Environmental Engineering Reference
In-Depth Information
SPACE SHUTTLE FUEL CELLS
The Space Shuttle Orbiter has three fuel cell power plants onboard
which are all reusable and restartable. The fuel cells are located under
the payload bay area in the forward portion of the orbiter's midfuselage.
The three fuel cells operate as independent electrical power sources, each
supplying its own isolated 28 volt dc bus. The fuel cell consists of a power
section, where the chemical reaction occurs, and an accessory section that
controls and monitors the power section's performance. In the power
section, where hydrogen and oxygen are converted into electrical power,
water and heat, there are 96 cells contained in three substacks. Manifolds
run the length of these substacks and distribute hydrogen, oxygen and
coolant to the cells. The cells contain an electrolyte consisting of potassium
hydroxide and water, an oxygen electrode (the cathode) and a hydrogen
electrode (the anode).
The accessory section monitors the reactant flow, removes waste
heat and water from the chemical reaction and controls the temperature of
the stack. The accessory section consists of the hydrogen and oxygen flow
system, the coolant loop and the electrical control unit.
Oxygen is routed to the cell's oxygen electrode, where it reacts with
the water and returning electrons to produce hydroxyl ions. The hydroxyl
ions migrate to the hydrogen electrode, where they enter into the hydrogen
reaction. Hydrogen is routed to the fuel cell's hydrogen electrode, where it
reacts with the hydroxyl ions from the electrolyte.
The resulting electrochemical reaction produces a flow of electrons to
provide the electrical power along with water and heat. The power is used
by the orbiter's electrical system. The oxygen and hydrogen are consumed
in the reaction in proportion to the orbiter's electrical power demand.
The excess water vapor is removed by an internal circulating
hydrogen system. Hydrogen and water vapor from the reaction exits the
cell stack and is mixed with hydrogen from the storage and distribution
system. It then enters a condenser, where waste heat from the hydrogen
and water vapor is transferred to the fuel cell coolant system. The resulting
temperature drop condenses some of the water vapor to water droplets.
A centrifugal water separator then extracts the liquid water and pressure-
feeds it to potable tanks in the lower deck of the pressurized crew cabin.
Water from the potable water storage tanks is used for crew consumption
and cooling the Freon-21 coolant loops. The remaining circulating
hydrogen is directed back to the fuel cell stack.
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