Fuel cells have long been used in the space program to provide electricity and drinking water for the astronauts. Terrestrial applications can be classified into categories of portable, stationary, or transportation power uses.
Fuel cells can power virtually anything that runs on electricity. Although a few products are ready for the market today, many more are on the verge of commercial production. Early products introduced to the market will include portable fuel cells to replace batteries in computers, wheel chairs and cell phones. A few larger stationary fuel cells are already available today, and they show tremendous potential for providing cost-effective distributed power, even in urban environments. Soon too, vehicles will run on clean fuel cells, as reflected in recent announcements by the President of the United States and major carmakers. [image: a fuel cell powered hydrogen sensing device.]
Since the invention of the "gas battery" by Robert Grove in 1839, several different fuel cell types have been developed. Each type has operating characteristics and temperatures that are suitable for certain applications but not for others. Fuel cell technologies are named for their electrolyte since the electrolyte defines the key properties, particularly operating temperature, of the fuel cell.
Phosphoric Acid Fuel Cells (PAFC) are the most commercially developed type of fuel cell and are in use in over 200 public buildings around the world. PAFCs use a phosphoric acid electrolyte contained in a Teflon-bonded silicone-carbide matrix and at 400º Fahrenheit, they operate at 36% efficiency for stand-alone and 85% efficiency in cogeneration.
Proton-Exchange Membrane Fuel Cells (also called Polymer Electrolyte Membrane Fuel Cells (PEM) provide a continuous electrical energy supply from fuel at high levels of efficiency and power density. PEMs provide a solid, corrosion free electrolyte, a low running temperature of 200º F, and fast response to power demand, making them the technology of choice to date for use in fuel cell vehicles.
Direct-Methanol Fuel Cells (DMFC) are similar to PEM fuel cells because they both use a polymer membrane as an electrolyte. But these units use methanol (or alcohol in some cases) as fuel and the anode catalyst draws the hydrogen directly from the fuel, eliminating the need for a fuel reformer. This has led some to consider this the chief competitor to the PEM cell for transportation. There is currently a large effort to develop DMFCs for battery replacement in cell phones, computers, and military use. Efficiencies are about 40% running at operating temperatures of 120-190º F, with higher efficiencies at higher temperatures.
Alkaline Fuel Cells (AFC) have been used on every NASA space mission. They use potassium hydroxide as the electrolyte and require no rare metal catalyst. Companies have reported efficiencies from 52% stand-alone and as high as 70% with cogeneration. They have a low operating temperature of 140º-200º F and can start generating at temperatures down to -40º Fahrenheit. Because the electrolyte is a liquid, if circulated, it can provide excellent cooling and water management within the cell.
Solid Oxide Fuel Cells (SOFC) use solid ceramic as their electrolyte. The ceramic electrolyte must be heated to 1800o F to allow the ions to pass through the ceramic (which is about the same temperature as a home furnace). There are many advantages to the SOFC. First, no fuel reformer is needed for fuels such as natural gas and propane, as the high-temperature system creates internal reformation. Secondly, there isn't an issue of fuel contamination, as with other system types that require relatively pure hydrogen. Thirdly, there are no caustic or molten electrolytes since the SOFC uses a solid ceramic electrolyte. Fourthly, the ceramic electrolyte is cheap and does not require rare metal catalyst. Finally, this fuel cell unit produces high-grade heat and is ideal for large-scale industrial use as well as home heating and hot water use. SOFCs are 50% - 60% efficient and can reach 80% - 90% with cogeneration.
Molten Carbonate Fuel Cells (MCFC) were developed in the 1960s to run on coal-based fuels. The liquid electrolyte is carbonate cells in a mixture of lithium carbonate and potassium carbonate and is contained in a porous and chemically inert lithium-based matrix, which makes it a good ionic conductor at the fuel cells' operating temperature of 1200° F. As a high temperature technology, carbonate fuel cells are capable of "internal reforming" -- that is, hydrogen molecules are stripped from the fuel stream within the fuel cell stack itself. Because of this feature, carbonate fuel cells are capable of operating directly on virtually any hydrocarbon fuel -- e.g. natural gas, wastewater digester gas, coal gas, methanol, diesel, etc. This significantly reduces the complexity of the design and eliminates the need for external reforming components upstream of the fuel cell stack.
Carbonate fuel cells are also one of the most efficient technologies available today -- approximately 50% electrical efficiency. Recovery of the 750° F cogenerated heat stream can push overall efficiencies to 80% or higher. This technology is well-suited for applications in the 200 kW to 10 MW size range. Carbonate fuel cells are available today for commercial field trials and are expected to become broadly available in the marketplace over the next two years.
Regenerative or Reversible Fuel Cells are the combination of a fuel cell and a hydrogen generator that also has hydrogen storage capability. Run in one direction, fuel cells act like electrical generators, but when run in reverse, they generate hydrogen and oxygen. This would allow a system to generate electricity for the electric power grid during periods of high demand, and generate fuels during periods of lower demand. They will likely replace rechargeable batteries in the future. Like a rechargeable battery, small regenerative fuel cells could be recharged at any time by plugging into a wall socket. The hydrogen produced can be stored in metal hydrides for later use in stand-alone electrical generation. Since the chemical reaction capacity in a fuel cell is not used up as with chemical batteries, regenerative fuel cells have an infinite recharge life, meaning you could them buy once and never face a disposal issue. Regenerative fuel cells can use any fuel cell to generate electricity, allowing the use of the most efficient and inexpensive technology.