There are five different types of fuel cell technology. A fuel cell type is determined by the composition of the electrolyte in the middle of the fuel cell. UTC Power has experience in all five areas. An explanation of each type of fuel cell and its attributes are below:
Phosphoric-acid fuel cells (PAFCs): Phosphoric acid fuel cells use liquid phosphoric acid as the electrolyte. The PureCell® 200 power plant, produced since 1991, is a PAFC power plant. The PureCell® 200 is highly efficient - total efficiency of 85 percent is achievable when waste heat produced by the fuel cell is used for co-generation. PAFC power plants are usually large and heavy and require warm-up time. Because of this, PAFCs are used mainly for stationary applications.
Proton Exchange Membrane fuel cells: PEM fuel cells, also known as polymer electrolyte fuel cells, are a type of fuel cell currently under development at most fuel cell companies. PEM fuel cells use a thin solid membrane as an electrolyte. These fuel cells deliver high power density and offer the advantages of low weight and volume, compared to other fuel cells. These fuel cells also operate at relatively low temperatures, around 175°F. Low temperature operation allows them to start quickly (less warm-up time), which makes them particularly well suited for transportation applications such as automobiles and fleet vehicles.
Alkaline fuel cells (AFCs): One of the oldest fuel cell types, alkaline fuel cell power plants were initially developed by UTC Fuel Cells, a unit of UTC Power, for the Apollo missions. An updated version was produced and is still in use to provide electrical power in today's Space Shuttle fleet. These power plants use potassium hydroxide. AFCs are very efficient, reaching efficiencies of 60 percent in space applications. However, AFC are susceptible to carbon contamination, so they require pure hydrogen and oxygen. Because of the requirement of pure fuels, terrestrial applications are limited.
Solid Oxide (SOFCs): Solid oxide fuel cells use a hard, non-porous ceramic compound as the electrolyte. SOFCs operate at very high temperatures — around 700 - 800°C. As a result they are highly efficient when their heat is recaptured for co-generation purposes, resulting in efficiencies of 65 - 70 percent. Size, heat output and a long start-up time make this fuel cell more suitable for stationary applications. SOFCs are at a relatively early stage of development compared to the other fuel cell technologies. United Technologies Research Center, the research arm of United Technologies Corp., is currently developing this technology.
Molten carbonate fuel cell: Molten carbonate fuel cells use an electrolyte composed of a molten carbonate salt mixture suspended in a porous, chemically inert ceramic lithium aluminum oxide (LiAlO2) matrix. These systems are large and operate at very high temperatures (in the range of 1,200°F). They are very efficient when the heat produced is used for co-generation. However, because MCFC fuel cells use a corrosive electrolyte, their durability is limited.
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