Fuel Cells
335
Moreover, higher temperature enhances stack efficiency through improved
cathode electrocatalyst activity. Higher temperature operation increases water
requirements, influencing water and thermal management. Higher temperature operation also adversely impacts the durability/performance of proton
exchange membrane materials. High pressure, low-cathode stoichiometry
operations facilitate water management by reducing water content required
for membrane and feed gas saturation. High pressure also enhances cell
voltage through reduction in cathode overpotential with increased oxygen
partial pressure and improved mass transport. Conversely, high pressure
increases parasitic compressor power consumption, often as much as 15–20%
of gross stack power. Reductions in cathode stoichiometry reduce humidification requirements and compressor parasitic power but at the cost of cell
voltage and stack gas distribution related stability.
To understand the push to higher temperature PEFCs for FCVs, it is desirable to compare the thermal requirements of FCVs with ICEVs. As a rule of
thumb, one-third of the system energy is removed each by the coolant and
the exhaust in ICEVs. Conversely, in a FCS, typically less than 10% of the
heat is rejected with the exhaust gases. In FCVs, waste heat is approximately
equal to the power generated. Thus, albeit being more efficient than ICEVs,
FCVs have similar or larger, coolant loads. In a PEFC stack, coolant temperature is significantly lower, typically 60–80°C, as compared to 120°C in ICEVs.
Since radiator performance is proportional to the initial temperature-difference, the driving force between the coolant and ambient temperature in an
ICEV has an approximately 2–4 times higher heat rejection capability during operation at elevated temperatures. Even a modest sized, FCS requires
a cooling system that taxes the bounds of normal automotive design. While
heat exchange does not present an insurmountable technical obstacle, considerable work remains to be done in optimizing thermal management hardware for FCSs.
Stack temperature and operating pressure are linked to the PEFC’s
water requirement. At elevated temperatures, the water feed required to
achieve saturation increases. At higher temperatures and lower pressures,
the humidification energy duty and associated condensate water requirement may affect the water and thermal management capabilities of an
automotive thermal system. Increased system pressure can offset the water
requirement but increased complexity and cost potentially reduce the efficiency. Alternative humidification schemes, such as water vapor transport
membranes or adsorption/desorption in a desiccant wheel, are promising.
It is hoped that PEFC development will lead to higher temperature, lower
humidification materials circumventing the need to transport large quantities of water through the system.
Presently, packaging constraints imposed by the system thermal requirements are a limiting step. Accordingly, either novel heat rejection concepts need
to be employed or stack temperatures need to be increased. Humidification
loads will also increase assuming the proton exchange mechanism to be
335
Moreover, higher temperature enhances stack efficiency through improved
cathode electrocatalyst activity. Higher temperature operation increases water
requirements, influencing water and thermal management. Higher temperature operation also adversely impacts the durability/performance of proton
exchange membrane materials. High pressure, low-cathode stoichiometry
operations facilitate water management by reducing water content required
for membrane and feed gas saturation. High pressure also enhances cell
voltage through reduction in cathode overpotential with increased oxygen
partial pressure and improved mass transport. Conversely, high pressure
increases parasitic compressor power consumption, often as much as 15–20%
of gross stack power. Reductions in cathode stoichiometry reduce humidification requirements and compressor parasitic power but at the cost of cell
voltage and stack gas distribution related stability.
To understand the push to higher temperature PEFCs for FCVs, it is desirable to compare the thermal requirements of FCVs with ICEVs. As a rule of
thumb, one-third of the system energy is removed each by the coolant and
the exhaust in ICEVs. Conversely, in a FCS, typically less than 10% of the
heat is rejected with the exhaust gases. In FCVs, waste heat is approximately
equal to the power generated. Thus, albeit being more efficient than ICEVs,
FCVs have similar or larger, coolant loads. In a PEFC stack, coolant temperature is significantly lower, typically 60–80°C, as compared to 120°C in ICEVs.
Since radiator performance is proportional to the initial temperature-difference, the driving force between the coolant and ambient temperature in an
ICEV has an approximately 2–4 times higher heat rejection capability during operation at elevated temperatures. Even a modest sized, FCS requires
a cooling system that taxes the bounds of normal automotive design. While
heat exchange does not present an insurmountable technical obstacle, considerable work remains to be done in optimizing thermal management hardware for FCSs.
Stack temperature and operating pressure are linked to the PEFC’s
water requirement. At elevated temperatures, the water feed required to
achieve saturation increases. At higher temperatures and lower pressures,
the humidification energy duty and associated condensate water requirement may affect the water and thermal management capabilities of an
automotive thermal system. Increased system pressure can offset the water
requirement but increased complexity and cost potentially reduce the efficiency. Alternative humidification schemes, such as water vapor transport
membranes or adsorption/desorption in a desiccant wheel, are promising.
It is hoped that PEFC development will lead to higher temperature, lower
humidification materials circumventing the need to transport large quantities of water through the system.
Presently, packaging constraints imposed by the system thermal requirements are a limiting step. Accordingly, either novel heat rejection concepts need
to be employed or stack temperatures need to be increased. Humidification
loads will also increase assuming the proton exchange mechanism to be