1. ASTEP Life in the AtacamaCarnegie Mellon Limits of Life in the Atacama: Investigation of Life in the Atacama Desert of Chile Power System Design James Teza The Robotics Institute Carnegie Mellon University

2. ASTEP Life in the AtacamaCarnegie Mellon Outline Introduction Current Configuration Requirements Components - Development Schedule Summary

3. ASTEP Life in the AtacamaCarnegie Mellon Current Configuration of Hyperion Design constraints 24 hour sun synchronous operation 24 hour insolation Relatively benign environment and terrain Primary power source – solar Battery storage – power leveling

4. ASTEP Life in the AtacamaCarnegie Mellon Current Configuration of Hyperion Batteries – sealed lead acid System bus voltage – 24 VDC Constrained by existing motor amplifiers and DC/DC converters Maximum power point trackers (MPPT) Support lead acid batteries only Provided no external control or data Not protected from over voltage fault conditions System loads cannot be switched Power system monitoring not accurate

5. ASTEP Life in the AtacamaCarnegie Mellon Current Configuration of Hyperion Solar power 320 Seimens Si cells on custom fabricated modules 8 detachable modules Fixed array with adjustable angle along roll axis 3.4 m 2 total array area

6. ASTEP Life in the AtacamaCarnegie Mellon Requirements – Atacama vs. Haughton Environment – less benign than Haughton Rougher terrain higher obstacle density larger obstacles Higher wind speeds Dust Greater thermal extremes Insolation Maxima higher in Atacama than Haughton Greater diurnal extremes (irradiance and angle) No requirement for sun synchrony but resource cognizance required Power budget will be driven by task and distance schedules Longer duration of operation

7. ASTEP Life in the AtacamaCarnegie Mellon Requirements – Goals Improved power efficiency Higher efficiency solar cells – smaller array Higher bus voltage – more efficient motors Higher battery storage efficiency – extend operating time, reduce battery mass Better vehicle stability Smaller array area – less wind loading and lower CG More accurate system monitoring Better instrumentation Better battery and system models Capable of entering low power mode(s) via selective load switching Reliable system capable of extended operation

8. ASTEP Life in the AtacamaCarnegie Mellon Requirements - Power Budget Note: does not assume overlapping of tasks

9. ASTEP Life in the AtacamaCarnegie Mellon Requirements - Power Budget Daily energy requirement based on proposed activity schedule

10. ASTEP Life in the AtacamaCarnegie Mellon Requirements – Power Budget Daily energy available

11. ASTEP Life in the AtacamaCarnegie Mellon Components – Solar cells Emcore advanced triple junction (ATJ) cells 27.7% average efficiency specified at AM0 Expect equal or higher efficiency at AM1.5 Thermal effect Greater dependence on spectra (air mass) Series (cascade) junctions InGa junction limits current with reduced UV 0 10 20 30 40 50 60 70 80 90 100 3005007009001100130015001700 Wavelength (nm) Quantum Efficiency (%) ATJ TJ Quantum efficiency (ATJ vs. TJ) GeInGa InGaAs

12. ASTEP Life in the AtacamaCarnegie Mellon Components – Solar cells Predicted Insolation Spectra

13. ASTEP Life in the AtacamaCarnegie Mellon Components - Solar cells Component testing Fabricate prototype array using Emcore ATJ cells for field tests Gather I sc, V oc and spectrophotometric data Perform initial testing in Phoenix, Arizona (winter ’02) First year field testing in Chile (’03) Actual efficiency TBD during year 1 component testing

14. ASTEP Life in the AtacamaCarnegie Mellon Components - PMAD Modify existing power system micro controller to provide increased capability Greater system monitoring accuracy Battery state Insolation Internal environment (temperature) Load switching for low power mode or modes Reliable hibernation mode Able to switch off main CPU and re-start system reliably Improve battery and system models

15. ASTEP Life in the AtacamaCarnegie Mellon Components - Batteries Sealed Lead Acid Relatively low energy density (25 to 40 Wh/kg, 33 Wh/kg for Hyperion) Relatively cheap Robust Relatively simple battery model Li-Ion High energy density (120 to 200 Wh/kg) Expensive (between 5 to 10 x cost of lead acid) Control of charge / discharge limits required Safety considerations More complex battery model

16. ASTEP Life in the AtacamaCarnegie Mellon Components - MPPT HTA Biel design MPPT Higher efficiency Communication via CANBus Open software Suitable with sealed lead acid and Li-ion battery systems Input and output protection

17. ASTEP Life in the AtacamaCarnegie Mellon Development – Issues Solar panel configuration Horizontal panel orientation Lower panel position Integrate panel with enclosure Thermal control Provide cooling Seal against dust High reliability in hostile environment

18. ASTEP Life in the AtacamaCarnegie Mellon Development - Schedule Year 1 Integrate on robot: PMAD Component testing in Chile: Advanced triple junction cells Component testing: Improved MPPT Li ion batteries High bus voltage components (new motor amplifiers, motors and DC/DC converters) Year 2 Integrate on robot: Advanced triple junction cells Improved MPPT Li ion batteries High bus voltage components

19. ASTEP Life in the AtacamaCarnegie Mellon Summary Improve system efficiency ATJ solar cells Improved MPPT Li batteries Higher bus voltage and components Improve vehicle stability Smaller array area, lower CG Improve power system monitoring and models Implement power system control Resolve issues panel configuration, thermal control, reliability