Electrical Power Technologies for Spacecraft: Options and Issues
Transcript of Electrical Power Technologies for Spacecraft: Options and Issues
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8/10/2019 Electrical Power Technologies for Spacecraft: Options and Issues
1/18
Copyright 1997, American Institute
of
Aeronautics
and
Astronautics, Inc.
Electrical Power TechnologiesforSpacecraft Op tionsandIssues
GaryL. Bennett*
Metaspace Enterprises
5000 ButteRoad
Etnm ett, Idaho 83617-9500
Abstract
Some of the principal new electrical power
technologies
for
spacecraft will
be
discussed.
Specifically,
new
solar array designs, photovoltaic
cells,batteries, and po wer m anagem ent and distribution
(PMAD) options will be discussed. Developments in
radioisotope power sources for outer planet missions
will be covered. For each of the principal technologies
for power sources (solar, nuclear), energy storage
(batteries, fuel cells, capacitors, flywheels) and PMAD
the ma in options will be discussed. Issues, both
technical and political/economic, will be highlighted.
Overall,
the
case will
be
made that there
are
some
exciting options available which can greatly improve
the performance of commercial, science, and military
spacecraft.
Introduction
Electrical power, like onboard propulsion and
structures, is an essential sp acecraft technology and can
be thought of as a
"utility".
Th e spacecraft will no t
function withoutit Intoday's competitive environment
where
spacecraft owners
are
trying
to get the
most
for
their money an d scientists are trying to get the most
payload
possible on the
smallest spacecraft there
is a
strong m otivation to reduce the m asses of these utilities.
Depending upon
the
mission,
the
electrical power
system can
consume
as
much
as 25
percent
of the
mass
of a spacecraft. Fortunately, there are a number of
exciting
new
electrical power technologies which will
enable just such reductions, sometimes
to as
much
as
50% ofstate-of-practice electrical power systems. This
paper, which builds upon an earlier general article
1
,
will discuss some
of
these options
and
some
of the
issues facing
future
electrical power systems.
Types
of
Spacecraft Electric Power Sources
The
classic
spacecraft electrical power system (EPS),as
illustrated
in
Figure
1,
consists
of a
power source
(usually photovoltaic
but
sometimes nuclear),
an
energy
storage device (usually rechargeable batteries bu t
sometimes fuel cellswith afuture possibility of using
Copyright 1998 by Gary L.
Bennett.
Published by the
American Institute
of
Aeronautics
and
Astronautics, Inc., w ith
permission.
*Fellow,
AIAA
capacitors an d flywheels), and the power management
and distribution (PMAD) subsystem (sometimes
referred
to as power conditioning and control
rt
subsystem or PCCS). Figur e 2 shows
diagrammatically
the
power source
an d
energy storage
options
for
spacecraft.
The
choice
of the
subsystems comprising
theelectrical
power system is largely dictated by the mission
requirements. A
qualitative overview
of the
trade space
can be seen in Figure 3 which is discussed below:
Photovoltaic power sources provide a long-term
source of
power
at a
known degradation rate.
Photovoltaic power sources coupled with rechargeable
batteries have been the usual choice for spacecraft
operating
in the
inner Solar System.
For short missions, energy storage systems may be
sufficient
to power the space system.
This
was the
practice in theearly d aysof thespace programan dthis
is how the crewed U.S. missions (e.g., Mercury,
Gemini,Apollo, Space Shuttle)
are
powered.
Nuclear power sources provide
a
long-term source
of
power and are very attractive for missions operating
where there is very little sunlight (e.g., outer Solar
System, polar regions
of
Mars, lunar nights, surface
o f
Venus)
or in hostile environments
(e.g.,
radiation
belts,
veryclose
to the
Sun).
Thechoiceof apower source shouldnot beseenas an
"either-or"choice, as spacecraft h ave been flown using
all three power sources (solar cells, batteries, and
radioisotope therm oelectric generators).
Generally, the parameter of principal interest for solar
power
sources and nuclear pow er sources is the specific
power,
i.e., watts per kilogram (We/kg). Fo r certain
applications
(e.g.,
low-Earth orbit, LEO) involving
solar power sources
the
area power density (watts
per
square meter
of the
solar array
or
We/m
2
)
is
also
important.
The parameters of interest for energy storage devices
(batteries, fuel
cells,
flywh eels, capacitors) include
specificenergy (watt-hoursperkilogram, We-h/kg)and
energyden sity (wa tt-hours per liter, We-h/1).
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1997,
American
Institute
ofA eronau t icsand
As t ronau t ics , Inc.
Some General
Issues
in
Spacecraft
Power Systems
As with any technology there areboth technical an d
political issues facing the spacecraft designer when
deciding on the elements of an electrical power
subsystem. The designer
will want
the highest
performance with thelowestmass andcost (including
life-cycle costs) bu tsafety an dre liability mustalsobe
considered.
With the highcostof spacecraft and launch
vehicles,
the
electrical
power system
(EPS)
must
be
highly
reliable.
Itmustbefault tolerant so
that
it does
no tcausealossof spacecraft functions.
Fault
tolerance
is essentialto any mission involvinghumans.
The twin goals of high reliability and
reduced
EP S
mass
pose areal challenge for
designers.
Historically
they
have
opted for conservative, proven
electrical
power subsystems
with
highermassescompared to the
newer, lower-mass technologies
which
often were
untested in
space.
This tensionbetween theso-called
"proven" technologies and the newer, better
technologies shouldbe seen as an opportunity for the
spacecraft
designers
and the
technologists
to work
cooperatively
to
realize
thebenefitsof
these lower
mass
technologies. (As an
aside,
it is
worth noting
that
sometimes
so-called"proven"
technologies
cease
to be
proven,
forcing
spacecraft designers to opt for newer
technologies that can be built. An example of this
situation occurred several years
ago when
problems
surfaced with
the
"NASA-Standard" nickel-cadmium
cells
thereby advancing the
newer
nickel-hydrogen
battery
technology.
3
)
For some applications growth potential is also
important. Can the
electrical
power subsystem grow
with anevolving
space
system
(involving,
fo rexample,
a humanpresenceon the
Moon
or
Mars)?
With these issues in
mind let's
consider
some
of the
power sources, energy storage devices and PMAD
designs. The reader interested in additional
technologies or more technical information is
referred
to the September-October 1996 issue (Volume 12,
Number
5) of theJournal o f
Propulsionand Power
an d
the
annual
Proceedings of the Intersociety Energy
Convers ion
Engineering
Conference
(TECEC).
Solar
Power
Sources
For the purposes of this paper we will consider two
principal
types
of
solarpower sources according
to the
system
used
to convert sunlight
into electricity
(see
Figure2):
Static,
i.e.,photovoltaic
Dynamic
PhotovoltaicSolar
Arrays
Th e
classic solar array
used
on most spacecraft is
usually composed
of a
large number
of
solar cells
mounted
on a
honeycomb
or fiberglass substrate. The
solar
cells are
connected
into the
appropriate
series-parallel electrical circuits
to
produce needed
power
and required
voltages
and curren ts. Toachieve
higher performance such
as a higher specific power
(watts/kilogram)meansusinghigh
efficiency
solar
cells
or
possibly
lower mass
cells
in combination with
low-massarray
materials.
The typical silicon
(Si)
solar
cell
in use today
(including those planned for use on the International
Space Station)
have efficiencies
on the order of 14%
although some promising overseas work reports that
18%-efficient
silicon solar
cells have
been
made.
Newer
cells
suchasgallium
arsenide
(GaAs),whichis
rapidlybecoming the
technology
o fchoice,and
indium
phosphide (InP) offer higher efficiencies (on theorder
of
18%)
an dbetter tolerancetoionizing
radiation
but at
increased
cost
(although as
thesenewercells
are
used
morethecostsare comingdown).
5
'
6
Galliumarsenide
cellswereused onM ars Pathfinder
(cruise
stage,lander
and Sojourner
rover),
an dtheyare in use on the NEAR
(Near Earth
Asteroid Rendezvous) spacecraft, and on
two
of the four panels on
Mars
Global Surveyor.
7
Other types of cells include
InP/InGaAs,
AlGaAs/Si,
GaAs/InGaAs, Al/GaAs/GaAs, InGaAs/InP,
AlGaAs/GaAs-InP/InGaAs, AlGaAs/GaAs/InGaAsP,
GaAs/GaSb, GaInP
2
/GaAs, GalnP/GaAs/Ge,
CuInGaSe^andZnO/CdS/uiP.
2
'
4
-
5
-
6
One
way
around
the
cost problem
is to use
less
expensive optical
systemstocon centratethe
sunlight
on
a
smaller
(hence,
less
expensive)cell. Th e
smaller
cells
are
easier
to
shield against ionizing radiation (e.g.,
protons and
electrons)
such as that found in the Van
Alien radiation
belts.
Experiments with combinations
of
cells
stacked
together
(e.g.
gallium arsenide on
gallium
antimonide) and
concentrators have
shown
efficiencies close
to 30% at 40
suns. Pointing
concentrator cells
toward the Sun
requires
more
attention
to
attitude
control
thanplanarsolararrays
do .
Efficiency is not the whole story, however. Under
NASA
and JPL
sponsorship,
TR Wdevelopeda flexible
blanket array
termed
the advanced photovoltaic solar
array (APSA) which canprovide a specific
power
of
130 We/kg using 14%-efficient thin silicon solar
cells
in
geosynchronous
Earth
orbit (GEO)
for
12-kWe
applications at beginning of life/* For comparison,
state-of-practice arrays
have
typically provided about
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10-25
We/kg.
A variation of
APSA
technology using
GaAs/Ge solarcellsinstead of S icellswillfind itsfirst
application on the Earth Observing
System
(EOS)
AM-1 spacecraft.
10
Earlier
flexible
blanket arrays
include
Milstar,
O lympus-1,
ERS-1,
a nd
Hubble Space
Telescope.
Th e International
Space
Station
will also
use
a
flexible blanket
array.
4
To carry
over
the benefits of
flexible
blanketarrays to
lower powers, AEC-Able
Engineering
Company has
developed the Ultraflex array which
uses
a round,
antenna-like
structure to put the blanket in tension.
Ultraflex promises specific powers
of
>100We/kg
for
1- to
2-kilowatt arrays.
11
The Department of Defense (DoD) has sponsored
development by AEC-Able Engineering of a
concentrator
array
known as
SCARLET
(solar
concentrator array with refractive linear element
technology) that is
based
on a
low-mass, low-cost
Fresnel
concentrator
system developed
originally by
Entech
underNASAand DoD
sponsorship.
SCARLET
offers the
potential
for array
specific
powersup to 100
We/kg.
SCARLET
will
be used to power NASA'sfirst
New Millennium spacecraft
(known
asDeep Space 1)
which
is being designed to test a number of new
technologies in an asteroid an d comet flyby mission.
(For
Deep
Space 1,becauseof the
tight schedule,
the
array will have
a
specific power
of 48
We/kg using
24%-efficient
solar cells made of gallium-indium
phosphide and gallium
arsenide
on a germanium
substrate (GaInP
2
/GaAs/Ge) but
this
is
still
about
twice
the
specific power
fo r
state-of-practice
arrays and a
factorof
seven reduction
in the
requiredsolar
cellarea.)
Deep Space
1
will
be the
first spacecraft
to be
powered
solely by multi-bandgap
solar
cells and by a
concentrator array. Deep Space
1
will show
the
synergy of power and propulsion because the2.6-kW e
SCARLET array will be used to power an ion
propulsion
system.
An innovative concentrator
array
that uses thin-film
reflectors toprovideabout1.5 concentration ofsunlight
ha s been developed by Astro
Aerospace
with USAF
and NASA
sponsorship.
The overall
array,
whichhas a
channel-like appearance, was
planned
to be used on
NASA's Clark spacecraft which was part of NASA's
Small Satellite Technology Initiative (SSTT).
This
array, termed Astro-Edge, promises over 50 We/kg.
Th e
Naval
Research
Laboratory
is
developing
a
similar
concept
with a concentration ratio of 2.5 that is
predicted to
produce
about100W e/kg.
4
Another innovative
approach
to array
design
is to use
inflatable cylindrical
tubes to deploy the
array instead
of
a
mast
or set of
rigid
panels. Fo r
certain
applications,
specific
powers>140We/kga re
expected.
Using
this
technology,
L'Garde,
Inc. ha s proposed a
concept,
termed
the Power Antenna,
that
combines
three functions in one structure:
electrical
power,
telecommunications,
and thermal management.
Another way to
reduce
themass of a solar array is to
use shape
memory alloy devices
an d ultra-light
composite materials with
thin-film
copper
indium
diselenide
photovoltaics. This technology,
which
is
being considered for thefirstEarth orbiting
mission
of
the New
Millennium
program, may yield specific
powers >150
W e/kg.
13
In fact, thin-film arrays
offer
the
potential
of
specific powers
in the
range
of
kilowatts
perkilogram .
5
The choice of
solar
array technology
depends
upon a
number of factors including the power
level
(some
innovativedesigns providetheirlargest
specific
powers
at
higher powers rather than lower powers); stowage
volume;
an d
environmental
conditions (including
ionizing radiation,
space
charging
effects,
atomic
oxygen exposure,
etc.).
Regardlessof the
array
design
or
the
cell type
it is
clear that existing advanced
technology can reduce the mass of state-of-practice
arrays by
over
afactorof two (and in
some
cases
more
like
a factor of
ten).
Even more
improvements
ar e
expectedin the
near
future.
Solar
Dynamic
Power
Another way of using the
Sun's power
is to heat a
working
fluid
that ca n
drive
a
turbine-alternator
much
the
same
way that most
U.S. electrical
power is
produced.
The component
technologies
for dynamic
power conversion ar e
fully
mature, having
been
developed
and
tested
for
more
than
20
years.
NASA's
Lewis Research Center has conducted the world's first
full-scale demonstration
of a complete
space-configured 2-kilowatt
solar
dynamic system
basedon the
closed Brayton
cycle in a relevant space
environment
Testing includes simulation of orbital
startups, both transient and steady-state
orbital
operation,
operation of the
heat receiver,
and hot
restarts andshutdowns. The preliminary resultso f this
testing
show
an overall solar dynamic on-orbit
efficiency
(defined as the
total electrical energy
produced over the orbit
divided
by the
solar insolation
energy received over the Sun portion of the orbit) of
14% to 16%. This compares very favorably with an
on-orbit system efficiency for traditional
photovoltaic-battery systemsof 4% to6%.
14
There
are
several operational advantages
for
solar
dynamic power
conversion compared to photovoltaic
systems. Unlike
photovoltaic
systems, solar dynamic
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system performance is not degraded by environmental
(ionizing) radiation.Ityieldsa stablepowerlevel
over
many
years
ofoperation.
Solar
dynamic powersystems
use a thermal energy storage system rather than
chemical
energy storage
system
toprovidethe heat to
continue running during eclipse periods. Thermal
energy storage,
which
is basically a
means
of storing
heat (usually in a molten
salt)
that is used todrive the
turbine-alternator
when the
solar dynamic
system is
shielded
from
the Sun, is potentially more enduring
than batteries which often have limited lifetimes in
low-Earthorbit(LEO) and
medium-Earthorbit
(MEO).
The
three-
to four-fold
efficiency advantage
of
solar
dynamic power systems over photovoltaic systems
results in a similar three- to four-fold
reduction
in the
solar collection area required for the solar dynamic
system at a
given
power. At LEO altitudes this
translates
to
lowera tmosphericdrag
for the
spacecraft;
thus, less
reboost propellant is required to
maintain
altitude.
Studies
conducted
for
Space
Station Freedom
showed substantially
lower
life
cycle costs for solar
dynamic power compared to
photovoltaic power.
Finally,
preliminary studies indicate
that
solar
dynamic
power
systems
using
advanced
technology receivers
and
concentrators
can
attain
system
specific
powers of
approximately 10
W e/kg
which is about the same
that
can be
attained
by advanced photovoltaic-battery
systems.
Issuesin SolarPower
With
on e exception (NEAR), the use of photovoltaic
solar arraysha s
been
restricted to spacecraft thathave
no t
traveled beyond
the
orbit
of Mars. A number of
factors have
led to this restriction, the
primary
on e
being that insolation falls off as the square of the
distance from the Sun. Thus, a
spacecraft
at Jupiter,
which is about
five
times as far from the Sun as the
Earth, will
"see"
25 times
less
sunlight For the
spacecraft to have the
same
power at Jupiter as it had at
Earth thesolararrayw ould have to be 25 times aslarge
as at Earth. Concentrators
don't
really solve this
problem
because
the
necessary
sunlight stillhas to be
collected
somehow
which
means the
array
has to be an
appropriate
size.
Studies
have shown that the use of
solar
arrays at the outerplanets(i ftheycan bemadeto
work
a t
all)generally
constrainsth e
spacecraft
to
point
at the Sun which means an
increase
in thepropulsion
subsystem mass to maintain tight attitude control with
three different alignments:
with the Earth for
communication;with
the Sun for
power;
and with the
planet to be studied. Two of these alignment goals
(Sun and Earth) become easierto
meet
as the
spacecraft
moves
farther
into
the outer
SolarSystem
but this is the
realm w here thearraybecomes largerandlarger.
Ionizing radiation is another problem that
must
be
addressed in the use of solar arrays. Jupiter, for
example, is notorious for its
radiation
belts which are
over
10 0
times
as
intense
as the V an Alien
belts.
Solar
cells,
particularly silicon cells,
are vulnerable to
ionizing radiation.
Finally,solarcells(especiallysilicon
cells) that
operate
in regionso fspacewith low solar illuminationand low
temperatures
(usually this
effect makes
itspresencefelt
at
>2.5
astronomical units, AUs)
are
subject
to a
performance
degrading
effect known by the acronym
LILT
which stands for
"low-intensity,
low
temperature". Some of theLILT degradationm ay be
caused by a kind of
corrosion
of theelectrical contacts.
Better cells
an d
fabrication techniques (including
the
use of different materials in the construction of the
contacts) coupledwiththe use of concentrator
lenses
to
raiseilluminationa ndtemperaturewillhelp.
Atthe
other extreme
is a
need
for
solar
cellswhich can
operate
at high temperatures. Anexampleof the need
for such high-temperature solarcells can be
found
in
the proposed Solar Probe mission to fly
within
four
solar radii
of the Sun where the sunlight is
3000 times
the intensity
at
Earth
(400 W/cm
2
) an d where
spacecraft surface temperatures canexceed 2000K .
Since this temperature exceeds thecapabilities of any
known
solar
array,
m ission plannersar e
considering
the
use of a disposable array for the early approach and
then
using
shielded
batteries for the closestapproach.
(The
ideal solution would be a
shielded
radioisotope
power
source
which
would
also
permit
an
extended
mission, but,
unfortunately,
politics ha s
once
again
hobbled
a proposed
space mission.)
Solarcells an d
arrays
capableo foperatingat high temperatures would
atleastalleviate
the power drain on the
batteries.
In
considering photovoltaic systems
for planetary
surfacepower
possible
solarspectrumchanges need
to
be
addressed.
Fortunately, the atmosphere of
Mars
is
quitethinand the Moon has noeffective
atmosphere
so
absorption is no t a problem.
Different insolationlevels
need
to be
considered
(Mars
receivesabout 43% of the
sunlight
Earth
receives;
some
have
saidthatagoodda y
on
M ars islikea bad day in
Philadelphia).
Duststorms
on
Mars can cloud the sky and cover the array.
Researchers at
NASA's
Lewis Research Center
have
examined
some of
theseissues
and identified
possible
solutions.
Fo r
example,
studies at
Lewishave shown
that
a
Martian
dust
storm will
produce a
diffuse light
which would
allow planar
solar arrays to function;
however,
this"white sky"
would
presentdifficulties
for
concentratorarrays.
17
'
18
If
solar
power
is to be used on hard
landersthen
arrays
thatcan
take
decelerationsas
high
a s
tens
of
thousands
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fj
of
G swill
beneeded.
Perhaps the toughest challengeforphotovoltaicpower
sourceswou ld be their use on the Moon
where
thelunar
nightstretchesto atleast 14Earth days. Sucha long
period of
darkness
requires good energy storage,
whether batteries
or
fuel cells.
A
realistic specific
power goal
for
advanced
photovoltaic arrays used for
human lunar bases
is at
least
30 0
We/kg, over twice
that of the existing advanced technology
described
earlier. Thin-film arrays might allow
achieving 1000
We/kg
if such
arrays
can be built to
operate
for long
periods
withoutdegrading.
Solar
dynamic
power
systems can
overcome
some of
the issues such
as radiation
damage
to
solar cells
an d
the
limited life
of chemical
energy
storage. But there
are
issues
in solardynamic
technology
that need to be
solved. One criticism often leveled at
solar
dynamic
power
(or
nuclear dynamic
power for
that matter)
is
that
the conversion systems
depend
on moving parts
that
are
assumed
to beintrinsically
lessreliable
and
less
long-livedthan
static conversion
systems
(suchas solar
cellsor thermoelectrics).
Thus, there is a need to evaluate the
reliability
of an
operational solar dynamic
system
on a spacecraft
Therewas a
plan
to testone on the Mir space station
but thatplan was
canceled.
Thereare studies now to
consider such
a test on the
International
Space
Station.
19
A
successful
space
test shouldremove
an y
lingering
doubts about
the
viability
of solar
dynamic
power.
Nuclear
Power
Nuclear power sources come in two
types
(radioisotopesand nu clearreactors)depending
upon
the
sourceof thethermalpower (seeFigure 2). Likesolar
powersources,nuclear powersources canproducetheir
electrical
power
by static conversion systems (e.g.,
thermoelectrics)
or by
dynamic
conversion
systems
(e.g.,
turbine-alternators).
Since 1961, the U.S. has
flown 44
radioisotope
thermoelectric generators
(RTGs) and one
reactor using
thermoelectric
conversion
to provide
power
for 25 space
systems.
Forty-one of these nuclear power
sources
on 23 space
systemsare
still
in
space
or on
other planetary
bodies.
In
ad dition,
the Mars
Pathfinder Sojourner
rover,
which
was launched on 4
December
1996, carried three
radioisotope heater units to
maintain
proper
temperatures
on the
coldsurface
o fMars.
1
'
20
Nuclear
powersources
provide a number ofattractive
featuresincluding long
life
time; relativeinvulnera bility
to the external
environment(e.g., radiation belts
an d
dust
storms); high
self-sufficiency (e.g., no need to
keepthe spacecraft
oriented
toward theSun);a nd high
reliability. All of theU.S.
nuclear
powersourcesflown
have
met or
exceeded their
specified prelaunch
requirements with most performing so well that
extendedmissionswereconducted.
Radioisotope
power
sources
Radioisotope
power
sources derive their power
from
the natural decay
of a
radioisotope. U.S.
radioisotope
power
sources use
plutonium -238with
a
half-life
on the
order of 87
years
meaning
that
the
decay
in thermal
power (which
is the
biggest
contributor to
electrical
power
decay)
is about 0.8% per year,
much less than
the decay of
moststate-of-practice
solarcells.
All
of the U.S. radioisotope
power
sources flown to
date have
used
static conversion,that
is,
they
haveused
thermoelectric materials to convert the heat generated
by
the
radioisotope directly into
useful
electrical
energy. Thermoelectric
conversion
ha s
worked well;
the
RTGson the
Pioneer
10 and
Pioneer
11 spacecraft
are functioning
after
more than 24 years in space.
While
efficiencies are low (typically around 10% for
the thermoelectric
elements
an d less than 7% for the
power source) the specific power can be raised by
operating at higher temperatures. For
example,
the
silicon-germanium-alloy
thermoelectric elements in use
on
Voyagers 1 and 2 ,
U.S.
A ir
Force
satellitesL ES 8
and
LES 9, Galileo, Ulysses, an d Cassini operate at
about
1000
C on the hot
junction
and
reject heat
at
almost 300 C. For the
300-W e-class
RTGs in u se on
Galileo, Ulysses,
and
Cassini,
the specific power is
about5.3 We/kg.
This
compares well
with
the
overall
specific
powers
of
existing
photovoltaic-battery
systems in Earth orbit and it is
much better than
an y
existing photovoltaic-battery system
thatcould
be
sent
tothe
outerSolar System.
NASA has
identified fourteen potential future
representative missions
that may use a radioisotope
power
source
(RPS).
Fo r thesefuture
NASAmissions
there
has been a
strong
drive to improve theconversion
efficiency
and the
specific
power of
radioisotopepower
sources.
This
drive
is
fueled
by a
need
to reduce the
mass of the
power source
for the
new, smaller
spacecraft being planned and to reduce the amount of
radioisotope
(andhencethecost).
21
NASA
and DoD have sponsored studies of
improved
static conversion
systems,
including
advanced
thermoelectric materials, thermophotovoltaics (TPV,
essentially solar
cells that
are sensitive to theinfrared
heat of the
radioisotope
heat
source), andalkali
metal
thermal-to-electric conversion (AMTEC, essentially a
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thermally
regenerated sodium
concentration
cell that
uses beta"-aluminasolid
electrolyte
ineffect toseparate
sodium ions and electrons, the latter traveling
through
an
external circuit
to
produce
the
power).
Both
conversion
systems o f f e r the promise of
radioisotope
power sources with efficiencies
an d
specific powers
abouttwicethose
of
existing
RTGs.
22
'
23
In supportoffutureouterplanetarymissionssuch as the
proposed Pluto Express andEuropa Orbiter missions,
NASA
and the Department of
Energy (DOE)
have
initiated
a
program
to
develop
an
advancedradioisotope
power
source
(ARPS) that can
provide
100 watts after
15 years
within
a mass of
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33 low-power (~l-2-kWe) nuclear reactors from 1967
to 1988 to power its radar
ocean
reconnaissance
satellites
(RORSATs).
31
Both theSNAP-lOAand the
RORSAT reactorsemployed thermoelectricelements to
convert the reactor's thermal
power
to electricity
because that
was a
proven
technology and consistent
with
the
power requirements
of the
respective
spacecraft.
In 1987 the former SovietUnion flew two
low-power (~5-6-kWe)
experimental
reactors
known
a s
TOPAZ (a Russian acronym for thermionic,
experimental, conversion in the active
zone)
to test a
thermionic conversion system. Thermionic conversion
operates
o n
principles similar
to the old
vacuum
tubes:
electrons are "boiled off a hot(-1600 C ) emitter,
captured by a
collector
and circulated to thespacecraft
for power.
One
reactor operated
for
about
six
months
the
other
for about a year which is consistent with the
typically shortlives
o ftherm ionic
reactors.
'
While
the SNAP-lOAflightprogramwas
under
way in
the 1960s,
the
U.S.
also sponsored research on a
30-60-kWe
mercuryRankine
conversion
systemknown
as SNAP 8. Two
SNAP
8 reactors
were
built and
tested;one
operated
for a year and the
second
for
7,000
hours. Both reactors
ha dproblems
with
the
zirconium
hydride
used to
moderate
the neutrons
(the
same
materialused
in SNAP-lOA and
TOPAZ).
In the early
1970s the
zirconium-hydride
reactor program was
reoriented for use
with
a 300-kWe
Brayton
cycle.
Other
concepts, including a liquid-metal-cooled fast
reactor with a potassium Rankine cycle,
boiling metal
reactor,gas-cooled reactor,an dthermionicreactorwere
studied in the
1960s
an d
early 1970s.
Th e
U.S. spent
over
$735 million
in
then-year
dollars on the
SNAP
reactor
program.
In today's
dollars
that
represents at
least$4
billion.
In the late 1970s, DOE's Los
Alamos
National
Laboratory
began
studies of a 100-kWe-class nuclear
reactor called
SPAR. By the
mid-1980s this evolved
into a joint DoD/DOE/NASA program
called
SP-100.
32
'
33
SP-100 was to be ageneric
reactor
power
system
that
could
be
scaled
from 10 kWe to
1000
kW e
to provide power for up to 10 years for a range of
missions includingspace-based
power,nuclear
electric
propulsion, an d
planetary
surface
power.
The
design
mass for the 100-kWe version was 4575 kg for a
specific power o f almost 22We/kg.
33
Th egovernment
program
office wisely decided to put the conversion
system external
to thereactorto
avoid
exposure to high
temperatures, high nuclear
radiation fluxes,
an d
potentially corrosive liquid me tals. In this
proven
design approach the SP-100 nuclear reactor could be
coupled
to a range of conversion systems
such
as
thermoelectric, out-of-core thermionic, Brayton,
Rankine,Stirling, or advanced staticsystems(e.g.,TPV
and AM TEC) for growth andflexibility.
By 1993, most of the nuclear component performance
development
work ha d
been
completed on SP-100.
Through
experiments the
reactor design
and computer
codes
were verified. Enou gh
nuclear fuel
ha d
been
fabricated to allow construction of a 100-kWe space
reactor
thermoelectric
power
system.
34
Budgetary
constraints an dchanging agency
priorities
then led to
the cancellation of the program, although testing
continueson thethermoelectric elementsand anactive
technology transferprogram w asinitiated. About$450
million was
spent
on the SP-100
program. Based
on
the
SNAP an d SP-100 experience
some experts
have
estimated that the cost to design, build,
ground-test,
qualify,
an dproducea
flight
reactor thatmeets all the
safety,
reliability,
performance, and
quality
requirements is probably on the order of $1 billion.
Some trade
pu blications
have
estimated
that the
former
SovietUnion
spent
similaramountsto
develop
its low er
powered
reactors.
' [It is
interesting
to
speculate
what might have
been accomplished in the
1980s
if a
truly focused,
goal-oriented reactor
program
ha d been
conducted
with
th ecombinedresourcesthatwerespent
on competing concepts (including an
estimated
$150
million
or more spent on theill-conceived
Timberwind
nuclearrocket).]
Under DoD initiatives the U.S. ha s looked at
other
reactorconcepts,including various thermionic reactors
and
so-called bimodal reactors
that
will provide both
power
and
limited nuclear propulsion.
To
date none
of
these
concepts
has
reached
th emanufacturing stageno r
are there any approved
missions
for them. DoD
also
led an
effort
to
purchase,
test,and fly
another Russian
thermionic
reactor
known
as
ENISY
in
Russia
bu t
called TOPAZ H
in the
U.S.
Unfortunately, the
Russian thermionic reactors
ar e
rather
low
powered
(~5-6 kWe)
fo r
theirmass (over 1000
kg) and
have
low
efficiencies
(
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American Institute of Aeronautics and Astronautics,
Inc.
IssuesinNuclear Power
Safety
is
often cited
as the
primary issue
associated
with
the use ofnuclear power sources in
outer space.
Th e
U.S.
has had three
accidents
(Transit 5BN-3,
Nimbus-Bl,and Apollo 13) involving
launch
vehicles
or
spacecraft carrying
nuclear
power sources.
However, none of these
accidents
was
caused
by the
nuclear power
sources and in each case the RTGs
performed as theywere
designed
to do, showing
that
it
is practical to
design,
build, and launch safe nuclear
power sources. The Russians havehad at least six
unplanned reentries
of
nuclear power sources: four
involving
reactors
and two involving radioisotopes
(including
N ovember's reentry of Mars
*96).
31
As far
as is
known
the
Russian
accidentswere all the
result
of
launch
vehicle
or upper
stage failures.
Th e
first
publicized Russian
accident,
Cosmos 954 which
reentered o ver
Canada in
1978,
led to the
1992
adoption
by the
U J S f.General Assembly
of a set of
principles
on
the use of nuclear power
sources
in outer space. The
U.S.
has stated it
believes
its
program
is consistent
with
the general goalsof theU .N. principlesbu tdiffers
with
them insomeparticulars where theU.S.approach has a
provenrecord.
37
From
the beginning of theU.S.
spacenuclear
program
in the 1950s, safety has
been
the principal
design
requirement.
Extensive tests
involving
high-speed
impacts,
projectiles, propellant fires,
an d
reentry
simulations
are combined
with
detailed probabilistic
risk
analyses
to
assess
the performance of
each
U.S.
nuclear power
source
before it is approved forlaunch.
Final
approval
authority for launchrestswithth eOffice
of
the
President
The Clinton administration in its
national space policy released in
September
1996
affirmed
that the nationmus t
m aintain
a spacenuclear
power capability.
Given the
lack
of
sunlight
in the
outer
solar system
and the
harsh environments
on the
Moonan dMars this isbotha
reasonable
and practical
decision. Now the Adm inistration andCongress must
be willingto
fund
the
long-leaddevelopment
items for
future nuclear power sources
in the
absence
of a
defined
mission because the nuclear power
development and qualification time often
exceeds
that
of
thespacesystem.
Another
area
of
concern
for the
radioisotope
power
source program is the availability of plutonium-238.
W ith the end of the coldwar, the
U.S.
has shut
down
its
production reactors
where plutonium-238
was
occasionally produced as a
sort
of
byproduct.
In
overcoming this deficiency, the U.S. has purchased
some
plutonium-238 from Russia. Other countries,
including Canada, France,
an d
Great Britain,
ca n
produce plutonium-238. (Plutonium-238
is
naturally
produced as a byproductin terrestrial nuclear reactors
bu t chemically separating it from activated
uranium-based fuel isvery
difficult).
Relatedto the availability of plutonium-238 is thecost
of the
fuel. Before
DOE
stopped
producing
plutonium-238
it proposed making NASA pay the
full
cost of
production.
This is akin to
asking
another
agency tosupportthe
already bu dgeted
infrastructure of
DOE;
it's
as if
NASA
askeda naircraft com panyto pay
the full costsof itswindtunnels. Since thisunbu dgeted
movecould
end up
costing
NASA
thousands
o f
dollars
per
gram (with
kilogram quantitiesbeing
needed)
there
wa s
reluctance to
accept that proposal.
In contrast,
DOE often gives
D oDnuclearma terial at nocostso, in
effect,
a double
standard
was in
place.
Historically,
DOE
and its
predecessoragencies
ha d
either
given the
fuel
to
NASA
or sold it at a reduced price
under
the
legislative charter
to
foster nuclear research
an d
development. The
issue
of w ho pays is
blurred
because
DOE is budgeted to run the facilities so the taxpayer
has already paid once. The best settlement would
include both the
legislative
charter to
clearly
establish
responsibility for budgeting and thenecessary
funds.
Asnotedearlier,the
cost
and
ava ilabilityissues
have in
part
driven NASA
and DOE to investigate more
efficient conversion systems
that
will reduce the
quantity
of
plutonium-238required.
However,
neither
NASA
nor DOE appear to be
devoting
the resources
necessary
to make the
advanced radioisotope power
source a
reality
in
time
for a
2001
or
2002
launch of the
proposed PlutoExpress or Europa
Orbiter.
To
show
thatit can operate
predictably
for 10 or 15 years a new
electrical
power
technology needs several years
of
ground testing. Several years of design, engineering
tests,an dma terials studies
mustcome
first. Rightn ow
the U.S.
has
only
two
thermoelectric elements
that
could be counted on for a long-term mission: the
silicon-germanium alloy used
on the
Voyagers,
LE S
8/9, Galileo, Ulysses, and Cassini and the
telluride-based alloys used, for example, on the
Pioneers
an d
Viking
Mars Landers. For a higher
efficiency
conversion
system
only the Brayton cycle
(undertestat LewisResearch
Center
as part of thesolar
dynamic
program) and
possibly
the
Stirling cycle
would
be
viable
candidates
with
the
Stirling
cycle
being
the
lowest mass option at 100
watts
o r
less. Based
on
earlier experiments andcertainterrestrial
applications,
a
Rankine
cycle
withan
organic
workingfluidmightalso
be made ready
for higher
powers.
A
small (5.9-kg)
mercury Rankine
turbine-alternator
system was
built
by
TR W
an d successfully tested in the late
1950s,
demonstrating the
capability
to
produce hundreds
of
watts
in a
small
package.
38
Withtoday'scapabilities
in
micromachinery,such
a
conversion systemshouldalso
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As t ronau t ics ,
Inc.
be considereda
candidate
for
futuresciencespacecraft
Small ARPS for landers an d
penetrators
will have to
survive high decelerations (2,000
to
80,000
Gs) at
Mars.
21
An issue not often considered with space
nuclear
reactors is the capacity factor,
i.e.,
howmuch energy
did it produce over time compared
with
theoutputhad
itoperatedat its
rated
capacity or
power.
The one
U.S.
space reactor flown wa s shut down by a spurious
voltage that triggered thereactor'ssafety system which
had been designed
for an
irreversible shutdown.
30
Oftenterrestrial reactors
are shut
downbecause
certain
trip
pointsare
reached
not for
crucial
safety
reasons.
In
the author's opinion a nuclear reactor is
more
likely to
shut
down
forreasonsother
than
safety-relatedevents.
Given thecom plexityof the
control
systemlogicfor a
fully automatic space reactor,
shutdowns
will certainly
occur. Thisbegs the question of providing the
backup
power when a reactor is temporarily shut down. A
completely
independent power
system
is
critical
to
maintaining human
bases. Fo rhigh-power
applications
this could
mean
having
two or
more independent
reactors. It also means that designers and operators
must
understand
the
reactor
and its
operation
as a
total
system (something notmuchin evidence atThree Mile
Island).
Energy
Storage
Both
mechanical
and chemical energy storage systems
have been considered. Mechanical energy storage
includes flywheels which could
be made
synergistic
with momentum
wheels
on spacecraft. The chemical
energy storage
systems
used in spacecraft typically
have been eitherbatteries
or
fuel cells.
Generally the
design and
systemgoals
can be
sum marized,
in order of
criticality,as: life,
reliability
andsafety,
efficiency
and
mass,
and reduced
unit
orlife-cyclecosts.
atteries
Batteriescome in twotypes: primary(used once) and
secondary (rechargeable).
Primary
batteries
have
been
an d
ar e
being
used on launch
vehicles
and a
lithium-based primary battery was used to power the
Galileo Probe
during its
descent
into the
Jovian
atmosphere a
year
ago and one is onCassini's
Huygens
Probe. Primaryba tteriesgenerallyoffer higherspecific
energies
than secondary
ba tteries
a ndtheyare
ideal
fo r
certain one-of-a-kind applications such as short-lived
planetary operations (e.g.,
penetrators).
Secondary
batteries are more
commonly
used in space and are
critical
to certain
surface
operations.
Spacecraft
operating in geosynchronous Earth orbit
(GEO) usua lly
experience
about 100
solar eclipses
per
year
so the
number
of
batterycharge/discharge
cyclesis
relatively low over a typical 10- to15-year design life.
Such batteriescaneasily operateat 80% or so of rated
capacity. Incontrast,spacecraft operatingin LEO may
experience 15 eclipses per day for
30,000
charge/dischargecyclesovera typicalfive-year design
life. Historically, such batteries
have
operated at less
than
25% ofrated
capacity
to
preserve
their
lifetimes.
Two principal battery
chemistries
are
being
used or
considered today: nickel-based and lithium-based
batteries.
There
are other systems, such as
sodium-sulfurbatterieswhichpromisea specific energy
of 100 We-h/kg for GEOap plications.
Nickel-based batteries,specifically nickel-cadmium and
nickel-hydrogen,
are used on
almost
all
operational
satellites today.
Nickel-cadmium
has been the
state-of-practice
battery used and state-of-the-art cells
can
produce on the order of 39
We-h/kg.
39
'
40
The
newer nickel-based battery that is replacing
nickel-cadmium, particularly in GEO applications and
on
spacecraft
requiring
powers inexcessof 1
kilowatt,
is nickel-hydrogen. State-of-the-art nickel hydrogen
cells can produce on the order of 50
We-h/kg.
Nickel-hydrogen
batteries
offer longer lifetimes and
greater depths of
discharge
in GEO
applications
than
nickel-cadmium batteries.
39
Recent improvements
in
the
electrolyte have
allowed
nickel-hydrogen batteries
to be considered for LEO applications with greater
depths of
discharge
and
longer
lifetimes. In fact, the
International Space Station will
use
nickel-hydrogen
batteries for
energy
storage
and will use the improved
electrolyte
nickel-hydrogen
batteries
for the
replacements.
The Bubble Space
Telescope currently
uses
an
early nickel-hydrogen
batterydesign.
A
nickel-hydrogen
battery,
based on a
two-cell,
common pressurevessel design, is in use on the Mars
Global Surveyor,
marking
the first use of
nickel-hydrogen
technology
on a planetary
spacecraft.
This design has also been selected for the first Ne w
Millennium
mission (Deep Space
1), Mars
'98,
and the
Discovery Stardust
mission. Earlier,
the Clementine
mission hadshownthe viability of the nickel-hydrogen
battery in the
single
pressure
vessel
design
for lunar
missions.
7
The
Iridium satcomswill
alsouse the
single
pressure
vessel design.
The
long-term
goal for nickel-hydrogenbatteries is to
achieve at
least
100
We-h/kg
of energy
storage
at the
celllevel. Thiscan be achieved by innovativedesigns
(there
are at least five different concepts
under
consideration,
one ofwhich
promises
76 W e-h/kg at the
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A st ronaut ics,Inc.
battery level) and by reducing the masses of the
3041 42
components.
^
For small satellites
which
do not have the
volume
availableorpowerdemands that would leadto the use
of nickel-hydrogen batteries,
nickel-metal hydride
batteries
may be the
answer.
In these batteries the
hydrogen is
stored
as a solid hydride instead of as
hydrogen gas which means theoverall volume of the
batterycan bereduced. Nickel-metalhydrideba tteries
also avoid such toxic
m aterials
as lead,cadmium, an d
mercuryfoundin
some
otherbatteries.
Lithium-based
batteries
are
being
considered for
planetary
spacecraft where
thecycle
lifetimes
required
are not asgreatas LEO or GEOsatellitesan d
where
the
spacecraft
powers may be lower
(typically
under 1
kilowatt)
but the
mass
budgeted for
energy storage
is
very
limited.
?
T h e three
principal
types
(and
theirpossiblespecificenergies)
being considered
a re
Lithium-ion
(-95
W e-h/kg)
Lithium
titanium
disulfide
(-135We-h/kg)
Lithium polymer
(-150
We-h/kg)
Lithium batteriesare environmentally
friendly
because
they
contain
no
caustics,
mercury,
lead
or cadmium.
Lithium-ion
cells
provide three times the
voltage
of
nickel-hydrogen cells which
means that a lithium-ion
battery
will
require
about
one-third
thenumbero fcells
required
by a nickel-hydrogen
battery
to
achieve
the
same voltage
which is
indicative
of the mass savings
with
lithium-ion technology.
>4 1
Both NASA an d
DoD are working with industry on
various
lithium
battery technologies,primarily lithium-ion.
In
summary, it can be confidently stated that
existing
advanced battery technologies can
reduce
the mass
allocated to state-of-practice
batteries
by at
least
25 %
an d
more
like
50%,
with
even further
reductions
envisioned.
FuelCells
Fuel cells have been used to
power
the
Gemini
and
Apollo spacecraft an d
they
are currently used to power
the
Space
Shuttle. Fuel
cells
would
be a good
candidate to power a
space
station during eclipse
periods
or a
lunar base during
the long
lunar night.
There
are
about
200 fuel cell
units (mostly using
phosphoric acid as the
electrolyte)
operating in 15
countries on Earth. Four (typed by electrolyte) are
being developed for terrestrial application in North
America: phosphoric acid (operates atabout200C);
proton exchange
membrane
(operatesat
about
80C);
molten
carbonate
(operates at
about
6 50
C);
an d
solid
oxide
(operates
at
about
1,000 C); Tw o
units
of a
phosphoric
acid
fuel
cell ran for about a
year each
powering a hospital in
Riverside,
California. A
megawatt-class
molten
carbonate fuel cellpower plant
has been
built
for the city of Santa
Clara,
California.
43
It
is possible todesign,
build,
an doperate
long-lived,
high-power
fuelcell
power
plants.
For planetary surface
power
applications,NASA
once
considered a goal of 1000We-h/kg of energy
storage
and a
lifetime
of over 20,000 hours to be desirable.
(For
terrestrial applications an
econom icallydrivengoal
of a
40,000-hour
life ha s
been
cited.) There was a
strong
interest
during the
Space Exploration
Initiative
in
using the
proton exchange membrane technology
which has a solid polymer electrolyte that can be
operated
at low temperatures to permit
fast startups.
Such
fuel
cellshave
been used
onbuses in Vancouver
and around
the Los
Angeles
airport.
43
Th e
proton
exchangemembrane
fuel cell is planned toreplacethe
phosphoric
acid fuel
cell
on
Space
Shuttle because it
meets
or
exceeds
all the requirements of the
phosphoric
acid fuel cella t a
lowercost
and lower
mass.
44
Th eJet PropulsionL aboratory,in
cooperationwith
a nd
sponsorship
by Lewis Research Center, ha s
successfully
assembled a Solar RegenerativeFuel Cell
Test Bed
Facility
at Edwards Air ForceBase. Th e
Naval Air
Warfare
Center has
provided
tw o 25-kWe
separately operatingsolar
arrays both made ofthin-film
cadmium
telluride (CdTe) material. In addition, the
facilityincludes a 25-kWe proton exchange
membrane
(PEM)
electrolysis unit,
four
5-kWe PEM
fuel
cells,
high-pressure
hydrogen
and oxygen storage
vessels,
high-purity water
storage
containers, and computer
monitoring,
control
and data acquisition.
This
facility
will allow JPL to
test
fuel
cells, electrolysis units,
photovoltaic
arrays, system
controls an d
integration
hardware in support of
planetary
and
Earth
surface
operations
(if
fully
funded).
Flywheels
Sparked by recent
advances
in
terrestrial
flywheel
technology,
there
has been a renewed
interest
in using
flywheelsfor energy
storage
in spacecraft.
These
new
terrestrial
flywheel
systems,
which
u secom posite
rotors
and
magnetic bearings toachievewheelspeedsgreater
than 60,000 revolutions per
minute,
can provide
specific energies greater than
66
We-h/kg
(at 75%
depth
ofdischarge),whichis
larger
than
presently
used
nickel-hydrogen
batteries.
In addition to the higher
specific
energy,flywheel
systems
offer
other
potential
advantages over
batteries:
(1)
they
are smaller in
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Inc.
volume
than nickel-hydrogen
batteries;
(2 )
manufacturing
lead
times
can be shorter
than
for most
batteries;
(3) there is no requirement for
taper
charging
which
can
consume
5% to 10% of the
solararraypower
for LE O missions; (4) there is no requirement for
reconditioning
or stringent
thermalcontrol
beyond that
of standard
electronic boxes; and (5) precise
measurements of the
available
energy can be
obtained
from
the wheel
speeds.
46
Fo r spaceapp lications there
would have
to be a scaling
down of the
terrestrial technology,
which has been
designed to provide tens of
kilowatts
for such
applications as
electric
vehicles an d uninterruptible
powersup plies. CurrentlyNASA, in
cooperation
with
USAF, is
considering
two prototypeflywheel
systems:
a small (250-to
300-We-h) flywheel
for small
spacecraft and a 3.25-kWe-h flywheel that could be
used
on International
Space Station.
To be
really
competitive
with
the
masses
of advanced
chemical
energy storage
systems,flywheel systemswillprobably
have to combine the functions of energy
storage
and
attitudecontrol (which has
beenperformed
by smaller
reaction
w heels).
46
Capacitors
Recently there has been renewed study of capacitors,
specifically chemical double layer (CDL) capacitor
banks to powerelectromechanicalactuators andelectric
thrusters.
47
49
Capacitorshave
the
advantage
of being
power dense
(but
limited by energy density)
while
batteries are energy dense bu t deficient in power
density. Some combination of batteries and capacitors
may
provide
the optimum energy
storage
for
those
applications requiring both
power
dense and
energy
denseoperation. Experiments
have
been run
showing
that
the CDL
system
can power
electric thrusters.
Technologists believe "It is reasonable toexpect
units
with
energydensitiesgreater
than
5Whr/kga nd
power
densities greater
than
a
kW/kg
to emerge from the
laboratories into
the
marketplace
in the near
future".
IssuesinEnergy S torage
Nickel-cadmiumba tteries
have
been limited
in
lifetime
by
degradation
of the
electrodes
and by hydrolysis of
the
separator
material.
Chemicals
(such as oxygen and
potassium hydroxide) in the battery can attack the
separator
ma terial.
Electricalshortingof the battery has
been caused by migration of cadmium
inside
the
battery. Both cadmium and one of thecom ponents of
the separator have
come
under environmental and
occupational health and
safety scrutiny.
In response,
the nickel-cadmium technologists have
been
working
on developing new separator materials and there is a
good
chance that
nickel-cadmium
batteries can be
improved
enough
to
compete
with existing
nickel-hydrogen batteries.
Theoretically, nickel-hydrogen batteries should have
specific energies of more than 40 0
We-h/kg.
In
practice, the
specific
energy ha s been
about
50
We-h/kg.
39
A
number
of
factors
including themassof
the
positive electrode,
th emassof the
pressure
vessel,
and
voltage
drops
within
the
battery
have led to
this
limitation.
To
improve
the performance of
nickel-hydrogen
batteries will require the design
changes and mass reductionsnotedearlier.
Lithium batteries
need to be controlled
carefully
to
prevent
overcharging
or overdischarging. (Some early
lithium
batteries exploded
bu t
this problem appears
to
be solved.) With
lithium
batteries (both
primary
an d
secondary)
being
consideredto
power
planetary
surface
penetrators
there is a requirement to
develop
lithium
batteries
which
can survive decelerations of
tens
of
thousands
of
G s.
7
Fuel cells
can
suffer from undesirable
chemical
reactions
that
interfere with reactions at the electrodes
an d from resistance heating within the electrode.
Ideally,fuel cells
should have high
chem ical reactivity
withno corrosion orsidereactions to
produce
thelarge
current
densities
needed for long lifetimes. If proton
exchange
membrane
fuel cells are to be used,
much
development
work
needs to be done to ensure long
lifetimes
of the
mem brane system.
Chemicalenergystoragesystems intendedfo rplanetary
surfaces must be able to withstand very low
temperatures (such a s
duringlunar
nights or on M ars or
on
the
satellites
of the outer
planets)
and
high
temperatures (such as on the surfaces of Venus or
Mercury).
To be
useful,
flywheel energystoragesystemswill have
to
be improved an d scaled
down
to meet spacecraft
requirements. This means further work on materials
and
magneticbearings.
Lifetesting is
clearly needed
to
determine
if all the
components will
last
Moreover,
flywheels
willhave to bedesignedto
accommodate
the
launch
environment.
A
systems-level approach
involving multiple sets of
flywheels
(and
probably
combining
the
functions
of
flywheels
an d
reaction
wheels) will be required to overcome torques and
possible
failures in a mass-competitive
manner.
Finally,
but noless
important,
flywheelsw ill have to be
fail safe especially against the consequences of a
catastrophic
failure such
as
seizures
or
destruction
of
therotor.
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Copyright
1997
American Instituteof
Aeronautics
and
Astronautics Inc.
Power Management and Distribution
Th e electrical tie that
binds
the power
source,
the
energy storage,and therestof the spacecraft
together
is
power management
an d
distribution
(PMAD).
Historically,
the PMAD subsystem has been composed
of
discrete parts mounted on planar printed circuit
boards leading to a mass fraction that ha s
been
increasing toward
10% of the dry
mass
of the
spacecraft.
50
This is unacceptable in today's
environment ofsmallerspacecraft.
A solution to
massive
PMAD subsystems is to
incorporate technologies developed in the commercial
electronicsworld where more and more functions are
being
incorporated
on smaller an d smaller chips. In
short, one can think of the new approach to power
management as
"PMAD
on a chip". JPL has already
begun this
approach in a
phased manner
starting
with
Cassiniwhich will use a new solid-statepower
switch
(sometimes called hybridization
technology).
7
' JPL
is extending this
work
to second-generation
power
switches,
high-performance
power converter modules,
an d
field
programmable
gate
arrays for command an d
telemetry
interfacing.
The overall goal is a complete
reconfiguration of the
basic building
blocks of the
PMAD
subsystem,
going from discrete
components to
hybrid systems tom ulti-chip modules. This approach
has the important effect of reducing the amount of
electrical cabling.
The overall
goal
is to
take PMAD
from state-of-the-art
150-We/kg systems to
>600
We/kg.
7
An
even more
revolutionary approach is being taken
under the New Millennium program.
Electronics
packaging, interconnections, an d data an d power
distribution
are
being integratedwiththoseparts
of the
spacecraft bearing mechanical loads and providing
thermal
control.
This approach,
referred to as
multifunctional structures, involves placing passive
electronic
components
within
the composite
structural
ma terials to
achieve
a 75 percentreductionin
electrical
harness
an d cable mass, a 50 percent
increase
in the
payload fraction, and a 40
percent
increase in the
internal
volume of the spacecraft. A modest
experiment
to
allow
an
in-flight test
of the
concept will
becarriedon the
first
New Millennium(Deep Space 1)
spacecraft. Bycontinuingtoworkon
this
revolutionary
approach it may be
possible
to
have
a completely
cableless
system
for the
third
Deep Space
mission.
7
'
13
-
50
'
51
Other technologies
that have
been developed or are
under
development include
new grounding
techniques
for
the high-voltage power systems (such as
International Space Station)
used
on LEO
missions
where
interactions
with ionospheric
plasma occur;
miniaturized microprocessors to condition and control
the electrical power; an innovative photovoltaic
regulator kit experiment
that
was to have
been
testeda s
part of
NASA's
small spacecraft technology initiative
(the Lewis
spacecraft)
an d
ground-based
test beds to
validate
ne wPMADtechnologies.
7
'
13
'
50
53
IssuesinPower M anagementand
istribution
The drivetoward
sma ller,
improved PMA D subsystems
for
th enew,smallerspacecraftappearsto bewellunder
way. Less attention is paid currently to PMAD for
future higher-powered
human habitats.
Terrestrial
experience shows
that
the use of alternating current
(AC)powerleads to smallerPMAD than does the use
of direct
current
DC). Commercialpassenger
aircraft,
for
example,
operate
with
400-HzA Cpower. Yetlittle
has
been
done
to develop a
space-qualified
AC
PMAD
forlarge power systems.
Anotherareawhichreceivedsome
attention
during the
SP-100 space reactorprogramwas thedevelopmento f
radiation-resistant,
high-temperature PMAD
components. Some
work was
done
developing
high-temperature silicon-carbide switches but more
needs
to be done if large nuclear power sources are
used. This isfirsto f all a m aterialsissue(i.e.,finding
materials that will work well under high
temperature
and
high
radiation) so
early
innovations can be
achieved at relatively low costs.
Concluding
Remarks
The
technology
exists
today to cut the
mass
of
state-of-practice electric
power
systems in half and in
some
cases
more. Technologists
are
continuing
topush
for more efficient and lighter
power components
an d
they
are developing innovative concepts that combine
power,
structures
and thermal control. However, this
effort needs continuing support, both
managerial
and
financial. This
effort also
must be
given
the time to
properly
develop
an dqualifythe newer
technologies.
As
mentioned before,
the choices for
electrical
power
systems should not be seen as "either-or" choices.
There is
room
for all of the options as requirements
evolve. For
example,
the
early
Transit
satellites
were
solar
an d battery powered but then small RTGs were
added for
extra power
then two
all-RTG
Transits were
flown. NASA
also
used
a
combination
of
nuclear,
solar,
and
ba tteries
on its first
n uclear mission (Nimbus
in). Some early
studies
for pow eringhuman-inhabited
lunar
bases showed the early outpost beginning with
photovoltaic arrays or solar dynamic modules
with
batteries
or regenerative fuel
cells
for energy
storage.
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The
overall specific power
for
such
systems is in the
range
of 1.5 to 3
We/kg.
As
these early studies
projected
the powerdemands to
rise
for
later
outposts,
nuclear reactors
could
be employed to provide
continuous day and
night power without
the
need
fo r
energy storage.
Th e
specific
power for the nuclear
reactor power
system can be in the
range
of 25 to 60
We/kg depending upon the power an d conversion
system.
But even with the nuclearreactor,the outpost
may want to maintain an
emergency solar-chemical
and/or
radioisotope
dynamic sourceofpowerfo rthose
occasionswhen
th e
reactor
isshutdown.
Finally,this
is an
exciting time
to be
working
in
space
power becauseof the
technical challenges
a ndbecause
of the chance to contribute in a measurableway to the
human
explorationofspace.
References
1. G. L. Bennett,"Electrical Power for Spacecraft
Opinions
andIssues",Space
Times,
Vol.36, No. 3 and
No. 4,
M ay-June
1997 and
July-August
1997.
2. H. W . Brandhorst, P. R. K.
Chetty,
M . J.
Doherty,
an d
G. L.
Bennett,
"Technologies for
Spacecraft
Electric Power Systems", Journal
of
Propulsion
and
Power ,
Vol.
12, No. 5, pp.
819-827,September-October
1996.
3. M. A. Manzo and S. W. Hall, "Aerospace
Nickel-Cadmium
Cell
Verification - An Update",
Proceed ings
of the Thirty-Second
Intersociety
Energy
Conversion
Engineering Conference,
Vol.1, pp.
99-107,proceedingsof a conference held inHonolulu,
Hawaii,27
July
- 1
August
1997 an dpublishedby the
American InstituteofChemicalEngineers,New York,
New York.
4. D. M.
Alien,
"A
Survey
of
Next Generation Solar
Arrays", AIAA 97-0086, prepared for the 35th
Aerospace
SciencesMeeting & Exhibit, held in Reno,
Nevada,
6-10January1997.
5. G. A.
Landis,
S. G . Bailey, and M . F. Piszczor, Jr.,
"RecentAdvances
in
Solar Cell Technology",Journal
o f Propuls ion and
Po w e r ,
Vol.
12, No. 5, pp.835-841,
September-October
1996.
6. K. A.Bermess,D. J.Friedman,S. R.Kurtz,A . E.
Kibbler, C.Kramer,and J. M . Olson,"High-Efficiency
GalnP/GaAs
Tandem
Solar Cells , Journal
of
Propu l s ion a n d P o w e r ,
Vol.
12, No. 5, pp. 842-846,
September-October 1996.
7. C. P. Bankston, "Progress in Spacecraft Electric
Power
System
Technologiesfor Deep SpaceM issions",
AIAA 97-0089,
prepared
for the 35th Aerospace
Sciences
Meeting
&
Exhibit, held
in
Reno, Nevada,
6-10
January
1997.
8. L. M. Fraas,G. R.G irard,J. E.Avery,B. A. Arau,
V.
S. Sundaram, A. G. Thompson, and J. M. Gee,
"GaSb
BoosterCellsforover 30%Efficient Solar-Cell
Stacks",
Journal o f
Ap plied
Physics ,Vol.66, No. 8, pp.
3866-3870,1989.
9. R. M. Kurland and P. M.
Stella, "The Advanced
Photovoltaic Solar
Array
Program Update",
Proceed ings
of the 3rd
European Space
P o w e r
Conference (Graz, Austria), European Space Agency
Publication
ESAWPP-054, Paris, France,August1993.
10. M. J. Herriage, R. M.Ku rland,C. D. Faust, E. M.
Gaddy, and D. J.
Keys,"EOS A M-1
G aAs/Ge
Flexible
Blanket
Solar
Array Verification Test
Program
Results ,Proceedings
of
the Thirty-Second
Intersociety
Energy
Convers ion Engineer ing
Conference,
Vol.
1,
pp . 556-562,
proceedings of a
conference
held in
Honolulu,
Hawaii, 27
July
- 1
August
1997and
published by the American Institute of Chemical
Engineers,
New York, New York.
11. P. A.
Jones,
T. J.Harvey,and M. V.
Douglas,
'The
UltraFlex SolarArray Qualification Testing Program",
Proceedings o f the30th
Intersociety
Energy Conversion
Engineer ing Conference, Vol. 1, pp.
347-351,
proceedings of a conference held in Orlando,Florida,
30 July- 4August 1995andpub lishedby the American
Society
of
Mechanical Engineers,
New
York,
New
York.
12. D. M.
Murphy
and D. M. Alien, "SCARLET
Development, Fabrication, an d Testing for the Deep
Space 1
Spacecra ft",
Proceedings of the
Thirty-Second
Intersociety Energy
Conversion
Engineering
Conference, Vol.
4, pp.
2237-2245, proceedings
of a
conference held in Honolulu, Hawaii, 27 July - 1
August1997
an d
published
by the
Am erican Institute
of
ChemicalEngineers,New York, New York.
13. A. B. Chmielewski etal.,"The New Millennium
Program Power Technology", Proceedings of the
31s t
Intersociety Energy Convers ion
Engineer ing
Conference, Vol. 4, pp.2193-2198, proceedings of a
conference held in
Washington,
D.C., 11-16 August
1996
and published by the Institute ofElectrical and
Electronics Engineers, Piscataway ,New
Jersey.
14. R. K.
Shaltens
and L. S.
Mason, "Early Results
from
Solar
Dynamic
Space Power
System Testing ,
Journal
of
Propu lsion
and Power ,
Vol.
12, No. 5, pp.
852-858,Septem ber-Octob er 1996.
13
-
8/10/2019 Electrical Power Technologies for Spacecraft: Options and Issues
14/18
Copyright 1997,
American
Inst i tu te o f Ae rona ut ics and Astrona ut ics , Inc .
15. J. E.
Randolph,
et
al., The
Technology
Development
Status
of the
Solar
Probe",Proceedings
o f th eSpace Technology andApplications International
Forum (STAIF-97), Part One,
pp .
123-130,
AIP
Conference
Proceedings 387, proceedings
of a
conference held in Albuquerque, New Mexico, 26-30
January 1997an dpu blished by the American
Institute
of Physics,Woodbury,New York.
16. G. A.
Landis, "Mars
DustRemoval
Technology",
Proceedings of the Thirty-Second
Intersociety
Energy
Convers ion
Engineering
Conference, Vol.
1, pp.
764-767,
proceedings
of a
conference
held in
Honolulu,
Hawaii, 27 July - 1August 1997an dpublishedby the
American
Institute
of
Chemical
Engineers,
N ew
York,
New York.
17. J. Appelbaum and G. A.
Landis,
"Photovoltaic
Arrays for
MartianSurface
Pow er",
Acta Astronau tica,
Vol.30, pp.
127-142,1993.
18. J.
Appelbaum,