Conventional power plants convert chemical energy into electrical energy in three steps:
- Production of heat by burning fuel
- Conversion of heat into mechanical energy
- Conversion of mechanical energy into electrical energy
The efficiency of the second step is limited (by the Second Law of Thermodynamics) to the Carnot efficiency, since the conversion of heat into mechanical energy occurs in a closed-cycle heat engine. An efficiency of about 41% can be reached by modern systems. A fuel cell is an electrochemical device that converts the chemical energy in fuels (e.g. hydrogen, methane, butane or even gasoline and diesel) into electrical energy. It exploits the natural tendency of oxygen and hydrogen to react to form water. The direct reaction is prevented by the electrolyte, which separates the two reactants. Therefore two half-reactions occur at the electrodes:
The ions are transported to the other electrode through the electrolyte. The fuel cell contains no moving parts and only four active elements: cathode, anode, electrolyte and interconnect; it is a simple and robust system. Fuel cells have a number of advantages compared to conventional electricity generation:
- Negligible air pollution (if fossil fuels are used, otherwise none)
- Reduced weight, especially in mobile applications
- 100% theoretical efficiency, 80% efficiency in high temperature turbine hybrid systems, that can use the generated heat
High efficiency in low power systems
- Constant efficiency at low load
- Flexible output with fast adjustment
- Low maintenance cost and very few moving parts (or none)
- Quiet or completely silent
Fuel cells have many interesting applications. This short video shows a demonstration fuel cell car. Note that hydrogen and oxygen being used up by the reactions.
Note: This animation requires Adobe Flash Player 8 and later, which can be downloaded here.