Fuel cell systems generate electricity not from the combustion process typical of auto engines and most giant power plants, but through an electrochemical reaction in which oxygen from air and a hydrogen-rich fuel combine to form electricity, with heat and water as byproducts. Batteries are another electrochemical process, but they only store a finite amount of energy. Fuel cells, in contrast, produce electricity continuously as long as fuel is supplied. In the case of high temperature fuel cells, they are not limited to pure hydrogen, and operate on readily available hydrocarbon fuels (e.g., natural gas, diesel, kerosene, propane, military JP-8, and renewable fuels such as ethanol, vegetable and plant oils, digester biogas (methane), ammonia, and biodiesel). They also produce waste heat useable for cogeneration (heat supply).
Individual fuel cells have two electrodes (the anode and the cathode) and an electrolyte. These are bipolar like batteries and can be stacked in multiples to increase the output of electricity. Systems that operate on ordinary fuels require a fuel reformer to convert hydrocarbon fuel into syngas, a hydrogen-rich gas which a fuel cell combines with oxygen to make electricity. Thermal integration of a fuel reformer and fuel cell stack is critical to achieving competitive system performance and operating costs.To ensure that a system is stable and reliable under all operating conditions, a complete fuel cell system must include components to ensure that, first, fuel, air, and water are available in a suitable form; and second, that electricity, heat, air and water are routed appropriately, using what the industry refers to as the balance of plant (BOP). Examples of these components include fuel and air management, inverters for DC-AC power conversion, electronic controls, and subsystems for heat recovery and water routing.