1st International Energy Conversion Engineering Conference, Portsmouth, Virginia, 17-21 August 2003

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FITS, THE LATEST AND GREATEST LIGHTWEIGHT SOLAR ARRAY FOR SPACE

Cary R. Clark, Bill Zuckermandel, Scott Enger, and Domenic Marcelli MicroSat Systems, Inc., 8130 Shaffer Parkway, Littleton, CO, 80111

ABSTRACT

Typical spacecraft solar array systems that use rigid cell technologies are very costly and mass inefficient. Improvements in their efficiency and stiffening structures have reached the point of diminishing return. Today’s commercial, military, large and small satellites demand a new, lighter weight, lower cost solar array technology for better payload mass fraction. MicroSat Systems, Inc. (MSI) has developed the fold integrated thin-film stiffened (FITS) solar array system to dramatically reduce cost and mass and enable the use of the newest and lightest thin-film photovoltaic (TFPV) technologies. FITS innovative design uses non-traditional, thin, foldable stiffeners to passively deploy and stiffen the array, resulting in greater than 50% improvement over the system metrics of state-of-the-art systems. At currently available TFPV efficiencies, MSI’s FITS system has a stowed volumetric power of greater than 45 kW/m3, and a specific power greater than 150 W/kg. This paper will describe MSI’s FITS solar array system, how it works, its inherent benefits and technology readiness, as well as future system metric improvement potential.

WHAT IS FITS AND HOW DOES IT WORK? MSI’s FITS solar array system is an integrated, passively deployed structure designed specifically for TFPV arrays. FITS technology extends the bounds of state-of-the-art (SOA) solar array systems by eliminating conventional rigid structures and mechanisms to maximize the lightweight and low stowage volume advantages of TFPV. This system uses multifunctional, foldable components with stored energy, providing deployment force and deployed stiffness, to meet the demanding mass, cost, and power requirements of today’s satellite programs. MSI's FITS concept is designed to replace conventional state-of-the-art photovoltaic solar array systems. Conventional spacecraft solar array deployments involve complex, heavy structure and deployment mechanisms to tension the array. The FITS system uses deployable stiffeners that are analogous to figures from a

children’s pop-up book. MSI has also developed a novel deployment mechanism called a “living hinge” that is integral to FITS and closely resembles a knee ligament, where there are no moving parts. These hinges provide the force to open the array and deploy the FITS stiffeners, which increase the area moment of inertia and create a rigid structure. The living hinges also maintain the array in the deployed configuration on orbit. Figure 1 illustrates the different deployment stages of the FITS solar array, the folding scheme and different types of stiffeners needed for outside, inside, and double-folded joints. First, the FITS array Z-folds away from the bus. Once the Z-fold deployment is complete, the Tri-fold deployment starts to deploy the outside 2 wings of the array. This completes the FITS solar array deployment sequence.

1. Z-FoldStart

2. Z-FoldComplete

3. Tri-foldStart

4. FITS Array Fully Deployed

FITS RibStiffeners

FITS Stiffeners

Center FITSStiffeners

Figure 1. FITS Configuration and Deployment

1st International Energy Conversion Engineering Conference17 - 21 August 2003, Portsmouth, Virginia

AIAA 2003-6034

Copyright © 2003 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

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BENEFITS OF MSI’s FITS SYSTEM FITS Parametric Assumptions In selecting a solar array system, the major system metrics that are important include: specific power (W/kg) and stowage volume (kW/m3). The present FITS system metrics assume 0.5 Hz minimum natural frequency at the array system level, 8.5% CIGS Stainless Steel substrate TFPV, and all structure and electrical cabling is included in mass calculations. Table 1 describes the components included in array calculations.

Blanket Assembly Space Coated TFPV CIGS Cells, Interconnects, Adhesives

FITS Structure AssemblyFITS Stiffeners, Misc. Stiffeners, Deployment Hinges, End Stiffeners

Electrical AssemblyBackwiring, Power Cabling, Diodes, Adhesives

CO

MP

ON

EN

T

Table 1. FITS Solar Array System Components

FITS System Metrics vs. SOA Heritage photovoltaic technologies and spacecraft solar array deployment and stiffening techniques have reached a point of diminishing returns for reducing mass and stowage volume. Current SOA solar array designs using rigid cell technology, such as SCARLET, have a specific power of 50 W/kg.1 The highest specific power tensioned flexible blanket system offered today is the AEC Able Engineering Ultraflex at 106 W/kg. In the past, it was thought that TFPV efficiencies must reach 13% or more for array performance to be equal to these available systems.2 It is also thought that the high packaging efficiency (kW/m3) of TFPV is not realizable in most structures, given the deployed area increase.2 The FITS solution refutes these assumptions due to its advantages of being lighter-weight and self-deployable while still meeting mission stiffness requirements. Current production level TFPV technology (8.5% cells) coupled with MSI’s innovative FITS deployment system exceeds today’s SOA system metrics at the array level due to the lightweight FITS structure. From Table 2, one can see that the FITS structure contributes a mere 0.09 kg/m2. This compares to that of AEC Able Engineering’s Ultraflex 3 kW design at 0.28 kg/m2.2

1 to 15 kW; Assume 8.5% CellsARRAY DESIGN W/kg (BOL) kg/m2

Blanket Assembly 266 0.36

FITS Structure Assembly NA 0.09

Electrical Assembly NA 0.08

1 to 15 kW Solar Array Design

181.75 0.53

Table 2. Current MSI FITS SA Design Metrics

Current MSI and Air Force Research Laboratory (AFRL) programs are addressing two factors that will improve specific power even further: lighter coatings and higher cell efficiencies. With these advances the FITS solar array system could reach specific powers of close to 300 W/kg. Table 3 shows the array system metric potential assuming 10% efficient cells on a stainless steel substrate.

1 to 15 kW; Assume 10% CellsARRAY DESIGN W/kg (BOL) kg/m2

Blanket Assembly 531 0.21

FITS Structure Assembly

NA 0.10

Electrical Assembly NA 0.09

1 to 15 kW Solar Array Design 276.31 0.41

Table 3. Near Term MSI FITS SA Design Metrics

With ever-increasing advances in TFPV efficiencies, and deposition onto polyimide substrates, the FITS system specific power could ultimately reach 600 W/kg, an improvement of 12 times over the current SOA. As cell efficiencies continue to improve, the packaging efficiency of the FITS systems can reach up to 90 kW/m3, due to smaller blanket size required and the innovative integration and folding scheme of the thin stiffening and self-deployment features. This packaging efficiency enables satellite manufacturers to achieve the high power required for present payloads without increasing the size of the launch vehicle.

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FITS Scalability Analysis Another advantage to the FITS system is scalability. MSI has completed scalability studies, design, and FEM analysis for 1000 Watt, 6 kWatt and 15 kWatt systems and their impact on the attitude determination and control system (ADCS) of a spacecraft bus. Figure 2 shows that each of these designs meets the stiffness requirement of 0.5 Hz.

D:\sdrc\models\test_bus.mf1RESULTS: 99- B.C. 1,NORMAL_MODE 29,STRAIN ENER_99MODE: 29 FREQ: 0.4857015STRAIN ENERGY - MAG MIN: 0.00E+00 MAX: 1.00E+00RESULTS: 49- B.C. 1,NORMAL_MODE 29,DISPLACEMEN_49MODE: 29 FREQ: 0.4857015DISPLACEMENT - MAG MIN: 0.00E+00 MAX: 3.74E+01 VALUE OPTION:ACTUAL

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D:\sdrc\models\fifteen_kilo.mf1RESULTS: 26- B.C. 1,NORMAL_MODE 1,STRAIN ENERG_26MODE: 1 FREQ: 0.4360792STRAIN ENERGY - MAG MIN: 0.00E+00 MAX: 5.35E-01RESULTS: 1- B.C. 1,NORMAL_MODE 1,DISPLACEMENT_1MODE: 1 FREQ: 0.4360792DISPLACEMENT - MAG MIN: 0.00E+00 MAX: 7.96E+00 VALUE OPTION:ACTUAL

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D:\sdrc\models\test_bus.mf1RESULTS: 375- B.C. 1,NORMAL_MODE 7,STRAIN ENERG_375MODE: 7 FREQ: 0.6810579STRAIN ENERGY - MAG MIN:-5.45E-18 MAX: 1.29E-01RESULTS: 325- B.C. 1,NORMAL_MODE 7,DISPLACEMENT_325MODE: 7 FREQ: 0.6810579DISPLACEMENT - MAG MIN: 1.86E-03 MAX: 8.65E+01 VALUE OPTION:ACTUAL

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D:\sdrc\models\test_bus.mf1RESULTS: 99- B.C. 1,NORMAL_MODE 29,STRAIN ENER_99MODE: 29 FREQ: 0.4857015STRAIN ENERGY - MAG MIN: 0.00E+00 MAX: 1.00E+00RESULTS: 49- B.C. 1,NORMAL_MODE 29,DISPLACEMEN_49MODE: 29 FREQ: 0.4857015DISPLACEMENT - MAG MIN: 0.00E+00 MAX: 3.74E+01 VALUE OPTION:ACTUAL

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D:\sdrc\models\fifteen_kilo.mf1RESULTS: 26- B.C. 1,NORMAL_MODE 1,STRAIN ENERG_26MODE: 1 FREQ: 0.4360792STRAIN ENERGY - MAG MIN: 0.00E+00 MAX: 5.35E-01RESULTS: 1- B.C. 1,NORMAL_MODE 1,DISPLACEMENT_1MODE: 1 FREQ: 0.4360792DISPLACEMENT - MAG MIN: 0.00E+00 MAX: 7.96E+00 VALUE OPTION:ACTUAL

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D:\sdrc\models\test_bus.mf1RESULTS: 375- B.C. 1,NORMAL_MODE 7,STRAIN ENERG_375MODE: 7 FREQ: 0.6810579STRAIN ENERGY - MAG MIN:-5.45E-18 MAX: 1.29E-01RESULTS: 325- B.C. 1,NORMAL_MODE 7,DISPLACEMENT_325MODE: 7 FREQ: 0.6810579DISPLACEMENT - MAG MIN: 1.86E-03 MAX: 8.65E+01 VALUE OPTION:ACTUAL

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D:\sdrc\models\test_bus.mf1RESULTS: 375- B.C. 1,NORMAL_MODE 7,STRAIN ENERG_375MODE: 7 FREQ: 0.6810579STRAIN ENERGY - MAG MIN:-5.45E-18 MAX: 1.29E-01RESULTS: 325- B.C. 1,NORMAL_MODE 7,DISPLACEMENT_325MODE: 7 FREQ: 0.6810579DISPLACEMENT - MAG MIN: 1.86E-03 MAX: 8.65E+01 VALUE OPTION:ACTUAL

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500 W Wing0.5 Hz

3 kW Wing0.5 Hz

7.5 kW Wing0.5 Hz

Figure 2. FITS Scalability Analysis Results

There is a delicate balance between deployed natural frequency and structural stability in a thin-

film solar array. Larger wings require more structure to maintain the same deployed natural frequency; however, with the FITS innovative folding scheme this does not eliminate the specific power or volumetric advantages of using TFPV.

FITS STRUCTURE TECHNOLOGY STATUS

Thru Small Business Innovation Research (SBIR) and MSI internal research and development (IR&D) funds, MSI has designed, fabricated, and tested all of the critical components needed to ensure FITS success. The success criteria were based on a collection of real space program requirements, including the TechSat 21 mission, to arrive at worst case loads. These critical components and tests include: § Living hinge force and stowage life testing § Carpenter hinge force and life testing § FITS structure characterization testing § Foam characterization & creep testing § TFPV and preload random vibe (RV) testing § Component thermal cycle testing Each of the critical components provides a critical function that is required for the successful deployment of the FITS solar array. MSI identified and then tested these critical components to verify that they meet the stringent mission goals and requirements that were identified previously. The living hinges were tested to verify that they provide the necessary Z-fold deployment force to deploy the array. This deployment force was also verified later at the subsystem level during the 500-W EDU deployment testing. The living hinges were also tested to verify that they meet the 12-month storage requirement in the stowed position and maintain a minimum force margin of 2 or more once deployed. The carpenter hinges were tested to verify that they provide the necessary Tri-fold deployment force to open the array wings upon successful Z-fold deployment. This deployment force was also verified later at the subsystem level during the 500-W EDU deployment testing. The carpenter hinge was also life cycle tested to verify its survivability over the mission life duration. The FITS structure was tested to verify that each section is stiff enough to transfer the load from one section to another during deployment and maintain this stiffness once deployed on orbit. The foam used to restrain the array during deployment was characterized to develop a force vs. deflection curve that included the time dependent creep

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inherent in the foam. The TFPV was random vibration (RV) tested in its stowed configuration, including the preload foam. I-V curves were generated for the TFPV cells pre and post RV testing to ensure no degradation occurred during the testing. Each of the components and adhesive joints that are used to fabricate the FITS TFPV solar array were thermal cycled and pull tested at the coupon level from –100 C to + 100 C for 100 cycles. Pictures of all of the critical component testing can be seen in Figure 3. All of the components met the designed acceptance criteria by a margin of 2 or more.

CIGS Solar CellRandom Vibe Testing

Living Hinge ForceAnd Stowage Life Testing

Carpenter HingeForce Testing

FITS StructureStiffness Testing

Foam CharacterizationTesting

Component ThermalCycle Testing

CIGS Solar CellRandom Vibe Testing

Living Hinge ForceAnd Stowage Life Testing

Carpenter HingeForce Testing

FITS StructureStiffness Testing

Foam CharacterizationTesting

Component ThermalCycle Testing

Figure 3. FITS Critical Component Testing

Upon successful completion of the critical component testing, MSI began fabrication and testing of other components, small demonstrator units, and eventually a 500-watt single wing engineering development unit (EDU). Figure 4 shows the Z-fold deployment testing and Figure 5 shows the Tri-fold deployment testing performed on the EDU in the MSI cleanroom.

Figure 4. Z-fold Deployment Testing at MSI

Figure 5. Tri-fold Deployment Testing at MSI

MSI has shown through testing that the 500-W FITS solar array successfully deploys using 0-g offload fixtures, meets the 0.5 Hz natural frequency requirement, and maintains a positive force margin of greater than 2 over extreme temperature range from –50C to +100C.

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Development efforts are continuing at MicroSat Systems on the FITS solar array. Some of these efforts can be seen in Figure 6, which shows many of the tests performed on the solar array components and the solar array EDU. Future plans include: random vibration testing, thermal vacuum testing, and thermal cycle testing of the EDU.

Thin-film conductorimpedance testing

End Stiffener AssemblyBend Radius Testing

Power CablingDevelopment

Subscale Demo Unit Subscale Demo UnitDeployment Testing

Full Size EDU Array

Natural Frequency Testing

Full Scale EDU Z-fold Deployment Testing

First Motion ColdDeployment

Figure 6. FITS Technology Developments

THIN-FILM PHOTOVOLTAIC (TFPV) CIGS TECHNOLOGY STATUS

Global Solar Energy (GSE) manufactures Copper-Indium-Gallium- diSelenide (CIGS) TFPV cells on both stainless steel and polyimide substrates and provides them to MSI. ITN Energy Systems, Inc., performs advanced CIGS TFPV research and development on coatings and interconnects to maximize the efficiency and survival of CIGS cells in space using MSI and ITN funds in conjunction with AFRL’s Duel Use Science & Technology (DUS&T) program. TFPV has exhibited very little degradation (~10%) from high radiation environments during radiation testing. ITN and MSI have developed CIGS space coatings, thermal coatings, and ESD mitigation techniques that allow for high voltage arrays. Cells that incorporate these coatings and developed interconnects are fabricated at MSI into modules and blankets specifically for space applications, shown in Figure 7.

Interconnected Cells

ESD/Thermal Coating

ESD Coating

Cross Section of 3 Cells

5 Bonded CIGS PV on Stainless Steel Substrate

Etched Structural Bond Area

Electrically Conductive Adhesive

High Strength Space Qualified

Dielectric Adhesive

Masked Electrical

Bond Area

Interconnected Cells

ESD/Thermal Coating

ESD Coating

Cross Section of 3 Cells

5 Bonded CIGS PV on Stainless Steel Substrate

Etched Structural Bond Area

Electrically Conductive Adhesive

High Strength Space Qualified

Dielectric Adhesive

Masked Electrical

Bond Area

Figure 7. GSE's Stainless Steel Substrate Cells, Interconnects & ESD Mitigation

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Further research at GSE and ITN is in progress to maximize the efficiency of production cells, improve monolithically integrated CIGS on polyimide, utilize a wider bandgap to de-emphasize thermal dependence, and develop multi-junctions for CIGS. From August of 2002 through the first Quarter of 2003, large strides have been made in the production of CIGS TFPV at GSE. Figure 8 shows the data MSI received from GSE showing the trend that cell efficiency is improving as well as the process. The chart gives the production efficiencies of stainless steel cells for August ’02, February ’03, and the best 5 consecutive runs in March ’03. GSE addressed certain CIGS process parameters that were responsible for variation in efficiency and steps to control these parameters were made. These parameters included web handling, effusion source performance, cross-web uniformity, difference between equipment sets, equipment reliability, and CIGS process control. As the ease of processing stainless steel cells was greater than that of polyimide cells, the polyimide production line was halted while these parameters were adjusted for stainless steel. GSE is producing roughly 5000 cells per day on stainless steel substrates. With the recent process improvements, GSE has produced cells that are over 12% efficient, AM1.5, which will improve the FITS CIGS solar array metrics even further in the near future. With the process improvements demonstrating a positive gain in cell production efficiency on stainless steel substrates, the polyimide line that was halted will be started again to begin producing high efficiency polyimide cells in July 2003.

0%

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Efficiency (%)

Nu

mb

er o

f C

ells

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Figure 8. CIGS Efficiency (AM1.5) Progress from Aug ’02 Through the First Quarter ’03

CONCLUSIONS MSI’s groundbreaking solar array deployment and stiffening system, FITS, allows for the advantages of TFPV to be realized at today’s current efficiencies. This is due to the innovative, lightweight deployment and stiffening structure utilized in the design. The FITS system increases the specific power (W/kg) and packaging efficiency (kW/m3) by up to 50% over today’s SOA systems. Figure 9 compares specific power of MSI’s FITS solar array using TFPV to that of SOA systems of today and tomorrow. With increasing array size, FITS packaging efficiency increases and all other system metrics are linearly scalable. Through continued development efforts at MSI, the FITS system and current TFPV efficiencies can provide valuable solutions to today’s spacecraft power needs.

Specific Power (W/kg), 3kW Array BOL

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FITS & TFPV Today = SOA & TFPV @ 15% (Tomorrow)

UF, 15% TFPV2

UF, 27% MJ2

TFPV, 8.5%

TFPV, 10%

TFPV, 20%

SOA

Figure 9. System Specific Power vs. Cell Efficiency. Ultraflex (UF) TFPV and multi-junction (MJ) data points taken from reference 2.

ACKNOWLEDGEMENTS Some of the efforts contained herein were sponsored by the Space Vehicles Directorate of the Air Force Research Laboratory, administered under contract F29601-02-C-0243. Thanks to Mr. Clay Mayberry and Dr. Paul Hausgen at the Air Force Research Laboratory for their support of the FITS solar array.

REFERENCES [1] Murphy, D. M., “The SCARLET Solar Array: Technology Validation and Flight Results,”

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Proceedings of the Deep Space 1 Technology Validation Symposium, Pasadena, CA, 2000. [2] Murphy, D. M.; Eskenazi, M. I.; White, S. F.; Spence, B. R., “Thin-film and Crystalline Solar Cell Array System Performance Comparisons,” Conference Record of the IEEE Photovoltaic Specialists Conference; 2002; p.782-787.