A fuel cell is a device that produces electricity through a chemical reaction from combining a fuel, usually hydrogen, with oxygen. In this reaction, electrons are freed from the hydrogen in the fuel cell by a catalyst, and gain energy from the chemical reaction binding hydrogen and oxygen. This provides a source for electrical current. The exhaust of hydrogen fuel cells consists simply of water, thus considerably reducing carbon emissions.

Fuel cells are currently used in spacecraft, and increasingly in ground transportation, with potential use everywhere electricity is required. Fuel cell systems can be differentiated into High Temperature Fuel Cells which operate at temperatures greater than 200°C, and Low Temperature Fuel Cell which operate at temperatures below 200°C.

High Temperature Fuel Cells have a shorter start up time and fast electrical response but require an expensive platinum-based catalyst. The difference in the physical and performance properties for fuel cells, which operate at either high or low temperatures, helps tailor their suitability for given applications.

There is a range of potential applications around public transport estates where fuel cells could play a role:

• Standby power: Diesel generators are often employed in a number of back-up power applications. There is interest in using fuel cells for on-site generation in a peak-shaving mode (i.e. to reduce the import of grid electricity during peak tariff periods)
• Uninterruptible power systems (UPS): A UPS is a power supply that includes a battery to maintain power in the event of a power outage. There are a number of potential applications for fuel cells to supply for example data centres
• Combined Heat and Power (CHP): Public transport buildings such as workshops, offices, stations and interchanges demand substantial heating, cooling and electricity. Sites possessing year-round thermal and electrical baseloads would be ideal applications for a small scale CHP fuel cell.