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Power Conversion and Control for Fuel Cell Systems in Transportation 301

12.5.1 Fuel Cell for Propulsion Systems
A fuel cell system designed for propulsion applications should match the weight, volume, power
density, start-up, and transient response with present-day ICE-based gasoline vehicles. Other
requirements are high performance over a short start-up time, better fuel economy, fast acceleration,
and the meeting of all the safety requirements. Expected lifetime and cost are also major consid-
erations [7]. A FCV drivetrain system usually consists of a fuel cell stack, power conditioner (or
DC–DC converter) to convert the variable output voltage of fuel cell stack to a fixed DC voltage,
propulsion inverter to obtain variable voltage and variable frequency AC power, propulsion motor,
and a transmission to transmit the power from the electric motor to the wheels as shown in Figure
12.6 [5, 7]. The fuel cell stack for propulsion application in the majority of vehicles is based on
PEMFC. The hydrogen fuel input to the fuel cell can be obtained from (1) electrolysis of water and
storage of hydrogen in pressurized cylinders, (2) metal hydrides such as sodium aluminum hydride
and lithium aluminum hydride, and (3) reformation of liquid fuels such as gasoline, methanol, and
other hydrocarbon-based fuels. Most of the early fuel cell demonstration vehicles were based on the
onboard reformer for producing hydrogen to fuel the fuel cell stack. Because of the reformer’s addi-
tional weight and volume, these vehicles were not efficient and had a poor performance. Automotive
manufacturers are presently focusing on direct hydrogen-based FCVs instead of onboard reformer-
based vehicles.
A battery is generally connected in parallel with the fuel cell system to enable the most efficient
usage of high power density of the battery and the inherently high energy density of the fuel cell.
During high power demand such as acceleration, batteries will supply the required power. During
normal driving operation such as cruising, the fuel cell supplies the required power. During low
power demand, batteries will be recharged. Therefore, based on the energy and power requirements,
the fuel cell and battery could be designed to supply cruising power and peak power, respectively.
The battery also assists in the rapid start-up of the fuel cell and protects it against cell reversal dur-
ing this operation. In addition, the battery supplies the peak power and enables the vehicle’s system
to respond faster for load changes and to capture the regeneration energy. Several other benefits of
using the battery are that the vehicle can be started without the preheating requirement of the fuel
cell and can be operated in pure electric mode until the fuel cell reaches its nominal output volt-
age level. The battery technology that has been found in most of the present-day FCVs is based on
lithium ion because of its high energy density, high power density, life cycle, etc.
Propulsion motor determines the propulsion system characteristics of the vehicle, operation of
the motor controller, and the ratings of the semiconductor devices of the power converters. The
main requirements for a propulsion motor are ruggedness, high torque-to-inertia ratio, high torque
density, wide speed range, low noise, little or no maintenance, small size, ease of control, and low
cost. Most of the present-day electric, hybrid, and fuel cell vehicles are based on the permanent

Power conditioner Electric
Fuel cell DC/DC motor
unit CAP converter CAP Inverter

Converter and inverter
Fuel cell control
controller

Control

FIGURE 12.6 A fuel cell propulsion system for an automobile.