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Alternative Fuels for Transportation
The lifetime of the PEM fuel cell generator is limited by membrane
degradation, it was limited to 1500 hours with last year’s technology; today
it seems possible to reach a few 1000 hours. However the reliability required
by railway applications is over 100,000 hours of working time, and for a tramway the stop–start cycle between stations is over 5 million (RAPSDRA 1995;
cycling time about 1 minute). Only PAFC reaches 40,000 hours, corresponding perhaps to 1,000,000 km, but at the moment they are used only for stationary applications.
9.9 Fuel Cell Vehicles Hybridization
Fuel cell generators are not well adapted to power variation. Research projects to introduce FC in mobile applications include battery energy storage to
deliver the traction power in combination with FC generators. Briefly, a storage unit can be employed together with FCs in order to:
• Reduce the size of the FC, the most expensive component, thus
reducing the drivetrain total cost. Projected cost of automotive storage systems is lower, even in case of mass production, than fuel
cells projected cost. Thus it is easy to predict that, for a given vehicle
performance, a hybrid power train will be cheaper than a pure fuel
cell, especially in those applications (urban cycles) where the ratio of
peak/average power is higher.
• Reduce power transients, not to reduce emissions as in motor generators, but to simplify the “balance of plant” configuration and to
enhance its efficiency: mainly in the case of FC vehicles with fuels
other than hydrogen, because the time constant of some subsystems (typically fuel processors) are much longer than required by
the driving cycle. In this case an electrical storage system acts like a
buffer during peak power, making a vehicle comfortably drivable.
• Reduce start-up power transients that may be important depending on the FC system configuration, for example, direct hydrogen
or methanol powered. Indeed, in case of cold start-up, the on board
energy storage can be used also to accelerate system heating and to
move the vehicle throughout this phase.
• Recover braking energy that moreover is produced and available
in the form of electricity (otherwise this energy will be dissipated
thermally). The amount of energy saved just with braking energy
recovering is in the range of 3.5–20%, enough to justify alone the
realization of a hybrid system.
Alternative Fuels for Transportation
The lifetime of the PEM fuel cell generator is limited by membrane
degradation, it was limited to 1500 hours with last year’s technology; today
it seems possible to reach a few 1000 hours. However the reliability required
by railway applications is over 100,000 hours of working time, and for a tramway the stop–start cycle between stations is over 5 million (RAPSDRA 1995;
cycling time about 1 minute). Only PAFC reaches 40,000 hours, corresponding perhaps to 1,000,000 km, but at the moment they are used only for stationary applications.
9.9 Fuel Cell Vehicles Hybridization
Fuel cell generators are not well adapted to power variation. Research projects to introduce FC in mobile applications include battery energy storage to
deliver the traction power in combination with FC generators. Briefly, a storage unit can be employed together with FCs in order to:
• Reduce the size of the FC, the most expensive component, thus
reducing the drivetrain total cost. Projected cost of automotive storage systems is lower, even in case of mass production, than fuel
cells projected cost. Thus it is easy to predict that, for a given vehicle
performance, a hybrid power train will be cheaper than a pure fuel
cell, especially in those applications (urban cycles) where the ratio of
peak/average power is higher.
• Reduce power transients, not to reduce emissions as in motor generators, but to simplify the “balance of plant” configuration and to
enhance its efficiency: mainly in the case of FC vehicles with fuels
other than hydrogen, because the time constant of some subsystems (typically fuel processors) are much longer than required by
the driving cycle. In this case an electrical storage system acts like a
buffer during peak power, making a vehicle comfortably drivable.
• Reduce start-up power transients that may be important depending on the FC system configuration, for example, direct hydrogen
or methanol powered. Indeed, in case of cold start-up, the on board
energy storage can be used also to accelerate system heating and to
move the vehicle throughout this phase.
• Recover braking energy that moreover is produced and available
in the form of electricity (otherwise this energy will be dissipated
thermally). The amount of energy saved just with braking energy
recovering is in the range of 3.5–20%, enough to justify alone the
realization of a hybrid system.