Spacecraft Power System Kyusyu Inst Tech HTV6 Mitsuru

Spacecraft Power System, Kyusyu Inst. Tech. HTV-6 Mitsuru Imaizumi Nov. 08, 2019

Contents 1. Solar cell utilization in space 2. Fundamentals of solar cells 3. Development history 4. Radiation degradation 5. Degradation prediction 6. Recent R&D activities 2

Contents 1. Solar cell utilization in space 2. Fundamentals of solar cells 3. Development history 4. Radiation degradation 5. Degradation prediction 6. Recent R&D activities 3

Solar Panels/Paddles GCOM-W 1 GOSAT WINDS 4

Solar Panels/Paddles HTV GMS-5 MMO 5

Solar Panels/Paddles Solar Paddle Series number: Generation voltage Output current Parallel number: Generation current Layout of solar cells in a panel 6

Space Solar Cells Buried bypass diodes Bypass diode Interconnector Cell size: 2× 2 cm 2 Size: 40 mm× 60 mm High efficiency Si solar cell Size: 37 mm× 76 mm In. Ga. P/Ga. As/Ge triple-junction solar cell 7

Space Solar Panel Cell size: 2× 2 cm 2 A coupon solar panel with Si solar cells for testing 8

Structure of Solar Panels Solar cell Coverglass Adhesive AR coating Electrodes Adhesive Polyimide film Aluminum Honeycomb CFRP Cross-sectional schematic of a solar panel (not to scale) 9

Contents 1. Solar cell utilization in space 2. Fundamentals of solar cells 3. Development history 4. Radiation degradation 5. Degradation prediction 6. Recent R&D activities 10

Current-Voltage Characteristics Current-voltage (I-V) characteristics of a solar cell 11

Output Parameters A typical current-voltage (I-V) curve 12

Equivalent Circuit of Solar Cells I Vd Id Rs = 0 Rsh = ∞ Iph V n=1 Ish Equivalent circuit of a (single-junction) solar cell 13

Contents 1. Solar cell utilization in space 2. Fundamentals of solar cells 3. Development history 4. Radiation degradation 5. Degradation prediction 6. Recent R&D activities 14

Space Solar Cells Buried bypass diodes Bypass diode Inter-connector Cell size: 2× 2 cm 2 Size: 40 mm× 60 mm High efficiency Si solar cell Size: 37 mm× 76 mm In. Ga. P/Ga. As/Ge triple-junction solar cell 15

Structure of Si Space Solar Cell Schematic cross-sectional structure of the “HES” Si space solar cell 16

Back-Surface Field Layer n-type Depletion Emitter Layer p-type Base no BSF layer n-type Emitter Depletion Layer p-type Base BSF Layer with BSF layer Effect of insertion of the back-surface field (BSF) layer 17

Back-Surface Reflection Layer Visible light Infrared (unabsorbed) light Solar Cell BSR Substrate no BSR layer with BSR layer Effect of insertion of the back-surface reflection (BSR) layer 18

Non-Reflection Surface Structure Reflection Texture ARC Solar Cell Flat surface Textured surface Effect of the textured non-reflection surface (NRS) structure 19

Efficiency of Space Solar Cells Historical improvement of the efficiency of space solar cells 20

Space Solar Cells Buried bypass diodes Bypass diode Inter-connector Cell size: 2× 2 cm 2 Size: 40 mm× 60 mm High efficiency Si solar cell Size: 37 mm× 76 mm In. Ga. P/Ga. As/Ge triple-junction solar cell 21

Structure of 3 J Space Solar Cell Cross-sectional structure of an In. Ga. P/Ga. As/Ge triple-junction (3 J) solar cell 22

Current-Voltage Charateristics of 3 J Cell A typical current-voltage (I-V) characteristics of an In. Ga. P/Ga. As/Ge 3 J solar cell 23

Efficiency of Space Solar Cells Historical improvement of the efficiency of space solar cells 24

Effect of Efficiency Improvement HES solar cell (η=17%) Area reduction of about 60% 3 J solar cell (η=27%) Effect of improvement of solar cell efficiency (paddle-size reduction) 25

Contents 1. Solar cell utilization in space 2. Fundamentals of solar cells 3. Development history 4. Radiation degradation 5. Degradation prediction 6. Recent R&D activities 26

Space Environment for Solar Cells 1. Radiation (High energy electrons/protons) 2. High/low temperature and thermal cycles 3. Ultraviolet 4. Charging/discharging 5. Mechanical vibration and sonic wave 6. Debris/meteoroids (particle) 27

Requirements for Space Solar Cells 1. Efficiency … but not at BOL but at EOL 2. Radiation resistance 3. Lightweight 4. Thermal (cycle) durability 5. Mechanical strength 6. Inexpensiveness 28

Structure of Solar Panel Solar cell Coverglass Adhesive AR coating Electrodes Adhesive Polyimide film Aluminum Honeycomb CFRP Cross-sectional schematic of a solar panel 29

Radiation Degradation of Output Cell size: 2× 2 cm 2 High fluence Degradation on I-V characteristics Beginning-of-life (BOL) and end-of-life (EOL) Degradation of solar cell output due to radiation exposure 30

Decrease in Output Parameters Absolute values Remaining factors Degradation trend of HES solar cell due to 10 Me. V proton irradiation 31

Contents 1. Solar cell utilization in space 2. Fundamentals of solar cells 3. Development history 4. Radiation degradation 5. Degradation prediction 6. Recent R&D activities 32

Prediction of Radiation Degradation Difference between ground tests and actual space to be considered Ground tests l Normal incidence Mono energy l No shielding l Actual space l Omni-directional incidence (4π) l Multi energy l Shielding effects Front: coverglass Back: substrate 33

Radiation Environment An example of the chronal change in dose rates of protons and electrons 34

Radiation Environment An example of the dependency of proton dose rates on proton energy 35

Degradation Curves An example of the dependency of degradation curves on proton energy 36

Relative Damage Coefficients Reference energy Electrons: 1 Me. V Protons: 10 Me. V An example of the relative damage coefficients for protons 37

Prediction of Radiation Degradation Matters to be considered and estimated 1. Reduction of energy and dose rate due to shielding effect by coverglass etc. 2. Effective penetration depth depending on incident angle. 38

Equivalent Fluence Calculation of the equivalent 1 Me. V electron fluence 1 Me. V electron / 10 Me. V proton conversion factor: k Isc Voc Pmax 4350 1280 2580 For HES cell 39

Prediction of Radiation Degradation An example of estimation of degradation rate from the equivalent 1 Me. V electron fluence 40

Contents 1. Solar cell utilization in space 2. Fundamentals of solar cells 3. Development history 4. Radiation degradation 5. Degradation prediction 6. Recent R&D activities 41

Recent R&D Activity on Space Solar Cells Realize a high specific power solar paddle 150 W/kg @BOL 100 W/kg @EOL Require 1. A high specific power solar cell in a sheet ( 1. 0 W/g) 2. An ultra-lightweight panel structure 42

Recent R&D Activity on Space Solar Cells In. Ga. As bottom-cell Ga. As middle-cell In. Ga. P top-cell In. G tomt o b s a. A cell In. Ga. P top-cell Ga. As bottom-cell In. Ga. As bottom-cell Ge substarte Drastic weight reduction and flexibilization of multi -junction high-efficiency space solar cells by removal of “heavy” Ge substrate 43

Recent R&D Activity on Space Solar Cells Current 3 J cell (150μm thick, 2. 2 grams) Developed 3 J cell (30μm thick, 0. 25 grams)

Recent R&D Activity on Space Solar Panels Front contact Thin 3 J cell Thin-film cell Back contact Supporting layer Laminating Film Interconnector P-side tab Cell-array sheet N-side tab Adhesive Lightweight panel Solar cell sheet Panel frame 45

Recent R&D Activity on Space Solar Cells 27. 4 cm 2, 330 mg, ~30μm AM 0, 136. 7 m. W/cm 2 @25ºC In. Ga. P/Ga. As/In. Ga. As Triple-junction Solar Cell Specific Power: 3. 6 W/g 46

Recent R&D Activity on Space Solar Sheets Thin coverglass Adhesive IMM-3 J Cell Adhesive Backsheet material Weight: ~30 g Cross section structure Thickness: ~ 0. 3 mm Specific Power: 0. 58 W/g 47

Recent R&D Activity on Space Solar Panels Demonstration panel has been mounted on a vacant panel slot on HTV-6 Space Demonstration Test 48

Recent R&D Activity on Space Solar Panels Stowed Deployed Lightweight Solar Paddle 49

Recent R&D Activity on Space Solar Panels Features： ü Lightweight, ü Compact, ü but Tough Curved, thin frame Lightweight and thin frame + SSSs applied with Velcro fasteners 50

Recent R&D Activity on Space Solar Panels Innovative Technology Demonstration Sat-1 51