Fuel Cells for Micro Air Vehicles

1. Fuel Cells for Micro Air Vehicles James C. Kellogg, Lesli Monforton, Danielle White, and Michael Vick Tactical Electronic Warfare Division Karen Swider Lyons and Peter Bouwman Chemistry Division Naval Research Laboratory, Washington DC Joint Service Power Expo, Tampa FL 5May2005

2. Demonstrate a Fuel Cell powered UAV Goal: 4 to 6 Hours of flight Solution: Hydrogen fuel cell (PEM) Overview • Potential Value: • Demonstrate fuel cells as a practical power source for a small UAVs • Two to ten-fold energy increase over batteries Polymer fuel cell Protonex Technology Corp • Key issues: • Fuel • Components selection • System design • System integration Successful Autonomous Vehicles BUILD THE VEHICLE AROUND THE POWER SOURCE Sail plane with fuselage modified to house hydrogen tank

3. Propulsion and power budget considered in design Weight budgets for 20 min flight “The emergence of mini UAVs for military applications” Montgomery and Coffey, Defense Horizons, Dec. 2002

4. Dragon Eye UAV Dragon Eye - • 8 LiSO2 D-cells - 680 g • Cruise at 110 W, climb at 300 W • Power = (160 W/kg) • Rated for 45 min flight DISADVANTAGES of battery power source • Limited energy (~200 Wh/kg) 680 g= 136 Wh • Rarely full use of energy (throw out after 20 min) • Primary battery and $ per replacement ADVANTAGES • Silent • Low heat signature • Attitude insensitive

5. Fuel source OPTION 1: hydrogen gas • Compressed hydrogen gas • Up to 10,000 psi in large bottles • Less pressure and %Hydrogen in smaller bottles (larger surface area to volume) ADVANTAGES • Responds immediately to change in load • Easy to handle/recharge in lab environment • No waste produced (only H2O) DISDAVANTAGES • Difficult logistics (supplying hydrogen to remote locations) • Safety Paintball canister 0.7 kg and 4500 psi for $365 0.94 kg with fill valve and regulators Specialty tanks designed for NASA may be needed for lower weight/higher pressure

6. Fuel Source OPTION 2: Chemical hydride • LiAlH4 + 2 H2O --> LiAlO2 + 4H2 • Theoretical: 6943 Wh/kg • Net system: ~3000 Wh/kg • Working with Trulite Inc to use LiAlH system with recuperated product water from fuel cell • ADVANTAGES: • High specific energy system • Easy to work with in lab environment • DISADVANTAGES: • Fuel system gains weight during flight • Reaction creates additional thermal • load • Logistics issues with refueling in field • Safety issue in humid environments • Waste disposal Fuel cell air/H2O H2 Chemical hydride

7. Fuel Cell Powered Micro UAV Step 1: Select an airframe • Estimate weight and power of fuel cell system • Test vehicle with batteries Step 2: Integrate and test the fuel cell system 100W Protonex Stack

8. Layout of fuel cell system • Hydrogen system • Storage Tank • Regulator • Pressure Relief • Purge Valve • Timer Circuit • Air supply • Pump • Humidifier • Cooling loop • Pump • Radiator

9. System components Radiator Regulators HP hydrogen tank (45ci) Humidifier Water pump Valve Timer Circuit Air Pumps

10. System Integration of Fuel Cell Improved Fuel Cell system/parts, 115 watts on the bench Radiator Fuel Cell Water Pump Humidifier Hydrogen HP Tank Flight weight: 4.0 pounds Air Pumps Valve/Regulators

11. centerfuselage nose Integration of parts into vehicle • Vehicle center of gravity • Set at 30% of chord • Hydrogen tank heaviest part of system • Pack air vehicle nose to offset weight of tank and regulator • Component placement can affect fuel cell performance • air flow • water through humidifier

12. Weight breakdown of fuel cell system Hydrogen tank, regulator, and air pumps dominate system weight Hydrogen fuel 1% Weight distribution in Phase I demonstration of PEM fuel cell system

13. Preparing the vehicle for flight • Hydrogen cylinder filled • Use cooler to prevent overheating • Load tank into vehicle • Systems check

14. The Fuel Cell Flyer Flight test, November 2004

15. Total weight of vehicle Starting plan: 3 lbs for air vehicle 3 lbs for fuel cell 1st generation vehicle: 2.2 lbs for air vehicle/batteries (1 kg) 4.6 lbs for fuel cell system (2.1 kg) • Approaches to lower weight of fuel cell system • Decrease pressure drop through fuel cell • Allows smaller air pump • Consider MEMS-type air pump • Use specialty hydrogen tank & regulator - lighter weight • Use chemical hydride system (properties TBD)

16. Summary of power source specs • Minimal signature from air pumps • Cost • $2500 fuel cell • $365 tank • $~500 Electronics & parts • $?? Refueling • Cycle life > life of plane • Logistics • Fuel availability and safety Power = 92 W Specific power = 44 W/kg fuel cell 30 W/kg total vehicle Energy = projected: 276 Wh 3 h flight for 3000 psi H2 90 Wh/kg Environment • Ambient humidity and temperature may affect performance No attitude sensitivity Next steps - achieve higher specific power and specific energy Add weight budget for mission equipment (video, etc).

17. Summary 8 h micro air vehicle flight • State-of-art batteries are inadequate • Fuel cells are a viable option for 8 to 12 h flights • Building the vehicle around the power source is key to success • Development team with expertise in fuel cells, air vehicles, and modeling • Weight of fuel cell systems must be reduced for use in tactical vehicles • Consider hydride fuels for PEMFCs • Possibility of butane-fueled SOFCs

18. Acknowledgements • Greg Ariff, Brian James Directed Technologies, Arlington VA • William Skrivan, Paul Sabin, Paul Osenar Protonex Technology Corporation, Southboro, MA • Timothy LaBreche, Aaron Crumm Adaptive Materials, Ann Arbor MI kellogg@suzie.nrl.navy.mil