An overview of the requirements, options, selection criteria and other considerations, and current status with regard to the energy storage subsystem (ESS) for the photovoltaic power system alternative for the space station is provided

Vanommering, G.

English

NASA Technical Reports Server

N87-11075 ,
ENERGY STORAGE SYSTEMS COMPARISON FOR THE SPACE STATION
G. van 0mmering
Ford Aerospace & Communications Corporation
Palo Alto, California
INTRODUCTION
The Space Station represents one of the largest space power system
applications under serious consideration at this time. Definition and
Preliminary Design studie_ under Phase B of the Space Station Program
are in process at most NASA centers and a large group of contractors.
In the Work Package 4 Power System studies, NASA Lewis Research Center
with contractor teams led by Rocketdyne (with major team members Ford
Aerospace, Harris, Garrett, and Sundstrand) and TRW (with General
Dynamics and General Electric) is defining the Power System. This
effort has progressed through conceptual design of various options,
and elimination of some options to a final selection process which is
now beginning.
This paper provides an overview of the requirements, options, selec-
tion criteria and other considerations, and current status with regard
to the energy storage subsystem (ESS) for the photovoltaic power
system alternative for the Space Station, presented from the perspec-
tive of Ford Aerospace as a member of the Rocketdyne team.
ENERGY STORAGE SYSTEM REQUIREMENTS
Technical Performance Requirements
The current requirements to which the power system is being designed
are summarized in Table I. Significant for the ESS is, beside the 75
kW base load, the contingency requirement which demands the ability to
support half the station load for a full orbit after eclipse
completion. This limits depth of discharge (DOD) to about 38-40 % for
nominal eclipse operation. Peaking support is not very severe in a
relative sense, but needs to be factored into DOD and contingency
capability for ESS sizing purposes. A design life of five years has
been used as a goal. Physical constraints for the ESS derive from the
9 x 9 x 9 foot envelope of the "utility centers" located Just outboard
of the transverse boom alpha joints, see Figure I.
Technology Readiness Requirements
Table 2 summarizes the technology readiness and risk implications of
the Space Station schedule, which has a goal of a 1992 Initial
Operating Capability (IOC) with growth beginning a few years later.
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https://ntrs.nasa.gov/search.jsp?R=19870001642 2020-03-23T15:00:31+00:00Z
For IOC the desired technology should be scalable to Space Station
proportions at low risk, while the growth Station permits more
advanced technologies for which both risk and payoff are higher.
IOC Cost Requirements
The cost of the IOC configuration of the Space Station is limited to
$ 8B. This must be accommodated by cost-effective design of each
subsystem, but in particular by ensuring that the overall system cost
is minimized, even if this means choosing a more costly option for a
given subsystem. As suggested in Table 3, cost of the ESS should not
be minimized in itself, but must be combined with cost impacts on
solar arrays, thermal control hardware, power management and distribu-
tion (PMAD) equipment, and others as a basis for overall system cost
optimization. These cost impacts result from ESS roundtrip
efficiency, heat dissipation, and electrical control requirements.
Commonality with other subsystems, platforms and free-flyers to save
development costs, and modularity and simplicity to save production
costs, are desirable to minimize IOC expenses.
Launch and Operations Cost Requirements
Launch cost is a key element of overall cost, and here mass impacts
must be accounted for that are caused by ESS selection and design in
other subsystems. Volume may in some cases be a stronger launch cost
driver than mass, and must be accounted for similarly. As summarized
in Table 4, other elements of life cycle cost include operational and
maintenance expenses. For the Space Station, operations costs are
affected strongly by the need to supply fuel to maintain orbit
altitude; this drives the ESS to high efficiency to minimize solar
array and radiator size. Replacement costs include considerations of
high reliability and wear-out life, and minimal cost of replacement.
SPACE STATION ESS OPTIONS
ESS Options Considered
Within the above framework of requirements, and as quantitatively as
possible, a range of ESS options has been evaluated. Table 5 shows
the major options considered. The first elimination round involved a
global Judgement of readiness of each technology and its ability to
meet the IOC date. This led to elimination of energy wheels, sodium-
sulfur batteries and hydrogen-halogen fuel cells. Bipolar Ni-H 2
batteries were borderline in readiness potential.
For some of the eliminated options an estimate of performance was
nevertheless done to estimate the potential for growth. Table 6,
discussed in more detail later, gives a comparison of some major
representatives of each ESS class. Designs are summarized below.
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Overview of ESSDesigns
Regenerative Fuel Cell. The alkaline regenerative fuel cell system
(RFCS) consists of four identical assemblies. Each includes a fuel
cell module (FCM), a water electrolysis module (WEM), a FCM accessory
section, and a WEM accessory section. The accessory sections contain
the valves, pumps, regulators, heat exchangers, etcetera, required for
RFCS operation. A set of hydrogen and oxygen tanks serves two of the
assemblies. The electrode areas of the FCM and WEM are sized to
provide a relatively high efficiency of 62%, which includes losses
associated with accessory section operation. Typical operating volt-
ages of the FCM and W_ stacks are 155 V.
IPV Ni-H9 Battery. The individual pressure vessel (IPV) Ni-Hp battery
option c_nsists of four batteries of 275 Ah capacity. Each b_ttery
has 105 cells of 275 Ah capacity in series, distributed over five
identical assemblies. These assemblies hold their 21 cells supported
on structural beams that carry heat pipes for efficient heat removal.
Twenty assemblies are held in two "oven-rack" type arrangements, one
per utility center. Typical discharge voltage is 133 V averaged over
the 35-minute, 40% DOD discharge.
Bipolar Ni-H o Battery. The bipolar Ni-H 2 battery uses the design
concept developed by Ford Aerospace and Yardney under NASA-LeRC
sponsorship. It consists of four batteries, each with three assem-
blies in parallel. The assemblies each consist of a pressure vessel
containing two cell stacks of 52 cells in series, with a capacity of
90 Ah. The cells have the long, rectangular configuration: about 12
cm wide by 160 cm long. Each assembly also contains redundant coolant
pumps and a heat exchanger interface.
Ni-Cd Battery. The Ni-Cd system consists of 16 batteries of 125 Ah
capacity and with 104 series cells. Each battery is divided into four
26-celi battery packs, mounted on a honeycomb panel with embedded heat
pipes. The 16 panels are mounted in "oven-rack" type arrangements in
the Station utility centers.
Na-S Battery. The sodium-sulfur (Na-S) battery, operating at 300 to
400eC, uses cell sizes close to those being produced currently. The
75-kW system would consist of four batteries each with four 87-kg
modules of 70 cells of 65 Ah capacity, delivering about 126 V on
discharge. Each module has a variable conductance radiator system on
its external surface. The modules are placed on the outside of the
utility module.
Energy Wheels. The energy wheel data shown represents a blend of
various approaches. This was necessary because of the extremely wide
range of characteristics reported for point designs for Space Station
flywheels.
33
ESS OPTIONS COMPARISON
Performance
Table 6 provides a comparison of ESS alternatives described above.
The alkaline H2-O 2 RFCS is used as the baseline in this comparison.
The RFCS has a much lower mass than the other feasible systems, the
Ni-Cd, IPV Ni-H2, and bipolar Ni-Hp batteries. However, its thermal
control equipment is considerably _eavler than that of the others,
because of the RFCS's relatively low roundtrip efficiency and its
resulting high heat rejection rate, albeit at a higher temperature.
In the case of the room-temperature systems it is also feasible to use
a common thermal control loop for the ESS and PMAD, which is difficult
to do with the RFCS. The roundtrip efficiency difference also results
in a solar array mass "credit" for the non-RFCS systems. When all the
impacts have been included, the RFCS has still the lowest mass, but
the other systems become more competitive.
By far the most attractive is the Na-S battery system; however, this
technology has not reached the maturity required for serious con-
sideration for the IOC Space Station. It provides low mass, high
efficiency, and minimal thermal support requirements due to the high
rejection temperature. With sufficient development its benefits may
be applicable to the growth station.
Maturity
Table 7 summarizes for the options with initial readiness potential
the estimated maturity level using the NASA I to 8 rating scale. The
levels shown here represent (abbreviated definitions):
4 - Critical function breadboard demonstration
5 - Component or brassboard model tested in relevant environment
6 - Prototype or engineering model tested in relevant environment
7 - Engineering model tested in space
8 - Baselined into production design
The rating for the alkallne/alkallne RFCS is dual: while the fuel cell
part has been demonstrated on the STS Orbiters with success, and can
be considered a prototype for the Space Station version, the elec-
trolyzer has so far been demonstrated only as a breadboard in the
laboratory and rates a 4 The IPV Ni-H_ battery has a dual rating of
6 for the qualification of smaller LEO _ells, and 5 for the slightly
lower maturity of the 275-Ah cells. A 220-Ah cell is being demon-
strated in December 1985 by Ford Aerospace and Yardney.
In addition to the maturity level, the degree of current development
activity, interest, and funding is an important factor in the assess-
ment of potential technology readiness. Qualitative estimates are
shown in Table 7.
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Combination of the maturity and activity estimates leads to the judge-
ment that only the alkline/alkallne RFCS, the Ni-Cd battery and the
IPV Ni-H 2 battery are viable options for Space Station energy storage.
Cost
Costs for the three surviving options are undergoing extensive refine-
ment and therefore quantitative values would be very preliminary.
However, a broad qualitative comparison can be made in the different
cost categories, and is expected to remain valid. Table 8 summarizes
the data. Development costs follow the maturity levels as expected.
Production costs are lowest for the Ni-H_ system due to low
complexity, moderate modularity and repl_cation. Ni-Cd batteries are
highest because of the large quantity of cells and battery packs. The
RFCS is intermediate due to greater complexity and lower modularity.
Solar array costs and thermal control system costs are somewhat higher
for the RFCS because of the greater heat rejection requirement. Launch
costs follow the net mass figures of Table 6. Overall, the IOC costs
appear lowest for Ni-H 2 batteries, with the RFCS not very far behind,
and Ni-Cd considerably more expensive.
Operations costs for the three options compare as follows. The drag-
related fuel costs will be higher for the RFCS due to the larger solar
arrays. Random failure occurences will be higher for the RFCS, but
the items to be replaced will be generally the accessory sections,
which are small and lightweight. While the replaced mass may thus be
less than for the batteries, the extravehicular activity repair events
are higher in number and therefore more costly.
SUMMARY AND CONCLUSION
Evaluation of ESS options for the Space Station has led to the selec-
tion of H^-O^ alkaline RFCS, IPV Ni-H^ batteries, and Ni-Cd batteries,
as potent_al_y able to meet requirements. Of these, the Ni-Cd bat-
teries are too heavy and too costly to be a serious contender. Ni-H 2
batteries appear somewhat lower in overall IOC cost and operational
costs, and are also favored slightly in non-quantltatlve criteria,
such as maintainability, safety, etcetera. The RFCS has a mass
advantage, but has an overall small disadvantage in IOC cost and
development risk.
The RFCS versus IPV Ni-H_ battery decision will be the subject of
further sensitivity and Trade studies to ascertain the potential
effects of evolution of requirements. The final selection is to be
made by March 1986 and will involve consideration of all Space Station
system impacts.
35
TABLE I. SPACESTATIONPOWERSYSTEMREQUIREMENTS
lOC NOMINALLOADPOWER : 75 xW
PEAKING: 100 KW FOR 15 MIN PER ORBIT
CONTINGENCY: 37.5KW FOR I FULL ORBIT (AFTERECLIPSE)
e GROWTH- NOMINALLOADPOWER : 300 KW
PEAKING: 350 KW FOR 15 MIN PER ORBIT
CONTINGENCY: 150 KW FOR I FULLORBIT (AFTERECLIPSE)
o IOCCOSTCONSISTENTWITH$ 8 BILLION(19875)TOTAL STATIONCOST
I MINIMALLIFECYCLECOST
UTILITY CENTER (1 OF 2) WITH ESS
GRATED
SERVICING AND
TEST FACILITY
- ESA MODULE
ERMAL RADIATOR
REMOTE
klRLOCK
.ARARRAY
- RCS THRUSTER
Figure 1. PHOTOVOLTAIC SPACE STATION CONFIGURATION
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TABLE2, SPACESTATIONENERGYSTORAGESUBSYSTEMPERFORMANCEREQUIREMENTS
IOC : 75 KW + CONTINGENCY+ PEAKS
e LOW RISKSCALE-UPOF CURRENTTECHNOLOGYFEASIBLE
e TECHNOLOGYREADINESSADEQUATETO MEET IOC SCHEDULE
o ABILITYTO MEET PEAKREQUIREMENTS
GROWTH: 300 KW + CONTINGENCY* PEAKS
o MEDIUMRISKSCALE-UPOF CURRENTTECHNOLOGYFEASIBLE
- MOREADVANCEDTECHNOLOGYPERMII-[ED
o TECHNOLOGYREADINESSADEQUATETO MEETGROWTHSCHEDULE
o ABILITYTO MEETPEAKREQUIREMENTS
TABLE3, SPACESTATIONENERGYSTORAGESUBSYSTEMIOC COSTREQUIREMENTS
0 COST CONSISTENTWITH $ 8 BILLIONIOC STATIONCOST
| LOW DEVELOPMENTCOST |
e HIGHMATURITYLEVEL
e HIGH MODULARITYLEVEL
e LOW COMPLEXITY
LOW PRODUCTIONCOST
o HIGHMODULARITYLEVEL
o LOW COMPLEXITY
| MINIMALADVERSEIMPACTON OTHERSYSTEMS/SUBSYSTEMS
e POWERGENERATIONSUBSYSTEM
e POWERMANAGEMENTAND DISTRIBUTION
o THERMALCONTROLSUBSYSTEM
0 - ETC -
| HIGHCOMMONALITY
o WITHOTHERSYSTEMS/SUBSYSTEMS
o WITHPLATFORMSAND FREE-FLYERS
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TABLE_, SPACESTATIONENERGYSTORAGESUBSYSTEMLIFECYCLECOSTREQUIREMENTS
| MINIMALLIFECYCLECOST
| MINIMALLAUNCHCOST
• LOW MASS
• LOW VOLUME
| MINIMALOPERATIONSCOST
• AUTOMATION
• MINIMALIMPACT.ONOTHERSUBSYSTEMS
- POWERGENERATIONSUBSYSTEMDRAG
MINIMALMAINTENANCE/REPLACEMENTCOST
• HIGHRELIABILITYAND LONGWEAR-OUTLIFE
e LOW REPLACEMENTCOST
LOW MEAN-TIME-TO-REPAIR
MODULARITY
LOW MASSAND VOLUME(LAUNCHCOST)
- LOW PRODUCTIONCOST
- THERMALCONTROLSUBSYSTEMDRAG
TABLE5. SPACESTATIONENERGYSTORAGEOPTIONS
BAI-rERYSYSTEMS
• NICKEL-CADMIUM
e NICKEL-HYDROGEN
• NICKEL-HYDROGEN
i NICKEL-HYDROGEN
o SODIUM-SULFUR
IPV
CPV
BIPOLAR
I REGENERATIVEFUELCELLSYSTEMS
e ALKALINE/ALKALINEHYDROGEN-OXYGEN
e ALKALINE-FC/SPE-EMHYDROGEN-OXYGEN
e SPE/SPEHYDROGEN-OXYGEN
o HYDROGEN-HALOGEN
0 ENERGYWHEELS(FLYWHEELS)
I • COULDBE READYFOR IOC
0 o CANNOTBE READYFOR IOC
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TABLE6, SPACESTATIONENERGYSTORAGEOPTIONSCHARACTERISTICSCOMPARISON
H2-O2 NI-H2 Nt-H2 NI-CD NA-S ENERGY
CHARACTERISTIC RFCS IPV BIPOLAR WHEELS
ROUND-TRIPEFFICIENCY(%) 62 80 82 80 85 85
DEPTH-OF-DISCHARGE(%) (40) 40 40 20 40 40
MASS (KG)
ENERGYSTORAGE 2300 4550 3600 9600 1400 6000
THERMALCONTROL 2100 I100 II00 II00 IO0 800
SOLARARRAYCREDIT - (270) (270) (270) (360) (360)
• TOTAL• 4400 5380 4430 10430 I140 6440
VOLUME(M3) 19 14 3 II 2 9
ECLIPSEHEATREJECTION(KW) 55 19 18 19 18 I0
TEMPERATURE(°C) 80 IO I0 IO 350 30
TABLE7, SPACESTATIONENERGYSTORAGEOPTIONSREADINESSAND ACTIVITYASSESSMENT
ENERGYSTORAGEOPTION M_TY
• ALK/ALKREGENERATIVE.FUELCELL 4/7
SPE/SPEREGENERATIVEFUELCELL 5
ALK/SPEREGENERATIVEFUELCELL 5
• NICKEL-CADMIUMBAI-[ERY $
• NICKEL-HYDROGENIPV BAI-[ERY 5/6
NICKEL-HYDROGENCPV BATTERY 4
NICKEL-HYDROGENBIPOLARBAI'IERY 4
• " SURVIVOR
HIGH
LOW
MED
HIGH
HIGH
LOW
MED
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TABLE8. SPACESTATIONENERGYSTORAGEOPTIONSQUALITATIVECOST COMPARISON
COSTELEMENT RFCS N]-H2 NI-CD
DEVELOPMENTCOST HIGHEST MEDIUM LOWEST
PRODUCTIONCOST MEDIUM LOWEST HIGHEST
SOLAR ARRAYCOST HIGHER BASIS BASIS
THERMALCONTROLCOST HIGHER BASIS BASIS
LAUNCHCOST LOWEST MEDIUM HIGHEST
OVERALLIOCCOST MEDIUM LOWEST HIGHEST
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Energy storage systems comparison for the space station

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