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T H E S P A C E S T A T I O N
P O W E R S Y S T E M
The Space Station represents the commitment of the United States to a
lasting future in space. This future will be ripe with intellectual
and technical challenges. It will hold vast opportunity for commercial
profit and preservation of the nation's economic vitality. It will be
both a research facility in space and a stepping-stone to long-term
human space exploration and discovery. The Space Station demonstrates
that America's significant achievements in space lie ahead of us, not
behind us. The Station also symbolizes our nation's desire to
cooperate with others in mutually beneficial civil space activities.
Canada, the European Space Agency, and Japan have already responded
positively to the U.S. invitation to participate in the development of
the Station. Formal agreements are being negotiated and are near
completion. If the negotiations are successful, their involvement will
lead to unprecedented international cooperation, toward the peaceful
exploration and utilization of the space environment.
During the early planning stages of the Space Station Program, before
the first engineer was allowed to set pencil to paper, two major
questions had to be answered:
o Who will use the Space Station?
o What resources will have to be
provided to those users?
A detailed survey of the technical community showed that five types of
experiments would most likely be performed on the Space Station.
o Observational sciences (astronomy and Earth
observations)
o Life sciences
o Materials sciences
o Servicing/repair, and
o Technology development/testing
The major resources these potential users demanded were found to be:
o Power
o Volume
o Crew time
The quantitative evaluation of these user requirements defined the
ground rules for the engineering studies that led to the system
definition and preliminary design of the Space Station baseline
configuration.
______________________________________________________________________
S P A C E S T A T I O N B A S E L I N E
C O N F I G U R A T I O N
Photovoltaic Power Array
\
[][][] [][][] High Gain [][][] [][][]
[][][] [][][] Antenna [][][] [][][]
[][][] [][][] Radiator \ |{ [][][] [][][]
[][][] [][][] \ __ ___ |__ [][][] [][][]
[][][] [][][] ## | |___|J | ## [][][] [][][]
[][][] [][][] ## |E | |E | ## [][][] [][][]
|\|\|\|\|\|\|\|\|\|\|\|==|M |___|M |==|\|\|\|\|\|\|\|\|\|\|\|
[][][] [][][] |__|___|__| [][][] [][][]
[][][] [][][] | | |H | [][][] [][][]
[][][] [][][] | |___|M | [][][] [][][]
[][][] [][][] |__|___|__| [][][] [][][]
[][][] [][][] / [][][] [][][]
[][][] [][][] US Laboratory Module [][][] [][][]
EM = European Module
JEM = Japanese Experiment Module
HM = Habitation Module
______________________________________________________________________
THE SPACE STATION POWER LEVEL
Electrical power, in many respects, is the most critical resource
aboard the Space Station. Electricity is essential to supporting human
life in space. It allows a multitude of systems on board Space Station
to operate, support, and produce. Whether electricity is used to power
life support systems, to run a furnace making crystals, to manage a
computerized data distribution system, or to operate a centrifuge,
electricity is the key.
The more electricity available, the more work possible, and the more
flexible the entire array of Space Station activities becomes. A
comfortable amount of power allows men and women to utilize their own
most precious resources: observation and innovation. Adequate power
allows a crew, in orbit, and a variety of researchers, using
telescience capabilities from the ground, the opportunity to make
instantaneous observations and responses. In a severely
power-constrained environment, flexibility and spontaneity are
diminished. This, in turn, limits the invaluable utility of a
permanent human presence in space.
In addition, power runs the infrastructure of the hardware and
software that supports the entire facility. For this reason, Space
Station power systems and power-level projections have been an
important focus of attention during Phase A and Phase B definition and
design stages.
The power level given as the ground rule in the "reference
configuration," the starting point for analytical studies during the
Space Station Concept Definition and Preliminary Design Phase (Phase
B), was 75 kW with growth capability to 300 kW. The Space Station
assembly sequence supplied 25 kW of photovoltaic power by the second
flight. This 25 kW of power would support general station-keeping
requirements and early payloads that would be provided during the
assembly phase of the program. An additional 50 kW of solar dynamic
power was planned downstream in the assembly sequence, raising the
total power supply to 75 kW, the baseline level for the permanently
manned phase. This figure was based on the projected needs of the
future Space Station user community and early estimates of the
housekeeping power.
A review by Congress of Space Station concluded that the preliminary
power of 25 kW was insufficient to adequately support early payloads.
As a result, the initial power level was increased to 37.5 kW of
photovoltaic power. With the addition of the 50 kW of solar dynamic
power intended for the later stage of the Station development, the
total power level for the program climbed to 87.5 kW. It should be
noted that the absence of a permanent crew in such a configuration
makes crew time the most critical parameter and severely limits the
kind of experiments that can be performed.
The Space Station review ordered by the NASA Administrator at the end
of Phase B resulted in several changes to the Phase B results,
including a reordering of the assembly sequence to allow for early
user operation and confirming power at 87.5 kW.
A subsequent cost review resulted in the "phased" approach to
construction of the Space Station. Early calculations of power needed
in this approach yielded 50 kW. Further examination of user and
housekeeping requirements, however, resulted in an increase of that
figure to 75 kW for Phase I and an additional 50 kW (125 kW total) for
a future Phase II.
THE SPACE STATION POWER SYSTEM
The only continuously available source of energy in this solar system
is the Sun. The Sun's energy is available in the form of light and
heat; however, spacecraft need electricity. Accordingly, NASA has
pioneered and is continuing to develop technologies to efficiently
convert the Sun's energy (light and heat) into electrical power.
Some materials, such as silicon and gallium arsenide, can directly
convert light to electricity. Hence, "solar cells" can be made from
these materials. The efficiency of energy conversion by this method is
not very high; it ranges from 5 to 10 percent. The cells, however, can
be assembled into "arrays" and these can be used to generate high
power levels. In fact, the 75 kW required for the Space Station Manned
Base and the power for the Polar Platform will be generated entirely
by solar arrays.
A spacecraft in orbit around the Earth is not always in direct
sunlight. Thus, energy has to be stored to provide a continuous source
of electricity. Storage is usually accomplished by using batteries,
which is the method of choice for Space Station. The Space Station
"photovoltaic power module" contains both the solar arrays and the
batteries.
______________________________________________________________________
SPACE STATION PROGRAM
S O L A R P O W E R O P T I O N S
(PHOTOVOLTAIC)
Sunlight
| | |
| | |
| | |
v v v
_________
|_________| Solar Cell
\ \
| Direct Current (DC)
| Electrical Power
__________ | ___________
| |____|_____| |
| Battery | | Power |
|__________| | Converter |
|___________|
\ \
Alternating Current (AC)
Electrical Power
(SOLAR DYNAMIC)
Receiver/Thermal Storage
\ ______
| | Sunlight
__________| | /
| |______| /
| ' /
| ' /
v ' /
| ' /
| ' /
| \_____|/____/ Mirrors
|
| ______________________
|________| |
| Turbine/Generator |
|______________________|
\ \
Alternating Current (AC)
Electrical Power
______________________________________________________________________
The photovoltaic power system is well understood and has the advantage
of being off-the-self technology. Its disadvantage is the large size
of the arrays required to generate sufficient power. In addition, the
large weight and relatively short lifetime (about five years) of the
batteries is a disadvantage.
The Space Station will operate in low Earth orbit (about 220 nautical
miles). In this, or any other near-earth orbit, there is a certain
amount of "drag," i.e. resistance to the progression of the
spacecraft. As a consequence, the spacecraft tends to slow down. This
results in a loss of altitude, a gradual progression towards an
ultimate de-orbit. To prevent the Station from eventually reentering
the atmosphere, periodic reboost of the spacecraft is necessary. This
requires a resupply of propellant: the larger the area, the larger the
drag, and the more reboost propellant is needed. Resupply of the
propellant is needed. Resupply of the propellant is part of the
life-cycle cost.
Decreasing the area of the spacecraft minimizes drag. The largest area
of the Space Station is the solar arrays. Early design concepts
indicated that a reduction in the area of solar arrays represented
life-cycle cost reduction.
However, a newer design concept has mitigated the increased life-cycle
costs associated with reboosting, by using a hydrogen fuel obtained
from surplus supplies of water. Therefore, the size of the solar array
no longer drives life-cycle costs as directly. Another source of
life-cycle cost is the need to replace the batteries after five years.
The use of a long-life energy storage system represents life-cycle
cost savings.
A solar dynamic power system might provide a solution for these
problems. This technology, far different from the photovoltaic system,
utilizes the Sun's heat instead of its light for the production of
power. Heat is collected in the focal point of a mirror. Power is then
generated exactly the same way as on an earthbound power station: by
heating a fluid, which in turn rotates a turbine. Since a
heat/gas-driven turbine is a much more efficient power converter than
a sunlight-driven solar cell, the mirror (the largest part of the
solar dynamic system) would have to be only one-fourth the area of a
solar array to generate the same amount of power from the Sun's light.
There are several different engines that can be used for the
generation of power within the solar dynamic system. They are similar
in that they are "closed cycle," i.e., they recycle the working fluid.
These engines are usually known by the names of their inventor. For
use on Space Station, the Brayton Cycle engine has been selected.
The energy storage device used for a solar dynamic power system is
superior to a photovoltaic system because heat is stored rather than
electricity. Heat is cheaper and far more simple to store for
subsequent use. Storage can be accomplished by taking advantage of the
heating, or fusion, of inorganic salts. On the sunny side of the
Earth, heat is absorbed by the salt and it melts. On the dark (cold)
side the salt freezes and gives up its heat to the working fluid of
the engine, ensuring continuous operation.
S U M M A R Y
An abundant supply of power is one of the top priorities for users of
the Space Station and therefore, of highest priority for the Space
Station Program. It was for this reason that the original "hybrid"
power system was chosen: it provided early power to the user by using
off-the-self photovoltaic/battery technology, then adding the more
"growable," but higher risk, solar dynamic system later. This concept
was revised in light of budget realities. By using only photovoltaic
modules in Phase I, NASA will be able to meet budget restrictions
without sacrificing the needs of the users. The ability to utilize
solar dynamic systems with lower life-cycle cost will be added in the
future as the Space Station evolves.
______________________________________________________________________
SOLAR POWER OPTIONS: ADVANTAGES DISADVANTAGES
----------------------------------------------------------------------
Photovoltaic o Large Data Base for o Limited Data on High
Small Rigid Arrays Voltage Arrays
with Batteries
o Tolerant of Pointing o High Life Cycle Cost
Errors
o Flexible Array Demon- o Development Risks on
strated Large Array & Energy
Storage
o Technology Well o Large Drag Area
Understood
----------------------------------------------------------------------
Solar Dynamic o High Efficiency o Limited Phase Change
Salt Data
o Terrestrial Data o High Development Cost
Base Than Photovoltaic
o Low Life Cycle Cost o More Sensitive to
--Low Drag Area Pointing Error than
--Low Production PV
Cost
o Not Demonstrated in
Space
----------------------------------------------------------------------
Hybrid o PV Power for Early o Requires Development
Station Buildup and Logistics Support
of Both Systems
o SD Low Cost Power as
Requirements Increase
o Low Life Cycle Cost
o Diverse Power Sources
----------------------------------------------------------------------
______
THE SPACE STATION POWER SYSTEM, NASA
