Fuel cells are energy conversion devices that produce electricity using a chemical process. The major difference between how a typical battery and a fuel cell convert energy is that a constant supply of fuel is fed into a fuel cell. Fuel cells can be used for many applications, including grid distribution, backup and stand-by power, automobiles, space programs, and portable electronics. They are known to be extremely reliable . Not one fuel cell in the National Aeronautics and Space Administration's (NASA) programs has ever failed. The major problem with Apollo 13, for example, was related to the oxygen tank, not the fuel cell itself .
How They Work
Fuel cells consist of an anode and a cathode, which are usually coated in platinum to best stimulate reactions between them with an electrolyte. The reaction splits electrons from the molecules of fuel at the anode, and the electrolyte only allows ions, molecules that have lost one or more electrons, to pass through.
Because individual fuel cells operate at a voltage of 1V or less (usually around 0.7V), fuel cells are linked together to obtain the desired voltage for an application, a process called stacking . An unlimited number of fuel cells can be stacked to create the voltages needed.
Fuel Cells vs. Batteries
Types of Fuel Cells
Fuel cells are classified largely by the kind of electrolyte they contain. The electrolyte determines both the fuel that can be used and the amount of heat that is released.
Alkaline fuel cells, one of the first fuel cell technologies developed, use hydrogen gas combined with pure oxygen instead of ambient air. This purification, while costly, increases efficiency. Alkaline fuel cells were first used in the Apollo space program to produce drinking water and electricity .
PEMFC (Polymer Electrolyte Membrane Fuel Cells)
The PEM fuel cell is the most popular fuel cell arrangement in transportation industry because it delivers high-power density compared to its low weight and volume, has a low operating temperature, and doesn’t produce harmful emissions. PEM-type fuel cells were used in the Gemini spacecraft. Batteries had provided spacecraft power in earlier missions, but longer flight durations required the use of fuel cells .
SOFC (Solid Oxide Fuel Cells)
SOFC are capable of converting fossil fuel to a hydrogen-rich gas, which eliminates the need to produce hydrogen externally. This attribute highly beneficial because producing hydrogen requires more energy, and hydrogen is not as readily available as other fossil fuels . The high operating temperature of SOFC makes them good candidates for cogeneration applications. Currently, SOFC are being developed for application as an on-board auxiliary power unit in vehicles .
MCFC (Molten Carbonate Fuel Cells)
MCFC also can convert fossil fuel to a hydrogen-rich gas, eliminating the need for external production of hydrogen. Because they are resistant to impurities in fuel, the fuel source does not have to be highly refined. Because MCFC have a high operating temperature, they are frequently coupled with a turbine to produce extra energy. They are used in large power plants, such as the 2 MW plant in Santa Clara, CA .
DMFC (Direct Methanol Fuel Cells)
DMFC run on hydrogen instead of methanol. Because it is a liquid, like gasoline, methanol is easier to transport and distribute with the infrastructure currently in place. Direct methanol fuel cell technology is relatively new compared with that of fuel cells powered by pure hydrogen. Research is underway to determine if DMFC are suitable for portable applications such as powering 3–20 W portable electronics .
Using fuel cells for large applications requires a large amount of on-site fuel storage. The fuel can be transported in through a pipeline or, if hydrogen is the fuel, manufactured locally. Hydrogen gas can be produced several ways. In steam-methane reforming, methane and steam create carbon dioxide and hydrogen gas. Electrolysis of water, also commonly used, involves passing a DC current through a container of water to create hydrogen and oxygen gases electrochemically. The biologic production of algae also generates hydrogen gas: feeding on special bacteria, the algae produce hydrogen gas and carbon dioxide.
Fuel cell power plants in the US are commonly used as backup or peaking plants. In 2012, for example, Toyota began construction of a 1 MW fuel cell power plant to offset the peak electric demand of the Toyota motor service headquarters, which enabled the company to sell unused power back to the grid .
Fuel cell-powered vehicles are also quieter than internal combustion vehicles because they contain many fewer parts to make noise than in an internal combustion engine that burns fuel. For the most part, the only noise from a fuel cell-powered vehicle makes is from the compressor.
Internal combustion engines require oil to operate the piston cylinder process properly. Since no moving parts are used in the production of power, fuel cell-powered vehicles do not
A potential problem with the design of fuel cell-powered automobiles is that water is needed to produce power. An engineer in the southern U.S. might not immediately identify this as an issue, but when the temperature of the air drops below 32°F the water can freeze and possibly damage the membrane and other components. This is a serious consideration in the northern half of the country where there are below freezing temperatures three or four months out of the year. Solutions to address this problem include: draining the water from the stack before shutting down the vehicle, or using new coolant systems and/or block heaters..
The efficiency of a fuel cell varies significantly depending on the type of fuel cell and the fuel it uses. Some types have been tested and improved for decades, but the testing phase for others has only just begun.
When a fuel cell is in operation, some of the input is used to create electrical energy, but a much larger portion is converted into thermal energy. Depending on the type of fuel cell, the temperature at the output would range from 80-2000°F. In a typical system this thermal energy would be wasted, but in a cogeneration system the waste heat is captured and used to heat water or air for a building. Total efficiencies of a cogeneration system can reach 85% .
Micro cogeneration, or small scale cogeneration, which is becoming popular, involves the installation of a fuel cell system in residential buildings. The electric output produced by the fuel cell offsets the amount of electricity purchased from the utility company, lowering the electric bill. The waste heat produced is used to heat water which reduces the load on the boiler. The use of fuel cells, which produce heat and power without harmful emissions, as a "green" technology fits well with current trends in our society .
Fuel cells have more advantages than the conventional combustion-based technologies currently used in many power plants and cars. Fuel cells produce smaller quantities of greenhouse gases and emit none of the air pollutants that create smog and cause serious health problems. Fuel cells using hydrogen, emit only heat and water as by-products. The Department of Energy (DOE) estimates that if just 10% of the vehicles in the U.S. were powered by fuel cells, the production of greenhouse gases would decrease by 60 tons per year, and the amount of air pollution particulates would be reduced by one million tons.
Many of the fuels used in fuel cells are renewable. For example, biogas is waste gas that can be captured from landfills or farms. Algae, when they feed on special bacteria, also create hydrogen gas (and carbon dioxide) through biologic production .
Physical sites that include fuel cells:
The College of New Jersey
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