Vehicle Electrification - February 21, 2013 - 15

February 21, 2013

traditional systems. UTC systems have also
shown excellent performance and startup at
ambient temperatures as well as start
capability and proven durability in subfreezing conditions.
As a result of the simplified BOP and cell
power levels, fuel cell systems have been
developed for UAVs that exceed 500 W/kg
for the system including both the fuel cell
and the BOP.

PEFC demonstration
To demonstrate a high-specific power PEFC,
the development of a UAV-specific design
was carried out by a team of researchers at
UTRC. The result was a fuel cell with a stack
specific power that exceeded 675 W/kg and
an overall system specific power above 500
W/kg, achieving the best-known specific
power in its class.
Operation of the fuel cell stack was at near
ambient pressure and required no external
humidification and cooling. The heat
generated by the stack was removed by
evaporation of liquid water contained within
the stack, eliminating the need for a heat
exchanger. This unique water-management
system significantly reduced the overall fuelcell system size, weight, and complexity
relative to traditional PEFC systems. During
operation, the overall efficiency of the system
exceeded 52%.
The fuel cell system was integrated into a
modified electric rotorcraft in the 1-2 kW
power class. The fuel cell was mounted in
the center of the aircraft, and the blower
protruded behind the stack. The highpressure hydrogen tank was mounted in the
nose of the aircraft. The brushless motor
and its power electronics were located
above the tank.
SAE Vehicle Electrification evsae.com

An off-the-shelf DOT-approved fuel tank
rated to 4500 psi was used for the flight. The
tank was connected to the fuel cell via
pressure regulators and a remote-controlled
valve. Refilling of the tank could be done in
less than a minute compared to the hour
required for recharging lithium polymer
batteries. The controls were provided with a
2.4-GHz radio control system, and the data
needed to monitor the well-being of the fuel
cell was transmitted back on a 900-MHz link
to a ground station.
The demonstration flight used a highpower-density PEFC stack. The remotecontrolled helicopter originally designed to
run on batteries had a rotor diameter of
1.83 m and a GTOM of 10 kg. The duration
of the PEFC-powered flight was
approximately 20 minutes.
The endurance of the test flight was limited
by the relative low hydrogen storage fraction
of 0.023 for the off-the-shelf DOT tank. The
PEFC system had excess power capability
for this application, which could extend the
endurance past 30 minutes by either adding a
second DOT hydrogen tank or acquiring a
larger custom-made lightweight tank.
Advanced batteries are capable of higher
power-to-weight, therefore, for short
endurances of only a few minutes, battery
systems can be lighter and thus carry more
payload.
However, battery systems are energy
limited and only able to provide limited
endurance, even with light payloads. For
payloads less than 1.5 kg, the fuel-cellpowered rotorcraft can provide improved
endurance over an advanced batterypowered version of the same rotorcraft.
This article is based of SAE technical paper 2012-01-2161 by Blake A.
Moffitt and Rachid Zaffou, United Technologies Research Center.

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Vehicle Electrification - February 21, 2013

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