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Fuel Cells Are Coming
Different types of fuel cells operate at different temperatures and use different fuels and different electrolytes to produce electricity. Each type of fuel cell has its strengths, limitations, and potential.
Alkaline fuel cells use an alkaline electrolyte, such as potassium hydroxide. These small fuel cells with very quick cathode reactions were originally used by NASA on space missions. They are becoming less costly and are now being used in hydrogen-powered vehicles.
Phosphoric acid fuel cells use a silicon-carbide matrix to contain a phosphoric acid electrolyte. These relatively large, heavy fuel cells are available commercially in limited quantities and are being used in hotels, hospitals, office buildings, and large vehicles, such as buses.
Proton exchange membrane fuel cells use a thin plastic membrane as the electrolyte. These relatively low-temperature cells can vary their output quickly to meet shifts in power demand. They are being developed for light-duty vehicles, buildings, and perhaps as battery replacements in other applications.
Molten carbonate salts are another electrolyte used in fuel cells. Molten carbonate fuel cells operate at very high temperatures and can use a variety of fuels. They are intended for major electric utility applications and may eventually power heavy machinery.
Solid oxide fuel cells, now in the final stages of precommercial development, use a hard ceramic material instead of a liquid electrolyte. These potentially long-lived, durable, stationary fuel cells show promise for use in industrial applications and electricity generating stations, as well as in small-scale commercial and residential settings.
Protonic ceramic fuel cells share many characteristics with solid oxide fuel cells, but the ceramic electrolyte material oxidizes fossil fuels directly to the anode, eliminating the reforming process that extracts hydrogen from fossil fuels. This new type of fuel cell has potential for large- and small-scale stationary electric power generation stations and for powering vehicles.
Direct methanol fuel cells are similar to proton exchange membrane fuel cells, but they don’t need a reformer because a catalyst in the anode draws the hydrogen directly from liquid methanol. The biggest potential for these relatively low-temperature fuel cells may be in powering tiny to midsize electronic appliances, such as clocks, cell phones, and laptop computers.
Regenerative fuel cells use a solar-powered electrolyzer to separate water into oxygen and hydrogen, creating electricity, heat, and more water. The water is recirculated into the solar-powered electrolyzer and the process begins again. NASA and several commercial companies are researching this new fuel cell technology because it could be used when fossil fuels are not available.
Zinc-air fuel cells use an air-permeable membrane and zinc to create electricity when oxygen and zinc are mixed in the presence of an electrolyte. When the fuel is used up, the system is connected to the commercial power grid and the process is reversed in about 5 minutes. These lightweight fuel cells can be used to power electric vehicles and electronic devices.
Portable battery replacement units producing up to 100 watts are available commercially at the high end of the competitive price range (figure 3). These units are much lighter and last longer than battery systems with similar output. Some manufacturers are producing stationary commercial units (figure 4). However, the cost of these large systems is substantially higher than for comparable diesel generators. Some vehicle manufacturers are producing limited quantities of fuel-cell-powered vehicles, and other manufacturers have tested fuel-cell-powered vehicles under commercial conditions.
Figure 3—The black box (foreground) attached to this commercial
video camera is a fuel cell manufactured by Jadoo Power Systems.
The fuel cell replaces much heavier batteries and is used whenever a
portable power source is needed. —Photo courtesy of Jadoo Power Systems,
Folsom, CA.
Figure 4—Steam vents from the top of a 200-kilowatt, model PC 25 fuel
cell
manufactured by UTC Fuel Cells. The unit can produce uninterruptible
power
for critical needs such as hospital life support equipment, computer
centers,
or emergency services, in addition to providing electrical power for
more ordinary uses.
—Photo courtesy of Concurrent Technologies Corp., Johnstown, PA.
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