336
Alternative Fuels for Transportation
constrained by current material characteristics are within the fuel cell stack.
These will require the stacks with high temperature MEAs at reduced humidification, and the system and application side where emphasis needs to be
placed on novel heat exchanger designs and development of advanced fuelcell air machinery. Fuel-cell specific membranes, compressors, expanders,
and humidification components are all in development. In the past decade
alone, the power density of the fuel cell stacks has increased by a factor of 10.
With the rapid pace of technological innovation and advancement in fuel-cell
materials, more elegant system solutions are expected to emerge.
11.5 Fuel-Cell Vehicles Versus Hybrid Vehicles
It is documented that PEFC-Ni/MH and PEFC-Li-ion battery powered
hybrid cars are more efficient than the FCVs (Britsche and Gutmann 2004).
Demirdoven and Deutch (2004) have stressed on the priority to deploy hybrid
cars against FCVs. Unlike ICEs, FCS exhibit higher efficiency at part loads.
This is particularly advantageous for automotive applications since the vehicles are mostly operated under part load conditions; the average demand on
standard U.S. drive cycles is <20% of the rated power of the engine. A recent
study suggests that the fuel economy of hydrogen FCVs could be ~three
times the fuel economy of the ICEVs (Ahluwalia et al. 2003).
Since the fuel cells are more efficient at part load than at full rated
power, hybridizing a FCV is different from hybridizing the ICEVs. One
motivation for hybridizing the FCVs is to improve their fuel economy
by recovering a portion of the braking energy. Hybridization can also
help if the energy storage device has higher specific power (kWkg –1 ) at
lower cost ($kW –1 ) in relation to the FCS to make the hybrid system lighter
and less expensive. Owing to the higher part load efficiency, even in a
hybrid configuration, it appears advantageous to preferentially operate
the FCS in a load-following mode and to use the power from the battery
on demand.
The automotive industry is pinning its hopes on two major automotive
technologies; namely, HEVs and FCVs. Fuel-cell system design is heavily
influenced by automotive requirements that are generally application specific. Engine peak-power is usually determined by the degree of acceleration, grade, or top speed performance targets. Maximum rated power may
be either a continuous requirement or for a finite duration lasting tens of
seconds. For the latter, hybridization, combining a down sized FCS designed
for continuous maximum-power with an energy buffer, typically a battery
bank, to augment the peak power may be an option. To date, early prototype light-duty passenger vehicles are designed between 50 and 80 kW, both
with and without hybridization. Full size bus applications are also focused
Alternative Fuels for Transportation
constrained by current material characteristics are within the fuel cell stack.
These will require the stacks with high temperature MEAs at reduced humidification, and the system and application side where emphasis needs to be
placed on novel heat exchanger designs and development of advanced fuelcell air machinery. Fuel-cell specific membranes, compressors, expanders,
and humidification components are all in development. In the past decade
alone, the power density of the fuel cell stacks has increased by a factor of 10.
With the rapid pace of technological innovation and advancement in fuel-cell
materials, more elegant system solutions are expected to emerge.
11.5 Fuel-Cell Vehicles Versus Hybrid Vehicles
It is documented that PEFC-Ni/MH and PEFC-Li-ion battery powered
hybrid cars are more efficient than the FCVs (Britsche and Gutmann 2004).
Demirdoven and Deutch (2004) have stressed on the priority to deploy hybrid
cars against FCVs. Unlike ICEs, FCS exhibit higher efficiency at part loads.
This is particularly advantageous for automotive applications since the vehicles are mostly operated under part load conditions; the average demand on
standard U.S. drive cycles is <20% of the rated power of the engine. A recent
study suggests that the fuel economy of hydrogen FCVs could be ~three
times the fuel economy of the ICEVs (Ahluwalia et al. 2003).
Since the fuel cells are more efficient at part load than at full rated
power, hybridizing a FCV is different from hybridizing the ICEVs. One
motivation for hybridizing the FCVs is to improve their fuel economy
by recovering a portion of the braking energy. Hybridization can also
help if the energy storage device has higher specific power (kWkg –1 ) at
lower cost ($kW –1 ) in relation to the FCS to make the hybrid system lighter
and less expensive. Owing to the higher part load efficiency, even in a
hybrid configuration, it appears advantageous to preferentially operate
the FCS in a load-following mode and to use the power from the battery
on demand.
The automotive industry is pinning its hopes on two major automotive
technologies; namely, HEVs and FCVs. Fuel-cell system design is heavily
influenced by automotive requirements that are generally application specific. Engine peak-power is usually determined by the degree of acceleration, grade, or top speed performance targets. Maximum rated power may
be either a continuous requirement or for a finite duration lasting tens of
seconds. For the latter, hybridization, combining a down sized FCS designed
for continuous maximum-power with an energy buffer, typically a battery
bank, to augment the peak power may be an option. To date, early prototype light-duty passenger vehicles are designed between 50 and 80 kW, both
with and without hybridization. Full size bus applications are also focused