Trends in Transportation - Hydrogen and Fuel Cells: Advances in Transportation and Power - page 153

The fuel cell coolant system uses a liquid fluorinated hydrocarbon
and transfers the waste heat from the cell stack through the fuel cell heat
exchanger of the fuel cell power plant to the Freon-21 coolant loop system
in the midfuselage. Internal control of the circulating fluid keeps the cell
stack at an operating temperature of approximately 200°F.
As the reactants enter the fuel cells, they flow through a preheater to
warm them from a cryogenic temperature to 40 degrees F or more. They
are also sent through a 6-micron filter and a two-stage, integrated dual gas
regulator module. The first stage of the regulator reduces the pressure of
the hydrogen and oxygen to 135-150 psia. The second stage reduces the
oxygen pressure to a range of 62-65 psia and keeps the hydrogen pressure
at 4.5-6 psia differential below the oxygen pressure. The regulated oxygen
lines are connected to an accumulator, which maintains an equalized
pressure between the oxygen and the fuel cell coolant. If the oxygen's and
hydrogen's pressure decreases, the coolant's pressure is also decreased to
prevent a large differential pressure inside the stack from deforming the
cell stack structural elements.
On leaving the dual gas regulator module, the incoming hydrogen
mixes with the hydrogen-water vapor exhaust from the fuel cell stack. This
saturated gas mixture is sent through a condenser, where the temperature
of the mixture is reduced, which condenses part of the water vapor to
form liquid water droplets. The liquid water is then separated from the
hydrogen-water mixture by the hydrogen pump/water separator.
A hydrogen pump circulates the hydrogen gas back to the fuel cell
stack, where some of the hydrogen is consumed in the reaction. The rest
flows through the fuel cell stack, removing the water vapor formed at the
hydrogen electrode. The hydrogen-water vapor mixture then combines
with the regulated hydrogen from the dual gas generator module, and the
loop starts again.
The oxygen from the dual gas regulator module flows directly
through two ports into a closed-end manifold in the fuel cell stack to
provide optimum oxygen distribution in the cells. All the oxygen that
flows into the stack is consumed, except during purge operations.
Fuel cells can also convert the energy in waste gases from water
treatment plants to electricity. In the future, fuel cells could be in automobiles
while allowing homeowners to generate electricity in garages.
Electric cars have proved less attractive to the mass market. The worry
of having to plug them in to recharge them is combined with the dread of
coming to a sudden halt in an inconvenient place when the batteries run