Fuel Cells
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using a high thermal capacity coolant and a radiator akin to ICEs. The
removed heat can also be used to heat the vehicle compartment.
As an alternative to the direct hydrogen supply to the fuel cell stack, a fuel
processor can be incorporated onboard the vehicle to produce a hydrogenrich gas stream from a liquid fuel such as methanol or gasoline. Although
major technical advances for onboard vehicle applications are still desired,
the concept is well proven and has been tested on a small size passenger
vehicle using methanol as the fuel. One of the most significant drawbacks of
the onboard fuel processors in vehicular applications, besides the packaging
issues, is the system time constant; slow dynamic response is mainly due to
the slow heat transfer processes, mass transfer, and mixing delays.
Three major subsystems currently requiring improvements are the air supply, thermal management, and water management. The most efficient operating pressure for current PEFC systems is about 207 kPa. This requires an air
supercharging mechanism at the cost of parasitic loads. Current superchargers used in FCSs have excessive parasitic loads, approaching 10–15 kW at a
peak power of 50 kW net, and are heavy, big, and costly. As pressure drops in
most of the stacks are only of the order of several psi, significant energy is still
contained in the cathode exhaust-gas. To recover this energy and to reduce
the total parasitic load on the supercharger, expanders are being developed as
part of the air supply subsystem. The present air supply subsystem requires
further improvement for successful applications of fuel cells in automotives.
Thermal management subsystems also require innovation and improvement. Two major difficulties facing thermal management are: (a) the large
radiator and fan required for removing the low-temperature waste heat from
the stack and (b) development of nonconducting coolants capable of operating at subfreezing temperatures. Due to the low-stack temperature of about
90°C at 3 atm operation, the radiator size for a 60 kW net power system will
be 1.5 times larger than that for an ICEV.
The automotive market will require higher temperatures and drier operating conditions (Beuscher, Cleghorn, and Johnson 2005). This requirement
is driven by the available radiator technology used to transport excess heat
away from the fuel cell stack (Oosterkamp et al. 2006). To maintain the current size of automotive radiators in PEFC vehicles, temperatures above 100°C
are desired for the fuel cell stack to increase the heat transfer efficiency of the
radiator. An additional advantage of higher temperatures is lower sensitivity to fuel impurities, particularly CO. System size and weight constraints
associated with water management require lower humidity conditions.
Operation under automotive drive cycle, with frequent stops and starts
including freeze tolerance, makes materials selection for FCVs even more
difficult. Proposed future operating conditions are challenging for current
PEFC membrane materials. High temperature and low humidity have detrimental effects on both performance and durability of currently used membranes. New membrane materials must be developed to reach the targets set
by the fuel-cell industry. To this end, the ionomer needs to be optimized to
331
using a high thermal capacity coolant and a radiator akin to ICEs. The
removed heat can also be used to heat the vehicle compartment.
As an alternative to the direct hydrogen supply to the fuel cell stack, a fuel
processor can be incorporated onboard the vehicle to produce a hydrogenrich gas stream from a liquid fuel such as methanol or gasoline. Although
major technical advances for onboard vehicle applications are still desired,
the concept is well proven and has been tested on a small size passenger
vehicle using methanol as the fuel. One of the most significant drawbacks of
the onboard fuel processors in vehicular applications, besides the packaging
issues, is the system time constant; slow dynamic response is mainly due to
the slow heat transfer processes, mass transfer, and mixing delays.
Three major subsystems currently requiring improvements are the air supply, thermal management, and water management. The most efficient operating pressure for current PEFC systems is about 207 kPa. This requires an air
supercharging mechanism at the cost of parasitic loads. Current superchargers used in FCSs have excessive parasitic loads, approaching 10–15 kW at a
peak power of 50 kW net, and are heavy, big, and costly. As pressure drops in
most of the stacks are only of the order of several psi, significant energy is still
contained in the cathode exhaust-gas. To recover this energy and to reduce
the total parasitic load on the supercharger, expanders are being developed as
part of the air supply subsystem. The present air supply subsystem requires
further improvement for successful applications of fuel cells in automotives.
Thermal management subsystems also require innovation and improvement. Two major difficulties facing thermal management are: (a) the large
radiator and fan required for removing the low-temperature waste heat from
the stack and (b) development of nonconducting coolants capable of operating at subfreezing temperatures. Due to the low-stack temperature of about
90°C at 3 atm operation, the radiator size for a 60 kW net power system will
be 1.5 times larger than that for an ICEV.
The automotive market will require higher temperatures and drier operating conditions (Beuscher, Cleghorn, and Johnson 2005). This requirement
is driven by the available radiator technology used to transport excess heat
away from the fuel cell stack (Oosterkamp et al. 2006). To maintain the current size of automotive radiators in PEFC vehicles, temperatures above 100°C
are desired for the fuel cell stack to increase the heat transfer efficiency of the
radiator. An additional advantage of higher temperatures is lower sensitivity to fuel impurities, particularly CO. System size and weight constraints
associated with water management require lower humidity conditions.
Operation under automotive drive cycle, with frequent stops and starts
including freeze tolerance, makes materials selection for FCVs even more
difficult. Proposed future operating conditions are challenging for current
PEFC membrane materials. High temperature and low humidity have detrimental effects on both performance and durability of currently used membranes. New membrane materials must be developed to reach the targets set
by the fuel-cell industry. To this end, the ionomer needs to be optimized to