Page 308 - Biofuels Refining and Performance

Fuel Cells 287

9.4.2 Air management
Besides fuel, a fuel cell also requires an oxidant (usually air). Depending
on the application and design, air provided to the fuel cell cathode can
be at a low pressure or a high pressure. High pressure of the air improves
the reaction kinetics and increases the power density and efficiency of
the stack. But increasing the air pressure reduces the water-holding
capacity of the air and therefore reduces the humidification require-
ments of the membrane (PEMFC). It also increases the power required
to compress the air to a high pressure and thereby reduces the net
power available. At present, most fuel cell stacks for stationary power
applications are designed for operating pressures in the range of 1–8
atm, while automotive fuel cell systems based on the PEMFC technol-
ogy are designed to operate at lower pressures of 2–3 atm to increase
power density and improve water management.

9.4.3 Water management
Water management is critical for fuel cell operation. Water is a product
of the fuel cell reaction, and it must be removed from the exhaust gas
for use in various operations such as fuel reformation and humidifying
reactant gases (to avoid drying out the fuel cell membrane). For auto-
motive applications, water condensed from the exhaust steam is recy-
cled for reforming and reactant humidification in a closed cycle to avoid
periodical recharging with water.

9.4.4 Thermal management
The reaction products of the electrochemical reaction in a fuel cell are
water, electricity, and heat. The heat energy released in a fuel cell stack
is approximately equal to the electrical energy generated and must be
managed properly to maintain the fuel cell stack temperature at the
optimal level. If this thermal energy (waste heat) is properly utilized,
it will considerably increase the efficiency of a fuel cell system. In low-
temperature (<200 C) fuel cells (PEMFC, AFC, and PAFC), the stack is
cooled by supplying excess air in low power (<200-W) systems, whereas a
liquid coolant (deionized water) is used for large-size systems. The waste
heat carried out by the coolant is utilized for cogeneration (space heat-
ing, water heating, etc.). In high-temperature (<600 C) fuel cell (MCFC
and SOFC) systems, all the heat of reaction is transferred to the reac-
tants to maintain the stack temperature at the optimal level. The ther-
mal energy of the high-temperature exhaust may be utilized to preheat
the incoming air stream, or in internal or external fuel reformer. The
high-temperature exhaust may also be used for cogeneration or elec-
tricity generation in a downstream gas turbine system.