Electrical Power System

1. Electrical Power System www.esmo.co.uk

2. Contents • What is ESMO • The Warwick EPS Team • EPS Architecture • Solar Arrays • Battery • PCDU • Simulations

3. European Student Moon Orbiter • Run by ESA, since 2006 • Includes over 20 European countries • Scheduled launch date 2014-2015 • Managed by SSTL • Experience

4. EPS Team 09/10 ESMO EPS Team 4

5. Electrical Requirements • Regulate solar array output. • Regulate battery charge levels. • Provide suitable bus for powering loads. • Power critical loads for entirety of mission life. • Power non-critical loads when necessary. • Protect against overcurrent and under voltage situations. • Ensure redundancy requirements are met.

6. Mechanical Requirements • Protect battery and EPS circuitry from damage and radiation • Thermal control to circuitry and solar cells • Validation of all designs against mission parameters • SolidWorks Simulation used for all analysis • Design tables implemented • Source components and solar panel assembly

7. EPS Architecture

8. Solar Panels 8

9. Solar Panels • Cells (256): • Type: AZUR SPACE 3G – 28% efficiency (integrated diode protection) • 2.6 V (open circuit) & 0.5 A (short circuit) • 40mm x 80mm • Panels (2): • 16 cells per string, 8 strings per panel • Estimated mass 5.5kg • Max voltage ≈ 41V per panel • Max power ≈ 164W (82W at 41V per section BOL)

10. Solar Panel Structure

11. Solar Cell Simulation • Thermal simulations to test for: • Excessive deformation • Stress concentrations

12. Panel Backing Structure • Cover plates: • Carbon Fibre used • Honeycomb: • Aluminium • Industry standard. • Different thermal properties, requiring modelling. • Stronger design and extra mass to compensate. • Carbon Fibre • Identical physical properties. • Expensive and difficult to obtain. • Significant redesign required.

13. Honeycomb Modelling

14. Battery

15. Battery - Specification • Characteristics: • Cells: • Voltage range: 2.5 – 4.2V • Capacity: 1.5Ah • Battery: • 28 cells provided by ABSL • 7s4p arrangement 18650HC • Li-ion cells capacity: 6Ah • Balanced Cell

16. Battery – Mechanical Mechanical Requirements: • First modal frequency > 300Hz • Max stress< Yield stress at launch g-force ~ 5g • Repetitive shock vibration, Grms < 30.4 - PENDING • Thermal expansion induced stress < Yield strength

17. Battery – Vibration Results • First modal frequency: 1373.1 Hz Failure mode deformation: Not to scale

18. Battery – G-Force Loading Results • Max stress: 3.1MPa Z axis X axis Y axis Deformation not to scale

19. Battery – Thermal Stress • Thermal stress: max. 1.8MPa 76K 340K Deformation not to scale

20. Battery – Thermal Requirements • Operating temperature, 273K-303K • Average operating temperature, 283K-293K • Determine need for heater • Regulated interface temperature, 278K – 303K • Multiple analysis performed for all scenarios

21. Battery – Thermal Response Steady State Analysis:

22. Battery – Thermal Response Transient:

23. PCDU

24. PCDU - Mechanical • Suitable casing required to contain and protect all EPS circuitry • Case Requirements: • Design Constraints • Size, Weight, Cost, Complexity, Thermal Performance, Strength/Stiffness, Electrical Routing • Environmental Factors • Radiation, Pressure, Vibration and Loading, Electrostatic

25. [51760000934 PCDU - Mechanical Design Process • PCDU structure last to be formalised within the EPS system • Iterative approach adopted • Integration of SSTL into ESMO project has led to further challenges • Requirements analysis undertaken by previous teams

26. [51760000934 PCDU - Mechanical Blade Based Design • Development of existing blade structure • 9 separate cards, main components spread for redundancy • Effective heat dissipation and simplicity • Large area for circuitry • Preliminary thermal analyses completed

27. [51760000934 PCDU - Mechanical Microtray design • Industry used design put forward by SSTL • Strict design requirements • Flight proven design • Significant reduction in circuitry area

28. [51760000934 PCDU - Mechanical • Microtrays to be part of larger satellite casing for all system circuitry • Reduction of radiation and shielding requirements

29. [51760000934 PCDU - Mechanical Electrical Architecture • Use of Microtrays only possible with revised circuitry • MPPT enables loss of BCR • Circuit integration possible, but with very tight margins • Subject to ongoing change

30. [51760000934 PCDU - Resonance Analysis • Modal Frequencies must be > 300Hz. • Simulation conducted on single and dual tray set up.

31. PCDU – Electrical Subsystems

32. Previous EPS Architecture

33. Regulation Options S3R MPPT

34. Regulation Trade off

35. Proposed Architecture

36. Power Management Unit Telecommand and telemetry module

37. Current limiters • Protect EPS and subsystems from faults in the loads • Non-critical loads • Latched Current Limiters (LCL) • Critical loads (OBDH and Comms) • Foldback Current Limiters (FCL) OR • Fused Line

38. Fuses Fuses Critical Load Protection • FCL – reduces power to subsystems • Fused Line – removes power permanently

39. FCL vs Fused Line Most Important Factors:

40. Simulations

41. EPS Electrical Simulation • Full electrical model in Simulink. • Two main purposes: • Trade-off decision: • MPPT vs S3R. • EPS performance under variations in: • Mission period. • Battery capacity. • Power requirements. • Battery charging efficiency.

42. EPS Simulation

43. EPS Simulation Mission Profile Solar Arrays PCDU Battery Solar Array Regulation Battery Charge Regulator Power Management Unit Power Distribution Unit (Current Limiters) Loads (Power Requirement)

44. Simulation Results Comparison of Delivered Power

45. Simulation Results Performance in De-spin Phase 15 Ah, 70% Power Increase 6 Ah, 20% Power Increase 6 Ah, 70% Power Increase

46. Simulation Results Performance in Geostationary Transfer Orbit 70% Increase in Power Requirement 100% Increase in Power Requirement

47. Simulation Results Performance in Lunar Operational Orbit 20% Increase in Power Requirement 30% Increase in Power Requirement

48. Simulation Results Effect of the Long Eclipse 12 Ah, 10% Power Increase 12 Ah, 20% Power Increase 6 Ah, 20% Power Increase

49. Conclusions - Simulation • MPPT shown to be desirable over DET. • 15 Ah battery may be needed in order to deal with de-spin requirements. • Battery charging efficiency is important. • Long eclipse in operational orbit should be avoided.

50. Conclusions - Electrical • Finalisation of power system architecture. • Solar array regulation trade-off. • MPPT preferred. • Current limiter trade-off. • Fused line for critical loads preferred. • Further work on BCR. • Determine requirement.