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
Overview
Demonstrate a Fuel Cell powered UAV
• Goal: 4 to 6 Hours of flight
• Solution: Hydrogen fuel cell (PEM)
Potential Value:
• Demonstrate fuel cells as a practical
power source for a small UAVs
• Two to ten-fold energy increase over
batteries
Key issues:
• Fuel
• Components selection
• System design
• System integration
Successful Autonomous Vehicles
BUILD THE VEHICLE AROUND
THE POWER SOURCE
Polymer fuel cell
Protonex
Technology
Corp
Sail plane with fuselage modified
to house hydrogen tank
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
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
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
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
H2
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
Chemical
hydride
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
Layout of fuel cell system
• Hydrogen system
–
–
–
–
–
Humidifier
Storage Tank
Regulator
Pressure Relief
Purge Valve
Timer Circuit
exhaust air
+ water
dry air
H2 in
wet air in
• Air supply
HP Tank
45 ci
cooling water out
– Pump
– Humidifier
• Cooling loop
– Pump
– Radiator
air out
purge H2
Fuel Cell
timer
Radiator
cooling
water
System components
Regulators
HP hydrogen tank (45ci)
Water pump
Radiator
Valve Timer Circuit
Humidifier
Air Pumps
System Integration of Fuel Cell
Improved Fuel Cell
system/parts, 115
watts on the bench
Radiator
Fuel
Cell
Water
Pump
Humidifier
Hydrogen
HP Tank
Air Pumps
Valve/Regulators
Flight weight:
4.0 pounds
Integration of parts into vehicle
nose
Component placement can
affect fuel cell performance
• air flow
• water through humidifier
center fuselage
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
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
Preparing the vehicle for flight
• Hydrogen cylinder filled
– Use cooler to prevent
overheating
• Load tank into vehicle
• Systems check
The Fuel Cell Flyer
Flight test, November 2004
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)
Summary of power source specs
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
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
No attitude sensitivity
Next steps - achieve higher specific power and specific energy
Add weight budget for mission equipment (video, etc).
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
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