PEM and SOFC Fuel Options
The fuel cell economy depends a lot on the fuels. Hydrogen is the most abundant element on earth but it mostly exists within other elements so hydrogen fuel must be manufactured by liberating it from other sources. Because hydrogen reacts cleanly with water as the only byproduct, it is seen as a significant alternative fuel of the future. Hydrogen contains the highest energy per weight basis. However, by volume, it contains a very small amount of energy and volume is what counts the most when fuel is stored and transported. Efficiently storing and liberating hydrogen from its many compounds are the primary challenges of using hydrogen as a fuel. Below are the most recognized and used processes. All are viable solutions depending on the application and typical user.
Metal Hydrides
One method for hydrogen storage is in a reversible metal hydride. Metal hydrides are metal alloys that stores hydrogen in a solid form under moderate temperature and pressure. Metal hydride canisters have the advantage of low operating energy, moderate pressure and temperature, and high volumetric density providing about twice the energy of conventional batteries while extending cycle life by 5 to 10 times.
Typical user: Commercial and military applications.
Chemical Hydrides
Hydrogen can also be stored by bonding it chemically to form chemical hydrides. In this case, the hydrogen is generated through a chemical reaction from a hydrogen rich compound. Chemical hydrides have the potential to provide energy that is nearly four times greater than today’s advanced batteries and provide additional volume saving.
Typical user: Commercial and military applications particularly those for portable power, where batteries cannot meet necessary energy requirements.
Fuel cells can also use more conventional hydrocarbon fuels by liberating the hydrogen from other fuel stocks via a reformer. Natural gas, propane and JP-8 are fuels with the most promise in terms of availability and energy density per volume.
Natural Gas
Most of the hydrogen produced today comes from natural gas. Reforming natural gas into hydrogen produces CO2, however, using the hydrogen in a fuel cell produces twice the output energy and half the Co2 as compared to traditional combustion.
Propane
Propane is produced as a by-product of natural gas processing and crude oil refining. The interest in propane as an alternative fuel stems mainly from its
domestic availability, high energy density, and clean-burning qualities. When liquid, propane has an energy density 270 times greater than the gaseous form. When used in a fuel cell system, propane can produce electrical power with an efficiency of at least 3 times that of conventional internal combustion generators, thus providing the user the ability to operate for 3 times as long as with currently available technologies.
Typical user: Due to its availability, natural gas and propane will be used mostly in commercial and consumer applications.
JP 8 (Jet Propulsion Fuel)
JP 8 is a hydrocarbon kerosene-based fuel that has logistics and safety advantages for military applications compared to compressed hydrogen. The energy density of JP8 is much higher compared to lower hydrocarbons (e.g. natural gas). A reforming technology is required to convert JP8 into a hydrogen-rich gas. Fuel cells operating on JP8 fuel will generate electric power at least 3 times the efficiency of conventional generators, lower noise and vibration signatures, and reduce maintenance requirements.
Typical user: Military applications.
