The major requirements and guidelines that affect the manned space station configuration and the power systems are explained. The evolution of the space station power system from the NASA program development feasibility phase through the current preliminary design phase is described. Several early station concepts are described and linked to the present concept. The recently completed phase B tradeoff study selections of photovoltaic system technologies are described. The present solar dynamic and power management and distribution systems are also summarized for completeness

Baraona, Cosmo R.

Sheibley, Dean W.

English

NASA Technical Reports Server

N87-29915
STATUS OF SPACE STATION POWER SYSTEM
Cosmo R. Baraona and Dean W. Sheibley
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
The manned space station is NASA'S next major program. It presents many
challenges to power system designers. The power system in turn is a major
driver on the overall configuration. In this paper the major requirements and
guidelines that affect the station configuration and the power system are
explained. The evolution of the space station power system from the NASA pro-
gram development-feasibility phase through the current preliminary design
phase is described. Several early station concepts are described and linked
to the present concept. The recently completed phase B tradeoff study selec-
tions of photovoltaic system technologies are described. The present solar
dynamic and power management and distribution systems are also summarized for
completeness.
BACKGROUND
The space station system is the next major step in the manned space pro-
gram. The space station will be a multipurpose facility that will enable
advancements in science, technology, and space transportation capabilities.
It will promote commercialization of space and open new avenues not yet fully
realized.
Numerous studies conducted in the 1960's and 1970's (ref. I) have helped
establish a role for a manned space station. Most unmanned satellites launched
since the beginning of the space age in 1957 have been powered by photovoltaic
systems based on silicon solar cells. A few deep space interplanetary missions
and manned spacecraft like Mercury, Gemini, and Apollo are the exceptions.
During this era technology has been developed for photovoltaic, solar dynamic,
and nuclear systems as well. The primary thrust of these developments has
been toward lighter weight, lower volume, higher efficiencies, longer life-
times, and reliability. These technologies and flight experiences formed the
starting point for establishing the feasibility of the current space station
and for defining its power system.
FEASIBILITY PHASE
The current Space Station Program can trace its roots back to 1981, when
technology steering committees were formed to identify candidate technolo-
gies. In early 1982, the Space Station Task Force was formed to determine the
feasibility of a space station (phase A in the program development process).
The task force analyzed the uses or missions for a manned space station. Spe-
cific missions to be performed were determined and studied extensively
(ref. 2). These studies showed that the station would serve as an assembly
https://ntrs.nasa.gov/search.jsp?R=19870020482 2020-03-20T10:01:52+00:00Z
facility, a storage depot, and a transportation node or way station for pay-
loads intended for higher Earth orbits or for interplanetary missions.
These diverse missions led to the space station concept shown in figure I.
It is composedof a mannedcore and an unmannedco-orbiting platform, both in
a 28.5° orbit. Another platform is in a polar orbit. A system of unmanned
vehicles for maneuvering payloads near the station or for transferring them to
other orbits is part of the space station system.
The mission analysis studies identified the total requirement for each
station element. Power levels were determined as a function of time from the
initial operational capability (IOC) through some future power level when the
station and the number of missions will have grown. These power requirements
have changed as the mission definition has evolved. The current user power
levels are shown in table i. User power means that all system losses for gen-
eration, storage, conditioning, and distribution have been taken into account.
Note that the IOC station power of 75 kW is about an order of magnitude higher
than that used on Skylab. Skylab, the first U.S. manned space station,
launched in 1973 is the largest (8-kW user power, 22-kW array power) solar
power system flown in space to date. The 75-kW requirement for the planned
space station is the most challenging factor facing the power system designer.
Additional challenges arise from programmatic requirements imposed on the
power system designer. These additional requirements are management and engi-
neering related. They include cost (both initial and life cycle), schedule,
technical development risk, weight, and safety requirements as most large
spacecraft projects do. However, the permanent nature of the space station
results in some new and unique requirements such as growth capability, main-
tainability, and commonality of hardware and software across all station ele-
ments. Commonality reduces development, qualification, and production costs.
It was an important factor in selecting the technologies for use on space sta-
tion. Replacement and growth of the station systems requires that they be
designed so that they can accept changes in technology (i.e., technology trans-
parency) yet still provide the same functions. Other considerations are the
station orbital altitude and decay, orbital assembly and buildup, lifetime,
logistics and control of spare parts, failure criteria, verification, contin-
gency requirements, load type and location, and interface.
In 1983 the task force took the results from the mission analysis studies
and synthesized them into several candidate space station configurations.
They also further studied and sharpened technology selection for all the sta-
tion systems including power.
As a result of the feasibility work NASA received approval to build the
space station and have it operational by 1994. The importance of drag area on
reboost cost and life-cycle cost coupled with the very large growth power
requirements (as high as 450 kW) resulted in the adoption of solar dynamic
generators with thermal energy storage in addition to photovoltaic arrays with
electrochemical energy storage for detailed study in the definition phase.
Definition Phase
The present space station configuration and the hybrid power system
(fig. 2) using both photovoltaic and solar dynamic technologies, were selected
in the definition (or phase B) studies, which began in 1984. Nuclear and
other power systems were ruled out on the basis of schedule, cost, risk, and
other factors. The size and drag area of the power system were major consider-
ations in selecting the overall space station geometry. This geometry must
allow the station and the power system to grow. It must minimize the effect
of the power system on viewing angles for experimenters and for communica-
tions. The space station and its power system must be controllable and struc-
turally sound. The maximumdegree of commonality between the station and
platform power systems was necessary to reduce costs. Most importantly, the
station must be passively controllable (i.e., gravity gradient stabilized).
From these diverse and sometimes contradictory requirements, the power-tower
and later the dual-keel configurations were developed and studied by NASA. At
the same time the NASALewis ResearchCenter, along with its two major phase B
contractors, TRWand Rocketdyne, studied numerouspower system types. These
phase B definition studies are described below.
Definition of Power System Configuration
Early in phase B six power system options were defined for study
(fig. 3). The IOC power level of 75 kW and the growth power level of 300 kW
were selected. The six cases were established on the basis of IOC power sys-
tem type (either solar dynamic (SD) or photovoltaic (PV)) and the method of
growing from 75 to 300 kW. Case i was all PV. Case 6 had minimum PV
(12.5 kW) at IOC and all SD at growth. An alI-SD system is not feasible
because power is needed on the first launch when the accurate Sun tracking
required for the SD system is not possible. Cases 2 to 5 had various propor-
tions of SD to PV. Commonality between the station and the platform was con-
sidered in these system studies.
The primary selection criterion for these system studies was both IOC and
life-cycle cost for the station and the platforms. Development, manufactur-
ing, verification testing, overhead, and launch costs for all the space sta-
tion system hardware and software were included. An especially important
life-cycle cost saving resulted from the reduced aerodynamic drag associated
with the SD system. This reduced drag allowed lower orbital altitude and
higher shuttle payload capacity.
As a result of these system studies the case 5 hybrid was selected. In
this case the PV portion of the power system generates 25 kW with four solar
array wings (array power, N57 kW). The station would also use nickel-hydrogen
batteries identical to those designed for the platform. This commonality of
hardware results in design and development cost savings for the Space Station
Program.
The SD portion of the case 5 power system generates about 50 kW. Overall
the technologies for the photovoltaic system are low risk and space proven,
whereas the solar dynamic technologies offer reduced drag and cost.
Photovoltaic System Technology Studies
Solar array. - Several array concepts were evaluated during the phase B
studies. They included planar arrays and concentrators. A planar array with
silicon cells was selected. This array design is similar to the NASA Office
of Aeronautics and Space Technology (OAST)flight experiment, OASTi, launched
in August 1984. If fully populated with cells, the array would have a power
output of 13 to 14 kWat the wing root. This flight experiment demonstrated
that this array type is technology ready and established that space station
planners can have a high degree of confidence in it. A more detailed descrip-
tion of the array and the flight experiment results can be found in
reference 3.
Energy storage system. - The PV system will store energy electrochemi-
cally. This stored energy is needed during the dark portion of the orbit and
for contingency purposes when the power system cannot produce or deliver
power. The phase B studies showed that the inherent storage capability or
residual energy of the electrochemical system was adequate to meet expected
contingency requirements. Building in greater contingency capability would be
unnecessarily expensive.
Energy storage options studied included nickel-cadmium (NiCd) batteries,
regenerative fuel cells (RFC), and nickel-hydrogen (NiH2) batteries. Although
NiCd batteries are established, flight-proven, low-risk devices, their low
depth of discharge results in high storage system weight. Space cells in
sizes up to i00 A-hr have been produced, so development risk would be low.
RFC's use a fuel cell and an electrolyzer to store energy in the form of
hydrogen and oxygen. In the dark portion of the orbit these elements are
recombined in the fuel cell to produce water and electricity. During the
lighted portion of the orbit excess array power is used to electrolyze the
water and "charge" the system with hydrogen and oxygen. The cycle is closed
so that the fluids are not consumed. RFC's are lighter than batteries and
allows storage of large amounts of contingency power with small changes in
tank volume. However, because RFC's are not as efficient as batteries
(60 vs 80 percent), the solar arrays must be larger. Also, RFC's are more com-
plex (i.e., contain pumps, valves, etc.) and not as reliable as batteries.
RFC's also have higher heat rejection needs. Reliability was a major consider-
ation for the platform, where 3 yr of operation without repair were required.
However, commonality between the station and the platform to reduce develop-
ment, resupply, and the cost of controlling spare parts was also considered.
The individual pressure vessel (IPV) type of NiH 2 battery has been used
in geosynchronous (GEO) spacecraft (fig. 4). (The bipolar NiH 2 battery has
low technology maturity and was screened out by the early tradeoff studies).
IPV, 3.5-inch-diameter, 50-A-hr GEO cells are in production. Other sizes and
capacities are available by using scaled-up versions of existing components.
The uncertainty with the NiH 2 battery stems from its charge-discharge cycle
life. GEO spacecraft experience only a fraction of the cycles that a LEO
spacecraft experiences. However, the Space Station Advanced Development Pro-
gram is beginning to test LEO cells with a goal of demonstrating 5-yr life-
times.
As a result of the phase B tradeoff studies IPV NiH 2 batteries were
selected for the platform. Weight, cost, reliability, development risk, and
schedule were the primary considerations. Nickel-hydrogen batteries weigh
about half as much and cost less than NiCd batteries and are more reliable
than RFC's. An identical IPV NiH 2 battery was also selected for the station
on the basis of cost and commonality with the platform. IPV NiH 2 was lower in
IOC cost and only slightly higher in life-cycle cost for the station.
4
The recent evolution of space station energy storage system selection is shown
in table II. The selection was strongly influenced by power level,
commonality, weight, and cost.
Solar Dynamic Technology Studies
The solar dynamic system consists of an offset parabolic concentrator mir-
ror and a receiver. The mirror focuses the Sun's heat into the receiver. The
receiver stores the heat in a salt (e.g., lithium hydroxide) and also trans-
fers it to a working fluid (e.g., toluene or helium-xenon gas). The heated
fluid drives a turbine that spins an alternator to generate electric energy.
The turbine also drives a pumpthat recirculates the working fluid. Excess
heat is rejected to space by a radiator.
In the tradeoff studies the two conversion cycles considered were closed
Brayton cycle (CBC)and organic Rankine cycle (ORC). These systems have not
been used in space, but a technology data base for the heat engines has
resulted from terrestrial and aircraft applications. Estimating costs, sched-
,,i^_ and other _^L=_tor° during the phase B tradeoff ....;_^_ _^-^_^-_
involved higher risk for these systems than for the PV system.
Design considerations for the SD system studied in phase B and being
worked on in the Advanced Development Program include low-gravity effects for
two-phase (gas-liquid) flow, heat flow and distribution in the receiver, life-
time for thermal energy storage (salt) capsules, weight and optical quality of
the concentrator, pointing accuracy (0.I °) for the mirror gimbals, atomic oxy-
gen protection, launch packaging, and on-orbit assembly. At the time of this
writing both the CBC and the ORC systems are still being considered. More
detailed study is required because cost and performance are nearly identical.
Power Management and Distribution Studies
The power management and distribution (PMAD) system must cope with
unknown load types and sizes as the station users change and increase in
number. Therefore the PMAD system must be user friendly and adaptable to
change and growth. The PMAD system for the space station must resemble a ter-
restrial utility power system rather than the PMAD system of previous space-
craft. Distribution voltages higher than the 28 V previously used are
mandatory to reduce losses.
During phase B distribution frequencies of dc, 400-Hz ac, and 20-kHz ac
were studied. Component efficiency, size, and weight as well as technology
readiness, availability of space components, acoustic noise, electromagnetic
interference, and plasma coupling were all considerations. After much
deliberation, 20 kHz was selected for the PMAD distribution frequency.
The overall PMAD architecture selected is a dual-ring system with multi-
kilowatt buses supplying power to load areas on the upper and lower keels and
the transverse boom. Buses supplying the manned modules are rated at 30 kW.
The PMAD system contains numerous switching and control assemblies as well as
a control system for sensing and commanding the loads. Isolators and power
controllers will sense faults and protect the system.
CONCLUDINGREMARKS
The present Space Station Program traces it roots back to 1981. The sta-
tion configuration and the power system for the present program have been stud-
ied extensively in the feasibility and definition phases.
The hybrid power system selected will meet the station and platform
requirements initially and into the future. The 25-kWphotovoltaic system
(57-kWarray power) will be larger than any system flown to date. The solar
dynamic system will facilitate economics and growth for the power system and
the station. The PMADsystem enables a growable, balanced utility type of
power system approach for maximumfriendliness for the station users.
The technologies selected for the photovoltaic, solar dynamic, and power
managementand distribution systems result in the lowest initial-operating-
system and life-cycle costs with acceptable development and schedule risk.
This hybrid system also meets programmatic and technical considerations driv-
ing the power system definition. The space station power system may set the
standard for future spacecraft power systems.
REFERENCES
I. Hook, W.R.: Space Stations - Historical Review and Current Plans. ASME
Winter Meeting, Phoenix, AZ, Nov. 14-19, 1982.
2. Space Station Mission Synthesis Workshop. NASAProceedings, Hampton, VA
Mar. 5-8, 1984.
3. Solar Array Flight Experiment, Final Report. Lockheed Missiles and Space
Co., LMSC-F087173,Apr. 1986.
TABLEI. - SPACESTATIONSYSTEM
POWEREQUIREMENTS
Element
Mannedcore:
IOCstationGrowth station
Polar platforms:IOC stationGrowth station
Co-orbiting platforms:IOC stationGrowth station
Average I Peak
User power, kW
75
3O0
8
15
6
23
I00
350
16
24
6
23
TABLE II. - EVOLUTION OF SPACE STATION ELECTROCHEMICAL ENERGY STORAGE SYSTEN
Date Station
solar
dynamic
system
Oct. 1985 0
Jan. 1986 37.5
Mar. 1986 50
Oct. 1986 50
Mar. 1987 (a)
Station Polar platform Co-orbiting
photo- platform
voltaic Average I Peak
system ]
Power level, kW
75 8 18 6
37.5 8 18 6
25 8 18 6
37.5 3.8 3.8 2
(a) (a) (a) (a)
Station Platform
Electrochemical
storage
H202 RFC NiCd
H202 RFC NiH 2
N1H2 [
NiR 2
NIH 2
Comments
Lacks commonality
Platform weight reliability
65 A-hr; 3.5-in. diameter
40 A-hr; minimum weight
62 _-hr; minimum cost
65 g-hr; 23 cells/pack (28 V);
2 packs/platform;
4 packs/station
Comnonality wlth orbital
manuevering vehicle, flight
telerobotlc services, and
mobile support center
aTo be determined.
ORIGtP_AL P:,i':: -_.
oFPOORQuALm;
FIGURE 1. - SPACE STATIOII COMPLEX, F.MLY 1980's.
FIGURE 2. - SPACE STATION IXIAL KEEL CONFIGURATION 1986.
ORIGINAL _'......
OF. POOR QUALITY
CASE INITIAL
OPERAT IONAL
CAPABIL ITY,
IOC
PHOTOVOLTAIC (PV)
AND SOLAR DYNAMIC
(SD) CAPABILITIES,
KWE
IOC PV
GROWTH PV
IOC PV
GROWTH SD
IOC 50 PV-25 SD
GROWTH SD
IOC 37.5 PV-57.5 SD
GROWTH SD
IOC 25 PV-50 SD
:GROWTH SD
IOC 12.5 PV-75 SD
GROWTH SO
GROWTH
lllfllll  lll
•::::I I-3I: ::t:!I
I_I
FIGURE 3. - CASES EVALUATED FOR SPACE STATION POWER SYSTEM.
FIGURE 4. - TYPICAL NICKEL-HYDROGEN CELL.
C-85_qq5
8


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