An overview is given of the uses of photovoltaic (PV) power in space. The contribution of PV systems on unmanned, low Earth orbit and inner planetary missions is noted. The development of PV technology along the two paths of high efficiency and high power is discussed. The importance of increasing the service life of PV systems is covered

Wasel, Robert A.

English

NASA Technical Reports Server

N87-26414
AN OVERVIEW OF PHOTOVOLTAIC APPLICATIONS IN SPACE
Robert A. Wasel
NASA Headquarters
Washington, D.C.
Power is certainly the backbone, the vital force of our program in space. As
on earth, there is little we can do without it. Of all our space power sources, by
far the most important is photovoltaic (PV) power (Fig. I). Radioactive thermo-
electric generators (RTG's) are also necessary for outer planetary missions where
solar power is less effective. Not shown here but potentially an important power
source of the future is the small nuclear reactor. Significant DOE and DOD funds
are being utilized to develop a system capable of producing over I00 KW of power
(SP-IO0) in the early 1990's. But little will occur in the near future to change
this predominance of PV power, in fact there are environmental concerns that could
lead to limitations on the use of RTG's, adding to the requirements for PV systems.
Fortunately for the satellite managers, PV system technology is improving at an
impressive rate. Considering all the advanced spacecraft technologies, it is cer-
tainly among the leaders in performance improvements. NASA is a major participant
in supporting PV technology and we are a primary user. The NASA PV effort is car-
ried out in three parts: cell research at our Lewis Research Center (LeRC); con-
centrator research at our Marshall Space Flight Center (MSFC); and the lightweight
array program at the Jet Propulsion Laboratory (JPL). LeRC acts as our Lead Center
for overall coordination. Significant improvements have been made in all three
areas and more are on the horizon.
PV systems contribute at all levels of NASA mission requirements (Fig. 2). Low
to intermediate power levels are required for unmanned low earth orbit and inner
planetary missions. Intermediate to high power levels are necessary for geosta-
tionary, lunar, outer planet, space station and electric propulsion thrusters.
Incidentally, electric propulsion offers an interesting symbiosis with PV if they
are selected for an outer planet mission. The PV system must be sized to produce a
given power level in the low-solar intensity at the operating area. This array
would be dramatically oversized in the early phase of the mission, near earth. The
surplus could be used to power an electric thruster, shortening the trip time or
increasing the payload. For example, if 500 w_tts are required at the end of a
year of orbiting in the radiation environment of Jupiter, the required array could
produce over 20 KW in the earth orbit for high efficiency electric thrusters.
NASA PV technology has developed along two paths (Fig. 3), one toward high
power, the other high efficiency. High power will be attained through concentrator
cells and reflecting arrays. Our goal here is to produce 200 watts/m 2, twice the
level demonstrated in the collapsible array flown on the Space Shuttle in the SEPS
experiment. For applications where weight is at a premium, we will use our most
efficient, radiation tolerant cells with advanced deployment systems to attain our
goal of 300 watts/Kg. JPL currently has designs in hand that are capable of 130
watts/Kg, almost twice the level of the SEPS flight test.
For high power requirements in earth orbit, like the Space Station, drag area
becomes a consideration. Figure 4 shows the size advantage that the advanced con-
https://ntrs.nasa.gov/search.jsp?R=19870016981 2020-03-20T09:50:46+00:00Z
centrator cells offer. By using these 5mmsquare GaAscells and efficient concen-
trator lenses, the required cell area is reduced, not only to less than half that
required for current planar silicon arrays, but to even less area than that pro-
jected for the large focusing dish liquid organic rankine cycle power generators
which are candidates for the Space Station.
In addition to the development of the lightest and highest efficiency arrays,
lifetime characteristics are increasing in importance. Current arrays, if exposed
to radiation, are certain to lose performance capability. Missions in the Van
Allen belts or near Jupiter may see 50%degradation during a year, meaning that the
initial array must be twice the normal size and weight. In this case an improve-
ment in cell radiation resistance is a direct payload increase. In Figure 5, note
that current silicon cells are almost destroyed by seven years in the radiation
belts. GaAsoffers a significant improvement, and Indium Phosphide (InP) is prob-
ably the best material tried to date. Of course the ultimate objective is a cell
that is completely unaffected by radiation, or one that anneals or heals itself at
ambient temperature.
The attendees at this meeting have made great strides in PV technology. Some
of these are shownon Figure 6, and a number of additional ones are in store,
perhaps to be reported at this meeting. What are the next mission opportunities?
Several are illustrated in Fig. 7. The Space Station and its "satellites," the
polar and co-orbiting platforms. These could be opportunities for advanced con-
cepts, but may be restrained by conservatism or modularity for interchange-
ability. RTG's have the spectre of radioactive-during-launch to contend with, but
are so remarkably efficient for outer-solar system missions that it would be a real
challenge to replace them. SDI applications are not yet defined, but are sure to
be interesting.
In addition to these, NASAhas been getting a great deal of encouragement from
advisory committees to expand its programs and research efforts. The prestigious
National Commissionon Space (NCOSChairman Dr. ThomasPaine) after noting that the
funding of SpaceR&Tat NASAhad dropped from over $800Min 1965 to less than $200M
in 1986, stated that "NASA'Sspace research and technology program should be trip-
led, moving from its current 2%of NASA'sbudget to 6%." In terms of our 1987 bud-
get this would be an increase from $170Mto over $500M. Other advisory committees
showed a remarkable concensus - that there should be a 2-3 times increase in NASA
Space R&T to correct technology deficiencies. This advise was received from the
Aeronautics and Space Engineering Board of the National Academyof Sciences, NASA's
Space Systems Technology Advisory Committee, the American Institute of Aeronautics
and Astronautics and the National Security Study Directive Team.
The NCOSalso recommendedan ambitious series of projects to be added to the
NASAmission plan. Fig. 8 shows the NASAfuture mission blueprint with the NCOS
additional proposed missions in boxes. NASAhas already responded to this NCOS
challenge and a proposal for a significant R&Dprogram increase is being considered
for FY 1988. Photovoltaic systems will obviously be an important part of any
acceleration of space technology effort. The Initial Operational Configuration
(IOC) Space Station will depend on PV systems for one half of its 75 Kwpower. The
system design is extremely conservative (about 25 w/kg), so a great deal of im-
provement is possible, ideally in time to contend for the "Growth" Station in the
late 1990's which will require several hundred kilowatts. The next major NASA
undertaking maybe a Lunar base, potentially in 2010. Next to life support, power
may be the most important issue. Power not only for the occupants and their exper-
2
iments but possibly for on-site operations such as propellant manufacturing. PV
systems would be a reasonable contender for generating someof this power, probably
in the hundreds of kilowatts, particularly if there is an efficient lightweight,
low cost, transportable concept by that time. It might be shipped in a roll like
a window shade and simply layed on the surface.
Meanwhile, the United States is not alone in space or in the development of PV
systems for space. Fig. 9 shows the USSRSalyut 7 manned space craft with an
appended module, currently in orbit as part of their manned station program.
Salyut 7 was launched in 1982 with silicon cell solar arrays capable of generating
4KW. Within the next two years, this solar array was supplemented by the small
extendable arrays shown beside the central array. These were reported to be gal-
lium arsenide (GaAs) cells, the first to be used operationally in space. The GaAs
arrays produced about 4 KW. This USSRcapability should be a very sobering
challenge to the attendees of this meeting, the technologists, researchers and
managersof PV programs in this country.
In summary, PV has made possible much of our scientific accomplishments in
space and with proper support and progress in the R&Dcommunity it will continue
its contributions.
POWERISTHEBACKBONE/VITALFORCEBEHIND
THESPACEPROGRAM
• PV IS THE PRIIqARY _ SOURCE
POWERPRODUCEDFOR NASA RISSIONS
(DOES NOT INCLUDE SPACE SHUTTLE)
Figure I.
PHOTOVOLTAIC POWER GENERATION IS APPLICABLE 
ACROSS THE NASA MISSION MODEL 
HISSION SUBSET POMR LEVEL SYSTEH ATTR!BUTES 
LW TO INTERIEDIATE LOU w0 LONG LIFE UmAWNED WEAR EARTH 
(LEO, HEO, E O )  AND 
PLMTARY 
SPACE STATION HIGH HIN IMM ARM0 LW HASS, 
LOW COST 
E O  PLATFORH I NTERED I ATE LONG LIFE. LW W S  
LUNAR BASE, M K D  INTEREDIATE TO HIGH LOU H A S ,  LONG LIFE. 
PLAKTARY PORTABILITY 
ELECTRIC PRWULSION 
ORBIT TRANSFER (OW) 
HIGH 
F i g u r e  2.  
DUAL TECHNOLOGY PATHS SPAN THE NASA MISSION MODEL 
CASCADEICONCENTRATOR HIGH EFFICIENCY RADIATION 
CEUS TOLERANT Ill-V CELLS 
ARRAY 
HIGH POWER 
200 wIm*  
SOA 
(SEPS) - - STACBEAM 
HIGH EFFICIENCY 
300 Wlkg  
F i g u r e  3. 
4 
THRUSTS 240 
o HIGH EFFICIENCY, SMALL AREA CELLS 
o LIGHTWEIGHT, EFFICIENT OPTICS 
o REDUCED POINTING REQUIREMENTS 
160 
o RIGID;.DEPLOYABLE. LIGHTWEIGHT ARRAYS 
ARRAY 
AREA, 80 
M* 
Si ADV. CONC. LOR C 
TECHNOLOGY WITH NiH2 STORAGE 
PLANAR Si: 
14 Sb CELL 
ADVANCED CONCENTRATORS: 
>30 96 CELL . 
> 94 96 OPTICAL EFFICIENCY 
F i g u r e  4 .  
COMPARISON OF SOLAR ARRAY CALCULATED OUTPUT AS FUNCTION 
OF ORBIT ALTITUDE BASED ON 1 MeV ELECTRON EQUIVALENT FLUENCES 
TIME IN ORBIT = 7 YEARS, CIRCULAR ORBIT, 3@ INCLINATION, T = 60 OC 
ARRAY 
S PEClFlC 
POWER, 
WIkg 
ALTITUDE, nm 
Figure 5. 
5 
SIGNIFICANTIMPROVEMENTSMADEANDMOREARECOMING
[I_ItOVElqENT
2 NIL SILICON CELL
6AAs_NTRATOIt CELL
SEP/OAST-1 ARRAY
I..kR6EAREASI CELL
SI6NIFICANCE
ILUIATIOII RESISTANCE, LI6H'I'NEIGHr ARRAYS
_,,20Z EFFICIENCY, LOWCOST/WATT
_'60Z W'K6 FLEXIBLE ARRAYTECHNOLOGY
LOMARRAYCOST
C_I_: >20ZI_ _LLS
CONCENTRAT_MMY
STACBEAR,APSA
RN)IATI011 BELT SURVIVABLE&qRAYS
HI6H EFFICIENCY, HIGH POW,:.R,RADIATION
TOLIERMCE
ULTRN.16HTMEI6HTARRAYS
Figure 6.
WHAT'SNEXT
• SPACE STATION - LARGE SYSTEM, BUT POOR PERFORMANCE (5 ]g/K6 10C)
• CO-ORBITIN6 AND POEM FREE-FLYERS (CONS'gLAINED BY ROINLNIITY?)
• RTG REPt.ACE]qENT?
• ELECTRIC PROPULSION ORBIT TRANSFER?
• SDI APPLICATIONS (HOUSEKEEPIN6, RAI)IATION/]LAST RESISTANCE)
Figure 7.
BLUEPRINTS TO THE FUTURE
i
I 1980's ] 1990"s I 2000"s 2010 s ' 2020's
!
ADVANCED TRAI_SLUNAR OW
SSME_ _ ,_,SPACEEAsEO• "N_S ]TRANS- ,_VA.CEOIMPROVEMENTS '_ OW SHUnLE REPLACEMENT _ TRANSFER VEHZCLE
SPACE-
CRAFT
COMET
RENDEZVOUS
IATURN ORBITER
_ AARS/COMET
MARINER _i/IN_R [ SAMPLE
MARK ,l _ yTURN
,AR,.o..v,.o.,'STEMS
PLANETARY
LARGE
SPACE
SYSTEMS
OUTER PLANET
-- ORBITERS A
LEO _ GEO
CONTINUOU|
COVERAGE
IOC GROWTH _LUNAR ORBIT] I
SPACE SPACE GEO / [_pAPJ:I_mT L_NAR [
STATION STATION PLATFORM ] .... _ BASE
_\_ _ I " _ _'" lllUl " REFLECTOR/ _ "_LMN/_
__ I,_-" PnOr_LLENT
"" MO.LE COMMUNICAT . V_, _l.E "_ _,_ I FZL_NT
IONS e_ .a.
EM 20M "M IPL_T OmPOST
©
FULL
Figure 8.
CONCEPTUAL DRAWING OF INTERIOR OF
KOSMOS 1443 TYPE MODULE SALYUT 71
AND A SOYUZ T FERRY CRAFT
Figure 9.


An overview of photovoltaic applications in space

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