Page 272 - Alternative Energy Systems in Building Design

248 FUEL CELL TECHNOLOGY

Figure 7.3 Operational diagram of a fuel cell. Courtesy of Ballard Engineering.

Water and air control In hydrogen and hydrant-type fuel cells, owing to the pro-
duction of water, the membrane must be dehydrated by means of precise evaporation at
the same rate at which water is produced. If water evaporates too quickly, the membrane
could dry and eventually crack, creating a gas “short circuit” where hydrogen and oxygen
could combine directly, generating excessive heat that will damage the fuel cell.
If the water evaporates too slowly, the electrodes could flood, preventing the reactants
from reaching catalytic reaction level. One methods used to control water in fuel cells
involves an electroosmotic pump that maintains a steady ratio between the amount of
reactants and the oxygen necessary to keep the fuel cell operating efficiently.

Temperature control Another challenge posed in the design of fuel cells is the
removal of large amounts of heat resulting from the exothermic reactions. If the combi-
nation of oxygen and hydrogen molecules (2H + O → 2H O) is not controlled, destruc-
2
2
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tion of the cell from thermal loading may occur. To prevent excessive thermal loading, the
temperature must be maintained at acceptable levels throughout fuel cell operation.
DURABILITY AND SERVICE LIFE OF FUEL CELLS
In stationary applications, when the power generated by fuel cells ranges in the hun-
dreds of kilowatts, life expectancies typically are required to exceed 40,000 hours of
reliable operation at temperatures of –35 to 40°C. In the automotive industry, fuel cells
are required to have a life span of 5000 hours, which is the equivalent of 150,000 miles
under extreme temperatures. Automotive engines are also required to start reliably at
–30°C. Table 7.1 outlines various types of fuel cell characteristics.