Fuel Cells
333
is either controlled with a back-pressure valve and/or expander or is left
unconstrained in ambient systems. Systems have been designed where the
stack pressure drop is used as a method for controlling the operating pressure. Typically the exhaust air is fed into a tail-gas combustor to serve as the
oxidant. In pressurized systems, either the cathode or combustor exhaust
may be fed to an expander for additional power generation. Common compressor types include twin screw, scroll, centrifugal turbo machines, and
roots blowers. In the system shown in Figure 11.2, discharge air from the
compressor is conditioned to stack temperature with a heat exchanger. As
with the anode, cathode humidification is performed by a deionized, waterfed membrane humidifier.
11.4.1.4 Stack Water and Thermal Management Subsystems
Although a FCS is the most efficient means of converting fuel to energy
onboard a vehicle, it produces considerable amounts of thermal energy.
Thermal loads are of the same magnitude as in conventional automotive
systems but at lower temperatures that require enhanced thermal rejection.
Heat exchangers are employed to carry out several key functions within the
FCS, namely, to reject fuel-cell heat, condition compressor discharge air to
stack temperature, condense water for stack humidification, cool power electronics, and to enhance energy recovery in expanders.
Water management is often reduced to the practice of providing the gasfed humidification required to maintain the performance of the fuel-cell
membrane during its life. Gas-feed humidification has been accomplished
by a number of means, including liquid water-fed membrane humidifiers,
direct cathode water injection, compressor water injection, humidification wheels, porous bipolar plates, and water vapor transport membranes.
Water for humidification is typically collected from the stack at the cathode
exhaust. Depending on pressure and temperature, a cathode exhaust condenser may be required to recover product water to maintain system water
neutrality.
Thermal management must also address the conductive and corrosive
nature of automotive coolants. Stack coolants must be nonconductive, corrosion cum freeze resistant, and should have a high-heat capacity and thermal conductivity without being excessively viscous. Deionized water is a
traditional coolant but glycol/water-based coolants have also been employed
with promise.
The aforesaid requirements present a significant challenge to system design
and hence FCV commercialization. In the system shown in Figure 11.2, the
stack coolant supplies the water and vaporization energy for the anode and
cathode inlet humidifiers. A cathode exhaust condenser is used to supply
make-up water to the humidifier.
333
is either controlled with a back-pressure valve and/or expander or is left
unconstrained in ambient systems. Systems have been designed where the
stack pressure drop is used as a method for controlling the operating pressure. Typically the exhaust air is fed into a tail-gas combustor to serve as the
oxidant. In pressurized systems, either the cathode or combustor exhaust
may be fed to an expander for additional power generation. Common compressor types include twin screw, scroll, centrifugal turbo machines, and
roots blowers. In the system shown in Figure 11.2, discharge air from the
compressor is conditioned to stack temperature with a heat exchanger. As
with the anode, cathode humidification is performed by a deionized, waterfed membrane humidifier.
11.4.1.4 Stack Water and Thermal Management Subsystems
Although a FCS is the most efficient means of converting fuel to energy
onboard a vehicle, it produces considerable amounts of thermal energy.
Thermal loads are of the same magnitude as in conventional automotive
systems but at lower temperatures that require enhanced thermal rejection.
Heat exchangers are employed to carry out several key functions within the
FCS, namely, to reject fuel-cell heat, condition compressor discharge air to
stack temperature, condense water for stack humidification, cool power electronics, and to enhance energy recovery in expanders.
Water management is often reduced to the practice of providing the gasfed humidification required to maintain the performance of the fuel-cell
membrane during its life. Gas-feed humidification has been accomplished
by a number of means, including liquid water-fed membrane humidifiers,
direct cathode water injection, compressor water injection, humidification wheels, porous bipolar plates, and water vapor transport membranes.
Water for humidification is typically collected from the stack at the cathode
exhaust. Depending on pressure and temperature, a cathode exhaust condenser may be required to recover product water to maintain system water
neutrality.
Thermal management must also address the conductive and corrosive
nature of automotive coolants. Stack coolants must be nonconductive, corrosion cum freeze resistant, and should have a high-heat capacity and thermal conductivity without being excessively viscous. Deionized water is a
traditional coolant but glycol/water-based coolants have also been employed
with promise.
The aforesaid requirements present a significant challenge to system design
and hence FCV commercialization. In the system shown in Figure 11.2, the
stack coolant supplies the water and vaporization energy for the anode and
cathode inlet humidifiers. A cathode exhaust condenser is used to supply
make-up water to the humidifier.