Fuel cells can generate power from a fraction of Watt to hundreds of kilowatts. Because of that, they may be used in almost every application where local electricity generation is needed. Applications such as automobiles, buses, utility vehicles, scooters, bicycles, and submarines have been already demonstrated. Fuel cells are ideal for distributed power generation at the level of individual home, building, or community, offering tremendous flexibility in power supply. In some cases both power and heat produced by a fuel cell may be utilized resulting in very high overall efficiency. As a backup power generator, fuel cells offer several advantages over either internal combustion engine generators (noise, fuel, reliability, maintenance) or batteries (weight, lifetime, maintenance). Small fuel cells are attractive for portable power applications, either as replacement for batteries (in various electronic devices and gadgets) or as portable power generators.

Fuel cell system design is not necessarily the same for each of these applications. On the contrary, each application, in addition to power output, has its own specific requirements, such as efficiency, water balance, heat utilization, quick start-up, long dormancy, size, weight, and fuel supply. Some of those application-specific requirements and corresponding design variations will be discussed in this chapter.