1. USAFA Department of Astronautics I n t e g r i t y - S e r v i c e - E x c e l l e n c e Astro 331 EPS—Design Lesson 20 Spring 2005

2. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 20 2 EPS—Design Objectives Objectives Objective 1. Know technologies used for solar panels and batteries Objective 2. Be able to perform a preliminary sizing of solar arrays for planar or body mounted configurations Objective 3. Be able to perform preliminary battery sizing Reading SMAD Chapter 11.4, Reeve’s Handout

3. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 20 3 EPS—Design Introduction Although the EPS design process can involve a variety of technologies for the power source and energy storage, most earth-orbiting missions of any significant duration usually find the right EPS combination is solar panels for the power source and batteries for energy storage. This lesson focuses on sizing the EPS for these predominant missions.

4. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 20 4 EPS—Design EPS Design Process

5. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 20 5 EPS—Design Driving Requirements

6. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 20 6 EPS—Design Solar Array Design Process Configurations Technologies I-V Curves Sizing

7. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 20 7 EPS—Design Solar Array Configurations

8. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 20 8 EPS—Design Solar Array Configurations Planar: used on 3-axis stabilized S/C Power output is proportional to area projected toward incident sunlight

9. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 20 9 Cylindrical Projected area of spinner is 1/  of surface area of cylinder sides Must account for orientation wrt sun EPS—Design Solar Array Configurations

10. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 20 10 Omnidirectional Equal projected area from any direction (sphere) Used by many smallsats or low power S/C (attitude doesn’t effect power generation) Projected area is ¼ of total surface area EPS—Design Solar Array Configurations

11. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 20 11 Inherent Degradation – loss of power from perfect case Shading of cells Temperature differential across solar array Real estate required for connections between cells EPS—Design Solar Array Configurations

12. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 20 12 From Spacecraft Systems Engineering, by Fortescue and Stark EPS—Design Assembly

13. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 20 13 EPS—Design Technologies What solar cell material we choose Considerations: Efficiency Cost Lifetime (radiation hardness) Operating temperature Ease of manufacturing (lay-up panels) … Choice is application specific!

14. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 20 14 EPS—Design Technologies

15. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 20 15 From Spacecraft Systems Engineering, by Fortescue and Stark EPS—Design Technologies

16. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 20 16 EPS—Design Solar Cell I-V Curves Cells have inherent electrical characteristics Current, voltage of cells affected by: Cumulated degradation (damage by particles) Temperature Intensity of sunlight Design Considerations Components on S/C require specific voltages, currents Solar cells must be combined in series + parallel to meet these power requirements Want to operate solar cells near peak efficiency Have to account for lifetime degradation → Must take into account when designing solar arrays

17. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 20 17 EPS—Design I-V Plot for Planar Array

18. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 20 18 EPS—Design Effects of Sun Angle on Solar Cell Current From Space Vehicle Design, by Griffin and French

19. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 20 19 EPS—Design Effects of Sun Distance on Solar Cell Current From Space Vehicle Design, by Griffin and French

20. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 20 20 EPS—Design Effect of Temperature on Solar Cell Performance From Space Vehicle Design, by Griffin and French

21. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 20 21 EPS—Design Solar Array Design Process

22. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 20 22 EPS—Design Solar Array Design Process

23. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 20 23 P sa = power generated by solar array P e and P d = S/C power loads during eclipse and daylight T e and T d = times each orbit spent in eclipse and daylight X d = efficiency getting power from S/A directly to loads (typically is 0.85) X e = efficiency getting power from S/A to charge battery and then from battery to the load (typical value is 0.65) (1&2) Calculate power output of Solar Arrays EPS—Design Solar Array Design Process

24. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 20 24 P o = power density output for cells (watts/m 2 ) Flux (or P i ) = input solar power density (watts/m 2 )  (or  ) = efficiency of solar cell material P BOL = power density S/A’s generate at beginning of life (watts/m 2 ) P EOL = power density at end of life (watts/m 2 ) I d = inherent degradation  = sunlight incidence angle (3&4) Determine size of arrays needed to generate power EPS—Design Solar Array Design Process

25. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 20 25 P EOL = power density generated at end of life (watts/m 2 ) L D = lifetime degradation Typical degradation/year: 0.0375 for silicon in LEO 0.0275 for GaAs in LEO (5) Account for degradation due to exposure to the space environment EPS—Design Solar Array Design Process

26. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 20 26 (6) Find size of solar array needed at end of life Substituting in previous equations: EPS—Design Solar Array Design Process

27. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 20 27 EPS—Design Battery Design Process

28. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 20 28 EPS—Design Battery Design Process

29. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 20 29 EPS—Design Battery Design Process

30. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 20 30 Equation for battery capacity: C r = total S/C battery capacity P e = average eclipse load (watts) T e = eclipse duration (hr) DoD = depth of discharge (0  DoD  1) N = number of batteries (need at least two if want some partial redundancy) n = transmission efficiency between battery and load (typical value is 0.9) EPS—Design Battery Design Process

31. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 20 31 Finding DoD: Use chart / tabular data to determine allowable DoD EPS—Design Battery Design Process

32. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 20 32 EPS—Design Battery Design Process

33. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 20 33 EPS—Design Battery Design Process

34. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 20 34 From Space Vehicle Design, by Griffin and French EPS—Design Battery Design Process Must be done under controlled conditions!

35. I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3 Jan 05 Lesson 20 35 From Spacecraft Systems Engineering, by Fortescue and Stark EPS—Design Battery Design Process