/i "

IS..], SPACE OPERATIONS CENTER

_. SYSTEM$ ANALYSE

I (CONTRACT NAS9-161,_ 1)

/_qALYSIS OF REGENERATIVE FUEL CELLS

D180-27160-!

FINAL REPORT

November, 1982

National Aeronautics and SpaceAdministration

LyndonB. JohnsonSpaceCenter _]_";'-_)cL_9_%._ -_

Houston,Texas ,_ _:_ _]

Boeing AerospaceCo.

P. O. Box 3999

Seattle, Washington 98124

,, (NA$A-_:_-1717tJ3) ANALXSIS 0¥ IiE.GBNEIt&_ZV3 Nsq-13671

EUJ_L C_LLS ¥_aa_. Eepol:t (Eouing &e£o:space

: ¢;o., Seattle. Wash.) I_I p HC AO7/d¥ AOI CSCL IO& Un.cl.a_s

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https://ntrs.nasa.gov/search.jsp?R=19840005603 2018-05-11T19:32:48+00:00Z
._ FOREWORD

This study was managedby the LyndonB. 3ohnsonSpace Center. Hoyt McBryar was

theNASA/3SC StudyTechnicalManager.Thisstudywas conductedby The Boeing ,,

AerospaceCompany, LargeSpaceSystemsGroup. The SOC studymanagerwas Gordon

W. Woodcock!thetedmica[staffengineerinmanager g was Dr.RichardOlson;thetask

managerwas SidneyW. Silverman;and SidneyGrosswas theWincipaJinvestigator.

Prepare isy, ,

SidneyG_oss
TABLE OF CONTENTS

Paragraph Page

1.0 INTRODUCTION 1

I.l SUMMARY 1

1.2 BACKGROUND 3

1.3 PROGRAM OB3ECTIVES 4

2.0 DEFINITION OF REQUIREMENTS AND PENALTIES 6

_ 3,0 EFFICIENCY CONSIDERATIONS 12

3.! THE IMPORTANCE OF HIGH EFFICIENCY 12

3,2 REGENERABLE FUEL CELL EFFICIENCY CONSIDERATIONS 15

3.3 BATTERY EFFICIENCY 28

4.0 ANALYSIS OF PAST REGENERABLE FUEL CELL STUDIES 36

4.1 CRITIQUE COMMON TO ALL STUDIES 36

__ 4,2 NORTH AMERICAN ROCKWELL STUDY 37

4,3 UNITED TECHNOLOGY STUDY 4!

__ 4.4 LOCKHEED STUDY _'

4.5 LIFE SYSTEMSSTUDY 49

4.6 McDONNELL DOUGLAS STUDY 56

4.7 TRW STUDY 6!

:" 4.$ PRC STUDY 7G

4.9 COMPARISON OF STUDY RESULTS 78

5,0 SOC REFERENCE RRGENERATIVE FUEL CELL SYSTEM 85

5.1 SYSTEM DESIGN 85

. 5.2 THERMAL DESIGN 89

5,3 DRY ELECTROLYSIS GAS 94

5.4 EMERGENCY POWER 95

5,5 ENERGY STORAGE INSTALLATION COMPARISON 99

5,6 DESIGN SUMMARY 99

6,0 ASSESSMENTOF HYDROGEN-HALOGEN REGENERATIVE SYSTEMS 105

6,1 INTRODUCTION AND SUMMARY !05

6.2 ANALYSIS OF HYDROGEN-HALOGEN SYSTEMS !05

6.3 ASSESSMENT Ill

7.0 EVALUATION OF IMPACT OF TECHNOLOGY ADVANCEMENTS 117

7.1 NEW SYSTEMS !17

7.2 NICKEL HYDROGEN BATTERIES 118

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TABLE OF CONTENTS (CONT.)

ParaRraDh

7.3 REGENERABLE FUEL CELL SYSTEMS [[9 ,!

7._ NICKEL CADMIUM BATTERIES 120

g.0 EVALUATION OF INTEGRATION WITH OTHER SUBSYSTEMS 121

$.1 INTEGRATION OPTIONS AND REQUIREMENTS 121

8.2 INTEGRATION WITH REACTION CONTROL SYSTEM 124

g.3 INTEGRATION WITH LIFE SUPPORT I25

g,; INTEGRATION SUMMARY 127

9.0 ASSESSMENTOF FUEL CELLS VS. BATTERIES t30

9.1 COMPARISON OF ATTRIBUTES I30

9.2 ASSESSMENT 132

10.0 EVALUATION OF PRIMARY FUEL CELLS USING SHUTTLE RESIDUALS 13_

10.I ANALYSIS 13._

-- l 1.0 CONCLUSIONS 139

i,'

!_ 12.0 RECOMMENDATIONS l_l

i!i 13.o 1' 3 'I

if,

L..

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1.0 INTRODUCTION

hl SUMMARY

This study analyzes and evaluates orbital energy storage systems for space stations,

using the Space Operations Center (SOC) as a point of reference. Energy storage

systems for spacecraft in the past generally have usednickel cadmium (Nl-Cd)

batteries for rechargeable systems, or hydrogen-oxygen fuel cells for relatively short

duration missions,suchas ApoLloor Shuttle. In this study, major attention is devoted

to the concept of a rechargeablefuel ceLl(RFC) system (Fig. l.l-I). A newer type of

rechargeable battery, the nickel hydrogen(Ni-H 2) battery, is also evaluated.

To help resolve the question as to the potential of the RFC system, a review was made

of past studies. These studiesshowedlarge variations in weight, cost and efficiency.

However, the variations are muchreduced whennormalized. Operations cost is the

most d_ficult to establish becausegroundrules varied considerably. However, the

general conclusion;n prior studies was that the RFC systemhas goodpotential for

space stations, and this conclusionwas found to be valid.

Hydrogen-bromineand hydrogen-chlorineregenerable fuel ceLlswere studied, andwere

found to have a potential for higher energy storage efficiency than the hydrogenoxygensystem. A reduction of up to l_ percent in solar array size may be possibleas

a result. These systemsare not yet developed,but further studyof them is

recommended.

Integration of the energy storage systemwith the reaction control systemoffers

weight savings possibilities. Water can be resupplied, electrolyzed on board, and the

product hydrogen and oxygen used as reactants for orbit makeup; this saves

considerable weight over the _se of hydrazine.

• An additionalopportunitywiththeregenerablefuelceLlsystemisintheuseof

".. residualhydrogenand oxygenfrom theShuttle.Thesefuelscan be usedinthefuel r

i cell, thus reducing or eliminating the size of the energy storage systemand the solar

array. With 19 evenly spaced Shuttle flights per year to the SOC, the solar array size

can be reducedby half; with _0 flights per year, nosolar array is required.

I! 1
RADIATOR

.,._.

Figure 1.1.1: Oellenerative Fuel Cell Orbital Energy StorageSystem- Block OiRgram
1.2 BACKGROUND

The backgrounduponwh;ch this study is cast is the SpaceOperations Center (SOC).

Studyof the SOC was :nnducted by Boeingin two parts: a PhaseA Systems Analysis

_: anda PhaseA Extension. The PhaseA study analyzed anddefined a manned space

station dedicated primarily to operational missions. It developedsystem design

requirements, and designand operational concepts. The PhaseA Extension

concentrated ondevelopment of missionmodelsand analysis of SOC utility. That

/ phaseof study consideredapplications scienceand technology missionsas wellas

i I_ operational missions. The SOC study was managedby the LyndonB. 3ohnsonSpace Centt:r, with Sam Nassiff as the S_JdyTechnical Manager. The final report includes

_ five documents:

Dt80-2675_-1 Voh l Executive Summary

D180-2678J-2 Voh 1I Programr_atics

DIS0-2678J-3 Voh [] Final Briefing

D 1g0-2678_-4 Vol. IV SOC System Analysis Report

D180-26_9_-2, SOC System Requirements

D180-26_95-3, SOC System Definition Report

_ Rev A

;

i_i Regenerable fuel cell (RFC) systemsare discussedat length in this study. They

_--_ produceelectricitdury ing spacecraft occultation in a conventional fuel cell mode by

I combining reactants, suchas hydrogenandoxygen, in an efficient electrochemical

: processto generate elec,. 'ty and the product water, buring sunlight, the processis 1 reversed to convert the dischargeproduct water electrochemically to the initial :

reactant state, typically hydrogengas and oxygengas. This regeneration mode can

take place in a separate unit, called the electro'-yzer, or in the fuel cell using

i

;i bifunctional electrodes. The useof an electroiyzer is of the greatest interest at this

I. time for space station applications.

: F,-

I!i,

The nickel hydrogenbattery is also a promisingcandidate for the energy storage

ii system. It b currently underdevelopment and is generally consideredto have good

!:i.............. promise for longerlife anddeeper depth-of-discharge capability than the conventional

!- nickel cadmium battery system.

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A number or studies have beenconductedover the past years, comparing regenerative

hydrogen-oxygenfuel cell systems with batteries to determine the applicability of

regenerable fuel cell systems to future multi-kilowatt spacecraft. Groundrules and

assumptionsall differed, but the results generally showeda weight advantage for the

regenerable fuel cell system. Weight alone is not a sufficient criterion, however, and

other factors must also be considered. This study was intendedto help resolve someof

the questionswith regard to the potential of this system.

1.3 PROGRAM OB_ZCTIVES

The general objective of this study program is to evaluate orbital energy storage

systemsfor SOC. A principal element is to compare andevaluate the previously

accomplishedpower studiestogether with their common conclusionsas to the

appLicabiU_yof regenerable fuel cells within the framework of various SOC

evolutionary design configurations. Study tasks for the program are as foUows.

1. Review and define energy storage requirements for the SOC. Define a reference

regenerative hydrogen-oxygen fuel cell system.

2. For the reference regenerative fuel cell system defined, determine the

important engineering parameters.

3. Categorize results from prior studies to identify differences andcommonaLities.

4. Develop a method to adjust for the differing groundrules and assumptionsin the

prior studies to obtain a commoncomparative basis.

_. Determine technology advancementsthat could influence systemscomparisons.

6. AssessH2-Br2 and H2-Ci 2 regenerative fuel cell systems.

7. Compare results of prior studiesand evaluate the impact of technology

advancements,

8. Evaluate integration of regenerative H2-O2 fuel cell systemswith other SOC

subsystems.
;°,

.._. 4

.... 9. Compare resenerative iuei cell systems with battery systems for SOC.

10. Document study re_ul_ and prepare tlnal report,

!.

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5
2.0 DEFINITION OF REQUIREMENTS AND PENALTIES

The followin8 requirements and penalties a_redefined for the SOCenergy storage

system.

Orbital Conditions

Altitude 370 km (200 NM) to 450 km (243 NM)

Inclination 25.5 degrees

Solar Cycle Sunlightduration - _ minutes

Occult duration - 37 minutes

BusVoltai|e

Regen. Fuel Cells 200 +2%, -20% dc

Batteries 200,10%, -30% dc

MIL 1_39 (Ret.) 28 +21.4%, -21.4% dc

Electric Power Requirementsf Normal Operation

See Figure 2.0-1. Load managementresults in less load duringoccultation than during

sunlight.

Electric Power Requirements -- EmerRencyOperation

See Figure 2.0-2

Equipment Cooling

Cold Plate Mounting and Cooling t I percent of equipment weight

(batteries & electronics)

Radiator area for batteries (_oc) t4 W/ft 2 radiation surface

Radiator area for electronics (20oc) t9 W/ft 2 radiation surface

Radiator weight 1.27 lb/tt 2 ot radiator (2 tt 2 radiation

surface/ft 2 radiator in plan view)

6

- ®
• I I j _ , ,

SUNLIGHT - W OCCUL.TED- W

I I I I I I

FULL. ELECTRIC LOADS .m,000 39_D0

SOC ELECTROL.Y?.ERFOR A8 REQUIRED --------

RECHARGE

INTEGRATED 8UBSYSq'EMS FIGURE 2._3 SUNL.IGHT ONLY

ELECTROL.YZER

I I

HALF EL.ECTRIC L.OADS 27,880 23,770

SO(: ELECTROLYZER FOR AS REQUIRED . .

"3 RECHARGE

"_ INTEGRATED SUBSYSTEMS FIGURE 2._3 SUNLIGHT ONLY

_2 ELEGTROL.YZER

.j'-

i

I i ii i I i I

_ GROWTH ELECTRIC L'OADS 60,000 39,_30

:: SOC EXPERIMENTS 40,000 30,000

ELL.ECTROL'YZERFOR A8 REQUIRED

RECHARGE

;.. INTEGRATED SUBSYSTEMS FIGURE 2._3 SUNL.IGHT ONLY

:. _ ELECTROL.YZER

Figure 20. I: Electn'cal Power Requiretnen_- Normal Operation

..o

-" 7

m
, EMERGENCY MDDE "A" - COMPI.ETE LOSSOF ONr SOLAR ARRAY WING. CAN LAST 80 DAYS.

. ARRAY' CAPABILITY IS REDUCED 50%, BUT LOADS ARE REDUCED

I B_J$(|_0 KW TO 10 KW). THEREFORE, REGENERABLE FUEL CELL MAY

! OPER,kTE IN A LEm EFFICIENT MANNER IF NEED BE.

.I

EMERGENCY MODE "B" - LOS_,OF ATTITUDE CONTROL/SOLAR ARRAY ORIENTATION. THIS

ti RESULTS IN A PARABOLIC OUTPUT OF SOLAR _'RRAY POWERWITH

TIME, BUT WITH PEAK OUTPUT APPROACHING _0 KW. THE

EFFECTIVE CHARGE DURATION IS REDUCED TO 38 MINUTES, AND

THE EFFECTIVE DISCHARGE DURATION IS INCRrr.ASEDTO 54

MINUTES. LOADS ARE REDUCED _ (50 KW TD 10 KW). THE

EMERGENCY CAN LAST 21 DAYS.

:_ EMEflGEiV_Y &qOOE"C" - COM M PLETE SHUTDOWN OF SOLAR ARRAY FOR ONE ORBIT (92 ,NUTEALLSUCT l. RICAL POWER ENTIREDFFLYENERGY

_- rrORASYSTEM GS FORONEORBIATAPOWER _VEDLF3,_W.

THiS CAN OCCUR IMMEDIATELY AFTER A NORMAI.LOAD OCCULTATION

._' DiSCHARGE. THE EFFECT OF THIS REOUIREMENT IS TO INCREASETHE

*ll CONTIGUOUS LOAD ON THE ENERGY STORAGE SYSTEM BY 19.S%OF

'; THE NORMAL LOAD.

,t

if" ENERGY STORAGE LOAD

_. EMERGENCY ....

_I DURATION FULL SOCOR HALF SOC "! GROWTH SOC

_k, I II l I i

lii EMERGENCY MODE "A" 80 DAYS g,000 W AVE, 4,500 W AVE,

_: 10,000 W Pr-.AK S,000 W PEAK

i!_.... EMERGENCMYODE"S" 21 DAYS 0,050WAVE, 4,50OWAVE.

_ 10.000 W PEAK 5,000 W PEAK

i'

_ I! EMERGENCY MODE "C" 92 MINUTES 3,000 W 1,S00W

11:,

i_ii Figure2.0.2: ElectricalPower Requirements- EmergencyOperation

];

'i

i 8
_UMBER OF ¢.LECTROLYZER

WATER

SYSTEMS REQUIR,_':)

IILECTROLYSlS ..... GAS STORAGE

RATE - LS/DAY CA_E NO. I - CASE NO. 2 - CRITERIA

INTE_I_ATION INTEGRATION

ii i q_mm

EMERGENCIES

FUEL CELL ELEG'TROLYZER PER ANALYSIS 3 A. B AND C

L.IFESUPPORT 17.8 L_/DAY EME_"(3EN_"_,

ELECTROLYZER [_ _ 2 , 3 A, F,A,

r I _ --,.----.

f

," 18.0 i.B/DAY

;! ORBIT MAINTENANCE L EMERGENCIES

'iI,:-, - ELECTROLYZER [_, 2 • 1 A,B. AND C

,! "

::-_ TOTAl,.: 7 TOTAL: 3

_> PEAK RE_.:IREMENT IS 20.2 I.B/DAY, WITH 27% AT 900 PSI

FOR GROWTH SOC. REQUIRED 27.2 LSIDAY FOR HALF SOC, REOUIRE 9.86 LB/DAY

[_ FOR GROWTH SOC, REQUIRED 28.7 LB/DAY FOR HALF _, REQUIRE 6.7 LB/DAY

)

11 Fiirum2.00: Elec_rolyzer/StorafeRequirement_for Integrated,,.¢ubsysmm- JFull r SOC

I

9
I!

Radiator area for fuel cell Temperature Watts/ft2radlator surface

60oc 37.2 .

70oc #2.9

80oc ¢9.0

90oc ,%%7

Power System Constraints onRqenerable Fuel Cell System

Number of fuel ceil modules, 3 busseswith minimum of 2 modules/buss,and

full SOC minimumof 6 modulestotal

Designto carry full load with 2 modulesout,

thoughefficiency may suffer

Design to emergency load with 2 modulesout.

Number of fuel ceils modules, 3 busseswith minimum of I module/bus

ili soc

Design to carry full loadwith t moduleout,

I_ thoughefficiency may suffer.

i! Design to load with 2 modulesout. emergency

t_

= i Number of fuel cell modules, No redundantmodulesrequired for science

growtht SOC equipment.

Designto carry full loadwith about 2_%

of the modules failed, thoughefficiency may

suffer.

Number of Electrolyzer Modules See Figure 2.0-3.

!_'_

i Power Supply/Controller Efficiency, (includinR transmissionloss)

"- Electrolyzer controller 99%

Battery Chargers 92%

10

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Solar Array Incremental Weight Penalty (weight _.. unit urray - generated power)

Haft SOC 30.6 lb/kW

FuU SOC 30.6 lb/kW

Growth SOC 30.6 lb/kW

0

Note" Penalty for solar array drag not included. This is a resupplypenalty that is

addressedseparately.

11

. " :;'.. . .........
3.0 EFFICIENCY CONSIDERATIONS

3.1 THE IMPORTANCE OF I-HGHEFFICIENCY

Energy storage efficiency is a key factor In the optimization oI any particular energy

storage system, and also in the choice between one systemand another. The

importance of this appears to have been mostly overlooked, and as a result the

minimum weight, or at least low weight designs,have often been favored in the past.

This is not to say that minimum weight designsare not worthwhile. Some missionscan

be visualized where low weight is paramount. However, for the general purposesolar

array-powered spacestation, high efficiency designsare compeWn$.

The reasonsfor underscoringthe importance of high efficiency are: (1) Cost can be

lowered by reducing solar array size for a given load, saving both onarray cost and on

fuel for orbit-makeup propulsion;(2) SpaceStation power capabLiitycan be effectively

i

increased by permitting more electrical payload for a given size of solar array

(alternative to iteml); (3) Life of the energy storage system is increaseddue to the

L: low current density designsneededfor high efficiency; (¢) Peak power and failure

b.

_7 modepower capabilities are increaseddueto the low current density designsneeded

it for high efficiency; (_) Attitude control performance is

improved.

L

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Solar Array Size andCost

i _: An efficient energy storage system reducesthe size of the solar array. This is shown

;_- inFigure 3.1-Ifor the 5OC requirements. Costsof solar arrays are expected tobe

'.2i considerablygreater than costs of energy storage systems,as shownin Figure 3.1-2.

, Note that there is a wide range of uncertainty onthese costs. As a reference point on

solar array costs, the cost of the 5kylab solar array was $600. per Watt. Although

_ there has been muchinterest in low cost solar arrays, it is questionablewhether there

_, is a sufficient market to allow large cost reductions. Large solar arrays possiblywill

;-_ also require complex, deployable structuresto provide stillness, andthe cost of this

!-4 wouldbe signif|cant. For these reasons,it is more important to try to reduce solar

i-_ array coststh._nIt is to try to reduce energy storagesystem cost. Thus,reduction of

ID solar array size shouldgive significant cost savings.

An alternative to solar array cost saving, due to improved efficiency, is to permit a

greater electrical payloadfor a given solar array si_e. Solar arrays of specific sizes

.... are likely to be built andqualified, andpower couldbe rigorously limited. The

12

l
130

110

jm

. _,t L

i,

Ii_.i $ I , I , I ,

i SPACECRAFTENERGYSTORAGEEFFICIENCY

: Figure 3.1.i. Effect of Efficiency on Solar Array Size

IL

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iiG
ORIGinAL PAGE g

OF POOR QUALITY

14W

NOTE:

A'i'11- OH, REQUIRE:

8OLAR ARRAY - M KW

ENERGY s'rOAAGE - 30.2 KW

1200 '' '

4OO

O II I

SOLAR ARRAY REGENERABLE NI-H2 NI-Cd FUEL CELLS

FigutB_ t.2: Esritnar_' Power Sysmm Co,_

14
problemsof large sized solar arrays are well out of proportion to the modest weight

involved.

PropulsionResupplyDue to Solar Array Drag

Significant quantities of propulsionfuel must be resuppliedregularly to offset the

effects of solar array drag, and maintain the spacestation within the selected orbit.

Inefficient energy storage systemsrequire greater solar area, and hence more

propulsionfuel. This is shownin Figure 3.1-3 for both hydrazine and hydrogen-oxygen

propellants. This penalty can be considerableover the life of the spacecraft.

Attitude Control Effects

Large solar arrays penalize the a_citude control system in several ways (Figure 3.1-4).

Of major concern is the fact that propellant weight consumptionriseswith array size

increase. Also, if a control moment _D,ro system is used, then the time between

desaturation events is decreased,which is an operational disadvantage. In addition,

pointing accuracy is reduced, which can be critical with somepayloads. ALlthese

att ltude control penalties are related to energy storage systemefficiency through its

effect on solar array size.

i 3.2 REG._qERABLE FUEL CELL EFFICIENCY CONSIDERATIONS

i ! Contributors to Inefficiency ii!

;! There are approximately ten possiblecontributors to energy storage system

ii inefficiency with the RFC system: (1) Fuel ceLlvoltage loss;(2) Fuel cell Faradaic

'- inefficiency; ()) Fuel cell ancillary power; (_) Fuel cell dischargeregulator power loss;

(5) Electrolyzer voltage loss;(6) Electrolyzer Faradaic inefficiency; (7) Electrolyzer

ancillary power (S) Electrolyzer input power regulator power loss;(9) Inefficient useof

solar array charging area; (10) Power consumptionfor temperature control. These

_,: items are discussed below.

' ?: It) Fuel Cell Voltage Loss. Typical discharge voltage for an alkaline fuel cell is shown

!,,. in Figure _.2-1. Voltage Is highestat low current densities,hence high efficiency

I would be favored at suchoperating conditions. A completely efficient dischargewould

! occur at the reversible voltage, which is associated with the Free Energy change, and

is 1.23 V at room temperature and lower at higher temperatures. The enthaipy

voltaget or thermoneutral voltage_ is approximately 1._ V at room temperature, so

15

:;4
ORIabI.AL PACE 1,_

OF pOOR CRJI_LrT'Y

12,01)0

BASIS:

• 8/A - lltW/F'I"2 B.O.L.

_S/A DEGRADATION - i1_10 YEARS

• N2H4 Isp - 230 SEC

10,000 • H2,O2 Isp - 380 $EC

0 _ ' I . I I I I

0.1 0.8 0.7 0.8 0.9 1.0

SPACECRAFT ENERGY STORAGE EFFICIENCY

Figure _ 1.3: Propulsion Resupply Due to Solar Array Draf

16
!i

PROPEI.I.ANT

WEIGHT

SOLARARRAYSIZE

TIME

IrrWEEN

DESATURATION

_ EVENTS

\!1

'"_; SOLARARRAY SIZE

POINTING

ACCURACY

i_ " _ ARRAY SIZE

,.:= Figure 3.1-4: Large So/mrArrays Penalize the Altitude Control System

it,:

t

' 17 I

x

}:
Figure 22-1: Typical Alkaline Fuel Cell Performanoe

ii'

!,

i

,b
even an ideal discharge at i.23 V will geneeate heat. Somepeopleprefer to use the

enthalphy voltage as the voltage reference for determining efficiency, but it doesnot

matter in the calculation of overall RFC system efficiency if a consistant basis is

used.

Using the reversible voltage as the basisfor idea. efficiency, the efficiency of fuel

cells is shownin Figure 3.2-2. Actual measuredopen circuit voltage is approximately

1.15 V, so this sets a practical upper limit to the efficiency.

_2) Fuel Cell Faradalc lnefficienc,/. The Faradalc, or current efficiency, is essentially

100 percent in alkaline fuel cells. Lossesare negligible, thoughat very high pressure

these lossesshouldbe measurable. With solid polymer electrolyte fuel cells, there is

always a current inefficiency, which is a self discharge due to cre_s diffusion of

hydrogenand oxygen. This loss Is shownin Figure 3.2-3, and is proportional to system

pressureand inversely proportional to thickness of the solid electrolyte NAFION. This

lossoccursat all times, irrespective of fuel cell discharge current, and it can become

an important limitation in a_empting to obtain maximum efficiency with solid

polymer electrolyte fuel cells. Thus, to minimize this losswith the solid polymer

electrolyte (acid electrolyte) system, oxygen andhydrogengases to the fuel cell could

be shut off during sunlight.

(3) Fuel Ceil Ancillary Power. Ancillary power to operate pumps,motors, controls

and other equipment varies with system size, but is on the order of 1.3 percent of the

power output. Figure 3.2-_ showsthe estimated ancillary power consumption. It is

expected that most of the ancillary load will be off during sun!ight; better definition is

neededto distinguishbetween occulted and sunlight ancillary loads.

_;) Fuel Cell Discharge Regulation. A regulator is sometimes includedto convert

output voltage to within an acceptable range. This is often done as an outgrowth of

other fuel cell applications where very tight voltage regulation was required.

However, fuel cell voltage regulation Is noworse than that of competingbatteries, and

thus generally neednot have output voltage regulation. Special voltage regulation

may be neededfor special loads,suchas voltage boosting for batteries, wherein this

penalty would be legitimate.

19
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•. 20oc • 1 ATM

0.0 I IIII'ACTICAI"LIMIT
®.

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PRESSURE-- PSI

F/gum 22.3: Cron Diffudon Loun from Solid PolymerElectrolyte Ceil System

21
'.
(5! Electrolyzer Voltage Loss. An ideal electrolyzer at room temperature would

operate at 1.23 Vdc. The higher voltage that must be used constitutes a loss. Typical

I

L voltage behavior for alkaline _.iectrolyzers is shown in Figure 3.2-:_. As with fuel cells, !

low current density gives the best voltage. Using the reversible voltage a_ the basis

for ideal efficiency (rather than enth=lpy voltage), the efficiency of electrolyzers is

shown In Figure 3.2-6. The actual measured open circuit voltase is approximately t.35

V, so this sets a practical upper limit to electroly=er efficiency. As with the fuel cell,

the enthaJpy voltage (thermoneutrai volt._Re)at room temperature is approximately

:i l._ V; operation above l._J V will generate heat, whereas operation below h#5 V will

require that heat be added to the electrolyzer to maintain constant temperature.

(6) Eiectrolyzer Fatadaic Inefficiency. The efficiency of alk_tline electrolyzers is

_ practically 100 percent, though there should be some small lossat very high pressures, (e.g., 3000 psi). Eiectrolyzers with solid polymer electrolyte are subject to Faradaic

I::i= losses due to cross diffusion of hydrogen and oxygen, similar to solid polymer fuel

)

1' ceils.

I

il,

; (7) Electrolyzer Ancillary power. Electrolyzer ancillary power to operate pumps,

_,...... controls and other equ,_pmentvaries with system size, But is on the order of [.0

; percent of the power level. Figure 3.2-# shows the estimated ancillary power

:_! consumption. As with fuel cells, better definition is needed to distinguish between

occulted and sunlight ancillary loads.

i_" (g) Electrolyzer lnpu.t Power Controller. An input power regulator is sometimes

I;_-.... included to control power to the electrolyzer. Though a controller is needed for

. switching functions and perhaps other purposes, it should not be necessary to regulate

i/_ the supply voltage, for that already is closely regulated by the solar array regulator.

i_: With the advanced technology expected for the space station, it should be practicai to

,.L!: vary the solar array control voltage, or to control bus voltage so as to obtain a desired

__-: current input to the electrolyzer. Also, all or part of the electrolyzer power can be

i_.; from a separate section of the solar array, so that the electrolyzer may operate at a

:_; voltage different than bus voltalse if there is an advantage to do so. Since the

!I'" controllcr need provide only switching and control logic functions, losses wall be slight,

--_ hence an efficiency of 99._ percent is used.

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,_ a w _ o _ q' •o

• L'IOA _, "IVhLNg.I,Od "1"1=10

24

,. - ............. _ __ : _ :.___._-:. _- _-_- _;71 77_ --_-._.:-_-_- 7-7_ _ T --_- ....... .
1.2 1,3 1.4 1.5 1.0 i.7

CHARGEPOTENTIAl.,-.VOl.TS

lI

t,!_ Figure 3,2.6: H2 - 02 Elecrrolyzer Efficiency

t

i1 ;

i,

"4

_' 25
(9) Inefficient Use of Solar Array Char_in_ Area. The solar array must be sized with i

a particular amount of area sized for or dedicated to charging batteries or, with a

RFC system, powering the electrolyzer. If the energy storage systemdoesnot make

maximum useof all that allocated power, then this becomesa losschargeableto the

energy storage system. Electrolyzers can be operated at constant current and

constant voltage, andthus shouldobtain very little, Lfany, lnefticiency from this

cau_.

(10) Power ConsumptionFor Temperature Control. The electrolyzers and fuel ceUs

are held at about i$_OF. If the electrolyzer operates at voltages that are below the

thermoneutraJ voltage or only a little aboveit, heat will have to be suppliedfor

temperature control. Since the fuel ceil and electrolyzer preferably are at the same

temperature, they can be fled together thermally with a small phasechangeheat

exchanger to maLnta_ntemperature control without comsumptLonof extra power. This

concept is shownin Section _.

Transfer of waste heat to the radiator requires energy, whether a pumpedUquid loop is

used,or heat pipes _re used. Heat pipesappear to be favored at this time, and sotheir

i: use hasbeenassumed. To maintain the close control desired, variable conductance

heat pipeswill be required. These require electric heaters to control the gas front

_ location, and lids heat penalty increaseswith the heat transfer distance from the

eiectroiyzer to the radiator. Becausethe heat load from the fuel ceil and electrolyzer

will be fairly predictable and at high temperature, the heater power is estimated to be

1.0 percent of the heat removed.

Voltage RangeEffects

An inherent characteristic of secondarybatteries is a relatively wide busvoltage

spreaddue to the large difference between charge and dischargevoltages. Since the

fuel cell and electrolyzer are separate units, only that unit which has the greater

voltage range need dictate busvoltage spread, this being the fuel cell. Thus,a typical

fuel cell systemwill have about half the voltage sl.read of a NL-H2 battery. Since

;_L. most equipment usersrequire their own internal power supplies,this makesthe design

of those power suppliesmore efficient. An estimate of the typical improvement in

efficiency of these loads is shownin Figure 3.2-7. It is seen that most of the loads

couldbe reducedone third of one percent usingthe tighter voltage regulation

attainable with fuel cells. It is believed that this potential saving wouldactually be

!i 26

q,

'! L
100"

!-- I

- I _

WITH RFC a I

i I

I REGENERABLE I

I FUEL CELLS I

I

I '

l N I

!

ibk'l'r'£HiES

t.

_:' 94

-Z;

.._:

-=5,.

_" I I I I I I I

•._ 1.0 1.1 1.2 1.3 1.4 1.6 1.6

_?: BUS VOLTAGE RATIO

Figure 3.2.7: Effect of Bus Volt Regulation on Power System Efficiency

• 27
attained. However, it is considered very unlikely that commitments could be obt_ned

to reduce loads, which would be necessaryin order to reduce the size of the electrical

power system. Therefore, it is concludedthat this advantage of the RFC system is

one that wilt showup as a performance improvement rather than as a saving in

electrical power system size.

Efficiency CaJculatJons

In the conductof RFC system design trades for 5OC, a system usingthe solid polymer

electrolyte fuel cell was designedwith an overa_ energy storage efficiency of 65

percent, and another systemwas designedwith an aJkalineelectrolyte with an overall

energy storage efficiency of 67 percent. For the co.di_on of both systemsweighing

#800 ib, the regenerative fuel ceU efficiency was 6# percent for the solid polymer

system, and 67 percent for the aJkaJinesystem. Neither of these data points may be

consideredoptimized. Slightly higherefficiencies were consideredpossible, but were

not pursued.

ALlowingfor 99.._percent efficiency with the elec_olyzer controller and 99 percent

equivalent efficiency for the heat pipe controller, the overaJJenergy storage

efficiency on these two calculated points is reduc_d to 63.0 percent for the solid

polymer electrolyte fuel ceil, and to 66.0 percent for the alkaline fuel cell. Thus, we

conclude that a design energy storageefficiency for the RFC systemof 60 percent is

l possible without unduedevelopmentrisk usingeither fuel ceLlsystem. As is shown

subsequently, this compareswith an energy storage designefficiency of _5 percent

with the Ni-H2 battery.

i'

3J BATTERY EFFICJENCY

Contribut.orsTo Inefficiency

There are approximately 9 possiblecontributors to energy storage system inefficiency

with batteries. (1) Dischargevoltage loss!(2) Charge voltage loss; (3) Charge current

inefficiency; (4) Battery charger inefficiency; (5) Inefficient useof solar array

charging area; (6) Battery dischargediodes;(7) Failed cell allowances(if de,igned for);

(8) Cell bipasselectronics (if used);(9) Power consumptionfor temperature control.

Theseitems are discussedbelow.

. 28

?i
1_i: I T_  ̧

(1) and (2) Dischart[e/Charxe Voltage Loss. These inefficiencies are best considered

together, becausethe reversible voltage of the Ni-Cd and Ni-H2 systemsvary with the

state of charge, and also becausea large entropy effect on voltage adds to the

complication. Figure 3.3-1 showstypical charge-dischargebehavior of a 50 AH Ni-H2

cell under cycle testing. The average charge potential is 1._5 V, and the average

discharge potentiaJ is 1.19 V. This results in a voltage efficiency of 76.77 percent.

Endof ILfeefficiency is expected to be worse. This voltage data, from Reference $, is

in agreement with other sources,for example, Reference 9.

_ (3) Charge Current Efficiency. The self dischargerate of Ni-H 2 batteries is much

i greaterthanwithNi-Cdbakeries(Figure3.3-2).As a consequencea, higherdegreeof

overchargeisneeded,resultining a highchargecurrentinefficiency.Itisnecessary

toselecttheoverchargeratio(AHin/AHout)basedon theneedsoftheweakestceU in

thebatteryand , basedon degradedcharacteristinear cs end oflife.Testsof_0 AH

NI-H2 ceUsinlow earthorbithaveshownrechargeratiosrangingfroma_out1.0_to

1.34with1.06beingtypicalofnew cells.A valueof 1.0gistakenasbeing

representa_vofe thetestingexperiencewithmaturecellswhile , discountingthose

examplesjudgedto be excessive.A sllghUyhigherdesignratioof 1.09isconsidered

necessaryforelectricalpower systemdesignto allowfordegradation,especiallyw_,th

large,200voltbatteries.

(4) Battery Charge Efficiency. Battery charge efficiency is sensitive to system

voltage, regulation complexity required, and to some degree.,to power level. For high

power battery chargers in a 200 volt system, a charger efficiency of 93 percent

efficiency was determined to be reasonable.

I (_)InefficienUset ofSolarArrayChargingArea. Ni-Cdand NI-H2 batteriersequirea

taper charge for long Life. Subjecting the nickel electrode 1:ohigh gassingrates is a

high stress; In fact, this is even usedsometimes as an accelerated life test. The

consequenceof the need to taper the charge is that the charging power level will not

be constant. Since the energy storage systemdoesnot use all the allocated charge

powerof thesolararray,thisbecomes a losschargeabletotheenergystoragesystem.

The efficiencyof thechargecanbe takenas theratioof theaveragechargepowerto

the maximum charge power, with care taken in the evaluation not to include the

i overcharge lossestwice. Even with constant current charging, without tapering, this

efficiency element is only approximately 94 percent. Becausethere is not a commonly

29

L
i..

..... _ _ _"

;= ."...

• 10

[,

' i

:- 2.0

.;

1.1

_ f

: ,,u 1.2

.. u __

1,0

0 29 40 O0 80 90

TIME (MIN.)

Figure 3.3.1: Typir.W Nick@!Hydrogen Cell Performance

3O

accepted charge control law for Ni-H2 batteries, experimental valuesof this

efficiency vary widely, approximately 76 percent to 94 percent. The problemhere is

the need to compromisebetween I_t,'ery life andsystem efficiency. Goodend-ofdischarge voltage himbeenobtained using a taper charge, which is beneficial but

inefficient. Taper charge hasalso provedbeneficial for Ni-Cd cells, but again this is

obtained at the expenseof efficiency. It is judged that a reasonableefficiency would

be 90 percent, though an even lower efficiency couldbe usedin the interest of

promoting long life.

(6) Battery Discharge Diodes. Spacecraft power systemscustomarUy provide

protection with paralleled batteries by the addition of one or more diodesin series

with each bakery. For a one volt dropin a 200 volt system, this would be only a 0._

percent loss.

_ I7) Failed CeLl ALlowance. Since a commonpremature failure mode of Ni-Cd and Ni-

_: H2 ceUs is by shorting, many times the system will be designedto accommodate about

_. _ or _ percent of the cells failing shorted. For the space station, it would be

is. worthwhile for the systemto continue to function properly with someshorted ceils,

It. but it is consideredunnecessaryto levy an efficiency penalty for this.

(8) Ceil 5_,BassElectronics, Electronic circuits are sometimesusedto bypassfailed t.

• _ cells in a battery string, allowing the battery to continue to function even though

:_'_ degraded. It is consideredunnecessaryto levy an efficiency penalty for this,

i_. (9) Power Consumptionfor Temperature Control. Heat pipes appear to be favored for

_-_ transferring waste battery heat froi_ the cold plate to the radiator. Variable

i_.- conductanceheat pipes are appropriatefor maintaining the close control of battery

_:_ temperature needed. Thesewill require heaters to control the gas front location.

i_T Because of the low temperature of these heat pipes (approx. 0oc), a relatively large

; ' amount of heat is required with today'stechnology (about _ to 8 percent of the peak

i_' heat load). Sincenew, high capacity heat pipe technologywill be neededfor the space

!

station, it is anticipated that these improvements will allow this heater load to be

reducedto 3 percent of the peak heat load.

• 32
Efficiency Calculations

Overall energy storage efficiency expected with Ni-H 2 batteries is summarized in

Figure 3.3-3. Typical performance is expected to result in an overall efficiency of

$7.7 percent which would be reflected in a typical design allowance of $$.2 percent.

Two tltirds of the losses are attributable to the battery Itself, whereas one third is due

to the battery's effect on the spacecraft system. This latter loss is often neglected in

system trade studies, and causes battery systems to project more favorably than is

justLfied. As shown previously, this $$ percent efficient design compares with a

selected design energy storage efficiency of 60 percent for the RFC system.

The Ni-H 2 system hasthe potential for improvement over the performance seenin

existing test data. It can be assumedthat the causesof the high premature failure

incidence observedon low earth orbit tests will be found andcorrected. Additionally,

it will be necessaryto reduce the voltage degradation rate. Figure 3.3-3 showsthe

estimated improvement potentiality, resulting in an overall designefficiency of 62

percent.

The data given in Figure ._.3-.3is more pessimistic toward nickel hydrogenceils than

someof the publisheddata, or other data which may be available to those concerned

[ with this system, indeed, the data taken early in cycling is much more favorable.

;i

:!; However, cell degradation eventually sets in and performance becomes much worse

, with time. This may be illustrated by the test data obtained by McDonnell Douglas on

il LEO cells of AF/Hughes design. The initial report on the first 2000 test cycles

_ suggestedthat the designwas good, with stable performance and with endof discharge

! voltage of 1.21 V andaboveat $0% DOD (Ref. 12). 7he later McDonnell Douglas data

I_ shows to 1.13thV,e wor withsecorr ningespondin effects gof_ocycling weringo(Rf eavf. e13). rage End dischaof rgdieschvol argtaegevolta . Algseo, hone as dropped of the

l

two cells at $0% DOD failed at near 10,000 cycles (1.8 years); suchearly failure is

_ consistent with the results of other testers at LEO conditions. Degradation

i_: phenomena must be given adequate consideration in determining the expected

I, efficiency alter long-term cyc/ing.

Ii'

i If the Nt-H 2 technology can be improved to eliminate early shorts andvoltage

"!:1 degradation, then the efficiency of the N:--H2 systemwouldbe muchimproved. We

'i have made the assumptionin this study that the shortingproblemwouldbe solved.

: However, voltage degradation is a fundamental problemand requiresa level of

!,

t

i

.!

i'!

i:i
'i

o ! ._._ ||__ ,,

.I

',-ii :g

( _ >_ __
understandingand research effort possibly beyondwhat is nowbeingpursued, At this

time, onecan only speculate onthe performanceat the endof four to seven yearsof

cycling, even if designimprovements are found and implemented,

Ni-Cd batteries have a higher efficiency than Ni-H2 bat,erie.,,, This is duein part to

greater designmaturity, and in part to the fact that lower dr.,pthsof discharge are

more appropriate for NI-Cd, Overall designefficiency is approximately 62,J percent

basedona recharge ratio of h06t an average charge voltaf_eof l,_ V, and an average

discharge voltage of 1o19V, Other factors are the same_¢swith Ni-H 2,

ml

35

L

(
_.0 ANALYSIS OF PAST REGENERABLE FUF_ CELL STUDIES

_.1 CRITIQUE COMMON TO ALL STUDIES

The following observationsapply to nearly all the studies, andthus are not cited

individually in the discussions.

Efficiency

High efficiency is a key attribute of spacestation energy storage systems. The

resulting reduction ih solar array size has many benefits, so that except for short lived

space stations or other special missions, it is advantageoustu sacrifice someinitial

launchweight. Benefits from reducedsolar array size are discussedin section 3.I, but

inc|ude: (I) Large size in itself is a problem, well out of proportion to the modest

weight involved;(2) Solararray costsare very high, especially the deployable structure

for the panels;(3) Orbit makeuppropulsionfuel dueto solar arre drag is a substantial

resupplyneed. High efficiency was often sacrificed in order to minimize initial launch

weight. An additional shortcoming was that the energy storageefficiency with

batteries tended to be determined too high for the reasonsdiscussedin Section 3.0.

ReliabLlit_

Reliability of spacecraft energy storage systemshasbeena continuousproblem, and

needsemphasisduring the formative yearsof spacestation power system

development. This needsto be an important consideration in systemselection.

Life Cycle Cost

As several studiesshowed,the total life cycle cost acsociatcdwith energy storage

vastly exceeds the equipmentpurchasecost. Thoughit is difficult to quantize total

cost, there shouldat least be qualitative life cycle cost comparisonsof contending

systems.

Dry Gas Electrolysis

The hydrogenand oxygen product gasesfrom the electrolysis of water, as conceivedin

the variolls studies,will have a high water vapor content. If all componentsin the

RFC sy.q*.emcouldbe assuredof long term isothermal operation, this wouldpresent no

probiems. However, temperature variability with occasional large excursionsis

commonplacein spacecraft. Thiscan causethe condensationor even freezing of

36

..... ,..... ': .-_, . .. • ........... ,.. . . // .. _ _ , ......... ,. ..........
._ _ water. Where w_ter electrolysis is used for reaction control fuel, the problem could

be very severe due to the long pipin 8 runs required. Thermodynamically it should not

:'_!i r_quire extra _f_ergy to electrolyze at! the w_l;er vapor. The technology for dry gas

_ electrolysis is not yet av_i!a_ble, though some experimenta_ work has been done.

! W__ejRhtl em ss Some weight items were omitted. Cold plates for mounting and cooling of the

batteries, electronic equipment, and other heat generation equipment were universally

omitted. This weight is expected to be approximately I 1 percent of the equipment so

mounted. Non-heat generating equipment or fluid-cooled equipment caJ_generally b_

mounted directly to structure without the need for cold plates. Radiator weights for

low temperature equipment, such as electronics and batteries, often was

/ underestimated due to selection of operating temperatures which were too high for

/ long life.

Tank Desil_n

ii' Varying design criteria were used for the sizing of the hydrogen and oxygen tanks.

i_! Tank weights of the older studies tended to be heavy, whereas the later studies reflect

improved tank technology, resulting in much lighter weights. In some cases the

_i requirement was imposed on the study contractor to use a safety factor of h_. This is

o_! the same factor of safety used on many of the shuttle pressurized tanks, such as the

_" KEVLAR-over-aJuminum gas bottles and the titanium propulsion tanks. These tanks

_ have good cycling capability, but additional allowance is needed for design to many

:_i cycles. A fracture mechanics design approach is needed for tanks for the space

_'" station to account for the effect of many cycles. Also, in some cases the tanks appear

_. not to have been sized with adequate consideration of ullage. Note the discussion in

!i. section _.J.

_ T. 4.2 NORTH AMERICAN ROCKWELL STUDY

;_ Objectives

!_.

!!,/ The energy storage part of this study (Reference 1) supports the Modular Space Station

.... study of North American Rockwell (now called Rockwell). The purpose of this study

segment was to prepare an electrical power system preliminary design and analysis,

_. based on the selected solar array primary power source and a RFC energy storage

system. This study was completed in 3anuary, 1972. That study did not provide data

37

, !i
in a form amenable to analysisof the type required for this report. A discussionof the

PhaseB study is given at the end of Section _,2.

Requirements

The Modular SpaceStation is a 6-man station with growth to 12 men. Orbit altitude is

240 to 270 nautical miles, with 270nautic_ miles baseline. Inclination is 28° to _o,

with _5o as the baseline. Missionduration was l0 years, based ona _-year initial

station, with the growth version lasting another J years. The loadsare nominally [l to

28 kW, with a 2_--houraverage of 19.64kW, and a "normal" requirement of 19.9_ kW.

An emergency mode is required at [.63 kW for 96 hours. During spacestation buildup

for 60 days, prior to solar array power being aVa_Lisble,powez must be providedby the

energy storage system. This power level is 3_ W average with peaksof 6_ W, and is

provided by operating the fuel cells continuouslywithout electrolysis of the product

water. Input voltage from either the solar array or the fuel cells is 112Vdc, but power

to usersis 120/208 Vac, V00Hz three phase,plus240/_16 Vac, 400 Hz three phase.

Major Findings

The major contribution of this studywas the definition of a RFC system to a

particular spacestation requirement. Figure 4.2-l summarizesthe major

characteristics of the RFC system. Life Systemsand Lockheeddid_ more detailed

studyof RFC systemsfor this particular spacestation, arid their studies _cediscussed

_! later in this report.

;! In Trades and Analysis, Volume 6 of Reference 1, backgroundinformation is provided

!i!_ on energy storage systemsfor the spacestation. Nickel cadmium battery technology

and regenerable fuel ceil technologyare discussedat length, includingproposed

_ solutions to someof the recognized problems. A trade study was conducted comparing

l

.. Ni-Cd batteries and the RFC system, and the RFC system was much the lighter. The

I: reported data for the electrical power subsystem(6-men) was: 22,932 ib using Ni-Cd

_: batteries, _nd 16,3_1 ib using the RFC system. However, energy storage weight

_ information is not brokenout. The RFC systemwasalso of lower cost: approximately

I:: $30 M vs. $37 M for NI-Cd batteries. The conclusionwas to select the RFC system.

ti', However, it was recognizedthat the designstraded were not optimum, andthat more

_'I optimized concept comparisonwouldbe needed.

i!

: [i.

, 38

:.!

i!

00000001-TSD01
Rr-c SYSTEM

ii i ;

1 LOADS -

BUILDUP (00 DAYS) 358 W AVE. 688 W PEAK, 298 KWHR

OPERATION 19.98 KW

REPLACEMENT U FE 2.26 YEARS

. _i, EFFICIENCY 52.5_

WEIGHT 4,898 POUNDS(NOTE: 996 POUND8

I$ INCLUOED FOR BUILDUP

REOUIREMENTS)

VOLUME 117.1F14

! sPSCIPiwCEjG_rr 2uoLe/KW

jr,

iL

Figure 4.2.1: Nor_ Americln Study -- _ctwd Summary

j.

%

--_L" 39

!1."

4.
Energyrequirementdur s ingspacecra3bui t ldupare29gkW-hrs,comparedwith163kWhrsforthe96 houremergency.Thus,thehighpressure(3000psi)hydrogenand oxygen

t_Jnkagiessizedby thebuilduprequirement.

Afterthesolararrayand theRFC systemareoperatingit, wouldbe worthwhileto

rechargethehighpressureoxygenandhydrogentanks.However,specialhighpressure

electrolyzers are neededfor this, since sizing the RFC electrolyzers to 3000 psi would

be heavy. The addition of two very small 3000psi electrolyzers might be a practical

solution.

North American Rockwell Conclusions

Use of an RFC system is practicable for spacestation energy storage. The fuel ceils

areespeclaUyusefulduringbuUdup,when theycan providepowerpriortopower being

availablefromthesolararray.

Critique

Thisisa pioneerapplicationf RFC systemstospacestationenergystorage.

Eiectrolyzertechnologywas initsformativestagesso, theweightinformationisnot

veryreliable.

The powerrequirementof 3JJwattsduringbuildupappearstobe verylow.Thereisa

!i

,L questionof whether the fuel ceils can operate at sucha low power level and maintain

_ theirown control.Much higherpowerlevelsare believed tobe needed,even ifonlyto

I_ maintaintemperaturecontrolfortheRFC systemandcriticalspacec:aftequipment.

L;

. Rockwell appearsto have failed to see an opportunity to capatalize ontheir RFC

?-

; desi the st gnattoionimprove buildupemer requirement. gency capability. After buildup Hydrogen is coand mpieoxygen ted, thistanka tankagegearecansizedbe uby_._

for reactants for emergency power, and should allow nearly double the allowable

,iil" emergency duration. However, if the emergency were to occur immediately after the :. buildup, the maximum quantity of reactants might not be availablefor the

,_, emergency.

!I Impact on North American Rockwell Conclusions

il Further analysiswouldalter the systemweight, but is not expected to change the

r. conclusionsthat an RFC systemis practicable for spacestation energy storage. If

40

....................... . ...... " ........... 00000001-TSD03
tqPI

I

t

I power duringstation buildupis required to be considerably larger than 355 watts, then

_I! it may be necessaryto use cryogenic reactant gases, or deploy the solar arrays much

i:.l_ earlier than 60 days.

i'_1 ¢.3 UNITED TECHNOLOGIES, POWF..RSYSTEMSDiWISION,STUDY

• Objectives

::_ The objectives of this study (Reference 2) were to define alkaline regenerable fuel cell

systems(RFC) for LEO space stations with power ranging from 35 kW to 250 kW.

_ Required wasa preliminary designconceptand performance characteristics. This

-_i study was completed in December, 1981.

_, M_n Requirements

Spacestation power levels to be considered were 35 kW, "'_ kW, and 250 kW. System

voltage was to be 120 Vdc _+1096.A 90 minute orbit with 36 minute occultation was

" postulated. A 2-hour emergencyduration was required. Specific weight of the solar

array was _5.15 ib/kW.

: Major Findinl_s b -

t The major contribution of this studywas the definition and analysisof RFC systems

!i usingstate-of-the-art technology. Figures _.3-1 to _..3-3summarize the significant

!- results of the designand analysis. No cost information wasprovided.

L

_!._ One of the findings was that the specific weight did not change much with power

systemsize (Fig. ;.)-1). Specific weights for the 35 kW and the 250 kW systemswere

_. 55.1 ib/kW and 51.1 lb/kW, respectively. With advancedtechnology, it was estimated

that the specific weight for a 100 kW systemwould decreasefrom 52.6 lb/kW to 35.1

ib/kW. This projected weight saving is partitioned as follows= (a) Replace inconel

oxygenand hydrogentanks with filament-wound tanks= 19.9%_(b) replace porous

nickel electrolyte reservoir plate in the fuel cell module with graphite=5..5%_(c)

improvedspace radiator= 6.#%! miscellaneous."2%! Total= 33%.

'1r .

!}" Componentreplacement life onthe fuel cell moduleswas determined to be 3.1 years. li

1 TLe critical life item identified was the pumps,with a replacement life of 1.1 years.

I

\

:i,:"" 41

!! ,
IOOKW

38 KW 100 KW ADVANCED 2B0 KW

TECHNOLOGY l, i • I i n

1

!, lAUNCH WEIGHT (LB) (NO WA) 1,927 5,257 3,606 12,778

I II I I I I

SPECIFICWEIGHT (LWKW) 68.1 62.6 38.1 61.1

I

- RF.SUPPLY-10 YRS liB) 3,000 8,6/0 ------ 12,500

i i I i

:' ENERGY STORAGE EFFICIENCY 0.80 0,604 --"-" 0,60

FUEL CELl. TEMP (°F) 140° 14_ 200° 140°

I I I I

,-

ELECTROLYZER TEMP (OF) 180o 180° 180° 180o

JI I I I

-:" FUEL CELL CURRENT DENSITY (AEF) 323 308 308 286

_ ,,

:" FUEL CELL V. 0.901 0.908 ---.-- 0.902

=',..... I I III I

' ELECTRQLY:'.;Eft CURRENT DENSITY (AtiF) 310 317 ---'--- 388

..o.

-_" l I I

-"_-_ ELEc'rRoLYZER V. 1,003 1,00? ------ 1,629

-.-._ SYSTEM PRESURE (PSi) 70- 200 70 - 200 70 • 200 70- 200

F/gum4.._I: United TechnologyStudy - _l_'md Summmry

-_,o . 42

' -)
TANKSAND REACTANTS 2B._

FUEl.CELL mM

ELECl"ROLYZERSYSTEM 17.8_

POWERCONDITIONER 12.4%

RADIATOR 18.7%

_ MIS4:ELLANEOUS m0.8%

100.0%

Figure 4.3.2: United Tachnology Study - Weight Dmakdown (100 KW)

_!i FUEL CELLMOOUI.U &l YEARS

_ EL_'rnoLV.OO..SS S,S 4.0VEA.S

!i: vAL.ves S.=vEA.s

I! PUMPS 1.1 YEARS

'(t

Ii, _. Figure 4.3,3: United T_,hnoloiw Study - Component Replacement Life

'.-.
Overall energy storage efficiency was approximately _0.4 percent. The fuel cell

system was designedto be fairly efficient (approx. 0.898 V/cell), but the water

electrolysis systemhad low efficiency (approx. 1.60 V/cell). Overall energy density

was calculated to be 38 W-Hr/lb, or _5.g W-Hr/lb excluding the radiators andpower

conditioning equipment.

United TechnologiesConclusions

Regenerable fuel ceU systemsfor spacestations are practicable and lightweight.

Technologyadvancementsshowthe potential of reducing weight 33 percent.

Critique

I The United Technolgiesdesign is a low weight design,thoughnot the minimum weight

universally preferable. High efficiency designs,though heavier, are judged as

o

: preferred for general spacestation needs.

One of the systems lossesshownis the power conditionerfor the electrolyzer, with an

efficiency of 9# percent. Raw solar array poweris seldomprovidedto the spacecraft

bus, for the power is usuallyregulated to constantvoltage by a shuntregulator or

° ' other type of controller. Thougha controller to doswitching and control functions to

the electrolyzer is desirable, this need not be a regulating power type of unit.

,_ The fuel cell module is designedto l#0OF (200OFadvanced design),and the

i! electrolyzer is designedto l$0OF. It wouldbe useful if both modulescouldbe designed

ti_ to the same temperature. A commonradiator couldthen be used,and excessheat

I_, from the one modulecouldbe usedfor temperature control of the other when needed.

'!i: Pumpswere identified as the critical life item, with a replacement life of hi years.

_il" This is misleading. The basisfor this was that hi years is the longest duration for

which there are life test data on fuel cell modulepumps. Since industrial pumpsof

_ many kindscan give long service without replacement, there is noreasonto believe

.... that a long life pumpcannot _lsobe designedfor aerospaceuse.

_i The hydrogenandoxygenweights were basedon the requirement that they be designed

i:: to a safety f_ctor of t._. This is the same designfactor usedfor many of the shuttle

I_: tanks, but may not be sufficient for the large numberof pressure cycles required for

: ..... 44
!i

I space stations. Note the discussion in section 4.1 on tank weights, and in section 5.5

I

tank optimization. ]

:1

The projected weight savings with advanced technology are large, and much dependent

:_.i: on tank weight analyses. The analyses of the metal and composite tanks were

apparently done separately, and different design criteria may have been used. It is

suggested that if interest develops in very lightweight systems, then tank weights

should be redetermined in enough detaU to eliminate any doubt, using fracture

mechanics design criteria. Also, though it is true that fuel cell weight can be reduced

by using graphite electrodes instead ot nickel electrodes and by operation at higher

temperature (200oF), these measures would require additional research and

development; even then, it is likely that there wUl be some sacrifice to life and

possibly also to performance. Nevertheless, the potential weight savings with the

= advanced, lightoweight technology are of great interest for applications which must be

...... optimized to low initial weight, for example, synchronous orbit.

' impact on United Teqhnology_s Conclusions i

i The design provided in this study is a light-weight, medium efficiency design. This

design philosophy is appropriate where minimum launch weight is paramount, perhaps

_" when a solar array of more than ample size is used, and when resupply cost is only a

minor consideration.._ higher efficiency, though heavier, energy storage system

(about 60% efficient) is judged to be preferred for general space station needs. This

_. WouLdincrease the w Jght and alter the performane of the described design. Also, due

__;. to the resulting lower current density with the fuel cell modules and electrolyzer

l_i" modules, a longer replacement interval should resuLt.

The power conditioner for the electrolyzer need not perform power rei_ulation

• functions, since this is accomplished by the solar array power regulator. The

efficiency of the power conditioner should therefore increase from 96 percent to

iil greater than 99 percent.

Operation of the fuel cell modules and the electroiyzer modules at the same

!ii: temperature will eliminate one of the two radiators plus accessories. Total radiator

i_ size will not change much, but system reitabi_ity will be improved. Minor changes in i

_ operating performance of the RFC system will also result.

i.

q.

45

I

.i
Ii®--

An evaluation shouldbe made of the expected pumpreplacement life, andthat value

shouldbe usedeven if not fully verified by life testing. Extra redundancyshouldbe

added, if necessary,to bring pumpreplacement life compatible with other

components. ,

Tanks probably needto be designedto a higher safety factor. This will increase

weight a small amount.

_._ LOCKHEED STUDY

Objectives

The principal objective of this study (Reference 3) was to develop technical materials

and methods for evaluation of regenerable fuel cells for the Modular SpaceStation.

The main output of the contract wasa Design Data Handbook,LMSC-Di:_9786, which

was intended to be a working document for useby electrical power system designers.

A secondaryobjective was ¢ocompare regenerable fuel cells with nickel cadmium

: batteries, and also to assessthe effects of integrating the electrical power system

!i_: with the environmental control/Itfe support and reaction control systems. This study

_: was performed in 1972 prior to the selection of the Shuttle fuel cell supplier.

{.

ii Requirements

!i: The main electrical power system requirements were for a power level of 10 to 3_ kW,

i with 2_ kW selected as the nominal level. Output power was selected as 112Vdc (+18

° i!I Vdc, -22 _/dc). Another requirement was the needto provide electrical power at a 3_

Watt level during a 60-day space station buildup without the availability of a solar

array.

Major Flndinlqs

.... The main output of this contract wasa data handbook. Equationswere formulated

, defining weight relationships, and these were programmed into a computer. One set of

output data was generated, with total system weight given for regenerable fuel cells

i!- and nickel cadmium batteries. Total equivalent weight penalty includes the solar

t array and drive, inverter, shuntregulator, charge controller or power controllers,

batteries, tankage, hydrogenand oxygen,fuel cells, electrol_ zers, andancillaries.

I

_........ 46

i? ........... =: =
I GD

Resultsbasedon a continuous25 kW systemare summarized in Figure _._-t. The

complete electrical power system with regenerable fuel cells is 25% _ighterthan with

nickel cadmium batteries. Subtractingout the weight of the solar array anddrive

system, the relative weight difference is even greater. The energy storage system

specific weight Is very high compared to todayts technology,calculated to be 38_ and

63l lb/kW for regenerative fuel cells _nd nickel cadmium batteries, respectively; it

shouldbe recognized howeverthat these values include inverters andpower

controllers.

One finding of the Lockheedstudywas that the Lockheed electroiyzer was prcposedas

the best oneof the available options. This concept requires electrolyzer units

circulating electrolyte with pumpsand bubbleseparator; two absorbentmatrices, on,=.

being for gas-liquid separation! dLfferential pressurecontrollers to maintain

=_: electrolyte equilibrium; a systemto sensegasin the liquid KOH electrolyte; a heat

'}: exchanger; a closed electrolyte reservoir; a water deionizer; a nitrogen purgesystem;

!I'

valves; and instrumentation and control with sensorsand transducers. This system

:?: operates at about 10oc, which is not compatible with the 60oc fuel cell temperature;

:_ this low electrolyzer temperature wouldalso require larger radiators for heat removal. :t

:_ Thus, it is evident that this is a complex system. This systemis no longer a viable

;-!: candidate, for its development never has materialized.

F t

i=i LockheedConclusions

!-: Lockheed concludedthat a regenerable fuel Cell electric power systemoffers up to a

i---

_: 26% weight savingover nickel-cadmium batteries. Development of the water

i electrolysis system wasdetermined to be the pacingitem, and a development program

_: was recommended. The Shuttle fuel cell was deemedappropriate, with modification

i_,., to 115 Vdc design.

.... Critique

,.. The Lockheed studywas donein 1972, andshouldbe viewed as a precursorstudywith

!..,.

.. many formative concepts. Thus,the electrolyzer systemis complexwith a large

-, numberof ancillaries. The weight modelgiven is nolonger valid, resulting in weights

:, considerablyheavier than those obtainedin current analyses. For example, the weight

difference between regenerative fuel cells and nickel cadmium batteries (6174 lb for

25 kW) is more than twice the weight of current regenerable fuel cell systems.

. _,

_ 47
REGENERABLE Ni-Cd BATTERY

FUEL CELL SYSTEM SYSTEM

I _ _

WEIGHT OF CONTINUOUS Z KW SYSTEM 8,400 KG 11,200 KG

WITH SOLAR ARRAY

WEIGHT OF SOLAR ARRAY SYSTEM 4,043 KG 4"043 KG

:>" WEIGHT OF ENERGY STORAGE SYSTEM 4,387 KG ?,187 KG

II

ENERGY STORAGE SPECIFICWEIGHT 384 I.B/KW B31 I.BIKW

I II

*'[I DEVELOPMENT COST _IL1M ------

,, Figure4.4.I: Lo_he_l Study- SelecaedSummary.,

ic.

!,

,!

-i

,i

i

p

i*i
One shortcam" of the study is the lack of weight breakdown information or sample

calculations. Calculations were doneby computer, and only total system weight was

reported.

Impact on Lockheed Conclusions

The energy storage system given in this study was an early design effort, and thus is

heavy compared with the more nlature technology available today. The major

concilJSlon was tl_t a RFC system for a space station would be lighter than a NI-Cd

1 battery system, and that conclusion still appears to be valid. Comparison of the RFC

, system and NI-H 2 batteries was not made, however. The Design Data Handbook is

considered too much out of date to be useful today.

! 4.5 LIFE SYSTEMS STUDY

t'_'i. Objectives

_; The objectives of this study (Reference _) were to evaluate the Modular Space Station

andtheapplicationaregenerf=ulelce.(Rvc)system. *nRFcenergy storage

i',_ system was to be defined, the RFC technology reviewed and characterized, and the

!!_ pacing technologies identified. Ni-Cd batteries were to be traded with the RPC

_ _! system. This study was completed in December, [972.

_: Main Requirements

_ The Modular Space Station, defined by North American Rockwell, is a 6-fnan station

_ili with growth to 12 men. Orbit altitude is 2_0 to 270 nautical miles, with 270 nautical

/_ miles baseline, inclination is 2go to _o, with _o baseline. Mission duration was t0

_

*{. yeats, based on a 5 years initial station with the growth version lasting another five

,

, years. of 1g.7 ThkWe,laoads peakareloadnomofinal 21lykWl_and kW ato2_30houkWr. avThe erageiniti

load al sof tation17._hakWs a. nomina An l load

: emergency use of expendables is required at h7_ kW for 96 hours. Voltage is 2_0/416

_._ Vac, ¢00 Hz, 3 phase, with t 12 Vdc inpul' from the solar array.

!I The major contribution of this study w_s the review and characterization of

i regenerable fuel cells, and their applicntlon to a particular space station requirement.

i_, Characterization of RFC systems is don,-,thoroughly enough that the report still

!

I

,' 49

L:.

.....2-- _ ....... ., _"_" ...."" "
iil:

!

i functions as a useful reference source,limited to 1972 technology. The pacing

_ technology Identified In the study was the water electrolysis subsystem.

Figures _._-t to _.J-3 showthe major characteristics o,_ the RFC system design, and

tabulate someof its key attributes. Since the poweroutput is 2_0/_t6 Vac, _00 14z,3-

phase, inverter= are required from the output of the fuel cells, which degradesoverall

energy storage efficiency. To permit efflcie,lcy comparisonswith dr _ystems i_hichdo

not re_|u=reirwerters, Figure _.j-i also showsenergy storageefficiency with and

without inverter=.

In the cource of this study, a numberof trades were conducted. Figure _.J-I

summarizes the trade betwe_,_Ni-Cd batteries anda RFC system. It was concluded

from this trade study that the RFC system is the lowest cost, has the lightest launch

weight, and requires the least amount of solar array. WhenonecomparesNi-Cd vs

RFC designssized for the peak load (Figure _.J-l), it is seenthat the RFC system

_=_i requires 9 percent larger solar array, but is _6g0 Pour,dslighter. Whenboth systems

f

t' are sized for the 2¢ hour average, they both are able to showa reduction in solar array

l

;: size with a concomitant increase in electric-,_lpower system v:eight. With sucha 2_-

'i

._ hour sizing criteria, the RFC system requires a solar array g percent larger than does

!i_ the Ni-Cd battery system_but the RFC systemis 9800 poundsUghter. On this basis,it

_ was reasonedthat it is practicable to designthe RFC system to the 2_ hour average,

_;- with capability providedfor peak loads,but it was judgedthat the Ni-Cd battery

;: system shouldbe siz_.,dfor the peak load. It is this rationale that resulted in the

_ conclusionthat the RFC was best in all major respects.

ti_.

ii' One of the trade studiesconducted was a determination of the best water electrolysis

t" design approach for an RFC system. This wa_ the weakest technologyarea in that

-i; time period, and there were a numberof competing technical approaches. The

ii competing water electrolysis conceptsdiffered in these major respects=

I,

t,!" t. Nature of the electrolyte -- alka!ine was preferred becauseweight was least and

il there is lesssensitivity to contamination.

!

I'

2. Method for electrolyte incorporation within the cell -- the selected method was

_ to hold the electrolyte in a porousmatrix.

],

'; 50
tl

NkCd b'Y'_r'EM RFC SYSTEM

i L me

LOAOII --

PEAK 21 KW 21 KW

24 HR AVE 17.4 ION 17.4 KW i Jl• ii

SOLAR ARRAYSZED FOR PEAK 7,790 FI"2 8,500 F1"2

SIZED FOR 24 HR AVE 0,980 FT2 ?.1140I1.2 i

ENERGY STORAGE WEIGHT -

SIZED FOR PEAK 9,172 IJ (4,482 LB REFERENCE)

SIZED FOR 24 HR AVE (12,012 LB REFERENCE) 2,818 LB

ENERGY STORAGE EFFICIENCY 0.825 0.826

t0.t4 W/O INVERTER) _S82 W/O INVERTER)

ii', I_ST -

i: 0EVEI.OPMENT 112.7M _4.7M

!I_ HARDWARE $7.5M 1.3M

i OPERATIONS mIIOJH_I _m.SM

ii ?AL ,.TM

!_ SPECIFICWEIGH? 438 LWKW 134 LB/KW

!, i i

1"

j: Figure4.5. t: Life SynwmlStudy - SelectedSummw'y

l:

- {!

!,,

.ll

_ |:'

f
REVISED DESIGNS

ORIGINAL M88

OPTIMISTIC MAINTAINABLE

WATER ELECTROLYSIS $UiI_ 1,288 1,(i08 4,016

FUEL CELL SUBSYSTEM 808 .'.e: .'O'-

,_ H2 STORAGE TANK 748 784 784

:; 0 2 STORAGE TANK 380 380 360

_.... 1420STORAGE TANK AND PUMP B0 80 80

" REACTANT 40 40 40

i'_

i :'.;, FIJJMBING, REGULATOR AND VALVES _ 262 282

t-'.

;_ MOUNTING AND SUPPORTS 386 386 388

INVERTERS, SEGUENCER$, WIRING 92 184 184

m m

TOTAL 4,044 I.B 4,492 I.B 6,000 LB

'i1

52
MAXIMUM SUSTAINED POWER 7.0 KW

MAXIMUM POWERWITHIN VOLTAGE LIMITS 10.0 KW

VOLTAGE lIMITS 112VOLTS (+6-11_I

MINIMUM POWER 200 WATTS

MINIMUM REACTANT SUPPLYPRESSURE (10PSIA

MAXIMUM COOLANT TEMPERATURE (TO FUEL CELL) 120°F

SPECIFIC REACTANT CONSUMPTION 0.82 lJ/KW-HR

CELL AREA 0.508 FT2

NUMBER OF CELTS 32/Sl"ACK

NUMBER OF STACKS/'/_ 4

ELECTROLYTE KOH

CURRENT DENSITY 123 (100- 380) AEF ;

OPERATING TEMPERATURE 1S0(190°F - 260°F)

OPERATING lIFE 10,000 HR (ADV. SHUTTLE FUEL CELL)

OVERLOAD 2 TIMES NOMINAL RATING

PER UNIT TOTAL OF 4 UNITS ii

WEIGHT, LE 202 808

UNIT DIMENSION8 (L x W x H) 13x 13x U IN -.----.

VOLUME, FT3 5.4 24

ACCEmORIES WT, L.B

8AI"I"ERIES 10 40

PLUMBING, REGULATOR AND VALVES 16 64

'i MOUNTING AND SUPPORTS 22 88

_ INVERTERS 5 20

_. SEQUENCERS 3 12

:! WIRING 1§

F/gum 4.5.3: MSS Fuel Doll ._bsysmm Characmri_/cs

ii:
z

!

3. Waste heat removal -- conduction to cooled fins was selected because it is

simple and avoids the use of pumps and phase separators associated with

circulating liquid systems.

t_. Makeup water addition -- a static feed gas phase method was selected to

eliminate gas-liquid separators and to minimize the effects of contamination.

Because the selected baseline was a 55o inclined orbit, there are cyclic variations in

the dark/light ratio. Since the electrolyzer must be sized for the highest dark/light

ratio, there is excess electrolyzer capability much of the time. Though this was

identified, no use was made of this capability in the study. It may be possible to

exploit this unique capability of the RFC system, particularly for electrolysis of water

for orbit makeup propulsion fuel.

An additional item looked at briefly was whether or not it would be worthwhile to

'I!: integrate the RFC system with other systems. It was concluded that integration of

h

'ti the hydrogen and oxygen systems is worthwhile for power, reaction control, and life

i_; support. The system was sized based on 2_hour averages.

Life Systems Conclusions

_:_ Life systems concluded that the RFC system is the best energy storage system for the

v:_: Modular Space Station. It was found to be superior to Ni-Cd batteries in weight, solar

.,!2_:,_" array size, and cost. It was also concluded that development of water electrolysis

Critique

The RFC system was determined to be better than the NI-Cd battery system because

it was judged that it would have been impractical to design the battery system for a

2#-hour average. The assumption made was that if the Ni-Cd battery were designed

: for a 2_-hour average, the large weight increase needed for the battery system would

not offset the saving in solar array size. However, no trade was made to verify that

i_ this is a correct conclusion. Fortunately, this is a moot point, for we know today that

!{ the efficiency of RFC systems can be comparable to that of batteries, approximately

! 60 percent to 67 percent.. Refinement in energy storage efficiency calculations would

_i!: result in a modest reduction in efficiency of the Ni-Cd battery system (shown to

/

;:" become 69._ percent without invcrters) and a modest increase in efficiency of the i:

,i ?"

54
!i® ®

RFC system (shown to become ,_8.3 percent without inverters). Thus, it would not be

possible for battery systems to have any advantage with respect to a smaller sized

solar array.

The cost estimates developed did not play a big role in the selection of the RFC

system. However, the estimates given do not appear to be vail& Assuming a shuttle

fuel cell as developed, development cost was $1#.7 M for the RFC system, and $13.7 M

for the Ni-Cd system. Considering that Ni-Cd technology is relatively mature, the

cost differential is too narrow. No substantiating deta_ was provided for this or the

other costs shown in Figure b.J-h

Not considered in this study was the nickel hydrogen (Ni-H 2) battery system, which

was not available at that time. If the expectations for higher allowable depth of

discharge and longer Life are realized, then this battery system could be better than

i the Ni-Cd battery system, which would affect the results of this study. The omission

of Ni-H 2 batteries from the Life systems study was not an oversight, but merely a

-- reflection of the state of technology ten years ago.

To lower the dew point of the product gases, the design involves throttling of the gases

from 20.4 arm. to h0 arm. This results in efficiency loss for the system; also, the dew

_t point reduction to 36OF may not be sufficiently low.

! i_'

!_i!i One of the design decisions was to integrate the hydrogen and oxygen system used for

'I"" the RFC with the hydrogen and oxygen systems used for reaction control and life

i _" support. It was expected that this would result in advantages in cost, reliability and

_ logistics. It is true that use of common gases for these systems would have important

i:li advantages, but there also are significant disadvantages to full integration of three major subsystems, namely, (1) the IL(e support system requires high pressure oxygen

_. for extra vehicular activity, and this possibly is not compatible with the optimum

i for RFC; (2) the several will have and pressure subsystems likely separate contractors,

:.; interface definition would be a serious obstacle throughout the program| (3) subsystem

" testing would be compromised and more costly.

Finally, it may be emphasized that progress has been made in the technology of RFC

I,

). systems in the ten years since this study was done. Reanalysis today in this, as well as

_ 55

_-2"

'ii(
all the other contractor studies,can be expected to result in improvements in all

phasesof the system.

Impact on Life SystemsConclusions

The conclusionthat an RFC system is tighter than a Ni-Cd battery system is expected

to be reinforced by any further analysis. Ni-H 2 batteries were not considered, soit is

not fair to say the RFC systemis best until those two systemsare properly traded.

Whether or not all hydrogenand oxygensystemsare integrated shouldhave little

effect on system weight, for integration doesnot save in weight of the major

componentsfor a low inclination orbit, suchas 23o. Integration or cross-couplingis an

excellent idea for use in the event of failures or emergencies for suchorbits, but the

decisionshouldhave little impact on weight or cost. For high inclination orbits such

as j_o, however, there can be a weight saving, for the periodic excesscapacity of the

eiectrolyzer can be used for electrolysis of water for orbit makeuppropulsionfuel.

Also, when the dark/light ratio becomeslow, the excessavailable power may be

planned uponfor experiments.

Life support systemsshouldnot be integrated with the electrical power systemunless

this is shownto be worthwhile by an in-depth study. However, there is little weight

impact of such a decisionto integrate or not integrate.

_i #.6 McDONNELL DOUGLAS STUDY

Objectives

The energy storage part of this study (Reference J) supportsa SpaceStation Systems

AnalysisStudy to evaluate alternatives for the Space Construction Base. The purpose

of this study segment was to evaluate energy storage alternatives to the nickel

cadmium battery system,operating with a solar array. Thisstudywas completed in

September 1977.

Requirements

The spacestation hasan electrical power level of 100 kW average. A peak powerof

_! #$0 kW is required for 15 minutesseveral times each1day. Emergency powerrequired

_l is _ kW for 180hours. The spaceconstruction basewas required in 198_, using

technology available in 1980. Missionlife was l0 years. The power systemguidelines

.!

56 L
I,

were for 800 Hz, t 15 Vac, though the energy storage section was considered to be dc

power.

Major Findings

Ni-Cd, Ni-H2, and RFC systems were compared. The RFC system was found to be the

-" Ughtest, having the longest life, and requiring the least amount of resupply. Cost of

_ the RFC system was 8 percent lower than the battery system, but the efficiency was

: also lower. Although the RFC system appeared to be the most attractive, it did not

meet the requirement of technology readiness in 19g0, needed for space construction

base operation in !95._.

Comparisons between the competing energy storage systems are summarized in Figure

;.6-1. In comparing Ni-Cd and Ni-H2 batteries, no clear advantage of one over the

__ other is apparent, for cost and life were equal The N|-H 2 battery system was Ughter,

;' with resulting lower resuppiy weight, but its efficiency was a little less than NI-Cd.

The RFC system was also found to be best for satisfying the emergency power

requirement of 5 kW for 180 hours. Both the alkaline and solid polymer iuel celt

technology appeared to be adequate for the RFC system.

:!_ Meeting the peak power requirement of 8_0 kW for l_ minutes several times each day

had a big impact on the battery energy storage systems, forcing discharge at very high

"_ rates. It was assumed that the batteries could tolerate the high discharge rates

- without additional weight.

,J

_" McDonneLl Douglas Conclusions

1. A solar array power source is better than other approaches.

_; 2. Ni-Cd batteries are best for energy storage within schedule restraints.

3. Ni-H 2 batteries _re ¢tso attractive, but are handicapped by lack of test data.

_ ;. The RFC system would be the best system, except that it could not be fully

_' developed by 1980.

- 57

it...
REGENERATIVE

ADVANCED

N i Cd Ni Cd Ni H2 FUEL CELL SYSTEM

i I i I

WEIGHT (KG| 17,470 10,73Q 7_13 2.748

SPECIFIC WEIGHT (LIVlCN) 388 237 159 81

10.YEAR RESUPPLY(KG) 41,919 25,748 17,386 2,994

RESUPPLY DOUELING TIME (YEARS) 4.2 4.2 4.2 9.2

EFFICIENCY 6E.7_ 60.8% 54.1%

DEPTH OF DISCHARGE 14,Mi 14,E_ 18,6% (33%}

LIFE (YEARS) 3,3 3.33 3.33 §

COSTDEVELOPMENT ,1. $1. SI0M S40M

HARDWARE $32M ID4M SliM SI0M

-|

it OPERATIDNS 4) ._M S42M .M

! IO-YEAR COST _ Sl37.5M Sl35.2M Sl16.7M

• OPERATIONS COSTSOBTAINED FROM TOTALS BY RATIOING ENERGY STORAGE FRACTION LESS

DRAG PROPELLANT TRANSR)RTATION

i{

!Iii Figure4.6.1: McDonnell DouglasStudy -. Selecn_lSummary

ll'

il

ii

i!"

}:

58

i °
Critique

Energy storage efficiency was established at 62 percent for Ni-Cd, 85.7 percent for

advancedNi-Cd, 60.8 percent for Ni-H2, and 5_.1 percent for the RFC system.

Becauseof the low depth of dischargeof the batteries (1_.5 to 18.6 percent), the NiCd and Ni-H 2 efficiencies reported are believed to be realistic. The improvement in

performance expected for advancedNi-Cd batteries is unlikely, for, althoughsome

research is continuing, there is very little emphasis today on the development of

• advancedNi-Cd bakeries. However, the RFC system efficiency of _#.1 percent is too

low, since overall efficiencies in excessof 62 to 6_ percent are possible.

The effect of high peak powerrequirementson the choice of spacestation energy

storage systems is important to determine, for variable power demandsare more

_/pical of spacecraft than are constant power demands. This study included a peak

power requirement of _50 kW for 15 minutes several times each day. This would

impact the selected Ni-Cd battery in the following ways:

h The discharge rate is at least four times greater than normal, which would

produce low voltages, probably lower than the allowable limit;

2. Maximum depth of discharge on the batteries would be increased, at least

_i" double. This increased depth of discharge in combinationwith the high discharge

_ rate would be expected to impact battery life!

,,!,- 3. Heavier batteries or more frequent resupplywould be required. The RFC

system, designedto low current density for high efficiency, is muchbetter suited

to suchpeak loads than are batteries.

A conservative depth of discha_'ge(18.6 percent) was selected _.-.'.tri,e Ni-H 2 batteries

!,. due to the lack o£ test data. Although even today there are not sufficient test data to

justify selection of Ni-H 2 batteries, the premiseshouldbe m_,.d=that this system will

eventually permit longer life and greater depth of discharge than Ni-Cd batteries.

i._,_ more attractive with respect to weight andresupply. Tentative rejection of NI-H 2

i! batteries wouldstill be justified on the basisof insufficient test data, however.

i,- 59

!!
_. A trade study was conductedon emergency _ _versystems to meet the requirement of

: _ kW for Ig0 hours. The normal battery compliment is not adequate, for the capacity

:. is exceeded. The study showedthat a RFC system is very attractive for this need.

: However, although the NI-Cd battery was selected for spacecraft energy storage, no

:_" extra weight increment can be identified for emergency power.

4"

i; The choice of a relatively low battery depth of discharge is judged to be a wise one in

principle. The choice of the best DOD is not entirely one of that is amenable to

i-

- analysis, but depends also on judgement. The reason is that the data on mean cell life

:_ are not adequate to determine whenthe first ceil failure is most likely to occurin a

battery. Low DOD is also consistant with the need for high reliability, and helps

!

t reduce the high risk of failure due to the large numberof cells in series.

The abovegeneral argument for low battery DOD also applies to the RFC system in

the form of low current density. However, with the RFC system, the low current

density hasadditional benefit_, particularly higher efficiency. The RFC system

designedin this study is muchlighter than batteries, and it would be appropriate to

increase RFC systemweight and obtain efficiencies equivalent to thoseobtained by

batteries.

Total10-yearcostforenergystoragerangedfrom$316M to $3#9M forallenergy

options.Care mustbe usedwiththesecostestimates,becausetheycontainother

power systemelementswhicharenotstrictlenyergystorage.The objectivewas to

allowcostcomparisons,ratherthanobtainabsolutcos e ts.

=ii Impact on McDonnell Douglas Conclusions

i. The Ni-Cd batteries appear to be undersized for the peak power requirement.

-_!! The peak power requirement makes the energy storage selection much more

'_ favorable to the RFC system.

2. Although someresearch is continuing onthe NI-Cd system, development of

If' advanced NI-Cd batteries is currently receiving relatively little emphasis,and

i the projected improvements are unlikely, Therefore, Ni-Cd batteries of 't

.4. advanceddesign cannot be considereda viable option. I

:"ii,, 60

;i, <

:-,p
3. An RFC system redesignedfor efficiency comparable to that of batteries would

require a weight increase, but the RFC system weight shouldstill be less than

that of batteries.

_. The RFC system apparently wouldhave beenselected if the schedulehad

permitted it.Since there isnotnow a schedulewhich must be met, the choice of

Nl-Cd batteries would not continue to apply.

_.7 TRIF STUDY

TRW Study S!atus

A final report has not yet beenreleased on the TRW study. Therefore, this review of

that study is basedonthe progressreport relating to energy storage, the mid-term and

...... fina_ oral reviews, and a publishedtechnicaJpaper. These are identified respectively

in references 6a, 6b, 6c, and 6d.

Objectives

The objectives of this studywere to define and assessmulti-hundred kW power system

) concepts and technology development requirements. Energy storage was oneof the

,_ items underthe electricad power system design;this review is Limited to that item.

Main Requirements

_i Task I requires that an electrical power system reference designbe estabLished. This

requires an analysis of energy storage selection and definition uf battery costs. The

l electrical power system has seventeen 16.2 kW channels to support 250 kW of payload

power plus 25 kW of housekeepingequipment. TRY/refers to this as a 2_0 kW system,

whereas in this report it is taken to be a 275 kW system. Each channel includesone

160-cel I, 150 ampere-hour Ni-H2 battery for energy storage. The orbit is 160

nautical miles, 28.5° inclination, with an occult duration of 36 minutes. Busvoltage is

wh220 Vdc _+20Vdc. Solar array costsare basedon a manufacturing cost of t30./Watt, ich reflects a prior solar array costing exercise.

Major Findings

An electrical power system was developedusing seventeen 16.2 kW channelsto support

the ioeds. Onebattery is devoted to each channel. This wasdone to avoid paralJ'_ling

high voltage batteries_ _d precludes the need for battery voltage matching reklulators.

61

I_i I-- ....... ®
Also, the isolation switch gear n_ednot operate at high voltage andhigh current.

Additionally, it is claimed that a higher efficiency systemwill result.

NI-Cd, NI-H29 and RFC systemswere compared,and Ni-H2 batteries were foundto be

the lightest andcheapestover'ill for a 30-year mission (Figures 4.7-! to _.7-5). For

short missions,less than five y-.ars, the three approacheswere very close in both

respects. The RFC system wouldbe the Llghtestapproachexcept for the propulsion

fuel to preclude orbita_ decay.

The cost of the electrical power systemwasdominated by the cost of Ni-H 2 batteries.

Initially, these cost half as rhuChas the solar array ($22._ M vs. $50.7 M), but by the

end of 30 years the battery cost was doublethat of the solar array ($118.6 M vs. $_0.7

M). It was concludedthat significant economic benefits remain in reducingelectrical

power systemcost, but that thi.-,requires considerableinvestment.

A _0 percent reduction In Ni-H 2 battery cost is projected for 1986by increasing cell

size from 50 AH to 150 AH. A 75 percent cost reduction is projected by the 1990'sby

increasing cell size to 250 AH with a commoncontainer design,with accompanying

improvements systemcost is inpossible llfe. Ibyt isredconc ucinluded g battthat ery acost greathrough ter reduction improved in energy technolostoragygthean by

_i,.. extending battery Life.

'l_ The optimumdepth of discharge for Ni-Cd and Ni-H 2 batteries was calculated to be 30

percent. This value was usedin the systemsanalyses. i

:_ TRW Conclusions

_:. h A power systemwith 17 separate channels is consideredbest. Each channel

2 would have its own battery.

-.R.

_. 2. Ni-H 2 batteries were foundto be the llghtest and cheapestfor a 30 year mission.

;_: 3. Batteries are the biggest power systemcost driver for a 30-year mission.

f! 4. A greater reduction in battery cost is possibleby technologyimprovementsthan

i_....... by extending battery life.

?

6Z
I i I iii

FUEL CELL IbLUS

PARAMETERS NICKEL CADMIUM NICKEL HYDROGEN

ELECI'ROL_SlS

| ii l i

svm. com,

ENERGYUORAG(MS) S 24= 101 1o1

SOLAARRRAY(MS) 10 1. 3.

ALTITUDE MAINTENANCE (MS) 63 51 99

THERMAL SUPPORT (MS) 6 5 S

DEVELOPMENT (MS) 1 6 60

4mnmm m ammm_

TOTAL (MS) 321 180 291

SYSTEM WEIGHT (30 YEARS}

,m .

_,' ENERGY STORAGE (L6) 364,(i00 96,000 44°500

:.;I_ SOLAR ARRAY liB) 14,700 14,100 27,LEO0

iI ALTITUDE MAINTENANCE (LB| I0e,800 .94,800 214,700

!i THERMAL SUPPORT liB) 3,100 2,600 5,200 d

_- m m emlmmmmmm

:7.

_, TOTAL (LB) 491,200 216_)0 2E2,000

I

!:. . ..

if'

iJ Figure 4.7.1: TRW Study - Energy Storage Cnmparisons

!L

1

i'

!Y

I

Ill,,•

Il

i

1

i'

i'_

h
400

_60lOWSYSTEM

ENERGYSTORAGEWEIGHTPLUSSUPPORT:
00 • i

210 KW SYSTEM

380 - ENERGY STORAGE C087 PLUSSUPPORT:

SOLAR ARRAY

3_0 .. HEAT EXCHANGERS

RADIATORS

;; _I ZO

i ¢: 1W

; 0

i '

._!

i ..

i=

_ 0

; , 0 8 10 16 20 25 30

..: SATELLITE OPERATIONAL LIFE - YEARS

i

"" Figure 4.7-3: TRW Study - Co_ Comparison
I

.... NICKEL NICKEL FUEL CELL PLUS

PARAMETER CADMIUM HYDROGEN ELECTROLYSIS UNIT

II I I I I I I ii

ENERGY STORAGE (WH) 306,000 460,000 200,000

DEPTH OF DISCHARGE (_) 49 33 75

AVERAGE L!FE (YR) 2.711 7 4.8 (STACKS)

1.1 (PUMPS)

CELL SIZE 100 AH 160 AH 270 AMPERES(FUEL CELLS)

1383AMPERES(ELECTROLYSIS UNITS)

CELLS/UNIT 170 160 308 FUEL CELLS

136 ELECTROLYSIS UNITS

UNIT QUANTITY 17 17 S FUEL CELLS*

8 ELECTROLYSIS UNITSe

HEAT LOSSES(KW) 26a. 32°° 79 (SUNLIGHT)

231 (ECLIPSE)

TEMPERATURE (°C) 0 Of 40/76

INPUT I_WER (KW) 229 220 430

Z_ SPECIFIC WEIGHT (WHJI.6) 10 26 38tt

WEIGHT

INITIAL (LB_ 33,600 18,O(]O §,77E

REPLACEMENTS(LB) 334,000 77,000 i38,709

COSTS

INITIAL (MS) 20.4 19.1 14.3

REPLACEMENTS(MS) 223 82 87

DLVELOPMENT (MS) 1 <S >60

• ONE UNIT ADDED FOR RELiABILiTY

•Q 9_MINUTE AVERAGE

? NOT OPTIMIZED

?1 NOT SCALABLE
°.

I • ,J I

FUEL CELL PLUS

NICKEL NICKEL

PARAMETER ELECl'ROLYSIS CADMIUM HYDROGEN

UNIT

• • I I II

" SOLAR ARRAY

POWER* (KW) 301 288

MFG COST (MS) 9.0 L? 17.0

LAUNCH (:O51"ee (MS) 9.4 9.1 17.7

AREA ($Q FI_ 21,(100 20,700 40,(100

i_ WEIGHT .e_ (LIB) 14,700 14,100 27,600

!_ HEAT EXCHANGERSml

_ WEIGHT (LB) 800 880 B13

MFG COST ? 2.4 MS 2.4 MS 220 KS

RADIATOR

WEIGHT tt (LB) 2,300 1,840 4,646

MFG COST (MS) 1.8 1.B 3.8

ALTITUDE MAINTENANCE (PROPULSION FUEL)

I

WEIGHT (LB) 108,800 104,500 214,700

LAUNCH COST (MS) B3 51 99

• BEGINNING OF LIFE, BASEDON 20_ DEGRADATION OVER 30 YEARS

ee BASED ON VOLUME REQUIREMENTS (NS KW/BHUI"rLE)

AT 48 W/KG (20.6 WILE)

1' AT $20.000 PER UNIT

tt PUMPED LIQUID RADIATORS. 5.B3 KGJM2

Figure4.7.6: TRWStudy - EnergyStorageSuDportP3rarnerers

66
It is not clear that the seventeenseparate power channel system is the best approach.

Unless the typical load is relatively small, it will not be practical to divide the loads

evenly between the seventeen channels. That will result in some batteries being

dischargedshallow, and others being dischargeddeeperthan designedfor. Also, since

each energy storage unit is a single-thread design-- onecharger and other batteryrelated equipment -- it is not clear how reliable the systemwill be, especially in view

ot the useof 160 cells in series. For the trade studyusing the RFC system, only four

fuel cell stackswere used;it is not clear how this couldbe consistent with the

seventeen channel concept.

!:i_l Much of the energy storage system results are closely associated with cost projections.

_! This is a difficult task, andit is difficult for people to believe in large projected cost

savingsfrom improved technology, which often is counter to their experience. Part of

the problem is the needto distinguish between the potential cost saving and what

might realistically be expected. This problemis made worseby the fact that, at least

in the preliminary reports available sofar, there is inadequate supportfor someof the

conclusions.

H we first addressthe quest.;-nof what is possible,then it is certainly possiblefor a

_ 40% reduction in Ni-H 2 battery cost for 1986by increasing size from _0 AH to 150

b AH, for there are real cost savingsobtainable with increased size. This is also a

i_ reasonableexpectation, with the qualification that sucha battery couldbe built in

i! 1986,but suchbatteries couldnot be useduntil several years later whenlife wouldbe

verified. A 7_% reduction in Ni-H 2 battery cost by the 1990'susing a common

t pressurevessel designis .,'alsopossible,for projections have been madeof cost

1. ruductions upto 8_%. What may be considereda reasonableexpectation is another

iii question, andmustconsider manyfactors. Solararraycostsof$30per wattdoesnot

_ even seem possible,but that item is beyondthe scopeof this evaluation.

!;

Ii_ The major cost emphasisin this studywas placed on the initial manufacturing cost of

!_, the batteries. This is very important in many spacecraft applications. However_ for

_!i spacestations, like the 30-year station evaluated, initial purchasecost of batteries is

! small comparedto the total life cycle cost, on the order of 6 percent. For example,

_, Ni-Cd batteries are relatively inexpensive,but the total life cycle cost is high because

_ battery life is short relative to the envisionedcycle life of Ni-H 2 batteries or the RFC

':_ 67

i! '

(4
system. Thus,technologyadvancementsmust not only include those factors relating

to low manufacturing cost, but must a/so result in significantly improved cycle life.

Life improvement is a much more difficult undertaking; it is a topic the i:ldustry could

never come to grips with on the Ni-Cd system_andit is risky to dependon

breakthroughsin cycle life with the Ni-H 2, In fact, the emphasison large cells can

run counter to llfe improvement, for large cells are expected to have higher failure

rates than smaller ceils. Consequently, if the assignedlife of 7 years at 33 percent

DOn with a 160-cell battery doesnot _lso materl_.!ize (in addition to the 75% cost

reduction), the total life cycle cost of the Ni-H 2 systemwill increase.

There are a numberof cost-related iterr.._which appear to be questionableor deserving

of commen,ttn the TRW study. These are not central to the general cost conclusions,

bu'tare worthwhile to identify."

a) Battery costsare prolected to be 1.52 times the cell cost. This compares

with 3.05 times the cell costsfor the NASA standard Ni-Cd 60 AHr

battery.

b) Dependenceoncost reduction by development of large cells may not be

werkable if _ 250 kW spacestation must evolve from a much smaller size,

_:. or if a muchsmaller size is ultimately selected as the baseline. Assume

i_ the TRW choice of a 15OAH cell is appropriate for a 250 kW system with b

_:, seventeen channels_if then a 50 kW system were required with $ channels,

i!i the ce_ size required wouldbe 6_ AH. This size is essentially state-of-theii_.... art; major cost savingsexpected from increased ceU size shouldbe

i_I neg'_[gibie, butsome costsavingsfroma common designapproachshouldbe

I_ expected.

c) TRW experience of $2300 per ceil for _0 AHr laboratory Ni-H 2 ceils was

usedas part of the basis for cost estimates, whereas the current budgetary

Ir, pricefortheAF-designed_0AHr LEO ceilsiscurrently$5000percell.

d) The optimum depthof dischargeforthebatterieswas determinedtobe q9

_ percent for Ni-Cd and 33 percent for Ni-H 2. This was basedonanalysisof

Ni-Cd data and Ni-H 2 projected cycle life data. This results from the I,

_. calculation that the product of meancycle life and DOD is greatest at

1 ')
q

• those dischargedepths. The problem is that we must consider not merely

• when the average cell fails, but whenthe first few cells fail in a battery.

" The average ceil life information doesnot help muchin this regard,

:. becausesamplesizes are too small to give good statistics. A lower depth

" of discharge must be usedto minimize the incidenceof early cell faUures

• in a battery. This is especially true when there is a large numberof cells

:;. in seriesin the battery. Also, lower DOD is consistentwith the needfor

: high reliability. As a further point, it shouldbe noted that 49% DOD on

!, Ni-Cd batteries is marginal with respect to recharging in low earth orbits;

: the cells cannot be charged quickly enough,especially whendegraded,

; unlessspeciallydesignedwith a sacrlfice in energy density. This

: unjustLfla_lyhigh DOD for Ni-Cd batteries is in part responsiblefor the

very low weights calculated for the Ni-Cd system.

._ Thiscritique doesnot take the position that costs cannotbe reduceddramatically, but

merely that insufficient evidence hasbeen provided to supportthat conclusion. We do

disagree, however, with the conclusionthat technology improvements wiU result in

- greater cost reductionsthan willimprovements that extend battery Life.

Costs of the RFC systemare very difficult to estimate. The hardware costs appear to"

• be low, but there are no bette.,"data available to make comparisons. The solar array

andaltitude maintenance costpenalties are high becauseof the low RFC system

efficiency used;these costs would be equal to thoseof batteries it the RFC System

were designedto the sameoverall efficiency. Perhapsthe costswill also even out. It

mustbe recognized that cost estimates of RFC systemswiU always be soft relative to

battery costsbecauseof limited applications and therefore the lack of a data base.

Thoughthe RFC system was analyzed to be the Lightest in-orbit energy storage

system, the lower afficlency assumedfor the RFC system resulted in a greater amount

of orbit makeupfuel, making the total weight heavier than for Ni-H 2 batteries,

Thoughenergy storage systemefficiency data are not given, this can be Inferred from

the associatedsolar array penalty (Figure #.7-J), which is 20,700 ft 2 for Ni-H 2 and

#0,600 ft 2 for the RFC system. These are measuresof relative inefficiency. Thus, if

the NI-H 2 sy_m efficiency were 70 percent, then the RFC systemefficiency would

be t_l percent. We now know that the energy storage efficiency of battery systems

and RFC systemscan be comparable, though that doesnot result in the minimum

69
®"

initial launch weight design for the RFC system. Thus, the RFC system weight data of

Figure 4.7-1 is not optimized to minimum total weight for 30-year life.

- Even though the TRW predicted battery costs are very low, they still are more costly

.iI than the solar array prediction. This is a consequenceof a low assumedsolar array

manufacturing cost of $30 !Jer watt. Comparing this with the $600 per watt cost for

the 5kylab solar array, anqJwith $900 per watt for more recent arrays, it is apparent

that this is an unusually'.owcost, and would require a major breakthroughto achieve.

° Impact on TRW Conclur.ions

.... t. The use of a seventeen-channel electric power system would appear to be usable

-. for battery energy storage, but not practical for RFC systemenergy storage. No

important impact is seen by reducing the numberof channelswere the RFC

system to be used.

2. Cost and weight of the battery systemsappearsto be too low. Refined analysis,

includingrevised assumptions,shouldcausebatteries to look less favorable, and

• RFC systems to look more favorable.

3. Solar array costs apear to be too low. Cost refinement shouldcause the entire

electrical power system to be more costly.

_.$ PRC 3Y.CrEM5 STUDY

Objective.,_

The purposeof this study (Reference 7) was to develop computer models which are

usedto determine the total ILf_cycle cost of electrochemical energy storage systems

for space stations. The effect of Jesignvariables on cost was also to be determined,

as well as system weight. This studywascompleted in September, 1981.

Requirements

Requirements for this studyare defined in a sp_,cification given in AppendixB of their

final report. Analyseswere to be made for both LrO and GEO orbits. The LEO orbit

was at _44 km with _6o inclination, with power levels r_nging from 2_ kW to 2_0 kW at

endof life. Missionduration w_s 30 years, with resupplyand maintenance provided.

The GEO missionwas for _ years operation without overhaulsor replacement of

7O J
hardware, and required autonomous deployment. Electrical power system voltage was

: 120 Vdc nominal for all missions.

: Major Findings

: The significant findings of this study are summarized in Figures _.g-I to _.g-3. The

"-i major output of this contract was the general_ion of two sets of computerized

....:. performance/cost models, one for battery subsystems, and one for regenerable fuel

ceU subsystems. These models permit analysis of the effect of design variations on

life cycle costs. A large number of runs was made and the results plotted, showing the

..... affects of many design variables. No other study has attempted such a comprehensive

- analysis. One result of these analyses is that the co_t of both Ni-Cd and Ni-H 2

batteries is more sensitive to design variables than is the cost of the regenerable fuel

cell system.

i:: For LEO missions, NI-Cd battery systems cost more than Ni-H 2 battery systems; this

appears to be related in part to the deeper depth of discharge assumed for Ni-H 2

batteries. Cost of the regenerable fuel cell system was roland to be approximately the

i- same as for NI-H 2 batteries. Weight of the LEO energy storage system was lightest

i:._ for RFC systems, being _100 kg compared with #_00 kg for Ni-H 2 and g200 kg for NiCd.

i)

Cost data generated show that the production or manufacturing cost of the energy

storage systems is almost insignificant compared to the total life cycle cost. The LEO

i data summarized in Figure _.$-t shows thatthe production cost is on the order of one

. twentieth the total cost, but it is even a smaller fraction than that because production

'7

---_ cost in this study also includes prelaunch and integration costs, space transport costs,

: ......... and space deployment and checkout costs: these costs are not conveniently separable,

i however. The Ni-H 2 battery and the RFC system have nearly equal energy storage

. subsystem life cycle costs ($131 M and $138 M respectively for _0 kW), bug the NI-H 2

: battery has lower total Ufe cycle cost ($_# M vs. $77_ M). This is due to the lower

: calculated efficiency of the RFC, resulting in a much greater solar array cost.

_ For GEO missions, total subsystem life cycle cost is about one sixth that of the LEO

•.... missions, and so the t_1 _Jlife cycle cost is also tess(Figure _.g-2). The RFC has the

_ least cost, especially at the production and energy storage subsystem levels, with only

/

i' 71
_i_ M_ Hi.,2 RFC

-_i ,.'_'" _ _ ,

-_..'._

_..,_. ; 000 24.S'& 43'& (80'&)

....:" EFFICIENCY (_.a • 02.11Xl 41.3%

_,: EI8 WEIGHT (KG) 8,231 4,488 4,102

::7:: LIFE (RESUPPLY) 7.1 YR 7.1 YR 7.1 YR

_:.,

SPECIFIC WT (LB/KW) 197.9 180.9

:_i':" COSTS(Ski)

DOTIkE 14.38 10.79 23.96

PRODUCTION 32.78 29.47 36.16

OPERATION AND MAINTENANCE 238,90 99.44 77.98

ELECTRICAL POWERSUBSYSTEM 288.16 130.70 138.10

lIFE CYCLE cob"r

cost INTERFACE

SOLAR ARRAY 408.19 412.04 587.83

:_ THERMAL 9.29 9.66 6.78

i PWRcoNO 3.17 1.99 1.99

TOTAL LiFE CYCLE COst (IM) 703.71 564.27 744.70

iii_! Figure 4.8.1: PRC 3yltems Study - Selected Summary, 50/¢W, LEO

r_

72

it
lEO (3_ YEARS) GEE (6 YEARS)

i l i !

i i Nl.Cdi Ni-H2 RFC Ni-Cd NI-It 2 RFC

DOD 24.8_ 40_ (80_ SE.2_ ?S.?'_ (80_)

ES8WEIGHT 4110KG 2178KG 20117KG 2726KG 17g8KG 448KG

COST(MILLIONS)

PROD'JCTION* 818.88 812.58 g20.90 144.88 $31.40 $18.31

F..88LIFECYCLE $1_LE7 $77.42 $81.04 $84.29 $38.86 _.68

TOTALLIFECYCI.E 8303.7'/ $318.28 $422.48 $72.21 $80.78 SE0.m

! i

• INCLUDES SPACETRANSPORTATION

Figunp 4.8.2: PR¢ Systems SnJdy, Comprison of LEO and GEE (25/_)V)

't,

!!;
BAI"rERIF.$ - TOTAL LIFE CYCLE COST, lEO

1. COST IS INSENSITIVE TO DOO UP TO 30_ DOD

t

)!, 2. olrrlMUM DESIGN LIFE 18APPROXIMATELY ? YEARS FOR Ni_xl AND 9 YEARS FOR NI-H2

l!i 3. COST DECREASESWITH INCREASING CELL SIZE (ALSO DISCHARGE CURRENT), AT I.EAS'r UP TO 280 AH

BATTERIES - TOTAL LIFE CYClE Cos'r, GEO

1. OPTIMUM CAPACITY FOR 26 KW I$ 7B AH

2. OPTIMUM 0OO I$ GREATER THAN E0_ FOR Ni_.d, AND IS 70%FOR NI-H2

REGENERABLE FUEL CELLS - TOTAL lIFE CYCLE COST, LEO

1. CO81_ ARE RELATIVELY INSENSITIVE TO CURRENT OENSITY OR ACTIVE AREA

11! 2. (:O811 ARE RELATIVELY INSENSITIVE TO DESIGN LIFE

_'- 3. OPTIMUM DISCHARGE VOLTAGE 150.7 V/CELL

REGENERABLE FUEL CELLS - TOTAL LIFE CYCLE COST, GEO

:_ I. COSTSARE RELATIVELY INSENSITIVE TO CURRENT DENSITY OR ACTIVE AREA

2. OPTIMUM FUEL CELL CwRRENT DENSITY IS 38Q MA/CM2

_. 3, OPTIMUM DISCHARGE VOLTAGE 180.? V/CELL

Figure 4.8.3: PRCStudy, DesignTrend, for Lowest Total Cycle Life Con

i*

i!

:],

' 7¢

)
i"" SJ

a small advantage at the total life cycle cost level. The we_,ghtadvantagecalculated

for the RFC system is spectacular for GEO missions,_0 ks .:s. 1500 kg for Ni-H2.

Hardly any analysis wa_made of the data generated. In our review of the PRC report,

a number of important trends were observed,and are summarized in Figure #.$-3.

Conclusionsof PRC

:! PRC elected to draw a minimum of conclusions,choosingrather to let the generated

data speak for itself. The conclusionsthey drew were. l) Energy storage system life

)!'

;. cycle costs for NI-Cd are about twice those for Ni-H2! 2) NI-H 2 and RFC Lifecycle

costs are comparable; 3) Battery parametersaremoresensitiveto life cyclecosts

than are fuel cell parameters.

' Critique

As PRC pointed out in their report, the effectiveness of the cost model prepared is

somewhatlimited at this time due to the absenceof goodemperical dal_ on

performance, physical characteristics, and costsfor NI-H 2 batteries and regenerable

- fuel cell systems. The fact that the data baseis thin for those two key systemsis no

ii_ fault of the study, but is an inherent limitation that must be recognized in usingthe

i study results.

1e'.

* _ One characteristic of this study is that in an effort to account for all the factors

o _- relating to cost, a highly detailed and complex modelwasgenerated. As a result,

J !i muchof the data andassumptionsusedin the model are buried within the computer

program and are not readily accessiblefor scrutiny by potential users. Thus, it is not

practicad to check on manyof the weight and cost elements or obtain breakdownsof

i! the results. For the calculated for GEO are high, but the

example, NI-H 2 weights

cause is unknown.

,_iii

One of the important results is that the total life cycle costs are considerablygreater

!_i thn the purchaseprice of the hardware. Costsfor operationsand maintenance have a

I: dominant influence on subsystemcosts, yet it seemsnot to be possibleto determine

why the operationsand maintenancecost for Ni-Cd exceedsthat fur Ni-H2.

Solar array costshave an overwhelmingimportance in the total life cycle cost,

dwarfing energy storage hardware costs. This is qualitatively correct_ thoughthe unit

cost information is either not given or is dif£icult to find. Solar array/size andcost

75

.....' .:_-_\-i_: .................... ,_
are directly tied to energy storage system efficiency. Thus, greater emphasison

i I system efficiency would have been warranted. The ca!culated e_fieiency of the

i regenerable fuel cell systemof _1.3 percent Is especially low. An independentanalysis

i"

i' of energy storage ef'diciencies showsthat battery and I_,FCefficiencies can be

:i comparable. A¢_umingthe RFC efficiency to be equal to that of the Ni-H 2 battery

_' for the 50 kW misd,on_the RFC total life cycle cost is reduced from $7_ M to $5#8

:i M, which Is comp_,rableto the $55_ M cost of the NI-H 2 system.

Solar array drag has a big impact on the need for resupply of orbital makeup

propellant. This was not consideredin the PRC study. However, since the efficiencies

- of the contendingenergy storage systemsshouldall be comparable, this shouldnot be

-- a large considerationin the comparisonof energy storage systems.

_,_ It wouldhave been preferable L_the PRC study hadincluded an analysis ,'_fthe data

"-_: generated. Interpretation of the data has largely b_-enleft to the.read_-r. The design

-/ trends we observedfor lowest total cycle life=c_st are given in Figure,.,#.8-3.

-:_ Comments onsome of these observedtrends _,reas follows:

,_

:. h The finding that very large battery cell sizes are best(at least 260 AH for LEr,_)

is questioned. Thiscritique doesnot take the position that the conclusion is

wrong, but merely that the arguments and analyses to supportit arp i,_5uffic}ent

to justify the conclusion. The finding stems from the expec_,edlarge reduction in

, operations andmaintenance cost with large size cells, _a_dis derived o_1the

-. assumptionthat failure rates, preventative maintenance, andoverhaul are

_ directly related to the number of modules,and not dependenton moduleor cell

_ size. This finding is questionedonthe basisthat (a) large cells are expected to

.- have higher failure rates than smaller ceils; (b) the questionis not yet resolved

_. whether it is best to promote reliability by redundancyat the cell level or at the

;' battery level; useof large cells tendsto lead to redundancyat the ceil level; (c)

. excessively large ceUswill reduce the numberof batteries below the minimum

,_ ileeded for safety! assuminga 50 kW system having three busseswith a rain[mum

_ of two batteries per bus,a minimumof _ix batteries is needed,with maximum

-_ cell size of approximately 85 AH. Recent events which lend somesupportto the

conclusionfavoring large cell sizes are-" a) developmentis beingstarted on the

commonpressurevessel Ni-H 2 multi-cell; and b) NASA-Lewis is showinggood

_ successin their developmentof a novel[ large capacity NI-H2 ceil.

76
2. The finding of an optimum fuel cell discharge voltage of 0.7 V/cell _.squestioned.

Lov.;voltage results from high current densityj which promotes low efficiency

ar,d shorterlife, and requires a larger solar array. Voltages in the range of 0.85

";o0.90 are consideredto be Inoreapproprb'tt¢,

3. The _indlngthat cost is relatively insensitive to furl cell current ce,;4sityor

active area is questioned. The argument is similar to that of item 2 above.

Impact on PRC Conclusions

h Caution must be exercisedJ;_the useof the cost information from the PRC

report. Someof the findings are Important and shouldbe followed up. However,

where these ,_indingsare usedas the basis for future R&D, the conclusioi',sshould

be corroborated by a separate analysis.

_. The conclusionth_.t Ni-Cd battery systemsare ultimately more costly than Ni-

_- H2 hingeson th_ expectation that Ni-H2 batteries can be dischargedto deeper

=_ii...... ,_ depths and will be longerlasting. This hasnot yet been prove.%but probably it

_ will eventua,_/ybe proven. With that qualification, this conclusionappearsto be

valid.

./I 3. The canclusionthst Ni-H2 and RFC Life cycle costsare comparable appearsfrom

:', other work to be valid, with the provisionthat both systemsbe designedwith

_i: comparable efficiency.

;i'" _. The conclusionthat battery parameters are more sensitive to life cycle costs

t than are fuel cell parameters cannot be refuted, but doesnot appear to be

!I reasonable. Considerable cost leverage is associatedwith operations awd

I: maintenance cost, and with solar array cost, both of which shouldbe sensitive to

' RFC designparameters.

i1;

;_:: _. The general finding that equipment purchasecostsrepresent only a small

I fraction of the total associated cost appearsto be valid.

_'." 6. The finding that RFC systemsare of lower cost andconsiderably lighter than

_::, batteries for GEO missionsis significant and appearsto be reasonable. Followup .,

..... studyshouldbe made to verify andquantify this conclusion.

77
_q

' 8.9 COMPARISON OF STUDY RESULTS

I_ Analysis Method

Analyses were made on the results of the studies by the seven contractc_rs.

Considerable diificu!ty was met with in trying to find common bases '¢omake

compari_0ns, The main problems were" I) the energy stora_e requirements differed,

not only in power levels and mission duration, but also in considerations such as power

i,I for station buildup: peak power and emergency power; 2_)the contractors had differing

•it_ usuallyVi OnwewesreWhatnotitemSprobviedienclud d.Shoeduldas part of each penalty; 3) data breakdowns

It was decided not to attempt to manipulate the data to compen=_te for all the

differences. Instead, it was determir,ed that a few key parameters would be

:/

established to normalize some of the data. Then, if irnpor_ant differences showed up

in the results, the reasons for those differences could be sought out independently in

• terms of differing requirements, differences in definition of an item, or simply as

'_ differences in a design or its expected cost.

I_ The following normalizing parameters are used" :) Energy Storage Specific Weight

;i (lb/kW). This consists of the launch weight of the energy storage system, divided by

;, the system rated power in kW; no associated penalties are included in this definition of

•!; the energy storage system ,_/ejght; 2) ResuppJy Doubling Time (years). This is the

= duration for the energy storage resupply weight to equal the energy storage system

!.

if_ launch weight; 3) Energy Storage Hardware Specific Cost ($/kW). The energy storage

ti system production cost for one space station is divided by the system rated power in

_ kW; _) Operations Cost, i0 years ($/kW). The cost of operations and maintenance is

!i divided by the system rated power in kW, and pro-rated for a 10-year duration.

In addition to the normalizing parameters, the reported energy storaFje efficiency is

j:-

_'f;: used. Where the inefficiency included tic to ac power losses, an adjustment w_s made t2

t, to exclude that loss, since it is not truly an energy storage loss. Development cost was

tli" also included without adjustment on *',e reported results.

.. Results of AnaJysis

The energy storage system selected as bes_ by each of the seven contractors is given

_ in Figure _.9-1. PRC did not make a choice of the best system, but indicated that Ni-

!

!,

78
U,:-

HI'

il

mi'

" NICKELCADMIUM NICKELHYDROGEN REGENERABLE

_; BA'I'rERY BA'I'rERY FUELCELLSYSTEM

i'

NORTH AMERICANROCKWELL O X

UNITEDTECHNOLOGIES X

LOCKHEED O X

LIFESYSTEMS O X

McDONNELLDOUGLAS X O O

TRW O X O

PRCSYSTEMS 0 * *

, X - SELECTEDAPPROACH

-:- O,,OTHERAPPROACHESEVALUATED

" • TWOTOPCONTENDERS;NOSELECTIONMADE

2

.- Figure ,_3-1: Energy Stor_ Sysmm _/ecmd

z.

79

.. ,:............... -. . o " ':!111==- ,, I .....L..II!! II ..... . .....
H2 batteries and the RFC system had comparable life cycle costs. Rockwell and

United Technology studied the RFC system, and Lockheed did not study the Ni-H2

system.

Results of studies by the seven contractors are compared in Figures #.9-2, _.9-3, and

_.9-_ for RFC systems, NI-Cd batteries, and NI-H2 batteries, respectively. It Is seen

that the attempt to normalize some o,c the data has been fairly succeuful, with most

of the parameters falUn 8 into a range of two-to-one or three-to-one. Operations cost

for each of the three systems is seen to be the most significant cost item, but this is

also the parameter with the greatest variation. This is attributed to the fact that

there were important differences L_the ways this cost was determined. Even so, the

reduced data give some idea on the costs that may be expected. It also points out that

the bi_est cost saving opportunity is in designing the systems for minimum operations

and maintenance activity.

The _oecific weight of the RFC systems also has high variability. Systems designed

long ago tend to be heavy, reflecting the early stage of RFC system technology.

Results of the NASA-sponsored studies should be viewed as information which wUJ

assist in making a decision on the best energy storage system for space stations. The

studies alone are not a sufficient basis upon which to decide this question. This can be

_ seen in Figure #.9-J showing the attributes that govern the selection of energy storage

_,, systems for space stations. The NA5A sponsored studies examined some of the

important attributesp but some others_ which could be more important, were for good

reason not part of the NASA studies. The comparison between RFC systems and

batteries is discussed further in Section 8.0.

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i 5.0 SOC REFERENCE FUEL C_! L SYSTEMS 5.1 SYSTEM DESIGN

Regenerative fuel cell systemswere studiedfor the 5OC to provide a point of

reference. In order to be sure the design would be credible, we obtained the assistance

of both General Electric Co. and United Technologies in the designandoptimization of

this system. Their help is gratefully acknowledged. General Electric's technologyIs

basedon_he solidpolymer electrolyte fuel cell with a separate solid polymer

electrolyte electrolyzer. United Technologies'approach is ba_edon the alkaline

electrolyte fuel cell. A separate alkaline electrolyzer made by LL_eScienceswouldbe

usedwith the alkaline fuel cell. Weconsider both the solidpolymer andthe alkaline

electrolyte systemsto be well suited to the spacestation, andbelieve that either one

wouldgive the neededlife and performance, i_oth General Electric Co. and United

Technologieshave documentedtheir con'_rib',Jtionsto this.study (References 10 and

!:. 11), and these reports provide additional analysis detail.

_- Rather than producea single design,a large numberof designswere establishedin 

order to investigate e.number of different objectives. One designof interest is one

which would be inter_,,angable with nickel hydrogenbatteries. Our analysis showed

that the energy storage efficiency with a nickel hydrogensystemshouldbe

approximately 5J percent, sowe configured a fuel cell designspecifically to that

efficiency. Actually, the regenerative fuel cell efficiency is 55.25 percent, but the

i'- energy storage efficiency is slightly lower at _#._ percent. This difference of 0.6

percent is minor andstill permits a valid comparison.

The 55 percent efficiency designwith solid polymer electrolyte weighs 2_18 lb, with

the following weight breakdown:

_' Radiator _53 lb

_:-_1

:_ _ Tanks and Reactants 187

": _" Fuel Cell System 1223

__;,.i._1 Electrolyzer System _.%_

::-LI ....

i_! Total 2_18 lb

y ";iq'

_.._!:, 85

(:

' ........ _s ' _".i ' • "

pB
_ • w j

I;

Ii: Operating conditions and designdata are summarized in Figure 5.l-l. For example, _ the designconsistsof six fuel cell modules,basedon the concept of three buseswith

I! two modulesper bus. This a_'rangementIs heavier than a design with fewer modules,

!;" but tolerates failures better.

A seconddesignproducedis a minimum weight design. This is not su_ested as the

• appropriate design for 5OC, for the low weight is obtained at the expenseof

efficiency. However, a minimum weight designmay be useful for somemissions,and

this designdata provides a useful comp=_.-isonwith the high efficiency designs.

: The minimum weight designthe solidpolymer electrolyte weighs 17## lbt with the

_:, following weight breakdown:

.,; Radiators $#8 Ib

• Tanks and Reactants 20#

i -t""

;_,,:__ Fuel Cell Subsystem 6#6

'.-: ElectrolysLsSubsystem 3#6

4

,%_ I

Total 17## lb

'_' This system has a regenerative fuel cell efficiency of #8.1 percent, and a slightly

;!-: lower energy storageefficiency of #7.# percent. Operating conditionsanddesigndata

;:.... are summariT-ed in Figure _.1-2.

As was discussed in Section 3, designs were established with regenerative fuel cell

i_, efficiencies of 65 percent and 67 percent for the solid polymer andalkaline electrolyte

" systems, resy_ectively, which would have energy storage efficiencies of 6# and 66

. percent, respectively. It was concludedthat a designenergy storage efficiency of 60

percent is possible without undue development risk using either fuel cell system. This

: comparesto an ener_T storageeffici_,ncy of _ percent with the nickel hydrogen

• battery basedon existingceg performance, or 62 percent basedonpotential design

_ improvements. To implement this philosor .I, a third design produced was a high

!:_ efficiency design. This hasa regenerative fuel cell efficiency of 62.8 percent, and an

._ energy storageefficiency of 61.9 percent.

i :; 86

;o
ORIGMNALPAGE •

OF POOR QUALIFY
ORV_,_iNAL PAGE 18

OF POOff QUALITY

NORI_d. BUS POt_R REQ" D 39. _ KW

BUS VOt.TN)E 180.00 VOt.TS

SOLAR ARRAY SPEC tdEIGHT 13.88 KGIKId 30._0 I-BS/KM

L,I_'tT PERIOD IN ORBIT 55.00 RIN.

DARK PERIOD IN ORIIIT 37,0¢ I'IIN.

ENEROY 8TORAOE CAPACITY 44.04 IqIN. •

FUEL CEI.I. OPERATINO CONDITION8

N[AN CEll, PRES$URE 206.85 KPA 30.00 PSIA

FIERN CEll. TEI'PI[RATURE 3"J5.37 K ISG.OQ F

CEIL, CURRENT Drd_ITY 161.46 FIA/_ICFt 150.00 RGF

CEll. VOt.Tl_4E ,.810 VOI-TS

_ NO. OF"CEI.I.S PER MODULE 223

NURBER OF NOOUU[S 6

": REI'IBR_IIE THICKNESS ._J4 PIN .010 IN

INDIVI_ ¢I[LJ. AREA .023 SQ Iq .245 SQFT

RQDULIS OU'/PUT POkiER 39.83 Kill

MQDUI.E OUTPUT VOI-TAOE 180.66 VOLTS

CEIL CURRENT,PMAL. I'IQDS 36.74 RRP$

CEIL. ¢URflF.NT EFFICIKY 99.2_ Y.

IqODIA.IS HEAT QEN. RATE 33.04 KN

_ b"I.ECTRQt.YZER OPERATING CONDITIONS

; MEAN CEll. PRESSURE 827.40 KPA 120.00 PSIA

_ IqEAN CE1J, TI_PERRTURE 355.37 K 180.00 F

CELl. CURRENT DENSITY 484.38 IqA/si;ICR 4,_),00 _SF

CEIL. VOLTAGE 1.584 VQt.TS

NO. OF CEILS Ft,_ IqODUI.E 126

NI_IBI_ OF RQDUI-Eb 3

IqlENBRANIETHICKNESS ._'_4 I_ .010 IN

INDIVIDU_. CELl. AREA .018 SQ 11 .199 SOFT

RODUtJES INPUT _ S4.36 KM

NODULE INPUT VOLTAGE 199.64 VOLTS

CELL CURRENT,PARAI- RODS _. 42 RIqPS

CLq,.l- CURRENT EFF I C I ENCY 98.56

;'IQDUI_S I,I_T GLEN. RATE 4.53 KM

SYSTEIq OPERATING CONDITIONS

SOLAR ARRAY OUTPUT POblER 54.91 Kill

IDEAl. REOEN FUEl. CEIL EFF. 50.03 _.

SYSTEM ENERGY STORAGE EFF. 4e.07

bI_TER PRODUCED-RODE C 12.22 KO 26.93 l-BS

H2 STORAGE TANK VOl-Ui_ 3.01 CU I't 106.21 CUFT

02 STORAGE TANK VOLUIqE 1.50 CU R 53. l I CUFT

WEI OHT SL_MARY

_OL.AR ARRAY 762 KO l&80 t.B$

.- SPACE RADIATORS 249 KG _4_ LB:_;

, H2,02 AND MATER TANKS 93 KO 204 L.B'_

FUEl. CELL SUBSYSTER 2.9:) KO 646 t,B._

ELECTR_,Y$1$ SUBSYSTEM 157 KO 346 L.BS

SYSTEM VARIABLE LAUNCH MEIOHT 1553 KO :)4:5 _I.BS

Figure 5.1.2: Regenerative Fuel Cell System Summary - Minimum Weight Design

:._

88
we TheigththirdFuelTanSubsys ksRasdoli tiaemReact tdorsbre polymer aakdown 3ni07tsW3it72177e hzdesigncell lecwei tr°lYgand htselb_797 pounds,with the fellowin8 blectrolysis Subsystem 1!q.l

i

Total ¢797 tb

°t Operating condition_and designdata are summarized in Figure 5.1-3.

J

i

_o Additional supportinganalysis for the three designsis providedin R(:ference 10. For

example, it is shownthat voltage regulation can be maintained with two of the sir, fuel

iil cell modules_ailed, and ability to survivethe three emergency conditionsis shown.

The Half SOC operates with three of the six fuel cell modules, but sincethe electrical

load is greater than half the fu!l $OC (60 percent of full 5OC), there is a sinai|

reduction in system efficiency usingequipment optimized for the Full SOC.

It was not practicable in this report to showthe results of all the analysesmadeon the

RFC systemfor 5OC. Data on the alkaline fuel cell systemwere in many ways

i: comparable to that generated for the solid polymer electrolyte system. The alkaline

!i; system estimates were in fact a IWde lighter, or viewed in another way, the alkaline

lil system was a little more efficient for the same weight. For example, for the

.. condition of both systemsweighing _800 lb, the regenerative fuel cell efficiency of the

l: solid polymer system was 6_ percent, compared with 67 percent for the alkaline

i/

i_ system.

I

i_:' _.2 THERMAL DF._IGN

1i

),

[.'

_ It can be seenin Figure _.2-1 that the heat generated by the fuel cells is considerably

greater than the heat generated by the electroiyzer, on the order of 15 to h This is a

consequenceof the thermodynamics of these reactions, as is illustrated in Figures 3.2-

2 and 3.2-6. Heat generation with fuel cells is al_'ays relatively high, but is reducedas

the cell operates more efficiently, at the higher voltages. Heat generation with

,T

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92
---i

0 c POOR QUALITY

Figure5+2.3,"Multilayer InsulationPerformance

-_ 93
Electrolyzers generate a lesser amount of heat, even crossing into the cooling regime

when the electrolyzer operates at very high efficiency) corresponding to low voltage.

This imbalance in heating behavio= could cause problems with high efficieDry designs.

in fact) some eiectroiyzer designs in the past have used relatively high voltages in part

to assure sufficient self-generation of heat to maintain proper temperature control.

The approach we have taken to this problem is illustrated in F_gure 5.2-2. Both the

fuel cell and eJectrolyzer modules dissipate heat to the radiator via a common phasechange heat exchanger. If the electrolyzer requires the addition of heat, it is obtained

from the heat storage capacity of the phase change material. Acetar,_ide, havins the

formula C2HsON) is a good heat transfer material for this purpose) with _ meitin_

point of I78OF r.nd a heat of fusion of I0_ 5TU/Ib. Only a few pounds would b_

required per heat exchanger.

The regenerative fuel ceil system is located outside the pressurized compartment in

the current packaging concept. Since the units operate at aproximately IgsoF) they

must be insulated to prevent overcooiing, especially the electrolyzer. The insulation

concern is not the multilayer insulation itself) which has superb thermal prol_erties)

but the effects of penetrations) mountings and other discontinuities. Figure 5.2-3

gives a correlation of the performance of a wide variety of prior multilayer insulation

applications, and showsthat in the worst case the RFC system may expect to have an

effective emittance of 0.0h For a t3'pical system operating at 185OF, this will result

in a heat loss of approximately 325 watts. This lossis small compared with the typical

system heat dissipation rate of 20 to 30 kW. However, it is comparable to the heat

deficit of the electroiyzer for the high efficiency design) which is approximately 250

watts. Though this is entirely manageable, it does point out the need to insulate the

electrolyzers especially well to minimize thermal difficulties.

_.3 DRY ELECTROLYSI-_ GAS

The hydrogen and oxygen product gases from the electrolysis of water will be nearly

saturated with water vapor unless steps are taken to reduce its level. If all

components in the RFC system could be assured of long term isothermal operation, a

high water content in the gases would present no problems. However, variable

temperatures in spacecraft are common, and could cause condensation or even

94

(
freezing of water. Where water electrolysis gases are used for reaction control fuel,

:,;' the problem could be very severe due _o the long piping runs required.

.,,,.

_- One concept for drying the gases is shown in Figure 5.3-I. Both the wet hydrogen and

" the wet oxygen gases are cooled in a heat exchanger which is coupled to a radiator by

a heat pipe. The condensed water isremoved by water separators, and the dried gases

are then routed to gas storage tanks via a regenerable heat exchanger to improve the

thermal efficiency of the process. The water collected will be saturated with oxygen

and hydrogen gases, so a catalytic deoxidizer unit is provided to combine these gases.

Though removal of water by condensation should be feasible, the process is somewhat

complex. An alternative approach is shown in Figure 5.3-2. Water is electrolyzed into

wet oxygen and wet hydrogen. These gases then continue into a water vapor

electrolyzer, which may either be a separate unit or a final stage of the electrolyzer.

Though it is feasible to electrolyze water vapor, this technology is not yet available.

J The major problem expected is not the ability to do electrolysis itself, but proper

,'i control of the process.

; The RFC system studied for the SOC presumed the use of a condensation system for

, water removal. A water vapor electrolyzer would have been preferable, but the

•_._ technology is not available. Hopefully this technology will be developed in the near ix'

: i

!_ future.

• "t

::> $.8 EMERGENCY POWER

e The designs previously described meet the e *.rgency requirements defined in Section

i_ 2.0. In emergency mode "A" the loss of one of the two solar array wings was

_ hypothesized. In Emergency mode "5", it was hypothesized that the solar array

_.:'. orientation control was damaged, resulting in cyclic power input from the array. In

-- emergency mode "C", full solar array power loss was presumed for one full orbit.

....., Total loss of solar array power for durations much longer than one orbit is considered

) : extremely remote. Nevertheless, the SOC would have provisions for an all-out

i

. survival emergency of 21 days. Electrical loads have not been determined for this

i-,= emergency, but would be an absolute minimum and are expected to have a total energy

95

"i
WATER FROM J

FUEL CELL

REGENERABLE H2 (WET) 0 2 (WET) REGENERABLE HEAT EXCHANGER HEAT EXCHANGER

H2 TANK H2 INTEGRATED 02 TANK

DESIGN

I imJ _ m 1rob am

Figure 5.3-1: Concept for Wamr Removal by Condensation

-, 96

( 3
O2

FUEL CELL H2

J PROOUCTWATER

ELECTROLYZER

WATER VAPOR

ELECTROLYZER

DRY 0 2 DRY H2

Figure £_3.2: Electrochemical Concept for Water Removal

97
demand less than the stored energy for the other emergencies defined. The 21 day

i duration i,_ based on the expected time required to effect a rescue.

Because of the emphasis required on safety, it =nay be worthwhile to provide additional

stored energy that would either extend the all-out emergency duration beyond 21 days)

or require a higher power level than that for an extended time. It is judged that it

would be appropriate to provide an additional t0-day emergency supply of continuous

power at t.5 kW. The RFC system has excellent capability for such emergency power)

especially when integrated with the orbit makeup reactant system. Orbit makeup can

be delayed up to 60 days) so the hydrogen and oxygen gas for orbit makeup can be

available for emergency power use. Tankage weight is the major penalty =equired.

i

•_: High pressure gas storage on the order of 2000 psi is preferred to minimize tank

volume° This will require use of an electrochemical oxygen pump) which is needed

_' anyway for life support oxygen) and an electrochemical hydrogen pump) which would

_: weigh about 6:> tb. Based on the requirements shown in Figure g.l-2 the design for this

additional emergen_;y would require approximately 965 pounds, as follows,"

_!i Gases Required Oxygen Hydrogen Sub Total

V Reactants for fuelcells 350 50 #00 Ibs

_,i, Oxygen for life support 173 -- 173

_. Total J23 50 573

;4

!i

: WeiRht Penalty Powe......__r Life Support

__ Oxygen tank -- power #20

_; Oxygen tank -- life support -- 208

_ Hydrogen tank #80 --

Hydrogen gas compressor 65 -- t.

4,

I; Total 965 lbs 208 lbs

98
5.5 ENERGY STORAGE INSTALLATION COMPARISON

An installation configuration was developed to show the main features of a

regenerable fuel cell energy storage system as compared to a nickel-hydrogen battery

sy3tem. Figures 5._-1 and 5._-2 compare the two installations on the service module

design that comprised a part of the SOC space station concept. This service module

was one of two in the complete $OC configuration and supplied half of the total

electric power tJsedby the SOC - 25 kW on the sunlit portion of the orbit and 20 kW on

the dark side. The reference SOC configuration is shown in Figure _._-3.

The battery version of the system utilized large numbers of individual Ni-H 2 ceils

organized into batteries. Details of the battery installation were not developed. The

! battery boxes shown represent the volume required for the battery installation.

The fuel cells and electrolyzers ca'_ be either of new design or derivatives of the

shuttle systems, Two RFC packages are installed on each service module, one on each

!!t side. Gas and water storage tanks and a box representing the appropriate power

!} electronics volume are also shown.

tl.

- ii The gas tanks shown are low-pressure tanks with a maximum pressure of about 100

l_it :t psia. The mass-optimum pressure for tank design is about _00 psia (3500 kpa) as shown

) in Figure 5.5-#. The configuration was drawn with low-pressure tanks to emphasize

;_: that packaging considerations w_ not force the use of high pressures.

!i Three regenerable fuel cell designs were prepared for the SOC because each design has

_ its own special merit. It is judged that the high efficiency design is the most

_: worthwhile, so that design is used in this sumt,lary. Note that engineering data on that

I_ design is orovided with in Figure 5.1-3. The weight summary is as follows."

ii:

1

i,

q

,!

I.

i

i:i

:' QQ

',i °"

_,,


Figure5,5.3: Reference$0C Configuration 1'2ServiceModules)

102
OIIIAL PAO[ -,,

103
I)jr

6296 efliciency regenerable fuel cell system 4796 lb

(without solar array)

tO-day errmrgency power and life support oxygen %5

system

Total 5761 lb

Due to the low current density used, a fuel cell manufacturer has estimated fuel cell

[ life to be about 10 years; electrolyzer life is assumed to be comparable. Life of

i ancillaries is estimated at 3 to 7 years, based on the range of life estimates made in

I recent studies of the RFC system) and discounting the l.t year pump life value as

I discussed previously.

11 A weigh1:cor,,parison with NI-H2 batteries is of interest. These batteries) without the i,

i':, t0-day emergency power system) based on a system energy storage efficiency of 55

i_ percent) and a 35 percent depth of discharge) results in a total weight penalty of

ii" approximately 5865 lb. This compares with 2gl$ lb for a 55 percent efficient RFC

:_ system) or 4796 ]b for a 62 percent efficiency RFC system. /

!/

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,i 104
6.0 ASSESSMENT OF HYDROGEN-HA_.OGEN REGENERATIVE FUEL CELL

SYSTEMS

6.1 INTRODUCTION AND SUMMARY

Relatively little serious consideration has been given to the possible role of hydrogenhalogen reg,:nerative fuel cell systems in future spacecraft. Contending ._,'stems are

the hydrogen-bromlne system and the hydrogen-chlorine system. Becaus_ of the

to_:icity of bromine and chlorine, our first impression was that these systems could not

survive the strict requirements on personnel hazards. However, space station studies

on the SOC showed that it would be preferable for any energy storage system,

whether batteries or fuel cells, to be installed outside the manned compartments. A

second concern was that suitab,e materials might not be available for use with these

highly corrosive materials, especially bromine. However, we determined that _ bunfiber composites and other composites appear to be suitable. Thus, toxicity anr'

materials are much reduced as concerns_ and so these sy3tems cat-,be giv n ser! ,',

consideration.

The natural competitor to hydrogen-halogen systems wou!d o= t|:e hydrogen-oxygen

system, which has a long successful history and has useful integration advantages. The

question then arises as to where might the hydrogen-halogen system have a use_'ul

advantage? Possible a-swers are in the improvement of weight, efficiency, life, or

reliability. As it turn, out, it is the efficiency which offers a principal advantage, and

this is reflected primarily as saving in solar array size. In this respect, the

hydrogen-halogen systems are unique, for with a few exceptions, no other fuel cell or

battery system has the potential for as high an efficiency as do the hydrogen halogen

systems. Whether this unique advantage is significant to Warrant R&D on these

systems then becomes the key question with these systems.

6.2 ANALYSIS OF HYDrOGEN-HALOGEN SYSTEMS

The good reversibility of the hydrogen-bromine system has long been known.

Development had been hampered by the lack of suitable membrane separators,

however. With the development of solid polymer membranes such as those *rom the

NAFION family_ laboratory work has been performed which characterizes these

systems, and their development for aerospace applications can be entertained. The

I05
hydrogen-chl,}rineleectrolyzerhas been appliedcommercially,and lullsystem

demonstr_Lic.... = "*.has ve been conducted on both systems. However, other

commercial applicationshave not appeared. General Electricisthedeveloperof these

_ystems.

Hydrogen-bromine cells use a solid polymer electrolyte which can function either in

the discharge mode or the electrolysis mode. The conservative design approach,

however, would be to use a separate discharge module and electrolysis module. Figure

6.2-1 showsthe electrolysis/discharge characteristics of these ceUs. The significgnt

difference between this behavior and that of the hydrogen-oxygen system is that the

electrolysis/discharge voltage difference approaches zero at zero current density for

hydrogen-bromine. It is this voltage difference which gives rise to system inefficiency

(othe- than ancillary power). Thus, the ideal system efficiency approaches 100% as

current density approaches zero.

Figure 6.2-2 shows the electrolysis/discharg_ characteristics of the hydrogen-chlorine

system. The behavior is similar, being linear with current density and approaching

zero voltage drop as current density approaches zero. The reversible voltage (voltage

" at zero current density) is a little higher, however.

The efficiency of these systems, excluding ancillaries, is shown in Figure 6.2-3. The

significant feature is that extremely high efficiency is attainable at low current

densities. It is not known whether there is any significant cross-dLffusion of hydrogen

and halogen through the solid electrolyte; this would cause a current inefficiency and

reduce overall efficiency. Also shown for comparison on this figure is some typical

performance of the advanced high temperature hydrogen-oxygen fuel cell using solid

ceramic electrolyte, under development by Westinghouse. This system also approaches

100 percent efficiency as current density approaches zero, and Lsone of the few other

systems to do so.

With all battery systems, the reversible voltage varies a little with concentration of

the electrolyte, the temperature, and variations in other com_tituents in the cell. With

most systems, the voltage sensitivity to those effects is so slight that it is usually is of

secondary importance. With the hydrogen-haiogenst however_ these are major effects

and show up strongly in voltage regulation for any application. Figure 6.2-_ shows the

dependence on acid concentration; as _he cells are discharged9 more acid (HCI or H6r)

T i I

0 100 2OO 3OO

CURRENT OENSITY - ASF

Figure 6.2.2" Performance of Regenerable Hydrogen.Chlorine Fuel Cell

108
109

.... - • -/t:.:_T-._.............................-,
1.4

OC

' * I I I I i I , I I I I I

1 3 5 7 9 11 13 16

ACIDMOL.ALITY

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_,- Figure _2-4: Helogen Fuel Cell Voltages Depend on Acid Concentration

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12"

;

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is formed, increasing its concentration, and lowering the reversible voltage. The

hydrogen-chlorine cell is especially sensitive to temperature as well, as shown in

Figure 6.2-5. Since temperature will typically rise during discharge, the lowering of

voltage with increased temperature intensifies the lowering of voltage with increasing

acidity, which also occurs during discharge. However, the strong sensitivity of

temperature and voltage can lend itself to an app_trant increase in efficiency if enough

heat is added during electrolysis to raise the temperature. This is shown in Figure 6.2-

6, and is a concept that can be considered only where thermal energy is readily

available or surplus. Of course, this extra heat ener,_y must be removed during

discharge, so a good heat sink must a/so be available. This unique property might be

exploited in certain special appUcations, but is unlikely to find general use for

spacecraft.

One approach that has been tested with hydrogen-bromine regenerable fuel cells is the

use of hydride storage for the hydrogen. This aJlows low volume for the hydrogen, and

has proved to be satisfactory during Boeing tests of this system. An analysis was

therefore made of the effect of hydride storage on system efficiency, since system

efficiency is known to be very important for the space station. The results of the

analysis are shown in Figure 6°2-7 for hydrogen-bromine. W_th gas sl:orage of

hydrogen, the ma_,r loss (ancillary losses were not considered) is due to electrode

polarization, which increases with current density. With hydr:_destorage, there is

always an additional loss because energy must be applied to the: hydride bed to drive

out the hydrogen. A further loss with hydride storage is the fact that the hydride bed

efficiency is not 100 percent, in that all of the heat provided does not go into the

hydrogen molecules, but some goes into the bed material. Hydricle storage with the

hydrogen-chlorine system is shown in Figure 6.2-g and shows behavior similar to that

of the hydrogen-bromine system. It is concluded from this analysis: that hydride

storage is not worthwhile for space stations because of the significant reduction in

efficiency that results.

KJ ASSESSMENT

Hydrogen-halogen systems offer the possibility of obtaining higher energy storage

efficiency than with batteries or regenerable hydrogen-oxygen fuel cells.

Theoretically, these systems could save up to about 20 percent of the sol._r array size

compared to a high efficient (60%) battery or RFC system. The practical upper limit

111
(J-" (+.)

112
113
BASIS:

• HYDRIDE BE_S HEATED ELECTRICALLY

• n T "THERMAL EFFICIENCY OF HYDRIDE FIEGENEHATION

• E°- 1.11V

1.0

0,8

il

W

0.2 -

0 I II I I I I I

0 100 2OO

CURRENT DENSITY *" AMPERES/FT2

Figure6.2.7: Efficiency of H2 - Br2 RegenerableFuel Cell Systemswith Hydride Storageof H2

114
BASIS:

• HYDRIDE BEDS HEATED ELECTRICALLY

• rlT = THERMAL EFFICIENCY OF HYDRIDE qEGENERATION

• E° = 1._V

1.0

GAS STORAGE OF H2

OA

HYDRIDE STORAGE nT - 1.0

__ HYDRIDE STORAGE nT - 0.8

0

0 100 200

CURRENT DENSITY -. AMPERES/FT2

Figure _2-8: Efficiency of H2 - CI2 Refenerable Fuel Cell Sysmms with Hydride Storage of H2

115

__\:" ' " __- _""" " - '__"'_'_-':-__:_-=_-_'_-"_-"...... _ '_ .................... ": " _............. . :- •: .......... -_:-_i ::: - -........ ::;_2,..,:;_.,.:_.i:_: ,.:_.;,_J2i.: :_ " : : _;
isexpected to be a 15 percentsaving.Low currentdensitydesignsare needed to

obtain the high energy storage efliciencies to permit such savings in solar a_ray size,

It is unknown what weights might be required, what realistic efficiencies might be

obtainable, and what serious technical 9roblems, if any, there might be with such new

systems, The fact that successful tests have been run on both systems shows that at

least the systems do work_ in the case o_ (he hydrogen-chlorine system_ the

electrolyzer portion is highly succe_fult aid this technology has been reduced to

commercial practice.

It is concluded that the potential for high energy storage efficiency is sufficiently

important that some follow-through should be carried out with these systems, This

work should include attempts to better define realistic overall efficiency. There is not

sufficient information now to make a good judgement as to which of the two systems

holds the most promise. An answer to that question should also be obtained on any

future work done with these systems,

116

L .)
7.0 TECHNOLOGY ADVANCEMENTS

This section summarizes the technology advanqements foreseen in the near future

which could affect the choice of the energy storage system for SOC.

7.1 NEW SYSTEMS

The number of new battery systems under investigation and development is

considerable. However, the requirements for low earth orbit spacecraft batteries are

verydemanding --many cycles,highcharge ariddischargerates,and highenergy

density. Thus, the number of candidates with promise is very much reduced.

A serious problem with new undeveloped syste ns that show promise is the very long

amount of time needed to develop a system t,; the point that it is usable. Fifteen

years is WpicaJ for commercial practice, and ionger time is often required for

aerospace applications. The nickel hydrogen b=ttery was invented in 1958, and

development in the U.5o started in the late 1960's. This concept was based on the use

of existing nickel electrodes from nickel cadmium technology, and existing hydrogen

electrodes from fuel ceil technology. Yet, the system is only now just beginning to be

ready for aerospace use for GEO, which is much less demanding than LEO.

Because of long lead times needed, existing battery and fuel cell systems will

command the most attention. The principal competitors are nickel cadmium batteries,

nickel hydrogen batteries, and hydrogen-oxygen fuel cells. Before dismissing all other

systems, however, brief mention should be made of ambient temperature lithium

batteries and high temperature be,cteriest both categories of which have attracted

much interest as possibly the next generation of high performance secondary batteries

for aerospace.

Inthe case of secondary ambient temperature lithium batteries, there is little to

support these hopes except the strong desire based on the good energy density of

primary lithium b_stteries. We need 5g00 cycles per year, and the lithium electrode is

capable of about 250 cycles. We need high charge and discharge rates, and the

ambient temperature lithium systems are low rate systems. We need one single data

point or analysis that shows promise, and we have none.

•.:: 117
Inthe case of the hightempera_u_re_ys_ l;ema,there ismore chanee o4 success,but even

thatisproblematic.The basicproblem i_thatthe weaknesses and failingsof the high

temperature systems coincide precisely with those criteria which are ol the greatest

importance in aerospace energy storage: long life) high reliability) high char&e and

discharge rate capability) and high efficiency. The geo-synchronous application feels

_he pressureol weight much more than does the LEO case)and many fewer cyclesare

needed, Thus, the GEO application is more likely to be developed and used first, as

has been the c_ with nickel-hydrogen batteries. A_ter success has been achieved for

GEO, then we may expect greater interest in applying this to LEO.

7.2 NICKEL HYDROGEN BATTERIES

The status and problems of nickel hydrogen batteries are discussed in other sections of

this report. In general, they have good promise, but the demonstrated life has not yet

been satisfactory for LEO, and the demonstrated energy storage efficiency after long

l cycling is not high. Work is continuing on improvement of this system, with efforts

i focusing primariI) on development of large ceils or multiple cells in one container in

'-i':,. orderto reduce weight and cost. Greater concentrationon the lifeproblem willbe J,

i', needed for space station use, for the system will see little use until the life issue is

i,. settled.

!_.

_, The Air Force has a program on nickel hydrogen cells (Ref. 1_) that is multi-faceted,

!

"' and this is improving the Ni-H 2 technology in several areas Computer programs h_ve i'

}_,. been developed to examine design variables, and these permit rapid focusing on the

i !, best designs. Ya_dney Corp. has a manufact_,ring technology program, one of the • objectives of which isto develop cost reduction approaches. A common pressure

!i. vessel design program with Hughes should improve weight energy density about 20

i_ percent over the conventional individual pressure vessel, and will also reduce volume; _.

I" capacity will be extended to 150-AH, and there are expectations that cost will also be

i:- reduced.

..:!. A novel development underway at NASA Lewis Research Center has taken a fresh

i: approach to the problems of nickel hydrogen ceil design, and initial results show _,ery

:!: good promise (Ref's. 15 and 16). This design uses bipolar construction in a common

_:,. pressure vessel, and has a projected energy density of 20-2# W-hr/lb. It resembles a

!' fuel cell system more closely than a conventional battery, havinl_ active cooling, a

118
separate catalyst surface for oxygen recombination) and an electrolyte o_.anagement

design which gives excellent electrolyte v_oium¢ t_olcra__qc_e_.Thi_s modular cgnce_pt ha=s

been tested, resulting in exceptionally unilorm cell-to-cell voltage) and a watt-hour

efficiency of 81 percent. The internal resistance is very low) and permits very high

peak power. This unique approach with bipolar construction has improved thermal and

oxygen management) and shows promise lot low weight, good electrical perlormance,

and low cost.

With the nickel hydrogen system there is the possibility that the present life and

reliabill_ uncertainties will soon be ,'esolved and verilied by lile _ests. It is possible

that would happen before verification could be obtained on the RFC system. When the

time tor decision comes, the system that is ready and has demonstrated life could

easily win out over other Systems that are not ready.

7.3 REGENERABLE FUEL CELL SYSTEMS

No major technology advancements in the RFC system are visualized in the near

(uture that would affect the choice of an energy storage system for SOC. Technology

improvement programs are underway with botll the alkaline electrolyte and the solid

polymer etectrolyte systems) but these are expected to result in gradual improvements

rather than large jumps. It is expected that a full systems test of the RFC system will

be conducted in the near future, and this will provide data needed for confide.rice in

the system. Proof is needed that the major components can be made to w_rk together

as a system.

Advances are being made in understanding the life-limiting effects with the alkaline

system. The identification of carbonate build=up with time as being a life-limiting

cause is a significant finding. Initial tests on potassium titanate as a substitue matrix

material have shown no loss of performance with time so far, thus suggesting that this

material has the potential for very long life.

Another area that could be meaningful would be the development of reliable static

methods for control of water) humidity) and perhaps other processes) thus placing less

reliance on dynamic equipment, Ancillaries probably are more prone to fail than the

fuel cells or the electrolyzers, and improvement of m,cillaries could make a significant

improvement in overall system life and reliability.

119
7.4 NICKEL CADMIUM BATTERIES

No major technology advancements in the nickel cadmium battery technology are

visualized in the near future that would affect the choice of an energy storage system

for SOC. Very little research is being devoted to this system, although the knowledge

that is being gained on the nickel electrode for nickel hydrogen batteries should

eventually have a beneficial effect on nickel cadmium batteries.

There are two additional developn.ents gradually being implemented that will improve

nickel cadmium battery performance over what has been seen in the past. The first of

these is the development of variable conductance, heat pipe-cooled battery mounting

plates. This will allow batteries to be cooled to a constant temperature nearly

;rrespective of the environment, degradation with timep or changes in battery load.

Temperature control has often been a problem for batteries in the past, for nickel

cadmium bad.aries are highly temperature sensitive.

The second development, especially with large power systems, is in the use of

sophisticated, computer-based control and diagnosis, including charge control.

Sophisticated logic and large memory can be readily available at low weight, and

there will. be little need to compromise the design or system operation in large power

systems of the future.

Although the Ni-Cd battery system is not expected to make major technology

advances, it still has attributes that could prove significant in selection of the energy

storage system for space stations. First, it is available and proven, and considerable

data, are accessible on its life and performance. Second, the initial cost of the system

is low_ and depending upon possible funding constraints and mission duration, this could

be more important than the total life cycle cost. If the life of the Ni-Cd system could

be significantly improved_ using, for example, advanced nickel electrode technology,

then it is possible that the Ni-Cd system could be a formidable competitor to the NiH 2 and RFC systems for space station energy storage.

120

q__ __
8.0 EVALUATION OF INTEGRATION WITH OTHER SUBSYSTEMS

8.1 INTEGRATION OPTIONS AND REQUIREMENTS

The baseline SOC reaction control system for orbit makeup uses hydrazine. An

alternative to this is to use hydrogen-oxygen propellant either as separate gases or as

transported water which would be electrolyzed on board. The life support system also

uses water, oxygen and hydrogen, and these gaseous systems can be integrated with

the hydrogen=oxygen regenerable fuel cell system. An additional possibility is the use

of primary fuel cells for power, using hydrogen and oxygen from shuttle residuals; this

rue: system can be integrated with the reaction control system.

:: The life support system ca, employ water electrolysis to provide Oxygen needed for

breathing and hydrogen needed for reduction of CO 2 in a Sabatier reactor. The

requirements for this are summarized in Flgu:e $.1-1. Typically I7.g ]b/day of water

'-_ will be electrolyzed, but this can increase during ex'_'ra-vehicular activity (EVA)to

':_ 20.2 tb/day. High pressure oxygen is also needed inte._mittently for EVA use.

:c The Full 5OC vehicle without a solar array is l_ge and requires 19.3._[b/day of

= Jr

electrolyzed water for orbit makeup, compared with 9.1 ]b/day for the Half SOC. The

solar array requires an additional 13.6_ ]b/day, for a total of 33 ]b/day for the Full

: SOC. This compares with approximately 32_ ]b/day of water electrolysis for the

:!-_ energy storage system. Thus, the energy storage requirement dominates the orbit

r.,

,- makeup requirement.

: Power needs during emergencies are expected to be between 1_00 W a_d 3000 W. The

fuel cells would be very efficient at this low power level, but we have assumed that

_._" the fuel cell ancillaries, which are on the order of 600 W, should not be cut back in

power. Assuming the worst case condition where the fuel cell must provide power

_.. continuously during light and dark, the hydrogen and oxygen consumption, given as the

. equivalent pounds of water, is 68.5 ]b/day for a 3000 W electrical load (plus the 600 W

load for fuel cell ancillaries); for a l_O0 W electrical load (plus the 600 W load for fuel

ceil ancillaries) _0 ]b/day are consumed. In addition, th_ life support consumption of

oxygen will be I5.8 lb/day, which is equivalent to the electrolysis of i7.g [b/day of

water, Hydrogen t_ndoxygen consumption during emergencies is summarized in Figure

$,1-2, based on Isp = 380 sec. It should be noted that some emergencies will be of a

121

:L --)

t.,
l_r ®.

122

v -_

.4

type where electrolysis of water during sunlit operation is possible, and others where it

is not possible. That distinction has not been made in this analysis, and the worst case

was assuKled.

11.2 INTEGRATION WITH [LF.ACTION CONTROL SYSTEM

Reactants for orbit ma_ceupare 33 ib/day using hydrogen and oxygen in stoichiometric

ratio, based on electrolysis of water. The Isp assumed was 3g0 sec. This results in

12,0#5 ]b/year of water transport to the SOC. Hydrazine, with an Isp of 230 sec.

would total 19,900 ]b/year. Thus, the use oi electrolyzed water would save 7555

lb/year in resupply for orbit maintenance.

:_ System pressure compat]bility with the integrated approach should not be a problem.

,- Hydrogen-oxygen thrusters have operated with rocket chambers at 100 psia and 50 psia

,!

and with a blow down to one third of these values. The minimum electrolysis gas

i. pressure we have considered for integrated systems is 120 psia.

'I_'

_" in addition to the 7gJ5 ]b/year weight saving, it is worthwhile to avoid the shipping,

'_'-i-_ handling and storage of hydrazine from the standpoint of safety. Hydrazine lines must .

i, be heated a.nd._p_ug_h. _h_a.tJ_sN_(2Emaily impor-t_t-<_onsideratiorr;-th_ tong _-.................................... .. ,..... .. . .P.ot_a_- ".....................

_, external line lengths required with the SOC exacerbates the problem, especially during

, power emergencies.

',)

,!

i'

if" Another feature of the hydrogen-oxygen system is the ability to provide very sinai!

,:;- impulse bits, as comp_Lredwith the hydrazine system; this obtains by gas release

.#

-,, without combustion. Factors in favor of hydrazine are (1) the thruster technology is

well developed; (2) hydrazine is a good source of nitrogen and hydrogen n_ded for life

_;. support; and (3) fuel processing is not required.

% The required electrolysis for orbit maintenance may b¢ _tamed either by dedicated

_ units or by integrating with the electrolyzers of the energy storage system.

,_ Integration would increase the normal 32# lb/day of water electrolysis by an additional

_ ]b/day, or 10.2 percent. Maintaining the same current density, the increased weight

of the 55 percent efficient energy storage system would be 56._ It), and for the 62

percent efficient system would be ! 16.2 lb. Integration in this way gives redundancy

from the multiple electrolyzers.

124
With dedicatedelectrolyzersthereisthe need to providesuitableredundancy, A

typicaldesignwould be one scaledup from the 18.0ib/dayunitdescribedinReference

[0, p. 27. Three units would be provided so that two failures could be endured.

Capacity would be increased from I8.0 lb/day to 33.0 lb/day, and current density

would be reduced to result in 500 ASF after the second failure, that is, 166.7 ASF

initially. The unit weight would scale tip from 142 [b to 221.3 Ibmand there would be

three units, for a total of 663.$ lb.

Since the current densities would be designed to be similar, electrolyzer module power

consumption by the electrolyzer celts would be no different whether the system were

integrated or not. However, ancillary power will be much incl.eased for the dedicated

units because of the relatively low po,.ver level. We expect ancillary power to increase

from approximately l._ percent with the enersy storage system to about _.0 percent

with the small, dedicated units. With dedicated electrolyzers, power consumption

would be 6.33 kW. Integrating with the energy storage system would save about 220

watts.

$.3 INTEGRATION WITH LIFE SUPPORT

The life support system requires the electrolysis of 17,g Ib/day. The trades and

rationale on integration of this with the RFC energy storage system are similar to that

of electrolysis for orbit maintenance reactants. Thus, three dedicated electrolyzers

would weight 383 lb versus 30.g lb for integration with the _J percent efficient energy

storage systemt or versus 63,4 Ib for integration with the 62 percent efficient system.

Whether or not the life support system were integrated with the energy storage

system_ means must be provided for the 900 psla needed for EVA. This can best be

done either by direct electrolysis to that pressure, or by means of an electrochemical

oxygen compressor. The electrochemical oxygen compressor has an advantage in that

it can be used also as a backup method for obtaining high pressure starting with either

the energy storage oxygen or the reaction control oxygen. This compressor weighs 6_

lb.

A more attractive integration system is exploitable if hydrogen and oxygen reactants

are used for orbit makeup. The concept, as shown in Figure g.3-l, is to use nonstoichiometric combustion in the thrusters and thus obtain a higher specific impulse

for propulsion. The oxygen that is saved can be used for life support. Thus_ if water

125

'V
_

126 I I
1,'

were electrolyzeadnd theproducthydrogenandoxygenusedas isforreactiocnontrol,

33 lb/daywouldbe required.By operatingatan oxygen/hydrogenmixtureratioof 5 to

l, the specific impulse is increasedfrom 380 sec. to _05 sec. and the reactant

requirement reducesfrom 33 lb/day to 30.82 lb/day.

The excessoxygenelectrolyzed in the non-stoichiometric concept is 10.3 lb/day and

meets muchof the daily oxygenneedof 15.g lb/day (basedon 17.g [b/day of water). It

shouldbe noted that the weight savingis 33.0 minus30.82, that Is, 2.tg lb/day or 795.7

[b/year. Note must be madeof the fact that in this concept there is title excess

hydrogenthat wouldbe available for reduction of CO2 in a Sabatier reactor.

However, there are other approachesto CO2 reduction, suchas a Boschreactor.

An attractive approachfor integration of the Lifesupport,energy storage, and

reaction control systemsis the opportunity to provide especially longduration

emergencycapability. Since large amountsof oxygenandhydrogenare neededfor

orbit makeup,a reserve of these gasescan be maintained at high pressureand be

available during emergenciesfor all three systems. ElectrochemicaJ pumpingis a

i!i_ simple, lightweight way to obtain the desiredhigh pressure,and oxygencompressionis

;_..... neededanyway for EVA. Tankage is the main penalty. For example, usingthe data in Is

_' Figure g.I-2, a 10-day emergency supplyof all the gasesneededfor orbit makeup, I._

t:, kW electric power, and life supportwouldrequire approximately 1900 lb of tanks; also

:i required would be 6_ lb for a hydrogencompressor. Postponingorbit makeup until

,; after the emergency would cut the tankageweight in half. Following an emergencyor

temporary useof these gases,the high pressurereserve can be replenishedonboard.

,,,. $._ INTEGRATION SUMMARY

Ii:" A summaryof the benefits and penalties of the several integration options is given in

;:: Figure g._-l. Conclusionswith regard to these optionsare as follows=

il:

It"

tt

t/" [. Electrolysis of orbit makeupwater to hydrogenandoxygenis preferable to the

i'::_ useof hydrazine. The weight of equipment and the electric power required are

_ modestcompared to the weight savingobtainable.

2. Electrolysis integration of orbit makeupwater v,ith the energy storagesystem

saves5_0 lb andappearsto be worthwhile.

;_: 127
A) H2_)2 VERSUS N2H4 FOR ORBIT MAKE-UP

N2H4 H2"O i 2

WEIGHT 1_.000 LB/YR 12,045 LB/YR (DELTA- 7,BEB)

POWER 0 _3 KW

B) H24) 2 FOR ORBIT MAKE-UP- DEDICATED UNITE VERSUS INTEGRATE WITH ENERGY STORAGE

DEDICATED INTEGRATED (DELTA)

i

883,8 LB m_ SYSTEM: 58_ LB,SAVE 220 W

e2_SY$'I'EM: 108.2 LB, SAVE 250W

C) H2-O2 FOR ORBIT MAKE.UP - NON-STOIC BURN WITH INTEGRATION WITH LIFE SUPPORTAND

ENERGY STORAGEVERSUSSTOIC BURN

STOIC BURN NON-$TOIC BURN AND INTEFRATION I

ELECTROLYZER: 6.3 KW S.9 KW (DELTA - 0.4 KW)

WATER: 12,048 LB/YR 11,248 LB (DELTA - 708.7 LB/YR)

Ol H2 AND 0 2 FOR LIFE SUPPORT- DEDICATED UNITE VERSUS INTEGRATION WITH ENERGY STORAGE

DEDICATED INTEGRATED (DELTA)

POWER: 3.4 KW 3,4 KW

ELECTROLYZER WEIGHT: 383 LB 58%SY$TEM: 30.8 LB

a% SYSTEM: 83.4 LB (SAVE0,_ KW)

0 2 COMPRESSORWEIGHT: 88 LB a LB

Figure 8.4. t: Summary of Integration Trades -We/ghr and Power

128
3. Non-stoichiometric combustionof hydrogenandoxygensaves nearly $00 lb/yr

and appearsto be worthwhile. This savingmay be contingent on development of

an atmospheric CO2 reduction processsuchas the Boschreactor.

_. Integration of life supportwater electrolysis with the electrolysis of the energy

storage system oilers a weight saving ot approximately 330 lb, It is judgedthat

this is not sufficient a weight saving to offset the advantages of a fully selfcontained life support system. However, water electrolysis by the energy

storage system shouldbe a backupto the life supportsystem.

:_"

f. An on-boamdreplenishable high pressurereserve of hydrogenand oxygenis a

worthwhile opportunity for an inte£rated emergency gas system for life support,

" energy storage, andorbit makeup. Ten days emergency can be providedfor with

- a weight penalty ot 920 lb for tanks; Lforbit makeup propulsioncan be delayed

,-_ until after the emergency, the penalty is halved.

4

:ji

.i

,.-i

•'T:

)

-t ;

"-I"

: :2...

129
e

!

t

9.0 ASSESSMENT OF FUEL CELL VS. BATTERIES

9.1 COMIP_N OF ATTRIBUTES

Results of the NASA-sponsored studies should be viewed as intormation which will

assist in making 8 decision on the best energy storage system for space stations. The

studies alone are not a sufficient basis upon which to decided this question. This can

be seen in Figure 4.9-5 showing the attributes that govern the selec.tion of energy

storage systems for space stations. The NASA-sponsored studies examined some of

the important attributes, but some others, which could be more important, were for

good reason not part of the NASA-sponsored studies.

Figure 9.1-1 gives a qualitative evaluation of attributes for the three candidate energy

storage systems. Low cost and low resupply weight were not evaluated because they

were considered to be closely Linked to long life and low weight. Comments on the

items in this figure are as follows:

Long Life. Long Life is mandatory. The Ni-H 2 battery is regarded as having good long

life potential. Both the nickel electrode and the hydrogen electrode have the intrinsic

capability of long life_ but the principal attempt to prove long life in low earth orbit

gave many early failures. This development is continuing, and is expected to succeed.

For the present, however, the Ni-H 2 is not yet credible for LEO. Fuel cells and

electrolysis ceils have shown a potential for long life at the ceil level. The solid

polymer electroi/te fuel cell is especially long lived, showing hardly any degradation

auEterfive years of operation. Lifetimes of both the solid electrolyte and alkaline

electrolyte fuel cells will be much extended by the low current density, which is

preferred for space station needs. Lack of full system tests remain a major

deficiency of the RFC system. It is clear that for both the Ni-H 2 battery and the

RFC system a better data base is needed drawn from long duration tests at read time

duty cycles.

ReliabLility. Both the Ni-H 2 and RFC reliability are unkown Early failures in LEO

tests raise concern with NI-H2 reiiability_ the chief concern with the RFC system is

its complexity. A detadied reliability analysis of the RFC system appears not to have

been attempted, and full-up tests are lacking.

130
NI-Cd NkH2 RFC

LONG LIFE

FAIR TESTSUNDERWAY TES'r_ UNDERWAY (CELL LEVEL)

MANDATORY

ATI'RIBUTES

REUABILITY

FAIR UNKNOWN UNKNOWN

_.SYSTEM)

LARGE'EMER¢3ENCY

POOR POOR GOOD POWERCAPABILITY

POSSIBLY

HIGH PEAK POWER

MANDATORY FAIR FAI R GOOD CAPAEILITY

AI"rTIIOUTES

STATION BUILDUP

POOR POOR GOOD CAPABILITY

INTEGRATION

NONE NONE GOOD ADVANTAGES

HIGH EFFICIENCY FAIR/GOOD FAIR/GOOD FAIR/GOOD

IMPORTANT

LOW COST --

., AI"TRIllUTES

LOWWEIGHT POOR FAIR FAIR/GOOD

Figure 9. 1-1: Evaluation _f Energy Storage Systems for St_ace Stations

131
Large Emergency Power Capability. Modest requirements have generally been set for

space station emergency power. This would be neened whenever primary power i_slost,

If there were a need or strong desire for a large emergency power capability, then the

RFC system would have a decided advantage.

High Peak Power Capability. BQth the Ni--H 2 and the Nt-Cd battery systems have good

peak power capability provided the peak load is brief. The H2-02 fuel cell has an

advantage in that it can be designed for quite high rates for tairly long durations with

only a modest weight penalty. This could be a significant attribute if the peak power

i requirements were similar to those defined in the McDonnell Douglas study, or if

: provisions were required for special high peak power military payloads.

i

Base Buildup Capability. The need for power during base buildup hinges on whether

the solar array can be deployed early, or whethe: deployment must be deferred for a

!_ long time, such _.sthe 60-day duration in the North American Rockwell study.

r'

i Intel_ration Advantages. The RFC system is the only one which has the option of

., integratiu,n. Oxygen and hydrogen tankage can be integrated with the reaction control

• or life support system, and the RFC electrolyzer can be used with the reaction control

•i" system. Integration may be either for the purpose of saving weight, or as a backup

• opera.rag mode.

J

.... Hil_h Efficienc},. High energy storage efficienc7 is very important in reducing solar

_i array size and cost, and in minimizing resupply of reactants to compensate for solar

K"

•_ array drag. All three'candidate energy storage systems can be designed for

._ii' comparable energy storage efficiency.

:_ Low Weight. The RFC system is lighter than batteries. However, this advantage may

not be sufficient if the Ni-H 2 is able to provide outstanding long life at deep depth of

discharge, as has been envisioned for it. !,

"; 9.2 ASSESSMENT

-" In assessing battet'ies vs. regenerative fuel cells for the space station, the objective at

_.. this time is not to choose the best system, but to define which ones have the best

: potential and should be developed.

132
The importance of long life and high reliability cannot be. emphasized too _trongly. If

the nickel hydrogen battery had proved out the early expectations and could provide

say 7 years of trouble-free life, it would be extremely attractive for the sp#ce station.

• We have actually moved backwards with the nickel hyd:ogen system, howewr, for now

i

there is a fair amount of poor performance data to attempt to overcome.

Nevertheless, the s,jstem hamthe potential for long life, and this potential should be

exploited. More emphasis on the fundamentals seems to be needed, aimed at

improving liiet rather than engineering advancements to improve energy density.

I:

i: Several comments are deserved in defense of the Ni-Cd This generally

system. system

took third place in the various space station energy storage studies, and was discussed

relatively tittle in this ama_;,sis. The reason for this is that the Ni-H 2 and the RFC

systems are regarded as having greater potential. The fact remain3, however, that if a

commitment had to be made today, it would have to be to the Ni-Cd system. It is the

omy one proven credible for LEO. We have a lot of experience with it, and though it

tC has limitations, we know how to deal with them. The sooner the commitment must be

i: made, the more likely it will be to the Ni-Cd system. There are at least two other

; conditions whereby the Ni-Cd systent might prove the best choise. One is the case

i!:: where, due to funding restraints, low first cost is much more important than total

!....... cost. The second is the case where either the space station or the power module is

' designed for only a few years.

The regenerative fuel cell system has some advmltages in addition to weight saving

h

i:i which are very attractive and make this system worthwhile of development. One

l?

I_i important advantage is the ability to function as a primary fuel cell as well as in the

, normal RFC mode. This gives it the capability to: a) provide a large amount of

i>

,,'_ emergency power on the order of days or weeks; b) operate temporarily (hours or days)

i!*

I: at power levels well above that of the solar array; and c) take advantage of the

_ opportunity tc use the large residuals expected from the shuttle. For example, with 19

i

)1:" evenly spaced shuttle flights per year and use of the shuttle residuals, the solar array

i' can be reduced to 50 percent of its normgl size.

t,,

!:r:i Low cmrent density designs are best for the fuel cell and electrolyzer in order to

L- :.: obtain high efficiency. In this way) the RFC system efficiency can equal or exceed

i; that of batteries. Moreover, the low energy density gives the capability to handle very

!,

' 133

'i
large peak current, or high steady-state loads if other units fail, and still remain

within voltage limits.

One of the most _mportant advantages of the low current density fuel ceil aesigns is

the opportunity for long life. Tests on electrodes suggest that fuel cells have an

inherent capability to outlive either the nickel cadmium or the nickel hydrogen

system. The lifesituationwithancillarieissnotclear,but thatismuch lessa

fundamental problem than the problem of electrodelife.

When faced with the question of whether or not to develop the RFC system, an

important issue is whether or not such a development would be worthwhile for

applications other than the space station. Here the RFC system has a potential that is

most enticing, and that is as an energy storage system for high power synchronous

orbit spacecraft. With low current density there is the potential to far exceed the life

of battery systems, and a l_-year system could be reasonable. Whereas the RFC

system is noticabiy lighter than batteries for LEO, there is a much greater weight

advantage for geo-synchronous orbit, and weight for that orbit is paramount. This

weight saving results from the fact that the electrolyzer can be sized quite small

because of the long recharge time available. Some indication of the weight saving can

be seen from the PRC study which showed that the RFC system would weigh 25

percent of the Ni°H 2 battery. Preliminary 8oeing estimates confirm that the RFC

system is by far the lighter.

In summary, it is concluded that the RFC system has the best potential in a variety of

areas and is deserving of development. Realization of this potent.ial lies in developing

and testing full systems, with emphasis on life and reliability. The nickel hydrogen

battery also has good potential and is deserving of development| realization of its

potential lies in fundamental work aimed at understanding and addressing the failure

and degradation mechanisms in the system.

134
i I0.0 ASSESSMENT OF PRIMARY ,:UEL CELLS USING SHUTTLE RESIDUALS

IO.l ANALYSIS

The shuttle is required to carry a greater amount of hydrogen and oxygen propulsion

fuel than is normally used on each mission. This results from the need to design for

worst-case conditions of engine performance, wind shear, trajectory error, and all

other factors that contribute to fuel consumption. As a consequence, _he typical

shuttle will arrive at the SOC with an excess of hydrogen and oxygen fuel. This fuel is

_ of high enough quality for use in fuel celts, for orbit makeup propulsion, or for life

support needs. Use of this fuel gives opportunities to save weight and cost of the

electrical power system, and is a further argument to eliminate hydrazine from the

° " alsoreSupplYcompetesCyCle' thisthefollowforln ingfuanel,alysis, it is recognized that the propulsion system

The hydrogen and oxygen in the Shuttle are in cryogenic form. This presents problems

,f

!/i in propellant transfer, especially in zero-G. A principal disadvantage of this approach

i_ is in the present lack of technology for zero-G transfer of cryogens. This could be a

i_i costly development.

!!1 We have looked at two approaches for the application of residuals to the SOC

• ill electrical power system. The first concept is to use the hydrogen and oxygen residuaJs

_; for all power needs, both sunlit and occulted. This eliminates the need for either a

!1 solar array or an energy storage system| a large quantity of fuel is required, however,

i for short duration used in the evolution of except InitiaJ, flights possibly a space

;: , station. The second concept is to use a primary fuel cell for the occulted period only.

_: This eliminates ti_e need for an energy storage system, and also reduces the solar array

_i! ,:t ; size significantly.

3

_11 The analysis was based on Rockwell data for the expected quantity of scavengable

cryogens. The mean weight of available LO2 is 6270 lb with a one sigma of +1582 lb.

The mean weight of available LH2 is 3078 lb with a one sigma of _+63#lb. Reactant

i. consumption of the fuel ceils was based on 0.8_ lb/kW-Hr_ based on a stoichometric

I'

i' Lratio. Excess delivered hydrogen is used for orbit makeup, and is not exploited ,n this

": I' analysis.

!,

i

!* 135
]7® ®

Figure lO.I-I shows the contimlous steady power (concept #1, no solar array) available

as a function of the frequency of Shuttle flights. Since the SOC has a 50 kW toad in

sunlight and a 39.2 kW load in the dark, the average load is #5.9 kW. At that power

level, approximately 50 Shuttle flights per year to the SOC are required to meet all

the power needs with fuel ceils. Also, it is seen that after about 6 to l0 flights per

year, the statistical deviations from the mean are insignificant.

For concept #2, in which the fuel cell provides only the nighttime power, the energy

requirement is reduced to 38 percent of the continuous level. Thus, only 19 flights per

year to the SOC are required for this condition. Since approximately half the solar

array is devoted to recharge of the energy storage system, there is a considerable

saving in array development and hardware cost.

Figure 10.1-2 gives a comparison of the energy storage systems weights, with

component weight breakdowns. This analysis is based on 50 kW continuous power, and

1,':_ does not reflect the nighttime power reduction to 39.2 kW as is the present SOC

The significant result from Figure 10.1-2 is the impressive reduction in weight

attained by either of the two fuel cell concepts, based on free fuel. Also shown is the

i!! weight comparison between batteries and the RFC system, with the result that the

!' RFC system is two thirds the weight of the NI-H 2 battery system.

• It is concluded from this analysis that use of Shuttle residuals is very attractive for

SOC. Weight is reduced significantly, there is less hardware to be developed, and the solar array size is reduced by a factor of two. Large cost reductions should be possible

ii:. if the technology for scavaging and transferring cryogen residuals can be made

_i!: feasible.

t:

!,

[,

],

_ 136

! ! .
S

• 3 SIGMA

0

0 S 10 15 20 28 30 38 40 48 S0

NO. OF 8Hu'rrLE FLIGHTS PER YEAR
]i I

__ ._. i_. I I. L,_I_

gg

• !

"_ _ I I i

__

138
I ....r

t 1.0 CONCLUSIONS

The conclusionsfrom this study are as follows:

h High energy storage efficiency shouldbe an important objectiv., in selection and

designof spacecraft power systems. This minimizes solar array size andcost,

and reduces resupplyfor orbit makeupfrom solar array drag.

2. Regenerable fuel cell systemscan be designedwith energy storage efficiency

that is equal to or higher than present nickel hydrogenbatter _-°-,

3. Evaluation of prior studieson spacestation energy storage systemsshowlarge

variations in weight, cost, and efficiency. Thesevariations are muchreduced

when normalized. Operations cost is the most difficult to establish because

groundrules varied considerably.

•... _. Prior studiesonspacestation energy storage systemsgenerally resulted in

,; preference for the regenerable fuel cell system becauseof a weight advantage.

;L.. Although those studies optimized the regenerable fuel ceil (RFC) systemsat too

I!2......

ii-. low an efficiency, the RFC system still will be lightest.

;i $. A high busvoltage (200 Vdc) may not be best for reliability of batteries, and

i!_! .: possiblyalso fuel cells, due to the limited numberof modulesandthe large b_

i:i_' numberof cells in series. Further evaluation of the optimum voltage is needed

_tii with regard to energy storage reliability.

_. 6. Hydrogen-halogenregenerable fuel cells have the potential for higher energy

il.._ storage efficiency than either batteries or hydrogen-oxygenfuel cell systems.

I!! Thiscan result in a reduction of upto l_ percent in solar array size. This is F

._i'i_. sufficiently worthwhile that further study shouldbe conductedof those systems.

'

.. 7. Electrolysis of water to yield H2 and 0 2 for propulsionfuel offers major weight

i' savingsover the useof hydrazine fuel.

, o

: S. Electrolysis integration of orbit makeupwater with the RFC energy storage

i systemsavesweight and appearsto be worthwhile.

_J

i' _ _ 139
I

9. Non-stoichiometricocmbustionofhydrogenandoxygensavesresupplyweight i

and appearstobe worthwhile.Thissavingmay be contingenont developmentof

an atmosphericCO 2 reductionprocesssuchas theBoschreactor.

10. Integrationf lifesupportwaterelectrolysis withtheelectrolysis oftheenergy

storage systemoffers a possibleweight saving_but it is judgedthat this is

overshadowedby the disadvantagesof integration.

II. BoththeNi-H2 and theRFC systemsareat a pointintheirdevelopmentwhere

thereisno clearsuperiorityof oneovertheotherinthekeyareasof longlife

and reliability. Bothhaveuniqueand worthwhileat"tributanes,d bothareworthy

ofdevelopment.The Ni-H2 systemhastheexpectationforachievinglonglife

withoutthecomplexityofancillaries, andengineeringdevelopmentsarebeing

carriedoutinseveralareasthatcouldapplyto spacestationsThe . regenerable

fuelcellsystemcanbe designedforhighefficiencyor low weight,andhasthe

potentiadforlonglife.AttributesoftheRFC systemwhichareclearlysuperior

r. tO theNi-H2 systemare:goodemergencycapabilitpoy,tentiaflorweightsaving

,_ by integration with other subsystems,the ability to take advantageof reactant

i!' residuidsfrom the shuttle, and the ability to service customer payloadsat

•!2. tempor_y, high power levels.

_.... t2. Energy storage equipment purchasecosts for spacestations represent only a

. small fraction of the total associatedcost. Therefore, in selecting systemsfor

_. R&D_ manufacturing cost differences between candidate energy storage systems

: shouldbe secondaryto system life andother performance attributes.

13. Use of hydrogenandoxygen residualsfrom the Shuttle is very attractive for use

in the RFC system. With 19 flights per year to the SOC, the solar array size can

be reduced by half| with _0 flights per year, nosolar array is required, provided

the residualscan be committed to the fuel cell.

I

,. 1i

!;i

t_

I

I

i

l,

!.

,i

L _ 140
®

4

[2.0 RECOMMENDATIONS

The following recommendationsare made:

1. The regenerable fuel cell system has been found to have good l_tential for space

stations. Therefore, emphasisshouldbe given to properly developthe

°, technology. Someof ".heappropriate tasksarea) Perform systemsama_ysesand designstudies, with identLfication and study

of the interfaces and spacecrMt systeminterrelationships.

;_; b) Develop energy-efficient dry gcs electrolysis for both the alka_ne and

solidpolymer electrolyte systems.

c) Develop technology for improvedreliability. Objectives shouldbe to

_ developpassiveoperation as muchas possible, reduce the complexity and

_ parts count of the ancillaries, and developlong Ufe componentswith

_ suitable redundancy.

2_" d) A test program shouldbe established to explore componentand system

_;_ behavior over a full range of environmentsandoperating conditions. A

_: dam base shouldbe establishedfor operating life and faUure modesboth at

='L the componentand systemlevels.

-_ 2. A water electrolysis/pcopulsion system shouldbe developed. This shouldinclude

integration with the power system. Non-stoichiometric operation shouldbe

_ evaluated, especially from the standpoint of massbalance over a wide range of

:" spacestation conditions.

_ 3. Hydrogen-halogen systemsshouldbe evaluated to determine if a high efficiency

-i__. system can be produced, An assessment should be made of the weight and

technical problems involved with these systems.

_:

.,:-. _.. The high temperature (ceramic electrolyte) hydrogen-oxygensystem shouldbe

evaluated to determine il a high efficiency RFC system is practicable. An

. 141
assessment should be made of the technical problems involved with these

systems.

5. Regenerative fuel cell systems should be studied for the GEO application.

Evaluations to be made should include realistic weight and ef:ficiency

improvement obtainable over batteries,

6. Water electrolysis/propulsion systems should be studied for the GEO application

for weight-savin 8 potential. Both dedicated electrolyzers and electrolyzers

integrated with the power system should be evaluated,

142
®

13.0 REFERENCES

I. Modular SpaceStation, Phase B Extension

SpaceDivision, North American Rockwell, 3anuary 1972

Report Nos. SD-71-217-4 and SD-71-217-6, NASA-CR-t 15_11.

2. R.E. Martin, "Electrochemical Energy Storage for an Orbiting Space Station",

Task t - State-of-the-Art Assessment and Performance Model, Contract NAS3-

21293 to United Technologies Corp, Topical Report FCR-3t42, December 19gI,

NASA Cr-165_36,

3. "Design Handbook for Regenerative Fuel Cell Study",

Lockheed Missiles & Space Co., Inc., November 1972,

o

ReportNo. LMSC-DI_9911.

4. R. Wynveen and F. Schubert, "Regenerative Fuel Cell Study", Life Systems, Inc.,

i_ December 1972, Contract No. NAS9-12509, Report Nos. LSI-ER-151-2 &-3.

It;

_: 5. "Space Station Systems Analysis Study, SCB Alternate EPS Evaluation",

il_'

4 _r McDonneil-Dougla_ Astronautics Co., August 1977,

_!, -- Report No. MDC G69_.

J

L?

I_ 6. "Space Power Distribution System Technology Study",

!_ TRW, Redondo Beach, California

i!!-I_- Under contractto MSFC, Contract No. NASg-3319g, 19g0/gh

(

'" a) April1980 ProgressReport,ReportNo.g720-3g*),May 16,1980.

I.

_ b) Mid-Term Oral Review Data Package, 3une 26, lgg0.

i7

it

_ c) Phase II Oral Review Data Package, Oct. 27, 1981.

d) C. Sollo, "Cost Optimized Multi-Hundred-KUowatt Power Distribution",

presented at 16th Intersociety Energy Conversion Engineering Conference,

August,1981.

,f

;,

,I 143

-I
7. F. Miller, P. Richardson, C. Grail, M. 3ordan, and V. Pattersan,

"Electrochemical Energy Storage Subsystems Study", PRC Systems Services Co.

Under contract to NASA-Lewis, Contract No. NAS3-21962, September 198t,

NASA-CR-I65_20-V-I and 2.

$. M. Ga_dei, Lockheed, "Nickel Hydrogen Life Cycle Test Results", The 1980

Goddard Space Flight Center Battery Workshop, NASA Conf. Publ. 2[77, pp. 3")9-

-_.07.

9. H. Thierfelder, "Nickel Hydrogen Celt Evaluation", General Electric Space

Division Report No. 795DS0_1, Dec. it, 1979.

10. "Analysis of SPE Regenerative Fuel CeU Systems for Space Operations Center",

General Electric Report ECOE$-73, May 26, 1982.

[ t. United Technologies, private communication, April, t982.

12. V. Mueller, McDonnell-Douglas, "Nickel Hydrogen Cell Tests", The £

I!::, 1980 Goddard Space Flight Center Battery Workshop, NASA CP-2177, pp. g09-gl$.

!_°-.

-!i

_, 13. M. 3. Mackowsi, "Cycling Test Progress Report", Addendum 9 to DN-E_-80-

I!:.

',',. 168, McDonnell-Douglas Astronautics Co., Feb. 1982.

)" I_. D. Warnock, "Life Test of _0 AH NI-H 2 Battery",The 1981 Goddard Space Flight

ii Center Battery Workshop", NASA CP-2217, pp. gg7499.

[_ l_. R.L. Cataldo and 3. 3. Smithrick, "Design of a 35-Kilowatt Bipolar Nickelt:i Hydrogen Battery for Low-Earth Orbit Application", NASA TM-g2$_#, Aug. 1982.

II 16. L.H. Thaller, "Nickel-Hydrogen Bipolar Battery Systems", NASA TM-Z2996,

Nov. 1982.

II ,

I ,

144