A summary is given of Air Force solar concentrator projects beginning with the Rockwell International study program in 1977. The Satellite Materials Hardening Programs (SMATH) explored and developed techniques for hardening planar solar cell array power systems to the combined nuclear and laser radiation threat environments. A portion of program dollars was devoted to developing a preliminary design for a hardened solar concentrator. The results of the Survivable Concentrating Photovoltaic Array (SCOPA) program, and the design, fabrication and flight qualification of a hardened concentrator panel are discussed

Geis, Jack W.

English

NASA Technical Reports Server

N87-26426
THE AIR FORCE CONCENTRAIING PHOTOVOLTAIC ARRAY PROGRAM
Jack W. Geis
AF Wright Aeronautical Laboratories
Wright Patterson AFB, Ohio
The history of the Air Force solar concentrator project goes back to FY77 when
an initial concentrator study program was conducted with Rockwell International
Space Systems. The conclusions drawn from the study were that solar concentrator
arrays have the potential to survive severe nuclear, particulate, and laser
radiation environments which no planar solar array of comparable weight could
tolerate. Concentrator arrays also offer the advantage over planar arrays of higher
power per unit area than conventional planar array designs because the higher
photovoltaic efficiency of the concentrator cells more than offsets the optical loss
of the concentrator system.
Cell costs can be reduced significantly compared to costs for planar cells
because a concentrator designed at a given Concentration Ratio (CR) will require
approximately I/CR the area of solar cells to produce the same electrical power
output. Because concentrator cells operate at higher efficiency and degrade less in
hazardous nuclear environments (concentrator shielding protects cells), the required
End-Of-Life (EOL) array size can be 30-60% smaller than that required by planar
solar cell arrays.
Satellite vehicles requiring high efficiency, low life cycle cost and maximum
W/m2 capability, and which must operate in high intensity natural radiation belts,
could profitably use concentrator array systems. The high performance, high
radiation resistance, but high cost gallium arsenide solar cell will find a natural
application in the concentrating photovoltaic array where the cost of the solar
cells themselves becomes secondary to other system costs.
A_UAI I$_I ours, , , ......
_,n_L/,,L funded TRW systems on the _+o11_te Materials _lard_ning Programs
(SMATH). Although the major effort under SMATH IV in the power hardening area was
to explore and develop techniques for hardening planar solar cell array power systems
to the combined nuclear and laser radiation threat environments, a portion of the
program dollars was devoted to developing a preliminary design for a hardened solar
concentrator. The TRW design that was selected was a Cassagrainian configuration
that utilized a narrow light acceptance angle to advantage for front side laser
protection, and a deployable metal foil shutter for backside protection. The
AFWAL/APL (Aero Propulsion Laboratory), as a result of the SNATH program, initiated
several efforts to design, fabricate, and test concentrating photovoltaic devices
which are survivable to hazardous nuclear and radiation environments. The first
contract was awarded to TRW Systems, Inc., "Multi-Threat Hardened Concentrating
Development" (F33615-81C-2055). The Laboratory has also sponsored concentrator
feasibility and development work with General Dynamics Corporation on contract
F33615-83C-2319, "SLATS Concentrator Development Program," and on contract
F33615-84C-2420, "SLATS PHASE II."
III
https://ntrs.nasa.gov/search.jsp?R=19870016993 2020-03-20T09:51:18+00:00Z
_lore recently efforts have been directed toward transitioning the exploratory
development efforts to advanced development through the initiation of the
"Survivable Concentrating Photovoltaic Array" (SCOPA)program. This program, which
is expected to begin in FY87, will result in the design, fabrication, test, flight
qualification, and subsequent flying of a 500 watt concentrator panel in a
low-altitude orbit. Ibis paper discusses the general program objectives and overall
performance goals of the program.
THESURVIVABLECONCENTRATINGPHOTOVOLTAICARRAY(SCOPA)PROGRAM
Concentrator development has progressed substantially under the above mentioned
contracts with TRWsystems and the general Dynamics Corporation. Estimated
performance capabilities for low to moderately hardened configurations are shownin
Table I (General Dynamics Corporation briefing to AFWAL/POOCApril, 1984).
Subsequent designs to meet increasingly higher laser irradiation threats will reduce
concentrator specific power and weight values due to higher mass density optical and
thermal radiator components, and higher temperatures for the concentrator cells. It
should also be noted here that post-laser irradiation performance levels of planar
arrays under low to moderate laser irradiation levels may decrease drastically, or
the array may fail catastrophically, compared to concentrators. The objectives of
the two Air Force sponsored programs were to develop designs that are survivable to
high level laser weapon, nuclear weapon, and background particulate irradiations,
while maintaining reasonable electrical performance.
Air Force plans for FY87 and beyond are to initiate a five-year, multi-million
dollar program to transition hardened concentrator elemental and modular technology
from exploratory development to advanced development, and subsequent flight test of
a hardened 500W concentrator panel. The program is divided into four phases: Phase
I - Critical Hardened Component Development and Preliminary Panel Design; Phase IIA
- Design, Fabrication, and Flight Qualification of Hardened Concentrator Panel;
Phase liB - Hardened Flight Panel Acceptance Testing and Spacecraft Integration; and
Phase IIC - Flight Support of Hardened Concentrator Panel.
PHASE I - CRITICAL HARDENED COMPONENT DEVELOPMENT
AND PRELIMINARY PANEL DESIGN
The intent is to award dual contracts for this 1 1/2 - 2 year effort. SCOPA
performance and survivability objectives have been discussed through numerous
meetings with various Satellite Program Offices (SPO's). As a result of these
meetings, there has arisen a general understanding of the need for near-term
development of space power systems that will be survivable to natural threats that
may exist in space in the 1990's and beyond. SCOPA can meet these threats, and has
been defined as d enablin 9 space power technology.
Stringent and demanding threat requirements support the development of two
competing concepts for Phase I to maximize the probability of program success.
Concepts proposed for SCOPA by the various bidders on this multi-source procurement
vary greatly in design, and each has its own strengths and weaknesses.
112
lhe major objectives of Phase I will be to establish confidence in concentrator
designs and fabrication/manufacturing processes necessary to meet the program
performance and survivability requirements and goals. Concentrator elements and
modules will be subjected to vacuumchamber irradiation exposures from continuous
wave and pulsed wave laser sources that simulate irradiation intensities and
exposure times of ground-based and space-based lasers. Modules will also be
subjected to thermal cycling, proton, electron, and space plasma environments such
as might be encountered in Mid-Earth-Orbit (MEO)and Low-Earth-Orbit (LEO). A 500 W
panel preliminary design will be accomplished near the end of Phase I. At the end
of Phase I the best concentrator concept will be carried into Phase II A.
PHASEIIA - DESIGN,FABRICATION,AND
FLIGHTQUALIFICATION
OFHARDENEDCONCENTRATORPANEL
Under Phase IIA the contractor will fabricate at least one hardened 500 W
panel, based on the test results and preliminary design work of Phase I. The panel
will represent structurally and thermally the characteristics of a multi-kilowatt
concentrator array. The contractor will conduct ground-based qualification of the
500 Wpanel. Qualification test guidelines will be based on a possible future space
shuttle launch with subsequent deployment in a LEOorbit for a one-year (nominal)
operation. Tests will include but not be limited to thermal, vibration, deployment
and dynamic/static loading, and electrical performance verification tests.
PHASEliB - HARDENEDFLIGHTPANELACCEPTANCET STING
AND
SPACECRAFTINTEGRATION
Under Phase liB, the contractor will refurbish the hardened concentrator panel
developed and tested under Phase IIA, or build another unit and perform flight
acceptance tests in accordance with acceptance test guidelines.
PHASEIIC - FLIGHTSUPPORTOFHARDENEDCONCENTRATOR
PANEL
The final phase of the program will be involved with providing the necessary
supporting personnel for experiment data collection, reduction, and analysis
resulting from a low-altitude "proof-of-concept" flight experiment. The experiment
will be designed to determine the operational envelope of hardened concentrating
photovoltaic arrays operating in space. Table II lists the Survivable Concentratin9
Photovoltaic Array Performance Verification Flight Experiment (AFWAL - 501)
characteristics, (AFWAL/POOC briefing to Space Test Program Tri-Service panel, June,
1986).
113
HARDENED CONCENTRATOR DESIGN
REQUIREMENTS AND GOALS
The achievement of the concentrator design requirements and goals will permit
the development of high performance, survivable, multi-kilowatt concentrator arrays
for operation in hazardous natural and artificiel space environments. In addition,
the achievement of these requirements and goals will demonstrate significant
survivability and performance improvements compared to conventionally hardened planar
se!ar arrays, and will satisfy the requirements of several Air Force satellite
systems.
In addition to continuous and pulsed wave laser, nuclear, and pellet threat
hardening requirements and goals, the concentrator must be survivable to natural
background radiation and space plasma environments as shown in Table III (AFWAL/POOC
SCOPA Requirements and Goals, 1986).
SCOPA requires that a concentrator system be capable of surviving, with no more
than 20% electrical power degradation, one exposure from each of the background
radiations described in Table III. The tests represent five-year fluences in a 5600
NMI orbit at 60% inclination. The test exposures are not to be cumulative.
The hardened arrays are to be designed for Beginning-Of-Life (BOL) performance
goals of 15W/kg (including pointing and tracking mechanism), and 120W/m 2. Although
the performance design goals are very conservative, the laser and nuclear
requirements and goals are not! The severe threat conditions imposed on the desiQn
result in a relatively heavy system, but the hardened design requirements and goals,
if achieved, will permit the future development of multi-kilowatt concentrating
photovoltaic arrays that will be survivable to artificial threat environments
projected for the 1990's and beyond, and to natural threat environments that may be
encountered in MEO missions and which cannot be accomplished with planar solar cell
arrays. Projections of perfoFmance imprcvements possible with concentrator systems,
when compared to planar array systems, are shown in Figure I and Figure 2
(Reference I).
CONCLUDING REMARKS
The SCOPA program is very ambitious, especially from a survivability
standpoint. However, a successful program will enable the Air Force to provide a
survivable space power system that will have the capability of performing a variety
of missions in hazardous environments, including those missions previously thought
incapable of being accomplished by photovoltaic arrays (e.g., 5600 NMI missions). In
addition, the concentrator will be capable of being designed as a lightweight system
(50-100W/kg) through the use of thinner, low mass density materials and
configurations, and may find applications in such missions as high power,
lightweight electric propulsion orbital transfer where weight is a critical factor.
RFFERENCES
I. Spacecraft Subsystem Survivability, R. M. Kurland, J. S. Archer,
W. R. Hardgrove, TRW Systems, Inc., 30 March 1986.
114
TABLEI - EFFECTOF IMPROVED
EFFICIENCYONOVERALLCONCENTRATORPE FORMANCE
Parameter Planar * Casse_rainian SLATS SLATSW/DUAL-BANDGAP
.98% .80% .92% .92%
.10%@65°C .20%@85°C .18%@85°C .25%@85°C
.86 .85 .95 .9
.76 .92 .92 .86
Optical Efficiency
Cell Efficiency
Packing Factor
Radiation Deg (5 yr,
600 NMi, 60° )
Net Array EOLEffic.
Specific Power
Specific Weight
6.4% 12.5% 14.5% 17.8%
87 W/m2 169 W/m2 196 W/m2 240 W/m2
18.8 W/kg 24.8 W/kg 48.8 W/kg 59.9 W/kg
* INSAI ARRAY
Note: Data presented are for 1984 concentrator designs, based on survivability
background radiation environment, and moderate laser irradiation threat environment
only.
TABLEII - SURVIVABLECONCENTRATING
PHOTOVOLTAICARRAYPERFORMANCEVERIFICATION
FLIGHTEXPERIMENT(AFWAL-501)
CHARACTERISTICS
0
0
0
0
0
0
0
Experiment only, free flyer out-of-space shuttle bay, or on another
spacecraft, up to 12 month in LEO
3-axis spacecraft stabi!iation to within ±! °
-20°C to +125°C, nominal thermal environment
500 W panel, 35 kg nominal, 454 X 92 cm
Panel pointing and tracking (± 1½°)
Temperature, current/voltage measurements
Sun sensor
o Mirror/cell degradation sensor
115
TABLEIII
GEOMAGNETICALLYTRAPPEDPROTONSANDELECTRONS
(OMNIDIRECTIONALF UENCES),AND
SPACECHARGE/SPACEPLASMAEFFECTSCONDITIONS
0
0
0
0
1Mev, 1X 1015 protons/cm 2
5 Mev, 8.6 X 1012 protons/cm 2
I0 Mev, 5 X I0 II protons/cm 2
These tests represent 5-year fluences in a 5600 NMi
orbit at 60 ° inclination
(a) PROTON TEST CONDITIONS
0
0
0
0
1Mev, 7.5 X 1013 electrons/cm 2
3 Mev, 7.5 X 1012 electrons/cm 2
5 Mev, 7.4 X 1011 electrons/cm 2
These tests represent 5-year fluences in a 5600 NMi
orbit at 60 ° inclination
(b) ELECTRON TEST CONDITIONS
103 - 106 electrons, and nitrogen (or argon) ions per cm 3
0.1 - 1.0 volt, individual particle energies
Test to be performed in 10-7 torn, with test article
biased + 500V relative to tank
(c) SPACE CHARGE/SPACE PLASMA EFFECTS TEST CONDITIONS
116
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The Air Force concentrating photovoltaic array program

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