NASA - National Aeronautics and Space Administration

Fuel cells are used in the space shuttle as one component of the electrical power system. Three fuel cell power plants, through a chemical reaction, generate all of the electrical power for the vehicle from launch through landing rollout.

Before launch, electrical power is provided by ground power supplies and the onboard fuel cell power plants. Each fuel cell power plant consists of a power section, where the chemical reaction occurs, and a compact accessory section attached to the power section, which controls and monitors the power section's performance. This is one of the three fuel cells that make up the generating system that provides electrical power to the space shuttle orbiter. The unit is a little more than a foot high and weighs approximately 200 pounds. Credit: NASA

The three fuel cell power plants are individually coupled to the reactant (hydrogen and oxygen) distribution subsystem, the heat rejection subsystem, the potable water storage subsystem, and the electrical power distribution and control subsystem. The fuel cell power plants generate heat and water as by-products of electrical power generation.

The excess heat is directed to fuel cell heat exchangers, where the excess heat is rejected to Freon coolant loops. The water is directed to the potable water storage subsystem.

More Information:

Read More About the Shuttle's Fuel Cell Power Plants at:
http://spaceflight.nasa.gov/shuttle/reference/shutref/orbiter/eps/pwrplants.html

The above excerpt was taken from NASA's Human Space Flight web site. http://spaceflight.nasa.gov/shuttle/reference/shutref/orbiter/eps/overview.html