its

manufacture. The technique used in the production of large

t

37
ORIGINAL; PAGE 13

OF ' POOR QUALITY

quantities of the homopolymer is a film blow. This technique

results in a biaxial orientation of the polymer within the plane

of the film. This orientation improves the polarization maximun

for the material in addition to increasing the dielectric

strength.

r^

Our present best estimate as to the intrinsic strength of

the copolymer is that it should be approximately equal to that of

the currently obtainable value for the homopolymer. This number

(300 megavolt/m) should not be mistaken for an ultimate limit for

pyroelectric polymers, however.

RADIATION EFFECTS

When polymers are subjected to ionizing radiation, whether

it be protons, neutrons, electrons, or photons, two main types of

reaction occur, crosslinking and degradation (33),(34). At low

dosage the polymer structure determines which reaction

predominates. Of primary concern for the operation of the

pyroelectric converter are the effects of crosslinking. We

presently assume that crosslinking will inhibit the rotation of

the pyroelectric polymer about the carbon back-bone. If such an

inhibition does take place the thermodynamically important phase

transition will be disturbed.

In order to get a feel for the orders of magnitude of the

parameters involved, consider the commercially important

irradiation of polyethylene film. This polymer is irradiated to a

level of about 10 megarad to achieve a high degree of

crosslinking (so as to improve its thermal stability and tensile

strength). One - rad - of radiation is equivalent to the deposition

of 100 erg per gram of irradiated material. The 10 megarad level

then corresponds to deposition of 100 Joule of energy into a

single gram of material (in SI units, 100,000 gray). Even though

this level of irradiation is sufficient to alter dramatically the

mechanical properties of polyethylene, it has been shown that the

heat of melting of this polymer is little affected by this level

of irradiation ( the heat of fusion is decreased by 10% and the

melting point is depressed by 2 C) (35).

Even though fluorocarbon polymers possess outstanding

chemical and thermal stability, they are usually clasd'ed among

the poorest in resistance to radiation. They are considered to

undergo degradation exclusively, and this degradation produces

corrosive products. If we include, materials having some

hydrocarbon groups (e.g. the copolymer), however, crosslinking

can occur. The radiation dose at which most useful properties are

lost ranges from a few megarad for polytetrafluoroethylene

(without hydrocarbon groups) to over 100 megarad for

hexafluoropropylene-vinylidene fluoride copolymers (with

hydrocarbon groups) (36).

We will next compare this irradiation to that which a polymer

is expected to experience in the near earth environment.

I 38
ORIG MAJ: PAaLr le

OF POOR QUALITY

The dose rate is given by,

-8 2 2

Dose(rad) = 1,6x10 dE/dx(MeV-cm /gm) X 'D (N/cm ) (9)

where dE/dx is the stopping power in the material and k is the

number of particles incident upon a square centimeter. The

stopping power for electrons is about 3 (at 100 keV), and for

protons it is about 50 (at 5 MeV) (37).

2

The flux and dose rate from cosmic rays is the order of 1/cm

-4

sec and less than 10 rad/hr (38). It would take ten million

years to accumulate 10 megarad from cosmic rays. A greater

radiation problem is. that associated with solar flars. A large

solar flare can produce a flux of energetic protons ( >5MeV) of

5 2

10 /cm-sec-steradian and a dose rate in excess of 100 rad./hr. On a

10 2

yearly basis• less than 10 protons/cm -year (8000 rad/yr) are

produced even during the peak of the 11-year solar flare cycle.

At this rate, the passage of one thousand years is necessary in

order to accumulate a potentially disturbing 10 megarad at GEO.

Drastically higher proton fluxes exist at lower altitudes.

8 2

At the peak, a.s many as 10 /cm -sec (1 meV and greater) protons

exist.

Figure 23 shows the estimated lifetime of a pyroelectric

sheet as a function of altitude. The estimate is based upon the

assumption that the life of a pyroelectric sheet will be

terminated after the sheet has received 100 megarads of radiation

(this is a conservative estimate according to the data of Wang

(39). The data for the radiation levels wcs taken from the Solar

Cell Radiation Handbook (37) for zero degree inclination (the

worst case) circa-lar orbits. The proton fluences given in the

handbook are reported in terms of 1 MeV electron equivalent (for

solar cells) fluence. Solar cells are more sensitive to 10 MeV

protons than they are to 1 MeV electrons on a per rad basis, and

this factor was taken into account when the fluences were

converted into rad units.

The solid dots

lifetime as limited

symbols indicate

electron and proton

only at low altitud

of Figure 23 represent the pyroelectric sheet

by the proton radiation. only. The open square

the lifetime limitations imposed by both

radiation. Note that electrons are important

es (below 3000 km).

39
—

10 2

3

1010

LOG ALTITUDE (KM)

Figure 23

Est'MatedI-'Oetime Of a pyroellectric sheet as a f

unction of altitude.

40

3

11

1(

000% 1 C

Lu

Lu

10,

LL

0

-j 10

ORIGMIAL, PACC. 13

OF Poop 01 ,Al _r

r
CND& e': al PAGE IS

Or DOOR QUALITY

The lifetimes for the pyroelectric sheet are in the range of

I to 100 years for all but the most severe region of the Van

Allen belt. At the peak radiation levels of the belt, the

lifetime of the pyroelectric sheet dips to 12 days. by

comparison, an unprotected solar cell would have its output power

cut in half in about 0.1 day at that same altitude.

In light of these findings it appears that more detailed

knowledge about the effects of radiation on the copolymer - and

other pyroelectric polymers - should be obtained.

ELECTRODES

An important question about this large area device is: what

will happen if/when a small region of the pyroelectric sheet

fails electrically? This is a crucial question since several

square meters of sheet will be electrically in parallel within a

given segment, and if one region fails as a short circuit then

the entire segment may be self shunted. Such localized failure

will certainly occur as dielectric breakdown of the plastic may

be cau6ed by small voids (from the manufacturing process),

pinholes, micrometeoroid punctures, or ionizing radiation damage.

The solution of this potential problem is the incorporation

of a technique which assures that point electrical failures

result in local open circuiting rather than short circuiting.

Researchers interested in studying the properties of various

electroactive polymers have already dealt with this problem. They

have solved this problem by using thin metallic (vapor deposited)

electrodes on their test samples. When a polymer sample with thin

electrodes (500 to 1000 Angstrom) suffers a localized dielectric

failure the current which pours through the"short" vaporizes the

electrode in that region.

It is possible that the self-healing approach may be

satisfactory for small areas (a few square centimeters) only. If

this turns out to be the case, then it may be be necessary to

incorporate some pattern of subdivision with the electroding of

each segment. The subdivided electrode patches might be connected

by narrow strips of conductor. The entire pattern would be

applied in a single plating process with a grid-like mask

defining the boundaries between patches. The boundary lines

should be kept small so as to minimize arcing from the insulator

to the plasma.

The electrodes should be black. The primary purpose for this

is, of course, to maximize the radiative heat transfer rates. A

secondary benefit of this surface preparation is that it

minimizes the optical radiation which actually reaches the

pyroelectric material bulk. Though the metallic electrode itself

intercepts much of the incident light, a few hundred Angstrom

electrode does have a finite transmittance.

41
'PAGE is

OF POOR QUALITY

Two possible methods of blackening the electrodes are

anodization, and 2)coating with carbon, Presently, the second

method is preferred since it results in a conductinZ outer

surface which is helpful in minimizing arcing. Other coatings are

also possible (40)

EFFICIENCY

The efficiency of the pyroelectric converter may be

approximated by the following equation,

W W

e losses

efficiency — — — — — — — — — — — — — x R x E ( 10 )

CxT + Q

s t

where W is the electrical energy output (density), W is the

e losses

sum of the several energy losses and includes the plasma loss,

the bulk conduction loss, and the hysteresis loss, C is the

volumetric heat capacity, T is the temperature swing, the

s

product CxT is called the normal lattice heat, Q is the

s t

transition heat corresponding loosely to the thermal energy

difference between the paraelectric and the ferroelectric

phases,R is the efficiency of -the thermal radiation cycling (and

reflects the heat loss, due to reradiation, during the heat-up of

the pyroelectric), and E is the efficiency of the power extractor

circuit.

With the following values of the parameters the efficiency

may be estimated. . a

W 6 x 10 J/m (see Eqn. (8)

e

W 23% of W

losses e

hysteresis loss 10%

plasma conduction loss = 3%

bulk conduction loss = 10%

6 3

C= 3xI0 J/m K (C is actally a function of T and B

but the important residuals of that

dependence are included in the Q

term) t

42
(jil OOR QUALITY

8 3

T = 50 K (thus the normal lattice heat is 1.5x10 J/m )

s

Q = (the Carnot limit of this heat)= (12.5%) X W

t e

7 3

4 ► 8xlO J/m (for a thermodynamically ideal cycle,

with lattice heat equal to zero, the transition heat

is determined by the Carnot limit on the efficiency)

R P 0.85 for the CS-2 design

a

and finally,

E = 0.90

With this selection of parameters the net system

pyroelectric conversion efficiency is estimated to be 2 percent

which is about one sixth of the Carnot limit. The principle

culprit for the low efficiency is the large value for the lattice

heat. The lattice heat may in principle be removed for

terrestrial pyroelectric converters by means of heat

regeneration. Regeneration is not included in the present design

of an extended s p ace pyroelectric converter so the only avenues

for system efficiency improvement are 1)reduction of the system

losses and 2)increases in the output electrical energy density.

Though system improvements seem likely as compared with the

conservatively estimated parameters, the greatest improvements

are expected with regard to the output characteristics of the

present and potential pyroelectric materials.

With a ten fold increase in the output energy density the

efficiency of the pyroelectric converter would become 8%. The

efficiency scales sublinearly with the increase in the output

energy density in this case, because the transition heat of

equation (10) also increases with the output energy density. The

sources of such an improvement in the output energy density are

crystallinity enhancement (current material is 50% crystalline),

crystal orientation (present material is not even uniaxially

oriented), increased dielectric strength (a byproduct of improved

dielectric strength is that the polarization is increased), and

new pyroelectric materials.

POWER TO MASS

We will discuss the power to mass in three different ways.

In each succeeding manner, more of the system will be included in

the power to mass accounting. In the first case only the 

pyroelectric sheet and its immediate supporting structure will be

considered. In the subsequent cases collector mass and, finally,

§ power extractor mass will be included.

c

43
44 

Oi ICINAW PAGE Iq

OF POOR QUAL rY

The power to mass parameters for the pyroelectric sheet and

supporting structure are given in Table X. The assumed values for

the energy density and efficiency were explained in the previous

subsections. The two columns correspond to the estimated near

term performance (in the left column) and a reasonable estimate

of performance potentially attainable in the future (shown in the

right column). The values in the table are based upon the

assumption that the pyroelectric sheet is 10 micrometer thick.

The mass per unit area is obtained by assuming a pyroelectric

sheet density of 2 and that the supporting structure mass is

equal to the pyroelectric sheet mass. This is in keeping with a

design rule of thumb which is attributed to Hedgepeth et al.

(41). The electrical energy per cycle and electrical power per

unit area follow from the information given earlier in the table.

The final entries in Table I show the very large output

power per unit mass which is estimated when only the pyroelectric

sheet and i ts immediate supporting structure are included.

t

Jk

a
OF POOR QUALITY

Table I Power to mass parameters for the pyroelectrc sheet

and supporting structure only.

_-----.-.—_---.--------------------- -----------^--^---..-- ----------

. ^6 _ 5

Energy Density 6x10 60x10

3

W/m )

Efficiency

Cylinder 1.2% 4.7%

CS-2 2% 8%

Volume per unit area

3 2

(m /m ) 10 10

Mass per unit area

2

(Kg/m ) 0.02 sheet 0.02 sheet

0.02 support 0.02 support

Electrical energy,

per unit area

2

w (J/m ) 60 600

Electrical power

per unit area

r 2

E ( watt/m )

Cylinde,;,

w

5 19

CS-2 31 132

Electrical power

per unit mass

r

(watt/Kg)

Cylinder 125 475

CS-2 775 3300

------ ----- ------ ---------------- ----------_ _---- ----^__

Table II shows the parameters corresponding to a power to

mass computation which includes the collector. The heat power

required for each of the configurations of Table I is found by

dividing the electrical power by the efficiency. The

corresponding collector area falls out by assuming an incident

solar flux of 1.4 kilowatt per square meter and an optical

collection efficiency of about 70 percent. The mass of the

collector assumes a collector area density in the range between

12 gm per square meter (41) and 200 gm per square meter (42).

45
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1 ^ ^

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46

0MOMIAL PAGIE is

OF POOR QUALITY

The inclusion of the collector mass brings the near term

power to mass ratios for the two designs to within a factor of

two of each other. The near term power densities are quite high

at about 100 watt per kilogram. These near term eatimat,.s compare

favorably against Cie advanced thin blanket photovoltaics which

promise 265 watt per kilogram (43). The improved pyroelectric

material designs at; still more attractive at 400 to 2500 watt

per kilogram.
ORIGINAL PAQE TO

OF POOR QUALITY

Table II Power to mass including collectors

------------------------------ ----=--------------------------

Heat power required

for each of the

c onf igurat ions

per unit area of

pyroelectric sheet

2

(watt thermal/m )

Cylinder 414 414

CS-2 1530 1650

Collector area per unit

pyroelectric sheet area

Cylinder 0 0 • G

CS-2 1.18 1.27

Added mass 

per unit pyroelectric 

sheet area 

2 

(Kg/m >

Cylinder 0 0

CS-2 0.24 0.013 to 0.26

Electrical power to 

mass

collectors¢

- including

(watt/kg)

Cylinder 125 475

CS-2 71 440 to 2500 

-------- --------------------- _ 

I 

Table III concerns the power to mass ratio with the addition

of the power extractor electronics. The.power extractor mass per 

unit electrical output shows a range of values. At the lower end

I

is a conservative rough estimate for the extractor described

earlier. At the high end is an accepted mass density for low 

i

voltage space power systems (44). Included in the second 

a 47
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^,

t

4

M

Y

i

j

A

ORIUNAL PAGE ^a

OF POOR QUALITY

row of the table is the mass of the radiator which is required to

reject the heat dissipated in the extractor electronics. The

assumed area density is taken from Schubert (44).

The final set of entries in Table III gives the electrical

power per unit mass for the entire system including the

pyroelectric sheet and its supporting structure, the r..ollectors,

and the power extraction electronics and associated radiator. The

impotcance of the mass of the extraction electronics to the

system ,.s evidenced by a decrease in the power to mass as

compared to Table II. An important consequence of this is that

the "improved material' design shows less of an enhancement in

performance over the near term design. On a total system power

to mass basis, however, the pyroelectric system appears more

attractive than photovoltaics since solar cells will also require

power conditioning ( of equal or greater mass per watt).

Table III Power to mass including' collectors and power

extractors

Power Extractor mass

per unit electrical

power ( 90% efficiency) 0.001 to 0.0022 0.001 to 0.0022

(Kg/watt electrical)

Power extractor radiator

mass per unit electrical

power 0-.0005 0.0005

(Kg/watt electrical)

Electrical power per unit

mass including collectors,

extractor, and extractor

radiator

(watt/Kg)

Cylinder 100 210 to 280

CS -2 62 200 to 580

48
49

ORIGINAL PAGE 1S

COST ESTIMATES OF POOR QUALITY

The task of estimating costs for a future spacecraft is

indeed a tricky one. I have studied the space power literature

and found quite a bit. of "apples versus oranges " comparisons. One

finds quite a number of estimates which are overly optimistic or

which do not include important system elements. Often times these

stripped down estimates are compared-with more complete systems

estimates of a competitive approach to show that the competitor

' is 'inferior." I have tried to avoid these pitfalls as much as

possible.

Tables IV through VIII show break-downs of the estimated

4 costs for the various components of pyroeiectric generator

systems. The first two tables correspond to the CS-2 design (at

assumed efficiencies of 2% and 8% respectively). The third and

fourth tables correspond to the simple cylinder design.

r

In Table IV the polymer cost is estimated to be about $160

per kilogram. This estimate includes the fact that the current

r price for 9 micron thick PVDF sheet (biaxially oriented) is $100

k per kilogram. In addition it is estimated that the synthesis of

the copolymer will be about 60 dollars per kilogram more

expensive than the homopolymer. The cost of the support structure

is taken as $600 per square meter (42). The metallized

concentrator/collector is estimated in this table to cost $1200

per square meter. The higher per area cost of the concentrator 

corresponds to the greater complexity of the curves required for

a collector of concentration ratio equal to 20 (42). The mass for 

the concentrator is also taken from French (42) in this case and

has the value of 0.2 kg per square meter. This is a more

conservative assumption as compared with the area mass density of

0.012 kg per square meter for the concentrators of Hedgepeth

(41).

T

YJ

T

YX

M.

iN
50

}

ORIGINAL PA;GC I5

OF POOR QUALITY

Table IV Estimated Cost Components for Pyroelectrtc Converters

Per Kilowatt of Electrical Output

(Assuming CS-2 design, efficiency--2%, 6 megajoule per

cubic meter electrical output)

5

2

$ Area (m ) Mass (kg)

96 32 0.64

640 64 0.32

19000 32 0.64

46000 38 7.6

30000 2

500 0.1 0.25

96000 11

Item

Polymer sheet

Electroding

Support structure

Metallized Collector

and structure

Electronics

Radiator for Electronics

Totals

Electrical power handling costs are listed under the heading

of electronics and are based on the figure of $30 per watt (45).

The mass for the power conditioner is assumed to be 2 kg per

kilowatt (44). The numbers for the radiator (for the rejection of

the heat generated by the power handling electronics) are based

upon an assumed extractor efficiency of 90%.Correspondins-toeac

kilowatt of electric output there will be 100 watts dissipated in

the extractor circuits. In order to radiate this 100 watts at 50

degrees Celsius will require 0.1 square meter of 2.5 kg

per square meter radiator. The estimated cost for a heat pipe

radiator is $5000 per square meter (42).

The costs before power conditioning, for the assumptions of

Table IV, total about $96,000 per kilowatt. This may be compared

with the estimate of $20,000 to $100,000 per kilowatt for large

photovoltaic systems without power conditioning (42). Power

conditioning costs for the photovoltaic system will be at least

equal to the costs for a pyroelectric system. The mass associated

with the photovoltaic system's power conditioning will likely be

double that for the pyroelectric system due to the low voltage

nature of the photovoltaics (* high current means lots of

copper-).

Notice that the estimated cost of the PY roelectric sheet

material is a trivial $96 per kilowatt. This number is only

representative of the basic sheet cost and does not include the
ORIGINAL PAGE IS

OF POOR QUALITY

costs of quality assurance inspections and tests, nor dots it

include overhead costs. However, the higher costs estimated for

the electroding and support structure should cover these

additional expense.

In order to put the previously mentioned costs in

perspective, current space photovoltaic "sheets" cost $100,000

per square meter (46) which is $500,000 per kilowatt.

Table V lists cost estimates in a more optimistic way. Here

the pyroelectric conversion efficiency is assumed to be 8% and

the electrical output, energy density is assumed to be 60

megajoule per cubic meter. Since the energy density has increased

by a factor of ten over that of Table IV the costs listed in

Table V for the sheet, electroding, and sheet support structure

all are seen to drop. Due to the higher assumed efficiency the

area and cost of the collector drop by a factor of four. The

collector mass also drops relative to that listed in Table IV by

at least a factor of four. The mass of the collector may be still

twenty fold lighter if a density of 0.012 kg per square meter

(41) is achieved.

Table V Estimated Cost Components for Pyroelectric Converters

Per Kilowatt of Electrical Output

(Assuming CS-2 design, efficiency= 8%, 60 megajoule per

cubic meter electrical output)

2

Item. $ Area (m ) Mass (kg)

Polymer sheet 23 7.6 0.15

Electroding 150 15 0.07

Support structure 4500 7.6 0.15

Metallized Collector

and structure 12000 9.7 0.1 to 2

Electronics "1000 1

Radiator for Electronics 500 0.1 0.25

Totals 18000 1.7 to 3.6

51 
:a 52

•^ N

ORIOINAL PAGE IS

OF POOR Oe IALITY

With the assumptions of Table V the cost for the

pyroelectric generator (w/o power conditioning) totals $17,000

which is less than the cost of the lower limit for the

photovoltaic approach.

The estimates for the mass and cost of the electronics are

also listed in a more optimistic tone in Table V (relative to

Table IV). The lighter power conditioning corresponds to an

advanced design which operates at higher frequencies than are

currently common. The reduced power conditioning cost of $1,000

per kilowatt is still much larger than the terrestrial cost of

about $100 per kilowatt (47).

Tables VI and VII are similar to Tables IV and V

respectively, where the system which is analyzed is now the

simple cylinder design instead of the CS-2 design. The collector

costs and masses are eliminated in these cases. This results in

systems which are fairly cost competitive with the CS-2 designs.

Thus the cylindrical approach still deserves consideration in

spite of its larger pyroelectric components. Certainly a

judgement as to cost differential between the two pyroelectric

designs cannot be drawn from Tables IV through VII as the

expected errors in the estimates are lar g er than the cost

differences.

Table VI Estimated Cost Components for Pyroelectric Converters

Per Kilowatt of Electrical Output

(Assuming cylinder design, efficiency 1.2%. 6 megajoule

per cubic meter electrical output)

z

K

t

Item

Polymer sheet

Electroding

Support structure

Metallized Collector

and structure

Electronics

Radiator for Electronics

Totals

2

$ Area (m ) Mass (kg)

650 200 4

4000 400 2

120000 200 4

0 0 0

30000 2

500 0.1 0.25

155000 12
COUGH AL, 6 Ac-v.F: 1S

OF POOR QUALITY

Table. VII Estimated Cost Compo..ents for Pyroelectri.c Converters

Per Kilowatt of Electrical Output

(Assuming cylinder design. efficiency=4.7%, 60 megajoule

percubic meter electrical output)

2

Item $ Area (m ) Puss (kg)

Polymer sheet 150 53 1.1

Electroding 1050 106 0.5

Support structure 32000 53 1.1

Metallized Collector

and structure 0 0 0

Electronics 1000 1

Radiator for Electronics 500 0.1 0.25

Totals 35000 4

Transportation costs must also be considered. The current

ahuttle cost (to LEO) is $340 per kilogram (48). For the

pyroelectric power systems, with their super light weight, this

transportation cost is small (less than 4% of the totals listed

in the tables) . 

Transportation to GEO, however, is a different matter. The

transportation cost to the higher altitudes cost $5,000 to

e $20,000 per kilogram (49). The total systems costs and 

transportation costs are listed in Table VIII.

rC

ap^

ti

^^

t ;,

 If

vp 

art

rF"','

Mt'

53

v.
>I z,

t ^^

i

i

k

9^^

r d

e^

^x

r.X

r

9.

lF.r

^i.t'

1

` 4

M

ORiGlNAL. PAGZ 15

OF P0m OIJALITY

Table VIII System and Transportation (to GEO) Cost in Dollars

System cost Transportation cost

(Assumi-ng CS-2 design, efficiency = 2%, 6 megajouleper

cubic meter electrical output) ,

96,000 55,000 to 220,000

(Assuming CS-2 design, efficiency=8%, 60 megajoule per

cubic meter electrical output)

18,000 8,500 to 72,000

(Assuming cylinder design, efficiency 1.2%, 6 megajoule

per cubic meter electrical output)

155,000 60,000 to 240,000

(Assuming cylinder design, efficiency=4.7X, 60 megajoule

percubic meter electrical output)

35,000— ^ ---- 20,000 to 80,000

For comparison, the cost of transporting an advanced

photovoltaic array of power density 250 watt per kilogram (43) (4

kg/kw plus 2 kg for power conditioning) to GEO would be $30,000

to $120.000. Therefore, the pyroelectric conversion approach, in

addition to its lower capital cost per kilowatts has the

potential advantage of being a third as expensive to transport to

high altitudes.

54
ORIGINAL PAGE IS

OF POOR QUALITY

Section 5

SPECIAL CONSIDERATION'

RADIATION HARDENED DESIGNS

Radiation hardening of the pyroelectric converter may be

feasible by at least two design approaches. The first idea is to

surround the pyroelectric disk (in the CS-2 desigc1) with a

shield against ionizing radiation (see Figure 2-4). A pancake

shaped can made of aluminum sheet 76 micrometer (0.003 inch)

thick would decrease the ionizing radiation flux substantially.

r The lifetime for the pyroelectric converter under the worst case

orbit conditions described earlier would be increased from 12

days to 1.5 years by the addition of such a shield. Greater

lifetime enhancements would be possible if thicker walled

shielding were utilized.

The aluminum can sheet would have a mass of about 410 gram

VW per square meter of pyroelectric sheet. Since the 2% efficient

pyroelectric converter would have a power density of 31 watt per

square meter, the can sheet mass would be 13 gram per watt (13

kg/kw). For the 8% efficient CS-2 design the area power density

should be 132 watt per square meter and, therefore the aluminum

sheet mass would be 3.1 kg/kw. As mentioned earlier, a rule of

thumb in designing the support for a large surface area is that

the structure mass will usually range from 50% to 100% of the

mass of the sheet which is to be supported. Assuming the upper

limit of this range for the support we find that the masses for

the shield and structure come to 26 'kg/kw and 6.2 kg/kw for the

2% and the 8% efficient designs respectively. These masses

should be compared with the system masses derived earlier. The

unshielded system mass (pyroelectric sheet, electroding,

pyroelectric sheet support structure, collector, power extraction

electronics, and radiator for electronics) for the 2% efficient

design was' 11 kg/kw. The unshielded system mass for the 8%

efficient design was 1.7 to 3.6 kg/kw. For both designs, the

added aluminum shield would dominate the system mass. Shielded

systems might weigh about three times as much as unshielded

systems.

It is interesting to note, however, that th.e (8% efficient)

shielded system's total mass of 6.9 to 9.8 kg/kw is very

attractive when compared with the 20 kg/kw (50) to 33 kg/kw (51)

for space reactors.

^t. a

55
ORIOIN.AL PAGE 19

OF POOR 01 11LITY

PYRCELECTRIC SHEET

ALUMINUM SHIELD 'CAN"

Figure 24 Ionizing radiation shield.

F 56
ORIGINAL Nor

.: 15 OF POOF? QUALITY

Heat flow for the "can" hardened design

The question immediately arises as to what effect the

ionizing radiation shield has upon the thermal radiation. To

address this question we wrote a simple computer code, similar to

the heat flow calculation described earlier, with the addition of

a surface interposed between the pyroelectric sheet and deep

space. The interposed surface which represents the aluminum

shield is assumed to reach a constant uniform temperature. Since

the outward radiating area of the shield is essentially equal to

the radiating area of the pyroelectric sheet # the temperature of

the shield is determined by the average input power per unit area

incident upo n the the pyroelectric (incident solar power times

Vconcentration ratio times the fraction of the pyroelectric sheet

illuminated) and the Stefan-Boltzmann equation. Figure 25, which

is similar to Figure 6, illustrates the average temperature and

^f amplitude of the temperature swing of the pyroelectric sheet for

the shielded case. The amplitude of the temperature swings is

nearly identical to that calculated for the unshielded case. An

important difference can be seen in the average temperature. In

the shielded case the temperature is higher. For example, the

temperature of the shielded pyroelectric sheet is 65 degree

higher at a concentration ratio of 8 than would be an unshielded

pyroelectric sheet. Thus the ionizing radiation shield also acts

as a partial insulator of thermal radiation. rut into different

terms, the implementation of a shield necessitates a reduction of

about a factor of two in the area power density for the same

operating temperature (in the 300 to 600 X range).

F The effect of the radiative heat flow considerations is to

double the mass per watt of the area dependent components of the

pyroelectric system. It may be possible to avoid this penalty, by

simply adding a gas (e.g. helium) to the interior of the can.

The convective heat transfer provided by the gas would help to

lower the average temperature of the pyroelectric sheet.

A second approach to hardening

Another method for hardening the pyroelectric converter to

the effects of ionizing radiation would be to encapsulate the

pyroelectric sheet. In this approach the shield is in direct

physical contact with the pyroelectric sheet. The most straight

forward application of this idea (say placing the aluminum sheets

of the previous method in contact with a 10 micrometer thick

pyroelectric polymer) would result in a drastic reduction of the

efficiency of the converter. The reason this is so is that the

added heat capacity of the shield will demand a much greater

input of thermal energy than is required to thermally cycle the

polymer sheet alone (the reduction in efficiency would be about a

factor of 9). If, however, the number of pyroelectric sheets is

increased so that the heat capacity of the pyroelectrics

dominates over that of the shield, the-n the efficiency will be

little effected.

M 57
J^P

P

AMPLITUDE

OF SWING

OR!W" ^.1. P^.q:: ' IS

OF POOR QUALITY

600

700

500

400

W

t ^

300 d

W

CL j 2 200

W

s ^

100

114"

0

1 2 4 8 16 32 64

CONCENTRATION RATIO

Figure 25 Averag. t o moeratur-^ '-- -e

e:ectric she-t :Dr the nshielded approach.

Lm
OC

4 OF Poop OUP- LITY

As an example, assume that the shield material will be the

some as the pyroelectric material (so as to minimize thermal

expansion mismatch problems). The shields should be of the order

d of 100 micromeer thick to achieve an attenuation of the ionizing

radiation wh..°'.ch is comparable to the previously described

example. If we require that the thermal loading of the

pyroelectric sheet shall not decrease the efficiency by more than

20t then the thickness of the pyroelectrically active material

must be at least 1 millimeter (1000 micrometer). The total

,r polymer sheet mass is now 120 larger than our standard 10

micrometer thick sheet. Excluding any consideration for support

structure the pyroelectric sheer would have a mass of about 20

kg/kw for a 6.7% (8°4/1.2 efficient design. Though support

` structure considerations might double this calculated mass per

unit power, a relaxation of either the efficiency constraint or

the amount of shielding constraint, may allow this hardening

method to be more competitive.

THERMAL ENERGY STORAGE

` Orbiting spacecraft generally require some sort of energy

R storage. For a satellite which derives its energy from

photovoltaies, the energy is stored in the electrical energy of a

charged up battery. In a pyroelectric system energy may also be

is stored in electric batteries. Alternatively, a pyroelectric

system may be coupled to a thermal storage element with the

partial or complete elimination of electrical batteries.

The energy density of the thermal storage system may exceed

that fox batteries. Consider the storage of sensible heat in

lithium. Lithium's heat capacity is 4 Joule /gm H in Mtg e range 973

to 1773 K (52). Over that temp p 40 utvMe range, 3200 Joule per gram

(3.2 megaJoule per kilogram) of thermal energy may bo stored in

lithium. If this heat energy is later converted by a 2%

efficient pyroelectric device, then the equivalent electrical

energy storage is 64 thousand Joule per kilogram (lb W-hr/kg).

With an 8% efficient pyroelectric system the equivalent

electrical energy density would be 26 . 0 thousand Joule per

Y kilogram (70 W-hr/kg). For comparison, the electrical storage

energy densities for NiCd and NiH2 batteries are 26 and 40 Whr/kg (and charging efficiencies of 76% and 65%) respectively

n

(51).

Thermal energy may also be stored by melting a material. The

heats of fusion for several materials are given by Severns and

Cobble (53) for melting transitions in the range of 1600 to 2100

K. High heats of fusion per kilogram are possible (in the range

of one to two megaJoule per kg) and the potential for isothermal

e heat transfer is attractive. As for any thermal storage which

uses a melting medium, heat transfer through the solid phase may

present difficulties. In'the previous case of the lithium 'storage

material, the lithium remains liquid during the entire thermal

cycle.

`

59
OR6C'.^NAL PACT,19

OF POOR T'AHTY

Figure 26 schematically illustrates a way i n which the

the,-_-il storage element may be incorporated into a pyroelectric

conversion system. Sunlight enters from the top and is

concentrated by a Cassegrainian - like mirror system. The thermal

storage element sits at the focus of the concentrator. When the

spacecraft i^; in "daylight" the thermal element collects heat

while simultaneously delivering, heat to the rotating pyroelectric

sheet. ( shown at the bottom of Fig. 26). While the spacecraft is

in the sunlight, there is a net gain of heat in the thermal

storage element. When the spacecraft passes into the shadow of

the earth, aperture ** A" is closed to avoid an unnecessary heat

loss. The retained heat in the thermal storage element provides

continuous radiative heating of the pyroelectric sheet. If the

thermal storage medium is of the `sensible heat" type (e.g.

lithium), then i t will he necessary to open the aperture « B" as

the medium cools if constant heat transfer from the medium to the

pyroelectric sheet is to be maintained. For the lithium case

discussed earlier • the area must change by a factor of eleven.

For an i„sotL.^ ,.,mal latent heat storage medium, the aperture may

remain essentially fixed.

It is well known that the requirement of electrical

batteries 'for an orbiting high power spacecraft has an important

impact upon the system mass. Such is also the case for the

thermal storage subsystem in a pyroelectric power supply. The

mass of batte l(is (either electrical or thermal) is determined by

amount of time that the spacecraft spends in the shadow of the

earth. At low orbit the percentage of orbital period spent in the

shadow is maximal. At higher orbits the percentage of time spent

in the shadow decreases but the amount of time remains fairly

-constant. For example, the shadow time for a 555 km orbit it) 0.59

hour while the shadow times at 5090 and 35800 km are 0.64 and 1.2

hour (for equatorial. orbits). Using 0.6 hour of storage and the

18 to 70 W-Hr/kg derived earlier for , the thermal storage

approach, we may see that 8.6 to 33 kilograms of lithium are

required per kilowatt. For comparison the pyroelectric system

mass per kilowatt ranged from 2 to 10. Thus thermal storage may

be expected to dominate over other component masses in orbiting

systems which require large amounts of power when the spacecraft

is in the earth t s shadow.

The addition of heavy batteries to heavy photovoltaics has

(along with compactness and survivablility considerations) led to

the apparent attractiveness of the nuclear reactors for 100

kilowatt plus space power systems. It is interesting to note that

a pyroelectric system including storage may ** weigh in ­ at about

10 kg per kw. Thus the pyroelectric system may be only half as

massive as an advanced reactor system (50).

6o
r r Y SUNLIGHT rrr

076TINAL PAGE IS

OF PQUR 011,4LITY

g

^ENTRAI

Q

THERMAL

STORAGE -

ELEMENT APERATURES

PYROELECTRIC

CYLINDER

0

c _ W, . r c l erma. s s?^= c^ 2 ? 1^ _ ^•o a

iu a roe:cc r Cvc -;eroi_u

system.

L
ORIGCIIA!^ » + t5

OF POOR QUAUTY

ION PROPULSION

Pyroelectric conversion systems are we,', suited for coupling

to ion propulsion devices. The most straign,:. • ward output of a

pyroelectric generator is high voltage direct current. The

majority of the power required by an ion propulsion system is

also high voltage dc. By contrast the standard output of a

conventional photovoltaic array is low voltage dc.

In a conventional power processing scheme for ion thrusters

several processing steps are involved. Starting with the low

voltage do output of a photovoltaic array, the power must first

be converted to alternating current. The second step is to

transform the ac to a higher voltage. The third step is to

rectify the *ac; power. Then the rectified power must be 'filtered.

And finally, the power must be modulated to control the amount of

power which is delivered to the ion propulsion system. This

r approach suffers from the serial nature of the efficiencies and

reliabilities of all of the steps in the process. An in-depth

ji- discussion of some of the design difficulties present in the

s= conventional power supplies for ion propulsion devices is

presented elsewhere (54).

^ h

The electrical output of a pyroelectric generator may

conveniently come preprocessed and ready for immediate

consumption by the ion thruster. Thus a pyroelectric driven ion

propulsion system may be constructed which eliminates most of the

' processing steps just mentioned. The pyroelectric converter

described earlier had an output do voltage of 400 V, but the

output voltage may easily be increased to 80% of the extraction 

r-' voltage (3000 V). The output of the pyroelectric generator may be

 matched to the high voltages of the ion thruster's high power

consumers, the screen and accelerator.

The resulting elimination of several steps in the p'ower

&. processing train nets a system which is improved with respect to

reliability and power to mass ratio. The electronics related to

the power extraction from a pyroelectric generator will ** weigh' 1

r to 2 kg/kw whereas the conventional power processing will

' correspond to 4 to 5 kg/kw.

ORBIT RAISING

An issue'which is somewhat related to ion propulsion is that

of raising the orbit of a spacecraft from LEO to higher altitude.

s

The general approach to the delivery of a very large spacecraft 

xh (e.g. the solar power satellite) to GEO is to first place the 

[11 parts of the craft in LEO, then assemble it in the absence of

gravity - . After assembly the large structure may be transported

to GEO by some sort of ferry. Photovoltaic arrays have been 

suggested as a possible power source for the orbit raising

" system. Unfortunately, the excursion from LEO to GEO is expec'ced.

Pto span several days to several weeks. The extended time spent in 

the radiation belts would severely damage the photovoltaic cells.

62
OF PO,

,4Li 'Y

Potentially the pyroelectric converter could be used to

power an orbit raising system. Figure 23 shows the estimated

lifetime of a pyroelectric sheet' as a function of altitude. The

lifetimes for the pyroelectric sheet are in the range of 1 to 100

years for all but the most severe region of the Van Allen belt.

At the peak radiation levels of the belt, the lifetime of the

pyroelectric sheet dips to 12 days. By comparison, an unprotected

solar cell would have its output power cut in half in about 0.1

day at that same altitude.

It appears therefore, that pyroelectric converters may have

the survivability to provide the power source for orbit raising

system. In addition; the pyroelectric systems can dispose of

damaged sheets as necessary (at extremely small mass penalty) in

order to extend the system lifetime even further.

TECHNOLOGICAL BARRIERS

The technological barriers which stand between the

conception" and the "realization" of pyroelectric conversion in

space are naturally numerous. At this early stage we can list

^. only the barriers which are more fundamental. The hurdles seem to

fall into four major categories; 1) demonstration of the

conversion cycle with the proposed class of polymers, 2)

achievement of improved dielectric strength of the material, 3)

demonstration of acceptable plasma power losses for low altitude 

by simulation, and 4) establishment of reasonable lifetime for

the pyroelectric material in the harsh environment of ' space. 

N

i

Though the type of pyroelectric r,.onversion cycle which we

have been discussing has been firmly established for ceramic

materials, the experiment to demonstrate the cycle with polymers

a (in particular the copolymer P(VDF-TrFE) ) has yet to be

performed. Though the measurements to date on the copolymer

(isothermal measurements of electric displacement versus applied

electric field at various temperatures) all indicate that this

material will indeed be a very good pyroelectric candidate, the

actual conversion cycle must be demonstrated. The demonstration 

will resolve the one remaining question as to the reversibility

of the ferroelectric to paraelectric transition under the

Condit ions of the thermodynamic cycle . 

The dielectric strength of the copolymer is currently 50 to

100 MV/m. This strength must be improved because it directly

effects the amount of electrical energy which may be extracted in

a pyroelectric conversion cycle. This issue has been discussed

in detail (in the Performance Section). It appears that this

problem is solvable.

TheP lasma power loss issue has also been described earlier.

Existing data indicate that the losses at higher altitudes will

not be important. At low earth orbit, however, the losses may

present a problem since the power losses may be as large as 10% 

63
64 

I

ORIGINAL INV.= 65

OF POOR QUALITY

to 50% of the output of the pyroelectric converter. One must

know both the performance of the pyroelectric material and the

plasma power loss before the importance of this parasite can be

quantified.

The pyroelectric conversion material must be able to operate

in the harsh space environment for reasonably long times if it is

to fly. The pyroelectric sheet must be able to survive thermalelectrical cycling in a sometimes arcing plasma while energetic

particles and x-rays and micrometeoroids are pelting it.. Optical

degradation will be prevented by the protective nature of the

electrodes, but to what extent the polymers can endure the space

environment is not yet known. Fortunately, the pyroelectric sheet

does not require as long a lifetime as its competitors. The

pyroelectric sheet will be lightweight, flexible, and

inexpensive. As a result, the pyroelectric sheet may be

considered as a consumable. When one sheet wears out it may be

ejected (or rolled up and stowed) and easily replaced with a new

one*

CONCLUSIONS

This study was very limited in scope and only allows a first

order assessment of pyroelectric conversion in space. However, it

5 is apparent that the pyroelectric approach may offer a lighter

s weight, lower cost alternative to power generation. Potential

plasma interactions and radiation degradations do not seem to

present any insurmountable difficulties.

Thermal energy storage appears to be feasible and

competitive with electrical battery storage. In power generating

systems the pyroelectric approach is expected to be substantially

lighter than both advanced photovoltaic and reactor

technologies. The pyroelectric systems studied will be area

E intensive, however.

Pyroelectric generators are expected to be resistant to

ionizing radiation and be well matched to ion propulsion systems.

As a result, this new form of generator may find application in

A orbit raising missions.

Pyroelectric conversion is attractive and further study is

recommended. Near term work should emphasize studies of the

conversion cycle, improvements in the dielectric strength of the

known pyroelectric polymer, and attempts to discover new

candidate materials.
ORIGINAL. ESA. ^2, Is

Section 6 OF POOR QUALITY

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